\r\n\tIt is a relatively simple process and a standard tool in any industry. Because of the versatility of the titration techniques, nearly all aspects of society depend on various forms of titration to analyze key chemical compounds.
\r\n\tThe aims of this book is to provide the reader with an up-to-date coverage of experimental and theoretical aspects related to titration techniques used in environmental, pharmaceutical, biomedical and food sciences.
Atrial flutter is between three important atrial arrhythmias resulting remarkable morbidities including heart failure and stroke. Atrial flutter has been defined as a macro-reentrant arrhythmia around an anatomic obstacle with an area larger than 2 cm2. Fast atrial activation inside the atria produces sinusoidal flutter waves at a rate of 240–320 bpm with no baseline isoelectric, most clearly visible in inferior and V1 leads in the surface ECG. Atrial flutter and fibrillation frequently coexist and atrial flutter can degenerate into atrial fibrillation. With regard to the mechanism of flutter (reentry), this atrial tachyarrhythmia is very amenable to Radiofrequency Ablation (RFA). In this chapter, clinical aspects of atrial flutter will be discussed in detail which includes classification, clinical manifestation, ECG and electrophysiological characteristics and medical or invasive management.
Overall, the incidence of AFL in the United States is 88 per 100,000 person-years. 15% of supraventricular arrhythmias are AFL and usually coexists with AF. More than 80% of patients who undergo RFA of typical AFL will have AF within the following 5 years. The incidence of AFL in men is more than twice that of women. Paroxysmal AFL can be seen in patients with no structural heart disease (SHD), whereas chronic AFL is frequently associated with underlying SHD, such as valvular disease or heart failure. Acute AFL may happen secondary to acute disease process, such as pericarditis, pulmonary embolism, exacerbation of lung disease, following heart or lung surgery, or myocardial infarction. [1]
AFL is defined as abnormal atrial activity inside a reentrant circuit with a diameter more than 2 cm2 at a high rate of 240–320 bpm which makes a continuous oscillation without an isoelectric baseline [2]. In contrast, focal atrial tachycardia (AT) is a rapid abnormal atrial rhythm originating from a “point source” with a baseline between P waves on ECG. The most practical classification is based on isthmus versus non-isthmus dependency (Diagram 1). According to the new classification, typical AFL is a macroreentrant atrial tachycardia that usually proceeds up the atrial septum (counterclockwise or CCW), down the lateral atrial wall, and through the CTI between the tricuspid valve annulus and inferior vena cava (IVC). It is also known as “common AFL” or “CTI-dependent AFL.” When the circuit rotates in the opposite direction, it is referred to as clockwise (CW) typical AFL or reverse typical AFL (Figure 1). Clockwise AFL is observed in only 10% of clinical cases. However, the flutter wave morphology might change in the presence of underlying atrial disease, prior surgery, or previous ablation which makes the flutter wave morphology not a reliable indicator of AFL type [2, 3].
Classification of atrial flutter (see the text for discussion).
Counterclockwise typical flutter (a), clockwise typical flutter (b), atypical flutter (c), atrial fibrillation (d), atrial flutter 1:1 in a patient on flecainide (e). In tracing d, the atrial waves amplitude in V1 is changing with irregular irregular RR interval.
Atypical flutter, or “non-CTI-dependent macroreentrant atrial tachycardia,” is attributed to those flutters that do not use the CTI originating in the right (RA) or left atrium (LA) [3]. In this group, different circuits have been described, including “perimitral flutter” reentry, LA roof dependent flutter and reentry around scars from previous surgery or ablation in atria. Obviously, these flutters are not amenable to ablation of the CTI, but common AFL often coexists with these atypical reentry circuits [4].
The CTI is bounded anteriorly by the tricuspid annulus and posteriorly by the ostium of the IVC and the eustachian ridge. The width and muscle thickness of CTI are variable, from several millimeters to around 3 cm in width and depth of 1 cm roughly. The CTI is wider in the lateral portion and thinner in the central portion. The central isthmus is concave and pouch-like in 47 and 45% of patients, respectively. The subeustachian isthmus is the area between the tricuspid annulus and the eustachian ridge which ends in IVC junction. The pectinates, spare the myocardium just in atrial part of the tricuspid valve and makes the smooth portion of the CTI which is referred to as the vestibular portion. Of note, the septal part of the CTI is adjacent to the posterior extensions of the AV node as well as the middle cardiac vein [5, 6]. This anatomic proximity explains the higher risk of AV block if ablation is done in the septal aspect. Also, the smooth vestibular portion around the tricuspid valve lies very close to the right coronary artery.
The patients with flutter sometimes are asymptomatic or may present with a variety of symptoms including palpitations, dyspnea, fatigue, dizziness or reduced functional class. However, it might be the first presentation of more serious conditions like acute pulmonary embolism, acute coronary syndrome or acute pulmonary edema. The severity of symptoms closely depends on the baseline left ventricular ejection fraction (LVEF), ventricular rate during the flutter and underlying SHD. As a common scenario, the patients present with a stroke or with decompensated heart failure secondary to tachycardia-induced cardiomyopathy. AFL occurs in nearly 25% of patients with AF.
The clinical presentation will dictate acute therapeutic approach which may include cardioversion or rate control strategy. Cardioversion (electrical or chemical) is usually the initial treatment of choice. Antiarrhythmic medication such as intravenous amiodarone, sotalol, have been reported with a high success rate of chemical cardioversion. These class III antiarrhythmics prolong the refractory period leading slower cycle length which could terminate AFL. Interestingly, intravenous ibutilide has been more effective than the formers up to 76% of patients. Electrical cardioversion at a low energy of 50 J has very high success rate. Overdrive atrial pacing by a catheter in RA or in preexisting pacemaker/defibrillator is an effective alternative option in terminating typical AFL. Anticoagulation by using the same criteria as for AF, prior to cardioversion should be considered [3]. In order to control the rate, oral or intravenous atrioventricular node (AVN) blockers such as verapamil, diltiazem, beta blockers, and digoxin, can be used. However, the rate control is difficult to achieve as opposed to AF.
If AFL occurs in the context of an acute disease process, long-term rhythm control medication is usually not required once the AFL is converted and the underlying pathologic process is eliminated. However, if there is a certain substrate for AFL recurrence such as enlarged RA or scar, medical suppression of AFL can be extremely difficult. Hence, the ablation procedure with highly successful rate and low complication risk is the approach of choice for typical AFL [7]. However, medication may be tried in some situation (i.e. patient preference). Several antiarrhythmic drugs have been somehow effective in AFL suppression, including class IC (flecainide and propafenone), and class III (usually sotalol and amiodarone) antiarrhythmics. In the absence of SHD, class IC agents are the first line medication. The antiarrhythmic agents should be combined with AVN blockers to avoid the risk of rapid ventricular rates. In fact, class IC drugs have a vagolytic effect on AVN. Although the atrial flutter rate will be slowed, more proportion of these atrial impulses will be conducted through AVN (enhanced conduction), by which net ventricular rate increases [3]. As a result, rapid 1:1 AV conduction is mostly seen if Class IC antiarrhythmic medication is not combined with AVN blockers such as beta blockers (Figure 1e). As mentioned, typical AFL is very amenable to ablation but AV junction ablation and pacemaker implantation may be indicated if rhythm and rate control strategies including ablation have failed in atypical flutter. The anticoagulant policy will be implemented based on the same guideline for AF.
In “typical” counterclockwise AFL, the wave of depolarization propagates through the lateral right atrium, then travels through the CTI in a lateral-to-medial direction. The wave of depolarization arrives at the inferior part of the interatrial septum, splits and propagates caudocephalically up the septum, finally traveling across the roof to arrive lateral RA to complete the circuit. At the same time, the depolarization wave propagates from inferior septum to the lateral wall of the left atrium. Flutter waves have constant morphology and polarity with same CL. In typical AFL, they are most visible in lead V1 and the inferior leads (II, III, aVF) with a sawtooth appearance. The propagation of depolarization wave is through interatrial septum which makes it positive or negative in inferior leads in clockwise (high to low) or counterclockwise (low to high) AFL, respectively. The wave includes a slow downsloping portion, with a sharp negative deflection, followed by a rapid positive deflection, merging to the next downsloping deflection (Figure 2). Because of constant activation inside the circuit, there is no electrically silent period and consequently no isoelectric period. This is as opposed to focal AT with silent periods between focal discharges (Figure 3). However, focal AT can produce a continuous wave pattern on ECG if the atrial CL is short enough to shorten the interval between atrial depolarization [4, 8].
The typical CCW flutter ECG characteristic (see the text).
The top diagram demonstrates the atrial activation sequence correlated with typical CCW atrial flutter. The slow conduction through CTI causes the flatter portion of flutter wave. The bottom diagram shows atrial activation sequence in focal AT with silent periods causing flat isoelectric line.
During CW AFL, the positive deflections in the inferior leads are nearly equal negative deflections in inferior leads making a sinusoidal pattern which makes it somehow difficult to recognize positive from negative deflections. So lead V1 is important in order to recognize clockwise typical AFL.
The atrial rate in AFL is typically 240–320 bpm, but it might be slower if conduction is slow inside the circuit due to scars from prior ablation or surgery. Also, antiarrhythmic medication can cause conduction delay. In these instances, distinct isoelectric intervals between flutter waves may be recognized similar to focal AT. The rapid ventricular response is occasionally seen in those patients with underlying anterogradely conducting bypass tracts or secondary to high sympathetic tone (e.g., exercise, sympathomimetic drugs) which enhance AV conduction [2]. Typically, the patients with flutter present with 2:1 AV conduction but variable AV conduction and higher grade AV block (e.g., 4:1 or 6:1) or AV dissociation (slow and fixed RR interval) may occur. P wave morphology is usually of limited anatomical value for precise circuit localization in other atypical forms of AFL. If there is any doubt on the initial ECG, infusion of adenosine or carotid massage may transiently decrease AV conduction, unmasking the flutter waves. Adenosine and digoxin increase the degree of AV block, but shorten atrial refractoriness and can cause the AFL to degenerate into AF. Of note, AF is sometimes confused with AFL when the atrial activity in lead V1 might look organized (see Figure 1d).
In case of AF, the cycle length of atrial waves is less than 200 ms, the RR interval is truly irregular (with precise measurement) and atrial activation is not usually organized in the limb leads. Also, there is no true relationship between the apparent “flutter waves” and QRS complexes, best seen in lead V1, and close inspection often reveals the atrial waves do not have the constant morphology and amplitude as seen in a real atrial flutter. These instances are usually defined as “coarse AF.” In rare examples, RA is in AFL but LA is in AF in which lead V1 might show more uniform morphology but the other leads manifest characteristics of fibrillation [9].
