Interfacial Coupling Strength Research Articles

Freeze-out of perturbation growth for shocked heavy fluid layers by eliminating reverberating waves

The shock wave accelerating a heavy fluid layer can induce reverberating waves that continuously interact with the first and second interfaces. In order to manipulate the perturbation growths at fluid-layer interfaces, we present a theoretical framework to eliminate the reverberating waves. A model is established to predict the individual freeze-out (i.e. stagnation of perturbation growth) for the first and second interfaces under specific flow conditions determined based on the shock dynamics theory. The theoretical model quantifies the controllable parameters required for freeze-out, including the initial amplitudes of the first and second interfaces, the interface coupling strength and the maximum initial layer thickness preventing the second interface's phase reversal. The effectiveness of the model in predicting individual freeze-out for the first and second interfaces is validated numerically over a wide range of initial conditions. The upper and lower limits of initial amplitudes for the freeze-out of the whole fluid-layer width growth are further predicted. Within this amplitude range, a slightly higher initial amplitude for the second interface is specified, effectively arresting the growth of the entire fluid-layer width before the phase reversal of the second interface.

Thermal bridging effect enhancing heat transport across graphene interfaces with pinhole defects

Interfacial thermal conductance (G) is of critical significance to the efficient thermal management of two-dimensional (2D) material devices. Despite the importance of defects engineering in tuning G, the mechanisms of heat transport across defective graphene interfaces remain unclear. In this study, we have identified and demonstrated the origins of the enhanced heat transport across pinhole defective graphene through our ultrafast pump-probe measurements and molecular dynamics (MD) simulations. In prior work, the enhanced G across defective graphene interfaces was commonly attributed to the improved overlap of phonon density of states (DOS) or interfacial coupling strength. We, however, observe a remarkable 2.5-fold increase in heat conduction and a distinctly different temperature dependence of G for defective graphene with the same superstrate and substrate material, compared to pristine graphene. In contrast, interfaces with different superstrate and substrate materials show only minor changes (<15%) in thermal conductance after introducing defects in the graphene layer. Through careful analysis of our MD simulations for various defective graphene interfaces, we conclude that, contradictory to common beliefs, it is the formation of direct contact thermal bridge, instead of the improvement in the phonon DOS overlap or interfacial coupling strength, that has a decisive effect and promotes heat transport across defective graphene interfaces. The differing impacts of the thermal bridge on G of defective graphene sandwiched by layers of the same and different materials are primarily attributed to the varying coupling strength between the substrate and superstrate. Our work provides important insights and physical understandings of the mechanisms of heat conduction across defective graphene interfaces.

Molecular dynamics simulation of thermal transport properties of phonons at interface of Au-TiO&lt;sub&gt;2&lt;/sub&gt; photoelectrode

Thermoplasmonics originating from the relaxation process of plasmon resonances in nanostructures can be utilized as an efficient and highly localized heat source in solar-hydrogen conversion, but there have been few researches on the interfacial heat transport properties of photoelectrode with the thermoplasmonics effect in a photoelectrochemical water splitting system. In this work, the effects of temperature, interfacial coupling strength and the addition of graphene layers on the interfacial thermal conductance of Au-TiO&lt;sub&gt;2&lt;/sub&gt; electrodes are investigated by the non-equilibrium molecular dynamics simulation, and the variation of interfacial thermal conductance is analyzed by the phonon density of states. The results show that the interfacial thermal conductivity is increased by 78.55% when the temperature increases from 300 to 800 K. This is related to the fact that more low-frequency phonons participate in the interface heat transport, allowing more heat to be transferred to TiO&lt;sub&gt;2&lt;/sub&gt; to promote the interface reaction. As the coupling strength of the Au-TiO&lt;sub&gt;2&lt;/sub&gt; interface increases, the interfacial thermal conductivity of the electrode increases and then tends to stabilize. The interfacial thermal conductivity can be optimized by increasing the degree of overlap of the phonon state densities of Au and TiO&lt;sub&gt;2&lt;/sub&gt;. The addition of a single layer of graphene can increase the interfacial thermal conductivity to 98.072 MW⋅m&lt;sup&gt;–2&lt;/sup&gt;⋅K&lt;sup&gt;–1&lt;/sup&gt;, but the addition of 2 and 3 layers of graphene can hinder interfacial heat transfer in Au and TiO&lt;sub&gt;2&lt;/sub&gt; due to the interaction between the layers of graphene. When adding graphene layer, medium-frequency phonons and high-frequency phonons are stimulated to participate in the interfacial heat transfer, but with the increase of the graphene layers, the number of low-frequency phonons in a range of 0—30 THz decreases, and these low-frequency phonons make the greatest contribution to the interfacial thermal conductivity. The obtained results are useful in regulating the thermal transport properties of the photoelectrode interface, which can provide new insights into and theoretical basis for the design and construction of composite photoelectrodes.

