Characterization of phase transitions in nanomaterials and multifunctional properties in architected nanolattices

Abstract

With the line width reaching 10 nm, we know that the Moore’s law already had been reached its limit. Nowadays, development of electronics focuses on multi-functionality and diversity as it faces with the 4th Industrial revolution. For example, many consumers want to buy smart phones that facilitate more functionality and the super-fast performance, which are convenient functions for people's daily lives including health care sensing for blood pressure, blood sugar level, and detecting gases, UV-radiation, hazardous chemicals. This thesis discusses the metal-insulator transition properties of a strongly correlated material, VO2 which is useful to maximize the multi-functionality of the future electronics, and illustrates the properties of nano-architected meta-structures, so called nanolattices under various stimuli, which are necessary for developing the multifunctional low dielectric constant (low-k) applications. The first part of the dissertation focuses on the roles of thermal heating and electric field on the metal-insulator transition (MIT) of the VO2 nanobeams with an altered strain rate. From the electrical measurements, the degree of strain on the nanobeam can be identified by the hysteresis – the larger the width of the hysteresis is, the higher the amount of strain is. The threshold voltage (VTH) versus temperature analyses suggest the transition mechanisms of the VO2 nanobeam – a Mott transition leads to the insulator-to-metal transition while a joule heating triggers the metal-to-insulator transition. Additionally, the stability of such insulating phases as M2 may be controllable by varying the applied voltage at the highly strained nanobeams. In the second part of the dissertation, a facile method for growing VO2 wires with controlled diameters by separating the formation of the liquidus V2O5 seed droplets from the evolution of the VO2 wire using oxygen gas is described. The kinetic analyses suggest that the growth proceeds via the VS (vapor−solid) mechanism, whereas the droplet determines the size and the location of the wire. In situ Raman spectroscopy combined with analyses of the electrical properties of an individual wire allowed us to construct a diameter−temperature phase diagram from three initial phases (i.e., M1, T, and M2), which were created by misfit stress from the substrate and were preserved at room temperature. The relation with resistivity−diameter and activation energy−diameter relations supported by theoretical modeling is also described. These carefully designed approaches enabled us to elucidate the details of the phase transitions over a wide range of stress conditions, offering an opportunity to quantify relevant thermodynamic and electronic parameters (including resistivities, activation energies, and energy barriers of the key insulating phases) and to explain the intriguing behaviors of the T phase during the MIT. In the third part of the dissertation, the fabrication methods and characterization of 3-D architected ultra low-k dielectric nanostructures for use in the universal interconnectors is described. These structures exhibit a dielectric constant of 1.08 and a Young’s modulus of 30 MPa, a yield strength of 1.07 MPa, when uniaxially compressed, and can mostly recover their original shape up to 98 % after being compressed by 50 % strain. In addition, the 3D nanoarchitectured ceramic shows a dramatic thermal stability, where the ultra-low temperature coefficients of dielectric constant (TCK) of 2.43 × 10-5 K-1 was achieved in the temperature ranging between 25 and 800 oC. Notably, the resulting nanolattice exhibited leakage current can be recovered after occurring breakdown under high electric field stress, indicating that the current paths can be changed by other substitutional routes using some of the laterally connected lines inside the octet geometry. In the last part of the dissertation, the correlation of electrical and mechanical properties of the ultra low-k dielectrics architected using the two-photon lithography direct laser writing is investigated. And the recovery of complete shortened dielectrics under the electrical breakdown is observed through the regaining strength for the given stress due to the mechanical recoverability. This behavior is related to the transition from bending and buckling to collapse. Surprisingly, the breakdown strength is regainable for each cycle due to the shape recoverability. The displacement for compression at which the breakdown occurs decreases with cycles due to non-recovered buckled beams. Such degradation of breakdown strength impacts the conduction mechanism of the leakage current, that transforms from Schottky emission (SE) to Poole-Frankel emission (P-FE). The breakdown does not occur up to 200 V at the k value of ~1.07 where the device should breakdown upon biasing based on the percolation model. The dielectric constants measured for 5 stress cycles also exhibit recoverability in a close relation with the breakdown behavior.