The document discusses photoluminescence, which is the emission of light from a material when it absorbs photons. There are three main steps in the photoluminescence process: excitation, relaxation, and emission. Excitation occurs when photons are absorbed and electrons are lifted to a higher energy state. Relaxation follows as electrons lose energy non-radiatively. Emission is the radiative decay of electrons as they return to the ground state, emitting photons of lower energy than those absorbed. The two main types of photoluminescence are fluorescence, which is a rapid emission, and phosphorescence, which is a slower emission.

1. Photo Luminescence
1
Presented By- Basant Kumar Sharma
2017PPH5335
Physics Department
MNIT Jaipur

2. INDEX:-
• What is luminescence & classification
• What is photoluminescence
• Process of photoluminescence
• it’s type
• Photoluminescence spectroscopy
• It’s application
2

3. LUMINESCENCE:-
 Greek word phosphor (light bearer) is usually used to describe
luminescent nature.
 Luminescence is spontaneous emission of light by a substance not
resulting from heat; it is thus a form of cold-body radiation. It can be
caused by chemical reactions, electrical energy, subatomic motions
or stress on a crystal.
 Luminescence is emission of light by certain materials when they are
relatively cool. It is in contrast to light emitted from incandescent
bodies, such as burning wood or coal, molten iron, and wire heated by
an electric current.
 This distinguishes luminescence from incandescence, which is light
emitted by a substance as a result of heating.
3

4. This chart shows the interrelationships between various forms of luminescence, particularly photoluminescence, which includes
fluorescence and phosphorescence. A few other types of luminescence relevant to gemology are shown as well (note that other
types of luminescence such as chemicaluminescence and bioluminescence are not included in this illustration). When the
scientific definition (in black) is distinctly different from the gemological usage (in red), both definitions are shown for clarity.
TYPES OF LUMINESCENCE:-
4
LUMINESCENCE
Emission of photons (UV, visible light,
infrared) by a material...
PHOTOLUMINESCENCE
…when activated by the absorption of UV
radiation, visible light, or infrared
…when activated by laser, typically at
cryogenic temperatures
TRIBOLUMINESCENCE
…when activated by friction
THERMOLUMINESCENCE
…when activated by heating
ELECTROLUMINESCENCE
…when activated by electric
current or field
CATHODOLUMINESCENCE
…when activated by electrons
(cathode rays)
FLUORESCENCE
Photoluminescence lasting less than 10 nanoseconds
The emission of visible light while a UV source is
turned on
PHOSPHORESCENCE
Photoluminescence lasting more than 10 nanoseconds
The emission of visible light after a UV source is
turned off

5. PHOTOLUMINESCENCE:-
 Photoluminescence, which occurs by virtue of electromagnetic
radiation falling on matter, may range from visible light through ultraviolet,
X-ray, and gamma radiation.
 Photoluminescence is a process in which a molecule absorbs a photon in
the visible region, exciting one of its electrons to a higher electronic
excited state, and then radiates a photon as the electron returns to a lower
energy state
 The phenomenon of temporary light absorption and subsequent light
emission is called Photoluminescence.
5

6. There are three main process happen in PL –
• Excitation
• Relaxation
• emission
PROCESS:-
6

7. The photo-excitation causes the material to jump to a higher electronic
state, and will then release energy, (photons) as it relaxes and returns to
back to a lower energy level. The emission of light or luminescence
through this process is photoluminescence, PL.
7

8.  Photoluminescence is a process in which a molecule absorbs a photon in the visible
region, exciting one of its electrons to a higher electronic excited state, and then radiates a
photon as the electron returns to a lower energy state (because excited states are
unstable). If the molecule undergoes internal energy redistribution after the initial photon
absorption, the radiated photon is of longer wavelength (i.e., lower energy) than the
absorbed photon.
emission
Excitation
Relaxation
emission
8

