Solution-processed silicon quantum dot photocathode for hydrogen evolution

The photoelectrochemical response of a photocathode made from a colloidal solution of boron (B) and phosphorus (P) codoped silicon (Si) quantum dots (QDs) 2-11 nm in diameters is studied. Since codoped Si QDs are dispersible in alcohol and water due to the hydrophilic surface, a photoelectrode with a smooth surface is produced by drop-coating the QD solution on an indium tin oxide substrate. The codoping provides high oxidation resistance to Si QDs and makes the electrode operate as a photocathode. The photoelectrochemical response of a Si QD photoelectrode depends strongly on the size of QDs; there is a transition from anodic to cathodic photocurrent around 4 nm in diameter. Below the size, anodic photocurrent due to self-oxidation of Si QDs is observed, while above the size, cathodic photocurrent due to electron transfer across the interface is observed. The cathodic photocurrent increases with increasing the size, and in some samples, it is observed for more than 3000 s under intermittent light irradiation.

Luminescent silicon nanoparticles for distinctive tracking of cellular targeting and trafficking

Developing therapeutic nanoparticles that actively target disease cells or tissues by exploiting the binding specificity of receptors presented on the cell surface has extensively opened up biomedical applications for drug delivery and imaging. An ideal nanoparticle for biomedical applications is required to report confirmation of relevant targeting and the ultimate fate in a physiological environment for further verification, e.g. to adapt dosage or predict response. Herein, we demonstrate tracking of silicon nanoparticles through intrinsic photoluminescence (PL) during the course of cellular targeting and uptake. Time-resolved analysis of PL characteristics in cellular microenvironments provides dynamic information on the physiological conditions where the silicon nanoparticles are exposed. In particular, the PL lifetime of the silicon nanoparticles is in the order of microseconds, which is significantly longer than the nanosecond lifetimes exhibited by fluorescent molecules naturally presented in cells, thus allowing discrimination of the nanoparticles from the cellular background autofluorescence in time-gated imaging. The PL lifetime is a physically intensive property that reports the inherent characteristics of the nanoparticles regardless of surrounding noise. Furthermore, we investigate a unique means to inform the lifespan of the biodegradable silicon nanoparticles responsive to local microenvironment in the course of endocytosis. A multivalent strategy of nanoparticles for enhanced cell targeting is also demonstrated with complementary analysis of time-resolved PL emission imaging and fluorescence correlation spectroscopy. The result presents the promising potential of the photoluminescent silicon nanoparticles toward advanced cell targeting systems that simultaneously enable tracking of cellular trafficking and tissue microenvironment monitoring.

Precise size separation of water-soluble red-to-near-infrared-luminescent silicon quantum dots by gel electrophoresis

Gel electrophoresis, which is a standard method for separation and analysis of macromolecules such as DNA, RNA and proteins, is applied for the first time to silicon (Si) quantum dots (QDs) for size separation. In the Si QDs studied, boron (B) and phosphorus (P) are simultaneously doped. Codoping induces a negative potential on the surface of a Si QD and makes it dispersible in water. Si QDs with different B and P concentrations and grown at different temperatures (950 °C-1200 °C) are studied. It is shown that native polyacrylamide gel electrophoresis can separate codoped Si QDs by size. The capability of gel electrophoresis to immobilize size-separated QDs in a solid matrix makes detailed analyses of size-purified Si QDs possible. For example, the photoluminescence (PL) studies of the dried gel of Si QDs grown at 1100 °C demonstrate that a PL spectrum of a Si QD solution with the PL maximum around 1.4 eV can be separated into more than 15 spectra with the PL maximum changing from 1.2 to 1.8 eV depending on the migration distance. It is found that the relationship between the PL peak energy and the migration distance depends on the growth temperature of Si QDs as well as the B and P concentration. For all the samples with different impurity concentrations and grown at different temperatures, a clear trend is observed in the relationship between the full width at half maximum (FWHM) and the peak energy of the PL spectra in a wide energy range. The FWHM increases with the increasing peak energy and it is nearly twice larger than those observed for undoped Si QDs. The large PL FWHM of codoped Si QDs suggests that excitons are further localized in codoped Si QDs due to the existence of charged impurities.

