Y.M. Wong

A novel microelectronic gas sensor utilizing carbon nanotubes for hydrogen gas detection

Abstract A novel microelectronic gas sensor utilizing carbon nanotubes (CNTs) in a thin-layered Pd/CNTs/n+-Si structure for hydrogen detection has been achieved. The sensor is fabricated on an n-type silicon wafer, which is needed as an ohmic supporting substrate. Multiwalled CNTs were grown selectively on the substrate via catalytic activation with microwave plasma enhanced chemical vapor deposition. The I–V characteristics of the sensor exhibit Schottky diode behavior at room temperature with marked sensitivity or current changes in the presence of hydrogen. Increasing detection sensitivity in hydrogen sensing was observed with increasing operating temperature. The results demonstrate that CNTs configured as a gas sensor has high sensitivity to hydrogen over a wide temperature range. Behaviors of the sensor in the presence of hydrogen and at elevated temperature were discussed. The successful utilization of CNTs in gas sensors may open a new door for the development of novel nanostructure gas-sensing devices.

CVD diamond anisotropic film as electrode for electrochemical sensing

Abstract The unique electrochemical properties of diamond such as a large working potential window, low background current and prolonged stability make it attractive for applications in electroanalysis. High quality and conductive diamond films are known to exhibit active voltammetric response without the need for surface pretreatment. This paper reports on the design, fabrication and characterization of CVD diamond film for electrochemical sensing. Two types of planar boron-doped diamond electrodes were achieved by plasma enhanced chemical vapor deposition (PECVD) using in situ gas phase doping method. The first utilizes the “as grown” diamond surface with randomly microstructured topology as a planar diamond electrode. The second utilizes a micropatterning technique to produce a well-defined pyramidal diamond tips array with good uniformity control. The fabrication process for the electrodes is described. The diamond microelectrodes were evaluated electrochemically for the detection of ferrocyanide, Fe(CN) 6 4− , using cyclic voltammetry. The results suggest that diamond electrodes can be used in electrochemical sensing application.

Uniformity conditioning of diamond field emitter arrays

The authors present recent advances in the uniformity conditioning of diamond field emitter arrays (DFEAs). Postfabrication conditioning procedures consisting of thermal annealing and high field/current operation have been examined. Nonuniformity due to varying contamination states of the emitters can be mitigated by moderate temperature (∼150–300°C) operation. Operating the emitters at elevated current levels was found to enhance the spatial uniformity in a self-limiting manner. The conditioning mechanism is most likely thermal-assisted field evaporation of the diamond nanotips, however, the nature of the dc tests does not definitively exclude back bombardment as a possible contributor. Pulsed testing is underway to remove this ambiguity, provide conditioning for high-density arrays, and demonstrate the operational current density limits of DFEAs.

Transistor characteristics of thermal chemical vapor deposition carbon nanotubes field emission triode

A study of thermal chemical vapor deposition (CVD) grown carbon nanotubes (CNTs) field emitters in a triode amplifier configuration is reported. The CNT transistor characteristics were examined by integrating gate and anode with CNT cathode in two structural forms, utilizing a TEM grid (transmission electron microscope specimen holder) as the gate electrode and a micropatterned CNT with self-aligned gate. The TEM-grid CNT triode displayed clearly gate-controlled current modulation behavior with distinct cutoff, linear, and saturation regions, and a reasonable gate turn-on field of ∼5.4V∕μm despite a large cathode-gate spacing of ∼120μm. The field emission result established the basic transistor characteristics of CNTs in a triode configuration. A CNT triode construct with a self-aligned gated fabrication technique was also developed to realize a monolithic triode structure with shorter gate-cathode spacing, lowering gate voltage, and enhancing emission current. The triode exhibited a significantly lower g...

Field emitter using multiwalled carbon nanotubes grown on the silicon tip region by microwave plasma-enhanced chemical vapor deposition

Carbon nanotubes (CNTs) were selectively grown on the proximity region of silicon tips by microwave plasma-enhanced chemical vapor deposition. The silicon substrate with silicon conical tips was sputtered with palladium to act as a nanocluster catalytic center for CNT nucleation. Curled and randomly oriented CNTs with diameters ranging from 150 to 200 nm were observed to grow selectively on surrounding areas of the silicon tips. Electron field emission tests show a low turn-on field of 3.2 V/μm. An emission current of 8.0 μA was achieved at ∼6.3 V/μm. These results show the ability to grow CNTs selectively on the silicon tip region from a catalytic metal covered surface, which may have practical applications.

Carbon nanotube growth from Cu–Co alloys for field emission applications

We have used Cu-Co alloy to generate uniform distribution of Co precipitates, and successfully grown CNTs by CVD on the Cu support.

Effect of Ti interlayer on the growth of carbon nanotubes on Si by microwave-heated chemical vapor deposition

The effect of Ti layer on the growth of aligned carbon nanotubes on Si using Pd, Ni, or Co as catalysts by microwave-heated chemical vapor deposition was systematically studied. For all growths, a thin Ti layer of 16 or 22 nm, a thin catalyst layer of 6–30 nm, a growth time of 15–45 min, and a growth temperature of 590 or 690 °C were varying deposition parameters. It was found that the growths with Ni or Co as the catalyst on Ti-coated Si could always produce well-aligned carbon nanotubes. However, a carbonaceouslike film was found to exist on the top of nanotubes in most Ni-catalyzed carbon nanotube films. In contrast, carbon nanotubes grown from Pd catalyst were generally not aligned. Furthermore, the nanotubes grown from Pd at 590 °C are crooked or twist, and very short.

Development of carbon nanotubes vacuum field emission differential amplifier integrated circuit

The device characteristics of the carbon nanotube (CNT) vacuum field emission (VFE) differential amplifier (diff-amp) is improved. The circuit-level characterization of the device was performed. SEM was used study the rectangular and circular arrays of the CNT triode array on the single chip VFE diff-amp

Carbon nanostructure field emission devices

In this paper, carbon nanostructures (CNS) utilizing high power (5 kW) and high working pressure (120 Torr) were synthesized using microwave plasma chemical vapor deposition (MPCVD), similar to the growth conditions for polycrystalline diamond synthesis. The effects of trimethylboron (TMB, 1000 ppm) addition, a typical p-type doping gas for CVD diamond, on the morphologies and field emission properties of the CNS were also investigated. This study is also extended to include fabrication of field emission triode structure with self-aligned gate. Field emission triode is a three-terminal device capitalizing on the proximity of the gate electrode to the electron emitting cathode to reduce the operating voltage of the vacuum device.

Effects of deposition and synthesis parameters on size, density, structure, and field emission properties of Pd-catalyzed carbon nanotubes synthesized by thermal chemical vapor deposition

The effects of deposition parameters and NH3 pretreatment on the size and distribution of Pd catalytic particles and subsequently their effects on the characteristics of the synthesized carbon nanotubes (CNTs) were systematically investigated. It was found that the size of Pd particles decreases and the particle density (total number of Pd particles per unit area) increases as the Pd film thickness decreases. Moreover, pretreatment of Pd film in NH3 gas promotes smaller Pd particles and higher particle density which is beneficial for CNT growth. The CNTs were synthesized by thermal chemical vapor deposition at 750 °C using methane (CH4) as the carbon source, and a mixture of Ar/H2 (80 vol %: 20 vol %) as a carrier gas with NH3 serving as a processing reagent. The incorporation of NH3 in CNT synthesis, per the specific pretreatment of catalytic film, has a distinct effect on the size and morphology of CNTs produced. The interrelation between processing, structure and emission behavior of CNTs produced with...