Ultrathin GeSe Nanosheets: from Systematic Synthesis, to Studies of

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Functional Nanostructured Materials (including low-D carbon)

Ultrathin GeSe Nanosheets: from Systematic Synthesis, to Studies of Carrier Dynamics and Applications for High Performance UV-Vis Photo-Detector Dingtao Ma, Jinlai Zhao, Rui Wang, Chenyang Xing, Zhongjun Li, Weichun Huang, Xiantao Jiang, Zhinan Guo, Zhengqian Luo, Yu Li, Jianqing Li, Shaojuan Luo, Yupeng Zhang, and Han Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19836 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Materials & Interfaces

Ultrathin GeSe Nanosheets: from Systematic Synthesis, to Studies of Carrier Dynamics and Applications for High Performance UV-Vis Photo-Detector Dingtao Ma†, Jinlai Zhao†, Rui Wang‡,§, Chenyang Xing‡, Zhongjun Li†, Weichun Huang‡, Xiantao Jiang‡, Zhinan Guo‡, Zhengqian Luo§, Yu Li‡, Jianqing Li†, Shaojuan Luo‡, Yupeng Zhang‡*, and Han Zhang‡* † Faculty of Information Technology, Macau University of Science and Technology, Taipa, Macau SAR 999078, P. R. China ‡ Collaborative Innovation Center for Optoelectronic Science and Technology and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, P. R. China § Department of Electronic Engineering, Xiamen University, Xiamen 361005, P. R. China *Email: [email protected]; Email: [email protected] Keywords: GeSe nanosheets, liquid phase exfoliation, carrier dynamics, photoelectrochemical, photo-detector

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Abstract: Owing to the attractive energy band properties, black phosphorus (BP)-analogue semiconductor, germanium selenide (GeSe), shows a promising potential applied for optoelectronic devices. Herein, ultrathin GeSe nanosheets were systematically prepared via a facile liquid phase exfoliation (LPE) approach, with controllable nano-scale thickness. Different from BP, ultrathin GeSe nanosheets exhibits a good stability under both liquid and ambient conditions. Besides, its ultrafast carrier dynamics was probed by transient absorption spectroscopy. We showed that the GeSe nanosheets-based photo-detector exhibits excellent photoresponse behaviors ranging from ultraviolet (UV) to the visible regime, with high responsivity and low dark current. Furthermore, the detective ability of such a device can be effectively modulated by varying the applied bias potential, light intensity and concentration of electrolyte. Generally, our present contribution could not only supply fundamental knowledge of GeSe nanosheets-based photoelectrochemical (PEC)-type device, but also offer a guidance to extend other possible semiconductor materials in the application of PEC-type photo-detector.


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Introduction Owing to the unprecedented physical, electronic, and chemical properties inherited from the ultrathin planar structures, the past decade have witnessed a huge development of twodimensional (2D) materials in various fields including electronics, optoelectronics, and sensors.1-6 Among them, the emerging black phosphorus (BP) has recently aroused great research interests. Compared with the graphene (high carrier mobility but zero band gap) and transition-metal dichalcogenides (TMDs) (low carrier mobility, with relatively large band gap of 1.5 eV~2.5 eV), their tunable direct band gap ranging from 0.3 eV to 1.5 eV, as well as high carrier mobility (up to 1000 cm2 V-1 s-1) provide more opportunities for innovative devices.7-14 However, their unstable behavior of easy degradation under ambient conditions, usually impedes their practical applications. In the meanwhile, mass production of black phosphorus remains a challenge, which normally leads to cost-ineffectiveness. Consequently, in order to meet with the blooming of optoelectronic devices, further explorations of other possible 2D materials with appropriate characteristics including proper band gap, air-stability, high carrier mobility and low cost are still mandatory. In contrast with the immense interests on other layered semiconductors like Group-III chalcogenides, IV-VI layered metal chalcogenides, i. e., MX (M = Ge, Sn; X = S, Se), seems to be received with less attention.15-22 Indeed, those chalcogenides also show great potentials in electronics and optoelectronics devices owing to their intriguing properties of high carrier mobility and planar anisotropy.23-25 Among them, 2D germanium selenides (GeSe) seems to be the most striking candidate for photodetector with the several following reasons: (1) Ge and Se elements are relatively earth-abundant and environment-friendly in nature; (2) Monolayer GeSe


