Effect of water and UV passivation on the luminescence of suspensions of silicon quantum dots
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- Vincent, J., Maurice, V., Paquez, X. et al. J Nanopart Res (2010) 12: 39. doi:10.1007/s11051-009-9708-9
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This article presents the evolution of the photo-luminescence (PL) of silicon quantum dots (QDs) with an average diameter of 5–6 nm dispersed in alcohol under different conditions. Two samples were considered after alcohol dispersion: freshly synthesized (kept in air for 2 days) QDs which do not exhibit luminescence and air-aged (kept in air for 2 years) QDs exhibiting red-IR luminescence. Experiments performed with addition of a small volume of water, followed by heating for different times showed that the oxidation occurs gradually until transforming totally the initial material in SiO2. The oxidation process does not enable the appearance of PL from the Si core for dispersed non-aged powders, while it results in a blue shift of the PL maximum intensity for the aged ones. The results obtained after UV illumination clearly indicate an effect of the UV irradiation on the luminescence of QDs dispersed in aqueous environment, and the treatments with acidic water lead to the conclusion of a possible enhancement of the PL by hydrogen passivation of the non-radiative defects. This result should be taken into account for post-production treatments and applications, more particularly, considering a controlled and safe use of luminescent Si QDs.
KeywordsSiliconNanoparticlesQuantum dotPassivationLuminescenceAqueous suspensionsEnvironmentEHS
Semiconductor quantum dots (QDs) show unique size-dependent optical absorption and photo-luminescence (PL) properties arising from the quantum confinement of the electron–hole pair below the Bohr radius (Yoffe 1993; Ledoux et al. 2002). In addition to the interesting properties of nanosized objects such as high quantum yield and low photobleaching (Li and Ruckenstein 2004), the possibility of tailoring the PL response with the particle size has induced a strong research effort on semiconductor QDs. For the last twenty years, various luminescent semiconductors QDs have been synthesized using a wide variety of techniques based on chemical or physical routes (Rogach 2008; Veinot 2006). One example described in the literature is the synthesis of cadmium selenium (CdSe) QDs which can be prepared with a perfectly controlled size and luminescence. However, for most applications of luminescent QDs materials, any hazardous toxicity cannot be accepted (Hardman 2006; Shiohara et al. 2004; Fujioka et al. 2008). Recent in-vitro studies seem to indicate that the cyto-toxicity of silicon QDs is far lower than cyto-toxicity exhibited by CdSe particles (Fujioka et al. 2008). Therefore, it seems possible to use Si QDs as safe agent for luminescent applications such as labeling (anti-counterfeiting, biological markers, pollution tracing, etc.).
Silicon is an indirect band–gap semiconductor and its optical characteristics, with respect to quantum confinement, have been intensively studied (Ledoux et al. 2002; Wolkin et al. 1999; Delerue et al. 1999). However, surface of silicon QDs is very reactive, and its PL properties are still a field of active discussions. Especially quenching of luminescence by defects seems to be the most important phenomenon leading to uncontrolled or non-reproducible luminescence from Si QDs.
Therefore, the control of the surface chemistry is a key point to develop applications of these nanoparticles. A possible way of surface passivation is to grow a silica shell at the surface. Moreover, the SiO2 shell makes Si QDs water easier to disperse and enables further functionalization of QDs surface by a well-developed chemistry (Niemeyer 2001; Burns et al. 2006). Indeed, Si nanoparticles oxidized by simple exposure to ambient atmosphere develop an amorphous silica shell (Yang et al. 2005; Huisken et al. 2003) and exhibit red PL easily detected by naked eye under UV illumination. This PL appears after an exposure time ranging from few days to weeks, depending on storage conditions, humidity, and so on. This process is efficient but slow, and cannot be controlled. It is known that the quality of the oxide layer depends greatly on the growth rate, its production way, with different behavior for air, water, or specific chemical aging (Morita et al. 1990; Bychto et al. 2008; Tardif et al. 2003; Mende et al. 1983; Cerofolini and Meda 1997). This induces different PL behaviors, and this point should be clarified to achieve controlled and stable luminescence properties.
