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In this work, stable blue-green luminescent colloidal silicon nanocrystals (SiNCs) are fabricated by nanosecond pulsed laser ablation of a silicon target in dimethyl sulfoxide (DMSO). Transmission electron microscopy and X-ray diffraction analysis have shown the formation of spherical silicon nanocrystals in the colloid with size range of 2-5 nm. Our results show that the DMSO stabilizes the silicon nanocrystals via oxide formations on the nanocrytals surfaces by a simple route of laser ablation and a schematic representation of the process is suggested. The colloid exhibits strong blue luminescent emissions in the spectral range of 455-465 nm when excited at wavelengths near the direct band gap of the silicon nanocrystal. The luminescent emission band shifts to longer wavelengths (green light) if the excitation wavelength increases toward the indirect band gap of the SiNCs. The oxidized SiNCs with quantum confinement effects are shown to be responsible for visible photoluminescence of the colloid. The observed blue-green emission of the colloid makes it a good candidate for display, solid-state lighting and biological luminescent based devices.
©2012 Optical Society of America
1. Introduction
Semiconductor nanocrystals exhibit novel interesting properties, in particular the visible photoluminescence (PL) that make them very attractive for intense research in nanotechnology. In recent years, the interest in synthesis of silicon nanocrystals has continuously grown due to the wide range of applications in fields of photonics [1–3], nanobiotechnology [4], electronics [5] and medicine [6]. Laser ablation in liquids has received much attention as an effective and simple technique for producing nanoparticles. Numerous studies have been reported related to synthesis of silicon nanocrystals [7–9] by irradiating intense laser light onto these materials dispersed in solvents. It has been shown in various studies that changing the surrounding solvent can provide different effects on the SiNCs such as stability, absorption spectrum, size distribution and surface chemistry [8–11]. This synthetic flexibility of physical and chemical properties is one of the reasons why SiNCs colloids are good candidates for luminescent based devices.
In the present study, our goal is to report the stable blue-green photoluminescence of colloidal silicon nanocrystals in dimethyl sulfoxide. This work shows that the nanosecond pulsed laser ablation of silicon wafer in the DMSO has the following significances. It introduces a simple one-step route for fabricating 2-5 nm sized SiNCs with surface oxide layer which not require any aging. The DMSO can easily react with the silicon nanocrystal surface and generates a cage causing stabilization of the colloid. The SiNCs-DMSO colloidal sample exhibits good photoluminescence stability in the blue-green spectrum region. Due to the luminescent properties of silicon nanocrystals and good properties of the DMSO in biological systems, the SiNCs-DMSO colloid is a good candidate to be investigated as luminescent materials for many applications [12–14]. The use of colloidal silicon nanocrystals has tremendous promise for achieving luminescence in the blue and green spectral regime, which is of great importance for, display and solid-state lighting applications. However, it is important to note that the state-of-the-art blue and green light-emitting diodes (LEDs) are based on III-Nitride technology [15–18], and significant recent advances have been accomplished in III-Nitride device technologies [19–23], resulting in high-efficiency blue and green LEDs.
First, we present the fabrication of colloidal silicon nanocrystals using nanosecond pulsed laser ablation of silicon wafer in the DMSO. Then, the colloid is characterized by linear absorption spectroscopy (UV-VIS), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The photoluminescent mechanisms of colloid are studied and our results show that PL emission is affected by both the sizes and surface states of the SiNCs.
2. Experimental section
To fabricate the SiNCs colloid, a crystalline silicon wafer (n-type) is placed in an open glass vessel in 10 cm3 of the DMSO and irradiated with fundamental harmonic beam (1064 nm) of a Q-switched Nd: YAG laser operating at 1 Hz with an 18 ns pulse width. The laser beam is focused by a lens having a focal length of 50 cm onto the wafer surface. The spatial profile of a laser pulse is Gaussian, with 300 µm (FW1/e2M) beam waist at the target. The silicon wafer is irradiated with laser fluence level of about 70 J/cm2 and the ablation is carried out for 2 hours. The prepared colloid has yellowish color (see Fig. 1 ) with silicon nanocrystals volume fraction of 2.1 × 10−4. The volume fraction,, of the nanocrytals colloid is defined by
Where, is the volume of the particles and is the volume of the liquid. The volume of the particle is , is the mass density and is the mass of particles dispersed in the liquid. The nanostructure size and morphology are studied using a transmission electron microscope (TEM). The crystal structure of the SiNCs are investigated by a Bruker D8ADVANCE X-ray diffractometer with high intensity Cu Kα radiation (λ = 1.5406 Å). The optical absorption spectra of the colloids are measured using an UV-Vis spectrometer (Perkin-Elmer, lamda 25). Fourier transform infrared (FTIR) spectroscopy (Bruker, IFS-66) is utilized to identify the surface composition of the SiNCs. The photoluminescence measurements of colloidal solutions are performed at room temperature and ambient atmosphere using a fluorophotometer (Perkin-Elmer, LS 45).
