Annealing effect on the physical and antibacterial properties of vacuum thermal evaporated Cu-doped ZnO thin films
D.Dergham1✉Emailddergham@cdta.dzEmailddergham1@gmail.com
S.Hassani1
F.Lekoui1
A.Boucheham2
J.Guitoum3
T.Bezzi4
L.Baghriche5
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Center for development of advanced technologies: Ionized Media & Laser Division AlgiersAlgeria2Research Center in Semiconductor Technology for EnergeticsAlgiersAlgeria
3Research Unit in Optics and Photonics (UROP-CDTA)El BezSetifAlgeria
4National Center for Biotechnology ResearchAli Mendjli Nouvelle VilleUV03 Bp E73ConstantineAlgeria
5National High School of Hydraulics29 road of SoumaâBlidaAlgeria
D.Dergham1, S.Hassani1, F.Lekoui1, A.Boucheham2, J.Guitoum3, T. Bezzi4, L.Baghriche5 1Center for development of advanced technologies: Ionized Media & Laser Division Algiers, Algeria
2 Research Center in Semiconductor Technology for Energetics, Algiers, Algeria
3Research Unit in Optics and Photonics (UROP-CDTA), El Bez, Setif Algeria
4National Center for Biotechnology Research, Ali Mendjli Nouvelle Ville, UV03 Bp E73, Constantine, Algeria
5 National High School of Hydraulics, 29 road of Soumaâ, Blida Algeria
ddergham@cdta.dz
ddergham1@gmail.com
Abstract
Cu-doped ZnO thin films were successfully deposited on glass substrates using vacuum thermal evaporation, employing a mixture of ZnO powder and copper grains as source materials. Following deposition, the films were annealed at temperatures of 300°C, 400°C, and 500°C to investigate the influence of thermal treatment on their structural, morphological, optical, electrical, and antibacterial properties.
Comprehensive characterization was carried out using X-ray diffraction (XRD) to determine crystal structure, atomic force microscopy (AFM) for surface morphology, UV-Vis spectroscopy and photoluminescence for optical properties, a four-point probe for electrical resistivity, and antibacterial assays to evaluate biological activity. Grain size was estimated using Scherer’s formula based on XRD data.
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XRD analysis confirmed that all films exhibit a polycrystalline hexagonal Wurtzite structure. Optical measurements revealed that film transmittance increased with annealing temperature up to 400°C, followed by a decrease at 500°C. A gradual decrease in optical band gap energy and an increase in root mean square roughness were observed with higher annealing temperatures. Additionally, grain size was found to increase with increasing annealing temperature. Antibacterial testing demonstrated effective inhibition of Escherichia coli growth on the surface of the Cu-doped ZnO films, confirming their potential for antimicrobial applications.Key Words:
Vacuum thermal evaporation
ZnO-Cu
annealing effect
Antibacterial properties
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2. Introduction
Zinc oxide (ZnO) thin films have attracted significant attention in recent years due to their importance as semiconductor materials in a wide range of applications, including optoelectronics, photovoltaics, gas sensors [1], solar cells, and the biomedical field [2].
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ZnO thin films can be deposited using various techniques, such as RF magnetron sputtering [3], chemical vapor deposition [4], spray pyrolysis [5], sol-gel method [6], and vacuum thermal evaporation [7]. Among these, vacuum thermal evaporation stands out for its simplicity and low cost. To enhance the electrical and optical properties of ZnO thin films, different elements have been introduced, including aluminum (Al) [8, 9], gallium (Ga) [10], manganese (Mn) [11], silver (Ag) [12], and rare earth elements such as europium (Eu) [13, 14] and erbium (Er) [15].Recently, doping ZnO with copper (Cu) has garnered growing interest, particularly at the nanoscale, due to Cu's potential to significantly alter the properties of ZnO [16]. However, there is still limited research on how annealing affects the antibacterial properties of Cu-doped ZnO thin films.
The objective of this study is to investigate the effect of annealing temperature on the structural, electrical, optical, and antibacterial properties of ZnO-Cu thin films. The films were deposited on glass substrates using vacuum thermal evaporation and subsequently annealed at 300°C, 400°C, and 500°C for one hour in ambient air.
