Reactions of some [C, N, O]-containing molecules with Si surfaces: Experimental and theoretical studies

Reaction of Hydrazine with Solution- and Vacuum-Prepared Selectively Terminated Si(100) Surfaces: Pathways to the Formation of Direct Si-N Bonds

The reactivity of liquid hydrazine (N₂H₄) with respect to H-, Cl-, and Br-terminated Si(100) surfaces was investigated to uncover the principles of nitrogen incorporation into the interface. This process has important implications in a wide variety of applications, including semiconductor surface passivation and functionalization, nitride growth, and many others. The use of hydrazine as a precursor allows for reactions that exclude carbon and oxygen, the primary sources of contamination in processing. In this work, the reactivity of N₂H₄ with H- and Cl-terminated surfaces prepared by traditional solvent-based methods and with a Br-terminated Si(100) prepared in ultrahigh vacuum was compared. The reactions were studied with X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy, and the observations were supported by computational investigations. The H-terminated surface led to the highest level of nitrogen incorporation; however, the process proceeds with increasing surface roughness, suggesting possible etching or replacement reactions. In the case of Cl-terminated (predominantly dichloride) and Br-terminated (monobromide) surfaces, the amount of nitrogen incorporation on both surfaces after the reaction with hydrazine was very similar despite the differences in preparation, initial structure, and chemical composition. Density functional theory was used to propose the possible surface structures and to analyze surface reactivity.

Exploration of M(100)-2×1 (M=Si, Ge) surface termination through hydrogen passivation using ethane and ammonia-borane derivatives: A theoretical approach

Termination process of Si(100)-2 × 1 as well as Ge(100)-2 × 1 reconstructed surfaces have been explored comprehensively through the dehydrogenation of ethane and ammonia-borane and their several analogues by employing density functional theory (DFT). From our study, it is evident that the termination of Si-surface via the dehydrogenation of aforementioned ethane and NH₃BH₃ derivatives is more feasible compared to Ge-surface. For ethane, the investigation shows that the substitution of non-participating hydrogens with +I group (electron donating) causes an enhancement in the kinetic and thermodynamic feasibility of the termination process, whereas the implementation of -I substituent (electron withdrawing) makes an adverse effect. While exploring the termination of Si- as well as Ge-surfaces through the dehydrogenation of NH₃BH₃ and its derivatives, it is noticed that from both the kinetic as well as thermodynamic perspectives, the termination processes are more feasible than that of ethane and its derivatives. We have further examined the detailed mechanism of each termination process by analyzing the geometrical parameters and NPA charges. From bonding evaluation, it is evident that the hydrogen abstraction from ethane by both the surfaces is symmetric in nature, where both the hydrogens show slightly positive charge. But for NH₃BH₃ the hydrogen abstraction process becomes asymmetric, where the boron associated hydrogen is abstracted as hydride by the electrophilic surface Si (Ge) and the hydrogen bonded with the N-centre is abstracted as proton by the nucleophilic surface Si (Ge). Overall, the present theoretical work reveals one of the efficient chemical processes for terminating Si as well as Ge(100)-2 × 1 reconstructed surfaces through the formation of non-polar SiH bonds.

Towards a comprehensive understanding of the Si(100)-2×1 surface termination through hydrogen passivation using methylamine and methanol: a theoretical approach

Using density functional theory, we explored the termination process of Si (100)-2 × 1 reconstructed surface mechanistically through the dehydrogenation of small molecules, considering methyl amine and methanol as terminating reagents. At first, both the terminating reagents form two types of adduct through adsorption on the Si (100)-2 × 1 surface, one in chemisorption mode and the other via physisorption, from which the dehydrogenation process is initiated. By analyzing the activation barriers, it was observed that termination of the Si-surface through the dehydrogenation is kinetically almost equally feasible using either reagent. We further examined in detail the mechanism for each termination process by analyzing geometrical parameters and natural population analysis charges. From bonding evaluation, it is evident that hydrogen abstraction from adsorbates on the Si-surface is asymmetric in nature, where one hydrogen is abstracted as hydride by the electrophilic surface Si and the other hydrogen is abstracted as proton by the neucleophilic surface Si. Moreover, it was also observed that hydride transfer from adsorbate to the Si-surface occurs first followed by proton transfer. Overall, our theoretical interpretation provides a mechanistic understanding of the Si (100)-2 × 1 reconstructed surface termination by amine and alcohol that will further motivate researchers to design different types of decorated semiconductor devices. Graphical Abstract Surface termination process of Si(100)-2×1 through formation of non-polar Si-H bonds via dehydrogenation of methylamine and methanol as terminating reagents.

