Denis V. Potapenko

Scanning tunneling microscopy study of titanium oxide nanocrystals prepared on Au(111) by reactive-layer-assisted deposition.

We report on an scanning tunneling microscopy study of the nanocrystallite phases of TiO(2) formed via reactive-layer-assisted deposition in ultrahigh vacuum. The synthesis used reaction of a thin layer of water, on a Au(111) substrate at 130 K, with low-coverage vapor-deposited Ti. The effects of annealing temperature and reactant coverage were investigated. Large-scale (>20 nm) patterns in the surface distribution of nanoparticles were observed with the characteristic length-scale of the pattern correlating with the thickness of the initial layer of H(2)O. The phenomenon is explained as being due to the formation of droplets of liquid water at temperatures between 130 and 300 K. After the surface was annealed to 400 K, the individual titania nanoparticles formed by this process had diameters of 0.5-1 nm. When the surface was annealed to higher temperatures, nanoparticles coalesced and for annealing temperatures of 900 K compact nanocrystals formed with typical dimensions of 5-20 nm. Three distinct classes of nanocrystallites were observed and their atomic structure and composition investigated and discussed.

Nanoscale Strain Engineering on the Surface of a Bulk TiO2 Crystal

Arrays of highly strained 5-25 nm-wide regions have been prepared on rutile TiO2(110) surface through a low energy Ar ion bombardment technique. Using scanning tunneling microscopy (STM) and an innovative STM tip-triggered nanoexplosion approach we show experimentally that the protrusions arise from subsurface Ar-filled pockets. Continuum mechanics modeling gives good estimates of the corresponding elastic deformation. Surface strain values of up to 4% have been deduced.

Preparation of TiO2 Nanocrystallites by Oxidation of Ti―Au(111) Surface Alloy

Ti-Au surface alloy oxidation is used to form nanocrystals of TiO(2) on Au(111). In situ scanning tunneling microscopy (STM) studies show that the approach yields arrays of 8-11 nm wide crystals with relatively narrow size dispersion and uniform crystallography. STM imaging shows that their crystallographic form is rutile with a triangular or hexagonal geometry. Scanning tunneling spectroscopy indicates that the crystals have a well-developed band gap, comparable to that in bulk TiO(2).

Controlling surface reactions with nanopatterned surface elastic strain.

The application of elastic lattice strain is a promising approach for tuning material properties, but the attainment of a systematic approach for introducing a high level of strain in materials so as to study its effects has been a major challenge. Here we create an array of intense locally varying strain fields on a TiO2 (110) surface by introducing highly pressurized argon nanoclusters at 6-20 monolayers under the surface. By combining scanning tunneling microscopy imaging and the continuum mechanics model, we show that strain causes the surface bridge-bonded oxygen vacancies (BBOv), which are typically present on this surface, to be absent from the strained area and generates defect-free regions. In addition, we find that the adsorption energy of hydrogen binding to oxygen (BBO) is significantly altered by local lattice strain. In particular, the adsorption energy of hydrogen on BBO rows is reduced by ∼ 35 meV when the local crystal lattice is compressed by ∼ 1.3%. Our results provide direct evidence of the influence of strain on atomic-scale surface chemical properties, and such effects may help guide future research in catalysis materials design.