Optimization of solution composition in hexagonal boron nitride crystal growth via the flux method

Doctor of PhilosophyDepartment of Chemical EngineeringJames H EdgarHexagonal boron nitride (hBN) is an ultrawide bandgap (>6 eV) semiconductor and 2D material that has attracted much attention due to its unique properties and applications in electronics, optoelectronics, and nanophotonics. In all of these applications, large, high quality single crystals of hBN are required and atmospheric pressure solution growth is a consistent method to achieve this. This study was undertaken to improve this process, accelerate its optimization, and enable creation of devices in a wide range of fields. A new methodology was developed to optimize the boron concentration in hBN solution growth using the CALPHAD (CALculation of PHAse Diagrams) method to rapidly predict the optimal boron concentration for a wide range of solvents. Comparison with experimental results validates its accuracy. Deviations from CALPHAD predictions, confirmed with crystal growth by reusing source material, suggest that the hBN crystal growth process from molten metal solutions is kinetically limited. Reusing source material also substantially improves the yield of boron to hBN, which is especially important when using expensive isotopically pure boron for growth of h10BN or h11BN. Increasing the nitrogen solubility of the solvent is often attributed to increasing crystal size, but this work digs deeper into the effects of this property. Five different solvents (Ni-Cr, Co-Cr, Fe, Fe-V, and Cu) were tested and the domain area and thickness of crystals they produced were compared versus their nitrogen solubility. The nitrogen solubility did not affect the hBN domain area but the crystal thickness increased with nitrogen solubility. This suggests that, so long as the boron concentration is properly optimized, similar domain sizes can be obtained from any solvent. Furthermore, the thickness of as-grown crystals may be engineered for specific applications by choosing a solvent that naturally grows hBN of the required thickness. Finally, the optimal boron concentration increased with the nitrogen solubility, provides a shortcut for optimizing future solvents, accelerating research. Crystal defects such as stacking faults, dislocations, and impurities are detrimental to device performance, thus it is important to understand their properties and how they can be avoided or eliminated. Oxygen impurities were greatly reduced in the solvent with the introduction of hydrogen gas while carbon impurities may need to react with oxygen to be removed. Regardless, the impurity content in hBN crystals grown from these solvents was below the detection limit of secondary ion mass spectrometry (SIMS) in all cases, suggesting the purity of the process is already sufficient. Three classes of defects were detected using cathodoluminescence (CL) spectroscopy: spots, invisible lines, and wrinkles, which were determined to be color centers, half-inserted planes, and plastic deformation of hBN single crystals, respectively. A combination of Raman, photoluminescence (PL), and CL spectroscopy indicates that hBN crystals grown from Ni-Cr and Co-Cr tend to have fewer defects than those grown from Fe, Fe-V, or Cu. Monoisotopic hBN is especially useful in applications where coherence of phonons is especially important such as sub-wavelength optical microscopy and quantum sensing. Previously, processes were developed to synthesize hBN enriched with either the 10B or 11B isotopes using naturally abundant 14N2. In this work, a new process was developed to extend the capabilities of solution growth to also produce hBN with the 15N isotope. Raman and PL spectroscopy on these crystals indicate that they are very high quality, on par with crystals grown with 14N. Furthermore, the effect of the reduced mass on the Raman shift of the E2ghigh peak and the energy of the phonon replicas is identified, which is in excellent agreement with theory

Isotope engineering for spin defects in van der Waals materials

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy center ($\mathrm{V}_{\mathrm{B}}^-$) in hexagonal boron nitride (hBN), we grow isotopically purified $\mathrm{h}{}^{10}\mathrm{B}{}^{15}\mathrm{N}$ crystals for the first time. Compared to $\mathrm{V}_{\mathrm{B}}^-$ in hBN with the natural distribution of isotopes, we observe substantially narrower and less crowded $\mathrm{V}_{\mathrm{B}}^-$ spin transitions as well as extended coherence time $T_2$ and relaxation time $T_1$. For quantum sensing, $\mathrm{V}_{\mathrm{B}}^-$ centers in our $\mathrm{h}{}^{10}\mathrm{B}{}^{15}\mathrm{N}$ samples exhibit a factor of $4$ ($2$) enhancement in DC (AC) magnetic field sensitivity. For quantum registers, the individual addressability of the $\mathrm{V}_{\mathrm{B}}^-$ hyperfine levels enables the dynamical polarization and coherent control of the three nearest-neighbor ${}^{15}\mathrm{N}$ nuclear spins. Our results demonstrate the power of isotope engineering for enhancing the properties of quantum spin defects in hBN, and can be readily extended to improving spin qubits in a broad family of van der Waals materials.Comment: 8+4+8 pages, 4+4+6 figure

trans-Acetyldicarbonyl(η5-cyclopentadienyl)[tris(3,5-dimethylphenyl)phosphane-κP]molybdenum(II)

The title compound, [Mo(C5H5)(C2H3O)(C24H27P)(CO)2], was prepared by reaction of [Mo(C5H5)(CO)3(CH3)] with tris(3,5-dimethylphenyl)phosphane. The complex exhibits a four-legged piano-stool geometry with trans-disposed acetyl and phosphane ligands. The molecular geometry is nearly identical to that of the triphenylphosphane derivative, but introduction of methyl groups on the aromatic phosphane substituents significantly impacts supramolecular organization. In the crystal, non-classical C—H...O interactions involving the acetyl carbonyl group lead to a chain motif along [010], and another set of C—H...O close contacts join inversion-related molecules

Polytypes of sp²-Bonded Boron Nitride

The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2-bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light

Isotope engineering for spin defects in van der Waals materials.

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy center ([Formula: see text]) in hexagonal boron nitride (hBN), we grow isotopically purified h10B15N crystals. Compared to [Formula: see text] in hBN with the natural distribution of isotopes, we observe substantially narrower and less crowded [Formula: see text] spin transitions as well as extended coherence time T2 and relaxation time T1. For quantum sensing, [Formula: see text] centers in our h10B15N samples exhibit a factor of 4 (2) enhancement in DC (AC) magnetic field sensitivity. For additional quantum resources, the individual addressability of the [Formula: see text] hyperfine levels enables the dynamical polarization and coherent control of the three nearest-neighbor 15N nuclear spins. Our results demonstrate the power of isotope engineering for enhancing the properties of quantum spin defects in hBN, and can be readily extended to improving spin qubits in a broad family of van der Waals materials

Bound states in the continuum and long-range coupling of polaritons in hexagonal boron nitride nanoresonators

Bound states in the continuum (BICs) garnered significant for their potential to create new types of nanophotonic devices. Most prior demonstrations were based on arrays of dielectric resonators, which cannot be miniaturized beyond the diffraction limit, reducing the applicability of BICs for advanced functions. Here, we demonstrate BICs and quasi-BICs based on high-quality factor phonon-polariton resonances in isotopically pure h11BN and how these states can be supported by periodic arrays of nanoresonators with sizes much smaller than the wavelength. We theoretically illustrate how BICs emerge from the band structure of the arrays and verify both numerically and experimentally the presence of these states and enhanced quality factor. Furthermore, we identify and characterize simultaneously quasi-BICs and bright states. Our method can be generalized to create a large number of optical states and to tune their coupling with the environment, paving the way to miniaturized nanophotonic devices with more advanced functions