Stoichiometric Doping of Highly Coupled Cu_2–xS Nanocrystal Assemblies

Abstract

The hole density of individual copper sulfide nanocrystals (Cu2–x S NCs) is determined from the stoichiometric mismatch (x) between copper and sulfide atoms. Consequently, the electronic properties of the material vary over a range of x. To exploit Cu2–x S NCs in devices, assemblies of NCs are typically required. Herein, we investigate the influence of x, referred to as the stoichiometric doping effect, on the structural, optical, electrical, and thermoelectric properties of electronically coupled Cu2–x S NC assemblies. The doping process is done by immersing the solid NC assemblies into a solution containing a Cu­(I) complex for different durations (0–10 min). As Cu+ gradually occupied the copper-deficient sites of Cu2–x S NCs, x could be controlled from 0.9 to less than 0.1. Consequently, the near-infrared (NIR) absorbance of Cu2–x S NC assemblies changes systematically with x. With increasing x, electrical conductivity increased and the Seebeck coefficient decreased systematically, leading to the maximal thermoelectric power factor from a film of Cu2–x S NCs at an optimal doping condition yielding x = 0.1. The physical characteristics of the Cu2–x S NC assemblies investigated herein will provide guidelines for exploiting this emerging class of nanocrystal system based on doping.

Electrochemical lithium doping of Cu2−xS nanocrystal assemblies for tuning their near infrared absorbance

“…The LSPR peak appearing at ∼1400 nm in the NIR region indicates abundant holes in the Cu 1.1 S NCs created because of the stoichiometry-dependent self-doping. 29 The X-ray diffraction (XRD) pattern of the as-synthesized Cu 1.1 S NCs matches that reported for Cu 1.3 S NCs by Manna et al 4 or Cu 1.1 S NCs by Kang et al , 28 which is ascribed to a set of peaks negatively shifted from the covellite phase (CuS) (Fig. 1c).…”

“…The Cu 2− x S NCs used in this study were synthesized using a hot-injection method based on a modified protocol described previously (step 1). 28 Copper( i ) chloride and sulfur powder were used as the copper and sulfur precursors, respectively. OAm was used as the reaction solvent as well as the ligands attached to the NC surface.…”

“…Cu 1.1 S NCs were synthesized by modifying a previously described method. 3,28 Copper(I) chloride (118.8 mg) was added to OAm (24 mL) in a three-neck round-bottom flask followed by degassing to form a copper precursor solution. Separately, sulfur powder (48.1 mg) was added to OAm (15 mL) in a three-neck roundbottom flask and degassed to form a sulfur precursor solution.…”

“…Furthermore, the ligand removal/exchange process for enhancing the electronic coupling between NCs can be conducted prior to post-treatment, which is critical for exploiting materials for thinfilm optoelectric devices. 27 For example, Lee et al 28 demonstrated that the LSPR properties or thermoelectric/electric properties of Cu 2Àx S NC assemblies can be tuned using a chemical agent that either provides Cu + to the lattice (e.g., tetrakiscopper hexafluorophosphate, [Cu(CH 3 CN) 4 ]PF 6 ) or extracts Cu + from the lattice (e.g., cerium ammonium nitrate, (NH 4 ) 2 [Ce(NO 3 ) 6 ]). By changing the duration of the chemical post-treatment, the physical properties of the Cu 2Àx S NC assemblies, including their LSPR characteristics, could be varied systematically with the contents of Cu + .…”

“…Furthermore, the ligand removal/exchange process for enhancing the electronic coupling between NCs can be conducted prior to post-treatment, which is critical for exploiting materials for thin-film optoelectric devices. 27 For example, Lee et al 28 demonstrated that the LSPR properties or thermoelectric/electric properties of Cu 2− x S NC assemblies can be tuned using a chemical agent that either provides Cu + to the lattice ( e.g. , tetrakiscopper hexafluorophosphate, [Cu(CH 3 CN) 4 ]PF 6 ) or extracts Cu + from the lattice ( e.g.…”