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Surface Analysis Microscopy and Spectroscopy

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1 Surface Analysis Microscopy and Spectroscopy
Fundamentals of Electrochemistry CHEM*7234 CHEM 720 Lecture 11

2 Areas of Application Microscopy Spectroscopy ELECTROCHEMISTRY ANALYSIS
SYNTHESIS Spectroscopy Microscopy Electrochemistry is finding significant application in all areas of science. One can perhaps divide application into two areas: that of analysis and that of synthesis. Electrochemistry is widely applied as an analytical tool; its use as a sensor is widely implemented. The CO sensor in your home is electrochemically based. The other application for electrochemistry is that of synthesis or materials fabrication. The largest single usage of electricity in North America is production of aluminum. Many other metals are refined electrochemically, including Cu and Ni. More exotic materials can be formed or processed electrochemically. In both of these areas, it proves helpful – and often crucial – to be able to probe the system to provide additional, complementary information in order to completely characterize an analyte. Often, the products of electrochemical syntheses need to be analyzed for other characteristics to fully understand the process and guide attempts to control it. Two important types of analyses are Spectroscopy and Microscopy. They help identify “WHAT” is present in the experiment and “HOW” is it arranged on the surface. This information is important for guiding our analysis of reaction mechanisms and our assessment of the success of a given process.

3 Current Microscopy Example
“Advanced Plating Chemistry for 65 nm Copper Interconnects” Semiconductor International May 2003 Mounding when filling trenches with electrochem deposited Cu using two component electrolyte system. As the semiconductor industry migrates towards Cu conductors (away from Al) and as the device dimensions shrink towards 65 nm, the Cu plating technology requires updating. The current technology involves an electrolyte with two components. One is a polymer which coats the surface and has an autocatalytic component. On flat surfaces, the autocatalyst is washed away, but inside trenches, the autocatalyst builds up in concentration, increasing the rate of deposition inside the trench so that it quickly fills. The problem then is that when the trench completes filling, there is still a concentration of catalyst and additional deposition occurs, leading to mounding on top of the trenches. The company developed an additional leveling component which did not enter the trenches and hence the catalytic acceleration continued inside the trench. But once the trench was filled, the additional leveling agent inhibited the catalytic process and produced uniform surface coverage. These FIB images demonstrate the “before” and “after” conditions of the surface. No mounding with three component system. Utilized Fast Ion Beam (FIB) Microscope for these images.

4 Current Spectroscopy Example
“Structural Studies in Lithium Insertion into SnO-B2O3 Glasses and Their Applications for All-Solid-State Batteries”, Katada et al., JES 150, A582 (2003) XRD NMR Mössbauer

5 Ex Situ vs. In Situ Ex Situ Experiment: An experiment performed on a sample after it has been removed from the location wherein it was formed. • wider range of experimental techniques available. One important way of categorizing different experiments is according to whether or not they can be performed in situ or whether an ex situ experiment is required. Any time we remove a sample from the environment where it has been found or formed, we risk altering its properties. Ones preference would be to study all samples exactly as they are found. That is often impossible as some analytical techniques require relocating the sample into a sample chamber and many are inherently destructive. Current efforts in analytical chemistry involve the redesign of experiments which have been necessarily ex situ but can be adapted to become in situ. In Situ Experiment: An experiment performed on a sample while it is still located in its native environment. • less risk of altering the sample’s true properties.

6 Challenges Analyzing Electrochemical Samples Ex Situ
Remove sample ? • loss of electrochemical control • loss of solvent, ion atmosphere • risk contamination, oxidation Electrochemical systems have some specific risks associated with their analysis outside of the electrochemical environment. First, the emersion of the sample from the electrolyte means that we lose the ability to control the potential at the surface of the sample. When we take the sample out, how do we know what the potential does? How can we be assured that the sample’s redox condition is unaffected by the emersion process? Also, it is obvious that the sample will lose the solvent molecules which blanket the sample, as well as all ions which are near the surface. How will the loss of these species affect the nature of the surface? And if we have tried to carefully produce a surface covered by some reduced species, how do we know that exposure to atmospheric oxygen will not suddenly oxidize them again? And what about simple contamination by atmospheric adsorption? All of these questions demand careful answers in the case of an ex situ analysis.

