Electronic structure of benzene adsorbed on single-domain Si(001)-(2×1): A combined experimental and theoretical study

Surface functionalization with nonalternant aromatic compounds: a computational study of azulene and naphthalene on Si(001)

Nonalternant aromatic π-electron systems show promises for surface functionalization due to their unusual electronic structure. Based on our previous experiences for metal surfaces, we investigate the adsorption structures, adsorption dynamics and bonding characteristics of azulene and its alternant aromatic isomer naphthalene on the Si(001) surface. Using a combination of density functional theory,ab initiomolecular dynamics, reaction path sampling and bonding analysis with the energy decomposition analysis for extended systems, we show that azulene shows direct adsorption paths into several, strongly bonded chemisorbed final structures with up to four covalent carbon-silicon bonds which can be described in a donor-acceptor and a shared-electron bonding picture nearly equivalently. Naphthalene also shows these tetra-σ-type bonding structures in accordance with an earlier study. But the adsorption path is pseudo-direct here with a precursor intermediate bonded via one aromatic ring and strong indications for a narrow adsorption funnel. The four surface-adsorbate bonds formed lead for both adsorbates to a strong corrugation and a loss of aromaticity.

The structure of deuterated benzene films adsorbed on the graphite (0001) basal plane: what happens below and above the monolayer coverage?

An exact description of the interactions in aromatic carbon systems is a key condition for the design of carbon based nanomaterials. In this paper we investigate the binding and adsorbate structure of the simplest prototype system in this class - the single aromatic ring molecule benzene on graphite. We have collected neutron diffraction data of the ordered phase of deuterated benzene, C6D6, adsorbed on the graphite (0001) basal plane surface. We examined relative coverages from 0.15 up to 1.3 monolayers (ML) in a temperature range of 80 to 250 K. The results confirm the flat lying commensurate (√7 × √7)R19.1° monolayer with lattice constants a = b = 6.5 Å at coverages of less than 1 ML. For this structure we observe a progressive melting well below the desorption temperature. At higher coverages we do neither observe an ordered second layer nor a densification of the structure by upright tilting of first layer molecules, as generally assumed up to now. Instead, we see the formation of clusters with a bulk crystalline structure for coverages only weakly exceeding 1 ML.

Efficient electronic coupling and improved stability with dithiocarbamate-based molecular junctions

Molecular electronic devices require stable and highly conductive contacts between the metal electrodes and molecules. Thiols and amines are widely used to attach molecules to metals, but they form poor electrical contacts and lack the robustness required for device applications. Here, we demonstrate that dithiocarbamates provide superior electrical contact and thermal stability when compared to thiols on metals. Ultraviolet photoelectron spectroscopy and density functional theory show the presence of electronic states at 0.6 eV below the Fermi level of Au, which effectively reduce the charge injection barrier across the metal-molecule interface. Charge transport measurements across oligophenylene monolayers reveal that the conductance of terphenyl-dithiocarbamate junctions is two orders of magnitude higher than that of terphenyl-thiolate junctions. The stability and low contact resistance of dithiocarbamate-based molecular junctions represent a significant step towards the development of robust, organic-based electronic circuits.

Two bonding configurations of acetylene on Si(001)-(2×1): A combined high-resolution electron energy loss spectroscopy and density functional theory study

Two coexisting adsorption states of molecularly adsorbed acetylene on the Si(001)-(2 x 1) surface have been identified by a combined study based on the high-resolution electron energy loss spectroscopy and density functional computations. Seven possible adsorbate-substrate structures are considered theoretically including their full vibrational analysis. Based on a significantly enhanced experimental resolution, the assignment of 15 C2H2- and C2D2-derived vibrational modes identifies a dominant di-sigma bonded molecule adsorbed on top of a single Si-Si dimer. Additionally there is clear evidence for a second minority species which is di-sigma bonded between two Si-Si dimers within the same dimer row (end-bridge geometry). The possible symmetries of the adsorbate complexes are discussed based on the specular and off-specular vibrational measurements. They suggest lower than ideal C(2v) and C(s) symmetries for on-top and end-bridge species, respectively. At low coverages the symmetry reductions might be lifted.

