Development of Sensors Based on Advanced Micro- and Nano-Structured Carbon Materials
Mendoza Centeno, Frank Willi
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The thesis is focused on the development of sensors based on advanced micro- and nano-structured carbon materials. In particular, we developed prototype diamond-based ultraviolet photodetectors and carbon nanotubes-based gas sensors. We describe the method of preparation and characterization of the active carbon-based materials and their structural and compositional characterizations. This is followed by the corresponding device fabrication and testing. The first part of the thesis briefly gives an introduction to our current understanding about carbon materials, with emphasis on synthetic diamond and bamboo-like carbon nanotubes, and to the materials’ properties that are useful for ultraviolet photodetectors and gas sensor applications. The rest of the thesis is organized as follows: In the Chapter 2, the room-temperature photosensitivity of sulfur-assisted micro- (MCD), submicro- (SMCD) and nano- (NCD) crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy and X-ray diffraction. The UV sensitivity and response time was studied for the three types of diamond materials using a steady state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region, as high as 109sec-1mV-1 range, linear response in a broad spectral range below 320 nm, photocurrents around ~10-5 A, and short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials’ photosensitivity. Chapter 3 expands on the usefulness of polycrystalline and nanocrystalline diamond films for UV sensor applications by exploring the field emission device configuration and the grain-size effect on the photo-response. Different grain sizes of crystalline diamond were grown to assess the changes to the photo-response behavior in the 200 to 300 nm wavelength range covering the band gap energy (5.5 eV or 225 nm). The comparison of the photo-response in the samples was evaluated by means of two types of contact electrode configurations. The first configuration operates in the well-known planar configuration (PC) with electrodes on S-assisted diamond surfaces while the second operates in an electron field emission (FE) mode at low electric field (0.7V/μm - 1.7V/μm) such that only ultraviolet light can trigger the emission current. Compared with samples in a planar configuration, the field emission performance of S-assisted diamond films can be greatly enhanced by the UV light illumination. Additionally, samples of interest measured in FE mode confirmed information about the grain boundary and negative electron affinity (NEA, ) which are believed are the responsible for the electrical response observed in samples. Chapter 4 describes chemical sensors based on tin dioxide-carbon nanotubes (SnO2-CNT) composite films that were fabricated by hot filament chemical vapor deposition (HF-CVD) technique. The composite films consist of SnO2 nanoparticles highly dispersed on the CNTs surface. Their resistivity is highly sensitive to the presence of adsorbates, which become easily attached or detached at room temperature and ambient pressure depending on their gas phase concentration. We systematically studied the sensitivity of the SnO2-CNT composite films for ethanol, methanol and H2S. The results were also compared to those for SnO2 and CNTs separately. It is shown that the SnO2-CNT composite films can detect ethanol, methanol and H2S down to ppm levels below OSHA’s permissible exposure limits at room temperature and ambient pressure. Moreover, they self-recover within one minute without requiring any heating or energy source. Chapter 5 describes another oxide-based material integration with carbon nanotubes under a new method. We developed to synthesize nano-size barium strontium titanate Ba0.7Sr0.3TiO3 (BST) maize bead-like structures conformally coated on Bamboo-like carbon nano-fibers (BCNTs). Initially BCNTs were fabricated by hot-filament chemical vapor deposition (HF-CVD) techniques on Cu substrates. Later, BST was deposited on BCNT/Cu by pulsed laser deposition (PLD) techniques. Surface morphology, cross-sectional image and topography of the BCNTs and BCNT-BST maize-like nanostructures were investigated by X-ray, field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy and tunneling electron microscopy (TEM) techniques. BST-BCNT hybrid structures provide an additional degree of freedom to interconnect the oxides, which in turn provides [3D] geometry for functionality. There had been a popular misconception that oxides cannot grow on CNT at high temperature; this long-standing problem has now been solved. The present unregistered nanostructures can be used as sensors; however, if these structures are made as registered arrays, they can be used as nonvolatile memory elements and high energy density capacitors. Finally, on Chapter 6, we give an overview of the experience gained through this research, and some suggestions for those who would like follow the research methods employ here. It provides experimental information learned through experience that may be helpful and avoid delays to the newer experimentalists.