Quantum-well and modified image-potential states in thin Pb(111) films: an estimate for the local work function

TL;DR

This study uses low-temperature STM and STS to analyze quantum-confined states in thin Pb(111) films on Si(111), estimating the local work function and examining tip shape effects on spectral features.

Contribution

It provides a novel method to estimate the local work function from quantum-well and image-potential states in thin Pb films using STM/STS data.

Findings

Estimated the local work function as approximately 3.8 eV.

Showed tip shape influences the number of observed IPS resonances.

Found that tip bluntness reduces the prominence of spectral maxima.

Abstract

Quantum-confined electronic states such as quantum-well states (QWS) inside thin Pb(111) films and modified image-potential states (IPS) above the Pb(111) films grown on Si(111)7$\times$7 substrate were studied by means of low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS) in the regime of constant current $I$. By plotting the position of the $n-$th emission resonances $U^{\,}_n$ versus $n^{2/3}$ and extrapolating the linear fit for the dependence $U^{\,}_n(n^{2/3})$ in the high-$n$ limit towards $n=0$, we estimate the local work function for the Pb(111) film: $W\simeq 3.8\pm 0.1\,$eV. We experimentally demonstrate that modifications of the shape of the STM tip can change the number of the emission peaks associated with the resonant tunnelling via quantized IPS levels for the same Pb terrace; however it does not affect the estimate of the local work function for the flat Pb terraces. We observe that the maxima in the spectra of the differential tunnelling conductance $dI/dU$ related to both the QWS and the modified IPS resonances are less pronounced if the STM tip becomes more blunt.