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is the only semiconductor with a direct band gap among those four alternatives, and its narrow band gap of 1.1-1.2 eV also implys photoresponse behaviors overlapping the solar spectrum; (3) GeSe also shows a high absorption coefficient of >104 cm-1. Besides, the calculated effective electron mass of GeSe is 0.31 mo,26 which is less than that of MoS2 (0.48 mo).27 The predicted electron mobility of monolayer GeSe even reaches an order of magnitude of 103 cm2 V-1 S-1 28, which is higher than that of WS229 and MoS230; (4) due to the limited dangling bonds on the surface, GeSe theoretically has a higher environmental stability than BP in practical applications.31 Till now, only few works about GeSe nanosheets-based photo-detectors have been reported. This should be mainly cuased by its high fragility of mono-crystalline flakes, and thus hidering its effective and mass production.26 Among them, Yoon et al. firstly proposed a vapor-condensation-recrystallization (VCR) method to synthesize GeSe nanosheets by using commercial bulk GeSe powder as the raw material.32 Besides, Vaughn II et al. put forward onepot solution-chemistry route to obtain GeSe nanosheets with an average thickness larger than 20 nm.33 Similarly, Mukherjee et al. employed chemical vapor deposition (CVD) to prepare GeSe crystals.34 It should be pointed out that all these methods are time-consuming and usually required accurate control of the technological parameters. Beyond these vapor-based methods, Zhao et al. developed a laser-thinning technique to fabricate GeSe nanosheets with different thicknesses.35 However, such method typically required a high vacuum condition, and the integrity of the crystal structure during the laser etching process was unwarrantable. Moreover, Ramasamy et al. reported a solution based strategy to synthesize GeSe nanosheets, while the thickness of product usually exceed 200 nm.36 Consequently, it is highly desirable to develop a facile and efficient approach to realize ultrathin GeSe nanosheets with high-quality and highyield, and systematically evaluate their potentials in photo-detector applications.


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Herein, liquid-phase exfoliation (LPE) approach had been employed to fabricate ultrathin GeSe nanosheets with high-yield and high-quality. The probe and bath sonications could easily separate the adjacent layers within GeSe crystal via expansing the inter-spacing and weakening the van der Waals interaction in liquid environments. The thickness and size distribution could be easily controlled by adjusting the centrifugation speed. Besides, femtosecond transient optical absorption spectroscopy was employed to investigate the transition process of photoexcited carriers of the GeSe nanosheets. In the following, the optoelectronic performances were systematically evaluated by applying it as photoanode in photoelectrochemical (PEC)-type photodetector. Such novel photodetector shows several competitive advantages when compared with the conventioal field-effect transistors based devices. They can be self-driven even without power supply in order to separate electrons and holes, which is required in the situation such as emergency cutoff or circuit. Additionally, such devices are considered to be low-cost, environmental-friendly and easily-assembled.37-40 Finally, our experimental results demonstrated that the bias potential, light wavelength, power intensity, and the concentration of electrolyte can significantly influence the photoresponse behaviors of GeSe nanosheets. Our contribution may provide the fundamental knowledge for constructing high performance GeSe nanosheets-based photodetector through the optimization of working conditions, inspiring the applications of other black phosphorus-analogue semiconductors.

