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A Hybrid Structure of 2D Layered GaTe with Au Nanoparticles for Ultra-sensitive Detection of Aromatic Molecules Pengqi Lu, Jiawei Lang, Zeping Weng, Arash Rahimi-Iman, and Huizhen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14121 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Materials & Interfaces

A Hybrid Structure of 2D Layered GaTe with Au Nanoparticles for Ultra-sensitive Detection of Aromatic Molecules Pengqi Lu1, Jiawei Lang1, Zeping Weng1, Arash Rahimi-Iman2, and Huizhen Wu1* Department of Physics and State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027 P.R. China. 2 Faculty of Physics and Materials Sciences Center, Philipps-Universität Marburg, 35032 Marburg, Germany. 1

*Email: [email protected] Abstract: Owing to a complex monocline structure and high-density of defects in monocrystalline GaTe, the performance of GaTe-based electronic devices is considerably compromised. Yet, the defects’ nature in GaTe could be a merit rather than a shortcoming in other realms. In our work, the density of defects in GaTe films is utilized for a facile decoration of Au nanoparticles (NPs), which allowed us to extend its application potential to the domain of surface enhanced Raman scattering (SERS) for the first time. Two-dimensional (2D) GaTe layered structures are prepared by mechanical exfoliation and high-density Au NPs are synthesized by immersion of 2D GaTe in HAuCl4 aqueous solution. By varying the immersion time, the sizes and coverage rate of Au NPs on GaTe can be elaborately tuned. Thanks to the defect nature of GaTe, the maximum coverage amounts to 98%. The hereby achieved Au-NPs–2D-GaTe hybrid structure demonstrates outstanding properties as a superior SERS substrate for ultra-sensitive detection of R6G aromatic molecules. Remarkably, the enhancement factor reaches up to 1.6 × 10ସ and the minimum detectable concentration is 10-11 M, undercutting that of recently reported Au-NPs–MoS2 SERS and Au-NPs–graphene SERS substrates which have the similar structure. With superior detection capability and facile preparation, Au-NPs–GaTe SERS substrates can become a perfect choice for the detection of aromatic molecules. Keywords: GaTe, Au nanoparticles, two dimensional materials, defects, surface enhanced Raman scattering, aromatic molecules detection. 1. Introduction Driven by the upsurge of graphene studies, many 2D-layered materials have been identified and studied with respect to their opto-electronic as well as structural properties. These 2D materials include transition-metal dichalcogenides, transition-metal oxides, BN, black phosphorus and the family of III-VI compounds (GaS, GaSe and GaTe). Many of those exhibit desirable and different properties which have been effectively utilized for photonics, biosensors, catalysis, and energy supply.1-7 Compared to well-studied graphene, BN, and MoS2, much less attention has been paid to 2D GaTe, which can be attributed to a possible high density of defects exhibited by monocrystalline GaTe.8-10 GaTe possesses a more complicated monoclinic structure compared to other layered materials ଷ as shown in Fig.1(a), featuring a low symmetry and the space group Cଶ୦ . From the view along the b-axis, two types of Ga-Ga bonds can be observed in one single layer: one third of those bonds are parallel to the layer plane and two third are perpendicular. Thus in a GaTe unit cell, there is only a two-fold rotational symmetry along the b-axis and no rotational symmetry perpendicular to the 1

