Interface Corrugation Research Articles

Surface plasmon-mediated photoluminescence boost in graphene-covered CsPbBr3 quantum dots

The optical properties of graphene (Gr)-covered CsPbBr3 quantum dots (QDs) were investigated using micro-photoluminescence spectroscopy, revealing a remarkable three orders of magnitude enhancement in photoluminescence (PL) intensity compared to bare CsPbBr3 QDs. To elucidate the underlying mechanisms, we combined experimental techniques with density functional theory (DFT) calculations. DFT simulations showed that the graphene layer generates interfacial electrostatic potential barriers when in contact with the CsPbBr3 surface, impeding carrier leakage from perovskite to graphene and enhancing radiative recombination. Additionally, graphene passivates CsPbBr3 surface defect states, suppressing nonradiative recombination of photo-generated carriers. Our study also revealed that graphene becomes n-doped upon contact with CsPbBr3 QDs, activating its plasmon mode. This mode resonantly couples with photo-generated excitons in the perovskite. The momentum mismatch between graphene plasmons and free-space photons is resolved through plasmon scattering at Gr/CsPbBr3 interface corrugations, facilitating the observed super-bright emission. These findings highlight the critical role of graphene as a top contact in dramatically enhancing CsPbBr3 QDs’ PL. Our work advances the understanding of graphene-perovskite interfaces and opens new avenues for designing high-efficiency optoelectronic devices. The multifaceted enhancement mechanisms uncovered provide valuable insights for future research in nanophotonics and materials science, potentially leading to breakthroughs in light-emitting technologies.

In-plane non-symmetric propagation of nonlinear elastic waves through a corrugated interface

The interaction between elastic waves and nonlinear materials can generate second harmonics, which makes inherent properties of linear systems can be broken in nonlinear ones. In this work, two half-spaces composed of nonlinear and linear solids are used to break the symmetric transmission, in which a corrugated interface is considered. Affected by interface corrugations, additional irregularly reflected and transmitted waves of the incident fundamental wave and its second harmonic can be generated. Based on Fourier series and Taylor expansion approximation, the amplitude ratios of irregular waves are derived. Numerical results show that the incident frequency and corrugation length can manipulate the propagation of irregular waves. Due to the asymmetry of material nonlinearity, second harmonics can enter the linear half-space form the nonlinear one, but the reverse transmission is not allowed. Both the theoretical analysis and experimental results can support the in-plane non-symmetric transmission in the two half-spaces, which is an extension of the nonreciprocal phenomena in nonlinear metamaterials.

Non–symmetric transmission of nonlinear elastic waves across a corrugated interface between two half–spaces

In this work, the symmetric transmission is broken in the system with a linear half–space combined by a nonlinear one with a corrugated interface. The interaction between nonlinear materials and elastic waves can generate second harmonics, so the non–symmetric transmission is realized. With the corrugation effects, irregularly reflected and transmitted waves are generated for both fundamental and second harmonics during the incident SH mode across the corrugated interface. Based on the Fourier series expansion method for the interface, the solution with the first order approximation of reflection and transmission coefficients are derived and the effects of the interface corrugation are discussed. The non–symmetric transmission is proved by both theoretical analysis and experiments. It is found that the reflected and transmitted waves are different when the fundamental wave incidents from the opposite direction. Compared with the positive incidence, there is no second harmonic in the linear half–space for the negative case and the generation of second harmonics in the nonlinear half–space is weaker.

Characterization of eigenstates interface-modulated in GaAs (631) multi-quantum well heterostructures

By taking advantage of the GaAs (631) corrugation self-assembled on top of multi-quantum well heterostructure interfaces, the modulation of the confined state wave functions (eigenstates) has been achieved, attaining quasi-one-dimensional or fractional dimension eigenstates. Two different theoretical approaches were used to compute the energy shift of subband optical transitions as a function of the interface corrugation geometrical configuration. For large nominal quantum well widths and small corrugation amplitude, the perturbation theory was employed, while a modified Lanczos algorithm assisted us to calculate the shifts when the corrugation amplitude was comparable to the nominal quantum well width. Experimentally, the heterostructures were grown by molecular beam epitaxy on (001) and (631) oriented substrates, where the quasi-one-dimensional ordering was reached by changing the As to Ga molecular beam fluxes ratio. It was found that the corrugated interfaces (i) break the wave function's in-plane symmetry, allowing transitions that, in principle, must be forbidden and (ii) induce blue shifts or red shifts in the order of 10 meV to the energy spectrum of the quantum wires depending on the lateral and vertical periodicities, exhibiting the presence of a lateral confinement system. The main result is the effective modulation of eigenstates through the interface corrugation control. Additionally, it was found that the interface modulation effect is greater for harmonic (n > 1) heavy (and light) hole subbands than for the ground states.

