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.

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.

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.

Analysis of contact resistance in single-walled carbon nanotube channel and graphene electrodes in a thin film transistor.

In this work, we present the experimental investigation on the contact resistance of graphene/single-walled carbon nanotube (SWCNT) junction using transfer length method with the simple equivalent circuit model. We find that p-n like junctions are formed in graphene/SWCNT transistors, and the contact resistance in the junction is observed to be ~ 494 and ~ 617 kΩ in case of metallic SWCNT (m-SWCNT) and semiconducting SWCNT (s-SWCNT), respectively. In addition, the contact resistance increases from 617 to 2316 kΩ as Vg increases from - 30 to - 10 V. Through our study, high carrier density induced from doping in both graphene and SWCNT leads to low contact resistance. This development of contact engineering, namely modulation of carrier density in the junction and contact length (Lcon) scaling shows the potential for all-carbon based electronics.

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.

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.