Low temperature carbon material deposition with photo-enhanced chemical vapor deposition

Photo-enhanced chemical vapor deposition (CVD) technique is investigated here for low temperature deposition of carbon nanotubes (CNTs) and hexagonal diamond. Most current deposition methods require high substrate temperature. Photo-enhanced CVD utilizes light energy to dissociate carbon containing precursor molecules and hence has a potential for low temperature deposition. CCl4, having a high absorption coefficient compared to other commonly employed hydrocarbons in the UV emission spectrum from a Xe arc lamp, is selected as a carbon precursor in this work. Extensive experimentation conducted by varying Al/Ni/Al catalyst layer thicknesses on SiO2 coated Si substrates, substrate annealing temperature in the range 350 - 450 ¡ÆC for 25 min, and chamber pressure in the range 0.22 - 10 Torr in ammonia ambient, yielded suitable catalyst layers of thicknesses 3/2/3, 5/1/5 and 5/3/5 nm and annealing pressure of 10 Torr. For photo-enhanced CVD deposition, experiments are conducted with various Ar/CCl4 flow ratio in 1.5 - 19 range, total chamber pressure in 3 - 10 Torr range, and substrate temperatures in 350 - 450 ¡ÆC range. Optimal condition for CNT deposition in this work is found to be 30 min at 400 ¡ÆC at 5 Torr total pressure with Ar/CCl4 ratio of 9 with 5/1/5 nm thick catalyst annealed at 400 ¡ÆC. Raman spectroscopy indicates MWCNT growth and I-V measurements yield sheet resistivity of 22 k§Ù/sq. The densest hexagonal diamond deposition is obtained at 450 ¡ÆC, 3 hr deposition time, at 10 Torr with Ar/CCl4 ratio of 2.3 with 5/3/5 nm thick catalyst annealed at 450 ¡ÆC. Lesser dense hexagonal diamond platelets are obtained at 450 ¡ÆC, 3 hr deposition time, at 10 Torr with Ar/CCl4 ratio of 2.3 with 3/2/3 nm thick catalyst annealed at 450 ¡ÆC. Based on the physical structures observed at various stages of growth in SEM images, a model is proposed for nucleation and subsequent growth of hexagonal diamond platelets with graphene layer playing role both during nucleation and during platelet growth. Raman spectroscopy and XPS results confirm the deposition material to be hexagonal diamond. The grown material is characterized with UV-Vis spectroscopy for optical and with a nanoindenter for electrical and mechanical properties

Localized Liquid-Phase Synthesis of Porous SnO_2 Nanotubes on MEMS Platform for Low Power, High Performance Gas Sensors

We have developed highly sensitive, low-power gas sensors through the novel integration method of porous SnO_2 nanotubes (NTs) on a micro-electro-mechanical-systems (MEMS) platform. As a template material, ZnO nanowires (NWs) were directly synthesized on beam-shaped, suspended microheaters through an in situ localized hydrothermal reaction induced by local thermal energy around the Joule-heated area. Also, the liquid-phase deposition process enabled the formation of a porous SnO_2 thin film on the surface of ZnO NWs and simultaneous etching of the ZnO core, eventually to generate porous SnO_2 NTs. Because of the localized synthesis of SnO_2 NTs on the suspended microheater, very low power for the gas sensor operation (<6 mW) has been realized. Moreover, the sensing performance (e.g., sensitivity and response time) of synthesized SnO_2 NTs was dramatically enhanced compared to that of ZnO NWs. In addition, the sensing performance was further improved by forming SnO_2–ZnO hybrid nanostructures due to the heterojunction effect

Comparative Study of Different Methods for Estimating Weibull Parameters: A Case Study on Jeju Island, South Korea

On Jeju Island, South Korea, an investigation was conducted to determine the best method for estimating Weibull parameters. Six methods commonly used in many fields of the wind energy industry were reviewed: the empirical, moment, graphical, energy pattern factor, maximum likelihood, and modified maximum likelihood methods. In order to improve the reliability of a research result, five-year actual wind speed data taken from nine sites with various topographical conditions were used for the estimation. Furthermore, the effect of various topographical conditions on the accuracy of the methods was analyzed and 10 bin interval types were applied to determine the most appropriate bin interval based on their performances. Weibull distributions that were estimated using these methods were compared with the observed wind speed distribution. Then the accuracy of each method was evaluated using four accuracy tests. The results showed that of the six methods, the moment method had the best performance regardless of topographical conditions, while the graphical method performed the worst. Additionally, topographical conditions did not affect the accuracy ranking of the methods for estimating the Weibull parameters, while an increase of terrain complexity resulted in an increase of discrepancy between the estimated Weibull distribution and the frequency of the observed wind speed data. In addition, the choice in bin interval greatly affected the accuracy of the graphical method while it did not depend on the accuracy of the modified maximum likelihood method

Localized Liquid-Phase Synthesis of Porous SnO₂ Nanotubes on MEMS Platform for Low-Power, High Performance Gas Sensors

