Department of Materials Science and Engineering, Centennial Campus, North Carolina State University, Raleigh, NC 27695-7907, USA.
Phys Chem Chem Phys. 2019 Jan 23;21(4):1700-1710. doi: 10.1039/c8cp04592e.
Nano- and microstructures of phase-pure cubic boron nitride (c-BN) are synthesized by employing nanosecond pulsed-laser annealing techniques at room temperature and atmospheric pressure. In a highly non-equilibrium synthesis process, nanocrystalline h-BN is directly converted into phase-pure twinned c-BN from a highly undercooled melt state of BN. By changing nucleation and growth rates, we have synthesized a wide range of sizes (90 nm to 25 μm) of c-BN. The electron diffraction patterns show the formation of twinned c-BN with [11[combining macron]1] as the twin axis. The twinning density in c-BN can be controlled by the degree of undercooling and quenching rates. The formation of twins predominantly occurs prior to the formation of amorphous quenched BN (Q-BN). Therefore, the defect density in nano c-BN formed under higher undercooling conditions is considerably larger than that in micro c-BN, which is formed under lower undercooling conditions. The temperature-dependent Raman studies show a considerable blue-shift of ∼6 cm-1 with a decrease in temperature from 300 to 78 K in nano c-BN as compared to micro c-BN. The size-effects of c-BN crystals in Raman spectra are modeled using spatial correlation theory, which can be used to calculate the correlation length and twin density in c-BN. It has also been found that the Raman blue-shift in nano c-BN is caused by anharmonic effects, and the decrease in Raman linewidth with decreasing temperature (300 to 78 K) is caused by three- and four-phonon decay processes. The bonding characteristics and crystalline nature of the synthesized c-BN are also demonstrated by using electron energy-loss spectroscopy and electron backscatter diffraction, respectively. We envisage that the controlled growth of phase-pure nano and microstructures of twinned c-BN and their temperature-dependent Raman-active vibrational mode studies will have a tremendous impact on low-temperature solid-state electrical and mechanical devices.
通过室温常压下的纳秒脉冲激光退火技术，合成了纯相立方氮化硼（c-BN）的纳米和微结构。在高度非平衡的合成过程中，纳米晶 h-BN 直接从 BN 的过冷熔体状态转化为纯相孪晶 c-BN。通过改变形核和生长速率，我们合成了一系列不同尺寸（90nm 至 25μm）的 c-BN。电子衍射图案显示出孪晶 c-BN 的形成，其孪晶轴为[11[combining macron]1]。c-BN 中的孪晶密度可以通过过冷度和淬火速率的控制来调节。孪晶的形成主要发生在非晶态淬火 BN（Q-BN）形成之前。因此，在较高过冷度条件下形成的纳米 c-BN 的缺陷密度明显大于在较低过冷度条件下形成的微 c-BN 的缺陷密度。温度依赖性拉曼研究表明，与微 c-BN 相比，纳米 c-BN 在 300 至 78 K 的温度范围内从 300 至 78 K 时，拉曼光谱有相当大的蓝移，约为 6cm-1。使用空间相关理论对 c-BN 晶体的拉曼光谱的尺寸效应进行建模，该理论可用于计算 c-BN 的相关长度和孪晶密度。还发现，纳米 c-BN 中的拉曼蓝移是由非谐效应引起的，拉曼线宽随温度降低（300 至 78 K）而减小是由三声子和四声子衰减过程引起的。电子能量损失谱和电子背散射衍射分别用于证明合成的 c-BN 的键合特性和晶体性质。我们预计，对纯相纳米和微结构的孪晶 c-BN 的控制生长及其温度相关的拉曼活性振动模式的研究将对低温固态电子和机械器件产生巨大影响。
