Synthesis of hexagonal boron nitride 2D layers using polymer derived ceramics route and derivatives

Hexagonal boron nitride (h-BN) is nowadays an increasingly attractive material, especially for two-dimensional material applications, due to its intrisic properties. However, its properties are highly dependent on the used synthesis approach. The polymer derived ceramics (PDCs) route allows elaboration of h-BN with tailored textural and structural properties. Here, we demonstrate the interest of the PDCs pathway for the synthesis of h-BN. Growth of h-BN single crystals with crystal sizes of a few microns at relatively low temperature and atmospheric pressure is successfully achieved from borazine precursor using PDCs. The crystallization is improved by additivation of 5 wt% of Li3N to the pre-ceramic polymer. Furthermore, by coupling PDCs with gas pressure sintering, starting from the same pre-ceramic polymer and 25 wt% of Li3N, the crystal size is enlarged up to hundreds of microns. The fabricated single crystals of pure h-BN can then be exfoliated into h-BN nanosheets. Finally, by combining PDCs with atomic layer deposition, functional BN nano-/hetero-structures are successfully synthesized from highly structured sensitive templates, making this ALD process a promising alternative for fabricating functional BN nanostructures.


Hexagonal boron nitride (h-BN)
Until its discovery as natural state in 2014 [1], boron nitride (BN) has only been known as a synthetic ceramic. It presents an elemental B/N ratio of 1. BN is isoelectronic of carbon with its constituents belonging to III and V groups. As carbon, BN displays a variety of structures, such as hexagonal (h-BN), cubic (c-BN), wurtzite (w-BN) and amorphous phases [2]. w-BN and c-BN are both sp 3 structures, the latter being the hardest material just after diamond, while h-BN evidences a sp 2 layered structure, in which the atomic planes are linked together by weak Van der Waals' interactions [3]. h-BN presents a low density (2.28) [4] and an electrical insulating behavior (indirect bandgap at 5.955 eV) [5] due to delocalized π-electrons on nitrogen atoms. Moreover, it displays a high thermal conductivity (up to 20 W m −1 K −1 ) [6] allowing heat dissipation, strong high temperature stability (900 • C-1000 • C under air and up to 2800 • C under inert atmosphere) and a chemical inertness without toxicity making it an interesting environmental-friendly material. Finally, its lamellar structure brings it a solid lubricant behavior with a low friction coefficient (0.2-0.7) [7,8]. The unique sum of all its properties makes h-BN a refractory ceramic, an additive powder in cosmetic, paints or dental cements and a lubricant in metallurgy [2,9,10]. At a lower scale, the extraordinary insulating and thermal-conducting properties of h-BN also ensures new applications in lightweight, high-performance mobile and flexible electronic systems [11,12]. With future applications in mind, it becomes urgent to develop miniaturized functional devices that can be used in relevant research fields, essentially in the electronic field [4]. For example, in its two-dimensional (2D) form, boron nitride is an excellent dielectric material that can be used as substrate for two-dimensional nanomaterials [13,14], intercalant in Van der Waals heterostructures [14,15], insulated gate dielectric [16] and counter electrode in dye-sensitized solar cells [17]. Thus, low dimensional h-BN have become among the most needed inorganic nanosystems [18]. Up to now, there are two main strategies to produce Boron Nitride NanoSheets (BNNSs), involving either a 'bottom-up' or 'top-down' approach [19]. Bottom-up routes mainly include Chemical Vapor Deposition (CVD) [20][21][22][23][24], whereas top-down ones rely on mechanical [19,25,26] or chemical [27] exfoliation from bulk h-BN crystals. Since many years, the Poymer Derived Ceramics (PDCs) route has been used for the synthesis of different kinds of boron nitride materials, from bulk to nanostructures [28][29][30][31]. Recently, it has demonstrated a high potential for the fabrication of bulk h-BN crystals that can be further exfoliated into BNNSs [32][33][34][35][36].

