Atomically thin semiconductors, such as the transition metal dichalcogenides, show great potential for nanoscale photodetection, energy harvesting, and nanophotonics. Here, we investigate the efficiency of energy transfer between colloidal quantum dots with a cadmium selenide core and cadmium sulfide shell and monolayer molybdenum diselenide (MoSe 2). We show that MoSe 2 effectively quenches the fluorescence of quantum dots when the two materials are in contact. We then separate the MoSe 2 and quantum dots by inserting variable thickness hexagonal boron nitride (h-BN) spacers and show that the efficiency at which the MoSe 2 quenches fluorescence decreases as the h-BN thickness is increased. For distances d, this trend can be modeled by a 1/d 4 decay, in agreement with theory and recent studies involving graphene. V

Monte Carlo simulation of terahertz quantum cascade laser structures based on wide-bandgap semiconductorsInvestigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells Due to their large optical phonon energies, nitride semiconductors are promising for the development of terahertz quantum cascade lasers with dramatically improved high-temperature performance relative to existing GaAs devices. Here, we present a rigorous Monte Carlo study of carrier dynamics in two structures based on the same design scheme for emission at 2 THz, consisting of GaN / AlGaN or GaAs/ AlGaAs quantum wells. The population inversion and hence the gain coefficient of the nitride device are found to exhibit a much weaker ͑by a factor of over 3͒ temperature dependence and to remain large enough for laser action even without cryogenic cooling.

We present a spiro-linked molecule 2,2',7,7'-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9'-spirobifluorene which acts as a secondary absorber in solid-state excitonic solar cells. Blending with a hole-transporting material 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene and used in conjunction with a near-infrared dye (termed TT1) results in an extended spectral response which yields a notable increase in short-circuit current and power conversion efficiency. This enhancement is due to both exciton energy transfer and also nanoscale charge generation in the blend via the formation of an excited state spiro-complex with charge transfer character.

Wide-bandgap semiconductors such as GaN / AlGaN and ZnO / MgZnO quantum wells are promising for improving the spectral reach and high-temperature performance of terahertz quantum cascade lasers, due to their characteristically large optical phonon energies. Here, a particle-based Monte Carlo model is developed and used to quantify the potential of terahertz sources based on these materials relative to existing devices based on GaAs/ AlGaAs quantum wells. Specifically, three otherwise identical quantum cascade structures based on GaN / AlGaN, ZnO / MgZnO, and GaAs/ AlGaAs quantum wells are designed, and their steady-state carrier distributions are then computed as a function of temperature. The simulation results show that the larger the optical phonon energies ͑as in going from the AlGaAs to the MgZnO to the AlGaN materials system͒, the weaker the temperature dependence of the population inversion. In particular, as the temperature is increased from 10 to 300 K, the population inversions are found to decrease by factors of 4.48, 1.50, and 1.25 for the AlGaAs, MgZnO, and AlGaN structure, respectively. Based on these results, the AlGaN and MgZnO devices are then predicted to be in principle capable of laser action without cryogenic cooling.

The authors investigate the use of electronic intersubband transitions in Ge/ SiGe quantum wells on SiGe ͑001͒ virtual substrates for the development of silicon-based long-wavelength quantum cascade lasers. These heterostructures can provide relatively strong quantum confinement in the Ge L valleys particularly if the SiGe layers are sufficiently thin so that L-to-⌬ intervalley scattering paths are suppressed. Numerical simulations indicate that low-threshold operation can be obtained from these devices, thanks to the nonpolar nature of SiGe. Furthermore, the tensor properties of the L-valley effective mass are favorable for the development of vertical emitting intersubband lasers.