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fundamentals of nano Flashcards

1
Q

what is a nanostructured material?

A

materials and or objects which have structural features less than 100nm in at least 1 dimension

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2
Q

classification types of nanomaterials

A

disordered or organised

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3
Q

2 general ways of making nanostructured materials

A

top down or bottom up

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4
Q

what is top down?

A

materials which are structured by a sequential processing and patterning approach (eg microelectronics)

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5
Q

what is bottom up?

A

materials whose structures evolve through (self)-organisation of small building units (hierarchical assembly)

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6
Q

examples of materials made by bottom up approach

A

block co-polymers, organic-inorganic nanocomposite materials, self-assembled dots and layers

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7
Q

why are nano materials interesting?

A

improved or new material properties intrinsic to nanoscopic dimension

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8
Q

types of nanomaterials

A
  1. single component materials and homogeneous structure
  2. single type of materials but hierarchical structure
  3. composite materials
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9
Q

examples of single component materials and homogeneous structure

A

0D - dots (C60 and Semiconductor Quantum Dots (CdSe))
1D - tubes (Carbon nanotubes and H2Ti3O7 nanotubes)
2D - sheets, films (graphene sheet, Al2O3 membrane)

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10
Q

examples of single type of material by hierarchical structure

A

polymer structure mimicking gecko feet - adhesive

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11
Q

what is biomimetics?

A

transfer principles from biological systems to artificial systems

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12
Q

where do gecko feet get their adhesive properties

A

adhesion forces from weak van der waals and capillary forces (spatula, setae, lamellae)

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13
Q

examples of composite nanomaterials

A

giant magnetoresistance/ tunnelling magnetoresistance
DNA modified Au nanoparticles
Mimicking nacre
biophysics - cell adhesion and spreading

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14
Q

basis of giant magnetoresistance

A

2 Ferromagnetic material layers separated by a layer of non-ferromagnetic material. If dipoles are >/> the electrical resistance is low, but if dipoles are >/< resistance is very high which is applied in magnetic field sensors for reading hard drives

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15
Q

explain DNA modified Au Nanoparticles

A

ss-DNA is attached to the metal nanoparticles which allows for particle aggregation when base pairing occurs linking the particles and changing the properties - eg developing colorimetric specific drug detectors

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16
Q

what is a plasmon

A

collective oscillations of electron, resonance frequency depends on size and environment of the particle.

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17
Q

how do nanostructure allow for elucidation of processes in living systems?

A

can use nanomaterials to mimic certain aspects - introduce a well measured array of certain things to determine exactly how certain properties work - eg gold nanoparticles in modelling cell adhesion and spreading. They can be used to make hcp array of binding sites to determine how density of binding sites of cells to their extracellular matrix changes its properties.

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18
Q

why do we want things on the nanoscale?

A
  1. simple downsizing is often useful - modern electronics (faster, cheaper, less energy)
  2. new properties of molecules at the nano scale
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19
Q

Types of forces

A

adhesion
viscous
friction
entropic

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20
Q

How would you measure interfacial forces?

A

AFM (atomic force microscopy)

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21
Q

Adhesion forces

A

eg van der waals

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22
Q

energy of van der waals

A

-Ad/12D (A = Hamaker Constant x10-19)

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23
Q

force of van der waals

A

-Ad/12D^2

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24
Q

mass of nanoparticle

A

(1/6)pip*d^3 (p = density, d = particle diameter)

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25
Q

kinetic energy

A

1/2 mv^2

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26
Q

gravitational force

A

F = mg (where m=mass of particle, g=gravitational force constant)

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27
Q

as size decreases, what happens to interfacial forces?

A

they begin to dominate and other forces need to be included to keep particles apart

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28
Q

how does energy of van der waals vary with molecule shape?

A

linearly for flat surface or spherical molecules

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29
Q

what is laminar flow?

A

fluid travelling smoothly in regular paths (small R)

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30
Q

what is turbulent flow?

