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Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell Quantum Dots

  • Amar Nath Yadav
  • Ashwani Kumar Singh
  • Kedar SinghEmail author
Chapter
  • 4 Downloads
Part of the Lecture Notes in Nanoscale Science and Technology book series (LNNST, volume 28)

Abstract

Semiconductor core/shell quantum dots (QDs) are composed of at least two semiconducting materials having a structure like an onion. In the recent past, the synthesis of these systems impelled significant progress, as by growing an epitaxial shell, we can easily tune basic optical properties such as fluorescence quantum yield, emission wavelength, and carrier lifetime. The significance of developing an epitaxial shell over the surface of core QDs is making the nanocrystal less sensitive to environmental changes and photo-oxidation. Another advantage is the enhancement of fluorescence quantum yield by passivating surface trap states of core QDs. These properties are essential for the application of semiconductor core/shell QDs in light-emitting diodes, solar cell, and biological labelling. This chapter discusses the synthesis and microstructural and optical properties of mostly II–VI semiconductor, core/shell QDs. Moreover, various applications of core/shell QDs in solar cells, light-emitting diode, and biomedical have been also discussed in detail.

Keywords

Core/shell quantum dots Photoluminescence Semiconductors Synthesis 

1 Introduction

In recent decades, semiconductor nanostructured materials are significantly cherished because they can link the gap between small molecules and bulk materials [1, 2, 3, 4]. The nanostructured materials show distinct optical and electronic properties when their size varies in the range of 1–100 nm. With the variation of dimension, they can be classified as (1) two-dimensional, e.g., nanosheets or thin films or quantum wells; (2) one-dimensional, e.g., quantum wires; and (3) zero-dimensional, e.g., quantum dots. Thus, compared to the bulk semiconductor material, a quantum dot (QD) is zero-dimensional and has a limited number of atoms, displaying discrete energy states (Fig. 1) [5, 6]. Bulk semiconductor materials have continuous valance and conduction energy states with composition-dependent bandgap (Eg), which is defined as minimal energy required to create an electron in conduction band from the valance band. In this process, the electron leaves a hole in valance band upon excitation energy higher than Eg. In the presence of the electric field, these opposite charge carriers may be mobilized and hence carry current. A bound state of electron-hole pair in their minimum energy state is called an exciton, and the distance between them is exciton Bohr radius (rB). Further, the excited-state electron relaxes to annihilate exciton via radiative recombination of electron-hole and emit their energy in the form of a photon [7]. Two major factors can influence the unique properties of QDs: quantum confinement and surface effects.
Fig. 1

Schematic representation of the idealized density of states for semiconductor nanostructure with reduced dimensionality (3d, 2d, 1d, and 0d represent three-dimensional, two-dimensional, one-dimensional, and zero-dimensional). For 3d bulk structure, the energy levels are continuous, whereas for 0d QDs energy levels are discrete. (Reprinted with permission from Ref. [5])

Quantum confinement: When the radius of semiconductor nanocrystal (r) becomes smaller or equal to exciton Bohr radius, i.e., r ≤ rB, then the motion of electrons and holes is spatially confined to the dimension of the QD. Also, in this condition, the energy difference between two levels of QD exceeds the value KBT (here KB is Boltzmann constant); this results in restriction of the mobility of electron and hole in the crystal dimensionality. In this regime, depending on the size of the nanocrystals, the QDs exhibit size-dependent absorption and emission with discrete electronic transitions [8, 9]. This effect is called quantum confinement effect. It can affect several properties of semiconductor QDs, including magnetic properties and conductivity. Nevertheless, the most exciting properties that arise from quantum confinement is the size- and shape-dependent optoelectronic properties.

Surface effect: As the size of nanocrystal reduces, their surface-to-volume ratio increases. This leads to an increment in the free energy of the NCs, which makes NCs more dynamic and reactive as compared to their bulk counterparts. Henceforth, it modifies the materials basic properties including solubility, reactivity, evaporation and melting temperatures, plasticity etc. It also makes the NCs to easily disperse in solvents media and opens up a possibility to functionalize or modify their surface. This is the most beautiful property of colloidal NCs that can be used to build optoelectronic devices and targeting bio drugs. However, for very smaller nanocrystal, the surface-to-volume ratio is very high; results in most of the bonds associated with the surface are unsatisfied [10]. The energy levels associated with surface states lie in between valance and conduction band. Further, these surface states behave as fast non-radiative de-exciton channels and trap the photo-generated charge carriers. As a result, the optical and optoelectronic properties of QDs are significantly influenced by surface trap states [11, 12]. Therefore, passivation or capping of the surface is essential for the evolution of luminescent and stable QDs. Surface passivation of QDs can be carried out by two types: (1) organically and (2) inorganically (Fig. 2) [13].
Fig. 2

Schematic representation of (a) organically passivated QD, (b) inorganically passivated QD (core/shell structure), and (c) energy band offsets of core/shell structure. (Reprinted with permission from Ref. [13])

Organic surface passivation involves organic molecules that bond with surface atoms and act as capping agent. The advantages of the organic passivation include monodispersity, colloidal suspension, and bioconjugation of QDs [14, 15, 16]. However, full coverage of surface atoms and simultaneous passivation of both cation and anion surface sites are still complicated because of shape distortion and larger size of the organic capping molecules [17, 18]. The second strategy is inorganically passivation, which involves full passivation of surface trap states. In this approach, surface passivation has been carried out by overgrowth of inorganic layer, particularly a second semiconductor. The resulting material is known as core/shell QDs [19]. In this case, fluorescence QY and photostability of the core/shell QDs drastically improved. It is also possible to tune the absorption and emission spectra of the material by choosing appropriate core and shell materials. The proper choice of shell material and its thickness are the essential terms that contribute to overall properties of QDs. If the core and shell structures have huge lattice mismatch, then this results in lattice strain that generates defect states within or at the core/shell interface. In addition, the thicker shell creates misfit dislocations, which also decreased fluorescence QY by the non-radiative process [20].

