A. M. Rao

Clemson University, Clemson, South Carolina, United States

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Publications (190)636.73 Total impact

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    ABSTRACT: While studying magnetism of d- and f-electron systems has been consistently an active research area in physics, chemistry, and biology, there is an increasing interest in the novel magnetism of p-electron systems, especially in graphene and graphene-derived nanostructures. Bulk graphite is diamagnetic in nature, however, graphene is known to exhibit either a paramagnetic response or weak ferromagnetic ordering. Although many groups have attributed this magnetism in graphene to defects or unintentional magnetic impurities, there is a lack of compelling evidence to pinpoint its origin. To resolve this issue, we systematically studied the influence of entropically necessary intrinsic defects (e.g., vacancies, edges) and extrinsic dopants (e.g., S-dopants) on the magnetic properties of graphene. We found that the saturation magnetization of graphene decreased upon sulfur doping suggesting that S-dopants demagnetize vacancies and edges. Our density functional theory calculations provide evidence for: (i) intrinsic defect demagnetization by the formation of covalent bonds between S-dopant and edges/vacancies concurring with the experimental results, and (ii) a net magnetization from only zig-zag edges, suggesting that the possible contradictory results on graphene magnetism in the literature could stem from different defect-types. Interestingly, we observed peculiar local maxima in the temperature dependent magnetizations that suggest the coexistence of different magnetic phases within the same graphene samples.
    Journal of Magnetism and Magnetic Materials 03/2016; 401:70-76. DOI:10.1016/j.jmmm.2015.10.012 · 1.97 Impact Factor
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    ABSTRACT: Submitted for the MAR15 Meeting of The American Physical Society Carbon composites with metal nanoparticles for Alcohol fuel cells LAKSHMAN VENTRAPRAGADA, Clemson University, R.S. SIDDHARDHA, Sri Sathya Sai Institute of Higher Learning, RAMAKRISHNA PODILLA, Clemson University, V.S. MUTHUKUMAR, Sri Sathya Sai Institute of Higher Learning, STEPHEN CREAGER, A.M. RAO, Clemson University, SAI SATHISH RAMA-MURTHY, Sri Sathya Sai Institute of Higher Learning — Graphene due to its high surface area and superior conductivity has attracted wide attention from both in-dustrial and scientific communities. We chose graphene as a substrate for metal nanoparticle deposition for fuel cell applications. There are many chemical routes for fabrication of metal-graphene composites, but they have an inherent disadvan-tage of low performance due to the usage of surfactants, that adsorb on their surface. Here we present a design for one pot synthesis of gold nanoparticles and simultaneous deposition on graphene with laser ablation of gold strip and functionalized graphene. In this process there are two natural advantages, the nanoparticles are synthesized without any surfactants, therefore they are pristine and subsequent impregnation on graphene is linker free. These materials are well characterized with electron microscopy to find their morphology and spectroscopic techniques like Raman, UV-Vis. for functionality. This gold nanoparticle decorated graphene composite has been tested for its electrocatalytic oxidation of alcohols for alkaline fuel cell applica-tions. An electrode made of this composite showed good stability for more than 200 cycles of operation and reported a low onset potential of 100 mV more negative, an important factor for direct ethanol fuel cells.
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    ABSTRACT: Metal nanoparticle-decorated low dimensional materials can synergistically combine the nonlinear optical properties of metallic/inorganic nanostructures for enhancing the optical limiting performance. While many materials exhibit excellent optical limiting performance at a relatively higher fluence (>9 J/cm2), there is a still a dearth of optical limiting materials for protecting low damage threshold (<1 J/cm2) photonic devices. Although metal nanoparticle-decorated graphene hybrids are expected to resolve this issue, the rehybridization of metal d-orbitals and graphene p-orbitals often lead to undesirable changes in graphene’s electronic structure which adversely affect the nonlinear optical performance. Here, we demonstrate that d-orbitals of Au nanoparticles exhibit little or no rehybridization with graphene and result in an enhanced optical limiting behavior at a low fluence of ∼0.4 J/cm2, which is lower than most metal decorated graphene, carbon nanotube nanocomposites and metal nanoparticles. This optical limiting performance at a lower fluence is attributed to the excellent photo-absorption of Au nanoparticles combined with rapid thermalization of excited carriers by graphene.
