[ASAP] Conformal Growth of Hexagonal Boron Nitride on High-Aspect-Ratio Silicon-Based Nanotrenches
Chemistry of MaterialsDOI: 10.1021/acs.chemmater.2c03568
Published in: "Chemistry of Materials".
Framework for engineering of spin defects in hexagonal boron nitride by focused ion beams. (arXiv:2303.06784v1 [cond-mat.mtrl-sci])
Hexagonal boron nitride (hBN) is gaining interest as a wide bandgap van der Waals host of optically active spin defects for quantum technologies. Most studies of the spin-photon interface in hBN focus on the negatively charged boron vacancy (VB-) defect, which is typically fabricated by ion irradiation. However, VB- fabrication methods often lack robustness and reproducibility when applied to thin flakes (less than 10 nm) of hBN. Here we identify mechanisms that both promote and inhibit VB- generation and optimize ion beam parameters for site-specific fabrication of optically active VB- centers. We emphasize conditions accessible by high resolution focused ion beam (FIB) systems, and present a framework for VB- fabrication in hBN flakes of arbitrary thickness for applications in quantum sensing and quantum information processing.
Published in: "arXiv Material Science".
Methane photooxidation with nearly 100% selectivity towards oxygenates: proton rebound ensures the regeneration of methanol
Restrained by uncontrollable dehydrogenation process, the target products of methane direct conversion would suffer from an inevitable overoxidation, which is deemed as one of the most challenging issues in catalysis. Herein, based on the concept of hydrogen bonding trap, we proposed a novel concept to modulate methane conversion pathway to hinder the overoxidation of target products. Taking boron nitride as a proof-of-concept model, for the first time, it is found that the designed N-H bonds can work as hydrogen bonding trap to attract electrons. Benefitting from it, the N-H bonds on the BN surface rather than C-H bonds in formaldehyde prefer to cleavage, greatly suppressing the continuous dehydrogenation process. More importantly, formaldehyde will combine with the released protons, which leads to a proton rebound process to regenerate methanol. As a result, BN shows a high methane conversion rate (8.5%) and nearly 100% product selectivity to oxygenates under atmospheric pressure.
Published in: "Angewandte Chemie International Edition".
Polarization dynamics of solid-state quantum emitters. (arXiv:2303.04732v1 [quant-ph])
Quantum emitters in solid-state crystals have recently attracted a lot of attention due to their simple applicability in optical quantum technologies. Color centers such as fluorescent defects hosted by diamond and hexagonal boron nitride (hBN) emit single photons at room temperature and can be used for nanoscale sensing. The atomic structure of the hBN defects, however, is not yet well understood. In this work, we fabricate an array of identical hBN emitters by localized electron irradiation. This allows us to correlate the dipole orientations with the host crystal axes. The angle of excitation and emission dipoles relative to the crystal axes are also calculated using density functional theory, which reveals characteristic angles for every specific defect. Moreover, we also investigate the temporal polarization dynamics and discover a mechanism of time-dependent polarization visibility and dipole orientation of color centers in hBN and diamond. This can be traced back to the excitation of excess charges in the local crystal environment. We therefore provide a promising pathway for the identification of color centers as well as important insight into the dynamics of solid-state quantum emitters.
Published : "arXiv Mesoscale and Nanoscale Physics".
Quantum sensing of paramagnetic spins in liquids with spin qubits in hexagonal boron nitride. (arXiv:2303.02326v1 [quant-ph])
Paramagnetic ions and radicals play essential roles in biology and medicine, but detecting these species requires a highly sensitive and ambient-operable sensor. Optically addressable spin color centers in 3D semiconductors have been used for detecting paramagnetic spins as they are sensitive to the spin magnetic noise. However, the distance between spin color centers and target spins is limited due to the difficulty of creating high-quality spin defects near the surface of 3D materials. Here, we show that spin qubits in hexagonal boron nitride (hBN), a layered van der Waals (vdW) material, can serve as a promising sensor for nanoscale detection of paramagnetic spins in liquids. We first create shallow spin defects in close proximity to the hBN surface, which sustain high-contrast optically detected magnetic resonance (ODMR) in liquids. Then we demonstrate sensing spin noise of paramagnetic ions in water based on spin relaxation measurements. Finally, we show that paramagnetic ions can reduce the contrast of spin-dependent fluorescence, enabling efficient detection by continuous wave ODMR. Our results demonstrate the potential of ultrathin hBN quantum sensors for chemical and biological applications.
