20 Photoluminescence (PL) Spectra

Dr. Anchal Srivastava

Contents of this Unit

1. Introduction

1.1 Luminescence

1.2 Photoluminescence (PL)

2. Fundamental Mechanism of PL

2.1 Absorption Spectra

2.2 Excitation and emission spectra

2.2.1 Non-Radiative Relaxation Process

2.2.2 Radiative processes -Fluorescence and Phosphorescence

3. Construction and working principle of PL

4. Applications of Photoluminescence

5. Summary

Learning Outcomes

After studying this module, you shall be able to learn
The nature of light and the basic definitions of photoluminescence and its importance. The recombination mechanism of photoluminescence
The basic idea about Fluorescence and Phosphorescence The working principle of photoluminescence
Identification of surface, interface, and impurity levels and band gap determination.

1. INTRODUCTION

Light is a form of energy whose behavior is described by the properties of both waves and particles. Some properties of electromagnetic radiation, such as its refraction when it passes from one medium to another are explained best by describing light as a wave. Other properties, such as absorption and emission, are better described by treating light as a particle. The exact nature of electromagnetic radiation remains unclear, as it has since the development of quantum mechanics in the first quarter of the 20th century. Nevertheless, the dual models of wave and particle behavior provide a useful description for electromagnetic radiation.

1.1 LUMINESENCE

Luminescence is a science closely related to spectroscopy, which is the study of the general laws of absorption and emission of radiation by matter. The existence of luminous organisms such as bacteria in the sea and in decaying organic matter, glow worms and fireflies have mystified and thrilled man since time immemorial. A systematic scientific study of the subject of luminescence is of recent origin, from the middle of nineteenth century. In 1852 English Physicist G.C.Stokes identified this phenomenon and formulated his law of luminescence now known as Stoke’s law, which states that the wavelength of the emitted light is grater than that of the exciting radiation. German physicist E. Wiedemann introduced the term ‘luminescence’ (weak glow) into the literature in 1888. The phenomenon of certain kinds of substance emitting light on absorbing various energies without heat generation is called luminescence. Luminescence is obtained under variety of excitation sources. The wavelength of emitted light is characteristic of the luminescent substance and not of the incident radiation.

A luminescing system is constantly expending energy to drive the emission process. The general term luminescence includes a wide variety of light emitting processes which derive their names from the varied sources of energy that power them. Photoluminescence, which includes fluorescence and phosphorescence, is one among many luminescent categories. To illustrate the diversity of luminescence emissions, the following represent some of the more commonly observed types of luminescence:

1. Electroluminescence: It is produced by the passage of an electric current through an ionized gas.

Example — a gas discharge lamp.

2. Radioluminescence: It obtains its energy from the high energy particles released via radioactive decay.

Example — a luminous radium watch dial.

3. Triboluminescence: It is derived from the Greek word tribo meaning to rub. It is emitted when certain

crystals are stressed, crushed, or broken. Example — certain types of sugar crystals.

4. Sonoluminescence: It is produced in liquids exposed to intense sound (compression) waves.

5. Chemiluminescence: It derives its energy from chemical reactions. It is the breaking of chemical bonds

that supplies the energy.

6. Bioluminescence: It can be considered a subdivision of chemiluminescence. It occurs when

chemiluminescent reactions take place in living systems and usually involves a ATP reaction. Example- light

emitted by fireflies and glow-worms.

7. Cathodoluminescence: It is light that is generated from the exposure of substances to cathode rays.

8. Photoluminescence: It derives its energy from the absorption of light energy (most commonly in the

wavelength ranges of infrared, ultraviolet, or visible light). Photoluminescence is further divided into the

categories of fluorescence, delayed fluorescence, and phosphorescence. Today they are defined via the

emission based quantum mechanical mechanism for the orbital angular momentum multiplicity of the

emitted electron (i.e. the singlet or triplet excited state). However, before the advent of quantum theory

photoluminescence was defined solely on the basis of empirical evaluation of the duration of emissive

lifetime.

I. Fluorescence: It is defined as a photoluminescent emission that arises from the singlet electronic state. To

the human eye fluorescence is observed only when the exciting light source shines on the radiator.

II. Phosphorescence: It is defined as a photoluminescent process that originates from the triplet electronic

state. Emissions from the triplet state are from 10 to 10,000 times longer than fluorescence; therefore, to the

eye these radiators appear to emit after the excitation radiation is removed.

III. Delayed fluorescence: It is a rare phenomenon whereby the electron responsible for the emission starts

out in the singlet state, crosses over to the triplet state, but eventually returns to the singlet state prior to

emission. The result is a singlet state emission of much longer lifetime than normal.

