Luminescent Silicon Research Articles

Luminescence and Magnetic Properties of Porous Silicon Investigated with Respect to Its Metal Filling

The influence of magnetic metal filling on the optical and on the magnetic properties of luminescent porous silicon (PSi) is presented. The metal deposits affect, depending on their amount, the optical behavior but also give rise to specific magnetic properties. Due to the metal filling of the porous silicon the photoluminescence is blue-shifted and furthermore an increase of its intensity is observed. The modification of the optical properties, especially of the photoluminescence and its life time and the magnetic characterization of the nanocomposits is elucidated. First the optical properties of the luminescent PSi are investigated, second Ni is deposited electrochemically within the porous silicon samples and subsequently the nanocomposite specimens are characterized optically again and also magnetically. The samples are structurally characterized by SEM, EDX and TEM. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm and the luminescence intensity is increased. The observed blue shift of the photoluminescence peak with increasing metal deposition time and the enhancement of the photoluminescence intensity as well as a decrease of the photoluminescence life time can be interpreted by a decrease in the radiative life time. The enhancement of the photoluminescence intensity is interpreted by the coupling of the silicon emitter to the Ni plasmons. Due to the direct vicinity of the emitter and the metal structure, only separated by a thin SiO2 native layer, coupling of the plasmonic modes with the emitter can occur [1]. Concerning the magnetic properties of the nanocomposite the embedded Ni structures can be superparamagnetic from the size of the pore diameters but due to the branched morphology the achieved deposits tend to be interconnected and thus do not offer necessarily a superparamagnetic behavior [2]. Also temperature dependent magnetization measurements give no hint for superparamagnetism. Field dependent magnetization measurements performed with the magnetic field applied perpendicular and parallel to the sample surface show a high magnetic anisotropy. It can be clearly seen that the samples offer a film-like behavior due to the interconnected Ni structures which is represented by the easy axis parallel to the surface. The magnetic properties of the Ni filled PSi also offer a dependence on the amount of deposited metal. An increase of the deposition time meaning an increase of the metal amount within the porous layer leads to more interconnected metal structures. This behavior is evidenced by the enhancement of the coercivity and the magnetic anisotropy. The presented systems are promising candidates for applications in optoelectronics and also for magneto optical integrated devices. [1] A. Huck, U.L. Anderson, Nanophotonics (2016), 5, 1. [2] G. Bertotti, Hysteresis in Magnetism, 1st Edition, Academic Press, 1998.

Si/SiO2 Core/Shell Luminescent Silicon Nanocrystals and Porous Silicon Powders With High Quantum Yield, Long Lifetime, and Good Stability

Most of the highly efficient luminescent silicon nanocrystals (SiNCs) reported to date consists of organically capped silicon cores. Here, we report a method of obtaining Si/SiO2 core/shell nanoparticles emitting at a peak energy of 1.5 eV with very high quantum yields (53-61%). The same method led to quantum yields of ~ 30% for porous silicon powder emitting at 1.9 eV. The SiNCs were very stable under continuous excitation for several hours. The lifetime at 1.5 eV was over 232 μs, the longest ever reported for SiNCs, consistent with the very high luminescence efficiency. The SiNCs were first fabricated by non-thermal plasma synthesis or anodization in the case of porous silicon. Then, a thin oxide shell (~ 1 nm) was grown using high-pressure water vapor annealing. This oxidation process allows for the growth of very good quality oxide with low defect concentration and low stress, resulting in very good surface passivation, which explains the very high quantum yields obtained.

Ultrasonication effect on size distribution of functionalized porous silicon microparticles

Sponge-like luminescent mesoporous silicon microparticles are produced from crystalline silicon with a simple and cheap top-down fabrication process which, however, offers scarce control of structural features such size, size distribution and pore dimensions of the final product. Herein, we report on a post-fabrication procedure based on ultrasonic treatment to get more homogeneous size distribution and much more control on the average dimension of the microparticles. We investigated the effect of variable ultrasonic treatments on the structural and optical properties of the microparticles. The structure was analysed both at micro and nano scale and for microparticles in dried state and in solution. We proved that few minutes of sonication strongly limits the heterogeneity of the sample size, without affecting the optical properties and porous structure. This simple post-fabrication step offers new opportunities for successful use of silicon microparticles in biomedicine.

