30 Core-Shell Semiconductor Nanocrystals (CSSNCs)

S.S. islam

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Introduction

 

Core-shell semiconducting nanocrystals are the materials whose properties are intermediary to those of individual atoms/molecules and bulk, semiconductor crystals. They have novel properties which dependent upon their size. A core-shell nanocrystal comprises a semiconducting QD core and a shell of different semiconductor. Core and shell usually comprise: type-II-VI, IV-IV, and III-V semiconductors. Typical core/shell examples include CdS/ZnS, CdSe/ZnS, etc. Organically passivated QDs exhibit low quantum fluorescence yield because of surface related trap states. The shell protects the nanocrystal against environmental variations, degradation caused by photo-oxidation, and is another means to control the properties of the crystal. The emission wavelength can be varied over a broad range by precisely controlling the size, shape, and composition of both the core and shell. This is more effective way to control emission properties than individually controlling the core or shell. The material has extensive applications in biological systems as well as optics.

 

 

Figure 1 TEM image of NaYF4:Yb,Tm nanoparticles coated with ZnO (top left), and the associated chemical maps confirming their chemical composition.

 

1. Background

 

Colloidal semiconductor nanocrystals or QDs, are ~1-10 nm in diameter and have organic ligands attached to their surfaces. Owing to the size dependent electrical and optical properties, these nanocrystals have extensive applications in nanoscaled photonics, photovoltaics, LEDs, etc. QDs are replacing organic dyes as fluorescent markers in biological imaging as well as biosensing because of their tiny size, adjustable emissions, and photostability.

 

Luminescence in QDs results from exciton decay (electron hole recombination), which might be radiative or nonradiative. In radiative decay, electrons relax from conduction band to valence band via emitting photons. The wavelength of the emitted photons is equal to the bandgap of the semiconductor. In nonradiative recombination, the electrons relax by releasing their energy by emitting phonon or Auger recombination. At nanoscale, quantum effects result in size dependent increase in the bandgap, and quantisation of energy levels. This quantisation of energy levels in QDs renders their electronic structure intermediary to individual atoms/molecules (having single HOMO-LUMO gap) and bulk semiconductors (having continuous energy bands).

 

Figure 2 Electronic structure of Bulk, QD, and single molecule.

 

The crystal structure of semiconductor nanocrystals is usually identical to their bulk counterparts, however, the lattice periodicity abruptly stops at the surface of the nanocrystals. Thus, surface atoms have lower coordination numbers than the atoms present in the interior of the nanocrystal. Due to this incomplete bonding (in comparison to the interior atoms), atomic orbitals point away from the surface, resulting in ‘dangling orbitals’, also known as unpassivated orbitals. The dangling orbitals present at the surface of the nanocrystal are localised and are slightly charged (positive or negative). A hypothetical band structure is assumed to be formed owing to the weak interactions among inhomogeneously charged energy states present at the surface. Electrons and holes can be trapped at the surface, when the energy of dangling orbital is within the bandgap of the semiconductor, e.g., in CdSe QDs, Cd dangling orbitals trap electrons whereas Se dangling orbitals trap holes. Moreover, charge carriers can also be trapped by any surface defects in the nanocrystal.

 

Due to the trapping of charge carriers of the surface of QDs, the probability of nonradiative decay increases, this results in a corresponding decrease in fluorescence quantum yield. To passivate these surface traps, organic ligands are used to coordinate with the surface atoms, e.g.,trioctylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO) are generally used to passivate the surface traps and for growth condition control in high quality CdSe QDs. Even though, this technique produces high quality QDs with uniform size, and good crystallinity, their quantum yield is ~5-15%.

 

2. Classification of Core-Shell nanocrystals

 

The properties of core-shell nanocrystals arise from relative alignment of conduction and valence band edges of core and shell materials. In the case of type-I semiconductor heterostructures, both the charge carriers (electron and hole) remain localised within the core, while for type-II, one carrier is localised in shell and the other in core.

Figure 3 Three types of core-shell nanocrystals. Upper and lower edges are the upper and lower energy edges of core (blue) and shell (red).

 

The core-shell structures can be classified in the following types:

 

a. Type-I

 

In type-I, core has smaller bandgap than the shell, and the conduction and valence band edges of core lie within the bandgap of shell. This results in confinement of both electrons and holes within the core. This is shown in Figure 4, where an exciton at CdSe/CdS interface occupies energy states in the CdSe core, corresponding to the lowest energy separation. Radiative recombination within the core emits the light of wavelength red-shifted relative to the uncoated CdSe. Typical examples include CdSe/CdS, CdSe/ZnS, InAs/CdSe, etc.

