Pingo Mutombo

Chemical structure imaging of a single molecule by atomic force microscopy at room temperature

Atomic force microscopy is capable of resolving the chemical structure of a single molecule on a surface. In previous research, such high resolution has only been obtained at low temperatures. Here we demonstrate that the chemical structure of a single molecule can be clearly revealed even at room temperature. 3,4,9,10-perylene tetracarboxylic dianhydride, which is strongly adsorbed onto a corner-hole site of a Si(111)–(7 × 7) surface in a bridge-like configuration is used for demonstration. Force spectroscopy combined with first-principle calculations clarifies that chemical structures can be resolved independent of tip reactivity. We show that the submolecular contrast over a central part of the molecule is achieved in the repulsive regime due to differences in the attractive van der Waals interaction and the Pauli repulsive interaction between different sites of the molecule.

Mapping the electrostatic force field of single molecules from high-resolution scanning probe images

How electronic charge is distributed over a molecule determines to a large extent its chemical properties. Here, we demonstrate how the electrostatic force field, originating from the inhomogeneous charge distribution in a molecule, can be measured with submolecular resolution. We exploit the fact that distortions typically observed in high-resolution atomic force microscopy images are for a significant part caused by the electrostatic force acting between charges of the tip and the molecule of interest. By finding a geometrical transformation between two high-resolution AFM images acquired with two different tips, the electrostatic force field or potential over individual molecules and self-assemblies thereof can be reconstructed with submolecular resolution.

Chemical identification of single atoms in heterogeneous III-IV chains on Si(100) surface by means of nc-AFM and DFT calculations.

Chemical identification of individual atoms in mixed In-Sn chains grown on a Si(100)-(2 × 1) surface was investigated by means of room temperature dynamic noncontact AFM measurements and DFT calculations. We demonstrate that the chemical nature of each atom in the chain can be identified by means of measurements of the short-range forces acting between an AFM tip and the atom. On the basis of this method, we revealed incorporation of silicon atoms from the substrate into the metal chains. Analysis of the measured and calculated short-range forces indicates that even different chemical states of a single atom can be distinguished.

Electronic and Chemical Properties of Donor, Acceptor Centers in Graphene

Chemical doping is one of the most suitable ways of tuning the electronic properties of graphene and a promising candidate for a band gap opening. In this work we report a reliable and tunable method for preparation of high-quality boron and nitrogen co-doped graphene on silicon carbide substrate. We combine experimental (dAFM, STM, XPS, NEXAFS) and theoretical (total energy DFT and simulated STM) studies to analyze the structural, chemical, and electronic properties of the single-atom substitutional dopants in graphene. We show that chemical identification of boron and nitrogen substitutional defects can be achieved in the STM channel due to the quantum interference effect, arising due to the specific electronic structure of nitrogen dopant sites. Chemical reactivity of single boron and nitrogen dopants is analyzed using force-distance spectroscopy by means of dAFM.

Temperature study of phase coexistence in the system Pb on an Si(111) surface

The temperature-dependent coexistence of Si 0.28 Pb 0.72 /Si( 111) and Si(111)-( 1 ×1)-Pb phases was investigated using scanning tunneling microscopy. An area of (I x 1)-Pb islands was monitored as a function of temperature ranging from room temperature up to 400°C. The movie strategy was applied in order to acquire successive images of a selected region. It was found that this area fluctuates quasi-periodically in time, with amplitude depending on temperature. The strongest fluctuations (up to 10% of the mean area value) were observed at 210°C. These fluctuations were restricted to the defect-free parts of the island perimeter, the rest of the ( 1 ×1)-Pb phase being stable. A further increase in temperature lead to a monotonous temperature-dependent decrease in ( 1 ×1)-Pb areas. This decrease was observed to be strongly tip-influenced at temperatures slightly above 210°C, while at higher temperatures the desorption of Pb atoms prevailed.

Characteristic Contrast in Δfmin Maps of Organic Molecules Using Atomic Force Microscopy

Scanning tunneling microscopy and atomic force microscopy can provide detailed information about the geometric and electronic structure of molecules with submolecular spatial resolution. However, an essential capability to realize the full potential of these techniques for chemical applications is missing from the scanning probe toolbox: chemical recognition of organic molecules. Here, we show that maps of the minima of frequency shift-distance curves extracted from 3D data cubes contain characteristic contrast. A detailed theoretical analysis based on density functional theory and molecular mechanics shows that these features are characteristic for the investigated species. Structurally similar but chemically distinct molecules yield significantly different features. We find that the van der Waals and Pauli interaction, together with the specific adsorption geometry of a given molecule on the surface, accounts for the observed contrast.

Ortho and Para Hydrogen Dimers on G/SiC(0001): Combined STM and DFT Study

The hydrogen (H) dimer structures formed upon room-temperature H adsorption on single layer graphene (SLG) grown on SiC(0001) are addressed using a combined theoretical-experimental approach. Our study includes density functional theory (DFT) calculations for the full (6√3 × 6√3)R30° unit cell of the SLG/SiC(0001) substrate and atomically resolved scanning tunneling microscopy images determining simultaneously the graphene lattice and the internal structure of the H adsorbates. We show that H atoms normally group in chemisorbed coupled structures of different sizes and orientations. We make an atomic scale determination of the most stable experimental geometries, the small dimers and ellipsoid-shaped features, and we assign them to hydrogen adsorbed in para dimers and ortho dimers configuration, respectively, through comparison with the theory.

Phase-sensitive lock-in imaging of surface densities of states

A new way of imaging the local density of states has been devised through a combination of the constant-height scanning tunnelling microscopy operational mode and lock-in techniques. We have obtained current images simultaneously with real space dynamical conductance maps (d I/d V) for energies around the Fermi level, on the Si(111)-(7 × 7) surface. We reconstructed the normalized dynamical conductance spectra—(d I/d V)/(I/V). Since the (d I/d V)/(I/V) curves are closely related to the local densities of states, we compared their sum over the unit cell to photoelectron spectra and theoretical calculations. We find that the results are in good agreement. Consequently, the extent of localization of surface electronic states at lattice positions was determined.

Interplay between Switching Driven by the Tunneling Current and Atomic Force of a Bistable Four-Atom Si Quantum Dot

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.

The effect of potassium on the adsorption of gold on the TiO2(110)-1 × 1 surface

Density functional theory (DFT) total-energy calculations have been used to investigate the effect of potassium on the adsorption geometry of gold on a TiO(2)(110)- 1 × 1 surface. The gold prefers to sit between the two bridge oxygen atoms above the sixfold titanium atom. The addition of potassium significantly affects the bonding geometry of the gold. Potassium displaces gold from the bridge site and causes its migration to the top of the fivefold titanium atom. Our calculations suggest that potassium is bonded to the bridging oxygen atoms, and to the sixfold titanium atom as well as to gold. This excludes the formation of a K(2)O-like compound at the surface.