Self-assembled monolayers (SAMs) are well-oriented molecular structures that are formed by the adsorption of an active site of a surfactant onto a substrate’s surface. Aromatic SAMs were used to modify anode/hole transport layer interface in order to achieve preferable barrier alignment and charge carrier injection from anode to an organic-based thin film material. Other functions of SAMs include current blocking layers or moisture penetration blocking layers, dipolar surface layers for enhanced charge injection, and modification of work function of a material such as graphene acting as a spacer to physically separate and electrically decouple it from the substrate. Additionally, SAM modification of graphene leads to its electronic passivation at layers’ edges, elimination of defects, and enhanced adhesion and stability. The surface modification with molecules capable of forming SAM is a fast, simple, low-cost, and effective technique for the development of novel materials especially for the production of electronic devices. The ability to modify its properties by SAM technique has opened up a wide range of applications in electronic and optoelectronic devices.
- self-assembled monolayer
- surface modification
- interface modification
- optoelectronic devices
The development of more efficient, multifunctional, and miniaturized electronic devices requires a continuous research in the area of suitable materials to meet the demand of the modern society. Self-assembled monolayers (SAMs), which are arranged spontaneously on surfaces of the substrate due to the chemical or physical interactions of molecules with a substrate, have gained considerable attention in the past decade. SAMs opened a way toward the miniaturization in microelectronics as well as tailoring the properties of surfaces such as graphene [1, 2, 3, 4, 5, 6].
This review focuses on researches committed over the past decade about the modification of graphene via SAM technique. In Section 2, a brief overview of the modification methods of graphene with SAMs was given. In Section 3, the applications of graphene modified by SAMs are summarized and discussed.
2. Modification of graphene with self-assembled monolayers
Graphene is a good candidate material for many applications due to its unique optical, mechanical, and electronic properties [7, 8, 9, 10]. Pristine graphene is a zero band gap material, chemically inert, and also insoluble in many organic and inorganic solvents [11, 12, 13]. Therefore, it cannot be used in many applications. In order to achieve commercial viability and provide to the specific needs of device applications, it is necessary to control the electrical properties of graphene without damaging its intrinsic properties. To fully utilize its potential, the surface and electronic structures of graphene have been modified via functionalization (post-synthesis) or doping (during in situ synthesis) methods [14, 15, 16]. Substitutional doping of graphene occurs during the synthesis by covalently bonding the chemical groups and thereby transforms carbon hybridization from sp2 to sp3 [17, 18, 19]. On the other hand, functionalization of graphene is a physical process involving the addition of functional species onto graphene sheets to manipulate van der Waals interactions occurring effectively among the graphene sheets. In the substitution process, electrons are exchanged between dopant molecules bonded onto the graphene surface and graphene. Surface charge transfer depends on the relative energy of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) of the dopant with respect to the Fermi level of graphene [16, 20, 21, 22, 23]. Many different chemical substances have been studied for the functionalization of graphene, and SAM technique is one of them. An ultrathin layer, SAM, can be uniformly formed on the surface by the attachment of the SAM-forming molecule through its functional group as a result of a covalent bond. This technique provides a suitable and flexible way to modify the surface properties of the substrate materials to some extent or completely depending on the molecular structure of acting as SAM-forming molecule. Recent reports have also shown that SAMs can significantly change the properties of graphene without sacrificing the intrinsic graphene performance. In order to modify the electronic properties of graphene by SAM technique, the usage of either electron donating or withdrawing terminal groups has been reported by Yan et al. . As shown in Figure 1, the threshold voltage (Vth) which corresponds to the neutrality point in graphene field-effect transistor (FET) devices shifted to positive values after the decoration of SiO2 substrates by using fluorinated molecules (F-SAMs), indicating p-type doping of the graphene layer. For the graphene FET device made by CH3-SAMs and H2N-SAMs, the negative Vth shift was observed, indicating p-type FET behaviors. This clearly indicates modification of the electronic properties of graphene which becomes available by suitably adapting the SAM techniques.
