Nanocrystal Shape and the Mechanism of Exciton Spin Relaxation (original) (raw)
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Relaxation in the Exciton Fine Structure of Semiconductor Nanocrystals
The Journal of Physical Chemistry C, 2009
A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of (1 and-2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin-orbit coupling effect.
Exciton spin dynamics in spherical CdS quantum dots
Physical Review B, 2010
Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k⋅p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time-and polarizationresolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.
Bright-exciton fine structure and anisotropic exchange in CdSe nanocrystal quantum dots
2006
We report on polarization-resolved resonant photoluminescence (PL) spectroscopy of bright (spin ±1) and dark (spin ±2) excitons in colloidal CdSe nanocrystal quantum dots. Using high magnetic fields to 33 T, we resonantly excite (and selectively analyze PL from) spin-up or spin-down excitons. At low temperatures (<4K) and above ∼10 T, the spectra develop a narrow, circularly polarized peak due to spin-flipped bright excitons. Its evolution with magnetic field directly reveals a large (1-2 meV), intrinsic fine structure splitting of bright excitons, due to anisotropic exchange. These findings are supported by time-resolved PL studies and polarization-resolved PL from single nanocrystals.
Spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanocrystals
Physical Review B, 2013
The spin dynamics in chemically synthesized CdSe/CdS core/shell nanocrystals (NCs) are studied by polarization-and time-resolved photoluminescence (PL) techniques in high magnetic fields and at low temperatures. Analysis of the recombination dynamics shows that the emission of thin-shell NCs is contributed by neutral excitons, while for thick-shell NCs it is dominated by charged excitons (trions). The sign of the PL polarization unambiguously demonstrates that these trions are negatively charged. A theoretical model of the PL polarization in an ensemble of randomly oriented NCs describes well magnetic field and time dependences of the PL polarization degree and allows us to determine the hole g factor in CdSe/CdS NCs, g h = −0.54. From direct measurements of the spin relaxation rate dependences on magnetic field and temperature, we identify the mechanism of the negative trion spin relaxation as two-phonon-assisted Raman scattering between the hole spin sublevels mixed by the applied magnetic field.
Spin-dependent exciton quenching and spin coherence in CdSe/CdS nanocrystals
2012
Large surface to volume ratios of semiconductor nanocrystals cause susceptibility to charge trapping, which can modify luminescence yields and induce single-particle blinking. Optical spectroscopies cannot differentiate between bulk and surface traps in contrast to spin-resonance techniques, which in principle avail chemical information on such trap sites. Magnetic resonance detection via spin-controlled photoluminescence enables the direct observation of interactions between emissive excitons and trapped charges.
Optical Spectroscopy of Dark and Bright Excitons in CdSe Nanocrystals in High Magnetic Fields
The Journal of Physical Chemistry C, 2017
Low temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spinrelaxation and the radiative lifetimes of excitons in wurtzite semiconductor (e.g. CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton-levels. We analyse these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting to quantify the exciton, electron and hole g-factors. 2
Physical Review B, 2006
The dynamics of exciton decay and spin dephasing in an ensemble of CdSe quantum dots grown in a glass matrix has been investigated by studying the polarized time-integrated four-wave mixing signal at 10 K. Quantum dots of equal size, but with different growth conditions, have been found to exhibit similar decay times but significantly ͑orders of magnitude͒ different spin depolarization times. We connect this effect to the difference in the quantum dot anisotropy. The anisotropy results in the splitting of bright J = 1 exciton states that causes the oscillation of circularly excited excitons between the two linearly polarized split states. In a quantum dot ensemble with a random distribution in the exciton level splitting, that results in the depolarization of the four-wave mixing signal observed in the scattering of − and + polarized light on the population grating created by two + pulses. The dependence of the quantum dot anisotropy on the growth conditions is discussed in terms of thermophysics of nanoparticle growth in glass.
Optical studies of spin relaxation in CdTe self-assembled quantum dots
physica status solidi (c)
We study exciton spin relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. The results show that by combining LO phonon -assisted absorption with circularly polarized resonant excitation, spin-polarized excitons may be photoexcited into the ground states of QDs. We find for both single CdTe QDs and large QD ensembles that when the exciton spin levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted, excitons maintain their spin polarization on a time scale much longer than the exciton recombination time (~300ps). A rate equation model allows us to estimate the spin relaxation time of the excitons in the CdTe QDs to be 4.8 ± 0.3 ns at T = 5 K.
Exciton spin relaxation in resonantly excited CdTe/ZnTe self-assembled quantum dots
Physical review. B, Condensed matter
We study the exciton spin relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. The experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon -assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of QDs. We find that for single symmetric QDs at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by QD shape asymmetry. Similar behavior is found in a large ensemble of CdTe QDs. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a QD become non-degenerate. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the exciton ground state during the excitation. J J π X π Y J J B=0 B=0 B≠0
ExcitonâExciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals
Advanced Materials, 2009
Semiconductor colloidal nanocrystals have been proposed as optically-active media for solution-processable optoelectronic devices, because they combine inexpensive, wet-chemistry synthesis with high photoluminescence quantum yield, large oscillator strength and size tuneability of optical transitions. Key to the success of nanocrystal-based devices is the possibility to design and consistently synthesize nanocrystals with desired properties. Size uniformity can be usually controlled within less than 5% uncertainty; surface capping, passivation and core/shell structures can lead to photoluminescence quantum yields exceeding 50%, optical gain and lasing. A new frontier in nanocrystal design has appeared with heterostructures allowing spatial separation of electron and hole wavefunctions, like in type-II CdSe/CdTe core/shell nanocrystals, through staggered conduction and valence band offsets. Charge separation inside nanocrystals is useful in photodetector and photovoltaic devices, quantum optics and low-threshold lasers. Exciton nonlinearities also depend on the degree of separation of electron and hole wavefunction. In type-II heterostructures, it has been demonstrated that charge separation can lead to a large repulsive exciton-exciton interaction. The resulting blueshift of the exciton-to-biexciton transition suppresses to a large extent resonant re-absorption of stimulated emission from single-exciton states, allowing net optical gain and lasing at excitations corresponding to less than one electron-hole pair per nanocrystal. In this regime, losses inherent to multiexciton recombinations are avoided, resulting in optical gain with a much longer lifetime, an essential step towards the demonstration of lasing under continuous wave operation.