Adsorption of Homotrifunctional 1,2,3-Benzenetriol on a Ge(100)-2 × 1 Surface

The adsorption of the homotrifunctional 1,2,3-benzenetriol on Ge(100)-2 × 1 has been investigated by density functional theory calculations, Fourier transform infrared spectroscopy, and X-ray-photoelectron spectroscopy. The results show that the adsorption can occur through OH dissociation of all three hydroxyl groups, and that all three reaction pathways are kinetically and thermodynamically favorable. A coverage-dependent analysis shows that at low coverage, the molecule reacts to form a mix of trifold and dually bound adsorbates. As the coverage increases, the reactions are limited to dissociative adsorption through single and dual attachments. Calculations on the three possible dually bound configurations further reveals that the dissociative adsorption of the third hydroxyl group is limited by geometrical constraints to only two reaction channels. Finally, the proximity between OH-groups in the molecule favors intra- and intermolecular hydrogen bonding, which stabilizes singly and dually bound adsorbate configurations and limits the reactivity of the functional groups.

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

The reaction of benzyl isocyanide, tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide with tetramesityldigermene (Mes₂ Ge=GeMes₂ ) was examined. Whereas the addition of benzyl isocyanide gave the C-NC activation product, Mes₂ Ge(CH₂ Ph)Ge(CN)Mes₂ , tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide did not give stable adducts, rather the rate of conversion of the digermene to the corresponding cyclotrigermane was accelerated. A comparison between the reactivity of the isocyanides with Mes₂ Ge=GeMes₂ and the Ge(100)-2×1 surface was made and some insights into the surface chemistry are offered.

The addition of nitriles to tetramesityldisilene: a comparison of the reactivity between surface and molecular disilenes

The addition of acetonitrile, propionitrile, and phenylacetonitrile to tetramesityldisilene (Mes2 Si=SiMes2 ) was examined. In general, 1,2,3-azadisiletines and the tautomeric enamines were formed, although a ketenimine was formed as the major product in the addition of phenylacetonitrile to the disilene. In the presence of LiCl, the mode of addition changed for both acetonitrile and propionitrile: insertion into the α-CH bond of acetonitrile and/or formation of the formal HCN adduct was observed. Preliminary investigations of the reactivity of the nitrile adducts are also reported. A comparison between the reactivity of nitriles with Mes2 Si=SiMes2 and the Si(100)-2×1 surface was made both in terms of the types of adducts formed and their reactivity. Some insights into the surface chemistry are offered.

Formation of stable nitrene surface species by the reaction of adsorbed phenyl isocyanate at the Ge(100)-2 × 1 surface

The reaction of phenyl isocyanate (PIC) following adsorption at the Ge(100)-2 × 1 surface has been investigated both experimentally and theoretically by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by [2 + 2] cycloaddition across the C═N bond of the isocyanate, as previously reported, but this initial product converts to a second product on the time scale of minutes at room temperature. The experimental and theoretical results show that the second product formed is phenylnitrene (C6H5N) covalently bonded to the germanium surface via a single Ge-N bond. This conclusion is further supported by FTIR spectroscopy experiments and density functional theory calculations using phenyl isocyanate-(15)N and phenyl-d5 isocyanate.

Dissociative adsorption of 3-chloropropyne on Si(111)-(7 × 7): binding and structure

To achieve silicon functionalization for the development of hybrid devices, multifunctional molecules may be employed to attach to the silicon surfaces. It is important to get a fundamental understanding about the molecule/silicon interface chemistry and the binding configuration. The surface chemistry of 3-chloropropyne (HC≡C-CH(2)Cl) on the Si(111)-(7 × 7) surface, as a model system for understanding the interaction of the multifunctional molecules with a silicon surface, was studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT). The 3-chloropropyne adsorbs molecularly on the silicon surface at 110 K. A chemical reaction clearly occurs such that 3-choloropropyne bonds onto the Si(111)-(7 × 7) surface at room temperature by forming C-Si linkage through the cleavage of C-Cl bond, and preserving the ethyne C≡C triple bond. This functionalized silicon surface may act as an intermediate for the growth of multiple organic layers by further attaching other functional molecules.

