Direct observation of narrow mid-infrared plasmon linewidths of single metal oxide nanocrystals

Vapor Infiltration Synthesis of Indium Sulfide Magic Size Cluster

The energetically favorable formation of atomically precise clusters, known as magic size clusters, in the solution phase enables a precision nanoscale synthesis with exquisite uniformity. We report the synthesis of magic size clusters via vapor infiltration of atomic layer deposition precursors directly in a polymer thin film. Sequential infiltration of trimethylindium vapor and hydrogen sulfide gas into poly(methyl methacrylate) leads to the formation of clusters with uniform properties consistent with a magic size cluster─In6S6(CH3)6. While an increase in cluster size might be expected with additional sequential infiltration cycles of the reactive In and S precursors, uniform properties consistent with magic size clusters form in multiple polymers under a range of processing conditions. Ultraviolet-visible absorption spectra of In6S6(CH3)6 are largely independent of the number of sequential infiltration cycles and exhibit air stability, both of which are attributed to an energetically favorable synthetic pathway that is evaluated with density functional theory.

Surface-Enhanced Infrared Absorption Spectroscopy by Resonant Vibrational Coupling with Plasmonic Metal Oxide Nanocrystals

Coupling between plasmonic resonances and molecular vibrations in nanocrystals (NCs) offers a promising approach for detecting molecules at low concentrations and discerning their chemical identities. Metallic NC superlattices can enhance vibrational signals under far-field detection by generating a myriad of intensified electric field hot spots between the NCs. Yet, their effectiveness is limited by the fixed electron concentration dictated by the metal composition and inefficient hot spot creation due to the large mode volume. Doped metal oxide NCs, such as tin-doped indium oxide (ITO), could overcome these limitations by enabling broad tunability of resonance frequencies in the mid-infrared range through independent variation of size and doping concentration. This study investigates the potential of close-packed ITO NC monolayers for surface-enhanced infrared absorption by quantifying trends in the coupling between their plasmon modes and various molecular vibrations. We show that maximum vibrational signal intensity occurs in monolayers composed of larger, more highly doped NCs, where the plasmon resonance peak lies at higher frequency than the molecular vibration. Using finite element and mutual polarization methods, we establish that near-field enhancement is stronger on the low-frequency side of the plasmon resonance and for more strongly coupled plasmonic NCs, thus rationalizing the design rules we experimentally uncovered. Our results can guide the development of optimal metal oxide NC-based superstructures for sensing target molecules or modifying their chemical properties through vibrational coupling.

Direct Determination of Carrier Parameters in Indium Tin Oxide Nanocrystals

We develop here a comprehensive experimental approach to independently determine charge carrier parameters, namely, carrier density and mass, in plasmonic indium tin oxide nanocrystals. Typically, in plasmonic nanocrystals, only the ratio between these two parameters is accessible through optical absorption experiments. The multitechnique methodology proposed here combines single particle and ensemble optical and magneto-optical spectroscopies, also using 119Sn solid-state nuclear magnetic resonance spectroscopy to probe the surface depletion layer. Our methodology overcomes the limitations of standard fitting approaches based on absorption spectroscopy and ultimately gives access to carrier effective mass directly on the NCs, discarding the use of literature value based on bulk or thin film materials. We found that mass values depart appreciably from those measured on thin films; consequently, we found carrier density values that are different from reported literature values for similar systems. The effective mass was found to deviate from the parabolic approximation at a high carrier density. Finally, the dopant activation and defect diagram for ITO NCs for tin doping between 2.5 and 15% are determined. This approach can be generalized to other plasmonic heavily doped semiconductor nanostructures and represents, to the best of our knowledge, the only method to date to characterize the full Drude parameter space of 0-D nanosystems.

Ultranarrow Mid-infrared Quantum Plasmon Resonance of Self-Doped Silver Selenide Nanocrystal