3. Experimental procedure
ZnO–Cu thin films were deposited on glass substrates via vacuum thermal evaporation. The precursor mixture for Cu-doped ZnO consisted of 100 mg of zinc oxide (ZnO) powder and 25 mg of copper (Cu) grains. Prior to deposition, the glass substrates were thoroughly cleaned with deionized water, followed by ultrasonic cleaning in acetone. The substrates were then dried and placed inside the vacuum deposition chamber. Simultaneously, the ZnO powder and Cu grains were mixed and loaded into a tungsten crucible, which was also placed inside the chamber.
The vacuum chamber was evacuated to a base pressure of 10⁻⁵ mbar using a diffusion pump. A variac was used to supply current to the tungsten crucible, facilitating the co-evaporation of ZnO and Cu materials. After deposition, the thin films were annealed in a furnace at three different temperatures 300°C, 400°C, and 500°C for one hour in ambient atmosphere.
To evaluate the properties of the deposited films, a series of characterization techniques were used, surface morphology was analyzed using Atomic Force Microscopy (AFM; Nanosurf Flex-AFM system), structural properties were studied via X-ray Diffraction (XRD; Philips X’Pert MPD) using Cu-Kα radiation (λ = 1.54 Å). The average crystallite size was estimated using the Scherer’s formula [17], based on the full width at half maximum (FWHM) of the XRD peaks, optical properties were examined using a UV–Visible spectrophotometer (OPTI ZEN 3220 UV) over the spectral range of 200–1100 nm, electrical resistivity of the films was measured using the four-point probe method, the antibacterial properties of the films were evaluated using the drop test method, with Escherichia coli (E. coli ATCC 25922) employed as a model Gram-negative bacterium commonly found in clinical environments. Before beginning the test, all glassware and sample substrates were sterilized in an autoclave at 120°C for 15 minutes. The samples were then placed in sterilized Petri dishes, and 100 µL of bacterial suspension (~ 10⁵ CFU/mL) was dropped onto the surface of each sample.
The samples were subsequently exposed to UV light (8 W, 365 nm; Philips) for 60 minutes at 37°C. After exposure, the surfaces were rinsed with 5 mL of sterile physiological saline to remove any remaining bacteria. The rinse solution was then plated onto nutrient agar, and 50 µL aliquots of each diluted solution were spread on the agar surface.
The agar plates were incubated at 37°C for 24 hours, after which the number of surviving bacterial colonies was counted. Each experiment was repeated three times, and the reported values represent the average of three measurements per trial.
4. Results and discussion
4.1. Surface Morphology
Figure 1 (A–D) presents atomic force microscopy (AFM) images of Cu-doped ZnO thin films deposited by vacuum thermal evaporation and annealed at various temperatures.
All images were acquired in contact mode over a scanning area of 5 µm × 5 µm. The films exhibit vertically aligned columnar structures perpendicular to the substrate surface, indicating a preferred c-axis orientation, consistent with previously reported findings. The surface morphology appears dense and uniform, further confirming this preferential orientation.
The root mean square (RMS) roughness values for the unannealed film and the films annealed at 300°C, 400°C, and 500°C are 12.3 nm, 18.45 nm, 22.5 nm, and 22.7 nm, respectively.
Figure 2 illustrates the variation in RMS roughness as a function of annealing temperature. A
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progressive increase in surface roughness is observed with rising annealing temperatures, which is attributed to enhanced grain growth and improved crystalline quality [18]. Lekoui et al. [12] linked this increase to the formation and agglomeration of ZnO nanocrystallites. According to Yang [19], this behavior is driven by the oxidation of thermally activated particles exhibiting high lattice.mobility, which agglomerate and stabilize into larger grains. These findings are further supported by the grain size calculations presented in Table 1.
Fig. 1
AFM images of ZnO-Cu A) unannealed, B, C, D) annealed at
Fig. 2
RMS variation versus annealing temperature
4.2. Structural properties
Figure 3 displays the X-ray diffraction (XRD) patterns of ZnO-Cu thin films. According to the ICSD database (card No. 065121), all samples exhibit a well-defined polycrystalline Wurtzite structure. Notably, the XRD pattern of the unannealed sample shows no discernible diffraction peaks, indicating its amorphous nature. Similar observations were reported by A. Zaier et al. [20], who found that ZnO layers deposited via thermal evaporation remained amorphous and began to crystallize upon annealing at 300°C.
