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Oxygen‐Activated Boron Nitride for Selective Photocatalytic Coupling of Methanol to Ethylene Glycol

2024-02-28T13:08:12+00:00February 28th, 2024|Categories: Publications|Tags: , |

Oxygen-activated boron nitride fibers (BNH) have been successfully synthesized and applied for the selective photocatalytic coupling of methanol to ethylene glycol (EG) reaction. The OB3 unit in the BNH plays an important role in the selective dehydrogenation and C−C coupling of methanol molecules, leading to excellent activity and selectivity of BNH for EG production. Abstract The controllable photocatalytic C−C coupling of methanol to produce ethylene glycol (EG) is a highly desirable but challenging objective for replacing the current energy-intensive thermocatalytic process. Here, we develop a metal-free porous boron nitride catalyst that demonstrates exceptional selectivity in the photocatalytic production of EG from methanol under mild conditions. Comprehensive experiments and calculations are conducted to thoroughly investigate the reaction mechanism, revealing that the OB3 unit in the porous BN plays a critical role in the preferential activation of C−H bond in methanol to form ⋅CH2OH via a concerted proton-electron transfer mechanism. More prominent energy barriers are observed for the further dehydrogenation of the ⋅CH2OH intermediate on the OB3 unit, inhibiting the formation of some other by-products during the catalytic process. Additionally, a small downhill energy barrier for the coupling of ⋅CH2OH in the OB3 unit promotes the selective generation of EG. This study provides valuable insights into the underlying mechanisms and can serve as a guide for the design and optimization of photocatalysts for efficient and selective EG production under mild conditions.

Published in: "Angewandte Chemie International Edition".

Polarized Ultrathin BN Induced Dynamic Electron Interactions for Enhancing Acidic Oxygen Evolution

2024-02-24T13:08:04+00:00February 24th, 2024|Categories: Publications|Tags: |

Developing ruthenium-based heterogeneous catalysts with an efficient and stable interface is essential for enhanced acidic oxygen evolution reaction (OER). Herein, we report a defect-rich ultrathin boron nitride nanosheet support with relatively independent electron donor and acceptor sites, which serves as an electron reservoir and receiving station for RuO2, realizing the rapid supply and reception of electrons. Through precisely controlling the reaction interface, a low OER overpotential of only 180 mV (at 10 mA cm−2) and long-term operational stability (350 h) are achieved, suggesting potential practical applications. In-situ characterization and theoretical calculations have validated the existence of a localized electronic recycling between RuO2 and BNNS. The electron-rich Ru sites accelerate the adsorption of water molecules and the dissociation of intermediates, while the interconnection between the O-terminal and B-terminal edge establishes electronic back-donation, effectively suppressing the over-oxidation of lattice oxygen. This study provides a new perspective for constructing a stable and highly active catalytic interface.

Published in: "Angewandte Chemie International Edition".

The Synthesis of Three‐Dimensional Hexagonal Boron Nitride as the Reinforcing Phase of Polymer‐Based Electrolyte for All‐Solid‐State Li Metal Batteries

2024-02-21T13:09:41+00:00February 21st, 2024|Categories: Publications|Tags: , |

A powdery h-BN with a micron-sized three-dimensional (3D) structure was developed with a facile NaCl-glucose-assisted approach based on reduced reaction temperature. It exhibited superior advantages in improving the electrochemical performance of polyethylene oxide (PEO)-based electrolytes compared with the commercial two-dimensional (2D) h-BN. Abstract Powdery hexagonal boron nitride (h-BN), as an important material for electrochemical energy storage, has been typically synthesized in bulk and one/two-dimensional (1/2D) nanostructured morphologies. However, until now, no method has been developed to synthesize powdery three-dimensional (3D) h-BN. This work introduces a novel NaCl-glucose-assisted strategy to synthesize micron-sized 3D h-BN with a honeycomb-like structure and its proposed formation mechanism. We propose that NaCl acts as the template of 3D structure and promotes the nitridation reaction by adsorbing NH3. Glucose facilitates the homogeneous coating of boric acid onto the NaCl surface via functionalizing the NaCl surface. During the nitridation reaction, boron oxides (BO4 and BO3) form from a dehydration reaction of boric acid, which is then reduced to O2-B-N and O-B-N2 intermediates before finally being reduced to BN3 by NH3. When incorporated into polyethylene oxide-based electrolytes for Li metal batteries, 5 wt % of 3D h-BN significantly enhances ionic conductivity and mechanical strength. Consequently, this composite electrolyte demonstrates superior electrochemical stability. It delivers 300 h of stable cycles in the Li//Li cell at 0.1 mA cm−2 and retains 89 % of discharge capacity (138.9 mAh g−1) after 100 cycles at 1 C in the LFP//Li full cell.

