Borazine Molecules Research Articles

Adsorption, Single‐Molecule Manipulation, and Self‐Assembly of Borazine on Ag(111)

AbstractThe interaction of borazine with metal supports and the concomitant surface chemistry play important roles in the synthesis of hexagonal boron nitride and the assembly of BN‐doped carbon nanostructures, thus making adsorbed borazine an intriguing model system. Herein, the first real space characterization of individual borazine molecules and highly ordered borazine self‐assemblies on solid supports, combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy, X‐ray photoelectron spectroscopy, and complementary density functional theory modeling is reported. Specifically, a weak, nondissociative adsorption of borazine with the ring aligned in parallel to the surface plane is observed on Ag(111) upon low temperature deposition. Borazine is found to favor hollow adsorption sites, which guide the assembly of intricate borazine assemblies including a porous, chiral honeycomb‐like network, and dense‐packed monolayer films. Additionally, a modification of the borazine adsorption configuration by STM‐based manipulation is demonstrated. Dehydrogenation of individual molecules by voltages pulses yields an upright standing borazine fragment bound via B to Ag. This study thus provides a comprehensive, single‐molecule level characterization of borazine adsorption and surface chemistry on a characteristic coinage metal support and may serve as a reference for advanced low‐dimensional materials based on functionalized borazines or including BN units as dopants.

Effects of Inorganic Substitutions and Different Metal Electrode Materials on Electronic Transport Properties of Organic Molecular Devices.

Incorporating inorganic components into organic molecular devices offers one novel alternative to address challenges existing in the fabrication and integration of nanoscale devices. In this study, using a theoretical method of density functional theory combined with the nonequilibrium Green's function, a series of benzene-based molecules with group III and V substitutions, including borazine molecule and XnB3-nN3H6 (X = Al or Ga, n = 1-3) molecules/clusters, are constructed and investigated. An analysis of electronic structures reveals that the introduction of inorganic components effectively reduces the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, albeit at the cost of reduced aromaticity in these molecules/clusters. Simulated electronic transport characteristics demonstrate that XnB3-nN3H6 molecules/clusters coupled between metal electrodes exhibit lower conductance compared to prototypical benzene molecule. Additionally, the choice of metal electrode materials significantly impacts the electronic transport properties, with platinum electrode devices displaying distinct behavior compared to silver, copper, and gold electrode devices. This distinction arises from the amount of transferred charge, which modulates the alignment between molecular orbitals and the Fermi level of the metal electrodes by shifting the molecular orbitals in energy. These findings provide valuable theoretical insights for the future design of molecular devices incorporating inorganic substitutions.

Probing the structural, bonding, electronic and magnetic properties of transition–metal borazine systems: Co m (borazine) n (m = 1, 2; n = 1–3)

ABSTRACT The geometrical structures of neutral and anionic Co m (borazine) n (m = 1, 2; n = 1–3) complexes have been determined by using density functional theory. The results indicate that most of the ground state structures for the complexes are similar to those of Co m (benzene) n , which might because borazine is isoelectronic and isostructural to benzene. The frontier molecular orbitals (FMOs) analyses show that their FMOs mainly arise from the 3d/4s electrons of cobalt atoms and the weak π-cloud of borazine molecule. Furthermore, the magnetic moments of complexes were studied and the results revealed that the Co atoms carry most of the magnetic moments. Comparing with the magnetic moment of a free Co atom (3.0μB), the magnetic moments of Co atom in most of Co m (borazine) n 0/- complexes are significantly reduced and even quenched except that the Co(borazine) remains unchanged. More importantly, there is a transition FM-to-AFM between neutral and anionic Co2(borazine)2. Finally, natural population analyses were performed to insightfully explore the reliable electronic structure properties.

