Lukas Pfaffmann

Charging of carbon thin films in scanning and phase-plate transmission electron microscopy

A systematic study on charging of carbon thin films under intense electron-beam irradiation was performed in a transmission electron microscope to identify the underlying physics for the functionality of hole-free phase plates. Thin amorphous carbon films fabricated by different deposition techniques and single-layer graphene were studied. Clean thin films at moderate temperatures show small negative charging while thin films kept at an elevated temperature are stable and not prone to beam-generated charging. The charging is attributed to electron-stimulated desorption (ESD) of chemisorbed water molecules from the thin-film surfaces and an accompanying change of work function. The ESD interpretation is supported by experimental results obtained by electron-energy loss spectroscopy, hole-free phase plate imaging, secondary electron detection and x-ray photoelectron spectroscopy as well as simulations of the electrostatic potential distribution. The described ESD-based model explains previous experimental findings and is of general interest to any phase-related technique in a transmission electron microscope.

Fabrication of polycrystalline Cu2ZnSnSe4 layers with strongly preferential grain orientation via selenization of Sn/Cu/ZnSe(001)/GaAs(001) structures

We report on a simple approach for the fabrication of polycrystalline Cu2ZnSnSe4 films on GaAs with strongly preferential grain orientation. Such layers are important for both fundamental spectroscopic investigations as well as studies of the impact of grain boundaries on the performance of solar cells. Cu2ZnSnSe4 (CZTSe) layers were fabricated by growing a Sn/Cu/ZnSe(001) stack on a GaAs(001) substrate in a molecular-beam epitaxy system followed by selenization in a tube furnace. Raman spectroscopy as well as X-ray diffraction measurements combined with a microstructural investigation indicate the presence of the kesterite phase. Further, X-ray analysis such as rocking curves and ϕ-scans proves a preferential grain orientation of the obtained CZTSe films in all three dimensions.

Lithium–air battery cathode modification via an unconventional thermal method employing borax

A novel, unconventional thermal treatment employing borax for preparing porous carbon materials is presented. The new method was used to prepare carbon felt electrodes for use in lithium–air batteries. The etching of the carbon fiber surface was found to be highly controllable by the amount of borax. The resulting felts were characterized by cyclic voltammetry (CV), secondary electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The borax treatment resulted in a change of the size, shape and orientation of the Li2O2 crystals formed during discharge.

Raman investigation of epitaxial Cu 2 ZnSnSe 4 layers from annealed Sn/Cu/ZnSe(001) precursors on GaAs(001)

We investigate the crystal unit cell orientation of epitaxial Cu2ZnSnSe4 films on GaAs(001) using polarized Raman spectroscopy. An epitaxial ZnSe(001) layer is grown in a molecular-beam epitaxy system followed by the deposition of Cu and Sn and annealing in Se atmosphere. This method results in an epitaxially ordered Cu2ZnSnSe4 film on GaAs as proven by Electron Backscatter Diffraction (EBSD) measurements and X-ray diffraction. Polarization-dependent Raman spectra of the layers not only exhibit the typical kesterite main modes but also intensity oscillations when the sample is rotated around the optical axis. These oscillations yield information about the orientation of the kesterite crystal unit cell.