Tuning the electronic properties of a clean TiO2(1 1 0) surface via repeated sputtering and annealing: A KPFM and LC-AFM study

K Cieślik,D Wrana,K Szajna,W Bełza,M Rogala,C Rodenbücher,P Dąbczyński,K Szot,F Krok

doi:10.1016/j.apsusc.2021.151303

Abstract

Repeated sputtering and annealing are standard preparation methods for obtaining a stoichiometric TiO2(110) surface for surface science experiments. However, both processes result in a reduction in TiO2 crystal when used separately, leading to the modification of the physical and chemical properties of oxide materials. Our investigation aims to determine how these two processes affect the electronic properties of the surface and subsurface regions at the nanometer scale. To accomplish this goal, we utilized local microscopy (Kelvin probe force microscopy and local-conductivity atomic force microscopy) and spectroscopy methods (X-ray photoelectron spectroscopy and secondary ion mass spectrometry). We found that repeated sputtering and annealing does, in fact, affect both the conductivity and work function of the surface. The work function, as well as conductivity, increase with increasing number of cycles, but then reach a plateau. Furthermore, we show that the way the surface is prepared, using multiple cycles or one cycle of equivalent ion-beam fluence, matters. We attribute the differences in the crystal properties to the dynamics of stoichiometric changes during sputtering and subsequent annealing which we illustrate using secondary ion mass spectroscopy, which shows that after multiple cycles the subsurface layer is modified, even though XPS shows a stoichiometric surface.

Highlights

Rutile titanium dioxide is one of the most widely-studied oxide materials, with applications ranging from photocatalysis [1], mem ristors [2], solar cells [3,4], sensors [5], and thin-film MOS transistors [6], to antibacterial additives for denim fabrics [7]
We have shown that the time-tested method of repeated sputtering and annealing does, restore the stoichiometric TiO2(1 1 0) surface, but at the cost of significantly altering its electronic properties
Even though the low-energy electron diffraction (LEED) images exhibit perfect diffrac tion patters and x-ray photoelectron spectroscopy (XPS) measurements indicate a stoichiometric surface, our Local-Conductivity Atomic Force Microscopy (LC-AFM) and Kelvin Probe Force Microscopy (KPFM) measurements clearly show that the modifica tions to the subsurface region drastically affect the electronic properties of the surface

Summary

Introduction

Rutile titanium dioxide is one of the most widely-studied oxide materials, with applications ranging from photocatalysis [1], mem ristors [2], solar cells [3,4], sensors [5], and thin-film MOS transistors [6], to antibacterial additives for denim fabrics [7]. To understand the processes that are the basis of such applications, controlled conditions are required, and in vestigations into the surfaces of monocrystals in ultra-high vacuum conditions can provide such needed model systems. Such studies can deliver an entirely new perspectives and contribute to the understanding and development of applications, as is the case, e.g., with photocatalysis [8]. Obtaining a suitable surface that can be used in such in vestigations is not trivial and many different methods for obtaining atomically-flat and stoichiometric surfaces exist. One of the most uti lized preparation methods is that of repeated sputtering and annealing

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