Applications of Graphdiyne on Optoelectronic Devices - American

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Applications of Graphdiyne on Optoelectronic Devices Tao Lin† and Jizheng Wang*,†,‡ †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Graphdiyne (GD) is a novel two-dimensional carbon material composed of sp and sp2-hybridized carbon atoms. This kind of carbon allotrope has attracted more and more attention not only because of the distinctive porous structure but also because of its intriguing electronic properties such as high mobility and conductivity, good field emission properties, and tunable natural band gap. In this review, some representative applications of GD on a variety of optoelectronic devices are described. Starting from the methods of introducing GD into the devices, we analyze the interactions between GD and other device components, summarize the general mechanism of how GD improves performance of the devices, and provide a glimpse into the future of GD at the end. KEYWORDS: graphdiyne, synthesis, electronic properties, applications, optoelectronic devices

1. INTRODUCTION Carbon can almost be said to be the most familiar element in our daily life, and it has always received particular attention in natural science research. Benefitting from the composition of identical atoms and the simple periodic structure, carbon allotropes can be an appropriate research subject. The four outer electrons filling the 2s and 2p states of carbon could form diverse hybridized orbital like sp3, sp2, and sp states. Among them, sp3-hybridized carbon atoms compose diamond and sp2hybridized carbon atoms are components of fullerene and graphene which come from graphite, and these carbon allotropes have been widely investigated. In contrast, sphybridized carbon atoms are scarcely able to form configurations with 2D or 3D periodic structure only by themselves. Graphdiyne (GD, or GDY in some references) is the first synthesized carbon material containing both sp and sp2hybridized carbon atoms,1 and enables us to study the properties of CC in carbon allotropes.2−6 Being composed of benzene rings linked by diacetylenic bond units, GD has larger uniformly distributed pores compared with graphene, and this makes it have many potential applications on gas separation,7−11 water desalination,12−15 H2 storage,16−18 ion batteries,19−23 etc. Meanwhile, benefitting from its active diacetylenic linkages, GD has stronger interactions with other materials in comparison to graphene, so that it could be more applicable to using as a catalyst support for photocatalytic degradation24−26 and hydrogen (or oxygen) evolution reaction.27−29 Besides the peculiar structure, the optoelectronic properties of GD are also intriguing. It was found through first-principles calculations that the single GD sheet exhibits high mobility (1 × 105 cm2/(V s) for electron and 1 × 104 cm2/(V s) for hole at room temperature), which is superior to that of the graphene nanoribbons and single-wall carbon nanotubes.30 More interestingly, different from © XXXX American Chemical Society

graphene, GD shows a natural band gap, and this makes it behave more like semiconductor rather than semimetal.31 As a promising carbon semiconductor material, GD has a wide range of applications on optoelectronic devices. In this review, we briefly discuss the synthesis and electronic properties of GD first, and then summarize the applications of GD on some common devices such as field-effect transistors (FETs), solar cells (especially for perovskite solar cells), photodetectors (PDs), photoelectrochemical (PEC) water splitting cells and memory devices. Here, we focus on the performance improvement of devices induced by GD, try to reveal the general mechanism behind it, and hope to present the broad application prospect of GD.

2. SYNTHESIS AND ELECTRONIC PROPERTIES As early as 1987, Baughman and Eckhardt had put forward a chemical structure of carbon called graphyne (GY) which is constructed by sp carbon atoms with the same number of sp2 carbon atoms in a 2D plane.32 The crystalline-state formation energy of GY appears to be much lower than any other carbon material which is mainly consisted of acetylenic groups. Interestingly, instead of GY, its family member GD, which contains one more acetylenic group between adjacent carbon hexagons (Figure 1), was first synthesized. Since Haley et al. reported the preparation and characterization of GD substructures in 1997,33 many efforts have been made to obtain stable, uniform and large-area GD film.34−36 Until 2010, Li et al. used hexaethynylbenzene (HEB) to synthesize GD on Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: February 12, 2018 Accepted: April 17, 2018

