Hydrothermal growth and structural studies of Si(1-x)Ge(x)O2 single crystals

The substitution of germanium in the α-quartz structure is a method investigated to improve the piezoelectric properties and the thermal stability of α-quartz. Growth of α-quartz type Si(1-x)Ge(x)O(2) single crystals was performed using a temperature gradient hydrothermal method under different experimental conditions (pressure, temperature, nature of the solvent, and the nutrient). To avoid the difference of dissolution kinetics between pure SiO(2) and pure GeO(2), single phases Si(1-x)Ge(x)O(2) solid solutions were prepared and used as nutrients. The influence of the nature (cristobalite-type, glass) and the composition of this nutrient were also studied. Single crystals were grown in aqueous NaOH (0.2-1 M) solutions and in pure water. A wide range of pressures (95-280 MPa) and temperatures (315-505 °C) was investigated. Structures of single crystals with x = 0.07, 0.1, and 0.13 were refined, and it was shown that the structural distortion (i.e., θ and δ) increases with the atomic fraction of Ge in an almost linear way. Thus, the piezoelectric properties of Si(1-x)Ge(x)O(2) solid solution should increase with x, and this material could be a good candidate for technological applications requiring a high piezoelectric coupling factor or high thermal stability.     

First principles calculations of electronic structures and metal mobility of Na(x)Si(46) and Na(x)Si(34) clathrates

Energetics, geometry, electronic band structures, and charge transfer for Na(x)Si(46) and Na(x)Si(34) clathrates with different degrees of cavity filling by sodium, and the mobility of the Na atom inside the different cavities are studied using first principles density functional calculations within the generalized gradient approximation. The stabilization of the clathrate lattice and the cell volume variation upon the inclusion of Na (which appears to move easily in the larger cavities of Na(x)Si(34), thus justifying the experimental observations) are discussed in connection with the onset of the repulsion between Na and Si for distances shorter than approximately 3.4 A. For all degrees of filling of the different cavities examined we find that the electron population of the s orbitals in the partially ionized Na atoms increases with a decrease in the size of the cavity, and that the Na states contribute significantly to the density of states at the Fermi level and thus influence the properties of these compounds.     

Synthesis and fundamental studies of (H3Ge)xSiH4-x molecules: precursors to semiconductor hetero- and nanostructures on Si

The synthesis of the entire silyl-germyl sequence of molecules (H(3)Ge)(x)SiH(4)(-)(x) (x = 1-4) has been demonstrated. These include the previously unknown (H(3)Ge)(2)SiH(2), (H(3)Ge)(3)SiH, and (H(3)Ge)(4)Si species as well as the H(3)GeSiH(3) analogue which is obtained in practical high-purity yields as a viable alternative to disilane and digermane for semiconductor applications. The molecules are characterized by FTIR, multinuclear NMR, mass spectrometry, and Rutherford backscattering. The structural, thermochemical, and vibrational properties are studied using density functional theory. A detailed comparison of the experimental and theoretical data is used to corroborate the synthesis of specific molecular structures. The (H(3)Ge)(x)SiH(4)(-)(x) family of compounds described here is not only of intrinsic molecular interest but also provides a unique route to a new class of Si-based semiconductors including epitaxial layers and coherent islands (quantum dots), with Ge-rich stoichiometries SiGe, SiGe(2), SiGe(3), and SiGe(4) reflecting the Si/Ge content of the corresponding precursor. The layers grow directly on Si(100) at unprecedented low temperatures of 300-450 degrees C and display homogeneous compositional and strain profiles, low threading defect densities, and atomically planar surfaces circumventing entirely the need for conventional graded compositions or lift-off technologies. The activation energies of all Si-Ge hydride reactions on Si(100) (E(a) approximately 1.5-2.0 eV) indicate high reactivity profiles with respect to H(2) desorption, consistent with the low growth temperatures of the films. The quantum dots are obtained exclusively at higher temperatures (T > 500 degrees C) and represent a new family of Ge-rich compositions with narrow size distribution, defect-free microstructures, and homogeneous, precisely tuned elemental content at the atomic level.     

