Photo-Detectors Based on Two Dimensional Materials

2D materials like transition metal dichalcogenides, black phosphorous, silicene, graphene are at the forefront of being the most potent 2D materials for optoelectronic applications because of their exceptional properties. Several application-specific photodetectors based on 2D materials have been designed and manufactured due to a wide range and layer-dependent bandgaps. Different 2D materials stacked together give rise to many surprising electronic and optoelectronic phenomena of the junctions based on 2D materials. This has resulted in a lot of popularity of 2D heterostructures as compared to the original 2D materials. This chapter presents the progress of optoelectronic devices (photodetectors) based on 2D materials and their heterostructures.


Introduction
Photodetectors are devices that sense the light and convert it into an electric current. Photodetectors are essential components of many devices that are a part of our day to day life [1][2][3][4][5]. Primarily, silicon (Si) has been a material of choice for photodetector applications. Such photodetectors are readily integrated with complementary metal oxide semiconductor (CMOS) technology. The aggressive scaling has reduced the cost of Si-based devices and expanded their range of applications. Though Si photodetectors have evolved and developed over the years. But their performance is limited by the indirect nature of the bandgap of Si. The absorption of Si is limited to the visible and near-infrared parts of the electromagnetic spectrum. Also, the indirect nature of Si′s bandgap leads to phonon generation to conserve the momentum during the light assisted transition of carriers from lower energy to higher energy. These phonons lead to scattering of the carriers and thereby reduce the efficiency of Si photodetectors. Also, Si as a material is not a good absorber of light in bulk form, further degrading Si photodetectors' efficiency. These limitations of Si photodetectors have prompted a quest in the research community for alternate materials. Two dimensional (2D) materials, among the class of novel materials for optoelectronic applications, have shown favorable characteristics. Features like direct nature and wide range of bandgap, atomically thin nature, efficient light-matter interaction, and heterostructures forming are interesting. The class of 2D materials encompasses materials like graphene, transition metal-di-chalcogenides (TMDCs), Xenes etc. 2d materials are artificially derived materials. These materials are derived from layered van der Waals solids. In van der Waals solids, the atomic arrangement is such that the constituent atoms are held together by covalent or ionic bonds giving rise to atomic layers, whereas these atomic layers are held together by van der Waals interactions. The weak nature of van der Waals forces makes it possible to cleave individual layers from these materials. It is possible to obtain a free-standing single atomic or few atomic layers via mechanical exfoliation [6,7] or liquid phase exfoliation [8,9]. Graphene, which is a single layer of carbon atoms arranged in a hexagonal manner, is regarded as the original 2D material. Over the years, it has been revealed that graphene possesses many appealing electronic, mechanical, optical and thermal properties. [10][11][12]. Interaction of light with graphene occurs over a broad bandwidth range (terahertz to ultraviolet wavelengths) because of semi-metallic/gapless nature. This makes graphene a candidate for wide spectral range photodetectors. The atomically thin nature of graphene limits its absorption coefficient [13][14][15]. Graphene absorbs only 2.3% of incident light (visible and ultraviolet), making this a primary limitation of graphene for photodetector applications. A high absorption coefficient is desirable for an optimum magnitude of photocurrent [16][17][18]. For the efficient operation of a photodetector, a longer lifetime of the photo exited carriers is desired. Graphene's gapless nature results in a shortened lifetime of photo-excited carriers, which further limits graphene photodetectors' performance. Beyond graphene, TMDCs have also attracted a lot of attention for optoelectronic applications over the past decade. One advantage of TMDCs over graphene is their semiconducting nature. TMDCs possess varied bandgaps, thus making them applicable for broadband photodetection. TMDCs can be represented by the general formula of MX2, where M represents a transition metal and X represents a chalcogenide atom. The arrangement of atoms in MX2 is such that the metal atom is sandwiched between the two chalcogenide atoms, as shown in Figure 1. TMDCs detect light at different wavelengths because of layer dependent bandgap [19][20][21]. Most of the TMDCs have a direct  nature of the bandgap, limiting the phonon scattering in TMDCs photodetectors, which leads to better efficiency [22]. 2D materials have localized electronic bands, leading to sharp peaks in the density of states (DOS) called Van Hove singularities at specific energies [22]. Generally, in 2D materials like TMDCs, these singularities reside near conduction and valence bands. This leads to an increased probability of electron-hole pair generation upon excitation with light [22,23]. TMDCs photodetectors show excellent light to current conversion with high responsivity [22]. Although TMDCs based photodetectors have shown an appealing development in their performance over the years, these devices are limited by slow response speed. Furthermore, TMDCs photodetectors are still behind the absorption efficiency of bulk Si photodetectors. Apart from these 2D materials, materials like silicene, phosphorene etc., have shown promising theoretical results as far as optoelectronic applications are considered.