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DOI: 10.1021/acsami.8b02671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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optoelectronic devices. Long et al. calculated the electronic structure of the GD sheet and obtained a direct band gap of 0.46 eV at Γ-point by using the density functional theory (DFT);30 shortly afterward, Luo et al. showed that the energy gap should be 1.10 eV after including the Coulomb interaction;31 since then, many investigations have verified the existence of the band gap, even though with different values come from different methods. In the meantime, the forbidden bandwidth was found to be affected by various factors such as the figuration30,48−50 and strain51−53 of GD and the doping of metal atoms54−57 and functional groups.4,5,58

3. APPLICATIONS ON OPTOELECTRONIC DEVICES 3.1. Field-Effect Transistors. The remarkable electrical properties of GD can be directly reflected in its applications on FETs. In studies of the contact barrier between GD and metals directed by Pan et al.,59 they used quantum transport calculations to investigate GD FET. In their opinion, the interactions between GD and metals would form the potential barrier (Schottky barrier and tunneling barrier), and they found that Al could provide an Ohmic contact with GD. According to the calculation results, the device is a typical n-type FET with the ON/OFF ratio of 104, and the current density of the 10 nm-channel-length GD FET at Vbias = 0.4 V and Vg = 0 V can reach 1.3 × 104 mA/mm. Even more encouraging, GD has already been used in practical devices. By doping GD, Cui et al. improved the carrier transport characteristics of FET based on P-o-FBDTPC8DTBTff (PFC; o-FBDTP, o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene; C8DTBTff, 5,6-difluoro4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole).60 They fabricated an FET device with the structure of top-contact bottom-gate by spin-coating GD/PFC on SiO2 covered silicon wafer and evaporating silver source/drain electrodes after that (10 μm for channel length and 5 μm for channel width). Figure 2a shows the transfer characteristics of GD/PFC and PFC (for reference) devices at VSD = 1 V. It can be seen that the threshold voltage of GD/PFC device is much lower than the reference device (absolute value: 0.5 V vs 2.75 V), and the improvement of ON/OFF ratio is obviously. Those differences are also reflected in the derived

Figure 1. Structure of GD. Reprinted with permission from ref 34. Copyright 2000 Wiley−VCH Verlag.

top of copper surface, and successfully fabricated large-area films.1 In this process, the copper foil can be both the catalyst and substrate. The area of the film reaches 3.61 cm2, and the device based on that film exhibits conductivity of 2.516 × 10−4 S/m at room temperature. After that, much research focused on the method improving the quality of GD film. Qian et al. deposited GD powder on ZnO nanorod arrays via a vapor−liquid−solid (VLS) growth process to synthesize GD nanofilms,37 and the films present high conductivity (2800 S/cm) and hole mobility (100 cm2/(V s)). Liu et al. synthesized monolayer GD analogs on silver surface through the chemical vapor deposition (CVD) process by using HEB as a precursor,38 and the FET device based on that film exhibits a p-type semiconducting feature with the corresponding conductivity of 6.72 S/cm. Matsuoka et al. used two kinds of solution which contained precursor and catalyst respectively to generate a liquid/liquid reaction interface and obtained multilayer GD with the thickness of 24 nm.39 Furthermore, they produced few-layer GD (thickness, 2.97 and 3.94 nm) regular hexagonal domains (diagonal size 1.51 μm) by using the similar gas/liquid interfacial synthesis, and revealed its ABC stacking with an interlayer distance of 0.34 nm. It is worth mentioning that heat treatment would be helpful in improving the quality of GD film. He et al. found that most GD nanoparticles can be removed from the surface of GD film after calcination at 400 °C for 2 h to construct uniform and smooth films;40 on the other hand, Zhong et al. showed that some functional groups can be removed from GD after annealing at 800 °C.3 Meanwhile, various morphologies of GD have been prepared. Li et al. fabricated GD nanotube (GDNT) arrays by virtue of an anodic aluminum oxide template.41 The wall thickness of the GDNTs can attain about 15 nm after annealing, and the GDNT arrays display intriguing field emission properties with the turn-on field and threshold field of 4.20 and 8.83 V/μm, respectively. Qian et al. constructed GD nanowires (GDNWs) by using the VLS growth process.42 The average conductivity of the GDNWs with the diameters about 30 nm is 1.9 × 103 S/m, and the average mobility obtained from fitting can achieve about 7.1 × 102 cm2/(V s). Zhou et al. synthesized GD nanowalls by adjusting catalyst distribution and monomer concentration,43 and the as-prepared GD nanowalls also exhibit good field emission properties (the turn-on field is 6.6 V/μm, and the threshold field is 10.7 V/μm). Moreover, similar GD nanostructures have been used in practical devices,44−47 and some of the applications will be introduced in more detail later. In addition to the excellent electrical properties, the natural band gap also makes GD to be an ideal material for

