Resonance Raman spectroscopy of twisted interfaces in turbostratic multilayer graphene.

Turbostratic multilayer graphene presents a unique system with a large number of twisted interfaces with variable twist angles. In this work, we have systematically studied the laser excitation energy dependence of the Raman modes of turbostratic graphene. The combination of 4 different laser energies is shown to be important to reveal the twist angles ranging from 5∘to 30∘present at the same lateral position of the sample. Rotational or R-modes and D-like modes are observed, which directly arise from additional momentum transfer from the potential of corresponding superlattices. Trends in their dispersion and intensity are discussed. The resonant window for laser excitation indicates lowered positions of the van Hove singularities. Furthermore, an anomalous broadening factor of 0.17-0.265 eV is estimated for the resonance window when compared to the literature on isolated twisted bilayer graphene. Interestingly, a weak dependence of the R-modes on the laser wavelength is also observed. Finally, the dispersion of the 2D modes is also presented.

Percolative proton transport in hexagonal boron nitride membranes with edge-functionalization.

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.

Dielectric response and proton transport in water confined in graphene oxide.

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.

Therapeutic effects of oral fluralaner in pet rabbits with severe sarcoptic mange (Sarcoptes scabiei).

Sarcoptic mange is one of the most severe, highly contagious, and fatal ectoparasitic infestations of rabbits. Fluralaner, an isoxazoline class of oral ectoparasiticide, is considered as a very potent acaricide. The present study aimed to evaluate the therapeutic efficacy of oral fluralaner in pet rabbits with severe spontaneous Sarcoptes scabiei infestation. A total of eight un-neutered pet rabbits, tested positive for S. scabiei by microscopy of skin scrapings, were enrolled. Seven rabbits had severe clinical infestation (score 5), while remaining one rabbit had moderate disease clinical signs (score <3). A single oral dose equivalent to 25 mg/kg of fluralaner was administered to each rabbit. On day 14 post-therapy, marked improvements in the skin lesions were observed; severely infested rabbits had a clinical score of 3, while the moderately infested rabbit had a score of 1. However, none of the rabbits tested negative for S. scabiei. On day 30 post-therapy, complete clinical recovery was recorded in all rabbits (Score 0), but, a complete parasitological clearance was not recorded except to the moderately infested rabbit. All rabbits were tested negative for S. scabiei on day 45 post-therapy. Therefore, a single oral dose of fluralaner at a 25 mg/kg was found to be effective in the treatment of severe sarcoptic mange in pet rabbits and no additional topical or systemic medications were needed. Further studies in a larger number of individuals with a bigger spectrum of disease severities (i.e. more moderate/mild) are needed to comprehensively document the safety and efficacy of this drug in mangy rabbits.

Intercalated water mediated electromechanical response of graphene oxide films on flexible substrates.

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-1for hydrated films and 8 to 22 cracks mm-1for 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.

Thermal transport across wrinkles in few-layer graphene stacks.

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/SiO2 substrates. The thermal conductivity of unwrinkled regions was measured to be, κ ∼ 165 W m-1 K-1. Measurements at the wrinkle sites revealed local enhancement of thermal conductivity, with κ ∼ 225 W m-1 K-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.

Chemical-free transfer of patterned reduced graphene oxide thin films for large area flexible electronics and nanoelectromechanical systems.

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.

High photoelectrochemical performance of reduced graphene oxide wrapped, CdS functionalized, TiO2 multi-leg nanotubes.

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.

A modified bulge test for in-situ study of ionic permeation properties of membranes under continuously tunable, uniform pressure.

The bulge test is a well-known material test to measure the mechanical properties of metal plates, thin films, and membranes. Also, two different experimental setups are needed to apply pressure and make a measurement. In this work, we describe a modified bulge test to both apply pressure and measure the electrical and ionic permeation properties of membranes in situ. A membrane, clamped at its periphery, with a circular window for measurement, is sandwiched between two liquids. The liquids serve dual purpose by facilitating the application of differential pressure and thus stress, by controlling the extent of immersion of the membrane in the liquid below the membrane, as well as enabling measurement of electrical and mass percolation properties. This was achieved with a stepper motor, a load cell, and a microcontroller. Relevant mathematical models are developed and discussed. Nafion was used to test and validate this approach, using electroimpedance spectroscopy in a 2-electrode configuration with gallium on both sides and in a 3-electrode configuration with electrolyte on one side and gallium on the other. Frequency-dependent response was modeled using equivalent circuits. The resistance of Nafion increases with the depth of immersion and therefore applied pressure. For Nafion in the 2-electrode configuration, conductivity was calculated to be ∼6.4 × 10-3 S/cm at the equilibrium position, where stress on the membrane is zero. This value matches well with existing literature values for partially hydrated Nafion. Also, it was observed that the response is symmetric about the equilibrium position.

