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.

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.

Monolayer graphene has been grown on crystallized Cu (111) films on standard oxidized Si 100 mm wafers. The monolayer graphene demonstrates high uniformity (>97% coverage), with immeasurable defects (>95% defect-negligible) across the entire wafer. Key to these results is the phase transition of evaporated copper films from amorphous to crystalline at the growth temperature as corroborated by X-ray diffraction and electron backscatter diffraction. Noticeably, phase transition of copper film is observed on technologically ubiquitous oxidized Si wafer where the oxide is a standard amorphous thermal oxide. Ion mass spectroscopy indicates that the copper films can be purposely hydrogen-enriched during a hydrogen anneal which subsequently affords graphene growth with a sole carbonaceous precursor for low defect densities. Owing to the strong hexagonal lattice match, the graphene domains align to the Cu (111) domains, suggesting a pathway for increasing the graphene grains by maximizing the copper grain sizes. Fabricated graphene transistors on a flexible polyimide film yield a peak carrier mobility ~4,930 cm 2 /Vs.

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.

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. 1-3 In these cases, power and size requirements are not critical, and such sensors tend to be large and bulky. But with the rapid expansion and prevalence of newer technologies like smart phones, cloud computing and the Internet of Things (IoT), mobile sensors are recognized as an essential component in future ubiquitous sensor networks. 4,5 The goals of IoT sensor networks require mobile and untethered sensors in quantities that negate any realistic possibility of individual sensor maintenance or battery replacement. The expectation is that the sheer number of future sensor devices, the "things" of IoT, will preclude human maintenance of individual nodes within large sensor networks. The implications for future device production are then twofold: the sensors must operate at low-power, and the cost of each device should be low enough that the expected orders of magnitude increase in sensor nodes is feasible. Much of current gas sensor research is therefore directed at the need for low-cost, low-power portable gas sensors, as well as integration with the technology platform best suited to meet that need: silicon complementary metal-oxide semiconductor (CMOS).Solid-state gas sensors cover a wide range of technologies, from microelectromechanical thermal and mass sensors to optical and chemiresistive sensors. 6,7 Of these, one of the most common is the chemisresistive sensor, whose relatively simple design and operation make it a strong candidate for CMOS integration.