Download or read book Investigating Photoinduced Structural Changes in Silicon Using Femtosecond Electron Diffraction written by Maher Harb and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Investigating Photoinduced Structural Changes in Si Using Femtosecond Electron Diffraction written by Maher Harb and published by . This book was released on 2009 with total page 212 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis reports on developing a femtosecond electron diffraction technique and its application in following the structural changes associated with photoexcited Si. Time resolved electron diffraction is capable in principle of capturing transient structures of photoinduced phenomena with femtosecond time resolution. In practice, however, the time resolution is severely limited by the broadening of electron pulses due to space-charge forces. Another technical difficulty lies in that diffraction in transmission geometry requires samples that are semi-transparent to electrons. These experimental challenges were dealt with using innovative engineering and software development, and novel nanofabrication methods. Subsequently, a robust highly automated experimental setup was established, and 50 nm free-standing membranes of polycrystalline Si were nanofabricated. These membranes were first excited below damage threshold with 387 nm light, at an absorbed fluence of 5.6 mJ/cm2. The observed loss of intensity and associated dynamics under these conditions are related to the Debye-Waller effect as the lattice heats up through electron-phonon processes. At high excitation levels, greater than ∼6% of the valence electron density, the crystalline structure of the lattice was lost in 500 fs. This observed time scale is indicative of an electronically driven order-to-disorder phase transition due to an abrupt change in the potential-energy landscape of the lattice. In addition, it was discovered that the high sensitivity of the electron diffraction process to shear-type deformation in single crystals makes the technique ideal to the generation and detection of coherent acoustic phonons. Longitudinal and shear acoustic phonons were subsequently resolved in (001)-oriented Si crystals. This study constitutes the first direct observation of shear phonons in symmetrically cut crystals, in sharp in contrast to the all-optical methods that rely on crystals asymmetrically cut at 20° to enhance the amplitude of the shear mode in order to detect its displacements.
Download or read book Femtosecond Electron Diffraction and Spectroscopic Studies of a Solid State Organic Chemical Reaction written by Hubert Jean-ruel and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Investigations on Interfacial Dynamics with Ultrafast Electron Diffraction written by Ryan A. Murdick and published by . This book was released on 2009 with total page 518 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Femtosecond Electron Diffraction Studies of Strongly driven Structural Phase Transitions microform written by Bradley John Siwick and published by National Library of Canada = Bibliothèque nationale du Canada. This book was released on 2004 with total page 204 pages. Available in PDF, EPUB and Kindle. Book excerpt: The new technique of femtosecond electron diffraction is developed and applied to the study of an ultrafast structural phase transition. A combination of numerical and analytical modeling is used to obtain a comprehensive understanding of the space-charge dominated propagation dynamics of high number density femtosecond electron pulses and to design a novel photoactivated electron gun. This electron source, capable of producing sub-500 femtosecond electron pulses with sufficient brightness to allow the study of even irreversible structural dynamics, is employed to investigate the strongly driven, laser-induced solid-to-liquid phase transition in polycrystalline aluminum. We are able to see, in real time, the loss of the long-range order present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations are present; this transition occurs in 3.5 ps for thin-film aluminum with an excitation fluence of 70 mJ/cm2. The sensitivity and temporal resolution was sufficient to capture the time-dependent atomic pair correlation function as the material evolved from the solid to liquid state. These observations provide an atomic level description of the melting process in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.