Results and discussion Ultrathin GeSe nanosheets were synthesized by LPE from bulk GeSe powder in Nmethylpyrrolidone (NMP) organic solvent, including the probe and bath sonication, and subsequent centrifugation treatment (Figure 1). During this process, van der Waals forces


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between the adjacent layers of bulk GeSe tend to break and the solvent-nanosheets interaction could balance the inter-sheet attractive forces to stabilize the dispersion of nanosheets. Detailed description can be found in the Experimental Section. The effect of various solvents on the exfoliation of bulk GeSe were also compared and discussed (Figure S1). And the comparison confirmed the best exfoliated result from the NMP solvent, which might be caused by the best matching of surface energies between the solvent and GeSe nanosheets. Besides, the influence of centrifugation speed on the thickness and size distribution of the GeSe nanosheets were also studied (Figure S2, S3). The crystal structure was characterized by employing the X-ray diffraction (XRD) measurements. As presented in Figure 2a, all the main reflections of GeSe nanosheets are in accordance with the Bragg position of GeSe (JCPDS: 48-1226), and no discernable oxide phase impurities were detected. And the decreased intensity of several diffraction peaks (e.g. (201), (111), and (311)) of GeSe nanosheets might be resulted from the formation of nanocrystals and/or defects on the exposed surface after exfoliation.36,37 UV-Vis spectroscopy was carried out to detect the optical property of GeSe nanosheets. As depicted in Figure 2b, GeSe nanosheets show a broad absorption acrossing the ultraviolet and visible regions with a gradual downward trend, especially in the range from 300 to 500 nm with a high absorption intensity, which indicates it a great potential in the application for UV-vis photodetectors. Besides that, the effect of thickness on the optical and electrical properties of the GeSe nanosheets were also studied (Figure S4, S5). The topographic morphology of GeSe nanosheets were conducted by using AFM measurements (Figure 2c). The averaged thickness was estimated to be around 4.3 ± 0.2 nm, corresponding to 3-layers in thickness based on the previously theoretical calculations.26,33,35 Besides, the morphology of GeSe nanosheets were observed by FESEM characterizations.


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Obviously, bulk GeSe (Figure S6a) changed to nanosheet-like structure after LPE (Figure S6b). Figure 2d displays the TEM image of the GeSe nanosheets, corresponding to elemental mapping images also prove the uniform distribution of Ge and Se elements. A clear crystal lattice spacing of ≈ 0.23 nm in the HRTEM image can be well assigned to the (400) plane of GeSe (Figure 2e). Besides that, the corresponding selected area electron diffraction (SAED) image (inset of Figure 2e) also demonstrated the high crystal quality of the as-prepared GeSe nanosheets.41,42 Similar to BP, the puckered structure of GeSe also consists double-layer slabs of covalent coordinated Ge-Se in a chair configuration and be separated by weak van der Waals forces, as illustrated in Figure 3a. According to the conservation of momentum and the group theory, totally 12 active Raman modes exist in GeSe of the D2h16 symmetry.43 However, only three vibrational modes Ag3, B3g1, and Ag1 (see in Figure 3b) can be readily detected when the incident laser is perpendicular to the GeSe plane. Besides, among the three typical Raman vibrational modes, B3g1 represents the out-of-plane mode, which is more sensitive to the layer number than the Ag1 and Ag3 modes.44 As shown in Figure 3c, the Raman peaks located at 76 cm-1, 142 cm-1, and 180.6 cm-1 are respectively corresponding to the Ag3, B3g1, and Ag1 modes, which is consistent with the previous reports.45 The comparison of Raman mapping of I(Ag3)/I(Ag1) for GeSe nanosheets and bulk counterpart was also studied (Figure S7). The corresponding histograms show that the value of I(Ag3)/I(Ag1) for GeSe nanosheets and bulk GeSe were both mainly distributed between 0.6~0.8, indicating the structural integrity of GeSe nanosheets even after exfoliation. Usually, the degradation of mechanical and electronic properties of the most 2D materials is caused by the oxidation under ambient conditions. Previous calculations have indicated that oxidation could lead to severe local deformation on monolayer GeSe, and thus affecting the optoelectronic properties.46 Herein, time-dependent Raman spectrum measurements