9. Mechanism:-
Electronically Excited State
• Atoms of different elements have a different number of electrons distributed into
several shells and orbitals. Electrons are a type of elementary particle. Electronic
transitions are responsible for luminescence . When the system absorbs energy,
electrons are excited and are lifted into a higher energetic state. Before excitation,
in the ground state, some of the electrons are in the so-called HOMO (Highest
Occupied Molecular Orbital). After they reach an excited state, they are in the
LUMO (Lowest Unoccupied Molecular Orbital) (see Fig). How this works exactly
will be explained using photoluminescence as a specific example.
• Different energetic states of an atom or molecule are known as "energy levels".
Depending on the molecule and atom, the electrons can only occupy discrete
energy levels since the energy is quantized, which means, energy can only be
absorbed and emitted in certain amounts . The difference between two levels can
be calculated with equation (where E2 is the higher energy level and E1 the lower
one).
ΔE = Ephoton ⇔ E2 – E1 = hν
ν = (E2 – E1)/h
λ = hc/(E2 – E1)
Deactivation of Electronically Excited States-
Such electronically excited states are unstable. Electrons drop back to their ground states. At the same time, the excitation
energy is released again. One distinguishes between radiative and non-radiative decay processes. Most of the time, the
decay is non-radiative, for example through vibrational relaxation, quenching with surrounding molecules, or internal
conversion (IC) .
Sometimes, a radiative decay can occur in form of fluorescence and phosphorescence. The energy is emitted as
electromagnetic radiation or photons. The emitted light has a longer wavelength and a lower energy than the absorbed light
because a part of the energy has already been released in a non-radiative decay process . This is the reason that an emission
in the visible spectrum can be achieved by excitation with non-visible UV-radiation. This shift towards a longer wavelength is
called Stokes shift .
9
Mechanism:-
Electronically Excited State
• Atoms of different elements have a different number of electrons distributed into
several shells and orbitals. Electrons are a type of elementary particle. Electronic
transitions are responsible for luminescence . When the system absorbs energy,
electrons are excited and are lifted into a higher energetic state. Before excitation,
in the ground state, some of the electrons are in the so-called HOMO (Highest
Occupied Molecular Orbital). After they reach an excited state, they are in the
LUMO (Lowest Unoccupied Molecular Orbital) (see Fig). How this works exactly
will be explained using photoluminescence as a specific example.
• Different energetic states of an atom or molecule are known as "energy levels".
Depending on the molecule and atom, the electrons can only occupy discrete
energy levels since the energy is quantized, which means, energy can only be
absorbed and emitted in certain amounts . The difference between two levels can
be calculated with equation (where E2 is the higher energy level and E1 the lower
one).
ΔE = Ephoton ⇔ E2 – E1 = hν
ν = (E2 – E1)/h
λ = hc/(E2 – E1)
Deactivation of Electronically Excited States-
Such electronically excited states are unstable. Electrons drop back to their ground states. At the same time, the excitation
energy is released again. One distinguishes between radiative and non-radiative decay processes. Most of the time, the
decay is non-radiative, for example through vibrational relaxation, quenching with surrounding molecules, or internal
conversion (IC) .
Sometimes, a radiative decay can occur in form of fluorescence and phosphorescence. The energy is emitted as
electromagnetic radiation or photons. The emitted light has a longer wavelength and a lower energy than the absorbed light
because a part of the energy has already been released in a non-radiative decay process . This is the reason that an emission
in the visible spectrum can be achieved by excitation with non-visible UV-radiation. This shift towards a longer wavelength is
called Stokes shift .
9

10. TYPES OF PL:-
PL
Fluorescence Phosphorescence
 spontaneous emissions of electromagnetic
radiation.
 glow of fluorescence stops right after the
source of excitatory radiation is switched off.
 Light production by the absorption of UV light
resulting in immediate emission of visible
light.
 Example-fluorescent dyes in detergent,
highlighter pens, fluorescent lighting
 ground state to singlet state and back.
 spontaneous emissions of electromagnetic
radiation.
 an afterglow with durations of fractions of a
second up to hours can occur.
 Light production by the absorption of UV light
resulting in the emission of visible light over
an extended period of time.
 Example-Objects coated with phosphors
 ground state to triplet state and back.
10

11. FLUORESCENCE:-
Jablonski diagram for fluorescence
11
 Fluorescent materials produce
visible or invisible light as a result of
incident light of a shorter
wavelength (i.e. X-rays, UV-rays,
etc.).

12. • In the Jablonski diagram for fluorescence (see Fig), the singlet spin state S0 is the ground state of
the electrons, and S1 and S2 are singlet excited states (the states are only used as an example in
this text and do not necessarily apply to certain atoms, molecules, etc.). Within those states, there
are several energy levels. The higher the level is, the more energy an electron possesses when
being in that level. In the case of singlet states, the electrons have antiparallel spins.
• The electrons are lifted from the ground state S0, for example, to an energy level of the second
excited state S2, when excited by electromagnetic radiation. After excitation stops, the electrons
only stay in that excited state for a short period of time (ca. 10−15
s) and then immediately start
falling back down into the ground state . In doing so, energy initially can be released to the
surroundings by vibrational relaxation. That means thermal energy is released by the motion of
the atom or molecule until the lowest level of the second excited state is reached.
•
The bigger gap between the second and first excited state is overcome by internal conversion.
That describes an electronic transition between two states while the spin of electrons is
maintained. Now, the electrons can relax further due to more vibrational relaxation until they
reach the lowest energy level of the S1 state.
•
Theoretically, the electrons could relax even further in a non-radiative way until they eventually
reach the ground state again. However, it can be the case that the last amount of energy is too
large to be released to the surroundings because the surrounding molecules cannot absorb this
much energy. Then, fluorescence occurs, which leads to an emission of photons possessing a
certain wavelength. The emission lasts only until the electrons are back in the ground state.
Since during all those transitions the electron spin is kept the same, they are described as spin-
allowed.
12

13. 13
Material Shows Fluorescence:-
• Vitamin B2 .
• Tonic water
• Highlighter
• pyranine.
• Banknotes,
• postage stamps
• credit cards
• Petroleum
• Uranium glass
• Rock salt.
• Fungus
• Athlete's Foot
• Canola oil.
• Some postage stamps

14. PHOSPHORESCENCE:-
Jablonski diagram for fluorescence
14

15. • For phosphorescence, things are a bit different (see Fig). There are again an S0 ground state
and the two excited states, S1 and S2. Additionally, there is an excited triplet T1 state which lies
energetically between the S0 and S1 state. The electrons again have antiparallel spins in the
ground state.
• Excitation happens in the same way as in fluorescence, namely through electromagnetic
radiation. The release of energy through vibrational relaxation and internal conversion while
maintaining the same spin is the same here, as well, but only until the S1 state is reached.
• Alongside the singlet states, a triplet state exists and so-called intersystem crossing (ISC) can
occur since the T1 state is energetically more favorable than the S1 state. This crossing, like
internal conversion, is an electronic transition between two excited states. But contrary to
internal conversion, ISC is associated with a spin reversal from singlet to triplet. Electrons in
the triplet state have parallel spins, which is noted as (↑↑) . This ISC process is described as
"spin-forbidden". It is not completely impossible – due to a phenomenon called "spin-orbit
coupling" – however, it is rather unlikely .
•
In the T1 state, non-radiative decay is possible as well. However, a transition between the
lowest energy level of the triplet state and the S0 state is not readily possible, because that
transition is spin-forbidden, too. Still, it can happen anyway with a small possibility. It causes
a rather weak emission of photons because the electron spin has to be reversed again. The
energy is trapped in this state for a while and can only be released slowly . After all energy has
been released, the electrons are back in the ground state .
15

16. Typically, Aromatic Molecules Shows phasphoroscence
–Quinine,
–Fluorescein,
–Rhodamine B,
–POPOP,
–Coumarin,
–Acridine Orange,
–Some Minerals
–Materials in low dimension
–Glass with Rare Earth Ions
16

17. 17

18. Photoluminescence spectroscopy:-
• The photoluminescence (PL) is a nondestructive spectroscopic technique
commonly used for the study of intrinsic and extrinsic properties of both bulk
semiconductors and nanostructures.
• Photoluminescence spectroscopy, often referred to as PL, is when light energy,
or photons, stimulate the emission of a photon from any matter.
• Photoluminescence spectroscopy is a contactless, versatile, nondestructive,
powerful optical method of probing the electronic structure of materials.
• The intensity and spectral content of this photoluminescence is a direct measure
of various important material properties.
• PL spectroscopy gives information only on the low lying energy levels of the
investigated system.
• During a PL spectroscopy experiment, excitation is provided by laser light with
an energy much larger than the optical band gap.
18
• Importance & facts-