Download or read book High Temperature Structure Formation and Surface Diffusion of Silver on Silicon Surfaces written by Dirk Wall and published by Cuvillier Verlag. This book was released on 2012-11-21 with total page 240 pages. Available in PDF, EPUB and Kindle. Book excerpt: Kurzbeschreibung Die vorliegende Arbeit beschäftigt sich mit Oberflächendiffusion und Strukturbildung an Oberflächen, speziell im Fall Silber auf Silizium. Unterschiedliche Methoden werden kombiniert um Si(001) und Si(111) Oberflächen als auch dazwischen liegende Orientierungen zu untersuchen. niedrigenergetische Elektronenmikroskopie (LEEM) und photoemissions Elektronenmikroskopie (PEEM) wurden verwendet um die Wachstumsdynamik und den Einfluss von Oberflächendiffusion auf die Strukturbildung an Oberflächen unter Ultrahochvakuum (UHV) Bedingungen zu untersuchen. Es wurden ein- und multi-kristalline Ag Inseln und selbstorganisierte Ag Drähte auf unterschiedlichen Si Oberflächen untersucht. Hierfür wurde Ag bei hohen Temperaturen auf Oberflächen aufgebracht, wobei die meisten Untersuchungen in-situ erfolgten. Die Struktur der Ag Inseln und Drähte und deren Orientierung zum Substrat wurde hauptsächlich mit niederenergetischer Elektronenbeugung an kleinen Bereichen (µ-LEED), hochauflösender niederenergetischer Elektronenbeugung (SPA-LEED) und Rasterelektronenmikroskopie (SEM) untersucht. Für die SEM Untersuchungen wurden die präparierten Proben aus dem UHV entnommen um sie in ein SEM zu transferieren und eine statistisch bessere Aussagekraft zu erreichen. Ag(001) und Ag(111) Inseln wurden bei Temperaturen von bis zu 700°C gewachsen. Mit steigender Wachstumstemperatur verändert sich die überwiegende Form der Inseln von hexagonal zu dreieckig. Die relative Drehung zum Substrat wurde Untersucht und mit einem modifizierten gitter-koinzidenz Modell (CSL) verglichen. Der Vergleich zeigt eine ausgesprochen gute Übereinstimmung der experimentellen Daten mit der Theorie, bei der praktisch alle Drehwinkel erklärt werden. Oberflächendiffusionsfelder wurden beim thermischen Zerfall und während der Desorption von Silberinseln untersucht. Um die Inseln bilden sich ein oder mehrere konzentrische rekonstruktionsbedingte Zonen. Ein einfaches kontinuum Diffusionsmodell zur Erklärung des Zerfallsmechanismus wird vorgestellt. Das Modell beinhaltet ein bereits zuvor präsentiertes Modell als einen Spezialfall und wurde in Zusammenarbeit mit J. Krug und I. Lohmar an der Universität zu Köln entwickelt. Unterschiedlichste Diffusionsparameter können mit diesem Modell bestimmt werden und stimmen sehr gut mit Literaturvergleichswerten überein. Der Zerfall der Inseln auf vizinalen Oberflächen kann nicht mehr mit diesem Modell erklärt werden, da eine Anisotropie auftritt, die die Rotationssymmetrie aufhebt. SPA-LEED Resultate zur Multistufenbildung und Facettierung sowie numerische Simulationen werden hinzugezogen und können mit Hilfe eines in der Literatur bekannten Modells praktisch alle experimentellen Daten erklären und so ein fast all-umfassendes Verständnis der Ursachen der Anisotropie erzeugen. Die Ergebnisse werden auch auf Ergebnisse zu Indium auf vizinalem Silizium angewendet und können auch hier die Überraschende Isotropie erklären. Außer den Inseln bilden sich auf Si(001) auch noch Drähte. Das Wachstum dieser Drähte wurde untersucht um eine Diskussion in der Literatur über die Ursache der Drahtbildung aufzuklären. Einkristalline Drähte wurden auf sehr genau orientiertem Si(001) und auf vizinalen Flächen präpariert. Alle Drähte orientieren sich entlang einer der beiden Hauptsymmetrierichtungen der Oberfläche. Ihr Wachstum ist thermisch aktiviert und erstaunlicherweise unabhängig von der Fehlneigung. Dennoch richten sich die Drähte mit zunehmender Fehlneigung und damit Stufendichte parallel zu den Stufenkanten aus. Die Resultate können jedoch die Diskussion ob Diffusionsanisotropie oder Verspannung die Ursache für das Drahtwachstum sind nicht aufklären, da diese zu stark ineinander überkoppeln. Dennoch kommen wir zu dem Entschluss, dass die Drahtausrichtung durch die zunehmende Diffusionsanisotropie verursacht wird. Description The present work deals with surface diffusion and structure formation, mainly for the case of Silver on Silicon surfaces. Various techniques are combined to investigate flat and vicinal surfaces oriented in the Si(001) and Si(111) directions as well as intermediate orientations. Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) were used to study the growth dynamics and diffusion involved in structure formation in ultrahigh vacuum (UHV) conditions. The investigated structure formation deals with single- and multi-crystalline Ag islands and self-organized Ag wires on various Si surfaces. Ag was deposited at elevated temperatures, while the investigations were mainly carried out in-situ. The structure of the grown Ag islands and wires was investigated with either small area low energy electron diffraction (µ-LEED), spot profile analyzing-low energy electron diffraction (SPA-LEED), or scanning electron microscopy (SEM). The SEM investigations were the only investigations, where the sample was extracted from the UHV and were carried out to improve the statistical significance of the data. Ag(001) and Ag(111) islands were grown at elevated temperatures of up to 700°C. With increasing growth temperature, the shape of the islands transformed from hexagonally shaped to triangular. The relative rotation to the substrate was investigated and compared to a modified coincidence site lattice approach (CSL) which agreed very well with the experimental results. Practically all of the significant rotation angles could be explained by the CSL model. Surface diffusion fields were investigated during the decay of islands in the process of desorption. These islands are surrounded by one or several concentric adsorbate induced reconstruction zones. A simple continuum diffusion model is presented, explaining the decay mechanism. The model contains a previously presented model as a special case and was developed in collaboration with J. Krug and I. Lohmar at the University of Cologne. Several diffusion parameters are extracted from the model and are in excellent agreement with values in literature. The decay of Ag islands