Morphological, optical and photovoltaic characteristics of MoSe2/SiOx/Si heterojunctions.

This work reports the effect of different processing parameters on the structural and morphological characteristics of MoSe2 layers grown by chemical vapour deposition (CVD), using MoO3 and Se powders as solid precursors. It shows the strong dependence of the size, shape and thickness of the MoSe2 layers on the processing parameters. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM) and the thickness of the deposited layers was determined by atomic force microscopy (AFM). Raman and photoluminescence (PL) spectroscopies were used to confirm the high quality of the MoSe2 layers. Surface composition was examined by photoelectron spectroscopy (XPS). Moreover, the MoSe2/SiOx/Si heterojunctions exhibit diode behaviour, with a rectification ratio of 10, measured at ±2.0 V, which is due to the p-i-n heterojunctions formed at the p-Si/SiOx/MoSe2 interface. A photovoltaic effect was observed with a short circuit current density (Jsc), open circuit voltage (VOC) and efficiency of -0.80 mA/cm2, 1.55 V and 0.5%, respectively. These results provide a guide for the preparation of p-i-n heterojunctions based on few-layer MoSe2 with improved photovoltaic response.

Highly sensitive thermoelectric touch sensor based on p-type SnO x thin film.

Here, the ability of using p-type tin oxide (SnO x ) thin films as a thermal sensor has been investigated. Firstly, the thermoelectric performance was optimized by controlling the thickness of the SnO x film from 60 up to 160 nm. A high Seebeck coefficient of +263 µV K-1 and electrical conductivity of 4.1 × 102 (S m-1) were achieved in a 60 nm thick SnO x film, due to a compact nanostructured film and the absence of the Sn metallic phase, which was observed for the thicker SnO x film leading to a typical thermoelectric transport properties of a n-type Sn film. Moreover, x-ray photoelectron spectroscopy revealed the co-existence of SnO (79.7%) and SnO2 (20.3%) phases in the 60 nm thick SnO x film, while the optical measurements revealed an indirect gap of 1.8 eV and a direct gap of 2.7 eV, respectively. The 60 nm-SnO x thin film have been tested as a thermoelectric touch sensor, achieving a Vsignal /Vnoise  ≈ 20, with a rise time <1 s. Therefore, this work provides an efficient way for developing highly efficient thermal sensors with potential use in display technologies.

Super-bandgap light stimulated reversible transformation and laser-driven mass transport at the surface of As2S3 chalcogenide nanolayers studied in situ.

The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during "laser irradiation"-"thermal annealing" cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion.

Growth of transition metals on cerium tungstate model catalyst layers.

Two model catalytic metal/oxide systems were investigated by photoelectron spectroscopy and scanning tunneling microscopy. The mixed-oxide support was a cerium tungstate epitaxial thin layer grown in situ on the W(1 1 0) single crystal. Active particles consisted of palladium and platinum 3D islands deposited on the tungstate surface at 300 K. Both metals were found to interact weakly with the oxide support and the original chemical state of both support and metals was mostly preserved. Electronic and morphological changes are discussed during the metal growth and after post-annealing at temperatures up to 700 K. Partial transition-metal coalescence and self-cleaning from the CO and carbon impurities were observed.

Depth profiling of ultra-thin alumina layers grown on Co(0001).

Epitaxial thin oxide layers were grown by simultaneous aluminum deposition and oxidation on a Co(0001) single crystal, and the metal-oxide interface between the substrate and the grown layer was studied using photoelectron spectroscopy. The oxide layers were composed of two kinds of chemically different layers. Angle-resolved measurements were used to determine the compositions of oxide sub-layers and to reveal their respective thicknesses. The topmost oxide layers were up to 0.23 nm thick, determined by analysis of O 1s and Co 2p(3/2) photoelectron spectra. The results of the analysis show that the interface layer is composed of a mixture of oxygen and cobalt atoms and its thickness is approximately 0.6 nm. The analysis of Co 2p(3/2), Al 2p(3/2) and O 1s core level binding energies confirmed the presence of CoO in the interface layer and Al(2)O(3) in the topmost oxide layer.

Investigation of the Ti/MgCl2 interface on a Si(111) 7 × 7 substrate.

