This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

▪ Abstract We review the rapid progress made in our understanding of the crystal properties of semiconductors and nanocrystals focussing on theoretical results obtained within the multiband effective mass approximation. A comparison with experiment shows these results are valid for nanocrystals down 22–26 A in diameter. The effect of the electron-hole Coulomb interaction on the optical spectra is analyzed. A theory of the quantum–size levels in wide gap (CdSe) and narrow gap semiconductors (InAs) is presented that describes the absorption spectra of these semiconductors well. A great enhancement of the electron-hole exchange interaction leads to the formation of the optically forbidden Dark Exciton in nanocrystals, which strongly affects their photoluminescence. A theory of the band-edge exciton fine structure is presented and applied to the study of the PL in CdSe nanocrystals. The effect of doping on nanocrystal spectra is considered. The enhancement of the short–range spin-spin interaction in Mn-doped ...

The Electronic Structure of Semiconductor Nanocrystals1

The dynamics and spectroscopy of silicon nanocrystals that emit at visible wavelengths were analyzed. Size-selective precipitation and size-exclusion chromatography cleanly separate the silicon nanocrystals from larger crystallites and aggregates and provide direct evidence for quantum confinement in luminescence. Measured quantum yields are as high as 50 percent at low temperature, principally as a result of efficient oxide passivation. Despite a 0.9—electron-volt shift of the band gap to higher energy, the nanocrystals behave fundamentally as indirect gap materials with low oscillator strength.

https://science.sciencemag.org/content/sci/262/5137/1242.full.pdf

Quantum confinement in size-selected, surface-oxidized silicon nanocrystals.

A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 °C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25−40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely...

Highly luminescent silicon nanocrystals with discrete optical transitions.

The luminescence of porous Si: the case for the surface state mechanism

The authors report time-resolved photoluminescence spectroscopy of highly porous silicon. Their results show that the luminescence is due to localized quantum-confined excitons in undulating crystalline silicon wires. The resonantly excited photoluminescence spectrum exhibits satellite structure due to momentum-conserving phonons of crystalline silicon. This provides a clear signature of the crystalline-silicon electronic band structure. The spin states of the localized exciton are split by the electron-hole exchange interaction. This splitting is manifested both in the strong dependence of the luminescence lifetime on temperature, and as an energy gap in the resonantly excited photoluminescence spectrum. The experimental splitting is in good agreement with the value calculated for a localized exciton in crystalline silicon.

https://iopscience.iop.org/article/10.1088/0953-8984/5/7/003/pdf

P D J Calcott

Papers

Identification of radiative transitions in highly porous silicon