XPS Core Research Articles

MASS LOSS AND FLAMMABILITY OF INSULATION MATERIALS USED IN SANDWICH PANELS DURING THE PRE-FLASHOVER PHASE OF FIRE

In this study, the mass-loss and flammability limits of different sandwich panels and their cores (PUR, PIR, stone wool, EPS and XPS) are studied separately using a special developed furnace. The focus is on the pre-flashover phase of fire (up to 400°C), because exceeding the lower flammability limit in this phase may lead to a smoke layer explosion, a hazardous situation for an offensive intervention by the fire brigade. The research has shown that the actual mass-loss of synthetic and stone wool based cores is comparable up to 300°C. From 300°C onwards, the mass-loss of PUR panels is significant. EPS and XPS cores become fluid before pyrolysis starts. Furthermore delamination of the panels can be observed at exposure to temperatures above 250°C for the synthetic and 350°C for the mineral wool panels. The lower flammability limits have been established experimentally at 39% m/m (PUR) and 36% m/m (PS) of the pyrolysis gasses on the air mass, respectively. For PIR and mineral wool no flammability limits could be established.

An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO2/Ni/Electrolyte Interfaces

The electrical and spectroscopic properties of the TiO2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2 surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO2/Ni electrodes and to contextualize the AP-XPS results.

Structural studies on molecular mechanisms of Nelfinavir resistance caused by non-active site mutation V77I in HIV-1 protease.

BackgroundThe human immunodeficiency virus (HIV-1) is a retrovirus causing acquired immunodeficiency syndrome (AIDS), which has become a serious problem across the world and has no cure reported to date. Human immunodeficiency virus (HIV-1) protease is an attractive target for antiviral treatment and a number of therapeutically useful inhibitors have been designed against it. The emergence of drug resistant mutants of HIV-1 poses a serious problem for conventional therapies that have been used so far. Until now, thirteen protease inhibitors (PIs), major mutation sites and many secondary mutations have been listed in the HIV Drug Resistance Database. In this study, we have studied the effect of the V77I mutation in HIV-PR along with the co-occurring mutations L33F and K20T through multi-nanosecond molecular dynamics simulations. V77I is known to cause Nelfinavir (NFV) resistance in the subtype B population of HIV-1 protease. We have for the first time reported the effect of this clinically relevant mutation on the binding of Nelfinavir and the conformational flexibility of the protease.ResultsTwo HIV-PR mutants have been considered in this study - the Double Mutant Protease (DBM) V77I-L33F and Triple Mutant Protease (TPM) V77I-K20T-L33F. The molecular dynamics simulation studies were carried out and the RMSD trajectories of the unliganded wild type and mutated protease were found to be stable. The binding affinity of NFV with wild type HIV-PR was very high with a Glide XP docking score of -9.3 Kcal/mol. NFV showed decreased affinity towards DBM with a docking score of -8.0 Kcal/mol, whereas its affinity increased towards TPM (Glide XP score: -10.3). Prime/MM-GBSA binding free energy of the wild type, DBM and TPM HIV-PR docked structures were calculated as -38.9, -11.1 and -42.6 Kcal/mol respectively. The binding site cavity volumes of wild type, DBM and TPM protease were 1186.1, 1375.5 and 1042.5 Å3 respectively.ConclusionIn this study, we have studied the structural roles of the two HIV-PR mutations by conducting molecular dynamics simulation studies of the wild type and mutant HIV-1 PRs. The present study proposes that DBM protease showed greater flexibility and the flap separation was greater with respect to the wild type protease. The cavity size of the MD-stabilized DBM was also found to be increased, which may be responsible for the decreased interaction of Nelfinavir with the cavity residues, thus explaining the decreased binding affinity. On the other hand, the binding affinity of NFV for TPM was found to be enhanced, accounted for by the decrease in cavity size of the mutant which facilitated strong interactions with the flap residues. The flap separation of TPM was less than the wild type protease and the decreased cavity size may be responsible for its lower resistance, and hence, may be the reason for its lower clinical relevance.

Pyrazole clubbed triazolo[1,5-a]pyrimidine hybrids as an anti-tubercular agents: Synthesis, in vitro screening and molecular docking study

A series of novel pyrazole linked triazolo-pyrimidine hybrids were synthesized and evaluated for their anti-tuberculosis activity against M.tb H37Rv strain. Some of the screened entities rendered promising anti-tb activity (MIC: 0.39μg/mL) and were found non toxic against Vero cells (IC50: ⩾20μg/mL). Further, the docking study against wild type InhA enzyme of Mycobacterium tuberculosis using Glide reproduced the most active inhibitors (J21 and J27) with lowest binding energies and highest Glide XP scores demonstrating efficient binding to the active pocket. Additionally, the enzyme inhibition assay and ADME prediction of the active proved to be an attest to the possibility of developing compound J27 as a potent anti-tubercular lead.

