Boron Nitride Heterostructures Research Articles

Borophenes made easy.

To date, the scalable synthesis of elemental two-dimensional materials beyond graphene still remains elusive. Here, we introduce a versatile chemical vapor deposition (CVD) method to grow borophenes, as well as borophene heterostructures, by selectively using diborane originating from traceable byproducts of borazine. Specifically, metallic borophene polymorphs were successfully synthesized on Ir(111) and Cu(111) single-crystal substrates and conjointly with insulating hexagonal boron nitride (hBN) to form atomically precise lateral borophene-hBN interfaces or vertical van der Waals heterostructures. Thereby, borophene is protected from immediate oxidation by a single hBN overlayer. The ability to synthesize high-quality borophenes with large single-crystalline domains in the micrometer scale by a straight-forward CVD approach opens up opportunities for the study of their fundamental properties and for device incorporation.

Ion-Selective Membrane-Coated Graphene-Hexagonal Boron Nitride Heterostructures for Field-Effect Ion Sensing.

An intrinsic ion sensitivity exceeding the Nernst–Boltzmann limit and an sp2-hybridized carbon structure make graphene a promising channel material for realizing ion-sensitive field-effect transistors with a stable solid–liquid interface under biased conditions in buffered salt solutions. Here, we examine the performance of graphene field-effect transistors coated with ion-selective membranes as a tool to selectively detect changes in concentrations of Ca2+, K+, and Na+ in individual salt solutions as well as in buffered Locke’s solution. Both the shift in the Dirac point and transconductance could be measured as a function of ion concentration with repeatability exceeding 99.5% and reproducibility exceeding 98% over 60 days. However, an enhancement of selectivity, by about an order magnitude or more, was observed using transconductance as the indicator when compared to Dirac voltage, which is the only factor reported to date. Fabricating a hexagonal boron nitride multilayer between graphene and oxide further increased the ion sensitivity and selectivity of transconductance. These findings incite investigating ion sensitivity of transconductance in alternative architectures as well as urge the exploration of graphene transistor arrays for biomedical applications.

Multi-spectral infrared camouflage through excitation of plasmon-phonon polaritons in a visible-transparent hBN-ITO nanoantenna emitter.

In an ideal platform for camouflage compatible cooling, the thermal emitter should be a spectrally selective antenna to radiate its heat buildup without being detected by thermal cameras. Moreover, to keep its visual appearance and to minimize solar induced heating, the structure should be visibly transparent. In this Letter, to achieve the visually invisible mid-infrared (MIR) camouflage-cooling feature, a metasurface design based on an indium-doped tin oxide (ITO)-hexagonal boron nitride (hBN) heterostructure is proposed. The proposed ITO-hBN nanoantenna shows spectrally selective broadband absorptions in near-infrared (NIR) and non-transmissive (MIR) windows, while it is dominantly non-emissive in other ranges. The camouflage ability of the structure in the targeted wavelengths is demonstrated using power calculations.

Ab initio study of the effect of 2D layer rippling on the electronic properties of 2D/H-terminated diamond (100) heterostructures

We report an ab initio study of the effect of rippling on the structural and electronic properties of the hexagonal Boron Nitride (hBN) and graphene two-dimensional (2D) layers and heterostructures created by placing these layers on the Hydrogen-terminated (H-) diamond (100) surface. Surprisingly, in graphene, rippling does not open a band gap at the Dirac point but does cause the Dirac cone to be shifted and distorted. For the 2D/H-diamond (100) heterostructures, a combined sampling and a clustering approach were used to find the most favorable alignment of the 2D layers. Heterostructures with rippled layers were found to be the most stable. A larger charge transfer was observed in the heterostructures with rippled hBN (graphene) than their planner counterparts. Band offset analysis indicates a Type-II band alignment for both the wavy and planar heterostructures, with the corrugated structure having stronger hole confinement due to the larger valence band offset between the hBN layer and the H-diamond (100) surface.Graphic abstract

Ab-Initio investigations of electronic and optical properties of Sn-hBN hetero-structure

In this manuscript, we have investigated the electronic and optical properties of Stanene and hexagonal Boron Nitride (Sn/h-BN) heterostructure using the Projected Augmented Wave (PAW) method within the framework of density functional theory (DFT). van der Waals forces are also considered to counter the inter-layer interactions between Sn and h-BN. The significant band gap opening has been observed in Sn/h-BN heterostructure compared to pristine Stanene (Sn). However, the band gap is much lesser than the Boron Nitride (h-BN). In order to use the proposed structure in futuristic opto-electronic devices, we have investigated the various optical parameters such as absorption coefficient, refractive index, extinction coefficient, reflectivity and dielectric constant of Sn/h-BN heterostructure along with monolayer Stanene and hexagonal Boron Nitride. As a significant finding of the results, a strong redshift has been observed in the absorption coefficient, extinction coefficient and reflectivity, also a strong absorption peak appears in ultra-violet region. The excellent optical properties of heterostructure make it viable for potential optical applications.

