Anthony J. Muscat

Water-Based Route to Ligand-Selective Synthesis of ZnSe and Cd-Doped ZnSe Quantum Dots with Tunable Ultraviolet A to Blue Photoluminescence

A water-based route has been demonstrated for synthesizing ZnSe and Cd-doped ZnSe (Zn(x)Cd(1-x)Se, 0 < x < 1) quantum dots (QDs) that have tunable and narrow photoluminescence (PL) peaks from the ultraviolet A (UVA) to the blue range (350-490 nm) with full-width at half-maximum (fwhm) values of 24-36 nm. Hydrazine (N(2)H(4)) was used to maintain oxygen-free conditions, allowing the reaction vessel to be open to air. The properties of the QDs were controlled using the thiol ligands, 3-mercaptopropionic acid (MPA), thiolglycolic acid (TGA), and l-glutathione (GSH). On the basis of optical spectra, linear three-carbon MPA attenuated nucleation and growth, yielding small ZnSe QDs with a high density of surface defects. In contrast, TGA and GSH produced larger ZnSe QDs with lower surface defect densities. The absorption spectra show that growth was more uniform and better controlled with linear two-carbon TGA than branched bifunctional GSH. After 20 min of growth TGA-capped ZnSe had an average diameter of 2.5 nm based on high-resolution transmission electron microscopy images; these nanocrystals had an absorbance peak maximum of approximately 340 nm (3.65 eV) and a band gap PL emission peak at 372 nm (3.34 eV). Highly fluorescent Zn(x)Cd(1-x)Se QDs were fabricated by adding a Cd-thiol complex directly to ZnSe QD solutions; PL peaks were tuned in the blue range (400-490 nm) by changing the Zn to Cd ratio. The Cd-bearing nanocrystals contained proportionally more Se based on X-ray photoelectron spectroscopy, and Cd-Se bonds had ionic character, in contrast to primarily covalent Zn-Se bonds.

Simple colloidal synthesis of single-crystal sb-se-s nanotubes with composition dependent band-gap energy in the near-infrared

We report the first synthesis of high-quality binary and ternary Sb(2)Se(3-x)S(x) nanotubes across the entire compositional range from x = 0 to 3 via a simple, low-cost, colloidal synthetic method of injection of Sb(III)-complex solution into a hot paraffin liquid containing Se, S, or a mixture thereof. In contrast to the classic rolling mechanism, the modular formation of the reported nanotubes follows a four-stage self-seeding process: (i) amorphous nanospheres, (ii) short crystalline nanotubes growing out of relatively large amorphous nanospheres, (iii) long crystalline nanotubes attached to small amorphous nanospheres, and (iv) single-crystal nanotubes. The obtained single-crystal nanotubes have tunable composition, orthorhombic phase, well-defined rectangular cross sections, and growth direction along [001], as revealed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction studies. UV-vis-NIR absorption spectroscopy reveals that the optical bandgap energy of the Sb(2)Se(3-x)S(x) (0 < or = x < or = 3) nanotubes increases quadratically with the sulfur concentration x with these bandgap energies falling in the range from 1.18 to 1.63 eV at the red edge of the solar spectrum. The present study opens a new avenue to low-cost, large-scale synthesis of high quality semiconductor nanotubes with technological applications in solar energy conversion and also for a wide range of optical nanodevices operating in the near-infrared.

A new route to self-assembled tin dioxide nanospheres: fabrication and characterization.

Nearly monodispersed self-assembled tin dioxide (SnO2) nanospheres with intense photoluminescence (PL) were synthesized using a new wet chemistry technique. Instead of coprecipitating stannous salts, bulk tin (Sn) metal was oxidized at room temperature in a solution of hydrogen peroxide and deionized water containing polyvinylpyrrolidone (PVP) and ethylenediamine (EDA). SnO2 nanocrystals were produced with diameters of approximately 3.8 nm that spontaneously self-assembled into uniform SnO2 nanospheres with diameters of approximately 30 nm. Analysis was performed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, UV-vis absorption spectroscopy, PL spectroscopy, and fluorescence lifetime measurements. The SnO2 nanospheres displayed room-temperature purple luminescence with an intense band at 394 nm (approximately 3.15 eV) and a high quantum yield of approximately 15%, likely as a result of emission from the surface states of SnO2/PVP complexes. The present study could open a new avenue to large-scale synthesis of self-assembled functional oxide nanostructures with technological applications as purple emitters, biological labels, gas sensors, lithium batteries, and dye-sensitized solar cells.

