S. I. Bozhko

Selecting the tip electron orbital for scanning tunneling microscopy imaging with sub-˚ angstrom lateral resolution

We report on scanning tunneling microscopy (STM) studies performed with single crystalline W[001] tips on a graphite(0001) surface. Results of distance-dependent STM experiments with sub-angstrom lateral resolution and density functional theory electronic structure calculations show how to controllably select one of the tip electron orbitals for high-resolution STM imaging. This is confirmed by experimental images reproducing the shape of the 5dxz, yz and 5dx2−y2 tungsten atomic orbitals. The presented data demonstrate that the application of oriented single crystalline probes can provide further control of spatial resolution and expand the capabilities of STM.

Fabrication of [001]-oriented tungsten tips for high resolution scanning tunneling microscopy.

The structure of the [001]-oriented single crystalline tungsten probes sharpened in ultra-high vacuum using electron beam heating and ion sputtering has been studied using scanning and transmission electron microscopy. The electron microscopy data prove reproducible fabrication of the single-apex tips with nanoscale pyramids grained by the {011} planes at the apexes. These sharp, [001]-oriented tungsten tips have been successfully utilized in high resolution scanning tunneling microscopy imaging of HOPG(0001), SiC(001) and graphene/SiC(001) surfaces. The electron microscopy characterization performed before and after the high resolution STM experiments provides direct correlation between the tip structure and picoscale spatial resolution achieved in the experiments.

Surface plasmon on topological insulator/dielectric interface enhanced ZnO ultraviolet photoluminescence

It has recently been predicted that the surface plasmons are allowed to exist on the interface between a topological insulator and vacuum. Surface plasmons can be employed to enhance the optical emission from various illuminants. Here, we study the photoluminescence properties of the ZnO/Bi2Te3 hybrid structures. Thin flakes of Bi2Te3, a typical three-dimensional topological insulator, were prepared on ZnO crystal surface by mechanical exfoliation method. The ultraviolet emission from ZnO was found to be enhanced by the Bi2Te3 thin flakes, which was attributed to the surface plasmon – photon coupling at the Bi2Te3/ZnO interface.

Writing with atoms: Oxygen adatoms on the MoO2/Mo(110) surface

AbstractWriting at the nanoscale using the desorption of oxygen adatoms from the oxygen-rich MoO2+x/Mo(110) surface is demonstrated by scanning tunnelling microscopy (STM). High-temperature oxidation of the Mo(110) surface results in a strained, bulk-like MoO2(010) ultra-thin film with an O-Mo-O trilayer structure. Due to the lattice mismatch between the Mo(110) and the MoO2(010), the latter consists of well-ordered molybdenum oxide nanorows separated by 2.5 nm. The MoO2(010)/Mo(110) structure is confirmed by STM data and density functional theory calculations. Further oxidation results in the oxygen-rich MoO2+x/Mo(110) surface, which exhibits perfectly aligned double rows of oxygen adatoms, imaged by STM as bright protrusions. These adatoms can be removed from the surface by scanning (or pulsing) at positive sample biases greater than 1.5 V. Tip movement along the surface can be used for controlled lithography (or writing) at the nanoscale, with a minimum feature size of just 3 nm. By moving the STM tip in a predetermined fashion, information can be written and read by applying specific biases between the surface and the tip.

Correlation between charge-transfer and rotation of C60 on WO2/W(110)

Understanding molecular switching between different charge states is crucial to further progress in molecule-based nano-electronic devices. Herein we have employed scanning tunnelling microscopy to visualize different charge states of a single C60 molecule within a molecular layer grown on the WO2/W(110) surface. The results obtained demonstrate that individual C60 molecules within the layer switch between neutral and negatively charged states in the temperature range of 220-260 K over the time scale of the experiment. The charging of the C60 causes changes in the local density of electron states and consequently a variation in tunnelling current. Using density functional theory calculations, it was found that the charged state corresponds to the negatively charged C60(-), which has accepted an electron. The switching of the molecule into the charged state is triggered continuously by tunnelling electrons when the STM tip is static above an individual C60 molecule with a bias applied. Molecular movement accompanies the molecules switching between these states.

Self-assembly of Fe nanocluster arrays on templated surfaces

The growth of Fe nanoclusters on the Ge(001) and MoO2/Mo(110) surfaces has been studied using low-temperature scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). STM results indicate that at low coverage Fe atoms self-assemble on both surfaces into well-separated nanoclusters, which nucleate at equivalent surface sites. Their size, shape, and the observed spatial separation are dictated by the substrate and depend on preparation conditions. Annealing the Fe nanoclusters on Ge(001) at 420 K leads to the formation of linear nanocluster arrays, which follow the Ge dimer rows of the substrate, due to cluster mobility at such temperature. In turn, linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature at a surface coverage greater than 0.5 monolayer. This is due to the more pronounced row pattern of the MoO2/Mo(110) surface compared to Ge(001). These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. The...

Nanoclusters and nanolines: the effect of molybdenum oxide substrate stoichiometry on iron self-assembly

The growth of Fe nanostructures on the stoichiometric MoO2/Mo(110) and oxygen-rich MoO2+x /Mo(110) surfaces has been studied using low-temperature scanning tunnelling microscopy (STM) and density functional theory calculations. STM results indicate that at low coverage Fe nucleates on the MoO2/Mo(110) surface, forming small, well-ordered nanoclusters of uniform size, each consisting of five Fe atoms. These five-atom clusters can agglomerate into larger nanostructures reflecting the substrate geometry, but they retain their individual character within the structure. Linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature when the surface coverage is greater than 0.6 monolayers. These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. Slightly altering the preparation procedure of MoO2/Mo(110) leads to the presence of oxygen adatoms on this surface. Fe deposition onto the oxygen-rich MoO2+x /Mo(110) surface results in elongated nanostructures that reach up to 24 nm in length. These nanolines have a zigzag shape and are likely composed of partially oxidised Fe formed upon reaction with the oxygen-rich surface.

Rotation dynamics of C60 molecules in a monolayer fullerene film on the WO2/W(110) surface near the rotational phase transition

The rotation dynamics of C60 molecules in monolayer fullerene films grown on the WO2/W(110) surface is studied by scanning tunneling microscopy. The formation of molecule clusters, which have a high libron vibration amplitude, is detected near the rotational phase transition temperature. The energy parameters that determine a change in the molecule orientation, namely, the energy difference between the nearest minima of the C60 molecule energy (30 meV) as a function of the molecule orientation and the potential barrier between them (610 meV), are determined. The results are discussed in terms of the mean-field approximation.

Application of single crystalline tungsten for fabrication of high resolution STM probes with controlled structure

The possibility to fabricate scanning tunneling microscopy (STM) probes with controlled electronic structure using single crystalline tungsten tips is discussed. High resolution power of oriented single crystalline probes is demonstrated in atomic and subatomic resolution STM studies of silicon, gallium telluride and graphite surfaces. The possibility of controllable selection of the tungsten tip atom electron orbitals responsible for the surface imaging in STM experiments is demonstrated.