Photoelectron spectroscopy with synchrotron radiation, low energy electron diffraction, and ion-scattering spectroscopy were used in order to study the Ti/MgCl(2) interface grown on an atomically clean Si(111) 7 × 7 substrate. A series of high resolution spectra after deposition of a thick MgCl(2) layer, step by step deposition of Ti and gradual annealing, indicated a very reactive interface even at room temperature. Strong interaction between the incoming Ti atoms and the MgCl(2) layer, leads to the formation of Ti(2+) and Ti(4+) oxidation states. The interfacial interaction continues even at multilayer Ti coverage mainly by the partial disruption of Mg-Cl bonds and the formation of Ti-Cl sites, rendering this interface a very promising UHV-compatible model of a pre-catalyst for olefin polymerization. After the final annealing, the MgCl(2) multilayers desorb while Ti remains on the surface forming a silicide layer on which Cl and Mg atoms are attached.

Pt-CeO2 coating of carbon nanotubes grown on anode gas diffusion layer of the polymer electrolyte membrane fuel cell.

The growing of carbon nanotubes on a gas diffusion layer (GDL) was investigated using electron microscopy and photoelectron spectroscopy. The 30 nm thick Pt doped CeO2 layers were deposited by (rf) magnetron sputtering using a CeO2-Pt target on a carbon diffusion layer overgrown by carbon nanotubes. The anode prepared in such a way was tested in the proton exchange membrane fuel cell. Hydrogen/air fuel cell activity measurements normalized to the amount of used Pt revealed high specific power (W mg(-1) Pt). The high activity of this anode with CNT-grown is explained by high specific area of the catalyst, high conductivity of CNT-GDL junction and high activity of platinum present in cationic state Pt2,4+. Very high specific power and low cost together with physical vapor deposition of catalyst makes this anode preparation promising for micro fabrication of fuel cells to power mobile systems.

Sn/Pt(110) bimetallic surfaces: formation and oxygen adsorption.

Submonolayer coverage of Sn on a Pt(110) surface was studied by photoemission and low-energy electron diffraction. Deposition of less than 0.6 ML at 300 K gives rise to a c(2 × 2) surface reconstruction with weak diffraction spots at the very beginning of growth, and no other LEED patterns were found at this temperature. A new (4 × 1) Sn/Pt(110) surface structure was observed after flashing to 570 K a coverage of 0.64 ML. The total Sn coverage decreased to 0.58 ML after flashing as some of the atoms diffused into deeper layers. Different Sn phases were identified on the (4 × 1) Sn/Pt(110) surface: two types of surface Sn atoms in different adsorption sites, a subsurface Sn-Pt intermetallic layer and Sn-Pt surface islands. To investigate chemical reactivity, 0.25 ML Sn/Pt(110) and 0.58 ML (4 × 1) Sn/Pt(110) surfaces were exposed to 1000 L of O(2) at 300 K. Analyses of the photoemission data provide evidence for the formation of tin oxide. The interaction with oxygen of the two surfaces is similar, independent of surface structure and the composition of the subsurface layers. The Sn concentration in the interface intermetallic layer is the main factor which influences the oxygen adsorption.

Interaction of tungsten with CeO2(111) layers as a function of temperature: a photoelectron spectroscopy study.

The interaction of tungsten with CeO(2)(111) layers grown on Cu(111) was studied in the temperature range between 300 and 870 K by photoelectron spectroscopy of the core levels and resonant valence band spectroscopy. The interaction was found to be very strong even at 300 K, leading to the formation of cerium tungstate Ce(6)WO(12) in which the metal atoms were in Ce(3+) and W(6+) chemical states. The growth was limited by the diffusion of W atoms into the ceria layer, so subsequent tungsten deposition led to formation of W suboxides with consecutively lower chemical oxidation states, i.e. W(4+), W(2+) and metallic W(0) with an almost negligible contribution of W(5+). Step-wise annealing of the layer showed that due to stimulated diffusion of tungsten into ceria at higher temperature, Ce(6)WO(12) was formed more easily. Larger W overlayer thicknesses needed higher annealing temperature to promote diffusion. The thickest sample studied, 1.4 nm W/CeO(2), was transformed by annealing to 870 K to the Ce(6)WO(12)/W system with a tungsten monoxide (WO) interface, whereas the rest of the tungsten was converted to the W(6 + ) oxidation state.