Kinetics and Mechanism for the Reaction of Hexafluoroacetylacetone with CuO in Supercritical Carbon Dioxide

A kinetic model and mechanism were developed for the heterogeneous chelation reaction of thin CuO films with hexafluoroacetylacetone (hfacH) in supercritical CO2. This reaction has relevance for processing nanoscale structures and, more importantly, serves as a model system to tune the reaction behavior of solids using supercritical fluids. Precise control over reaction conditions enabled accurate etching rates to be measured as a function of both temperature [(53.5-88.4) +/- 0.5 degrees C] and hfacH concentration (0.3-10.9 mM), yielding an apparent activation energy of 70.2 +/- 4.1 kJ/mol and an order of approximately 0.6 with respect to hfacH. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the CuO surface, and a maximum etching rate of 24.5 +/- 3.1 A/min was obtained. Solvation forces between hfacH and the dense CO2 permitted material removal at temperatures more than 100 degrees C lower than that of the analogous gas-phase process. In the low concentration regime, the etching reaction was modeled with a three-step Langmuir-Hinshelwood mechanism. Small amounts of excess water nearly doubled the reaction rate through the proposed formation of a hydrogen-bonded hfacH complex in solution. Further increases in the hfacH concentration up to 27.5 mM caused a shift to first-order kinetics and an adsorption-limited or Rideal-Eley mechanism. These results demonstrate that relatively modest increases in concentration can prompt a heterogeneous reaction in supercritical CO2 to switch from a mechanism most commonly associated with a low-flux gas to one emblematic of a high-flux liquid.

Synthesis of two-dimensional single-crystal berzelianite nanosheets and nanoplates with near-infrared optical absorption

The solar cell industry requires convenient and inexpensive fabrication of semiconductor nanostructures as highly efficient absorptive layers with low-cost, environmentally benign, heavy-metal-free (i.e., free from Hg, Cd, and Pb) and suitable band gap near 1 eV features. In this paper, we demonstrate the synthesis of two-dimensional single-crystal berzelianite (Cu2−xSe) nanosheets (in-plane diameter-to-thickness ratio ∼100) and nanoplates (in-plane diameter-to-thickness ratio ∼10) via a simple, “green” and environmentally benign method of injecting Cu(I)-complex precursor into Se-solution in paraffin liquid. Unlike the previous syntheses of binary chalcogenide nanostructures such as CdSe, the current strategy for berzelianite synthesis does not use expensive and toxic phosphine ligands such as trioctylphosphine (TOP). The products were characterized by a range of methods, such as X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction, revealing that the products have the cubic phase and high-quality single-crystal two-dimensional nanostructure. UV-Vis-NIR absorption spectroscopy reveals that the nanosheets and nanoplates show obvious absorption onsets at 0.89 eV and 0.80 eV, respectively, and strong optical absorption peak at 1.70 eV and 1.62 eV, covering the whole red range of the solar spectrum. The present study opens a new avenue to “green” and low-cost controllable synthesis of binary chalcogenides with technological applications in solar energy conversion and also in a wide range of photonic devices operating in the near-infrared.

Moisture Absorption and Reaction in BPSG Thin Films

As-deposited (AD) and annealed (500, 750, and 900°C) borophosphosilicate glass (BPSG) films were characterized during aging, baking, and etching using transmission Fourier transform infrared spectroscopy and ellipsometry. BPSG films contained oxides such as Si-O, P=O, P-O, and B-O as well as hydroxyl groups such as SiO-H, HOH, PO-H, and BO-H in a variety of local bonding environments, which became more uniform as the annealing temperature was increased. The water content in the BPSG films increased steadily during storage at ambient conditions. Based on bond strength, polarity, thermodynamics, and FTIR data, the B-O bond is the primary site for water adsorption on the surface of the film. Water absorption within the film was consistent with a reaction-limited model. Water reacted readily with P-O groups forming P=O and PO-H, which H bonds strongly within the film. The slower reaction with P=O moieties is proposed as the rate-limiting step for water absorption. Annealing after deposition strengthened the Si-O lattice, which reduced the affinity to absorb water. Etching rates ranged from 1 to 10 A/s on the films studied. A 200°C bake desorbed water from the surface layer of the films and increased the reaction rate between water and P=O and B-O to form PO-H and BO-H groups. The bulk etching rate was not affected by baking, but the induction time needed to start etching increased to 31 ± 1, 22 ± 2, and 74 ± 24 s for the AD, 500 and 750°C annealed films, respectively, and increased from 45 ± 5 to 72 ± 5 s for the 900°C annealed film.

