Electronic properties of adsorbates on GaAs(001)-c(2x8)/(2x4)

A systematic experimental and theoretical study was performed to determine the causes of oxide-induced Fermi level pinning and unpinning on GaAs(001)-c(2 x 8)/(2 x 4). Scanning tunneling spectroscopy (STS) and density functional theory (DFT) were used to study four different adsorbates' (O(2), In(2)O, Ga(2)O, and SiO) bonding to the GaAs(001)-c(2 x 8)/(2 x 4) surface. The STS results revealed that out of the four adsorbates studied, only one left the Fermi level unpinned, Ga(2)O. DFT calculations were used to elucidate the causes of the Fermi level pinning. Two distinct pinning mechanisms were identified: direct (adsorbate induced states in the band gap region) and indirect pinnings (generation of undimerized As atoms). For O(2) dissociative chemisorption onto GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was only indirect, while direct Fermi level pinning was observed when In(2)O was deposited on GaAs(001)-c(2 x 8)/(2 x 4). In the case of SiO on GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was a combination of the two mechanisms.     

Molecular chain properties of poly (N-isopropyl acrylamide)

A series of poly( N-isopropyl acrylamide) (PNIPAM) samples with molecular weight ranging from 2.23×10~4 to 130×10~4 and molecular weight distribution M_w/M_n≤1.28 were obtained by free radical polymerization and repeat precipitation fractionation. The molecular weight M_w, second virial coefficient A_2 as well as the mean-square-root radius of gyration 〈S~2〉 for PNIPAM samples in tetrahydrofuran (THF) were determined by light scattering, and the relations were estimated at A_2 ∞ M_w~0.25) and 〈S~2〉~(1/2)=1.56×10~(-9) M_w~(0.56). The intrinsic viscosity for THF solution and methanol solution of PNIPAM samples was measured and the Mark-Houwink equations were obtained as [η]=6.90×10~(-5) M~(0/73) (THF solution) and [η]=1.07×10~(-4) M~(0.71) (methanol solution). The above results indicate that both THF and methanol are good solvents for PNIPAM. The limit characteristic ratio C_∞ for PNIPAM in the two solutions was determined to be 10.6 by using Kurata-Stockmayer equation, indicating that the f     

Effects of resonance occupation on dynamical electronic properties of adsorbates

A great variety of phenomena encountered in the studies of adsorption systems is, in one way or another, determined by the dynamics and the energetics of electronic transitions in adsorbates. Being intrinsically of a quantum nature, these transitions reflect the properties of the unperturbed species, as well as those of the interactions between the adsorbates and substrates that lead to adsorption. A typical feature of chemisorption systems is the occurrence of adsorbate valence electronic resonances which are degenerate to the substrate valence bands. The presence of a resonance may give rise to changes in the properties of the adsorbate electronic transitions relative to the corresponding gas phase characteristics. These changes should, in turn, manifest themselves in a number of the properties of adsorbates, which can be studied by modern surface sensitive experimental methods. In this article, we first briefly review the characteristics of the adsorbate electronic transitions involving valence resonances. Using this as a prerequisite, we present examples of the physical phenomena and events, such as van der Waals scattering from adsorbates and the measurements of the adsorbate spectra by electronic spectroscopies, which can be interpreted by invoking the effects of fractionally occupied valence resonances on the electronic transitions in chemisorbed species.     

两种影响生态环境的亚硝基取代烷烃的电子结构

采用紫外光电子能谱实验(PES)和量子化学方法对两种影响生态环境的亚硝基取代烷烃RONO[R=(CH3)2CH,(CH3)3CO]的电子结构进行了分析和讨论。实验得到两种化合物的第一电离能分别为10.37eV和10.12eV,结合从头算自洽场分子轨道(abinitioSCFMO)计算和外壳层格林函数法(OVGF)计算对PES进行了分析和指认。研究表明化合物中取代基效应对电离能存在明显的影响;外层格林函数法计算得到电离能与实验吻合很好;同时发现在外层格林函数计算结果中由于考虑相关能,得到的分子轨道存在能级顺序的交错。实验和理论计算结果进一步证实亚硝基取代烷烃是产生烷基氧自由基(RO^.)的很好的源物种,这为深入研究由它们产生的对环境破坏有着重要影响的相应自由基奠定了基础。     

Molecular cube of Re(II) and Mn(II) that exhibits single-molecule magnetism

A pseudocubic, paramagnetic cluster of ReII and MnII, [{MnCl}4{Re(triphos)(CN)3}4], has been prepared, and its magnetic properties have been investigated. Antiferromagnetic coupling is observed between the "S = 1/2" ReII and S = 5/2 MnII centers resulting in an effective S = 8 ground state. AC susceptibility studies reveal that the molecule is a single-molecule magnet with an effective barrier for magnetization reversal of Ueff = 8.8 cm-1.     

