Direct Observation of Plasmon-Induced Interfacial Charge Separation in Metal/Semiconductor Hybrid Nanostructures by Measuring Surface Potentials

Abstract

Plasmon-induced interfacial charge separation (PICS) is one of the key processes responsible for the improved conversion efficiencies of energy-harvesting devices that incorporate metal nanostructures. In this Letter, we reveal a mechanism of PICS by visualizing (with nanometer-scale resolution) and characterizing plasmon-exciton coupling between p-type poly(pyrrole) (PPy) nanowires (NWs) and Ag nanoparticles (NPs) using light-irradiated Kelvin probe force microscopy (KPFM). Under blue-light irradiation, the Ag NPs are expected to donate electrons to the PPy NWs via a hot electron injection process. However, in this Letter, we observe that under blue-light irradiation the plasmonically and excitonically excited electrons in the semiconductor back-transfer to the metal. The PICS in this system can be explained by comparing it with a similar one where Au NPs are attached to n-type ZnO NWs; we observed a net electron transfer from the Au NPs to the ZnO NWs (an upward band bending is formed at the interface of the two materials, presumably obstructing electron back-transfer). Indeed, energy band matching between the metal and the semiconductor components of hybrid nanostructures influences PICS pathways. These experimental findings and our proposed mechanism consistently explain the PICS occurring in the PPy NW-Ag NP system with important implications on explaining their cooperative optoelectronic activities.

Optically Governed Dynamic Surface Charge Redistribution of Hybrid Plasmo‐Pyroelectric Nanosystems

“…Under blue light illumination, an electron transfer pathway similar to the nonpoled counterparts is established. Interestingly, the green light illumination also resulted in the increase of Δ V cpd , which signifies the back‐transfer of electrons from BTO to Ag . As the back‐transfer of electrons is not observed in nonpoled nanosytem, we postulate that negative poling has facilitated a steeper energy band that favors the electron back transfer.…”

“…In order to elucidate the influence of light on the poled hybrid BTO–Ag nanosystem, KPFM equipped with light source was employed to probe in situ changes in the charge density based on surface potential at a nanometer‐level spatial resolution. Here, the contact potential difference ( V cpd ) between the tip and the sample surface is calculated by: ( W tip − W sample )/ e , where W tip and W sample represent the respective work function of the tip and sample respectively, and e is the elementary charge (1.6 × 10 −19 C) . To prevent the tip‐induced band bending effect, the KPFM tip was operated at a hover height of 30 nm above the sample.…”

“…As the Ag + molar ratio increases, we enter region R2 where the photogeneration of electron–hole pairs in BTO starts to dominate and gradually occupies the empty surface state. Since the charging capability is linearly correlated to the size of the metallic nanoparticle, the pyroelectric response in R2 also increased linearly. Lastly, in R3, as the Δ W is reaching its plateau (Figure S7, Supporting Information), the charge separation also exhibits a similar pattern.…”

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

“…125 As they point out, there is still no consensus regarding which mechanism is at play for the reported systems, and novel probing techniques are constantly recruited for this task. [324][325][326][327] One of the consequences of the plasmonic response is local heat generation through electron-phonon scattering, 318 which has made a distinct contribution to successful photocatalytic CO 2 methanation of Rh/TiO 2 , for example. 318 It has been suggested recently that the catalytic activity of the generated 'hotelectrons', i.e., non-thermal charge carriers in the metallic nanoparticles (which are responsible for the photocatalytic enhancement reported for Cu-Ru NPs 328 and others 312,315,317 ) disregards the local heating effect of the plasmonic structures, and that the measured enhanced photocatalytic effects should be ascribed, in lieu, to thermal effects (well-described using a simple Arrhenius model).…”

Two‐step hydrothermal fabrication of Na 0.23 TiO 2 nanofibers and enhanced photocatalysis after loaded with gold or silver determined by surface potentials

“…Recently, SKPM was used to characterize the work function of metallic samples and the surface potential of semiconductor samples . In this paper, the surface potential of NTO loaded with Au was measured by an SPM equipped with SKPM to determine the charge distribution under light illumination with different wavelengths and then to reveal the photocatalytic mechanism.…”