Design and Development of an in-house Scanning Tunneling Microscope System

ABSTRACT The invention of Scanning Tunneling Microscope (STM) by Binnig and Rohrer in 1982 eliminated the use of optical lenses and replaced the conventional optical microscopes with a new class of microscopes called the Scanning Probe Microscopes (SPM). Because of their unique characteristics such as higher resolution and acquisition of nano level images without affecting the physical properties of the sample, they have found wide applications in a variety of scientific disciplines such as biology, material science and electrochemistry. After considerable advancements in instrumentation, the STM has evolved as a nanomanipulation and nanofabrication tool. It operates in two modes: constant current mode and constant height mode. In constant current mode, the feedback parameter is the tunneling current based on which the voltage applied to the piezoelectric actuator is varied. Hence, the tip height is varied in accordance with this tunneling current. In the constant height mode, however, the height is maintained at a constant value and hence the voltage applied to the piezoelectric actuator is adjusted (PZT). Unlike constant current mode, it is the tunneling current which changes according to the surface profile and the local electronic structure of the tip and the sample. The present research is an effort in designing and fabricating an in-house STM to be operated in the constant current mode by interfacing various subsystems. The various subsystems constituting the experimental setup mainly include a micro positioner, a nano stager, STM Electronics, and STM head. The fabrication process involved testing and verification of a suitable preamplifier for providing the feedback signal, design of the STM head and development of a computer automated system in order to facilitate the acquisition of signals related to a micro positioner which acts as the coarse positioner. The software control consists of ControlDesk¨ as the front end and Simulink¨ as the backend. An optical subsystem in the form of a high resolution camera that has been interfaced facilitates visual monitoring and development of dual stage control of the fine as well as coarse positioners. The ability of the STM to acquire images at the nano level is attributed to the tip to sample interaction based on quantum mechanical tunneling. To better understand the aspects of STM, the present work also traces the development of theoretical modeling of the tip-sample interaction and the conceptual design of other classes of microscopes belonging to the SPM family. Certain hardware limitations associated with the data acquisition board need to be addressed in order to acquire nanolevel images. The future scope of the research would include development and testing of various types of controllers on the STM test bed

INTEGRATING PHASE CHANGE MATERIALS IN BUILDING MATERIALS: EXPERIMENTATION, CHARACTERIZATION AND NUMERICAL SIMULATION

Phase Change Materials (PCM) have many commercial applications, including in buildings to reduce cooling and heating loads. However, the lack of research has prevented their use as a common building material. This research is part of a larger project that aims at developing a set of practical design guidelines for integrating PCM in buildings. A review of and classification of common PCM available on the market is provided. The thesis then focuses on the experimental characterization of the thermo-physical properties of PCM-integrated building materials and the development of a numerical model for the thermal analysis of buildings with PCM. Different types of PCM are mixed in plaster, a building material used for wall surface finish. The main thermal properties of interest are the thermal conductivity, latent heat, and melting and solidification temperatures. Differential scanning calorimetry is conducted to determine the phase change temperature and the latent heat. A thermopile is used to measure thermal conductivity. A Finite Element numerical model of a wall assembly is used to conduct parametric studies and predict building energy consumption