Abstract Optical Characterization of Localized Surface Plasmon Resonances in Doped Metal Oxide Nanocrystals by Robert Walker Johns Doctor of Philosophy in Chemistry University of California, Berkeley Professor Tanja Cuk, Co-Chair Professor Delia Milliron, Co-Chair Electronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, for metallic behavior. The ways in which the electronic structure of the semiconductor and of the dopants used to make them metallic hybridize substantially changes the plasmonic behavior. Choice of dopant element, dopant placement within the nanocrystal, and dopant interaction with other defects in the lattice all lead to changes in the observed optical properties of these nanocrystals. Here, the methods for optical characterization of LSPRs in doped metal oxides are discussed with particular attention directed at how undetermined heterogeneous contributions to ensemble measurements lead to misattributing inhomogenous broadening to poor plasmonic performance. Electronic damping in these materials is incredibly low compared to coinage metals, and they tout the added benefit of spectral tuning through chemical composition rather than morphology. The result is a class of materials that can both have their optical response tuned separately from other application relevant factors like nanocrystal size, and yield high performance LSPR for directing far-field radiation to the near-field. Learning that doped metal oxides have high quality factor LSPR was found through the first single nanocrystal measurements of LSPR made in the mid-IR through the use of near-field optics to interrogate these nanocrystals separately. The result was uncovering substantial nanocrystal-to-nanocrystal variation within batches of nanocrystals making ensemble measurements appear to have broad LSPR, while in fact these materials have high quality factors individually. These measurements were enabled by broadband synchrotron based scattering type- scanning near field optical microscopy (s-SNOM). Broadband s-SNOM in the IR can yield the single nanocrystal optical spectrum and dielectric function of an isolated signal nanocrystal when the proper considerations are made to backgrounding signal over such a wide spectral range. The methodology as well as new understanding of the materials learned through this instrumentation advance are outlined. Finally, the lessons learned about the properties of LSPR in doped metal oxides from single nanocrystal measurements are extended to an adaptation of applying Mie theory to the nanocrystal dielectric function in order to assign reasonable dielectric constants to nanocrystals even from ensemble optical measurements over any energy range, not just those obtained from mid-IR s-SNOM. Further, these advances in assigning optical density to an ensembles of doped metal oxide nanocrystals are applied to understanding how energy relaxation out of the LSPRs occurs in these materials through the use of the two-temperature model, using constants obtained from NIR ultrafast transient absorption measurements. The low free carrier concentrations of metal oxide nanocrystals lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide nanocrystals may be ideal for applications wherein untoward heat generation may disrupt the application’s overall performance, such as solar energy conversion and photonic gating.

Plasmonically active materials have the unique ability to use photons to drive a collective multi-electronic oscillatory response. On the nanoscale, this plasmon response gives rise to absorption features previously unseen in bulk materials. This brilliant optical effect has been seen for centuries; suspensions of metallic nanocrystals have been used as a way to achieve beautiful coloration in glassware and art. The nature of this phenomena has only recently been explained in the last century, however, the physics behind the relaxation of electrons driven by this response, and how to exploit them, still desire elucidation. Here, the energetic pathways of electronic absorption and relaxation in plasmonically-active doped semiconductor nanocrystals are studied using spectroscopic and computational methods. We explore the material-dependent properties of the localized surface plasmon resonance in doped metal-oxide nanoparticles, and how to optimize a material for a desired effect. We find that compared to their metallic counterparts, metal oxide nanoparticles have the unique ability to absorb near-infrared light while elevating their electrons to exceedingly high energies. The intense changes in electronic temperatures result in various optical and thermal changes necessary for applications such as electron transfer, biological phototherapies, and optical switching. Next, observable variations to the material’s extinction profile driven by plasmon excitation, whether absorption or reflectivity, are detected using ultrafast spectroscopic methods. The changes are due to alterations in the nanocrystal’s dielectric function due to heating of its electronic and lattice temperatures. We are able to successfully model the ultrafast response of these materials by determining several material constants, that allows us to predict how different materials will behave under plasmon excitation. Lastly, utilization of these plasmonically-active charge carriers for photocatalytic processes is explored. Knowledge of the physics behind how plasmonically-driven electrons respond to photoexcitation allows us to confidently move forward complexing these semiconductors with organic molecules with the goal of directing electron and/or hole transfer with low-energy photons. We find there is much to explore in this area, as the preliminary data suggests plasmonically-enhanced multiphoton absorption by organic semiconductors. The fundamentals of plasmon resonances in semiconductor nanoparticles is vast, yet current research, including this work, suggests their future as a photoactive material is bright

