Photonics
Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films You-Rong Tao, Xing-Cai Wu,* and Wei-Wei Xiong
Two
new flexible visible-light photodetectors based on ZrS3 nanobelts films are fabricated on a polypropylene (PP) film and printing paper, respectively, by an adhesive-tape transfer method, and their light-induced electric properties are investigated in detail. The devices demonstrate a remarkable response to 405 to 780 nm light, a photocurrent that depends on the optical power and light wavelength, and an excellent photoswitching effect and stability. This implies that ZrS3 nanobelts are prospective candidates for high-performance nanoscale optoelectronic devices that may be practically applied in photodetection of visible to near infrared light. The facile fabrication method is extendable to flexible nanodevices with different nanostructures.
1. Introduction One-dimensional (1D) nanostructures have become the focus of worldwide research because of their potential use in high-performance nano-devices based on their high surface to volume ratios, their rationally designed surface, and unique physical properties.[1–12] Photodetectors are essential elements in many fields, for instance, imagining techniques, optical communications, and optoelectronic integrated circuits. Photodetectors based on various 1D nanostructures, such as ZnO,[13] ZnSe,[14] CdS,[15] CdS1-xSex,[16] In2Ge2O7,[17] Ga2O3,[18] graphene,[19] Ta3N5[20] nanobelts, WS2 nanotubes,[21] ZnS/ZnO biaxial nanobelts,[22] branched ZnS–ZnO heterostructure nanowires,[23] ZnO-SnO2 heterojunction nanofibers,[24] GaAs/AlGaAs core–shell nanowires,[25] singlelayer MoS2 nanobelts,[26] and so on have been extensively investigated. However, it is still a challenge to develop highperformance and broad-response photodetectors. Transition-metal trichalcogenides MX3 (where M is Ti, Zr, Hf, Mo, W, V, Nb, or Ta; X is S, Se, or Te) are structurally and chemically well-defined compounds. They possess a pseudo one-dimensional structure whereby an infinite chain of trigonal prismatic (MX6) units extends parallel to the b-axis and shares upper and lower faces. The chains showing strong Prof. Y.-R. Tao, Prof. X.-C. Wu, W.-W. Xiong Country Key Laboratory of Mesoscopic Chemistry of MOE School of Chemistry and Chemical Engineering Nanjing University Nanjing 210093, China E-mail: [email protected] DOI: 10.1002/smll.201401376 small 2014, DOI: 10.1002/smll.201401376
ionic covalent (or metallic) bonding are separated by a relatively large distance due to interchain bonding, so they show a marked anisotropy in most of their physical properties.[27] Zirconium trisulfide (ZrS3) is a member of this family of compounds. ZrS3 crystals are p-type semiconductors with a roomtemperature resistivity of 15 Ω cm, a direct optical bandgap of 2.56 eV at room temperature, and two indirect optical bandgaps of 2.055 eV (Eb) and 2.058 eV (E⊥b) at liquid helium temperature (below 4.2 K).[27] Although ZrS3 nanobelts have been synthesized,[28] nanodevices based on ZrS3 nanobelts have not been produced to date. Therefore, designing nanodevices based on ZrS3 nanostructures will largely push forward applications of ZrS3 and other transition-metal trichalcogenide nanostructures in the optoelectronic fields. Li et al. first made a visible photodetector based on an individual ZrS2 nanobelt, and they discovered an excellent photosensitivity.[29] Here we used a ZrS3-nanobelt film to fabricate flexible visiblelight photodetectors on polypropylene and on a paper substrate, respectively. The devices demonstrate a tunable spectral selectivity, wide-range photoresponse, high-speed response, and excellent environmental stability. In addition, a complex lithography process is not necessary to fabricate these photodetectors and they are also able to bear external mechanical forces, suggesting a wide range of potential applications.
2. Results and Discussion 2.1. Preparation and Characterization of the ZrS3 Nanobelts An X-ray diffraction (XRD) pattern of a ZrS3 nanobelt film on zirconium foil is shown in Figure 1a. All the diffraction peaks can be readily indexed to the monoclinic ZrS3 phase
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Figure 1. a) XRD pattern, b) low-magnification SEM image, and c) high-magnification SEM image of ZrS3 nanobelt film. d) TEM image of ZrS3 nanobelts, e) TEM image of an individual ZrS3 nanobelt (inset shows its SAED pattern), and f) HRTEM image of the individual ZrS3 nanobelt (inset shows the corresponding FFT pattern).
