Bi2S3-xOx nanowire photodetector with broadband response from ultraviolet to near-infrared range

Journal Pre-proof Single Bi2S3/Bi2S3-xOx nanowire photodetector with broadband response from ultraviolet to near-infrared range Yufeng Liu, Peng Chen, Guozhang Dai, Weitao Su, Yan Sun, Jingshan Hou, Na Zhang, Guoying Zhao, Yongzheng Fang, Ning Dai PII:

S1386-9477(19)31843-0

DOI:

https://doi.org/10.1016/j.physe.2020.114041

Reference:

PHYSE 114041

To appear in:

Physica E: Low-dimensional Systems and Nanostructures

Received Date: 8 December 2019 Revised Date:

5 February 2020

Accepted Date: 21 February 2020

Please cite this article as: Y. Liu, P. Chen, G. Dai, W. Su, Y. Sun, J. Hou, N. Zhang, G. Zhao, Y. Fang, N. Dai, Single Bi2S3/Bi2S3-xOx nanowire photodetector with broadband response from ultraviolet to near-infrared range, Physica E: Low-dimensional Systems and Nanostructures (2020), doi: https:// doi.org/10.1016/j.physe.2020.114041. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Single

Bi2S3/Bi2S3-xOx

Nanowire

Photodetector

with

Broadband Response from Ultraviolet to Near-infrared Range Yufeng Liu,∗a,b Peng Chen,a Guozhang Dai,*c Weitao Su,d Yan Sun,b Jingshan Hou,a Na Zhang,a Guoying Zhao,a Yongzheng Fang,*a Ning Daib a

School of Materials Science and Engineering, Shanghai Institute of Technology,

Shanghai 200235, P. R. China. b

State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics,

Chinese Academy of Sciences, Shanghai 200083, P.R. China. c

School of Physics and Electronics, Central South University, Changsha, Hunan

410083, P. R. China. d

College of Materials and Environmental Engineering, Hangzhou Dianzi University,

Hangzhou, 310018, P. R. China.

∗

Corresponding author. Tel.: +86 21 60873117, fax: +86 21 60873117.

E-mail addresses: [email protected] (Yufeng Liu) ∗

Corresponding author. Tel.: +86 731 88836457, fax: +86 731 88836457.

E-mail addresses: [email protected] (Guozhang Dai) ∗

Corresponding author. Tel.: +86 21 60873555, fax: +86 21 60873555.

E-mail addresses: [email protected] (Yongzheng Fang)

Abstract Constructing heterojunction in single nanowire (NW) can improve the band alignments of heterogeneous interface to promote the optoelectronic performance of materials. In this paper, Bi2S3/Bi2S3-xOx heterojunction NWs are in situ constructed by surface oxidation technique to fill the vacancies of sulfur with oxygen atoms into Bi2S3 NWs. According to the first principle calculation, Bi2S3-xOx shell possesses a narrower band gap of 1.32 eV than that of Bi2S3 core, which aids NW further to enlarge the response range of near infrared spectrum. Moreover, n-type Bi2S3 converts to p-type Bi2S3-xOx owing to the incorporation of oxygen atoms in the surface of the NWs based on the calculated electronic structures. The band alignment of Bi2S3/Bi2S3-xOx heterojunction contributes to transform the carriers between the interface of Bi2S3 and Bi2S3-xOx. As a result, the single Bi2S3 NW based photodetector presents a broadband response from ultraviolet light of 325 to near-infrared light of 1064 nm. Moreover, the detectivity of photodetector is up to 1011 Jones for the visible light of 405, 442 and 642 nm. This investigation demonstrates an efficient broadband photodetector based on Bi2S3/Bi2S3-xOx NWs, in which provides an effective route to develop the high performance of optoelectronic devices for the other NWs.