 

 

Figure 4 Energy level diagram of type-I core-shell heterostructure.

 

b. Reverse Type-I

 

In this case, the bandgap of core is larger than that of shell, and the edges of conduction and valence bands of shell lie within the bandgap of core. Lowest available exciton energy separation occurs when charge carries are localised in shell. The wavelength of the emitted light can be tuned by varying the thickness of the shell. Typical examples include CdS/HgS, ZnSe/CdSe, etc.

 

c. Type-II

 

In this structure, both conduction and valence band edges of core are both either higher or lower than those of the shell. Figure 5 shows a typical example of this case, where the band edges of the core (ZnTe) are higher than the shell (CdSe) band edges. Lowest energy separation of exciton occurs when hole is confined in the valence band of core (ZnTe), and electron is confined in conduction band of shell (CdSe). The wavelength of the emitted light is determined by the energy difference between these occupied states (indicated by red arrow in Figure 5). Thus, the wavelength of emitted light is lower than either of the individual bandgaps, resulting in considerable red-shift in the emission wavelength in compatriot to the unpassivated core. Typical examples include ZnTe/CdSe, CdS/ZnSe, etc.

 

Figure 5 Type-II core-shell ZnTe (2.26 eV)/CdSe (1.74 eV) structure.

 

 

d. Doped core-shell Semiconductor Nanocrystals

 

Doping can be used to effectively alter the optical properties of QDs. The impurity content in QDs produced via colloidal synthesis is lower than their bulk counterparts. The most popular applications of QDs which involve doping include magnetic memory and spin based electronics. Doping has been found to strongly influence the magnetic properties of core-shell nanocrystals. For example, Doping the shell with Mn in CdSe/ZnS made the nanocrystal paramagnetic.

 

3. Synthesis

 

Core-shell nanostructures have been prepared via various wet chemical methods including chemical precipitation, sol-gel, micro-emulsion, and inverse micelle formation. Chalcogenide core-shell nanoparticles have been prepared from these techniques with emphasising better control over size, shape, and size distribution. To control the optical properties of these nanoparticles, various support matrices have been used, e.g., glass, zeolite, polymer, fatty acids, etc.

 

Additionally, nanoparticles of sulfide, selenide, and telluride have been prepared via Langmuir-Blodgett film method. Electrochemical technique is preferred over wet chemical methods, due to the possibility of using aqueous solvents instead of toxic organic solvents, formation of conformal deposits, deposition at room temperature, inexpensive, precise control over composition and thickness of the semiconductor coating on metal nanoparticles. Nonetheless, electrochemical synthesis of core-shell structures is rather difficult because of the challenges involved in producing electrically addressable arrays of nanoparticles. There have been recent reports about the electrochemical synthesis of core-shell CdS and CuI nanoparticles on a three-dimensional nanoelectrodes via layer-by-layer deposition of alternate layers of nanoparticles and polyoxometalate (POM).

 

Colloidal synthesis of core-shell semiconductor nanocrystals can be performed with proper control of the reaction kinetics. This method provides great control over size and shape of the semiconductor nanoparticles and various types of nanostructures have been grown with this technique, such as dots, tubes, wires, etc. which demonstrate exciting electrical and optical size dependent properties. Owing to the synergistic effects of close contact between core and shell, core-shell nanoparticles offer novel properties and improved functions, not observed in individual particles.

 

It is possible to control the size of core and thickness of shell during synthesis process, e.g., volume of H2S gas influences the size of CdSe core nanoparticles during synthesis, such that the size of the core decreases with an increase in the volume of H2S. Also, the size can be reduced by rapid cooling of the products. The thickness of shell depends upon the quantity of shell material added during coating process.

 

4. Characterization

 

The wavelength of the emitted light increase with an increase in either the size of core, or thickness of shell. To passivate the relaxation pathways and increase radiative states, the interface between the core and shell can be modified. Since the nanoparticles exhibit size dependent bandgap arising from quantum confinement, the photoluminescence color from the nanoparticles, can be tuned from blue to red by altering the size of the nanoparticles. Thus, the luminescence colour as well as purity can be tuned by adjusting the size and shape of the nanoparticles. Nonetheless, the surface traps limit the quantum yield and luminescence brightness of the core-shell nanoparticles.

 

Core-shell semiconductor nanocrystals can be characterised by UV-Vis absorption spectra, X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectroscopy.

 

5. Applications

 

The key property of core-shell nanostructures is that the quantum dot cores exhibit strong fluorescence, which is essential property in their optical and biomedical applications. Further, the shells can also be manipulated to control the bulk properties, e.g. solubility, reactivity, etc.

