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.

Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized surface plasmon resonance (LSPR) in the infrared range of the electromagnetic spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics enabled by doping, or via electrochemical or photochemical charging. Tuning plasmonic properties through carrier density modulation suggests potential applications in smart optoelectronics, catalysis and sensing. Here, we elucidate fundamental aspects of LSPR modulation through dynamic carrier density tuning in Sn-doped InO (Sn:InO) NCs. Monodisperse Sn:InO NCs with various doping levels and sizes were synthesized and assembled in uniform films. NC films were then charged in an in situ electrochemical cell and the LSPR modulation spectra were monitored. Based on spectral shifts and intensity modulation of the LSPR, combined with optical modelling, it was found that often-neglected semiconductor properties, specifically band structure modification due to doping and surface states, strongly affect LSPR modulation. Fermi level pinning by surface defect states creates a surface depletion layer that alters the LSPR properties; it determines the extent of LSPR frequency modulation, diminishes the expected near-field enhancement, and strongly reduces sensitivity of the LSPR to the surroundings.

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.

Localized surface plasmon resonance phenomena have recently been investigated in unconventional plasmonic materials such as metal oxide and chalcogenide semiconductors doped with high concentrations of free carriers. We synthesize colloidal nanocrystals of Cs x WO 3 , a tungsten bronze in which electronic charge carriers are introduced by interstitial doping. By using varying ratios of oleylamine to oleic acid, we synthesize three distinct shapes of these nanocrystalshexagonal prisms, truncated cubes, and pseudosphereswhich exhibit strongly shape-dependent absorption features in the near-infrared region. We rationalize these differences by noting that lower symmetry shapes correlate with sharper plasmon resonance features and more distinct resonance peaks. The plasmon peak positions also shift systematically with size and with the dielectric constant of the surrounding media, reminiscent of typical properties of plasmonic metal nanoparticles.

Metallic nanostructures can manipulate light-matter interactions to induce absorption, scattering, and local heating through their localized surface plasmon resonances. Recently, plasmonic behavior of semiconductor nanocrystals has been investigated to stretch the boundaries of plasmonics farther into the infrared spectral range and to introduce unprecedented tunability. However, many fundamental questions remain regarding characteristics of plasmons in doped semiconductor nanocrystals. Field enhancement, especially near features with high curvature, is essential in many applications of plasmonic metal nanostructures, yet the potential for plasmonic field enhancement by semiconductor nanocrystals remains unknown. Here, we use the discrete dipole approximation (DDA) to understand the dependence of field enhancement on size, shape, and doping level of plasmonic semiconductor nanocrystals. Indium-doped cadmium oxide is considered as a prototypical material for which faceted cube-octohedral nanocrystals have been experimentally realized; their optical spectra are compared to our computational results. The computed extinction spectra are sensitive to changes in doping level, dielectric environment, and shape and size of the nanocrystals, providing insight for materials design. High-scattering efficiencies and efficient local heat production make 100 nm particles suitable for photothermal therapies and simultaneous bioimaging. Meanwhile, single particles and dimers of nanocrystals demonstrate strong, shape-and wavelength-dependent near-field enhancement, highlighting their potential for applications in infrared sensing, imaging, spectroscopy, and solar conversion. ■ INTRODUCTIONThe ability to localize electromagnetic waves to the size of the nano object through localized surface plasmon resonances (LSPRs) gives rise to a manifold of applications and biologyrelated challenges, such as sensing, 1−7 enhanced spectroscopies, 8−17 or photothermal therapy. 4,22,23 In metallic nanocrystals (NCs), the LSPR is mostly limited to the visible part of the spectrum and determined at the stage of synthesis. The LSPR frequency range of metallic nanostructures can be further extended to the near-infrared (NIR), but this requires larger sized particles with complex shapes such as nanorods 16 (>50 nm in length) or nanoshells 17,18 (>60 nm in diameter). The ability to modify the plasmon resonance of NCs less than 20 nm in size to precise resonant absorption lines and with resonances within the biological window in the NIR 19 would be beneficial for numerous applications, such as enhancement spectroscopies in the near-infrared, 5,6,20 sensing, or photothermal therapies. 21,22 More recently, intense interest has been focused on a new type of plasmonic nanomaterials offering exactly these tunable properties, namely doped semiconductor NCs. 23,24 These are comprised of vacancy doped semiconductors such as copper chalcogenides, 25−27 tungsten oxides, 28,29 or doped metal oxide nanocrystals. 23,24,30−32 The greatest advantage is, however, the p...

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.