Optical Properties Of Si Nanocrystals Research Articles

Direct observation of the distribution of impurity in phosphorous/boron co-doped Si nanocrystals

Doping in Si nanocrystals is an interesting topic and directly studying the distribution of dopants in phosphorous/boron co-doping is an important issue facing the scientific community. In this study, atom probe tomography is performed to study the structures and distribution of impurity in phosphorous/boron co-doped Si nanocrystals/SiO2 multilayers. Compared with phosphorous singly doped Si nanocrystals, it is interesting to find that the concentration of phosphorous in co-doped samples can be significantly improved. Theoretical simulation suggests that phosphorous–boron pairs are formed in co-doped Si nanocrystals with the lowest formation energy, which also reduces the formation energy of phosphorous in Si nanocrystals. The results indicate that co-doping can promote the entry of phosphorous impurities into the near-surface and inner sites of Si nanocrystals, which provides an interesting way to regulate the electronic and optical properties of Si nanocrystals such as the observed enhancement of conductivity and sub-band light emission.

Optical properties of Si nanocrystals in SiO2 matrix synthesized by reactive pulsed laser deposition

Si ion implantation into SiO2 is widely used to synthesize specimens of SiO2 containing supersaturated Si. We also prepared specimens of supersaturated Si in SiO2 by reactive pulsed laser deposition (PLD) in an oxygen atmosphere. After high temperature annealing of these specimens induces the formation of embedded luminescent Si nanocrystals in SiO2. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to enhance the photoluminescence and to achieve low temperature formation of Si nanocrystals have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluence of 7.5 x 1016 ions/cm2. We also prepared Si nanocrystals embedded in a SiO2 by reactive pulsed laser deposition (PLD) in an oxygen atmosphere by using conventional PLD system with 2nd-harmonic YAG laser (532 nm, 10 Hz, 80 J/cm2) under controlled low oxygen pressure. Samples were subsequently irradiated with an excimer-UV lamp. After the process, the samples were rapidly thermal annealed before furnace annealing (FA). Photoluminescence spectra were measured at various stages at the process. We found that the luminescence intensity is enhanced with excimer-UV irradiation and RTA. Moreover, effective visible photoluminescence is found to be observed even after FA below the annealing temperature at 1000 °C, only for specimens treated with excimer-UV lamp and RTA. We will make clear the similarities of photoluminescence with the way of preparation techniques.

Tight-binding calculations of the optical properties of Si nanocrystals in a SiO2 matrix.

We develop an empirical tight binding approach for the modeling of the electronic states and optical properties of Si nanocrystals embedded in a SiO2 matrix. To simulate the wide band gap SiO2 matrix we use the virtual crystal approximation. The tight-binding parameters of the material with the diamond crystal lattice are fitted to the band structure of β-cristobalite. This model of the SiO2 matrix allows us to reproduce the band structure of real Si nanocrystals embedded in a SiO2 matrix. In this model, we compute the absorption spectra of the system. The calculations are in an excellent agreement with experimental data. We find that an important part of the high-energy absorption is defined by the spatially indirect, but direct in k-space transitions between holes inside the nanocrystal and electrons in the matrix.

Synthesis, optical properties and theoretical modelling of discrete emitting states in doped silicon nanocrystals for bioimaging.

The creation of multiple emission pathways in quantum dots (QDs) is an exciting prospect with fundamental interest and optoelectronic potential. For the first time, we report multiple emission pathways in semiconductor nanocrystals (NCs) where the number of emission pathways desired is controlled by the number of dopant atoms per quantum dot. The origin of additional emission pathways is explained by interactions between dopant states and NC energy levels. Density functional theory (DFT) calculations of undoped 2.3 nm silicon (Si NCs) and the same NCs doped with 2 interstitial Cu atoms show good agreement to experiment. Such calculations provide valuable data to explain the changes in optical transitions due to the Cu dopant in terms of transition energies, quantum yield and dopant position as a function of dopants per NC. Changes in the optical properties of Si NCs induced by dopant concentration include extended excitation range and enhanced absorption coefficients, emission redshifts of up to 60 nm, and a two-fold increase in quantum yields up to 22%. The optical properties of doped NCs lead to significant bioimaging improvements illustrated by in vitro cell imaging, including redshifted excitation wavelengths away from natural autofluorescence and enhanced fluorescent signals.

