Synthesis Of Silicon Quantum Dots Research Articles

Plasma electrochemical synthesis of silicon quantum dots

Environmentally friendly and fast synthesis of silicon quantum dots (SiQDs) is realized with the assistance of plasma. The precursors used are N-(β-aminoethyl)-γ-aminopropyl trimethoxysilane (DAMO) and citric acid. When the excitation wavelength was 370 nm, the photoluminescence emission peak of the SiQDs appeared at 452.5 nm. The optimization of precursor concentration, reaction time and other parameters can effectively improve the quantum yield (QY) of SiQDs. The results show that the amidation and condensation of DAMO and citric acid plays an important role in the improvement of QY, as this means that more fluorescent molecules are produced and therefore QY is increased. This paper increases QY from 4.23% to 23.9%, providing a promising way to improve QY even more.

(Invited) Photophysics of QDs Enabled By Gas-Phase PECVD Growth

We have been exploring doping and alloying of semiconductor quantum dots (QDs) and the impact of chemical composition on the QD primary photochemical events including charge generation, separation, and recombination. Dopant incorporation and alloying without phase segregation are significant challenges using conventional colloidal growth methods. Nonthermal plasma-enhanced chemical vapor deposition (PECVD) growth is an attractive approach to overcome these challenges since it is controlled by kinetics rather than thermodynamics.In this presentation, we will describe the PECVD synthesis of silicon quantum dots (Si QDs) that provides the ability to incorporate high concentrations (~1020 cm–3) of either p-type (B:Si) or n-type (P:Si) dopants. We also can use this synthetic technique to prepare co-doped QDs containing both P and B atoms in the same PB:Si QD. Whereas the optical absorption of singly P:Si and B:Si QDs exhibit plasmonic features characteristic of a high concentration of dopants, the co-doped PB:Si QDs instead exhibit photophysics that resemble those of intrinsic Si QDs, for example emission lifetimes of ~100 μs, but with size-dependent optical transitions that are tunable even below the energy of bulk Si (1.12 eV). Finally, we will describe new work on III–V semiconductors, in particular gallium nitride (GaN) QDs, that also exhibit tunable photophysics and serve as useful model systems for photochemically-driven small molecule conversion.

Low-Cost Synthesis of Silicon Quantum Dots with Near-Unity Internal Quantum Efficiency.

As a cost-effective batch synthesis method, Si quantum dots (QDs) with near-infrared photoluminescence, high quantum yield (>50% in polymer nanocomposite), and near-unity internal quantum efficiency were fabricated from an inexpensive commercial precursor (triethoxysilane, TES), using optimized annealing and etching processes. The optical properties of such QDs are similar to those prepared from state-of-the-art precursors (hydrogen silsesquioxane, HSQ) yet featuring an order of magnitude lower cost. To understand the effect of synthesis parameters on QD optical properties, we conducted a thorough comparison study between common solid precursors: TES, HSQ, and silicon monoxide (SiO), including chemical, structural, and optical characterizations. We found that the structural nonuniformity and abundance of oxide inherent to SiO limited the resultant QD performance, while for TES-derived QDs this drawback can be avoided. The presented low-cost synthetic approach would significantly favor applications requiring high loading of good-quality Si QDs, such as light conversion for photovoltaics.

Synthesis of silicon quantum dots using chitosan as a novel reductor agent

In the present paper we report a novel synthesis method of silicon quantum dots (SiQDs) using 3-Aminopropyltriethoxysilane (APTES) as silicon precursor and low molecular weight chitosan (CS) as reducing agent. The obtained SiQDs have a hydrodynamic diameter of 2.3 nm, water dispersible, presents blue emission band at 434.5 nm (2.85 eV) with a Commission Internationale de l’Eclairage 1931 (CIE1931) chromaticity coordinates (x = 0.1665, y = 0.1222), the experimental absorbance of the SiQDS was measured and the band gap (Eg) was estimated through PerkinElmer’s method, the obtained value was 3.1 eV and a positive ζ-potential of + 35 mV, resulting in photonics, microelectronics, and biotechnological potential applications.

Cost-Effective Synthesis of Silicon Quantum Dots

Silicon (Si) nanomaterials with bright luminescence in the visible region are promising materials for use as the next-generation light source in displays, lighting, and biomedical imaging. A scalab...

Critical assessment of wet-chemical oxidation synthesis of silicon quantum dots.