Figure 2. (a) Transfer ISD−Vg curves for GD/PFC and PFC films. (b) Nyquist plots for GD/PFC and PFC films at zero bias. Reprinted with permission from ref 60. Copyright 2017 American Chemical Society. B

DOI: 10.1021/acsami.8b02671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Key J−V Parameters of Perovskite Solar Cells Incorporated with GDY QDs in Different Waysa incorporating ways GDY-coated TiO2 GDY-coated MAPbI3 GDY-treated MAPbI3 GDY-doped MAPbI3 GDY-doped Spiro-OMeTAD GDY-optimized device a

JSC (mA/cm2)

VOC (V) 1.097 1.101 1.098 1.094 1.098 1.124

(1.082) (1.082) (1.085) (1.081) (1.079) (1.084)

21.50 21.32 20.96 21.75 21.88 22.48

(21.03) (21.03) (21.05) (21.06) (21.01) (21.03)

FF (%) 75.8 77.5 77.7 76.7 76.3 78.7

(74.8) (74.7) (74.7) (74.9) (75.1) (75.3)

PCE (%) 17.88 18.20 17.89 18.26 18.33 19.89

(17.02) (16.99) (17.06) (17.06) (17.03) (17.17)

The parameters of the reference device of the same batch are in the parentheses.

mobility μ: 0.69 cm2/(V s) for the composite film and 0.002 cm2/(V s) for the raw film. The reasons for increased mobility of the device base on GD/PFC could be attributed to three aspects: first, GD flakes would form “fast lanes” within the conduction channel due to its high carrier mobility; second, carrier could move from PFC to GD via intramolecular interactions, and this would enhance conductivity of the film; finally, the ability of attracting electron for acetylene linkages in GD is strong, so that the introduction of GD in PFC would increase carrier mobility. Furthermore, it can be derived from electrochemical impedance spectroscopy (EIS, Figure 2b) that the doping of GD increases carrier lifetime (from 4.94 to 7.25 μs) and diffusion length (from 0.16 to 3.6 μm). 3.2. Solar Cells. As the fairly high charge carrier mobility and the intrinsic bandgap above-mentioned, GD can be used in photovoltaic devices via various ways. Because the large-area films of GD were synthesized, it was introduced into the organic solar cells. In 2011, Du et al. incorporated GD into the polymer solar cells by doping it into the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-buytyric acid methylester (PCBM) active layer.61 The devices were fabricated with the configuration of ITO/Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/P3HT:PCBM:GD (x wt %) (60 nm)/Al (100 nm), and GD was mixed into the active layer with different doping ratios. The optimized weight ration of GD is 2.5%, and the best device presents an opencircuit voltage (VOC) of 0.64 V, a short-circuit current density (JSC) of 10 mA/cm2, a fill factor (FF) of 0.55 with a corresponding power conversion efficiency (PCE) of 3.52%. The JSC, FF and PCE are all enhanced significantly compared with the reference device (VOC = 0.63 V, JSC = 7.6 mA/cm2, FF = 0.47, and PCE = 2.25%). They inferred that the enhanced photocurrent mainly due to the additional electrons transport paths which are constructed by the percolation of GD in the active layer. These percolation paths provide more efficient electron transport and suppress charge recombination. However, overmuch GD would be aggregated to form defects and result in a deleterious effect. They also demonstrated that the doping of GD enhance the conductivity of P3HT film. Hybrid metal halide perovskites have been widely studied in recent years for a variety of optoelectronic devices. Their handy fabrication method, efficient utilization of solar energy and balanced carrier mobility make them one of the most promising materials to replace silicon as the active layer for photovoltaic device, and the PCE of the solar cell incorporated with perovskites has exceeded 20%. However, since perovskite film is always consisted a mass of crystalline grain with different size, shape and orientation, the defects formed on the grain boundary and/or the interface between active layer and other layers would be harmful for the performance of the devices. To improve the crystallization and obtain the large-size grain has been a general research focus of the community; meanwhile, an

