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Scanning tunneling microscopy (STM) is used to examine reactions of 1,3-cyclohexadiene with silicon dimers on bare silicon (100) surface. In addition to the intradimer [2+2] and [4+2] reaction products, two different [2+2] conformers are identified.

The chemisorption of a series of diatomic molecules on the Si(100) surface was studied through scanning tunneling microscopy (STM). Dissociative chemisorption was found to occur and the adatoms produced were observed at two kinds of surface sites. The stability of these sites was examined by exciting the adsorbate system through a tip voltage pulse. The excitation caused the adatoms to undergo a rearrangement that depends both on the nature of the adspecies and on the site being stimulated.

The general concept of etching has undergone minimal modifications over the years. Despite the introduction of semiconductor-based devises, the primary means by which the process is carried out remains virtually the same- and that is exposing the surface of a hardened wax, without piercing it through.

The dynamics of dangling bond (DB) diffusion was studied after deuterium desorption from the silicon (100)-2x1 surface. At elevated temperatures, paired DB sites produced after desorption unpaired as deuterium atoms hopped from adjacent dimers. Below 620 kelvin, the unpaired configuration most commonly observed corresponded to two DBs on adjacent silicon dimers. At higher temperatures, unpaired DBs executed one-dimensional walks along the dimer rows, and recombination was observed with the same partner after walks lasting many minutes. The frequency and extent of these excursions increased with temperature. Above 660 kelvin, complete dissociation was observed and was sometimes followed by recombination by means of partner exchange. The implications for low-temperature materials growth are discussed., Hydrogen desorption is an essential step in the growth of materials by chemical vapor deposition (CVD). Desorption regenerates surface DBs to provide reactive sites for further growth (1). However, the [...]

There has been substantial analysis of the possible impact of climate change on water supply, especially with respect to runoff and river flows. Less attention has been given to urban water use. Little is known of the suitability of various water use forecasting models for predicting climate impacts or of the best procedures for assessing this issue. This paper will: (1) demonstrate the feasibility of a scenario approach to describing possible changes in climate, (2) evaluate the IWR-MAIN model as a source of plausible water use forecasts given uncertain future climate, (3) test the effectiveness of conservation and pricing interventions in reversing the postulated effects of climate change, and (4) assess the significance of climate change for future urban water management. Other possible responses to climate change, such as supply augmentation, are not explicitly considered. Using data for the Washington (DC) metropolitan area, the study reveals problems with IWR-MAIN version 5.1 when used for this purpose, but results in a reasonable assessment of the possible water use consequences of climate change. Variation in future water use due to climate uncertainty was found to be moderate compared to other uncertain influences, and well within reach of feasible policy interventions.

An experimental opening of the mouth of San Elijo Lagoon (San Diego County, Calif.) was conducted in order to evaluate its effect on microbial water quality of adjacent recreational ocean waters. Water quality was most negatively impacted right after opening of the lagoon, and after any significant rain event. At the surizone site closest to the lagoon mouth, 40% of the samples (within a three-month period) exceeded a total coliform bacteria density of 1,000 organisms per 100 ml, out of compliance with the California Ocean Plan water-contact standard. The fecal coliform standard in the Ocean Plan was attained at all surfzone sites, while the objective of a geometric mean enterococci density, of 24 organisms per 100 ml (over a 30-day period) was exceeded at the surizone site closest to the lagoon mouth. A survey of sources of fecal loading to the lagoon identified runoff from a storm drain as a likely source of contamination to the western basin of the lagoon, and ultimately the ocean surizone. Recommendations include conducting a detailed sanitary survey and controlling sewer overflows from establishments in the area before a permanent lagoon enhancement project is implemented., San Elijo Lagoon is a coastal estuary located approximately 20 miles north of San Diego. The wetland is composed of about 530 acres divided into three basins by the Interstate [...]

Chlorine atoms strongly chemisorbed at dangling bond sites on the Si(100)-(2 x 1) surface are observed by scanning tunneling microscopy (STM) to hop between adjacent sites. The origin of this [...]

