Size-tunable UV-luminescent silicon nanocrystals

Rational ligand design for enhanced carrier mobility in self-powered SWIR photodiodes based on colloidal InSb quantum dots

Solution-processed colloidal III-V semiconductor quantum dot photodiodes (QPDs) have potential applications in short-wavelength infrared (SWIR) imaging due to their tunable spectral response range, possible multiple-exciton generation, operation at 0-V bias voltage and low-cost fabrication and are also expected to replace lead- and mercury-based counterparts that are hampered by reliance on restricted elements (RoHS). However, the use of III-V CQDs as photoactive layers in SWIR optoelectronic applications is still a challenge because of underdeveloped ligand engineering for improving the in-plane conductivity of the QD assembled films. Here, we report on ligand engineering of InSb CQDs to enhance the optical response performance of self-powered SWIR QPDs. Specifically, by replacing the conventional ligand (i.e., oleylamine) with sulfide, the interparticle distance between the CQDs was shortened from 5.0 ± 0.5 nm to 1.5 ± 0.5 nm, leading to improved carrier mobility for high photoresponse speed to SWIR light. Furthermore, the use of sulfide ligands resulted in a low dark current density (∼nA cm-2) with an improved EQE of 18.5%, suggesting their potential use in toxic-based infrared image sensors.

Water-borne, durable and multicolor silicon nanoparticles/sodium alginate inks for anticounterfeiting applications

Recently, water-borne fluorescent inks have attracted extensive attention in anti-counterfeiting applications due to their convenient implementation and eco-friendliness. However, due to poor service durability, the latent authorization information from the inks is easily damaged, and even disappears when encountering water. Moreover, most of the existing fluorescent inks are monochromic, toxic, and allergic to skin, thus are unsuitable for their sustainability during real-life applications. Herein, this work presents environment-friendly, durable, and multicolor fluorescent anti-counterfeiting silicon nanoparticles (SiNPs)/sodium alginate (SA) inks. The multicolor SiNPs are synthesized by a one-pot method with defined morphologies and optical properties. Subsequently, SA is employed as the binder to prepare the fluorescent inks with optimized rheological properties. Practicability results show that the SiNPs/SA inks not only exhibit excellent printability, but also impart authentic information with superior covert performance. More notably, spraying solution of calcium dichloride can further improve fluorescent fastnesses of the SiNPs/SA inks by ionic crosslinking.

Microscopic Proof of Photoluminescence from Mechanochemically Synthesized 1-Octene-Capped Quantum-Confined Silicon Nanoparticles: Implications for Light-Emission Applications

Silicon nanoparticles (SiNPs) have been explored intensively for their use in applications requiring efficient fluorescence for LEDs, lasers, displays, photovoltaic spectral-shifting filters, and biomedical applications. High radiative rates are essential for such applications, and theoretically these could be achieved via quantum confinement and/or straining. Wet-chemical methods used to synthesize SiNPs are under scrutiny because of reported contamination by fluorescent carbon species. To develop a cleaner method, we utilize a specially designed attritor type high-energy ball-mill and use a high-purity (99.999%) Si microparticle precursor. The mechanochemical process is used under a continuous nitrogen gas atmosphere to avoid oxidation of the particles. We confirm the presence of quantum-confined NPs (<5 nm) using atomic force microscopy (AFM). Microphotoluminescence (PL) spectroscopy coupled to AFM confirms quantum-confined tunable red/near-infrared PL emission in SiNPs capped with an organic ligand (1-octene). Using micro-Raman-PL spectroscopy, we confirm SiNPs as the origin of the emission. These results demonstrate a facile and potentially scalable mechanochemical method of synthesis for contamination-free SiNPs.

Synthesis of Renal-Clearable Multicolor Fluorescent Silicon Nanodots for Tumor Imaging and In Vivo H2O2 Profiling

Fluorescent silicon nanodots have shown great prospects for bioimaging and biosensing applications. Although various fluorescent silicon-containing nanodots (SiNDs) have been developed, there are few reports about renal-clearable multicolor SiNDs. Herein, renal-clearable multicolor fluorescent SiNDs are synthesized by using silane molecules and organic dyes through a facile one-step hydrothermal method. The fluorescence of the resulting SiNDs can be tuned to blue (bSiNDs), green (gSiNDs), and red (rSiNDs) by simply changing the categories of silane reagents or dye molecules. The as-prepared SiNDs exhibit strong fluorescence with a quantum yield up to 72%, excellent photostability, and good biocompatibility with 12 h renal clearance rate as high as 86% ID. These properties enabled the SiNDs for tumor fluorescence imaging and H₂O₂ imaging in living cells and tissue through in situ reduction reaction-lighted fluorescence of the nanoprobe. Our results provide an invaluable methodology for the synthesis of renal-clearable multicolor SiNDs and their potential applications for fluorescence imaging and biomarker sensing. These SiNDs are also promising for various biological and biomedical applications.