Electronic Structure of Methoxy-, Bromo-, and Nitrobenzene Grafted onto Si(111)

The properties of Si(111) surfaces grafted with benzene derivatives were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The investigated materials were nitro-, bromo-, and methoxybenzene layers (-C(6)H(4)-X, with X = NO(2), Br, O-CH(3)) deposited from diazonium salt solutions in a potentiostatic electrochemical process. The UPS spectra of the valence band region are governed by the molecular orbital density of states of the adsorbates, which is modified from the isolated state in the gas phase due to molecule-molecule and molecule-substrate interaction. Depending on the adsorbate, clearly different emission features are observed. The analysis of XPS intensities clearly proves multilayer formation for bromo- and nitrobenzene in agreement with the amount of charge transferred during the grafting process. Methoxybenzene forms only a sub-monolayer coverage. The detailed analysis of binding energy shifts of the XPS emissions for determining the band bending and the secondary electron onset in UPS spectra for determining the work function allow one to discriminate between surface dipole layers--changing the electron affinity--and band bending, affecting only the work function. Thus, complete energy band diagrams of the grafted Si(111) surfaces can be constructed. It was found that silicon surface engineering can be accomplished by the electrochemical grafting process using nitrobenzene and bromobenzene: silicon-derived interface gap states are chemically passivated, and the adsorbate-related surface dipole effects an increase of the electron affinity.

Atomic Structures of Benzene and Pyridine on Si(5 5 12)-2 × 1

The adsorption structures of benzene and pyridine on Si(5 5 12)-2 x 1 were studied at 80 K by using a low-temperature scanning tunneling microscope and density functional theory calculations. These structures are different from those observed on low-index Si surfaces: benzene molecules exclusively bind to two adatoms, that is, with di-sigma bonds between carbon atoms and silicon adatoms, leading to the loss of benzene aromaticity; in contrast, pyridine molecules interact with adatom(s) through either Si-N dative bonding or di-sigma bonds. Dative bonding configurations with pyridine aromaticity are the dominant adsorption features and are more stable than di-sigma bonding configurations. Thus the dative bonding of nitrogen-containing heteroaromatic molecules provides a strategy for the controlled attachment of aromatic molecules to high-index surfaces.

Surface Chemistry of Monochlorinated and Dichlorinated Benzenes on Si(100)2×1: Comparison Study of Chlorine Content and Isomeric Effects

Using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), the room temperature (RT) adsorption and thermal evolution of monochlorobenzene (MCB) and 1,3-dichlorobenzene (1,3-DCB) on Si(100)2x1 have been investigated and compared with that of 1,2-dichlorobenzene (1,2-DCB) reported previously. Like 1,2-DCB, the C 1s features observed at 284.6 (C(1)) and 286.0 eV (C(2)) for both MCB and 1,3-DCB could be attributed to the C-H and C-Cl bonds, respectively. The C(1)/C(2) intensity ratios for MCB (5.0) and 1,3-DCB (2.0) are found to follow the stoichiometric ratios of the C-H to C-Cl bonds for MCB and 1,3-DCB, respectively, indicating that both MCB and 1,3-DCB adsorb on Si(100)2x1 molecularly with negligible C-Cl dissociation at RT, in marked contrast to the partial C-Cl dissociation found for 1,2-DCB. Unlike 1,2-DCB with two discernible Cl 2s features at 270.3 and 271.2 eV, a single Cl 2s feature at 271.2 eV is observed for MCB and 1,3-DCB, in accord with the single local chemical environment for Cl. The TPD results show that MCB undergoes molecular desorption exclusively, similar to that found for benzene. Both molecular desorption and recombinative HCl desorption are found for 1,3-DCB, similar to that for 1,2-DCB. Despite the different Cl contents and relative Cl locations on the benzene ring, both MCB and 1,3-DCB exhibit RT adsorption behavior remarkably similar to that of benzene. To explain the C-Cl dissociation observed for 1,2-DCB, we propose a possible transition state involving the Cl atoms located at more physically compatible positions with the surface Si dimers in order to facilitate the conversion of 1,2-DCB (preferentially over 1,3-DCB) to dissociated products at RT. However, the thermal evolution of 1,3-DCB is closer to that of 1,2-DCB than that of MCB and benzene. The breakage of C-Cl bonds is found to occur at a relatively low temperature of 425 K, which suggests a relatively low activation barrier for the dechlorination of 1,3-DCB adspecies. Calculated energetics for 1,4-DCB on Si(100)2x1 shows that double dechlorination is not as favorable a process as those for 1,2-DCB and 1,3-DCB.

Chemical Reactions and Adsorption Geometries of Pyrrole on Ge(100)

The adsorption structures of pyrrole (C(4)H(5)N) on a Ge(100) surface at various coverages have been investigated with both scanning tunneling microscopy (STM) and ab initio density-functional theory (DFT) calculations. Three distinct features are observed in the STM images at low coverages. The comparison of the STM images with the simulation reveals that the most dominant flowerlike feature with a dark side is that the adsorbed pyrrole molecules with H dissociated form bridges between two down Ge atoms of neighboring Ge dimer rows through N-Ge bonding and beta-carbon-Ge interaction. The flowerlike feature without a dark side is also observed as a minority, which is identified as nearly the same structure as the most dominant one where a dissociated H is out of the feature. The third feature showing bright protrusions may be due to a C- and N-end-on (CN) configuration, where the pyrrole molecule is located on one dimer row. At higher coverages, the number of localized configurations increases.

