Direct, Transfer-Free Growth of Large-Area Hexagonal Boron NitrideFilms by Plasma-Enhanced Chemical Film Conversion (PECFC) ofPrintable, Solution-Processed Ammonia BoraneTianqi Liu,† Kasun Premasiri,‡ Yongkun Sui,§ Xun Zhan,# Haithem A. B. Mustafa,∥,⊥ Ozan Akkus,∥,⊥

Christian A. Zorman,§ Xuan P. A. Gao,‡ and R. Mohan Sankaran*,†

†Department of Chemical and Biomolecular Engineering, ‡Department of Physics, §Department of Electrical Engineering andComputer Science, ∥Department of Mechanical and Aerospace Engineering, and ⊥Center for Applied Raman Spectroscopy, CaseWestern Reserve University, Cleveland, Ohio 44106, United States#Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois atUrbana−Champaign, Urbana, Illinois 61801, United States

*S Supporting Information

ABSTRACT: Synthesis of large-area hexagonal boron nitride (h-BN) films for two-dimensional (2D) electronic applicationstypically requires high temperatures (∼1000 °C) and catalyticmetal substrates which necessitate transfer. Here, analogous toplasma-enhanced chemical vapor deposition, a nonthermal plasmais employed to create energetic and chemically reactive states suchas atomic hydrogen and convert a molecular precursor film to h-BN at temperatures as low as 500 °C directly on metal-freesubstratesa process we term plasma-enhanced chemical filmconversion (PECFC). Films containing ammonia borane as aprecursor are prepared by a variety of solution processing methodsincluding spray deposition, spin coating, and inkjet printing and reacted in a cold-wall reactor with a planar dielectric barrierdischarge operated at atmospheric pressure in a background of argon or a mixture of argon and hydrogen. Systematiccharacterization of the converted h-BN films by micro-Raman spectroscopy shows that the minimum temperature fornucleation on silicon-based substrates can be decreased from 800 to 500 °C by the addition of a plasma. Furthermore, thecrystalline domain size, as reflected by the full width at half-maximum, increased by more than 3 times. To demonstrate thepotential of the h-BN films as a gate dielectric in 2D electronic devices, molybdenum disulfide field effect transistors werefabricated, and the field effect mobility was found to be improved by up to 4 times over silicon dioxide. Overall, PECFC allowsh-BN films to be grown at lower temperatures and with improved crystallinity than CVD, directly on substrates suitable forelectronic device fabrication.

KEYWORDS: boron nitride (BN), two-dimensional (2D) material, plasma, chemical vapor deposition (CVD)

■ INTRODUCTION

Hexagonal boron nitride (h-BN) is an insulating material withexcellent chemical, mechanical, and thermal properties.Combined with its layered structure and potential to beproduced as an atomically flat and thin film, h-BN is ideal forintegration with two-dimensional metallic and semiconductingmaterials in electronic devices.1 Encapsulation by h-BN allowsdevices to be isolated from the environment and has beenshown to provide excellent protection to thermal2 andphotodegradation.3 In addition, because of its close latticematch (sometimes termed “white graphene”), h-BN has beenemployed as a substrate to epitaxially grow high-qualitygraphene.4,5

Thin films of h-BN have been typically produced by twoapproaches: exfoliation from a bulk crystal via mechanicalcleavage6 or sonication in liquid7,8 and chemical vapordeposition (CVD).9,10 Exfoliation is capable of producing

high quality single crystals of layered materials, but the size ofmaterial produced is relatively small (<10 mm), the yield ispoor, and the process is not controllable with respect to thethickness or number of layers. While methods such as pulsedlaser deposition,11 reactive magnetron sputtering,12 and ionbeam sputtering have been reported for large-area andthickness-controlled growth,13 CVD remains the mostdesirable scalable synthesis technique.14 Initial reports ofCVD of h-BN employed metals as a catalytic substrateincluding nickel (Ni),9 copper (Cu),10,15 platinum (Pt),16 andiron (Fe).17 For electronic applications, this necessitatestransfer to a suitable substrate that introduces mechanicaldeformation18 and chemical impurities19 on the h-BN surface.

Received: October 1, 2018Accepted: November 21, 2018Published: November 21, 2018

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.8b17152ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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A smaller number of studies have demonstrated metal-freegrowth of h-BN films directly on Si-based substrates.20,21

However, adsorption of the vapor precursor, nucleation, andgrowth rate are sensitive to the substrate, leading to poorcrystallinity.22

