Endohedral nitride cluster fullerenes: Formation and spectroscopic analysis of L3−xMxN@C2n (0≤x≤3; N=39,40)

The class of endohedral fullerenes is demonstrated in its large variety by the new type of nitride cluster fullerenes. These endohedral fullerenes were obtained as the most abundant fullerenes in the soot extract. This is reached by the concept of reactive gas atmosphere in arc burning fullerene production. The chemical reactions in the gas atmosphere during the production are described. Different nitride cluster fullerenes were produced with selectivity up to 90% by using reactive gas addition to the cooling gas of the arc burning process. Fullerenes prepared by this method are Sc₃N@C₈₀, Sc_3−xEr_xN@C₈₀ (x=1,2,3), Sc₃N@C₇₈, Y₃N@C₈₀, Ho₃N@C₈₀ and Tb₃N@C₈₀.By studying Vis/NIR spectra, it is demonstrated that nitride cluster fullerenes are generally large band-gap endohedrals. Therefore, the M₃N@C₈₀ structures are very stable and suitable for applications. This stability is caused by a charge transfer from the cluster to the carbon cage and the formation of a M₃N–carbon cage bond as well as covalent metal–nitrogen bonds. Infrared spectroscopy of M₃N@C₈₀ was used to study the metal and cage size influence on the structure of the nitride cluster.     

TiY2N@C80电子结构的第一性原理研究

采用第一性原理方法研究了TiY₂N@C₈₀分子的几何、振动和电子性质．理论计算结果表明TiY₂N@C₈₀分子的电子结构性质明显与Sc₃N@C₈₀和Y₃N@C₈₀的不同,而与TiSc₂N@C₈₀相近．对TiY₂N@C₈₀分子进行载流子掺杂时,其磁性     

Influence of electric field annealing on atom diffusion in Cu/Ta/Si stacks

In this letter, we present a quantitative analysis of the influences caused by an electric field annealing on interface atom diffusion in a Cu/Ta/Si stack at a range of temperatures 450～650 °C. The results indicate that the external electric field has a remarkably accelerated effect on Cu atom diffusion in the Ta layer and the failure of Ta as the diffusion barrier. The preexponent D ₀ and the activation energy Q for Cu atom diffusion in the Ta layer were both decreased with the application of an external electric field. The activation energy for electric field annealed stacks is 1.22 eV, which is lower than that for annealed stacks (1.58 eV). The accelerating effect is mainly attributed to the perturbation of the electric state of the defects in the interface and grain interior.     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er₃N@C₈₀ and Sc₃N@C₈₀ on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250-300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Temperature induced band gap shrinkage in Cu2GeSe3: Role of electron-phonon interaction

The ternary semiconducting compound Cu₂GeSe₃ has been investigated for optical properties with photoacoustic spectroscopy. Optical absorption spectra of Cu₂GeSe₃ is obtained in the range of 0.76-0.81 eV photon-energy at temperatures between 80 and 300 K. The thermal variation of band gap energy has been examined from the optical absorption spectra at different temperatures. The temperature induced band gap shrinkage has been explained on the basis of electron-phonon interaction. Varshni's empirical relation in conjunction with Vina and Passler model is taken into consideration for data fitting. The Debye temperature was calculated approximately as 240 K. The acoustic phonons with a characteristic temperature as 160 K corresponding to effective mean frequency have been attributed to the thermal variation of the energy gap.     

Dielectric relaxation of samarium aluminate

A ceramic SmAlO₃ (SAO) sample is synthesized by the solid-state reaction technique. The Rietveld refinement of the X-ray diffraction pattern has been done to find the crystal symmetry of the sample at room temperature. An impedance spectroscopy study of the sample has been performed in the frequency range from 50 Hz to 1 MHz and in the temperature range from 313 K to 573 K. Dielectric relaxation peaks are observed in the imaginary parts of the spectra. The Cole–Cole model is used to analyze the dielectric relaxation mechanism in SAO. The temperature-dependent relaxation times are found to obey the Arrhenius law having an activation energy of 0.29 eV, which indicates that polaron hopping is responsible for conduction or dielectric relaxation in this material. The complex impedance plane plot of the sample indicates the presence of both grain and grain-boundary effects and is analyzed by an electrical equivalent circuit consisting of a resistance and a constant-phase element. The frequency-dependent conductivity spectra follow a double-power law due to the presence of two plateaus.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Li<Superscript>+</Superscript> ion conductivity and transport properties of LiYP<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

A lithium yttrium diphosphate LiYP₂O₇ was prepared by a solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern suggests the formation of the single phase desired compound with monoclinic structure at room temperature. The infrared and Raman spectrum of this compound was interpreted on the basis of P₂O₇ ^4? vibrations. The AC conductivity was measured in the frequency range from 100 to 10⁶ Hz and temperatures between 473 and 673 K using impedance spectroscopy technique. The obtained results were analyzed by fitting the experimental data to the equivalent circuit model. The Cole–Cole diagram determined complex impedance for different temperatures. The angular frequency dependence of the AC conductivity is found to obey Jonscher’s relation. The temperature dependence of σ _AC could be described in terms of Arrhenius relation with two activation energies, 0.87 eV in region I and 1.36 eV in region II. The study of temperature variation of the exponent(s) reveals two conduction models: the AC conduction dependence upon temperature is governed by the correlated barrier hopping (CBH) model in region I (T < 540 K) and non-overlapping small polaron tunneling (NSPT) model in region II (T > 540 K). The near value of activation energies obtained from the equivalent circuit and DC conductivity confirms that the transport is through ion hopping mechanism dominated by the motion of the Li⁺ ion in the structure of the investigated material.     

Chemical spray pyrolysis deposition and characterization of p-type CuCr1−xMgxO2 transparent oxide semiconductor thin films

A chemical spray pyrolysis technique for deposition of p-type Mg-doped CuCrO₂ transparent oxide semiconductor thin films using metaloorganic precursors is described. As-deposited films contain mixed spinel CuCr₂O₄ and delafossite CuCrO₂ structural phases. Reduction in spinel CuCr₂O₄ fraction and formation of highly crystalline films with single phase delafossite CuCrO₂ structure is realized by annealing at temperatures ?700 °C in argon. A mechanism of synthesis of CuCrO₂ films involving precursor decomposition, oxidation and reaction between constituent oxides in the spray deposition process is presented. Post-annealed CuCr_0.93Mg_0.07O₂ thin films show high (?80%) visible transmittance and sharp absorption at band gap energy with direct and indirect optical band gaps 3.11 and 2.58 eV, respectively. Lower (∼450 °C) substrate temperature formed films are amorphous and yield lower direct (2.96 eV) and indirect (2.23 eV) band gaps after crystallization. Electrical conductivity of CuCr_0.93 Mg_0.07O₂ thin films ranged 0.6-1 S cm⁻¹ and hole concentration ∼2×10¹⁹ cm⁻³ determined from Seebeck analysis. Temperature dependence of conductivity exhibit activation energies ∼0.11 eV in 300-470 K and ∼0.23 eV in ?470 K region ascribed to activated conduction and grain boundary trap assisted conduction, respectively. Heterojunction diodes of the structure Au/n-(ZnO)/p-(CuCr_0.93Mg_0.07O₂)/SnO₂ (TCO) were fabricated which show potential for transparent wide band gap junction device.     

Band alignment at the In2S3/Cu2ZnSnS4 heterojunction interface investigated by X-ray photoemission spectroscopy

The band alignment at the In₂S₃/Cu₂ZnSnS₄ heterojunction interface is investigated by X-ray photoemission spectroscopy. In₂S₃ is thermally evaporated onto the contamination-free polycrystalline Cu₂ZnSnS₄ surface prepared by magnetron sputtering. The valence band offset is measured to be 0.46 ± 0.1 eV, which matches well with the valance band offset value 0.49 eV calculated using “transitivity” method. The conduction band offset is determined to be 0.82 ± 0.1 eV, indicating a ‘type I’ band alignment at the heterojunction interface.     

Na+-ion conductivity of double phosphate Na3Sc2(PO4)3 in the region of the β-γ transition

The Na⁺-ion conductivity σ of double phosphate Na₃Sc₂(PO₄)₃ in the region of the β-γ transition has been studied using impedance spectroscopy (1–10⁶ Hz). The polycrystalline sample of Na₃Sc₂(PO₄)₃ has been prepared by solid-phase synthesis and ceramic technology. It has been found that, upon the β-γ transition, the conductivity σ of Na₃Sc₂(PO₄)₃ suffers a ～1.5-fold jump at 470 ± 2 K upon heating and a ～2.5-fold jump at 430 ± 4 K upon cooling (the temperature hysteresis of the jump in σ is 40 K). For double sodium-scandium phosphate γ-Na₃Sc₂(PO₄)₃ in the superionic state, σ attains 0.07 S/cm at 700 K and the ion transport activation enthalpy is 0.42 ± 0.02 eV.     

Modulation of strain,electric field and organic cation rotation on the band gap and electronic structures of organic-inorganic hybrid perovskite CH3NH3PbI3

Band gap modulation engineering is an important step in the application of optoelectronic materials. In this paper, the first-principles calculations were carried out to study the influence of strain, external electric field, spatial orientation of organic cation on the band gaps and electronic structures of organic-inorganic hybrid halide perovskites CH₃NH₃PbI₃. The results show that both the uniform strain and the tetragonal deformation can modulate the band gap obviously. The electric field of 0.2 V/Å is the critical point of the band gap modulation. The band gap increases when an electric field is applied from 0 to 0.2 V/Å. The electric field above 0.2 V/Å will cause the band gap to decrease. The spatial orientation of the organic cation also has modulation influence on the band gap of CH₃NH₃PbI₃, but has no effect on the direct semiconductor characteristics. The above results will be helpful to study the band gap modulation of other organic-inorganic hybrid halide perovskites.     

Anomalous temperature dependence of the band gap in AgGaSe2 and AgInSe2

The lowest band gaps of AgGaSe₂ and AgInSe₂ single crystals in the temperature range from 90 to 300 K were determined from photoconductivity measurements. Below (above ≈ 120 K in AgInSe₂ and ≈ 125 K in AgGaSe₂ the temperature coefficient of the band gap is +5 × 10⁻⁴ eV/K (−1.5 × 10⁻⁴ eV/K) and +1.1 × 10⁻⁴ eV/K (−4.28 × 10⁻⁴ eV/K), respectively. The positive value is explained with the lattice dilation effect being the dominant mechanism for the band gap variation at the temperatures less than ≈ 120–125 K.     

Semiconducting properties of CuIn5S8 single crystals I. Electrical properties

The electrical resistivity and Hall coefficient of n-type CuIn₅S₈ single crystals were measured in the temperature range from 80 K–500 K. The energy gap at 0 K was determined to be 1.4 eV. The donor levels at 0.017 eV and 0.09 eV below the conduction band are identified. The mobility data are analysed assuming scatterings by acoustic and polar optical phonons and ionized impurities.     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Ionic thermocurrents in sodium fluoride thin film

Ionic thermocurrent (ITC) in NaF thin film deposited between gold electrodes has been investigated in situ in the temperature region 300–500 K. The thermally stimulated polarization current (TSPC) and thermally stimulated depolarization current (TSDC) obtained under low electric field pf polarization (E_p) at a lower temperature of polarization (T_p) exhibited current-peak temperatures (T_M) about 420 K and 450 K. The activation energies associated with the first and the second current peaks were 1.15 + 0.05 eV and 0.65 + 0.02 eV, which are assigned to cation vacancy blocking at the grain boundary barrier and the electrode contact interface barrier respectively. Polarization under a strong electric field (E_p > MV m₋₁) at T_p > 480 K causes a quasi-stable shift of the ITC peak positions to higher temperature. The current peak positions and magnitudes then depend upon the polarity of the electrode metal during polarization, and activation energies associated with the first peak positions are 0.90 + 0.02 eV and 0.79 + 0.02 eV for positive and negative bias at the counter electrode respectively, which may be attributed to the clustering of the cation vacancies and the dislocation networks at the interfaces.     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters – oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index – were found to be 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.