Electron-Energy Loss Spectroscopy By S.NARAYANASWAMY 11/17/20161

Contents Introduction EELS Instrumentation EELS Spectrum Summary References Source of an electron Electron-Specimen Interaction Magnetic Prism Spectrometer Energy filters Zero loss peak (ZLP) Low loss region High loss region 11/20/20162

The Bohr model of an atom The radium r n of the orbit of quantum number n is To calculate ionization energy of lowest energy electron Physical Principles of Electron Microscopy by R.F.Egerton 11/20/20163

EELS instrumentation is incorporated into TEM When electron interacts with specimen, it loses energy in different ways. This loses energy contains information about the sample. So, It can be separated by magnetic prism and analyzed to find the details of their bonding/valence state, the nearest-neighbor atomic structure, their dielectric response, the free electron density, the band gap, and the specimen thickness. Introduction to EELS 11/20/20164

Instrumentation of EELS Spectrometer : To be used to record and measure the EELS spectrum. To collect the spectrum we need a recording device in the dispersion plane of the Spectrometer The energy-loss spectrum is recorded using either serial or parallel collection detector. Energy filters : To filter out electrons of specific energy The post-column Gatan Image Filter (GIF) The in-column filter Spectrometer and Energy Filters 11/20/20165

Source of Electrons Filament Tungsten/Lanthanum hexaboride In the thermoionic electron gun, electrons are emitted from heated filament and then accelerated towards anode. Strong electric field was created to extract electron from the filament Field emitting tip has strong curvature 11/20/20166

Serial Recording of the Energy-Loss Spectrum The serial recording system consist of a scintillator (emitting visible photon in the response to each incident electron ) and photo multiplier tube (PMT) which convert some of these photons into electrical pulses. This method is inefficient, requiring longer recording times to avoid excessive statistical noise at high energy loss. It is preferable for recording low-loss spectra 11/20/20167

Parallel recording of energy loss data A parallel-recording system utilizes a position-sensitive detector that is exposed to a broad range of energy selecting slit. Because there is no slit, the detective quantum efficiency (DQP)of the recording system is the same as that of detector. As a result spectrum can be recorded in shorter times and with less radiation dose than with serial acquisition for the same noise content. Earlier photographic film was used as parallel-recording medium. Now it was replaced by Silicon diode array such as photodiode array (PDA) or charge-coupled diode (CCD) array. It is mainly for the spectroscopy of ionization edges at high energy loss where electron intensity is low 11/20/20168

Electron energy-loss spectrometers (EELS) Latest PEELS (Enfina) uses a CCD detector instead of a photodiode array Fig : serial-collection 11/20/20169

The magnetic prism The electrons are selected by a variable entrance aperture The electrons travel down a ‘drift tube’ through the spectrometer and are deflected through >= 90 0 by the magnetic field. Electrons that have lost energy are deflected further than those suffering zero loss. A spectrum is thus formed in the dispersion plane, consisting of a distribution of electron intensity (I) versus energy loss (E). You can see that this process is closely analogous to the dispersion of white light by a glass prism shown in the inset. 11/20/201610

The Electron-Energy Loss Spectrum Fig: An EELS Spectrum EELS Spectrum Zero loss peak Low loss region (<50eV) Plasmon peak High loss region (>50eV) Ionization edges 11/20/201611

Low loss region (<50eV) The low-loss region up to an energy loss of 50 eV contains electrons which have interacted with the weakly bound, outer-shell electrons of the atoms. It can be used to determine dielectric constant, band gap and measure the local composition of materials. The cut-off energy for the low-loss spectrum is 50 eV. 11/20/201612

Plasmon Peak A transmitted electron produces an oscillation of electron density at an angular frequency Where, n e = density of atomic electron which are electrostatically coupled within solid: the conduction electrons in a meta and the valence electrons in a semiconductor or insulator. The plasmon wake produces a backward force on the electron, causing it to lose an average energy (E p ) which is in the range 3-30eV for most solids Even though the value of Ep is not element-specific, plasmon spectroscopy has been used to measure the local composition of binary alloys, based on the shift of Plasmon energy with composition The collective oscillation (resonance) of many outer-shell electrons is known as a plasmon excitation 11/20/201613

High-Energy Loss Spectra The high-loss portion of the spectrum above 50 eV contains information from inelastic interactions with the inner or core shells. These interactions provide direct elemental identification in a manner similar to XEDS and other information, such as bonding and atomic position. The ionization losses precisely because the process is characteristic of the atom involved and so the signal is a direct source of chemical information, just like the characteristic X-ray. 11/20/201614

Energy level of an atom 11/20/201615

The full range of possible edges in EELS The below link provides X-ray transition energies database 11/20/201616

Ionization edges How to identify ionization edges in the spectrum? There is a discrete increase in the slope of the spectrum. This is value is edge onset i.e Ec the critical ionization energy. Quantification is easy with K and L edges. Up to Z=13 (Al), use K edge because any L edges occur at very low energy. Above Z=13, can use either K or L edges. 11/20/201617

EELS spectrum Ni L edge shows two sharp peaks, which are L3 and L2 edges 11/20/201618

Quantitative Analysis In the spectrum, corresponding ionization edges showing the qualitative presence of of Ti, C and N. These can be identified as TiC and TiN. The Ti-nitride and Ti-Carbide 11/20/201619

Quantitative Analysis Assumptions The electrons contributing to the edge have only undergone a single ionization even, then we can obtain an expression for P k. Since, considering single scattering, the exponential term is unity. Approach to quantify K edge. But same approach can be used for all edges Where, I K is the K-edge intensity above the background, P K Probability of ionization I T Total transmitted intensity N is number of atoms per unita area of the specimen (of thickness t) that contribute to the K-edge We can measure the absolute number of atoms per unit area of the specimen by measuring the intensity above background in the K edge and dividing it by the total intensity in the spectrum multiplied by the ionization cross section The K-edge intensity above background, I K, is related to the probability of ionization, PK, and the total transmitted intensity, I T by 11/20/201620

Quantitative Analysis Earlier equations have to be modified due to following reasons To separate the edge intensity, you will need to fit, extrapolate, and then subtract a background model 11/20/201621

Sample thickness From a spectrum, the integral of the ZLP will give Io, while the integral of the entire spectrum will give It. The inelastic mean free path represents the mean distance between inelastic scattering events for these electrons. When you regard inelastic scattering as a random event, the probability of n-fold inelastic scattering follows a Poisson distribution. Plural scattering occurs when a significant fraction of incident electrons that pass through a sample are scattered inelastically more than once 11/20/201622

Sample thickness t/λ=−ln(I o /I t ) Generally log-ratio method used to assess the sample thickness of sample Where, I 0 is zero loss intensity I t is total transmitted intensity t/λ is the mean number of scattering events per incident electron. 11/20/201623

SUMMARY Introduction to electron-energy loss spectroscopy and it’s uses Instrumentation of EELS which comprises Source of an electron Electron-Specimen Interaction Magnetic Prism Spectrometer Energy filters EELS spectrum which comprises zero-loss peak, low-loss region and high-loss region have been studied. Determination of composition and sample thickness by quantitative techniques 11/20/201624

References Physical Principles of Electron Microscopy by R.F.Egerton R.F.Egerton and M.Malac,“EELS in the TEM”, Journals of Electron Microscopy and Related Phenomenon,143(2005), F Hofer, FP Schmidt, W Grogger and G Kothleitner “Fundamentals of electron energy-loss spectroscopy”. Transmission electron microscopy by williams and carter 11/20/201625

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