The infrared quantum plasmon resonance (IR QPR) of nanocrystals (NCs) exhibits the combined properties of classical and quantum mechanics, potentially overcoming the limitations of conventional optical features. However, research on the development of localized surface plasmon resonance (LSPR) from colloidal quantum dots has stagnated, owing to the challenge of increasing the carrier density of semiconductor NCs. Herein, we present the mid-IR QPR of a self-doped Ag2Se NC with an exceptionally narrow bandwidth. Chemical modification of the NC surface with chloride realizes this narrow QPR bandwidth by achieving a high free-carrier density in the NC. The mid-IR QPR feature was thoroughly analyzed by using various experimental methods such as Fourier transform (FT) IR spectroscopy, X-ray photoelectron spectroscopy, and current-voltage measurements. In addition, the optical properties were theoretically analyzed using the plamon-in-a-box model and a modified hydrodynamic model that revealed the effect of coupling with the intraband transition and the limited nature of electron density in semiconductor NCs. Integrating the quantum effect into the plasmonic resonance reduces the peak bandwidth to 19.7 meV, which is an extremely narrow bandwidth compared with that of the LSPR of conventional metal oxide or metal chalcogenide NCs. Our results demonstrate that self-doped silver selenide quantum dots are excellent systems for studying mid-IR QPR.

Hierarchically Doped Plasmonic Nanocrystal Metamaterials

Assembling plasmonic nanocrystals in regular superlattices can produce effective optical properties not found in homogeneous materials. However, the range of these metamaterial properties is limited when a single nanocrystal composition is selected for the constituent meta-atoms. Here, we show how continuously varying doping at two length scales, the atomic and nanocrystal scales, enables tuning of both the frequency and bandwidth of the collective plasmon resonance in nanocrystal-based metasurfaces, while these features are inextricably linked in single-component superlattices. Varying the mixing ratio of indium tin oxide nanocrystals with different dopant concentrations, we use large-scale simulations to predict the emergence of a broad infrared spectral region with near-zero permittivity. Experimentally, tunable reflectance and absorption bands are observed, owing to in- and out-of-plane collective resonances. These spectral features and the predicted strong near-field enhancement establish this multiscale doping strategy as a powerful new approach to designing metamaterials for optical applications.

Optical Modeling of Plasmonic Nanoparticles with Electronically Depleted Layers

Doped metal oxide (MO) nanocrystals (NCs) are well-known for the localized surface plasmon resonance in the infrared range generated by free electrons in the conduction band of the material. Owing to the intimate connection between plasmonic features and the NC's carrier density profile, proper modeling can unveil the underlying electronic structure. The carrier density profile in MO NCs is characterized by the presence of an electronically depleted layer as a result of the Fermi level pinning at the surface of the NC. Moreover, the carrier profile can be spatially engineered by tuning the dopant concentrations in core-shell architectures, generating a rich plethora of plasmonic features. In this work, we systematically studied the influence of the simulation parameters used for optical modeling of representative experimental absorption spectra by implementing multilayer models. We highlight in particular the importance of minimizing the fit parameters by support of experimental results and the importance of interparameter relationships. We show that, in all cases investigated, the depletion layer is fundamental to correctly describe the continuous spectra evolution. We foresee that this multilayer model can be used to design the optoelectronic properties of core-shell systems in the framework of energy band and depletion layer engineering.

Colloidal Approaches to Zinc Oxide Nanocrystals

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Simultaneous Imaging of Dopants and Free Charge Carriers by Monochromated EELS

Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO₃ (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a previously unobserved plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage by EELS and transport measurements on the bulk single crystals of BLSO. These results not only represent a practical approach for studying heterogeneities in solids and understanding structure-property relationships at the nanoscale, but also demonstrate the possibility of infrared plasmon tuning by leveraging nanoscale doping texture.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Shape- and Size-Dependent Refractive Index Sensing and SERS Performance of Gold Nanoplates

Plasmonic sensors are promising for ultrasensitive chemical and biological analysis. Gold nanoplates (Au NPLs) show unique geometrical structures with high ratios of surface to bulk atoms, which display fascinating plasmonic properties but require optimization. This study presented a systematic investigation of the influence of different parameters (shape, aspect ratio, and resonance mode) on localized surface plasmon resonance properties, refractive index (RI, n) sensitivities, and surface-enhanced Raman scattering (SERS) enhancement ability of different types of Au NPLs through finite-difference time-domain (FDTD) simulations. As a proof of concept, triangular, circular, and hexagonal Au NPLs with varying aspect ratios were fabricated via a three-step seed-mediated growth method by the experiment. Both FDTD-simulated and measured experimental results confirm that the RI sensitivities increase with the aspect ratio. Furthermore, choosing a lower order resonance mode of Au NPLs benefits higher RI sensitivities. The SERS enhancement abilities of Au NPLs also predicted to be highly dependent on the shape and aspect ratio. The triangular Au NPLs showed the highest SERS enhancement ability, while it drastically decreased for circular Au NPLs after the rounding process. The SERS enhancement ability gradually became more intense as the hexagonal Au NPLs overgrown on circular Au NPLs with increasing volumes of HAuCl₄ solution. The results are expected to help develop effective biosensors.

