3.1. Scanning Electron Microscopy (SEM)
SEM images were made with the sem phenom pro X equipment. The Fig. 1 shows SEM images of (a) MZN-1 and (b) MZN-5 thin films. In the cross section in the thin solid film shown in Fig. 1(a), fibers of different lengths and crystal flakes-like fibers create layers of crystalline conglomerates overlapping each other and regions with non-uniformly distributed voids are appreciated. However, in the samples Fig. 1(b), a compact and uniform surface can be seen presenting crystalline flakes-like limited by very thin crystalline lines. The surface morphology of materials undergoes modification by the effect of thermal treatment and is governed by the desorption of some contaminants adhered to the surface and the volume of the sample. Generally, a drastic dependence on the experimental conditions related to the crystalline growth parameters used is associated. It is evident that the calcination temperature is a key and necessary parameter to understand in greater depth the conditions to find the optimal experimental conditions in the morphological properties when applying the present technique of chemical synthesis. The calcination temperature increases the surface energy in the layers and also tends to become higher which caused smaller particles to fuse with the neighboring particles thus, forming larger crystallite size [17].
3.2. Difracción de Rayos-X (XRD)
The XRD characterization technique was applied in this report to examine the structural properties of the compounds. Figure 2 shows the XRD diffractograms of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 layers. We perform the angular sweep (2θ) located at range 2θ ~ 20°-70°. The crystallographic planes of reflection examined are assigned at (002) and (103) crystalline planes, which correspond to the Wurtzite crystalline phase, according to the standards (JCPDS36-1451). The recorded planes present full width at half maximum (FWHM) difference, a structural phenomenon commonly associated with the creation of nanocrystals with small grain size (GS) dimensions, as well as amorphous regions [18]. The crystal plane oriented at the preferential direction (002) is associated with the surface energy density of greater minimum relative crystal energy stability. Each Zn2+ cation layer is driven to grow along the exposed closest packed plane (002), leading to compact and planar ZnO-QDs deposition [19]. Therefore, the FWHM shown in the crystallographic planes is associated with an increase in the calcination temperature. The microstructural behavior can be tentatively justified and associated with the stages involved in the nucleation phenomenon and crystalline growth. Although the nucleation phenomenon is complex, interest should not be lost in finding the correlation between chemical and physical parameters that formally govern the morphological phenomenon. The crystals precipitate on the pores' surface, modify the pore space morphology, and reduce their flow and transport properties [20]. However, the phenomenon of nucleation followed by growth and, in turn, the creation of fine nanoparticles with different GS averages, governed by the calcination temperature [21]. The microstructural behavior is directly related to the average grain size (GS) calculated using the Scherer’s equation [22]
$$D=\frac{k\lambda }{\beta Cosϴ}$$
1
Were λ is the wavelength of X-ray radiation (~ 0.154 Å), k the Scherer’s constant (k ~ 0.9), θ (in radians) the characteristic X-ray radiation and β is FWHM of the crystalline plane (in radians). Using the recorded experimental data of the FWHM obtained from the XRD experimental results, the average GS was calculated [23]. Table 2 shows grain size (GS), calcination temperature, FWHM (β), microstrain (ε), dislocation density (δ) of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 layers. Calcination temperature is a key parameter that obeys the microstructural properties of layers. Tentatively, the microstructural behavior can be considered by the sliding effect of (Zn2+) cations, (O2−) ions, atoms and/or molecules and other non-specific rearrangements, which generates deformation of the crystal lattice. These design principles are based on the assumption of compositional and structural rigidity, as measured crystallographically. It is necessary to mention that it is a phenomenon that difficult to predict with certainty. It is interesting to investigate through a theoretical-experimental analysis and to propose it at each stage carried out in the calcination treatment. It was found that the growth of the crystals and the transition from nano to a micro-size occurred at ~ 700°C [24].
Table 2
Grain size (GS), calcination temperature, FWHM (β), microstrain (ε), dislocation density (δ) of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 layers.
