Direct, Transfer-Free Growth of Large-Area Hexagonal Boron Nitride Films by Plasma-Enhanced Chemical Film Conversion (PECFC) of Printable, Solution-Processed Ammonia Borane

Abstract

Synthesis of large-area hexagonal boron nitride (h-BN) films for two-dimensional (2D) electronic applications typically requires high temperatures (∼1000 °C) and catalytic metal substrates which necessitate transfer. Here, analogous to plasma-enhanced chemical vapor deposition, a nonthermal plasma is employed to create energetic and chemically reactive states such as atomic hydrogen and convert a molecular precursor film to h-BN at temperatures as low as 500 °C directly on metal-free substratesa process we term plasma-enhanced chemical film conversion (PECFC). Films containing ammonia borane as a precursor are prepared by a variety of solution processing methods including spray deposition, spin coating, and inkjet printing and reacted in a cold-wall reactor with a planar dielectric barrier discharge operated at atmospheric pressure in a background of argon or a mixture of argon and hydrogen. Systematic characterization of the converted h-BN films by micro-Raman spectroscopy shows that the minimum temperature for nucleation on silicon-based substrates can be decreased from 800 to 500 °C by the addition of a plasma. Furthermore, the crystalline domain size, as reflected by the full width at half-maximum, increased by more than 3 times. To demonstrate the potential of the h-BN films as a gate dielectric in 2D electronic devices, molybdenum disulfide field effect transistors were fabricated, and the field effect mobility was found to be improved by up to 4 times over silicon dioxide. Overall, PECFC allows h-BN films to be grown at lower temperatures and with improved crystallinity than CVD, directly on substrates suitable for electronic device fabrication.

Plasma-Induced Fabrication and Straining of MoS2 Films for the Hydrogen Evolution Reaction

“…MoS 2 thin films were synthesized by a process which we term plasma-enhanced chemical film conversion (PECFC), similar to that previously reported for h-BN . Ammonium tetrathiomolybdate (ATM, Sigma-Aldrich) was first dissolved in 10 mL of dimethylformamide (DMF, Fisher Scientific) at 0.1 M with 0.1 g of 10000 MW linear polyethylenimine (L-PEI, Sigma-Aldrich) and sonicated at 60 °C for 1 h to completely dissolve the polymer.…”

Plasmas for additive manufacturing

“…To address these limitations, a plasma‐based process known as plasma‐enhanced chemical film conversion (PECFC) has been developed and successfully used to obtain printable h‐BN at temperatures as low as 650°C on noncatalytic substrates. [ 138 ] The process involves the deposition of an ammonia borane precursor either by spin casting, spray coating, or inkjet printing, followed by atmospheric pressure DBD treatment in either Ar or a gas mixture of Ar and H 2 and simultaneous external heating of the substrate; a photo of an inkjet printed and converted film and its corresponding micro‐Raman spectral intensity map confirming h‐BN are shown in Figure 11a. Materials analysis showed that the crystallinity of the films grown by PECFC at 650°C are comparable to films grown by CVD on catalytic substrates at >1,000°C.…”

“…(b) Schematic diagrams of molybdenum disulfide (MoS 2 )‐based FET devices with and without plasma‐converted h‐BN layers (left) and corresponding I ds – V g transfer curves showing higher carrier mobility with h‐BN (right). (a and b) reprinted with permission from Liu et al [ 138 ] (c) Micro‐Raman spectra of MoS 2 films synthesized by thermal and plasma conversion at 500°C (left) and atomic force microscopy image of MoS 2 film synthesized by plasma conversion after a second thermal annealing step at 1,000°C, reprinted with permission from Liu et al [ 139 ] The spectra for the precursor before conversion is included for reference. The E 1 2g and A 1g peaks indicative of MoS 2 are only observed in the plasma‐converted film…”

Wafer‐scale vertical van der Waals heterostructures

“…[73][74][75] As long as the stable and homogeneous nanosheet ink is arranged (the ink applicability is judged by parameter Z, affected by ink viscosity, surface tension, density and nozzle diameter 76,77 ), the large-scale vdWHs arrays can be fabricated simply by printing the materials layer by layer through a printer. 78,79 A vertical Schottky barrier (SB) transistors array was successfully constructed in this way by printing indium−tin−oxide (ITO), indium−gallium−zinc−oxide (IGZO), reduced graphene oxide (rGO) and ion gel layer on a 4-in. Si wafer in sequence ( Figure 1E,F).…”