Investigation of warm dense matter using time-resolved X-ray absorption spectroscopy with third- and fourth- generation light sources

The advent of high-power lasers has provided insights into laboratory high energy density (>10¹¹ J/m³) physics. In particular, the properties of warm dense matter (WDM) with temperatures of 10⁴–10⁶ K and near-solid densities is a research area that has garnered significant interest recently. However, owing to the high temperatures and pressures associated with WDM, the measurement of fundamental properties is difficult, and insufficient data has been a significant setback in WDM research. Herein, we review recent developments in time-resolved X-ray absorption spectroscopy with synchrotron and X-ray free electron lasers for WDM research. Various physical properties, such as atomic bonding, electronic structures, electron–phonon coupling, and thermal conductivity of various elements in WDM conditions are investigated via this noble X-ray technique at various time scales from 100 ps to 100 fs.     

Effects of carbon doping on chemical states of amorphous Ge2Sb2Te5, measured with synchrotron radiation

We fabricated and analyzed the chemical states of carbon-doped (5.2–13.2 at.%) Ge₂Sb₂Te₅ thin films on Si substrates using high-resolution, X-ray photoelectron spectroscopy with synchrotron radiation. Thin films were completely amorphous and their phase-change temperature was 150 °C higher than for un-doped GST. As the carbon doping concentration increased, new chemical states of Ge 3d with 29.9 eV and C 1s with 283.7 eV core-levels were observed. The doped carbon was bonded only with Ge in GST and doping was saturated at 8.7 at.%.     

Time-resolved spectroscopy of Al, Ti, and Mo X pinch radiation using an X-ray streak camera

Time-resolved spectroscopic measurements of the radiation emitted from Al, Ti, and Mo X pinches have been made with time resolution. The radiation is emitted from micropinch plasmas with sizes of order in times in the 10- range. Spectra implied that dense, plasmas were produced, such as a lifetime, 1.5- electron temperature and near solid-density Ti plasma. The experimental systems and analysis methods are described in detail, including line ratio calculations for μm-scale Ti and Al plasmas with ion densities of 10¹⁹-10²⁴ cm⁻³ and electron temperatures.     

Application of spectroscopic photoemission and low energy electron microscope to high-k gate dielectrics: Relationship between surface morphology and electronic states during Hf-silicide formation

We have applied the spectroscopic photoemission and low energy electron microscope to study high-k gate dielectrics and have performed the following in situ operations during ultrahigh vacuum annealing: real-time observation of surface morphology and microregion photoelectron spectroscopy measurements. Changes in surface morphology and electronic states were consistent with the models previously reported in the case of HfO₂/Si. No clear differences between void regions and nonvoid regions have been observed in microregion photoelectron spectra for poly-Si/HfO₂/Si, regardless of phase separation in real space. These results have suggested that the initial void formation occurs in about 100-nm wide regions for both HfO₂/Si and poly-Si/HfO₂/Si.     

Thermal stability and chemical bonding states of AlOxNy/Si gate stacks revealed by synchrotron radiation photoemission spectroscopy

Annealing-temperature dependence of the thermal stability and chemical bonding states of AlO_xN_y/SiO₂/Si gate stacks grown by metalorganic chemical vapor deposition (MOCVD) using new chemistry was investigated by synchrotron radiation photoemission spectroscopy (SRPES). Results have confirmed the formation of the AlN and AlNO compounds in the as-deposited samples. Annealing the AlO_xN_y samples in N₂ ambient in 600-800 °C promotes the formation of SiO₂ component. Meanwhile, there is no formation of Al-O-Si and Al-Si binding states, suggesting no interdiffusion of Al with the Si substrate. A thermally induced reaction between Si and AlO_xN_y to form volatile SiO and Al₂O is suggested to be responsible for the full disappearance of the Al component that accompanies annealing at annealing temperature of 1000 °C. The released N due to the breakage of the Al-N bonding will react with the SiO₂ interfacial layer and lead to the formation of the Si₃-N-O/Si₂-N-O components at the top of Si substrate. These results indicate high temperature processing induced evolution of the interfacial chemistry and application range of AlO_xN_y/Si gate stacks in future CMOS devices.     

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three (iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl₂ group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

Cu adsorption on pyrite (1 0 0): Ab initio and spectroscopic studies

Surface optimised S 2p photoelectron spectra show that both surface S²⁻ monomers and (S-S)²⁻ dimers are present at pyrite (1 0 0) fracture surfaces. In order to determine which sulfur species are involved in Cu adsorption, fresh pyrite surfaces were exposed to Cu²⁺ in solution. The S 2p spectra suggest that both types of S surface species are involved in the mechanism of Cu adsorption (activation). Ab initio density functional theory was used to model Cu adsorbed onto pyrite (1 0 0) to support the interpretation of the spectroscopy. Mulliken population analysis confirms the charge distribution suggested by the core line shifts as observed in the photoelectron spectra. The ab initio calculations were consistent with a two-coordinate bond between Cu(I), a surface S monomer and a surface S dimer.     