1.1 Classification of Core/Shell Quantum Dots

Based on the energy bandgap offset, semiconductor core/shell QDs are generally categorized into three groups; type I, reverse type I, and type II (Fig. 3). In type I, the shell material has wider bandgap than the core material, i.e., valance and conduction band edge of the core lies in between shell material so that the charge carriers (electrons and holes) are restricted within the core. Further, in reverse type I, the bandgap of the core material is larger than the shell material. In this case, the photo-generated charge carriers partially or completely delocalized in the shell, and by changing shell thickness, the emission wavelength can be tuned. Finally, in type II, either valance or conduction band edge of shell material lies within the bandgap of the core material. Consequently, upon photoexcitation, the charge carriers are spatially isolated in a distinct region of core/shell heterostructure [19, 21].
Fig. 3

Schematic diagram of band alignment for different types of core/shell structure: (a) type I, (b) type II, (c) quasi type II, and (d) reverse type I. The wave function represented by blue color is stands for electron wave function, whereas red color represents hole wave function. (Reprinted with permission from Ref. [21])

In addition to above-discussed types of core/shell structure, there is an intermediate one identified as quasi type II (or type I1/2) core/shell QDs. The most studied quasi type II system is CdSe/CdS core/shell QDs, although it is a type I core/shell structure. However, the energy offset of the electron is minimal to confine it in the CdSe core, and subsequently the electronic wave function delocalized over the whole nanocrystal, while hole wave function remnant inside the core of the QD [19, 21].

2 Synthesis of Core and Core/Shell Semiconductor QDs

Core/shell QDs have been mostly synthesized by two-step process: first the synthesis of core QDs and second subsequent shell growth process.

2.1 Synthesis of Core QDs

Colloidal semiconductor QDs have been generally synthesized by using three components such as solvents, precursor, and organic surfactant. In an inert atmosphere, heating of reaction mixture up to the required temperature changes the precursor into monomers (active atomic and molecular species) and is called nucleation. Further, the monomers are converted into nanocrystals whose continuous growth depends on the surfactant molecules. Thus, two steps are required for the formation of nanocrystals: (1) nucleation and (2) growth [22, 23].

In more than two decades, a large number of reviews have been reported related to the synthesis of II–VI and the other semiconductor QDs [13, 19, 21, 22, 23, 24]. In general, two methods of non-injection and injection have been used for the synthesis of aqueous and nonaqueous semiconductor QDs by varying solvents, temperatures, and precursors (Fig. 4).
Fig. 4

Schematic illustration of nucleation and growth mechanism in nanocrystals based on La Mer model [23]. (Reprinted with permission from Ref. [22])

2.1.1 Injection Method

In 1993, Murray et al. developed a traditional approach for the synthesis of semiconductor QDs by fast injection of the precursor into a hot solvent [25]. In a typical synthesis protocol, they first prepared Cd and Se precursor solutions by mixing Cd(CH3)2 and Se powder into tri-n-octyl phosphine (TOP) solvent, distinctively. These precursors were further injected quickly into hot trioctylphosphine oxide (TOPO) solution in a three-neck flask and under an inert (N2) atmosphere. Here, TOPO acts as stabilizing agent and allows the reaction mixture to heat at a high temperature generally up to 320 °C. This synthesis technique was a model for the preparation of CdSe, CdS, and CdTe QDs with different sizes 1.5–11.5 nm.

Later in 2001, Peng et al. have used CdO as a Cd precursor in place of Cd(CH3)2. The reason behind is toxicity, explosive, and expensiveness of Cd(CH3)2 in comparison to CdO [26, 27]. Moreover, the quality of CdSe QDs remains the same as in the case of Cd(CH3)2. Well ahead, due to environment issue, many efforts have been made for the synthesis of II–VI semiconductor QDs using “green” hot injection synthesis method [28, 29]. After that, the expensive and coordinating solvent TOPO has been replaced by low-cost and non-coordinating solvent octadecane (ODE) [30, 31, 32]. Further, Deng et al. simplified the reaction by using low-cost solvent oleic acid instead of TOP/TOPO [33].

As described above, the hot injection method is based on a fast injection of precursors into a hot solution containing another precursor followed by an instant homogenous reaction. In this method, it was hard to control the precise reaction temperature upon injection of the precursor solution. Thus, scale-up synthesis and reproducibility are difficult to achieve using this method. Therefore, for large-scale synthesis, another method “non-injection method” has been developed.

2.1.2 Non-injection Method

In this method, all reagents are mixed in a three-neck flask, and nucleation and growth are originated either chemically, thermally, or by physical impact (e.g., microwave irradiation) [34, 35, 36]. First, Pradhan et al. have developed a versatile and easy method for the synthesis of extremely good metal sulfide (CdS) nanoparticles using one-spot and low-temperature process [37]. In this method, heating of metal xanthate in hexadecylamine (HDA) gives rise to metal sulfide NPs even at low temperature 70 °C. Moreover, by adjusting reaction temperature, NPs of various sizes could be obtained using this method. Later, Coe and his collaborators synthesized high-quality and monodispersed CdS QDs by one-spot non-injection method [34]. In this method, they have introduced two nucleation initiators, namely, tetraethylthiuram (I1) and 2,2′-dithiobisbenzothiazole (I2), in the reaction mixture. So separate nucleation and growth are accomplished, and the quality of CdS QDs is quite comparable with the injection method.

Besides the abovementioned non-injection method, another non-injection way is single-source inorganic cluster approach. In this method, the single-source precursor (e.g., Li4[Cd10Se4(SPh)16] for CdSe) was added in dodecylamine or hexadecylamine at 80 °C under N2. Further, after dissolving the cluster, the reaction mixture was heated up to 220 °C for the growth of QDs. From this method, nucleation at relatively low temperature can be achieved, and reaction can be scaled to large quantities (1–50 g/L). Further, other QDs such as CdS, ZnSe, and CdSe/ZnS also can be prepared via this method [38].