Love-Type Wave in PMN-PT Single Crystal Layered Structures with Periodic Undulations

Love-type wave propagation has been studied in a layered structure consisting of a thin layer of PMN-PT single crystal deposited on an elastic half-space. The weakly periodic undulations of the upper surface of the layered system and the common interface between the PMN-PT layer and the elastic substrate are considered. The PMN-PT single crystal is poled along the [011]_c direction so that the macroscopic symmetry is orthonormal mm2. The dispersion relations for electrically open and shorted circuits are obtained. The effects of various parameters including corrugation, undulatory, position and electrical boundary conditions on dispersion properties of Love-type waves are discussed. The results show that corrugations of the surface and the interface play a dominant role in the propagation of a Love-type wave, especially, the interfacial corrugation can produce an anomalous dispersion in the case of an electrically shorted condition. The surface corrugation enhances the phase velocity of a Love-type wave while the interface corrugation reduces the phase velocity in the range of relatively smaller wavenumber, and these variations gradually tend to vanish with increase in wavenumber. The undulatory parameter and the position parameter present a secondary impact on the dispersion behaviors of a Love-type wave compared with the corrugation parameters, both of them slightly enhance the phase velocity. The obtained results can offer some fundamentals for improving the efficiency and sensitivity of the interface response in the design and application of piezoelectric SAW devices.

Grid refinement in the three-dimensional hybrid recursive regularized lattice Boltzmann method for compressible aerodynamics.

Grid refinement techniques are of paramount importance for computational fluid dynamics approaches relying on the use of Cartesian grids. This is especially true of solvers dedicated to aerodynamics, in which the capture of thin shear layers require the use of small cells. In this paper, a three-dimensional grid refinement technique is developed within the framework of hybrid recursive regularized lattice Boltzmann method (HRR-LBM) for compressible high-speed flows, which is an efficient collide-stream-type method on a compact D3Q19 stencil. The proposed method is successfully assessed considering several test cases, namely, an isentropic vortex propagating through transition interface, shock-vortex interaction with intersection between grid refinement interface and shock corrugation, and transonic flows over three-dimensional DLR-M6 wing with seven levels of grid refinement.

The influence of the atomic scale interface roughness on the GMR effect in Fe/Cr multilayers

In this study, we analysed Fe/Cr multilayers that had been modified by Bi, In, and Pb surfactants. Their structure, morphology and interface roughness were investigated using low-energy electron diffraction, Auger electron spectroscopy (AES), X-ray reflectivity and conversion electron Mössbauer spectroscopy (CEMS). The magnetic and magnetotransport properties of the multilayers were studied by measuring the Kerr effect and four-point magnetoresistance, respectively.We observed that the Bi surfactant caused a slight interface smoothening while the introduction of In and Pb surfactants had a negative influence on the epitaxial growth of Fe and Cr, which were associated with an increase of interface roughness. These changes did not destroy the layer continuity, which was confirmed by AES measurements. The magnetic measurements revealed an antiferromagnetic coupling of the Fe layers for the as-deposited sample as well as for the Bi and Pb-modified multilayers and its partial reduction for the In-modified sample. We observed that the increase of interface roughness that had originated from atom intermixing resulted in a reduction of the giant magnetoresistance. In these investigations in which CEMS studies were used, we were able to distinguish between the interface roughness that was caused by atom intermixing and interface corrugation.

Experimental investigation of air–water turbulent swirling flow of relevance to phase separation equipment

Single and two-phase turbulent swirling flow inside a circular pipe in the presence of a conical bluff body is studied using high speed photography and laser-Doppler velocimetry. The geometrical configuration is relevant to liquid–gas separating equipment. Several flow regimes were identified and, in particular, the results show the presence of a spiral axial flow for low values of gas–liquid ratio (GLR). High speed photographs indicated that the flow consists of a gas core surrounded by a liquid annulus and forms a liquid–gas interface corrugations of capillary nature. The tip of the gas core becomes unstable and ejects gas bubbles in a spiraling stream. The velocity measurements indicated an increase of turbulent fluctuations in the liquid layer in the vicinity of the interface. The liquid layer maintained a Rankine vortex, while a weak forced vortex was obtained for higher GLR.

