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Photoluminescence Spectroscopy


Photoluminescence refers to a form of luminescence that results from photoexcitation. Simply, photoluminescence occurs when a material emits light after absorbing a photon from an external light source. In photoluminescence spectroscopy, you can measure the intensity of emitted light as a function of wavelength by using an optical spectrometer. We recommend that you also use a high energy light source to stimulate all of the available electrons into their excited energy state.

There are several ways of categorising phenomena within photoluminescence. It is firstly important to consider that both radiative and non-radiative emissions will occur within your sample. As a form of radiative emission, photoluminescence can then be separated into two main classifications - fluorescence and phosphorescence. These both have different energy transition pathways, and therefore very different lifetimes. One main way of quantifying photoluminescence efficiency is by measuring the photoluminescent quantum efficiency (PLQE) of a sample. You could also measure its photoluminescent quantum yield (PLQY).

For measuring bulk material properties, you will need a high energy excitation source and a spectrometer (such as the Ossila Optical Spectrometer). Your excitation source will work to excite electrons into their higher energy states. As they relax, the electrons will then emit photons of a lower energy, which can be detected by your optical spectrometer.

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Photoluminescence Theory


Absorbance spectroscopy tells us that if the band structure allows it, unpaired electrons in the ground state of a material can be excited by a photon into a higher energy state. This only occurs if the energy of the photon is larger than the gap between the energy levels.

If an electron is excited into a higher energy vibrational state, it will lose some of this energy to thermalisation, lattice vibrations or through the emission of phonons. Essentially, the electron will quickly relax to the S1 energy state.

Jablonski diagram illustrating photoluminescence energy transition
Jablonski diagram illustrating photoluminescence energy transition.

Photoluminescence occurs when this electron relaxes back into the S0 state, emitting a lower energy photon. Another equivalent way to view this process is as the recombination of an electron and an electron vacancy (often referred to as a "hole"). The emitted light corresponds to the energy difference between the S1 and S0 through the photon energy equation.

 

Equation - energy of a photon is equal to plancks constant times the speed of light, all divided by the photons wavelength

 

Radiative and Non-Radiative Relaxation

Excited electrons in a material can lose energy through radiative or non-radiative emissions. In radiative emission, the excited electron relaxes, releasing a photon. However, in non-radiative relaxation, this energy is lost in other ways.

Photoluminescence is an example of radiative emission. Examples of non-radiative emission include the thermalisation of electrons which are excited to vibrational energy levels. It can also be recognised in the process of fluorescence quenching. This energy gets lost as heat, through vibrational interactions or through atomic collisions.

In the study of semiconductors or other electronic devices, these transitions are often referred to as radiative or non-radiative recombination. This means the same thing as radiative or non-radiative emission but focuses on a different part of the emission. Here, radiative recombination is viewed as the recombining of an electron and hole pair, which releases a photon. This is an important distinction to make, because depending on what your electronic device will do, your desired amount of radiative and non-radiative recombination will differ.

Want more information about non-radiative relaxation? See fluorescence quenching and non-radiative relaxation

Stokes Shift

When a photon of greater energy than the energy gap (i.e. S0 → S1 transition) is absorbed, an electron will be excited to a higher vibronic (electronic and vibrational) energy level. The probability of a transition happening between these vibronic states is given by the Franck-Condon principle. This relies on the relative overlap of the wavefunctions of the two states: the greater the overlap, the greater the probability of the transition happening. The Franck-Condon principle applies to both the absorption and emission processes.

Once excited, the electron will generally relax back down to the lowest vibrational level of the S1 state through non-radiative relaxation. It will then relax through photoluminescence back to the electronic ground state, S0, thus emitting a photon. Again, this is recognised according to the Franck-Condon principle.

Essentially, the energy of this emitted photon will be less than the photon absorbed. This is because of the non-radiative energy loss that occurs as the electron transitions through the vibrational energy levels. This principle is more commonly known as Stokes shift.