Synthesis and characterization of taper- and rodlike si nanowires on Si(x)Ge(1-x) substrate

Taper- and rodlike Si nanowires (SiNWs) are synthesized successfully on Si and Si(0.8)Ge(0.2) substrates. The growth mechanisms of taper- and rodlike SiNWs are proposed to be oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) growth, respectively. For taperlike SiNWs annealed at 1200 degrees C for 3 h, the emission peaks are found at 772, 478, and 413 nm. On the other hand, for rodlike SiNWs annealed at 1200 degrees C for 4 h, emission peaks are found at 783, 516, and 413 nm. From the field-emission measurements, the taperlike Si nanowires exhibit superior field-emission behavior with a turn-on field of 6.3-7.3 V/mum. The field enhancement, beta, has been estimated to be 700 and 1000 at low and high fields, respectively. The excellent field-emission characteristics are attributed to the perfect crystalline structure and the taperlike geometry of the Si nanowires.     

Crystallization of single phase (K,Na)-clinoptilolite

A single phase (K, Na)-clinoptilolite was hydrothermally crystallized without seed crystals from a reactant mixture of (K, Na)-aluminosilicate gel slurry through homogeneous mixing at 150 °C for 144 h. Compositions of the reactant mixtures and reaction temperatures to obtain the clinoptilolite were restricted within narrow limits in the case of syntheses without seed crystals, while the compositions and temperatures were expanded into wide ranges in the case of syntheses with 1 wt.-% of seed crystals. Coexistence of Na⁺- and K⁺-ions with appropriate ratios in the reactant mixture, and homogeneous mixing of this mixture, were indispensable for the crystallization of clinoptilolite. The crystals obtained were characterized by X-ray diffraction, chemical analysis, thermal analysis (DTA/TG), electron microscopy, and electron diffraction. The crystals were assigned as clinoptilolite, not heulandite, based on their chemical compositions and thermal stability.     

Electrodeposition of Ge, Si and Si x Ge 1-x from an air- and water-stable ionic liquid

The electrodeposition of Ge, Si and, for the first time, of Si(x)Ge(1-x) from the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) containing GeCl(4) and/or SiCl(4) as precursors is investigated by cyclic voltammetry and high-resolution scanning electron microscopy. GeCl(2) in [Py(1,4)]Tf(2)N is electrochemically prepared in a two-compartment cell to be used as Ge precursor instead of GeCl(4) in order to avoid the chemical attack of Ge(iv) on deposited Ge. Silicon, germanium and Si(x)Ge(1-x) can be deposited reproducibly and easily in this ionic liquid. Interestingly, the Si(x)Ge(1-x) deposit showed a strong colour change (from red to blue) at room temperature during electrodeposition, which is likely to be due to a quantum size effect. The observed colours are indicative of band gaps between at least 1.5 and 3.2 eV. The potential of ionic liquids in Si(x)Ge(1-x) electrodeposition is demonstrated.     

Electron correlation effects at semiconductor surfaces and interfaces: Si(111)-5x5, Si(111)-7x7 and Sn/Ge(111)

Electron correlation effects associated with the dangling bond surface states of Si(111)-5×5, Si(111)-7×7 and Sn/Ge(111)-3×3 are analyzed. In all the cases, extensive LDA-calculations are performed and effective two-dimensional Hamiltonians are deduced. Our analysis of these Hamiltonians shows that: (a) the Si(111)-5×5 surface states exhibits a metal-insulator transition; (b) the Si(111)-7×7 surface shows important similarities with the Si(111)-5×5 case, but it has a dangling bond surface band having a metallic character; (c) finally, the Sn/Ge(111)-3×3 dangling bond surface bands also shows important correlation effects that are found, however, not to affect the metallic character of the surface bands.     

