Ultrathin, High-Aspect-Ratio Bismuth Sulfohalide Nanowire Bundles for Solution-Processed Flexible Photodetectors

Abstract

In this study, a novel synthesis of ultrathin, highly uniform colloidal bismuth sulfohalide (BiSX where X = Cl, Br, I) nanowires (NWs) and NW bundles (NBs) for room-temperature and solution-processed flexible photodetectors are presented. High-aspect-ratio bismuth sulfobromide (BiSBr) NWs are synthesized via a heat-up method using bismuth bromide and elemental S as precursors and 1-dodecanethiol as a solvent. Bundling of the BiSBr NWs occurs upon the addition of 1-octadecene as a co-solvent. The morphologies of the BiSBr NBs are easily tailored from sheaf-like structures to spherulite nanostructures by changing the solvent ratio. The optical bandgaps are modulated from 1.91 (BiSCl) and 1.88 eV (BiSBr) to 1.53 eV (BiSI) by changing the halide compositions. The optical bandgap of the ultrathin BiSBr NWs and NBs exhibits blueshift, whose origin is investigated through density functional theory-based first-principles calculations. Visible-light photodetectors are fabricated using BiSBr NWs and NBs via solution-based deposition followed by solid-state ligand exchanges. High photo-responsivities and external quantum efficiencies (EQE) are obtained for BiSBr NW and NB films even under strain, which offer a unique opportunity for the application of the novel BiSX NWs and NBs in flexible and environmentally friendly optoelectronic devices.

Keywords: flexible devices; nanobundles; nanocrystal inks; nanowires; semiconductors.

Figure 1

a,b) SEM and c,d) TEM images of BiSBr NWs. e) HRTEM image of BiSBr NWs along the [210] zone axis. The inset represents the FFT of the HRTEM image. f) XRD pattern of the BiSBr NWs. g) Atomistic model of orthorhombic BiSBr. h) Large‐scale synthesis of BiSBr NWs to obtain 52.29 g in a single reaction batch. The reaction was performed at 120 °C for 3 min. i) STEM image of a single BiSBr NW and EDS elemental mapping images of j) Bi, k) S, and l) Br in a single BiSBr NW.

Figure 2

a,b) SEM and c–f) TEM images of BiSBr NBs. g) HRTEM image of a single NW view along [110] zone axis. Inset represents FFT of HRTEM image. h) XRD pattern of the BiSBr NWs. i) Dark‐field STEM and EDS mapping images of j) Bi, k) S, and l) Br.

Figure 3

a) SEM and b) TEM images of BiSCl NWs. c) SEM and d) TEM images of BiSI NWs. e,f) SEM images of spherulite BiSI NBs. g) Photograph of colloidal solutions of BiSCl (orange), BiSBr (red), and BiSI (dark brown) NWs dispersed in toluene. h) Kubelka–Munk plot of BiSCl, BiSBr, and BiSI NWs. i) The crystal structure and j) the calculated band dispersion of 1 × 1 chain (left), 2 × 2 chain (middle), and bulk (right) BiSBr. The dark cyan, yellow, and green spheres in i) represent Bi, S, and Br atoms, respectively. The magenta and cyan arrows in j) represent the direct and indirect transitions, respectively. k) The calculated band gap at the optimized lattice parameters. The direct (indirect) band gap at the optimized structure of the 1 × 1 chain, 2 × 2 chain, and bulk BiSBr are denoted by the large filled (empty) markers of the blue circle, orange triangle, and green diamond, respectively. The optimized c lattice parameter (along the chain direction) is 4.110 Å, 4.068 Å, and 4.062 Å for the 1 × 1 chain, 2 × 2 chain, and bulk BiSBr, respectively. For 1 × 1 BiSBr, the evolution of direct (blue solid line) and indirect (blue dashed line) band gaps was also evaluated as a function of c considering the uniaxial strain ε from −1.5% (compressive) to 1% (tensile). The optimized lattice parameter c for 1 × 1 BiSBr (${\mathrm{c}}_{1\times 1}^{\mathrm{opt}}$) is denoted by the blue vertical dotted line.

Figure 4

SEM and TEM images of BiSBr NBs synthesized at 120 °C using ODE:DT volume ratios of a and b) 90:10, c and d) 70:30, e and f) 40:60, and g and h) 10:90, and BiSBr NBs synthesized at 150 °C using ODE:DT volume ratios of i and j) 90:10, k and l) 70:30, m and n) 40:60, and o and p) 10:90. All reactions were performed for 3 min.

Figure 5

a) Schematic of the fabrication of photodetectors by sequential electric field alignment and solid‐state ligand exchange. Current (I)–voltage (V) characteristics of flexible photodetectors fabricated using b) BiSBr NWs and c) NBs before and after 0.8% strain application under dark and illumination conditions. Photocurrent characteristics were examined under 532 nm laser illumination at a light intensity of 0.7 mW cm^{− 2}. d) I–V curves of the BiSBr NB‐based flexible photoconductor under a strain of 0.8% under dark and 532 nm laser illumination conditions at different light intensities ranging from 0.0026 to 3.25 mW cm⁻². e) Light intensity‐dependent photocurrent of the BiSBr NB‐based photoconductor under 0.8% strain and 532 nm illumination at 1 and 10 V.