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Observation of the “Dark Exciton” in CdSe Quantum Dots

Observation of the “Dark Exciton” in CdSe Quantum Dots. N. Nirmal et al., Phys. Rev. Lett. 75, 3728 (1995). Itoh Lab Takanobu Yamazaki. Contents. Introduction Motivation Experiments & Results Summary. Exciton. Quantum dot. CdSe behavior. FLN measurement. Magnetic field dependence.

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Observation of the “Dark Exciton” in CdSe Quantum Dots

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  1. Observation of the “Dark Exciton” in CdSe Quantum Dots N. Nirmal et al., Phys. Rev. Lett. 75, 3728 (1995) Itoh Lab Takanobu Yamazaki

  2. Contents • Introduction • Motivation • Experiments & Results • Summary Exciton Quantum dot CdSe behavior FLN measurement Magnetic field dependence

  3. E E n=∞ photon … n=2 n=1 (1s) G Eg k luminescence recombination k electron hole Exciton Band structure of semiconductor Exciton energy diagram Conduction band Eg Valence band

  4. DOS DOS Energy Artificial atom Quantum dot (QD) Quantum dot　・・・　a nanoparticle of semiconductor Full fluorescence Buik crystal (Broadening due to size distribution) Intensity Fluorescence line of single size QDs Energy QD Energy QD size small large

  5. bulk crystal CdSe behavior Smaller size crystal quantum dot (QD) Optically active singlet singlet 1s 1s triplet Optically passive triplet 0.13 meV The enhancement of electron - hole exchange interaction 12.5 meV (12Å) Exchange interaction depends on overlap of the electron and hole wave function Radiative lifetime long short ~1μs ~1 ns

  6. motivation An exciton confined in the CdSe QD shows the long lifetime compared to that in the bulk crystal. This long lifetime is related to the dark exciton. By applying an external magnetic field, the dark exciton is mixed with the bright exciton. (mixing) How is the dark exciton in the quantum dot related to the exciton dynamics ? Fluorescence line narrowing (FLN) measurement Magnetic field dependence measurement

  7. Fluorescence line narrowing (FLN) measurement Refer) M.Nirmal,et.al, phys.Rev.B 50, 2293 Quantum dot size small large Absorption energy high low Generally, the sample has a size distribution. Excitation on the red edge of the sample absorption To measure the exciton fine structure Selectively excite the largest dots

  8. Me2Cd & TOP + TOPSe & TOP Me : methyl group TOP : trioctylphosphine CdSe QD sample Organometallic synthesis ・Dot size changes as time passes. ・We can control the dot size by removing some at proper time. 　（15~115Å） ・size distribution < 5% ・high quantum yield 0.1~0.9

  9. Energy diagram Stokes shift Thermal relaxation singlet triplet excitation luminescence GND Crystal size large small Stokes shift 2 meV 20 meV Solid line : FLN spectra Dashed line : laser line Experimental result - 1 FLN spectra (10K) TEM data ZPL (zero LO phonon line) Intensity (a.u.) Stokes shift Energy (meV)

  10. : Experimental values Experimental result - 1 Comparison between experimental value and theoretical value Increment of energy splitting due to confinement effect on a relative motion of an electron and hole. 10K Stokes Shift (meV) Discrepancy between experiment and theory for small radius There are additional contributions by phonons For example… Exction-acoustic phonon coupling The size dependence of energy splitting is clarified. Radius (Å) Solid line : theoretical size dependent splitting between singlet and triplet

  11. Experimental result - 2 Magnetic field dependence of the FLN spectra triplet LO phonon ZPL 1PL (one phonon line) GND 10 T magnetic field 0 T ZPL / 1PL weak strong 0 T Relaxation of triplet state needs LO phonon assist. Normalized to the zero field 1 phonon line (1PL) a = 12Å Increasing magnet field Excitation energy : 2.467 eV (at the band edge) Triplet exciton is mixed with optically active singlet exciton.

  12. Experimental result - 2 Dark exciton lifetime in a magnetic field 0 T 10 T Magnetic field lifetime short long The emission comes primarily from the triplet state. ( ⇒ Thermalization processes are efficient.) Quantum yield remains essentially constant. Pump energy : 2.736 eV Emission decay : 2.436 eV (at the luminescence peak) Short lifetime originates from an enhancement of the radiative rate by mixing. a=12Å

  13. Summary • The authors measured size dependence of FLN spectra in CdSe QDs. 　　⇒The band edge exciton structure (energy splitting between the bright and dark exciton state) is clarified. • They measured magnetic field dependence of FLN spectra and luminescence decays. 　　⇒An external magnetic field mixes the dark exciton with the bright exciton states and allows its recombination. The authors confirmed the presence and the dynamics of the dark exciton state.

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