Nanostructuring mechanical cracks in a flexible conducting polymer thin film for ultra-sensitive vapor sensing.

The swelling of electrically conducting polymer films upon absorption of vapors like alcohol or moisture is widely known. However, this swelling leads to feeble changes in charge transport characteristics. We demonstrate a colossal enhancement (from ∼6% to 108%) in the vapor-induced resistance change for a representative system, poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS). This is achieved when the films are nanostructured by strain-induced quasi-periodic parallel cracks, which is then followed by crack engineering. The cracks are nanostructured such that the charge carrier percolation pathways are nearly turned off in the absence of alcohol vapor or at low humidity. These percolation pathways are restored upon alcohol vapor or humidity exposure. When used as an alcohol sensor, this system shows ultra-high sensitivity of 106 for methanol vapors, when compared to ethanol vapors (2 × 102). When used as a humidity sensor in the range 60-100% RH, a resistance ratio of 1.5 × 102 is realized. The different extent of response to alcohol vapors and humidity is attributed to the dominance of the surface ionic conduction process in the former. These sensing characteristics are achieved with short response and recovery time (<5 s). The developed sensing platform outperforms commercial portable breath analyzers. While cracks have been utilized for developing ingenious strain sensors in the literature, here we demonstrate an approach based on the same that substantially amplifies vapor response.

Probing the electric double-layer capacitance in a Keggin-type polyoxometalate ionic liquid gated graphene transistor.

A variety of device applications has been proposed using polyoxometalate-based ionic liquids. However, the assembly of large polyoxometalate ions on surfaces and the associated interfacial properties are not well understood, particularly since the assembly is influenced by steric effects and stronger ion-ion interactions. In this study, graphene transistors gated with a polyoxometalate-based ionic liquid were probed with in situ Raman spectroscopy and charge transport studies. The ionic liquid comprised Cu-substituted lacunary Keggin anions, [PW11O39Cu]5-, which were surrounded by tetraoctyl ammonium cations, (C32H68N)+. The application of gate voltage caused these ions to assemble at the interface with graphene, which resulted in a shift of the Fermi level of the graphene monolayer grown on a copper foil. The shift was determined by the quantum capacitance, Cq, of graphene in series with the electric-double layer capacitance. Estimates of the electric-double layer thickness, spatial density of the ions and temporal rate of the assembly of the electric double-layer were obtained. This study provides insights into the microscopic understanding of the electric double-layer formation at the graphene interface.

Breakdown of water super-permeation in electrically insulating graphene oxide films: role of dual interlayer spacing.

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.

Swelling kinetics and electrical charge transport in PEDOT:PSS thin films exposed to water vapor.

We report the swelling kinetics and evolution of the electrical charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films subjected to water vapor. Polymer films swell by the diffusion of water vapor and are found to undergo structural relaxations. Upon exposure to water vapor, primarily the hygroscopic PSS shell, which surrounds the conducting PEDOT-rich cores, takes up water vapor and subsequently swells. We found that the degree of swelling largely depends on the PEDOT to PSS ratio. Swelling driven microscopic rearrangement of the conducting PEDOT-rich cores in the PSS matrix strongly influences the electrical charge transport of the polymer film. Swelling induced increase as well as decrease of electrical resistance are observed in polymer films having different PEDOT to PSS ratio. This anomalous charge transport behavior in PEDOT:PSS films is reconciled by taking into account the contrasting swelling behavior of the PSS and the conducting PEDOT-rich cores leading to spatial segregation of PSS in films with PSS as a minority phase and by a net increase in mean separation between conducting PEDOT-rich cores for films having abundance of PSS.

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

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

Photo-electrochemical properties of graphene wrapped hierarchically branched nanostructures obtained through hydrothermally transformed TiO2 nanotubes.

