Andrew V. Teplyakov

An NEXAFS investigation of the reduction and reoxidation of TiO2(001)

Abstract The near-edge X-ray absorption fine structure (NEXAFS) technique was applied to characterize the oxidation states of titanium cations on TiO 2 (001) surfaces reduced by argon-ion bombardment and reoxidized by thermal treatment. Although many characterization studies of reduced TiO 2 have been performed, none of these has applied both surface-sensitive (electron yield) and bulk-sensitive (fluorescence yield) NEXAFS to characterize reduced TiO 2 (001) single-crystal surfaces. The fluorescence yield NEXAFS of polycrystalline samples of the suboxides TiO and Ti 2 O 3 were used as standards to fingerprint reduced cations on the TiO 2 (001) surface. NEXAFS has allowed us to estimate the concentration of oxygen and titanium in the near-surface region of reduced and reoxidized samples. The results of this study demonstrate that oxygen is preferentially removed during ion bombardment, that the depth of the altered layer is comparable to the ion penetration depth, and that the electronic environment of cations in the altered layer is comparable to that of cations in titanium suboxides. The extents of reduction calculated from NEXAFS results for reduced and reoxidized surfaces as a function of annealing temperature compare favorably with those previously determined by analysis of Ti 2p XPS data.

Diels–Alder reactions of butadienes with the Si(100)-2×1 surface as a dienophile: Vibrational spectroscopy, thermal desorption and near edge x-ray absorption fine structure studies

The mechanism and intermediates of a Diels–Alder-type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2×1 surface, which was theoretically predicted by Doren and Konecny, have been investigated. The reactions of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene were studied using multiple internal reflection infrared spectroscopy, thermal desorption spectrometry, and near edge x-ray absorption fine structure (NEXAFS) measurements. The results show that the compounds physisorb on Si(100)-2×1 at cryogenic temperature. Infrared studies of the room temperature adsorption of both dienes indicate that reaction leads to the formation of stable, chemisorbed Diels–Alder adducts. By NEXAFS measurements on 2,3-dimethyl-1,3-butadiene, we determine that the angle between the π orbitals of the reaction product and the Si(100)-2×1 surface is near 40°. Upon heating, the chemisorbed butadienes primarily decompose to form adsorbed carbon and hydrogen at the surface. Hydrogenation of chemisorbed butadie...

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 (CH3CH2NH2) and aniline (C6H5NH2) 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 (((CH3)2N)4Ti, 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.

Semiconductor surface functionalization for advances in electronics, energy conversion, and dynamic systems

Semiconductors have played a tremendous role in the development of electronics since the inception of the electronics revolution more than 60 years ago. Over this period, the performance of semiconductors relied on the development of robust and reliable surface passivation and functionalization schemes. As the size of the individual components in microelectronics has decreased, the role of surface chemistry has become even more important. Moreover, in the development of fields such as sensing and energy conversion, the surface chemistry of the component semiconductor materials has often driven the functionality of devices and applications. Available functionalization chemistries take advantage of the localized and covalent nature of the semiconductor surfaces to form organic layers that can passivate the surface, assemble nanopatterns, influence subsequent deposition, or change the nature of interfacial electron transfer. Despite an established toolkit already available for semiconductor surface functiona...

–NH– Termination of the Si(111) Surface by Wet Chemistry

For over a quarter of a century the hydrogen-terminated Si(111) single-crystalline surface has been the gold standard as a starting point for silicon surface modification chemistry. However, creating a well-defined and stable interface based on Si-N bonds has remained elusive. Despite the fact that azides, nitro compounds, and amines do lead to the formation of surface Si-N, each of these modification schemes produces additional carbon- or oxygen-containing functional groups that in turn react with the surface itself, leaving contaminants that affect the interface properties for any further modification protocols. We describe the preparation of a Si(111) surface functionalized predominantly with Si-NH-Si species based on chlorination followed by the room temperature ammonia treatment utilizing NH(3)-saturated tetrahydrofuran (THF). The obtained surface has been characterized by infrared spectroscopy and X-ray photoelectron spectroscopy. This analysis was supplemented with DFT calculations. This newly characterized surface will join the previously established H-Si(111) and Cl-Si(111) surfaces as a general starting point for the preparation of oxygen- and carbon-free interfaces, with numerous potential applications.

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.

NEXAFS and TPD studies of molecular adsorption of hydrocarbons on Cu(100): segmental correlations with the heats of adsorption

Abstract Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CHn groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between π-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increase the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon.

Adsorption of ethylene on the Ge(100)-2×1 surface: Coverage and time-dependent behavior

Studies of the adsorption and thermal chemistry of ethylene on the Ge(100)-2×1 surface have been performed. The results of multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy and temperature programmed desorption (TPD) show that ethylene chemisorbs molecularly on the Ge(100)-(2×1) surface at room temperature. Infrared spectroscopy gives evidence for only one adsorbate configuration on this surface at room temperature, consistent with a structure in which ethylene bridges across a germanium dimer. However, TPD measurements show two molecular desorption features at almost all coverages, indicating that at least two adsorption states can be formed. Further shifts in one of the peaks with both coverage and time, paralleled by changes in the vibrational spectrum, suggest the presence of attractive intermolecular interactions or cooperative effects. The complex time- and coverage dependence of ethylene adsorption on Ge(100)-2×1 is analyzed using a two-state kinetic model.

Vibrational mode-softening of alkanes on clean and modified Cu and Mo surfaces: absence of a simple correlation with thermal desorption temperatures

Although the strength of the CH…M interactions between alkanes and metal surfaces has often been correlated with the vibrational frequencies of so-called “softened” ν(CH) modes, this correlation is still considered to be controversial in many cases. In this letter, we report vibrational and thermal desorption results to demonstrate that there is no simple correlation between the softened ν(CH) frequencies and the thermal desorption temperatures of hydrocarbon molecules on Cu and Mo surfaces. In order to determine whether general trends exist, we have performed systematic studies in which we varied the nature of both alkane molecules and metal substrates. We have compared the degree of CH mode softening for different types of hydrocarbons, including linear, branched, and cyclic alkanes. We have also studied the effect of varying the substrate symmetry by comparing the adsorption of cyclohexane on Cu(100), Cu(110), and Cu(111). Furthermore, we have investigated the effect of chemical modification on the extent of mode softening by comparing the adsorption of normal cyclohexane (C6H12) on Cu(111) and Cu3Pt(111), and of deuterated cyclohexane (C6D12) on Mo(110) and carbide-modified Mo(110).

Mechanisms of adsorption and decomposition of metal alkylamide precursors for ultrathin film growth

Atomic layer deposition film growth is usually characterized by the presence of a transient (nonlinear) regime, where surface reactions of precursors take place on the substrate, resembling the first stages of chemical vapor deposition and affecting the composition of the forming interface. Here, the adsorption and decomposition of tetrakis(dimethylamido)titanium, Ti[N(CH3)2]4, tetrakis(dimethylamido)zirconium, Zr[N(CH3)2]4, tetrakis(dimethylamido)hafnium, Hf[N(CH3)2]4, pentakis(dimethylamido)tantalum, Ta[N(CH3)2]5, and bis(t-butylimido)-bis(dimethylamido)tungsten, [(CH3)3CN]2W[N(CH3)2]2, on a silicon substrate are investigated using density functional methods. These alkylamides are widely used for deposition of both diffusion barriers and high-permittivity (high-κ) materials. Adsorption is found to be dissociative, with scission of metal-ligand bonds being more feasible than scission of N–C bonds, suggesting that decomposition of ligands is not favored at low temperatures. However, decomposition through ...