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, I-III-VI, and III–V semiconductors, with configurations such as CdS/ZnS, CdSe/ZnS, CuInZnSe/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
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
- Description
- Examples
Reverse Type I
Description
In the reverse type I configuration, the core has a wider bandgap than the shell, and the conduction and valence band edges of the shell lie within those of the core. The lowest available exciton energy separation occurs when the charge carriers are localized in the shell. Changing the shell thickness tunes the emission wavelength.Examples
CdS/HgS, CdS/CdSe, ZnSe/CdSe and MgO/ZnOType II
Description
In the type II configuration, the valence and conduction band edge of the core are both lower or higher than the band edges of the shell. An example of a type II is shown in figure X, ZnTe /CdSe. The lowest energy separation of the electron and the hole will occur when the hole is confined in the ZnTe core valence band and the electron is confined in the CdSe shell conduction band. The emission wavelength will be determined by the energy difference between these occupied states, as shown by the red arrow, which will be at a lower energy than either of the individual bandgaps. The emission wavelength can be significantly red shifted compared to the unpassivated core.Examples
ZnTe/CdSe, CdTe/CdSe, CdS/ZnSeDoped 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.