Surface Chemistry of Ternary Nanocrystals: Engineering the Deposition of Conductive NaBiS₂ Films

Abstract

The ability to engineer the surface chemistry of complex ternary nanocrystals is critical to their successful application in photovoltaic, thermoelectric, and other energy conversion devices. For many years, several studies have shed light into the surface chemistry of unary and binary semiconductor nanocrystals, as well as their surface modification with monodentate and multidentate ligands in a variety of applications. In contrast, our understanding of the surface chemistry and ligand modification of ternary and other complex multinary nanocrystals remains relatively limited. Recently, our group reported the synthesis of colloidal NaBiS2 semiconductor nanocrystals with sizes tunable between 2-60 nm, and a light absorption edge of ca. 1.4 eV. Here, we use a combination of infrared and nuclear magnetic resonance spectroscopies to show that the as-made NaBiS2 nanocrystals are capped by oleylamine and neodecanoate ligands. We investigate biphasic liquid-liquid exchange as a means to replace these native ligands with either carboxylate-terminated lipoic acid or with small iodide ligands, leading in both cases to solubility in polar solvents-such as methanol, water, and dimethylformamide. We also investigate a layerby-layer, biphasic solid-liquid exchange approach to prepare films of NaBiS2 nanocrystals capped with halide ligands-iodide, bromide, chloride. Upon exchange and removal of the native ligands, we show that the resistance of NaBiS2 nanocrystal films greatly decreases, with their measured conductivity being comparable to that of films made of isostructural PbS nanocrystals, which have been used in solar cells. Lastly, we report the first solar cell device made of NaBiS2 nanocrystal films with a limited power conversion efficiency (PCE) of 0.07. Further nanostructuring and ligand optimization may enable the preparation of much more efficient energy conversion devices based on NaBiS2 as well as other non-toxic and Earth-abundant, biocompatible multinary semiconductors.

Photoconductive Thin Films Composed of Environmentally Benign AgBiS2 Nanocrystal Inks Obtained through a Rapid Phase Transfer Process

“…From the dark I – V curves of these lateral devices (Figure a), the conductivity (σ) was obtained by the relation σ = ρ –1 , with ρ = R ( A / L ), R is the resistance in ohms, L is the channel length between contacts, and A is the cross-sectional film area defined by the film thickness (∼94 nm) and channel width. The films showed σ ∼ 3 × 10 –8 S/cm, comparable to early PbS QD films generated in SSLE processes with dithiols and recently published work on dip-coated, ternary NaBiS 2 QD thin films . A modest photocurrent of ∼1 nA at 10 V bias was observed under wide-area steady-state illumination at 444 nm (power density ∼2.7 mW/cm 2 ).…”

Ternary ACd4P3 (A = Na, K) Nanostructures via a Hydride Solution-Phase Route

“…Recent reports on the colloidal synthesis of LiZnPn (Pn = P, Sb) , and NaBiE 2 (E = S, Se) , nanocrystals, as well as solid-state alkali zinc antimonides , rely on highly reducing conditions and reactants. Inspired by this approach, we have synthesized ACd 4 P 3 (A = Na, K) nanostructures by injecting CdMe 2 into a hot solution of the corresponding, commercially available alkali metal hydride (AH) in tri-n-octylphosphine (TOP) at 240 °C, followed by heating at 330 °C (Scheme ).…”

“…Determining an appropriate balance between synthetic conditions and precursor reactivity is key for successful ternary formation . Understanding this balance can help decode the complexity of nucleation, growth, and phase evolution of multinary nanocrystals. , Examples include I–IV–VII 3 (halide perovskites), − I–III–VI 2 , − and CZTS. − Relatively few examples exist of multinary phosphides, including LiZnV (V = P, Sb) nanocrystals, , and doped Ni x Co 2– x P, Co x Fe 2– x P, and Fe x Ni 2– x P nanocrystals.…”

Solution-Grown Ternary Semiconductors: Nanostructuring and Stereoelectronic Lone Pair Distortions in I–V–VI2 Materials

“…All spectra also show vibrational modes that are characteristic of oleylamine bound to the nanocrystal surface, including a ν(NH 2 ) stretch at ∼3300 cm −1 , a (N−H) bending shoulder at ∼1600 cm −1 , and a ν(C−N) stretch at ∼1100 cm −1 . 21 In contrast to free oleylamine, which displays two ν s (NH 2 ) and ν as (NH 2 ) frequencies at IR, the presence of the single stretch indicates the NH 2 headgroup may be hydrogen bonded with nearby ligands�such as S 2− or COO − �that may also be present on the nanocrystal surface. 21,61 In addition to hydrocarbons and amine, the IR spectra of the as-made NaSbS 2 , NaBiS 2 , and NaBiSe 2 nanocrystals�but not LiSbSe 2 and NaSbSe 2 , which were prepared in the absence of carboxylates�show two strong bands in the ∼1400−1650 cm −1 range, which are characteristic of carboxylate vibrational modes.…”

“…58 Alternatively, this slightly off stoichiometry could be due to the presence of small amounts of Sb-containing ligands, such as unreacted SbPh 3 , which may remain bound to the NaBiS 2 nanocrystal surface�see solidstate NMR measurements below. 21,50,59 Nanocrystals of NaSbSe 2 show a cuboidal morphology with a wider diameter distribution of 140 ± 120 nm. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) provides valuable information about the thermal stability of APnCh 2 nanocrystals.…”