Time-resolved and temperature-dependent photoluminescence (PL) spectroscopy of ternary compound copper indium disulfide (CuInS2, or CIS) core materials, CIS/ZnS coreshells, and quaternary compound ZnCuInS2 (ZnCIS) revealed their optical properties with spectral, temporal, and thermal characteristics, which were closely linked to surface-related recombination, and shallow or deep defect-related donor-acceptor transitions. The PL peaks of semiconductor nanocrystals (SNCs) with sizes near Bohr radius displayed at ∼775 nm for CIS, ∼605 nm for CIS/ZnS, and ∼611 nm for ZnCIS. The spectral blue shift and spectral narrowing with CIS/ZnS and ZnCIS are assigned to the increased spatial confinement and surface regularity with the etching of core materials. Both the shorter lifetime at surface-trapped states or interface states and the longer lifetime at intrinsic defect-related states of CIS, CIS/ZnS, and ZnCIS SNCs were widely distributed across the entire PL spectral region. The surface or interface-trapped electrons were thermally active even at low temperatures, but the electrons at intrinsic defect-related states were relatively stable, which was attributable to the strong Coulomb energy between the charge carriers.
Au nanoparticles exhibited both negative and positive nonlinear absorptions with ground-state plasmon bleaching and free-carrier absorption that could be origins of the saturable and reverse-saturable optical properties. Au/Ag coreshells displayed only positive nonlinear absorption and reverse-saturable optical properties as a function of excitation intensity at the edge of surface-plasmon resonance, which implies no ground-state plasmon bleaching and the existence of two-photon absorption.
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.
The nonlinear refraction coefficient of silica aerogel was estimated to be ∼−1.5×10−15 m2/W (∼−3.67×10−9 esu) with a signal-beam z-scan spectroscopy. The third-order nonlinear refraction coefficient of nanostructure silica aerogel was almost five orders larger than the nonlinear refraction (χ3) of bulk material. The large nonlinear refraction with high nonlinear figure of merit is an ideal optical property for nonlinear optical applications.
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