Structural, optical and electrical properties of Nd-doped SnO2 thin films fabricated by reactive magnetron sputtering for solar cell devices

The use of photon conversion layers is an interesting way to improve the overall efficiency of solar cells. Herein we report on Nd-doped SnO2 thin films with photon management property inserted further into CIGS based solar cells. The functionalized layers were deposited by reactive magnetron sputtering whose structural, optical and electrical properties were tuned by varying the deposition temperature. Careful analysis of the structure using XRD and XPS showed that the tetragonal rutile SnO2 phase can be obtained at a deposition temperature as low as 100°C. Transparency was found to be as high as 90% for all layers while the absorption edge is found to increase when increasing the deposition temperature up to 300°C. The photoluminescence measurements under 325nm UV laser excitation showed that 100°C is needed for the optical activation of the rare earth. Despite the small amounts of Nd (around 0.62at%), intense and narrow emission bands have been collected in the Near Infrared Region (NIR) which are characteristics of Nd3+ ions whose the ionic state was confirmed by the 3d XPS core levels. Thus, the emission spectra cover a good part of the spectrum useful to the solar cell. Photoluminescence excitation spectroscopy experiments were also carried out on Nd:SnO2 samples to get insights on the energy transfer. By exciting in the deep UV from 250 to 400nm intense Nd emission was collected giving an experimental evidence of the down-shifting process through a resonant energy transfer from the SnO2 host matrix to the Nd3+ ions. Hall Effect measurements showed that the n-type character and good conductivity of the Nd doped SnO2 films can be correlated to the highest optical activity of Nd in the matrix. An optimal condition is found for the Nd-doped SnO2 film grown at 300°C for which the highest PL and the best electrical data were measured. Finally, we show that the implementation of such optimized Nd–SnO2 films on CIGS based solar cells serving as a transparent conducting oxide and a down shifting converter results in the best power conversion efficiency.

Insights on the structural perturbations in human MTHFR Ala222Val mutant by protein modeling and molecular dynamics

Methylenetetrahydrofolate reductase (MTHFR) protein catalyzes the only biochemical reaction which produces methyltetrahydrofolate, the active form of folic acid essential for several molecular functions. The Ala222Val polymorphism of human MTHFR encodes a thermolabile protein associated with increased risk of neural tube defects and cardiovascular disease. Experimental studies have shown that the mutation does not affect the kinetic properties of MTHFR, but inactivates the protein by increasing flavin adenine dinucleotide (FAD) loss. The lack of completely solved crystal structure of MTHFR is an impediment in understanding the structural perturbations caused by the Ala222Val mutation; computational modeling provides a suitable alternative. The three-dimensional structure of human MTHFR protein was obtained through homology modeling, by taking the MTHFR structures from Escherichia coli and Thermus thermophilus as templates. Subsequently, the modeled structure was docked with FAD using Glide, which revealed a very good binding affinity, authenticated by a Glide XP score of −10.3983 (kcal mol−1). The MTHFR was mutated by changing Alanine 222 to Valine. The wild-type MTHFR-FAD complex and the Ala222Val mutant MTHFR-FAD complex were subjected to molecular dynamics simulation over 50 ns period. The average difference in backbone root mean square deviation (RMSD) between wild and mutant variant was found to be ~.11 Å. The greater degree of fluctuations in the mutant protein translates to increased conformational stability as a result of mutation. The FAD-binding ability of the mutant MTHFR was also found to be significantly lowered as a result of decreased protein grip caused by increased conformational flexibility. The study provides insights into the Ala222Val mutation of human MTHFR that induces major conformational changes in the tertiary structure, causing a significant reduction in the FAD-binding affinity.

Formation of Ge0 and GeOx nanoclusters in Ge+-implanted SiO2/Si thin-film heterostructures under rapid thermal annealing

The results of X-ray photoelectron spectra (XPS valence band and core levels) measurements for Ge+ implanted SiO2/Si heterostructures are presented. These heterostructures have a 30nm thick Ge+ ion implanted amorphous SiO2 layer on p-type Si. The chemical-state transformation of the host-matrix composition after Ge+ ion implantation and rapid thermal annealing (RTA) are discussed. The XPS-analysis performed allows to conclude the formation of Ge0 and GeOx clusters within the samples under study. It was established, that the annealing time strongly affects the degree of oxidation states of Ge-atoms.

Electronic and surface properties of Ga-doped In2O3 ceramics

The limit of solubility of Ga2O3 in the cubic bixbyite In2O3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250°C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga2O3 as compared to In2O3. However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga+ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga+. However experiments on ion bombarded Ga2O3, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga+ at the surface, show that the chemical shift between Ga+ and Ga3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga+ is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.

Growth of an Ultrathin Zirconia Film on Pt3Zr Examined by High-Resolution X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, Scanning Tunneling Microscopy, and Density Functional Theory.

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 × 10–7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO2 films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO2 films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O–Zr–O) films on the alloy; only a small area fraction (10–15%) is covered by ZrO2 clusters (thickness ∼0.5–10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt3Zr substrate by ZrO2, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO2 films are between those of metallic Zr and thick (bulklike) ZrO2. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrOx is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO2 films or metal/ZrO2 interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators.

Pit assisted oxygen chemisorption on GaN surfaces.

A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

Structural basis for complementary and alternative medicine: Phytochemical interaction with non-structural protein 2 protease-a reverse engineering strategy.

To understand the druggability of the bioactive compounds from traditional herbal formulations "Nilavembu Kudineer" and "Swasthya Raksha Amruta Peya" to heal chikungunya virus (CHIKV) infection. The efficiency of twenty novel chemical entities from "Nilavembu Kudineer" and "Swasthya Raksha Amruta Peya" to inhibit CHIKV infection in silico were evaluated. Ligands were prepared using Ligprep module of Schrödinger. Active site was identified using SiteMap program. Grid box was generated using receptor grid generation wizard. Molecular docking was carried out using Grid Based Ligand Docking with Energetics (GLIDE) program. Molecular docking studies showed that among twenty compounds, andrographoside, deoxyandrographoside, neoandrographolide, 14-deoxy-11-oxoandrographolide, butoxone and oleanolic acid showed GLIDE extra precision (XP) score of -9.10, -8.72, -8.25, -7.38, -7.28 and -7.01, respectively which were greater than or comparable with chloroquine (reference compound) XP score (-7.08) and were found to interact with the key residues GLU 1043, LYS 1045, GLY 1176, LEU 1203, HIS 1222 and LYS 1239 which were characteristic functional unit crucial for replication of CHIKV. The binding affinity and the binding mode of chemical entities taken from herbal formulations with non-structural protein 2 protease were understood and our study provided a novel strategy in the development and design of drugs for CHIKV infection.

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5eV and thermionic electron emission (TE) has been detected at temperatures less than 500°C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5×10−5A/cm2 at 500°C. In contrast, the initial TE current density of a film aged for 18months was ~1.8×10−9A/cm2 at 500°C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3×10−4mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2h of exposure with the sample at 500°C, the surface exhibited a stable emission current density of ~2.3×10−6A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5×10−8mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400°C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200meV, and the TE intensity increased by a factor of ~100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.

Photoemission study of copper phthalocyanine growth on hydrogen-terminated surface: Si(100)2 × 1–H

Copper phthalocyanine molecules have grown at room temperature under ultra high vacuum on hydrogen-terminated surface Si(100)2×1–H. Chemical and electronic properties of the interface were investigated by ultraviolet and X-ray photoemission spectroscopy (UPS, XPS). ResultsResults indicated the existence of an interfacial dipole of 0.36±0.05eV and a band bending of 0.40±0.05eV. These were evidenced by shifts of XPS core levels and change of the vacuum level position. During the growth, the work function was found to decrease from 4.5eV for the substrate to 3.74eV for the highest coverage (40monolayers). This band bending was also due to the shift of the highest occupied molecular orbital.The interfacial dipole was correlated to a rearrangement of molecules on the surface. An energy level diagram of the interface was deduced from a combination of the XPS and UPS results.

Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N2+ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N2+ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N2+ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of SixNy on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N2+ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.

Comparative molecular docking studies of novel 3, 5-disubstituted thiazolidinedione analogs as HIV-1-RT inhibitors

Molecular docking studies were performed on 30 novel thiazolidinediones using Glide, FlexX and Scigress Explorer Ultra programs and the X-ray crystallographic structure of HIV-1 Reverse transcriptase (PDB code 1RT2) to study the binding mode of these analogs. The experimental conformation of the bound ligand TNK 651 was precisely reproduced by the docking procedures as demonstrated by low (<3.00 A) root mean square deviations. Results of this study indicated that most of the compounds dock into the active site of 1RT2 enzyme with good docking scores comparable to the bound compound TNK 651. The docking analysis of the best docked molecules 29 (Glide XP score −11.76) and 22 (FlexX score −20.77) showed significant interactions with active-site amino acid residues and formed H bond interactions with the key amino acid residues, while the molecule 1 in Scigress Explorer Ultra exhibited the highest score of −59.49.

