Surface and Interface • When phases exist together, the boundary between two of them is known as an interface. • The properties of the molecules forming the interface are often sufficiently different from those in the bulk of each phase. • The term surface is used when referring to either a gas–solid or a gas–liquid interface.

1. Surfaces and Interfaces
Nanotechnology
Foothill DeAnza Colleges

2. Surfaces to Ponder
Triply periodic minimal balance surfaces with cubic symmetry
New Geometries for New Materials http://metalrg.iisc.ernet.in/~lord/

3. Overview
• Importance of surfaces
– What is a surface?
– Surface structure
– Surface processes
– Surface interfaces
– Surfaces in nature
– Measuring surfaces
– Modifying surfaces

4. Importance of Surfaces
• Surfaces are a primary ‘point of contact’
– Materials contact each other at surfaces
• Catalysis of surface mediated reactions
• Where many biological reactions occur
– Perhaps where life began
• Tribology - friction, lubrication and wear
• Most metal corrosion occurs at surfaces

5. Biosphere – Our ‘Surface’
All important things happen at a surface – and almost all of life on earth!

6. Surfaces Defined
• Discontinuity between material phases:
– Solid / air
– Solid / liquid
– Solid / solid
– Liquid / air
– Liquid / liquid
– Liquid / solid
• Molecules and colloids / particles have surfaces,
surface charges, etc. This is what drives proteins
to spontaneously fold (surface energy with water)

7. Surfaces and Phases
• Surfaces exist at phases
– Free energy must be minimized
• Energy drives most surface reactions
– Passivation
– Oxidation
– Adsorption of hydrocarbon and water
– Reconstruction and reorientation

8. Water Phase Diagram
http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

9. CO2 Phase Diagram
http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

10. Heterogeneous
Surface Structure
Different length scales involved during solidification. In the left image the thickness of
the temperature diffusion layer (largest scale). In the middle image the mass diffusion
layer is shown; at this scale the microstructures in the solid region can be seen. In the
image at right the height deviations of the interface on the smallest scale.
http://www.uni-regensburg.de/Fakultaeten/nat_Fak_I/Mat8/lst/spp/projectSPP1095solidification.html

11. Real Surfaces Explained
• Discontinuities create an interface
• Dangling bonds, attractive / repulsive
forces, unit cell cleavage planes
• Interfaces often form passivation layers
• Surfaces can scatter electrons
• Materials can fail at interfaces
– Can be cohesive / adhesive failures

12. Surface Structure Database
• The Surface Structure
Database (SSD) is the only
complete critical compilation
of reliable crystallographic
information now available on
surfaces and interfaces.
SSD brings instant access
to detailed text and
graphical displays of over
1250 experimentally-
determined atomic-scale
structural analyses. http://www.nist.gov/srd/nist42.htm

13. Silicon Surface Planes
• Model of the ideal surface
for Si{111}1x1.
The open and closed
circles represent Si atoms
in the first and second
layers, respectively.
Closed squares are fourth-
layer atoms exposed to the
surface though the double
double-layer mesh.
The dashed lines indicated
the surface 1x1 unit-cell.
http://www.matscieng.sunysb.edu/leed/trunc.html

14. http://w3.rz-berlin.mpg.de/~reuter/highlights/2003/highlight_karsten.html
Surface Structure

15. Si Surface Reconstruction
Schematic diagram of a covalent semiconductor with (a) an unrelaxed vacancy
involving four dangling bonds and (b) a relaxed vacancy with no dangling bonds
http://www.mtmi.vu.lt/pfk/funkc_dariniai/sol_st_phys/defects.htm

17. Structure of Silicon Surface
Measured using STM
http://www.chm.ulaval.ca/chm10139/
Scanning tunnelling microscope
image of a Si surface, ~0.3° off
(100) orientation showing the type
A steps (Si dimers parallel to
steps) and type B steps (Si dimers
perpendicular to steps).
Uppermost part of the surface is
at lower right, with downward tilt
to upper left. Scale is ~110 nm
square (Prof. Max Lagally).

18. Structure of Si Surface
STM image of the Si(1 1 1)(7×7) structure is shown at the top, covering a region of four surface unit meshes
(the surface unit mesh is denoted by the bold lines on the left). Below is shown a schematic diagram, in
plan view, of the DAS model of this surface; the bold lines again show the surface unit mesh but for clarity
the model shows some of the atoms in the edges of adjacent surface unit meshes. In this diagram the
adatoms imaged as the asperities in STM are shown as large pink spheres, while the dimerised Si atoms
are shown as pale blue. The red spheres show un-dimerised Si atoms in this same layer. The Si atoms in
the layer below are shown green, while those in deeper layers are dark blue. Notice that in the right-hand
(unfaulted) half of the unit mesh these lower atoms lie directly below those in the outermost two layers.

19. Surfaces of Interest
• Silicon: Si-OH, carbon
• Metal: M-OH, carbon
• Polymer: reconstruction / orientation
• Liquid: liquid interface / SAMs
• Molecular
– Proteins, lipid walls, etc.

