Surface photovoltage dynamics at passivated silicon surfaces: influence of substrate doping and surface termination.

We have monitored the temporal evolution of the band bending at controlled silicon surfaces after a fs laser pump excitation. Time-resolved surface photo-voltage (SPV) experiments were performed using time resolved photoemission spectroscopy with time resolution of about 30 ns. To disentangle the influence of doping and surface termination on SPV dynamics, we compare the results obtained on two surface terminations: the water saturated (H,OH)-Si(001) surface and the thermally oxidized Si(001) one. The SPV dynamics were explored as a function of laser fluence and as a function of time for the two surface terminations at given doping levels. The return to equilibrium involves a characteristic time in the 0.1 µs to 10 µs range, depending on the surface termination and bulk doping. Exploring several laser fluences, different SPV regimes were found for the two surface terminations at given doping levels. For low laser fluence the SPV dynamic follows the commonly accepted thermionic model. At higher fluence, the SPV signal reaches a saturation value, and if the fluence is further increased, the decay time of the SPV increases and can no longer be explained by a thermionic model alone.

Spin/valley pumping of resident electrons in WSe2 and WS2 monolayers.

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.

Subpicosecond metamagnetic phase transition in FeRh driven by non-equilibrium electron dynamics.

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350 ± 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.

Unraveling intrinsic correlation effects with angle-resolved photoemission spectroscopy.

Interaction effects can change materials properties in intriguing ways, and they have, in general, a huge impact on electronic spectra. In particular, satellites in photoemission spectra are pure many-body effects, and their study is of increasing interest in both experiment and theory. However, the intrinsic spectral function is only a part of a measured spectrum, and it is notoriously difficult to extract this information, even for simple metals. Our joint experimental and theoretical study of the prototypical simple metal aluminum demonstrates how intrinsic satellite spectra can be extracted from measured data using angular resolution in photoemission. A nondispersing satellite is detected and explained by electron-electron interactions and the thermal motion of the atoms. Additional nondispersing intensity comes from the inelastic scattering of the outgoing photoelectron. The ideal intrinsic spectral function, instead, has satellites that disperse both in energy and in shape. Theory and the information extracted from experiment describe these features with very good agreement.

Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission.

We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of FeRh during the first-order phase transition, identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi edge is a fingerprint of the magnetic status of the system that is independent of the long-range ferromagnetic alignment of the magnetic domains. We use this feature to follow the phase transition induced by a laser pulse in a pump-probe experiment and find that the magnetic transition occurs in less than 50 ps and reaches its maximum in 100 ps.

CO oxidation activity of Pt, Zn and ZnPt nanocatalysts: a comparative study by in situ near-ambient pressure X-ray photoelectron spectroscopy.

The investigation of nanocatalysts under ambient pressure by X-ray photoelectron spectroscopy gives access to a wealth of information on their chemical state under reaction conditions. Considering the paradigmatic CO oxidation reaction, a strong synergistic effect on CO catalytic oxidation was recently observed on a partly dewetted ZnO(0001)/Pt(111) single crystal surface. In order to bridge the material gap, we have examined whether this inverse metal/oxide catalytic effect could be transposed on supported ZnPt nanocatalysts deposited on rutile TiO2(110). Synchrotron radiation near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) operated at 1 mbar of O2 : CO mixture (4 : 1) was used at a temperature range between room temperature and 450 K. To tackle the complexity of the problem, we have also studied the catalytic activity of nanoparticles (NPs) of the same size, consisting of pure Pt and Zn nanoparticles (NPs), for which, moreover, NAP-XPS studies are a novelty. The comparative approach shows that the CO oxidation process is markedly different for the pure Pt and pure Zn NPs. For pure Pt NPs, CO poisoned the metallic surfaces at low temperature at the onset of CO2 evolution. In contrast, the pure Zn NPs first oxidize into ZnO, and trap carbonates at low temperature. Then they start to release CO2 in the gas phase, at a critical temperature, while continuously producing it. The pure Zn NPs are also immune to support encapsulation. The bimetallic nanoparticle borrows some of its characteristics from its two parent metals. In fact, the ZnPt NP, although produced by the sequential deposition of platinum and zinc, is platinum-terminated below the temperature onset of CO oxidation and poisoned by CO. Above the CO oxidation onset, the nanoparticle becomes Zn-rich with a ZnO shell. Pure Pt and ZnPt NPs present a very similar activity towards CO oxidation, in contrast with what is reported in a single crystal study. The present study demonstrates the effectiveness of NAP-XPS in the study of complex catalytic processes at work on nanocatalysts under near-ambient pressures, and highlights once more the difficulty of transposing single crystal surface observations to the case of nanoobjects.

Commissioning of a multi-beamline femtoslicing facility at SOLEIL.

The investigation of ultrafast dynamics, taking place on the few to sub-picosecond time scale, is today a very active research area pursued in a variety of scientific domains. With the recent advent of X-ray free-electron lasers (XFELs), providing very intense X-ray pulses of duration as short as a few femtoseconds, this research field has gained further momentum. As a consequence, the demand for access strongly exceeds the capacity of the very few XFEL facilities existing worldwide. This situation motivates the development of alternative sub-picosecond pulsed X-ray sources among which femtoslicing facilities at synchrotron radiation storage rings are standing out due to their tunability over an extended photon energy range and their high stability. Following the success of the femtoslicing installations at ALS, BESSY-II, SLS and UVSOR, SOLEIL decided to implement a femtoslicing facility. Several challenges were faced, including operation at the highest electron beam energy ever, and achievement of slice separation exclusively with the natural dispersion function of the storage ring. SOLEIL's setup also enables, for the first time, delivering sub-picosecond pulses simultaneously to several beamlines. This last feature enlarges the experimental capabilities of the facility, which covers the soft and hard X-ray photon energy range. In this paper, the commissioning of this original femtoslicing facility is reported. Furthermore, it is shown that the slicing-induced THz signal can be used to derive a quantitative estimate for the degree of energy exchange between the femtosecond infrared laser pulse and the circulating electron bunch.

