Newly discovered van der Waals materials like MoS, WSe, hexagonal boron nitride (h-BN), and recently CN have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. In this paper we investigate correlations between h-BN structural features and emitter location from bulk down to the monolayer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters' spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photodynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long-lived metastable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability, and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications.

Abstractvan Hove singularities (VHS's) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids. At the critical point the Fermi surface connectivity changes, and topological properties undergo a transition.Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the atomic registry of neighboring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here we have addressed magnetotransport when the chemical potential crosses the twist angle induced VHS in twisted bilayer graphene. A topological phase transition is experimentally disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a nonzero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity.Keywords: Twisted bilayer graphene, van Hove singularity, Moiré superlattice, topological transition. 3VHS's have been frequently invoked as the source of rich physics. 1 We exemplary refer here to their influence on superconductivity. Near the van Hove filling the electron−phonon coupling may get enhanced thereby enabling superconductivity or affecting its stability. 2 They can also mediate nematic or other deformations of the Fermi surface in the presence of interactions and trigger Landau type symmetry breaking phase transitions through a Pomeranchuk instability. 3 In bulk materials the chemical potential is not readily tunable across a wide enough energy range to explore the physics by a VHS in a systematic manner.Conventional and electrolyte gating techniques applicable to low-dimensional materials seem suitable for such purposes. Graphene has been considered a promising candidate material for such studies and, stimulated by the controversially debated prospect of turning it into a superconductor, has been looked at intensively up to high densities. 4,5 Its π-conduction and valence band possess a VHS due to the merger of Dirac cone states emanating from the K and K′ symmetry points in the Brillouin zone. However, its energy is in essence determined by the large intersublattice hopping parameter of 2.7 eV apparently placing it out of reach for even the most potent in situ controllable gating techniques based on electrolytes.From high-temperature superconductors it is well-known that VHS's are common in quasi two-dimensional layered materials where they arise as a result of the interlayer coupling. 6−8 In van der Waals heterostructures this interlayer coupling can be tuned by changing the atomic registry between neighboring layers. 9−11 It has been confirmed in scanning tunneling microscopy and spectroscopy that in twisted bilayer graphene where the crystallographic axes of the neighboring layers are misaligned by a rotation angle θ, a VHS exists across a broad range of twist angles. 12−14 These occur at controllable and, most of all,...

The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong affinity and graphitic gates that screen charge impurity disorder provided access to a plethora of interesting physical phenomena by drastically boosting the device quality. The encapsulation is accompanied by a self-cleansing effect at the interfaces. The otherwise predominant charged impurity disorder is minimized and random strain fluctuations ultimately constitute the main source of residual disorder. Despite these advances, the fabricated heterostructures still vary notably in their performance. While some achieve record mobilities, others only possess mediocre quality. Here, we report a reliable method to improve fully completed van der Waals heterostructure devices with a straightforward post-processing surface treatment based on thermal annealing and contact mode AFM. The impact is demonstrated by comparing magnetotransport measurements before and after the AFM treatment on one and the same device as well as on a larger set of treated and untreated devices to collect device statistics. Both the low temperature properties as well as the room temperature electrical characteristics, as relevant for applications, improve on average substantially. We surmise that the main beneficial effect arises from reducing nanometer scale corrugations at the interfaces, i.e. the detrimental impact of random strain fluctuations.

Graphene, being an atomically thin conducting sheet, is a candidate material for gate electrodes in vacuum electronic devices, as it may be traversed by low-energy electrons. The transparency of graphene to electrons with energies between 2 and 40 eV has been measured by using an optimized vacuum-triode setup. The measured graphene transparency equals ∼60% in most of this energy range. Based on these results, nano-patterned sheets of graphene or of related two-dimensional materials are proposed as gate electrodes for ambipolar vacuum devices.

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