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. The I-scan technique characterized the nonlinear transmittance properties of MoS2 nanoflakes at the peak and valley of closed Z-scan trace as a function of excitation intensity.

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. The coherence loss analysis revealed that the exciton dephasing time of WSe2 atomic layers is due to mainly the temperature rather than the excitation intensity.

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. In this process, TOP does not make any detectable structural changes to PQDs. Besides, TOP can effectively enhance the stability of PQDs against long-term storage, temperature, UV irradiation, and polar solvents. This unusual emission recovery and stability enhancement by TOP shall promote the understanding of particle surface conditions and the development of PQD devices.

Piezoelectricity in WSe2/MoS2 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. The output voltages for a MoS2/WSe2 partial vertical heterostructure with a size of 3.0 nm × 1.5 nm were 0.137 V and 0.183 V under 4% and 8% tensile strains, respectively. The output voltage of an AB-stacking MoS2/WSe2 heterostructure was larger than that of an AA-stacking heterostructure under 4% tensile strain due to the contribution of intrinsic piezoelectricity and symmetric out-of-plane conditions. The AB-stacking has a lower formation energy and better structural stability compared to AA-stacking. The large output voltage of nanoscale partial or full vertical heterostructures of 2D WSe2/MoS2 atomic layers in addition to the increased output voltage through the series connection of multiple nanoscale piezoelectric devices will enable the realization of nano-electromechanical systems (NEMS) with TMDC heterostructure atomic layers.

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. Increased scattering and spontaneous decay probabilities explains the amplified electron-phonon interaction when the phonon energy is large. The change in exciton formation entropy can be increased ~3-fold while the bandgap is managed through the electron-phonon coupling strength.

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 (〈hω〉) 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 1.5 eV bandgap change was observed for weak electron-phonon coupling (s = 2) as well as with the strong electron-phonon coupling (s = 30). However, the weak electron-phonon coupling leads to a linear decrease in the bandgap energy as a function of temperature above ~85 K while the strong coupling exhibits similar behavior after ~60 K. Narrower (wider) tunability of the bandgap results from the larger (smaller) acoustic phonon energy for a constant electron-phonon coupling strength. The slope of negative entropy of exciton formation is large (small) at lower (higher) temperature. The management of the electron-phonon interaction as well as the average acoustic phonon energy indicates the ability to control the bandgap.

Piezoelectricity enhancement and bandstructure modification of atomic defect-mediated MoS2 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 (e11) of MoS2 is ∼298 pC m-1. For the single atomic shift of Mo of 20% along the armchair direction, the piezoelectric coefficient (e11) of MoS2 with 5 × 5 unit cells was enhanced up to 18%, and significantly modified the band structure. The single atomic shift in the MoS2 monolayer also induced new energy levels inside the forbidden bandgap. The defect-induced energy levels for a Mo atom shift along the armchair direction are relatively deeper than that for a S atom shift along the same direction. This indicates that the piezoelectricity and band structure of MoS2 can be engineered by a single atomic shift in the monolayer with multi unit cells for piezo- and opto-electric applications.

Excitation wavelength independent visible color emission of carbon dots.

Carbon dots (CDs) usually emit a strong blue light and excitation wavelength dependent long wavelength lights. This significantly limits their applications because one has to use a series of different excitation light sources to get different colors and the long wavelength emissions are usually very weak. We found that one type of CDs synthesized from p-phenylenediamine could emit various long wavelength lights (green to red) independent of the excitation wavelength when dispersed in different solvents. The photoluminescence quantum yields of the same CDs were 10-35% in different solvents for different color emissions. Based on this solvent-color effect, we further mixed the same CDs with different polymers to form solid CD films for various color emissions, and these film emissions were also excitation wavelength independent. Multicolor LEDs were demonstrated with the same CDs in solution and solid film states for color displays.

Plasmon-Coupled CdSe/ZnS and CdTe/CdS/ZnS Coreshells for Hybrid Light Emitting Devices.

Plasmon-coupled CdSe/ZnS and CdTe/CdS/ZnS coreshells are investigated for their optoelectronic applications because of their high color purity, wide optical tunability, large PL enhancement, and compact and easy integration into electronic devices. The quantum confinement of carriers within quantum dots (QDs) with sizes near the exciton Bohr radius (CdSe ~ 5.8 nm, CdTe ~ 7 nm) exhibits the features of discrete energy states and blue-shift from the bulk bandgap (CdSe ~718 nm, CdTe ~ 863 nm) in the optical spectrum. While the fluorescence from the QDs is attributable to the exciton carrier recombination, large PL enhancement and fast emission time is achieved through plasmon-exciton coupling via the Coulomb interaction. Large PL enhancement of QDs in the vicinity of plasmonic particles was observed and attributed to the reduction of the non-radiative decay rate and large local field enhancement. The large PL enhancement and wide optical tunability along with high color purity from plasmon-coupled QDs enables the realization of hybrid LEDs.