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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Chemical Vapor Deposition-Grown Hexagonal Boron Nitride and Graphene for Tandem Dielectric Capacitive Polymer Devices

To address the low energy density of polymeric capacitors, this work explores how tandem combinations of antagonistic properties can enhance energy storage. We introduce multilayer architecture integrating chemical vapor deposition (CVD)-grown ≈2 nm hexagonal boron nitride (hBN) and graphene with ferroelectric polyvinylidene fluoride (PVDF) and linear polyetherimide (PEI) layers to suppress leakage paths, reduce loss, and promote Maxwell–Wagner–Sillars polarization. Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and plasma focused ion beam (PFIB) analyses confirm robust material integration. In conclusion, for the tandem device, E BD ≈600 V/μm, while individual layers show lower values (PVDF|Graphene (Gr)|PVDF ≈ 50 V/μm and PEI|hBN|PEI ≈ 275 V/μm), yielding an overall ≈6000% enhancement and demonstrating the effectiveness of the 2D multilayer design.

Chemical vapor deposition (CVD)↗
DOE OSTI2018

Origin and roles of oxygen impurities in hexagonal boron nitride epilayers

Photoluminescence emission spectroscopy and electrical transport measurements have been employed to study the origin and roles of oxygen impurities in hexagonal boron nitride (h-BN) epilayers grown on sapphire substrates. The temperature dependence of the electrical resistivity revealed the presence of a previously unnoticed impurity level of about 0.6 eV in h-BN epilayers grown at high temperatures. The findings suggested that in addition to the common nitrogen vacancy (V N ) shallow donors in h-BN, oxygen impurities diffused from sapphire substrates during high temperature growth also act as substitutional donors (O N ). The presence of O N gives rise to an additional emission peak in the photoluminescence spectrum, corresponding to a donor-acceptor pair recombination involving the O N donor and the C N (carbon occupying nitrogen site) deep level acceptor. Moreover, due to the presence of O N donors, the majority charge carrier type changed to electrons in epilayers grown at high temperatures, in contrast to typical h-BN epilayers which naturally reflect “p-type” character. The results offered a more coherent picture for common impurities/defects in h-BN as well as a better understanding of the growth mediated impurities in h-BN epilayers, which will be helpful for finding possible ways to further improve the quality and purity of this emerging material.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Phonon-Mediated Quasiparticle Lifetime Renormalizations in Few-Layer Hexagonal Boron Nitride

Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of non-volatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide energy bandgap semiconductor, showing immense potential as a platform for lowdimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phonon-mediated and consistent with both single- and multiple-phonon scattering events. Our findings thus unveil a so-far unknown many-body state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Atomistic Insights into Nucleation and Formation of Hexagonal Boron Nitride on Nickel from First-Principles-Based Reactive Molecular Dynamics Simulations

Atomistic-scale insights into the growth of a continuous, atomically thin hexagonal boron nitride (hBN) lattice from elemental boron and nitrogen on Ni substrates were obtained from multiscale modeling combining density functional theory (DFT) and reactive molecular dynamics. The quantum mechanical calculations focused on the adsorption and reaction energetics for the hBN building-block species, i.e., atomic B, N, BxNy (x, y = 1, 2), on Ni(111) and Ni(211), and the diffusion pathways of elemental B and N on these slab model surfaces and in the sublayer. B can diffuse competitively on both the surface and in the sublayer, while N diffuses strictly on the substrate surface. The DFT data were then used to generate a classical description of the Ni–B and Ni–N pair interactions within the formulation of the reactive force field, ReaxFF. Using the potential developed from this work, the elementary nucleation and growth process of an hBN monolayer structure from elemental B and N is shown at the atomistic scale. The nucleation initiates from the growth of linear BN chains, which evolve into branched and then hexagonal lattices. Subsequent DFT calculations confirmed the structure evolution energetically and validate the self-consistency of this multiscale modeling framework. On the basis of this framework, the fundamental aspects regarding crystal quality and the role of temperature and substrates used during hBN growth can also be understood.

