Semiconductor quantum dot (QD) superlattices, which are periodically ordered three-dimensional (3D) array structures of QDs, are expected to exhibit novel photo-optical properties arising from the resonant interactions between adjacent QDs. Since the resonant interactions such as long-range dipole-dipole Coulomb coupling and short-range quantum resonance strongly depend on inter-QD nano space, precise control of the nano space is essential for physical understanding of the superlattice, which includes both of nano and bulk scales. Here, we study the pure quantum resonance in the 3D CdTe QD superlattice deposited by a layer-by-layer assembly of positively charged polyelectrolytes and negatively charged CdTe QDs. From XRD measurements, existence of the periodical ordering of QDs both in the lamination and in-plane directions, that is, the formation of the 3D periodic QD superlattice, was confirmed. The lowest excitation energy decreases exponentially with decreasing the nano space between the CdTe QD layers and also with decreasing the QD size, which is apparently indicative of the quantum resonance between the QDs rather than a dipole-dipole Coulomb coupling. The quantum resonance was also computationally demonstrated and rationalized by the orbital delocalization to neighboring CdTe QDs in the superlattice.

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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We have investigated the temperature dependence of an energy transfer ͑ET͒ between CdSe/ZnS quantum dots ͑QDs͒ measuring photoluminescence dynamics in a bilayer structure consisting of differently sized QDs. In the bilayer structure, an effective direct ET from donor QDs to acceptor QDs is realized. The temperature dependence of the observed ET rate can be classified into two categories. Under the condition that the thermal energy ͑k B T͒ is comparable to the splitting energy between the bright-and dark-exciton states and above, the observed ET rate is dominated by the thermal population behavior of the bright-exciton state: the ET rate increases with an increase in temperature. On the other hand, in the lower temperature region, the observed ET rate is almost constant, which may be due to a breakdown of the thermal equilibrium between the lower-lying dark-exciton state and the bright-exciton state.

In quantum dot superlattices, wherein quantum dots are periodically arranged, electronic states between adjacent quantum dots are coupled by quantum resonance, which arises from the short-range electronic coupling of wave functions, and thus the formation of minibands is expected. Quantum dot superlattices have the potential to be key materials for new optoelectronic devices, such as highly efficient solar cells and photodetectors. Herein, we report the fabrication of CdTe quantum dot superlattices via the layer-by-layer assembly of positively charged polyelectrolytes and negatively charged CdTe quantum dots. We can thus control the dimension of the quantum resonance by independently changing the distances between quantum dots in the stacking (out-of-plane) and in-plane directions. Furthermore, we experimentally verify the miniband formation by measuring the excitation energy dependence of the photoluminescence spectra and detection energy dependence of the photoluminescence excitation spectra.

In the published version of our paper, the expression for k ET is incorrect ͑the left hand column of page 4͒. This equation should bewhere PVA and bilayer are the decay time of the bright exciton state in the PVA film sample and that in the bilayer structure, respectively.It is noted that they are just typographical errors and that the calculations in the paper are based on the correct equation.

Quantum dot (QD) superlattices, periodically ordered array structures of QDs, are expected to provide novel photo-optical functions due to their resonant couplings between adjacent QDs. Here, we computationally demonstrated that electronic structures and phonon dynamics of a QD superlattice can be effectively and selectively controlled by manipulating its interior nanospace, where quantum resonance between neighboring QDs appears, rather than by changing component QD size, shape, compositions, etc. A simple H-passivated Si QD was examined to constitute one-, two-, and three-dimensional QD superlattices, and thermally fluctuating band energies and phonon modes were simulated by finite-temperature ab initio molecular dynamics (MD) simulations. The QD superlattice exhibited a decrease in the band gap energy enhanced by thermal modulations and also exhibited selective extraction of charge carriers out of the component QD, indicating its advantage as a promising platform for implementation in solar cells. Our dynamical phonon analyses based on the ab initio MD simulations revealed that THz-frequency phonon modes were created by an inter-QD crystalline lattice formed in the QD superlattice, which can contribute to low energy thermoelectric conversion and will be useful for direct observation of the dimension-dependent superlattice. Further, we found that crystalline and ligand-originated phonon modes inside each component QD can be independently controlled by asymmetry of the superlattice and by restriction of the interior nanospace, respectively. Taking into account the thermal effects at the finite temperature, we proposed guiding principles for designing efficient and space-saving QD superlattices to develop functional photovoltaic and thermoelectric devices.

Photoluminescence (PL) enhancement and quenching using localized surface plasmons in metal nanoparticles (NPs) are important factors to control optical properties of semiconductor quantum dots (QDs). However, the PL enhancement and quenching have not been simultaneously observed in previous works, which lead to the situation that the mechanisms for controlling the enhancement and quenching are still controversial. In this work, we have investigated the PL properties in bilayer structures consisting of Au NPs and CdSe QDs in order to realize the precise control of the PL enhancement and quenching of the QDs. It was demonstrated that both the separation distance between Au NPs and CdSe QDs and the surface density of Au NPs in the bilayer structure are key factors for controlling the balance between the PL enhancement and quenching. The experimental results were quantitatively analyzed, taking account of the energy transfer from CdSe QDs to Au NPs and local electric field effects due to Au NPs.