In situ study of two-dimensional dendritic growth of hexagonal boron nitride

Hexagonal boron nitride, often entitled the ‘white graphene’ because of its large band gap, is one of the most important two-dimensional (2D) materials and frequently investigated in context with stacked arrays of single 2D layers, so called van der Waals heterostructures. Here, we concentrate on the growth of hBN on the coinage metal surface Cu(1 1 1). Using low energy electron microscopy and diffraction, we investigate the self-terminated growth of the first layer in situ and in real time. Most prominently, we find dendritic structures with three strongly preferred growth branches that are mostly well aligned with the Cu(1 1 1) substrate and exhibit a three-fold symmetric shape. The observation of dendritic structures is very surprising since hBN was found to grow in compact, triangular-shaped islands on many other metal substrates, in particular, on transition metal surfaces where it shows a much stronger interaction to the surface. We explain the unexpected dendritic growth by an asymmetry of the bonding energy for the two possible ways a borazine molecule can attach to an existing hBN island, namely either with one of its boron or one of its nitrogen atoms. We suggest that this asymmetry originates from different dehydrogenation states of the adsorbed borazine molecules and the hBN islands. We call this mechanism ‘Dehydrogenation Limited Aggregation’ since it is generic in the sense that it is merely based on different dehydrogenation energies for the involved building blocks forming the 2D layer.

Formation of a monolayer h-BN nanomesh on Rh (111) studied using in-situ STM

As a member of the 2D family of materials, h-BN is an intrinsic insulator and could be employed as a dielectric or insulating inter-layer in ultra-thin devices. Monolayer h-BN can be synthesized on Rh (111) surfaces using borazine as a precursor. Using in-situ variable-temperature scanning tunneling microscopy (STM), we directly observed the formation of h in real-time. By analyzing the deposition under variable substrate temperatures and the filling rate of the h-BN overlayer vacant hollows during growth, we studied the growth kinetics of how the borazine molecules construct the h-BN overlayer grown on the Rh surface.

Theoretical study of the effects of substituents (F, Cl, Br, CH3, and CN) on the aromaticity of borazine.

This paper presents a theoretical study of the effects of substituents (F, Cl, Br, CH3, and CN) on the aromaticity of borazine (B3N3H6), using density functional theory (DFT) and the Hartree-Fock (HF) method. The calculations to optimize the geometries, structural properties, and vibrational frequencies were performed using the same 6-311G(d,p) and 6-311++G(d,p) basis sets, comparing the methods with experimental results. In the analysis of the NICSZZ values, it was found that that replacing the hydrogen atoms by halogen atoms (F, Cl, and Br) and CH3 reduced the aromaticity of the borazine molecule, while use of the CN group resulted in NICSZZ values (0.9-2.0Å) very close to those of borazine, presenting the following order of increasing aromaticity: B3N3H3-(Br)3 < B3N3H3-(Cl)3 < B3N3H3-(F)3 < B3N3H3-(CH3)3 < B3N3H6 ~ B3N3H3-(CN)3. All the spectra of the compounds showed only the presence of transition peaks distant from the UV region, reflecting the large energy difference between the HOMO and LUMO orbitals. After the substitution of the borazine ring, all the compounds presented an intensification of the spectrum, with a shift of the maximum absorbance toward red, indicative of a bathochromic effect. There was a direct inverse relation between the energy gap and the maximum wavelength of the compounds.

Microanalysis of single-layer hexagonal boron nitride islands on Ir(111)

Large hexagonal boron nitride (hBN) single-layer islands of high crystalline quality were grown on Ir(111) via chemical vapor deposition (CVD) and have been studied with low-energy electron microscopy (LEEM). Two types of hBN islands have been observed that structurally differ in their shape and orientation with respect to iridium, where the former greatly depends on the iridium step morphology. Photoemission electron microscopy (PEEM) and IV-LEEM spectroscopy revealed that the two island types also exhibit different work functions and bindings to iridium, which provides an explanation for differences in their shape and growth modes. In addition, various temperatures were used for the CVD synthesis of hBN, and it was found that at temperatures higher than ≈950°C boron atoms, originating either from decomposed borazine molecules or disintegrated hBN islands, can form additional compact reconstructed regions. The presented results are important for advancement in synthesis of high-quality hBN and other boron-based layered materials, and could therefore expedite their technological implementation.

Epitaxial growth of a single-crystal hybridized boron nitride and graphene layer on a wide-band gap semiconductor.

