Core–shell semiconductor nanocrystal
Core–shell semiconducting nanocrystals are a class of materials which have properties intermediate between those of small, individual molecules and those of bulk, crystalline semiconductors. They are unique because of their easily modular properties, which are a result of their size. These nanocrystals are composed of a quantum dot semiconducting core material and a shell of a distinct semiconducting material. The core and the shell are typically composed of type II–VI, IV–VI, and III–V semiconductors, with configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe Organically passivated quantum dots have low fluorescence quantum yield due to surface related trap states. CSSNCs address this problem because the shell increases quantum yield by passivating the surface trap states. In addition, the shell provides protection against environmental changes, photo-oxidative degradation, and provides another route for modularity. Precise control of the size, shape, and composition of both the core and the shell enable the emission wavelength to be tuned over a wider range of wavelengths than with either individual semiconductor. These materials have found applications in biological systems and optics.
Background
Colloidal semiconductor nanocrystals, which are also called quantum dots, consist of ~1–10 nm diameter semiconductor nanoparticles that have organic ligands bound to their surface. These nanomaterials have found applications in nanoscale photonic, photovoltaic, and light-emitting diode devices due to their size-dependent optical and electronic properties. Quantum dots are popular alternatives to organic dyes as fluorescent labels for biological imaging and sensing due to their small size, tuneable emission, and photostability.The luminescent properties of quantum dots arise from exciton decay which can proceed through a radiative or nonradiative pathway. The radiative pathway involves electrons relaxing from the conduction band to the valence band by emitting photons with wavelengths corresponding to the semiconductor's bandgap. Nonradiative recombination can occur through energy release via phonon emission or Auger recombination. In this size regime, quantum confinement effects lead to a size dependent increasing bandgap with observable, quantized energy levels. The quantized energy levels observed in quantum dots lead to electronic structures that are intermediate between single molecules which have a single HOMO-LUMO gap and bulk semiconductors which have continuous energy levels within bands
Semiconductor nanocrystals generally adopt the same crystal structure as their extended solids. At the surface of the crystal, the periodicity abruptly stops, resulting in surface atoms having a lower coordination number than the interior atoms. This incomplete bonding results in atomic orbitals that point away from the surface called "dangling orbitals" or unpassivated orbitals. Surface dangling orbitals are localized and carry a slight negative or positive charge. Weak interaction among the inhomogeneous charged energy states on the surface has been hypothesized to form a band structure. If the energy of the dangling orbital band is within the semiconductor bandgap, electrons and holes can be trapped at the crystal surface. For example, in CdSe quantum dots, Cd dangling orbitals act as electron traps while Se dangling orbitals act as hole traps. Also, surface defects in the crystal structure can act as charge carrier traps.
Charge carrier trapping on QDs increases the probability of non-radiative recombination, which reduces the fluorescence quantum yield. Surface-bound organic ligands are typically used to coordinate to surface atoms having reduced coordination number in order to passivate the surface traps. For example, tri-n-octylphosphine oxide and trioctylphospine have been used to control the growth conditions and passivate the surface traps of high quality CdSe quantum dots. Although this method provides narrow size distributions and good crystallinity, the quantum yields are ~5–15%. Alkylamines have been incorporated into the TOP/TOPO synthetic method to increase the quantum yields to ~50%.
The main challenge in using organic ligands for quantum dot surface trap passivation is the difficulty in simultaneously passivating both anionic and cationic surface traps. Steric hindrance between bulky organic ligands results in incomplete surface coverage and unpassivated dangling orbitals. Growing epitaxial inorganic semiconductor shells over quantum dots inhibits photo-oxidation and enables passivation of both anionic and cationic surface trap states. As photogenerated charge carriers are less likely to be trapped, the probability for excitons to decay through the radiative pathway increases. CdSe/CdS and ZnSe/CdSe nanocrystals have been synthesized that exhibit 85% and 80–90% quantum yield, respectively.