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were performed to examine the possible oxidation of GeSe nanosheets in air. For Raman characterizations, GeSe nanosheets were firstly loaded on the membrane filter and then exposed in air for various times. As shown in Figure 3d, no obvious change in Raman spectra can be found, and no position shift or intensity decrease of B3g1 Raman peak has been observed even after 14 days, depicting that the as-prepared GeSe nanosheets is stable in the air. In addition, time-dependent UV-Vis absorption tests (Figure 3e) also show that GeSe nanosheets could present a strong absorbance without significant reduction, even after two weeks, indicating that GaSe possesses an excellent long term stability different from BP. Such good stability might be resulted from the large activation energy for the chemisorption of O2 molecules on the surface of GeSe nanosheets, leading to the high oxidation resistance.47 The ultrafast carrier dynamics of the GeSe nanosheets were studied by femtosceond transient optical absorption spectroscopy. As shown in Figure S8, GeSe nanosheets shows similar transient absorption signals in various liquid environments, including H2O, ethanol, and IPA. To simplify the experiments, we herein just selected the IPA solvent as testing environment and do a comprehensive analysis. Such studies are expected to provide a basic understand about the band structure of the GeSe nanosheets, as well as the transition process of photoexcited carriers. Figure 4a shows the temporally and spectrally resolved transient absorption signal of GeSe nanosheets. In view of the intense bleaching effect caused by the high intensity pump light at a wavelength of 400 nm, and the strong noise signals at both long and short wavelength side, we therefore choose a visible region ranging from 500 nm~650 nm for mechanism analysis. The transient absorption spectra of the GeSe nanosheets at different delay times are shown in Figure 4b. It is clearly that the transient absorption signals tend to decrease followed with the increasing delay time. Such a phenomenon could be attributed to further movement of excited electrons. On


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one hand, the excited electrons are likely to decay back to ground state again through radiation and non-radiative transition. On the other hand, the excited electrons might be transited to the higher energy levels.48 In this case, the increasing number of escaped excited electrons led to a gradual decrease in absorption of probe light. Besides, the wide range of absorption signals also implies that the excited state absorption (ESA) predominate within the carrier transfer process. As can be seen in Figure 4c, this wide range of ESA signals may be attributed to lots of excited electrons (E1) absorb the energy of photons and transfer to higher energy levels (E2). On this basis, the transient absorption signals of the GeSe nanosheets were studied by the global fitting. Among them, the kinetic curves measured at the probe wavelength of 550 and 650 nm, were also shown and discussed in Figure S9. As depicted in Figure 4d, there is only one dominated decay kinetic. This kinetic curve can be fitted with the following exponential function: △A = A1 e-t/τ1+A2 e-t/τ2

(1)

Where, A1 and A2 represent the amplitude of each component; t is defined as the delay time between the pump light and probe light while 1 and 2 are the time constants corresponding to the lifetime of the GeSe nanosheets. Typically, the fast component refers to the carrier relaxation in conduction band, or the carrier get trapped by localized defect and surface state, while the slow one is relevant with the carrier recombination process.49 The corresponding fitting result demonstrates a fast and a slow component with the lifetime of 3.8 ± 1.5 ps and 57.7 ± 3.9 ps, respectively, which reveals that the carrier recombination behavior dominates the carrier transfer process in this region. Such a fast recovery time also demonstrates that GeSe nanosheets have a great potential applied for high-speed photo-detector.


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To evaluate the optoelectronic properties of GeSe nansheets, PEC-type photodetector with three electrodes system based on GeSe nanosheets was constructed, as shown in Figure 5a. Under the illumination of simulated light, the photogenerated electrons of GeSe nanosheets could be effectively transported to the ITO electrode. Meanwhile, the corresponding holes would move to the counter electrode through the KOH electrolyte. In addition, such liquid environment could also restrain the oxidation procedure and thus ensuring the stable photoresponse behaviors of GeSe nanosheets-based photodetector (Figure 5b). In this work, the critical parameters including photocurrent density (Iph), photoresponsivity (Rph), specific detectivity (D*), as well as the external quantum efficiency (EQE) are respectively calculated. The photoresponse performance of the GeSe nanosheets-based photodetector under various working conditions was evaluated by using the following equations: Iph = (Ilight - Idark) / S