Though the field of 2D materials is still developing, the early results of optoelectronic devices based on these materials are very promising. The unique properties of 2D materials have ushered in a lot of theoretical and experimental research for optoelectronic applications over the past decade or so. This has led to the proposal of numerous photodetectors based on 2D materials both theoretically and experimentally. This chapter aims at presenting an insight into the novel photodetectors based on 2D materials. Section 2 offers a discussion on photodetectionchanisms in 2D materials. Section 3 presents a discussion on photodetectors based on 2D materials and their heterostructures; Section 4 presents a brief summary of the chapter and future scope of 2D materials for photodetector applications.

Photodetection mechanisms in 2D materials
Generally, photocurrent generation mechanisms are divided into three categories, viz. photovoltaic effect, photo-thermoelectric effect, and photo-bolometric effect. In the photovoltaic effect, a built-in electric field results in the separation of the electrons and holes. This built-in electric field may be generated due to a Schottky barrier at the metal-semiconductor interface. Photodetectors working under this mechanism are called photodiodes. In the photo-thermoelectric effect, a non-uniform light source is used. This light source leads to non-uniform heating of the channel, resulting in a temperature gradient within the channel. Due to this temperature gradient, carriers move from the high-temperature region to the low-temperature region. The migration of the carriers leads to their accumulation in the low-temperature region, which results in a potential. The photo-bolometric effect is based on uniform heating of the material under illumination. This uniform heating results in a change in the resistivity of the material. This effect is directly proportional to the variation of the material's conductivity and the increment in temperature caused by light irradiation. In contrast to the photo-thermoelectric effect, the photo-bolometric effect does not drive the current but only changes the intensity of the current under external bias and illumination.Another unique mechanism observed in optoelectronic devices like photodetectors is internal photoemission (IPE). IPE involves photoinjection of electrons from an emitter/source (metal or semiconductor) into the conduction band of a collector/drain (semiconductor or insulator) in a BJT/FET. The holes are photoinjected into the valence band of the collector/drain and is called as hole photoemission [24]. In IPE, an optical excitation of electrons in the metal to an energy above the Schottky barrier is involved. These excited electrons are then transported to the conduction band of the semiconductor. The Initial theory of IPE was proposed by Fowler [25,26]. However, this theory does not take the thickness of the Schottky Light-Emitting Diodes and Photodetectors -Advances and Future Directions 4 metal layer into consideration. Over the years the original theory of IPE has been refined largely resulting in much better assessment of the performance of the devices based on this effect [27,28].

Graphene photodetectors
Graphene is regarded as the original 2D material and has a hexagonal arrangement of atoms. Graphene has a planar geometry contrary to some other 2D materials like Xenes (silicene, germanene stanene etc.). The Xenes, in general, have a buckled geometry wherein the two sub lattices of the hexagonal lattice are slightly displaced with respect to each other. Graphene can absorb light with a wavelength ranging from ultraviolet to mid-infrared [29,30]. Graphene has small optical absorption due to its atomically thin nature, limiting the photoresponsivity of the photodetectors based on it. A graphene photodetector exhibited a bandwidth of 500 GHz and a photoresponsivity of 0.5 mAW −1 [31]. A metal-graphene-metal (MGM) photodetector having asymmetric electrodes has been investigated for extended operating frequency. This device shows an external photoresponsivity of 6.1 mAW −1 .