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(201) plane (Fig.1(a)). So far most discovered 2D materials have high crystal symmetry5,6,11,12 and very few studies were reported about highly anisotropic 2D system. The low crystal symmetry of monocline GaTe indicates that it could become a significant representative of this kind of structures for scientific research and technical applications related with anisotropic physical properties. Recently, a few reports about 2D GaTe films have been published with the focus on their applications in field-effect transistors and optoelectronic devices.10,13,14 However, the performance of such 2D GaTe devices gets considerably compromised by the high-density defect nature of GaTe films. Thus, the reduction of defects in GaTe films has become a primary goal in order to realize reasonably good device performance.10 The question arises whether one can exploit the high-density defects of GaTe in 2D-layered structures tailored for certain applications, where defect-related physics can be rather seen as a merit than a shortcoming. For example, surface decoration of GaTe can be pursued in order to functionalize the material, which could be realized in case of few-layer 2D GaTe with metallic nanoparticles (NPs) and is a promising approach to open an alternative route to the functionalization of GaTe.15-18 Moreover, the high density of defects in GaTe which impedes its applications in electronic devices can herein be a great advantage over MoS2 or graphene because the formation of Au NPs is closely related to the existence of surface defects,15,16,19 ultimately making Au decoration much more facile and effective. In our work, a simple two-step process is employed to decorate 2D GaTe films with Au NPs and the achieved hybrid structure presents a superior surface-enhanced Raman spectroscopy (SERS) capability with ultra-high sensitivity of aromatic-molecules detection, which has constantly been one of the most popular research hotspots in realms of chemistry and biology.20-23 The produced Au-NPs–GaTe hybrid shows strong Raman enhancement of rhodamine 6G (R6G) molecules even at the resonance excitation wavelength (532nm). The limiting detectable concentration is found to be as low as 1 × 10ିଵଵ M, being considerably lower than that of Au-NPs–MoS2 SERS substrates15 and recently reported Au-NPs–graphene SERS substrates,16 which is partially attributed to the high density of defects in 2D GaTe. Such a result indicates that Au-NPs–GaTe hybrid structures can become a promising candidate for ultra-sensitive detection of aromatic molecules, opening up an avenue for applying 2D GaTe in a wide variety of sensing or detection fields, including for instance bio-molecule detection, biomedical monitoring schemes, and pesticide detection. 2. Results and Discussion

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Figure 1. (a) Left panel: Schematic representation of the GaTe crystal structure. Right panel: Top view along the direction perpendicular to the (201) plane (up) and the view along the b-axis (down). (b) A typical AFM image of 2D GaTe (top) with corresponding line profile (bottom). High-quality few-layer GaTe films can be facilely obtained by repeatedly peeling a bulk GaTe crystal with Scotch tape. Fig.1(b) shows a typical atomic force microscopy (AFM) image and the corresponding line profile, which clearly reveals the thickness of the GaTe film ranging from 4 to 6 layers. A corresponding optical image is provided in Figure S1, in which the parts with different layer number can be recognized through the different surface color (optical contrast). Fig.2(a) shows a piece of mechanically exfoliated few layer GaTe film transferred onto a SiO2(300nm)/Si substrate. Black arrows indicate the direction in which the layer number increases as displayed by the varying surface colors. The large and plain area indicated by black lines is the 2D GaTe which we needed for decoration of Au NPs. The deep green color of the surface, which is similar to the color of the diamond-marked part shown in Figure S1, indicates that the chosen area’s thickness is 6 layers. The very small thickness is also verified by Raman and photoluminescence measurements, and the results are shown in Fig.2(b). The Raman spectrum exhibits a distinct peak at 118.3 cm-1, while the Raman signal around 112 cm-1 is very weak, clearly demonstrating the few-layer nature of the GaTe film. Furthermore, the peak of photoluminescence (PL) at 746nm indicates that the film is thinner than 10nm.14 The as-prepared few layer GaTe films are then decorated with Au NPs via a simple immersion process as described in the Methods Section. With optical microscopy observation, a direct visual change of surface color was obtained after the immersion process, i.e. the light-green part of the 2D GaTe changed into a pronounced green color (e.g. seen at the spot marked by a black star in Fig.2(a and c)). This modification is ascribed to the formation of Au NPs on the surface as verified by the results of X-ray photoelectron spectroscopy (XPS) measurement shown in Fig.2(d). The blue curve in Fig.2(d) shows the XPS spectrum of the unprocessed 2D GaTe . The strongly coupled peaks at 574 and 584.5 eV are attributed to Te 3d orbits while the peaks at 21.5 eV and 107.5 eV are assigned to Ga 3d and 3p orbits, respectively.24 The peaks at 165.5 and 193.0 eV are the Auger peaks of Ga. Besides, all other peaks can be attributed to the orbits of Te, C, and O elements as marked in Fig.2(d). Such a result corresponds well with the known elements in a GaTe film. The orange curve represents the XPS spectrum after immersion of the GaTe film in HAuCl4 aqueous solution, in which two strongly coupled peaks emerge (at 84.5 and 88.0 eV, as 3



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well as at 335.5 and 354.0 eV), which are attributed to Au 4f and 4d orbits respectively, declaring the existence of Au element on the GaTe surface. Besides, the peaks of unprocessed GaTe can also be observed in this spectrum with reduced intensity, indicating that the GaTe film is not completely covered by Au NPs, otherwise signals of Te and Ga are not detectable by XPS because the sizes of Au NPs are bigger than 10 nm. To figure out detailed morphology after Au NPs coating, a scanning electron microscopy (SEM) was employed.

