Size dependence of exciton fine structure in CdSe quantum dots

Norris, David J.; Éfros, Al. L.; Rosen, Matthew S.; Bawendi, Moungi G.

doi:10.1103/physrevb.53.16347

Cited by 515 publications

(548 citation statements)

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“…The quenching of the luminescence lifetime with decreasing size accompanied by an enhancement of the luminescence for large crystals can be observed this way [44]. Photoluminescence excitation and fluorescence line narrowing spectroscopies can be used to examine the exciton fine structure [45]. Magnetophotoluminescence spectroscopy is applied to examine the influence of the nanocrystal asymmetry on the emission of single threedimensionally confined biexcitons, where the impact of electron-hole exchange interaction on the fine structure and the polarization properties of optical transitions can be revealed [46].…”

Section: Chapter 3 Methodsmentioning

confidence: 99%

Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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Colloidal CdSe nanorods are nanoparticles synthesized in solution. The synthesis allows the growth of CdSe nanorods with well defined diameters and aspect ratios. They have many potential applications in the field of optoelectronics and biotechnology. The nanorods can be epitaxially covered with a graded CdS/ZnS shell of a few monolayers in thickness. The shell leads to an increased quantum efficiency and improved photostability of the nanorods. However, the lattice mismatch between the nanorod core material and the shell material introduces strain into the core lattice. In this work Raman spectroscopy, accompanied by ab-initio calculations, is used to determine the amount of this strain, the exciton-phonon coupling strength in the nanorods and to investigate confinement effects. The longitudinal optical phonons in a nanorod are confined to the nanorod volume. The confinement of a phonon wavefunction leads to a neutralization of the q = 0 rule and the phonon frequency is found to depend on the nanorod diameter. The coupling strength between longitudinal optical phonons and excitons is investigated. It also depends on the nanorod diameter. The total coupling strength is much lower than in bulk material due to a decrease of the influence of the Coulomb interaction in nanoparticles. However, the coupling strength is found to rise for decreasing diameters. This is due to the increasing contributions of higher frequency phonons for smaller nanoparticle sizes. A radial breathing mode with a diameter dependent frequency is deduced from ab-initio calculations and its existence in nanorods is verified experimentally. The diameter-dependence of the modes' frequency can be used to estimate the nanorod diameter from a Raman measurement. In core-shell structures, the coverage of a CdSe nanorod with a ZnS shell leads to a compressive strain of the CdSe core due to the smaller lattice parameter of ZnS compared to CdSe. Ab-initio calculations show that all bonds of the CdSe core are shortened. The bonds in the lateral direction are much more strongly compressed than the bonds parallel to the c-axis. The amount of strain is estimated from the Raman spectra. The compressive nature of the shell decreases for thicker nanorods. The exciton wave function changes with the modified boundary from air to ZnS. This is reflected in a altered exciton-phonon coupling strength and can be monitored in the Raman spectra. ZusammenfassungCdSe Nanorods sind Nanopartikel, die in einer kolloidalen Lösung synthetisiert werden. Für diese existieren viele mögliche Anwendungen im Bereich der Optoelektronik und der Biotechnologie. Der Durchmesser und die Länge der Nanorods lässt sich durch die Syntheseparameter festlegen. Die Nanorods können in eine epitaktische Hülle aus einem Halbleitermaterial mit einer größeren Bandlücke, ZnS, eingebettet werden. Diese Hülle verbessert die optischen Eigenschaften und ermöglicht eine weitere Funktionalisierung der Oberfläche. Der Unterschied der Gitterparameter zwischen ZnS und CdSe führt jedoch zu Verspannung...

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Section: Chapter 3 Methodsmentioning

confidence: 99%

Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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“…The electronÀphonon coupling is weak for excitonic emission and is reflected by a small HuangÀRhys parameter S (for CdSe, S is about 0.1À0.2). 33 In the configurational coordinate diagram, the offset in equilibrium distance between the ground state and excited state parabola is very small. Hence the crossing point is at very high energies, and for systems with a HuangÀRhys parameter below 1, no thermal quenching due to thermally activated crossover has been observed.…”