2.2 Synthesis of Semiconductor Core/Shell QDs

For the growth of the shell, two points are most important: (1) choice of shell material and (2) thickness of the shell material. In the former one, generally, semiconductors with small lattice mismatch have been chosen for the core and shell structure. If both structures have a huge lattice mismatch, then it results in lattice strain and generates defect states at the core/shell structure or within the shell. The defect states can act as trap states for the charge carrier and can decrease PL QY [20]. Table 1 represents a list of material parameters for some selected bulk semiconductors.
Table 1

Material parameter for bulk II–VI semiconductors [19]

Semiconductor materials

Structure (300 K)

Type

Egap (eV)

Lattice parameter (Å)

Density (kg m−3)

CdSe

Wurtzite

II–VI

1.74

4.3/7.01

5810

CdTe

Zinc blende

II–VI

1.43

6.48

5870

CdS

Wurtzite

II–VI

2.49

4.136/6.714

4820

ZnSe

Zinc blende

II–VI

2.69

5.668

5266

ZnTe

Zinc blende

II–VI

2.39

6.104

5636

ZnS

Zinc blende

II–VI

3.61

5.41

4090

The second point is the thickness of shell material, which also plays a vital role in the properties of core/shell QDs. If the shell thickness is skinny, then passivation of surface traps of core QDs may be incomplete. On the other hand, if the shell thickness is very thick, then it results in the formation of defect states due to lattice strain. During shell growth, number monolayers of shell material are deposited on the surface of the core QDs. The required amount of shell thickness can be calculated as follows.

The volume of the shell material for “m” monolayers of shell thickness can be calculated as
$$ {V}_{\mathrm{shell}}=\frac{4}{3}\Pi \left[{\left({r}_c+m\times {d}_{\mathrm{ML}}\right)}^3-{r}_c^3\right] $$
where Vshell is the volume of the shell material, rc is the radius of core QDs, and dML is the shell thickness for one monolayer (nm).
If nshell is the amount of shell material (in moles) required to deposit “m” mL shell, then
$$ {n}_{\mathrm{shell}}=\frac{V_{\mathrm{shell}}\times {D}_{\mathrm{core}}\times {N}_{\mathrm{A}}\times {n}_{\mathrm{QD}}}{MW_{\mathrm{core}}} $$
where Dcore is the density of core material, NA is the Avogadro number, nQD is the number of moles of core QDs in solution, and MWcore is the molecular weight of core material [39].

After calculations of the precise amount of shell thickness, deposition of shell has been carried out by a technique called successive ionic layer adsorption and reaction (SILAR) method [40]. By this technique, we can deposit one monolayer at a time by injecting cationic and anionic precursors into the core solution. After depositing one layer successively, one can deposit many monolayers of shell thickness on the surface of core QDs. By using this method, monodisperse and highly luminescent semiconductor core/shell QDs could be synthesized.

2.2.1 Synthesis and Characterization of Type I Core/Shell QDs

Type I core/shell QDs have been synthesized for the motive to increase fluorescence QY and to improve photostability of the QDs. The most studied II–VI semiconductor in type I is CdSe/ZnS core/shell QDs. First, Guyot-Sionnest group have grown ZnS shell of one to two monolayers onto 3 nm CdSe QDs by using growth temperature 300 °C, and, in this case, they have found QY to be 50% [41]. For the synthesis of this system, they have used a hot injection method. First, they prepared Cd and Se stock solutions by dissolving dimethylcadmium and selenium shot in TOP. Further, using Cd and Se stock solutions, CdSe nanocrystal was synthesized. The growth of ZnS shell has been achieved by injection of Zn and S stock solutions which were developed by diethylzinc (Me2Zn) and hexamethyldisilathiane [S(TMS)2] in TOP solution. Later, Bawendi group have reported a depth study on CdSe/ZnS core/shell QDs using low-temperature (140–220 °C) synthesis [42]. Moreover, using HDA into TOPO/TOP, Weller group have been able to control the growth kinetics of CdSe/ZnS core/shell QDs and achieved lower particle size distribution with high QY up to 60% [43]. Moreover, the blue spectral region is also accessible using this material with extremely small size as reported by Kundra and co-authors [44]. They first synthesized CdSe “magic-sized” cluster in the solution of TOP, DDA, and nonionic acid at 80 °C and further sequential growth of ZnS shell. In another report, Jun and Jang obtained similar spectral range with overgrowth of ZnS shell at 300 °C by using zinc acetate and octanthiol as a precursor [45]. In this case, due to the low reactivity of precursors, the shell material spread into the core results in a remarkable blue shift of emission peak observed around 470 nm with QY up to 60%. In another work on the same material by Zhang group, they reached QY up to 95% by TOP-SILAR method (Fig. 5) [46]. This high QY is obtained by growing three monolayers of ZnS shell and even maintained after six monolayers of shell thickness (Fig. 6c).
Fig. 5

Growth mechanism of multilayered CdSe/ZnS core/shell QDs by TOP-SILAR synthesis method. (Reprinted with permission from Ref. [46])

Fig. 6

TEM images of (a) CdSe core and (b) CdSe/ZnS core/shell QDs after three monolayers of Zn and S precursors. Insets of these figures are their high-resolution transmission electron microscope (HRTEM) image. (c) Variation of QY with increasing shell thickness, where the core is synthesized with three emitting colors green, orange, and red, respectively. Inset is photograph of CdSe/ZnS core/shell QDs under UV light with three monolayers of shell thickness. (Reprinted with permission from Ref. [46])