We have developed highly sensitive, low-power gas sensors through the novel integration method of porous SnO₂ nanotubes (NTs) on a micro-electro-mechanical-systems (MEMS) platform. As a template material, ZnO nanowires (NWs) were directly synthesized on beam-shaped, suspended microheaters through an in situ localized hydrothermal reaction induced by local thermal energy around the Joule-heated area. Also, the liquid-phase deposition process enabled the formation of a porous SnO₂ thin film on the surface of ZnO NWs and simultaneous etching of the ZnO core, eventually to generate porous SnO₂ NTs. Because of the localized synthesis of SnO₂ NTs on the suspended microheater, very low power for the gas sensor operation (<6 mW) has been realized. Moreover, the sensing performance (e.g., sensitivity and response time) of synthesized SnO₂ NTs was dramatically enhanced compared to that of ZnO NWs. In addition, the sensing performance was further improved by forming SnO₂–ZnO hybrid nanostructures due to the heterojunction effect

Self-Powered Gas Sensor Based on a Photovoltaic Cell and a Colorimetric Film with Hierarchical Micro/Nanostructures

We report a new type of self-powered gas sensors based on the combination of a colorimetric film with hierarchical micro/nanostructures and organic photovoltaic cells. The transmittance of the colorimetric film with micro/nanostructures coated with N,N,N&apos;,N&apos;-tetramethyl-p-phenylenediamine (TMPD) changes by reacting with NO2 gas, and it is measured as a current output of the photovoltaic cell. For this purpose, materials for the organic photovoltaic cells were carefully chosen to match the working wavelength of the TMPD. Micropost arrays and nanowires increase the surface area for the gas reaction and thus improve the transmittance changes by NO2 gas (6.7% change for the plain film vs 27.7% change for the film with hierarchical micro/nanostructures to 20 ppm of NO2). Accordingly, the colorimetric device with the hierarchical structures showed a response of ��I/I0 = 0.27-20 ppm of NO2, which is a 71% improvement compared to that of the plain sensing film. Furthermore, it showed a high selectivity against other gases such as H2S and CO with almost negligible responses. Since the current output change of the photovoltaic cell is utilized as a sensor signal, no extra electrical power is required for the operation of gas sensors. We also integrated the sensor device with an electrical module and demonstrated a self-powered gas alarm system.11Nsciescopu

Micropatterning of metal oxide nanofibers by electrohydrodynamic (EHD) printing towards highly integrated and multiplexed gas sensor applications

Integration of heterogeneous sensing materials in microelectronic devices is essential to accomplish compact and highly integrated environmental sensors. For this purpose, a micro-patterning method of electrospun metal oxide nanofibers based on electrohydrodynamic (EHD) printing process was developed in this work. Several types of metal oxide (SnO_2, In_2O_3, WO_3 and NiO) nanofibers that were produced by electrospinning, fragmented into smaller pieces by ultrasonication, and dissolved in organic solvents were utilized as inks for the printing. Constant or pulsed wave bias consisting of base and jetting voltages were applied between a nozzle and a substrate to generate a jetting of nanofiber solutions. Several parameters for EHD printing such as pulse width, inner diameter of the nozzle, distance from the nozzle to the substrate, and stage speed, were optimized for accurate micro-patterning of electrospun nanofibers. By using optimized printing parameters, microscale patterns of electrospun nanofibers with a minimum diameter less than 50 μm could be realized. Gas sensors were fabricated by EHD printing on the microelectrodes and then used for the detection of toxic gases such as NO_2, CO and H_2S. Four kinds of metal oxides could detect down to 0.1 ppm of NO_2, 1 ppm of H_2S and 20 ppm of CO gases. Also, heterogeneous nanofiber gas sensor array was fabricated by the same printing method and could detect NO_2 using the sensor array platform with microheaters. Furthermore, microscale patterns of nanofibers by EHD printing could be applied to the suspended MEMS platform without any structural damage and this sensor array could detect NO_2 and H_2S gases with 20 mW power consumption

Multiplexed Gas Sensor Based on Heterogeneous Metal Oxide Nanomaterial Array Enabled by Localized Liquid-Phase Reaction

A novel method for the selective and localized synthesis of nanomaterials and their in situ integration based on serial combination of localized liquid-phase reaction has been developed for the fabrication of heterogeneous nanomaterial array. This method provides simple, fast and cost-effective fabrication process by using well-controlled thermal energy and therefore solves the challenging problems of assembly and integration of heterogeneous nanomaterial array in functional microelectronic devices. We have fabricated a parallel array of TiO₂ nanotubes, CuO nanospikes, and ZnO nanowires, which exhibited adequate gas sensing response. Furthermore, we could approximately determine individual gas concentrations in a mixture gas consisting of 0–2 ppm of NO₂ and 0–800 ppm of CO gas species by analyzing multiple data from an array of heterogeneous sensing nanomaterials