Experimental observation of quantum oscillation of surface chemical reactivities

Abstract

Here we present direct observation of a quantum reactivity with respect to the amounts of O2 adsorbed and the rates of surface oxidation as a function of film thickness on ultrathin (2-6 nm) Pb mesas by scanning tunneling microscopy. Simultaneous spectroscopic measurements on the electronic structures reveal a quantum oscillation that originates from quantum well states of the mesas, as a generalization of the Fabry-Pé rot modes of confined electron waves. We expect the quantum reactivity to be a general phenomenon for most ultrathin metal films with broad implications, such as nanostructure tuning of surface reactivities and rational design of heterogeneous catalysts.Pb(111) ͉ quantum size effects ͉ scanning tunneling microscopy ͉ surface reactivity

Van Hove singularities as a result of quantum confinement: The origin of intriguing physical properties in Pb thin films

“…More interestingly, the anisotropy of the m-band of a Pb film is smaller than for the band of the bulk, the reason for which needs more detailed studies. We can explain very strong thickness-dependent physical properties observed in Pb thin films [5][6][7][8][9][10][11][12][13][14][15][16][17] in terms of the 1-D-like DOS singularity developing from the 2-D band bending: it is simply because the electronic structure near EF is now dominated by the singularity. Although we have only studied 23 and 24 ML Pb films, we know where the VHS of other Pb films is from previous STS measurements [16].…”

“…In this regard, Pb thin films represent a peculiar system. In this system, quantum size effects have been observed to drive quantum oscillation of many properties with a period of 2 ML: examples include the superconducting transition temperature T c [5][6], thermal stability [11], thermal expansion coefficient [12], electron-phonon coupling [13], local work function [14], surface reactivity [15] and Kondo effect [16]. These have been explained by an oscillation in electronic density of states (DOS) near the Fermi level (E F ) [5][6][7][8][9][10][11][12][13][14][15][16][17], particularly in the bottom of subbands that pass though the Fermi level [18,19].…”

“…In this system, quantum size effects have been observed to drive quantum oscillation of many properties with a period of 2 ML: examples include the superconducting transition temperature T c [5][6], thermal stability [11], thermal expansion coefficient [12], electron-phonon coupling [13], local work function [14], surface reactivity [15] and Kondo effect [16]. These have been explained by an oscillation in electronic density of states (DOS) near the Fermi level (E F ) [5][6][7][8][9][10][11][12][13][14][15][16][17], particularly in the bottom of subbands that pass though the Fermi level [18,19]. More interestingly, the oscillations in the properties of Pb persist for large film thicknesses, and quantized electronic states have even been observed at film thicknesses up to 45 ML [14], a much larger thickness [5][6][7][8][9][10][11][12][13][14][15][16][17] than observed for other films [2][3][4].…”

“…The electronic structure of Pb films with different thicknesses has been studied by photoemission spectroscopy and scanning tunneling spectroscopy (STS) [5,12,[14][15][16]. Many of these studies have revealed that the oscillations in the DOS at E F are consistent with the observed properties of the films-for instance, higher superconducting transition temperature (T c ) [5], surface reactivity [15] and Kondo temperature [16]. However, why the Pb films exhibit much stronger DOS oscillations than other 2-D systems is still an open question.…”

Quantum-well states with image state character for Pb overlayers on Cu(111)

“…4,[6][7][8]10 As a consequence of the quantization of the electronic structure, many properties of metal films show a quantum size effect, [3][4][5][6]14,15 i.e., a (oscillatory) dependence on the film thickness D, where D can be changed only by the discrete amount given by the interlayer spacing a. Because of the fundamental and practical interest quantum size effects have been studied in detail for the work function and surface energy, [16][17][18][19][20] chemical reactivity, 21 magnetic coupling, 5,[22][23][24] Rashba effect, 25,26 etc.…”