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 ...

Surface brightens up Si quantum dots: direct bandgap-like size-tunable emission

“…8 In the past, optical investigations of chemically synthesized Si-QDs have revealed nanosecond emission tunable in the visible. [12][13][14][15][16] However, the microscopic origin of this emission has been a matter of debate due to: (i) diversity of results obtained by different groups (see, e.g., the introductory remarks in Ref. 11); (ii) the fact that emission characteristics of many organic solvents and compounds used for, or being produced, in the synthesis are very similar to the presumed Si-QD emission; 17,18 (iii) difficulties to assess the size effects on optical properties due to lack of techniques for synthesis of batches with well-defined sizes; (iv) lack of theoretical models for larger organically terminated Si-QDs that could correlate size and surface effects with the observed fast emission rates and emission spectra (ab-initio calculations being available only for ultra-small alkyl-terminated nanoclusters with d QD ,1.5 nm).…”

The effect of alkyl chain length on the level of capping of silicon nanoparticles produced by a one-pot synthesis route based on the chemical reduction of micelle

“…The origin of the photoluminescence observed in Figure 5a) is complicated by the combination of both indirect and direct band gap transitions present in SiNPs (Holmes et al, 2001). However, there is strong theoretical evidence suggesting that 1-2 nm SiNPs with a hydrogen or carbon-terminated surface have direct band gap optical transitions that lead to photoluminescence in the blue region (Zhou et al, 2003, Warner et al, 2005b.…”

Quantum dots wide absorption pattern, photo stability and greater quantum yields provides a gate way to power full applications in nano medicine and nano biotechnology

“…Water stabilized quantum dots shows an improved colloidal stability and solubility [11]. The resultant bio imaging will have a great detailing and brightness which can be used to check trafficking events in live cells.…”