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Influence of chromium concentration on the structural, electronic structure, optical and temperature dependent magnetic properties of ZnS nanocrystals

  • Soumendra Ghorai
  • Nirmalendu Patra
  • Dibyendu Bhattacharyya
  • Shambhu Nath Jha
  • Bishwajit Ray
  • Sandip Chatterjee
  • Anup K. GhoshEmail author
Article
  • 33 Downloads

Abstract

The effect of Cr dopant on the structural, optical, magnetic properties and local electronic structure of aqueous synthesis derived Zn1−xCrxS diluted magnetic semiconductor nano crystals have systematically investigated. The nano crystalline structure and crystallite size have been estimated by X-ray diffraction measurements with Rietveld refinement and high-resolution transmission electron microscopy. Effective increase of the lattice parameter has been observed in doped samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder and defects in the host lattice. The tetrahedral coordination of the sulfur ions surrounding the zinc ions has been studied by FTIR analysis. The decrease of energy band gap for Cr doped samples has observed. Blue emission has been observed by photo luminescence spectroscopy due to defect formation (Cri) in Cr-doped samples. The local electronic structures of Zn and Cr sites are thoroughly studied by synchrotron based X-ray absorption spectroscopy comprising of both X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). EXAFS studies indicate the presence of secondary phase within the sphalerite lattice of diluted Zn1−xCrxS nanocrystals while XANES studies exhibit single pre-edge feature. The amplitude of such pre edge is found to be independent of Cr amount on doped ZnS nanocrystals. The results demonstrated that diluted Cr3+ ions are substituted on the host ZnS nanocrystal. The Cr doped ZnS sample shows paramagnetism at room (300 K) and at low (5 K) temperature. The Cr–S bonds are the crucial premise for paramagnetic ordering.

1 Introduction

The semiconductor nano crystals (NCs) have been studied vastly to explore their distinctive physical properties [1, 2]. The engineering of micrometer to nanometer scale semiconductor materials with spherical morphologies have great application in fundamental and applied research [1, 2, 3, 4]. The incorporation of semi magnetic impurities into semiconducting lattice has an immense effect on optical, magnetic or other physical properties of the semiconductor and on their device application (optoelectronics, spintronics etc.) [3, 4]. Such properties can be tuned by chemical reaction (ambient and surface) condition of nano crystals (NCs) [5]. Among the well-studied host II–VI semiconductors, the zinc sulfide (ZnS) is an assuring material for device applications because of its wide energy band gap [2, 3]. The chromium (Cr) doped ZnS nano crystals have great attention for extraordinary properties in nano scale forms to develop in optical devices [2, 3, 4, 5]. Incorporation of Cr2+ in ZnS with molecular beam epitaxy has been reported by Ichino et al. [6], and Cr3+ has been reported with the laser ablation method [7]. Among three oxidation states of chromium, the Cr3+ state is favored one due to partially filled energy level.

There are scanty reports available regarding structural, optical, local structure and mechanism of magnetism in Cr doped ZnS host. Kaur et al. have shown that the local structure of Cr doped ZnS with Cr K-edge X-ray absorption spectroscopy spectra [8, 9]. Zeng et al. have concluded the paramagnetic nature of the Cr doped ZnS sample with trivalent oxidation state and purple color luminescence emission [10]. Reddy et al. have observed the effect of annealing temperature on structural, optical and magnetic properties of Cr doped ZnS nano particles [11]. Zhang et al. have shown ferromagnetism of Cr doped ZnS nano sheets with a Curie temperature above room temperature with first principle computational calculation [12]. Chawla et al. have pointed out the super paramagnetic nature of Cr doped ZnS nano particles with spin ordering and single domain magnetism of those samples at low temperature [13]. Car et al. have reported the local structure of diluted Cu doped ZnS nano crystals with significant fraction of Cu defect [14].

Since few reports are available on local structure, adequate information are unavailable on EXAFS based local structures of Zn1−xCrxS nano crystals (NCs). Here, we have presented a systematic study on the local structure of Zn1−xCrxS around Zn ion and Cr ions using synchrotron based X-ray absorption spectroscopy that include extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES). The structural information has also been complemented with results of X-Ray Diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) measurements. Optical properties of the prepared samples have been studied by the Ultra-violet visible (UV–Vis) spectroscopy and photoluminescence spectroscopy. Magnetic properties have been observed by superconducting quantum interference device magnetometer (SQUID). The quantum confined Cr doped ZnS nano crystals discussed here with combination of magnetic behavior and blue luminescence properties can be used for optical devices, spintronics and biomedical application.