Optical properties of Si nanocrystals formed with laser pulse annealing

Nano- and femtosecond laser pulse annealings were applied for crystallization of thin amorphous Si films and amorphous Si nanoclusters in silicon-rich nitride and oxide films. Laser assisted formation of amorphous Si nanoclusters in the non-stoichiometric dielectric films was observed. Non-thermal effects in crystallization of amorphous Si by femtosecond annealing were supposed. This approach is applicable for the creation of dielectric films with Si nanoclusters on non-refractory substrates.

Enhancing crystallization of silicon nanocrystal embedded in silicon-rich oxide by ion beam-assisted sputtering

This study examined the material and optical properties of Si nanocrystals (NCs) embedded in Si-rich oxide (SRO) films prepared through ion beam-assisted sputtering (IBAS). Transmission electron microscopy and grazing-incidence X-ray diffraction revealed that IBAS improved the formation of the Si NCs in the SRO films. The size and density of Si NCs were predominantly controlled by IBAS with varying anode voltage. The photoluminescence levels of the SRO films were enhanced, which was associated with the quantum confinement effect of the Si NCs. The benefits of an Ar ion beam used on the SRO films are discussed in this paper. The results indicate that IBAS is a promising approach for the development of highly efficient Si-based optoelectronic devices.

Investigation of Photoluminescence Mechanisms from SiO2/Si:SiO2/SiO2 Structures in Weak Quantum Confined Regime by Deconvolution of Photoluminescence Spectra.

Si nanocrystals embedded in SiO2 matrix were prepared by co-sputtering method followed by a post annealing process in N2 ambient. By fixing sputtering parameters, the effects of annealing time and annealing temperature on the optical properties of Si nanocrystals are investigated. Origin and evolution of the photoluminescence (PL) in weak quantum confinement regime are discussed in the light of X-ray diffraction, Fourier transform infrared, and temperature dependent photoluminescence measurements. For all samples, the PL peaks tend to decompose to four Gaussian peaks in which attributed to the radiative defects in SiO2 matrix, nc-Si/SiO2 interface related localized defects, localized states in the amorphous Si band gap and quantum confinement of excitons in smaller nanocrystals. Considering the observation of luminescence and its decomposition tendency in nanocrystals with average sizes larger than exciton's Bohr radius the necessity to distinguish between the role of smaller and larger nanocrystals in the PL mechanisms is discussed. Furthermore, possible origin of the interface related localized states in particular Si=O double bonds in the nc-Si/SiO2 interface and that of radiative defects in SiO2 matrix are discussed.

Modeling optical properties of silicon clusters by first principles: From a few atoms to large nanocrystals

Time dependent density functional tight binding (TDDFTB) method is implemented with sparse matrix techniques and improved parallelization algorithms. The method is employed to calculate the optical properties of various Si nanocrystals (NCs). The calculated light absorption spectra of small Si NCs from TDDFTB were found to be comparable with many body perturbation methods utilizing planewave basis sets. For large Si NCs (more than a thousand atoms) that are beyond the reach of conventional approaches, the TDDFTB method is able to produce reasonable results that are consistent with prior experiments. We also employed the method to study the effects of surface chemistry on the optical properties of large Si NCs. We learned that the optical properties of Si NCs can be manipulated with small molecule passivations such as methyl, hydroxyl, amino, and fluorine. In general, the shifts and profiles in the absorption spectra can be tuned with suitably chosen passivants.

Ab initio study on the effect of structural relaxation on the electronic and optical properties of P-doped Si nanocrystals

In contrast to the conventional doping of bulk silicon (Si), the doping of Si nanocrystals (NCs) that are often smaller than 5 nm in diameter may lead to serious structural changes. Since the electronic and optical properties of Si NCs are intimately associated with their structures, it is critical to understand how doping impacts the structures of Si NCs. By means of ab initio calculation we now compare 1.4 nm phosphorus (P)-doped Si NCs without structural relaxation and those with structural relaxation. Structural changes induced by structural relaxation are manifested by the stretching and compressing of bonds and apparent variations in bond angles. With the increase of the concentration of P structural changes induced by structural relaxation become more serious. It is found that structural relaxation makes differences in the energy-level schemes of P-doped Si NCs. Structural relaxation also causes the binding energy of an electron in a P-doped Si NC to more significantly increase as the concentration of P increases. With the increase of the concentration of P structural relaxation leads to more pronounced changes in the optical absorption of P-doped Si NCs.

