Single-layer Heterostructures Research Articles

Two-Dimensional Magnetic Semiconducting Heterostructures of Single-Layer CrI3-CrI2.

Single-layer heterostructures of magnetic materials are unique platforms for studying spin-related phenomena in two dimensions (2D) and have promising applications in spintronics and magnonics. Here, we report the fabrication of 2D magnetic lateral heterostructures consisting of single-layer chromium triiodide (CrI3) and chromium diiodide (CrI2). By carefully adjusting the abundance of iodine based on molecular beam epitaxy, single-layer CrI3-CrI2 heterostructures were grown on Au(111) surfaces with nearly atomic-level seamless boundaries. Two distinct types of interfaces, i.e., zigzag and armchair interfaces, have been identified by means of scanning tunneling microscopy. Our scanning tunneling spectroscopy study combined with density functional theory calculations indicates the existence of spin-polarized ground states below and above the Fermi energy localized at the boundary. Both the armchair and zigzag interfaces exhibit semiconducting nanowire behaviors with different spatial distributions of density of states. Our work presents a novel low-dimensional magnetic system for studying spin-related physics with reduced dimensions and designing advanced spintronic devices.

Mechanical Properties of All MoS2 Monolayer Heterostructures: Crack Propagation and Existing Notch Study

The outstanding thermal, optical, electrical and mechanical properties of molybdenum disolphide (MoS2) heterostructures make them exceptional candidates for an extensive area of applications. Nevertheless, despite considerable technological and academic interest, there is presently a few information regarding the mechanical properties of these novel two-dimensional (2D) materials in the presence of the defects. In this manuscript, we performed extensive molecular dynamics simulations on pre-cracked and pre-notched all-molybdenum disolphide (MoS2) heterostructure systems using ReaxFF force field. Therefore, we study the influence of several central-crack lengths and notch diameters on the mechanical response of 2H phase, 1T phase and composite 2H /1T MoS2 monolayers with different concentrations of 1T phase in 2H phase, under uniaxial tensile loading at room temperature. Our ReaxFF models reveal that larger cracks and notches decrease the strength of all 2D MoS2 single-layer heterostructures. Additionally, for all studied crack and notch sizes, 2H phase of MoS2 films exhibits the largest strength. Maximum tensile stress of composite 2H/1T MoS2 nanosheet with different concentrations are higher than those for the equivalent 1T phase, which implies that the pre-cracked composite structure is remarkably stronger than the equivalent 1T phase. The comparison of the results for cracked and notched all-MoS2 nanosheet heterostructures reveal that the load bearing capacity of the notched samples of monolayer MoS2 are higher than the cracked ones.

Photodegradation Protection in 2D In-Plane Heterostructures Revealed by Hyperspectral Nanoimaging: The Role of Nanointerface 2D Alloys.

Single-layer heterostructures exhibit striking quasiparticle properties and many-body interaction effects that hold promise for a range of applications. However, their properties can be altered by intrinsic and extrinsic defects, thus diminishing their applicability. Therefore, it is of paramount importance to identify defects and understand 2D materials' degradation over time using advanced multimodal imaging techniques. Here we implemented a liquid-phase precursor approach to synthesize 2D in-plane MoS2-WS2 heterostructures exhibiting nanoscale alloyed interfaces and map exotic interface effects during photodegradation using a combination of hyperspectral tip-enhanced photoluminescence and Raman and near-field nanoscopy. Surprisingly, 2D alloyed regions exhibit thermal and photodegradation stability providing protection against oxidation. Coupled with surface and interface strain, 2D alloy regions create stable localized potential wells that concentrate excitonic species via a charge carrier funneling effect. These results demonstrate that 2D alloys can withstand extreme degradation effects over time and could enable stable 2D device engineering.

