We show both gas pressure and species sensing capabilities based on the electrothermal effect of a multiwalled carbon nanotube (MWCNT). Upon exposure to gaseous environments, the resistance of a heated MWCNT is found to change following the conductive heat-transfer variances of gas molecules. To realize this mechanism, a suspended MWCNT is constructed by synthesis and assembly in localized chemical vapor deposition that is accomplished within seconds via real-time electrical feedback control. Vacuum pressure sensitivity and gas species differentiability are observed and analyzed. Such MWCNT electrothermal sensors are compact, fast and reversible in responses, and fully integratable with microelectronics.Carbon nanotube (CNT) gas sensors based on fundamentally different mechanisms have been demonstrated, 1 including electrical conductance or capacitance changes 2-4 or using the sharp tips of the CNT as gas ionization sensors. 5 Limiting factors for sorption-based CNT sensors include low adsorption energies, diffusion kinetics, response speed, selectivity, and reversibility. We report on the electrothermal effect of a single multiwalled carbon nanotube (MWCNT) suspended between two silicon microbridges. When a Joule-heated MWCNT is exposed to different pressures, the electrical resistance change follows the conductive heat-transfer variances of gas molecules. The MWCNT is grown on silicon using the local synthesis method and is accomplished within tens of seconds via real-time electrical feedback control. Our findings suggest that the MWCNT electrothermal mechanism has applications to both gas sensing and species differentiation, with advantages of compactness, fast and reversible responses, low power consumption, and being fully integratable with microelectronics.The electrothermal gas sensing mechanism is schematically illustrated in Figure 1a. A single MWCNT is suspended between two silicon microstructures and heated by electrical current in the system. Energy conservation calls for total heat generation equal to the summation of heat conduction to the two microstructures (W C ), heat transfer via gases (W G , shown in the figure as W gas1 and W gas2 representing the case of two types of gases of different thermal conductivity values), and heat radiation (W R ). Both gas pressure and species can affect the heat-transfer process and result in MWCNT temperature changes and therefore resistance changes.To realize this architecture, we use a complementary metal oxide semiconductor (CMOS)-compatible, in situ controlled synthesis, assembly, and integration process previously reported in the literature. 6-9 Synthesis and assembly of carbon nanotubes has been heavily investigated with various device demonstrations using high-temperature synthesis processes 10,11 and labor-intensive assembly steps, 12,13 but assembly and heterogeneous integration of CNTs with microelectronics such as standard CMOS circuitry are still problematic. Many issues still need to be resolved, such as the synthesis and accurate placement...

A rapid yet simple methodology to synthesize carbon nanotubes (CNTs) in a room temperature environment has been demonstrated using an inductive heating system. Substrates of either heavily doped silicon or nickel-coated, lightly doped silicon have been used to synthesize CNTs using Fe as the catalyst. Aligned carbon nanotubes with growth rates as high as 200μm∕min have been achieved in less than 1min. Transmission electron micrographs illustrated average diameters of 8 and 6.8nm for CNTs grown under average temperatures of 760 and 910°C, respectively. This system allows the synthesis of CNTs that is easy to set up, fast, clean, and inexpensive.

ZnO nanowires have been rapidly synthesized using inductive heating in a room temperature environment. Nanowires with random and aligned orientations were grown on silicon and 4H-SiC (0001) substrates in less than 5min, respectively, using ZnO/graphite as the solid source powder. Scanning electron microscopy showed nanowire diameters of 20–120nm and lengths up to 5μm, and transmission electron microscopy verified the single-crystalline lattice of the nanowires. Electrical properties were studied by connecting a single ZnO nanowire in the field-effect transistor configuration. This demonstration further illustrates the feasibility of a simple and fast nanoscale synthesis using inductive heating for nanomaterial synthesis.

A novel local vapor transport technique via induction heating is presented to enable selective, localized synthesis and self-assembly of nanowires, providing a simple and fast method for the direct integration of nanowires into functional devices. The single-crystalline zinc oxide (ZnO) nanowires are grown locally across the silicon-on-insulator microelectrodes within minutes, and the enhancement of gas sensing of ZnO nanowires is demonstrated under ultraviolet (UV) illumination at room temperature. Experiments indicate that when suspended nanowires are exposed to UV light, a twelve-fold increase in conductance and a near five-fold improvement in oxygen response are measured. Furthermore, the UV-enhanced transient responses exhibit a two-level photocurrent decay attributed to carrier recombination and oxygen readsorption. As such, the local vapor transport synthesis and UV-enhanced sensing scheme could provide a promising approach for the construction of miniaturized and highly responsive nanowire-based gas sensors.

Localized heating has emerged as a viable technique for the site specific synthesis of one-dimensional ͑1D͒ nanostructures. By localizing the heat source, the extent of chemical vapor deposition synthesis reactions can be confined to well-defined, microscale regions. Resistive heating has been extensively used to realize highly localized regions of elevated temperature while maintaining a microelectronics-compatible thermal environment elsewhere. Other localized heating methods are being pursued as well. Overall, the approach is simple, flexible, and robust, and offers unique opportunities in 1D nanostructure synthesis, characterization, and integration. Herein, the recent progress of these techniques is reviewed and discussed.