Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy

A simple, reliable method for preparation of bulk Cr tips for Scanning Tunneling Microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7x7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations

Interface coupling in Au-supported MoS2–WS2 heterobilayers grown by pulsed laser deposition

Van der Waals heterostructures of transition metal dichalcogenides (TMDs) are promising systems for engineering functional layered 2D materials with tailored properties. In this work, we study the growth of WS2/MoS2 and MoS2/WS2 heterobilayers by pulsed laser deposition (PLD) under ultra-high vacuum conditions. Using Au(111) as growth substrate, we investigated the heterobilayer morphology and structure at the nanoscale by in-situ scanning tunneling microscopy. Our experiments show that the heterostructure growth can be controlled with high coverage and thickness sensitivity by tuning the number of laser pulses in the PLD process. Raman spectroscopy complemented our investigation, revealing the effect of the interaction with the metallic substrate on the TMD vibrational properties and a strong interlayer coupling between the MoS2 and WS2 layers. The transfer of the heterobilayers on a silica substrate via a wet etching process shows the possibility to decouple them from the native metallic substrate and confirms that the interlayer coupling is not substrate-dependent. This work highlights the potential of the PLD technique as a method to grow TMD heterostructures, opening to new perspectives in the synthesis of complex 2D layered materials

Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon

A novel form of amorphous carbon with sp-sp2 hybridization has been recently produced by supersonic cluster beam deposition showing the presence in the film of both polyynic and cumulenic species [L. Ravagnan et al. Phys. Rev. Lett. 98, 216103 (2007)]. Here we present a in situ Raman characterization of the low frequency vibrational region (400-800 cm-1) of sp-sp2 films at different temperatures. We report the presence of two peaks at 450 cm-1 and 720 cm-1. The lower frequency peak shows an evolution with the variation of the sp content and it can be attributed, with the support of density functional theory (DFT) simulations, to bending modes of sp linear structures. The peak at 720 cm-1 does not vary with the sp content and it can be attributed to a feature in the vibrational density of states activated by the disorder of the sp2 phase.Comment: 15 pages, 5 figures, 1 tabl

Vibrational - Electrical Properties Relationship in Donor Doped TiO2 by Raman Spectroscopy

Transparent conducting TiO2, obtained by Nb or Ta doping of the anatase structure, is gaining increasing attention for the development of transparent electrodes. Usually, regardless the deposition technique, a crystallization process in reducing atmosphere is necessary to achieve large mobility; in addition, electrical and optical properties are also strongly sensitive to the oxygen deposition pressure. These facts reveal that the defect chemistry of donor doped TiO2 is not trivial and involves a strict interplay among extrinsic dopant atoms, oxygen vacancies and ‘electron killer’ defects such as Ti vacancies and O interstitials. We here present a Raman characterization of donor-doped TiO2 films synthesized under several deposition and post-annealing conditions, employing different doping levels and dopant elements (i.e. Ta and Nb). Correlations between structure, crystallinity, shift and width of Raman peaks and electrical properties are shown and discussed. In particular, a clear relationship between the shift of the Eg(1) anatase Raman mode and the charge carrier density is found, while the B1g(1) mode connected to Ti-Ti vibrations is significantly affected by the extrinsic doping level. In this complex framework Raman spectroscopy can provide an invaluable contribution towards understanding the material structure and its influence on the functional properties

Two-dimensional TiOx nanostructures on Au(111): a Scanning Tunneling Microscopy and Spectroscopy investigation

We investigated the growth of titanium oxide two-dimensional (2D) nanostructures on Au(111), produced by Ti evaporation and post-deposition oxidation. Scanning tunneling microscopy and spectroscopy (STM and STS) and low-energy electron diffraction (LEED) measurements characterized the morphological, structural and electronic properties of the observed structures. Five distinct TiOx phases were identified: the honeycomb and pinwheel phases appear as monolayer films wetting the gold surface, while nanocrystallites of the triangular, row and needle phases grow mainly over the honeycomb or pinwheel layers. Density Functional Theory (DFT) investigation of the honeycomb structure supports a (2 x 2) structural model based on a Ti-O bilayer having Ti2O3 stoichiometry. The pinwheel phase was observed to evolve, for increasing coverage, from single triangular crystallites to a well-ordered film forming a (4*sqrt(7) x 4*sqrt(7))R19.1° superstructure, which can be interpreted within a moire-like model. Structural characteristics of the other three phases were disclosed from the analysis of high-resolution STM measurements. STS measurements revealed a partial metallization of honeycomb and pinwheel and a semiconducting character of row and triangular phases