7 Challenges Analyzing Electrochemical Samples In Situ
photons • the electrolyte solution can strongly absorb the various probe particles which might be used to perform different analyses. • cell design needs to account for refraction electrons ions The problem with analyzing a sample in situ is that the solvent and the other ions can interfere with many analytical experiments. Many spectroscopic experiments employ electrons or ions as probes; these would not be able to traverse even the thinnest solvent layer to interact with the surface. While photons are able to pass through many solvents, IR radiation or UV radiation can be strongly absorbed by apparently transparent solutions. Aqueous solutions absorb these wavelengths. Other solutes, the inert electrolyte and other counter ions may also be a problem. So while some photon based spectroscopies are conceivable, it is still important to consider increased absorption problems for which we must account. Also, in designing cells for such analyses, we need to remember that we have to worry about gravity (liquid solutions will pour out) and we have to account for refraction effects. But this is just part of good experimental design. The real kicker is to come up with ways to minimize the absorptive interferences of the solvent.

8 Microscopy • What is the structure of the surface of the sample?
• Resolution: Lateral, Vertical • Contrast mechanism • Dynamic Range • Ex situ or In situ 1 cm 1 mm 1 µm 1 nm 1 Å Atoms Molecules Viruses Computer Circuits Red Blood Cells Hair Microscopy can helps to describe the structure of the surface of a sample. This information is often critical for understanding the mechanisms of forming the sample or the observed performance of the material. Important questions to ask regarding a microscopic technique are regarding its ultimate resolution; what is the smallest feature size that it can resolve? Resolution is usually different when comparing the determination of lateral resolution (resolution in the plane of the sample) and vertical resolution. It is important to distinguish the capabilities of each technique in this regard. The nature of the contrast mechanism is important; what is about a sample gives rise to a change in signal for the technique? Knowledge of this helps to plan an experiment and to select the right experiment for a particular sample. Next, one needs to know the dynamic range of the instrument. Techniques that have a high resolution are often not able to image large objects or feature sizes that vary over several orders of magnitude. Finally we need to know whether or not the technique can be applied in situ to our sample or whether an ex situ examination will be required.

9 Lateral Resolution AFM OM SAM SEM IM STM
Here are some of the techniques we will examine and a comparison of their lateral resolution capabilities. AFM OM SAM 1 cm 1 mm 1 µm 1 nm 1 Å Lateral Resolution. Optical Microscopy is usually what one thinks of when studying microscopy. It is prevalent and widely available. Its resolution is limited by the diffraction of light, which is limited to about 1/2 a wavelength of the light being diffracted through the optical lensing elements of the microscope. While this is several hundred nanometers, in practice it is challenging to get good optical resolution down to the 1 µm range. The next most widely available technique would be that of Scanning Electron Microscopy. Here we are limited by the diffraction of electron waves. Since these are in the Ångstrom range, there are instruments with resolution of a few nanometers quite widely deployed. Recent reports have described a new instrument able to observe atoms (a few Ångstroms in size) but this is not available for use yet. Each instrument achieves high resolution by focusing the electron beam as tightly as possible. The minimum spot size achievable by focusing the electron beam defines the resolution of a particular instrument. Scanning Auger Microscopy (SAM) is closely related to SEM, except that the scattering in the sample leads to a “smearing” of the incident beam, and broadens the effective spot size. Its resolution general lags behind that for SEM. Probe Microscope techniques like STM and AFM have resolution that is controlled by the sharpness of the probe tip. This can be in the range of a few nanometers, but the differing contrast mechanisms mean that STM can easily achieve atomic resolution while atomic resolution for AFM is difficult to achieve. Interference Microscopy is a newer technique that employs optical wavelength light. Its lateral resolution is controlled exactly like that of optical microscopy and it has the same limitations. (Its virtues come elsewhere.) SEM IM STM

10 Vertical Resolution AFM SAM OM SEM IM STM
Here are the same techniques, comparing their vertical resolution. AFM SAM OM 1 cm 1 mm 1 µm 1 nm 1 Å Vertical resolution is very different. OM, SEM, and SAM do not give much vertical information at all. While it is rather straightforward to derive quantitative lateral data, these techniques do not provide for quantitation in the vertical direction. It is possible to tilt samples and image their sides, then height information can be obtained. However, in a specific image, the vertical information is very poor. Hence, these techniques have much better lateral resolution than vertical resolution. By contrast, probe microscopies and the IM have much better vertical resolution than lateral resolution. The interference microscope, even though it is an optical technique, has a resolution limit approaching 1 nm. The AFM and the STM have vertical resolutions in the sub-atomic range of tens of picometers. The best instruments can resolve down to a few hundred femtometers. SEM IM STM

11 Dynamic Range OM AFM STM IM SEM SAM
Not only do we need image small things, but large things too. What is the largest field of view that the instrument can provide? What is the range of features, largest to smallest, that can be observed? OM AFM STM The resolution limit defines the lower limit of the dynamic range of these instruments. The upper limit is controlled by how large is the field of view that can be imaged at once. OM is able to image from macroscopic features down to its resolution limit. The scanning electron techniques generally have a maximum field of view of a few mm. The probe microscopes are quite limited in their maximum field of view. Also, the limitations are different for the lateral and vertical directions. Lateral limits range from a few µm to over 100 µm. By contrast, the range in the vertical direction is limited to around a few µm. These limits are often determining factors in assessing the applicability of a probe microscope technique for a particular application. The IM has a very wide range. It is able to image features varying in vertical displacement of over a mm. The lateral range is set by its maximum field of view which is around 5 mm. 1 cm 1 mm 1 µm 1 nm 1 Å IM SEM SAM

12 Diffraction vs. Scanning
Two approaches to image formation Diffraction: Incident wave scatters from surface features, interfering with itself and forming a diffraction pattern. When diffracted wave is refocused, it produces an image of the surface. •Entire image formed simultaneously •Resolution limited by wavelength Scanning: Incident wave focused to a small point and rastered across surface. Signal is acquired from each point on surface. •Image formed sequentially •Resolution determined by spot size There are two principal methods for forming microscopic images. When a particle wave is incident on a surface, the wave scatters from the various features. The wave interferes with itself as it scatters from different points on the object. This interference patterns is called a diffraction pattern and it faithfully encodes all of the information about the structure. Some non-microscopic techniques image the diffraction pattern and extract structural information from it directly. It does not, however, present an image of the surface. Instead, the diffraction pattern can be refocused and an image of the surface results. The image is formed directly in this technique but the resolution is limited by the wavelength of the particles involved. A competing procedure takes the probe particle beam and focuses it to a fine point. The response of the surface to this probe beam is recorded and is scaled to provide a signal corresponding to that point on the surface. This beam is then rastered across the surface, the surface response being recorded at each point. The sequentially determined responses make up an image of the surface. The resolution in this case is limited by the size of the focused spot. This is dependent upon the wavelength of the particles also, but not in the same way as the diffraction process.

