Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of adsorbed SiH(3) radical precursor dissociation on surfaces of MD-grown a-Si:H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55 eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20 eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si:H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50 eV. The results are consistent with experimental measurements of a-Si:H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si:H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH((s)), becomes increasingly more dominant as the temperature is increased.     

Thermally activated mechanisms of hydrogen abstraction by growth precursors during plasma deposition of silicon thin films

Hydrogen abstraction by growth precursors is the dominant process responsible for reducing the hydrogen content of amorphous silicon thin films grown from SiH(4) discharges at low temperatures. Besides direct (Eley-Rideal) abstraction, gas-phase radicals may first adsorb on the growth surface and abstract hydrogen in a subsequent process, giving rise to thermally activated precursor-mediated (PM) and Langmuir-Hinshelwood (LH) abstraction mechanisms. Using results of first-principles density functional theory (DFT) calculations on the interaction of SiH(3) radicals with the hydrogen-terminated Si(001)-(2x1) surface, we show that precursor-mediated abstraction mechanisms can be described by a chemisorbed SiH(3) radical hopping between overcoordinated surface Si atoms while being weakly bonded to the surface before encountering a favorable site for hydrogen abstraction. The calculated energy barrier of 0.39 eV for the PM abstraction reaction is commensurate with the calculated barrier of 0.43-0.47 eV for diffusion of SiH(3) on the hydrogen-terminated Si(001)-(2x1) surface, which allows the radical to sample the entire surface for hydrogen atoms to abstract. In addition, using the same type of DFT analysis we have found that LH reaction pathways involve bond breaking between the silicon atoms of the chemisorbed SiH(3) radical and the film prior to hydrogen abstraction. The LH reaction pathways exhibit energy barriers of 0.76 eV or higher, confining the abstraction only to nearest-neighbor hydrogens. Furthermore, we have found that LH processes compete with radical desorption from the hydrogen-terminated Si(001)-(2x1) surface and may be suppressed by the dissociation of chemisorbed SiH(3) radicals into lower surface hydrides. Analysis of molecular-dynamics simulations of the growth process of plasma deposited silicon films have revealed that qualitatively similar pathways for thermally activated hydrogen abstraction also occur commonly on the amorphous silicon growth surface.     

Interactions between radical growth precursors on plasma-deposited silicon thin-film surfaces

We present a detailed analysis of the interactions between growth precursors, SiH3 radicals, on surfaces of silicon thin films. The analysis is based on a synergistic combination of density functional theory calculations on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of film growth on surfaces of MD-generated hydrogenated amorphous silicon (a-Si:H) thin films. In particular, the authors find that two interacting growth precursors may either form disilane (Si2H6) and desorb from the surface, or disproportionate, resulting in the formation of a surface dihydride (adsorbed SiH2 species) and gas-phase silane (SiH4). The reaction barrier for disilane formation is found to be strongly dependent on the local chemical environment on the silicon surface and reduces (or vanishes) if one/both of the interacting precursors is/are in a "fast diffusing state," i.e., attached to fivefold coordinated surface Si atoms. Finally, activation energy barriers in excess of 1 eV are obtained for two chemisorbed (i.e., bonded to a fourfold coordinated surface Si atom) SiH3 radicals. Activation energy barriers for disproportionation follow the same tendency, though, in most cases, higher barriers are obtained compared to disilane formation reactions starting from the same initial configuration. MD simulations confirm that disilane formation and disproportionation reactions also occur on a-Si:H growth surfaces, preferentially in configurations where at least one of the SiH3 radicals is in a "diffusive state." Our results are in agreement with experimental observations and results of plasma process simulators showing that the primary source for disilane in low-power plasmas may be the substrate surface.     

Ab initio chemical kinetics for SiH3 reactions with Si(x)H2x+2 (x = 1-4)

Gas-phase kinetics and mechanisms of SiH(3) reactions with SiH(4), Si(2)H(6), Si(3)H(8), and Si(4)H(10), processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH(3) reactions with these silanes occur by H abstraction, leading to the formation of SiH(4) + Si(x)H(2x+1) (silanyl) radicals. For both Si(3)H(8) and n-Si(4)H(10) reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH(4) + s-Si(3)H(7) and SiH(4) + s-Si(4)H(9), respectively. In the i-Si(4)H(10) reaction, tertiary Si-H abstraction has the lowest barrier producing SiH(4) + t-Si(4)H(9). In addition, direct SiH(3)-for-X substitution reactions forming Si(2)H(6) + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH(3) reactions with the analogous CH(3) reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si(4)H(10) isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.     

Interstitialcy diffusion of oxygen in tetragonal La2CoO(4+δ)

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La(2)CoO(4+δ). The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La(2)CoO(4+δ). Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principles-based calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling-untilting of the CoO(6) octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800-1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT- and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31-0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden-Popper structures.     