The second material in the type I system is CdSe/CdS core/shell QDs. In this case, the lattice mismatch between core and shell material is only 4% and exhibits different band alignment (holes have larger band offset than electrons). A detailed study on CdSe/CdS core/shell QDs with varying core diameters 2.3–3.9 nm and QY up to 50% was first reported by Peng and co-authors [47]. In this system, due to the delocalization of electronic wave function over the whole core/shell, the absorption and emission spectra get more shifted in comparison to the CdSe/ZnS system. The precursors used in this system were dimethylcadmium and bis(trimethylsilyl sulfide) with solvent TOPO. Later, using SILAR technique the same group synthesized the same material by air-stable precursors cadmium oleate and elemental sulfur in the low-cost, low toxic, and high boiling solvent octadecane [40]. The SILAR technique was further extended to synthesize “giant” core/shell QDs as reported by Klimov group for the synthesis of giant CdSe/CdS core/shell QDs [48]. The shell thickness was 6 nm by growing 19 monolayers of CdS, and the QY was achieved up to 40%. Recently, Manna group have reported the synthesis of extremely luminescent giant CdSe/CdS core/shell QDs by a fast continuous injection method [49]. In this report, they have synthesized CdSe core of diameter between 2.8 and 5.5 nm and CdS shell thickness of 7–8 nm (~20 monolayers of CdS). Interestingly, the QY was maintained up to 90% by using this defect-free QD. The purpose of growth of the giant shell was to make nanocrystal independent from environment and surface chemistry. Further, a thicker shell protects the QDs from photobleaching and photoblinking. However, due to their larger size, they have poor size distribution and broad PL emission. Therefore, another method was required that could meet all the criteria. Later, Bawendi group synthesized high-quality CdSe/CdS core/shell QDs by optimizing growth rate of the shell material, using cadmium oleate and octanthiol as shell precursors [50]. The obtained core/shell QDs have very narrow emission peak with high PL QY (Fig. 7) and high uniformity (Fig. 8). Moreover, in contrast to previous studies, in this case, photoblinking too much suppressed with growing a relatively thin shell (2.4 nm).
Fig. 7

Optical properties of CdSe/CdS core/shell QDs. a–d: UV-visible absorption (blue) and photoluminescence (PL) (red) spectra of four different CdSe/CdS core/shell QDs synthesized with different CdSe core diameters of (a) 2.7 nm, (b) 3.4 nm, (c) 4.4 nm, and (d) 5.4 nm. (e and f) Temporal evolution of sample as shown in figure (c) during shell growth: (e) emission, (f) absorption, (g) PL QY, and (h) FWHM of PL peak. Green square shows the original photoluminescence QY of CdSe QDs. (Reprinted with permission from Ref. [50])

Fig. 8

TEM images of (a) CdSe core with diameter 4.4 nm and CdSe/ZnS core/shell QDs with thickness (b) 0.8 nm, (c) 1.6 nm, and (d) 2.4 nm. Scale bar: 50 nm. (Reprinted with permission from Ref. [50])

CdSe/ZnSe is another type I core/shell system, where electrons conduction band offset is larger than the holes valance band offset. The lattice mismatch, in this case, is significant as 6.3%; however, the anion-type structure is suitable for epitaxial shell growth. In the early time of synthesis, the QY of this system is very low (<1%) [51]. Afterward, the synthesis method has been modified using zinc stearate rather than diethylzinc as a zinc precursor and TOP-Se (Se dissolved in TOP) as a selenium precursor [52, 53]. Narrow emission line widths with high QY up to 85% were obtained using this method. Further, in another study, the concentration of ZnSe precursor solution was varied to observe changes in optical spectra due to lattice strain [54].

In CdSe/CdS and CdSe/ZnSe core/shell QDs, the emission wavelength can be tunable with high fluorescence QY. However, for the high stability of optical properties against photoblinking and photodegradation, the better choice of shell material is zinc sulfide (ZnS). It is discussed above that, using a smaller size of CdSe/ZnS core/shell QDs, it was possible to obtain blue and green emission. In this context, materials other than CdSe have been developed for the better spam of UV/blue/green and near-infrared spectrum. QD using alloyed structure is an alternative for the tuning of emission by changing materials composition rather than the particles size. For example, in Cd1 − xZnxSe QDs, the emission wavelength can be tuned by changing the value of x. Subsequently, the alloy Cd1 − xZnxSe core with ZnS shell has been grown using well-established precursors diethylzinc and air-stable zinc diethyl xanthate [55, 56].

Shen et al. have described synthesis of high-quality Cd1 − xZnxSe/ZnS core/shell QDs by using a new method called “nucleation at low temperature/shell growth at high temperature” [57]. The obtained core/shell QDs are monodispersed with high QY (up to 100%), high color purity (FWHM<25 nm), and emission tunability from 400 to 470 nm of the optical window (Fig. 9). Moreover, the resulting QDs have excellent chemical and photochemical stability, and method can be easily applied to large-scale synthesis up to 37 g in batch synthesis.Since after shell growth the absorption and emission spectra were shifted toward longer wavelength side. Such red shift, in this case, can be explained in terms of delocalization of electronic wave function into the surrounding shell and reduction in quantum confinement effect. Further, the size of the core/shell QDs varies in between 5.7 and 10.8 nm with core mean diameter of 4.8 nm (Fig. 10). Recently, the same group successfully controlled ZnS shell thickness on the ZnCdSe core QDs [58]. Moreover, by growing the shell thickness up to ten monolayers, the external quantum efficiency of 17% was achieved.
Fig. 9

Evolution of (a) absorption and emission spectra of Cd1 − xZnxSe core and Cd1 − xZnxSe/ZnS (x = 0.25) core/shell QDs. (b) Normalized PL spectra of Cd1 − xZnxSe core and Cd1 − xZnxSe/ZnS core/shell QDs (at different x) and inset image is corresponding UV-illumination photos. (Reprinted with permission from Ref. [57])