Jablonski diagram for a stokes shift
Jablonski diagram illustrating the mechanism that results in a Stokes shift between the absorption (blue arrow) and emission (red arrow) peaks in organic molecules.
Stokes shift of BODIPY-Br
Example of Stokes shift. The normalised absorbance (blue) and photoluminescence (red) spectra of a thin film of BODIPY-Br, a fluorescent small molecule, dispersed in polystyrene. Here, there is a Stokes shift of 18 nm or 77 meV.

The above spectrum shows the absorbance and photoluminescence spectra of BOPIDY-Br in polystyrene. You can see in this spectrum that the maximum absorbance peak is blue shifted compared to the photoluminescence peak. Here, you can recognize an example of the stokes shift.

It is important to note that electrons tend to first relax non-radiatively to S1 and then radiatively from S1 to S0. Therefore, photoluminescence spectroscopy only measures the transitions between S1 to S0. This is known as Kasha's rule, that the photoluminescence spectrum obtained from a material is entirely independent of the excitation wavelength.

Fluorescence or Phosphorescence

Fluorescence is a type of photoluminescence in which a material absorbs a photon and almost immediately emits a lower energy photon. Atomically, this process consists of only singlet-singlet transitions, or completely allowed transitions. Therefore, fluorescence emission takes place over a short timescale. This type of PL happens while the sample is being illuminated.

In phosphorescence, an electron absorbs a photon and is excited into a higher energy level. Again, this electron then relaxes into the ground state, emitting a photon. However, this relaxation takes much longer than it does for fluorescence. This is because phosphorescence requires a singlet-triplet transition. These transitions are "forbidden", which means that although these transitions can occur, they are rare. Therefore, phosphorescent emissions have a much longer lifetime than fluorescent emissions. This is also why phosphorescent materials will glow after illumination has stopped.

You can use fluorescent materials for a wide range of applications in bioluminescent technology, microscopy, and medicine. They are also effective as chemical labels. You can use phosphorescent materials for a number of different "glow-in-the-dark" applications (paints, toys, etc) as they will continuously emit light after illumination. These were also useful in second generation OLED devices, while both phosphorescence and fluorescence play a key part in thermally activated delayed fluorescence.

To find out more information about fluorescence and phosphorescence, see Fluorescence or Phosphorescence

Fluorescence Quenching

Fluorescence quenching refers to processes which can reduce the photoluminescence quantum yield (the amount of fluorescence) by allowing the electron to relax to a lower energy state non-radiatively.

Quenching mechanisms include:

  • Vibrational relaxation and internal conversion.
  • Intersystem crossing.
  • Förster resonance energy transfer.
  • Dexter electron transfer.
  • Radiative energy transfer.
To find out more information about fluorescence quenching, see Fluorescence Quenching and Non-Radiative Relaxation

Thermally Activated Delayed Fluorescence (TADF)

Thermally Activated Delayed Fluorescence (TADF) is a process for generating fluorescence from electrons in the triplet state. Fluorescence from triplet states is forbidden, however, electrons in TADF emitters can use surrounding thermal energy to transition electrons from the excited triplet to the excited singlet state. This is referred to as ‘reverse intersystem crossing’. It is a slow system which leads to the resulting fluorescence to be ‘delayed’.

TADF has significant implications for OLEDs as it allows higher photoluminescence quantum yields to be achieved.

To find out more information about TADF, see Thermally Activated Delayed Fluorescence (TADF)

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How to Measure Photoluminescence


To measure the photoluminescence of a material, you will need an optical spectrometer, and a monochromatic, low wavelength light source (such as a continuous wave laser, or UV light source). Alternatively, you can use a broadband light source in combination with a filter to select the excitement energy suitable for your sample. You will also need a filter to omit the original excitation signal from your final measurement, as this can swamp the signal intensity.

A typical photoluminescence setup for spectroscopy.

