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Synthesis of Quantum Dots

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We synthesize colloidal semiconductor nanocrystals, which are called ‘Quantum Dots’. Our research aims at structural engineering of quantum dots for their use in optoelectronic applications such as light-emitting device, luminescent solar concentrator by a quantum dot and etc.

Fig 1. Representative photo of synthesized nanocrystals and their transmission electron microscope image.

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We can control their dimension, shape, composition, and heterostructures on demand. In our lab, lots of semiconductors made of binary compounds such as CdSe, CdS, ZnTe, ZnSe, ZnS, InP, InAs and their alloys can be synthesized by our students and facilities. In combination with spectroscopic analysis with lots of collaborators in the world, we discover the secrets of quantum dots.

Fig 2. Novel heterostructure quantum dots with near-unity photoluminescent quantum yield

Optoelectronic Devices using Quantum Dots

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Fig 1. Representative device fabrication method

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We fabricate optoelectronic device by using QDs synthesized in our lab. There are many applications using QDs. For example, QD solar cell, QD LSC, QD detector, QD laser and QD-LEDs are well known applications. Based on  narrow spectrum line width of QDs, QD-LEDs produce very pure red, green and blue light. Based on this characteristics, QD-LEDs are promising applications. In our group, we not only fabricate device, but also characterize and analyze device operation. This analysis provides deep understanding on operation mechanism of QD-LEDs and suggests design protocol for high performance QD-LEDs.

Fig 2. Device stability testing system

Optical Characterization of Quantum Dots

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Fig 1. PL-decay dynamics

Many features of the single exciton fluorescence have been extensively studied for aiding theories to applications and understanding phenomena within QDs. Beyond the essential optical characterizations, such as absorption and emission spectroscopy, we have extended the range of characterization techniques to explore carrier dynamics over time scales ranging from microsecond to picosecond. Time-resolved photoluminescence (TrPL) spectroscopy of QD ensembles reveals new features when the QD is excited with a pulse laser.

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Fig 2. PL intermittency(“blinking’) in single QD 

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Observation of individual QDs gives us a chance to learn about new phenomena, which cannot be obtained from ensemble spectroscopy. The focus is on the mechanisms underlying the dynamic inhomogeneities—spectral diffusion and fluorescence intermittency—observed in the emission properties of single QDs. Individual dots have the tendency of turning on and off with a statistical pattern, and the phenomena can be recorded via a highly sensitive single-photon detector.

 

 

These measurements offer a glance into the capacity of single QD spectroscopy in unraveling the intricacies of single semiconductor QD optical dynamics. The narrow emission spectra of a QD have been obtained from electron-multiplying charge-coupled devices (EMCCDs). Spectral diffusion refers to discrete and continuous changes in the emitting wavelength as a function of time. With a Hanbury-Brown-Twiss setup with two single-photon detectors, we also measure single QD biexciton physics.

Fig 3. Two-photon correlation measurements

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