Boron Nitride Substrate Research Articles

Organic Higher-Order Topological Insulators: Heterotriangulene-Based Covalent Organic Frameworks.

The ability to design and control the chemical characteristics of covalent organic frameworks (COFs) offers a new avenue for the development of functional materials, especially with respect to topological properties. Based on density functional theory calculations, by varying the core units through the choice of bridging groups [O, C═O, CH2, or C(CH3)2] and the linker units [acetylene, diacetylene, or benzene], we have designed heterotriangulene-based COFs that are predicted to be two-dimensional higher-order topological insulators (TIs). The higher-order TI characteristics of these COFs are identified via their topological invariants and the presence of in-gap topological corner modes and gapped edge states. The frontier molecular orbital energies of the building moieties play an important role in determining the size of the higher-order TI gap, which we find to be highly dependent on linker units. We also examined the deposition of the COFs on a boron nitride substrate to assess the feasibility of experimental observation of a higher-order TI phase in the organic layer. This work thus provides new insights into heterotriangulene-based COFs and guidance for the exploration of purely organic topological materials.

Observation of well-defined Kohn-anomaly in high-quality graphene devices at room temperature

Due to its ultra-thin nature, the study of graphene quantum optoelectronics, like gate-dependent graphene Raman properties, is obscured by interactions with substrates and surroundings. For instance, the use of doped silicon with a capping thermal oxide layer limited the observation to low temperatures of a well-defined Kohn-anomaly behavior, related to the breakdown of the adiabatic Born–Oppenheimer approximation. Here, we design an optoelectronic device consisting of single-layer graphene electrically contacted with thin graphite leads, seated on an atomically flat hexagonal boron nitride substrate and gated with an ultra-thin gold layer. We show that this device is optically transparent, has no background optical peaks and photoluminescence from the device components, and no generation of laser-induced electrostatic doping (photodoping). This allows for room-temperature gate-dependent Raman spectroscopy effects that have only been observed at cryogenic temperatures so far, above all the Kohn-anomaly phonon energy normalization. The new device architecture, by decoupling graphene optoelectronic properties from the substrate effects, allows for observing quantum phenomena at room temperature.

Molecular beam epitaxy growth of MoTe2 on hexagonal boron nitride

Hexagonal boron nitride has already been proven to serve as a decent substrate for high quality epitaxial growth of several 2D materials, such as graphene, MoSe2, MoS2 or WSe2. Here, we present for the first time the molecular beam epitaxy growth of MoTe2 on atomically smooth hexagonal boron nitride (hBN) substrate. Occurrence of MoTe2 in various crystalline phases such as distorted octahedral 1T’ phase with semimetal properties or hexagonal 2H phase with semiconducting properties opens a possibility of realization of crystal-phase homostructures with tunable properties. Atomic force microscopy studies of MoTe2 grown in a single monolayer regime enable us to observe impact of growth conditions on formation of various structures: flat grains, net of one-dimensional structures, quasi continuous monolayers with bilayer contribution. Comparison of the distribution of the thickness with Poisson distribution shows that tested growth conditions favorite formation of grains with monolayer thickness. The diffusion constant of MoTe2 grown on hBN can reach order of 10−6 cm2/s for typical growth conditions. Raman spectroscopy results suggest a coexistence of various phases with domination of 2H MoTe2 for samples grown at lower temperatures. XPS measurements confirm the stoichiometry of MoTe2.

Giant nonlinear Hall effect in strained twisted bilayer graphene

Recent studies have shown that moir\'e flat bands in twisted bilayer graphene (TBG) can acquire nontrivial Berry curvatures when aligned with a hexagonal boron nitride substrate, which can be manifested as a correlated Chern insulator near the 3/4 filling. In this Letter, we show that the large Berry curvatures in the moir\'e bands lead to a strong nonlinear Hall (NLH) effect in strained TBG with general filling factors. Under a weak uniaxial strain $\ensuremath{\sim}0.1%$, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\text{\AA{}}$, exceeding the values of previously known nonlinear Hall materials by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain, and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for a twist angle near the magic angle due to the strong susceptibility of nearly flat moir\'e bands to symmetry breaking induced by strains, which can even induce a topological band inversion. Our results establish TBG as a promising platform for investigating nonlinear effects such as the NLH effect, the nonlinear Nernst effect, and the nonlinear thermal Hall effect due to its giant Berry curvature dipole.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe2, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe2 can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., − 2 to − 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe2 was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p–n diode using a 0.8 nm-thick MoTe2 flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10−4. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Boron Nitride- and Graphene-Supported Trimetallic Yolk–Shell and Hollow Nanoparticles