Liu, Song↗
DOE OSTI2021

Moiré commensurability and the quantum anomalous Hall effect in twisted bilayer graphene on hexagonal boron nitride

The quantum anomalous Hall (QAH) effect is sometimes observed in twisted bilayer graphene (tBG) when it is nearly aligned with an encapsulating hexagonal boron nitride (hBN) layer. We propose that the appearance or absence of the QAH effect in individual devices could be related to commensurability between the graphene/graphene and graphene/hBN moiré patterns. We identify a series of points in the (θ GG , θ GBN ) twist-angle space at which the two moiré patterns are commensurate, allowing moiré band theory to be applied, and we show that the band Chern numbers are in this case sensitive to a rigid in-plane hBN displacement. Given this property, we argue that the QAH effect is likely only when (i) the (θ GG , θ GBN ) twist-angle-pair is close enough to a commensurate point that the two moiré patterns yield a supermoiré pattern with a sufficiently long length scale, and (ii) the supermoiré has a percolating topologically nontrivial QAH phase. For twist angles far from commensurability, the hBN layer acts as a source of disorder that can destroy the QAH effect. Our proposal can explain a number of current experimental observations. Finally, further experimental studies that can test this proposal more directly are suggested.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Growth Dynamics of Millimeter-Sized Single-Crystal Hexagonal Boron Nitride Monolayers on Secondary Recrystallized Ni (100) Substrates

The outstanding physical properties of 2D materials have sparked continuous research interest in exploiting these materials for next-generation high-performance electronic and photonic technology. Scalable synthesis of high-quality large-area 2D hexagonal boron nitride (h-BN) is a crucial step toward the ultimate success of many of these applications. In this work, a synthetic approach in which secondary recrystallized Ni (100) substrates underwent a carburization process, followed by the growth of h-BN in a molecular beam epitaxy system is designed. The h-BN growth dynamics is studied by tuning different growth parameters including the substrate temperature, and the boron and nitrogen source flux ratio. With assistance from density functional theory calculations, the role of interstitial C atoms in promoting h-BN growth by enhancing the catalytic effect of the transition metal, which lowers the nucleation activation energy barrier, is rationalized. Furthermore, through the control of the growth parameters, a single-crystal h-BN monolayer domain as large as 1.4 mm in edge length is achieved. In addition, a high-quality, continuous, large-area h-BN single-layer film with a breakdown electric field of 9.75 MV cm -1 is demonstrated. The high value of the breakdown electric field suggests that single-layer h-BN has extraordinary dielectric strength for high-performance 2D electronics applications.

2D↗
DOE OSTI2014

Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride

We present a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach, showing good electronic quality with mobilities ranging from 17 000 cm{sup 2} V{sup −1} s{sup −1} at room temperature to 49 000 cm{sup 2} V{sup −1} s{sup −1} at 4.2 K, and entering the quantum Hall regime below 0.5 T. This method provides a strong and useful tool for the fabrication of future high quality layered crystal devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Temperature-dependent Spectral Emission of Hexagonal Boron Nitride Quantum Emitters on Conductive and Dielectric Substrates

We report a reduction in the linewidth and suppression of spectral diffusion of quantum emitters in hexagonal boron nitride supported on a conductive substrate. In this work, we observe a temperature-dependent reduction in the spectral emission linewidth for CVD-grown and exfoliated crystals on conductive ITO relative to those seen on silicon dioxide (SiO 2 ) substrates. We show that the inhomogeneous linewidth can be suppressed by 45% as a result of using a conductive substrate. We investigate the zero-phonon line profile at temperatures ranging from 4 to 300 K and decompose the effects of thermal broadening and spectral diffusion at each temperature by Voigt fitting. The temperature dependence of homogeneous and inhomogeneous components of the broadening is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Single Atoms Anchored in Hexagonal Boron Nitride for Propane Dehydrogenation from First Principles

We report single-atom catalysts embedded in N-doped graphene have attracted great interest recently, but the hexagonal boron nitride (h-BN) is much less explored as a support. Using first principles density function theory and molecular dynamics, here we investigate the stability of Pt, Au, and Ru single atoms anchored at B and N vacancies on h-BN. We find that Pt and Ru single atoms are much more stable than Au on h-BN. We further examine propane dehydrogenation on these single-atom catalysts and find that Pt 1 at the B vacancy in h-BN and Ru 1 at the N vacancy in h-BN show excellent activity for propane dehydrogenation, as evidenced by low energy barriers for both dehydrogenation steps. Our work suggests that Pt and Ru single atoms anchored at vacancy sites in h-BN could be promising for propane dehydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Elementary excitations of single-photon emitters in hexagonal boron nitride