Control of electronic band profiles through depletion layer engineering in core-shell nanocrystals

Fermi level pinning in doped metal oxide (MO) nanocrystals (NCs) results in the formation of depletion layers, which affect their optical and electronic properties, and ultimately their application in smart optoelectronics, photocatalysis, or energy storage. For a precise control over functionality, it is important to understand and control their electronic bands at the nanoscale. Here, we show that depletion layer engineering allows designing the energetic band profiles and predicting the optoelectronic properties of MO NCs. This is achieved by shell thickness tuning of core-shell Sn:In₂O₃-In₂O₃ NCs, resulting in multiple band bending and multi-modal plasmonic response. We identify the modification of the band profiles after the light-induced accumulation of extra electrons as the main mechanism of photodoping and enhance the charge storage capability up to hundreds of electrons per NC through depletion layer engineering. Our experimental results are supported by theoretical models and are transferable to other core-multishell systems as well.

Deep image restoration for infrared photothermal heterodyne imaging

Infrared photothermal heterodyne imaging (IR-PHI) is an all-optical table top approach that enables super-resolution mid-infrared microscopy and spectroscopy. The underlying principle behind IR-PHI is the detection of photothermal changes to specimens induced by their absorption of infrared radiation. Because detection of resulting refractive index and scattering cross section changes is done using a visible (probe) laser, IR-PHI exhibits a spatial resolution of ∼300 nm. This is significantly below the mid-infrared diffraction limit and is unlike conventional infrared absorption microscopy where spatial resolution is of order ∼5μm. Despite having achieved mid-infrared super-resolution, IR-PHI's spatial resolution is ultimately limited by the visible probe laser's diffraction limit. This hinders immediate application to studying samples residing in spatially congested environments. To circumvent this, we demonstrate further enhancements to IR-PHI's spatial resolution using a deep learning network that addresses the Abbe diffraction limit as well as background artifacts, introduced by experimental raster scanning. What results is a twofold improvement in feature resolution from 300 to ∼150 nm.

Imaging Infrared Plasmon Hybridization in Doped Semiconductor Nanocrystal Dimers

Carrier-doped semiconductor nanocrystals (NCs) offer strong plasmonic responses at frequencies beyond those accessible by conventional plasmonic nanoparticles. Like their noble metal analogues, these emerging materials can harness free space radiation and confine it to the nanoscale but at resonance frequencies that are natively infrared and spectrally tunable by carrier concentration. In this work we combine monochromated STEM-EELS and theoretical modeling to investigate the capability of colloidal indium tin oxide (ITO) NC pairs to form hybridized plasmon modes, providing an additional route to influence the IR plasmon spectrum. These results demonstrate that ITO NCs may have greater coupling strength than expected, emphasizing their potential for near-field enhancement and resonant energy transfer in the IR region.

Antimony Nanopolyhedrons with Tunable Localized Surface Plasmon Resonances for Highly Effective Photoacoustic-Imaging-Guided Synergistic Photothermal/Immunotherapy

Antimony (Sb), a typical group VA semimetal, has rarely been studied both experimentally and theoretically in plasmonic photothermal therapy, possibly due to the lack of effective morphology-controllable methods for the preparation of high-quality Sb nanocrystals. In this study, an effective ligand-guided growth strategy to controllably synthesize Sb nanopolyhedrons (Sb NPHs) with ultrahigh photothermal conversion efficiency (PTCE), good photothermal stability, as well as biocompatibility is presented. Furthermore, the modulation effect of different morphologies on localized surface plasmon resonance (LSPR) of Sb NPHs in experimentation is successfully observed. When the resonance frequency of the Sb NPHs is matched well with the excitation wavelength (808 nm), the PTCE of the Sb NPHs is as high as 62.1%, which is noticeably higher compared to most of the reported photothermal agents. The Sb NPHs also exhibit good photothermal stability. In addition, Sb-NPHs-based multifunctional nanomedicines are further constructed via loading 1-methyl-d-tryptophan on PEGylated Sb NPHs for a highly efficient photoacoustic-imaging-guided synergistic photothermal/immune-therapy of tumors in vivo. This work can stimulate further theoretical and experimental investigations of Sb NPHs and other semimetal nanomaterials regarding their LSPR properties and inspire various potential applications of semimetals in biomedicine and sensors.

