Boron Nitride Superlattice Research Articles

Theoretical study of heat transfer across biphenylene/h-BN superlattice nanoribbons

Controlling thermal conductivity of nanostructures is a key element in manufacturing tailor-made nanodevices for thermoelectric applications. Moreover, superlattice nanostructures have been demonstrated to be useful in achieving minimal thermal conductivity for the employed nanomaterials. In this work, we model two-dimensional biphenylene, a recently-synthesized sp2-hybridized allotrope of carbon atoms, for the implementation of a biphenylene/hexagonal Boron-Nitride (biphenylene/h-BN) superlattice nanoribbons. The effects of the length of ribbon and its superlattice period (lp) on the thermal conductivity are explored using molecular dynamics simulations. We calculated the length-independent intrinsic thermal conductivity (Kα) of the superlattice nanostructure, which was approximately 68% and 55% lower than the thermal conductivity of pristine h-BN and biphenylene nanosheets, respectively. The superlattice period largely determines the minimum thermal conductivity, which was at 64.1 W m−1k−1 for a period value of lp = 2.51 nm. This work opens a new window to tune and/or minimize thermal conductivity in nanoribbons when designing thermoelectric and thermal insulation materials for favorable applications.

Phonon thermal transport in graphene/h-BN superlattice monolayers

The superlattice monolayers composed of periodically assembled graphene and hexagonal boron nitride (h-BN) superlattices by strong covalent bonds have aroused great interest in academia and industry due to their adjustable physical properties, which are expected to play an important role in the modulation of material properties, especially in thermal transport properties. In general, thermal transport properties highly rely on the characteristic of nanostructures. Thus, understanding the impact of structural characteristics on physical properties is essential for the design of high-performance materials. In this study, we report our results from three different methods including non-equilibrium molecular dynamics, homogeneous non-equilibrium molecular dynamics, and spectral heat current decomposition to explore the phonon thermal transport properties in different graphene/h-BN superlattice monolayers, which are constructed by periodically stitched and equal-sized graphene and h-BN superlattices. We find that the thermal conductivities decrease first and then increase with the increase of periodic length, corresponding to a transition from coherent transport to incoherent transport, and a minimum thermal conductivity at a certain period is identified. These results provide a theoretical validation of a possible control on thermal properties at the nanoscale, which may have potential applications in thermoelectric devices once upon experimental validation.

Lattice relaxation and substrate effects of graphene moiré superlattice

When two two-dimensional (2D) materials with different lattice constants or with different rotation angles are superimposed, a moiré superlattice can be constructed. The electronic properties of the superlattice are strongly dependent on the stacking configuration, twist angle and substrate. For instance, theoretically, when the rotation angle of twisted bilayer graphene is reduced to a set of specific values, the so-called magic angles, flat bands appear near the charge neutrality, and the electron-electron interaction is significantly enhanced. The Mott insulator and unconventional superconductivity are detected in the twisted bilayer graphene with a twist angle around 1.1°. For a moiré pattern with a large enough periodicity, lattice relaxation caused by an interplay between van der Waals force and the in-plane elasticity force comes into being. The atomic relaxation forces atoms to deviate from their equilibrium positions, and thus making the system reconstructed. This review mainly focuses on the effects of the lattice relaxation and substrates on the electronic properties of the graphene superlattices. From both theoretical and experimental point of view, the lattice relaxation effects on the atomic structure and electronic properties of graphene-based superlattices, for example, the twisted bilayer graphene, twisted trilayer graphene, graphene-hexagonal boron nitride superlattice and twisted bilayer graphene-boron nitride superlattice are discussed. Finally, a summary and perspective of the investigation of the 2D material superlattice are presented.

Structural Determination of a Graphite/Hexagonal Boron Nitride Superlattice Observed in the Experiment.