In brief, RFA is performed by creating lesions across the critical isthmus to achieve the complete and stable bi-directional block across CTI. The procedure includes linear lesion, finding the residual conducting gaps to eliminate, and ultimately the end point is the confirmation of bidirectional block across CTI. Typically, three catheters are used for ablation of typical AFL. They include the ablation catheter, multipolar coronary sinus (CS) catheter, and a duo-decapolar halo catheter. Halo catheter is positioned around tricuspid annulus so that proximal poles positioned in upper interatrial septum and roof, mid poles positioned from high to low in the lateral RA anterior to the crista terminalis and distal poles positioned in lateral inferior at 6–7 o’clock in the LAO view in fluoroscopy. In fact, the distal poles will record the middle and lateral part of the CTI [10, 11].
It’s very crucial to distinguish between isthmus-dependent and non-isthmus-dependent flutters in order to perform curative ablation. Hence, AFL should be induced to confirm the diagnosis and make sure the AFL is isthmus dependent prior to ablation. AFL can usually be induced with programmed electrical stimulation (PES) from different locations; however, it is usually performed with pacing from proximal coronary sinus which can induce CCW typical flutter. Isoproterenol infusion (0.5–4 μg/min) may be needed to facilitate tachycardia induction. Burst pacing or atrial extra stimulus at the shorter CL/ coupling intervals will more likely induce AF which is often self-terminating. However, AF can be sustained around 10% of cases needing cardioversion in EP lab. The importance of AF induction in these patients with no clinical history of AF is uncertain [10, 12].
In typical AFL, intracardiac electrogram (EGM) shows bipolar electrograms with constant CL, polarity and morphology with sequential atrial activation characteristic of a macroreentry in which the atrial CL is very stable and less than <15% as opposed to focal AT [13]. The endocardial recordings of the interatrial septum, upper anterolateral, and CTI indicates low-voltage atrial electrical activity cover 100% of the tachycardia cycle length (TCL) (Figure 4). In focal AT, the atrial activation covers less than 50% of TCL, even if only RA atrial activation recordings are taken account. The CTI is the zone of delayed conduction required for establishing reentry. Some studies show slow conduction area is probably located in the medial part of CTI in older patients versus the lateral part of CTI in younger people [7, 14]. With consideration of depolarization propagation around the circuit in typical isthmus-dependent AFL, the atrial activation sequence is predictable. The onset of flutter waves in the surface ECG is simultaneous with activation of the septal atrial electrogram which is His atrial recording in clockwise and the atrial recording of proximal CS in counterclockwise AFL [9].
Electrocardiogram and endocardial electrogram of CCW typical flutter (b), CW (reverse) typical flutter (a) and focal atrial tachycardia (c). Arrows show the atrial activation sequence. The halo catheter (RA channel)) has 10 bipolar electrograms recorded from the distal (low lateral right atrium or RA 1–2) to the proximal (high right atrium or RA 19–20) poles of the halo catheter positioned around the tricuspid annulus. The distal pole of RA 1–2 is at 7 o’clock and the proximal pole of RA 19–20 is at 1–2 o’clock. CS 9–10 electrograms are recorded from the CS proximal positioned at the CS ostium, ABL d (distal) electrogram is recorded from ablation catheter positioned in the cavotricuspid isthmus. Green shading shows the portion of the tachycardia cycle covered by the activation; which is around 100% in atrial flutter and less than 50% in focal atrial tachycardia (see the text).
Entrainment is the essential maneuver to confirm the diagnosis. It determines whether AFL is CTI dependent [12].
Overdrive atrial pacing is performed at a CL of 10–20 ms shorter than the TCL to entrain. As shown in Figure 5, if the pacing site is farther from the reentrant circuit, it takes longer for the impulse to get the circuit and entrain the tachycardia. By definition, the entrainment is a continuous resetting of a reentrant circuit with an excitable gap by a series of stimuli with following characteristics:
During pacing at shorter CL, all P waves (and intracardiac atrial electrograms) are accelerated to the pacing rate
During pacing at progressively faster cycle lengths, progressive fusion of flutter waves (surface ECG) or intracardiac EGM occurs
The same tachycardia (same TCL and atrial activation sequence) resumes upon termination of pacing.
The right diagram illustrates the entrainment concept. The time from the stimulus site to the circuit is equal to X. The whole circuit time is equal to Y, so the time required for the stimulus to travel from stimulus site to get the circuit, turn around the circuit, returning back to the stimulus site is equal to 2X + Y. In the left tracing, TCL =200, overdrive pacing from ablation catheter positioned at CTI is performed at CL = 190 ms which entrains the tachycardia. The PPI (208 ms in this example) is measured from the stimulus to the first potential appears in ablation catheter channel. PPI-TCL = 8 ms which confirms the ablation catheter is inside the circuit at the CTI; hence, the flutter is CTI dependent.
The post-pacing interval (PPI) is the time between the last pacing stimulus that entrained the AFL and the next recorded atrial EGM at the same pacing site (same catheter through which pacing was done). Obviously, PPI will be shorter if the pacing stimulus site is closer to the circuit. Accordingly, if PPI is equal or within 20 ms of TCL, it means the pacing stimulus site is inside the circuit. In order to assess whether AFL is isthmus (CTI) dependent, an ablation catheter is placed in CTI and paced at shorter CL to entrain AFL. If PPI is within 20 ms of TCL, it indicates that CTI is in the circuit which confirms AFL is isthmus (CTI) dependent. If the pacing site is outside of the circuit, the PPI will be equal to TCL plus the time required for the stimulus to travel from the pacing site to the flutter circuit and return back to the stimulus site [12, 14, 15] (Figure 5).
Typically, a 4-mm irrigated steerable ablation catheter is used in order to deliver point-by-point RF applications across CTI. Adequacy and quality of lesions depend on proper contact, local blood flow, enough power, and tissue thickness. A guiding sheath (e.g. SR0 or ramp sheath) can help stabilizing the catheter position and prevent sliding off the line of ablation during RF application. At first, the catheter is advanced into the RV in RAO view, then is dragged back gradually until the EGM shows small atrial and large ventricular electrograms. The relative electrogram size of the ventricular and atrial signals helps to estimate the approximate location of catheter tip (e.g. the A/V ratio is around 1:4 at the ventricular side of the tricuspid annulus, and 4:1 near the IVC). The tip of the ablation catheter is finely adjusted in midway between the interatrial septum (CS Ostium as a landmark) and lateral RA lateral in the LAO view (at around 6 or 7 o’clock). The first RF application is delivered at the tricuspid annulus with small AV ratio. After each RF application lasting for 30–60 s, the atrial electrogram voltage is reduced and may become fragmented; then the catheter is dragged back around 4 mm until EGM shows new sharp atrial electrogram (not far field), and the next RF burn is delivered. This sequence is repeated until EGM shows minimal or no atrial electrograms which implies that the catheter tip has reached the IVC border [10, 16, 17].
Firstly, the eustachian ridge is a “floppy” structure that comprises the posterior border of CTI and separates the IVC from the inferior RA, and sometimes prevents complete ablation of the posterior part of CTI with simple dragging of the catheter. In such situation, the catheter tip should be curled back, in order to get access to the most posterior part of CTI and the floor of pouch created by “Eustachian ridge”(Figure 6). It is important to keep in mind that AV block can occur in approximately 1% of cases, particularly during ablation of the medial side of CTI (5 o’clock) which is close to the septum. Observation of changing variable conduction of 2:1–3:1 or appearance of relatively regular QRS complexes should warrant possibility of damage to AVN and consequent high-grade AV block. So the risk of AV block is less likely if ablation is done far away septum [18, 19]. In AFL recurrence and re-do cases, 3D mapping system and intracardiac echocardiography will be helpful to figure out the complex anatomy and hence to ablate effectively the gaps.
In some difficult cases, the eustachian ridge separates the IVC from the inferior RA and creates a pouch. In order to get access to the pouch floor to complete the line of the block, the ablation catheter should be curled back as shown on the left panel.
With respect to isthmus anatomy and the presence of potential pouches, conduction gaps frequently remain despite continuous lesions. Locating and ablating residual gaps is mandatory to achieve complete bidirectional block and to prevent AFL recurrence. The residual gaps can be detected via local electrograms including fractionated EGM potential or the isoelectric interval between double potentials [20] (Figure 7).
Identification of residual gaps in the ablation line. An interval separating the two components of double potentials recorded along the ablation line in the CTI during CS pacing, after RF ablation. The first component (potential a) is produced once the stimulus impulse reaches the one side of the ablation catheter. If there is a gap along the line of ablation, the impulse still can travel through the gap to another side of the ablation catheter, producing second potential (B). When the tip of catheter moves toward to the site of the gap, the time required for an impulse to reach the tip of the catheter on the other side gets shorter, which leads to shortening of the interval between two components. Finally, at the site of gap, the two signals will be fused and the double potentials disappear. This approach with moving the catheter through the line can detect the gap. The last diagram shows completion of block line in which the impulse should turn around the RA (far away CTI) that will obviously increase the time required for the impulse to reach spot B; thus, the interval between two components will be prolonged.
Confirmation of bidirectional block is traditionally considered as the endpoint of AFL ablation. The created lesion can recover conduction so bidirectional block should be verified with the current maneuver as the endpoint of RF ablation and repeated after 20–30 min monitoring [21].
Pacing from the medial side of the ablation line (e.g. from CS proximal) is performed and atrial activation sequence is evaluated (Figure 8). In the presence of medial to lateral block across CTI, atrial depolarization wave must propagate caudocephalically up the septum and travel down to the lateral RA to arrive at distal poles of halo catheter. So the distal poles of halo catheter are the last poles which record the atrial potentials (Figure 8b). In order to assess the lateral to medial block across CTI, pacing from lateral to the ablation line is performed (e.g. halo distal poles or tip of ablation catheter at 8:00 o’clock). In case of lateral to medial block, atrial depolarization wave will propagate superiorly up the lateral RA and travel down the interatrial septum to reach the CS ostium, recorded at proximal CS (Figure 8a). Figure 8c shows the atrial activation during CS proximal pacing prior to medial to lateral isthmus block which demonstrates the collision of the cranial and caudal right atrial wave fronts in the mid-lateral RA (RA 5–6).
Right atrial endocardial electrograms recorded during CSp pacing from distal halo catheter or RA 1–2 after ablation (a) and from CSp pacing before (c) and after (b) ablation of CTI during sinus rhythm (see the text for discussion).
CS proximal or low lateral RA is usually paced in order to measure conduction interval across CTI. Obviously, the interval is the time from the stimulus pacing artifact from one side of the isthmus (e.g. CS proximal) to the atrial electrogram recorded on the other side (e.g. distal halo poles). More than 50% prolongation of this interval or an absolute interval time of 150 ms or more is in favor of CTI block [22, 23].
