Single-layer Hexagonal Boron Nitride Research Articles

Computational analysis of device-to-device variability in resistive switching through single-layer hexagonal boron nitride and graphene vertical heterostructure model

Abstract We quantify the device-to-device variations in resistive switching by considering a single-layer hexagonal boron nitride and graphene junction as a model. Then, we mimic the variations in the surface of a two-dimensional material in terms of defects and interface states by changing the distance between single-layer hexagonal boron nitride and graphene. We use density functional theory as a methodology to perform simulations at the atomic scale. The results show that the distance affects the current–voltage characterization results and that creating ultra uniform structures is important to reduce the device-to-device variability. These results are crucial to understand the reliability and accuracy of device-to-device variations in memory devices and mimic the neural dynamics beyond the synaptic cleft.

Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators.

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

Interplay of Kinetic Limitations and Disintegration: Selective Growth of Hexagonal Boron Nitride and Borophene Monolayers on Metal Substrates.

The CVD growth of bielemental 2D-materials by using molecular precursors involves complex formation kinetics taking place at the surface and sometimes also subsurface regions of the substrate. Competing microscopic processes fundamentally limit the parameter space for optimal growth of the desired material. Kinetic limitations for diffusion and nucleation cause a high density of small domains and grain boundaries. These are usually overcome by increasing the growth temperature and decreasing the growth rate. In contrast, the nature of molecular precursors with limited thermal stability can result in dissociation and preferential desorption, leading to an undesired or ill-defined composition of the 2D-material. Here we demonstrate these constraints in a combined low-energy electron diffraction and low-energy electron microscopy study by examining the selective formation of single-layer hexagonal boron nitride (hBN) and borophene on Ir(111) using a borazine precursor. We derive a temperature-pressure phase diagram and apply classical nucleation theory to describe our results. By considering the competing processes, we find an optimum growth temperature for hBN of 950 °C. At lower temperatures, the hBN island density is increased, while at higher temperatures the precursor disintegrates and borophene is formed. Our results introduce an additional aspect that must be considered in any high-temperature growth of bielemental 2D-materials from single molecular precursors.

Predicting mechanical properties of defective h-BN nanosheets using Data-Driven models

Machine learning (ML) methods have assisted different fields of science in overcoming challenging problems. In this work, with the aid of ML and deep learning algorithms, predicting the mechanical properties of defective single-layer hexagonal boron nitride (h-BN) nanosheets is accelerated compared to conventional atomistic simulations. To provide the required data for ML models, molecular dynamic (MD) simulation is used to determine the Young modulus, ultimate tensile strength, and fracture strain of these 2D nanostructures under the influence of various defect features, namely density, type, structure, and distribution. The obtained MD results exhibit strong correlation between the mechanical properties of defective h-BN monolayers and the introduced defect characteristics. Accuracy of the results is supported by the analysis of variance (ANOVA) and correlation coefficient between independent and dependent variables. In the next step, three ML algorithms including SVR, Random Forest and XGBoost, and three artificial neural network (ANN) models with different hidden layers are trained to accomplish our goals. Among all the developed models, an ANN design with four hidden layers performs better with the highest R2 score of 0.86. Our findings provide significant assets for defect engineering by accelerating the mechanical properties prediction of defective h-BN monolayers compared to traditional MD simulations.

Incommensurability and negative thermal expansion of single layer hexagonal boron nitride

The emerging field of straintronics, i.e., the control and utilization of the strain state of 2D-materials, is of great importance for their technological development, specifically in view of their future incorporation into van der Waals heterostructures. To gain fundamental insight into structural peculiarities of two-dimensional systems, single layer hexagonal boron nitride (hBN) grown on Ir(111) by chemical vapor deposition was used as a prototypical model system: High-resolution reciprocal space mapping reveals the incommensurate nature of the material system by measuring the hBN in plane lattice parameter with high precision, facilitated by the moiré magnification effect in electron diffraction. In a growth temperature (Tg) regime of 700–1150°C an average lattice parameter of 2.496 ± 0.006 Å was found. Eventually, careful disentanglement of the hBN’s and substrate’s behavior for rising Tg allowed the determination of a negative thermal expansion coefficient of αhBN = −2.4 ± 1.2 × 10-6 K−1 for free-standing hBN.

Mesoscopic 2D molecular self-assembly on an insulator

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly—where organic molecules act as building blocks and where surfaces play the role of supporting templates—allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Geometrical and magnetic properties of small titanium and chromium clusters on monolayer hexagonal boron nitride.

