Summary of 4471 Session 5: Simulations and Surfaces

1
Summary of 4471 Session 5Simulations and
Surfaces

• More on numerical simulation techniques
• Extracting information from Monte Carlo
  calculations (e.g. energy, heat capacity, free
  energy)
• Comparison of molecular dynamics and Monte Carlo
  methods
• Interatomic interactions beyond the pair potential

• Structure of (crystalline, clean) surfaces
• Two-dimensional crystallography
• Low Energy Electron Diffraction (LEED)
• The silicon (001) surface as an example of a
  surface reconstruction driven by local bonding
  changes

2
4471 Session 7 Nanotechnology

• A survey of possibilities for nanotechnology
• Ways of making and characterising nanoscale
  structures
• Lithography (conventional, electron-beam, soft)
• Scanning probe microscopy
• Self-assembly and directed assembly
• Some electronic properties of nanoscale systems
• Coulomb blockade
• Conductance quantization

3
Richard Feynmans 1959 Lecture

• Richard Feynman at the 1959 annual meeting of the
  American Physical Society

But I am not afraid to consider the final
question as to whether, ultimately---in the great
future---we can arrange the atoms the way we
want the very atoms, all the way down! What
would happen if we could arrange the atoms one by
one the way we want them?
4
What is Nanotechnology?

• A set of tools and ideas for the manipulation and
  control of matter in the size range between
  0.1nmand 1?m
• Corresponds to the range of sizes between current
  electronics and atomic/molecular dimensions

5
Possible applications in electronics

• Current CMOS electronic technology may be
  approaching fundamental limits in hardware
  performance and cost
• New types of electronic components (e.g. wires,
  transistors) operating at smaller length scales
• Completely new ways of manipulating information
  (e.g. using reorientable magnetisation of small
  magnetic particles)
• New ways of coupling light to electronic
  processes (e.g. using patterns on the scale of
  the optical wavelength)

6
Possible applications in biomedicine

• Understanding of the function of biomolecules -
  particularly the cooperation between them, and
  their function in cell membranes (difficult to
  study by conventional crystallography)
• Controlling interaction of cells with their
  environment (e.g. tissue culture,
  biocompatibility of implants)

7
Richard Feynmans 1959 Lecture

• Richard Feynman at the 1959 annual meeting of the
  American Physical Society

Another thing we will notice is that, if we go
down far enough, all of our devices can be mass
produced so that they are absolutely perfect
copies of one another. We cannot build two large
machines so that the dimensions are exactly the
same. But if your machine is only 100 atoms high,
you only have to get it correct to one-half of
one percent to make sure the other machine is
exactly the same size---namely, 100 atoms high!
8
Methods for producing structure on the nanoscale

• How do we pattern matter on the nanometer
  lengthscale?
• Using layer-by-layer growth
• By interaction with a beam of light or
  particles
• By interaction with a scanning probe tip
• By using contact with a stamp or mask
• By exploiting molecules natural tendency to
  order as a result of their mutual interactions

9
Optical or UV lithography

• Standard method for current generation
  semiconductor device processing (CMOS)
• Use a resist whose susceptibility to etching is
  affected by light
• Resolution depends on wavelength of light used
  current (2001) standards for fabrication 0.15?m

Activated resist
Chemical etch (e.g. HF)
10
Electron beam lithography

• Just as have higher spatial resolution in imaging
  with shorter-wavelength electron microscopes,
  have higher resolution in patterning too
• Sensitive to electrons because can induce free
  radical formation (promoting resist removal) or
  crosslinking (preventing resist removal)

11
Electron beam lithography

• Possible to produce feature sizes down to about
  5nm using this technique
• Figure shows 5nm metallic line on silicon surface
  (Welland et al., Cambridge)

12
Soft lithography - nanoimprint lithography

• Can print a structure directly on to a soft
  surface (e.g. a polymer) from a hard mould
  (e.g. a metal surface prepared by e-beam
  lithography)

13
Soft lithography - nanoimprint lithography

• Get a variety of structures e.g. holes and pillars

14
Soft lithography - lithographically induced
self-assembly (LISA)

• Apply a large electric field between a mask and a
  polymer film
• Polymer film spontaneously grows up towards mask
• Pillars form when mask-polymer separation between
  200nm and 800nm
• Works because polymer attracted to high-field
  region

Mask
Polymer film
15
The scanning probe idea

• Get very high spatial resolution by
• Scattering very short-wavelength waves

Sample
16
The scanning probe idea

• Get very high spatial resolution by
• Scattering very short-wavelength waves and
  detecting them a long way away (e.g. electron
  microscopy, neutron or X-ray diffraction)

Sample
17
The scanning probe idea

• Get very high spatial resolution by
• Scattering very short-wavelength waves and
  detcecting them a long way away (e.g. electron
  microscopy, neutron or X-ray diffraction)
• Bringing a small detector up to the sample

Sample
18
The scanning probe idea

• Get very high spatial resolution by
• Scattering very short-wavelength waves and
  detcecting them a long way away (e.g. electron
  microscopy, neutron or X-ray diffraction)
• Bringing a small detector up to the sample and
  arranging for a very localised interaction
  between them

Sample
19
The scanning probe idea

• Get very high spatial resolution by
• Scattering very short-wavelength waves and
  detcecting them a long way away (e.g. electron
  microscopy, neutron or X-ray diffraction)
• Bringing a small detector up to the sample and
  arranging for a very localised interaction
  between them

Scan detector across sample
Sample
20
The STM(Scanning Tunnelling Microscope)

• Electrons tunnel across small (few Å) vacuum gap
  between tip and sample.
• Relies on sensitivity of tunnelling to
  tip-surface distance (hence localised
  interaction).
• Normal mode of operation is constant-current
  feedback loop keeps current constant as tip is
  scanned across surface.

21
Tersoff-Hamann Theory

• Assume
• Tip-sample tunnelling probability small (so
  perturbation theory can be applied)
• Spherically symmetric tip
• Initial state for tunnelling is an s state on tip
• Fermis golden rule for rates in quantum physics
  then gives conductance

22
Tersoff-Hamann Theory (2)

• Write the matrix element in terms of the current
  operator as
• Assuming S lies in a region of constant
  potential, and that we tip wavefunction is an
  exponentially decaying s-wave, we can do all the
  integrals to get

23
What does this mean?

• Conductance proportional to probability of
  finding highest-energy electrons outside the
  sample near the tip
• The STM measures the local density of states
  (under certain conditions)

Surface
Tip
?
rtip
24
Atomic manipulation with the STM the ground state
Atom on surface

• Can use presence of tip to affect the potential
  energy of atoms on or near the surface
• Allows movement of individual atoms along the
  surface (parallel process)...

Potential energy
Distance along surface
25
Atomic manipulation with the STM the ground state

• Can use presence of tip to affect the potential
  energy of atoms on or near the surface
• Allows movement of individual atoms along the
  surface (parallel process)...

STM tip
Potential energy
Distance along surface
26
Atomic manipulation with the STM the ground state

• Can use presence of tip to affect the potential
  energy of atoms on or near the surface
• Allows movement of individual atoms along the
  surface (parallel process)...

Potential energy
Distance along surface
27
Atomic manipulation with the STM the ground state

• Can use presence of tip to affect the potential
  energy of atoms on or near the surface
• Allows movement of individual atoms along the
  surface (parallel process)...

Potential energy
Distance along surface
28
Atomic manipulation with the STM the ground state

• Can use presence of tip to affect the potential
  energy of atoms on or near the surface
• Allows movement of individual atoms along the
  surface (parallel process)...

Potential energy
Distance along surface
29
Atomic manipulation example Xe atoms on Ni at
T4K

• Individual Xe atoms manipulated by the parallel
  process at T4K
• STM tip moves up over atoms, showing that
  electrons tunnel more easily through them than
  through vacuum

Don Eigler et al (IBM Almaden)
30
Atomic manipulation example Xe atoms on Ni at
T4K

• Individual Xe atoms manipulated by the parallel
  process at T4K
• STM tip moves up over atoms, showing that
  electrons tunnel more easily through them than
  through vacuum

Don Eigler et al (IBM Almaden)
31
STM manipulation example molecular abacus

• Produced from C60 molecules (about 5Å across)
• Can be pushed along with the STM tip

Jim Gimzewski et al (IBM Zurich)
32
STM manipulation use of electronic forces

• Can use the electronic state to manipulate atomic
  positions in various ways
• The electron wind effect (electrons transfer
  momentum to atoms)
• This is believed to be the physics behind the
  atomic switch (on and off states correspond to
  atom on tip and on surface)

e-
e-
Atom on surface
Surface
Force
33
STM manipulation use of electronic forces

• Can also exploit transient change of chemical
  environment as a tunnelling electron passes
  through the system
• Temporary occupation of antibonding electronic
  states can lead to desorption of atoms (DIET-
  desorption induced by electronic transitions)

Potential energy
Antibonding state occupied by tunnelling electron
Distance from surface
Electronic ground state
34
STM manipulation use of electronic forces

• Example removal of H atoms from a passivated
  Si(001) surface
• Conducting line of reactive bonds, one atom
  wide
• Behaves like an atomic wire

H atoms removed here
Hitosugi et al, Tokyo University and Hitachi
35
Single-molecule vibrations

• Study vibrations of individual molecules and
  individual bonds by looking at phonon emission by
  tunnelling electrons

Wilson Ho et al., UC Irvine
36
Single-molecule vibrations

• Study vibrations of individual molecules and
  individual bonds by looking at phonon emission by
  tunnelling electrons
• New possibilities for inducing reactions by
  selectively exciting individual bonds.