Polymer derived ceramics (PDCs) route
The PDCs has been first mentioned in the sixties by Chantrell and Popper [37] for the design and the synthesis of advanced non-oxide ceramics with a compositional and structural homogeneity [29]. One of the most appealing advantage is that, starting from molecular or preceramic polymeric precursors, specific shapes including ceramic fibers, films or composite materials, which cannot be easily obtained by conventional powder technology, can be produced. Preceramic polymers can indeed be processed or shaped using conventional polymer-forming techniques, and further converted into dense ceramics using thermal treatments [31]. Therefore, a number of precursors have been developed for the PDCs route in order to tune the shaping properties of the preceramic polymer. Concerning the synthesis of h-BN, borane-based and borazine-based compounds are mainly used. Their synthesis is reproducible and their ceramisation can be obtained with a high yield and high purity.
For many years, our lab works on synthesis of h-BN by the PDCs route giving rise to different shapes (fibers, coatings, foams, nanocages…) [38][39][40][41][42][43]. More recently, our research efforts have been focused on the improvement of the PDCs route for h-BN nanomaterials fabrication. In this contribution, we will present results obtained with PDCs alone, PDCs combined with Gas Pressure Sintering (GPS) and with Atomic Layer Deposition (ALD).

Synthesis using PDCs alone
Growth of large-area h-BN nanosheets at relatively low temperature and atmospheric pressure conditions is successfully achieved from borazine precursor using the PDCs route either without (h-BN 0 ) or with 5 wt.% (h-BN 5 ) Li 3 N added as crystallization agent to the pre-ceramic mixture. Ceramization is realized at 1400 • C under inert atmosphere to obtain BN material.
The morphology of the h-BN samples is investigated using Scanning Electron Microscopy (SEM). Figures 1(a) and (b) display typical SEM micrographs of the as-synthesized h-BN 0 and h-BN 5 samples, respectively. It can be observed that the h-BN 0 sample exhibits agglomerated and irregular grained nanocrystals comprising fractures and particles in jagged shapes and edges. On the other hand, the h-BN 5 sample reveals agglomerated, smooth-edged and well-defined platelike crystals with estimated diameter around 2 µm. Morphological properties of the h-BN samples is ascertained using Transmission Electron Microscopy (TEM). Low magnification TEM micrographs illustrate that addition of Li 3 N generates samples mainly composed of overlapping sheet-like structures, with crystal size less than 1 µm (figure 1(d)), whereas growth without crystallization agent leads to the formation of an amorphous mass of undefined shape and dimensions (figure 1(c)). Better crystallization in presence of Li 3 N is confirmed by both Raman spectroscopy and x-ray Diffraction (XRD) data.
Composition and crystallinity of the synthesized samples are examined by powder XRD (figure 2(a)). Diffraction patterns, indexed using the JCPDS 00-034-0421 h-BN reference file, show formation of not well-crystallized h-BN 0 sample, as indicated by the very broad and undefined (002) and (100) peaks (figure 2(a)(i)). On the other hand, the h-BN 5 diffractogram reveals the formation of two phases of boron nitride: hexagonal (h-BN) and rhombohedral (r-BN, JCPDS card n • 00-045-1171) ones (figure 2(a)(ii)). Furthermore, improved crystallinity of h-BN nanosheets is evidenced by symmetrical sharpening of the (002) peak, as well as the appearance of (004), (110), and (112) peaks. Addition of Li 3 N leads to reaction with polyborazilene (PBN) forming Li 3 BN 2 , which is known to dissolve BN [44]. Therefore, B-N nuclei would form and would agglomerate, grow and crystallize into BN domains. However, the preferential crystallization into h-BN or r-BN remains challenging.
The synthesized materials are investigated using Raman spectroscopy. Figure 2   well-crystallized h-BN, and sharpening of the peak prove that addition of crystallization promoter permits improving the crystallinity of the BNNSs [25,45,46]. This is further confirmed by the value of the full width at half maximum (FWHM) of the E 2g peak determined to be 16.8 cm −1 and 39.4 cm −1 for h-BN 5 and h-BN 0 , respectively. Indeed, the lower is the FWHM value, the higher is the crystalline quality of the sample. Watanabe et al report a FWHM of~7 cm −1 for high-purity h-BN single crystals [25,45,46]. Raman data are thus in good agreement with the obtained SEM, TEM and XRD results, indicating the influence of the addition of Li 3 N to the crystallinity of the BNNSs.