2 Experimental section

2.1 Synthesis of samples

Chemicals were used as received, without further any purification. Zinc acetate (min assay ~ 98%), sodium sulfide (min assay ~ 58–62%), polyvinyl pyrolodine (PVP, K-30 and min assay ~ 11.5–12.8% N), Chromium chloride (CrCl3,anhydrous, min assay ~ 99%) were obtained from HIMEDIA while Ethanol (absolute for analysis) was obtained from Merck. The Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals (NCs) were synthesized by aqueous synthesis procedure have named as pure ZnS, 0.5Cr:ZnS, 1Cr:ZnS, 2Cr:ZnS, 4Cr:ZnS, 6Cr:ZnS for different Cr concentration x = 0, 0.5%, 1%, 2%, 4% and 6%, respectively. Water-dispersive nano crystals (NCs) have been prepared by capping the NCs with a hydrophilic polymer polyvinyl pyrolodine (PVP) as a stabilizer. To describe the synthesis procedure briefly, in a two necks round bottom flask, appropriate amount 4 mmol zinc acetate, 5 µmol PVP and required amount of CrCl3 were dissolved in 100 ml de-ionized water under stirring at room temperature for 1 h. Subsequently the flask was shifted to a hot plate maintained at temperature of 200 °C (± 0.5 °C). To it, 100 ml aqueous solution of Na2S was added drop wise under vigorous stirring and the reaction was continued for 2 h. Then the reaction temperature was slowly cooled down to room temperature. Excess amount of polymers was washed out by repeated re-dispersion and centrifugation (~ 12000 rpm for 30 min) with de-ionized water followed by the same with ethanol. Finally, formed NCs were dried under vacuum oven at 50 °C for 12 h. The white (pure ZnS) and shallow grayish (Cr doped ZnS) powder were obtained on grinding the dried precipitate.

2.2 Characterization

Structural characterization of NCs has been performed by an X-ray diffractometer (Model: Miniflex-600, Rigaku, Japan) using CuKα radiation (λ = 1.5406 Å) at room temperature. The XRD patterns were collected between the range 20° to 80° with a scanning rate of 10° per minute and step size of 0.02° (2θ). Rietveld refinement has been carried out by considering Pseudo-Voigt function [15, 16] by using ‘Fullproff Suite’ software [17], and the fcc unit cell of ZnS with (216) space group as starting model. The transmission electron microscope (TEM), high resolution TEM (HRTEM) image and selected area electron diffraction (SAED) patterns were taken with a ‘Technai G2 Twin field emission transmission electron microscope’ (FEI, Netherland) operated at an accelerating voltage of 200 kV, current 12 µA. The size of nanocrystallites are analyzed by ‘Image-J’ software [18], and observed the diameters of well dispersed nano crystals in TEM micrographs. Raman spectra have been taken with a micro Raman system ‘LABRAM-HR visible’ (Model: HR800, Horibra Jobin–Yvon) with 632.81 nm laser as a exciting source with a acquisition time 100 s and 9 mW power laser head to avoid structural deformation of samples due to laser heating. ‘Perkin Elmer Instrument’ has been used for Fourier-transform infrared spectroscopy measurements with mixing appropriate amount of KBr and samples. ‘Perkin Elmer Instrument’ (Lambda-25, USA) has been used to get absorption spectrum with step size of 1 nm and the samples are well dispersed in bi-distilled water. The PL emission measurements of powder NCs samples have been performed by employing the ‘Edinburgh spectro fluorometer’ (Model: FLS 900P) equipped with a 450 W Xenon flash lamp at room temperature with ‘HS RED’ PMT detector and integration time of 1 s.

Observation of local atomic structure of prepared samples have been done by X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The room temperature XANES and EXAFS measurement of the prepared samples have been carried out at the Energy Scanning EXAFS beam line (BL-9) at Indus-2 Synchrotron source (2.5 GeV, 100 mA and with beam size 0.5 mm × 0.5 mm) at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. The beam line operates in the photon energy range of 4–25 keV and offers a typical resolution (\( \Delta E/E \)) of 10−4 at 10 keV photon energy.

For the present set of samples, the EXAFS measurements at the Zn K-edge (9659 eV) was performed in transmission mode and at the Cr K-edge (5989 eV) measurements were performed in fluorescence mode and the final data has been taken by averaging over four scans. To obtain a reasonable intensity at K-edge, appropriate weights of the powdered samples have been mixed thoroughly with cellulose powder (total weight 100 mg) and pressed to get 10 mm diameter (circular), 1 mm thick homogenous pellets. These pellets have been encapsulated in tape to attach the sample holder.

The field depended magnetic measurements were carried out on powdered samples by using a 7 T superconducting quantum interference device (SQUID; MPMS Quantum Design, Model: 1802) magnetometer [10, 19]. Magnetization vs. magnetic field measurements have carried out within the range of magnetic field + 20000 Oe to − 20000 Oe at 300 K and 5 K temperature under pressure of 3.5 KPa. For field dependent magnetization 0.5075 g 6Cr:ZnS sample and 0.3157 g pure ZnS sample have taken for measurements.

