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Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared
Accepted Manuscript Title: Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared Author: Jia-Jing Wu You-Rong Tao Yi Wu Xing-Cai Wu PII: DOI: Reference:
S0925-4005(16)30322-7 http://dx.doi.org/doi:10.1016/j.snb.2016.03.029 SNB 19834
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
22-1-2016 27-2-2016 7-3-2016
Please cite this article as: Jia-Jing Wu, You-Rong Tao, Yi Wu, Xing-Cai Wu, Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared
Jia-Jing Wu, You-Rong Tao, Yi Wu, Xing-Cai Wu* Key Laboratory of Mesoscopic Chemistry of MOE, and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. *
Corresponding author. Fax: +86 25 83317761.
E-mail: [email protected] (X.C.Wu).
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Highlights ●We synthesized the SnS2 ultrathin nanosheets by an ultrasonic chemical method for first time. ●The photodetector was fabricated on the basis of the SnS2 ultrathin nanosheet film. ●The photodetectors showed decent photoresponse from ultrviolet to near infrared.
Abstract Ultrathin SnS2 nanosheets are synthesized for the first time by a simple ultrasonic method, and then fabricated onto a SiO2/Si substrate to form nanosheet-based phototransistor which exhibits a broad photoresponse from 254 to 980 nm, dependence of photocurrent on optical power and wavelength, fast-response, and long-term stability. Under illumination of 532-nm light with an optical power of 19.3 mW/cm2 (0.68 nW), the photoswitch current ratio (PCR) is about 8.7, while the photoresponsivity, external quantum efficiency, and detectivity are 0.65 mA/W, 0.15%, and 1.13×108 J, respectively. Compared with the reported SnS2-based photodetectors, the SnS2nanosheet phototransistor shows an enhanced photosensitive performance. Keywords: Tin disulfide, Nanosheets, Ultrasonic synthesis, Photodetectors.
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1. Introduction The discovery of graphene in 2004 has led to a booming effort towards research on twodimensional (2D) materials [1]. The research becomes now one of the most active fields because of the wide applications of the materials in Li-ion batteries [2, 3], field-effect-transistors (FETs) [4], biomedical monitoring [5], gas/humidity sensors [6], solar cells [7], photocatalysis [8], photodetectors [9], and so on. The 2D materials include graphene [6], hexagonal boron nitride (hBN) [10], black phosphorus [11], metal dichalcogenides (MDCs, for example, MoS2, WS2, TiS2, ReS2, SnSe2, HfS2, SnS2 etc.) [12–19], metal chalcogenides [20–26], metal oxides [27–29], and so on. Because the layered structures of the MDCs are easily exfoliated into monolayer or fewlayer structures (2D materials) to the extent that the band gaps of the materials change, the 2D materials have been fabricated into various photosensitive devices [30–33]. Tin disulfide (SnS2) is also a layer-structure compound (CdI2-type) where the atoms within the S-Sn-S layer are held together by strong covalent forces while Van der Waals interaction enables stacking of the layers, so it can be also exfoliated to ultrathin nanostructures. SnS2 is an n-type semiconductor, nontoxicity, low cost, and high chemical stability, making it an important optoelectronic material [34–38]. Recently, SnS2-based optoelectronic devices have drawn intensive attention. Song et al. reported the top-gated field-effect-transistor (FET) based on few-layers SnS2 nanosheet with higher carrier mobility via the function of dielectric screening from mediate-κ dielectric Al2O3 [39]. Huang et al. reported a FET based on monolayer SnS2 flake by exfoliating SnS2 crystals and explored its potential application in photodetection [40]. Su et al. obtained the thin crystal arrays of SnS2 at predefined locations, introducing a new idea for large-scale production by chemical vapor deposition [41]. However, the broadband photodetector based on SnS2
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nanosheets is rarely mentioned. Here we report synthesis, phototransistor, and broadband photoresponses of the SnS2 nanosheets. In the past years, a large number of SnS2 micro-/nano-structures have been synthesized such as hierarchical microspheres [42], hollow spheres [43], nanowires [44], nanorods [45], nanotubes [46] and nanoflakes [47]. Recently, a few preparation methods have been developed for preparing ultrathin SnS2 nanosheets including mechanical exfoliation [40], chemical vapor deposition [41], and solvothermal synthesis [48]. However, mechanical exfoliation is limited to yields and reproducibility; the solvothermal synthesis needs spend a long reaction time. Herein we directly synthesize ultrathin SnS2 nanosheets with a thickness of ~3.4 nm by a facile ultrasonic chemical method. In ultrasonic process, thioacetamide serves as sulfur source, and enthanol and deionized water as solvent, and stannic chlorides as tin source. Tin disulfide is easy to form ultrathin structure due to ultrasound cavitation effect. Subsequently, ultrathin SnS2 nanosheets are fabricated into a phototransistor on a SiO2/Si wafer. It shows good photoresponse to wavelengths from 254 to 980 nm. Under a 532-nm light illumination with an optical power of 19.3 mW/cm2 (0.68 nW), the photoswitch current ratio (PCR) is 8.7, so the phototransistor is a decent photodetector.
2. Experimental method 2.1.
Preparation and characterization
SnS2 nanosheet microspheres were synthesized via a facile ultrasonic chemical method. In a typical process, 1.7525 g SnCl4·5H2O (C. P.) and 0.7513 g thioacetamide (A. R.) were put into 100 mL beaker, and then 40 mL enthanol was added, and first stirred vigorously for 5 min with
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glass rod. Subsequently the mixtures were sonicated by using a 120 W ultrasonic cleaner for 15, 30, 45, 60, 90, and 120 min, respectively. After the reactions finished, the resulting yellow dispersion was centrifuged by centrifuge at 3000 r. p. m., alternately rinsed with deionized water and ethanol, until the solution was neutral, and in vacuum dried at 60℃ for 3 h. The as-prepared products were characterized by a X-ray diffraction (XRD, Shimadzu XR−D6000 with graphite monochromatized Cu Kα1 radiation), a field-emission scanning electron microscope (FESEM, Hitachi S−4800) with energy-dispersive X-ray spectrometer (EDX), a high-resolution transmission electron microscope (HRTEM, JEM−2100), and UV3−600 spectrophotometer (Shimadzu UV−3600). Atomic force microscopy (AFM) was performed in ScanAsyst mode with NP-S oxide sharpened silicon nitride tips using a Bruker Innova microscope instrument which signal voice in Z-direction is lower than 40 pm. 2.2. Microfabrication and measurements of photodetector Firstly, the as-prepared samples (prepared within 60 min.) were dispersed in ethanol, then ultrasonically dispersed for 10 min, afterwards dropped on a SiO2 (300 nm)/Si substrate, and finally dried naturally in air to form film. Second, the metal electrodes were fabricated on the top of the SnS2 nanosheets via a standard photolithography process. In a typical photolithography process, photoresist was first covered onto the SiO2/Si substrates by spin coating and then baked. After that, the photoresist was exposed to ultraviolet light engraving machine with a mask, followed by developing in positive photoresist developer solution. Next, Ti/Au (10 nm/100 nm) metal was deposited on the surface of the substrate in high vacuum turbo evaporator apparatus. Finally, lift-off process was carried out to obtain phototransistor with an electrode separation of 3 μm. The current–voltage (I–V) and the current-time (I–t) characteristics of the photodetector were recorded by Model CRX-4K Cryogenic Probe Station (Lake Shore Inc.) and Keithley 236
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source meter (Keithley Instruments Inc.) A spectroscopic response ranging from 350 to 1000 nm was measured using a 300 W Xenon lamp (HSX-UV300), and a multi-grating monochromator (71SW151) with order sorting filters was used. Optical power was measured with FZ-A radiometer. The spectral response was recorded under illumination of different wavelength laser. Laser power was measured with a LP1 laser power meter. The experiments have been done at room temperature in air except specially proposing condition.
3. Results and discussion 3.1. Structure characterization An XRD pattern of the as-prepared products is shown in Fig. 1a, which can be indexed to hexagonal SnS2 (JCPDS Card No. 75 – 0367; a = 0.3620 nm and c = 0.5850 nm). Based on the least square method calculation to the XRD pattern, the cell parameters of the nanosheets are a = 0.3611 nm and c= 0.5927 nm, almost coinciding with above JCPDS card. No peaks of impurities are detected, so they are higher purity. The EDX spectrum (Fig. S1) shows that the atomic ratio of Sn and S is 32.61: 67.39, closing to 1: 2, demonstrating the formation of SnS2. In addition, no impure peaks were observed except C and O (C and O comes from adhesive of sample desk), so the sample is higher purity. Morphology of the products is observed by a FESEM, as shown in Fig.1b, c, which displays that they are curled plate-like nanosheet with the length of several micrometer, and Viewed from the side in Fig.1 c, the thickness of the nanosheet is about 11 nm. TEM image further confirms that they are nanosheets, as shown in Fig. 1d. Fig. 1e reveals HRTEM image of a SnS2 nanosheet, which lattice fringe spacings of 0.27 and 0.32 nm can be assigned to the (011) and (100) planes of the SnS2, respectively. As shown in Fig. 1f, the
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corresponding selected area electron diffraction (SAED) pattern can be still indexed as hexagonal SnS2 phase, and indicating its polycrystalline structure. 3. 2. Growth process To understand the growth process, SnS2 nanosheets were synthesized at different reaction time (15 to 120 min). When the ultrasonic reaction passed by 15 min, the solution began to turn into yellow and generated SnS2 nanoparticles with the diameter of about 120-180 nm, as shown in Fig. 2a. Here only two low XRD peaks for the samples appeared, showing that the crystallinity of the products is no good (Fig.2f, 15 min.) When the reaction passed by 30 min., the products still were nanoparticles and the sizes almost did not change, as shown in Fig. 2b. Here intensity of the XRD peaks slightly increased (Fig.2f, 30 min.). When the reaction passed by 45 min., many nanosheets formed (Fig. 2c). Here the intensity of XRD peaks obviously increased (Fig. 2f, 45 min.). When the reaction passed by 90 min., the nanosheets became bigger, as shown in Fig. 2d. Here the XRD peaks got clearer, exhibiting that the crystallinity became better (Fig.2f, 90 min.). When the reaction passed by 120 min, we obtained high-crystalline SnS2 nanosheets. The morphology and the XRD pattern were shown in Fig. 2e and f (120 min.), respectively. AFM images can further confirm the size of the products, as shown in Fig. 3a–e. The thickness ranges from 3.0 to 3.7 nm when the reaction time increases from 15 to 120 min. As is mentioned above, a 2D no-perferred oriented attachment growth mechanism may be proposed [20], which is different from 2D oriented attachment growth process of PbS nanosheets [21]. Because the SnS2 nanosheets are grown throughout the ultrasonic process, small SnS2 nanocrystals expand with random orient in plane. Meanwhile, the vertical direction attachment cannot be well carried out. Therefore, the nanosheets are made up of various nanoparticles, i. e. forming polycrystals. Such morphology can be attributed to the electrostatic interaction between the nanocrystals [120].
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Fig.4a is UV absorbance spectrum of the SnS2 nanosheets (prepared within 60 min.) which shows existence of light absorbance of the products from 253 to 1200 nm, but there are only weak absorbance after 800 nm. Based on energy gap formula [49, 50], the indirect energy gap of the nanosheets is 1.39 eV (Fig. 4b). 3.3. Photoresponses of the phototransistors The as-prepared SnS2 nanosheets are fabricated into nanosheet-film phototransistors on a SiO2/Si wafer by a micro-fabrication process, as shown in inset of Fig. 5a. The AFM image of the nanofilm is shown in Fig. S2, and the thickness of the nanofilm is about 170 nm. The film is composed of ultrathin nanosheets, and the effective illuminated area is ~4.9 × 10-8 cm2. Responsitivity to light wavelength measured by Xe lamp almost coincides with UV absorbance spectrum, as shown in Fig. 5a, indicating that photoresponse originate from the carrier excitation in the semiconducting SnS2 nanosheet rather than the contact region with Schottky barriers. Fig. 5b, c demonstrates the I–V curves of the phototransistor under dark and light illumination with different wavelengths, which shows photosensitive properties from 254 to 980 nm. The nonlinearity and asymmetry of the curves can be attributed to Schottky contact between the nanosheet and electrodes. Responsivity (Rλ), external quantum efficiency (EQE), and detectivity (D*) are critical parameters for a photodetector, which reflect the photosensitivity of the photodetector. They can be determined from the following equations [51, 52]: Rλ=ΔI/ (PS); EQE= Rλhc/ (eλ); D*= Rλ/ (2e.Idark/S)0.5, where ΔI is the difference between the light and dark currents; P is the incident light power; S is the effective illuminated area; h is Planck’s constant; C is the speed of light; λ represents the light wavelength; and e is the electronic charge. Based on the equations, Rλ for 254, 365, 405, 532, 650, 780, 850, and 980 nm wavelengths at a bias of 5 V are 1950.4, 5.23, 0.695, 23.9,11.3,1.28,0.92, and 0.18 mA/W, respectively. It shows that the
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response is the biggest for 254 nm (4.88 eV), and the second for 532 nm (2.26 eV), and the least for 980 nm (1.27 eV). The corresponding EQE are 955%, 1.75%, 0.22%, 5.52%, 2.21%, 0.20%, 0.15%, and 0.0184%, respectively, while the corresponding D* are 2.02×1010, 5.33×107, 7.39×106, 2.48×108, 1.17×108, 1.35×107, 9.93×106, and 3.5×106 Jones, respectively. The wavelength-dependent response corresponds well with the fact that higher excitation energy can enhance the conversion of photoelectrons, so 254 nm-light has highest photoresponse. Fig. 5d is the I–V curves of the device to various light intensity under 405-nm light illumination, which demonstrates that the photocurrents depend on the light intensities, and gradually increase as the light intensities increase. Under 405-nm light illumination with an optical power of 233.2 mW/cm2 (11.4 nW), and at a bias of 5 V, the PCR is ~12. Fig. 5e shows an I–t characteristic under illumination of 532-nm light with an optical power of 19.3 mW/cm2 (0.68 nW). Here the PCR (Ion/off) is about 8.7, while the R532 nm, EQE, and D* are 0.65 mA/W, 0.15%, and 1.13×108 J, respectively. Fig. 5f, g show the I–t characteristics of the device to the 405 nm incident radiation with an optical power of 162 mW/cm2 (7.94 nW) at a bias voltage of 5 V with the photoswitch periods of 0.5 and 50 s, respectively. As shown in Fig. 5f, the PCR, R405 nm, EQE, and D* are ~5, 0.02 mA/W, 6.1×10-3%, and 1.8×106 J, respectively, and the response time (rise time/decay time) is 0.4 /0.4 s. As shown in Fig. 5g, the PCR, the R405 nm, EQE, and D* are ~6.4, 0.034 mA/W, 1.03×10-2%, and 2.74×106 J, respectively, and the response time (rise time/decay time) is 0.4/0.2 s. The time taken for the current to increase from 10% to 90% of the peak value or vice versa is defined as the rise time and decay time, respectively [53]. Upon illumination, the photocurrent increases to a stable value, and then dramatically decreases to its initial value as light is turned off, thereby showing excellent stability and reproducible characteristics, and allowing the device to act as a high quality photosensitive switch. Different photoswitch periods result in different
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Ion/off and rise/decay times, because the increase and decay of photocurrent need enough time [54]. From the plots, we can observe that the photocurrents are not too stable, which is attributed to environment factors such as temperature, air flow, noise as well as slight vibrations [55]. Fig. 5h shows the time-dependent currents of the device under 405-nm light illumination of 162 mW/cm2 power with a photoswitch period of 5 s at the bias of 3, 5, and 7 V. Upon illumination, the currents rapidly switch from ~0.05 pA to ~0.15, 0.26, and 0.5 pA, respectively, and then sharply returns to its “OFF-state” when the light is turned off. A nearly identical fast response is observed for multiple cycles, indicating the sensitivity and excellent stability of the device. The responses in air and vacuum are measured under 405-nm light illumination of 171.3 mW/cm2 power, as illustrated in Fig. 5i. At an applied voltage of 5 V, the current (0.713 pA) in vacuum is about 1.44 times higher than that (0.495 pA) in air under dark, while the current in vacuum is 12.2 pA and is about 3.42 times higher than that (3.57 pA) in air under 405-nm light illumination. In all, the current in vacuum is higher than in air regardless of dark or illumination. It can be explained by an oxygen adsorption/desorption mechanism. Because SnS2 is an n-type semiconductor, the electrons are dominant carriers in the nanosheets. In air, oxygen molecules with larger electronegativity are adsorbed on the surface of the nanosheets and capture free electrons from the conduction band (O2 + e-→O2-) so that the conductivity reduces. In vacuum, oxygen adsorption becomes rare so that the electrons return to conduction band, so the current increases. Here oxygen acts as a trap for electrons. Under light illumination, electron-hole pairs are generated on the large scale. The holes migrate to the surface to combine with O2- to form O2 to desorb, resulting in an increase in the electron concentration so that the photocurrent increases. In order to understand the stability of the device, after the device is exposed in air for two months, the I–t curves are measured under 405-, 850-, and 980-nm illumination, as shown in Fig.
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6a–c, respectively. The PCRs are 10.6, 1.3, and 1.1, respectively. The photosensitivity almost remains, so the device is stable in air. The crucial parameters for a few photodetectors are summarized in Table 1. As can be seen from Table 1, the SnS2 nanosheet film photodetector shows a broader light response. The responsivity under 532 nm/0.68 nW light illumination is much lower than those of CdTe nanoribbon [56] and few-layer SnS2 photodetectors [40], but the PCR is much higher than those of the either. So the SnS2 nanosheet film phototransistor shows an enhanced photoresponse.
4. Conclusions In summary, we prepared ultrathin SnS2 nanosheets with a thickness of about 3.4 nm by a facile ultrasonic chemical method for the first time. The indirect energy gap of the sample is 1.39 eV. The SnS2-nanosheet phototransistor shows an excellent photoresponse from UV to NIR. The photoresponse to 980 nm is reported for the first time. So the phototransistor is an excellent photodetector and the ultrathin SnS2 nanosheet is an excellent optoelectrical material. Acknowledgements We thank for the financial support from the National Science Foundations of China (No. 21171091 and 20671050).
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References [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666 − 669. [2] J. W. Seo, J. T. Jang, S. W. Park, C. Kim, B. Park, J. Cheon, Two-dimensional SnS2 nanoplates with extraordinary high discharge capacity for lithium ion batteries, Adv. Mater. 20 (2008) 4269 − 4273. [3] Y. Du, Z. Yin, X. Rui, Z. Zeng, X. J. Wu, J. Liu, Y. Zhu, J. Zhu, X. Huang, Q. Yan, H. Zhang, A facile, relative green, and inexpensive synthetic approach toward large-scale production of SnS2 nanoplates for high-performance lithium-ion batteries, Nanoscale 5 (2013) 1456 − 1459. [4] K. Kim, J. Y. Choi, T. Kim, S. H. Cho, H. J. Chung, A role for graphene in silicon-based semiconductor device, Nature 479 (2011) 338 − 344. [5] T. Kosugi, K. Murakami, S. Kaneko, Thin-film structures for photovoltaics, Mater. Res. Soc. Symp. Proc. 485 (1998) 273−278. [6] A. K. Geim, K. S. Novoselov, The rise of grapheme, Nat. Mater. 6 (2007) 183−191. [7] F. R. Tan, S. C. Qu, J. Wu, K. Liu, S. Y. Zhou, Z. G. Wang, Preparation of SnS2 colloidal quantum dots and their application in organic/inorganic hybrid solar cells, Nano Res Lett. 6 (2011) 1− 8. [8] Y. C. Zhang, L. Yao, G. S. Zhang, D. D. Dionysiou, J. Li, X. H. Du, One-step hydrothermal synthesis
of
high-performance
visible
light-driven
SnS2/SnO2
nanoheterojunction
photocatalyst for the reduction of aqueous Cr(VI), Appl. Catal. B: Environ. 144 (2014) 730 − 738.