Single-photon emitters serve as building blocks for many emerging concepts in quantum photonics. The recent identification of bright, tunable and stable emitters in hexagonal boron nitride (hBN) has opened the door to quantum platforms operating across the infrared to ultraviolet spectrum. Although it is widely acknowledged that defects are responsible for single-photon emitters in hBN, crucial details regarding their origin, electronic levels and orbital involvement remain unknown. Here we employ a combination of resonant inelastic X-ray scattering and photoluminescence spectroscopy in defective hBN, unveiling an elementary excitation at 285 meV that gives rise to a plethora of harmonics correlated with single-photon emitters. We discuss the importance of N π* anti-bonding orbitals in shaping the electronic states of the emitters. Importantly, the discovery of elementary excitations in hBN provides fundamental insights into quantum emission in low-dimensional materials, paving the way for future investigations in other platforms.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Volatile and Nonvolatile Resistive Switching Coexistence in Conductive Point Hexagonal Boron Nitride Monolayer

Recently, we demonstrated the nonvolatile resistive switching effects of metal–insulator–metal (MIM) atomristor structures based on two-dimensional (2D) monolayers. However, there are many remaining combinations between 2D monolayers and metal electrodes; hence, there is a need to further explore 2D resistance switching devices from material selections to future perspectives. This study investigated the volatile and nonvolatile switching coexistence of monolayer hexagonal boron nitride (h-BN) atomristors using top and bottom silver (Ag) metal electrodes. Utilizing an h-BN monolayer and Ag electrodes, we found that the transition between volatile and nonvolatile switching is attributed to the thickness/stiffness of chain-like conductive bridges between h-BN and Ag surfaces based on the current compliance and atomristor area. Computations indicate a “weak” bridge is responsible for volatile switching, while a “strong” bridge is formed for nonvolatile switching. Further, the current compliance determines the number of Ag atoms that undergo dissociation at the electrode, while the atomristor area determines the degree of electric field localization that forms more stable conductive bridges. The findings of this study suggest that the h-BN atomristor using Ag electrodes shows promise as a potential solution to integrate both volatile neurons and nonvolatile synapses in a single neuromorphic crossbar array structure through electrical and dimensional designs.

2D material↗
DOE OSTI2019

Liquids with Lower Wettability Can Exhibit Higher Friction on Hexagonal Boron Nitride: The Intriguing Role of Solid–Liquid Electrostatic Interactions

Here, we investigate the wetting and frictional behavior of polar (water and ethylene glycol) and nonpolar (diiodomethane) liquids on the basal plane of hexagonal boron nitride (hBN) using molecular dynamics simulations. Our results for the wettability of water on the hBN basal plane (contact angle 81°) are in qualitative agreement with the experimentally deduced mild hydrophilicity of the hBN basal plane (contact angle 66°). We find that water exhibits the lowest wettability, as quantified by the highest contact angle, but the highest friction coefficient of (1.9 ± 0.4) × 10 5 N-s/m 3 on the hBN basal plane among the three liquids considered. This intriguing finding is explained in terms of the competition between dispersion and electrostatic interactions operating between the hBN basal plane and the three liquids. We find that electrostatic interactions do not affect the wetting behavior appreciably, as quantified by a less than 3° change in the respective contact angles of the three liquids considered. On the other hand, electrostatic interactions are found to increase the friction coefficients of the three liquids in contact with hBN to different extents, indicating that despite the increased friction of water on hBN, relative to that on graphene, nonpolar liquids may exhibit similar friction coefficients on hBN and graphene. Our findings reveal that the increase in the friction coefficient, upon incorporation of solid–liquid electrostatic interactions, is brought about by a greater increase in the solid–liquid mean-squared total lateral force, as compared to a smaller reduction in the decorrelation time of the solid–liquid force.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Blue-light-emitting color centers in high-quality hexagonal boron nitride

Light emitters in wide-band-gap semiconductors are of great fundamental interest and have potential as optically addressable qubits. Here we describe a unique color center in high-quality hexagonal boron nitride (h-BN) with a sharp emission line at 435 nm. The emitters are activated and deactivated by electron beam irradiation and have spectral and temporal characteristics consistent with atomic color centers weakly coupled to lattice vibrations. The emitters are conspicuously absent from commercially available h-BN and are present in only ultrahigh-quality h-BN grown using a high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these emitters originate from impurities or related defects specific to this unique synthetic route. Lastly, our results imply that the light emission is activated and deactivated by electron beam manipulation of the charge state of an impurity-defect complex.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Macroscale and Nanoscale Photoelectrochemical Behavior of p-Type Si(111) Covered by a Single Layer of Graphene or Hexagonal Boron Nitride