Operational and environmental conditions regulate the frictional behavior of two-dimensional materials

The friction characteristics of single-layer h-BN, MoS2, and graphene were systematically investigated via friction force microscopy measurements at various operational (e.g., normal force and sliding speed) and environmental (e.g., relative humidity and thermal annealing) conditions. The low friction characteristics of these single-layer materials were clearly observed from the normal force-dependent friction results, and their interfacial shear strengths were further estimated using a Hertz-plus-offset model. In addition, speed-dependent friction characteristics clearly demonstrated two regimes of friction as a function of sliding speed – the first is the logarithmic increase in friction with sliding speed regime at sliding speeds smaller than the critical speed and the second is the friction plateau regime at sliding speeds greater than the critical speed. Fundamental parameters such as effective shape of the interaction potential and its corrugation amplitude for these single-layer materials were characterized using the thermally-activated Prandtl-Tomlinson model. Moreover, friction of single-layer h-BN, MoS2, and graphene was found to increase with relative humidity and decrease with thermal annealing; these trends were attributed to the diffusion of water molecules to the interface between the single-layer materials and their substrates, which leads to an increase in the puckering effect at the tip-material interface and interaction potential corrugation. The enhanced puckering effect was verified via molecular dynamics simulations. Overall, the findings enable a comprehensive understanding of friction characteristics for several classes of two-dimensional materials, which is important to elucidate the feasibility of using these materials as protective and solid-lubricant coating layers for nanoscale devices.

Influence of metal support in-plane symmetry on the corrugation of hexagonal boron nitride and graphene monolayers

Predicting the properties of two-dimensional (2D) materials as graphene and hexagonal boron nitride (h-BN) monolayers after their growth on any given substrate is a major challenge. While the influence of the electron configuration of the atoms of the underlying surface is well-understood, the effect of substrate geometry still remains unclear. The structural properties of h-BN monolayers grown on a rectangularly packed Rh(110) surface were characterized in situ by ultrahigh vacuum scanning tunneling microscopy and were compared to those that this material exhibits when grown on substrates showing different crystallographic orientations. Although the h-BN monolayer grown on Rh(110) was dominated by a unique quasiunidimensional moire pattern, suggesting considerable interface interaction, the moire corrugation was unexpectedly smaller than those reported for strongly interacting interfaces with hexagonal-terminated substrates, owing to differences in the possible binding landscapes at interfaces with differently oriented substrates. Moreover, a rule was derived for predicting how interface corrugation and the existence and extent of subregions within moire supercells containing favorable sites for orbital mixing between h-BN monolayers and their supports depend on substrate symmetry. These general symmetry considerations can be applied to numerous 2D materials, including graphene, thereby enabling the prediction of how substrate choice determines the properties of these materials. Furthermore, they could also provide new routes for tuning 2D material properties and for developing nanotemplates showing different geometries for growing adsorbate superlattices.

Interparticle Interactions in Colloidal Systems: Toward a Comprehensive Mesoscale Model.

Intermolecular interactions control the collective behavior of colloidal clusters. The overwhelming majority of literature focuses on cohesive attributes of intermolecular forces as they govern the jamming process. However, the overlooked sliding component plays a critical role in the slow relaxation dynamics of colloidal aggregates and the emergence of discontinuous shear thickening in dense suspensions. Here, we use crystalline calcium-silicate-hydrate (C-S-H) as a model system to explore synergistic cohesive and sliding interactions. We use the free energy perturbation approach to reconstruct potential of mean force profile between two finite-sized nanolayers in an aqueous environment. We show that sliding free energy barriers decay exponentially as the separation distance increases. The characteristic length scale of the decay is related to the interface corrugation. We introduce a simple yet effective mechanism to capture the sliding behavior of colloids with surface roughness. Moreover, we develop a global free energy landscape model by juxtaposing cohesive and sliding interactions. This model enables us to measure the height of energy barriers, which is essential to elucidate deformation mechanism and dynamics of colloidal aggregates. For cohesive colloids, our approach predicts a sublinear relation between applied normal and shear stress at the onset of sliding that is in contrast to Amontons' laws of friction. We demonstrate that the sublinear trend is due to the adhesion and nature of soft contact at the nanoscale. The proposed framework provides a new route to draw a more realistic picture of intermolecular interactions in nanoparticulate systems such as geomaterials, cementitious systems, complex colloidal assemblies, and dense suspensions.

