Photon Energy Region Research Articles

Quasiparticle electronic structure and optical spectra of single-layer and bilayer PdSe2 : Proximity and defect-induced band gap renormalization

The fundamental properties of recently synthesized single- and bilayer ${\mathrm{PdSe}}_{2}$ are investigated using accurate many-body perturbation GW theory to quantitatively examine their electronic structure and explain the insufficiency of previously reported experimental and theoretical results. Including electron-hole interactions responsible for exciton formation, we solve the Bethe-Salpeter equation on top of the ${\mathrm{GW}}_{0}$ approximation to predict the optical properties. The fundamental quasiparticle band gaps of single- and bilayer ${\mathrm{PdSe}}_{2}$ are 2.55 and 1.89 eV, respectively. The optical gap of monolayer ${\mathrm{PdSe}}_{2}$ reduces significantly due to a large excitonic binding energy of 0.65 eV comparable to that of ${\mathrm{MoSe}}_{2}$, while an increase of the layer number decreases the excitonic binding energy to 0.25 eV in bilayer ${\mathrm{PdSe}}_{2}$. The giant band gap renormalization of \ensuremath{\sim}36--38% in the bilayer (BL) ${\mathrm{PdSe}}_{2}$/graphene heterostructure has a high impact on the construction of ${\mathrm{PdSe}}_{2}$-based devices and explains the experimentally observed band gap. The small value of the experimental optical gap of single-layer (SL) ${\mathrm{PdSe}}_{2}$ (1.3 eV) can be explained by the presence of Se vacancies, which can drop the Tauc-estimated optical gap to \ensuremath{\sim}1.32 eV. The absorption spectra of both mono- and bilayer ${\mathrm{PdSe}}_{2}$ cover a wide region of photon energy, demonstrating promising application in solar cells and detectors. These findings provide a basis for a deeper understanding of the physical properties of ${\mathrm{PdSe}}_{2}$ and ${\mathrm{PdSe}}_{2}$-based heterostructures.

Photoionization and dissociation study of 2-methyl-2-propen-1-ol: Experimental and theoretical insights

The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultraviolet synchrotron radiation in the photon energy region of 8.0 $-$ 15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C $_4$ H $_8$ O $^+$ , C $_4$ H $_7$ O $^+$ , C $_3$ H $_5$ O $^+$ , C $_4$ H $_7$ $^+$ , C $_4$ H $_6$ $^+$ , C $_4$ H $_5$ $^+$ , C $_2$ H $_4$ O $^+$ , C $_2$ H $_3$ O $^+$ , C $_3$ H $_6$ $^+$ , C $_3$ H $_5$ $^+$ , C $_3$ H $_3$ $^+$ , CH $_3$ O $^+$ , CHO $^+$ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d, p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO.

Photon attenuation parameters for some tissues from Geant4 simulation, theoretical calculations and experimental data: a comparative study

Mass attenuation coefficients, effective atomic numbers, effective electron densities and Kerma relative to air for adipose, muscle and bone tissues have been investigated in the photon energy region from 20 keV up to 50 MeV with Geant4 simulation package and theoretical calculations. Based on Geant4 results of the mass attenuation coefficients, the effective atomic numbers for the tissue models have been calculated. The calculation results have been compared with the values of the Auto- $$Z_{\text {eff}}$$ program and with other studies available in the literature. Moreover, Kerma of studied tissues relative to air has been determined and found to be dependent on the absorption edges of the tissue constituent elements.

Single photoionization of the Kr-like Rb ii ion in the photon energy range 22–46.0 eV

Abstract Single photoionization cross-sections for Kr-like Rb+ ions are reported in the energy (wavelength) range from 22 (564 Å) to 46 eV (270 Å). Theoretical cross-section calculations for this trans-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and from the dual laser plasma technique, at respectively 40 and 35 meV FWHM energy resolution. In the photon energy region 22–32 eV the spectrum is dominated by excitation auto-ionizing resonance states. Above 32 eV the cross-section exhibits classic Fano window resonances features, which are analysed and discussed. Large-scale theoretical photoionization cross-section calculations, performed using a Dirac Coulomb R-matrix approximation are benchmarked against these high-resolution experimental results. Comparison of the theoretical work with the experimental studies allowed the identification of resonance features and their parameters in the spectra in addition to contributions from excited metastable states of the Rb+ ions.

