Aruna Velamakanni
University of Texas at Austin
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Featured researches published by Aruna Velamakanni.
Science | 2009
Xuesong Li; Weiwei Cai; Jinho An; Seyoung Kim; Dongxing Yang; Richard D. Piner; Aruna Velamakanni; Inhwa Jung; Emanuel Tutuc; Sanjay K. Banerjee; Luigi Colombo; Rodney S. Ruoff
Growing Graphene The highest quality graphene samples, single-atom-thick layers of carbon, are suspended flakes exfoliated from graphite, but these samples are very small in size (square micrometers). For many electronics applications, larger areas are needed. Li et al. (p. 1312, published online 7 May) show that graphene grows in a self-limiting way on copper films as large-area sheets (one square centimeter) from methane through a chemical vapor deposition process. The films, which are mainly one layer in thickness, can be transferred to other substrates and have electron mobilities as high as 4300 square centimeters per volt second. Predominantly single-layer graphene films grow in a self-limited manner on copper and can be transferred to other substrates. Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.
Science | 2008
Weiwei Cai; Richard D. Piner; Frank J. Stadermann; Sungjin Park; Medhat A. Shaibat; Yoshitaka Ishii; Dongxing Yang; Aruna Velamakanni; Sung Jin An; Meryl D. Stoller; Jinho An; Dongmin Chen; Rodney S. Ruoff
The detailed chemical structure of graphite oxide (GO), a layered material prepared from graphite almost 150 years ago and a precursor to chemically modified graphenes, has not been previously resolved because of the pseudo-random chemical functionalization of each layer, as well as variations in exact composition. Carbon-13 (13C) solid-state nuclear magnetic resonance (SSNMR) spectra of GO for natural abundance 13C have poor signal-to-noise ratios. Approximately 100% 13C-labeled graphite was made and converted to 13C-labeled GO, and 13C SSNMR was used to reveal details of the chemical bonding network, including the chemical groups and their connections. Carbon-13–labeled graphite can be used to prepare chemically modified graphenes for 13C SSNMR analysis with enhanced sensitivity and for fundamental studies of 13C-labeled graphite and graphene.
ACS Nano | 2011
Shanshan Chen; Lola Brown; Mark Levendorf; Weiwei Cai; Sang Yong Ju; Jonathan Edgeworth; Xuesong Li; Carl W. Magnuson; Aruna Velamakanni; Richard D. Piner; Junyong Kang; Jiwoong Park; Rodney S. Ruoff
The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.
Nano Letters | 2010
Xuesong Li; Carl W. Magnuson; Archana Venugopal; Jinho An; Ji Won Suk; Boyang Han; Mark Borysiak; Weiwei Cai; Aruna Velamakanni; Yanwu Zhu; Lianfeng Fu; Eric M. Vogel; Edgar Voelkl; Luigi Colombo; Rodney S. Ruoff
The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.
ACS Nano | 2010
Yanwu Zhu; Meryl D. Stoller; Weiwei Cai; Aruna Velamakanni; Richard D. Piner; David J. Chen; Rodney S. Ruoff
Graphite oxide was exfoliated and dispersed in propylene carbonate (PC) by bath sonication. Heating the graphene oxide suspensions at 150 degrees C significantly reduced the graphene oxide platelets; paper samples comprising such reduced graphene oxide platelets had an electrical conductivity of 5230 S/m. By adding tetraethylammonium tetrafluoroborate (TEA BF(4)) to the reduced graphene oxide/PC slurry and making a two-cell ultracapacitor, specific capacitance values of about 120 F/g were obtained.
Macromolecular Rapid Communications | 2010
Sun Hwa Lee; Daniel R. Dreyer; Jinho An; Aruna Velamakanni; Richard D. Piner; Sungjin Park; Yanwu Zhu; Sang Ouk Kim; Christopher W. Bielawski; Rodney S. Ruoff
A method for growing polymers directly from the surface of graphene oxide is demonstrated. The technique involves the covalent attachment of an initiator followed by the polymerization of styrene, methyl methacrylate, or butyl acrylate using atom transfer radical polymerization (ATRP). The resulting materials were characterized using a range of techniques and were found to significantly improve the solubility properties of graphene oxide. The surface-grown polymers were saponified from the surface and also characterized. Based on these results, the ATRP reactions were determined to proceed in a controlled manner and were found to leave the structure of the graphene oxide largely intact.