1.2 PHOTOLUMINESENCE

Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence.

Photo-excitation causes electrons within a material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process). The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process.

All solids, including semiconductors, have so-called “energy gaps” for the conducting electrons. In order to understand the concept of a gap in energy, first consider that some of the electrons in a solid are not firmly attached to the atoms, as they are for single atoms, but can hop from one atom to another. These loosely attached electrons are bound in the solid by differing amounts and thus have much different energy. Electrons having energies above a certain value are referred to as conduction electrons, while electrons having energies below a certain value are referred to as valence electrons. This is shown in the diagram where they are labeled as conduction and valence bands. The word band is used because the electrons have a multiplicity of energies in either band. Furthermore, there is an energy gap between the conduction and valence electron states. Under normal conditions electrons are forbidden to have energies between the valence and conduction bands.

Figure 1 : Schematic diagram showing PL process

If a light particle (photon) has energy greater than the band gap energy, then it can be absorbed and thereby raise an electron from the valence band up to the conduction band across the forbidden energy gap. (see figure 1) In this process of photo-excitation, the electron generally has excess energy which it loses before coming to rest at the lowest energy in the conduction band. At this point the electron eventually falls back down to the valence band. As it falls down, the energy it loses is converted back into a luminescent photon which is emitted from the material. Thus the energy of the emitted photon is a direct measure of the band gap energy, Eg. The process of photon excitation followed by photon emission is called photoluminescence.

The importance of photoluminescence

In most photoluminescent systems chromophore aggregation generally quenches light emission via aggregation-caused quenching (ACQ). This means that it is necessary to use and study fluorophores in dilute solutions or as isolated molecules. This in turn results in poor sensitivity of devices employing fluorescence, e.g., biosensors and bioassays. However, there have recently been examples reported in which luminogen aggregation played a constructive, instead of destructive role in the light-emitting process. This aggregated-induced emission (AIE) is of great potential significance in particular with regard to solid state devices. Photoluminescence spectroscopy provides a good method for the study of luminescent properties of a fluorophore.

2. FUNDAMENTAL MECHANISM OF PHOTOLUMINISENCE

The phenomena which involve absorption of energy and subsequent emission of light are classified generically under the term luminescence. Phosphors are luminescent materials that emit light when excited by radiation, and are usually microcrystalline powders or thin-films designed to provide visible color emission. After decades of research and development, thousands of phosphors have been prepared and some of them are widely used in many areas. Excitation by absorbance of a photon leads to a major class of technically important luminescent species which fluoresce or phosphoresce. In general, fluorescence is “fast” (ns time scale) while phosphorescence is “slow” (longer time scale, up to hours or even days). For convenience, the topic of photoluminescence (PL) will be broadly divided into that based on relatively large-scale inorganic materials, mainly exhibiting phosphorescence, and that of smaller dye molecules and small-particle inorganic (“nanomaterials”), which can either fluoresce or phosphoresce.

2.1 Absorption Spectra

Figure 2: Schematic picture of absorption process

The quantity of interest in atomic absorption measurements is the amount of light at the resonant wavelength which is absorbed as the light passes through a cloud of atoms. As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way. By measuring the amount of light absorbed, a quantitative determination of the amount of analyte element present can be made. The use of special light sources and careful selection of wavelength allow the specific quantitative determination of individual elements in the presence of others.

2.2 Excitation and emission spectra

Figure 3: schematic picture of excitation and decay process

The atom is made up of a nucleus surrounded by electrons. Every element has a specific number of electrons which are associated with the atomic nucleus in an orbital structure which is unique to each element. The electrons occupy orbital positions in an orderly and predictable way. The lowest energy, most stable electronic configuration of an atom, known as the ‘‘ground state’’, is the normal orbital configuration for an atom. If energy of the right magnitude is applied to an atom, the energy will be absorbed by the atom, and an outer electron will be promoted to a less stable configuration or ‘‘excited state’’. As this state is unstable, the atom will immediately and spontaneously return to its ground state configuration. The electron will return to its initial, stable orbital position, and radiant energy equivalent to the amount of energy initially absorbed in the excitation process will be emitted. The process is illustrated in Figure 3. Note that in Step 1 of the process, the excitation is forced by supplying energy. The decay process in Step 2, involving the emission of light, occurs spontaneously.

The wavelength of the emitted radiant energy is directly related to the electronic transition which has occurred. Since every element has a unique electronic structure, the wavelength of light emitted is a unique property of each individual element. As the orbital configuration of a large atom may be complex, there are many electronic transitions which can occur, each transition resulting in the emission of a characteristic wavelength of light, as illustrated in Figure 4.

