1 Introduction

The II–VI compound semiconductors are widely studied due to their promising applications in nano-dimensional structures. Cadmium sulfide (CdS) is a semiconductor compound that was initially investigated as a fluorescent material for image reproduction [1, 2]. Nowadays, CdS is widely employed in solar cells, optoelectronic devices, lasers, gas sensors, biological, and pharmaceutic products [3,4,5,6,7]. Recently, Li et al. [8] published a review on the nanostructured cadmium sulfide as a promising semiconductor for profitable optoelectronic devices. According to this report, CdS nanocrystals can be utilized to prepare functional lasers, waveguides, photovoltaics, among others. There has been an increasing interest in nanostructured CdS due to a wide variety of industrial applications such as photocatalysis [9], photoelectrochemistry [10], as an antimicrobial [11], ferromagnetism [12], and medical imaging [13]. CdS has been synthesized in three different crystalline structures: cubic zinc blende (ZB), hexagonal wurtzite (WZ), and cubic rock salt (RS). As nanocrystals, WZ and ZB phases are stable at standard conditions, while RS is stable only at high pressures. However, recently, the WZ-to-RS transformation in nanocrystalline flower-shaped CdS at atmospheric pressure has been reported [14]. The direct band gap (Eg) of nano-dimensional WZ and ZB phases can oscillate between 2.42 to 3.85 eV depending on the crystallite size and morphology [15]. Indirect Eg values of the RS structure are located between 0.8 and 1.7 eV in the pressure range 0 ≤ P ≤ 70 GPa [16, 17]. The RS indirect Eg values are also dependent on the size of the nanoparticles (NP’s) [18]. Due to the wide variety of CdS applications, the study of the properties associated with the different crystalline phases has become a fundamental task [3], especially in nanoparticles [19].

Stable ZB and WZ-CdS phases have been extensively used in different applications with no clear evidence of a preferred structure. For instance, ZB shows improved performance for water splitting in ZnO/CdS nanocomposites [20], while WZ does the same for the CdS/Cd(OH)2 nanostructure [21]. In the case of photoluminescence (PL), some authors have obtained an enhancement in PL using WZ [22] and others with ZB [23]. Gaiardo et al. reported that ZB-CdS nanoparticles are ideal for chemoresistive gas sensors [24], and Li et al. found that WZ-CdS nanoparticles are excellent candidates for detectors in photoelectrochemical sensing of gases [25]. Also, some authors have reported that WZ, ZB, and WZ + ZB phases are suitable materials for devices such as LEDs [26], solar cells [27], photocatalysts [28], etc. With respect to the ZB ↔ WZ phase transformation, the disorder generated by the phase change can produce non-expected behaviors, especially in CdS or CdSe, due to the low energy of transition (~ 2 meV) [29]. This fact indicates that special care must be considered when planning an experiment since some perturbation could trigger a phase transformation.

Nanocrystalline CdS thin films have been synthesized by different techniques such as chemical bath deposition (CBD) [30], spray pyrolysis [31], precipitation [32], thermal evaporation [33], vapor sublimation [34], etc. In this work, we report the optical properties during the ZB to the WZ phase transition of CdS-nanoparticulated thin films grown by CBD. The samples were grown at different temperatures in the 60 to 97 °C interval, using a microwave oven to heat the growth solution. The phase transition occurs in the bath temperature (Tb) 93–95 °C range, which is an abrupt change of structure. Band gap, refractive index, and photoluminescence effects from the ZB-to-WZ transformation are discussed. A blue shift is observed in the band gap, expected for spherical NP´s in the intermediate quantum confinement (IQC) [35, 36].

