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The micellar properties of dodecyltrimethylammonium bromide (DTAB) in water and methanol water mixtures at different temperatures have been studied by conductivity and surface tension measurements. The critical micelle concentrations (CMC), degree of ionization (α), standard Gibbs free energy of micellization (), standard enthalpy of micellization (), standard entropy of micellization () and free energy of transfer () were evaluated from conductivity data. The CMC, maximum excess surface concentration ( ), area occupied per surfactant molecule ( ), surface pressure at the CMC ( ), packing parameter (P) and standard free energy interfacial adsorption ) were estimated from surface tension measurements. The CMC of DTAB was found to increase with increasing volume fraction of methanol and increasing temperature. Thermodynamic parameters and surface properties revealed that the addition of methanol changes the relevant physicochemical properties which affect the process of micellization.

The monolayers of semiconducting transition metal dichalcogenides host strongly bound excitonic complexes and are an excellent platform for exploring many-body physics. Here we demonstrate a controlled kinetic manipulation of the five-particle excitonic complex, the charged biexciton, through a systematic dependence of the biexciton peak on excitation power, gate voltage, and temperature using steady-state and time-resolved photoluminescence (PL). With the help of a combination of the experimental data and a rate equation model, we argue that the binding energy of the charged biexciton is less than the spectral separation of its peak from the neutral exciton. We also note that while the momentum-direct radiative recombination of the neutral exciton is restricted within the light cone, such restriction is relaxed for a charged biexciton recombination due to the presence of near-parallel excited and final states in the momentum space.

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The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultra-thin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through inter-layer dipole-dipole coupling induced non-radiative Förster resonance energy transfer (FRET) from SnSe2 underneath which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the inplane orientation of dipoles in these layered materials. The FRET driven uniform single-and twophoton luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers, and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications. KEYWORDS: MoS2, WS2, SnSe2, van der Waals heterostructure, photoluminescence enhancement, two-photon luminescence, Förster Resonance Energy Transfer (FRET), charge transfer.transfer across WS2/MoSe2 hetero-bilayer stack through FRET across higher order exciton transitions. Nonetheless, the donor's absorption is still constrained by its physical thickness at the monolayer limit despite the high efficiency of FRET at the closest possible physical separation.Here, we demonstrate enhanced PL of monolayer MoS2 (and WS2) across its vdW heterojunction with multi-layer SnSe2 via FRET with single and two-photon excitation. Counteracting the charge transfer across highly staggered conduction bands of MoS2 and SnSe2, MoS2 single-photon luminescence (1P-PL) shows ~14-fold enhancement at room temperature with resonant excitation and ~5-fold enhancement with non-resonant excitation, while two-photon luminescence (2P-PL) of MoS2 shows up to ~7.5-fold enhancement with non-resonant excitation. Even with the insertion of few-layer hBN between MoS2 and SnSe2, the 1P-PL enhancement persists up to 5 times with resonant excitation. We demonstrate modulation of the degree of the PL enhancement by systematic parameter variation, including donor material, acceptor material, their thickness, physical separation between donor and acceptor, sample temperature, and excitation wavelength which corroborate FRET aided PL enhancement. We emphasize the intrinsic advantage of realizing FRET with SnSe2 as a donor and elucidate the impact of multiple parameters on the luminescence enhance...

The possibility of electrical manipulation and detection of charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS 2 /monolayer graphene vertical heterojunction, we report inter-layer charge transport from top fewlayer graphene to bottom monolayer graphene, mediated by coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature, the resulting change in the WS 2 bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anomalously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultra-fast formation of trion through exciton-trion coherent coupling, followed by its fast inter-layer transport. The demonstration of coherent formation, high stabilization, vertical transportation and electrical detection of trions marks a step towards room temperature trionics. 1 arXiv:1903.06437v1 [cond-mat.mes-hall]

Precise measurements on the density of sodium lauryl sulfate in pure water and in methanol (1) + water (2) mixed solvent media containing (0.10, 0.20, 0.30, and 0.40) volume fractions of methanol at (298.15, 308.15, 318.15, and 323.15) K are reported. The concentrations are varied from (0.3 3 10 À1 to 1.3 3 10 À1 ) mol 3 kg À1 . The results were almost constant on the partial molar volumes with increasing surfactant concentration. Also, the partial molar volumes are found to increase with increasing temperature over the entire concentration range investigated in a given mixed solvent medium and are found to decrease with increasing methanol content in the solvent composition.

The dielectric constant of water vapor was determined at 9280 Mc by a cavity comparison method at 10 temperatures ranging from 32 to 103°C. In addition, observations were made at 24,800 Mc at the single temperature of 24.5°C. Although the vapor pressure was raised to within 10 percent of the saturation value at 24.5°C and 103°C, the variation of (ε′−1)/(ε′+2) with pressure remained linear, contrary to observations made at radiofrequencies. These data are discussed in relation to the questions of association and adsorption of the vapor. The present results are in agreement with those obtained at radiofrequencies (sufficiently below saturation pressures) and do not show the dispersion reported in an early microwave experiment. Results obtained here and those obtained by others are used to determine mean values for the constants in the Debye equation: namely, P=[3.96±0.32]+[(2.077±0.016)104/T]. This gives a dipole moment of (1.846±0.005)10−18 esu.