(2)

Rph = Iph / Pλ

(3)

D* = Rph × S1/2 / (2q·Idark)1/2

(4)

EQE = h·c·Rph / (q·λ)

(5)

Where, Ilight, Idark and S represent the current density with/without the light irradiation, and effective loading area of ITO glass, respectively; Pλ refers to the light power density; q is defined as the electron charge with the value of 1.60×10-19 C; h is Planck’s constant with the value of 6.63×10-34 J·S; c is the speed of light with the value of 3×108 m·s-1; λ denotes the excitation wavelength. Figure 6 shows the typical photoresponse behavior under the irradiation of simulated light. The comparison of linear sweep voltammetry curves (Figure S10) for GeSe nanosheets in a range from 0 to 1.0 V, demonstrates a positive response to simulated light. The long-term on/off


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switching behavior of GeSe nanosheets with different bias potentials were explored (Figure 6a). Interestingly, a stable photoresponse behavior was clearly observed even without any bias potential supporting, which demonstrates the self-powered capability of the GeSe nanosheetsbased photo-detector. Besides, the photocurrent density can be effectively adjusted through the control of bias potential. Typically, the photocurrent density of the GeSe nanosheets-based photo-detector enhanced from 2.1 to 4.4 and 7.1 μA cm-2, with the applied bias potential increasing from 0 to 0.3 and 0.6 V, respectively. Beyond that, the influence of light power intensity on the photocurrent density of GeSe nanosheets were also carried out (Figure S11 and Figure 6b). The photocurrent density displayed a gradual improvement along with the increasing light power intensity. Particularly, a three times enhancement in photocurrent density can be achieved when the light power intensity increased from 26 mW cm-2 to 122 mW cm-2. The calcuated detectivity D* value of GeSe nanosheets-based photo-detector in Table S1 are in the order of magnitude of 1010 Jones, which is higher than that of 2D Te nanosheets (106 Jones)52 and Bi2S3 nanosheets (107 Jones)40, demonstrating the smaller dark current of GeSe nanosheetsbased photo-detector. As another vital factor for practical application, the cycling and temporal stabilities of the GeSe nanosheets-based photodetector were also investigated before and after storing 7 days, and 14 days, as shown in Figure S12. Our results demonstrated that such device exhibits a quite good cyclic stability, while the photocurrent density weaken gradually after longterm storage can be ascribed to the partial peeling of the effective material from the ITO glass.52 The response time (tres) and recovery time (trec) of the GeSe photodetector is derived from the time interval for the rise and decay from 10% to 90% and from 90% to 10% of its peak value, respectively. For instance, the value of tres and trec in 0.1 M KOH with a bias potential of 0.3 V were calculated to be 0.2 s and 0.3 s, respectively (Figure S13), which are much lower than that


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of previous reported BP-based photodetector36,37. Furthermore, the effect of bias potential on the photoresponse behavior of the GeSe nanosheets-based photodetector was also performed. As shown in Figure 6c, the photocurrent density shows a positive relationship with bias potential. Concretely, such GeSe photo-detector can achieved a photocurrent density of 8.5 μA cm−2 at 0.8 V, comparing with the value of 1.4 μA cm−2 at 0 V. Combined with the result in Figure S14, the significant sensitivity of GeSe nanosheets toward the external bias potential was revealed. Figure 6d presents the 2D plot of the photocurrent density map as a function of the bias potential and concentration of electrolyte. Obviously, the photocurrent density exhibits sensitive dependence on bias potential, increasing from 2.35 μA cm−2 at 0 V to 22.6 μA cm−2 at 1.0 V, with a large span value of 20.2 μA cm−2 in 0.7 M electrolyte. Besides, the value of Iph changed from 4.5 to 19.3 μA cm−2 at a bias potential of 0.5 V, along with the concentration of KOH electrolyte increasing from 0.1 to 0.7 M, implying a similar sensitive behavior of the concentration of electrolyte on the performance of GeSe nanosheets-based photo-detector. Typically, both of bias potential and concentration of electrolyte play vital roles in regulating the photoresponse behaviors of GeSe nanosheets-based photo-detector. On one side, bias potential could promote the separation ratio of the photogenerated holes and electrons. On the other side, the increasing concentration of the electrolyte could also contributes to accelerate the PEC reaction, resulting in high photocurrent density. Considering the broad absorption of GeSe nanosheets, we further investigate the photoresponse properties of GeSe based photo-detector under the illumination at different wavelength lights (λ= 350, 365, 380, 400, 475, 550, and 650 nm). Figure 7a shows the long-term on/off tests under various wavelengths of light under fixed power density of level IV (i.e., 118 mW cm-2, see in Table S2). Interestingly, GeSe based photo-detector displays a stable on/off switching