Silicon surface functionalization targeting Si-N linkages

Silicon substrates have been a fascinating topic of fundamental and applied research for well over 50 years. They have attracted even more attention over the last couple of decades with advances in chemical functionalization that made oxide-free silicon surfaces a reality. Fundamentally new electronic properties and chemical reactivity became available, and the focus of chemical research turned more toward targeting specific chemical bonds and functionalities on silicon. Although thermodynamics clearly drives most processes under ambient conditions toward the formation of an oxide layer, kinetic control of the oxidation processes and thermodynamic tricks based on gaining stability of surface monolayers with high-density assembly have allowed for the formation of stable Si-C bonds and Si-O-C linkages on oxide-free silicon crystals. This feature article targets recent advances in making Si-N linkages on the same oxide-free single crystals. It covers the range of chemical approaches to achieving this goal and offers possible chemistry that can take advantage of the systems produced. The present status of the field and the future directions of its development will be considered.

Disulfide passivation of the Ge(100)-2 × 1 surface

Understanding the bonding of sulfur at the germanium surface is important to developing good passivation routes for germanium-based electronic devices. The adsorption behavior of ethyl disulfide (EDS) and 1,8-naphthalene disulfide (NDS) at the Ge(100)-2 × 1 surface has been studied under ultrahigh vacuum conditions to investigate both their fundamental reactivity and their effectiveness as passivants of this surface. X-ray photoelectron spectroscopy, multiple internal reflection-infrared spectroscopy, and density functional theory results indicate that both molecules adsorb via S-S dissociation at room temperature. Upon exposure to ambient air, the thiolate adlayer remains intact for both EDS- and NDS-functionalized surfaces, indicating the stability of this surface attachment. Although both systems resist oxidation compared to the bare Ge(100)-2 × 1 surface, the Ge substrate is significantly oxidized in all cases (17-57% relative to the control), with the NDS-passivated surface undergoing up to two times more oxidation than the EDS-passivated surface at the longest air exposure times studied. The difference in passivation capability is attributed to the difference in surface coverage on Ge(100)-2 × 1, where EDS adsorption leads to a saturation coverage 17% higher than that for NDS/Ge(100)-2 × 1.

Tuning the reactivity of semiconductor surfaces by functionalization with amines of different basicity

Surface functionalization of semiconductors has been the backbone of the newest developments in microelectronics, energy conversion, sensing device design, and many other fields of science and technology. Over a decade ago, the notion of viewing the surface itself as a chemical reagent in surface reactions was introduced, and adding a variety of new functionalities to the semiconductor surface has become a target of research for many groups. The electronic effects on the substrate have been considered as an important consequence of chemical modification. In this work, we shift the focus to the electronic properties of the functional groups attached to the surface and their role on subsequent reactivity. We investigate surface functionalization of clean Si(100)-2 × 1 and Ge(100)-2 × 1 surfaces with amines as a way to modify their reactivity and to fine tune this reactivity by considering the basicity of the attached functionality. The reactivity of silicon and germanium surfaces modified with ethylamine (CH(3)CH(2)NH(2)) and aniline (C(6)H(5)NH(2)) is predicted using density functional theory calculations of proton attachment to the nitrogen of the adsorbed amine to differ with respect to a nucleophilic attack of the surface species. These predictions are then tested using a model metalorganic reagent, tetrakis(dimethylamido)titanium (((CH(3))(2)N)(4)Ti, TDMAT), which undergoes a transamination reaction with sufficiently nucleophilic amines, and the reactivity tests confirm trends consistent with predicted basicities. The identity of the underlying semiconductor surface has a profound effect on the outcome of this reaction, and results comparing silicon and germanium are discussed.