(JCPDS Card No. 30–1498; a = 0.5124 nm, b = 0.3624 nm, c = 0.898 nm, and β = 97.28°), so it is clear that the film is pure. Field-emission scanning electron microscopy (SEM) was used to record its morphology. Figure 1b shows the lowmagnification SEM image of the film on Zr foil, and Figure 1c is the high-magnification SEM image, revealing that the films consists of high-yield nanobelts with a typical size of about 0.85 micrometers in width, 70 nm in thickness, and several tens of micrometers in length. The belt-like structure was further confirmed by transmission electron microscopy (TEM), as shown in Figure 1d–f. Figure 1e exhibits a TEM image of an individual ZrS3 nanobelt, and the inset at the upper right corner is its corresponding selected-area electron diffraction (SAED) pattern. Figure 1f shows a high-resolution
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TEM (HRTEM) image of the individual nanobelt. The lattice fringe spacings of 0.37 nm and 0.51 nm correspond to the d-values of the (010) and (100) planes of the above monoclinic ZrS3 phase, respectively. The corresponding fastFourier transform (FFT) pattern (inset in Figure 1f) demonstrates that the nanobelt grew along the [010] direction.
2.2. Fabrication and Photosensitivity of the Flexible Film Photodetectors To fabricate the flexible ZrS3 nanobelt-film based photodetector, a transparent adhesive tape with a polypropylene (PP) film substrate (WingTai Tape, China) was
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Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films
Scheme 1. Schematic illustration of ZrS3-nanowire film transferred onto PP substrate and an overall device structure.
first attached to the surface of the as-grown nanobelt mats under a certain pressure. By peeling the PP film off the Zr substrate, the nanobelts film was successfully transferred
to the PP substrate, as illustrated in Scheme 1. Two copper wires with diameter of 70 µm were put on top of the ZrS3 film, then a rectangular frame was added, after this Ti/Au (10 nm/100 nm) metal was evaporated on the surface of the film, and then the two copper wires and the frame were lifted off, revealing a flexible photodetector with two channels (device 1, inset in Figure 2a). Figure 2a demonstrates a channel of 91.06 µm between the electrodes on the surface of device 1. To measure the photosensitivity of this device, monochromatic light was used to perpendicularly illuminate the surface and the corresponding I–V curves were recorded. Figure 2b shows the I–V curves of the photodetector (device 1) exposed to light of different wavelengths and under dark conditions. The nonlinearity and asymmetry of the curves indicate a non-ohmic contact between the Ti/Au
Figure 2. a) Optical micrograph of ZrS3-nanobelt photodetector (device 1) with two channels with a width of 91.06 µm between the electrodes. Inset is an optical image of the photodetector on a PP film substrate. b) I–V curves of the photodetector illuminated with light of different wavelengths, and under dark conditions. c) I–V curves of the photodetector under 405-nm light illumination with different optical power. d) Photocurrent–optical power curves of the photodetector under 405-nm light illumination at different bias voltage. e) Spectral photoresponse of device 1 measured at a bias of 5 V at different wavelengths ranging from 350 to 1000 nm. small 2014, DOI: 10.1002/smll.201401376
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Figure 3. a) Time-dependent response of the photodetector (device 1) recorded at different bias voltages under 405-nm light illumination with an on–off period of 1 s. b) Time-dependent response of the photodetector (device 1) measured at a bias voltage of 5 V under 405-nm light illumination with an on–off period of 50 s. c) Enlargement of the part outlined in (a). d) Enlargement of the part outlined in (b).
electrodes and the nanobelts. Compared to the dark state, currents at the same bias voltage are higher under illumination of light within the wavelength range of 405 nm to 780 nm, revealing a photosensitivity of the device from the visible light to the near infrared (NIR). As shown in Figure 2b, the photocurrent induced by 405-nm light at 45 mW cm-2 is much higher than that of 780-nm light at 49.9 mW cm-2 at the same voltage, so the photosensitivity does not only depend on the optical intensity but also on the wavelength of the light. Figure 2c exhibits the I–V curves of device 1 illuminated by 405-nm light of various optical powers. The corresponding photocurrent–optical power curves are plotted in Figure 2d at a bias of 1, 3, and 5 V. These curves can be fitted with: Iph =αPθ, where Iph is the photocurrent (A), this is, the difference between the currents under illumination and in the dark, α is a coefficient, θ is the exponent, and P is the optical power (mW cm−2). Fitting of the curves resulted in Iph = 1.46 × 10−11P0.34, Iph = 4.82 × 10−11P0.356, and Iph = 8.66 × 10−11P0.361 for a bias of 1, 3, and 5 V, respectively. The non-unity exponent is a result of the complex processes of electron–hole generation, trapping, and recombination in the semiconductor.[30] This clearly indicates the high selectivity and sensitivity of the ZrS3-nanobelt-film photodetector. Figure 2e shows a photoresponse curve of device 1 at a bias of 5 V at different wavelengths from 350 to 1000 nm, showing that the photocurrent gradually reduces with increasing wavelength above 500 nm, so the cutoff wavelength for strong photosensitivity is about 850 nm.
