Andrew P. Purdy

Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders

We present photoluminescence (PL) studies of GaN and ZnO nanocrystallites and powders. Our studies show that in addition to the intrinsic photoluminescence characteristics, the photoluminescence properties of the porous media are also a strong function of conditions such as ensemble size and powder density, ultraviolet-laser excitation power, and vacuum state. PL redshifts up to 120 meV were observed for GaN and ZnO crystallites and were attributed to laser heating and heat trapping in the ensemble. The electron-phonon interaction model for GaN indicated ensemble temperature ∼550 K, which is consistent with the finding obtained via high-temperature PL and Raman experiments. The PL in the vacuum state exhibited a significant redshift, ∼80 meV relative to that in air, and the PL of a dense ZnO pellet was found to resemble that of the bulk more than does a loose powder. The PL analyses indicated an excitonic emission at room temperature for both GaN and ZnO crystallites with intensity saturation occurring fo...

Lithium dicyanamide, its reactions with cyanuric chloride, and the crystal structures of LiN(CN)2(MeCN)2 and LiCN(C5H5N)2

Abstract Reactions of dicyanamides with cyanuric chloride were investigated as possible routes to carbon nitrides. Anhydrous lithium dicyanamide (1) was synthesized from NaN(CN), and LiCl in THF. Compound 1 was found to react with cyanuric chloride to form a yellow product, which forms an amorphous lithium-carbon nitride with an approximate formula CN0.7Li0.2 upon thermolysis to 700°C. Upon heating to 1000°C, the product decomposes with the distillation of lithium cyanide. Reactions between cyanuric chloride and trimethylstannyl dicyanamide were also investigated. Lithium dicyanamide was also found to trimerize and polymerize at 280–320°C. Additionally, two LiCN complexes were structurally characterized. The bis (acetonitrile) adduct of 1 crystallizes with the lithium atoms linking the cyano nitrogens into infinite LiNCNCN chains, and the bis(pyridine) adduct of LiCN consists of infinite zig-zag LiCN chains.

Copper diffusion in organic polymer resists and inter-level dielectrics

Abstract In future generation microelectronic devices, it is anticipated that back end of the line processing may incorporate interconnects composed of Cu metal, and inter-level dielectrics composed of organic polymers. In this study, we examined a number of commercially available and new NRL synthesized organic polymer systems towards Cu metal incorporation in the film during application. All the polyimides examined transported Cu, in large part due to the ability of the solvent, 1-methyl-2-pyrrolidinone (NMP), to dissolve Cu. We report examples where the solvent is largely responsible for Cu incorporation into the polymer film, and examples where the polymer and not the solvent is responsible for Cu transport into the film polymethyl methacrylate. Three preparations examined in this study, polystyrene, Teflon AF, and 1,3,5-tris (2-allyloxy-hexafluoro-2-propyl) benzene/poly methyl hydrosiloxane, were found to resist Cu diffusion.

Preparation of Phase Pure Cubic Gallium Nitride, c-GaN, by Ammonothermal Conversion of Gallium Imide, {Ga(NH)3/2}n

Gallium imide, {Ga(NH)3/2}n, was sealed in quartz tubes with anhydrous NH3 and NH4X (X=Cl, Br, I) and heated vertically in an autoclave with the hot-zone (bottom) temperature ranging from 300–530°C. With NH4I mineralizer, the imide was converted to primarily the zinc-blende phase of GaN (c-GaN), usually in the form of micron sized or smaller tetrahedra. With NH4Cl mineralizer, hexagonal GaN nanoparticles formed instead, and NH4Br facilitated the conversion of imide to irregular h-GaN nanoparticles at lower temperatures, and larger particles above 500°C. At the higher temperatures, chemical transport took place and GaN crystals grew on the walls at the middle to the top of the tube. Most of the deposited GaN was cubic, usually in a heavily etched triangular prismatic morphology. Several experiments were performed in a Pt lined pressure vessel in order to demonstrate that the synthesis method can be scaled up.

Synthesis and structure of tri-tert-butoxyzincates

Abstract The reaction between 3 eq MOCME 3 (M = Na, K) and ZnCl 2 in ethereal solvents affords the dimeric zincates M 2 [Zn(OCMe 3 ) 3 ] 2 ( 1 M = Na, 2 M = K). The sodium derivative begins to sublime under vacuum at 50°C, and the potassium salt starts to sublime under vacuum at 90°C. Both zincates are soluble in hydrocarbon solvents. The X-ray crystal structures of both compounds were determined. The centrosymmetric structures consist of a (ZnOCMe 3 ) 2 ring with four terminal OCMe 3 groups. An alkali metal ison is positioned above and below the Zn 2 O 2 plane, each bonded to three oxygen atoms.

Structure and properties of heterometallic alkoxides containing copper(I)

Abstract The heterometallic Cu I alkoxides Li 4 Cu 4 (OCMe 3 ) 8 ( 1 ) and Ba 2 Cu 4 (OCEt 3 ) 8 ( 3 ) were synthesized from the component single metal alkoxides in anhydrous THF solution. Only 1 sublimes without decomposition under vacuum (150°C); 3 is non-volatile. Crystal structures were determined for 1, 3 and the previously reported Na 4 Cu 4 (OCEt 3 ) 8 ( 2 ). Compounds 1 and 2 have nearly identical M 4 Cu 4 O 8 cores consisting of two puckered M 2 O 2 rings related by a centre of symmetry and joined at the oxygens by two bridging copper atoms, and at the M edges by two nearly linear OCuO units. Compound 3 has a Ba 2 (OR) 2 ring containing the centre of symmetry and is bridged by two Cu 2 (OR) 3 units that are nearly perpendicular to the Ba 2 O 2 ring. The two BaCu 2 O 3 planes in 3 are parallel, but separated by 0.48 A. Hydrolysis of compounds 1–3 in THF afforded an orange or orange-brown solid that was converted to the mixed metal oxide by thermolysis, but copper and Cu 2 O were also produced from the Li 4 Cu 4 (OCMe 3 ) 8 hydrolysis product.

