Quantum size effects on the work function of metallic thin film nanostructures

Abstract

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant κ, resulting in a very rich phenomenon of "quantum oscillations" in κ as a function of thickness, L, and bias voltage, V s . More specifically, we find that the phase of the quantum oscillations in κ vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (jV s j < 0.03 V) the measurement of κ vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of κ vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.STM | quantum well states | Pb T he work function, the minimum energy required to move an electron from a solid into the vacuum, is the most fundamental material parameter in surface science. It plays a key role, for example, in the photo-electric effect, one of the first phenomena through which quantum mechanics unveiled itself. The work function is the result of a complex interplay between quantum mechanics and forces on the atomic scale. Recent studies suggest the exciting possibility of controlling the work function through quantum engineering of electronic structures at the nanoscale (1-5). Such efforts are, however, still in their infancy, and there are many puzzles and contradictions in the observations so far that hinder further progress (1-5). In this report, we show direct evidences for quantum size effects on the work function in ultrathin Pb films. We further establish the direct correlation of this quantity with the behavior of quantum well states. With the ability to control the growth of metallic thin films with atomic layer precision, one can anticipate the possibility of tuning the work function of metallic thin films through the quantum size effects, thus influencing chemical processes on surfaces.It should be noted that theoretical investigations of quantum size effects (QSE) on the work function (Φ) were carried out several decades ago (6). By using a jellium model and continuously varying the film thickness, Schulte showed that the QSE leads to work function oscillations as a function of layer thickness. Moreover, it was found that the Φ oscillations are directly related to the energy locations of the quantum well states (QWS). In particular, whenever a QWS channel crosses the Fermi level (E F ), there is a negative cusp in Φ as a function of film thickness (L), which results from the sudden increase of the surface dipole due to the additional electronic ...

Electronically stabilized nanowire growth

“…The main access road to these Friedel oscillations in a film is through calculations. Experimentally, direct access is impossible, but QSE have been observed in electron reflectivity for electrons above the vacuum level, both in integral measurements [34][35][36] and in position-resolved experiments [37][38][39][40][41] . However, because of their unique geometry, the Ir wires would offer a unique and viable opportunity to access the relationship between size selection and Friedel oscillations in a most direct manner.…”

Anomalous phase relations of quantum size effects in ultrathin Pb films on Si(111)

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Another example is the quantum size effect on the work function of metallic thin films, a phenomenon predicted 4 decades ago, 14 but addressed experimentally only very recently. [15][16][17][18][19] Among all material systems, Pb on Si(111) has been the most intensively investigated one. This is due to the nearly halfinteger phase matching between the Fermi wavelength and the lattice spacing along [111], leading to bi-layer quantum oscillation phenomena for many physical properties which have been observed experimentally.…”

Atomic-scale epitaxial aluminum film on GaAs substrate

“…Driven by the huge potential of the quantized electronic/metallic states for various applications, searching for a practical and reliable method for preparing an atomic-scale metal film has been a long-standing challenge for material scientists and engineers. [13][14][15][16][17][18][19][20][21] To prepare an atomically uniform and thin metal film, small area (∼µm 2 ) and low temperature deposition in an ultra-high vacuum (UHV) chamber are commonly applied. Extensively studied metals such as silver and lead reveal quantum size effect on chemical reactivity, film stability, superconductivity, and electrical conductivity, etc.…”

“…Extensively studied metals such as silver and lead reveal quantum size effect on chemical reactivity, film stability, superconductivity, and electrical conductivity, etc. 9,[14][15][16][17][18][19][20][21] To prevent the samples from oxidation, the films were mostly in-situ characterized in a UHV chamber, which makes them almost not possible for practical applications. 8,9 An over layer on top of the films to avoid oxidation is feasible.…”