II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study

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Highlights

  • Coating of a shell over the bare quantum dots alters their properties.

  • Investigation of the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs.

  • Introduction of magnetism in core/shell QDs on doping with transition metal ions.

  • Doping with transition metal ions alters several magnetoelectronic properties of the semiconductor.

Abstract

This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn2+ to Fe2+ to Co2+ to Ni2+ to Cu2+. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg2+ ions. As we move from Mn2+ to Hg2+, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe2+ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.

Introduction

Recent advances in the synthesis of highly monodisperse nanocrystallites have paved the way for numerous spectroscopic studies assigning the quantum dot (QD) electronic states and mapping out their evolution as a function of size. QDs, due to their reduced dimensions, have very high surface to volume ratios, with roughly 80% of the atoms residing on the surface. Hence, their optical and structural properties are significantly affected by the atoms present on the surface. The presence of atoms on the surface with unsaturated valencies, called surface defects, leads to the generation of surface trap-states acting as non-radiative recombination sites, which degrade the fluorescence quantum yield of the QDs [1]. On passivating the QD surface with organic ligands, these trap states get reduced, but still the complete passivation of surface defects does not occur. In order to passivate the surface in a more complete manner, a secondary layer of semiconductor may be epitaxially grown over the surface surrounding the core semiconductor. This secondary layer is called the shell, and the resulting structure is called a core/shell QD or simply a core/shell structure. The quantum yield has been shown to increase up to ten times, with increased resistance toward photo-oxidation on overgrowing shells around the semiconductor material [2].

The material for an overgrowing shell layer depends upon the properties we require from the resultant structure. Based on this, we have three types of core/shell systems, type I core/shell, reverse type I core/shell and type II core/shell systems (Fig. 1). These all differ in the alignment of the valence and conduction bands between the core and shell. Type I core/shell systems are the most common, where the shell is composed of the higher band gap semiconductor material, confining the exciton (an electron-hole pair) to the core. These types of systems are employed when high quantum yield is desired, e.g. CdSe/ZnS. In reverse type-I systems, a shell of a narrower band gap semiconductor is grown on the higher band gap core. Here, the charge carriers are partially delocalized from the core to the shell. Such types of systems are required when red shifting of the fluorescence spectrum is required, e.g. CdS/CdSe and ZnSe/CdSe. In type-II core/shell systems, the lowest energy states for the electrons and holes are in different semiconductors, setting an energy gradient at the interfaces tending to spatially separate electrons and holes, e.g. ZnTe/CdSe and CdTe/CdSe. These types of systems are majorly employed in photovoltaic technologies [3].

It has been observed that nanocrystallites passivated with higher band gap semiconductor materials exhibit improved photoluminescence quantum yields [4,5]. The choice of the shell material highly depends upon both the alignment of its valence and conduction bands relative to those of the core material. It is also necessary to match the band alignment and lattice parameters between the core and shell, since new defects are formed at the interface if the lattice mismatch between core and shell is too great, which reduces the fluorescence quantum yield [1].

Core/shell type composite QDs exhibit novel properties, making them attractive from both an experimental and practical point of view [[5], [6], [7], [8], [9], [10], [11], [12]]. Overcoating nanocrystallites with higher band gap inorganic materials has been shown to improve the photoluminescence quantum yields by passivating the surface nonradiative recombination sites. Some examples of core/shell QD structures reported earlier include CdS on CdSe and CdSe on CdS [10], ZnS grown on CdS [12], ZnS on CdSe, and the inverse structure [7], CdS/HgS/CdS QD quantum wells [9], ZnSe overcoated CdSe [6], and SiO2 on Si [8,11]. (CdSe)ZnS nanocrystallites at room temperature show upto 50% fluorescence quantum yield [5]. Various core/shell structures have been studied computationally also; high-quality CdSe/CdS core/shell QDs have been studied to understand the effects of oxygen on the photoluminescence of QDs at both the single dot and ensemble (on substrate and in solution) levels [13]. In another study [14], the third-order optical nonlinearity in CdSe and CdSe–CdS core-shell QDs with particle sizes in the range 4.4–5.2 nm has been investigated by the z-scan technique and the quantum confinement effect has been discussed extensively. CdTe@HgTe Core@Shell nanostructures have been produced [15] by a partial Hg2+ → Cd2+ cation exchange reaction in colloidal CdTe nanocrystals of ∼4–6 nm size. A study of CdSe/CdS core-shell NCs using density functional theory was carried out [16] to understand the dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. TDDFT studies have been performed [17] to describe the effects of morphology on the optical response of QDs. The microstructural and optical properties of CdSe/CdS/ZnS core-shell-shell QDs have been determined [18]. Type-I CdSe/ZnS core/shell QDs have been found to effectively sensitize single crystal TiO2 electrodes and are maintained to work in a regenerative mode in an aerated iodide electrolyte for more than 20 h. On the other hand, core CdSe QDs degrade promptly in the same electrolyte probably due to the formation of CdI2 [19].