Here, we report the metal-free growth of large-area,continuous, highly crystalline h-BN films by a plasma-assisted,precursor-film conversion process which we term plasma-enhanced chemical film conversion (PECFC). In contrast toCVD, a molecular precursor for h-BN, ammonia borane, isinitially deposited on a substrate from solution and thenconverted which circumvents barriers associated withadsorption and nucleation. A similar strategy was reported byPark et al. using borazine oligomer as a precursor film andthermally converting at 1026 °C but required a metal (Ni)substrate.23 Our approach is highly versatile, and we show thatfilms can be prepared by a range of solution processingmethods including spray deposition, spin coating, and inkjetprinting to control the film thickness and geometry. Followingdeposition, the films are converted by plasma-assisted heatingin a home-built, cold-wall reactor equipped with a planar,atmospheric-pressure dielectric barrier discharge (DBD).Analogous to plasma-enhanced CVD,24 plasma-enhancedatomic layer deposition,25,26 and plasma-enhanced molecularbeam epitaxy,27,28 the nonthermal plasma excites anddissociates the gas to create energetic and reactive speciesthat interact with the film and lower the growth temperature.Systematic comparison of films synthesized by PECFC and bythermal conversion alone using Raman spectroscopy show thatthe minimum temperature for h-BN nucleation is decreasedfrom 800 to 650 °C in a background of argon (Ar) and to 500°C in a mixture of Ar and hydrogen (H2). Moreover, theplasma increases the crystallinity of the synthesized h-BN, asassessed by the full width at half-maximum (FWHM) of theE2g Raman scattering peak which decreases from >40 cm−1 toas low as 13 cm−1 and atomic resolution scanning transmissionelectron microscopy (STEM). We suggest that a key role ofthe plasma is the generation of atomic hydrogen (H),confirmed by optical emission spectroscopy, which throughsurface reactions such as hydrogen abstraction and strain reliefenables nucleation at lower temperatures and growth of highcrystalline quality h-BN. The average dielectric constant of theh-BN films was measured using metal−insulator−metal devicesto be 3.8, comparable to bulk values. Molybdenum disulfide(MoS2) field effect transistors (FET) fabricated with PECFCh-BN films exhibited carrier mobilities 4 times higher thansilicon dioxide as the gate dielectric. These properties arecomparable to h-BN grown on metal substrates and the bestreported to date for metal-free substrates.

■ EXPERIMENTAL SECTIONDeposition of Ammonia Borane Precursor Film. Three

approaches to preparing ammonia borane films from solution wereutilized: spray coating, spin coating, and inkjet printing. In all cases,ammonia borane, which is a molecular precursor and a solid powderat room temperature, was completely dissolved in polar solvents atconcentrations well below its solubility limit of >5 M. For spraycoating, ammonia borane (Sigma-Aldrich, 97% purity) was dissolvedin ethanol (Fisher Scientific, 200 proof) at 0.1 M. The spray coatingsetup consisted of a simple commercial airbrush (Master ModelG233) powered by compressed Ar gas at 12 psi and a substrate placedon a hot plate heated to 60 °C, separated by a distance of 15 cm. Forspin coating, ammonia borane was dissolved in dimethylformamide(Fisher Scientific) at various concentrations from 0.075 to 0.25 M

with average Mn 5000 linear poly(ethylenimine) (Sigma-Aldrich) andsonicated at 50 °C for 1 h to completely dissolve the polymer. Thespin coater was typically operated at 2000 rpm for 10 s and then 3000rpm for 50 s, and following spin coating, films were dried in airovernight. For inkjet printing, 0.031 g of ammonia borane wasdissolved in a mixture of 10 mL of ethanol and 3 mL of ethyleneglycol (Fisher Scientific). The ethylene glycol was needed to adjustthe viscosity and measured to be 10−12 cP (Brookfield DV-E).Before printing, the solution was filtered through a 0.5 μm syringefilter to remove any precipitate. Printing was performed with apiezoelectric inkjet printer (DMP 3000, Fujifilm Dimatix) equippedwith 16 nozzles. Requisite patterns were preprogrammed, and thedrop size, drop spacing, nozzle driving waveform, and number of printlayers could be tailored from the computer. The typical substrate forall deposition approaches was Si which was pretreated in an O2plasma at 150 W for 90 s to remove organic residue and improve thesurface wettability.

Synthesis of h-BN by PECFC. PECFC was performed in ahomemade atmospheric-pressure reactor consisting of a DBD andcold-wall substrate heater. The DBD was formed in a planar electrodegeometry between a 2 in. diameter powered aluminum electrodecovered by a ceramic and a grounded aluminum electrode, separatedby a gap of 3 mm. The reactor was backfilled with either Ar or an80:20 mixture of Ar and H2 by continuously flowing gas at a flow rateof 100 sccm, controlled by digital mass flow controllers, beginning atleast 1 h before each experiment to purge and remove background air.The plasma was ignited and sustained by high-voltage alternativecurrent (Model PVM500, Information Unlimited) typically at 20−30kHz and 8.5 kV peak-to-peak voltage (Figure S1, SupportingInformation). To heat the substrate, a 1 in. diameter microheater(MHI, Inc.) served as the bottom electrode, and the temperature wascontrolled by a variable ac voltage regulator and measured by a hightemperature infrared thermometer (Model 42545, Extech). In thisstudy, all films were converted with or without plasma for 1 h.