In this study, the stability of yolk–shell and hollow Ni@PdAu and PdAu@Ni nanoparticles in free state and supported on boron nitride and graphene substrates was investigated at 300 K using molecular dynamics simulations. To this end, analyses of several parameters such as relative stability, surface energy, coordination number, number of surface atoms, bond length, displacement vector, and atomic strain were employed. Results show that the yolk–shell and hollow Ni@PdAu nanoparticles are more stable than yolk–shell and hollow PdAu@Ni nanoparticles in free and supported states due to the less surface energy. Furthermore, results show that the hollow nanoparticles are more stable than yolk–shell nanoparticles in all states. In addition, results show that the void and hollow core in yolk–shell and hollow nanoparticles are unstable and collapse due to the contraction and expansion of the core and shell, which lead to the formation of the core–shell and quasi–Janus structures in hollow and yolk–shell nanoparticles, respectively. Moreover, nanoparticles create a quasi–ellipsoid structure with low density and high coordination number on the boron nitride. The yolk–shell and hollow nanoparticles are the most stable on the boron nitride due to the smaller displacement and variation of the atomic bond length and lower atomic strain, which are caused by the strong nonbonded interaction, good atomic match, and placement of the Ni, Au, and Pd atoms on the middle position of the boron nitride hexagonal structure. Generally, this study demonstrated that the stability of yolk–shell and hollow nanoparticles can be tuned by the substrate–nanoparticle interaction.

Chemical vapor deposition of sp2-boron nitride on mechanically polished pyrolytic boron nitride substrates

Nanocrystalline sp2-boron nitride (BN) thin films have been grown for 2 and 4.5 h on mechanically polished polycrystalline pyrolytic boron nitride substrates at 1060 °C via chemical vapor deposition using diborane (B2H6) and ammonia (NH3) as the B- and N sources, respectively. The use of an N/B gas phase ratio of 200 resulted in sp2-BN thin films having a stoichiometric surface chemistry, as determined by x-ray photoelectron spectroscopy. Cross-sectional transmission electron microscopy revealed the initial growth of ∼4 nm thick partially ordered [0001] sp2-BN layers regardless of the crystallographic orientation of the sets of layers in the substrate. A transition zone then formed within the deposited films that contained randomly oriented polycrystalline grains and that culminated in the formation of protrusions. Atomic force microscopy verified an increase in roughening of the surfaces of the films with an increase in growth time and the associated thickness.

Excitonic fractional quantum Hall hierarchy in moiré heterostructures

We consider fractional quantum Hall states in systems where two flat Chern number $C=\ifmmode\pm\else\textpm\fi{}1$ bands are labeled by an approximately conserved valley index and interchanged by time reversal symmetry. At filling factor $\ensuremath{\nu}=1$ this setting admits an unusual hierarchy of correlated phases of excitons, neutral particle-hole pair excitations of a fully valley-polarized orbital ferromagnet parent state where all electrons occupy a single valley. Excitons experience an effective magnetic field due to the Chern numbers of the underlying bands. This obstructs their condensation in favor of a variety of crystalline orders and gapped and gapless liquid states. All these have the same quantized charge Hall response and are electrically incompressible, but differ in their edge structure, orbital magnetization, and hence valley and thermal responses. We explore the relevance of this scenario for moir\'e heterostructures of bilayer graphene on a hexagonal boron nitride substrate.

Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates.

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2nm and a typical bandgap of ≈1eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. Thisstudy provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

Phonon-Limited Mobility in h-BN Encapsulated AB-Stacked Bilayer Graphene.

