The structural and optical properties of supercontinuum emitting Si nanocrystals prepared by laser ablation in water

Abstract

Laser ablation of Si(100) targets immersed in deionized water at room temperature was studied by applying the second harmonic of a ns pulsed Nd:YAG laser and by employing different methods for characterization of the resulting nanocrystals (NCs). Spontaneous Raman scattering showed first order Raman spectra of the Si NCs, which varied in their Raman shift and width. The Raman data were analyzed using a phonon confinement model, involving three-dimensional confinement and lognormal size distributions for the nanocrystallites constituting the samples, indicating the presence of particles greater than ∼2 nm diameter. High-resolution transmission electron microscopy and electron diffraction of the as-prepared samples showed NCs with diameters greater than ∼1.75 nm and quantum dots, corresponding to Si/SiOx. The optical properties of the Si NCs were studied with cathodoluminescence (CL) spectroscopy for sample temperatures in the 50–300 K range, which exhibited a supercontinuum emission ranging from the near ultraviolet to the red regions. The emission is characterized by four major emission bands, peaked at 1.88, 2.18, 2.64 and 3.44 eV, which practically remain at the same positions as the temperature is varied. These bands are assigned to excitonic recombination in the Si NCs, whose electron/hole transition energies are consistent with the quantum confinement of carriers in four size-dispersed groups of Si NCs. The influence of electron beam irradiation on the passivation of the NCs was examined, showing quenching of the CL as a function of electron-beam exposure time. This quenching is attributed to desorption of hydrogen, disruption of the NC surface passivation, and formation of nonradiative recombination channels involving surface states in the gap. The approach used in this study demonstrates the possibility of growing nano-scale Si crystals with the potential for incorporation into optoelectronic devices or for use in biomedical applications.