Published : "arXiv Mesoscale and Nanoscale Physics".
Hexagonal boron nitride nanophotonics. (arXiv:2303.02804v1 [physics.optics])
A global trend to miniaturization and multiwavelength performance of nanophotonic devices drives research on novel phenomena, such as bound states in the continuum and Mietronics, as well as the survey for high-refractive index and strongly anisotropic materials and metasurfaces. Hexagonal boron nitride (hBN) is one of promising materials for the future nanophotonics owing to its inherent anisotropy and prospects of high-quality monocrystals growth with atomically flat surface. Here, we present highly accurate optical constants of hBN in the broad wavelength range of 250-1700 nm combining the imaging ellipsometry measurements scanning near-field optical microscopy and first-principle quantum mechanical computations. hBN’s high refractive index, up to 2.75 in ultraviolet (UV) and visible range, broadband birefringence of 0.7, and negligible optical losses make it an outstanding material for UV and visible range photonics. Based on our measurement results, we propose and design novel optical elements: handedness-preserving mirrors and subwavelength waveguides with dimensions of 40 nm operating in the visible and UV range, respectively. Remarkably, our results offer unique opportunity to bridge the size-gap between photonics and electronics.
Published in: "arXiv Material Science".
The ideal wavelength for daylight free-space quantum key distribution. (arXiv:2303.02106v1 [quant-ph])
Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle demonstrations to commercially available systems. One of the major bottlenecks is the limited communication distance in fiber networks due to the exponential signal damping. To bridge intercontinental distances, low Earth orbit satellites transmitting the quantum signals over the atmosphere can be used. These free-space links, however, can only operate during the night, as the sunlight otherwise saturates the detectors used to measure the quantum states. For applying QKD in a global quantum internet with continuous availability and high data rates, operation during daylight is required. In this work, we model a satellite-to-ground quantum channel for different quantum light sources to identify the optimal wavelength for free-space QKD in ambient conditions. Daylight quantum communication is possible within the Fraunhofer lines or in the near-infrared spectrum, where the intrinsic background from the sun is comparably low. The highest secret key rate is achievable at the Ca II Fraunhofer line. We also propose a true single photon source based on a color center in hexagonal boron nitride coupled to a microresonator that can implement such a scheme. Our results can also be applied in roof-to-roof scenarios and are therefore relevant for near-future quantum networks.
Published in: "arXiv Material Science".
Exciton-assisted electron tunneling in van der Waals heterostructures. (arXiv:2303.01544v1 [cond-mat.mes-hall])
The control of elastic and inelastic electron tunneling relies on materials with well defined interfaces. Van der Waals materials made of two-dimensional constituents form an ideal platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage ($I-V$) measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here, we use a novel tunneling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride (hBN) with an adjacent TMD monolayer and observe prominent resonant features in $I-V$ measurements. These resonances appear at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunneling pathway, we demonstrate that this phonon-exciton mediated tunneling process does not require any charge injection into the TMD. This work demonstrates the appearance of optical modes in electrical transport measurements and introduces a new functionality for optoelectronic devices based on van der Waals materials.
Published in: "arXiv Material Science".
Creation of single vacancies in hBN with electron irradiation. (arXiv:2303.00497v1 [cond-mat.mes-hall])
Understanding electron irradiation effects is vital not only for reliable characterization of materials using transmission electron microscopy, but increasingly also for the controlled manipulation of two-dimensional materials. Knock-on displacements due to elastic electron backscattering are theoretically straightforward to model, and appear to correctly describe damage in pristine graphene. For semiconducting MoS$_2$, some experimental and theoretical progress has been recently made, indicating that a combination of elastic and inelastic effects appears to be needed to explain experiments. For insulating hexagonal boron nitride (hBN), however, much less is currently known. We measure the displacement cross sections of suspended monolayer hBN using aberration-corrected scanning transmission electron microscopy in near ultra-high vacuum at primary beam energies between 50 and 90~keV. We find damage rates below 80~keV up to three orders of magnitude lower than previously measured at hBN edges under comparatively poorer residual vacuum conditions where chemical etching appears to have been the dominant damage mechanism. Notably, we are now able to show that it is possible to create single vacancies in hBN using electron irradiation, and resolve that boron are almost twice as likely as nitrogen to be ejected below 80~keV. Moreover, any damage at such low energies cannot be explained by pure elastic knock-on, even when accounting for vibrations of the atoms. We thus develop a theoretical description that accounts for a lowering of the displacement threshold energy due to valence ionization resulting from inelastic scattering of probe electrons, and model this using charge-constrained density functional theory molecular dynamics. Our findings show
Published : "arXiv Mesoscale and Nanoscale Physics".