Magnetic clusters on an insulating substrate are potential candidates for spin-based quantum devices. Here we investigate the geometric, electronic, and magnetic structures of small Ti and Cr clusters, from dimers to pentamers, adsorbed on a single-layer hexagonal boron nitride (h-BN) sheet within the framework of density functional theory. The stable adsorption configurations of the Ti clusters and Cr clusters composed of the same number of atoms are found to be totally different from each other. The difference in their bonding mechanisms has been revealed by the density of states and the charge density difference of the corresponding adsorption systems. While chemical bonds are formed between the Ti atoms and the supporting sheet, the Cr clusters are found in the physisorption state on the substrate. In addition, it is shown that the h-BN sheet is energetically favorable for building three-dimensional Ti clusters. These findings support the use of h-BN as a suitable decoupling substrate for manipulation of quantum spin states in small transition metal (TM) clusters and fabrication of devices based on them.

Graphene and 2D Hexagonal Boron Nitride Heterostructure for Thermal Management in Actively Tunable Manner.

Thermal management is a critical task for highly integrated or high-power semiconductor devices. Low dimensional materials including graphene and single-layer hexagonal boron nitride (BN) are attractive candidates for this task because of their high thermal conductivity, semi-conductivity and other excellent physical properties. The similarities in crystal structure and chemistry between graphene and boron nitride provide the possibility of constructing graphene/BN heterostructures bearing unique functions. In this paper, we investigated the interfacial thermal transport properties of graphene/BN nanosheets via non-equilibrium molecular dynamics (NEMD) simulations. We observed a significant thermal rectification behavior of these graphene/BN nanosheets, and the rectification ratio increased with the system length increases up to 117%. This phenomenon is attributed to the mismatch of out-of-plane phonon vibration modes in two directions at the interface. In addition, we explored the underlying mechanism of the length dependence of the thermal transport properties. The results show promise for the thermal management of this two-dimensional heterostructure in an actively tunable manner.

Growth and characterization of uniformly distributed triangular single-crystalline hexagonal boron nitride grains on liquid copper surface

Single-layer hexagonal boron nitride (h-BN) is a two-dimensional (2D) material with a wide band gap and extraordinary mechanical, thermal and optical properties. It has promising applications in optoelectronics, electronics and photovoltaics. Low-pressure (150 Pa) chemical vapor deposition (LPCVD) was used to prepare uniformly distributed single-crystalline triangular h-BN grains and continuous film on liquid copper surface. A series of microscopic and spectroscopic methods were performed to characterize the morphology, crystalline structure and quality, component and thickness. It was found that h-BN grains turned into circles at higher background pressure (1000 Pa), which was due to the change in the growth mechanism from equilibrium controlled to deposition controlled. Small islands formed on primary h-BN grains because of higher concentration of precursor product. Compared to the higher background pressure, the synthetic h-BN films at low background pressure exhibited larger domain size, lower nucleation density, and no adlayer growth. Due to randomly distributed polar h-BN grains, the grain boundaries formed along the h-BN domains after merging. Typical patterns formed via the coalescence of triangular h-BN grains indicating the merging mode of growth, including edge-to-edge and edge-to-point modes. This work provides a pathway for the preparation of uniformly distributed single-crystalline h-BN grains and an in-depth understanding of the growth and merging process on liquid Cu surface.

Path-integral Monte Carlo simulations on the thermodynamic properties of single-layer hexagonal boron nitride

We investigate the quantum effects and anharmonicity on the thermodynamical properties of single-layer hexagonal boron nitride (SL-hBN) using quantum path-integral Monte Carlo (PIMC) and classical Monte Carlo (CMC) simulations in the isothermal–isobaric ensemble at zero pressure. The quantum effects are extracted from the differences between the results obtained from the PIMC and CMC calculations. Our numerical simulations show that the out-of-plane motion of the atoms plays a decisive role in the negative thermal expansion of the SL-hBN crystal and the quantum zero-point vibrational energy greatly enhances the negative thermal expansion coefficients. We demonstrate for the first time the anisotropic thermal expansions in SL-hBN crystal manifested in the different linear thermal expansion coefficients in the zig-zag and armchair directions. Such an anisotropic effect can be significant at high temperatures. From an analysis of the atomic kinetic and potential energy contributions in the thermal vibration energy of the crystal lattice, we find that the anharmonicity of the SL-hBN crystal lies on the ground of the asymmetric thermal vibrations of the boron and nitrogen atoms and their coupling in the same unit cell of the hexagonal lattice.

