No description available.

No description available.

We report on the characteristics of a ZnO based metal insulator semiconductor (MIS) diode comprised of a heterostructure of n-ZnO nanorods/n-GaN. The MIS structure consisted of unintentional - doped n type ZnO nanorods grown on n-GaN sample using hydrothermal synthesis at low temperature (100°). The ZnO nanorod layer was vertically grown from the GaN sample, having the diameter 100nm and length 2µm. Then, an insulator layer for electrical isolation was deposited on the top of ZnO nanorod layer by using spin coating method. A metal layer (gold) was finally deposited on the top. The I-V dependences show a rectifying diode like behavior with a leakage current of 2.10⁻⁵ A and a threshold voltage of about 3V. Depend on the thickness of the insulator, the I-V dependences of the n-ZnO/n-GaN heterostructure was varied from rectifying behavior to Ohmic and nearly linear. / Singapore-MIT Alliance (SMA)

No description available.

The bis(2-pyridylthio)methyl ligand, [Bptm], offers a synthetically convenient alternative to a variety of multidentate ligands, including most notably [Tptm] (tris(2-pyridylthio)methyl) and [BptmSTol] (bis(2-pyridylthio)(p-tolylthio)methyl), and, in contrast with [Tptm], necessarily coordinates to metal centers in a κ³ fashion. As such, numerous [Bptm] complexes of zinc have been synthesized and structurally characterized. In Chapter 1, we describe the reaction of the protonated ligand [Bptm]H with the homoleptic zinc compounds Me₂Zn and Zn[N(SiMe₃)₂]₂ to afford, respectively, [Bptm]ZnMe and [Bptm]ZnN(SiMe₃)₂; the latter has been used as a starting point for a wide range of reactivity.Most notably, the terminal zinc hydride, [Bptm]ZnH, can be accessed via either (i) metathesis of the zinc siloxide, [Bptm]ZnOSiPh₃, with either PhSiH₃ or HBpin, or (ii) direct metathesis of the zinc amide [Bptm]ZnN(SiMe₃)₂ with HBpin; the latter reactivity is not precedented and offers a novel approach for the synthesis of molecular zinc hydrides. Both [Bptm]ZnN(SiMe₃)2 and [Bptm]ZnH provide access to a variety of monomeric derivatives, including the zinc halides [Bptm]ZnX (X = Cl, Br, I) and the zinc isocyanate [Bptm]ZnNCO; the latter can be accessed directly via (i) metathesis of [Bptm]ZnH with Me₃SiNCO or (ii) a multistep reaction of [Bptm]ZnN(SiMe₃)₂ with CO₂. [Bptm]ZnH also undergoes insertion of CO₂ into its Zn—H bond to afford the zinc formate, [Bptm]ZnO₂CH, in which the formate moiety exhibits a monodentate binding mode in the solid state. This reactivity enables it to serve as a catalyst for the hydrofunctionalization of CO₂; specifically, [Bptm]ZnH catalyzes the hydrosilylation of CO₂ by (RO)₃SiH (R = Me, Et) at elevated temperatures to afford the respective silyl formates (RO)3SiO₂CH, as well as the hydroboration of CO₂ by HBpin at room temperature to afford the boryl formate HCO₂Bpin. In the absence of CO₂, [Bptm]ZnH also catalyzes the reduction of HCO₂Bpin to the methanol level, MeOBpin. Similarly, [Bptm]ZnH serves as an effective catalyst for the hydrosilylation and hydroboration of a variety of ketones and aldehydes. In all cases, hydroboration is more facile than the corresponding hydrosilylation. The [Bptm]Zn system has been investigated computationally, and the kinetics of insertion of CO₂ into the Zn—H bond of [Bptm]ZnH as well as the thermodynamics of the catalytic cycle have been examined. Further mechanistic studies examine two noteworthy spectroscopic features of the system, namely rapid exchange (i) between the zinc and boryl formates [Bptm]ZnO₂CH and HCO₂Bpin, as well as (ii) between [Bptm]ZnH and [Bptm]ZnO₂CH. Both