UV light modulated synaptic behavior of MoTe2/BN heterostructure

Electrical synaptic devices are the basic components for the hardware based neuromorphic computational systems, which are expected to break the bottleneck of current von Neumann architecture. So far, synaptic devices based on three-terminal transistors are considered to provide the most stable performance, which usually use gate pulses to modulate the channel conductance through a floating gate and/or charge trapping layer. Herein, we report a three-terminal synaptic device based on a two-dimensional molybdenum ditelluride (MoTe2)/hexagonal boron nitride (hBN) heterostructure. This structure enables stable and prominent conductance modulation of the MoTe2 channel by the photo-induced doping method through electron migration between the MoTe2 channel and ultraviolet (UV) light excited mid-gap defect states in hBN. Therefore, it is free of the floating gate and charge trapping layer to reduce the thickness and simplify the fabrication/design of the device. Moreover, since UV illumination is indispensable for stable doping in MoTe2 channel, the device can realize both short- (without UV illumination) and long- (with UV illumination) term plasticity. Meanwhile, the introduction of UV light allows additional tunability on the MoTe2 channel conductance through the wavelength and power intensity of incident UV, which may be important to mimic advanced synaptic functions. In addition, the photo-induced doping method can bidirectionally dope MoTe2 channel, which not only leads to large high/low resistance ratio for potential multi-level storage, but also implement both potentiation (n-doping) and depression (p-doping) of synaptic weight. This work explores alternative three-terminal synaptic configuration without floating gate and charge trapping layer, which may inspire researches on novel electrical synapse mechanisms.

Tunable Goos-Hänchen Shift Surface Plasmon Resonance Sensor Based on Graphene-hBN Heterostructure.

In this paper, a bimetallic sensor based on graphene-hexagonal boron nitride (hBN) heterostructure is theoretically studied. The sensitivity of the sensor can be improved by enhancing the Goos–Hänchen (GH) shift in the infrared band. The theoretical results show that adjusting the Fermi level, the number of graphene layers and the thickness of hBN, a GH shift of 182.09 λ can be obtained. Moreover, sensitivity of 2.02 × 105 λ/RIU can be achieved with monolayer graphene, the thickness of gold layer is 20 nm, silver layer is 15 nm, and the hBN thickness of 492 nm. This heterogeneous infrared sensor has the advantages of high sensitivity and strong stability. The research results will provide a theoretical basis for the design of a new high-sensitivity infrared band sensor.

Measuring and Tuning the Potential Landscape of Electrostatically Defined Quantum Dots in Graphene.

We use Kelvin probe force microscopy (KPFM) to probe the carrier-dependent potential of an electrostatically defined quantum dot (QD) in a graphene/hexagonal boron nitride (hBN) heterostructure. We show that gate-dependent measurements enable a calibration scheme that corrects for uncertainty inherent in typical KPFM measurements and accurately reconstructs the potential well profile. Our measurements reveal how the well changes with carrier concentration, which we associate with the nonlinear dependence of graphene's work function on carrier density. These changes shift the energy levels of quasi-bound states in the QD which we can measure via scanning tunneling spectroscopy (STS). We show that the experimentally extracted energy levels closely compare with wave functions calculated from the reconstructed KPFM data. This methodology, where KPFM and STS data are simultaneously acquired from 2D materials, allows the quasiparticle response to an electrostatic potential to be determined in a self-consistent way.

Controlling relaxation dynamics of excitonic states in monolayer transition metal dichalcogenides WS2 through interface engineering

Transition metal dichalcogenides (TMDs) are known to support complex excitonic states. Revealing the differences in relaxation dynamics among different excitonic species and elucidating the transition dynamics between them may provide important guidelines for designing novel excitonic devices. Combining photoluminescence and reflectance contrast measurements with ultrafast pump-probe spectroscopy at cryogenic temperatures, we herein study the relaxation dynamics of neutral and charged excitons in a back-gate-controlled monolayer device. Pump-probe results reveal quite different relaxation dynamics of excitonic states under different interfacial conditions: while neutral excitons have a much longer lifetime than trions in monolayer WS2, the opposite is true in the WS2/hexagonal boron nitride (h-BN) heterostructure. It is found that the insertion of the h-BN layer between the TMD monolayer and the substrate has a great influence on the lifetimes of different excitonic states. The h-BN flakes can not only screen the effects of impurities and defects at the interface but also help establish a non-radiative transition from neutral excitons to trions to be the dominant relaxation pathway, at cryogenic temperature. Our findings highlight the important role that the interface may play in governing the transient properties of carriers in 2D semiconductors and may also have implications for designing light-emitting and photo-detecting devices based on TMDs.

