>From: "Science-Week" <[EMAIL PROTECTED]> >Organization: Science-Week >To: [EMAIL PROTECTED] >Date: Wed, 12 Apr 2000 10:54:02 -0600 >X-Distribution: Moderate >Subject: ScienceWeek BULLETIN April 12, 2000 >Reply-to: [EMAIL PROTECTED] >Priority: normal >X-mailer: Pegasus Mail for Win32 (v3.01d) > >ScienceWeek BULLETIN - April 12, 2000 >**************************************************************** > >This is SW BULLETIN, a free publication published each Monday by >the Editors of SCIENCE-WEEK, the weekly Email research digest. >The single report that appears here is from a back issue of >ScienceWeek. A subscription to the full SCIENCE-WEEK is only >US$20 per year. Subscription info at URL: >http://www.scienceweek.com/subinfo.htm > >**************************************************************** >Table of Contents of the current issue of ScienceWeek will be >found near the end of this file. >**************************************************************** > >This Week's Report: > >ON SINGLE-MOLECULE PHYSICS AND CHEMISTRY >Only a few decades ago, most scientists believed that individual >molecules would not come within the domain of experimental >observations within their lifetime, if ever, and that the >statistical ensemble properties of molecules were therefore the >only properties of relevance. That view has now undergone a >dramatic alteration as a consequence of technological advances, >and there is much excitement evident in many laboratories over >the prospects of single-molecule explorations in physics, >chemistry, and biology. >... ... C. Bai et al (4 authors at 3 installations, CN US) >present a short review of recent work in single-molecule physics >and chemistry, the authors making the following points: > 1) The authors point out that when Richard Feynman (1918- >1988) was bothered while looking through one of the first >*scanning tunneling microscopes, he was upset to have been >interrupted because seeing the images of singe atoms was a >"religious experience". For many generations of scientists, the >molecule was both the concrete ultimate entity upon which our >understanding of the everyday world was based, and at the same >time an elusive intellectual construct whose very existence could >only be inferred circumstantially by experiments on macroscopic >samples. Thus, seeing an individual atom or molecule in motion >brings immediate emotional impact to this central concept of >modern thought. > 2) The authors ask: "When is molecular individuality >important?" The new possibility of studying single molecules is >important because molecular individuality does finally come into >play when the molecule is a complex entity. This may occur >because the molecule itself may have an intricate internal >structure -- e.g., a biomolecule -- resulting in a complex energy >landscape. Alternatively, the molecule may be part of a complex >environment that substantially changes the behavior of the >molecule. Here, distinguishing different molecules at different >locales is crucial for understanding the system as a whole. >Biomolecules in living cells are examples of this. Even simple >inorganic molecules on structured surfaces or in disordered >systems such as viscous liquids or glasses provide situations in >which molecular individuality matters. In all of these cases, the >capability of studying an individual molecule over time can >provide new insights unavailable by straightforward experiments >on macroscopic populations of molecules. > 3) With the aid of *scanning probe microscopy, direct >observations of entire arrays of atoms, molecules, and the fine >structures of molecular aggregates have become possible. The >ability to precisely control probes permits the study of long- >range structures made by molecules lying on surfaces. However, >although pretty pictures of such systems are easy to construct, >obtaining quantitative characteristics of surface-bound molecules >is not entirely straightforward, and the rigorous interpretation >of scanning probe microscopy images requires substantial >theoretical as well as experimental effort. > 4) The authors conclude: "We are only at the beginning, but >it is clear there is much to be discovered of a fundamental >nature about complex molecules viewed as individuals. Perhaps >equally important will be the idea of single molecule control. >Now that experiments interact with molecules at an individual >level, we can try to control them as individuals, not as >populations. A molecule under active control by an adaptive >environment will be a new beast. Such tamed molecules may well >resemble much more the elegant engineered machinery of everyday >experience than the unruly, wild molecules we are used to >studying today." >----------- >C. Bai et al: Single molecule physics and chemistry. >(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11075) >QY: Chunli Bai, Institute of Chemistry, The Chinese Academy of >Sciences, Beijing 100080 CN. >----------- >Text Notes: >... ... *scanning tunneling microscopes: First available in the >early 1980s, this technique involves an atomically sharp metal >tip brought in atomic proximity (e.g., 0.5 to 1 nanometer) to a >flat surface so that electrons can *tunnel between the two >systems. Recording the atomic modulation of the atomic structure >which scanning the tip across the surface allows one to image >adsorbed species and surface morphologies. >... ... tunnel: Tunneling is a quantum mechanical phenomenon >involving an effective penetration of an energy barrier resulting >from the width of the barrier being less than the wavelength of >the particle. >... ... *scanning probe microscopy: A general term comprising all >atomic-level probe techniques. See background material below. >------------------- >Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 3Dec99 >[For more information: http://scienceweek.com/swfr.htm] >------------------- >Related Background: >ON THE NANOSCALE SCIENCE OF SINGLE MOLECULES >In