XPS characterization of single crystalline SrLaGa3O7:Nd

Core-level XPS studies of single crystalline SrLaGa3O7 (SLGO) doped with Nd have been performed for the first time. The detailed analysis of the main XPS core lines of SLGO:Nd included the comparative literature data for the selected oxide compounds containing La and/or Sr, Nd or Ga cations. In particular, the binding energies (BEs) of the Ln 3d5/2 core levels (Ln=La, Nd) in SLGO:Nd were found consistent with the relevant ones for the La- and Nd-based oxides considered, thus indicating that they represent the ground final states of La3+ and Nd3+ ions (respectively) in the crystal lattice after photoexcitation. Analogous consistency has been found for the XPS-derived BEs of the Sr 3d5/2 and Ga 2p3/2 core levels related to the Sr2+ and Ga3+ cationic states in SLGO and the corresponding oxide compounds. Generally, the binding energies of the deep core levels of cations in SLGO:Nd and the other oxides considered are mainly determined by their common oxygen ligand, irrespectively of the crystalline structure.

Structural and surface study of calcium glyceroxide, an active phase for biodiesel production under heterogeneous catalysis

Calcium diglyceroxide has been reported as a very active phase in the transesterification of triglycerides with methanol for biodiesel production. This work reports on the determination of the crystal structure of Ca diglyceroxide by Patterson-based direct methods from synchrotron X-ray powder diffraction (XRD) data in combination with characterisation by Differential Thermal Analysis-Thermogravimetry (DTA-TG), Scanning Electron Microscopy (SEM) and X-ray Photoelectron and Fourier Transform-Infrared spectroscopies (XPS and FT-IR). Its crystal structure is formed by molecular tetramers held together by a complex H-bond network. Both the XRD structural determination and the O1s XPS core level indicate the presence of a basic non-protonated O− anion at the surface of Ca diglyceroxide. Along with the presence of surface lipophilic CHx units, this O− anion may be the origin of this molecule’s high activity relative to CaO.

Molecular docking studies of novel thiazolidinedione analogs as HIV-1-RT inhibitors

Molecular docking studies were performed on 280 novel thiazolidinediones by Glide, FlexX, and Scigress Explorer Ultra programs and the X-ray crystallographic structure of HIV-1 Reverse transcriptase (PDB code 1RT2) to study the binding mode of these analogs. The experimental conformation of the bound ligand TNK 651 was precisely reproduced by the docking procedures as demonstrated by low (<3.00 A) root mean square deviations. Results of this study indicated that most of the compounds dock into the active site of 1RT2 enzyme with good docking scores comparable to the bound compound TNK 651. The docking analysis of the highest active molecule 232 (Glide XP score −12.47) and 55 (FlexX score −29.71) shows significant interaction with active site amino acid residues and H-bond interactions with the key amino acid residues.

The Influence of Ce on the Electrical Resistivity and Electronic Structure in the Gd&lt;sub&gt;1-x&lt;/sub&gt;Ce&lt;sub&gt;x&lt;/sub&gt;In&lt;sub&gt;3&lt;/sub&gt; Compounds

All studied Gd1-xCexIn3 compounds crystallize in the cubic AuCu3 - type of crystal structure. The influence of Gd/Ce substitution is reflected in the linear increase of the unit cell parameter. The temperature dependence of the electrical resistivity ρ(T) strongly depends on the Ce content. For compounds with x≤0.2 a typical metallic behavior has been observed. In contrary, for Ce-rich compounds (x≥0.5) a characteristic Kondo-type behaviour has been noticed. The analysis of cerium XPS core level lines reveals the occurrence of possible Ce intermediate valence.

Structure-based design of rhodanine-based acylsulfonamide derivatives as antagonists of the anti-apoptotic Bcl-2 protein

A series of novel rhodanine-based acylsulfonamide derivatives were designed, synthesized, and evaluated as small-molecule inhibitors of anti-apoptotic Bcl-2 protein. These compounds exhibit potent antiproliferative activity in three human tumor cell lines (Hep G2, PC-3 and B16-F10). Among them, the most potent compounds 10 and 11 bind to Bcl-2 with a Ki of 20 and 25nM, respectively. Docking studies demonstrated that these two compounds orient similarly at the binding site of Bcl-2, and the calculated binding affinities (Glide XP score) of compound 10 is more negative than that of compound 11. The binding interactions of compounds with high binding affinity to Bcl-2 protein were analyzed.