This paper describes the initial phase of an investigation to develop a methodology for estimating the total costs of a dam failure. This methodology takes into account the fundamental issues of accounting stance, damage to fixed assets, loss of income, intangible losses, and loss of life and limb. A review and analysis of the English-language literature on historical floods and dam failures was used as the starting point for developing a framework for evaluating dam-failure costs. Cost-related information is portrayed by type and collection priority in a data-collection matrix to guide an on-site investigation. This matrix was tested by evaluating five major dam failures that occurred in the United States during the past 20 years, and significant limitations in the current data base were identified. Once validated by trial data collections involving future events, the methodology might be used to assist in conducting a probabilistic safety assessment of a dam, or to develop more rational public policies aimed at risk mitigation.

Author(s): John J. Boland [1] Silicon-based electronics continues to obey the dictum known as Moore's law: that the density of transistors on an integrated circuit -- a rough measure of [...]

Hydrogen atoms can selectively eliminate strained bonds that form during the growth of amorphous silicon films. By periodically interrupting the growth and exposing the grown material to hydrogen, the film composition can be varied continuously from a non-equilibrium amorphous structure to that of a crystalline solid. Furthermore, by tuning the hydrogen exposure it is possible to discriminate between Si-Si bonds formed on different substrates, thereby allowing substrate-selective growth. The evolution of the film structure during hydrogen exposure is directly observed by scanning tunneling microscopy, and a model describing the role of hydrogen is presented., The ability to tailor structural and electronic properties is a fundamental goal of materials science. This necessitates the development of techniques that allow direct control of the chemical bonding in [...]

Density functional theory calculations are used to order the predicted frontier wave functions in terms of energy for a single Si dimer unit across the surface Brillouin zone. The calculated frontier states are found to be a valuable tool in describing reactivity on Si(100) that is consistent with experiment and provides a rational means to predict allowed reaction products on Si(100).

Identification of the Products from the Reaction of Chlorine with the Silicon(111)-(7X7) Surface SCANNING TUNNELING MICROSCOPY has developed into a powerful tool for the study of surfaces. The strength of [...]

The conductive properties of vertically aligned germanium nanowires, with mean diameters of 50 and 100 nm, within anodized aluminum oxide (AAO) templates are characterized by conductive atomic force microscopy (C-AFM) and macrocontact measurements. C-AFM was used to determine the electrical transport properties of individual nanowires within the arrays, while macrocontacts were used to measure the mean current-voltage characteristics of groups of nanowires.

The density functional theory (DFT) is used to study different possible bonding geometries of 1,3-cyclohexadiene (1,3-CHD) on the Si(100)-2x1 surface and to compare these results with the STM experiments. The results obtained are analyzed in terms of the total energy values and the detailed optimized geometries.

Intuitively one decomposes climate variables such as solar irradiation and ambient temperature into their deterministic and stochastic components. The deterministic component comprises a sum of contributions at various frequencies, thereby defining the climate of the location. The stochastic component comprises the fluctuations about this component, giving the day-to-day weather variations. Boland (Solar Energy 60(6) (1997) 359) shows that on a day-to-day basis both components must be included in building thermal performance simulations to give sensible results. It is shown in this paper that on an hour-to-hour basis the stochastic component may be disregarded in building thermal, solar process heat and photovoltaic system performance without significantly affecting the results. Since the hourly stochastic component of the solar irradiation is very difficult to model, this result greatly simplifies the construction of synthetic climate data sets.

A chemically induced dimer configuration was prepared on the silicon (Si) (100) surface and was characterized by scanning tunneling microscopy (STM) and spectroscopy (STS). These prepared dimers, which are essentially unfired and differ both electronically and structurally from the dynamically tilting dimers normally found on this surface, are more reactive than normal dimers. For molecular hydrogen ([H.sub.2]) adsorption, the enhancement is about [10.sup.9] at room temperature. There is no appreciable barrier for the [H.sub.2] reaction at prepared sites, indicating the prepared configuration closely approximates the actual dimer structure in the transition state. This previously unknown ability to prepare specific surface configurations has important implications for understanding and controlling reaction dynamics on semiconductor surfaces., The reaction of [H.sub.2] with the Si(100)-2 X 1 surface is perhaps the best characterized and most important adsorbate-semiconductor system (1), but even the dynamics of [H.sub.2] on Si(100)-2 X [...]