Solution Synthesis, Optical Properties, and Bioimaging Applications of Silicon Nanocrystals

Understanding and unlocking the potential of semiconductor nanocrystals (NCs) is important for future applications ranging from biomedical imaging contrast agents to the next generation of solar cells and LEDs. Silicon NCs (Si NCs) have key advantages compared with other semiconductor NCs due to silicon's high natural abundance, low toxicity and strong biocompatibility, and unique size, and surface dependent optical properties. In this Account, we review and discuss the synthesis, surface modification, purification, optical properties, and applications of Si NCs. The synthetic methods used to make Si NCs have improved considerably in the last 5-10 years; highly monodisperse Si NCs can now be produced on the near gram scale. Scaled-up syntheses have allowed scientists to drive further toward the commercial utilization of Si NCs. The synthesis of doped Si NCs, through addition of a simple elemental precursor to a reaction mixture or by the production of a single source precursor, has shown great promise. Doped Si NCs have demonstrated unique or enhanced properties compared with pure Si NCs, for example, magnetism due to the presence of magnetic metals like Fe and Mn. Surface reactions have reached a new level of sophistication where organic (epoxidation and diol formation) and click (thiol based) chemical reactions can be carried out on attached surface molecules. This has led to a wide range of biocompatible functional groups as well as a degree of emission tuneability. The purification of Si NCs has been improved through the use of size separation columns and size selective precipitation. These purification approaches have yielded highly monodisperse and pure Si NCs previously unachieved. This has allowed scientists to study the size and surface dependent properties and toxicity and enabled the use of Si NCs in biomedical applications. The optical properties of Si NCs are complex. Using a combination of characterization techniques, researchers have explored the relation between the optical properties and the size, surface functionalization, and preparation method. This work has led to a greater fundamental understanding of the unique optical properties of Si NCs. Si NCs are being studied for a wide range of important applications, including LEDS with tunable electroluminescence ranging from NIR to yellow, the encapsulation of Si NCs within micelles terminated with proteins to allow targeted in vivo imaging of cells, Si NC-polymer hybrid solar cells, and the use of Si NCs in battery anodes with high theoretical capacity and good charge retention.

Optical Bandgap Tunability of Silicon Nanocrystals Fabricated by Inductively Coupled Plasma CVD for Next Generation Photovoltaics

Superior optical properties of Si-nanocrystals (Si-NCs) compared with bulk Si, particularly tunability of bandgap by controlling size, can be exploited for realizing next-generation Si tandem solar cells. In view of this, optical bandgap tunability of Si-NCs fabricated by Inductively Coupled Plasma Enhanced Chemical Vapor Deposition (ICPCVD) is presented. The SiOx<;2/SiO2 superlattice approach was used for realizing Si-NCs with tight size control. Deposition time of SiOx sublayer and, hence, the related thickness (TSRO), was used as a variable parameter to realize Si-NCs of varying sizes. Formation of Si-NCs was verified by transmission electron microscopy and Raman spectroscopy. Using XPS analysis, the stoichiometry parameter x was estimated to be 0.82 for SiOx sublayer. The optical bandgap ETauc estimated using Tauc analysis was observed to be tunable from 1.57 to 2.52 eV as the size of Si-NCs was varied from 5.8 (±0.5) to 2 (±0.4) nm, respectively.

Temperature dependent optical properties of Si nanocrystals embedded in SiO2 matrix

The optical properties of Si nanocrystals (nc-Si) with different sizes (2∼5 nm) embedded in a SiO2 matrix synthesized by the SiOx/SiO2 superlattice approach were studied in a temperature range from room temperature to 600 K by spectroscopic ellipsometry. The Maxwell–Garnett effective medium approximation and Lorentz oscillator model were employed to extract the dielectric function of nc-Si. The results show that the temperature dependence of optical properties of nc-Si is sensitive to its size. An empirical expression of Varshni approximation was obtained to characterize the band-gap energy change of nc-Si at different temperatures.

Size-Dependent Optical Properties of Si Nanocrystals Embedded in a Silica Matrix

We report on a numerical investigation of the size distribution effect on the optical properties of Si nanocrystals embedded in a silica matrix. Firstly, the dielectric function of the composite film is accurately evaluated. Next, a modified version of the Maxwell-Garnett theory is introduced. This model allows the consideration of the size distribution of the Si nanocrystals. We show that a small change in the size dispersion value may induce a sensitive modification in the line shape of the dielectric function. Our results can be used to explain the divergences observed in the literature regarding the dielectric function of Si nanocrystals.