Sample | (hkl) | 2\({\theta }\) | β | Grain Size (nm) | Microstrain \(\epsilon \times\)10− 3 | Dislocation density δ(lines/m2) |
MZN-1 | (100) | 33.04 | 0.32 | 25.1 | 0.42 | 1.57\(x{10}^{15}\) |
(002) | 34.89 | 0.84 | 3.6 | 3.65 | 7.49\({x 10}^{16}\) |
MZN-2 | (100) | 33.01 | 042 | 19.7 | 0.54 | 2.56\(x{10}^{15}\) |
(002) | 33.95 | 2.10 | 3.9 | 2.8 | 6.41\({x 10}^{16}\) |
MZN-3 | (100) | 33.01 | 0.37 | 21.8 | 0.49 | 2.09\(x{10}^{15}\) |
(002) | 33.92 | 1.29 | 6.4 | 1.72 | 2.43\({x 10}^{16}\) |
MZN-4 | (100) | 33.02 | 0.38 | 21.5 | 0.47 | 2.13\(x{10}^{15}\) |
(002) | 33.95 | 1.73 | 4.7 | 2.31 | 4.37\({x 10}^{16}\) |
MZN-5 | (100) | 33.08 | 0.39 | 21.1 | 0.50 | 2.22\(x{10}^{15}\) |
(002) | 33.83 | 1.89 | 6.9 | 1.57 | 2.05\({x 10}^{16}\) |
3.3. Spectroscopic analysis of the Fourier transforms in the Infrared (FTIR)
Figure 3 shows FTIR spectra of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 layers. It is inferred that the samples have absorption bands located at ~ 462 cm− 1, ~ 670 cm− 1, ~ 1090 cm− 1, ~ 1158 cm− 1, ~ 1325 cm− 1, ~ 1390 cm− 1, ~ 1545 cm− 1 and ~ 3256 cm− 1. Vibration modes located at ~ 400–1600 cm− 1 assigned to at \({CO}_{3}^{2-}\) anion [25].
We have reported absorption bands at ~ 400–3600 cm− 1 range as associated to vibrations in \({CO}_{3}^{2-}\) anions and the progenitor reagent that generates this ion in aqueous solution [26]. The absorption mode situated at ~ 1390 cm− 1 is connected with the asymmetric stretching vibrations of \({CO}_{3}^{2-}\) is assigned to the bending out plane vibration modes. In all FTIR spectra, an intense band located at ~ 1323 cm− 1, which were attributed to asymmetric stretching vibrations of the \({CO}_{3}^{2-}\) groups. Such absorption band, due to CH3COO− fragmenting (was induced by our working conditions (pH, stirring time growth. For MZN-1, MZN-2, MZN-3 and MZN-4 thin solid films, a broad absorption band in the ~ 3256 cm− 1 region can be attributed to stretching of the -OH groups of defective sites and the physically adsorbed water (H2O) molecules (or hydrated) adsorbed on the hydroxide surface. The band located at ~ 1123 cm− 1 shifted to high wavenumbers due to a large number of point defects at the surface of nanoparticles, which are characteristic defects reported for these nanocrystals [27] The absorption mode at ~ 472 cm− 1 corresponds to metal-oxygen (ZnO stretching vibrations) vibration mode. The absorption band at ~ 1005 cm− 1 is ascribed to the stretching vibration of -C-N bond of the primary amine or to the stretching vibration of the -C-O bond of the primary alcohol. Generally, these by-products are associated with the hydrolysis of zinc acetate di-hydrate (Zn(CH3COO)2), and the alkaline medium of chemical synthesis. The absorption bands situated at ~ 1090 cm− 1, ~ 1325 cm− 1, and ~ 1390 cm− 1 ascribed to primary, secondary alcohol in-plane bend or vibration. The absorption band at ~ 1595 cm− 1 is ascribed to the vibration modes of aromatic nitro compounds and alkyl ascribed to the stretching vibration of hydroxyl compounds. The sample labeled according to the MZN-5 symbology; the absorption band assigned to the H2O molecules almost completely disappears due to the calcination temperature. In addition, a decrease in the relative intensity of the bands identified with the carbonate ion and contaminants incorporated in the crystalline growth can be seen.
