Basics: Metallic Quantum Wells

When the dimension of a metallic system enters the nanoscale regime, its electronic structure acquires a discrete component to its spectrum. These quantum size effects are ubiquitous and mediate phenomena such as the interlayer exchange coupling in magnetic multilayers, magic numbers in thin film and island growth, conductance anomalies in epitaxial films and chemisorption properties. The formation of so-called metallic quantum well (MQW) states is schematically illustrated at the left. Significant electron reflectivity at the Cu/transition metal (TM) interface causes states to be confined to the Cu overlayer. These states change energy as the thickness of the Cu overlayer increases and modify the density of states at the Fermi level as they pass through it.

       Recent Highlights: Metallic Quantum Wells

Chemisorption modified by quantum confinement of electrons
As suggested by the discussion above, as the thickness of a metallic quantum well changes, MQW states change energy. The figure below on the left shows a series of inverse photoemission spectra obtained from the Cu/fccFe/Cu(100) system as a function of increasing film thickness. The tick marks show that MQW states move upward with increasing thickness.  In particular, there is a strong modulation of the intensity, reflecting a modulation of the density of states (DOS) as MQW states pass through the Fermi level (EF).


The center figure above shows carbon monoxide (CO) temperature programmed desorption spectra from Cu MQWs grown on top of the fccFe films.  The peak desorption temperature (TD) is a measure of the strength of the CO-metal bond.  The non-monotonic change in TD shows that the CO-MQW bond strength modulates as a function of MQW thickness.  The figure above on the right shows that there is a direct correlation between changes in TD and  modulations in the DOS at the EF caused by the MQW states.  These results demonstrate that quantum confinement of electrons is a viable way to modify chemisorption at surfaces, and may be used to modify surface reactions, catalysis, electronic friction, or molecular self assembly.

Read more: Quantum size effect induced modification of the chemisorption properties of thin metal films ; A. G. Danese, F. G. Curti *, and R. A. Bartynski, Physical Review B 70, 165420 (2004)

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