Flexible ultraviolet–visible photodetector based on HfS3 nanobelt film

Accepted Manuscript Flexible ultraviolet-visible photodetector based on HfS3 nanobelt film You-Rong Tao, Jin-Qiang Chen, Jia-Jing Wu, Yi Wu, Xing-Cai Wu PII:

S0925-8388(15)31440-7

DOI:

10.1016/j.jallcom.2015.10.184

Reference:

JALCOM 35739

To appear in:

Journal of Alloys and Compounds

Received Date: 18 August 2015 Revised Date:

16 October 2015

Accepted Date: 20 October 2015

Please cite this article as: Y.-R. Tao, J.-Q. Chen, J.-J. Wu, Y. Wu, X.-C. Wu, Flexible ultraviolet-visible photodetector based on HfS3 nanobelt film, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.10.184. 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.

ACCEPTED MANUSCRIPT Abstract graphical

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Flexible ultraviolet-visible photodetector based on HfS3 nanobelt film

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You-Rong Tao, Jin-Qiang Chen, Jia-Jing Wu, Yi Wu, Xing-Cai Wu*

Key Laboratory of Mesoscopic Chemistry of MOE, and School of Chemistry and Chemical Engineering,

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Nanjing University, Nanjing 210093, China.

Abstract

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HfS3 nanobelts with a width of 700 −70 nm, a thickness of about 10 −30 nm and a length of more than 10 µm, were directly synthesized at 650
for 5 h by a facile chemical vapor transport (CVT) method. The direct and indirect optical energy gaps of the nanobelts were measured as 2.19 and 1.73 eV, respectively. The nanobelts have good photoluminescence (PL), i. e. there are strong emissions at 483,

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540 and 600 nm under excitation at 400 nm. The nanobelts were dispersed in enthanol and adhered to a transparent polypropylene (PP) film by double-side adhesive tape, and then two separate electrodes (Ti/Au) were evaporated on the surface with a mask, removing the mask to form a flexible

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photodetector. The detector demonstrated an excellent photoresponse from ultraviolet to visible light.

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Under illumination of 405 nm with 6.32 mW, the light on/off current ratio is 12, and response time is less than 0.2 s. It suggested that the HfS3 nanobelts are promising for applications in optoelectronic nanodevices, and the flexible photodetector can be used in practical photodetection. Keywords: Transition-metal trichalcogenides, Nanobelts, Gas-solid reaction, Photoluminescence, Flexible photodetector.

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Corresponding author. Fax: +86 25 83317761.

E-mail: [email protected] (X.C.Wu).

1. Introduction

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In the past decades, one-dimensional (1D) nanostructures including nanowires [1, 2], nanorods [3], nanotubes and nanobelts [4−7], have been extensively applied in electronic and optical nanodevices such as field emitters [6,7], gas sensors [8, 9], light emission diodes [10], solar cells [11], field-effect

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transistors (FETs) and photodetectors [2, 5, 12] because of their large surface-to-volume ratios and low dimensionality. Photodetector is an important photosensitive device which is widely used in image

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sensing, communications, environmental monitoring, remote control, day and night surveillance, and chemical/biological sensing [13−15]. Therefore, it is significant subject to probe for new materials to construct new devices all time. Over the several years, photodetectors based on individual 1D nanostructure were fabricated to improve photosensitivity by lowering dark currents [16, 17], however,

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the production technique is complicated, so a few heterostructure films were used to make photodetectors such as CdTe-Si nanowire film [18], graphene oxide-ZnO film [19], CdSSe nanowirechip [20], SrTiO3/TiO2 nanowire arrays [21] and so on. With the demand for flexible devices, new high-

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performance flexible photodetectors are becoming mainstreams [22−24]. Transition metal chalcogenides (TMCs) are a type of excellent photoelectric semiconductors which are

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usually used in ultraviolet to visible photodetectors such as ZnS, CdS, CdSe and so on [25, 26]. Transition metal dichalcogenides (TMDCs) are a sort of layer-structure semiconductors or metals, some of which have high-performance photosensitivity from visible to near infrared such as ZrS2 [27], MoS2 [28], WS2 [29] and so on. Transition metal trichalcogenides (TMTCs) are also a sort of layer-structure semiconductors or metals [30]. Recent research shows that ZrS3 [31, 32], HfS3 [33], TiS3 [34], ZrSe3 and HfSe3 [35] also have good visible-light sensitivity. Our group once prepared HfS3 nanobelts by a chemical vapour deposition (CVT) at 650oC for 10 h, and fabricated the individual HfS3 nanobelt on a SiO2/Si

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ACCEPTED MANUSCRIPT substrate to form phototransistor, and discovered good visible-light sensitivity [33]. However, it is worth exploring subject whether HfS3 nanobelts can be prepared in shorter time and fabricated into flexible photodetector. Here we realized to prepare HfS3 nanobelts by the same method within 5 h, and fabricated the nanobelt films into flexible photodetector on polypropylene (PP) film, and studied on ultraviolet-

UV-vis absorbance of the nanobelts were investigated in detail.