20. Surface Processes
• Passivation
– Oxide formation
– Adventitious carbon
• Reconstruction
– Crystalline
– Polymer orientation
• Adsorption of gases and water vapor
– Both can lead to surface passivation

21. The net effect of this situation is the presence of free energy at the surface.
The excess energy is called surface free energy and can be quantified as a
measurement of energy/area. It is also possible to describe this situation as
having a line tension or surface tension which is quantified as a force/length
measurement. Surface tension can also be said to be a measurement of
the cohesive energy present at an interface. The common units for surface
tension are dynes/cm or mN/m. These units are equivalent. Solids may also
have a surface free energy at their interfaces but direct measurement of its
value is not possible through techniques used for liquids. Polar liquids, such
as water, have strong intermolecular interactions and thus high surface
tensions. Any factor which decreases the strength of this interaction will
lower surface tension. Thus an increase in the temperature of this system
will lower surface tension. Any contamination, especially by surfactants, will
lower surface tension. Researchers should be very cautious about the issue
of contamination. http://www.ksvinc.com/surface_tension.htm
Surface Free Energy

22. Surface Energetics
• The unfavorable contribution to the total (surface)
free energy may be minimized in several ways:
1.By reducing the amount of surface area
exposed – this is most common / fastest
2.By predominantly exposing surface planes
which have a low surface free energy
3.By altering the local surface atomic geometry
in a way which reduces the surface free energy

23. Surface Tension
• The molecules in a liquid have a certain
degree of attraction to each other. The
degree of this attraction, also called
cohesion, is dependent on the
properties of the substance. The
interactions of a molecule in the bulk of
a liquid are balanced by an equally
attractive force in all directions. The
molecules on the surface of a liquid
experience an imbalance of forces i.e. a
molecule at the air/water interface has a
larger attraction towards the liquid
phase than towards the air or gas
phase. Therefore, there will be a net
attractive force towards the bulk and the
air/water interface will spontaneously
minimize its area and contract.
http://www.ksvinc.com/LB.htm

24. • The storage of energy at the
surface of liquids. Surface tension
has units of erg cm-2 or dyne cm-
1. It arises because atoms on the
surface are missing bonds.
Energy is released when bonds
are formed, so the most stable
low energy configuration has the
fewest missing bonds. Surface
tension therefore tries to minimize
the surface area, resulting in
liquids forming spherical droplets
and allowing insects to walk on
the surface without sinking.
Surface Tension
http://scienceworld.wolfram.com/physics/SurfaceTension.html

25. Surface Tension in Action
http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

26. How do Molecules
Bond to Surfaces?
• There are two principal modes of adsorption of molecules on surfaces:
• Physical adsorption ( Physisorption )
• Chemical adsorption ( Chemisorption )
• The basis of distinction is the nature of the bonding between the
molecule and the surface. With:
• Physical adsorption : the only bonding is by weak Van der Waals -
type forces. There is no significant redistribution of electron density in
either the molecule or at the substrate surface.
• Chemisorption : a chemical bond, involving substantial rearrangement
of electron density, is formed between the adsorbate and substrate.
The nature of this bond may lie anywhere between the extremes of
virtually complete ionic or complete covalent character.
http://www.chem.qmul.ac.uk/surfaces/scc/

27. Adsorption / Self Assembly
Processes on Surfaces
• Physisorption
– Physical bonds
• Chemisorption
– Chemical bonds
• Self-Assembled Monolayers (SAMs)
– Alkane thiols on solid gold surfaces
– Self assembly of monolayers

28. Chemi / Physi - Adsorption
The graph above shows the PE curves due to physisorption and chemisorption separately -
in practice, the PE curve for any real molecule capable of undergoing chemisorption is best
described by a combination of the two curves, with a curve crossing at the point at which
chemisorption forces begin to dominate over those arising from physisorption alone. The
minimum energy pathway obtained by combining the two PE curves is now highlighted in
red. Any perturbation of the combined PE curve from the original, separate curves is most
likely to be evident close to the highlighted crossing point.
http://www.chem.qmul.ac.uk/surfaces/scc/scat2_4.htm

29. Adsorption Model of CO
Chemisorbed on a Metal Surface
• A trace of the bonding in the
chemisorbed CO reveals that the 2*
interaction with the surface d is
responsible for a good part of the
bonding. (a) Forward donation from
the carbonyl lone pair 5 to some
appropriate hybrid on a partner
metal fragment. (b) Back donation
involving the 2* of CO and a d
orbital, xz, yz of the metal. Shading
corresponds to a positive phase of
the wave function, and no shading
corresponds to a negative phase of
the wave function. Alternatively,
shading may also mean a wave
function with a positive sign, and no
shading means the same wave
function with a negative sign.
http://www.chm.ulaval.ca/chm10139/peter/figures4.doc

