3.3 Absorption and photoluminescence spectroscopy

Absorption and photoluminescence spectroscopy are key techniques for studying quantum dots. They reveal crucial info about electronic structure, optical properties, and energy levels. These methods help scientists understand how quantum dot size, composition, and shape affect light absorption and emission.

By measuring how quantum dots interact with light, researchers can fine-tune their properties for various applications. The spectra provide insights into quantum confinement effects, allowing precise control over optical characteristics. This knowledge is essential for developing advanced quantum dot technologies.

Principles of Absorption and Photoluminescence Spectroscopy

Absorption Spectroscopy

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• Absorption spectroscopy measures the absorption of light by quantum dots as a function of wavelength
  • Provides information about the electronic structure and optical properties of quantum dots
• In absorption spectroscopy, light is passed through a sample containing quantum dots, and the transmitted light intensity is measured
• The absorbance is calculated using the Beer-Lambert law, which relates the absorbance to the concentration and the path length of the sample

Photoluminescence Spectroscopy

• Photoluminescence spectroscopy measures the emission of light from quantum dots after they have been excited by a light source
  • Provides information about the radiative recombination processes and the energy levels involved in the emission
• In photoluminescence spectroscopy, quantum dots are excited using a light source with a specific wavelength, and the emitted light is collected and analyzed using a spectrometer
  • The photoluminescence spectrum shows the intensity of the emitted light as a function of wavelength
• The Stokes shift, which is the difference between the absorption and emission peak wavelengths, is an important parameter in studying quantum dots using absorption and photoluminescence spectroscopy
  • Provides information about the energy relaxation processes occurring within the quantum dots (phonon interactions, surface states)

Interpreting Quantum Dot Spectra

Absorption Spectra Features

• The absorption spectrum of quantum dots typically shows a series of distinct peaks corresponding to different electronic transitions
  • The lowest energy peak corresponds to the first exciton transition, which is the transition from the ground state to the first excited state
• The position of the absorption peaks depends on the size, composition, and shape of the quantum dots
  • Smaller quantum dots have absorption peaks at shorter wavelengths (higher energies) compared to larger quantum dots (CdSe, PbS)
• The presence of defects or surface states in quantum dots can lead to additional features in the absorption spectra, such as trap state absorptions or broadening of the peaks

Photoluminescence Spectra Features

• The photoluminescence spectrum of quantum dots usually shows a single, narrow emission peak
  • The position of the emission peak depends on the size, composition, and surface properties of the quantum dots (InP, perovskite)
• The full width at half maximum (FWHM) of the emission peak is a measure of the size distribution and homogeneity of the quantum dots
  • Narrower emission peaks indicate a more uniform size distribution and higher quality of the quantum dots
• The presence of defects or surface states in quantum dots can lead to additional features in the photoluminescence spectra, such as trap state emissions or broadening of the peaks

Quantum Dot Size and Wavelength

Quantum Confinement Effect

• The absorption and emission wavelengths of quantum dots are strongly dependent on their size due to the quantum confinement effect
  • As the size of the quantum dots decreases, the confinement of the charge carriers (electrons and holes) increases, leading to an increase in the band gap energy
• The relationship between the quantum dot size and the band gap energy can be described by the Brus equation, which takes into account the effective mass of the charge carriers and the dielectric constant of the material

Size-Dependent Optical Properties

• Smaller quantum dots have a larger band gap energy, resulting in absorption and emission at shorter wavelengths (higher energies)
  • Conversely, larger quantum dots have a smaller band gap energy, leading to absorption and emission at longer wavelengths (lower energies)
• The tunability of the absorption and emission wavelengths by varying the quantum dot size is a unique property that makes quantum dots attractive for various applications (optical devices, displays, biomedical imaging)
• The relationship between the quantum dot size and the absorption and emission wavelengths can be experimentally verified using size-dependent absorption and photoluminescence measurements
  • The spectra are recorded for quantum dots of different sizes (2 nm, 4 nm, 6 nm)

Factors Affecting Quantum Dot Properties

Composition and Doping

• The composition of the quantum dots, including the choice of semiconductor material and any alloying or doping, significantly influences their absorption and photoluminescence properties
  • Different materials have different band gap energies, effective masses, and dielectric constants, which affect the electronic structure and optical properties of the quantum dots (CdSe, InP, PbS)
• Doping quantum dots with impurities can introduce additional energy levels within the band gap, modifying the absorption and emission characteristics

Shape and Surface Effects

• The shape of the quantum dots, such as spherical, rod-like, or tetrahedral, can impact their absorption and photoluminescence characteristics
  • The shape affects the degree of quantum confinement in different dimensions and can lead to anisotropic optical properties (nanorods, nanoplatelets)
• Surface passivation and ligand chemistry play a crucial role in the absorption and photoluminescence properties of quantum dots
  • Proper surface passivation can minimize surface defects and trap states, which can otherwise lead to non-radiative recombination and reduced photoluminescence efficiency (core-shell structures, organic ligands)

Environment and Defects

• The surrounding environment, such as the solvent or matrix in which the quantum dots are dispersed, can influence their absorption and photoluminescence properties
  • Dielectric screening, surface interactions, and energy transfer processes can modify the optical response (polymer matrices, colloidal suspensions)
• The presence of defects, impurities, or strain within the quantum dots can introduce additional energy levels or modify the band structure, affecting the absorption and photoluminescence spectra
  • Minimizing defects and ensuring high crystalline quality are important for optimizing the optical properties of quantum dots

Measurement Conditions

• The excitation wavelength and intensity used in photoluminescence measurements can impact the observed emission spectra
  • Higher excitation intensities can lead to multiexciton generation, while resonant excitation can result in narrower emission linewidths
• Temperature can affect the absorption and photoluminescence properties of quantum dots through thermal broadening of the energy levels and temperature-dependent non-radiative recombination processes
  • Low-temperature measurements are often used to study the intrinsic optical properties of quantum dots with reduced thermal effects (cryogenic temperatures, liquid nitrogen)