of these exchange processes have been investigated with variable-temperature NMR spectroscopy; in particular, the former exchange resolves at low temperatures and can be confirmed by exchange spectroscopy. In addition to the aforementioned monomeric zinc halides [Bptm]ZnX (X = Cl, Br, I), the dimeric bridging zinc fluoride {[Bptm]Zn(μ-F)}₂ has been synthesized via reaction of Me3SnF with either [Bptm]ZnN(SiMe₃)₂ or [Bptm]ZnH, as outlined in Chapter 2. The dimeric nature of the fluoride in contrast with the other monomeric halides can be attributed to the significant polarity of the Zn—F bond. {[Bptm]Zn(μ-F)}2 also reacts with Me₃SiCF₃ to afford an unusual instance of a structurally characterized zinc trifluoromethyl complex, [Bptm]ZnCF₃. Chapter 3 discusses cadmium analogues to the [Bptm]Zn system, which provide a comparison and a contrast both with their zinc counterparts as well as with previously reported [Tptm]Cd complexes. While the cadmium amide [Bptm]CdN(SiMe₃)2 may be synthesized in a manner corresponding to that for its zinc analogue, the siloxides {[Bptm]Zn(μ-OSiR₃)}₂ (R = Me, Ph) form dimers that are distinct from the monomeric [Bptm]ZnOSiPh₃ and [Tptm]CdOSiPh₃, although similar to {[Tptm]Cd(μ-OSiMe₃)}₂. The distinctions between the [Bptm]Zn and [Bptm]Cd siloxides have been investigated computationally, indicating that the cadmium species show a thermodynamic preference for dimer formation, which can be attributed to the larger atomic radius of cadmium relative to zinc. Attempts to synthesize a cadmium hydride are interrupted by a Schlenk-type equilibrium giving way to the bis(ligand) complex [Bptm]2Cd and CdH₂, which in turn decomposes to Cd and H2. However, spectroscopic studies indicate that under CO₂, [Bptm]CdN(SiMe₃)₂ and HBpin react to trap a cadmium hydride species as the bridging formate derivative, [Bptm]Cd(μ-O₂CH)₂Bpin. The interaction of nitrogen-rich ligands with main group metals is further probed in Chapter 4, which describes the investigation of the coordination of 2,2’:6,2”-terpyridine (terpy) to magnesium compounds. Most prominently, unsubsituted terpy forms an adduct, terpyMg[N(SiMe₃)₂]₂, with the monomeric form of the magnesium amide {Mg[N(SiMe₃)₂]₂}₂. The adduct reacts with halide donors to form a series of mixed amide-halide complexes, terpyMg[N(SiMe₃)]X (X = Cl, Br, I), as well as a mixed amide-azide complex, terpyMg[N(SiMe₃)₂]N₃. These complexes represent the first instances of neutral monomeric terpyMg compounds that feature unsubstituted terpyridine. Structural comparisons of these complexes with one another as well as with comparable compounds are undertaken. Complexes of terpy with cadmium and zinc analogues, terpyCd[N(SiMe₃)₂]₂ and terpyZn [N(SiMe₃)₂]₂, are explored further, and DFT calculations are used to explore the strength of the interactions between the ligand and the metals in each case. Finally, in Chapter 5, attention is given to the recently reported zinc bromide complex featuring a zwitterionic carboxylate ligand, (Cbp)2ZnBr₂. The structure reported for this complex features several anomalous features, including abnormally long Zn—Br and Zn—O bonds, unusually small atomic displacement parameters for Zn, and a high R-value. This information led us to synthesize and investigate the cadmium counterpart, (Cbp)₂CdBr₂; we find that the cadmium complex possesses nearly identical structural parameters to the reported zinc complex, and when the cadmium is refined as zinc, the displacement parameter problems are reproduced. Therefore, we conclude that the reported structure is in fact that of (Cbp)₂CdBr₂, and report a revised structure for (Cbp)₂ZnBr₂.