Some of the essential advantages of graphene photodetectors are high speed, ultra-broadband frequency range, and compatibility to circuits [32]. Compared to conventional semiconductors, graphene photodetectors show low photoresponsivity, which proves to be a significant drawback of such photodetectors. To overcome this and the other drawbacks, some techniques have been proposed to improve graphene photodetectors' optical absorption. For example, the use of nanostructured plasmonics leads to enhanced light concentration in the device via plasmonics resonance [33,34]. This helps in improving the local electric field [33,34]. Apart from enhancing the quantum efficiency, the plasmonics can also help in achieving multicolor detection [35]. A graphene photodetector possessing plasmonics nanoantennas sandwiched between two graphene layers shows a quantum efficiency of up to 20%. Though this method may offer quantum efficiency improvements, it reduces the device's operational bandwidth as the nanostructures' resonance determines the working wavelength in these systems.
Another method to improve graphene photodetectors' photoresponsivity is to integrate quantum dots with graphene [36]. The photoresponsivity and photodetection gain of such a device are 10 7 AW −1 and 10 8 , respectively. The presence of quantum dots in this device helps the photo-excited carriers (electrons or holes) to reach the graphene sheet while trapping the opposite type of carriers (holes or electrons). This leads to a phenomenon known as field-effect doping. Graphene photodetectors using PbS quantum dots have also been fabricated [37]. The device portrays a photoresponsivity of 10 7 AW −1 . Graphene-quantum dot photodetectors are limited by factors like low operational speed and low operating bandwidth.
Another method to improve the photoresponse in graphene photodetectors is to use micro-cavities [38][39][40][41][42]. Such photodetectors are characterized by high speed, high efficiency, ultra-wide bandwidth and high photoresponsivity. The disadvantage of using micro-cavities is that the device's dimensions are relatively large compared to traditional photodetectors [41].

Molybdenum disulfide (MoS 2 ) photodetectors
MoS 2 in its monolayer form has exciting properties like high carrier mobility 200 cm 2 V −1 s −1 [ [45], strong light-matter interaction [8,44], mechanical flexibility, chemical stability and ease of processing etc. Such exciting features of MoS 2 in its monolayer and few-layer forms make it the most widely studied 2D semiconductor for optoelectronic applications. A photodetector having a typical field-effect transistor (FET) configuration was first reported by Yin et al. [46]. The device comprises of a mechanically exfoliated monolayer of MoS 2 monolayer nanosheet as the effective region. The device shows a unique response with a cut-off wavelength of 670 nm. The cut-off wavelength is consistent with the bandgap of MoS 2 in its monolayer form (1.8 eV). The maximum responsivity of this device is 7.5 mAW −1 along with a response speed of 50 ms. A similar photodetector/phototransistor was reported by Lopez-Sanchez et al. [47]. Again, this device is based on an exfoliated MoS 2 monolayer but has an improved responsivity of 800 mAW −1 and a cut-off wavelength of 680 nm [47]. The model of the device is shown in Figure 2. The improvement in the device performance is attributed to improved mobility of the carriers, quality of the contacts and positioning technique. Apart from improved responsivity, the device portrays a low noise equivalent power (NEP) of 1.5 × 10 −15 WHz −1/2 . Such a low value of NEP is associated with a low value of dark current.
Furthermore, the dark current in this device is limited by the bandgap of MoS 2 , which reduces the role of thermally excited carriers. However, the device is relatively slow in its response time, which is of the order of several seconds. Though the response time can be reduced (to 0.6 s) by using short pulses on the gate terminal to remove trapped charges, the response time is still considerable compared to other devices [46] [50].
