3-1- Structural properties
Fig. 3 presents the XRD GaN nanoparticles synthesized and drop-casted on quartz substrate at different ablation energies (1000, 1200, 1400, 1600, 1800, and 2000mj). The diffraction peaks in the XRD pattern are matched with standard cubic and wurtzite hexagonal structures of the GaN crystal. The nanoparticles prepared for 1000mj exhibit h-GaN rise at 2θ =34.64 reflected from (002) plane and c-GaN peak at 2θ =40.22 reflected from (111) plane cubic phase has affected the peak intensity and the sharpness of the hexagonal phase. The sample of 1200mj displays h-GaN nanoparticles at 2θ = 34.64 and 37.98, which are reflected from (002) and (100) planes. The intensity and the sharpness of the peak begin to increase with increasing the ablation energy. The third sample of 1400mj shows two peaks of h-GaN nanoparticles at 2θ = 34.64, and 37.98 reflected from (002) and (100) planes, the intensity of the peak increased due to increase the structure crystallization. The fourth sample of 1600mj displays h-GaN nanoparticles at 2θ = 34.64 and 37.98, which is reflected from (002) and (100) planes; this sample has the highest peak due to the excellent quality of h-GaN nanoparticles. The fifth sample of 1800mj shows two peaks of h-GaN nanoparticles at 2θ = 34.64 and 37.98, which reflected from (002) and (100) planes, the peak intensity begins to decrease due to decreasing the structure crystallization, and the sixth sample of 2000 mJ shows two peaks of h-GaN nanoparticles at 2θ = 34.64 and 37.98 which reflected from (002) and (100) planes, the intensity of the peak is less than the fifth sample. This indicates high intensity in response to the excellent quality of h-GaN nanoparticles synthesized in the fourth sample at low temperature.
Furthermore, it is shown that the intensity of the peaks for the XRD pattern increased with increasing laser ablation energy due to increasing of the grain size and concentration of ablated material and due to enhancement of crystal quality until (1600 mJ) that has the highest intensity peak then the intensity peak back to decrease as the laser ablation increase.
3-2- Morphological properties
3-2-1 AFM results
This section includes a profound discussion of the results. Fig.4 presents AFM images of the GaN nanophotonic; the thickness of the outer layer affected by the laser ablation energy where at 1000 mJ the thickness was 37 nm and increase at 1200 mJ to be 40 nm and then decrease at 1400 mJ and 1600 mJ that have the less thickness than other due to high regular crystals distribution and the small rang of grain size and then back to increase at 1800 mJ with 62nm because of the decrease in the regularity of the distribution of the crystal and then back to decrease at 2000 mJ to be 27 nm due to regular crystals distribution .
Fig. 5 presents the change in the root mean square and roughness and grain size of the GaN nanophotonic, with the difference in the pulsed laser ablation energy. The GaN nanoparticles prepared for 1000mj exhibit high root mean square (23.40nm) and roughness (3.750nm). The sample of 1200 mJ displays root mean square (16.00nm) and roughness of (4.533nm). The sample of 1400mj and 1600mj shows the highest roughness. The smallest root mean square due to the excellent quality of the structure crystallization and high regular crystals distribution, the roughness back to decrease, and the root mean square increase cause the reduction in the structure crystallization and regular crystals distribution 1800 mJ and 2000mJ .
Fig. 6 represents the relation between the grains density and the grains size at different ablation energies. The GaN nanoparticles prepared for 1000mj exhibit a wide range of grain size (27. 29 nm - 116.6nm) with grains density (0.002). The sample 1200 mJ display a range of grain size (12.57 nm-103.7nm), the range of grain size decrease and the grains density increase. The samples of 1400 mJ and1600 mJ show the smallest range of grain size (4.361nm-58.88nm) and (9.183nm-69.44nm) and the highest grains density due to high regular crystals distribution, then the density of the grain back to decrease at 1800 mJ because of the decrease in the regularity of the distribution of the crystal and then back to increase at 2000 mJ due to regular crystals distribution .
Fig. 7 presents frequency with the grain size of GaN/quartz nanostructures at different ablation energies. The surface topography of GaN nanophotonic as observed from the AFM micrographs proves that the grains are uniformly distributed within the scanning area (78nm × 78nm), with individual columnar grains extending upward. This surface characteristic is quoted from the topographic image, which is uniform, smooth and homogeneous at 1400 mJ. The third sample of 1400 mJ has Gaussian distribution due to the excellent quality of the structure crystallization. The fourth sample of 1600 mJ also has Gaussian distribution with a broader scope than (1400 mJ) sample. Other samples show the ununiform distribution of particle size, which leed to optical and electrical properties inhomogeneous.
3-2-2 FESEM results
Fig. 8 presents the FESEM images and the EDX spectrum of GaN nanoparticles for the six samples synthesized under different ablation energies. These samples were built using a drop cast on the quartz substrate. The EDX spectra of GaN nanoparticles have both elements Ga, and N. The [Ga]/N ratio depends on laser ablation energy. At 1000 mJ, FESEM image shows the beginning of the crystallization process, and the ratio of [Ga] / [N] was 5. The second sample at 1200 mJ exhibits an increase in the structure crystallization. The ratio of [Ga] / [N] was 3.77. The third sample of 1400 mJ shows improves crystallization than the second sample; the ratio of [Ga] / [N] was 3.633. The fourth sample at 1600 mJ shows a good crystallization quality, the ratio of [Ga] / [N] was 4.19. The fifth sample at 1800 mJ exhibits several particles to be agglomerated; the ratio of [Ga] / [N] was 3. The sixth sample at 2000 mJ shows an increase in the number of agglomerated particles with increasing the laser ablation energy, the ratio of [Ga] / [N] was 3.45.
Moreover, FESEM images showed that the crystallization of GaN nanoparticle increased with increasing laser ablation energy due to increased grain size, the concentration of ablated material becoming higher, and the enhancement of crystal quality until (1600 mJ), and this in good agreement with XRD result. Increasing the laser ablation energy above 1600 mJ will make the particles agglomerated, and the number of agglomerated particles will increase with increasing the laser ablation energy.
3-2-3 TEM results
TEM was performed to obtain a submicroscopic image of the nanoparticles of less than 100nm. Fig.9a shows the 1000 mJ indicates that the GaN nanoparticles have quasi-spherical particles with grain size varies from 27.29nm to116.6nm. Fig.9b at 1000 mJ, the sample exhibited a good crystallization quality with the lowest grain size ranging from 4.361nm to 58.88nm. Fig.9c prepared for 1800 mJ shows grain size varies from 4.196nm to 88.51nm; the particles began to agglomerate with increasing the laser ablation energy. The TEM images support the FESEM images of GaN. The FESEM with EDX and TEM also validated the GaN nanoparticles' chemical purity earlier shown with the XRD data. Therefore, it can be inferred that ablation energy is crucial in the synthesis GaN nanoparticles.
3-3- Optical properties
Fig. 10 demonstrates the photoluminescence (PL) spectra of the prepared GaN under different laser ablation energies. The energy of the incident photon ( ) as a function of the wavelength (λ) was calculated using equation (1)
The first GaN nanoparticle prepared for 1000 mJ presents an energy gap of 3.38 eV at the wavelength 366nm. The second sample prepared for 1200mj exhibits an energy gap of 3.41 eV at the wavelength 363nm. The energy gap increases with increasing the ablation energy, and the wavelength will have a blue shift. The third sample prepared for 1400 mJ presents an energy gap of 3.83 eV at the wavelength 323 nm; this sample has the highest energy gap due to the small piratical size and the blue shift. The fourth sample prepared for 1600 mJ exhibits an energy gap of 3.71 eV at 334 nm. The energy gap began to decrease with increasing the ablation energy that will ablate large piratical size, and the wavelength will have redshift. The fifth sample prepared for 1800 mJ exhibit an energy gap of 3.54 eV at the wavelength 350 nm; the wavelength will have a redshift, and the energy gap decrease. The sixth sample prepared for 2000mj present the smallest energy gap of 3.34 eV at the wavelength 371nm; the wavelength will have redshift, the photoluminescence (PL) investigation proved the AFM result.
Furthermore, It can be seen that increasing the ablation energy will remove the larger particle size, and therefore, the wavelength will have a red shift, So decreasing the ablation energy will make a blue shift in the wavelength and increase the energy gap until 1400mj that has the highest peak of power then at 1200 and 1000 mJ the energy gap decrease and wavelength will have redshift due to the core-shell phenomenon.