Core–shell semiconductor nanocrystal architecture was initially investigated in the 1980s, followed by a surge of publications on synthetic methods the 1990s.
Classification of core–shell semiconductor nanocrystals
Core–shell semiconductor nanocrystal properties are based on the relative conduction and valence band edge alignment of the core and the shell. In type I semiconductor heterostructures, the electron and holes tend to localize within the core. In type II heterostructures, one carrier is localized in the shell while the other is localized in the core.Type I
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Reverse Type I
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Type II
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Doped core-shell semiconductor nanocrystals
Doping has been shown to strongly affect the optical properties of semiconductor nanocrystals. Impurity concentrations in semiconductor nanocrystals grown using colloidal synthesis, however, are typically lower than in their bulk counterparts. There has been interest in magnetic doping of CSSNCs for applications in magnetic memory and spin-based electronics. Dual-mode optical and magnetic resonance imaging has been explored by doping the shell of CdSe/ZnS with Mn, which caused the CSSNC to be paramagnetic.Synthesis
In synthesizing core shell nanoparticles, scientists have studied and found several wet chemical methods, such as chemical precipitation, sol-gel, microemulsion and inverse micelle formation. Those methods have been used to grow core shell chalcogenide nanoparticles with an emphasis on better control of size, shape, and size distribution. To control the growth of nanoparticles with tunable optical properties, supporting matrices such as glasses, zeolites, polymers or fatty acids have been used. In addition, to prepare nanoparticles of sulfides, selenides and tellurides, the Langmuir–Blodgett film technique has been used successfully. In comparison to wet chemical methods, electrochemical synthesis is more desirable, such as the use of aqueous solvents rather than toxic organic solvents, formation of conformal deposits, room-temperature deposition, low cost, and precise control of composition and thickness of semiconductor coating on metal nanoparticles. However, owing to the difficulty of preparing electrically addressable arrays of nanoparticles, the use of electrochemical techniques to produce core-shell nanoparticles was difficult. Recently, Cadmium Sulfide and Copper iodide was electrochemically grown on a 3-D nanoelectrode array via layer-by-layer depositing of alternating layers of nanoparticles and Polyoxometalate.Core–shell semiconductor nanocrystals can be grown by using colloidal chemistry methods with an appropriate control of the reaction kinetics. Using this method which results in a relatively high control of size and shape, semiconductor nanostructures could be synthesized in the form of dots, tubes, wires and other forms which show interesting optic and electronic size-dependent properties. Since the synergistic properties resulting from the intimate contact and interaction between the core and shell, CSSNCs can provide novel functions and enhanced properties which are not observed in single nanoparticles.
The size of core materials and the thickness of shell can be controlled during synthesis. For example, in the synthesis of CdSe core nanocrystals, the volume of H2S gas can determine the size of core nanocrystals. As the volume of H2S increases, the size of the core decreases. Alternatively, when the reaction solution reaches the desired reaction temperature, rapid cooling can result in smaller core sizes. In addition, the thickness of shell is typically determined by the added amount of shell material during the coating process.
Characterization
An increase in either the core size or shell length results in longer emission wavelengths. The interface between the core and shell can be tailored to passivate relaxation pathways and form radiative states. The size dependence of the band gap in these nanoparticles due to the quantum confinement effect has been utilized to control the photoluminescence color from blue to red by preparing nanoparticles of varying sizes. By manipulating the size or shape of the nanoparticles, the luminescence colors and purity can be controlled. However, the quantum yield and the brightness of luminescence of the CSSNCs is ultimately limited and it cannot be controlled because of the presence of surface traps.UV-vis absorption spectra, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy are the techniques typically used to identify and characterize CSSNCs.
