Communication pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Single Crystal Growth of Millimeter-Sized Monoisotopic Hexagonal Boron Nitride Song Liu,† Rui He,‡ Lianjie Xue,†,§ Jiahan Li,† Bin Liu,† and James H. Edgar*,† †
Department of Chemical Engineering, Kansas State University, Durland Hall, Manhattan, Kansas 66506, United States Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409, United States § Department of Physics, Kansas State University, Cardwell Hall, Manhattan, Kansas 66506, United States ‡
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S Supporting Information *
ABSTRACT: Hexagonal boron nitride (hBN) with a single boron isotope have many enhanced physical, thermal and optical properties compared to the most common hBN with the natural distribution of boron (19.9 at. % 10B and 80.1 at. % 11B). These property differences can significantly improve the device performance in applications, such as neutron detectors, nanoscale electronics, and optical components. In this study, a new method for the growth of large-scale, high-quality monoisotopic hBN single crystals, i.e., h10BN and h11BN, was developed. hBN single crystals were grown using a nickel−chromium solvent and pure boron and nitrogen sources at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 1 mm. Raman measurements demonstrate that the crystals produced with this method are pure hBN phase with low defect density, and the spectral peaks vary with the boron isotope concentrations. X-ray photoelectron spectroscopy spectra show that the B−N bond in h11BN is slightly stronger than that in h10BN. The ability to produce crystals in this manner opens the door to isotopically engineering the properties and performance of hBN devices.
H
hBN, and Vuong et al. 24 found the boron isotope concentration slightly changes the energy bandgap of hBN. Although monoisotopic hBN single crystals are advantageous in many applications, its synthesis method, to the best of our knowledge, has not been reported. Our group has previously demonstrated the atmospheric growth of largescale and high-quality hBN single crystals with natural abundance (hNaBN) from Ni−Cr and Fe−Cr solvents and a hot-pressed hBN source.25,26 These crystals showed intense ultraviolet luminescence peak of 5.77 eV at room temperature. A narrow Raman peak width of 7.8 cm−1 at 1366 cm−1, which is among the smallest values reported in the literature, indicates excellent crystal quality. However, the isotopically pure hBN cannot be synthesized through same method, as the hot-pressed BN source contains boron with the natural isotope distribution. Here, we report on the atmospheric growth of millimetersized and high-quality monoisotopic hBN single crystals from a Ni−Cr flux. Isotopically enriched boron powders, i.e., 10B and 11 B, and nitrogen gas were used as the precursors. The crystal sizes, shapes and morphologies were determined by optical microscopy. Raman spectra and X-ray photoelectron spectroscopy (XPS) were utilized to evaluate the quality and surface composition of the hBN crystals. The monoisotopic hexagonal boron nitride crystals were grown from high-purity elemental 10B (Ceradyne, 99.22 at. %) and 11B (Ceradyne, 99.41 at. %) powders by using metal flux method. Figure 1a illustrates a schematic of the experimental setup for the crystal growth, where a single-zone furnace with
exagonal boron nitride (hBN) has recently demonstrated great promise for optoelectronics, nanophotonics, and fundamental physics applications due to its strong anisotropy and unique properties. For example, hBN has been envisioned to be a promising candidate for far-ultraviolet (FUV) light-emitting devices for implementing compact UV optoelectronics.1,2 Moreover, as a natural hyperbolic material, hBN possesses high confinement and low loss of phonon polaritons,3,4 which enables many potential applications such as hyperlenses,5,6 negative refraction,7,8 nanolithography,9 and thermal radiation enhancement.10 In addition, hBN is an excellent substrate and dielectric layer for 2D material-devices, as it can significantly increase the carrier mobility in graphene11,12 and decrease hysteresis in MoS2 field-effect transistors.13 Furthermore, because of its superb stability and impermeability at ambient conditions, hBN is an excellent encapsulant to protect 2D materials from detrimental environmental effects.14,15 Typically, the hBN utilized in most of current research has the natural distribution of boron isotopes, which is 19.9 at. % 10 B and 80.1 at. % 11B. As the isotope composition can impact several properties in solid materials,16,17 it is of interest to investigate the novel properties and applications of monoisotopic hBN, i.e., h10BN and h11BN. Given that 10B has a capture cross-section of 3840 b for thermal neutrons, h10BN has been proposed to be a promising candidate for neutron detectors.18,19 Both experimental and theoretical studies20−22 have shown that the thermal conductivity of hBN can be significantly enhanced in isotopically pure materials, as the phonon-isotope scattering in the crystal is much weaker than that in naturally abundant hBN. Most recently, Giles et al.23 demonstrated that the polariton lifetime in our monoisotopic hBN samples increases 3-fold over the naturally abundant © XXXX American Chemical Society
Received: June 19, 2018 Revised: September 11, 2018 Published: September 13, 2018 A
DOI: 10.1021/acs.chemmater.8b02589 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
Figure 1. Schematic illustration showing (a) the experimental system for monoisotopic hBN growth and (b) the procedure of hBN growth.
12-in. heating length was used. The starting materials, 48 wt % Ni (Alfa Aesar, 99.70%), 48 wt % Cr (Alfa Aesar, 99.86%), and 4 wt % B mixed powders, were loaded into an alumina crucible and placed in a 1.5-in. diameter alumina tube equipped in the furnace. Prior to the growth, the furnace was evacuated to 50 mTorr and then filled with N2 and H2 gases (ultrahigh purity grade) to a constant pressure of 850 Torr. During the reaction process, the N2 and H2 gases continuously flowed through the system at rates of 125 and 5 sccm, respectively. All the nitrogen elements in the hBN crystal originated from the flowing N2 gas. Adding H2 gas minimizes the concentration of oxygen and carbon impurities in the hBN crystal. The heating cycle started with a quick ramping (200 °C/h) from room temperature to 1550 °C. After a dwell time of 24 h at 1550 °C, the hBN crystals were precipitated on the metal surface by cooling at a rate of 0.5 °C/h to 1525 °C, and then the system was quickly quenched to room temperature. The schematic of the growth procedure is illustrated in Figure 1b. As noted in previous studies of hBN crystal growth,27,28 a mixture of nickel and chromium was used as the solvent because of their high solubility for boron and nitrogen, respectively. In pure hot nitrogen, chromium reacts to form Cr2N and CrN while nickel remains unreacted. The use of pure boron sources makes this process different from the natural hBN crystal growth using a hot-pressed hBN source. Specifically, as the source materials are heated, boron first reacted with nitrogen to form boron nitride before the metal melted and then dissolved into the solvent. A saturated solution was formed by keeping the metal flux at 1550 °C for 24 h, which allowed the crystal precipitation during the slow cooling due to the oversaturation. Figure 2a illustrates the optical images of monoisotopic hBN, i.e., h10BN and h11BN, crystals grown onto the Ni−Cr metal surface, showing that the crystals are clear and colorless. To obtain free-standing hBN crystals, we used thermal release tape to adhere and peel the crystals from the metal surface, and then heated the tape at 150 °C for 10 s to release the adhesive and transfer the crystals onto a substrate. The inset figures show the free-standing hBN flakes, around 7 mm across and 10-μm thick, exfoliated from the metal surface. Figure 2b shows magnified optical micrographs of those hBN flakes on a glass substrate. The largest single crystal domain size is about 1 mm across. The size of the monoisotopic hBN crystals we synthesized here is comparable with our previously prepared hBN crystals with natural B abundance (hNaBN).25,26 The ratio of the two boron isotopes in the crystals were quantified using secondary ion mass spectrometry (SIMS), which demonstrated that the 10B and 11B purities in h10BN and h11BN are 98.7 at. % and 99.2 at. %, respectively.
Figure 2. (a) Photographs of monoisotopic hBN, i.e., h10BN and h11BN, crystals on top of Ni−Cr metal surface. (Inset) The freestanding hBN flakes exfoliated from the metal surfaces. (b) Optical micrographs of free-standing hBN flakes on a glass substrate.
Figure 3 displays the Raman spectra from h10BN, h11BN, and h BN crystals in both shear and intralayer modes (see Supporting Information for more details on the measurements). The low-frequency spectra in Figure 3a, which is attributed to the rigid shearing oscillation between adjacent layers, show that the peaks in h10BN, h11BN, and hNaBN are at 53.4 cm−1, 52.0 and 52.5 cm−1, respectively. The hNaBN peak is between the peaks for h10BN and h11BN, since it is an isotopic mixture of both. As NaB includes 80.1% of 11B, the h11BN and hNaBN peaks are close to each other. The full width at half-maximum (fwhm) value for the all three peaks is the same, i.e., 1.3 cm−1. The high-frequency spectra in Figure 3b, which corresponds to the intralayer E2g vibrational mode between in-plane boron and nitrogen, also show an intermediate peak value of 1366.1 cm−1 for hNaBN, in which the peaks for h10BN and h11BN are positioned at 1393.3 and 1357.4 cm−1 respectively. Note that the fwhm in h10BN and h11BN (3.1 and 3.3 cm−1) is much narrower than that in hNaBN (7.8 cm−1), which originates from the isotope disorder in the naturally abundant hBN crystals. The small values of fwhm in both shear and intralayer modes indicate the high quality of the monoisotopic hBN crystals. As the mass difference of boron isotopes can have an influence on the electron density distribution24 and chemical kinetics,29 one could anticipate an isotopic effect on the chemical bonding in hBN. X-ray photoelectron spectroscopy (XPS) was carried out to investigate this effect (detailed procedures are described in the Supporting Information). Na
Figure 3. Raman spectra of (a) shear mode and (b) intralayer mode from h10BN, h11BN and hNaBN crystals grown with Ni−Cr metal flux. B
DOI: 10.1021/acs.chemmater.8b02589 Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
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Communication
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02589.
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Details of Raman and XPS measurements (PDF)
AUTHOR INFORMATION
Corresponding Author
*(J.H.E.) E-mail:
[email protected]. ORCID
Song Liu: 0000-0002-3046-3335 Lianjie Xue: 0000-0003-0222-3905 Bin Liu: 0000-0001-7890-7612 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the Materials Engineering and Processing program of the National Science Foundation, Award Number CMMI 1538127, and the II−VI Foundation is greatly appreciated. R.H. acknowledges support by the National Science Foundation CAREER award (Grant No. DMR1760668). SIMS measurements were performed by Evans Analytical Group as part of a work-for hire agreement. The authors are thankful to Dr. Siyuan Zhang for her support of the XPS measurements.
Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of (a) N 1s and B 1s from h10BN, h11BN, and hNaBN crystals grown with Ni−Cr metal flux. (b) N 1s and B 1s peak positions versus 11B concentration in hBN.
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Figure 4a shows the XPS spectra of respective the N 1s and B 1s peaks from h10BN, h11BN and hNaBN crystals. The binding energies for N 1s and B 1s in h Na BN (blue curve) are 398.1 and 190.5 eV, respectively, which are in good agreement with previously reported literature values.30−32 Note that the peak position for both the N 1s and B 1s in h11BN is slightly higher than that in h10BN, and the hNaBN peaks are located between them. Plotting the N 1s and B 1s peak positions versus 11B concentration (Figure 4b) shows that both the N 1s and B 1s binding energies linearly increase with the 11B concentration, which suggests that the B−N bond in h11BN is slightly stronger than that in h10BN. This trend is similar to a previous study on the carbon doping in MgB2 that showed the 11B−Mg bond is stronger than the 10B−Mg bond due to the isotope effect, which leads to the smaller amount of C substituting for 11B in the lattice.33 In conclusion, we have successfully synthesized monoisotopic hBN single crystals with millimeter size at atmospheric pressure. The hBN crystals produced this way show high quality in Raman spectra. A clear spectral shift from the natural hBN in both shear and intralayer modes is found in monoisotopic hBN, and the broad E2g peak in natural hBN results from the isotopic disorder. The XPS spectra reveal that the B−N bond in h11BN is stronger than that in h10BN. Our newly developed growth method could pave the way for exploring the novel properties and applications of monoisotopic hBN.
REFERENCES
(1) Watanabe, K.; Taniguchi, T. Jahn-Teller effect on exciton states in hexagonal boron nitride single crystal. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 193104. (2) Watanabe, K.; Taniguchi, T.; Niiyama, T.; Miya, K.; Taniguchi, M. Far-ultraviolet plane-emission handheld device based on hexagonal boron nitride. Nat. Photonics 2009, 3, 591−594. (3) Caldwell, J. D.; Kretinin, A. V.; Chen, Y.; Giannini, V.; Fogler, M. M.; Francescato, Y.; Ellis, C. T.; Tischler, J. G.; Woods, C. R.; Giles, A. J.; et al. Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nat. Commun. 2014, 5, 5221. (4) Li, P.; Dolado, I.; Alfaro-Mozaz, F. J.; Casanova, F.; Hueso, L. E.; Liu, S.; Edgar, J. H.; Nikitin, A. Y.; Vélez, S.; Hillenbrand, R. Infrared hyperbolic metasurface based on nanostructured van der Waals materials. Science 2018, 359, 892−896. (5) Liu, Z.; Lee, H.; Xiong, Y.; Sun, C.; Zhang, X. Far-field optical hyperlens magnifying sub-diffraction-limited objects. Science 2007, 315, 1686. (6) Salandrino, A.; Engheta, N. Far-field subdiffraction optical microscopy using metamaterial crystals: Theory and simulations. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 075103. (7) Yao, J.; Liu, Z.; Liu, Y.; Wang, Y.; Sun, C.; Bartal, G.; Stacy, A. M.; Zhang, X. Optical negative refraction in bulk metamaterials of nanowires. Science 2008, 321, 930. (8) Hoffman, A. J.; Alekseyev, L.; Howard, S. S.; Franz, K. J.; Wasserman, D.; Podolskiy, V. A.; Narimanov, E. E.; Sivco, D. L.; Gmachl, C. Negative refraction in semiconductor metamaterials. Nat. Mater. 2007, 6, 946−950. (9) Ishii, S.; Kildishev, A. V.; Narimanov, E.; Shalaev, V. M.; Drachev, V. P. Sub-wavelength interference pattern from volume
C
DOI: 10.1021/acs.chemmater.8b02589 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials plasmon polaritons in a hyperbolic medium. Laser Photonics Rev. 2013, 7, 265−271. (10) Guo, Y.; Cortes, C. L.; Molesky, S.; Jacob, Z. Broadband superPlanckian thermal emission from hyperbolic metamaterials. Appl. Phys. Lett. 2012, 101, 131106. (11) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol. 2010, 5, 722−726. (12) Lee, K. H.; Shin, H.; Lee, J.; Lee, I.; Kim, G.; Choi, J.; Kim, S. Large-scale synthesis of high-quality hexagonal boron nitride nanosheets for large-area graphene electronics. Nano Lett. 2012, 12, 714− 718. (13) Vu, Q. A.; Fan, S.; Lee, S. H.; Joo, M.; Yu, W. J.; Lee, Y. H. Near-zero hysteresis and near-ideal subthreshold swing in h-BN encapsulated single-layer MoS2 field-effect transistors. 2D Mater. 2018, 5, 031001. (14) Lee, G.; Cui, X.; Kim, Y. D.; Arefe, G.; Zhang, X.; Lee, C.; Ye, F.; Watanabe, K.; Taniguchi, T.; Kim, P.; et al. Highly stable, dualgated MoS2 transistors encapsulated by hexagonal boron nitride with gate-controllable contact, resistance, and threshold voltage. ACS Nano 2015, 9, 7019−7026. (15) He, R.; Zhong, S.; Kim, H. H.; Ye, G.; Ye, Z.; Winford, L.; McHaffie, D.; Rilak, I.; Chen, F.; Luo, X.; et al. Dimensionality-driven orthorhombic MoTe2 at room temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, No. 041410, DOI: 10.1103/ PhysRevB.97.041410. (16) Plekhanov, V. G. Isotope effects in lattice dynamics. Phys.