Figure 3: Energy transitions

The process of excitation and decay to the ground state is involved in all three fields of atomic spectroscopy. Either the energy absorbed in the excitation process or the energy emitted in the decay process is measured and used for analytical purposes. In atomic emission, a sample is subjected to a high energy, thermal environment in order to produce excited state atoms, capable of emitting light. The energy source can be an electrical arc, a flame, or more recently, plasma. The emission spectrum of an element exposed to such an energy source consists of a collection of the allowable emission wavelengths, commonly called emission lines, because of the discrete nature of the emitted wavelengths. This emission spectrum can be used as a unique characteristic for qualitative identification of the element. Atomic emission using electrical arcs has been widely used in qualitative analysis.

Emission techniques can also be used to determine how much of an element is present in a sample. For a ‘‘quantitative’’ analysis, the intensity of light emitted at the wavelength of the element to be determined is measured. The emission intensity at this wavelength will be greater as the number of atoms of the analyte element increases. The technique of flame photometry is an application of atomic emission for quantitative analysis.

PL in solids is classified in view of the nature of the electronic transitions producing the luminescence. In the case of PL a molecule absorbs light of wavelength, decays to lower energy excited electronic state and then emits light of wavelength λ2 as it radiatively decays to its ground electronic state. Generally the wavelength of emission is longer than the excitation wavelength, but in resonance emission, absorbed wavelength is equal to emission wavelength. Luminescence bands can be either fluorescence or phosphorescence, depending on the average lifetime of the excited state, which is much longer for phosphorescence than fluorescence. The relative broadness of the emission band is related to the relative difference in equilibrium distance between the excited emitting state and the ground electronic state. PL of a molecular species is different from emission from an atomic species. In the case of atomic emission both the excitation and emission are at the resonance wavelengths, in contrast excitation of a molecular species usually results in an emission that has a longer wavelength than the excitation wavelength. PL can occur in gas, liquid and solid phases. An energy level diagram as in Figure 4 can illustrate the radiative and non-radiative transitions that lead to the observation of molecular photoluminescence

Figure 4. Partial energy level diagram of a photoluminecent molecule. S1 & S2 are singlet states and T1 the triplet states

The spin multiplicities of a given electronic state can either a singlet paired electrons) or a triplet (unpaired electrons). The ground electronic state is normally a singlet state and is designated as S0. Excited electronic states are either singlet (S1, S2) or triplet (T1) states. When the molecule absorbs light an electron is promoted within 10-14 – 10-11 seconds from the ground electronic state to an excited state that posses the same spin multiplicity as the ground state. This excludes a triplet-excited state, as the final state of electronic absorption because the selection rules for electronic transition dictates the spin state should be maintained upon excitation.

A plethora of radiative and non-radiative processes usually occur following the absorption light en route to the observation of molecular luminescence.

2.2.1. Non-radiative relaxation processes

(a) Vibrational relaxation: -Excitation usually occurs to higher vibrational level of the target-excited state. The excited molecules normally relax rapidly to the lowest vibrational level of the excited electronic state. These non-radiative processes are called vibrational relaxation. It occurs within 10-14 to 10-12s a time much shorter than the typical luminescence lifetime. So such processes occur prior to luminescence.

(b) Internal conversion: – If the molecule is excited to a higher energy excited singlet state than S1 (like S2 in Figure 4), a rapid non-radiative relaxation usually occurs to the lowest energy singlet excited state (S1). Relaxation processes between electronic states of like spin multiplicity such as SI and S2 are called internal conversion. It normally occurs on a time scale of 10^-12 s

(c) Intersystem crossing: – Non- radiative relaxation processes can also occur between excited states of different spin multiplicity. Such relaxation process is known as intersystem crossing. The relaxation from S1 to T1 in Figure 4 is an example of intersystem crossing

(d) Non-radiative de-excitation: – The above mentioned non-radiative processes occur very rapidly and release small amount of energy .The rest of the energy is dissipated either radiatively, by emission of photons, or non-radiatively by the release of thermal energy The non-radiative decay of excitation energy which leads to the decay of excited molecule to the ground electronic state is called nonradiative de-excitation. The amount of energy released in the form of heat is very small and cannot be measured experimentally. The evidence for non-radiative deexcitation process is the quenching of luminescence. In solid-state luminescent materials the crystal vibrations (phonons) provide the mechanism for nonradiative de-excitation.