Synthesis and Hydrolysis of Hybridized Silicon Alkoxide: Si(OEt)x(OBut)4-x. Part I: Synthesis and Identification of the Si(OEt)x(OBut)4-x

Using silicon tetrachloride (SiCl₄), tert-butanol (t-BuOH), ethanol (EtOH) and NH₃, the hybridized silicon ethoxide 3-tert-butoxide (Si(OEt)_x(OBu^t)_4-x) was synthesized and the configuration of the material was investigated by FT-IR,¹ H and ¹³C NMR and gas chromatogram/mass spectrum (GC/MS) techniques. The results confirm that both the ethoxy and the tert-butoxy groups have been attached to silicon atoms. Furthermore, the alkoxy group types and their relative amounts in the alkoxide were also determined by ¹H and ¹³C NMR and GC/MS.     

Synthesis and characterization of four new europium group XIV chalcogenides: K(2)EuTSe(5) and KEuTS(4) (T = Si, Ge)

Four europium group XIV chalcogenides have been synthesized using the reactive flux method: K(2)EuTSe(5) (I, II) and KEuTS(4) (III, IV) where T = Si, Ge. K(2)EuSiSe(5), I, crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 11.669(3) A, b = 9.844(2) A, c = 8.917(2) A, beta = 91.583(5) degrees, and Z = 4. K(2)EuGeSe(5), II, crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 11.8056(3) A, b = 9.9630(1) A, c = 8.9456(1) A, beta = 91.195(1) degrees, and Z = 4. Both K(2)EuSiSe(5) and K(2)EuGeSe(5) are semiconductors with optical band-gaps of approximately 2.00 and 1.84 eV, respectively. Raman spectroscopy shows vibrations from the (TSe(5))(4-) (T = Si, Ge) unit. KEuSiS(4), III, crystallizes in the monoclinic space group P2(1) with cell parameters a = 6.426(4) A, b = 6.582(5) A, c = 8.566(7) A, beta = 107.83(6) degrees, and Z = 2. KEuGeS(4), IV, crystallizes in the monoclinic space group P2(1) with cell parameters a = 6.510(2) A, b = 6.649(2) A, c = 8.603(3) A, beta = 107.80(2) degrees, and Z = 2. Band-gap analysis shows that both compounds are semiconductors with optical band-gaps of 1.72 and 1.71 eV, respectively. The Raman spectrum of KEuGeS(4) shows the vibrations of the (GeS(4))(4-) unit. Fluorescence spectroscopy confirms the presence of Eu(III) in III and IV instead of Eu(II) as in I and II. These four crystalline products were formed under equivalent stoichiometric reaction conditions. The fact that two different products are observed can be used to understand the relationship between the oxidative and reductive potentials within these flux reactions.     

Voltammetric studies in (K,Na)SCN eutectic melt

Current-potential curves are given for flow of solution containing cyanideion through a silver packed-bed electrode. It is shown that under the proper experimental conditions analyses of the free cyanide ion may be carried out. It is also shown that, within certain concentration limits, analyses of mixtures of CN^? and Cl^? may be carried out.     

Electronic properties of Si and Ge atoms doped in clusters: In(n)Si(m) and In(n)Ge(m)

Electronic properties of silicon and germanium atom doped indium clusters, In(n)Si(m) and In(n)Ge(m), were investigated by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of the anions. Size dependence of ionization energy and electron affinity for In(n)Si(1) and In(n)Ge(1) exhibit pronounced even-odd alternation at cluster sizes of n = 10-16, as compared to those for pure In(n) clusters. This result shows that symmetry lowering with the doped atom of Si or Ge results in undegeneration of electronic states in the 1d shell formed by monovalent In atoms.     