Two types of single-beam deflection and asymmetric transmission in photonic structures without interface corrugations.

We study single-beam deflection and asymmetry in transmission, two aspects of the same phenomenon that appear in the topologically simple, nonsymmetric, photonic crystal (PhC)-based structures without corrugations at the interfaces. Strong diffractions enabling efficient blazing, i.e., redistribution of the incident wave energy in favor of the desired higher diffraction order(s), can be achieved owing to the defect-like layer(s) embedded in a regular slab of PhC. The main features, together with the peculiarities of the two basic transmission types and relevant coupling and deflection scenarios, are discussed, for one of which a part of the PhC works in the evanescent-wave regime. Performances are suggested, in which efficient single-beam deflection and asymmetry in transmission can be obtained even when the irregular layer is deeply embedded. More than 97% of the incident wave energy can be converted into a single deflected beam that is associated with the first negative diffraction order, even though the entire structure is nonsymmetric and the diffractive element is located at some distance from the incidence interface.

Diffraction inspired unidirectional and bidirectional beam splitting in defect-containing photonic structures without interface corrugations

It is shown that strong diffractions and related dual-beam splitting can be obtained at transmission through the nonsymmetric structures that represent two slabs of photonic crystal (PhC) separated by a single coupled-cavity type defect layer, while there are no grating-like corrugations at the interfaces. The basic operation regimes include unidirectional and bidirectional splitting that occur due to the dominant contribution of the first positive and first negative diffraction orders to the transmission, which is typically connected with different manifestations of the asymmetric transmission phenomenon. Being the main component of the resulting transmission mechanism, diffractions appear owing to the effect exerted by the defect layer that works like an embedded diffractive element. Two mechanisms can co-exist in one structure, which differ, among others, in that whether dispersion allows coupling of zero order to a wave propagating in the regular, i.e., defect-free PhC segments or not. The possibility of strong diffractions and efficient splitting related to it strongly depend on the dispersion properties of the Floquet-Bloch modes of the PhC. Existence of one of the studied transmission scenarios is not affected by location of the defect layer.

Surface breakup of a non-turbulent liquid jet injected into a high pressure gaseous crossflow

We employ detailed numerical simulations to understand the physical mechanism underlying the surface breakup of a non-turbulent liquid jet injected transversely into a high pressure gaseous crossflow under isothermal conditions. The numerical observations reveal the existence of shear instability on the jet periphery as the primary destabilization mechanism. The temporal growth of such azimuthal instabilities leads to the formation of interface corrugations, which are eventually sheared off of the jet surface as sheet-like structures. The sheets next undergo disintegration into ligaments and drops during the surface breakup process. The proposed instability mechanism is inherently an inviscid mechanism, contrary to the previously suggested mechanism of surface breakup (known as “boundary layer stripping”), which is relied on a viscous interpretation. The numerically obtained length and time scales of the shear instabilities on the jet laterals are compared with the results of Behzad et al. (2015) on temporal linear stability analyses of a jet in crossflow at near the nozzle. The stability characteristics of the most amplified modes (i.e., the wavenumber and the corresponding growth rate) obtained from the numerical simulations and the stability analyses are in good agreement.

Azimuthal shear instability of a liquid jet injected into a gaseous cross-flow

AbstractWe investigate azimuthal instabilities which exist on the periphery of a non-turbulent liquid jet injected transversely into a gaseous cross-flow. We predict that the temporal growth of such instabilities may lead to the formation of interface corrugations, which are eventually sheared off of the jet surface (known as the jet ‘surface breakup’). In this study we employ temporal linear stability analyses to understand the nature of these instabilities. The analysis is based on a continuous formulation of momentum equations in which the jet and cross-flow are considered to be slightly miscible at the vicinity of the interface. We identify the shear instability as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as ‘boundary-layer stripping’), which is based on a viscous interpretation. The results show that the wavelengths of instabilities increase by moving away from the jet windward stagnation point toward the leeward point. We also investigate the influence of the jet-to-cross-flow density ratio on the flow stability and find that a higher ratio leads to formation of instabilities with higher wavenumbers on the jet surface. The results show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow.

