Shape-Dependent Field Enhancement and Plasmon Resonance of Oxide Nanocrystals

Abstract

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...

Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures

“…[ 29 ] As expected, the octahedron ICO has two LSPR features in the NIR (2507 nm and 3898 nm) that become one peak (3296 nm) when in a spherical morphology. This follows the same trend observed in Cs x WO 3 and is in agreement with the Drude-like free electron model of ICO described by Agrawal et al, [ 19 ] which uses Lorentz-Mie theory and DDA to predict LSPR properties based on particle shape and free-carrier concentration.…”

“…[ 17 ] Only recently have metal chalcogenides of varying shapes been developed, such as doped tungsten oxide (prisms, cubes and spheres) [ 15 ] and copper telluride (plates, cubes and rods). [ 18 ] Recent work by Agrawal et al [ 19 ] highlights the importance of morphology by calculating the infl uence of n on the LSPR of an indium doped cadmium oxide (ICO) sphere, cube, rounded cube, and octahedron. [ 19 ] Varying the particle morphology within the same system allows researchers to make shape comparisons with respect to LSPR that were previously only possible in noble metals, which enables the development of new classes of devices with plasmon-enhanced functionalities spanning multiple frequencies in the electromagnetic spectrum.…”

Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of Solar Transmittance

“…By contrast, a nanocrystal film made without the block copolymer shows a broad, redshifted optical response that can be explained by strong LSPR coupling among the nanocrystals, which are densely packed in three dimensions. 23 The mesostructured film has a clear aspect without haziness in both colored and bleached states, consistent with the mesopore dimensions lying far below the visible wavelengths. The specific charge capacity of the nanocrystal framework is more than double that of a dense nanocrystal film (16.2 vs 7.5 mC/cm 2 ) with the same loading, facilitating strong modulation of the LSPR.…”

A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite

“…Plasmonic semiconductor oxide nanocrystals such as Al-doped ZnO (AZO), [1][2][3] indium-doped ZnO (IZO), 3,4 Ga-doped ZnO (GZO), 3 Sn-doped In 2 O 3 (ITO), [6][7] Sb-doped SnO 2 (ATO) 8,9 or others [10][11][12][13][14] have attracted growing attention due to their applicability in many optoelectronic applications, such as nearinfrared selective electrochromic devices, 15 (flexible) displays 11 and polymer light emitting diodes. 4 They also demonstrate excellent bio-sensing and chemical sensing capabilities 16 .…”