Koungmin Ryu

Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics

We report the implementation of continuous, highly flexible, and transparent graphene films obtained by chemical vapor deposition (CVD) as transparent conductive electrodes (TCE) in organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to transparent substrates, and evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous nature of the CVD graphene films, which led to minimal surface roughness ( approximately 0.9 nm) and offered sheet resistance down to 230 Omega/sq (at 72% transparency), much lower than stacked graphene flakes at similar transparency. In addition, solar cells with CVD graphene and indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene terephthalate (PET) substrates and were confirmed to offer comparable performance, with power conversion efficiencies (eta) of 1.18 and 1.27%, respectively. Furthermore, CVD graphene solar cells demonstrated outstanding capability to operate under bending conditions up to 138 degrees , whereas the ITO-based devices displayed cracks and irreversible failure under bending of 60 degrees . Our work indicates the great potential of CVD graphene films for flexible photovoltaic applications.

Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications

Preseparated, semiconductive enriched carbon nanotubes hold great potential for thin-film transistors and display applications due to their high mobility, high percentage of semiconductive nanotubes, and room-temperature processing compatibility. Here in this paper, we report our progress on wafer-scale processing of separated nanotube thin-film transistors (SN-TFTs) for display applications, including key technology components such as wafer-scale assembly of high-density, uniform separated nanotube networks, high-yield fabrication of devices with superior performance, and demonstration of organic light-emitting diode (OLED) switching controlled by a SN-TFT. On the basis of separated nanotubes with 95% semiconductive nanotubes, we have achieved solution-based assembly of separated nanotube thin films on complete 3 in. Si/SiO(2) wafers, and further carried out wafer-scale fabrication to produce transistors with high yield (>98%), small sheet resistance ( approximately 25 kOmega/sq), high current density ( approximately 10 microA/microm), and superior mobility ( approximately 52 cm(2) V(-1) s(-1)). Moreover, on/off ratios of >10(4) are achieved in devices with channel length L > 20 microm. In addition, OLED control circuit has been demonstrated with the SN-TFT, and the modulation in the output light intensity exceeds 10(4). Our approach can be easily scaled to large areas and could serve as critical foundation for future nanotube-based display electronics.

TRANSPARENT ELECTRONICS BASED ON TRANSFER PRINTED CARBON NANOTUBES ON RIGID AND FLEXIBLE SUBSTRATES

We report high-performance fully transparent thin-film transistors (TTFTs) on both rigid and flexible substrates with transfer printed aligned nanotubes as the active channel and indium-tin oxide as the source, drain, and gate electrodes. Such transistors have been fabricated through low-temperature processing, which allowed device fabrication even on flexible substrates. Transparent transistors with high effective mobilities (approximately 1300 cm(2) V(-1) s(-1)) were first demonstrated on glass substrates via engineering of the source and drain contacts, and high on/off ratio (3 x 10(4)) was achieved using electrical breakdown. In addition, flexible TTFTs with good transparency were also fabricated and successfully operated under bending up to 120 degrees . All of the devices showed good transparency (approximately 80% on average). The transparent transistors were further utilized to construct a fully transparent and flexible logic inverter on a plastic substrate and also used to control commercial GaN light-emitting diodes (LEDs) with light intensity modulation of 10(3). Our results suggest that aligned nanotubes have great potential to work as building blocks for future transparent electronics.

Devices and chemical sensing applications of metal oxide nanowires

Metal oxide nanowires, with special physical properties, are ideal building blocks for a wide range of nanoscale electronics, optoelectronics, and chemical sensing devices. This article will describe the state-of-the-art research activities in metal oxide nanowire applications. This paper consists of three main sections categorized by metal oxide nanowire synthesis, electronic and optoelectronic devices applications, and chemical sensing applications. Finally, we will conclude this review with some perspectives and outlook on the future developments in the metal oxide nanowire research area.