Thin Film Characterization. The synthesized h-BN films werecharacterized by atomic force microscopy (AFM) (Dimension 3100,Veeco), micro-Raman spectroscopy (LabRam HR800, Horiba Jobin-Yvon, 633 nm laser excitation), scanning electron microscopy (SEM)(Helios Nanolab 650, FEI), optical profilometry (NewView7300,Zygo), X-ray diffraction (XRD) (Discover D8, Bruker, Co source),transmission electron microscopy (TEM) (Tecnai F30, FEI, 300 kV),STEM (Themis Z advanced probe aberration corrected analyticalSTEM, Thermo, 300 kV), and electron energy loss spectroscopy(EELS) (Tecnai F30, FEI, 300 kV). Wide-field Raman imaging of apatterned h-BN film was performed by a custom-made system thatfiltered photons specific to the E2g peak of h-BN at 1372 cm−1 using akinematically tunable band-pass filter. The field of view wasilluminated by a 532 nm laser (GSLR-532-2.5W, Lasermate Group,Inc.) that provided a near flat-top profile and an image was capturedby a high quantum efficiency 2D CCD (iKon-M 934, Andor). Thedifference between this peak image and a baseline image, which wasobtained with a band-pass filter at 1248 cm−1 chosen near the valleyof the E2g peak, resulted in a final Raman image of the E2g intensity.Each image was averaged from 15 scans with a 10 s integration timefor a total acquisition time of 150 s.

For XRD and micro-Raman characterization, h-BN films weresynthesized by converting drop-casted films of 0.1 M ammonia boranein ethanol on Si and quartz substrates, respectively. For TEM, h-BNfilms were synthesized by converting spin-coated ammonia boranefilms on Si substrates and transferred to a carbon-coated Cu TEMgrid by etching away the Si substrate in 30 wt % KOH solution for 3 hand picking up the floating, free-standing film.

Metal−Insulator−Metal (MIM) Capacitor Device Fabricationand Analysis. Metal−insulator−metal (MIM) capacitor devices tomeasure the relative dielectric constant (εr) were fabricated as follows:First, h-BN films were synthesized by PECFC or thermal conversionfrom ammonia borane films spin coated onto gold-coated Sisubstrates. A second metal electrode (50 nm Al) was then evaporatedon top of the h-BN films by physical vapor deposition (PVD) througha mask with 50 × 50 μm2 openings. The capacitances were measured

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from multiple devices across the film by a LCR meter (ModelE4980A, Keysight Technologies).MoS2 FET Device Fabrication and Characterization. MoS2

FET devices were fabricated on h-BN films synthesized by PECFCdirectly on nm p-doped Si substrates with 300 nm thick SiO2. Few-layer MoS2 flakes were obtained by mechanical exfoliation usingScotch tape. After transferring to the h-BN surface, metal contacts(Ni/Ti 60/25 nm) were deposited by electron beam evaporation(Angstrom Engineering Evovac) through glider TEM grids (Ted PellaG200HS) which served as stencil masks.Electrical measurements were performed at ambient conditions

with a Lakeshore probe station, a Keithley 228A voltage/currentsource, and a Stanford Research Systems SR570 low-noise currentpreamplifier. Standard dc techniques were used to obtain current−voltage and gate transfer curves.

■ RESULTS AND DISCUSSION

Figure 1a schematically illustrates the process flow that wasdeveloped for PECFC of h-BN. Ammonia borane was chosenas the precursor because it is nontoxic and nonflammable and,thus, a safe compound, and serves as a single moleculeprecursor for h-BN having the prerequisite 1:1 B:Nstoichiometry. Previously, h-BN thin films have beensynthesized using ammonia borane by CVD, with ammoniaborane heated and brought into the reactor as a vapor.10,22,29

The ability to pyrolytically decompose ammonia borane to h-BN has also been known for some time, but as a powder.30

Here, we prepared the ammonia borane as a thin film bysolution processing methods and then converted it by acombination of plasma treatment and heating. Three differentapproaches to film deposition were explored: (1) spraydeposition, (2) spin coating, and (3) inkjet printing. Thesolution containing ammonia borane was slightly altered ineach of these cases to enable deposition. Spray deposition wasthe simplest with ammonia borane dissolved in pure ethanol.In the case of spin coating, a polymer, linear polyethylenimine(L-PEI), was critical to act as a binder and obtain a continuousfilm; the polymer was in situ removed by the conversionprocess as long as the temperature was >400 °C.31 Inkjetprinting using a piezoelectric head required ethylene glycol tosufficiently increase the viscosity. Other approaches todeposition from solution should also be possible with verylittle restriction on the solvent, other than solubility, andsubstrate, other than temperature, reflecting the versatility ofthis general methodology. Following preparation of theammonia borane film, the conversion to h-BN was performedby treating with a planar atmospheric-pressure dielectric barrierdischarge (DBD) while heating the substrate in a home-built,cold-wall reactor. The size of the substrates was only limited byour DBD, which is an easily scalable plasma with commercialapplications in large-area and roll-to-roll surface treatment.32