No description available.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

The thesis outlines the development of novel cationic zinc complexes for application in lactide polymerization catalysis. These complexes were characterized spectroscopically and crystallographically, and where appropriate their efficacy as catalysts for the polymerization of lactide was evaluated. The strongly donating, neutral chelating ligands employed in this study were prepared by installation of either one or two phosphinimine donors on a dibenzofuran backbone. An efficient synthetic methodology was then developed for the synthesis of cationic complexes of the formula [LZnE+][BAr4-], wherein E = C2H5, CH3, Ph, C6F5, OAc, OC6F5, or methyl-(D,L)-lactate, and AR = Ph, C6F5, or m-(CF3)2-C6H3. Only the cationic zinc-lactate species were found to be highly active polymerization catalysts. Tuning of the steric and electronic properties of the ligand resulted in the discovery of zinc-lactate systems that promote rapid and well-controlled polymerization of lactide under mild conditions, marking the first well-defined cationic metal catalysts to do so. / xxiv, 254 leaves : ill. ; 29 cm + 1 CD-ROM

Zinc dialkyldithiophosphates (ZDTPs) are widely incorporated in lubricant formulations as anti-oxidant and anti-wear additives. Recent years have seen the emergence of considerable research aimed at defining the mode of action of these compounds. Despite this, the mechanistic action of ZDTPs in their capacity as both anti-wear and anti-oxidant agents remains ill defined. Previous investigations have used a wide variety of techniques, however, electrochemical methods have been poorly exploited. This thesis describes application of electrochemical techniques to the study of zinc ndibutyldithiophosphate (Buⁿ-ZDTP) in an investigation of the value of such methods as a tool for ZDTP analysis. A comprehensive study of Bun-ZDTP redox activity under standard electrochemical conditions has been implemented. Using cyclic voltammetry, Buⁿ-ZDTP has been shown to be oxidised in an irreversible 2 electron transfer process. The diffusion coefficient of this species in DMF has been calculated via rotating disc electrode (RDE) voltammetry. Buⁿ-ZDTP reduction proceeds via nucleation of zinc at the electrode surface. The limiting current of the 2 electron reduction process is lower than previously observed for Buⁿ- ZDTP oxidation. This occurs since zinc deposition is confined to growth centres which effectively limit the electroactive area of the electrode. A more thorough investigation of zinc nucleation at the electrode surface was undertaken using chronoamperometry. Transients plotted in dimensionless form were compared to theoretical models of nuclear growth. The potential of atomic force microscopy as a promising technique for the imaging of ZDTP filming action has been demonstrated. Ex-situ atomic force microscopy was conducted of electrode surfaces following reduction in Buⁿ-ZDTP solution under cyclic and RDE voltammetric conditions. Development of a variable elevated temperature apparatus for the study of Buⁿ-ZDTP under conditions more typical of an engine environment has been described. Application of the system to a wide range of electrochemical problems was demonstrated as an illustration of the advantages of this novel experimental approach. Studies of N, N, N/, N/-tetramethyl-p-phenylenediamine and tris(4-bromophenyl)amine oxidation yielded diffusion co-efficients (D) at elevated temperatures from which activation parameters for diffusion were obtained. Variation of D with temperature was compared to predicted theory according to the Stokes-Einstein and Wilke-Chang relationships. Studies of ortho-bromonitrobenzene and 9-chloroanthracene reduction demonstrated the elevated temperature apparatus to be a viable method for the interrogation of rapid kinetic processes. Both compounds are reduced according to an ECE mechanistic scheme where halide bond cleavage constitutes the chemical step. Since the use of microelectrodes permits "outrunning" of bond cleavage kinetics to give an n_eff of less than two, rate constants at varying temperatures were evaluated. Arrhenius plots were used to deduce activation parameters of halide bond cleavage in both compounds. Preliminary studies of Buⁿ-ZDTP redox activity under conditions more typical of an engine environment are discussed. Application of the novel microelectrode apparatus to an elevated temperature study of Buⁿ-ZDTP is described. Voltammetric investigations in toluene, a resistive solvent which serves as a convenient model for oil, are discussed.

Thesis (Ph.D.)--McMaster University (Canada), 1991. / Source: Dissertation Abstracts International, Volume: 54-02, Section: B, page: 1040.