hBN Surface Research Articles

An experimental investigation of epoxy‐based hybrid composites with hexagonal boron nitride and short sisal fiber as reinforcement for high performance microelectronic applications

AbstractIn the present article, an investigation is presented on epoxy‐based composites where the discontinuous phases are microsized boron nitride and sisal fiber (SF). Both the reinforcing materials are surface modified before incorporating them into the epoxy matrix. Hexagonal boron nitride (hBN) surface is treated by silane‐coupling agent, whereas the aqueous NaOH solution is used to modify the surface of SF. The effect of fillers on the physical, mechanical, thermal, and dielectric properties of hybrid composites is studied through experimentation. The result shows that the inclusion of hBN increases the thermal conductivity of epoxy appreciably and dielectric constant marginally, while the inclusion of SF reduces the thermal conductivity marginally and dielectric constant appreciably. The maximum thermal conductivity of 1.88 W/m‐K is obtained for the combination of 30 wt% hBN and 3 wt% SF. For the same combination, the dielectric constant is 4.57 at 1 GHz, which is almost similar to neat epoxy. Also, other properties like compressive strength, hardness, glass‐transition temperature, and coefficient of thermal expansion improve when combinations of ceramic filler and natural fiber were incorporated in the epoxy matrix. Due to outstanding comprehensive properties, epoxy/hBN/SF composites found potential application in wide microelectronic applications.

Memory effect of vertically stacked hBN/QDs/hBN structures based on quantum-dot monolayers sandwiched between hexagonal boron nitride layer

The characteristics of a flexible write-once-read-many-times (WORM) memory fabricated with monolayered 0-dimensional (0D) CdSe–ZnS quantum dots (QDs) layers sandwiched between two insulating 2-dimensional (2D) hexagonal boron nitride (hBN) multilayers were investigated by electrical measurement method. The hBN/QDs monolayer/hBN structure was fabricated in a vertical stacked structure using a technique which control the formation of the QDs monolayer. QDs monolayer was formed by electrostatic interaction between the negative charge group on the CdSe–ZnS QDs surface and the positive charge group on the hBN surface. The device has a WORM characteristic due to the presence of QDs in the current-voltage (I–V) measurement. When a bias is applied, carriers were initially trapped by tunneling due to the QDs and then a conductive filament was formed in the hBN, which were not detrapped and exhibit characteristics of write-once-read-many-times memory. The maximum ON/OFF ratio of the current for the devices was as large as 4 × 10, and the endurance was 5 × 10 4 cycles, and a retention time was larger than 1 × 10 5 s. In order to explain the carrier transport mechanism and conductive filament of the WORM memory device caused by QDs, it through various methods such as I–V fitting data, simulation, and conductive AFM. Unlike the conventional conductive filament mechanism, through random diffusion such as Ag filament, the Au/hBN/QD/hBN/ITO/PET structures implemented a consistent conductive filament using Au metal and QDs active layer.

Molecular insights from theoretical calculations explain the differences in affinity and diffusion of airborne contaminants on surfaces of hBN and graphene

Exposed surfaces of two-dimensional (2D) materials are susceptible to the adsorption of various molecules including airborne contaminants, which can affect their performance in real applications. Hexagonal boron nitride (hBN) is structurally the closest relative to graphite and its single layer form to graphene. The adsorption of organic molecules to graphene was subject of extensive research, however, little is known about interaction ofadsorbates to hBN surface. We studied the affinity of organic molecules to the surface of hBN by inverse gas chromatography. The adsorption enthalpies of polar molecules including acetonitrile, nitromethane, ethanol, and acetone exhibited strong coverage dependency up to 20 % of a monolayer. Density functional theory and molecular dynamics calculations were employed to understand and interpret experimentally measured adsorption enthalpies. The calculations revealed that the strong affinity of polar molecules at low coverage was due to adsorption on steps and edges of hBN. The calculated surface diffusion barriers of all molecules were rather low, i.e., below 0.5 kcal/mol (except for benzene and cyclohexane), and molecules adsorbed on the surface behaved like a 2D gas. The results demonstrated that coupling inverse gas chromatography with computer simulations can provide vital insights into the mechanism of adsorption at the molecular level.

High Osmotic Power Generation via Nanopore Arrays in Hybrid Hexagonal Boron Nitride/Silicon Nitride Membranes.