3.4. UV-Vis spectroscopy
The spectral analysis of Transmittance (%T), absorbance (α) of all samples, is performed in the UV-Vis region situated at range ~ 300–700 nm (~ 4.13–1.77 eV). Figure 3 shows of %T vs. Wavelength (λ) spectrum of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 thin solid films. The optical behavior in %T presents a gradual relative increase of ~ 20–85% due to the increase in the calcination temperature. In particular, MZN-3, MZN-4 and MZN-5 samples, present a drastic jump at %T with respect to MZN-1, MZN-2 thin films, with a %T minimum. The abrupt jump in the %T located at ~ 325 nm (~ 3.81 eV) is tentatively associated with the charge-transfer from the Zn2+ to Zn3+ cation, processes between electron-rich and electron-poor counterparts, typically resulting in a new absorption band at a higher λ [28]. This discontinuity in the optical curve of the %T is difficult to obtain; the behavior is interesting and requires a careful study that we will return to later.
Figure 4 shows the absorbance (α) vs. Wavelength (λ) spectrum of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5. The absorption bands (AB) located at ~ 365 nm (~ 3.97 eV) observed with higher magnification in films MZN-4 and MZN-5 samples, are assigned to the fundamental electronic transition CB→VB of ZnO [29]. The absorption bands (AB) located at ~ 365 nm (~ 3.97 eV) observed with higher magnification at MZN-4 and MZN-5 are assigned to the fundamental CB→VB electronic transition of ZnO [10]. In MZN-4 and MZN-5 samples, showing a significant shift towards greater λ at AB, detectable within the error threshold. The reaches a relative maximum of ~ 80% localized at ~ 350 nm (~ 3.54 eV), then remains unchanged for λ > 350 nm. It is observed in the samples MZN-1, MZN-2, MZN-3, α registered is relatively low; this is associated with intrinsic defects. It is observed at MZN-1, MZN-2, MZN-3 layers, α registered is relatively low, and is associated with intrinsic defects [30]. The optical phenomenon is associated with intrinsic crystallization failures in the chemical synthesis process presented here. The variations in the relative intensity of the ABs are correlated with the superficial and volumetric morphology associated with energy levels created by intrinsic crystalline defects, free charge transfer from the passage of Zn2+(3d10) to Zn3+(3d9) cation, stacking failures, microstrain, non-crystalline compaction, homogeneous, faults in grain boundaries etc. The phenomenon is associated with the gradual increase of the GS, calcination temperature, FWHM (β), microstrain (ε), and dislocation density (δ) with modification of intrinsic crystalline defects, critically modifying the optical properties. The optical phenomenon can in principle be associated with the crystalline disorder and chaotic fault creation, resulting in the creation of discrete energy levels and deep traps associated with free charge carriers, thus polarizing the electronic cloud. An AB situated at the visible range that is not apparent in the separated materials emerges when the interface is formed. Interestingly, photoexcitation of this interface band or band-to-band results in a counterintuitive photovoltaic response when a supra/sub-band-gap light is shone [31]. The optical phenomenon can be associated with the crystalline disorder and chaotic fault formation, resulting in the creation of discrete energy levels and deep traps associated with free charge carriers, thus polarizing the electronic cloud. However, MZN-4, MZN-5 thin films, a significant difference is manifested in the region located at ~ 350–375 nm (~ 3.54–3.30 eV) is registered. A sudden jump of α located at UV-region, specifically situated at ~ 365 nm (~ 3.39 eV), is observed at MZN-4, MZN-5 layers. On the other hand, we have that the ionic chemical bond for ZnO is the result of the mixture of dsp orbitals and it is possible to tentatively associate it with the charge transfer phenomenon: Zn2+→Zn3+ + e− (defect-emission free) in this material [32].
The band gap (Eg) energy of the material related to the optical absorption, according to the interesting Tauc eq.
$$\alpha h\nu =A{(h\nu -{E}_{g})}^{x}$$
2
Where A is a constant, α absorbance coefficient, hν energy of the incident photon, x for electronic direct band transition assigned the value of 1/2. The graphical method to numerically quantify Eg consists of drawing a straight line that intersects the axis of energy (α = 0). Figure 6 displays (αhv)2 vs. E(eV) spectrum of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 thin films and Fig. 7 shows the spectrum of Eg(eV) vs, calcination temperature (°C) of all the thin solid films. Reports of the optical behavior of the fundamental electronic transition CB→CV of ZnO, which is often attributed to direct recombination in the inverted electron-hole plasma [33], identified and assigned to the phenomenon of recombination of energetic excitons are tightly bound and dominate the optical response even at room temperature [34]. The MZN-1, MZN-2 samples, present the slope associated with the Eg defined according to the aforementioned graphic method. In the case of MZN-3, MZN-4 and MZN-5 layers, two electronic transitions very close to each other. As a first approximation, the optical behavior is associated with charge transfer, considering the Zn(pd) and O(sp) orbitals very close to each other. The intra-electronic transition phenomenon reduced to a Zn2+→Zn3+ + e− (defect-emission free) [32]. Tentatively, we propose charge transfer of the Zn2+ and Zn3+ cation is an optical phenomenon observed here. However, this optical behavior requires a detailed study to describe in more depth [35].
