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Vol. 9. Issue 1.
Pages 364-372 (January - February 2020)
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Vol. 9. Issue 1.
Pages 364-372 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.065
Open Access
Enhanced photoluminescence effects in nanostructured cubic CdS matrix doped with Cu2+ obtained by chemical Bath deposition
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J.I. Contreras-Rascóna, J. Díaz-Reyesb,*, A. Flores-Pachecoc, L.E. Serrano-de la Rosad, P. del Ángel-Vicentee, R. Lozada Moralesf, M.E. Álvarez Ramosc, P. López-Salazarg
a Benemérita Universidad Autónoma de Puebla, Complejo Regional Centro, Campus San José Chiapa. 2 Sur, Ciudad Modelo, Puebla 75010, Mexico
b Centro de Investigación en Biotecnología Aplicada, Instituto Politécnico Nacional. Ex-Hacienda de San Juan Molino. Km. 1.5. Tepetitla, Tlaxcala 90700, Mexico
c Posgrado en Nanotecnología, Departamento de Física, Universidad de Sonora. Apdo. Postal 1626. Hermosillo, Sonora 83000, Mexico
d Benemérita Universidad Autónoma de Puebla, Laboratorio Central. Ciudad Universitaria. Puebla, Puebla 72570, Mexico
e Instituto Mexicano del Petróleo, Dirección de Investigación y Posgrado. Eje Central Lázaro Cárdenas 152. Ciudad de México 07730, Mexico
f Benemérita Universidad Autónoma de Puebla, Facultad de Ciencias Fisicomatemáticas. Av. San Claudio y Av. 18 Sur. Col. San Manuel, Ciudad Universitaria. Puebla, Puebla 72570, Mexico
g Centro de Investigación en Dispositivos Semiconductores, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla. Av. 14 Sur y San Claudio. Edif. IC5, Ciudad Universitaria. Puebla, Puebla 72570, Mexico
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Table 1. The results of the elementary quantitative analysis of the glass substrate and the thin films obtained by X-ray fluorescence spectroscopy are presented.
Abstract

In the present work, we highlight the enhancements of the photoluminescent properties of the binary II-VI nanocomposite thin film semiconductor cadmium sulphide (CdS) doped with the metallic ions Cu2+ obtained by low-temperature chemical bath deposition (CBD). The doping percentage of the CdS matrix was around 1.74% determined by X-ray fluorescence spectroscopy. The most intense photo-electronic transitions of Cd 3d5/2 (404.94 eV), S 2p3/2 (161.52 eV) and Cu 2p3/2 (933.27 eV) were detected by X-ray photoelectron spectroscopy. The crystallographic study shows that the preferential growth planes are cubic (111) both in the matrix and the doped samples. HRTEM micrographs exhibit the reduction of the particle size of the CdS matrix from 5.87 to 4.76 nm in the doped sample, which confirms the quantum confinement effect. The first optical effect of the doped CdS was noticed in Raman spectroscopy by a frequency shift of the first longitudinal optical mode (1LO-CdS) from 306 to 302 cm−1, related with the shrinking of the nanoparticle, also a great improvement in the sensibility of the characterization by the surface effects of the Cu2+ metallic ion. Photoluminescence was measured in the temperature range of 258–298 K, which showed more recombination emissions of energetic excitons as consequence of the decrease in particle size and the defects created by the Cu2+ metallic ion in the doped sample in whole the range of investigated temperatures. Also the pronounced stimulation of the luminescent spectra of the semiconductor compound at room temperature.

Keywords:
CBD-CdS
Doping copper
Raman spectroscopy
XPS
XRD
Surface effects
Photoluminescence
Full Text
1Introduction