alternative way to surmount the disadvantage is to modify the surface and the grain boundary of perovskite layer. Recently, Zhang et al. used the GD quantum dots (GD QDs) to improve the performance of the perovskite solar cells, and investigated the effect of GD QDs combined with each layer.62 The configuration of the fabricated perovskite solar cells is substrate/fluorine-doped tin oxide (FTO)/TiO2/CH3NH3PbI3 (MAPbI3)/2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD)/Au, and GD QDs were coated and/or doped to TiO2, MAPbI3 and Spiro-OMeTAD. The key J−V parameters of the devices were summarized in Table 1; the AFM images of as-deposited film and the contact angle of water droplets were presented in Figure 3. For the optimized device with 0.002 mg/mL GD-coated TiO2 film as the electron transport layer (ETL), the VOC, JSC, FF and PCE are all enhanced compared with the reference device of the same batch (Table 1). Although the small amount of GD QDs could not influence the surface morphology, the water contact angle is obviously increased (Figure 3). This

Figure 3. AFM images of as-deposited film, and the contact angle of water droplets presented in the insets: (a) bare TiO2 film, (b) perovskite film, (c) spiro-OMeTAD film; (d) UVO-treated GD-coated TiO2 film; (e) GD-coated perovskite film; (f) GD-doped SpiroOMeTAD film; (g) perovskite film fabricated on UVO-treated GDcoated TiO2; (h) GD-doped perovskite film; and (i) GD-treated perovskite film. Reprinted with permission from ref 62. Copyright 2017 Wiley−VCH. C

DOI: 10.1021/acsami.8b02671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces would be beneficial to the dispersing process of the perovskite upon the TiO2 film. In consequence, the perovskite could form more uniformly pinhole-free continuous film, which would be beneficial to charge transport. Similarly, the coating of the GD QDs on the MAPbI3 film (GD-coated MAPbI3) or the using of the GD QDs CB solution as an antisolvent (GD-treated MAPbI3) would also increase the water contact angle on the surface of the perovskite film (Figure 3). Again, because of the efficient charge transfer in the GD QDs-modified film, the key J−V parameters of the device with 0.002 mg/mL GD-coated/ treated MAPbI3 film are generally superior to the reference device of the same batch (Table 1). Interestingly, when they doped the GD QDs into the MAPbI3 as an additive, they obtained larger grains compared to the reference perovskite (the average grain size increase from ≈140 nm to ≈220 nm). The MA+ cation may form hydrogen bonds with the GD, and the improvement of crystallization is induced by suppressing nucleation and slowing down the crystallization process. As expected, the performance of the device with 0.002 mg/mL GD-doped MAPbI3 film is better than the reference device of the same batch (Table 1). More specifically, the using of the GD QDs solution (coating or treating) would deposit the GD on the perovskite grain boundary and passivate the surface traps, whereas the doping would make the GD QDs become a component of the perovskite and form the lager grains. For the hole transport layer (HTL), the effects of doping the GD QDs into Spiro-OMeTAD are same with the situation of the ETL and the active layer. The contact angle of water droplets on the surface of the Spiro-OMeTAD film is increased (Figure 3), and the VOC, JSC, FF and PCE of the optimized device with 0.01 mg/mL GD-doped Spiro-OMeTAD film are enhanced obviously (Table 1). As mentioned above, the percolation paths which are formed by GD and the polymer matrix could improve the carrier transport property. Finally, the PCE of the GD-optimized perovskite solar cells (based on GD-coated TiO2 film as the ETL, with GD-coated (GD-treated and GD-doped) MAPbI3 film as the active layer, and GD-doped SpiroOMeTAD film as the HTL) increases from 17.17% to 19.89% compared with the reference device (Table 1). Meanwhile, the enhanced hydrophobicity of each layer indicated by the increasing of the contact angle could prevent water entry so as to improve the device stability. To simplify the fabrication process and reduce the cost of perovskite solar cells, many efforts focus on the substitution materials of ETL and HTL, and there are also some works about GD. Kuang et al. built the ETL composed of PCBM doped with GD in perovskite solar cells (the weight ration of GD is 0.75%),63 and the device structure was ITO/ PEDOT:PSS/CH3NH3PbI3−xClx/PCBM:GD/C60/Al (Figure 4a, b). The device using PCBM:GD as ETL yielding a PCE = 14.8% (average PCE = 13.9%) with VOC = 0.969 V, JSC = 23.4 mA/ cm2, and FF = 0.654, compared with the reference device using pure PCBM interlayer (VOC = 0.989 V, JSC = 22.3 mA/cm2, FF = 0.615, and PCE = 13.6% (average PCE = 10.8%)). Except for the declining VOC which is due to an approximately 200 mV surface potential decrease, other parameters are enhanced. SEM measurements showed that the coating of PCBM:GD film could smoothen the perovskite layer, and this would reduce the trap states by passivating the grain boundary. The increasing of electrical conductivity for PCBM after doped with GD is indicated by conductive atomic force microscope (c-AFM) measurements, and space charge limited current measurements