3.5. Photoluminescence
The experimental Photoluminescence (PL) spectroscopic technique is applied to investigate the emission bands (EB) associated with intrinsic crystalline defects (vacancies, interstices, and the introduction of color centers) and the optical behavior of Eg in inorganic single crystals. The EB registered at UV-Vis region by PL is considered an optical phenomenon originated by radiative recombination of acceptors and charge donors, which significantly affects charge photogeneration [36]. Figure 7 shows the PL spectrum of (a) MZN-1, (b) MZN-2, (c) MZN-3, (d) MZN-4 and MZN-5 samples. The Gaussian curve of thin films, presents a non-symmetric geometry dependency (or asymmetry) at EB. The asymmetric optical behavior is interpreted geometrically as the superposition of two or more signal bands, for this reason, we proceed to carry out the deconvolution and investigate its origin. We use a differently colored fringe on the symmetrical Gaussian curve in order to facilitate quick identification of the corresponding emission band. The deconvolution of the asymmetric Gaussian curve generates three symmetric EB of different relative intensities located in the region ~ 400–700 nm (~ 3.10–1.77 eV). Crystal defects of ZnO have been identified and extensively investigated, assigning their origin with crystallographic characteristics commonly to zinc (VZn) and oxygen (VO) vacancies among other crystal faults [37, 38, 39, 40]. Table 4 compiles the electronic transitions and intro transitions of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 thin films. In all the samples, EB located in the UV-Vis region is observed, these show the overlap between them, which is typical in asymmetric emission signals, which are strongly governed by the blue (BE) and yellow emission bands (GE). These defects, of course, produce all the residual emissions in addition to the desired laser line [41]. The deconvolution of the Gaussian curve allows for observing symmetric emission signals. The deconvolution of the Gaussian curve shows an oscillatory and gradual increase in optical intensity [41, 42]. It is possible to determine approximately the correlation that exists between the optical behavior and the intrinsic crystalline defects (see Table 1). All thin solid films, present a broad emission band identified as a blue emission (BE) band associated with zinc interstices (Zni) located at ~ 450–550 nm (~ 2.75–2.25 eV) and zinc vacancies (VZn), located at ~ 525–750 nm (~ 2.63–1.65 eV) [42]. Symmetrical emission signals assigned at emission band: green (GE) situated at range ~ 492–520 nm (~ 2.50–2.38 eV), yellow (YE) at ~ 570–600 nm (~ 2.17–2.06 eV) and red (RE) at ~ 770 nm (~ 1.61 eV). The energy level corresponding to Zni lies just below the conduction band, and it can trap photo excited electron (e−) followed by its radiative recombination with holes (h+) in the valence band [43]. These defects are associated with doubly ionized vacancies ( \({V}_{0}^{++}\)) and attributed to oxygen vacancies (VO) and oxygen interstices (Oi). The BE is related to the transition of the Vzn or oxygen antisites (Ozn) in the conduction band (CB). The GE band assigned to the electronic transition of deep defects from VO and from Zni to BV, is probably caused by the transition of localized surface charge donors. The YE band is related to oxygen interstices (Oi), attributed to the stoichiometric imbalance created by excess oxygen, and the RE band with VO is located below the ZnO conduction band. The relative decrease in crystalline defects is associated with the creation of deep levels assigned to RE and located at ~ 681–770 nm (~ 1.82–1.61 eV).
Table 4
Electronic transitions and intra-transitions of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 layers.
