Milo Holt

Synthesis of High Quality Monolayer Graphene at Reduced Temperature on Hydrogen-Enriched Evaporated Copper (111) Films

We report new findings on the chemical vapor deposition (CVD) of monolayer graphene with negligible defects (≥95% negligible defect-peak over 200 μm × 200 μm areas) on evaporated copper films. Compared to copper foils used in the CVD of graphene, several new unexpected results have been observed including high-quality monolayer synthesis at temperatures <900 °C, a new growth window using a hydrogen-free methane precursor for low-defects, and electron microscope evidence of commensurate growth of graphene grains on underlying copper grains. These thermal, chemical, and physical growth characteristics of graphene on copper films can be attributed to the distinct differences in the dominant crystal orientation of copper films (111) versus foils (100), and consequent dissimilar interplay with the precursor gas. This study suggests that reduced temperature, hydrogen-free synthesis of defect-negligible monolayer graphene is feasible, with the potential to shape and scale graphene grains by controlling the size and crystal orientation of the underlying copper grains.

25 GHz Embedded-Gate Graphene Transistors with High-K Dielectrics on Extremely Flexible Plastic Sheets

Despite the widespread interest in graphene electronics over the past decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this article, we report detailed studies on the electrical and mechanical properties of vapor synthesized high-quality monolayer graphene integrated onto flexible polyimide substrates. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobilities of 3900 cm(2)/V·s, and importantly, 25 GHz cutoff frequency, which is more than a factor of 2.5 times higher than prior results. Mechanical studies reveal robust transistor performance under repeated bending, down to 0.7 mm bending radius, whose tensile strain is a factor of 2-5 times higher than in prior studies. In addition, integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene.

Enhanced sensitivity of graphene ammonia gas sensors using molecular doping

We report on employing molecular doping to enhance the sensitivity of graphene sensors synthesized via chemical vapor deposition to NH3 molecules at room temperature. We experimentally show that doping an as-fabricated graphene sensor with NO2 gas improves sensitivity of its electrical resistance to adsorption of NH3 molecules by about an order of magnitude. The detection limit of our NO2-doped graphene sensor is found to be ∼200 parts per billion (ppb), compared to ∼1400 ppb before doping. Electrical characterization and Raman spectroscopy measurements on graphene field-effect transistors show that adsorption of NO2 molecules significantly increases hole concentration in graphene, which results in the observed sensitivity enhancement.

INVERSION OF THE ELECTRICAL AND OPTICAL PROPERTIES OF PARTIALLY OXIDIZED HEXAGONAL BORON NITRIDE

By acoustically irradiating pristine, white, electrically insulating h-BN in aqueous environment we were able to invert its material properties. The resulting dark, electrically conductive h-BN (referred to as partially oxidized h-BN or PO-hBN) shows a significant decrease in optical transmission (>60%) and bandgap (from 5.46 eV to 3.97 eV). Besides employing a wide variety of analytical techniques (optical and electrical measurements, Raman spectroscopy, SEM imaging, EDS, X-Ray diffraction, XPS and TOF-SIMS) to study the material properties of pristine and irradiated h-BN, our investigation suggests the basic mechanism leading to the dramatic changes following the acoustic treatment. We find that the degree of inversion arises from the degree of h-BN surface or edge oxidation which heavily depends on the acoustic energy density provided to the pristine h-BN platelets during the solution-based process. This provides a facile avenue for the realization of materials with tuned physical and chemical properties that depart from the intrinsic behavior of pristine h-BN.

3D integrated monolayer graphene–Si CMOS RF gas sensor platform

Integration of a complementary metal-oxide semiconductor (CMOS) and monolayer graphene is a significant step toward realizing low-cost, low-power, heterogeneous nanoelectronic devices based on two-dimensional materials such as gas sensors capable of enabling future mobile sensor networks for the Internet of Things (IoT). But CMOS and post-CMOS process parameters such as temperature and material limits, and the low-power requirements of untethered sensors in general, pose considerable barriers to heterogeneous integration. We demonstrate the first monolithically integrated CMOS-monolayer graphene gas sensor, with a minimal number of post-CMOS processing steps, to realize a gas sensor platform that combines the superior gas sensitivity of monolayer graphene with the low power consumption and cost advantages of a silicon CMOS platform. Mature 0.18 µm CMOS technology provides the driving circuit for directly integrated graphene chemiresistive junctions in a radio frequency (RF) circuit platform. This work provides important advances in scalable and feasible RF gas sensors specifically, and toward monolithic heterogeneous graphene–CMOS integration generally.Chemical sensing: monolithic CMOS integration of chemi-resistive graphene junctionsThe combination of monolayer graphene with a silicon-based CMOS platform results in a monolithically integrated gas sensor. A team led by Deji Akinwande at the University of Texas at Austin developed a graphene-based gas sensing platform that leverages the remarkable gas sensitivity of graphene and the well-established advantages of silicon technology. Using a commercially available 0.18 μm CMOS radio-frequency (RF) circuit, the authors fabricated graphene chemi-resistive junctions atop the CMOS readout circuit, whereby the interaction between gas molecules and graphene can be read as an output frequency while minimizing the number of post-processing steps. The built-in RF functionalities enable a direct wireless connection with the transducer and offer reduced flicker noise, as opposed to direct-current sensors.

Design and Verification of a PECVD Fabricated Multi-Layer Nano-Scale Photovoltaic Device

This paper summarizes the results of computational and experimental studies of an enhanced thin film solar structure. The cell structure consists of a reflective aluminum layer beneath an 80nm absorbing layer of amorphous silicon, coated with a top layer of transparent and conductive indium tin oxide (ITO). The structure is mounted on a glass substrate. We first use constrained optimization techniques along with numerical solvers of the electromagnetic equations to specify the layer thicknesses of the design for maximized efficiency. Numerical analysis suggests that solar absorptivity in the thin film silicon can be enhanced by a factor of 2. The proposed design is then fabricated using Plasma Enhanced Chemical Vapor Deposition techniques, along with a control sample of bare silicon absorber for comparison. AFM imaging and spectrophotometry experiments are applied to estimate the realized thin film dimensions, deposition error, unwanted oxidation volume and the resulting reflectivity spectra. Comparisons of the measured and simulated reflectivity spectra of the fabricated cells, as well as Monte Carlo simulations based on incorporating random geometry errors in the numerical simulations suggest that the measured spectra are in accordance with the expected curves from simulations.Copyright

Wafer-scale synthesis and transfer of high quality monolayer graphene for nanoelectronics

Recent progress in synthesis of large area mono- or few layer graphene on copper catalyst[1] has enabled various fascinating sensor, optical and electrical applications.[2–4] The idea for graphene to be employed for next generation nanoelectronics, however, require high quality films with wafer-scale uniformity and low defect density. Here we report our progress on wafer-scale synthesis and transfer of low-defect monolayer graphene for field-effect transistors that will enable graphene nanoelectronics.