Nanopores embedded in two-dimensional (2D) nanomaterials are a promising emerging technology for osmotic power generation. Here, coupling our new AFM-based pore fabrication approach, tip-controlled local breakdown (TCLB), with a hybrid membrane formed by coating silicon nitride (SiN) with hexagonal boron nitride (hBN), we show that high osmotic power density can be obtained in systems that do not possess the thinness of atomic monolayers. In our approach, the high osmotic performance arises from charge separation induced by the highly charged hBN surface rather than charge on the inner pore wall. Moreover, exploiting TCLB's capability of producing sub 10 nm pore arrays, we investigate the effects of pore-pore interaction on the overall power density. We find that an optimum pore-to-pore spacing of ∼500 nm is required to maintain an efficient selective transport mechanism.

Van der Waals Bound Organic/2D Insulator Hybrid Structures: Epitaxial Growth of Acene Films on hBN(001) and the Influence of Surface Defects.

Combining 2D materials with functional molecular films enables the fabrication of van der Waals bound organic/inorganic hybrids that are of interest for future device architectures. Recently, the 2D dielectric hexagonal boron nitride (hBN) has received particular attention since exfoliation allows the preparation of crystalline layers which have been utilized as ultrathin dielectrics in electronic devices. Here, we have studied the formation and structure of molecular films of the prototypical organic semiconductors pentacene (PEN) and perfluoropentacene (PFP) on hBN. Special attention was paid to the influence of substrate surface defects on the film formation by comparing molecular films that were grown on hBN substrates of various quality, including single crystals (representing the most ideal surface), briefly ion bombarded substrates, and exfoliated flakes. While X-ray diffraction (XRD) yields precise information about the crystalline structure of films grown on (large) single crystals, it is hardly applicable to analyze the films formed on exfoliated flakes because of their small size. Here, we demonstrate that in the case of flakes detailed structural analyses of the molecular films are possible by combining atomic force microscopy (AFM) with microspot UV/vis spectroscopy and optical polarization microscopy. On well-ordered hBN single crystal surfaces both acenes form very smooth and epitaxial crystalline films where molecules adopt a recumbent orientation (even in 100 nm thick films). By contrast, both materials adopt an upright molecular orientation and different polymorphs on defective hBN surfaces and reveal distinctly different film morphologies. On exfoliated flakes, PFP shows a film structure similar to that on the hBN single crystals, while PEN films exhibit a structure as on defective hBN substrates. In addition, a pronounced decoration of defect steps, which are probably created by the exfoliation process, was observed for PEN leading to the formation of tall and extended fibers where molecules adopt a recumbent orientation. The present study reveals different robustness in film growth on exfoliated hBN flakes for various molecules, which has to be considered in their device integration, especially with regard to their optoelectronic properties such as light absorption or charge transport, which depend critically on the molecular orientation and crystalline order.

Energetically stretching proteins on patterned two dimensional nanosheets

Driven by increasing interests in sequencing proteins, several single molecule methods (such as the measurement of electron tunneling currents through each amino acid) have been recently proposed and demonstrated in experiment. While promising, they are still in early stages and challenges such as the lack of protein stretching and large thermal fluctuations of amino acids remain to be solved. Here, we show that a patterned two dimensional (2D) in-plane heterostructure comprising both graphene and hexagonal boron nitride (hBN) domains can be utilized to stretch the intrinsically disordered Aβ42 protein because of the chemical potential difference between the graphene and hBN surfaces. Additionally, thanks to the physisorption of the flexible Aβ42 protein on the 2D surface, thermal fluctuations of amino acids are substantially reduced. Therefore, this stretched protein conformation with amino acids adsorbed on the 2D surface may facilitate protein sequencing using the high resolution atomic force microscopy (AFM) or scanning tunneling microscope (STM), with improved signal-to-noise ratios. We further show that the 2D-heterostructure-based protein electrophoresis can help detect the phosphorylation in a single protein molecule. We expect that the patterned 2D heterstructure could serve as a new versatile platform for various single-protein analyses.