Luminescent Si quantum dots in flexible and semitransparent membranes for photon down converting material

In this work, luminescent silicon quantum dots (Si-QDs) in flexible and semitransparent porous silicon-polymer hybrid materials exhibiting intense visible luminescence, observed with the naked eye, are reported. Si-QDs within free-standing porous silicon membranes (PSiM) were obtained and transferred to polyethylene terephthalate (PET) substrates and covered with polymetilacrylate (PMMA), all at room temperature. The effect of the thickness (2.46–13.36 μm) and porosity (79–85%) of the PSiM on their photoluminescence (PL) properties are analyzed. PSiM of 2.46 μm thickness shows the most intense PL emission, observed with the naked eye under UV excitation. The transmittance between 450 and 900 nm reduces only ∼4.9% from that one of the PET substrate when the porosity is about 85%. The size of the Si-QDs, analyzed by Raman spectroscopy, reduces from 4.58 to 3.10 nm when the porosity is increased from 79 to 85%, and as a consequence a PL blue-shift is observed. The PL property of the flexible PMMA/PSiM/PET structures remains stable and breaking strength after several cycles of bending at different angles. Therefore, this work provides an adequate method for the development of hybrid luminescent, flexible and stable PSiM structures for its application as a photon-down converting material.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Magnetic Characteristics of Ni-Filled Luminescent Porous Silicon.

The aim of the presented work is to combine luminescent porous silicon (PSi) with a ferromagnetic metal (Ni) to modify on the one hand the photoluminescence by the presence of metal deposits and on the other hand to influence the optical properties by an external magnetic field. The optical properties are investigated especially with respect to the wavelength-shift of the photoluminescence due to the metal filling. With increasing metal deposits within PSi the photoluminescence peak is blue-shifted and furthermore an increase of the intensity is observed. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm for a deposition time of 15 min. Field dependent magnetic measurements performed with an applied field parallel and perpendicular to the surface, respectively, show a magnetic anisotropy which is in agreement with a thin film. This film-like behavior is caused by the interconnected Ni structures due to the branched porous silicon morphology. The coercivity increases with increasing metal deposition from about 150 Oe to about 450 Oe and also the magnetic anisotropy is enhanced with the growth of metal deposits. Within this work the influence of the magnetic metal filling on the optical properties and the magnetic characterization of the nanocomposites are discussed. The presented systems give not only rise to optoelectronics applications but also to magneto optical integrated devices.

Luminescent Porous Silicon Nanoparticles for Continuous Wave and Time-Gated Photoluminescence Imaging.

Luminescent porous silicon nanoparticles (LpSiNPs) display red-orange photoluminescence (PL) that provides large penetration depth for precise deep-tissue imaging and diagnostics. Herein, we describe in detail the fabrication process of porous silicon nanoparticles (pSiNPs), activation of photoluminescence, quantum yield measurement, and PL imaging. LpSiNPs perform as imaging probe for both the continuous wave and time-gated PL imaging.

Optical Characterization of Luminescent Silicon Nanowires

Visible photoluminescence (PL) at room temperature from silicon nanowires (Si NWs) prepared by using the metal-assisted chemical-etching (MACE) technique is reported. The morphology and the luminescence properties of Si NWs are characterized by using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), and luminescence spectroscopy. TEM images of the luminescent Si NWs reveal that the surfaces of the Si NWs are very rough, with a few nano-sized silicon particles being attached to the Si NWs. Luminescent Si NWs are optically characterized by PL and Raman measurements. Temperature-dependent PL measurements are measured at temperatures from 5 K to room temperature to determine the origin of the PL. The PL intensity decreases and the wavelength of the PL is blue-shifted as the temperature is increased. The Raman spectra of luminescent Si NWs reveal quantum confinement of the Si NWs.

One-pot green synthesis of highly luminescent silicon nanoparticles using Citrus limon (L.) and their applications in luminescent cell imaging and antimicrobial efficacy

Strongly luminescent and water dispersible silicon nanoparticles were synthesised by one-pot synthesis from venyl-trimethoxy-silane and (3-aminopropyl)-triethoxy-silane using both chemical and green reducing agents, sodium ascorbate and Citrus limon (L.) respectively. Unlike sodium ascorbate, the reduction using lemon extracts produces smaller nanoparticles with average size of 20 and 57 nm. The antimicrobial efficacy of the nanoparticles is examined against various microorganisms and is noted that these particles possessed antibacterial property if used in concentrations above the threshold value of 1.5 μg/ml. In addition, these silicon nanoparticles emit intense blue-green emission which is successfully utilised for the luminescent cell imaging of human white blood cells and mice epithelial cells.