Control of the phonon band gap with isotopes in hexagonal boron nitride. (arXiv:2302.13415v1 [cond-mat.mtrl-sci])
The isotopic mass of constituent elements of materials has a well-known effect on the energy of vibrational modes. By means of monochromated scanning transmission electron microscopy we have experimentally studied the phonon bandstructure of hexagonal BN, where a phonon band gap appears between in-plane optical phonon modes and the lower energy part of the phonon spectrum. The size of the phonon band gap can be manipulated by the isotopic mass of the boron. While in $^{11}$BN the phonon band gap is about 7 meV wide, in $^{10}$BN the gap nearly closes, being an order of magnitude smaller (below 0.5 meV). This opens exciting options for manipulating terahertz wave propagation through isotopically structured devices having otherwise no interfaces between chemically distinct components.
Published in: "arXiv Material Science".
[ASAP] Paramagnetic States in Oxygen-Doped Boron Nitride Extend Light Harvesting and Photochemistry to the Deep Visible Region
Chemistry of MaterialsDOI: 10.1021/acs.chemmater.2c01646
Published in: "Chemistry of Materials".
Cavity Moir’e Materials: Controlling Magnetic Frustration with Quantum Light-Matter Interaction. (arXiv:2302.11582v1 [cond-mat.str-el])
Cavity quantum electrodynamics (QED) studies the interaction between light and matter at the single quantum level and has played a central role in quantum science and technology. Combining the idea of cavity QED with moir’e materials, we theoretically show that strong quantum light-matter interaction provides a way to control frustrated magnetism. Specifically, we develop a theory of moir’e materials confined in a cavity consisting of thin polar van der Waals crystals. We show that nontrivial quantum geometry of moir’e flat bands leads to electromagnetic vacuum dressing of electrons, which produces appreciable changes in single-electron energies and manifests itself as long-range electron hoppings. We apply our general formulation to a twisted transition metal dichalcogenide heterobilayer encapsulated by ultrathin hexagonal boron nitride layers and predict its phase diagram at different twist angles and light-matter coupling strengths. Our results indicate that the cavity confinement enables one to control magnetic frustration of moir’e materials and might allow for realizing various exotic phases such as a quantum spin liquid.
Published in: "arXiv Material Science".
Quantum sensing and imaging with spin defects in hexagonal boron nitride. (arXiv:2302.11169v1 [quant-ph])
Color centers in hexagonal boron nitride (hBN) have recently emerged as promising candidates for a new wave of quantum applications. Thanks to hBN’s high stability and 2-dimensional (2D) layered structure, color centers in hBN can serve as robust quantum emitters that can be readily integrated into nanophotonic and plasmonic structures on a chip. More importantly, the recently discovered optically addressable spin defects in hBN provide a quantum interface between photons and electron spins for quantum sensing applications. The most well-studied hBN spin defects so far, the negatively charged boron vacancy ($V_B^-$) spin defects, have been used for quantum sensing of static magnetic fields, magnetic noise due to spin fluctuations, temperature, strain, nuclear spins, RF signals, and beyond. In particular, hBN nanosheets with spin defects can form van der Waals (vdW) heterostructures with 2D magnetic or other materials for in situ quantum sensing and imaging. This review summarizes the rapidly evolving field of nanoscale and microscale quantum sensing with spin defects in hBN. We introduce basic properties of hBN spin defects, quantum sensing protocols, and recent experimental demonstrations of quantum sensing and imaging with hBN spin defects. We also discuss methods to improve their sensitivity. Finally, we envision some potential development and applications of hBN spin defects.
