Palanisamy Ramesh

Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups

The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon-carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.

Effect of nitrophenyl functionalization on the magnetic properties of epitaxial graphene.

Graphene displays unprecedented electronic properties including room-temperature ballistic transport and quantum conductance, and because of its small spin-orbit interaction, graphene has the potential to function as the building block of future spintronic devices. Theoretical calculations indicate that a defective graphene sheet will be simultaneously semiconducting and magnetic; thus it would act as a room-temperature magnetic semiconductor. Recently, ferromagnetic ordering at room temperature has been observed by magnetometry measurements on bulk samples of reduced graphene oxide.

Dependence of the thermal conductivity of two-dimensional graphite nanoplatelet-based composites on the nanoparticle size distribution

We report the thermal conductivities of graphite nanoplatelet-epoxy composites prepared by exfoliation of natural graphite flakes of varying lateral sizes. We found that utilization of natural graphite flakes of the optimum lateral dimensions (∼200-400 µm) as a starting material for exfoliation significantly enhanced the thermal conductivity of the composite. In order to understand this enhancement we developed a procedure for evaluation of the particle size distribution of graphite nanoplatelets and correlated the measured distributions with the resulting thermal conductivities. In order to expand the scope of our study we applied our statistical and thermal analysis to commercially available graphite nanoplatelet materials.

Functionalized Single-Walled Carbon Nanotube-Based Fuel Cell Benchmarked Against US DOE 2017 Technical Targets

Chemically modified single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization were utilized for the fabrication of SWNT thin film catalyst support layers (CSLs) in polymer electrolyte membrane fuel cells (PEMFCs), which were suitable for benchmarking against the US DOE 2017 targets. Use of the optimum level of SWNT -COOH functionality allowed the construction of a prototype SWNT-based PEMFC with total Pt loading of 0.06 mgPt/cm2 - well below the value of 0.125 mgPt/cm2 set as the US DOE 2017 technical target for total Pt group metals (PGM) loading. This prototype PEMFC also approaches the technical target for the total Pt content per kW of power (<0.125 gPGM/kW) at cell potential 0.65 V: a value of 0.15 gPt/kW was achieved at 80°C/22 psig testing conditions, which was further reduced to 0.12 gPt/kW at 35 psig back pressure.

Electro-oxidized Epitaxial Graphene Channel Field-Effect Transistors with Single-Walled Carbon Nanotube Thin Film Gate Electrode

We report the effect of electrochemical oxidation in nitric acid on the electronic properties of epitaxial graphene (EG) grown on silicon carbide substrates; we demonstrate the availability of an additional reaction channel in EG, which is not present in graphite but which facilitates the introduction of the reaction medium into the graphene galleries during electro-oxidation. The device performance of the chemically processed graphene was studied by patterning the EG wafers with two geometrically identical macroscopic channels; the electro-oxidized channel showed a logarithmic increase of resistance with decreasing temperature, which is ascribed to the scattering of charge carriers in a two-dimensional electronic gas, rather than the presence of an energy gap at the Fermi level. Field-effect transistors were fabricated on the electro-oxidized and pristine graphene channels using single-walled carbon nanotube thin film top gate electrodes, thereby allowing the study of the effect of oxidative chemistry on the transistor performance of EG. The electro-oxidized channel showed higher values for the on-off ratio and the mobility of the graphene field-effect transistor, which we ascribe to the availability of high-quality internal graphene layers after electro-oxidation of the more defective top layers. Thus, the present oxidative process provides a clear contrast with previously demonstrated covalent chemistry in which sp(3) hybridized carbon atoms are introduced into the graphitic transport layer of the lattice by carbon-carbon bond formation, thereby opening an energy gap.

Enhanced photosensitivity of electro-oxidized epitaxial graphene

We report the enhanced photosensitivity of epitaxial graphene (EG) after electrochemical oxidation in nitric acid. The onset of photoconductivity appears at a photon energy of ∼1.7 eV while the responsivity reaches 2.5 A/W at a wavelength of 470 nm (blue light, energy 2.64 eV) and further increases to 200 A/W in the UV spectral range (3.5 eV, 350 nm). The observed photoresponse is attributed to the formation of deep traps at the electro-oxidized EG interface, which release charge carriers under illumination and prolong the life time of the photocarriers. Potential applications of electro-oxidized EG in ultraviolet photodetection are discussed.

Carbon nanotube free-standing membrane of Pt/SWNTs as catalyst layer in hydrogen fuel cells

A simple and promising fuel-cell architecture is demonstrated using a carbon nanotube free-standing membrane (CNTFSM) made from Pt supported on purified single-walled carbon nanotubes (Pt/SWNT), which act as the catalyst layer in a hydrogen proton exchange membrane fuel cell without the need for Nafion in the catalyst layer. The CNTFSM made from Pt/SWNT at a loading of 0.082 mg Pt cm–2 exhibits higher performance with a peak power density of 0.675 W cm–2 in comparison with a commercially available E-TEK electrocatalyst made of Pt supported on XC-72 carbon black, which had a peak power density of 0.395 W cm–2 at a loading of 0.084 mg Pt cm–2 also without Nafion in the catalyst layer.

Oxidized Graphite Nanoplatelets as an Improved Filler for Thermally Conducting Epoxy-Matrix Composites

We report a 40% improvement of the thermal conductivity of graphite nanoplatelets–epoxy composites by chemical functionalization of graphite nanoplatelets utilizing nitric acid treatment, which also serves to enhance the spreadability of the material. FTIR and Raman spectroscopy confirmed the presence of a variety of oxygen functional groups at the edges and basal plane of the functionalized graphite nanoplatelets, which contributed to improved interaction with the polymer matrix. A comparative statistical analysis of the particle size distributions in pristine and functionalized graphite nanoplatelets based on scanning electron microscopy showed an increasing degree of exfoliation of the functionalized material. We compare the performance of the functionalized graphite nanoplatelets and carbon nanotubes as fillers in the polymer matrix and discuss the prospects for utilization of graphite nanoplatelets-based thermal interface materials in electronic packaging.

Effect of functionalization on the electrostatic charging, tunneling, and Raman spectroscopy of epitaxial graphene

The authors report the effects of radical functionalization on the electrostatic force microscopy (EFM), the scanning tunneling spectra (STS), and Raman spectroscopy of epitaxial graphene. The EFM studies show the existence of layer-dependent trapped charges in the pristine graphene. The uniform enhancement of energy gap is observed through STS. Raman spectra show nonuniformly distributed D-band intensities throughout the functionalized sample as a result of the inhomogeneous distribution of covalent bonds to the graphene sheets. The functionalization chemistry has a marked effect on the homogeneity of the electrostatic charge and leads to an increase of the energy of the band gap.

Ruthenium oxide - single walled carbon nanotube composite based high energy supercapacitor

Ruthenium oxide (RuO2) nanoparticles were supported on the outer surface of single walled carbon nanotubes (SWNTs) using chemical reduction technique. The nanocomposite was employed as an electrode material for charge storage capacitor using ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte solution. The electrochemical charge storage properties of the composite were investigated using two electrode cyclic voltammetry and galvanostatic charge-discharge techniques. The specific capacitance and energy density of the RuO2-SWNT nanocomposite electrode material based on the fabricated coin cell supercapacitor were measured to be as high as 174 F/g and 74 Wh/kg at current density of 1 A/g.