In order to further enhance the properties of these core/shell QDs, transition-metal ion doped semiconductor nanocrystals (NCs) have attracted much attention in the past two decades due to their promising applications in light-emitting devices [20,21], biological imaging [22,23], and spin-based electronics [24]. The structural and electronic properties are tremendously affected on introduction of an inorganic shell outside the QD which may act as the core. The results of some studies [25] suggest that the complete coverage of the CdS core with the wider energy band gap ZnS enhances the photoluminescence of the CdS core. Hence, in the present work, we have coated the toxic core of CdS with a shell of the less toxic ZnS in order to enhance its optical, electronic and magnetic properties further. The experimental findings [26] suggest that uniform shell coverage is obtained only for a graded CdS/ZnS core/shell material and is found to be critical for achieving near 100% quantum yield.

In line with this, we have developed a theoretical model of core/shell QDs, which comprises a core and an inorganic shell formed from the chalcogenides of the zinc triad. The idea is to form a stable model to make it interact with other molecules of scientific interest in order to sense them. Core/shell structures of QDs are the latest trend in the manifestation of reduced dimensionality.

Several experimental groups are enthusiastically pursuing the synthesis of magnetic semiconducting nanoparticles and the production of magnetically doped II-VI semiconductors. Doping of impurities into semiconductor nanocrystals in a controlled manner is essential for producing and enhancing optical and magnetic functionalities. This has opened new gates for scientists to explore how these dopants can influence semiconductor nanocrystals. Transition metal ions are unique dopants, because doping with these ions has the effect of injecting localized luminescence centers and electronic spins into semiconductors. As a result, semiconductor nanocrystals doped with transition metal ions are the key materials for future optronic and spintronic devices [27]. Incorporation of dopants into semiconductor nanocrystals has been used to tailor the optical, electronic, and magnetic properties of nanocrystals [[28], [29], [30], [31], [32], [33], [34], [35]]. Much effort has been poured into realizing doping II-VI semiconductor QDs with impurities, especially to obtain dilute magnetic semiconductors (DMS). Undoped nanocrystals are highly fluorescent, the fluorescence depending upon the size. The lasers based on this emission are inefficient. Several approaches can improve this situation [36], and one way is to incorporate dopants that can provide carriers. Dopants in nanocrystals lead to phenomena which are not found in bulk semiconductors, as the electronic states in nanocrystals or QDs are confined to a small volume and in all the three dimensions. Historically, a major motivation for the exploration of transition metal doped semiconductors has been the use of luminescence activators such as Mn2+ or Cu2+ for sensitized photoluminescence and electroluminescence applications [24,[37], [38], [39]].