Figures 1b and 1c show optical images of two 2 × 1 cm2 Sisubstrates with ammonia borane films deposited by spincoating before and after PECFC, respectively. The slightlywhite appearance of the film after PECFC is indicative of h-BNformation. Optical images of spray-coated films show similarcontinuous, large-area coverage and color change followingPECFC (Figure S2). By adjusting the concentration ofammonia borane in the spin-coating solution, we could controlthe converted film thickness from ∼3 to 100 nm (Figure S3).SEM images of spray-coated and spin-coated films convertedby PECFC are shown in Figures S4 and S5. Figure 1d showsan AFM image of a converted spin-coated film that has athickness of 2.8 nm. We note that spin coating produced the

thinnest and smoothest films, followed by spray deposition(>100 nm) and inkjet printing (>10 μm). To confirm theconversion of ammonia borane to h-BN and assess thecrystallinity, the films were characterized by XRD. The XRDspectra in Figure 1e of the films before and after PECFC showthat the as-deposited ammonia borane, which exhibits itscharacteristic (110), (101), and (211) peaks, is completelyconverted to h-BN, which exhibits its characteristic (002) and(004) peaks. The strong presence of only these peakscorresponding to h-BN indicates that the multilayers in theplasma-converted film are well-aligned.17 Figure 1f shows anoptical image of a patterned film in the shape of the letter “C”standing for Case Western Reserve University produced by

Figure 1. (a) Schematic illustration of process flow for synthesis of h-BN films by PECFC. The ammonia borane precursor was depositedfrom solution by one of three approaches: (1) spray coating using anairbrush, (2) spin coating, or (3) inkjet printing using a piezoelectrichead. Optical images of ammonia borane films deposited on Sisubstrates by spin coating (b) before and (c) after PECFC. (d) AFMimage of h-BN film synthesized by spin coating and PECFC in Ar/H2at 800 °C. (e) XRD of ammonia borane film deposited on quartzsubstrates before and after PECFC. (f) Optical image of h-BN filmsynthesized by inkjet printing as a letter “C” and PECFC in Ar/H2 at800 °C. (g) Background corrected Raman mapping image of the E2gpeak in the field of view shown in (f).

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PECFC of inkjet-printed ammonia borane. The correspondingRaman mapping image of the E2g peak characteristic of h-BNin Figure 1g (see Figure S6 for details) confirms thatcontrolled patterns of h-BN can be directly (nonlitho-graphically) obtained.To more carefully assess the crystalline quality of the

PECFC-grown h-BN, we performed micro-Raman spectrosco-py and systematically compared plasma-assisted heating andheating alone. Raman spectroscopy provides clear evidence ofammonia borane from its prominent scattering peaks at 782,2281, 2378, and 3254 cm−1 and h-BN from its scattering peakat 1366 cm−1 corresponding to the in-plane optical phononmode with E2g symmetry (Figure S7). Figure 2a shows a seriesof representative micro-Raman spectra of ammonia boranefilms deposited by drop casting on Si substrates afterconversion in an Ar background at different temperatureswith and without plasma treatment. Additional spectracollected from multiple spots on each sample are shown inFigure S8. We note that in agreement with XRD the Ramanspectra did not show any peaks corresponding to ammoniaborane. In addition, the presence of the Raman scattering peakcorresponding to the underlying Si substrate indicated that theentire film was probed (see, for example, Figure S6),confirming that the ammonia borane was converted through-out the film depth in all of our processed samples. In theabsence of a plasma, the E2g peak appears at temperatures >800°C. Previous reports for CVD-grown h-BN have shown thatnucleation required >900 °C on Si-based substrates,22 which ismoderately higher than metal substrates, which can nucleate at∼750 °C,33 because of the lower catalytic activity of nonmetals.We suggest that the slightly lower minimum temperatureachieved here is because there is no adsorption barrier bybeginning with ammonia borane on the substrate. Based on thepeak intensity which is proportional to the film thickness,34 theconversion of ammonia borane to h-BN also exhibits thickerfilm growth than CVD of h-BN from ammonia borane on Si-based substrates.21,22 Further lowering of the minimumtemperature for h-BN nucleation to 650 °C and increase inthe peak intensity at a given temperature (800 °C) areobserved for PECFC. The peak also appears to be narrower infilms grown by PECFC than thermal conversion, which isrelated to the film crystallinity and will be discussed inadditional detail. The conversion of ammonia borane films toh-BN was also performed in a background mixture of Ar andH2 (20%) (Figure 2b and Figure S9). Adding H2 did notchange the minimum temperature for h-BN nucleation bythermal conversion but, in the case of PECFC, decreased evenmore to 500 °C. Moreover, the peak intensity was found toincrease at comparable temperatures (650 °C) (compareFigure 2b to Figure 2a).The position and shape of the E2g Raman peak are related to

the h-BN film structure (i.e., crystallinity) and thickness. As thethickness decreases, the peak shifts upward (>1366 cm−1). Wenote that our films were all significantly thicker than where thisshift would occur (<3−4 layers).26 As previously stated, thepeak intensity is also related to the film thickness but is affectedby crystallinity as well. Alternatively, the FWHM has beenfound to show minimal correlation with film thickness,34 butagain only for very thin films,35 and can be used to obtaininformation about film crystallinity.20 In support, we comparedRaman spectra for films prepared by drop casting with spincoating which are significantly thinner (∼80 nm) and foundthat while the peak intensity decreased as expected, the

