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.

This paper reports a scalable approach to achieve spatially selective graphene functionalization using multiscale wrinkles. Graphene wrinkles were formed by relieving the strain in thermoplastic polystyrene substrates conformally coated with fluoropolymer and graphene skin layers. Chemical reactivity of a fluorination process could be tuned by changing the local curvature of the graphene nanostructures. Patterned areas of graphene nanowrinkles and crumples followed by a single-process plasma reaction resulted in substrates with regions having different fluorination levels. Notably, conductivity of the functionalized graphene nanostructures could be locally tuned as a function of feature size without affecting the mechanical properties.

Binding graphene with auxiliary nanoparticles for plasmonics, photovoltaics, and/or optoelectronics, while retaining the trigonal-planar bonding of sp hybridized carbons to maintain its carrier-mobility, has remained a challenge. The conventional nanoparticle-incorporation route for graphene is to create nucleation/attachment sites via "carbon-centered" covalent functionalization, which changes the local hybridization of carbon atoms from trigonal-planar sp to tetrahedral sp. This disrupts the lattice planarity of graphene, thus dramatically deteriorating its mobility and innate superior properties. Here, we show large-area, vapor-phase, "ring-centered" hexahapto (η) functionalization of graphene to create nucleation-sites for silver nanoparticles (AgNPs) without disrupting its sp character. This is achieved by the grafting of chromium tricarbonyl [Cr(CO)] with all six carbon atoms (sigma-bonding) in the benzenoid ring on graphene to form an (η-graphene)Cr(CO) complex. This nondestructive functionalization preserves the lattice continuum with a retention in charge carrier mobility (9% increase at 10 K); with AgNPs attached on graphene/n-Si solar cells, we report an ∼11-fold plasmonic-enhancement in the power conversion efficiency (1.24%).

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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