Keywords: Bi2S3 nanowire, photodetector, bandgap alignment

Introduction High performance functional devices based on semiconductor nanostructure have been the popular research focus in the field of optoelectronic integrated circuit (OEIC) owing to their unique characteristics, such as remarkable surface effect, quantum size and confinement effects etc.1-5 One dimensional nanowires (NWs) as one of typical candidates have been extensively studied in the devices of photodetector, solar cell, photocatalysis, laser diode, light emitting diode and gas sensor etc.6-11 However, in contrast with multiple NWs array, thin films and bulk counterparts, single NW usually exhibit poor photoelectronic performance although it can be facilely integrated in one small chip due to its minisize.12-16 Therefore, for their practical applications, one of the principal objectives is to promote the performance of the nano-devices and simultaneously maintain the unique properties of NWs. Among the numerous strategies, to construct core/shell heterojunction NWs not only can modify the surface properties of NWs, but also improve the band structure of the interface between heterojunctions to promote the transport and separation of carriers, which are considered as one of the most efficient ways to improve the performance of NW-based functional devices.17-20 As a typical binary V-VI semiconductors, bismuth chalcogenides of Bi2Ch3 (Ch=S, Se, Te) are a category of distinctive materials, which have been researched in the

fields

of

topological

insulator,

photocatalysis,

thermoelectric

device,

photodetectors and solar cells owing to their environment-benign chemical compositions and dramatic optical, electrical and photoelectric conversion

characteristics.21-25 Among the category, Bi2Se3 and Bi2Te3 based devices present excellent characteristic of near-infrared detection.26-28 However, Bi2S3 based photodetectors have relatively normal performance although these devices are fabricated via combining multiple nanocrystals or nanorods, assembling thin films or forming Au or MoS2 heterojunction.29-34 Especially, there is rare study on single Bi2S3 NW photodetectors with ohmic contact, in which the relatively low photodetection performance need to be improved by constructing Schottky junction.35 Moreover, most of the Bi2S3 based photodetectors only response prominently for a certain wavelength or a narrow spectral range, although Bi2S3 is provided with optimal band gap of 1.3-1.7 eV, high absorption coefficient of 104-105 cm-1 and reasonable incident photon to electron conversion efficiency.29-35 Therefore, it is a challenge to clarify the mechanism of photoelectric detection and improve the performance of single Bi2S3-based photodetector. In this paper, Bi2S3/Bi2S3-xOx heterojunction NWs are in-situ structured by surface oxidation technique to fill the vacancies of sulfur with oxygen atoms on Bi2S3 NWs, in which the band gap of Bi2S3-xOx shell is narrower than that of Bi2S3 core, aiding further to enlarge the near infrared spectral response range of NWs. Moreover, the band gap alignment the Bi2S3/Bi2S3-xOx heterojunction is propitious to transform the carriers between the interface of Bi2S3 core and Bi2S3-xOx shell after the surface of n-type Bi2S3 convert to p-type Bi2S3-xOx shell owing to the surface oxidization of the NWs. As a result, the photodetector based on the single Bi2S3 heterojunction NW presents a broadband response from 325 to 1064 nm, a response time less than 1 ms

under the incident light of 442 nm and an excellent detectivity up to 1011 Jones for the incident light of 405, 442 and 642 nm. The results indicate that the Bi2S3/Bi2S3-xOx heterojunction NW are a promising potential candidate in nanoscale electronic and optoelectronic devices.

Experimental Section Chemicals All chemicals were used as received without further purification. Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, analytical grade), thiourea (CH4N2S, analytical grade), lithium hydroxide (LiOH, analytical grade) were all purchased from J&K chemical LTD (Shanghai). Synthesis of Bi2S3 heterojunction NWs Bi2S3 heterojunction NWs were synthesized via a high concentration alkaline hydrothermal method. Typically, 10 ml of deionized water, 5 mmol of bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O) and 25 mmol of thiourea (CH4N2S) were in turn added into a 50 mL beaker and dissolved under stirring. Then, 12 g of lithium hydroxide (LiOH) powders was added into the beaker and stirred for 5 minutes to form a black mixed solution. The mixture was transferred into a 25 ml Teflon-lined stainless-steel autoclave in succession. The autoclave was sealed and maintained at 200 oC for 72 h before it is cooled to room temperature in air. Finally, the precipitates were taken out and washed 5 times with deionized water and the products were subsequently dried in an oven at 70 oC for 10 h. Fabrication of single Bi2S3 NW photodetector

