Center for Relativistic Laser Science

   IBS Center for Molecular Spectroscopy and Dynamics

n  SPEAKER
Dr. Young-Shin Park (Department of Chemical & Biomolecular Engineering, KAIST)

n  TITLE
Compositionally graded CdSe/CdxZn1-xSe colloidal quantum dots

n  ABSTRACT
Semiconductor colloidal quantum dots (QDs) have been explored as promising nanomaterials for light-emission application such as light-emitting diodes (LEDs) and display due to their favorable properties at room temperature: size-tunable emission colors based on quantum confinement effect, very high brightness (quantum yield up to ~ 100%), and inexpensive solution-based processability. One of long-standing issues that remain unresolved in QD studies is the control of nonradiative Auger recombination of multicarrier states, where the electron-hole recombination energy is not converted to a photon, but instead transferred to a third carrier. Due to Auger process, biexciton lifetime is very fast (typically, < 100ps) and makes biexciton state nearly non-emissive (quantumyield ~ a few % only). Thus, Auger recombination is normally known to beresponsible for, for example, efficiency roll-off in LEDs and photoluminescence(PL) intermittency in single-dot level.​ ​Among trials to suppress Augerprocess in heterostructured QDs, an effective way is to make the​ ​core-shellpotential transition ‘soft’, instead of ‘sharp’ core-shell interface, as widely reported in​ ​various publications.

In this talk, I will discuss the newly developed CdSe/CdxZn1-xSe QDs, wherein the CdSe core is surrounded by a compositionally graded shell, leading to ‘soft’ interface along the radial direction. We demonstrated that biexciton quantum yield of CdSe/CdxZn1-xSe QDs is as high as ~ 45% along with an extended biexciton lifetime (tBX > 1 ns). When these QDs are applied in devices, we achieved nearly roll-off-free behavior in LEDs and theoretical-limit thresholds in QD lasers. Interestingly, in single-level studies, these structures exhibit a highly stable emission energy (s.d. < 1 meV) along with non-blinking emission. In addition, a verynarrow, subthermal roomtemperature linewidth (~20 meV) is observed, which isattributed to strong suppression of excitonphonon interaction. These remarkablespectral characteristics make these compositionally graded QDs well suited forpractical realization of single-photon sources.

n  DATE AND VENUE
December 13, 2021 (Monday, 16:00 - 17:00)  

Seminar Room B (119), KU R&D Center

n  LANGUAGE
Korean

n  INVITED BY
Professor Kwang Seob Jeong