-Usp. 2003, 46, 689−715. (17) Kogan, V. Isotope effects in the structural properties of solids. Phys. Usp. 1963, 5, 951−975. (18) Li, J.; Dahal, R.; Majety, S.; Lin, J.; Jiang, H. Hexagonal boron nitride epitaxial layers as neutron detector materials. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 654, 417−420. (19) Doan, T.; Majety, S.; Grenadier, S.; Li, J.; Lin, J.; Jiang, H. Fabrication and characterization of solid-state thermal neutron detectors based on hexagonal boron nitride epilayers. Nucl. Instrum. Methods Phys. Res., Sect. A 2014, 748, 84−90. (20) Chang, C.; Fennimore, A.; Afanasiev, A.; Okawa, D.; Ikuno, T.; Garcia, H.; Li, D.; Majumdar, A.; Zettl, A. Isotope effect on the thermal conductivity of boron nitride nanotubes. Phys. Rev. Lett. 2006, 97, 085901. (21) Lindsay, L.; Broido, D.; Reinecke, T. Phonon-isotope scattering and thermal conductivity in materials with a large isotope effect: A first-principles study. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 144306. (22) Cuscó, R.; Artús, L.; Edgar, J. H.; Liu, S.; Cassabois, G.; Gil, B. Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, 155435. (23) Giles, A. J.; Dai, S.; Vurgaftman, I.; Hoffman, T.; Liu, S.; Lindsay, L.; Ellis, C. T.; Assefa, N.; Chatzakis, I.; Reinecke, T. L.; et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 2018, 17, 134−139. (24) Vuong, T.; Liu, S.; Van der Lee, A.; Cuscó, R.; Artús, L.; Michel, T.; Valvin, P.; Edgar, J.; Cassabois, G.; Gil, B. Isotope engineering of van der Waals interactions in hexagonal boron nitride. Nat. Mater. 2018, 17, 152−158. (25) Hoffman, T. B.; Clubine, B.; Zhang, Y.; Snow, K.; Edgar, J. H. Optimization of Ni−Cr flux growth for hexagonal boron nitride single crystals. J. Cryst. Growth 2014, 393, 114−118. (26) Liu, S.; He, R.; Ye, Z.; Du, X.; Lin, J.; Jiang, H.; Liu, B.; Edgar, J. H. Large-Scale Growth of High-Quality Hexagonal Boron Nitride Crystals at Atmospheric Pressure from an Fe−Cr Flux. Cryst. Growth Des. 2017, 17, 4932−4935. (27) Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Hexagonal Boron Nitride Single Crystal Growth at Atmospheric Pressure Using Ni− Cr Solvent. Chem. Mater. 2008, 20, 1661−1663.
(28) Liu, S.; van Duin, A. C.; van Duin, D. M.; Liu, B.; Edgar, J. H. Atomistic Insights into Nucleation and Formation of Hexagonal Boron Nitride on Nickel from First-Principles-Based Reactive Molecular Dynamics Simulations. ACS Nano 2017, 11, 3585−3596. (29) Chkhartishvili, L. S. Isotopic effects of boron. Trends Inorg. Chem. 2009, 11, 105−167. (30) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bombem, J. Handbook of X-ray photoelectron spectroscopy; Perkin-Elmer Co.: Eden Prairie, MN, 1992. (31) Miyamoto, Y.; Rubio, A.; Cohen, M. L.; Louie, S. G. Chiral tubules of hexagonal BC2N. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 4976−4979. (32) Park, K.; Lee, D.; Kim, K.; Moon, D. Observation of a hexagonal BN surface layer on the cubic BN film grown by dual ion beam sputter deposition. Appl. Phys. Lett. 1997, 70, 315−317. (33) Cheng, F.; Liu, Y.; Ma, Z.; Al Hossain, M. S.; Somer, M. The isotope effect of boron on the carbon doping and critical current density of Mg11B2 superconductors. J. Mater. Chem. C 2017, 5, 663− 668.
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DOI: 10.1021/acs.chemmater.8b02589 Chem. Mater. XXXX, XXX, XXX−XXX