A previous study reported an observed unidentified graphite/hexagonal boron nitride (hBN) superlattice structure in special multilayer heterojunction devices via cross-sectional transmission electron microscopy [Haigh S. J. et al., Nat. Mater. 2012, 11, 764-767]. In this letter, we designed and confirmed two possible graphite/hBN superlattice structures (AA and Ab), which were probably the structures observed by the aforementioned experiment. The formation enthalpies of both structures were negative, indicating that they could be successfully synthesized as the previous experiment reported. The results also showed that both structures possessed dynamic stability and elastic stability. Importantly, the theoretical interlayer distances of AA and Ab were 3.34 and 3.30 Å, respectively, which were consistent with the experimental value of 3.32 Å. The X-ray diffraction patterns and Raman spectra of both structures were simulated to aid in distinguishing them. This study on the atomic structure of the graphite/hBN superlattice lays a foundation for further research and application of this material.

Tunable Negative Poisson's Ratio in Van der Waals Superlattice.

Negative Poisson's ratio (NPR) materials are functional and mechanical metamaterials that shrink (expand) longitudinally after being compressed (stretched) laterally. By using first-principles calculations, we found that Poisson's ratio can be tuned from near zero to negative by different stacking modes in van der Waals (vdW) graphene/hexagonal boron nitride (G/h-BN) superlattice. We attribute the NPR effect to the interaction of pz orbitals between the interfacial layers. Furthermore, a parameter calculated by analyzing the electronic band structure, namely, distance-dependent hopping integral, is used to describe the intensity of this interaction. We believe that this mechanism is not only applicable to G/h-BN superlattice but can also explain and predict the NPR effect in other vdW layered superlattices. Therefore, the NPR phenomenon, which was relatively rare in 3D and 2D materials, can be realized in the vdW superlattices by different stacking orders. The combinations of tunable NPRs with the excellent electrical/optical properties of 2D vdW superlattices will pave a novel avenue to a wide range of multifunctional applications.

The dimensionality effect on phonon localization in graphene/hexagonal boron nitride superlattices

Phonon localization, a largely elusive phenomenon, has a great promise for improving current applications like thermoelectric materials and thermal barrier coatings. By freezing the out-of-plane atomic motion in graphene/hexagonal boron nitride (hBN) superlattices and random multilayers, i.e aperiodic superlattices, we are able to completely isolate the effect of the third dimension on phonon scattering and localization in 2D materials. In particular, we find much more prominent phonon heat conduction and localization when atomic motions in the third dimension are frozen. Rigorous spectral phonon transmission and scattering phase space analyses reveal that the phase-breaking anharmonic scatterings can significantly hinder the occurrence of phonon localization. Phonon participation ratio calculations further reveal that the flexural modes—arising from the degree of freedom of the third dimension—are rather extended, in contrast to the vastly localized in-plane modes in graphene/hBN random multilayers. These two factors altogether greatly obstruct the observation of coherent phonon localization in realistic materials. This work will be useful for guiding the search for nanostructures possessing significant phonon localization behaviors.

Fabrication of folded bilayer-bilayer graphene/hexagonal boron nitride superlattices

Carbon-based superlattices represent a novel class of quantum metamaterials having promising prospects. Here, we show the first fabrication of folded bilayer-bilayer graphene (fBBLG)/hexagonal boron nitride (hBN) superlattices. In the transport properties at low temperatures, satellite peaks are observed due to the crystallographic angle alignment of fBBLG and hBN. The peculiar resistance peaks appear away from the charge neutrality point and the hBN-induced satellites, which may be signatures of possible competing orders. Our results suggest the emergence of a unique electronic band-structure in the fBBLG, providing a way for investigating the correlated electron phenomena by performing energy-band engineering with superlattice structures.

Topological valley currents in bilayer graphene/hexagonal boron nitride superlattices

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a “hot spot” at the charge-neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 kΩ, which obeys a scaling relation. This nonlocal resistance evolves from an analog of the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way for developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking.