Ultrathin ZrO2 films on Si-rich SiC(0 0 0 1)-(3 × 3): Growth and thermal stability

The growth and thermal stability of ultrathin ZrO₂ films on the Si-rich SiC(0 0 0 1)-(3 × 3) surface have been explored using photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The films were grown in situ by chemical vapor deposition using the zirconium tetra tert-butoxide (ZTB) precursor. The O 1s XAS results show that growth at 400 °C yields tetragonal ZrO₂. An interface is formed between the ZrO₂ film and the SiC substrate. The interface contains Si in several chemically different states. This gives evidence for an interface that is much more complex than that formed upon oxidation with O₂. Si in a 4+ oxidation state is detected in the near surface region. This shows that intermixing of SiO₂ and ZrO₂ occurs, possibly under the formation of silicate. The alignment of the ZrO₂ and SiC band edges is discussed based on core level and valence PES spectra. Subsequent annealing of a deposited film was performed in order to study the thermal stability of the system. Annealing to 800 °C does not lead to decomposition of the tetragonal ZrO₂ (t-ZrO₂) but changes are observed within the interface region. After annealing to 1000 °C a laterally heterogeneous layer has formed. The decomposition of the film leads to regions with t-ZrO₂ remnants, metallic Zr silicide and Si aggregates.     

Chemistry of l-proline on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The surface chemistry of proline is explored on Pd(1 1 1) using a combination of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. Proline adsorbs on Pd(1 1 1) at temperatures of 250 K and below into second and subsequent layers prior to the saturation of the first layer, where approximately 70% of the adsorbed proline is present in its zwitterionic form. Molecular proline desorbs between ∼315 K and ∼333 K depending on coverage. When adsorbed at ∼300 K, only the first monolayer is formed, and the proline is present as zwitterions, oriented such that all of the carbons are detected equally by XPS. Proline decomposes by scission of the C-COO bond, where the carboxylate moiety desorbs as carbon monoxide and carbon dioxide, while the nitrogen-containing moiety desorbs as to HCN, and evolves pyrrole at ∼390 K, pyrrolidine at ∼410 K, and final species that desorbs at ∼450 K that cannot be unequivocally assigned but may be 2-butenenitrile (CH₃-CHCH-CN), 3-butenenitrile (CH₂CH-CH₂-CN), 2-methyl-2-propenenitrile (CH₂C(CH₃)-CN) or cyclopropanecarbonitrile.     

ToF-SIMS studies of Bacillus using multivariate analysis with possible identification and taxonomic applications

In this paper we discuss the application of ToF-SIMS with an Au₃⁺ primary ion beam, combined with principal components analysis (PCA) and discriminant function analysis (DFA) for the identification of individual strains of two Bacillus species. The ToF-SIMS PC-DFA methodology is capable of distinguishing bacteria at the strain level based on analysis of surface chemical species. By classifying the data using hierarchical cluster analysis (HCA) we are able to show quantitative separation of species and of these strains. This has taxonomic implications in the areas of rapid identification of pathogenic microbes isolated from the clinic, food and environment.     

Proline adsorption on TiO2(1 1 0) single crystal surface: A study by high resolution photoelectron spectroscopy

The surface chemistry and binding of dl-proline were investigated on the oxidised (stoichiometric) and reduced (sub-stoichiometric) TiO₂(1 1 0) single crystal surfaces. TiO₂ was chosen as the substrate as it best represents the surface of a biomedical implant, which bio-molecules interact with during the healing of bone/teeth fractures (molecular recognition). High resolution X-ray photoelectron spectroscopy (HR-XPS) studies of the C1s and N1s regions revealed that dl-proline is present in two forms (dissociated and zwitterionic) on the oxidised TiO₂ surface. On TiO₂(1 1 0) surfaces reduced by Ar⁺ sputtering, a significant increase in the amount of zwitterionic proline at the surface was detected when compared with the oxidised surface. Study of the temperature effect showed that in both cases the zwitterionic structure was the less stable structure. The reason for its relative instability appears to be thermodynamic.     

Planar nano-block structures Tin+1Al0.5Cn and Tin+1Cn (n = 1, and 2) from MAX phases: Structural,electronic properties and relative stability from first principles calculations

Structural, electronic properties and relative stability of quasi-two-dimensional (2D) free-standing planar nano-block (NBs) structures Ti_n+1Al_0.5C_n and Ti_n+1C_n (n = 1 and 2), which can be prepared using the recently developed procedure of exfoliation of corresponding NBs from MAX phases, were examined within first principles calculations in comparison with parent MAX phases Ti₃AlC₂ and Ti₂AlC. We found that in general Ti_n+1C_n and Ti_n+1Al_0.5C_n NBs retain the atomic geometries of the corresponding blocks of the MAX phases, but some structural distortions for the NBs occur owing to the lowering of the coordination number for atoms in the external Ti sheets of the nano-block structures. Our analysis based on their cohesive and formation energies reveals that the stability of the nano-block structures increases with index n (or, in other words, with a growth of the number of Ti–C bonds), the Al-containing NBs becoming more stable than the “pure” Ti–C NBs. Our data show that the magnetization of the simulated planar nano-block structures can be expected; so, for the Ti₃C₂ nano-block the most stable will be the spin configuration, where within each external Ti sheet the spins are coupled ferromagnetically together with antiferromagnetic ordering between opposite external titanium sheets of this nano-block.     