M. M. de Almeida J, Pastoriza-Santos I, C. C. Coelho L. Advances in Plasmonic Sensing at the NIR-A Review

Surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) are among the most common and powerful label-free refractive index-based biosensing techniques available nowadays. Focusing on LSPR sensors, their performance is highly dependent on the size, shape, and nature of the nanomaterial employed. Indeed, the tailoring of those parameters allows the development of LSPR sensors with a tunable wavelength range between the ultra-violet (UV) and near infra-red (NIR). Furthermore, dealing with LSPR along optical fiber technology, with their low attenuation coefficients at NIR, allow for the possibility to create ultra-sensitive and long-range sensing networks to be deployed in a variety of both biological and chemical sensors. This work provides a detailed review of the key science underpinning such systems as well as recent progress in the development of several LSPR-based biosensors in the NIR wavelengths, including an overview of the LSPR phenomena along recent developments in the field of nanomaterials and nanostructure development towards NIR sensing. The review ends with a consideration of key advances in terms of nanostructure characteristics for LSPR sensing and prospects for future research and advances in this field.

Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion

The greenhouse effect arises when thermal radiation is forced to undergo absorption and re-emission many times before escaping, while sunlight transmits largely unimpeded. Although this effect is responsible for global warming, it is generally weak in solid-state materials because radiation can be easily overpowered by other modes of heat transfer. Here, we report on the use of infrared plasmonic nanoparticles to enhance the greenhouse effect in transparent mesoporous materials. Local surface plasmon resonances in transparent conducting oxide nanoparticles (TCO NPs) selectively shorten the mean free path of thermal photons while maintaining high solar transmittance. The addition of a small amount of TCO NPs (<0.1% by volume) nearly halves the heat losses at 700 °C. This leads to an experimentally demonstrated effective thermal emittance of ∼0.17 at 700 °C, which is the lowest reported value to date, among all selective surfaces and transparent insulating materials measured at 650 °C or above. Our results show that plasmon-enhanced greenhouse selectivity (PEGS) is a promising mechanism for spectral control of radiative heat transfer, and more specifically, for conversion of minimally concentrated sunlight into high-temperature heat.

Dual-Mode Infrared Absorption by Segregating Dopants within Plasmonic Semiconductor Nanocrystals

When aliovalent dopants are sufficiently segregated to the core or near the surface of semiconductor nanocrystals, charge carriers donated by the dopants are also segregated to the core or near the surface, respectively. In Sn-doped indium oxide nanocrystals, we find that this contrast in free charge carrier concentration creates a core and shell with differing dielectric properties and results in two distinctly observable plasmonic extinction peaks. The trends in this dual-mode optical response with shell growth differ from core/shell nanoparticles composed of traditional plasmonic metals such as Au and Ag. We developed a model employing a core/shell effective medium approximation that can fit the dual-mode spectra and explain the trends in the extinction response. Lastly, we show that dopant segregation can improve sensitivity of plasmon spectra to changes in refractive index of the surrounding environment.

Magnetoplasmon Resonances in Semiconductor Nanocrystals: Potential for a New Information Technology Platform

Interaction between light and plasmon oscillations in semiconductor nanocrystals has received significant attention in recent years driven, in part, by the possibility of coupling between plasmonic and semiconducting properties. Such coupling could lead to a variety of new applications in plasmonics, photonics, and optoelectronics. In this Concept we discuss the methods for generation of localized surface plasmon resonances in colloidal semiconductor nanocrystals and their unique magneto-optical properties. Different means of introducing free charge carriers, including aliovalent doping, non-stoichiometry, and external charging, are first compared and contrasted. The resulting plasmons can be manipulated using circularly polarized light and external magnetic field, allowing for the formation of the magnetoplasmon modes. The concept of using these magnetoplasmon modes as a new degree of freedom for controlling excitonic states and charge-carrier polarization is introduced and discussed. We also highlight some notable recent examples of controlling plasmon-exciton interactions and comment on their implications for future research in sustainable information technology.

Approaches to mid-infrared, super-resolution imaging and spectroscopy

This perspective highlights recent advances in super-resolution, mid-infrared imaging and spectroscopy. It provides an overview of the different near field microscopy techniques developed to address the problem of chemically imaging specimens in the mid-infrared "fingerprint" region of the spectrum with high spatial resolution. We focus on a recently developed far-field optical technique, called infrared photothermal heterodyne imaging (IR-PHI), and discusses the technique in detail. Its practical implementation in terms of equipment used, optical geometries employed, and underlying contrast mechanism are described. Milestones where IR-PHI has led to notable advances in bioscience and materials science are summarized. The perspective concludes with a future outlook for robust and readily accessible high spatial resolution, mid-infrared imaging and spectroscopy techniques.

