## 1 Introduction  
Semiconductor microparticles (MPs) and nanoparticles (NPs) exhibit interesting electronic, optical and magnetic properties, which depend on a preferential orientation of electron and nuclear spins in those particles. These properties are essential for their biomedical applications. Spatial confinement of charge carriers (electron and holes) in a semiconductor nanostructure results in an increase of the spin-lattice relaxation time. Going from itinerant to immobile, fully-localized electrons, while inducing the hyperfine dephasing, can be also beneficial in quenching the spin-lattice relaxation. The dynamic nuclear polarization in semiconductor nano- and microstructures opens fascinating prospects for creation of new efficient contrast agents in magnetic resonance imaging (MRI), which is a powerful diagnostic tool in biomedicine. Perspective applications of silicon (Si) MPs and NPs in hyperpolarized \({}^{29}\)Si MRI are discussed in Reference [1]  
Si nanocrystals (nc-Si) in electrochemically prepared porous Si are found to be a sensitizer of the photoinduced generation of singlet oxygen, which is a highly reactive form of molecular oxygen. The photosensitized singlet oxygen generation by nc-Si with adsorbed molecular oxygen is explained by spin-dependent energy transfer from excitons confined in nc-Si to molecular oxygen in the ground triplet state. This type of oxygen generation is promising for application of nc-Si based NPs and MPs in photodynamic therapy (PDT) of cancer [2]. Both the bioimaging and therapeutic functionality of Si NPs can be combined in an approach called as _theranostics_ (therapy + diagnostics) of cancer, which is recently an urgent task of modern nanomedicine [3]. Importantly, nanostructuring of semiconductor materials provides an avenue for a significant control over the dynamic characteristics [4; 5], including those which determine the spin relaxation [6]. For instance, spin-lattice relaxation times derived in germanium quantum wells (Ge QWs) (Figure 1) approach 5 \(\upmu\)s and are substantially longer than the one reported for conduction electrons in bulk Ge at the same temperatures (0.02 to 0.9 \(\upmu\)s). By spatially separating conduction band electrons residing in the Ge QW from their parent donor atoms embedded in the SiGe barriers, the impurity-induced bottleneck pertaining to experiments utilizing bulk Ge wafers was prevented and long-lived spins were resolved despite the low temperature operation. As emphasized in Reference [7], the relative strength of modulation of the hyperfine coupling with nuclei by lattice vibrations and the spin-orbit interaction are different for various materials. In Si, where the spin-orbit interaction is much weaker than in GaAs, the dominant spin-flip mechanism in the case of the phonon-assisted transitions between the Zeeman levels of the usual impurity was found to be the modulation of the hyperfine coupling with nuclei by lattice vibrations. _The magnitude of the_ calculated _spin-phonon relaxation rates_ in parabolic lens-shaped III-V quantum dots due to the phonon modulation of the spin-orbit interaction in presence of an external magnetic field through the deformation-potential  
Figure 1: (**a**) Color-coded intensity map of the conduction electron spin resonance peak in 17 nm Ge quantum wells (QWs) as a function of the microwave power (\(P\)). (**b**) Peak-to-peak amplitude (\(A_{\text{pp}}\)) of the peaks shown in (**a**) versus \(P\). (**c**) Values of spin-lattice relaxation time \(T_{1}\) and ensemble dephasing time \(T_{2}^{*}\) obtained from electron spin resonance peaks in QWs with different thickness. (Reprinted from Reference [6], © (2016) Springer Nature. This article is licensed under a Creative Commons Attribution 4.0 International License [[http://creativeccommons.org/licenses/by/4.0](http://creativeccommons.org/licenses/by/4.0)]([http://creativeccommons.org/licenses/by/4.0](http://creativeccommons.org/licenses/by/4.0))).