13 Vibration Isolation Buildings vibrate (motors, air conditioners, walking, vehicles). Resonances between 1 and 100 Hz. Amplitudes in micron range. • build microscope rigid • couple to building loosely • provide multiple stages with alternate rigid/loose coupling • shield acoustically for very precise measurements Vibration is a fundamental problem of all microscopical experiments. It will blur all images since the excitation, detection, and sample regions can move with respect to each other during the course of the experiment. Most buildings have vibrational resonances in the 1 to 100 Hz range and amplitudes in the micron range. The effect of vibrations is mitigated by constructing the microscope on stages which are connected to each other alternately as rigid or loose connections. The rigid connections permit only the transmission of high frequency vibrations while the loose connections permit only low frequency vibrations. It is somewhat like having alternately high pass and low pass filters. Often this is achieved by building the microscope to be as small and rigid as possible, and then mounting the entire system on pneumatically supported legs. Sometimes an additional stage using soft springs is included. While these procedures can reduce mechanical vibration, acoustical vibrations can also couple to the system and circumvent these stages. Including a surrounding of acoustic absorbing material is often necessary for the best work.

14 Optical Microscopy • a diffraction experiment • basic lens components
• coarse/fine focus • Mon/Bin/Tri ocular schemes • working distance • adjust interpupillary distance • quantitation with reticle • image recording The most obvious tool for microscopy is the optical microscope. It is quite easy to study a sample ex situ. Refraction at the air-solution interface would complicate an in situ measurement but it would not be impossible. A couple of excellent sites to learn more about optical microscopy (WAY more than you need for this class) are these sites below. The first is very comprehensive and seems to cover the most modern techniques. There are a lot of very useful java tutorials throughout the site. The second is a down-to-earth look at what you should look for in a small student microscope before purchasing. We want to touch on the basic lens components of a microscope and make you aware of the more fundamental features. Be aware of the presence of both a fine and a coarse focus adjustment, and the fact that there are monocular, binocular, and trinocular microscopes available. Microscopes can be fabricated with either short or long working distances - the distance from the end of the objective lens to the sample. With binocular instruments, the adjustment of the eye piece focus and the interpupillary distance is important for observing a good image comfortably. Quantitation is achieved with a reticle etch ed into one of the eye pieces. Images can be recorded on film or electronically to a CCD camera. A good web site for a brief introduction to optical microscopes can be found below.

15 Optical Microscopy continued
Select the correct combination of lenses for your task. I think that physical and analytical chemists short change the potential value of optical microscopy. Some modern instruments and techniques can probably provide some valuable and unique data regarding their systems. Objective lenses are designed to correct for aberrations: Achromats are the lowest level of correction and Apochromats are the highest and most sophisticated. Eyepieces are important for comfort and can have a reticle (graticle) etched inside to impose a scale on an image for measurement purposes.

16 Optical Microscopy Resolution
• Rayleigh equation d = 0.61 (l / N.A.) d is distance between objects that can still be distinguished l is wavelength of light N.A. is numerical aperture of lens = n sin(Qvertex) The Rayleigh equation is the most cited equation for describing the resolving power of an optical system which depends upon diffraction effects. Here d is the distance that two objects must be separated by in order to still be distinguishable. The wavelength of light that is irradiating the object is involved and the numerical aperture is unique to the lens involved. The numerical aperture is the sin of the vertex angle of the most divergent rays that can be focused by the lens, multiplied by the index of refraction of the medium in which the acceptance cone is located. Numerical aperture ranges from 0.1 to <1 for in air use. Oils with large indices of refraction can be used to increase the N.A. of a system which improves resolution. Theoretical resolutions are clearly still in the range of a couple of hundred nanometers. Q

17 Scanning Electron Microscopy
Electron Gun Secondary Electron Detector SEM requires a vacuum system so that the electrons can make the trip. A focused electron beam irradiates a sample. Secondary electrons are ejected from the sample with a flux amplitude that depends upon the nature of the material and the angle of the feature’s surface from the incident beam. An electron multiplier is used to detect and amplify this flux of electrons. The incident electron beam is rastered across the sample and secondary electron intensity as a function of beam position is used to create the Sem image. Vacuum Chamber

18 SEM Focusing Column Steering Quadrupole 1 Thermal Field Emitter Lens 1
2 Lens 2 Sample Extractor Here are the principal features of the typical SEM column. The TFE can be as high as V. Beam Acceptance Aperture Deflection Octupole Beam Blanking Plates Suppressor Assembly

19 SEM Experiment Trochodiscus longispinus in OM and SEM. Note improved depth of field and resolving capability of the SEM experiment.