3 Results and discussion

3.1 X-ray absorption spectroscopy

Synchrotron based X-ray absorption spectroscopy (XAS) studies have been carried out at the Zn K-edge and Cr K-edge of the Zn1−xCrxS nanocrystals to get information about the local structure and local environment of all the samples. The XANES study gives information about the oxidation state, chemical environment and structural symmetry while EXAFS probes the local structure around the respective absorbing atoms and ensure the identification of presence of secondary phase. These give the insight to understand the correlation between optical and structural properties.

In order to compare absorption features quantitatively at the Zn K-edge (9659 eV), the normalized XANES spectra of the Zn1−xCrxS NCs samples and standard pure ZnS (Zn2+) nano crystals (NCs) have been shown in the Fig. 1a.
Fig. 1

a Normalized XANES data at Zn K-edge, b normalized EXAFS data at Zn K-edge, c\( k^{2} \) weighted \( \chi (k) \) spectra at Zn K-edge and d experimental \( \chi (R) \) vs. \( R \) data and best fit theoretical plot (solid line) at Zn K-edge of the Zn1−xCrxS nanocrystals

Figure 1a exhibits that in Zn1−xCrxS NCs samples first inflection point (K absorption edge) at Zn edge matches with that of standard pure ZnS NCs. This manifests that in all the samples Zn remains in +2 state and after doping, the oxidation state of Zn atoms remains unperturbed. The absorption at Zn K-edge arises due to transition of electron from Zn \( 1s \to 4p \) shell and no pre-edge has been observed due to fully filled outer valence ‘d’ shell. A noticeable variation in the white line intensity has been observed which may be due to variation in unoccupied electron density of states in the outer most orbital. The XANES spectra of all synthesized samples show identical features manifesting a similar kind of structural symmetry around the Zn atom and exclude the possibility of formation of zinc oxide in the samples.

Figure 1b shows the normalized absorption (\( \mu (E) \) vs.\( E \)) spectra while Fig. 1c shows the \( k^{2} \) weighted \( \chi (k) \) spectra at Zn K-edge of the Zn1−xCrxS NCs samples. In order to fit experimentally obtained data the standard pure ZnS NCs structure with refined lattice parameter a = 5.35 Å (obtained from Rietveld refinement) has been used as a starting model and respective atomic scattering paths have been generated accordingly. According to this model, Zn atom located at \( (0,0,0) \) position is surrounded by 4 S atoms in the first co-ordination shell at an atomic bond distance of 2.32 Å giving rise to a tetrahedral symmetry and also surrounded by 12 Zn atoms in next co-ordination sphere at a distance of 3.78 Å. For fitting the data the \( k^{2} \) weighted \( \chi (k) \) spectra has been taken within the \( k \) range of 2 to 9 Å−1 for Fourier transform and \( \chi (R) \) vs. \( R \) data are generated, which are then fitted in the \( R \) space up to 3 Å. During fitting co-ordination number (CN) of each co-ordination shell have been kept constant and only the atom-to-atom bond distance (\( R \)) and Debye–Waller factor (\( \sigma^{2} \)) (which gives information about the mean square fluctuation in the atomic bond length) have been varied as fitting parameters. Figure 1d shows the phase uncorrected Fourier transformed \( \chi (k) \) vs. \( R \) spectra of the experimentally obtained data fitted with the theoretically generated model at Zn K-edge of Zn1−xCrxS NCs samples. As mentioned above that at the Zn K-edge, the first co-ordination shell comes at a distance of 2.32 Å that contributes to the first intense peak appearing within the \( R \) range of 1.2 to 2.6 Å distance in the Fourier transformed \( \chi (k) \) vs. \( R \) spectra. However, no significant peak due to the second co-ordination shell has been identified after 2.5 Å, which are due to the higher disorder in that shell. The best fit results at the Zn K-edge EXAFS data of the Zn1−xCrxS NCs samples have been summarized in Table 1.
Table 1

EXAFS best fitted structural parameters (\( R \) and \( \sigma^{2} \)) values at Zn K-edge of the Zn1−xCrxS nanocrystals

Sample

Parameters

Zn–S × 4

\( R \)(Å)

\( \sigma^{2} \)2)

\( R_{factor} \)

Pure ZnS

2.27 ± 0.01

0.008 ± 0.006

0.007

0.5Cr:ZnS

2.27 ± 0.01

0.003 ± 0.001

0.006

1Cr:ZnS

2.28 ± 0.01

0.006 ± 0.001

0.005

2Cr:ZnS

2.27 ± 0.01

0.003 ± 0.001

0.008

4Cr:ZnS

2.27 ± 0.01

0.003 ± 0.002

0.015

6Cr:ZnS

2.28 ± 0.01

0.004 ± 0.001

0.006

The fit parameters for Zn K-edge data are obtained assuming pure ZnS structure at Zn site

The result shows that with increasing doping concentration, there is no variation in the first co-ordination shell (Zn–S) of Zn1−xCrxS NCs samples and it matches with the bond distance (2.27 Å) of the synthesized pure ZnS NCs samples. However, the significant disorder of Zn–Zn co-ordination has observed as average number of neighboring atoms becomes very smaller due to the nano size effect of nanocrystals and at the surface a large fraction of the Zn atoms located which has extra distortions in Zn–Zn co-ordination as average number of Zn–Zn pair becomes negligible. Now in order to get the clarification about the above mentioned observations the EXAFS data have also been analyzed at Cr K-edges (5989 eV).