2. Experimental procedures

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2. 1. Preparation and Characterization of HfS3 nanobelts

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visible (UV-vis) photosensitivity of the photodetector in detail. Meanwhile, the photoluminescence and

HfS3 nanobelts were directly synthesized by chemical vapor transport method. In a typical

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procedure, a hafnium powders (99.9%, 100.0 mg) and sulfur sublimate powders (99.5%, 53.9 mg) were completely mixed with a molar ratio of 1:3. Then the mixture was sealed in a quartz tube (Ф 6 mm × 12 cm) under vacuum ( ca. 10-2 Pa) and was placed in a conventional horizontal furnace (temperature gradient: ca. 10 K cm-1) with the powers positioned at the center of the furnace. After the furnace was

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heated to 650
at a rate of 10
/min., and was keep at the temperature for 5 h, the furnace was cooled to room temperature, and an end of the quartz tube was cut off, the HfS3 nanobelts were taken out. The morphology and structure of the products were determined by field-emission scanning electron

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microscopy (SEM, Hitachi S-4800) with energy-dispersive X-ray spectrometer (EDX) ， X-ray diffraction with Cu Kα radiation (XRD, Shimadzu XRD-6000), and High-resolution transmission

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electron microscopy (HRTEM, JEOL2010) with selected area electron diffraction (SAED), respectively. In addition, the UV-vis absorbance and photoluminescence (PL) spectra of the products were recorded by a UV-3600 spectrophotometer (Shimadzu UV-3600) and a fluorescence spectrometer (RF-5301PC, Shimadzu), respectively.

2. 2. Device fabrication and characterization Appropriate amount of HfS3 nanobelts were suspended in ethanol by sonication for 5 min., and then the dispersion were dropped on a transparent polypropylene (PP) film with a double-side adhesive tape, 3

ACCEPTED MANUSCRIPT and dried naturally at room temperature. A copper wire with a diameter of about 0.6 mm was used as a partition, and a rectangular paper frame was covered on the film, and a Ti/Au (10/100 nm) was deposited, and then the copper wire and paper frame were lifted off so that flexible device was obtained. The current–voltage (I–V) and current−time (I−t) characteristics of the photodetector were measured by

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Model CRX-4K Cryogenic Probe Station (Lake Shore Inc.) and keithley 2636A. The spectral response for wavelengths was recorded by using different wavelength laser device, and a 300 W Xe lamp (HSX UV300) with a multi-grating monochromator (71SW151). Light was chopped by an electric shutter to

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measure the I−t curves of the device. All measurements were carried out in air at room temperature

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except special designation.

3. Results and discussion

Fig. 1(a) shows the power X-ray diffraction (XRD) pattern of the products obtained at 650
for 5 h， which can be completely indexed as monoclinic HfS3 (PCPDFWIN no. 29−0655; a = 0.5092 nm, b =

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0.3595 nm, c = 0.8967 nm, β = 97.38o) by comparing d-value of the products with standard X-ray diffraction card of HfS3. The cell parameters are almost agreement with the results (JCPDS no 65−2348, a

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= 0.5086 nm, b = 0.3592 nm, c = 0.8979 nm, β = 97.42o) in Ref. 33 and 36. No impure peak is observed, so the HfS3 is pure. Fig. 1(b) and (c) reveal high- and low-magnification images of the nanobelts,

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respectively, which exhibits that the nanobelts have a width of 700 −70 nm, a thickness of about 10 −30 nm, and a length of more than 10 µm. The size is obviously less than the results (thickness of 60 −120 nm, and length of 2 − 5 mm) of Ref. 36. Fig. 1(d) is the EDX spectrum of the nanobelts, exhibiting atomic ratio of Hf and S with 1: 2.54. Fig. 1(e) shows the TEM image of a single nanobelt, and inset is corresponding SAED pattern which further confirms that it is monoclinic HfS3 (PCPDFWIN no. 29−0655), and it grows along [010] direction. Fig. 1(f) displays the HRTEM image of the single nanobelt corresponding to Fig.1 (e). The lattice fringe space is 0.50 nm, approaching d-value of (100) space of HfS3

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ACCEPTED MANUSCRIPT (PCPDFWIN no. 29−0655). It shows that HfS3 nanobelts can be prepared at 650
within 5 h. The reaction time is much less than the reported 15 days [36] and 10 h [33]. Fig. 2(a) displays the PL spectra of the HfS3 nanobelts. The plot on the left is an excitation spectrum while one on the right is an emission spectrum at excitation of 400 nm. The PL exhibits emission peaks at

