Band structure analysis in SiGe nanowires

One of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modification of the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological applications.     

Control of nonstoichiometric defects in manganese oxides by self-propagating high-temperature synthesis

Nonstoichiometric compounds of Mn_1−δO were self-propagating high-temperature synthesized by ignition of thoroughly mixed powders of Mn, NaClO₄, and MnO without additional external heating. The mixing ratio was systematically varied to control δ. Single phase Mn_1−δO products containing nonstoichiometric defects were obtained for 0 < δ ≤ 0.025. With increase in δ, Mn₃O₄ content of the products is increased. Double phases of Mn_1−δO and Mn₃O₄ were obtained at 0.025 < δ ≤ 0.150. The lattice parameter, a, of the single phase Mn_1−δO was well explained by the linear equation: a = −0.0667δ + 4.448.     

Hydrogen impurities in oxides and halides

The effect of H-impurity on the properties of vitreous SiO₂ is presented. Thermogravimetric analyses of the volatility of pure ZrF₄ using two different carriers, N₂ and CO, show that outgas H₂O should be taken into account. Consequently, it is shown that the intrinsic hydrolyzed state of pure ZrF₄ is significant. It is proposed that the stronger hydrogen bonded fluoride glasses will have generally a higher content of H-impurity than the oxide (SiO₂) counterpart.     

Semi-empirical atomistic study of point defect properties in BCC transition metals

We have constructed a set of embedded atom method (EAM) potentials for Fe, Ta, W and V and used them in order to study point defect properties. The parametrizations of the potentials ensure that the third order elastic constants are continuous and they have been fitted to the cohesive energies, the lattice constants, the unrelaxed vacancy formation energies and the second order elastic constants. Formation energies for different self-interstitials reveal that the split dumbbell is the most stable configuration for Fe while for Ta, W and V we find that the split dumbbell is preferred. Self-interstitial migration energies are simulated using the nudged elastic band method and for Fe and W the migration energies are found to be in good agreement with experimental and ab initio data. Migration energies for Ta and V self-interstitials are found to be quite low. The calculated formation, activation and migration energies for monovacancies are in good agreement with experimental data. Formation energies for divacancies reveal that the second nearest neighbor divacancy is more energetically favorable than nearest neighbor divacancies and the migration energies indicate that nearest neighbor migration paths are more likely to occur than second nearest neighbor migration jumps. For Fe, we have also studied the influence of the pair potential behavior between the second and third nearest neighbor on the stability of the split dumbbell, which revealed that the higher the energy level of the pair potential is in that region, the more stable the split dumbbell becomes.     

Disorder, band offsets and dopability of transparent conducting oxides

N-type transparent conducting oxides are based on ionic oxides with s-like cation conduction bands. The effect of disorder on their conduction band states is found to be small, because angular disorder has no effect on s states. Aliovalent impurities give rise to shallow states at the conduction band, which leads to an absence of a conduction band tail of localized states. This leads to a higher electron mobility than in typical p state amorphous materials like a-Si, the ability to move the Fermi level well into the conduction band and an absence of electrical instability as in a-Si:H. The band offsets are used to suggest appropriate oxide dielectrics for their thin transistors.     

Charge trapping during constant current stress in Hf-doped Ta2O5 films sputtered on nitrided Si

Metal-insulator-silicon structures containing Hf-doped Ta₂O₅ dielectric films sputtered on rapid thermally nitrided Si are shown to have very good reliability properties. Stress-induced leakage currents are low, both at low and at high-fields. It is found that charge trapping during the stress is the dominant wear-out mode for very long stress times of 500 s even for injected current densities J_s as high as 100 mA/cm². Stress curves approach saturation at long stress time, indicating that the trap generation rate is very low, even compared to the observed reduced trapping at pre-existing traps.Applying a trapping kinetics model, two trapping sites with characteristic trapping times τ₁ = 3.2 s and τ₂ = 49 s were determined and attributed to pre-existing defects in the bulk Hf:Ta₂O₅ layer and not in the interfacial SiO_xN_y layer. It was found that both τ₁ and τ₂ do not depend on J_s, which may be explained by the presence of a mechanism of charging the active sites through field activated emission of charge from them.     

Synergistic band convergence and defect engineering boost thermoelectric performance of SnTe

As an eco-friendly thermoelectric material,SnTe has attracted extensive attention.In this study,we use a stepwise strategy to enhance the thermoelectric performance of SnTe.Firstly,AgCl is doped into SnTe to realize band convergence and enlarge the band gap of AgCl-doped SnTe.AgCl-doping also induces dense point defects,strengthens the phonon scattering,and reduces the lattice thermal conductivity.Secondly,Sb is alloyed into AgCl-doped SnTe to further optimize the carrier concentration and simultaneously reduce the lattice thermal conductivity,leading to improved thermoelectric dimensionless figure of merit,ZT.Finally,(Sn0.81Sb0.19Te)0.93(AgCl)0.07 has approached the ZT value as high as～0.87 at 773 K,which is 272％higher than that of pristine SnTe.This study indicates that stepwise AgCl-doping and Sb-alloying can significantly improve thermoelectric performance of SnTe due to synergistic band engineering,carrier concentration optimization and defect engineering.     

