Chitra Gurnani

Synthesis, characterisation and structures of thio-, seleno- and telluro-ether complexes of gallium(III)

The reactions of GaX3 (X = Cl, Br or I) with SMe2, SeMe2 and TeMe2 (L) in non-coordinating solvents produces only the pseudo-tetrahedral [GaX3L], which have been characterised by IR, Raman and multinuclear NMR (1H, 71Ga, 77Se or 125Te) spectroscopy, and by the crystal structure of [GaCl3(SeMe2)]. The 71Ga NMR resonances show small low frequency shifts for fixed halides as the neutral donors change from S --> Se --> Te. Bidentate ligands including MeS(CH2)2SMe, PhS(CH2)2SPh, MeSe(CH2)2SeMe, nBuSe(CH2)2Se(n)Bu and MeTe(CH2)3TeMe (L-L) also produce complexes with 4-coordinate gallium centres, [(GaX3)2(mu-L-L)], confirmed by the crystal structures of [(GaI3)2(mu-MeS(CH2)2SMe)], [(GaCl3)2(mu-PhS(CH2)2SPh)] and [(GaCl3)2(mu-nBuSe(CH2)2Se(n)Bu)]. The structural data are consistent with the weaker Lewis acidity of the gallium as the halide co-ligands become heavier. Multinuclear NMR studies suggest that in chlorocarbon solutions partial dissociation of the ligands occur, which increases with the halide co-ligand Cl < Br < I. The o-xylyl dithioether, o-C6H4(CH2SMe)2, despite being pre-organised for chelation, also forms [(GaCl3)2(mu-L-L)]. The corresponding diselenoether complex decomposes in solution with C-Se bond cleavage to form the selenonium salt [o-C6H4CH2Se(Me)CH2][GaCl4], which was structurally characterised. The ditelluroether o-C6H4(CH2TeMe)2 undergoes rapid C-Te bond fission and rearrangement upon reaction with GaCl3, and the telluronium species [o-C6H4CH2Te(Me)CH2]+ and [MeTe(CH2(o-C6H4)CH2TeMe)2]+ have been identified by ES+ mass spectrometry from their characteristic isotope patterns.

Synthesis, characterisation and structures of thio-, seleno- and telluro-ether complexes of indium(III) halides

The indium(III) halo-bridged octahedral dimers [InX(2)(L-L)(mu-X)(2)InX(2)(L-L)] (X = Cl: L-L = MeS(CH(2))(2)SMe, MeSe(CH(2))(2)SeMe, (n)BuSe(CH(2))(2)Se(n)Bu), the ionic trans-[InX(2)(L-L)(2)][InX(4)] (X = Cl: L-L = (i)PrS(CH(2))(2)S(i)Pr; X = Br: L-L = MeS(CH(2))(2)SMe, (i)PrS(CH(2))(2)S(i)Pr, MeSe(CH(2))(2)SeMe), cis-[InCl(2)(thiamacrocycle)][InCl(4)] (thiamacrocycle = [12]aneS(4) or [14]aneS(4)) and the neutral, octahedral [InCl(3)([9]aneS(3))] and [InCl(3){MeC(CH(2)SMe)(3)}] were obtained in good yield by the reaction of 1:1 molar ratios of InX(3) with the ligand in anhydrous CH(2)Cl(2) solution. The distorted tetrahedral [InX(3)(Me(2)Se)] (X = Cl, Br or I) and [InX(3)(Me(2)Te)] (X = Br or I) were obtained from 1:3 and 1:2 molar ratios respectively of InX(3) and Me(2)E (E = Se or Te) also in CH(2)Cl(2). The ligand-bridged, distorted tetrahedral dimers [(InCl(3))(2){micro(2)-o-C(6)H(4)(CH(2)SMe)(2)}] and [(InCl(3))(2){micro(2)-MeTe(CH(2))(3)TeMe}] are formed even from a 1:1 In:ligand ratio. Key structure types were confirmed from crystal structures of [InCl(2){RSe(CH(2))(2)SeR}(micro-Cl)(2)InCl(2){RSe(CH(2))(2)SeR(2)}] (R = Me or (n)Bu), trans-[InX(2){(i)PrS(CH(2))(2)S(i)Pr}(2)][InX(4)] (X = Cl or Br), trans-[InBr(2){MeSe(CH(2))(2)SeMe}(2)][InBr(4)], cis-[InCl(2)([14]aneS(4))][InCl(4)] and [InBr(3)(Me(2)Se)]. The bulk complexes have been characterised by IR and Raman spectroscopy and microanalyses, while (1)H, (77)Se{(1)H} and (125)Te{(1)H} NMR spectroscopy show that the compounds are extremely labile in solution and undergo rapid dynamic exchange equilibria. Comparisons are drawn between these structurally rather diverse In(III) chalcogenoether complexes and the corresponding Ga(III) species (all of which are neutral and involve distorted tetrahedral coordination). The reaction of TlCl(3) with Me(2)E (E = Se or Te) shows that chlorination of Me(2)E rather than adduct formation occurs, while no reaction occurred between TlCl(3) and Me(2)S, consistent with Tl(III) being a very poor Lewis acid.

Area Selective Growth of Titanium Diselenide Thin Films into Micropatterned Substrates by Low-Pressure Chemical Vapor Deposition.

The neutral, distorted octahedral complex [TiCl4(SenBu2)2] (1), prepared from the reaction of TiCl4 with the neutral SenBu2 in a 1:2 ratio and characterized by IR and multinuclear (1H, 13C{1H}, 77Se{1H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe2, films onto SiO2 and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe2 (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe2 films is 3.36 ± 0.05 × 10–3 Ω·cm with a carrier density of 1 × 1022 cm–3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO2/TiN patterned substrates. TiSe2 is selectively deposited within the smallest 2 μm diameter TiN holes of the patterned TiN/SiO2 substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 μm hole size, whereas their lengths/widths continue to increase.

