4.3 Charge carrier trapping and surface states

Quantum dots are tiny semiconductor particles with unique properties. In this section, we'll explore how charge carriers (electrons and holes) can get trapped in these dots. We'll also look at surface states, which are energy levels that exist on the dot's surface.

Trapping 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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  Charge carrier trapping controlled by polymer blend phase dynamics - Journal of Materials ... 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 recombination (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 surface passivation, 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 thermal activation, 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