Byoung-Kye Kim

Investigation of the humidity effect on the electrical properties of single-walled carbon nanotube transistors

We investigated the effect of humidity on the electrical transport properties of single-walled carbon nanotube field effect transistors (SWNT-FETs). Water molecules are found to behave as electron donors to the nanotube: Current through the p-type carbon nanotube device is found to decrease under a modest humidity, and starts to increase as the humidity increases over 65%, which is believed to be due to the opening of electron channels. Through first principles calculations based on the density functional theory, we found that water molecules can donate electrons to the carbon nanotube. Moreover, a hydrogen-bonded water monolayer will be formed around the nanotube at fully covered conditions. We suggest that this result could provide a systematic understanding of the humidity effect on SWNT-FETs, which has been believed to be essential in the development of nanotube-based sensors.

The effect of metal cluster coatings on carbon nanotubes

The electrical transport and chemical sensing properties of single-walled carbon nanotube field effect transistors (SWNT-FETs) coated with metal clusters have been investigated. The source–drain current passing through an SWNT-FET coated with Pd nanoparticles showed no change over a range of gate voltages. Nevertheless, the magnitude of the current was still sensitive towards NO2, NH3 and H2 exposure. The Pd nanoparticles coating on the nanotube generated hole carriers, which either became diluted upon NH3 or H2 adsorption, or enhanced upon NO2 adsorption. Unlike the ohmic behaviour demonstrated by SWNT-FETs coated with Pd nanoparticles, the transfer characteristics of SWNT-FETs coated with Al nanoparticles revealed Schottky barrier formations at the metal–nanotube contacts. Here, the conductance through the nanotube decreased, while the device sensitivity towards NO2 and NH3 gases improved greatly. We suggest that coating SWNT-FETs with metal nanoparticles could be exploited for the development of highly sensitive nanotube-based molecular sensors.

Electrical properties of polyaniline nanofibre synthesized with biocatalyst

Polyaniline (PANI) nanofibres were synthesized using a biocatalyst (recombinant Coprinus cinereus peroxidase) instead of toxic chemical oxidants. Relatively uniform nanofibres with 50?100?nm diameter were easily obtained with this method, and the doping state of the PANI nanofibre could be controlled either with 1N camphorsulfonic acid (CSA) or with 30% NH4OH. Doped (or dedoped) PANI nanofibres were deposited on pre-patterned Au electrodes for electrical characterization. Completely dedoped PANI behaves as an insulator, while a larger current, by more than four?orders of magnitude, was observed from doped PANI nanofibres. A?weak p-type gate effect was observed for PANI nanofibre devices as well. As one could expect from the easy doping nature of PANI, PANI nanofibre devices show high sensitivity toward dedoping (NH3) gases, thereby demonstrating the possibility of using enzyme-synthesized PANI nanofibre devices as sensitive chemical sensors.

Controllable modification of transport properties of single-walled carbon nanotube field effect transistors with in situ Al decoration

We use an in situ Al decoration technique to control the transport characteristics of single-walled carbon nanotube field effect transistors (SWNT-FETs). Al nanoparticle decoration in a high vacuum caused the devices to change from p-type to n-type FETs, and subsequent exposure to the ambient atmosphere induced a gradual recovery of p-type character. In comparison with the bare SWNT-FETs under high vacuum, the channel-open devices with decorated Al particles exhibited reduced current under ambient conditions. However, selective Al decoration only at the contact resulted in an improved p-type current in ambient air.

Carbon nanotube diode fabricated by contact engineering with self-assembled molecules

The authors report the construction of carbon nanotube Schottky diodes by covering a selectively exposed area of the electrode with self-assembling molecules. Two self-assembling molecules with different polarities, 2-aminoethanethiol and 3-mercaptopropionic acid, were used to modify the Fermi level lineup at the selected contact. The devices showed p-type behavior with symmetric I-V showing clear rectifying behavior after treatment of one contact with 2-aminoethanethiol. Their experiment, in conjunction with the results of ab initio electronic structure calculations, suggests that the diode action stems from the asymmetric Fermi level lineup between the bare and engineered contacts.

