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In this paper, we report on the measurement of the optical properties (absorption and scattering coefficients) of photoluminescent turbid media using a homemade integrating sphere setup equipped with a tunable monochromatic light source. The hemispherical reflectance and transmission data are analyzed with the radiative transfer equation using a Monte Carlo simulation-based lookup table to obtain the optical properties of the sample. The results are compared with the optical properties received from a classical integrating sphere setup equipped with a broadband white light source. The additional light of the photoluminescence generates artifacts within the optical properties, which are not present using a monochromatic light source. Additionally, a batch of samples with a broad range of scattering coefficients and dye concentrations were prepared and characterized with the aforementioned setup. The findings can help to generate a digital twin with the optical properties of the sample, which improves the physically based rendering and the design of, e.g., white-light LEDs. Dental restoration and photodynamic therapy also benefit from determination of the optical properties of photoluminescent turbid media. [ABSTRACT FROM AUTHOR]

In this paper, we introduce the phenomenon of light driven diffusioosmotic long-range attraction and repulsion of porous particles under irradiation with UV light. The change in the inter-particle interaction potential is governed by flow patterns generated around single colloids and results in reversible aggregation or separation of the mesoporous silica particles that are trapped at a solid surface. The range of the interaction potential extends to several times the diameter of the particle and can be adjusted by varying the light intensity. The "fuel" of the process is a photosensitive surfactant undergoing photo-isomerization from a more hydrophobic trans-state to a rather hydrophilic cis-state. The surfactant has different adsorption affinities to the particles depending on the isomerization state. The trans-isomer, for example, tends to accumulate in the negatively charged pores of the particles, while the cis-isomer prefers to remain in the solution. This implies that when under UV irradiation cis-isomers are being formed within the pores, they tend to diffuse out readily and generate an excess concentration near the colloid's outer surface, ultimately resulting in the initiation of diffusioosmotic flow. The direction of the flow depends strongly on the dynamic redistribution of the fraction of trans- and cis-isomers near the colloids due to different kinetics of photo-isomerization within the pores as compared to the bulk. The unique feature of the mechanism discussed in the paper is that the long-range mutual repulsion but also the attraction can be tuned by convenient external optical stimuli such as intensity so that a broad variety of experimental situations for manipulation of a particle ensemble can be realized. [ABSTRACT FROM AUTHOR]

We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH2 groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length. [ABSTRACT FROM AUTHOR]

Using atmospheric-pressure chemical vapor deposition, we have synthesized vanadium disulfide (VS 2 ) flakes with a metallic 1T phase that display nanoscale spiral surface ripples. To understand the origin of these chiral patterns in these transition metal dichalcogenides, tip-enhanced Raman spectroscopy and Kelvin probe force microscopies were jointly used to investigate their crystal structure, possible oxidation, and electronic properties, respectively. We found that the surface corrugation consists of small crystalline domains with distinct orientations. The change in local orientation is observed concomitantly with a spectral shift of the lattice modes of VS 2 and results in the formation of grain boundaries between the domains with distinct orientation. Additionally, the periodic surface structure is modulating the work function of VS 2 by 14 meV.

We report on tungsten disulfide (WS 2 ) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS 2 flake, further highlighting how its multidomain properties can be finely tuned using CVD.