12
[9] H. Li, J. Wu, Z. Yin, H. Zhang, Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and WSe2 nanosheets, Acc. Chem. Res. 47 (2014) 1067−1075. [10] K. K. Kím, A. Hsu, X. T. Jía, S. M. Kím, Y. S. Shí, M. Hofmann, D. Nezích, J. F. Rodriguez-Níeva, M. Dresselhaus, T. Palacíos, J. Kong, Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition, Nano Lett. 12 (2012) 161−166. [11] M. Buscema, D. J. Groenendijk, S. I. Blanter, G. A. Steele, H. S. J. vander Zant, A. Castellanos-Gomez, A fast and broadband photoresponse of few-layer black phosphorus field-effect transistors, Nano Lett. 14 (2014) 3347 − 3352. [12] Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey, H. Zhang, Single-layer semiconducting nanosheets: high-yield preparation and device fabrication, Angew. Chem. Int. Ed. 50 (2011) 11093−11097. [13] Q. N. Cui, H. Zhao, Coherent control of nanoscale ballistic currents in transition metal dichalcogenide ReS2, ACS Nano 9 (2015) 3935− 3941. [14] X. Zhou , L. Gan , W. M. Tian , Q. Zhang , S. Y. Jin , H. Q. Li , Y. Bando, D. Golberg , T. Y. Zhai, Ultrathin SnSe2 flakes grown by chemical vapor deposition for high-performance photodetectors, Adv. Mater. 27 (2015), 8035 – 8041. [15] J. Jang, S. Jeong, J. Seo, M. C. Kim, E. Sim, Y. Oh, S. Nam, B. Park, J. Cheon, Ultrathin zirconium disulfide nanodiscs, J. Am. Chem. Soc. 133 (2011) 7636 − 7639. [16] X. Huang, Z. Y. Zeng, H. Zhang, Metal dichalcogenide nanosheets: preparation, properties and applications, Chem. Soc. Rev. 42 (2013) 1934 − 1946. [17] K. Xu, Z. X. Wang, F. Wang, Y. Huang, F. M. Wang, L. Yin, C. Jiang, J. He, Ultrasensitive phototransistors based on few-layered HfS2, Adv. Mater. 27 (2015) 7881–7887.
13
[18] Y. Huang, K. Xu, Z. X. Wang, T. A. Shifa, Q. S. Wang, F. Wang, C. Jiang, J. He, Designing the shape evolution of SnSe2 nanosheets and their optoelectronic properties. Nanoscale 7 (2015) 17375–17380. [19] Y. Huang, H. X. Deng, K. Xu, Z. X .Wang, Q. S. Wang, F. M. Wang, F. Wang, X. Y. Zhan, S. S. Li, J. W. Luo, Highly sensitive and fast phototransistor based on large size CVD-grown SnS2 nanosheets, Nanoscale 33 (2015) 140931–4099. [20] Z. Y. Tang, Z. L. Zhang, Y. Wang, S. C. Glotzer, N. A. Kotov, Self-assembly of CdTe nanocrystals into free-floating sheets, Science 314 (2006) 274−278. [21] C. Schliehe, B. H. Juarez, M. Pelletier, S. Jander, D. Greshnykh, M. Nagel, A. Meyer, S. Foerster, A. Kornowski, C. Klinke, Ultrathin PbS sheets by two-dimensional oriented attachment, Science 329 (2010) 550 − 553. [22] L. Li, H. Lu, Z. Y. Yang, L. M. Tong, Y. Bando, D. Golberg, Bandgap-graded CdSxSe1x nanowires for high-performance field-effect transistors and solar cells, Adv. Mater. 25 (2013) 11091113. [23] G. H. Chen, W. L. Wang, C. D. Wang, T. Ding, Q. Yang, Controlled synthesis of ultrathin Sb2Se3 nanowires and application for flexible photodetectors, Adv. Sci. 2 (2015) 1500109. [24] G. H. Chen, W. Li, Y. Q. Yu, Q. Yang, Fast and low-temperature synthesis of one dimensional (1D) single-crystalline SbSI microrod for high performance photodetector, RSC Adv. 5(2015) 21859–21864. [25] G. H. Chen, Y. Q. Yu, K. Zheng, T. Ding, W. L. Wang, Y. Jiang, Q. Yang, Fabrication of ultrathin Bi2S3 nanosheets for high- performance, flexible, visible–NIR photodetectors, Small 11(2015) 2848–2855.
14
[26] Q. Yang, K. B. Tang, C. R. Wang, D. Y. Zhang, Y. T. Qian, The Synthesis of SnS2 nanoflakes from tetrabutyltin precursor, J. Solid State Chem. 164 (2002) 106–109. [27] F. Xu, X. Y. Zhang, Y. Wu, D. P. Wu, Z. Y. Gao, K. Jiang, Facile synthesis of TiO2 hierarchical microspheres assembled by ultrathin nanosheets for dye-sensitized solar cells, J. Alloys Comp. 574 (2013) 227 − 232. [28] C. T. Gao, X. D. Li, X. P. Zhu, L. L. Chen, Y. Q. Wang, F. Teng, Z. X. Zhang, H. G. Duan, E. Q. Xie, High performance, self-powered UV-photodetector based on ultrathin, transparent, SnO2-TiO2 core-shell electrodes, J. Alloys Comp. 616 (2014) 510 − 515. [29] L. F. Hu, L. M. Wu, M. Y. Liao, X. H. Hu, X. S. Fang, Electrical transport properties of large, individual NiCo2O4 nanoplates, Adv. Funct. Mater. 22 (2012) 998 –1004. [30] Z. Y. Yin, H. Li, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen, H. Zhang, Single-layer MoS2 phototransistors, ACS Nano 6 (2012) 74−80. [31] N. Perea-López, A. L. Elías, A. Berkdemir, A. Castro-Beltran, H. R. Gutiérrez, S. M. Feng, R. T. Lv, T. Hayashi, F. López-Urías, S. Ghosh, B. Muchharla, S. Talapatra, H. Terrones, M. Terrones, Photosensor device based on few-layered WS2 films, Adv. Funct. Mater. 23 (2013) 5511− 5517. [32] W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G. B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, S. Kim, High-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infrared, Adv. Mater. 24 (2012) 5832 − 5836. [33] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Ultrasensitive photodetectors based on monolayer MoS2, Nat. Nanotechnol. 8 (2013) 497−501. [34] T. Jiang, G. A. Ozin, New directions in tin sulfide materials chemistry, J. Mater. Chem. 8 (1998) 1099 −1108.
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[35] Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, F. Lei, T. Yao, J. He, S. Wei, Freestanding tin disulfide single-layers realizing efficient visible-light water splitting, Angew. Chem. Int. Ed. 51 (2012) 8727 − 8731. [36] G. Radovsky, R. P. Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert, R.Tenne, Synthesis of copious amounts of SnS2 and SnS2/SnS nanotubes with ordered superstructures, Angew. Chem. Int. Ed. 50 (2011) 12316 − 12320. [37] E. C. Greyson, J. E. Barton, T. Odom, Tetrahedral zinc blende tin sulfide nano and microcrystals, Small 2 (2006) 368 − 371. [38] H. J. Geng, Y. J. Su, H. Wei, M. H. Xu, L. M. Wei, Z. Yang, Y. F. Zhang, Controllable synthesis and photoelectric property of hexagonal SnS2 nanoflakes by triton X-100 assisted hydrothermal method, Mater. Lett. 111 (2013) 204 − 207. [39] H. S. Song, S. L. Li, L. Gao, Y. Xu, K. Ueno, J. Tang, Y.B. Cheng, K. Tsukagoshi, Highperformance top-gated monolayer SnS2 field-effect transistors and their integrated logic circuits, Nanoscale 5 (2013) 9666 − 9670. [40] Y. Huang, E. Sutter, J. T. Sadowski, M. Cotlet, O. L. A. Monti, D. A. Racke, Mahesh. R. Neupane, Darshana. Wickramaratne, R. K. Lake, B. A. Parkinson, P. Sutter, Tin disulfide－ an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics, ACS Nano 8 (2014) 10743−10755. [41] G. X. Su, V. G. Hadjiev, P. E. Loya , J. Zhang, S. D. Lei , S. Maharjan, P. Dong, P. M. Ajayan, J. Lou, H. B. Peng, Chemical vapor deposition of thin crystals of layered semiconductor SnS2 for fast photodetection application, Nano Lett. 15 (2015) 506 − 513.
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[42] J. Zai, X. Qian, K. Wang, C. Yu, L. Tao, Y. Xiao, J. Chen, 3D-hierarchical SnS2 micro/nano-structures: controlled synthesis, formation mechanism and lithium ion storage performances, CrystEngComm. 14 (2012) 1364 − 1375. [43] J. Xia, G. Li, Y. Mao, Y. Li, P. Shen, L. Chen, 3D-hierarchical SnS2 micro/nano-structures: controlled synthesis, formation mechanism and lithium ion storage performances, CrystEngComm 14 (2012) 4279 − 4283. [44] Y. T. Lin, J. B. Shi, Y. C. Chen, C. J. Chen, P. F. Wu, Synthesis and characterization of tin disulfide (SnS2) nanowires, Nano Res Lett. 4 (2009) 694 − 698. [45] P. Wu, N. Du, H. Zhang, J. Liu, L. Chang, L. Wang, D. Yang, J. Z. Jiang, Layer-stacked tin disulfide nanorods in silica nanoreactors with improved lithium storage capabilities, Nanoscale 4 (2012) 4002 − 4006. [46] A. Yella, E. Mugnaioli, M. Panthöfer, H. A. Therese, U. Kolb, W. Tremel, Bismuthcatalyzed growth of SnS2 nanotubes and their stability, Angew. Chem. Int. Ed. 48 (2009) 6426 − 6430. [47] J. Ma, D. Lei, L. Mei, X. Duan, Q. Li, T. Wang, W. Zheng, Hydrothermal growth of SnS2 hollow spheres and their electrochemical propertie, CrystEngComm. 14 (2012) 832−836. [48] T. J. Kim, C. Kim, D. Son, M. Choi, B. Park, Novel SnS2-nanosheet anodes for lithium-ion batteries, J. Power Sources 167 (2007) 529 − 535. [49] Q. X. Gao, X. F. Wang, Y. R. Tao, X. C. Wu, Photoluminescence properties of zirconia nanobelts with trace Sm3+ ions, Sci. Adv. Mater. 4 (2012) 327331. [50] C. Jauberthie-Carillon, C. Guillemin, Dependence of the indirect energy-gap of gap on quasi-hydrostatic pressure and phase-transition, J. Phys.: Condens. Matter. 1 (1989) 68076816.