Wilson Ho et al., UC Irvine
37
Scanning Force Microscopy (SFM)

• We would like to
• be able to image insulating (as well as
  conducting) surfaces
• measure forces, as well as currents, on the
  atomic scale, in order to
• learn more about them
• control the manipulation process
• The solution scanning force microscopy (SFM)

38
Scanning force microscopy

• Measure deflection of small cantilever on which
  tip is mounted, by deflection of a laser beam

39
Scanning force microscopy

• It used to be thought that contact mode would
  give the best resolution, but the interpretation
  is complicated by strong mechanical interactions
  between the tip and the sample

Alex Shluger et al, CMMP, UCL
40
Scanning force microscopy

• Most recent development is non-contact force
  microscopy tip vibrates above sample and only
  approaches briefly

41
Scanning force microscopy

• Allows truly atomic-resolution force microscopy
  images to be obtained for the first time.

Defects on surface
Defects migrate
Ernst Meyer et al, Basel
42
Scanning force microscopy

• Allows truly atomic-resolution force microscopy
  images to be obtained for the first time.

Atomic step on surface
Ernst Meyer et al, Basel
43
Scanning force microscopy

• Understanding the physics behind the formation of
  these images is complicated...

Image of NaCl island
Simulated tip scan
Ernst Meyer et al, Basel Adam Foster and Alex
Shluger, CMMP, UCL
44
Other ways of producing structure with SPM

• Find a local chemical reaction promoted by the
  presence of a tip - for example oxidation
• or exposure of a resist (as in e-beam
  lithography)

45
Other ways of producing structure with SPM

• Find a local chemical reaction promoted by the
  presence of a tip - for example oxidation
• or exposure of a resist by the local electron
  current (as in e-beam lithography)

46
Self-assembly

• Exploit chemical forces to produce organization
  into desired patterns
• Inspired by biology (and soap!) e.g. spontaneous
  formation of bilayer membranes (living cells and
  soap films)

Hydrophilic headgroups (polar)
Hydrophobic tails (non-polar)
47
Self-assembly

• Generate films on metal surfaces by a similar
  method end tail part of molecule with an S-H
  group that reacts with gold
• Head group can now be arbitrary (e.g. a
  biological antibody or antigen)

Headgroup
C-S-Au bonds
Gold substrate
48
Quantum dots and huts

• Also get spontaneous self-organization in other
  ways, for example during strained growth of one
  material on another when their lattice parameters
  differ

49
Examples of atomic-scale lines

• Lines of Si ad-dimers formed by annealing
  (heating) the Si-rich SiC(001) surface
• Self-assembly, probably mediated by long-range
  elastic interactions between the lines

50
Directed growth

• Try to combine the idea of control (as in
  lithography) and spontaneous formation of an
  ordered structure (as in self-assembly) by
  directed growth that is spontaneous following
  some initiation event
• For example, use an SPM initiation (slow,
  expensive, can only be done at a limited number
  of sites) followed by a self-propagating chemical
  reaction

51
Molecular device Self-directed wire growth

• Lines of molecules can be grown on silicon by a
  self-directed process
• Follows use of STM tip to produce a single
  unpaired electron in a dangling bond

Lopinski et al, Nature 406 48 (2000)
52
Molecular device Self-directed wire growth
53
Molecular device Self-directed wire growth

• Do the resulting wires conduct? Watch this
  space...

54
Richard Feynmans 1959 Lecture

• Richard Feynman at the 1959 annual meeting of the
  American Physical Society

When we get to the very, very small world---say
circuits of seven atoms---we have a lot of new
things that would happen that represent
completely new opportunities for design. Atoms on
a small scale behave like nothing on a large
scale, for they satisfy the laws of quantum
mechanics. So, as we go down and fiddle around
with the atoms down there, we are working with
different laws, and we can expect to do different
things. We can manufacture in different ways. We
can use, not just circuits, but some system
involving the quantized energy levels, or the
interactions of quantized spins, etc.
55
Electronic and magnetic properties of nanosystems

• Electronic and magnetic properties of nanoscale
  structures differ from bulk (because electrons
  and other excitations experience the nanoscale
  structure, on the same scale as their own de
  Broglie wavelength, and are confined)
• They also differ from conventional molecules,
  because the structures are in intimate contact
  with their environment and so the systems are
  open