The properties of 2D MoS 2 are distinctly dependent on the method of preparation. Zheng et al. reported a phototransistor based on chemical vapor deposition (CVD) grown MoS 2 [51]. This device has a maximum responsivity of 2200 AW −1 in vacuum operating at a wavelength of 532 nm. The same device shows a responsivity of 780 AW −1 in air. The cause for such a decrease in responsivity is the adsorbates. Due to the large surface-to-volume ratio of MoS 2 , many adsorbates migrate from ambient air to the surface of MoS 2 and the MoS 2 /substrate interface. These adsorbents act as p-type dopants, leading to carrier scattering and degraded carrier mobility and responsivity in air. The photoresponse could also get affected (decreased) as the adsorbents may act as recombination centers for photoexcited carriers [52].
Perea-Lopez et al. have also fabricated a photodetector based on CVD-grown MoS 2 monolayer [53]. The reported device shows a relatively lower responsivity of 1.1 mAW −1 at an illuminating wavelength of 514.5 nm [53]. Such a considerable variation in the two devices' responsivity shows the significant role of contact resistance in these devices. Another study has put CVD-grown few-layer MoS 2 to use for a photodetector [54]. The performance of the device has been evaluated under harsh conditions with a wavelength of 532 nm [54]. Even at 200°C, the device portrays a photocurrent to dark current ratio of 10. Photodetectors based on MoS 2 employing other methods of synthesis like liquid exfoliation [55], solution synthesis [56] and magnetron sputtering [57] have also been reported. As compared to mechanically exfoliated and CVD grown MoS 2 based devices, these devices show degraded values of responsivities.
In photodetectors, based on monolayer and bilayer MoS 2, both photoconductive and photogating effects were observed to contribute to the photocurrent [58]. Different response times were observed for the two effects, respectively, making it possible to identify their independent contribution to the photocurrent. The photogating effect shows an obvious dependence on the gate voltage and is a slow process. The slowness of this effect comes from the longer lifetime of the trapped charges at the MoS 2 -SiO 2 interface. In contrast, the photoconductive effect has a negligible dependence on the gate voltage and is a fast process. The fast response of the photoconductive effect arises from the mid-gap states due to structural defects in MoS 2 . The photoconductive response can be studied independently by varying the illuminating light faster than the photo-gating effect.
In view of the average performance of MoS 2 photodetectors, several techniques have been proposed to improve their performance [59][60][61][62][63][64][65][66]. One such technique proposed by Leu et al. involves micro-patterning and localized modification of the MoS 2 layer [59]. The device is operated at an illuminating wavelength of 532 nm. The local modification is achieved by surface oxidation and oxygen doping. A photodetector based on such a modified MoS 2 layer shows improved photoresponse with a responsivity increase of several folds [59]. Kwon et al. proposed a photodetector based on multilayer MoS 2 with a bottom gate configuration [60]. As compared to previously reported global gate counterparts, the device shows much-improved photocurrent [49,60,67]. The purpose of a bottom gate in such a device is to impose a large tunnel barrier at ungated channel regions, which helps accumulate holes, thereby reducing the potential barrier for free electrons. Once the potential barrier is reduced, there is an increase in the electron depletion region's thermionic current. Furthermore, photocurrent improvement in the accumulation region arises due to decreased tunnel barrier for photoexcited holes. Also, the dark current is suppressed because of the series resistance from ungated areas. Consequently, the responsivity shows huge improvements and attains a value of 342.6AW −1 . Kufer et al. fabricated a MoS 2 photodetector, wherein HfO 2 encapsulates the MoS 2 layer. Upon encapsulation, it was seen that the electronic and optoelectronic properties of multilayer MoS 2 photodetector improved [61]. The encapsulated MoS 2, along with negligible hysteresis in the transfer characteristics, showed an enhanced n-type behavior. Encapsulation decreases the number of surface adsorbents, which eventually leads to improved performance. Encapsulation results in an increase in the mobility of carriers and a decrease in the contact resistance. These two effects, in combination, give rise to an increased response speed and responsivity. The device's responsivity can be tuned by the gate voltage and ranges from 10 to 10 4 AW −1 .

Other TMDCs photodetectors
Apart from MoS 2 , other TMDCs have been utilized for photodetector applications. These include MoSe 2 , WS 2 , WSe 2 , MoTe 2 , ReS 2 and ReSe 2 . This section presents photodetectors based on these materials.