A

fluid undergoes irregular fluctuations or mixing (large R)

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31
Q

What is R?

A

a dimensionless number that quantises the ratio of inertial forces to viscous forces acting on a liquid moving through a channel

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32
Q

What is the equation for R

A

(density p * average velocity *channel diameter L) / viscosity n.

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33
Q

what is kinematic viscosity?

A

n/p

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34
Q

at what viscosity does mixing not occur?

A

high viscosity

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35
Q

What is Amontons law of friction?

A

F=uL (friction force = friction coefficient * normal force)

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36
Q

what is tau (t) in friction?

A

shear stress

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37
Q

What is Fs

A

static friction

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38
Q

What is Fk

A

dynamic friction

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39
Q

what is the stick slip phenomenon

A

When two materials are experiencing friction, the movement isn’t smooth - it has periods where it sticks to build up energy (Fs) then slides to the next bit it sticks (Fk)

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40
Q

Is static or dynamic friction higher or lower

A

Fs > Fk

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41
Q

How does friction change with number of layers of lubricant?

A

F increases as number of layers decreases

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42
Q

what is hydrodynamic lubrication?

A

lots of layers of lube - so smooth, low friction

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43
Q

what is boundary lubrication?

A

very thin layer of lube - not able to establish full fluid condition

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44
Q

can the difference in size of Fs and Fk change?

A

Yes - the difference between the two forces increases with decreasing number of layers of lubricant

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45
Q

equation for statistical entropy

A

S=k*lnW

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46
Q

two ways for a polymer to bind to a surface

A

physisorption or chemisorption

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47
Q

what is a mushroom regime?

A

low coverage of polymers on a surface (more balled up) separation between polymer molecules so that nearest neighbours don’t overlap

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48
Q

what is a brush regime?

A

high coverage of polymers on the surface so that the shapes of the polymers are perturbed due to neighbour influence

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49
Q

when two polymer coated surface are in contact, what happens when d>/=2dp?

A

no interaction between the polymers

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50
Q

when two polymer coated surface are in contact, what happens when d<2dp?

A

polymer molecules compressed so a smaller volume so entropy decreases - causes steric repulsion

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51
Q

what are the main forces behind AFM?

A

attractive VdW and repulsive exchange interaction

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52
Q

how does AFM detect stuff?

A

by reflecting light off the cantilever and reflecting it onto a quadrant photodiode to detect cantilever movement

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53
Q

what does the cantilever z axis do?

A

gives topography of the surface

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54
Q

2 types of imaging mode for AFM

A

scanning surface line-by-line

1) set signal value (force or current) and feed back into z
2) record a signal at a fixed value of z

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55
Q

2 types of spectroscopy mode for AFM

A

using a defined top position

1) measure a signal as a function of distance
2) keep Z constant and measure the signal as a function of input

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56
Q

2 types of AFM measurement modes

A

contact vs non-contact/tapping

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57
Q

explain contact mode (AFM)

A

set a constant photodiode value (2-4) and measure the forces. The cantilever will bend to maintain the photodiode value to show the topography and energy dissipation

58
Q

advantage and disadvantage of contact mode (AFM)

A

adv: straightforward, good imaging stability
disadvantage: tip-wear, high lateral forces - deformation of soft materials

59
Q

explain tapping/non-contact mode (AFM)

A

oscillate tip at resonance frequency of the cantilever. As vibrating cantilever gets closer to the surface, interactive forces change its frequency of vibration

60
Q

advantage and disadvantage of non-contact mode (AFM)

A

adv: non-damaging, avoids lateral forces
disadvantage: imaging stability, reduced sensitivity in liquids

61
Q

what equation can be used for non-contact mode (AFM)

A

f(o) = sqrt(k/m)

frequency of cantilever = sqrt (force constant of cantilever/mass of cantilever)

62
Q

4 examples of AFM imaging

A

1: topographic imaging of micro and nanostructure
2: ultrahigh resolution using tapping mode
3: friction force microscopy
4: force spectroscopy (removing molecules from a surface

63
Q

what different properties can AFM measure?