Wet-chemical synthetic procedures are powerful strategies to afford fluorescent silicon quantum dots (Si QDs) in a versatile and scalable manner. However, development of Si QDs is still hampered by a lack of control over photoluminescence emission, in addition to synthesis and characterization complexities. The wet-chemical Si QD synthesis by oxidation of magnesium silicide (Mg2Si) with bromine (Br2) was revisited and a control reaction was carried out where the silicon source was omitted. Both reaction conditions resulted in substantial quantities of fluorescent material. Moreover, a comparative analysis of their optical properties (UV-vis/fluorescence) revealed no apparent differences. Other characterization techniques also confirmed the resemblance of the two materials as 1H NMR, FTIR and XPS spectra were nearly identical for both samples. Elemental analysis revealed the presence of only 2 wt% silicon in the Si QD sample. No evidence was found for the formation of significant amounts of Si QDs via this wet-chemical procedure.

Correction to "Synthesis of Silicon Quantum Dots with Highly Efficient Full-Band UV Absorption and Their Applications in Anti-Yellowing and Resistance of Photodegradation".

UV absorbers are very effective in the fields of antiyellowing, resistance of photocatalytic degradation, and sunscreen cosmetics. However, commercialized UV absorbers have the drawbacks of toxicit...

Synthesis of Silicon Quantum Dots with Highly Efficient Full-Band UV Absorption and Their Applications in Antiyellowing and Resistance of Photodegradation.

UV absorbers are very effective in the fields of antiyellowing, resistance of photocatalytic degradation, and sunscreen cosmetics. However, commercialized UV absorbers have the drawbacks of toxicity, low absorption efficiency, transparency, etc. Here, we report for the first time silicon quantum dots as full-band UV absorbers. The NH-refunctionalized silicon quantum dots with high-performance UV absorption were successfully synthesized under the synergistic effect of sodium citrate and ethanediamine, and the (NH, OH)-functionalized silicon quantum dots (SiQDs) with full-band UV absorption can be achieved by reregulating -NH2 and -OH groups on the surface. The as-prepared (NH, OH)-functionalized SiQDs exhibited good stability and underwent treatment of varying pH and temperature. Furthermore, experimental results demonstrated that compared to commercial water-soluble organic UV absorbers, the (NH, OH)-functionalized SiQDs showed better antiyellowing performance for polyurethane and resistance of photocatalytic degradation for rhodamine B, and presented huge application potential in sunscreen cosmetics. Finally, the UV absorption mechanism of SiQDs was explained to be mainly related to Γ → Γ direct band gap transition, which absorb UV light and release it as thermal radiation.

Novel two-stage method for the synthesis of silicon quantum dots embedded on ZnO matrix

At the present work, a novel two-stage method for the synthesis of zinc oxide (ZnO) films with silicon quantum dots embedded (SiQDs-ZnO) is reported. The experimental procedure consisting on ZnO matrix produced by the sol-gel technique and silicon quantum dots (Si-QDs) synthesized by a green synthesis is described. The incorporation of the Si-QDs on the ZnO films was obtained by spin coating followed by a post-deposition air annealing at 400 °C. The XPS results show that the Si-QDs were successfully embedded in the ZnO matrix. The effect of the tunable bandgap (Eg) of the ZnO with the addition of Si-QDs was obtained which is consistent with the Burstein-Moss effect. The enhancement of the photoluminescence (PL) of the ZnO films with the increment of Si content is presented.

The influence of local Si[sbnd]C bonding density on the photoluminescence of Si-QDs upon thermal annealing the hydrogenated amorphous Si-rich silicon carbide thin films

Non-stoichiometric hydrogenated amorphous silicon carbide thin films (α-SiC:H) were deposited by plasma-enhanced chemical vapor deposition. The samples were subsequently post-annealed at 750, 900, 1050, and 1200°C, respectively. Photoluminescence (PL) was measured by fluorescence spectrometer at room temperature. Infrared absorption was carried out by Fourier transform infrared absorption. Chemical compositions were analyzed by X-ray photoelectron spectroscopy. The synthesis of silicon quantum dots (Si-QDs) was characterized by Raman scattering spectroscopy and directly by high-resolution transmission electron microscope. PL measurements revealed that there were complicatedly shifted sub-bands upon the thermal annealing temperature increase. The behaviors of these shifted sub-bands showed converse trends as that of the local SiC bonding densities. A possible influence for the PL by the evolvement of the local SiC bonding densities and the synthesis of Si-QDs in the α-SiC:H samples is supposed.

