3.4 Quantum yield and fluorescence lifetime

Quantum yield and fluorescence lifetime are key properties of quantum dots that determine their light-emitting efficiency and excited state dynamics. These characteristics are crucial for applications in bioimaging, sensing, and optoelectronics, where bright and stable fluorescence is essential.

Understanding and optimizing quantum yield and fluorescence lifetime enables researchers to tailor quantum dots for specific uses. Factors like size, composition, and surface chemistry influence these properties, allowing for fine-tuning of quantum dot performance in various technological applications.

Quantum yield and fluorescence lifetime of quantum dots

Definition and significance

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  Electron transition pathways of graphene oxide quantum dots unraveled by emission wavelength ... View original

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  Nitrogen-doped carbon quantum dots for fluorescence detection of Cu2+ and electrochemical ... View original

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• Quantum yield is the ratio of the number of photons emitted to the number of photons absorbed by a quantum dot
  • Represents the efficiency of the fluorescence process
  • A higher quantum yield indicates a greater proportion of absorbed photons being converted into emitted photons (CdSe/ZnS quantum dots can have quantum yields up to 90%)
• Fluorescence lifetime is the average time a quantum dot remains in the excited state before returning to the ground state through the emission of a photon
  • Typical fluorescence lifetimes of quantum dots range from a few nanoseconds to tens of nanoseconds (CdSe quantum dots: 10-30 ns, InP quantum dots: 20-80 ns)
  • Provides information about the excited state dynamics and the rate of radiative and non-radiative processes
• Quantum yield and fluorescence lifetime are important photophysical properties that characterize the fluorescence behavior of quantum dots
  • Determine the suitability of quantum dots for various applications (bioimaging, sensing, displays)
  • Enable the optimization of quantum dot properties for specific purposes (high brightness, long-term stability)

Relationship between quantum yield and fluorescence lifetime

• Quantum yield and fluorescence lifetime are interconnected properties
  • A high quantum yield is often associated with a longer fluorescence lifetime
  • Quantum dots with fewer non-radiative recombination pathways exhibit higher quantum yields and longer lifetimes
• The relationship between quantum yield (QY) and fluorescence lifetime (τ) can be expressed as: $QY = k_r / (k_r + k_{nr})$, where $k_r$ is the radiative rate constant and $k_{nr}$ is the non-radiative rate constant
  • The radiative rate constant ($k_r$) determines the intrinsic fluorescence lifetime ($τ_0 = 1/k_r$)
  • The presence of non-radiative processes ($k_{nr}$) reduces the quantum yield and shortens the observed fluorescence lifetime ($τ = 1/(k_r + k_{nr})$)

Measuring quantum yield and fluorescence lifetime

Quantum yield measurement techniques

• Comparative method: Compares the fluorescence intensity of the quantum dots to that of a standard fluorescent dye with a known quantum yield
  • Requires the use of a reference dye with similar excitation and emission wavelengths (Rhodamine 6G, Coumarin 153)
  • The quantum yield is calculated based on the relative fluorescence intensities and the known quantum yield of the reference dye
• Absolute quantum yield measurement: Uses an integrating sphere to collect all the emitted photons and determine the ratio of emitted to absorbed photons
  • Integrating sphere is a hollow spherical cavity with a highly reflective inner surface (Spectralon, Teflon)
  • Accounts for all the photons emitted in different directions, eliminating the need for a reference dye
  • Provides a direct measurement of the absolute quantum yield

Fluorescence lifetime measurement techniques

• Time-correlated single-photon counting (TCSPC): Records the time delay between the excitation pulse and the arrival of the emitted photon at the detector
  • Uses a pulsed laser source and a high-speed single-photon detector (photomultiplier tube, avalanche photodiode)
  • Constructs a histogram of photon arrival times, representing the fluorescence decay curve
  • The fluorescence lifetime is extracted by fitting the decay curve with an exponential function
• Time-resolved fluorescence spectroscopy techniques: Provide high temporal resolution for measuring ultrafast fluorescence dynamics
  • Fluorescence upconversion: Mixes the fluorescence signal with a short laser pulse in a nonlinear crystal to generate a sum-frequency signal, enabling femtosecond time resolution
  • Streak camera: Converts the temporal profile of the fluorescence signal into a spatial profile on a detector, allowing for picosecond time resolution

Factors influencing quantum yield and fluorescence lifetime

Size and composition effects

• The size and composition of quantum dots significantly affect their quantum yield and fluorescence lifetime
  • Smaller quantum dots generally exhibit higher quantum yields and shorter lifetimes due to enhanced quantum confinement and reduced surface-to-volume ratio (CdSe quantum dots: QY increases from 10% to 50% as size decreases from 5 nm to 2 nm)
  • The bandgap and electronic structure of the quantum dot material influence the radiative and non-radiative recombination rates (InP quantum dots have longer lifetimes than CdSe quantum dots due to a larger bandgap)
• Core-shell quantum dot structures can improve the quantum yield and stability by passivating surface defects and confining charge carriers (CdSe/ZnS, InP/ZnS)
  • The shell material acts as a barrier, reducing the interaction of charge carriers with surface defects and the surrounding environment
  • Proper band alignment between the core and shell materials is crucial for efficient charge carrier confinement and high quantum yield