Fig. 10

(a) TEM image for Cd1 − xZnxSe core QDs. TEM images of Cd1 − xZnxSe/ZnS core/shell QDs, (b) shell growth for 5 min, (c) shell growth for 15 min, (d) shell growth for 45 min, (e) shell growth for 90 min. Figures (fj) represent HRTEM images corresponding to a–e images. (Reprinted with permission from Ref. [57])

In addition to the above discussion, the emission wavelength in blue and near-UV spectral region was generally obtained by core material with larger bandgap. As far as we know, the first core/shell structure historically was reported by Spanhel et al. in 1987 [59]. It was treatment on 4–6 nm CdS core QDs with sodium hydroxide (NaOH) and cadmium perchlorate [Cd(ClO4)] in aqueous media, making cadmium hydroxide shell. Initially, the PL QY of CdS QDs was 1%, but after surface modification, the PL QY quickly rose to 50% with narrow emission line widths. Later, in organic media, ZnS shell is grown on CdS core using the organometallic method with S(TMS)2 and dimethylcadmium as precursors. The emission wavelength spans over 460–480 nm with PL QY of 20–30% [60]. Moreover, Reiss group developed a new approach for the synthesis of CdS/ZnS core/shell QDs [61]. In this method, the reagent described above is further replaced by air-stable precursors like zinc stearate and zinc ethylxanthate. The subsequent CdS/ZnS core/shell QDs were found to be monodispersed, tunable emission (range of 440–480 nm) and QY up to 35–45%. In another study, manganese has been used as a dopant in CdS host and the ZnS shell was either grown by the reverse micelle or organometallic route [62, 63, 64]. In 2009, Peng group developed a new approach “thermal cycling coupled single precursor” (TC-SP) for the synthesis of high-quality CdS/ZnS core/shell QDs [65]. In contrast to classical SILAR, this is also a one-spot method; however, the growth temperature considerably reduced, and the technique was simple than SILAR.

Meanwhile, ZnSe/ZnS core/shell QDs are also synthesized using organometallic precursors by Manna group [66]. The observed emission wavelength was in the range of 400 nm, with FWHM of 20 nm and QY of 17%. Further, a new method was adopted by using alternative precursors like ZnO and TOPSe for the synthesis of ZnSe core and zinc laurate and TOPS for ZnS shell [67]. The reaction mixture was carried out at 180 °C in HDA and the final product ZnSe/ZnS QDs have QY up to 30%. However, most of the above method entails the use of TOP or TBP as a precursor solvent. It well accepted that alkylphosphines (TOP or TBP) are very expensive, hazardous, unstable, and air-unstable solvent. Therefore, a green approach, i.e., phosphine-free synthesis, will be needed to synthesize these core/shell QDs. Afterward, Dong et al. described a very-low-temperature (<150 °C), facile, and two-step process to prepare water-soluble ZnSe/ZnS core/shell QDs [68]. The precursors used for the shell growth were zinc acetate and thiourea in MPA solvent. The emission wavelength was tuned in the range of 390–460 nm with QY up to 45% by growing three monolayers of shell thickness (Fig. 11). The diameter of core/shell QDs is found to be 5.5 nm after three monolayers of ZnS shell thickness on 3.3 nm ZnSe core.
Fig. 11

(a) Absorption and emission spectra of ZnSe core and ZnSe/ZnS core/shell QDs with increasing shell thickness. (b) PL QY and FWHM at different ZnS shell monolayers. (Reprinted with permission from Ref. [68])

In this context, a green synthesis was described by Li group, who grow ZnS shell over ZnSe core by two converse injection methods [69]. In this report, zinc and selenium precursors were prepared by using octadecane (ODE: a phosphine-free solvent) to synthesize ZnSe QDs. After that, stock solutions for shell growth were prepared by dissolving ZnO in oleic acid and paraffin oil and sulfur powder in ODE. The optical properties using this method retain the same as in the case of phosphine solvent used QDs. In another report by the same group were synthesized (via low-temperature/high-temperature shell growth) highly stable and violet-blue emitting ZnSe/ZnS core/shell QDs [70]. First zinc stock solution was prepared by mixing ZnO in oleic acid and paraffin oil at 300 °C. Similarly, selenium stock solution was adopted by dissolving Se powder in ODE at 220 °C. These two stock solutions were used to synthesize ZnSe core QDs in paraffin oil at 280 °C. Finally, shell growth of ZnS was carried out at 320 °C by injecting Zn stock solution and octanethiol in a solution containing ZnSe QDs. The obtained core/shell QDs retain excellent optical properties such as high QY (up to 83%), high color saturation (FWHM between 12 and 20 nm), tunable emission (violet to blue, 400–455 nm), and superior chemical/photostability.

Cadmium telluride (CdTe) QD is another material in the II–VI group, which has lower bandgap (1.5 eV) in comparison to CdSe (1.76 eV). This material is a good candidate for application in red-infrared-emitting LEDs and other optical devices [19]. Even if the described synthesis method in the case of CdSe also could be adopted to CdTe QDs, however, some little efforts also were chosen to synthesize this QD in organic media [71]. Meanwhile, Gao group observed a substantial enhancement in PL QY up to 85% by the illumination of thioglycolic acid (TGA)-capped CdTe QDs [72]. As a matter of fact, the improvement in PL QY was due to photodegradation of TGA rather than CdTe QDs. In this case, the sulfide ions are released from TGA during illumination and form a shell of CdS on the surface of CdTe QDs reacting with Cd surface atoms. After that, similar to the previous study, the CdS shell was grown on CdTe by microwave-assisted and ultrasonic irradiation, respectively [73, 74]. In a report by Gu et al., they have proposed a one-spot aqueous synthesis of highly luminescent CdTe/CdS core/shell QDs [75]. The CdS shell was grown in a crude solution of MPA-capped CdTe QDs by using thiourea (acts as sulfur source), which further reacted with excess Cd to form core/shell structure. The QY was found to be high as 75%, and the stability of QDs quite improved.