Figure 2a shows the normalized XANES spectra of the Zn1−xCrxS NCs samples along with the standard Cr2O3 and Cr2(SO4)3 samples measured at Cr K-edge. It has been observed that the energy of Cr K-edge in the doped NCs samples remains little higher than that of Cr2O3 and Cr2(SO4)3 samples [20]. Figure 2a shows the variation in the pre-edge features of all the standards and Cr doped ZnS NCs samples. The pre-edge region of the Cr2O3 sample shows a double pre-edge feature (Fig. 3), which has been reported by other workers [21]. The origin of this doublet feature is related to the quadrupolar \( 1s \to 3d \) transition due to the hybridization of the Cr ‘3d’ orbital with the ‘p’ orbital of the surrounding oxygen ligands. However, Zn1−xCrxS NCs samples and Cr2(SO4)3 sample show only a single pre-edge feature (Fig. 3). The origin of such single pre-edge is feature possibly due to the tetrahedral point symmetry broken leading to the local mixing of ‘3d’ orbital and ‘4p’ orbitals states [22], which acquiescent a local dipole transition into ‘3d’ state. Hence, mixing of localized orbital states creates ‘3d4p’ single states with maintaining same energy [23], and the pre-edge feature exhibits a single \( 1s \to 3d4p \) transition. The amplitude of such pre-edge does not depend on the content of Cr ions in the ZnS host. Therefore, the presences of such ‘d/p’ mixing symmetry are allowed in Zn1−xCrxS NCs samples. The nonlocal peak arises in pre-edge region of the doped samples due to transition of ‘4p’ state with hybridization into ‘3d’ band through nonmetal sulfur ions to the nearest metal neighbors, so the final state is less localized and less influenced by the core hole potential as the localized ‘3d’ state [24].
Fig. 2

a Normalized XANES data at Cr K-edge, b normalized EXAFS data at Cr K-edge, c\( k^{2} \) weighted \( \chi (k) \) spectra at Cr K-edge, d experimental \( \chi (R) \) vs. \( R \) data and best fit theoretical plot (solid line) at Cr K-edge of the Zn1−xCrxS nanocrystals

Fig. 3

Pre edge feature of the Cr doped ZnS nanocrystals with respect to Cr2O3 and Cr2(SO4)3 samples

Figure 2b shows the normalized absorption (\( \mu (E) \) vs. \( E \)) spectra while Fig. 2c shows \( k^{2} \) weighted \( \chi (k) \) spectra at Cr K-edge of the Zn1−xCrxS NCs samples. For fitting the data, standard pure ZnS structure was used replacing Zn by Cr atom as the central absorbing atom however the data could not be fitted with the above model, however, the experimentally obtained data could be fitted by considering the Cr2(SO4)3 structure [22]. Figure 2d shows the experimentally obtained data fitted with the theoretically generated model. The best-fit parameters obtained from EXAFS measurements have been summarized in Table 2. From Fig. 2d it is quite evident that there is only presence of 6 Cr–O and 6 Cr–S bonds in the doped samples, which contribute to the first and second co-ordination shells at atomic bond distances of 1.98 Å and 3.20 Å respectively. It should also be noted that no significant presence of Cr–Cr bond was observed in the Zn1−xCrxS NCs samples due to the higher structural disorder in the samples and these bonds do not appear significantly in the \( \chi (R) \) vs. \( R \) spectra. The Fourier transformed \( \chi (R) \) vs. \( R \) spectra (experimental) fitted with the theoretically generated model has been shown in Fig. 2d and the best fit values of EXAFS parameters have been shown in Table 2.
Table 2

EXAFS best fitted structural parameters (\( R \) and \( \sigma^{2} \)) values at Cr K-edge of the Zn1−xCrxS nanocrystals and Cr K-edge of Cr2O3 and Cr2(SO4)3 reference samples

Sample

Parameters

Cr–O × 6

Cr–S × 6

Cr–Cr × 4

 

\( R \) (Å)

\( \sigma^{2} \)2)

\( R \) (Å)

\( \sigma^{2} \)2)

\( R \) (Å)

\( \sigma^{2} \)2)

\( R_{factor} \)

