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

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 materiala € 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 materialsWork supported by the NIMS (AA002 and AF006 projects), by the MEXT KAKENHI Grant Number 26104540, by the Charles University (GAUK 339311) and by the Spanish MINECO (projects PLE2009-0061, MAT2011- 023627 and CSD2010-00024). Computer time was provided by the Spanish Supercomputing Network (RES, Spain) at the MareNostrum III Supercomputer (BCS, Barcelona), and by the PRACE initiative (project RA0986) at the Curie Supercomputer (CEA, France). O.S and V.M. thank the Charles University-NIMS International Cooperative Graduate School Program. J.W.R. thanks NIMS for funding through the NIMS Internship Program and ICIQ for his ICIQ Fellowshi

Charakterizace modelových CeO2 a TiO2 systémů pomocí rastrovací tunelové mikroskopie a mikroskopie atomárních sil

Atomic scale characterization of materials is important for the fundamental understanding of their properties. Here, model systems of industrially relevant cerium and titanium oxides are characterized with the combination of the Scanning Tunneling Microscopy (STM) and Non Contact Atomic Force Microscopy (NC AFM). Cerium oxide model systems are represented by fully oxidized and partially reduced ultra-thin ceria films supported on copper single crystal. Interaction of the model ceria systems with catalytically important adsorbates (water, methanol) is studied on atomic scale. Titanium oxide model systems are represented by pentacene and C60 molecules adsorbed on the surface of bulk titania in anatase polymorph. Organic layers on titania are studied with intramolecular resolution with the help of the newly developed Double pass scanning mode of NC AFM. The atomic contrast formation mechanisms in STM and NC AFM on ceria and anatase surface are presented. Powered by TCPDF (www.tcpdf.org)Charakterizace materialu na atomarni urovni je nepostradatelna pro zakladni vyzkum jejich vlastnosti. V teto praci jsou charakterizovany modelove systemy technologicky dulezitych oxidu ceru a titanu kombinaci rastrovaci tunelove mikroskopie (STM) a bezkontaktni mikroskopie atomarnich sil (NC AFM). Modelove systemy oxidu ceru jsou ve forme tenkych vrstev oxidovaneho a redukovaneho oxidu ceru na monokrystalu medi. Je studovana interakce techto modelovych systemu s adsorbaty dulezitymi pro heterogenni katalyzu (voda, metanol) na atomarni urovni. Modelove systemy oxidu titanu jsou ve forme molekul C60 a pentacenu adsorbovanych na povrchu monokrystalu oxidu titanu - anatase. Tyto organicke tenke vrstvy na oxidu titanu jsou studovany s intramolekularnim rozlisenim pomoci nove vyvinute merici metody dvoupruchodoveho rastrovani v NC AFM. Je predstaveno take vysvetleni vzniku atomarniho kontrastu na povrsich oxidu ceru a titanu v STM i NC AFM. Powered by TCPDF (www.tcpdf.org)Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Investigating model CeO2 and TiO2 systems by means of Scanning Tunneling Microscopy and Atomic Force Microscopy

Atomic scale characterization of materials is important for the fundamental understanding of their properties. Here, model systems of industrially relevant cerium and titanium oxides are characterized with the combination of the Scanning Tunneling Microscopy (STM) and Non Contact Atomic Force Microscopy (NC AFM). Cerium oxide model systems are represented by fully oxidized and partially reduced ultra-thin ceria films supported on copper single crystal. Interaction of the model ceria systems with catalytically important adsorbates (water, methanol) is studied on atomic scale. Titanium oxide model systems are represented by pentacene and C60 molecules adsorbed on the surface of bulk titania in anatase polymorph. Organic layers on titania are studied with intramolecular resolution with the help of the newly developed Double pass scanning mode of NC AFM. The atomic contrast formation mechanisms in STM and NC AFM on ceria and anatase surface are presented. Powered by TCPDF (www.tcpdf.org

Counting Molecules: Python based scheme for automated enumeration and categorization of molecules in scanning tunneling microscopy images

Scanning tunneling and atomic force microscopies (STM/nc-AFM) are rapidly progressing to offer unprecedented spatial resolution of a diverse array of chemical species. In particular, they are employed to characterize on-surface chemical reactions by directly examining precursors and products. Chiral effects and self-assembled structures can also be investigated. This open source, modular, python based scheme automates the categorization of a variety of molecules present in medium sized (10$\times$10 to 100$\times$100 nm) scanned probe images

