Ab initio study of the electronic structures and conduction properties of some novel low band-gap donor-acceptor polymers

The results of the electronic structures and conduction properties of four novel donor-acceptor polymers based on polysilole, obtained on the basis of ab initio Hartree-Fock crystal orbital method using their optimized geometries, are reported. The repeat unit of these polymers consists of bicyclopentadisilole unit bridged by an electron-accepting group Y(Y=CCH₂ in PSICH, CO in PSICF, CCF₂ and CC(CN)₂ in PSICN). All the polymers on the basis of their geometries and π-bond order values are found to have benzenoid-type electronic structures. Comparison of the important electronic properties such as ionization potential, electron affinity and band-gap of these polymers indicates PSICN to be the best candidate for intrinsic conductivity and reductive (n-) doping while PSICH is predicted to be the best candidate for oxidative (p-) doping. All these polymers are estimated to have band-gap values ranging between 1 and 2 eV. The low band-gap values of these polymers are rationalised on the basis of the patterns of their frontier orbitals.     

On the electronic structure and conduction properties of polymeric quasi-one-dimensional model superlattices: Study of Type I and Type II-staggered superlattices

The electronic spectrum of the various quasi-one-dimensional model compositional superlattices (copolymers) (A_mB_n)_x belonging to class of Type I and Type II-staggered superlattices have been calculated in the tight binding approximation using the direct numerical approach. The trends in their electronic structure and conduction properties as a function of (i) composition (m/n), (ii) block sizes m and n and (iii) arrangement of the blocks in the copolymer chain are discussed. The results obtained are important guidelines for designing copolymers with tailor made conduction properties.     

Theory of band gap bowing of disordered substitutional II–VI and III–V semiconductor alloys

For a wide class of technologically relevant compound III?CV and II?CVI semiconductor materials AC and BC mixed crystals (alloys) of the type A _x B_1?x C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A _x B_1?x C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight-binding models for the pure AC and BC systems. The electronic properties of the A _x B_1?x C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1 ? x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA) We apply this treatment to the II?CVI system Cd _x Zn_1?x Se, to the III?CV system In _x Ga_1?x As and to the III-nitride system Ga _x Al_1?x N.     

Dependence of interconfiguration excitation energies on local environment,composition and temperature in EuA2−xBx compounds

The dependence of the energy released by removing an electron from the 4? level to the conduction band on environment, composition and temperature was deduced from Mossbauer measurements on EuA_2-xB_x compounds (A = Rh or Ir, B = Pt or Al). For each system, this energy is equal to α + βx - λm + T, where α, β and λ are constants. m is the number of B neighbors of an Eu ion. The sizes of the unit cells are consistent with the recoilless absorption data.     

Structural and electronic properties of GaAsBi

The structural and electronic properties of the GaAs_1−xBi_x ternary alloy are investigated by means of two first principles and full potential methods, the linear augmented plane waves (FPLAPW) method and a recent version of the full potential linear muffin-tin orbitals method (FPLMTO) which enables an accurate treatment of the interstitial regions. In particular, we have found that the maximal GaBi mole fraction x for which GaBi_xAs_1−x remains a semiconductor is probably around x=0.5. The electronic properties of (GaAs)_m/(GaBi)_n quantum well superlattices (SLs) have also been calculated and it is found that such SLs are semiconductors when m is larger or equal to n.     

FP-LMTO study of structural,electronic and optical properties of wurtzite (CdS)n/(CdSe)n superlattices

In this paper, we have conducted a first-principles study of the structural, electronic and optical properties of (CdS)_n/(CdSe)_n superlattices (where n is numbers of monolayers) in the wurtzite phase (B4), using the Full-Potential Linear Muffin-Tin Orbital (FP-LMTO) method within the Local Density Approximation (LDA) technique, in order to describe the exchange correlation energy. The calculated electronic properties indicate that all (CdS)_n/(CdSe)_n superlattices configurations, possess a semiconductor behavior with same energy gaps. We have seen more carefully and accurately that the different superlattices configurations have no effect on the electronic properties; in particular, we did not observe any dependence between the band gap behavior and the used layers.     

