Photoluminescence Spectroscopy for studying Electron-Hole pair recombination in semiconductors for photoelectrochemical water splitting

Photoluminescence Spectroscopy for studying Electron-Hole pair recombination in semiconductors for photoelectrochemical water splitting

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  PHOTOLUMINESCENCE SPECTROSCOPY -- BY RUNJHUNDUTTA
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  PHOTOLUMINESCENCE Photoluminescence is thespontaneous emission of light from a material under optical excitation. This light can be collected and analyzed spectrally, spatially and also temporally. The term embraces both fluorescence and phosphorescence, which differ in the time after irradiation over which the luminescence occurs. Photoluminescence is excited by illumination of the device with light which has a photon energy above the band gap energy in semiconductors.
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  Principle of PhotoluminescenceSpectroscopy
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  SPECTROFLUOROMETER-PL  Basic Principle Photoluminescence spectroscopy is a contactless, versatile, nondestructive, powerful optical 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. 4
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  Photo excitation causeselectrons within the 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 non radiative 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. PL spectroscopy gives information only on the low lying energy levels of the investigated system.
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  Experimental Set Up Aspectrofluorometer is an analytical instrument used to measure and record the fluorescence of a sample. While recording the fluorescence, the excitation, emission both wavelength may be scanned. With additional accessories, variation of signal time, temperature, concentration, polarization, or other variables may be monitored. Fluorescence spectrometers use laser sources, which contains wavelength selectors, sample illumination, detectors and corrected spectra.
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  Block diagram ofFluorescence Spectrometer
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  Illuminator source • Thecontinuous light source is 150 W ozone free xenon arc lamp. • Light from the lamp is collected by a diamond turned elliptical mirror, and then focused on the entrance slit of the excitation monochromator. • The lamp housing is separated from the excitation monochromator by a quartz window. • This vents heat out of the instrument, and protects against the unlikely occurrence of lamp failure. • Resolution over the entire spectral range and minimize spherical aberrations diffraction.
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  Monochromators • It containstwo monochromators : Excitation monochromator and Emission monochromator. • They use all reflective optics to maintain high resolution over the entire spectral range, and minimize spherical aberrations and diffraction. Reflection Gratings • A grating disperses the incident light by means of its vertical grooves. • A spectrum is obtained by rotating the gratings contain 1200 grooves mm−1, and are blazed at nm (excitation) at 500 nm (emission). • Each grating is coated with MgF2 for protection against oxidation.
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  Slits • The entranceand exit ports of each monochromator have continuously adjustable slits. • The width of the slits on the excitation monochromator determines the band pass of light incident on the sample. • The emission monochromator’s slits control the intensity of the fluorescence signal recorder by the signal detector. • When setting slit width, the trade off is intensity of signal versus spectral resolution. • The wider the slits are, the more light falls on the sample and detector, but the resolution decreases. • The narrower slits are, the higher the resolution gets but at the expense of signal. Shutters • An excitation shutter is located just after the excitation monochromator’s exit slit. • The shutter protects sample from photo bleaching or photo degradation from prolonged exposure to the light source. • An emission shutter is placed just before the emission monochromator’s entrance and protects the detector from bright light.
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  Sample compartment • Thesample compartment accommodates various optional accessories, as well as fiber optic bundles to take the excitation beam to a remote sample and return the emission beam to the emission monochromator. Detectors • The signal detector is a photon counting detector. This detector is an R928P photomultiplier which sends the signal to a photon counting module. • The reference detector monitors the xenon lamp, in order to correct for wavelength and time dependent output of the lamp. This detector is a UV enhance silicon photodiode, which is just before the sample compartment. Computer Control • The entire control of the originates in PC with software and is transmitted through a serial link. • On start up, the system automatically calibrates and presents itself for new experiments or routines instantly called from memory.
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  In semiconductor systems,the most common radiative transition is between states in the conduction and valence bands, with the energy difference being known as the bandgap. During a PL spectroscopy experiment, excitation is provided by laser light with an energy much larger than the optical band gap. 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. 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.
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  The respective ratesof 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. Thus photoluminescence is a process of photon excitation followed by photon emission and important for determining band gap, purity, crystalline quality and impurity defect levels of semiconducting material. It also helps to understand the underlying physics of the recombination mechanism.