This maneuver is used to assess CW and CCW block across CTI. At first, pacing is performed close to the ablation line, then pacing site is moved away from first pacing spot (Figure 9). For example, to check CCW (lateral to medial) block, the pacing is done from halo distal poles (Halo 1,2) and the time from stimulus artifact to the atrial electrogram recorded on the proximal CS is measured. Then pacing site is done farther from ablation line (Halo 3,4). If there is CCW, the measured interval will be shortened on the latter spot (Halo 3,4). When CTI conduction is intact, conduction occurs via a counterclockwise wavefront across the CTI to reach proximal CS, so the measured interval will be longer once it is paced from Halo 3,4 compared to Halo 1,2 [24].
The method of differential pacing to evaluate bidirectional CTI block (see the text).
Electroanatomical 3D mapping can also be used to confirm conduction block across CTI. For example, when CCW block is present, Halo 1,2 pacing results in an activation wavefront directing in a CW pattern and CTI immediately medial to the ablation line will be the last part in the circuit which will be activated. If CTI is intact, with pacing from Halo 1,2 the activation wavefront travels rapidly through the CTI, with the upper septum will be the last part of activation.
RF ablation of the typical AFL is relatively safe, with an average complication rate of 3% which includes mostly peripheral vascular injury (0.5%). The risk of serious complication is very low which include complete heart block (0.3%), tamponade, myocardial infarction due to damage to the right coronary artery, stroke and pulmonary embolism (0.1%). The recurrence rate of AFL has been reduced by using irrigated tip catheters and is around half of the recurrence in standard RF ablation (7 vs 14%). The occurrence of AF or atypical AFL is dramatically high at around 70% in long-term follow up [25, 26]
Atypical AFL frequently demonstrates attenuated flutter waves which help to distinguish from typical flutter. They are classified as atypical right or left atrial flutter [27] (Figure 10). Prompt identification of these AFL types will maximize the success rate of ablation.
Types of atypical flutter. Top left, Intraisthmus reentry. Top right, lower loop reentry. Bottom left, perimitral reentry. Bottom right, incisional (around ASD patch) reentry.
Lower loop reentry is a form of CTI-dependent AFL in which the circuit is around IVC. The eustachian ridge and lower crista terminalis usually cause a breakdown in wavefront conduction across CTI; consequently, impulse revolves around the IVC instead of the tricuspid annulus. It is mostly identified during 3D activation mapping [28].
The circuit of intraisthmus reentry is bounded by the medial side of CTI and CS ostium. The previous ablation at the CTI might predispose and perpetuate this reentry. The EGMs are usually similar to typical AFL but entrainment shows the lateral CTI is not inside the circuit (long PPI) whereas the medial side of CTI presents in the circuit (short PPI). The mapping of the region between proximal CS and medial CTI usually shows fractionated or double potentials which are a good target for ablation. A linear lesion across the medial CTI usually breaks the circuit [27, 28].
These circuits arise around a low-voltage area, incision, patch or scar in the lateral or posterolateral RA. These areas usually develop after the atriotomy and surgery for the congenital disease. 3D activation mapping is an excellent modality to identify this type of AFL circuits [28].
In this type of AFL, the wavefront activation propagates around the superior vena cava (SVC) and travels through a conduction gap in the crista terminalis.
The coexistence of two circuits is known as dual-loop reentry. The activation wavefront can propagate through both circuits intermittently. In practice, they are identified when the ablation of one circuit leads to change in atrial activation sequence suggesting a transition to the other circuit [27].
LA flutter often coexists with AF. It is usually secondary to AF ablation (up to 50%) or open heart surgery for the valvular disease. The central obstacle of the AFL circuits is low-voltage or scar areas in the LA detected by electroanatomic 3D mapping [29].
Whenever the flutter wave morphology in ECG is not characteristic of typical AFL, left atrial flutter must be considered. A characteristic finding in LA flutter is a dominantly positive broad deflection in lead V1. The combination of attenuated deflections in the frontal leads with a dominantly positive deflection in V1 also suggests that origin of flutter is probably in LA. Uncommonly, negative flutter deflections in the inferior leads might be seen in left AFL mimicking typical AFL (pseudo-typical flutter). However, typical AFL demonstrates positive overshoot immediately following the negative deflection. This positive deflection is as a result of inferiorly down activation of the lateral RA. Lack of this sharp positive deflection raises suspicion of atypical AFL. In the presence of low voltage areas, the electrical impulse traversing the isthmus (protected within scar) might generate low voltage potentials, demonstrated on the surface ECG as an iso-electric interval. In addition, a small portion of the atria is being activated during the silent isoelectric period. Therefore, the isoelectric interval strongly supports the presence of a slow conducting isthmus, although its absence does not exclude it. For example, if intracardiac CS electrograms coincide with an isoelectric interval, it suggests CS region may be involved in the reentrant circuit, indirectly implies the flutter might be left sided in origin. Likewise, the electrograms of CTI region coincide with the isoelectric period between the negative flutter waves in typical AFL [30, 31].
In this type of AFL, the reentrant circuit arises around the mitral annulus. 3D voltage mapping often shows low-voltage or scar areas on the posterior LA which act as a boundary of this circuit. Most of the patients have a past history of AF ablation [29].
In addition to CS and Halo catheter, the transseptal puncture is performed to insert a catheter in LA (usually irrigated tip ablation catheter) for the full study. In order to confirm LA flutter, a systematic approach is used. The first step is the exclusion of CTI dependent AFL. Coronary sinus activation from proximal to distal can suggest that AFL origin is from the RA; however, CS activation sequences are not very valuable in LA flutter diagnosis. If EGMs recorded throughout RA (i.e. atrial electrograms recorded on all Halo poles) covers more than 50% of the TCL, it is another clue for RA AFL. Another helpful maneuver is entrainment at multiple sites in the RA and comparison of their PPIs. For instance, if PPI is shorter in septum compared to lateral RA or CTI, it might be suggestive for LA flutter. In fact, the gradient of PPI in LA flutter typically is longest (more than 30 ms) in the lateral RA and remarkably shorter in the mid and distal coronary sinus. However, roof or anterior LA flutter might show long PPI in mid to distal CS [28, 29, 32]. The 3D mapping system is often necessary to perform the full activation and voltage mapping for localization and effective ablation of the reentrant circuit [33].
Atrial flutter is relatively common atrial arrhythmia with the nearly similar morbidity and mortality to atrial fibrillation. However, it’s highly amenable to RF ablation. This procedure has emerged as therapy of choice in the light of highly successful rate and low complication risk. The knowledge of the anatomy and electrophysiology of the atrial flutter circuit is essential to choosing the optimal site for elimination of reentry. An electroanatomic 3D mapping system is highly recommended to perform the full activation and voltage mapping in order to localize the circuit and critical isthmus targeted for effective ablation.
I thank my wife, Mojgan for her continued inspiration and my daughter, Diana for her tremendous support to complete this chapter. But most importantly, I want to thank my son Arshia for his great technical support and his help, editing this chapter.
None.
Since the successful exfoliation of graphene [1], a group of materials with two-dimensional structures have revived and are attracting explosive interests from a variety of fields, including transistors [2], photodetectors [3], chemical sensors, memories, and artificial synapses [4, 5]. This is benefited from the versatile properties, of 2D materials defined not only by their crystal structure (1 T, 2H, etc.) but also by their layer number, i.e., the electrical conductivity and optical bandgaps [6]. The transition metal chalcogenides (TMDs) of 1 T or 1 T’ phase usually manifest metallic behavior, while in 2H phase, they are semiconductor and can be transformed into insulator by field-effect modulation [7]. Meanwhile, monolayer MoS2, WSe2, and MoTe2 are transformed into direct band semiconductor with greatly improved photoluminescence yield compared to their indirect bulk form, rendering the further fabrication of light emitting diodes [8, 9]. The recent appearance of 2D ferroelectric materials from direct chemical synthesis or atom doping has further enriched the physical properties of 2D semiconductors [10, 11]. These rapid evolution of 2D materials with diverse physical and chemical properties motivates enduring efforts to explore various property tuning and integration strategies in functional devices, e.g., via chemical doping, alloying, or constructing heterostructures [12].
An indispensable feature of the 2D materials is their van der Waals interlayer coupling, which is weak enough compared to covalent or ionic bonding to enable mechanical or electrochemical exfoliation [13]. The exfoliated 2D materials in monolayer or few layer thicknesses can then be artificially stacked, either laterally or vertically, making heterostructures in various forms that are not possible in conventional semiconductors with 3D crystal lattice (Si, III-V, and oxides) due to the lattice mismatch. The great flexibility in assembling 2D materials thus renders unprecedented opportunity in discovering novel nanoscale transport phenomenon [14] and carrier dynamics and stimulates the exploration of 2D functional devices via deliberately designing the heterostructures. In optoelectronics, this enabled the tailoring of charge separation characteristics of photogenerated electron–hole pairs in semiconductors [15], thereby allowing innovated designs of heterostructured transistors [16, 17], tunneling diode for photodetection [18, 19], and further optoelectronic memories with float gate structures [20].
In this chapter, we first introduce the basic design of heterostructures for optoelectronics and the pick-transfer methods for their artificial assembly and then discuss the recent progress in fabricating novel 2D vdW heterostructures for functional devices. In view of the rapid progress in this field, the chapter is not intended to cover all aspects of the field but focus on optoelectronic-related application, typically photodiode and phototransistors for photodetection and optoelectronic memories that integrate both light sensing and memory function.
The interfacial energy band alignment in heterostructures governs the carrier dynamics in devices and therefore determines directly their functional performances. Depending on the relative positions of conduction band and valance band of constituting materials, there are generally three types of band alignments, including type I (straddling gap), type II (staggered gap), type III (broken gap), as illustrated in Figure 1a [21]. The different band offsets make them perform differently in optoelectronic devices [22]. In type I alignment, the bandgap of a semiconductor is located within the bandgap of another one; thus both electrons and holes tend to relax in the first narrow bandgap semiconductor. It is therefore widely used in light emitting diodes for higher light illumination efficiency by confining electron and hole pairs within the narrow bandgap semiconductor [23]. In contrary, in type II alignment, both the conduction band minimum (CBM) and valance band maximum (VBM) are higher or lower than the other, which forces electrons and holes residing in different semiconductors. The separation of electron–hole pairs in type II aligned heterostructures allows the fabrication of rectifying diodes with photovoltaic effects and is usually adopted for photoelectric detectors that transform incident light into electrical signals [3]. In the case of type III band alignment, the bandgap of a semiconductor lies outside of the other one, with its CBM lower than the VBM of the other. There is no more forbidden gap at the interface compared to the bulk semiconductor. Such type III band alignment is useful in tunneling field-effect transistors with large current density [24].
