M. K. Kavitha

Bio: M. K. Kavitha is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Graphene & Graphene oxide paper. The author has an hindex of 3, co-authored 4 publications receiving 13 citations.

Anomalous charge transport in reduced graphene oxide films on a uniaxially strained elastic substrate

Abstract: We investigate temperature-dependent charge transport in reduced graphene oxide (rGO) films coated on flexible polydimethylsiloxane (PDMS) substrates which are subject to uniaxial strain. Variable strain, up to 10%, results in an anisotropic morphology comprising of quasi-periodic linear array of deformations which are oriented perpendicular to the direction of strain. The anisotropy is reflected in the charge transport measurements, when conduction in the direction parallel and perpendicular to the applied strain are compared. Temperature dependence of resistance is measured for different values of strain in the temperature interval 80–300 K. While the resistance increases significantly upon application of strain, the temperature-dependent response shows anomalous decrease in resistance ratio R 80 K/R 300 K upon application of strain. This observation of favorable conduction processes under strain is further corroborated by reduced activation energy analysis of the temperature-dependent transport data. These anomalous transport features can be reconciled based on mutually competing effects of two processes: (i) thinning of graphene at the sites of periodic deformations, which tends to enhance the overall resistance by a purely geometrical effect, and (ii) locally enhanced inter-flake coupling in these same regions which contributes to improved temperature-dependent conduction.

Correlating Chemical Structure and Charge Transport in Reduced Graphene Oxide for Transparent Conductor and Interconnect Applications

Abstract: Graphene oxide is solution-process able and widely tunable semiconductor which has potential applications as transparent electrode materials in organic solar cells as well as chemical sensors and interconnects. It is also amenable to micro-patterning with laser-induced heating. Density of defects and water permeation are among the major factors affecting the degree of conduction in reduced graphene oxide. The wide tunability of conduction is related to the nature and density of defects. In this work, we correlate micro structural parameters of reduced graphene oxide with parameters obtained from our transport studies. The quantitative estimation of defect density from IR and Raman spectroscopy is, however, non-trivial. Here, we outline a procedure for carefully extracting the defect density for graphene oxide films subject to varied degrees of thermal reduction. This density of defect scatterers is then correlated with transport length scales extracted from frequency and temperature dependent charge transport studies. Furthermore, the effect of uniaxial strain on the dc and frequency-dependent ac transport in graphene oxide films is studied towards their potential application as hole-transporting layer for flexible solar cells.

Graphene: Polymer composites as moisture barrier and charge transport layer toward solar cell applications

Abstract: Graphene: polymer composite based electrically conducting films are realized by a facile solution processable method. Ultraviolet Photoelectron Spectroscopy (UPS) measurements on the composite films, reveal a low work function of reduced graphene oxide (rGO) obtained from hydrazine hydrate reduction of graphene oxide (GO). We suggest that the low work function could potentially make rGO: PMMA composite suitable for electron conducting layer in perovskite solar cells in place of traditionally used expensive PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) layer. Further, we demonstrate from the gravimetric experiments conducted on rGO: PMMA films, that the same coating is also resistant to moisture permeation. This latter property can be used to realize a protective coating layer for perovskite films, which are prone to moisture induced degradation. Thus, dual functionality of rGO-PMMA films is demonstrated towards integration with perovskite solar cells. Architecture of perovskite solar cell based on these concepts is proposed.

Graphene interfaced perovskite solar cells: Role of graphene flake size

Abstract: Graphene interfaced inverted planar heterojunction perovskite solar cells are fabricated by facile solution method and studied its potential as hole conducting layer. Reduced graphene oxide (rGO) with small and large flake size and Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) are utilized as hole conducting layers in different devices. For the solar cell employing PEDOT:PSS as hole conducting layer, 3.8 % photoconversion efficiency is achieved. In case of solar cells fabricated with rGO as hole conducting layer, the efficiency of the device is strongly dependent on flake size. With all other fabrication conditions kept constant, the efficiency of graphene-interfaced solar cell improves by a factor of 6, by changing the flake size of graphene oxide. We attribute this effect to uniform coverage of graphene layer and improved electrical percolation network.Graphene interfaced inverted planar heterojunction perovskite solar cells are fabricated by facile solution method and studied its potential as hole conducting layer. Reduced graphene oxide (rGO) with small and large flake size and Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) are utilized as hole conducting layers in different devices. For the solar cell employing PEDOT:PSS as hole conducting layer, 3.8 % photoconversion efficiency is achieved. In case of solar cells fabricated with rGO as hole conducting layer, the efficiency of the device is strongly dependent on flake size. With all other fabrication conditions kept constant, the efficiency of graphene-interfaced solar cell improves by a factor of 6, by changing the flake size of graphene oxide. We attribute this effect to uniform coverage of graphene layer and improved electrical percolation network.

Efros-Shklovskii Variable Range Hopping in Reduced Graphene Oxide Sheets

Abstract: We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.

Thickness-dependent Crack Propagation in Uniaxially Strained Conducting Graphene Oxide Films on Flexible Substrates.