Molecular dynamics study of the interfacial thermal transport properties of the graphene/GaN heterostructure

The performance of interfacial thermal transport in heterostructures determines the reliability of micro- and nano-scale devices. In this study, a molecular dynamics methods is used to investigate the interfacial thermal transport properties of graphene/GaN sandwich heterostructure. The effects of temperature, defect, and size on the interface thermal conductance at the heterostructure are analyzed. It is found that the interface thermal conductance increases with temperature; at 1100 K, the interface thermal conductance of the 3-layer graphene heterostructure increases by 61%. This increase is mainly attributed to the enhanced lattice vibrations at higher temperature, which excite more out-of-plane phonons. The presence of minor vacancy defects in GaN leads to an increase in interface thermal conductance, reaching a maximum of 0.0357 GW/(m&lt;sup&gt;2&lt;/sup&gt;·K) at the defect rate of 20%. This enhancement is believed to be due to additional thermal transport pathways created by the defects. However, as the defect rate increases further, the interface thermal conductance begins to decrease, which is thought to be due to a reduction in interfacial coupling strength. With an increase in the number of GaN layers from 8 to 24, the interface thermal conductance decreases, the change attributes to a reduction in the number of phonons participating in the thermal transport across the interface. Conversely, increasing the number of graphene layers from 2 to 6 initially increases and then decreases the interface thermal conductance. This behavior is linked to initial improvements in phonon matching and coupling strength, and followed by increases in phonon scattering and localization. The results of this study provide a theoretical basis for the regulation of interfacial thermal transport in microelectronic devices.

Thermal transport mechanism of 4H–SiC/SiO2 heterostructures: a molecular dynamics study

Silicon carbide (SiC) is widely used in high-frequency, high-speed, and high-power applications such as power electronics, rail transportation, new energy vehicles, and aerospace. However, the thermal properties of the SiC/SiO2 interface, which is commonly found in SiC-based devices, are not yet fully understood. This study aims to investigate the influence of temperature and interface coupling strength on the interface thermal resistance (ITR) of 4H-SiC/SiO2 using non-equilibrium molecular dynamics simulations. Both crystalline and amorphous SiO2, as well as two interface contact modes (Si-terminated and C-terminated), have also been considered. The results reveal that the ITR of 4H-SiC/SiO2 is significantly affected by the interface coupling strength and contact modes. Under strong interface coupling conditions, the ITR for Si-terminated and C-terminated contacts modes of 4H-SiC/SiO2 interfaces are 8.077 × 10−10 m2KW−1 and 6.835 × 10−10 m2KW−1, respectively. However, under weak interface coupling conditions, these values increase to 10.142 × 10−10 m2KW−1 and 7.785 × 10−10 m2KW−1, respectively. Regardless of whether SiO2 is crystalline or amorphous, the ITR of the 4H-SiC/SiO2 interface exhibits a similar trend with increasing temperature (from 300 to 700 K). Additionally, the ITR of the amorphous SiO2 interface is smaller than that of the crystalline SiO2 interface under both strong and weak coupling conditions. To gain insights into the heat transport mechanism, the phonon density of states was analyzed to examine the phonon spectral characteristics under varying coupling strengths. These findings have implications for enhancing the thermal management and heat dissipation of SiC devices, providing a framework for controlling interface phonon scattering, and informing the thermal design of nanodevices.

Tuning the Inter-Nanoplatelet Distance and Coupling Strength by Thermally Induced Ligand Decomposition.

