Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings

Abstract

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO2) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

Emerging Methods for Controlling Hot Carrier Excitation and Emission Distributions in Nanoplasmonic Systems

“…Even hot electrons in plasmonic applications with sufficient energy to transfer into the SiO 2 conduction band (∼3.5 eV above the Au Fermi level), for instance, will travel only a few nanometers before decaying below the escape barrier. 35,54 Thus, dielectric coatings can serve to block photocurrents over a wide range of excitation energies. By contrast, semiconductor coatings are often utilized as energy filters, with lower-energy Schottky barriers (∼1.1 eV for an Au-TiO 2 junction 8 ) that can selectively transmit hot electron or hot hole photocurrents.…”

“…This was also demonstrated recently by Medeghini et al, using photoelectron velocity mapping and correlated scanning electron microscopy to resolve directional emission from thinner defect regions in nominally uniform silica coatings (Figure 2c). 35 The anisotropic coatings influence both nanoscale spatial photocurrent distributions as well as their corresponding vector momentum distributions. Carefully designed synthetic or lithographic coatings could thus be utilized to enhance the directivity of nanocathode emitters (and emitter arrays), influence photocurrent distributions in nanoelectronic systems, or help define nanometer-scale active sites in photocatalytic systems.…”

“…Insulators such as SiO 2 have long been used to block charge transfer in semiconductor devices, with the wide bandgaps serving as barriers to low-energy carriers around the metal Fermi level or semiconductor conduction band edge. Even hot electrons in plasmonic applications with sufficient energy to transfer into the SiO 2 conduction band (∼3.5 eV above the Au Fermi level), for instance, will travel only a few nanometers before decaying below the escape barrier. , Thus, dielectric coatings can serve to block photocurrents over a wide range of excitation energies. By contrast, semiconductor coatings are often utilized as energy filters, with lower-energy Schottky barriers (∼1.1 eV for an Au-TiO 2 junction) that can selectively transmit hot electron or hot hole photocurrents. ,, These are particularly useful for charge separation in nanocatalysis and photovoltaic energy harvesting .…”

“…26,32 In this Perspective, we survey recent work and highlight general design principles for promoting surface-vs bulk-like photoexcitations, while also looking at opportunities to enhance ballistic transfer via tailored hot carrier spatial and momentum distributions. 34 Conversely, we also examine opportunities for blocking or regulating hot carrier transfer with thin dielectric coatings, 35 which can be utilized to eliminate unwanted chemical transformations in surface/tip-enhanced Raman spectroscopy 36 or to distinguish between carrier-induced and thermal effects in plasmonenhanced photocatalysis, which remains a subject of some debate. 37−39 Another broad set of emerging nanoplasmonic applications relies on both the efficient ultrafast generation and emission of photoexcited electrons into free space, with tailored kinetic energy and vector momentum distributions.…”

“…In photovoltaic and photocatalytic applications, internal quantum yields for ballistic charge transfer are often less than 1%, − while enhanced efficiencies have been demonstrated via direct surface photoexcitation. , In this Perspective, we survey recent work and highlight general design principles for promoting surface- vs bulk-like photoexcitations, while also looking at opportunities to enhance ballistic transfer via tailored hot carrier spatial and momentum distributions . Conversely, we also examine opportunities for blocking or regulating hot carrier transfer with thin dielectric coatings, which can be utilized to eliminate unwanted chemical transformations in surface/tip-enhanced Raman spectroscopy or to distinguish between carrier-induced and thermal effects in plasmon-enhanced photocatalysis, which remains a subject of some debate. − …”