Session A30: Graphene: Growth, Properties and Devices

The effects of copper substrate structure and impurities on the quality of graphene growth

Since we discovered growth of mono-layer graphene on Cu substrates, most researchers use the same 99.8\% pure foil from Afla-Aesar as sited in our original publication. We have investigated several other copper substrates for their suitability for graphene growth. We find that the purity and thickness of the copper foil have measureable effects on the quality of the graphene and growth parameters needed to obtain large mono-layer coverage. We will present our findings and summarize the effects that we have seen. Our methods for determining graphene quality include SEM, scanning micro-Raman, and AFM.   

Graphene Films Grown on Insulating Substrates

We report a method of direct CVD growth of carbon films on quartz substrates. The films are grown at temperatures from 650 to 1200 $^{\circ}$C in a graphite container filled with methane. Films grown at 1200 $^{\circ}$C reveal clear G and 2D Raman bands characteristic of graphene. A combination of Raman, absorption and electrical measurements allows us to conclude that carbon films grown by this method are polycrystalline graphene, large areas of which may be composed of single carbon layer. Sheet resistivity of these graphene films is low enough to make them interesting objects for electronic applications. Advantages of our synthetic approach include simplicity and the ability to deposit films on any insulating substrate, which can stand temperature of at least 650 $^{\circ}$C. Thus far, no factors limiting the area of deposition and uniformity of the deposited graphene films have been identified.   

Electron Stimulated Decomposition of Acetylene as a Precursor for Graphene

We report here on the deposition of carbon via C$_2$H$_2$ dissociation by electron beam irradiation and thermal decomposition. The substrates investigated include sapphire, silicon, ALD deposited Al$_2$O$_3$/SiO$_2$, and GaN/sapphire. Raman analyses show that on C-plane sapphire both thermal decomposition and electron beam stimulated dissociation of C$_2$H$_2$ deposit carbon successfully. On other substrates these methods were inactive, showing the decomposition of C$_2$H$_2$ on sapphire is catalytic. We tested different annealing times and C$_2$H$_2$ pressures, gauging absorption saturation with RHEED. Samples exposed to 15 min. C$_2$H$_2$ adsorption during 400 eV electron irradiation and then annealed for 2 hr. to above 600$^{\circ}$C in high vacuum showed the greatest proportion of sp2 to sp3 bonding by Raman analysis. The Raman spectra also suggest hydrogen adsorption, which may hinder further sp2 bonding. Annealing samples in a hydrogen atmosphere does not change their Raman spectra, suggesting hydrogen saturation. \noindent Partial support from the University of Minnesota Institute for Renewable Energy and the Environment   

Electrical and Raman characterizations of chemical vapor deposited (CVD) graphene grains and grain boundaries

We performed Raman spectroscopy and electrical transport studies on graphene grains grown on copper foils by ambient pressure CVD. These grains are found to be hexagonally-shaped with edges macroscopically parallel to zig-zag directions as evidenced by scanning tunneling microscopy and transmission electrical microscopy. After the grains are transferred to SiO$_{2}$/Si, Raman spectroscopy and mapping are performed. The intensity of the D peak (I$_{D}$) is negligibly small over most grain area with the notable exception of a few isolated spots, attributed mostly as nucleation centers. We show Raman mapping is a convenient tool to identify grain boundaries, which show large I$_{D}$. Simultaneous measurements of both intra-grain and inter-grain electronic transport were performed on merged grains. We found the inter-grain resistivity to be always larger than the intra-grain resistivity. Low temperature inter-grain magneto-resistance (R$_{xx}$(B)) displays a prominent weak localization (WL) feature, which was not observable or was much weaker for intra-grain R$_{xx}$(B). Our observation indicates that grain boundaries are major sources of intervalley scattering and strongly affect electron transport in polycrystalline CVD graphene.   