Hierarchically structured nanomaterials play an important role in both light absorption and separation of photo-generated charges. In the present study, hierarchically branched TiO2 nanostructures (HB-MLNTs) are obtained through hydrothermal transformation of electrochemically anodized TiO2 multi-leg nanotubes (MLNT) arrays. Photo-anodes based on HB-MLNTs demonstrated 5 fold increase in applied bias to photo-conversion efficiency (%ABPE) over that of TiO2 MLNTs without branches. Further, such nanostructures are wrapped with reduced graphene oxide (rGO) films to enhance the charge separation, which resulted in ∼6.5 times enhancement in %ABPE over that of bare MLNTs. We estimated charge transport (η tr) and charge transfer (η ct) efficiencies by analyzing the photo-current data. The ultra-fine nano branches grown on the MLNTs are effective in increasing light absorption through multiple scattering and improving charge transport/transfer efficiencies by enlarging semiconductor/electrolyte interface area. The charge transfer resistance, interfacial capacitance and electron decay time have been estimated through electrochemical impedance measurements which correlate with the results obtained from photocurrent measurements.

Wrinkle and crack-dependent charge transport in a uniaxially strained conducting polymer film on a flexible substrate.

We investigate charge transport in poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) films on functionalized polydimethylsiloxane (PDMS) substrates under varying uniaxial strain up to 16%. Strong anisotropy in transport is observed at a large applied strain (ε > 4%), which is understood in terms of an extrinsic process, involving a change in density of cracks from a few cracks per mm at ε = 4% to >100 cracks per mm at ε = 16%. The quasi-periodic cracks are aligned perpendicular to the stretching direction. A strain-history dependent response of the resistance of PEDOT:PSS films cycled through a uniaxial strain up to 4% is also observed, for current paths which are both parallel and perpendicular to the direction of stretching. The resistance-strain plots of strained PEDOT:PSS films for the second and subsequent few strain cycles follow the reverse path of the previous strain cycle. This unique strain-history dependence of resistance helps to identify the source of resistance changes at a low strain (ε < 4%). We demonstrate that the out-of-plane uniaxial wrinkle arrays that appear in a direction parallel to stretching have the same hysteresis response as the resistance, and therefore wrinkle formation governs the low-strain resistance changes. These phenomena are extensively investigated with dc-conductivity and frequency-dependent-ac-conductivity measurements, and surface morphological studies of the films under various applied strains. Our work quantitatively identifies the contributions of wrinkles and cracks to the change in resistance of PEDOT:PSS under an applied strain.

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

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.

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

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.

Graphene Oxide Modified TiO2 Micro Whiskers and Their Photo Electrochemical Performance.

Harnessing the solar energy and producing clean fuel hydrogen through efficient photo-electrochemical water splitting has remained one of the most challenging endeavors in materials science. The core problem is to develop a suitable photo-catalyst material that absorbs a significant part of the solar spectrum and produces electron-hole pairs that can be easily separated without recombination. In the recent times, the composite of Titanium dioxide with graphene have been investigated to explore the advantages of both class of materials. Here we report on the photo-electrochemical properties of reduced graphene oxide functionalised TiO2 whiskers. The TiO2 whiskers are obtained from potassium titanium oxide (KTi8O16) synthesized through hydrothermal technique followed by ion exchange method and heat treatment. Graphene oxide was deposited on the as prepared TiO2 whiskers using hydrothermal method. As formed samples were characterized by Raman spectroscopy to confirm the presence of reduced graphene oxide (RGO) attached to TiO2 whiskers. Comparative photo electrochemical studies were carried out for TiO2 and reduced graphene oxide modified TiO2 whiskers. Among these, RGO modified TiO2 whiskers show significantly higher photo current density possibly due to enhancement in charge separation ability and longer electron life times.

Estimating the thermal expansion coefficient of graphene: the role of graphene-substrate interactions.

The temperature-dependent thermal expansion coefficient of graphene is estimated for as-grown chemical vapor deposited graphene using temperature-dependent Raman spectroscopy. For as-grown graphene on copper, the extent of thermal expansion mismatch between substrate and the graphene layer is significant across the entire measured temperature interval, T = 90-300 K. This mismatch induces lattice strain in graphene. However, graphene grown on copper substrates has a unique morphology in the form of quasi-periodic nanoripples. This crucially influences the profile of the strain in the graphene membrane, which is uniaxial. An estimate of the thermal expansion coefficient of grapheme α(T) is obtained after consideration of this strain profile and after incorporating temperature-dependent Grüneisen parameter corrections. The value of α(T), is found to be negative (average value, -3.75 × 10(-6) K(-1)) for the entire temperature range and it approaches close to zero for T < 150 K. For graphene wet-transferred to three kinds of substrates: copper, poly-dimethylsiloxane, and SiO2/Si, the Raman shifts can largely be modeled with lattice expansion and anharmonic contributions, and the data suggests limited interfacial interaction with the substrate.