Review of magnetic features observed in (A,A′)Ni2B2C solid solutions

The nickel-borocarbides ANi₂B₂C [A=Y, Ln (lanthanide), An(actinide)], crystallizing in the body-centred tetragonal LuNi₂B₂C-type structure, are classified according to the existence or coexistence of superconducting and antiferromagnetic states (AF). The magnetic features observed in polycrystalline (A,A′)Ni₂B₂C solid solutions, adopting the same crystal structure, are reviewed and discussed. Published data on the magnetism in (A,Ln)Ni₂B₂C systems (ANi₂B₂C nonmagnetic, A=Y,La,Lu) indicate a gradual rise in the threshold content, x(m), in (Y_1−xLn_x)Ni₂B₂C (Ln=Gd,Tb,Dy,Ho,Er) for the establishment of AF states. (A,A′)Ni₂B₂C systems with magnetic end compounds show gradual variation in magnetic features when A and A′ are both heavy Ln. The behaviour of (A,A′)Ni₂B₂C systems of light A (Pr or U) and heavy A′ (Dy or Tm) depends on the magnetic structures of the end compounds. In intermediate compositions, incomplete moment compensation in (Pr,Dy)Ni₂B₂C decreases T_N, while different moment directions in the end compounds in (U,Dy)Ni₂B₂C lead to a directional frustration of ordered moments. Such a frustration in (U,Tm)Ni₂B₂C is related to different magnetic structures of the end compounds.     

Hydrogen storage capabilities of the most stable isomers of NanBm (m+n=6) clusters

The most stable isomers of NanBm(m+n=6) clusters and their hydrogen storage properties are investigated by means of density functional theory with a 6-311+G(d) basis set. To study the hydrogen storage properties,all of the stable structures of Na n BmHx (m+n=6) clusters have been optimized. It shows that boron atoms of Na n B m are separated from the other boron atoms,and form satellite BHx (x=3,4) clusters around the centre,which attach to the system by a bridging bond of a hydrogen atom or an Na atom. Compared with the hydrogen storage capabilities,the Na3B3 has the highest hydrogen storage capacity among Na n B m clusters. The binding energies,interaction energies of hydrogen atom with Na n B m clusters and second difference in energy of Na3B3Hx clusters have been calculated. The results show that the stability of the Na n B m H x clusters present an odd-even oscillatory effect,as the number of H atoms increases.     

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

Thermodynamic instability of Ag/Au and Cu/Pd metal superlattices

We show how the formation energies of A_pB_q superlattices with arbitrary periods p and q and layer orientation Ĝ can be predicted via a 'cluster expansion' technique, given the formation energies of short period structures from first-principles calculations. We predict both bulk and epitaxial energies as well as the energies of the fully intermixed (alloyed) superlattices. Applications to Ag/Au and Cu/Pd superlattices illustrate our method, as well as a global classification scheme for superlattice stability.     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

Coherent potential approximation calculations for electronic spectra of one-dimensional quaternary alloys

The coherent potential approximation is used to investigate the electronic structure of quaternary alloys of the types A_xC_yB_{1 − x − y}D and A_xB_{1 − x}C_yD_{1 − y}. Formalisms are developed for and application is made to the calculation of the electronic state densities for both types of alloys in the one-dimensional, one-site per atom, nearest-neighbor tight-binding approximation.     

Second harmonic generation in generalized Thue-Morse ferroelectric superlattices

In this paper the second harmonic generation (SHG) in generalized Thue-Morse (GTM(m, n)) ferroelectric superlattices is studied. Under the small-signal approximation, the SHG spectra in both real and reciprocal spaces are investigated. It is found that: (1) only when the structure parameters l, l_A, and l_B are all chosen to be proper, can SHG in GTM(m, n) ferroelectric superlattices be generated; (2) for Family A of generalized Thue-Morse, GTM(m, 1) ferroelectric systems, with the increase of parameter m, the intense peaks of SHG concentrate on the long wavelength (the fundamental beam (FB) wavelength is within ), but for Family B of generalized Thue-Morse, GTM(1, n) ferroelectric superlattices, with the increase of parameter n, the intense peaks of SHG concentrate on the middle wavelength ; and (3) for GTM(m, 1) ferroelectric superlattices, the bigger the m, the stronger the relative integral intensity (RII) of SHG would be, but for GTM(1, n) ferroelectric systems, the bigger the n, the weaker the RII of SHG would be.     

