Self-trapping nature of Tl nanoclusters on Si(111)-7$\times$7 surface

TL;DR

This study reveals that Tl nanoclusters on Si(111)-7x7 surface are highly mobile and self-trapped within faulted half unit cells at room temperature, with multiple metastable phases influencing their electronic and structural properties.

Contribution

It provides the first detailed investigation combining spectroscopy and first principles calculations showing the self-trapping and metastability of Tl nanoclusters on Si(111)-7x7 surface.

Findings

Tl nanoclusters form within faulted half unit cells at RT.

Multiple metastable phases exist for Tl nanoclusters.

Tl atoms are highly mobile and self-trapped, affecting surface states.

Abstract

We have investigated electronic and structural properties of thallium (Tl) nanoclusters formed on the Si(111)-7$\times$7 surface at room temperature (RT) by utilizing photoemission spectroscopy (PES) and high-resolution electron-energy-loss spectroscopy (HREELS) combined with first principles calculations. Our PES data show that the state S2 stemming from Si restatoms remains quite inert with Tl coverage $\theta$ while S1 from Si adatoms gradually changes, in sharp contrast with the rapidly decaying states of Na or Li nanoclusters. No Tl-induced surface state is observed until $\theta$=0.21 ML where Tl nanoclusters completely cover the faulted half unit cells (FHUCs) of the surface. These spectral behaviors of surface states and a unique loss peak L$_2$ associated with Tl in HREELS spectra indicate no strong Si-Tl bonding and are well understood in terms of gradual filling of Si dangling bonds with increasing $\theta$. Our calculational results further reveal that there are several metastable atomic structures for Tl nanoclusters at RT transforming from each other faster than 10$^{10}$ flippings per second. We thus conclude that the highly mobile Tl atoms form self-trapped nanoclusters within FHUC at RT with several metastable phases. The mobile and multi-phased nature of Tl nanoclusters not only account for all the existing experimental observations including the fuzzy scanning tunneling microscope images and a dynamical model proposed by recent x-ray study but also provides an example of self-trapping of atoms in a nanometer-scale region.