Vertical and lateral heterogeneous structures of two-dimensional (2D) materials have paved the way for pioneering studies on the physics and applications of 2D materials. A hybridized hexagonal boron nitride (h-BN) and graphene lateral structure, a heterogeneous 2D structure, has been fabricated on single-crystal metals or metal foils by chemical vapor deposition (CVD). However, once fabricated on metals, the h-BN/graphene lateral structures require an additional transfer process for device applications, as reported for CVD graphene grown on metal foils. Here, we demonstrate that a single-crystal h-BN/graphene lateral structure can be epitaxially grown on a wide-gap semiconductor, SiC(0001). First, a single-crystal h-BN layer with the same orientation as bulk SiC was grown on a Si-terminated SiC substrate at 850 °C using borazine molecules. Second, when heated above 1150 °C in vacuum, the h-BN layer was partially removed and, subsequently, replaced with graphene domains. Interestingly, these graphene domains possess the same orientation as the h-BN layer, resulting in a single-crystal h-BN/graphene lateral structure on a whole sample area. For temperatures above 1600 °C, the single-crystal h-BN layer was completely replaced by the single-crystal graphene layer. The crystalline structure, electronic band structure, and atomic structure of the h-BN/graphene lateral structure were studied by using low energy electron diffraction, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy, respectively. The h-BN/graphene lateral structure fabricated on a wide-gap semiconductor substrate can be directly applied to devices without a further transfer process, as reported for epitaxial graphene on a SiC substrate.

Discovery of low energy pathways to metal-mediated B[double bond, length as m-dash]N bond reduction guided by computation and experiment.

This manuscript describes a combination of DFT calculations and experiments to assess the reduction of borazines (B-N heterocycles) by η6-coordination to Cr(CO)3 or [Mn(CO)3]+ fragments. The energy requirements for borazine reduction are established as well as the extent to which coordination of borazine to a transition metal influences hydride affinity, basicity, and subsequent reduction steps at the coordinated borazine molecule. Borazine binding to M(CO)3 fragments decreases the thermodynamic hydricity by >30 kcal mol-1, allowing it to easily accept a hydride. These hydricity criteria were used to guide the selection of appropriate reagents for borazine dearomatization. Reduction was achieved with an H2-derived hydride source, and importantly, a pathway which proceeds through a single electron reduction and H-atom transfer reaction, mediated by anthraquinone was uncovered. The latter transformation was also carried out electrochemically, at relatively positive potentials by comparison to all prior reports, thus establishing an important proof of concept for any future electrochemical B[double bond, length as m-dash]N bond reduction.

“Magic” Surface Clustering of Borazines Driven by Repulsive Intermolecular Forces

It's a kind of magic: Hydroxy pentaaryl borazine molecules self-assemble into small clusters (see structure) on Cu(111) surfaces, whereas with symmetric hexaaryl borazine molecules large islands are obtained. Simulations indicate that the observed "magic" cluster sizes result from long-range repulsive Coulomb forces arising from the deprotonation of the B-OH groups of the hydroxy pentaaryl borazine.

Electron Localization Function as a Measure of Electron Delocalization and Aromaticity

The Electron Localization Function (ELF) has played in the last time an important role in understanding the special characteristics of the chemical bond. The chemical interpretation of the ELF as an indicator of the regions of the space where it is most probable to find a localized electron pair has been of great value in order to understand some complex chemical bonds. In this work, the ELF has been used to study the delocalization and aromatic character of a diversity of molecules. It is shown that whereas the analysis of the total ELF does not provide clear information about aromaticity, the separation of the function on its σ and π parts yields indeed valuable information about it. Moreover, it is possible to construct a quantitative scale of aromaticity. It is also shown that the use of the ELF to understand aromaticity is complementary to other methodologies. The study includes mono substituted benzene derivatives, cyclic organic compounds, borazine molecule and the mechanism of acetylene trimerization. Keywords: Electron localization, ELF, Aromaticity, ELF TOPOLOGICAL CONCEPTS, Localized orbitals, ellipticity, B3LYP, CYCLOADDITION REACTIONS, layer chromatography (TLC), Spectroscopy, Atoms in Molecules (AIM), Fisher carbenes