One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon Nanoparticles.

Preparation of highly luminescent glasses involves expensive and complicated processes and usually requires high temperature. In this work, we show that luminescent silicon (Si) nanoparticle (NP)-embedded silicate gel glasses can be developed under near-ambient conditions by a remarkably simple, one-pot strategy, without using any sophisticated instrumentation or technique. Simultaneous hydrolysis and reduction of (3-aminopropyl)triethoxysilane leads to the formation of colloidal Si nanocrystals that can be transformed to a glassy phase upon slow evaporation followed by freezing. Structural investigations reveal the formation of a sodium silicate gel glass framework having discernible shear bands, along with embedded Si NPs. High photoluminescence quantum yield (ca. 35-40%), low glass-transition temperature ( Tg ≈ 66-73 °C), strain-tolerant mechanical stability, and inexpensive preparation make the glass attractive for applications as display materials and photonic converters.

Luminescent Silicon Nanocrystals Prepared by Laser-Assisted Synthesis in Liquid for Imaging and Photovoltaic Applications

Silicon nanocrystals (SiNCs) with photoluminescence in the visible spectral range were synthesized by laser irradiation of silicon wafers in solutions. Laser irradiation leads to the formation of well-dispersed Si–NCs with an average diameter of about 5 nm, which upon excitation with 300−450 nm light exhibited room temperature luminescence tunable in the range from 410 to 650 nm. The results showed that photoluminescence (PL) is affected by both sizes and surface states of the Si–NCs. The results obtained allow to conclude that the synthesized nanoparticles (NPs) can be suitable materials for applications in fluorescence imaging along with photovoltaics.

Luminescent silicon oxycarbide thin films obtained with monomethyl-silane by hot-wire chemical vapor deposition

The use of silicon-based materials can represent enormous advantages to develop optoelectronic devices. The preparation of luminescent silicon oxycarbide thin films deposited by the hot-wire CVD technique using monomethyl-silane as single and safer source of silicon and carbon atoms is here reported. The conditions for deposition that allow obtaining an intense emission on these thin films in a wide region of the visible spectrum at low deposition temperature without further thermal annealing are presented. When the oxygen flow rate was increased, a transition from silicon carbide to silicon oxycarbide was observed. X-ray diffraction confirms the presence of nanocrystalline material. Measurements showed that the origin of the photoluminescence may be to a combination of quantum confinement effects and defects in the silicon oxycarbide matrix, mainly those related to oxygen deficient centers and hydrogen and carbon-related defects. The obtained results are promising for the development of light emitting devices compatible with current technologies at low cost.

High photoluminescence quantum yields generated from N-Si-O bonding states in amorphous silicon oxynitride films.

We investigated the high absolute photoluminescence quantum yields (PL QYs) from tunable luminescent amorphous silicon oxynitride (a-SiNxOy) films. The PL QY of 8.38 percent has been achieved at PL peak energy of 2.55 eV in a-SiNxOy systems, which is higher than those of reported nanocrystal-Si embedded silicon nitride films. The existence of N-Si-O bonding states was confirmed by performing FTIR, XPS and EPR measurements. The PL QY is proportional to the concentration of Nx defects, indicating the dominant contribution of luminescent N-Si-O bonding states in radiative recombination processes. Particularly, we precisely monitored the ns-PL lifetimes evolution profile versus detected emission wavelengths, and further verified that the N-Si-O bonding states are responsible for highly efficient PL.

