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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 Exciton Quantum dot CdSe behavior • Motivation • Experiments & Results FLN measurement Magnetic field dependence • Summary Exciton Band structure of semiconductor Exciton energy diagram E E … n=∞ Conduction band n=2 n=1 (1s) G photon Eg Eg recombination luminescence Valence band k k electron hole Quantum dot (QD) Quantum dot ・・・ a nanoparticle of semiconductor Full fluorescence (Broadening due to size distribution) Intensity DOS Buik crystal Fluorescence line of single size QDs Energy QD DOS Energy QD size small Energy Artificial atom large CdSe behavior Smaller size crystal quantum dot (QD) bulk crystal Optically active singlet triplet 1s singlet 1s 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 short ~1 ns long ~1μs 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 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 CdSe QD sample Organometallic synthesis Me2Cd & TOP + TOPSe & TOP ・Dot size changes as time passes. ・We can control the dot size by removing some at proper time. （15~115Å） Me : methyl group TOP : trioctylphosphine ・size distribution < 5% ・high quantum yield 0.1~0.9 Experimental result - 1 FLN spectra (10K) TEM data Crystal size large small Stokes shift 2 meV 20 meV Intensity (a.u.) ZPL (zero LO phonon line) Energy diagram Thermal relaxation Stokes shift Stokes shift singlet triplet excitation Energy (meV) Solid line : FLN spectra Dashed line : laser line GND luminescence 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. Stokes Shift (meV) 10K Discrepancy between experiment and theory for small radius There are additional contributions by phonons For example… Exction-acoustic phonon coupling Radius (Å) : Experimental values Solid line : theoretical size dependent splitting between singlet and triplet The size dependence of energy splitting is clarified. Experimental result - 2 Magnetic field dependence of the FLN spectra triplet LO phonon ZPL 1PL (one phonon line) GND magnetic field ZPL / 1PL Normalized to the zero field 1 phonon line (1PL) a = 12Å Excitation energy : 2.467 eV (at the band edge) 0T 10 T 0T weak strong Relaxation of triplet state needs LO phonon assist. Increasing magnet field Triplet exciton is mixed with optically active singlet exciton. Experimental result - 2 Dark exciton lifetime in a magnetic field Magnetic field lifetime 0T long 10 T short The emission comes primarily from the triplet state. ( ⇒ Thermalization processes are efficient.) Pump energy : 2.736 eV Emission decay : 2.436 eV (at the luminescence peak) a=12Å Quantum yield remains essentially constant. Short lifetime originates from an enhancement of the radiative rate by mixing. 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.