Gang Xiong

Carrier dynamics in α‐Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α‐Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α‐Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d‐d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.

In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy

We report photoelectron emission microscope observations of the thermal martensitic transformation in a CuZnAl shape memory alloy. The phase transformation appears at 48°C during heating and at 42°C upon cooling. The transformation is marked by a sharp change in photoelectron intensity, as well as a significant displacement and reorientation of surface features. The difference in the photoelectron intensity before and after the transformation is attributed to a change in work function of about 0.2eV. Photoemission electron microscopy provides real-time information on microstructural changes and phase-dependent electronic properties.

Synthesis and photoexcited charge carrier dynamics of β-FeOOH nanorods

β-FeOOH nanorods of dimensions of 15nm diameter and 200nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation display three time scales. The first is a subpicosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2ps. The trapped electrons and holes persist for significantly longer times (at least tens of picoseconds), similar to previous results from α-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.

Study of copper diffusion through a ruthenium thin film by photoemission electron microscopy

Photoemission electron microscopy is used to study copper diffusion through a ruthenium thin film. The photoemission electron microscopy images display a large contrast between Cu and Ru due to the differences in work function, making photoemission electron microscopy an ideal methodology to study thin film diffusion in real time. Between 175 and 290°C, Cu mainly diffuses through defect sites in the thin Ru film. Uniform diffusion of Cu through the Ru film begins at approximately 300°C. The results are confirmed by x-ray photoemission spectroscopy depth profiling and scanning electron microscopy–energy dispersive x-ray spectroscopy analysis.

Real Time Study of Cu Diffusion Through a Ru Thin Film by Photoemission Electron Microscopy (PEEM)

We demonstrate the efficacy of Photoemission Electron Microscopy (PEEM) as a tool to detect metal diffusion processes at nanoscale spatial resolution in real time. For a sample comprising a nominally 1nm physical vapor-deposited (PVD) Ru thin film covering a thick Cu substrate, we have observed the appearance of bright features on a dark background as the temperature is monotonically increased and irradiated with photons from a Hg are lamp. These bright features are the result of a lower work function due to Cu diffusion through the Ru films.