2 Experimental details

The aqueous solutions were prepared by using ~ 625 ml of ultrapure deionized water. All the reagents employed were high purity: (a) Cd(CH3COO)2·2H2O, 1.372 × 10–3 mol, 243.2 mg, (b) CH3CO2NH4, 27.56 × 10–3 mol, 1413 mg, (c) NH4OH, 28–30%, 30 ml, and (d) NH2CSNH2, 6.124 × 10–3 mol, 310 mg. Glass substrates Corning 2947 were ultrasonically cleaned in a residue-free detergent solution and subsequently immersed in a chromic acid solution for 24 h. Then they were placed in an aqueous nitric acid solution (1:3) at 75 °C for 3 h and rinsed in deionized water. Finally, the substrates were dried with N2. The growing reactor was a commercial microwave oven, and the temperature was regulated using an N1100 Novus controller. The samples were deposited at different bath temperatures (Tb): 60, 70, 80, and 90 °C and for Tb’s between 90 and 97 °C with 1 °C steps. For each Tb, the temperature was automatically controlled within ± 0.2 °C [30]. This control is achieved since the microwaves heat up uniformly and rapidly all the volume of the aqueous solution, which is continuously stirring. The deposition time was varied from ½ h for the film deposited at 97 °C, up to 24 h for Tb = 60 °C, to obtain similar thicknesses for the samples (200–250 nm). X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max-2100 diffractometer (CuKα radiation, 0.15406 nm), equipped with a thin film attachment. The thickness was measured by utilizing a Dektak II profilometer. Surface morphology was observed by a scanning electron microscope (SEM) Philips XL30-ESE. Transmission electron microscopy (TEM) images were obtained in a Jeol 2010 microscope working at 200 keV; small pieces of the CdS films were detached from the substrate and put onto the TEM grid. Selected area electron diffraction (SAED) patterns were analyzed with the Diamond 3.0 software. Photoluminescence measurements were performed by a 0.5 m spectrometer (Jovin-Ivon) equipped with a 325 nm line of a He–Cd laser. An ellipsometer (Jobin-Ivon Uvisel) was used for the measurements in the 1.5 to 4.5 eV interval. The ellipsometry results were compared with the theoretically calculated spectra using the software available in the ellipsometer (Psi-Delta 2.0). All the experimental characterization was performed at room temperature (RT).

3 Results and discussion

Figure 1 displays the XRD patterns in the range 20 ≤ 2θ ≤ 60 degrees for the set of eleven films. According to the XRD data, the CdS nanoparticles (NP’s) in the films grow with cubic symmetry for 60 ≤ Tb < 94 °C, and a hexagonal symmetry for 94 < Tb ≤ 97 °C. At the critical temperature (Tbc = 94 °C), the films grow with a mix of both crystalline phases. The (002) is the preferential orientation of the films for WZ and the (111) for ZB is observed in diffractograms. In the hexagonal structure, small zones grow with cubic symmetry and vice versa, whose proportion depends on the closeness to Tbc. Figure 1 shows that the XRD intensity increases with Tb, showing an increase of crystallinity with temperature in both ZB and WZ phases.

Fig. 1
figure 1

XRD patterns in the 20 ≤ 2θ ≤ 60 degrees of all the CdS samples analyzed

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In Fig. 2 (I), a 36 nm × 42 nm TEM image shows the NP dimensions of the crystallites in the film grown at 94 °C, where both ZB and WZ phases of CdS coexist. The inset (a) displays the SAED pattern of this film, where the presence of both ZB and WZ crystalline phases are revealed by the diffracted rings, as noted in insets (b) and (c), respectively. The nanocrystals observed in Fig. 2(I) show the (111) and the (002) interplanar spacings of the ZB and the WZ phases, respectively, which support the preferential orientation observed in XRD. In Fig. 2 (II), a SEM image shows the surface morphology of the film grown at Tb = 94 °C, which illustrates a homogeneous film with few isolated particles (average diameter ~ 200 nm) on the top. The more probable stacking on the amorphous substrate is (002) and (111) planes for WZ and ZB crystalline structures, respectively, since the difference of stacking energy is only 8 meV/atom [37].

Fig. 2
figure 2

TEM image of the CdS sample prepared at Tbc = 94 °C (I). The coexistence of both ZB and WZ phases can be observed. The electron diffraction pattern of this sample is displayed in the inset (a). Insets (b, c) reveal that the sample has both WZ and ZB crystalline phases. SEM image of this sample surface (II)

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CdS NP’s average crystal size (ACS) as a function of Tb, obtained from the full width at half maximum (FWHM) of the XRD reflections, and calculated by the Scherrer equation, is displayed, the error bar of Tb is ± 0.2 °C in Fig. 3 (right). ACS is approximately constant (~ 6 nm) for Tb < 91 °C and increases abruptly after this temperature up to ~ 20 nm. The direct band gap energy (Eg) of the films (Fig. 3 left) was determined by optical absorbance spectra and calculated using the Tauc’s formula (not shown here). An abrupt decrease of Eg is observed for 91 °C < Tb < 94 °C (Tbc), which corresponds to the increase of the ACS. Eg shows a minimum during the phase transition at Tbc = 94 °C. This behavior has been observed previously for the solid–solid ZB → WZ transformation of CdS and is explained by the fact that the CdS lattice has the maximum disorder at this point [38, 39]. In these reports, ZB-CdS was transformed to WZ-CdS through thermal annealing in an Ar + S2 atmosphere in the temperature range of 60–550 °C. The gradual transformation occurs within an interval of ~ 100 °C, where Tbc was found to be at 300 °C. In the present case, almost all the ZB → WZ transition occurs in the Tb range 93–95 °C (15.0 ≤ ACS ≤ 19 nm), which is within ~ 0.2 meV. The phase transformation occurs very abruptly since it is not a solid–solid transformation. At least four factors are involved in the variation of Eg as a function of the Tb: disorder [40], strain [32], lattice structure [41], and intermediate quantum confinement [42, 43]. IQC has been extended up to ACS of CdS NP’s reported in this work because of the non-stoichiometry character of the films. CdS thin films prepared by CBD has an excess of Cd [44], which increases the Eg [45, 46].