Cr2O3

1.99 ± 0.01

0.005 ± 0.001

2.92 ± 0.02

0.009 ± 0.002

0.004

Cr2(SO4)3

1.95 ± 0.01

0.002 ± 0.001

3.19 ± 0.01

0.015 ± 0.005

0.003

0.5Cr:ZnS

1.97 ± 0.01

0.002 ± 0.001

3.29 ± 0.02

0.005 ± 0.003

0.008

2Cr:ZnS

1.97 ± 0.01

0.002 ± 0.001

3.30 ± 0.02

0.008 ± 0.004

0.004

4Cr:ZnS

1.97 ± 0.01

0.004 ± 0.001

3.30 ± 0.02

0.009 ± 0.003

0.005

6Cr:ZnS

1.98 ± 0.01

0.006 ± 0.001

3.31 ± 0.02

0.011 ± 0.003

0.006

The fit parameters for Cr K-edge data are obtained assuming Cr2(SO4)3 structure at Cr site

Table 2 shows that with increase in Cr doping concentration the Cr–O and Cr–S bond lengths remain almost unchanged, though the Cr–S bond appears little higher than that of the pure ZnS NCs sample. Since the ionic radii of Cr3+ and Zn2+ ions are almost similar [25], the Cr–S bond distances should be equal to the Zn–S bond distances if Cr atoms properly replace the Zn atoms on ZnS matrix. However, from the above EXAFS analysis, it has been clearly observed that Cr–S bond distances in the samples are higher than the Zn–S bond distance in ZnS NCs. This observation along with the appearance of a Cr–O peak at ~ 1.3 Å, clearly indicates to the formation of Cr2(SO4)3 like phase in the Cr doped ZnS samples. This confirms that Cr ions remain in an oxidation state of + 3 in the doped samples.

This thermodynamic stability causes due to octahedral co-ordination of ‘d3’ metal ion, has ground state and half filled ‘t2g’ orbital, to charge compensation in ZnS lattice [24]. The Cr3+ is a ‘d3’ metal ion can favors octahedral co-ordination structure. So presence of Cr3+ in Cr2(SO4)3 phase is favorable for stabilization to occupy octahedral site formed by Zn ion vacancy.

3.2 X-ray diffraction

The Rietveld refinement of X-ray diffraction (XRD) patterns for Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nano crystalline samples have been shown in Fig. 4a. Figure 4a shows that all diffraction peaks of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) NCs samples match with the standard Bragg positions of cubic ZnS. The lattice parameter (a) and cell volume (V = a3) have been estimated from the Rietveld refinement of X-ray diffraction data (Fig. 4b). The increase in lattice parameters is observed due to doping of Cr ions.
Fig. 4

a Rietveld refinement of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) NCs samples, b the variation of lattice parameter and cell volume (inset) estimated from Rietveld refinement and c the variation of crystallite size

The variation of crystallite size estimated from Scherrer’s equation [26] has been plotted in Fig. 4c. From Fig. 4c, it has been observed that there is no appreciable change in the crystallite size. The broadened of peaks (FWHM) in XRD patterns have been arisen due to formation of tiny size nano crystals of the pure ZnS as well as Zn1−xCrxS NCs. Close observation of the XRD pattern indicates that the doping of Cr ion into ZnS does not lead to appearance of any extra peak or disappearance of any peak of standard cubic pure ZnS. Hence, no secondary or impurities phase arises due to Cr doping. The microstrain (non-uniform strain) in the samples arises due to dislocation and disorder layer near the surface, which leads to peak broadening. Parameters related to this microstrain can be estimated from the Williamson–Hall (W–H) plot (Fig. 5a) [27, 28], and ‘size-strain plot’ (SSP) (Fig. 5b) [29] which are shown in Table 3. The variation of microstrain obtained from W–H plot and SSP plot are shown in Fig. 5c.
Fig. 5

a W–H plot, b size–strain plot of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals samples and c variation of microstrain of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals

Table 3

Estimated nano crystallite size and micro strain for different synthesized samples

Sample

Nano crystallite size (nm)

Micro strain

From Scherrer’s equation

W–H plot

Size–strain plot

W–H plot

Size–strain plot

Pure ZnS

2.39

2.39

3.94

61.45 × 10−4

99.79 × 10−3

0.5Cr:ZnS

2.30

2.30

3.22

64.90 × 10−4

91.43 × 10−3

1Cr:ZnS

2.41

1.71

2.93

45.82 × 10−4

68.70 × 10−3

2Cr:ZnS

2.65

1.16

2.86

10.47 × 10−4

36.39 × 10−3

4Cr:ZnS

2.52

1.06

2.81

10.00 × 10−4

56.23 × 10−3

6Cr:ZnS

2.61

1.89

3.07

36.67 × 10−4

37.95 × 10−3

3.3 Transmission electron microscopy

Microstructure of the nano crystals has been studied by using transmission electron microscopy (TEM) measurements. It has been found that the pure ZnS nanocrystals have a distribution of size between 1.6 nm to 3.2 nm while 6Cr:ZnS nanocrystals it is between 1.7 nm to 3.2 nm. It should be pointed out that there is no appreciable change in size with Cr-doping. TEM micrographs (Fig. 6) also show that all nanocrystals are nearly spherical in shape and smooth in surface. The inter-planner spacing for (111) plane of the pure ZnS NCs and the 6Cr:ZnS NCs have been estimated from HRTEM patterns which are 0.309 nm and 0.310 nm. The SAED patterns indicate that all the nanocrystals exist in poly crystalline nature.
Fig. 6