20 Electron Reemission Elastically scattered SEM Backscattered e–
Inelastically scattered Secondary electron emission Elastically scattered electrons have the same energy as the incident electron beam. They can be detected by introducing a field to repel all slower electrons. These electrons are used in diffraction imaging experiments, such as LEED. The backscattered electrons have an energy of about 80% of the incident beam. They are emitted more or less specularly. These can be used to form SEM images. The inelastiically scattered electrons have interacted strongly with the substrate electrons. They have lost all information of their initial direction and phase. These include electrons which are emitted from the Auger, photoelectron, and e-h pair recombination processes. These mainly have spectroscopic value, but can be used for imaging also, but not within SEM. The secondary electrons arise because the incident beam locally charges the substrate with excess electrons. The substrate sheds this excess charge by emitting low energy electrons, more or less isotropically from the surface. These electrons are commonly used to form an SEM image. Relative Intensity Fraction of Incident Beam Energy

21 BSE vs. 2° Detection Both can be used, different information, different detection scheme. BSE Specular reflection Higher energy Encode some chemical information 2° Electrons Isotropic emission Very low energy Better structural contrast SEM electron detectors can be oriented and operated differently to preferentially detect either secondary or backscattered electrons.

22 Excitation Depth Profile
Incident electron beam penetrates mm into sample. Different emission mechanisms arise from different depths. BSE X-ray SED CL Cathodoluminescence, which arises from the recombination of electron-hole pairs generated by the incident beam, consists of visible photons. These can only escape the sample when emitted very close to the surface. Hence, the CL detector is very surface specific. The secondary electrons are a a very low energy; a gentle bias on the face of the SED will attract them into it. They come from the near surface region, perhaps 10’s of nm deep. The X-ray photons are able to escape from a considerable depth and are used for providing some chemical information. The backscattered electrons can also escape from a considerable depth. The BSE image, since its origin is closer to that of the source of the x-rays, more closely describes the x-ray map and is better for correlation than the SED image.

23 Atomic Number Dependence
The probability of an incident electron being scattered varies as the square of the atomic number of the atom and inversely as the incident kinetic energy. • Greater depth of penetration for low Z materials (e.g. Al vs. W) • BSE emission branch increases with Z Low Z High Z Equation d prop to W V2/Z rho

24 SEM Example Microstructural Development and Surface Characterization of Electrodeposited Nickel/Yttria Composite Coatings, Cunnane et al., JES 150, C356 (2003) Changing the Y content in the Ni electrolyte bath from 1 to 5 g/L. Preferential growth directions are altered as the nucleation rates are changed by the co-depositing material.

25 Scanning Auger Microscopy
Uses same e-beam source as SEM. Energy analyzes electrons emitted in eV range (higher than secondary, lower than backscattered). Provides unique atomic identity information. Very surface specific (10 nm) Chemical maps of surfaces Some of the electrons which are emitted from the region of the sample excited by the incident electron beam have arisen from the Auger process occurring in atoms on the surface. These electrons are energy analyzed and the spectrum is unique to every atom. Auger spectroscopy can identify every atom from Li on up (cannot do H). Hence, it gives very specific atomic identity information rather than the general small/large Z of BSE SEM. By correlating a specific signal with the excitation beam’s location, a chemical map of a surface can be produced. Auger process. Chemical maps. Hemispherical Electron Analyzer. Secondary scattering in samples.