Multidimensional Nanoparticle Characterization through Ion Mobility-Mass Spectrometry

Multidimensional techniques that combine fully or partially orthogonal characterization methods in a single setup often provide a more comprehensive description of analytes. When applied to nanoparticles, they have the potential to reveal particle properties not accessible to more conventional 1D techniques. Herein, we apply recently developed 2D characterization techniques to nanoparticles using atmospheric-pressure ion mobility-mass spectrometry (IM-MS), and we demonstrate the analytical capability of this approach using ultraporous mesostructured silica nanoparticles (UMNs). We show that IM-MS yields a 2D particle size-mass distribution function, which in turn can be used to calculate not only important 1D distributions, i.e. particle size distributions, but also nanoparticle structural property distributions not accessible by other methods, including size-dependent particle porosity and the specific pore volume distribution function. IM-MS measurement accuracy was confirmed by measurement of NIST-certified polystyrene latex particle standards. For UMNs, comparison of IM-MS results with TEM and N₂ physisorption yields quantitative agreement in particle size and qualitative agreement in average specific pore volume. IM-MS uniquely shows how within a single UMN population, porosity increases with increasing particle size, consistent with the proposed UMN growth mechanism. In total, we demonstrate the potential of IM-MS as a standard approach for the characterization of structurally complex nanoparticle populations, as it yields size-specific structural distribution functions.

Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm^-1 for 2.5-3.9-μm-long NWs reveal a series of resonances due to the Fabry-Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry-Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry-Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Properties, fabrication and applications of plasmonic semiconductor nanocrystals

We highlight three widely explored oxide-based plasmonic materials, including H_xMoO_3−y, H_xWO_3−y, and Mo_xW_1−xO_3−y, and their applications in catalysis.

Plasmonic fiber optic hydrogen sensor using oxygen defects in nanostructured molybdenum trioxide film

Hydrogen is one of the most promising candidates for fulfilling the next energy demands in transportation, aerospace, heating, and power generation. Due to its highly explosive nature, hydrogen leakage sensors are considered a critical industrial need. We propose a room-temperature, high-sensitivity hydrogen sensor using oxygen defect-induced plasmonic features. The proposed sensing probe utilizes nanostructured α-MoO₃ thin film as the sensing material in which free carriers and plasmonic properties are induced in response to hydrogen exposure. A notable blue spectral shift of 70.6 nm is observed in response to hydrogen gas exposure from 150 ppm to 2000 ppm, which confirms the sensor's capability for efficient detection of low hydrogen concentrations. The sensor's sensitivity, linearity, and reversibility are experimentally investigated through a simple optical setup.

ITO@SiO2 and ITO@{M6Br12}@SiO2 (M = Nb, Ta) nanocomposite films for ultraviolet-near infrared shielding

Transparent optical thin films for energy saving applications have recently gained substantial prominence for functional window processes. In this study, highly visible transparent nanocomposite films with ultraviolet (UV) and near-infrared (NIR) blocking capabilities are reported. Such nanocomposite films, prepared by electrophoretic deposition on ITO-coated glass, are composed of indium tin oxide (ITO) nanocrystals (9 nm) and octahedral metal atom clusters (1 nm, Nb₆ or Ta₆) embedded into silica nanoparticles (∼80 nm). The functional silica nanoparticles were prepared by a reverse microemulsion process. The microstructural characterization proved that ITO nanocrystals are centered in the silica nanoparticles, whereas the metal atom clusters are homogeneously distributed in the silica matrix. The optical absorption spectra of these transparent nanocomposite films exhibit distinct and complementary contributions from their ITO nanoparticles and metal atom clusters (absorption in the UV range) and from the ITO layer on silica.

Clear and transparent nanocrystals for infrared-responsive carrier transfer

Infrared-light-induced carrier transfer is a key technology for ‘invisible’ optical devices for information communication systems and energy devices. However, clear and colourless photo-induced carrier transfer has not yet been demonstrated in the field of photochemistry, to the best of our knowledge. Here, we resolve this problem by employing short-wavelength-infrared (1400–4000 nm) localized surface plasmon resonance-induced electron injection from indium tin oxide nanocrystals to transparent metal oxides. The time-resolved infrared measurements visualize the dynamics of the carrier in this invisible system. Selective excitation of localized surface plasmon resonances causes hot electron injection with high efficiency (33%) and long-lived charge separation (~ 2–200 μs). We anticipate our study not only provides a breakthrough for plasmonic carrier transfer systems but may also stimulate the invention of state-of-the-art invisible optical devices.