a Low magnification TEM, b HRTEM and c SAED images of pure ZnS nano crystals; d low magnification TEM, e HRTEM and f SAED images of 6Cr:ZnS nano crystals

3.4 Raman spectroscopy

Raman spectroscopy study has been employed to observe the crystal structure and the local structural changes in Zn1−xCrxS nano crystals (Fig. 7). Moreover, signatures of acoustic and optical modes is well established by their polarization characteristics Raman study [30, 31].The two major bands are observed at 262 cm−1 and 342 cm−1, which are identified transverse optical (TO) and longitudinal optical (LO) modes, respectively. From Fig. 7, it has been observed that there is no appreciable shift in both the Raman modes viz. TO and LO modes. Therefore, this observation reveals that the crystal structure and the local symmetry in the doped nanocrystals are same as that of the pure ZnS NCs sample. It also suggests that the nature of samples remain unperturbed due to Cr doping (Table 4)
Fig. 7

Room temperature Raman spectra of the Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals at excitation wavelength 632.81 nm

Table 4

The symmetry assignments of observed Raman peaks

Vibration frequency (cm−1)

Pure ZnS

1 Cr:ZnS

2 Cr:ZnS

4 Cr:ZnS

6 Cr:ZnS

Assignments

Process

262

261

263

263

264

TO

First order

342

343

343

344

344

LO

First order

3.5 FTIR spectroscopy

The Fourier transformed infrared spectroscopy (FTIR) is resemblance of functional groups present in a compound, inter-molecular or intra-molecular interactions and the molecular geometry. Normally, the band frequencies within 700 cm−1 could be attributed to the bonds between inorganic elements. The most prominent band at 468 cm−1 and 658 cm−1 are assigned to the stretching vibrations of Zn–S bonds, in the tetrahedral co-ordinations. This observation suggests that the tetrahedral co-ordinations is stronger in this system and also confirms zinc blende structure formation of the nano crystals [32]. In Fig. 8 the bands at around 470 cm−1 and 656 cm−1 (due to tetrahedral co-ordinations) remain unaffected by Cr doping which suggests that Cr ions enter only into the tetrahedral site (interstitial). From Table 5, the peak around 1260 cm−1 to 1272 cm−1 is absent in pure ZnS, which indicating that the doped Cr ion affected the structure of ZnS nano crystals, and may be attribute to Cr–S stretching [33]. In other words, these stretching vibration arises due to Cr dopant related vibration in Zn1−xCrxS NCs. From these peaks, it is confirmed that Cr ions are placed into zinc (Zn) sites. The peak at 780 cm−1 shows symmetrical stretching vibrations of C–S band. A broad absorption peak at ~ 3408 cm−1 is attributed to the –OH group (exhibiting asymmetric vibration) of H2O, which indicates the existence of water absorbed on the surface of nanocrystalline NCs powders. Due to surface hydroxyl groups, these synthesized colloids can be dispersed into polar and non-polar solvents (e.g. water, ethanol), and the dispersions show moderate stability. Moreover, the surface hydroxyls can provide functional groups to react with functional organic molecules with interesting optical properties (e.g., dyes, cluster compounds), which may generate new and potential organic–inorganic materials [32, 34].
Fig. 8

Room temperature FTIR spectra of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals

Table 5

Different vibrational modes of FTIR spectra of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) samples

Pure ZnS

0.5Cr:ZnS

1Cr:ZnS

2Cr:ZnS

4Cr:ZnS

6Cr:ZnS

Modes (cm−1)

