Atomic Force Microscopy/Scanning Tunneling Microscopy 3

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Along the line profile highlighted in Fig. This weak contrast in the AFM topography, although compatible with the calculations, is clearly not enough for the identification. These combined features provide strong support to identify the defect highlighted in Fig. Such defects may correspond to the presence of interstitial atoms 15 or hydrogen incorporated into the bulk A direct comparison between these two types of images is in principle difficult due to the different acquisition scheme These calculated images correspond to different isosurfaces of the local density of states of the anatase surface integrated in a 0.

Larger isosurface values correspond to larger current set points, and thus, to scanning closer to the surface. The images reveal that when scanning with STM at distances far from the surface, the current maxima are essentially spherical and centered around the Ti 5 c atoms, but when scanning at higher set points, the current maxima widen out due to an increasing contribution from the O 2 c atoms.

The contribution of the different atomic species to the STM tunnelling current is available from the corresponding line profiles green line in the images , which are compared in Fig. The simulated topography corrugation depends strongly on the tip—surface distance. Previous conventional STM measurements and theoretical results 45 indicate a significant contribution of the O 2 c atoms to the images, which we also obtain for simulated STM images close to the surface.

Those theoretical STM images 45 were calculated at a close distance 2. The larger bias voltage also enhances the role of the O 2 c atoms, as it is shown in the projected density of states of the surface atoms see Supplemental Fig. Larger isosurface values correspond to STM scans closer to the surface as indicated by the arrow.

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The blue red vertical lines correspond to the position of the Ti 5 c O 2 c atoms. In summary, our results demonstrate the benefits of combined AFM and averaged tunnelling current imaging for the study of oxide surfaces at the atomic scale, and specially for the characterization of defects and adsorbates that play a crucial role in the catalytic and energy-harvesting applications of these materials.

We have applied these techniques for the discrimination and simultaneous imaging of different atomic species—with O 2 c Ti 5 c dominating the AFM averaged tunnelling current channel—at the TiO 2 anatase surface and showed tunnelling current acquisition within the surface band gap, where standard STM imaging is challenging.

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First-principles simulations provide an explanation for the small tip-surface interactions in terms of an hydroxyl-terminated tip and reveal the role of dynamic tip relaxation effects in contrast formation during AFM imaging. Assisted by key differentiating traits extracted from our first-principles AFM and STM calculations, we have identified candidates for the most common surface defects, such as subsurface oxygen vacancies and surface hydroxyls. Our STM calculations also shed light on the role of the tip—surface separation and the bias voltage on the tunnelling current detected over the anatase surface, explaining the contrast observed in the averaged tunnelling current images with respect to conventional topographic STM images previously reported.

The findings reported here provide the foundation for future work on anatase, including the thorough characterization of a large amount of purposefully created defects.

More importantly, they pave the way for the study of more complex anatase systems related to water splitting and organic photovoltaics solar energy conversion , including the adsorption geometries and binding sites of organic dyes and other photoactive molecules like pentacene and buckminsterfullerene, as well as metal dopants to enhance hydrogen production. Platinum—iridium-coated silicon cantilevers PPP-NCLPt, Nanosensors, Switzerland were instantaneously excited to their first mechanical resonant frequency keeping the oscillation amplitude constant.

The determination of the corresponding K and A values are described elsewhere The absence of any tip or surface modification during the spectroscopic acquisition was carefully checked In standard AFM imaging and force spectroscopic measurements, the long-range electrostatic interaction was minimized by compensating the CPD. Topographic effects on Z due to the spectroscopic acquisition were compensated 50 , assuring a common origin for curves measured at different locations with respect to the surface plane. The tip—surface interatomic forces were obtained by subtracting an appropriate fit 47 , 48 over the long-range interaction region to the total force.

Atomic Force Microscopy/Scanning Tunneling Microscopy 3

For the comparison of the experimental and theoretical forces in Fig. This method prevents a slight tip retraction from the surface on increasing the electrostatic force, approximately keeping the same tip—surface separation for images acquired at different bias voltages. This procedure often results in the tip picking up surface material—holes of a few nanometers diameter were normally detected at the surface after the tip conditioning—in an environment where most of the UHV residual gas consists of hydrogen molecules.

Natural single crystals of anatase cut exhibiting a polished surface were purchased from Surface Net GmbH www. The anatase natural crystals used in this work have a variety of impurities that may differ in concentration and nature from one crystal to another. With our sample mounting and surface preparation protocol, we have not observed the formation of surface structures assigned to segregation of impurities Water dosage was carried out by exposing a clean anatase surface to 0.

Ultra-clean Milli-Q water—further purified by several freeze—pump—thaw cycles—was introduced into the UHV system as vapour via a leak valve. This U value is compatible with recent theoretical results The surface was modelled as a Smooth force curves were obtained by fitting a Morse potential function modified by a polynomial to the theoretical force data points.

OSA | Advantages of photon scanning tunneling microscope combined with atomic force microscope

How to cite this article: Stetsovych, O. Atomic species identification at the anatase surface by simultaneous scanning tunnelling and atomic force microscopy. Fujishima, A.

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Electrochemical photolysis of water at a semiconductor electrode. Nature , 37—38 Titanium dioxide photocatalysis. C 1 , 1—21 O'Regan, B. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature , — Shi, E. TiO2 photocatalysis and related surface phenomena.

Henderson, M. A surface science perspective on TiO2 photocatalysis.

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Direct visualization of defect-mediated dissociation of water on TiO2 Sader, J.