CdSe nanoplatelets (NPLs) are promising 2D semiconductors for optoelectronic applications, in which efficient charge transport properties are desirable. It is reported that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. Combining optical absorption, transmission electron microscopy, and thermogravimetric analysis, it is revealed that the thermal decomposition of ligands (e.g., cadmium myristate) governs the inter-NPL distance and thus the inter-NPL electronic coupling strength. Employing ultrafast terahertz spectroscopy, it is shown that this enhanced electronic coupling increases both the free carrier generation efficiency and the short-range mobility in NPL solids. The results show a straightforward method of controlling the interfacial electronic coupling strength for developing functional optoelectronic devices through thermal treatments.

Proximity-induced superconductivity in type-II Weyl semimetal NbIrTe4

Heterostructures between conventional superconductors and materials with different electronic ground states have emerged as a powerful method for exploring the exotic superconducting properties induced by the proximity effect. Here, we investigate Andreev transport through the interface between an s-wave superconductor Nb and a type-II Wely semimetal NbIrTe4. The differential conductance measurement reveals an anomalous zero-bias conductance peak and prominent subgap structures at low temperatures. Furthermore, we found that these subgap structures are not only related to the interface coupling strength but also influenced by the thickness of the NbIrTe4 flake. For thin devices (≤100 nm), the differential conductance spectra only exhibit a single-gap structure. While in thicker devices (∼150 nm), we observed the distinct double-gap structure, which is likely to originate from the proximity-induced superconductivity gap on the bulk and surface of the NbIrTe4 flakes. These results can provide a good reference for understanding the superconducting phase in type-II Weyl semimetals and take a step toward its future application in the field of superconducting electronics.

Enhanced low-field positive magnetoresistance and magnetic anisotropy in La0.7Sr0.3MnO3 films grown on (001) Si

Strong anisotropy is observed in the magnetization and magnetoresistance (MR) of a series of La0.7Sr0.3MnO3 (LSMO) films grown on (001) oriented Si using pulsed sputtering plasma. Magnetic anisotropy (MA) of LSMO films is in the order of 106 erg/cm3, comparable to or larger than that of the other heterostructures. Analysis using the Stoner-Wohlfarth model and the density functional theory (DFT) indicates that the crystalline and induced anisotropies are almost compensating with each other, and observed MA is of the same order of shape anisotropy. The rise in MA with the LSMO thickness is attributed to the shape anisotropy and interfacial spin reordering. These LSMO films exhibit positive and negative MR, even though the LSMO is well known for colossal negative MR. The 12.4 nm thick LSMO film shows a maximum ∼7 % in-plane low-field positive MR, which increases to ∼12 % for 20.0 nm thick LSMO, but the variation of out-of-plane positive MR is ∼ ± 0.8 %. The maximum in-plane positive MR occurs at ∼ ± 0.2 kG (switching field), which corresponds to the coercive field. In contrast, the out-of-plane positive MR switching field is relatively higher, ∼ ± 5 kG, 25 times larger than the in-plane switching field. The anisotropy in positive MR and its switching field is due to the strong MA, modulated by the charge transfer-induced interfacial exchange coupling strength, confirmed using the DFT. The MA and anisotropy in MR with the dual sign of MR are intriguing phenomena; their tunability could pave the way for advanced technology in magnetic devices.

Photoinduced spin-polarized transport and controllable valley-triplet pairing states in a silicene-superconductor junction

We theoretically investigate the spin-polarized subgap transport and valley-triplet pairing states in a silicene-superconductor junction. By using the nonequilibrium Green’s function, it is found that the subgap conductance is strongly affected by the spin polarization induced by an off-resonant circularly polarized light. More importantly, the presence of valley-mixing scattering at the interface could result in a valley-triplet Andreev reflection (AR) process, where the incident electrons and reflected holes come from the same valley. We also find that the amplitude of the valley-triplet AR is controllable by tuning the intensity of the light, the position of the Fermi level, and even the interface coupling strength. Particularly, the fully valley-polarized conductance spectrum shows distinctive behaviors, which is beneficial for us to verify and detect the possible valley-triplet pairing states as well as the spin/valley polarizations in silicene. Our results may pave the way for the applications of silicene in spin-valleytronics.