Thermal annealing of the deposited layers promotes recrystallization and leads to the formation of a polycrystalline microstructure.
All annealed films exhibit a strong preferential orientation along the (002) crystallographic plane at 2θ = 34.49°, indicating that the ZnO-Cu thin films are polycrystalline with a hexagonal Wurtzite structure and display a pronounced c-axis orientation. When the samples are annealed at 300°C, two diffraction peaks appear at 34.49° and 36.53°, corresponding to the (002) and (101) orientations, respectively. As the annealing temperature increases to 400°C and 500°C, these peaks become more intense and shift toward higher angles, indicating an improvement in the crystallinity of the layers with increasing annealing temperature.
Several factors may contribute to this enhancement in crystallinity during thermal annealing, including the reduction of defects and the associated internal stress, the relaxation of external stress resulting from lattice mismatch between the substrate and the film, and grain growth facilitated by the coalescence process [21, 22]. The shift of X-ray diffraction (XRD) peaks toward higher angles can be attributed to residual stresses within the films. These stresses, which tend to shift the diffraction peaks to higher angles, can be reduced through annealing [23]. In a similar study by Ali Rahmati et al. [24], the observed peak shifts were attributed to the substitution of Zn by Cu in the ZnO wurtzite structure. Moreover, additional ZnO diffraction peaks at 31.82°, 47.36°, and 56.80° were observed in the films annealed at 400°C and 500°C. Notably, the peak at 31.82° shifted to 31.55°, and a new peak appeared at 62.63° after annealing at 500°C. It is also noteworthy that no copper oxide (CuO) phase was detected in the XRD patterns, suggesting that copper atoms are incorporated into the ZnO hexagonal lattice by
substituting zinc atoms.
Fig. 3
XRD spectra of ZnO-Cu unannealed and annealed at 300°C, 400°C, 500°C
The crystallite size was estimated using the Scherrer equation, based on the full width at half maximum (FWHM) of the principal diffraction peak [25]:
D=
Where λ, β, θ are the X-Ray wavelength (1.54 A°), the full width at half maximum (FWHM) and the Bragg diffraction angle.
As shown in Fig. 4, the crystallite size of ZnO–Cu increases with annealing temperature: it is approximately 11.4 nm at 300°C, 13.0 nm at 400°C, and 13.5 nm at 500°C.
This trend indicates that higher annealing temperatures promote crystal growth, which can be attributed to enhanced recrystallization of ZnO nanoparticles.
Additionally, the incorporation of Cu dopant atoms into the ZnO lattice causes anisotropic distortion—expanding the lattice parameters in the a and b directions, while slightly contracting the c axis. This distortion reduces the FWHM of the diffraction peaks, thereby contributing to an apparent increase in crystallite size.
These observations are consistent with previous reports. T. Ivanova et al. [26] found that crystallite size increases with annealing temperature, and T. Saidani et al. [18] similarly reported a linear increase in grain size, attributing it to enhanced atomic mobility and crystallite coalescence at elevated temperatures.
To gain further insight into the microstructure of ZnO-Cu films, the lattice parameter c was calculated from X-ray diffraction (XRD) data using Bragg’s law:
……………………………….1
The crystalline plane distance was calculated
………………………………………2
For hexagonal crystal systems, the lattice parameters are related to the interplanar spacing by:
………………….……3
The lattice parameter c corresponding to the (002) diffraction peak can also be calculated directly from Equations (2) and (3). For the (002) orientation (i.e., h = k = 0, l = 2), the c parameter simplifies to:
Where c is the lattice constant calculated for each sample. The calculated c values for the ZnO-Cu films annealed at 300°C, 400°C, and 500°C are 0.51939 nm, 0.52005 nm, and 0.51860 nm, respectively.
A slight decrease in c values is observed with increasing annealing temperature, suggesting lattice deformation along the c-axis. Such deformations are typically attributed to internal stresses arising from the mismatch in thermal expansion coefficients between the substrate and the thin film.
The minor variation in lattice parameters among the Cu-doped ZnO films may be due to the substitution of Zn atoms by Cu atoms at different lattice sites and the formation of complex defects such as [CuZn Zni]x in Cu-ZnO [27].
