## 1 Introduction
Later it was shown in a number of works [53; 54; 55; 56; 57; 58] performed on systems composed of two different QD species that the luminescence of the smaller dots (donors) is quenched by the large dots (acceptors), whose emission in turn is enhanced. These studies demonstrated a dependence of the FRET effect on the NC density and spatial arrangement. Results of other experiments performed on multilayer SiO\({}_{x}\)/SiO\({}_{2}\) structures [59; 60; 61], porous Si [62], three-dimensional (3D) ensembles of Si [63; 64] crystallites were interpreted as a manifestation of exciton migration via FRET-type mechanisms. The effect has a potential interest for photonics (e.g., photoluminescence upconversion in QD ensembles [65]), sensing [66], lighting and energy harvesting (e.g., by unidirectional energy transfer in size-gradient layered QD assemblies [53; 54] or fractal aggregates [67; 68]). We shall discuss it below, in particular, addressing the question whether the FRET rates and length scales can be tuned by the photonic environment [69]. The influence of the environment on the exciton-related optical properties of NCs is the third topic that will be discussed in this overview. The most common effect is the energy transfer between a photoexcited QD and surrounding materials, which can be reversible or not. It has the same physical nature as FRET. In most cases, it is responsible for the photoluminescence (PL) quenching [70; 71]. However, the environment can be used for engineering the photonic density of states (DOS) in the vicinity of the QDs, often referred to as the Purcell effect [72]. Observed for the first time in QDs about twenty years ago [73], it has recently been discussed with respect to radiative decay rates of embedded point emitters [74]. Moreover, strong near-field effects associated with localized surface plasmon resonance (LSPR) in metallic (nano-)structures can enhance PL emission [38; 75]. Moreover, one can think of energy transfer from a recombining QD exciton to propagating surface plasmons that would carry the energy over a large distance and then eventually transfer it to another QD (by creating an exciton), thus allowing for a long-range exciton transport between two dots, much more efficient than if it occurred directly. Below we consider the influence of a flat interface between two media on the PL emission and FRET rates for a QD emitter located in the vicinity of the interface (We will not cover here the broad topic of interactions between QDs and metallic nanostructures supporting _localized_ surface plasmons because it deserves a separate article. Refs. [36; 37; 76; 77] can provide an introduction to this topic.). The article is organized as follows. In Section 2, we introduce basic notions of radiative transitions and discuss their rates (i.e., probabilities per unit time) for NCs of materials possessing direct or indirect band structure. In particular, the rates are calculated for intrinsic and doped silicon NCs, which are either hydrogen-coated or halogenated. Section 3 is devoted to non-radiative transitions and considers the Auger recombination, dangling-bond traps and phonon-assisted relaxation of hot carriers. In Section 4, multiple exciton generation initiated by a highly excited electron or hole is considered. Section 5 is dedicated to the exciton transfer processes between two QDs via the FRET mechanism. In Section 6, emission decay and FRET rates near a plane interface between two dielectrics or a dielectric and a metal are discussed. The last section is left for summary and conclusions.