Band positions and alignments for 2D materials and heterostructures. (a) Heterostructures of type I, II and III interfacial band alignments, reproduced with permission from Ref. [21], Copyright 2016 American Physical Society. (b) Conduction and valance band positions of selected 2D materials collected from literatures.
Since the conduction band is usually related to the cations while valance band is related to the anions, designing the band offsets is traditionally mostly achieved in superlattices of semiconductor alloys with widely tuned bandgap and suppressed lattice mismatches, e.g., AlxGa1−xAs/GaAs [25]. However, by using 2D materials, the lattice mismatch between adjacent heterostructured materials is in principle eliminated due to the weak interlayer coupling via van der Waals force. Various 2D materials of different energy band structures and gaps, e.g., graphene, MXenes, black phosphorous (BP), TMDs, and hexagonal boron nitride (h-BN), can thus be artificially stacked to make multiple kinds of heterostructures [13, 26]. Figure 1b illustrates the energy bandgap position of several 2D semiconductors [21, 27, 28]. Due to the zero-bandgap characteristic of graphene, it could not be directly used for high on–off switching devices, e.g., transistors, diodes, but is often used as electrode contacts for its ultrahigh carrier mobility >10,000 cm2 V−1 s−1 [29]. Recently, other 2D semiconductors have been found as alternatives, with widely distributed bandgaps from 0.2 eV to 2–3 eV [4, 6]. The electron affinity also varies largely from 3 to <5 eV, thus rendering the possibility to make all kinds of heterostructures (types I, II, III) with different band offsets, i.e., by choosing suitable 2D semiconductors. For example, WSe2/SnS2 constitutes a type III heterostructure, while MoS2/WSe2 forms a typical type II structure. Notably, the number of stacked layers is also not limited to two but can be facilely increased for multilayer heterostructures for tunneling diodes or device encapsulation [13]. The continuously increasing 2D material family thus incubates infinite possibilities in 2D heterostructures and extremely rich functions.
The deterministic transfer of two-dimensional materials constitutes a crucial step toward the fabrication of heterostructures based on the artificial stacking of two-dimensional materials. To stack multiple 2D materials into heterostructure, one needs to transfer 2D materials into a specified position on substrate. This is usually done under an optical microscope, in which one could identify the ultrathin 2D materials through their slight color contrast with substrates. A 3D moving stage is usually equipped to fine adjust the stacking position of each layer, as indicated in Figure 2a [30]. So far, a lot of methods and processes have been developed to achieve high-quality assembly of 2D materials in devices and multilayer heterostructures. For 2D materials initially grown on substrates, e.g., graphene on copper, MoS2 on sapphire, they are usually first etched free from the substrates via polymer (typically poly(methyl methacrylate), known as PMMA)-assisted handling and wet-chemical etching processes [31]. However, the residual of PMMA and wet etching chemicals often deteriorate material performance and also degrades the cleanness of stacked interface, which can be serious in multilayered heterostructures. All-dry transfer of 2D materials is thus desired for making high-performance devices.
Setup and typical dry transfer processes for 2D vdW stacking. (a) Schematic of the experimental setup and (b) the processes employed for the all-dry transfer process, reproduced with permission from Ref. [30], Copyright 2014 IOP publishing Ltd. (c) Schematic of the van der Waals (vdW) technique for polymer-free assembly of layered materials, reproduced with permission from Ref. [33], Copyright 2013 Science.
To make a heterostructure based on vertical stacking, 2D materials can be typically exfoliated from single crystals by Scotch tape and then transferred to viscous elastomer stamp (poly dimethyl siloxane, PDMS), as illustrated in Figure 2b [30]. The transparent PDMS stamp is then used to handle the exfoliated 2D flakes. Under optical microscope, it is then aligned to a target position, e.g., the position of already transferred 2D layer. The position of the stamp is then fine-tuned in all three dimensions to approach the target substrate, until the full contact. It is then slowly peeled off from substrate leaving 2D material behind. Sometimes slight heating of the substrate is necessary to reduce the viscosity of PDMS stamp and promote the successful transfer of 2D material onto target substrate. Instead of the common used PDMS stamps, thermal release tapes can be also used as the handle [32]. Though no wet-chemical etching processes is adopted in the above procedures, the surface of PDMS is full of silane groups and may contaminate the 2D material during transfer and make the contacting substrate hydrophobic. It may therefore deteriorate the material and interface quality in device.
An improved polymer-free method was reported by Wang et al., which adopted the clean h-BN as the buffer layer to attach graphene (Figure 2c) [33]. This is based on the stronger interaction between graphene and h-BN compared to SiO2, so that elastomer stamps with h-BN layer could pick up graphene layer from substrate. Note that the graphene layer is initially transferred onto SiO2 substrate by tape exfoliation; both the top and bottom surface are free from polymer residuals due to the fresh exfoliation when peeling off the tape. Through this method, the graphene layer during all transfer processes is protected by h-BN and thus could form clean interfaces with both the top and bottom h-BN layers. The as-prepared h-BN-encapsulated graphene manifests unprecedented room temperature mobility up to 140,000 cm2 V−1 s−1, with long ballistic transport distance over 15 μm at 40 K, demonstrating the ultrahigh interface quality formed in such polymer-free transfer methods.
Recently, the pick-transfer methods have been also modified to transfer metal electrodes onto 2D materials, avoiding the interdiffusion of elements within the contact interface with 2D materials from traditional physical deposition of metal electrodes (via magnetic sputtering, thermal evaporation, etc.) [34]. Importantly, the formed electrical contact with MoS2 using different metal electrodes displayed ideal Schottky barriers defined by the work function difference between metal and MoS2, which have not been achieved in conventional Si devices. It is therefore undoubted that the versatile usages of pick-transfer methods in assembling 2D devices hold vast potential in reforming existing technologies from many aspects.
There are several kinds of photodetectors that convert incident light signal to electrical signals, including detectors that rely on photoelectric effect, pyroelectric effect, and photothermal effects. Among the various detectors, the photoelectric detectors exhibit fast response dynamics based on simple separation of electron–hole pairs and are mostly used in commercial products. The photoelectric detectors can be further categorized into photodiode and phototransistors. In photodiodes, the photogenerated electron–hole pairs are separated by the built-in electric field in space charge region, while in phototransistors, an external electric field is applied to generate photodetection gain >100% for highly sensitive detection.
In 2D heterostructures, both photodiodes and phototransistors can be built up by vdW stacking of different materials. Because of the presence of band offset at the interface, heterostructured junctions tend to enable efficient charge separation compared to homojunctions, which requires deliberate control of their p and n doping states. In this section, we discuss several typical heterostructures in type I–III band alignments and their behavior in photodetection.
To fabricate heterostructured diode, one kind of 2D material is exfoliated and transferred onto the other one. For the charge separation in vertical direction, type II band alignment is desired. However, this is not naturally obtained, especially when one adopts a narrow bandgap semiconductor for infrared applications, e.g., BP. However, since the work function of ultrathin 2D materials can be dramatically modulated by electrostatic methods, the behavior of 2D diodes was demonstrated tunable by applying gate voltages [35, 36, 37]. As illustrated in Figure 3a, BP and WSe2 form type I band alignments [38], with slight conduction band offset (~0.1 eV). When increasing the back-gate voltage from negative to positive, the WSe2 layer is tuned sequentially from p to i and n states by the injection of gate-coupled electrons and then forming, respectively, p–p, pi, and p-n junctions with the p-typed BP. Further increasing gate bias also tunes BP to n type and results in n-n junction. Accordingly, these junctions manifest different rectification ratios under gate bias. Figure 3b displays the forward and reverse channel current (at Vds = 1 V and −1 V); the different onset threshold gate voltage under forward and reverse bias results in a window of −30 V < Vg < 10 V, in which high rectification ratio is obtained by the formation of p-n junction. The widely tuned doping characteristic of 2D bipolar semiconductor thus renders feasible modification of the diode characteristics in the device via various kinds of field effects, including using ionic liquids and ion gels [39].
2D Heterostructures of different interfacial band alignments and their characteristics. (a) The type I band alignment between BP and WSe2 and (b) the appearance of various junction behaviors (p-p, p-n, n-n) under gate modulation, reproduced with permission from Ref. [38], Copyright 2017 Wiley-VCH. (c) Schematic of the type II heterostructure based on n-type MoS2 and p-type GaTe, and (d) the photovoltaic characteristic under light irradiation, reproduced with permission from Ref. [37], Copyright 2015 American Chemical Society. (e) Schematic diagram of a type III WSe2/SnS2 heterostructure and (f) its IV characteristic under dark and light illumination (550 nm), reproduced with permission from Ref. [19], Copyright 2018 Wiley-VCH.
By choosing appropriate 2D semiconductors, p-n junctions can be formed directly without gate bias. Wang et al. reported such diode based on p-typed gate and n-type MoS2 [37]. It displays apparent photovoltaic effect under light illumination, as indicated in Figure 3c. The extracted ideal factor of the junction is as low as 2 at room temperature, corresponding to Shockley-Read-Hall (SRH) recombination-dominated carrier loss during transport. So far, various kinds of p-n junctions have been made based on such type II band alignments, including BP/MoS2 [38], MoS2/MoTe2 [40], MoS2/WSe2 [41], etc. The open circuit voltage by photovoltaic effect in such type II band heterostructures is limited by the interfacial bandgap determined by the lower conduction and higher valance band. It is therefore usually less than the maximum Voc attainable in p-n junction of each component. However, an essential benefit of such heterostructure is the formation of photodiode without deliberate efforts in controlling the p and n-type doping. A strong evidence of the formation of type II band alignment is that the photoluminescence at the junction area is quenched due to the separation of electron–hole pairs at the interface. Another benefit of such type II heterostructure is based on the interlayer transition, which supports sub-bandgap photodetection [42]. For example, MoS2/WS2 heterojunction displays near-infrared response that is beyond both the bandgap limits of MoS2 and WS2 [43].
Tunneling diodes can be formed by heterostructures of type III band alignment [24]. In the case of WSe2/SnS2 heterostructure, due to the high electron affinity of SnS2, type III heterostructure is formed with direct interband transition between valance band of WSe2 and the conduction band of SnS2 [19]. The diode initially displayed high rectification ratio >104 for low dark current under reverse bias, whereas under illumination, the device exhibits dramatically increased light current by direct tunneling, resulting in high responsivity >200 AW−1 and excellent detectivity >1013 Jones. Further exploration of the kind of heterostructure using other 2D materials with different bandgap may have the potential to make high-performance tunneling photodiodes for infrared. The heterostructure of narrow bandgap BP and larger bandgap MoS2 has been used to realize multi-value inverters with high gains >150 based on gate-modulated tunneling current [44].
In addition to the two-layer stacking, multilayered heterostructures have been also developed as tunneling diodes. Figure 4a illustrates such a heterostructure based on vertically stacked graphene/MoS2/graphene [35]. Because of the work function between top and bottom graphene (due to the unidentical substrate doping effect), the multilayer displayed photovoltaic separation of electron–hole pairs under illumination, reaching a Voc~0.3 V under additional gate bias. The device also exhibits wavelength-dependent responsivity that is related to the absorption in MoS2 (as indicated in Figure 4b), demonstrating the working principle of the multilayer junction. By using graphene at both the bottom and top of the junction made by MoS2 and WSe2, Lee et al. demonstrated an efficient p-n junction at the ultimate atomic thin thickness with improved collection of photoexcited carriers [36]. Figure 4c and d illustrate the structure and IV characteristic of the device. Under illumination, tunneling-assisted interlayer recombination of the majority carriers dominates the electronic and optoelectronic behavior of the junction. Alternatively, sandwiching graphene within WSe2 and MoS2 can make a broadband photodetector up to 2 μm based on the absorption of graphene [45]. Such interlayer tunneling can be suppressed by inserting an insulating h-BN layer. Vu et al. fabricated a tunneling heterostructure based on graphene/h-BN/MoS2 in the configuration shown in Figure 4e [18]. The dark current in device is greatly suppressed by blocking direct tunneling. However, under illumination, photogenerated carriers may overcome the barrier and contribute significant photocurrent via Fowler-Nordheim (FN) tunneling. Notably, to balance the photodetection performance in terms of the responsivity and detectivity, the thickness of h-BN is optimal ~5–7 nm, as indicated in Figure 4f. The thicker the h-BN layer, the lower probability of FN tunneling and thus lesser photocurrent, while the thinner h-BN results in large dark current by direct tunneling, therefore less detectivity in photodetection.
Various kinds of vertical heterostructures. (a) Schematic of Gr/MoS2/Gr heterojunction, which displays photovoltaic separation of electron–hole pairs and (b) the extracted external quantum efficiency (EQE) under different light power and wavelengths, reproduced with permission from Ref. [35], Copyright 2013 Nature Publishing Group. (c) The schematic diagram of vertical p-n junction made by MoS2/WSe2 sandwiched within two graphene layers and (d) their IV characteristic in dark and illumination, reproduced with permission from Ref. [36], Copyright 2014 Nature Publishing Group. (e) Schematic of a tunneling diode based on graphene/h-BN/MoS3 heterostructure and (f) its photodetection performance when using h-BN with thickness within 1–22 nm, reproduced with permission from Ref. [18], Copyright 2016 American Chemical Society.
In photodiodes, the photodetection gain is limited due to the maximum attainable quantum efficiency (photon-to-electron conversion efficiency) less than unity [46]. Hence, photodiodes are less sensitive and are usually operated under reverse bias or self-driven mode without external bias. In comparison, when integrating such heterostructure into a photoconductor configuration, phototransistors can be made with high sensitivity based on the photoconductive gain and vertical photovoltaic effects. The photodetection gain originates from the separation of electron–hole pairs at the heterostructure interface, with one kind of carrier accumulated in the 2D high mobility channel, therefore yielding amplified photoconductive gains by the ratio of injected charges compared to the inherent carrier concentration in 2D channel [47]. A representative example is PbS quantum dot (QD)-sensitized graphene, in which the QDs and 2D surface are coupled by vdW interaction (Figure 5a) [48]. Upon illumination, holes are injected into graphene and transport there with dramatically increased mobility compared to QDs that have large amount of grain boundaries and surface states. In this way, ultrahigh responsivity >107 A/W has been demonstrated in such hybrid photodetectors. Notably, based on the gate-modulated Fermi level in graphene, the charge injection from PbS QDs to graphene can be extensively tailored. As indicated in Figure 5b, the attained responsivity is sensitive to the applied gate bias; under Vg = 4 V, the photoresponse gain is tuned even to zero by eliminating the interfacial charge transfer. Such widely tuned gain is potentially useful for intentionally selected sensitivity levels for a detector. However, due to the zero-bandgap nature of graphene, hybrid detectors with graphene as the channel exhibit large dark current and low detectivity. Alternatively, other 2D semiconductors, such as MoS2 and WSe2, have been also explored as the channel, yielding improved on–off ratio in detector [47, 49].
Phototransistors based on various heterostructures. (a) The schematic of PbS quantum dots sensitized graphene for infrared photodetection; (b) the back-gate-modulated responsivity of the hybrid photodetector, reproduced with permission from Ref. [48], Copyright 2012 Nature Publishing Group. dependence of the responsivity on the different wavelength. (c) Configuration of a vertically stacked BP/WSe2 heterostructure and (d) its wavelength-dependent gain and detectivity, reproduced with permission from Ref. [17], Copyright 2017 Elsevier Ltd. (d) Detectivity of various photodetector versus wavelength of the incident laser. (e) Illustration of the organic/inorganic vdW heterostructured phototransistor based on ZnPc-decorated MoS2 and (f) its photoresponse behavior, which is greatly improved compared to photoconductors that suffer persistent photoconductance, reproduced with permission from Ref. [16], Copyright 2018 American Chemical Society.
In addition to colloidal quantum dots, 2D heterostructures based on vertically stacked 2D layers can also make up phototransistors. A narrow bandgap semiconductor can be stacked on another 2D material for extended photodetection spectra. As illustrated in Figure 5c, BP is stacked on a WSe2 channel [17]. The photoexcited carriers in BP by near-infrared photons are separated by the type II interface, with electrons injected to WSe2. The amount of injected charge is related to the junction capacitance and the photovoltage built across the junction. In Figure 5d, the photodetection gain in such device reaches 102 at 1500 nm, which is considerably larger than the photodiodes (<1) by the amplification mechanism in phototransistor. Therefore, the specific detectivity of the device reaches 1010–1014 Jones at the measured wavelength (400–1500 nm) range. Longer wavelength results in low gain and detectivity due to the decrease of light absorption. Instead of BP, a lot of other 2D materials has been also explored to construct such heterostructured phototransistor, in which the photovoltaic separation of photocarriers can be used to gate the semiconductor channel and amplify the photoconductive gain.
Without complicated stacking processes, 2D vdW heterostructures can be also made by combining organic small molecules with 2D material. As illustrated in Figure 5e, Huang et al. recently reported such a vdW phototransistor based on Zinc phthalocyanine (ZnPc, a π-conjugated planar molecule)-decorated monolayer MoS2, which is achieved by simple solution treatment [16]. The formed junction displayed apparent rectification characteristic at the out-plane direction by forming type II band alignment and p-n junction. As a result, the detector displayed remarkably improved response speed and optimal responsivity (Figure 5f) with proper Al2O3 passivation. Other molecules, such as pentacene, have been also used to modify the performance of 2D semiconductors (MoS2, ReS2, etc.) in addition to response dynamics but also the response spectra [50, 51]. Considering the huge library of 2D materials and organic molecules, it is believed such hybrid heterostructure holds special promise in achieving scalable high-performance photodetections, in which using existing pick-transfer procedures is apparently challenging.
Optoelectronic memory can transform incident optical signals into stored electric charges [52]. Considering the light program signals can be free from interferences, the optoelectronic memories are particularly attractive for realizing high-throughput data storage, e.g., in parallel computing [53]. A typical optoelectronic memory is consisted of light sensing part and charge storage component, which could be feasibly realized using multilayered 2D stacking. Compared to the conventional 3D counterparts, the 2D devices have the advantages in having high on–off ratio by the ultrathin channel, the conductance of which can be feasibly modulated via slight amount of trapped charges. According to the charge trapping mechanism, in the following we describe two kinds of optoelectronic memories, based on, respectively, the charge trapping in (i) defect energy states or (ii) float gates.
The ultrathin nature makes 2D semiconductors highly suitable as the readout channel in memory, as their conductance can be modulated greatly by slight charge trapping, including by the inherent trap states in devices. In literatures, the prepared MoS2 often exhibits midgap trap states [54], and the device also suffers from interface defect states, e.g., at the interface with SiO2 [55], which may capture some charges under gate modulation by the shifted Fermi level EF (the trap states below EF are prone to be filled with electrons, while those states above EF tend to be empty). This usually results in large hysteresis in field-effect devices and different conduction states after positive and negative gate stress. However, the limited density of trap states restricted the on–off switch in memory. Lee et al. reported an improved device by introducing localized electronic states in MoS2 using tailored SiO2 substrate with functional silanol groups (Si-OH)(Figure 6a) [56], which exhibit strong polar interaction and causes local potential fluctuation in energy band. The device is composed of thin MoS2 layer on SiO2 substrate, using the back-Si as the gate. The conduction state is reset by using positive gate bias (80 V) and then programmed using light exposure under gate bias (20 V). Applying VG = 80 V fills the traps with electrons, resulting in OFFstates of channel when removing VG, while light exposure releases the trapped electrons by generating electron–hole pairs that promote the charge release. The device manifested highly linear readout charges programmed by light exposure time. However, since the trapped charges can be thermally activated to conduction/valanced band for trapped electrons/holes, the programmed states exhibit transient change of conduction states after initial program (Figure 6b), and the charge readout is slow ~seconds.
2D optoelectronic memories. (a) Schematic of a 2D memory based on MoS2 on tailored SiO2 surface and (b) the light exposure time programmed memory states in the device, reproduced with permission from Ref. [56], Copyright 2017 Nature Publishing Group. (c) An infrared memory based on vdW heterostructure of PbS/MoS2 and (d) the schematic of its band alignment, reproduced with permission from Ref. [57], Copyright 2018 Science. (e) The schematic and optical image (inset) of an optoelectronic memory based on WSe2 on h-BN, (f) shows the device conductance change during electrical erase and optical program, reproduced with permission from Ref. [58], Copyright 2018 Nature Publishing Group.
The above optoelectronic memory works under visible light excitation due to the bandgap limit of MoS2. Wang et al. reported an infrared memory using the vdW heterostructure of MoS2/PbS [57], which is sensitive to 1550 nm radiation with the sensitization of narrow bandgap PbS thin flakes epitaxially grown on MoS2 (as illustrated in Figure 6c). The charge trapping is based on the electron injection into MoS2 by the generation of large amount of photoexcited electrons in PbS under light illumination, as indicated by the energy band diagram shown in Figure 6d. However, the device exhibits low resistance change by light exposure and transient conductance variation after program, due to the eventual recombination of electron–hole pairs in dark, which drive carrier distribution to equilibrium. Also, the program speed is directly determined by photon energy and the overall incident power, as the former governs the energy of photoexcited carriers (whether it is sufficient to overcome the interfacial potential barrier to be injected into the other side) and the latter determines the number of excited carriers. Alternatively, the charge trapping in defect states in dielectric materials tends to exhibit long retention time. Xiang et al. constructed a nonvolatile memory using WSe2 transferred on insulate h-BN layer (Figure 6e) [58]. The inherent defect states in h-BN are able to trap photoexcited carriers in WSe2, therefore enabling optoelectronic memory operation. The memory is operated under the simultaneous light exposure and gate bias, thus to force the charge trapping into the midgap states of h-BN. Because of the large bandgap of h-BN, the trapped charges can hardly move, and the resulted memory exhibits long-term retention characteristics for more than 104 s. Such optoelectronic memory can be feasibly transformed into multi-bit memory, by using either the amplitude of gate bias or the light irradiation power, wavelength, and pulse number as the input (Figure 6f). However, slight temporal change of conductance is still observed due to the recombination of photogenerated electron–hole pairs in WSe2 itself. Nevertheless, the strategy has been exploited to develop artificial optoelectronic synapses, the overall weight of which is less sensitive to the single-unit device but to the average of multiple connections [59].