Corrugation reduction in periodically inverted GaAs by molecular beam epitaxy growth using arsenic dimers

Molecular beam epitaxy (MBE) regrowth of GaAs on periodically inverted GaAs(100) substrates using arsenic dimers (As2) was studied. We found that we can reduce corrugations on the regrown GaAs by MBE growth using As2 at considerably higher substrate temperatures (∼450 °C) than in the case of growth using arsenic tetramers (∼300 °C). This result indicates that As2 MBE regrowth can be used to fabricate highly efficient GaAs/AlGaAs wavelength conversion waveguiding devices owing to a reduction in the propagation losses caused by interface corrugations. We also estimated the diffusion lengths of Ga atoms coexisting with As2 on GaAs(100) surfaces from experimental thickness variations on the basis of a diffusion equation analysis.

Atomic scale structure investigations of epitaxial Fe/Cr multilayers

Abstract Fe/Cr multilayers were deposited by molecular beam epitaxy on the MgO(1 0 0) substrate. Structural properties of the samples were analyzed by low energy electron diffraction, high resolution transmission electron microscopy (HRTEM), as well as by X-ray reflectivity, conversion electron Mossbauer spectroscopy (CEMS) and Auger electron spectroscopy. Investigations revealed multilayered system built of well-ordered Fe and Cr thin films with (1 0 0) orientation. A high geometrical perfection of the system, i.e. planar form of interfaces and reproducible thickness of layers, was also proven. Fe/Cr interface roughness was determined to be 2–3 atomic layers. CEMS studies allowed to analyze at atomic scale the structure of buried Fe/Cr interfaces, as well as to distinguish origin of interface roughness. Roughnesses resulting from interface corrugations and from the Fe–Cr interdiffusion at interfaces were observed. Fe/Cr multilayers showed strong antiferromagnetic coupling of Fe layers.

Morphology evolution in strain-compensated multiple quantum well structures

Morphology evolution in (In,Ga)As-Ga(As,P) strain-compensated multilayer structures is studied. The effects of nanoscale interface corrugation and phase separation are evident after the third period of the multilayer structure and become more pronounced with each new stack until the sixth period. Then, the interface stabilizes pointing to the formation of strain-balanced equilibrium interface structure. The epitaxial structure remains defect-free up to the maximum number (twenty) of periods studied. In a structure with a high lattice mismatch between the neighboring layers, In0.40Ga0.60As/GaAs0.85P0.15, clusters of dislocations are revealed already in the third period. The observed phenomena are critical for proper engineering of optoelectronic devices.

Analytical solution for the diffraction of an electromagnetic wave by a graphene grating

An analytical method for diffraction of a plane electromagnetic wave at a periodically modulated graphene sheet is presented. Both interface corrugation and a periodic change in the optical conductivity are considered. Explicit expressions for reflection, transmission, absorption and transformation coefficients in arbitrary diffraction orders are presented. The dispersion relation and decay rates for graphene plasmons of the grating are found. Simple analytical expressions for the value of the bandgap in the vicinity of the first Brillouin zone edge are derived. The optimal amplitude and wavelength, guaranteeing the best matching of the incident light with graphene plasmons are found for the conductivity grating. The analytical results are in a good agreement with first-principles numeric simulations.

The effect of impedance contrast upon surface motion due to scattering of plane harmonic P, SV, and Rayleigh waves by a randomly corrugated elastic inclusion

Scattering of a plane harmonic P, SV, or Rayleigh wave by a corrugated elastic inclusion completely embedded in a two-dimensional isotropic half space is investigated by using a direct boundary integral equation method. The corrugated scatterer is generated by superimposing a random perturbation of arbitrary amplitude to a smooth elliptical shape. The probability density function for the randomly corrugated inclusions is assumed to be of uniform or normal types. The displacement fields are evaluated along the half-space surface for a range of impedance contrasts, frequencies, five incident waves for circular- and elliptic-based rough inclusions. Subsequently, the roughness factor is introduced in terms of the surface response for the rough and the corresponding smooth inclusions. The results clearly show that this factor strongly depends upon the impedance contrast of the materials, the basic inclusion geometry, the type of incident wave, and the frequency. The peak values of the roughness factor are observed for the near-grazing SV incidence. Furthermore, the impedance contrast of the materials has a non-uniform effect upon the surface motion. While the smallest roughness factor is consistently observed for the minimal impedance contrast the peak values of the same factor may take place for different non-minimal impedance contrasts. For the range of frequencies considered here, the type of the random interface corrugation (random-uniform vs. random-normal) has a minimal effect upon the roughness factor.