Defect structure-electronic property correlations in transition metal dichalcogenide grain boundaries.

Grain boundaries (Gb) in transition metal dichalcogenides are a rich source of interesting physics as well as a cause of concern because of its impact on electron transport across them in large area electronic device applications. Here, using first principles calculations, we show that beyond the conventional definition of grain boundaries based on misorientation, the defect structure present at the grain boundaries plays a significant role in defining the local electronic properties. We observed that even the standard 5-7 defect ring has differing electronic characteristics depending on its internal configuration. While the 5-7 ring presents shallow defect states, and induce long range strain fields with the local bandgap increasing up to 32.7%, the other commonly observed 4-8 defect rings introduce only mid-gap states, induce smaller strain fields with no observable bandgap change. The results show the seminal character of the individual defect structures at grain boundaries, and that their relative density can be used to determine the overall physico-chemical properties of the grain boundary.

On the unique temperature-dependent interplay of a B-exciton and its trion in monolayer MoSe2.

Plasmonics in metal nanoparticles can enhance their near field optical interaction with matter, promoting emission into selected optical modes. Here, using Ga nanoparticles with carefully tuned plasmonic resonance in proximity to MoSe2 monolayers, we show selective photoluminescence enhancement from the B-exciton and its trion with no observable A-exciton emission. The nanoengineered substrate allows for the first direct experimental observation of the B-trion binding energy in semiconducting monolayers. Using temperature-dependent photoluminescence measurements, we show the following features of the MoSe2 B-exciton family: (i) the trion binding energy has an observable temperature dependence with a decreasing trend towards low temperatures and (ii) the exciton-trion emission ratio varies non-monotonically with temperature with a steep increase in the trion emission at lower temperatures. Using detailed models, we identify the particle size required for selective excitation and describe the underlying physical processes. This opens newer avenues for selectively promoting excitonic species and tuning the effective particle lifetimes in monolayer semiconductors. These results demonstrate the excellent plasmonic properties of Ga nanoparticles, which along with facile processing techniques makes it an attractive alternative to the prevalent noble metal plasmonics having applications in flexible/stretchable materials and textiles.

Thermodynamic Window for Size-Controlled Pore Formation in Graphene for Large-Scale Molecular Sieves.

Nanopores in graphene monolayers are a promising option for molecular separation applications, such as desalination and carbon capture. Graphene's atomic thickness allows for an optimal balance between molecular selectivity and permeability, while its chemical stability and robust mechanical properties make it appealing for a wide range of commercial applications. However, scaling to large areas with controlled pore size distribution is an open challenge in ultrathin membranes. Here, using first-principles calculations, we identify a suitable thermodynamic window in a chemical vapor deposition system for directly growing graphene monolayers with a controlled pore size distribution. As an example, our calculations show that a postgrowth annealing step with a supersaturation range of 19.7-25 kJ/mol at 1000 K results in the creation of a controllable pore density at graphene grain boundaries, with pore sizes falling within the range of 5-8 Å. Such pores isolate hydrated Cl ions from water molecules, effectively desalinating seawater. Thus, it allows the design of targeted synthesis of large-scale 2D layers for membrane applications.

Enhanced Cooper-Pair Injection into a Semiconductor Structure by Resonant Tunneling.

We demonstrate enhanced Andreev reflection in a Nb/InGaAs/InP-based superconductor-semiconductor hybrid device resulting in increased Cooper-pair injection efficiency, achieved by Cooper-pair tunneling into a semiconductor quantum well resonant state. We show this enhancement by investigating the differential conductance spectra of two kinds of samples: one exhibiting resonant states and one which does not. We observe resonant features alongside strong enhancement of Cooper pair injection in the resonant sample, and lack of Cooper pair injection in the nonresonant sample. The theoretical modeling for measured spectra by a numerical approach agrees well with the experimental data. Our findings open a wide range of directions in condensed matter physics and in quantum technologies such as superconducting light-emitting diodes and structures supporting exotic excitations.

Quantum capacitance of coupled two-dimensional electron gases.

Quantum capacitance effect is observed in nanostructured material stacks with quantum limited density of states. In contrast to conventional structures where two-dimensional electron gases (2DEG) with reduced density of states interact with a metal plate, here we explore the quantum capacitance effect in a unique structure formed by two 2DEG in a graphene sheet and AlGaN/GaN quantum well. The total capacitance of the structure depends non-linearly on the applied potential and the linear density of states in graphene leads to enhanced electric field leakage into the substrate causing a dramatic 50% drop in the overall capacitance at low bias potentials. We show theoretical projections of the quantum capacitance effect in the proposed device stack, fabricate the structure and provide experimental verification of the calculated values at various temperatures and applied potentials. The wide swing in the total capacitance is sensitive to the chemical potential of the graphene sheet and has multiple applications in molecular sensing, electro-optics, and fundamental investigations.