2.2.2 Synthesis and Characterization of Type II Core/Shell QDs

Type II core/shell QDs could be recognized by recording optical spectra because in this case huge spectral shift was observed in absorption as well as in emission spectra after shell growth. Bawendi group described the first protocol for the preparation of type II CdTe/CdSe and CdSe/ZnTe core/shell QDs and studied their optical properties [76]. The emission wavelength was varied in the spectral region from 700 to 1000 nm by changing CdTe core size and CdSe shell thickness. In this structure, the average decay lifetime significantly increased in the core/shell structure (57 ns) in comparison to core QDs (9.6 ns), but the QY was low as 4%. Further, the synthesis of CdTe/CdSe core/shell QDs was carried out without using organometallic precursors [77]. The shell precursors in this method were used as CdO and TOPSe in TOP, whereas for core QDs CdO and TOPTe. The QY approaches up to 40% with growth of shell thickness of 0.5 nm. In a report on the same material using SILAR technique, Chin et al. observed very high QY up to 80% and the formation of the anisotropic structure (pyramids and multi-pods) during shell growth [78]. Xia and co-authors carried out the aqueous synthesis of CdTe/CdSe core/shell QDs, where CdCl2 and Na2SeSO3 were used as the shell precursor [79]. The prepared QDs had high stability, moderate QY up to 20%, and tunable emission near-infrared region.

Further, different shells of CdTe, CdSe, and CdS were grown onto ZnTe core by Xie et al., and they studied their properties [80]. The precursors were prepared by dissolving cadmium oleate, TOPTe, TOPSe, and sulfur in octadecane. The obtained precursors were further dissolved in crude ZnTe core QDs to form core/shell QDs. The PL emission was spammed in the range of 500–900 nm with QY up to 30%. Moreover, after lowering the shell growth temperature from 240 °C to 215 °C, the same group observed a transition from pyramidal to tetrapod using ZnTe/CdSe core/shell QDs in another report [81]. Similar tradition is also described by Alivisatos group in another paper [82].

Furthermore, Klimov group have developed a synthesis procedure for highly luminescent type II CdS/ZnSe core/shell QDs [83]. For the synthesis of this system, first of all, as-synthesized CdS QDs were added in the mixture of octadecylamine and ODE. Further, ZnSe shell has been grown by adding precursors zinc oleate and TOPSe in the as-prepared solution. The emission wavelength was tuned from 500 to 650 nm, with varying core radius and shell thickness. Moreover, Smith and co-authors have tuned the optical and electronic properties of colloidal QDs by lattice strain [39]. They deposited compressive shell of ZnS, ZnSe, ZnTe, CdS, and CdSe onto a CdTe core to form lattice-mismatched QDs. These materials were synthesized by two-step organometallic method in a high-temperature coordinating solvent, as described in previous studies. The band structure of obtained core/shell QDs changes from type I to type II behavior, which was characterized by a large spectral shift in absorption and emission spectra as well as an increment in excited-state lifetimes (Fig. 12 a and b). In particular, the emission wavelength was tuned from visible to near-infrared (500–1050 nm), and PL QY varied in between 25 and 60%.
Fig. 12

(a) Absorption and emission spectra of CdTe core QDs (size 1.8 nm) and CdTe/ZnSe core/shell QDs with different shell thickness. (b) HRTEM and fast Fourier transform of CdTe core QDs (top) and CdSe/ZnSe core/shell QDs with 6 mL of shell thickness (bottom). (c) HRTEM of CdSe/ZnSe core/shell QDs with 6 mL of shell thickness. (Reprinted with permission from Ref. [39])

Bang et al. pioneered a synthesis of nontoxic, water-soluble, and highly stable ZnTe/ZnSe core/shell QDs [84]. The emission wavelength of the QDs was in the range of 500–580 nm and PL QY was reached up to 60%. For the synthesis of this core/shell structure, zinc and selenium precursors were prepared by dissolving diethylzinc and selenium pellets in TOP, respectively. After that, ZnTe core QDs were loaded in QDE, and HDA under N2. Finally, these precursors were added dropwise in bare CdTe QDs for 6 h, and final shell growth was achieved at a temperature in between 200 and 250 °C.

2.2.3 Synthesis and Characterization of Reverse Type I Core/Shell QDs

A reverse type I core/shell QD is also called “inverted” core/shell QDs because in this case, the core material has a larger bandgap than the shell material. Peng group developed a synthesis of inverted CdS/CdSe core/shell QD by SILAR technique [85]. The emission wavelength was tuned from 520 to 650 nm and QY from 20% to 40% by growing another shell of CdS onto CdS/CdSe core/shell QDs. Later, Klimov group described a continuous transformation of type I to type II and back to the type I structure in ZnSe/CdSe core/shell QDs [86]. These transformations depend on the core size and shell thickness, where electron and hole wave functions delocalized in the distinct region of core/shell structure. The emission wavelength was varied in the range of 430–600 nm with QY in between 60% and 80%. Moreover, the same material is synthesized by using cadmium oxide (dissolved in oleic acid and ODE) and TOPSe as the shell precursors [87]. The emission color was varied from violet to red (417–678 nm) with QY up to 85% by changing CdSe shell thickness onto 2.8 nm ZnSe core (Fig. 13).
Fig. 13

Absorption and corresponding normalized emission spectra of ZnSe/CdSe core/shell QDs with increasing monolayers of CdSe shell (a) 0, (b) 0.1, (c) 0.2, (d) 0.5, (e) 1, (f) 2, (g) 4, (h) 6. (Reprinted with permission from Ref. [87])

3 Applications of Semiconductor Core/Shell QDs

The semiconductor core/shell QDs can be used in many applications because of its excellent properties including tunability of bandgap by size, high PL QY with narrow FWHM, longer excited-state lifetime, and robust stability. Here, we have discussed some selected applications of QDs in solar cells, light-emitting diodes (LEDs), and biomedical.

