Journal
ACS NANO
Volume 7, Issue 11, Pages 10233-10244Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nn4045358
Keywords
STM contrast; DFT simulation; metal oxide; tip asymmetry; tip chemistry; tip oxidation
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Funding
- U.S. Department of Energy [DE-FG02-06ER15834]
- National Science Foundation through the Yale Materials Research Science and Engineering Center [MRSEC DMR-1119826, DMR-0520495]
- Materials World Network program [MWN DMR-0806893]
- Spanish MINECO [MAT2008-02953-E, MAT2011-023627, CSD2010-00024]
- UAM-Banco Santander Program of Collaboration with the USA
- Deutsche Forschungsgemeinschaft (DFG) [TRR 061]
- U.S. Department of Energy (DOE) [DE-FG02-06ER15834] Funding Source: U.S. Department of Energy (DOE)
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A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2 root 2 x root 2)R45 degrees missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip-sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen-terminated tips show complex contrasts with pronounced asymmetry and tip-sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip-sample distances. In contrast, oxygen-terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip-sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters.
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