It is well known that strain has a significant impact on the microstructure and properties of ZnO films. Therefore, analyzing the strain in ZnO:Cu films is essential. The strain along the c-axis
was calculated using the expression provided by Chen et al. [28]:
……………..………..………….5
Where c0 is the lattice parameter of ZnO without deformation (c0 = 0.5205 nm), the results indicate compressive stresses with values of -0.002132, -0.000864, and − 0.00365, for layers annealed at 300°C, 400°C and 500°C respectively.
According to Hoffman’s relation [29], for hexagonal crystals, the stress in the plane of the film can be calculated using the biaxial strain model.
……………………6
Where
= 209.7 GPa,
= 121.1 GPa,
= 105.1 GPa, and
= 210.9 GPa are constants of the elastic modulus of ZnO [7], the stress values of the films at different annealing temperatures are listed in Table.1. The obtained results exhibit a positive values, indicating that the deposited films are under tensile type stresses.
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Annealing temperature | Theta (°) | FWHM | Grain size | c(nm) | εz | σfilms(GPa) |
|---|---|---|---|---|---|---|
300°C | 17.29 | 0.7298 | 11.4 | 0.51939 | -0.002132 | 0,967 |
400°C | 17.21 | 0.6401 | 13 | 0.52005 | -0.000864 | 0,392 |
500°C | 17.27 | 0.6163 | 13.5 | 0.51860 | -0.00365 | 1,655 |
Fig. 4
Variation of grain size as a function of annealing temperature
4.3. Optical properties
Optical analysis of the ZnO–Cu thin films deposited on glass substrates was performed using transmittance (T %) and photoluminescence measurements.
Figure 4 shows the variation of optical transmittance spectra as a function of annealing temperature.
In the UV region, the films exhibit low transmittance. However, annealing the samples at 300°C, 400°C, and 500°C leads to an increase in transmittance, reaching a maximum at 400°C. This improvement in transmittance can be attributed to the reaction of zinc with oxygen during the annealing process, which promotes the formation of the ZnO phase. Additionally, the enhancement in crystallinity due to annealing also contributes to the increased transmittance.
Sheng Xu et al. [30] reported a similar trend, attributing the transmittance improvement with increasing annealing temperature to enhanced crystallinity of ZnO films under thermal treatment. Zein K. [31] observed the same behavior, explaining the increase in transmittance as a result of defect reduction at higher annealing temperatures. Since structural defects act as light-scattering centers, their reduction leads to improved optical transparency. Nevertheless, at 500°C, a slight decrease in transmittance is observed, likely due to defect formation or grain growth saturation at elevated temperatures.
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Fig. 5
Optical transmittance spectra of ZnO-Cu thin films unannealed
and annealed at 300°C, 400°C, 500°C
3.4 Photoluminescence analysis
Figure 6 shows the photoluminescence (PL) spectra of ZnO-Cu thin films annealed at various temperatures, excited using a 325 nm wavelength. For the non-annealed samples, a broad emission peak is observed ranging from 380 nm (3.26 eV) to 409 nm (3.03 eV). The broadness of this peak decreases with increasing annealing temperature up to 300°C. At higher annealing temperatures of 400°C and 500°C, the peak becomes more intense and shifts towards shorter wavelengths.
The ultraviolet (UV) emission originates from the radiative recombination of free excitons, corresponding to the near-band-edge (NBE) emission of ZnO. This NBE emission is associated with ZnO films having fewer crystal defects, whereas the deep-level emission arises from intrinsic defects within the ZnO lattice [32]. Similar peak positions have been reported by Yong Liu et al. [33].
The observed blue shift of the emission peak at higher annealing temperatures can be attributed to the increased concentration of free electrons generated by the substitution of Zn ions with Cu ions. These free electrons occupy energy levels near the bottom of the conduction band. Upon excitation, excitons occupy higher energy states within the conduction band, and their radiative recombination results in the UV emission peak shifting towards shorter wavelengths.
