K. Dohnalová

Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals

Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

Step-like enhancement of luminescence quantum yield of silicon nanocrystals

Carrier multiplication by generation of two or more electron-hole pairs following the absorption of a single photon may lead to improved photovoltaic efficiencies and has been observed in nanocrystals made from a variety of semiconductors, including silicon. However, with few exceptions, these reports have been based on indirect ultrafast techniques. Here, we present evidence of carrier multiplication in closely spaced silicon nanocrystals contained in a silicon dioxide matrix by measuring enhanced photoluminescence quantum yield. As the photon energy increases, the quantum yield is expected to remain constant, or to decrease as a result of new trapping and recombination channels being activated. Instead, we observe a step-like increase in quantum yield for larger photon energies that is characteristic of carrier multiplication. Modelling suggests that carrier multiplication is occurring with high efficiency and close to the energy conservation limit.

Silicon quantum dots: surface matters

Silicon quantum dots (SiQDs) hold great promise for many future technologies. Silicon is already at the core of photovoltaics and microelectronics, and SiQDs are capable of efficient light emission and amplification. This is crucial for the development of the next technological frontiers-silicon photonics and optoelectronics. Unlike any other quantum dots (QDs), SiQDs are made of non-toxic and abundant material, offering one of the spectrally broadest emission tunabilities accessible with semiconductor QDs and allowing for tailored radiative rates over many orders of magnitude. This extraordinary flexibility of optical properties is achieved via a combination of the spatial confinement of carriers and the strong influence of surface chemistry. The complex physics of this material, which is still being unraveled, leads to new effects, opening up new opportunities for applications. In this review we summarize the latest progress in this fascinating research field, with special attention given to surface-induced effects, such as the emergence of direct bandgap transitions, and collective effects in densely packed QDs, such as space separated quantum cutting.

Brightly Luminescent Organically Capped Silicon Nanocrystals Fabricated at Room Temperature and Atmospheric Pressure

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.

On the origin of the fast photoluminescence band in small silicon nanoparticles

Colloidal suspensions of small silicon nanoparticles (diameter around 2nm) with fast and efficient ultraviolet-blue photoluminescence (PL) band are fabricated by enhanced electrochemical etching of Si wafers. The detailed study of photoluminescence excitation spectra in a wide range of excitation photon energies (270-420nm) reveals specific behavior of the Stokes shift of the fast PL band that agrees well with theoretical calculation of optical transitions in small silicon nanocrystals and is distinct from emission of silicon dioxide defects.

Optical gain in porous silicon grains embedded in sol-gel derived SiO2 matrix under femtosecond excitation

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm−1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission.

White-emitting oxidized silicon nanocrystals: Discontinuity in spectral development with reducing size

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.

Time-resolved photoluminescence spectroscopy of the initial oxidation stage of small silicon nanocrystals

In this paper we study the influence of progressing oxidation on the photoluminescence spectra of small silicon nanocrystals (SiNCs). H-terminated SiNCs exhibit only a fast approximately nanosecond photoluminescence component at ∼525 nm, quenched and redshifted to ∼550 nm by progressing oxidation. At the same time a new approximately microsecond photoluminescence component appears, intensity of which progressively increases and its peak position redshifts continuously from 575 up to 660 nm. We interpret our observations in terms of the quasidirect core electron-hole pair recombination quenched by the ultrafast trapping into the oxygen-related surface/interface states, forming within the band gap due to oxidation.

Ultrafast photoluminescence in silicon nanocrystals studied by femtosecond up-conversion technique

Photoluminescence dynamics in silicon nanocrystals measured by a femtosecond up-conversion technique are reported. The samples were prepared by embedding porous silicon grains in a sol-gel derived SiO2 matrix. Efficient initial relaxation of the excess energy of photoexcited carriers with the effective rate ⩾3.8eV∕ps was observed. A fast decay component (400fs) of the photoluminescence signal was found and interpreted in terms of quenching the interior exciton radiative recombination by carrier trapping on the nanocrystal surface. The ultrafast photoluminescence dynamics are followed by a microsecond decay of the stretched-exponential type.

Closely packed luminescent silicon nanocrystals in a distributed-feedback laser cavity

Silicon nanocrystals (Si-ncs) of sufficiently small size, emitting luminescence at short wavelengths (which implies the occurrence of quasi- direct radiative recombination) and being densely packed in a planar thin film (which ensures short stimulated emission (StE) lifetime) can become a suitable active material for the observation of StE in the visible region. In this paper, we describe a fabrication method of nanostructures of this type, based on enhanced electrochemical etching of silicon wafers followed by embedding porous silicon grains into an SiO2 matrix. Further, we report on time-resolved photoluminescence spectra and optical gain measurements performed via the variable-stripe-length and the shifting-excitation-spot methods. Finally, we realize a transient wavelength-tunable distributed-feedback-laser (DFL) cavity with inserted densely packed Si-ncs as an active medium. We demonstrate an increase in emission intensity on the blue emission wing (below 600nm), which is spectrally shifting in accordance with the cavity tuning. We also present a mathematical model of the DFL cavity enabling us to simulate the experimental