Ondřej Cibulka

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 at the F-band of oxidized silicon nanocrystals

In this paper we present time-resolved optical gain spectroscopy using the variable stripe length technique in combination with the shifting excitation spot technique under pumping with nanosecond laser pulses. Measurements reveal positive optical gain on a nanosecond time scale at 430 nm (F-band), accompanied by spectral narrowing and a threshold behaviour of the amplified spontaneous emission as a function of the excitation intensity. We show that the presence of the slow-red (S-band) emission component critically influences the observation of stimulated emission from the F-band.

Silicon nanoparticle formation depending on the discharge conditions of an atmospheric radio-frequency driven microplasma with argon/silane/hydrogen gases

Atmospheric radio-frequency driven non-equilibrium microplasma jets in an argon/silane/hydrogen gas mixture are characterised and analysed with respect to the reaction pathway, which leads to the formation of silicon nanoparticles. Optical emission spectroscopy is used to obtain initial information about possible plasma chemistry processes and high-time-resolution images uncover the mode of operation of the discharge. It is demonstrated that the effect of the way the electric field is applied (parallel or perpendicular to the gas flow), the gas flow magnitude, and varying the gas mixture can result in three different operation modes—filamentary plasma with a stationary filament, diffuse-like plasma with the filament changing its position, and a diffuse non-filamentary plasma—being formed in the one millimetre inner diameter tube with ring electrodes, which apply an electric field parallel to the gas flow (a parallel-field plasma). An electric field applied perpendicular to the gas flow (a cross-field plasma) results only in a homogeneous diffuse discharge with low plasma density. The nanoparticles synthesised in the microplasma jet are studied by scanning electron microscopy and dynamic light scattering. The experimental results reveal that the silane precursor can very probably be fully dissociated in the parallel-field plasma and particles with sizes almost independent of silane concentration are generated. In contrast, silane is only weakly fragmented in the cross-field plasma and negative ions are formed. Particle size reacts very sensitively to silane concentration in this case and is a result of a condensation of radicals or ions on the particle surface.

Enhanced photoluminescence extraction efficiency from a diamond photonic crystal via leaky modes

Two-dimensional photonic crystal can be exploited as the top part of a light source in order to increase its extraction efficiency. Here, we report on the room-temperature intrinsic photoluminescence (PL) behavior of a nanocrystalline diamond (NCD) layer with diamond columns prepared on the top and periodically ordered into the lattice with square symmetry. Angle-resolved far-field measurements in the Γ–X crystal direction of broadband visible PL revealed up to six-fold enhancement of extraction efficiency as compared to a smooth NCD layer. A photonic band diagram above the lightcone derived from these measurements is in agreement with the diagram obtained from transmission measurements and simulation, suggesting that the enhancement is primarily due to lights coupling to leaky modes.

Time-resolved measurements of optical gain and photoluminescence in silicon nanocrystals

In this paper, we present time-resolved optical gain spectroscopy using a combination of the variable stripe length and the shifting excitation spot techniques under pulsed nanosecond excitation at 355 nm. Optical gain measurements in the temporal detection window of 10 ns width, coincident with the excitation pulse, revealed induced absorption losses, whereas measurements with a different detection gate width and delay in two main photoluminescence components (a fast band at ~430 nm decaying in nanoseconds and a slow band at ~620 nm decaying in microseconds) show a positive optical gain of the order of tens of cm-1.