Observation of the "Dark Exciton" in CdSe Quantum Dots

Nirmal, M.; Norris, David J.; Kuno, Masaru; Bawendi, Moungi G.; Efros, Alexander L.; Rosen, M.

doi:10.1103/physrevlett.75.3728

Cited by 785 publications

(828 citation statements)

References 19 publications

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“…In contrast, semiconductor nanocrystals emit light with a decay time in the order of a few tens of nanoseconds (~30-100 nsec) at room temperature. 17 The fluorescence decay is slower than the autofluorescence background decay, but fast enough to maintain a high photon turnover rate. Consequently, semiconductor nanocrystals might be ideal probes for spectrally multiplexed, time-gated cellular detection with enhanced selectivity and sensitivity.…”

Section: Introductionmentioning

confidence: 99%

Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

et al. 2001

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We describe the synthesis of water-soluble semiconductor nanoparticles, and discuss and characterize their properties. Hydrophobic CdSe/ZnS core/shell nanocrystals with a core size between 2 and 5 nm are embedded in a siloxane shell and functionalized with thiol and/or amine groups. Structural characterization by AFM indicates that the siloxane shell is 1-5 nm thick yielding final particle sizes of 6-17 nm, depending on the initial CdSe core size. The silica coating does not significantly modify the optical properties of the nanocrystals. Their fluorescence emission is about 32-35 nm FWHM, and can be tuned from blue to red with quantum yields up to 18%, mainly determined by the quantum yield of the underlying CdSe/ZnS nanocrystals. Silanized nanocrystals exhibit enhanced photochemical stability over organic fluorophores. They also display high stability in buffers at physiological conditions (>150 mM NaCl). The introduction of functionalized groups onto the siloxane surface would permit the conjugation of the nanocrystals to biological entities. 2 IntroductionRecent progress in wet chemistry has established a robust route towards the synthesis of highly luminescent semiconductor nanocrystals having sizes ranging from 1.5 to 8 nm. 1,2 By judiciously controlling the growth conditions, the size, and even the shape of II-VI nanocrystals can be accurately tailored. 3,4 The ability to control these parameters has a profound impact in materials science since it can be harnessed for engineering assemblies of nanometer scale units with novel characteristics. [5][6][7][8] At this point, the prominent focus has been the optical properties of these semiconductor nanocrystals. They are governed by strong quantum confinement effects and, therefore, are size dependent. 2,9,10 The absorption onset and fluorescence emission shift to larger energy with decreasing size.Moreover, while the absorption spectrum is a continuum from the bandgap into the UV, the emission pattern is narrow, symmetric, and does not depend on the excitation frequency. Several different sizes of nanocrystals can thus be excited simultaneously with a single excitation source, resulting in well-resolved colors of emission. Further passivation of the nanocrystal surface by a thin shell of a higher band gap material does not significantly modify the absorption and emission features but increases the nanoparticle quantum yield up to 50-70%. [11][12][13] The passivation shell also imparts an efficient photochemical stability, so that the photobleaching is reduced, and the number of photons a single nanocrystal can emit dramatically increases. The ability to discriminate many different colors simultaneously under long-term excitation holds great promise for fluorescent labeling technologies, especially in biology. 14,15 In this respect, organic dye molecules suffer from several limiting factors.First, their narrow absorption bands make it difficult to excite several colors with a single 3 excitation source. In addition, due to the large spectral overlaps between t...

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Section: Introductionmentioning

confidence: 99%

Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

et al. 2001

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“…Thus if the magnetic field reduces the ratio of allowed to unallowed components of a hole level, the exciton will become "dark" and have a longer decay time. 28 Calculations indicate that these changes are small, typically much smaller than a factor of two. 29 These changes in vb mixing do not appear to account for the order of magnitude increase observed in the decay time.…”

