Change Of Second Harmonic Generation Intensity Research Articles

Two-Dimensional MXene Flakes with Large Second Harmonic Generation and Unique Surface Responses.

MXenes are a family of two-dimensional (2D) materials with broad and varied applications in biology, materials science, photonics, and environmental remediation owing to their layered structure and high surface area-to-volume ratio. MXenes have exhibited significant nonlinear optical characteristics, which have been primarily explored in the context of photonics applications, yet the second-harmonic generation (SHG) behavior of MXenes remains an unexplored aspect of their optical properties. Herein, we demonstrate and quantify large second-order responses of 2D Ti3C2Tx MXenes both in aqueous solutions and on a silicon substrate for the first time. MXene flakes showed strong second-harmonic scattering (SHS) in a dilute suspension with a sensitivity of less than 0.1 μg/mL. Angle-dependent SHS experiments further found that the second-order responses originate from coherent 2D dipole radiation. Through confocal and atomic force microscopies, we found that the intense SHG signal from free-standing MXene flakes increases exponentially with decreasing thickness, while two-photon fluorescence increases linearly with thickness. The second-order susceptibility of the MXenes was determined to be 3.6 pm V-1 with a thickness of 10 nm, almost twice of that for an often-used SHG crystal, beta barium borate. We further explored surface properties of the MXene sheets by investigating the SHS responses upon addition of organic dye molecules to the system. It was found that the adsorption of crystal violet (CV) obeys a Langmuir adsorption model while the addition of malachite green (MG) resulted in almost no change in SHG intensity, even though the adsorption capacities for both CV (61.3 ± 1.7 mg/g) and MG (54.8 ± 2.8 mg/g) are similar. Such a stark difference in adsorption characteristics between cationic organic CV and MG dyes is likely due to their distinct orientational orderings on the MXene surfaces. This work opens many possibilities for the further employment of the family of 2D materials in photonics, optics, and surface catalysis applications.

Second harmonic generation properties of Ca3(O3C3N3)2 ‐Sr3(O3C3N3)2 solid solutions

Solid solutions of the second harmonic generation (SHG) materials Ca3(O3C3N3)2 (CCY) and Sr3(O3C3N3)2 (SCY) were prepared via exothermic solid state metathesis reactions from appropriate amounts of the corresponding metal chlorides and potassium cyanate at 525 °C. The change in SHG intensity caused by the successive cation substitution is reported. Differential thermal analyses are used to explore the SCY–K(OCN) phase diagram as a medium for the growth of SCY crystals.

Binding of the Anti-Cancer Drug Daunomycin to DNA Probed by Second Harmonic Generation

Second harmonic generation (SHG) was used to selectively probe DNA-drug interactions without the need for chemical labels or invasive detection methods. In particular, the binding constant of the anticancer drug daunomycin to a recognition triplet sequence in a 33-mer of double stranded DNA was determined. The SHG method, which is interface selective, probed the binding of daunomycin to DNA that was tethered to the surface of colloidal microparticles suspended in aqueous solution. Probing biomolecule coated colloids is expected to yield larger SH signals and provides experimental flexibility as compared to experiments performed at planar interfaces. The change in SHG intensity as daunomycin was added to the microparticle solution was fit to a Langmuir binding model, which yielded an equilibrium constant of 2.3 (±0.7) × 10(5) M(-1); the corresponding Gibbs free energy change at 20 °C is -7.2 ± 0.2 kcal/mol. Control experiments established that daunomycin preferentially binds to DNA at the recognition sequence. The equilibrium was found to be unaffected by the presence of free DNA in solution, and hyper-Rayleigh scattering from bulk molecules did not change with increasing daunomycin concentration. The extracted equilibrium constants are in agreement with the range of reported values found in the literature.

Phase transitions and polarizations in epitaxial BaTiO3/SrTiO3 superlattices studied by second-harmonic generation

BaTiO 3 / SrTiO 3 superlattices are found to have a critical periodicity of 30 SrTiO3 unit cells, below which the room-temperature symmetry is mm2 orthorhombic. The orthorhombic-tetragonal phase-transition temperature is shifted upward from the bulk value (<3 K and normally not observed at finite temperature) of the quantum paraelectric pure strontium titanate to temperatures above ambient (599 K for 30/30-unit-cell superlattices). Antidipole patterns appear in superlattices with unit cells below 10/10. A model is presented that accounts quantitatively for the change in second-harmonic generation intensity with superlattice periodicity from 4 unit cells to 50.

The adsorption of triethylgallium on GaAs(100) at 300 K: adsorption kinetics from optical second harmonic generation transients

Data for the adsorption of triethylgallium on GaAs(100) obtained using the technique of optical second harmonic generation (SHG) are analysed using the assumption that the non-linear second-order surface susceptability Xs (2) varies as a linear function of surface coverage. Equations are generated which relate the change in SHG intensity with coverage for both a non-dissociative and a dissociative Langmuir adsorption isotherm. Solution of these non-linear equations yields parameters related to the sticking coefficient and the saturation coverage. For the system studied here, the best fit is achieved using a non-dissociative Langmuir model. However the fitting process reveals that if the assumptions are valid, it must be concluded that the system does not follow Langmuir kinetics but adopts a more complex mechanism in which the sticking coefficient increases slightly with increasing coverage.