Luminescence of semiconductors is nowadays based on very firm background of solid state physics. The purpose of this book is to introduce the reader to the study of the physical principles underlying inorganic semiconductor luminescence phenomena. It guides the reader starting from the very introductory definitions over luminescence of bulk semiconductors and finishing at the up-to-date luminescence spectroscopy of individual nanocrystals. The book thus set the aim of filling the gap between general textbooks on semiconductors and dedicated advanced monographs. At the beginning, important knowledge of the solid state like lattice vibrations, exciton–phonon interaction and the concept of configurational coordinate are reviewed. Self-contained chapters are then devoted to exciton luminescence processes, effects of high optical excitation, and to an overview of the essentials of electroluminescence. Apart from spontaneous luminescence, special attention is paid to stimulated emission and investigation of optical gain. Considerable space is given also to optical processes in low-dimensional semiconductor structures. The book has been written by experimentalists and is destined primarily for experimentalists, too. Visual approach using schemes and graphs is used frequently instead of rigorous mathematical derivation. The chapter devoted to experimental techniques of luminescence spectroscopy is rich in content. Whenever it makes sense, the accent is put on how to extract from the appearance of luminescence emission spectrum (shapes of emission lines, their behaviour with varying experimental parameters) as much information on microscopic origin of luminescence as possible. The book cannot be regarded as a comprehensive monograph on semiconductor luminescence; selected examples from extremely rich literature only have been chosen to illustrate the text.

Silicon nanocrystals are an extensively studied light-emitting material due to their inherent biocompatibility and compatibility with silicon-based technology. Although they might seem to fall behind their rival, namely, direct band gap based semiconductor nanocrystals, when it comes to the emission of light, room for improvement still lies in the exploitation of various surface passivations. In this paper, we report on an original way, taking place at room temperature and ambient pressure, to replace the silicon oxide shell of luminescent Si nanocrystals with capping involving organic residues. The modification of surface passivation is evidenced by both Fourier transform infrared spectroscopy and nuclear magnetic resonance measurements. In addition, single-nanocrystal spectroscopy reveals the occurrence of a systematic fine structure in the emission single spectra, which is connected with an intrinsic property of small nanocrystals since a very similar structure has recently been observed in specially passivated semiconductor CdZnSe nanoparticles. The organic capping also dramatically changes optical properties of Si nanocrystals (resulting ensemble photoluminescence quantum efficiency 20%, does not deteriorate, radiative lifetime 10 ns at 550 nm at room temperature). Optically clear colloidal dispersion of these nanocrystals thus exhibits properties fully comparable with direct band gap semiconductor nanoparticles.

We introduce a general method which allows reconstruction of electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. A comprehensive study of band structure of a realistic nanocrystal is given including full geometric and electronic relaxation with the surface passivating groups. In particular, we combine this method with large scale density functional theory calculations to obtain insight into the luminescence properties of silicon nanocrystals of up to 3 nm in size depending on the surface passivation and geometric distortion.We conclude that the band structure concept is applicable to silicon nanocrystals with diameter larger than ≈ 2 nm with certain limitations. We also show how perturbations due to polarized surface groups or geometric distortion can lead to considerable moderation of momentum space selection rules.

The three-dimensional photonic crystals used in this study were synthetic opals, composed of submicron silica spheres, close-packed in a face-centered cubic structure with a period of 200 nm, that exhibit photonic stopbands around 600 nm. We present measurements of the optical gain of CdS quantum dots (QDs) embedded inside the interstitials between the silica spheres. Unlike the usual gain spectra of CdS QDs in glass matrices, which display maximum gain at energies of the first quantum-confined transitions, for QDs embedded in photonic crystals the gain maximum is shifted toward the high-frequency edge of the photonic stopband (2.2 eV) far below the absorption edge of the semiconductor (2.5 eV). Studies of temperature, intensity, and orientation dependencies of the gain spectra allow one to ascribe the observed effect to gain enhancement caused by multiple coherent Bragg scattering of light in the periodic photonic crystal.

We discuss applicability of the variable stripe length method to experimental investigation of optical gain in a luminescent layer that behaves like a planar waveguide. We show that an interplay between the output direction of guided light modes and the numerical aperture of the collection optics may lead to an artifact manifesting itself as an apparent but false gain. We propose a way to circumvent this inconvenience by using a “shifting excitation spot” complementary measurement. The method is demonstrated on a layer of Si nanocrystals embedded into a synthetic silica plate.

Luminescence of disordered (amorphous) semiconductors is due to a different microscopic mechanism compared to those being active in the luminescence of crystalline counterparts with long-range order. Electron and hole tail states, originating from dangling bonds, play the decisive role. Features typical for the amorphous semiconductor luminescence are discussed, namely: peculiar temperature dependence driven by the demarcation energy and distribution of luminescence decay times. Two theoretical models describing spectral shape of the emission band are examined: the phonon broadening model and the disorder broadening model. Theoretical band shapes are compared with experimental spectra found in amorphous elemental semiconductors. The concept of geminate and non-geminate electron–hole pairs is briefly debated. Multiple nonradiative recombination paths are mentioned as well as luminescence of impurities and defects.

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect‐gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect‐gap nature. Here, we propose a solution to this long‐standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E(k) band‐structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole‐allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.

Small oxidized silicon nanocrystals of average sizes below 3.5 nm are prepared using modified electrochemical etching of a silicon wafer. Modifications introduced in the etching procedure together with postetching treatment in H2O2 lead to a decrease in the nanocrystalline core size and also, to some extent, to changes in the surface oxide. The interplay between these two factors allows us to blueshift the photoluminescence (PL) spectrum from 680 down to 590 nm, which is accompanied by changes in PL dynamics. This continual development, however, stops at about 590 nm, below which abrupt switching to fast decaying blue emission band at about 430 nm was observed. Discontinuity of the spectral shift and possible relation between both bands are discussed.