FWHM was unchanged (Figure S10). The relationshipbetween crystallinity and FWHM is attributed to the domainsize of the crystallites and for h-BN is given by La = 1417/ Γ1/2− 8.7 where La is the domain size and Γ1/2 is FWHM of the E2gRaman peak.36 Therefore, larger crystalline domains lead tonarrower peaks or smaller FWHM. Figure 2c summarizes ouranalysis of the E2g peak FWHM for all of the h-BN filmsconverted by PECFC and heating alone at different temper-atures in Ar and in Ar/H2. The FWHM for thermallyconverted films are found to fall between ∼26 and 61 cm−1,while PECFC films are significantly reduced to between ∼13and 19 cm−1, indicating that the addition of a plasma increases

Figure 2. Representative micro-Raman spectra of h-BN filmssynthesized by PECFC (blue) and thermal conversion (red) in abackground of (a) Ar and (b) Ar and H2. (c) Summary of the FWHMof the E2g Raman peaks for PECFC (blue) and thermally converted(red) h-BN films in Ar (solid) and Ar and H2 (open). The error barscorrespond to the standard error calculated from the first standarddeviation by collecting multiple spectra at different locations on thefilms. Literature values of FWHM reported for CVD-grown h-BN onmetal and Si substrates are included for comparison. Reference detailsare provided in Table S1.

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the average crystallite size (Table S1). For both thermalconversion and PECFC, increasing the temperature and addingH2 are found to enhance film crystallinity. The lowest achievedFWHM of 13 cm−1 for PECFC at 800 °C in Ar/H2 comparesfavorably with the best literature values reported to date forCVD-grown h-BN on metal substrates (∼7−21 cm−1) and isremarkably lower than CVD-grown h-BN on Si substrates(32−44 cm−1).The role of a nonthermal plasma in the conversion process is

to create energetic and reactive species in the gas phase thatinteract with the ammonia borane film. The energy of thespecies can be much higher than the background temperatureand thus enhance h-BN nucleation and growth at lowertemperatures than required by heating alone, supporting ourobservations by Raman characterization. To characterize thespecies produced by the plasma, we implemented opticalemission spectroscopy (OES), which is a nonintrusivetechnique that relies on radiative transitions. Figure 3a showsemission spectra collected from the DBD formed at the twoprocess conditions studied: in Ar and a mixture of Ar and H2.We note that the light was collected near the center of thedischarge and is representative of the plasma volume withoutany spatial significance. The intense lines observed in Arbetween ∼700 and 800 nm correspond to Ar neutrals

(metastables). No lines corresponding to Ar ions which aretypically between ∼400 and 500 nm were detected. This is notsurprising because of the high operating pressure of 1 atm,which leads to quenching of the more energetic Ar ions.37,38

Additionally, the DBD is inhomogeneous, made up offilamentary microdischarges ∼100 μm in diameter that arerandom in space and time.39,40 The Ar ions are most probablycontained in these bursts which were not captured by oursteady-state optical system. The spectrum for Ar/H2 shows onekey difference: the appearance of a line at 654 nmcorresponding to Hα, the most intense of the emissionspectrum of atomic hydrogen in the visible known as theBalmer series.Combining the OES and Raman results, we present the

following picture for conversion of ammonia borane films to h-BN. Figure 3b shows the thermal decomposition mechanismthat has been previously reported for ammonia borane.30 Forboth the thermally converted and PECFC films, the temper-atures are higher than 500 °C, and the films should decomposesimilar to the first two steps, first forming polyiminoborane andsecond forming polymimonoborane, evolving H2 in both steps.The key difference lies in the third step to form semicrystallineh-BN that was previously found to require 1150−1500 °C.Here, we find that h-BN nucleation by heating alone occurs at

Figure 3. (a) Optical emission spectra of DBD formed in Ar (black) and Ar/H2 (red). The line corresponding to atomic hydrogen (Hα) isindicated. (b) Previously reported reaction mechanism for thermal decomposition of ammonia borane30 with the potential role of atomic hydrogen(H) in the final step of conversion to h-BN indicated in red. (c) Illustration of potential mechanism for nucleation and growth of polycrystalline h-BN from nanoparticles (top left image) to larger and larger crystal domains (top middle and top right images). Potential key steps involving Hincluding abstraction (bottom left) and insertion (bottom middle) reactions that lead to highly crystalline h-BN (bottom right) are also shown.