Instead of charge trapping in random trap states, float gate structure exhibits well-described charge trapping characteristics and long-term retention characteristics [60]. The charge trapping can also be triggered by light irradiation to the light sensing semiconductor channel or float gate. Using 2D materials, the float gate structure can be assembled by h-BN as the insulate barrier and 2D semiconductors as the channel.
Figure 7a displays the initial 2D float memory based on graphene and MoS2 separated by h-BN [61]. The device usually has the structure of a field-effect transistor but with an additional float gate inserted between the source-drain channel and the control gate. The memory behavior of the device by using MoS2 as the channel and graphene as the float gate is shown in Figure 7b. The charge trapping is based on the quantum tunneling under gate bias, which induces FN tunneling by lowering the effective tunneling barrier with trigonal potential profile in the insulate h-BN layer. Alternatively using graphene as the channel results in low on–off ratio due to the zero-bandgap nature of graphene, by what the graphene channel can hardly be turned off. Notably, the thickness of h-BN is critical for the float memory, as too thin h-BN results in direct tunneling loss of charges and poor retention behavior, while too thick h-BN is good for retention but requires high operation voltages. The optimal thickness of h-BN is ~6–10 nm. The thin thickness of h-BN enables efficient tunneling of channel conductance by the float gate potential, as indicated in the inset of Figure 7b. Instead of graphene and MoS2, many other 2D semiconductors have been explored for the float memory, including WSe2, ReS2, BP, etc. [63, 64, 65, 66]. They all displayed high on–off ratio up to 107, which is likely to benefit multi-bit storage.
Several representative 2D float gate heterostructures. (a) The schematic configuration of a float gate memory based on MoS2/h-BN/graphene (b) shows the hysteresis memory behavior using back-Si gate, and the inset depicts the MoS2 conductance modulated by float gate potential, reproduced with permission from Ref. [61], Copyright 2013 Nature Publishing Group. (c) Schematic of the semi-float gate device base on graphene/h-BN/WSe2 heterostructure for the formation of lateral diode and (d) its IV characteristic showing rectification behavior by the formation of p-n junction, reproduced with permission from Ref. [62], Copyright 2017 Nature Publishing Group. (e) A two-terminal optoelectronic memory based on vdW heterostructure of MoS2/h-BN/graphene (f) displays its IV characteristics in dark and light illumination, showing the light programmed on and off states under positive bias, and the memory states are electrically erased using large negative bias, reproduced with permission from Ref. [20], Copyright 2017 Wiley-VCH.
Because of the excellent tunability of charges in 2D channel, the float gate structure has been reformed into semi-float and two-terminal structures. Figure 7c shows a semi-float gate device with WSe2 as the channel [62], in which the graphene as float gate spans half of the channel. Thus, the charge trapping in graphene only modulates the carrier concentration in partially the overlapped region. Taking advantage of the ambipolar characteristic of WSe2, the gate region can be tuned either p- or n-doped, forming the lateral pn diodes or Schottky diodes with apparent rectification behavior (Figure 7d). A special advantage of such device is their reconfigurable device behavior on demand. The device structure can be further simplified into two-terminal structures by removing the control gate, which usually is the back-Si gate [20]. Figure 7e displays a schematic structure of such two-terminal float memory. The charge tunneling can be realized by applying enough source-drain bias as indicated in Figure 7f. Because of the nonuniform electric field in channel, the potential drop between drain and float gate is sufficient to induce charge injection into float gate. After applying negative Vds, electrons are injected into graphene, resulting in off state when reading at Vds > 0. However, shining light to the device releases the trapped charges and recovers the initial state. Thus, the memory can be electrically erased and programmed by light exposure. Due to the absorption limit of MoS2, the device is only programmable with wavelengths <650 nm. By controlling the light dose with power and duration, the device manifests 18 states, rendering potential application for multi-bit purposes. However, an essential drawback of such two-terminal device is the high power consumption during electrical erase, as high source-drain current is present compared to the negligible leakage current via gate coupling.
The various heterostructures by versatile 2D stacking have enabled the blossom of 2D optoelectronic devices. There is also an emerging of optoelectronic programmed logic elements using the flexible gate coupling in ultrathin thickness [67]. The pathway toward multifunctional 2D devices seems very promising to stimulate indispensable applications based on continuously expanding family of 2D materials.
As a summary, in this chapter, we have introduced various types of 2D heterostructures for both photodetection and optoelectronic memory, both of which extensively take advantage of the feasible field-effect modulation to the optoelectronic properties of 2D materials. In the past few years, we have witnessed the marvelous revolution of design and construction of functional devices using diverse 2D materials and feasible vdW stacking methods. The progress will undoubtedly continue given the remarkable flexibility of stacking 2D material in atomic thickness, which had been extremely challenging for 3D materials. However, one shall expect critical breakthroughs are necessary before their practical applications, especially in the large-scale fabrication of vdW devices, and the development of indispensable functions compared to the existing ones in consumer markets.
This work was supported by National Natural Science Foundation of China (Grant No. 61804059).
The authors declare no conflict of interest.
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\\n\\n2.4 The Corresponding Author (on their own behalf and on behalf of each Co-Author) will not assert any rights under the Copyright, Designs and Patents Act 1988 to object to derogatory treatment of the Chapter as a consequence of IntechOpen's changes to the Chapter arising from translation of it, corrections and edits for house style, removal of problematic material and other reasonable edits.
\\n\\n3. CORRESPONDING AUTHOR'S DUTIES
\\n\\n3.1 When distributing or re-publishing the Chapter, the Corresponding Author agrees to credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen. The Corresponding Author warrants that each Co-Author will also credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen, when they are distributing or re-publishing the Chapter.
\\n\\n3.2 When submitting the Chapter, the Corresponding Author agrees to:
\\n\\nThe Corresponding Author will be held responsible for the payment of the Open Access Publishing Fees.
\\n\\nAll payments shall be due 30 days from the date of the issued invoice. The Corresponding Author or the payer on the Corresponding Author's and Co-Authors' behalf will bear all banking and similar charges incurred.
\\n\\n3.3 The Corresponding Author shall obtain in writing all consents necessary for the reproduction of any material in which a third-party right exists, including quotations, photographs and illustrations, in all editions of the Chapter worldwide for the full term of the above licenses, and shall provide to IntechOpen upon request the original copies of such consents for inspection (at IntechOpen's option) or photocopies of such consents.
\\n\\nThe Corresponding Author shall obtain written informed consent for publication from people who might recognize themselves or be identified by others (e.g. from case reports or photographs).
\\n\\n3.4 The Corresponding Author and any Co-Author shall respect confidentiality rights during and after the termination of this Agreement. The information contained in all correspondence and documents as part of the publishing activity between IntechOpen and the Corresponding Author and any Co-Author are confidential and are intended only for the recipient. The contents may not be disclosed publicly and are not intended for unauthorized use or distribution. Any use, disclosure, copying, or distribution is prohibited and may be unlawful.
\\n\\n4. CORRESPONDING AUTHOR'S WARRANTY
\\n\\n4.1 The Corresponding Author represents and warrants that the Chapter does not and will not breach any applicable law or the rights of any third party and, specifically, that the Chapter contains no matter that is defamatory or that infringes any literary or proprietary rights, intellectual property rights, or any rights of privacy. The Corresponding Author warrants and represents that: (i) the Chapter is the original work of themselves and any Co-Author and is not copied wholly or substantially from any other work or material or any other source; (ii) the Chapter has not been formally published in any other peer-reviewed journal or in a book or edited collection, and is not under consideration for any such publication; (iii) they themselves and any Co-Author are qualifying persons under section 154 of the Copyright, Designs and Patents Act 1988; (iv) they themselves and any Co-Author have not assigned and will not during the term of this Publication Agreement purport to assign any of the rights granted to IntechOpen under this Publication Agreement; and (v) the rights granted by this Publication Agreement are free from any security interest, option, mortgage, charge or lien.
\\n\\nThe Corresponding Author also warrants and represents that: (i) they have the full power to enter into this Publication Agreement on their own behalf and on behalf of each Co-Author; and (ii) they have the necessary rights and/or title in and to the Chapter to grant IntechOpen, on behalf of themselves and any Co-Author, the rights and licenses expressed to be granted in this Publication Agreement. If the Chapter was prepared jointly by the Corresponding Author and any Co-Author, the Corresponding Author warrants and represents that: (i) each Co-Author agrees to the submission, license and publication of the Chapter on the terms of this Publication Agreement; and (ii) they have the authority to enter into this Publication Agreement on behalf of and bind each Co-Author. The Corresponding Author shall: (i) ensure each Co-Author complies with all relevant provisions of this Publication Agreement, including those relating to confidentiality, performance and standards, as if a party to this Publication Agreement; and (ii) remain primarily liable for all acts and/or omissions of each such Co-Author.
\\n\\nThe Corresponding Author agrees to indemnify and hold IntechOpen harmless against all liabilities, costs, expenses, damages and losses and all reasonable legal costs and expenses suffered or incurred by IntechOpen arising out of or in connection with any breach of the aforementioned representations and warranties. This indemnity shall not cover IntechOpen to the extent that a claim under it results from IntechOpen's negligence or willful misconduct.
\\n\\n4.2 Nothing in this Publication Agreement shall have the effect of excluding or limiting any liability for death or personal injury caused by negligence or any other liability that cannot be excluded or limited by applicable law.
\\n\\n5. TERMINATION
\\n\\n5.1 IntechOpen has a right to terminate this Publication Agreement for quality, program, technical or other reasons with immediate effect, including without limitation (i) if the Corresponding Author or any Co-Author commits a material breach of this Publication Agreement; (ii) if the Corresponding Author or any Co-Author (being an individual) is the subject of a bankruptcy petition, application or order; or (iii) if the Corresponding Author or any Co-Author (being a company) commences negotiations with all or any class of its creditors with a view to rescheduling any of its debts, or makes a proposal for or enters into any compromise or arrangement with any of its creditors.
\\n\\nIn case of termination, IntechOpen will notify the Corresponding Author, in writing, of the decision.
