Role of Bi3+ ions on structural, optical, photoluminescence and electrical performance of Cd0.9-xZn0.1BixS QDs

In this work, the Cd0.9-xZn0.1BixS QDs with different compositions of Bi3+ ions (0 ≤ x ≤ 0.05) were synthesized using a facile chemical route. The prepared QDs were characterized for analyzing the structural, morphological, elemental, optical, band gap, photoluminescence and electrochemical properties. XRD results confirmed that the Cd0.9-xZn0.1BixS QDs have a cubic structure. The mean crystallite size was increased from ~ 2 to ~ 5 nm for the increase of Bi3+ ions concentration. The optical transmittance behavior was decreased with increasing Bi3+ ions. The scanning electron microscope images showed that the prepared QDs possessed agglomerated morphology and the EDAX confirmed the presence of doped elements as per stoichiometry ratio. The optical band gap was slightly blue-shifted for initial substitution (Bi3+  = 1%) of Bi3+ ions and red-shifted for further increase of Bi3+ compositions. The optical band gap was ranged between 3.76 and 4.0 eV. High intense red emission was received for Bi3+ (1%) doped Zn:CdS QDs. The red emission peaks were shifted to a higher wavelength side due to the addition of Bi3+ ions. The PL emission on UV-region was raised for Bi3+ (1%) and it was diminished. Further, a violet (422 nm) and blue (460 nm) emission were received for Bi3+ ions doping. The cyclic voltammetry analysis showed that Bi3+ (0%) possessed better electrical properties than other compositions of Bi3+ ions.


Introduction
The Quantum dots (QDs) consist of semiconductor nanoparticles which dimension varies from 1 to 10 nm [1][2][3]. The combination of (III-V) and (II-VI) semiconductors is getting significant than the IV semiconductors. QDs are more photo-stable, higher signal-to-noise ratio, sharp and narrow emission spectra, longer fluorescence and higher photo-resistance compare to conventional organic dyes. These unique properties of QDs are attracted much attention in the field of biomedical imaging and optoelectronic devices like sensors, photoconductive cells, photovoltaic cells and solar cells, etc. Quantum dots play a substantial role in the imaging and labeling techniques, especially CdS, ZnSe, ZnS are having much significance [4]. CdS QDs based fluorescence sensors find novel application in modern technology [5]. In particular, the fluorescence emission wavelength from deep red (DR) to near-infrared (NIR) region is highly required for bio-imaging sensor designing and other bio-medical applications. Self-induced fluorescence interference, reduced light scattering, a high degree of penetration depth and less tissue damage are the important aspects due to the minimized emission bands. The DR and NIR region may be achieved by the various combination of QDs [6][7][8].
For the past two decades, CdS and ZnS series elements have been widely used to develop optical, optoelectronic device applications. CdS find a distinct advantage on luminescent and optoelectronic applications [9]. Cd-Zn-S QDs need some advantage since fast recombination of charge carriers. But these materials are having good efficiency for photolysis depends on the integration with appropriate dopants like Ni, Bi, Ba, Sb, Mn, Co, Fe, etc. Among these transition metals, bismuth is a good candidate to enhance the optical, electrical and magnetic properties (ZnBiS, Bi:CdS, Bi:CdZnS, Bi 2 O 3 , BiOCl, BiVO 4 , Bi 2 WO 6 , and Bi 2 MoO 6 ) [10][11][12][13]. We have already reported the structural, morphological and photoluminescence properties of Cd 0.8 Zn 0.2 S nanomaterials with the influence of Fe 2+ ions. It was noticed that the PL spectra were blue-shifted for the increase of Fe 2+ concentrations. At the higher wavelength (λ ~ 650 nm) side the band emission (DLE) move towards the redshift. It is required that a higher fluorescence emission band (λ > 650 nm) for biomedical and other deep red band emission applications, these may overcome by using bismuth materials [14]. So far few reports have been published on bismuth-doped Cd-Zn-S QDs for optical property activity exploration. There are many kinds of synthesis that were carried out to get CdS nanoparticles like sol-gel, hydrothermal, solvothermal, microwaveassisted method, sonochemical and co-precipitation method [15][16][17]. The co-precipitation method is identified as a simple and cost-effective method to synthesize nanoparticles in Mass quantity [18]. Hence we have selected the co-precipitation method for the preparation of the proposed combination of CdS nanoparticles. The present investigation deals with the synthesis of bismuth-doped cadmium-zinc-sulfide QDs (Cd 0.9-x Zn 0.1 Bi x S QDs, where x = 0, 0.01, 0.03 and 0.05) using the coprecipitation method.  