Color-switchable hybrid dots/hydroxyethyl cellulose ink for anti-counterfeiting applications

Many anti-counterfeiting inks have been explored recently, most of them are commonly involved in weak fastness, high cost and long-term toxicity, impeding their real-life applications. Herein, an environment-friendly and inexpensive anti-counterfeiting ink with excellent fastness is reported. The untifake ink is developed by combining hybrid dots (silicon/carbon) with hydroxyethyl cellulose (HEC) binder. Interestingly, the HEC binder can effectively prevent from aggregation-induced quenching of hybrid dots. Subsequently, the customized patterns are successfully transferred onto different surfaces of various substrates including cotton fabric, cellulosic paper, glass, metal, silicon wafer and PET film, using the as-prepared ink by screen-printing technique, exhibiting that the hybrid dots/HEC ink possesses widespread practicability. Notably, fluorescent color of these patterns can be switchable by adjusting environmental pH-value, further imparting the as-prepared ink with excellent covert performance. This new fluorescent hybrid dots/HEC ink will be promising candidates for high-level anti-counterfeiting applications including food packaging, apparel and documents.

Photophysical properties of ball milled silicon nanostructures

Luminescent silicon nanocrystals (SiNCs) have attracted scientific interest for their potential use in LEDs, displays, lasers, photovoltaic spectral-shifting filters and for biomedical applications. A lot of efforts have been made to improve the radiative emission rate in SiNCs, mostly using quantum confinement, strain and ligands. Existing methods, however, are not easily upscalable, as they do not provide the high material yield required for industrial applications. Besides, the photoluminescence (PL) efficiency of SiNCs emitting in the visible spectral range also remains very low. Hence, there is a need to develop a low-cost method for high material yield of brightly emitting SiNCs. Theoretically, strain can be used alongside quantum confinement to modify the radiative emission rates and band-gaps. In view of that, high-energy ball milling is a method that can be used to produce large quantities of highly strained SiNCs. In this technique, balls with high kinetic energy collide with the walls of a chamber and other balls, crushing the particles in between, followed by welding, fracture and re-welding phenomena, reducing the particle size and increasing strains in the samples. In this study, we have used high-energy ball milling in an inert gas atmosphere to synthesize SiNCs and study their photophysical properties. The induced accumulation of high strain, quantum confinement and possibly also impurities in the SiNCs resulted in visible light spectrum PL at room temperature. This method is low cost and easily up-scalable to industrial scale.

Spectral tuning of colloidal Si nanocrystal luminescence by post-laser irradiation in liquid

We report a simple technique to tune the luminescence spectra of blue-emitting colloidal silicon nanocrystals (Si-ncs) to the ultraviolet region via post-laser irradiation.

Decoupling the effects of confinement and passivation on semiconductor quantum dots

Semiconductor (SC) quantum dots (QDs) have recently been fabricated by both chemical and plasma techniques for specific absorption and emission of light. Their optical properties are governed by the size of the QD and the chemistry of any passivation at their surface. Here, we decouple the effects of confinement and passivation by utilising DC magnetron sputtering to fabricate SC QDs in a perfluorinated polyether oil. Very high band gaps are observed for fluorinated QDs with increasing levels of quantum confinement (from 4.2 to 4.6 eV for Si, and 2.5 to 3 eV for Ge), with a shift down to 3.4 eV for Si when oxygen is introduced to the passivation layer. In contrast, the fluorinated Si QDs display a constant UV photoluminescence (3.8 eV) irrespective of size. This ability to tune the size and passivation independently opens a new opportunity to extending the use of simple semiconductor QDs.

Temperature-dependent photoluminescence of surface-engineered silicon nanocrystals

In this work we report on temperature-dependent photoluminescence measurements (15–300 K), which have allowed probing radiative transitions and understanding of the appearance of various transitions. We further demonstrate that transitions associated with oxide in SiNCs show characteristic vibronic peaks that vary with surface characteristics. In particular we study differences and similarities between silicon nanocrystals (SiNCs) derived from porous silicon and SiNCs that were surface-treated using a radio-frequency (RF) microplasma system.