recent years, experiments on individual molecules using >scanning probe microscopies [*Note #1] have demonstrated a >diversity of physical, chemical, mechanical, and electronic >phenomena. These techniques have permitted deeper insight into >the quantum electronics of molecular systems and have provided >unique information about the conformational and mechanical >properties of these systems. Concomitant developments in >experimentation and theory have allowed a diverse range of >molecules to be studied, molecules varying in complexity from >simple diatomic systems to biological macromolecular systems. >... ... J.K. Gimzewski and C. Joachim (2 installations, CH FR) >present an extensive review of current single-molecule research, >the authors making the following points: 1) The very nature of >proximal probe methods encourages exploration of the nanoworld >beyond conventional microscopic imaging. Scanning probes now >allow us to perform "engineering" operations on single molecules, >atoms, and bonds, thereby providing a tool that operates at the >ultimate limits of fabrication. These techniques have also >enabled explorations of molecular properties on an individual >basis as opposed to explorations restricted to the statistical >properties of large populations of molecules. 2) The >nanomechanical properties of individual molecules take the form >of vibrations, rotations, conformational changes, and >translations. *Inelastic tunneling processes, probe-tip-induced >forces, and Brownian motion have been found to drive mechanical >responses in individual molecules, and these aspects are the >focus of current research. The important role of thermal noise at >room temperature in nanoscale systems suggests that future >technologies for building small energy-efficient devices will >need to use ambient temperature fluctuations rather than fight >against them. 3) Future developments in single-molecule nanoscale >science call for a close integration of chemistry, biology, >physics, and technology in terms of synthesis, theoretical >modeling, and advanced scanning probe microscope techniques. >Although scanning probe microscopy has been shown to be an >ultimate probe for investigating the properties of individual >molecules, it is still an open question whether these techniques >have the intrinsic capabilities to be useful fabrication tools in >technology. The recent development of massive micromechanical >arrays of thousands of scanning probe microscopy probes suggests >that such a possibility is becoming more real each day. >----------- >J.K. Gimzewski and C. Joachim: Nanoscale science of single >molecules using local probes. >(Science 12 Mar 99 283:1683) >QY: James K. Gimzewski [[EMAIL PROTECTED]] >----------- >Text Notes: >... ... *Note #1: The general approach in scanning probe >microscopy research is illustrated by consideration of two major >techniques, scanning tunneling microscopy (STM) and atomic force >microscopy (AFM). In scanning tunneling microscopy, an atomically >sharp metal tip is brought in atomic proximity (e.g., 0.5 to 1 >nanometer) to a flat surface so that electrons can *tunnel >between the two systems. The probe is slowly moved across the >surface and raised and lowered so as to keep the tunneling >current constant. A computer-generated contour map of the surface >is thus produced. The technique can resolve individual atoms, but >requires electrically conducting materials. In atomic force >microscopy, a tip is fixed to a cantilever whose position is >monitored while the tip scans the surface. The force between the >tip and the surface determines the position of the cantilever. >When recorded in atomic resolution, the image represents a map of >atomic forces at the surface. The advantage of atomic force >microscopy is that the probed surface does not need to be >electrically conducting. >... ... *tunnel: "Tunneling" is a quantum mechanical >phenomenon involving an effective penetration of an energy >barrier resulting from the width of the barrier being less than >the wavelength of the particle. >... ... *Inelastic tunneling processes: In general, an >"inelastic" process is a process which results in a permanent >change in the properties of a system. In this context, the term >"inelastic tunneling process" refers to a technique involving the >input of energy into a single-molecule system to selectively >excite chemical bonds or to perform spectroscopic studies of the >system. >------------------- >Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 21May99 >------------------- >Related Background: >ON REACTIONS ON SEMICONDUCTOR SURFACES >Chemical reactions on surfaces are important in many areas of >science and technology. On metal surfaces, the electronic states >of the surface atoms are spatially extended and can therefore be >easily shared with those of reactive species, the result a >dramatic influence on the structure of these species as they >approach the surface. In contrast, bonding on semiconductor >surfaces is largely covalent, and surface electronic states tend >to be spatially localized. ... ... Harry E. Ruda (University of >Toronto, CA) presents a short review of current research >concerning reactions on semiconductor surfaces, the author making >the following points: 1) Understanding of the interactions of >species with semiconductor surfaces has been considerably >advanced by the widespread use of *scanning tunneling microscopy >(STM), which can probe, with atomic resolution, the spatial >extent of electron density on a surface. 2) Bias-dependent >scanning tunneling microscopy studies, in which images are taken >at different voltages between the STM tip and the sample, allow >the determination of the energy spectra of surface electronic >states, and in special cases enable discrimination between >different chemical species. 