Role of interface and valley-mixing scattering in a ferromagnetic silicene/superconductor junction

We theoretically investigate the subgap transport in a ferromagnetic silicene/superconductor junction and highlight the role of the interface. It is demonstrated that the subgap conductance spectra for the continuous and discontinuous interface models are significantly different. In particular, the intravalley Andreev reflection (AR) may occur at the discontinuous interface, and then the subgap conductance will be remarkably enhanced when the silicene sheet is highly valley polarized. This novel AR resulted from the valley-mixing scattering, which is absent in the continuous interface model. The influence of the interface coupling strength on the subgap conductance is also computed. Our results can be directly tested experimentally using scanning tunnel microscope measurements and/or point-contact spectroscopy.

Probing the interfacial coupling in ternary van der Waals heterostructures

Good interfacial coupling between each constituent of van der Waals Heterostructures (vdWHs) is the prerequisite for the distinguished performance of related devices. Since vdWHs-based devices commonly consist of three or more constituents, an effective evaluation of interfacial coupling quality in multiple heterointerfaces is critical during the device fabrication process. Here, in ternary vdWHs composed of hBN, graphene (Gr) and transition metal dichalcogenide (TMD) flakes, which are essential building blocks for low-dimensional vdWHs-based electronic and optoelectronic devices, we realized probe and quantification of the interfacial coupling by low-frequency Raman spectroscopy under resonant excitation through the C exciton energy in TMD constituents. Based on the frequencies of emerging interlayer vibration modes in hBN/TMD/Gr ternary vdWHs, the interfacial coupling force constants of hBN/TMD and TMD/Gr interfaces are estimated as parameters to quantitatively evaluate the interfacial coupling strength at the corresponding interfaces. Moreover, the interfacial coupling strength at Gr/hBN interface is also successfully revealed in Gr/hBN/MoTe2 ternary vdWHs, which cannot be directly characterized from Gr/hBN binary vdWH due to its unobservable interlayer modes with weak electron-phonon coupling. This general strategy can be further extended to probe and quantify the interfacial coupling quality in polynary vdWHs and related devices.

A comparative study of interfacial thermal conductance between metal and semiconductor

To understand and control thermal conductance of interface between metal and semiconductor has now become a crucial task for the thermal design and management of nano-electronic and micro-electronic devices. The interfacial alignments and electronic characteristics of the interfaces between metal and semiconductor are studied using a first-principles calculation based on hybrid density functional theory. The thermal conductance of interfaces between metal and semiconductor were calculated and analyzed using diffuse mismatch model, acoustic mismatch model and nonequilibrium molecular dynamics methods. Especially, according to nonequilibrium molecular dynamics, the values of thermal conductance were obtained to be 32.55 MW m−2 K−1 and 341.87 MW m−2 K−1 at C–Cu and Si–Cu interfaces, respectively. These results of theoretical simulation calculations are basically consistent with the current experimental data, which indicates that phonon–phonon interaction play a more important role than electron–phonon interaction during heat transport. It may be effective way to improve the interfacial thermal conductance through enhancing the interface coupling strength at the metal–semiconductor interface because the strong interfacial scattering plays a role in suppressing in the weaker interface coupling heterostructure, leading to the lower thermal conductance of interfaces. This could provide a beneficial reference for the design of the Schottky diode and thermal management at the interfaces between metal and semiconductor.

Characterization and regulation of Ni3Si//Fe3O4@PVDF magnetoelectric composites

Exploring new magnetoelectric (ME) composites and improving their forming methods are keys to overcome the development bottlenecks. This study constructed a novel pseudo 2–2 connectivity Ni3Si//Fe3O4@PVDF composite based on lamellar Ni3Si template under the pulsed magnetic fields and investigated the influences of Fe3O4 nanoparticles (NPs) content and the magnetic flux density peak (MP) on the ME properties. Ni3Si template provides additional stress for the magnetic-electric conversion and significantly enhances the ME properties. The increase of Fe3O4 NPs content leads to the improvement of interfacial strain transfer and the decrease of effective contact area synchronously. Therefore, ME coefficients of 3.63, 7.61 and 2.43 mV/(cm·Oe) were achieved with Fe3O4 NPs contents of 4.8 wt%, 7.9 wt% and 16.7 wt%. The pulsed magnetic field can increase the ordering of Fe3O4 NPs, but the induced particle aggregation reduces the interfacial coupling strength. If the MP is 4.5 T, the maximum ME coefficient is 8.35 mV/(cm·Oe), which is 2.27 times of that without magnetic field. But when MP increases to 8.9 T, the maximum ME coefficient is only 4.1 mV/(cm·Oe). Therefore, it is feasible to adjust the ME characteristics by varying the content of Fe3O4 NPs and the MP of the pulsed magnetic fields.