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behavior at almost all wavelengths, especially in 350, 365, 380, 400, and 475 nm. Such results are also consistent with the observation in the UV-vis spectra (Figure 2b). Besides, such photodetector also performed a fast response and recovery characteristics under different light power densities (Figure S15). With the light power density increasing from level I to II, III, IV, and VI, photo-response of such a device gradually increased at every wavelengths, which was consistent with the results measured under the simulated light irradiation (Figure 7b). Compared with the Iph values obtained under simulated light, the lower values in each wavelength might be attributed to the decreasing power density (Figure 7c). Notably, it was found that such GeSe nanosheets-based photo-detector performed a higher sensitivity to short wavelength regime, especially in 350, 365, and 380 nm (see in Figure 7d). To evaluate the photoelectronic performance of GeSe PEC-type photodetector, the comparison with other previous works were performed, which includes the PEC- and FET-type devices. As shown in Table 1, such GeSe PEC-type device shows a much higher responsivity (43.6-76.3 μA W-1) and shorter response time (0.2 s) when compared with the other BP38, InSe50, Bi2S340, SnS55, and Te52 nanosheets PEC-type devices. It is worth noting that such response speed is also comparable with the MoS253, WSe251, GeSe36, and GeS56 nanosheets FET-type devices. Although the responsivity of such PEC-type device is smaller than that of FET-type one, their attractive characteristics of lowcost, environmental-friendly and simply packaging would also make it the promising alternatives. In conclusion, typical photoresponse performances of such GeSe nanosheets under various working conditions solidly suggest its great potential in PEC-type photodetector.

Conclusion


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In summary, ultrathin GeSe nanosheets with controllable thickness was successfully prepared by a facile and environmental-friendly LPE method. We demonstrated that such GeSe nanosheets has a high stability both in liquid and air conditions, and a fast recovery speed during the carrier transfer process. As the photoanode applied for the PEC-type photo-detector, our results demonstrated that such device exhibits high photocurrent density and responsivity, especially in the UV-vis region. In addition, the effects of applied bias potential, light power density, and concentration of electrolyte on photoresponse behavior were also systematically revealed. It is believed that such GeSe nanosheets-based PEC-type photo-detector could be a promising photonics device, and also providing a basic guidance for the development of other possible semiconductor materials in this research field.

EXPERIMENTAL SECTION Materials: GeSe powder (99.9%) was pruchased from Shanghai Yiji technology co. LTD. Nmethyl-2-pyrrolidone (NMP, 99.5%), isopropyl alcohol (IPA, 99%), dimethyl formamide (DMF, 99.9%), and ethanol organic solvent, as well as the poly(vinylidene fluoride) (PVDF) binder were pruchased from Sigma Aldrich. Preparation of GeSe nanosheets: In this work, the GeSe nanosheets were synthesized by liquidphase exfoliation method. Briefly, 600 mg bulk GeSe powder was directly added into 200 ml NMP solvent to form the GeSe/NMP suspension with an initial concentration of 3 mg/ml. Then the suspension was firstly treated with probe sonication process with a energy power of 600 W, followed by bath sonication with a energy power of 500 W for 6 h. It should be noted that both the probe and bath sonication process were kept at a constant environment of 5 oC. To obtain the GeSe nanosheets, the above resulting GeSe/NMP solution was firstly centrifuged at 2000 rpm for


