Oxidized Silicon Nanocrystals Research Articles

Physical properties enhancement of porous silicon treated with In2O3 as a antireflective coating

Abstract In this work, we investigate the effect of Indium Oxide (In2O3) on the microstructural, optical, optoelectrical, and electrical properties of Porous Silicon (PS) layer. PS film was prepared by electrochemical anodization technique. In2O3 thin film was coated onto PS layer by using simple chemical immersion method. The investigation showed a significant enhancement in the optoelectronic property of PS coated with In2O3. The In2O3/PS layer was thermally annealed to improve the efficiency of the photovoltaic cells. Surface morphology and chemical composition modification of In2O3/PS were analyzed by atomic force microscopy (AFM) and Fourier transfer infrared (FTIR) spectroscopy. Optical reflectivity of sample In2O3/PS decreases significantly from 30% to 2.4% owing to an improvement in the light absorption. The optical parameters such as refractive index (n) and extinction coefficient (k), real and imaginary dielectric constants (e1 & e2), as well as the components percentage and thickness of samples were determined by analyzing the TanΨ and CosΔ ellipsometric parameters. Photoluminescence (PL) analysis of PS layer treated with In2O3 before and after annealing indicated a bleu emission shifts due to the quantum confinement effect of the oxidized silicon nanocrystals. Annealed PS coated In2O3 resulted a significant improvement in the minority carrier lifetime (τeff) from 2 to 16.4 μs. The optoelectronic and electronic quality of the treated PS layer with In2O3 was enhanced noticeably compared to the untreated porous layer, making it an ideal candidate for the solar cell applications.

Impact of Silicon Nanocrystal Oxidation on the Nonmetallic Growth of Carbon Nanotubes.

Carbon nanotube (CNT) growth has been demonstrated recently using a number of nonmetallic semiconducting and metal oxide nanoparticles, opening up pathways for direct CNT synthesis from a number of more desirable templates without the need for metallic catalysts. However, CNT growth mechanisms using these nonconventional catalysts has been shown to largely differ and reamins a challenging synthesis route. In this contribution we show CNT growth from partially oxidized silicon nanocrystals (Si NCs) that exhibit quantum confinement effects using a microwave plasma enhanced chemical vapor deposition (PECVD) method. On the basis of solvent and a postsynthesis frgamentation process, we show that oxidation of our Si NCs can be easily controlled. We determine experimentally and explain with theoretical simulations that the Si NCs morphology together with a necessary shell oxide of ∼1 nm is vital to allow for the nonmetallic growth of CNTs. On the basis of chemical analysis post-CNT-growth, we give insight into possible mechanisms for CNT nucleation and growth from our partially oxidized Si NCs. This contribution is of significant importance to the improvement of nonmetallic catalysts for CNT growth and the development of Si NC/CNT interfaces.

Silicon excess and thermal annealing effects on structural and optical properties of co-sputtered SRO films

In this work, the structural and optical properties of co-sputtered SRO (Silicon Rich Oxide) films are studied as a function of the annealing temperature and the change in the Si (Silicon) content. Si-excess of 4.4–16.9at.% were obtained by a variation of RF-power densities applied to Si target from 1.97 to 3.94W/cm2, respectively. The Si-excess calculated by XPS promotes the formation of Si clusters within SRO films where the size depends on the Si-excess quantity. After annealing, ultra-small oxidized silicon nanocrystals (Si-NCs) and agglomeration of Si-excess, promotes the possible creation of Si=O, centers like Eδ′ and ODC defects at the surface of Si-NCs. Therefore, two broad Photoluminescence (PL) bands are observed in the red (1.5–1.7eV) and blue (2–2.9eV) region. A high emission intensity in red region is observed in sample with 5.9at.% of Si-excess, which could be related with a complete activation of defects (Si=O) favored by the dominant average size of the Si-NCs (1.68nm). The density and average size of Si-NCs increase according to Si-excess, whereby a decrease in PL intensity is observed. Thus, a considerable Si-excess (>10at.%) is not necessary to obtain the maximum PL emission in red region.