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As shown in Figure 3a, the current of device 1 as a function of time was measured at different bias voltages under alternating dark and illumination conditions with a photoswitching period of 1 s. Here a 450-nm light at 45 mW cm−2 was used. The switching behavior of the current could clearly be observed, that is, the current ramped to a high value (ON state) under illumination and resumed to the low value (OFF state) under dark conditions. The current rises with increasing bias voltage but the ratio of the on–off current (Ion/off) does not change much. For example, when a bias voltage of 8, 5, 3, and 2 V was applied, Ion/off was 1.34, 1.71, 1.64, and 1.62, respectively, and the response time (rise time/decay time) was 0.49/0.84, 0.39/0.88, 0.35/0.78, and 0.34/0.45 s, respectively. The time needed 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. Figure 3b reveals the dependence of the current on the illumination time with light of 405 nm wavelength at 12.1 mW cm−2 for a photoswitching period of 50 s at a bias of 5 V, showing the photosensitive stability of device 1. Here the dark current was about 109 pA, the current under illumination was about 377 pA, and Ion/off was about 3.5. The rise and decay times were about 13 and 28 s, respectively. The enlargement of the parts outlined in Figure 3a and b are shown in Figure 3c and d, respectively. Different photoswitch periods resulted in different Ion/off and rise/decay times, because the increase and decay of the photocurrent require enough time.[31]
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Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films
Figure 4. a) Optical micrograph of ZrS3-nanobelt photodetector (device 2) with a channel of 582.19 µm between the two electrodes. The inset is an optical image of the device on a paper substrate. b) I–V curves of the photodetector illuminated with light of different wavelengths, and under dark conditions. c) Time-dependent response of device 2 measured in air at different bias voltages under 405-nm light (45 mW cm−2) illumination in air. d) Time-dependent response of device 2 measured at a bias voltage of 5 V under 405-nm light (12.1 mW cm−2) illumination. e) Enlargement of the part outlined in (c) (time-dependence response at bias of 3 V). f) Enlargement of the part outlined in (d).
To fabricate device 2, a procedure similar to fabricating device 1 was carried out. A double adhesive tape with a paper substrate (WingTai Tape, China) was used to transfer the ZrS3 nanobelt film. A copper wire with a diameter of 0.5 mm served as a mask, and then a flexible photodetector with a channel of 582.19 µm was made, as shown in the inset of Figure 4a. Figure 4a demonstrates the channel of 582.19 µm between the electrodes on the surface of device 2. Figure 4b small 2014, DOI: 10.1002/smll.201401376
shows the I–V curves of device 2 exposed to light of wavelengths from 405 to 780 nm and under dark conditions, also demonstrating the photosensitivity of the device from visible light to NIR. Figure 4c shows the current of device 2 as a function of time at different bias voltages under alternating dark and illumination conditions with a photoswitching period of 1 s. Ion/off was 1.47 and 1.46, respectively, and the response time (rise time/decay time) was 0.27/0.42 and 0.38/0.42 s at
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chemisorption/desorption on the nanobelts. In air, oxygen molecules are adsorbed on the nanobelts surface and they can capture free electrons from the nanobelts (forming O2−), creating a low-conductivity depletion layer near the surface. When the device is illuminated, electron–hole pairs are generated. The holes migrate to the surface to recombine with O2− thus forming O2 which desorbs from the surface, resulting in an increase in electron concentration, so that the photocurrent increases.[32] Figure 5b exhibits the dependence of the current on time of device 1 in vacuum and air under 405-nm light illumination at 55.6 mW cm-2 with a photoswitching period of 1 s at a bias of 5 V. Ion/off was 1.94 and 2.0 in vacuum and air, respectively, and the response time (rise time/decay time) was 0.29/0.34 and 0.45/0.89 s, respectively. Obviously, the ratio of the photoswitching current was similar, but the response speed in vacuum was faster than that in air. The same research results were also found for device 2.
3. Conclusions
Figure 5. a) The current–voltage (I–V) curves and b) time-dependent response of device 1 illuminated with light of 405-nm wavelength measured under air and vacuum (2.0 Pa) conditions at room temperature. The I–V characteristics were measured using a Model CRX-4K Cryogenic Probe Station (Lake Shore Inc.).
bias of 8 and 3 V, respectively. Figure 4d reveals the dependence of the current on the time under 405-nm light illumination at 16 mW cm−2 with a photoswitching period of 50 s at a bias of 5 V, which shows the photosensitive stability of device 2. Here the dark current was about 1.02 nA, the current under illumination was about 1.47 nA, and the Ion/off was about 1.44. The rise and decay times were about 20 s and 32 s, respectively. Figure 4e and f represent the enlargement of the parts outlined in Figure 4c,d, respectively, further exhibiting the photoswitching behavior under the above two conditions. In summary, both detectors, whether on a PP or a paper substrate, show good photosensitivities.