Published in: "Angewandte Chemie International Edition".

Retrieving optical parameters of emerging van der Waals flakes. (arXiv:2305.13994v2 [physics.optics] UPDATED)

2023-05-30T02:29:37+00:00May 30th, 2023|Categories: Publications|Tags: |

High-quality low-dimensional layered and van der Waals materials are typically exfoliated, with sample cross sectional areas on the order of tens to hundreds of microns. The small size of flakes makes the experimental characterization of their dielectric properties unsuitable with conventional spectroscopic ellipsometry, due to beam-sample size mismatch and non-uniformities of the crystal axes. Previously, the experimental measurement of the dielectrirc permittivity of such microcrystals was carried out with near-field tip-based scanning probes. These measurements are sensitive to external conditions like vibrations and temperature, and require non-deterministic numerical fitting to some a priori known model. We present an alternative method to extract the in-plane dielectric permittivity of van der Waals microcrystals, based on identifying reflectance minima in spectroscopic measurements. Our method does not require complex fitting algorithms nor near field tip-based measurements and accommodates for small-area samples. We demonstrate the robustness of our method using hexagonal boron nitride and {alpha}-MoO3, and recover their dielectric permittivities that are close to literature values.

Published in: "arXiv Material Science".

Comprehensive scheme for identifying defects in solid-state quantum systems. (arXiv:2305.17889v1 [quant-ph])

2023-05-30T02:29:28+00:00May 30th, 2023|Categories: Publications|Tags: , |

A solid-state quantum emitter is one of the indispensable components for optical quantum technologies. Ideally, an emitter should have a compatible wavelength for efficient coupling to other components in a quantum network. It is therefore essential to understand fluorescent defects that lead to specific emitters. In this work, we employ density functional theory (DFT) to demonstrate the calculation of the complete optical fingerprints of quantum emitters in the two-dimensional material hexagonal boron nitride. These emitters are of great interest, yet many of them are still to be identified. Our results suggest that instead of comparing a single optical property, such as the commonly used zero-phonon line energy, multiple properties should be used when comparing theoretical simulations to the experiment. This way, the entire electronic structure can be predicted and quantum emitters can be designed and tailored. Moreover, we apply this approach to predict the suitability for using the emitters in specific quantum applications, demonstrating through the examples of the Al$_{text{N}}$ and P$_{text{N}}$V$_{text{B}}$ defects. We therefore combine and apply DFT calculations to identify quantum emitters in solid-state crystals with a lower risk of misassignments as well as a way to design and tailor optical quantum systems. This consequently serves as a recipe for classification and the generation of universal solid-state quantum emitter systems in future hybrid quantum networks.

Published in: "arXiv Material Science".

Distinguishing different stackings in layered materials via luminescence spectroscopy. (arXiv:2305.17554v1 [cond-mat.mtrl-sci])

2023-05-30T02:29:23+00:00May 30th, 2023|Categories: Publications|Tags: |

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from “indirect”, finite-momentum, excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed.

Published in: "arXiv Material Science".

Flat bands in bilayer graphene induced by proximity with polar $h$-BN superlattices. (arXiv:2305.09749v2 [cond-mat.mes-hall] UPDATED)

2023-05-29T02:30:22+00:00May 29th, 2023|Categories: Publications|Tags: , , |

Motivated by the observation of polarization superlattices in twisted multilayers of hexagonal boron nitride ($h$-BN), we address the possibility of using these heterostructures for tailoring the properties of multilayer graphene by means of the electrostatic proximity effect. By using the combination of first-principles and large-scale tight-binding model calculations coupled via the Wannier function approach, we demonstrate the possibility of creating a sequence of well-separated flat-band manifolds in AB-stacked bilayer graphene at experimentally relevant superlattice periodicities above $sim$30 nm. Our calculations show that the details of band structures depend on the local inversion symmetry breaking and the vertical electrical polarization, which are directly related to the atomic arrangement. The results advance the atomistic characterization of graphene-based systems in a superlattice potential beyond the continuum model.

Published in: "arXiv Material Science".