Additionally, a broad emission peak centered at 534 nm (2.32 eV) is observed, which is attributed to impurities and defects in the crystal structure. This defect-related emission is believed to arise from oxygen vacancies, Cu impurities, and Zn interstitials. J. Arul Mary et al. [34] reported a similar emission peak at 537 nm, attributing it to defect levels associated with oxygen vacancies or zinc interstitials. A.A. Othman et al. [35] also observed this peak and ascribed it to the recombination of conduction band electrons with holes trapped at oxygen vacancy sites (VO).
Fig. 6
Photoluminescence spectra of ZnO-Cu thin films unannealed
and annealed at 300°C, 400°C and 500°C
3.5.
Optical band gap
The optical band gap of the Cu doped ZnO thin films unannealed and annealed at different temperatures was estimated by employing the Tauc equation [36]
Where
a constant, hν is the photon energy and Eg is the optical band gap.
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The optical band gap was determined by extrapolating the linear portion of the on the axis abscissa (
) Fig. 7, and it’s variation as a function of photon energy (
is illustrated in Fig. 8. As observed, the optical band gap of the unannealed films is 3.86 eV, which is higher than the commonly reported band gap of undoped ZnO (3.36 eV). A similar result was reported by H. Abdullah et al. [36].
In general, an increase in carrier concentration leads to a widening of the band gap due to the filling of the lower-energy states in the conduction band — a phenomenon known as the Burstein–Moss effect [37]. Similar observations were made by Laya Dejam et al. [38] in ZnO films doped with Al and co-doped with Al–Cu.
Annealing ZnO-Cu samples at 300°C, 400°C and 500°C leads to a decrease in the band gap, it passes to 3.78 eV, 3.48 eV and 3.45 eV respectively, this reduction of the gap with the annealing temperature is due to the growth of the grain size.
Fig. 7
Eg variation of ZnO-Cu Versus Annealing temperature
Fig. 7
Variation of band gap of Cu-doped ZnO samples as a function of annealing temperature
3.6. Antibacterial properties
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The antibacterial properties of ZnO-Cu samples—both unannealed and annealed at various temperatures—against the Gram-negative bacterium Escherichia coli are presented in Fig. 9.Bacterial colonies are clearly visible on the agar plates corresponding to the unannealed samples, as well as those annealed at 300°C and 400°C. However, with an increase in annealing temperature to 500°C, complete bacterial inhibition is observed. This suggests that higher annealing temperatures enhance the antibacterial activity of the ZnO-Cu samples, likely due to multiple contributing factors. The antibacterial effect can be attributed to the presence of Zn²⁺, Cu, Cu⁺, and Cu²⁺ ions, which are known cytotoxins to microorganisms. These ions are released as a result of the dissociation of ZnO and ZnO-Cu structures. Similar findings were reported by Iman A. et al. [39]. Furthermore, the antibacterial mechanisms of ZnO nanoparticles have been described in the literature [40] through the following processes:
1.
Direct interaction of ZnO nanoparticles with bacterial cells, leading to penetration of the cell membrane and potential disruption of the cell wall.
2.
Release of Zn²⁺ ions, which can inhibit active transport processes and interfere with enzyme systems associated with the bacterial cell wall.
3.
Generation of reactive oxygen species (ROS) on the ZnO surface, inducing oxidative stress that ultimately leads to bacterial cell death.
Conclusion
In this study, Cu-doped ZnO thin films were successfully deposited on glass and silicon substrates using the vacuum thermal evaporation method. The influence of annealing temperature on the structural, morphological, and optical properties of the films was systematically investigated.
Atomic Force Microscopy (AFM) analysis revealed that the surface roughness of the as-deposited films was approximately 12.3 nm. Upon annealing, the roughness increased, reaching 22.5 nm and 22.7 nm for samples annealed at 400°C and 500°C, respectively.
X-ray diffraction (XRD) analysis confirmed the polycrystalline nature of all films, with an improvement in crystallinity observed as the annealing temperature increased. The grain size increased from 11 nm in the unannealed samples to approximately 13–13.5 nm in the samples annealed at 400°C and 500°C.
Optical measurements showed that the transmittance of the films increased upon annealing at 400°C, but subsequently decreased at 500°C. Additionally, the optical band gap decreased with increasing annealing temperature.
Antibacterial tests demonstrated a significant reduction in bacterial colonies on the surface of the films, confirming their antibacterial activity.
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