Thomas J Preston

Soldrevet Chemistry · Sustainable Development

In modern Li-based batteries, alloying anode materials have the potential to drastically improve the volumetric and specific energy storage capacity. For the past decade silicon has been viewed as a “Holy Grail” among these materials; however, severe stability issues limit its potential. Herein, we present amorphous substoichiometric silicon nitrid...

Amorphous silicon nanoparticles were synthesized through pyrolysis of silane gas at temperatures ranging from 575 to 675 C. According to the used temperature and silane concentration, two distinct types of particles can be obtained: at 625 C, spherical particles with smooth surface and a low degree of aggregation, but at a higher temperature (650 C...

With its high strength, high thermal stability, low density, and high electrical resistance, silicon‑nitride‐based ceramics have been widely used as gate insulating layers, oxidation masks, and passivation layers. Employing SiN nanomaterials in anode applications also improves rate performances and cycling stability of the lithium‐ion batteries. Ho...

The formation of higher order silanes during monosilane pyrolysis is studied using a millimeter scale pyrolysis reactor combined with a tailormade gas chromatography–mass spectrometry (GC–MS) setup. We use monosilane diluted in hydrogen and investigate the effects of monosilane inlet concentration (0–100%) and reactor temperature (450–530 °C) on th...

The needs for rapid deployment of solar panels are myriad. Here I discuss some of the global challenge, but dive into silicon refining using pyrolysis of silane (SiH4). The presentation take a mixed perspective of sociology and physical chemistry.

Silicon in a form of nanoparticles has attracted a significant interest in the field of lithium-ion batteries, due to the enormous capability of lithium intake. While attracting substantial attention for next-generation lithium-ion batteries, the large volume changes due to lithiation/delithiation during charge/discharge significantly restrict wide...

Despite a decade of extensive research, silicon, as material for anodes of lithium ion batteries, still has not become a practical solution. Suffering from enormous expansion/contraction during lithiation/delithiation cycles, silicon undergoes rapid self-destruction resulting in battery failure. To counteract the degradation several options has bee...

We develop and demonstrate an advanced combination of gas chromatography and mass spectrometry (GC-MS) for detection of higher order silanes. In a recent contribution we have successfully identified the three tetrasilane isomers n-tetrasilane (n-Si 4 H 10), silyltrisilane (i-Si 4 H 10) and cyclotetrasilane (cyclo-Si 4 H 8), as well as the four pent...

There is a deficit of ways to detect higher order silane isomers during silane pyrolysis. Thus, a novel instrument utilizing gas chromatography-mass spectrometry (GC-MS) for detection of higher order silanes has been developed. The instrument enables us to separate higher order silane species using gas chromatography before they are introduced to t...

Thermal decomposition experiments with monosilane diluted in hydrogen have been conducted in a free-space reactor with an extendable reaction zone, allowing for easy variation of residence time. Reactor effluent was analyzed by gas-chromatography combined with mass-spectrometry (GC-MS). The applied analysis technique enables detection of silanes wi...

Velocity map imaging (VMI) measurements and quasi-classical trajectory (QCT) calculations on a newly developed, global potential energy surface (PES) combine to reveal the detailed mechanisms of reaction of Cl atoms with n-pentane. Images of the HCl (v = 0, J = 1, 2 and 3) products of reaction at a mean collision energy of 33.5 kJ mol(-1) determine...

Today's most frequently used production path for polysilicon and solar grade silicon (SG-Si) is based on thermal decomposition of trichlorosilane (SiHCl 3) in a Siemens-type reactor. Due to the batch-wise mode of its operation and its requirement for heating and cooling, the Siemens technology is an energy demanding procedure. As an alternative to...

Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br∗), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br∗ a...

Thermal decomposition of monosilane (SiH 4) is an important process step in the production of poly-Si in fluidized bed reactors (FBRs) and centrifugal chemical vapor deposition reactors (CCVDRs). These two reactor technologies both offer energy-saving alternatives to the Siemens reactor, which is the most commonly used reactor technology in today's...

Production of high-purity silicon for solar cells and electronics can be done through several routes. Although the trichlorosilane-based Siemens process is dominating the industry today there is reason to believe that monosilane-based processes and upgraded metallurgical silicon will take over in the long run. This paper gives a short description o...

Dynamics of collisions between structured molecular species quickly become complex as molecules become large. Reactions of methane with halogen and oxygen atoms serve as model systems for polyatomic molecule chemical dynamics, and replacing the atomic reagent with a diatomic radical affords further insights. A new, full-dimensional potential energy...

The dynamics of the reaction of chlorine atoms with alkenes are contrasted in the gas and liquid phases. A combination of experimental measurements and computational simulations distinguishes direct hydrogen-atom abstraction to make HCl from addition to the C=C bond to make chloroalkyl radicals. A proposed roaming pathway from the chloroalky radica...

We report a theoretical investigation of the CH4 + Cl hydrogen abstraction reaction in the framework of empirical valence bond (EVB) theory. The main purpose of this study is to benchmark the EVB method against previous experimental and theoretical work. Analytical potential energy surfaces for the reaction have been developed on which quasi-classi...

Quasi-classical trajectory calculations on a newly constructed and full-dimensionality potential energy surface (PES) examine the dynamics of the reaction of Cl atoms with propene. The PES is an empirical valence bond (EVB) fit to high-level ab initio energies and incorporates deep potential energy wells for the 1-chloropropyl and 2-chloropropyl ra...