Synthesis using PDCs combined with gas pressure sintering (GPS)
As demonstrated, addition of a crystallization promoter allows improving the quality of the obtained h-BN.
To synthesize large size high-quality h-BN crystals, the PDCs route is coupled with a sintering process, i.e. the gas pressure sintering (GPS). From similar mixture of PBN and 25 wt% of Li 3 N, after GPS process, shiny transparent material is collected either from the top inner surface or the bottom of the crucible (figures 3(c) and 4(c)). The transparent crystals located on the top inner surface of the crucible show very well defined hexagonal shapes up to dozens of microns, with smooth surfaces and sharp edges (figures 3(a) and (b)). They are stacked in a layered 2D structure and show different orientations, which could be due to growth conditions. Actually, as mentioned in previous studies [47], the reaction occuring between PBN and Li 3 N leads to an intermediate Li 3 BN 2 compound at about 860 • C. At the melting temperature of Li 3 BN 2 (920 • C), the compound would evaporate inside the crucible, and reach its top inner surface [47] that would act as nucleation seed allowing the BN crystallization. This implies that the crystallization occurs by nucleation from the vapor phase.  Concerning the sample collected from the bottom of the crucible, SEM characterization shows different features with larger flakes up to hundreds of microns without hexagonal shape but still stacked into layered structure (figures 4(a) and b). These characteristics are close to the ones previously observed for synthesis of h-BN combining PDCs route with spark plasma sintering process [34]. In that case, we can assume that the growth occurs in the liquid phase and the proposed crystallization mechanism would be similar to one of the previous studies reported by Bezrukov [48] and other groups [47,49,50]. At around 600 • C-700 • C, Li 3 BN 2 formed by reaction between Li 3 N and preceramic BN, melts and infiltrates amorphous BN and dissolves it, leading to highly crystallized hexagonal boron nitride.
Chemical analyses conducted by Energy Dispersion Spectroscopy (EDS) (figures 3(d) and 4(d)) show an elemental composition of 52.6 at% B and 47.4 at% N (top inner part) and 51.3 at% B and 48.6 at% N (bottom part), confirming the synthesis of h-BN. The oxygen detected is only 0.1 at%, which is out of the EDS instrument sensitivity and accuracy.
Structural characterization by means of XRD and Raman spectroscopy shows exactly the same features whether it is on top and bottom parts. Figure 5(a) presents the XRD pattern of the sintered product.  Compared with JCPDS 00-034-0421 file, all h-BN characteristic peaks can be assigned in the diffractogram, without any unexpected peaks. The diffraction pattern confirms the hexagonal structure with a lattice constant c of 6.66 Å. The integrated intensity ratio between (100) and (002) peak is about 0.21, which is slightly higher than the theoretical value of 0.15 for polycrystalline h-BN. This means that the sintering conditions promote growth along the c-axis.
Raman spectrum ( figure 5(b)), also displays very good signature of h-BN with well-defined single peak located at 1366 cm −1 , very close to the reference value of 1366.2 cm −1 [51]. This Raman peak, with a E 2g symmetry, corresponds to the in-plane shearing mode of h-BN. One can notice that the measured FWHM value is only 7.6 cm −1 , close to that of the best quality h-BN single crystals fabricated by High Pressure High Temperature (HPHT) method (7.3 cm −1 ) [52]. This low FWHM value is a further proof of a highly crystallized h-BN structure.
In order to demonstrate the ability of these crystals to be used as BNNSs supply, exfoliation of nanosheets has been performed by ultra-sonication in ethanol medium. The obtained nanolayers have been then deposited on a TEM copper grid (300 mesh) covered with holey carbon. Figure 6 presents representative micrographs obtained on exfoliated h-BN. Low resolution TEM shows h-BN nanosheets (figure 6(a)) as proven by Selected Area Electron Diffraction (SAED) (inset figure 6(a)) which clearly exhibits hexagonal bright dots, representative of single crystal. It is possible to determine the number of layers by counting the dark lines separated by interatomic distance (3.33 Å) on the edges of the sheet. Figure 6(b) confirms that they are very thin with an average number of layers around 3-4. The High Resolution Transmission Electron Microscopy (HRTEM) performed on a fine zone of one sheet shows the perfect atomic in-plane arrangement characteristic from the hexagonal lattice ( figure 6c).
In this part, we have demonstrated for the first time the synthesis of h-BN crystals by the unique combination of PDCs route and GPS method. The chemical and structural quality of the samples has been evidenced by use of EDS, XRD and Raman spectroscopy. This hybrid process allows synthesizing h-BN crystals with large lateral size (from tens to hundreds of microns) that can be used as BNNSs supply as proven by TEM.