Selective reactions and adsorption geometries of a multifunctional molecule: cis-2-butene-1,4-diol on Si(100)-2 x 1

The adsorption geometry of cis-2-butene-1,4-diol (BEDO, HOCH(2)CH=CHCH(2)OH) on Si(100)-2 x 1 was studied using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Bias-voltage-dependent STM images exhibited features characteristic of two distinct BEDO adsorption geometries. One feature was a bright protrusion located on the center of a single dimer, indicating an on-top configuration. The low bias-voltage STM image clearly showed dark features indicative of the formation of Si-H bonds on adjacent dimers in the same dimer row. The other feature was a bright protrusion bridged on end between two adjacent dimers in the same dimer row, indicating an end-bridge configuration. Accompanying this feature, two dark features attributed to Si-H bonds were observed on opposite positions to the bridged protrusion. From the XPS results, the Si 2p core level spectra revealed that the dimer atoms are involved in the formation of Si-O and Si-H bonds. On the other hand, carbon K-edge NEXAFS spectra showed that the C=C bond does not participate in the adsorption reaction and remains as an unreacted group. Collectively, the experimental results indicate that the adsorption of BEDO on Si(100)-2 x 1 occurs through the formation of two Si-O bonds via nucleophilic reaction between the two OH groups of BEDO and two Si-Si dimers. Importantly, the maintenance of the C=C bond means that the C=C functional group can be utilized as a new reaction site for further surface chemical reactions.

Functionalization of the semiconductor surfaces of diamond (100), Si (100), and Ge (100) by cycloaddition of transition metal oxides: a theoretical prediction

The viability of functionalization of the semiconductor surfaces of diamond (100), Si (100), and Ge (100) by traditional [3 + 2] cycloaddition of transition metal oxides has been predicted using effective cluster models in the framework of density functional theory. The cycloaddition of transition metal oxides (OsO(4), RuO(4), and MnO(4)(-)) onto the X (100) (X = C, Si, and Ge) surface is much more facile than that of other molecular analogues including ethylene, fullerene, and single-walled carbon nanotubes because of the high reactivity of surface dimers of X (100). Our computational results demonstrate the plausibility that the well-known [3 + 2] cycloaddition of transition metal oxides to alkenes in organic chemistry can be employed as a new type of surface reaction to functionalize the semiconductor X (100) surface, which offers the new possibility for self-assembly or chemical functionalization of X (100) at low temperature. More importantly, the chemical functionalization of X (100) by cycloaddition of transition metal oxides provides the molecular basis for preparation of semiconductor-supported catalysts but also strongly advances the concept of using organic reactions to modify the solid surface, particularly to modify the semiconductor C (100), Si (100), and Ge (100) surfaces for target applications in numerous fields such as microelectronics and heterogeneous photocatalysis.

Binding of glycine and L-cysteine on Si(111)-7 x 7

The adsorption of glycine and l-cysteine on Si(111)-7 x 7 was investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The observation of the characteristic vibrational modes and electronic structures of NH3+ and COO- groups for physisorbed glycine (l-cysteine) demonstrates the formation of zwitterionic species in multilayers. For chemisorbed molecules, the appearance of nu(Si-H), nu(Si-O), and nu(C=Omicron) and the absence of nu(O-H) clearly indicate that glycine and l-cysteine dissociate to produce monodentate carboxylate adducts on Si(111)-7 x 7. XPS results further verified the coexistence of two chemisorption states for each amino acid, corresponding to a Si-NH-CH2-COO-Si [Si-NHCH(CH2SH)COO-Si] species with new sigma-linkages of Si-N and Si-O, and a NH2-CH2-COO-Si [NH2CH(CH2SH)COO-Si] product through the cleavage of the O-H bond, respectively. Glycine/Si(111)-7 x 7 and l-cysteine/Si(111)-7 x 7 can be viewed as model systems for further modification of Si surfaces with biological molecules.

