Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell Quantum Dots
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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.
KeywordsCore/shell quantum dots Photoluminescence Semiconductors Synthesis
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.
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 . 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 .
1.1 Classification of Core/Shell Quantum Dots
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].
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 . 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 .
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 . 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 . 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 .
2.2 Synthesis of Semiconductor Core/Shell QDs
Material parameter for bulk II–VI semiconductors 
Structure (300 K)
Lattice parameter (Å)
Density (kg m−3)
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.
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 . 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
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%) . 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 .
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].
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 . 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% . Moreover, Reiss group developed a new approach for the synthesis of CdS/ZnS core/shell QDs . 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 . 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.
In this context, a green synthesis was described by Li group, who grow ZnS shell over ZnSe core by two converse injection methods . 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 . 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 . 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 . Meanwhile, Gao group observed a substantial enhancement in PL QY up to 85% by the illumination of thioglycolic acid (TGA)-capped CdTe QDs . 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 . 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 . 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 . 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 . 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 . 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 . 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 . Similar tradition is also described by Alivisatos group in another paper .
Bang et al. pioneered a synthesis of nontoxic, water-soluble, and highly stable ZnTe/ZnSe core/shell QDs . 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
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 2009, Lie et al. used first-time core/shell QDs as a sensitizer in solar cell applications . 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 . 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% .
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) . 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 . Thus, charge transport materials play a primary role in the performance and stability of QLEDs.
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 . 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.
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 . 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 . Yang et al. developed a sensitive biosensor for probing the interaction of clofazimine with protein . 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 .
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 .
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 . 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 . In other work, Yang et al. prepared quercetin (QE)-loaded CdSe/ZnS core/shell QDs as antibacterial and anticancer nano-drug .
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 . 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.
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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