Sandra C. Hernández

High-quality uniform dry transfer of graphene to polymers.

In this paper we demonstrate high-quality, uniform dry transfer of graphene grown by chemical vapor deposition on copper foil to polystyrene. The dry transfer exploits an azide linker molecule to establish a covalent bond to graphene and to generate greater graphene-polymer adhesion compared to that of the graphene-metal foil. Thus, this transfer approach provides a novel alternative route for graphene transfer, which allows for the metal foils to be reused.

Direct mechanochemical cleavage of functional groups from graphene

Mechanical stress can drive chemical reactions and is unique in that the reaction product can depend on both the magnitude and the direction of the applied force. Indeed, this directionality can drive chemical reactions impossible through conventional means. However, unlike heat- or pressure-driven reactions, mechanical stress is rarely applied isometrically, obscuring how mechanical inputs relate to the force applied to the bond. Here we report an atomic force microscope technique that can measure mechanically induced bond scission on graphene in real time with sensitivity to atomic-scale interactions. Quantitative measurements of the stress-driven reaction dynamics show that the reaction rate depends both on the bond being broken and on the tip material. Oxygen cleaves from graphene more readily than fluorine, which in turn cleaves more readily than hydrogen. The technique may be extended to study the mechanochemistry of any arbitrary combination of tip material, chemical group and substrate.

Epitaxial Growth of III–Nitride/Graphene Heterostructures for Electronic Devices

Epitaxial GaN films were grown by metal organic chemical vapor deposition (MOCVD) on functionalized epitaxial graphene (EG) using a thin (~11 nm) conformal AlN nucleation layer. Raman measurements show a graphene 2D peak at 2719 cm-1 after GaN growth. X-ray diffraction analysis reveals [0001]-oriented hexagonal GaN with (0002) peak rocking curve full width at the half maximum (FWHM) of 544 arcsec. The FWHM values are similar to reported values for GaN grown by MOCVD on sapphire. The GaN layer has a strong room-temperature photoluminescence band edge emission. Successful demonstration of GaN growth on EG opens up the possibility of III–nitride/graphene heterostructure-based electronic devices and promises improved performance.

Achieving clean epitaxial graphene surfaces suitable for device applications by improved lithographic process

It is well-known that the performance of graphene electronic devices is often limited by extrinsic scattering related to resist residue from transfer, lithography, and other processes. Here, we report a polymer-assisted fabrication procedure that produces a clean graphene surface following device fabrication by a standard lithography process. The effectiveness of this improved lithography process is demonstrated by examining the temperature dependence of epitaxial graphene-metal contact resistance using the transfer length method for Ti/Au (10 nm/50 nm) metallization. The Landauer-Buttiker model was used to explain carrier transport at the graphene-metal interface as a function of temperature. At room temperature, a contact resistance of 140 Ω-μm was obtained after a thermal anneal at 523 K for 2 hr under vacuum, which is comparable to state-of-the-art values.

Modifying Surface Energy of Graphene via Plasma-Based Chemical Functionalization to Tune Thermal and Electrical Transport at Metal Interfaces

The high mobility exhibited by both supported and suspended graphene, as well as its large in-plane thermal conductivity, has generated much excitement across a variety of applications. As exciting as these properties are, one of the principal issues inhibiting the development of graphene technologies pertains to difficulties in engineering high-quality metal contacts on graphene. As device dimensions decrease, the thermal and electrical resistance at the metal/graphene interface plays a dominant role in degrading overall performance. Here we demonstrate the use of a low energy, electron-beam plasma to functionalize graphene with oxygen, fluorine, and nitrogen groups, as a method to tune the thermal and electrical transport properties across gold-single layer graphene (Au/SLG) interfaces. We find that while oxygen and nitrogen groups improve the thermal boundary conductance (hK) at the interface, their presence impairs electrical transport leading to increased contact resistance (ρC). Conversely, functionalization with fluorine has no impact on hK, yet ρC decreases with increasing coverage densities. These findings indicate exciting possibilities using plasma-based chemical functionalization to tailor the thermal and electrical transport properties of metal/2D material contacts.