A DFT study for silicene quantum dots embedded in silicane: controllable magnetism and tuneable band gap by hydrogen

This paper presents a design for silicene quantum dots (SiQDs) embedded in silicane. The shape and size of an embedded SiQD are managed by hydrogen atoms. A first-principles method was used to evaluate the magnetism as well as the electronic and structural properties of embedded SiQDs of various shapes and sizes. The shape of the embedded SiQD determined its electronic structure as well as the dot size. Moreover, the magnetic properties of SiQDs in silicane were highly shape dependent. The triangular SiQDs were all magnetic, some small parallelogram SiQDs were nonmagnetic, and all others were antiferromagnetic; almost all hexagonal SiQDs were nonmagnetic. An unequal number of bare Si atoms at the A and B sites was identified as a critical factor for establishing magnetism in embedded SiQDs. The tip of a triangular SiQD enhanced the magnetic moment of the dot. The parallelogram SiQD with two tip atoms appeared as a magnetic needle and has potential for use in spintronic applications. SiQDs embedded in silicane can be used in the design of silicon-based nanoelectronic devices and binary logic based on nanoscale magnetism.

Silver nanoparticles stabilized with a silicon nanocrystal shell and their antimicrobial activity

The antimicrobial activity of a hybrid nanoparticle (NP) composed of a silver (Ag) NP core decorated with silicon (Si) nanocrystals (NCs) on the exterior (Ag/Si NPs) is evaluated. The shell of Si NCs effectively protects the surface of Ag NPs, thus the particles are more stable in water and in air compared to conventional organic-capped Ag NPs. The bacterial growth kinetic analysis reveals that the Si NC shell does not suppress the release of Ag ions from the Ag NP surface due probably to the porous structure. For the antimicrobial coating application, a thin film of the hybrid Ag/Si NPs is produced by drop coating the solution on a cover glass. Thanks to the Si NC shell, agglomeration of Ag NPs in the film is prevented and the film shows a very similar optical absorption spectrum to that of the solution. The film exhibits a larger zone of inhibition in an agar diffusion assay of Escherichia coli compared to a film produced from organic-capped Ag NPs.

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.

Photoluminescence, infrared, and Raman spectra of co-doped Si nanoparticles from first principles

Co-doped silicon nanoparticles (NPs) are promising for the realization of novel biological and optoelectronic applications. Despite the scientific and technological interest, the structure of heavily co-doped Si NPs is still not very well understood. By means of first principles simulations, various spectroscopic quantities can be computed and compared to the corresponding experimental data. In this paper, we demonstrate that the calculated infrared spectra, photoluminescence spectra, and Raman spectra can provide valuable insights into the atomistic structure of co-doped Si NPs.

Si nanocrystal solution with stability for one year

Colloidal silicon nanocrystals (SiNCs) are a promising material for next-generation nanostructured devices. High-stability SiNC solutions are required for practical use as well as studies on the properties of SiNC. Here, we show a solution of SiNCs that was stable for one year without aggregation. The stable solution was synthesized by a facile process, i.e., pulsed laser ablation of a Si wafer in isopropyl alcohol (IPA). The long-term stability was due to a large ζ-potential of −50 mV from a SiNC passivation layer composed of oxygen, hydrogen, and alkane groups, according to the results of eight experiments and theoretical calculations. This passivation layer also resulted in good performance as an additive for a conductive polymer film. Namely, a 5-fold enhancement in carrier density was established by the addition of SiNCs into an organic conductive polymer, poly(3-dodecylthiophene), which is useful for solar cells. Furthermore, it was found that fresh (<1 day) and aged (4 months) SiNCs give the same enhancement. The long-term stability was attributed to a great repulsive energy in IPA, whose value was quantified as a function the distance between SiNCs.

A stable nanocrystal for one year without aggregation in a liquid is synthesized by one-step, one-pot, and one-hour process.