Plasmonic nanostructure materials have been widely investigated recently because of their considerable potential for applications in biological and chemical sensors, nano-optical devices and photothermal therapy. Compared to metal nanocrystals (NCs), doped semiconductor NCs with tunable localized surface plasmon resonance (LSPR) from near-infrared (NIR) mid-infrared (MIR) region bring more opportunities to the applications of plasmonics. Magnetoplasmonic nanostructures which could be utilized in multifunctional devices also have attracted attention due to the combination of plasmonic and magnetic properties and the manipulation of light with external magnetic fields. In this research, indium oxide (In2O3) as a typical n-type semiconductor with high mobility and carrier concentration is selected as the host lattice for doping, and molybdenum (Mo) and tungsten (W) which are transition metal elements from the same group as dopants. Colloidal molybdenum-doped indium oxide (IMO) NCs and tungsten-doped indium oxide (IWO) NCs with varying doping concentrations have been successfully synthesized, and their plasmonic and magneto-optical properties have been explored. Similarities and differences between IMO NCs and IWO NCs were discussed. Both IMO and IWO NCs have shown good tunability of plasmon resonance in the MIR range approximately from 0.22 eV to 0.34 eV. 9.2 % IMO NCs show the strongest LSPR at 0.34 eV and the maximum free electron concentration of 1.1x1020 cm-3, and 1.5 % IWO NCs exhibit the strongest LSPR at 0.33 eV with the free electron concentration of 0.94x1020 cm-3. The magneto-optical properties were studied by magnetic circular dichroism (MCD) spectroscopy. The variable-temperature-variable-field MCD spectra that coincide with the band gap absorption, indicate the excitonic splitting in the NCs. A robust MCD intensity at room temperature suggests intrinsic plasmon-exciton coupling and carrier polarization induced by plasmon, which might be phonon-mediated. A decrease in MCD signal with temperature and the saturation-like field dependence of MCD intensity for IMO and IWO NCs may related to the different oxidation states of the dopant ions since the reduced 5+ oxidation states can exhibit the Curie-type paramagnetism. IMO and IWO NCs show the coupling between exciton and plasmon in a single-phase which opens a possibility for their application in electronics and photonics. Moreover, magnetoplasmonic modes provide a new degree of freedom for controlling carrier polarization at room temperature in practical photonic, optoelectronic and quantum-information processing devices.

Degenerately doped semiconductor nanocrystals (NC) exhibit a localized surface plasmon resonance (LSPR) that falls in the near- to mid-IR range of the electromagnetic spectrum. Unlike metal, the metal oxide LSPR characteristics can be further tuned by doping, and structural control, or by in situ electrochemical or photochemical charging. Here, we illustrate how intrinsic NC attributes like its crystal structure, shape and size, along with band structure and surface properties affects the LSPR properties and its possible applications. First, the interplay of NC shape and the intrinsic crystal structure on the LSPR was studied using model systems of In:CdO and Cs:WO3, the latter of which has an intrinsic anisotropic crystal structure. For both systems, a change of shape from spherical to faceted NCs led to as anticipated higher near field enhancements around the particle. However, with Cs:WO3, presence of an anisotropic hexagonal crystal structure, leads to additional strong LSPR band-splitting into two distinct peaks with comparable intensities. Second, plasmon-molecular vibration coupling, as a proof of concept for sensing applications, was shown using newly developed F and Sn codoped In2O3 NCs to couple to the C-H vibration of surface-bound oleate ligands. A combined theoretical and experimental approach was employed to describe the observed plasmon-plasmon coupling, the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor, and the observed Fano lineshape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption, and reflection. Third, plasmon modulation through dynamic carrier density tuning was investigated using thin films of monodisperse ITO NCs with various doping level and sizes along with an in situ electrochemical setup. From the combination of the in-situ spectroelectrochemical analysis and optical modeling, it was found that often-neglected semiconductor properties, such as band structure modification upon doping and surface chemistry, strongly affect the LSPR modulation behavior. The influence of band structure and effects like Fermi level pinning by surface defect states were shown to cause a surface depletion layer that alters the LSPR properties, namely the extent of LSPR modulation, near field enhancement, and sensitivity of the LSPR to the surrounding.

Colloidal Metal Oxide Nanoparticles: Synthesis, Characterization and Applications is a one-stop reference for anyone with an interest in the fundamentals, synthesis and applications of this interesting materials system. The book presents a simple, effective and detailed discussion on colloidal metal oxide nanoparticles. It begins with a general introduction of colloidal metal oxide nanoparticles, then delves into the most relevant synthesis pathways, stabilization procedures, and synthesis and characterization techniques. Final sections discuss promising applications, including bioimaging, biosensing, diagnostic, and energy applications—i.e., solar cells, supercapacitors and environment applications—i.e., the treatment of contaminated soil, water purification and waste remediation. Provides the most comprehensive resource on the topic, from fundamentals, to synthesis and characterization techniques Presents key applications, including biomedical, energy, electronic and environmental Discusses the most relevant techniques for synthesis, patterning and characterization

Nanoplasmonics is a young topic of research, which is part of nanophotonics and nano-optics. Nanoplasmonics concerns to the investigation of electron oscillations in metallic nanostructures and nanoparticles. Surface plasmons have optical properties, which are very interesting. For instance, surface plasmons have the unique capacity to confine light at the nanoscale. Moreover, surface plasmons are very sensitive to the surrounding medium and the properties of the materials on which they propagate. In addition to the above, the surface plasmon resonances can be controlled by adjusting the size, shape, periodicity, and materials' nature. All these optical properties can enable a great number of applications, such as biosensors, optical modulators, photodetectors, and photovoltaic devices. This book is intended for a broad audience and provides an overview of some of the fundamental knowledges and applications of nanoplasmonics.

This book highlights the theoretical foundations of and experimental techniques in photothermal heating and applications involving nanoscale heat generation using gold nanostructures embedded in various media. The experimental techniques presented involve a combination of nanothermometers doped with rare-earth atoms, plasmonic heaters and near-field microscopy. The theoretical foundations are based on the Maxwell’s and heat diffusion equations. In particular, the working principle and application of AlGaN:Er3+ film, Er2O3 nanoparticles and β-NaYF4:Yb3+,Er3+ nanocrystals for nanothermometry based on Er3+ emission are discussed. The relationship between superheated liquid and bubble formation for optically excited nanostructures and the effects of the surrounding medium and solution properties on light absorption and scattering are presented. The application of Er2O3 and β-NaYF4:Yb3+,Er3+ nanocrystals to study the temperature of optically heated gold nanoparticles is also presented. In closing, the book presents a new thermal imaging technique combining near-field microscopy and Er3+ photoluminescence spectroscopy to monitor the photothermal heating and steady-state sub-diffraction local temperature of optically excited gold nanostructures.