Applications
One of the most important properties of core–shell semiconducting nanocrystals is that their cores, which are quantum dots, fluoresce, which is important in their biomedical and optical applications. The shells are highly modular, and thus the bulk properties, such as solubility and activity of the CSSNCs can be changed.Biomedical applications
The properties desired of CSSNCs when using them for biological applications include high quantum yield, narrow fluorescence emission, broad absorption profile, stability against photobleaching, 20 second fluorescent lifetime, and high brightness. High quantum yields mean that minimal energy will need to be put into the quantum dot to induce fluorescence. A narrow fluorescence emission allows for multiple colors to be imaged at once without color overlap between different types of CSSNCs. Having a broad absorption profile allows multiple CSSNCs to be excited at the same wavelength and thus, multiple CSSNCs could be imaged simultaneously. Having a 20-second fluorescent lifetime allows for time-resolved bioimaging. The utility of CSSNCs is that they can be a complement to organic fluorophores. CSSNCs are less susceptible to photobleaching, but less is known about them compared to organic fluorophores. CSSNCs have 100–1000 times the two-photon fluorescence efficiency as organic dyes, exemplifying their value.In the cases where CSSNCs are used in biological medium, the core is a quantum dot and the shell can be an organic molecule or biological ligands, such as a DNA, that are used for biocompatibility and targeting. The shell can also be an organic molecule to which a biological molecule is later conjugated, furthering the modularity of core–shell structure. The most popular core/shell pair used is CdSe core with ZnS or CdS shell, which improves the quantum yield and protects against photobleaching compared to that of the core material alone. The size of the CSSNC is directly correlated to the color of fluorescence, so being able to control particle size is desirable. However, it is generally unknown how the shell molecules, and salt concentration, pH, and temperature of the media affect the CSSNCs’ properties and remains empirical.
In vitro cell labeling
Because multiple colors can be imaged, CSSNCs’ ability to be used in cell labeling is of growing importance. However, it can be difficult to get CSSNCs across the cell membrane. This has been achieved via endocytosis, direct microinjection, and electroporation, and once in the cell, they become concentrated in the nucleus and can stay there for extended periods of time. Once CSSNCs are inside cells, they remain even after cellular division and can be imaged in both mother and daughter cells. This particular technique was shown using Xenopus embryos. Another example of CSSNCs is seen in their tracking ability; when cells are gown on a 2D matrix embedded with CSSNCs, cells uptake the CSSNCs as they move, leaving a trail seen as the absence of CSSNCs. This means that the mobility of cells can be imaged, which is important since the metastatic potential of breast tissue cells has been shown to increase with mobility. Also, it has been shown that five different toxins can be detected using five different CSSNCs simultaneously.In a move toward environmentally friendlier and less toxic CSSNCs, Si quantum dots with various shells have been developed. Si is 10 times safer than Cd and current work is focused on making Si more water-soluble and biocompatible. In particular, Si quantum dots with poly and allylamine shells have been used in cell labeling. Other in vitro uses include flow cyclometry, pathogen detection, and genomic and proteomic detection.
In vivo and deep tissue imaging
Because CSSNCs emit in the near-infrared region of the electromagnetic spectrum, imaging them is not complicated by autofluorescence of tissue, which occurs at higher frequencies, and scattering effects. This has been used in the mapping of sentinel lymph-nodes in cancer surgery in animals. Lymph nodes 1 cm deep were imaged and the excised nodes with CSSNC accumulation were found to have the highest probability for containing metastatic cells. In addition, CSSNCs have been shown to remain fluorescent in cells in vivo for 4 months. To track and diagnose cancer cells, labeled squamous carminoma cell-line U14 cells were used and fluorescent images could be seen after 6h. CSSNCs conjugated to doxorubicin were also used to target, image, and sense prostate cancer cells that express the prostate-specific membrane antigen protein. Using a cancer-specific antibody conjugated to QDs with polymer shells is the most popular in tumor targeted imaging.The main disadvantage of using CSSNCs for in vivo imaging is the lack of information about their excretion and toxicity. The typical cores used show DNA damage and toxicity toward liver cells, but using shells seems to diminish this effect. The use of other substances in the core, such as rare-earth elements and Si, are being explored to reduce toxicity. Other disadvantages include limited commercial availability, variability in surface chemistry, nonspecific binding, and instrument limitation.