SAMs have been widely used to eliminate the Schottky barrier at the materials’ interface by modifying the interface between graphene and its support substrate which is creating p-n junctions in graphene [25, 26, 27]. Sojoudi et al. used 3-aminopropyltriethoxysilane (APTES) and perfluorooctyltriethoxysilane (PFES) for the modification of the interface between transferred CVD-graphene films and its supporting dielectric . Thus, n-type and p-type graphene was created by treating APTES and PFES, respectively. As shown in Figure 2, APTES and PFES are patterned on the same dielectric, thus creating a graphene p-n junction. They demonstrated that substrate functionalizing with these SAMs resulted in p-n junctions with controlled position and height.
Recent studies have shown that graphene functionalization has been enhanced significantly by electrochemical control [29, 30, 31, 32]. Fermi level of graphene can be shifted and also increased its reactivity toward aggressive chemicals by application of electrochemical potential. The groups of Treossi and Palermo used electrochemical approach, a fast and simple method, to functionalize graphene . The electrochemical potential allowed good control over the morphology of the 4-docosyloxy-benzenediazonium tetrafluoroborate (DBT) suspended layer, which does not react randomly on the substrate, and also transformed the physisorption into a covalent chemisorption.
3. Applications of graphene/SAM structure
Graphene has well-known electrical properties, which may be utilized in various applications such as biosensors, environmental sensors, electronic and optical devices, etc. Modification with SAM can result in the change of properties at the interface, such as friction coefficient. The monolayer can also be utilized as a physical spacer, in order to enhance the sensing properties of the layer underneath. Apart from these applications, SAMs can be used in order to synthesize graphene, both by electrostatical assembly of graphene flakes onto the surface and by providing the carbon atoms that is required to assemble graphene from their own molecular structure.
The enhancement effect of SAM is widely used in the field of biosensor and biomedical. Shi et al. formed a SAM from 4-aminothiophenol (4-ATP) solution on gold substrate and placed the prepared substrate in graphene oxide (GO) solution vertically. Coupling reagents ethylene dichloride and N-hydroxysuccinimide (EDC and NHS, respectively) were added to promote the reactions between –NH2 groups of SAM molecules and –COOH groups of GO. After the device fabrication, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out, and ultrasensitive electrochemical Hg ion sensing properties were shown . Sun et al. prepared GO and fluorescent SAMs with thrombin aptamer (TBA) fixed onto the outermost layer and used CdTe quantum dots as a fluorescence probe, which allow sensing biomolecules by interfacial energy transfer from CdTe in SAMs to GO. The measured fluorescence spectra showed that the increase of the intensity was related to the concentration of the target molecule (Figure 3) and the linear correlation was obtained by using GO which was necessary for defining the device as a sensor .
Thakur et al. prepared a graphene-based FET sensor using α-ethyl-tryptamine (AET) to add functionality to the electrodes. This approach enables the electrostatical adsorption of functional groups that are present in GO monolayers, such as carboxylic acid and epoxide, by the amine end of AET. The selectivity and response time showed great results, being able to detect a single
As a selectivity study, Khan et al. suggested an ITO-based, APTES-modified sensor that contained electrochemically reduced GO. Positively charged amino groups (NH3+) within APTES have interacted with the negatively charged carboxyl and hydroxyl groups within GO. This electrostatic interaction has allowed deposit of GO onto ITO/APTES electrode. It was observed that the electronic transport is improved with the modification of the ITO surface with SAM, and the addition of GO onto the ITO/SAM surface increased the conductivity. Therefore, the resulting device had the property of sensing the oxidation peak of tyramine due to selective behavior of electrode toward electrooxidation. The sensor had also stability and selectivity tests, which it stayed stable for 3 weeks (with the loss of <5% activity) and as selective for human urine, commercial milk, and beer sample. Final conclusions ended up with that the device had sensing capabilities for dopamine and ascorbic acid as well, and it is ready for commercial applications .