Figure 2. (a) An optical image of a few-layer GaTe film. The number of layers increases in the direction marked by the black arrows. The plain part encircled by black lines was chosen for Raman and PL characterizations. (b) Raman and PL spectra of the GaTe film shown in Fig.2(a). (c) An optical image of the GaTe film shown in Fig.2(a) after immersion in HAuCl4 aqueous solution. The black star marks the part of the film where the color changes from bright to regular green after the immersion, indicating NP coverage. (d) XPS data of the 2D GaTe before and after the immersion process, which shows the adding up of Au signatures.

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Figure 3. (a)-(i) show the surface of few-layer GaTe films after immersion in HAuCl4 solution for 0, 10, 30, 60, 120, 240, 480, 1920, and 3840s, respectively. The insets of (b) - (e) show the size distribution as well as the mean diameter of Au NPs. The inset of (f) shows the immersion-time-dependent coverage of Au NPs on the GaTe films. Fig.3(a) shows the surface of a GaTe film without immersion in HAuCl4 solution. The GaTe surface is very clean and no covering can be seen by SEM. All other eight images from Fig.3 (b) to (i) show the surface of the few-layer GaTe films immersed in HAuCl4 solution for 10, 30, 60, 120, 240, 480, 1920, and 3840s, respectively. The Fig. 3(b) clearly demonstrates that small Au NPs with high density are formed on the surface within a short reaction time of 10s. With prolonged reaction time, Au NPs become larger with the mean diameter increased from 12.44 nm (10s) to 29.73nm (120s) as shown in the insets of Fig.3(c) to (e). When immersion time increases further to 240s or much longer, there are no more isolated NPs on the surface of GaTe, instead they coalesce together and cover most of the area on the GaTe surface as displayed in Fig.3(f) to (i). As can be seen in the inset of Fig.3(f), the coverage of Au NPs increases rapidly from 0% to 95% in the first 240s, then turns to a slow increase to 98% from 240 to 3840s. Such a high coverage of Au NPs can be ascribed to the high density of defects in the few-layer GaTe, declaring the advantage of GaTe over other 2D materials for decoration with Au NPs. Besides, the surface morphology of Au NPs-GaTe films also had a good consistency as shown in Figure S3. The growth mechanism of Au NPs is illustrated in the Figure 4. The existence of Ga vacancy (VGa) defects in GaTe films plays a key role in the formation of Au NPs. The GaTe utilized in this work is of p-type conduction (Figure S2(b)), in agreement with the first principles calculation of formation energy that most defects are VGa.10 Due to the existence of VGa, the outward Te atoms 5



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which had bonded with the bygone Ga atoms become negative ions in this case. It has been reported that unbound sulfur S2- can actively reduce Au3+ in the reaction of AuCl3 and Na2S.25, 26 The formation of Au NPs on the laser processed MoS2 films was attributed to the existence of S2-.15 A similar reaction is understood here to happen on the GaTe surface in the form of 3Te-+Au3+=3Te+Au, which leads to the nucleation and formation of Au NPs.

Figure 4. The schematic diagram of growth mechanism of Au nanoparticles on GaTe with Ga vacancy defects. Thanks to the defect nature of GaTe, as shown in Fig.3, Au-NPs–GaTe hybrid structures can be facilely prepared with a wide and elaborate tunability of Au coverage from 0% to 98%. However, the question might arise here whether the highly Au NPs covered GaTe films have any practical application. SERS has been widely utilized in the fields of chemistry, biology, and biomedicine in the past decades. However, a challenge remains to fabricate an excellent SERS substrate for the detection of some molecules, such as nonthiolated aromatic molecules, because of the low adsorption capability of these molecules onto the plasmonic metals.27,28 However, the adsorption of nonthiolated molecules onto the 2D materials is much more efficient as a result of the π-π interaction between molecules and 2D materials.15,28,29 Herein the Au-NPs–GaTe hybrid structure is utilized as a superior SERS substrate in the realm of aromatic molecules detection and feasible realization of ultra-high sensitivity for the detection of R6G molecules is demonstrated. Au-NPs–GaTe hybrid structures were coated with R6G through immersion in R6G aqueous solution (10-4M) for 20 min, followed by rinsing with ethanol and drying by N2 gas. As a comparison, original GaTe films and bare SiO2/Si substrates were processed with the same procedure. Raman scattering measurements were conducted under the resonant wavelength excitation of R6G (532nm). As shown in Fig.5(a), strong and high signal-to-noise ratio Raman signals are observed in the spectrum of the R6G/Au NPs/GaTe substrate, while only a few weak Raman peaks can be seen in the spectrum of the R6G/GaTe substrate. When collecting Raman signals from the R6G/SiO2/Si substrate, no vibration modes are seen. These interesting results indicate the significance of Au-NPs–GaTe hybrid structures on the Raman signal enhancement. Since aromatic molecules in general are not adsorbed efficiently on metallic surface,27 they are expected to be adsorbed on the surface of GaTe due to the π-π interaction between molecules and GaTe,15,28,29 and get Raman intensity enhanced under the effect of surface plasmons induced by Au NPs. The Raman enhancement of R6G on metal-2D material substrates normally arises from electromagnetic enhancement resulting from the strongly-localized surface plasmon resonance and 6