Ionic liquid-assisted synthesis of multicolor luminescent silica nanodots and their use as anticounterfeiting ink

Here we propose a simple route for the fabrication of silica nanodots which are strongly photoluminescent in both solution and the solid state based on the use of ionic liquids (ILs). It is found that the ILs not only provides the environment for the reaction but also contributes to the quantum yield (QY) of the silica nanodots. In particular, the produced silica nanodots also displayed excitation-dependent photoluminescence and temperature sensitive properties. Based on the unique optical properties, the as-prepared nanomaterial was used for anticounterfeiting application and the results demonstrated the great potential of the silica nanodots alone or combined with other fluorescent material of unicolor for an improved anticounterfeiting technology. This simple approach and the resulting outstanding combination of properties make the prepared silica nanodots highly promising for myriad applications in areas such as fluorescent anticounterfeiting, optoelectronic devices, medical diagnosis and biological imaging.

Silicon nanoparticles obtained via a low temperature chemical “metathesis” synthesis route and their lithium-ion battery properties

Silicon (Si) nanoparticles have been prepared by a “metathesis” reaction of magnesium silicide (Mg₂Si) and zinc chloride (ZnCl₂) in an autoclave at 300 °C.

Silicon quantum dots: surface matters

Silicon quantum dots (SiQDs) hold great promise for many future technologies. Silicon is already at the core of photovoltaics and microelectronics, and SiQDs are capable of efficient light emission and amplification. This is crucial for the development of the next technological frontiers-silicon photonics and optoelectronics. Unlike any other quantum dots (QDs), SiQDs are made of non-toxic and abundant material, offering one of the spectrally broadest emission tunabilities accessible with semiconductor QDs and allowing for tailored radiative rates over many orders of magnitude. This extraordinary flexibility of optical properties is achieved via a combination of the spatial confinement of carriers and the strong influence of surface chemistry. The complex physics of this material, which is still being unraveled, leads to new effects, opening up new opportunities for applications. In this review we summarize the latest progress in this fascinating research field, with special attention given to surface-induced effects, such as the emergence of direct bandgap transitions, and collective effects in densely packed QDs, such as space separated quantum cutting.

Colloidal silicon quantum dots: synthesis and luminescence tuning from the near-UV to the near-IR range

This review describes a series of representative synthesis processes, which have been developed in the last two decades to prepare silicon quantum dots (QDs). The methods include both top-down and bottom-up approaches, and their methodological advantages and disadvantages are presented. Considerable efforts in surface functionalization of QDs have categorized it into (i) a two-step process and (ii) in situ surface derivatization. Photophysical properties of QDs are summarized to highlight the continuous tuning of photoluminescence color from the near-UV through visible to the near-IR range. The emission features strongly depend on the silicon nanostructures including QD surface configurations. Possible mechanisms of photoluminescence have been summarized to ascertain the future challenges toward industrial use of silicon-based light emitters.

The effects of electronic coupling between capping molecules and quantum dots on the light absorption and emission of octyl, styryl, and 4-ethynylstyryl terminated silicon quantum dots

Optical properties of silicon quantum dots (Si QDs) are greatly influenced by their size and surface chemistry.

Preparation and characterization of decyl-terminated silicon nanoparticles encapsulated in lipid nanocapsules

In this Article, we report on the encapsulation of decyl-modified silicon nanoparticles (decyl-SiNPs) into ∼80 nm lipid nanocapsules (LNCs). The decyl-SiNPs were produced by thermal hydrosilylation of hydride-terminated SiNPs (H-SiNPs) liberated from porous silicon. Various techniques, including Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), UV-vis absorption, dynamic light scattering (DLS), and photoluminescence (PL), were used to characterize their size, shape, colloidal, and optical properties. The results indicate that these nanocapsules feature controllable size, good dispersity, high loading rate of SiNPs, colloidal stability in various media, and bright PL. The PL of decyl-SiNPs loaded LNCs was stable upon heating to 80 °C, but was sensitive to basic solutions due to proton-gated emission of the SiNPs arranged at the LNCs interface between the oil phase and the hydrophilic polyethylene glycol moieties of the surfactant. These luminescent nanocapsules are therefore promising candidates as cellular probes for fluorescence imaging. In addition, it was found that TEM imaging of small-sized decyl-SiNPs could be greatly improved by preliminary negative staining of TEM grids with phosphotungstic acid.

Silicon nanoparticle based fluorescent biological label via low temperature thermal degradation of chloroalkylsilane

A simple low temperature colloid-chemical synthetic method is reported for size controlled synthesis of hydrophobic silicon nanoparticles in the 1-10 nm range. These silicon nanoparticles show size dependent tunable visible emission from blue to red with fluorescence quantum yield in the range of 6-13%. These silicon nanoparticles can be subjected to extensive surface chemistry without significant loss of their fluorescence properties. The as-synthesized red emitting nanoparticles have been transformed into water soluble functional nanoprobes of 18 nm hydrodynamic diameter and 5% fluorescence quantum yield and used as fluorescent biological labels.