Infrared-light-induced carrier transfer is a key technology for ‘invisible’ optical devices, but making materials with the right properties remains a challenge. Here, the authors fabricate a clear and colourless material which converts infrared light to an electrical signal or energy based on a localized surface plasmon resonance, with implications for the development of invisible optical devices.

Identifying site-dependent reactivity in oxidation reactions on single Pt particles

IR nanospectroscopy measurements revealed the influence of oxidizing reaction conditions on the reactivity of different surface sites on Pt particles.

Catalytic nanoparticles are heterogeneous in their nature and even within the simplest particle various surface sites exist and influence the catalytic reactivity. Thus, detailed chemical information at the nanoscale is essential for understanding how surface properties and reaction conditions direct the reactivity of different surface sites of catalytic nanoparticles. In this work, hydroxyl-functionalized N-heterocyclic carbene molecules (NHCs) were anchored to the surface of Pt particles and utilized as chemical markers to detect reactivity variations between different surface sites under liquid and gas phase oxidizing conditions. Differences in the chemical reactivity of surface-anchored NHCs were identified using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 20 nanometers. By conducting IR nanospectroscopy measurements, along with complementary spatially averaged IR and X-ray spectroscopy measurements, we identified that enhanced reactivity occurred on the particles' periphery under both gas and liquid phase oxidizing conditions. Under gas phase reaction conditions the NHCs' hydroxyl functional groups underwent preferential oxidization to the acid along the perimeter of the particle. Exposure of the sample to harsher, liquid phase oxidizing conditions induced modification of the NHCs, which was mostly identified at the particle's periphery. Analysis of X-ray absorption spectroscopy measurements revealed that exposure of the sample to oxidizing conditions induced aromatization of the NHCs, presumably due to oxidative dehydrogenation reaction, along with reorientation of the NHCs from perpendicular to parallel to the Pt surface. These results, based on single particle measurements, demonstrate the high reactivity of surface sites that are located at the nanoparticle's periphery and the influence of reaction conditions on site-dependent reactivity.

Nanoscale Optical Microscopy and Spectroscopy Using Near-Field Probes

Light-matter interactions can provide a wealth of detailed information about the structural, electronic, optical, and chemical properties of materials through various excitation and scattering processes that occur over different length, energy, and timescales. Unfortunately, the wavelike nature of light limits the achievable spatial resolution for interrogation and imaging of materials to roughly λ/2 because of diffraction. Scanning near-field optical microscopy (SNOM) breaks this diffraction limit by coupling light to nanostructures that are specifically designed to manipulate, enhance, and/or extract optical signals from very small regions of space. Progress in the SNOM field over the past 30 years has led to the development of many methods to optically characterize materials at lateral spatial resolutions well below 100 nm. We review these exciting developments and demonstrate how SNOM is truly extending optical imaging and spectroscopy to the nanoscale.

Plasmon-induced carrier polarization in semiconductor nanocrystals

Spintronics ¹ and valleytronics ² are emerging quantum electronic technologies that rely on using electron spin and multiple extrema of the band structure (valleys), respectively, as additional degrees of freedom. There are also collective properties of electrons in semiconductor nanostructures that potentially could be exploited in multifunctional quantum devices. Specifically, plasmonic semiconductor nanocrystals^3-10 offer an opportunity for interface-free coupling between a plasmon and an exciton. However, plasmon-exciton coupling in single-phase semiconductor nanocrystals remains challenging because confined plasmon oscillations are generally not resonant with excitonic transitions. Here, we demonstrate a robust electron polarization in degenerately doped In₂O₃ nanocrystals, enabled by non-resonant coupling of cyclotron magnetoplasmonic modes ¹¹ with the exciton at the Fermi level. Using magnetic circular dichroism spectroscopy, we show that intrinsic plasmon-exciton coupling allows for the indirect excitation of the magnetoplasmonic modes, and subsequent Zeeman splitting of the excitonic states. Splitting of the band states and selective carrier polarization can be manipulated further by spin-orbit coupling. Our results effectively open up the field of plasmontronics, which involves the phenomena that arise from intrinsic plasmon-exciton and plasmon-spin interactions. Furthermore, the dynamic control of carrier polarization is readily achieved at room temperature, which allows us to harness the magnetoplasmonic mode as a new degree of freedom in practical photonic, optoelectronic and quantum-information processing devices.