17
[51] Z. Liu, G. Chen, B. Liang, G. Yu, H. T. Huang, D. Chen and G. Z. Shen, Fabrication of high-quality
ZnTe
nanowires
toward
high-performance
rigid/flexible
visible-light
photodetectors, Opt. Express, 21 (2013) 7799 − 7810. [52] P. A. Hu, L. F. Wang, M. Yoon, J. Zhang, W. Feng, X. N. Wang, Z. Z. Wen, J. C. Idrobo, Y. Miyamoto, D. B. Geohegan, Highly responsive ultrathin GaS nanosheet photodetectors on rigid and flexible substrates, Nano Lett. 13 (2013) 1649 −1654. [53] W. Tian, C. Zhang, T. Y. Zhai, S. L. Li, X. Wang, J. W. Liu, X. Jie, D. Q. Liu, M. Y. Liao, Y. Koide, D. Golberg, Y. Bando, Flexible ultraviolet photodetectors with broad photoresponse based on branched ZnS-ZnO heterostructure nanofilms, Adv. Mater. 26 (2014) 3088−3093. [54] J. A. Garrido, E. Monroy, I. Izpura, E. Muńoz, Photoconductive gain modeling of GaN photodetectors, Semicond. Sci. Technol. 13 (1998) 563 − 568. [55] Z. Liu, T. Luo, B. Liang, G. Chen, G. Yu, X. M. Xie, D. Chen, G. Z. Shen, High-detectivity InAs nanowire photodetectors with spectral response from ultraviolet to near-infrared, Nano Res. 6 (2013) 775−783. [56] X. Xie, S. Y. Kwok, Z. Z. Lu, Y. K. Liu, Y. L. Cao, L. B. Luo, J. A. Zapien, I. Bello, C. S. Lee, S. T. Lee, W. J. Zhang, Visible-NIR photodetectors based on CdTe nanoribbons, Nanoscale 4 (2012) 2914 − 2919. [57] W. W. Xiong, J. Q. Chen, X. C. Wu, J. J. Zhu, Visible light detectors based on individual ZrSe3 and HfSe3 nanobelts, J. Mater. Chem. C. 3 (2015) 1929 − 1934.
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Biographies Jiajing Wu received her bachelor degree of Chemical Engineering and Technology in He'nan Normal University, China in 2014. Now she is working for her MS degree, majoring in Physical Chemistry, in Nanjing University. Her research interests include the synthesis and application of metal chalcogenides nanomaterials. Yourong Tao is an associate professor. Now she is working in the School of Chemistry and Chemical Engineering, Nanjing University. Her research interests are focused on nanomaterial chemistry, synthesis and application of semiconductor nanocrystals. Yi Wu is an undergraduate student at the School of Chemistry and Chemical Engineering, Nanjing University, China. Her research interests are focused on nanomaterial chemistry, synthesis and application of semiconductor nanocrystals. Xingcai Wu is a professor at the School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, and Nanjing University, China. He obtained his Ph D degree from Institute of Solid State Physics, Chinese Academy of Science in 2001. His research interests concentrate on functional nanomaterial control synthesis and photoelectrical property, and he has published more than eighties papers in peer-refereeing international journals.
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Figure captions Fig. 1. (a) XRD pattern,(b) low-magnification FESEM image,(c) high-magnification FESEM image, and (d) TEM image of SnS2 nanosheets prepared ultrasonically for 60 min. (e) HRTEM image and (f) corresponding SAED pattern of the single SnS2 nanosheet. Fig. 2. (a–e) FESEM images of SnS2 nanosheets prepared within 15, 30, 45, 90, and 120 min, respectively. (f) XRD patterns of the SnS2 nanosheets corresponding to the above reaction time. Fig. 3. AFM images of the products prepared in (a) 15, (b) 30, (c) 45, (d) 60, (e) 90, and (f) 120 min. Insets show the corresponding height files. Fig. 4. (a) UV spectrum of the SnS2 nanosheets. (b) (αhν)1/2 –hν curve of SnS2 nanosheets. Fig. 5. (a) Responsivity to wavelength of photodetector (inset: microphotograph of device). (b, c) I–V curves of the device illuminated by different-wavelength light, and under dark conditions. (d) I–V curves of the device under 405-nm light illuminations with different optical powers. (e) I–t curve of the device under 532-nm-light illumination with an on–off period of 1s at a bias of 5 V. (f, g) I–t curves of the device at a bias of 5 V under 405-nm-light illumination with an on–off period of 0.5 and 50 s. (h) I–t curves of the device recorded at different bias voltages under 405 nm-light illuminations with an on–off period of 5 s. (i) I–V curves of the device under 405-nm light illumination and dark, in air and vacuum. Fig. 6. The time-dependent responses of the device measured at a bias of 5 V under (a) 405-nm, (b) 850-nm, and (c) 980-nm light illumination with an on-off period of 1s.
20
(a)
SnS2 :JCPDS No.75-0367
112
012
111
110
100
011
Intensity (a.u.)
001
(b)
10 μm 10
20
30
40
50
60
70
80
2 (degree)
(d)
(c)
1 μm
100 nm
(e)
(f)
0.32 nm
(100) (011) 2 μm
Fig. 1
0.27 nm
Wu et al.
21
(a)
(b)
500 nm
500 nm
150 nm
(e)
500 nm
(f)
1 μm
120 min 90 min
Intensity (a.u.)
(d)
(c)
45 min 30 min 15 min 10
20
30
40
50
60
70
80
2(degree)
Fig. 2
Wu et al.
22
3.12nm
(b)
0 -1 0.2 0.4 0.6 0.8 1.0 1.2
(e)
2
3.405nm
0 -1 0.0 0.3 0.6 0.9 1.2 1.5
Distance (m)
Height (nm)
Height (nm)
1.5
2.0
4
3
-2
1.0
2 1 0
2.99nm
-1 -2 -3 0.0
0.5
1.0
1.5
2.0
Distance (m)
4
1
(c)
3.06nm
Distance (m)
Distance (m)
(d)
3
3
(f)
2 1 0
3.27nm
-1 -2 0.0 0.7 1.4 2.1 2.8 3.5 4.2
Distance (m)
Height (nm)
1
3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0.5
Height (nm)
2
Height (nm)
Height (nm)
(a)
3
3 2 1 0 -1
3.73nm
-2 0.0 0.6 1.2 1.8 2.4 3.0
Distance (m)
Fig. 3. Wu et al.
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0.7
1.6
0.5
(h)1/2 (a.u.)
Absorbance
0.6
0.4 0.3 0.2 0.1 0.0
(b)
1.8
(a)
1.4 1.2 1.0 0.8 0.6
1.39 eV
0.4 200
400
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800
Wavelength (nm)
1000
1200
0.2 1
2
3
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5
h (eV)
Fig. 4. Wu et al
24
0.6
(d)
405 nm,1.5 mW/cm2
-2
405 nm,4.17 mW/cm2 405 nm,162 mW/cm2
-4
405 nm,233.2 mW/cm2 dark
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
(g)
-2 0 2 Voltage (V)
4
-1.0 -4
-2 0 2 Voltage (V)
4
-6
6
0.5
5
10 15 Time (s)
20
25
1000 Time (s)
1500
2000
4
6
0.15 0.10 0.05 0.00
0
4
15 7 V, 5 s 3 V, 5 s 5 V, 5 s
(h)
10
0.4 0.3 0.2
8
12 16 Time (s)
20
24
(i)
5 0 Dark, air
-5
405 nm, 171.3 mW/cm2, air 405 nm, 171.3 mW/cm2, vacuum
-10
0.1
500
0 2 Voltage (V)
405 nm,0.5 s,162 mW/cm2
(f)
0.2
5 V, 50 s, 405 nm,162 mW/cm2
-2
0.20
0.3
0.0
-4
0.25
532 nm,19.35 mW/cm2,1 s
(e)
0.1
6
254 nm, 6 W/cm2 365 nm, 3.86 W/cm2 Dark 850 nm, 12.17 mW/cm2 980 nm, 48.9 mW/cm2
-0.5
0.4
Current (pA)
-4
0.0
Current (pA)
0
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Current (pA)
Current (pA)
2
(c)
1.0
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Current (pA)
(b)
Current (pA)
Responsivity (A/W)
Wavelength (nm)
6
Current (pA)
1.5 2
650 nm, 20.42 mW/cm 2 780 nm, 48.32 mW/cm 2 405 nm,162 mW/cm 2 532 nm, 19.35 mW/cm Dark
Current (pA)
30 25 20 0.6 15 0.5 10 3 μm 5 0.4 0 0.3 -5 -10 0.2 -15 0.1 -20 -25 0.0 -6 300 400 500 600 700 800 900 10001100
(a)
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Dark, vacuum
100
150 200 Time (s)
250
-15 -6
-4
-2 0 2 Voltage (V)
4
6
Fig 5. Wu et al.
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405 nm, 266.8 mW/cm2 , 1s
1.6
Current (pA)
Current (pA)
100 80 60 40
1.35
850 nm, 9.44 mW/cm2 ,1s
(b)
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1.5
Current (pA)
(a)
120
1.4 1.3 1.2 1.1
5
10
15 20 Time (s)
25
30
980 nm, 62.97 mW/cm2,1s
1.25 1.20 1.15 1.10
20 0
(c)
1.05
5
10 15 Time (s)
20
25
5
10 15 Time (s)
20
25
Fig 6. Wu et al.
