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: Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to characterize the atomic level structure of electronic properties, reactivity and wear of metal dichalcogenide materials that are or have potential as solid state lubricants. Single crystals Of MoS2, Ni(x)Mo(l-x) S2, MoS(2-x)Se(x) and MoS(2-x)Te(x) have been prepared to determine how chemical modifications affect the local structure and electronic properties of this lubricant. STM images of Ni-doped MoS2 show localized electronic states due to the Ni atoms, while images of Se- and Te-doped materials indicate that anion substitution is electronically delocalized. AFM studies of Te-doped MoS2 show, however, that the tellurium dopants form atomic scale structural protrusions that may reduce sliding friction. AFM has also been used to characterize nanometer scale wear and oxidation on MoS2 and NbSe2 surfaces. In atmosphere at room-temperature AFM studies showed that NbSe2 wears approximately three times faster than MoS2. Furthermore, oxidation studies demonstrated that NbSe2 was significantly more reactive than MoS2 with molecular oxygen. These results indicate that the intrinsic stability of the MoS2 surface make it an effective lubricant, AFM was also used to elucidate the growth of MoO3 on the surface Of MoS2 during oxidation, and to study wear properties of these MoO3 crystallites. The AFM tip was used to define lines with 10 nm resolution in MoO3 and to manipulate distinct MoO3 structures on the MoS2 surface. In addition, metal- substitution in TaS2 has been studied systematically using STM and theoretical methods.

Überall in der Biologie kommen polyvalente Wechselwirkungen vor. Sie zeichnen sich durch die gleichzeitige Bindung mehrerer Liganden einer biologischen Einheit an mehrere Rezeptoren einer anderen biologischen Einheit aus (oberer Teil der Graphik) und haben eine Reihe von Charakteristika, die monovalenten Wechselwirkungen fehlen (unten). Besonders im Verbund können polyvalente Wechselwirkungen viel stärker sein als entsprechende monovalente Wechselwirkungen, und sie können die Basis für das Verständnis fördernder und hemmender biologischer Wechselwirkungen liefern, die sich grundsätzlich von denen in monovalenten Systemen unterscheiden.

Crack tip stress and deformation fields are analyzed for tensile-loaded ideally plastic crystals. The specific cases of (0 1 0) cracks growing in the [1 0 1] direction, and (1 0 1) cracks in the [0, 1, 0] direction, are considered for both fcc and bcc crystals which flow according to the critical resolved shear stress criterion. Stationary and quasistatically growing crack fields are considered. The analysis is asymptotic in character; complete elastic-plastic solutions have not been determined. The near-tip stress state is shown to be locally constant within angular sectors that are stressed to yield levels at a stationary crack tip, and to change discontinuously from sector to sector. Near tip deformations are not uniquely determined but fields involving shear displacement discontinuities at sector boundaries are required by the derived stress state. For the growing crack both stress and displacement must be fully continuous near the tip. An asymptotic solution is given that involves angular sectors at the tip that elastically unload from, and then reload to, a plastic state. The associated near-tip velocity field then has discontinuities of slip type at borders of the elastic sectors. The rays, emanating from the crack tip, on which discontinuities occur in the two types of solutions are found to lie either parallel or perpendicular to the family of slip plane traces that are stressed to yield levels by the local stresses. In the latter case the mode of concentrated shear along a ray of discontinuity is of kink type. Some consequences of this are discussed in terms of the dislocation generation and motion necessary to allow the flow predicted macroscopically.

Abstract : Significant effort has been placed on several areas. First, we have carried out systematic studies of the growth of carbon nitride materials as a function of carbon reactant energetics, nitrogen flux and growth temperature and have quantitatively analyzed the structure and local bonding in these materials. The emphasis of these studies has been to determine unambiguously conditions that can produce sp3-bonding since this is required for a superhard coating. Significantly, we have show recently that there is a nitrogen-driven sp3 to sp2 structural transformation in the carbon nitride materials as nitrogen content is increased about 15 atomic percent. With these limits worked out we have also prepared films with optimal C-N ratios for hardness, and investigated their nanotribological properties by force microscopy. In addition, we have carried out theoretical cluster calculations to understand the origin of this sp3 to sp2 transition. Significantly, these calculations demonstrate that the transition is an intrinsic property of carbon nitride system since (1) the sp2 bonded structure becomes thermodynamically favored for 15% nitrogen and (2) the barrier between sp3 and sp2 structures also reduces significantly for 15% nitrogen.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTLaser-Assisted Catalytic Growth of Single Crystal GaN NanowiresXiangfeng Duan and Charles M. LieberView Author Information Department of Chemistry and Chemical Biology Harvard University, Cambridge, Massachusetts 02138 Cite this: J. Am. Chem. Soc. 2000, 122, 1, 188–189Publication Date (Web):December 18, 1999Publication History Received18 October 1999Published online18 December 1999Published inissue 1 January 2000https://pubs.acs.org/doi/10.1021/ja993713uhttps://doi.org/10.1021/ja993713urapid-communicationACS PublicationsCopyright © 2000 American Chemical SocietyRequest reuse permissionsArticle Views5248Altmetric-Citations755LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Catalysts,Crystal structure,Crystallization,Nanowires,Nitrides Get e-Alerts

A Griffith-type energy balance for crack growth leads to paradoxical results for solids that are modelled as elastic/plastic continua, since such solids provide no energy surplus in continuous crack advance to equate to a work of separation [1]. An alternative proposed recently is that the finite size of the fracture process zone be taken into account, and a simple way of doing this is to define a crack tip energy release rate Gδ based on the work of quasi-static removal of stresses from the prospective crack surfaces over a finite crack growth step δa [2]. Recent finite element results for energy releases during crack growth are reviewed in this way and an analytical solution, based on the Dugdale-Bilby-Cottrell-Swinden (DBCS) crack model, is developed for Gδ when δ a is small compared to plastic zone size. This solution and the finite element results are in remarkably good agreement and, based on the analytical solution, we develop the asymptotic formula for the value of the J integral required for crack growth, J = .70 ζ exp(.43/ζ) Gδ, with ζ = (1-v2)σy 2δa/EGδ, and this formula is valid whenever ζ is smaller than, approximatley, 0.15; σy is the yield stress. A defect of the DBCS model, however, is that unlike the finite element results, it makes no distinction between the criterion for onset of growth and that for continuing growth in the case of highly ductile solids.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSelf-assembly based on the cyanuric acid-melamine latticeChristopher T. Seto and George M. WhitesidesCite this: J. Am. Chem. Soc. 1990, 112, 17, 6409–6411Publication Date (Print):August 1, 1990Publication History Published online1 May 2002Published inissue 1 August 1990https://pubs.acs.org/doi/10.1021/ja00173a046https://doi.org/10.1021/ja00173a046research-articleACS PublicationsRequest reuse permissionsArticle Views2046Altmetric-Citations212LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts