ABSTRACT:The influence of defects on the local structural, electronic, and chemical properties of a surface oxide on Cu(100) were investigated using atomic resolution three-dimensional force mapping combined with tunneling current measurements and ab initio density functional theory. Results reveal that the maximum attractive force between tip and sample occurs above the oxygen atoms; theory indicates that the tip in this case terminates in a Cu atom. Meanwhile, simultaneously acquired tunneling current images emphasize the positions of Cu atoms, thereby providing species-selective contrast in the two complementary data channels. One immediate outcome is that defects due to the displacement of surface copper are exposed in the current maps even though force maps only reflect a well-ordered oxygen sub-lattice. The exact nature of the defects is confirmed by the simulations, which also reveal that the arrangement of the oxygen atoms is not disrupted by the copper displacement. The experimental force maps uncover in addition a position-dependent modulation of the attractive forces between the surface oxygen and the copper-terminated tip, which is found to reflect the surface's inhomogeneous chemical and structural environment. As a consequence, the demonstrated method has the potential to directly probe how defects affect surface chemical interactions.3
Cataloged from PDF version of article.A comprehensive analysis of contrast formation\ud
mechanisms in scanning tunneling microscopy (STM) experiments on\ud
a metal oxide surface is presented with the oxygen-induced\ud
(2√2\ud
√2)R45 missing row reconstruction of the Cu(100) surface\ud
as a model system. Density functional theory and electronic\ud
transport calculations were combined to simulate the STM imaging\ud
behavior of pure and oxygen-contaminated metal tips with structurally\ud
and chemically different apexes while systematically varying\ud
bias voltage and tip sample distance. The resulting multiparameter\ud
database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large\ud
number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find\ud
that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygenterminated\ud
tips show complex contrasts with pronounced asymmetry and tip sample distance dependence. Considering the structural and chemical\ud
stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip sample distances. In contrast, oxygenterminated\ud
tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip sample distances. Our results provide a conclusive\ud
understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by\ud
properly selecting imaging parameters
Understanding the connection of graphene with metal surfaces is a necessary step for developing atomically precise graphene-based technology. Combining high-resolution STM experiments and DFT calculations, we have unambiguously unveiled the atomic structure of the boundary between a graphene zigzag edge and a Pt(111) step. The graphene edges minimize their strain by inducing a 3-fold edge-reconstruction on the metal side. We show the existence of an unoccupied electronic state that is mostly localized on the C-edge atoms of one particular graphene sublattice, which could have implications in the design of graphene based devices.
We assemble bistable silicon quantum dots consisting of four buckled atoms (Si4-QD) using atom manipulation. We demonstrate two competing atom switching mechanisms, downward switching induced by tunneling current of scanning tunneling microscopy (STM) and opposite upward switching induced by atomic force of atomic force microscopy (AFM). Simultaneous application of competing current and force allows us to tune switching direction continuously. Assembly of the few-atom Si-QDs and controlling their states using versatile combined AFM/STM will contribute to further miniaturization of nanodevices.
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