Abstract: We demonstrate that crack propagation in uniaxially strained reduced graphene oxide (rGO) films is substantially dependent on the film thickness, for films in the sub-micron regime. rGO film on flexible polydimethylsiloxane (PDMS) substrate develop quasi-periodic cracks upon application of strain. The crack density and crack width follow contrasting trends as film thickness is increased and the results are described in terms of a sequential cracking model. Further, these cracks also have a tendency to relax when the strain is released. These features are also reflected in the strain-dependent electrical dc and ac conductivity studies. For an optimal thickness (3-coat), the films behave as strain-resistant, while for all other values it becomes strain-responsive, attributed to a favorable combination of crack density and width. This study of the film thickness dependent response and the crack propagation mechanism under strain is a significant step for rationalizing the application of layered graphene-like systems for flexible optoelectronic and strain sensing applications. When the thickness is tuned for enhanced extent of crack propagation, strain-sensors with gauge factor up to ∼470 are realized with the same material. When thickness is chosen to suppress the crack propagation, strain-resistive flexible TiO2- rGO UV photoconductor is realized.

Multiscale Charge Transport in van der Waals Thin Films: Reduced Graphene Oxide as a Case Study

Abstract: Large area van der Waals (vdW) thin films are assembled materials consisting of a network of randomly stacked nanosheets. The multiscale structure and the two-dimensional (2D) nature of the building block mean that interfaces naturally play a crucial role in the charge transport of such thin films. While single or few stacked nanosheets (i.e., vdW heterostructures) have been the subject of intensive works, little is known about how charges travel through multilayered, more disordered networks. Here, we report a comprehensive study of a prototypical system given by networks of randomly stacked reduced graphene oxide 2D nanosheets, whose chemical and geometrical properties can be controlled independently, permitting to explore percolated networks ranging from a single nanosheet to some billions with room-temperature resistivity spanning from 10-5 to 10-1 Ω·m. We systematically observe a clear transition between two different regimes at a critical temperature T*: Efros-Shklovskii variable-range hopping (ES-VRH) below T* and power law behavior above. First, we demonstrate that the two regimes are strongly correlated with each other, both depending on the charge localization length ξ, calculated by the ES-VRH model, which corresponds to the characteristic size of overlapping sp2 domains belonging to different nanosheets. Thus, we propose a microscopic model describing the charge transport as a geometrical phase transition, given by the metal-insulator transition associated with the percolation of quasi-one-dimensional nanofillers with length ξ, showing that the charge transport behavior of the networks is valid for all geometries and defects of the nanosheets, ultimately suggesting a generalized description on vdW and disordered thin films.

Phonon spectra of a two-dimensional solid dusty plasma modified by two-dimensional periodic substrates

Abstract: Phonon spectra of a two-dimensional (2D) solid dusty plasma modulated by 2D square and triangular periodic substrates are investigated using Langevin dynamical simulations. The commensurability ratio, i.e., the ratio of the number of particles to the number of potential well minima, is set to 1 or 2. The resulting phonon spectra show that propagation of waves is always suppressed due to the confinement of particles by the applied 2D periodic substrates. For a commensurability ratio of 1, the spectra indicate that all particles mainly oscillate at one specific frequency, corresponding to the harmonic oscillation frequency of one single particle inside one potential well. At a commensurability ratio of 2, the substrate allows two particles to sit inside the bottom of each potential well, and the resulting longitudinal and transverse spectra exhibit four branches in total. We find that the two moderate branches come from the harmonic oscillations of one single particle and two combined particles in the potential well. The other two branches correspond to the relative motion of the two-body structure in each potential well in the radial and azimuthal directions. The difference in the spectra between the square and triangular substrates is attributed to the anisotropy of the substrates and the resulting alignment directions of the two-body structure in each potential well.

Laser‐Assisted Rapid Fabrication of Large‐Scale Graphene Oxide Transparent Conductors

Abstract: Although graphene films are the main candidate for replacing scarce and brittle metal‐oxide transparent conductors, the current challenges on graphene deposition and patterning are severely limiting their commercial application. Here, materials and methods that allow large‐scale, efficient, and low‐cost fabrication of highly transparent conductors in a few minutes are demonstrated. First, a low‐cost graphene oxide (GO) solution through spray coating is deposited, followed by simultaneous reduced graphene oxide reduction and thinning through a low‐cost nanosecond fiber laser. It is shown that a 1064 nm master oscillator power amplifier laser enhances the conductivity of GO coated sample by ≈60 times, with excellent optical transparency of 90%. The laser parameters are adjusted, and creation of various shades of transparency, conductivity, and a full ablation are demonstrated. This way, complex GO‐based circuits can be produced rapidly. Compared to the existing techniques that require chemical vapor deposition, and chemical/thermal reduction, this technique is considerably more accessible, replicable, and can open doors for various applications in optoelectronics, energy storage/harvesting, and sensing. The electrical, chemical, and optical properties of the samples are characterized before and after laser treatment, through optical profilometry, scanning electron microscopy, and (UV, Raman, and energy dispersive X‐ray) spectroscopy and applications in transparent conductors, and laser‐patterned high‐resolution and miniaturized interdigitated sensors are shown.