In this context, we present the results obtained on the passivation of Si QDs synthesized by laser pyrolysis of silane focusing on the particular role of water oxidation on Si QDs dispersed in pure ethanol. In order to study the passivation of Si QDs by simple chemical water oxidation and its influence on luminescence properties, several sets of experiments were performed on freshly synthesized (kept in air for 2 days) and air-aged (kept in air for 2 years) Si QDs nanoparticles. The nanoparticles were dispersed in alcohol, small amounts of water were added to the suspension, and the resulting mixtures were heated for different times. These experiments have shown the gradual oxidation of Si QDs by water and the different behaviors of freshly synthesized and air-aged QDs for the different treatment durations. Further studies of the PL stability versus addition of small amount of acid and UV irradiations illustrate the strong photochemical reactivity of aged Si QDs.
Pyrolysis of silane SiH4 molecules by CO2 laser is an efficient process to synthesize high-quality Si QDs (Ehbrecht et al. 1995; Li et al. 2003; Lacour et al. 2007). By controlling the silane flow rate, the laser pulse intensity and duration, one can tailor the particle size with a quasi mono-disperse size distribution. The main advantage of this technique is its high production rate ≈200 mg/h for particles with a small diameter (down to 3 nm). The detailed description of the laser-driven aerosol reactor used in our experiment has been reported elsewhere (Lacour et al. 2007).
After production, the Si QDs powders are collected without preventing contact with air. From a dark brown powder showing no PL we obtained after aging in air for 2 years a slightly orange powder exhibiting red-IR luminescence after dispersion in alcohol. This powder is referenced as aged (a), while the powder aged for 2 days is referenced as freshly synthesized (fs) powder.
Si QDs are dispersed in absolute ethanol (AnalaR, Normapur, VWR) by ultra-sonication for a duration of 10 min with the following concentrations [Si QDs] = 10 mg/L, leading to perfectly stable colloidal solutions (no sedimentation or flocculation is observed within years). Experiments were performed by adding a small quantity of deionized (DI) water (Millipore), [Water] = 5 vol.%, to the dispersions. They were heated in a closed vessel at a constant temperature T of 120 °C for duration from t = 4 min to 20 h. In a first approximation, we assume that the heating results mainly in increased kinetics. For experiments using acidic water, we added some tens of mL of HCl (0.03 M) (Sigma Aldrich) or some drops of nitric acid (37%) (Merck). In general, acid dispersions undergo sedimentation within half an hour, and the doses have been adapted to prevent this phenomenon. After thermal treatment, the particles remained in the same solvent (water/alcohol).
After treatments, the PL spectra were measured by illumination of the dispersions using standard quartz with 1*1 cm2 cross section containers with a 6 W UV lamp emitting at 254 nm. The PL signal was collected through an optical fiber connected to a CCD with an acquisition system (spectrometer Ocean Optics USB 2000 covering the range 348–1138 nm). The acquisition time was set to 10 s for each sample which enables direct comparison of the spectra. FTIR measurements were recorded with a Perkin Elmer FTIR2000 spectrometer using the KBr pellet method. Absorption measurements were performed on a Spectronic-Unicam UV-500. Transmission Electron Microscopy (TEM) micrographs were obtained on a Philips CM12 microscope. X-Ray Diffraction patterns (XRD) were obtained on a Bragg–Brentano diffractometer (Siemens D 5000) in the θ–2θ geometry.
Results and discussion
The absorption measurements of the freshly dispersed suspensions obtained from air-aged luminescent nanoparticles (Fig. 3b) clearly show an absorption threshold at a lower wavelength compared to Fig. 3a. This confirms the presence of small Si core. Similar to the evolution observed in Fig. 3a, a shift of the threshold toward shorter wavelengths is observed for duration lower than 30 min. For longer treatment durations (about 1 h), the absorption increases, and the threshold is shifted toward higher wavelength. In addition to the peaks previously observed, these samples show a small absorbance contribution at 280 nm which may be attributed to silica defects (Salh 2007; Robertson 1988). TEM Micrograph (Fig. 4b) shows that after 20 h of treatment, the material presents a strong agglomeration, and it is no more possible to distinguish Si QDs. The increased absorption of the suspension for durations equal or longer than 1 h and the appearance of the 280 nm step may be related to this agglomerated structure.