Bandgap manipulation of hBN by alloying with aluminum: absorption properties of hexagonal BAlN. (arXiv:2305.15810v1 [cond-mat.mtrl-sci])

2023-05-28T08:31:04+00:00May 28th, 2023|Categories: Publications|Tags: , , |

The versatile range of applications for two-dimensional (2D) materials has encouraged scientists to further engineer the properties of these materials. This is often accomplished by stacking layered materials into more complex van der Waals heterostructures. A much less popular but technologically promising approach is the alloying of 2D materials with different element compositions. In this work, we demonstrate a first step in manipulating the hBN bandgap in terms of its width and indirect/direct character of the optical transitions. We present a set of aluminum alloyed hexagonal boron nitride (hBAlN) samples that were grown by metal organic vapor phase epitaxy (MOVPE) on 2-inch sapphire substrates with different aluminum concentration. Importantly, the obtained samples revealed a sp$^2$-bonded crystal structure. Optical absorption experiments disclosed two strong peaks in the excitonic spectral range with absorption coefficient $alpha sim 10^6$ cm$^{-1}$. Their energies correspond very well with the energies of indirect and direct bandgap transitions in hBN. However, they are slightly redshifted. This observation is in agreement with predictions that alloying with Al leads to a decrease of the bandgap energy. The observation of two absorption peaks can be explained in terms of mixing electronic states in the K and M conduction band valleys, which leads to a significant enhancement of the absorption coefficient for indirect transitions.

Published in: "arXiv Material Science".

Modeling Interlayer Interactions and Phonon Thermal Transport in Silicene Bilayer. (arXiv:2305.15423v1 [cond-mat.mtrl-sci])

2023-05-28T08:31:00+00:00May 28th, 2023|Categories: Publications|Tags: , , |

We develop an accurate interlayer pairwise potential derived from the $textit{ab-initio}$ calculations and investigate the thermal transport of silicene bilayers within the framework of equilibrium molecular dynamics simulations. We find that the electronic properties are sensitive to the temperature with the opening of band gap in the $Gammarightarrow M$ direction at the room temperature. The calculated phonon thermal conductivity of bilayer silicene is surprisingly higher than that of the monolayer silicene, contrary to the trends reported for other class of 2D materials like graphene and hBN bilayers. We attribute this counterintuitive result to the higher velocity of LA$_1$/LA$_2$ phonon modes arising from the interlayer interaction effects and buckling, inherent to silicene bilayer. Interestingly, the thermal conductivity of both the mono- and bilayer silicene decreases with temperature as $kappasim T^{-0.9}$ because of the strong correlations between heat current decay characteristic timescales and temperature ($tausim T^{-0.75}$). The mechanisms underlying phonon thermal transport in silicene bilayer are further established by analyzing the temperature induced changes in acoustic group velocity.

Published in: "arXiv Material Science".

Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites. (arXiv:2304.08474v1 [cond-mat.mtrl-sci])

2023-04-18T02:29:39+00:00April 18th, 2023|Categories: Publications|Tags: , |

Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.

Published in: "arXiv Material Science".

Effect of Environmental Screening and Strain on Optoelectronic Properties of Two-Dimensional Quantum Defects. (arXiv:2304.05612v1 [cond-mat.mtrl-sci])

2023-04-13T02:29:27+00:00April 13th, 2023|Categories: Publications|Tags: , |

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that the environmental screening causes lowering of the GW gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the BSE Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

Published in: "arXiv Material Science".

Near-Intrinsic Photo- and Electroluminescence from Single-Walled Carbon Nanotube Thin Films on BCB-Passivated Surfaces. (arXiv:2304.05649v1 [physics.app-ph])

2023-04-13T02:29:21+00:00April 13th, 2023|Categories: Publications|Tags: , |

Their outstanding electrical and optical properties make semiconducting single-walled carbon nanotubes (SWCNTs) highly suitable for charge transport and emissive layers in near-infrared optoelectronic devices. However, the luminescence spectra of SWCNT thin films on commonly used glass and Si/SiO2 substrates are often compromised by broadening of the main excitonic emission and unwanted low-energy sidebands. Surface passivation with a commercially available, low dielectric constant, cross-linked bis-benzocyclobutene-based polymer (BCB) enhances the emission properties of SWCNTs to the same level as hexagonal boron nitride (h-BN) flakes do. The presence of BCB suppresses sideband emission, especially from the Y1 band, which is attributed to defects introduced by the interaction of the nanotube lattice with oxygen-containing terminal groups of the substrate surface. The facile and reproducible deposition of homogeneous BCB films over large areas combined with their resistance against common solvents and chemicals employed during photolithography make them compatible with standard semiconductor device fabrication. Utilizing this approach, light-emitting (6,5) SWCNT network field-effect transistors are fabricated on BCB-treated glass substrates with excellent electrical characteristics and near-intrinsic electroluminescence. Hence, passivation with BCB is proposed as a standard treatment for substrates used for spectroscopic investigations of and optoelectronic devices with SWCNTs and other low-dimensional emitters.