3 were taken as per stoichiometry ratio and dissolved using 50 ml DI water in a separate beaker which contains 50 ml double distilled water, respectively. These solutions were prepared under continuous stirring for 30 min. These solutions were added one by one in a beaker where on magnetic stirring. The pH value of the solution was standardized using ammonia solution and the pH value was maintained at 9. The stirring rate was 600 rpm and the duration was 4 h. Upon the successful completion of the reaction, a precipitate was obtained. In the purification process, the precipitate was filtered out, washed with deionized water and methanol to remove the impurities if any. To get nanopowders, the samples were kept in a furnace at 70 °C for 8 h for drying and properly pulverized using agar mortar to get the homogeneous fine size. The same procedure was repeated for synthesizing samples with other compositions of Bi and Cd to get Cd 0.9-x Zn 0.1 Bi x S QDs (x = 0, 0.01, 0.03, 0.05).

Characterization
We have prepared the nanoparticles of Cd 0.9-X Zn 0.1 Bi X S (x = 0.00, 0.01, 0.03 and 0.05) and characterized for structural, compositional, elemental, optical and electrochemical analysis of the samples. For aforesaid investigations we used X-ray diffractometer (Model: Rigaku C/max-2500) for recording the X-ray diffractions (using Cu-Kα radiation: wavelength is 1.54056 Å) from 20˚ to 80˚ with the step angle 0.02˚ per min, Scanning electron microscope (Model: JEOLJSM 6390) for Morphological and elemental analysis, FT-IR spectrometer (Model: Perkin Elmer, Make: Spectrum RXI) for molecular vibrational study, UV-spectrometer ( Model: lambda 35, Make: Perkin Elmer)) for UV-vis optical absorption, transmittance, band gap analysis, PL spectrometer (Model: F-2500 Make: Hitachi) photoluminescence emission, Cyclic voltameter (Model: VersaS-TAT MC electrochemical system, Make: Princeton Applied Research, USA) for CV analysis and Nyquist plot to explore the electrical properties. The studies were taken using a three-electrode cell at room temperature in the 3 wt. % NaCl solution. The niobium mesh covered with platinum was taken a role as a counter electrode. The saturated calomel electrode (SCE) was served as a reference electrode. The electrochemical impedance analysis was recorded at the constant dc potential of 0.7 V under the dark condition

Structural analysis
The average crystallite size (D), lattice parameter (a, b & c) micro-strain (ε) and crystal structure of Bi 3+ -doped Cd 0.9 Zn 0.1 S QDs samples were analyzed by using XRD. Figure 1 shows XRD spectrum of Cd 0.9-x Zn 0.1 Bi x S QDs with different Bi 3+ compositions (0 ≤ x ≤ 0.05). The observed three diffraction peaks are corresponding to the lattice planes of (111), (220) and (311) and confirmed that the Cd 0.9-x Zn 0.1 Bi x S QDs possessed a cubic structure. The diffraction peaks corresponding to the cubic phase were matched with JCPDS file number 10-454 [19]. A high intense peak was observed around 2θ = 26.80˚ corresponding to the (111) plane for Cd 0.9-x Zn 0.1 S QDs. While the other samples (x = 0.01, 0.03 and 0.05) diffraction peaks were located at 2θ = 26.6, 26.94 and 26.65 respectively, with the same (111) plane direction. From the XRD peaks, a shift towards the lower value of 2θ and peak intensity is decreased. The shifting of 2θ is ascertained to ionic radius of bismuth (Bi 3+ = 1.03 Å) is greater than cadmium (Cd 2+ = 0.95 Å) and Zinc (Zn 2+ = 0.74 Å) [15]. This observation indicates that the separation of the neighboring lattice plane is longer than those of pure Cd 0.9-x Zn 0.1 S. The variation of peak intensity concerning to Bi 3+ doping concentration is presented in Fig. 2a.