CMOS-Analogous Wafer-Scale Nanotube-on-Insulator Approach for Submicrometer Devices and Integrated Circuits Using Aligned Nanotubes

Massive aligned carbon nanotubes hold great potential but also face significant integration/assembly challenges for future beyond-silicon nanoelectronics. We report a wafer-scale processing of aligned nanotube devices and integrated circuits, including progress on essential technological components such as wafer-scale synthesis of aligned nanotubes, wafer-scale transfer of nanotubes to silicon wafers, metallic nanotube removal and chemical doping, and defect-tolerant integrated nanotube circuits. We have achieved synthesis of massive aligned nanotubes on complete 4 in. quartz and sapphire substrates, which were then transferred to 4 in. Si/SiO(2) wafers. CMOS analogous fabrication was performed to yield transistors and circuits with features down to 0.5 mum, with high current density approximately 20 muA/mum and good on/off ratios. In addition, chemical doping has been used to build fully integrated complementary inverter with a gain approximately 5, and a defect-tolerant design has been employed for NAND and NOR gates. This full-wafer approach could serve as a critical foundation for future integrated nanotube circuits.

Carbon Nanotube Transistor Circuits: Circuit-Level Performance Benchmarking and Design Options for Living with Imperfections

1D carbon nanotube FET (CNFET)-based circuits offer 4.6times faster FO4 speed and 12times energy-delay product improvement over 32nm node Si CMOS (including diameter and doping variations), provided circuits can be built that are immune to misaligned and metallic nanotubes. A design technique that guarantees correct logic operation in the presence of misaligned nanotubes is also presented.

2,4,6-Trinitrotoluene (TNT) Chemical Sensing Based on Aligned Single-Walled Carbon Nanotubes and ZnO Nanowires

2010 WILEY-VCH Verlag Gmb Chemical sensors based on one-dimensional (1D) nanostructures have attracted a great deal of attention because of their exquisite sensitivity and fast response to the surrounding environment. In addition, both carbon nanotubes and metal oxide nanowires are promising candidates for building an electronic nose (e-nose) system. Among these materials, semiconductor single-walled carbon nanotubes (SWNTs) are molecular-scale wires composed entirely of surface atoms, which should be ideal for the direct electrical detection and are expected to exhibit excellent sensitivity to surrounding chemical and biological species. Kong et al. initially utilized SWNT field-effect transistors (FETs) to detect nitrogen dioxide (NO2) and ammonia (NH3), and demonstrated a detection limit of 2 ppm for NO2 and 0.1% for NH3. [11] Subsequently, such SWNT-based chemical sensors have been applied to detect a wide variety of chemicals and the detection limits have been significantly improved. Qi et al. fabricated large arrays of functionalized SWNTsensors, and the detection limit of NO2 was lowered to 100 ppt. [12] In addition, metal oxide nanowires have been widely studied and demonstrated with great potential for chemical sensing applications. Recently, due to the threat of terrorism and the need for homeland security, significant progress has been achieved in the detection of both explosives and nerve agents, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), hexogen (DRX), and dimethyl methylphosphonate (DMMP). One of the leading candidates is 1D nanostructure-based chemoresistors or FETs. Snow et al. and Wang et al. have reported the detection of DMMP at ppb level by using SWNT and SnO2 nanowire-based chemical sensors, respectively. However, to our knowledge, there were only a few reports on the use of 1D nanostructure-based chemoresistors and FETs for detecting explosives, and the detection mechanism is still unclear. In addition, electronic devices fabricated on mechanically flexible substrates have recently attracted enormous attention, due to the proliferation of handheld applications in portable electronics, aerospace science, and civil engineering. Currently, conventional microfabrication techniques or printing methods can be applied to SWNTs on plastic substrates to form devices, allowing inexpensive mass-production and conformable electronics. In this paper, we report the transfer of aligned semiconductor SWNTs onto cloth fabric and successful fabrication of flexible SWNTchemical sensors, which have great potential for wearable electronics. These SWNT chemical sensors exhibited good sensitivity of trace chemical vapors, including 8 ppb TNT and 40 ppb NO2, at room temperature. Besides, to realize the concept of an electronic nose (e-nose) system for explosives, we also fabricated ZnO nanowire-based chemical sensors, which showed a detection limit of 60 ppb for TNT molecules at room temperature. To our knowledge, this is the first TNT sensor built on the basis of metal oxide nanowires. In addition, the detection limit of our chemical sensors is close to the limit of 1.5 ppb TNT set by the U.S. Occupational Safety and Health Administration. The flexible TNT sensors can find immediate applications in systems that demand mechanical flexibility, light weight, and high sensitivity. The fabrication of flexible SWNTchemical sensors started with the synthesis of SWNTs on quartz substrates using a chemical vapor deposition (CVD) method, which have been reported by us and other groups. After growth, we adapted a facile method to transfer the aligned nanotubes from the original substrate to fabric. In brief, a 100-nm-thick gold film was first deposited on the original substrate with aligned SWNTs, followed by applying a thermal tape to peel off the gold film and nanotubes from the growth substrate. The gold film with SWNTs on the thermal tape were pressed against a piece of textile fabric, which was pre-coated with polyethylene at elevated temperature and then transferred from thermal tape onto textile fabric, which had a 50-nm Ti layer as back-gate electrode and 2-mm-thick SU-8 as gate dielectric layer. The thermal tape was released, and KI/I2 gold etchant was then applied to remove gold films. Finally, Ti (0.5 nm) and Pd (40 nm) were deposited on the transferred SWNTs as source/drain electrodes. A schematic diagram of a flexible SWNT chemical sensor is shown in Figure 1a. Figure 1b shows an optical photograph of flexible aligned SWNT FETs on a textile fabric. It can be clearly seen from the SEM image (right) that the nanotubes bridge the two electrodes. Figure 1c displays the current–gate-voltage (I–Vg) characteristics of a typical flexible transistor on fabric before and after electrical breakdown. The device showed significant improvement for the

