Photoluminescence PDF

## Photoluminescence ### Photoluminescence:- - The phenomenon of temporary light absorption and subsequent light emission is called Photoluminescence. - Photoluminescence, which occurs by virtue of electromagnetic radiation falling on matter, may range from visible light through ultraviolet, X-ray...

## Photoluminescence ### Photoluminescence:- - The phenomenon of temporary light absorption and subsequent light emission is called 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. ### Process:- - There are three main processes that happen in photoluminescence: - Excitation - Relaxation - Emission - 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. - A diagram depicting the process is included. ### Mechanism:- - 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). - 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 the following equation (where $E_2$ is the higher energy level and $E_1$ the lower one): > $$ΔE = E_{photon} = E_2 - E_1 = hv $$ > $$v = (E_2-E_1)/h$$ > $$λ = hc/(E_2 – Ε_1)$$ - 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. ### Types of Photoluminescence:- #### Photoluminescence - Includes fluorescence and phosphorescence #### Fluorescence - Includes spontaneous emissions of electromagnetic radiation. - The 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 #### Phosphorescence - Includes 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 ### Photoluminescence spectroscopy:- #### Importance & facts- - 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. #### PL spectroscopy facts- - 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. ### Experimental Setup:- - Includes a labelled diagram of the experimental setup for Photoluminescence spectroscopy. ### Analyses of Samples Fingerprints Captured By PL Spectra:- - Features of PL spectra and what they reveal - Characteristics PL frequencies: Composition - Changes in Frequency of PL peaks: Stress/Strain State Symmetry - Polarization of PL peak: Orientation - Width of PL peak: Quality - Intensity of PL peak: Amount - One broad peak may be superposition of two or several peaks: De-convolution is needed ### 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. - A graph of PL vs Absorption Spectra is included ### Examples of PL Spectra - Fluorescence of different size particles of ZnS coated CdSe - Increasing size - Fluorescence emission wavelength - Size dependent fluorescence - Normalized PL intensity vs Wavelength of Si NPs - PL Intensity vs Wavelength of various Ge:Er:ZnO conditions ### Photoluminescence connection with nanomaterial:- - 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. - A Diagram of the photoluminescence connection with nanomaterial is included. ### 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.