Structural evolution and effective improvement of emission quantum yields for silicon nanocrystals synthesized by femtosecond laser ablation in HF-contained solution

Stable luminescent colloidal silicon (Si) nanocrystals (NCs) with sufficient surface protection are prepared through femtosecond laser ablation in organic solvent containing diverse concentrations of HF solution. The average size of Si NCs shows the decreasing tendency from 6.5 to 2.7 nm when the concentration of HF varies from 0 to 11.1 vol% (volume ratio). In line with the structural evolution, UV–visible absorption, photoluminescence (PL) excitation spectra, and time-resolved PL, we propose that room temperature blue emission peaks at 412 and 440 nm originate from alkyl-related radiative recombination centers. The enhanced PL quantum yield of colloidal Si NCs from 16.3% to 76.5% has been attributed to the effective passivation and suppression of non-radiative defect centers with increasing HF concentration from 0 to 11.1 vol%.

Atomically precise cluster-based white light emitters $$^{\S }$$ §

Materials emitting white luminescence are receiving increasing attention due to their potential applications in electroluminescent devices, information displays and fluorescent sensors. To produce white light, one must have either three primary colors, blue, green and red or two colors, blue and orange. In this paper, we have used thiol/phosphine protected red luminescent silver nanoclusters (Ag NCs), $$[\hbox {Ag}_{29}(\hbox {BDT})_{12}(\hbox {PPh}_{3})_{4} ]^{3\hbox {-}}\, (\hbox {BDT}=1,\!3\hbox {-benzenedithiol})$$ , $$[\hbox {Au}_{\mathrm{x}}\hbox {Ag}_{29\hbox {-}\mathrm{x}}(\hbox {BDT})_{12} (\hbox {PPh}_{3})_{4}]^{3\hbox {-}}$$ and $$\hbox {Ag}_{29}(\hbox {LA})_{12}$$ ( $$\hbox {LA}= \hbox {lipoic acid}$$ ) as one of the fluorophores for white light emission. These clusters are mixed with blue luminescent silicon nanoparticles (Si NPs) and green luminescent fluorescein isothiocyanate (FITC). The mixtures show white luminescence with CIE coordinates of (0.31, 0.34), (0.33, 0.35) and (0.29, 0.31) which are in good agreement with pure white light (0.33, 0.33). The other clusters with yellow, blue, orange, etc., luminescence can also be used to make white light. This work provides a prospective pathway for white light emission based on atomically precise noble metal NCs. Synopsis Monolayer protected noble metal nanoclusters such as $$[\hbox {Ag}_{29}(\hbox {BDT})_{12}(\hbox {PPh}_{3})_{4}]^{3\hbox {-}}\, (\hbox {BDT}=1,3\hbox {-benzenedithiol})$$ , $$[\hbox {Au}_{\mathrm{x}}\hbox {Ag}_{29\hbox {-}\mathrm{x}}(\hbox {BDT})_{12} (\hbox {PPh}_{3})_{4}]^{3\hbox {-}}$$ and $$\hbox {Ag}_{29}(\hbox {LA})_{12}$$ ( $$\hbox {LA}= \hbox {lipoic acid}$$ ) are used as red luminophores which can produce white light emission when mixed with blue luminescent silicon nanoparticles (Si NPs) and green luminescent fluorescein isothiocyanate (FITC). The mixtures produce white luminescence with CIE (Commission Internationale d’Eclair) coordinates of (0.31, 0.34), (0.33, 0.35) and (0.29, 0.31), respectively. All-cluster-based white light emission is indeed possible.

Luminescence Properties of Porous Silicon Influenced By Magnetic Metal Filling

In this work the influence of magnetic metal filling on the photoluminescence of luminescent porous silicon is presented. The metal deposits affect the optical properties but also give rise to specific magnetic behavior. Due to the metal filling of the porous silicon the photoluminescence is blue-shifted and furthermore an increase of the intensity is observed. The influence of the magnetic metal filling on the optical properties (photoluminescence, decay time) is discussed and the magnetic characterization of the nanocomposits is presented. First the optical properties of the luminescent PSi are investigated, second Ni is deposited within the porous silicon samples and subsequently the nanocomposite specimens are characterized optically again and also magnetically. The samples are structurally characterized by SEM, EDX and TEM. The optical properties are investigated with respect to the shift of the photoluminescence and their decay times due to the metal filling. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm and the luminescence intensity is increased. Concerning the magnetic properties of the nanocomposite the embedded Ni structures can be superparamagnetic from the size of the pore diameters but due to the branched morphology the achieved deposits tend to be interconnected and thus do not offer necessarily a superparamagnetic behavior. Also temperature dependent magnetization measurements give no hint for superparamagnetism. Field dependent magnetization measurements performed with the magnetic field applied perpendicular and parallel to the sample surface show a high magnetic anisotropy. It can be clearly seen that the samples offer a film-like behavior due to the interconnected Ni structures which is represented by the easy axis parallel to the surface. In the frame of this work the optical characterization of luminescent PSi with respect to its photoluminescence compared with Ni filled samples is discussed in detail as well as the corresponding magnetic properties of the nanocomposites. Furthermore the influence of an external magnetic field on the optical properties is elucidated. The presented systems are promising candidates for applications in optoelectronics and also for magneto optical integrated devices.