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chemical enhancement including charge transfer as well as molecular resonance.15,30 Well known as an excellent plasmonic metal, Au NPs on the GaTe surface could generate a strong electric field resulting from plasmon resonance at interstitial sites of the Au NPs, influencing the R6G molecules absorbed on the GaTe. Scaled as fourth power of the local field near the Au NPs, the enhancement can be very strong.30 Charge transfer could happen between R6G molecules and Au NPs or GaTe, and since the R6G molecules are resonantly excited via the 532nm laser, the effect of surface enhanced resonance Raman spectroscopy (SERRS) should be another impact factor.31 The effects of molecular resonance and charge transfer are coupled and according to the coupled resonance theory, the Raman bands of R6G around 618 and 780 cm-1 could get their intensity enhanced via a vibronic coupling mechanism allowing a nearby transition,32 which is well consistent with our Raman spectra. Therefore, the conclusion can be made that SERS here can be attributed to a combined effect of surface plasmon, molecular resonance and charge transfer.

Figure 5. (a) Raman spectra of the compared R6G/SiO2, R6G/GaTe/SiO2, and R6G/Au NPs/GaTe/SiO2 samples. Intense SERS signals are observed in R6G/Au NPs/GaTe/SiO2. The weak Raman signals of R6G/GaTe/SiO2 are marked by the black rectangles. (b) Intensity of R6G Raman signals versus different immersion time of GaTe in HAuCl4 solution. The inset shows the Raman spectrum of the sample for an immersion time of 960s. (c) Intensity of Raman signals as a function of concentration of R6G. The inset shows the Raman spectra of the samples immersed in 1×10-10M and 1×10-11M R6G solution (see Raman spectra of all the prepared concentrations of R6G in Figure S6). Raman peaks of R6G are indicated by the orange and blue triangles. (d) Raman signals of R6G on the Au NPs/GaTe/SiO2 SERS substrate recorded for stability testing. 7



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The orange one was measured at the initial stage, while the purple one was measured after exposure to atmosphere for two months. The intensity of SERS signals is influenced by the morphology of Au NPs on the GaTe surface, such as density and size, which can be tuned by changing the immersion time of GaTe in HAuCl4 solution as mentioned above. We firstly tested the uniformity of Raman signals when collected from various locations of the structures (Figure S4) as well as the reproducibility of the SERS signals (Figure S5). Then, a systematic immersion experiment was conducted to optimize the immersion time. Nine samples prepared using immersion times 10, 30, 60, 120, 240, 480, 960, 1920, and 3840s were all treated equally with R6G solution and their Raman spectra were measured. The intensity of the Raman peak at 1652.1cm-1 is plotted as a function of the immersion time in Fig.5(b). As can be seen, the intensity increases rapidly at the initial stage, then tends to be saturated and reaches its maximum at 960s. When immersion time is further increased, Raman signal intensity becomes gradually decreased. This phenomenon can be readily explained by the change of Au coverage. Obviously, the initial increase of SERS signal results from the enhancement of strong coupling between Au NPs and R6G molecules as the density of Au NPs on the GaTe surface becomes higher with prolonged immersion time. However, as R6G molecules are adsorbed on the GaTe surface, a higher Au coverage with prolonged immersion time also implies a smaller GaTe area available for the adsorption of R6G. The gradually decreased amount of adsorbed R6G molecules shall counteract the enhancement resulted from plasmon coupling, leading to the SERS intensity saturation and eventual decrease of Raman intensity15. The inset of Fig.5(b) shows the optimized Raman signals when the immersion time is 960s, in which a small feature at 1228.9cm−1 is clearly revealed, in strong contrast to the most reported SERS substrates that could not detect the weakest peak under the same R6G concentration28,33,34. The corresponding enhancement factor (EF) is concluded to be 1.6 × 10ସ according to the calculation,

