Influence of Gamma-irradiation on the Structural, Morphological, and Optical Properties of β-H₂Pc Nanocrystalline Films: Implications for Optoelectronic Applications

Beta Metal-free Phthalocyanine (β-H₂Pc) powder was obtained from Eastman Kodak, USA, and used as received. Thin films of β-H₂Pc were deposited onto quartz and glass substrates using the thermal evaporation method (Edwards Co., model E306-A, England) under high vacuum conditions (10⁻⁴ Pa) for optical and structural measurements, respectively. The β-H₂Pc powder was heated slowly in a molybdenum boat to deposit the thin films, while the substrate temperature was kept at room temperature. The film thickness (152 nm) and deposition rate (2.5 Å/s) were controlled using a quartz crystal thickness monitor (Edwards Co., model FTM6, England) attached to the work chamber.

The β-H₂Pc powder and thin films were exposed to γ-rays using a ⁶⁰Co irradiator chamber (Indian cell GC 4000 Å) at room temperature. The samples were placed in a drawer and irradiated with a dose rate of 10 kGy/100 min, with total doses ranging from 20 kGy to 100 kGy. The thin films were exposed for varying durations to achieve a series of different integrated absorbed doses. The pristine β-H₂Pc thin film was irradiated at the National Center for Radiation Research and Technology (NCRRT) in Cairo, Egypt.

The obtained x-ray diffraction tracings (XRD) were used to study the structural properties. A Shimadzu x-ray diffractometer (XRD 6000) with a monochromatic CuK_α radiation target and Ni filter (λ = 0.15406 nm) was used for the characterizations. The x-ray tube was operated at a voltage of 40 kV and a current of 30 mA. The diffraction tracings were recorded at a scanning speed of 8°/min in the normal 2θ range of 4° to 90°. The morphology of the samples was examined using the scanning electron microscope (JEOL JSM-636 OLA, Japan) at potential 15 kV and 20 kV. The particle size distribution was obtained using the image-J software program.

The Fourier transform infrared (FTIR) spectrum of β-H₂Pc has been obtained on a Bruker (Alpha II) spectrophotometer equipped with a Platinum diamond ATR module. The spectral resolution was 2 cm⁻¹. The spectrum was recorded in the 400⁻¹–4000 cm⁻¹ region at room temperature. The photoluminescence (PL) spectra of the samples were obtained using a Cary Eclipse fluorescence spectrophotometer. The spectra were recorded in the wavelength range of 300 nm–900 nm, and the scan rate was 600 nm/min.

To measure the reflectance, absorbance, and transmittance of the β-H₂Pc thin films, a double-beam spectrophotometer (JASCO Co., model V-670, Japan) was used. The spectral range used for the measurements was 190 nm–2500 nm. The absorption and transmission scans were carried out with the films placed at a normal incidence of light and a blank quartz substrate as a reference. The reflection scan was conducted at an incident angle of 5° using an Al-mirror as a reference.

The preferred orientation of β-H₂Pc thin films was evaluated by the texture coefficient (TC) parameter according to the following expression¹⁸:where \({I}_{0(hkl)}\), and \({I}_{(hkl)}\) denote the standard and the measured relative intensities of the individual crystal planes (hkl), and N represents the number of the most intens peaks. The standard peak intensity for the reflection (hkl) was taken from the JCPDS card No. 37-1844.

The mean crystallite size (D) of β-H₂Pc thin films was estimated to check the nanostructure using Debye–Scherrer’s formula. Also, other microstructural parameters such as the microstrain (ε), the average dislocation density (δ), and the number of crystallites per unit surface area (χ) were obtained from the full width at half maximum of highly intense peak (β) in radian using the following relations¹⁷:where λ is the x-ray wavelength, θ is the diffraction angle, and d is the film thickness.

