Graphdiyne Ultrathin Nanosheets for Efficient Water Splitting

Graphdiyne (GDY) is an emerging 2D carbon material that exhibits unusual structures and properties. Therefore, growing heterogeneous materials on the surface of GDY is very attractive to achieve efficient energy utilization. Here, a simple method for the controllable synthesis of ultrathin charge-transfer complexes (CTs) of nickel with terephthalic acid nanosheets on GDY is reported. This catalyst shows record-high oxygen evolution reaction (OER) activity with an overpotential of only 155 mV to deliver a current density of 10 mA cm⁻² in an alkaline electrolyte. Density functional theory calculations reveals that a strong p–d coupling effect in the GDY–CT interface region enhances the overall electronic activity, resulting in fast reversible redox-switching with a low electron-transfer barrier. Experimental characterization confirms that GDY plays a key role in modulating the morphological and electronic structures to accelerate the OER rate. These findings are expected to contribute to the design of more efficient catalysts for the realization of efficient hydrogen energy technologies.     

Electronic spectra of quasi-regular heterostructures: simple versus realistic models

The electronic spectra of quasi-regular systems are investigated via simple one-dimensional, one-band models and compared with those obtained by means of realistic empirical tight-binding models, particularly for the Fibonacci sequence case.     

Quantum reflection pole method for determination of quasibound states in semiconductor heterostructures

The quantum reflection pole method (QRPM) is introduced for determining quasibound state eigenenergies and their lifetimes in symmetric, asymmetric, biased, and unbiased quantum heterostructures. In the QRPM the single-band effective-mass Schrödinger equation is solved without using complex arithmetic. Calculations are much simpler to perform than with previous methods. Further, results are found to be in excellent agreement with other rigorous techniques.     

Current distribution in the integer quantum Hall effect: The role of bulk states

The role of bulk and edge currents in a two-dimensional electron gas under the conditions of the integer quantum Hall effect (IQHE) was studied by means of an inductive coupling to Hall bar geometry. From this study we conclude that the extended states at the bulk of the sample below the Fermi energy are capable of carrying a substantial amount of Hall current. For Hall bar geometry sample with a back gate we demonstrated that injected current can be pushed from one edge to another by reversing the direction of the external magnetic field.     

Fourier-inversion and wavelet-transform methods applied to X-ray reflectometry and HRXRD profiles from complex thin-layered heterostructures

We show that X-ray scattering techniques can be used for the assessment of individual layer thicknesses inside complicated semi-conductor heterostructures dedicated to the opto-electronic domain. To this end, we propose methods to overcome two main drawbacks coming from: (1) the complexity of the X-ray profiles and, hence the difficulty to use model-dependent tools such as fitting procedures and (2) large dynamics in intensity due to numerous high diffraction superlattice peaks from superlattices which limit the use of the model-independent Fourier-inversion method.We demonstrate first the reliability of the Fourier-inversion method applied to high-resolution X-ray diffraction profiles curve from quantum well infrared photodetectors heterostructures, complementary to the model-dependent fitting tools. Then, a wavelet-transform-based procedure has been successfully used on X-ray reflectometry profiles containing intense SL Bragg peaks.     

Splitting and shift of hot magnetophonon resonance peaks in a two-dimensional electron gas

The splitting of the magnetophonon resonance peaks in a two-dimensional electron system is investigated as function of the electric field (or average electron velocity) for different values of the broadening of the Landau levels. We found that for small broadening the maxima in the magnetophonon oscillations are split into two peaks. A new physical interpretation is presented for this splitting which is based on the separate contributions of LO-phonon absorption and emission processes. A shift of the resonance maxima is found when the broadening of the Landau levels is large. A new explanation is given for the apparent temperature and electron density dependence of the optical phonon frequency in heterostructures as determined from magnetophonon resonance experiments. A mechanism is proposed which is able to produce the observed shift in the resonant position and which is consistent with an interaction with the optical phonon mode of the bulk material.     

Nonequilibrium green function techniques applied to hot electron quantum transport

During the last few years considerable effort has been devoted to deriving quantum transport equations for semiconductors under extreme conditions (high electric fields, spatial quantization in one or two directions). Here we review the results obtained with nonequilibrium Green function techniques as formulated by Baym and Kadanoff, or by Keldysh. In particular, the following topics will be discussed: (i) Systematic approaches to reduce the transport equation governing the correlation function to a transport equation for the Wigner function; (ii) Approximations reducing the nonmarkovian quantum transport equation to a numerically tractable form, and results for model semiconductors; (iii) Recent progress in extending the formalism to inhomogeneous systems; and (iv) Nonequilibrium screening. In all sections we try to direct the reader's attention to points where the present understanding is (at best) incomplete, and indicate possible lines for future work.     

Stimulated emission and optical gain in CdTe/CdMnTe graded index separate confinement heterostructures

Stimulated emission and optical gain in CdTe/CdMnTe graded index separate confinement quantum wells have been investigated as a function of optical excitation powers and temperatures. Maximum gain of about 100 cm^-1 is obtained at 95K for a single quantum well under 2-3 kW/cm² excitation. This value allows to design laser cavities compatible with the microgun pumped laser device concept. The temperature dependence of the gain still remains a problem (T₀ = 110K).