Charudatta Galande

Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide

Graphene oxide (GO) has drawn tremendous interest as a tunable precursor in numerous areas, due to its readily manipulable surface. However, its inhomogeneous and nonstoichiometric structure makes achieving chemical control a major challenge. Here, we present a room-temperature based, controlled method for the stepwise reduction of GO, with evidence of sequential removal of each organic moiety. By analyzing signature infrared absorption frequencies, we identify the carbonyl group as the first to be reduced, while the tertiary alcohol takes the longest to be completely removed from the GO surface. Controlled reduction allows for progressive tuning of the optical gap from 3.5 eV down to 1 eV, while XPS spectra show a concurrent increase in the C/O ratio. This study is the first step toward selectively enhancing the chemical homogeneity of GO, thus providing greater control over its structure, and elucidating the order of removal of functional groups and hydrazine-vapor reduction.

Quasi-Molecular Fluorescence from Graphene Oxide

Aqueous dispersions of graphene oxide (GO) have been found to emit a structured, strongly pH-dependent visible fluorescence. Based on experimental results and model computations, this is proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds, formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of graphene. Sharp and structured emission and excitation features resembling the spectra of molecular fluorophores are present near 500 nm in basic conditions. The GO emission reversibly broadens and red-shifts to ca. 680 nm in acidic conditions, while the excitation spectra remain very similar in shape and position, consistent with excited state protonation of the emitting species in acidic media. The sharp and structured emission and excitation features suggest that the effective fluorophore size in the GO samples is remarkably well defined.

Carbon nanotube-nanocup hybrid structures for high power supercapacitor applications.

Here, we design and develop high-power electric double-layer capacitors (EDLCs) using carbon-based three dimensional (3-D) hybrid nanostructured electrodes. 3-D hybrid nanostructured electrodes consisting of vertically aligned carbon nanotubes (CNTs) on highly porous carbon nanocups (CNCs) were synthesized by a combination of anodization and chemical vapor deposition techniques. A 3-D electrode-based supercapacitor showed enhanced areal capacitance by accommodating more charges in a given footprint area than that of a conventional CNC-based device.

Intraband conductivity response in graphene observed using ultrafast infrared-pump visible-probe spectroscopy

Graphene, a monolayer of carbon atoms arranged in a hexagonal pattern, provides a unique two-dimensional (2D) system exhibiting exotic phenomena such as quantum Hall effects, massless Dirac quasiparticle excitations and universal absorption&conductivity. The linear energy-momentum dispersion relation in graphene also offers the opportunity to mimic the physics of far-away relativistic particles like neutron stars and white dwarfs. In this letter, we perform a counterintuitive ultrafast pump-probe experiment with high photon energies to isolate the Drude-like intraband dynamics of photoexcited carriers. We directly demonstrate the relativistic nature of the photoexcited Dirac quasiparticles by observing a nonlinear scaling of the response with the density of photoexcited carriers. This is in striking contrast to the linear scaling that is usually observed in conventional materials. Our results also indicate strong electron-phonon coupling in graphene, leading to a sub-100 femtosecond thermalization between high energy photoexcited carriers and optical phonons.

Direct imaging of charge transport in progressively reduced graphene oxide using electrostatic force microscopy.

Graphene oxide (GO) has emerged as a multifunctional material that can be synthesized in bulk quantities and can be solution processed to form large-area atomic layered photoactive, flexible thin films for optoelectronic devices. This is largely due to the potential ability to tune electrical and optical properties of GO using functional groups. For the successful application of GO, it is key to understand the evolution of its optoelectronic properties as the GO undergoes a phase transition from its insulating and optically active state to the electrically conducting state with progressive reduction. In this paper, we use a combination of electrostatic force microscopy (EFM) and optical spectroscopy to monitor the emergence of the optoelectronic properties of GO with progressive reduction. EFM measurements enable, for the first time, direct visualization of charge propagation along the conducting pathways that emerge on progressively reduced graphene oxide (rGO) and demonstrate that with the increasing degree of reduction, injected charges can rapidly migrate over a distance of several micrometers, irrespective of their polarities. Direct imaging reveals the presence of an insurmountable potential barrier between reduced GO (rGO) and GO, which plays the decisive role in the charge transport. We complement charge imaging with theoretical modeling using quantum chemistry calculations that further demonstrate that the role of barrier in regulating the charge transport. Furthermore, by correlating the EFM measurements with photoluminescence imaging and electrical conductivity studies, we identify a bifunctional state in GO, where the optical properties are preserved along with good electrical conductivity, providing design principles for the development of GO-based, low-cost, thin-film optoelectronic applications.

Experimental Determination of the Electronic Density of States for Graphene Oxide

CPS is unique in that it specifically measures absorption due to the formation of charge carriers, while being uninfluenced by absorption due to vibrational states of the lattice, or non-mobile impurity states. The set-up is shown in Figure 1. The GO sample lies on top of an ITO coated quartz slide, which is anchored to a copper plate within an evacuated optical cryostat. Electrical contact is made between the copper plate and a gold pad deposited near the edge of the ITO layer. The sample is illuminated with light from an optical parametric amplifier which emits a series of 120 fs light pulses at a 1 kHz repetition rate. Light absorption occurs in the GO film, producing electron-hole pairs. The ITO acts as an acceptor for negative photoexcited charge, causing the electron-hole pairs to separate across the GO / ITO interface. This produces an ac voltage, whose frequency is equal to the laser repetition rate. An applied DC potential causes the negative charge to be attracted to the ITO/quartz interface, amplifying the capacitive photocurrent signal. Figure 2 shows a sample capacitive photocurrent spectrum, while Fig. 3 shows the DOS extracted from the data. Three peaks are observed at approximately 0.7, 1.6 and 3.2 eV (marked as ▲, ●, and ■). Strong minima occur at 2.1 eV, and beyond 4 eV, indicating gaps in the density of states. Similar results are observed for all measured GO samples. The DOS extracted from the data consists of the π / π* peaks, plus three additional mid gap states at -0.5 eV, 0.4 eV and 1.5 eV. For comparison, the density of states for graphene is also shown. The π and π* peaks are asymmetric with respect to the Fermi energy due to interaction with the substrate / environment. 3 The ■ transition in the CPS correlates with the 320 nm shoulder observed in the absorbance spectra of GO, and attributed to n-π* transitions of C=O. This suggests that the -0.5 eV state is the non-bonding orbital of the oxygen atoms. 5

Local charge transfer doping in suspended graphene nanojunctions

We report electronic transport measurements in nanoscale graphene transistors with gold and platinum electrodes whose channel lengths are shorter than 100 nm and compare them with transistors with channel lengths from 1 μm to 50 μm. We find a large positive gate voltage shift in charge neutrality point (NP) for transistors made with platinum electrodes but negligible shift for devices made with gold electrodes. This is consistent with the transfer of electrons from graphene into the platinum electrodes. As the channel length increases, the disparity between the measured NP using gold and platinum electrodes disappears.