EF =

୍౩౛౨౩ ୍౨౛౜

୒

× ୒ ౨౛౜ (see SI for calculation details).35 ౩౛౨౩

As demonstrated in Fig.5(b), SERS signal reaches its maximum when immersion time of GaTe in HAuCl4 solution is 960s. The synthesized Au-NPs–GaTe hybrid substrates were then immersed into R6G solution with varied concentration from 1×10–11M to 1×10–3M for 20 min to explore its quantitative detection performance of the molecules, which is one of the important practical applications of SERS substrates. As expected, Raman signals decrease with the reduction of R6G concentration. The intensity of the Raman peak at 1652.1cm-1 is plotted as a function of the concentration in Fig.5(c), which corresponds well to the Langmuir model.29 It can be seen that distinct Raman signals can be detected for R6G solution at concentration levels as low as 10-11M. This limiting concentration is lower than that of recently reported Au-NPs–MoS2 SERS and Au-NPs–graphene SERS substrates15,16 which have the similar structure, indicating the superiority of few-layer GaTe utilized in the SERS realm. Though there remain a few SERS substrates with even higher EF or lower detection limit, these structures are complex and often artificially developed through nano processing with expensive fabrication cost, such as Ag/graphene/Au sandwiched structures,36 substrates with artificial nanocavity arrays,37 artificial Au NPs arrays on graphene.17 As a result, the promising potential of few-layer GaTe films serving as SERS substrates can also be exploited further with some specially designed structures. Noble Au-NPs–GaTe substrates are stable in atmosphere. After exposing Au-NPs–GaTe hybrid structures 8


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in atmosphere for two months, the Raman intensity of R6G on it remains at about 60 percent as shown in Fig.5(d). Besides R6G, Methylene blue (MB) and rhodamine101 (R101) molecules were tested similarly. The detection limit of them is found to be 1×10–10M (see SI), declaring again the promising potential of Au-NPs–GaTe in aromatic molecules detection. Considering the unique characters of the Au-NPs–GaTe hybrid structures discovered in this work, the performance of a specially designed 2D layered GaTe based SERS substrate is possibly superior to that of artificially developed graphene based SERS substrates. 3. Conclusion In conclusion, we have employed the defects nature of few-layer GaTe for decoration of Au NPs and demonstrated the capability of applying 2D GaTe in hybrid structures as SERS substrates. 2D few-layer GaTe based Raman active surfaces are prepared via a mechanical exfoliation procedure and a consecutive immersion in HAuCl4 solution. The effective formation of Au NPs on the GaTe surface is confirmed by XPS and SEM measurements. By varying the immersion time, the size and coverage of Au NPs can be elaborately tuned and a maximum coverage of 98% achieved, highlighting the advantage of few-layer GaTe with regard to other 2D materials concerning decoration with Au NPs. The developed Au-NPs−2D-GaTe hybrid structure is then utilized as a superior SERS substrate for R6G molecules. The maximum EF and corresponding limiting detectable concentration is found to be 1.6 × 10ସ and 10-11M, respectively, using an immersion time of 960s. Combining facile preparation and outstanding performance, such SERS substrates based on 2D GaTe can become a promising candidate for ultra-sensitive detection of aromatic molecules. Methods section Few-layer GaTe flakes were mechanically exfoliated from bulk GaTe (99.999%, Aladdin) using Scotch tape and transferred to SiO2(300nm)/Si substrate. A traditional optical microscope (Olympus, BX53) was employed to observe the surface color. The thickness was examined by an atomic force microscope (AFM, Multimode-8) under the contact mode. All the Raman and PL spectra were measured under 532-nm continuous-wave laser excitation using an Andor SR-500i-B1 spectrograph. All parameters involved in the measurements kept same, such as the power density of excitation, exposure time and number of accumulations. For decoration of Au NPs, the as-prepared 2D GaTe/SiO2 substrates were immersed into 0.2mg/ml HAuCl4 (AR, Sinopharm Chemical Reagent Co., Ltd) aqueous solution in a beaker. The temperature of solution was controlled by a thermostat. After immersion in solution for different periods, the substrates were taken out, rinsed with ethanol and dried by N2 gas. X-ray photoelectron spectroscopy (VG ESCALAB MARK II) was employed to analyze the components of NPs. The morphology of the NPs was examined with a scanning electronic microscope (SEM, Hitachi S4800). For SERS characterization, Au-NPs–GaTe substrates were immersed in R6G (99%, Aladdin), R101 (98%, Aladdin) and MB (90%, Meryer) aqueous solutions for 20min, then rinsed with ethanol and dried by N2 gas. Supporting Information An optical image of GaTe film utilized in AFM measurement, GaTe FET and its output characteristics, consistency of surface morphology among samples, influence of temperature, 9



