Hu Zhi-Xin, Kong Xianghua, Qiao Jingsi, Normand Bruce, Ji Wei
Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices, Renmin University of China, Beijing 100872, China.
Nanoscale. 2016 Feb 7;8(5):2740-50. doi: 10.1039/c5nr06293d.
Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D systems. Indeed, these couplings call into question whether the stacking of FLBP can be governed only by vdW interactions, which is of crucial importance for epitaxy and property refinement. Here, we perform a theoretical investigation of the vibrational properties of FLBP, which reflect directly its interlayer coupling, by discussing six Raman-observable phonons, including three optical, one breathing and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface phenomena, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from the electronic hybridization of the lone electron-pairs of FLBP, rather than from vdW interactions. This exceptional interlayer coupling mechanism controls the stacking stability of BP layers and thus opens a new avenue beyond vdW epitaxy for understanding the design of 2D heterostructures.
将二维（2D）材料堆叠成多层或异质结构，即所谓的范德华（vdW）外延，是按需调整其性能的一个重要自由度。少层黑磷（FLBP）是一种在纳米和光电子应用中具有很高潜力的材料，其层间耦合似乎比石墨烯和其他二维系统要强得多。实际上，这些耦合让人质疑FLBP的堆叠是否仅受范德华相互作用的控制，而这对外延和性能优化至关重要。在此，我们通过讨论六种拉曼可观测声子，包括三种光学声子、一种呼吸模式和两种剪切模式，对FLBP的振动特性进行了理论研究，这些振动特性直接反映了其层间耦合。随着样品厚度的增加，我们发现每种光学模式的频率出现异常红移，但扶手椅剪切模式出现蓝移。我们的计算还表明，由于异常表面现象，声子分支会发生分裂，并且存在强烈的声子 - 声子耦合。通过计算单轴应力效应、原子间力常数和电子密度，我们有力地证明了这些特性是由FLBP孤对电子的电子杂化产生的强且高度定向的层间相互作用导致的，而非范德华相互作用。这种特殊的层间耦合机制控制着BP层的堆叠稳定性，从而为理解二维异质结构的设计开辟了一条超越范德华外延的新途径。
