Lecture 11 Flashcards
What analytical methods do you use to get information about the chemical composition of a material?
Interaction of the materials with radiation:
- IR: Infrared Spectroscopy
- UV/Vis: Ultraviolet/Visible Spectroscopy
- EDX: Energy Dispersive X-Ray
Spectroscopy - XPS: X-Ray Photoelectron Spectroscopy
What analytical methods do you use to get the structural information of a material?
Atoms-bond environment and arrangements, crystal structures, phases and assemblies
- NMR: Nuclear Magnetic Resonance Spectroscopy
- XRD: X-Ray Diffraction
- SAXS: Small Angle X-Ray Scattering
What analytical methods do you use to get the thermal properties of a material?
- DSC: Differential Scanning Calorimetry
- TGA: Thermogravimetric Analyses
What analytical methods do you use to get other special properties of a material?
Special properties:
- Magnetic and Electrical techniques
- Surface Charges: Zeta Potential measurements
- Gas adsorption techniques
- …
What information does microscopy give?
size, shape, morphology
(topography)
The term chemical morphology implies both the chemistry of shape and the shape of chemicals
What are the 3 microscopic techniques that we will focus on?
- Optical microscopy (LM)
- Scanning probe microscopy (SPM)
- Electron microscopy (EM)
What are the 4 pillars needed for microscopy?
- Interaction with the sample: Here we need to define what is the type of interaction that will occur with our sample
- Control: Here we set to control the said interaction. For example in EM the interaction with the sample is electrons and to control them we can change the energy they were shot at.
- Resolution:
- Diffraction rings formed at high magnification: airy discs
- If the separation between the two disks exceeds their radii, they are
resolvable. The limit at which two airy disks can be resolved into separate entities is often called the Rayleigh criterion. (This is depicted in digital notes)
We can use the diffraction limit d as a measure for the resolving power
d = λ/2nsin(a) = λ/2*NA
λ- Wavelength of the light
n- Lenses refractive index
NA - Numerical aperture
a - angle of light
- Contrast: In microscopy, contrast refers to the difference in light intensity between a specimen and its background, allowing distinct features of the specimen to be seen more clearly. Higher contrast makes it easier to distinguish the details of a sample, especially when using light microscopes. Adjusting the contrast helps in visualizing transparent or colourless samples that might otherwise be difficult to see.
Note that relative resolution and contrast is based on the same concept for the microscopic techniques discussed here
How does the LM set-up work?
An optical microscope setup functions by using light to magnify small objects, enabling detailed examination. Here’s how a typical setup works:
- Illumination: A light source, usually built into the microscope’s base, illuminates the specimen from below. This light can be adjusted for intensity using a field diaphragm, which helps control the light’s spread and focus on the specimen.
- Condenser: Light is focused on the specimen using a condenser lens located beneath the stage, improving image clarity and contrast. For specific techniques, the condenser can be adjusted to alter lighting modes, such as phase contrast or dark-field illumination.
- Stage: The stage holds the slide in place, often with mechanical controls to move the slide smoothly. It has a central hole allowing light to pass through and illuminate the specimen from below.
- Objective lenses: These lenses, located blow the stage, are the primary magnifiers. Modern microscopes have multiple objectives of varying magnification levels (5x to 1000x), which can be rotated into place as needed.
- Eyepiece (or ocular lens): The eyepiece further magnifies the image produced by the objective lens. Typically, the final magnification is a combination of the objective and eyepiece magnification (e.g., 40x objective combined with a 10x eyepiece yields 400x magnification).
- Focusing system: Coarse and fine focus knobs adjust the distance between the objective lens and the specimen, allowing for precise focusing of the image.
What are the four pillars of LM?
- Interaction: Light is used as the interaction parameter. Once light is shown into a material different phenomenons are observed: Transmission, reflection, refraction, diffraction, adsorption, and scattering (this is depicted in the digital notes)
- Control: We can control multiple variables here, such as what phenomena of light to measure, or the intensity of the light shot
- Resolution: One thing to note here. Due to the light being limited to the visible spectrum, and given that the equation for the resolving power depends on the wavelength, we will limit the resolving power. IMP: Resolution of 1 um
- Contrast: Some examples are depicted in the notes to show how contrast can play a role in light microscopy.