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483, 540 and 600 nm, and there is a strongest peak at 540 nm, so it is a green-light-emission material. Fig. 2(b) is the UV-vis absorption spectrum of the HfS3 nanobelts, showing that there is strong absorption from 200 to 700 nm, and the strongest absorption is at 425 nm. Based on direct and indirect energy gap

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formulae [37, 38], the direct energy gap (DEG) is 2.19 eV, as shown in Fig. 2(c), while indirect energy gap (IEG) is 1.73 eV, as shown in inset of Fig. 2(c). Both are less than the DEG (3.0 eV) and IEG (2.1 eV)

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of the reported HfS3 [30]. Fig. 2(d) shows optical micrograph of the flexible photodetector. Space between two electrodes is 602.1 µm on the surface, and inset displays bended photograph of the flexible device. To investigate the photoelectric properties of the device, the photoresponses, photocurrents, and photoswitch were measured under monochromatic light of different wavelength. Fig. 3(a) compares the

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current−voltage (I−V) curves of the photodetector exposed to the light of different wavelengths (365, 405, 532, and 650 nm) and under dark condition, revealing senses of the device to ultraviolet-visible light. The photoresponsivities from 350 to 700 nm at an applied voltage of 5.0 V are further measured by

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monochromatic light of Xe lamp, as presented in Fig. 3(b). It shows good responses from 350 to 650 nm, and the best response is at about 400 nm, and the cut-off wavelength is about 700 nm. The responsivity

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(Rλ) can be determined from the following equation, respectively: Rλ =Iph/P, Where Iph is the difference between the photo-excited current and dark current, P is the light power irradiated on the materials, and λ is the exciting wavelength. Fig. 3(c) shows dependence of current on voltage under illumination of 405nm light with different optical powers, showing that the currents increase with the optical powers increasing. The corresponding photocurrents−optical power curves are plotted as Fig. 3(d) at a bias of 3 and 5 V. Both the curves can be fitted with Iph = αPθ, where Iph is the photocurrent (nA), α is a coefficient, θ is the exponent, and P is the optical power (mW). Fitting of the curves resulted in Iph = 0.214P0.7 and Iph 5

ACCEPTED MANUSCRIPT = 0.359P0.71 for a bias of 3 and 5 V, respectively. The non-unity exponent is a result of the complex process of electron-hole generation, trapping, and recombination in the semiconductor [39]. Fig. 3(e) and (f) exhibit the dependence of current on time (I−t) of the photodetector under 405-nm light illumination of 6.32 mW at a bias of 5 V with a photoswitch period of 1 and 50 s, respectively, revealing that Ion/Ioff (on-

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off current ratio of light) is 8.2 and 12, and response time (rise time/recovery time) is 0.18/0.19 and 0.2/0.2 s, respectively. 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. It shows that the photodetector has high on/off

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ratio and good response. The light on/off ratio (12 at a period of 50 s) is much less than that (337.5) of the individual nanobelt photodetector [33], but such high on/off ratio (12) can meet applicable demands.

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To understand atmosphere effect on the device, the responses of the device in air and vacuum were also investigated. As shown in Fig. 4(a), the dark current of the device in air is slightly greater than that in vacuum, so the HfS3 could be attributed to a p-type semiconductor, because the increase of oxygen concentration in air results in the increase of hole concentration so that conductivity increases [33], but the

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change is too little. Under illumination of light，the light-illuminated current in air is greater than that in vacuum due to similar reason. As shown in Fig. 4(b), though the photocurrent in air is greater than that in vacuum, because the dark current in air is also greater than that in vacuum, the photoswitch current ratio

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(5.4) in air is still less than that (5.7) in vacuum, and the response time are 0.17/0.15 s (in air) and

ambient.

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0.15/0.15 s (in vacuum), respectively. So the photodetector is almost not affected by air or vacuum

To explore durability of the flexible photodetector, the I−V and I−t curves were measured under 405nm light illumination after the device was placed for 2 months and bended 500 times, as shown in Fig. 5 (a) and (b), respectively. Fig. 5(a) shows that the device is not destroyed, and has strong photosensitivity. Fig. 5 (b) reveals the photoswitch properties after the device was bended. The light on/off current ratio is still 12, and rise and decay time is 0.2 and 0.19 s, respectively. Therefore, the flexible device has high stability and durability. 6

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4. Conclusions In summary, the HfS3 nanobelts were fast prepared at 650℃ within 5 h for first time, and the PL phenomenon of the nanobelts was discovered for first time. The HfS3 nanobelt film was fabricated to the

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flexible photodetector for first time. The photodetector demonstrated excellent sensitivity to UV-vis light, and photosensitivity is not so much affected by atmosphere. After the photodetector was placed for 2 months and bended 500 times, it still kept high photodetectivity and fast response, showing high durability

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and stability. The results suggested that the photodetector could be applied in practical photodetection.