Published in: "arXiv Material Science".

Mechanically reconfigurable van der Waals devices via low-friction gold sliding | Science Advances

2023-04-11T10:20:09+00:00April 11th, 2023|Categories: Publications|Tags: |

Abstract Interfaces of van der Waals (vdW) materials, such as graphite and hexagonal boron nitride (hBN), exhibit low-friction sliding due to their atomically flat surfaces and weak vdW bonding. We demonstrate that microfabricated gold also slides with low friction on hBN. This enables the arbitrary post-fabrication repositioning of device

Published in: "Science Advances".

Lightwave-controlled band engineering in quantum materials. (arXiv:2303.13044v1 [cond-mat.mes-hall])

2023-03-24T02:29:26+00:00March 24th, 2023|Categories: Publications|Tags: |

Stacking and twisting atom-thin sheets create superlattice structures with unique emergent properties, while tailored light fields can manipulate coherent electron transport on ultrafast timescales. The unification of these two approaches may lead to ultrafast creation and manipulation of band structure properties, which is a crucial objective for the advancement of quantum technology. Here, we address this by demonstrating a tailored lightwave-driven analogue to twisted layer stacking. This results in sub-femtosecond control of time-reversal symmetry breaking and thereby band structure engineering in a hexagonal boron nitride monolayer. The results practically demonstrate the realization of the topological Haldane model in an insulator. Twisting the lightwave relative to the lattice orientation enables switching between band configurations, providing unprecedented control over the magnitude and location of the band gap, and curvature. A resultant asymmetric population at complementary quantum valleys lead to a measurable valley Hall current, detected via optical harmonic polarimetry. The universality and robustness of the demonstrated sub-femtosecond control opens a new way to band structure engineering on the fly paving a way towards large-scale ultrafast quantum devices for real-world applications.

Published in: "arXiv Material Science".

Bias sputtering of granular $text{L1}_0$-FePt films with hexagonal boron nitride grain boundaries. (arXiv:2303.11411v1 [cond-mat.mtrl-sci])

2023-03-22T02:29:21+00:00March 22nd, 2023|Categories: Publications|Tags: |

In this paper, we present an experimental study of $text{L1}_0$-FePt granular films with crystalline/amorphous boron nitride (BN) grain boundary materials for heat assisted magnetic recording (HAMR). It is found that an adequate RF substrate bias yields the formation of hexagonal boron nitride (h-BN) nanosheets in grain boundaries, facilitating the columnar growth of FePt grains during sputtering at high temperatures. The h-BN monolayers conform to the side surfaces of columnar FePt grains, completely encircling individual FePt grains. The resulting core-shell FePt/h-BN nanostructures appear to be highly promising for HAMR application. The high thermal stability of h-BN grain boundaries allows the deposition temperature to be as high as 800oC such that high order parameters of FePt $text{L1}_0$ phase have been obtained. For the fabricated FePt/h-BN thin film, excellent granular microstructure with FePt grains of 6.5nm in diameter and 11.5nm in height has been achieved along with good magnetic hysteresis properties.

Published in: "arXiv Material Science".

Coherent imaging and dynamics of excitons in MoSe$_2$ monolayers epitaxially grown on hexagonal boron nitride. (arXiv:2303.10697v1 [cond-mat.mtrl-sci])

2023-03-21T02:29:35+00:00March 21st, 2023|Categories: Publications|Tags: |

Using four-wave mixing microscopy, we measure the coherent response and ultrafast dynamics of excitons and trions in MoSe$_2$ monolayers grown by molecular beam epitaxy on thin films of hexagonal boron nitride. We assess inhomogeneous and homogeneous broadenings in the transition spectral lineshape. The impact of phonons on the homogeneous dephasing is inferred via the temperature dependence of the dephasing. Four-wave mixing mapping, combined with the atomic force microscopy, reveals spatial correlations between exciton oscillator strength, inhomogeneous broadening and the sample morphology. The quality of coherent optical response of the epitaxially grown transition metal dichalcogenides becomes now comparable with the samples produced by mechanical exfoliation, enabling coherent nonlinear spectroscopy of innovative materials, like magnetic layers or Janus semiconductors.