Predicting phonon scattering and tunable thermal conductivity of 3D pillared graphene and boron nitride heterostructure

In this work, we have studied the transfer of phonon energy in hybrid structure of pillared graphene and boron nitride film. The obtained results showed that, due to the occurrence of ballistic transfer with the elongation of heterostructure, thermal conductivity was increased. We also found that the amount of interfacial thermal conductivity was changed by changing the direction of thermal flux and the application of thermal flux along the direction of pillared graphene nanostructure was more favorable. We observed that increase of temperature from 100 to 700 K increased the amounts of interfacial thermal conductance (ITC) in C-N and C-B models by 19 and 10%, respectively. Since defects are inevitable during the fabrication and growth of these structures, in this work, we investigated the concentration and arrangement of these defects at joint interface and found that increase of defect concentration decreased thermal flux and ITC and also type of defects had a significant effect on thermal rectification. Also, the detection of these defects in a controlled manner can help making thermal conductivity tunable. We evaluated the effect of tensile from 0 to 10% in heterostructure with and without vacancy defects and found that ITC was decreased by 45% and 30% and temperature jump was increased by 80% and 45% at interface, respectively. In addition, to further analyze the obtained results, phonon vibration power spectra were also examined. Finally, by using Von Mises stress criterion, stress distribution and concentration through sheets and interface in the presence of mechanical strains and various defects were investigated.

Effect of overlayer–substrate interaction on the coalescence behaviors of in-plane graphene/hexagonal boron nitride heterostructures

In-plane graphene/hexagonal boron nitride (h-BN) heterostructures show promising applications in novel two-dimensional electronic and optoelectronic devices. The quality of graphene/h-BN (G-BN) domain boundaries, which plays a critical role in device performances, depends on their coalescence. Here, the coalescing mechanism determined by the overlayer–substrate interaction and the growth dynamics during the heterostructure synthesis were studied by in situ surface imaging measurements. In-plane G-BN heterostructures were grown (h-BN first and graphene then) on Pt(111) and Ru(0001) surfaces by chemical vapor deposition. Oxygen intercalation acted as a probe reaction to investigate the coalescing behaviors of the G-BN domain boundaries. Oxygen atoms can intercalate from the outer graphene edges and cross the G-BN domain boundaries into the interior h-BN zone on Pt(111) surfaces. On Ru(0001) surfaces, only interior h-BN domains could be intercalated from G-BN domain boundaries, while no intercalation occurred on outer graphene domains. This shows that graphene can seamlessly stitch with h-BN on Pt(111) but not on Ru(0001), because the weaker overlayer–substrate interaction on Pt(111) compared to that on Ru(0001) makes the G-BN coalescing interface flatter and allows stitching at an identical height. High-quality in-plane G-BN heterostructures are expected to grow on weakly interacting surfaces rather than on strong ones.

Origin of Monochromatic Electron Emission From Planar-Type Graphene/ h -BN/ n -Si Devices

We previously reported highly monochromatic electron emission from the planar-type electron emission devices based on a graphene/hexagonal boron nitride (h-BN) heterostructure. In this paper, the electron energy distribution (EED) of these devices is examined to clarify the mechanism of monochromatic electron emission. We find that the monochromaticity of the electron beam depends significantly on the electronic structure of the substrate material; for the devices with an n-type silicon substrate, the narrowest FWHM of the electron beam is 0.18 eV, whereas that of devices with a metallic (Nb) substrate is 0.33 eV. At the same time, simulations considering the electron scattering by phonons acceptably reproduced the shape of each EED spectrum considering the small energy loss due to out-of-plane acoustic phonon modes in h-BN. Thus, the monochromatic electron emission from the $\mathrm{graphene}/h$-$\mathrm{BN}/n$-$\mathrm{Si}$ device is ascribed to a combination of the narrow energy distribution of electrons at the conduction band of the n-$\mathrm{Si}$ substrate and small phonon energy of the h-BN insulating layer. These features also realize the excellent emission properties in addition to the monochromaticity of the beam, such as a high emission current density of $9.3{\mathrm{A}/\mathrm{cm}}^{2}$, insensitivity to environmental pressure up to 10 Pa, and long lifetime of more than 7 days with little decay.