The development of II-VI semiconductor nanocomposites doped with metallic ions has gained a lot of attention in different research groups given the possibilities of modification of the structural, optical and electrical properties, thanks to the quantum confinement effect [1]. Particularly, using the copper ion Cu2+, remarkable changes in the optical properties of the nanocomposite were noticed [2]. Some synthesis techniques to obtain this kind of semiconductor are spray pyrolysis (SP) [3], successive ionic layer adsorption and reaction (SILAR) [4] and chemical bath deposition (CBD). Being this last one the most relevant in the fabrication of this kind of materials. This technique employs a controlled chemical reaction to deposit a thin film through a complexing agent [5], it was chosen because does not require high vacuums or high temperatures. There is no emission of toxic gases, and the preparation of the solutions is carried out at microscale, so that it is environmentally friendly [6], the films can be grown in large areas. This in addition to a good reproducibility [7] makes CBD a feasible technique for mass production. Particularly, cadmium sulphide (CdS) is a direct wide band gap semiconductor. With applications in optoelectronic devices in wavelengths between blue and green in the visible spectrum [8]. In addition, it is used as part of the active layers p- or n-type of Thin Film Transistors (TFT) [9]. One of the most popular applications of CdS is as the window layer in high performance copper, indium, gallium, selenium thin film photocells, which achieved a worldwide efficiency record of 16.5% using CBD-CdS [10] and an outstanding performance of 18.6% in CdS/CdTe semiconductor-based commercial products [11]. Those excellent results are in part consequence of a peculiarity of this kind of binary nanocomposites, which increases the lifetime of its excess carriers [12]. Under special conditions, like doping with copper, p-type CdS films can be obtained as consequence of the shift of the excess carriers from n- to p-type that also increases the conductivity up to 9 magnitude orders [13]. The key factor to the switch of p-type excessive carriers is due to the raise of the position of the valence band edge. The incorporation of dopants occurs at the surface of the grown crystal; the formation of compensating centres will be mostly determined by the conditions on the surface and not by the volumetric conditions, consequently, could be enough to increase the valence band edge at the surface. One possible route to achieve this is through surface segregation that usually occurs if atoms of very different size are available simultaneously. So, it becomes energetically favourable for the larger atom to occupy surface sites where there is more space available than in the volumetric section of the semiconductor [14]. CdS photocells activated by Cu-doping exhibit enhanced photovoltaic effects [4]. All of this, incentives the continuous work in this kind of nanocomposites to pursue further enhancements.

2Experimental details2.1Thin film deposition

The nanocomposite CdS matrix was obtained using the chemical bath deposition technique, with a deposit temperature of 20 ± 2 °C without special atmosphere. Aqueous solutions of CdCl2 (0.01 M, 20 mL) as Cd+2 ions precursor, NaOH (0.05 M, 10 mL), NH4(NO3)2 (0.5 M, 15 mL) as complexing agent, SC(NH2)2 as S2− ions precursor that were prepared separately with deionized water (18 MΩ), which were poured and mixed in the mentioned order keeping a neutral pH during reaction. Before introducing the soda-lime glass substrates to the chemical bath, a 48 h cleaning process was performed on them using an H2O−CH3CH2OH–K2Cr2O7 solution to get better adherence of the nanocomposite film. The chemical bath deposition is performed in a PolyScience refrigerated circulating bath with a temperature controller with ±0.005 °C precision. During the reaction, a colour change in the solution was observed, from light green when the reaction starts, to metallic yellow, obtaining thin films with a uniform metallic yellow with good stoichiometry and controlled thickness, achieved in a deposit time of 60 min. The films are removed from the chemical bath and cleaned with deionized water. The Cu-doped nanocomposite was obtained by the addition of the metallic ion Cu precursor Cu(NO3)2 (0.1 M, 0.05 mL) before the complexing reaction. After a visual inspection, two representative samples were chosen: S0 for the CdS matrix and S1 for the cadmium sulphur doped with copper.

2.2Characterization

All characterizations are performed on thin film presentation, where the thickness of the films was measured using a Philips PZ2000 200 mm laser ellipsometer. The atomic composition and imaging were obtained with a JEOL JSM-7800 F SEM with an EDS detector Bruker XFlash 6/60 installed, adjusted at 1 kV as acceleration voltage for the caption of the micrographs, 3.5 and 10 kV acceleration voltages were needed to obtain a reliable quantitative composition analysis for the samples S0 and S1, respectively. Surface morphology studies were performed on a VG-Thermo-Fisher XR3 XPS, using an Mg k-alpha X-ray source filament (1253.6 eV) with 15 kV polarization. Semi-quantitative elemental analyses were measured using the QUANT-EXPRESS (Fundamental Parameters) method in the range of sodium (Na) to Uranium (U) in a sequential X-ray Fluorescence spectrometer of dispersive wavelength of 1 kW brand BRUKER model S8 TIGER. The detector type is by Blink (heavy elements) and Flow (light elements). The X-ray source is a Rhodium (Rh) tube. In addition, the Goniometer is of high precision for theta and 2 theta angles. X-ray diffraction analysis of the films was done on a Bruker D8-Discover diffractometer with parallel beam geometry using a copper k-alpha source of 1.54 Å operating at 40 kV, 40 mA. Transmission electron microscopy (TEM) micrographs were obtained on a Tecnai G2 T20 TEM, operating at 300 kV. Raman spectroscopy was performed on Horiba LabRam HR Micro Raman, with a 488 nm Omnichrome series 43 laser installed. Photoluminescence studies were executed in an Agilent Cary Eclipse Spectrofluorometer with 250–1100 nm operating range, with ±0.2 nm resolution.