High-T c Cooper-pair injection in a semiconductor-superconductor structure.

We observe Andreev reflection in a YBCO-GaN junction through differential conductance spectroscopy. A strong characteristic zero-bias peak was observed and persisted up to the critical temperature of the superconductor with a smaller superconducting order parameter Δ âˆ¼ 1 meV. The presence of Andreev reflection with the small Δ in comparison to its value for high-T c superconductors forms an important milestone toward demonstration of superconducting proximity in high-T c/semiconductor junctions. Experimental results were then compared to the theoretical model with good agreement. Efficient injection of Cooper pairs into direct bandgap semiconducting structures, together with high transition temperature of YBCO, can pave the way to novel optoelectronics and quantum optical studies of high-T c materials.

Enhanced two-photon amplification in superconductor-semiconductor plasmonic waveguides.

We theoretically demonstrate significant enhancement of two-photon amplification by using a superconductor for both a Cooper-pair source and surface plasmon-polariton mode guiding. Cooper-pair-based gain active region restriction to the superconductor-semiconductor interface limits its potentially highly efficient two-photon gain process. Using the superconductor layer for a plasmonic waveguide structure allows strong photon confinement while reducing design and fabrication constraints. This results in three orders of magnitude enhancement of the superconducting two-photon gain (TPG) compared to superconductor-based dielectric waveguides. Moreover, a superconducting TPG produced by a plasmonic waveguide increases with carrier concentration, meeting practical device requirements. Our results pave the way for efficient two-photon amplification realization in nanoscale devices.

Observation of 2D semiconductor P-type dark-exciton lifetime using two-photon ultrafast spectroscopy.

We report direct measurements of intrinsic lifetimes of P-type dark-excitons in MoS2 monolayers. Using sub-gap excitation, we demonstrate two-photon excited direct population of P-type dark excitons, observe their scattering to bright states and decay with femtosecond resolution. In contrast to one-photon excitation schemes, non-monotonic density variation in bright exciton population observed under two-photon excitation shows the indirect nature of its population and competing decay pathways. Detailed modeling of different recombination pathways of bright and dark excitons allows experimental measurement of 2P dark → 1S bright exciton scattering rates. These insights into the dark states in a MoS2 monolayer pave the way for novel devices such as quantum memories and computing.

Reversible defect engineering in graphene grain boundaries.

Research efforts in large area graphene synthesis have been focused on increasing grain size. Here, it is shown that, beyond 1 µm grain size, grain boundary engineering determines the electronic properties of the monolayer. It is established by chemical vapor deposition experiments and first-principle calculations that there is a thermodynamic correlation between the vapor phase chemistry and carbon potential at grain boundaries and triple junctions. As a result, boundary formation can be controlled, and well-formed boundaries can be intentionally made defective, reversibly. In 100 µm long channels this aspect is demonstrated by reversibly changing room temperature electronic mobilities from 1000 to 20,000 cm2 V-1 s-1. Water permeation experiments show that changes are localized to grain boundaries. Electron microscopy is further used to correlate the global vapor phase conditions and the boundary defect types. Such thermodynamic control is essential to enable consistent growth and control of two-dimensional layer properties over large areas.

Andreev Reflection in a Superconducting Light-Emitting Diode.

We experimentally demonstrate Cooper-pair injection into a superconducting light-emitting diode by observing Andreev reflection at the superconductor-semiconductor interface, overcoming the contradicting requirements of an electrically transparent interface and radiative recombination efficiency. The device exhibits electroluminescence enhancement at the quasi-Fermi energy at temperatures below Tc. The theoretically predicted conductance and electroluminescence spectra based on Cooper-pair injection into the semiconductor correspond well to our experimental results. Our findings pave the way for practical superconductor-semiconductor quantum light sources.

Nanoscale High-Tc YBCO/GaN Super-Schottky Diode.

We demonstrate a high-temperature nanoscale super-Schottky diode based on a superconducting tunnel junction of pulsed-laser-deposited YBCO on GaN thin films. A buffer-free direct growth of nanoscale YBCO thin films on heavily doped GaN was performed to realize a direct high-Tc superconductor-semiconductor junction. The junction shows strongly non-linear I-V characteristics, which have practical applications as a low-voltage super-Schottky diode for microwave mixing and detection. The V-shaped differential conductance spectra observed across the junction are characteristic of the c-axis tunneling into a cuprate superconductor with a certain disorder level. This implementation of the super-Schottky diode, supported by the buffer-free direct growth of nanoscale high-Tc thin films on semiconductors, paves the way for practical large-scale fabrication and integration of high-Tc-superconductor devices in future technologies.