PDCs route applied to atomic layer deposition (ALD) of boron nitride
PDCs and PDCs coupled with GPS are highly promising for production of self-standing h-BN single crystals that can be exfoliated into BNNSs. In order to widen the range of applications, it can be necessary to directly synthesize the BN nanosheets with a controlled number of layers. In this regards, ALD appears suitable. Indeed, it is a deposition technique producing highly reproducible, conformal and homogeneous thin films even on high aspect ratio substrates. Briefly, it is based on a reaction between precursor materials, separated into successive surface reactions. The reactants are kept separated and react with surface species in a self-limiting manner, i.e. without gas phase reaction. Each surface reaction is separated by a purge step to remove the unreacted precursor and the by-product. The succession of self-limited reactions and purges constitutes a cycle. The thickness of the deposit is simply determined by the number of cycles applied, allowing its control at the atomic level. ALD processes exist for a large variety of material classes, such as metals, oxides, nitrides, sulfides and phosphates [53]. Few processes have been developed for BN deposition. They are mainly based on use of ammonia and/or boron halide [54]. In 2018, a low temperature process based on electron enhanced ALD was reported. Even though this approach is promising for depositing crystalline BN on flat substrates, it is unsuitable for structured substrates [55].
Offering a large panel of precursors and allowing pre-ceramic polymerization at low temperature, adaptation of the PDCs route to ALD appears as an elegant approach. Indeed, it permits developing a novel two-step ALD route that consists in: (1) deposition of a pre-ceramic polymer intermediate by ALD, (2) followed by its conversion into dense BN using a thermal annealing. Such two-step ALD approach has been previously reported for MoS 2 . An amorphous organometallic intermediate is indeed deposited prior its conversion into crystalline 2D MoS 2 by thermal treatment [56].
As precursors must fulfill several requirements such as thermal stability, sufficient vapor pressure and inertness towards themselves, trichloroborazine (TCB), a borazine derivative, reacted with hexamethylsilazane (HMDS) are chosen as ALD precursors. During the 1st ALD step, the pre-ceramic intermediate called polyborazine, [B 3 N 4 H 4 ] n , is deposited at low temperature (80 • C) with a controlled thickness under release of trimethylsilyl chloride ((CH 3 ) 3 SiCl). During the 2nd step, the formed amorphous polyborazine film is thermally converted into dense BN layers under release of H 2 and NH 3 .
Due to the low temperature of the 1st step, various substrates, including polymer templates, are coated in a reproducible manner using the ALD route, demonstrating the versatility of the approach. Characteristic linear relationship between the thickness of the intermediate as well as of the final BN film and the number of cycles is reported, the growth per cycle (GPC) being 0.19 nm cycle −1 [57]. Uniform and conformal BN thin films or nanostructures, when templates are used, are successfully fabricated after thermal annealing under controlled atmosphere. XPS analysis reveals a close stoichiometry with a B/N ratio of 0.95 and no Cl contamination is noted [57]. Depending on the substrate, BN layers display either amorphous, turbostratic (t-BN) or hexagonal phase as visible in figure 7. HRTEM image recorded from BN coated Si (100) wafer with native oxide after annealing at 1000 • C ( figure 7(b)) reveals the presence of very few lattice fringes indicating   7(a)). On the other side, crystalline materials are observed after ALD on PC membrane (figure 7(c)) and PAN fibers ( figure 7(d)) which both act as templates. Inverse structures are thus fabricated using ALD followed by annealing at 1400 • C partially under NH 3 to remove the soft template. Nanotube arrays and mats are respectively obtained with controlled thickness of the walls. Furthermore, carbon materials like amorphous carbon fibers are uniformly coated with polyborazine layer as nicely shown by Scanning Transmission Electron Microscopy (STEM) and EDS elemental mapping distribution (figures 8(a)-(d)). The deposited pre-polymer layer leads then to either amorphous or t-BN (figures 8(e) and f) film depending on the annealing temperature. Besides the influence of the heat treatment, difference in term of crystallinity is also noted as a function of the nature of the polymer support. Interaction between the organic support and the precursor as well as the creation of a polymer/inorganic interface can take place resulting in precursor infiltration into the polymeric chains [58,59] and higher GPC value is noted. In the present case, reactant infiltration impacts the structure of the final material. Indeed, according to the respective GPC value, i.e. 0.28 and 0.20 nm cycle −1 on PAN and PC, and TEM characterization, no significant precursor diffusion seems to take place in case of ALD on PC and high crystalline BN material is fabricated ( figure 7(c)), whereas less-crystallized t-BN ( figure 7(d)) results from PAN template due to TCB precursor interaction with the soft template.
Enabling use of highly structured sensitive templates, our ALD approach is successful for synthesizing functional BN nano-/hetero-structures. In particular, the fabricated BN nanostructures show great potential in water treatment. While only weak hydrophilicity and high lipophilicity are displayed by BN films on SiO 2 /Si surface, unwoven BN nanotube mats exhibit super-hydrophobicity as well as good lipophilicity and good stability towards aging under air and drastic pH conditions. They demonstrate efficient as filter to separate water from organic solvents and excellent performances for organic absorption with reusable properties [60]. Moreover, BN films reveal suited as protective coating. BN coated carbon nanotubes exhibit improved oxidation resistance by near 100 • C compared to the uncoated carbon material as demonstrated by thermogravimetric analysis (TGA) (figure 9) [61].
To conclude, two-step ALD process of BN based on polymer derived ceramics chemistry has been successfully reported. The process is versatile and represents a promising alternative to the existing processes. Despite the high temperature second step, the two-step ALD approach based on PDCs chemistry appears promising for fabricating h-BN nanostructures with controlled thickness. Indeed, the ALD step occurring at low temperature (80 • C) widen the range of substrates and paves the way to fabrication of functional BN nanostructures that require highly structured polymer template.

