Self-assembled monolayers (SAMs) of functionalized alkanethiolates on gold are a well-characterized system for studying the interfacial properties of organic materials. We have used SAMs as models for the surfaces of organic polymers and used mem to study the adsorption of proteins onto organic materials. We have formed SAMs from mixtures of alkanethiols in which one alkanethiol is hydrophobic and the other is terminated by a short (2 ≤ n ≤ 17) oligomer of poly(ethylene oxide). These “mixed” SAMs effectively resist the adsorption of fibrinogen from moderately concentrated (1 mg/mL) solutions. Protein adsorption begins when < 5% of the accessible area of the surface consists of hydrophobic groups. These findings suggest that real protein-resistant monolayers must present an almost defect-free surface of oligo(ethylene oxide) groups in order to eliminate adsorption.

We are exploring the requirements for enzyme-catalyzed reactions on small molecules tethered to the surfaces of organic monolayers. Despite considerable effort toward understanding enzymatic processes in solution1, the chemistry of enzymes at interfaces has not been studied. Increasingly sophisticated methods of surface modification, including self-assembly2 and photolithographic3 techniques, raise intriguing prospects for enzymatic surface chemistry. This paper describes our initial investigations of the proteolysis of a dipeptide substrate covalently tethered to the surface of a polydiacetylene film using the enzyme, subtilisin BPN'.

A technique commonly used as the final stage of cleaning gold electrode surfaces is to electrochemically form and reduce a monolayer of oxide on the surface. We have investigated the changes on the Au(111) surface during this procedure using in situ electrochemical scanning tunneling microscopy.

Treatment of gallium arsenide with sulfur-containing media has been shown to improve GaAs surface electronic properties. However, there is still considerable controversy regarding the chemical nature of the surface film which results from the sulfidation, and of the basis of the electronic improvement and of the decay in the improved electronic properties with time. We have investigated the surface chemistry of the chemical sulfidation treatment of GaAs with Na2S-9H2O and the electrochemical sulfidation treatment of GaAs with Na2S-9H2O-ethylene glycol. Using surface infrared spectroscopy (SIRS), we have studied the film formed on the surface after the treatments and its behavior with time. Results show that the film on the GaAs surface contains sulfur which is often associated with oxygen, that this film slowly reacts in air to form unexpected species, e.g. sodium carbonate and sulfur-oxygen group-containing compounds, and that sulfur and oxygen are non-uniformly distributed.

The principles for an electrochemical digital etching method for compound semiconductors are described and initial results reported. The method is designed to allow atomic level control over the etching process, resulting in the removal of a bilayer of the compound for each cycle. An atomic layer of one element is removed at one potential and then an atomic layer of the second element is removed at a second potential to complete one cycle. The results reported here are for the etching of CdTe. For CdTe, Te is stripped by reduction to Te2- while Cd is stripped by oxidation to Cd2+. Underpotentials are chosen so that only the top atomic layer of an element is removed. Potentials sufficient to strip the elemėnt from the bulk of the CdTe substrate are avoided. Application of the method should involve the use of a simple electrochemical cell, with solution convection. The substrate is placed in the cell and a square wave applied, where each cycle results in the dissolution of a bilayer of the compound. The two potentials of the square wave correspond to underpotential stripping potentials for Cd and Te respectively. Directions for the future development of this etching method are discussed.

Cadmium sulfide particulate films have been generated at monolayer interfaces. Controlled and slow infusion of hydrogen sulfide onto compressed monolayers prepared from cadmium arachidate resulted in the formation of covalent metal sulfide bonds at a large number of sites at the monolayer-aqueous interface. The initial nucleation resulted in the downward growth of well-separated, metal-sulfide microclusters which grew in height and width and coalesced into interconnected arrays of semiconductor particles.

- We have designed and built a Low Energy Electron Microscope for surface and interface studies in Ultra High Vacuum. In this paper we present major features of the design, and some of our results on surfactant mediated Ge growth on Si(001).

X-ray diffraction has found an increasing use in the characterization of surface structures. Due to the high penetration depth of X-rays, the technique is also very suitable for the study of buried interfaces. We will give a general outline of the technique, and then discuss two examples concerning epitaxial growth.