where λ is the wavelength of X-ray diffraction, k is a constant taken to be 0.9, θ is the Bragg angle and β is denoted the full width at half maximum (FWHM) of the diffraction peak. The Table 1 reports the average crystallite size, lattice constant, FWHM, d-value, 2θ position and micro-strain value of Cd 0.9 -xZn 0.1 Bi x S ( x = 0, 0.01, 0.03, 0.05) QDs. From the given data, for the increase of x-value from 0.01 to 0.05, the average crystallite size (D 111 ) was increased. It is also cleared from the decrease in the β value of XRD as shown in Table 1. [21]. The variation of XRD primary peak intensity with Bi 3+ dopant concentration was shown in Fig. 2a. When Bi 3+ ions doped (1%) reduced XRD peak intensity because the initial incorporation of large-sized dopant ion into the host lattice produced some distortion. The replacement of Cd 2+ ions by Bi 3+ ions produced defect states. Bi 3+ (3%) doping increased the crystallinity than 1% doped nanocrystals which indicate an elevation in peak intensity. The average crystallite size is small for this concentration of Bi 3+ . While increasing Bi 3+ concentration to 5% reduces the peak intensity indicates the loss of crystallinity due to the distortion produced in the lattice. The creation of sulfur vacancy produced more distortion for the higher doping composition of Bi 3+ ions. A similar decrease of peak intensity due to Bi 3+ doping ZnO was reported in the literature [22]. While Bi 3+ doped with ZnS nanoparticles the 2theta angle shifts lower side. the similar shift due to Bi 3+ was reported in the literature [23] The dislocation density (δ) is measured using the relation, δ = 1/D 2 . The crystal size of Cd 0.9-x Zn 0.1 Bi x S QDs was increased up to 5.98 nm after Bi 3+ doped (x = 0.05). The dislocation density was decreased by increasing the Bi 3+ concentration as shown in Fig. 2b, which may be attributed to the increase of the crystal structure [24]. The lattice parameters of Cd 0.9-x Zn 0.1 Bi x S QDs are found to lie in the range of 3.28 Å (for x = 0) to 3.34 Å (for x = 0.05). The lattice parameter of Cd 0.9-x Zn 0.1 Bi x S QDs increased with the increase of Bi 3+ ions content, which can be attributed to the replacement of Cd 2+ shown in Fig. 2c. It was noticed that the unit cell volume (V = a 3 ) increased for Bi 3+ doped samples, in agreement with ionic radii of Bi 3+ (R Bi ) larger than ionic radii of Cd 2+ (1) D hkl = k cos (R Cd ). This confirmed the expansion of the crystalline plane spacing due to the substitution of Bi 3+ ions into Cd-Zn-S lattice [25]. It is confirmed that lattice constants were seen to increase at slightly increased crystallite sizes. The lattice micro-strain of Bi 3+ doped Cd 0.9-x Zn 0.1 S QDs obtained using the below-given formula [26].
(2) Micro − strain ( ) = cos 4 It is also observed from the Table 1 that the micro-strain decreases with increasing Bi 3+ ions concentrations. The obtained micro-strain value was gradually decreased with the increase of Bi 3+ ions (x = 1-5) as shown in Fig. 2d. The decrease of micro-strain with increasing Bi 3+ ions due to the decrease in the dislocation density (δ), FWHM and the increase of crystallite size (D). The lattice defects like δ and ε showed a decreasing trend with increasing Bi 3+ doping concentrations, which may be due to the improvement of  Table 1 The variation of peak position (2θ), FWHM (β) value, d-value, cell parameter 'a' , average crystallite size (D) and micro-strain (ε) of Cd 0.9-x Zn 0.1 Bi x S (x = 0% to 5%) nanoparticles  Fig. 3a. We could find spherical-shaped small-sized fine particles. We couldn't find the particles size since the particle are aggregated like a chain. The SAED pattern is given in Fig. 3b. We can view three concentric rings corresponding to (111) (220) (311) plane respectively. The diffraction patterns are in good agreement with XRD results.