Fig. 1 Image of the SiNCs colloid prepared by nanosecond laser ablation of silicon wafer in the DMSO.
3. Results and discussion
The morphology and size distribution of the silicon nanocrystals are studied by TEM and the measurements conducted just after laser ablation as shown in Fig. 2 . Average SiNCs radius is found to be about 3.5 nm, with a standard deviation of 1.5 nm. Figure 3 depicts the X-ray diffraction pattern of the SiNCs film which shows diamond crystalline structure with well-resolved diffraction peaks for the silicon (111), (220) and (311) planes. The result indicates that silicon nanocrystals are fabricated by laser ablation of a silicon target in the DMSO. The average size of SiNCs film is calculated about 13 nm by the Debey-scherer formula [24] using the line broadening of (220) reflection. It should be noticed that the 3.5 nm average size of SiNCs observed by TEM is associated with the colloidal sample. Apparently, in fabrication of the SiNCs film due to multiple coating and subsequently drying at the 200 °C resulted into aggregation of the silicon nanocrystals according to the XRD data.
Fig. 2 TEM image and size distribution of the SiNCs colloid prepared by nanosecond laser ablation of silicon wafer in the DMSO.
Fig. 3 X-ray diffraction pattern of the SiNCs thin film which shows diffraction peaks for the silicon (111), (220) and (311) planes.
Figure 4 shows comparison of UV-VIS optical absorption spectra of the fresh SiNCs colloid (solid curve) with one obtained after one month (dashed curve). The results indicate that the colloid absorption spectrum remains unchanged and the silicon nanocrystals agglomeration does not occur in the colloid. The UV-VIS absorption spectrum of the SiNCs colloid shows a gradual increase in the absorbance with decreasing the wavelength from the on-set wavelength of 475 nm (2.6 eV) which shows the absorption band edge of the indirect band gap of silicon nanocrystals. It should be pointed out that bulk silicon exhibits an indirect band gap of 1.1 eV and a direct band gap transition 3.4 eV [25]. The silicon nanocrystals direct band gap is obtained 332 nm (3.7 eV) using the method suggested in [26], for the nanocrystals average size of 3.5 nm the ΔΕg is calculated to be about 0.33 eV. This value is found to be in good agreement with the value of 355 nm (3.5 eV) which is obtained using the UV-VIS optical absorption spectrum of the SiNCs (see Fig. 4).
Fig. 4 Absorption spectrum of the colloidal SiNCs in the DMSO at different time after production. The absorption spectrum is remained stable for one month.
Since the DMSO contains a large amount of water, it is expected that silicon nanocrystal after formation reacts with the water and produce oxide layer on its surface [27,28]. Interestingly, the UV-VIS optical absorption spectrum shows stability of the SiNCs colloid. This can be attributed to the interaction of water with silicon oxygen bond through hydrogen bonding. Then, water hydrogen bonds to the DMSO and generates a cage which causes stabilization of the SiNCs colloid. The formation of the Si-O-Si bond on the nanocrystals surfaces and presence of DMSO is analyzed by means of FTIR spectroscopy of the SiNCs powder, as shown in Fig. 5 . This spectrum shows pronounced bands located at 1100 and 804 cm−1 due to Si-O-Si stretching vibration, and the band observed at about 1027 cm−1 arises primarily from the S-O stretching vibration of the DMSO molecule, respectively [29,30]. However, as reported in [31], a Si = O double bond is more likely to be formed and may stabilize the interface, since it requires neither a large deformation energy nor an excess element. From all the presented data, we propose the following schematic representation of the interaction between the DMSO and SiNCs as shown in Fig. 6 .
Fig. 5 FTIR spectrum of the SiNCs powder, Shows the presence of the oxide layer on the surface of the silicon nanocrystal.
The origin of the photoluminescence in silicon nanocrystals has been shown to be consistent with the general perception of quantum confinement effects. Recently, it has been reported that the surface chemistry plays an important role in the luminescence process of the silicon nanocrystals, especially the presence of oxide on the nanocrystal surface [30–34]. Here, the nanosecond pulsed laser ablation of a silicon target in the DMSO has been used to produce silicon nanocrystals with Si = O surface states. This may contribute as radiative centers in photoluminescent process. The photoluminescent property of the SiNCs colloid is investigated at two different excitation wavelength ranges, one about the silicon nanocrystals direct band gap (350-375 nm) and the other one is about the silicon nanocrystal indirect band gap (400-475 nm). This may allow us to understand the PL emission mechanisms of the SiNCs colloid.