Gamma-Radiation Monitoring of Luminescent Porous Silicon for Tumor Imaging

Photoluminescent macroporous silicon is studied for tumor imaging application. Gamma-radiation is used for modification of morphology of pores and their size. It is shown by Raman and photoluminescence study that after the irradiation of gamma rays new nanocrystallites of smaller diameter in comparison to the initial porous silicon appeared.

Luminescence of silicon nanoparticles from oxygen implanted silicon

Oxygen with a kinetic energy of 20 keV is implanted in a silicon wafer (100) at different fluences, followed by post-implantation thermal annealing (PIA) performed at temperatures ranging from 1000 to 1200 °C, in order to form luminescent silicon nanoparticles (SiNPs) and also to reduce the damage induced by the implantation. As a result of this procedure, a surface SiOx layer (with 0 < x < 2) with embedded crystalline Si nanoparticles has been created. The samples yield similar luminescence in terms of peak wavelength, lifetime, and absorption as recorded from SiNPs obtained by the more conventional method of implanting silicon into silicon dioxide. The oxygen implantation profile is characterized by elastic recoil detection (ERD) technique to obtain the excess concentration of Si in a presumed SiO2 environment. The physical structure of the implanted Si wafer is examined by grazing incidence X-ray diffraction (GIXRD). Photoluminescence (PL) techniques, including PL spectroscopy, time-resolved PL (TRPL), and photoluminescence excitation (PLE) spectroscopy are carried out in order to identify the PL origin. The results show that luminescent SiNPs are formed in a Si sample implanted by oxygen with a fluence of 2 × 1017 atoms cm−2 and PIA at 1000 °C. These SiNPs have a broad size range of 6–24 nm, as evaluated from the GIXRD result. Samples implanted at a lower fluence and/or annealed at higher temperature show only weak defect-related PL. With further optimization of the SiNP luminescence, the method may offer a simple route for integration of luminescent Si in mainstream semiconductor fabrication.

Optical Properties of Nanoporous Silicon in the Presence of Magnetic Nanostructures

This work aims to combine luminescent microporous silicon with a ferromagnetic metal to create a nanocomposite system with modifiable photoluminescence due to the presence of metal deposits as well as to influence the optical behavior by an external magnetic field. Due to the metal filling of the porous silicon (PSi) the peak of the photoluminescence is blue-shifted and furthermore an increase of the intensity is observed. Magnetic characterization of the samples shows a film-like behavior with the magnetic easy axis parallel to the surface which is due to interconnections of the metal deposits. The influence of the magnetic metal filling on the optical properties is discussed and the magnetic characterization of the samples is presented.

Excitation mechanism of Tb3+ in a-Si3N4:H under sub-gap excitation

We studied a sample of Tb-doped a-Si3N4:H prepared by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD). The sample has an optical gap E04 = 4.7 ± 0.3 eV and refractive index n (at 632 nm) = 1.81 ± 0.01. Room temperature photoluminescence was measured under sub-gap excitation. Both characteristic a-Si3N4:H and Tb3+ photoluminescence peaks were detected in the sample as deposited. Annealing at 300 °C maximizes the Tb3+ photoluminescence lines. At higher annealing temperatures the Tb3+ photoluminescence decreases while the host photoluminescence increases. The Tb3+ photoluminescence is inversely correlated with the density of Si-H bonds in the sample. The results indicate that silicon dangling bonds are involved in the excitation of the Tb3+ ions. We propose a new efficient non-radiative recombination path to the static disorder model that explains the luminescence of amorphous silicon and alloys: the Auger excitation of a rare earth ion near a silicon dangling bond. The model provides a very good explanation of the excitation and does not require the presence of nanostructures.