Molecular flexibility as a factor affecting the surface ordering of organic adsorbates on metal substrates

The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using alpha,omega-dihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (alpha-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.     

Density Functional Theory Study of the Point Defects on KDP (100) and (101) Surfaces

Surface defects are usually associated with the formation of other forms of expansion defects in crystals, which have an impact on the crystals’ growth quality and optical properties. Thereby, the structure, stability, and electronic structure of the hydrogen and oxygen vacancy defects (V_H and V_O) on the (100) and (101) growth surfaces of KDP crystals were studied by using density functional theory. The effects of acidic and alkaline environments on the structure and properties of surface defects were also discussed. It has been found that the considered vacancy defects have different properties on the (100) and (101) surfaces, especially those that have been reported in the bulk KDP crystals. The (100) surface has a strong tolerance for surface V_H and V_O defects, while the V_O defect causes a large lattice relaxation on the (101) surface and introduces a deep defect level in the band gap, which damages the optical properties of KDP crystals. In addition, the results show that the acidic environment is conducive to the repair of the V_H defects on the surface and can eliminate the defect states introduced by the surface V_O defects, which is conducive to improving the quality of the crystal surface and reducing the defect density. Our study opens up a new way to understand the structure and properties of surface defects in KDP crystals, which are different from the bulk phase, and also provides a theoretical basis for experimentally regulating the surface defects in KDP crystals through an acidic environment.     

Room-temperature kinetics of short-chain alkanethiol film growth on Ag(111) from the vapor phase

We have used time-of-flight (TOF) direct recoiling spectroscopy (DRS) to follow propanethiol adsorption at 300 K from the vapor phase on an Ag(111) surface, for exposures ranging from 10(-1) to 10(5) L. Results show that the adsorption proceeds with changes in the sticking coefficient, consistent with at least three phases. At low exposures, the alkanethiol molecules adsorb with high probability at defect sites, followed by a slower growth mode that essentially covers the whole surface. A third change in the sticking coefficient is associated with the final saturation stage, corresponding to a thicker layer related to molecules in a more upright orientation. The adsorption kinetics for hexanethiol is similar to that of propanethiol but taking place at higher rates, stressing the importance of the hydrocarbon chain length in the growth process. ISS-TOF measurements during thermal desorption show that most of the C, H, and S go away together, suggesting that the molecules adsorb and desorb from flat regions without S-C bond cleavage. Fitting the desorption maximum at 450 K with a first-order desorption curve gives a desorption energy of 1.43 eV. A small final S content that is correlated with the initial Ag(111) surface roughness is observed after desorption.     

Mechanism and kinetics properties for the reaction: Chloroethane with atomic O (P)

In this paper, we present direct dynamics calculations for the multiple-channel reaction of CH₃CH₂Cl with atomic O (³P) in a wide temperature range (200–3000 K), based on canonical variational transition state theory including small curvature corrections. Four distinct saddle points, one for α-abstraction and three for β-abstraction, have been located for this reaction. The potential energy surface information has been calculated at the MP2/6-311G(d,p) level. The energies along the minimum energy path have been further improved by single-point energy calculations at the G3MP2 level. In the β-abstraction channel, Jahn–Teller effect has been found. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths for all channels have been discussed and compared. The calculated total rate constants match the available experimental values reasonable well over the measured temperature range. The results show the variational effect can be negligible and the small curvature tunneling contribution plays an important role for the calculation of the rate constant. At low temperature α-abstraction may be the major reaction channel, while β-abstraction will have more contribution to the whole reaction rate as the temperature increase.     

Growth and characterization of unidirectional (100) KDP single crystal by Sankaranarayanan-Ramasamy (SR) method

Unidirectional (100) potassium dihydrogen orthophosphate (KDP) single crystals were grown by Sankaranarayanan-Ramasamy (SR) method. The (100) oriented seed crystals were mounted at the bottom of the glass ampoules and the crystals of 20mm diameter, 30 mm height and 15 mm diameter, 65 mm height were grown by SR method. The grown crystals were characterized by high-resolution X-ray diffractometry anlaysis, UV-vis spectroscopy, dielectric and microhardness studies. The high-resolution X-ray diffractometry anlaysis indicates that the crystalline perfection is excellent without having any very low angle internal structural grain boundaries. The SR method-grown unidirectional KDP has 15% higher transmittance compared to conventional method-grown crystals. The dielectric constant was higher and the dielectric loss was less in SR method-grown crystal. The crystals grown by SR method have much higher hardness value than conventional method-grown crystals. The quality of the crystal grown by SR method is better than conventional method-grown crystal.     