Like MoS 2 , monolayer MoSe 2 has several alluring properties, such as a direct bandgap of 1.5 eV [68], enhanced photoluminescence (PL) [69] and considerable binding energy of excitons [70]. Improvements in the synthesis of MoSe 2 via mechanical exfoliation [71,72] and CVD methods [73][74][75] [76,77]. MoSe 2 monolayers for the phototransistors were prepared via CVD methods. The responsivities of the phototransistors are of the order of mAW −1 , which is lower than the CVD-grown MoS 2 monolayer counterparts by a few orders [51]. However, if the density of the charge impurities and defects are reduced, an improved photoresponse of the order of tens of milliseconds is expected. The responsivity of MoSe 2 based devices can be improved by using a CVD-grown multilayer MoSe 2 [78]. But the improvement comes at the cost of degraded response speed [78]. A phototransistor based on a few-layer MoSe 2 has been fabricated by Abderrehmane et al. [72]. MoSe 2 layers were obtained by mechanical exfoliation methods [72]. This device has a response time of tens of milliseconds and a responsivity of 97.1 AW −1 operating at a wavelength of 532 nm.
Photodetectors based on monolayer and few-layer WS 2 obtained via different synthesis methods have been reported [79,80]. The photoresponse of CVD-grown few-layer WS 2 has been studied by Parea-Lopez et al. [81]. The photoresponse reportedly shows a high dependence on photon energy [81]. The responsivity and response speed of the device are reported to be 92 μAW −1 and 5 ms, respectively at a wavelength of 457-647 nm. The dependence of multilayer WS 2 devices' responsivity was observed to depend on the surrounding gaseous environment by Huo et al. [82]. The responsivity shows an increase when the environment changes from vacuum (tens of AW −1 ) to NH 3 (884 AW −1 ) at a wavelength of 633 nm. The increased responsivity is a consequence of the charge transfer between the NH 3 gas molecule and WS 2 . The doping level of WS 2 gets modified by the charge transfer, which eventually increases the lifetime of photoexcited carriers and hence the responsivity. Another study conducted by Lan et al. showed a similar surrounding dependent performance of WS 2 devices [83]. The device showed a decrease in its responsivity from 18.8 mAW −1 in vacuum to 0.2 μAW −1 in air.
Monolayer and few-layer WSe 2 has also been studied for photodetector applications. Zheng [85]. The device exhibits the responsivity and response speed of 7AW −1 and 10 μs, respectively at an illuminating wavelength of 532 nm. Other reports involving graphene contacts and doping of a few-layer WSe 2 have been observed to improve the performance of WSe 2 photodetectors [66,86,87].
A newly introduced 2D material, MoTe 2 has excellent electronic and optoelectronic properties, due to which it has received a lot of attention recently [88][89][90]. Yin et al. have reported a phototransistor based on exfoliated few-layer MoTe 2 [91]. A study of the effect of different metal contacts on the electrical properties of the MoTe 2 phototransistor is presented. The device attains a responsivity of 2.56 × 10 3 under optimum conditions under an illumination of 473 nm laser.
Re-dichalcogenides are different from the majority of other layered TMDCs due to their high crystal symmetry. ReS 2 and ReSe 2 , in their distorted 1 T in-plane structure are anisotropic semiconductors [92]. The electrical, mechanical, and optical properties of these materials are extremely anisotropic, rendering these materials interesting for optoelectronic and electronic applications. The bandgap and carrier mobility of ReSe 2 was found to be dependent on the layer thickness by Yang et al. [93]. This allows modification of the electronic and optoelectronic properties of ReS 2 devices. A monolayer ReSe 2 phototransistor has an exceptional photoresponse with responsivity and response time of 95 AW −1 and tens of milliseconds, respectively [93]. The operating wavelength for the device is chosen to be 633 nm. Just like MoS 2 and WS 2 devices, the photoresponse of ReSe 2 devices is also found to be dependent on the surroundings [50,51,82,83,94]. The charge transfer between the surrounding gas and ReSe 2 consequently affects the device performance. This charge transfer alters the doping in ReSe 2 along with the carrier lifetime [94]. One way to avoid this dependence of performance on surroundings is to use encapsulation or passivation. Though the ReSe 2 photodetectors/phototransistors show promising results but an obvious disadvantage of these devices is that the current after removing the illuminating light can not return to dark current levels. This disadvantage is a consequence of the slow recombination rate of the photoexcited carriers. However, this issue may be solved by applying short pulses at the gate terminal to reset the device [36].
