Surface Of Silicon Nanocrystals Research Articles

Microstructure and optical properties of porous silicon after plasma assisted nitridation

AbstractPorous silicon samples have been subjected to plasma treatments under ammonia atmosphere. The samples were characterized by IR, UV‐Visible reflectance and stationary photoluminescence spectroscopies. A deep modification concerning both surface and bulk properties of silicon nanocrystals is clearly observed. In particular, the nanocrystals surface is covered by amino groups, as checked by IR spectroscopy, whilst bulk modification can be inferred by UV‐Visible reflectance and luminescence characterizations, in particular for high porosity. The results demonstrate that plasma assisted nitridation yields to a controllable modification of the optical and chemical properties of porous silicon, without need of thermal annealing and/or wet chemical processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Singlet oxygen inhibits nonradiative defects in porous silicon

Experimental evidences are presented for the first time that singlet oxygen generated ex situ acts as an inhibitor of nonradiative recombination in porous silicon (PSi). This effect is observed on a pristine PSi as well as on degraded porous layers quenched by ozone adsorption. A photoluminescence (PL) enhancement produced by singlet oxygen is accompanied with only slight oxidation of a PSi. We assume that the observed effect on PL efficiency is due to gentle selective oxidation of single defects on silicon nanocrystal surface.

Effect of the granule size in porous silicon on the photosensitization efficiency of molecular oxygen on the surface of silicon nanocrystals

Photoluminescence is used to study the effect of the granule size in porous silicon on the generation efficiency of the excited state of molecular oxygen (1O2) on the surface of silicon nanocrystals. The generation efficiency is found to increase as the granule size becomes smaller than 100 nm, which can be explained by a change in the conditions of exciton diffusion along a network of silicon nanocrystals.

Investigation of the generation of singlet oxygen in ensembles of photoexcited silicon nanocrystals by electron paramagnetic resonance spectroscopy

The generation of singlet oxygen is investigated and its concentration upon photoexcitation of silicon nanocrystals in porous silicon layers is determined using electron paramagnetic resonance spectroscopy. The relaxation times of spin centers, i.e., silicon dangling bonds, in vacuum and in an oxygen atmosphere in the dark and under illumination of the samples are measured for the first time. It is revealed that the spin-lattice relaxation in porous silicon is retarded as compared to that in a single-crystal substrate. From analyzing experimental data, a microscopic model is proposed for interaction of oxygen molecules in the triplet state and spin centers at the surface of silicon nanocrystals. The results obtained have demonstrated that porous silicon holds promise for the use as a photosensitizer of molecular oxygen in biomedical applications.

Inside Front Cover: Plasma‐Assisted Synthesis of Silicon Nanocrystal Inks (Adv. Mater. 18/2007)

AbstractThe inside cover shows a photo and a schematic of a two‐step nonthermal plasma process used to produced silicon nanocrystal inks, that is, stable colloidal solutions of silicon nanocrystals. In the first (upper) plasma, nanocrystals are formed through plasma‐induced dissociation of silane molecules, leading to nanocrystal nucleation through chemical clustering. In the second (lower) plasma step, organic ligands are attached in‐flight to the silicon nanocrystal surfaces. The collected powder of surface functionalized silicon nanocrystals readily forms stable colloidal solutions in nonpolar solvents, report Lorenzo Mangolini and Uwe Kortshagen on p. 2513.

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I2 iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I2 molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ∼1018−1019 cm−3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I2 molecules.

EPR diagnostics of the photosensitized generation of singlet oxygen on the surface of silicon nanocrystals

Electron paramagnetic resonance spectroscopy is used to determine the concentration of excited (singlet) molecular oxygen photosensitized by silicon nanocrystals. The generation of singlet oxygen in the porous silicon layers is studied quantitatively at various oxygen pressures and exciting light intensities with the use of the technique proposed. The experimental results indicate that silicon nanoclusters as the photosensitizers of singlet oxygen are promising for biomedical applications.

Dynamics of photosensitized formation of singlet oxygen by porous silicon in aqueous solution

Generation of singlet oxygen due to energy transfer from photoexcited silicon nanocrystals in D2O is demonstrated. It is shown that the singlet oxygen generation efficiency, i.e., the intensity of near-infrared emission from singlet oxygen gradually decreases when Si nanocrystals are continuously irradiated in O2-saturated D2O. The mechanism of the photodegradation of the photosensitizing efficiency is studied using photoluminescence and infrared absorption techniques. Experimental results suggest that the interaction of photogenerated singlet oxygen with the hydrogen-terminated surface of silicon nanocrystals results in photo-oxidation of silicon nanocrystals, and the surface oxides reduce the photosensitizing efficiency. It is also demonstrated that photo-oxidation of porous silicon in O2-saturated water results in a strong enhancement of the photoluminescence quantum yield of porous Si.

Highly Enhanced Efficiency and Stability of Photo- and Electro-luminescence of Nano-crystalline Porous Silicon by High-Pressure Water Vapor Annealing

The effects of high-pressure water vapor annealing (HWA) on the photoluminescence (PL) of quantum-sized silicon materials have been studied for as-anodized or electrochemically oxidized heavily doped n-type porous Si (PS) and electrochemically oxidized p-type PS. PL efficiency and stability are markedly enhanced in all HWA-treated samples. PL peak wavelength remains almost unchanged except for n+-type PS in which a large redshift in the PL emission band was observed due to a structural nonuniformity in the depth direction. The high PL efficiency and stability of HWA-treated PS are attributed mainly to a suppression of nonradiative defect density by complete passivation of silicon nanocrystal surfaces. On the basis of these results, HWA is employed to improve the electroluminescence (EL) characteristics. The device fabricated using an HWA-treated anodized pn+ junction shows improved EL efficiency and stability.

Magnetic properties of nanoporous Si powder

Results of magnetic susceptibility and electron paramagnetic resonance studies of porous silicon powder (g = 2.0055) obtained by electrochemical technique are presented. In comparison with diamagnetic bulk Si (χd = –2.29 × 10–7 cm³/g), the porous Si powder exhibits paramagnetic properties (χp = 1.1 × 10–5 cm³/g) with weak share of ferromagnetism (χf = 1.85 × 10–5 cm³/g). Centres of 'dangling covalent bonds' type on the surface of silicon nanocrystals were registered to be responsible for the EPR spectra. In suggestion of uniform distribution of the paramagnetic centres in powder bulk its density was estimated to be 3.5 × 1022 cm-3.

Time-resolved photoluminescence studies of the energy transfer from excitons confined in Si nanocrystals to oxygen molecules

The formation of singlet oxygen due to the energy transfer from excitons confined in silicon nanocrystals to oxygen molecules is studied using time-resolved photoluminescence spectroscopy. The process of the excitation of oxygen molecules from the ground triplet state to the second excited singlet state is studied at low temperatures, where oxygen molecules are physisorbed on the surface of silicon nanocrystals and at room temperature in gaseous oxygen ambient and in oxygen-saturated water. The low temperature measurements reveal that the energy transfer time is the shortest for the resonant energy transfer. The involvement of one energy-conserving transversal optical phonon results in about 40% increase of the energy transfer time. The excitation rate of oxygen dimers is found to be similar to that measured for oxygen molecules. At room temperature, the time of the energy transfer to oxygen molecules is about $17\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. The photosensitizing efficiency of silicon nanocrystals at room temperature is found to be as high as 80% for gaseous oxygen ambient and for oxygen-saturated water.

Photoluminescence fatigue effect in luminescent porous silicon induced by photosensitized molecular oxygen

We report on the mechanism of photodegradation of hydrogenated porous silicon luminescence in ambients containing molecular oxygen. Energy transfer from excitons confined in silicon nanocrystals assembling porous silicon layers to molecular oxygen results in the generation of highly chemically reactive singlet oxygen molecules. Their subsequent interaction with the hydrogenated surface of silicon nanocrystals results in its photooxidation and creation of additional nonradiative defects, i.e. the luminescence fatique effect. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of adsorption of the donor and acceptor molecules at the surface of porous silicon on the recombination properties of silicon nanocrystals

The effect of adsorption of the donor and acceptor molecules on the spectra of photoluminescence and electron spin resonance (ESR) of microporous silicon is studied. It is found that photoluminescence of microporous silicon is quenched, the photoluminescence peak shifts to shorter wavelengths, and the intensity of the ESR signal increases after adsorption of molecules of nitrogen dioxide and pyridine. The results obtained are interpreted using a model of radiative excitonic recombination in porous silicon that takes into account the formation of both the charged (NO2)− and (C5H5N)+ complexes and defects (e.g., dangling bonds at the silicon surface) at the surface of silicon nanocrystals.

Photodegradation of porous silicon induced by photogenerated singlet oxygen molecules

We report on the mechanism of photodegradation of porous silicon luminescence in ambient containing molecular oxygen. Energy transfer from excitons confined in silicon nanocrystallites to molecular oxygen results in the generation of highly chemically reactive singlet oxygen molecules. The subsequent interaction of singlet oxygen molecules with the hydrogenated surface of silicon nanocrystals results in its photooxidation and the creation of additional nonradiative defects, i.e., the luminescence fatigue effect.

Interaction of nitrogen dioxide molecules with the surface of silicon nanocrystals in porous silicon layers

The methods of infrared absorption spectroscopy and electron paramagnetic resonance are used for studying the effect of adsorption of NO2 molecules, which are strong acceptors of electrons, on the electronic and optical properties of silicon nanocrystals in mesoporous silicon layers. It is found that the concentration of free charge carriers (holes) in silicon nanocrystals, which exhibits a nonmonotonic dependence on the NO2 pressure, sharply increases in the presence of these molecules. At the same time, a monotonic increase in the concentration of dangling silicon bonds (Pb1 centers) is observed. A microscopic model proposed for explaining this effect presumes the formation of donor-acceptor pairs P+b1-(NO2)− on the surface of nanocrystals, which ensure an increase in the hole concentration in nanocrystals, as well as Pb1 centers, which are hole-trapping centers. The proposed model successfully explains a substantial increase in photoconductivity (by two or three orders of magnitude) in the layers of porous silicon in the presence of NO2 molecules; the increment in the concentration of free charge carriers is detected within an order of magnitude of this quantity. The results can be used in designing electronic and luminescence devices based on silicon nanocrystals.

The Oxidation Behavior of Silicon Nanocrystals in the Submonolayer Region

AbstractFourier transform infrared spectroscopy is used to determine the time evolution of oxygen incorporation onto the surface of silicon nanocrystals. Oxygen concentrations up to one monolayer are investigated. The temporal progress of surface oxidation of Si nanocrystals in porous silicon shows a linear dependence on the square root of the oxidation time. This is similar to the oxidation of bulk Si and mesoporous silicon.

Red- and blue-light emission from free-standing porous silicon

We report the observation of strong red- and blue-light emission in free-standing porous-silicon samples prepared from n + substrates at different anodization current densities. The surface morphology of the free-standing samples has been analyzed by means of atomic-force microscopy. Upon excitation with nanosecond pulses at room temperature, both blue and red luminescence bands appear, peaked around 3.2 and 2.0 eV, respectively. An extensive study of the time-resolved behavior of the photoluminescence signal reveals different dynamical features for the two spectral regions. The observed long decay time (several μs) of the red band reflects the predominant effect of non-radiative processes and is consistent with models based on excition diffusion through the interconnected silicon nanocrystals (quantum dots) skeleton. The relatively fast blue-band decay time (≈400 ns) is shown to be related to non-radiative recombination through trap states at the silicon nanocrystal surface.