Preparation of Si nanoparticles from bamboo leaves and measurement of their photoluminescence and electroluminescence spectra

Bamboo leaves were cut into pieces smaller than 3 cm and immersed in 10% HCl solution for 2 h to remove the metal elements. After the HCl treatment, the small pieces of bamboo leaves were washed with water and dried at RT, approximately 25 °C. Dried specimens were incinerated in ambient air at 700 °C for 3 h to extract the SiO₂ particles. A mixed powder of 1 g SiO₂ particles from the bamboo leaves and 1 g Mg powder was heated at 650 °C for 2 h in N₂ gas with an electric tube furnace to reduce the SiO₂ particles. After the Mg reduction process, the reduced specimens were immersed in HCl solution (10% HCl:99.5% EtOH = 1:9) for 12 h to remove the MgO formed during the reduction process. The HCl solution containing Si nanoparticles were filtered under reduced pressure using a 5 μm membrane filter to separate the specimens into residue specimens and filtrate solutions. The Si nanoparticle filtrate specimens were dialyzed using cellulose tube with 1.25 nm pores to remove MgCl. We prepared two type specimens of Si nanoparticles from bamboo leaves—Si nanoparticles of the residues and Si nanoparticles of the filtrate.

Figure 1 shows a schematic of the fabricated EL device with Si nanoparticles from bamboo leaves. Only the Si nanoparticles of the filtrate specimens was used as the optical active layer of the EL devices. The Si nanoparticles of the filtrate specimens were wedged between the fluorine-doped tin oxide transparent electrode. Polytetrafluoroethylene tape was placed around the Si particle active layer as the insulating layer.

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Fourier transform IR (FTIR) spectroscopy (FT-IR-4300, Jasco) was performed to analyze each step of the sample preparation process from the bamboo leaves to the Si nanoparticles.

In the measurement of PL and EL spectra, a spectrometer (SP 2150, Princeton Instruments) and electronically cooled CCD (PIXIS 256, Princeton Instruments) was used. A 405 nm line of GaN laser (TC35, NEOARK) was used as the excitation source for the PL measurements and DC stabilized power supply (GPD-2303S, GW Instek) was used for the EL measurements.

Figure 2 shows the FTIR spectra of the specimens during the preparation of Si nanoparticles from bamboo leaves and the corresponding photograph of the specimens.

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From the FTIR spectra of the untreated bamboo leaves [Fig. 2(a)] and bamboo leaves after HCl treatment [Fig. 2(b)], absorption peaks at approximately 2900 cm⁻¹ correspond to the C–H bonds. The absorption peak at approximately 1100 cm⁻¹ is ascribed to the Si–O bonds (approximately 1060 cm⁻¹) and C–O bonds (approximately 1040 cm⁻¹), which could not be clearly separated.

After the incineration at 700 °C, the absorption peak at approximately 2900 cm⁻¹ disappeared, indicating that the organic material originally existing in the bamboo leaves were disappeared [Fig. 2(c)].

Meanwhile, the absorption peak at approximately 1100 cm⁻¹, which is attributed to the Si–O bonds remained after the incineration process. This result indicates that the white powder in Fig. 2(c) is SiO₂ particles that originally exist in the bamboo leaves. The yield of the SiO₂ powder in the bamboo leaves is approximately 19 wt% in dry weight, which is comparable to that of rice husks.

Figure 2(d) shows the FTIR spectra of the specimens after the Mg reduction process. The absorption peak corresponding to the Si–O bonds disappeared, indicating the successful reduction of SiO₂ powder (deoxidized). Figure 2(e) presents the FTIR spectra and photograph of the specimens after the HCl treatment. The absorption peaks at approximately 1100 cm⁻¹ that correspond to the Si–O bonds were observed again. These results indicate the formation of Si particles by the Mg reduction process and oxidation of the surface of the Si particles by the HCl treatment. Thus, these results denote that the brown powder in Figs. 2(d) and 2(e) correspond to Si particles formed by the Mg reduction process.

However, size of the Si particles from bamboo leaves is not clear at the present. In our previous study, Si particles were formed from rice husks by the same Mg reduction process. As results of TEM and TED observations, nanometer-sized Si particles with crystalline structure were confirmed. ¹⁸⁾ Therefore, nanometer-sized Si particles were probably formed from bamboo leaves because the both specimens were prepared by the same Mg reduction process.

Figures 3(a) and 3(b) show the Si nanoparticles of the residue and filtrate specimens from bamboo leaves, respectively. The photographs were obtained through dielectric filters that cut light with a wavelength of less than 430 nm.

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When the Si nanoparticles made from bamboo leaves were excited by the 405 nm line laser, both specimens emitted red luminescence from the sample point (excited part). These luminesce could observed by the naked eyes under a white room light.

Figure 3(c) shows the PL spectra of the Si nanoparticles made form bamboo leaves by the Mg reduction process. The PL intensity of the Si nanoparticles of the filtrate was approximately seven times higher than that of the residue. Based on these results, the difference in the PL intensity is considered to be ascribed to the size distribution of the Si nanoparticles in the specimens. The most widely known luminescent mechanism of the Si nanoparticles are quantum size effect. ^37,38) In this case Si nanoparticles could emit IR-visible luminescence when their size is below the several nanometer scale. Therefore, the Si nanoparticles of the filtrate have relatively high concentrations of smaller particles that contribute to luminescence, resulting in their higher PL intensity.

Figure 3(d) shows the normalized spectra of Fig. 3(c). Broad PL spectra with the peak wavelength at approximately 850 nm were observed from the Si nanoparticles of the residue. Similarly, the Si nanoparticles of the filtrate have a broad PL spectra with the peak wavelength located at approximately 750 nm, which is shorter than that of the residue.

The wavelength of the PL spectra of semiconductor nanoparticles depends on the size distribution of particles, whereby shorter wavelength luminescence is attributed to smaller particles, and a longer wavelength luminescence is ascribed to larger particles. Therefore, with the distribution of small Si nanoparticles in the filtrate than in the residue, the PL spectra of the Si nanoparticles of filtrate has a shorter wavelength.

Luminescent properties of Si nanoparticles from bamboo leaves were well explained by the quantum size effect. However, the possibility of luminescence from the localized state in SiO_x and surface of the particles could not be rule out completely at the present. Therefore, further investigation about properties of Si nanoparticles from bamboo leaves such as luminescent lifetime and particle size distribution are required.

Figure 4(a) shows the EL device fabricated using the Si nanoparticles made from bamboo leaves as the active layer. With a voltage of 15 V was applied to the EL device, a red EL was observed from the Si nanoparticles in the active layer [Fig. 4(a)']. However, the EL intensity is considerably weaker, which could not be observed by the naked eyes.

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Figure 4(b) shows the PL and EL spectra of the Si nanoparticles in the active layer of the EL device. The PL peak intensity of the Si nanoparticles is approximately 30 times higher than that of the EL spectra. The normalized spectra of Fig. 4(b) is shown in Fig. 4(c). Broad EL and PL spectra were obtained with the peak wavelengths at approximately 700 nm. Compared to the PL spectra, the EL spectra have a slightly higher intensity with the wavelengths of more than 800 nm. The difference in the EL and PL spectral shape can be ascribed to the excitation method. For the EL spectra, the larger size of the Si nanoparticles with a lower energy gap tends to be excited by the electric field in the device.

Although the EL intensity using the Si nanoparticles made from bamboo leaves is not sufficient for current applications, the preparation of luminescent Si nanoparticles made from bamboo leaves have the potential for the high-value-added recycling.