Diode performance of silica nanoparticles extracted from Pleurosira laevis diatom frustules

In this work, we measured the I-V characteristics of silica nanoparticles (SNPs) extracted from the Pleurosira laevis diatom and deposited on top of a p-type Si(111) wafer. The electric response of the SNPs-based diode is found to be more sensitive to thermal and optical power than the fresh frustules-based diode by about 3 and 5 times, respectively. Moreover, the chemically processed SNPs exhibit better diode parameters, i.e. for them the ideality factor is closer to 1, the series resistance is 3 times lower, and the shunt resistance is 4 times higher than those of the fresh frustules silica. It is stimulating to use the extracted SNPs in innovative electronic and optoelectronic applications as an abundant, cheap, and easy-to-process material.


Introduction
The ultimate benefit of transforming natural wastes into useful materials used in technological applications is an eternal state-of-the-art methodology in material science and technology. This is because it is a fair adjustment for both the biological and technological mutual benefits. This strategy applies fairly to the natural microalgae diatoms, which are found in fresh and marine water. The diatom frustule is of a siliceous nature (Marella et al. 2020), consisting of two valves connected by a system of circumferential girdle bands. Each valve and girdle band develop inside the cell in a separate compartment termed a silica deposition vesicle (SDV) (Heintze et al. 2020). The mechanisms for silica morphogenesis in valve SDVs have been reported by many researchers (Kröger and Poulsen 2008;Tesson and Hildebrand 2010;Pawolski et al. 2018). The taxonomy of frustule can be classified according to their shape based on the type of symmetry exhibited by the cells, as it is radial in centric diatoms and bilateral in pennates (Rea et al. 2017). Their frustule cell wall is of mostly amorphous micro and nano silica designed into two-and three-dimensional nanopatterns with different morphological diversity (Bradbury 2004;Nassif and Livage 2011). These macro/nano-scale patterns were attractive in many fields of bionanotechnology. Thanks to their large surface area, biocompatibility, high pore volumes, ordered porous channels, optical properties, and high thermal stability (Stefano et al. 2009;Yuliarto et al. 2009;Li et al. 2011;Wei et al. 2011), diatoms' silica are used as a platform for nanopatterning (Blättler et al. 2006;Christman et al. 2006;Mendes et al. 2007;Erickson et al. 2008;Mishra et al. 2017), medical and environmental applications (Weatherspoon et al. 2005;Gordon et al. 2009;Losic et al. 2009;Tommasi et al. 2010;Ferrara et al. 2014;Tommasi et al. 2014), biosensing (Stefano et al. 2005Viji et al. 2014), and drug delivery systems (Chao et al. 2014).
Owing to the diatoms' frustule natural intricate 3D-structure silica-based templates, they have the advantage of easy integration with conventional, well-established processing methods in the semiconductor industry (Lin et al. 2010;Zimmerman et al. 2014). Recently, micro silica of various diatom types was investigated as a possible photovoltaic material (Fuhrmann et al. 2004;Jeffryes et al. 2011) and as an anode material for Li-ion batteries (Wang et al. 2012;Campbell et al. 2016;Zong et al. 2018;Cui et al. 2019). However, using diatom frustules in optoelectronic device applications was limited due to their huge dielectric properties arising from their natural amorphous structure. Therefore, many efforts have been underway to modify the frustules' silica to be more suitable for many applications. In particular, transforming these micro-organisms into nanoparticles is a significant advancement in silica nanoparticles (SNPs) production for its cheap and mass production (Weatherspoon et al. 2005;Cai et al. 2005;Dudley et al. 2006;Losic et al. 2006;Townley et al. 2007;Lee et al. 2007;Lee et al. 2008;Jeffryes et al. 2008).
In this work, we transformed the as-extracted Pleurosira laevis diatom frustules into SNPs via simple chemical treatment. Subsequently, we characterize and track any changes in their structural, optical, and electrical properties by utilizing extensive high-resolution techniques and measurements, respectively. The novelty of the current research lies in the ability to assemble diodes based on the fresh Pleurosira laevis diatom frustules and the extracted SNPs on top of a p-Si(111) wafer and measure, for the first time, their I-V characteristics under different conditions of illumination and temperature.

