Jong Seung Hwang

Fabrication of single electron transistors with molecular tunnel barriers using ac dielectrophoresis technique

We demonstrate the fabrication and characterization of single electron transistors composed of Au nanoparticles and insulating molecular tunnel barriers. We fabricated these devices by forming a 1,8-octanedithiol self-assembled monolayer on Au nanogap electrode pairs and by bridging them with colloidal Au nanoparticles using ac dielectrophoresis. We observed the typical characteristics of these single electron transistors at the temperature from 4.2 to 300 K. The measured data were consistent with the orthodox theory of Coulomb blockade.

Microwave Characterization of a Single Wall Carbon Nanotube Bundle

Microwave (MW) characteristics of a single bundle of single wall carbon nanotubes (SWCNTs) were obtained in the frequency range from 0.1 to 40 GHz. A tapered, sandglass shaped coplanar waveguide (CPW) structure was fabricated, and the input and output signal lines in the neck of the CPW were connected by a single bundle of SWCNTs. The positioning of the SWCNT was done by AC dielectrophoresis and further deposition of Au at the contacts was done by selective electrochemical plating. The MW characteristics of those CPWs were measured, and the characteristics of the CPW with the SWCNTs exhibited clear differences from that of the bare CPW. From the de-embedding process using an equivalent circuit, we found that the inductance of the SWCNTs was 1.2 nH.

Transport Properties of a DNA-Conjugated Single-Wall Carbon Nanotube Field-Effect Transistor

We performed electrical transport measurements on a DNA-conjugated single-wall carbon nanotube (SWCNT) field-effect transistor (FET) at various temperatures (Ts). At T = 300 K, the gate action of the SWCNT FET clearly exhibited n-type behavior, while the bare SWCNT FET showed usual p-type characteristics. The value of the drain current measured from the DNA-conjugated SWCNT FET decreased rapidly with the decrease of T. We fit the T-dependence of the DNA-conjugated SWCNT FET with a simple back-to-back Schottky diode model and found a slight decrease of ΦB with decreasing T. A possible explanation for the type conversion is the chemical modification of the CNT surface as a result of the conjugation process.

Electrical Transport Properties of Au-Doped Deoxyribonucleic Acid Molecules

Deoxyribonucleic acid (DNA) molecules were doped with Au atoms and their electrical transport properties were measured. The Au doping was carried out by incubating a mixture of HAuCl43H2O and DNA solutions. The binding of Au atoms to DNA bases was identified using Fourier transform infrared spectroscopy and X-ray photoemission spectroscopy. The Au-doped DNA molecules were deposited on nanoelectrodes and the presence of the molecules between the electrodes was determined by both scanning electron microscopy and atomic force microscopy. Measurement of the current-voltage characteristics showed that the Au-doped DNA molecules exhibited a higher conductivity than undoped DNA molecules. Detailed analysis of the chemical composition shows that there is a strong possibility of reliably controlling the conductivity of DNA molecules using this method.

Single-String Carbon Nanotube Field Effect Transistors Fabricated by Two-Step Dielectrophoresis

Carbon nanotube field effect transistors (CNT FETs) with a long, single channel are an essential ingredient for gas and bio sensors, because a single spot modification of the channel can change the conductivity of the whole device. Herein, the two-step dielectrophoresis (DEP) technique was used to fabricate single string, single wall CNT FETs with a length longer than 10 µm. The single string FET showed an on/off ratio and transconductance which are larger than those of the network FET on the same substrate. The observed characteristics were explained by a circuit simulation. We also demonstrated that our method could be applied to flexible substrates.

Faraday's Induction Experiment in Nano-Transformers

Faradays induction experiment in a nanometer scale was performed. Various nano-transformers with the smallest coupling area of approximately 1 mum2 were fabricated, and the induced open circuit voltages were measured. The output responses showed linear dependences on both the magnitude and the frequency of the input current. This suggested that the nano-transformer behaved as a linear transformer. The extracted mutual inductance value was approximately 30 nH and it was several orders larger than the one expected from 1 mum2 coupling area.

Magnetic bead detection using nano-transformers

A novel scheme to detect magnetic beads using a nano-scale transformer with a femtoweber resolution is reported. We have performed a Faradays induction experiment with the nano-transformer at room temperature. The transformer shows the linear output voltage responses to the sinusoidal input current. When magnetic beads are placed on the transformer, the output responses are increased by an amount corresponding to the added magnetic flux from the beads when compared with the case of no beads on the transformer. In this way, we could determine whether magnetic beads are on top of the transformer in a single particle level.

Organic Electrolyte Based Pulsed Nanoplating and Fabrication of Carbon Nanotube Network Transistors

The formation of gold nanocontacts was performed using a pulsed electrochemical plating technique. The effect of various plating variables on the surface roughness of the plated electrodes was studied in the high frequency regime where the reduction reaction of gold complex becomes the bottleneck process. We demonstrated the selective contact formation of single wall carbon nanotube network field effect transistors (FETs) with this technique. The fabricated FETs exhibit usual p-type behavior with the performance comparable to usual network FETs.

DC Transport Characteristics of Lambda DNA Molecules and Effect of RF Signals

We investigated DC and RF transport characteristics of lambda DNA molecules. Our DNA film device with a coplanar waveguide structure exhibited reliable high-frequency characteristics and sufficiently small leakage currents. The DC gate response of the DNA transistor showed nonlinear transport properties and resistance decreased as VG became increasingly negative. It suggests that lambda DNA behaves as a p-type semiconductor. The RF pulse response of the lambda DNA transistor showed an increase in average current with increase in the frequency. The observed frequency characteristics can be explained by AC heating, and our observation suggests that the conductivity of DNA molecules increases with temperature.