Photodetector based on single Bi2S3 heterojunction NW was fabricated with silver paste as electrodes, forming Ag-Bi2S3 heterojunction NW-Ag lateral structure. The process of fabricated device is described as follows: high quality Bi2S3 heterojunction NWs were transferred onto the polyethylene terephthalate substrate. The straight NWs with clean surface were chosen to make devices. Then, two drops of silver paste were bounded with the two end sides of Bi2S3 heterojunction NW severing as the two electrodes. Finally, a thin layer of polydimethylsiloxane was used to package the device, which can prevent contamination or corrosion of the NW. Characterization Field emission scanning electron microscopy (FESEM) images were acquired from a FEI Sirion 200 with energy dispersive X-ray (EDS) analysis. Low magnification, high-resolution transmission electron microscopy (TEM) images and STEM elemental mapping images were obtained on a JEOL-2010F instrument operated at an accelerating voltage of 200 kV. X-ray diffraction of the Bi2S3 NWs were performed on Bruker D8 Focus with a monochromatized source of Cu Kα1 radiation (λ=0.15405 nm) at 1.6 kW (40 kV, 40 mA). Micro Raman and PL measurements excited at laser wavelength of 532nm were conducted on a home built Raman/PL system, consisting of an inverted microscope (Ti eclipse, Nikon) and a Raman spectrometer (iHR320, Horiba) attached with a CCD detector (Syncerity, Horiba). The nanowires were dispersed on a glass coverslip with a thickness of 0.17mm. UV-vis absorbance spectrum was recorded from 250 to 2000 nm on an Agilent Cary 5000 spectrophotometer with a scanning velocity of 300 nm min-1.

X-ray photoelectron spectroscopy (XPS) was carried out with ESCALAB 250 X-ray photoelectron spectrometer for the analysis of surface elemental composition. The photodetector based on single NW was excited by a He-Cd laser (325 nm, 442 nm) and a multichannel fiber coupled laser source with a wavelength of 405 nm, 642 nm, 808 nm and 1064 nm, whose light power density can be adjusted directly and measured by a thermopile powermeter. Some neutral density filters placing between light sources and devices were used to realize varies power density under illumination. I-V/I-T characteristics of the device were measured by a low-noise voltage/current preamplifier (Stanford Research Systems, model SR560/SR570) in conjunction with a computer-controlled measurement system.

Results and discussion The surface morphology of Bi2S3 NWs are presented by FESEM in Figures 1(a,b). The whole visible nanomaterials exhibit wirelike extrinsic feature with the length of 94.3±7.5 µm and the diameter of 122.6±10.3 nm, respectively. Besides, there is a fluctuant surface as seen in HRTEM of a single NW (Figures 1(c,d)), although it seems to be smooth by zooming out single NW (Figure 1(b)). The fluctuant parts present distinct crystalline domians as shown in Figure 1(c), which is confirmed by HRTEM in the surface of the single NW (Figure 1(d)). In addition, the obvious lattice fringes with the interplanar spacing of 2.79 Å can be seen from HRTEM (Figure 1(d)) in the inside of the single NW, which corresponds to (211) plane of Bi2S3 with orthorhombic structure. Besides, there is a distinct crystalline layer of–

1.93 nm depth in the surface of the single NW, which forms an obvious

interface labeled with red dotted line. The UV-vis-NIR absorption spectrum is used to evaluate the optical band gap of the Bi2S3 NWs as shown in Figure 1(e), which indicates that Bi2S3 NWs show intense light absorption from ultraviolet, visible light to near infrared region (250-1050 nm). The band gap energy (Eg) is determined by extrapolating linear region from the (αhυ)2 (ie. the square of the absorption coefficient α multiplied by the photon energy hυ) versus hυ according to equation (1). (αhυ)2 = A(hυ − Eg)

(1)

where α is the absorption coefficient, Eg is the band gap of the Bi2S3 NWs, A is a constant, hυ is photon energy. The value of Eg for Bi2S3 NWs is 1.29 eV according to the UV-vis-NIR absorption spectrum and equation (1), which agrees well with the reported Eg values of Bi2S3 NWs as in the references. [29-33] XRD analysis is employed to verify the phase and crystallization of Bi2S3 NWs. The XRD pattern of Bi2S3 NWs is shown in Figure 1(f), in which the red bars represent the standard XRD spectrum (JCPDS card No. 75-1306). The major peaks at 15.78°, 17.83°, 22,46°, 25.04°, 28.69°, 31.82°, 35.67°, 40.11°, 43.28°, 47.18°, 48.37°, 62.78°, 67.01°, 73.13°and 76.43° correspond to (020), (210), (220), (130), (230), (221), (240), (141), (250), (350), (060), (171), (180), (281) and (303) planes of Bi2S3 NWs, respectively. Thus, the results indicate that Bi2S3 has orthorhombic crystal structure and Pbnm62 space group. However, the identification of the distinct crystalline layer of–

1.93 nm depth in the surface of the single NW still can’t be

distinguished according to XRD.

