Water‐Soluble Photoluminescent Silicon Quantum Dots

Abstract

The quantum confinement of excitons in semiconductor quantum dots leads to interesting optical properties that can be exploited in a range of photonic applications including biological fluorescence imaging [1][2][3][4][5][6] and optoelectronic devices. [7][8][9][10][11][12][13] Quantum dots are becoming popular as replacements for fluorescent dyes in biological fluorescence imaging because of their superior stability against photobleaching. To date, considerable emphasis has been placed on using CdSe quantum dots with a ZnS shell as biological chromophores since they emit light that can be tuned throughout the visible spectrum.[14] However, concerns have been raised about the toxicological issue of using cadmium in biological systems.[15] In particular, Derfus et al. showed that CdSe quantum dots without a ZnS shell were toxic to liver cells after exposure to UV light.[15] The potential biocompatibility of silicon makes photoluminescent silicon quantum dots an ideal candidate for biological fluorescence imaging and should eliminate any potential toxicology problems that might arise from having a CdSe core. [16,17] Strong quantum confinement in silicon increases the probability of radiative recombination through the direct band gap transitions and reduces phonon-assisted indirect band gap transitions. [18] In silicon, this requires the physical dimensions of the quantum dots to be on the order of or less than the bulk exciton Bohr radius of 4 nm. [19,20] Quantum dot sizes of less than 8 nm are easily achieved using wet chemistry techniques. [19][20][21][22] The remarkably successful advances in the synthesis of the semiconductor groups II/VI and III/V have not been applied to silicon as a consequence of the relatively high temperatures required to degrade the precursor and to produce highly crystalline quantum dots. The greatest success in producing silicon quantum dots with strong quantum confinement to date has been by the solution-phase reduction of silicon salts. [19,21,22] The solution-phase synthesis of silicon quantum dots has previously been reported by Kauzlarich and co-workers [21,22] through the use of a variety of reducing agents, by Korgel and co-workers [19,23] through the use of high temperatures and pressures, and by Wilcoxon et al. by using micelles. [20,24] The current problem associated with the simpler room-temperature syntheses [20][21][22] is the large size distribution produced. The large size distribution prevents a simple interpretation of the optical spectra. Post-synthesis treatments for narrowing the size distribution of the silicon quantum dots, such as highpressure gas chromatography (HPGC), have revealed sharp features in the absorption spectra and narrow photoluminescence spectra attributed to direct band gap transitions.[20] We have recently reported the synthesis of 1-2-nm silicon quantum dots that were surface-passivated by 1-heptene. These dots had a narrower size distribution than previously reported and gave a strong blue photoluminescence. [25] For silicon quantum dots to be ...

Soft X‐ray induced oxidation on acrylic acid grafted luminescent silicon quantum dots in ultrahigh vacuum

“…Several groups have attempted to make the QDs water soluble by various routes [14][15][16][17][18]. Warner et al [19] produced allylamine-capped Si-QDs using a Pt catalyst. The resulting Si-QDs were water-soluble and exhibited strong blue photoluminescence (PL) with a rapid rate of recombination.…”

Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions

“…Recently, several groups, including ourselves, have prepared alkyl-passivated silicon nanocrystals. [5][6][7][8][9][10][11][12][13][14][15][16] These are SiNCs whose surface is capped by a monolayer of saturated hydrocarbon molecules and anchored to the Si core via covalent Si-C bonds. Although the particles contain a little oxide from their preparation, the alkyl monolayer protects the Si core and they are not oxidized further under ambient conditions.…”

Cationic Silicon Nanocrystals with Colloidal Stability, pH‐Independent Positive Surface Charge and Size Tunable Photoluminescence in the Near‐Infrared to Red Spectral Range

“…The use of solid lipid nanoparticles by Henderson et al and phospholipid micelles by Erogbogbo et al are prime examples 13, 14, 15. Electrostatic stabilization strategies rely on ionic ligands to prevent flocculation, commonly used ones include olefins with terminal carboxylic acids and terminal primary amines 8, 12, 15, 16, 17, 18, 19, 20. Korgel and co‐workers demonstrated a strategy using amphiphilic polymers to encapsulate ncSi in 2010 21…”

“…For example, ncSi based on steric stabilization strategies typically have larger hydrodynamic diameters ranging from 20 to 100 nm, and incorporates many ncSi in one nanoparticle, whereas ncSi based on electrostatic strategies typically have diameters below 5 nm and are dispersible as isolated nanocrystals 12, 15, 17, 21. In general, hydrodynamic diameters below about 5 nm are desirable since nanoparticles below this threshold can be cleared through urinary excretion to minimize potential toxicity effects 4…”