Quantum dots are tiny semiconductor particles with unique properties. In this section, we'll explore how charge carriers ( and ) can get trapped in these dots. We'll also look at surface states, which are energy levels that exist on the dot's surface.
and surface states can greatly affect how quantum dots behave. They impact things like how long charges last and how bright the dots glow. Understanding these processes is key to making better quantum dots for use in electronics and other cool applications.
Charge carrier trapping in quantum dots
Concept and mechanisms of charge carrier trapping
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Top images from around the web for Concept and mechanisms of charge carrier trapping
Charge carrier trapping controlled by polymer blend phase dynamics - Journal of Materials ... View original
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Spin-polarized charge trapping cell based on a topological insulator quantum dot - RSC Advances ... View original
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Charge carrier trapping controlled by polymer blend phase dynamics - Journal of Materials ... View original
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Spin-polarized charge trapping cell based on a topological insulator quantum dot - RSC Advances ... View original
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Charge carrier trapping localizes electrons or holes in quantum dots due to defects, impurities, or surface states
Trapped charge carriers are confined to specific energy levels within the bandgap, reducing their mobility and participation in optical and electronic processes
Trapping occurs through mechanisms like electron capture by defect states (structural defects, impurities), hole capture by surface states (unsatisfied chemical bonds, adsorbed species), or Auger (energy transfer to a third charge carrier)
Factors influencing charge carrier trapping
The extent and nature of charge carrier trapping depend on the size, composition, and surface chemistry of the quantum dots and the surrounding environment
Smaller quantum dots have a higher surface-to-volume ratio, making them more susceptible to surface-related trapping effects
The composition of the quantum dot (core material, shell material) influences the electronic structure and the presence of defect states
Surface chemistry, including ligand coverage and , plays a crucial role in controlling the density and nature of surface traps
Surface states in charge dynamics
Nature and origin of surface states
Surface states are electronic states that exist at the surface or interface of quantum dots, arising from unsatisfied chemical bonds, structural defects, or adsorbed species
They introduce additional energy levels within the bandgap of the quantum dot, altering the electronic structure and charge carrier dynamics
Surface states can act as trap sites for charge carriers, leading to the localization of electrons or holes at the surface
Role of surface states in charge carrier dynamics
Surface states participate in charge carrier trapping and detrapping processes, influencing the rates of carrier capture and release
Interaction between charge carriers and surface states can lead to non-radiative recombination, reducing the photoluminescence quantum yield and carrier lifetime
Passivation of surface states, through surface modification (ligand exchange, shell growth) or ligand exchange, can mitigate the detrimental effects on charge carrier dynamics
Mechanisms of trapping and detrapping
Charge carrier trapping mechanisms
Electron capture by defect states involves the localization of electrons in energy levels associated with structural defects or impurities within the quantum dot
Hole capture by surface states occurs when holes are trapped at the surface due to unsatisfied chemical bonds or adsorbed species
Auger recombination is a non-radiative process where the energy from electron-hole pair recombination is transferred to a third charge carrier, leading to its excitation and potential trapping
Detrapping processes and factors
Detrapping of charge carriers can occur through , where trapped carriers gain sufficient energy to overcome the potential barrier and escape the trap state
The rates of charge carrier trapping and detrapping depend on the depth of the trap states (shallow or deep traps), temperature, and carrier concentration
Higher temperatures provide more thermal energy for detrapping, while higher carrier concentrations increase the probability of trapping events
The competition between trapping and detrapping processes determines the overall charge carrier dynamics and the associated optical and electronic properties
Impact of surface states on quantum dots
Influence on optical properties
Surface states can significantly impact the photoluminescence quantum yield and carrier lifetime of quantum dots
Non-radiative recombination pathways introduced by surface states reduce the photoluminescence quantum yield and shorten the carrier lifetime
Surface states can cause spectral broadening or shifts in the absorption and emission spectra due to the additional energy levels within the bandgap
Effects on electronic properties
Trapped charge carriers at surface states act as scattering centers, reducing the mobility of free charge carriers and hindering efficient charge transport
Surface states influence the energy level alignment at interfaces between quantum dots and other materials, impacting charge injection and extraction processes in optoelectronic devices (solar cells, LEDs)
Strategies to passivate or minimize the impact of surface states, such as surface modification (ligand exchange, shell growth) or ligand optimization, are crucial for optimizing the optical and electronic properties of quantum dots