2.3. Photoresponsivity Dependence on the Ambient Pressure To study the photosensitive mechanism of the device, the responses of device 1 in air and vacuum were investigated, as illustrated in Figure 5a. The current in vacuum (2.0 Pa) was about 4.7 times higher than that under ambient conditions at a bias of 4 V under dark, whereas the current in vacuum (2.0 Pa) was about 1.3 times higher than that under ambient conditions at a bias of 2.5 V under 405-nm light illumination at 60.6 mW cm−2. In other words the current in vacuum is higher than that in air. This confirms the existence of oxygen
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We have developed novel nanobelt-film-based photodetectors from a film of ZrS3 nanobelts. The photodetectors show a high spectral selectivity, and a wide-range and fastspeed photoresponse for visible light to NIR. Compared to a single nanowire photodetector, the nanobelt-film-based photodetectors show stability and durability. The film is easily integrated with flexible substrates. The fabrication method is simple and practical, and can easily be extended to fabricating other types of photodetector.
4. Experimental Section Material Preparation and Characterization: ZrS3-nanobelt films were synthesized via a chemical-vapor transport (CVT) method. In a typical procedure, firstly, zirconium (Zr) foil (99.8%, 2.885 mmol; size of 32 mm × 5 mm × 0.2 mm) and sulfur powder (99.8%, 0.5769 mmol) were sealed in a quartz tube under vacuum (6.0 × 10−2 Pa) and placed in a conventional horizontal furnace (temperature gradient: 10 K cm−1) with the Zr foil positioned at the center of the furnace. After the furnace was heated to 650 °C within 1 h and holding the temperature for 5 h, the furnace was cooled down to room temperature, and then the Zr foil with ZrS3 nanobelts was extracted. The as-prepared ZrS3 nanobelts were characterized by X-ray diffraction (XRD, Shimadzu XRD-6000 with graphite monochromatized Cu Kα1 radiation), a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800), and a high-resolution transmission electron microscope (HRTEM, JEM-2100). Device Measurements: The current–voltage (I–V) and the current-time (I–t) characteristics were recorded using an SM-4 probe system and Keithley 236 source meter (Keithley Instruments Inc.) The spectral response was recorded under laser illumination of different wavelengths. The spectroscopic response ranging from 350 to 1000 nm was measured using a 300 W Xe lamp (HSXUV300), and a multi-grating monochromator (71SW151) with ordered sorting filters was used. The wavelength was controlled and adjusted by applied software. The optical intensity was adjusted by applying currents and an aperture. The optical power
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Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films
was measured using an FZ-A radiometer (made in Beijing Normal University). The experiments were all carried out at room temperature in air except where mentioned.
Acknowledgements We acknowledge the financial support from the National Science Foundations of China (No. 21171091 and 21335004).
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[16] L. Li, H. Lu, Z. Y. Yang, L. M. Tong, Y. Bando, D. Golberg, Adv. Mater. 2013, 25, 1109. [17] L. Li, P. S. Lee, C. Y. Yan, T. Y. Zhai, X. S. Fang, M. Y. Liao, Y. Koide, Y. Bando, D. Golberg, Adv. Mater. 2010, 22, 5145. [18] L. Li, E. Auer, M. Y. Liao, X. S. Fang, T. Y. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 2011, 3, 1120. [19] B. Chitara, L. S. Panchakarla, S. B. Krupanidhi, C. N. R. Rao, Adv. Mater. 2011, 23, 5419. [20] X. C. Wu, Y. R. Tao, L. Li, Y. Bando, D. Golberg, Nanotechnol. 2013, 24, 175 701. [21] C. Y. Zhang, S. Wang, L. J. Yang, Y. Liu, T. T. Xu, Z. Y. Ning, A. Zak, Z. Y. Zhang, R. Tenne, Q. Chen, Appl. Phys. Lett. 2012, 100, 243 101. [22] L. F. Hu, J. Yan, M. Y. Liao, H. J. Xiang, X. G. Gong, L. D. Zhang, X. S. Fang, Adv. Mater. 2012, 24, 2305. [23] 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, Adv. Mater.2014, 26, 3088. [24] W. Tian, T. Y. Zhai, C. Zhang, S. L. Li, X. Wang, F. Liu, D. Q. Liu, X. K. Cai, K. Tsukagoshi, D. Golberg, Adv. Mater. 2013, 25, 4625. [25] E. M. Gallo, G. Chen, M. Currie, T. McGuckin, P. Prete, N. Lovergine, B. Nabet, J. E. Spanier, Appl. Phys. Lett. 2011, 98, 241 113. [26] a) Z. Y. Yin, H. Li, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen, H. Zhang, ACS Nano 2012, 6, 74; b) O. Lopez-Sanchz, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Nat. Nanotechnol. 2013, 8, 497. [27] S. K. Srivastava, B. N. Avasthi, J. Mater. Sci. 1992, 27, 3693. [28] a) Y. L. Zhang, X. C. Wu, Y. R. Tao, C. J. Mao, J. J. Zhu, Chem. Commun. 2008, 26 832; b) Q. X. Gao, X. F. Wang, X. C. Wu, Y. R. Tao, J. J. Zhu, Microporous Mesoporous Mater. 2011, 143, 333; c) Q. X. Gao, X. F. Wang, Y. R. Tao, X. C. Wu, Sci. Adv. Mater. 