Feng et al. studied a new type of electrochemical aptasensor for highly sensitive detection of adenosine triphosphate (ATP) and adenosine deaminase (ADA) activity. For modification, 6-mercaptohexanol (MCH) was used. Fabricated sensor could sense different molecules with varying concentrations and had amplified electrochemical signals, due to the facilitation of electron transfer of the functionalized graphene (Figure 5) . Yeh et al. demonstrated graphene field-effect transistors (G-FETs) for high-mobility applications that were both rigid and flexible. First, graphene was synthesized by CVD method and then transferred onto silicon substrates. The graphene-coated silicone samples were soaked into the solution of hexamethyldisilazane (HMDS) used as SAM-forming molecule for 17 h in a partially covered beaker. The samples were turned into G-FET devices using e-beam lithography and functionalized. As a result, they showed that modification of substrates for graphene electronic devices through patterned SAM arrays of HDMS can enhance the electrical properties of G-FETs by limiting and screening the interactions between graphene and hydrophilic polar groups attached. The HMDS-modified G-FET devices they built could detect cancer biomarker CSPG4 in serum samples at a concentration of as low as 0.01 fM, which is five orders of magnitude more sensitive than a conventional colorimetric assay .
Chiu et al. modified a gold electrode using 1-octadecanethiol (ODT) as SAM-forming molecule and then immersed it into GO aqueous solution. After the completion of the coating process, the GO film could be turned into electrochemically reduced GO (ERGO) films. The resulting film showed sensing properties, and the process variables can control the residual oxygen functionalities and conductivity in GO sheets in order to provide tools for sensitive immunoassay detection (Figure 6) . Bhardwaj et al. prepared a biological sensor that contained covalently bonded GO array with a layer of SAM formed by 4-aminothiophenol on Au electrodes. After functionalizing the surface with lipase enzyme, the sensor could bind with tributyrin and found to be linearly correlated with concentration between certain limits .
In a study of binding mechanism, Wang et al. reported that included CVD-grown graphene, hexagonal boron nitride (hBN), and OTS as SAM-forming molecules. They also used a stamp-like micropatterning process in order to give SAM layer a pattern before the coating of the graphene, which affected the binding mechanism of target molecules (Figure 7) .
One of the several properties of SAM layers is to alter the working conditions of the bare device. Aydin et al. investigated the properties of a diode that contained graphene, 4″bis(diphenylamino)-1, 1′:3″-terphenyl-5′carboxylic acids (TPA), and 4,4-di-9H-carbazol-9-yl-1,1′:3′1′-terphenyl-5′carboxylic acid (CAR) aromatic SAMs (Figure 8). The decrease in the Schottky barrier height and series resistance by forming a suitable interface between Si and graphene is also noted. The graphene forms π-π interaction with aromatic SAMs as well. The theoretical calculations of the study also confirmed that CAR molecules showed better results than TPA, due to the more compatible energy levels .
Kim et al. studied the electrical transport characteristics of graphene transistors functionalized with a SAM of cationic molecules. They used 1-aminoethanethiol as SAM molecule onto mechanically exfoliated graphene, and the device is completed after the electrical contact electrodes were fabricated by e-beam lithography (Figure 9). Finally they found that work-function engineering with SAMs results in a larger doping effect than the simple adsorption of amine-containing molecules onto the graphene channels (Figure 10) . Jung et al. fabricated graphene hybrid film consisted of graphene/APTES/polyethylene terephthalate (PET) for enhancement of the conductivity. Basically, a CVD-grown graphene film is transferred onto an APTES layer on PET substrate. This process enhanced the conductivity of graphene effectively. As a result of the study, the carrier density of graphene was improved from 9.3 × 1012 to 1.16 × 1013/cm2 with transmittances of 82% and 86%, depending on the process parameters .