Initial evaluation and comparison of plasma damage to atomic layer carbon materials using conventional and low Te plasma sources

The ability to achieve atomic layer precision is the utmost goal in the implementation of atomic layer etch technology. Carbon-based materials such as carbon nanotubes (CNTs) and graphene are single atomic layers of carbon with unique properties and, as such, represent the ultimate candidates to study the ability to process with atomic layer precision and assess impact of plasma damage to atomic layer materials. In this work, the authors use these materials to evaluate the atomic layer processing capabilities of electron beam generated plasmas. First, the authors evaluate damage to semiconducting CNTs when exposed to beam-generated plasmas and compare these results against the results using typical plasma used in semiconductor processing. The authors find that the beam generated plasma resulted in significantly lower current degradation in comparison to typical plasmas. Next, the authors evaluated the use of electron beam generated plasmas to process graphene-based devices by functionalizing graphene with...

Plasma-Based Chemical Modification of Epitaxial Graphene

In this work, the treatment of epitaxial graphene on SiC using electron beam generated plasmas produced in mixtures of argon and oxygen is demonstrated. The treatment imparts oxygen functional groups on the surface with concentrations ranging up to about 12 at.%, depending on treatment parameters. Surface characterization of the functionalized graphene shows incorporation of oxygen to the lattice by disruption of ∏-bonds, and an altering of bulk electrical properties.

Plasma-Modified, Epitaxial Fabricated Graphene on SiC for the Electrochemical Detection of TNT

Using square wave voltammetry, we show an increase in the electrochemical detection of trinitrotoluene (TNT) with a working electrode constructed from plasma modified graphene on a SiC surface vs. unmodified graphene. The graphene surface was chemically modified using electron beam generated plasmas produced in oxygen or nitrogen containing backgrounds to introduce oxygen or nitrogen moieties. The use of this chemical modification route enabled enhancement of the electrochemical signal for TNT, with the oxygen treatment showing a more pronounced detection than the nitrogen treatment. For graphene modified with oxygen, the electrochemical response to TNT can be fit to a two-site Langmuir isotherm suggesting different sites on the graphene surface with different affinities for TNT. We estimate a limit of detection for TNT equal to 20 ppb based on the analytical standard S/N ratio of 3. In addition, this approach to sensor fabrication is inherently a high-throughput, high-volume process amenable to industrial applications. High quality epitaxial graphene is easily grown over large area SiC substrates, while plasma processing is a rapid approach to large area substrate processing. This combination facilitates low cost, mass production of sensors.

Improved Chemical Detection and Ultra-Fast Recovery Using Oxygen Functionalized Epitaxial Graphene Sensors

Oxygen functionalized epitaxial graphene (OFEG) sensors are demonstrated toward the sensing of polar chemical vapors at room temperature. The electrical characteristics of the sensor show an increase in resistance upon exposure to polar protic chemicals while the resistance decreased for polar aprotic vapors The average response and recovery times of the OFEG sensor to all analyte vapors are 10 and 100 s, respectively. In comparison, non-functionalized epitaxial graphene (NFEG) sensors show similar response times as OFEG, but with extremely long recovery rates in the range of ~1.5-2 hours. The dipole moment of the chemical is found to have a strong impact on the magnitude of the response for both OFEG and NFEG which increased with the increasing dipole moment from 2.0 D to 4.1 D. However, OFEG exhibits significantly higher sensitivity (twofold increase) to all polar chemicals over NFEG sensors. For example, exposing OFEG to n-methyl-2-pyrrolidone vapors produces a 45% change in resistance, in comparison to a 27% resistance change displayed by NFEG sensors. The noise spectral density of NFEG follows a typical 1/f dependence upon exposure to di-methylformamide vapors but with a lower change in noise from 1 × 10-18A2/Hz to 1 × 10-17 A2/Hz at 1 Hz. In contrast, OFEG displays a unique 1/f2 behavior at lower frequency range (1-10 Hz) with a significant change to the sensor noise from 3 × 10-18A2/Hz to 2 × 10-15 A2/Hz.

Detection of polar chemical vapors using epitaxial graphene grown on SiC (0001)

Epitaxial graphene grown on SiC (0001) showed significant changes in electrical resistance upon exposure to polar protic and polar aprotic vapors in the ambient atmosphere. The dipole moment of these chemicals was found to have a strong impact on the magnitude of the sensor response, which increases with increasing dipole moment. Using the combination of low-frequency noise and Hall measurements, we demonstrate that the chemical sensing mechanism in epitaxial graphene is based on fluctuations of the charge carrier density induced by vapor molecules adsorbed on the surface of the graphene.