A

topography, friction, electrical conductivity, work function surface potential, magnetic forces, mechanic properties

64
Q

What are boundary conditions?

A

psi=0 at x=0 and x=L because potential is infinitely high for L

65
Q

what is the energy of a particle in a 1D box?

A

E=n^2h^2/8mL^2

66
Q

what is the wave function of a particle in a 1D box?

A

psi = (2/L)^2sin(npi*x/L)

67
Q

what is density of states?

A

number of states N(E) per unit volume in an energy interval E, E+dE

68
Q

general equation for DOS

A

=1/V(dN/dE)

69
Q

DOS equation for 3D box

A

1/VdN/dE = pi/4(8m/h^2)^3/2 * sqrtE

70
Q

DOS equation for 2D box

A

1/L^2dN/dE = pi/4(8m/h^2)

71
Q

DOS equation for 1D box

A

1/L*dN/dE = 1/2 (8m/h^2)^1/2 * 1/sqrtE

72
Q

what is the energy dependence of DOS depending on dimensionality of system?

A 
3D = sqrtE
2D = no dependence on E
1D = 1/sqrtE
73
Q

explain 3D DOS dependence on E

A

a continuous increase of DOS as E increases

74
Q

explain 2D DOS dependence on E

A

constant DOS with a jump at each quantised energy level from confinement in one dimension

75
Q

explain 1D DOS dependence on E

A

DOS peaks at each quantised level.

76
Q

When E

A

that when the probability amplitudes, A1 and B1 of a wave are the same (aka the wave is reflected), there is a remaining probability that the wave passes through the potential step into region 2. This is due to heisenberg’s uncertainty principle. Penetration dependent on magnitude of dx if the particle has sufficient energy to surmount the potential step.

77
Q

When E>V for a potential step, what does this tell you about reflection of the particle?

A

Even if the wave is over the energy barrier and is energetically not reflected back, the particle could experience potential discontinuity which has a finite probability that it gets reflected.

78
Q

When E

A

Tunneling.

79
Q

When E>V for a potential barrier, what does this tell you about reflection of the particle?

A

that the electron has finite probability of being reflected at each potential (scattering).

80
Q

how does barrier thickness affect transmission of electrons?

A

thin barrier - continuous increase of transmission

thick barrier - oscillating transmission due to interference effects from reflected waves.

81
Q

how can applying a voltage over a barrier affect its current

A

for low V, dependence of I on applied bias.

I is exponentially dependent on width of barrier - very quick decay of current as thickness of barrier increases.

82
Q

what type of spectroscopy uses tunnelling effects?

A

scanning tunneling microscope

83
Q

3 key parts of an STM

A

1: tunneling tip - conductive wire, ideally atomically sharp at the tip
2: piezoceramic element - applying a voltage expands/contracts the element thus allowing extremely precise movement of tip in x,y,z.
3: electronics - measuring tunneling current and controlling movement of tip.

84
Q

what mode of action does STM use?

A

constant current imaging mode - current set to fixed value (1pA-1nA at V=10mV to 2V). While scanning, feedback loop maintains set value by varying Z

85
Q

Why is the resolution of STM so high?

A

I=Io exp(-1.02sqrtoa)

basically a very small change in energy/work-function can have a large effect on the current.

86
Q

what is the current of STM determined by?

A

the local density of states (LDOS) and tunneling probability

87
Q

how do different types of functional groups affect the tunneling barrier?

A

aromatic - lowers barrier

aliphatic - raies barrier as tunneling harder to access

88
Q

how does DOS affect tunneling barrier?

A

high DOS - lots of electrons to tunnel

low DOS - lower statistical weight.

89
Q

What is quantum confinement?

