Temperature Dependence of Photoluminescence Dynamics in Colloidal CdS Quantum Dots

Kim, D.; Mishima, T.; Tomihira, K.; Nakayama, Masaaki

doi:10.1021/jp8009172

Cited by 41 publications

(54 citation statements)

References 35 publications

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“…26 The A-QDs of CdS with a diameter of 5.0 nm were synthesized by a colloidal method as reported in Ref. 27. The D-QDs of CdS were prepared by a combination of a sizeselective photoetching and a surface modification technique with a Cd͑OH͒ 2 layer, 27,28 which enabled us to prepare sizecontrolled CdS QDs with high PL efficiency.…”

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confidence: 99%

Experimental verification of Förster energy transfer between semiconductor quantum dots

et al. 2008

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In recent years, energy transfer ͑ET͒ using semiconductor quantum dots ͑QDs͒ is getting increased attention. However, it has been postulated that ET between QDs is based on the Förster model, which is a wellestablished model of ET mechanism in organic dye systems, without verification. In this work, we have investigated ET mechanism in colloidal CdS QDs measuring photoluminescence dynamics of a bilayer structure consisting of differently sized CdS QDs. In the bilayer structure, the distance between the monolayer of donor QDs and that of acceptor QDs was controlled precisely by a spacer layer that is layer-by-layer assembly of polyelectrolytes. The bilayer structure enabled us to systematically measure the spacer-layer dependence of photoluminescence dynamics reflecting the ET process between QDs. It is demonstrated that ET between the donor and acceptor QDs is conclusively dominated by the dipole-dipole interaction, which verifies the appropriateness of the Förster model.Since the first report of quantum size effects in semiconductor doped glasses 1 and also in colloidal solutions 2 in the early 1980s, semiconductor quantum dots ͑QDs͒ have attracted considerable attention. So far, many studies have been conducted from a scientific viewpoint to understand the intrinsic nature of physical/chemical properties of QDs, as well as from interest in the application to new functional materials. 3-6 A turning point in QD studies was the development of monodispersed colloidal QDs having high photoluminescence ͑PL͒ yield with use of rapid injection of organometallic precursors into hot coordinating solvents ͑hot-injection method͒. 7,8 The breakthrough in synthesizing the new class of colloidal QDs have led to an explosive increase in QD studies and opened up possibilities for various applications such as biomolecular imaging, 9 QD lasing, 6,10 and QD solar cells. 11,12 Randomly dispersed QDs have been a major target in most of the studies so far. The dynamical process of resonant ET between CdSe QDs was reported in recent years. 13,14 This opened up a new aspect in photophysics of semiconductor QDs and stimulated studies on QD-based energy transfer ͑ET͒ processes employing QDs as energy donors in QDbioconjugate systems 9 and QD-organic dye systems, 15,16 as well as ET between QDs. 17,18 All of the studies, however, have postulated that ET employing QDs is based on the Förster model 19 that is a typical ET mechanism between organic molecules. It should be noted that the appropriateness of the Förster model for explanation of ET in QDs has not been verified until now. For the experimental clarification of the ET mechanism, it is essential to measure PL dynamics in a well-designed sample structure in which the distance between QDs was precisely controlled.How can we control the distance between QDs? We have focused on a layer-by-layer ͑LBL͒ assembly. 20-22 LBL is a simple and powerful technique allowing the realization of a multilayer structure that is controlled at a molecular level. This technique is based on the sequential a...

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confidence: 99%

Experimental verification of Förster energy transfer between semiconductor quantum dots

et al. 2008

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“…Our work highlights the interesting regime of intermediate CdS sizes and bridges the gap between a few reports on small and large CdS NCs. [15][16][17]26 The results presented here are an important input to the controversial debate about the appropriate theoretical modeling of such NC materials 27,29,30 and they contribute to the fundamental understanding of the emission properties of CdS NCs and other NCs with small spin-orbit coupling.The electron and hole level structures of II-IV NCs that result in their characteristic absorption and emission properties have been studied intensively in the last years. Common to many NCs is the relatively large Stokes shift between absorption and emission energies.…”