Effects of annealing on structural and morphological properties of e-beam evaporated AgGaSe2 thin films

Polycrystalline AgGaSe₂ thin films were deposited by using single crystalline powder of AgGaSe₂ grown by vertical Bridgman-Stockbarger technique. Post-annealing effect on the structural and morphological properties of the deposited films were studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA) measurements. XRD analysis showed that as-grown films were in amorphous structure, whereas annealing between 300 and 600 °C resulted in polycrystalline structure. At low annealing temperature, they were composed of Ag, Ga₂Se₃, GaSe, and AgGaSe₂ phases but with increasing annealing temperature AgGaSe₂ was becoming the dominant phase. In the as-grown form, the film surface had large agglomerations of Ag as determined by EDXA analysis and they disappeared because of the triggered segregation of constituent elements with increasing annealing temperature. Detail analyses of chemical composition and bonding nature of the films were carried out by XPS survey. The phases of AgO, Ag, Ag₂Se, AgGaSe₂, Ga, Ga₂O₃, Ga₂Se₃, Se and SeO₂ were identified at the surface (or near the surface) of AgGaSe₂ thin films depending on the annealing temperature, and considerable changes in the phases were observed.     

Surface chemistry of NO and NO2 on the Pt(1 1 0)-(1 × 2) surface: A comparative study

The surface chemistry of NO and NO₂ on clean and oxygen-precovered Pt(1 1 0)-(1 × 2) surfaces were investigated by means of high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At room temperature, NO molecularly adsorbs on Pt(1 1 0), forming linear NO(a) and bridged NO(a). Coverage-dependent repulsive interactions within NO(a) drive the reversible transformation between linear and bridged NO(a). Some NO(a) decomposes upon heating, producing both N₂ and N₂O. For NO adsorption on the oxygen-precovered surface, repulsive interactions exist between precovered oxygen adatoms and NO(a), resulting in more NO(a) desorbing from the surface in the form of linear NO(a). Bridged NO(a) experiences stronger repulsive interactions with precovered oxygen than linear NO(a). The desorption activation energy of bridged NO(a) from oxygen-precovered Pt(1 1 0) is lower than that from clean Pt(1 1 0), but the desorption activation energy of linear NO(a) is not affected by the precovered oxygen. NO₂ decomposes on Pt(1 1 0)-(1 × 2) surface at room temperature. The resulted NO(a) (both linear NO(a) and bridged NO(a)) and O(a) repulsively interact each other. Comparing with NO/Pt(1 1 0), more NO(a) desorbs from NO₂/Pt(1 1 0) as linear NO(a), and both linear NO(a) and bridged NO(a) exhibit lower desorption activation energies. The reaction pathways of NO(a) on Pt(1 1 0), desorption or decomposition, are affected by their repulsive interactions with coexisting oxygen adatoms.     

A study of the interaction between perylene and the TiO2(1 1 0)-(1 × 1) surface-based on XPS, UPS and NEXAFS measurements

The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO₂(1 1 0)-(1 × 1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO₂(1 1 0)-(1 × 1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO₂(1 1 0)-(1 × 1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO₂ substrate due to weak van der Waals interactions.     

Spectroscopy of X2Y4 (D2h) molecules: tensorial formalism adapted to the O(3)⊃D2h chain, Hamiltonian and transition moment operators

A tensorial formalism adapted to the case of the X₂Y₄ molecules with D_2h symmetry has been developed in the same way as in the previous works on XY₄ (T_d) and XY₆ (O_h) spherical tops and XY₅Z (C_4v) symmetric tops. Here, we use the O(3)⊃D_2h group chain. All the coupling coefficients and formulas for the computation of matrix elements are given for this chain and used in the case of the Hamiltonian and transition moment operators.     

Francisella tularensis detection using magnetic labels and a magnetic biosensor based on frequency mixing

A biosensor that uses resonant coils with a special frequency-mixing technique and magnetic beads as detectable labels has been established for the detection of Francisella tularensis, the causative agent for tularemia. The detection principle is based on a sandwich immunoassay using an anti-Ft antibody for immunofiltration immobilized to ABICAP^® polyethylene filters, and biotinylated with streptavidin-coated magnetic beads as labels. The linear detection range of this biosensor was found to be 10⁴–10⁶ cfu F. tularensis lipopolysaccharide (LPS) per ml. Tested sample matrices were physiological PBS buffer and rabbit serum.     

Ground state rotational spectrum of nitrogen trifluoride: The K = 3 splittings of NF3 and NF3

The ground state rotational spectrum of the ¹⁴NF₃ and ¹⁵NF₃ isotopic species of nitrogen fluoride has been observed in the ∼450-810 GHz frequency range. This investigation allowed us to improve the rotational parameters for both isotopologues. In particular, for the first time the K = 3 line splitting parameter and the sextic centrifugal distortion constants have been determined for ¹⁵NF₃.