Synchrotron Radiation-based X-ray Photoelectron Spectroscopy Research Articles

Surface science and liquid phase investigations of oxanorbornadiene/oxaquadricyclane ester derivatives as molecular solar thermal energy storage systems on Pt(111).

The transition to renewable energy sources comes along with the search for new energy storage solutions. Molecular solar thermal systems directly harvest and store solar energy in a chemical manner. By a suitable molecular design, a higher overall efficiency can be achieved. In this study, we investigate the surface chemistry of oxa-norbornadiene/quadricyclane derivatives on a Pt(111) surface. Specifically, we focus on the energy storage and release properties of molecules that are substituted with ester moieties of different sizes. For our model catalytic approach, synchrotron radiation-based x-ray photoelectron spectroscopy measurements were conducted in ultra-high vacuum (UHV) and correlated with the catalytic behavior in the liquid phase monitored by photochemical infrared reflection absorption spectroscopy. The differences in their spectral appearance enabled us to unambiguously differentiate the energy-lean and energy-rich isomers and decomposition products. Next to qualitative information on the adsorption motifs, temperature-programmed experiments allowed for the observation of thermally induced reactions and the deduction of the related reaction pathways. We analyzed the selectivity of the cycloreversion reaction from the energy-rich quadricyclane derivative to its energy-lean norbornadiene isomer and competing processes, such as desorption and decomposition. For the 2,3-bis(methylester)-substitution, the cycloreversion reaction was found to occur between 310 and 340K, while the thermal stability limit of the compounds was determined to be 380K. The larger 2,3-bis(benzylester) derivatives have a lower apparent adsorption energy and a decomposition onset already at 135K. In the liquid phase (in acetonitrile), we determined the rate constants for the cycloreversion reaction on Pt(111) to k = 5.3 × 10-4s-1 for the 2,3-bis(methylester)-substitution and k = 6.3 × 10-4s-1 for the 2,3-bis(benzylester) derivative. The selectivities were of >99% and 98% for the two molecules, respectively. The difference in the catalytic behavior of Pt(111) for both derivatives is less pronounced in the liquid phase than in UHV, which we attribute to the passivation of the Pt(111) surface by carbonaceous species under ambient conditions.

Surface Studies on the Energy Release of the MOST System 2-Carbethoxy-3-Phenyl-Norbornadiene/Quadricyclane (PENBD/PEQC) on Pt(111) and Ni(111).

Novel energy-storage solutions are necessary for the transition from fossil to renewable energy sources. Auspicious candidates are so-called molecular solar thermal (MOST) systems. In our study, we investigate the surface chemistry of a derivatized norbornadiene/quadricyclane molecule pair. By using suitable push-pull substituents, a bathochromic shift of the absorption onset is achieved, allowing a greater overlap with the solar spectrum. Specifically, the adsorption and thermally induced reactions of 2-carbethoxy-3-phenyl-norbornadiene/quadricyclane are assessed on Pt(111) and Ni(111) as model catalyst surfaces by synchrotron radiation-based X-ray photoelectron spectroscopy (XPS). Comparison of the respective XP spectra enables the distinction of the energy-rich molecule from its energy-lean counterpart and allows qualitative information on the adsorption motifs to be derived. Monitoring the quantitative cycloreversion between 140 and 230 K spectroscopically demonstrates the release of the stored energy to be successfully triggered on Pt(111). Heating to above 300 K leads to fragmentation of the molecular framework. On Ni(111), no conversion of the energy-rich compound takes place. The individual decomposition pathways of the two isomers begin at 160 and 180 K, respectively. Pronounced desorption of almost the entire surface coverage only occurs for the energy-lean molecule on Ni(111) above 280 K; this suggests weakly bound species. The correlation between adsorption motif and desorption behavior is important for applications of MOST systems in heterogeneously catalyzed processes.

