Dehydrative Cyclocondensation Reactions on Hydrogen-Terminated Si(100) and Si(111): An ex Situ Tool for the Modification of Semiconductor Surfaces

Reactive Simulations of Silica Functionalization with Aromatic Hydrocarbons

Reactive molecular dynamics simulations are used to model the covalent functionalization of amorphous silica with aromatic hydrocarbons. Simulations show that the surface density of silanol-terminated phenyl, naphthyl, and anthracenyl molecules is lower than the maximum value calculated based on molecule geometry, and the simulation densities decrease faster with the number of aromatic rings than the geometric densities. The trends are analyzed in terms of the surface-silanol bonding configurations, tilt angles, local conformational ordering, and aggregation of surface-bound molecules under steady-state conditions. Results show that the surface density is affected by both the size and symmetry of the aromatic hydrocarbons. The correlations among bonding, orientation, and surface density identified here may guide the selection or design of molecules for functionalized surfaces.

Solution Chemistry to Control Boron-Containing Monolayers on Silicon: Reactions of Boric Acid and 4-Fluorophenylboronic Acid with H- and Cl-terminated Si(100)

The reactions of boric acid and 4-fluorophenylboronic acid with H- and Cl-terminated Si(100) surfaces in solution were investigated. X-ray photoelectron spectroscopy (XPS) studies reveal that both molecules react preferentially with Cl-Si(100) and not with H-Si(100) at identical conditions. On Cl-Si(100), the reactions introduce boron onto the surface, forming a Si-O-B structure. The quantification of boron surface coverage demonstrates that the 4-fluorophenylboronic acid leads to ∼2.8 times higher boron coverage compared to that of boric acid on Cl-Si(100). Consistent with these observations, density functional theory studies show that the reaction of boric acid and 4-fluorophenylboronic acid is more favorable with the Cl- versus H-terminated surface and that on Cl-Si(100) the reaction with 4-fluorophenylboronic acid is ∼55.3 kJ/mol more thermodynamically favorable than the reaction with boric acid. The computational studies were also used to demonstrate the propensity of the overall approach to form high-coverage monolayers on these surfaces, with implications for selective-area boron-based monolayer doping.

Reaction of BCl3 with H- and Cl-terminated Si(1 0 0) as a pathway for selective, monolayer doping through wet chemistry

The reaction of boron trichloride with the H and Cl-terminated Si(100) surfaces was investigated to understand the interaction of this molecule with the surface for designing wet-chemistry based silicon surface doping processes using a carbon- and oxygen-free precursor. The process was followed with X-ray photoelectron spectroscopy (XPS). Within the reaction conditions investigated, the reaction is highly effective on Cl-Si(100) for temperatures below 70°C, at which point both surfaces react with BCl₃. The XPS investigation followed the formation of a B 1s peak at 193.5 eV corresponding to (B-O)_x species. Even the briefest exposure to ambient conditions lead to hydroxylation of surface borochloride species. However, the Si 2p signature at 102 eV allowed for a confirmation of the formation of a direct Si-B bond. Density functional theory was utilized to supplement the analysis and identify possible major surface species resulting from these reactions. This work provides a new pathway to obtain a functionalized silicon surface with a direct Si-B bond that can potentially be exploited as a means of selective, ultra-shallow, and supersaturated doping.

29,31- H Phthalocyanine Covalently Bonded Directly to a Si(111) Surface Retains Its Metalation Ability

The reaction of metal-free phthalocyanine molecules with a chlorine-terminated Si(111) surface is investigated to produce a phthalocyanine functionality directly attached to a semiconductor surface, without additional linkers or layers. The carefully prepared Cl-Si(111) surface provides an oxygen-free substrate that is reacted with 29,31- H phthalocyanine (H₂Pc) in a wet-chemistry process resulting in HCl elimination. The in situ metalation of this H₂Pc-modified silicon surface with cobalt is confirmed, suggesting that the produced functionality is chemically active. These processes are investigated by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and time-of-flight secondary ion mass spectrometry supplemented by density functional theory calculations. The morphology of the surface is monitored by atomic force microscopy. The combined spectroscopic, microscopic, and theoretical investigations demonstrate that additional linkers are not required for phthalocyanine attachment to occur, as the direct attachment can take place by forming Si-N bonds, and that the resulting surface species can participate in a metalation process.

Universal Calibration of Computationally Predicted N 1s Binding Energies for Interpretation of XPS Experimental Measurements

Computationally predicted N 1s core level energies are commonly used to interpret the experimental measurements obtained with X-ray photoelectron spectroscopy. This work compares the application of Koopmans' theorem to core electrons using the B3LYP functional with two commonly used basis sets, analyzes the factors relevant to the comparison of the computational with experimental data, and presents several correlations that allow an accurate prediction of the N 1s binding energy. The first correlation is obtained with a series of known nitrogen-containing functional groups on well-characterized organic monolayers. This approach can then be reliably extended to a number of nitrogen-containing chemical systems on silicon surfaces in which the nature of the chemical environment of nitrogen atoms had only been proposed based on a number of analytical techniques. In most of those cases, the XPS analysis is consistent with the proposed structures, but is not always sufficient for conclusive assignments. Third, it was attempted to also include N-containing systems on metals. Despite the admittedly oversimplified approach taken in this case (the metal surface is approximated by a single atom), the observed correlations are still experimentally useful, although in this case significant outliers are found. Finally, previously published correlations between experimental and theoretical C 1s data were reexamined, yielding a set of correlations that allow experimentalists to predict C 1s and N 1s XPS spectra with high accuracy.