Energy-Level Alignment of a Hole-Transport Organic Layer and ITO: Toward Applications for Organic Electronic Devices.

2,2',6,6'-Tetraphenyl-4,4'-dipyranylidene (DIPO-Ph4) was grown by vacuum deposition on an indium tin oxide (ITO) substrate. The films were characterized by atomic force microscopy as well as synchrotron radiation UV and X-ray photoelectron spectroscopy to gain an insight into the material growth and to better understand the electronic properties of the ITO/DIPO-Ph4 interface. To interpret our spectroscopic data, we consider the formation of cationic DIPO-Ph4 at the ITO interface owing to a charge transfer from the organic layer to the substrate. Ionization energy DFT calculations of the neutral and cationic species substantiate this hypothesis. Finally, we present the energetic diagram of the ITO/DIPO-Ph4 system, and we discuss the application of this interface in various technologically relevant systems, as a hole-injector in OLEDs or as a hole-collector interfacial layer adjacent to the prototypical OPV layer P3HT:PCBM.

Protected surface state in stepped Fe (0 18 1).

Carbon (C) surface segregation from bulk stabilizes the Fe(0 18 1) vicinal surface by forming a c(3[Formula: see text] × [Formula: see text] reconstruction with C zig-zag chains oriented at 45° with respect to the iron surface steps. The iron surface electronic states as measured by high resolution ARPES at normal emission with polarized synchrotron radiation split in two peaks that follow distinct energy dispersion curves. One peak follows the dispersion of the carbon superstructure and is photoexcited only when the polarization vector is parallel to the steps, the second peak disperses similarly to the pristine Fe(0 0 1) surface. Such surface electronic structure is robust as it persists even after coating with an Ag overlayer. The robustness of this surface electronic structure and its similarity with that of the clean Fe(0 0 1) surface make this system of interest for magnetic and spintronic properties such as magneto tunnel junctions based on Fe/MgO interface.

Pump-probe experiments at the TEMPO beamline using the low-α operation mode of Synchrotron SOLEIL.

The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated to pump-probe experiments. Among others, the low-α mode operation is characterized by shorter pulse duration and represents the natural bridge between 50 ps synchrotron pulses and femtosecond experiments. Here, the capabilities in low-α mode of the experimental set-ups developed at the TEMPO beamline to perform pump-probe experiments with soft X-rays based on photoelectron or photon detection are presented. A 282 kHz repetition-rate femtosecond laser is synchronized with the synchrotron radiation time structure to induce fast electronic and/or magnetic excitations. Detection is performed using a two-dimensional space resolution plus time resolution detector based on microchannel plates equipped with a delay line. Results of time-resolved photoelectron spectroscopy, circular dichroism and magnetic scattering experiments are reported, and their respective advantages and limitations in the framework of high-time-resolution pump-probe experiments compared and discussed.

The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling.

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

Compelling experimental evidence of a Dirac cone in the electronic structure of a 2D Silicon layer.

The remarkable properties of graphene stem from its two-dimensional (2D) structure, with a linear dispersion of the electronic states at the corners of the Brillouin zone (BZ) forming a Dirac cone. Since then, other 2D materials have been suggested based on boron, silicon, germanium, phosphorus, tin, and metal di-chalcogenides. Here, we present an experimental investigation of a single silicon layer on Au(111) using low energy electron diffraction (LEED), high resolution angle-resolved photoemission spectroscopy (HR-ARPES), and scanning tunneling microscopy (STM). The HR-ARPES data show compelling evidence that the silicon based 2D overlayer is responsible for the observed linear dispersed feature in the valence band, with a Fermi velocity of comparable to that of graphene. The STM images show extended and homogeneous domains, offering a viable route to the fabrication of silicene-based opto-electronic devices.

Tunable Doping in Hydrogenated Single Layered Molybdenum Disulfide.

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in monolayered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements.

van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties.

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a binding energy of about -0.73 eV relative to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about m*/m0 = -1.1 was determined. By coupling the ARPES data with high-resolution X-ray photoemission spectroscopy measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow a deeper understanding of the interlayer interactions and the electronic structure of the GaSe/graphene vdW heterostructure.

Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer.

The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field.

Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures.

Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

Case studies on the formation of chalcogenide self-assembled monolayers on surfaces and dissociative processes.

This report examines the assembly of chalcogenide organic molecules on various surfaces, focusing on cases when chemisorption is accompanied by carbon-chalcogen atom-bond scission. In the case of alkane and benzyl chalcogenides, this induces formation of a chalcogenized interface layer. This process can occur during the initial stages of adsorption and then, after passivation of the surface, molecular adsorption can proceed. The characteristics of the chalcogenized interface layer can be significantly different from the metal layer and can affect various properties such as electron conduction. For chalcogenophenes, the carbon-chalcogen atom-bond breaking can lead to opening of the ring and adsorption of an alkene chalcogenide. Such a disruption of the π-electron system affects charge transport along the chains. Awareness about these effects is of importance from the point of view of molecular electronics. We discuss some recent studies based on X-ray photoelectron spectroscopy that shed light on these aspects for a series of such organic molecules.

Stable room-temperature ferromagnetic phase at the FeRh(100) surface.

Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at ~400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We find that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that support the experimental findings.

High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001).

The van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)(1/2) dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm(2)·V(-1)·s(-1) at 4 K.