Fig. 3
figure 3

Direct band gap energy (Left) and average crystal size (Right) as a function of Tb

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The refractive index (n) was calculated from ellipsometry measurements (Fig. 4). Close to Tbc, the n values suffer a pronounced decrease. The energy band gap and the refractive index are related to the threshold of absorption and transparency of the incident photons, respectively [47]. In a phase transition, a change of the refractive index occurs due to the modification of the physical properties. Even though the ZB and WZ structures have similar optical properties, the disorder at the critical point provokes a change in the degree of polarization, originating Eg and n to decrease [48, 49]. The thickness (τ) of the films was measured to be in the 210–250 nm range. In general, n follows a similar behavior to τ which is due to the direct relationship between both parameters in thin films [50]. In the vicinity of Tbc, a sharp decay in n is observed due to the disordered nature of these films. Vacancies and interstitial defects in the vicinity of Tb alter the average interplanar spacing among the different crystalline planes, causing a change in the porosity of the films and so the n [51].

Fig. 4
figure 4

Refractive index (n) (Left) and thickness (Right) vs. Tb for all the studied CdS thin films

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In many cases, the photoluminescence (PL) spectrum of CdS thin films, containing native defects, are fitted with three main emissions: (i) upper near edge emission (violet) from surface states due to oxidation caused by the aqueous solutions [52,53,54], (ii) conduction band (CB) to interstitial Cd (ICd) (green emission), as aforementioned, undoped CdS prepared by chemical bath commonly contains an excess of Cd [44], even though, S and Cd vacancies (V) and IS can also be present with lower density [44, 54, 55], and (iii) defect-to-defect transition emissions (yellow and red) [30, 56, 57]. PL spectra, in the 1.3 ≤ hν ≤ 3.5 eV range of CdS thin films, are displayed in Fig. 5a for all the films. For Tb’s in the 90–97 °C interval (Fig. 5b), the PL spectra can be fitted using three bands: red emission in 1.83–1.90 eV (653—678 nm), green emission in 2.35–2.43 eV (510–528 nm), and blue violet band in 2.85–2.93 eV (423–435 nm). These spectra present a similar behavior except for Tbc = 94 °C. The PL intensity at this temperature is higher than other films and an additional emission peak (blue) emerges.

Fig. 5
figure 5

PL spectra of the CdS films prepared at different Tb’s (a) and for the films prepared at 90 °C ≤ Tb ≤ 97 °C (b)

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Figure 6a displays the main transitions within the forbidden band due to native defects in CdS, compiled from some reports [58,59,60], in particular, interstials (I) and vacancies (V) of Cd and S. The more probable radiative transitions which originate the PL bands of Fig. 5 are schematically depicted in Fig. 6a, where dotted lines represent emissions that are enhanced at Tbc = 94 °C. In Fig. 6a, emissions from the surface oxide to defects are not included. The difference of transitions from exciton level and CB are not appreciated at RT. The deconvolution of the three PL spectra for Tbc and those close to Tbc is depicted in Fig. 6b. The deconvolution-fitting spectra using Gaussian curves were performed to define the three bands aforementioned. However, for Tbc = 94 °C, the green band (~ 2.4 eV) splits into two bands. A new PL band appears at hν = 2.53 eV (blue). The authors of this work have associated this new band to the arising of interstitial Cd (ICd) donor defect, since a PL band at hν ~ 2.25 eV is associated to ICd’s in microcrystalline CdS, which splits into two levels under ZB-to-WZ phase transition [38, 39]. The presence was identified with ICd’s, which arise during the phase transition. The transition with band maximum at hν ~ 2.25 eV has been reported to be caused by ICd-to-VCd transitions [61, 62]. In the case of the CdS NP’s, the emission has been shifted to 2.53 eV, i.e., from ICd’s to CB (light-blue dotted line in Fig. 6a). Observe that other transitions from ICd’s (green and yellow dotted lines) are also included as the FWHM of the light-blue band of Fig. 6b is very wide. During the phase transition, IS and VS are present, but in less proportion since ICd’s are produced in higher density because Cd2+ ions have lesser binding energy than S2− ions [38, 63]. Moreover, Cd2+ ions have smaller ionic radius (109 pm) than S2− (170 pm), which allows a higher mobility of Cd2+ and, consequently, generating a higher density of ICd’s and VCd’s. In this way, emissions from Cd defects were enhanced. Furthermore, it can be seen in Fig. 5a that the overall intensity of the PL bands for Tb = 94 °C is higher than those of the other samples due an increment of the defects density.