Conclusion
PDCs route has proven to be an effective route for synthesizing high quality h-BN materials. In particular, addition of a crystallization promoter, like Li 3 N, allows good crystallinity at reduced temperature and atmospheric pressure. Moreover, use of GPS permits to enlarge the lateral size of BNNS up to hundreds of µm, while maintaining the crystal quality. Finally, ALD process based on PDCs route demonstrates to be an interesting approach for fabricating thin BN pre-ceramic films on various substrates that leads to well-defined BN nano-/hetero-structures.
These synthesized high quality boron nitride nanomaterials can find applications in various areas as micro-electronics, photonics and environmental domains. To conclude, the PDCs pathway is a versatile and efficient chemical method that can be combined with other techniques allowing tuning the shape and properties of the BN material.

Synthesis using PDCs alone
Borazine is prepared from a reaction between ammonium sulphate ((NH 4 ) 2 SO 4 , ⩾99%, Aldrich) and sodium borohydride (NaBH 4 , 98% purity, Aldrich) in tetraethylene glycol dimethyl ether or tetraglyme (C 10 H 22 O 5 , ⩾99%, Alfa Aesar) solvent, as reported by Wideman et al [62]. After the purification of borazine via distillation, a colourless pre-ceramic polymer, the PBN is obtained through the poly-condensation of borazine at 55 • C inside a pressure-sealed system under argon for 5 d [29,38,39]. Inside a glove box under argon atmosphere, lithium nitride (Li 3 N, 99.4%, Alfa Aesar), at a 5 wt% ratio, is added to PBN, as a crystallization agent. The mixture is homogenized via stirring for 10 min, after which the suspension is heated to 200 • C in an alumina crucible and kept for 1 h to give a solid-state polymer [32,33,63]. Finally, the stabilized mixture is annealed for 1 h at 1400 • C (1 • C min −1 ) under inert nitrogen (N 2 , 98%, Air Liquide, France) atmosphere. As a control experiment, a similar procedure is followed using a pre-ceramic mixture comprised of PBN only. Finally, after cooling the furnace to room temperature, the h-BN samples are collected and labelled as h-BN 5 and h-BN 0 , for samples prepared with 5 wt.% of Li 3 N and without, respectively.