Excitation Wavelength- and Medium-Dependent Photoluminescence of Reduced Nanostructured TiO2 Films

The performance of TiO2 nanomaterials in solar energy conversion applications can be tuned by means of thermal treatments in reducing atmospheres, which introduce defects (such as oxygen vacancies), allowing, for instance, a better charge transport or a higher photocatalytic activity. The characterization of these defects and the understanding of their role are pivotal to carefully engineer the properties of TiO2, and, among various methods, they have been addressed by photoluminescence (PL) spectroscopy. A definitive framework to describe the PL properties of TiO2, however, is still lacking. In this work, we report on the PL of nanostructured anatase TiO2 thin films, annealed in different atmospheres (oxidizing and reducing), and consider the effects of different excitation energies and different surrounding media on their PL spectra. A broad PL signal centered around 1.8–2.0 eV is found for all the films with UV excitation in air as well as in vacuum, while the same measurements in ethanol lead to a blueshift and to intensity changes in the spectra. On the other hand, measurements with different sub-bandgap excitations show PL peaking at 1.8 eV, with an intensity trend only dependent on the thermal treatment and not on the surrounding medium. The results of PL spectroscopy, together with electron paramagnetic resonance spectroscopy, suggest the critical role of oxygen vacancies and Ti3+ ions as radiative recombination centers. The complex relationship between thermal treatments and PL data in the explored conditions is discussed, suggesting the importance of such investigations for a deeper understanding on the relationship between defects in TiO2 and photoactivity

Morphology-driven electrical and optical properties in graded hierarchical transparent conducting Al:ZnO

Graded Al-doped ZnO layers, constituted by a mesoporous forest like system evolving into a compact transparent conductor, were synthesized by Pulsed Laser Deposition with different morphology to study the correlation with functional properties. Morphology was monitored by measuring the resulting surface roughness and its effects on electrical conductivity (especially carrier mobility, which significantly decreases with increasing roughness) allow to discuss the limitations in conduction mechanisms. Significant changes in light scattering capability due to variations in morphology are also investigated and discussed to study the correlation between morphology and functional properties.Comment: 11 pages, 4 figure

Structure-dependent optical and electrical transport properties of nanostructured Al-doped ZnO

The structure-property relation of nanostructured Al-doped ZnO thin films has been investigated in detail through a systematic variation of structure and morphology, with particular emphasis on how they affect optical and electrical properties. A variety of structures, ranging from compact polycristalline films to mesoporous, hierarchically organized cluster assemblies, are grown by Pulsed Laser Deposition at room temperature at different oxygen pressures. We investigate the dependence of functional properties on structure and morphology and show how the correlation between electrical and optical properties can be studied to evaluate energy gap, conduction band effective mass and transport mechanisms. Understanding these properties opens the way for specific applications in photovoltaic devices, where optimized combinations of conductivity, transparency and light scattering are required.Comment: 8 pages, 9 figure

Enhancing Light Harvesting by Hierarchical Functionally Graded Transparent Conducting Al-doped ZnO Nano- and Mesoarchitectures

A functionally graded Al-doped ZnO structure is presented which combines conductivity, visible transparency and light scattering with mechanical flexibility. The nano and meso-architecture, constituted by a hierarchical, large surface area, mesoporous tree-like structure evolving in a compact layer, is synthesized at room temperature and is fully compatible with plastic substrates. Light trapping capability is demonstrated by showing up to 100% improvement of light absorption of a low bandgap polymer employed as the active layer.Comment: 21 pages, 6 figures, submitted to Solar Energy Materials and Solar Cell