When two-dimensional atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals can start influencing each other's electronic properties. Of particular interest is the situation when the periodicity of the two crystals closely match and a moiré pattern forms, which results in specific electron scattering, reconstruction of electronic and excitonic spectra, crystal reconstruction, and many other effects. Thus, formation of moiré patterns is a viable tool of controlling the electronic properties of 2D materials. At the same time, the difference in the interatomic distances for the two crystals combined, determines the range in which the electronic spectrum is reconstructed, and thus is a barrier to the low energy regime. Here we present a way which allows spectrum reconstruction at all energies. By using graphene which is aligned simultaneously to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern, which can have arbitrarily small wavevector and, thus results in spectrum reconstruction at arbitrarily low energies. We demonstrate that the strength of such a potential relies crucially on the atomic reconstruction of graphene within the differential moiré super-cell. Such structures offer further opportunity in tuning the electronic spectra of two-dimensional materials.Introduction: Van der Waals heterostructures allow combining different two-dimensional (2D) materials into functional stacks(1, 2), which has already produced a range of interesting electronic(3, 4) and optoelectronic(5-8) devices and resulted in observation of exciting physical phenomena. The large variety of the heterostructures is mainly due to the large selection of 2D materials. However, the assembly of van der Waals heterostructures allow one extra degree of freedom: apart from the selection of the sequence of the 2D crystalsthe individual crystals can be differently oriented with respect to each other. Previously such control over the rotational alignment between crystals resulted in the observation of the resonant tunnelling(9-11), renormalisation of exciton binding energy(12) insulating(13) and superconducting(4) states. 01129a). J.Y. and A.M. acknowledge the support of EPSRC Early Career Fellowship EP/N007131/1.

Two-dimensional (2D) hexagonal boron nitride (hBN) is a wide-bandgap van der Waals crystal with a unique combination of properties, including exceptional strength, large oxidation resistance at high temperatures and optical functionalities. Furthermore, in recent years hBN crystals have become the material of choice for encapsulating other 2D crystals in a variety of technological applications, from optoelectronic and tunnelling devices to composites. Monolayer hBN, which has no center of symmetry, has been predicted to exhibit piezoelectric properties, yet experimental evidence is lacking. Here, by using electrostatic force microscopy, we observed this effect as a strain-induced change in the local electric field around bubbles and creases, in agreement with theoretical calculations. No piezoelectricity was found in bilayer and bulk hBN, where the centre of symmetry is restored. These results add piezoelectricity to the known properties of monolayer hBN, which makes it a desirable candidate for novel electromechanical and stretchable optoelectronic devices, and pave a way to control the local electric field and carrier concentration in van der Waals heterostructures via strain. The experimental approach used here also shows a way to investigate the piezoelectric properties of other materials on the nanoscale by using electrostatic scanning probe techniques.Piezoelectricity is an important property of noncentrosymmetric crystals that allows conversion of mechanical strain into electric field, and vice versa. [1] Recently, two-dimensional (2D) crystals have shown to be a unique platform to investigate and exploit such property for many reasons. First, they have the ability to sustain large strain (up to 10%) before rupture or plastic deformation, [2] while this is challenging to achieve in 3D crystals. Second, many crystals are found to be piezoelectric only when reduced to two-dimensionality. This is the case of semiconducting transition metal dichalcogenides, in which inversion symmetry is broken only in their 2D forms, as recently observed in single-layer MoS2. [3] Furthermore, 2D crystals are likely to show areas of non-uniform strain near corrugations or bubbles that naturally form on substrates. [4] In such areas, strainedinduced local charge densities, ߩ, are expected to appear owing to the local variation in polarization, ܲ , since ߩሺ‫ݎ‬ሻ = −ߘ • ܲሺ‫ݎ‬ሻ. [5] 2D hexagonal boron nitride (hBN) is a van der Waals crystal with remarkable properties [2a, 6] and is an essential component of many new 2D technologies. [7] Recently, single-layer hexagonal boron nitride (hBN) has been theoretically predicted to be piezoelectric due to its broken inversion symmetry. [5,8] Boron and Nitrogen atoms in hBN are arranged in a honeycomb lattice similarly as graphene, but the presence of different elements in the two sublattices of its unit cell makes it non-centrosymmetric. On the other hand, its bilayer and bulk counterparts recover the inversion symmetry and, therefore, no piezoelectricity is expected. [5,8] Here we r...

Abstract. Two-dimensional materials are characterised by a number of unique physical properties which can potentially make them useful to a wide diversity of applications. In particular, the large thermal conductivity of graphene and hexagonal boron nitride has already been acknowledged and these materials have been suggested as novel core materials for thermal management in electronics. However, it was not clear if mass produced flakes of hexagonal boron nitride would allow one to achieve an industrially-relevant value of thermal conductivity. Here we demonstrate that laminates of hexagonal boron nitride exhibit thermal conductivity of up to 20 W/mK, which is significantly larger than that currently used in thermal management. We also show that the thermal conductivity of laminates increases with the increasing volumetric mass density, which creates a way of fine tuning its thermal properties.

Aim: The genetic background of orthostatic blood pressure dysregulation remains poorly understood. Since the renin-angiotensin system plays an important role in blood pressure regulation and response to position change, we hypothesized that angiotensin-converting enzyme (ACE) and ACE2 genetic polymorphisms might contribute, at least partially, to orthostatic blood pressure dysregulation in hypertensive patients. Methods: Two tag single nucleotide polymorphisms (SNPs) of ACE2 and ACE I/D were genotyped in 3630 untreated hypertensive patients and 826 normotensive subjects. Orthostatic hypertension was defined as an increase in systolic blood pressure of 20 mmHg or more and orthostatic hypotension as a drop in blood pressure of 20/10 mmHg or more within three minutes of assumption of upright posture. Results: Female and male patients had similar rates of orthostatic hypertension (16.5% vs 15.3%) and hypotension (22.5% vs 23.8%). No significant differences were detected in the minor allele frequency of ACE2 rs2106809, rs2285666, or ACE I/D in either female or male patients with orthostatic hypertension (15.1%, 22.7%, 19.6%, respectively), hypotension (13.8%, 25%, 16.5%), or normal orthostatic blood pressure response (14.4%, 21.9%, 15.8%) in additive, dominant or recessive models after adjustment for confounders (all P>0.05). The orthostatic changes in systolic and diastolic blood pressure were also comparable among patients carrying different genotypes. Similar results were observed in normotensive subjects. Conclusion: These data provide no support for the involvement of ACE or ACE2 in the genetic predisposition to orthostatic hypotension or hypertension.