Determination of electron inelastic mean free path of three transition metals from reflection electron energy loss spectroscopy spectrum measurement data

The energy loss functions (ELFs) of three transition metals (Cr, Co and Pd) have been derived from reflection electron energy loss spectroscopy spectrum with a theoretical analysis of the measured data [Xu et al., J. Appl. Phys. 123, 043306 (2018)]. In this work, we update our previous ELFs in a wider photon energy region (0–200 eV) with a better accuracy, which is verified by sum rules and a root-mean-square deviation. The electron inelastic mean free paths (IMFPs) of Cr, Co and Pd have been calculated with the obtained ELFs by adopting a dielectric response theory. We employ both the single-pole approximation and full Penn algorithm for the calculation of IMFPs, and the calculated results are compared with other references.

Infrared spectroscopy on electronic structures of platinum-group metal pernitrides MN2 (M = Ru, Rh, Ir, and Pt)

The electronic structures of platinum-group metal pernitrides MN2 (M = Ru, Rh, Ir, and Pt) were investigated via synchrotron radiation infrared spectroscopy and first-principles calculations. Measured reflectance spectra of marcasite-type RuN2 and RhN2 showed Drude-like responses, approaching 1 as the photon energy was decreased, whereas reflectance of arsenopyrite-type IrN2 and pyrite-type PtN2 became ∼0.3 in the low photon energy region with a few features. These findings agreed well with the predictions of the metallic nature of marcasite-type RuN2 and RhN2 and the semiconducting properties of arsenopyrite-type IrN2 and pyrite-type PtN2, respectively. The measured reflectance spectra were also reasonably consistent with the calculated optical responses. The band gaps of IrN2 and PtN2 were estimated to be 0.8 and 2.1 eV, respectively, via first-principles calculation with a modified Becke–Johnson (MBJ) potential for the exchange potential.

Эпитаксиальный рост cульфида цинка методом молекулярного наслаивания на гибридных подложках SiC/Si

In this work, we obtained epitaxial films of zinc sulfide on silicon by the ALD method. To avoid the interaction between silicon and zinc sulphide on the silicon surface, a high-quality buffer layer of silicon carbide ~ 100 nm thick was preliminarily synthesized by chemical substitution of atoms. The diffraction of high energy electrons showed that the ZnS layers are epitaxial. Using ellipsometric methods, it has been proved that the grown ZnS layers are transparent in the photon energy region up to 3 eV, which is of crucial importance for applications in optoelectronics.

Giant thermal effect of vibration modes of single-crystalline alanine

Amino acids are the most basic molecules of living bodies. The temperature dependence of the polarized optical conductivity [σ(ω)] spectrum of single-crystalline L-alanine, which has the simplest molecular structure among amino acids with chirality, has been measured in the photon energy region of 1 meV–30 eV. The polarized σ(ω) spectra suggest the strong anisotropy of vibration modes owing to the strong anisotropic crystal structure. Some peaks of these vibration modes have been observed to show extremely strong temperature dependence. In this paper, we picked up one peak in the THz region that has the largest temperature dependence and the thermal effect can be explained by a temperature-dependent anharmonic potential model.

Interference Induced Enhancement of Magneto-Optical Effect in Pt/TbCo Hetero-Structured Films

Magnetic films with a heavy metal layer show strong interfacial interaction of spin-orbit. Spin-orbit interaction is one of the key technologies for spintronics. In this paper, we measured magneto-optical Kerr spectra of Pt/TbCo hetero-structure films on a thermally oxidized silicon substrate (0.3 mm); A: Pt (3 nm)/TbCo (6 nm)/Pt (3 nm), B: Si3N4 (10 nm)/TbCo (6 nm)/Pt (3 nm), and C: Pt (3 nm)/TbCo (6 nm)/Si3N4 (10 nm). Magneto-optical Kerr spectra of each sample were measured with a wavelength range of 300–700 nm, and were compared to the simulated spectra using the effective refractive index method. In the sample A, which has a symmetric structure, the simulated spectra are consistent with the measured ones. On the other hand, in the samples B and C, with an asymmetric structure, there are some differences between the simulated spectra and the measured ones in a lower photon energy region. This may be caused by interfacial effects of the spin-orbit interaction.