Published in: "arXiv Material Science".

Stacking and Thickness Effects on Cross-Plane Thermal Conductivity of Hexagonal Boron Nitride. (arXiv:2303.10627v1 [cond-mat.mtrl-sci])

2023-03-21T02:29:33+00:00March 21st, 2023|Categories: Publications|Tags: , |

Recently, the in-plane thermal transport in van der Waals (vdW) materials such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) has been widely studied. Whereas, the cross-plane one is far from sufficient. Based on the non-equilibrium molecular dynamics simulations and Boltzmann transport equation, here we reveal the stacking and thickness effects on the cross-plane thermal conductivity (K) of h-BN. We find that K can be significantly modulated by both the stacking structure and thickness (d) of h-BN, which is unexpected from the viewpoint of its smooth in-plane structure and weak interlayer interaction. In the small thickness region (d

Published in: "arXiv Material Science".

Stacking order effects on the energetic stability and electronic properties of $n$-doped graphene/h-BN van der Waals heterostructures on SiC(0001). (arXiv:2303.08191v1 [cond-mat.mtrl-sci])

2023-03-16T02:30:28+00:00March 16th, 2023|Categories: Publications|Tags: , , , |

Heterostructures made of stacked 2D materials with different electronic properties are studied for their potential in creating multifunctional devices. Graphene (G) and hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively researched, including recent studies on synthesizing h-BN on graphene/SiC(0001) templates. These studies suggest that h-BN encapsulation could occur in addition to vdW epitaxy. This work presents theoretical research on G/h-BN heterostructures on SiC(0001) with a carbon buffer layer. The results show an energetic preference for h-BN encapsulation below a single layer of graphene: G/h-BN/SiC in bilayer systems and G/h-BN/G/SiC in trilayer systems. Electronic structure calculations reveal that the linear energy band dispersion of graphene is maintained in bilayer systems but with Dirac points at different energy positions due to the varied electron doping level of graphene. In trilayer systems, the doping level of graphene also depends on the stacking order. The electronic band structure of G/h-BN/G/SiC features two Dirac points below the Fermi level but with different energies. Less stable systems like G/G/h-BN/ and h-BN/G/G/SiC display parabolic bands near the Fermi level. Additional structural characterizations were performed based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near-edge spectroscopy (XANES) to aid future experimental spectroscopy in these graphene/h-BN vdW systems.

Published in: "arXiv Material Science".

Insulators at Fractional Fillings in Twisted Bilayer Graphene Partially Aligned to Hexagonal Boron Nitride. (arXiv:2303.08246v1 [cond-mat.mes-hall])

2023-03-16T02:29:46+00:00March 16th, 2023|Categories: Publications|Tags: , , |

At partial fillings of its flat electronic bands, magic-angle twisted bilayer graphene (MATBG) hosts a rich variety of competing correlated phases that show sample to sample variations. Divergent phase diagrams in MATBG are often attributed to the sublattice polarization energy scale, tuned by the degree of alignment of the hexagonal boron nitride (hBN) substrates typically used in van der Waals devices. Unaligned MATBG exhibits unconventional superconductivity and correlated insulating phases, while nearly perfectly aligned MATBG/hBN exhibits zero-field Chern insulating phases and lacks superconductivity. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) to observe gapped phases at partial fillings of the flat bands of MATBG in a new intermediate regime of sublattice polarization, observed when MATBG is only partially aligned ($theta_{Gr-hBN}$ $approx$ 1.65$^circ$) to the underlying hBN substrate. Under this condition, MATBG hosts not only phenomena that naturally interpolate between the two sublattice potential limits, but also unexpected gapped phases absent in either of these limits. At charge neutrality, we observe an insulating phase with a small energy gap ($Delta$ < 5 meV) likely related to weak sublattice symmetry breaking from the hBN substrate. In addition, we observe new gapped phases near fractional fillings $nu$ = $pm 1/3$ and $nu$ = $pm 1/6$, which have not been previously observed in MATBG. Importantly, energy-resolved STS unambiguously identifies these fractional filling states to be of single-particle origin, possibly a result of the super-superlattice formed by two moir'e superlattices. Our observations emphasize the power of STS in distinguishing single-particle gapped phases from many-body

Published in: "arXiv Material Science".

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