## 1 Introduction
For ZnSe/ZnS core-shell QDs it was revealed, in particular, that thermodynamical (slow) growth of the ZnS-shell on the colloidal ZnSe-core leads to the quantum yield increase because of decreasing amount of the traps at the core-shell interface [24]. The maturity of this technology of synthesis of semiconductor nanocrystals (We are not aware of successful colloidal chemistry synthesis of matrix-free silicon nanocrystals. However, they can be fabricated by other methods [25; 26; 27] and processed in a way similar to colloidal NCs of other semiconductors. [28] They will be discussed below.) is witnessed by the incorporation of colloidal QDs into real-world products such as colour displays [29] and light-emitting diodes for lighting [30], both already commercialized by large companies such as Samsung [31] and OSRAM [32], respectively (see in [33] for a recent review covering both colloidal and epitaxial QDs). More on the scientific research side, a broad variety of nanostructures can be prepared using colloidal NCs as building blocks, in particular, multilayer structures of QDs of different average size, deposited on different substrates [12]. Combining these structures with other materials, such as organic dielectrics [34], epitaxial quantum well (QW) heterostructures [35], metallic nanoparticles [36; 37], patterned metallic surface [38] or graphene [39; 40] can result in new interesting effects and applications, which physics is related to the coupling of QD excitons with elementary excitations (such as surface plasmons or QW excitons) in the surrounding materials. One interesting possibility is the excitation of colloidal QDs by pumping energy through a nearby epitaxial QW, as demonstrated in [35]. Another research topic with QDs, popular in the past two decades is the modification of their emission properties caused by the strong light-matter interaction, which has been achieved for a variety of composite structures that include point emitters embedded in Fabry-Perot microcavities, micropillars and photonic crystals as recently reviewed in [41]. In this article, we overview several aspects of the exciton-photon interactions in nanocrystal QDs with focus on the role of non-radiative processes and environment, which affect the light emission. The QD photoluminescence is a result of competition between the radiative and various non-radiative processes, such as capture on dangling bonds, multi-phonon intra-band relaxation, Auger recombination, etc. In the past decade, a growing interest has been paid to the multi-exciton dynamics in nanocrystals because of their high potential for photovoltaic applications. Such processes as carrier multiplication (or multi-exciton generation) in nanocrystals, as well as the Auger recombination (which is just a fast reverse process), are widely discussed in the literature [23; 42; 43; 44] and we will consider these effects below. Special attention is traditionally paid to nanostructured silicon [25; 45; 46; 47; 48] because of its widest use in microelectronics, high purity, natural abundance, low cost, and non-toxicity; we shall also dedicate some space to the exciton-photon interactions and main non-radiative processes mentioned above, in Si NCs and related nanostructures. As a rule, one deals with nanocrystal _ensembles_ (rather than with isolated QDs). Therefore, non-radiative energy exchange between the nanocrystals is possible via the Forster-type [49] and Dexter-type [50] exciton migration. Both mechanisms were originally proposed for fluorophores. The second one, based on the exchange interaction between electrons located on different sites can be neglected in the presence of allowed dipole transitions [50]. Recently, a third, so-called exciton tandem tunneling mechanism was proposed [51], which is specific of connected nanocrystals. The most universal process named Forster Resonant Energy Transfer (FRET) was first observed for QDs by [PERSON] et al. in specially designed films containing two different sizes of nanocrystals acting as donors and acceptors, respectively [52].