Keith T. Wong

Periodic trends in organic functionalization of group IV semiconductor surfaces.

Organic functionalization of group IV semiconductor surfaces provides a means to precisely control the interfacial properties of some of the most technologically important electronic materials in use today. The 2 x 1 reconstructed group IV (100) surfaces in ultrahigh vacuum, in particular, have a well-defined surface that allows adsorbate-surface interactions to be studied in detail. Surface dimers containing a strong sigma- and weak pi-bond form upon reconstruction of the group IV (100) surfaces, imparting a rich surface reactivity, which allows useful analogies to be made between reactions at the surface and those in classic organic chemistry. To date, most studies have focused on single substrates and a limited number of adsorbate functional groups. In this Account, we bring together experimental and theoretical results from several studies to investigate broader trends in thermodynamics and kinetics of organic molecules reacted with group IV (100)-2 x 1 surfaces. By rationalizing these trends in terms of simple periodic properties, we aim to provide guidelines by which to understand the chemical origin of the observed trends and predict how related molecules or functionalities will react. Results of experimental and theoretical studies are used to show that relative electronegativities and orbital overlap correlate well with surface-adsorbate covalent bond strength, while orbital overlap together with donor electronegativity and acceptor electron affinity correlate with surface-adsorbate dative bond strength. Using such simple properties as predictive tools is limited, of course, but theoretical calculations fill in some of the gaps. The predictive power inherent in periodic trends may be put to use in designing molecules for applications where controlled attachment of organic molecules to semiconductor surfaces is needed. Organic functionalization may facilitate the semiconductor industrys transition from traditional silicon-based architectures to other materials, such as germanium, that offer better electrical properties. Potential applications also exist in other fields ranging from organic and molecular electronics, where control of interfacial properties may allow coupling of traditional semiconductor technology with such developing technologies, to biosensors and nanoscale lithography, where the functionality imparted to the surface may be used directly. Knowledge of thermodynamic and kinetic trends and the fundamental basis of these trends may enable effective development of new functionalization strategies for such applications.

What a difference a bond makes: the structural, chemical, and physical properties of methyl-terminated Si(111) surfaces.

The chemical, electronic, and structural properties of surfaces are affected by the chemical termination of the surface. Two-step halogenation/alkylation of silicon provides a scalable, wet-chemical method for grafting molecules onto the silicon surface. Unlike other commonly studied wet-chemical methods of surface modification, such as self-assembly of monolayers on metals or hydrosilylation on silicon, the two-step method enables attachment of small alkyl chains, even methyl groups, to a silicon surface with high surface coverage and homogeneity. The methyl-terminated Si(111) surface, by comparison to hydrogen-terminated Si(111), offers a unique opportunity to study the effects of the first surface bond connecting the overlayer to the surface. This Account describes studies of methyl-terminated Si(111), which have shown that the H-Si(111) and CH3-Si(111) surfaces are structurally nearly identical, yet impart significantly different chemical and electronic properties to the resulting Si surface. The structure of methyl-terminated Si(111) formed by a two-step halogenation/methylation process has been studied by a variety of spectroscopic methods. A covalent Si-C bond is oriented normal to the surface, with the methyl group situated directly atop a surface Si atom. Multiple spectroscopic methods have shown that methyl groups achieve essentially complete coverage of the surface atoms while maintaining the atomically flat, terraced structure of the original H-Si(111) surface. Thus, the H-Si(111) and CH3-Si(111) surface share essentially identical structures aside from the replacement of a Si-H bond with a Si-C bond. Despite their structural similarity, hydrogen and methyl termination exhibit markedly different chemical passivation. Specifically, CH3-Si(111) exhibits significantly greater oxidation resistance than H-Si(111) in air and in aqueous electrolyte under photoanodic current flow. Both surfaces exhibit similar thermal stability in vacuum, and the Si-H and Si-C bond strengths are expected to be very similar, so the results suggest that methyl termination presents a greater kinetic barrier to oxidation of the underlying Si surface. Hydrogen termination of Si(111) provides nearly perfect electronic passivation of surface states (i.e., less than 1 electronic defect per 40 million surface atoms), but this electronic passivation is rapidly degraded by oxidation in air or under electrochemical conditions. In contrast, methyl termination provides excellent electronic passivation that resists degradation due to oxidation. Moreover, alkylation modifies the surface electronic structure by creating a surface dipole that effectively changes the electron affinity of the Si surface, facilitating modification of the charge-transfer kinetics across Si/metal or Si/electrolyte junctions. This Account also briefly describes recent studies of mixed monolayers formed by the halogenation/alkylation of silicon. Mixed monolayers allow attachment of bulkier groups that enable secondary chemistry at the surface (e.g., attachment of molecular catalysts or seeding of atomic layer deposition) to be combined with methyl termination of remaining unreacted surface sites. Thus, secondary chemistry can be enabled while maintaining the chemical and electronic passivation provided by complete termination of surface atoms with Si-C bonds. Such studies of mixed monolayers demonstrate the potential use of a wet-chemical surface modification scheme that combines both chemical and electronic passivation.

Strong Carbon-Surface Dative Bond Formation by tert-Butyl Isocyanide on the Ge(100)-2 × 1 Surface

Carbon dative bond formation between an organic molecule and a semiconductor surface is reported here for the first time. Our studies show that the adsorption of tert-butyl isocyanide on the (100) surface of germanium, measured using Fourier transform infrared spectroscopy, temperature-programmed desorption, and density functional theory calculations, occurs via formation of a dative bond to the surface through the isocyanide carbon. The experimentally observed adsorption energy of 26.8 kcal/mol is the largest among any organic molecule dative bonded on the Ge(100)-2 × 1 surface studied to date. The dative-bonded adsorbate is characterized by a N≡C stretching frequency significantly blue-shifted from that of the free molecule. Moreover, the adsorbate N≡C vibrational frequency red-shifts back toward that of the free molecule upon increasing coverage. These spectroscopic effects are attributed to σ-donation of the isocyanide lone pair electrons to the surface.

Vibrational dynamics and band structure of methyl-terminated Ge(111)

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH3-Ge(111) and CH3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.

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.