Ultrasmooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport in interfaced transition metal dichalcogenides (TMDs), and amplify the effect of quantum capacitance in field-effect gating. All of the current processes to realize h-BN-based heterostructures involve transfer or exfoliation. Rational chemistries and process techniques are still required to produce large-area, transfer-free, directly grown TMDs/BN heterostructures. Here, we demonstrate a novel boron-oxygen chemistry route for oxide-assisted nucleation and growth of large-area, uniform, and ultrathin h-BN directly on oxidized substrates (B/N atomic ratio = 1:1.16 ± 0.03 and optical band gap = 5.51 eV). These intimately interfaced, van der Waals heterostructures of MoS2/h-BN and WS2/h-BN benefit from 6.27-fold reduced roughness of h-BN in comparison to SiO2. This leads to reduction in scattering from roughness and charged impurities, and enhanced carrier mobility verified by an increase in electrical conductivity (5 times for MoS2/h-BN and 2 times for WS2/h-BN). Further, the heterostructures are devoid of wrinkles and adsorbates, which is critical for 2D nanoelectronics. The versatile process can potentially be extrapolated to realize a variety of heterostructures with complex sandwiched 2D electronic circuitry.

Hexagonal boron nitride (h-BN) is an ideal platform for interfacing with two-dimensional (2D) nanomaterials to reduce carrier scattering for high-quality 2D electronics. However, scalable, transfer-free growth of hexagonal boron nitride (h-BN) remains a challenge. Currently, h-BN-based 2D heterostructures require exfoliation or chemical transfer of h-BN grown on metals resulting in small areas or significant interfacial impurities. Here, we demonstrate a surface-chemistry-influenced transfer-free growth of large-area, uniform, and smooth h-BN directly on silicon (Si)-based substrates, including Si, silicon nitride (SiN), and silicon dioxide (SiO), via low-pressure chemical vapor deposition. The growth rates increase with substrate electronegativity, Si < SiN < SiO, consistent with the adsorption rates calculated for the precursor molecules via atomistic molecular dynamics simulations. Under graphene with high grain density, this h-BN film acts as a polymer-free, planar-dielectric interface increasing carrier mobility by 3.5-fold attributed to reduced surface roughness and charged impurities. This single-step, chemical interaction guided, metal-free growth mechanism of h-BN for graphene heterostructures establishes a potential pathway for the design of complex and integrated 2D-heterostructured circuitry.

Since 2D transition metal dichalcogenides (TMDs) exhibit strain-tunable bandgaps, locally confining strain can allow lateral manipulation of their band structure, in-plane carrier transport and optical transitions.

Clay minerals are used in variety of applications ranging from composites to electronic devices. For their efficient use in such areas, understanding the effect of surface-active agents on interfacial properties is essential. We investigated the role of surface ions in the adsorption of water molecules by using a muscovite mica surface populated with two different, H(+) and K(+), surface ions. A series of grand canonical Monte Carlo (GCMC) simulations at various relative vapor pressures (p/p0) were performed to obtain the water structure and adsorption isotherm on the H(+)-exposed mica (H-mica) surface. The obtained results were compared to the recent simulation data of water adsorption on the K(+)-exposed mica (K-mica) surface reported by Malani and Ayyappa (Malani, A.; Ayappa, K. G. J. Phys. Chem. B 2009, 113, 1058-1067). Water molecules formed two prominent layers adjacent to the H-mica surface, whereas molecular layering was observed adjacent to the K-mica surface. The adsorption isotherm of water on the K-mica surface was characterized by three stages that corresponded to rapid adsorption in the initial regime below p/p0 = 0.1, followed by a linear development regime for p/p0 = 0.1-0.7 and rapid film thickening for p/p0 ≥ 0.7, whereas only latter two regimes were observed in the H-mica system. In addition, the film thickness of adsorbed water molecules for p/p0 < 0.7 was lower as compared to that for the K-mica surface and comparable beyond. The film thickness obtained from the MC simulations was in excellent agreement with the interferometry experimental data of Balmer et al. (Balmer, T. E.; Christenson, H. K.; Spencer, N. D.; Heuberger, M. Langmuir 2008, 24, 1566-1569). It was observed that the hydration behaviors of the two ions were completely different and depended on the size of their hydration shell and their ability to form hydrogen bonds. The behavior of water adsorption between these two cases was illustrated using the water density distribution, orientational distributions, hydrogen bonding, and isosteric heats of adsorption.

