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 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.

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using QDSLs such as solar cells, [7][8][9][10] photodetectors, [11][12][13] and field effect transistors. [14,15] Thus far, the formation of minibands in QDSLs has been discussed in terms of charge transport properties and its temperature dependence. [5,6,[16][17][18] It is also possible to investigate the formation of minibands based on their optical properties because absorption and photoluminescence (PL) properties change when minibands are formed. Such optical measurements are advantageous for evaluating the intrinsic properties of QDSLs because they do not require any preparation of device structures such as electrode junctions.Several methods have been proposed for arranging QDs synthesized by hot injection method: solvent evaporation, solvent destabilization, and assembly at air-liquid interfaces. [19][20][21][22] However, it is difficult to make QDs close to each other enough to induce the quantum resonance because ligands used in the hot injection method such as trioctylphosphine oxide [23] and octadecylamine [24] are too long. One approach to solving the problem is to exchange the long ligands with short ligands such as ethanedithiol, [25] ethanediamine, [16] or metal chalcogenide complexes. [5] However, QDs need to be closer to each other without ligand exchange to investigate the optical properties of QDSLs because the ligand exchange process degrades the PL properties. [26] The use of water-soluble QDs is a promising method to make QDs close. [27][28][29][30][31] It is possible to make QDs closer without ligand exchange because water-soluble QDs can be originally synthesized using short ligands such as N-acetyl-l-cysteine (NAC), [30] thioglycolic acid, [27][28][29] and mercaptopropionic acid. [27,29] In our previous study, we reported that CdTe QDSLs can be fabricated by the layer-by-layer (LBL) assembly of NAC-capped CdTe QDs and polyelectrolytes, and that the quantum resonance was clearly observed between adjacent CdTe QDs. [32,33] Further, it was recently demonstrated that the dimensions of the quantum resonance can be controlled by independently changing the distances between QDs in the stacking (out-of-plane) and in-plane directions. [33] We also experimentally observed the formation of minibands by studying the excitation energy dependence of the PL spectra and the detection energy dependence of the PL-excitation spectra. In this paper, we reveal excitonic dynamics in CdTe QDSLs wherein the 1D, 2D, and 3D quantum resonance occurs with the miniband formation by systematically investigating the temperature dependence of absorption, PL spectra, and PL decay profiles. The formation of minibands in quantum dot (QD) superlattices (SLs) dramatically increases the mobility of carriers, giving a new way to apply QDs for optoelectronic devices. In previous studies on QDSLs, only a few studies have investigated the temperature dependence of the photoluminescence (PL) properties of QDSLs focusing on the formation of minibands. Here, a new model is proposed that simultaneously considers the extended and loc...