Nanomaterial-Based Plasmon-Enhanced Infrared Spectroscopy

Surface-enhanced infrared absorption (SEIRA) has attracted increasing attention due to the potential of infrared spectroscopy in applications such as molecular trace sensing of solids, polymers, and proteins, specifically fueled by recent substantial developments in infrared plasmonic materials and engineered nanostructures. Here, the significant progress achieved in the past decades is reviewed, along with the current state of the art of SEIRA. In particular, the plasmonic properties of a variety of nanomaterials are discussed (e.g., metals, semiconductors, and graphene) along with their use in the design of efficient SEIRA configurations. To conclude, perspectives on potential applications, including single-molecule detection and in vivo bioassays, are presented.

Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO Nanocrystals

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonance in the mid-infrared. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the low, free-electron density affects the E-field localization and plasmon coupling in NC films when compared to metal nanoparticles (NP). In this article, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface-attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide NC.

ITO nanoparticles enhanced upconversion luminescence in Er3+/Yb3+-codoped silica glasses

Upconversion (UC) materials have shown many applications in the solar cell industry, biomedical imaging, and LED lighting. For the first time, we report enhanced UC in Er³⁺/Yb³⁺-codoped silica glasses induced by the energy transfer between rare earth ions and indium tin oxide nanoparticles (ITO NPs), introduced by an in situ growth approach. The enhancements of the intensities of the emissions of red and green light were all more than 10 fold and in some cases up to 42 fold. This work in our opinion has contributed a novel method and materials for UC enhancement in Er³⁺/Yb³⁺-codoped silica glasses.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

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.

Minimum Line Width of Surface Plasmon Resonance in Doped ZnO Nanocrystals

The optical response of ZnO nanocrystals (NCs) doped with Al (Ga) impurities is calculated using a model that incorporates the effects of quantum confinement, dielectric mismatch, surface, and ionized impurity scattering. For dopant concentrations of a few percent, the NC polarizability is dominated by a localized surface plasmon resonance (LSPR) in the infrared (IR) which follows the Drude-Lorentz law for NC diameter above ∼10 nm but is strongly blue-shifted for smaller diameters due to quantum confinement effects. The intrinsic width of the LSPR peak is calculated in order to characterize plasmon losses induced by ionized impurity scattering. Widths below 80 meV are found in the best cases, in agreement with the lowest values recently measured on single NCs. These results confirm that doped ZnO NCs are very promising for the development of IR plasmonics. The width of the LSPR peak strongly increases when dopants are placed near the surface of the NCs or when additional fixed charges are present.

Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals

Dynamic optical control of infrared (IR) transparency and refractive index is achieved using boron-doped silicon nanocrystals excited with mid-IR optical pulses. Unlike previous silicon-based optical switches, large changes in transmittance are achieved without a fabricated structure by exploiting strong light coupling of the localized surface plasmon resonance (LSPR) produced from free holes of p-type silicon nanocrystals. The choice of optical excitation wavelength allows for selectivity between hole heating and carrier generation through intraband or interband photoexcitation, respectively. Mid-IR optical pumping heats the free holes of p-Si nanocrystals to effective temperatures greater than 3500 K. Increases of the hole effective mass at high effective hole temperatures lead to a subpicosecond change of the dielectric function, resulting in a redshift of the LSPR, modulating mid-IR transmission by as much as 27%, and increasing the index of refraction by more than 0.1 in the mid-IR. Low hole heat capacity dictates subpicosecond hole cooling, substantially faster than carrier recombination, and negligible heating of the Si lattice, permitting mid-IR optical switching at terahertz repetition frequencies. Further, the energetic distribution of holes at high effective temperatures partially reverses the Burstein-Moss effect, permitting the modulation of transmittance at telecommunications wavelengths. The results presented here show that doped silicon, particularly in micro- or nanostructures, is a promising dynamic metamaterial for ultrafast IR photonics.

