Core–shell semiconductor nanocrystal
Core–shell semiconducting nanocrystals (CSSNCs) are a class of materials which have properties intermediate between those of small, individual molecules and those of bulk, crystalline semiconductors.
[3] CSSNCs address this problem because the shell increases quantum yield by passivating the surface trap states.
These nanomaterials have found applications in nanoscale photonic, photovoltaic, and light-emitting diode (LED) 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 (recombination of electron hole pairs) which can proceed through a radiative or nonradiative pathway.
[3] 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[7] Semiconductor nanocrystals generally adopt the same crystal structure as their extended solids.
Weak interaction among the inhomogeneous charged energy states on the surface has been hypothesized to form a band structure.
Charge carrier trapping on QDs increases the probability of non-radiative recombination, which reduces the fluorescence quantum yield.
For example, tri-n-octylphosphine oxide (TOPO) and trioctylphospine (TOP) have been used to control the growth conditions and passivate the surface traps of high quality CdSe quantum dots.
Steric hindrance between bulky organic ligands results in incomplete surface coverage and unpassivated dangling orbitals.
[4] Growing epitaxial inorganic semiconductor shells over quantum dots inhibits photo-oxidation and enables passivation of both anionic and cationic surface trap states.
Core–shell semiconductor nanocrystal properties are based on the relative conduction and valence band edge alignment of the core and the shell.
The emission wavelength due to radiative electron-hole recombination within the core is slightly redshifted compared to uncoated CdSe.
[20][21] Dual-mode optical and magnetic resonance (MR) imaging has been explored by doping the shell of CdSe/ZnS with Mn, which caused the CSSNC to be paramagnetic.
[24] To control the growth of nanoparticles with tunable optical properties, supporting matrices such as glasses, zeolites, polymers or fatty acids have been used.
[24] 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.
Recently, Cadmium Sulfide (CdS) and Copper iodide (CuI) was electrochemically grown on a 3-D nanoelectrode array via layer-by-layer depositing of alternating layers of nanoparticles and Polyoxometalate (POM).
[41][42][43] When an extracting solvent is introduced to the as-synthesized CSNC solution, due to the partition coefficient, CSNCs and impurities are redistributed to different phases.
[47][48][49] However, as purification of CSNCs via gel-electrophoresis is highly time-consuming, recently, nano-scientists are shifting towards more advanced free-flow electrophoresis (FFE)[50] and electrophoretic deposition (EPD) techniques.
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.
This has been achieved via endocytosis (the most common method), direct microinjection, and electroporation, and once in the cell, they become concentrated in the nucleus and can stay there for extended periods of time.
CSSNCs conjugated to doxorubicin were also used to target, image, and sense prostate cancer cells that express the prostate-specific membrane antigen protein.
CSSNC LEDs constructed using multiple layers of CSSNCs resulted in poor conduction, charge imbalance, low luminescence efficiency, and a large number of pinhole defects.
[58] Specifically, the core–shell motif is desirable for use in LEDs because of their electroluminescence and photoluminescence quantum efficiencies and their ability to be processed into devices easily.
This is because these wavelengths maximize the perceived power and they lie outside of the National Television System Committee standard color triangle.
However, type II CSSNCs, CdS/ZnSe, were used in optical amplification from stimulated emission of single-exciton states, eliminating Auger recombination.
This has the advantage that lasing threshold could be lowered under continuous wave excitation, enhancing the potential of CSSNCs as optical gain media.
Type II CSSNCs separate the electrons and holes of the exciton pair, which leads to a strong electric field and thus, reducing absorption losses.
By mixing the appropriate amounts of the different sizes of CSSNCs, the entire visible range with narrow emission profiles and high photoluminescence quantum yields can be achieved.
[62] ZnO-TiO2 core-shell nano-structures were synthesized with fast electron transport and high surface area combining the properties of ZnO nanorods and TiO2 nano particles.