Figure 1 FESEM (a, b), TEM (c), HRTEM (d), absorbance spectrum (e) and XRD (the red bars represent the standard XRD spectrum (JCPDS card No. 75-1306)) (f) of Bi2S3 NWs. The elemental mapping of single NW is tested to analyze the distribution of each element in NW as shown in Figure 2. In accordance with STEM images of single NW, Bi and S elements (Figures 2(b,c)) are uniform distributed in the whole NW. Besides, the mapping of Bi and S elements almost coincide with the surface profile of NW. In addition, relative weaker mapping signal of O element also appears in the single NW (Figure 2(d)), which indicates there is oxygen element in the single NW, although the contents of oxygen element are less. The existence of oxygen element in the surface of NWs is further confirmed by XPS. Figures S1(a-c) are XPS of Bi 4d, Bi 4f and S 1s of Bi2S3 NWs, respectively.

Two narrow and symmetric peaks at 441.2 eV and 465.3 eV deriving from Bi 4d5/2 and 4d3/2 (Figure S1(a)), correspond to Bi (III) with a peak splitting of 23.9 eV. There are two main peaks at 158.0 eV and 163.3 eV with a peak split of 5.3 eV deriving from Bi 4f7/2 and 4f5/2 as shown in Figure S1(a), in which one shoulder peak at 160.6 eV arising from S 2p3/2. The bonding energy of S 1s locates at 529.4 and 531.4 eV can be seen in Figure S1(c). Besides, there is a peak at 225.1 eV in Figure S1(d), which deriving from O 1s. All these confirm that the crystalline products are composed of Bi, S and O elements in the surface of NWs, which is in accordance with the above analysis results. In addition, EDS also prove the existence of O elements in Bi2S3 NWs as shown in Figure S2. Therefore, the distinct crystalline layer of–

1.93 nm depth in the surface of the

single NW can result from the incorporation of oxygen atoms in the surface of NW, which can be understood as follows. The Bi2S3 is synthesized by a high concentration alkaline hydrothermal method, in which crystal growth occurs in order from dissolution to nucleation, aggregation, and then recrystallization to form NWs at the high temperature.[36-38] However, the reaction time is prolonged to 72 h during the Bi2S3 NWs formed, which brings about the surface of Bi2S3 NWs being oxidized to form Bi2S3-xOx layer under the condition of high pH value. The following reaction summarizes a postulated mechanism for the surface of Bi2S3 NWs: 2 Bi3++(3-x) S2-+2x OH

Bi2S3-xOx+x H2O

(2)

Micro Raman spectrum is performed to confirm the information of the molecular vibration and rotation of the single NW (Figure 2(e)). There are four characteristic

peaks at 148.4, 272.5, 632.1 and 1082.3 cm-1 originating from the single NW. The other two peaks labeled star originate from the glass substrate. The positions of the characteristic peaks at 272.5 and 632.1 cm-1 agree with the reported results.[39-41] However, there are red shifts of 19.6 and 117.3 cm-1 for the characteristic peaks at 148.4 and 1082.3 cm-1 in contrast with Bi2S3,[39] which indicate there are different the states of molecular vibration and rotation in Bi2S3-xOx. The results might be relative with the incorporation of oxygen atoms in the surface of NW. In addition, a near-infrared luminescence signal of 887.3 nm is detected from the single NW by Raman spectrometer, which derives from the radiative electrons transition of interband in single NW.

Figure 2 STEM image of (a) and Bi, S, and O elemental mappings of STEM-EDS (b-d), Raman spectrum (e) and luminescence (f) of single Bi2S3 NW. The bar is 400 nm.