Adsorption structures of benzene on a Si(5512)-2×1 surface: A combined scanning tunneling microscopy and theoretical study

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2x1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-sigma bonds with the substrate and of two C[Double Bond]C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03-1.20 eV on the adatoms and 0.22 eV on the honeycomb chains.

Picometer-Scale Electronic Control of Molecular Dynamics Inside a Single Molecule

Tunneling electrons from a low-temperature (5 kelvin) scanning tunneling microscope were used to control, through resonant electronic excitation, the molecular dynamics of an individual biphenyl molecule adsorbed on a silicon(100) surface. Different reversible molecular movements were selectively activated by tuning the electron energy and by selecting precise locations for the excitation inside the molecule. Both the spatial selectivity and energy dependence of the electronic control are supported by spectroscopic measurements with the scanning tunneling microscope. These experiments demonstrate the feasibility of controlling the molecular dynamics of a single molecule through the localization of the electronic excitation inside the molecule.

Cycloaddition of Benzene on Si(100) and Its Surface Conversions

A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-sigma-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (-29.0 kcal/mol), although one of the two tetra-sigma-bonded structures (-26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels-Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a (relatively strongly bound) van der Waals complex (-8.9 kcal/mol). However, the net activation barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-sigma-bonded structures (that are energetically comparable to the [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol. However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product to the tetra-sigma-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product) but also kinetically very stable (large barriers with respect to the isomerization to other products).

Thermal Chemistry of Styrene on Si(100)2×1 and Modified Surfaces: Electron-Mediated Condensation Oligomerization and Posthydrogenation Reactions

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.

Mechanisms of molecular manipulation with the scanning tunneling microscope at room temperature: chlorobenzene/si(111)-(7 x 7)

We report a systematic experimental investigation of the mechanism of desorption of chlorobenzene molecules from the Si(111)-(7 x 7) surface induced by the STM at room temperature. We measure the desorption probability as a function of both tunneling current and a wide range of sample bias voltages between -3 V and +4 V. The results exclude field desorption, thermally induced desorption, and mechanical tip-surface effects. They indicate that desorption is driven by the population of negative (or positive) ion resonances of the chemisorbed molecule by the tunneling electrons (or holes). Density functional calculations suggest that these resonant states are associated with the pi orbitals of the benzene ring.

Inducing desorption of organic molecules with a scanning tunneling microscope: theory and experiments

A scanning-tunneling microscope has been used to induce efficient local desorption of benzene from Si(100) at low currents (<100 pA), sample biases (approximately -2.4 V) and temperatures (22 K). A theoretical model based upon first principles electronic structure calculations and quantum mechanical wave packet dynamics describes this process as occurring via transient ionization of a pi state of the adsorbed molecule. This model accounts for the unexpected efficiency and sharp threshold of the yield.

Cycloaddition chemistry of organic molecules with semiconductor surfaces

Recent investigations have shown that cycloaddition reactions, widely used in organic chemistry to form ring compounds, can also be applied to link organic molecules to the (001) surfaces of crystalline silicon, germanium, and diamond. While these surfaces are comprised of Si=Si, Ge=Ge, and C=C structural units that resemble the C=C bonds of organic alkenes, the rates and mechanisms of the surface reactions show some distinct differences from those of their organic counterparts This article reviews recent studies of [2 + 2], [4 + 2] Diels-Alder, and other cycloaddition reactions of organic molecules with semiconductor surfaces and summarizes the current understanding of the reaction pathways.

CONTROLLEDMOLECULARADSORPTION ONSILICON: Laying a Foundation for Molecular Devices

This review is about understanding and controlling organic molecular adsorption on silicon. The goal is to provide a microscopic picture of structure and bonding in covalently attached molecule-silicon surface systems. The bias here is that an unprecedented, detailed understanding of adsorbate-surface structures is required in order to gain the control necessary to incorporate organic function into existing technologies or, eventually, to make new molecule-scale devices. A discussion of recent studies of adsorbate structure is presented. This includes simple alkenes, polyenes, benzene, and carene adsorbed on Si(100). Also included is a discussion of wet chemical procedures for forming alkyl and alkoxy covalently functionalized silicon. These discussions are presented together with comments on the related issues of adsorption dynamics and nano-scale manipulation in an effort to point the way toward principles and procedures that will allow the hybrid properties of organic molecules and surfaces to be harnessed.