The weak acoustic phonon scattering in graphene monolayer leads to high mobilities even at room temperatures. We identify the dominant role of the shear phonon mode scattering on the carrier mobility in AB-stacked graphene bilayer, which is absent in monolayer graphene. Using a microscopic tight-binding model, we reproduce experimental temperature dependence of mobilities in high-quality boron nitride encapsulated bilayer samples at temperatures up to ∼200 K. At elevated temperatures, the surface polar phonon scattering from boron nitride substrate contributes significantly to the measured mobilities of 15 000 to 20000 cm^{2}/Vs at room temperature and carrier concentration n∼10^{12} cm^{-2}. A screened surface polar phonon potential for a dual-encapsulated bilayer and transferable tight-binding model allows us to predict mobility scaling with temperature and band gap for both electrons and holes in agreement with the experiment.

Theory of competing charge density wave, Kekulé, and antiferromagnetically ordered fractional quantum Hall states in graphene aligned with boron nitride

We investigate spin and valley symmetry-broken fractional quantum Hall phases within a formalism that naturally extends the paradigm of quantum Hall ferromagnetism from integer to fractional quantum Hall states, allowing us to construct detailed phase diagrams for a large class of multi-component states. Motivated by recent experiments on Graphene aligned with a Boron Nitride substrate, we predict a sequence of transitions realized by increasing the magnetic field, starting from a sub-lattice polarized state to a valley coherent Kekule charge density wave state and further to an anti-ferromagnetic phase. Moreover for filling fractions such as $\nu=\pm 1/3$, we predict that the system undergoes a transition at low fields, that not only differ by the spin-valley orientation of the fractionally filled flavors but also by their intrinsic fractional quantum Hall nature. This transition is from a Laughlin-like state to a two component Halperin-like state both with a charge density wave order. Moreover for $\nu=\pm 1/3,\pm 2/3$, we predict a "canted Kekule density phase"(CaKD) where the spinors of integer and fractionally occupied components have different orientations in the valley Bloch sphere, in contrast to the Kekule state for the integer quantum Hall state at neutrality where both occupied components have the same orientation in the valley Bloch sphere.

Inside Back Cover

This cover shows the growth of a benchmark organic semiconductor, rubrene, on hexagonal boron nitride (h‐BN) substrate via physical vapor deposition from the initial amorphous phase to the final crystalline phase; the role of temperature in such transition and the epitaxy relationship between rubrene and h‐BN are revealed. With the increase of substrate temperature, the critical thickness of amorphous‐crystalline‐transition decreases and the morphology of crystalline phase also evolves from porous to terrace‐like. The high ordering film demonstrates good charge transport ability with a p‐type field‐effect mobility of >1 cm2·V–1·s–1. More details are given in the article by Chi et al. on page 1298—1304.image

Electric fields and substrates dramatically accelerate spin relaxation in graphene

Electrons in graphene are theoretically expected to retain spin states much longer than most materials, making graphene a promising platform for spintronics and quantum information technologies. Here, we use first-principles density-matrix (FPDM) dynamics simulations to show that interaction with electric fields and substrates strongly enhance spin relaxation through scattering with phonons. Consequently, the relaxation time at room temperature reduces from microseconds in free-standing graphene to nanoseconds in graphene on hexagonal boron nitride (hBN) substrate, the order of magnitude typically measured in experiments. Further, inversion symmetry breaking by hBN introduces a stronger asymmetry in electron and hole spin lifetimes, than predicted by the conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from the conventional DP model are stronger for in-plane spin relaxation, resulting in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a maximum close to the Dirac point. These FPDM results, independent of symmetry-specific assumptions or material-dependent parameters, also validate recent modifications of the DP model to explain such deviations. Overall, our results indicate that spin-phonon relaxation in the presence of substrates may be more important in graphene than typically assumed, requiring consideration for graphene-based spin technologies at room temperature.