Monobridged Si2H4

The rotational spectrum of a new monobridged isomer of Si(2)H(4), denoted here as H(2)Si(H)SiH, has been detected by Fourier transform microwave spectroscopy of a supersonic molecular beam through the discharge products of silane. On the basis of high-level coupled cluster theory, this isomer is calculated to lie only 7 kcalmol above disilene (H(2)SiSiH(2)), the most stable isomeric arrangement of Si(2)H(4), and to be fairly polar, with a calculated dipole moment of mu = 1.14 D. The rotational spectrum of H(2)Si(H)SiH exhibits closely spaced line doubling, characteristic of a molecule undergoing high-frequency inversion. Transition state calculations indicate that inversion probably occurs in two steps: migration of the bridged hydrogen atom to form silylsilylene, H(3)SiSiH, and then internal rotation of the SiH(3) group, followed by the reverse process. The potential energy surface for this type of inversion is quite shallow, with a barrier height of only 2-3 kcalmol. Searches for the rotational lines of silylsilylene, calculated to be of comparable stability to H(2)Si(H)SiH but about five times less polar (mu = 0.23 D), have also been undertaken, so far without success, even though strong lines of H(2)Si(H)SiH have been detected. The favorable energetics and high polarity of monobridged Si(2)H(4) with respect to either disilene or silylsilylene make it a plausible candidate for radioastronomical detection in sources such as IRC + 10216, where comparably large silicon molecules such as SiS, SiC(3), and SiC(4) have already been discovered.     

Density functional theory study of one-dimensional growth of styrene on the hydrogen-terminated Si(001)-(3 x 1) surface

Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.     

Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)-(2 x 2)

First-principles pseudopotential calculations, within a simple dynamically constrained scheme, have been performed to investigate the reaction of 0.25 ML coverage of SiH4 and Si2H6 with the Si(001)-(2 x 2) surface. The silane molecule (SiH4) is adsorbed on to the surface at a number of different sites (on dimer, interrow, or intrarow) with varying barrier heights. Two distinct structures, which are similar in energy, arise from the initial dissociative reaction SiH4-->SiH3(silyl) + H, where the dissociated species are adsorbed either on the same dimer components or on adjacent dimer components. Several further decays of silyl from SiH4 are presented in two separate regimes of high and low ambient hydrogen coverages. The decomposition of silyl can form two different bridging structures: an on top or an intrarow bridging structure in both of the two hydrogen coverage regimes. The disilane molecule (Si2H6) is also adsorbed upon this surface with varying energy barriers, resulting in a dissociation reaction where two SiH3 species are adsorbed on one dimer or in an adjacent dimer configuration. Plausible energy reaction paths for the above models are presented. The stability of the SiH2 species is also discussed.     

Surface diffusion and incorporation of adatom in Co/Al (0 0 1) system

The surface diffusion and the incorporation of an adatom on early stage of Co/Al (001) interface formation was investigated using the density functional theory (DFT). The energy barrier for the surface diffusion (migration of Co adatom to an adjacent hollow site passing the bridge site) was calculated as 1.01 eV. Large displacement of neighboring Al atoms was accompanied by the surface diffusion of Co adatom. For incorporation process, the energy barrier was 0.39 eV, only 38.6% of the barrier for surface diffusion and the energy gain of the system was 0.43 eV. After the incorporation process was completed, the Co adatom and Al atoms formed seven Co-Al bonds of highly coordinated B2-like configuration.     

Equilibrium properties of Ag clusters on Pt(111)

Molecular dynamics simulations (MD) and Embedded Atom Method (EAM) potentials are employed to investigate the properties of small Ag clusters deposited on the (111) surface of Pt. The computation of the dissociation energies reveals the most stable structures up to 19 atoms, while MD simulations allow to extract the dynamical diffusion barrier for adatom, dimer and trimer. We obtain dissociation energies in the range of 0.3 eV for the trimer and 0.45 eV for the heptamer and a migration energy as low as 0.05 eV for the adatom.     

Theoretical study of the Cp2Zr-catalyzed hydrosilylation of ethylene. Reaction mechanism including new sigma-bond activation

