1. Microscopy • What is the structure of the surface of the sample? Atoms
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.

2. 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

3. 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.

4. 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

5. 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

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

7. 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

8. 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.

9. 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.

10. Scanning Probe Microscopy:
Introduction and principles

11. 1) General concept of Scanning Probe Microscopy (SPM)
2) Atomic Force Microscopy
3) Scanning tunneling Microscopy
4) Scanning Near Field Optical Microscopy
5) Examples

12. 1.- SPM Concept
SPM: “Scanning probe microscope”
Microscope (“Nanoscope”) in which the image acquisition is based on the control and acquisition of the vertical movement of a ultra sharp needle that scans the surface that we want to visualize

13. 1.1 Atomic scale manipulation

14. History of SPM
1981: Invention of STM. IBM Zurich. G. Binnig and H. Rohrer
1986: Invention of AFM. IBM Zurich-Stanford. G. Binnig, C. Quate
1990: Atomic manipulation of Atoms (D. Eigler, IBM Almaden)
1990: Nanolithography (J. Dagata, NIST)

17. Tip – sample interact Interaction: Tunneling current
Forces between tip and surface
Electrical field
Magnetic field
...

18. Concept of scanning probe microscope
Basic elements of an SPM
Needle with a very sharp tip
Piezoelectric actuators for the fine displacement in X, Y, Z
Positioning system
Electronic control of tip sample distance
Scanning electronic system
Acquisition, visualization and control by means of a computer

20. Piezoelectric actuators for scanning the tip (surface) over the surface (tip)

21. AFM image of a thin and strained silicon layer

22. Exemples d’imatges que es poden obtenir
STM image of a Si(111) surface

23. AFM image of DNA fragment on Mica

24. 2) AFM: Atomic Force Microscope
Principle of operation
Forces between tip and surface
Operation mode
Probes for AFM

26. Tip and cantilever for AFM

27. Aproximately: 2·Dh/L = Dx/D
Fotodiode Dx
Laser D Dh L
Aproximately: 2·Dh/L = Dx/D
If minimum Dx = 0.1 mm, Dh minimum = 0.1 mm·L/D
Typically, L= 100 mm, D= 10 cm
Dh minimum = 0.5 Å

28. STM: Scanning tunneling microscope

29. Tunneling current: distance dependence

31. Tunneling current: voltage dependence
Density of electronic surface sates

32. STM tip Preparation method Initial material: W, Pt/Ir or Au wire
Phase 1
Electrochemical sharpening or
Mechanical sharpening
Phase 2 (Tip-clean in UHV)
Heating
Electrical field sharpening

34. Scanning Electrochemical Microscope

35. Ultramicroelectrode

36. Positive Feedbck

37. Negative Feedback

38. Scanning near field optical microscope (SNOM)
Principle of operation
© Xavier Borrisé
© Xavier Borrisé

39. Scanning near field optical microscope (SNOM)
© Xavier Borrisé

40. Propagació de la llum en guies d’ona integrades
Scanning near field optical microscope (SNOM)
Propagació de la llum en
guies d’ona integrades
Detecció de fluorescència en molècules
© Niek van Hulst. MESA Research Institute
© Xevi Borrisé. ECAS-UAB

41. SPM in relation with other microscopes
© Xavier Borrisé

42. 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

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

44. 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.

45. Control Electronics Feedback Electronics Error Signal Z-piezo
Set Point —
Difference
Current Amplifier
Logarithmic Amplifier
Sample

46. 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).

47. 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.

48. 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

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

50. 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

51. 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

52. 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.

53. 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

54. 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

55. Cantilevers
For contact mode
For LFM and non-contact mode

56. 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.

57. 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.

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

59. 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

60. 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.

61. 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

62. 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

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

64. 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.

65. 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.

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

67. 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.

68. 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.

69. 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?

70. 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

71. 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.

72. 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

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

74. 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.

75. 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).

76. X-ray Source: Synchrotron

77. 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.

78. XPS Spectrum #1
Spectrum for Yttrium

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

80. 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

81. 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

82. 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.

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