Enhanced photothermal effect of surface oxidized silicon nanocrystals anchored to reduced graphene oxide nanosheets

We demonstrate the coupling of the photothermal effects of silicon nanocrystals and graphene oxide (GO) dispersed in water. Using laser irradiation (532nm or 355nm) of suspended Si nanocrystals in an aqueous solution of GO, the synthesis of surface oxidized Si-reduced GO nanocomposites (SiOx/Si-RGO) is reported. The laser reduction of GO is accompanied by surface oxidation of the Si nanocrystals resulting in the formation of the SiOx/Si-RGO nanocomposites. The SiOx/Si-RGO nanocomposites are proposed as promising materials for photothermal therapy and for the efficient conversion of solar energy into usable heat for a variety of thermal and thermomechanical applications.

Do Excited Silicon–Oxygen Double Bonds Emit Light?

Oxidation of the surface of well-passivated silicon nanocrystals introduces defects which dramatically affect the optical properties of the material. One such defect is the silicon–oxygen double bond, which has been implicated as the source of the unusual particle-size-independent S-band photoluminescence of oxidized silicon nanocrystals. Herein, we investigate the photodynamics of this defect by application of a first-principles nonadiabatic molecular dynamics approach to a cluster model containing a silicon–oxygen double bond. Upon excitation, pyramidalization occurs about the double-bonded silicon atom, leading to a conical intersection between the ground and first excited state. This conical intersection facilitates nonradiative decay, resulting in the internal conversion of 7% of the excited population in the first picosecond after excitation. Extrapolation to longer times suggests that nonradiative decay via conical intersection proceeds faster than the microsecond photoluminescence lifetime of sili...

A complex study of the fast blue luminescence of oxidized silicon nanocrystals: the role of the core

Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.

Communication: Non-radiative recombination via conical intersection at a semiconductor defect

Localization of electronic excitations at molecule-sized semiconductor defects often precedes non-radiative (NR) decay, and it is known that many molecules undergo NR decay via conical intersection. Herein, we report the direct simulation of fast and efficient NR decay via a conical intersection at a known semiconductor defect. It is suggested that this silicon epoxide defect may selectively quench photoluminescence (PL) in small silicon nanocrystals (band gap > ~2.8 eV), and thus influence both the observed PL yield and PL energy of oxidized silicon nanocrystals.

Study of defect-related light emission in oxidized silicon nanocrystals

Luminescence from surface oxidized silicon nanocrystals (Si-nc) is an intriguing phenomenon with great potential for applications. Full consensus on the source of optical emission is not yet reached despite the huge research effort devoted to this topic. In this paper we report new evidence supporting a defect-related emission mechanism. In particular we propose a defect-based model to interpret the photoluminescence (PL) emission from partially oxidized Si-nc after thermal annealing in air. The PL emission is first studied in the time-dependent regime, using an approach based on discrete light-emitters. The results of this analysis are then exploited for elucidation of continuous wave (CW) data. Using our model the dependence of the PL emission spectrum on the excitation power and the shortening of the time decays at increasing pump power can be explained in terms of the different saturation behavior of luminescent defects. Moreover a rate equation approach for the coupled system composed by the Si-nc core and the associated luminescent defects is used to interpret the luminescence rise-time data under pulsed excitation. In this way the consistency of our picture is verified. Attribution of luminescent defects to non-bridging hole centers combined with oxygen vacancies in the oxide layer capping the Si-nc allows for a preliminary understanding of the increase of luminescence emission intensity and of the variation of its spectral features with the thermal annealing conditions.

Light emission from silicon nanocrystals

The main experimental results of studies of the photoluminescence of silicon nanocrystals and theoretical methods developed for the description of optical processes occurring in them are reviewed. Special attention is focused on silicon nanocrystals in the SiO2 matrix that were the object of most of the studies. Two fundamental theoretical methods described in detail are the multiband effective-mass method and the tight-binding method which have found wide application in simulating various processes occurring in nanostructures. A phenomenological model for excitons self-trapped on the surface of oxidized silicon nanocrystals, which has been recently developed on the basis of experimental results obtained by femtosecond spectroscopy, is reported.

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.

Surface and Core Electronic Structure of Oxidized Silicon Nanocrystals

Ab initio restricted Hartree‐Fock method within the framework of large unit cell formalism is used to simulate silicon nanocrystals between 216 and 1000 atoms (1.6–2.65 nm in diameter) that include Bravais and primitive cell multiples. The investigated properties include core and oxidized surface properties. Results revealed that electronic properties converge to some limit as the size of the nanocrystal increases. Increasing the size of the core of a nanocrystal resulted in an increase of the energy gap, valence band width, and cohesive energy. The lattice constant of the core and oxidized surface parts shows a decreasing trend as the nanocrystal increases in a size that converges to 5.28 Ǻ in a good agreement with the experiment. Surface and core convergence to the same lattice constant reflects good adherence of oxide layer at the surface. The core density of states shows highly degenerate states that split at the oxygenated (001)‐(1 × 1) surface due to symmetry breaking. The nanocrystal surface shows smaller gap and higher valence and conduction bands when compared to the core part, due to oxygen surface atoms and reduced structural symmetry. The smaller surface energy gap shows that energy gap of the nanocrystal is controlled by the surface part. Unlike the core part, the surface part shows a descending energy gap that proves its obedience to quantum confinement effects. Nanocrystal geometry proved to have some influence on all electronic properties including the energy gap.

Silicon nanocrystals as matrix material for the desorption of biomolecule–water complexes

A new technique for the efficient production of biomolecule–water complexes, which may be employed in mass spectrometry and laser spectroscopy experiments, is described. Using native oxidized silicon nanocrystals as matrix material for biomolecules in laser desorption experiments, an enhanced efficiency for the formation of biomolecule–water complexes is observed. It is found that the water molecules are supplied by the silicon oxide shells covering the silicon nanocrystals. The new technique is demonstrated for the formation of tryptophan–water complexes with up to six water molecules.

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.

Surface-Related States in Oxidized Silicon Nanocrystals Enhance Carrier Relaxation and Inhibit Auger Recombination

We have studied ultrafast carrier dynamics in oxidized silicon nanocrystals (NCs) and the role that surface-related states play in the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs. Transient photoinduced absorption techniques have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption (PA) signal and an identical decay, suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NC.

Two-photon excitation of luminescence in pyrolytic silicon nanocrystals

Two-photon-excited luminescence after femtosecond near-infrared (NIR) excitation was observed in chemically oxidized silicon nanocrystals (Si-nc) produced by laser-assisted pyrolysis of silane. The luminescence time decay and emission spectrum resulted in good agreement with those after blue excitation. The underlying two-photon absorption coefficient was also measured by the open-aperture z-scan technique, obtaining a value of 12.5 cm/GW, which is intermediate between the values reported in the literature for bulk silicon and for very small Si-nc.

Ab initio calculation of the vibrational modes ofSiH4,H2SiO,Si10H16,and Si10H14O

We have studied the normal modes of hydrogenated and oxidized silicon nanocrystals, namelySiH4 (silane),H2SiO (silanone),Si10H16, andSi10H14O. The smallclusters (SiH4 and H2SiO) have been used for convergence tests and their bond-lengths and frequencies have beencompared with experimental and theoretical reference data. For the large clusters(Si10H16 and Si10H14O) we have investigated the vibrational density of states, where we have identified theoxygen-related spectral features. The vibrational modes have also been analyzed withrespect to the displacement patterns. The calculations have been carried out within thedensity-functional and density-functional perturbation theory using the local-densityapproximation.

Ultrafast transient photoinduced absorption in silicon nanocrystals: Coupling of oxygen-related states to quantized sublevels

The authors have studied transient photoinduced absorption in single monolayers of oxidized silicon nanocrystals. Transient photoinduced absorption measurements along with optical absorption and photoluminescence (PL) emission reveal that the light-absorption process takes place in defects related to strong PL emission, suggesting that the photoexcited carriers are in oxygen-related interface states. They have time-resolved ultrafast relaxation paths in oxygen-related states and quantized sublevels, which have important implications in the understanding of fundamental optical properties for this system.

Strong low-temperature anti-Stokes photoluminescence from coupled silicon nanocrystals

Abstract We report efficient low-temperature anti-Stokes photoluminescence (upconverted photoluminescence=UPL) at resonant excitation of coupled silicon nanocrystals (NCs) in porous silicon (PSi). This photoluminescence is absent in strongly oxidized porous silicon and oxidized silicon nanocrystals. The UPL is a result of resonant excitation of electron–hole (e–h) pairs spatially separated in neighboring crystals with different band-gap. It is generated by the excitation of a second e–h pair in the larger of the two crystals and Auger ejection of a carrier into the smaller one, with the larger band-gap.

Photoluminescence spectrum and dynamics in oxidized silicon nanocrystals: A nanoscopic disorder system.

Photoluminescence spectrum and dynamics in oxidized silicon nanocrystals: A nanoscopic disorder system.