19. • The photo excited carriers consist of electrons and holes, which relax toward
their respective band edges and recombine by emitting light at the energy of the
band gap.
• The quantity of the emitted light is related to the relative contribution of the
radiative process.
• Radiative transitions in semiconductors may also involve localized defects or
impurity levels therefore the analysis of the PL spectrum leads to the
identification of specific defects or impurities, and the magnitude of the PL
signal allows determining their concentration.
• The respective rates of radiative and nonradiative recombination can be
estimated from a careful analysis of the temperature variation of the PL
intensity and PL decay time.
• At higher temperatures nonradiative recombination channels are activated and
the PL intensity decreases exponentially.
19

20. Experimental Setup:-
20

21. 21
Spectrofluorometer Schematic

22. 22

23. 23
Spectrofluorometer - two monochromators for excitation or Fluorescence scanning:-

24. 24
•Illumination source
–Broadband (Xe lamp)
–Monochromatic (LED, laser)
•Light delivery to sample
–Lenses/mirrors
–Optical fibers
•Wavelength separation (potentially for both excitation and emission)
–Monochromator
–Spectrograph
•Detector
–PMT
–CCD camera
Major Components For Fluorescence Instrument

25. Characteristics PL
frequencies
Changes in Frequency
of PL peaks
Polarization of PL peak
Width of PL peak
Intensity of PL peak
Composition
Stress/Strain State
Symmetry/
Orientation
Quality
Amount
One broad peak may
be superposition of
two or several peaks:
De-convolution is
needed
Analyses Of Samples Fingerprints Captured By PL Spectra :-
25

26. Difference b/w PL spectrum and absorption spectrum :-
 absorption spectrum measures transitions from the ground state to excited
state, while photoluminescence deals with transitions from the excited state to
the ground state.
 The period between absorption and emission is typically extremely short.
 An excitation spectrum is a graph of emission intensity versus excitation
wavelength which looks very much like an absorption spectrum.
26

27. 27
Types of Photoluminescence Spectroscopy -
1. PL Spectroscopy
 Fixed frequency laser
 Measures spectrum by scanning
spectrometer
2. PL Excitation Spectroscopy (PLE)
 Detect at peak emission by varying frequency
 Effectively measures absorption
3. Time-resolved PL Spectroscopy
 Short pulse laser + fast detector
 Measures lifetimes and relaxation processes

28. 28
Examples Of PL Spectra
Si NPs
NPs size dependent
fluorescence

29. 29
Applications of PL Spectroscopy
 PL spectroscopy is not considered a major structural or
qualitative analysis tool, because molecules with subtle
structural differences often have similar fluorescence spectra
 Used to study chemical equilibrium and kinetics
 Fluorescence tags/markers
 Important for various organic-inorganic complexes
 Sensitivity to local electrical environment polarity,
hydrophobicity
 Track (bio-)chemical reactions
 Measure local friction (micro-viscosity)
 Track solvation dynamics
 Measure distances using molecular rulers: fluorescence
resonance energy transfer (FRET)
 Band gap of semiconductors
 Nanomaterials characterization

30. Photoluminescence connection with nanomaterial:-
30
 The photoluminescence of nc-Si nanocrystals (5 nm in size) have been investigated. The shape
and spectral position of maxima in the photoluminescence and IR transmission spectra are
theoretically described. It is shown that nc-Si particles consist of a Si core and a SiO2 shell.
The existence of surface Si-O and Si-H states in Si nanocrystals enhances photoluminescence.

Figure
(a,b) High resolution
transmission electron
microscope (HRTEM) cross-
section images of
different proportional scale for
the 600 C annealed Ge:Er:ZnO
film;
(c) A diffraction pattern from the
film;
(d) Size distribution of nc-Ge in
600 C annealed Ge:Er:ZnO film;
(e) The relation of emission
energy versus radius R of nc-
Ge.

31. 31
CONCLUSIONS:-
 Luminescence spectroscopy provides complex information about the
defect structure of solid
- importance of spatially resolved spectroscopy
- information on electronic structures
 There is a close relationship between specific conditions of mineral
formation or alteration, the defect structure and the luminescence
properties (“typomorphism”)
 Useful for determining semiconductor band gap, excitation energy
etc.
 For the interpretation of luminescence spectra it is necessary to
consider several analytical and crystallographic factors, which
influence the luminescence signal

32. 32