To systematically explore the optoelectronic properties of the single NWs, unique photodetectors were fabricated using the as-grown individual samples as shown in the inset of Figure 3(a). The typical light power density dependent photocurrent-voltage (I-V) curves of photodetector based on single Bi2S3 NW under the incident light of 442 nm are shown in Figure 3(a). I-V curves for the different light power densities are approximately straight and symmetric, which indicates there is good ohmic contact between the NW and the Ag electrodes. Moreover, the photocurrents of the photodetector based the single Bi2S3 NW increase distinctly with the increase of light power density from dark condition to 175.25 mW/cm2 under a bias voltage of 4 V. Figure 3(b) shows the time dependent response of NW photodetector under 442 nm light irradiation with the various light intensity, which is measured by periodically turn on and off in the air with a 1 s cycle at a bias voltage 4 V. With the increase of light intensity, the photocurrent of the device reaches to stable on states with the values from 73.24 (0.18) to 891.1 nA (175.25 mW/cm2) and then quickly declined (19.98 nA) while the light is off. Moreover, the intensity of photocurrent for each cycle is almost equal, which indicates the device based on single NW possesses excellent reversible photostability undergoing repeated switches. The maximum value of Ion/Ioff ratio is up to 44.6 under the light intensity of 175.25 mW/cm2. The detail relation between the light intensity and photocurrent is shown in Figure 3(c), which increase in the form of power function fitted by ‫ = ݕ‬a‫ ݔ‬௕ , where a=139.9, b=0.36. The photoresponsivity (R) is the generated photocurrent per unit incident light

power on the effective area of a photodetector, reflecting the photoelectric conversion performance of a device under the incident light. The photoresponsivity (R) of the photodectector are calculated by the equation (3) as follows: ܴ=

ூ೗೔೒೓೟ ିூ೏ೌೝೖ ௉೔ ∗஽∗௅

(3)

where ‫ܫ‬௟௜௚௛௧ and ‫ܫ‬ௗ௔௥௞ are photocurrent and dark current of the device, ܲ௜

is

incident optical power, D and L are width and length of photosensitive area respectively. The calculated R values are 2908.9, 2133.1, 751.1, 379.4, 154.8 and 49.7 A/W corresponding to the different incident light density of 0.18, 0.49, 2.2, 6.875, 27.5 to 175.25 mW/cm2 respectively, which can be fitted by a power function of ‫ = ݕ‬a‫ ݔ‬௕ , where a=1279.0, b=-0.50 as shown in Figure 4d. The maximum value of R reaches nearly 3*103 A/W under the incident light density of 0.18 mW/cm2, which indicate the photodetector based on single Bi2S3 NW displays excellent photosensitivity for weak light signal. Furthermore, it can be seen that the photoresponsivity of the single NW-based photodetector is decreasing with the increase of the light intensity. The reasons are as follows. A large number of electron–hole pairs in the device are generated under the excitation of the incident light. When the light intensity is lower than 0.18 mW/cm2, the photocurrent increases nearly linearly. However, with the incident light continuously increasing, the photon-generated electron–hole pairs are saturated, bringing about slow increase of the photocurrent. Therefore, the photoresponsivity decreases with the increase of the light intensity when the light intensity exceeds 0.18 mW/cm2. Besides the photoresponsivity (R), the external quantum efficiency (EQE) is a

crucial parameter to judge the performance of a photodetector. EQE is related to the number of electron-hole pairs excited by absorbed photons, which can be expressed by the following equation: ‫= ܧܳܧ‬

ூ೗೔೒೓೟ ିூ೏ೌೝೖ ௤

(௉

௛௩

೔ ∗஽∗௅

)

(4),

where ‫ܫ‬௟௜௚௛௧ and ‫ܫ‬ௗ௔௥௞ is photocurrent of the device, q is unit charge quantity, ܲ௜

is

optical power, ℎ‫ ݒ‬is the photon energy, D and L are width and length of photosensitive area respectively. From the equation (4), the calculated value of EQE is up to 8.1*103 under the incident light of 442 nm with the light intensity of 0.18 mW/cm2, which indicate the photodetector based on single Bi2S3 NW processes excellent phototsensitivity for weak light signal again. Response speed is also a significant parameter to assess the photosensitivity of photodetector. The response of the short cycle by the amplified sections of a 60.702-60.705 s and a 60.727-60.730 s range is measured (442 nm, 175.25 mW/cm2 at 4 V) as shown in Figures 3(e, f), in which the photocurrent increases quickly from light off state to on state. The corresponding raise time and decay time are 0.47 ms and 0.94 ms, respectively, which indicate the photodetector based on single NW possess fast response speed.