Growth Behavior of Rubrene Thin Films on Hexagonal Boron Nitride in the Early Stage

Comprehensive SummaryTwo‐dimensional materials, with an in‐plane ordered and dangling‐bonding‐free surface, are ideal substrates for fabricating high‐ quality crystalline thin films. Here, we show a systematic study on the growth of a benchmark organic semiconductor, rubrene, on hexagonal boron nitride (h‐BN) substrate via physical vapor deposition from the initial amorphous phase to the final crystalline phase; the role of temperature in such transition and the epitaxy relationship between rubrene and h‐BN are revealed. With the increase of substrate temperature, the critical thickness of amorphous‐crystalline‐transition decreases and the morphology of crystalline phase also evolves from porous to terrace‐like. When substrate temperature reaches >100°C, the critical thickness reduces to only 0.5 nm and a precise layer‐by‐layer growth from the very first layer is achieved, which is quite rare for rubrene growing on other substrates. The high ordering can be attributed to the fine epitaxy relationship between rubrene films and the h‐BN surface lattice, and this film demonstrates good charge transport ability with a p‐type field‐effect mobility of >1 cm2·V–1·s–1.

Electronic Structure of Graphene on the Hexagonal Boron Nitride Surface: A Density Functional Theory Study

Poor electron-related cutting current in graphene-based field-effect transistors (FETs) can be solved by placing a graphene layer over a hexagonal boron nitride (BN) substrate, as established by Giovannetti et al. and other researchers. In order to produce high-quality results, this investigation uses 2 × 2 cells (~2.27% mismatch), given that larger cells lead to more favourable considerations regarding interactions on cell edges. In this case, the substrate-induced band gap is close to 138 meV. In addition, we propose a new material based on graphene on BN in order to take advantage of the wonderful physical properties of both graphene and BN. In this new material, graphene is rotated with respect to BN, and it exhibits a better mismatch, only ~1.34%, than the 1 × 1-graphene/1 × 1-BN; furthermore, it has a very small bandgap, which is almost zero. Therefore, in the bands, there are electronic states in cone form that are like the Dirac cones, which maintain the same characteristics as isolated graphene. In the first case (2 × 2-graphene/2 × 2-BN), for example, the resulting band gap of 138 meV is greater than Giovannetti’s value by a factor of ~2.6. The 2 × 2-graphene/2 × 2-BN cell is better than the 1 × 1-graphene/BN one because a greater bandgap is an improvement in the cutting current of graphene-based FETs, since the barrier created by the bandgap is larger. The calculations in this investigation are performed within the density functional theory (DFT) theory framework, by using 2 × 2-graphene/2 × 2-BN and 13 × 13-graphene/23 × 23-(0001) BN cells. Pseudopotentials and the generalized gradient approximation (GGA), combined with the Perdew–Burke–Ernzerhof parametrization, were used. Relaxation is allowed for all atoms, except for the last layer of the BN substrate, which serves as a reference for all movements and simulates the bulk BN.

Effect of boron nitride defects and charge inhomogeneity on 1/f noise in encapsulated graphene

Low frequency 1/f noise is investigated in graphene, encapsulated between the hexagonal boron nitride (hBN) substrate in dual gated geometry. The overall noise magnitude is smaller as compared to graphene on the Si/SiO2 substrate. The noise amplitude in the hole doped region is independent of carrier density, while in the electron doped region, a pronounced peak is observed at Fermi energy, EF∼90 meV. The physical mechanism of the anomalous noise peak in the electron doped region is attributed to the impurity states originating from the Carbon atom replacing the nitrogen site in the hBN crystal. Furthermore, the noise study near the Dirac point shows a characteristic “M-shape,” which is found to be strongly correlated with the charge inhomogeneity region near the Dirac point.