DNA Translocation through Vertically Stacked 2D Layers of Graphene and Hexagonal Boron Nitride Heterostructure Nanopore.

Cost-effective, fast, and reliable DNA sequencing can be enabled by advances in nanopore-based methods, such as the use of atomically thin graphene membranes. However, strong interaction of DNA bases with graphene leads to undesirable effects such as sticking of DNA strands to the membrane surface. While surface functionalization is one way to counter this problem, here, we present another solution based on a heterostructure nanopore system, consisting of a monolayer of graphene and hexagonal boron nitride (hBN) each. Molecular dynamics studies of DNA translocation through this heterostructure nanopore revealed a surprising and crucial influence of the heterostructure layer order in controlling the base specific signal variability. Specifically, the heterostructure with graphene on top of hBN had nearly 3-10× lower signal variability than the one with hBN on top of graphene. Simulations point to the role of differential underside sticking of DNA bases as a possible reason for the observed influence of the layer order. Our studies can guide the development of experimental systems to study and exploit DNA translocation through two-dimensional heterostructure nanopores for single molecule sequencing and sensing applications.

CdS@h-BN heterointerface construction on reduced graphene oxide nanosheets for hydrogen production

Solar light driven hydrogen evolution is a prospective strategy to solve the energy problem. However, the poor photoresponse, low photocurrent density and easy electron-hole recombination remarkably restrict the HER activity of the conventional photocatalyst. Herein, we constructed a CdS@hexagonal boron nitride (h-BN) heterostructure on reduced graphene oxide (rGO) to synthesize a ternary CdS@h-BN/rGO catalyst via a structural reconstruction strategy. The CdS@h-BN heterointerface promoted the electrons transferring to the CdS active centers from the valence band of h-BN under photoinduction, overcoming the wide bandgap defect of pristine h-BN. The easily photoexcited property of CdS cocatalyst resulted in superhigh photocurrent density (2.7·μA cm−2) under simulated solar light, which was 3.3-fold greater of pristine CdS NPs. Meanwhile, the interfacial interactions of three components remarkably restrained the carriers recombination. Hence, the resultant ternary hybrid catalyst presented remarkably improved photoresponse and interfacial conductivity, thereby the optimal catalyst represented remarkably enhanced HER activity (6465.33 μmol h−1 g−1) under simulated solar light, which was about 10.9-fold greater of the pristine CdS NPs and 218.3-fold greater of the optimal h-BN/rGO hybrid. About 19.8 % of apparent quantum yield was achieved under illumination of λ =420 nm, and the excellent regenerability and durability were represented during six cycles and 20 h of continued illumination. Thereby, this study provides a prospective idea for constructing low-cost nonprecious metal based photocatalyst with highly efficient HER performance.

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

Research on correlation of mechanical behavior of multilayer in-plane graphene/hexagonal boron nitride heterostructures in the presence of Stone-Wales defects and interlayer sp3 bonds with multiple physical fields

The integration of vertical and in-plane heterostructures will generate unexpected structures that may trigger novel physical properties. By introducing interlayer sp3 bonds, the multilayer in-plane graphene/h-BN heterostructures are gradually changed to “quasi three-dimensional” structure. Different cases of mechanical properties of quasi three-dimensional structure have been investigated by MD simulations, including the different orientations of SW defect, different sp3 bonds fraction and distance between sp3 bond and SW defect. The results show the delamination failure and in-plane failure appear in multilayer staggered stacked heterostructure subjected external force due to complex out-of-plane stresses (interlayer bonds and vdW interaction) and in-plane stresses. The adverse coupling effect of SW defects and sp3 bond on mechanical properties of quasi three-dimensional structure can be expedited by the interconnection of multiple physical fields (tensile strain and temperature). SW defect has no effect on the Young’s modulus of the quasi three-dimensional but can affect greatly the tensile stress and strain. The sp3 bond will produce a “defect amplification effect” on SW defects, and the “defect amplification effect” decreases with the increase of the distance between them. Our findings suggest a feasible approach to control performance and stability graphene and other two-dimensional materials by introducing defect coupling method and changing spatial configuration angle.

