Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Doped metal oxides are plasmonic materials that boast both synthetic and postsynthetic spectral tunability. They have already enabled promising smart window and optoelectronic technologies and have been proposed for use in surface enhanced infrared absorption spectroscopy (SEIRA) and sensing applications. Herein, we report the first step toward realization of the former utilizing cubic F and Sn codoped InO nanocrystals (NCs) to couple to the C-H vibration of surface-bound oleate ligands. Electron energy loss spectroscopy is used to map the strong near-field enhancement around these NCs that enables localized surface plasmon resonance (LSPR) coupling between adjacent nanocrystals and LSPR-molecular vibration coupling. Fourier transform infrared spectroscopy measurements and finite element simulations are applied to observe and explain the nature of the coupling phenomena, specifically addressing coupling in mesoscale assembled films. The Fano line shape signatures of LSPR-coupled molecular vibrations are rationalized with two-port temporal coupled mode theory. With this combined theoretical and experimental approach, we describe the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor and further explain the basis of the observed Fano line shape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption and reflection. This study therefore illustrates various factors involved in determining the LSPR-LSPR and LSPR-molecular vibration coupling for metal oxide materials and provides a fundamental basis for the design of sensing or SEIRA substrates.

Defects may tend to make crystals interesting but they do not always improve 10 performance. In doped metal oxide nanocrystals with localized surface plasmon resonance 11 (LSPR), aliovalent dopants and oxygen vacancies act as centers for ionized impurity scattering 12 of electrons. Such electronic damping leads to lossy, broadband LSPR with low quality factors,

Metal oxides, when electronically doped with oxygen vacancies, aliovalent dopants, or interstitial dopants, can exhibit metallic behavior due to the stabilization of a substantial charge carrier concentration within the material. As a result, localized surface plasmon resonances (LSPRs) occur in nanocrystals of conducting metal oxides. Through deliberate choice of both the host material and the defect, these resonances can be tuned across the entirety of the near- and mid-infrared regions of the electromagnetic spectrum. Optical modeling has revealed that the defects present have profound impacts on charge carrier mobility and electronic structure, and in some cases, choosing one dopant over another is an important trade-off for optimizing plasmonic performance. These materials are distinct from classical metals in that one can tune their LSPR in energy and intensity through their elemental composition independently of any particular size or nanocrystal morphology. In addition, the LSPR in these materials is highly modulable through external stimuli over substantial spectral windows. As a result, these materials uniquely provide a responsive plasmonic material that can offer optimal nanocrystal arrangements and morphology without compromising the intended resonance frequency for light concentration at any infrared wavelength.

Infrared-responsive doped metal oxide nanocrystals are an emerging class of plasmonic materials whose localized surface plasmon resonances (LSPR) can be resonant with molecular vibrations. This presents a distinctive opportunity to manipulate light–matter interactions to redirect chemical or spectroscopic outcomes through the strong local electric fields they generate. Here we report a technique for measuring single nanocrystal absorption spectra of doped metal oxide nanocrystals, revealing significant spectral inhomogeneity in their mid-infrared LSPRs. Our analysis suggests dopant incorporation is heterogeneous beyond expectation based on a statistical distribution of dopants. The broad ensemble linewidths typically observed in these materials result primarily from sample heterogeneity and not from strong electronic damping associated with lossy plasmonic materials. In fact, single nanocrystal spectra reveal linewidths as narrow as 600 cm−1 in aluminium-doped zinc oxide, a value less than half the ensemble linewidth and markedly less than homogeneous linewidths of gold nanospheres.

The optical extinction spectra arising from localized surface plasmon resonance in doped semiconductor nanocrystals (NCs) have intensities and lineshapes determined by free charge carrier concentrations and various mechanisms for damping the oscillation of those free carriers. However, these intrinsic properties are convoluted by heterogeneous broadening when measuring the spectra of ensembles. We reveal that the traditional Drude approximation is not equipped to fit spectra from a heterogeneous ensemble of doped semiconductor NCs and produces fit results that violate Mie scattering theory. The heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by accounting for ensemble heterogeneity and near-surface depletion. The HEDA model is applied to tin-doped indium oxide NCs for a range of sizes and doping levels, but we expect it to be employed for any isotropic plasmonic particles in the quasistatic regime. It captures individual NC optical properties and their contributions to the ensemble spectra, thereby enabling the analysis of intrinsic NC properties from an ensemble measurement. Quality factors for the average NC in each ensemble are quantified and found to be notably higher than those of the ensemble. Carrier mobility and conductivity derived from the HEDA fits matches that reported in the bulk thin-film literature.

The electronic structures of n-type ZnO nanocrystals formed via photochemical reduction and by aliovalent doping with aluminum are investigated using timedependent density functional theory. Connections between the density functional theory results and a simple quantummechanical particle-in-a-spherical-potential model are highlighted. Molecular orbitals obtained from density functional theory reveal the often-invoked S-, P-, D-, ... type "super" orbitals used to characterize the absorption spectra of these materials. ■ INTRODUCTIONColloidal semiconductor quantum dots (QDs) containing excess delocalized charge carriers play important roles in the development of devices for solar energy conversion, 1,2 IR plasmonics, 3−8 information processing, 9 and other technologies. Such n-or p-type semiconductor QDs have been prepared using remote doping, 10−15 photodoping, 3,13,16−22 aliovalent doping, 5,21,23−28 or electrochemical oxidation and reduction. 29−31 In most cases, aliovalent doping of colloidal semiconductor nanocrystals to yield band-like charge carriers has proven difficult because only a small fraction of dopant ions lead to charge carriers. 21,28,32 Recently, high-quality colloidal Al 3+ -doped ZnO (Al 3+ :ZnO) nanocrystals have been reported in which Al 3+ acts as an ionized shallow donor. 26 In these Al 3+ :ZnO nanocrystals, electronic absorption spectroscopy reveals excess band-like electrons, similar to those in photodoped ZnO (e − :ZnO) nanocrystals. 3,17−20,22,33−35 Explicit comparison of the electron paramagnetic resonance (EPR) and electron absorption spectra of Al 3+ :ZnO and e − :ZnO nanocrystals shows the two species are nearly indistinguishable. 21 Despite these similarities, however, they show qualitatively different chemical reactivity; Al 3+ :ZnO is completely stable against oxidation by O 2 , whereas e − :ZnO rapidly oxidizes when exposed to air. 13,[16][17][18]36,37 Consequently, while it is possible to determine the number of conduction band (CB) electrons per nanocrystal in photodoped ZnO nanocrystals via anaerobic titration with mild oxidants, 17,19,34 the stability of CB electrons in Al 3+ :ZnO nanocrystals prevents such characterization. Instead, the number of CB electrons in Al 3+ :ZnO nanocrystals has been estimated via EPR and absorption spectroscopies. NIR absorption increases, for example, as more electrons are added to the ZnO nanocrystals or as more Al 3+ is incorporated. 21Here we report the theoretical characterization of the lowenergy (ultraviolet/visible/near-infrared) electronic transitions of photodoped ZnO and Al 3+ :ZnO QDs using time-dependent hybrid density functional theory (TDDFT). We examine the electronic structures of the QDs using DFT, comparing the density of states for the two types of n-type QDs. We explore the connection between our computed DFT results and the simple quantum mechanical particle in a spherical potential model. 19,38,39 This theoretical characterization allows for direct comparison of the electronic structures of these two systems, offering uniq...

The tunability of the localized surface plasmon resonances of doped metal oxides also impact their thermal relaxation.