It is clearly revealed from the earlier studies that doping with transition metals with half and more than half filled d-orbitals is of great significance in causing an alteration in their own properties as well as in the properties of the host moiety. Mn doping has been extensively investigated, such as Mn doped CdS/ZnS in order to reveal the surface effects and position-dependent properties of the doped core/shell QDs [40,41], tunable magnetic exchange interactions in Mn2+ doped inverted core-shell ZnSe/CdSe nanocrystals [42], enhanced photoluminescence from Mn2+-doped CdS nanocrystals capped with ZnS (CdS:Mn/ZnS core/shell) [43], photoluminescence dynamics dependent on the Mn2+ ion concentration in Mn2+-doped CdS/ZnS core/shell nanocrystals [27], Mn-doped CdS core/shell nanocrystals [44], an application of Mn2+-doped ZnS core/shell QDs in detection of silver ions [45], synthesis of non-toxic, less expensive Mn:ZnSe/ZnS core/shell and Mn:ZnSeS shell-alloyed doped nanocrystals [46], Mn2+ doping in MnS/ZnS core/shell QDs [47], and a green synthetic approach of low-toxic Mn:ZnSe/ZnS core/shell luminescent nanocrystals to label antibodies for its potential application in the detection of human immunoglobulin G (IgG) [48], Recently, it has been found that the thermal stability of the Mn-doped QD emissions significantly depends on the shell thickness and the host band gap [49]. Bui & Nguyen [50] have proved that Mn-doped ZnSe/ZnS core/shell QDs could be successfully applied in the detection of Escherichia coli and methicillin-resistant Staphylococcus aureus and this has inordinate potential for use in the food industry to avoid occurrences of food-borne infections. Doping with other transition metals has also been done, such as Fe doping in ZnS core/shell (ZnS:Fe/ZnS) nanocrystals [51]. Fe3+ doped QDs have been investigated [52] to study the effect of doping on the magnetic and optical properties, and Co doping in ZnO/ZnOcore/shell colloidal QDs [53]. Cu doping has also been extensively done such as in Zn1-xCdxS QDs and their core/shell structure [54], ZnS core/shell nanocrystals [55], and inverted ZnSe/CdSe core/shell nanocrystals [56], and highly photostable CdSe/CdS:Al core/shell QDs [57].

The dopants chosen for the present work include transition metal atoms from manganese to zinc, along with mercury. The aim is to extend our study to explore and investigate the deviations/amplification in the properties of core/shell nanocrystals on doping the CdS/ZnS core/shell QDs at the atom centered position of the QDs. One of the candidate methods for fabricating nanocrystals doped with impurity ions is that of constructing a core/shell structure, i.e. core semiconductor nanocrystals doped with impurities are coated with a non-doped semiconductor shell layer [43,[58], [59], [60]].

Another reason for choosing doping in the core of the QD is that a nonmagnetic coating is used routinely for magnetic core stabilization and surface functionalization for biomedical applications [61]. Also, an antiferromagnetic coating over a ferromagnetic core led to exchange bias and improvements in the thermal stability of the core [62]. Compared with these two different types of core/shell systems, a bimagnetic core/shell one, where both the core and shell are strongly magnetic (ferro- or ferri-magnetic) is less studied, yet more interesting due to its potential in electromagnetic and permanent magnetic applications [63,64]. In such a system, the intimate contact between the core and shell leads to effective exchange coupling and therefore cooperative magnetic switching, facilitating the fabrication of nanostructured magnetic materials with tunable properties. Thus, an interesting magnetic nanoparticle system is that of core/shell structured nanoparticles in which the magnetic core is coated with a layer of a nonmagnetic, antiferromagnetic, or ferro/ferrimagnetic shell. Recently, it has been demonstrated that the eco-friendly copper doped InP/ZnSe core/shell QD (InP/ZnSe:Cu) is a substitute to traditional Cd/Pb-based QDs in light-emitting technologies [65]. Composition-tunable Cu-doped ZnInS/ZnS nanocrystals were synthesized [66], which show that a change in the Zn/In ratio tunes the percentage assimilation of Cu in nanocrystals.