Size-dependent optical properties of Si nanocrystals embedded in amorphous SiO2 measured by spectroscopic ellipsometry

Silicon nanocrystals (Si NCs) with average diameters ⟨dNC⟩=3, 5 and 8 nm embedded in amorphous SiO2 synthesized by Si ion implantation have been characterized by spectroscopic ellipsometry. The dielectric function of the Si NCs has been extracted from SE data using a Bruggeman effective medium approximation and a Gauss–Lorentz oscillator model, taking into account the size dispersion of the nanoparticles. The dielectric function is found to depend strongly on average NC size. Although electronic critical point transitions of bulk silicon are discernible in NCs down to 3 nm diameter, the E1 resonance weakens with decreasing NC size, nearly disappearing for 3 nm diameter, while the dominant E2 transition blueshifts. In addition, a non-bulk-Si-like resonance intermediate between E1 and E2 is observed.

Comparative study of the nonlinear optical properties of Si nanocrystals fabricated by e‐beam evaporation, PECVD or LPCVD

AbstractThe nonlinear optical properties of silicon nanocrystals (Si‐nc) embedded in oxide matrices have been studied in samples obtained by three different technological process: e‐beam evaporation, plasma enhanced or low pressure chemical vapor deposition (PECVD and LPCVD, respectively). Z ‐scan measurements were performed in all the samples at 1064 nm by using ns‐pulses of a Nd:YAG laser. Similar nonlinear refractive index was found in the three systems, while differences in the nonlinear absorption were found as a function of the deposition method.The nonlinear results have been evaluated in terms of the crystalline and amorphous fractions from Raman scattering and photoluminescence (PL) measurements. While PL measurements show a emission spectrum similar for all the samples, Raman spectra reveal a sizeably different Si crystalline/amorphous ratio, depending on the deposition method. Therefore, the interface between the SiO2 matrix and Si‐nc plays a crucial role in determining the nonlinear optical response of the Si‐nc rich layers. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Size, oxidation, and strain in smallSi/SiO2nanocrystals

The structural, electronic, and optical properties of Si nanocrystals of different size and shape, passivated with hydrogens, OH groups, or embedded in a ${\text{SiO}}_{2}$ matrix are studied. The comparison between the embedded and free, suspended nanocrystals (NCs) shows that the silica matrix produces a strain on the embedded NCs, which contributes to determine the band gap value. By including the strain on the hydroxided nanocrystals, we are able to reproduce the electronic and optical properties of the full $\text{Si}/{\text{SiO}}_{2}$ systems. Moreover, we found that while the quantum confinement dominates in the hydrogenated nanocrystals of all sizes, the behavior of hydroxided and embedded nanocrystals strongly depends on the interface oxidation degree, in particular for diameters below 2 nm. Here, the proportion of NC atoms at the $\text{Si}/{\text{SiO}}_{2}$ interface becomes relevant, producing surface-related states that may affect the quantum confinement appearing as inner band gap states and then drastically changing the optical response of the system.

Electronic and optical properties of Si and Ge nanocrystals: An ab initio study

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge35H36 nanocrystal, and compare the results with those for the corresponding Si35H36 nanocrystals and the co-doped Si33BPH36.

Nonlinear optical properties of Si nanocrystals embedded in SiO2 prepared by a cosputtering method

Nonlinear optical properties of Si nanocrystals (Si-ncs) doped SiO2 prepared by a cosputtering method were studied by z-scan technique in a femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) were strongly enhanced compared to those of bulk Si and found to be about ∼2×10−13 cm2/W and ∼0.8 cm/GW, respectively. In the photon energy region from 1.48 to 1.65 eV, the n2 and β spectra followed the absorption spectra and no enhancement was observed in the band-edge photoluminescence region. In the diameter range of 2.7–5.4 nm, the size dependence of n2 coincided well with that calculated by a pseudopotential approach, suggesting that the discrete energy states of Si-ncs are responsible for the observed enhanced optical nonlinearity.

Influence of SiO2 matrix on electronic and optical properties of Si nanocrystals

Based on ab initio density functional theory we present electronic propertiesand the optical response for Si nanocrystals embedded in amorphousSiO2 networks. Quasi-spherical dots with diameters from 0.8 to 1.6 nmare investigated. The results for Si nanocrystals embedded inSiO2 are compared with corresponding results for hydrogenated Si nanocrystals of the same size.The calculations show the influence of the interface between nanocrystal and matrix on theelectronic properties. The results are compared with recent experimental data anddiscussed in detail. As striking features, strong reductions of the gaps and their diametervariation are predicted due to the oxide presence. Electronic confinement mainly influencesthe absorption edge while at higher photon energies only broad peaks at almost fixedpositions occur.