Self-assembly of single layer V2O5 nanoribbon/graphene heterostructures as ultrahigh-performance cathode materials for lithium-ion batteries

Two-dimensional (2D) heterostructures composed of the single layer V2O5 and graphene achieve good chemical diffusion of Li+, high conductivity and cycling stability. Hence, a simple and novel bottom-up approach has been proposed to build the true single layer V2O5 nanoribbon/graphene heterostructures. The single layer V2O5 nanoribbon, which has a thickness of 800 p.m. and are approximate 80 nm in wide, is combined with graphene as sandwich-like heterostructures through the layer-by-layer assembly method. The distinct heterostructures can effectively shorten the practical diffusion pathway for Li+ and improve the electrical conductivity of the overall electrode. Therefore, the as-prepared ultrathin V2O5 nanoribbon/graphene heterostructures exhibit excellent reversible capacity and stable cycling performance as a cathode material for lithium storage. Under the current density of 10 C, the electrode made from the ultrathin V2O5 nanoribbon/graphene heterostructures has an ultrahigh initial discharge capacity of 225 mAh g−1 (with corresponding coulombic efficiency of 99.6%) and a superior capacity retention of 92.8% after 600 charge and discharge cycles. In all, rapid ion and electron diffusions, which are the kinetic demands of alternative electrode material for lithium storage, will be satisfied by our single layer heterostructures.

Thermal Conductivity of Graphene-hBN Superlattice Ribbons

Superlattices are ideal model systems for the realization and understanding of coherent (wave-like) and incoherent (particle-like) phonon thermal transport. Single layer heterostructures of graphene and hexagonal boron nitride have been produced recently with sharp edges and controlled domain sizes. In this study we employ nonequilibrium molecular dynamics simulations to investigate the thermal conductivity of superlattice nanoribbons with equal-sized domains of graphene and hexagonal boron nitride. We analyze the dependence of the conductivity with the domain sizes, and with the total length of the ribbons. We determine that the thermal conductivity reaches a minimum value of 89 W m−1K−1 for ribbons with a superlattice period of 3.43 nm. The effective phonon mean free path is also determined and shows a minimum value of 32 nm for the same superlattice period. Our results also reveal that a crossover from coherent to incoherent phonon transport is present at room temperature for BNC nanoribbons, as the superlattice period becomes comparable to the phonon coherence length. Analyzing phonon populations relative to the smallest superlattice period, we attribute the minimum thermal conductivity to a reduction in the population of flexural phonons when the superlattice period equals 3.43 nm. The ability to manipulate thermal conductivity using superlattice-based two-dimensional materials, such as graphene-hBN nanoribbons, opens up opportunities for application in future nanostructured thermoelectric devices.

Phenomenological description of strain relief in step-graded metamorphic buffer layers based on In x Al1 − x As ternary solutions

Spatial distributions of the residual elastic strains in layers of step-graded metamorphic buffers of two different designs, grown via molecular beam epitaxy on the basis of In x Al1 − x As ternary solutions, are obtained by means of reciprocal space mapping. It is shown that with allowance for work hardening, which affects strain relief in buffer layers and increases the strain in dislocation-free layers, the mechanism of strain relief in the final buffer steps, and the residual elastic strain in a buffer dislocation-free layer, are governed by the same phenomenological law as in a single-layer heterostructure.

Comparative analysis of strain fields in layers of step-graded metamorphic buffers of various designs

Spatial distribution of residual elastic strain in the layers of two step-graded metamophic buffers of various designs, grown by molecular beam epitaxy from ternary InxAl1–xAs solutions on GaAs(001) substrates, is obtained using reciprocal space mapping by three-axis X-ray diffractometry and the linear theory of elasticity. The difference in the design of the buffers enabled the formation of a dislocation-free layer with different thickness in each of the heterostructures, which was the main basis of this study. It is shown that, in spite of the different design of graded metamorphic buffers, the nature of strain fields in them is the same, and the residual elastic strains in the final elements of both buffers adjusted for the effect of work hardening subject to the same phenomenological law, which describes the strain relief process in single-layer heterostructures.

Comparative Analysis of Strain Fields in Step-graded Buffers of Different Design, based on InxAl1-xAs Ternary Solutions

Two hetero structures with the step-graded buffers of different design grown on (001) GaAs substrates by molecular beam epitaxy were employed to reveal applicability of an extension of phenomenological approach developed for the description of strain relief in single layer hetero structures to multilayer thin film systems. Difference in the design of buffers provided to the formation of dislocation free layers of different thickness. The determination of the residual strains in the epitaxial layers was done using reciprocal space mapping performed with a tripleaxes X-ray diffractometer Smart Lab 9 kW and the following processing of data obtained within the linear theory of elasticity. It was established that, despite the different design of buffers the character of strain spatial distributions in them was similar. It gives possibility to attract a phenomenological rule to describe the strain relief in the final constructive elements of both hetero structures. A correction for a work hardening in the phenomenological rule governing the strain relief in single layer hetero structures was performed.

