Quantitative Scanning Transmission Electron Microscopy of Thick Samples and of Gold and Silver Nanoparticles on Polymeric Surfaces
Haadf stem, tem, gold, nanoparticles, multislice simulations, z contrast, atomic number contrast, fib
Transmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces to provide a path for predictability and control of these systems. This dissertation aims to provide fundamental understanding of the surface chemistry of Electroless Metallization onto Polymeric Surfaces (EMPS) through characterization with TEM. The research focuses on a single EMPS system: deposition of Ag onto the cross-linked epoxide "SU8", where Au nanoparticles act as nucleation sites for the growth of Ag nanoparticles on the polymer surface. TEM cross sections were analyzed to investigate the morphology of the Au nanoparticles and to determine the thicknesses of the Ag nanoparticles and of the Ag layers. A method for the direct measurement of the volume and thickness of nanomaterials has been developed in the project using High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). The morphology of Au and Ag NPs has been studied to provide reliable statistics for 3-D characterization. Deposition rates have been obtained as a function of metallization conditions by measuring the composition and thickness of the metal for EMPS. In the present work a calibration method was used to quantify the sensitivity of the HAADF detector. For thin samples a linear relationship of the HAADF signal with the thickness of a material is found. Cross-sections of multilayered samples provided by Triquint Semiconductors, FL, were analyzed as calibration standards with known composition in a TECNAI F30 transmission electron microscope to study the dependence of the HAADF detector signal on sample thickness and temperature. Dynamical diffraction processes play an important role in electron scattering for larger sample thicknesses. The HAADF detector intensity is not linearly dependent on sample thicknesses for thick samples. This phenomenon involves several excitation processes including Thermal Diffuse Scattering (TDS) which depends on temperature-dependent absorption coefficients. Multislice simulations have been carried out by Python programming using the scattering parameters (2) available in the literature. These simulations were compared with experimental results. Wedge-shaped Focused Ion Beam (FIB) samples were prepared for quantitative HAADF-STEM intensity measurements for several samples and compared with these simulations. The discrepancies between the simulated and experimental results were explained and new sets of absorptive parameters were calculated which correctly account for the HAADF-STEM contrasts. A database of several pure elements is compiled to illustrate the absorption coefficients and fractions of scattered electrons per nanometer of the sample. In addition, the wedge-shaped FIB samples were used for studying the HAADF-STEM contrasts at an interface of a high- and a low-density material. The use of thick samples reveals an increased signal at the interfaces of high- and low-density materials. This effect can be explained by the transfer of scattered electrons from the high density material across the interface into the less-absorbing low-density material. A ballistic scattering model is proposed here for the HAADF-STEM contrasts at interfaces of thick materials using Python. The simulated HAADF-STEM signal is compared with experimental data to showcase the above phenomenon. A detailed understanding of the atomic number contrast in thick samples is developed based on the combination of experimental quantitative HAADF-STEM and simulated scattering. This approach is used to describe the observed features for Ag deposition on SU8 polymers.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dutta, Aniruddha, "Quantitative Scanning Transmission Electron Microscopy of Thick Samples and of Gold and Silver Nanoparticles on Polymeric Surfaces" (2014). Electronic Theses and Dissertations. 6677.