3Results and discussion

The measurement of the thickness by ellipsometry of both samples ensures a good control of the surface growth of the film. Fig. 1 shows the thickness distribution of thin film of the typical sample S0, which has a mean thickness of 100.99 nm, with a standard deviation of ±7.68 nm of 100 points measured from the surface. The thickness distribution in the case of doped sample S1 had a size distribution with an average thickness of 97.55 nm and a standard deviation of ±2.61 nm of the same number of measured points.

Fig. 1.

It shows the thin film thickness distribution of the sample S0.

(0.19MB).

Fig. 2 presents two X-ray fluorescence spectra obtained for the sample with the highest retention (sample S1), lowest retention (sample S0). The emission lines Kα and Kß of sulphur can be observed at around 2.30 and 2.46 keV, respectively. Around 3.12 and 3.31 keV the emission lines Lα and Lß of cadmium appear and around 0.94 keV the emission line of copper Lα1 and in 8.04 and 8.89 keV the lines Kα and Kß of copper. The following elements silicon (Si), calcium (Ca), sodium (Na), magnesium (Mg), aluminium (Al), potassium (K), sulphur (S), iron (Fe) were registered on the optical glass substrate, strontium (Sr) whose emission lines are presented as delivered by the X-ray fluorescence equipment. The experimental molar fractions of the chemical elements present in the glass substrate and the thin films are summarized in Table 1. Given that the unitary cell of CdS has two atoms of cadmium and two atoms of sulphur, the atomic weight of the ideal unit cell is ∼288.95 u that corresponds to 22.19% sulphur atoms and 77.81% cadmium atoms. In comparison, the quantitative X-ray fluorescence analysis shows a ratio of 61.77% Cd and 38.23% S in the sample S0, which provides a nanocomposite rich in sulphur. The cadmium percentage decreases and also sulphur has a slightly decrease in the doped sample S1, that has 1.74% of atomic copper proportion. These stoichiometry differences could be attributed to lattice defects like sulphur vacancies (VS2-), interstitial sulphur (IS2-), interstitial cadmium (ICd2-), interstitial oxygen  (IO1-). Due to the sulphur excess and the subsequent deficit of cadmium, interstitial sulphur and cadmium vacancies could be found in the sample S1. The diminishing of Cd in the sample, it is indicative of the substitution of cadmium atom for the copper ion that enters to the lattice, also the oxygen observed in the matrix S0 was depleted in the doped sample S1 with the inclusion of possible interstitial copper atoms.

Fig. 2.

X-ray fluorescence quantitative analysis of samples S0 and S1.

(0.18MB).
Table 1.

The results of the elementary quantitative analysis of the glass substrate and the thin films obtained by X-ray fluorescence spectroscopy are presented.

Chemical element  Atomic number (Z)  Impurity molar fraction in the glass substrate  Elements molar fraction in S0  Elements molar fraction in S1 
Cd  48  –  61.77%  61.03% 
16  0.10%  38.23%  37.23% 
Cu  29  0.00%  –  1.74% 
Si  14  29.80%  –  – 
Ca  20  6.13%  –  – 
Na  11  5.96%  –  – 
Mg  12  1.63%  –  – 
Al  13  0.59%  –  – 
19  0.40%  –  – 
Fe  26  978 PPM  –  – 
Sr  38  410 PPM  –  – 
Zr  40  30.2 PPM  –  – 

