Manu Jaiswal

In this paper, a wet-dry hybrid technique to transfer patterned reduced graphene oxide (rGO) thin film to arbitrary substrates at predetermined locations without using any chemicals is reported. The transfer process involves water-assisted delamination of rGO, followed by dry transfer to an acceptor substrate using viscoelastic stamp. Patterned reduced graphene oxide films are transferred to silicon dioxide (SiO2/Si) substrate to begin with. Subsequently, the method is deployed to transfer rGO to different polymer substrates such as poly(methyl methacrylate) (PMMA), and crosslinked poly(4-vinylphenol) (c-PVP), which are commonly used as gate dielectric in flexible electronic applications. The credibility of the transfer process with precise spatial positioning on the target substrate leads to fabrication of freely suspended reduced graphene oxide membrane towards nanoelectromechanical systems (NEMS) based devices such as nanomechanical drum resonators.

Recent studies on twisted bilayer and multi-layer graphene have opened new avenues of research in condensed matter physics. These artificially made superlattices showed very intriguing electrical properties, including superconductivity at certain commensurate rotation angles between the stacked layers. Even though electrical properties are studied to some extent, the effect of rotation angle on the thermal properties has not received adequate attention. In particular, the thermal expansion of these systems is not well understood. In this direction, we have studied thermal expansion coefficient of few-layer graphene samples with rotational misorientation. These samples were synthesized using chemical vapor deposition technique from a solid carbon precursor. The Raman spectra of these samples indicated the presence of rotational stacking faults, which was confirmed by selected area electron diffraction measurements. Finally, the in-plane thermal expansion coefficient of this system was obtained using temperature dependent Raman spectroscopy for T = 4.5 to 300K. Thermal expansion coefficient was determined to be negative in this temperature range and the corresponding room temperature value was obtained to be ≈ -3.47 × 10-6 K-1. This value is comparable to that of few- layer graphene. We suggest that the thermal expansion of multilayer graphene with rotational stacking faults follows the thermal expansion of the individual few-layer crystallites rather than showing bulk graphitic behavior.

Graphene oxide (GO) membranes possess a hierarchical microstructure, with well-ordered crystalline lamellae combining to form a macroscopic membrane. Water can intercalate in GO either in the sub-nanometer interlayer spaces or in the gaps between the lamellae known as voids; distinguishing the contribution of these two has been challenging. Addressing this challenge, we systematically study various properties of GO membranes exposed to controlled humidity levels ranging from 0% to 90% RH. Thickness-dependent dynamic vapor sorption is used to quantify the water content under different humidity environments. Complementing the vapor sorption studies, the AC impedance response of the GO membrane is determined at different humidity values. Our findings suggest that (a) most water gets absorbed in interlayer spaces at low humidity (<25% RH), (b) the fraction of water in the void spaces increases with RH%, (c) the lower bound for the dielectric constant of confined water is estimated to be ϵwater > 17, and (d) the conductivity increases by 5 to 6 orders of magnitude over a narrow range of water content (13 wt% to 31 wt%). The rapid increase in conductivity over a narrow range of water content suggests a percolative process for the protons. The dielectric constant estimates suggest that confined water behaves distinctly differently in a hydrophilic environment than in a hydrophobic one.

Absorption of visible light and separation of photogenerated charges are two primary pathways to improve the photocurrent performance of semiconductor photoelectrodes. Here, we present a unique design of tricomponent photocatalyst comprising of TiO2 multileg nanotubes (MLNTs), reduced graphene oxide (rGO) and CdS nanoparticles. The tricomponent photocatalyst shows a significant red-shift in the optical absorption (∼2.2 eV) compared to that of bare TiO2 MLNTs (∼3.2 eV). The availability of both inner and outer surfaces areas of MLNTs, the visible light absorption of CdS, and charge separating behavior of reduced graphene oxide layers contribute coherently to yield a photocurrent density of ∼11 mA cm-2 @ 1 V vs. Ag/AgCl (100 mW cm-2, AM 1.5 G). Such a high PEC performance from TiO2/rGO/CdS photoelectrode system has been analyzed using diffused reflectance (DRS) and electrochemical impedance (EIS) spectroscopy techniques. The efficient generation of charge carriers under light irradiation and easy separation because of favourable band alignment, are attributed to the high photoelectrochemical current density in these tricomponent photocatalyst systems.