The above-described results confirm the oxidation of Si QDs in aqueous environment for both air-aged and freshly synthesized powders. The absorption threshold of the spectra is progressively shifted toward shorter wavelengths when the treatment duration is increased. This may indicate a gradual oxidation of the Si QDs (Lockwood et al. 1994; Meier et al. 2007) for durations lower than 1 h. For longer treatment duration, the material is fully oxidized (Fig. 4b). Concerning the agglomeration problem, a different chemical behavior of freshly synthesized and air-aged nanoparticles is observed for durations longer than 1 h.
From these PL results, we can infer that the water-induced oxidation of the Si QDs leads systematically to some luminescence originating from silica defects with an intensity rising with the treatment duration. We use powders that show a quite good mono-dispersity, considering that typical size dispersion for samples synthesized in similar experimental conditions was measured at ± 10%. Therefore, for duration smaller than 1 h, we exclude the possibility that the oxidation occurs first for very small particles, and it seems most likely that the treatment results in a rough core/shell structure with defect density rising as the oxidation layer increases. Despite the core/shell structure which may appear through the oxidation of silicon, no PL from Si core could be identified for freshly synthesized powders indicating the presence of radiative defects and therefore a low quality of the passivation layer.
These results demonstrate that the PL peak position of suspensions of air-aged powders can be tuned from 790 to 700 nm with oxidation treatment and UV exposure. In order to explain the increase in PL intensities when suspensions of air-aged nanoparticles are illuminated with UV, one possibility is to consider a luminescence originating from quantum confinement. In this hypothesis, non-radiative defects which reduce the PL intensity are progressively passivated during UV exposure. As many defects are dangling bonds, passivation is likely to occur by bonding of the incorporated hydrogen (Morse and Pianetta 2004; Cartier et al. 1993), which may originate from the dissociation of the water under UV radiation. Therefore, more efficient passivation leads to increased PL intensity. In addition, according to the literature dealing with bulk or layered silicon, some other mechanisms can be involved. In air with oxygen and UV, an oxidation of silicon surface gives a stable SiO2 layer with a good stability (Kasi and Liehr 1992). The exposition of H+ without UV does not affect the native oxide layer (Crossley et al. 1995) in air. The article by Morse and Pianetta (2004) gives some experimental observations of increasing oxidation rate of silicon surface in water by the UV light at 254 nm compared to the water-only exposition. According to these authors, the oxidation of silicon surface comes from the Si surface radical generated by UV light (photoreaction) which reacts with the H2O or O2 to form the oxide layer. Concerning nanoparticles, there are a few reports (Savchyn et al. 2008), and more detailed studies are necessary to understand the mechanisms involved in the passivation of free standing silicon nanoparticles and evolution of PL under UV light.
These acidic water experiments may confirm the fact that H+ (Cartier et al. 1993) brought by the acid plays a role in non-radiative defect passivation. Interestingly, once the dispersions have been treated with acid, the PL wavelength remains unchanged at 790 nm. Note that this wavelength is close to the position of the PL peak measured on dry powder (815 nm), indicating strong surface effects that shift the PL nanoparticles in suspension (PL of freshly dispersed nanopowders is measured at 765 nm (Fig. 5b). Using the model of Delerue et al. (1999), the shift from 790 to 700 nm can be interpreted in terms of size evolution. In these conditions, the water oxidation of aged Si QDs would be a quite low growth process with a rate in the order of ~1 nm/h.
Finally, the interaction of Si QDs with UV illumination demonstrates the high reactivity of luminescent Si QDs. Its potential interaction with the surrounding during exposure should be clarified by further experiments especially the role of H+ that may be unavoidable in biological environments.
Air-aged and freshly synthesized Si QDs powders dispersed in alcohol were heated in the presence of water for different durations. For both dispersions, the oxidation occurs gradually until full transformation of the initial material in SiO2, as reflected by the PL at 430 nm. However, for long oxidation durations, air-aged powder dispersions show a tendency to agglomeration that is not observed for non-aged powders. From the luminescence point of view, the oxidation leads to the appearance of a typical SiO2 defect luminescence for both dispersions. In air-aged dispersion, PL of silicon can be tuned from 790 to 700 nm. The results obtained under UV illumination indicate that the UV illumination affects the QDs radiative properties. Several mechanisms involving radical chemistry can be invoked. Treatments made with acidic water lead us to assume that H+ may also be a passivation agent. This point should be taken into account for further studies on luminescent Si QDs applications.