Surface effects and passivation

• Surface defects and trap states can reduce the quantum yield and prolong the fluorescence lifetime by providing non-radiative recombination pathways for the excited charge carriers
  • Dangling bonds, atomic vacancies, and surface ligand detachment can create trap states that quench the fluorescence (CdSe quantum dots with surface defects can have quantum yields below 10%)
  • Trap states can lead to delayed fluorescence and multi-exponential decay behavior
• Surface passivation with appropriate ligands or shell materials can enhance the quantum yield and stability of quantum dots
  • Ligands (oleic acid, trioctylphosphine oxide) can coordinate to the quantum dot surface, passivating defects and preventing aggregation
  • Inorganic shell materials (ZnS, CdS) can provide a physical barrier and electronic passivation, reducing the influence of the external environment on the quantum dot properties

Environmental factors

• The dielectric environment surrounding the quantum dots, such as the solvent or matrix, can influence the quantum yield and lifetime
  • Changes in the local electric field can affect the radiative and non-radiative recombination rates (quantum dots in high-dielectric constant solvents like water exhibit reduced quantum yields compared to those in organic solvents)
  • Interactions between the quantum dots and the surrounding molecules can lead to quenching or energy transfer processes, altering the fluorescence properties
• Temperature affects the quantum yield and lifetime of quantum dots
  • Higher temperatures generally lead to increased non-radiative recombination and reduced quantum yield (CdSe/ZnS quantum dots show a 50% decrease in quantum yield when the temperature is raised from 25°C to 80°C)
  • Thermal activation of surface traps and increased electron-phonon coupling at higher temperatures contribute to the reduction in quantum yield and lifetime

Importance of quantum yield and fluorescence lifetime in applications

Bioimaging and sensing

• High quantum yield is crucial for applications requiring bright and efficient fluorescence, such as bioimaging
  • A strong signal-to-noise ratio is necessary for sensitive detection and high-resolution imaging of biological samples (quantum dots with quantum yields above 50% are preferred for in vivo imaging)
  • Quantum dots with high quantum yields enable longer imaging times and reduced excitation power, minimizing phototoxicity and photobleaching
• Long fluorescence lifetime is advantageous for time-gated imaging and sensing applications
  • Allows for the suppression of short-lived background fluorescence (autofluorescence) and improved signal specificity (CdSe/CdS quantum dots with lifetimes >20 ns are used for time-gated imaging of cells and tissues)
  • Enables the development of fluorescence lifetime imaging microscopy (FLIM) techniques, providing additional contrast and environmental sensitivity

Energy-related applications

• In solar cells and light-emitting diodes (LEDs), high quantum yield is essential for efficient light harvesting and emission
  • Quantum dots with high quantum yields can enhance the power conversion efficiency of solar cells by absorbing a wide range of photon energies and reducing thermalization losses (PbS quantum dots with quantum yields >80% are used in quantum dot solar cells)
  • Quantum dot LEDs (QLEDs) require quantum dots with high quantum yields to achieve bright and pure color emission, with reduced energy losses (CdSe/ZnS quantum dots with quantum yields >90% are employed in QLEDs)
• Short fluorescence lifetimes are desirable for fast charge transfer and reduced recombination losses
  • In quantum dot solar cells, short lifetimes facilitate the rapid extraction of photogenerated charge carriers, minimizing recombination and improving the device performance
  • In QLEDs, short lifetimes enable fast radiative recombination and high brightness, while minimizing non-radiative losses and efficiency roll-off at high current densities

Environmental and chemical sensing

• The sensitivity of quantum yield and lifetime to the local environment makes quantum dots valuable probes for sensing applications
  • Changes in the quantum yield or lifetime can be used to detect the presence of analytes or monitor biological processes (pH-sensitive CdSe/ZnS quantum dots exhibit a 50% decrease in quantum yield when the pH changes from 7 to 4)
  • Quantum dots can be functionalized with specific ligands or receptors to achieve selective sensing of target molecules (DNA, proteins, metal ions)
• Ratiometric sensing approaches based on the combination of quantum yield and lifetime measurements can provide enhanced sensitivity and reliability
  • The use of two different types of quantum dots with distinct fluorescence properties allows for the development of ratiometric sensors that are insensitive to fluctuations in excitation intensity or detector efficiency
  • Förster resonance energy transfer (FRET) between quantum dots and organic dyes or other quantum dots can be employed for ratiometric sensing, where the energy transfer efficiency depends on the distance and interaction between the donor and acceptor