Kang Sungwoo, Kim Yongwook, Jang Eunjoo, Kang Youngho, Han Seungwu
Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Korea.
Inorganic Material Lab, Samsung Advanced Institute of Technology, Samsung Electronics, Gyeonggi-do 16678, Korea.
ACS Appl Mater Interfaces. 2020 May 13;12(19):22012-22018. doi: 10.1021/acsami.0c02904. Epub 2020 Apr 29.
The emission linewidth of a semiconducting nanocrystal (NC) significantly affects its performance in light-emitting applications, but its fundamental limit is still elusive. Herein, we analyze the exciton-phonon coupling (EPC) from Huang-Rhys (HR) factors using ab initio calculations and compute emission line shapes of CdSe NCs. When surface traps are absent, acoustic modes are found to dominate EPC. The computed linewidths are mainly determined by the size of NCs, being largely insensitive to the shape and crystal structure. Linewidths obtained in this work are much smaller than most measurements on homogeneous linewidths, but they are consistent with a CdSe/CdZnSe (core/shell) NC [Park, Y.-S.; Lim, J.; Klimov, V. I. 2019 18, 249-255]. Based on this comparison, it is concluded that the large linewidths in most experiments originated from internal fields by surface (or interface) traps or quasi-type II band alignment that amplifies EPC. Thus, the present results on NCs with ideal passivation provide the fundamental minimum of homogeneous linewidths, indicating that only the CdSe/CdZnSe NC has achieved this limit through well-controlled synthesis of shell structures. To further verify the role of internal fields, we model NCs with charged surface defects. We find that the internal field significantly increases HR factors and linewidths, in reasonable agreement with experiments on single cores. By revealing the fundamental limit of the emission linewidths of quantum dots, this work will pave the way for engineering quantum dots with an ultrasharp spectrum.
半导体纳米晶体（NC）的发射线宽对其发光应用性能有显著影响，但其基本极限仍不清楚。在此，我们使用从头算计算从黄-里斯（HR）因子分析激子-声子耦合（EPC），并计算CdSe NCs的发射线形。当不存在表面陷阱时，发现声学模式主导EPC。计算得到的线宽主要由NCs的尺寸决定，对形状和晶体结构基本不敏感。本工作中获得的线宽比大多数关于均匀线宽的测量值小得多，但与CdSe/CdZnSe（核/壳）NC [Park, Y.-S.; Lim, J.; Klimov, V. I. 2019 18, 249 - 255]一致。基于这种比较，得出结论：大多数实验中的大线宽源于表面（或界面）陷阱或准II型能带排列产生的内场，这些内场会放大EPC。因此，本研究中关于具有理想钝化的NCs的结果提供了均匀线宽的基本最小值，表明只有CdSe/CdZnSe NC通过精心控制壳结构的合成达到了这一极限。为了进一步验证内场的作用，我们对具有带电表面缺陷的NCs进行建模。我们发现内场显著增加了HR因子和线宽，这与单核的实验结果合理一致。通过揭示量子点发射线宽的基本极限，这项工作将为设计具有超锐光谱的量子点铺平道路。