The absolute values of transmittance, T(λ), and reflectance, R(λ), are taken from the spectrophotometer and were corrected for canceling the absorbance and reflectance of the quartz substrate using a computer software package based on the subsequent relations¹⁹:where I_ft and I_q are the intensities of light passing through the thin film-quartz system and the reference quartz, respectively, and R_q is the reflectance of the quartz substrate.where I_fr and I_m are the intensities of the reflected light from the sample and the Al-mirror reference, respectively, and R_m is the Al-mirror reflectance.

The absorption coefficient, α, the absorption index, k, and the refractive index, n, of the thin films can be estimated from the absolute values of T(λ) and R(λ) using a computer program according to the following equation²⁰

The value of energy gap E_g and electronic transition type can be determined using the following formula²¹:where B is a constant and s is an exponent used for identifying the transition.

The main Dispersion parameters are completely described through the Wemple and DiDomenico approximation in terms of a single oscillator model according to the following expression²²:where, E_d, E_o and E are the dispersion energy, the oscillator energy, and the photon energy, respectively.

The refractive index can be expressed as a function of the wavelength by the following Eq. ²³where ε_L is the high-frequency lattice dielectric constant, N/m^* is the ratio of the free carrier concentration to the effective mass, e is the elementary electronic charge, ε_o is the permittivity of free space, and c is the speed of light.

The imaginary ε₂ and real ε₁ parts of the complex dielectric constant are determined using the following relations²⁴:

The dielectric loss tangent or the dissipation factor tan(δ) is evaluated by the following expression²⁵:

Both the volume energy loss function (VELF) and surface energy loss function (SELF) is given by the following Eq. ²⁶:

The real part σ₁ and the imaginary part σ₂ of the complex optical conductivity are given by²⁷:where ω is the angular frequency.

Miller developed a simple relation that relates the third-order nonlinear optical susceptibility χ⁽³⁾ to the static refractive index n₀ through the following relation²⁸:

Also, the nonlinear refractive index (n₂) is evaluated according to Miller’s rule by using the following formula²⁹:

The XRD pattern of β-H₂Pc in powder form was studied in Fig. 1a to investigate the effect of γ-irradiation with different dosages. The picture showed a polycrystalline nature with different preferred orientations. A hump was observed in the pattern at higher γ-irradiation doses. This hump could be attributed to the formation of defects or imperfections in the crystal structure of the powder.³⁰ These defects could be due to the creation of lattice defects and atomic vacancies that can alter the crystal structure of the powder upon gamma irradiation. The defects generated in the β-H₂Pc powder may cause changes in its electronic and optical properties, which can have significant consequences for its applications.

The XRD pattern of β-H₂Pc film subjected to γ-irradiation with different doses is presented in Fig. 1b The film exhibited a preferential orientation in the (001) direction with a hump of amorphism, indicating that it belongs to a nano-sized structure. The presence of the diffraction hump confirms the amorphous nature of the film. The position of the (001) peak was not affected by changing the γ-irradiation doses, but its intensity decreased with increasing gamma dose. While the hump of amorphism increased. The decrease in the intensity of the (001) peak with increasing gamma dose can be attributed to the radiation-induced disorder in the crystal lattice of the film. The gamma radiation can generate a large number of free radicals and ion pairs in the film, which may cause bond breakage and the formation of defects in the crystal lattice. These defects can act as scattering centers, which can reduce the intensity of the diffraction peak.³¹

The texture coefficient (TC) of plane which is the texture of a certain plane, whose deviation from the standard sample involves the preferred growth was estimated from Eq. 1. The calculated values of the TCs of β-H₂Pc film as a function of γ-irradiation doses of the diffraction peak (001) are displayed in Fig. 4b. It is clearly seen that the TC value for (001) plane decreases with increasing the irradiation doses. The lower values of texture coefficient reveals that the films have poor crystallinity.³² Moreover, the gamma radiation can also induce grain boundary formation, which can result in a decrease in the average grain size of the crystallites. The decrease in the grain size can further reduce the intensity of the diffraction peak and increase the hump of amorphism.

