8.2 Electronic and optical properties of quantum dots

Quantum dots are tiny semiconductor particles with unique electronic and optical properties. Their behavior is governed by quantum confinement, which restricts electron movement and creates discrete energy levels. This leads to size-dependent emission and tunable bandgaps.

These nanostructures exhibit fascinating phenomena like Stokes shift, blinking, and Auger recombination. Understanding these properties is crucial for developing applications in displays, solar cells, and biomedical imaging. Quantum dots offer exciting possibilities for next-gen technologies.

Quantum Confinement and Bandgap Engineering

Fundamentals of Quantum Confinement

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• Quantum confinement occurs when particle size approaches de Broglie wavelength
• Restricts electron movement in one or more dimensions
• Leads to discrete energy levels instead of continuous bands
• Affects electronic and optical properties of nanostructures
• Strength of confinement depends on particle size and material properties

Bandgap Engineering and Size-Dependent Emission

• Bandgap engineering involves manipulating electronic band structure
• Allows tuning of optical and electrical properties
• Size-dependent emission results from quantum confinement effects
• Smaller quantum dots exhibit blue-shifted emission
• Larger quantum dots produce red-shifted emission
• Enables creation of tunable light sources for various applications (displays, biomedical imaging)

Excitons in Quantum Dots

• Excitons consist of bound electron-hole pairs
• Quantum confinement enhances exciton binding energy
• Bohr radius of excitons affected by dot size
• Strong confinement regime occurs when dot size is smaller than Bohr radius
• Weak confinement regime exists when dot size exceeds Bohr radius
• Exciton dynamics influence optical properties and recombination processes

Optical Properties and Phenomena

Stokes Shift and Blinking

• Stokes shift represents energy difference between absorption and emission
• Caused by energy loss through non-radiative relaxation
• Larger Stokes shift in quantum dots compared to bulk materials
• Blinking involves intermittent fluorescence emission
• Results from charging and discharging of quantum dots
• Affects single-molecule spectroscopy and imaging applications
• Can be mitigated through surface passivation and core-shell structures

Auger Recombination and Multiple Exciton Generation

• Auger recombination involves energy transfer between carriers
• Occurs when multiple excitons are present in quantum dot
• Leads to non-radiative recombination and reduced quantum efficiency
• Rate increases with decreasing dot size due to enhanced carrier interactions
• Multiple exciton generation produces multiple electron-hole pairs from single photon
• Enhances photovoltaic efficiency in quantum dot solar cells
• Requires high-energy photons and efficient carrier extraction