Localized Surface Plasmon Resonance Modulation Research Articles

Redox Chemistries for Vacancy Modulation in Plasmonic Copper Phosphide Nanocrystals.

Copper phosphide (Cu3-xP) nanocrystals are promising materials for nanoplasmonics due to their substoichiometric composition, enabling the generation and stabilization of excess delocalized holes and leading to localized surface plasmon resonance (LSPR) absorption in the near-IR. We present three Cu-coupled redox chemistries that allow postsynthetic modulation of the delocalized hole concentrations and corresponding LSPR absorption in colloidal Cu3-xP nanocrystals. Changes in the structural, optical, and compositional properties are evaluated by powder X-ray diffraction, electronic absorption spectroscopy, 31P magic-angle spinning solid-state nuclear magnetic resonance spectroscopy, and elemental analysis. The redox chemistries presented herein can be used to access nanocrystals with LSPR energies of 660-890 meV, a larger range than has been possible through synthetic tuning alone. In addition to utilizing previously reported redox chemistries used for copper chalcogenide nanocrystals, we show that the largest structural and LSPR modulation is achieved using a divalent metal halide and trioctylphosphine. Specifically, nanocrystals treated with zinc iodide and trioctylphosphine have the smallest unit-cell volume (295.2 Å3) reported for P63cm Cu3-xP, indicating more Cu vacancies than have been previously observed. Overall, these redox chemistries present valuable insight into controlling the optical and structural properties of Cu3-xP.

Dynamically modulating the mid-infrared localized surface plasmon resonance of Al-doped ZnO nanocrystals

The localized surface plasmon resonance (LSPR) regulation of semiconductor oxide nanocrystals (NCs) in the near-infrared region (0.8–2.5 μm) of the solar spectrum has attracted much attention for its potential in applications such as electrochromism and sensing. However, the LSPR modulation properties of doped semiconductor oxide NCs in the mid-infrared region (2.5–25 μm) have rarely been reported. Here, we synthesized aluminum-doped zinc oxide (AZO) NCs with various nominal Al doping contents (1–20at%). The LSPR absorption peaks of these films were regulated between ∼3.8 and 6.57 μm depending on the doping content. AZO NC films with 1at% nominal Al doping and a 1316 nm film thickness exhibited the highest electrochemical infrared absorption regulation, achieving 35.9% regulation at 3–5 μm and 32.5% at 7.5–13 μm. The infrared absorption regulation mechanism of the AZO NCs involved changes in the LSPR absorption intensity caused by electron injection/extraction into/from the surface depletion layer of the AZO NCs. After applying different voltages, the AZO NC film on the Al substrates exhibited different apparent temperatures, demonstrating its potential applications in infrared displays and camouflage.

Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals.

Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR), revealing fundamental aspects of their photophysics and enabling dynamic optical applications. We synthesized and chemically reduced dopant-segregated Sn-doped In2O3 NCs, investigating the influence of radial dopant segregation on LSPR modulation and near-field enhancement (NFE). We found that core-doped NCs show large LSPR shifts and NFE change during chemical titration, enabling broadband modulation in LSPR energy of over 1000 cm-1 and of peak extinction over 300%. Simulations reveal that the evolution of the LSPR spectra during chemical reduction results from raising the surface Fermi level and increasing the donor defect density in the shell region. These results establish dopant segregation as a useful strategy to engineer the dynamic optical modulation in plasmonic semiconductor NC heterostructures going beyond what is possible with conventional plasmonic metals.

Investigating the Role of Surface Depletion in Governing Electron-Transfer Events in Colloidal Plasmonic Nanocrystals

Doped metal oxide nanocrystals (NCs) attract immense attention because of their ability to exhibit a localized surface plasmon resonance (LSPR) that can be tuned extensively across the infrared region of the electromagnetic spectrum. LSPR tunability triggered through compositional and morphological changes during synthesis (size, shape, and doping percentage) is becoming well-established, while the principles underlying dynamic, postsynthetic modulation of LSPR are not as well understood. Recent reports have suggested that the presence of a depletion layer on the NC surface may be instrumental in governing the LSPR modulation of doped metal oxide NCs. Here, we employ postsynthetic electron transfer to colloidal Sn-doped In2O3 NCs with varying sizes and Sn doping concentrations to investigate the role of the depletion layer in LSPR modulation. By measuring the maximum change in the LSPR frequency after NC reduction, we determine that a large initial volume fraction of the depletion layer in NCs results in a broad modulation of the LSPR energy and intensity. Utilizing a mathematical Drude fitting model, we track the changes in the electron density and the depletion-layer volume fraction throughout the chemical doping process, offering fundamental insights into the intrinsic NC response resulting from such electron-transfer events. We observe that the maximum change in electron density that can be induced through chemical doping is independent of Sn concentration, and subsequently, the maximum total electron density in the presence of excess reductant is independent of the NC diameter and is dependent only on the as-synthesized Sn doping concentration. This study establishes the central role that surface depletion plays in the electronic changes occurring in the NCs during postsynthetic doping, and the results will be instrumental in advancing the understanding of optical and electrical properties of different colloidal plasmonic NCs.