In summary, the XRD patterns of both β-H₂Pc powder and the film showed gamma irradiation-induced structural changes in the samples. These changes may result from the radiation-induced disorder in the crystal lattice of the samples, the formation of defects, and the reduction in the average grain size. These findings are consistent with the results reported in previous studies on the effect of gamma irradiation on other organic materials.

Figure 2 illustrates the impact of γ-irradiation with various doses on the microstructural parameters of β-H₂Pc films. The results show that the mean crystallite size of the films increased with the increasing dose of gamma radiation. In contrast, the microstrain, the average dislocation density, and the number of crystallites per unit surface area decreased as the γ-irradiation doses increased. This behavior can be attributed to the merging of defects during irradiation, leading to the formation of larger grains. The reduction in the microstrain and the average dislocation density suggests a decrease in the lattice defects, which could be due to the enhancement and increment in the crystallinity of the films. Therefore, γ-irradiation of β-H₂Pc films resulted in significant changes in their microstructure.

Scanning electron microscopy (SEM) was used to study the surface morphology of pristine and γ-irradiated β-H₂Pc at different doses, as shown in Fig. 3. The former displays a two-dimensional surface morphology image, while the latter shows the histogram of the average particle size distribution over the entire surface of β-H₂Pc irradiated with 60 kGy (as a representative example). The data indicates that the average particle size increases with increasing γ-irradiation doses. This behavior could be attributed to the coalescence of smaller grains to form larger ones.

The mean grain size was calculated and plotted as shown in Fig. 4a to confirm the effect of γ-irradiation doses on β-H₂Pc. The graph clearly shows an increase in the grain size as the doses increase, except for a significant drop at 100 kGy. This could be attributed to the saturation of grain growth due to the limited number of β-H₂Pc molecules available for crystal growth. Additionally, at high doses, the radiation-induced defects can promote amorphization, which leads to a decrease in grain size. This is consistent with previous studies that have reported a decrease in grain size at high radiation doses due to the increased amorphization caused by the collision cascades of high-energy particles.

XRD and SEM analyses showed consistent results, with both techniques revealing an increase in grain size and a decrease in microstrain and lattice defects with increasing doses of gamma radiation. SEM images also confirmed an increase in the average particle size of the β-H₂Pc films with increasing radiation doses.

These findings suggest that gamma radiation can be used to tailor the microstructure and surface morphology of β-H₂Pc films for potential applications in various fields such as optoelectronics, photovoltaics, sensors, and catalysis. For example, changes in the crystallinity and grain size of the β-H₂Pc films can affect their charge transport properties, making them suitable for use in electronic devices such as organic field-effect transistors. Similarly, modifications in the surface morphology can enhance the sensitivity of β-H₂Pc films to chemical or biological species, making them useful for sensing applications. The use of gamma radiation to tailor the microstructure and surface morphology of β-H₂Pc films can therefore open up new avenues for the development of advanced functional materials.

FTIR spectra were utilized in the study to evaluate the chemical stability of β-H₂Pc after exposure to γ-irradiation. The spectra for both pristine and irradiated β-H₂Pc powders were obtained (Fig. 5). The results indicate that while γ-irradiation has a significant impact on the structure and surface morphology of β-H₂Pc, the molecular structure of β-H₂Pc remained stable after exposure to γ-irradiation except for the disappearance of the hydroxyl (OH) functional group peak. The presence of the hydroxyl group O-H in the pristine sample may indicate water absorption from the surrounding atmosphere. However, γ-irradiation was observed to have a positive impact on the material by removing the hydroxyl group without affecting the main functional groups of β-H₂Pc. Furthermore, the FTIR spectra of the pristine and irradiated powders were quite similar with insignificant differences in band intensities, indicating the molecular stability of β-H₂Pc. These findings suggest that β-H₂Pc has good radiation resistance and can be utilized in various applications, particularly in medical applications such as cancer treatment.