56
Atomic manipulation example quantum corals

• Coral (circle of iron atoms on copper surface)
  gradually assembled by moving atoms across surface

Don Eigler et al (IBM Almaden)
57
Atomic manipulation example quantum corals

• Coral (circle of iron atoms on copper surface)
  gradually assembled by moving atoms across
  surface
• When circle complete, ripples observed within it

Don Eigler et al (IBM Almaden)
58
Atomic manipulation example quantum corals

• Coral (circle of iron atoms on copper surface)
  gradually assembled by moving atoms across
  surface
• When circle complete, ripples observed within it

Don Eigler et al (IBM Almaden)
59
Atomic manipulation example quantum corals

• Ripples do not arise from shape of surface
• Come from presence of electron standing wave
  quantum states
• This affects the local density of states and
  produces the apparent ripples

Don Eigler et al (IBM Almaden)
60
Atomic manipulation example quantum corals

• Shape of ripple pattern depends on shape of coral
  - its quite different for a rectangle

Don Eigler et al (IBM Almaden)
61
Coulomb blockade

• When a metallic nanoparticle is almost isolated
  from its surroundings, there is a non-negligible
  charging energy to add an electron
• This charging energy can block current flow in
  a certain voltage range

62
Coherent transport

• Another difference compared with current flow on
  the macroscopic scale transport in small
  structures is coherent (occurs as the result of a
  single quantum process)
• As a result conventional formulae, such as the
  series and parallel addition of resistances, no
  longer hold
• Must be replaced by a way of thinking involving
  two new quantities the transmission coefficient
  and the Greens function

63
Coherent transport STM of benzene on the
graphite surface

• Molecule appears triangular in the STM, even
  although its true shape is hexagonal
• Arises from quantum mechanical interference (like
  double slit experiment)

64
Origin of the interference

• There are no benzene states at the Fermi energy
• Tunnelling takes place through highest occupied
  and lowest unoccupied molecular states, some
  distance away in energy

• These two routes for charge transport
  (corresponding to positive and negative transient
  charging) can interfere

65
How the interference works

• Bonding orbital same sign on adjacent carbon pz
  orbitals

-
-
Bonding
66
How the interference works

-

• Bonding orbital same sign on adjacent carbon pz
  orbitals
• Antibonding orbital opposite signs on adjacent
  pz orbitals

-
-

-

Bonding
Antibonding
(? is molecular energy gap)
67
How the interference works

-

• Bonding orbital same sign on adjacent carbon pz
  orbitals
• Antibonding orbital opposite signs on adjacent
  pz orbitals
• Transport is controlled by the Green function

-
-

-

Bonding
Antibonding
68
How the interference works

-

• Direct transmission through an atom into the
  substrate the two contributions cancel out
  because the energy denominators have opposite
  signs

-
-

-

Bonding
Antibonding
69
How the interference works

-

• Transmission involving a hop along the molecular
  bond electron picks up an extra sign change in
  the antibonding state and produces constructive
  interference

-
-

-

Bonding
Antibonding
70
Conductance quantization
Conductance

• When transmission probability in a particular
  channel is close to unity, get quantization
  of conductance in units of e2/h
• Happens in specially grown semiconductor wires
  grown by e-beam lithography, or in metallic
  nanowires

Extension
Jacobsen et al. (Lyngby)
71
Conductance quantization

• Such nanowires can be produced by pulling an STM
  tip off a surface, or simply by a break
  junction in a macroscopic wire

Jacobsen et al. (Lyngby)
72
Conductance quantization

• Such nanowires can be produced by pulling an STM
  tip off a surface, or simply by a break
  junction in a macroscopic wire
• Understood on the basis of simultaneous changes
  in atomic and electronic structure

Jacobsen et al. (Lyngby)
73
Extreme nanotechnology single-molecule
electronics

• Experiments now possible on the conductance
  properties of individual molecules

Langlais et al. 1999
74
Extreme nanotechnology single-molecule
electronics

• Experiments now possible on the conductance
  properties of individual molecules
• Those chosen for conducting applications are
  invariably conjugated

Langlais et al. 1999
75
Extreme nanotechnology single-molecule
electronics

• Experiments now possible on the conductance
  properties of individual molecules
• Those chosen for conducting applications are
  invariably conjugated

Langlais et al. 1999
76
Molecular device Example Molecular Transducer

• Transducer made from single C60 molecule
• Conductance of molecule changes as it is
  pressed by the tip

Jim Gimzewski et al (IBM Zurich)
77
Summary and Conclusions

• A variety of methods now available to manipulate
  and control matter on the atomic and molecular
  scale
• Focus is now on novel properties of the resulting
  structures, potential for applications, and on
  combining lithography and directed growth for
  mass production