Recently, group II-VI semiconducting materials exhibit great scientific and technological significance within different family of semiconductors because of their numerous practical applications in science and technology [1–4]. Quantum dots (QDs) are semiconducting material of nano-scale dimensions with distinctive properties determined by its size. Yang et al.  investigated N and S heavily doped carbon quantum dot (CQDs) with size around 1–6 nm which had better photoluminescence (PL) behaviour and better sensitive to Cu2+ and Hg2+, respectively. Selenium added graphene QDs  with size around 1–5 nm possessed a quantum field of 0.29 and a PL lifetime of 3.44 nm which guaranteed a better selectivity for the fluorescent switch. Wang et al.  reported the controlled PL behaviour in sulfur doped GQDs by tuning the S concentrations. Huang and their co-workers  developed a weak electrolyte-based electrochemical method to enhance the oxidation and cutting process and achieved a better yield of GQDs.
Porous based graphene (PGN) showed an outstanding performance in PGN based detectable molecule separation or other biomedical applications . Zhu and their co-workers  demonstrated that graphene oxide can be oxidized and cut into GQDs by hydroxyl radicals. Back-gated field‐effect transistors made of single‐layer C3N exhibited an on–off current ratio reaching 5.5 × 1010 . The challenges of developing solid‐state PL CQDs induced a great curiosity among research people . Nitrogen based CD probes can also distinguish tumor cells from normal cells and be used to evaluate their proliferation activity .
Among the different semiconducting materials, ZnS is the significant one due to its unique properties  and applications [15–17]. The two most common forms of ZnS are zinc blende and wurtzite structure where the energy gap varied between 3.72–3.77 eV [18–19]. Moreover, ZnS have the elevated transmittance in the higher wavelength and significantly huge exciton binding energy and so helpful for manipulating well-organized optoelectronic device applications [20–22]. By its unique features and characteristics, ZnS can be employed in an extensive spectrum of different applications like solar cells, photo-voltaics, sensors, electroluminescence devices, modern emitting diodes, and lasers [23, 24].
The intentional doping of metal ions into ZnS is fundamental to modify the optical nature of semiconducting materials by building innovative quantum states between two bands of semiconductors [25, 26]. ZnS is believed as a suitable host material for optically dynamic elements like transition metal (TM) ions, which can fascinatingly provoke a impressive changes in energy level structures, surface activities, visual and electrical characteristics and stimulates its transition probabilities [27–29]. In opto-electronic applications, it is essential that the doping element enclose a deep energy level and elevated voltage stability. Therefore, TM ions can be chosen as a suitable element due to the extraordinary properties accessible by an inclusion into ZnS basic lattice [29, 30].
The modified and improved property of ZnS by the addition of TM element has created an immense curiosity in the possibility to alter and elevate their optical and micro-structural properties for their scientific and industrial applications . Among the various possible TM ions, manganese (Mn2+) is considered as one of the best and significant doping agents owing to its extraordinary luminescence characters using in optical sensors, phosphors, lasers, fluorescence bio-imaging and displays [3, 33]. The ionic radius and the ionic charge of Mn2+ ion is realistically close to Zn2+ ion. Moreover, Mn2+ occupies the various sites of ZnS lattice and stimulates the considerable modification in its optical, structural, microstructure, electronic properties [34–36]. Up to now, an immense deal of hard work has been paid to inspect the optical nature and structure of Mn doped ZnS semiconducting material due to its excellent thermal and photo-stability.
Bhargava et al.  achieved the superior quantum efficiencies and better lifetime shortening by Mn in ZnS lattice. Kripal  studied the luminescence and conductivity nature of Mn substituted ZnS nanostructures and they noticed that the photo-sensitivity was enhanced by Mn2+ into ZnS system. A noticeable increase in visible transmittance and a small decline in resistivity by Mn2+ in ZnS were found by Jrad . Goudarzi et al.  described that the insertion of Mn occupied the substitutional position of Zn and modified the energy level of ZnS which promotes the elevated luminescence properties. The luminescence and photo-catalytic studies carried out by Nasser  in Mn / ZnS described that the inclusion of Mn2+ stimulates the charge separation and activate the photo-catalytic activity of Mn doped ZnS. From the literature it is understood that the higher percentage of Mn produced the secondary phase generation. To avoid the secondary phase generation, Mn level is optimized as 3% (Zn0.97Mn0.3S).
The adding of two or more elements through ZnS causes a significant progression in photoluminescence, energy gap, size and charge transport properties. Doping at higher percentage with no secondary phase or metallic cluster is achieved by the adding two or more suitable TM ions . In this work, Cr is chosen as second doping element because it can easily enters into ZnS lattice  and enhanced the stability of ZnS . The higher doping percentage without secondary phase is achieved by Cr doping. The Cr percentage is limited to 2% and it is attained by fixing the appropriate of quantity of precursors. Moreover, the addition of Cr3+ through ZnS stimulates the different special defect sites in the lattice owing to the dissimilarities among Cr3+ and Zn2+ ions. Poornaprakash reported the disappearance of blue emission bands and appearance of RTFM by Cr addition in ZnS . Recently, Aqeel et al.  described that Cr added ZnS performed the superior photocatalytic behaviour and hence they may use as an effective one for the elimination of environmental pollutants.
Lattice contraction and the enhancement of energy levels were reported by Reddy et al.  in ZnS with increase of Cr concentrations. Yang et al.  noticed the strong higher wavelength emissions within 515–560 nm in Co2+, Cu2+ dual doped ZnS nanostructure. The PL intensity of dual doped material is considerably superior than ZnS. Liu et al.  reported the red radiation emission in Mn-Cd dual doped ZnS nanostructures. Yang et al.  described the PL characteristics of Ni2+ / Mn2+ doped ZnS. The similar PL studies were made on Mn, Pr co-doped ZnS , Co-Cu doped ZnS , Cu-Mn doped ZnS nanostructures .
The optical, structural, PL and magnetic studies on Mn single doped ZnS [31–41], Cr single added ZnS [43–47] and some other TM dual doped ZnS [48–53] were reported in the literature. But, the comprehensive investigation of optical, morphological and emission characters on Cr, Mn dual doped ZnS is almost scanty. In this work, Zn0.97Mn0.03S and Zn0.95Mn0.03Cr0.02S quantum dots (QDs) were synthesized by co-precipitation route. The structural, spectral, morphological and PL properties of the Mn2+/Cr3+ doped ZnS QDs are characterized systematically to achieve the better insight about the crystal nature, morphology, bonding nature, energy gap and emission properties.