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.

Two-dimensional layered materials have been used as matrices to study the structure and dynamics of trapped water and ions. Here, we demonstrate unique features of proton transport in layered hexagonal boron nitride membranes with edge-functionalization subject to hydration. The hydration-independent interlayer spacing indicates the absence of water intercalation between the h-BN sheets. An 18-fold increase in water sorption is observed upon amine functionalization of h-BN sheet edges. A 7-orders of magnitude increase in proton conductivity is observed with less than 5% water loading attributable to edge-conduction channels. The extremely low percolation threshold and non-universal critical exponents (2.90 ≤ α ≤ 4.43), are clear signatures of transport along the functionalized edges. Anomalous thickness dependence of conductivity is observed and its plausible origin is discussed.

The presence of twist angles between layers of two-dimensional materials has a profound impact on their physical properties. Turbostratic multilayer graphene is a system containing a distribution of rotational stacking faults, and these interfaces also have variable twist angles. In this work, we examine the influence of turbostratic single-layer graphene content on the in-plane thermal conductivity of a defect free multilayer graphene system with low defect density. Detailed Raman mode analysis is used to quantify the content of turbostratic single-layer graphene in the system while complementing insight is obtained from selected area electron diffraction studies. Thermal transport in these systems is investigated with Raman optothermal technique supported with finite element analysis simulations. Thermal conductivity of AB-stacked graphene diminishes by a factor of 2.59 for 1% of turbostratic single-layer graphene content, while the decrease at 19% turbostratic content is by an order in magnitude. Thermal conductivity broadly obeys the relation, κ∼exp(−F), where F is the fraction of turbostratic single-layer graphene content in the system.

We investigate thermal transport in three-dimensional graphene aerogel networks at elevated temperatures. The aerogels are solution-processed from graphene-oxide flakes using amine-based linkers and then partially reduced to impart stability in the chemical structure at elevated temperatures. Thermal conductivity of the system is estimated using steady-state electrothermal technique in vacuum in the temperature interval from 30 to 200 °C. The thermal conductivity value is κ ∼ 0.2 W/mK at room temperature, and is found to be weakly dependent on temperature across the entire temperature interval. To examine the microscopic origin of this stable response, the thermal conductivity estimates are complemented with insights from temperature-dependent transient electrothermal response. We show that the temperature stable thermal insulation behaviour observed in this system can be attributed to two factors: point-defect scattering at the flake level from the remnant oxygen-functionalities which dominates over Umklapp scattering processes, and another contribution that arises from interfacial thermal resistance between flakes. The partial reduction thus achieves a delicate balance between imparting chemical stability while also retaining the dominance of point-defect phonon scattering, where the latter contributes to temperature stable thermal conductivity.

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.

Wrinkles significantly influence the physical properties of layered 2D materials, including graphene. In this work, we examined thermal transport across wrinkles in vertical assemblies of few-layer graphene crystallites using the Raman optothermal technique supported by finite-element analysis simulations. A high density of randomly oriented uniaxial wrinkles were frequently observed in the few-layer graphene stacks which were grown by chemical vapor deposition and transferred on Si/SiO2substrates. The thermal conductivity of unwrinkled regions was measured to be,κ∼ 165 W m−1K−1. Measurements at the wrinkle sites revealed local enhancement of thermal conductivity, withκ∼ 225 W m−1K−1. Furthermore, the total interface conductance of wrinkled regions decreased by more than an order of magnitude compared to that of the unwrinkled regions. The physical origin of these observations is discussed based on wrinkle mediated decoupling of the stacked crystallites and partial suspension of the film. Wrinkles are ubiquitous in layered 2D materials, and our work demonstrates their strong influence on thermal transport.