 

Biomedical Applications

 

The desirable properties of core-shell nanoscrystals for their biomedical applications are: high quantum yield, narrow fluorescence emission, broad absorption spectrum, photostability, moderate fluorescent lifetime (~20s), and high brightness.

 

High quantum yield implies minimum energy required to induce fluorescence in the QD. Narrow fluorescence emission is important to image multiple colours simultaneously with no colour overlap between different type of core-shell QDs. Broad absorption spectrum allows multiple QDs to be simultaneously excited at same wavelength. This allows simultaneous imaging of multiple QDs. Time-resolved imaging requires around 20 s fluorescent lifetime. Additionally, the core-shell QDs are compatible with organic fluorophores. These QDs are stable against photo-bleaching, however, their performance vis-a-vis fluorophores needs further investigations. Besides, two-photon fluorescence efficiency of core-shell QDs is around 100 to 1000 times than that of the organic dyes. For biological applications, the quantum dot core can be covered by a biocompatible shell (such as DNA) for specific targeting. In addition to this, an organic molecule can also be used as the shell, with which another biomolecule may be conjugated at a later stage. Typical examples of core-shell materials used are: CdSe/ZnS, CdSe/CdS, etc. The addition of shell protects the core from photobleaching, as compared to using core material alone. Fluorescence color can be tuned by controlling the particle size. Nonetheless, further study on the effects of shell molecules, pH, temperature, etc. in the properties of the core-shell nanoparticles is required.

 

In-vitro Labeling

 

Owing to the possibility of imaging multiple colours, core-shell QDs are important for cell labelling. An important challenge to use core-shell nanocrystals in cell labelling is the difficulties involved in passing them through the cell membrane. Various techniques have been adopted to achieve this, such as endocytosis, direct microinjection, and electroporation. After entering the cell, these nanoparticles aggregate in the nuclei and remain there for prolonged time spans. Since core-shell nanoparticles stay in the cells even after the cell division, both mother and daughter cells can be easily imaged using them. This has been demonstrated in the case of Xenopus embryos.

 

Core-shell nanocrystals can also be used for tacking. This is achieved by growing cells on a 2D matrix, which is pre-embedded with core-shell nanocrystals. As cells move, they uptake these nanoparticles, thereby leaving a trail which can be seen in the form of the absence of the nanocrystals. In this way, the mobility of the cells can be observed, which is important since the metastatic potential of breast tissue cells has been shown to increase with mobility. Furthermore, five different core-shell nanoparticles have been shown to simultaneously detect five different toxins.

 

Environmentally friendly and less toxic core-shell structures have been prepared by using silicon QDs coated with different shells. Silicon is ten times safer than cadmium and present work is focused to make silicon more hydrophilic and biocompatible. Particularly, silicon QDs with poly(acrylic acid) and allylamine shells have been used in cell labelling. Other in vitro uses are flow cyclometry, pathogen detection, and genomic and proteomic detection.

 

In vivo and Deep Tissue Imaging

 

Core-shell QDs exhibit emission in NIR (700 to 900 nm) region of the electromagnetic spectrum, thus their imaging is not affected by the autofluorescence from the tissues (400 to 600 nm), and scattering effects. This has been used to map sentinel lymph-nodes in cancer surgery in animals. In a study, 1 cm deep lymph nodes were imaged and the excised nodes with nanoparticles accumulation were found to have the highest probability for containing metastatic cells. Core-shell QDs have been demonstrated to remain fluorescent within the cells in vivo for up to 4 months. To track and diagnose cancer cells, labeled squamous carminoma cell-line U14 cells were used and fluorescent images were observed after 6h. Core-shell QDs conjugated to doxorubicin have been also used to target, image, as well as detect prostate cancer cells which express the prostate-specific membrane antigen proteins. Conjugating nanoparticles (having polymer shells) to a cancer-specific antibody is the most frequently used tumor targeted imaging.

 

An important concern for use of core-shell nanoparticles to image in vivo is regarding their excretion and toxicity. There have been reports showing DNA damage and toxicity towards liver cells. Besides, the use of shells have shown to reduce these ill-effects. Presently, core made from rare-earth elements as well as silicon are being studied for reducing toxicity. Other concerns are limited commercial availability, variations in surface chemistries, non-specific binding, and instrument limitations.

 

Optics

 

The optical properties of the core-shell QDs are decided by their bandgap, which can be tuned by modifying their size, shape and composition. Thus, by tuning these attributes of the core-shell QDs, they can be optimised for use in optical devices like LEDs, photodetectors, lasers, photodiodes, photovoltaics, etc.

 

you can view video on Core-Shell Semiconductor Nanocrystals (CSSNCs)

 

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