Cyclotron Resonance in Graphene at Ultrahigh Magnetic Fields

To investigate the effects of intentional and unintentional doping on the conduction properties of CVD-grown large-area graphene, we have performed high-field cyclotron resonance (CR) measurements on graphene. We accessed ultrahigh magnetic fields using the Single-Turn Coil System at NHMFL-Los Alamos, which can produce peak fields over 300 T in $\sim $2.5 $\mu $s pulses. We investigated magneto-infrared transmission at 10.6 $\mu $m in pulsed ultrahigh magnetic fields up to 170 T for a variety of graphene samples on KRS-5 substrates with different levels of doping. Circularly polarized CO$_{2}$ light was used to determine the carrier type of the doping, and temperature-dependent measurements were also performed. We observed a clear CR peak at $\sim $50 T corresponding to the $n$ = 1 to $n$ = 2 Landau level transition, which indicates that the Fermi energy measured from the Dirac point has to be $\sim $250-400 meV.   

Measurement of nanomechanical properties of suspended graphene membranes

Since graphene was first isolated from graphite, its unique properties have been intensively investigated in various ways. Recently, a method to grow large-area, uniform monolayer graphene has been realized by chemical vapor deposition (CVD) on metal substrates. In this respect various properties of CVD-grown graphene need to be studied and compared with those measured from mechanically exfoliated graphene. In this talk we report mechanical measurement of suspended graphene membranes made by CVD on copper foils. Monolayer graphene was transferred onto through holes to make suspended graphene membranes. Bulge testing with uniform pressure was done on those membranes to extract the mechanical properties of CVD-grown monolayer graphene. Moreover, nanoindentation was performed on those suspended graphene membranes and the result is compared with that obtained by bulge testing.   

Thermal Conductivity of CVD grown graphene

When suspended, CVD grown graphene has a high thermal conductivity (k) of 2,500$\pm $1100 W/mK near 350 K. But for practical applications, graphene would be attached to a substrate. Previously it was reported that the CVD grown graphene supported on Si/SiO2 has a k value as low as 370+650/-320 W/mK in ambient. We find that the k of CVD grown graphene on glass varies in a range of 1100 - 2000 W/mK and depends on the growth parameters. The k of graphene is measured by a differential thermocouple technique and compared with that obtained by scanning thermal microscopy. Moreover, the samples grown in ambient pressure have shown higher k compared to the graphene grown at low pressure.   

Electrochemistry of individual monolayer graphene sheets

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene, indicating that the redox reactions occur on relatively clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We also demonstrate real-time electrochemical detection of molecular desorption from graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the kinetics of interactions between molecules and graphene.   

Mechanical and Electrical Properties of Polycrystalline Graphene

Graphene grown by chemical vapor deposition (CVD) has enabled large scale fabrication of graphene-based devices [1]. We apply transmission electron microscopy and AFM techniques to identify individual grain boundaries [2]. This further allows the direct investigation of mechanical and electrical properties of polycrystalline graphene in correlation with its grain structure. We used atomic force microscopy in order to induce and image tearing along individual grain boundaries and find a decreased mechanical strength in CVD graphene compared with pristine exfoliated graphene [3]. Our electrical measurements of CVD graphene devices show that charge mobility is sensitive to different growth conditions. However, we found that average grain size is not directly correlated with the charge mobility, suggesting that grain boundaries are not necessarily a dominating factor. \\[4pt] [1]. Li, X. \textit{et al}. \textit{Science }\textbf{2009}$,$ 1312-1314. \\[0pt] [2]. Huang, P \textit{et al.} \textit{arxiv }\textbf{2010,} 1009.4714v1. \\[0pt] [3]. Lee, C. \textit{et al.} \textit{Science }\textbf{2008}, 385-388.   

Transparent and Flexible Large-scale Graphene-based Heater

We report the application of transparent and flexible heater with high optical transmittance and low sheet resistance using graphene films, showing outstanding thermal and electrical properties. The large-scale graphene films were grown on Cu foil by chemical vapor deposition methods, and transferred to transparent substrates by multiple stacking. The wet chemical doping process enhanced the electrical properties, showing a sheet resistance as low as 35 ohm/sq with 88.5 {\%} transmittance. The temperature response usually depends on the dimension and the sheet resistance of the graphene-based heater. We show that a 4x4 cm$^{2}$ heater can reach 80 $^{\circ}$C within 40 seconds and large-scale (9x9 cm$^{2})$ heater shows uniformly heating performance, which was measured using thermocouple and infra-red camera. These heaters would be very useful for defogging systems and smart windows.   