Radial Dopant Placement for Tuning Plasmonic Properties in Metal Oxide Nanocrystals

Doped metal oxide nanocrystals that exhibit tunable localized surface plasmon resonances (LSPRs) represent an intriguing class of nanomaterials that show promise for a variety of applications from spectroscopy to sensing. LSPRs arise in these materials through the introduction of aliovalent dopants and lattice oxygen vacancies. Tuning the LSPR shape and energy is generally accomplished through controlling the concentration or identity of dopants in a nanocrystal, but the lack of finer synthetic control leaves several fundamental questions unanswered regarding the effects of radial dopant placement, size, and nanocrystalline architecture on the LSPR energy and damping. Here, we present a layer-by-layer synthetic method for core/shell nanocrystals that permits exquisite and independent control over radial dopant placement, absolute dopant concentration, and nanocrystal size. Using Sn-doped In₂O₃ (ITO) as a model LSPR system, we synthesized ITO/In₂O₃ core/shell as well as In₂O₃/ITO core/shell nanocrystals with varying shell thickness, and investigated the resulting optical properties. We observed profound influence of radial dopant placement on the energy and linewidth of the LSPR response, noting (among other findings) that core-localized dopants produce the highest values for LSPR energies per dopant concentration, and display the lowest damping in comparison to nanocrystals with shell-localized or homogeneously distributed dopants. Inactive Sn dopants present on ITO nanocrystal surfaces are activated upon the addition of a subnanometer thick undoped In₂O₃ shell. We show how LSPR energy can be tuned fully independent of dopant concentration, relying solely on core/shell architecture. Finally, the impacts of radial dopant placement on damping, independent of LSPR energy, are explored.

Shape Controlled Plasmonic Sn Doped CdO Colloidal Nanocrystals: A Synthetic Route to Maximize the Figure of Merit of Transparent Conducting Oxide

The synthesis of different anisotropic shaped (eight different shapes) Sn⁴⁺ doped CdO (Sn:CdO) colloidal nanocrystals (NCs) by precise tuning of precursor reactivity and proper choice of capping agent is reported. In all these systems, formation of Sn:CdO quantum dots (QDs) of 2-3 nm is identified at very early stage of reaction. The colloidally stable QDs act as a continuous source for the formation of primary nanoparticles that can be transformed selectively into specific type of nanoparticle morphology. The specific facet stabilization of fcc (face centered cubic)CdO is predicted by particular choice of ligand. Fine tuning of plasmonic absorbance band can be achieved by variation of Sn⁴⁺ doping concentration. Different anisotropic Sn:CdO NCs exhibit interesting shape dependent plasmonic absorbance features in NIR region. High quality crack free uniform dense thin film has been deposited on glass substrate to make high quality transparent conducting oxide (TCO) coatings. figure of merit of TCO can be maximized as high as 0.523 Ω^-1 with conductivity of 43 600 S cm^-1 and visible transmittance of ≈85% which is much higher than commercially available tin doped indium oxide and other transparent electrodes.

Control of the compensating defects in Al-doped and Ga-doped ZnO nanocrystals for MIR plasmonics

The MIR plasmon of degenerate ZnO nanocrystals is tuned by the direct control of oxygen during their synthesis.

Defect Engineering in Plasmonic Metal Oxide Nanocrystals

Defects may tend to make crystals interesting but they do not always improve performance. In doped metal oxide nanocrystals with localized surface plasmon resonance (LSPR), aliovalent dopants and oxygen vacancies act as centers for ionized impurity scattering of electrons. Such electronic damping leads to lossy, broadband LSPR with low quality factors, limiting applications that require near-field concentration of light. However, the appropriate dopant can mitigate ionized impurity scattering. Herein, we report the synthesis and characterization of a novel doped metal oxide nanocrystal material, cerium-doped indium oxide (Ce:In2O3). Ce:In2O3 nanocrystals display tunable mid-infrared LSPR with exceptionally narrow line widths and the highest quality factors observed for nanocrystals in this spectral region. Drude model fits to the spectra indicate that a drastic reduction in ionized impurity scattering is responsible for the enhanced quality factors, and high electronic mobilities reaching 33 cm(2)V(-1) s(-1) are measured optically, well above the optical mobility for tin-doped indium oxide (ITO) nanocrystals. We investigate the microscopic mechanisms underlying this enhanced mobility with density functional theory calculations, which suggest that scattering is reduced because cerium orbitals do not hybridize with the In orbitals that dominate the bottom of the conduction band. Ce doping may also reduce the equilibrium oxygen vacancy concentration, further enhancing mobility. From the absorption spectra of single Ce:In2O3 nanocrystals, we determine the dielectric function and by simulation predict strong near-field enhancement of mid-IR light, especially around the vertices of our synthesized nanocubes.