Controlled synthesis of uniform multilayer hexagonal boron nitride films on Fe2B alloy

Two-dimensional (2D) hexagonal boron nitride (h-BN) is highly appreciated for its excellent insulating performance and absence of dangling bonds, which could be employed to maintain the intrinsic properties of 2D materials. However, controllable synthesis of large scale multilayer h-BN is still very challenging. Here, we demonstrate chemical vapor deposition (CVD) growth of multilayer h-BN by using iron boride (Fe2B) alloy and nitrogen (N2) as precursors. Different from the self-limited growth mechanism of monolayer h-BN on catalytic metal surfaces, with sufficient B source in the bulk, Fe2B alloy promotes the controllable isothermal segregation of multilayer h-BN by reacting with active N atoms on the surface of the substrate. Microscopic and spectroscopic characterizations prove the high uniformity and crystalline quality of h-BN with a highly orientated layered lattice structure. The achievement of large scale multilayer h-BN in this work would facilitate its applications in 2D electronics and optoelectronics in the future.


Introduction
Two-dimensional (2D) materials have attracted enormous attention for future electronic and optoelectronic device integrations. [1][2][3] Hexagonal boron nitride (h-BN), a 2D insulator with a wide band gap ($6 eV), is in general highly desired for practical device applications, such as gate dielectric layers, tunnelling barriers or encapsulations. [4][5][6][7] It is widely regarded as an ideal dielectric substrate for 2D materials because of its atomic atness and absence of dangling bonds, which could maintain the intrinsic properties of the overlying 2D materials. 3,5,6,8 Moreover, single phonon emission from isolated point defect sites in h-BN has been investigated recently, which demonstrates potential in quantum photonics and quantum computation. [9][10][11] So far, many efforts have been devoted to synthesize highquality h-BN lms. In recent years, layered h-BN akes, integrated with other layered materials for 2D heterostructure devices, were mechanically exfoliated from high quality h-BN bulk crystal, which was obtained under extremely high temperature and pressure for several days. 12,13 However, the limited size and inhomogeneous thickness hinder further development of 2D electronics and photonics. As the most promising technique, chemical vapor deposition (CVD) was employed to achieve large-area monolayer and few layer h-BN on metal or dielectric substrates by using ammonia borane (NH 3 $BH 3 ) or borazine (B 3 N 3 H 6 ) as the precursor. 14-23 However, the inhomogeneous thickness, poor crystallinity, toxicity of precursors and complicated procedures limits the application in practical 2D devices. High quality, thickness uniformity, low cost and facile synthesis remain extremely challenging for compound 2D layered materials such as h-BN.
Here, we report a new method to synthesize multilayer h-BN on Fe 2 B alloy with N 2 as a reactant. It is much easier to control the ow rate by using gaseous N 2 as a precursor than solid precursors (NH 3 $BH 3 or B 3 N 3 H 6 ). Comparing with previous approaches, the new synthesis method yields large scale and high quality multilayer h-BN at ambient pressure. The surface morphology, crystalline quality and defect distributions were investigated by various characterizations. Fe 2 B, an alloy not only supplies boron source directly, but also reacts with N 2 to form multilayer h-BN. The entire growth process breaks the traditional self-limited growth mechanism on catalytic metal substrates and successfully obtains multilayer h-BN on Fe 2 B alloy. Controllable synthesis of h-BN lm establishes a signicant approach for further rational catalyst design for other compound 2D materials.

Materials and methods
Multilayer h-BN was synthesized on Fe 2 B alloy using ambient pressure CVD. Firstly, a Fe 2 B substrate was heated to 1300 C and maintained for 60 minutes under Ar and H 2 atmosphere at ow rates of 300 and 50 sccm, respectively. Then, N 2 and H 2 atmosphere at ow rates of 300 sccm and 50 sccm were introduced at 1300 C for 60 minutes to facilitate h-BN thin lm growth. Finally, the furnace was cooled down to room temperature naturally under Ar and H 2 ows.
The as-grown multilayer h-BN lms were transferred to arbitrary substrates through chemical transfer process. The morphology of multilayer h-BN on Fe 2 B alloy was characterized by scanning electron microscope (SEM, Zeiss Supra 55, operated at 3 kV). Atomic force microscope (AFM, Bruker Icon, Tapping mode) was employed to determine the thickness of multilayer h-BN. Raman spectra were obtained in the spectral range of 1100-1700 (cm À1 ) by using a laser excitation of 532 nm. The stoichiometry of B and N of h-BN was evaluated by X-ray photoemission spectroscopy (XPS, species, Al Ka). The phase structure of h-BN thin lm was identied by X-ray diffraction (XRD, Bruker discover D8). Transmission electron microscope (TEM, JEM-2100F, operated at 200 kV) images and selected area electron diffraction (SAED) conrm the high crystalline quality of as-prepared h-BN. Photoluminescence (PL) mapping and PL spectra were collected by using a laser excitation of 532 nm (WITec Alpha 300R, 5 mW).