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30 min to obtain the supernatant, and followed by another centrifugation at 14000 rpm for 30 min to obtain the precipitate of GeSe nanosheets. The as-prepared GeSe nanosheets were dried at 80 oC under vacuum overnight. Preparation of ITO@GeSe photodetector electrode: The dried GeSe nanosheets were redispersed in 1 ml of PVDF/DMF solution (with a concentration of 15 mg/100 ml) to form the suspension. Then the as-prepared suspension was directly dropped onto the conductive side of a ITO-coated glass, and GeSe nanosheets-coated electrode (ITO@GeSe) was dried at 80 oC under vacuum overnight. Characterization: The morphology and microstructure of the GeSe nanosheets were examined by HRTEM (Tecnai G2 F30) and AFM (Dimension Edge, Bruker, America), respectively. The structure and Raman spectra were collected on X-ray diffraction (Bruker, D8 Advance with CuKα radiation) and Raman microscope (DXR Thermo-Fisher Scientific) with an excitation wavelength of 532 nm. Optical absorption performances were measured by a UV-vis spectrum in the range of 280-800 nm. Optoelectronic measurements: The optoelectronic properties of GeSe nanosheets were evaluated in a PEC measurement system (as shown in Figure S16). A three-electrode system including ITO@GeSe photoanode as working electrode (as shown in Figure S17), platinum wire photocathode as counter electrode and a saturated calomel reference electrode were constructed in alkaline electrolyte with various concentration of KOH aqueous solution. Linear sweep voltammetry (LSV) and Amperometric current-time (i-t) curves were recorded by employing an electrochemical workstation (CHI660E). A 500 W Xenon arc lamp (CHF-XM 500) was working as a light source and placed 20 cm away from the reaction vessel. Simulated light (mixed light


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from 300 to 800 nm) and light with wavelengths of 350, 365, 380, 400, 475, 550, and 650 nm (obtained by a certain optical filter) were used to irradiate GeSe nanosheets.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; *Email: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript. D. Ma, J. Zhao and R. Wang contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The research is partially supported by the National Natural Science Fundation of China (Grant No. 61435010), Science and Technology Planning Project of Guangdong Province (Grant No. 2016B050501005), Science and Technology Innovation Commission of Shenzhen (Grant No. KQTD2015032416270385), Student Innovation Development Foundation of Shenzhen University (PIDFP-ZR2017002), and the Science and Technology Development Fund (Grant No. 007/2017/A1), Macao SAR, China.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional details of experimental resultss: the effect of solvent and centrifugation speed on the exfoliated GeSe nanosheets, the effect of thickness on the electrical and optical properties of GeSe nanosheets, the studies of femtosecond transient optical absorption spectroscopy, other supplementary photoresponse tests, the stability tests, the digital photograph of PEC-type device and GeSe photoanode, the comparison of GeSe nanosheets-based photodetector with previous works.

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Figure 1. Schematic illustration of ultrathin GeSe nanosheets fabricated by liquid-phase exfoliation.


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Figure 2. (a) XRD patterns of the bulk GeSe and GeSe nanosheets. (b) UV-Vis absorption spectrum of the GeSe nanosheets dispersed in NMP solvent. (c) AFM image of the GeSe nanosheets and the corresponding height profiles. (d) TEM and (e) corresponding HRTEM images of the GeSe nanosheets (inset: the corresponding SAED image).


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Figure 3. (a) Side view of the crystal structure of GeSe; (b) Atomic displacement of the Raman active modes in GeSe; (c) Raman spectrum of the GeSe nanosheets; (d) Time-dependent Raman spectrum of GeSe nanosheets after different storage times; (e) Time-dependent UV-vis absorption spectra of the GeSe nanosheets in NMP.


