1 Introduction

Recent research into semiconductor nanoparticles has made it clear that II-VI semiconductor nanoparticles are becoming increasingly relevant to successful applications due to their large bandgap as well as their unique optical and chemical properties. One of the most common applications of these materials is in the field of optoelectronics, spintronics, and light-emitting devices [1,2,3]. To achieve the fundamental role of technological applications and advanced research materials, such as solar cells, several transition metals can be doped into semiconductor nanocrystals [4, 5]. As examples of transition metals that can be doped into semiconductor nanocrystals are copper (Cu), vanadium (V), manganese (Mn), and cobalt (Co). CdS is one of the most promising materials to be used in the future, with a band gap at room temperature of 2.42 eV for the hexagonal phase and 2.57 eV for the cubic phase. Several applications of CdS nanoparticles have been proposed including photocatalysis, sensitive devices, and high-efficiency solar cells [6,7,8]. CdS semiconductor materials doped with Mn exhibit enhanced properties. In contrast, a CdS semiconductor material not doped with Mn is an n-type that can be converted to a p-type by adding Mn [9, 10]. A variety of methods can be used to prepare CdS nanoparticles. Chemical co-precipitation has been used to prepare highly doped CdS nanoparticles with Mn dopants [11]. This magnetic-doped material (CdS:Mn) can also be used as a magneto-optic material [12, 13]. There are some doped elements that are capable of forming quantum dots or nanocrystals. Additionally, they can provide diluted magnetic properties [14], half-metallic behavior [15, 16], quantum dots [17], and nanocrystal cores [18]. A study conducted by P. Maity et al. [19] reported that CdS diluted magnetic semiconductor quantum dots can be considered a promising optoelectronic as well as magnetic material, which can be employed as an active material for prolonged use in quantum opto-spintronics with tunable efficiency. S. Suresh studied the dielectric properties of CdS nanoparticles [20]. The dielectric constant of CdS nanoparticles was found to be much larger than that of the bulk CdS and it was decreasing with increasing temperature. AC electrical conductivity was found to increase with an increase in the temperatures and frequency. Due to the extreme scarcity of research that investigates the electrical properties of Mn doped CdS, in this work the structural, compositional, and dielectric properties of CdS:Mn nanoparticles prepared by chemical co-precipitation was studied.

2 Experimental methods and techniques

2.1 Materials

Prior to the experiment, all glassware was thoroughly washed with deionized water and dried overnight in the oven. Throughout this study, deionized water was used. Sodium sulfide (Na2S), cadmium acetate Cd(CH3COO)2, and manganese chloride (MnCl3) were purchased from Sigma-Aldrich and used as received without further purifications.

2.2 Preparation of CdS: Mn nanocomposites

A co-precipitation method was used to prepare CdS. Cadmium acetate Cd (CH3COO)2 (200 mM) was dissolved in deionized water and stirred magnetically for 30 min at 80 °C. A solution of Na2S (200 mM) was then added to the solution. The mixture was stirred at 80 °C for 180 min. A centrifuge was used to collect the precipitates, which were then washed with deionized water and dried at 100 °C for eight hours. A similar process was used to prepare Mn-doped CdS nanoparticles. Different concentrations of MnCl3 were dissolved in deionized water, corresponding to 3%, 5%, and 7%. Different concentrations of MnCl3 solution were added to a solution of Cd (CH3COO)2 (200 mM). In addition, Na2S (200 mM) was added to the solution obtained. A stirring process was performed at 80 °C for 180 min. The sample was collected using centrifugation, washed with deionized water, and dried for eight hours at 100 °C.

2.3 Characterization

To study the morphology and composition of the samples, a scanning electron microscope (JEOL JSM-6360 LA, Japan) was used at an accelerating voltage of 30 kV. The surface functional group structure of the samples was studied using Fourier Transform Infrared (FTIR, Bruker Vertex 80, Bruker Co., Germany). Analyzing the samples was conducted using a confocal Raman microscope, (WITec 300 R alpha, made in Germany). The laser beam used in this experiment was 523 nm. A wide range of dielectric properties was measured using broadband dielectric spectroscopy (Alpha High-Resolution Analyzer (Novocontrol GmbH) supported by Quatro temperature controls with high-temperature stability).

3 Results and discussion

3.1 Morphological properties

In previous work [21], X ray diffraction analysis demonstrated single phase of CdS doped with Mn with average crystallite size 3.7 nm. A scanning electron microscope (SEM) was used to investigate the morphology of pure CdS nanoparticles and Mn-doped CdS (3%) nanoparticles. Figure (1: a) shows the agglomeration of CdS nanoparticles in the SEM image. On the other side, Mn-doped CdS nanoparticles demonstrate reducing the agglomeration and the observed porosity in the sample, which enhances the surface area, as shown in Fig. 1b.