Theoretical and spectroscopic study of the reaction of diethylhydroxylamine on silicon(100)-2×1

Incorporating diversity into structures constructed from the organic modification of silicon surfaces requires the use of molecules that contain multiple substituents of different types. In this work we examine the possible dissociation pathways of diethylhydroxylamine (DEHA, (C(2)H(5))(2)NOH) on the surface of clean silicon(100)-2x1 using cluster and planewave computational methods and high resolution electron energy loss spectroscopy. Our computational results show that DEHA initially forms a strongly-bound complex with the surface via a dative N-Si bond. A low-barrier O-H bond scission then occurs yielding a surface silicon dimer capped by the (C(2)H(5))(2)NO and H fragments. Calculated and measured vibrational spectra support the computed reaction mechanism.

Origin of nonlocal interactions in adsorption of polar molecules on Si(001)-2 x 1

Using density functional theory slab calculations, we have investigated (i) the origin of nonlocal interactions occurring in the adsorption of small polar molecules (H2O,NH3,CH3OH,CH3NH2) on the clean Si(001)-2 x 1 surface and (ii) the nonlocal effects on two-dimensional arrangement of adsorbates. Our results show the adsorption properties are significantly altered in the presence of adsorbates on an adjacent dimer along a row. We have identified that the coverage dependent behavior arises from a combination of (i) surface polarization change, (ii) adsorbate-induced charge delocalization, (iii) adsorbate-adsorbate repulsion, and (iv) hydrogen bonding. The nucleophilic-electrophilic molecular adsorption involves charge delocalization to neighboring dimers along a row, which in turn undermines molecular adsorption on the neighboring dimers. Nonlocal effects associated with polar interactions with neighboring dimers and adsorbates vary with adsorption system. While such polar interactions are unimportant in CH3OH adsorption, hydrogen bonding and adsorbate-adsorbate repulsion play an important role in determining the adsorption structures of H2O and NH3CH3NH2, respectively. In addition, the electrostatic attraction with the buckled-up Si atoms of adjacent dimers contributes to stabilization of H2O, NH3, and CH3NH2 adsorption. We also discuss kinetic effects on two-dimensional ordering of adsorbates, in conjunction with surface phase transition and adsorption-dissociation rates.

Mechanism and Regioselectivity for the Reactions of Propylene Oxide with X(100)-2×1 Surfaces (X = C, Si, Ge): A Density Functional Cluster Model Investigation

We have performed density functional cluster model calculations to explore the mechanism and regioselectivity for the reactions of propylene oxide with X(100)-2x1 surfaces (X = C, Si, and Ge). The computations reveal the following: (i) the reactions on Si(100) and Ge(100) are barrierless and highly exothermic; (ii) the reactions on X(100) (X = Si and Ge) are initiated by the formation of a dative-bonded precursor state followed by regioselective cleavage of the C2-O bond (C2 directly connected to the methyl-substituent) in propylene oxide, giving rise to a five-membered ring surface species; and (iii) the reaction on C(100), although highly exothermic, requires a large activation energy and would be kinetically forbidden at room temperature.

Carboxylic acid chemistry at the Ge(100)-2 x 1 interface: bidentate bridging structure formation on a semiconductor surface

The reactions of acetic acid, acetic-d3 acid-d, and formic acid with the Ge(100)-2 x 1 surface have been investigated using multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The infrared and photoelectron data provide experimental evidence for an O-H dissociation product at 310 K. DFT calculations indicate that the O-H dissociation pathway is significantly favored, both kinetically and thermodynamically, over other potential reaction pathways. All of the carboxylic acids studied exhibit unexpected vibrational modes between 1400 and 1525 cm(-1), which are attributed to the presence of a bidentate bridging structure where both oxygen atoms interact directly with the surface.

Predicting Facile Epoxidation of the Diamond (100) Surface by Dioxiranes and Subsequent Ring-Opening Reactions with Nucleophiles

By means of density functional theory coupled with effective cluster models, we have theoretically predicted the viability of epoxidation of the diamond (100) surface by organic dioxiranes. In addition, subsequent ring-opening reactions of the as-formed epoxide surface species with some nucleophiles, including water, ammonia, and alcohol, have also been explored. The facile epoxidation of diamond (100) by dioxiranes presents a new alternative for oxidation of the diamond (100) surface. More importantly, the as-formed epoxide-like surface species would be a useful springboard for further functionalizations of the diamond surface given the well-known abundant chemistry of organic epoxides. Therefore, this approach provides another new route to chemical functionalization of the diamond surface, which is potentially useful for leading to the improvement of diamond behavior and constructing new hybrid diamond-based materials for wide potential applications in many fields. In perspective, implications for other theoretical work are also discussed.