Structural Engineering of Zinc‐Blend/Wurtzite BN Superlattices

AbstractBy constructing Zinc‐Blende (ZB)/Wurtzite (WZ) boron nitride (BN) (ZB1‐x/WZx‐BN) superlattices (SLs), their electronic structure and mechanical properties were investigated by first‐principles techniques based on density functional theory. It is found that the band structure is significantly modulated by the ratio of WZ phase in ZB/WZ‐BN SLs. With the increase of WZ ratio, the band gap of SLs decreases and then increases dramatically. More importantly, transformation from type‐I to type‐II band alignment between ZB and WZ can be tuned by adjusting the ratio of WZ to be >0.6. In addition, a universal expression of the band gap is proposed to reveal a nonlinear dependence of the band gap within the structure of SLs as induced by from the interface effect achieved via structural transformation. The elastic constants results revealed that the change of ZB/WZ ratio poses no significant effect on the mechanical properties. The effect of structure modulation on band structure and mechanical properties is significantly important for future engineering of novel semiconductor SLs.

Impacts of in-plane strain on commensurate graphene/hexagonal boron nitride superlattices

Due to atomically thin structure, graphene/hexagonal boron nitride (G/hBN) heterostructures are intensively sensitive to the external mechanical forces and deformations being applied to their lattice structure. In particular, strain can lead to the modification of the electronic properties of G/hBN. Furthermore, moiré structures driven by misalignment of graphene and hBN layers introduce new features to the electronic behavior of G/hBN. Utilizing ab initio calculation, we study the strain-induced modification of the electronic properties of diverse stacking faults of G/hBN when applying in-plane strain on both layers, simultaneously. We observe that the interplay of few percent magnitude in-plane strain and moiré pattern in the experimentally applicable systems leads to considerable valley drifts, band gap modulation and enhancement of the substrate-induced Fermi velocity renormalization. Furthermore, we find that regardless of the strain alignment, the zigzag direction becomes more efficient for electronic transport, when applying in-plane non-equibiaxial strains.

Electrochemical functionalization and in-situ deposition of the SAA@rGO/h-BN@Ni electrode for supercapacitor applications

Abstract Functionalization and electrodeposition of reduced graphene oxide/hexagonal boron nitride (rGO/h-BN) superlattice was carried out by a novel one-step electrochemical process. The sulfanilic acid azochromotrop (SAA) functionalized and electrodeposited nickel foam (SAA@rGO/h-BN@Ni) electrode showed high specific capacitance of ∼1300 F g −1 . An asymmetric supercapacitor cell (ASC) using SAA@rGO/h-BN@Ni as positive and thermally reduced GO as negative electrode showed ∼80% capacitance retention after 10,000 charge–discharge cycles. The ASC showed low relaxation time constant of ∼0.47 ms along with high energy density (∼95.3 W h kg −1 ) and power density (∼7200 W kg −1 ) ensuring the utility of SAA@rGO/h-BN@Ni electrode for supercapacitor application.

Phonon wave interference in graphene and boron nitride superlattice

The thermal transport properties of the graphene and boron nitride superlattice (CBNSL) are investigated via nonequilibrium molecular dynamics simulations. The simulation results show that a minimum lattice thermal conductivity can be achieved by changing the period length of the superlattice. Additionally, it is found that the period length at the minimum shifts to lower values at higher temperatures, and that the depth of the minimum increases with decreasing temperature. In particular, at 200 K, the thermal conductivities of CBNSLs with certain specific period lengths are nearly equal to the corresponding values at 300 K. A detailed analysis of the phonon spectra shows that this anomalous thermal conductivity behavior is a result of strong phonon wave interference. These observations indicate a promising strategy for manipulation of thermal transport in superlattices.

Continuum modeling of thermal transport in superlattices and layered materials for new energy matierlas