Tuning interfacial charge transfer in atomically precise nanographene-graphene heterostructures by engineering van der Waals interactions.

Combining strong light absorption and outstanding electrical conductivity, hybrid nanographene-graphene (NG-Gr) van der Waals heterostructures (vdWHs) represent an emerging material platform for versatile optoelectronic devices. Interfacial charge transfer (CT), a fundamental process whose full control remains limited, plays a paramount role in determining the final device performance. Here, we demonstrate that the interlayer vdW interactions can be engineered by tuning the sizes of bottom-up synthesized NGs to control the interfacial electronic coupling strength and, thus, the CT process in NG-Gr vdWHs. By increasing the dimensions of NGs from 42 to 96 sp2 carbon atoms in the polyaromatic core to enhance the interfacial coupling strength, we find that the CT efficiency and rate in NG-Gr vdWHs display a drastic increase of one order of magnitude, despite the fact that the interfacial energy driving the CT process is unfavorably reduced. Our results shed light on the CT mechanism and provide an effective knob to tune the electronic coupling at NG-Gr interfaces by controlling the size-dependent vdW interactions.

Polarization‐Driven‐Orientation Selective Growth of Single‐Crystalline III‐Nitride Semiconductors on Arbitrary Substrates

AbstractIII‐nitride semiconductor films are usually achieved by epitaxial growth on single‐crystalline substrates (sapphire, silicon (Si), and silicon carbide). It is important to grow these films on non‐epitaxial substrates of interest such as polycrystalline substrates for exploring novel applications in electronics and optoelectronics. However, single‐crystalline III‐nitride films with uniform orientation on non‐epitaxial substrates have not yet been realized, due to the lack of crystallographic orientation of the substrates. Here, this work proposes a strategy of polarization‐driven‐orientation selective growth (OSG) and demonstrate that single‐crystalline gallium nitride (GaN) can in principle be achieved on any substrates. Taking polycrystalline diamond and amorphous‐silicon dioxide/Si substrates as typical examples, the OSG is demonstrated by utilizing a composed buffer layer consisting of graphene and polycrystalline physical vapor deposited (PVD) aluminium nitride (AlN). The polarization of the PVD AlN can effectively tune the strength of interfacial orbital coupling between AlN nuclei and graphene at different rotation angles, as confirmed by atom‐scale first‐principles calculations, and align the AlN nuclei to form a uniform orientation. This consequently leads to continuous single‐crystalline GaN films. The ability to grow single‐crystalline III‐nitrides onto any desired substrates would create unprecedented opportunities for developing novel electronic and optoelectronic devices.

Transition from incoherent to coherent phonon thermal transport across graphene/h-BN van der Waals superlattices

The van der Waals (vdW) superlattice, obtained by applying the concept of the periodic superlattice to two-dimensional materials using low-energy vdW physical assembly, is undoubtedly an instrumental avenue for the modulation of material properties. In the field of nanoscale thermal transport, the influence of the periodic structure of superlattice on the wave-particle phonon transport regime arouses substantial interests from the standpoint of basic physics and applied science. In the Graphene/h-BN vdW superlattice, we have found the wave-particle crossover of phonon transport, which is reflected in the transition from incoherent to coherent regime as the interface density increases. The analysis reveals that the increased thermal conductivity owing to coherent transport effects will amply compensate for the progressively increasing interface phonon scattering throughout this process. In addition, due to the stronger effects of the above two aspects, the superlattices with higher interface density are more sensitive to changes in temperature and interface coupling strength, which are manifested in the rate of change in thermal conductivity caused by their alteration, respectively. These results establish an in-depth understanding of coherent phonon transport while exploring the possibility of phonon wave-particle crossover in vdW superlattices, providing guidance for related thermal management based on phonon engineering.