Formation of Surface-Bound Acyl Groups by Reaction of Acyl Halides on Ge(100)−2×1

We have investigated the reaction of a series of acyl halides, including acetyl chloride, acetyl bromide, acetyl-d3 chloride, benzoyl chloride, and pivaloyl chloride, on Ge(100)-2x1 with multiple internal reflection infrared (MIR-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Infrared spectra following saturation exposures of acetyl chloride and acetyl bromide to Ge(100)-2x1 at 310 K are nearly identical, both exhibiting strong nu(C=O) stretching peaks near 1685 cm-1 and no vibrational modes in the nu(Ge-H) region. These data provide strong evidence for the presence of a surface-bound acetyl group on Ge(100)-2x1, which results from a C-X dissociation reaction (where X=Cl, Br). For acetyl chloride, DFT calculations predict that the barrier to C-Cl dissociation is only 1 kcal/mol above a chlorine-bound precursor state and is considerably smaller than barriers leading to the [2+2] C=O cycloaddition and alpha-CH dissociation products. In addition to the C-X dissociation product, both infrared and photoelectron results point to the presence of a second structure for acetyl halides where the oxygen of the surface-bound acetyl group donates charge to a nearby surface atom. This interaction is not observed for benzoyl chloride and pivaloyl chloride.

Functionalization of diamond (100) by organic cycloaddition reactions of nitrenes: a theoretical prediction

[structure: see text] We predict the viability of organic cycloadditions of nitrenes onto the diamond (100) surface. This new type of surface reaction can be employed to functionalize diamond surface at low temperature, which might introduce new functionalities to the diamond surface for novel applications in a diversity of fields.

Selective attachment of benzaldehyde on Si(100)-2×1: Structure, selectivity, and mechanism

The interaction of benzaldehyde with the Si(100) surface has been investigated as a model system for understanding the interaction of conjugated pi-electron systems with semiconductor surfaces. Vibrational features of chemisorbed benzaldehyde unambiguously demonstrate that the carbonyl group directly interacts with the Si surface dangling bonds, evidenced in the disappearance of the C=O stretching mode around 1713 cm(-1) coupled with the retention of all vibrational signatures of its phenyl ring. X-ray photoemission spectroscopy shows that both C 1s and O 1s binding energies of the carbonyl group display large downshifts by 1.9 and 1.3 eV, respectively. Vibrational and electronic results show that the covalent attachment of benzaldehyde on Si(100) occurs in a highly selective manner through the direct interaction of both C and O atoms of the carbonyl group with a Si=Si dimer to form a four-membered Si-C-O-Si ring at the interface, leaving a nearly unperturbed phenyl ring protruding into vacuum. This conclusion is further confirmed by the observation of a predominant protrusion for benzaldehyde adsorbed on Si(100)-2 x 1 in scanning tunneling microscopy experiments, consistent with the predication of density-functional theory calculation.

Organic functionalization of diamond (100) by addition reactions of carbene, silylene, and germylene: a theoretical prediction

We present a theoretical prediction of the facile cycloadditions of carbene, silylene, and germylene onto the diamond (100) surface, a new type of surface reaction that can be employed to functionalize diamond surface at low temperature. This finding renders the plausibility that the diamond surface can be chemically modified by the well-known carbene addition chemistry, which might introduce new functionalities to the diamond surface for novel applications in a diversity of fields.

Formation of a tetra-sigma-bonded intermediate in acetylethyne binding on Si(100)-2 x 1

The covalent binding of acetylethyne on Si(100)-2 x 1 has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS spectra of chemisorbed monolayers show the absence of the C=O, C[triple bond]C, and C(sp)-H stretching modes coupled with the appearance of C=C (at 1580 cm(-1)) and C(sp2)-H (at 3067 cm(-1)) stretching modes. This demonstrates that both of the C=O and CC groups of acetylethyne directly participate in binding with silicon surfaces to form C-O and C=C bonds, respectively, which is further confirmed by the XPS studies. A tetra-sigma-binding configuration through two [2 + 2]-like cycloaddition reactions in acetylethyne binding on Si(100) is proposed to account for the experimental observation. The cycloadduct containing a C=C double bond may be employed as an intermediate for further in situ chemical syntheses of multilayer organic thin films or surface functionalization.