A full interionic potential for Na(1+x)Zr(2)Si(x)P(3-x)O(12) superionic conductors

Most inorganic solids are made up of octahedral and tetrahedral units interconnected to give an infinite framework. Use of computer simulation to study these materials has not been as prevalent as in the organic or biomolecules. Na(1+x)Zr(2)Si(x)P(3-x)O(12) is a typical inorganic solid with ZrO(6) octahedra and (Si/P)O(4) tetrahedra which are shown along with a few Na(+) sites marked M1, M2, and M3. We report here a full interionic potential which reproduces the structure and conductivity of these solids. This augurs well for the study of other inorganic solids.     

Electron-precise/deficient La(5-x)Ca(x)Ge4 (3.4 < or = x < or = 3.8) and Ce(5-x)Ca(x)Ge4 (3.0 < or = x < or = 3.3): probing low-valence electron concentrations in metal-rich Gd5Si4-type germanides

We report for the first time the syntheses of electron-precise/deficient alloys, Ln5-xCaxGe4 (Ln = La, Ce; x = 3.37, 3.66, 3.82 for La; x = 3.00, 3.20, 3.26 for Ce), in the metal-rich R5Tt4 Zintl system (R = rare earth metal; Tt = Si, Ge). The new alloys extend the phase width from electron-rich to open-shell electron-deficient region in the metal-rich Zintl system and demonstrate possible occurrence of varied electron deficiencies in Zintl phases without structural changes, as a result of other existing structure-forming factors.     

Synthesis and Hydrolysis of Hybridized Silicon Alkoxide: Si(OEt)x(OBut)4-x. Part II: Hydrolysis of the Si(OEt)x(OBut)4-x

The hydrolyzation behavior of the Si(OEt)_x(OBu^t)_4-x synthesized in part I was investigated by gas chromatogram/mass spectrum (GC/MS) technique. In the Si(OEt)_x(OBu^t)_4-x, Si(OEt)(OBu^t)₃ showed more rapid hydrolysis than Si(OEt)₂(OBu^t)₂, especially in the initial stages of the hydrolyzation. The dimer and trimer formations along with the sol-gel course were followed during the hydrolyzation process.     

Synthesis and characterization of transition-metal zintl phases: K(10)NbInAs(6) and K(9)Nb(2)As(6)

The two title compounds were prepared by direct reactions of the corresponding elements at high temperature. The structures were determined by single-crystal X-ray diffraction: K(10)NbInAs(6), monoclinic, P2(1)/n, Z = 2, a = 9.107(1) A, b = 8.2878(8) A, c = 15.139(1) A, beta = 91.112(9) degrees; K(9)Nb(2)As(6), monoclinic, P2(1)/c, Z = 2, a = 9.348(1) A, b = 9.113(1) A, c = 12.798(1) A, beta = 95.98(1) degrees. They contain isolated dimers made of edge-sharing tetrahedra of [NbAs(4)] and [InAs(4)] in the former, NbInAs(6)(10)(-), and only [NbAs(4)] in the latter, Nb(2)As(6)(9)(-). Magnetic measurements show that K(10)NbInAs(6) is diamagnetic, i.e., a d(0) transition-metal Zintl phase, while K(9)Nb(2)As(6) exhibits a Curie-Weiss behavior consistent with the presence of one unpaired electron. The latter defines K(9)Nb(2)As(6) as a mixed-valence (presumably of type III) transition-metal Zintl phase, only the third example of such phases.     

基于溶液配位化学过程调控的中空单晶氧化铁纳米材料

本文简要综述了本课题组近年来在单晶氧化铁空心纳米结构制备与生长机理研究方面的相关工作.通过采用磷酸根与硫酸根的双阴离子调节,结合反应时间、反应温度等条件的改变,实现了对α-Fe2O3单晶空心纳米结构形貌、尺寸和表面结构的良好调控,并提出一种针对非层状结构纳米环/纳米管新颖生长机理,即"配位辅助的定向溶解机理".以所制备的α-Fe2O3为前驱体,采用气相还原方法,成功制备得到了系列单晶Fe3O4和γ-Fe2O3管状及环状纳米结构,并对单个Fe3O4纳米环的磁学行为进行了考察.