Results and discussion
The schematic of ambient-pressure CVD setup for multilayer h-BN growth was depicted in Fig. 1a (details are given in Materials and methods). To demonstrate the growth dynamics, Fe 2 B alloys were annealed at 1100 C, 1200 C and 1300 C for 60 minutes, respectively. SEM images exhibit h-BN nucleus, isolated h-BN akes and continuous h-BN lm synthesized at different growth temperatures (Fig. 1d-f). Corresponding AFM images and height proles in the inset verify the thickness of asgrown h-BN transferred on SiO 2 (300 nm)/Si substrate. By introducing N 2 gas as a reactant, uniform multilayer h-BN lms over large areas formed on Fe 2 B substrate. Fig. 1b displays the Raman spectra of multilayer h-BN lms prepared at various growth temperatures. It could be observed that the position of characteristic E 2g band presents a slightly shi (from 1372 to 1366 cm À1 ) with the increasing of growth temperature, indicating the increase of thickness. 24 According to earlier reports, the full-width at half maximum (FWHM) of the Raman E 2g band is associated with the crystallinity of h-BN. [24][25][26] The FWHM of E 2g phonon mode decreased with growth temperature increasing (Fig. 1c). The h-BN synthesized at 1300 C shows the minimum FWHM value of $15 cm À1 , which is much smaller than that synthesized by existing catalytic epitaxial approaches. 25 Multilayer h-BN lm obtained at 1300 C was further characterized to analyse its morphology and crystalline quality. Continuous and uniform h-BN lm grown on Fe 2 B alloy was displayed in Fig. 2a. Layered structure was veried primarily by measuring the edge of h-BN lm. To further evaluate the thickness uniformity of h-BN lms on Fe 2 B substrate, the asgrown multilayer h-BN was transferred on a SiO 2 (300 nm)/Si substrate. The uniform optical contrast conrms the smooth surface and high uniformity of thickness (Fig. 2b). The thickness of the lm is around 30 nm according to the height prole on the edge of h-BN (Fig. 2c). Also, Raman mapping of E 2g phonon mode depicts the homogeneity thickness of asprepared h-BN multilayers (Fig. 2d). Moreover, we investigated the composition and crystal structure of h-BN lm. The survey XPS spectrum of h-BN lm, displayed in Fig. 2e, indicates that the stoichiometry of B and N atoms is 1 : 1.06. Further, The XRD spectrum displays sharp (0002) diffraction peak at 26.8 and (0004) diffraction peak at 55.2 , which signies the layers of h-BN are well aligned with the c axis (Fig. 2f). 18 The Inorganic Crystal Structure Database (ICSD) (h-BN, 340421) was used for  phase identication. The lattice constant was calculated to be 0.33 nm, which is in good agreement with reported values. 18 As previous reports, NH 3 $BH 3 or B 3 N 3 H 6 was always used to synthesize monolayer or few-layer h-BN on metal substrates. [14][15][16][17]19,21,23 For monolayer h-BN grown on Cu-Ni alloy, B 3 N 3 H 6 precursor undergoes decompose, polymerization and dehydrogenation on the surface of the substrate (Fig. 3d). 19 SEM image displays the morphology of h-BN domains and TEM image signies the monolayer nature of as-grown h-BN ( Fig. 3f  and g). The surfaces do not assist the synthesis of multilayer h-BN due to the self-limited growth effect when monolayer h-BN covered the Cu-Ni alloy. 19 For multilayer h-BN grown on Fe 2 B alloy, N 2 would rstly decompose into active nitrogen atoms with the catalysis of Fe 2 B substrate. 27 Then, active nitrogen atoms would react with Fe 2 B to form initial B-N molecular. 18,27 High concentration B-N molecules would segregate and induce the nucleation of h-BN isothermally. Finally, h-BN nucleus would grow and coalesce into multilayer h-BN lms on the surface of Fe 2 B alloy. The controllable thickness would be realized by optimizing growth temperature or reaction time (Fig. 3a). SEM image of as-grown multilayer h-BN displays the uniform and smooth surface morphology and TEM image indicates multi-layered h-BN nanostructure (Fig. 3b and c). Further, E 2g phonon mode of multilayer h-BN in the Raman spectrum shows much stronger intensity than the E 2g band of monolayer h-BN (Fig. 3e). Comparing with the two synthesis approaches mentioned above, the success of thickness controllable growth is mainly due to the change of h-BN growth mechanism from surface-mediated growth to isothermal segregation on Fe 2 B substrate by using nitrogen as feedstock.
The crystalline quality and atomic structure of the multilayer h-BN is further characterized by TEM. The low-magnied TEM view in Fig. 4a displays continuous and uniform h-BN lm suspended on Cu mesh grid. High resolution TEM image exhibits the lattice fringe of h-BN multilayers (Fig. 4b). Corresponding selected area electron diffraction (SAED) presents only one set of hexagonally arranged spots, rather than ringshaped patterns, assuring the highly oriented stacking order. 20 Moreover, Fig. 4c reveals a honeycomb structure with an interatomic distance of about 0.25 nm, in good agreement with bulk h-BN. 28 Moreover, as earlier theoretically and experimentally analysis, point defects would induce non-resonant absorption and emission within the band gap of materials. 9,[29][30][31] Here, PL spectroscopy was used to investigate point defects distributions in h-BN lm. 9,30,31 PL measurement was carried out at room temperature with a 532 nm sub-bandgap excitation laser. Fig. 4d illustrates the defect distributions in h-BN ake transferred on Si substrate. Optical emissions are accumulated on the edge of as-prepared h-BN multilayers, which may be introduced by the transfer and annealing processes. Typical PL spectra were extracted and tted by Lorentzian tting (Fig. 4e). Three peaks centred at 2.13 eV, 1.97 eV and 1.81 eV were observed, which is consisted with zero phonon line (ZPL) and corresponding one-and two-optical-phonon sidebands (PSB), respectively. 9,30,31 In addition, Fig. 4f displays a histogram of the emission energy values measured for 40 individual emitters. More than 85% of the emitters were measured to have a ZPL energy localized in a very narrow spectral window, (2.13 AE 0.1) eV. Such a small spectra variability of the ZPLs may be caused by strain and/or local lattice symmetry distortions. 31 Moreover, it is observed that no optical emission was found on the large area of h-BN region, indicating the high quality of as-grown h-BN multilayers.

Conclusion
In this work, uniform and continuous multilayer h-BN has been obtained isothermally through the reaction between Fe 2 B alloy and N 2 via CVD method. The thickness could be mediated by controlling diffusion rate of nitrogen in the catalyst at different growth temperatures. Raman shi at 1366 cm À1 and sharp (0002) diffraction peak in the XRD pattern conrm the crystallinity of h-BN. TEM measurements revealed that the h-BN has highly orientated layered lattice structure. Comparing with  monolayer h-BN growth on catalytic metal surfaces, this method shows its unique mechanism for achieving multilayer h-BN, which has potential on the synthesis of other layered 2D heterostructures. Therefore, the present achievement of highquality multilayer h-BN synthesis would give strong impact on ultrathin dielectric-, support-, or barrier-layer in particular for integrated electronics and photonics of 2D materials.

Conflicts of interest
There are no conicts to declare.