3.1 Solar Cells

In the beginning, II–VI (e.g., CdSe, etc.) and IV–VI (e.g., PbSe, etc.) QDs were used as light absorber (sensitizer) in the solar cell [88, 89]. Now, the research extends on core/shell QDs because of its high stability against photobleaching and easy alternation of its basic properties including bandgap, charge separation, and exciton recombination rate. All three types of core/shell QDs can be used in solar cell applications. However, much better efficiency in the solar cell performance has been found by using type II core/shell QDs as a sensitizer.

In typical QD-sensitized solar cell, there are a photoanode, liquid electrolytes, and a counter electrode (Fig. 14a) [90]. The photoanode part consists of a light absorber (QDs) and a large bandgap semiconductor crystalline (e.g., TiO2 or ZnO, etc.) substituted on FTO (fluorine-doped tin oxide) or ITO (indium-doped tin oxide) glass. Upon light excitation, electrons and holes are generated in the conduction and valance band of the QD, respectively. Further, the photo-generated electrons are quickly injected into the conduction band of the TiO2 (used as electrons acceptor). The electrons transferred to the FTO substrate through TiO2 and then to counter electrode through an external wire [90]. In the meantime, the oxidized QD was neutralized by reduced species of redox couple in the electrolyte, whereas electrons reduced the oxidized species of redox couple from the counter electrode. The circuit is completed and the photocurrent is measured. The photovoltaic performance of QD-solar cells can be determined from the J-V curve and IPCE test. The J-V graph has been used to find power conversion efficiency (PCE) and is related through power density of incident light (Pin) by the equation as follows:
$$ \mathrm{PCE}=\frac{I_{\mathrm{mp}}.{V}_{\mathrm{mp}}}{P_{\mathrm{in}}}=\frac{J_{\mathrm{sc}}.{V}_{\mathrm{oc}}.\mathrm{FF}}{P_{\mathrm{in}}} $$
where Imp is the photocurrent, Vmp is the photovoltage, Jsc is the short-circuit density, Voc is the open-circuit voltage, and FF is the fill factor defined by the ratio of specific value Pmax to Voc and Jsc (all factors can be found from Fig. 14b) [90].
Fig. 14

(a) Schematic diagram for device structure of QD-sensitized solar cells (QDSCs). (b) J-V characteristic graph for QDSCs. (Reprinted with permission from Ref. [90])

In 2009, Lie et al. used first-time core/shell QDs as a sensitizer in solar cell applications [91]. They observed PCE of 4.22% by using inverted type I CdS/CdSe core/shell QDs. After that, core/shell QDs have been commonly used in the solar cell. Further, Parkison et al. have used type I CdSe/ZnS core/shell QDs to sensitize TiO2, and, in this case, the stability of photoanode significantly improved [92]. Moreover, in the case of type I CdSeTe/ZnS core/shell QDs, the PCE of solar cells has been found to be 9.48%, whereas bare CdSeTe QD shows a lower PCE of 8.02% [93].

Z. Ning et al. used the first-time type II ZnSe/ZnS core/shell QDs as a sensitizer in the solar cell [94]. In this case, they have found a very high photon-to-current conversion efficiency due to the improvement of charge separation character. Using type II CdTe/CdS QDs to sensitize TiO2 electrode, Yu et al. recorded PCE of 3.8% [95]. Q. Meng group used a microwave-assisted aqueous synthesis method to prepare alloys CdSexTe1 − x/CdS core/shell QDs and recorded PCE of 5.04% [96]. Furthermore, J. Wang et al. have shown a very high PCE value of 6.76% using type II CdTe/CdSe core/shell QDs linked with the TiO2 mesoporous electrode. The reason for the high PCE value was the suppression of the electron-hole recombination rate and acceleration of electron injection from QD to TiO2 [97]. Moreover, Zhong et al. explored type II ZnTe/CdSe core/shell QDs as a sensitizer and showed PCE of 7.17% (Fig. 15) [98].
Fig. 15

Photovoltaic performance of ZnTe/CdSe core/shell QDs, compared with CdTe/CdSe and CdSe QDs: (a) J-V curve and (b) IPCE curve. (Reprinted with permission from Ref. [98])

3.2 Light-Emitting Diodes (LEDs)

In recent years, light-emitting diodes using QD (QLEDs) gain much attention of scientists due to its high brightness (~200,000 cd m−2), high color purity (FWHM <30 nm), low operating voltage (< 2 V), and easy process-ability [99, 100, 101]. The QLEDs generally consist of a cathode, electron transport layers (ETLs), QD layers, hole transport layers (HTLs), and an anode (Fig. 3a) [102]. When the potential applied, electrons and holes are injected into charge transport layers (ETLs and HTLs) from cathode and anode, respectively. Further, the charge carriers are injected into the QDs where they recombine radiatively [102]. Thus, charge transport materials play a primary role in the performance and stability of QLEDs.