Fig. 6.
figure 6

a Electron transitions in CdS native defects, dashed emissions are enhanced only when Tb = 94 °C (defects indicate other defects like antisites, surface, etc.). b Deconvolution, using three or four Gaussian curves, of the PL spectra of the samples prepared with Tb = 93, 94 and 95 °C.

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The position of the maxima of the three PL bands as a function of Tb is displayed in Fig. 7. The center of the blue band as a function of Tb is similar to the Eg vs. Tb behavior since Eg, donor, and acceptor levels depend on the size of NP’s, which are subjected to internal stress from surface tension which, in turn, depends on the crystal size [64,65,66]. The green and red bands only show small depresions in the region close to Tbc.

Fig. 7
figure 7

Position of the blue, green, and red PL bands maximum obtained from the deconvolution of the spectra of all the samples

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Figure 8 shows the PL spectra in the region from 2.7 to 3.7 eV of the samples deposited at Tb from 91 to 96 °C. PL bands of this zone are radiative states located above the CB, which become accessible when the samples are excited with higher energy than the sample Eg [67, 68]. These signals are emitted with very small intensity, two orders of magnitude less than visible emissions, because most excited electrons decay rapidly to CB due to the phonons generation [68]. The discrete emissions have been also observed by Narasimman et al. in Zn-doped CdS, but the origin was not identified [69]. Notice in Fig. 8 that the three bands indicated in the bottom of the plot (Tb = 95 °C) by  asterisks are also present in the spectra of samples of Tb equal to 91, 92, and 96 °C. These bands split into two bands for Tb equals to 93 and 94 °C. This fact is the result that the high energy levels either split in two levels because of disorder, or the levels are at different energy in the ZB and WZ phases. The average value of the splitting energy in  Fig. 8 is ~ 0.07 eV, which is close to ΔEg = EgWZ − EgZB. Soltani et al. report [70] that EgWZ = 2.635 eV, EgZB = 2.573 eV, ΔEg = 0.062 eV, based on experimental measurements. The inset of Fig. 8 shows the absorption spectrum of the sample in the same energy region. Most of the absorbance spectra in that region are perturbed by noise; however, in some cases, some optical absorption peaks are observed, which could be related with these PL bands. These emissions seem to be the intra-atomic energy transitions of the Cd atom: (a) 5p3P0, (b) 5p3P1, and (c) 5p3P2 to 5s1S0, with energy transitions 3.74, 3.80, and 3.95 eV, respectively [71]; these values have an average value of 0.06 eV. Then, the emissions reported in this work come from Cd atoms transitions. This result suggests that the atomic orbitals of Cd are influenced by the internal crystal field of the corresponding lattice, due to different structural symmetry.

Fig. 8
figure 8

PL in the high-energy region (2.7–3.7 eV)  for the samples in the 91-96 °C range. The inset exhibits the absorbance of the sample grown at 95 °C  in the 3.2–3.6 eV interval

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4 Conclusions

The influence of the ZB → WZ phase transformation on the optical properties of CdS thin films has been analyzed. The NP’s CdS thin films were deposited on glass by chemical bath deposition, using microwaves in the temperature range of 60 ≤ Tb ≤ 97 °C. The nanoparticulate CdS films grow in the cubic zinc blende crystalline phase for 60 ≤ Tb < 94 °C, in the hexagonal wurtzite structure for 94 < Tb ≤ 97 °C, and a mix of phases at the critical temperature (Tbc = 94 °C). The ZB → WZ transition occurs in the 93 < Tb < 95 °C interval. The effect of the phase transformation on the optical properties is to displace the band gap to lower energies as the effect of the disorder, which in turn modifies the refractive index and PL emissions similarly. At the phase transition, the PL spectrum exhibits an additional band due to a high density of interstitials and vacancies of Cd, which arise due to the coexistence of both phases. High-energy emissions from the Cd atom intra-atomic transitions also show a split at the critical temperature.