Midterm/last set of slides info

Question 1

What is an x-ray

Answer 1

high energy (1-100keV) electromagnetic radiations that are between 0.01nm to 10 nm in wavelength

Question 2

How are x-rays generated?

Answer 2

When electrically charged particles are decelerated?

Question 3

Why are x-rays interesting for the purpose of biomedical instrumentation?

Answer 3

Because they have a wavelength that is similar spacing of atoms in matter

Question 4

What is a crystal?

Answer 4

any solid material characterized by a near-perfect repeating arrangement of molecules (unit cell)that forms a lattice

Question 5

When does diffraction occur?

Answer 5

When a wave hits an object and the path is changed of the wave but the wavelength stays constant

Question 6

How is crystallography done and what does it use?

Answer 6

It is done using x-ray diffraction, it uses this to find the arrangement of atoms

Question 7

What is braggs law and what is it used for?

Answer 7

ngamma=2dsintheta, it is used to find interference in the measurement

Question 8

What can diffraction study?

Answer 8

Can study all states of matter as long as they have a wavelength similar to the distance between the atomic or molecular structures of interest

Question 9

What are the applications of XRD?

Answer 9

Measure the average spacing between layers or rows of atoms

Determine the orientation of a grain or crystal

Find the crystal structure of an unknown material

Measure size, shape, internal stress of small crystalline structures

Question 10

What are the advantages of XRD?

Answer 10

It is a non-destructive method that is well studied and mathematically based, the signal is averaged over a large volume

Question 11

What are the disadvantages of XRD?

Answer 11

Weak interaction, there is noise, imaging is not possible

Question 12

What should happen to XRD samples to be prepared?

Answer 12

the surfaces should be smooth after polishing, they should be thermally annealed to get rid of surface deformations

Question 13

What does TEM stand for? What is it based on?

Answer 13

Transmission Electron Microscope, based on the physics principle that moving electrons act as waves

Question 14

What is TEM patterned after and what is the difference?

Answer 14

TEM is similar to the light microscope yet electrons are used instead of light

Question 15

How are electrons in TEM focused?

Answer 15

By one or two electron lenses that focus the electrons

Question 16

What dimensions are seen in TEM and what does brightness demonstrate?

Answer 16

TEM is a 2 dimensional method, brightness corresponds to the electron-dense region of the sample

Question 17

What dimensions are seen in SEM, how is this image made?

Answer 17

SEM is 3 dimensional, it uses 2-3 spots of electrons, it is then reflected off of the specimen to give the image, due to the method, it is often lower resolution.

Question 18

What is STEM?

Answer 18

A combination of the two methods, it determines the presence of atomic elements in a sample, STEM can distinguish columns of atoms in a crystal.

Question 19

What is the difference between HRTEM and HAADF?

Answer 19

HRTEM uses a conventional coherent beam of electronics while HAADF uses incoherently scattered electrons due to thermal diffusion

Question 20

What happens when XRD is done on crystals smaller than 100 nm?

Answer 20

Peak broadening can occur and this is not good data for finding the angle 2theta

Question 21

How does an electron microscope work?

Answer 21

Light source makes a vibrational motion that sends energy towards the object, after this the object generates a new series of waves towards due to diffraction, the new waves interact with the original and this is what creates the image

Question 22

What does the edge of an electron microscope image look like?

Answer 22

It appears to have a band or many fringes that are called fresnel fringes

Question 23

Where does spherical aberration occur and what is it from?

Answer 23

It occurs in electron microscopes, it occurs in both glass and spherical lenses, it happens when the outer rays are passing more than the ones along the axis

Question 24

Where does chromatic aberration occur?

Answer 24

It occurs in electron microscopes and it bends the lower energy electrons more strongly

Question 25

What is astigmatism?

Answer 25

it is a non-uniform magnetic field

Question 26

What is distortion?

Answer 26

changes in the magnetic field across the field

Question 27

What is the resolution limit for biological for biological and polymeric specimens?

Answer 27

About 1/10 of the specimen thickness

Question 28

What do thicker specimens need for good resolution?

Answer 28

Increased voltage

Question 29

What decides the method of electron microscopy that you use?

Answer 29

information needed, time restraints, available tools, type of sample

Question 30

What are the requirements for an electron microscope sample?

Answer 30

must be thin (electron transparent) stable under an electron beam and non magnetic

Question 31

How can safety be ensured?

Answer 31

Using MSDS, follow instructions, and know what to do if something goes wrong

Question 32

What are the 6 steps of sample prep for SEM/TEM

Answer 32

chemical fixation, wash, dehydration, mounting, coating, imaging

Question 33

What is often used for chemical fixation?

Answer 33

Glutaraldehyde

Question 34

What is added to the centrifuge to “wash” the sample?

Answer 34

PBS