Fig. 1
figure 1

SEM images of of CdS (a) and 3% Mn-doped CdS nanoparticles (b)

Full size image

The elemental compositions of CdS nanoparticles and CdS enriched with Mn (3%) were determined using energy dispersive X-ray analysis (EDX). As shown in Fig. 2a, based on the EDX analysis of the Cd:S atomic ratio, the atomic ratio of Cd:S is (53.08: 46.92). As a result, it can be concluded that the CdS nanoparticles prepared are nearly stoichiometric. Furthermore, it can be seen from Fig. 2b that the atomic ratios of (Cd: S: Mn) are (53.32: 43.44: 1.24) for CdS that has been doped with Mn (3%). These results concluded that Mn is doped into CdS nanoparticles.

Fig. 2
figure 2

EDAX spectra of CdS (a) and 3% Mn-doped CdS nanoparticles (b)

Full size image

3.2 Fourier transform infrared

To investigate the vibrational and functional groups present in the prepared samples, FTIR measurements were performed at room temperature in the wavenumber range of 4000–400 cm−1. Figure 3 shows the FTIR spectra of CdS nanoparticles without Mn and those Mn-doping (3%, 5%, and 7%). A broad and weak transmission band was observed around 3200–3600 cm−1, indicating OH stretching vibration of water molecules and moisture. The low-intensity transmission peak around 2800–2900 cm−1 was assigned to C–H stretching. A medium peak is seen around 1626 cm−1 as a result of H–O-H bending vibrations within its atoms [22]. It is believed that the strong band positions in the range of 1016–1140 cm−1 may be attributed to stretching vibrations of the sulfate groups. There is a sharp peak around 606–615 cm−1 corresponding to the stretching of Cd–S in the samples. It has been found that pure CdS shows its Cd–S bond at 613 cm–1 but when doped with Mn, there is a small shift to 615 cm–1 which means that There were no significant changes observed in CdS when doped with Mn [23]. Cd–S bonds with Mn incorporation or without could show a shift in frequency for a variety of reasons as particle sizes.

Fig. 3
figure 3

FTIR spectra of undoped and Mn-doped CdS nanoparticles

Full size image

3.3 Raman spectroscopy

Figure 4 illustrates the Raman spectrum of CdS nanoparticles and Mn-doped CdS nanocomposite at room temperature. The Raman spectra of CdS exhibit an overtone series of longitudinal optical phonon vibrations, which correspond to the 1LO and 2LO peaks [24]. A longitudinal optical mode (LO) is observed in bulk CdS at 305 cm−1 [25]. The weak overtone mode of CdS nanoparticles synthesized in this work appeared at 603 cm−1 and the strong fundamental mode was observed at 303 cm−1. There was a change in these bands in Mn-doped CdS nanoparticles to a strong fundamental mode at 301 cm−1 and a weak overtone mode at 601 cm−1. In addition, the 1LO mode of the Mn-doped CdS peaks has an asymmetric broadening toward the lower frequency side. This phonon softening and broadening of peaks may be attributed to surface optical phonons (SOP) and confinement effects [26, 27]. The intensity ratio of the 2LO to 1LO modes (2LO/1LO) gives exciton–phonon coupling strength. This intensity ratio of CdS nanoparticle (0.834) slightly decreases as Mn2+-doped CdS (0.818) due to the particle size increases. This result is in concurrence with the findings previously documented by Romčević et al. [28].

Fig. 4
figure 4

Raman spectra of CdS nanoparticles and Mn-doped CdS nanoparticles

Full size image

3.4 Electrical properties

3.4.1 Dielectric constant

It is possible for materials to polarize at specific frequencies due to their dielectric properties [29]. Figure 5a–d illustrates the frequency dependence of the real part of the dielectric constant \((\varepsilon{\prime} )\) of CdS and Mn-doped CdS nanoparticles at different temperatures (30, 70, 110, and 150 °C). As frequency increased, the dielectric constant decreased in all samples and became constant for at all ranges of temperatures. CdS and Mn-doped CdS nanoparticles have dielectric constants in the range of 105 orders. A dielectric constant \((\varepsilon{\prime} )\) of 30 °C, 70 °C, and 110 °C increases when Mn is present at a percentage of 3%. Nevertheless, it decreases as Mn concentration increases to a minimum value below 5%. As Mn increases by 5%, its value decreases by 102 orders. At 150 °C, it is observed that the dielectric decreases with the addition of Mn. This may be due to many factors, including the speed of response mechanisms of different polarizations (electronic, atomic, interfacial, and dipole orientations) with the applied field. In contrast, dipoles have difficulty orienting themselves as the frequency increases. As a consequence, their oscillations are delayed back into the field, which results in a decline with increasing frequency [30, 31].