2012, 4, 327; d) H. Jin, D. Cheng, J. X. Li, X. J. Cao, B. X. Li, X. F. Wang, X. Y. Liu, X. D. Zhao, Solid State Sci. 2011, 13, 1166. [29] L. Li, X. S. Fang, T. Y. Zhai, M. Y. Liao, U. K. Gautam, X. C. Wu, Y. Koide, Y. Bando, D. Golberg, Adv. Mater. 2010, 22, 4151. [30] a) A. Rose, Concepts in Photoconductivity and Allied Problems, Krieger Publishing Company, New York, NJ, USA 1978; b) R. S. Chen, H. Y. Chen, C. Y. Lu, K. H. Chen, C. P. Chen, L. C. Chen, Y. J. Yang, Appl. Phys. Lett. 2007, 91, 223 106. [31] J. A. Garrido, E. Monroy, I. Izpura, E. Munoz, Semicond. Sci. Technol. 1998, 13, 563. [32] a) T. Gao, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 2005, 86, 173 105; b) J. Zou, Q. Zhang, K. Huang, N. Marzari, J. Phys. Chem. C 2010, 114, 10 725.
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Received: May 18, 2014 Revised: June 27, 2014 Published online:
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Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films You-Rong Tao, Xing-Cai Wu,* and Wei-Wei Xiong
Two
new flexible visible-light photodetectors based on ZrS3 nanobelts films are fabricated on a polypropylene (PP) film and printing paper, respectively, by an adhesive-tape transfer method, and their light-induced electric properties are investigated in detail. The devices demonstrate a remarkable response to 405 to 780 nm light, a photocurrent that depends on the optical power and light wavelength, and an excellent photoswitching effect and stability. This implies that ZrS3 nanobelts are prospective candidates for high-performance nanoscale optoelectronic devices that may be practically applied in photodetection of visible to near infrared light. The facile fabrication method is extendable to flexible nanodevices with different nanostructures.
1. Introduction One-dimensional (1D) nanostructures have become the focus of worldwide research because of their potential use in high-performance nano-devices based on their high surface to volume ratios, their rationally designed surface, and unique physical properties.[1–12] Photodetectors are essential elements in many fields, for instance, imagining techniques, optical communications, and optoelectronic integrated circuits. Photodetectors based on various 1D nanostructures, such as ZnO,[13] ZnSe,[14] CdS,[15] CdS1-xSex,[16] In2Ge2O7,[17] Ga2O3,[18] graphene,[19] Ta3N5[20] nanobelts, WS2 nanotubes,[21] ZnS/ZnO biaxial nanobelts,[22] branched ZnS–ZnO heterostructure nanowires,[23] ZnO-SnO2 heterojunction nanofibers,[24] GaAs/AlGaAs core–shell nanowires,[25] singlelayer MoS2 nanobelts,[26] and so on have been extensively investigated. However, it is still a challenge to develop highperformance and broad-response photodetectors. Transition-metal trichalcogenides MX3 (where M is Ti, Zr, Hf, Mo, W, V, Nb, or Ta; X is S, Se, or Te) are structurally and chemically well-defined compounds. They possess a pseudo one-dimensional structure whereby an infinite chain of trigonal prismatic (MX6) units extends parallel to the b-axis and shares upper and lower faces. The chains showing strong Prof. Y.-R. Tao, Prof. X.-C. Wu, W.-W. Xiong Country Key Laboratory of Mesoscopic Chemistry of MOE School of Chemistry and Chemical Engineering Nanjing University Nanjing 210093, China E-mail: [email protected] DOI: 10.1002/smll.201401376 small 2014, DOI: 10.1002/smll.201401376
ionic covalent (or metallic) bonding are separated by a relatively large distance due to interchain bonding, so they show a marked anisotropy in most of their physical properties.[27] Zirconium trisulfide (ZrS3) is a member of this family of compounds. ZrS3 crystals are p-type semiconductors with a roomtemperature resistivity of 15 Ω cm, a direct optical bandgap of 2.56 eV at room temperature, and two indirect optical bandgaps of 2.055 eV (Eb) and 2.058 eV (E⊥b) at liquid helium temperature (below 4.2 K).[27] Although ZrS3 nanobelts have been synthesized,[28] nanodevices based on ZrS3 nanobelts have not been produced to date. Therefore, designing nanodevices based on ZrS3 nanostructures will largely push forward applications of ZrS3 and other transition-metal trichalcogenide nanostructures in the optoelectronic fields. Li et al. first made a visible photodetector based on an individual ZrS2 nanobelt, and they discovered an excellent photosensitivity.[29] Here we used a ZrS3-nanobelt film to fabricate flexible visiblelight photodetectors on polypropylene and on a paper substrate, respectively. The devices demonstrate a tunable spectral selectivity, wide-range photoresponse, high-speed response, and excellent environmental stability. In addition, a complex lithography process is not necessary to fabricate these photodetectors and they are also able to bear external mechanical forces, suggesting a wide range of potential applications.
