Browsing by Author "Huynh, Lam K."

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A computational study on the adsorption configurations and reactions of SiHx(x=1-4) on clean and H-covered Si(100) surfaces
(2016-11-30)
Le, Thong N-M
;
Raghunath, P.
;
Huynh, Lam K.
;
Lin, M. C.
;
應用化學系
Possible adsorption configurations of H and SiHx (x= 1 4) on clean and H-covered Si(100) surfaces are determined by using spin-polarized DFT calculations. The results show that, on the clean surface, the gas-phase hydrogen atom and SiH3 radicals effectively adsorb on the top sites, while SiH and SiH2 prefer the bridge sites of the first layer. Another possibility for SiH is to reside on the hollow sites with a triple-bond configuration. For a partially H-coverd Si(100) surface, the mechanism is similar but with higher adsorption energies in most cases. This suggests that the surface species become more stable in the presence of surface hydrogens. The minimum energy paths for the adsorption/migration and reactions of H/SiHx species on the surfaces are explored using the climbing image-nudged elastic band method. The competitive surface processes for Si thin-film formation from SiHx precursors are also predicted. The study reveals that the migration of hydrogen adatom is unimportant with respect to leaving open surface sites because of its high barriers (>29.0 kcal/mol). Alternatively, the abstraction of hydrogen adatoms by H/SiHx radicals is more favorable. Moreover, the removal of hydrogen atoms from adsorbed SiHx, an essential step for forming Si layers, is dominated by abstraction rather than the decomposition processes. (C) 2016 Elsevier B.V. All rights reserved.
Ab Initio Chemical Kinetics for the HCCO plus H Reaction
(2016)
Pham-Cam Nam
;
Raghunath, P.
;
Huynh, Lam K.
;
Xu, Shucheng
;
Lin, M. C.
Ketenyl radical (HCCO) is an important hydrocarbon combustion intermediate. The mechanisms and kinetics for the reaction of HCCO (X(2)A \'\') with H(S-2) occurring on both singlet and triplet surfaces have been studied by a combination of ab initio calculations and rate constant predictions at the CCSD(T)/6-311++ G(3df,2p)//CCSD/6-311++G(d,p) level of theory. The kinetics and product branching ratios have been investigated in the temperature range of 297-3000 K by variational transition state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories for the production of CH2(a(1)A(1)) + CO(X-1 Sigma(+)) and CH2((XB1)-B-3) + CO(X-1 Sigma(+)). Our prediction for the primary product CH2(a(1)A(1)) + CO(X-1 Sigma(+)) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed by k(1) (T) = 8.62 x 10(-11)T(0.16)exp(-20/T) cm(3) molecule(-1) s(-1). For the production of CH2((XB1)-B-3) + CO(X-1 Sigma(+)), the rate constant k(2) can be represented as k(2)(T) = 7.63 x 10(-16)T(1.56)exp(-386/T) cm(3) molecule(-1) s(-1). The predicted product branching ratios for the reaction are in close agreement with experimental data as well. We also predicted the heat of formation at 0 K for (HCCO)-H-2, (CCO)-C-3, and (CCO)-C-1 by CCSD(T)/6-311++ G(3df,2p), CBS-QB3, and G2M; the values are in good agreement among one another. Specifically, the CCSD(T) values are:Delta H-f degrees(HCCO, X2A \'\') = 42.52 +/- 0.70; Delta H-f degrees(CCO, (XSg)-S-3) = 91.50 +/- 0.54; and Delta H-f degrees(CCO, a(1)Delta) = 110.22 +/- 0.54 kcal/mol.