Optical Nonlinearity of Emerging ZrS2 and HfS2 Semiconductors

Compatible with existing processing technology, chemical vapor deposition method is used to synthesize ZrS₂ and HfS₂ films a a large scale. The nonlinear optical properties are characterized by Z-scan measurement with femtosecond pulses at 800 nm. The results show that saturable absorption happens in ZrS₂ owing to the larger ground state absorption than the excited state absorption, while reverse saturable absorption appears in HfS₂ due to the two-photon absorption. The figure of merit values of ZrS₂ (≈4.30 ± 0.12 × 10⁻¹⁵ esu cm) and HfS₂ (≈6.0 ± 1.4 × 10⁻¹⁵ esu cm) are much larger than those of MoS₂ and graphene in ultrafast nonlinear optical performance at the wavelength of 800 nm.     

Study of the electronic structure and spectral features of TiS2,ZrS2 and HfS2 by the Xα discrete variational method

In order to study the electronic structure of TiS₂, ZrS₂ and HfS₂, selfconsistent charge X_α discrete variational calculations of clusters MeS₆^8- (Me = Ti, Zr, Hf) have been performed. The results obtained are consistent with band structure calculations. The covalency of the chemical bonding is shown to increase in the series TiS₂-ZrS₂-HfS₂, which corresponds to the atomization energy in the series. The results are applied in the interpretation of X-ray emission, photoelectron and optical spectra.     

Evidence for Ruelle's turbulent crystal?

The nuclear quadrupole interaction at¹⁸¹Ta on Hf/Zr-sites in ZrS₂, HfS₂, HfSe₂, and HfTe₅ exhibits anomalies as a function of temperature, indicative of lattice instabilities. We interpret our observations as being due to static or low frequency structural distortions perpendicular to the layers and take these as evidence for Ruelle's turbulent crystal.     

Indirect absorption edge of ZrS2 and HfS2

The tranmission spectra and the wavelength-modulated transmission spectra of ZrS₂ and HfS₂ were measured simultaneously over the temperature range from 1.9 to 300K, and structures in the spectra due to the indirect allowed transitions including excitonic effects were clearly observed. Comparing with the band structure calculations, these correspond to Γ⁻₂ å L ⁺₁ and Γ⁻₂ å M⁺₁ transitions.     

Low voltage organic field effect transistors with a poly(hexylthiophene)–ZnO nanoparticles composite as channel material

We report on solution processable organic field effect transistors prepared using a poly(3‐hexylthiophene)–ZnO nanoparticles composite as channel semiconductor material and cross‐linked polyvinyl alcohol as gate insulator. Our transistors show a field effect mobility of 0.35 ± 0.06 cm²/V s, threshold voltage of –1.30 ± 0.11 V, and I_on/I_off ratio of (1.0 ± 0.1) × 10³. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron-impact ionization and dissociation of C<Subscript>2</Subscript>D<Superscript>+</Superscript>

Absolute cross sections for electron-impact single ionization, dissociative excitation and dissociative ionization of the ethynyl radical ion (C₂D⁺)^+) have been measured for electron energies ranging from the corresponding reaction thresholds to 2.5 keV. The animated crossed electron-ion beam experiment is used and results have been obtained for the production of C₂D²⁺, C²⁺, C₂⁺_2^+ , CD⁺, C⁺ and D⁺. The maximum of the cross section for single ionization is found to be (2.01 ± 0.02) × 10^-17 cm², at the incident electron energy of 105 eV. Absolute total cross sections for the various singly charged fragments production are observed to decrease by a factor of almost three, from the largest cross-section measured for C⁺, over C₂⁺_2^+ and CD⁺ down to that of D⁺. The maxima of the cross sections are obtained to be (14.5 ± 0.5) × 10^-17 cm² for C₂⁺_2^+, (12.1 ± 0.1) × 10^-17 cm² for CD⁺, (27.7 ± 0.2) × 10^-17 cm² for C⁺ and (11.1 ± 0.8) × 10^-17 cm² for D⁺. The smallest cross section is measured to be (1.50 ± 0.04) × 10^-18 cm² for the production of the doubly charged ion C²⁺. Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. Kinetic energy release distributions of dissociation fragments are seen to extend from 0 to 6 eV for the heaviest fragment C₂⁺_2^+, up to 11.0 eV for CD⁺, 14.2 eV for C⁺ and 11.2 eV for D⁺ products.     

A photoelectron spectroscopic study of the second ionisation of the CF(X2Π) radical

The CF(X²Π) radical has been re-investigated with vacuum ultraviolet photoelectron spectroscopy. Two bands were observed and assigned to the ionisations CF⁺(X¹Σ⁺)←CF(X²Π) and CF⁺(a³Π)← CF(X²Π). The first band, which has been observed previously, has an adiabatic ionisation energy of (9.11±0.02) eV and a vertical ionisation energy of (9.55±0.02) eV. For the second band, three vibrational components were observed with the first, the most intense, at an ionisation energy of (13.94±0.02) eV. Analysis of the vibrational structure in the two observed bands allowed ω_e and r_e to be determined as 1810±30 cm⁻¹ and 1.154±0.005 Å, respectively for the first ionic state, CF⁺(X¹Σ⁺), and 1614±30 cm⁻¹ and 1.213±0.005 Å, respectively for the second ionic state, CF⁺(a³Π). Comparison of the ionisation energies and spectroscopic constants obtained has been made with values obtained from recent multi-reference configuration interaction calculations.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Electrical transport measurements in a quasi-onedimensional semiconductor ZrS3

We have used the technique of chemical vapour transport to prepare needle shaped single crystal of ZrS₃. Results of the measurements of d.c. resistivity. Hall coefficient and thermoelectric power of the temperature range 100–500 K are reported. All the samples exhibited semiconducting behaviour with a room temperature resistivity of about 15 Ω-cm and an activation energy of 0.20±0.02 eV. Room temperature thermoelectric power is -850 μVK^?1 and the dominant carriers are electrons. The thermoelectric power varies as (1/T), a behaviour associated with a typical semiconductor. Mobility at low temperatures is limited by ionized impurity scattering and is given by μ₁ = 6.5 × 10^?2T^3/2 cm²V^7-1 sec^?1. At high temperatures, phonon scattering is dominant and the mobility is given by μ₂ = 1.35 × 10⁺⁵T^?32 cm²V^?1 sec^?1.     

Migration and stabilization of hydrogen atoms in KCl

Stabilities of hydrogen atoms at the interstitial (H⁰_i), cation and anion sites (H⁰_sc and H⁰_sa) in KCl have been studied with ESR and thermoluminescence. The close pair between H⁰_i and S^? formed by UV-irradiation of KCl:SH^? recombines first with the activation energy of 0.17±0.02 eV following the emission of light and then H⁰_i becomes mobile with the energy of 0.20±0.02 eV. The energies determined from the decay of ESR signal intensity of H⁰_sc and H⁰_sa are 0.24±0.02 eV and 0.43±0.03 eV, respectively.     

A.C. electrical conductivity of potassium perchlorate

A.C. electrical conductivity of pottasium perchlorate (KP) has been measured in the temperature range 25–325°C at frequencies ranging from 50–500 Hz using an automated technique. Three regions with activation energies. 1.08 ± 0.02, 0.66 ± 0.01 and 0.39 ± 0.04eV are observed in ln(σT) vs 1/T plots at frequencies < - 50 Hz. Dielectric loss measurements on KP samples doped with CrO₄^2? and SO₄^2? ions show the presence of impurity-vacancy complexes with reorientation energies of 0.99 ±0.02 and 1.07 ± 0.01 eV respectively. The results are interpreted in terms of a defect model which highlights the role of various conduction and relaxation mechanisms in the a.c. electrical properties of the material.     

Some nuclear and atomic properties of201Hg determined by conversion electron spectroscopy

Conversion electron measurements with an electrostatic spectrometer proved the existence of the 1,565±6 eV transition in²⁰¹Hg. The conversion intensity ratios,N ₁/N ₂ =1.2±0.2,N ₁/N ₃=1.1±0.2,N ₂/N ₃=0.92±0.15,N ₄/N ₃=0.03± 0.02 andN ₅/N ₃=0.04 ±0.02 were determined. These values agree with our calculations for the M1±E2 multipolarity with theE2/M1 mixing ratioδ ²=(l.l±0.3)xl0^?4 and exclude all pure multipolarities withL≦4. The total conversion coefficient for the aboveM1 +E2 mixture was evaluated to be (4.7±0.7)× 10⁴. The reducedB(M1, 1/2→3/2) probability was derived to be (3.9 ±1.2) × 10^?3 (e?/2Mc)². The natural widths of theN-subshell conversion lines in mercury were found to beΓ(N ₁)=8.3± 1.5,Γ(N ₂) =5.8±1.5 and Γ(N ₃) =6.5±1.0 eV. Monte Carlo calculations of electron scattering in matter yielded the conversion line shapes in qualitative agreement with the experiment.     

Weak correlation between isomer excitation probabilities and the ground state deformation parameters of nuclei

The photoexcitation of isomer ^157m Gd (11/2-, 426 keV, T _1/2 = 18.5 μs) in reaction (γ, γ′) was studied on the pulsed nonferromagnetic betatron at St. Peterburg State University. Only two strong intermediate structures (IntS) with E ₁ = 4.0 ± 0.1 MeV, (σΓ)₁^iso = 5 eV b and E ₂ = 5.3 ± 0.1 MeV, (σΓ)₂^iso = 40 eV b, respectively, were found. Comparison of the spectra IntS of isomers ^157m Gd and ^181m Ta demonstrates the weak correlation between isomer excitation probabilities and ground states deformation parameters.     

Photoelectrochemical properties of texture‐controlled nanostructured α‐Fe2O3 thin films prepared by AACVD

Nanostructured α‐Fe₂O₃ thin film electrodes were deposited by aerosol‐assisted chemical vapour deposition (AACVD) for photoelectrochemical (PEC) water splitting on conducting glass substrates using 0.1 M methanolic solution of Fe(acac)₃. The XRD analysis confirmed that the films are highly crystalline α‐Fe₂O₃ and free from other iron oxide phases. The highly reproducible electrodes have an optical bandgap of ～2.15 eV and exhibit anodic photocurrent. The current–voltage characterization of the electrodes reveals that the photocurrent density strongly depended on the film morphology and deposition temperature. Scanning electron microscopy (SEM) analysis showed a change in the surface morphology with the change in deposition temperature. The films deposited at 450 °C have nanoporous structures which provide a maximum electrode/electrolyte interface. The maximum photocurrent density of 455 µA/cm² was achieved at 0.25 V vs. Ag/AgCl/3M KCl (～1.23 V vs. RHE) and the incident photon to electron conversion efficiency (IPCE) was 23.6% at 350 nm for the electrode deposited at 450 °C. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Evidence for ammonia oxidation and the ionic nature of ammoniated titanium disulfide

In order to elucidate the nature of Lewis bases in layered transition metal dichalcogenides (TX₂), ammoniated titanium disulfide, (NH₃)_yTiS₂ with 0.4 ? y ? 0.6, has been synthesized using stoichiometric TiS₂ and investigated by X-ray diffraction, vapor pressure measurements, thermogravimetric analysis, differential scanning calorimetry, and SQUID magnetrometry. These intercalation compounds lose ammonia rapidly at ambient temperature to form primarily a stage II structure. Upon heating, about half of the ammonia is weakly bound (deintercalation enthalpy ?2.6 kcal/mol) and leaves the TiS₂ host at low temperatures (?75°C), whereas the other half is bound more strongly (deintercalation enthalpy ?6.3 kcal/mol) and deintercalates only at higher temperatures (?150°C). The more strongly bound ammonia is identified as NH₄⁺, and an electron is donated to the TiS₂ conduction band for each NH₄⁺-ion present. Hence, the ionic formulation (NH₄⁺)_y' (NH₃)_y'TiS₂^{y' -} best describes these materials, where the NH₄⁺ ions are analogous to alkali-metal cations M⁺ in metal-ammonia intercalation complexes. The presence of a substantial concentration of NH₄⁺ provides a simple explanation of the unusual observation that the C₃ axis of NH₃ is parallel to the TiS₂ layers. The results of this study show that redox reactions do indeed play an important role in ammonia intercalation into TiS₂ and probably also into other TX₂ hosts.     

Low-temperature specific heats of ZrSe3 and ZrS3

The specific heats of the semiconducting compounds ZrSe₃ and ZrS₃ have been measured in the temperature range between 8 and 200 K. At the lowest values of T, the ZrSe₃ measurements can be analyzed in terms of the familiar Debye T³ law yielding a characteristic Debye temperature at absolute zero θ_D(0) = 110 ± 2 K in satisfactory agreement with the value extracted from the available experimental sound velocities. In contrast, the low-temperature ZrS₃ measurements do not behave according to the Debye law. This is interpreted as resulting from a strong mixing of low-lying optical modes with the acoustical branches, and reflects a more pronounced one-dimensional character of ZrS₃ compared to ZrSe₃.     

Luminescence and decay times of CsI:In+

Luminescence spectra of single crystals of CsI:In⁺ excited in the A(304 nm), B(288 nm), C(268 nm) and D(257 nm) absorption bands have been studied in the temperature range 4.2–300 K. Excitation in the A band at 4.2 K gives rise to the principal emission at 2.22 eV accompanied by a partly-overlapping weak band at 2.49 eV. An additional emission band at about 2.96 eV is observed on excitation in the B, C or D bands. Yet another emission band located at 2.67 eV is excited only in the D band. The relative intensities of the bands are very sensitive to excitation wavelength as well as to temperature. The origin of all these bands is assigned in terms of a model for the relaxed excited states (RES). All the luminescence spectra were resolved into an appropriate number of skew-Gaussian components. Moments analysis leads to a value of (1.35 ± 0.02) × 10¹³ rad s^-1 for the effective frequency (ω_eff) of lattice vibrations coupled to the RES. At the lowest temperature, the radiative decay times of each of the intracenter emission bands (2.22, 2.49 and 2.96 eV) show a slow decay ( ～ 10–100 μs) and a fast decay ( ～ 10–100 ns). The 2.96 eV band, which is assigned to an emission process which is the inverse of the D-band absorption, exhibits a single decay mode ( ～ 10 μs). The intrinsic radiative decay rates (k₁, k₂), the one-phonon transition rate (K) and the second-order spin-orbit splitting (D) for the RES responsible for the principal emission are: k₁ = (6.0±-0.3)×10³ s^-1, k₂ = (1.33±-0.06)×10⁵ s^-1, K = (2.4±-0.4)×10⁷ s^-1 and D = (13.8±-0.5) cm^-1.     

Polycrystalline GaSb films prepared by the coevaporation technique

Gallium antimonide (GaSb) films were deposited onto fused silica and n-Si (100) substrates by coevaporating Ga and Sb from appropriate evaporation sources. The films were polycrystalline in nature. The size and the shape of the grains varied with the change in the substrate temperature during deposition. The average surface roughness of the films was estimated to be 10 nm. Grain boundary trap states varied between 2×10¹² and 2.2×10¹² cm^?2 while barrier height at the grain boundaries varied between 0.09 eV and 0.10 eV for films deposited at higher temperatures. Stress in the films decreased for films deposited at higher temperatures. XPS studies indicated two strong peaks located at ～543 eV and ～1121 eV for Sb 3d_3/2 and Ga 2p_3/2 core-level spectra, respectively. The PL spectra measured at 300 K was dominated by a strong peak located ～0.55 eV followed by two low intensity peaks ～0.63 eV and 0.67 eV. A typical n-Si/GaSb photovoltaic cell fabricated here indicated V _oc～311 mV and J～29.45 mA/cm², the density of donors (N _d)～3.87×10¹⁵ cm^?3, built in potential (V _bi)～0.48 V and carrier life time (τ)～28.5 ms. Impedance spectroscopy measurements indicated a dielectric relaxation time ～100 μs.     

High performance organic planar heterojunction solar cells by controlling the molecular orientation

A highly efficient planar heterojunction OSC based on zinc phthalocyanine (ZnPc)/fullerene (C₆₀) by controlling the orientation of the ZnPc by using copper iodide (CuI) as the interfacial layer is reported. The proportion of face-on ZnPc molecules was increased significantly on the CuI layer compared to the layer without the CuI layer, which was analyzed with wide-angle X-ray scattering (WAXS) and optical absorption. The power conversion efficiency (PCE) of the orientation controlled planar heterojunction OSC was remarkably enhanced to 3.2 ± 0.1% compared with 1.2 ± 0.1% of the conventional OSCs without the control of the molecular orientation. By inserting the 3-nm-thick CuI layer, J_SC, V_OC and FF have increased from 4.6 ± 0.2 to 8.9 ± 0.2 mA cm^?2, from 0.48 ± 0.01 to 0.59 ± 0.02 V, and from 0.56 ± 0.01 to 0.61 ± 0.02, respectively. V_OC enhancement is discussed with the result of the ultraviolet photoemission spectra (UPS) measurements.     

Band alignment at sputtered ZnSx O1–x/Cu(In,Ga)(Se,S)2 heterojunctions

Valence band offsets ΔE_VBM at ZnS_x O_1–x/Cu(In,Ga)(Se,S)₂ (CIGSSe) heterojunctions have been studied by photoemission spectroscopy (XPS, UPS) as a function of composition x in sputtered ZnS_x O_1–x films. In the composition range from ZnO to ZnS we found ΔE_VBM between –(2.1 ± 0.3) eV and –(0.8 ± 0.4) eV, respectively. Considering the optical band gaps, the conduction band offsets ΔE_CBM range from –(0.1 ± 0.3) eV to +(1.4 ± 0.4) eV. These results suggest that sputtered ZnS_x O_1–x is suitable as substitution for the CdS buffer and ZnO window layers in standard chalcopyrite‐based solar cells. Current–voltage characteristics of the solar cells have been investigated as a function of the composition x. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)