Section: Articlementioning

confidence: 99%

High-Temperature Luminescence Quenching of Colloidal Quantum Dots

et al. 2012

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A high luminescence efficiency is an important property of colloidal quantum dots (QDs), and quantum yields higher than 90% have been reported for coreÀshell QDs. 1 High efficiencies are especially important for application of QDs as luminescent biolabels, 2 in QD lasers, 3 in spectral converters for warm white LEDs, 4,5 electroluminescent devices, 6 and solar concentrators. 7 Luminescence efficiencies are strongly temperature-dependent. 8 Extensive temperature-dependent luminescence studies for colloidal QDs have been conducted at cryogenic temperatures (0.3À300 K). 9À15 In this temperature region, interesting effects were observed, including a prolonged lifetime below 20 K related to brightÀdark state splitting, 11,16 thermally activated quenching due to surface defect states, 9,10,17 and temperature antiquenching assigned to a phase transition in the capping layer. 14,15 However, the luminescence properties of QDs above room temperature (RT) are hardly investigated, and yet, for most applications in luminescent devices, the working temperature is higher than 300 K. An interesting example is the recent application of QDs as color converters in warm-white LEDs, 18 in which QDs serve as narrow band red emitters under excitation with blue light from a (In,Ga)N LED. The narrow emission bandwidth renders QDs superior to classical phosphors based on broad band emission from luminescent ions. 19 In high-power LEDs for general lighting applications, the heat generated in the pÀn junction and phosphor converter layer leads to temperatures as high as 150À200°C in the layer applied on top of the blue diode. 20 To avoid these high temperatures, the QD phosphor layer can be placed in a more remote configuration. Still, temperatures in such a configuration are expected to be well above 50°C due to heat dissipation of the QDs themselves (excess energy from converting the blue into red light). Clearly, the quenching of QD luminescence at elevated temperatures is relevant for application of QDs in luminescent devices, and a better insight in the quenching behavior is needed.Despite its importance, research on luminescence temperature quenching above RT is very limited for QDs. It is theoretically expected for a QD to have a very high luminescence quenching temperature (T q ). Three generally accepted mechanisms for thermal quenching involve thermally activated crossover from the excited state to the ground state, multiphonon relaxation, and thermally activated photoionization. The first mechanism is generally depicted in a simple configurational coordinate diagram. 8,21 The energy difference between the minimum * Address correspondence to a.meijerink@uu.nl.Received for review July 18, 2012 and accepted September 14, 2012. Published online 10.1021/nn303217qABSTRACT Thermal quenching of quantum dot (QD) luminescence is important for application in luminescent devices. Systematic studies of the quenching behavior above 300 K are, however, lacking. Here, high-temperature (300À500 K) luminescence studies are reported for highly ef...

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“…Theoretical analyses showed that the rate of radiative recombination decreases as the size increases in regions ͑1͒ and ͑3͒, whereas it behaves oppositely in region ͑2͒. 5,6,8,22,23 According to the computational result for a quantum dot including the excitonpolariton effect, the size-dependent recombination rate is 8…”

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confidence: 99%

Time-resolved photoluminescence of the size-controlled ZnO nanorods

Hong

Joo

Park

et al. 2003

Applied Physics Letters

131

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Size dependence of the time-resolved photoluminescence ͑TRPL͒ has been investigated for the ZnO nanorods fabricated by catalyst-free metalorganic chemical vapor deposition. The nanorods have a diameter of 35 nm and lengths in the range of 150 nm to 1.1 m. The TRPL decay rate decreases monotonically as the length of the nanorods increases in the range of 150 to 600 nm. Decrease of the radiative decay rate of the exciton-polariton has been invoked to account for the results. There has been a great interest in low-dimensional lightemitting nanostructures for highly integrated photoelectronic device applications. As the size of a nanostructure approaches the exciton Bohr radius, optical properties critical to device applications, such as bandgap and photoluminescence ͑PL͒ lifetime, are affected greatly by the quantum confinement effect. [1][2][3] PL lifetime is related to light-matter interaction; that is, the radiative decay of the exciton polariton and various nonradiative processes, such as leak by deep-level traps, lowlying surface states, and multiphonon scattering. There are several studies on the light-matter interaction in lowdimensional nanostructures of semiconductors. 4 -8 The exciton polariton luminescence is, however, quite sensitive to the concentration of defects and structural factors of the nanostructures, and it is not easy to separate the radiative recombination and nonradiative processes.In this letter, we report time-resolved PL ͑TRPL͒ of the size-controlled ZnO nanorods. ZnO is a wide-bandgap semiconductor enabling ultraviolet light-emitting diodes and photonic devices. It shows efficient PL at room temperature due to the large exciton binding energy of ϳ60 meV. 9 ZnO can be fabricated in many different nanostructures, such as quantum dots, 10-12 nanowires, 13-18 and quantum wells, 19 and we have shown in a previous report that high quality singlecrystal nanorods can be grown without using any metal catalyst. 14 The samples were grown on Al 2 O 3 (00•1) substrates by catalyst-free metalorganic vapor phase epitaxy. Details of the ZnO nanorod fabrication has been described elsewhere. 20 A series of different size ZnO nanorods was prepared by controlling the growth time. Care has been taken to ensure the same condition other than the growth time to keep the impurity concentration constant. Samples with the growth time from 30 to 120 min were selected for the PL study. The sizes of the nanorods thus selected were determined to be 29-40 nm in diameter and 150-1100 nm in length by field-emission scanning electron microscopy, as summarized in Table I. The length of the rods increases almost linearly by the growth time with a small standard deviation. The diameter, however, stays nearly constant, with somewhat larger uncertainty.For the optical characterization of the ZnO nanorods, time-integrated PL ͑TIPL͒ and TRPL were measured. Output of a Kerr lens mode-locked Ti:sapphire laser at 700 nm was doubled to 350 nm to serve as an excitation source in both TIPL and TRPL experiments. TRPL was measured by ...

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Size dependence of exciton fine structure in CdSe quantum dots

Cited by 515 publications

References 34 publications

Optical phonons in colloidal CdSe nanorods

Optical phonons in colloidal CdSe nanorods

High-Temperature Luminescence Quenching of Colloidal Quantum Dots

Time-resolved photoluminescence of the size-controlled ZnO nanorods

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