Synthesis using PDCs combined with GPS
The synthesis of PBN is made following the procedure described above. Then 25 wt% lithium nitride (Li 3 N, 99.5%, Aldrich) micro powder is dispersed as crystallization promoter into pure homemade liquid PBN. The dispersion is slowly heated in a glove box filled with argon from room temperature to 200 • C, and hold at this temperature for 1 h leading to a solid-state polymer avoiding further oligomer evaporation. The resulting solid is placed into an alumina crucible to be heated under N 2 to 650 • C (3 • C min −1 ), and kept at this temperature for 1 h. After heat treatment, the mixture is grinded into powder in the glove box.
For GPS experiment, the preceramic powder is firstly filled and enclosed in a pure BN crucible (5 mm internal diameter and 5 mm height) in a glove box and transferred into the GPS chamber. Before sintering, the chamber is vacuumed twice to remove the air residue. When starting the experiment, the chamber is filled with Ar and pressure is increased up to 95 bar. While keeping the pressure constant, the temperature ramps up to 2000 • C in 3 h. The furnace maintains this temperature for a 3-5 h dwelling. The pressure is kept until the end of dwelling.

PDCs route applied to ALD of BN
Depositions take place in a home-made ALD working in exposure mode. 2,4,6-tri(chloro)borazine (laboratory from ref [57]) and hexamethyldisilazane (ROTH, 98%) are used as reactants. Their canisters are, respectively, kept at 70 and 30 • C. Deposition occurs at 80 • C with an opening time of 1 s and 0.05 s for TCB and HMDS, respectively. Each pulse is followed by a residence and purge time of 10 s. Pure nitrogen is used as carrier gas at a constant flow rate of 5 sccm. Films are deposited on Si(100) wafer with native oxide. Millipore polycarbonate membranes, electrospun polyacrylonitrile fibers and carbon fibers (synthesized at the laboratory) are also coated using a simple home-made ALD reactor made of glassware [57]. All as-deposited thin films are post-annealed in a tubular furnace under inert atmosphere, and for polymer support partially under NH 3 . All transfers are performed under controlled atmosphere to avoid exposure to air before annealing.

Characterization techniques
The synthesized BN materials are characterized by optical microscopy using a Zeiss Axiophot Photomicroscope as well as by SEM using a FEG-SUPRA Zeiss 55VP microscope operating at 800 V with an Oxford X-Max Silicon Drift EDS detector and a Zeiss Merlin VP compact microscope operated at 5 kV and equipped with a SDD OXFORD X-Max EDS detector. Samples were prepared without carbon or platinum coating. TEM, HRTEM and high-angle annular dark-field STEM investigations are carried out on 300 mesh copper TEM grids covered with a holey carbon support film using a JEOL 2100F and JEOL 2010 F microscopes both operated at 200 kV and equipped with silicon drift detector (Oxford Instrument) for EDS. Cross-section of boron nitride thin film on Si substrate is realized using mechanical polishing followed by ion etching using Precision Ion Polishing System (PIPS) 691 from Gatan. In addition to TEM studies, the crystalline quality of the BN materials is characterized by powder XRD and Raman spectroscopy. XRD is carried out with a Bruker D8 Advance diffractometer, using CuK radiation source, λ = 1.5418 Å, and the Raman spectra are recorded using a Labram HR800 spectrometer (HORIBA Jobin-Yvon) with 532 nm laser excitation wavelength. XPS is performed, using a PHI Quantera SXM spectrometer with Al Kα monochromatism radiation, to characterize the composition of the deposited material. Sample is kept on the holder using carbon paste. The raw XPS data are corrected using the binding energy of the C-C bond at 284.5 eV and fitted with Gaussian-Lorentzian curves. Finally, the thickness of the deposited ALD BN films on the wafer substrates are measured by ellipsometry spectroscopy at an incidence angle of 70 • using a Sopra GES-5E spectrometer. Data fit is realized with the 'Winell II' software using Cauchy dispersion law. The protective property towards oxidation of the BN coating is evaluated by thermogravimetric analysis using a TGA/DSC2 Mettler Toledo thermobalance with a temperature ramp of 10 • C min −1 until 1100 • C under air.