    Optical Materials 12/2014; 39. DOI:10.1016/j.optmat.2014.11.023 · 1.98 Impact Factor
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    ABSTRACT: As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar(®) 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively.
    Review of Scientific Instruments 09/2014; 85(9):095118. DOI:10.1063/1.4895679 · 1.61 Impact Factor
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    ABSTRACT: A well-known limitation in the fabrication of metal-graphene composite has been the use of surfactants that strongly adsorb on the surface and reduce the performance of the catalyst. We demonstrate here a novel one-pot synthesis of gold nanoparticles by laser ablation of gold strip and in-situ decoration on graphene substrate. Not only the impregnation of nanoparticles was linker free, but also the synthesis by itself was surfactant-free. The composite materials were well characterized morphologically and functionally using electron microscopy, X-ray and electron diffraction, Raman spectroscopy, Zeta potential, electrochemical measurements and UV-Visible spectroscopic techniques. This linker-free gold-graphene based composite has been employed for catalytic applications pertaining to electrooxidation. We have explored the use of this composite as a binder-free electrode in electrocatalytic oxidation of methanol and ethanol in alkaline medium. Additionally, the onset potential for ethanol oxidation was found to be more negative, −100 mV, an indication of its promising application in direct ethanol fuel cells.
    Electroanalysis 08/2014; 26(8). DOI:10.1002/elan.201400244 · 2.14 Impact Factor
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    D Saini · H Behlow · R Podila · D Dickel · B Pillai · M J Skove · S M Serkiz · A M Rao ·
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    ABSTRACT: Despite their wide spread applications, the mechanical behavior of helically coiled structures has evaded an accurate understanding at any length scale (nano to macro) mainly due to their geometrical complexity. The advent of helically coiled micro/nanoscale structures in nano-robotics, nano-inductors, and impact protection coatings has necessitated the development of new methodologies for determining their shear and tensile properties. Accordingly, we developed a synergistic protocol which (i) integrates analytical, numerical (i.e., finite element using COMSOL®) and experimental (harmonic detection of resonance; HDR) methods to obtain an empirically validated closed form expression for the shear modulus and resonance frequency of a singly clamped helically coiled carbon nanowire (HCNW), and (ii) circumvents the need for solving 12th order differential equations. From the experimental standpoint, a visual detection of resonances (using in situ scanning electron microscopy) combined with HDR revealed intriguing non-planar resonance modes at much lower driving forces relative to those needed for linear carbon nanotube cantilevers. Interestingly, despite the presence of mechanical and geometrical nonlinearities in the HCNW resonance behavior the ratio of the first two transverse modes f2/f1 was found to be similar to the ratio predicted by the Euler-Bernoulli theorem for linear cantilevers.