Sodium fluoride thin films by chemical vapor deposition

Abstract Thin films of sodium fluoride have been deposited for the first time by chemical vapor deposition of the fluorinated metal organic precursors sodium hexafluoroisopropoxide, sodium perfluoro- t -butoxide, sodium hexafluoroacetylacetonate and sodium heptafluoro-octadionate. The best quality films were obtained using sodium hexafluoroisopropoxide in vacuo at a substrate temperature of 250–300°C.

Electrical and ionic conductivity effects on magic-angle spinning nuclear magnetic resonance parameters of CuI

We investigate experimentally and theoretically the effects of two different types of conductivity, electrical and ionic, upon magic-angle spinning NMR spectra. The experimental demonstration of these effects involves (63)Cu, (65)Cu, and (127)I variable temperature MAS-NMR experiments on samples of γ-CuI, a Cu(+)-ion conductor at elevated temperatures as well as a wide bandgap semiconductor. We extend previous observations that the chemical shifts depend very strongly upon the square of the spinning-speed as well as the particular sample studied and the magnetic field strength. By using the (207)Pb resonance of lead nitrate mixed with the γ-CuI as an internal chemical shift thermometer we show that frictional heating effects of the rotor do not account for the observations. Instead, we find that spinning bulk CuI, a p-type semiconductor due to Cu(+) vacancies in nonstoichiometric samples, in a magnetic field generates induced AC electric currents from the Lorentz force that can resistively heat the sample by over 200 °C. These induced currents oscillate along the rotor spinning axis at the spinning speed. Their associated heating effects are disrupted in samples containing inert filler material, indicating the existence of macroscopic current pathways between micron-sized crystallites. Accurate measurements of the temperature-dependence of the (63)Cu and (127)I chemical shifts in such diluted samples reveal that they are of similar magnitude (ca. 0.27 ppm/K) but opposite sign (being negative for (63)Cu), and appear to depend slightly upon the particular sample. This relationship is identical to the corresponding slopes of the chemical shifts versus square of the spinning speed, again consistent with sample heating as the source of the observed large shift changes. Higher drive-gas pressures are required to spin samples that have higher effective electrical conductivities, indicating the presence of a braking effect arising from the induced currents produced by rotating a conductor in a homogeneous magnetic field. We present a theoretical analysis and finite-element simulations that account for the magnitude and rapid time-scale of the resistive heating effects and the quadratic spinning speed dependence of the chemical shift observed experimentally. Known thermophysical properties are used as inputs to the model, the sole adjustable parameter being a scaling of the bulk thermal conductivity of CuI in order to account for the effective thermal conductivity of the rotating powdered sample. In addition to the dramatic consequences of electrical conductivity in the sample, ionic conductivity also influences the spectra. All three nuclei exhibit quadrupolar satellite transitions extending over several hundred kilohertz that reflect defects perturbing the cubic symmetry of the zincblende lattice. Broadening of these satellite transitions with increasing temperature arises from the onset of Cu(+) ion jumps to sites with different electric field gradients, a process that interferes with the formation of rotational echoes. This broadening has been quantitatively analyzed for the (63)Cu and (65)Cu nuclei using a simple model in the literature to yield an activation barrier of 0.64 eV (61.7 kJ/mole) for the Cu(+) ion jumping motion responsible for the ionic conductivity that agrees with earlier results based on (63)Cu NMR relaxation times of static samples.

Impact of ultraviolet-laser heating on the photoluminescence of ensembles of GaN microcrystallites

We present optical analysis concerning the redshift of the photoluminescence (PL) of ensembles of GaN microcrystals. We found that the extent of the redshift depends on the laser power as well as on the size of the ensemble. For ensembles of ∼30 μm, the laser power in our experimental specification impacted the PL energy and caused a redshift of up to 120 meV. This phenomenon was not observed for a small ensemble of ∼1 μm or less. For the small ensemble, the PL redshift was negligible and depended weakly on the laser power; similar behavior was found in GaN thin film. The above findings were observed in the PL of GaN microcrystalline of wurtzite as well as the cubic structure. Our results point to a laser heating event occurring in the large ensemble; the emitted scattered light is confined among the microcrystallites thus causing heating. For a small ensemble, the light has a higher probability of diffusing outside the enclosure, and thus no laser heating occurs.

Ultraviolet Raman scattering of GaN nanocrystallites: Intrinsic versus collective phenomena

Resonant Raman scattering in wurtzite structured GaN nanocrystallites of various morphologies were studied. The polar mode A1(LO) exhibited Frohlich-type resonant Raman scattering whose characteristics were found to depend weakly on the morphology of the crystallites. In contrast, the UV-laser heating and heat retention in the porous media of a crystallite ensemble were discovered to drastically modify the Raman properties: A Raman thermal redshift was observed that might mask any redshift due to the confinement effect. The thermal redshift was found to depend on the laser power and on the ensemble size. An ensemble temperature on the order of 550K was inferred from the electron–phonon interaction model, a result that was verified via Raman scattering experiments at the elevated temperature regime. For a small ensemble that contains ∼10–20 crystallites and with nominal laser-heating effect, the Raman line shape was found to have mainly a Lorentzian component indicative of phonon-lifetime broadening mechan...