Bio-electron transfer modulated localized surface plasmon resonance biosensing with charge density monitoring

The analysis of reactant at different regions of the bioreaction interface is significant for the study on the influence of interface condition on bioreaction. In this study, we proposed a localized surface plasmon resonance (LSPR) biosensing platform for local charge density monitoring and corresponding analytes detection based on the bio-electron transfer modulation of plasmon resonance. Core-shell nanocomposites of polyaniline coated gold nanoparticles were synthesized for the enhanced sensitivity of plasmon resonance to applied electric potential. Tin-doped indium oxide (ITO) substrates modified with the nanocomposites were used as LSPR chip for optical and electrochemical measurements simultaneously. The charge sensitivity of LSPR was verified with external electric potential modulation theoretically and experimentally. Through layer-by-layer self-assembly immobilization of glucose oxidase (GOD) on the LSPR chips, the charge transfer monitoring during the bioreaction of glucose catalysis was further demonstrated based on the bio-electron transfer modulation of LSPR. By equivalent circuit method, the charge density of the LSPR chip were detected with optical extinction peak shifts, and the limit of detection was about 0.51 μC/cm2. This bio-electron transfer modulated LSPR provides a promising approach for the detection of spatial charge densities and the evaluation of bioreaction substances at different region of single chip.

Direct Monitoring of Cell Membrane Vesiculation with 2D AuNP@MnO2 Nanosheet Supraparticles at the Single‐Particle Level

AbstractWe herein demonstrate robust two‐dimensional (2D) UFO‐shaped plasmonic supraparticles made of gold nanoparticles (AuNPs) and MnO2 nanosheets (denoted as AMNS‐SPs) for directly monitoring cell membrane vesiculation at the single‐particle level. Because the decorated MnO2 nanosheets are ultrathin (4.2 nm) and have large diameters (230 nm), they are flexible enough for deformation and folding for parceling of the AuNPs during the endocytosis process. Correspondingly, the surrounding refractive index of the AuNPs increases dramatically, which results in a distinct red‐shift of the localized surface plasmon resonance (LSPR). Such LSPR modulation provides a convenient and accurate means for directly monitoring the dynamic interactions between 2D nanomaterials and cell membranes. Furthermore, for the endocytosed AMNS‐SPs, the subsequent LSPR blue‐shift induced by etching effects of reducing molecules is promising for exploring the local environment redox states at the single‐cell level.

Direct Monitoring of Cell Membrane Vesiculation with 2D AuNP@MnO2 Nanosheet Supraparticles at the Single‐Particle Level

We herein demonstrate robust two-dimensional (2D) UFO-shaped plasmonic supraparticles made of gold nanoparticles (AuNPs) and MnO2 nanosheets (denoted as AMNS-SPs) for directly monitoring cell membrane vesiculation at the single-particle level. Because the decorated MnO2 nanosheets are ultrathin (4.2 nm) and have large diameters (230 nm), they are flexible enough for deformation and folding for parceling of the AuNPs during the endocytosis process. Correspondingly, the surrounding refractive index of the AuNPs increases dramatically, which results in a distinct red-shift of the localized surface plasmon resonance (LSPR). Such LSPR modulation provides a convenient and accurate means for directly monitoring the dynamic interactions between 2D nanomaterials and cell membranes. Furthermore, for the endocytosed AMNS-SPs, the subsequent LSPR blue-shift induced by etching effects of reducing molecules is promising for exploring the local environment redox states at the single-cell level.

Noble Metal Nanoparticle-Based Multicolor Immunoassays: An Approach toward Visual Quantification of the Analytes with the Naked Eye.

Noble metal nanoparticle-based colorimetric sensors have become powerful tools for the detection of different targets with convenient readout. Among the many types of nanomaterials, noble metal nanoparticles exhibit extraordinary optical responses mainly due to their excellent localized surface plasmon resonance (LSPR) properties. The absorption spectrum of the noble metal nanoparticles was mostly in the visible range. This property enables the visual detection of various analytes with the naked eye. Among numerous color change modes, the way that different concentrations of targets represent vivid color changes has been brought to the forefront because the color distinction capability of normal human eyes is usually better than the intensity change capability. We review the state of the art in noble metal nanoparticle-based multicolor colorimetric strategies adopted for visual quantification by the naked eye. These multicolor strategies based on different means of morphology transformation are classified into two categories, namely, the etching of nanoparticles and the growth of nanoparticles. We highlight recent progress on the different means by which biocatalytic reactions mediated LSPR modulation signal generation and their applications in the construction of multicolor immunoassays. We also discuss the current challenges associated with multicolor colorimetric sensors during actual sample detection and propose the future development of next-generation multicolor qualification strategies.

Competition between Depletion Effects and Coupling in the Plasmon Modulation of Doped Metal Oxide Nanocrystals.