    Scientific Reports 07/2014; 4:5542. DOI:10.1038/srep05542 · 5.58 Impact Factor
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    ABSTRACT: A facile surfactant free laser ablation mediated synthesis (LAMS) of gold-graphene composite is reported here. The material was characterized using transmission electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, powdered X-ray diffraction, Raman spectroscopy, Zeta potential measurements and UV-Visible spectroscopic techniques. The as-synthesized gold-graphene composite was effectively utilized as catalyst for decolorization of 4 important textile and laser dyes. The integration of gold nanoparticles (AuNPs) with high surface area graphene has enhanced the catalytic activity of AuNPs. This enhanced activity is attributed to the synergistic interplay of pristine gold’s electronic relay and π-π stacking of graphene with the dyes. This is evident when the Rhodamine B (RB) reduction rate of the composite is nearly twice faster than that of commercial citrate capped AuNPs of similar size. In case of Methylene blue (MB) the rate of reduction is 17000 times faster than uncatalyzed reaction. This synthetic method opens door to laser ablation based fabrication of metal catalysts on graphene for improved performance without the aid of linkers and surfactants
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 05/2014; 133C. DOI:10.1016/j.saa.2014.05.069 · 2.35 Impact Factor
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    ABSTRACT: Single filaments of HexTow® IM7-12K carbon fiber were subjected to tensile measurements on a device which applies a known stress σ, and measures the resulting strain ∊, and the change in resistivity Δρ. Young's modulus E, the resistivity ρ, the piezoresistivity Δρ/ρ∊, and the nonlinearity in the stress-strain relation δ, were determined to be 264.1 ± 16.0 GPa, 1.5 ± 0.1 × 10-3 Ω cm, 1.3 ± 0.1, and -4.96 ± 0.23, respectively. The values obtained for Young's modulus and the resistivity of the fiber are in reasonable agreement with the values reported by the manufacturer. To the best of our knowledge, this is the first report of a measurement of a third-order elastic constant of a single filament of HexTow® IM7-12K. Given the high elastic strains attainable in these fibers and the negative value of δ, the usual calculation of E from a linear fit to the stress-strain data leads to an incorrect higher value of E. According to the accepted thermodynamic definition of the elastic constants, one must use the initial slope of the stress-strain curve to evaluate E. We also observed that the glue used to secure the fiber has an influence on the apparent modulus of the fiber.
    Journal of Materials Engineering and Performance 02/2014; 23(3). DOI:10.1007/s11665-013-0826-2 · 1.00 Impact Factor
  • J. Zhu · L. Oliveira · R. Podila · S. Neeleshwar · Y. Y. Chen · J. He · M. Skove · A. M. Rao ·
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    ABSTRACT: Recently we investigated the magnetic behavior of as-prepared and sulfur doped chemically exfoliated graphene nano-platelets (GNPs) and single walled carbon nanotubes (SWCNTs). The doping was achieved by annealing desired carbon nanostructures with 0, 1.0, 1.5 and 3 at% sulfur in an evacuated quartz tube at 1000 ^oC for 1 day, followed by multiple rinsing in alcohol and drying in vacuum to remove excess sulfur. The isothermal M vs. H as well as the temperature-dependent M vs. T measurements were obtained using a vibrating sample magnetometer. We found that sulfur doping drastically changes the magnetic behavior of the as-prepared samples (both SWCNTs and GNPs). The results of zero-field-cooling (ZFC) and field-cooling (FC) in M vs. T measurements indicated the existence of large amount of coupled super-paramagnetic domains, along with antiferromagnetic domains. The saturation magnetization decreased in S doped GNPs, while a contrasting trend was observed in S doped SWCNTs. The role of edge states and structural defects in carbon nanostructures in the observed magnetic properties will be discussed.
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    P. Puneet · R. Podila · S. Zhu · J. He · M. J. Skove · T. M. Tritt · A. M. Rao ·

    Advanced Materials 12/2012; · 17.49 Impact Factor
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    R Podila · P Vedantam · P C Ke · J M Brown · A M Rao ·
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    ABSTRACT: The binding of proteins to a nanostructure often alters protein secondary and tertiary structures. However, the main physical mechanisms that elicit protein conformational changes in the presence of the nanostructure have not yet been fully established. Here we performed a comprehensive spectroscopic study to probe the interactions between bovine serum albumin (BSA) and carbon-based nanostructures of graphene and single-walled carbon nanotubes (SWNTs). Our results showed that the BSA "corona" acted as a weak acceptor to facilitate charge transfer from the carbon nanostructures. Notably, we observed that charge transfer occurred only in the case of SWNTs but not in graphene, resulting from the sharp and discrete electronic density of states of the former. Furthermore, the relaxation of external α-helices in BSA secondary structure increased concomitantly with the charge transfer. These results may help guide controlled nanostructure-biomolecular interactions and prove beneficial for developing novel drug delivery systems, biomedical devices and engineering of safe nanomaterials.