Ambient air effects on electrical characteristics of GaP nanowire transistors

Gallium phosphide (GaP) nanowire transistors were fabricated in back-gated structure, and their electrical characteristics were measured systematically in both air and vacuum. The transistors turn on typically between −5 and −7V in ambient air. However, a large threshold voltage (Vth) shift, ∼10V, toward negative gate bias was observed in vacuum. After the transistors were exposed to air for 48h, Vth returned to the similar value in ambient air, implying a reversible process. The rate of Vth shift slows down when they were exposed to N2 in comparison with that of air. The shift of Vth is believed to be related to the charge transfer from the surface of GaP nanowire to the physically adsorbed OH or oxygen. In addition, the observed Vth shift from the GaP nanowire transistors can be explained by the conventional n-channel depletion mode metal-oxide-semiconductor field-effect transistor.

Electrical properties of individual single-crystalline gallium phosphide nanowires with an outer oxide shell

High-quality single-crystalline GaP nanowires were grown by a simple vapour deposition method and their electrical and opto-electric transport properties were studied. Structural studies showed that the GaP nanowires consisted of a core?shell structure with a single-crystalline GaP core and an outer gallium oxide (GaOx) layer of thickness ?nm. The individual GaP nanowires exhibited n-type field effects with an on/off ratio as high as 105 and their carrier mobilities are in the range of about 10?22?cm2?V?1?s?1 at room temperature. When the devices were exposed to an ultraviolet (UV) light source, the current in the nanowires increased abruptly to more than 103 times, and this was possibly due to carrier generation in the nanowires and de-adsorption of adsorbed O2? ions on the GaOx surface shell. The nanowires also showed good reversible switching actions between the high-?and low-resistance states.

Single nucleotide polymorphism detection using au-decorated single-walled carbon nanotube field effect transistors

We demonstrate that Au-cluster-decorated single-walled carbon nanotubes (SWNTs)may be used to discriminate single nucleotide polymorphism (SNP). Nanoscale Au clusters were formed on the side walls of carbon nanotubes in a transistor geometry using electrochemical deposition. The effect of Au cluster decoration appeared as hole doping when electrical transport characteristics were examined. Thiolated single-stranded probe peptide nucleic acid (PNA) was successfully immobilized on Au clusters decorating single-walled carbon nanotube field-effect transistors (SWNT-FETs), resulting in a conductance decrease that could be explained by a decrease in Au work function upon adsorption of thiolated PNA. Although a target single-stranded DNA (ssDNA) with a single mismatch did not cause any change in electrical conductance, a clear decrease in conductance was observed with matched ssDNA, thereby showing the possibility of SNP (single nucleotide polymorphism) detection using Au-cluster-decorated SWNT-FETs. However, a power to discriminate SNP target is lost in high ionic environment. We can conclude that observed SNP discrimination in low ionic environment is due to the hampered binding of SNP target on nanoscale surfaces in low ionic conditions.

Designing the Carbon Nanotube Field Effect Transistor Through Contact Barrier Engineering

Through recent publications, as reviewed in this article, we have determined the effects of contact barrier change on the electrical transport properties of carbon nanotube field-effect transistors. To analyze the Fermi level alignment and the Schottky barrier at the contact, we used the first-principles electronic structure calculations of different types of metal electrodes with various bonding configurations. In parallel, we have used various experimental techniques to engineer the contact barrier: decorations of metal nanoparticles, the self-assembled monolayers of molecules, and protein nanoparticles. We investigated the changes in the electron transport properties of the nanotube transistors in relation to the adjustment of the contact barrier. Overall reviews of these studies are presented here, and a few potential applications are also suggested.

Electrical sorting of carbon nanotube transistors for mass-producible bio-sensors

This paper reports useful fabrication scheme customized for biosensing application by adopting the Collins approach of electrical breakdown (Collins, 2001) with some modification to improve the fabrication cost and reduce fabrication time.The electrical breakdown scheme was modified to increase the yield of electrical sorting and reduce sorting time, which is very important in real mass-production. CNTFETs were fabricated using the patterned catalyst growth technique, and source and drain electrodes were defined using photolithography and lift-off process. Then electrical sorting was performed in ambient condition.