most intensively studied research areas in the development of high-performance separation membranes.Most 2D materials typically have a layered geometry where the atoms are linked by strong in-plane covalent bonds, while the two adjacent layers are held together by van der Waals forces. [7] Exfoliated nanosheets typically show a high surface-area-tothickness ratio, which is instrumental in improving the adsorption capacity or the ion selectivity, both of which can lead to a better separation performance. In addition to highly effective ion-rejection properties, 2D membranes are also known to possess excellent water permeability. Due to these properties, many 2D materials such as nm-thick graphitic carbon nitride (g-C 3 N 4 ) nanosheets, [8] MoS 2 sheets, [9] WS 2 nanosheets, [6] graphene, [10] Mxene nanosheets, [11] and graphene oxide (GO), [12] have been recently used as the building blocks for fabricating ultrathin layered membranes for separation applications. These 2D membranes have shown highly efficient size-selective ion separation and high water permeance.Hexagonal boron nitride (h-BN), so-called "white graphene," is however one of the promising 2D materials that has not been utilized to its full potential in ion-separation applications. For efficient ion separation, the membrane should show high ion selectivity, which is primarily dependent on the channel size and surface charge on the membrane. The h-BN material is known to have a high surface charge density resulting from the adsorption of the hydroxyl ions on the surface defect sites. [13] It also shows excellent oxidation and corrosion resistance, which is an important property for wastewater treatment applications. The high chemical stability of h-BN also enables it to be resistant to chemical cleaning, which is frequently needed during separation processes. Qin et al. reported high ionic conductivities for h-BN nanofluidic channels, prepared by the one-step BN exfoliation method and amine functionalization. [14] More recently, Chen et al. developed a 2D h-BN membrane by functionalizing h-BN flakes (h-BNF) with amino groups to overcome its poor water dispersibility. The functionalized h-BNF membranes demonstrated fast solvent transport and good ion-rejection properties, based on molecular sieving mechanism. [15] However, these membranes do not show charge-based (Donnan) exclusion, due to the amine functionalization of the 2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h-BNF) for highly efficient ion separation is reported. The ion-rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h-BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10 −3 L m m −2 h −1 bar −1...

By leveraging the Van Hove singularity induced enhancement in optical absorption, a photovoltaic cell is designed with WS2 on graphene atop n-Si to enhance the power conversion efficiency.

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Percolating network of mixed 2D nanomaterials (2DNs) can leverage the unique electronic structures of different 2DNs, their interfacial doping, manipulable conduction pathways, and local traps. Here, we report on the percolation mechanism and electro-capacitive transport pathways of mixed-platelet network of hexagonal boron nitride (hBN) and reduced graphene oxide (rGO), two isostructural and isoelectronic 2DNs. The transport mechanism is explained in terms of electron hopping through isolated hBN defect traps between rGO (possibly via electron tunneling/hopping through "funneling" points). With optical bandgaps of 4.57 and 4.08 eV for the hBN-domains and 2.18 eV for the rGO domains, the network of hBN with rGO exhibits Poole-Frenkel emission-based transport with mean hopping gap of 1.12 nm (∼hBN trilayer) and an activation barrier of ∼15 ± 0.7 meV. Further, hBN (1.7 pF) has a 6-fold lower capacitance than 1:1 hBN:rGO, which has a resistance 2 orders of magnitude higher than that of rGO (1.46 MΩ). These carrier transport results can be applied to other multi-2DN networks for development of next-generation functional 2D-devices.