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Table 1 Comparisons of the SnS2-nanosheet film photodetector with others’
Response Range
Wavelength/
Bias Voltage
Power
(V)
(mA/W)
Vis-NIR
400 nm/637 Wcm-2
5
1.2104
Single-layer MoS2
Vis
550 nm/80W
1
0.42
Black phosphorus
Vis-NIR
640/10 nW
0.2
4.8
Vis
532 nm/5.3 mWcm-2
5
12
SnS2 nanoplate
Vis
457 nm/8μW
-2
Few-layer SnS2
Vis
532 nm/ 0.1μW
SnS2 nanosheets
UV-NIR
SnS2 nanosheets
UV-NIR
Photodetector
CdTe nanoribbon
HfSe3 nanobelt
R
Ion/Ioff
~1.6
RiseRef. time/decay -time
1.1/3.3 s
56
50 ms
37
1 ms
11
2.2
0.4 s
57
~4.0
~7.4
12 /17 μs
41
5
1.0105
~1.1
44 ms
40
532 nm/0.68 nW
5
0.65
8.7
0.36 s
This work
405 nm/ 7.94 nW
5
3.410-2
6.4
0.4/0.2 s
This work
~16
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S0925-4005(16)30322-7 http://dx.doi.org/doi:10.1016/j.snb.2016.03.029 SNB 19834
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
22-1-2016 27-2-2016 7-3-2016
Please cite this article as: Jia-Jing Wu, You-Rong Tao, Yi Wu, Xing-Cai Wu, Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrathin SnS2 nanosheets of ultrasonic synthesis and their photoresponses from ultraviolet to near-infrared
Jia-Jing Wu, You-Rong Tao, Yi Wu, Xing-Cai Wu* Key Laboratory of Mesoscopic Chemistry of MOE, and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. *
Corresponding author. Fax: +86 25 83317761.
E-mail: [email protected] (X.C.Wu).
1
Highlights ●We synthesized the SnS2 ultrathin nanosheets by an ultrasonic chemical method for first time. ●The photodetector was fabricated on the basis of the SnS2 ultrathin nanosheet film. ●The photodetectors showed decent photoresponse from ultrviolet to near infrared.
Abstract Ultrathin SnS2 nanosheets are synthesized for the first time by a simple ultrasonic method, and then fabricated onto a SiO2/Si substrate to form nanosheet-based phototransistor which exhibits a broad photoresponse from 254 to 980 nm, dependence of photocurrent on optical power and wavelength, fast-response, and long-term stability. Under illumination of 532-nm light with an optical power of 19.3 mW/cm2 (0.68 nW), the photoswitch current ratio (PCR) is about 8.7, while the photoresponsivity, external quantum efficiency, and detectivity are 0.65 mA/W, 0.15%, and 1.13×108 J, respectively. Compared with the reported SnS2-based photodetectors, the SnS2nanosheet phototransistor shows an enhanced photosensitive performance. Keywords: Tin disulfide, Nanosheets, Ultrasonic synthesis, Photodetectors.
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1. Introduction The discovery of graphene in 2004 has led to a booming effort towards research on twodimensional (2D) materials [1]. The research becomes now one of the most active fields because of the wide applications of the materials in Li-ion batteries [2, 3], field-effect-transistors (FETs) [4], biomedical monitoring [5], gas/humidity sensors [6], solar cells [7], photocatalysis [8], photodetectors [9], and so on. The 2D materials include graphene [6], hexagonal boron nitride (hBN) [10], black phosphorus [11], metal dichalcogenides (MDCs, for example, MoS2, WS2, TiS2, ReS2, SnSe2, HfS2, SnS2 etc.) [12–19], metal chalcogenides [20–26], metal oxides [27–29], and so on. Because the layered structures of the MDCs are easily exfoliated into monolayer or fewlayer structures (2D materials) to the extent that the band gaps of the materials change, the 2D materials have been fabricated into various photosensitive devices [30–33]. Tin disulfide (SnS2) is also a layer-structure compound (CdI2-type) where the atoms within the S-Sn-S layer are held together by strong covalent forces while Van der Waals interaction enables stacking of the layers, so it can be also exfoliated to ultrathin nanostructures. SnS2 is an n-type semiconductor, nontoxicity, low cost, and high chemical stability, making it an important optoelectronic material [34–38]. Recently, SnS2-based optoelectronic devices have drawn intensive attention. Song et al. reported the top-gated field-effect-transistor (FET) based on few-layers SnS2 nanosheet with higher carrier mobility via the function of dielectric screening from mediate-κ dielectric Al2O3 [39]. Huang et al. reported a FET based on monolayer SnS2 flake by exfoliating SnS2 crystals and explored its potential application in photodetection [40]. Su et al. obtained the thin crystal arrays of SnS2 at predefined locations, introducing a new idea for large-scale production by chemical vapor deposition [41]. However, the broadband photodetector based on SnS2
3
nanosheets is rarely mentioned. Here we report synthesis, phototransistor, and broadband photoresponses of the SnS2 nanosheets. In the past years, a large number of SnS2 micro-/nano-structures have been synthesized such as hierarchical microspheres [42], hollow spheres [43], nanowires [44], nanorods [45], nanotubes [46] and nanoflakes [47]. Recently, a few preparation methods have been developed for preparing ultrathin SnS2 nanosheets including mechanical exfoliation [40], chemical vapor deposition [41], and solvothermal synthesis [48]. However, mechanical exfoliation is limited to yields and reproducibility; the solvothermal synthesis needs spend a long reaction time. Herein we directly synthesize ultrathin SnS2 nanosheets with a thickness of ~3.4 nm by a facile ultrasonic chemical method. In ultrasonic process, thioacetamide serves as sulfur source, and enthanol and deionized water as solvent, and stannic chlorides as tin source. Tin disulfide is easy to form ultrathin structure due to ultrasound cavitation effect. Subsequently, ultrathin SnS2 nanosheets are fabricated into a phototransistor on a SiO2/Si wafer. It shows good photoresponse to wavelengths from 254 to 980 nm. Under a 532-nm light illumination with an optical power of 19.3 mW/cm2 (0.68 nW), the photoswitch current ratio (PCR) is 8.7, so the phototransistor is a decent photodetector.
2. Experimental method 2.1.
Preparation and characterization
SnS2 nanosheet microspheres were synthesized via a facile ultrasonic chemical method. In a typical process, 1.7525 g SnCl4·5H2O (C. P.) and 0.7513 g thioacetamide (A. R.) were put into 100 mL beaker, and then 40 mL enthanol was added, and first stirred vigorously for 5 min with
4
glass rod. Subsequently the mixtures were sonicated by using a 120 W ultrasonic cleaner for 15, 30, 45, 60, 90, and 120 min, respectively. After the reactions finished, the resulting yellow dispersion was centrifuged by centrifuge at 3000 r. p. m., alternately rinsed with deionized water and ethanol, until the solution was neutral, and in vacuum dried at 60℃ for 3 h. The as-prepared products were characterized by a X-ray diffraction (XRD, Shimadzu XR−D6000 with graphite monochromatized Cu Kα1 radiation), a field-emission scanning electron microscope (FESEM, Hitachi S−4800) with energy-dispersive X-ray spectrometer (EDX), a high-resolution transmission electron microscope (HRTEM, JEM−2100), and UV3−600 spectrophotometer (Shimadzu UV−3600). Atomic force microscopy (AFM) was performed in ScanAsyst mode with NP-S oxide sharpened silicon nitride tips using a Bruker Innova microscope instrument which signal voice in Z-direction is lower than 40 pm. 2.2. Microfabrication and measurements of photodetector Firstly, the as-prepared samples (prepared within 60 min.) were dispersed in ethanol, then ultrasonically dispersed for 10 min, afterwards dropped on a SiO2 (300 nm)/Si substrate, and finally dried naturally in air to form film. Second, the metal electrodes were fabricated on the top of the SnS2 nanosheets via a standard photolithography process. In a typical photolithography process, photoresist was first covered onto the SiO2/Si substrates by spin coating and then baked. After that, the photoresist was exposed to ultraviolet light engraving machine with a mask, followed by developing in positive photoresist developer solution. Next, Ti/Au (10 nm/100 nm) metal was deposited on the surface of the substrate in high vacuum turbo evaporator apparatus. Finally, lift-off process was carried out to obtain phototransistor with an electrode separation of 3 μm. The current–voltage (I–V) and the current-time (I–t) characteristics of the photodetector were recorded by Model CRX-4K Cryogenic Probe Station (Lake Shore Inc.) and Keithley 236
5
source meter (Keithley Instruments Inc.) A spectroscopic response ranging from 350 to 1000 nm was measured using a 300 W Xenon lamp (HSX-UV300), and a multi-grating monochromator (71SW151) with order sorting filters was used. Optical power was measured with FZ-A radiometer. The spectral response was recorded under illumination of different wavelength laser. Laser power was measured with a LP1 laser power meter. The experiments have been done at room temperature in air except specially proposing condition.
3. Results and discussion 3.1. Structure characterization An XRD pattern of the as-prepared products is shown in Fig. 1a, which can be indexed to hexagonal SnS2 (JCPDS Card No. 75 – 0367; a = 0.3620 nm and c = 0.5850 nm). Based on the least square method calculation to the XRD pattern, the cell parameters of the nanosheets are a = 0.3611 nm and c= 0.5927 nm, almost coinciding with above JCPDS card. No peaks of impurities are detected, so they are higher purity. The EDX spectrum (Fig. S1) shows that the atomic ratio of Sn and S is 32.61: 67.39, closing to 1: 2, demonstrating the formation of SnS2. In addition, no impure peaks were observed except C and O (C and O comes from adhesive of sample desk), so the sample is higher purity. Morphology of the products is observed by a FESEM, as shown in Fig.1b, c, which displays that they are curled plate-like nanosheet with the length of several micrometer, and Viewed from the side in Fig.1 c, the thickness of the nanosheet is about 11 nm. TEM image further confirms that they are nanosheets, as shown in Fig. 1d. Fig. 1e reveals HRTEM image of a SnS2 nanosheet, which lattice fringe spacings of 0.27 and 0.32 nm can be assigned to the (011) and (100) planes of the SnS2, respectively. As shown in Fig. 1f, the
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corresponding selected area electron diffraction (SAED) pattern can be still indexed as hexagonal SnS2 phase, and indicating its polycrystalline structure. 3. 2. Growth process To understand the growth process, SnS2 nanosheets were synthesized at different reaction time (15 to 120 min). When the ultrasonic reaction passed by 15 min, the solution began to turn into yellow and generated SnS2 nanoparticles with the diameter of about 120-180 nm, as shown in Fig. 2a. Here only two low XRD peaks for the samples appeared, showing that the crystallinity of the products is no good (Fig.2f, 15 min.) When the reaction passed by 30 min., the products still were nanoparticles and the sizes almost did not change, as shown in Fig. 2b. Here intensity of the XRD peaks slightly increased (Fig.2f, 30 min.). When the reaction passed by 45 min., many nanosheets formed (Fig. 2c). Here the intensity of XRD peaks obviously increased (Fig. 2f, 45 min.). When the reaction passed by 90 min., the nanosheets became bigger, as shown in Fig. 2d. Here the XRD peaks got clearer, exhibiting that the crystallinity became better (Fig.2f, 90 min.). When the reaction passed by 120 min, we obtained high-crystalline SnS2 nanosheets. The morphology and the XRD pattern were shown in Fig. 2e and f (120 min.), respectively. AFM images can further confirm the size of the products, as shown in Fig. 3a–e. The thickness ranges from 3.0 to 3.7 nm when the reaction time increases from 15 to 120 min. As is mentioned above, a 2D no-perferred oriented attachment growth mechanism may be proposed [20], which is different from 2D oriented attachment growth process of PbS nanosheets [21]. Because the SnS2 nanosheets are grown throughout the ultrasonic process, small SnS2 nanocrystals expand with random orient in plane. Meanwhile, the vertical direction attachment cannot be well carried out. Therefore, the nanosheets are made up of various nanoparticles, i. e. forming polycrystals. Such morphology can be attributed to the electrostatic interaction between the nanocrystals [120].