\\n\\n6. INTECHOPEN’S DUTIES AND RIGHTS
\\n\\n6.1 Unless prevented from doing so by events outside its reasonable control, IntechOpen, in its discretion, agrees to publish the Chapter attributing it to the Corresponding Author and any Co-Author.
\\n\\n6.2 IntechOpen has the right to use the Corresponding Author’s and any Co-Author’s names and likeness in connection with scientific dissemination, retrieval, archiving, web hosting and promotion and marketing of the Chapter and has the right to contact the Corresponding Author and any Co-Author until the Chapter is publicly available on any platform owned and/or operated by IntechOpen.
\\n\\n6.3 IntechOpen is granted the authority to enforce the rights from this Publication Agreement, on behalf of the Corresponding Author and any Co-Author, against third parties (for example in cases of plagiarism or copyright infringements). In respect of any such infringement or suspected infringement of the copyright in the Chapter, IntechOpen shall have absolute discretion in addressing any such infringement which is likely to affect IntechOpen's rights under this Publication Agreement, including issuing and conducting proceedings against the suspected infringer.
\\n\\n7. MISCELLANEOUS
\\n\\n7.1 Further Assurance: The Corresponding Author shall and will ensure that any relevant third party (including any Co-Author) shall, execute and deliver whatever further documents or deeds and perform such acts as IntechOpen reasonably requires from time to time for the purpose of giving IntechOpen the full benefit of the provisions of this Publication Agreement.
\\n\\n7.2 Third Party Rights: A person who is not a party to this Publication Agreement may not enforce any of its provisions under the Contracts (Rights of Third Parties) Act 1999.
\\n\\n7.3 Entire Agreement: This Publication Agreement constitutes the entire agreement between the parties in relation to its subject matter. It replaces and extinguishes all prior agreements, draft agreements, arrangements, collateral warranties, collateral contracts, statements, assurances, representations and undertakings of any nature made by or on behalf of the parties, whether oral or written, in relation to that subject matter. Each party acknowledges that in entering into this Publication Agreement it has not relied upon any oral or written statements, collateral or other warranties, assurances, representations or undertakings which were made by or on behalf of the other party in relation to the subject matter of this Publication Agreement at any time before its signature (together "Pre-Contractual Statements"), other than those which are set out in this Publication Agreement. Each party hereby waives all rights and remedies which might otherwise be available to it in relation to such Pre-Contractual Statements. Nothing in this clause shall exclude or restrict the liability of either party arising out of its pre-contract fraudulent misrepresentation or fraudulent concealment.
\\n\\n7.4 Waiver: No failure or delay by a party to exercise any right or remedy provided under this Publication Agreement or by law shall constitute a waiver of that or any other right or remedy, nor shall it preclude or restrict the further exercise of that or any other right or remedy. No single or partial exercise of such right or remedy shall preclude or restrict the further exercise of that or any other right or remedy.
\\n\\n7.5 Variation: No variation of this Publication Agreement shall be effective unless it is in writing and signed by the parties (or their duly authorized representatives).
\\n\\n7.6 Severance: If any provision or part-provision of this Publication Agreement is or becomes invalid, illegal or unenforceable, it shall be deemed modified to the minimum extent necessary to make it valid, legal and enforceable. If such modification is not possible, the relevant provision or part-provision shall be deemed deleted.
\\n\\nAny modification to or deletion of a provision or part-provision under this clause shall not affect the validity and enforceability of the rest of this Publication Agreement.
\\n\\n7.7 No partnership: Nothing in this Publication Agreement is intended to, or shall be deemed to, establish or create any partnership or joint venture or the relationship of principal and agent or employer and employee between IntechOpen and the Corresponding Author or any Co-Author, nor authorize any party to make or enter into any commitments for or on behalf of any other party.
\\n\\n7.8 Governing law: This Publication Agreement and any dispute or claim (including non-contractual disputes or claims) arising out of or in connection with it or its subject matter or formation shall be governed by and construed in accordance with the law of England and Wales. The parties submit to the exclusive jurisdiction of the English courts to settle any dispute or claim arising out of or in connection with this Publication Agreement (including any non-contractual disputes or claims).
\\n\\nLast updated: 2020-11-27
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The Corresponding Author (acting on behalf of all Authors) and INTECHOPEN LIMITED, incorporated and registered in England and Wales with company number 11086078 and a registered office at 5 Princes Gate Court, London, United Kingdom, SW7 2QJ conclude the following Agreement regarding the publication of a Book Chapter:
\n\n1. DEFINITIONS
\n\nCorresponding Author: The Author of the Chapter who serves as a Signatory to this Agreement. The Corresponding Author acts on behalf of any other Co-Author.
\n\nCo-Author: All other Authors of the Chapter besides the Corresponding Author.
\n\nIntechOpen: IntechOpen Ltd., the Publisher of the Book.
\n\nBook: The publication as a collection of chapters compiled by IntechOpen including the Chapter. Chapter: The original literary work created by Corresponding Author and any Co-Author that is the subject of this Agreement.
\n\n2. CORRESPONDING AUTHOR'S GRANT OF RIGHTS
\n\n2.1 Subject to the following Article, the Corresponding Author grants and shall ensure that each Co-Author grants, to IntechOpen, during the full term of copyright and any extensions or renewals of that term the following:
\n\nThe aforementioned licenses shall survive the expiry or termination of this Agreement for any reason.
\n\n2.2 The Corresponding Author (on their own behalf and on behalf of any Co-Author) reserves the following rights to the Chapter but agrees not to exercise them in such a way as to adversely affect IntechOpen's ability to utilize the full benefit of this Publication Agreement: (i) reprographic rights worldwide, other than those which subsist in the typographical arrangement of the Chapter as published by IntechOpen; and (ii) public lending rights arising under the Public Lending Right Act 1979, as amended from time to time, and any similar rights arising in any part of the world.
\n\nThe Corresponding Author confirms that they (and any Co-Author) are and will remain a member of any applicable licensing and collecting society and any successor to that body responsible for administering royalties for the reprographic reproduction of copyright works.
\n\nSubject to the license granted above, copyright in the Chapter and all versions of it created during IntechOpen's editing process (including the published version) is retained by the Corresponding Author and any Co-Author.
\n\nSubject to the license granted above, the Corresponding Author and any Co-Author retains patent, trademark and other intellectual property rights to the Chapter.
\n\n2.3 All rights granted to IntechOpen in this Article are assignable, sublicensable or otherwise transferrable to third parties without the Corresponding Author's or any Co-Author’s specific approval.
\n\n2.4 The Corresponding Author (on their own behalf and on behalf of each Co-Author) will not assert any rights under the Copyright, Designs and Patents Act 1988 to object to derogatory treatment of the Chapter as a consequence of IntechOpen's changes to the Chapter arising from translation of it, corrections and edits for house style, removal of problematic material and other reasonable edits.
\n\n3. CORRESPONDING AUTHOR'S DUTIES
\n\n3.1 When distributing or re-publishing the Chapter, the Corresponding Author agrees to credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen. The Corresponding Author warrants that each Co-Author will also credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen, when they are distributing or re-publishing the Chapter.
\n\n3.2 When submitting the Chapter, the Corresponding Author agrees to:
\n\nThe Corresponding Author will be held responsible for the payment of the Open Access Publishing Fees.
\n\nAll payments shall be due 30 days from the date of the issued invoice. The Corresponding Author or the payer on the Corresponding Author's and Co-Authors' behalf will bear all banking and similar charges incurred.
\n\n3.3 The Corresponding Author shall obtain in writing all consents necessary for the reproduction of any material in which a third-party right exists, including quotations, photographs and illustrations, in all editions of the Chapter worldwide for the full term of the above licenses, and shall provide to IntechOpen upon request the original copies of such consents for inspection (at IntechOpen's option) or photocopies of such consents.
\n\nThe Corresponding Author shall obtain written informed consent for publication from people who might recognize themselves or be identified by others (e.g. from case reports or photographs).
\n\n3.4 The Corresponding Author and any Co-Author shall respect confidentiality rights during and after the termination of this Agreement. The information contained in all correspondence and documents as part of the publishing activity between IntechOpen and the Corresponding Author and any Co-Author are confidential and are intended only for the recipient. The contents may not be disclosed publicly and are not intended for unauthorized use or distribution. Any use, disclosure, copying, or distribution is prohibited and may be unlawful.
\n\n4. CORRESPONDING AUTHOR'S WARRANTY
\n\n4.1 The Corresponding Author represents and warrants that the Chapter does not and will not breach any applicable law or the rights of any third party and, specifically, that the Chapter contains no matter that is defamatory or that infringes any literary or proprietary rights, intellectual property rights, or any rights of privacy. The Corresponding Author warrants and represents that: (i) the Chapter is the original work of themselves and any Co-Author and is not copied wholly or substantially from any other work or material or any other source; (ii) the Chapter has not been formally published in any other peer-reviewed journal or in a book or edited collection, and is not under consideration for any such publication; (iii) they themselves and any Co-Author are qualifying persons under section 154 of the Copyright, Designs and Patents Act 1988; (iv) they themselves and any Co-Author have not assigned and will not during the term of this Publication Agreement purport to assign any of the rights granted to IntechOpen under this Publication Agreement; and (v) the rights granted by this Publication Agreement are free from any security interest, option, mortgage, charge or lien.
\n\nThe Corresponding Author also warrants and represents that: (i) they have the full power to enter into this Publication Agreement on their own behalf and on behalf of each Co-Author; and (ii) they have the necessary rights and/or title in and to the Chapter to grant IntechOpen, on behalf of themselves and any Co-Author, the rights and licenses expressed to be granted in this Publication Agreement. If the Chapter was prepared jointly by the Corresponding Author and any Co-Author, the Corresponding Author warrants and represents that: (i) each Co-Author agrees to the submission, license and publication of the Chapter on the terms of this Publication Agreement; and (ii) they have the authority to enter into this Publication Agreement on behalf of and bind each Co-Author. The Corresponding Author shall: (i) ensure each Co-Author complies with all relevant provisions of this Publication Agreement, including those relating to confidentiality, performance and standards, as if a party to this Publication Agreement; and (ii) remain primarily liable for all acts and/or omissions of each such Co-Author.
\n\nThe Corresponding Author agrees to indemnify and hold IntechOpen harmless against all liabilities, costs, expenses, damages and losses and all reasonable legal costs and expenses suffered or incurred by IntechOpen arising out of or in connection with any breach of the aforementioned representations and warranties. This indemnity shall not cover IntechOpen to the extent that a claim under it results from IntechOpen's negligence or willful misconduct.
\n\n4.2 Nothing in this Publication Agreement shall have the effect of excluding or limiting any liability for death or personal injury caused by negligence or any other liability that cannot be excluded or limited by applicable law.