Figure 7 depicts the room temperature PL emission spectrum of the colloidal silicon nanocrystals in the DMSO. The PL exhibits prominent blue-green emissions in the range of 455-520 nm at different applied excitation wavelengths ranging from 350 to 475 nm. Figure 8 shows comparison of PL emission spectra of the fresh SiNCs colloid (solid curve) with one obtained after one month (dashed curve) at excitation wavelengths of 375 and 435 nm. The results indicate photoluminescence stability of the colloid for both blue and green spectrum regions. By increasing the applied excitation wavelength above the 475 nm, the colloid does not show any significant PL emission. Note that the UV-VIS absorption spectra of the SiNCs colloid shows a gradual increase in the absorbance with decreasing the wavelength from the on-set wavelength of 475 nm which shows the absorption band edge of the indirect band gap of the SiNCs (see Fig. 4). These results indicate that the silicon nanocrystals play major role in absorption of light in PL process of the colloid, since by using the excitation wavelength above the silicon nanocrystals absorption band edge, no significant PL intensity has been observed.
Fig. 7 PL emission spectrum of the colloidal SiNCs in the DMSO at different excitation wavelength range (a) 350-375 nm and (b) 400-475 nm.
Fig. 8 Stability of PL emission spectrum of the colloidal SiNCs in the DMSO is obtained at the excitation wavelengths of (a) 375 nm and (b) 435 nm.
Figure 7(b) indicates that by increasing the excitation wavelength within the range of 400 t0 475 nm, the PL emission band shows red-shift from the 475 to 520 nm. In addition, at longer applied excitation wavelengths, the PL emission peak intensity also decreases. This may be attributed to the size distribution of the silicon nanocrystals [26], because at longer excitation wavelength only the larger particles that have a smaller bandgap will contribute to the emission photons. Similarly, at a shorter excitation wavelength the PL emission is related to contributions of smaller nanocrystals dispersed in the colloid. In presence of silicon nanocrystals in the DMSO, quantum confinement effects start to play an important role and can fully alter the luminescence properties of the colloid [35,36]. When the carriers are confined in the nanocrystals the wave function of an electron and a hole can partially overlap even in an indirect bandgap semiconductor such as silicon, giving rise to quasi-direct transitions and thus increasing the probability of a radiative recombination of the pair of carriers. Thus, the quantum confinement effect is dominant mechanism for photoluminescent of the SiNCs colloid at excitation wavelengths about the indirect band gap of silicon nanocrystals.
The strong blue-light emission is observed at about 455-465 nm while using an excitation wavelength range of 350-375 nm (see Fig. 7(a)). It should be pointed out that this excitation wavelength range is located near the direct band gap of the silicon nanocrytals. The observed blue-light emission cannot be simply ascribed to the quantum confinement effect of the silicon nanocrystals with size rang 2-5 nm. Because the blue emission via the direct electron-hole recombination across the г- г direct band gap occurs for silicon nanocrystals with a diameter below 2 nm [30]. Note that the colloid contains a few numbers of SiNCs with a diameter less than 2 nm as shown in the insert part of Fig. 2. Thus, the contribution of these nanocrystals may not only describe the strong blue emission of the colloid. In addition, we have not observed any pronounced blue emission peak shift with changing the excitation wavelength from 350 to 375 nm. This also confirms that the quantum confinement effect is not the only mechanism for the strong blue emission of the SiNCs colloid. Some oxide-related defects might be formed within the surface oxide layer of the SiNCs which might act as the radiative centers for the recombination of electron-hole pairs giving rise to the strong blue-light emission [30–34]. At about 500 nm, a shoulder on the peak of blue emission is also observed which is attributed to the quantum confinement effect (see Fig. 7(a)). It seems that at excitation wavelength range near the direct band gap, the luminescent mechanisms are originated from both of the quantum confinement effect and the oxide surface states /defects of the SiNCs.
4. Conclusion
The silicon nanocrystals with average size 3.5 nm are fabricated by laser ablation of a silicon wafer in the DMSO. The colloid remains stable and a schematic presentation for interaction of the DMSO and the SiNCs is suggested. We have investigated the luminescent properties of the SiNCs colloid and stable blue-green emission is observed. Our results suggest that the green photoluminescent emission of colloid is due to the quantum confinement size effect of the silicon nanocrystals. The observation of strong blue emission of the colloid is originated from the quantum confinement effect and the oxide surface states /defects of the SiNCs. The observation of stable blue-green luminescent of the SiNCs-DMSO colloid makes it a good candidate for many applications such as lighting, sensing and biological imaging.
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