First-Principles Study of Defected Single Layer Hexagonal Boron-Nitride (h-BN)

The novel properties of pristine h-BN, oxygen (O) atom impurity defects in h-BN (h-B(N-O) and h-(B-O)N), one boron (1B) atom vacancy defect in h-BN (h-BN_1B) and one nitrogen (1N) atom vacancy defect in h-BN (h-BN_1N) materials are investigated by spin-polarized density functional theory (DFT) using computational tool Quantum ESPRESSO. We found that they are stable materials. From the band structure calculations, we found that all the considered systems are wide bandgap materials. The bandgap energy of pristine h-BN, impurity defects h-B(N-O) and h-(B-O)N, and vacancy defects h-BN_1B and h-BN_1Nmaterials have values 4.98 eV, 4.19 eV& 2.47 eV,and 4.84 eV& 3.62 eV respectively. Also, it is found that h-B(N-O) and h-BN_1N materials have n-type Schottky contact while h-(B-O)N and h-BN_1B materials have p-type Schottky contact. From the analysis ofdensity of states (DOS) and partial density of states (PDOS) calculations, we found that non-magnetic pristine h-BN changes to magnetic h-B(N-O) andh-(B-O)N materials due to presence of impurity defects,and h-BN_1B andh-BN_1N materials due to presence of vacancy defects. Magnetic moments of h-B(N-O), h-(B-O)N, h-BN_1B and h-BN_1N materials are 1.00 µB/cell, 0.94 µB/cell, 3.00 µB/cell and 1.00 µB/cell respectively. They are obtained due to unpaired up and down spins state of electrons in 2p orbital of B and N atoms in the structures.

An Atomistic-Based Nonlinear Plate Theory for Hexagonal Boron Nitride.

Through the continuity of the DREIDING force field, we propose, for the first time, the finite-deformation plate theory for the single-layer hexagonal boron nitride (h-BN) to clarify the atomic source of the structure against deformations. Divergent from the classical Föppl-von Karman plate theory, our new theory shows that h-BN’s two in-plane mechanical parameters are independent of two out-of-plane mechanical parameters. The new theory reveals the relationships between the h-BN’s elastic rigidities and the atomic force field: (1) two in-plane elastic rigidities come from the bond stretching and the bond angle bending; (2) the bending rigidity comes from the inversion angle and the dihedral angle torsion; (3) the Gaussian rigidity only comes from the dihedral angle torsion. Mechanical parameters obtained by our theory align with atomic calculations. The new theory proves that two four-body terms in the DREIDING force field are necessary to model the h-BN’s mechanical properties. Overall, our theory establishes a foundation to apply the classical plate theory on the h-BN, and the approach in this paper is heuristic in modelling the mechanical properties of the other two-dimensional nanostructures.

Wafer-scale, epitaxial growth of single layer hexagonal boron nitride on Pt(111)

Single-layer hexagonal boron nitride is produced on 2 inch Pt(111)/sapphire wafers. The growth with borazine vapor deposition at process temperatures between 1000 and 1300 K is in situ investigated by photoelectron yield measurements. The growth kinetics is slower at higher temperatures and follows a tanh2 law which better fits for higher temperatures. The crystal-quality of hexagonal boron nitride (h-BN)/Pt(111) is inferred from scanning low energy electron diffraction (x-y LEED). The data indicate a strong dependence of the epitaxy on the growth temperature. The dominant structure is an aligned coincidence lattice with 10 h-BN on 9 Pt(1 × 1) unit cells and follows the substrate twinning at the millimeter scale.

The Effect of Magnetic Field on Optical Conductivity of Hexagonal Boron-Nitride Sheet

The main purpose of this work is to investigate the effect of external magnetic field on the optical absorption of single-layer hexagonal boron nitride. The optical absorption of the system can be found by calculating the imaginary part of the dielectric function. The optical absorption is proportional to from the real part of the dynamical electrical conductivity function. Based on linear response theory, dynamical conductivity function can be calculated in terms of the current-current correlation function. In this study, the tight-binding model Hamiltonian has been applied to study the electronic band spectrum of the system. Also, the energy spectrum of electrons is calculated in the presence of a magnetic field along perpendicular to the plane. The Green’s function approach has been explored to obtain current-current correlation function and optical conductivity. The results show applying magnetic field has significant effects on dependence of optical absorption of boron nitride system on photon frequency and temperature.

Wafer-Scale Integration of Graphene-Based Photonic Devices.

Graphene and related materials can lead to disruptive advances in next-generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (≥5000 cm2 V–1 s–1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapor deposition (CVD) of single layer graphene (SLG) matrices comprising up to ∼12000 individual single crystals, grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ∼80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ∼5000 cm2 V–1 s–1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ∼0.25, 0.45, 0.75, 1 dB V–1 for device lengths ∼30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hexagonal boron nitride (hBN) is used to protect SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. The processes are compatible with full automation. This paves the way for large scale production of graphene-based photonic devices.