What are other advanced techniques of microscopy?
Modern light microscopes allow more than just observation of
transmitted light image from a sample.
The techniques mentioned are:
- Fluorescence microscopy
- Ultraviolet microscopy
- Near-Infrared microscopy
- Confocal microscopy
What is fluorescence microscopy?
Fluorescence microscopy is a specialized type of microscopy that uses fluorescence, rather than reflection or absorption of light, to generate an image. In this technique, the specimen is labeled with fluorescent dyes or proteins called fluorophores. When these fluorophores are excited by light of a specific wavelength (usually in the ultraviolet or blue range), they emit light of a longer wavelength (in the visible range), which is then captured to create an image. This method is particularly useful for studying the localization and dynamics of specific molecules within cells, tissues, or biological samples.
What is Ultraviolet microscopy?
Ultraviolet (UV) microscopy is a type of optical microscopy that uses ultraviolet light instead of visible light to examine specimens. Developed in the early 20th century, it is used to achieve higher resolution than traditional light microscopy, thanks to the shorter wavelength of UV light. This allows for finer detail to be captured.
What is near-infrared microscopy?
Near-infrared (NIR) microscopy uses light in the near-infrared spectrum (700–2500 nm) to image biological tissues and materials. Its key advantage is deeper tissue penetration compared to visible light, making it ideal for non-invasive imaging, such as visualizing tumors or internal biological processes. NIR microscopy often combines with fluorescence imaging to improve contrast and resolution, particularly for in vivo studies.
What is confocal microscopy? IMP!!!
Confocal microscopy is an advanced optical imaging technique that improves contrast and resolution by using point-by-point scanning and focusing. Unlike traditional widefield microscopy, which illuminates and collects light from the entire specimen, confocal microscopy uses a laser to focus light on a specific point in the sample and detects only the light coming from that precise focal plane. A key feature is the use of a pinhole aperture in front of the detector, which blocks out-of-focus light, enabling clear, high-contrast images of thin sections within a thick sample. This technique allows for the creation of detailed 3D reconstructions of specimens by scanning layer by layer.
This is depicted in digital notes.
What is scanning probe microscopy?
-Powerful technique to study the morphology and local properties of solid surfaces based on interactions between
- High resolution can be achieved: down to the atomic scale!
- No special sample preparation, but we do need the right probe depending on the forces we can measure: magnetic, current, electrostatic, light, etc.
- Note that SPM is a SURFACE TECHNIQUE meaning it is limited to 2D images
What is the physical working principle of SPM?
SPM is based on a probe scanning the surface of the material. This probe will measure the different interactions it will have with the surface. Those interactions are termed as parameter P.
The said parameter P is then going to be a function of the distance between the surface and the probe. This distance is termed parameter Z. Analysis of P and Z allow for the construction of an image.
This is depicted in notes
What are the 4 pillar forces of SPM?
*Interaction with sample: forces
- Control
- magnification, focus
- The way we can control the forces depends on the specific techniques for SPM which are discussed later
- Resolution
- resolution within the nanoscale. It is very powerful. Once we reach the nanoscale the image tends to become a bit more ruff.
*Contrast: Just note again that it is a SURFACE TECHNIQUE, this means that often you will see a clear background in the 2D images produced.
What are the piezo tubes?
In scanning Probe Microscopy (SPM), piezo tubes are crucial components that provide precise control over the probe’s movement. They enable fine positioning and adjustments in the three-dimensional space, particularly in the Z-axis, allowing for accurate imaging of surfaces at the nanoscale. The fast response times of piezo tubes are essential for rapid scanning, and their stability helps minimize noise, enhancing image quality.
Piezo tubes are used in various SPM techniques, including Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM), where they ensure the probe maintains a consistent distance from the sample surface to achieve detailed measurements and imaging.
Note that they can also change there size under the influence
of an electrical field.
This is depicted in the notes
What is the biggest issue faced by SPM?
It is the design of the probe. This determines the resolution of the image constructed.