Because of the anisotropic crystal structure, ReS 2, in particular, can be utilized to detect polarized light [95]. The model of one such photodetector is shown in Figure 3. The responsivity of ReS 2 photodetectors can be largely improved up to the levels of 3.97 × 10 3 to 1.18 × 10 6 by electron doping [96] under illumiation of a 1064 nm laser. Besides improved responsivity, the device portrays a broad range of wavelength detection and fast response speed of the order of tens of milliseconds. Significant enhancement in both the electronic and optoelectronic properties of ReS 2 via O 2 plasma treatment was observed by Shin et al. [97]. The device exhibits a high responsivity of 2.5 × 10 7 AW −1 at a laser illumination of 405 nm, which is the highest obtained for a 2D semiconductor based back gated photodetector. Such a high responsivity is a consequence of large thickness (30 nm) and direct bandgap of ReS 2 layers. The response time is observed to be inversely proportional to the plasma treatment duration. Prolonged plasma treatment leads to the formation of trap states within the bandgap of ReS 2 . Such trap states result in enhanced recombination rates of photoexcited carriers, which consequently reduce the response time.
In summary, TMDCs photodetectors/phototransistors show a widely varying performance. Responsivities and the response times range from 10 −7 AW −1 to 10 7 Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org /10.5772/intechopen.95559 and 10 −5 to 10 3 , respectively. Generally, trap states affect the performance of TMDCs photodetectors. An increase in responsivities is observed at the existence of the trap states in TMDCs and/or at TMDC-dielectric interfaces. However, the response speed is found to decrease because of these trap states. Other factors that affect the TMDCs photodetectors/phototransistors are synthesis methods, number of layers, contact resistance and surrounding environment.

Black phosphorous photodetectors
Phosphorous, in its elemental nature, can exist in many forms. One such form of phosphorous is called black phosphorous (BP). With a formation energy of −395 KJmol −1− black phosphorous is a thermodynamically stable form of phosphorous at room temperature. Black phosphorous is similar to graphite in its appearance, properties and structure. Black phosphorous sheets have a puckered geometry [98]. Black phosphorous was first successfully exfoliated in 2014 and has received a lot of attention since then [99,100]. In its monolayer form, the phosphorous atoms form covalent bonds with three adjacent atoms, which results in a wrinkled honeycomb structure. The corresponding layers are held together by van der Waals forces [101]. Unlike graphene, black phosphorous is a semiconductor with a direct bandgap. Due to its strong anisotropic interaction with electrons and photons, black phosphorous is a strong candidate for electronic and optoelectronic applications.
The bandgap's direct nature in black phosphorous makes it easy for the carriers to transit to excited states, as there are negligible chances of phonon scattering [102]. The photoelectric characteristics of a black phosphorous FET were studied by Buscema et al. [103]. Figure 4 shows the model of the device. The device operates at a wavelength ranging from visble to NIR part of the spectrum. The device shows an On/Off ratio of 10 along with an electron mobility of 0.5 cm 2 V −1 s −1 . Wavelength ranging from visible to NIR results in a photocurrent generation in the proposed device. The responsivity exhibits a typical increase with a decreasing wavelength and attains a maximum value of 4.8 mAW −1 . Chen et al. used a sandwich of hBN-BP-hBN to demonstrate a photodetector with a widely tuneable infrared wavelength range [104]. The device shows an absorption of 3% at a wavelength of 3.4 μm, and the absorption of the device was observed to decrease with increasing wavelength. Furthermore, it was observed that the light absorption decreases with an increase in vertical electrical bias. Due to the vertical bias, the bandgap shrinks, giving rise to an increase in carrier concentration. The high carrier concentration results in decreased photo-carrier lifetime and degraded performance of the device. The hBN layer aims to prevent the black phosphorous from oxidation and provide a clean interface.