Fabrication and Characterization of Pre-Defined Few-Layer Graphene

Graphene is one of the most well-known two-dimensional (2D) materials that has attracted significant interest due to its unique electrical and optical properties. Being a van der Waals substrate, the fabrication of few-layered graphene by stacking a pre-defined number of graphene monolayers is essential in the field. The thickness can influence the interface interaction and therefore tune the surface electronic properties. In the study, we demonstrate a bottom-up synthesis of pre-defined few-layer graphene on SiC substrate using the thermal decomposition method and carefully characterize its thickness by the non-damageable synchrotron-radiation-based X-ray photo-electron spectroscopy (SR-XPS). By varying the photon energy, we acquire different probe depths, resulting in the different intensity ratios of graphene to SiC substrate, which is then used to estimate the thickness of the few-layer graphene. Our calculation demonstrates that the thermal decomposition method in the study can repeatedly fabricate graphene samples with expected thickness. We further compare the obtained few-layer graphene to the single-layer graphene and HOPG using the scanning tunneling microscopy (STM) technique. Our work provides accurate methods for fabricating and characterizing pre-defined few-layer graphene, providing essential knowledge in future graphene-based thin film electronics.

Investigations into Spin- and Unpolarized Secondary Electron-Induced Reactions in Self-Assembled Monolayers of Cysteine.

Cysteine is the simplest thiolated, chiral amino acid and is often used as the anchor for studies of self-assembled monolayers (SAMs) of complex biomolecules such as peptides. Understanding the interaction of SAMs of cysteine with low-energy secondary electrons (SEs) produced by X-rays can further our understanding of radiation damage in biomolecules. In particular, if the electrons are polarized, chiral-selective chemistry could have bearing on the origin of homochirality in nature. In the present paper, we use synchrotron radiation-based X-ray photoelectron spectroscopy to determine the changes that occur in the bonding of self-assembled layers of cysteine on gold as a result of soft X-ray irradiation. To investigate the possibility of chiral selectivity resulting from the interaction of low-energy, spin-polarized SEs (SPSEs), measurements were conducted on cysteine adsorbed on a 3 nm-thick gold layer deposited on a CoPt thin-film multilayer with perpendicular magnetic anisotropy. Time-dependent measurements of the C 1s, N 1s, O 1s, S 2p, and Au 4f core levels are used to follow the changes in surface chemistry and determine reaction cross-sections as a function of SE exposure. Analysis of the data results in cross-sections in the range of 5-7 Mb and suggests possible reaction pathways. Changing the magnetization direction of the CoPt multilayer produces SPSEs with opposite polarity. Some evidence of spin-dependent reactions is indicated but is inconclusive. Possible reasons for the discrepancy are posited.

Testing the activation temperature of non-evaporable Ti-Zr-Hf-V getter films by XPS

Developing new non-evaporable getter (NEG) films with low activation temperature is the key to developing next generation high-energy particle accelerators with the ultimate degree of chamber vacuum. Here, a quaternary alloy Ti-Zr-Hf-V getter film with a uniform thickness of 1.64μm is fabricated by DC magnetron sputtering. The activation temperature of the Ti-Zr-Hf-V getter film was detected by measuring the binding energy of Ti, Zr, Hf and V at different temperatures through in situ synchrotron radiation-based X-ray photoelectron spectroscopy. The Ti-Zr-Hf-V getter films are found to be activated at a temperature of 150 ∘C. In addition, the binding energy of Hf 4f at 130 ∘C is basically the same as that at 300 ∘C. The presence of the Hf element reduces the activation temperature of the obtained films.

Synthesis of Monolayer Blue Phosphorus Enabled by Silicon Intercalation.

The growth of entirely synthetic two-dimensional (2D) materials could further expand the library of naturally occurring layered solids and provide opportunities to design materials with finely tunable properties. Among them, the synthesis of elemental 2D materials is of particular interest as they represent the chemically simplest case and serve as a model system for exploring the on-surface synthesis mechanism. Here, a pure atomically thin blue phosphorus (BlueP) monolayer is synthesized via silicon intercalation of the BlueP-Au alloy on Au(111). The intercalation process is characterized at the atomic scale by low-temperature scanning probe microscopy and further corroborated by synchrotron radiation-based X-ray photoelectron spectroscopy measurements. The evolution of the band structures from the BlueP-Au alloy into Si-intercalated BlueP are clearly revealed by angle-resolved photoemission spectroscopy and further verified by density functional theory calculations.