The general condition for the design of the probe is for it to be the same size as the features it is going to measure. This presents difficulties when measuring features on the nanoscale, as we must construct a probe of the same size.
An example of how we can reuse our probe is shown in the notes but it isn’t important.
What are the two types of scanning probe microscopy that we are going to focus on?
- Scanning Tunnelling Microscopy (STM)
- Atomic Force Microscopy (AFM)
What is STM?
Scanning Tunneling Microscopy (STM) is a high-resolution imaging technique used to visualize surfaces at the atomic level. It works on the principle of quantum tunneling, where electrons can tunnel between a sharp metallic tip and a conductive surface when they are very close together (within a few nanometers).
The current between the tip (metal
needle) and a conducting sample is measured as a function of Z = tip-sample distance
What is the control mechanism found in STM?
- STM Constant current: The current applied is set to be kept constant. This is done by a continuous feedback system adjusts the tip’s height (Z) based on the tunneling current
- STM constant height: the STM tip is set to a specific distance from the surface. As the tip moves across the surface, it measures variations in the tunnelling current, which reflects changes in the electronic density of states of the material. The resulting data can be used to generate an image that highlights surface features, defects, and molecular arrangements.
note that constant height is done for a smooth surface, to avoid the breaking of the Tip.
This is depicted in the notes
What is AFM?
Atomic Force Microscopy (AFM) is a sophisticated imaging technique utilized for analyzing surfaces at the nanoscale. It operates by scanning a sharp tip attached to a flexible cantilever over the surface of a sample. As the tip moves closer to the surface, it detects intermolecular forces, allowing it to create detailed topographical maps of the material being examined. This capability enables researchers to visualize surface features at atomic resolutions
The fundamental working principle of AFM involves measuring the forces between the tip and the sample, which allows the instrument to “feel” the surface’s contours. When the cantilever approaches the surface, it experiences attractive forces, transitioning to repulsive forces upon contact. The deflection of the cantilever is measured using a laser beam that reflects off its back, and a feedback mechanism maintains a constant force during scanning.
Interactive force (attractive or repulsive) between the tip and the
sample varies with Z = tip-sample distance.
What are the two control mechanisms found in AFM?
- Constant force - a technique that maintains a consistent force between the AFM tip and the sample surface during scanning. The cantilever’s deflection is controlled using a feedback loop based on Hooke’s law, which states that the force exerted is proportional to the cantilever’s deflection. This mode enables the simultaneous measurement of surface topography and additional properties, such as friction. This is considered to be an AFM contact mode
- Constant distance: involves scanning a sample surface while keeping the AFM tip at a fixed height above it. In the distance mode, the tip does not compensate for height variations, making it suitable for studying delicate samples that might be damaged by direct contact. This is considered to be an AFM semi-contact mode.
This is depicted in the notes
What are some applications of AFM?
- They are the perfect too to give information about soft materials, such as block copolymer
- It can also give information about hybrid molecules like SiO2 and polymers
What is a special feature of SPM?
It can manipulate the surface of a material via a very sharp tip. For example, atoms force the electrons in a copper surface to specific quantum states.
What are the two types of electron microscopy we consider?
- Scanning Electron Microscope (SEM)
- Transmission Electron Microscope (TEM)
What are the 4 pilar forces of Electron microscopy?
- Interaction with sample: Electrons
- Control: The energy of the electron beam can be controlled
- Resolution: Depends on each type discussed. Both are going to have a better resolution than LM. This is because the wavelength of the electron is much much smaller than the wavelength of visible light. To take full advantage of the shorter wavelength of the electron a
combination of beam voltage, aperture size and lenses is needed.
Contrast
Bright-field
Darkfield
Cross polarized
*Cross-polarized contrast microscopy is a technique that utilizes polarized light to enhance the visibility of birefringent materials—substances that exhibit different refractive indices based on the light’s polarization direction.
How does the setup for scanning electron microscopy work?
- Electron Gun: The process begins with the electron gun, which generates a beam of electrons. This gun typically uses a tungsten filament or a lanthanum hexaboride (LaB6) crystal to produce electrons through thermal or field emission. These electrons are then accelerated towards the sample by a positively charged anode.