The interfacial properties of MgCl(2) thin films grown on Ti(0001).

Photoelectron spectroscopy with synchrotron radiation (SRPES), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ion-scattering spectroscopy (ISS) were used in order to study the MgCl(2)/Ti(0001) interface. A clear hexagonal LEED pattern confirmed the presence of a quite large grain of Ti(0001) on the substrate while no new superstructure was formed after deposition of MgCl(2) either at room or at elevated temperatures. A series of high resolution spectra after step by step MgCl(2) deposition and gradual annealing indicated strong interaction between MgCl(2) and the substrate while ISS measurements showed that there is no migration of Ti atoms into the deposit layers. Additional quantities of deposited MgCl(2) grew stoichimetrically on top of the chemically active interface. Annealing at approximately 350 degrees C caused clustering of the MgCl(2) multilayer and TPD results showed that they desorbed stoichimetrically at temperatures between 360 and 380 degrees C. The interfacial TiCl(x)Mg(y) species dissociated by the disruption of the Cl-Mg bonds at temperatures higher than 400 degrees C and metallic Mg evaporated. The Cl atoms remained attached on the Ti surface but they did not form any ordered structure even after annealing at 730 degrees C. The present results indicate the occurrence of charge transfer at the Ti/MgCl(2) interface through the Cl ligands and provide valuable information for catalyst design.

Platinum-doped CeO2 thin film catalysts prepared by magnetron sputtering.

The interaction of Pt with CeO(2) layers was investigated by using photoelectron spectroscopy. The 30 nm thick Pt doped CeO(2) layers were deposited simultaneously by rf-magnetron sputtering on a Si(001) substrate, multiwall carbon nanotubes (CNTs) supported by a carbon diffusion layer of a polymer membrane fuel cell and on CNTs grown on the silicon wafer by the CVD technique. The synchrotron radiation X-ray photoelectron spectra showed the formation of cerium oxide with completely ionized Pt(2+,4+) species, and with the Pt(2+)/Pt(4+) ratio strongly dependent on the substrate. The TEM and XRD study showed the Pt(2+)/Pt(4+) ratio is dependent on the film structure.

Interaction of oxygen with Au/Ti(0001) surface alloys studied by photoelectron spectroscopy.

The interaction of oxygen with gold adsorbed on Ti(0001) was studied by synchrotron radiation photoelectron spectroscopy. Two kinds of surfaces were explored: as-deposited 0.38, 1.16 and 1.85 monolayer (ML) thick Au overlayers on the Ti(0001) surface, and the same samples after thermal treatment, which resulted in the formation of Au-Ti intermetallic surfaces. The Ti 3p core level was strongly affected by reaction with oxygen, while the Au 4f core level showed only minor changes other than a decrease in intensity. The Ti 3p peak was fitted with several components which were identified as Ti atoms in different oxidation states, namely TiO, Ti(2)O(3), TiO(2) and Ti-OH. Titanium oxide phase formation is accompanied by Au-Ti bond dissociation and outward diffusion of Ti. The presence of an Au-Ti intermetallic phase on the Ti(0001) surface promotes oxidation of the Ti atoms.

Excitons at the B K edge of boron nitride nanotubes probed by x-ray absorption spectroscopy.

We have performed a near-edge x-ray absorption fine-structure (NEXAFS) investigation of multi-walled boron nitride nanotubes (BNNTs). We show that the one-dimensionality of BNNTs is clearly evident in the B K edge spectrum, while the N K edge spectrum is similar to that of layered hexagonal BN (h-BN). We observe a sharp feature at the σ* onset of the B K edge, which we ascribe to a core exciton state. We also report a comparison with spectra taken after an ammonia plasma treatment, showing that the B K edge becomes indistinguishable from that of h-BN, due to the breaking of the tubular order and the formation of small h-BN clusters.

A study of tungsten oxide nanowires self-organized on mica support.