We present dynamic structural studies of thin films and their interface with underlying substrates using real-time x-ray diffraction. Using synchrotron light we have observed, in real-time, interface dynamics in semiconductor systems such as GexSi(i−x)/Si. The measurements show that under large temperature changes thin epitaxial layers may behave cooperatively to modify the overall strain profile. Dynamic behavior is exhibited in a series of discontinuities in the perpendicular lattice constant of the overlayer.

Using grazing incidence x-ray scattering, we have studied incommensurate structures of Pb adlayers on Cu(110) surface and epitaxial Al films grown on Si(111) surface. Similar diffuse scattering profiles were found in both cases, which can be fitted with a Gaussian-plus-Lorentzian lineshape. The results are attributed to a overlayer structure with pinned misfit dislocations at the interface.

The effects of hydrogen adsorption on the growth process and structures of Ag thin films on Si(111)-7×7 surfaces has been studied. The growth process and film structures are investigated by low energy electron diffraction(LEED) and low energy ion scattering spectroscopy of time of flight mode(TOF-ICISS). The hydrogen adsorbed on the surface is investigated by low energy recoil detection analysis(TOF-ERDA). We have found that Ag thin films deposited onto hydrogen covered Si(111) surfaces grow with a mode definitely different from that on clean surfaces.

Interface structures between Si(111) and thick noble metal overlayers are studied by grazing-angle-incidence x-ray diffraction and crystal truncation rods. The 7×7 reconstruction is only preserved under a Ag film deposited at room temperature. This capped 7×7 structure changed to a 1×1 structure upon annealing over 250°C. A thick overlayer of room temperature deposited Au film destroyed the 7×7 reconstruction and changed the interface structure to 1×1. Our results are compared to a thick Cu/Si(111) interface structure1.

The results of specular and diffuse x-ray scattering studies of multilayers are discussed. We show here that such studies can yield detailed statistical information about the interfacial roughness and morphology. Results on a GaAs/AlAs multilayer are presented and the data is analyzed within the Born approximation.

Previous work has recently employed x-ray interferometry for the unique determination of the profile structures of ultrathin Langmuir-Blodgett multilayer films of Cd-arachidate and of tethered protein monolayers on the surface of Ge/Si multilayer substrates [1, 2]. These studies utilized the inorganic substrate as the reference structure for the interferometrie phasing of the meridional x-ray diffraction I (Qxy=0A-1, QZ) from the inorganic-organic composite structure. The substrates, fabricated by magnetron sputtering, contained only broad profile features (≥20 A), thereby limiting the spatial resolution of the organic profile structures so-determined. Molecular beam epitaxy (MBE) permits the fabrication of the multilayer reference structure with profile features as narrow as a single atomic monolayer, thereby providing delta-function-like features in the reference structure. The reference structure can then be tailored such that the autocorrelation function of the inorganic-organic composite profile structure [obtained by a Fourier transform of its meridional diffraction I (Qxy=0A-1, Qz) data without phase information] contains only the organic profile structure itself over a particular range of the profile coordinate z. This approach for uniquely determining the unknown profile structure of the organic overlayer is x-ray holography by analogy to simple off-axis holography with much longer wavelength radiation. We have initially utilized MBE fabricated Ge/Si multilayer substrates of the type N(Ge2Si30), e.g., for N=2 or 3 superlattice unit cells, each containing two Ge monolayers and thirty Si monolayers, to thereby determine the profile structures of four different organic overlayers, namely a) self-assembled alkylsiloxane monolayers, b) Langmuir-Blodgett cadmium alkylcarboxylate monolayers, c) a Langmuir-Blodgett Cd-arachidate head-to-head bilayer deposited on a)-above and, d) a covalently tethered protein monolayer.

We performed in-situ x-ray reflectivity measurements to study the growth dynamics of gold sputter deposited onto silicon using an x-ray scattering chamber equipped with a faced magnetron source where the substrate is held at a right angle relative to the sputtering guns. By operating the guns at low power (1 watt) and under 20 mTorr Ar, we could control the gold deposition rate to less than 1Å/sec. The observed x-ray reflectivity for gold deposited onto a silicon substrate at 300 K and 400 K is consistent with nucleated island growth for average gold particle sizes less than 50 Å. Above 50 Å, the reflectivity data indicates that the gold film uniformly covers the silicon surface, and that as the film thickness is increased the gold-vacuum interface gets progressively rougher. Detailed analysis of room temperature data is in progress, as is the temperature dependence on the roughness of the gold vacuum interface.

