Design and Fabrication of Suspended 1D Carbon Micro/Nanostructures for Thermally-Driven Environmental Sensors

Abstract

Recently, suspended one-dimensional (1D) micro/nanostructure–based environmental sensors have attracted significant attention due to their high surface-to-volume ratio, high aspect ratio, and small footprint. In particular, such structures are highly desirable in thermally driven environmental sensors because they enable efficient heating and enhanced sensitivity. However, conventional fabrication methods—both top-down and bottom-up—often suffer from high costs, complex etching steps, and poor reproducibility. To address these challenges, this dissertation proposes a novel wafer-scale nanofabrication platform that utilizes suspended carbon nanobackbones and a built-in shadow mask. First, suspended 1D carbon nanostructures were formed through Carbon-microelectromechanical system (C-MEMS), wherein pre- patterned polymer microstructures were pyrolyzed and underwent significant volume reduction to achieve nanoscale dimensions. Crucially, the built-in shadow mask enabled the selective deposition of functional materials (e.g., heaters, thermopiles) onto the suspended nanostructures using only standard microscale alignment, effectively preventing electrical connection through the substrate without requiring complex etching processes. This platform was successfully applied to develop two types of high-performance environmental sensors: 1. Thermal Conductivity Detector (TCD)–type gas sensor: A suspended 1D nanoheater was realized through selective deposition of a gold layer onto a carbon nanowire backbone. The sensor demonstrated an ultrafast thermal time constant (< 1 μs) owing to its minimal thermal mass and high aspect ratio. This rapid response enabled duty-cycled operation, reducing power consumption to the nanowatt range (240 nW), representing a ~1000-fold reduction compared to constant-power operation. Furthermore, an advanced etching-free fabrication process based on a carbon nanogrid architecture was developed, further simplifying the manufacturing steps while preserving high sensitivity to gases such as H₂, He, and Ar. 2. Thermopile-type IR Sensor: A suspended carbon IR-absorber-based sensor was fabricated by integrating Ni/Ge thermopiles onto a suspended carbon IR absorber. The pyrolyzed carbon served a dual role: as a robust mechanical backbone with low thermal conductivity (~1.2 W/m·K), maximizing the temperature gradient, and as a broadband IR absorber superior to conventional silicon-based materials. The sensor geometry, including the number of thermopile elements, was optimized using Finite Element Method (FEM) simulations. The resulting device exhibited high sensitivity and broadband detection capabilities suitable for non-contact temperature monitoring. In conclusion, this dissertation demonstrates the development of suspended 1D nanostructures for thermally driven environmental sensing. The resulting sensors exhibit high power efficiency and high sensitivity, confirming the potential of this platform for next-generation portable and wearable environmental monitoring systems.