Leggett - nanochemistry Flashcards
What is the main problem for “nanobot” nanoparticles?
Brownian Motion - Very large forces acting on very small particles
Why can classic optical microscopy not be used for characterisation of nanoparticles?
Optical resolution limit.
Limit is approx. lambda/2 for the wavelength of light used.
Visible light is ca. 400-700 nm so resolution limit for optical microscopy is approx. 200nm
What must be taken into account when working at small length scales?
- Surface effects become more dominant
- Effects ignored at macro scale become important
- High surface area to volume ratio of nps influence structures and reactivities
- electrons may tunnel through thin insulators
- optical properties change when particle size is smaller than the wavelength of light
Resolution limit formula
R = (0.61 lambda)/(n_r sin(alpha))
lambda = wavelength
n_r - refractive index of medium
n_rsin(alpha) = numerical aperture of lens
How does a scanning electron microscope work?
Electron gun fires electrons between a condenser lens (in a vacuum), this is focused by an objective lens through scan coils and through an aperture, onto the sample.
Electron beam interacts with the sample matter and electrons are scattered, leading to:
- Secondary electrons, with a yield of delta per primary electron (often delta > 1); mainly used for imaging
- Backscattered electrons, yield eta (often delta»_space; eta)
- X-rays and Auger electrons
- Auger electron yield is small, but x-rays have energy characteristics of the element from which they are emitted –> use for elemental analysis
What advantages does TEM have over SEM
Thin samples may be analysed in transmission mode
- Superior resolution
- Higher acceleration voltage; 100-400 keV vs 1-30 keV for SEM
- High voltages allow better resolution and deeper penetration
- May obtain electron diffraction
- Imaging contrast comes from mass contrast (thicker/more dense regions absorb more
Probability of tunnelling equation
T = A exp(-2kl)
How does TEM image a surface?
Large separation between tip and surface - little overlap
Small separation between tip and surface - effective overlap
This leads to current flow in external circuit
Tunnel current depends exponentially on the separation, decreasing by a factor of 10 for every 1A separation. –> basis of high resolution
Piezoelectric crystal detects very small movements
BUT sample must conduct electricity
How do you perform constant height TEM?
Hold tip at constant height
measure variation in tunnel current as surface is scanned
Image shows variation in tunnel current
How do you perform constant current TEM?
- Measure instantaneous current
- Adjust piezo-voltage to move tip until preset value of current is achieved
- Image shows variation in v_z, approx. topography
- Most widely used mode in practice
Piezoelectric crystal is key to control of motion
Practical issues with TEM
Tip exerts mechanical load on sample:
- Increases with i_t (current)
- may be large at small gaps
- May damage delicate materials
How is the TEM tip made?
(1) Mechanical - Cut Pt/Ir wire with wire cutters
(2) Electrochemical - Etch wire in caustic solution until end drops off to give a sharp tip
What problems can occur for the TEM tip?
Tip profile can become convoluted with the surface topography
Leads to self-imaging to give a regular, repeating geometric feature
Square or Double tip
How to use Scanning Tunnelling Microscopy for spectrosocpy
STm probes the Local Density Of States (LDOS) at the surface - i.e. the tunnel current depends on the surface electronic structure at high resolution and the electronic structure of the tip.
–> Changing the bias voltage changes electronic states involved in tunnelling
What is the basis of lithography and nanomanipulation?
Tip exerts a mechanical force Problems: - sweeping of adsorbates - Scratches inorganic materials - Damage to tip BUT can be exploited Voltage pulse: +ve: field emission from sample to tip -ve: redeposit atom --> leads to mechanical sliding (but only occurs at 4K)
Describe the principles of Atomic Force Microscopy
Sharp tip attached to a flexible cantilever is rastered across a sample
As the interaction forces between the tip and surface changes, the deflection of the lever changes
Typically measured by deflecting a laser off the back of the cantilever onto a photodetector
–> No requirement for conductivity
Hooke’s law equation
F = -kx
AFM design criteria
Small spring constant (k) - large deflection for small F, but need a high resonant frequency to avoid noise
Need micro-engineered, high stiffness material
Use Si3N4 (contact mode) or Si (non-contact/tapping mode)
Measure deflection of laser beam from back face of cantilever
Usually have triangular cantilever and pyramidal tip
Lennard-Jones Potential
epsilon = 4E {(sigma/r)^12 - (sigma/r)^6}