Assignments

468

474

475

476

477

477

Zn–S band I

Stretching vibrations

658

659

660

661

661

661

Zn–S band II

Stretching vibrations

780

781

781

780

780

782

C–S band

Symmetrical stretching vibrations

918

919

919

910

930

920

C–H band

Bending due to vinyl group

1011

1012

1032

1067

1077

1102

C–N–C Band

Stretching vibrations

1112

1115

1114

1115

1116

1112

N–C Band

Stretching vibrations

1262

1260

1259

1275

1272

New

Cr–S band

Stretching vibrations

1384

1385

1384

1384

1384

1384

C–OH band

Stretching vibrations

1458

1458

1459

1458

1458

1459

C–H2 band

Scissoring/in plane bending

1638

1633

1633

1632

1632

1633

CH=CH2 Band

Wagging mode

2350

2351

2346

2344

2346

2346

O=C=O band

Stretching vibrations

2924

2925

2930

2930

2931

2931

C=CH2 band

Bond stretching

3412

3414

3435

3422

3428

3429

O–H band

Asymmetric vibrations

3.6 UV–visible spectroscopy

UV–Vis absorption spectra of the pure ZnS NCs and the Zn1−xCrxS NCs samples have been plotted in Fig. 9. From Fig. 9, an absorption peak centered at around 323 nm is observed for the pure ZnS NCs sample and the energy band gap is estimated from first excitonic transition. A gradual blue shift has been observed with increase in Cr doping. As a consequence the energy band gap of nano crystals decreases with increase in doping concentration of Zn1−xCrxS NCs. The ‘sp-d’ spin exchange interactions between the band electrons (in conduction and valence bands) of ZnS and the localized ‘d’ electrons of Cr ions are main reasons for decrease in energy band gap of these samples [35]. This decrease in the energy band gap can be also interpreted with p-d repulsion and hybridization, where sulfur (S) ‘3p’ electron density decreases while ‘3d’ electron density of Cr increases below the valence band. However, the Cr ‘3d’ electrons are strongly bound than S ‘3p’ electrons, and push towards the lower binding energy causes to decrease of the energy band gap [36]. The decrease in the optical absorption band edge with increase of Cr donor density is due to increase in extra charge in conduction band and strength of the interaction potentials between doped Cr donor electrons and the host pure ZnS nanocrystal which result in band gap shrinkage [37].
Fig. 9

UV–visible spectra of Zn1−xCrxS (\( 0 \le x \le 0.06 \)) nanocrystals and variation of energy band gap (inset) of nanocrystals samples

3.7 Photoluminescence spectroscopy

Photoluminescence (PL) spectroscopy is used to investigate the intrinsic and extrinsic defects in semiconducting materials and to study the optical properties. The PL spectrum is ascribed to the presence of inherent peaks due to distributed defect states on the surface of nanostructured system. Observed asymmetrically broadened PL is due to several recombination site, point defects and dislocation. In the room temperature PL spectra (Fig. 10a) of Zn1−xCrxS NCs with 320 nm excitation wavelength, the band-edge emissions of the nano crystals can be clearly observed at about 365 nm. The origin of this peak is due to recombination between intrinsic defects centers in ZnS NCs such as zinc vacancy acceptor center and sulfur donor centre. The blue emission at around 462 nm of such samples is similar to that of reported by Chawla et al. [13]. For the pure ZnS NCs, the first peak is observed in the violet region at around 443 nm and other peak at around 463 nm in blue region (Fig. 10b). The violet band edge emission at 375 nm in the Cr doped ZnS NCs samples originate from radiative recombination of free excitons through an exciton–exciton collision process as well as with self-activated emission involving donor-accepter pair [37, 38]. The hump like peaks at about 400 nm and 540 nm are arisen (Fig. 10a) due to surface impurities and dangling bonds in Zn1−xCrxS NCs. Several blue or green emission peaks have been observed in the wavelength region of 465 nm for all Zn1−xCrxS NCs samples, which are associated with the recombination of free charge carriers and surface defect sites of the nano crystals, which are discussed by different workers [10, 39].
Fig. 10

a The room temperature PL spectra of Zn1−xCrxS nanocrystals samples and b PL spectra of pure ZnS nanocrystals samples observed by exciting at 320 nm

3.8 Magnetic properties

To study the magnetic behavior of the prepared nanocrystals, the field depended magnetic measurements were carried out on a representative sample, 6Cr:ZnS NCs, magnetizations vs. applied field (M–H) measurements have performed at 5 K and 300 K (Fig. 11a). From Fig. 11b it is observed that for pure ZnS NCs behaves as diamagnetic material at 300 K. The pure ZnS NCs shows diamagnetism due to absence of unpaired electrons in ‘d’ orbitals.
Fig. 11

a Field dependent magnetization (M–H plot) measured in the range ± 20000 Oe at 5 K and 300 K of 6Cr:ZnS NCs and b field dependent magnetization of pure ZnS NCs measured in ± 70000 Oe at 5 K and 300 K

The Cr doped ZnS sample (6Cr:ZnS) exhibits certain hysteresis at room temperature (300 K) and at low temperature (5 K) that supports the fact that sample has some magnetization. From XRD data we were unable to give proof of presence of any secondary phase, while XANES study indicates the presence of Cr2(SO4)3 phase in the Zn1−xCrxS NCs. There are several contradictory reports available on the origin of magnetism in Cr doped ZnS NCs samples, though none of the mechanisms are universally accepted still now [9, 10, 11, 12, 13]. Paramagnetism in the doped sample at room temperature (300 K) and low temperature (5 K) may arises due to (i) magnetic impurities as the intrinsic property of the doped nano crystals [9, 12], (ii) extended defects present in nano crystals [9, 12], (iii) magnetic disorder [8, 9], (iv) clustering of some Cr related nano scale secondary phase (Cr2S3,Cr2O3 etc.) [10], and (v) formation of metallic Cr precipitation [9, 10]. From the EXAFS study presence of Cr2(SO4)3 has been evidenced rather than possibility of any other secondary phase (Cr2O3, Cr2S3, Cr metal etc.). The 6Cr:ZnS NCs shows paramagnetism due to presence of unpaired electrons in ‘d3’ orbitals.