One of the primitive methods to improve the performance of TMDC photodetectors is to use doping. Accordingly, Keng et al. demonstrated an n-type and p-type black phosphorous photodetectors [105]. The concentration of the dopants was found to be dependent on the thickness of the black phosphorous layer. The device shows a responsivity of 1.4 × 10 4 AW −1 for a device with a black phosphorous thickness of 10 nm [105].
Using a transparent substrate opens up the possibility of novel device designs. Miao et al. have fabricated a photodetector based on multilayer black phosphorous on polyimide film substrate [106]. The device shows a responsivity of 53 AW −1 . It is observed that when the device is illuminated by infrared light, enhanced scattering of the carriers with the phonons occurs, which eventually degrades the carrier mobility and the performance of the device. However, such behavior is not observed when a SiO 2 /Si substrate is used instead of polyimide film.

Photodetectors based on 2D-heterostructures
The ever-growing evolution and development of 2D materials have led to the formulation of 2D van der Waals heterostructures. Based on these heterostructures, several photodetectors have been reported recently. Apart from their high degree of integration, these devices exhibit excellent performance. The electronic structure and properties induced between these 2D heterostructures' layers show promising characteristics as far as electronic and optoelectronic applications are concerned. 2D heterostructures/heterojunctions are essential building blocks of modern  electronic devices [107]. The band structures of the constituent 2D materials of these heterostructures undergo considerable changes due to electrostatic interactions. Xue et al. have fabricated a MoS 2 /WS 2 vertical heterostructures based photodetector [108]. Mo, S and WO 3 were used to prepare the MoS 2 /WS 2 heterojunction. The device shows a high rectification along with a considerable responsivity of 2.3 AW −1 . The characteristics of the device are evaluated at an illuminatring light of 450 nm. The interfacial built-in electric field prompts the separation of the photogenerated carriers [109]. On transferring the heterojunction to the polydimethylsiloxane (PMDS) substrate, a decrease in photocurrent is observed due to trapping states between the heterojunction and the substrate [47]. Duan et al. demonstrated a heterojunction diode based on WSe 2 /MoS 2 heterojunction [110]. The heterojunction was obtained by transferring the exfoliate MoS 2 to a physical vapor deposition (PVD) grown WSe 2 monolayer. A significant rectification ratio, along with high external quantum efficiency (EQE), was observed at an operating wavelength of 514 nm. It is noteworthy to mention that the EQE of the device is much higher than what is achieved in a lateral doped WSe 2 p-n homojunction [111]. Such a behavior is a consequence of the much better charge separation at the vertically stacked junction interface. Peng et al. have also reported a heterojunction between MoS 2 and WSe 2 [112]. The MoS 2 /WSe 2 heterojunction is obtained by mechanical exfoliation and transfer methods. A high charge transfer of 99% from WSe 2 to MoS 2 is observed in a very short time of 470 fs [112]. The device shows promising characteristics for sub-picosecond applications.
Apart from the semiconducting materials based heterostructures, graphene has also been utilized for heterostructures formation. Graphene may not be suitable for photodetector applications independently due to its zero bandgap and high light transmittance. Yu et al. formulated a photodetector based on MoTe 2 /graphene heterostructures [113] as shown in Figure 5. MoTe 2 multilayer serves as a light active material in the said heterostructure, and graphene monolayer serves as an efficient transport path for photo-excited carriers. The heterostructure shows better performance as compared to individual graphene and MoTe 2 based devices. MoTe 2 /graphene photodetectors work on the principle of photogating effect. Due to this photogating effect, electrons are trapped in localized states of MoTe 2 and holes are shifted towards the graphene layer. The high carrier mobility of graphene allows for a quick extraction of the holes injected into the graphene layer. This results in an enhanced photocurrent in the device. The device shows exceptional values of photoconductive gain and responsivity.
