Long range piezoelectricity effects in van der Waals heterobilayer systems beyond 1000 atoms.

Twist angle is a relevant design and control component for the piezoelectric coefficients of van der Waals (vdW) heterostructures. This theoretical work assesses in high detail the impact of the twist angle on the piezoelectricity of two-dimensional (2D) heterobilayer systems. We expand the density-functional based tight-binding method to predict the piezoelectric coefficients of twisted and corrugated 2D heterobilayer structures with more than 1000 atoms. We showcase the method on hexagonal III-V/transition metal dichalcogenide vdW heterosystems. Our calculations yield a periodic relationship between the in-plane piezoelectric coefficients and the corresponding twist angles, indicating the tunability of the in-plane piezoelectricity. In contrast, the out-of-plane piezoelectricity is not twist angle dependent, but nonlinearly changes with the average interlayer distance.

Quantum Mechanical Modeling of Reaction Rate Acceleration in Microdroplets.

Organic reactions in microdroplets can be orders of magnitude faster than their bulk counterparts. We hypothesize that solvation energy differences between bulk and interface play a key role in the intrinsic rate constant increase and test the hypothesis with explicit solvent calculations. We demonstrate for both the protonated phenylhydrazine reagent and the hydrazone transition state (TSB) that molecular orientations which place the charge sites at the surface confer high energy. A pathway in which this high-energy form transforms into a fully solvated TSB has a lower activation energy than bulk by some 59 kJ/mol, a result that is consistent with experimental rate acceleration studies.

Probing scattering mechanisms with symmetric quantum cascade lasers.

A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.

Indirectly pumped 3.7 THz InGaAs/InAlAs quantum-cascade lasers grown by metal-organic vapor-phase epitaxy.

Device-performances of 3.7 THz indirect-pumping quantum-cascade lasers are demonstrated in an InGaAs/InAlAs material system grown by metal-organic vapor-phase epitaxy. The lasers show a low threshold-current-density of ~420 A/cm2 and a peak output power of ~8 mW at 7 K, no sign of parasitic currents with recourse to well-designed coupled-well injectors in the indirect pump scheme, and a maximum operating temperature of Tmax ~100 K. The observed roll-over of output intensities in current ranges below maximum currents and limitation of Tmax are discussed with a model for electron-gas heating in injectors. Possible ways toward elevation of Tmax are suggested.