H-terminated Si Research Articles

Catalytic Stamp Lithography for Sub-100 nm Patterning of Organic Monolayers

A stamp-based nanoscale patterning technique of organic monolayers, termed catalytic stamp lithography, is described. The surface of poly(dimethylsiloxane) was patterned with catalytic Pd nanoparticles (NPs) via the use of self-assembled block copolymers. Using this catalytic stamp, catalytic hydrosilylations of terminal alkenes/alkynes were performed on H-terminated Si(111) or Si(100) surfaces to create nanoscale patterns of organic monolayers. Since the reaction takes place exclusively underneath the patterned Pd NPs (localized catalysis), the pattern formation is less susceptible to ink diffusion and stamp deformation, even at this sub-100 nm scale. A range of different molecular inks can be utilized to produce monolayer patterns of different chemical functionalities, and the stamps can be reused multiple times. The potential utility of this kind of chemically patterned surfaces as the basis for more complicated nanoarchitectures was demonstrated through gold nanoparticle capture, with a thiol-terminated nanopatterned silicon surface.

Chemical routes to molecular SAMs on H-Si(100) with distinct and well-defined redox potentials

New or rarely reported synthetic routes have been applied to produce electroactive hybrid materials from ferrocene (Fc) derivatives with a known and pre-established variation in the tethering arm bound to Si. The series is characterized for having the shortest molecular link for a direct covalent attachment to H-terminated Si(100). Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetilyde anion have been applied for the first time in order to obtain the full series of possible unsaturations in the C–C lateral chain bound to Si: ethyl ( EtFC/Si), vinyl ( VFC/Si) and ethynyl group ( EFC/Si). The redox potentials of these three molecule/surface hybrids, measured by cyclic voltammetry, are respectively 0.059, 0.136 and 0.251 V vs. Ag/Ag +, and increase with the extent of unsaturation in the tethering arm. XPS and electrochemical measurements have been used to assess the chemical nature and electronic properties of the hybrids.

Temperature Dependence of Competitive Reaction of Iodine Ions on H-Terminated Si(111) Surface in a Concentrated HI Solution

The surface reactions of iodine ions on nearly atomically flat, H-terminated Si(111) surfaces, when immersed in 7.6 M HI in a temperature range of −20 to 63 °C, were investigated by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and core-level shift spectra. It was revealed that the following two reactions simultaneously occurred, which were competitive with each other: (1) substitution reaction of Si−H bonds with Si−I bonds and (2) cluster formation by the condensation of SiHxI4−x or their oligomer species, which were generated by surface etching reaction. The ratio of those competitive reactions was strongly dependent on the temperature of the HI solution. At the high temperature, the former reaction was enhanced, whereas the latter reaction was suppressed. Inversely, at the low temperature, the latter reaction was relatively enhanced, resulting in that the ordered rodlike clusters were formed on the Si(111) surface. These results were explained by the fact that the former and latter reactions were mainly enhanced by the H2O and dissolved oxygen, respectively, and the concentration of dissolved oxygen was drastically decreased in the HI solution at high temperature, compared with that at low temperature. It was also revealed that the order and shape of the clusters formed by the condensation of SiHxI4−x or their oligomer species produced by etching reaction were strongly dependent on the temperature of the HI solution.

Site-specific adsorption of metallic and biological nanoparticles on nanostructured silicon surfaces

The (photo)electrochemical preparation of nanostructures on single crystalline Si surfaces is described and surfaces are characterized by tapping mode atomic force microscopy (TM-AFM), Kelvin probe microscopy (KPM), high-resolution electron energy loss spectroscopy (HREELS), and synchrotron radiation photoelectron spectroscopy (SRPES). The H-terminated Si(111) surface and the step-bunched Si surface that exhibits multiatomic bilayer (BL) steps are prepared. HREELS demonstrates the low contamination levels achieved with the used in-system combined electrochemistry/surface analysis apparatus. With a miscut in the direction, the surfaces prepared show a zigzag structure of the atomic terraces. These unique features provide the substrate for the (electro)deposition of platinum and for the immobilization of enzymes. Pt deposition, of relevance for the development of photovoltaic or fuel-generating solar cells, occurs predominantly at the edges of steps. On step-bunched surfaces, where KPM shows negative charging along the step edges and SRPES evidences the presence of an accumulation layer, the Pt nanoislands are considerably smaller than on H-terminated surfaces that are in flat band situation before deposition. SRPES analysis of the chemical and electronic condition after electrodeposition shows silicon oxide formation on both surfaces, suboxidic species, and the typical asymmetric Pt 4f core level lines. The results are discussed based on the details of the Pt deposition process at Si surfaces. As biomolecule, the enzyme reverse transcriptase (RT) of the avian myeloblastosis virus (AMV) was used. Immobilization is observed in TM-AFM experiments at the negatively charged step edges of the step-bunched surface. It is attributed to a superposition of attractive van der Waals and electrostatic interactions. The latter is related to the rather large Debye length of the carrier solution (∼4 nm) and the overall positive charge of the RT where the pH of the carrier solution (pH 7) is smaller than that for the isoelectric point (IP) of 8.3.