Branching Ratio Measurements of the Predissociation of 12C16O by Time-Slice Velocity-Map Ion Imaging in the Energy Region from 106 250 to 107 800 cm-1.

Photodissociation of CO is a fundamental chemical mechanism for mass-independent oxygen isotope fractionation in the early Solar System. Branching ratios of photodissociation channels for individual bands quantitatively yield the trapping efficiencies of atomic oxygen resulting into oxides. We measured the branching ratios for the spin-forbidden and spin-allowed photodissociation channels of 12C16O in the vacuum ultraviolet (VUV) photon energy region from 106 250 to 107 800 cm-1 using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to four 1Π bands and three 1Σ+ bands of 12C16O were identified and investigated. The branching ratios for the product channels C(3P) + O(3P), C(1D) + O(3P), and C(3P) + O(1D) of these predissociative states strongly depend on the electronic and vibrational states of CO being excited. By plotting the branching ratio of the spin-forbidden dissociation channels versus the excitation energy from 102 500 to 110 500 cm-1 that has been measured so far, the global pattern of the 1Π-3Π interaction that plays a key role in the predissociation of CO is revealed and discussed.

Optical properties of $$\hbox {TlGa}(\hbox {S}_{{x}}\hbox {Se}_{1-x})_{2}$$ TlGa ( S x Se 1 - x ) 2 layered mixed crystals $$(0 \le x \le 1)$$ ( 0 ≤ x ≤ 1 ) : Absorption edge and photoluminescence study at $$T = 10\hbox { K}$$ T = 10 K

Transmittance measurements of $$\hbox {TlGa}(\hbox {S}_{{x}}\hbox {Se}_{1-x})_{2}$$ layered mixed crystals $$(0 \le \, x \le 1)$$ were performed in the 1.80–2.80 eV photon energy range at $$T = 10 \hbox { K}$$ . Band-gap energies of the studied crystals were estimated by means of the derivative spectra of transmittance and photon energy dependence of absorption coefficient. The compositional dependence of direct band-gap energy at $$T = 10\hbox { K}$$ revealed that as sulphur (selenium) composition is increased (decreased) in the mixed crystals, the direct band-gap energy increases from 2.19 eV ( $$x = 0$$ ) to 2.67 eV ( $$x = 1$$ ). Photoluminescence spectra of $$\hbox {TlGa}(\hbox {S}_{{x}}\hbox {Se}_{1-x})_{2}$$ mixed crystals were investigated in the photon energy region of 2.00–3.10 eV at $$T = 10\hbox { K}$$ . The observed bands were assigned to the transitions of electrons from conduction band to the shallow acceptor levels in the band gap. The shift of the PL bands to higher energies with increasing sulphur content was revealed. In addition, the composition ratio of the mixed crystals was obtained from the energy-dispersive spectroscopy experiments.

Composition-tuned optical absorption and luminescence of TlGaxIn1-xS2 layered mixed crystals at T = 10 K

Transmittance on TlGaxIn1-xS2 mixed crystals (0 ≤ x ≤ 1) was carried out in the 2.35–2.75 eV photon energy range at T = 10 K. Band gap energies of crystals were obtained utilizing the derivative spectra of transmittance and photon energy dependence of absorption coefficient. The dependence of direct band gap energy on composition at T = 10 K revealed that as gallium composition is elevated in the mixed crystals, the direct band gap energy increases from 2.56 to 2.67 eV.Photoluminescence spectra of TlGaxIn1-xS2 crystals were studied in the photon energy region of 2.30–2.95 eV at T = 10 K. The observed bands were attributed to transitions of electrons from shallow donor levels to valence band. The shift of PL bands to higher energies with increasing gallium content was revealed. Furthermore, composition ratio of crystals was determined from energy dispersive spectroscopy experiments.