2. Results and Discussion 2.1. Preparation and Characterization of the ZrS3 Nanobelts An X-ray diffraction (XRD) pattern of a ZrS3 nanobelt film on zirconium foil is shown in Figure 1a. All the diffraction peaks can be readily indexed to the monoclinic ZrS3 phase
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Figure 1. a) XRD pattern, b) low-magnification SEM image, and c) high-magnification SEM image of ZrS3 nanobelt film. d) TEM image of ZrS3 nanobelts, e) TEM image of an individual ZrS3 nanobelt (inset shows its SAED pattern), and f) HRTEM image of the individual ZrS3 nanobelt (inset shows the corresponding FFT pattern).
(JCPDS Card No. 30–1498; a = 0.5124 nm, b = 0.3624 nm, c = 0.898 nm, and β = 97.28°), so it is clear that the film is pure. Field-emission scanning electron microscopy (SEM) was used to record its morphology. Figure 1b shows the lowmagnification SEM image of the film on Zr foil, and Figure 1c is the high-magnification SEM image, revealing that the films consists of high-yield nanobelts with a typical size of about 0.85 micrometers in width, 70 nm in thickness, and several tens of micrometers in length. The belt-like structure was further confirmed by transmission electron microscopy (TEM), as shown in Figure 1d–f. Figure 1e exhibits a TEM image of an individual ZrS3 nanobelt, and the inset at the upper right corner is its corresponding selected-area electron diffraction (SAED) pattern. Figure 1f shows a high-resolution
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TEM (HRTEM) image of the individual nanobelt. The lattice fringe spacings of 0.37 nm and 0.51 nm correspond to the d-values of the (010) and (100) planes of the above monoclinic ZrS3 phase, respectively. The corresponding fastFourier transform (FFT) pattern (inset in Figure 1f) demonstrates that the nanobelt grew along the [010] direction.
2.2. Fabrication and Photosensitivity of the Flexible Film Photodetectors To fabricate the flexible ZrS3 nanobelt-film based photodetector, a transparent adhesive tape with a polypropylene (PP) film substrate (WingTai Tape, China) was
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films
Scheme 1. Schematic illustration of ZrS3-nanowire film transferred onto PP substrate and an overall device structure.
first attached to the surface of the as-grown nanobelt mats under a certain pressure. By peeling the PP film off the Zr substrate, the nanobelts film was successfully transferred
to the PP substrate, as illustrated in Scheme 1. Two copper wires with diameter of 70 µm were put on top of the ZrS3 film, then a rectangular frame was added, after this Ti/Au (10 nm/100 nm) metal was evaporated on the surface of the film, and then the two copper wires and the frame were lifted off, revealing a flexible photodetector with two channels (device 1, inset in Figure 2a). Figure 2a demonstrates a channel of 91.06 µm between the electrodes on the surface of device 1. To measure the photosensitivity of this device, monochromatic light was used to perpendicularly illuminate the surface and the corresponding I–V curves were recorded. Figure 2b shows the I–V curves of the photodetector (device 1) exposed to light of different wavelengths and under dark conditions. The nonlinearity and asymmetry of the curves indicate a non-ohmic contact between the Ti/Au
Figure 2. a) Optical micrograph of ZrS3-nanobelt photodetector (device 1) with two channels with a width of 91.06 µm between the electrodes. Inset is an optical image of the photodetector on a PP film substrate. b) I–V curves of the photodetector illuminated with light of different wavelengths, and under dark conditions. c) I–V curves of the photodetector under 405-nm light illumination with different optical power. d) Photocurrent–optical power curves of the photodetector under 405-nm light illumination at different bias voltage. e) Spectral photoresponse of device 1 measured at a bias of 5 V at different wavelengths ranging from 350 to 1000 nm. small 2014, DOI: 10.1002/smll.201401376
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Figure 3. a) Time-dependent response of the photodetector (device 1) recorded at different bias voltages under 405-nm light illumination with an on–off period of 1 s. b) Time-dependent response of the photodetector (device 1) measured at a bias voltage of 5 V under 405-nm light illumination with an on–off period of 50 s. c) Enlargement of the part outlined in (a). d) Enlargement of the part outlined in (b).