Effects of Different Metal-Molecule Interface Conformations on the Electronic Transport in Molecular Junctions

Based on nonequilibrium Green’s function and first-principles calculation, we investigate the electronic transport of borazine molecule with different metal-molecule interface conformations, namely bridge and top site. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the current will be increased with the different adsorption sites; especially the enhancement of current is more obvious when molecule is located at the bridge site. Furthermore, a negative differential resistance under applied bias can be observed when the molecule is located at the top site. The mechanism of negative differential resistance is mainly induced by the resonance peak around the Fermi energy in top adsorption site.

Nitroborazines as potential high energy materials: density functional theoretical calculations

As part of a search for new high energy density materials, we used density functional theoretical calculations to determine the thermochemical properties of various nitro-substituted borazine molecules. Optimized geometries, vibrational frequencies and spectra, and enthalpies of formation and combustion were determined for nitroborazine, dinitroborazine, trinitroborazine, and methyltrinitroborazine with substituents on either the boron atoms or the nitrogen atoms of the parent borazine ring. Our results indicate that the specific enthalpy of combustion ranged from 4 to 11 kJ g(-1), with increasing substitution of nitro groups lowering the energy of combustion per unit mass.

Theoretical Study of Main-Group Metal−Borazine Sandwich Complexes

Using density functional theory within the generalized gradient approximation, we have theoretically studied the formation of neutral metal-aromatic complexes R1-M and R1-M-R2, where M is either neutral lithium, calcium, or gallium and R1 or R2 is benzene or borazine. We first find that calcium atom is an effective mediator for cooperative formation of a sandwich complex with borazine, while others are not. When benzene and borazine are mixed in the presence of calcium, a 1:2:1 mixture of benzene-calcium-benzene, borazine-calcium-benzene, and borazine-calcium-borazine is expected. An "A"-shaped structure is predicted for homo- and heterocomplexes of borazine with partial B-B and B-C bonds, while two rings are planar in the case of homocomplexes of benzene. Our analysis of the electron density distributions in HOMO-1 to LUMO in terms of orbital symmetry in conjunction with analysis of l,m-projected electronic local density of states shows that this correlates with the charge transfer and the interaction of pi states of the rings mediated by empty d-states of Ca, which is ultimately related to the polarity of the B-N bond. We find that there is a large accumulation of electron density on particular atoms upon complex formation, predicting characteristic behavior in electron-transfer reaction and nucleophilic reaction different from those for pure benzene or borazine molecule. The hetero-sandwich complex is of particular interest due to its asymmetrical distribution of excess electrons.

Low-temperature adsorption and decomposition of borazine on the Si(100)-(2 × 1) surface

Borazine adsorption at a surface temperature ( T s) of − 120°C and the thermal decomposition of borazine have been investigated on the Si(100)-(2 × 1) surface using temperature-programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The initial reactive sticking coefficient ( S 0) at this temperature was measured as 0.08. Borazine dissociates by breaking BH and/or NH bonds to form silicon monohydride species, SiH, even at −120°C because of the highly reactive silicon dangling bonds. The monohydride then decomposes at 550°C to desorb H 2. After the initial decomposition stage, subsequent borazine molecules (α 1-borazine) can coadsorb intact with the partially decomposed borazine (β-borazine). α 1-borazine eventually desorbs molecularly with a peak temperature of 110°C. Taking into account the coexistence of both α 1- and β-borazine, the saturation coverage of borazine (θ BZ) was estimated as 1.2 molecules per surface Si atom. After borazine adsorption and subsequent high-temperature annealing (900°C), the surface contains boron and nitrogen originating from decomposed borazine. This surface is less reactive (“poisoned”) towards borazine adsorption. Above 900°C, the nitrogen- and boron-containing Si surface further decomposes to evolve SiN or Si 2N.

Determination of the values of the mean-square amplitudes of vibration of the borazine molecule

Determination of the values of the mean-square amplitudes of vibration of the borazine molecule