Discrete Valence-Electron States in Thin Metal Overlayers on a Metal

Abstract

We have measured photoelectron energy spectra from thin Na and Ba overlayers on Cu(lll). The spectra show extremely narrow adsorbate-induced peaks, much narrower than observed for any other adsorbate system. These features arise from electrons trapped in the potential well between the vacuum barrier and the Cu(lll) surface which has a high electron reflectivity for energies within the band gap producing the necks of the Cu Fermi surface.For zero parallel wave vector, a thin film of freeelectron-like metal bounded by vacuum has discrete electron energy levels with energies given by E~n 2 h 2 n 2 l 2ma 2 if we assume unrealistic infinite barriers and a thickness of a. In contrast there will be a continuum of energies for the electrons if the film is placed on a metal substrate such that the electrons are able to move across the interface between substrate and adsorbate. For several systems studied, the electrons in the overlayer form an electron gas, which already at around full monolayer coverage has a density and a vacuum barrier similar to that of a thick sample of the overlayer metal. 1 " 4 Na adsorbed on Cu(lOO) provides an example of this behavior. At around full monolayer coverage the photoemission spectrum, which is due to the surface photoelectric effect, is similar to that obtained for thick Na films. 4 For free-electron metals adsorbed on Cu(lll) the overall properties of the overlayer are similar to that for Cu(l 00). There is, however, one important difference. For electrons in the overlayer with energies and wave vectors within the necks of the Cu Fermi surface, the substrate erects a barrier which, as will be discussed further below, is not, for these electrons, drastically different from the vacuum barrier. For this range of energies and k\\ values there is thus a chance of observing states having discrete energies. The purpose of the present Letter is to point out that such states can be and, indeed, have already been observed.In our early work on Na-covered Cu(l 11) we observed a narrow peak close below the Fermi edge in photoemission spectra recorded in the normal direction at around full monolayer thickness of Na. 5 This peak was poorly understood with regard to both the ejection mechanism and the states involved. The later breakthrough in the understanding of the surface photoelectric effect, 4,6,7 the recent calculations of electronic structure of monolayer films of free-electron-like metals, 8,9 the calculations of energies for states in the image-potential region of lowindex Cu surfaces, 10,11 together with the present data on Ba-covered Cutlll] provide a framework for understanding of the narrow emission peaks observed for Ba and Na overlayers. Of particular interest with the Ba adsorbate is that, in contrast to Na, distinct new structure is observed in the photoemission spectra when the layer thickness is increased beyond the monolayer stage.Ba is evaporated onto the Cu(lll) crystal from a heated boron nitride crucible. The photoelectron energy spectra reveal a Ba-coverage dependence for t...

Image potential states of supported metallic nanoislands

“…Present results compare well with available experimental and theoretical data. 7,20,25,26 Thus, at ⌫ , the 1 ML Na on Cu͑111͒ exhibits a quantum well state ͑QWS͒ at −127 meV below the Fermi level and a series of ISs with energies E n=1 = −992 meV, E n=2 = −215 meV, and E n=3 = −90 meV with respect to the vacuum level. Here, the ISs are labeled by their principal quantum number n. The entire ISs series is in the projected band gap of Cu͑111͒ because of the −1.53 eV shift of the vacuum level as induced by the Na monolayer.…”

“…1,3,4 Not only the nanostructure modifies the states which are already present at the surface, but new nanostructure-specific states can also emerge as has been demonstrated for, e.g., adatom chains, 5 nanoislands, 6 and adlayer structures. 7,8 Presently, the phenomena controlling the properties of confined states are rather well understood leading to their quantitative descriptions. 4,9,10 The energies of the states are found to follow the general trends predicted by the "particle in a box" picture and their lifetimes are determined by the scattering at the boundaries of the confining structure.…”

Quantum-well-induced ferromagnetism in thin films

“…In a Stoner picture the corresponding increase in the state density at the Fermi level may lead to onset of a magnetism. An alternative explanation, which will be discussed in the present work, is based on the quantumwell ͑QW͒ states [8][9][10][11] that are formed in the overlayer film and that moves through the Fermi level as the film thickness increases whereby the state density at the Fermi level is periodically enhanced and suppressed. The QW concept is particularly fruitful when considering possible magnetism in overlayer films consisting of more than a single monolayer.…”

Decay of electronic excitations at metal surfaces

“…Various techniques have been used to measure binding energies and dispersion of QWS. Angle-resolved photoemission spectroscopy was used to study QWS in alkali-metal overlayers on different metals [151,156,157], in silver overlayers on Fe(1 0 0) [152,158,159] and on V(1 0 0) [153,160,161]. Unoccupied QWS in overlayers have been studied with inverse photoemission [162][163][164][165][166][167][168][169][170][171] and with 2PPE spectroscopies [4,[172][173][174][175].…”