The Synthesis and Structural Properties of Crystalline Silicon Quantum Dots upon Thermal Annealing of Hydrogenated Amorphous Si-Rich Silicon Carbide Films

Silicon quantum dots (QDs) embedded in non-stoichiometric hydrogenated silicon carbide (SiC:H) thin films have been successfully synthesized by plasma-enhanced chemical vapor deposition and post-annealing. The chemical composition analyses have been carried out by x-ray photoelectron spectroscopy (XPS). The bonding configurations have been deduced from Fourier transform infrared absorption measurements (FTIR). The evolution of microstructure with temperature has been characterized by glancing incident x-ray diffraction (XRD) and Raman diffraction spectroscopy. XPS and FTIR show that it is in Si-rich feature and there are a few hydrogenated silicon clusters in the as-grown sample. XRD and Raman diffraction spectroscopy show that it is in amorphous for the as-grown sample, while crystalline silicon QDs have been synthesized in the 900°C annealed sample. Silicon atoms precipitation from the SiC matrix or silicon phase transition from amorphous SiC is enhanced with annealing temperature increase. The average sizes of silicon QDs are about 5.1 nm and 5.6 nm, the number densities are as high as 1.7 × 1012 cm−2 and 3.2 × 1012 cm−2, and the crystalline volume fractions are about 58.3% and 61.3% for the 900°C and 1050°C annealed samples, respectively. These structural properties analyses provide an understanding about the synthesis of silicon QDs upon thermal annealing for applications in next generation optoelectronic and photovoltaic devices.

The influence of annealing temperature on the synthesis of silicon quantum dots embedded in hydrogenated amorphous Si-rich silicon carbide matrix

Hydrogenated amorphous silicon carbide thin films (a-SiC:H) were prepared by plasma-enhanced chemical vapor deposition (PECVD) and thermal annealed at temperatures of 900, 1050, and 1200°C, respectively. The influence of annealing temperature on the silicon quantum dot (QD) synthesis was investigated by Raman scattering spectroscopy, X-ray diffraction spectroscopy, and high-resolution transmission electron microscopy. The influence of annealing temperature on the chemical bonding configurations was revealed by Fourier transform infrared absorption microscopy. The element ratios of the as-deposited sample were deduced by X-ray photoelectron spectroscopy. Results reveal that the samples are in silicon-rich nature. Silicon in the as-deposited sample and the 900°C annealed sample are amorphous. When the annealing temperature is increased to 1050°C, crystal silicon QDs have come into being. The calculated number density is about 2.15±0.03×1012cm−2 and more than 80±3% of the silicon QDs fall within a narrow size range of 2–3nm. When the annealing temperature is increased to 1200°C, the average size of crystal silicon QDs is tuned from 2.6 to 3.2nm, while the crystallinity is enhanced from 56.7±2.5 to 67.1±1.5%. We attribute the influence of annealing temperature on the synthesis of silicon QDs to be dependent on the evolution of chemical bonding configurations and the agglomeration of silicon atoms from the host matrix.

Low temperature synthesis of silicon quantum dots with plasma chemistry control in dual frequency non-thermal plasmas

The advanced materials process by non-thermal plasmas with a high plasma density allows the synthesis of small-to-big sized Si quantum dots by combining low-temperature deposition with superior crystalline quality in the background of an amorphous hydrogenated silicon nitride matrix. Here, we make quantum dot thin films in a reactive mixture of ammonia/silane/hydrogen utilizing dual-frequency capacitively coupled plasmas with high atomic hydrogen and nitrogen radical densities. Systematic data analysis using different film and plasma characterization tools reveals that the quantum dots with different sizes exhibit size dependent film properties, which are sensitively dependent on plasma characteristics. These films exhibit intense photoluminescence in the visible range with violet to orange colors and with narrow to broad widths (∼0.3-0.9 eV). The observed luminescence behavior can come from the quantum confinement effect, quasi-direct band-to-band recombination, and variation of atomic hydrogen and nitrogen radicals in the film growth network. The high luminescence yields in the visible range of the spectrum and size-tunable low-temperature synthesis with plasma and radical control make these quantum dot films good candidates for light emitting applications.

Synthesis of silicon quantum dots showing high quantum efficiency.

Quantum efficiencies of Si quantum dots (QDs) have been investigated from the reaction of magnesium silicide and ammonium chloride. The change of quantum yield and optical characterization of Si QDs are measured depending on the reaction time. Highly luminescent Si QDs were obtained as the reaction time increased. Absorption measurement indicated that the Si QDs consisted of only silicon and hydrogen atom. Optical characterizations of Si QDs were measured by UV-Vis and PL spectroscopy. The size distribution and orientation of Si QDs were measured by TEM and XRD. TEM image displays the spherical Si QDs with the size of 3-4 nm. As the reaction time increased, Si QDs grew and their emission wavelength shifted to the longer wavelength. The monotonic shift of the PL as a function of excitation wavelength resulted in the excitation of different sizes of QDs that had different optical transition energies. Photoluminescence quantum yields exceeding 60% have been achieved.