Transient electronic and vibrational absorption spectroscopy unravel the mechanisms and dynamics of bimolecular reactions of CN radicals with acetone in deuterated chloroform solutions. The CN radicals are produced by ultrafast ultraviolet photolysis of dissolved ICN. Two reactive forms of CN radicals are distinguished by their electronic absorptio...

Solvent-solute interactions influence the mechanisms of chemical reactions in solution, but the response of the solvent is often slower than the reactive event. Here, we report that exothermic reactions of fluorine (F) atoms in d3-acetonitrile and d2-dichloromethane involve efficient energy flow to vibrational motion of the deuterium fluoride (DF)...

Transient electronic absorption measurements with 1 ps time resolution follow XeF2 photoproducts in acetonitrile and chlorinated solvents. Ultraviolet light near 266 nm promptly breaks one Xe-F bond, and probe light covering 320-700 nm monitors the products. Some of the cleaved F atoms remain in close proximity to an XeF fragment and perturb the el...

Reactions between Cl atoms and propene can lead to HCl formation either by direct H abstraction or through a chloropropyl addition complex. Barring stabilizing collisions, the chloropropyl radical will either decompose to reactants or form HCl and allyl products. Using velocity-map imaging to measure the quantum state and velocity of the HCl produc...

Transient absorption spectroscopy is used to follow the reactive intermediates involved in the first steps in the photochemistry initiated by ultraviolet (266-nm wavelength) excitation of solutions of 1,5-hexadiene, isoprene and 2,3-dimethylbut-2-ene in carbon tetrachloride or chloroform. Ultraviolet and visible bands centred close to 330 nm and 50...

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have ps time resolution and monitor the C-H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C-H stretching transitions of these isomers, in wh...

Using ultrafast pump-probe spectroscopy, we study vibrationally driven bimolecular chemistry in the condensed phase. Our current focus is on the abstraction of hydrogen by the bromine atom from cyclohexane. This reaction has a barrier close in energy to that of a C-H stretch and also avoids isomer and complex complications seen in other systems. We...

Photolysis of chloroiodomethane (CH2ClI) in cryogenic matrices followed by recombination of the nascent radical pair produces an isomer (CH2Cl–I) that features a halogen-halogen (Cl–I) bond. Using ultrafast laser pulses, it is possible to follow the formation of this isomer by transient electronic absorption in low-temperature matrices of N2, CH4,...

Iso-CH2Cl-I is formed in a cryogenic matrix using ultraviolet excitation of it stable precursor, chloroiodomethane (CH2ClI). Excitation of the isomer is effected by a pulse of light from an ultrafast, and its subsequent dynamics are monitored with a second pulse of light.

Photodissociation of species entrained in solid matrices potentially leads to reassociation of the newly formed fragments. After fixing CH_2ClI in various atomic and molecular matrices, we measure ultrafast transient absorptions to monitor the photolysis of the precursor and isomerization to form iso-CH_2ClI. We probe the two lowest energy electron...

We follow up on the previous talk on ultrafast timescale studies of the isomerization between CH_2ClI and CH_2Cl-I in cryogenic matrices. We establish a population of CH_2Cl-I in cryogenic matrices and then pump the two lowest electronic absorption features of CH_2Cl-I near 435 nm and 800 nm. Then we study the formation of CH_2ClI and CH_2Cl-I by p...

We have followed the evolution of the bromine species that arise from the photolysis of bromoform in the condensed phase. Solvent caging promotes the formation of iso-bromoform (CHBr_2-Br), which can then release a Br atom by breaking the newly formed Br-Br bond. This ejected Br can form a van der Waals complex (Br-CHBr_3) with a nearby un-photolyz...

Ultrafast photolysis of bromoform (CHBr(3)) with a 267 nm pulse of light followed by broadband transient electronic absorption identifies the photoproducts and follows their evolution in both neat bromoform and cyclohexane solutions. In neat bromoform, a species absorbing at 390 nm appears promptly and decays with a time constant of 13 ps as anothe...

The bromine radical is an interesting species to interrogate with vibrationally mediated chemistry. It forms a long-lived complex in the liquid phase, and hydrogen abstraction from a solvent molecule is endothermic. Photolysis of a bromine precursor forms an iso-compound within 1 ps, which decays to a radical-solvent complex on a ˜10 ps timescale....

Ultrafast transient absorption experiments monitor the reaction of CN radicals with 16 different alkane, alcohol, and chloroalkane solutes in CH(2)Cl(2) and with a smaller number of representative solutes in CHCl(3) and CH(3)CCl(3). In these experiments, 267-nm photolysis generates CN radicals, and transient electronic absorption at 400 nm probes t...

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational...

Includes vita. Thesis (B.S.) in Chemistry--University of Maine, 2006. Includes bibliographical references (leaves 95-97).

We perform direct MD simulations of the formation of mixed XenArm clusters (500

Motivated by the recent experiments of the Swedish group [M. Tchaplyguine, R. R. Marinho, M. Gisselbrecht et al., J. Chem. Phys. 120, 345 (2004)], we simulate the photoelectron spectra of pure xenon and argon clusters. The clusters are modeled using molecular dynamics with Hartree-Fock-dispersion type pair potentials while the spectrum is calculate...