In the present work, firstly, the cores of cadmium chalcogenides were constructed with the shells comprising the respective chalcogenides of the zinc triad. This work includes the following core/shell structures: CdS/ZnS, CdS/HgS, CdSe/ZnSe, CdSe/HgSe, CdTe/ZnTe and CdTe/HgTe. All these core/shell structures have been compared with similar sized bare uncapped non-stoichiometric QDs comprising the core without any shell. Then, we chose a few core/shell systems and interchanged their core and shell materials in order to examine the effect of interchanging the core and shell materials on the properties of these systems. In the subsequent section, the CdS/ZnS core/shell QD, which has a core of CdS and a ZnS shell and is a well-known wide band gap semiconductor, with a diameter of 15 Å and a binding energy −77.69 eV, has been specifically chosen as the most stable non-stoichiometric core/shell QD amongst all the core/shell nanocrystals considered for doping with transition metal ions. We have restricted our study to doping of only the atom at the center of the atom-centered core/shell QDs. Detailed comparison with the undoped analogous CdS/ZnS QDs has been done in order to have an in-depth knowledge of the effect of doping with these single transition metal ions on the structural, electronic and magnetic properties.

First-principles density functional (DF) calculations [67] were performed by means of the DMol3 code [[68], [69], [70], [71], [72], [73], [74]], obtained from Accelrys Inc. in the Materials Studio 4.4 package. High quality numerical functions on an atom-centered grid were used as the atomic basis in these calculations. The basis set used was DNP, which is of double zeta quality plus polarization functions, and is the numerical equivalent of the Gaussian basis, 6-31G** [74].

All the structures were optimized using DFT–semilocal pseudopotentials (DSPP) [72] to define the cores, using the GGA-PBE (generalized gradient approximation-Perdew Burke Ernzerhof) functional [75]. Hirshfeld's method was used to carry out charge partitioning [76] and the covalent bond orders were calculated using Mayer's procedure [77].

Section snippets

DFT study of various core/shell QDs

In traditional type QDs, a low band gap core material is coated with a second semiconductor with a wider band gap than the core material [78,79]. It has been shown that coating a core semiconductor with a larger band gap material of the same kind improves the properties of nanocrystals [80,81]. As mentioned earlier, we have selected a core of the cadmium chalcogenides, with the shells comprising the respective chalcogenides of the zinc triad. These structures were compared with similar sized

Effect of interchange of core and shell materials

Here, we have constructed spherical, atom-centered and non-stoichiometric, passivated colloidal nanoclusters of diameter 15 Å having charge +6. These structures were fully optimized (Figs. 2 and S1) in order to study the differences in the structural and electronic properties produced on interchanging the core and shell materials.

The following three cases have been considered:

  • (i)

    CdS/CdSe and CdSe/CdS

  • (ii)

    CdS/ZnS and ZnS/CdS

  • (iii)

    CdSe/ZnSe and ZnSe/CdSe

Here also, as we saw earlier, three different kinds of

Doping of CdS/ZnS core/shell quantum dots with transition metal ions

We have considered seven transition metal dipositive ions, i.e. Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ and Hg2+ in order to maintain the overall charge on the core/shell system and to study their role in enhancing the structural and electronic properties of the CdS/ZnS core/shell QDs for its use in various fields such as spintronics, in integrated novel magnetoelectronic devices such as spin-LEDs, mid IR lasing, etc. Zn2+ and Hg2+, being d10 ions, belong to a closed shell system; therefore, they

Spin multiplicities

We first determined the most stable spin state of the transition metal ions used for doping. The most stable spin state in case of the Mn2+ doped CdS/ZnS core/shell QD is the sextet spin state, which is 1.36 eV lower in energy than the doublet spin state, and 1.32 eV lower than the quartet spin state (Table 6). This observation is easily attributable to the additional stability achieved by Mn2+ because of the presence of half-filled d-orbitals in the sextet case. The size of manganese is not

Conclusions

In this paper, we have thoroughly investigated the modifications in properties brought about by coating a II-VI QD with another material of the same family. Thus, we have considered shells in which the cation is varied, or the anion is varied, and, in each case, significant variations in the bonding characteristics are brought about, depending on the shell material. Interchanging the material of the core with that of the shell also brings about profound changes in the properties, significant

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing Senior Research Fellowships to PM, RT and JS. Also, monetary support from “Delhi University's system to build up Research by providing resources to Faculty” is gratefully acknowledged.

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