Band structure and gaps of triangular graphene superlattices

The general properties of long wavelength triangular graphene superlattices are studied. It is shown that Dirac points with and without gaps can arise at a number of high-symmetry points of the Brillouin zone. The existence of gaps can lead to insulating behaviour at commensurate fillings. Strain and magnetic superlattices are also discussed.     

Band gap study on zinc-doped cadmium sulphide

Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.     

A simple preparative method for porous hollow-ware stacked cobalt oxides and their catalytic properties

β-Co(OH)₂ hollow-ware stacked nanosheets have been synthesized at low temperature (60 °C) through a simple surfactant-free technique. The factors on the formation of the hollowware-like stacked nanosheets have been studied and a possible formation mechanism is proposed. Thermally stable porous Co₃O₄ with the same morphology was prepared by thermal decomposition of the as-prepared Co(OH)₂. The catalytic properties of the porous Co₃O₄ and Au/Co₃O₄ are reported.     

Taming the ground-state and optical properties of transition metal oxides

We have implemented the so-called local density approximation (LDA) plus the Hubbard parameter U method (LDA+U), in a formalism based on the projector augmented wave (PAW) method and calculated the ground-state properties of NiO and its optical properties. Good agreement with the measured ground-state antiferromagnetic magnetic moment and optical properties are obtained for U=5 eV. The enhancement of the O 2p near the top of the valence states increases with the value of U. At intermediate U, the nature of the band gap is a mixture of charge transfer and Ni d → d excitations.     

Synthesis of nanocrystalline rare earth oxides by glycothermal method

The reaction of yttrium acetate hydrate in 1,2-propanediol at 300 °C yielded a product containing acetate groups and glycol moieties. From this product, Y₂O₃ was directly crystallized at 400 °C without the formation of a carbonate oxide phase. The thus-obtained Y₂O₃ samples had a small crystallite size (2.2 nm) and significantly large surface area (280 m²/g). Other nanocrystalline rare earth (Gd-Yb) oxides were also obtained by this method.     

Band alignment of metal-oxide-semiconductor structure by internal photoemission spectroscopy and spectroscopic ellipsometry

In this paper, we will provide an overview of the internal photoemission (IPE) and the significance of this technique when combined with spectroscopic ellipsometry (SE) to investigate the interfacial electronic properties of heterostructures. In particular, the main interest is focused on the electron transport mechanism and properties at and near the interface of the technologically important metal-oxide-semiconductor (MOS) devices. Not until recently, IPE and SE have become important metrology tools in band offset characterization for the MOS materials. The most common and straightforward application of IPE and SE is to determine how the Fermi level of the metal, and the conduction and valence bands of the semiconductor align with those of the oxide of the MOS structure. For demonstration, we will present the results recently obtained on a set of MOS devices consisting of metal gate / high-k dielectric stack / Si and III-V high mobility substrate. The examples include [TaN/TaSiN] metal gate / [HfO₂/SiO₂] dielectric stack / Si substrate and Al metal gate / Al₂O₃ dielectric / In_xGa_1 − xAs substrate.     

Metal-insulator transitions in reduced molybdenum oxides Sm4Mo18O32 and Nd4Mo18O32

Synthesis, structure and electronic properties are reported on single crystals of Sm₄Mo₁₈O₃₂ and Nd₄Mo₁₈O₃₂. The triclinic crystal structure has three distinctly different Mo metal atom clusters (Mo₂, Mo₄ and Mo₆), which extend in complex chains. The temperature dependence of the electrical resistivity displays a broad minimum around 150-200 K with a pronounced increase in the 30-50 K range. However, unlike the related monoclinic reduced Mo oxides, A₄Mo₁₈O₃₂ (A = Y, Gd-Yb), there is no true metal-insulator transition in the Sm and Nd analogs. We discuss these observations in terms of correlations among the Mo clusters.     