Large‐Scale Fast Fluid Dynamic Processes for the Syntheses of 2D Nanohybrids of Metal Nanoparticle‐Deposited Boron Nitride Nanosheet and Their Glycolysis of Poly(ethylene terephthalate)

AbstractOwing to the unique mass and heat transfer, fluidic‐flow control systems or reactors can potentially provide advantages for organic and inorganic syntheses compared to the static reactors. In this study, it is demonstrated that a fluid dynamic reactor based on Taylor–Couette (T–C) flow provides high‐throughput processes for the exfoliation of hexagonal boron nitride (hBN), syntheses of metal‐nanoparticle‐(NP)‐deposited hBN nanohybrids, and their glycolysis reactions of poly(ethylene terephthalate) (PET). The mechanical shear force and dynamic mixing behavior of the T–C flow lead to a stable colloidal suspension of exfoliated hBN sheets with a high exfoliation yield of 77.9% and high production rate of 0.48 g h−1. A fast synthesis of metal NPs (Pd, Pt, Ag, and RuO2 NPs) onto the hBN surface is achieved by using the controlled T–C flow within only 2 min owing to the efficient mass and heat transfer of the T–C flow. The T–C flow considerably reduces the reaction time (30 min) and temperature (100 °C) for the Pd/hBN‐catalyst‐based glycolysis reaction of PET to bis(2‐hydroxyethyl) terephthalate compared to those of the conventional static reaction that is performed above 200 °C for 120 min.

Direct observation of water-mediated single-proton transport between hBN surface defects.

Aqueous proton transport at interfaces is ubiquitous and crucial for a number of fields, ranging from cellular transport and signalling, to catalysis and membrane science. However, due to their light mass, small size and high chemical reactivity, uncovering the surface transport of single protons at room temperature and in an aqueous environment has so far remained out-of-reach of conventional atomic-scale surface science techniques, such as scanning tunnelling microscopy. Here, we use single-molecule localization microscopy to resolve optically the transport of individual excess protons at the interface of hexagonal boron nitride crystals and aqueous solutions at room temperature. Single excess proton trajectories are revealed by the successive protonation and activation of optically active defects at the surface of the crystal. Our observations demonstrate, at the single-molecule scale, that the solid/water interface provides a preferential pathway for lateral proton transport, with broad implications for molecular charge transport at liquid interfaces.

Complete confinement and extraordinary propagation of Dyakonov-like polaritons in hBN

Two Dyakonov-like polaritons (DLPs) were found in the hexagonal boron nitride (hBN). DLP-I lies in the lower reststrahlen band (RB-I) where the hBN is a hyperbolic material, but DLP-II exists in a narrower frequency region where the hBN is an elliptical material, adjacent to the higher reststrahlen band (RB-II). After uniformly considering the DLP electromagnetic fields on the two sides of the hBN surface, we found that the two DLPs are highly confined in the hBN, and even are completely confined in a certain wave-vector. And also, the Poynting vector of either DLP in the hBN seriously deviates from its wave vector and the directional difference outside and inside the hBN dramatically changes with the distance from the surface. The above extraordinary results can support some potential applications in the mid-infrared range, such as high-efficient energy conduction, strictly manipulating infrared-ray direction and infrared-filtering as well as continuous light-splittingfrom micro-meter to nano-meter dimensions.

Defect-Controlled Nucleation and Orientation of WSe2 on hBN: A Route to Single-Crystal Epitaxial Monolayers.

A defect-controlled approach for the nucleation and epitaxial growth of WSe2 on hBN is demonstrated. The WSe2 domains exhibit a preferred orientation of over 95%, leading to a reduced density of inversion domain boundaries (IDBs) upon coalescence. First-principles calculations and experimental studies as a function of growth conditions and substrate pretreatment confirm that WSe2 nucleation density and orientation are controlled by the hBN surface defect density rather than thermodynamic factors. Detailed transmission electron microscopy analysis provides support for the role of single-atom vacancies on the hBN surface that trap W atoms and break surface symmetry leading to a reduced formation energy for one orientation of WSe2 domains. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe2 monolayer films on hBN were achieved. Low-temperature photoluminescence (PL) measurements and transport measurements of back-gated field-effect transistor devices fabricated on WSe2/hBN films show improved optical and electrical properties compared to films grown on sapphire under similar conditions. Our results reveal an important nucleation mechanism for the epitaxial growth of van der Waals heterostructures and demonstrate hBN as a superior substrate for single-crystal transition-metal dichalcogenide (TMD) films, resulting in a reduced density of IDBs and improved properties. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

Dynamic Phase Diagram of Catalytic Surface of Hexagonal Boron Nitride under Conditions of Oxidative Dehydrogenation of Propane.

