Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371.
Nano Lett. 2011 Jun 8;11(6):2407-14. doi: 10.1021/nl200773n. Epub 2011 May 23.
We report on Raman spectroscopy of few quintuple layer topological insulator bismuth selenide (Bi2Se3) nanoplatelets (NPs), synthesized by a polyol method. The as-grown NPs exhibit excellent crystalline quality, hexagonal or truncated trigonal morphology, and uniformly flat surfaces down to a few quintuple layers. Both Stokes and anti-Stokes Raman spectroscopy for the first time resolve all four optical phonon modes from individual NPs down to 4 nm, where the out-of-plane vibrational A(1g)(1) mode shows a few wavenumbers red shift as the thickness decreases below 15 nm. This thickness-dependent red shift is tentatively explained by a phonon softening due to the decreasing of the effective restoring force arising from a decrease of the van der Waals forces between adjacent layers. Quantitatively, we found that the 2D phonon confinement model proposed by Faucet and Campbell cannot explain the red shift values and the line shape of the A(1g)(1) mode, which can be described better by a Breit–Wigner–Fano resonance line shape. Considerable broadening (17 cm(–1) for six quintuple layers) especially for the in-plane vibrational mode E(g)(2) is identified, suggesting that the layer-to-layer stacking affects the intralayer bonding. Therefore, a significant reduction in the phonon lifetime of the in-plane vibrational modes is probably due to an enhanced electron–phonon coupling in the few quintuple layer regime.
我们报告了通过多元醇法合成的少数五重层拓扑绝缘体碲化铋（Bi2Se3）纳米板（NPs）的拉曼光谱。所生长的 NPs 表现出极好的结晶质量、六边形或截顶三角形态，以及均匀平坦的表面，直至少数五重层。斯托克斯和反斯托克斯拉曼光谱首次在单个 NPs 中分辨出所有四个光学声子模式，直至 4nm，其中面外振动 A(1g)(1)模式随着厚度降至约 15nm 以下表现出几个波数的红移。这种厚度相关的红移可以通过由于相邻层之间的范德华力减小而导致的有效恢复力减小导致的声子软化来初步解释。定量地，我们发现，Faucet 和 Campbell 提出的 2D 声子限制模型不能解释 A(1g)(1)模式的红移值和线形状，而 Breit–Wigner–Fano 共振线形状可以更好地描述它。特别对于面内振动模式 E(g)(2)，发现相当大的展宽（对于六重层约为 17cm(-1)），这表明层间堆积会影响层内键合。因此，面内振动模式的声子寿命可能由于少数五重层范围内的电子-声子耦合增强而显著降低。