Characterization of intermetallic phases and oxides formed in annealed Ni/Al multilayer structures

Two different multilayer structures composed of ten alternating Ni and Al thin films were sputter deposited on Si (111) substrates. These multilayers with individual Ni and Al thin film thicknesses of about 25 nm and 38 nm and of 25 nm and 13 nm, respectively, have the average compositions of Ni_0.50Al_0.50 and Ni_0.75Al_0.25. The samples were heat treated in a differential scanning calorimeter instrument with a constant heating rate of 40 °C min ⁻¹ in Ar from room temperature to 550 °C. The compositions of as-deposited and heat-treated samples were studied with high-resolution Auger electron spectroscopy (AES) rotational depth profiling. X-ray photoelectron spectroscopy (XPS) analyses show an excess of Ni in both annealed samples. X-ray diffraction measurements of annealed multilayers show the formation of Ni₂Al₃ and NiAl₃ phases in the Ni_0.50Al_0.50 sample and the presence of Ni₃Al and Ni A1₃ phases with some excess of Ni in the Ni_0.75Al_0.75 sample. AES and XPS investigations of the reacted layers after 15 min annealing in air at 500 °C disclose considerably different surface oxide thin films: on the Ni_0.50Al_0.50 layer the oxide thin film consists of Al₂O₃ with a small amount of NiO, whereas that on the top of the Ni_0.75Al_0.25 layer is thicker and consists of NiO on top and some Al₂O₃ below.     

石墨烯能带结构调控的第一性原理研究

采用第一性原理方法,系统研究了单轴应变状态下纳米级孔洞和层数对石墨烯能带结构的影响,结果表明,其带隙不但与层数的奇偶性有关,而且对单轴应变的大小和孔洞存在与否非常敏感,这些均可归因于层间/层内结构的不对称性。     

Defect behavior in oxides: Insights from modern atomistic simulation methods

Oxide ceramics are important for a large number of technological applications. In most cases, the behavior of defects determines the properties of the oxide that make it appealing. Thus, understanding defect properties – thermodynamic and kinetic – is central for optimizing structure/property relationships for oxides. Here, we provide a perspective on the use of modern computational capabilities to interrogate defect properties in complex oxides. We focus on three aspects: the screening of multiple defect reactions in a large set of oxide chemistries to determine dominant defect structure, the interaction of ionic and electronic defects, and the kinetic properties of defects. These examples serve to illustrate the types of insights that can be gained when applying new methodologies, combined with modern computational resources, to advance the understanding of these materials.     

Microwave hydrothermal synthesis of nanoporous cobalt oxides and their gas sensing properties

In this paper, the precursors were synthesized by microwave hydrothermal method using Co(NO₃)₂·6H₂O as raw material, CO(NH₂)₂ and KOH as precipitants, respectively. The precursors and calcined products were characterized by XRD, FESEM, and BET-BJH. The results show that both constituent and synthetic condition can determine the products morphology. When using KOH as precipitant, hollow Co₃O₄ nanorings were obtained whose precursor was synthesized at 140 °C for 3 h and calcined at 500 °C in air for 2 h. While using CO(NH)₂, Co₃O₄ like-nanochains were obtained whose precursor was synthesized at 110 °C for 1 h and calcined at 420 °C in air for 2 h, and Co₃O₄ nanosheets were obtained while their precursor was synthesized at 140 °C for 3 h and calcined at 500 °C in air for 2 h. The sensitivity test of Co₃O₄ to alcohol reveals that the hollow Co₃O₄ nanorings show the best sensitivity, porous Co₃O₄ like-nanochains are superior to that of the porous nanosheets.     

Scalable route to mesoporous iron oxides and their Cr(VI) ions uptake capacity study

Here we demonstrate a simple and scalable synthetic route for preparing porous iron oxides with tunable porosity characteristics by using the high-energy mechanical ball milling technique. Nanocomposites composed of networked hematite encapsulated in silica were produced. Silica was utilized as scaffolds to form nanocomposites and then was etched off hydrothermally in a NaOH solution. The formation mechanism of porous iron oxides, effects of volume ratio of silica to magnetite, milling period, calcination temperature and concentration of NaOH solution were studied systematically. The formation process was monitored by applying a variety of techniques, such as scanning and transition electron microscopes (SEM and TEM), and X-Ray diffractometer (XRD). SEM and TEM studies revealed that a jelly-like solid-state nanocomposite of silica and iron oxide was produced after milling process, and networked iron oxide structures encapsulated in silica displayed a high mass contrast and their grain size decreased with elongating milling period. Barrett–Joyner–Halender (BJH) method analysis indicated that the pore width and Brunauer–Emmett–Teller (BET) specific surface area of iron oxide decreased with the increase of milling period and volume ratio of silica to iron oxide, and porosity increased with increasing volume ratio of silica to iron oxide. Magnetic measurement revealed the superparamagnetic nature of the mesoporous materials at room temperature. Further, absorption capacity of Cr (VI) ions onto mesoporous magnetite was checked to evaluate its toxic ions uptake ability.