Fig. 5
figure 5

a–d Real part of the dielectric constant \((\varepsilon{\prime} )\) of CdS and Mn-doped CdS nanoparticles at temperatures (30, 70, 110, and 150 °C) as a function of frequency

Full size image

The imaginary dielectric part \((\varepsilon^{^{\prime\prime}} )\) can provide a variety of information, including dispersion energy, polarization, and changes in the material structure. Figure 6a–d illustrates the correlation between the imaginary part of the dielectric constant (dielectric loss) and the frequency of CdS and Mn-doped CdS nanoparticles at various temperatures. Dielectric loss decreases with increasing frequency, but increases with increasing temperature, except at 150 °C. The imaginary part values increase at low frequencies due to the material conductivity and free charge carrier motion [32, 33]. On the basis of the dielectric constant's behavior with temperature, it can be seen that the imaginary part \((\varepsilon^{^{\prime\prime}} )\) of the dielectric constant increases up to 70 °C and then decreases for the samples (Mn = 3% and 7%). However, the sample Mn = 5% increased linearly with temperature.

Fig. 6
figure 6

a–d Imaginary part of the dielectric constant \((\varepsilon^{^{\prime\prime}} )\) of CdS and Mn-doped CdS nanoparticles at temperatures (30, 70, 110, and 150 °C) as a function of frequency

Full size image

3.4.2 AC electrical conductivity (\(\sigma_{AC}\))

Figure 7a–d illustrates the change in AC conductivity (\(\sigma_{AC}\)) with frequency for CdS and Mn-doped CdS nanoparticles at various concentrations (3, 5, and 7%) and temperatures (30, 70, 110, and 150 °C). As can be seen from the figures, the conductivity increases linearly with frequency, with the exception of T = 150 °C, where a plateau can be observed at low frequencies. A similar relationship exists between the AC conductivity and the dielectric constant of the samples, where the sample with Mn = 5% displayed the lowest conductivity values, and the sample with Mn = 3% represented the highest conductivity values at different temperatures. The AC conductivity of these samples was enhanced at 150 °C with an increase in Mn concentration due to their small crystallite sizes. According to Šalkus et al. (2014), and Bellino et al. (2006), the increase in total electrical conductivity at room temperature can be attributed to the reduction in average grain size [34, 35]. This behavior may be explained by the predominance of grain boundary conduction in nanomaterial structures. The reduction in grain size is often accompanied by an increase in grain boundary ionic diffusivity [36]. Also, as a result of an increase in temperature, charge carriers move more freely, resulting in an increase in AC conductivity. According to Jonscher's power law [37], conductivity and frequency are related in the following way:

$$\sigma_{AC} = A\omega^{s}$$
Fig. 7
figure 7

a–d AC electrical conductivity (\(\sigma_{AC}\)) of CdS and Mn-doped CdS nanoparticles at temperatures (30, 70, 110, and 150 °C) as a function of frequency

Full size image

In this equation, \(A\) is a complex parameter that is weakly affected by temperature, \(\omega\) is the angular frequency, and \(s\) is an exponent which indicates how much interaction between mobile ions and lattices exists at varying temperatures. In addition, conductivity increases with frequency at hopping frequencies due to the relaxation of charge carriers. At high temperatures, this phenomenon is caused by polaron hopping of charged species, and at high temperatures, the hopping frequency is related to a higher frequency region. However, as temperatures rise, the charge accumulates at grain boundaries with sufficient energy to overcome the barrier. An index \(s\) corresponds to a back hopping rate divided by a site relaxation time. Back hopping occurs whenever an ion returns to its first site due to either a bad site or Coulomb-repulsive interactions. When \(s\) = 1, there is a greater probability of hopping motion, while \(s\) > 1 implies re-orientational motion, and the ions are well-located. In a similar manner, when the mobile ion is well-localized, the dipole can orient itself in the direction of the applied electric field while re-orientational motion is expressed [38, 39]. Furthermore, the temperature dependence of the frequency exponents of CdS and Mn-doped CdS is discussed. As the temperature increases, the value of the frequency exponent decreases. This is in accordance with the correlated barrier hopping model CBH [40, 41]. In AC conductivity, Mn elements are doped through lattice vibrations, resulting in phonons that reduce carrier mobility. Mn doping caused by defects or impurities in CdS may also contribute to the reduction in AC conductivity at high temperatures [42]. It is a very important aspect of AC conductivity in Mn-doped CdS. In terms of AC conductivity, its behavior is similar to that of the dielectric constant. The reason for this is that the value rises with temperature up to 70 °C, but then declines, whereas the sample Mn = 5% rises linearly with temperature.