Figure 3 (a) The photocurrent as a function of incident light intensity (at 442 nm) for Bi2S3 NWs. (b) Time response of Bi2S3 NWs under different intensity of incident light (442nm, at 4 V, with 5 cycles). (c) The relation between the light intensity and photocurrent and (d) the relation between the light intensity and the photoresponsivity. (e, f) The amplified sections of 60.702-60.705 and 60.727-60.730 s range corresponding to light-off to on and light-on to off transitions

Besides the visible incident light of 442 nm, the time dependent response of the single NW photodetector from ultraviolet to near-infrared incident light also are tested as shown in Figure 4. There are remarkable photoresponse under the other five incident lights of 325, 405, 642, 808 and 1064 nm as shown in Figures 4(a-e) and Figures S(3-7(a)). In detail, the calculated photoresponsivity of the photodetector are 974.4, 1193.6, 2908.9, 2247.6, 944.6 and 43.0 for incident lights of 325, 405, 642, 808 and 1064 nm as shown in Figure S8. There are lower dark currents of 4.35, 5.53, 7.32 and 4.6 nA for incident lights of 405, 642, 808 and 1064 nm. In addition, the

photodetector based on single NW display notable photosensitivity for the five incident lights, in which the raise times and decay times are 0.4 ms and 3.6 ms, 1.6 and 3.6ms, 2.4 and 7 ms, 1.5 and 3.7 ms, 0.22 and 0.59 s as shown in Figures S(3-7(b)). In addition, the calculated detectivity under six incident lights of 325, 405, 442, 642, 808 and 1064 nm are 4.43×1010, 1.01×1011, 1.15×1011, 1.69×1011, 6.17×1010 and 3.54×109 Jones as shown in Figure 4(f), which indicate there is relatively lower detectivity of 1010 order under unltraviolet incident light. Then it increases to 1011 orders for visible incident light form 405, 442 and 642 nm. Subsequently, the detectivity decreases to 1010 order while near-infrared incident light of 808 nm illuminates, in succession declining to 109 order under the near-infrared incident light of 1064 nm. It is obvious that the detectivity at the regions of unltraviolet and near-infrared light are lower than those of at the region of visible incident light, which presents similar tendency with the absorbance spectrum. The results indicate that the decrease of absorbance for photons results in the detectivity declining. Especially for near-infrared incident light of 1064 nm, the energy of near-infrared photon is 1.17 eV, which is less than the band gap value of Bi2S3 NW (1.29 eV), failing to excite the electrons from the valence band to conduction band of Bi2S3 NWs. Therefore, the detectivity decreases 102 times from visible incident light region (1011 Jones) to near-infrared incident light of 1064 nm (109 Jones). It is obvious that the photodetector based on Bi2S3 heterojunction single NW presents broader spectral response region. Moreover, in consideration of single NW of Bi2S3 in the work, there

are relatively high photodetection performance referring to the photodetectors based on the thin films, nanosheets, nanocrystals and composites of Bi2S3 etc. in the reported results as shown in Table 1.

Figure 4 (a-e) The photocurrent as a function of incident light intensity (at 325, 405, 642, 808 and 1064 nm) for the single Bi2S3 NW respectively. (f) Photodetectivity of the single Bi2S3 NW for 325, 405, 642, 808 and 1064 nm. Table 1 Comparison of parameters between the single Bi2S3 NW-based photodetector in this paper and other Bi2S3 materials based photodetectors.

Photodetector

Wavelength

Bi2S3 nanostructures Bi2S3 nanorods

475-650 nm

Ion/Ioff

Rise time

Decay time

Responsivity

Detectivity

Refs.

1.1

50 ms

240 ms

[29]

13

0.3 s

0.7 s

[30]

Bi2S3 nanocrystals

White

Bi2S3 nanosheets

405-633 nm

Bi2S3@MoS2

650 nm

40

6

10 us

100 ms

120 A/W

1011 Jones

[31]

350 us

4.4 A/W

1011 Jones

[32]

567

Bi2S3 NR@Au

560nm

1066

Single Bi2S3 NW

532 nm

9.8

Single Bi2S3 NW

325-1064 nm

44.6

13.3 A/W 2310 A/W 1s

0.47 ms

[33] 1013 Jones

1s

0.94 ms

[34] [35]

2908.9 A/W

1011 Jones

this paper

The composition and microscopic crystal and electronic structures influence the photoelectric characteristics of materials.[42] To clarify the mechanism of the photodetector based on single heterojunction NW, the electronic structures of the Bi2S3 and Bi2S3-xOx are simulated and calculated by the first principle calculation as shown in Figures 5(a, c) and Figure S9(a). While there are no sulfur vacancies in the lattice of Bi2S3, the Fermi energy level of Bi2S3 locates at the precise middle between the valance band and conduction band, which is an intrinsic semiconductor with the band gap of 1.47 eV according to the band structure and density of states (DOS) as shown in Figures S9(a,b). While there are S vacancies in the lattice of Bi2S3, the Fermi level shifts to the conduction band minimum, forming the electronic structure and density of state (DOS) (Figure 5(b)) of n-type semiconductor. The band gap of Bi2S3 with S vacancies is 1.43 eV, in which CBM and VBM locate at 0.35 and -1.08 eV. Moreover, there is a deep energy level (red line in Figure 5(a)) originating from S vacancies in the electronic structure of Bi2S3, which cause a weaker signal of DOS (Figure 5(b)). In general, Bi2S3 is usually n-type semiconductor owing to the existence of sulfur

vacancies in the lattice of Bi2S3 as reported in the previous works. [29-33] In the process of high pH value chemical reaction for 72 h in the paper, oxygen atoms firstly prefer to fill the positions of missing sulfur vacancies in the lattice of Bi2S3, instead of expelling and occupying the positions of the existent sulfur atoms in the lattice of Bi2S3, which results in Bi2S3 being oxidized to form Bi2S3-xOx in the surface of NWs. While the surface of Bi2S3 NW is oxidized under the condition of high temperature, O atoms fill the vacancies of S, in which O 2s and O 2p orbitals are involved into the valence band maximum (VBM) and the conduction band minimum (CBM) respectively. Due to the different the ionic radius and electronegativity for oxygen and sulfur atoms, the Fermi energy level of Bi2S3-xOx approaches the middle of the CBM and VBM, slightly shifting to VBM as shown in the band structure and DOS (x=0.667) (Figures 5(c,d)), which results in the electronic structure of Bi2S3-xOx not being intrinsic semiconductor, but forming p-type electronic structure. In addition, the band structures and DOS of Bi2S3-xOx for x=0.333, 1, 1.333 and 1.5 are presented in Figure S10 (a-h), which indicates that the value of band gap decreases from 1.47, 1.34 to 1.32 eV while x increase from 0, 0.333 to 0.667, respectively. On the contrary, the band gap increases to 1.68 eV while x is up to 1. With the sequential increase of the contents of oxygen atoms in the surface of NW, the band gap decrease from 1.68, 1.42 to 0.90 eV once again while x are 1, 1.333 and 1.5.

Figure 5 Band structures (a, c) and DOS (b, d) of Bi2S3 with S vacancies and Bi2S3-xOx (x=0.67) calculated by the first principle calculation.

Therefore, the crystal structure of Bi2S3 can be constructed as Figure 6(a), in which the position labeled red circle represents a vacancy of sulfur in the lattice of Bi2S3. On the surface of the Bi2S3 NW, the S vacancies are substituted by O atoms while Bi2S3 is oxidized to form Bi2S3-xOx by LiOH under the condition of high temperature. The S vacancy labeled red circle will be filled with O atoms. Thus, the NWs form the heterojunction with Bi2S3 core and Bi2S3-xOx shell. The band alignment of the heterojunction interface in NW can be illuminated according to the calculation results of the first principle calculation as the Figure 6(b). There are S vacancies in Bi2S3 to form n-typed energy band structure (1.43 eV), which cause Fermi energy band to approach the CBM of Bi2S3 NW. However, the surface of NW is oxidized by LiOH under the condition of high temperature, resulting in the formation of Bi2S3-xOx shell with band gap of 1.32 eV. Therefore, that the nonequilibrium carriers are produced by absorbing the incident photons, transport in the band alignment of Bi2S3/Bi2S3-xOx NW as indicated in Figure 6(b). The electrons travel from the CBM of

Bi2S3-xOx to the CBM of Bi2S3 of Bi2S3-xOx. And the holes transport from the VBM of Bi2S3 to VBM of Bi2S3-xOx. The band alignment prefers the nonequilibrium carriers to separate to acquire high photoresponse for the NW. In addition, the band gap of Bi2S3-xOx is 1.32 eV, which is smaller than that of Bi2S3 (1.43 eV), broadening the range of absorbance spectra for NW. It brings about the single NW photodetector presents the higher photoelectric response and detectivity from ultraviolet, visible light to near infrared region. According to the structure of device based on single heterojunction NW, the electrodes are on the surface of NM. Therefore, the electrical test presents the results of photoconductive properties for the shell of p-type Bi2S3-xOx in the surface of NW. Although there is not the rectification effect for the electrical test of NW, the electrical characteristics of p-type Bi2S3-xOx is modulated by n-type Bi2S3 due to the rectification effect between the interface of p-n junction after irradiating. As shown in Figure 6, the photovoltage cause the electrons and holes move to the side of n-type Bi2S3 and p-type Bi2S3-xOx respectively, which results in the concentration of holes in the surface of Bi2S3-xOx increasing. However, due to without electrode on surface of NW, the aggregating holes are scattered to transport along longitudinal direction, where two-terminal electrodes collect them, enhancing the photoresponse of the overall two-terminal device.

Figure 6 (a) Crystal structure of Bi2S3 with the vacancy of sulfur. (b) The band gap alignment of Bi2S3/Bi2S3-xOx heterojunction under the nonequilibrium condition.

Conclusions In summary, Bi2S3/Bi2S3-xOx heterojunction NWs are in-situ synthesized by surface oxidation technique to fill the vacancy of sulfur with oxygen atoms on Bi2S3 NWs in the paper. The band gap of Bi2S3-xOx shell is smaller than that of Bi2S3 core, which aid further to enlarge the near infrared spectral response range of NWs. Moreover, the band gap alignment the Bi2S3/Bi2S3-xOx heterojunction is propitious to transform the carriers between the interface of Bi2S3 core and Bi2S3-xOx shell in that the surface of n-type Bi2S3 convert to p-type Bi2S3-xOx shell owing to the surface oxidization of the NWs. Finally, the single Bi2S3 core/shell nanowire photodetector with broad spectral response from 324, 405, 442, 642, 808 to 1064 nm is acquired. The detectivity of single Bi2S3 NW based photodetector for the visible incident light of 405, 442 and 642 nm is up to 1011 Jones.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) (grant numbers 51672177, 51402335, 51472162), Young and

Middle-aged Technology Talents Development Foundation of Shanghai Institute of Technology

(grant

number

ZQ2018-17)

and

the

Program

of

Shanghai

Academic/Technology Research Leader (grant number 19XD1434700).

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Highlight • In this paper, Bi2S3/Bi2S3-xOx NWs are in situ constructed by surface oxidation technique to fill the vacancies of sulfur with oxygen atoms into Bi2S3 NWs. • The band alignment of Bi2S3/Bi2S3-xOx heterojunction contributes to transform the carriers between the interface of Bi2S3 core and Bi2S3-xOx shell. • The single Bi2S3 NW based photodetector presents a broadband response from ultraviolet light of 325 to near infrared light of 1060 nm. • Moreover, the detectivity of single NW based photodetector is up to 1011 Jones for the visible light of 405, 442 and 642 nm.

Author statement I have made substantial contributions to the conception or design of the work, or acquisition, analysis, or interpretation of data for the work. And I have drafted the work or revised it critically for important intellectual content. I have approved the final version to be published. I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The authors declare no competing financial interest. All persons who have made substantial contributions to the work reported in the manuscript, including those who provided editing and writing assistance.

Conflict of Interest We would like to submit the manuscript entitled ‘Single Bi2S3/Bi2S3-xOx Nanowire

Photodetector

with

Broadband

Response

from

Ultraviolet

to

Near-infrared Range’ by Peng Chen, Guozhang Dai, Weitao Su, Yan Sun, Jingshan Hou, Na Zhang, Guoying Zhao, Yongzheng Fang, Ning Dai and myself, to Physica E of Low-dimensional Systems and Nanostructures as a research article. This work has not been published before, nor is it under consideration for publication elsewhere. We have no conflicting commercial interest, other than being inventors of patents and patent disclosures that have been filed based on the technology described here. We have all read and discussed the manuscript, for which I am the corresponding author. Sincerely yours, Yufeng Liu ([email protected]) School of Materials Science and Engineering Shanghai Institute of Technology Phone: +86-20-60873117 Fax: +86-20-60873117