26 The Auger Process Measure kinetic energy of ejected electron. Incident
Another higher level electron is ejected to carry away excess energy. Measure kinetic energy of ejected electron. Incident Electron Beam (5 kV) Vacuum Level M The Auger process is a three electron concerted process. An incident electron beam hits an atom and ejects a core electron, leaving a hole in a highly excited atom. This state of excitation is quenched by another electron falling into that hole. The binding energy of the first electron is always very much greater than that of the second, so that there is still considerable excess energy in the system. This extra energy is shed by the ejection of a third electron from another shell. The residual kinetic energy of the ejected electron is measured and is a unique quantity determined by the binding energies of the three electrons and the work function of the materials involved. An Auger process is identified by the three letters of the atomic shells that participate in the process. Above is a KLL Auger transition. L Eject core electron Higher electron falls into hole K Ekinetic = E(K) - E(L) - E(L)

27 Auger Spectra Here are the Auger spectra for Ti and Cu. Imagine electrodepositing Cu on a Ti substrate. By tuning the energy analyzer to check the intensity at 920 eV (for Cu), at 418 eV (for Ti), and at 600 eV (for background) at each pixel at which the electron beam scans, the computer can then create a chemical map which would show the distribution of Cu and Ti.

28 SAM Resolution BS electrons are also scattered into the neighbouring regions of the sample with sufficient energy to further excite atoms not in the original excitation volume. Spatial resolution degraded 2 to 5 times over that of the corresponding SEM resolution. In the excitation process, the backscattered electrons also find their way into neighbouring regions of the substance and can excite electrons and induce an Auger process in regions of the sample that were not in the original excitation volume. The Auger signal then arises from an apparently larger region of the sample than the incident electron beam directly excites. This degrades the spatial resolution in the SAM process by a factor of 2 to 5 compared to the same SEM experiment.

29 Scanning Tunneling Microscopy
Tunneling gap ~ 5 Å Probe Tip Tunneling Current 10 pA - 10 nA This is the most famous of the probe microscope techniques. In these cases, we control resolution by manufacturing a probe whose end is of the dimension we seek. We bring the tip within a few Ångstroms of the surface. A small bias voltage (10 mV - 2 V) is applied between the tip and the sample and an electrical current flows by electrons tunneling across the gap between the two conductors. The tunneling current magnitude depends upon the gap voltage, the gap distance, and the local density of electron states. Tunneling Electron Current Sample

30 Tunneling Mechanism Sample Tip DOS DOS EF VBias EF d IT  exp(-2kd)

31 Density of States Every substance has a complex electronic structure. At every energy, there are a certain number of electronic states. The number is so large for bulk material, that one reports the number of states per unit energy – the Density of States or DOS. Tunneling can occur between states of the same energy; the electron’s energy does not change during the tunneling event.

32 Control Electronics Feedback Electronics Error Signal Z-piezo
Set Point Difference Current Amplifier Logarithmic Amplifier Sample

33 Resolution Lateral Vertical R ∆x
Lateral: Model indicates that Tip with R = 1000 Å actually focuses 90% of current in a circle of radius 45 Å. A 100 Å tip goes to 14 Å. In practice, 2Å resolution is routine. Vertical: exponential dependence means that 1 Å change in height corresponds to almost 1 order of magnitude difference in current. Noise floor is in 10’s of femtometer range. (Atom is several Ångstroms in diameter).

34 In Situ Electrochemical STM
There’s still a vacuum gap, even in water! Shield tip to minimize faradaic processes. Melted wax or plastic to coat shank of tip. Expose last few nanometers only. Tunneling current must be large compared to faradaic current.

35 STM Example #1 Monitored molecular orientation on surface in real time
Adlayer of 1,10-phenanthroline on Cu(111) in acidic solution Itaya, et al. J.E.S. 150 E266 (2003). Monitored molecular orientation on surface in real time

36 Scanning Electrochemical Microscope (SECM)
Create an ultramicroelectrode and use the faradaic current as the control signal. Signal modulated by proximity to surface.