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confidence: 82%

Radiative lifetimes and orbital symmetry of electronic energy levels of CdS nanocrystals: Size dependence

et al. 2010

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Using time-resolved photoluminescence spectroscopy, we studied the electronic levels of semiconductor nanocrystals ͑NCs͒ with small spin-orbit coupling such as CdS. Low temperature radiative lifetimes indicate that the lowest-energy transition changes from orbital allowed to orbital forbidden with decreasing NC size. Our results are well explained by a size-dependent hierarchy of s-and p-orbital hole levels that is in agreement with theoretical predictions. Around the critical NC radius of ϳ2 nm we observe an anticrossing of s-and p-orbital hole levels and large changes in transition rates.Size tunability of optical properties and flexibility of surface chemistry make semiconductor nanocrystals ͑NCs͒ ͑Refs. 1 and 2͒ attractive fluorophores for a wide range of applications including solid-state lighting, 3,4 lasing, 5,6 and fluorescent labeling. 7,8 High emission efficiencies with quantum yields larger than 50% were demonstrated, 9-11 despite the optically passive character of the lowest-energy transition in many NCs. 12-20 The high emission yields at room temperature ͑RT͒ are obtained because of thermally populated excited transitions that are characterized by high oscillator strengths. Understanding such important optical properties such as radiative lifetimes requires a detailed knowledge of the electronic level structure. Extensive studies of CdSe NCs led to the concept of dark and bright excitons 12-14 that allowed explaining radiative lifetime measurements. 20-23 Significantly less experimental studies exist on radiative lifetimes and the electronic level structure of NCs made of other semiconductors, including CdS. 15-17 Besides the applicationrelevant emission in the blue and UV spectral range, 24-26 the importance of CdS NCs arises from the distinct differences in the electronic band structure between CdS and CdSe. Specifically, CdS is a representative of semiconductors ͑e.g., sulphides and phosphides͒ with very small spin-orbit coupling ⌬ SO in contrast to CdSe which has a large spin-orbit coupling.In this Brief Report, we discuss radiative lifetimes and the electronic level structure of CdS NCs ͑as an example of a small ⌬ SO material system͒ using time-resolved photoluminescence ͑PL͒ measurements. We determined temperatureand size-dependent radiative lifetimes of CdS NCs and analyzed them with a model that takes into account two lowestenergy transitions between s-orbital electron and s-or p-orbital hole levels. We show that the radiative lifetime in certain NCs is accelerated at low temperatures ͑LTs͒. Moreover, we demonstrate experimental evidence that the orbital symmetry of the hole ground state in CdS NCs depends on the NC size, a phenomenon that has only been predicted theoretically. 27,28 Around the critical NC size, our measurements indicate an anticrossing of the s-and p-orbital hole levels that is accompanied by significant changes in the transition rates. Our work highlights the interesting regime of intermediate CdS sizes and bridges the gap between a few reports on small and large CdS NCs. [...

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“…At low pump power (2 μW, Figure 6a), the increase in temperature promotes the nonradiative relaxation process of I 2 emission, leading to faster time constant τ 1 . The rate of this process can be described by the thermal activation energy in the Boltzmann distribution 74 1 τ nonrad (T)…”

Section: Articlementioning

confidence: 99%

Dynamics of Bound Exciton Complexes in CdS Nanobelts

et al. 2011

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Intrinsic defects such as vacancies, interstitials, and anti-sites often introduce rich luminescent properties in II-VI semiconductor nanomaterials. A clear understanding of the dynamics of the defect-related excitons is particularly important for the design and optimization of nanoscale optoelectronic devices. In this paper, low-temperature steady-state and time-resolved photoluminescence (PL) spectroscopies have been carried out to investigate the emission of cadmium sulfide (CdS) nanobelts that originates from the radiative recombination of excitons bound to neutral donors (I(2)) and the spatially localized donor-acceptor pairs (DAP), in which the assignment is supported by first principle calculations. Our results verify that the shallow donors in CdS are contributed by sulfur vacancies while the acceptors are contributed by cadmium vacancies. At high excitation intensities, the DAP emission saturates and the PL is dominated by I(2) emission. Beyond a threshold power of approximately 5 μW, amplified spontaneous emission (ASE) of I(2) occurs. Further analysis shows that these intrinsic defects created long-lived (spin triplet) DAP trap states due to spin-polarized Cd vacancies which become saturated at intense carrier excitations.

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Temperature Dependence of Photoluminescence Dynamics in Colloidal CdS Quantum Dots

Cited by 41 publications

References 35 publications

Experimental verification of Förster energy transfer between semiconductor quantum dots

Experimental verification of Förster energy transfer between semiconductor quantum dots

Radiative lifetimes and orbital symmetry of electronic energy levels of CdS nanocrystals: Size dependence

Dynamics of Bound Exciton Complexes in CdS Nanobelts

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