The experimental demonstration of an optical data storage technique in this section makes use of a graphene layer placed on top of a copper substrate. Here, graphene's micro dimensions were used to achieve large storage densities. Graphene is applied to the copper substrate via chemical vapour deposition (CVD). It serves as the foundation upon which nanoholes are utilised to engrave information. Due to the plasmonic effect, the nanoholes made on top of the graphene sheet alter the reflectance of incident light. Photodetectors are able to detect and interpret this reflected light. High heat is used to erase data since graphene heals itself after being damaged. The applications for high-density data storage would greatly benefit from this technology.
a. Synthesis of Graphene filim uing CVD
Graphene of the highest quality is produced by this method of synthesis. Graphene can be created by CVD on a variety of substrates, including copper, nickel, cobalt, silicon, etc. In comparison to other techniques, the CVD synthesis of graphene on copper substrates provides a number of benefits, including affordability, scalability, and the capacity to create graphene films of superior quality. By breaking down the precursor gas into carbon atoms, which then adsorb on the copper substrate to create a graphene layer, the high temperature and low pressure conditions in the CVD process encourage the formation of graphene. Two steps were used to complete the graphene CVD synthesis on copper substrates. The copper foil was cleaned in the first step by being annealed for 30 minutes at 1000°C in a hydrogen environment. Purge the surface of any contaminants. In the subsequent phase, a quartz tube reactor containing copper foil was filled with a precursor gas that was a combination of methane and hydrogen. The reactor's temperature and pressure were both kept constant at 1000°C and 1 atm, respectively. In this, methane and hydrogen were used as the growth gases for chemical vapour deposition. The CVD setup is depicted in Fig. 9. The substrate utilised was 0.025mm thick alfa aesar copper foil. Figure 10 depicts the low-pressure system that was employed in this study. The method used was the conventional CVD process. In a tank, a 95:5 Argon + Hydrogen mixture is utilised instead of pure hydrogen for safety reasons. Compared to pure hydrogen and methane, this mixture enables the growth to be carried out at a significantly higher pressure. Figure 11 illustrates the copper-based graphene that was produced
b. Nanohole drilling using laser ablation
The manufacturing and structuring of graphene can be accomplished using the promising laser ablation approach. High-energy laser pulses are employed in laser ablation to vaporise a target substance into grapheme .In the laser ablation process, a solid is exposed to a powerful continuous wave (CW) or pulsed laser beam as shown in Fig. 12 in order to remove atoms from it. This technique can be applied to prepare material surfaces in a micro- and nano-controlled manner, lifting atoms to drill nano range small deep holes through very hard materials, and other applications. Actually, the ablation is the thermal removal of atoms.
Once the laser system is turned on, the Continuous Wave (CW) laser beam refers to a continuous output, whereas the pulsed laser refers to a brief output with a duration of milliseconds to femtoseconds. A pulsed laser beam has a higher overall output power than a continuous-wave laser beam while maintaining a similar average output power
A solid surface will absorb the laser energy and heat up when it is subjected to a CW or long-pulsed laser beam. The outcome is an acceleration of the thermal motion of the particles. These particles evaporate or sublimate and turn into vaporised particles when the absorbed energy is greater than the threshold sublimation energy. This indicates that the exposed area has been abraded. The vaporised particles per unit area per second are used to express the laser ablation rate
N˙=ρd/τm
Where τ = The duration time of the laser pulse,
ρ = The density of the target,
d = The thickness of the ablated material,
m = The average mass of ablated atoms.
The damage threshold is the point at which a carbon lattice hole is produced by a single laser pulse exposure. According to Roberts et al. (2011), the damage threshold was determined for various pulse durations and is depicted in Figure.13. The energy fluence between 50 fs and 1.6 ps had a relatively constant FTH of 200 mJ/cm2, which is the threshold at which graphene was destroyed. This fits well with the theoretically expected damage threshold for rapid destruction to a graphitic film (Jeschke, Garcia, & Bennemann, 2001). For a shorter pulse, graphene can withstand an intensity that is significantly higher. The intensity damage threshold is 2.7X 1012 Wcm2 for a duration of 50 fs. Comparatively speaking, the femtosecond damage threshold is substantially greater than the point at which the CW laser results in 106 Wcm2 of lattice alteration has been measured.