Published in: "arXiv Material Science".
Detection of paramagnetic spins with an ultrathin van der Waals quantum sensor. (arXiv:2302.10560v1 [cond-mat.mes-hall])
Detecting magnetic noise from small quantities of paramagnetic spins is a powerful capability for chemical, biochemical, and medical analysis. Quantum sensors based on optically addressable spin defects in bulk semiconductors are typically employed for such purposes, but the 3D crystal structure of the sensor inhibits the sensitivity by limiting the proximity of the defects to the target spins. Here we demonstrate the detection of paramagnetic spins using spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material which can be exfoliated into the 2D regime. We first create negatively charged boron vacancy (V$_{rm B}^-$) defects in a powder of ultrathin hBN nanoflakes ($
Published : "arXiv Mesoscale and Nanoscale Physics".
Symmetry of the Hyperfine and Quadrupole Interactions of Boron Vacancies in a Hexagonal Boron Nitride. (arXiv:2302.08831v1 [cond-mat.mtrl-sci])
The concept of optically addressable spin states of deep level defects in wide band gap materials is successfully applied for the development of quantum technologies. Recently discovered negatively charged boron vacancy defects (VB) in hexagonal boron nitride (hBN) potentially allow a transfer of this concept onto atomic thin layers due to the van der Waals nature of the defect host. Here, we experimentally explore all terms of the VB spin Hamiltonian reflecting interactions with the three nearest nitrogen atoms by means of conventional electron spin resonance and high frequency (94 GHz) electron-nuclear double resonance. We establish symmetry, anisotropy, and principal values of the corresponding hyperfine interaction (HFI) and nuclear quadrupole interaction (NQI). The HFI can be expressed in the axially symmetric form as Aperp = 45.5 MHz and Apar = 87 MHz, while the NQI is characterized by quadrupole coupling constant Cq = 1.96 MHz with slight rhombisity parameter n = (Pxx – Pyy)/Pzz = -0.070. Utilizing a conventional approach based on a linear combination of atomic orbitals and HFI values measured here, we reveal that almost all spin density (84 %) of the VB electron spin is localized on the three nearest nitrogen atoms. Our findings serve as valuable spectroscopic data and direct experimental demonstration of the VB spin localization in a single two dimensional BN layer.
Published in: "arXiv Material Science".
Exotic single-photon and enhanced deep-level emissions in hBN strain superlattice. (arXiv:2302.07614v1 [cond-mat.mtrl-sci])
The peculiar defect-related photon emission processes in 2D hexagonal boron nitride (hBN) have become a topic of intense research due to their potential applications in quantum information and sensing technologies. Recent efforts have focused on activating and modulating the defect energy levels in hBN by methods that can be integrated on a chip, and understanding the underlying physical mechanism. Here, we report on exotic single photon and enhanced deep-level emissions in 2D hBN strain superlattice, which is fabricated by transferring multilayer hBN onto hexagonal close-packed silica spheres on silica substrate. We realize effective activation of the single photon emissions (SPEs) in the multilayer hBN at the positions that are in contact with the apex of the SiO2 spheres. At these points, the local tensile strain induced blue-shift of the SPE is found to be up to 12 nm. Furthermore, high spatial resolution cathodoluminescence measurments show remarkable strain-enhanced deep-level (DL) emissions in the multilayer hBN with the emission intensity distribution following the periodic hexagonal pattern of the strain superlattice. The maximum DL emission enhancement is up to 350% with a energy redshift of 6 nm. Our results provide a simple on-chip compatible method for activating and tuning the defect-related photon emissions in multilayer hBN, demonstrating the potential of hBN strain superlattice as a building block for future on-chip quantum nanophotonic devices.
Published in: "arXiv Material Science".