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a lower temperature of 800 °C. We note that our conversionwas performed on thin films whereas the previous study wasfor a bulk powder of ammonia borane. Similar to metals inCVD of h-BN, there may be a small catalytic effect of thesubstrate. Nucleation is further enhanced by the addition ofplasma which lowers the temperature to 650 °C in Ar and 500°C in Ar/H2. In Ar alone, we suggest that Ar ions andmetastables are involved in bond breaking and forming,particularly in the third step to dissociate polyaminoboraneand evolve hydrogen. Low-energy ions (<20 eV) have beenpreviously shown to be capable of displacing lattice atoms andtransferring energy to aid in crystallization of thin films whileavoiding undesirable sputtering.41,42 When H2 is added, H mayplay a critical role in nucleating and growing h-BN. Theimportance of H in thin film growth is general and well-known.Two examples that we refer to in this context are CVD ofsilicon and diamond. In the case of Si, H has been shown tofacilitate crystallization at lower temperatures by abstraction ofhydrogen, either from the surface or from silicon hydrides withovercoordinated Si atoms, and insertion into strained Si−Sibonds.43 In the case of diamond, the growth is morecomplicated because the formation of diamond competeswith the formation of non-diamond (graphitic) carbon.Nonetheless, H is similarly involved in abstraction of hydrogento create surface radical sites that nucleate diamond as well asstabilize the diamond nuclei44 by suppressing surfacereconstruction45 and remove (etch) the non-diamond nuclei.46

The above description can be summarized by the followingproposed mechanism which is illustrated in Figure 3c: (i)nucleation of h-BN nanoparticles via decomposition ofpolyiminoborane and, importantly, hydrogen abstraction byeither heating alone for thermal conversion, or Ar ions,metastables, and/or H for PECFC; (ii) growth of the h-BNnuclei into larger crystals aided by heating alone for thermalconversion, or Ar ions, metastables, and/or H for plasmaconversion. The role of H is of particular importance since itleads to the lowest minimum temperature for h-BN nucleationand the largest crystallite size. In (i), hydrogen abstraction byH could allow the removal of hydrogen at lower temperaturesand the creation of free radicals that promote nucleation of h-BN. In (ii), H could interact with the already formed h-BNcrystal nuclei through a combination of abstraction andinsertion reactions. Analogous to CVD of Si and diamond,these reaction pathways could promote amorphous tocrystalline transitions and the growth of larger crystallitesizes in the films grown by PECFC. We note that H2 could alsohave other effects on film conversion such as reducingoxidation which could explain its impact on the size of thegrown crystals for thermal conversion (see Figure 2c).However, the highly reactive H has a stronger effect whichmay explain why there is no change in the minimumtemperature for h-BN nucleation with the addition of H2 inthermal conversion. This also suggests that H is moreimportant for nucleation (i) than growth (ii), which iscorroborated by the stronger effect it has on temperature ascompared to crystallite size (see Figure 2c). A similar argumenthas been made in the growth of Si where nucleation has anactivation energy of 5 eV while growth has an activation energyof 2.7 eV, and thus, plasma treatment and the generation of Hare more important to creation of seed nuclei than the growthof the larger crystal from the seeds.47

To assess the surface flatness of our films, our smoothestfilms prepared by converting spin-coated solutions were

characterized by AFM. Figure 4 shows representative AFMimages of h-BN synthesized by PECFC in Ar/H2 at 500 °C

(Figure 4a), PECFC in Ar/H2 at 800 °C (Figure 4b,c), andthermal conversion in Ar/H2 at 800 °C (Figure 4d). Thesurface morphologies of the two PECFC films are comparable,with the surface roughness slightly reduced from a RMS valueof ∼1 to 0.5 nm by increasing the growth temperature from500 to 800 °C. Additionally, we synthesized films on a maskedSi substrate and measured the thickness at multiple locationsalong the film edge. The thicknesses were found to vary by0.06 nm (Figure S11). The flatness confirms the high quality ofthe PECFC films and are similar to the lowest report for CVD-grown h-BN films which is important for electronic deviceapplications.17 In comparison, the thermally converted h-BNfilms were found to be much rougher characterized by a RMSroughness value of ∼20 nm.The atomic-scale morphology and structure of h-BN films

synthesized by PECFC and thermal conversion were examinedby TEM and STEM. In agreement with AFM analysis, thefilms were highly continuous, stable to transfer, and free-standing, permitting suspension on TEM grids (Figure S12a).Low-magnification TEM images of the PECFC films showedcrystals as large as ∼50 nm (Figure S12b). The interlayerdistance was measured to be 0.34 nm from a cross-sectionalTEM image, close to the expected value of bulk h-BN (FigureS12c).48 The high-magnification STEM images in Figures 5aand 5b of PECFC films grown at 500 and 800 °C, respectively,reveal regions containing lattice fringes with hexagonalarrangement that are relatively disorder-free and continuous,confirming the high quality of our h-BN films and supportingRaman analysis. The selected area electron diffraction (SAED)in the inset of Figure 5b for the PECFC film grown at 800 °Cdisplays a single hexagonal pattern, indicating that the films arehighly ordered. The (−1100) and (01−10) lattice planes couldbe identified in the (0001) zone axis from the SAED. Incomparison, the STEM image of a thermally converted film inFigure 5c reveals poor crystallinity, characterized by nano-meter-sized grains (<5 nm) surrounded by amorphous regions,similar to previous reports of metal-free h-BN growth.22 TheSAED pattern shown in the inset is consistent with apolycrystalline film. The trend in the crystal sizes betweenPECFC and thermally converted films is generally consistentwith the FWHM analysis of the Raman spectra (Table S1).However, the sizes of the crystalline regions observed in TEM

