Photoluminescence (PL)
Introduction:
In the process of photoluminescence, electrons in a material are excited to higher energy levels by absorbing energy from incident photons of specific wavelengths. Once in an excited state, the electrons must return to their ground state (equilibrium state) by emitting light (photon energy). This emission appears as a spectrum in terms of wavelength, typically ranging from 400 to 900 nm. It should be noted that in this method, photons can excite electrons in the sample with energy higher than the band gap. Therefore, in photoluminescence analysis, we need a laser or lamp with energy higher than the sample's band gap. This analysis is non-destructive and is often used to study electrical and optical properties, as well as exploring the characteristics of semiconductors, quantum dots, and surface impurities.
Photoluminescence refers to the emission of light from a material after it has absorbed photons. It is a key property in various fields, including material science, chemistry, and semiconductor physics. Photoluminescence can be categorized based on different criteria:
Types Based on Excitation Source:
Fluorescence: Emission occurs almost immediately after excitation, typically within nanoseconds. The emitted light has a longer wavelength than the absorbed light.
Phosphorescence: Emission occurs over a longer time scale (microseconds to hours) due to the involvement of triplet states, which are usually "forbidden" transitions, leading to delayed emission.
Although both fluorescence and phosphorescence fall under photoluminescence, they differ in fundamental properties:
Fluorescence:
Time: Fluorescence occurs rapidly, with light emission happening almost immediately after photon absorption, usually within nanoseconds.
Mechanism: Involves electron transitions from a higher energy state to a lower energy state. Once the electron returns to the ground state, light is emitted.
Duration: Fluorescence ceases as soon as the excitation source is removed.
Phosphorescence:
Time: Phosphorescence lasts significantly longer, from microseconds to hours or even days after the excitation source is removed.
Mechanism: Involves electron transitions to a triplet state, which is a "forbidden" state. This makes the return to the ground state slower, leading to delayed light emission.
Duration: Phosphorescence persists even after the excitation source is removed, making it suitable for applications requiring continuous light emission.
In summary:
Fluorescence: Immediate emission, short-lived.
Phosphorescence: Delayed emission, long-lasting.
Both types have applications in fields such as lighting, material identification, and more.
Fluorescence and phosphorescence techniques are widely used in chemistry and biology. Fluorescence is highly sensitive and provides rapid results, making it suitable for detecting low concentrations of molecules. Phosphorescence, on the other hand, allows for signal recording over longer periods after illumination stops, which enhances the study of material optical properties. Both methods contribute to scientific research and the understanding of chemical and biological interactions.
Principle and Mechanism
Photoluminescence is one of the most well-known methods in luminescence science, where a material is excited by photons (incoming light). In this process, electrons move to higher energy levels (excited state), and upon returning to lower energy levels, they release the absorbed energy in the form of emitted photons (light generation).
To excite a sample, such as semiconductors, a beam with energy higher than the band gap is directed at the sample, causing excitation and pushing it to a higher electronic state. The sample then begins to emit light as it returns to the ground (relaxed) state. More precisely, electronic excitation occurs between two main energy levels, E₁ and E₂. When an electron absorbs energy from a photon, it is excited to energy level E₂, and after approximately eight to ten seconds, it emits a photon with energy equivalent to the difference between these two levels (E₂ - E₁) and returns to the initial energy level. This phenomenon of light emission or luminescence is referred to as photoluminescence (PL).
The emitted light from the sample is analyzed using a spectrometer to determine the intensity of the emitted wavelengths. This method is used to study the electronic and optical properties of semiconductors and molecules. As mentioned earlier, photoluminescence includes both fluorescence and phosphorescence. The difference between phosphorescence and fluorescence lies in the time interval between photon absorption and light emission, known as emission duration. If the excitation time is less than 10⁻⁸ seconds, the phenomenon is considered fluorescence, whereas if it is greater than 10⁻⁸ seconds, it is considered phosphorescence. In other words, in phosphorescence, excitation and emission occur over a longer period, as previously discussed.
When a sample emits light after being exposed to a laser, both photoluminescence (PL) and Raman scattering occur. The intensity of PL emission can be significantly higher. As a result, for samples with strong PL, Raman analysis may not be possible at the used wavelength. To understand Raman scattering, refer to the relevant section. The quantity and type of PL depend on the material and the laser wavelength. Undesired phosphorescence interference can be minimized by selecting the appropriate laser wavelength.
The emitted light from the sample is analyzed using a spectrometer to determine the intensity of the emitted wavelengths. This method is used to study the electronic and optical properties of semiconductors and molecules.
Advantages of PL Analysis:
Non-destructive and contactless method.
Studying optical and structural properties of semiconductors and molecules.
Measurement of band gap energy.
Determining the concentration of luminescent molecules in a solution.
Investigating drug release and delivery mechanisms.
Differences in Instruments:
Fluorescence Spectrometers: Designed specifically for fluorescence measurements, using strong light sources (such as xenon lamps or lasers) to excite materials. These instruments include filters to select the appropriate wavelength of emitted light.
Photoluminescence Spectrometers: May include fluorescence measurement capabilities but are also designed to analyze materials exhibiting phosphorescence. These instruments require additional features to detect phosphorescence, such as the ability to measure long-lived emissions.
Why Use Different Instruments?
Precision: Fluorescence and phosphorescence differ in emission duration. Fluorescence instruments need high precision to measure very short times between excitation and emission, while phosphorescence instruments must be capable of measuring emissions over extended periods.
Different Applications: Some applications require only fluorescence analysis, while others require phosphorescence or both, leading to specialized instrument designs.
Efficiency: Specialized instruments can be more efficient in measuring a specific type of radiation. For example, fluorescence-specific instruments may be optimized for sensitivity to emitted light, making them more suitable for those applications.
Interpretation of PL Results
Interpreting photoluminescence analysis results involves several key aspects, which provide insights into the optical and electronic properties of the studied materials:
Emission Intensity:
Indicates the amount of light emitted from the sample.
High intensity suggests greater efficiency in absorbing and re-emitting energy as light.
Wavelength of Emission:
Provides information about electronic transitions and band gap energy.
Different wavelengths indicate different electronic states or material properties.
Band Gap Measurement:
The energy gap between the highest occupied and lowest unoccupied energy levels.
Larger band gaps require higher excitation energy.
Important Notes:
PL Curve Analysis: Specific peaks in the PL curve may indicate the presence of different components or chemical interactions.
Environmental Factors: Conditions like temperature and pH can affect PL intensity and emission wavelength.
Comparison of PL Results: Used to compare properties of different samples, helping evaluate the impact of chemical or environmental changes on material behavior.
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