Manu Jaiswal

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.

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.

Conventional graphene oxide (GO) is characterized by low sp2 content in a sp3 rich matrix, which is responsible both for electrical insulation and water super-permeation. Upon reduction, electrical conduction is achieved at the expense of water permeation ability. Here, we demonstrate that charge conduction and water permeation can be simultaneously restricted in a functionalized form of GO. Gravimetric studies reveal that diffusion of water vapor through a glassy polymer membrane is arrested by loading a hydrophobic form of GO (H-GO) in the polymer matrix, even as such, water inhibition cannot be realized by substantially increasing the thickness of the bare polymer. As an application, the ability of the coating to impede the degradation of methyl ammonium lead iodide films under high humidity conditions is demonstrated. At the same time the H-GO film has a resistance over 107 times higher when compared to thermally reduced GO of similar sp2 fraction. We attribute this unique behavior to the presence of a sub-micron matrix of GO with simultaneous presence of large (∼9.5 Å) and small (∼4.7 Å) interlayer spacing. This leads to disruption of the spatially distributed percolation pathways for electrical charge, and it also serves to block the nanocapillary networks for water molecules.

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.

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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.

The confinement of water between sub-nanometer bounding walls of layered two-dimensional materials has generated tremendous interest. Here, we examined the influence of confined water on the mechanical and electromechanical response of graphene oxide films, prepared with variable oxidative states, casted on polydimethylsiloxane substrates. These films were subjected to uniaxial strain under controlled humid environments (5 to 90% RH), while dc transport studies were performed in tandem. Straining resulted in the formation of quasi-periodic linear crack arrays. The extent of water intercalation determined the density of cracks formed in the system thereby, governing the electrical conductance of the films under strain. The crack density at 5% strain, varied from 0 to 3.5 cracks mm-1 for hydrated films and 8 to 22 cracks mm-1 for dry films, across films with different high oxidative states. Correspondingly, the overall change in the electrical conductance at 5% strain was observed to be ∼5 to 20 folds for hydrated films and ∼20 to 35 folds for the dry films. The results were modeled with a decrease in the in-plane elastic modulus of the film upon water intercalation, which was attributed to the variation in the nature of hydrogen bonding network in graphene oxide lamellae.

Interaction of water and water-based solvents with graphene oxide (GO) has attracted much attention, due to the ability of GO to serve as a highly effective water filtration membrane. In this work, we study the evolution of the structure of GO in a partially reduced form, before and after being hydrated in high humidity conditions. X-ray diffraction (XRD) studies reveal that progressive thermal reduction leads to the increase in the microstructural disorder in the stacking of GO flakes. However, upon hydration of partially reduced GO, microstructural ordering is revealed. This ordered state is characterized by two XRD peaks with substantially smaller full-width-at-half-maximum (FWHM), when compared to the pre-hydration state. The peak corresponding to the sp 3 regions has larger d-spacing of ∼9.7 Å and an FWHM ∼6 times smaller compared to pre-hydration state, while the other peak corresponds to the ordered sp 2 regions with a d-spacing of ∼3.3 Å, observed at the characteristic graphitic peak position. Gravimetry studies on suspended films reveal both accelerated and diminished water permeation rates upon annealing when compared to unreduced GO films, which can be attributed to void-assisted permeation in the microstructurally disordered films. The hydrated films in a similar way show a permeation behavior that involves either the increase or decrease in water permeation rates in comparison with pre-hydrated samples. We reconcile to the gravimetry outcomes by suggesting the possibilities of both super-permeating channels and void assisted permeation, and the contribution of each of the mechanisms to the permeation flux.

We study the effect of geometrical confinement of reduced graphene oxide (rGO) films on strain-induced wrinkling patterns, which strongly affects their electromechanical response. Quasi-periodic wrinkle patterns are characteristics of uniaxially strained large-area thin films, both free-standing and supported on soft substrates. The universality of their scaling behaviour is known across a wide spectrum of length scales ranging from curtains to atomically thin crystals. Distinct wrinkle patterns oriented orthogonal to those on large-area films are observed in narrow micro-stripes of electrically conducting rGO films on polydimethylsiloxane (PDMS) substrates. Furthermore, as the width is reduced from 100 μm to 5 μm, the onset of the electromechanical response progressively shifts to lower strain, and importantly a breakdown is observed beyond a critical strain of less than 2.5%. Morphology studies suggest this irreversible response to arise as a consequence of local delamination of the strained micro-stripes subsequent to crack propagation. In contrast, large-area films have subdued electromechanical response which is also characterized by reversibility despite cracking. The altered electromechanical response, when going from large-area films to narrow micro-stripes, is reconciled as a consequence of the peculiar wrinkle pattern in the strained films arising from the free edge of the micro-stripes.