Reactions of Hydrazoic Acid on TiO2 Nanoparticles: an Experimental and Computational Study

This article reports the results of a computational and experimental study on the reaction of hydrazoic acid, HN3, adsorbed on 15-20 nm TiO2 particle films. Experimentally, FTIR spectra of HN3(a) have been measured by varying HN3 dosage, UV irradiation time and surface annealing temperature. Three sharp peaks, related to v(a)(NNN) of HN3(a) and N3(a) with different configurations in the 2000-2200 cm(-1) region, and a broad band absorption, related to associated and isolated HN(a) and HO(a) adsorptions in the 3000-3800 cm(-1) region, have been detected. Computationally, molecular structures, vibrational frequencies and adsorption energies of possible adsorbates including HN3 and its derivatives, N3, N2, NH, and H, have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. On the basis of the experimental and computational results, the peak appeared at 2075 cm(-1), which increases at a faster rate with HN3 exposure time, is attributed to a stable adsorbate, N3-Ti(a), with the predicted adsorption energy, E(ads) = 13 kcal/mol. The peak at 2118 cm(-1), which survives at the highest surface temperature in the heating experiment, is attributable to the most stable adsorbate, Ti-N2N(H)-O(a) with E(ads) = 36 kcal/mol. The peak at 2170 cm(-1), which vanishes most readily in all of the aforementioned experiments, is related to less stable molecular adsorbates, end-on HN3-Ti(a) with E(ads) = 5 kcal/mol and side-on HN(N2)-Ti(a) with E(ads) = 8 kcal/mol. A potential energy diagram for the formation of various absorbates with their transition states has been established for the HN3/TiO2 system. On the basis of the predicted desorption energies, the four most stable products of the HN3 reaction on TiO2 are H-O(a), 118 kcal/mol; HN-O(a), 85 kcal/mol; Ti-N2N(H)-O(a), 36 kcal/mol; and N3-O(a), 19 kcal/mol.

Cycloaddition reactions of cyanogen (C2N2) on the Si(100)-2×1 surface

Multireference as well as density functional theories in combination with the surface integrated molecular orbital molecular mechanics were adopted to study the surface reactions of cyanogens on Si(100)-2x1 surface. Three different products were identified as minima in the initial surface reaction. Among these, the [2+2] product is both kinetically easily accessible and thermodynamically the most stable. Therefore, it can be considered as the experimentally found strongly bound surface species. Unlike other conjugated systems, the [4+2] product is less stable than the [2+2] product. Subsequent surface isomerization studies revealed that kinetically favorable channels exist between the initially formed low-temperature species and the high-temperature species, indicating that surface morphology changes gradually as a function of surface temperature. Theses two channels eventually lead to the same final surface products, which is consistent with experiment. Current study shows that the subsequent surface isomerizations are the key reactions to better understand the complex surface structures and their properties.

Selective bonding of pyrazine to silicon(100)-2×1 surfaces: The role of nitrogen atoms

The covalent binding of pyrazine on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS) and x-ray photoelectron spectroscopy. Experimental results clearly suggest that the attachment occurs exclusively through the bonding of the two para-nitrogen atoms with the surface without the involvement of the carbon atoms, as evidenced from the retention of the (sp2) C-H stretching mode in HREELS and a significant down shift of 1.6 eV in the binding energy of N 1s. The binding mechanism for pyrazine on Si(100) demonstrates that reaction channels for heteroatomic aromatic molecules are strongly dependent on the electronic properties of the constituent atoms.

Diradical mechanism for the [2 + 2] cycloaddition of ethylene on Si(100) surface

Density functional cluster model calculations have been performed to explore the reaction mechanism for the adsorption of ethylene on Si(100). It is shown that the [2 + 2] cycloaddition of ethylene on a Si=Si dimer of Si (100) surface follows a diradical mechanism, via a pi-complex precursor and a singlet diradical intermediate, and the rate-determining step for the overall reaction is the formation of the diradical intermediate.