Electrochemical growth of ultraflat Au(111) epitaxial buffer layers on H–Si(111)

Thin metal layers deposited on a substrate (buffer layers) are widely employed in many fields of research and in microelectronic industry. In this work we present x-ray diffraction measurements and atomic force microscopy observations which demonstrate that high quality ultrathin Au(111) epitaxial films can be electrodeposited from a gold chloride solution on a well-defined H-terminated vicinal Si(111) surface. In addition, the obtained films present no apparent grain boundary. This result, which seems in conflict with classical growth theory, requires applying unconventional electrodeposition conditions in which a strong hydrogen evolution reaction is promoted.

Adsorption of metal impurities on H-terminated Si surfaces and their influence on the wetchemical etching of Si

We use first-principles methods to investigate the adsorption of Cu, Pb, Ag, and Mg onto aH-terminated Si surface. We show that Cu and Pb can adsorb strongly whileAg and Mg are fairly inert. In addition, adsorption states of two types are seento exist for Pb. We also study the clustering energetics of Cu and Pb on thesurface and find that while Cu clusters eagerly, Pb may prefer to form only smallclusters of a few atoms. This kind of behavior of impurities is incorporated inkinetic Monte Carlo simulations of wet etching of Si. The simulation results agreewith experiments supporting the idea that micromasking by Cu clusters and Pbatoms is the mechanism through which these impurities affect the etching process.

Nonlocal Conductance Modulation by Molecules: Scanning Tunneling Microscopy of Substituted Styrene Heterostructures on H-Terminated Si(100)

One-dimensional organic heterostructures consisting of contiguous lines of CF3- and OCH3-substituted styrene molecules on silicon are studied by scanning tunneling microscopy and ab initio simulation. Dipole fields of OCH3-styrene molecules are found to enhance conduction through molecules near CF3-styrene/OCH_{3}-styrene heterojunctions. Those of CF3-styrene depress transport through the nearby silicon. Thus the choice of substituents and their attachment site on host molecules provide a means of differentially tuning molecule and substrate transport at the molecular scale.

Growth and interface of HfO2 films on H-terminated Si from a TDMAH and H2O atomic layer deposition process

Hf O 2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis(dimethyl)amino hafnium and H2O precursors at a substrate temperature of 200–325°C. The initial stage of film growth on OH- and H-terminated Si(100) surfaces is investigated using Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). The authors observe an initial growth barrier on the Si–H surface for the first approximately four process cycles, where film growth is more efficient on the OH-terminated surface. Both starting surfaces require about 15cycles to reach a steady growth rate per cycle, with the OH-terminated surface displaying a slightly higher growth rate of 2.7×1014Hf∕cm2 compared to 2.4×1014Hf∕cm2 for Si–H. Combining the RBS and SE data we conclude that the films deposited on the OH-terminated surface are denser than those deposited on the Si–H surface. Angle-resolved XPS measurements reveal the formation of an ∼8Å interfacial layer after four ALD cycles on the H-terminated surface for a deposition temperature of 250°C, and transmission electron microscopy verifies that the thickness of the interfacial layer does not change substantially between the 4th and the 25th process cycles. The interfacial layer appears to depend weakly on the deposition temperature from 200to325°C, ranging from 6.9to8.4Å.

The impacts of surface conditions on the vapor-liquid-solid growth of germanium nanowires on Si (100) substrate

The impacts of surface conditions on the growth of Ge nanowires on a Si (100) substrate are discussed in detail. On SiO2-terminated Si substrates, high-density Ge nanowires can be easily grown. However, on H-terminated Si substrates, growing Ge nanowires is more complex. The silicon migration and the formation of a native SiO2 overlayer on a catalyst surface retard the growth of Ge nanowires. After removing this overlayer in the HF solution, high-density and well-ordered Ge nanowires are grown. Ge nanowires cross vertically and form two sets of parallel nanowires. It is found that nanowires grew along ⟨110⟩ directions.