3) Coupled with computer modeling, >which can provide increasingly realistic descriptions of the >pertinent underlying physics, scanning tunneling microscopy >studies are providing the information needed to understand and >control the interactions of chemical species on semiconductor >surfaces. The author concludes: "This research will have a >profound influence on nanotechnology, slated to become the >cornerstone of coming generations of semiconductor devices and >circuitry." >----------- >Harry E. Ruda: Reactions on semiconductor surfaces. >(Science 29 Jan 99 283:646) >QY: Harry E. Ruda [[EMAIL PROTECTED]] >------------------- >Summary by SCIENCE-WEEK [http://scienceweek.com] 9Apr99 >[For more information: http://scienceweek.com/swfr.htm] > >[SW Bulletin 12 Apr 00] > >**************************************************************** > >Contents of the Current Issue of ScienceWeek: > >April 14, 2000 -- Vol. 4 Number 15 >----------------------------------------------- >1. Neurobiology: >On the Biological Basis of Memory >--------------------------------- >Although the idea that processes underlying new memories >initially persist in a fragile state and then consolidate over >time still guides research on memory, neurobiologists do not yet >have a consensus theory concerning the biological basis of memory >formation. (Includes related background material.) > >2. Neurobiology: >Functional Regeneration of Sensory Axons in Adult Spinal Cord >------------------------------------------------------------- >Natural repair of severed connections between the spinal cord and >spinal nerves does not occur in humans, but in the past decade >there has been much progress in understanding the mechanisms of >nerve fiber regeneration. There is now new evidence of functional >regeneration of dorsal root sensory axons in adult mammalian >spinal cord after treatment by local introduction of neurotrophic >factors. (Includes related background material.) > >3. Medical Biology: >On New Approaches to Human Aging >-------------------------------- >The resolution of all causes of death currently written on the >death certificates of those older than 65 will result only in an >increase in life expectancy of approximately 15 years. Any major >increase in human life span will require knowledge of the >fundamental biology of the aging process. (Includes related >background material.) > >4. Earth Sciences: >Ice-Core Evidence of Abrupt Climate Changes >------------------------------------------- >Records of abrupt changes in Earth's climate are particularly >clear in high-resolution ice cores, which can preserve histories >of local climate, regional climate, and broader climate -- all on >a common time scale. (Includes related background material.) > >5. Astrobiology: >Evidence for Sufficient Energy for a Biosphere on Europa >-------------------------------------------------------- >Disequilibrium chemistry in the ice cover of Europa's oceans, >driven by charged particles accelerated in Jupiter's >magnetosphere, should produce enough organic and oxidant >molecules to fuel a substantial Europan biosphere. (Includes >related background material.) > >6. Astrophysics: >On Stars, Brown Dwarfs, and Superplanets >---------------------------------------- >As recently as 1994, brown dwarfs were "theoretical" stars, with >no brown dwarfs considered to be unambiguously identified. During >the past few years, dozens of brown dwarfs have been discovered, >and it is now apparent they are as numerous as ordinary stars. >(Includes related background material.) > >In Focus: On Carbon (Includes related background material.) > >Total text 86K ASCII bytes > >**************************************************************** > >You will always find the table of contents of the current issue >of ScienceWeek at the SW website: http://www.scienceweek.com >The website also includes a search engine to search the SW >archive of back issues, with free access to over 1000 reports. > >================================================================ > >If you have questions or comments about SW BULLETIN, >send Email to: [EMAIL PROTECTED] >Claire Haller, Managing Editor > >**************************************************************** > >What you are now reading is SW BULLETIN, a free publication >sponsored by ScienceWeek. SW BULLETIN is published on Mondays, >delivered by Email, and also posted each week on the SW website. >Each week the Bulletin provides an in-depth report on a single >topic of general scientific interest, the text usually amplified >by notes and background material from ScienceWeek. Anyone can >receive SW BULLETIN free via Email. To subscribe to SW Bulletin, >transmit SUB BULLETIN as the subject of an Email message to: >[EMAIL PROTECTED] To unsubscribe, transmit REMOVE BULLETIN >to the same address. (If you are unsubscribing, your request must >originate from the Email address at which you receive SWB. >Otherwise, send a special message providing that address.) > >SCIENCE-WEEK, the main publication, is now in its 3rd year. >ScienceWeek is an Email digest of new research in the sciences. >Each weekly issue contains in-depth summaries, explicating >texts, glossaries, and related background reports. A one year >subscription to ScienceWeek is only US$20 per year. A free >sample issue and subscription details are available at the SW >website: http://www.scienceweek.com > >Copyright >Copyright (c) 2000 ScienceWeek >All Rights Reserved > >We encourage you to share SW BULLETIN with colleagues who may >have an interest in its contents. SW BULLETIN may be >redistributed for non-commercial purposes, in printed or >electronic form, as long as the contents of the publication are >not changed in any way, the document is not offered for sale, and >the document is complete (including front and end matter). >================================================================ > >SCIENCEWEEK, a weekly Email research digest devoted to improving >communication between the sciences, and between scientists, >science educators, and science policy-makers. www.scienceweek.com >-----end file > >