Applied Physics Letters | 2009
Yanwu Zhu; Weiwei Cai; Richard D. Piner; Aruna Velamakanni; Rodney S. Ruoff
Transparent conducting films have been fabricated in one step, combining self-assembly and chemical reduction of graphene oxide platelets dispersed in water. The films are of centimeter scale and their thickness can be controlled by the concentration of the graphene oxide suspension. The optical transmittance values at a wavelength of 550 nm were 87% and 96% for the films made from 1.5 and 0.5 mg/ml suspensions, respectively, and have sheet resistances of 11.3 and 31.7 kΩ/◻. Scanning and transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy were used to characterize the films.
215th ECS Meeting | 2009
Xuesong Li; Weiwei Cai; In Hwa Jung; Jin Ho An; Dongxing Yang; Aruna Velamakanni; Richard D. Piner; Luigi Colombo; Rodney S. Ruoff
Graphene and few-layer graphene films exhibit unique properties and show promise as electronic devices as well as for passive applications. A significant challenge however is the synthesis of large area graphene and/or multilayer films. A method that is showing promise is the growth of graphene on metal substrates by chemical vapor deposition. In this paper we report on a method to grow large-area graphene films on Cu foils and graphene transfer methods to other substrates. The transferred graphene films were characterized by optical, scanning and transmission electron microscopy, Raman and UV-VIS spectroscopy, and four-point probe electrical measurements. The data shows that graphene can be grown directly on Cu substrates by chemical vapor deposition. The graphene films transferred to SiO2/Si substrates show low defects as determined by the near absence of the “D” band at 1350 cm -1 in the Raman spectrum. In addition, the films were found to have high optical transmittance and electrical conductivity.
ACS Nano | 2010
Aruna Velamakanni; Carl W. Magnuson; K. J. Ganesh; Yanwu Zhu; Jinho An; Paulo J. Ferreira; Rodney S. Ruoff
There has been no attempt to date to specifically modify the nodes in carbon nanotube (CNT) networks. If the nodes can be modified in favorable ways, the electrical and/or thermal and/or mechanical properties of the CNT networks could be improved. In an attempt to influence the performance as a transparent conductive film, gold nanoparticles capped with the amino acid cysteine (Au-CysNP) have been selectively attached at the nodes of multiwalled carbon nanotubes (MWCNTs) networks. These nanoparticles have an average diameter of 5 nm as observed by TEM. FTIR and XPS were used to characterize each step of the MWCNT chemical functionalization process. The chemical process was designed to favor selective attachment at the nodes and not the segments in the CNT networks. The chemical processing was designed to direct formation of nodes where the gold nanoparticles are. The nanoparticles which were loosely held in the CNT network could be easily washed away by solvents, while those bound chemically remained. TEM results show that the Cys-AuNPs are preferentially located at the nodes of the CNT networks when compared to the segments. These nanoparticles at the nodes were also characterized by a novel technique called diffraction scanning transmission electron microscopy (D-STEM) confirming their identity. Four-probe measurements found that the sheet resistance of the modified CNT networks was half that of similarly transparent pristine multiwalled CNT networks.
Langmuir | 2010
Aruna Velamakanni; Jahn R. Torres; K. J. Ganesh; Paulo J. Ferreira; J. S. Major
We describe the controlled assembly of silane-based copolymers on various interfaces that have surface silanol groups. This assembly occurs as a result of the formation of very robust siloxane bonds (Si-O-Si) due to a condensation reaction between the alkoxysilane groups of the polymers and surface hydroxyl groups of the substrates. Deposition of these copolymers is not self-limiting; therefore, they could not be assembled into discrete monolayers. However, UV-visible data collected as a function of deposition cycle reveals a linear relationship, confirming the deposition of a constant amount of polymer in each deposition cycle. A linear variation of layer thickness with deposition cycles is also observed. The assembled polymer layers are found to be very robust and resistant even when exposed to piranha solution for several hours.