Two-dimensional (2D) materials may enable a general approach to the introduction of a dipole at a semiconductor surface as well as control over other properties of the double layer at a semiconductor/liquid interface. Vastly different properties can be found in the 2D materials currently studied due in part to the range of the distribution of density-of-states. In this work, the open-circuit voltage (V oc ) of p-Si–H, p-Si/Gr (graphene), and p-Si/h-BN (hexagonal boron nitride) in contact with a series of one-electron outer-sphere redox couples was investigated by macroscale measurements as well as by scanning electrochemical cell microscopy (SECCM). The band gaps of Gr and h-BN (0–5.97 eV) encompass the wide range of band gaps for 2D materials, so these interfaces (p-Si/Gr and p-Si/h-BN) serve as useful references to understand the behavior of 2D materials more generally. The value of Voc shifted with respect to the effective potential of the contacting solution, with slopes (ΔV oc /ΔE Eff ) of -0.27 and -0.38 for p-Si/Gr and p-Si/h-BN, respectively, indicating that band bending at the p-Si/h-BN and p-Si/Gr interfaces responds at least partially to changes in the electrochemical potential of the contacting liquid electrolyte. Additionally, SECCM is shown to be an effective method to interrogate the nanoscale photoelectrochemical behavior of an interface, showing little spatial variance over scales exceeding the grain size of the CVD-grown 2D materials in this work. The measurements demonstrated that the polycrystalline nature of the 2D materials had little effect on the results and confirmed that the macroscale measurements reflected the junction behavior at the nanoscale.

2D material↗
DOE OSTI2023

Revealing Variable Dependences in Hexagonal Boron Nitride Synthesis via Machine Learning

Wafer-scale monolayer two-dimensional (2D) materials have been realized by epitaxial chemical vapor deposition (CVD) in recent years. To scale up the synthesis of 2D materials, a systematic analysis of how the growth dynamics depend on the growth parameters is essential to unravel its mechanisms. However, the studies of CVD-grown 2D materials mostly adopted the control variate method and considered each parameter as an independent variable, which is not comprehensive for 2D materials growth optimization. Herein, we synthesized a representative 2D material, monolayer hexagonal boron nitride (hBN), on single-crystalline Cu (111) by epitaxial chemical vapor deposition and varied the growth parameters to regulate the hBN domain sizes. Furthermore, we explored the correlation between two growth parameters and provided the growth windows for large flake sizes by the Gaussian process. Here, this new analysis approach based on machine learning provides a more comprehensive understanding of the growth mechanism for 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Off-Stoichiometric Restructuring and Sliding Dynamics of Hexagonal Boron Nitride Edges in Conditions of Oxidative Dehydrogenation of Propane

Boron-containing materials, such as hexagonal boron nitride (h-BN), recently shown to be active and selective catalysts for the oxidative dehydrogenation of propane (ODHP), have been shown to undergo significant surface oxyfunctionalization and restructuring. Although experimental ex situ studies have probed the change in chemical environment on the surface, the structural evolution of it under varying reaction conditions has not been established. Herein, we perform global optimization structure search with a grand canonical genetic algorithm to explore the chemical space of off-stoichiometric restructuring of the h-BN surface under ambient as well as ODHP-relevant conditions. A grand canonical ensemble representation of the surface is established, and the predicted 11B solid-state NMR spectra are consistent with previous experimental reports. In addition, we investigated the relative sliding of h-BN sheets and how it influences the surface chemistry with ab initio molecular dynamics simulations. Furthermore, the B–O linkages on the edges are found to be significantly strained during the sliding, causing the metastable sliding configurations to have higher reactivity toward the activation of propane and water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Catalytic reduction of carbon dioxide to methanol over defect-laden hexagonal boron nitride: insights into reaction mechanisms

Abstract We present a density functional theory-based mechanistic understanding of CO 2 hydrogenation to value-added products on a nitrogen-vacancy (V N ) defect in hexagonal boron nitride ( dh -BN). Activation occurs through back-donation to the π * orbitals of CO 2 from the frontier orbitals (defect state) of the h- BN sheet that are localized near a nitrogen-vacancy. Subsequent hydrogenation to methanol (CH 3 OH) and formic acid (HCOOH) proceed through vacancy-facilitated co-adsorption of hydrogen and CO 2 . More importantly, our reaction pathway analyses complimented by microkinetic modeling indicate that dh -BN is potentially a low-temperature, selective catalyst for CO 2 reduction to methanol. Our findings are in agreement with experiments conducted in a mechanical reactor that show high selectivity towards methanol formation for CO 2 hydrogenation on defect induced h- BN.

Jiang, Tao (ORCID:0000000304197288)↗