Multiscale modelling of heat conduction in all-MoS2 single-layer heterostructures

We developed a combined atomistic-continuum multiscale modeling to explore the effective thermal conductivity of all-MoS2 single-layer heterostructures.

Intervalley scattering in GaAs/AlGaAs quantum wells and quantum cascade lasers

The results of simulations of Γ−X scattering in GaAs/AlGaAs quantum wells are presented, discussing the importance of the mole fraction, doping density, and lattice and electron temperatures in determining the scattering rates. A systematic study of Γ−X scattering in GaAs/Al x Ga 1− x As heterostructures, using a single quantum well to determine the importance of well width, molar concentration x, lattice temperature, and doping density, has been performed. After this we consider a double quantum well to determine the role of intervalley scattering in the transport through single-layer heterostructures, i.e. Γ−X− Γ scattering compared with Γ− Γ scattering. Finally, we estimate the relative importance of intervalley scattering in a GaAs-based quantum-cascade laser device and compare it with other relevant scattering mechanisms important to describe carrier dynamics in the structure. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures.

Spin ordering: two different scenarios for the single and double layer structures in the fractional and integer quantum Hall effect regimes

AbstractWe investigate the ground state competition at the transition from the spin unpolarized to spin ordered phase at filling factor ν =2/3 in single layer heterostructure and at ν =2 in double layer quantum well. To trace the quantum Hall phase we use the minimum in the dissipative conductivity σ xx We observe two different transition scenarios in two investigated situations. For one of them we propose a qualitative explanation, based on the domain structure evolution in the vicinity of the transition point. The origin for the second scenario, corresponding to the experimental situation at ν =2 in double layer quantum well, still remains unclear. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain and compositional profile determination in ion bombarded heterostructures by the complementary use of RBS/channeling and high resolution X-ray diffraction

Damage creation in In 0.517Ga 0.483As/InP single layer heterostructures and superlattices during bombardment with 8 MeV As ions was investigated by RBS/channeling and high resolution X-ray diffraction (HRXRD). Important defect migration during ion bombardment through the interface was observed, which leads to defect depletion in the InP region close to interface. Large defect accumulation was found in InGaAs on the other side of the interface leading to the substantial increase of strain in this layer. A similar effect was observed in a superlattice composed of the same materials. HRXRD analysis revealed that the strain build up is much lower in the InP layers than in the InGaAs ones.

Misfit Dislocations in Epitaxial Heterostructures: Mechanisms of Generation and Multiplication

In this paper, the regularities of dislocation pattern formation in single layer heterostructures with low lattice mismatch (f < 1%) are reviewed. The main attention is focused on the recently suggested mechanisms of dislocation nucleation and multiplication. An evolution of dislocation pattern from regular flat misfit dislocation network to the dense three-dimensional one was found to proceed during the epitaxial growth. Experimentally found regularities of dislocation structure formation are treated in terms of the influence of the nature of materials contacting at the interface upon the dislocation behavior. Special emphasis is given to the role of microsegregations and self-point defects in the alloy matrix in dislocation generation, propagation and multiplication.

Work-hardening effects in the lattice relaxation of single layer heterostructures

A modelization of the strain relaxation in single heteroepitaxial layers is presented in this letter. The calculations consider the energetic variations of the heteroepitaxial structure when introducing one new dislocation into the existing interfacial array of fixed misfit dislocations without continuous readjustment of the spacing array. The interaction energy of the new dislocation with both lattice mismatch and dislocation array is shown to be the limiting factor that controls the mechanism of strain relaxation at the saturation stage of relaxation. The model is shown to be in good agreement with the lattice relaxation behavior of previously published strain/thickness data.

