RESUMEN
The impact of ferroelectric polarization on the chemical and electronic properties of atomically thin layers of non-polar chromium oxide deposited on positively and negatively poled LiNbO3(0001) was studied. Chromium(III) oxide readily forms on LiNbO3; however, annealing at high temperatures was required to maintain well-ordered films as the thickness increased. Prolonged heating at these temperatures caused Cr diffusion into the LiNbO3 substrate. Comparing Cr 2p X-ray photoelectron spectroscopy (XPS) peak positions as a function of temperature and substrate polarization revealed no evidence of shifts from the peak positions expected for Cr2O3. The lack of any band offset between Cr2O3 on the oppositely poled surface suggests that charge compensation of the ferroelectric substrate occurs at least predominantly at the surface of the film, as opposed to the film-substrate interface. No evidence of shifts due to oxidation or reduction of the Cr was observed indicating that charge compensation did not involve a change in the ionic state of the Cr. Exposing the films to reactive oxygen species emitted from an oxygen plasma, however, caused a distinct high binding energy shoulder on the Cr 2p3/2 XPS peaks that could be associated with oxygen adsorption on surface Cr and concomitant oxidation to Cr(5+). This feature was used to gauge the concentration of O adatoms on the surfaces as a function of temperature for oppositely poled substrates; these measurements did not reveal any significant polarization dependence for oxygen desorption. Further, temperature programmed desorption measurements for a Cr2O3 film on α-Al2O3 showed a similar trend in O2 desorption. Therefore, it is concluded that the reactivity of Cr2O3 toward O is at least largely independent of substrate polarization despite data suggestive of charge compensation at the film surfaces.
RESUMEN
An unusual distribution of particle sizes has been observed following the formation of molybdenum particles by argon ion sputtering. Many of the molybdenum particles produced by sputtering at the threshold pressure for particle formation in the vapor appear to be single crystalline cubes. There are two prominent peaks in the edge length distribution of the cubes, one centered at 4.8 nanometers with a halfwidth of approximately 1.3 nanometers and the other at 17.5 nanometers. The peak for the larger cubes is approximately square and has a total width of 7.0 nanometers. Evidence is presented that the larger cubes are formed by a 3 by 3 by 3 self-arrangement of the smaller cubes, which contain approximately 7000 atoms. Self-arrangement in inorganic structures is normally only observed when the building blocks are atoms, molecules, or clusters of less than 100 atoms.
RESUMEN
It has been recognized since the 1950s that the polar and switchable nature of ferroelectric surfaces can potentially lead to polarization direction-dependent surface chemistry. Recent theoretical studies and advances in growing high quality epitaxial ferroelectric thin films have motivated a flurry of experimental studies aimed at creating surfaces with switchable adsorption and catalytic properties, as well as films whose polarization direction switches depending on the gas phase environment. This research news article briefly reviews the key findings of these studies. These include observations that the adsorption strengths, and in certain cases the activation energies for reactions, of polar molecules on the surfaces of ferroelectric materials are sensitive to the polarization direction. For bare ferroelectric surfaces, the magnitudes of these differences are not large, but are still comparable to the energy barrier required to switch the polarization of approximately 10 nm thick films. Highlights of a recent study where chemical switching of a thin film ferroelectric was demonstrated are presented. Attempts to use the ferroelectric polarization to influence the behavior of supported catalytic metals will also be described. It will be shown that the tendency of the metals to cluster into particles makes it difficult to alter the chemical properties of the metal surface, since it is separated from the ferroelectric by several layers of metal atoms. An alternate approach to increasing the reactivity of ferroelectric surfaces is suggested that involves modifying the surface with atoms that bind strongly to the surface and thus remain atomically dispersed.