Quantum Electronics 44 (6) 552 – 555 (2014) © 2014 Kvantovaya Elektronika and Turpion Ltd Abstract. Broadband femtosecond spectroscopy has been used to study two types of hydrophilic silicon nanoparticles: (1) photolumi- nescent, passivated with deuterium and oxidised in fully deuterated dimethyl sulphoxide, and (2) nonluminescent (control samples hav- ing a similar crystalline core), passivated with hydrogen and oxi- dised in dimethyl sulphoxide. We have found significant differences in ultrafast spectral – temporal induced absorption dynamics between the two types of nanoparticles in the energy range corre- sponding to their calculated band gap. The observed distinction is due to the considerably higher oxidation rate of silicon on the sur- face of the deuterated samples in comparison with the undeuterated ones and with the associated increase in the number of photolumi- nescence centres on the surface of the nanoparticles. In the samples containing self-trapped exciton (STE) energy states responsible for the photoluminescence in the red spectral region, carrier capture at these levels and carrier relaxation to the ground state have charac- teristic times in the femtosecond range. In the samples free of STE states, excited carriers relax to the conduction band bottom in a characteristic time of several picoseconds. Keywords: femtosecond spectroscopy, silicon quantum dots, charge carrier relaxation. 1. Introduction Because of the explosive growth in medical and biological applications of red fluorescent markers, the search for quan- tum dots (QDs) free of toxic ions has now become a critical issue. In connection with this, there is increased interest in crystalline nanosilicon-based QDs (Si-QDs). Silicon is known to be nontoxic and, moreover, it is one of the elements of life because it enters into the composition of a number of enzymes and proteins. The optical properties of Si-QDs are determined by both quantum confinement and silicon surface modifica- tion [1]. The red luminescence band of Si-QDs was interpreted in terms of an oxygen-related defect level, lying in the band gap of the silicon nanocrystals at nanoparticle diameters under 3 nm [2]. To date, many techniques for the synthesis of silicon nanoparticles with bright red and infrared lumines- cence have been proposed [3 – 5]. One of the most technologi- cally feasible techniques among them is the chemical synthesis of Si-QDs from silicon monoxide [6 – 8], which was used in this study. Unfortunately, the photoluminescence quantum yield in Si-QDs is still rather low, which is mainly due to the nonradiative relaxation of excited carriers. A deep under- standing of the processes responsible for the reduced lumines- cence efficiency is critical for the ability to improve the optical properties of silicon nanoparticles. Femtosecond absorption spectroscopy provides information about primary femto- and picosecond nonradiative relaxation channels for photoexcita- tion, and controlled engineering approaches enable targeted nanoparticle synthesis with the aim of verifying one mecha- nism or another. In this paper, we report an experimental study of the influ- ence of oxygen-related defect levels on the relaxation rate of excited carriers in Si-QDs. To this end, broadband femtosec- ond spectroscopy was used to gain insight into carrier excita- tion and relaxation processes in two types of hydrophilic Si-QDs having identical crystalline cores: (1) luminescent, passivated with deuterium and oxidised in fully deuterated dimethyl sulphoxide (DMSO-D6) and (2) nonluminescent, passivated with hydrogen and oxidised in dimethyl sulphox- ide (DMSO). 2. Experimental The nanocrystalline Si core used in the synthesis of the hydro- philic Si-QDs was prepared by the thermal annealing of sili- con monoxide [6 – 8] at 400 °C. The average silicon core diam- eter in the nanoparticles was 2.5 ± 0.5 nm (as determined by small-angle X-ray scattering [8]). The core size was essentially independent of the SiO annealing temperature when it was within 900 °C. Hydrophilic nanosilicon sols were prepared by a process described elsewhere [7], with the following modifi- cations: The SiO shell surrounding the Si core after annealing was etched by hydrofluoric acid-d (DF solution in D 2 O) to give a deuterated material (D – Si-QDs) or by hydrofluoric acid (HF) to give a deuterium-free material (H – Si-QDs). To remove the acids, the nanoparticles were first washed with D 2 O and H 2 O, respectively. In the final washing and drying Effect of deuterium substitution for hydrogen in surface functionalisation of hydrophilic nanosilicon particles on their spectral and dynamic properties V.O. Kompanets, S.V. Chekalin, S.G. Dorofeev, N.N. Kononov, P.Yu. Barzilovich, A.A. Ischenko V.O. Kompanets, S.V. Chekalin Institute of Spectroscopy, Russian Academy of Sciences, Fizicheskaya ul. 5, Troitsk, 142190 Moscow, Russia; e-mail: kompanetsvo@isan.troitsk.ru, chekalin@isan.troitsk.ru; S.G. Dorofeev Department of Chemistry, M.V. Lomonosov Moscow State University, Vorob’evy gory, 119991 Moscow, Russia; e-mail: dorofeev_sg@mail.ru; N.N. Kononov A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, 119991 Moscow, Russia; e-mail: nnk@kapella.gpi.ru; P.Yu. Barzilovich Institute of Problems of Chemical Physics, Russian Academy of Sciences, prosp. Akad. Semenova 1, 142432 Chernogolovka, Moscow region, Russia; e-mail: bpy@icp.ac.ru; A.A. Ischenko Lomonosov Moscow State University of Fine Chemical Technologies, prosp. Vernadskogo 86, 119579 Moscow, Russia; e-mail: aischenko@yasenevo.ru Received 3 March 2014; revision received 20 March 2014 Kvantovaya Elektronika 44 (6) 552 – 555 (2014) Translated by O.M. Tsarev PACS numbers: 42.62.Fi; 78.47.jd; 78.67.Hc DOI: 10.1070/QE2014v044n06ABEH015457