electrodes and the nanobelts. Compared to the dark state, currents at the same bias voltage are higher under illumination of light within the wavelength range of 405 nm to 780 nm, revealing a photosensitivity of the device from the visible light to the near infrared (NIR). As shown in Figure 2b, the photocurrent induced by 405-nm light at 45 mW cm-2 is much higher than that of 780-nm light at 49.9 mW cm-2 at the same voltage, so the photosensitivity does not only depend on the optical intensity but also on the wavelength of the light. Figure 2c exhibits the I–V curves of device 1 illuminated by 405-nm light of various optical powers. The corresponding photocurrent–optical power curves are plotted in Figure 2d at a bias of 1, 3, and 5 V. These curves can be fitted with: Iph =αPθ, where Iph is the photocurrent (A), this is, the difference between the currents under illumination and in the dark, α is a coefficient, θ is the exponent, and P is the optical power (mW cm−2). Fitting of the curves resulted in Iph = 1.46 × 10−11P0.34, Iph = 4.82 × 10−11P0.356, and Iph = 8.66 × 10−11P0.361 for a bias of 1, 3, and 5 V, respectively. The non-unity exponent is a result of the complex processes of electron–hole generation, trapping, and recombination in the semiconductor.[30] This clearly indicates the high selectivity and sensitivity of the ZrS3-nanobelt-film photodetector. Figure 2e shows a photoresponse curve of device 1 at a bias of 5 V at different wavelengths from 350 to 1000 nm, showing that the photocurrent gradually reduces with increasing wavelength above 500 nm, so the cutoff wavelength for strong photosensitivity is about 850 nm.
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As shown in Figure 3a, the current of device 1 as a function of time was measured at different bias voltages under alternating dark and illumination conditions with a photoswitching period of 1 s. Here a 450-nm light at 45 mW cm−2 was used. The switching behavior of the current could clearly be observed, that is, the current ramped to a high value (ON state) under illumination and resumed to the low value (OFF state) under dark conditions. The current rises with increasing bias voltage but the ratio of the on–off current (Ion/off) does not change much. For example, when a bias voltage of 8, 5, 3, and 2 V was applied, Ion/off was 1.34, 1.71, 1.64, and 1.62, respectively, and the response time (rise time/decay time) was 0.49/0.84, 0.39/0.88, 0.35/0.78, and 0.34/0.45 s, respectively. The time needed 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. Figure 3b reveals the dependence of the current on the illumination time with light of 405 nm wavelength at 12.1 mW cm−2 for a photoswitching period of 50 s at a bias of 5 V, showing the photosensitive stability of device 1. Here the dark current was about 109 pA, the current under illumination was about 377 pA, and Ion/off was about 3.5. The rise and decay times were about 13 and 28 s, respectively. The enlargement of the parts outlined in Figure 3a and b are shown in Figure 3c and d, respectively. Different photoswitch periods resulted in different Ion/off and rise/decay times, because the increase and decay of the photocurrent require enough time.[31]
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201401376
Flexible Visible-Light Photodetectors with Broad Photoresponse Based on ZrS3 Nanobelt Films
Figure 4. a) Optical micrograph of ZrS3-nanobelt photodetector (device 2) with a channel of 582.19 µm between the two electrodes. The inset is an optical image of the device on a paper substrate. b) I–V curves of the photodetector illuminated with light of different wavelengths, and under dark conditions. c) Time-dependent response of device 2 measured in air at different bias voltages under 405-nm light (45 mW cm−2) illumination in air. d) Time-dependent response of device 2 measured at a bias voltage of 5 V under 405-nm light (12.1 mW cm−2) illumination. e) Enlargement of the part outlined in (c) (time-dependence response at bias of 3 V). f) Enlargement of the part outlined in (d).