Atomic modeling of surface photovoltage: Application to Si(1 1 1):H

The dependence of surface photovoltages on the wavelength of light are obtained for the first time from an atomic model and ab initio calculations. Photovoltages follow from a time-dependent density matrix treatment using a basis set of Kohn–Sham orbitals and a steady state solution for the time-dependent density matrix. An application to a H-terminated Si(1 1 1) surface gives the main features of calculated photovoltage versus incident photon energies in agreement with experimental trends. Our treatment can be implemented for a wide class of photo-electronic materials relevant to solar energy capture.

Depth distribution of Cs implanted into Si at steady-state during dual beam ToF-SIMS profiling

The steady-state depth distributions of Cs atoms implanted into a H-terminated Si surface at different energies (250 eV to 2 keV, 45° incidence angle) were studied. In situ ToF-SIMS depth profiles of the Si surface preloaded with Cs were obtained immediately after the Cs implantation using low energy Xe + (350 eV, 45°) and O 2 + (500 eV, 45°) sputter beams and a Ga + analysis beam (15 keV, 45°). The crater depth was measured ex situ by a profilometer to calibrate the depth scale. All the Cs profiles exhibited a surface peak attributed to segregation and a maximum under the surface for both Xe + and O 2 + sputtering beams. The relative height of the surface segregation peak, the position of the maximum and the width of the Cs profiles depend on the energy of the Cs primaries. Depth profiles of adsorbed/sputter deposited Cs obtained next to the implantation craters exhibited no surface peak. A dynamic TRIM code was used to obtain the depth distribution of the implanted Cs at steady-state. The results of the simulations were compared to those obtained experimentally.

Extracting maximum information from polarized surface vibrational spectra: Application to etched, H-terminated Si(110) surfaces

A general method to maximize the information extracted from polarized surface absorption spectra is developed and applied to the study of etched Si(110) surfaces. In essence, this technique transforms spectra from the experimental reference frame, which is defined by the direction of the surface electric field during irradiation by s- and p-polarized light, into a more appropriate Cartesian reference frame defined by the surface normal and the plane of incidence. If the Cartesian reference frame is aligned with high symmetry directions of the system, significant spectral simplification can result. This analysis relies on the well-known boundary conditions on interfacial electric fields and is independent of any adsorbate screening or the effective dielectric constant of the adsorbate layer. The validity of this analysis is demonstrated on the spectra of NH4F-etched, H-terminated Si(110). The transition dipole moments of the symmetric and antisymmetric Si[Single Bond]H stretch modes associated with flat terraces are polarized along the [110] and [001] directions, respectively. Two additional modes with transition dipoles polarized along the [001] and [110] directions are assigned to defect species associated with microfaceting and other surface roughness. Data taken in two different experimental geometries are shown to be in excellent quantitative agreement, confirming the validity of the technique. Additionally, the measured adsorbate layer dielectric constant is in good agreement with previously reported values for hydrogen-terminated silicon surfaces.

Theoretical calculation on the reaction of alkene molecules on H-terminated Si(100)-3×1 surface

A study on mechanisms of radical initiated surface chain reaction of ethylene molecule on H-terminated Si(100)-3 × 1 has been carried out in a supercell approach by using density functional theory and ab initio molecular dynamic method. On the H-terminated Si(100)-3 × 1 surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy from the nearest silicon site than from the next-nearest silicon site. From the intermediate state to the final state, the transition state has bigger activation energy. Ab initio molecular dynamics (MD) shows that the H-abstraction on Si(100)-3 × 1 surface bound organic group with a carbon-centered radical is very easy to be obtained from the transition state, and it also shows that the C…H bond at methyl group is formed in a very short MD time, and the Si…C bond between the Si surface and the alkyl chain oscillates with time evolution on Si(100)-3 × 1 surface.

Adsorption of Disilyne [ Si ( H2 ) Si ] and Disilene ( H2SiSiH2 ) on the Si ( 100 ) -2 × 1 Surface and on the H-Terminated Si ( 100 ) -2 × 1 Surface

Disilyne and disilene adsorption on the surface and the H-terminated surface was investigated by density functional theory calculations using the and cluster models of the surface. It was found that on the surface, a positive activation energy does not exist for the adsorptions of both disilyne and disilene. On the H-terminated surface, disilyne adsorbs through a positive activation energy around at the Becke-3 Lee–Yang–Parr functional and the 6–31G(d) basis set level. No stable disilene adsorption structure on the H-terminated surface was found, which indicates that disilene rarely reacts with the H-terminated surface. For the adsorption of disilyne on the H-terminated surface, the adsorption energy calculated using the cluster model is in reasonable agreement with that calculated using the cluster model .