Interfacial thermal conductance of graphene/MoS2 heterointerface

The thermal transport performance and thermal rectification characteristics of graphene/MoS2 heterointerface (G/M) are comprehensively studied in this paper. The influence of three different types of vacancy defects on thermal conductance is simulated using Molecular Dynamics method. We find that the single C-vacancy defects greatly enhance the interfacial thermal conductance (G) by 90% at the defect concentration of 5%, resulting from the enhanced interface coupling strength. We also find the temperature plays an important role in G. The interfacial thermal conductance of pristine G/M heterointerface increases of by 80% from 300 K to 700 K. In addition, an increase in both temperature and single C-vacancy defects concentration result in enhanced G/M thermal rectifier effect, and the thermal rectification ratio (TR) can reach 8% because of the superior interface coupling of M/G over G/M. It provides theoretical support for the application of graphene/MoS2 heterojunctions in heat rectifier devices.

N‐Doped and P‐Doped Graphene on MgO (111): A First‐Principles Study

Herein, first‐principles density functional theory is used to investigate the atomic structure and electrical properties of the contact between primitive graphene, N‐doped graphene, P‐doped graphene, and MgO (111) surface. It has been found that the energy band of the graphene/Mg‐terminated MgO (111) interface exhibits p‐type characteristics, and the energy band of the graphene/O‐terminated MgO (111) interface exhibits n‐type characteristics. N‐doped or P‐doped can control the energy band structure of the interface. The interaction between the graphene and Mg‐terminated MgO (111) interface is a weak physical interaction, and the interaction between the graphene and O‐terminated MgO (111) surface is a strong chemical interaction. N‐doped or P‐doped can promote the charge transfer between the interfaces, and further enhancing the interfacial coupling strength. This result is helpful to understand the interaction between graphene and MgO interface and provides theoretical guidance for the practical application of graphene/MgO interface‐based devices.

Spontaneous magnetization reversal and positive exchange bias effect in CoFeSiB/FeCrO3 heterostructures

The exchange bias (EB) phenomenon is an interface-driven effect; it depends on the interface spin configuration, which in turn is immensely related to antiferromagnetic spin configuration. Here, we chose an antiferromagnet, FeCrO3, with a corundum structure with randomly occupied cations. We report EB in the bilayer heterostructures that are composed of an amorphous CoFeSiB ferromagnet interfaced with an epitaxial FeCrO3 layer fabricated in-situ by pulsed laser deposition. The bilayers exhibit crossover from conventional negative EB to positive EB from room temperature down to 5 K. The crossover occurs for a cooling field that exceeds the interfacial coupling strength at the interface. Systems that show positive EB effects sometimes also display spontaneous magnetization reversal; the present bilayers showed spontaneous EB below 50 K. Temperature and field-dependent magnetisation studies reveal that the prepared heterostructure consists of uncompensated interface spins due to the disorder in the FeCrO3 layer that gets frozen at low temperatures, pins the spins in the ferromagnetic layer and causes the spontaneous EB effect.

Electron-magnon coupling and quasiparticle lifetimes on the surface of a topological insulator

The fermionic self-energy on the surface of a topological insulator proximity coupled to ferro- and antiferromagnetic insulators is studied. An enhanced electron-magnon coupling is achieved by allowing the electrons on the surface of the topological insulator to have a different exchange coupling to the two sublattices of the antiferromagnet. Such a system is therefore seen as superior to a ferromagnetic interface for the realization of magnon-mediated superconductivity. The increased electron-magnon-coupling simultaneously increases the self-energy effects. In this paper we show how the inverse quasiparticle lifetime and energy renormalization on the surface of the topological insulator can be kept low close to the Fermi level by using a magnetic insulator with a sufficient easy-axis anisotropy. We find that the antiferromagnetic case is most interesting from both a theoretical and an experimental standpoint due to the increased electron-magnon coupling, combined with a reduced need for easy-axis anisotropy compared to the ferromagnetic case. We also consider a set of material and instrumental parameters where these self-energies should be measurable in angle-resolved photoemission spectroscopy experiments, paving the way for a measurement of the interfacial exchange coupling strength.