Figure 12 shows the inverse susceptibility (χ−1) as a function of temperature. The plot of inverse susceptibility (χ−1) vs. temperature exhibits linear behavior passing through the origin. This imply that the 6Cr:ZnS NCs is paramagnetic nature following the Curie’s law. Hence, at low and high temperature, the M-H relationship of a paramagnet could be better explained by the Langevin theory of paramagnetism [10]. Moreover, sufficient Cr–S bonds are crucial premise for achieving paramagnetic order in 6Cr:ZnS nano crystals.
Fig. 12

Temperature dependent magnetization (M–T plot) measured in magnetic field 1000 Oe and measured data of χ−1 (inverse susceptibility) vs. temperature of 6Cr:ZnS NCs (inset Fig.)

4 Conclusion

Cr doped ZnS nano crystals have been synthesized successfully by aqueous synthesis method. The results observed from XRD and Raman studies indicate that Cr ions incorporation does not cause any change in the crystal structure of the ZnS lattice. The energy band gaps of the doped samples and photoluminescence intensity of blue emission decrease with an increase in Cr doping concentrations. From XANES data of Cr K-edge, it can be concluded that the oxidation state of Cr remains in the doped samples is + 3. The results obtained at Cr K-edges suggests that in doped samples, Cr atoms form Cr2(SO4)3 phase. The single pre edge feature in Cr doped ZnS nano crystals are originated due to 3d4p orbital states and 1 s → 3d4p transition. The amplitude of pre-edge is found to be independent of Cr amount in doped ZnS nano crystals. EXAFS data indicates that interstitial Cri sites are not responsible luminescent emission. At room temperature and low temperature, Cr doped ZnS nano crystals show paramagnetic nature. Cr–S bonds are the crucial premise for paramagnetic order in Cr doped ZnS nano crystals. The quantum confined Cr doped ZnS nano crystals with blue fluorescence emission properties are important for optical device and biomedical applications.

5 Supplementary materials

See supporting information for Refined parameters of Zn1−xCrxS from Rietveld refinement, Refined structural profile parameters for Zn1−xCrxS from Rietveld refinement, Details of EXAFS data reduction and fitting procedure, XRD (experimental data) of undoped and doped samples, TEM (low resolution) of Cr doped ZnS nanocrystal in sectional view, Comparable normalized XAS data at Zn K-edge of Zn metal foil along with pure ZnS, 6Cr:ZnS nanocrystals, Comparable normalized XAS data at Cr K-edge of Cr metal foil along with 6Cr:ZnS nanocrystals, Cr2O3, Cr2(SO4)3, Magnetization data of 2Cr:ZnS nanocrystals.

Notes

Acknowledgements

S.Ghorai acknowledges to Council of Scientific and Industrial Research (CSIR), Govt. of India for Junior Research Fellowship & Senior Research Fellowship. AKG is thankful to DST-FIST program; to DST-PURSE program; to UGC-UPE program; to UGC-CAS program. AKG is also thankful to DST, India; DAE-BRNS; CSIR and UGC Govt. of India for financial support (Grant No.: SR/S2/CMP-0038/2008; 2011/37P/11/BRNS/1038-103(1302)/13/EMR-II, and Grant no. F.No.42-787/2013 (SR), respectively). We acknowledge to Dr. A. Banerjee for magnetic measurements and Dr. V. Sathe for Raman measurements UGC-DAE, Indore; to “Central Instrument Facility Centre” (CFIC), IITBHU for providing XRD, HRTEM measurements.

Funding

This investigation was funded by DST, DAE-BRNS and UGC, India (Grant No.: SR/S2/CMP-0038/2008, Grant no. 2011/37P/11/BRNS/1038-1 and Grant No. F. No. 42-787/2013 (SR), respectively).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

10854_2019_1524_MOESM1_ESM.docx (1.7 mb)
Supplementary material 1 (DOCX 1706 kb)

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© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Physics, Institute of ScienceBanaras Hindu UniversityVaranasiIndia
  2. 2.Atomic & Molecular Physics Division Bhabha Atomic Research CentreMumbaiIndia
  3. 3.Department of Chemistry, Institute of ScienceBanaras Hindu UniversityVaranasiIndia
  4. 4.Department of PhysicsIndian Institute of Technology (BHU)VaranasiIndia

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