\n\n5. TERMINATION
\n\n5.1 IntechOpen has a right to terminate this Publication Agreement for quality, program, technical or other reasons with immediate effect, including without limitation (i) if the Corresponding Author or any Co-Author commits a material breach of this Publication Agreement; (ii) if the Corresponding Author or any Co-Author (being an individual) is the subject of a bankruptcy petition, application or order; or (iii) if the Corresponding Author or any Co-Author (being a company) commences negotiations with all or any class of its creditors with a view to rescheduling any of its debts, or makes a proposal for or enters into any compromise or arrangement with any of its creditors.
\n\nIn case of termination, IntechOpen will notify the Corresponding Author, in writing, of the decision.
\n\n6. INTECHOPEN’S DUTIES AND RIGHTS
\n\n6.1 Unless prevented from doing so by events outside its reasonable control, IntechOpen, in its discretion, agrees to publish the Chapter attributing it to the Corresponding Author and any Co-Author.
\n\n6.2 IntechOpen has the right to use the Corresponding Author’s and any Co-Author’s names and likeness in connection with scientific dissemination, retrieval, archiving, web hosting and promotion and marketing of the Chapter and has the right to contact the Corresponding Author and any Co-Author until the Chapter is publicly available on any platform owned and/or operated by IntechOpen.
\n\n6.3 IntechOpen is granted the authority to enforce the rights from this Publication Agreement, on behalf of the Corresponding Author and any Co-Author, against third parties (for example in cases of plagiarism or copyright infringements). In respect of any such infringement or suspected infringement of the copyright in the Chapter, IntechOpen shall have absolute discretion in addressing any such infringement which is likely to affect IntechOpen's rights under this Publication Agreement, including issuing and conducting proceedings against the suspected infringer.
\n\n7. MISCELLANEOUS
\n\n7.1 Further Assurance: The Corresponding Author shall and will ensure that any relevant third party (including any Co-Author) shall, execute and deliver whatever further documents or deeds and perform such acts as IntechOpen reasonably requires from time to time for the purpose of giving IntechOpen the full benefit of the provisions of this Publication Agreement.
\n\n7.2 Third Party Rights: A person who is not a party to this Publication Agreement may not enforce any of its provisions under the Contracts (Rights of Third Parties) Act 1999.
\n\n7.3 Entire Agreement: This Publication Agreement constitutes the entire agreement between the parties in relation to its subject matter. It replaces and extinguishes all prior agreements, draft agreements, arrangements, collateral warranties, collateral contracts, statements, assurances, representations and undertakings of any nature made by or on behalf of the parties, whether oral or written, in relation to that subject matter. Each party acknowledges that in entering into this Publication Agreement it has not relied upon any oral or written statements, collateral or other warranties, assurances, representations or undertakings which were made by or on behalf of the other party in relation to the subject matter of this Publication Agreement at any time before its signature (together "Pre-Contractual Statements"), other than those which are set out in this Publication Agreement. Each party hereby waives all rights and remedies which might otherwise be available to it in relation to such Pre-Contractual Statements. Nothing in this clause shall exclude or restrict the liability of either party arising out of its pre-contract fraudulent misrepresentation or fraudulent concealment.
\n\n7.4 Waiver: No failure or delay by a party to exercise any right or remedy provided under this Publication Agreement or by law shall constitute a waiver of that or any other right or remedy, nor shall it preclude or restrict the further exercise of that or any other right or remedy. No single or partial exercise of such right or remedy shall preclude or restrict the further exercise of that or any other right or remedy.
\n\n7.5 Variation: No variation of this Publication Agreement shall be effective unless it is in writing and signed by the parties (or their duly authorized representatives).
\n\n7.6 Severance: If any provision or part-provision of this Publication Agreement is or becomes invalid, illegal or unenforceable, it shall be deemed modified to the minimum extent necessary to make it valid, legal and enforceable. If such modification is not possible, the relevant provision or part-provision shall be deemed deleted.
\n\nAny modification to or deletion of a provision or part-provision under this clause shall not affect the validity and enforceability of the rest of this Publication Agreement.
\n\n7.7 No partnership: Nothing in this Publication Agreement is intended to, or shall be deemed to, establish or create any partnership or joint venture or the relationship of principal and agent or employer and employee between IntechOpen and the Corresponding Author or any Co-Author, nor authorize any party to make or enter into any commitments for or on behalf of any other party.
\n\n7.8 Governing law: This Publication Agreement and any dispute or claim (including non-contractual disputes or claims) arising out of or in connection with it or its subject matter or formation shall be governed by and construed in accordance with the law of England and Wales. The parties submit to the exclusive jurisdiction of the English courts to settle any dispute or claim arising out of or in connection with this Publication Agreement (including any non-contractual disputes or claims).
\n\nLast updated: 2020-11-27
\n\n\n\n
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I am also a member of the team in charge for the supervision of Ph.D. students in the fields of development of silicon based planar waveguide sensor devices, study of inelastic electron tunnelling in planar tunnelling nanostructures for sensing applications and development of organotellurium(IV) compounds for semiconductor applications. I am a specialist in data analysis techniques and nanosurface structure. I have served as the editor for many books, been a member of the editorial board in science journals, have published many papers and hold many patents.",institutionString:null,institution:{name:"Sheffield Hallam University",country:{name:"United Kingdom"}}},{id:"54525",title:"Prof.",name:"Abdul Latif",middleName:null,surname:"Ahmad",slug:"abdul-latif-ahmad",fullName:"Abdul Latif Ahmad",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"20567",title:"Prof.",name:"Ado",middleName:null,surname:"Jorio",slug:"ado-jorio",fullName:"Ado Jorio",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Universidade Federal de Minas Gerais",country:{name:"Brazil"}}},{id:"47940",title:"Dr.",name:"Alberto",middleName:null,surname:"Mantovani",slug:"alberto-mantovani",fullName:"Alberto Mantovani",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"12392",title:"Mr.",name:"Alex",middleName:null,surname:"Lazinica",slug:"alex-lazinica",fullName:"Alex Lazinica",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/12392/images/7282_n.png",biography:"Alex Lazinica is the founder and CEO of IntechOpen. After obtaining a Master's degree in Mechanical Engineering, he continued his PhD studies in Robotics at the Vienna University of Technology. Here he worked as a robotic researcher with the university's Intelligent Manufacturing Systems Group as well as a guest researcher at various European universities, including the Swiss Federal Institute of Technology Lausanne (EPFL). During this time he published more than 20 scientific papers, gave presentations, served as a reviewer for major robotic journals and conferences and most importantly he co-founded and built the International Journal of Advanced Robotic Systems- world's first Open Access journal in the field of robotics. Starting this journal was a pivotal point in his career, since it was a pathway to founding IntechOpen - Open Access publisher focused on addressing academic researchers needs. Alex is a personification of IntechOpen key values being trusted, open and entrepreneurial. Today his focus is on defining the growth and development strategy for the company.",institutionString:null,institution:{name:"TU Wien",country:{name:"Austria"}}},{id:"19816",title:"Prof.",name:"Alexander",middleName:null,surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/19816/images/1607_n.jpg",biography:"Alexander I. Kokorin: born: 1947, Moscow; DSc., PhD; Principal Research Fellow (Research Professor) of Department of Kinetics and Catalysis, N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow.\r\nArea of research interests: physical chemistry of complex-organized molecular and nanosized systems, including polymer-metal complexes; the surface of doped oxide semiconductors. He is an expert in structural, absorptive, catalytic and photocatalytic properties, in structural organization and dynamic features of ionic liquids, in magnetic interactions between paramagnetic centers. The author or co-author of 3 books, over 200 articles and reviews in scientific journals and books. He is an actual member of the International EPR/ESR Society, European Society on Quantum Solar Energy Conversion, Moscow House of Scientists, of the Board of Moscow Physical Society.",institutionString:null,institution:{name:"Semenov Institute of Chemical Physics",country:{name:"Russia"}}},{id:"62389",title:"PhD.",name:"Ali Demir",middleName:null,surname:"Sezer",slug:"ali-demir-sezer",fullName:"Ali Demir Sezer",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/62389/images/3413_n.jpg",biography:"Dr. Ali Demir Sezer has a Ph.D. from Pharmaceutical Biotechnology at the Faculty of Pharmacy, University of Marmara (Turkey). He is the member of many Pharmaceutical Associations and acts as a reviewer of scientific journals and European projects under different research areas such as: drug delivery systems, nanotechnology and pharmaceutical biotechnology. Dr. Sezer is the author of many scientific publications in peer-reviewed journals and poster communications. Focus of his research activity is drug delivery, physico-chemical characterization and biological evaluation of biopolymers micro and nanoparticles as modified drug delivery system, and colloidal drug carriers (liposomes, nanoparticles etc.).",institutionString:null,institution:{name:"Marmara University",country:{name:"Turkey"}}},{id:"61051",title:"Prof.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:null},{id:"100762",title:"Prof.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"St David's Medical Center",country:{name:"United States of America"}}},{id:"107416",title:"Dr.",name:"Andrea",middleName:null,surname:"Natale",slug:"andrea-natale",fullName:"Andrea Natale",position:null,profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:null,institutionString:null,institution:{name:"Texas Cardiac Arrhythmia",country:{name:"United States of America"}}},{id:"64434",title:"Dr.",name:"Angkoon",middleName:null,surname:"Phinyomark",slug:"angkoon-phinyomark",fullName:"Angkoon Phinyomark",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/64434/images/2619_n.jpg",biography:"My name is Angkoon Phinyomark. I received a B.Eng. degree in Computer Engineering with First Class Honors in 2008 from Prince of Songkla University, Songkhla, Thailand, where I received a Ph.D. degree in Electrical Engineering. My research interests are primarily in the area of biomedical signal processing and classification notably EMG (electromyography signal), EOG (electrooculography signal), and EEG (electroencephalography signal), image analysis notably breast cancer analysis and optical coherence tomography, and rehabilitation engineering. I became a student member of IEEE in 2008. During October 2011-March 2012, I had worked at School of Computer Science and Electronic Engineering, University of Essex, Colchester, Essex, United Kingdom. In addition, during a B.Eng. I had been a visiting research student at Faculty of Computer Science, University of Murcia, Murcia, Spain for three months.\n\nI have published over 40 papers during 5 years in refereed journals, books, and conference proceedings in the areas of electro-physiological signals processing and classification, notably EMG and EOG signals, fractal analysis, wavelet analysis, texture analysis, feature extraction and machine learning algorithms, and assistive and rehabilitative devices. I have several computer programming language certificates, i.e. Sun Certified Programmer for the Java 2 Platform 1.4 (SCJP), Microsoft Certified Professional Developer, Web Developer (MCPD), Microsoft Certified Technology Specialist, .NET Framework 2.0 Web (MCTS). 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