- Condenser Lenses: After the electron beam is generated, it passes through condenser lenses that focus the beam down to a diameter of about 0.4 to 5 nm. This focusing is critical for achieving high-resolution.
- Deflection Coils: The focused electron beam is directed using deflection coils located in the electron column, allowing the beam to raster across the sample in a controlled manner, scanning it line by line.
- Vacuum System: SEM operates under a high vacuum environment to prevent the scattering of electrons by air molecules. This vacuum is achieved using mechanical and high-vacuum pumps, and samples must be solid and fit within the chamber.
- Interaction with Sample: As the electron beam strikes the sample, various interactions occur, producing secondary electrons (SE) and backscattered electrons (BSE). These emitted electrons are collected by detectors, allowing for the formation of images based on the electron signals received.
- Detection and Imaging: The detectors are strategically positioned to capture emitted electrons, which provide information about the surface topography and composition. The intensity of the detected electron signal is translated into an image displayed on a monitor, where bright areas indicate higher electron emission.
This is depicted in notes under flashcard 8
What are the conditions required to prepare a sample for SEM?
- Small-medium sized samples
- Stable under a vacuum (again the vacuum is needed to prevent electrons from interacting with the air molecules)
- The sample must be electrically conductive, at least at the surface. This is to avoid the accumulation of electrons on the surface that will luminesce together at one point giving bad images. Another reason why the sample must be conductive is to prevent their destruction.
non-conductive materials can be coated with an electrically conductive material: Carbon, Gold,
Silver, etc…
A photo of what the sample is put in is shown in the notes
What is the interaction volume in SEM?
The interaction volume in scanning electron microscopy (SEM) refers to the three-dimensional region within the sample where interactions between the incident electron beam and the sample material occur. This volume is critical in determining the types of signals that are generated and subsequently collected for imaging and analysis.
What are the key aspects of the interaction volume?
- Dimensions: The size and shape of the interaction volume in scanning electron microscopy (SEM) depend on factors such as the energy of the incident electrons, the atomic number of the sample, and the angle at which the electrons strike.
- Composition: Within the interaction volume, various interactions occur, including elastic scattering (which generates backscattered electrons) and inelastic scattering (which produces secondary electrons and X-rays). The sample’s composition also affects the interaction volume; for example, materials with higher atomic numbers can allow deeper penetration of the electron beam
- Signal Generation: The interaction volume is vital for determining the signals collected during SEM analysis. Secondary electrons provide detailed topographical information, while emitted X-rays offer insights into the sample’s elemental composition. Understanding this volume is crucial for accurately interpreting SEM images.
- Practical Implications: Knowledge of the interaction volume is essential for optimizing SEM techniques. It influences choices regarding electron beam energy and specimen preparation, helping researchers achieve desired imaging resolution and contrast .
What is the depth of field in SEM?
The depth of field (DOF) in scanning electron microscopy (SEM) refers to the range of distance within a sample that appears acceptably sharp in an image. A larger depth of field means that a greater range of the sample can be in focus simultaneously, which is particularly important for imaging three-dimensional structures.
Not to be confused here. SEM is still a SURFACE TECHNIQUE, no matter what you do you are still going to get a 2D image so be very careful. Thus even if the depth of field image might look like it gives insight about the structure of the material. It doesn’t. don’t be fooled
What does the Depth of field depend on?
- On the voltage applied (i.e. energy of the electron beam). The higher the voltage the larger the depth
- On how conductive the material is. The more conductive the larger the depth of field
How is magnification controlled in SEM?
Magnification is controlled by the ratio of the “dimension” of the area observed in the sample and the area of observation in the screen
What is the resolution of SEM determined by?
The resolution is determined by the electrons used:
- Electrons wavelength
- Characteristics of the electron beam produced by the
electron-optical set-up
Note that for SEM the maximum resolution is 1um (i think)
What is the tricky thing about topography in SEM?
As stated before SEM depends on the interaction between the sample and the electron beam. Now if a sample isn’t symmetrical, at different angles it will have different interaction volumes. Thus when electron beams hit the sample at different angles, there will be areas (the one with the larger interaction field) that will deflect the electron beam more, causing them to become brighter.
