Surface-Ligand-Controlled Enhancement of Carrier Density in Plasmonic Tungsten Oxide Nanocrystals: Spectroscopic Observation of Trap-State Passivation via Multidentate Metal Phosphonate Bonding

Abstract

The localized surface plasmon resonance (LSPR) properties of nanocrystals (NCs) allow manipulation of optical responses by controlling their morphology, free carrier density, and local dielectric environment. In this context, semiconductor NCs, in which plasmonic properties arise due to various types of doping, provide unique opportunities in tailoring LSPR properties for a wide range of applications as viable alternatives to expensive noble metal NCs. Although extensive works have been done to control the LSPR properties of semiconductor NCs via doping, the role of surface ligand chemistry in the enhancement of LSPR properties remains poorly understood. Incomplete passivation of surface atoms creates dangling bonds and surface trap states that together could compromise the free carrier density and thus optoelectronic properties. Here, we report the impact of metal–ligand bonding interactions on the free electron density (N e) and the LSPR response of monoclinic, sub-stoichiometric, and two-dimensional tungsten oxide (WO3–x ) nanoplatelets (NPLs). The LSPR properties of WO3–x NPLs arise from the presence of free electrons in the conduction band as a result of oxygen vacancies in the monoclinic crystal. In situ surface passivation of unpurified colloidal WO3–x NPLs with X-type alkylphosphonate (R-PO3 2–) produces an LSPR peak in the near-infrared region of the electromagnetic spectrum. X-ray photoelectron, electron paramagnetic, and Raman spectroscopic data support the presence of a tridentate PO3–W3 bonding motif that allows increased passivation of shallow surface trap states, leading to an experimentally determined N e value of 8.4 × 1022 cm–3. Furthermore, experimentally determined bonding characteristics are correlated with density functional theory calculations. The effect of the high N e values of NPLs on their refractive index sensitivity is also evaluated. Together, the knowledge gained regarding surface-ligand-chemistry-controlled manipulation of the plasmonic properties in semiconducting metal oxide NPLs and the high N e values of WO3–x NPLs achieved may result in sizable advancement in various LSPR-driven applications such as sensing and energy storage and conversion schemes.

Inorganic–Organic Interfacial Electronic Effects in Ligand-Passivated WO3–x Nanoplatelets Induce Tunable Plasmonic Properties for Smart Windows

“…32,33 We showed that TD-PA is a unique surface passivating ligand that not only increased the passivation sites per ligand compared to alkylcarboxylate ligands allowing greater passivation of undercoordinated near-neighbor tungsten atoms in highly deficient WO 3−x NPLs but also substantially improved the LSPR properties of colloidal WO 3−x NPLs. 30 Our selected ligands, Ethyl-PA, Phenyl-PA, and NO 2 -BA provide a wide variety of structural diversity such as length of the alkyl chain, conjugation, and electron-withdrawing substitution. Colloidal synthesis and ligand exchange of WO ) and carboxylate (−CO 2 − ) binding head groups (i.e., TD-PA and MA); (2) ligands with different backbones but an identical, −PO 3 2− binding head group (i.e., TD-PA, Phenyl-PA, and Ethyl-PA); and (3) ligands with aromatic πconjugation but two different binding head groups (i.e., NO 2 -BA and Phenyl-PA).…”

“…The lateral dimensions of NPLs are determined to be ∼14 and 4.7 nm in length and width, respectively (Figure 1B,C). Based on our recent work, 30 we do not anticipate any morphological changes in the NPL during the ligand treatment. We used purified samples to prepare thin films.…”

“…) binding head group containing ligands compared to bidentate binding carboxylate (−CO 2 − ) ligands. 30 Therefore, tridentate phosphonate-tungsten bonding provides stronger interactions than bidentate carboxylate-tungsten bonding. The increased surface passivation and stronger electronic interactions of phosphonate ligands stabilize surface W and prevent the transfer of the conduction band electrons to trap sites, thus increasing the free carrier density.…”

“…Furthermore, appropriate surface-ligand engineering provided additional means to increase free electron concentration (8.4 × 10 22 cm −3 ) and thus pushed the LSPR wavelength toward the visible region. 30 Together, ligand-induced alteration of free electrons in WO 3−x NCs is controlled by the interfacial chemical interaction between inorganic NC and organic ligands. In this context, the polarizability of the metal−ligand bond should govern the optoelectronic and dielectric properties of ligand-passivated WO 3−x NCs.…”

“…The LSPR wavelength of these NPLs can be tuned between 1200 and 1600 nm by controlling the oxygen deficiency, where a free electron concentration of 4.13 × 10 22 cm –3 provided an LSPR peak position of ∼1200 nm. Furthermore, appropriate surface-ligand engineering provided additional means to increase free electron concentration (8.4 × 10 22 cm –3 ) and thus pushed the LSPR wavelength toward the visible region . Together, ligand-induced alteration of free electrons in WO 3– x NCs is controlled by the interfacial chemical interaction between inorganic NC and organic ligands.…”