Method of Determining the Elastic Characteristics of Graphene and Other 2D Nanoallotropes

To calculate and design nanoelectronic and nanophotonic devices, in which graphenes and graphene-like 2D nanoallotropes of carbon, silicon, and binary III–V compounds are used, knowledge of the elastic characteristics and piezoelectric, photoelastic, and other properties of 2D materials, which depend on them, is of great significance. In this work, a simple method suitable for engineering calculations of determining the isothermal values of the force constants, elastic rigidities, Young’s modulus, and Poisson coefficient for 2D nanoallotropes of elements of Group IV of the periodic table and binary III–V compounds is proposed. The method is based on the Harrison method of bonding orbitals modified by S.Yu. Davydov and the R. Keating model for describing the elastic properties of such materials. It is shown that the method makes it possible to perform evaluative calculations of the elastic properties for known synthesized 2D crystal structures as well as for structures “constructed” theoretically. It is established that single-layer hexagonal boron nitride and other binary III–V compounds in the form of 2D nanoallotropes of various types of symmetry, which are piezoelectrics in addition, can be promising materials for nanoelectronics along with graphene. This extends the spectrum of the possible practical applications of the studied 2D materials (C, Si, BN, AlN, GaN, AlP, and GaP) both with a graphene-like and more complex structure. The results of this work can be used when developing the acoustic delay lines of the terahertz-frequency range, piezoelectric transducers for exciting and receiving elastic waves in nanoscale 2D acoustic lines, as well as piezoelectric sensors.

Thermomechanical Response of Supported Hexagonal Boron Nitride Sheets of Various Thicknesses

Raman spectroscopy is employed to investigate the temperature dependence of the E2g phonon mode of single-layer, few-layer (FL), and bulk hexagonal boron nitride (hBN) sheets, situated over Si/SiO2 (90 nm). Depending on the sample, two temperature regimes are recorded. In the low to mid range of the examined temperatures, the monolayer and FL samples exhibit significantly higher slopes compared to the thicker ones because of the thermal expansion of the underlying substrate. In the high-temperature region, all the examined samples show almost the same temperature slope, indicating slippage of the hBN sheets relative to the substrate and providing strong evidence that the slopes of the monolayer, FL, and bulk hBN are quite similar of about −0.020 cm–1/K. This is further justified from the full width at half-maximum versus T of the studied samples and the observed similarities of the thermal expansion coefficient (TEC) of one to three layers and bulk hBN, revealing a comparable level of anharmonicity for the E2g mode from the monolayer up to bulk hBN. Moreover, using a finite element method, we have determined the TEC of the underling substrate and the strain induced by TEC mismatch between the single-layer hBN and the substrate. Consequently, the intrinsic frequency shift of the E2g mode for the monolayer hBN upon temperature change is extracted. Finally, the vibrational response of monolayer hBN upon temperature is also examined by means of computational simulations, and satisfactory agreement with the experiment is obtained.

Equilibrium shape of single-layer hexagonal boron nitride islands on iridium

Large, high-quality layers of hexagonal boron nitride (hBN) are a prerequisite for further advancement in scientific investigation and technological utilization of this exceptional 2D material. Here we address this demand by investigating chemical vapor deposition synthesis of hBN on an Ir(111) substrate, and focus on the substrate morphology, more specifically mono-atomic steps that are always present on all catalytic surfaces of practical use. From low-energy electron microscopy and atomic force microscopy data, we are able to set up an extended Wulff construction scheme and provide a clear elaboration of different interactions governing the equilibrium shapes of the growing hBN islands that deviate from the idealistic triangular form. Most importantly, intrinsic hBN edge energy and interaction with the iridium step edges are examined separately, revealing in such way the importance of substrate step morphology for the island structure and the overall quality of 2D materials.

Optical absorption of single-layer hexagonal boron nitride in the ultraviolet

In this paper we theoretically describe the absorption of hexagonal boron nitride (hBN) single layer. We develop the necessary formalism and present an efficient method for solving the Wannier equation for excitons. We give predictions for the absorption of hBN on quartz and on graphite. We compare our predictions with recently published results (Elias et al 2019 Nat. Commun. 10 2639) for a monolayer of hBN on graphite. The spontaneous radiative lifetime of excitons in hBN is also computed. We argue that the optical properties of hBN in the ultraviolet are very useful for the study of peptides and other biomolecules.

Substitutional Si impurities in monolayer hexagonal boron nitride

We report the observation of substitutional silicon atoms in single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning transmission electron microscopy. The images reveal silicon atoms exclusively filling boron vacancies. Density functional theory is used to study the energetics, structure, and properties of the experimentally observed structure. The formation energies reveal SiB+1 as the most stable configuration. In this case, the silicon atom elevates by 0.66 Å out of the lattice with unoccupied defect levels in the electronic bandgap above the Fermi level. Our results unequivocally show that heteroatoms can be incorporated into the h-BN lattice opening the way for applications ranging from single-atom catalysis to atomically precise magnetic structures.