Cubic Nonlinearity of Graphene-Oxide Monolayer.

The cubic nonlinearity of a graphene-oxide monolayer was characterized through open and closed z-scan experiments, using a nano-second laser operating at a 10 Hz repetition rate and featuring a Gaussian spatial beam profile. The open z-scan revealed a reverse saturable absorption, indicating a positive nonlinear absorption coefficient, while the closed z-scan displayed valley-peak traces, indicative of positive nonlinear refraction. This observation suggests that, under the given excitation wavelength, a two-photon or two-step excitation process occurs due to the increased absorption in both the lower visible and upper UV wavelength regions.

Exciton Dephasing in Tungsten Diselenide Atomic Layer.

An intrinsic exciton dephasing is the coherence loss of exciton dipole oscillation, while the total exciton dephasing originates from coherence loss due to exciton-exciton interaction and excitonphonon coupling. In this article, the total exciton dephasing time of tungsten diselenide (WSe₂) atomic layers was analyzed as functions of excitation intensity with exciton-exciton coupling strength and temperature with exciton-phonon coupling strength. It was hypothesized that the total exciton dephasing time is shortened as the exciton-exciton interaction and the exciton-phonon coupling are increased.

Cubic Nonlinearity of Molybdenum Disulfide Nanoflakes.

The cubic optical nonlinearity of molybdenum disulfide (MoS₂) nanoflakes was characterized by Z-scan and I-scan with resonant excitation. The excitation source was a ~6 ns laser at 532 nm with a 10 Hz repetition rate. The open and closed Z-scan analyzed the nonlinear absorption and nonlinear refraction properties of MoS₂ nanoflakes.

Emission Recovery and Stability Enhancement of Inorganic Perovskite Quantum Dots.

Inorganic lead halide perovskite quantum dots (PQDs), especially red emission PQDs, are well-known to easily lose their luminescence emission with time, which shows from strong emission of fresh PQDs to no emission of aged PQDs. Here, we demonstrate that trioctylphosphine (TOP) can effectively and instantly recover the luminescence emission of aged red PQDs, making the "dead" PQDs "reborn". Furthermore, TOP also works to improve the emission intensity of freshly synthesized PQDs.

Piezoelectricity in WSe/MoS heterostructure atomic layers.

A two-dimensional heterostructure of WSe2/MoS2 atomic layers has unique piezoelectric characteristics which depend on the number of atomic layers, stacking type and interlayer interaction size. The van der Waals heterostructure of p- and n-type TMDC atomic layers with different work functions forms a type-II staggered gap alignment. The large band offset of the conduction band minimum and the valence band maximum between p-type WSe2 and n-type MoS2 atomic layers leads to large electric polarization and piezoelectricity.

Bandgap Tunability of Transition Metal Dichalcogenide Atomic Layers.

The temperature-dependent bandgap of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) is analyzed using the O'Donnell and Chen relation with parameters including the average acoustic phonon energy (〈ħω〉) and the electron-phonon coupling strength (s). Wider (narrower) tunability of the bandgap results from the larger (smaller) electron-phonon coupling strength for a constant acoustic phonon energy. A 1.

Exciton Formation Entropy Changes in Transition Metal Dichalcogenide Atomic Layers.

The atomic layers of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) are of great interest in the areas of photonics and optoelectronics due to the correlation between valley orbital, spin, and optical helicity; the compositional tuning of exciton bandgaps in visible and near-infrared spectra; and the bandgap modification from indirect for bilayer or multilayer to direct for monolayer. The derivative of the O'Donnell and Chen relation is analyzed as a function of temperature and gives the relationship between the change in entropy of exciton formation and the bandgap energy. The analysis suggests the change in entropy of exciton formation with higher energy phonons (~100 meV) is constant until ~90 K while lower energy phonons (~10 meV) approaches a constant value of -2skB between ~250 K and ~300 K where s is the strength of electron-phonon interaction and kB is the Boltzmann constant.

Piezoelectricity enhancement and bandstructure modification of atomic defect-mediated MoS monolayer.

Piezoelectricity appears in the inversion asymmetric crystal that converts mechanical deformation to electricity. Two-dimensional transition metal dichalcolgenide (TMDC) monolayers exhibit the piezoelectric effect due to inversion asymmetry. The intrinsic piezoelectric coefficient (e) of MoS is ∼298 pC m.

Label-free and non-contact optical biosensing of glucose with quantum dots.

We present a label-free, optical sensor for biomedical applications based on changes in the visible photoluminescence (PL) of quantum dots in a thin polymer film. Using glucose as the target molecule, the screening of UV excitation due to pre-absorption by the product of an enzymatic assay leads to quenching of the PL of quantum dots (QDs) in a non-contact scheme. The irradiance changes in QD PL indicate quantitatively the level of glucose present.