The Cp(2)Zr-catalyzed hydrosilylation of ethylene was theoretically investigated with DFT and MP2-MP4(SDQ) methods, to clarify the reaction mechanism and the characteristic features of this reaction. Although ethylene insertion into the Zr-SiH(3) bond of Cp(2)Zr(H)(SiH(3)) needs a very large activation barrier of 41.0 (42.3) kcal/mol, ethylene is easily inserted into the Zr-H bond with a very small activation barrier of 2.1 (2.8) kcal/mol, where the activation barrier and the energy of reaction calculated with the DFT(B3LYP) method are given and in parentheses are those values which have been corrected for the zero-point energy, hereafter. Not only this ethylene insertion reaction but also the coupling reaction between Cp(2)Zr(C(2)H(4)) and SiH(4) easily takes place to afford Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with activation barriers of 0.3 (0.7) and 5.0 (5.4) kcal/mol, respectively. This coupling reaction involves a new type of Si-H sigma-bond activation which is similar to metathesis. The important interaction in the coupling reaction is the bonding overlap between the d(pi)-pi bonding orbital of Cp(2)Zr(C(2)H(4)) and the Si-H sigma orbital. The final step is neither direct C-H nor Si-C reductive elimination, because both reductive eliminations occur with a very large activation barrier and significantly large endothermicity. This is because the d orbital of Cp(2)Zr is at a high energy. On the other hand, ethylene-assisted C-H reductive elimination easily occurs with a small activation barrier, 5.0 (7.5) kcal/mol, and considerably large exothermicity, -10.6 (-7.1) kcal/mol. Also, ethylene-assisted Si-C reductive elimination and metatheses of Cp(2)Zr(H)(CH(2)CH(2)SiH(3)) and Cp(2)Zr(CH(2)CH(3))(SiH(3)) with SiH(4) take place with moderate activation barriers, 26.5 (30.7), 18.4 (20.5), and 28.3 (31.5) kcal/mol, respectively. From these results, it is clearly concluded that the most favorable catalytic cycle of the Cp(2)Zr-catalyzed hydrosilylation of ethylene consists of the coupling reaction of Cp(2)Zr(C(2)H(4)) with SiH(4) followed by the ethylene-assisted C-H reductive elimination.     

Mechanism for degradation of Nafion in PEM fuel cells from quantum mechanics calculations

We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH(●), H(●)) only when in the presence of H(2), O(2), and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H(2) (or H(+)) and O(2) in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S-C bond to form H(2)SO(4) plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H(2) crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C-F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF(2), SCF(2)) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism.     

Chemical and electrical passivation of single-crystal silicon(100) surfaces through a two-step chlorination/alkylation process

Single-crystal Si(100) surfaces have been functionalized by using a two-step radical chlorination-Grignard (R = MgCl, R = CH3, C2H5, C4H9, C6H5, or CH2C6H5) alkylation method. After alkylation, no chlorine was detectable on the surface by X-ray photoelectron spectroscopy (XPS), and the C 1s region showed a silicon-induced peak shift indicative of a Si-C bond. The relative intensity of this peak decreased, as expected, as the steric bulk of the alkyl increased. Despite the lack of full alkyl termination of the atop sites of the Si(100) surface, functionalization significantly reduced the rate of surface oxidation in air compared to that of the H-terminated Si(100) surface, with alkylated surfaces forming less than half a monolayer of oxide after over one month of exposure to air. Studies of the charge-carrier lifetime with rf photoconductivity decay methods indicated a surface recombination velocity of <30 cm s(-1) for methylated surfaces, and <60 cm s(-1) for Si surfaces functionalized with the other alkyl groups evaluated. Soft X-ray photoelectron spectroscopic data indicated that the H-Si(100) surfaces were terminated by SiH, SiH2, and SiH3 species, whereas Cl-Si(100) surfaces were predominantly terminated by monochloro (SiCl and SiHCl) and dichloro (SiCl2 and SiHCl2) Si species. Methylation produced signals consistent with termination by Si-alkyl bonding arising from SiH(CH3)-, SiH2(CH3)-, and Si(CH3)2-type species.     

H2 dissociation on individual Pd atoms deposited on Cu(111)

We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.     

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.     

A DFT study of SiH(4) activation by Cp(2)LnH

A theoretical study of SiH(4) activation by Cp(2)LnH complexes for the entire series of lanthanides has been carried out at the DFT-B3PW91 level of theory. The reaction paths corresponding to H/H exchange and silylation, formation of Cp(2)Ln(SiH(3)), have been computed. They both occur via a single-step sigma-bond metathesis mechanism. For the athermal H/H exchange reaction, the calculated activation barrier averages 1.8 kcal.mol(-)(1) relative to the precursor adduct Cp(2)LnH(eta(2)-SiH(4)) for all lanthanide elements. The silylation path is slightly exogenic (DeltaE approximately -6.5 kcal.mol(-1)) with an activation barrier averaging 5.2 kcal.mol(-1) relative to the precursor adduct where SiH(4) is bonded by two Si-H bonds. Both pathways are therefore thermally accessible. The H/H exchange path is calculated to be kinetically more favorable whereas the silylation reaction is thermodynamically preferred. The reactivity of this familly of lanthanide complexes with SiH(4) contrasts strongly with that obtained previously with CH(4). The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent.     

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.     