To evaluate the suitability of β-H₂Pc for optoelectronic applications, it is important to study its photoluminescence (PL) characteristics. Figure 6a displays the emission spectrum of both pristine and irradiated β-H₂Pc at room temperature (300 K). The emission spectrum was generated using a 300 nm excitation source. It exhibits a strong and broad band of emission in the visible region, corresponding to orange light. The peak of the emission was observed at around 602.02 nm. This high broadband is the result of the direct electronic transition from the first excited state to the ground state accompanied by phonon energy within the inter-band. This phenomenon is responsible for the observed band in the PL spectrum.

Moreover, Fig. 6b reveals the proposed energy level diagram of β-H₂Pc. This diagram depicts the energy gap, the electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the orange emission peak correspond to the electron transitions from the LUMO levels to levels higher than the top of the HOMO levels.

The intensity and broadening of the PL spectra are dependent on the γ-irradiation doses, but all peaks of emission are nearly centered at the same wavelength. As the γ-irradiation doses increase, the intensity of the emission band decreases, indicating the generation of defects in the substance. This is because γ-radiation can create defects in the crystal lattice, leading to a decrease in the PL intensity. This observation is consistent with the results reported in previous studies.³³

Organic materials with high nonlinear optical susceptibility are highly desirable in the development of advanced optoelectronic devices. The nonlinear optical refractive index provides valuable insight into the interaction of light with these materials. The ability of a material to generate a nonlinear optical response, particularly at the third harmonic of the incident light, can be quantified by its nonlinear third-order optical susceptibility χ⁽³⁾ and nonlinear optical refractive index n₂. Both parameters can be evaluated using Eqs. 21 and 22. The obtained values for these parameters for both pristine and irradiated β-H₂Pc films are presented in Table I, along with corresponding phthalocyanine compounds. Observations reveal that β-H₂Pc films exhibit significantly higher χ⁽³⁾ and n₂ values when compared with values reported for other materials in the literature. This indicates that β-H₂Pc films have great potential for various nonlinear optical applications, making them a promising candidate for optoelectronic devices such as solar cells and Organic Light Emitting Diodes (OLEDs). The higher χ⁽³⁾ and n₂ values of β-H₂Pc films may be attributed to the presence of defect centers induced by gamma irradiation, which enhances local polarizabilities and leads to an increase in nonlinear optical response.

The spectral distribution of χ⁽³⁾ and n₂ versus photon energy for both pristine and γ-irradiated β-H₂Pc films is shown in Fig. 15a and b, which displays similar behavior with three peaks and an irradiation dependency. The presence of peaks in the nonlinear optical susceptibility at specific photon energies indicates the existence of distinct physical mechanisms contributing to the overall nonlinear response of the material. In the case of β-H₂Pc films, the three peaks at 0.75 eV, 1.60 eV, and 2 eV are attributed to different physical mechanisms. The peak at 0.75 eV is associated with the one-photon resonant electronic transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The peak at 1.60 eV could be attributed to the two-photon resonant electronic transitions. The peak at 2 eV could be explained by the combination of one-photon and two-photon electronic transitions with a significant contribution from the two-photon absorption process. The presence of multiple peaks in the nonlinear optical susceptibility provides important information about the underlying physical mechanisms contributing to the nonlinear response of the material. By understanding the physical mechanisms involved, researchers can tailor the material's properties to optimize its performance for specific applications.

Additionally, the results illustrated in Fig. 15c reveal that the values of χ⁽³⁾ and n₂ increase proportionally with the gamma irradiation dose. The enhancement in these values can be attributed to the formation of defect centers within the material due to external gamma irradiation. These defect centers can significantly increase the local polarizabilities of the material, which results in a larger nonlinear optical response. As a result, β-H₂Pc films are highly suitable for various nonlinear optical applications. The significance of these effects can be observed in various fields such as telecommunications, materials science, and bio-photonics.