Jinsup Lee

Enhanced mechanical properties of graphene/copper nanocomposites using a molecular-level mixing process.

RGO flakes are homogeneously dispersed in a Cu matrix through a molecular-level mixing process. This novel fabrication process prevents the agglomeration of the RGO and enhances adhesion between the RGO and the Cu. The yield strength of the 2.5 vol% RGO/Cu nanocomposite is 1.8 times higher than that of pure Cu. The strengthening mechanism of the RGO is investigated by a double cantilever beam test using the graphene/Cu model structure.

Strengthening effect of single-atomic-layer graphene in metal–graphene nanolayered composites

Graphene is a single-atomic-layer material with excellent mechanical properties and has the potential to enhance the strength of composites. Its two-dimensional geometry, high intrinsic strength and modulus can effectively constrain dislocation motion, resulting in the significant strengthening of metals. Here we demonstrate a new material design in the form of a nanolayered composite consisting of alternating layers of metal (copper or nickel) and monolayer graphene that has ultra-high strengths of 1.5 and 4.0 GPa for copper-graphene with 70-nm repeat layer spacing and nickel-graphene with 100-nm repeat layer spacing, respectively. The ultra-high strengths of these metal-graphene nanolayered structures indicate the effectiveness of graphene in blocking dislocation propagation across the metal-graphene interface. Ex situ and in situ transmission electron microscopy compression tests and molecular dynamics simulations confirm a build-up of dislocations at the graphene interface.

Uniform Graphene Quantum Dots Patterned from Self-Assembled Silica Nanodots

Graphene dots precisely controlled in size are interesting in nanoelectronics due to their quantum optical and electrical properties. However, most graphene quantum dot (GQD) research so far has been performed based on flake-type graphene reduced from graphene oxides. Consequently, it is extremely difficult to isolate the size effect of GQDs from the measured optical properties. Here, we report the size-controlled fabrication of uniform GQDs using self-assembled block copolymer (BCP) as an etch mask on graphene films grown by chemical vapor deposition (CVD). Electron microscope images show that as-prepared GQDs are composed of mono- or bilayer graphene with diameters of 10 and 20 nm, corresponding to the size of BCP nanospheres. In the measured photoluminescence (PL) spectra, the emission peak of the GQDs on the SiO(2) substrate is shown to be at ∼395 nm. The fabrication of GQDs was supported by the analysis of the Raman spectra and the observation of PL spectra after each fabrication step. Additionally, oxygen content in the GQDs is rationally controlled by additional air plasma treatment, which reveals the effect of oxygen content to the PL property.

Simple Preparation of High-Quality Graphene Flakes without Oxidation Using Potassium Salts

Graphene is a 2D sheet of sp 2 -hybridized carbon with interesting properties, including exceptionally high thermal/electrical conductivity, surface area, and mechanical strength. [ 1–5 ] Many fabrication methods have been proposed to obtain graphene so as to utilize these interesting properties. The approaches to fabricate graphene can be roughly categorized into two classes: i) top-down exfoliation of multilayer graphene or graphite by breaking pi-bonding between carbon atoms, [ 6–10 ] and ii) bottom-up formation of sp 2 -bonding between carbon atoms in a monolayer. [ 11–16 ] Mechanical and chemical exfoliation methods fall into the fi rst category, while chemical vapor deposition (CVD) and epitaxial growth from silicon carbide (SiC) belong to the second. Flake-types of graphene promise potential applications in the fi elds of transparent electrodes, energy storage, electromagnetic (EM) shields, etc. [ 17–19 ] Dispersed graphene fl akes are mostly produced by a chemical exfoliation method, through chemical oxidation and reduction, known as Hummers’ method. [ 20 ] Hummers’ method is a lowcost process applicable to mass production. However, the quality of the achieved graphene is often below desired levels, mainly due to the presence of residual oxygen, even after a suffi cient reduction process. [ 21 ] Here, we fi rstly introduce a new method to acquire high-quality graphene fl akes by simply using metal salts without oxidation. Graphite intercalation compounds (GICs) are typically formed by the insertion of atomic or molecular species, called intercalants, between layers in a graphite host. The formation of GICs has been an active fi eld of research especially in relation to lithium ion batteries because graphite can store a large amount

Self-Assembly-Induced Formation of High-Density Silicon Oxide Memristor Nanostructures on Graphene and Metal Electrodes

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.

Intrinsic Photoluminescence Emission from Subdomained Graphene Quantum Dots

The photoluminescence (PL) origin of bright blue emission arising from intrinsic states in graphene quantum dots (GQDs) is investigated. The bright PL of intercalatively acquired GQDs is attributed to favorably formed subdomains composed of four to seven carbon hexagons. Random and harsh oxidation which hinders the energetically favorable formation of subdomains causes weak and redshifted PL.