The high-resolution XPS surface analyses are shown in Fig. 3. The matrix S0 shows strong, symmetrical and clean signals from cadmium 3d doublet 3d3/2 (411.69 eV) and 3d5/2 (404.94 eV), which have a slight widening that are illustrated in Fig. 3a typical of the CdS II-VI binary [15]. The XPS spectra of the 2p doublet of sulphur illustrated on Fig. 3b show a chemical shift of 2.4 eV at lower binding energies from elemental sulphur [16] shown in the photoelectron peaks of 2p1/2 (162.67 eV) and 2p3/2 (161.52 eV) also characteristic of cadmium sulphide [17]. The first surface effect of the copper doping in the sample S1 was observed in the reduction of the charge-effect ionization from 3.92 eV found in the matrix, at 2.89 eV in the doped sample, as is shown in Fig. 4. The copper ions were found in the first nanometres of the film, as possible CuS compounds with a 2p doublet with binding energies 2p1/2 (953.25 eV) and 2p3/2 (933.27 eV) [18], see Fig. 3c.

Fig. 3.

High-resolution photoelectron spectra of the binding energy ranges of (a) cadmium in S0, (b) sulphur in S0 and (c) copper in S1.

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Fig. 4.

Adventitious C 1s (284.8 eV) charge effect shift of the doped sample versus the undoped matrix.

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X-ray diffraction measurements confirm a high crystallinity degree in the CdS matrix, sample S0, displayed in Fig. 5a with a preferential growth plane (111) of the cubic zinc blende type phase, indexed in the crystallography open database [19] entry (COD#1011251). The background signal is an expected effect due to the amorphous substrates. Fig. 5b displays the polycrystalline structure that was found in the doped sample S1, with the growth planes (1–10) and (01-1) of the triclinic CuO9S phase (COD#9014405), also the planes (111) of cubic CdS and (210) of cubic CuS2 phase (COD#9000742) confirming the Cu-S interaction observed in XPS spectrum. Additionally, the peaks observed below 10° in the diffractogram are related to the chalcanthite structure (CuO9S) (COD#96-901-4406), which is a segregate that formed before the Cu2+ is incorporated into the matrix. The monocrystalline nature of the undoped CdS thin film can be observed in the TEM micrograph shown in Fig. 6. Bragg’s law states that the interplanar distances are associated with the angle where the constructive interference of the incoming X-rays occurs in crystals, with the relationship given by the Bragg’s condition equation:? ??? = 2??????????. In the case of the sample S0, an interplanar distance of 3.35 Å that has the same value of 2θ of the (111) CdS cubic phase. A typical particle diameter of around 5.87 nm was measured, so that this semiconductor can be classified as a nanocomposite. For the Cu-doped sample, the effects of doping are clear in TEM the micrographs of the sample S1 shown in Fig. 7, where the quantum confinement is evident due the reduction of the particle diameter to an average size of 4.76 nm. In addition, the change to a polycrystalline structure of the doped sample is confirmed in the micrographs, with different crystalline domains that are clearly visible in figure.

Fig. 5.

X-ray diffractograms of the samples (a) S0 and (b) S1.

(0.23MB).
Fig. 6.

It is shown a HRTEM micrograph of the sample S0, in which the size of the nanocrystals and the interplanar distance are shown.

(0.49MB).
Fig. 7.

Three HRTEM micrographs of the S1 sample obtained from different areas are shown, where the nanocrystals that make up the thin film observed are illustrated.

(1.44MB).

