Deposition of Graphene Nanoparticles Prepared by Laser Ablation Method Mixed with Chlorophyll Dye on Silicon Nanowire Substrate as a Detector

INTRODUCTION 
Silicon-based materials have garnered considerable interest because of their unique properties and potential applications in broad areas, including electronics [1–6], thermoelectric [7], solar energy harvesting [8–11], and biotechnology [12, 13]. With a room temperature band gap of 1.12 eV, silicon promises efficient solar energy harvesting across the entire solar spectrum from ultraviolet (UV) to near infrared (IR). For photodetector, nanostructured materials typically exhibit much better activity than their bulk counterparts because of their large surface areas and short charge carrier diffusion distances [10, 14–17].
Graphene, a single atomic layer of a honeycomb lattice of carbon atoms, has recently become the central focus of material research for fundamental studies because of its potential applications in diverse areas [18–25]. The covalently bonded carbon lattice can exhibit excellent chemical stability and function as a natural protective barrier. For example, it has been shown that graphene can function as an effective passivation layer to protect metal surfaces from oxidation [26, 27]. Additionally, graphene can exhibit excellent electrical transport properties and facilitate charge separation and transport in semiconductors and their interfaces [28–30]. Lastly, single or few-layer graphene sheets exhibit high optical transparency at visible wavelengths [31], which will not affect the light absorption of underlying materials. Together, these combined attributes make graphene an excellent candidate as a novel protection material and charge- mediating layer for SiNW photocatalysts. 
On the other hand, chlorophyll is a cheap and easily extractable most abundant bio-molecule in the photosynthetic system of green plants, bryophytes, algae, and bacteria [32–34]. Chl. possesses two strong absorption bands, one is Soret-band in the range of 410 nm-450 nm and other is Q-band in the range of 600 nm-700 nm [32,34,35]. The spectral characteristics of Chl. are explained by Gouterman four orbital model [36,37], which are HOMO-1 (highest occupied molecular orbital), HOMO, LUMO (lowest unoccupied molecular orbital) and LUMO+1. Not only light absorption, Chl. also plays a crucial role in the transfer of absorbed photon energy in terms of excitons through resonance energy transfer to the reaction centers of photosystems [33–35]. Chl. contains porphyrin ring with a central magnesium (Mg) atom surrounded by four nitrogen atoms, which is mainly responsible for light-harvesting and energy transfer [33,34]. Efforts have already been made to utilize this excellent property of Chl. in different kinds of devices such as dye-sensitized solar cells [32,38-40], photo detector [41,42] and photoelectrode [43–46]. But the reported efficiencies are not up to the mark due to the absence of proper photo-excited charge transfer system, which can be escalated with the introduction of suitable material like graphene [32,41]. 

MATERIALS AND METHODS
General Consideration 
A JSM-6510LV scanning electron microscopy (FESEM) measured the morphology of the samples (Type - S-1640 HITACHI company Japan). The sample structure was analyzed using (Cu Kα) radiation (λ=1.5406 Å) in reflection geometry with a Shimadzu 6000 X-ray diffractometer (made in JAPAN). For some pure samples and all doped samples, Mid-IR spectra, from (4000 - 400 cm-1), were obtained using FTIR-Spectrometer, supplied by ALPHA (Made in Germany). KBr powder was mixed with powder samples in order to determination of the spectra for FTIR. A UV-Vis diffused reflectance spectroscope was used to obtain the optical properties (CECIL CE 7200, ENGLAND) of all prepared materials.

Preparation of Graphene/chlorophyll/silicon nanowires (GNPs/Chl./SiNWs) nanocomposite
In the first stage, Graphene nanoparticles (GNPs) were produced by Q-Switched Nd: YAG pulsed laser in liquids (PLAL-Method). after placing the graphite pellet in a clean beaker and submerging it in 5 mL of deionized water, it is removed using laser pulses with a fundamental wavelength of 1064 nm and an energy of 200 mJ/pulse, all at a repetition rate of 5 Hz and 200 pulses. As shown in the Fig. 1.
In the second stage, the chlorophyll (Chl.) was prepared from spinach as shown in the Fig. 1, the and Chlorophyll Extraction was prepared as Ten milliliters of 80% acetone were used to grind five grams of freshly chopped leaves. Then, for five minutes, it was centrifuged at 5000–10000 rpm. Once the supernatant was moved, the process was repeated until the residue lost all of its color [47]. 
In the third stage, silicon nanowire (SiNWs) was prepared as shown in the Fig. 1 as follows: The Si (p type) wafer was cleaned with acetone and isopropyl alcohol, and dried by using air gun. Si wafer (p type) were initially ultrasonically degreased in acetone and ethanol for 10 minutes, respectively. After each cleaning step, the Si pieces were thoroughly rinsed with DI water. The cleaned Si pieces were coated with Ag nanoparticle film by electroless metal deposition method in aqueous solution of 5% HF and 0.02 M AgNO3 for 1 minute, and then immediately introduced in the electrochemical cell that exposing 1.0 cm 2 of the surface to the aerated 10% HF aqueous solution. Silver glue was used on the back side of the Si substrate in contact with the copper plate to establish an ohmic contact. Subsequently, Si substrate is electrically connected to graphite rod through an external copper wire. he first peak corresponds to the silicon nanowires, while the latter two peaks are attributed to graphene. Notably, shifts in peak positions suggest effects related to the size distribution of the nanowires. This study underscores the interactions between silicon nanowires and graphene, enhancing our understanding of their combined properties in nanocomposite applications.

RESULTS AND DISCUSSION
X-ray Diffraction Analysis
Fig. 2 illustrated the XRD of GNPs/Chl./SiNWs nanocomposite at pH=3, the main peaks locate at 28.2°, 32.6°, 35.2, 38.5°, 39.3° and 48.5° corresponding to (111), (100), (100) (110)  and (220) planes of cubic silicon structure and it is identical to what the researcher mentioned in the references [48,49,50]. but the peaks locate at 10.4°, 26.6° and 42.9 corresponding to (001), (002) and (100) planes of graphene oxide and graphene [51,52]. A diffraction angle of 22.1° was observed, possibly due to the presence of SiO2. 

Morphological investigations of nanocomposite
When a silicon surface is nanostructured, low reflection and strong absorption of visible light can be obtained. Such a silicon is called a black silicon and can be made by different techniques leading to different surface structures. By using the chemical etching (EMACE) technique [53], a very thin nanowire can be obtained Fig. 3. This kind of structure is very interesting for making detectors because electrons are conducted in the axis of the wires, while photons are well absorbed for a large range of incident angles. 
The mean of dimeter of silicon nanowires is about 135.7 nm was also calculated using the image J program as shown in the Fig. 4.
Fig. 5 shows FESEM images of graphene nanoparticles (GNPs), it is clear that the graphene is formed in the form of spherical particles and in different sizes and homogeneously distributed, as is clear from the FESEM images in Fig. 4. The mean of dimeter of graphene nanoparticles is about 18.8 nm was also calculated using the image J program as shown in the Fig. 6.
Fig. 7 shows FESEM images of GNPs/Chl./SiNWs nanocomposite, it is clear that also, the graphene is formed in the form of spherical particles in different sizes and concentrated in small and large groups. The mean of dimeter of GNPs/Chl./SiNWs nanocomposite is about 23.6 nm as shown in the Fig. 8.

Fourier Transform Infrared (FTIR) Analysis
Fig. 9 show the spectrum of GNPs/Chl. nanocomposite shows prominent peaks at 3399 cm−1, which corresponds to O–H stretching vibrations of carboxylic acids [54]. Characteristic absorption peaks appearing at 1694, 1369 and 1230 cm−1 in the FTIR spectra showed the presence of carboxyl C=C, alkoxy C–O stretching vibrations, and epoxy C–O, respectively [54,55,56,57].

UV-Diffused Reflectance Spectroscopy
The fabrication of SiNWs is a surface modification technique that aims to minimize the reflection of incident light and increase the absorption as much as possible. The antireflective nature of the SiNWs has drawn attention since one of the major energy loss mechanisms of solar cells is optical reflection; the utilization of these nanostructures in photosensitive devices may eliminate the need for antireflective coatings. Fig. 10 shows the variation in reflectance (R) depending on the wavelength (λ) of incident radiation on the Si substrate [58]. The measurement was obtained by illuminating samples with radiation varying from the ultraviolet region (UV) to the infrared region (IR) with wavelengths ranging from 200 to 1100 nm. The reflectivity of Si wafers is quite high, exceeding 88% (maximum) and 65% (minimum) in the ultraviolet (UV) regions and decreasing in the IR region to 48% [58]. That is, there is no peak or absorption edge in the reflectivity spectrum of a silicon wafer in the visible region of the electromagnetic spectrum.
Fig. 11 was showed the reflectance of GNPs/Chl./SiNWs nanocomposite is much lower than that of their bare Si wafer counterpart and it is similar to what is stated in the reference [58]. It is clear that there is a slope between 400-700 nm, and this behavior is different from silicon wafer (i.e., the longer the wavelength, the lower the reflectivity and the higher the absorbance) due to the effect of graphene and chlorophyll deposited on the silicon nanowire. This simply indicates that the sample is very sensitive to visible compared to silicon wafer. This wavelength range (400 -700 nm) is essential for photosensitive device applications. The maximum and minimum reflectance of the sample is approximately 59% and 54% respectively in the UV range while the maximum and minimum reflectance of the sample is approximately 52% and 31% respectively in the visible range (400-700) nm. The results reveal that modifying the surface morphology of the Si substrate to form SiNW successfully reduced the reflection loss of incident radiation over a broad spectral range. This remarkable property suggests that SiNW arrays are a good candidate for antireflective surfaces and absorption materials in photovoltaic cell [58]. 