Ligand-controlled growth of ZnSe quantum dots in water during Ostwald ripening

A strong ligand effect was observed for the aqueous-phase growth of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The QDs were made by injecting Se monomer at room temperature followed by a ramp to 100 °C. The ramp produced a second, more gradual increase in the concentrations of both Zn and Se monomers fed by the dissolution of QDs below the critical size. The dissolution process was followed using measurements of the mass of Zn in QDs and in the supernatant by inductively coupled plasma optical emission spectroscopy (ICP-OES). Despite the flux of monomers, there was little growth in the QDs of average size based on UV-vis absorption spectra, until the temperature reached 100 °C, when there was a period of rapid growth followed by a period of linear growth. The linear growth stage is the result of OR as the total mass of Zn in QDs and in the solvent remained constant. The growth data were fit to a continuum model for the limiting case of surface reaction control. The rate is proportional to the equilibrium coefficient for ligand detachment from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was the most tightly bound to the surface and produced the lowest growth rate of (1.5-2) × 10(-3) nm/min in the OR stage, whereas thiolactic acid (TLA) was the most labile and produced the highest growth rate of 3 × 10(-3) nm/min. Methyl thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in between these values. Ligands containing electron-withdrawing groups closer to the S atom and branching promote growth, whereas longer, possibly bidendate, ligands retard it. Mixed ligand experiments confirmed that growth is determined by ligand bonding strength to the QD. Photoluminescence spectroscopy showed that the more labile the ligand, the more facile the repair of surface defects during the exposure of the QDs to room light.

Condensation of silanol groups in porous methylsilsesquioxane films using Supercritical CO/sub 2/ and alcohol cosolvents

Fourier transform infrared (FTIR) spectroscopy, goniometry, and electrical measurements were used to investigate the effect of adding alcohol and carboxylic acid cosolvents to supercritical carbon dioxide (scCO/sub 2/) to condense silanol groups in blanket porous methylsilsesquioxane (p-MSQ) films (JSR LKD 5109). The aliphatic C1-C6 alcohols removed approximately 50% more hydrogen-bonded silanol (SiO-H) groups than pure scCO/sub 2/, leaving isolated silanol groups on the surfaces of the pores. Acetic acid removed H-bonded silanols but left fewer isolated moieties. On a molar basis, n-propanol, isopropanol, and n-butanol removed the largest percentage of silanols per molecule of cosolvent. These cosolvents were also among the lowest vapor pressure cosolvents studied, making them the most environmentally acceptable. As-received ashed ultralow-k MSQ had a contact angle of less than 10/spl deg/ and a dielectric constant of 3.5/spl plusmn/0.1. After processing in a mixture containing 7% n-propanol and scCO/sub 2/, the contact angle was 15/spl deg/ and the dielectric constant decreased to 3.2 /spl plusmn/ 0.1. The surface was hydrophilic after processing in mixtures of cosolvents and scCO/sub 2/ because of the isolated silanol groups on the surface. A comparison of the trends across the alcohol series indicates that cosolvent addition to scCO/sub 2/ increased the solubility of water in the supercritical fluid mixture compared to pure scCO/sub 2/. Within the same class of molecules, the solubility of the cosolvent in the supercritical fluid is a more important selection criterion than the solubility of water in the cosolvent.

The effect of site distribution on desorption kinetics: carbon monoxide from Ni(100)

Abstract The effect of binding site distribution of CO on its desorption kinetics from Ni(100) was investigated using temperature program desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Desorption of CO was studied from either surfaces presaturated at 100 K and annealed to 250 K (θ co = 0.59 ML), 342 K (θ co = 0.49 ML), and 414 K (θ co = 0.21 ML) or by dosing to similar coverages without anneal at 100 K. The TPD spectra indicate that the initial desorption kinetics are significantly different for the two methods of preparation. Desorption kinetic parameters, E d and v d , obtained using the threshold temperature program desorption (TTPD) analysis without anneal were 114 ± 2 kJ mol and 8.1 × 10 15 s −1 at 0.18 ML and 32 ± 2 kJ mol and 10 7 s −1 at 0.50 ML. At both of these coverages CO is initially distributed nearly equally between atop and bridged sites. Annealing the adlayer led to the dominance of atop sites in the adsorbate population, yielding less coveragesensitive desorption kinetics. At θ co = 0.21 ML, E d and v d were 128 ± 4 kJ mol and 3.9 × 10 16 s −1 and at 0.50 ML they were 92 ± 3 kJ mol and 1.3 × 10 14 s −1 .

Transport of monomer surfactant molecules and hindered diffusion of micelles through porous membranes

Abstract Diffusion of Triton X-100 through Celgard 2500 membranes was examined. The pore permeability for monomers was 5.0 × 10 −6 cm 2 /sec and it was measured for upstream concentrations below the CMC value of 2.29 × 10 −4 M at 30°C. This value is close to the monomer diffusion coefficient in bulk suggesting that the monomers do not experience significant hindrance due to the pore walls. The permeability of the surfactant drops abruptly within a narrow range of reservoir solution concentrations in the vicinity of the CMC. At concentrations 10 × CMC, the permeability coefficient becomes constant and equal to 3.9 × 10 −7 cm 2 /sec which is the pore permeability for the Triton X-100 micelles. Compared to the diffusion coefficient of micelles in bulk water, the transport of micelles is hindered by the pore walls. In a 10-fold concentration range the micellar pore permeability is practically constant indicating no large change in micelle size. The chemical equilibrium model applied to surfactant diffusion in pores shows reasonable agreement over the entire range of the experimental data for reservoir concentrations from one-fifth times the CMC to 100 times the CMC.