Degenerately doped semiconductor nanocrystals (NCs) exhibit strong light-matter interactions due to localized surface plasmon resonance (LSPR) in the near- to mid-infrared region. Besides being readily tuned through dopant concentration introduced during synthesis, this LSPR can also be dynamically modulated by applying an external electrochemical potential. This characteristic makes these materials candidates for electrochromic window applications. Here, using prototypical doped indium oxide NCs as a model system, we find that the extent of electrochemical modulation of LSPR frequency is governed by the depletion width and the extent of inter-NC LSPR coupling, which are indirectly controlled by the dopant density, size, and packing density of the NCs. The depletion layer is a near-surface region with a sharply reduced free carrier population that occurs whenever the surface potential lies below the Fermi level. Changes in the depletion width under applied bias substantially control the spectral modulation of the LSPR of individual NCs and also modify the inter-NC LSPR coupling, which additionally modulates the LSPR absorption on the NC film scale. Here, we show that both of these effects must be considered primary factors in determining the extent of LSPR frequency modulation and that the dominant factor depends on NC size. For a constant doping concentration, depletion effects govern LSPR modulation for smaller NCs, while LSPR coupling is prevalent in larger NCs. Consequently, as the size of the NCs is increased while keeping the doping concentration constant, we observe a reversal in the sign of the LSPR frequency modulation from positive to negative.

Impacts of surface depletion on the plasmonic properties of doped semiconductor nanocrystals.

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 In2O3 (Sn:In2O3) NCs. Monodisperse Sn:In2O3 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.

Localized surface plasmon resonance modulation of totally encapsulated VO2/Au/VO2 composite structure

We design and fabricate a totally encapsulated VO2/Au/VO2 composite structure which is aimed to improve the tunability of the localized surface plasmon resonance (LSPR) peak. In this work, the structure will ensure all the Au NPs’ resonant electric field area is filled with VO2. The modulation range of the totally encapsulated structure is larger than that of the semi-coated structure. To further improve the modulation range, we also explore the VO2 thickness dependence of the structure’s LSPR modulation. With the increase of the top layer VO2 thin film thickness, the modulation range becomes larger. When the thickness is about 80 nm, the absorption peak achieves a largest shift of 112 nm. FDTD solution and equivalent model of series capacitor are used to explain the phenomenon. These results will contribute to the area of metamaterial electromagnetic wave absorber and other fields.

Optically-Triggered Nanoscale Memory Effect in a Hybrid Plasmonic-Phase Changing Nanostructure

Nanoscale devices, such as all-optical modulators and electro-optical transducers, can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials. Here we demonstrate all-optical control of a nanoscale memory effect in such a heterostructure by coupling the localized surface plasmon resonance (LSPR) of gold nanodisk arrays to a phase-changing material (PCM), vanadium dioxide (VO2). By latching the VO2 in a distinct correlated metallic state during the insulator-to-metal transition (IMT), while concurrently exciting the hybrid nanostructure with one or more ultraviolet optical pulses, the entire phase space of this correlated state can be accessed optically to modulate the plasmon response. We find that the LSPR modulation depends strongly but linearly on the initial latched state, suggesting that the memory effect encoded in the plasmon resonance wavelength is linked to the strongly correlated electron states of the VO2. The continuous, linear variation of ...

Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene

We investigate the effect of new redox molecular switches based on oligothiophene deposited on gold nanoparticles (AuNPs) as thin electroactive layers in the 5–80 nm thickness range. In doing so, we compare systems based on physisorbed electroactive layers (weak electronic coupling) with those based on covalently bonded layers (strong electronic coupling), and we investigate orientation and thickness effects. Two different deposition methods were used. The first is based on bithiophene electropolymerization and the second on diazonium salt electroreduction. In both cases, redox switching of the electroactive layer makes is possible to tune the plasmonic properties of the AuNPs, and the layer thickness has a strong impact on the amplitude of the localized surface plasmon resonance (LSPR) modulation. LSPR modulation upon redox switching also depends on the electronic coupling regime between the AuNP and the organic layer. Indeed, the apparent real part of the dielectric constant seen by the AuNP is larger w...

Study on the localized surface plasmon resonance properties of bimetallic gold and silver three-layered nanotubes

The localized surface plasmon resonance (LSPR) properties of bimetallic gold and silver three-layered nanotubes such as SiO2-Ag-Au and SiO2-Au-Ag nanotubes have been investigated with the variations of Au or Ag shell thickness and size of the core by means of Finite Difference Time Domain (FDTD) method. With increase in the core size, it is found that the LSPR peaks show red-shift. We also observe with increasing the thickness of the Au or Ag shell that the LSPR peaks show blue-shift. Furthermore, the modulation of LSPR due to Ag shell is more significant compared with that due to Au shell. We have ascribed the variations of the LSPR peaks to the plasmon hybridization and the competition between the changes of conduction and oscillation electrons.