2.4 Size and shape control of quantum dots

Quantum dots are tiny semiconductor particles with size-dependent properties. Their optical and electronic characteristics can be fine-tuned by controlling their size and shape during synthesis. This allows for customization of quantum dots for specific applications.

Mastering size and shape control is crucial for creating quantum dots with desired properties. Various synthesis parameters and post-processing techniques can be used to achieve precise control, enabling the development of advanced materials for diverse technologies.

Quantum dot size and properties

Relationship between quantum dot size and optical/electronic properties

Top images from around the web for Relationship between quantum dot size and optical/electronic properties

• 

  Adjusting the band structure and defects of ZnO quantum dots via tin doping - RSC Advances (RSC ... View original

  Is this image relevant?

• 

  Tuning the Optical Properties of Silicon Quantum Dots via Surface Functionalization with ... View original

  Is this image relevant?

• 

  Theoretical and Experimental Investigation of Quantum Confinement Effect on the Blue Shift in ... View original

  Is this image relevant?

• 

  Adjusting the band structure and defects of ZnO quantum dots via tin doping - RSC Advances (RSC ... View original

  Is this image relevant?

• 

  Tuning the Optical Properties of Silicon Quantum Dots via Surface Functionalization with ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Relationship between quantum dot size and optical/electronic properties

• 

  Adjusting the band structure and defects of ZnO quantum dots via tin doping - RSC Advances (RSC ... View original

  Is this image relevant?

• 

  Tuning the Optical Properties of Silicon Quantum Dots via Surface Functionalization with ... View original

  Is this image relevant?

• 

  Theoretical and Experimental Investigation of Quantum Confinement Effect on the Blue Shift in ... View original

  Is this image relevant?

• 

  Adjusting the band structure and defects of ZnO quantum dots via tin doping - RSC Advances (RSC ... View original

  Is this image relevant?

• 

  Tuning the Optical Properties of Silicon Quantum Dots via Surface Functionalization with ... View original

  Is this image relevant?

1 of 3

• Quantum dots exhibit size-dependent optical and electronic properties due to the quantum confinement effect
  • Energy levels become discrete as the size of the quantum dot decreases
  • Bandgap energy increases as quantum dot size decreases, leading to a blue shift in absorption and emission spectra (shorter wavelengths)
• The size of quantum dots determines the wavelength of light they absorb and emit
  • Smaller quantum dots absorb and emit shorter wavelengths (higher energy)
  • Larger quantum dots absorb and emit longer wavelengths (lower energy)
• The exciton Bohr radius is a characteristic length scale that determines the onset of quantum confinement effects in quantum dots
  • Typically ranges from 1-10 nm for most semiconductor materials (CdSe, InP)

Applications of size-dependent properties

• Size-dependent properties of quantum dots enable their use in various applications
  • Color-tunable light-emitting diodes (LEDs) with a wide range of emission colors
  • Solar cells with enhanced light absorption and energy conversion efficiency
  • Biological imaging with targeted, fluorescent labeling of cells and tissues
  • Quantum dot lasers with tunable emission wavelengths
  • Quantum dot displays with improved color gamut and energy efficiency

Controlling quantum dot size

Synthesis parameters for size control

• The size of quantum dots can be controlled by adjusting reaction conditions during synthesis
  • Temperature: higher temperatures lead to larger quantum dots, lower temperatures result in smaller quantum dots
  • Reaction time: longer times allow for growth of larger quantum dots, shorter times yield smaller quantum dots
  • Precursor concentration: higher concentrations lead to larger quantum dots, lower concentrations result in smaller quantum dots
• Surfactants and capping agents can be used to control size and prevent aggregation
  • Passivate the quantum dot surface and limit their growth (oleic acid, trioctylphosphine oxide)

Post-synthesis size selection techniques

• Size-selective precipitation narrows the size distribution of quantum dots
  • Separates quantum dots based on their size-dependent solubility in different solvents (methanol, acetone)
• Chromatography techniques (size-exclusion, ion-exchange) can separate quantum dots by size
• Electrophoresis can separate quantum dots based on their size-dependent charge-to-mass ratio
• Centrifugation can be used to isolate quantum dots of a specific size range

Quantum dot shape and behavior

Influence of quantum dot shape on properties

• The shape of quantum dots affects their electronic structure, optical properties, and surface chemistry
  • Leads to different behaviors and applications compared to spherical quantum dots
• Anisotropic quantum dots (nanorods, nanoplatelets) exhibit shape-dependent quantum confinement effects
  • Results in unique optical and electronic properties (linearly polarized emission, enhanced absorption)
• Quantum dot shape influences the polarization of absorbed and emitted light
  • Nanorods exhibit linearly polarized emission along their long axis

Shape-dependent applications

• Different quantum dot shapes can be advantageous for specific applications
  • Nanorods for polarized light emission in displays and photonic devices
  • Nanoplatelets for high-efficiency light-emitting diodes and lasers due to reduced auger recombination
  • Tetrapods for improved charge transport in solar cells and photodetectors
  • Core-shell structures for enhanced photostability and quantum yield in bioimaging

Precision in quantum dot fabrication

Challenges in size and shape control

• Achieving precise control over quantum dot size and shape is crucial for obtaining desired properties
  • Can be challenging due to the complex interplay of synthesis parameters (temperature, time, concentration)
• Polydispersity of quantum dot samples limits their performance and reproducibility in applications
  • Polydispersity refers to the distribution of sizes and shapes within a batch

Strategies for improving size and shape control

• Optimizing reaction conditions (temperature, time, precursor ratios) for better control
• Using advanced synthesis methods for more uniform quantum dots
  • Microwave-assisted synthesis enables rapid, homogeneous heating
  • Continuous flow reactors provide better control over reaction conditions and mixing
• Employing in situ monitoring techniques to track quantum dot growth in real-time
  • Absorption and photoluminescence spectroscopy
  • Small-angle X-ray scattering (SAXS)
• Post-synthesis size and shape selection techniques to narrow the distribution
  • Size-selective precipitation, chromatography, electrophoresis
• Surface functionalization and ligand exchange to enhance stability and dispersibility
  • Tailors surface properties for specific applications (water solubility, biocompatibility)

Characterization techniques for assessing size and shape

• Transmission electron microscopy (TEM) provides direct imaging of quantum dot size and shape
  • High-resolution TEM (HRTEM) can reveal atomic-level structure and defects
• Dynamic light scattering (DLS) measures the hydrodynamic size distribution of quantum dots in solution
• Spectroscopic methods (absorption, photoluminescence, Raman) probe the optical properties related to size and shape
  • Photoluminescence spectroscopy reveals the emission wavelength and quantum yield
  • Raman spectroscopy can detect phonon modes related to size and shape
• X-ray diffraction (XRD) determines the crystal structure and average size of quantum dots
• These characterization techniques guide the optimization of quantum dot synthesis and post-processing