Britnell et al. demonstrated a photodetector based on the heterostructures of a few-layer TMDCs and graphene [114]. The device's performance depends on the encapsulation of one or more layers of TMDC sheets with graphene. The device has a sandwich structure wherein the TMDC photoactive layer is encapsulated between the top and bottom graphene electrodes. Because of the transparency of graphene, the illuminating light can reach efficiently to the TMDC layer. An appreciable photocurrent is observed when the illuminating light impinges on the overlapped regions of graphene and TMDC. The direction of the photocurrent aligns with the direction of the built-in electric field resulting from the gate voltage. This allows to modulate the photocurrent through gate voltage. Due to graphene, the extraction of charges is swift, thus reducing the recombination rate of photo-excited carriers. A similar structure is reported by Duan et al. as well [115]. The device consists of a vertical sandwich of graphene-MoS 2 -graphene heterojunction. Similar to the device reported by Britnell et al. [114], the top and bottom layers of graphene act as electrodes, whereas the middle MoS 2 layer acts as the barrier layer. Upon illuminating the MoS 2 layer, the electron-hole pairs get separated asymmetric potentials at the graphene/MoS 2 interface, which leads to an appreciable photocurrent [115]. A graphene-WSe 2 -graphene heterostructure based photodetector is reported by Massicotte et al. [116]. The heterostructure is packaged with hBN layers. The device exhibits an ultra-fast response time of 5.5 ps.
Apart from graphene, another novel material called silicene has received a lot of attention in recent years. Silicene, regarded as the 'silicon version of graphene,' also has a hexagonal structure [117][118][119]. Silicene is the single-layer version of graphene, having the constituent Si atoms arranged in a hexagonal form via covalent bonds [22,23]. Silicene shares many properties of graphene, like zero bandgap, high mobility of carriers and the presence of a Dirac cone in its band structure. Apart from these excellent electronic properties, one advantage of silicene over graphene is its expected integration with the present state of the art Si-based technology. Kharadi et al. have proposed a photodetector based on silicene/MoS 2

Figure 6.
Model of Si/MoS 2 heterostructure based photodetector [22]. heterostructure [22]. The model of the device is shown in Figure 6. Due to the high mobility of carriers in silicene, it is used as a high-velocity transport path for the photo-excited carriers. Illuminating the device's active region with a light of 650 nm results in the electron-hole pair generation. The electron-hole pairs are separated at the silicene/MoS 2 heterostructure interface due to the built-in electric field generated by a combined effect of charge transfer between silicene and MoS 2 and the gate voltage. Apart from an appreciable photoconductive gain of 2.5 × 10 11 , the device exhibits considerable values of responsivity (5.66 × 10 5 AW −1 ) and detectivity (4.76 × 10 10 Jones). Table 1. presents the characteristics of the optoelectronic devices based on different 2D materials. In general it can be seen that the light sensitive devices based on 2D materials have shown a steady increase in the performance over the years. Depending on the bandgap of the material used, the photosensitive device can be used in different wavelength regions of the electromagnetic spectrum.

Summary
This chapter has summarized the present advances of the photodetectors based on 2D materials. With the ongoing research on 2D materials and their heterostructures, the class of 2D materials may mature to a large extent as far as electronic applications in general and optoelectronic applications, in particular, are concerned. Despite the promising results, certain gaps need to be bridged for the swift development of 2D material-based devices. First, convenient and costeffective methods for the synthesis of high-quality 2D materials should be explored and developed. Second, several new properties of 2D materials are yet to be fully explored and understood. An exhaustive effort should be focused on exploring and understanding these properties. Third, more effort should be made to formulate the application-specific heterostructures of 2D materials. The electronic and optoelectronic applications may receive a heavy push upon concurrent improvements in material growth processes and fabrication methods.
The future applications of 2D materials depend on effective integration with the present Si-based technology. Materials like silicene, germanene etc., have brought a fresh breath to 2D materials' integration with Si-based technology. Though high-performance optoelectronic applications of 2D materials have been realized, there is still a lot of room for improvement. In general, one may not be surprised if wide-spread 2D material based applications are seen in the commercial market in the near future.
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