2.2.2. Radiative processes -Fluorescence and Phosphorescence

Fluorescence refers to the emission of light associated with a radiative transition from an excited electronic state that has the same spin multiplicity as the ground electronic state. The radiative transition S1to S0 in Figure 4 represents fluorescence. Since fluorescence transitions are spin allowed they occur very rapidly and average lifetimes of the excited states responsible for are typically less than 10^-6 s. Electronic transitions between states of different spin multiplicity are spin forbidden, however it becomes more probable when spin orbit coupling increases. The net result of spin orbit coupling is the mixing of excited singlet and triplet states. This mixing removes the spin forbidden nature of the transitions between pure singlet and pure triplet states. Therefore if intersystem crossing populates the triplet-excited state then luminescence might occur from the triplet state to the ground state. Phosphorescence refers to the emission of light associated with a radiative transition from an electronic state that has a different spin multiplicity from that of ground electronic state. The radiative transition T1 to S0 in Figure 4 represents the phosphorescence. Since phosphorescence transitions are spin forbidden they occur slowly and so the average lifetime for such emission typically ranges from 10^-6 to several seconds. Phosphorescence is also known as ‘delayed fluorescence.’

3. CONSTRUCTION AND WORKING PRINCIPLE OF PHOTOLUMINISENCE

The electronic structure of the sample can be characterized with the photoluminescence (PL) method. In a PL measurement, electron-hole pairs are generated by photons of light. Typically, the excitation photon energy is in the range of 0.3^–6 eV, depending on the bandgap of the material under investigation. The electrons and holes recombine either by radiative or by non-radiative processes. First electrons and holes relax to the band edges or the states in quantum structures by rapid non-radiative scattering processes. By measuring the spectrum of this luminescence, valuable information from the band structure and the carrier states in quantum structures can be extracted.

A schematic diagram of the PL measument setup used in this work is shown in figure 7. The sample is located in a closed cycle helium cryostat, in which the temperature can be varied between 9 K and room temperature. Most of the PL measurements in this thesis were made at 9 K. An argon ion laser (wavelengths 488 nm, 514 nm) or a frequency doubled Nd:YVO laser (532 nm) is used for excitation. The absorption coefficient of GaAs at these wavelengths is of the order of 10⁵ cm⁻¹, which means that absorption and carrier generation occur mainly in the substrate below the quantum structure. The full width at half maximum of the intensity distribution of the focused laser beam on the sample surface is typically 100 μm.

Figure 7: Schematic illustration of the continuous wave photoluminescence setup.

The luminescence is collected and focused with two lenses into a 0.5 m monochromator. A liquid nitrogen cooled germanium pin-diode is used as a detector. The standard locks in -technique is used to improve the signal-noise ratio of the detection. A computer with an AD converter is used to control the monochromator and collect the measurement data.

4. APPLICATIONS OF PHOTOLUMINISENCE

Apart from the detection of light emission patterns, photoluminescence spectroscopy is of great significance in other fields of analysis, especially semiconductors.

Band gap determination

Band gap is the energy difference between states in the conduction and valence bands, of the radiative transition in semiconductors. The spectral distribution of PL from a semiconductor can be analyzed to nondestructively determine the electronic band gap. This provides a means to quantify the elemental com- Position of compound semiconductor and is a vitally important material parameter in quenching solar cell device efficiency.

Impurity levels and defect detection

Radiative transitions in semiconductors involve localized defect levels. The photoluminescence energy asso-ciated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration. The PL spectrum at low sample temperatures often reveals spectral peaks associated with impurities contained within the host material. Fourier transform photoluminescence microspectroscopy, which is of high sensitivity, provides the potential to identify extremely low concentrations of intentional and unintentional impurities that can strongly affect material quality and device performance.

Recombination mechanisms

The return to equilibrium, known as “recombination”, can involve both radiative and nonradiative processes. The quantity of PL emitted from a material is directly related to the relative amount of radiative and nonradiative recombination rates. Nonradiative rates are typically associated with impurities and the amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Thus, analysis of photoluminescence can qualitatively monitor changes in material quality as a function of growth and processing conditions and help understand the underlying physics of the recombination mechanism.

Surface structure and excited states

The widely used conventional methods such as XRD, IR and Raman spectroscopy are very often not sensitive enough for supported oxide catalysts with low metal oxide concentrations. Photoluminescence, however, is very sensitive to surface effects or adsorbed species of semiconductor particles and thus can be used as a probe of electron-hole surface processes.

5. SUMMARY

In this module you study Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence.

Suggested Reading

The luminescence of biological systems, Johnson, F.H., Amer. Assoc., Adv. Sci., Washington, D.C. (1955).
Fluorescence and phosphorescence analysts, Hercules, D.M., Editor, Wiley-Interscience Publishers, New York, London, Sydney (1965).
Photoluminescence of solutions, Parker, C.A., Elsevier Publishing Co., New York (1968).
Theory and interpretation of fluorescence and phosphorescence, Becker, R., Wiley-Interscience (1969).

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