Umesha Mogera

Binding of DNA nucleobases and nucleosides with graphene.

Interaction of two different samples of graphene with DNA nucleobases and nucleosides is investigated by isothermal titration calorimetry. The relative interaction energies of the nucleobases decrease in the order guanine (G)>adenine (A)>cytosine (C)>thymine (T) in aqueous solutions, although the positions of C and T seem to be interchangeable. The same trend is found with the nucleosides. Interaction energies of the A-T and G-C pairs are somewhere between those of the constituent bases. Theoretical calculations including van der Waals interaction and solvation energies give the trend G>A approximately T>C. The magnitudes of the interaction energies of the nucleobases with graphene are similar to those found with single-walled carbon nanotubes.

Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow

Measuring humidity in dynamic situations calls for highly sensitive fast response sensors. Here we report, a humidity sensor fabricated using solution processed supramolecular nanofibres as active resistive sensing material. The nanofibres are built via self- assembly of donor and acceptor molecules (coronene tetracarboxylate and dodecyl methyl viologen respectively) involved in charge transfer interactions. The conductivity of the nanofibre varied sensitively over a wide range of relative humidity (RH) with unprecedented fast response and recovery times. Based on UV-vis, XRD and AFM measurements, it is found that the stacking distance in the nanofibre decreases slightly while the charge transfer band intensity increases, all observations implying enhanced charge transfer interaction and hence the conductivity. It is demonstrated to be as a novel breath sensor which can monitor the respiration rate. Using two humidity sensors, a breath flow sensor was made which could simultaneously measure RH and flow rate of exhaled nasal breath. The integrated device was used for monitoring RH in the exhaled breath from volunteers undergoing exercise and alcohol induced dehydration.

High‐Mobility Field Effect Transistors Based on Supramolecular Charge Transfer Nanofibres

Self-assembled charge transfer supramolecular nanofibres of coronene tetracarboxylate (CS) and dodecyl substituted unsymmetric viologen derivative (DMV) behave as active channel in field effect transistors exhibiting high mobility. These devices work in ambient conditions and can regenerate in the presence of a single drop of water.

Dynamic self-assembly of charge-transfer nanofibers of tetrathiafulvalene derivatives with F4TCNQ.

One-dimensional charge-transfer nanostructures were constructed by the supramolecular coassembly of amphiphilic (Amph-TTF) and hydrophobic (TDD-TTF) tetrathiafulvalene (TTF) donor derivatives with the acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ), in appropriate solvent composition mixtures. Microscopic analyses show that TDD-TTF retains its self-assembled fibrillar morphology even in the charge-transfer state, whereas Amph-TTF undergoes a spherical to nanorod transition upon coassembly. Time-dependent optical spectroscopy studies have shown a spontaneous change in molecular organization in TDD-TTF-based donor-acceptor costacks, which suggests a dynamic behavior, in contrast to the kinetically stable amphiphilic TTF assemblies. We have also tried to get an insight into the observed time-dependent change in molecular packing of these nanostructures through spectroscopic analyses by commenting on whether the TTF-TCNQ pair is cofacially arranged or present in the classical herringbone (orthogonal) fashion. Furthermore, our two-probe electrical measurements showed that these charge-transfer fibers are conducting. A supramolecular approach that yields 1D charge-transfer nanostructures of donor and acceptor molecules will be an alternative to existing crystalline substances with high conductivity and hence can be a viable tool for nanoelectronics.

Highly Decoupled Graphene Multilayers: Turbostraticity at its Best.

The extraordinary properties of graphene are truly observable when it is suspended, being free from any substrate influence. Here, a new type of multilayer graphene is reported wherein each layer is turbostratically decoupled, resembling suspended graphene in nature, while maintaining high degree of 2D crystallinity. Such defect-free graphene multilayers have been made over large areas by Joule heating of a Ni foil coated with a solid hydrocarbon. Raman spectra measured on thick flakes of turbostratically single layer graphene (T-SLG) (100-250 nm) have shown characteristics similar to suspended graphene with very narrow 2D bands (∼16 cm(-1)) and I2D/IG ratios up to 7.4, importantly with no D band intensity. Electron diffraction patterns showed sets of diffraction spots spread out with definite angular spacings, reminiscent of the angular deviations from the AB packing which are responsible for keeping the layers decoupled. The d-spacing derived from X-ray diffraction was larger (by ∼0.04 Å) compared to that in graphite. Accordingly, the c-axis resistance values were three orders higher, suggesting that the layers are indeed electronically decoupled. The high 2D crystallinity observed along with the decoupled nature should accredit the observed graphene species as a close cousin of suspended graphene.

In-Situ GISAXS Study of Supramolecular Nanofibers having Ultrafast Humidity Sensitivity

Self assembled nanofibers derived from donor-acceptor (D-A) pair of dodecyl methyl viologen (DMV) and potassium salt of coronene tetracarboxylate (CS) is an excellent material for the development of organic electronic devices particularly for ultrafast response to relative humidity (RH). Here we have presented the results of in-situ grazing incidence small angle x-ray scattering (GISAXS) measurements to understand aridity dependent self reorganization of the nanofibers. The instantaneous changes in the organization of the nanofibers was monitored with different equilibrium RH conditions. Additionally formation of nanofibers during drying was studied by GISAXS technique – the results show two distinct stages of structural arrangements, first the formation of a lamellar mesophase and then, the evolution of a distorted hexagonal lattice. The RH dependent GISAXS results revealed a high degree of swelling in the lattice of the micelles and reduction in the distortion of the hexagonal structure with increase in RH. In high RH condition, the nanofibers show elliptical distortion but could not break into lamellar phase as observed during formation through drying. This observed structural deformation gives insight into nanoscopic structural changes of the micelles with change in RH around it and in turn explains ultrafast sensitivity in its conductivity for RH variation.

Twisted multilayer graphene exhibiting strong absorption bands induced by van Hove Singularities

Twisted bilayer graphene exhibits several angle-dependent properties due to the emergence of the van Hove Singularities in its density of states. Among them, twist-angle-dependent optical absorption has gained a lot of attention due to its presence in the visible spectral region. However, observation of such absorption is experimentally tricky due to large transmittance. In this study, we use highly decoupled twisted multilayer graphene to observe such absorption in the visible region using a simple spectrometer. A large number of twisted graphene layers in the system enable observation of such absorption evident in the visible region; the absorption band position correlates with the twist angle measured using selective area electron diffraction pattern as well as predictions from theory. While the Raman spectra were akin to those of the decoupled graphene system, at specific twist angle of

Charge‐Transfer Nanostructures through Noncovalent Amphiphilic Self‐Assembly: Extended Cofacial Donor‐Acceptor Arrays

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