Spin dynamics in electrochemically charged CdSe quantum dots

Abstract

We use time-resolved Faraday rotation to measure coherent spin dynamics in colloidal CdSe quantum dots charged in an electrochemical cell at room temperature. Filling of the 1Se electron level is demonstrated by the bleaching of the 1Se-1S 3/2 absorption peak. One of the two Landé g-factors observed in uncharged quantum dots disappears upon filling of the 1Se electron state. The transverse spin coherence time, which is over 1 ns and is limited by inhomogeneous dephasing, also appears to increase with charging voltage. The amplitude of the spin precession signal peaks near the half-filling potential. Its evolution at charging potentials without any observable bleaching of the 1Se-1S 3/2 transition suggests that the spin dynamics are influenced by low-energy surface states.PACS numbers: 78.67. Hc, 78.47.+p, 71.35.Pq, In recent years, chemically synthesized semiconductor quantum dots (QDs) have been used in a wide variety of applications from QD-lasers 1,2,3,4 and light emitting devices 5,6 to fluorescent labels. 7,8 The size-tunable spectrum of energy levels and the ease of colloidal nanocrystal synthesis make these nanocrystals versatile systems for the study of confined charges; the ability to optically excite carriers has spurred work on coherent spin dynamics in colloidal QDs revealing multiple Landé g-factors and spin coherence times reaching several nanoseconds at room temperature.9,10 Advances in electrochemical charging of such nanocrystals have given researchers exquisite control over the electronic state of the QD and have opened new avenues in the study of state-filling effects in colloidal QDs. 11,12,13 In this paper we study the effects of electrochemical charging on the spin dynamics of optically excited carriers in an ensemble of CdSe QDs. The filling of QD energy levels affects properties such as the transverse spin lifetime and the Larmor precession frequencies, revealing the dependence of QD spin coherence on surface states and the externally controlled electron occupation.Stearic acid capped 6-nm diameter CdSe nanocrystal QDs are synthesized and purified by standard colloidal chemistry methods 14 and are dissolved in a 9:1 (by volume) hexane:octane mixture. Following the methods of Guyot-Sionnest and co-workers, 12,13 close-packed CdSe nanocrystal films are formed by drop-casting the QDs onto a quartz slide covered with a transparent indiumtin-oxide (ITO) electrode modified with 4-aminobutyldimethylmethoxysilane linker molecules. Dipping these films into a 5mM solution of 1, 7-diaminoheptane in anhydrous methanol followed by a bake for 3 hours at 70• C cross-links the 'loosely' packed QDs into tightly closepacked arrays. The nanocrystal-coated slide is then immersed in a 0.1 M electrolyte solution of tetrabutylammonium tetrafluoroborate dissolved in anhydrous N,Ndimethylformamide inside a fluorometer cell. A Pt counter electrode and a Ag pseudo-reference electrode complete a three-terminal electrochemical cell with the ITO working electrode. The preparation of the QD film and assembly of the ce...

Magneto-optical properties of trions in non-blinking charged nanocrystals reveal an acoustic phonon bottleneck

“…Finally, the data from three-peaked trions originate from prolate NCs, which are increasingly dominated by light-hole properties and thus should be more dispersed 8 . Overall, our measurements provide a direct route to determining single-charge g-factors, which now enables unambiguous assignment of g-factors extracted from indirect ensemble measurements 6,7 .…”

“…Charged NCs serve as the ideal system for investigating charge-carrier spin properties. To date, such properties have only been indirectly investigated in CdSe NC ensembles 6,7 yielding results that could not be reconciled with theoretical estimates 8 . In a charged NC, both the ground state and the band-edge excited state have an angular momentum determined by a single-charge carrier.…”

Hole–spin dephasing time associated with hyperfine interaction in quantum dots

“…However, experimental constraints for n-doped samples are slightly different and make the final task much more difficult. The main experimental difference is given by the fact that the lifetime of photo-created trions is in the order of 1 ns, and during this time the electron spin evolves with a dephasing time in the order of 500ps; for holes, a much longer dephasing time is expected, and then no significant evolution during lifetime should be observed, as confirmed by several experimental studies [30,31].To get experimental information on the hole spin dynamics, pump-probe experiments on samples containing p-doped QDs are more appropriate, such as the measurement of the photoinduced Faraday or Kerr rotation [32,33], or of the photo-induced circular dichroism [27,34].In these experiments an initial hole spin polarization is created by a resonant excitation of charged trions and subsequent transfer of their spin polarization to the hole spin. The observed 14 Faraday or Kerr rotation is related to the component of the spin polarization along the light propagation direction (z direction).…”

Spin dephasing of doped electrons in charge-tunable InP quantum dots: Hanle-effect measurements

“…A long spin-dephasing time T 2 * , which is the lifetime of coherent superposition of spin-up and spin-down states, is absolutely imperative for the implementation of spin-based quantum computing and storage. Although a few studies have been made on spin-dephasing relaxation in charged QDs, [23][24][25][26] studies about the mechanisms of spin dephasing are still at the elementary stage.…”