Figure 4. Schematic illustration of the fabricated perovskite solar cells for (a) with PCBM doped with GD as ETL, (c) with P3HT modified with GD as HTL. (b, d) Corresponding J−V characteristic curves. (a, b) Reprinted with permission from ref 63. Copyright 2015 American Chemical Society. (c. d) Reprinted with permission from ref 64. Copyright 2015 Wiley−VCH.

demonstrated that the electron mobility of PCBM:GD increases from 2.98 × 10−4 cm2/(V s) to 5.32 × 10−4 cm2/ (V s). Meanwhile, photoluminescence (PL) spectra proved that as electron transport layer, PCBM:GD can extract electron more efficiently. Xiao et al. fabricated the perovskite solar cells (FTO/TiO2/CH3NH3PbI3/P3HT:GD/Au, Figures 4c, d) to investigate the effects of GD in HTL (the weight ration of GD is 5%).64 As previously mentioned, the doping of GD into P3HT could decrease the resistance and increase the conductivity. Furthermore, the HOMO level of P3HT would be lowered by GD (from −4.7 eV to −4.9 eV) due to the π−π stacking between them, which could improve the charge transfer property of HTL. The pristine P3HT-based perovskite solar cell presents VOC = 0.932 V, JSC = 18.3 mA/cm2, FF = 0.676, and PCE = 11.53%, whereas the P3HT:GD-based device can present VOC = 0.941 V, JSC = 21.7 mA/cm2, FF = 0.713, and PCE = 14.58%. Besides applying to photovoltaic devices based on polymer and perovskite, GD can also be used in some other solar cells. For dye-sensitized solar cells (DSSCs), Ren et al. demonstrated both experimentally and by DFT calculations that the platinum nanoparticles combined with GD nanosheet (PtNP- GDNS) can be used as counter electrode.65 DFT calculation results manifested that the interaction between GD and PtNP, which makes the Pt atoms positive polarized could improve the adsorption of the I3−. Additionally, as the C atoms have strong electronegativity, there would form I• which could capture electrons easier than I2, and this may promote the transition from I3− to I−. These results were confirmed by cyclic voltammetry (CV) analyses and EIS tests, which indicate that the PtNP-GDNS could catalyze the reduction reaction of I3−. The DSSC with a structure of FTO/TiO2/ cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)rut-henium(II) (N719 dye)/electrolyte (0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.05 M iodine, 0.5 M lithium iodide and 0.5 M 4tertbutylpyridine (TBP) in 3-methoxypropionitrile)/PtNPGDNS exhibits VOC = 0.80 V, JSC = 14.13 mA/cm2, FF = D

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OFF state; and ON state comes from the photogenerated holes discharge the oxygen ions, which increases the conductivity. However, the response time would be long due to the slow adsorption/desorption processes. Jin et al. incorporated GD into PDs based on ZnO via three different ways (Figure 6).