Size-dependent color tuning of efficiently luminescent germanium nanoparticles

It is revealed that rigorous control of the size and surface of germanium nanoparticles allows fine color tuning of efficient fluorescence emission in the visible region. The spectral line widths of each emission were very narrow (<500 meV). Furthermore, the absolute fluorescence quantum yields of each emission were estimated to be 4-15%, which are high enough to be used as fluorescent labeling tags. In this study, a violet-light-emitting nanoparticle is demonstrated to be a new family of luminescent Ge. Such superior properties of fluorescence were observed from the fractions separated from one mother Ge nanoparticle sample by the fluorescent color using our developed combinatorial column technique. It is commonly believed that a broad spectral line width frequently observed from Ge nanoparticle appears because of an indirect band gap nature inherited even in nanostructures, but the present study argues that such a broad luminescence spectrum is expressed as an ensemble of different spectral lines and can be separated into the fractions emitting light in each wavelength region by the appropriate postsynthesis process.

Newly synthesized silicon quantum dot-polystyrene nanocomposite having thermally robust positive charge trapping

Striving to replace the well known silicon nanocrystals embedded in oxides with solution-processable charge-trapping materials has been debated because of large scale and cost effective demands. Herein, a silicon quantum dot-polystyrene (SiQD-PS) nanocomposite (NC) was synthesized by post-functionalization of hydrogen-terminated silicon quantum dots (H-SiQDs) with styrene using a thermally induced surface-initiated polymerization approach. The NC contains two miscible components: PS and SiQD@PS which, respectively, are polystyrene and polystyrene chains-capped SiQDs. Spin-coated films of the nanocomposite on various substrate were thermally annealed at different temperatures and subsequently used to construct metal-insulator-semiconductor (MIS) devices and thin film field-effect transistors (TFTs) having a structure of p-Si++/SiO2/NC/pentacene/Au source-drain. Capacitance-voltage (C-V) curves obtained from the MIS devices exhibit a well-defined counterclockwise hysteresis with negative fat band shifts, which was stable over a wide range of curing temperatures (50-250 °C). The positive charge trapping capability of the NC originates from the spherical potential well structure of the SiQD@PS component while the strong chemical bonding between SiQDs and polystyrene chains accounts for the thermal stability of the charge trapping property. The transfer curve of the transistor was controllably shifted to the negative direction by varying applied gate voltage. Thereby, this newly synthesized and solution processable SiQD-PS nanocomposite is applicable as charge trapping materials for TFT based memory devices.

One-step Melt Synthesis of Water Soluble, Photoluminescent, Surface-Oxidized Silicon Nanoparticles for Cellular Imaging Applications

We have developed a versatile, one-step melt synthesis of water-soluble, highly emissive silicon nanoparticles using bi-functional, low-melting solids (such as glutaric acid) as reaction media. Characterization through transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy shows that the one-step melt synthesis produces nanoscale Si cores surrounded by a silicon oxide shell. Analysis of the nanoparticle surface using FT-IR, zeta potential, and gel electrophoresis indicates that the bi-functional ligand used in the one-step synthesis is grafted onto the nanoparticle, which allows for tuning of the particle surface charge, solubility, and functionality. Photoluminescence spectra of the as-prepared glutaric acid-synthesized silicon nanoparticles show an intense blue-green emission with a short (ns) lifetime suitable for biological imaging. These nanoparticles are found to be stable in biological media and have been used to examine cellular uptake and distribution in live N2a cells.

Colloidal Si nanocrystals: a controlled organic-inorganic interface and its implications of color-tuning and chemical design toward sophisticated architectures

The optical use of colloidal silicon nanocrystals (Si NCs) has gained increasing attention for its possible contributions to building a sustainable society that ideally uses resources and energy with high efficiency without causing damage to the environment or human health. Si wafers (E(g) ≈ 1.1 eV) dominate modern microelectronics as an impressive electronic material, but they exhibit relatively poor optical performance owing to an indirect bandgap structure. Interestingly, however, full control of the size distribution and surface chemistry of the NCs yields size-dependent light emission in a very wide range from near-ultraviolet through visible to near-infrared wavelengths. In addition to such unique luminescence properties, Si exhibits a high chemical affinity to covalent linkages with carbon, oxygen, and nitrogen, thereby producing almost unlimited variations in organic-Si NCs architectures hybridized at the molecular level. To achieve this goal, I note some parameters, including interfacial chemistry, that are emerging as important elements for increasing our understanding of the effect of quantum confinement in nanostructured Si and for realizing efficient fluorescence emission. This article covers new aspects of derivatives of Si NCs in applications that utilize their optical absorption and emission features.