We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

Circular polarized luminescence (CPL) is essential to chiral sciences and photonic technologies, but the achievement of circular polarized room-temperature phosphorescence (CPRTP) remains a great challenge due to the instability of triplet state excitons. Herein, we found that dual CPL and CPRTP were demonstrated by hybrid chiral photonic films designed by the coassembly of cellulose nanocrystals (CNCs), poly(vinyl alcohol) (PVA), and carbon dots (CDs). Tunable photonic band gaps were achieved by regulating the ratio of CNC/PVA in the hybrid films, leading to tunable CPL with invertible handedness, tunable wavelengths, and considerable dissymmetric factors (g lum ) up to −0.27. In particularly, triplet excitons produced by CDs were stable in the chiral photonic crystal environment, resulting in tunable right-handed CPRTP with long lifetimes up to 103 ms and large RTP dissymmetric factors (g RTP ) up to −0.47. Moreover, patterned films with multiple polarized features were demonstrated by a mold technique.

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This letter reports a giant magnetocaloric effect (GMCE) in a novel series of materials based on the shape memory alloy Ni 2 MnGa. The origin of an enhanced GMCE is traced to the coincidence of a first-order magnetic transition and its attendant structural phase transition with a second-order magnetic transition. This coincidence is achieved by careful compositional tuning and is a technique which provides a criterion for enhancing the GMCE in this system. Thus, for Ni 55.2 Mn 18.6 Ga 26.2 , we report an entropy change S m = −20.4 J kg −1 K −1 at 317 K in a field of 5 T. This shape memory system also has the added advantage of being formed from inexpensive, nontoxic constituents. With a working temperature at and above room temperature, it appears to be a most promising candidate for practical room temperature magnetic refrigeration.

Based on the first-principles band structure calculations, we investigate the effects of hydrostatic pressure on the conventional insulator (CI) Sb2Se3 and predict that it undergoes a topological quantum phase transition from a CI to a non-trivial topological insulator at a critical pressure value. The pressure induced topological quantum phase transition is confirmed by calculating the evolution of the bulk energy gap as a function of pressure, the inversion of energy band structure and the Z2 topological invariant as well as the existence of the Dirac-like topological surface states. Our predictions can be tested by both spectroscopy and transport experiments.