Raman spectroscopy 
The common properties of graphene in Raman spectroscopy are known to be in the wavelength range of (800–3000) cm−1, which is in accordance with the electronic characteristics of graphite materials. Three prominent peaks, referred to as the G, D, and 2D bands, are located at approximately (1580, 1350, and 2700 ) cm−1 [59–61]. At the middle of the Brillouin zone, the G band associates with the E2g phonon [58]. Another fingerprint peak in graphite material with a concentration of faults is the disorder-induced D band [60]. The double resonant Raman scattering with two-phonon emissions is the source of a 2D peak, which is the second order of the D peak [59,61] as shown in Fig. 12.
For silicon nanowires, various recent papers reported Raman spectra of SiNWs. They report peak positions of 500-510 cm-1 for wires 10-15 nm in diameter [62-72].
It is noted through the Raman spectrum Fig. 12 that the silicon bare peak appears at      (520cm-1 ),but in the nanocomposite, three peaks appeared with a higher intensity at (520,1382,1598)cm-1 respectively. Where the first peak belongs to the silicon nanowire and the other two peaks belong to graphene. There is a shift in the peaks which is a result of the size distribution of the nanowires, some of which may be of small diameters.

Photodetector Parameters Measurement
The Spectral Responsivity (Rλ)
The spectral responsivity of structures in the (350-1000) nm wavelength region at a (3)V bias voltage was calculated using Eq. 1.

The spectral responsivity graphs of the Ag/PSi/GNPs/Chl./Ag structure constructed are displayed as a function of wavelength in Fig. 13. According to the Fig. 13, the Ag/PSi/GNPs/Chl./Ag spectral responsivity curve has many response peaks, with the absorption edge of silicon responsible for the maximum peak at (900) nm, which is consistent with earlier research [73]. 

Specific Detectivity (D*)
The performance of the detector is correlated with specific detectivity, an essential parameter for a photodetector that indicates the lowest detectable power. The specific detectivity was calculated using Eq. 2. 

The specific detectivity for Ag/PSi/GNPs/Chl./Ag photodetectors is plotted against wavelength in Fig. 14. This graph demonstrate the direct relationship between particular detectivity and responsivity. It was discovered that the specific detectivity was (1.01×1012) cm.Hz1/2W-1 for Ag/PSi/GNPs/Chl./Ag photodetector at wavelength (650) nm.

Quantum Efficiency 
The quantum efficiency was calculated using Eq. 3. 

The quantum efficiency of the Ag/PSi/GNPs/Chl./Ag structure as a function of wavelength (350–1000) nm is displayed in Fig. 15. The highest peak quantum efficiency was 18.9% at 650 nm, based on the data we collected. This is due to increased absorption in this area, which raises spectrum sensitivity and, eventually, quantum efficiency by generating more carriers in the depletion region.

CONCLUSION
The Q-Switched Nd-YAG pulsed laser has the ability to generate graphene and graphene oxide nanoparticles. The process of mixing chlorophyll with graphene and graphene oxide nanoparticles lead to an increase in the size of graphene nanoparticles because of aggregation and chlorophyll molecules may be interaction with graphene nanoparticles. The results reveal that modifying the surface morphology of the Si substrate to form SiNW successfully reduced the reflection loss of incident radiation over a broad spectral range. This remarkable property suggests that GNPs/Chl./SiNW nanocomposite are a good candidate for antireflective surfaces and absorption materials in photovoltaic cell. From Raman spectrum, the observed peak shifts and the emergence of additional peaks suggest that the size distribution of the silicon nanowires influences the spectral characteristics, underscoring the complex interactions between the components in the nanocomposite material. GNPs/Chl./SiNW nanocomposite was also used in the application of a detector with an efficiency of up to18.9% at 650 nm, based on the data we collected. This is due to increased absorption in this area, which raises spectrum sensitivity and, eventually, quantum efficiency by generating more carriers in the depletion region.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.