Visible Light Driven Hot‐Electron Injection by Pd Nanoparticles: Fast Response in Metal–Semiconductor Photodetection

Abstract

multidisciplinary research interests. For instance, based on optical design, dimer NPs can obtain the enhancement factor over 10 14[1] therefore highly beneficial to the applications such as surface enhanced Raman spectroscopy (SERS) in which the detection accuracy is approaching to single molecular level. [2] Also, the intermittent photothermal effect of Au NPs that induced by pulsed laser has been demonstrated in ultrasonic wave generation [3,4] and utilized as laser-driven liquid pump via light-mechanical energy conversion, jetting the liquid with tunable characters. Whereas, in optoelectronic research, the important role of hot-electron generation by plasmon nonradiative decay has been gradually recognized and widely investigated in metal-semiconductor system with regards of the mechanism and dynamics. [5,6] According to the transparent spectral studies, hot-electron generated by LSPR decay can inject into the conductive band of neighbor semiconductor within 200 fs, [7,8,9] and via the recently proposed direct interfacial charge transfer transition mechanism, the injection can be further speeded up to even 30 fs, [10] presenting an encouraging mechanism to be applied in ultrafast photodetection. Meanwhile, the transparent injection efficiency was reported as high as 50%, [8,11] which is quite favorable for the energy conversion applications based on semiconductors including photo detection, photocatalysis, and photovoltaics. Although the transparent studies exhibit great potentiality for optoelectronics, in the applications at steady state, the actual efficiency was observed much lower. For example, Au/ TiO x system possess an injection efficiency over 40% according to the study based on pump-probe absorption spectrum, [8] however, the Au/TiO x plasmonic nanodiode can only achieve the IPCE of less than 3% [12] in sharp comparison. One of the important dynamic inducement lies on the material electronic structure. As the free electron of Au come from s-orbit, the plasmon resonance is mainly consisted of intraband transition absorption. [13,14] However, as the amount of s-electron is relatively small, the electron export can easily induce large decrease in Fermi energy level of electrons, therefore, result in a rising energy barrier and hinder the further electron injection. [8,15,16] Also, as the hot electrons originated by Au interband absorption Hot-electron injection induced by plasmon decay enables ultrafast electron transfer in femtosecond scale and therefore endows metallic nanoparticles (MNPs) promising potentials in high-speed optoelectronics. With much higher density of states next to its Fermi level, palladium (Pd) can more efficiently launch hot electrons according to the theoretical prediction, as compared to the conventional plasmonic NPs. In this work, the optical properties of Pd NPs as well as the plasmon induced hot-electron injection are investigated through confocal Kelvin probe force microscopy. Analysis based on surface potential redistribution suggests Pd NPs can initiate dense hot-electro...

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