Ionic conduction in BaxCe0.8Er0.2O3−α

Ba_xCe_0.8Er_0.2O_3−α (x=0.98–1.03) solid electrolytes were synthesized by high temperature solid-state reaction. X-ray diffraction (XRD) patterns showed that the specimens were a single perovskite-type orthorhombic phase. The conduction properties of the specimens were studied electrochemically in the temperature range of 600–1000 °C. The influence of nonstoichiometry in the specimens with x≠1 on conduction properties was investigated and compared with that in the specimen with x=1. These three specimens showed a good protonic conduction under wet hydrogen, a mixed conduction of oxide-ion and electronic hole under dry higher oxygen partial pressure, and a mixed conduction of proton, oxide-ion and electronic hole under wet higher oxygen partial pressure. Both of the protonic and oxide-ionic conductivities increased with increasing barium content in the specimens under wet hydrogen and dry air, respectively.     

星形两嵌段共聚高分子薄膜微相分离的耗散粒子动力学模拟

利用耗散粒子动力学模拟研究了两类典型的星形两嵌段共聚高分子薄膜的微相分离行为. 结果表明,(A_x)₄(B_y)₄体系的相图较(A_xB_y)₄体系具有明显的对称性,且前者比后者更易发生相分离,与体相中的情况相近,并可归因于两类星形高分子在结构上的差异;组成相同的星形共聚高分子体系在体相与薄膜中所形成的介观结构之间存在对应关系;减小薄膜厚度与加大A-B组分间排斥强度均有利于增强所形成结构的有序性. 同样是缘于分子结构差异,两类星形高分 子薄膜中的均方回旋半径随体系组分分率的变化规律几乎相反.     

Improving dosimetric properties of tellurite glasses

Thermoluminscence (TL) properties of quaternary tellurite glass in the form 80(TeO₂)–5(TiO₂)–(15−x)(WO₃)–(x)A_nO_m where A_nO_m=Nb₂O₅, Nd₂O₃, Er₂O₃ and x mol% have been measured. TL main dosimetry peak for each produced glass sample were investigated for ⁶⁰Co gamma rays. Dosimetric properties of the quaternary tellurite glasses have been measured as a function of different compositions of the glass system in different rare earth oxides concentration by using thermoluminescence (TL) detection technique.     

Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)_n/(BAs)_n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.     

(La0.7Sr0.3MnO3 )m(BiFeO3)n 超晶格结构的导电机理

利用射频磁控溅射的方法在SrTiO₃(001) 基片上制备了(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构.对所制备的超晶格结构进行了50—150℃温度范围内的电流-电压测试分析.结果表明,随着BiFeO₃薄膜的厚度减小,温度的升高,(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构的电流变大.进一步根据介质导电模型对(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构的导电特性做了分析.在温度较低或者电场较弱时,所制备的(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构表现为欧姆导电,而在高温,高电场的情况下,其导电行为由空间电荷限制电流机理主导. 关键词： 超晶格薄膜 多铁 空间电荷限制电流     

Resonant electron tunneling in GaN/Ga1−x AlxN(0001) strained structures with spontaneous polarization and piezoeffect

Electron tunneling through the GaN/Ga_1?x Al_xN(0001) wurtzite strained structures is investigated by the pseudopotential and scattering matrix methods. It is shown that the results of multiband calculations at low aluminum concentrations (x<0.3) are adequately described within the single-valley model in the envelope wave function method accounting for the dependences of the effective mass on the energy and strain. Upon electron tunneling through two-barrier structures, sharp resonance peaks are observed at a barrier thickness of several monolayers and the characteristic collision time in the resonance region is equal to ～1 ps. The internal electric fields associated with spontaneous and piezoelectric polarizations lead to a “red” or “blue” shift in the resonance energy according to the thickness and location of barriers with respect to the polar axis. In the (GaN)_n(Ga_1?x Al_xN)_m superlattices, the internal fields can form the Stark ladder of electronic states at a small number of ultrathin layers even in the absence of external fields.