Samples and Techniques
Fresh samples were collected from the sampling site in the Wadi El-Rayan area at El-Fayum Governorate, Egypt. Fresh Diatom Frustules were rinsed with distilled water to remove the excess of salts adhering to them. Silica Nanoparticles (SNPs) samples were extracted by chemically processing the fresh frustules with a strong H 2 SO 4 and HNO 3 mixture as described by Taylor et al. (2007). Typically, 5 mL of supernatant diatom frustules were mixed with 5 mL of a 2:1 ratio of concentrated H 2 SO 4 and HNO 3 (> 90%), respectively. Then, the mixed solution of strong acids and diatoms was stirred at 1000 rpm, keeping the temperature at nearly 90 ºC for 3 h. To facilitate the experimental measurements, emulsions of the fresh and chemically processed samples were prepared by dispersing 0.02 g of each sample powder in 10 mL of ethylene glycol.
The transmission electron microscope (TEM) images and electron diffraction (ED) were recorded using a JEOL® JEM-2100LaB6 apparatus operating at 120 kV and equipped with a 4 Megapixel Gatan® CCD camera (Orius SC200B). X-ray photoemission spectroscopy (XPS) was used to investigate the chemical configuration of the different atomic species in both samples, using a non-monochromatic Al-Kα x-ray source (E = 1486.6 eV) and a SPECS Phoibos 100 electron analyzer. The spectra were scanned for each finely powdered sample spread on carbon tapes under 10 − 10 mbar of UHV conditions. The binding energies of Si2p and O1s core levels of each sample were tracked. The spectra were calibrated with the Si2p core level of a standard Si wafer. Optical absorption spectra, in the 190-900 nm wavelength range, were collected at room temperature with a Perkin-Elmer Lambda 25/35 double-beam spectrophotometer. For electrical properties measurements, two droplets of each sample emulsion were deposited on top of the p-type Si(111) wafer surface using the spin-coating technique, as it is a simple and easy method to process our samples, and then we left the samples to dry. Copper (Cu) wires were used as the ohmic metallic contacts and the I-V curves were measured using a Keithley electrometer and quick I-V measurement software (Keysight B 2900). More details about the I-V setup and conditions can be found in our previous work (Osama et al. 2021).

Shape and Structure
High-resolution transmission electron microscope (HR-TEM) images were recorded at different magnification powers to characterize the shape and size of Pleurosira laevis diatom frustules before and after the chemical treatment as shown in Figs. 1 and 2, respectively. In Fig. 1 (a-c), we can see the microstructure of the fresh Pleurosira laevis diatom frustules. The dimensions of the frustules extend to a micrometer scale with a clear hexagonal porous structure in the fresh silica. The distance between the pores is about 0.8 mm. The electron diffraction (ED) pattern on a selected area of the fresh sample, shown in Fig. 1d, resembles an amorphous structure in the absence of any diffraction rings around the central ring. After the strong acidic treatment, the shape of the diatom frustules is utterly transformed into SNPs with long tubes (> 200 nm) and small spheres, both of which have diameters of about 20-25 nm, see Fig. 2 (a-c). The selected area ED pattern for the extracted SNPs exhibits a 1 3

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A. Ashery et al. well-defined hexagonal quartz structure. The lattice spacing of these SNPs is about 3.20 Å, which is relatively smaller than the lattice spacing of quartz 3.35 Å (Maurice and Huggins 1922).

Chemical Configuration
To explore the chemical configuration of the diatoms before and after the strong acidic treatment, we measured their x-ray photoemission spectra as shown in Fig. 3a & b, respectively. Both spectra of fresh and chemically processed frustules, show a broad Si2p core level peak centring at 103.7 eV, belonging to the oxidation state of quartz SiO 2 (Moulder et al. 1993). However, the Si2p peak of the fresh frustules is broader by about 0.3 eV than that of the processed ones. This is probably a result of the multiple chemical configurations of the frus- Diode performance of silica nanoparticles extracted from Pleurosira… tules silica due to the presence of the living organs, which have disappeared after the acid treatment, or due to their amorphous structure. The same broadening-difference observation is also noticed in the O1s core level between the fresh and processed frustules. In detail, we can see two distinct peaks for the O1s of the fresh diatoms (Fig. 3a). The first peak of the O1s core level is located at about 532.7 eV, belonging to the oxygen state of the SiO 2 configuration (Moulder et al. 1993), whereas the second peak, located at about 529.5 eV, belongs to the O − 2 oxidation state of organic living or any environmental effect (Moulder et al. 1993). The position of the first peak is slightly shifted to 533 eV and the intensity of the second peak is hugely dimensioned in the chemically processed frustules with respect to the fresh ones, see Fig. 3b. Again, this is a confirmation of the success of the acidic treatment to extract pure SNPs.

Optical Absorption
To explore the optical properties of both fresh and processed Pleurosira laevis frustules, we measured the UV-visible optical absorption spectra for both samples as shown in Fig. 4. Clearly, the fresh sample shows some optical transitions in the blue range of the spectra, i.e. 400-500 nm, with an an extra transition in the red region at about 680 nm. These optical transitions might be due to the presence of some living organs in the fresh sample since they are not observed in the extracted SNPs as shown in Fig. 4. Consequently, these extra optical transitions might be the reason why the optical absorption of the fresh sample is higher than that of the SNPs sample. This, in turn, affects the natural colours of both samples, making the fresh sample appear with a green colour, whereas the chemically processed frustule appears with a brown colour, see the photographic insets of both samples in Fig. 4. Quartz silica is well known for its transparency due to its insulating bandgap (which is below 140 nm, i.e. ≥ 9 eV, which is in the far ultraviolet) (Maj 1988). In our case, the main band transitions of the silica of both fresh and chemically processed frustules are noticed at ≤ 300 nm, i.e. above ≥ 4.15 eV in the near ultraviolet region.