Molecular recognition properties of FN3 monobodies that bind the Src SH3 domain

We have constructed a phage-displayed library based on the human fibronectin tenth type III domain (FN3) scaffold by randomizing residues in its FG and BC loops. Screening against the SH3 domain of human c-Src yielded six different clones. Five of these contained proline-rich sequences in their FG loop that resembled class I (i.e., +xxPxxP) peptide ligands for the Src SH3 domain. The sixth clone lacked the proline-rich sequence and showed particularly high binding specificity to the Src SH3 domain among various SH3 domains tested. Competitive binding, loop replacement, and NMR perturbation experiments were conducted to analyze the recognition properties of selected binders. The strongest binder was able to pull down full-length c-Src from murine fibroblast cell extracts, further demonstrating the potential of this scaffold for use as an antibody mimetic.     

Miscibility and spherulite growth kinetics in the poly(ethylene oxide)/poly(benzyl methacrylate) system

This study for the first time discovered miscibility in the binary blend of semicrystalline poly(ethylene oxide) (PEO) with amorphous poly(benzyl methacrylate) (PBzMA). Differential scanning calorimetry, optical and scanning electron microscopy, and infrared spectroscopy were performed to characterize and demonstrate miscibility in the PEO/PBzMA system. The glass‐transition behavior and Fourier transform infrared results suggest that the intermolecular interactions between the pairs were likely nonspecific and at best comparable to those among the same constituent component. The melting‐point depression study yielded χ = −0.1, indicating a relatively low interaction strength. It is concluded that the phase behavior of the blend was miscibility with nonspecific interactions, mostly a matched polar–polar intermolecular attraction. PEO spherulitic crystallization in the blend is discussed to support the miscibility behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 562–572, 2000     

Water-in-Oil Dispersion for KH2PO4 (KDP) Crystal CMP

Chemical mechanical polishing (CMP) has become an essential process in the manufacturing of advanced microelectronic devices. More recently, CMP has also been applied to the process of other advanced materials such as optical crystals and thin films. Typically, a CMP slurry is formulated as an aqueous dispersion which may contain abrasive particles, activating agent, passivating agent, surfactant, etc. Due to its sensitivity to water, hygroscopic crystals must not be processed with aqueous based slurry. In this study, a new abrasive-free system based on water-in-oil microemulsion was investigated to address this challenge. More specifically, a dispersion made of dodecanol, Triton X-100, and water was studied for its potential application in KH₂PO₄(KDP) crystal processing. In this unique polishing system, water molecules are caged into micelles so the reaction between KDP and water is controlled. As a result, the static etch rate of the substrate surface is minimized. During polishing process, the frictional action between crystal surface and pad leads to the release of reactive water molecules. The material removal is, thus, enhanced. In this paper, the techniques used to characterize such abrasive-free system were first introduced. The water-in-oil structures were characterized and confirmed by conductivity, dynamic lighting scattering and dynamic nuclear magnetic resonance (NMR) measurements. The performance of this system on the process of KDP crystals was then discussed. The static etch rate and the material removal rate in polishing process were measured under various conditions in order to elucidate the polishing mechanism. Finally, the potential application of such a novel nonaqueous polishing system in CMP beyond KDP crystals is discussed.     

Molecular triangle of palladium(II) and its anion binding properties

The ligand 4(3H)-pyrimidone (Hpm) forms the complexes trans-[PdCl(2)(Hpm)(2)] and [Pd(PP)(Hpm)(2)](CF(3)SO(3))(2) (PP = Ph(2)PCH(2)PPh(2) or Ph(2)P(CH(2))(3)PPh(2)), with the neutral ligand (Hpm), and a bowl-like molecular triangle, [(Pd(bu(2)bipy)(mu-pm))(3)](3+) (bu(2)bipy = 4,4'-di-tert-butyl-2,2'-bipyridine), with the deprotonated ligand (pm). This triangular complex acts as a host for binding of several anionic guests.     

Factors that affect the tautomeric equilibrium of hydroxyazopyridine derivatives

The ratio of azo and quinonehydrazone forms of 3-hydroxy-6-phenylazopyridine derivatives was determined by IR and electronic spectroscopy. The formation of two crystalline modifications of 2-methoxy-3-hydroxy-6-phenylazopyridine is explained by crystallization of the molecules either in the form of dimers of predominantly the quinonehydrazone form or in the form of polyassociates of the azo form, which are formed by a strong hydrogen bond.Communication VII from the series Structures and Properties of Dyes, See [1] for communication VI.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1364–1370, October, 1974.