Quantum solid phase and Coulomb drag in two-dimensional electron-electron bilayers of MoS2. (arXiv:2302.07067v1 [cond-mat.mes-hall])
Coulomb drag experiments can give us information about the interaction state of double-layer systems. Here, we demonstrate anomalous Coulomb drag behaviours in a two-dimensional electron-electron bilayer system constructed by stacking atomically thin MoS2 on opposite sides of thin dielectric layers of boron nitride. In the low temperature regime, the measured drag resistance does not follow the behaviour predicted by the Coulomb drag models of exchanging momenta and energies with the particles in Fermi-liquid bilayer systems. Instead, it shows an upturn to higher and higher values. We investigate quantum solid/fluid phases and the Kosterlitz-Thouless/Wigner two-dimensional quantum melting transition in this bilayer system and describe this interesting phenomenon based on thermally activated carriers of quantum defects from the formation of the correlation-induced electron solid phases with enhanced stabilization by the potential due to the boron nitride dielectric layers.
Published : "arXiv Mesoscale and Nanoscale Physics".
Lasing of Moir’e Trapped MoSe$_2$/WSe$_2$ Interlayer Excitons Coupled to a Nanocavity. (arXiv:2302.07046v1 [cond-mat.mes-hall])
Moir’e trapped interlayer excitons (IXs) in heterobilayer transition metal dichalcogenides currently attract strong interest due to their potential for non-classical light generation, coherent spin-photon interfaces and exploring novel correlated phases of electrons. Here, we report lasing of moir’e trapped IXs by integrating a pristine hBN-encapsulated MoSe$_2$/WSe$_2$ heterobilayer in a high-Q ($>10^4$) nanophotonic cavity. We control the detuning between the IX line and the cavity mode with a magnetic field and measure the dipolar coupling strength to the cavity mode to be $78 pm 4 mathrm{mu eV}$, fully consistent with the 82 $mathrm{mu eV}$ predicted by theory. The emission from the cavity mode shows clear threshold-like behaviour. We observe a superlinear power dependence accompanied by a narrowing of the linewidth as the distinct features of lasing. The onset and prominence of these threshold-like behaviours are significant at resonance whilst weak off-resonance. Our results show that a lasing transition can be induced in interacting moir’e trapped IXs with macroscopic coherence extending over the lengthscale of the cavity mode. Such systems raise interesting perspectives for low-power switching and synaptic nanophotonic devices using 2D materials.
Published : "arXiv Mesoscale and Nanoscale Physics".
Cryogenic nano-imaging of second-order moir’e superlattices. (arXiv:2302.05487v1 [cond-mat.mes-hall])
Second-order superlattices form when moir’e superlattices of similar dimensions interfere with each other, leading to even larger superlattice periodicities. These crystalline structures have been engineered utilizing two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN) under specific alignment conditions. Such specific alignment has shown to play a crucial role in facilitating correlation-driven topological phases featuring the quantized anomalous Hall effect. While signatures of second-order superlattices have been found in transport experiments, any real-space visualization is lacking to date. In this work, we present cryogenic nanoscale photovoltage (PV) measurements that reveal a second-order superlattice in magic-angle twisted bilayer graphene (MATBG) closely aligned to hBN. This is evidenced by long-range periodic photovoltage modulations across the entire sample backed by corresponding electronic transport features. Our theoretical framework shows that small strain- or twist-angle variations can lead to a drastic shift between a local one-dimensional, square or triangular superlattices. Our real-space observations shed new light on the mechanisms responsible for breaking spatial symmetries in TBG and pave an avenue to engineer long-range superlattice structures in 2D materials.
Published : "arXiv Mesoscale and Nanoscale Physics".
Radiation pressure backaction on a hexagonal boron nitride nanomechanical resonator. (arXiv:2302.04291v1 [physics.optics])
Hexagonal boron nitride (hBN) is a 2D material with excellent mechanical properties hosting quantum emitters and optically active spin defects, several of them being sensitive to strain. Establishing optomechanical control of hBN will enable hybrid quantum devices that combine the spin degree of freedom with the cavity optomechanical toolbox. In this letter, we report the first observation of radiation pressure backaction at telecom wavelengths with a hBN drum-head mechanical resonator. The thermomechanical motion of the resonator is coupled to the optical mode of a high finesse fiber-based Fabry-P’erot microcavity in a membrane-in-the-middle configuration. We are able to resolve the optical spring effect and optomechanical damping with a single photon coupling strength of $g_0 = 710$ Hz. Our results pave the way for tailoring the mechanical properties of hBN resonators with light.
Published : "arXiv Mesoscale and Nanoscale Physics".