Partially oxidized surfaces of hexagonal boron nitride (hBN) and several metal borides are unexpectedly excellent catalysts for oxidative dehydrogenation of alkanes to olefins, but the nature of the active site(s) on these B-containing interfaces remains elusive. We characterize the surface of the partially oxidized B-rich hBN surface under reaction conditions from first principles. The interface has thermal access to multiple different stoichiometries and multiple structures of each stoichiometry. The size of the thermal ensemble is composition-dependent. The phase diagram of the interface constructed on the basis of the statistical ensembles of many accessible states is very different from the one based on global minima. Phase boundaries shift and blur, and phases consist of several stoichiometries and structures. The BO layer transiently exposes the reactive -B═O motifs in the metastable states. The fluxionality and structural diversity emerging under reaction conditions must be taken into account in theoretically descriptions of the catalytic interface.

Aligned van der Waals Coupled Growth of Carbon Nanotubes to Hexagonal Boron Nitride

Abstract1D carbon nanotubes (CNTs) are grown on hexagonal boron nitride (hBN) surfaces. The nanotubes show clear preference to align to specific crystal directions of the hBN substrate. Raman spectra confirm that the nanotubes consist of sp2 carbon, while nanomanipulation shows that they are van der Waals coupled to the underlying hBN substrate. Scanning conductance and electric force microscopy show that the CNTs have significantly greater electrical conductance compared to the hBN. This integrated aligned growth of materials with similar lattices, yet having dissimilar dimensionality and electrical conducting properties, provides a promising route to achieving intricate nanoscale electrical circuits on high‐quality insulating substrates.

High-Resolution Volumetric Adsorption and Molecular Dynamics of then-Alkanes on the Surface of Hexagonal Boron Nitride

High-resolution, volumetric adsorption isotherm measurements of methane through octane on hexagonal boron nitride (hBN) were performed in the vicinity of their bulk triple-point temperatures. It is observed that the adsorption behavior can be classified into three regimes depending on the molecular geometry and chain length; i.e., quasispherical, cigarlike, and rodlike. The film growth is observed to change from six discrete steps, i.e., layer-by-layer like, to two discrete steps, i.e., incomplete wetting as the molecular length increases. The heat of adsorption and differential entropy and enthalpy were calculated from these isotherm data for monolayer, bilayer, and where possible, thicker films. It is found that the monolayer heat of adsorption exhibits a smooth increase with increasing chain length. Molecular dynamics simulations were performed to provide complementary microscopic insight into the adsorption properties of these alkanes on hBN. Concentration profiles (C(z)) normal to the hBN surface pla...

Synthesis of Ultrathin Graphdiyne Film Using a Surface Template.

Graphdiyne is predicted to have a natural band gap and simultaneously possesses superior carrier mobility, which makes it potential for electronic devices. Synthesis of ultrathin graphdiyne film is highly demanded. In this work, we proposed an approach for synthesis of ultrathin graphdiyne film using graphene as a surface template, which can induce confined reaction on substrate. With all-carbon, conjugated, atomically flat structure, graphene has a strong interaction with the graphdiyne system, resulting the formation of continuous flat ultrathin graphdiyne film with thickness less than 3 nm. Raman spectra, grazing incidence X-ray diffraction, and transmission electron microscopy characterization all confirmed the features of graphdiyne. Furthermore, this strategy was also extended to the hexagonal boron nitride (hBN) surface with resembling structure, serving as a perfect dielectric layer. Field-effect transistor devices based on graphdiyne film grown on hBN were fabricated directly, and electrical transport measurements demonstrate the good conductivity with p-type characteristics of the as-obtained graphdiyne film.