Controlling the nanostructure of bismuth telluride by selective chemical vapour deposition from a single source precursor

High quality, nanostructured Bi2Te3, with an unprecedented degree of positional and orientational control of the material form on the nanoscale, is readily obtained by low pressure chemical vapour deposition using a new molecular precursor. This system offers a convenient method that delivers key structural requirements necessary to improve the thermoelectric efficiency of Bi2Te3 and to develop the nascent field of topological insulators.

A Dicationic Iminophosphane

A novel dicationic system containing a PN fragment has been synthesized and structurally characterized. According to the solid-state analysis and theoretical investigation, the dicationic iminophosphane resonance form is the most appropriate description for the dication. However, the contribution from the phosphorus mononitride resonance form is not negligible.

Niobium(V) and tantalum(V) halide chalcogenoether complexes – towards single source CVD precursors for ME2 thin films

A series of pentavalent niobium and tantalum halide complexes with thio-, seleno- and telluro-ether ligands, [MCl5(E(n)Bu2)] (M = Nb, Ta; E = S, Se, Te), [TaX5(TeMe2)] (X = Cl, Br, F) and the dinuclear [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta), has been prepared and characterised by IR, (1)H, (13)C{(1)H}, (77)Se, (93)Nb and (125)Te NMR spectroscopy, as appropriate, and microanalyses. Confirmation of the tantalum(V)-telluroether coordination follows from the crystal structure of [TaCl5(TeMe2)], which represents the highest oxidation state transition metal complex with telluroether coordination structurally authenticated. The Ta(V) monotelluroether complexes are much more stable than the Nb(V) analogues. In the presence of TaCl5 the ditelluroether, CH2(CH2Te(t)Bu)2, is decomposed; one of the products is the dealkylated [(t)BuTe(CH2)3Te][TaCl6], whose structure was determined crystallographically. Crystal structures of [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta) show ligand-bridged species. The complexes bearing β-hydrogen atoms on the terminal alkyl substituents have also been investigated as single source reagents for the deposition of ME2 thin films via low pressure chemical vapour deposition. While the tantalum complexes proved to be unsuitable, the [NbCl5(S(n)Bu2)] and [NbCl5(Se(n)Bu2)] deposit NbS2 and NbSe2 as hexagonal platelets onto SiO2 substrates at 750 °C and 650 °C, respectively. Grazing incidence and in-plane X-ray diffraction confirm both materials adopt the 3R-polytype (R3mh), and the sulfide shows preferred orientation with the crystallites aligned predominantly with the c axis perpendicular to the substrate. Scanning electron microscopy and Raman spectra are consistent with the X-ray data.

Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition

Arrays of individual single nanocrystals of Sb2Te3 have been formed using selective chemical vapor deposition (CVD) from a single source precursor. Crystals are self-assembled reproducibly in confined spaces of 100 nm diameter with pitch down to 500 nm. The distribution of crystallite sizes across the arrays is very narrow (standard deviation of 15%) and is affected by both the hole diameter and the array pitch. The preferred growth of the crystals in the <1 1 0> orientation along the diagonal of the square holes strongly indicates that the diffusion of adatoms results in a near thermodynamic equilibrium growth mechanism of the nuclei. A clear relationship between electrical resistivity and selectivity is established across a range of metal selenides and tellurides, showing that conductive materials result in more selective growth and suggesting that electron donation is of critical importance for selective deposition.

Preparation and structures of tellurium(IV) halide complexes with thioether coordination.

The first chalcogenoether complexes of Te(iv) chloride and bromide are prepared by reaction of the thioether with a suspension of TeX(4) in anhydrous CH(2)Cl(2), and the products characterised by IR, Raman, (1)H and (125)Te{(1)H} NMR spectroscopy and microanalysis. The structures of the distorted octahedral chelate complexes [TeX(4){RS(CH(2))(2)SR}] (X = Cl or Br; R = Me or (i)Pr) and the centrosymmetric halo-bridged dimers [{X(3)(Me(2)S)Te}(2)(micro-X)(2)] (X = Cl or Br) involving Te(iv) are reported and the structures interpreted in terms of a three-centre-four-electron bonding model, with weak, secondary Te-S interactions. The structure of a unique Te(ii) thioether complex, [TeCl{(i)PrS(CH(2))(2)S(i)Pr}][Te(2)Cl(9)] obtained as a decomposition product from a sample of [TeCl(4){(i)PrS(CH(2))(2)S(i)Pr}], in which a Te(ii) thioether cation and Te(iv) chloride anion are weakly associated via micro(2)- and micro(3)-bridging Cl ligands, is also described. In this case distortions in the coordination environment at the Te(ii) ion are attributed to the effects of the Te-based lone pairs.

Compositionally tunable ternary Bi2(Se1−xTex)3 and (Bi1−ySby)2Te3 thin films via low pressure chemical vapour deposition

The inherently rapid ligand substitution kinetics associated with the novel and chemically compatible precursors, [MCl3(EnBu2)3] (M = Sb, Bi; E = Se, Te), enable CVD growth of ternary Bi2(Se1−xTex)3 and (Bi1−ySby)2Te3 thin films with very good compositional, structural and morphological control, for the first time. X-ray diffraction data follow Vegards law and Raman bands shift linearly with the atom substitutions, indicating very well-distributed solid solutions.