Surface morphological investigations
The surface morphology of Cd 0.9-x Zn 0.1 Bi x S (x = 0, 0.01, 0.03, 0.05) QDs is examined by Scanning Electron Microscopy. SEM images and their corresponding EDAX spectra of Cd 0.9-x Zn 0.1 Bi x S QDs with different x values are presented in Fig. 4a-d. The QDs particle size and its distribution mainly depend on relative rates of nucleation of particles and the agglomeration rate. In Fig. 4a pure Cd 0.1 Zn 0.9 S QDs reveal lamellar and agglomerate microstructure features. Agglomerated structures occur during the synthesis process and local strain in the nanocrystal, its results in non-homogenous nanoparticles are distributed inside the sample [27]. Figure 4b shows the microstructure of Cd 0.89 Zn 0.1 Bi 0.01 S QDs (x = 0.01); the image observed an almost uniform spherical shape. Moreover, it shows the large cluster of particles having smaller dimensions and also the occurrence of voids can be observed on the surface. Figure 4c presents Cd 0.87 Zn 0.1 Bi 0.03 S QDs (x = 0.03); it can be seen that the QDs particles have a good interfacial bond formed between Cd 0.1 Zn 0.9 S and Bi 3+ ions. Furthermore, it clearly shows a surface topography morphological change in the microstructures of the sample.  Figure 5a shows the UV-vis absorption spectra of Cd 0.9-x Zn 0.1 Bi x S QDs (x = 0, 0.01, 0.03 and 0.05) with recorded in the wavelength range of 300-600 nm. All the prepared samples show a very strong and steep absorption edge due to light scattering at a high concentration of the QDs. While the steep edge indicates that the UV-vis light absorption due to the transition from impurity levels formed by Bi 3+ doped into Cd 0.9-x Zn 0.1 S QDs. It can be seen that high absorption intensity at an electromagnetic wavelength range of λ < 350 nm, and it is rapidly decreased in the wavelength range of 300 nm ≤ λ ≤ 350 nm. When the wavelength is greater than 350 nm, the absorption value is very small. As Bi 3+ ions increases notable increase absorption intensity and absorption edge shifting towards lower wavelength (λx = 1,λx = 3,λx = 5 < λx = 0 ~ 340 nm, i.e., blue shift) side is due to quantum size effect and alloy composition formation and this phenomenon is well-known Burstein-Moss effect that denotes blue shift is formed with increasing doping concentrations [29,30]. And also this shift related to band edge indicates that the band gap of the light in response and it can be controlled with Cd 2+ : Bi 3+ ratio in Cd 0.9-x Zn 0.1 Bi x S QDs. The prepared QDs The transmission loss is due to light scattering at centers of the Cd 0.9-x Zn 0.1 Bi x S QDs. The decrease in optical transmission may be associated with the loss of light due to sulfur vacancies.

Optical energy band gap (E g )
The optical energy band gap (E g ) of Cd 0.9-x Zn 0.1 Bi x S QDs (0 ≤ x ≤ 0.05) is derived from Tauc relation by analyzing optical absorption data. This expression is related to the absorption coefficient as a function of incident photon energy [31].