Figure 4. AFM images of h-BN films synthesized by (a) PECFC inAr/H2 gas at 500 °C, (b, c) PECFC in Ar/H2 gas at 800 °C, and (d)thermal conversion in Ar/H2 gas at 800 °C.

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(∼50 and ∼5 nm for PECFC and thermally converted,respectively) is smaller than the domain sizes estimated fromthe FWHM (∼100 and ∼25 nm for PECFC and thermallyconverted, respectively), indicating that the crystallites thatmake up the film could break up during the transfer process.To confirm the composition of the film, electron energy lossspectroscopy (EELS) was carried out. The representativeEELS spectrum in Figure 5d clearly shows the presence of theB and N K-shell ionization edges at ∼190 and ∼400 eV,respectively. We estimated the stoichiometry of boron andnitrogen from the integrated areas of the corresponding peaksto be 0.93, which is reasonably close to the expected 1:1 ratio.The distinct π* and σ* energy-loss peaks confirm that the filmcontains sp2-hybridized bonds, and the sp3:sp2 ratio was

evaluated by the “two-window intensity ratio” method49 to be98% sp2, confirming the h-BN phase.The dielectric properties of the PECFC films were evaluated

by fabricating metal−insulator−metal (MIM) capacitorstructures to obtain the relative dielectric constant, εr. Figure6a shows a schematic diagram of the MIM device whichconsisted of gold-coated Si as one metal electrode, the h-BNfilm which was directly synthesized on top of the electrode byPECFC of a spin-coated ammonia borane film, and an Al layerevaporated on top of the h-BN film by physical vapordeposition (PVD). Devices with varying thicknesses of h-BNwere synthesized by adjusting the ammonia borane concen-tration in the spin-coating solution (Figure S3), and the filmthickness was measured after conversion by either profilometry(Figure S13) or AFM. The averages of the capacitancesmeasured from multiple devices as a function of h-BNthickness for PECFC and thermal conversion, both grown inAr/H2 at 800 °C, are shown in Figure 6b. Two sets of errorbars are shown: the horizontal error bars correspond todifferences in the film thickness at different locations and thevertical error bars correspond to differences in the capacitancemeasurements from multiple devices. By fitting the data inFigure 6b (see the Supporting Information for details), we canextract an average dielectric constant, εr, of 3.8 for h-BNsynthesized by PECFC and 2.8 for thermally converted h-BN.The significantly higher relative dielectric constant with theaddition of a plasma is consistent with the improved crystallinestructure of the h-BN films found by Raman analysis. Thesevalues are comparable to those previously reported for bulksingle-crystals of h-BN of ∼3−450 and CVD h-BN of 2−5.17,51To demonstrate the potential of PECFC h-BN films for

electronic device applications with 2D materials, MoS2 deviceswere fabricated. Figure 7a shows a schematic diagram of theFET device structure (bottom) and a correspondingrepresentative AFM image (top). A 2.8 nm thick h-BN filmwas first synthesized by PECFC in Ar/H2 at 800 °C from aspin-coated solution of ammonia borane on a 300 nm SiO2/p++ Si substrate. A few layered MoS2 flake was then mechanicallyexfoliated and transferred onto the h-BN film. Ni/Ti contactswere finally lithographically deposited by electron beamevaporation. We also fabricated MoS2 FETs without h-BNon the SiO2/Si substrate for comparison. MoS2 FETs withthermally converted h-BN films could not be fabricatedbecause of their large roughness (Figure S14).

Figure 5. STEM annular bright field (ABF) images of h-BN filmssynthesized by (a) PECFC in Ar/H2 gas at 500 °C, (b) PECFC inAr/H2 gas at 800 °C, and (c) thermal conversion in Ar/H2 gas at 800°C. Insets in (b) and (c) show the corresponding selected areaelectron diffraction (SAED) patterns. (d) EELS showing B and N K-edges consisting of π* and σ* peaks.

Figure 6. (a) Schematic diagram of metal−insulator−metal (MIM) capacitor device consisting of gold-coated Si, h-BN, and Al used to measurerelative dielectric constant (εr) of synthesized h-BN films. (b) Capacitance of MIM devices as a function of h-BN film thickness for PECFC andthermal conversion, both grown in Ar/H2 gas at 800 °C.