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Figure 4. The transient absorption signals of the GeSe nanosheets measured by femtosceond transient optical absorption spectroscopy. (a) Temporally and spectrally resolved transient absorption signal of the GeSe nanosheets. (b) Transient absorption spectra of the GeSe nanosheets in various delay times. (c) Schematic diagram of the carriers transfer process in GeSe nanosheets. (d) The global fitting curve of the GeSe nanosheets. The fitting results reveal a fast and a slow component with the lifetime of 3.8 ± 1.5 ps and 57.7 ± 3.9 ps, respectively. Pump wavelength: ~400 nm; Pump power: ~1 mW.


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Figure 5. (a) Typical PEC system with three electrodes for evaluating the photoresponse behaviors of GeSe nanosheets-based photodetector in KOH electrolyte. (Working electrode, reference electrode and counter electrode refer to GeSe nanosheets deposited on ITO glass, saturated calomel electrode and the platinum wire, respectively.) (b) Schematic illustration of the rapid separation and movement of photogenerated carrier under illumination of simulated light.


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Figure 6. Optoelectronic performances of GeSe nanosheets under illumination of simulated light. (a) On/off cyclic stability tests under different external potential with 0, 0.3, and 0.6 V, in 0.1 M KOH electrolyte. (b) Normalized photocurrent density in GeSe nanosheets photodetector under different light power densities, in 0.1 M KOH electrolyte with a 0.3 V bias potential. (c) Photoresponse of GeSe nanosheets under different external bias potentials from 0 to 0.8 V, in 0.1 M KOH electrolyte. (d) Photocurrent density map as a function of both applied bias potential and concentration of KOH aqueous electrolyte.


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Table 1. Comparison of the characteristic parameters of the present GeSe nanosheet-based photodetector with previously reported photodetectors.

Materials

Measurement conditions

Response time

Responsivity

Ref.

GeSe nanosheets

0.1 M KOH , 0.3 V

0.2 s

43.6-76.3 μA W-1

This work

BP nanosheets

0.1 M KOH , 0 V

0.5 s

1.9-2.2 μA W-1

[38]

InSe nanosheets

0.2 M KOH , 1V

5s

3.3 μA W-1

[50]

Bi2S3 nanosheets

0.1 M KOH , 0.6 V

0.1 s

52 μA W-1

[40]

Te nanosheets

0.1 M KOH , 0.6 V

70 ms

1.0-1.3 μA W-1

[52]

MoS2 nanosheets

Schottky-contact

4s

880 A W-1

[53]

10 s

180 A W-1

[51]

Vds= 8 V, Vg= -70 V Schottky-contact

WSe2 nanosheets

Vds= 2 V, Vg= -60 V

Few-layer BP

FET, Vds= 0.3 V, Vg= 0V

1 ms

4.8 mA W-1

[54]

GeSe nanosheets

FET, Vds= 2 V, Vg= 0 V

0.15 s

870 A W-1

[36]

SnS

0.1 M Na2SO4 , 0.6 V

0.3 s

17.8 μA W-1

[55]

GeS

FET, 530 nm, Vds= 5 V

0.85 s

139.9 A W-1

[56]

SnS2

FET, 450 nm, Vds= 10 V

42 ms

2 A W-1

[57]


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Figure 7. Optoelectronic performances of GeSe nanosheets-based photodetector under the illumination of various wavelength of light at a fixed voltage of 0.3 V in 0.5 M KOH electrolyte. (a) Cyclic stability tests under various wavelength of light (350, 365, 380, 400, 475, 550, and 650 nm). (b) Cyclic stability tests under various wavelength of light (350, 365, 380, 400, 475, 550, and 650 nm) with increasing incident power densities. Profiles of (c) Iph values and (d) Rph values as a function of Pλ, respectively.


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TOC figure


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Ultrathin GeSe Nanosheets: from Systematic Synthesis, to Studies of

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