The effects of the crystalline orientation of Cu domains on the formation of nanoripple arrays in CVD-grown graphene on Cu

Defect structures such as boundaries, ripples and wrinkles in graphene have been considered as main causes reducing the electrical properties of graphene. Among them, the formation of a periodic nanoripple array and surface roughening intrinsically occurs as graphene grows on the surface of a metal catalyst during chemical vapor deposition, which results in anisotropic charge transport and limits the possible sheet resistance. In this study, we observed that among the various growth factors, the crystalline orientation of Cu domains can play an important role in the occurrence of periodic surface roughening. With the exception of Cu (111) domain, the surfaces of Cu domains are considerably rippled to a particular direction with abundant terrace structure and step edges. Such ripples occur to relax the strain from a large lattice mismatch between graphene and Cu lattice at a high temperature during the CVD process, which remain as rippled regions of graphene after wet transfer. However, a relatively flat surface is observed in the graphene transferred from hexagonal Cu (111) domain. Additional conductivity mapping also reveals that graphene from Cu (111) domain shows highly homogeneous current distribution. On the other hand, degraded conductivity on rippled regions introducing anisotropic transport of current is observed in the graphene from Cu domains except Cu (111) domain. We believe that current observation can contribute to the preparation of graphene with flat structure simply by controlling the crystalline orientation of Cu.

Superplasticity in PM 6061 Al alloy and elimination of strengthening effect by reinforcement in superplastic PM aluminum composites

Plastic-flow behavior of a powder-metallurgy (PM) processed 6061 matrix alloy has been investigated in a wide range of elevated temperature between 430 and 620°C. It was found that the 6061 Al alloy exhibits superplasticity in a relatively wide range of temperature from 520 to 620°C at a high strain rate of 10 2 s 1 . Deformation behavior of the present alloy could be divided into three regions when the presence of threshold stress for plastic flow was assumed. They are DL controlled grain boundary sliding, DL controlled dislocation climb creep and powder-law breakdown, respectively. When temperature is as high as 590°C, however, the activation energy increases significantly higher than that for self-diffusion in aluminum and flow stress decreases further than normally expected. This phenomenon is likely attributed to the presence of liquid phase above 590°C. Comparison of the data in Region I below 610°C with those for a number of superplastic aluminum composites indicates that strengthening effect by reinforcement does not exist. Several speculations including diffusional relaxation in vicinity of reinforcements were made to explain this phenomenon.

High-angle tilt boundary graphene domain recrystallized from mobile hot-wire-assisted chemical vapor deposition system.

Crystallization of materials has attracted research interest for a long time, and its mechanisms in three-dimensional materials have been well studied. However, crystallization of two-dimensional (2D) materials is yet to be challenged. Clarifying the dynamics underlying growth of 2D materials will provide the insight for the potential route to synthesize large and highly crystallized 2D domains with low defects. Here, we present the growth dynamics and recrystallization of 2D material graphene under a mobile hot-wire assisted chemical vapor deposition (MHW-CVD) system. Under local but sequential heating by MHW-CVD system, the initial nucleation of nanocrystalline graphenes, which was not extended into the growth stage due to the insufficient thermal energy, took a recrystallization and converted into a grand single crystal domain. During this process, the stitching-like healing of graphene was also observed. The local but sequential endowing thermal energy to nanocrystalline graphenes enabled us to simultaneously reveal the recrystallization and healing dynamics in graphene growth, which suggests an alternative route to synthesize a highly crystalline and large domain size graphene. Also, this recrystallization and healing of 2D nanocrystalline graphenes offers an interesting insight on the growth mechanism of 2D materials.

Tuning the electrode work function via a vapor-phase deposited ultrathin polymer film

In organic electronic devices, the capability of controlling the work function (WF) of electrodes is crucial in order to facilitate charge injection/collection. In this work, we introduce a new method to improve the electron injection characteristics of electrodes by depositing an ultrathin, electron-donating polymer layer, poly(dimethylaminomethyl styrene) (PDMAMS), via initiated chemical vapor deposition (iCVD). The deposition process enabled the ultrathin PDMAMS layer to be coated on top of the electrode surfaces with a wide range of hydrophilicity, without requiring any surface pretreatment. WF reduction was observed in various electrode materials including Au, Cu, indium tin oxide (ITO), and graphene surfaces, due to the strong electron donating property of the polymeric layer. The newly integrated WF-controlled Au and Cu electrodes were applied to organic thin film transistors (OTFTs), and a substantial improvement in electron mobility (from 4.75 × 10−6 cm2 V−1 s−1 to 0.89 cm2 V−1 s−1) was observed, due to the facilitated electron injection from the modified air-stable electrodes. The ultrathin layer was also coated onto the ITO cathode of inverted organic solar cells (IOSCs), resulting in around 3.5 fold improvement in the device efficiency. Furthermore, an effective n-type doping of graphene was successfully demonstrated where the Dirac point of graphene systematically shifted after the film deposition, which enabled the production of a graphene inverter with distinct on/off configurations. Since the iCVD can be used to form ultrathin films free of dewetting issues, deposition of ultrathin PDMAMS via iCVD can be a new powerful method to control the interface of various electrodes and the active channel for organic electronics.