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The linear output curves (Ids vs Vds) in Figure 7b confirmthat Ohmic contacts were formed between the Ti metal layerunderlying Ni and the MoS2 films in devices with h-BN. Theapplication of a gate voltage is found to change theconductance as expected and is consistent with n-typesemiconductor behavior, which has been linked in MoS2 totrapped charge in the form of adsorbates or defects at the gateinsulator interface.52,53 Figure 7c shows a comparison ofrepresentative transfer curves (Ids vs Vg) for MoS2 FETs on h-BN and on only SiO2. The inclusion of h-BN is found toincrease both the conductance and transconductance by morethan an order of magnitude because of reduced Coulombscattering from the charge traps in SiO2 as has been previouslyreported.54,55 Field-effect mobilities of 4−5 devices (FigureS15a,b) were extracted from the transfer curves (see theSupporting Information for details), and a histogram is shownin Figure 7d. We used an average dielectric constant, εr, of 3.8obtained from our capacitance measurements for h-BN and 3.9for SiO2. It should be noted that as the thickness of h-BNapproaches a monolayer, the dielectric constant increases;56

however, for the multilayer films with a thickness of 2.8 nmused here, the dielectric constant would increase by only ∼30%and change the mobility by only ∼0.2%. The average mobilityfor devices with h-BN was estimated to be 14.4 ± 1.2 cm2 V−1

s−1, more than 4 times higher than the average mobility of 3.2± 0.4 cm2 V−1 s−1 for devices with only SiO2. The mobilityimprovement of MoS2 FETs on the PECFC h-BN films overonly SiO2 is comparable to previously reported devices withCVD-grown h-BN.17,57 We also note that the devices with h-BN qualitatively exhibited better reproducibility. Because thethickness of the h-BN film is only 2.8 nm, the gate capacitance,

Cgate, for the h-BN-covered SiO2 is only ∼1% lower than thatof the bare 300 nm SiO2 (note that Cgate = 1/(CSiO2

−1 +ChBN

−1)). Therefore, the improvement does not come from achange in gate capacitance and confirms an improved interfaceas compared to that between MoS2 and SiO2. We note that animportant distinction from previous work is that here the h-BNis grown directly (no transfer) on the SiO2, which reduces thenumber of device processing steps.

■ CONCLUSIONSIn summary, the combination of plasma-assisted heating andconversion of a precursor film has been demonstrated toproduce large-area, continuous, high crystalline quality h-BNfilms directly on metal-free substrates. The approach isfundamentally different from traditional CVD and circumventssubstrate interactions that can influence nucleation, growth,and adsorption, allowing the film thickness or geometry to becontrolled by a variety of deposition methods including spraycoating, spin coating, and inkjet printing. The generation ofenergetic species such as Ar ions and metastables and H in anonthermal plasma lowers the minimum growth temperatureto as low as 500 °C and increases the film crystallinity. Themethod is generic and could be applicable to other layeredmaterials as well such as MoS2

31 to similarly enable low-temperature, large-scale growth of 2D materials on arbitrarysubstrates.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b17152.

Figure 7. (a) Schematic diagram of MoS2/h-BN FET device. The inset shows an AFM image of a real device. (b) Ids−Vds characteristics of a MoS2/h-BN FET at various gate voltages from 0 to 100 V. (c) Ids−Vg transfer curves of MoS2/h-BN (red) and MoS2/SiO2 (green) FET devices. (d)Respective calculated carrier mobilities of MoS2/h-BN and MoS2/SiO2 devices.

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Charge−voltage waveforms for dielectric barrier dis-charge (DBD), optical images of spray-coated filmsbefore and after PECFC, spin-coated film thicknessbefore and after PECFC as a function of solutionprecursor concentration, SEM images of spray-coatedand spin-coated films, micro-Raman spectra of inkjetprinted films, micro-Raman spectra of ammonia boraneprecursor and bulk h-BN reference, micro-Ramanspectra from different spots on films processed in Arand Ar and H2 by PECFC and thermal conversion,micro-Raman spectra of dropcast vs spin-coated films,FWHM analysis of micro-Raman spectra, AFM analysisof film thickness, TEM analysis of films, opticalprofilometry of films, dielectric constant calculationfrom capacitor measurements, optical micrographs ofMoS2 FET devices, transfer curves of MoS2 FET devices,and FET mobility calculation (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail mohan@case.edu.

ORCIDYongkun Sui: 0000-0001-6994-7196R. Mohan Sankaran: 0000-0002-9399-4790NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Department of Energy BasicEnergy Sciences (DOE-BES) Grant DE-AC02-09CH11466.K.P and X.G acknowledge support by the National ScienceFoundation (NSF) under Grant DMR-1151534. H.A.B.M. andO.A. acknowledge support by NSF under Grant MRI-1531035.We thank J. Toth for help with setup and electricalcharacterization of the DBD, J. Cole and H. Ishida for helpwith micro-Raman spectroscopy, and S. Bhattacharya for helpwith AFM characterization. STEM imaging was carried out inthe Frederick Seitz Materials Research Laboratory at theUniversity of Illinois.

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