3.4.3 Electric modulus

Studies have been done to inhibit high-loss effects at low frequencies on the electric modulus formalism. The electric modulus plays a crucial role in determining the bulk response of the material. It is discussed as a consequence of eliminating electrode polarization and inhibiting conduction relaxation responses. Considering the complex electric modulus of the material, it is evident that the carriers are dynamically relaxed [43]. It is necessary to have a complex electric modulus in order to understand electrical relaxation, which is determined by the following relationship [33, 44]:

$$M^{ * } (\omega ) = 1/\varepsilon^{ * } (\omega ) = M^{\prime} (\omega ) + iM^{{\prime\prime}} (\omega ) = \varepsilon^{\prime} \varepsilon^{{\prime}2} + \varepsilon^{{\prime\prime}2} + i\varepsilon^{{\prime\prime}} \varepsilon^{{\prime}2} + \varepsilon^{{\prime\prime}2}$$

The complex electric modulus \(M^{ * }\) (complex relative permittivity) of the system represents the real (\(M{\prime}\)) and imaginary (\(M^{^{\prime\prime}}\)) parts of the complex electric modulus. The complex electric modulus can be determined by measuring the real and imaginary parts of the dielectric. Figure 8a–d illustrates the electric modulus versus frequency of CdS and Mn-doped CdS at different temperatures and Mn (3, 5, and 7%) concentrations. According to the figure, \(M^{\prime}\) values increase slightly with the temperature of all samples and exhibit similar behavior with increasing frequency. There is a very slight increase in value with increasing frequency when Mn is incorporated, and the value reaches its maximum at high frequencies[45]. This can be seen in the electrode polarization phenomenon that is neglected in materials [46].

Fig. 8
figure 8

a–d Real part of the electric modulus (\(M{\prime}\)) of CdS and Mn-doped CdS nanoparticles at temperatures (30, 70, 110, and 150 °C) as a function of frequency

Full size image

Figure 9a–d illustrates the relationship between the imaginary part of the electric modulus \(M^{^{\prime\prime}}\) and the applied frequency. In the case of CdS and Mn-doped CdS at low frequencies, the values of \(M^{^{\prime\prime}}\) are close to zero regardless of the Mn concentration or the temperature. Additionally, this figure illustrates that \(M^{^{\prime\prime}}\) increased for all samples up to Fmax, then decreased again except those containing 5% Mn as the frequency increased at T = 30 °C. With the exception of samples containing Mn = 5%, peak positions increase with increasing frequency and decrease with decreasing Mn concentrations. As Mn content increased in samples Mn = 5%, peak positions shifted to lower frequencies. Therefore, Mn = 5% has the lowest conductivity, which is in accordance with Fig. 9a–d. Shifts at high frequencies are caused by an increase in conductivity. In the F < Fmax region, the ions moved a long way between sites, thus offering excellent hopping performance. When F > Fmax, carriers are trapped in potential wells and move short distances as a result of relaxation [47, 48]; at Fmax, long-range mobility transitions to short-range mobility [49, 51]. According to Fig. 9a–d, the peak position of \(M^{^{\prime\prime}}\) shifts to a high frequency and then to a low frequency up to 150 °C, where sample Mn = 5%, shows a peak shifting to a high frequency with temperature which consistent with the conductivity and dielectric measurements.

Fig. 9
figure 9

a–d Imaginary part of the electric modulus (\(M^{^{\prime\prime}}\)) of CdS and Mn-doped CdS nanoparticles at temperatures (30, 70, 110, and 150 °C) as a function of frequency

Full size image

4 Conclusions

A chemical precipitation method was used to prepare CdS and Mn-doped CdS nanocomposite at different Mn concentrations (Mn = 3%, 5%, and 7%). SEM analysis indicated that Mn doping decreased particle size. According to the FTIR results, Cd-S bands in the range (606–615 cm–1) of CdS and Mn-doped CdS nanocomposite changed due to particle size changes caused by Mn doping. Raman spectra of CdS show a longitudinal optical mode (LO) at 305 cm−1 and a weak overtone at 603 cm−1. In Mn-doped CdS, these bands changed to 301 cm−1 and 601 cm−1 due to surface optical phonons (SOPs) and the confinement effects. At temperatures ranging from 298 to 423 K, the change of the real \((\varepsilon{\prime} )\), imaginary \((\varepsilon^{^{\prime\prime}} )\) parts of the dielectric constant and AC conductivity \((\sigma_{AC} )\) was analyzed at frequencies ranging from 0.1 Hz to 1 MHz. At temperatures (30, 70, and 110 °C), AC conductivity and dielectric constant increased at 3% Mn content, but decreased at 5% Mn content; these parameters decreased at 3% Mn content at 150 °C.