HAADF STEM and Ab Initio Calculations Investigation of Anatase TiO2/LaAlO3 Heterointerface

Mahabul Islam,Goran Dražić,Giorgio Rossi,Regina Ciancio,Sandeep Kumar Chaluvadi,Pasquale Orgiani,Piu Rajak,Daniel Knez

doi:10.3390/app12031489

Abstract

The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO2 remains a challenging issue. In particular, in TiO2 ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO2, from extrinsic effects, such as those arising from structural defects, dislocations, and the presence of competing phases at the film/substrate interface. It is, therefore, mandatory to unambiguously ascertain the structure of the TiO2/substrate interface. In this work, by combining high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), density functional theory calculations, and multislice image simulations, we have investigated the nature of strainless anatase TiO2 thin films grown on LaAlO3 substrate. In particular, the presence of oxygen vacancies in anatase TiO2 has been proved to stabilize the formation of an extra alloy layer, Ti2AlO4, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO2 directly on LaAlO3 substrate has required the deposition of a TiOx extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO2 to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO2/LAO hetero-interfaces to engineer the 2DEG formation in anatase TiO2 for tailored applications.

Highlights

Over the last decades, novel phenomena and functionalities at artificial heterointerfaces have been attracting extensive interest in both materials science and fundamental condensed matter physics
We provide an in-depth investigation of the film/substrate interface of anatase TiO2 thin films grown on LAO (001) substrates by pulsed laser deposition (PLD)
The corresponding selected area electron diffraction (SAED) pattern shown in Figure 1b indicates very good crystalline matching between the TiO2 film and the LAO

Summary

Introduction

Novel phenomena and functionalities at artificial heterointerfaces have been attracting extensive interest in both materials science and fundamental condensed matter physics. Interface electronic reconstruction [5] are prominent examples of exotic phenomena emerging at heterointerfaces between oxide layers with properties differing from either constituent, suggesting new possible platforms for a future generation of technological applications [6]. In this regard, understanding the physical mechanisms responsible for the formation of a 2DEG at the surface of insulating oxides or at their interface remains one of the most challenging issues. Having access to a very high-quality single-unit-cell (and sub-unit-cell) layer of a given material is the only way to provide conclusive evidences on the dimensionality issue

Methods

Results

Conclusion