Z3 Charge Density Wave of Silicon Atomic Chains on a Vicinal Silicon Surface

An ideal one-dimensional electronic system is formed along atomic chains on Au-decorated vicinal silicon surfaces but the nature of its low temperature phases has been puzzled for last two decades. Here, we unambiguously identify the low temperature structural distortion of this surface using high resolution atomic force microscopy and scanning tunneling microscopy. The most important structural ingredient of this surface, the step-edge Si chains are found to be strongly buckled, every third atoms down, forming trimer unitcells. This observation is consistent with the recent model of rehybridized dangling bonds and rules out the antiferromagnetic spin ordering proposed earlier. The spectroscopy and electronic structure calculation indicate a charge density wave insulator with a Z3 topology making it possible to exploit topological phases and excitations. Tunneling current was found to substantially lower the energy barrier between three degenerate CDW states, which induces a dynamically fluctuating CDW at very low temperature

Pentacene/TiO2 Anatase Hybrid Interface Study by Scanning Probe Microscopy and First Principles Calculations

The understanding and control of the buried interface between functional materials in optoelectronic devices is key to improving device performance. We combined atomic resolution scanning probe microscopy with first-principles calculations to characterize the technologically relevant organic/inorganic interface structure between pentacene molecules and the TiO2 anatase (101) surface. A multipass atomic force microscopy imaging technique overcomes the technical challenge of imaging simultaneously the corrugated anatase substrate, molecular adsorbates, monolayers, and bilayers at the same level of detail. Submolecular resolution images revealed the orientation of the adsorbates with respect to the substrate and allowed direct insights into interface formation. Pentacene molecules were found to physisorb parallel to the anatase substrate in the first contact layer, passivating the surface and promoting bulk-like growth in further organic layers. While molecular electronic states were not significantly hybridizedby the substrate, simulations predicted localized pathways for molecule-surface charge injection. The localized states were associated with the molecular lowest unoccupied molecular orbital inside the oxide conduction band, pointing to efficient transfer of photo-induced electron charge carriers across this interface in prospective photovoltaic devices. In uncovering the atomic arrangement and favorable electronic properties of the pentacene/anatase interface, our findings testify to the maturity and analytic power of our methodology in further studies of organic/inorganic interfaces.Peer reviewe

Imaging Three-Dimensional Surface Objects with Submolecular Resolution by Atomic Force Microscopy

Submolecular imaging by atomic force microscopy (AFM) has recently been established as a stunning technique to reveal the chemical structure of unknown molecules, to characterize intramolecular charge distributions and bond ordering, as well as to study chemical transformations and intermolecular interactions. So far, most of these feats were achieved on planar molecular systems because high-resolution imaging of three-dimensional (3D) surface structures with AFM remains challenging. Here we present a method for high-resolution imaging of nonplanar molecules and 3D surface systems using AFM with silicon cantilevers as force sensors. We demonstrate this method by resolving the step-edges of the (101) anatase surface at the atomic scale by simultaneously visualizing the structure of a pentacene molecule together with the atomic positions of the substrate and by resolving the contour and probe-surface force field on a C₆₀ molecule with intramolecular resolution. The method reported here holds substantial promise for the study of 3D surface systems such as nanotubes, clusters, nanoparticles, polymers, and biomolecules using AFM with high resolution

Creation and annihilation of mobile fractional solitons in atomic chains

The movement of fractionalized phase defects, that can be considered as fractional solitons promising for future information technology, is observed in atomic chains formed along step edges of silicon surfaces, solitons may serve as robust, topologically protected information carriers in future information technology Localized modes in one-dimensional (1D) topological systems, such as Majonara modes in topological superconductors, are promising candidates for robust information processing. While theory predicts mobile integer and fractional topological solitons in 1D topological insulators, experiments so far have unveiled immobile, integer solitons only. Here we observe fractionalized phase defects moving along trimer silicon atomic chains formed along step edges of a vicinal silicon surface. By means of tunnelling microscopy, we identify local defects with phase shifts of 2 pi/3 and 4 pi/3 with their electronic states within the band gap and with their motions activated above 100 K. Theoretical calculations reveal the topological soliton origin of the phase defects with fractional charges of +/- 2e/3 and +/- 4e/3. Additionally, we create and annihilate individual solitons at desired locations by current pulses from the probe tip. Mobile and manipulable topological solitons may serve as robust, topologically protected information carriers in future information technology.11Nsciescopu

Resolving Ambiguity of the Kondo Temperature Determination in Mechanically Tunable Single-Molecule Kondo Systems

Determination of the molecular Kondo temperature (TK) poses a challenge in most cases when the experimental temperature cannot be tuned to a sufficient extent. We show how this ambiguity can be resolved if additional control parameters are present, such as magnetic field and mechanical gating. We record the evolution of the differential conductance by lifting an individual molecule from the metal surface with the tip of a scanning tunneling microscope. By fitting the measured conductance spectra with the single impurity Anderson model we are able to demonstrate that the lifting tunes the junction continuously from the strongly correlated Kondo-singlet to the free spin-1/2 ground state. In the crossover regime, where TK is similar to the temperature of experiment, the fitting yields ambiguous estimates of TK varying by an order of magnitude. We show that analysis of the conductance measured in two distinct external magnetic fields can be used to resolve this problem