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confidence: 99%

Inhibited recombination of charged magnetoexcitons

et al. 1998

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Time-resolved photoluminescence measurements show that the decay time for charged excitons in a GaAs twodimensional electron gas increases by an order of magnitude at high magnetic fields. Unlike neutral excitons, the charged exciton center-of-mass is spatially confined in a "magnetically-adjustable quantum dot" by the cyclotron orbit and the quantum well. The inhibited recombination is explained by a reduced phase coherence volume of the magnetically-confined charged excitons.PACS numbers: 71.35. Ji, 8.47.+p, 73.20.D Charged excitons or "trions" were first identified in optical absorption experiments on electron-doped CdTe quantum well (QW) structures through their polarization properties in a magnetic field. 1 The negatively charged exciton (X − ) transition in a narrow QW was manifest in the spectra as a peak lying several meV below the uncharged exciton (X 0 ) peak. Although both X 0 and X − transitions may had been observed previously in PL spectra of GaAs QWs, the high electron density precluded their identification. 2 In hindsight, it is not surprising that the recently identified X − is often the most common exciton found in a system with excess electrons, similar to the X 0 in a system without excess electrons. An X 0 in the presence of excess electrons becomes polarized by a nearby electron and binds the electron by a dipolar attraction. The properties of X − transitions in GaAs QWs have been explored in several recent experimental 3-7 and theoretical [8][9][10][11][12] studies.An interesting facet of the charged exciton that has yet to be explored is the confinement produced by the cyclotron motion in a magnetic field. The X − complex (two electrons plus one hole) is singly-charged and a magnetic field confines the X − center-of-mass motion to a cyclotron orbit, unlike the neutral exciton (X 0 ) which is free to move in a magnetic field. This will be referred to as the magnetically-confined charged exciton (MCX). A magnetic field applied perpendicular to a twodimensional (2D) QW effectively confines the exciton to a quantum dot (QD) whose size is adjustable with magnetic field. The 3D MCX volume, defined roughly by the QW width and the area of the cyclotron orbit in the plane of QW, is inversely proportional to the perpendicular magnetic field, V M CX ∝ 1/B ⊥ . At high magnetic fields this volume is typically smaller than QDs currently available via patterned nanostructures.The purpose of the present study is to examine the radiative recombination of excitons confined in these MCX QDs. Exciton recombination times were determined by measuring the photoluminescence (PL) decay times in low-density GaAs/AlGaAs electron gases in magnetic fields to 18 T, at temperatures 0.5-7 K. At low temperatures, the X − decay time was found to increase by an order of magnitude for increasing perpendicular magnetic field. In contrast, the recombination is rapid for both the X − in fields applied in the plane of the QW and for the uncharged X 0 . In the latter two cases the exciton is not confined to a QD. The linear d...

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“…[8][9][10] In this description, the zeroth order single particle states are expressed as eigenstates of the total angular momentum F = J + L where J and L are the angular momenta of the Bloch function and envelope functions. The electron-hole exchange interaction and the anisotropy associated with the axial crystal field and shape distortion are treated perturbatively.…”

Section: Electronic Structure In Doped Spherical Nanocrystalsmentioning

confidence: 99%

“…The electron-hole exchange interaction and the anisotropy associated with the axial crystal field and shape distortion are treated perturbatively. [8][9][10] To this "standard model" of the CdSe NC, which is reviewed in the Supporting Information, we add a perturbative treatment of the Coulomb center potential associated with a charged defect inside a NC or at the NC surface.…”

Section: Electronic Structure In Doped Spherical Nanocrystalsmentioning

confidence: 99%

“…At the same time, the radiative decay time of the dark exciton in CdSe NCs was measured to be on the order of 1 µs at low temperatures at which only the dark exciton is populated. 4,5,8 Its activation is connected with various phonon or dangling bond assisted mechanisms of the dark exciton recombination. 11 In wurtzite CdSe NCs, both the hexagonal crystal field and the NC shape asymmetry contribute to splitting the hole levels.…”

Section: Introductionmentioning

confidence: 99%

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Photoluminescence Enhancement through Symmetry Breaking Induced by Defects in Nanocrystals

2017

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We present a theoretical model for the effect of symmetry breaking introduced by the doping of semiconductor nanocrystals with Coulomb impurities. The presence of a Coulomb center breaks the nanocrystal symmetry and affects its optical properties through mixing of the hole spin and parity sublevels, breaking the selection rules responsible for the exciton dark state in undoped nanocrystals. After reviewing the effects on the exciton fine structure and optical selection rules using symmetry theory, we present a perturbative model to quantify the effects. We find that the symmetry breaking proceeds by two mechanisms: First, mixing by even parity terms in the Coulomb multipole expansion results in an exciton fine structure consisting of three optically active doublets which are polarized along x, y and z axes with a ground optically passive dark exciton state, and second, odd parity terms which break inversion symmetry significantly activate optical transitions which are optically forbidden in the 1

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Observation of the "Dark Exciton" in CdSe Quantum Dots

Cited by 785 publications

References 19 publications

Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

Synthesis and Properties of Biocompatible Water-Soluble Silica-Coated CdSe/ZnS Semiconductor Quantum Dots

Inhibited recombination of charged magnetoexcitons

Photoluminescence Enhancement through Symmetry Breaking Induced by Defects in Nanocrystals

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