on vicinal Si substrates no longer yields concentric circular zones, but the zones become anisotropic, and the model can no longer be applied due to the no lack of rotational symmetry. A model from the literature is used to explain the data in combination with SPA-LEED results on multi-step formation and faceting and numerical simulations. Only a combination of all these techniques is capable of a thorough and all-embracing explanation of surface diffusion. The results are compared to the system of Indium on vicinal Si(001) surfaces. Here, in contrast to Ag on vicinal Si(001), no anisotropy is found and the drawn picture can also explain the surprising diffusion isotropy. Among the islands that were used for the diffusion investigations, on Si(001), wires form. The growth of these single crystalline wires was investigated and an attempt has been taken to clear an ongoing discussion about the cause of the wire formation. The single crystalline wires were grown on flat and vicinal Si(001) surfaces. All wires align to one of the two principal directions of the substrate. Their growth is thermally activated and surprisingly independent of the substrate vicinality. The wires align with the step edges as the sample vicinality and with it the step density is increased. The results cannot lead to a clear decision on which of the discussed phenomena diffusion anisotropy or strain are the cause for self-organized wire formation on vicinal Si(001) surfaces. We can, however, come to the conclusion, that the wire alignment is much more closely linked to the diffusion anisotropy than the formation itself. We therefore state, that the diffusion anisotropy is a possible cause for the wire alignment, restricting the wire growth to one of the possible two directions with increasing diffusion anisotropy.
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1994 with total page 892 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book International Aerospace Abstracts written by and published by . This book was released on 1999 with total page 974 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Physics Briefs written by and published by . This book was released on 1994 with total page 1248 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Chemical Abstracts written by and published by . This book was released on 2002 with total page 2692 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book American Doctoral Dissertations written by and published by . This book was released on 2000 with total page 816 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Dissertation Abstracts International written by and published by . This book was released on 2006 with total page 854 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Book of Abstracts written by and published by . This book was released on 2000 with total page 778 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book 4D Electron Microscopy written by Ahmed H. Zewail and published by World Scientific. This book was released on 2010 with total page 359 pages. Available in PDF, EPUB and Kindle. Book excerpt: Structural phase transitions, mechanical deformations, and the embryonic stages of melting and crystallization are examples of phenomena that can now be imaged in unprecedented structural detail with high spatial resolution, and ten orders of magnitude as fast as hitherto. No monograph in existence attempts to cover the revolutionary dimensions that EM in its various modes of operation nowadays makes possible. The authors of this book chart these developments, and also compare the merits of coherent electron waves with those of synchrotron radiation. They judge it prudent to recall some important basic procedural and theoretical aspects of imaging and diffraction so that the reader may better comprehend the significance of the new vistas and applications now afoot. This book is not a vade mecum - numerous other texts are available for the practitioner for that purpose.
Download or read book The Engineering Index Annual written by and published by . This book was released on 1992 with total page 2264 pages. Available in PDF, EPUB and Kindle. Book excerpt: Since its creation in 1884, Engineering Index has covered virtually every major engineering innovation from around the world. It serves as the historical record of virtually every major engineering innovation of the 20th century. Recent content is a vital resource for current awareness, new production information, technological forecasting and competitive intelligence. The world?s most comprehensive interdisciplinary engineering database, Engineering Index contains over 10.7 million records. Each year, over 500,000 new abstracts are added from over 5,000 scholarly journals, trade magazines, and conference proceedings. Coverage spans over 175 engineering disciplines from over 80 countries. Updated weekly.
Download or read book Ceramic Abstracts written by and published by . This book was released on 1994 with total page 972 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Femtosecond Laser Micromachining written by Roberto Osellame and published by Springer Science & Business Media. This book was released on 2012-03-05 with total page 485 pages. Available in PDF, EPUB and Kindle. Book excerpt: Femtosecond laser micromachining of transparent material is a powerful and versatile technology. In fact, it can be applied to several materials. It is a maskless technology that allows rapid device prototyping, has intrinsic three-dimensional capabilities and can produce both photonic and microfluidic devices. For these reasons it is ideally suited for the fabrication of complex microsystems with unprecedented functionalities. The book is mainly focused on micromachining of transparent materials which, due to the nonlinear absorption mechanism of ultrashort pulses, allows unique three-dimensional capabilities and can be exploited for the fabrication of complex microsystems with unprecedented functionalities.This book presents an overview of the state of the art of this rapidly emerging topic with contributions from leading experts in the field, ranging from principles of nonlinear material modification to fabrication techniques and applications to photonics and optofluidics.