Non-volatile programmable homogeneous lateral MoTe2 junction for multi-bit flash memory and high-performance optoelectronics

Flash memories and semiconductor p-n junctions are two elementary but incompatible building blocks of most electronic and optoelectronic devices. The pressing demand to efficiently transfer massive data between memories and logic circuits, as well as for high data storage capability and device integration density, has fueled the rapid growth of technique and material innovations. Two-dimensional (2D) materials are considered as one of the most promising candidates to solve this challenge. However, a key aspect for 2D materials to build functional devices requires effective and accurate control of the carrier polarity, concentration and spatial distribution in the atomically thin structures. Here, a non-volatile opto-electrical doping approach is demonstrated, which enables reversibly writing spatially resolved doping patterns in the MoTe2 conductance channel through a MoTe2/hexagonal boron nitride (h-BN) heterostructure. Based on the doping effect induced by the combination of electrostatic modulation and ultraviolet light illumination, a 3-bit flash memory and various homojunctions on the same MoTe2/BN heterostructure are successfully developed. The flash memory achieved 8 well distinguished memory states with a maximum on/off ratio over 104. Each state showed negligible decay during the retention time of 2,400 s. The heterostructure also allowed the formation of p-p, n-n, p-n, and n-p homojunctions and the free transition among these states. The MoTe2 p-n homojunction with a rectification ratio of 103 exhibited excellent photodetection and photovoltaic performance. Having the memory device and p-n junction built on the same structure makes it possible to bring memory and computational circuit on the same chip, one step further to realize near-memory computing.

High-order minibands and interband Landau level reconstruction in graphene moiré superlattices

The propagation of Dirac fermions in graphene through a long-period periodic potential would result in a band folding together with the emergence of a series of cloned Dirac points (DPs). In highly aligned graphene/hexagonal boron nitride (G/hBN) heterostructures, the lattice mismatch between the two atomic crystals generates a unique kind of periodic structure known as a moir\'e superlattice. Of particular interests is the emergent phenomena related to the reconstructed band-structure of graphene, such as the Hofstadter butterfly, topological currents, gate dependent pseudospin mixing, and ballistic miniband conduction. However, most studies so far have been limited to the lower-order minibands, e.g. the 1st and 2nd minibands counted from charge neutrality, and consequently the fundamental nature of the reconstructed higher-order miniband spectra still remains largely unknown. Here we report on probing the higher-order minibands of precisely aligned graphene moir\'e superlattices by transport spectroscopy. Using dual electrostatic gating, the edges of these high-order minibands, i.e. the 3rd and 4th minibands, can be reached. Interestingly, we have observed interband Landau level (LL) crossinginducing gap closures in a multiband magneto-transport regime, which originates from band overlap between the 2nd and 3rd minibands. As observed high-order minibands and LL reconstruction qualitatively match our simulated results. Our findings highlight the synergistic effect of minibands in transport, thus presenting a new opportunity for graphene electronic devices.

Thermal annealing effect on the electrical quality of graphene/hexagonal boron nitride heterostructure devices

The development of the dry transfer method provides an abundant platform to construct various heterostructures of two-dimensional materials. However, the surface and interface cleanliness are essential to realize high electronical performance of heterostructures devices. Here, we demonstrated thermal annealing effect on the mobility and electrical transport properties of graphene on hexagonal boron nitride heterostructures devices. With different annealing temperature recipes for graphene on hexagonal boron nitride devices, we found annealing temperature at 300 °C can clean resist residual and achieve high mobility. Atomic force microscopy results also present a clean surface and small average root mean square roughness as low as 210 pm. Well defined oscillations and plateaus of electrical transport at low magnetic field indicate a high-quality graphene surface.

Comprehensive Electrostatic Modeling of Exposed Quantum Dots in Graphene/Hexagonal Boron Nitride Heterostructures.

Recent experimental advancements have enabled the creation of tunable localized electrostatic potentials in graphene/hexagonal boron nitride (hBN) heterostructures without concealing the graphene surface. These potentials corral graphene electrons yielding systems akin to electrostatically defined quantum dots (QDs). The spectroscopic characterization of these exposed QDs with the scanning tunneling microscope (STM) revealed intriguing resonances that are consistent with a tunneling probability of 100% across the QD walls. This effect, known as Klein tunneling, is emblematic of relativistic particles, underscoring the uniqueness of these graphene QDs. Despite the advancements with electrostatically defined graphene QDs, a complete understanding of their spectroscopic features still remains elusive. In this study, we address this lapse in knowledge by comprehensively considering the electrostatic environment of exposed graphene QDs. We then implement these considerations into tight binding calculations to enable simulations of the graphene QD local density of states. We find that the inclusion of the STM tip’s electrostatics in conjunction with that of the underlying hBN charges reproduces all of the experimentally resolved spectroscopic features. Our work provides an effective approach for modeling the electrostatics of exposed graphene QDs. The methods discussed here can be applied to other electrostatically defined QD systems that are also exposed.