Acknowledgements

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We greatly appreciate the financial support from National Natural Science Foundation of China (NSFC) (No. 21171091 and 20671050).

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. (a) X-ray diffraction pattern of the HfS3 nanobelts. (b, c) SEM images of the HfS3 nanobelts. (d) EDX spectrum of the sample. (e) Transmission electronic microscope (TEM) image of a single nanobelt

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(inset is corresponding electron diffraction pattern). (f) HRTEM image of the single nanobelt. Fig. 2. (a) PL spectra of the HfS3 nanobelts (left: excitation at 540 nm; right: emission at 400 nm). (b) UV-vis absorbance of the HfS3 nanobelts. (c) (αhν)2− hν curve of the HfS3 nanobelts on the basis of Fig.

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2(b), and inset is (αhν)1/2 − hν curve corresponding to Fig. 2(b). (d) Optical micrograph of the flexible photodetector. Inset is its bended photograph.

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Fig. 3. (a) I−V curves of the photodetector illuminated under different wavelength light and dark. (b) Responsivity of the photodetector to different wavelengths at a bias voltage of 5 V. (c) I−V curves of the photodetector illuminated by 405-nm light of different optical powers. (d) Photocurrents vs. optical powers curves of the photodetector illuminated under 405-nm light at a bias voltage of 3 and 5 V. (e) I−t

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curve of the photodetector illuminated under 405-nm light with an on-off period of 1 s. (f) I−t curve of the photodetector illuminated under 405-nm light with an on-off period of 50 s.

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Fig. 4. (a) I–V curves of the photodetector with an illumination of 405-nm light and dark in air and vacuum (2.0 Pa). (b) Time-dependent responses of the photodetector in air and vacuum (2.0 Pa) at a bias

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of 5 V under 405-nm light illumination with an on-off period of 0.5 s. Fig. 5. (a) I–V curves measured under 405-nm light illumination, and (b) I–t curve measured at a bias of 1 V under 405-nm light illumination with an on-off period of 1 s after the photodetector was placed for 2 months and bended 500 times.

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0.2

1.2

Current (nA)

1.0

6

V olta g e (V ) 1.6

550

SC

Photocurrent (nA)

D a rk 0 .4 9 5 m W 3 .3 m W 3 .6 2 m W 4 .0 2 m W 5 .1 5 m W 5 .2 3 m W -4

500

5 V 3 V

(d )

1.2

405 nm

M AN U

Current (nA)

1.4

(c)

-6

450

W a ve le n g th (n m )

V o lta ge (V ) 1 .6 1 .4 1 .2 1 .0 0 .8 0 .6 0 .4 0 .2 0 .0 -0 .2 -0 .4 -0 .6 -0 .8 -1 .0 -1 .2 -1 .4 -1 .6

RI PT

1.5

Fig 3. Wu et al

14

80 0

100 0

ACCEPTED MANUSCRIPT

1.5

RI PT

(a)

0.5 0.0 -0.5

SC

Current (nA)

1.0

Air, 405 nm,5.26 mW Vacuum, dark Vacuum,405 nm,5.26 mW Air, dark

-1.0

-6

-4

M AN U

-1.5 -2

0

2

4

6

Voltage (V)

1.8

(b)

1.6

Vacuum, 405 nm, 5.26 mW Air, 405 nm, 5.26 mW

1.2 1.0

TE D

C urrent (nA )

1.4

0.8 0.6 0.4 0.2

AC C

EP

0

2

4

6

Time (s)

Fig 4. Wu et al

15

8

10

12

ACCEPTED MANUSCRIPT

3

(a) After 2 months and bending 500 times

2

Current (nA)

RI PT

4

405 nm, 8.11 mW Dark

1 0 -1

-3 -4 -6

-4

-2

0

0.8

Current (nA)

4

6

1V, 405 nm, 8.11 mW

(b)

1.0

2

M AN U

Voltage (V)

SC

-2

0.6 0.4

TE D

0.2 0.0

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30

Fig 5. Wu et al

AC C

EP

Time (s)

16

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Highlights ●We synthesized pure HfS3 nanobelts by chemical vapor transport in a short time. ●The nanobelts have good photoluminescence under the excitation at 400 nm. ● A flexible UV-vis photodetector was fabricated on the basis of HfS3 nanobelt film. ●The photodetector showed high photoresponse and high selectivity.