Investigations on interfacial dynamics with ultrafast electron diffraction.

摘要

An ultrafast electron diffractive voltammetry (UEDV) technique is introduced, extended from ultrafast electron diffraction, to investigate the ultrafast charge transport dynamics at interfaces and in nanostructures. Rooted in Coulomb-induced refraction, formalisms are presented to quantitatively deduce the transient surface voltages (TSVs), caused by photoinduced charge redistributions at interfaces, and are applied to examine a prototypical Si/SiO2 interface, known to be susceptible to photoinduced interfacial charging The ultrafast time resolution and high sensitivity to surface charges of this electron diffractive approach allows direct elucidation of the transient effects of photoinduced hot electron transport at nanometer (∼2 nm) interfaces. Two distinctive regimes are uncovered, characterized by the time scales associated with charge separation. At the low fluence regime, the charge transfer is described by a thermally-mediated process with linear dependence on the excitation fluence. Theoretical analysis of the transient thermal properties of the carriers show that it is well-described by a direct tunneling of the laser heated electrons through the dielectric oxide layer to surface states. At higher fluences, a coherent multiphoton absorption process is invoked to directly inject electrons into the conduction band of SiO2, leading to a more efficient surface charge accumulation. A quadratic fluence dependence on this coherent, 3-photon lead electron injection is characterized by the rapid dephasing of the intermediately generated hot electrons from 2-photon absorption, limiting the yield of the consecutive 1-photon absorption by free carriers.;The TSV formalism is extended beyond the simple slab geometry associated with planar surfaces (Si/SiO2), to interfaces with arbitrary geometrical features, by imposing a corrective scheme to the slab model. The validity of this treatment is demonstrated in an investigation of the charge transfer dynamics at a metal nanoparticle/self-assembled monolayer (SAM)/semiconductor interconnected structure, allowing for the elucidation of the photo-initiated charging processes (forward and backward) through the SAM, by monitoring the deflection of the associated Bragg peaks in conjunction with the UEDV extended formalism to interpret the surface voltage.;The design, calibration, and implementation of a molecular beam doser (MBD), capable of layer-by-layer coverage is also presented, with preliminary investigations on interfacial ice.;With the development of UEDV and implementation of the MBD, continued investigations of charge transfer in more complex interfaces can be explored, such as those pertinent to novel solar-cell device technology, as their quantum efficiencies are usually strongly dependent on an interfacial charge transfer process. As UEDV is inherently capable of probing charge and atomic motion simultaneously, systems that exhibit phenomena that are attributable to strong coupling of the atomic and electronic degrees of freedom are of particular interest for future investigations with UEDV, such as optically induced electronic phase transitions and colossal field switching in functional oxides.