High-temperature molecular beam epitaxy of hexagonal boron nitride layers

The growth and properties of hexagonal boron nitride (hBN) have recently attracted much attention due to applications in graphene-based monolayer thick two dimensional (2D)-structures and at the same time as a wide band gap material for deep-ultraviolet device (DUV) applications. The authors present their results in the high-temperature plasma-assisted molecular beam epitaxy (PA-MBE) of hBN monolayers on highly oriented pyrolytic graphite substrates. Their results demonstrate that PA-MBE growth at temperatures ∼1390 °C can achieve mono- and few-layer thick hBN with a control of the hBN coverage and atomically flat hBN surfaces which is essential for 2D applications of hBN layers. The hBN monolayer coverage can be reproducible controlled by the PA-MBE growth temperature, time and B:N flux ratios. Significantly thicker hBN layers have been achieved at higher B:N flux ratios. The authors observed a gradual increase of the hBN thickness from 40 to 70 nm by decreasing the growth temperature from 1390 to 1080 °C. However, by decreasing the MBE growth temperature below 1250 °C, the authors observe a rapid degradation of the optical properties of hBN layers. Therefore, high-temperature PA-MBE, above 1250 °C, is a viable approach for the growth of high-quality hBN layers for 2D and DUV applications.

The effect of water absorption on the dielectric properties of polyethylene hexagonal boron nitride nanocomposites

The effect of water absorption on the dielectric response of polyethylene/hexagonal boron nitride nanocomposites has been studied by dielectric spectroscopy. The nanocomposites have been prepared with hBN concentrations ranging from 2 wt% to 30 wt%. Fourier transform infrared spectroscopy and thermogravimetric analysis revealed a very small amount of hydroxyl groups on the surface of hBN. Mass loss measurements showed that the nanocomposites did not absorb any water under ambient and dry conditions while there was some water absorption under wet conditions. The dielectric spectroscopy results showed a broad relaxation peak, indicative of different states of water with water shells of different thickness, which moved to higher frequencies with increasing water content. However, the dielectric losses were significantly lower than the losses reported in the literature of nanocomposites under wet conditions. In addition, all the absorbed water was successfully removed under vacuum conditions which demonstrated that the interactions between the water and the nanocomposites were very weak, due to the hydrophobic nature of the hBN surface. This is a highly useful property, when considering these materials for applications in electrical insulation.

Slidable atomic layers in van der Waals heterostructures

We report the preparation and manipulation of slidable atomic layers in clean, incommensurate van der Waals (vdW) heterostructures. Monolayer and multilayer WS2 grains are grown on graphite and hexagonal boron nitride (hBN) via chemical vapor deposition, and these grains can slide smoothly on graphite and hBN surfaces by manipulation with a tip. Furthermore, this sliding process allows the suspension, tearing, stacking, and connection of the atomic layers. These results demonstrate a novel approach for developing a wide variety of atomic-layer heterostructures with tunable interlayer coupling and an ideal system for studying the superlubricity of incommensurate, highly clean vdW contacts.

Interaction of CO with an hBN surface doped with Ti and Pt: A First Principles Study

We studied the possible catalytic effect that a transition metal atom could have after being adsorbed in a surface of hexagonal boron nitride (hBN). We considered platinum and titanium, performing ab-initio calculations, including molecular dynamics at 300K, within the Density Functional Theory. We considered an hBN surface either with a vacancy of a Boron atom, or with a vacancy of a Nitrogen atom, and we found that both titanium and platinum are absorbed at the place of the vacancy for both cases considered. Afterwards, we found that this decoration of the hBN surface indeed has a catalytic effect on the adsorption of a carbon monoxide molecule. Possible desorption was explored, at 800 K. To perform the calculations, the Quantum ESPRESSO package code was used. The generalized gradient corrected Perdew-Burke-Ernzerhof (PBE) approximation was used for the exchange and correlation functional.

Understanding and controlling the substrate effect on graphene electron-transfer chemistry via reactivity imprint lithography

Graphene has exceptional electronic, optical, mechanical and thermal properties, which provide it with great potential for use in electronic, optoelectronic and sensing applications. The chemical functionalization of graphene has been investigated with a view to controlling its electronic properties and interactions with other materials. Covalent modification of graphene by organic diazonium salts has been used to achieve these goals, but because graphene comprises only a single atomic layer, it is strongly influenced by the underlying substrate. Here, we show a stark difference in the rate of electron-transfer reactions with organic diazonium salts for monolayer graphene supported on a variety of substrates. Reactions proceed rapidly for graphene supported on SiO(2) and Al(2)O(3) (sapphire), but negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces, as shown by Raman spectroscopy. We also develop a model of reactivity based on substrate-induced electron-hole puddles in graphene, and achieve spatial patterning of chemical reactions in graphene by patterning the substrate.