Gan et al. studied a new method for controlling H- and J-stacking in self-assembly. They used the property of GO and reduced graphene oxide (RGO) in order to control the self-assembly of perylene (Py). This new method suggested new materials for photosensitive applications in optoelectronics and liquid-junction solar cells . Wieghold et al. demonstrated optoelectronic function in almost monolayer molecular architecture. The active layer consisted of a self-assembled terrylene diimide (TDI) derivative dye that formed a bicomponent supramolecular network with melamine. They found photocurrents of 0.5 nA and open-circuit voltages of 270 mV employing 19 mW/cm2 irradiation intensities at 710 nm. They also estimated an incident photon to current efficiency of 0.6% at 710 nm with a contact area of 9.9 × 102 mm2, meaning the opening up to the intriguing possibilities in bottom-up optoelectronic device fabrication with molecular resolution .
Kang et al. presented a new efficient and stable RGO doping method by employing two different types of alkylsilane compounds to produce p-type or n-type doping of RGO: (tridecafluoro-1,1,2,2,-tetrahydrooctyl) trichlorosilane (FTS) and APTES, respectively (Figure 11). The tunable electrical property of the SAM-functionalized RGO was utilized in developing source/drain (S/D) electrodes in bottom contact OFETs. The OFET device performance was improved significantly upon the use of FTS-RGO S/D electrodes . Park et al. devised a method to optimize the performance of OFETs by controlling the work functions of graphene electrodes by functionalizing the surface of SiO2 substrates with N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) as SAM molecules (Figure 12). In order to facilitate the electron injection, the work function of graphene is lowered to 3.9 eV in NH2-SAM-modified SiO2. The method of performance optimization of graphene-based OFETs which utilizes work-function engineering by functionalizing the substrate with SAMs is described and shows ∼10 times enhancements in properties regarding the charge carrier mobility and the on-off ratio of OFETs (Figure 13) . The same group also used APTES as SAM molecule and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as doping agent in order to modify the properties of graphene and examined the thickness of the F4-TCNQ related to the electrical properties of bilayer graphene FETs on NH2-SAM-modified SiO2/Si substrates. A single-gate bilayer graphene FET with high-current on/off current ratios is successfully fabricated, and tunable bandgap was demonstrated. (Figure 14) .
Gan et al. discovered a weak organic acid, 3,4,9,10-perylene tetracarboxylic acid (PTCA), and utilized to synthesize a supramolecular nanocomplex containing PTCA-graphene that showed wide voltage window and ultrahigh specific capacitance .
In terms of environmental sensor, Sun et al. developed a sensor for the detection of Pb2+ ions by constructing quartz/GO/PDDA/G5/PDDA/GQDs@GSH system which included GO, glutathione-functionalized graphene quantum dots (GQDs@GSH), poly(diallydimethylammonium) chloride (PDDA), and G5 (G-rich DNA) (Figure 15). The group showed that the detection concentration of the sensor for Pb2+ ions is as low as 2.2 nM .
3.3 Interface modifier
Pfaffeneder-Kmen et al. used 4-mercaptophenol as SAM molecule in order to produce homogeneous GO coatings. This process allowed the modification of the surface for contact angle to be low enough in order to achieve complete wetting that causes GO suspension to adhere onto the Au surface (Figure 16a) . Larisika et al. used APTES as SAM-forming layer on SiO2 substrates in order to enhance the adsorption of GO before the dip-coating process. The resulting sensor had potential as a high-performance and low-voltage operating graphene device . Kamiya et al. investigated protein adsorption to graphene films which modificates the surface of substrates with two different SAMs in aqueous environment, octadecyltrichlorosilane (OTS) and APTES. They employed latex bead projection method for patterning SAMs and observed that the protein adsorption behavior on the graphene flakes is attached to partially SAM-modified SiO2/Si substrates in aqueous environment (Figure 16b–c). It was also found that a high-density of agglomerated avidin molecule clusters was formed in the OTS-supported graphene areas, whereas a low-density of large clusters was formed in the SiO2 and APTES-supported ones. For the high-performance graphene biosensors with small nonspecific adsorption of protein molecules, the suggested technique can control protein adsorption phenomena on graphene surfaces .