Synthesis of silicon quantum dots in zinc silicate matrix by low-temperature process: Optical, structural and electrical characterization

Silicon nano-crystallites dispersed in an insulating matrix of zinc silicates have been obtained by a two-step process, consisting of sputtering deposition of several alternated amorphous silicon and zinc oxide thin layers, followed by a chemical reaction carried out under vacuum at comparatively low temperature (from 450°C to 560°C) between stratified reagents. This low cost, low temperature and scalable process has to be considered promising in quantum dot fabrication technology. Several characterization techniques have been employed and large evidence has been found for the presence of silicon and zinc oxide nano-crystallites showing quantum confinement effect and related band gap enlargement. Photoluminescence phenomenon has been observed in the range 550–850nm and a consistent explanation has been given in terms of exciton radiative recombination on trap states at the interface between Si dot and silicate matrix. Electrical measurements have given evidence for an expected material anisotropy where large difference between lateral and perpendicular resistivity values has been found. A technological strategy for transport property improvement has been prospected.

Liquid-phase plasma synthesis of silicon quantum dots embedded in carbon matrix for lithium battery anodes

Silicon quantum dots embedded in carbon matrix (SiQDs/C) nanocomposites were prepared by a novel liquid-phase plasma assisted synthetic process. The SiQDs/C nanocomposites were demonstrated to show high specific capacity, good cycling life and high coulmbic efficiency as anode materials for lithium-ion battery.

Synthesis of Silicon Quantum Dots Functionalized Chemically with Monosaccharides and Their Use in Biological Fluorescence Imaging

Abstract Novel water-dispersible, pH-stable, and biocompatible quantum dots were synthesized through the surface modification of silicon quantum dots (SiQDs) with monosaccharides. The presence of the monosaccharide on the surface of the SiQDs was recognized by fluorescence measurements using its complementary lectin (concanavalin A). The fluorescence cell imaging was carried out by using SiQDs functionalized chemically with d-glucosamine, and it was shown that the SiQDs developed in this study are applicable as blue chromophores for biological fluorescence imaging.

Method for fabricating third generation photovoltaic cells based on Si quantum dots using ion implantation into SiO2

In this paper, we report on the synthesis of silicon quantum dots for photovoltaic applications by means of ion implantation followed by annealing. Nucleation was achieved by implanting Si+ ions into SiO2 thin films, previously thermally grown on a Si(100) substrate, and annealing to 1100 °C. Passivation was used for photoluminescence (PL) measurements. The thickness of the oxide layer, the stoichiometry of the implanted layer, and the depth profiles of the implanted ions were determined for all samples by both Rutherford backscattering spectroscopy (RBS) and ellipsometry techniques. Characterization by transmission electron microscopy (TEM) indicates that the diameter of the silicon quantum dots (Si-QDs) varies from 2 to 4 nm, which is less than the Bohr radius of bulk crystalline Si(∼5 nm). Optical and electrical properties have been investigated by PL and I-V measurements. When passivated silicon nanocrystals (Si-nc) embedded into SiO2 are excited using a 450 nm diode laser, they exhibit a strong PL emission in the range of 650-1000 nm. Based on these investigations, p-type Si-QDs/n-type c-Si junctions were fabricated and electrically characterized in the dark as well as under an AM1.5G terrestrial solar spectrum for nonimplanted, as-implanted, and implanted-annealed samples for different implantation fluences. The electrical curves of the structures under illumination demonstrate the photovoltaic behavior of the Si-QDs. Despite the weak light conversion of these devices, these results remain very promising and offer potentially unprecedented, vast improvements to third generation solar cells.

Synthesis and On-line Size Control of Silicon Quantum Dots

The synthesis of silicon quantum dots is performed in the [3–5 nm] range using CO2 laser pyrolysis of SiH4. This size range is particularly relevant for potential applications in photovoltaic devices and biomedical imaging. The laser pyrolysis technique offers convenient control of the synthesis parameters in the case of nanoparticle production. However, controlling the size of small silicon objects remains difficult. The original approach consists here in a time-control of the energy injected into the reaction by gating the laser. The laser gate-on duration is adjusted in the range of 10 to 80 μs while keeping the average power constant. In parallel, supersonic expansion and on-line time-of-flight mass spectrometry are performed for on-line size characterization. A monotonic increase of the size as a function of the gate-on duration is observed for several SiH4 volume concentrations. The results are discussed qualitatively.

Sized controlled synthesis, purification, and cell studies with silicon quantum dots

This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using (1)H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales.