They are called dots because they are tiny, so small that it iseffectively concentrated into itself/a single point. They are 0Dsemiconductors, and have been referred to as ‘artificial atoms’. Drummen, G., 2010: the term ‘quantum dots’ denotes nanocrystallinesemiconducting fluorophores, whose excitons are confined in all three spatialdimensions – quantum confinement: strict confinement of electrons and holeswhen the nanoparticle radius is below the exciton Bohr radius – and havetypical diameters of 2-20nm. Tytus, et al.
2008:Self-assembled QDs are formed as a result of strain between two lattices withsignificantly mismatched lattice constants (Stransly-Krastanowmethod). In quantum dots, the core determinestheir optical properties, and the properties of the shell strengthens theirphotostability. The structural properties of QDs: Semi-conducting crystallites, with dimensions smaller than the exciton Bohr radius Crystal-like self-assembly into ordered superlattices The physical (physics) properties of QDs: Physical properties: fluorescent properties, such as high quantum yield, photostability, broad absorption spectra, and size-dependent emission-tunability Light-harvesting applications (e.g. photodetectors or solar cells) Three limiting charge carrier localization regimes in core/shell semiconductors.
The energy of the bulk conduction and valence band gaps sets the potential energy of the charge carriers, while the effective mass from the bulk band structure determines the kinetic energy. The wave functions of the lowest-energy electron and hole states can be seen. The band edges are the limits of the core and shell. The charger carriers tend to localizein the part of the hetero-NC with the lowest potential energy. Paramagnetic QDs: certain materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. They have relative magnetic permeability slightly bigger than 1 (i.
e. small positive magnetic susceptibility) and hence attracted to magnetic fields. Diamagnetic materials are repelled by magnetic fields, and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Asthe excitation photon energy increases (and wavelength decreases) the QDabsorption likewise increases suggesting that extremely large effective Stoke’sshift (spectral intervals between excitations and emission maxima) arepossible. Large molar absorption cross-sections = effective brightness perprobe particle is superior The chemical properties of QDs: Inorganic crystallites and organic surfactants Core-shell structures; core material coated with shell material Surface functionalization and development of flexible bioconjugation techniques.Chemical adaptation of the surface notonly renders it water-soluble, but allows biocompatilization andfunctionalization, but also eliminates photobleaching by physically excludinginteraction of the excited state particle with molecular oxygen and thusprevents formation of ROS, such as singlet oxygen. The optical properties of QDs: Strong light absorbance Size-tunable emission Bright fluorescence/high quantum yield Narrow symmetric emission bands High photostability Low photobleaching ratesAnd broad absorption spectrum allowsthe simultaneous excitation of QDs of all sizes by a single excitation lightsource in the UV to violet part of the spectrum.
Drummen, G., 2010 Different types of QDs: Type I hetero-NC (e.g. CdSe/ZnS), both charge carriers co-localize in one part. These trap electrons and holessimultaneously. They have contravariant band layout. Type I 1/2 (e.g.
CdSe/CdS), one charge carrier delocalizes over the entire NC while the other one is localized in one part. Type II (e.g. CdSe/ZnTe), two charge carriers are spatially separated, each in a different part, forming a spatially indirect exciton.
These empty dots attract only type ofcharged carrier and repel the other. They have covariant band layout. Conduction bandsNative ligands have been exchanged with small organic or inorganiclinkers. The native ones had wide gaps between their highest occupied andlowest unoccupied molecular orbital (HOMO and LUMO respectively) limitedeffective interparticle coupling. Physical size of band gap determines the photon’s emissionwavelength. Bandgap energy is inversely proportional to the square of the sizeof the QD. Quantum confinement effects: the bandgap (or HOMO-LUMO gap) of thesemiconductor nanocrystal increased with decreasing size, while discrete energylevels arise at the band-edges.
The energy separation between the band-edgelevels also increases with decreasing size. The colour of the luminescencechanges from red to blue as the QD diameter is reduced from 6 to 2nm. Drummen, G., 2010: QD fluorescence: absorption of a photon higher inenergy than the spectral bandgap of the core semiconductor, resulting inelectron excitation to the conduction band, generating an electron-hole pair(exciton). Broad absorption spectrumachieved due to long lifetime (10-40ns), as it increases the probability ofabsorption at shorter wavelengths. Energy-band structure is what gives the different electricalcharacteristics in different materials. Electrons can move from the valence tothe conduction band, but only if they can satisfy that minimum amount of energythat they require (either by absorbing a phonon -heat- or a photon -light-). Temperature dependence of the energy bandgapThe energy bandgap of semiconductors tends to decrease as thetemperature is increased.
This is because: the interatomic spacing increaseswhen the amplitude of the atomic vibrations increases due to the increasedthermal energy. This effect is quantified by the linear expansion coefficientof a material. An increased interatomic spacing decreases the potential seen bythe electrons in the material, which in turn reduces the size of the energybandgap. A direct modulation of the interatomic distance, such as by applyinghigh compressive (tensile) stress, also causes an increase (decrease) of thebandgap.
Quantum dots are semiconductornanocrystals varying in size from 2-10nm (10-50 atoms) in diameter. Theyexhibit quantum mechanical effects allowingthem to mimic atomic properties. Bandgapis the energy that they require for their electronsto become excited. Small dot (blue) =larger bandgap (i.e.
electrons require a lot of energy to enter the excited state) Big dot (red) = smaller bandgap.High energy = high frequency = small wavelength.