During this process, the high energy photons from the excitation source excite electrons within the sample into higher energy states. For most luminescent samples, light will almost immediately be re-emitted in the form of low energy photons. Passing this light through a filter removes the excitation peak. A spectrometer will then be able to detect the emitted photons via an optical fibre or from the direct beam.

When measuring photoluminescence, we also recommend that you measure the absorbance spectra of your sample. These measurements should be complementary as your absorbance measurements will therefore require less optimisation. This will then give you an idea of where you should expect to see the photoluminescence peaks.

Equipment Setup

One important thing to remember about photoluminescence is that photons are emitted in all directions. This means that, unlike in absorbance and transmittance spectroscopy, there is no "ideal" place to put your spectrometer. It is important that you optimise the arrangement of your system to work best for your sample. This optimal set-up will depend on factors such as the amount of photoluminescence emitted by your sample, your film thickness or sample concentration as well as the amount of background light in your lab environment. Remember that if you are measuring at only one angle, you are only ever detecting a fraction of the total photoluminescence. Therefore, photoluminescence intensity values will not be absolute. If your sample is in the form of a thin film, you can assume that the majority of photoluminescence will be emitted from the largest surface area of the sample (i.e. the front and back of the film). Therefore, it is beneficial to put the samples in the pathway of the excitation source, as you would do in absorbance spectroscopy.

Photoluminescence Quantum Efficiency

Photoluminescence quantum efficiency (or photoluminescent quantum yield) compares the number of photons emitted by a sample to the number of photons that have been absorbed. You can use this measurement to determine the suitability of your sample for use in laser pumping, solar cells or in LED technology. It will also inform you about the optical properties of your material, and about the amount of non-radiative recombination there is in your sample.

Typically, photoluminescence quantum yield measurements use an integrating sphere fibre coupled with an optical spectrometer. It is important to place your sample at the centre of the sphere at a slight angle. This is so that the light isn't directly reflected back out of the entrance port and irradiated by a monochromatic light source such as a laser. You should also ensure that the photon energy of your light source is higher than that of the sample emission. This makes sure that the sample has a non-zero absorption at the incident wavelength.

The inside of the sphere is coated with a diffuse, white reflective layer, which, through multiple reflections, distributes the light isotopically over the sphere's inner surface. Outgoing light, which consists of both source photons and sample emission, is then collected by the fibre.

Finally, in order to achieve a reliable measurement, you will need to take a control measurement. If your sample is a solution, your control should be a cuvette containing the solvent used. If you are using a thin film, it should be a blank substrate. A typical PLQE setup is shown in the figure below.

Photoluminescence spectroscopy setup
Typical PLQE setup. Light from a monochromatic source - here, a laser - is focused onto a sample inside an integrating sphere. The laser light and the light emitted by the sample are reflected multiple times from the diffuse inner surface of the sphere. This is collected by an optical fibre, which sends the light to a spectrometer.

You can then compare your two spectra to calculate the amount of light absorbed and emitted by the sample. In the example PLQE data shown below, a portion of the excitation source is absorbed (red shaded region) when the sample is placed inside the sphere. Some of this light is re-emitted at longer wavelengths (blue shaded region). The PLQE, Φ, is then given by

 

PLQE equation

 

In practice, this is usually calculated by integrating the two shaded regions.

Example PLQE data
Example PLQE data. The red and blue solid lines show the spectrum from the sphere without and with the sample, respectively. The red and blue shaded regions show the light absorbed and emitted by the sample, respectively. The PLQE of the sample is given by the blue shaded area (which has been unrealistically scaled up for ease of viewing) divided by the red shaded area (x100).

If the sample's absorption and PL spectra overlap, it can reabsorb some of the photons that it emits after they've been reflected from the sphere's inner surface. In this case, the PL emission in the integrating sphere measurements will have a different shape, with a reduction in emission at the high energy end - this is illustrated in the figure below. Due to this we recommend using a self-absorption correction that uses the actual PL spectrum [1].