Usually, type I core/shell QDs have been used in QLEDs because wider bandgap shell material confines the exciton to the core of the QDs and passivates the surface defect states [41, 42, 47, 102, 103, 104, 105]. This results in an enhancement in external quantum efficiency (EQE) since it is directly related to PL QY and stability of the QDs. Meanwhile, increment in PL QY does not mean improvement in the electroluminescence (EL) performance because Augur recombination in charged QD and interparticle energy transfer between different QDs can lead to decrease in EL efficiency. These processes are directly affected by the structure of the core/shell interface, and thus structure modification becomes an essential issue [102]. For example, Yang et al. have observed better EL properties for the ZnSe-rich intermediate shell in comparison to CdS-rich intermediate shell in the case of CdSe/ZnS core/shell structure [106]. The results can be interpreted in terms of a low carrier injection barrier of ZnSe-rich QDs in comparison to the CdS-rich intermediate shell (Fig. 16).
Fig. 16

(a) Schematic illustration of device structure for QD light-emitting diodes (QLEDs). (b) Energy-level diagram of a typical QLED displaying charge injection from the cathode and anode. (Reprinted with permission from Ref. [102])

Another way to make QDs with high stability and less blinking (or non-blinking) is to grow a thicker shell or multi-shell over the surface of QDs [48, 107]. Thicker shell in QDs can enhance PL QY of the charged QDs and suppressed the charge fluctuation. The enhancement in these properties of thick core/shell QDs can lead to a drastic improvement in device performance. Recently, Hao et al. have observed excellent luminescence properties using a thick shell of ZnS over the surface of CdSe [108]. The green-emitting thick CdSe/ZnS core/shell QDs were synthesized by the TOP-SILAR method.

3.3 Applications in Biology

QDs have the upper hand over the organic fluorophores because of their size, surface chemistry, spectral properties, large one- and two-photon absorption cross sections, and stability, making it an excellent material to investigate for in vitro and in vivo detection/imaging in biology. QDs with stokes shift, tunable emission wavelengths, and high absorption coefficients can be created simply by altering the size or composition without compromising the biocompatibility of the QDs. Starting from the late 1990s, the QDs are playing an essential role in the field of biology for biomolecular imaging, photodynamic therapy, sensing, gene and drug delivery, and many more [109, 110, 111]. Selected applications of semiconductor QDs in the biomedical field have been given below.

3.3.1 Biosensing

Semiconductor QDs are excellent material for their applications in sensing due to their tunable bandgap along with their surface modification ability. It is essential and beneficial to detect a specific biomarker for early-stage detection and therapy of disease. Various biological and chemical species have been detected by surface functionalization of QDs. Luminescence quenching of QDs is the primary technique used for biosensing.

First of all, using CdSe/ZnS core/shell QDs coupled with biomolecules, Chan and Nie developed a biosensor for the detection of ultrasensitive nonisotopic [109]. A sensor to investigate heme iron absorption Geng et al. used hemin-conjugated CdSe/ZnS core/shell QDs as a probe in the biological setup [112]. Yang et al. developed a sensitive biosensor for probing the interaction of clofazimine with protein [113]. To make this sensor, they have developed CdZnSeS/ZnS alloyed core/shell QDs as energy donors in Förster resonance energy transfer (FRET) applications. They have used a direct ligand method for capping of multifunctional polymer and further functionalized with cyanine 3-labelled human serum albumin [113].

3.3.2 Gene and Drug Delivery

These days there is a growing trend of gene and drug delivery in the research field of nanomedicine. Drug-formulated QDs can provide superiorities like a long lifetime, targeted drug delivery, and enhanced cellular uptake. Various new nano-drugs and nanoprobes have been designed for in vivo and in vitro study by using the versatility of bioconjugated QDs [114].

Recently, Olerile et al. have used paclitaxel (PTX: a drug for various human cancers), in CdTe/CdS/ZnS core/shell QDs along with nanostructured lipid carrier for the cancer therapy [115]. This drug (PTX) along with hybrid silica nanocapsules was further loaded with ZnSe:Mn/ZnS core/shell QDs by Gu group for chemotherapy and fluorescence imaging [116]. In other work, Yang et al. prepared quercetin (QE)-loaded CdSe/ZnS core/shell QDs as antibacterial and anticancer nano-drug [117].

3.3.3 Therapy

QDs either separately or in combination with various known photosensitizers such as porphyrins, phthalocyanines, organic dyes, inorganic complexes, etc. have been investigated in photodynamic therapy (in vitro and in vivo both). Photosensitized singlet oxygen and other reactive oxygen species (ROS) produced through photosynthesis involving the transference of excitation energy is the basic step involved in photodynamic therapy [118]. However, QDs are known to have poor efficiency in producing singlet oxygen which hinders the active application of QDs in photodynamic therapy. Later on, it was found that the conjugation of standard photosensitizers like chlorine e6 with QDs results in an almost 70% increase in efficiency. In this scenario, photoactivation of QDs provides an efficient energy transfer to sensitized and followed by intersystem crossing efficiently and consequently energy transfer to oxygen. Nevertheless, the scientific community is still facing a major challenge which arises due to cytotoxicity of cadmium-based QDs in developing QD-based in vivo imaging and photodynamic therapy. Furthermore, various other issues particularly the surface charge, surface functional groups, and the size decide the renal clearance of QDs implemented in vivo.

4 Summary and Prospects

This chapter mostly emphasizes how the chemical synthesis and its challenges with a broad perspective have been developed in recent decades for II–VI semiconductor core/shell QDs. Further, we intend to address its potential application in the various research fields. The chemical-based synthesis and its ability to precisely control shell thickness make highly luminescent semiconductor core/shell QDs for multiple applications, including in solar cells, LEDs, biomedical, and many more. Due to the enormous successes of semiconductor QDs in the recent past, it can be expected the synthesis of a large number of new materials with excellent properties in the next few years. In particular, to compete Cd-based QDs, the future research will be focused on the synthesis of lead-free perovskite and I-III-VI ternary QDs. This should lead to novel discoveries in biomedical applications such as biosensors, targeted drug delivery, cell levelling, and bioimaging.

Notes

Acknowledgments

Authors thank “Science and Engineering Research Board (SERB), Government of India,” for financial assistance under the project number EEQ/2016/000652. ANY thanks UGC, New Delhi, for junior research fellowship.

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© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020

Authors and Affiliations

  • Amar Nath Yadav
    • 1
  • Ashwani Kumar Singh
    • 1
  • Kedar Singh
    • 1
    Email author
  1. 1.School of Physical SciencesJawaharlal Nehru UniversityNew DelhiIndia

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