The single-shot damage threshold for femtosecond pulses is seen to be extremely clearly defined. Up to a certain intensity value, the lattice remains largely unchanged, but above that value, it is completely ablated..
c. Uv analysis
The optical characteristics and behaviours of plasmonic and nonplasmonic objects can differ significantly, as shown by UV analysis. Metallic nanostructures that exhibit surface plasmon resonance (SPR) at particular light wavelengths, resulting in improved absorption, scattering, and electromagnetic field localisation, are known as plasmonic structures. SPR peaks, which correspond to the resonant wavelengths at which the structures interact with light most intensely, can be found in plasmonic structures using UV analysis. These peaks' size, form, composition, and surrounding environment, among other things, can affect their intensity and shape. However, nonplasmonic structures can still interact with UV light in other ways, such as through absorption or reflection, even if they don't show SPR. Dielectric structures, for instance, can show powerful absorption or owing to their high refractive index or surface shape, scattering at specific UV wavelengths. Nonplasmonic structures' absorption or reflection spectra can be discovered through UV analysis, which can offer details about their optical characteristics and prospective uses.
Figures 14 to 17 depict UV analysis graphs of holed and unholed graphene. It is clear from comparing Figs. 14 and 15 that the absorption changed significantly after the holes were engraved. Sharp peaks and absorption up to a range of 1500 nm are both characteristics of pure graphene sheet. Strong absorption peak at 265 nm, which is associated with the carbon-carbon bond pi-pi* transition in the graphene lattice. The transitions between the pi and pi* orbitals of the carbon atoms in the graphene lattice are responsible for the relatively modest absorption band we noticed in the UV-B region (280–320 nm). Third, in the visible region, we found a relatively faint absorption band. Graphene's transitions between its conduction and valence bands are responsible for the wavelength range (400–700 nm).
. The holed one, however, exhibits absorption up to a wavelength of 700 nm and lacks any strong peaks that would suggest a narrow spectrum.. A number of intriguing aspects of nanoholed graphene's optical behaviour were discovered through UV investigation. The transitions between the pi and pi* orbitals of the carbon atoms in the graphene lattice are first seen as a prominent absorption peak at about 280 nm. When compared to the peak of pure graphene, this peak is slightly redshifted, indicating that the presence of nanoholes has changed the electronic structure. Additionally, we noticed absorption peaks at about 450 and 650 nm, which are attributed to graphene's transitions between its valence and conduction bands. Third, we noticed that nanoholed graphene's overall absorption was significantly lower than that of pristine graphene, indicating that it is more transparent
d. Raman spectroscopy
Since the interactions between the layers have a major impact on the phonon characteristics, Raman spectroscopy has been extensively utilized to estimate the number of layers in graphene.Raman spectroscopy can detect the vibrational modes in graphite and few-layer graphene, including not only the naturally-occurring stacking but also intermediate states like twisted magic-angle graphene layers, which are tuned by the interaction between layers with different stacking orders. I2D/IG and ID' both exhibit a dependence on defect density, while ID/IG and ID' exhibit a dependence on layer count. Without being able to see individual flakes, they can be utilized as a quick and accurate calculation to measure the lateral dimensions and thickness of graphene statistically(Li et al., 2023).
Raman spectroscopy offers a wealth of data on a variety of material properties, including disorder, edge and grain boundaries, thickness, doping, strain, and thermal conductivity. Phonon dispersion can be used to interpret the graphene's Raman spectra. Single-layer graphene has a unit cell made up of two carbon atoms and six phonon dispersion bands. Some of the vibrations are longitudinal, while others are transverse. On the spectrum, G, G', and D bands will appear depending on the layer and deformations. Figure 18 depicts the graphene spectrum without holes, while Fig. 19 depicts the spectrum with holes. Strong G and G' bands are displayed in Fig. 18. But after drilling holes, the peaks' sharpness became disoriented.
Graphene is a potential material for a variety of applications because the existence of nanoholes in its lattice can cause major changes in its electrical and mechanical characteristics.