Reabsorption by sample in PLQE measurement
Example of reabsorption by sample in PLQE measurement. The green line represents the missing part of the PL spectrum and the green shaded area represents the reabsorbed photons.

The PLQE of a material depends on its electronic and vibrational energy levels. When a photon is absorbed, the excited electron can either decay radiatively or non-radiatively. If it decays radiatively, a photon will be released. If it decays non-radiatively, however, a photon will not be released. The PLQE is hence related to the radiative (fluorescence) rate, kr, and the sum (Σ) of all the non-radiative, knr, rates through

 

PLQE is related to the radiative (fluorescence) rate and sum of non-radiative rates

 

Φ is strongly affected by the molecules' environment - for example, solvent polarity. It can also be affected by aggregation. Aggregation can occur when molecules form complexes in the ground state and the electronic properties of the molecule are altered. This can result in broadened or shifted absorption and emission spectra and often results in an increase in non-photoluminescent pathways and therefore a decreased PLQE. This is the case with aromatic molecules (those that contain planar, cyclic structures, such as benzene), which undergo π-π stacking, and when hydrophobic molecules are placed in polar solvents. Additionally, the close proximity of molecules in the solid state also increases the effects of aggregation if the length scales are short enough to allow energy transfer and intramolecular charge transfer to occur.

For this reason, thin films often have significantly reduced PLQEs compared to solutions. It is possible to reduce the effects of aggregation in films by dispersing the molecules in a transparent polymer matrix such as polystyrene or poly(methyl methacrylate) (PMMA). This is done simply by adding the polymer to the dye solution and processing as normal - for example, through spin coating. The polymer will help to space out the dye molecules, reducing intermolecular interactions and reducing aggregation. By increasing the concentration of polymer relative to the dye, aggregation can be further reduced; however, this is accompanied by a reduced absorption, which may not be desirable.

Steady-State vs. Time-Resolved Photoluminescence Spectroscopy

Steady-state photoluminescence (or steady state photoluminescence) only measures photoluminescence intensity as a function of wavelength at a given point in time. Therefore, you can use steady-state spectroscopy when the photoluminescence intensity of your sample doesn't change with continued illumination. It is also appropriate to use when the fluorescence intensity of your sample changes after at least >1 second.

Time-resolved photoluminescence spectroscopy (or time-resolve photoluminescence spectroscopy) can be used to study fast radiation processes occurring over a very short time scale (from milliseconds to picoseconds). Here, rather than the constant illumination used in steady-state photoluminescence, you excite the sample with a short, high energy pulse and detect photoluminescence over time.

You will need very sophisticated equipment to perform time-resolved photoluminescence, including an extremely sensitive detector and a dark-room environment. Often, you may need to bring the temperature of your samples down to extremely cold temperatures to extend the lifetime of these excited states long enough for them to be observed. For this, you would need to use other specific equipment such as cryostat.

Things to remember when measuring photoluminescence

  1. For solid samples, you will find that photoluminescence measurements are more sensitive to minor defects or changes in structure than absorbance measurements. For example, the introduction of trap states in a semiconductor material can quench the luminescence completely, while not really changing the bulk absorbance properties.

  2. You can increase the integration time of your measurement in order to increase the size of your signal. However, ensure that the highest signal is still below the saturation level of the detector.
  3. You can reduce the signal-to-noise ratio by averaging your spectra over several measurements. This is referred to as spectral averaging.
  4. You can adjust the gain if you need to amplify the signal from your sample, for example if the photoluminescence signal is particularly weak. Although, keep in mind that this will also amplify noise level in your signal.
  5. If you are measuring multiple samples, ensure that your setup stays the same throughout. Factors that can affect photoluminescence intensity include the source intensity and wavelength, sample orientation, detector angle and distances between the excitation source, sample, and detector. If you change these factors between measurements, your results may not be comparable.
  6. Without using an integrating sphere, your measured photoluminescence intensities will not be reliable. You can compare emission intensities between samples if all other factors are kept the same. However, do not rely on these measurements. If you want to confirm these results, you can use another technique such as electron quantum efficiency measurements. Alternatively, you can normalise the spectra to highlight changes in emission peak shape and location without taking into consideration peak intensity.