Wavelength (nm) | E(eV) | Defect | |
457 | 2.77 | \({Zn}_{i}\) | |
466 | 2.66 | \({Zn}_{i}\) | |
469 | 2.64 | \({Zn}_{i}\) | |
474 | 2.67 | \({Zn}_{i}\) | |
488 | 2.54 | \({V}_{z}\) | |
510 | 2.24 | \({V}_{z}\) | |
565 | 2.19 | \({V}_{o}\) | |
525 | 2.36 | \({V}_{o}\) | |
576 | 2.15 | \({V}_{o}\) | |
608 | 2.03 | \({V}_{o}\) | |
625 | 1.98 | \({V}_{o}\) | |
671 | 1.84 | \({V}_{o}^{++}\) | |
3.6. Hall Effect
ND(cm-3) charge carrier concentration, resistivity and mobility are recorded using Hall effect equipment. We found that the MZN-1, MZN-2, MZN-3 and MZN-4 layers show various crystalline defects (see Table 2), in a lower relative proportion compared to the MZN-5 sample. Table 4 shows the compilation of the experimental results recorded at room temperature (RT) of the electrical properties of all solid thin films. The existence of native defects of the material avoids the realization of the electrical measurement in the sample MZN-1, while in the samples MZN-2, MZN-3, MZN-4, MZN-5 with increasing temperature of the heat treatment the intrinsic defects of the material decrease which makes possible the measurement of the electrical properties, which there is no representative variation due to the temperature range. The types of crystalline defects in the specific application should be defined or differentiated according to whether they are desirable or undesirable.
Table 4
Effect Hall of MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5 thin solid film.
Sample | MZN-5 | MZN-4 | MZN-3 | MZN-2 | MZN-1 |
ND (cm− 3) | \({1.45\times 10}^{14}\) | \({1.33\times 10}^{14}\) | \({1.26\times 10}^{14}\) | \({1.11\times 10}^{14}\) | ------- |
Mobility (cm2/Vs) | \({64.1}^{}\) | 60.2 | 52.7 | 48.9 | ------- |
Resistivity (Ω-cm) | \({6.48\times 10}^{2}\) | \({7.8\times 10}^{2}\) | \({8.77\times 10}^{2}\) | \({2.5\times 10}^{3}\) | ------- |
Type | n | n | n | n | ------- |
3.7. Application of the thin solid film in the construction of the Schottky diode of ZnO-Qd´s and Qd´s ZnO.
According to the results reported in the previous work of ZnO-Qd's [15], it can be considered an improvement in the electrical properties when using the ZnO-Qd's material, such as ND, mobility, and decrease in resistance, as shown in the Table 5. To compare the behavior and effect of ZnO-Qd's material Vs ZnO Qd's are applied as Schottky diode, using PEDOT:PSS as "p" type material.
Table 5
Comparison of QD's ZnO and ZnO-QD's electric properties thin film.
Material | ND (\({{c}{m}}^{-3}\)) | Mobility (\({{c}{m}}^{2}\)/V.s) | Resistance (Ω-cm) |
Qd´s ZnO | 1.1x1013 | 6.1 x101 | 2.3 x103 |
ZnO-Qd´s | 1.5x1014 | 6.43 x101 | 6.48 x102 |
The results obtained in Fig. 10 show a P-n junction between the ZnO QD's and the Pedot: PSS, only in the sample at 60°C there is no P-n junction due to the large number of defects in the films, but when the films are heat treated at 100–220°C, P-n junction is achieved although the graph does not look well formed, due to the high resistivity and low concentration of carriers. [15].
Figure 11 show the current-voltage (I-V) MZN-1, MZN-2, MZN-3, MZN-4 and MZN-5. In the ZnO-QDs films, in this analysis, the intrinsic crystalline defects are an important parameter that is associated with the response of the I-V curve. According to Table 4, it is possible to relate the structural behavior of the nanocrystals associated with the electrical phenomena examined here. The experimental results are registered considering the optimum calcination temperature in the crystalline growth and adequate to obtain the optimum concentration of structural defects. The behavior that considerably governs the Schottky diode responds that in all films it is not possible to obtain the p-n junction. However, in the MZN-5 samples, it is observed in particular that the I-V curve presents the optimal behavior, which is associated with the optimal reduction of intrinsic crystalline defects and the optimal obtaining (p-n junction) of the Schottky diode curve.