Ermakova et al. used the zinc complex of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP) as an organic promoter for the interfacial adsorption of GO. The ZnTPyP/GO/ZnAc2 bilayer was transferred as whole onto silicon slide by vertical LB method. As conclusion, they informed the stabilizing effect of GO which both anchored the assembled organic layer on the surface. By this result, the final multilayer structures showed large-area uniformity, especially for applications in catalysis, sensors, optoelectronics, etc. .
Hui et al. utilized a SAM molecule, 11-mercapto-1-undecanol (11-MU), for the blocking of Au surface, discovering the effect of the electrochemical kinetics of external-sphere redox mediators. For this purpose, metal electrodes were buried in the subsurface of continuous double-layer graphene electrodes. Modified Au tip exhibited a substantial decrease in its electron transfer kinetics .
In terms of mechanical modification, Bai et al. used APTES-SAM molecules between Si substrate and ceria/GO composite films in order to alter the mechanical properties of the final film. The friction coefficient was reduced drastically, and the anti-wear lifetime was longer than the GO films (Figure 17) . Li et al. investigated and compared three different structures: titanium substrate, APTES-SAM, and GO-APTES nanolayer. The modifications changed the wettability, adhesion, and friction forces of the final structures (Figure 18) .
Li et al. used APTES as an intermediate coupling agent for chemisorption of RGO sheets on titanium alloy substrate. The results showed that the prepared films contain chemisorptive bonds on the substrate. Tribological results indicated that the prepared APTES-RGO film decreased the friction coefficient while improving antifriction properties of titanium alloy under dry friction (Figure 19a) . Ou et al. utilized two different SAM layers including APTES and OTS in order to study lubrication coating properties. First, an APTES-SAM with amine (―NH2) outer groups was covalently anchored onto the Si wafer. GO sheets were chemo-grafted onto the APTES-SAM surface. Finally, an OTS outer layer was assembled onto the GO surface (Figure 19b). In order to increase the hydrophobicity of the film, an outer layer of OTS-SAM is introduced, which reduced the friction and boosted the anti-wear life .
Lee et al. used four different SAMs, APTES, (3-aminopropyl)trimethoxysilane (APTMS), (3-glycidyloxypropyl)trimethoxysilane (GPTMS), and triethoxymethylsilane (MTES), in order to modify GO and silane-functionalized GO (sGO). The samples were prepared using epoxy and tested mechanically. The results showed that sGO, especially those containing amine functional groups, can strengthen the interfacial bonding between the carbon fibers and epoxy adhesive (Figure 20) . Liu et al. used negatively charged poly(sodium 4-styrenesulfonate) (PSS)-mediated graphene sheets (PSS-GS) and the positively charged polyethyleneimine (PEI) repeatedly layer by layer, in order to obtain a multilayered film. Both the layer thickness and layer numbers were changed in order to determine the relation. Their good tribological properties under the given testing conditions suggested that the use as a low-friction and anti-wear coating layer is possible (Figure 21) .
Kong et al. reported the influence of chemically reduced graphene oxide sheets (CRGOs) on the electrochemical performance through methyl or carboxylic acid-terminated SAMs (Figure 22). Modified gold electrode and immobilization of the CRGOs on a SAM-treated surface showed effective enhancement of the heterogeneous electron transfer (ET) of the SAM due to the tunneling effect. They also reported that the kinetics of electron transfer activity between the CRGOs/SAM/Au electrode and redox species in the solution was attributed to charge transfer being confined to CRGOs with different interactions with ―CH3 and ―COOH terminated thiols, which can influence the electron transfer efficiency and the rate of charge transfer due to different electron transfer pathways .
