Il Ha Lee

Reduction-Controlled Viologen in Bisolvent as an Environmentally Stable n-Type Dopant for Carbon Nanotubes

Various viologens have been used to control the doping of single-walled carbon nanotubes (SWCNTs) via direct redox reactions. A new method of extracting neutral viologen (V(0)) was introduced using a biphase of toluene and viologen-dissolved water. A reductant of sodium borohydride transferred positively charged viologen (V(2+)) into V(0), where the reduced V(0) was separated into toluene with high separation yield. This separated V(0) solution was dropped on carbon nanotube transistors to investigate the doping effect of CNTs. With a viologen concentration of 3 mM, all the p-type CNT transistors were converted to n-type with improved on/off ratios. This was achieved by donating electrons spontaneously to CNTs from neutral V(0), leaving energetically stable V(2+) on the nanotube surface again. The doped CNTs were stable in water due to the presence of hydrophobic V(0) at the outermost CNT transistors, which may act as a protecting layer to prevent further oxidation from water.

Adaptive Logic Circuits with Doping-Free Ambipolar Carbon Nanotube Transistors

A CMOS-like inverter was integrated by using ambipolar carbon nanotube (CNT) transistors without doping. The ambipolar CNT transistors automatically configure themselves to play a role as an n-type or p-type transistor in a logic circuit depending on the supply voltage (V(DD)) and ground. A NOR (NAND) gate is adaptively converted to a NAND (NOR) gate. This adaptiveness of logic gates exhibiting two logic gate functions in a single logic circuit offers a new opportunity for designing logic circuits with high integration density for next generation applications.

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnels equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.

Majority Carrier Type Conversion with Floating Gates in Carbon Nanotube Transistors

A charge trapping layer can serve not only for designing multilevel nonvolatile memory but also for reversible type conversion from p- to n-type in carbon nanotube channels. Young Hee Lee and co-workers demonstrate here that reversible conversion from p- to n-type can be robustly realized in CNT fi eld-effect transistors by changing the polarity of trapped charges.

FIELD EMISSION AND APPLICATION OF CARBON NANOTUBES

The mechanism of field emission from a metal surface was well explained based on the quantum mechanics in early 20th century. Since then, various materials have been studied for field emitters. However, so far, we have been using only limited materials as a field emitter and an application in some area requires further scientific understandings and technological advancements. In this paper, we review the current status of researches in field emission and emission phenomena of carbon nanotubes (CNTs). This may include current saturation induced by gas adsorbates, screening effects, high current emission, degradation of emitter, and field enhancement factor. We also introduce the present status in the development of various CNT-based field emission devices and discuss their performances. In this part, various potential applications such as field emission display, ionization gauge, X-ray gun, and lamp will be presented.

Ferroelectric SrBi4Ti4O15 thin films with high polarization grown on an IrO2 layer

Ferroelectric strontium bismuth titanate (SrBi4Ti4O15) thin films with a high remanent polarization were produced by a chemical solution deposition method. Pt and IrO2 layers were used as substrates. It was found that ferroelectric SrBi4Ti4O15 films can be successfully fabricated on IrO2: They demonstrate a saturated hysteresis loop at 5 V with remanent polarization (Pr) of 19 μC/cm2 and coercive field (Ps) of 116 kV/cm. SrBi4Ti4O15 films grown on IrO2 show larger and denser grains and controlled surface morphology. The grains are random oriented, while those of films on Pt are mainly c-axis oriented. It is concluded that the high remanent polarization of the films grown on IrO2 originates from the relatively high concentration of a- and b-axis orientations.

HUMIDITY-ASSISTED SELECTIVE REACTIVITY BETWEEN NO2 AND SO2 GAS ON CARBON NANOTUBES

In spite of the technical importance of detecting environmental SOx and NOx gases, a selective detection has not been realized because of their similar chemical properties. In this report, adsorption and desorption of SO2 and NO2 gas on carbon nanotubes are investigated in terms of different humidity levels at room temperature. A random-network single walled carbon nanotube (SWCNT) resistor is constructed by a dip-pen method using a SWCNT/dichloroethane (DCE) solution. In the case of SO2 gas adsorption, the resistance increases at high humidity level (92%) and shows no obvious change at low humidity levels. On the other hand, in the case of NO2 gas adsorption, the resistance always decreases independent of moisture levels. Our density functional theory (DFT) calculations show that this selective behavior originates from cooperative charge compensation between the SO2–nH2O complex and the p-type CNT resistor. The change of response time and recovery time with different moisture levels is further investigated. This humidity-assisted gas reaction provides a simple route to detect these two gases selectively.

Carbon Nanotube Doping Mechanism in a Salt Solution and Hygroscopic Effect: Density Functional Theory

The mechanism of doping carbon nanotubes (CNTs) with a salt solution was investigated using the density functional theory. We propose that the anion-CNT complex is a key component in doping CNTs. Although the cations play an important role in ionizing CNTs as an intermediate precursor, the ionized CNTs are neutralized further by forming a stable anion-CNT complex as a final reactant. The anion-CNT bond has a strong ionic bonding character and clearly shows p-type behavior by shifting the Fermi level toward the valence band. The midgap state is introduced by the strong binding of carbon and anion atoms. These localized charged anion sites are highly hygroscopic and induce the adsorption of water molecules. This behavior provides a new possibility for using anion-functionalized CNTs as humidity sensors.

The rapid growth of vertically aligned carbon nanotubes using laser heating

Growth of densely packed vertically aligned carbon nanotubes (VA-CNTs) using laser-induced chemical vapor deposition with visible laser (lambda = 532 nm) irradiation at room temperature is reported. Using a multiple-catalyst layer (Fe/Al/Cr) on quartz as the substrate and an acetylene-hydrogen mixture as the precursor gas, VA-CNT pillars with 60 microm height and 4 microm diameter were grown at a high rate of around 1 microm s(-1) with good reproducibility. It is demonstrated that the fabrication of uniform pillar arrays of VA-CNTs can be achieved with a single irradiation for each pillar using LCVD with no annealing or preprocessing of the substrate. Here, laser fast heating is considered the primary mechanism facilitating the growth of VA-CNT pillars. Field emission characteristics of an array of VA-CNT pillars were then examined to investigate their potential application in vacuum electronic devices.

Direct printing of aligned carbon nanotube patterns for high-performance thin film devices

The aligned assembly of carbon nanotubes (CNTs) on substrate presents a significant bottleneck in the fabrication of high-performance thin film devices. Here, we report a direct printing method to prepare laterally aligned thick CNT patterns over large surface regions. In this method, CNT forests were grown selectively on specific regions of one substrate, and the forest patterns were transferred on another SiO2 substrate in a laterally aligned formation while keeping their original shapes. The degree of alignment was characterized via electrical measurement and polarized Raman spectroscopy. Furthermore, we demonstrated high-performance field-effect transistors and gas sensors using our method.