Raman spectroscopy of graphene on different substrates and influence of defects

We show the evolution of Raman spectra with number of graphene layers on different substrates, SiO$_{2}$/Si and conducting indium tin oxide (ITO) plate. The G mode peak position and the intensity ratio of G and 2D bands depend on the preparation of sample for the same number of graphene layers. The 2D Raman band has characteristic line shapes in single and bilayer graphene, capturing the differences in their electronic structure. The defects have a significant influence on the G band peak position for the single layer graphene: the frequency shows a blue shift upto 12 cm$^{-1}$ depending on the intensity of the D Raman band, which is a marker of the defect density. Most surprisingly, Raman spectra of graphene on the conducting ITO plates show a lowering of the G mode frequency by $\sim$ 6 cm$^{-1}$ and the 2D band frequency by $\sim$ 20 cm$^{-1}$. This red-shift of the G and 2D bands is observed for the first time in single layer graphene.Comment: 6 pages, 8 figure

FACTORS INFLUENCING DELAYED RELEASE FOLLOWED BY RAPID PULSE RELEASE OF DRUGS FROM COMPRESSION COATED TABLETS FOR COLON TARGETING

Objective: This work was undertaken to develop colon targeted tablets that can minimize premature release of ibuprofen (IBP) and metronidazole (MNZ) in a lag period of 7h during which the tablets are likely to remain in the upper gastro-intestinal tract, and produce rapid pulse release within 1-5 h after the lag period when the tablets could be located in the colon with or without intervention of colonic microflora.Methods: Core tablets of ibuprofen and metronidazole containing different amounts of tri-sodium citrate (TSC) as osmogen were compression coated with locust bean gum (LBG) and carboxymethyl LBG (CMLBG). In vitro drug release studies were performed in a dynamic pH shift condition with or without rat cecal matters. The release of the drugs were also monitored at different hydrodynamic conditions.Results: In vitro release studies revealed that increase in the amount of TSC, decrease in coat-weight and change in hydrodynamic conditions influenced the drug release considerably. While LBG coated tablets under the stated conditions failed to provide complete release of the drugs in 12 h, CMLBG coated tablets produced complete release rapidly in the post lag period minimizing the release in the initial 7 h. Presence of rat cecal matter in dissolution medium further accentuated the release of the drugs from CMLBG compression coated tablets in the post lag period.Conclusion: The study revealed that tablets containing appropriate amount of osmogen in the core and compression coated with suitable amount of CMLBG may be suitable for colon targeting of drugs even in the absence of colonic microflora

QUICK/SLOW BIPHASIC RELEASE OF A POORLY WATER SOLUBLE ANTIDIABETIC DRUG FROM BI-LAYER TABLETS

Objective: The objective of the present work is to develop a bi-layer tablet consisting of an Immediate Pulse Release [IPR] layer and a sustained release [SR] layer that can produce a distinct biphasic release having two different drug release rates. The IPR layer is intended to release a fraction of the dose rapidly at a faster rate and the SR layer is meant for slow release of the remaining dose at a slower rate for a desired period of time.Methods: The quantitative determination was carried out by UV spectrophotometer. Solid dispersion was prepared by melt method. IPR layer was prepared by direct compression method, SR layer was prepared by wet granulation method. In-vitro drug release study from tablets was carried out in USP II tablet dissolution rate test apparatus. FTIR, DSC, XRD studies were performed.Results: 89% of the incorporated drug was released within 30 min in acid solution of pH 1.2 from the IPR tablet prepared with the highest amount of gelucire due to solid state transformation of the drug. The SR layer (SR8) comprising of SAL, CG and CMC produced prolonged drug release (70% in 10 h). The optimized IPR3 layer and SR layers were compressed to form bi-layer tablets from which 23-37% drug was released immediately in 30 min, and the remaining drug was released slowly for 7 to 10 h depending upon the compositions of the tablets.Conclusion: This study revealed that bi-phasic release of GPZ consisting of an initial quick release and subsequent slow release could be achieved by formulating bi-layer tablets using the existing tablet technology, and such formulation may be able to control hyperglycaemia effectively for a longer period of timeÂ

Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

Strong electron-phonon interaction which limits electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in-situ Raman scattering from single layer MoS$_2$ electrochemically top-gated field effect transistor (FET), we show softening and broadening of A$_{1g}$ phonon with electron doping whereas the other Raman active E$_{2g}^{1}$ mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why A$_{1g}$ mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single layer MoS$_2$-based FETs, which have a high on-off ratio and are of enormous technological significance.Comment: 5 pages, 3 figure

Site-specific stable deterministic single photon emitters with low Huang-Rhys value in layered hexagonal boron nitride at room temperature

Development of stable room-temperature bright single-photon emitters using atomic defects in hexagonal-boron nitride flakes (h-BN) provides significant promises for quantum technologies. However, an outstanding challenge in h-BN is creating site-specific, stable, high emission rate single photon emitters with very low Huang-Rhys (HR) factor. Here, we discuss the photonic properties of site-specific, isolated, stable quantum emitter that emit single photons with a high emission rate and unprecedented low HR value of 0.6 at room temperature. Scanning confocal image confirms site-specific single photon emitter with a prominent zero-phonon line at ~578 nm with saturation photon counts of 105 counts/second. The second-order intensity-intensity correlation measurement shows an anti-bunching dip of ~0.25 with an emission lifetime of 2.46 ns. Low-energy electron beam irradiation and subsequent annealing are important to achieve stable single photon emitters

Electron-Hole Asymmetry in the Electron-phonon Coupling in Top-gated Phosphorene Transistor

Using in-situ Raman scattering from phosphorene channel in an electrochemically top-gated field effect transistor, we show that its phonons with A$_g$ symmetry depend much more strongly on concentration of electrons than that of holes, while the phonons with B$_g$ symmetry are insensitive to doping. With first-principles theoretical analysis, we show that the observed electon-hole asymmetry arises from the radically different constitution of its conduction and valence bands involving $\pi$ and $\sigma$ bonding states respectively, whose symmetry permits coupling with only the phonons that preserve the lattice symmetry. Thus, Raman spectroscopy is a non-invasive tool for measuring electron concentration in phosphorene-based nanoelectronic devices

pH dependent chemical stability and release of methotrexate from a novel nanoceramic carrier

Considering the pH dependent chemical stability of anticancer drug methotrexate (MTX), the present communication reports a new approach for intercalation of the same in a nanoceramic vehicle, magnesium aluminium layered double hydroxide (LDH), by ex situ anion exchange method at pH 7.00, using 0.3 M ammonium acetate solution for dissolution of the drug. This simple method ensures maximum stability of the drug at the above said pH, with no degradation byproduct (e.g., N-10-methyl folic acid formed due to alkaline hydrolysis) under the given experimental conditions, compared to the similar approach, using 0.1 M sodium hydroxide solution, reported in our earlier work. Importantly, the above method leads to an enhanced drug loading of 32.3 wt%, compared to our previous reports. The cumulative release profile of MTX from LDH-MTX formulation in phosphate buffer saline (PBS) at pH 7.4 exhibited burst release initially which was taken care of by imparting a unique coating of poly(D,L-lactideco-glycolide, PLGA) on the LDH-MTX nanostructure that reduces the toxicity due to local accumulation. Hence, the superiority of the above for use in cancer chemotherapy, over the conventional drug-polymer system has been established w.r.t the drug release profile and a possible hypothesis of the same has been suggested. The half maximal inhibitory concentration (IC50) of the MTX drug used in this study has been determined and the same has been used to estimate the time dependent (24, 48, 72 and 96 h) efficacy of the MTX loaded samples with/without polymer coating, on human colon tumour cells (HCT-116)

Formation of p-n junction in polymer electrolyte-top gated bilayer graphene transistor

We show simultaneous p and n type carrier injection in bilayer graphene channel by varying the longitudinal bias across the channel and the top gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 $\mu F/cm^2$, a value about 125 times higher than the conventional SiO$_2$ back gate capacitance. Unlike the single layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top and back gate geometry is estimated.Comment: 16 pages, 6 figure