0.56, and PCE = 6.35%, and is better than the reference device (VOC = 0.76 V, JSC = 12.87 mA/cm2, FF = 0.55, and PCE = 5.39%). Colloidal quantum dots (CQDs) solar cells are also promising next-generation photovoltaic devices, and Jin et al. deployed GD as the anode buffer layer in PbS CQD solar cells.66 An architecture of ITO/ZnO/PbS-TBAI (TBAI: tetrabutylammoniumiodide)/PbS-EDT (EDT: 1, 2-ethanedithiol)/GD/Au was used (Figure 5), and the device displays

Figure 5. (a) Schematic illustration of the fabricated PbS CQD solar cells with GD anode buffer layer. (b) Corresponding J−V characteristic curves. Reprinted with permission from ref 66. Copyright 2016 Wiley−VCH. Figure 6. Schematic illustration and corresponding ON/OFF switching properties of the fabricated PDs based on: (a) the ZnO film, (b) the GD/ZnO bilayer film, (c) the GD:ZnO film and (d) the GD:ZnO/ZnO bilayer film. Reprinted with permission from ref 67. Copyright 2016 Wiley−VCH.

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VOC = 0.654 V, JSC = 22.83 mA/cm , FF = 72.14% and PCE = 10.64%, superior to the reference device without GD (VOC = 0.650 V, JSC = 21.74 mA/cm2, FF = 67.34%, and PCE = 9.49%). Estimated from impedance spectroscopy (IS) data, the longer effective carrier lifetime and the lager recombination resistance of the device with GD compared with the reference device indicate that the charge transfer property should be improved by the GD buffer layer. Transient photovoltage (TPV) measurements also confirm that GD could enhance the transport of carriers by reducing the recombination rate. 3.3. Photodetectors. PDs are another kind of optoelectronic devices which could convert optical signal into electrical signal, and have wide applications in many areas. As the operating principle of PDs is similar to solar cells, it is feasible to introduce GD into PDs to improve their performance. Jin et al. demonstrated the application of GD:ZnO nanocomposites in fabricating UV PDs.67 Response/recovery time (rise/decay time) are key parameters for PDs, and the improvement of the device sensitivity has always been a research direction. For conventional ZnO NPs, a low-conductivity depletion region is formed by the adsorption of oxygen molecules, which leads to

Figure 6 also shows the corresponding ON/OFF switching properties of the devices. It can be seen that both the rise time and decay time of the PDs incorporating with GD are shorter than the reference device, especially for the GD:ZnO PD and GD:ZnO/ZnO bilayer PD which both exhibit similar rise time of ≈6.1 s and decay time of ≈2.1 s (the rise time and decay time of the reference device are 32.1 and 28.7 s, respectively). The shortening of the rise/decay time can be attributed to the PN junction formed between GD and ZnO at the ZnO NP/ GD NP interface. In this situation, the ON/OFF switching of the device is manipulated by the depletion region width of PN junction, and the response could be very fast. 3.4. Photoelectrochemical Water Splitting Cells. As a kind of electrode modification material, GD can also be used in PEC water splitting cells. Li et al. introduced GD to improve the hole transfer in the photocathode of PEC cell.45 In their research, 4-mercaptopyridine surface-functionalized CdSe QDs E

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ACS Applied Materials & Interfaces were adsorbed on GD nanowalls through π−π stacking, and the assembled CdSe QDs/GD was used as photocathode to fabricate the PEC cell together with Pt wire as counter electrode (Figure 7a). For CdSe QDs/GD system, the

Al2O3 core−shell NP layers in the polyimide (PI) layer as carrier traps, and the structure of the device (Figure 8a) is Ag

Figure 7. Schematic illustration of the PEC cell consisting of (a) CdSe QDs/GD photocathode and (b) GD/BiVO4 photoanode, and corresponding migration process of the photogenerated excitons. (a) Reprinted with permission from ref 45. Copyright 2016 American Chemical Society. (b) Reprinted with permission from ref 46. Copyright 2016 Wiley−VCH.