Interpreting Photoluminescence Spectra

There is plenty of information you can find out about your sample using photoluminescence spectroscopy. Often, you will have to take several factors into account to see the whole picture. Some of the key features to examine include:

Peak Shift in PL spectrum

The location of photoluminescence peaks is the first thing to notice when studying photoluminescence spectra. You can work out the transition energy from the wavelength of the emission maxima using the photon-energy equation. If this peak shifts to a higher or lower wavelength than expected, this could indicate a change the bulk material properties such as the material band gap. Movement of these emission peaks could also be due to aggregation of molecules, such as for the formation of excimers, or due to increased conjugation in a polymer.

Photoluminescence Intensity

Photoluminescence intensity can show many things about the optical properties of your sample (the amount of radiative vs. non-radiative processes, the amount of quenching occurring, if there is re-absorbance occuring within your sample). Without using of an integrating spere, measurements of photoluminescence intensity should be used comparatively, not quantitatively. However, by using an optical spectrometer, you can measure if there is notable change in photoluminescence intensity in your sample compared to a control.

Additionally, if you have used photoluminescent materials to label certain molecules in your system, you can measure the abundance of these molecules during or after an event through PL intensity. In this case, photoluminescence will be linearly proportional to the concentration of fluorophores in your sample, solution, or culture.

Full Width Half Maximum

The full width half maximum of your sample can supply useful information about its homogeneity. If the linewidth of your peak gets broader, the fluorescing material is more polydisperse or inhomogeneous. In colloidal dispersions (such as luminescent quantum dots), an increase in polydispersity could imply a broader range of colloid size, shape or the presence of defects or imperfections in your colloids. In thin films, an increase in line width could suggest an increase in the number of defects, trap states or non-radiative recombination centres within the film.

Evolving Peaks

You may notice that as you continually illuminate your sample, the photoluminescence intensity increases or decrease with time. There are many things can cause this. For example, decreasing photoluminescence intensity with time could imply there is photoluminescence quenching happening in your sample. If the sample is a thin film in contact with a transport layer, this can be used to measure how well the charge can be extracted from the absorbent layer. On the other hand, it has been observed that PL intensity can increase with time in semiconductors due to interstitial vacancies. Initially, the first electrons that are excited fill these traps states, and the amount of radiative recombination is low. However, over time these trap states remain occupied, and electrons can relax radiatively from the S1 state. Therefore, the amount of radiative recombination increases. Depending on time frame of this intensity change, this can be measured with steady-state (lifetimes >1 s) or time-resolved photoluminescence (picoseconds-milliseconds) spectroscopy.

Example of Photoluminescence Spectroscopy


Jablonski diagram for a stokes shift
Absorbance spectra of BODIPY-Br/polystyrene thin films with varying dye concentration.

Here is an example of photoluminescence of BODIPY with increasing dye concentration. There is plenty you can tell about this molecule from the photoluminescence peak placement and intensities. Increasing the concentration of BOPIDY-Br from 1% to 5% increases photoluminescence intensity. After this, photoluminescence intensity decreases with increased concentration. We also see an increase in photoluminescence at around 600 nm, as fluorophore concentration increases.

We also found that the emission maxima shifted with increased concentration. All these factors indicate that there is aggregation occurring within the solution with increased dye concentration, in particular that excimers are forming with increased BODIPY-Br concentration.

  

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References


  1. T.-S. Ahn, et al. Rev. Sci. Instrum. 78, 086105 (2007).

Contributing Authors


  • Kirsty McGhee
  • Mary O'Kane
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