Young-Kyun Kwon

Unusually High Thermal Conductivity of Carbon Nanotubes

Combining equilibrium and nonequilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity lambda of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value, lambda approximately 6600 W/m K, for an isolated (10,10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of lambda are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite.

Linker-free directed assembly of high-performance integrated devices based on nanotubes and nanowires

Advanced electronic devices based on carbon nanotubes (NTs) and various types of nanowires (NWs) could have a role in next-generation semiconductor architectures. However, the lack of a general fabrication method has held back the development of these devices for practical applications. Here we report an assembly strategy for devices based on NTs and NWs. Inert surface molecular patterns were used to direct the adsorption and alignment of NTs and NWs on bare surfaces to form device structures without the use of linker molecules. Substrate bias further enhanced the amount of NT and NW adsorption. Significantly, as all the processing steps can be performed with conventional microfabrication facilities, our method is readily accessible to the present semiconductor industry. We use this method to demonstrate large-scale assembly of NT- and NW-based integrated devices and their applications. We also provide extensive analysis regarding the reliability of the method.

Fractional Quantum Conductance in Carbon Nanotubes

Using a scattering technique based on a parametrized linear combination of atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of multiwall carbon nanotubes. We find that interwall interactions not only block some of the quantum conductance channels, but also redistribute the current nonuniformly over individual tubes across the structure. Our results provide a natural explanation for the unexpected integer and noninteger conductance values reported for multiwall nanotubes by Stefan Frank et al. [Stefan Frank et al., Science 280, 1744 (1998)].

Universal Parameters for Carbon Nanotube Network-Based Sensors: Can Nanotube Sensors Be Reproducible?

Carbon nanotube (CNT) network-based sensors have been often considered unsuitable for practical applications due to their unpredictable characteristics. Herein, we report the study of universal parameters which can be used to characterize CNT network-based sensors and make their response predictable. A theoretical model is proposed to explain these parameters, and sensing experiments for mercury (Hg(2+)) and ammonium (NH(4)(+)) ions using CNT network-based sensors were performed to confirm the validity of our model.

Electronic structures of one-dimensional metal–molecule hybrid chains studied using scanning tunneling microscopy and density functional theory

The electronic structures of self-assembled hybrid chains comprising Ag atoms and organic molecules were studied using scanning tunneling microscopy (STM) and spectroscopy (STS) in parallel with density functional theory (DFT). Hybrid chains were prepared by catalytic breaking of Br-C bonds in 4,4″-dibromo-p-terphenyl molecules, followed by spontaneous formation of Ag-C bonds on Ag(111). An atomic model was proposed for the observed hybrid chain structures. Four electronic states were resolved using STS measurements, and strong energy dependence was observed in STM images. These results were explained using first-principles calculations based on DFT.

Band gap control of small bundles of carbon nanotubes using applied electric fields: A density functional theory study

Electrostatic screening between carbon nanotubes (CNTs) in a small CNT bundle leads to a switching behavior induced by electric field perpendicular to the bundle axis. Using a first-principles method, we investigate the electronic structures of bundles consisting of two or three CNTs and the effects of the electric field applied perpendicular to the bundle axis. The applied field causes band gap closure in semiconducting bundles, while a gap opening occurs in metallic ones, which enables considerable modulation of bundle conductivity. The modulation effect originates from symmetry breaking due to electrostatic screening between the adjacent tube walls.

“Textured” Network Devices: Overcoming Fundamental Limitations of Nanotube/Nanowire Network-Based Devices†

Single-walled carbon nanotubes (swCNTs) and nanowires are strong candidate materials for next-generation devices such as high-mobility field-effect transistors (FETs), ultrasensitive sensors, and so on. One approach for practical device applications can be thin-film devices based on nanotube/ nanowire networks. However, such network-based devices have been suffering from various fundamental limitations. For example, transistors based on swCNTnetworks usually have a poor on–off ratio due to metallic swCNTs in the network channels. Furthermore, nanotube/nanowire network-based devices in general exhibit low mobility and conductivity with nanoscale channel width due to the poor scaling behavior of percolated network channels. Herein,

Electrical transport in small bundles of single-walled carbon nanotubes: Intertube interaction and effects of tube deformation

We report a combined electronic transport and structural characterization study of small carbon nanotube bundles in field-effect transistors (FETs). The atomic structures of the bundles are determined by electron diffraction using an observation window built in the FET. The electrical transport of single-walled nanotube bundles depends on the structure of individual tubes, deformation due to intertube interaction, and the orientation with respect to the electric field. Ab initio simulations show that tube deformation in the bundle induces a band gap opening in a metallic tube. These results show the importance of intertube interaction in electrical transport of bundled carbon nanotubes.

The potential of carbon-based memory systems

It seems likely that density concerns will force the DRAM community to consider using radically different schemes for the implementation of memory devices. We propose using nano-scale carbon structures as the basis for a memory device. A single-wall carbon nanotube would contain a charged buckyball. That buckyball will stick tightly to one end of the tube or the other. We assign the bit value of the device depending on which side of the tube the ball is. The result is a high-speed, non-volatile bit of memory. We propose a number of schemes for the interconnection of these devices and examine some of the known electrical issues.

Structure Controlled Synthesis of Vertically Aligned Carbon Nanotubes Using Thermal Chemical Vapor Deposition Process

Due to their unique one-dimensional nanostructure along with excellent mechanical, electrical, and optical properties, carbon nanotubes (CNTs) become a promising material for diverse nanotechnology applications. However, large-scale and structure controlled synthesis of CNTs still have many difficulties due to the lack of understanding of the fundamental growth mechanism of CNTs, as well as the difficulty of controlling atomic-scale physical and chemical reactions during the nanotube growth process. Especially, controlling the number of graphene wall, diameter, and chirality of CNTs are the most important issues that need to be solved to harness the full potential of CNTs. Here we report the large-scale selective synthesis of vertically aligned single walled carbon nanotubes (SWNTs) and double walled carbon nanotubes (DWNTs) by controlling the size of catalyst nanoparticles in the highly effective oxygen assisted thermal chemical vapor deposition (CVD) process. We also demonstrate a simple but powerful strategy for synthesizing ultrahigh density and diameter selected vertically aligned SWNTs through the precise control of carbon flow during a thermal CVD process.