Zinc oxide nanoparticles and activated charcoal-based nanocomposite electrode for supercapacitor application

In the present work, zinc oxide (ZnO) nanoparticles (NPs) have been synthesized at room temperature using co-precipitation method for the use as an electrode material by forming a composite with activated charcoal (AC) powder for its application in electrochemical supercapacitor. For synthesizing of ZnO nanoparticles, dry zinc chloride (ZnCl2) and sodium hydroxide (NaOH) have been used as the initial precursors. The obtained ZnO nanopowder has been calcined at 400, 500 and 600 °C for 2 h. The nanoparticles have been characterized by powder X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The nanocomposite has also been investigated by synchrotron radiation-based X-ray photoelectron spectroscopy (XPES). Supercapacitor cells (symmetrical) have been fabricated by making use of different composition of zinc oxide (ZnO) nanoparticle-activated carbon (AC). Electrochemical properties of the prepared nanocomposite electrodes and fabricated supercapacitor cells have been characterized using a.c. impedance, cyclic voltammetry (CV) and charge–discharge (CD) techniques by using 6 M KOH and 1 M NaOH as the electrolytes. The optimized composition is 1:1 (mass ratio) of ZnO and activated charcoal, and 600 °C for 2 h is an optimized temperature for synthesis of nanoparticles. The specific capacitance of the supercapacitor cells is stable up to 2000 cycles at 100 mV cm−2, which show that the device has good electrochemical reversibility and cycle life with 6 M KOH and 1 M NaOH electrolyte.

Mo-Al co-doped VO2(B) thin films: CVD synthesis, thermal sensitive properties, synchrotron radiation photoelectron and absorption spectroscopy study

Endowing vanadium dioxide (VO2) thin films with excellent thermal sensitive performance is greatly desired for application in uncooled infrared (IR) detectors. Here, we demonstrate a strategy of co-doping by using a simple and feasible CVD method to synthesize Mo-Al-doped VO2(B) thin films with high temperature-coefficient-of-resistance (TCR, −3.6%/K) and favorable square resistances (16.2 kΩ). Both the TCR and resistance are superior to that of undoped and Al-doped VO2(B) thin film, and the TCR is larger than that of Mo-doped VO2(B) thin film. The underlying microscopic mechanism for the performance improvement might be that Mo-Al co-doping creates less lattice deformation, strain and inhomogeneity of carrier concentration due to dimension and charge compensation than that of single-doping and the partly release of 3d electrons around V4+-V4+ pairs in co-doping films. Besides, the special two-dimensional octahedral structure of monoclinic (C2/m) VO2(B) favors the strain control with doping. The films have been subjected to synchrotron radiation based X-ray photoelectron spectroscopy (SRPES) and X-ray absorption near edge structure (XANES) techniques to study the local atomic and electronic structure around V ions. The results indicated that the dopants of Mo and Al were incorporated into the VO2 lattice and occupied some of the V lattice sites, resulting in the formation of the V1-x-yMoxAlyO2 substitution solid solution. V L-edge and O K-edge XAS results identically verified that Mo-Al co-doping might create the least lattice deformation in films. Hence, the high performance of films proves that the co-doping strategy is suitable for uncooled infrared detector applications.

Analysis of nitrogen species in titanium oxynitride ALD films

Titanium oxynitride films are prepared by plasma enhanced atomic layer deposition method using two different precursors and nitrogen sources. Synchrotron radiation-based X-ray photoelectron spectroscopy and X-ray absorption spectroscopy are used to characterize the nitrogen species incorporated within these films depending on the deposition parameters. It is found that nitrogen atoms in these films are differently bonded. In particular, it can be distinguished between TiON and TiN bonding configurations and molecular nitrogen species caused by precursor fragments.

Graphene-Templated Growth of Pd Nanoclusters

Graphene grown on Rh(111) was used as a template for the growth of Pd nanoclusters. Using high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy, we studied the deposition of Pd on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster growth mode. The formation of clusters with 3 nm diameter was confirmed by low-temperature scanning tunneling microscopy measurements. The investigation of the thermal stability of the Pd particles showed three characteristic temperature regimes: Up to 550 K restructuring of the particles takes place, between 550 and 750 K the clusters coalesce into larger agglomerates, and finally between 750 and 900 K Pd intercalates between the graphene layer and the Rh surface.

Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy

Plasmons are investigated in free nanoscale Na, Mg, and K metal clusters using synchrotron radiation-based x-ray photoelectron spectroscopy. The core levels for which the response from bulk and surface atoms can be resolved are probed over an extended binding energy range to include the plasmon loss features. In all species the features due to fundamental plasmons are identified, and in Na and K also those due to either the first order plasmon overtones or sequential plasmon excitation are observed. These features are discussed in view of earlier results for planar macroscopic samples and free clusters of the same materials.