37 Scanning Force Microscopy
Depends on forces (repulsive or attractive) between atoms. Reflected light To Position Sensitive Detector Diode laser Major extension of the STM technique. Also called Atomic Force Microscopy (AFM). Sharpened Cantilevered Tip

38 Position Sensitive Detector
4-Quadrant Photodiode (current in each quadrant changes with light intensity) 2 1 1+2-(3+4) = 0 1+2-(3+4) < 0 1+2-(3+4) < 0 and 1+3-(2+4) > 0 4 3

39 Contact Mode SFM Repulsive force between surface atoms and tip atoms, lead to cantilever deflection, altering of relected beam path. Sample is rastered and moved vertically to maintain constant cantilever deflection. Can damage delicate samples.

40 Lateral Force Mode SFM Frictional force measurement. During scan, frictional forces on surface will tend to twist the cantilever. Use Signal = (2+4) as feedback/imaging signal. Chemically sensitive: –CH3 covered surface vs. –COOH covered surface

41 Non-Contact Mode SFM Important when dealing with delicate samples.
Can achieve atomic resolution. Vibrate tip at resonant frequency (100’s of kHz). As tip approaches surface, the attractive forces between the substrate and the tip alter the resonance condition. For feedback/imaging • frequency shift • phase shift • damping

42 Cantilevers For contact mode For LFM and non-contact mode

43 SFM Example The Electrochemical Reaction of Lithium with Tin Studied By In Situ AFM, Dahn et al., JES 150, A419 (2003). Li is driven into Sn electrochemically which leads to a swelling of the Sn grains. SFM images were used to measure the grain sizes as the potential changed, contributing to a model rgarding Li incorporation in the Sn film.

44 Interference Microscopy
Visible wavelength optical microscope. Also called Non-contact Profilometry. Nanometer resolution vertical to surface. Uses interferometry to measure surface profile. Large dynamic range. The IM is an excellent microscopical tool. It has the vertical resolution of an SFM with a dynamic range that greatly exceeds the probe microscopes. Instrument. Interference technique. Computational process. VSI mode. PSI mode. Angle of acceptance. Terraced surface vs. rough surfaces.

45 Interference Fringes In-phase reflections are bright; out-of-phase are dark Top view First reflecting surface Structured reflecting surface Side view

46 Imaging Process Interferometer
Recombined, reflected light is directed to image plane of CCD camera. Points on surface that are separated from lens by an integer number of wavelengths is bright; those a half-integer are dark. Objective Lens

47 Imaging Process continued
Interference is strong only when reflected light is in focus; the sample-lens distance is at the focal position. Scan sample-lens distance around the focal length. Each pixel will strongly modulate its intensity when the lens reaches the focal position corresponding to each point on the surface. High resolution position information comes from a linear variable differential transformer (LVDT) connected to the lens scanning drive.

48 Vertical Scanning Interferometry
VSI and PSI Modes Vertical Scanning Interferometry Phase Shifting Interferometry Scan objective over range of µm. Record image frames sequentially. Search each pixel through frames and locate frame where intensity modulation is greatest. Assign height information by correlating frame number to LVDT. Alter optical path length in series of steps. This causes fringe pattern to shift laterally. The series of shifted fringe patterns are combined to form interferograms from which height information is calculated

49 Rough vs. Terraced Surfaces
Interference can occur only if light is reflected back into objective lens. If surface angle is inclined beyond acceptance angle of lens, no interference is observed. Lens Angle 2.5x obj. 2° 10x obj. 10° 50x obj. 25° O.K. Missed data Terraced surface

50 IM Example Preparing Au substrates on mica for use in forming nanostructured electrodes from self-assembled monolayers. Heat treatment created mounds on surface.

51 Raman Imaging Microscopy
Raman spectroscopy is molecular vibrational spectroscopy. Microscope uses a focused laser beam as the excitation source. The detector can be tuned to look for a particular spectral peak and this can be used to produce a chemical map - now based on molecular and not just atomic features.

52 Raman Effect Incident laser impinges on sample. Scattered light is shifted slightly to longer wavelengths; small amount of photon energy is left in molecules to excite vibrations. This scattered light, looking for loss of energy, correlates with molecular vibrational spectrum.

53 Mapping Distribution of beclamethasone dipropionate (BDP) and salbutamol in an allergy medication. Particle size is important for effectiveness.

54 Imaging Much faster than mapping. Uses bandpass filters instead of dispersive grating detection. Entire image passes through filter and exposed to CCD camera at once. Image keyed to the radiation intensity passing through the bandpass. This is selected for a particular molecular transition. Raman image can pick out the 5 differfent layers very easily. From a forensics study of a car.

55 Raman Example Fuel cell development. Troubled by contamination with NO+ in solid oxide fuel cell electrolyte, which poisoned process. IR is weak and overlapped by CO2.

56 Spectroscopy What is on the surface? (Atoms or Molecules or Bulk)
What is the structure of the surface layer? How are they oriented? What is their oxidation state? How do these properties change with potential? with time? with additional participants in the electrolyte solution?

57 Energy Dispersive X-ray Spectroscopy (EDX)
Done in conjunction with SEM. Name shifting to EDS. Add an X-ray detector. Emitted X-rays identify atomic species in excitation volume. Detector analyzes X-ray photons by energy, rather than wavelength. Can be used to chemically map a surface. Can also be done in wavelength dispersion mode. Higher resolution (10 eV compared to 100 eV), but more complex. Getting better. Also higher sensitivity. Order of magnitude better. Energy dispersion is in contrast to wavelength dispersion. It highlights the difference

58 EDS Detector Cooled in LN2 temps, Si crystal converts X-ray photon into charge by ionization. Charge is integrated through the FET and is proportional to X-ray energy.

59 WDS Detector Concave mirror crystal is key to the process. Can be LiF, thallium acid phthalate, or multilayered structures such as W/C, W/Si, or Mo/B. MoS2

60 EDX Example A cast iron sample SEM C map Si Map Fe map

61 X-ray Photoelectron Spectroscopy
Irradiate sample with monochromatic X-ray beam and energy analyze the photoelectrons which are ejected. (Kind of opposite of EDS). High resolution (< 1eV) allows chemical state identification (Si, Si2+, Si4+, SiO2 compared to SiTe2. Vacuum required to be able to detect the electrons. New instruments can focus X-ray to a few µm in diameter. The beam can be scanned to do imaging XPS.

62 X-ray Source: Anode Electron beam (15 kV) strikes an anode (Mg or Al). Emits x-rays. Tuned to maximize for narrow emission range (example, Mg Ka).

63 X-ray Source: Synchrotron

64 Electron Energy Analyzer
Hemispherical analyzer. Electron lens systems adjusts incoming electron energy to particular kinetic energy. Only specific energy passes through the hemispherical path to reach detector. Detector is electron multiplier. Can be multichannel.

65 XPS Spectrum #1 Spectrum for Yttrium

66 XPS Spectrum #2 Ag spectrum showing the spin-orbit splitting of the 3d peaks. The instrumental linewidth of 0.82 eV is also shown.

67 IR Spectroscopy Vibrational information about molecules. Valuable because of surface selection rules P-polarized: electric vector amplified at surface. S-polarized: electric vector cancels at surface. Ep Ep Es Es Phase shifts 180° upon reflection

68 SNIFTIRS Working Electrode Thin Film Electrolyte (2 µm) ZnSe prism
Subtractively Normalized Interfacial Fourier Transform Infared Spectroscopy i d d Working Electrode Thin Film Electrolyte (2 µm) ZnSe prism

69 PM FTIRRAS BaF2 prism Kerr Cell Electrolyte (D2O) µm Organic layer nm
Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy BaF2 prism Kerr Cell Electrolyte (D2O) µm Organic layer nm Electrode surface Electronically modulate polarization at 150 kHz.

70 PM FTIRRAS Spectrum - Pyridine
Pyridine bound to Au(111) changes orientation with cell potential

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