Large Area Chemical Vapor Deposition Graphene Photodetectors

We investigate large area graphene photodetectors based on graphene grown by Chemical Vapor Deposition on Cu foils and then transferred to SiO$_{2}$/Si wafers. Through scanning photocurrent microscopy (SPM) at 532 nm, we compare the performance of CVD fabricated devices using Ti/Pd/Au, Au, and Pt graphene metal junctions with those from literature fabricated through mechanical exfoliation. Our initial experiments show that photocurrent from CVD graphene is about an order of magnitude smaller than devices in literature. Non-idealities related to material properties, defects, and transfer related inhomogenities are believed to be the cause of the discrepancy. These effects are studied through concurrent registration of atomic force microscopy, optical microscopy, Raman Microscopy, and SPM. In addition to intrinsic material property effects, fabrication related issues of graphene-metal junctions are also explored.   

Solution-gated Field Effect Transistors based on CVD grown Graphene for chemical and bio sensing applications

Graphene holds great potential for bioelectronic applications and, more specifically, for fast high-sensitivity pH measurements and biosensing. Its monolayer structure (just one carbon atom thick) in combination with its very high carrier mobility enable very high transconductance, low noise and biocompatibility which are key parameters for chemical sensors with electronic readout. In fact, single molecule detection has already been demonstrated in graphene gas sensors. In this paper we report on the fabrication and characterization of solution-gated field effect transistors (SGFET) arrays based on CVD grown graphene films on copper that can operate in various liquid environments. These devices exhibit transconductances around 20 $\mu$Siemens, which highlights their excellent sensitivity. We also performed some pH sensing experiments and demonstrated that the transfer characteristics of the GFET are pH dependent with a pH sensitivity of 14 mV/pH. These results drive the way for chemical and bio-sensing by functionalized graphene, which is the aim of our future work.   

PECVD silicon nitride gate dielectrics and band-gap engineering in graphene devices

We found that silicon nitride can provide excellent coverage of graphene in field-effect transistors while preserving its good carrier mobilities, without the need of a seed layer. Moreover, the silicon nitride film has the advantage of higher dielectric constant and higher surface polar optical phonon energy (i.e. less remote phonon scattering in the graphene channel) compared to silicon oxide. The breakdown strength in silicon nitride is high as well. The effect of a perpendicular electric field on the band-structure of different numbers of graphene layers used as channels of the transistor was also studied and the induced band-gap or band-overlap was obtained accounting for the effects of the variation of the surface potential near the Dirac/neutrality point.   

Graphene: Atomically thin protective coating

We explore the properties of graphene as a cathodic coating to protect copper substrates from oxidation and further corrosion. High-quality and large area graphene films are grown on copper substrates by chemical vapor deposition. Samples were thermally oxidized in an oxygen-rich environment. X-ray photoelectron spectroscopy (XPS) characterization of a Graphene/copper and bare copper samples reveals the absence of oxidized copper at the graphene/copper interface indicating that the graphene monolayer protects the underlying copper. We also determine the protective properties of graphene in aqueous media using electrochemical characterization techniques. First, we use Electrochemical Impedance Spectroscopy (EIS) to show that graphene coated substrates lower frequencies (1Hz) exhibit impedance values 2 orders of magnitude higher compared to bare Cu substrates. Cyclic voltammetry also shows that a monolayer of graphene significantly reduces the oxygen reduction, thus exhibiting little charge transfer at the solid-liquid interface. Finally, we use Tafel analysis to estimate that the corrosion rate exhibited by Graphene/Cu is $\sim7$ times lower than that of bare Cu substrates.   

Ordered carbon nanotube growth on graphene and few-layer graphene

Carbon nanotubes are grown on graphene and few-layer graphene films through chemical vapor deposition. The nanotube growth is found to depend on the thickness of the few-layer graphene films. The thinnest films show significant alignment of the nanotubes with the crystallographic axes of the graphene. This alignment is compared to the orientation of the crystallographic etch tracks, permitting the orientation of the nanotubes to be determined. Related nanotube/graphene structures will also be presented and discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.