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Fig.4a is UV absorbance spectrum of the SnS2 nanosheets (prepared within 60 min.) which shows existence of light absorbance of the products from 253 to 1200 nm, but there are only weak absorbance after 800 nm. Based on energy gap formula [49, 50], the indirect energy gap of the nanosheets is 1.39 eV (Fig. 4b). 3.3. Photoresponses of the phototransistors The as-prepared SnS2 nanosheets are fabricated into nanosheet-film phototransistors on a SiO2/Si wafer by a micro-fabrication process, as shown in inset of Fig. 5a. The AFM image of the nanofilm is shown in Fig. S2, and the thickness of the nanofilm is about 170 nm. The film is composed of ultrathin nanosheets, and the effective illuminated area is ~4.9 × 10-8 cm2. Responsitivity to light wavelength measured by Xe lamp almost coincides with UV absorbance spectrum, as shown in Fig. 5a, indicating that photoresponse originate from the carrier excitation in the semiconducting SnS2 nanosheet rather than the contact region with Schottky barriers. Fig. 5b, c demonstrates the I–V curves of the phototransistor under dark and light illumination with different wavelengths, which shows photosensitive properties from 254 to 980 nm. The nonlinearity and asymmetry of the curves can be attributed to Schottky contact between the nanosheet and electrodes. Responsivity (Rλ), external quantum efficiency (EQE), and detectivity (D*) are critical parameters for a photodetector, which reflect the photosensitivity of the photodetector. They can be determined from the following equations [51, 52]: Rλ=ΔI/ (PS); EQE= Rλhc/ (eλ); D*= Rλ/ (2e.Idark/S)0.5, where ΔI is the difference between the light and dark currents; P is the incident light power; S is the effective illuminated area; h is Planck’s constant; C is the speed of light; λ represents the light wavelength; and e is the electronic charge. Based on the equations, Rλ for 254, 365, 405, 532, 650, 780, 850, and 980 nm wavelengths at a bias of 5 V are 1950.4, 5.23, 0.695, 23.9,11.3,1.28,0.92, and 0.18 mA/W, respectively. It shows that the
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response is the biggest for 254 nm (4.88 eV), and the second for 532 nm (2.26 eV), and the least for 980 nm (1.27 eV). The corresponding EQE are 955%, 1.75%, 0.22%, 5.52%, 2.21%, 0.20%, 0.15%, and 0.0184%, respectively, while the corresponding D* are 2.02×1010, 5.33×107, 7.39×106, 2.48×108, 1.17×108, 1.35×107, 9.93×106, and 3.5×106 Jones, respectively. The wavelength-dependent response corresponds well with the fact that higher excitation energy can enhance the conversion of photoelectrons, so 254 nm-light has highest photoresponse. Fig. 5d is the I–V curves of the device to various light intensity under 405-nm light illumination, which demonstrates that the photocurrents depend on the light intensities, and gradually increase as the light intensities increase. Under 405-nm light illumination with an optical power of 233.2 mW/cm2 (11.4 nW), and at a bias of 5 V, the PCR is ~12. Fig. 5e shows an I–t characteristic under illumination of 532-nm light with an optical power of 19.3 mW/cm2 (0.68 nW). Here the PCR (Ion/off) is about 8.7, while the R532 nm, EQE, and D* are 0.65 mA/W, 0.15%, and 1.13×108 J, respectively. Fig. 5f, g show the I–t characteristics of the device to the 405 nm incident radiation with an optical power of 162 mW/cm2 (7.94 nW) at a bias voltage of 5 V with the photoswitch periods of 0.5 and 50 s, respectively. As shown in Fig. 5f, the PCR, R405 nm, EQE, and D* are ~5, 0.02 mA/W, 6.1×10-3%, and 1.8×106 J, respectively, and the response time (rise time/decay time) is 0.4 /0.4 s. As shown in Fig. 5g, the PCR, the R405 nm, EQE, and D* are ~6.4, 0.034 mA/W, 1.03×10-2%, and 2.74×106 J, respectively, and the response time (rise time/decay time) is 0.4/0.2 s. The time taken for the current to increase from 10% to 90% of the peak value or vice versa is defined as the rise time and decay time, respectively [53]. Upon illumination, the photocurrent increases to a stable value, and then dramatically decreases to its initial value as light is turned off, thereby showing excellent stability and reproducible characteristics, and allowing the device to act as a high quality photosensitive switch. Different photoswitch periods result in different
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Ion/off and rise/decay times, because the increase and decay of photocurrent need enough time [54]. From the plots, we can observe that the photocurrents are not too stable, which is attributed to environment factors such as temperature, air flow, noise as well as slight vibrations [55]. Fig. 5h shows the time-dependent currents of the device under 405-nm light illumination of 162 mW/cm2 power with a photoswitch period of 5 s at the bias of 3, 5, and 7 V. Upon illumination, the currents rapidly switch from ~0.05 pA to ~0.15, 0.26, and 0.5 pA, respectively, and then sharply returns to its “OFF-state” when the light is turned off. A nearly identical fast response is observed for multiple cycles, indicating the sensitivity and excellent stability of the device. The responses in air and vacuum are measured under 405-nm light illumination of 171.3 mW/cm2 power, as illustrated in Fig. 5i. At an applied voltage of 5 V, the current (0.713 pA) in vacuum is about 1.44 times higher than that (0.495 pA) in air under dark, while the current in vacuum is 12.2 pA and is about 3.42 times higher than that (3.57 pA) in air under 405-nm light illumination. In all, the current in vacuum is higher than in air regardless of dark or illumination. It can be explained by an oxygen adsorption/desorption mechanism. Because SnS2 is an n-type semiconductor, the electrons are dominant carriers in the nanosheets. In air, oxygen molecules with larger electronegativity are adsorbed on the surface of the nanosheets and capture free electrons from the conduction band (O2 + e-→O2-) so that the conductivity reduces. In vacuum, oxygen adsorption becomes rare so that the electrons return to conduction band, so the current increases. Here oxygen acts as a trap for electrons. Under light illumination, electron-hole pairs are generated on the large scale. The holes migrate to the surface to combine with O2- to form O2 to desorb, resulting in an increase in the electron concentration so that the photocurrent increases. In order to understand the stability of the device, after the device is exposed in air for two months, the I–t curves are measured under 405-, 850-, and 980-nm illumination, as shown in Fig.
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6a–c, respectively. The PCRs are 10.6, 1.3, and 1.1, respectively. The photosensitivity almost remains, so the device is stable in air. The crucial parameters for a few photodetectors are summarized in Table 1. As can be seen from Table 1, the SnS2 nanosheet film photodetector shows a broader light response. The responsivity under 532 nm/0.68 nW light illumination is much lower than those of CdTe nanoribbon [56] and few-layer SnS2 photodetectors [40], but the PCR is much higher than those of the either. So the SnS2 nanosheet film phototransistor shows an enhanced photoresponse.
4. Conclusions In summary, we prepared ultrathin SnS2 nanosheets with a thickness of about 3.4 nm by a facile ultrasonic chemical method for the first time. The indirect energy gap of the sample is 1.39 eV. The SnS2-nanosheet phototransistor shows an excellent photoresponse from UV to NIR. The photoresponse to 980 nm is reported for the first time. So the phototransistor is an excellent photodetector and the ultrathin SnS2 nanosheet is an excellent optoelectrical material. Acknowledgements We thank for the financial support from the National Science Foundations of China (No. 21171091 and 20671050).
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References [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666 − 669. [2] J. W. Seo, J. T. Jang, S. W. Park, C. Kim, B. Park, J. Cheon, Two-dimensional SnS2 nanoplates with extraordinary high discharge capacity for lithium ion batteries, Adv. Mater. 20 (2008) 4269 − 4273. [3] Y. Du, Z. Yin, X. Rui, Z. Zeng, X. J. Wu, J. Liu, Y. Zhu, J. Zhu, X. Huang, Q. Yan, H. Zhang, A facile, relative green, and inexpensive synthetic approach toward large-scale production of SnS2 nanoplates for high-performance lithium-ion batteries, Nanoscale 5 (2013) 1456 − 1459. [4] K. Kim, J. Y. Choi, T. Kim, S. H. Cho, H. J. Chung, A role for graphene in silicon-based semiconductor device, Nature 479 (2011) 338 − 344. [5] T. Kosugi, K. Murakami, S. Kaneko, Thin-film structures for photovoltaics, Mater. Res. Soc. Symp. Proc. 485 (1998) 273−278. [6] A. K. Geim, K. S. Novoselov, The rise of grapheme, Nat. Mater. 6 (2007) 183−191. [7] F. R. Tan, S. C. Qu, J. Wu, K. Liu, S. Y. Zhou, Z. G. Wang, Preparation of SnS2 colloidal quantum dots and their application in organic/inorganic hybrid solar cells, Nano Res Lett. 6 (2011) 1− 8. [8] Y. C. Zhang, L. Yao, G. S. Zhang, D. D. Dionysiou, J. Li, X. H. Du, One-step hydrothermal synthesis
of
high-performance
visible
light-driven
SnS2/SnO2
nanoheterojunction
photocatalyst for the reduction of aqueous Cr(VI), Appl. Catal. B: Environ. 144 (2014) 730 − 738.