Both high-efficient thermoelectric materials and thermal insulating coatings requiring low thermal conductivities, layered materials and superlattices prove to be an efficient multiscale material design for such requirements. The interfaces are artificially introduced to scatter thermal phonons, thus hindering thermal transport. Very recently, it has been found that interface modulation can further reduce the thermal conductivity. All of the recent advances originate from highly demanding numerical computations. An efficient estimate of the thermal properties is important for fast and/or high-throughput calculations. In this article, the phonon transport on layered material is studied theoretically for general purposes, based on the fact that long-wavelength phonons contribute dominantly in general. According to the Debye hypothesis, the classical wave equation can describe phonon transport very well. This fact has been very recently used to model phonon transport carbon nanotubes, which justifies the applicability of continuum mechanics for nanomaterials. Furthermore, Kronig and Penny have solved the electron transport on periodic lattices. In a very similar way, for the periodic layered materials and superlattices, with Floquet and linear attenuation theory, the wave equations with and without damping are solved analytically. The wave equation decouples to Helmholtz equations in each direction with periodic excitation functions. In this paper, we propose to model the phonon transport by using Matthew-Hill equation, with which we can obtain the phonon spectrum (i.e. phonon dispersion relation). The proposed theory is justified by two-dimensional (2D) graphene/hexagon boron nitride superlattice and three-dimensional (3D) silicon/germanium superlattices. Like the carbon nanotube cases, using this continuum-mechanics method, we can reproduce the previous numerical results very quickly compared with using published molecular dynamics and density functional theory The effects of interface modulation and phonon localization are shown over full phase space, which further enables the calculating of both high and low bounds of thermal conductivity for all possible superlattices and layered materials. In order to model real interfaces, with considering possible mixing and transition due to other mechanisms, we use the smooth transition function, which is further modeled via sinusoidal series. Very interestingly, interface grading is shown to erase band gaps and delocalize modes. This fact has been seldom reported and can be helpful for designing real materials. Likewise, we take phonon damping (equivalent to inter-phonon scattering) into account by adding damping into the wave equation. It is observed that phonon damping smears the originally sharp boundaries of phonon phase space. In this way, evanescent phonons and transporting phonons can be treated simultaneously on the same footing. The proposed method can be used for modeling the efficient and general thermal materials

Electric Field Tunable Band Gap in Bi-Axially Strained Graphene/Hexagonal Boron Nitride Super-Lattice

In this work, we have reported the tunable band gap of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) monolayer arranged in B3 super-lattice form, as a function of an external electric field perpendicular to the super-lattice plane and an applied external bidirectional stress from the first principle Density Functional Theory. To reduce the effect of substrate related performance degradation and open a band gap, graphene placed on h-BN has been reported in literature. To tailor the band gap,perpendicular electric field and biaxial stress were applied in this work. It was observed that band gap increases from compressive to tensile biaxial strain when no electric field was applied. With the application of 5×109V/m external electric field a band gap of 144.5 meV was found which is promising for logic FET with a trade off in degradation in Fermi velocity and higher electron effective mass.

Resolving anomalous strain effects on two-dimensional phonon flows: The cases of graphene, boron nitride, and planar superlattices

The physics of thermal transport on strained, two-dimensional (2D) materials graphene, boron nitride, and their superlattices is analyzed by molecular dynamics, lattice dynamics, and numerical solutions to Boltzmann transport equation. The thermal conductivity of these materials is found to be highly sensitive to tensile strain, and the strain dependence itself is also highly dependent on the sample total length. Both finite-sized systems (varying from \ensuremath{\sim}100 to 300 nm long) as well as infinitely long systems are considered. In contrast to the typical reduction of thermal conductivity with strain exhibited by bulk 3D materials, the thermal conductivity initially increases to a peak value, after which it then decreases with further strain. Modal decomposition of the phonon spectrum shows that the nonmonotonic behavior arises from a competition between in-plane softening and flexural stiffening of phonons. The length sensitivity arises from the nature of the linear/quadratic dispersion of the in-plane/flexural modes and their distinct scattering selection rules: longer systems favor out-of-plane flexural phonon stiffening while smaller systems favor in-plane phonon softening. Additionally, we show that this competition occurs in concert with a strain-induced transition in the nature of the phonon flow from ballistic dominant to diffusive dominant. Overall these effects give rise to a complex dependence of thermal conductivity on strain and sample size.

Electric Field Tunable Band Gap in Bi-Axially Strained Graphene/Hexagonal Boron Nitride Super-Lattice

In this work, we have reported the tunable band gap of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) monolayer arranged in B3 super-lattice form, as a function of an external electric field perpendicular to the super-lattice plane and an applied external bidirectional stress from the first principle Density Functional theory (DFT). Graphene with its intrinsic high electron mobility poses itself very promising for electronic device application, but absence of band gap and substrate induced mobility degradation limits its application in logic transistor. To reduce the effect of substrate related performance degradation graphene placed on h-BN has been reported in literature. Three super-lattice structures of this bilayer system have been proposed in literature where B3 is reported to have the greatest intrinsic band gap opening among the three but this is not sufficient for logic transistor. To tailor the band gap perpendicular electric field and biaxial stress was applied in this work. It was observed that band gap increases (65.45 meV – 84.5 meV) from compressive to tensile biaxial strain (-5% to +5%) when no electric field was applied. With the application of 5×109 V/m external electric field a band gap of 144.5 meV was found which is promising for logic FET. However, tensile stress and external electric field causes degradation in Fermi velocity and produces higher electron effective mass. Figure 1

First-Principles Study of the Transport Properties of Graphene-Hexagonal Boron Nitride Superlattice.

Hexagonal boron nitride (hBN) is a promising material to be integrated with graphene for high-performance graphene based electronics. We investigate the electronic, thermal, and thermoelectric transport properties of graphene-hexagonal boron nitride (G-hBN) superlattice by using the first-principles density functional calculations combined with the non-equilibrium Green's function formalism. The results show that a gap of 0.2 eV is opened in the band structure of the G-hBN superlattice due to the sublattice symmetry broken, the conductance and corresponding electron thermal conductance are both reduced. The phonon thermal conductance is also reduced due to the interlayer interactions, which linearize the flexural phonon modes and reduce the corresponding phonon density of states. Compared with those of graphene, though the electronic and phonon transport are both reduced, while the Seebeck coefficient is greatly enhanced. Finally, the thermoelectric figure of merit ZT of the G-hBN superlattice is enhanced 44% that of graphene. Our findings provide instructional information for future applications of graphene in electronics design.

Ripples, strain, and misfit dislocations: structure of graphene-boron nitride superlattice interfaces.

In recently synthesized two-dimensional superlattices of graphene and boron nitride, the atomic structure of the interface is complicated by a 2% lattice mismatch between the two materials. Using atomistic and continuum analysis, we show that the mismatch results in a competition between two strain-relieving mechanisms: misfit dislocations and rippling. For flat superlattices, beyond a critical pitch the interface is decorated by strain-relieving misfit dislocations. For superlattices that can deform out-of-plane, optimal ripple wavelengths emerge.

Numerical Study of Graphene Superlattice-Based Photodetectors

The optical properties of 1-D superlattices formed by armchair graphene nanoribbons embedded in hexagonal boron nitride superlattices (BNSLs) are studied. A set of tight-binding (TB) parameters is proposed, which gives results in excellent agreement with first-principle calculations. Based on the tight-binding model, it is demonstrated that in BNSLs, a larger bandgap opening is achieved than in hydrogen-passivated superlattices (HSL). The optical properties of BNSL and HSL are studied by employing the nonequilibrium Green function method and are verified with first-principle calculations. The role of line-edge roughness on the optical properties of such devices is carefully investigated.

Graphene–boron nitride superlattices: the role of point defects at the BN layer

We investigate, by means of first-principles calculations, the role of hBN point defects on the energetical stability and electronic structure of heterostructures composed of graphene atop hBN, rotated at angles of 13.17°, 9.43° and 7.34°. We consider, as possible point defects, boron and nitrogen vacancies and antisites, substitutional oxygen at the nitrogen site ON, substitutional carbon dimers, and nitrogen interstitials. The electronic and structural properties of all defects were analyzed. Among these, the most stable is ON, with negative formation energies at several possible rotation angles and chemical environments. Under such conditions, ON doping can raise the Fermi level of the neutral system by as much as 1 eV relative to graphene’s Dirac point, reaching the band crossing between adjacent Dirac cones at the M point of the heterostructure Brillouin zone. This could lead to interesting electronic transport properties without the need for electrostatic doping.