photogenerated holes from CdSe QDs could inject into GD efficiently since the HOMO energy levels of CdSe QDs and GD are about −5.67 and −5.49 eV, respectively. This is benefit for the transfer and collection of holes, and would decrease charge recombination. In addition, the introduction of GD could decrease contact resistance, because it would raise the HOMO level of CdSe QDs. The faradic efficiency up to 95% was obtained. Meanwhile, Xin et al. fabricated a GD/BiVO4 photoanode for PEC cell by the direct synthesis of GD nanowalls on BiVO4 electrode (Figure 7b).46 Similarly, PL spectroscopy of samples manifests that the deposition of GD on the surface of BiVO4 could significantly enhance the extraction of photogenerated carriers from BiVO4, and this certainly suppresses the undesirable charge recombination process. Moreover, EIS implies that the introduction of GD could decrease charge transfer resistance and reduce recombination rate. The faradic efficiency can be calculated to be 85% during a 4 h test. It should be noted that for the both situations mentioned above, owing to the efficient charge transfer, GD could eliminate the excess photogenerated carriers in active materials. This would improve the efficiency of photoelectrochemical water splitting, more importantly, it could improve the stability of the electrodes by preventing them from being photocorrosion. 3.5. Memory Devices. Memory devices have always been a research hotspot in the electronics field, as they will play a decisive role in the next-generation of digital technology. Since the excellent electrical properties of GD, it would have a wide range of applications in this area. Jin et al. employed GD in constructing memory devices.68 They obtained multilevel memory states by inserting discontinuous GD NP and Al−

Figure 8. (a) Schematic illustration of the fabricated RRAM. (b) Corresponding I−V characteristic curves. Reprinted with permission from ref 68. Copyright The Royal Society of Chemistry and the Chinese Chemical Society 2017.

bottom electrode (BE)/PI/GD/PI/Al−Al2O3/PI/Al top electrode (TE). As shown in Figure 8b, there are one OFF state and two ON states: it remains in the OFF state until the applied bias reaches 1.7 V, which is the voltage that the device turns to the first ON state, and the final ON state is realized by increasing the bias to 3.1 V. The memory mechanism should be attributed to carrier tunneling and trapping effects, and the charge transport process could be described by thermionic emission conduction (TEC) theory. To be more specific, when the bias voltage is below 1.7 V, electrons could barely inject into the device due to the potential barrier between the electrode and PI (high resistance state); when the bias voltage is between 1.7 and 3.1 V, electrons would fill the traps of the GD NPs, nevertheless, they could not inject into the Al cores owing to the higher Al2O3 tunneling barrier shells (intermediate resistance state); when the bias voltage is over 3.1 V, electrons could also fill the traps of the Al−Al2O3 NPs (low resistance state). It is worth noting that the ON states cannot be turned OFF by applying a negative bias, which indicates that those carrier traps are deep.

4. SUMMARY AND OUTLOOK This review summarizes some typical applications of GD on optoelectronic devices since the large-area GD films were F

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successfully synthesized. Beginning with the synthesis and electronic properties of GD, we discuss the methods and effects of introducing GD into devices by doping it into active layer, incorporating it with electrode and using it as buffer layer. Theoretical and experimental investigations have shown that GD exhibits outstanding charge mobility and conductivity, in addition, it has a natural band gap. These make GD have great potential application value on semiconductor devices. In our opinion, the performance improvement of devices with GD could be attributed mainly to two aspects: on the one hand, the deposition of GD on a certain active layer by coating and/or treating would modify the surface and reduce the trap states, so as to suppress the charge recombination and decrease the series resistance thereby improve the contact with electrodes or other layers; on the other hand, the interfacial interactions of GD with other materials coming from doping would induce effective charge transfer, and this could either increase the conductivity of the films or make GD become carrier traps to eliminate the excess charges. As mentioned in the introduction, GD has many unique advantages that make it become a promising 2D carbon material. For instance, with the uniformly distributed pores and active diacetylenic linkages, GD could behave like cytomembranes, which control the transport of ions and provide a platform for the reactions of proteins. Moreover, the natural band gap which could be affected by external factors opens up new areas like sensors for GD. Certainly, there are also some obstacles for the development of GD, and the key challenge at present we deemed is to realize the standardized synthesis and large-scale industrial production, which would enable GD to be investigated more widely. Finally, we consider that the bright prospect of GD may come from efforts in three ways: first, synthesizing high quality GD film to achieve the excellent electronic properties theoretical predicted; second, constructing various GD nanostructures to meet the demand of multiple use; and third, taking advantages of GD both in structure and electronic characteristics to expand the application range.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jizheng Wang: 0000-0003-0477-4145 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Basic Research Program of China (2013CB632103), the National Natural Science Foundation of China (61474094, 61405208), National Key Research Program of China (2016YFA0200104), 973 Program (2014CB643600 and 2014CB643503), and The Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200).

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DOI: 10.1021/acsami.8b02671 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX