Introduction

Group V-VI binary pnictogen chalcogenide semiconductors, typically crystallizing into a one-dimensional nanostructure in layer-structures parallel to the growth direction, have attracted a lot of attention owing to their specific properties and potential application in various fields, namely, photovoltaic, thermoelectric and electric devices1,2,3,4,5,6. In particular, antimony selenide (Sb2Se3) has promising characteristics, such as narrow band gap (approximately 1 eV), high chemical stability and high Seebeck coefficient (1800 μVK−1). These properties are attributable to a fast transition from amorphous to crystalline state. Despite its potential in various applications, research on the synthesis and application of the Sb2Se3 nanostructures has been limited by challenges, such as its low thermoelectric power factor(α2σ), low spectral response (Rλ) and low external quantum efficiency(EQE), which result in low electrical conductivity(σ, 10−6 ~ 10−2 Ω−1m−1) in bulk state5,7,8,9,10,11,12,13,14,15,16,17,18.

Nonetheless, a few groups have synthesized and studied Sb2Se3 nanomaterials to improve its electrical conductivity19,20,21,22,23,24,25,26. For example, Golberg et al. synthesized single-crystalline Sb2Se3 nanowires using a hydrothermal method and investigated its field emission and photoconductive properties. Single crystalline Sb2Se3 nanowires, synthesized for 72 h at 180°C exhibited remarkable response to 600 nm of specific visible light with response time of 0.3 sec. However, their time-consuming synthetic process, low electrical conductivity and low photosensitivity factor (K = ILight/IDark) still hamper practical applications of the Sb2Se3 nanowires22.

Here we suggest new synthetic process of single-crystal Sb2Se3 nanowires to overcome these challenges.

Sb2Se3 nanowires were synthesized by injection of chemicals into hot solvent (called Hot Injection method). This is one of the most common methods to produce nano-structured materials such as Q-dots, metal alloy nanoparticles and metal oxide nanoparticles, as size controlled nanoparticles can be easily prepared27,28,29. Using this hot injection method, we could not only produce Sb2Se3 nanowires quickly, but could also control the diameter of the nanowires.

Using an additional process, Sb2Se3 nanowires decorated with Ag2Se nanoparticles (Ag2Se-decorated Sb2Se3 nanowires) were also developed. Ag2Se nanoparticles increased the electrical conductivity and also improved photosensitivity of the Sb2Se3 nanowires30,31,32,33,34,35,36,37.

Results

Characterization of nanowire

Figures 1a shows representative transmission electron microscopy (TEM) image of the intrinsic Sb2Se3 nanowires grown using the hot injection method. Individual nanowires had a diameter of 80–100 nm and a typical length of several micrometers. A high-resolution TEM image and corresponding electron diffraction pattern of the nanowires, as shown in Figure 1(b), revealed that as-synthesized nanowire is single crystal without any detectable crystal defect. The powder X-ray diffraction (XRD) pattern of the intrinsic Sb2Se3 nanowires had five prominent peaks, which were indexed to the (120), (230), (221), (240) and (061) planes, corresponding to the orthorhombic crystal structure of Sb2Se3 with Pmma space group (Figure 1 c).

Figure 1
figure 1

(a) TEM image of Sb2Se3 nanowires; (b) high-resolution TEM images of Figure 1(a) and corresponding SAED pattern; (c) X-ray diffraction patterns of the nanowires.

Diameter of nanowires is a critical factor, especially in semiconductor nanowires, as their electronic properties strongly depend on diameter of the wire. For instance, GaN nanowires showed various photocurrent responses between 1 nA and 100 μA according to their diameter38. In our study, the diameter of Sb2Se3 nanowires could be controlled. Figure 2 shows transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of various diameter of Sb2Se3 nanowires, which were controlled by altering the precursor and surfactant ratio. Oleic acid was used as the surfactant in our study. Generally, high concentration of surfactant results in smaller sizes, including diameter, thickness and length. Three diameter of nanowires were obtained, which were 200–300 nm, 80–100 nm and 50 nm, with precursor and surfactant ratios of 1:2, 1:4 and 1:20, respectively. An increase in oleic acid resulted in narrow diameter Sb2Se3 nanowires. In addition, SEM images, which correlated well with the TEM images, confirmed that the nanowires synthesized had smooth surfaces.

Figure 2
figure 2

(a), (c), (e) TEM image of diameter -controlled Sb2Se3 nanowires; (b),(d),(f) SEM image of diameter -controlled Sb2Se3 nanowires, respectively. Three different nanowire diameter s were obtained, which were 200–300, 80–100 and 50 nm, with precursor and surfactant ratios of 1:2, 1:4 and 1:20, respectively.

To improve the electrical conductivity of Sb2Se3 nanowires, silver precursor (AgNO3) was added to the nanowire solution and nanowires decorated with silver nanoparticles were expected. However, we obtained Ag2Se-decorated Sb2Se3 nanowires. Nonetheless, Ag2Se-decorated Sb2Se3 nanowires were also expected to have better conductivity and photosensitivity than bare Sb2Se3 nanowires. Figure 3a clearly shows that each Ag2Se nanoparticle was incorporated onto the Sb2Se3 nanowires. The high-resolution scanning TEM (HR-STEM) image of the Ag2Se-decorated Sb2Se3 nanowires shows that Ag2Se nanoparticles and Sb2Se3 nanowire have different lattice structures and a visible interface (Figure 3b). The HR-STEM image further supports on the synthesis of high crystallinity Ag2Se nanoparticles and Sb2Se3 nanowires. The d spacing is 2.58 Å and 1.98 Å, which is agrees well with the distance of the [121] lattice plane of Ag2Se and [002] lattice plane of Sb2Se3, respectively. Additionally, the unblemished nanowire surface was convincing evidence that selenium of the Ag2Se nanoparticles originated from excess selenium precursor in the reaction mixture. To confirm the origin of selenium, we removed excess selenium precursor from the reaction mixture and added Ag precursors. We could not obtain the Ag2Se decorated Sb2Se3 nanowires with this reaction condition. The powder X-ray diffraction (XRD) pattern of the Ag2Se-decorated Sb2Se3 nanowires shows that all diffraction patterns can be indexed to the peaks of Ag2Se(JCPDS 71-2410)and Sb2Se3(JCPDS 15-0861) without any visible peak from impurities(Figure 1c).

Figure 3
figure 3

(a) TEM image of Ag2Se decorated Sb2Se3 nanowires; (b) high-resolution scanning TEM images of Figure 3(a) with the distance between the lattice planes; (c) corresponding X-ray diffraction patterns.

To find the optimum reaction condition, especially the reaction temperature, the same reaction was conducted at different temperatures. The optimum temperature of the reaction was found to be 100°C. When the temperature was increased above 150°C, the Ag precursor was sufficiently activated to oxidize the as-synthesized Sb2Se3 nanowires and resulted in Ag2Se nanoparticle-decorated Sb2Se3 nanowires (150°C, Supporting Information Figures S1a and, S1b) and more dissolved nanowires (200°C, Supporting Information Figure S1d and, S1e). The reactivity of the silver precursor decreased at 80°C and no reaction occurred. Extremely high reaction temperature at above 300°C resulted in Ag nanoparticles and a trace of Ag2Se nanoparticles (Supporting Information Figure S1f and, S1g).

The selenium and silver ratio in the reaction mixture had also a significant effect on the surface morphology of the nanowires. As the amount of Ag increased, more nanoparticles attached to the surface of the nanowires, which then turned coarse. SEM images in Supporting Information Figure S2 show the surface change from bare Sb2Se3, 200/1, 100/1 and 50/1 ratios of Se/Ag, respectively.

Interestingly, the size of Ag2Se nanoparticles on the Sb2Se3 nanowires could be also controlled by the diameter of the Sb2Se3 nanowires. An increase in the diameter of Sb2Se3 nanowires resulted in larger Ag2Se nanoparticles. The reason was unclear, however, it can be postulated that larger curvature formed from the increase in diameter of the nanowires allowed the Ag2Se nanoparticles to attach more easily onto the nanowire surface and also allowed for larger sized nanoparticles to be formed. Detailed TEM image of Ag2Se nanoparticles on nanowire surface can be found in the Supporting Information Figure S3.

In order to investigate the elemental distribution in the Ag2Se-decorated Sb2Se3 nanowires, scanning electron microscopy (SEM) and mapping analysis were performed. SEM mapping images of Se, Ag and Sb elements are shown in Figure 4. Se atoms were distributed evenly in both, the wire and dot positions. Ag distribution was restricted to the specific areas corresponding to the positions of Ag2Se nanoparticles on the Sb2Se3 nanowires. Ag atoms were found in the nanowire positions, the reason was not clear. EDX-STEM elemental analysis was performed to confirm the results. The O2-labeled areas in Supporting Information Figure S4, shows that the dot areas were composed of Ag and Se elements, while Sb and Se elements were found in the nanowire region.

Figure 4
figure 4

SEM image of the Ag2Se decorated Sb2Se3 nanowires and the corresponding EDX mapping images of Se, Ag and Sb elements, respectively.

Measurement of Photo-device

Photocurrent responses of individual nanowires using 655nm irradiation were measured under ambient conditions at room temperature. Figures 5a and b are the schematic illustration and SEM image, respectively, of a single nanowire device. To fabricate the nanowire device, the nanowires are spin-coated using nanowire/hexane solution and SiO2/Si substrate, after which, identical parallel electrode patterns with channel length of 4 μm were randomly positioned using the conventional lithography technique. The electrical properties were measured either before or after scanning a single nanowire device using SEM.

Figure 5
figure 5

Photoconductive properties of Sb2Se3 nanowires.

(a) Schematic illustration and (b) SEM image of single nanowire device. (c) I–V curves of 100 nm diameter Sb2Se3 single nanowire in the dark condition and under 655 nm light illumination (15 mW cm−2). (d) Time dependent photocurrent response of Sb2Se3 nanowire with the periodic (~10 s) incidence of 655 nm light at an applied voltage of 3 V.

Figure 5c shows the I–V curves of a single Sb2Se3 nanowire device with a diameter of 100 nm measured under dark as well as in presence of 655 nm illumination (15 mW·cm−2). Photo-response properties were measured when the light was turned on and off, at 10 sec intervals (Figure 5d). From the I–V curve, the electrical conductivity of the 100 nm diameter Sb2Se3 single nanowire in dark condition was 7.6 × 10−2Ω −1 m−1 which was similar to the bulk Sb2Se316,39. Electrical conductivity of the Sb2Se3 nanowire increased under the 655 nm irradiation condition. At 3 V bias, the current under dark conditions was 450 pA, which increased to 34 nA with illumination.

The photosensitivity factor, K was defined as K = Ilight/Idark, where, Ilight and Idark correspond to current measured with a 655 nm laser turned on and off, respectively. The K factor of the intrinsic Sb2Se3 nanowire was found to be 75, which was comparable to previous results22. (where K value was 15, with power density of 1.68 mWcm−2, under 615 nm illumination). However, the conductivity was 3 times the previous result. The Sb2Se3 nanowires used in this study, which were generated using the hot injection method were expected to be more resistant to oxidation and contained fewer surface defects (e.g. dangling bonds) than hydrothermally grown Sb2Se3 nanowire due to passivating ligands. Moreover, we used relatively thick (Ti (20 nm)/Au (100 nm)) metal electrodes with very slow deposition rates (1 ~ 2 Å/sec) and a thermal evaporator, which we presumed would allow for better Ohmic contact between the nanowire and the metal electrode.

As shown in Figure 6, time responses of three different diameters of the Sb2Se3 nanowire (100 nm, 200 nm and 400 nm) are also compared. When the Sb2Se3 wires are irradiated using same light source (655 nm, 6.5 mW·cm−2) photocurrent response increased with the diameter and the ratio of the magnitude of the photocurrent response in 100, 200 and 400 nm wires were 1, 7 and 21, respectively.

Figure 6
figure 6

Time-dependent photocurrent response of the Sb2Se3 nanowires with 3 different diameter.

(100 nm (blue), 200 nm (red) and 400 nm (black)). Light incidence is 655 nm (5.6 mWcm−2) pulsed light with a period of 30 s. Applied voltage is 3 V.

The photocurrent in the nanowire for a given photon energy can be expressed as40

In addition, the power absorbed in the photoconductor (Pabs) and the photoconductive gain (G) can be expressed as

Where, Popt is the incident optical power, I0 is the illumination intensity, A ( = πdl/2) is the exposed surface area of the nanowire and τt is the carrier transit time of the nanowire.

In bulk Sb2Se3 material, the increase in photocurrent is proportional to the nanowire surface area (1:2:4 for 100 nm:200 nm:400 nm) due to light absorption. However, in the Sb2Se3 nanowire, the increase in photocurrent was greater than the light absorbed, probably because of the increase in photoconductive gain due to the nanowire structure. In semiconductor nanowires, a depletion space charge layer forms due to the surface state and Fermi-level pinning, which allows for physical separation of the electron and the hole within the nanowire. (Supporting Information Figure S5)38,40 Until the critical diameter of the nanowire is reached, the depletion layer remains fully depleted and the recombination barrier increases. This may prolong the life of photo-generated carriers and may further increase the photocurrent as the nanowire diameter increases. (Supporting Information Figure S5).

Figures 7a shows the I–V curves of a device fabricated using Ag2Se-decorated Sb2Se3 single nanowire with 100 nm diameter, measured both in dark and under 655 nm illumination (15 mWcm−2). The photo-response property of the Ag2Se-decorated nanowire was also measured when the light was turned on and off at 10 sec intervals (Figure 7b). Compared with the pure Sb2Se3 nanowires of similar size (100 nm diameter) but without Ag2Se decoration, the current under dark conditions increased approximately 50 times (from 450 pA to 22.8 nA at 3 V) and photocurrent increased approximately 7 times (from 34 nA to 228 nA at 3 V). The K factor of Ag2Se-decorated Sb2Se3 nanowires was 10. The addition of Ag2Se decoration to the Sb2Se3 nanowire may increase conductivity under dark, due to the decrease in thickness of the depletion layer. The Fermi level of Ag2Se, a narrow band gap (0.15 eV) semiconductor with properties such as electric conductivity, superionic conductivity and giant magnetoresistance, is higher than that of Sb2Se3 (Supporting Information, Figure S6a)41,42. After the Ag2Se decoration, the electrons from the Ag2Se particles might flow to the Sb2Se3 nanowire surface, thus decreasing the thickness of the depletion layer near the surface due to charge redistribution (Supporting Information Figure S6b). When lights are irradiated, the light absorption efficiency can be increased in the Ag2Se decorated nanowires by particle induced light scattering43. The increase in light absorption and light scattering by Ag2Se may contribute to photocurrent in addition to the intrinsic photocurrent generated in the Sb2Se3 nanowire. The amount of the photocurrent in the decorated nanowire is 10 times than that of the dark current. The K factor of the decorated nanowire is smaller than undecorated nanowire (from 55 to 10) due to the thinner surface depletion layer. The decrease of the degree of surface depletion might increase the intrinsic (dark) conductivity as well as increased the portion of the charge recombination under light irradiation. However, the responsivity (Rres = Iph/Iirr A, where Iph is the background substituted photocurrent (Iillumination-Idark) and Iirr is the irradiance of the incident light and A is the area of the device) of the Sb2Se3 nanowire and Ag2Se decorated Sb2Se3 nanowire are ~560 A/W and ~3400 A/W which is very high and is due to the increased conductivity of our wet-chemically grown Sb2Se3 nanowires.

Figure 7
figure 7

Photoconductive properties of Ag2Se decorated Sb2Se3 nanowires with 100 nm diameter.

(a) I–V curves of single nanowire in the dark condition and under 655 nm light illumination (15 mWcm−2). (b) Time dependent photocurrent response nanowire with the periodic (~10 s) incidence of 655 nm light at an applied voltage of 3 V.

Discussion

In summary, we report the synthesis of diameter-controlled Sb2Se3 nanowires by injection of antimony precursor into hot solvent containing selenium precursor as well as production of Ag2Se-decorated Sb2Se3 nanowires, by adding silver precursor (AgNO3) to the nanowire solution. The presence of Ag2Se nanoparticles on the nanowires was confirmed using TEM, XRD, SEM mapping and EDX-STEM elemental analysis. Photocurrent response of individual nanowires using 655 nm light irradiation was measured under ambient conditions at room temperature. The photo-device comprising of 100 nm Sb2Se3 nanowire had an electrical conductivity of approximately 7.6 × 10−2 Ω−1 m−1 in dark conditions and K factor of 75. The current obtained in dark conditions was 450 pA, which increased to 34 nA with light. In addition, as the diameter of the Sb2Se3 nanowire increased, the photocurrent increased non-linearly. Under identical conditions, the photo-device comprising of Ag2Se-decorated 100 nm Sb2Se3 nanowires had a 50-fold increase in dark current from 450 pA to 22.8 nA and 7-fold increase of photocurrent from 34 nA to 228 nA in comparison to bare Sb2Se3 nanowires.

Methods

Materials

Selenium metal powder was purchased from Strem Chemical Inc. Oleic acid, 1-octadecene and silver nitrate were purchased from Sigma-Aldrich. All chemicals were used without further purification.

Synthesis of intrinsic Sb2Se3 nanowires

Selenium metal (0.3 mmol, 24 mg), oleic acid (0.9 mmol, 256 mg) and 10 ml of 1-octadecene were added into a 50 ml three-neck flask and the mixture was degassed under vacuum at 100°C for 1h. To prepare antimony precursor solution, 0.3 mmol of antimony chloride (68 mg, Aldrich), 0.9 mmol of stearic acid (256 mg, Aldrich) and 4 ml of 1-octadecene (Aldrich) in another flask were degassed at room temperature for 1 h and purged with nitrogen. The antimony precursor solution was quickly injected into the selenium and surfactant solution at 250°C under nitrogen atmosphere, the mixture was stirred at the same temperature for 10 min and then cooled to room temperature. Sb2Se3 nanowires were precipitated from the reaction by adding ethanol and separated by centrifugation.

Synthesis of Ag2Se decorated Sb2Se3 nanowires

To attach Ag2Se nanoparticles onto the surface of Sb2Se3 nanowires, 3 μmol of silver nitrate (0.51 μg, Aldrich) was added into the above Sb2Se3 nanowires solution at room temperature. Subsequently, the temperature was raised to 100°C, the mixture was stirred at this temperature for 90 min under nitrogen atmosphere and then cooled to room temperature. Sb2Se3 nanowires with Ag2Se nanoparticles attached were precipitated from the reaction by adding ethanol and separated by centrifugation.

Photo-device fabrication

Photodevices were fabricated from bare Sb2Se3 nanowires as well as Ag2Se-decorated Sb2Se3 nanowires. Nanowires dispersed in hexane were spread on the SiO2 (100 nm)/Si wafer and then O2 plasma ashing proceed by Reactive Ion Etching(RIE) to remove residual organic species. Photolithography and thermal evaporation techniques were used to deposit the Ti (20 nm)/Au (100 nm) electrodes in contact with the individual nanowires. The channel length of the photodevice was 4 μm. I–V and photoswitching are measured with a 655 nm laser (15 mWcm−2) at room temperature under air.

Characterization

The nanowires dispersed in hexane were spread on a copper grid and a silicon wafer for measurement using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. TEM image and STEM-EDX were taken with a FEI Tecnai G2 F30 Super-Twin transmission electron microscope operating at 300 kV. SEM image and X-ray mapping data were obtained with a JSM-6700F field emission scanning electron microscope at 30 kV operating voltage, equipped with an INCA energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) patterns were taken using a Rigaku Ultima III diffractometer equipped with a rotating anode and Cu Kα radiation source (λ = 0.15418 nm). Electrical characterization of the nanowire devices was performed using a probe station (MSTECH MST-6000C, Korea) and the Keithley SCS-4200 system.