Vibrational Raman spectroscopy in nanocomposite semiconductors is a non-destructive technique that can give information about crystallinity and surface conditions. Crystalline samples show narrow Raman shift peaks, while polycrystalline or amorphous materials show broad Raman peaks. The Raman spectra of both samples are shown in Fig. 8, which have a good crystallinity. After adjusting the Raman spectrum of sample S0 using the Lorentz line shape as is shown in Fig. 8a, it was possible to detect three CdS-like optical longitudinal modes at 306 cm−1 (1LO), 607 cm−1 (2LO) and 913 cm−1 (1LO + 2LO) [20]. The small peak found around 156 cm−1 can be attributed to the sulphur molecular vibrations [21]. Fig. 8b illustrates the Raman spectrum of the doped sample S1 showing the downshifts of all longitudinal optical modes at 301.63 cm−1 (1LO), 598.53 cm−1 (2LO) and 899 cm−1 (1LO + 2LO), which can be attributed to lower frequencies of vibrations as a consequence of the higher density and smaller particle size of the doped nanocomposite. The formation of two new I-VI compounds, cupric sulphide (CuS) with optical longitudinal modes at 260 cm−1 (1LO), 520 cm−1 (2LO) [22] and cupric oxide (CuO) with B1g symmetry modes at 344 and 688 cm−1 were detected [20]. A similar effect to the surface-plasmon-resonance effect like the used on Surface Enhanced Raman Scattering (SERS) [23] is shown in Fig. 9, which could be associated with some remaining copper atoms located in the surface of the nanofilm. From the TEM micrographs (Figs. 6 and 7), a mean particle radius of 2.94 nm for the undoped sample S0 and 2.38 nm for the Cu doped sample S1 confirming the results obtained by X-ray diffraction, which in the doped sample the particle size is smaller than the exciton Bohr radius of CdS of 2.8 nm [24]. Although, slightly higher radius values, 3 nm, have also been reported [25], so the independent confinement of electrons and holes takes place and leads to a blue shift in the optical properties [24].

Fig. 8.

It illustrates the Raman spectra of the samples (a) S0 and (b) S1.

(0.34MB).
Fig. 9.

Raman spectra of both samples without background removal are shown.

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Fig. 10 illustrates the photoluminescence spectra measured at different temperatures of the samples. Fig. 10a shows the photoluminescence spectra of the undoped sample. The photoluminescent characteristic shows a remarkable emission stimulation that can be seen in Fig. 10b given by the creation of new potential barriers attributed the grain boundaries of the polycrystalline nature of the doped sample, which has been previously reported in thin films with small grain size [26]. That remains relatively stable in the whole range of investigated temperatures (258–298 K). The photoluminescence spectrum at the lowest study temperature of the sample S0 shown in Fig. 11a, it exhibits the first identifiable peak at 1.50 eV in the region of low energies, so it is associated with deep residual impurities. The peak at 1.58 eV is related to the sulphur vacancy (VS2-)[27]. The peak observed around 1.76 eV could be attributed to the electron-phonon interaction and size distribution of the nanoparticles, which has been previously associated with the recombination of trap surface states [23,28]. The peaks observed at 2.15 and 2.42 eV have been previously attributed to the interstitial cadmium (ICd2+) and interstitial oxygen (IO1-) defects [27], respectively. At 2.55 eV the main excitonic recombination (D0X) emission of CdS [20] was found. Finally, the most energetic emission at 2.75 eV found in the undoped sample can be related to the quantum confinement associated with the presence of some particles with radius values below the value of the Bohr excitonic radius, as shown in the HRTEM micrograph illustrated in Fig. 7. The photoluminescence spectrum measured at the lowest study temperature of the doped sample S1, which is shown in Fig. 11b, has a considerable reduction of approximately 80% of the interstitial defects of cadmium and oxygen compared to the matrix, which has the same behaviour observed in the EDS analysis. The presence of multiple peaks of high energy after the 2.55 eV main exciton recombination (D0X), with values of 2.60, 2.75 and 2.85 eV that are evidence of the increase of the nanoparticle population into the quantum confinement regimen, consequence of the interactions with the Cu2+ copper ion.

Fig. 10.

Photoluminescence spectra of the samples (a) S0 and (b) S1 measured in the temperature range of 258–298 K.

(0.44MB).
Fig. 11.

It shows the deconvolution of the photoluminescence spectra measured at 258 K of samples: (a) S0 and (b) S1.

(0.45MB).
4Conclusions

The influence of the metallic ion Cu2+ in the structural and optical characteristics are quite remarkable due to the noticeable changes in the response of the light stimulation with more energetic emissions in the optical characterizations. These quantum effects are due to the nanometric nature of the material, allowing the possible applications of this wide bandgap nanostructured semiconductor in photo-electronic devices working in the blue part of the visible range like light emitting diodes, which can be synthesised by a low energy consuming technique like chemical bath deposition. That also it has good reproducibility and growth control with minimal environmental impact.

Acknowledgements

The authors want to thank to the Department of Physics Nanotechnology Postgraduate Program of University of Sonora and to CONACyT through grant 2015-01-255791, which strengthen the scientific and human endeavour of the authors.

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