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Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy.

Stetsovych O, Todorović M, Shimizu TK, Moreno C, Ryan JW, León CP, Sagisaka K, Palomares E, Matolín V, Fujita D, Perez R, Custance O - Nat Commun (2015)

Bottom Line: Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments.Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects.Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

View Article: PubMed Central - PubMed

Affiliation: 1] National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan [2] Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, Praha 8, Czech Republic.

ABSTRACT
Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the material's band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

No MeSH data available.


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Distance dependence of the STM topography on the TiO2 (101) anatase surface.(a) Images of the clean surface calculated using the Tersoff-Hamann theory with the one of the large positive bias voltages used in our experiments (+0.8 eV). These images correspond to different isosurfaces (10−5 to 10−8 e bohr−3) of the local density of states of the anatase (101) surface, integrated in a 0.8 eV energy window from the conduction band minimum. Larger isosurface values correspond to STM scans closer to the surface as indicated by the arrow. (b) Line profiles corresponding to the green lines in the images and revealing the dependence of the STM topographic corrugation and the relative contribution of the different chemical species with the tip-surface separation distance. The blue (red) vertical lines correspond to the position of the Ti5c (O2c) atoms.
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f6: Distance dependence of the STM topography on the TiO2 (101) anatase surface.(a) Images of the clean surface calculated using the Tersoff-Hamann theory with the one of the large positive bias voltages used in our experiments (+0.8 eV). These images correspond to different isosurfaces (10−5 to 10−8 e bohr−3) of the local density of states of the anatase (101) surface, integrated in a 0.8 eV energy window from the conduction band minimum. Larger isosurface values correspond to STM scans closer to the surface as indicated by the arrow. (b) Line profiles corresponding to the green lines in the images and revealing the dependence of the STM topographic corrugation and the relative contribution of the different chemical species with the tip-surface separation distance. The blue (red) vertical lines correspond to the position of the Ti5c (O2c) atoms.

Mentions: We now consider the relation between the <It> and the topographic STM images of the anatase (101) surface. A direct comparison between these two types of images is in principle difficult due to the different acquisition scheme31. However, we provide theoretical evidence that makes our <It> measurements and previous STM observations compatible1314151617181920333442 under a common framework. Figure 6a shows simulated STM images of the clean surface computed with one of the large positive bias voltages used in our experiments (+0.8 V). These calculated images correspond to different isosurfaces of the local density of states of the anatase (101) surface integrated in a 0.8 eV energy window from the conduction band minimum. 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 Ti5c atoms, but when scanning at higher set points, the current maxima widen out due to an increasing contribution from the O2c 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. 6b. The simulated topography corrugation depends strongly on the tip–surface distance. The result closest to our experiments (isosurface of 10−7 e bohr−3) is associated with a STM imaging mode dominated by the Ti5c atoms. The latter corresponds to <It> images acquired typically between 4 to 5 Å above the surface, with deconvoluted tunnelling current values31 of a few tenths of a nanoampere (see Supplementary Fig. 1). Previous conventional STM measurements and theoretical results45 indicate a significant contribution of the O2c atoms to the images, which we also obtain for simulated STM images close to the surface. Those theoretical STM images45 were calculated at a close distance (2.5 Å) above the surface but with a bias of 1.5 eV, imitating the experimental bias voltages used in conventional STM imaging of this surface. The larger bias voltage also enhances the role of the O2c atoms, as it is shown in the projected density of states of the surface atoms (see Supplemental Fig. 5) in which the O2c contribution is negligible near the minimum of the conduction band but increases for energies above +0.6 eV. This small contribution of the O2c atoms to the local density of states at the bias voltages used in our experiments enhances the contribution of the Ti5c atoms to our <It> images even when scanning relatively close to the surface.


Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy.

Stetsovych O, Todorović M, Shimizu TK, Moreno C, Ryan JW, León CP, Sagisaka K, Palomares E, Matolín V, Fujita D, Perez R, Custance O - Nat Commun (2015)

Distance dependence of the STM topography on the TiO2 (101) anatase surface.(a) Images of the clean surface calculated using the Tersoff-Hamann theory with the one of the large positive bias voltages used in our experiments (+0.8 eV). These images correspond to different isosurfaces (10−5 to 10−8 e bohr−3) of the local density of states of the anatase (101) surface, integrated in a 0.8 eV energy window from the conduction band minimum. Larger isosurface values correspond to STM scans closer to the surface as indicated by the arrow. (b) Line profiles corresponding to the green lines in the images and revealing the dependence of the STM topographic corrugation and the relative contribution of the different chemical species with the tip-surface separation distance. The blue (red) vertical lines correspond to the position of the Ti5c (O2c) atoms.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4491188&req=5

f6: Distance dependence of the STM topography on the TiO2 (101) anatase surface.(a) Images of the clean surface calculated using the Tersoff-Hamann theory with the one of the large positive bias voltages used in our experiments (+0.8 eV). These images correspond to different isosurfaces (10−5 to 10−8 e bohr−3) of the local density of states of the anatase (101) surface, integrated in a 0.8 eV energy window from the conduction band minimum. Larger isosurface values correspond to STM scans closer to the surface as indicated by the arrow. (b) Line profiles corresponding to the green lines in the images and revealing the dependence of the STM topographic corrugation and the relative contribution of the different chemical species with the tip-surface separation distance. The blue (red) vertical lines correspond to the position of the Ti5c (O2c) atoms.
Mentions: We now consider the relation between the <It> and the topographic STM images of the anatase (101) surface. A direct comparison between these two types of images is in principle difficult due to the different acquisition scheme31. However, we provide theoretical evidence that makes our <It> measurements and previous STM observations compatible1314151617181920333442 under a common framework. Figure 6a shows simulated STM images of the clean surface computed with one of the large positive bias voltages used in our experiments (+0.8 V). These calculated images correspond to different isosurfaces of the local density of states of the anatase (101) surface integrated in a 0.8 eV energy window from the conduction band minimum. 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 Ti5c atoms, but when scanning at higher set points, the current maxima widen out due to an increasing contribution from the O2c 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. 6b. The simulated topography corrugation depends strongly on the tip–surface distance. The result closest to our experiments (isosurface of 10−7 e bohr−3) is associated with a STM imaging mode dominated by the Ti5c atoms. The latter corresponds to <It> images acquired typically between 4 to 5 Å above the surface, with deconvoluted tunnelling current values31 of a few tenths of a nanoampere (see Supplementary Fig. 1). Previous conventional STM measurements and theoretical results45 indicate a significant contribution of the O2c atoms to the images, which we also obtain for simulated STM images close to the surface. Those theoretical STM images45 were calculated at a close distance (2.5 Å) above the surface but with a bias of 1.5 eV, imitating the experimental bias voltages used in conventional STM imaging of this surface. The larger bias voltage also enhances the role of the O2c atoms, as it is shown in the projected density of states of the surface atoms (see Supplemental Fig. 5) in which the O2c contribution is negligible near the minimum of the conduction band but increases for energies above +0.6 eV. This small contribution of the O2c atoms to the local density of states at the bias voltages used in our experiments enhances the contribution of the Ti5c atoms to our <It> images even when scanning relatively close to the surface.

Bottom Line: Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments.Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects.Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

View Article: PubMed Central - PubMed

Affiliation: 1] National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan [2] Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, Praha 8, Czech Republic.

ABSTRACT
Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the material's band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

No MeSH data available.


Related in: MedlinePlus