To fabricate device 2, a procedure similar to fabricating device 1 was carried out. A double adhesive tape with a paper substrate (WingTai Tape, China) was used to transfer the ZrS3 nanobelt film. A copper wire with a diameter of 0.5 mm served as a mask, and then a flexible photodetector with a channel of 582.19 µm was made, as shown in the inset of Figure 4a. Figure 4a demonstrates the channel of 582.19 µm between the electrodes on the surface of device 2. Figure 4b small 2014, DOI: 10.1002/smll.201401376
shows the I–V curves of device 2 exposed to light of wavelengths from 405 to 780 nm and under dark conditions, also demonstrating the photosensitivity of the device from visible light to NIR. Figure 4c shows the current of device 2 as a function of time at different bias voltages under alternating dark and illumination conditions with a photoswitching period of 1 s. Ion/off was 1.47 and 1.46, respectively, and the response time (rise time/decay time) was 0.27/0.42 and 0.38/0.42 s at
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chemisorption/desorption on the nanobelts. In air, oxygen molecules are adsorbed on the nanobelts surface and they can capture free electrons from the nanobelts (forming O2−), creating a low-conductivity depletion layer near the surface. When the device is illuminated, electron–hole pairs are generated. The holes migrate to the surface to recombine with O2− thus forming O2 which desorbs from the surface, resulting in an increase in electron concentration, so that the photocurrent increases.[32] Figure 5b exhibits the dependence of the current on time of device 1 in vacuum and air under 405-nm light illumination at 55.6 mW cm-2 with a photoswitching period of 1 s at a bias of 5 V. Ion/off was 1.94 and 2.0 in vacuum and air, respectively, and the response time (rise time/decay time) was 0.29/0.34 and 0.45/0.89 s, respectively. Obviously, the ratio of the photoswitching current was similar, but the response speed in vacuum was faster than that in air. The same research results were also found for device 2.
3. Conclusions
Figure 5. a) The current–voltage (I–V) curves and b) time-dependent response of device 1 illuminated with light of 405-nm wavelength measured under air and vacuum (2.0 Pa) conditions at room temperature. The I–V characteristics were measured using a Model CRX-4K Cryogenic Probe Station (Lake Shore Inc.).
bias of 8 and 3 V, respectively. Figure 4d reveals the dependence of the current on the time under 405-nm light illumination at 16 mW cm−2 with a photoswitching period of 50 s at a bias of 5 V, which shows the photosensitive stability of device 2. Here the dark current was about 1.02 nA, the current under illumination was about 1.47 nA, and the Ion/off was about 1.44. The rise and decay times were about 20 s and 32 s, respectively. Figure 4e and f represent the enlargement of the parts outlined in Figure 4c,d, respectively, further exhibiting the photoswitching behavior under the above two conditions. In summary, both detectors, whether on a PP or a paper substrate, show good photosensitivities.
2.3. Photoresponsivity Dependence on the Ambient Pressure To study the photosensitive mechanism of the device, the responses of device 1 in air and vacuum were investigated, as illustrated in Figure 5a. The current in vacuum (2.0 Pa) was about 4.7 times higher than that under ambient conditions at a bias of 4 V under dark, whereas the current in vacuum (2.0 Pa) was about 1.3 times higher than that under ambient conditions at a bias of 2.5 V under 405-nm light illumination at 60.6 mW cm−2. In other words the current in vacuum is higher than that in air. This confirms the existence of oxygen
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We have developed novel nanobelt-film-based photodetectors from a film of ZrS3 nanobelts. The photodetectors show a high spectral selectivity, and a wide-range and fastspeed photoresponse for visible light to NIR. Compared to a single nanowire photodetector, the nanobelt-film-based photodetectors show stability and durability. The film is easily integrated with flexible substrates. The fabrication method is simple and practical, and can easily be extended to fabricating other types of photodetector.
4. Experimental Section Material Preparation and Characterization: ZrS3-nanobelt films were synthesized via a chemical-vapor transport (CVT) method. In a typical procedure, firstly, zirconium (Zr) foil (99.8%, 2.885 mmol; size of 32 mm × 5 mm × 0.2 mm) and sulfur powder (99.8%, 0.5769 mmol) were sealed in a quartz tube under vacuum (6.0 × 10−2 Pa) and placed in a conventional horizontal furnace (temperature gradient: 10 K cm−1) with the Zr foil positioned at the center of the furnace. After the furnace was heated to 650 °C within 1 h and holding the temperature for 5 h, the furnace was cooled down to room temperature, and then the Zr foil with ZrS3 nanobelts was extracted. The as-prepared ZrS3 nanobelts were characterized by X-ray diffraction (XRD, Shimadzu XRD-6000 with graphite monochromatized Cu Kα1 radiation), a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800), and a high-resolution transmission electron microscope (HRTEM, JEM-2100). Device Measurements: The current–voltage (I–V) and the current-time (I–t) characteristics were recorded using an SM-4 probe system and Keithley 236 source meter (Keithley Instruments Inc.) The spectral response was recorded under laser illumination of different wavelengths. The spectroscopic response ranging from 350 to 1000 nm was measured using a 300 W Xe lamp (HSXUV300), and a multi-grating monochromator (71SW151) with ordered sorting filters was used. The wavelength was controlled and adjusted by applied software. The optical intensity was adjusted by applying currents and an aperture. The optical power
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was measured using an FZ-A radiometer (made in Beijing Normal University). The experiments were all carried out at room temperature in air except where mentioned.
Acknowledgements We acknowledge the financial support from the National Science Foundations of China (No. 21171091 and 21335004).
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Received: May 18, 2014 Revised: June 27, 2014 Published online:
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