Margapoti et al. reported the experimental studies of resonance oscillation in the current density of a SAM-graphene hybrid system. They used Au surface functionalized with a mixture of 4-(1-mercapto-6-hexyloxy) azobenzene as SAM treatment and 6-mercapto-1-hexanol as spacer molecules. A reversible change in conductivity was observed after transforming the molecular configuration from trans to cis (Figure 23) .
3.5 Graphene synthesis by SAM
It is also quite possible to synthesize high-quality graphene by using SAM technique as assembling surfaces. Xie et al. reported a simple and environmentally friendly approach for synthesizing a composite of graphene/gold nanoparticles (3DG/Au NPs) in one step. The selected molecule for SAM treatment is 4-aminothiophenol. They immersed the SAM-coated glass substrate into the GO-HAuCL4 solution, and the resulting sample showed catalytic reduction properties .
Yin et al. successfully bonded monolayer of CRGO nanosheets on Si substrates chemically and investigated their possible applications in Raman scattering (Figure 24). The assembly of large-scale and uniform graphene sheet was performed by the electrostatic absorption between the ―NH2 groups of APTES that was coated onto the Si surface and residual groups on RGO. They also concluded that the mildly RGO substrate has the optimum graphene-enhanced Raman scattering (GERS) performance among all the CRGO substrates. The π-π stacking and the residual polarized oxygen groups on CRGO surface were mainly responsible for the excellent GERS effect of mildly RGO substrate .
Jing et al. reported a facile approach to the direct synthesis of graphene sheets based on the SAM technique. They used a thiophene-terminated silane molecule, triethoxy-(6-(thiophen-3-yloxy)-hexyl)silane (TEHS), as carbon source, and by heating the cross-linked polythiophene SAM up to 1000°C under high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate . Xie et al. investigated the electrochemical behaviors of graphene sheets attached to a SAM (n-octadecyl mercaptan) on a gold electrode (Figure 25). Their studies revealed that the heterogeneous electron transfer blocked by the SAM layer can be restored by graphene sheets, and the graphene-/SAM-modified Au electrode had a smaller interfacial capacitance, as compared with that of a bare one .
Three innovative methods are introduced by Nayak et al. for functionalizing bioinert ceramic. These methods include immobilizing rGO onto the surface using appropriate SAMs. Among these methods, activated ―COOH groups of rGO and ―NH2 functionalized zirconia shows the most effective immobilization. They also concluded that further immobilization makes it possible to customize the surface properties for the desired application and could be easily performed on graphene-modified ceramics .
Following Geim and Novoselov study about graphene, it has been the most exciting and promising material discovered in the last few years. Due to its outstanding properties, it has enormous potential to develop many applications ranging from nanoelectronics to nanobiotechnology [71, 72]. However, tuning the physical and chemical properties of graphene, which possess high sheet resistance and zero bandgap, is crucial to the realization of graphene-based technologies. Furthermore, adsorption of graphene onto a solid surface is important for the device applications since it provides good charge transfer. Functionalization of the surface with SAMs is useful for controlling the electronic properties of the graphene layer and improving adsorption, paving the way to develop commercial applications.
In recent years, the interaction between SAMs with different chemical properties and graphene has been investigated by many research groups. For instance, SAMs with thiol group is most widely studied for electronic applications due to having delocalized π-electrons. These SAMs can also modify electrodes for electrochemical sensor applications. For example, SAM-decorated Au probes can electrostatically adsorb graphene derivatives, both functionalizing the sensor and improving the sensing ability. These researches demonstrate that SAMs are a utility for improving the performance of experimental devices. It is considered widely that overcoming the difficulties in the production of large-area graphene sheets may allow developing commercial applications.
In this chapter, we have demonstrated the functionalization, modification, and synthesis of graphene by SAMs and reviewed different applications. This chapter also demonstrates how to utilize the SAM-modified graphene and that SAM modification can improve the possibility of its usage in many applications.
Conflict of interest
The authors declare no conflict of interest for this publication.