Sol-Gel Derived Cds Nanocrystalline Thin Films: Optical and Photoconduction Properties

Abstract High-quality CdS nanocrystalline thin films were grown by sol-gel spin coating method at different solution temperatures on glass substrates. As-deposited films exhibited nanocrystalline phase with hexagonal wurtzite structure and showed good adhesion and smooth surface morphology. It was clearly observed that the crystallinity of the thin films improved with the increase in solution temperature. Crystallites sizes of the films also increased and were found to be in the range of 10 mm to 17 nm. The influence of the growth mechanism on the band and sub-band gap absorption of the films was investigated using UV-Vis and photothermal deflection spectroscopy (PDS). The band gap values were calculated in the range of 2.52 eV to 2.75 eV. The band gap decreased up to 9 % with the increase in solution temperature from 45 °C to 75 °C. Absorption coefficients estimated by PDS signal showed the significant absorption in low photon energy region of 1.5 eV to 2.0 eV. The dark and illuminated I-V characteristics revealed that the films were highly photosensitive. The results demonstrated the potential applications of sol-gel grown CdS nanocrystalline thin films as photoconductors and optical switches.

Blueshift of absorption edge and photoluminescence in Al doped ZnO thin films

ABSTRACTPure and Al doped ZnO thin films are fabricated on quartz substrates by sol–gel method, and then analyzed by X-ray diffraction (XRD), transmittance spectra, and photoluminescence (PL) measurements respectively. XRD results reveal that all the thin films have a preferential c-axis orientation. With the increase of Al doping, however, the peak position of the (002) plane is shifted to a low 2θ value. On the other hand, the data of spectrometer transmittance are obtained, with which the band gap energies of Al doped ZnO films are calculated by a linear fitting method. The band gap is found to be broadening, and the absorption edge has an obvious blueshift to the shorter wavelength with increasing dopant concentration. PL measurement is also conducted, and deep-level (DL) emission and near band edge (NBE) emission are observed in pure ZnO thin films. But DL emissions are depressed when Al is doped into thin films. And the peak of NBE emission has a blueshift to the region of higher photon energy as the Al concentration increases, a performance which tallies with observations through the optical transmittance data. The study demonstrates that the blueshift of optical properties in the ZnO:Al films, can nevertheless be easily manipulated and managed by controlling dopant concentration, a big plus to the said films in their applications in broadband UV photodetectors with highly tunable wavelength resolution.

Absolute Photoionization Cross Section for Fe6+ to Fe10+ Ions in the Photon Energy Region of the 2p–3d Resonance Lines

Abstract Resonant single photoionization cross sections of Fen+ (n = 6 to 10) ions have been measured in absolute values using a merged-beams setup at the SOLEIL synchrotron radiation facility. Photon energies were between about 710 and 780 eV, covering the range of the 2p–3d transitions. The experimental cross sections are compared to calculations we performed using a multi-configuration Dirac–Fock code and the OPAS code dedicated to radiative opacity calculations. Comparisons are also done with the Chandra X-ray observatory NGC 3783 spectra and with the results of previously published calculations.

Stabilizing benzene-like planar N6 rings to form a single atomic honeycomb BeN3 sheet with high carrier mobility.

It is a longstanding quest to use the planar N6 ring as a structural unit to build stable atomic sheets. However, unlike C6H6, the neutral N6 ring is unstable due to the strong repulsion of the lone-pair of electrons. Using first-principles calculations and the global structure search method, we show that the N6 unit can be stabilized by the linkage of Be atoms, forming a h-BeN3 honeycomb monolayer, in which the geometry and the π-molecular orbitals of the N6 rings are well kept. This sheet is not only energetically, dynamically and thermally stable, but also can withstand high temperatures up to 1000 K. Band structure calculation combined with a group theory analysis and a tight-binding model uncover that h-BeN3 has a π-band dominated band structure with an indirect band gap of 1.67 eV. While it possesses a direct band gap of 2.07 eV at the Γ point lying in the photon energy region of visual light, its interband dipole transition is symmetrically allowed so that electrons can be excited by photons free of phonons. Based on deformation potential theory, a systematic study of the transport properties reveals that the h-BeN3 sheet possesses a high carrier mobility of ∼103 cm2 V-1 s-1, superior to the extensively studied transition metal dichalcogenide monolayers. We further demonstrate that this sheet can be rolled up into either zigzag or armchair nanotubes. These nanotubes are also dynamically stable, and are all direct band gap semiconductors with carrier mobility comparable to that of their 2D counterparts, regardless of their chirality and diameter. The robust stability and novel electronic and transport properties of the h-BeN3 sheet and its tubular derivatives endow them with great potential for applications in nanoelectronic devices.

Polarization observables in few nucleon systems with CLAS

The CEBAF Large Acceptance Spectrometer (CLAS), housed in Hall-B at the Thomas Jefferson National Accelerator Facility provides us with the experimental tools to study strongly-interacting matter and its dynamics in the transition from hadronic to partonic degrees of freedom in nuclear interactions. In this paper we discuss the progress made in understanding the relevant degrees of freedom using polarisation observables of deuteron photodisintegration in the few-GeV photon-energy region. We also address progress made in studying the interaction between Hyperons and Nucleons via polarisation observables, utilising high-statistics experiments that provided us with the large data samples needed to study final-state interactions, as well as perform detailed studies on initial-state effects. The polarisation observables presented here provide us with unique experimental tools to study the underlying dynamics of both initial and final-state interactions, as well as the information needed to disentangle signal from background contributions.

Weakening of excitonic screening effects in TixZn1-xO thin films

Excitonic transitions of TixZn1-xO thin films have been studied using spectroscopic ellipsometry in the photon energy range of 0.5–6.5eV. ZnO-based thin films were deposited on Si substrates by DC-unbalanced magnetron sputtering with Ti concentrations of 0, 1, and 3at.%. By using a combination of Tauc-Lorentz, Drude, and Gaussian oscillators, the complex dielectric function and its derivatives were obtained and analyzed. In pure ZnO, the excitonic transitions were observed at ~3.35eV through critical point analysis of the complex dielectric function. Mid-gap states, which acted as an electronic screening of excitons, were found in the photon energy region of 1.50–3.35eV. Upon 1at.% Ti doping, the amplitude of the mid-gap states decreased from 0.26 to 0.04 while the amplitude of excitonic transitions increased from 0.22 to 0.28, as compared to the pure ZnO. We explain such phenomena due to the weakening of excitonic screening effects. The results show that Ti plays an important role in the electronic structure and excitonic effects of ZnO.

Experimental and theoretical study on the dissociative photoionization of methyl methacrylate

The photoionization of methyl methacrylate and dissociation of its cation have been investigated by tunable vacuum ultraviolet synchrotron radiation coupled with time-of-flight mass spectrometer in the photon energy region of 9.0–15.5 eV. The ionization energy of methyl methacrylate and the appearance energies (AEs) for major fragments, C5H6O+, C4H5O+, C3H4O+, C3H3O+ and are determined to be 9.76, 10.30, 10.66, 10.51, 11.17, 10.51, 10.74, 12.88, 12.73, 12.52, and 12.82 eV, respectively, by measurement of the photoionization efficiency curves. Possible formation pathways of the major fragments are proposed based on comparison of experimental AEs and energies predicted by ab initio G3B3 calculations. Transition states and intermediates involved in the dissociation channels are also located. The majority of the proposed channels occur through isomerization prior to dissociation. Hydrogen shift and ring closing/opening are found to be the dominant processes during photofragmentaion of methyl methacrylate.

Design of the new soft X-ray beamline for in situ analysis of energy materials at National Synchrotron Radiation Laboratory

A new bending magnet based soft X-ray beamline for in situ analysis of energy materials at National Synchrotron Radiation Laboratory will be constructed and the optical design is presented. This beamline will mainly focus on nanomaterials and their applications in energy conversion, storage, and catalysis studies. A varied-line-spacing plane grating monochromator is employed to cover the photon energy region of 60–1000 eV by two plane varied line spacing gratings with central groove densities of 840 l/mm and 1400 l/mm respectively. The designed energy resolution power E∕ΔE is about 1300∼3000. The calculated flux is higher than 1×1010 phs/s under 300 mA ring beam current for the whole covered photon energy. This beamline will provide a powerful and efficiency instrument in the future for users doing research on nanomaterials, energy materials and catalytic science.