The structural and vibrational characteristics of nanoholed graphene are well understood thanks to the Raman spectroscopy research. The increased D band intensity in graphene with nanoholes shows that the nanoholes are causing more structural instability. This is in line with other research on various flaws in graphene, which also demonstrates a rise in the D band's intensity. The insertion of hydrogen into the graphene lattice is indicated by the existence of extra peaks that match to the C-H stretching modes in graphene, possibly as a result of the chemical etching procedure employed to make the nanoholes
The Raman spectra of a graphene layer without holes are compared to (Childres, Jauregui, Park, Caoa, & Chena, 2013) in the absence of a reliable database.
The Stokes phonon energy shift brought on by laser irradiation contributes to two main peaks in perfect graphene: G (1580 cm-1), a primary in-plane vibrational mode, and 2D (2690 cm-1), a second-order overtone of a separate in-plane vibration, D (1350 cm-1). These D and 2D peak positions are dispersive, depending on the laser excitation energy. The expansion of the crystal lattice, which lowers the energy of the Raman phonons, is thought to be the source of high electron concentration or low hole concentration, which results in a shift in the position of the 2D peak with higher electron concentration and a decrease in Asymmetry in the G peak position's doping effect.hole concentration
The 2D peak's intensity changes as a result of doping of graphene as well. The crystal lattice's expansion and contraction can alter intensity in addition to doping. Stretching of the lattice, which would also result in a drop in phonon energy, is what causes the redshift of the Raman spectrum. If this produced strain is uniaxial, it splits the G peak into two distinct features, which corresponds to the vibrational mode splitting into two distinct ones along axes perpendicular to the curvature and parallel to it, respectively. The peak locations of the Raman spectra may alter as a result of temperature changes. The anharmonic coupling will increase as the temperature rises, and thermal expansion causes a linear redshift in the spectrum. peak 2D and G. Figure 20 compares the CVD graphene produced by the two processes in (Yoon, Thiyagarajan, Ahn, & Jang, 2015).
Nearly identical surface-enhanced Raman spectra to those discussed in the prior case. A specific focus is required for the GERS (graphene-enhanced Raman spectrum). Graphene oxide, nanomesh graphene, and other materials are frequently used in GERs. GERS is influenced by a number of variables, including the thickness of the graphene layers, the density of the probe molecules, the distance between the graphene and the probe molecules, the interference effect, and molecular alignment. The GERS signal between the probe molecules and the graphene substrate also includes
e. Graphene as a storage Device
Graphene sheet on a copper substrate makes up the fundamental structure, as depicted in Fig. 21. Through simulation, various substrate materials are tested. Copper is chosen even though it has a higher reflectivity than gold, aluminium, and copper because producing graphene on copper only requires one CVD step and is inexpensive. Glass can easily be coated with graphene, but its poor reflectivity makes it unsuitable for use as a substrate
Reflectivity can be altered via holes. Nanoholes can be used to represent binary data. For instance, holes could stand in for "zeroes" and nonhole areas for "ones," or vice versa.The suggested plan is depicted in Fig. 22.
As discussed in previoous sections data writing is carried out via laser writing of holes. Graphene can be penetrated using holes created by femtosecond high-intensity laser irradiation. Data is typically written to optical memory using pulse width modulation techniques. The data reading is done directly using the laser light. Depending on whether there are holes or not, the reflected lighting will vary. Using photodetectors, the reflected light can be decoded to read data.
Data erase makes use of the self-healing mechanism in graphene as explained in section 3.7. Nano holes created on the graphene surface automatically vanish at high temperatures. This mechanism can be used for erasing data.
● They are more energy-efficient since optical memory uses the majority of its energy only when writing data. Once the data has been written, the optical disc drive will shut off. Therefore, the optical disc system is the best choice for data archiving and internet backup due to its benefits like low cost and low energy consumption.
● The studies often yielded retardance levels of 40 nm for reading.
Traditionally, one bit of data is stored in each memory cell. However, multiple bits can be stored in a single cell thanks to multiplex technology. As a result, there is room to enhance the overall storage capacity. This method can be used with materials that are sensitive to aspects of light other than intensity, such as polarisation, wavelength, and space fluorescence. This method allows the signal to be read over numerous separate channels allowing for data multiplexing
● Using 50 nm holes, 0.25GB/second may be read using a 500 nm diffraction-limited laser with a 5 ns reading time.
Making multidimensional memories is possible because graphene exhibits bire fringing.