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uniformity of Au NPs-GaTe film, Reproducibility of the SERS signals, SERS spectra of all concentration of R6G, calculation of enhancement factor, detection of R101 and MB molecules. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (Nos. 61290305 and 11374259). References (1) Wang, Q.; Kourosh, K.; Andras, K.; Jonathan, N. C.; Michael, S. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nano 2012, 7, 699-712. (2) Yu, Y.; Huang, S.; Li, Y.; Stephan N. S.; Yang, W.; Cao, L. Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553-558. (3) Dean, C. R.; Young, A.F.; Meric, I.; Lee, C.; Wang, L.; orgenfrei, S.S; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron nitride substrates for high-quality graphene electronics. Nat. Nano, 2010, 5, 722-726. (4) He, S.; Liu, K.; Su, S.; Yan, J.; Mao, X.; Wang, D.; He, Y.; Li, L.; Song, S.; Fan, C. Graphene-Based High-Efficiency Surface-Enhanced Raman Scattering-Active Platform for Sensitive and Multiplex DNA Detection. Anal. Chem. 2012, 84, 4622-4627. (5) Hu, P.; Wang, L.; Yoon, M.; Zhang, J.; Feng, W.; Wang, X.; Wen, Z.; Idrobo, J. C.; Miyamoto, Y.; Geohegan, D. B.; Xiao, K. Highly Responsive Ultrathin GaS Nanosheet Photodetectors on Rigid and Flexible Substrates. Nano Lett. 2013, 13, 1649-1654. (6) Late, D. J.; Liu, B.; Luo, J.; Yan, A.; Matte, H. S. S. R.; Grayson, M.; Rao, C. N. R.; Dravid, V. P. GaS and GaSe Ultrathin Layer Transistors. Adv. Mater. 2012, 24, 3549-3554. (7) Li, R.; Schneider, L. M.; Heimbrodt, W.; Wu, H.; Koch, M.; Rahimi-Iman, A.; Gate Tuning of Förster Resonance Energy Transfer in a Graphene - Quantum Dot FET Photo-Detector. Scientific Reports 2016, 6:28224. (8) Cui, Y.; Caudel, D. D.; Bhattacharya, P.; Burger, A.; Mandal, K. C.; Johnstone, D.; Payne, S. A. Deep levels in GaTe and GaTe:In crystals investigated by deep-level transient spectroscopy and photoluminescence. J. Appl. Phys. 2009, 105, 053709. (9) Yamamoto, A.; Syouji, A.; Goto, T.; Kulatov, E.; Ohno, K.; Kawazoe, Y.; Uchida, K.; Miura, N. Excitons and band structure of highly anisotropic GaTe single crystals. Phys.Rev. B 2001, 64, 035210. (10) Wang, Z.; Xu, K.; Li, Y.; Zhan, X.; Safdar, M.; Wang, Q.; Wang, F.; He, J. Role of Ga Vacancy on a Multilayer GaTe Phototransistor. ACS Nano,; 2014, 8, 4859-4865. (11) Wu, S.; Huang,C.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. Vapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization. ACS Nano 2013, 7, 2768-2772. (12) Xu, K.; Wang, Z.; Du, X.; Safdar, M.; Jiang, C.; He, J. Atomic-layer triangular WSe2 sheets: synthesis and layer-dependent photoluminescence property. Nanotechnology 2013, 24, 465705. (13) Wang, Z.; Safdar, M.; Mirza, M.; Xu, K.; Wang, Q.; Huang, Y.; Wang, F.; Zhan, X.; He, J. High-performance flexible photodetectors based on GaTe nanosheets. Nanoscale 2015, 7, 7252-7258. (14) Hu, P.; Zhang, J.; Yoon, M.; Qiao, X.; Zhang, X.; Feng, W.; Tan, P.; Zheng, W.; Liu, J.; Wang, X.; Idrobo, J. C.; Geohegan, D. B.; Xiao, K. Highly sensitive phototransistors based on two10


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Hybrid Structure of 2D Layered GaTe with Au Nanoparticles for

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