We studied the finite size effect on the metal-insulator phase transition and the accompanying tetragonal to monoclinic structural phase transition of VO2 films grown by MOCVD. X-ray diffraction measurements and electrical conductivity measurements were done as a function of temperature for VO2 films with out-of plane particle size ranging from 60–310Å. Each VO2 film was grown on a thin TiO2 buffer layer, which in turn was grown by MOCVD on a polished sapphire (1120) substrate. The transition was found to be first order. As the out-of plane particle size becomes larger, the transition temperature shifts and the transition width narrows. For the 60Å film the transition was observed at ∼61°C with a transition width of ∼10°C, while for the 310Å film the transition temperature was ∼59°C and the transition width ∼2°C. We also observed thermal hysteresis for each film, which became smaller with increasing particle size.

Epitaxial α-Fe films deposited on H-terminated Si(111) surfaces by electron beam evaporation in ultra high vacuum were studied using ion channeling/Rutherford Backscattering spectrometry (RBS). A 700 Å thick Fe film deposited on a Si(1 11) substrate at room temperature has a surface minimum yield, χmin, of 22% at 2 MeV energy of 4He+ ions along Si <111>. A rapidly increasing yield in the aligned spectrum with the depdi of the film shows the presence of defects throughout the film. The sharp lower energy edge of the Fe signal indicates the absence of an amorphous region or a suicide layer at the interface. This establishes epitaxial growth of iron film on Si(111), a result supported by x-ray diffraction and transmission electron microscopy studies. A 700 Å thick iron film deposited at 300 °C shows a surface minimum yield of 12%. Ion channeling/ backscattering spectra of 1400 Å and 3000 Å thick Fe films deposited at 300 °C on Si(111) substrates show a χmin of about 3% for both films, indicating a substantial reduction in the defect density in the thicker Fe films. Etching of Si(111) substrates in either a 10% HF solution or a NH4F solution does not change the crystalline quality of the iron films significantly.

Ultrathin ferromagnetic Fe films have been grown on an Au(OOl) surface. Using the angular profile measurements of High-Resolution LEED (HRLEED), we show the inho-mogeneities at the interface due to atomic place exchange between Fe and Au atoms can drastically change the angular intensity distributions of diffraction beams. The contributions from the inhomogeneities on the surface can be properly decoupled from the total angular intensity distributions. Through the energy dependent angular profile analysis of diffraction beams, we have quantitatively determined the degree of inhomogeneous atomic intermixing at elevated annealing temperatures.

We have investigated the growth mode of iron on clean Cu(100), Cu (110) and Cu(111) surfaces using high resolution LEED. We studied the effect of substrate temperature on the growth of epitaxial fee iron. Iron metal was evaporated from an electron beam source under UHV conditions and deposited on the copper single crystal surfaces at three different temperatures (173 K, 300 K and 473 K). A high resolution LEED system was used to measure the diffraction spot profiles. The samples were studied as a function of temperature after deposition on the copper substrates. The clean Cu surfaces were also measured and used as references for the Fe:Cu system. The diffraction spot profiles show Gaussian line shape before and after the Fe deposition. The effect of iron grown on the Cu surface is to reduce the coherent length. This effect is observed for all the copper surfaces. The intensity of the diffraction spots exhibit a sudden increase above 470 K, simultaneously a larger coherent length is observed. This effect occurs for all copper surfaces. The epitaxial growth of Fe is better on Cu (100) than on the other surfaces. The best epitaxial growth is obtained for the highest substrate temperature. A careful Auger study reveals that the anomalous increase observed on the intensity of the diffraction spots is related to the evaporation of iron from the copper surface. The spot profiles at low temperature are very broad and are suggestive of very poor epitaxy, random islands growth with a high probability of bec iron inclusions.

We investigate the room temperature growth of evaporated Au thin films on both clean and dirty single crystal Ge (111) substrates. The annealing behavior of these films was then examined under low and high temperatures.