Nanoprobe interferometer for vapor mapping in a microscopic space

Abstract

Water vapor in the air, indicated by the relative humidity, is a fundamental physical quantity that influences various fields in science, industry, and society. In particular, with the continuous growth of micro/nanotechnology, vapor in a microscopic handling with very small amounts of water has attracted considerable attention in both physical/chemical phenomena and in technological applications, for example, micro-/nanofluidics, nanofabrication, and micro-robotics. Therefore, vapor mapping has long been an unavoidable task and its technological advancements are highly demanded. Recently, advanced humidity sensors (i.e. hygrometers) with great performance such as high sensitivity, fast response time, and device-configuration compatibility, have been developed by utilizing various nanomaterials to quantitatively measure the amount of vapor. However, most hygrometers still have difficulties for detecting water vapor in microscopic spaces due to the fundamental limitations of conventional hygrometers such as lack of accessibility and positioning accuracy. Therefore, the development of hygrometers with high spatial-resolution and on-demand positioning control in precision that can be accessed in microscopic spaces remains a challenging task that goes beyond the limitations of conventional hygrometers. In this dissertation, a novel approach to map water vapor distribution in a microscopic space is presented based on a scanning probe hygrometry. The key strategy is to develop a freestanding nanoprobe interferometer consisting of a nanowire probe interferometer, fabricated by three-dimensional (3D) nanoprinting of a humidity-responsive nanowire on a tapered optical fiber. Unlike conventional hygrometers, which are limited to surface and surrounding environments sensing, the scanning of the nanoprobe interferometer, can overcome a variety of critical issues such as the precise positioning and access into microscopic spaces due to the advantage of freestanding nanoprobe geometry. First, for fundamental development of nanoprobe interferometer, the nanowire probe interferometer was developed for measuring humidity in a microscopic space. Spatial mapping of humidity was successfully presented by directly observing the interferometric response of the nanoprobe on humidity within a humidity-responsive nanowire during the scanning process. Here, multiscale spatial mapping of humidity with versatile scanning resolutions from ~ 100 nm to a few mm is demonstrated based on scanning of the nanoprobe interferometer. The implication of this work is that it provides novel nanoscale metrology, which is able to answer fundamental questions about humidity-related research and applications in physics, chemistry and biomedicine fields. Second, for applied research of nanoprobe interferometer, the mapping of water vapor inside a microscopic space was demonstrated, for studying humidity-related phenomena, i.e. evaporation. Novel experimental determination of evaporation characteristics from a water meniscus in a micro-capillary was presented, based on quantitative measurement of vapor concentration by scanning the nanoprobe interferometer. In this study, it was found that evaporation flux from the meniscus is always higher at the wall than at the center. In addition, the dependence of the meniscus geometry (e.g. diameter) and ambient conditions (e.g. ambient humidity) on evaporation dynamics was studied based on quantitative measurement of evaporation flux from the meniscus. This work would pave the way for exploring various questions related to microscopic and geometrically constrained evaporation dynamics that have been experimentally inaccessible so far.

Chapter 1. Introduction 1 1.1 Research motivation 1 1.2 Critical issue 2 1.3 Dissertation objective 3 References 4 Chapter 2. Literature survey 6 2.1 Nanomaterial-based humidity sensors 6 2.1.1 Nanowire based humidity sensors 6 2.1.2 Thin-film based humidity sensors 8 2.2 Three dimensional (3D) nano-printing: meniscus guided approach 9 Figures 11 References 12 Chapter 3. Nanoprobe interferometer for scalable humidity mapping 14 3.1 Introduction 15 3.2 Methods 17 3.3 Results and discussions 19 3.4 Conclusions 25 Figures 26 References 37 Chapter 4. Nanoprobe interferometer for vapor mapping in a microscopic space 42 4.1 Introduction 43 4.2 Methods 45 4.3 Results and discussions 46 4.4 Conclusions 56 Figures 57 Tables 74 References 76

Chapter 5. Summary 81