where α is the absorption coefficient, h is a Planck constant (6.626 × 10 − 34 m 2 kg/s), hυ refers to the incident photon energy, A is a constant, E g is the band gap energy and n is a constant. The optical absorption index (n) value depends on the type of transition and it is equal to 0.5, 1.5, 3 respectively for direct allowed, direct forbidden, indirect allowed or indirect forbidden transition. Here it takes a direct allowed transition, Hence the n value was taken as 0.5. The extrapolating straight lines of the graph (αhυ) 2 vs (hυ) to intercept at photon energy x-axis (α = 0) gives the value of the energy band gap (Eg) shown in Fig. 5b. It has been shown that pure Cd 0.9-x Zn 0.1 S QDs have E g value equal to 3.92 eV which is slightly higher than 3.8 eV reported in the literature [32]. The higher E g (4 eV) value was observed for x = 0.01, which may due to the size effect. Generally, The quantum confinement effect influences the blue shift of band gap. But in the present case size effect dominates the quantum confinement effect, because the particle size was increased due to the incorporation of dopant. On the other hand, the larger band gap of Bi 3+ ions initially produces many donor levels in the Cd 0.9-x Zn 0.1 Bi x S QDs. Hence the Fermi energy (EF) level is shifted more away from the valence band and increases the E g value. With a further increase in the doping concentration, the E g value slightly (redshift) as a function of Bi 3+ ion concentration. The blue shift is blocking off the QDs mainly arises from the low energy transitions in optical band-to-band transitions. The observed decreased energy gap values are E g = 3.88 eV and 3.76 eV corresponding to x = 0.03 and 0.05. The decrease of the energy gap is due to Bi 3+ ions, which increases the internal pressure because Bi 3+ ion has a larger ionic size than Cd 2+ and also due to the presence of layered morphology with lower carrier density [33].

Photoluminescence properties of Cd 0.9-x Zn 0.1 Bi x S QDs
The room temperature photoluminescence spectra of Cd 0.9-x Zn 0.1 Bi x S QDs with different Bi 3+ compositions (x = 0, 1, 3 and 5) are illustrated in Fig. 7. The measurement was performed wavelength range is 350-750 nm with excitation wavelength 350 nm. From the PL spectrum, it was observed that five intensity fluorescence emission bands, three weak peaks are on the lower wavelength side and two strong emission peaks are on the higher wavelength side. That emission bands are near ~ 365, ~ 422, ~ 454, ~ 648 and ~ 712 nm corresponding to Bi 3+ doped Cd 0.9-x Zn 0.1 S QDs. The PL1 band corresponding to the undoped Cd 0.9 Zn 0.1 S QDs sample, which exhibits a broad emission peak centered at ~ 365 nm in the ultraviolet luminescence region. This near band edge emission (NBE) is generally attributed to the recombination of free excitons (e − h + ) present in the Cd 0.9 Zn 0.1 S QDs lattices and also due to the transition of an electron trapped in the sulfur vacancy to the valence band. This UV emission peak (~ 365 nm) intensity is suitable for the treatment of psoriatic and skin diseases. The Bi 3+ doped Cd 0.9 Zn 0.1 S sample shows that the next two broad excitation bands between 420 and 470 nm, which peak centered at ~ 422 nm and ~ 456 nm. These peaks are ascertained to the localized energy level introduced by Bi 3+ dopant ion, which acts as an acceptor impurity and the emission arises from the sulfur vacancy to the Bi 3+ dopant level. This violet-blue emission peak ~ 422 nm originates from the transition of the shallow trap level (STL) [36]. The peak centered at ~ 456 nm may be attributed to a higher level of excitonic emission caused by the size effect. The intensity of blue color emission band at 456 nm becomes high toward the higher Bi 3+ doping concentration (x = 5) and there is no excitation peaks are observed at 422 nm and 456 nm corresponding x = 0. The photoluminescence properties of these structures are investigated aiming at the field of blue and violet-blue light-emitting diodes (LEDs) [37].   On a longer wavelength side (λ ≥ 600 nm) a defect related to deep-level emission (DLE) was produced. It was observed that the photoluminescence (PL) emission from the Cd 0.9 Zn 0.1 S QDs can be easily tuned from the red (648 nm) to deep red (712 nm) region of visible light by adding Bi 3+ doping concentration (x = 1, 3 and 5) into Cd 0.9 Zn 0.1 S QDs [38]. This strong and narrow emission peak may be due to band to band transition. It can be seen that the PL emission spectra of all samples, the Gaussian shape did not change significantly except for the PL emission intensity. Not change in Gaussian shape due to the end of chemical reaction to stabilize the synthesized nanocrystals. Moreover, the PL spectra of Cd 0.9 Zn 0.1 S: Bi 3+ sample show a strong emission at x = 1 corresponding wavelength is 712 nm. Further adding Bi 3+ doping into the Cd 0.9 Zn 0.1 S sample the peak slightly shifted towards a longer wavelength (Redshift ~ 1.75 nm) side and emission intensity decreased due to the quenching phenomenon [39]. This small red shift in PL peak was confirmed from XRD. The PL peak decreased with the increase of Bi 3+ doping amount, indicating that the recombination of photo-generated electron-hole pairs in bulk crystal defects was efficiently depressed. This observed change in the PL emission wavelength can be highly beneficial the imaging screen applications [40].