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  PL spectrum isquite different from absorption spectrum in the sense that 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. The value of wavelength at which the molecules absorbs energy can be used as the excitation wavelength which provide a more intense emission at a red shifted wavelength, with a value usually twice of the excitation wavelength.
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  Characteristics PL frequencies Changes inFrequency of PL peaks Polarization of PL peak Width of PLpeak Intensity of PL peak Composition Stress/Strain State Symmetry/ Orientation Quality Amount ANALYSES OF SAMPLES FINGERPRINTS Captured by PL Spectra One broad peak may be superposition of two or several peaks: De- convolution is needed  Number of peaks  Peak Intensities  Peak position  FWHM  Peak shape
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  Transitions & TimeScales, Energy Scale
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  DECAY PROCESSES  Internalconversion: • Movement of electron from one electronic state to another without emission of a photon e.g. S2 S1 lasts about 1012 sec  Predissociation internal conversion: • Electron relaxes into a state where energy of that state is high enough to rupture the bond.  Vibrational relaxation (1010-1011sec): • Energy loss associated with electron movement to lower vibrational state without photon emission.  Intersystem crossing: • Conversion from singlet state to a triplet state. e.g. S1 toT1  External conversion: • A nonradiative process in which energy of an excited state is given to another molecule (e.g. solvent or other solute molecules). • Related to the collisional frequency of excited species with other molecules in the solution. • Cooling the solution minimizes this effect.
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  In PL-excitation (PLE)measurements, the intensity is recorded as a function of excitation photon energy. Under a condition of fast intra-band relaxation, PLE is equivalent to linear absorption spectra.
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   Recombination mechanisms The return to equilibrium, also known as "recombination," can involve both radiative and nonradiative processes. The amount of PL emission and its dependence on the level of photo- excitation and temperature are directly related to the dominant recombination process.  Material quality  Nonradiative processes are associated with localized defect levels.  Material quality can be measured by quantifying the amount of radiative recombination.
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  PL spectra andtheir intensity dependencies can allow to:  Determine the band gap energy and/or the wavelength of maximum gain  Determine the composition of ternary or quaternary layers  Determine impurity levels (which can e.g. Lead to the emission of light with longer wavelengths than otherwise possible)  Investigate recombination mechanisms  Measure the purity and crystalline quality of semiconductors.  Determine the minority carrier lifetime of semiconductors like GaAs. {Time-resolved photoluminescence (TRPL)}  Determine the band gap, exciton life time,exciton energy, bi-exciton, etc. of semiconductor and other functional materials.  Determine the properties, e.g. Structure and concentration, of the Emitting species.
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  Defects activated photoluminescencein two dimensional semiconductors: Interplay between bound, charged and free excitons Tongay etal 2013
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  Figure a. PL spectrummeasured over the temperature range from 77 K to 300 K of a monolayer MoS2 after being annealed to 500uC. b. The PL at 77 K with different excitation laser power. Both a and b were taken in the presence of N2 gas (50 Torr). c. Integrated PL intensity of bound exciton (XB) and free exciton (X0) as a function of excitation laser power.
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  Figure a. PL spectrumof a monolayer MoS2 at 300 K in the presence of N2 or in vacuum before and after irradiation. b. The same taken at 77 K in vacuum. c. PL spectrum taken at 77 K in N2 on WSe2 and MoSe2 monolayers before and after the irradiation.
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  CONCLUSIONS FROM RESULTOF PL SPECTRA Since this PL peak appears after the irradiation and its intensity increases at higher doses, we attribute it to radiative recombination of bound excitons (XB), i.e Neutral excitons (X0) bound to defects. Peak appears relatively stronger and broader than in the irradiated samples, possibly due to a higher density of point defects and defect clusters created by annealing. Vacancies generation by particle irradiation is a series of highly non-equilibrium, random and isolated events, while thermal annealing is much slower and may facilitate formation of vacancy clusters with different configurations. These defect complexes with different clustering configurations may have different exciton binding energies, thus broadening the observed defect PL peak.
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   Intensity ofXB exhibits a sub-linear laser power dependence with a tendency to saturate at high excitation powers, whereas the free exciton intensity (X0) scales linearly without any sign of saturation .  As a result, the overall PL spectrum is mostly dominated by the defect peak, XB, at low excitation intensities, and the X0 line becomes observable only at high excitation intensities  Field of 2d semiconductors optical quality at room temperature (pl intensity and sharpness) cannot be used as criteria to assess the crystal quality of the monolayers. The defective monolayers yield even stronger PL intensity at room temperature. However, the PL spectrum at 77 K immediately tells the difference between the pristine and defective monolayers.  The occurring of the defect PL peak was instantaneously reversible when the chamber is purged with or pumped out of N2. This implies that the interaction between the defect sites and the N2 gas molecules is weak (physisorbed), but dictates the optical emission of the material.
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  Anion vacancies aspoint defects drastically modify the optical properties of monolayer tmds in such a way that: (1) The overall integrated PL intensity is enhanced, (2) A new, defect related peak is observed below the bandgap. These effects are prominent at low temperatures and in gas environments (such as N2). The new peak originates from bound excitons that are formed by localizing excitons at the defect sites. The overall enhancement in the PL intensity is attributed to an electronic effect of the defects: the gas molecules drain free electrons from the material via these defect sites, causing a transition of exciton population from charged to neutral (both free and bound) excitons
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  Rare earth dopedTiO2-CdS and TiO2-CdS composites with improvement of photocatalytic hydrogen evolution under visible light irradiation Peng_2012
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  Photoluminescence spectra ofvarious photocatalysts upon 420 nm excitation
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  CONCLUSION FROM RESULTOF PL SPECTRA  To reveal the effect of RE3 þ-doping on the separation of photogenerated electron hole pairs, the PL spectra of Pt/TiO2-CdS and Pt/RE/TiO2-CdS were examined.  The PL spectra of Pt/TiO2-CdS and Pt/RE/TiO2-CdS are similar and show a broad light emission signal. However, the PL emission intensity of Pt/RE/TiO2-CdS is much weaker than that of Pt/TiO2-CdS.  Their PL intensity follows in this order: Pt/La3þ/ TiO2 - CdSoPt/Eu3þ/TiO2-CdSoPt/Er3þ/TiO2-CdSoPt/ Gd3þ/TiO2- CdSoPt/TiO2-CdS. This is contrary to their photoactivity order.
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  TiO2 cement-based materials: Understandingoptical properties and electronic band structure of complex matrices Relinque etal 2016
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  Normalized PL emissionspectra of cement based samples and pure photocatalyst (P25)
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   The photon-excitednon-equilibrium carriers in semiconductor recombine mainly via two routes—radiative and non-radiative recombination.  Generally, the photoluminescence emissions on semiconductor materials are originated from the radiative recombination of photo-generated electrons and holes, leading to photoluminescence signals.  In the other photoluminescence process, the excited electrons firstly transfer from the conduction band to different sub-bands, e.g. surface oxygen vacancies or defects, via non-radiative transition, and subsequently transfer from the sub- bands to the valance band via radiative transition with the release of photoluminescence signals.  These defects promote the charge separation and restrain the electron–hole recombination which in turn increases the lifetime of the photo-induced charge carriers.  The corresponding photoluminescence signals can be observed in the visible region.  All of these results confirmed that the cement matrix disorder through the creation of defect levels resulted in modification of absorption characteristics.
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  TL Spectra results: TiO2-photocatalyst mainly consist of four emission bands, a strong violet emission centered at 419 nm (2.96 eV), at violet-blue with a maximum at 446 nm (2.78 eV), a blue-green band at 483 nm (2.57 eV), and a weak green band at 527 nm (2.35 eV).  The photoluminescence signals observed at 419 nm (2.96 eV) and 446 nm (2.78 eV) are due to band–band transitions for anatase and rutile TiO2crystal phases, respectively.  The two peaks at 483 nm (2.57 eV) and 527 nm (2.35 eV) of TiO2photocatalyst were attributed to the transition from the oxygen vacancies with two trapped electrons and one trapped electron to the valence band of TiO2, respectively.  A observed widening of the band at 457 nm (2.71 eV) on TiO2-photocatalyst is also ascribed to the radiative recombination of excitons of the shallow traps identified with oxygen vacancies and Ti3+.
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