ChemInform Abstract: Chemical Bonding of Water to Metal Surfaces Studied with Core‐Level Spectroscopies

AbstractReview: 106 refs.

Dielectric properties of single crystalline PrO2(111)/Si(111) heterostructures: Amorphous interface and electrical instabilities

Single crystalline PrO2(111)/Si(111) heterostructures are flexible buffers for global Ge integration on Si. A combined materials science–electrical characterization is carried out to study the influence of postdeposition annealing in 1 bar oxygen at 300–600 °C on the dielectric properties of PrO2(111)/Si(111). The materials science transmission electron microscopy and x-ray reflectometry studies reveal that postdeposition oxidation of the PrO2(111)/Si(111) boundary results in an amorphous interface (IF) layer, which grows in thickness with temperature. Nondestructive depth profiling synchrotron radiation-based x-ray photoelectron spectroscopy and x-ray absorption spectroscopy methods demonstrate that this amorphous IF layer is composed of two Pr-silicate phases, namely, with increasing distance from Si, a SiO2-rich and a SiO2-poor Pr silicate. The electronic band offset diagram shows that the wide band gap dielectric Pr silicate results in higher band offsets with respect to Si than the medium band gap dielectric PrO2. The electrical characterization studies by C-V measurements show that (a) well-behaved dielectric properties of the PrO2(111)/IF/Si(111) are achieved in a narrow postdeposition oxidation window of 400–450 °C and that (b) defects are distributed over the Pr-silicate IF layer. Temperature-dependent J-V studies report furthermore that the formation of the single crystalline PrO2/amorphous Pr-silicate bilayer structure on Si(111) results in (a) improved insulating properties and (b) strong electrical instability phenomena in the form of a Maxwell–Wagner instability and dielectric relaxation.

Chemical bonding of water to metal surfaces studied with core-level spectroscopies

The nature of the contact layer of water on surfaces is of relevance for many practical fields, including corrosion, electrochemistry, environmental science and heterogeneous catalysis. Here we focus on the geometric and electronic structure of the water contact layer on transition metal surfaces and the interaction between the water monolayer and the surface. By combining synchrotron radiation-based X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) techniques with density functional theory (DFT) computational methods we obtain element-specific information on the partial local density of states, local atomic structure, geometrical parameters and molecular orientation, allowing general principles for water–metal interaction to be derived.

Growth of praseodymium oxide on Si(111) under oxygen-deficient conditions

Surface science studies of thin praseodymium oxide films grown on silicon substrates are of high interest in view of applications in such different fields as microelectronics and heterogeneous catalysis. In particular, a detailed characterization of the growth and the final structure of the films are mandatory to achieve a fundamental understanding of such topics as oxygen mobility and defect structure, and their role for the electronic and chemical properties. In this paper, the MBE growth of praseodymium oxide films on Si(111) substrates was investigated at low-deposition rates (0.06 nm/min) and low-oxygen partial pressures $(\text{p}({\text{O}}_{2})<1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{mbar})$. To obtain insight into the structure and chemical composition of the growing film, spot profile analyzing low-energy electron diffraction (SPA-LEED), transmission electron microscopy, and synchrotron radiation-based x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) were applied. SPA-LEED reveals the formation of an initial closed layer followed by continuous roughening and formation of ordered three-dimensional structures. This result is in contrast to observations at higher-deposition rates, were a layer-by-layer growth was reported. XAS and XPS provide evidence that a continuous reaction takes place in the growing ${\text{Pr}}_{2}{\text{O}}_{3}$ film leading to the formation of silicate and silicide structures within the film. Combining all data, a consistent picture of the deposition of praseodymium oxide on Si(111) emerges which clearly shows that in contrast to higher-throughput molecular beam epitaxy conditions the reactivity of the growing film strongly influences the growth behavior at low-deposition rates and low pressures.

Adsorption and structure of methyl mercaptoacetate on Cu(1 1 1) surface by XPS and NEXAFS spectroscopy

Methyl mercaptoacetate (MA) on Cu(1 1 1) surface was investigated using synchrotron radiation-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. MA adsorbs on the surface via thiolate formation and weak interaction of the carbonyl group with the surface. Two different adsorption states previously reported for methanethiolate and ethanethiolate were confirmed, besides atomic sulfur. NEXAFS measurements support gauche-type conformation of MA whose skeleton lies on the surface.