12
[9] H. Li, J. Wu, Z. Yin, H. Zhang, Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and WSe2 nanosheets, Acc. Chem. Res. 47 (2014) 1067−1075. [10] K. K. Kím, A. Hsu, X. T. Jía, S. M. Kím, Y. S. Shí, M. Hofmann, D. Nezích, J. F. Rodriguez-Níeva, M. Dresselhaus, T. Palacíos, J. Kong, Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition, Nano Lett. 12 (2012) 161−166. [11] M. Buscema, D. J. Groenendijk, S. I. Blanter, G. A. Steele, H. S. J. vander Zant, A. Castellanos-Gomez, A fast and broadband photoresponse of few-layer black phosphorus field-effect transistors, Nano Lett. 14 (2014) 3347 − 3352. [12] Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey, H. Zhang, Single-layer semiconducting nanosheets: high-yield preparation and device fabrication, Angew. Chem. Int. Ed. 50 (2011) 11093−11097. [13] Q. N. Cui, H. Zhao, Coherent control of nanoscale ballistic currents in transition metal dichalcogenide ReS2, ACS Nano 9 (2015) 3935− 3941. [14] X. Zhou , L. Gan , W. M. Tian , Q. Zhang , S. Y. Jin , H. Q. Li , Y. Bando, D. Golberg , T. Y. Zhai, Ultrathin SnSe2 flakes grown by chemical vapor deposition for high-performance photodetectors, Adv. Mater. 27 (2015), 8035 – 8041. [15] J. Jang, S. Jeong, J. Seo, M. C. Kim, E. Sim, Y. Oh, S. Nam, B. Park, J. Cheon, Ultrathin zirconium disulfide nanodiscs, J. Am. Chem. Soc. 133 (2011) 7636 − 7639. [16] X. Huang, Z. Y. Zeng, H. Zhang, Metal dichalcogenide nanosheets: preparation, properties and applications, Chem. Soc. Rev. 42 (2013) 1934 − 1946. [17] K. Xu, Z. X. Wang, F. Wang, Y. Huang, F. M. Wang, L. Yin, C. Jiang, J. He, Ultrasensitive phototransistors based on few-layered HfS2, Adv. Mater. 27 (2015) 7881–7887.
13
[18] Y. Huang, K. Xu, Z. X. Wang, T. A. Shifa, Q. S. Wang, F. Wang, C. Jiang, J. He, Designing the shape evolution of SnSe2 nanosheets and their optoelectronic properties. Nanoscale 7 (2015) 17375–17380. [19] Y. Huang, H. X. Deng, K. Xu, Z. X .Wang, Q. S. Wang, F. M. Wang, F. Wang, X. Y. Zhan, S. S. Li, J. W. Luo, Highly sensitive and fast phototransistor based on large size CVD-grown SnS2 nanosheets, Nanoscale 33 (2015) 140931–4099. [20] Z. Y. Tang, Z. L. Zhang, Y. Wang, S. C. Glotzer, N. A. Kotov, Self-assembly of CdTe nanocrystals into free-floating sheets, Science 314 (2006) 274−278. [21] C. Schliehe, B. H. Juarez, M. Pelletier, S. Jander, D. Greshnykh, M. Nagel, A. Meyer, S. Foerster, A. Kornowski, C. Klinke, Ultrathin PbS sheets by two-dimensional oriented attachment, Science 329 (2010) 550 − 553. [22] L. Li, H. Lu, Z. Y. Yang, L. M. Tong, Y. Bando, D. Golberg, Bandgap-graded CdSxSe1x nanowires for high-performance field-effect transistors and solar cells, Adv. Mater. 25 (2013) 11091113. [23] G. H. Chen, W. L. Wang, C. D. Wang, T. Ding, Q. Yang, Controlled synthesis of ultrathin Sb2Se3 nanowires and application for flexible photodetectors, Adv. Sci. 2 (2015) 1500109. [24] G. H. Chen, W. Li, Y. Q. Yu, Q. Yang, Fast and low-temperature synthesis of one dimensional (1D) single-crystalline SbSI microrod for high performance photodetector, RSC Adv. 5(2015) 21859–21864. [25] G. H. Chen, Y. Q. Yu, K. Zheng, T. Ding, W. L. Wang, Y. Jiang, Q. Yang, Fabrication of ultrathin Bi2S3 nanosheets for high- performance, flexible, visible–NIR photodetectors, Small 11(2015) 2848–2855.
14
[26] Q. Yang, K. B. Tang, C. R. Wang, D. Y. Zhang, Y. T. Qian, The Synthesis of SnS2 nanoflakes from tetrabutyltin precursor, J. Solid State Chem. 164 (2002) 106–109. [27] F. Xu, X. Y. Zhang, Y. Wu, D. P. Wu, Z. Y. Gao, K. Jiang, Facile synthesis of TiO2 hierarchical microspheres assembled by ultrathin nanosheets for dye-sensitized solar cells, J. Alloys Comp. 574 (2013) 227 − 232. [28] C. T. Gao, X. D. Li, X. P. Zhu, L. L. Chen, Y. Q. Wang, F. Teng, Z. X. Zhang, H. G. Duan, E. Q. Xie, High performance, self-powered UV-photodetector based on ultrathin, transparent, SnO2-TiO2 core-shell electrodes, J. Alloys Comp. 616 (2014) 510 − 515. [29] L. F. Hu, L. M. Wu, M. Y. Liao, X. H. Hu, X. S. Fang, Electrical transport properties of large, individual NiCo2O4 nanoplates, Adv. Funct. Mater. 22 (2012) 998 –1004. [30] Z. Y. Yin, H. Li, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen, H. Zhang, Single-layer MoS2 phototransistors, ACS Nano 6 (2012) 74−80. [31] N. Perea-López, A. L. Elías, A. Berkdemir, A. Castro-Beltran, H. R. Gutiérrez, S. M. Feng, R. T. Lv, T. Hayashi, F. López-Urías, S. Ghosh, B. Muchharla, S. Talapatra, H. Terrones, M. Terrones, Photosensor device based on few-layered WS2 films, Adv. Funct. Mater. 23 (2013) 5511− 5517. [32] W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G. B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, S. Kim, High-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infrared, Adv. Mater. 24 (2012) 5832 − 5836. [33] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Ultrasensitive photodetectors based on monolayer MoS2, Nat. Nanotechnol. 8 (2013) 497−501. [34] T. Jiang, G. A. Ozin, New directions in tin sulfide materials chemistry, J. Mater. Chem. 8 (1998) 1099 −1108.
15
[35] Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, F. Lei, T. Yao, J. He, S. Wei, Freestanding tin disulfide single-layers realizing efficient visible-light water splitting, Angew. Chem. Int. Ed. 51 (2012) 8727 − 8731. [36] G. Radovsky, R. P. Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert, R.Tenne, Synthesis of copious amounts of SnS2 and SnS2/SnS nanotubes with ordered superstructures, Angew. Chem. Int. Ed. 50 (2011) 12316 − 12320. [37] E. C. Greyson, J. E. Barton, T. Odom, Tetrahedral zinc blende tin sulfide nano and microcrystals, Small 2 (2006) 368 − 371. [38] H. J. Geng, Y. J. Su, H. Wei, M. H. Xu, L. M. Wei, Z. Yang, Y. F. Zhang, Controllable synthesis and photoelectric property of hexagonal SnS2 nanoflakes by triton X-100 assisted hydrothermal method, Mater. Lett. 111 (2013) 204 − 207. [39] H. S. Song, S. L. Li, L. Gao, Y. Xu, K. Ueno, J. Tang, Y.B. Cheng, K. Tsukagoshi, Highperformance top-gated monolayer SnS2 field-effect transistors and their integrated logic circuits, Nanoscale 5 (2013) 9666 − 9670. [40] Y. Huang, E. Sutter, J. T. Sadowski, M. Cotlet, O. L. A. Monti, D. A. Racke, Mahesh. R. Neupane, Darshana. Wickramaratne, R. K. Lake, B. A. Parkinson, P. Sutter, Tin disulfide－ an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics, ACS Nano 8 (2014) 10743−10755. [41] G. X. Su, V. G. Hadjiev, P. E. Loya , J. Zhang, S. D. Lei , S. Maharjan, P. Dong, P. M. Ajayan, J. Lou, H. B. Peng, Chemical vapor deposition of thin crystals of layered semiconductor SnS2 for fast photodetection application, Nano Lett. 15 (2015) 506 − 513.
16
[42] J. Zai, X. Qian, K. Wang, C. Yu, L. Tao, Y. Xiao, J. Chen, 3D-hierarchical SnS2 micro/nano-structures: controlled synthesis, formation mechanism and lithium ion storage performances, CrystEngComm. 14 (2012) 1364 − 1375. [43] J. Xia, G. Li, Y. Mao, Y. Li, P. Shen, L. Chen, 3D-hierarchical SnS2 micro/nano-structures: controlled synthesis, formation mechanism and lithium ion storage performances, CrystEngComm 14 (2012) 4279 − 4283. [44] Y. T. Lin, J. B. Shi, Y. C. Chen, C. J. Chen, P. F. Wu, Synthesis and characterization of tin disulfide (SnS2) nanowires, Nano Res Lett. 4 (2009) 694 − 698. [45] P. Wu, N. Du, H. Zhang, J. Liu, L. Chang, L. Wang, D. Yang, J. Z. Jiang, Layer-stacked tin disulfide nanorods in silica nanoreactors with improved lithium storage capabilities, Nanoscale 4 (2012) 4002 − 4006. [46] A. Yella, E. Mugnaioli, M. Panthöfer, H. A. Therese, U. Kolb, W. Tremel, Bismuthcatalyzed growth of SnS2 nanotubes and their stability, Angew. Chem. Int. Ed. 48 (2009) 6426 − 6430. [47] J. Ma, D. Lei, L. Mei, X. Duan, Q. Li, T. Wang, W. Zheng, Hydrothermal growth of SnS2 hollow spheres and their electrochemical propertie, CrystEngComm. 14 (2012) 832−836. [48] T. J. Kim, C. Kim, D. Son, M. Choi, B. Park, Novel SnS2-nanosheet anodes for lithium-ion batteries, J. Power Sources 167 (2007) 529 − 535. [49] Q. X. Gao, X. F. Wang, Y. R. Tao, X. C. Wu, Photoluminescence properties of zirconia nanobelts with trace Sm3+ ions, Sci. Adv. Mater. 4 (2012) 327331. [50] C. Jauberthie-Carillon, C. Guillemin, Dependence of the indirect energy-gap of gap on quasi-hydrostatic pressure and phase-transition, J. Phys.: Condens. Matter. 1 (1989) 68076816.