Effect of surface preparation on the morphology of ZnO nanorods

The effects of Si substrate orientation and surface treatment on the morphology and density of Zinc oxide (ZnO) nanorods were investigated. The size and density of ZnO nanorods were influenced by Si substrate orientation and surface preparation. ZnO nanorods synthesized on the ideally H-terminated Si(1 1 1) prepared with an NH 4F solution resulted in the biggest size and the lowest density. It is suggested that the smoother surface of the Si substrate and lattice shape match with a larger atomic distance result in the increase of the ZnO seedlayer's grain size, which in turn enhances the size of ZnO nanorods grown on it. The optical properties of the ZnO nanorods were affected by their size and crystallinity. The smallest ZnO nanorods with a preferential c-axis orientation synthesized on the HF-treated Si(1 1 1) surface showed the highest intensity ratio of UV to visible emission, and the biggest ZnO nanorods synthesized on the N 2-sparged NH 4F-treated Si(1 1 1) surface showed the lowest intensity ratio of UV to visible emission. Therefore, it can be concluded that Si substrate orientation and surface preparation significantly affect the optical properties of ZnO nanorods.

The chemisorption of polyaromatic hydrocarbons on Si(1 0 0)H dangling bonds

The adsorption of polyacene molecules on a H-terminated Si(0 0 1)-2 × 1 surface where a few hydrogen atoms have been extracted is presented using the semi-empirical ASED+ method. To scale up the qualitative ASED+ method, the adsorption of benzene and pentacene on a clean silicon surface is first compared with DFT calculations together with their adsorption on a fully hydrogenated Si(1 0 0) surface. When a few hydrogen atoms have been selectively removed from the SiH(0 0 1) surface, ASED+ demonstrates a difficult chemisorption for the polyacenes series on one dangling bond and a large interaction on two dangling bonds compensating for the large molecule deformation required. The influence of the nearest H atoms when the molecule is adsorbed on two dangling bonds of a passivated Si(0 0 1) surface is very small. On a clean and on a partially hydrogenated surface, all the polyacenes need to overcome an energy barrier of 0.3–0.4 eV to reach a chemisorption state.

Theoretical prediction of 1-D molecular lines on the Si(001) surface

We propose a facile method for the self-directed growth of ID molecular lines on the H-terminated Si(001) surface. This method employing a single H-free dimer instead of a single DB greatly enhances the stability of the radical intermediate, thereby facilitating the chain reaction for ID molecular lines. The proposed method will stimulate experiments for fabrication of ID molecular lines which are strongly attached to the Si(001) substrate.

Ab-Initio Study on Adsorption and Diffusion of Au Atoms on Clean Si(001) and H-Si(001) Surface

Performing ab-initio total-energy calculations to investigate the adsorption and diffusion processes of the Au atoms with both the clean Si(001)-(1×1) and H-terminated Si(001)-(2×1) surfaces. It was found that, on the clean Si(001)-(1×1) surface, the most stable adsorption sites for Au atoms are middle part of four Si atoms, while on H-terminated Si(001)-(2×1) surface, the most stable sites are the middle part of a Si-Si dimer. The result showed that surface hydrogenation make most stable site transfer and affect the adsorption of Au on Si(001) surface.

UV-induced immobilization of tethered zirconocenes on H-terminated silicon surfaces

A tethered ethylenebis(indenyl) zirconocene was covalently immobilized on H-terminated Si(111) surfaces using UV-mediated alkene hydrosilylation, thus making possible the development of structured catalytic surfaces with highly controlled properties.

Radical Chain Reaction of Alkene Molecule on H-terminated Si(100)-2×1 Surface

应用密度泛函理论（DFT）和从头计算分子动力学方法（abinitioMD）研究了不饱和烯烃在氢终止的Si（100）-2×1表面的自由基链反应.计算表明,自由基链反应中重要的一步是烷烃链自由基的氢抽提过程,硅表面上邻近位置（the nearest neighbor,NN）的氢抽提比次邻近位置（thenext-nearest neighbor,NNN）的氢抽提有一稍低的能垒.从头算分子动力学显示,过渡态的烷烃自由基与氢终止Si（100）-2×1表面上的氢原子能够很容易形成C—H键,完成一个氢抽提过程,同时在硅表面产生下一个孤电子,继续引发链反应.