Electrochemical study
Cyclic Voltammetry (CV) is a primary method to study the developed current in an electrochemical cell under the voltage that goes beyond the anticipated value by the Nernst equation. Figure 8 shows the cyclic voltammograms of Bi 3+ -doped Cd 0.9 Zn 0.1 S QDs. The cathodic current density (J pc ) and anodic current density (J pa ) are taken for the present investigation in this study. The high (J pc ) current peak value was obtained for Bi = 0% incorporated into Cd 0.9 Zn 0.1 S QDs. The high value of peaks observed either in positive and negative current values for Bi = 0%. ΔE P is referred to as peak to peak separation value. The value of ΔE P is calculated by the given relation [41] The crystallite nature of the materials with low resistivity indicates the fast charge transfer kinetics. The low ΔE P values deduced an enhanced catalytic activity [42].
The Nyquist plots were plotted for the samples used in this work and they were depicted in Fig. 9. From the analysis, the maximum diameter value of the semicircle was (4) ΔE P = E pc − E pa observed for Bi 3+ = 5% and it was further moved towards a very high-frequency side. The semi-circle area of Bi = 5% was increased than the other compositions of Bi. The semicircle reaches a higher percentage for Bi 3+ = 5%. The R ct represents the charge transfer resistance for the semi-circle on the higher frequency side. Where, R S is denoted as series resistance for the intercept on the real axis near the higher frequency side [43]. The proposed equivalent circuit for these elements is shown in Fig. 9. From the figure, the R s and R ct values of Bi 3+ = 0% doped Cd 0.9 Zn 0.1 S QDs very lower than the other compositions. As low resistance was exhibited by Bi 3+ = 0% doped Cd 0.9 Zn 0.1 S QDs, this combination offered an excellent electrical feature.

Conclusion
The Cd 0.9-x Zn 0.1 Bi x S QDs have been synthesized by a controlled co-precipitation method at room temperature with different (x = 0, 0.01, 0.03, 0.05) values. XRD studies revealed that the samples had a cubic structure and the average crystallite size was varied from 2.13 nm to 5.98 nm for the various doping concentration of Bi 3+ ions. SEM images showed that the surface morphology of the prepared QDs possessed agglomerate microstructure and morphology modified from unequally sized grains to nearly equally sized grains with the increase of dopant concentration. The elemental analysis (EDAX) and FT-IR studies confirmed the presence of elements as they are expected. The UV-vis absorption intensity increased with the increase of Bi 3+ concentration. High absorption intensity (Iab) was observed in the wavelength range 300 nm ≤ λ ≤ 350 nm. Optical band gap (Eg) values of doped CdS QDs were different from the bulk CdS nanocrystal. The Eg value of Cd 0.9-x Zn 0.1 Bi x S QDs could be tailored from 3.76 to 4 eV by varying the doping ratio of Bi 3+ ions. Bi 3+ -doped Cd 0.9 Zn 0.1 S, the emission intensity was maximum for 1% doping. The PL emission for Cd 0.9-x Zn 0.1 Bi x S QDs have been received in violet, blue and red colour regions and the emission peak position was shifted to higher wavelength side due to the incorporation of Bi 3+ ions. The incorporation of Bi 3+ ions increases the electrical resistivity of Zn:CdS QDs. As Bi 3+ -doped Zn:CdS QDs exhibited excellent optical and photoluminescence properties the doping ratio of Bi 3+ (1%), this combination shall be selected for the fabrication of red-emitting diodes, labelling and imaging applications.

Conflict of interest
The authors declare that they have no competing interests.
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