17
[51] Z. Liu, G. Chen, B. Liang, G. Yu, H. T. Huang, D. Chen and G. Z. Shen, Fabrication of high-quality
ZnTe
nanowires
toward
high-performance
rigid/flexible
visible-light
photodetectors, Opt. Express, 21 (2013) 7799 − 7810. [52] P. A. Hu, L. F. Wang, M. Yoon, J. Zhang, W. Feng, X. N. Wang, Z. Z. Wen, J. C. Idrobo, Y. Miyamoto, D. B. Geohegan, Highly responsive ultrathin GaS nanosheet photodetectors on rigid and flexible substrates, Nano Lett. 13 (2013) 1649 −1654. [53] W. Tian, C. Zhang, T. Y. Zhai, S. L. Li, X. Wang, J. W. Liu, X. Jie, D. Q. Liu, M. Y. Liao, Y. Koide, D. Golberg, Y. Bando, Flexible ultraviolet photodetectors with broad photoresponse based on branched ZnS-ZnO heterostructure nanofilms, Adv. Mater. 26 (2014) 3088−3093. [54] J. A. Garrido, E. Monroy, I. Izpura, E. Muńoz, Photoconductive gain modeling of GaN photodetectors, Semicond. Sci. Technol. 13 (1998) 563 − 568. [55] Z. Liu, T. Luo, B. Liang, G. Chen, G. Yu, X. M. Xie, D. Chen, G. Z. Shen, High-detectivity InAs nanowire photodetectors with spectral response from ultraviolet to near-infrared, Nano Res. 6 (2013) 775−783. [56] X. Xie, S. Y. Kwok, Z. Z. Lu, Y. K. Liu, Y. L. Cao, L. B. Luo, J. A. Zapien, I. Bello, C. S. Lee, S. T. Lee, W. J. Zhang, Visible-NIR photodetectors based on CdTe nanoribbons, Nanoscale 4 (2012) 2914 − 2919. [57] W. W. Xiong, J. Q. Chen, X. C. Wu, J. J. Zhu, Visible light detectors based on individual ZrSe3 and HfSe3 nanobelts, J. Mater. Chem. C. 3 (2015) 1929 − 1934.
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Biographies Jiajing Wu received her bachelor degree of Chemical Engineering and Technology in He'nan Normal University, China in 2014. Now she is working for her MS degree, majoring in Physical Chemistry, in Nanjing University. Her research interests include the synthesis and application of metal chalcogenides nanomaterials. Yourong Tao is an associate professor. Now she is working in the School of Chemistry and Chemical Engineering, Nanjing University. Her research interests are focused on nanomaterial chemistry, synthesis and application of semiconductor nanocrystals. Yi Wu is an undergraduate student at the School of Chemistry and Chemical Engineering, Nanjing University, China. Her research interests are focused on nanomaterial chemistry, synthesis and application of semiconductor nanocrystals. Xingcai Wu is a professor at the School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, and Nanjing University, China. He obtained his Ph D degree from Institute of Solid State Physics, Chinese Academy of Science in 2001. His research interests concentrate on functional nanomaterial control synthesis and photoelectrical property, and he has published more than eighties papers in peer-refereeing international journals.
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Figure captions Fig. 1. (a) XRD pattern,(b) low-magnification FESEM image,(c) high-magnification FESEM image, and (d) TEM image of SnS2 nanosheets prepared ultrasonically for 60 min. (e) HRTEM image and (f) corresponding SAED pattern of the single SnS2 nanosheet. Fig. 2. (a–e) FESEM images of SnS2 nanosheets prepared within 15, 30, 45, 90, and 120 min, respectively. (f) XRD patterns of the SnS2 nanosheets corresponding to the above reaction time. Fig. 3. AFM images of the products prepared in (a) 15, (b) 30, (c) 45, (d) 60, (e) 90, and (f) 120 min. Insets show the corresponding height files. Fig. 4. (a) UV spectrum of the SnS2 nanosheets. (b) (αhν)1/2 –hν curve of SnS2 nanosheets. Fig. 5. (a) Responsivity to wavelength of photodetector (inset: microphotograph of device). (b, c) I–V curves of the device illuminated by different-wavelength light, and under dark conditions. (d) I–V curves of the device under 405-nm light illuminations with different optical powers. (e) I–t curve of the device under 532-nm-light illumination with an on–off period of 1s at a bias of 5 V. (f, g) I–t curves of the device at a bias of 5 V under 405-nm-light illumination with an on–off period of 0.5 and 50 s. (h) I–t curves of the device recorded at different bias voltages under 405 nm-light illuminations with an on–off period of 5 s. (i) I–V curves of the device under 405-nm light illumination and dark, in air and vacuum. Fig. 6. The time-dependent responses of the device measured at a bias of 5 V under (a) 405-nm, (b) 850-nm, and (c) 980-nm light illumination with an on-off period of 1s.
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3.12nm
(b)
0 -1 0.2 0.4 0.6 0.8 1.0 1.2
(e)
2
3.405nm
0 -1 0.0 0.3 0.6 0.9 1.2 1.5
Distance (m)
Height (nm)
Height (nm)
1.5
2.0
4
3
-2
1.0
2 1 0
2.99nm
-1 -2 -3 0.0
0.5
1.0
1.5
2.0
Distance (m)
4
1
(c)
3.06nm
Distance (m)
Distance (m)
(d)
3
3
(f)
2 1 0
3.27nm
-1 -2 0.0 0.7 1.4 2.1 2.8 3.5 4.2
Distance (m)
Height (nm)
1
3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 0.5
Height (nm)
2
Height (nm)
Height (nm)
(a)
3
3 2 1 0 -1
3.73nm
-2 0.0 0.6 1.2 1.8 2.4 3.0
Distance (m)
Fig. 3. Wu et al.
23
0.7
1.6
0.5
(h)1/2 (a.u.)
Absorbance
0.6
0.4 0.3 0.2 0.1 0.0
(b)
1.8
(a)
1.4 1.2 1.0 0.8 0.6
1.39 eV
0.4 200
400
600
800
Wavelength (nm)
1000
1200
0.2 1
2
3
4
5
h (eV)
Fig. 4. Wu et al
24
0.6
(d)
405 nm,1.5 mW/cm2
-2
405 nm,4.17 mW/cm2 405 nm,162 mW/cm2
-4
405 nm,233.2 mW/cm2 dark
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
(g)
-2 0 2 Voltage (V)
4
-1.0 -4
-2 0 2 Voltage (V)
4
-6
6
0.5
5
10 15 Time (s)
20
25
1000 Time (s)
1500
2000
4
6
0.15 0.10 0.05 0.00
0
4
15 7 V, 5 s 3 V, 5 s 5 V, 5 s
(h)
10
0.4 0.3 0.2
8
12 16 Time (s)
20
24
(i)
5 0 Dark, air
-5
405 nm, 171.3 mW/cm2, air 405 nm, 171.3 mW/cm2, vacuum
-10
0.1
500
0 2 Voltage (V)
405 nm,0.5 s,162 mW/cm2
(f)
0.2
5 V, 50 s, 405 nm,162 mW/cm2
-2
0.20
0.3
0.0
-4
0.25
532 nm,19.35 mW/cm2,1 s
(e)
0.1
6
254 nm, 6 W/cm2 365 nm, 3.86 W/cm2 Dark 850 nm, 12.17 mW/cm2 980 nm, 48.9 mW/cm2
-0.5
0.4
Current (pA)
-4
0.0
Current (pA)
0
-6 -6
0.5
Current (pA)
Current (pA)
2
(c)
1.0
0.5
4
Current (pA)
(b)
Current (pA)
Responsivity (A/W)
Wavelength (nm)
6
Current (pA)
1.5 2
650 nm, 20.42 mW/cm 2 780 nm, 48.32 mW/cm 2 405 nm,162 mW/cm 2 532 nm, 19.35 mW/cm Dark
Current (pA)
30 25 20 0.6 15 0.5 10 3 μm 5 0.4 0 0.3 -5 -10 0.2 -15 0.1 -20 -25 0.0 -6 300 400 500 600 700 800 900 10001100
(a)
0.7
Dark, vacuum
100
150 200 Time (s)
250
-15 -6
-4
-2 0 2 Voltage (V)
4
6
Fig 5. Wu et al.
25
405 nm, 266.8 mW/cm2 , 1s
1.6
Current (pA)
Current (pA)
100 80 60 40
1.35
850 nm, 9.44 mW/cm2 ,1s
(b)
1.30
1.5
Current (pA)
(a)
120
1.4 1.3 1.2 1.1
5
10
15 20 Time (s)
25
30
980 nm, 62.97 mW/cm2,1s
1.25 1.20 1.15 1.10
20 0
(c)
1.05
5
10 15 Time (s)
20
25
5
10 15 Time (s)
20
25
Fig 6. Wu et al.
26
Table 1 Comparisons of the SnS2-nanosheet film photodetector with others’
Response Range
Wavelength/
Bias Voltage
Power
(V)
(mA/W)
Vis-NIR
400 nm/637 Wcm-2
5
1.2104
Single-layer MoS2
Vis
550 nm/80W
1
0.42
Black phosphorus
Vis-NIR
640/10 nW
0.2
4.8
Vis
532 nm/5.3 mWcm-2
5
12
SnS2 nanoplate
Vis
457 nm/8μW
-2
Few-layer SnS2
Vis
532 nm/ 0.1μW
SnS2 nanosheets
UV-NIR
SnS2 nanosheets
UV-NIR
Photodetector
CdTe nanoribbon
HfSe3 nanobelt
R
Ion/Ioff
~1.6
RiseRef. time/decay -time
1.1/3.3 s
56
50 ms
37
1 ms
11
2.2
0.4 s
57
~4.0
~7.4
12 /17 μs
41
5
1.0105
~1.1
44 ms
40
532 nm/0.68 nW
5
0.65
8.7
0.36 s
This work
405 nm/ 7.94 nW
5
3.410-2
6.4
0.4/0.2 s
This work
~16
27