A

when one or more dimensions of a nanoparticle are made very small so it approaches the size of an exiton in the bulk material called the Bohr exciton radius.
idea is to trap electrons and holes within a small area to give new electronic properties

90
Q

At what scales do quantised energies become continuous

A

quantised: nanoscale–> mesoscopic –> macroscopic : continuum

91
Q

what is the classical model for diffusive transport?

A

electrons moving through a conductor experience scattering events due to (eg) impurities or lattice imperfections

92
Q

how can you use scattering events to define distance or time?

A

a large time between scattering events gives a high conductivity

93
Q

what are 3 characteristic length scales between scattering events?

A

elastic mean free path
inelastic mean free path
phase coherence length

94
Q

explain elastic mean free path

A

distance between 2 elastic collisions which is determined by the time between collision and average velocity

95
Q

explain inelastic mean free path

A

scattering events associated with energy transfer. electron phonon scattering, electron-electron scattering.

96
Q

explain phase coherence length

A

measure of the distance before the phase of the electron is randomised.

97
Q

what is Ohm’s law?

A

I=U/R

98
Q

what is the resistance of a conductor?

A

R=p*L/A (p = resistivity, L = sample length, A = cross-sectional area)

99
Q

why does shrinking of electronics eventually become problematic?

A

Ohmic resistance - at a smaller scale there is more heat to get rid of - harder when things are much smaller

100
Q

what 2 factors account for deviation from purely geometrical scaling of resistance

A

1) scattering at surfaces

2) scattering at grain boundaries

101
Q

what is resistivity cf resistance?

A

resistivity is a material constant whereas resistance is geometry dependent.

102
Q

what is electormigration?

A

when the current is so high that atoms move

103
Q

what is diffuse scattering?

A

in a thick film with an uneven edge - scattering in many directions
extent of diffuse scattering determines how the film thickness affects conductivity - higher diffuse scattering, higher resistivity

104
Q

in a thin film how does thickness of film affect conductivity

A

conductivity down resistance increase

105
Q

what is ballistic transport and how is it modelled?

A

the electron travels through the sample without any elastic scattering event.
when the mean free path of the electron is much longer than the medium through which it travels

106
Q

what is elastic scattering?

A

kinetic energy of a particle is conserved in the centre of mass frame bu its direction of propagation is modified.

107
Q

how can current flow through a system be interpreted in terms of electrons?

A

probability of electrons being transmitted through a system

108
Q

What does transmission of an electron depend on

A

its energy

109
Q

what is the fermi dirac distribution function?

A

1/(1+ exp(E-u/kt))

110
Q

what is the landauer butter formula?

A

Itot = 2e^2/h * V * sum of |T|^2

111
Q

what can we say about current based on channels from one potential reservoir to another

A

current is quantised and given by number of channels available

112
Q

what is the equation for quantum of conductance?

A

Go = 2e^2/h

113
Q

what is the equation for quantum of resistance?

A

Ro = 1/Go

114
Q

Give some examples of systems that require conductance on the nanoscale

A

semiconductor heterojunction
metal contact
conduction through molecules

115
Q

explain a semiconductor heterojunction

A

a thin layer of mobile electrons between two materials. apply a voltage to the gate electrodes leaving an electron depleted area under the electrodes which makes a channel for the electrons to travel between the electrodes

116
Q

how does varying of gate voltage affect current in a semiconductor heterojunction?

A

causes current to change in disrete steps

117
Q

explain how current changes in retracting a metal tip from metal surface

A

a nanowire is generated on retraction of the tip. the contact gets thinner until it breaks - this causes discrete changes in current

118
Q

what affects conduction through molecules

A

determined by twist angle - therefore about interaction of pi orbitals on rings

119
Q

2 ways to control metal-molecule-metal contact

A

1) STM based - tip in contact with surface + retracted, when tip-surface contact breaks, molecules adsorb in gap
2) lithographically fabricated break junction - pushing rod + 2 supporting rods push on material until small break occurs. Repeat many times for stats and to extract reliable info as molecule between contact can adopt different geometries so different conductivities.

