Thiocyanate-Capped Nanocrystal Colloids: Vibrational Reporter of Surface Chemistry and Solution-Based Route to Enhanced Coupling in Nanocrystal Solids

Abstract

Ammonium thiocyanate (NH(4)SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH(4)SCN is also effective in replacing ligands on NCs after their assembly into the solid state. The spectroscopic properties of this ligand provide unprecedented insight into the chemical and electronic nature of the surface of the NCs. Spectra indicate that the thiocyanate binds to metal sites on the NC surface and is sensitive to atom type and NC surface charge. The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 10(3) Ω(-1) cm(-1) for Au NC thin films deposited from solution. NH(4)SCN treatment of PbTe NC films increases the conductivity by 10(13), allowing the first Hall measurements of nonsintered NC solids, with Hall effect mobilities of 2.8 ± 0.7 cm(2)/(V·s). Thiocyanate-capped CdSe NC thin films form photodetectors exhibiting sensitive photoconductivity of 10(-5) Ω(-1) cm(-1) under 30 mW/cm(2) of 488 nm illumination with I(photo)/I(dark) > 10(3) and form n-channel thin-film transistors with electron mobilities of 1.5 ± 0.7 cm(2)/(V·s), a current modulation of >10(6), and a subthreshold swing of 0.73 V/decade.

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

“…The exchange was performed in nitrogen atmosphere based on our previously published procedure 16,20 , and optimized for each individual batch of NCs. Typically, 1 ml of NH 4 SCN ranging in concentration from 100 to 250 mM in acetone was combined with 2 ml of CdSe NCs dispersed in hexane at an optical density of 5-10 cm À 3 at the first excitionic absorption feature.…”

“…To prepare the NC dispersion for spincoating, monodisperse CdSe NCs with as-synthesized long insulating ligands were treated in solution with ammonium thiocyanate in a nitrogen glovebox 16,20 , replacing the long ligands with the compact thiocyanate ligands, while maintaining solution dispersibility. The thiocyanate-exchanged NCs were redispersed in dimethylformamide and spincast atop the flexible substrates to form uniform, crack-free, randomly close-packed NC thin-film semiconducting channels.…”

“…Until recently, this has posed a significant challenge to using these colloidal inks as technologically viable electronic materials for devices 12 and integrated circuitry 13 . Advances in ligand chemistry have shown that the original, long ligands used in synthesis can be replaced by shorter inorganic ligands [14][15][16][17] either in solution, and still maintain solution dispersibility and thin-film processability, or in thin-film solids. These novel ligands preserve the discrete, size-dependent features of quantum confinement and enhance electronic coupling between the NCs in thin films.…”

Materials processing strategies for colloidal quantum dot solar cells: advances, present-day limitations, and pathways to improvement

“…Steps toward this vision including the rapidly advancing field of QDs terminated, in the solution phase, using short capping ligands, examples of which include metal chalcogenide complexes [40,53,54] and simple chalcogenide ions [55] by the Talapin group, thiocyanates [56] by Kagan and Murray, and short alkylthiols, with a trialkylsilylated chalcogenide-driven ligand exchange, by the Milliron group. [57] Excellent field-effect mobilities (1-10 cm 2 /V s range) have been reported, as have promising photoluminescence behavior.…”

An Enhanced UV–Vis–NIR an d Flexible Photodetector Based on Electrospun ZnO Nanowire Array/PbS Quantum Dots Film Heterostructure

“…However, the study on the photodetection performance of such heterostructures is still rare. Moreover, to maintain the stability of QDs in solvents, very long and insulating ligands are used to cap the surface of QDs,28 which would hinder the charge transfer between NWs and QDs 29…”