Self-organized straight nanowires of WO3 have been epitaxially grown on muscovite mica in low super-saturation conditions. Morphology, structure and chemical composition of the prepared nanostructures have been investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). SEM permits us to observe nanowire networks and substrate electron channeling patterns (ECP) simultaneously and thus to determine crystallographic direction of nanowire growth. The experimental results show that straight WO3 nanowires are orientated preferentially parallel to two of three high symmetry crystallographic directions of mica [100] and [110] or [100] and [Formula: see text]. XRD and XPS measurements indicated self-assembly of very thin nanowires of hexagonal tungsten bronze in the first stage of growth followed by the formation of stoichiometric WO3. The growth mechanism has been studied as a function of different preparation conditions with special focus on the role of potassium ions present on the mica surface. Based on obtained results a growth model of tungsten oxide nanowires on mica is proposed.

A resonant photoemission applied to cerium oxide based nanocrystals.

Cerium 4f level occupation determines the properties of cerium oxide based catalysts in a significant way. The Ce 4f level of nanosized cerium oxide particles was investigated with the use of resonant photoelectron spectroscopy in the Ce 4d-4f photoabsorption region. A strong interaction of ceria with different additives, e.g. Pd and Sn, led to a partial Ce4+-->Ce3+ transition that was observed as a significant resonance enhancement of 4f photoemission intensity. Increases of the CO oxidation catalytic activity were observed simultaneously. The ratio of resonance enhancement of Ce photoemission intensity DCe(3+)/DCe(4+) was used to monitor Ce(3+) and Ce(4+) state occupation. The relative parameter DCe(3+)/DCe(4+) was found to be particularly useful in the case of photoemission studies of nanopowder ceria catalysts.

Interaction of Au with CeO2(111): A photoemission study.

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO(2)(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO(2)(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce(4+)-->Ce(3+) reduction, observed as a resonant enhancement of the 4f level of the Ce(3+) species. This can be explained by possible creation of a new Au(+) ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 degrees C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold.

Palladium interaction with CeO(2), Sn-Ce-O and Ga-Ce-O layers.

Using photoemission, we have studied the interaction of palladium with thin layers of stoichiometric ceria (Ce(4+) character) and two mixed oxides, Ga-Ce-O and Sn-Ce-O, where cerium in the Ce(3+) oxidation state is present. Palladium was found to partially reduce the CeO(2) layer by introducing oxygen vacancies most probably in the vicinity of the growing Pd particles. In mixed oxide systems palladium very strongly interacts with both added metals-gallium and tin-leading to a breaking of metal-ceria bonds and the establishment of Pd-Ga(Sn) intermetallic compounds. As a consequence the ceria reoxidizes back to a Ce(4+) oxidation state.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys.

The reaction of oxygen at low pressure with the Sn/Pd(110) system has been examined by photoelectron spectroscopy using synchrotron radiation. The c(2 × 2) and (3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers (ML) Sn coverage and a 1.75 ML Sn overlayer on the Pd(110) surface after flashing to 470 K were studied. The Sn 4d core level is strongly affected by O(2) adsorption while the Pd 3d core level shows very little change other than a decrease in intensity. Starting with a 10 L dose of oxygen, prominent changes in the spectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4d core levels indicates that oxidation proceeds with the formation of well-defined states of Sn, which were identified as a Pd-Sn-O interface layer, SnO and SnO(2) oxides. The valence band spectra confirm this assignment. The Sn(2+) and Sn(4+) component signals originate from the topmost surface layer, i.e. tin atoms in more highly oxidized states constitute the topmost surface layer on top of the Pd-Sn-O interface. The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxide formation; the Sn-O phase formation is accompanied by Pd-Sn bond dissociation.

Photoelectron spectroscopy characterization of diamond-like carbon films.

The electronic states of diamond-like hydrogenated carbon (DLC) films were studied by synchrotron radiation photoelectron spectroscopy. The valence band spectra measured at different excitation energies show the gradual emergence of the p-pi band in relation to the sample annealing and ion bombardment amorphization. The p-pi band of the annealed DLC was characterized by localized p(z) states, while the formation of the amorphous carbon surface was accompanied by appearance of the delocalized p(z) states, which reduce the optical gap. A simple approach permitting the extraction of the 2p band shape from the photoelectron spectra is proposed.