Assessing alkyl-, silyl-, and halo-substituent effects on the electron affinities of silyl radicals

Neutral anion energy differences for a large class of alpha-substituted silyl radicals have been computed to determine the effect of alkyl, silyl, and halo substituents on their electron affinities. In particular, we report theoretical predictions of the adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) for a series of methyl-, silyl-, and halo-substituted silyl radical compounds. This work utilizes the carefully calibrated DZP++ basis set, in conjunction with the pure BLYP and OLYP functionals, as well as with the hybrid B3LYP, BHLYP, PBE1PBE, MPW1K, and O3LYP functionals. Bromine has the largest effect in stabilizing the anions, and the BLYP/DZP++ AEA for SiBr(3) is 3.29 eV. The other predicted electron affinities are for SiH(3) (1.37 eV), SiH(2)CH(3) (1.09 eV), SiH(2)F (1.54 eV), SiH(2)Cl (1.94 eV), SiH(2)Br (2.05 eV), SiH(2)(SiH(3)) (1.77 eV), SiH(CH(3))(2) (0.92 eV), SiHF(2) (1.86 eV), SiHCl(2) (2.53 eV), SiHBr(2) (2.67 eV), Si(CH(3))(3) (0.86 eV), SiF(3) (2.66 eV), SiCl(3) (3.21 eV), Si(SiH(3))(3) (2.25 eV), and SiFClBr (3.13 eV). For the five silyl radicals where experimental data are available, the BLYP functional gives the most accurate determination of AEAs; the average absolute error is 0.04(1) eV, whereas the corresponding errors for the O3LYP, MPW1K, PBE1PBE, B3LYP, OLYP, and BHLYP functionals are 0.05(8), 0.06(0), 0.06(3), 0.08(5), 0.11(5), and 0.15(3) eV, respectively.     

Synthesis and fundamental studies of (H3Ge)xSiH4-x molecules: precursors to semiconductor hetero- and nanostructures on Si

The synthesis of the entire silyl-germyl sequence of molecules (H(3)Ge)(x)SiH(4)(-)(x) (x = 1-4) has been demonstrated. These include the previously unknown (H(3)Ge)(2)SiH(2), (H(3)Ge)(3)SiH, and (H(3)Ge)(4)Si species as well as the H(3)GeSiH(3) analogue which is obtained in practical high-purity yields as a viable alternative to disilane and digermane for semiconductor applications. The molecules are characterized by FTIR, multinuclear NMR, mass spectrometry, and Rutherford backscattering. The structural, thermochemical, and vibrational properties are studied using density functional theory. A detailed comparison of the experimental and theoretical data is used to corroborate the synthesis of specific molecular structures. The (H(3)Ge)(x)SiH(4)(-)(x) family of compounds described here is not only of intrinsic molecular interest but also provides a unique route to a new class of Si-based semiconductors including epitaxial layers and coherent islands (quantum dots), with Ge-rich stoichiometries SiGe, SiGe(2), SiGe(3), and SiGe(4) reflecting the Si/Ge content of the corresponding precursor. The layers grow directly on Si(100) at unprecedented low temperatures of 300-450 degrees C and display homogeneous compositional and strain profiles, low threading defect densities, and atomically planar surfaces circumventing entirely the need for conventional graded compositions or lift-off technologies. The activation energies of all Si-Ge hydride reactions on Si(100) (E(a) approximately 1.5-2.0 eV) indicate high reactivity profiles with respect to H(2) desorption, consistent with the low growth temperatures of the films. The quantum dots are obtained exclusively at higher temperatures (T > 500 degrees C) and represent a new family of Ge-rich compositions with narrow size distribution, defect-free microstructures, and homogeneous, precisely tuned elemental content at the atomic level.     

The ability of silylenes to bind excess electrons: electron affinities of SiX(2), and SiXY species (X,Y=H,CH(3),SiH(3),F,Cl,Br)

Recently, Ishida and co-workers have isolated silylene radical anions via the one-electron reduction of isolable cyclic dialkylsilylenes, discovering these corresponding radical anions to be relatively stable at low temperatures. Herein we report theoretical predictions of the adiabatic electron affinities (AEA), vertical electron affinities, and vertical detachment energies of a series of methyl, silyl, and halosubstituted silylene compounds. This research utilizes the carefully calibrated DZP++ basis with the combination of the popular nonhybrid and hybrid DFT functionals, BLYP, B3LYP, and BHHLYP. The level of theory employed and the ensemble of species under study confirm the ability of silylenes to bind excess electrons with Si(SiH(3))(2) being the most effective, having a predicted AEA of 1.95 eV. While it is known that methyl substituents have a diminishing effect on the computed electron affinities (EAs), it is shown that fluorine shows an analogous negative effect. Similarly, previous suggestions that Si(CH(3))(2) will not bind an electron appear incorrect, with EA[Si(CH(3))(2)] predicted here to be 0.46 eV.