120
Q

what is the statistical approach to molecular conductance measurements by STM break junction?

A

substrate/STM tip immersed in molecule containing solution or molecule is pre-adsorbed on substrate
current measured as function of tip distance from substrate

121
Q

what are the 4 statistical aspects in nanoscale materials?

A

fluctuations, brownian motion, reaction rates and molecular motion

122
Q

how do size of energy fluctuations scale with size of a system?

A

bigger system, smaller fluctation

from dE/ = 1/sqrt(#molecules)

123
Q

what is brownian motion?

A

random motion of small objects due to bombardment within molecules from all sides

124
Q

what is the langevin equation?

A

the force experienced by a particle
Fp = mdv/dt=-av+f(t) where a=friction coefficient = 6pin*a (a=radius, n = viscosity) and f(t) = random force from collisions with molecules

125
Q

how does mean square displacement vary with time?

A

it increases linearly

126
Q

Kramer’s reaction rate theory

A

extension of transition state theory describing exponential dependence of chemical reactions on temperature as known from Arrhenius law. Basis is stochastic motion of particles in a potential well and reactions occur via fluctuations.

127
Q

what is Kramers theory k+/-

A

k=wawb/2pi*gamma * exp(Eb/kt)

128
Q

what is the equation for FRET efficiency?

A

E = na/(na+gamma*nd) where n=#detected photons and gamma=correction factor

129
Q

give an example of Kramers theory

A

fluctuation induced conformational change of a DNA molecule - fluorescence signal is fluctuating between 2 states.

130
Q

what is a molecular machine?

A

molecular system converting chemical, photochemical or electrochemical energy into motion

131
Q

what are the 3 essential ingredients of molecular motion

A

topological entanglement
mechanical bonds
isomerisable unsaturated bonds

132
Q

give two examples of compounds that undergo molecular motion

A

catenanes - two loops together - rotate by change in ox state
rotaxanes - loop round a stick -oxidation and reduction moves ring along the structure from fn group to fn group.

133
Q

what are some challenges faced by artificial molecular motion?

A

1) Brownian motion
2) organisation of systems to be able to harness mechanical motion
3) proof that what molecules are supposed to do is actually happening

134
Q

what is the main necessity for a nano car and 2 key reactions that must happen

A

need to ensure rotation is uni-directional

need isomerisation and helix inversion

135
Q

what is a thermal ratchet

A

an attempt to generate directed motion
by switching from asymmetric “saw-tooth” potential to flat potentials using electron fields or chemical reactions - driving particle to minima at each type of potential mimics motion.

136
Q

why, even in an asymmetric potential, is there an equal probability of a particle moving in the x or -x direction?

A

a consequence of the 2nd law of thermodynamics - the entropy of an isolated system in thermal equilibrium is constant and has reached its max. value.
aka no spontaneous formation of gradients

137
Q

how does kinesis move?

A

ATP/ADP conversion moves the “feet” by diffusion (not energy used to propel it)

138
Q

are all nanomaterials potentially toxic?

A

nanostructured fixed structures (eg thin film coatings, microchip electronics) are unlikely to cause harm - the concern is about free/released particles.

139
Q

how do nanoparticles enter the body?

A

skin, lungs, gastrointestinal tract

140
Q

what makes a nanoparticle toxic?

A

physicochemical properties (size, aggregation, composition, crystallinity, surface functionalisation), particle-cellular interactions, biokinetics, individual’s genetic complement, routes/degrees of exposure. logistics

141
Q

what are some factors that need to be considered when comparing nanoparticle vs bulk structure toxicity?

A

difference in surface areas, different crystal structures, coatings?, difference in surfeiting acidity, property design - the particles vs bulk material were probably made for v different applications.

nano (2 decks)

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