Electric Conduction
The possibility of using both the fresh and chemically processed frustules silica in electronic applications has been examined by measuring the I-V characteristics for their assembled diodes at different illumination and temperature conditions. The I-V characteristics of both samples at dark and light conditions are shown in Fig. 5. As we can see, the electric current density of the extracted SNPs under illumination condition is higher by about 4 times than that of the fresh frustules. For example, at V = 1.5 V, the current of the fresh sample is about 0.1 mA and 0.2 mA under dark and illumination conditions, respectively. However, the current of the extracted SNPs is about 0.1 mA under dark condition and increases up to 0.6 mA under illumination condition. Also, at 2 volts, the fresh sample expresses a current increase from 0.3 to 0.4 mA, whereas the SNPs sample expresses a current increase from 0.2 to 2.0 mA from dark to illumination conditions, respectively. Thus, the current increase from dark to illumination condition is just about 2 times for fresh frustules silica, whereas it is about 10 times increase for the extracted SNPs in their best conditions. Although the optical absorption of the fresh sample is higher than that of the SNPs one, its ability to transport the charges is much less. In other words, we can say that the electric conduction sensitivity of the extracted SNPs to the presence of light is vastly better than that of the fresh frustules. This is might be correlated to the amorphous structure of the fresh frustules that lacks the charge transport within the fresh silica, despite the high crystalline structure of the silica nanoparticles achieved by the strong acid treatment that facilitate the charge transport within the SNPs.
Moreover, the effect of temperature on the electrical conduction of both samples has been investigated by measuring the electrical current vs. the applied voltage at three different temperatures of 300, 350, and 400 K for each sample. The electric current density of both samples under different temperature conditions is more in favour of the SNPs than of the fresh frustules as shown in Fig. 6. The electrical conductivity of the SNPs extracted by chemically processing Pleurosira laevis frustules, Fig. 6b, is vastly enhanced by annealing, up to 3 times higher than that of the fresh frustules, Fig. 6a. This significant enhancement in the electric conduction of the SNPs is again due to their purified crystalline structure with respect to the amorphous fresh frustules that are contaminated with the living organic body. It is worth mentioning that the diode characteristics of silica nanoparticles are not extensively examined in the literature. Interestingly, the enhancement of conduction sensitivity to light and temperature in our SNPs is much stronger than previously reported (Ken and Andronikov 2014).
Calculating the diode characteristics as described before (Osama et al. 2021;Gencer Imer et al. 2019;Karabulut et al. 2022), using the semi-log curves shown in Fig. 6c and d, we can see that generally, all diode factors of extracted SNPs are much better than those of the fresh frustules silica, see Table 1. In detail, the shunt resistance (R sh ) of the SNPs is almost 4 times higher than that of the fresh frustules' silica at high temperatures, see Table 1 and Fig. 7a. Besides, its decay upon rising the temperature exhibits a linear behaviour, which is slower than that of the fresh sample, which exhibits an exponential decay. On the other hand, the series resistance (R s ) shown in Fig. 7b of the extracted SNPs is almost half that of the fresh Diode performance of silica nanoparticles extracted from Pleurosira… frustules, making it better for any electronic application. Similarly, the same behaviour has been observed in the ideality factor (I f ), which is closer to the ideal value of about 1 at 400 K  . 6 The I-V curves for (a) fresh Pleurosira laevis frustules and (b) chemically extracted silica nanoparticles (SNPs) at different temperatures of 300, 350, and 400 K. The current of the SNPs is almost 3 times higher than that of the fresh frustules at the same voltage and temperature. (c) and (d) are the corresponding semi-log curves of (a) and (b), respectively.
in the case of the SNPs than in the case of the fresh frustules' silica, see Table 1 and Fig. 7c. However, both samples have almost the same values of the barrier height (B h ) and its linear temperature dependence, Fig. 7d. All these improved diode parameters are affirmative indicators of how much better the SNPs, which are processed chemically from Pleurosira laevis diatom frustules, are than the freshly extracted frustules.

Conclusion
In conclusion, we have comprehensively investigated how the strong-acid treatment was successful in turning the fresh Pleurosira laevis diatom frustules into highly pure crystalline SNPs. These extracted SNPs exhibit higher electrical conductivity compared to their fresh frustules' silica counterparts. Interestingly, the electric conduction of the SNPs is higher by about 3 and 5 times than that of the fresh frustules' silica under heating or illumination conditions, respectively. Moreover, the diode characteristic parameters, e.g. R sh , R s , and I f of the extracted SNPs are better than those of the fresh frustules' silica. Thus, with a simple and mass-productive treatment, we were able to transform the Pleurosira laevis diatom frustules' silica into pure, better-conductive SNPs that can be high-potential active materials for innovative electronic and optoelectronic devices.