    The Journal of Physical Chemistry C 10/2012; 116(41):22098-22103. DOI:10.1021/jp3085028 · 4.77 Impact Factor
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    ABSTRACT: Current research efforts are aimed at controlling the electronic properties via doping graphene. Previously, dopant-induced changes in the Fermi velocity were observed to result in an effectively downshifted Raman peak below the G Prime -band for n-doped carbon nanotubes. However, in the case of N-doped graphene, we find that several Raman features vary depending upon both dopant concentration and its bonding environment. For instance, only pyridinic/pyrrolic dopants were observed to result in intense D/D Prime -bands with a concomitant downshift in the G Prime -band. Here, we correlate x-ray photoelectron measurements with Raman spectra to elucidate effects of dopant bonding configuration on vibrational properties of graphene.
    Applied Physics Letters 09/2012; 101(12-12). DOI:10.1063/1.4752736 · 3.30 Impact Factor
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    ABSTRACT: Bismuth is a fascinating material system owing to its unusual Fermi surface topology, which depends on size and temperature. Theoretical calculations predict that Bi should undergo a semimetal-to-semiconductor transition as at least one of its dimensions becomes < 50 nm. This prediction was experimentally confirmed by infrared (IR) absorption spectra, which is largely underlain by transitions between the L (electron) and T (hole) pockets of the Fermi surface. In this work, however, we report that in our nanosize samples, the observed IR peak positions are practically independent of temperature, which is hard to reconcile with the predicted behavior of the L–T transition. To help elucidate the origin of these IR peaks, we performed a careful analysis of the IR spectra of Bi nanorods, as well as those of bulk Bi, Bi samples prepared under different conditions and Bi2(CO3)O2 using Fourier transform infrared and photoacoustic spectroscopy measurements. We propose that the observed IR peaks in Bi nanorods arise from the oxygen–carbon containing secondary phases formed on the surface of Bi rather than from the Bi itself. We believe that secondary phases must be taken into account on a general basis in modeling the IR spectra of Bi and that the scenario that ascribes these IR peaks solely to the L–T transitions may not be correct. The results reported herein may also impact the research of Bi-based thermoelectric nanostructures and bulk materials.
    Nano brief reports and reviews 05/2012; 07(02). DOI:10.1142/S1793292012300034 · 1.09 Impact Factor
  • B. Anand · R. Podila · R. Philip · A.M. Rao · S.S.S. Sai ·
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    ABSTRACT: Using open aperture z-scan measurements in tandem with XPS measurements and Raman spectroscopy, we show that nonlinear optical properties of single-walled carbon nanotubes can be tailored by substitutional doping with Boron.
    Fiber Optics and Photonics (PHOTONICS), 2012 International Conference on; 01/2012
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    ABSTRACT: The recent realization of silicon core optical fibers has the potential for novel low insertion loss rack-to-rack optical interconnects and a number of other uses in sensing and biomedical applications. To the best of our knowledge, incoherent light source based rapid photothermal processing (RPP) was used for the first time to anneal glass-clad silicon core optical fibers. X-ray diffraction examination of the silicon core showed a considerable enhancement in the length and amount of single crystallinity post-annealing. Further, shifts in the Raman frequency of the silicon in the optical fiber core that were present in the as-drawn fibers were removed following the RPP treatment. Such results indicate that the RPP treatment increases the local crystallinity and therefore assists in the reduction of the local stresses in the core, leading to more homogenous fibers. The dark current-voltage characteristics of annealed silicon optical fiber diodes showed lower leakage current than the diodes based on as-drawn fibers. Photons in UV and vacuum ultraviolet (VUV) regions play a very important role in improving the bulk and carrier transport properties of RPP-treated silicon optical fibers, and the resultant annealing permits a path forward to in situ enhancement of the structure and properties of these new crystalline core optical fibers.
    Journal of Applied Physics 12/2011; 110(9-110):093107 - 093107-6. DOI:10.1063/1.3660270 · 2.18 Impact Factor
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    ABSTRACT: Due to their unusual electronic and vibrational properties, single walled carbon nanotubes (SWCNTs) with sub-nanometer diameters d∼0.5–0.9nm have recently gained interest in the carbon community. Using UV–Vis–NIR spectroscopy and ultra-centrifugation, we have conducted a detailed study of the π plasmon energy (present at∼5–7eV) in sub-nm SWCNTs as a function of the size of the bundle. We find that the energy of the π plasmon peak E varies with the bundle diameter Dh as E=(-0.023eV)∗ln(Dh/do)+5.37eV, where do=0.5nm and corresponds to the smallest tube diameter.1In order to make our empirical relation dimensionally correct, we divide the SWCNT bundle diameter Dh with the smallest tube diameter (do=0.5 nm) present in the sub-nm SWNTs. This value for do is constant in the empirical relation and was determined from a careful Raman and photoluminescence measurements.1 This is compared with the same data for HiPCo and Carbolex SWCNTs of larger diameter (1–1.4nm) confirming a clear dependence of E on the bundle size, which is present in addition to the previously reported dependence of E on SWCNT diameter d.
    Carbon 10/2011; 49(12):3803-3807. DOI:10.1016/j.carbon.2011.04.072 · 6.20 Impact Factor
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    ABSTRACT: Here, we report rich and new resonant Raman and infrared (IR) spectral features for several sub-nanometer diameter single wall carbon nanotubes (sub-nm SWCNTs) samples grown using chemical vapor deposition technique operating at different temperatures. We find that the high curvature in sub-nm SWCNTs leads to (i) an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron–phonon coupling, (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and (iii) a clear observation of the IR modes. Furthermore, an analytical approach which includes the effects of curvature into the overlap integral and the energy gap between the van Hove singularities is discussed. Lastly, we show that the phonon spectra for sub-nm SWCNTs obtained from the molecular dynamic simulations which employs a curvature-dependent force field concur with our experimental observations.Graphical abstractResearch highlights► We find that the high curvature in sub-nm SWCNTs leads to the following interesting features in the Raman and IR spectra: ► an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron-phonon coupling, ► an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and ► a clear observation of the infra-red modes.
    Carbon 03/2011; 49(3):869-876. DOI:10.1016/j.carbon.2010.10.033 · 6.20 Impact Factor
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    ABSTRACT: High molecular weight polyaniline / multi-walled carbon nanotube composite films were fabricated using solution processing. Composite films with various weight percentages of multiwalled carbon nanotubes were fabricated. Physical properties of these composites were analyzed by thermogravimetric analysis, tensile testing, and scanning electron microscopy. These results indicate that the addition of multiwalled nanotubes to polyaniline significantly enhances the mechanical properties of the films. In addition, metal–semiconductor (composite) (MS) contact devices were fabricated, and it was observed that the current level in the films increased with increasing multiwalled nanotube content. Furthermore, it was observed that polyaniline containing one weight percent of carbon nanotubes appears to be the most promising composition for applications in organic electronic devices.
    MRS Online Proceeding Library 01/2011; 791. DOI:10.1557/PROC-791-Q10.3
  • S. L. Ren · Ying Wang · M. Holden · A. M. Rao · F. Ni · W. Lee · G. T. Hager · P. C. Eklund ·

    MRS Online Proceeding Library 01/2011; 247. DOI:10.1557/PROC-247-367
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    ABSTRACT: Low-temperature electrical conductivity and Raman scattering are studied as characterization tools for activated carbon fibers, which have a high density of defects and a huge specific surface area. The transport mechanism at low temperature is governed by variablerange hopping, as in other strongly disordered systems. From the Raman spectra obtained, we deduce that the long phenolic fibers are more disordered than the acrylic fibers and that increased specific surface area corresponds to increased disorder. The average in-plane microcrystallite size is about 20–30 Å.
    MRS Online Proceeding Library 01/2011; 209. DOI:10.1557/PROC-209-335

Publication Stats

12k Citations
636.73 Total Impact Points


  • 2000-2014
    • Clemson University
      • Department of Physics and Astronomy
      Clemson, South Carolina, United States
  • 1988-2007
    • University of Kentucky
      • • Center for Applied Energy Research
      • • Department of Physics & Astronomy
      Lexington, Kentucky, United States
  • 1993
    • University of Georgia
      • Department of Chemistry
      Атина, Georgia, United States
  • 1991-1993
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, MA, United States