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Open Manoharan 999805608.Pdf The Pennsylvania State University The Graduate School College of Engineering INFLUENCE OF STRAIN ON THE PHYSICAL PROPERTIES OF MATERIALS AT THE NANOSCALE A Dissertation in Mechanical Engineering by Mohan Prasad Manoharan © 2011 Mohan Prasad Manoharan Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2011 The dissertation of Mohan Prasad Manoharan was reviewed and approved* by the following: M. Amanul Haque Associate Professor of Mechanical Engineering Dissertation Adviser Chair of Committee Alok Sinha Professor of Mechanical Engineering Adri van Duin Associate Professor of Mechanical Engineering Tony Jun Huang Associate Professor of Engineering Science and Mechanics Karen A. Thole Professor of Mechanical Engineering Head of the Department of Mechanical and Nuclear Engineering * Signatures are on file in the Graduate School ii Abstract At the nanoscale, materials properties differ substantially from that at the bulk scale, opening new avenues for technological applications and basic science research. Such size effects arise from dimensional and microstructural constraints, especially when specimen size coincides with the critical fundamental length scales for various physical properties. While the state of the art practice is to investigate the size effects on „individual‟ properties (mechanical or electrical or thermal and so on), the focus of this research is to explore the size effects on the „coupling‟ among these domains. In particular, the effect of mechanical strain on various physical properties of materials at the nanoscale is studied. This is motivated by the hypothesis that very small elastic strain could be engineered in micro and nanoscale systems to „tune‟ materials properties, which is not possible at the bulk scale using strain as a parameter. The objective of this research is to study the influence of strain on various material properties at the nanoscale, such as crystal structure, thermal and electrical conductivity, electronic bandgap and tribological properties through experimental characterization. While characterization of nanoscale materials in single domains remains the state of the art, coupled domain studies usher even stiffer challenges. This is because in addition to the difficulties in nanoscale specimen preparation, handling and properties measurement, meticulous attention has to be given to the boundary conditions for each of the domains. Another desired feature of the experimental setup is the capability for in situ high resolution microscopy so that microstructural details as well as experimental accuracy are achieved. A major contribution of this research is the development of iii microfabricated integrated systems to perform coupled domain characterization of small scale specimens in situ in thermal (infra-red), micro-Raman and electron microscopes. In addition, conceptual modeling based on the experimental characterization was developed to explain the observed phenomena on the basis of existing nanoscale materials theories, or if none exist, suitable scientific hypothesis were proposed. The salient contributions of this research are summarized below. a) Uniaxial tensile testing was performed on 4 - 6 nm thick amorphous carbon thin film specimens in situ in a scanning electron microscope. The study revealed size effect on the Young‟s modulus, which is traditionally a scale independent property. The size effect is explained on the basis of the increased contribution of surface elastic properties (surface stress) at the nanometer length-scale. b) Significant stress-induced crystallization was observed in about 5 nm thick ion beam deposited amorphous platinum thin films. At 3 % tensile strain, electron diffraction patterns clearly show irreversible transformation to face-centered cubic (FCC) structure even at room temperature. It is proposed that the externally applied stress, in addition to the large tensile residual stress in the films provided the activation energy needed to nucleate crystallization, while subsequent grain growth occurred through enhanced atomic and vacancy diffusion as an energetically favorable route towards stress relaxation at the nanoscale. iv c) While it is widely accepted that amorphous materials do not exhibit any size or deformation effects on thermal conductivity, about one order of magnitude reduction in conductivity was experimentally observed at strains of up to 2.4 % for 50 nm thick freestanding amorphous silicon nitride thin films. To explain this unusually strong strain–thermal transport coupling, it is proposed that in silicon nitride, vibration localization due to very strong Si-N bonds decreases hopping mode thermal transport. d) The friction coefficient between individual zinc oxide nanowires and silicon substrate in ambient conditions was measured to be about two orders of magnitude higher than bulk values, even under zero externally applied normal loads. This anomalous behavior is explained by the compliant nature of the nanowires and the presence of molecularly thin surface moisture layers. e) Effect of strain on the electronic band gap of single zinc oxide nanowires was studied under a micro-Raman microscope by acquiring the photoluminescence spectra. The bandgap of the nanowires was found to reduce by 25 meV per 1 % of tensile strain applied. This red shift of the near-band-edge emission is largely due to the lowering of the conduction band edge instead of valence band edge. f) The thermal and electrical conductivity of polyaniline thin films was measured by the 3-omega method and four-point I-V method respectively. The conductivities were found to reduce with increasing tensile bending strains, due to the appearance of cracks in the film. v Table of Contents List of Figures viii List of Tables xi Acknowledgements xii Chapter 1 Introduction 1 1.1 Material properties at the nanoscale ................................................................ 2 1.2 Multi-domain coupling in nanoscale materials ................................................ 6 1.3 Challenges in materials characterization at the nanoscale ............................... 8 1.4 Research contributions summary ..................................................................... 9 1.5 Outline of dissertation .................................................................................... 15 Chapter 2 Literature Review 16 2.1 Characterization of mechanical properties at the nanoscale .......................... 16 2.2 Characterization of thermal conductivity at the nanoscale ............................ 24 2.3 Characterization of electrical conductivity at the nanoscale .......................... 30 2.4 Characterization of tribological properties at the nanoscale .......................... 32 Chapter 3 Mechanical Properties of Glassy Carbon Thin Films 36 3.1 Introduction .................................................................................................... 36 3.2 Properties of glassy carbon ............................................................................ 37 3.3 Synthesis of glassy carbon thin films............................................................. 38 3.4 Experimental setup to measure Young‟s modulus ......................................... 39 3.5 Experimental results – Young‟s modulus ...................................................... 44 3.6 Size effect at the nanoscale ............................................................................ 47 Chapter 4 Effect of Strain on Atomic Structure of Platinum Thin Films 49 4.1 Phase transformations .................................................................................... 49 4.2 Experimental setup for in situ TEM study ..................................................... 51 4.3 Experimental results – amorphous to crystalline transformation .................. 55 4.4 Stress-induced crystallization in FIB-deposited platinum ............................. 58 Chapter 5 Strain dependence of Thermal Conductivity of Silicon Nitride Thin Films 65 5.1 Introduction .................................................................................................... 65 5.2 Device Design and Fabrication ...................................................................... 68 5.3 Experimental Procedure and Analysis ........................................................... 72 5.4 Results and Discussion .................................................................................. 78 vi Chapter 6 Tribological properties of individual Zinc Oxide Nanowires 86 6.1 Introduction .................................................................................................... 86 6.2 Experimental setup......................................................................................... 87 6.3 Experimental results and discussion .............................................................. 92 Chapter 7 Strain dependence of Bandgap in individual Zinc Oxide Nanowires 101 7.1 Introduction .................................................................................................. 101 7.2 Experimental setup and results .................................................................... 103 7.3 Discussion of results .................................................................................... 105 Chapter 8 Strain dependence of Thermal and Electrical Conductivity of Polyaniline Thin Films 109 8.1 Introduction .................................................................................................. 109 8.2 Experimental setup
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