Usually, edges are brighter due to them having larger interaction volumes.
This is depicted in the notes.
How does the setup for TEM work?
- Electron Source: The process begins with an electron source that generates a beam of electrons. This can be a tungsten filament (thermionic emission) or a field emission gun, both of which require a high vacuum to operate effectively.
- Condenser Lenses: These lenses focus the electron beam onto the specimen. The first condenser lens (C1) shapes the electron beam, while the second condenser lens (C2) further refines it, allowing control over the beam’s diameter and intensity at the specimen.
- Condenser Aperture: Located in the beam path after the condenser lenses, this aperture controls the diameter of the electron beam that strikes the specimen, thereby influencing the intensity and contrast of the image produced.
- Sample Stage: The sample is mounted on a holder that allows precise positioning within the electron beam. The specimen needs to be very thin (typically less than 100 nm) to permit electrons to transmit through it, enabling imaging.
- Objective Lens: This lens is crucial as it focuses the transmitted electrons to form a magnified image of the specimen. The objective lens operates within a strong magnetic field (about 2 Tesla) to achieve high resolution.
- Objective Aperture: Situated in the back focal plane of the objective lens, it helps enhance contrast by selecting which electrons contribute to the final image, typically excluding scattered electrons.
- Intermediate Lenses: These lenses, including the first and second intermediate lenses, help magnify the image further and allow for adjustments in imaging mode or diffraction mode.
- Projector Lens: This strong lens finalizes the image before it reaches the viewing screen or detector. It produces the final magnified image of the specimen or a diffraction pattern on the display.
- Selected Area Aperture (SAA): This aperture allows the user to isolate a specific area of the specimen for imaging or diffraction, enhancing the analysis of that region.
- Imaging: The transmitted electrons are collected by a detector, often a charge-coupled device (CCD) or a phosphor screen, which converts the electron signal into a visual image.
This is depicted in notes.
Note that a simpler version of TEM is also shown in note underFlashcard 8
How is the sample prepared for TEM?
For TEM analysis, samples must be extremely thin, typically less than 150 nanometers in thickness. Thin samples allow electrons to transmit through rather than scatter off, which is critical for generating clear images. Sample preparation may involve slicing materials into ultra-thin sections, chemical staining, or embedding in resin. This is a laborious process!
This is depicted within the notes.
What are important things to note about TEM?
- It is a bulk technique, as the electrons pass fully through the same, imaging the full volume of the material and yielding 3D images
- The fact that it has so many lenses tells us that it will have very high resolution (It does literally at the nanoscale making it arguably the best resolution microscopy, only compared with SPM)
- it can only show how atoms are linked together but it doesn’t show the chemical composition
- it shows electron diffraction in crystal lattices.
How are electron diffraction patterns in cystal lattices shown by TEM?
In transmission electron microscopy (TEM), electron diffraction is a crucial technique for analyzing the crystallographic structure of materials. When a beam of high-energy electrons passes through a thin sample, the electrons interact with the periodic atomic arrangements within the crystal lattice. This interaction leads to the diffraction of electrons, producing characteristic patterns that can reveal information about the crystal structure, such as lattice spacing and symmetry.
Note that objective aperture must be removed!
This is depicted within the notes
What is Cryo-TEM?
Cryo-transmission electron microscopy (CryoTEM) is an advanced imaging technique that allows for the detailed visualization of nanostructures while preserving their native state. This method involves rapidly freezing samples in vitrified ice, which prevents structural changes that can occur with traditional fixation methods.
Key benefits of CryoTEM include its ability to analyze the internal morphology of soft materials, such as liposomes and nanoparticles, without chemical alteration. By combining CryoTEM with electron tomography, researchers can generate three-dimensional reconstructions of complex structures, providing insights into their organization and behaviour.
Up to < 2 nm resolution for (cryo-conditions)!
This depicted in the notes
What we will mainly be tested on? (imp)
- Identify and describe the different techniques
- From an image we need to determine what technique was used
- Sometimes the technique will be coupled with a type of material. Most likely one with a silicon oxide core.