Metal Contact Engineering and Registration-Free Fabrication of Complementary Metal-Oxide Semiconductor Integrated Circuits Using Aligned Carbon Nanotubes

Complementary metal-oxide semiconductor (CMOS) operation is very desirable for logic circuit applications as it offers rail-to-rail swing, larger noise margin, and small static power consumption. However, it remains to be a challenging task for nanotube-based devices. Here in this paper, we report our progress on metal contact engineering for n-type nanotube transistors and CMOS integrated circuits using aligned carbon nanotubes. By using Pd as source/drain contacts for p-type transistors, small work function metal Gd as source/drain contacts for n-type transistors, and evaporated SiO(2) as a passivation layer, we have achieved n-type transistor, PN diode, and integrated CMOS inverter with an air-stable operation. Compared with other nanotube n-doping techniques, such as potassium doping, PEI doping, hydrazine doping, etc., using low work function metal contacts for n-type nanotube devices is not only air stable but also integrated circuit fabrication compatible. Moreover, our aligned nanotube platform for CMOS integrated circuits shows significant advantage over the previously reported individual nanotube platforms with respect to scalability and reproducibility and suggests a practical and realistic approach for nanotube-based CMOS integrated circuit applications.

Device study, chemical doping, and logic circuits based on transferred aligned single-walled carbon nanotubes

In this paper, high-performance back-gated carbon nanotube field-effect transistors based on transferred aligned carbon nanotubes were fabricated and studies found that the on/off ratio can reach 107 and the current density can reach 1.6μA∕μm after electrical breakdown. In addition, chemical doping with hydrazine was used to convert the p-type aligned nanotube devices into n-type. These devices were further utilized to demonstrate various logic circuits, including p-type metal-oxide-semiconductor inverters, diode-loaded inverters, complementary metal-oxide-semiconductor inverters, NAND, and NOR gates. This approach could work as the platform for future nanotube-based nanoelectronics.

Scalable Light-Induced Metal to Semiconductor Conversion of Carbon Nanotubes

Coexistence of metallic and semiconducting carbon nanotubes in as-grown samples sets important limits to their application in high-performance electronics. We present the metal-to-semiconductor conversion of carbon nanotubes for field-effect transistors based on both aligned nanotubes and individual nanotube devices. The conversion process is induced by light irradiation, scalable to wafer-size scales and capable of yielding improvements in the channel-current on/off ratio up to 5 orders of magnitude in nanotube-based field-effect transistors. Inactivation of metallic nanotubes in the channels was achieved as a consequence of a diameter-dependent photochemical process that led to a controlled oxidation of the nanotube sidewall and, hence, stronger localization of pi-electrons. Optimization of irradiation conditions yields nearly 90% of depletable nanotube field-effect transistors.