Evolution of CH3NO2/Si interfacial chemistry under reaction conditions: a combined experimental and theoretical study

Dissociative adsorption of CH₃NO₂ onto a Si(100)-2 × 1 surface is studied using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations.

Dehydrohalogenation Condensation Reaction of Phenylhydrazine with Cl-Terminated Si(111) Surfaces

Formation of stable organic-inorganic contacts with silicon often requires oxygen- and carbon-free interfaces. Some of the general approaches to create such interfaces rely on the formation of a Si-N bond. A reaction of dehydrohalogenation condensation of Cl-terminated Si(111) surface with phenylhydrazine is investigated as a means to introduce a simple function to the surface using a -NH-NH₂ moiety as opposed to previously investigated approaches. The use of substituted hydrazine allows for the formation of a stable structure that is less strained compared to the previously investigated primary amines and leads to minimal surface oxidation. The process is confirmed by a combination of infrared studies, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry investigations. Density functional theory is utilized to yield a plausible surface reaction mechanism and provide a set of experimental observables to compare with these data.

Multifunctional silicon surfaces: reaction of dichlorocarbene generated from Seyferth reagent with hydrogen-terminated silicon (111) surfaces

Insertion of dichlorocarbene (:CCl2), generated by decomposition of the Seyferth reagent PhHgCCl2Br, into the Si-H bond of a tertiary silane to form a Si-CCl2H group is an efficient homogeneous, molecular transformation. A heterogeneous version of this reaction, between PhHgCCl2Br and a silicon (111) surface terminated by tertiary Si-H bonds, was studied using a combination of surface-sensitive infrared and X-ray photoelectron spectroscopies. The insertion of dichlorocarbene into surface Si-H bonds parallels the corresponding reaction of silanes in solution, to produce surface-bound dichloromethyl groups (Si-CCl2H) covering ∼25% of the silicon surface sites. A significant fraction of the remaining Si-H bonds on the surface was converted to Si-Cl/Br groups during the same reaction, with PhHgCCl2Br serving as a halogen atom source. The presence of two distinct environments for the chlorine atoms (Si-CCl2H and Si-Cl) and one type of bromine atom (Si-Br) was confirmed by Cl 2p, Br 3d, and C 1s X-ray photoelectron spectroscopy. The formation of reactive, halogen-terminated atop silicon sites was also verified by reaction with sodium azide or the Grignard reagent (CH3MgBr), to produce Si-N3 or Si-Me functionalities, respectively. Thus, reaction of a hydrogen-terminated silicon (111) surface with PhHgCCl2Br provides a facile route to multifunctional surfaces possessing both stable silicon-carbon and labile silicon-halogen sites, in a single pot synthesis. The reactive silicon-halogen groups can be utilized for subsequent transformations and, potentially, the construction of more complex organic-silicon hybrid systems.

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.

Metallic nanostructure formation limited by the surface hydrogen on silicon

Constant miniaturization of electronic devices and interfaces needed to make them functional requires an understanding of the initial stages of metal growth at the molecular level. The use of metal-organic precursors for metal deposition allows for some control of the deposition process, but the ligands of these precursor molecules often pose substantial contamination problems. One of the ways to alleviate the contamination problem with common copper deposition precursors, such as copper(I) (hexafluoroacetylacetonato) vinyltrimethylsilane, Cu(hfac)VTMS, is a gas-phase reduction with molecular hydrogen. Here we present an alternative method to copper film and nanostructure growth using the well-defined silicon surface. Nearly ideal hydrogen termination of silicon single-crystalline substrates achievable by modern surface modification methods provides a limited supply of a reducing agent at the surface during the initial stages of metal deposition. Spectroscopic evidence shows that the Cu(hfac) fragment is present upon room-temperature adsorption and reacts with H-terminated Si(100) and Si(111) surfaces to deposit metallic copper. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to follow the initial stages of copper nucleation and the formation of copper nanoparticles, and X-ray energy dispersive spectroscopy (XEDS) confirms the presence of hfac fragments on the surfaces of nanoparticles. As the surface hydrogen is consumed, copper nanoparticles are formed; however, this growth stops as the accessible hydrogen is reacted away at room temperature. This reaction sets a reference for using other solid substrates that can act as reducing agents in nanoparticle growth and metal deposition.

Density functional theory study on the reaction mechanisms of bis(cyclopentadienyl)magnesium with hydrogenated and hydroxylated Si(100)-(2x1) surfaces

The mechanisms for reaction between bis(cyclopentadienyl)magnesium and Si(100)-(2x1) surface are investigated with the aid of density functional theory calculations. The reactions on hydrogenated and hydroxylated Si surfaces are compared to understand the dominated initial reaction during atomic layer deposition of MgO on Si surface. The overall reaction energy and activation barrier are calculated for each reaction. It is found that the reaction of bis(cyclopentadienyl)magnesium with OH-terminated Si surface is both kinetically and thermodynamically more favorable than that with H-terminated Si surface.

Molecular transport junctions with semiconductor electrodes: analytical forms for one-dimensional self-energies

Analytical self-energies for molecular interfaces with one-dimensional, tight-binding semiconductors are derived, along with analytical solutions to the electrode eigensystems. These models capture the fundamental differences between the transport properties of metals and semiconductors and also account for the appearance of surface states. When the models are applied to zero-temperature electrode-molecule-electrode conductance, junctions with two semiconductor electrodes exhibit a minimum bias threshold for generating current due to the absence of electrode states near the Fermi level. Molecular interactions with semiconductor electrodes additionally produce (i) non-negligible molecular-level shifting by mechanisms absent in metals and (ii) sensitivity of the transport to the semiconductor-molecule bonding configuration. Finally, the general effects of surface states on molecular transport are discussed.