Transient Reflection Spectroscopy on Ultrafast Interlayer Charge Transfer Processes in a MoS2/WSe2van der Waals Heterojunction

Understanding the dynamical process of charge transfer (CT) across van der Waals heterostructures (VdWHs) is vitally important for improving photovoltaic conversion efficiency at the atomically thin interfaces. Here, ultrafast dynamics of VdWHs constructed by vertically stacking monolayer molybdenum disulfide (MoS2) onto tungsten diselenide (WSe2) with a multilayer hexagonal boron nitride (hBN) substrate were investigated by transient reflection measurements. MoS2 and WSe2 were selectively excited to obtain direct evidence of the CT processes. It is interesting to note that the bleaching band of MoS2/WSe2 is composed of two different contributions with different dynamics under excitation at 650 nm: direct excitation-induced bleaching of WSe2 (with a decay time constant of ∼63 ps) and bleaching signals arising from photo-induced hole transfer (with a decay time constant of ∼303 ps). The interlayer CT process was found to occur on the subpicosecond time scale, resulting in long-lived interlayer spatially separated charge carriers (approximately hundreds of picoseconds). These results imply potentially high photovoltaic conversion efficiency of these heterostructures.

Domain wall competition in the Chern insulating regime of twisted bilayer graphene

We consider magic-angle twisted bilayer graphene (TBG) at filling $\nu=+3$, where experiments have observed a robust quantized anomalous Hall effect. This has been attributed to the formation of a valley- and spin-polarized Chern insulating ground state that spontaneously breaks time-reversal symmetry, and is stabilized by a hexagonal boron nitride (hBN) substrate. We identify three different types of domain wall, and study their properties and energetic selection mechanisms via theoretical arguments and Hartree-Fock calculations adapted to deal with inhomogeneous moir\'e systems. We comment on the implications of these results for transport and scanning probe experiments.

Proximity Effect of Epitaxial Iron Phthalocyanine Molecules on High-Quality Graphene DevicesSupported by the National Natural Science Foundation of China (Grant No. 11674150), the Key-Area Research and DevelopmentProgram of Guangdong Province (Grant No. 2019B010931001), the Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06D348), the SUSTech CRF, and the Hong Kong RGC (Grant Nos. 16300617 and C6012-17E).

Depositing magnetic insulators on graphene has been a promising route to introduce magnetism via exchange proximity interaction in graphene for future spintronics applications. Molecule-based magnets may offer unique opportunities because of their synthesis versatility. Here, we investigate the magnetic proximity effect of epitaxial iron phthalocyanine (FePc) molecules on high-quality monolayer and bilayer graphene devices on hexagonal boron nitride substrates by probing the local and nonlocal transport. Although the FePc molecules introduce large hole doping effects combined with mobility degradation, the magnetic proximity gives rise to a canted antiferromagnetic state under a magnetic field in the monolayer graphene. On bilayer graphene and FePc heterostructure devices, the nonlocal transport reveals a pronounced Zeeman spin-Hall effect. Further analysis of the scattering mechanism in the bilayer shows a dominated long-range scattering. Our findings in graphene/organic magnetic insulator heterostructure provide a new insight for use of molecule-based magnets in two-dimensional spintronic devices.

Efficient ReSe2 Photodetectors with CVD Single-Crystal Graphene Contacts.

Rhenium-based 2D transition metal dichalcogenides such as ReSe2 are suitable candidates as photoactive materials for optoelectronic devices. Here, photodetectors based on mechanically exfoliated ReSe2 crystals were fabricated using chemical vapor deposited (CVD) graphene single-crystal (GSC) as lateral contacts. A “pick & place” method was adopted to transfer the desired crystals to the intended position, easing the device fabrication while reducing potential contaminations. A similar device with Au was fabricated to compare contacts’ performance. Lastly, a CVD hexagonal boron nitride (hBN) substrate passivation layer was designed and introduced in the device architecture. Raman spectroscopy was carried out to evaluate the device materials’ structural and electronic properties. Kelvin probe force measurements were done to calculate the materials’ work function, measuring a minimal Schottky barrier height for the GSC/ReSe2 contact (0.06 eV). Regarding the electrical performance, I-V curves showed sizable currents in the GSC/ReSe2 devices in the dark and under illumination. The devices presented high photocurrent and responsivity, along with an external quantum efficiency greatly exceeding 100%, confirming the non-blocking nature of the GSC contacts at high bias voltage (above 2 V). When introducing the hBN passivation layer, the device under white light reached a photo-to-dark current ratio up to 106.