Seoung-Ki Lee

High-Performance Flexible Graphene Field Effect Transistors with Ion Gel Gate Dielectrics

A high-performance low-voltage graphene field-effect transistor (FET) array was fabricated on a flexible polymer substrate using solution-processable, high-capacitance ion gel gate dielectrics. The high capacitance of the ion gel, which originated from the formation of an electric double layer under the application of a gate voltage, yielded a high on-current and low voltage operation below 3 V. The graphene FETs fabricated on the plastic substrates showed a hole and electron mobility of 203 +/- 57 and 91 +/- 50 cm(2)/(V x s), respectively, at a drain bias of -1 V. Moreover, ion gel gated graphene FETs on the plastic substrates exhibited remarkably good mechanical flexibility. This method represents a significant step in the application of graphene to flexible and stretchable electronics.

Chemical Vapor Deposition-Grown Graphene: The Thinnest Solid Lubricant

As an atomically thin material with low surface energy, graphene is an excellent candidate for reducing adhesion and friction when coated on various surfaces. Here, we demonstrate the superior adhesion and frictional characteristics of graphene films which were grown on Cu and Ni metal catalysts by chemical vapor deposition and transferred onto the SiO(2)/Si substrate. The graphene films effectively reduced the adhesion and friction forces, and multilayer graphene films that were a few nanometers thick had low coefficients of friction comparable to that of bulk graphite.

High-performance graphene-based transparent flexible heaters.

We demonstrate high-performance, flexible, transparent heaters based on large-scale graphene films synthesized by chemical vapor deposition on Cu foils. After multiple transfers and chemical doping processes, the graphene films show sheet resistance as low as ∼43 Ohm/sq with ∼89% optical transmittance, which are ideal as low-voltage transparent heaters. Time-dependent temperature profiles and heat distribution analyses show that the performance of graphene-based heaters is superior to that of conventional transparent heaters based on indium tin oxide. In addition, we confirmed that mechanical strain as high as ∼4% did not substantially affect heater performance. Therefore, graphene-based, flexible, transparent heaters are expected to find uses in a broad range of applications, including automobile defogging/deicing systems and heatable smart windows.

All Graphene-Based Thin Film Transistors on Flexible Plastic Substrates

High-performance, flexible all graphene-based thin film transistor (TFT) was fabricated on plastic substrates using a graphene active layer, graphene oxide (GO) dielectrics, and graphene electrodes. The GO dielectrics exhibit a dielectric constant (3.1 at 77 K), low leakage current (17 mA/cm(2)), breakdown bias (1.5 × 10(6) V/cm), and good mechanical flexibility. Graphene-based TFTs showed a hole and electron mobility of 300 and 250 cm(2)/(V·s), respectively, at a drain bias of -0.1 V. Moreover, graphene TFTs on the plastic substrates exhibited remarkably good mechanical flexibility and optical transmittance. This method explores a significant step for the application of graphene toward flexible and stretchable electronics.

Coplanar-Gate Transparent Graphene Transistors and Inverters on Plastic

Transparent flexible graphene transistors and inverters in a coplanar-gate configuration were presented for the first time using only two materials: graphene and an ion gel gate dielectric. The novel device configuration simplifies device fabrication such that only two printing steps were required to fabricate transistors and inverters. The devices exhibited excellent device performances including low-voltage operation with a high transistor-on-current and mobility, excellent mechanical flexibility, environmental stability, and a reasonable inverting behavior upon connecting the two transistors.

Graphene Films for Flexible Organic and Energy Storage Devices.

Graphene and its derivatives have been the subject of extensive research in fundamental science and have viable applications in current and future technology. The exceptionally high electronic and thermal conductivity, optical transparency, and high specific surface area, combined with excellent mechanical flexibility and environmental stability leave graphene poised to be a material of the future. This perspective introduces the importance of graphene electrodes, discusses the synthesis of graphene and transfer onto desired substrates and the role of graphene in electrodes for a broad range of flexible devices such as photovoltaic, electronic, and electrochemical energy storage.

Mechanical and environmental stability of polymer thin-film-coated graphene.

A uniform polymer thin layer of controllable thickness was bar-coated onto a chemical vapor deposition (CVD) grown monolayer graphene surface. The effects of this coating layer on the optical, electric, and tribological properties were then investigated. The thin polymer coating layer did not reduce the optical transmittance of the graphene films. The variation in the sheet resistance of the graphene films after the coating depended on the interaction between polymer and graphene. The top coating layer can maintain the high conductivity of chemical doped graphene films under long-term ambient conditions compared with uncovered doped samples. Friction tests demonstrated that the polymer coating layer can enhance both the friction force and the coefficient of friction of the graphene films and protect the graphene against damage in the repeated sliding processes.

Preparation of Three-Dimensional Graphene Foams Using Powder Metallurgy Templates

A simple and scalable method which combines traditional powder metallurgy and chemical vapor deposition is developed for the synthesis of mesoporous free-standing 3D graphene foams. The powder metallurgy templates for 3D graphene foams (PMT-GFs) consist of particle-like carbon shells which are connected by multilayered graphene that shows high specific surface area (1080 m(2) g(-1)), good crystallization, good electrical conductivity (13.8 S cm(-1)), and a mechanically robust structure. The PMT-GFs did not break under direct flushing with DI water, and they were able to recover after being compressed. These properties indicate promising applications of PMT-GFs for fields requiring 3D carbon frameworks such as in energy-based electrodes and mechanical dampening.

Double-layer CVD graphene as stretchable transparent electrodes.

The stretchability of CVD graphene with a large area is much lower than that of mechanically exfoliated pristine graphene owing to the intrinsic and extrinsic defects induced during its synthesis, etch-out of the catalytic metal, and the transfer processes. This low stretchability is the main obstacle for commercial application of CVD graphene in the field of flexible and stretchable electronics. In this study, artificially layered CVD graphene is suggested as a promising candidate for a stretchable transparent electrode. In contrast to single-layer graphene (SLG), multi-layer graphene has excellent electromechanical stretchability owing to the strain relaxation facilitated by sliding among the graphene layers. Macroscopic and microscopic electromechanical tensile tests were performed to understand the key mechanism for the improved stretchability, and crack generation and evolution were systematically investigated for their dependence on the number of CVD graphene layers during tensile deformation using lateral force microscopy. The stretchability of double-layer graphene (DLG) is much larger than that of SLG and is similar to that of triple-layer graphene (TLG). Considering the transmittance and the cost of transfer, DLG can be regarded as a suitable candidate for stretchable transparent electrodes.

Photo-patternable ion gel-gated graphene transistors and inverters on plastic

We demonstrate photo-patternable ion gel-gated graphene transistors and inverters on plastic substrates. The photo-patternable ion gel can be used as a negative photoresist for the patterning of underlying graphene as well as gate dielectrics. As a result, an extra graphene-patterning step is not required, which simplifies the device fabrication and avoids a side effect arising from the photoresist residue. The high capacitance of ion gel gate dielectrics yielded a low voltage operation (~2 V) of the graphene transistor and inverter. The graphene transistors on plastic showed an on/off-current ratio of ~11.5, along with hole and electron mobilities of 852 ± 124 and 452 ± 98 cm(2) V(-1) s(-1), respectively. In addition, the flexible graphene inverter was successfully fabricated on plastic through the potential superposition effect from the drain bias. These devices show excellent mechanical flexibility and fatigue stability.