Strained-layer analysis with high-energy ion channeling

Channeling with high-energy He or H ions offers enhanced angular resolution since the characteristic angle for channeling decreases with increasing energy. In this study channeling experiments with high-energy He ions (4–15 MeV) have been performed with the Eindhoven AVF cyclotron, carefully controlling important parameters such as beam divergence and target alignment. We applied high-energy ion channeling using a three-axis goniometer with high angular accuracy for strain analysis in Si/Si 1 − x Ge x /Si single-layer heterostructures with x = 0.175 and 0.05. The strain is derived from axial angular scans through the 〈110〉 and 〈111〉 directions in the {100} and {110} planes, respectively. The obtained scans are interpreted using the ion-trajectory simulation program LAROSE [J.H. Barrett, Phys. Rev. B 3 (1971) 1527]. The perpendicular lattice constant of the deformed Si 1 − x Ge x layers was determined with an accuracy comparable to the accuracy reached with high resolution X-ray diffraction.

Calculation of compositional dependence of stresses in GaInAs/GaAs strained multilayer heterostructures

A theoretical model was proposed to calculate the stress in strained multilayer heterostructures. In this model, each composed crystal layer is divided into many imaginary thin layers. The face force, moment and strained balance were considered over all the imaginary thin layers with coherent interfaces. The accurate bending moment for multilayer heterostructures was derived by expanding Davidenkov's expression of a two-phase composite layer, and it was used in the model. Using this model, the stresses at 25°C were calculated for both the GaInAs/GaAs strained multilayer heterostructure and the GaInAs/GaAs single layer heterostructure, and were compared with each other. In this calculation, the relative thickness of GaInAs layers in the GaInAs/GaAs strained multilayer heterostructure is set up to be equal to that of the GaInAs layer in the GaInAs/GaAs single layer heterostructure. The stress in both the structures was calculated over the entire GaInAs layer composition range. The relative stress in the GaInAs/GaAs bottom heterointerface of the multilayer heterostructure is smaller than that in the GaInAs/GaAs heterointerface of the single layer heterostructure especially for In-rich GaInAs. However, the absolute stress in the GaInAs layer at the bottom heterointerface of the multilayer heterostructure is larger than that in the GaInAs layer at the heterointerface of the single layer heterostructure over the entire GaInAs composition range. The difference between the stresses of both heterostructures increases as the thickness of the GaInAs layer increases. The total thickness of GaInAs layers in the multilayer heterostructure corresponding to a constant interfacial relative stress decreases as the Ga composition in the GaInAs layers decreases and it abruptly decreases when the Ga composition is smaller than 0.4. The relative stress in the GaInAs/GaAs bottom heterointerface of the multilayer heterostructure decreases as the thickness of the GaAs barrier layer increases, but the absolute stress in the GaInAs layer at the GaInAs/GaAs bottom heterointerface of the multilayer heterostructure increases as the thickness of the GaAs barrier layer increases. The effect of the thickness of the GaAs barrier layer on the relative and absolute stresses increases as the thickness of the GaInAs layers increases. These results imply that misfit dislocations due to the interfacial total stress are more easily introduced into the single layer heterostructure than into the multilayer heterostructure, and the band structure due to the absolute stress is more easily transformed in the multilayer heterostructure than in the single layer heterostructure.

Elastic distortion field in single layer heterostructures in the presence of misfit dislocations

The elastic distortion field in (001)-grown lattice mismatched heterostructures of diamond or zinc-blende structure having misfit dislocations at the interface is calculated within the linear elasticity theory. Not only the misfit dislocation densities in the two 〈110〉 directions but also the distribution of the possible Burgers vectors are taken into consideration. A transition layer where the elastic field is appreciably laterally nonuniform extends from the interface up to a distance of the same order of the mean dislocation spacing. It is shown that this transition layer affects x-ray diffraction measurements. Beyond this region, the elastic distortion field is uniform and is found to depend only on the mean values of the Burgers vectors associated with the two dislocation distributions. In particular it is shown that in general the strain field, i.e., the symmetric part of the elastic distortion field, is not biaxial. The three independent parameters describing the lattice deformations in the epilayer depend not only on the projections on the interface of the edge components but also on the screw components of the Burgers vectors. The predictions of this model are tested on InxGa1−xAs/GaAs samples analyzed by means of channeling and double crystal x-ray diffractometry. The unbalance of the dislocation densities in the two 〈110〉 directions measured by the dechanneling behavior of the samples agrees rather well with the prediction of the model on the basis of the measured asymmetries of the strain field.