30. Structure of Polymeric Surfaces
• Atomic force microscopes
are ideal for visualizing the
surface texture of polymer
materials. In comparison to a
scanning electron
microscope, no coating is
required for an AFM. Images
A, B, and C are of a soft
polymer material and were
measured with close contact
mode. Field of view:
4.85 µm × 4.85 µm
http://www.pacificnanotech.com/polymers_single.html

31. Polymer Surface Orientation
• AFM of polymer surface
showing molecular
orientation.
• Note the change in scale
of the scanning
measurement.
• Polymers can ‘reorient’
over time to reduce
surface energy (like a
self-assembly process)
http://www.msmacrosystem.nl/3Dsurf/Shots/screenShots.htm

32. Ozone Treated Polypropylene
• Ozone treated
polypropylene showing
the affect of energetic
oxygen etching of the
polymer, and loss of fine
structural filaments.
• AFM images and force
measurements show
increase in surface
energy, as well as an
increase in surface
ordering of the filaments.
http://publish.uwo.ca/~hnie/sc2k.html

33. Oxide Layers on Alloys
• Schematical side view
projection of best-fit
results for (a) the
NiAl(110) surface and (b)
the Al2O3/NiAl(110)
interface, showing the
rippling of the topmost
surface layer. The atomic
arrangement in the oxide
structure has not yet
been determined.
http://www.esrf.fr/info/science/highlights/2001/surfaces/SURF2.html

34. Metal-oxide Interfaces in
Magnetic Tunnel Junctions
http://shell.cas.usf.edu/~oleynik/research-projects.html

35. Surface Interfaces
• Every interface has
two surfaces
– Solid / air
– Solid / liquid
– Solid / solid
– Liquid / air
– Liquid / liquid
– Liquid / solid
Interesting things happen at interfaces! Like the start of life!
~99% of living organisms live in the top 1cm of the ocean

36. Forces at Interfaces
• Van Der Val's forces
• Surface tension
• Interfacial bonding
• Hydrophobic / hydrophilic interactions
• Surface reconstruction / reorientation
• Driven by, or are part of ‘excess surface
free energy’ which must be minimized.

37. Importance of Interfaces
• Chemical reactions occur at interfaces
– Particularly corrosion
• Scattering energy
– Electrons
– Light
– Phonons
• An interface is actually two surfaces

38. Constant current STM image
of a GaAs (110) surface
• Constant current STM
image of a GaAs (110)
surface highly doped
with Zn acceptors at T =
4.7 K. The acceptors
appear as triangle
features. Both gallium
(light blue to yellow)
and arsenic (dark blue)
atoms are observed.
(sample voltage : +1.6
V current, : 80 pA).
http://www.omicron.de/index2.html?/rom/coloured_sem_images/~Omicron

39. Defects at Interfaces
• Missing atoms
– Defects and holes
• Extra atoms
– Surface segregation
• Dangling bonds
– Disrupted electronic properties
• Dimensional issues
– Lattice mismatch / shelves

40. Atomic resolved non-contact AFM
imaging of Ge / Si(105) surface
• High-resolution noncontact
atomic force microscope (AFM)
images were successfully taken
on the Ge(105)-1x2 structure
formed on the Si(105) substrate
and revealed all dangling bonds
of the surface regardless of their
electronic situation, surpassing
scanning tunneling microscopy,
whose images strongly deviated
from the atomic structure by the
electronic states involved.
http://www.omicron.de/index2.html?/rom/coloured_sem_images/~Omicron

41. Cohesive / Adhesive
Failure at Interfaces
• Cohesive failure occurs within a layer
• It can be from material weakness
• Or simply less strong than adhesion
• Adhesive failure occurs between layers
• It can arise form contamination, or poor
adhesion, or simply the strength of
adhesion was greater than the material

42. Cohesive Failure
Material A
Material B
Material B
Material fails cohesively within B

43. Adhesive Failure
Material A
Material B
Material fails adhesively between A and B

44. Adhesive Failure (Craze)
Schematic representation of
the structure at the crack tip
in a crazing material are
shown at three length
scales. It is assumed that
only material A crazes. The
whole of the craze consists
of lain and cross-tie fibrils.
http://www.azom.com/details.asp?ArticleID=2089

45. Surface Reactions
• Oxidation
• Surface diffusion
• Diffusion and oxidation
• Adventitious carbon bonding
– Hydrocarbons from the atmosphere
• Surface rearrangement
– Polymers may reorient to minimize energy

46. A Typical Surface
Solid material like silicon or aluminum
Oxide layer of about 15 to 20 Angstroms
Hydrocarbon layer of about 15 to 20 Angstroms
Hydrocarbons and water rapidly adsorb to a metal or
Silicon surface. Oxides form to a thickness of about 15
To 20 Angstroms, and hydrocarbons to a similar thickness.
This is part of the normal surface passivation process.

47. Langmuir-Blodgett Films
• Definition of LB films
– History and development
• Construction with LB films
• Building simple LB SAMs
• Nano applications of LB films
– Surface derivatized nanoparticles
– Functionalized coatings in LB films

48. Langmuir-Blodgett Films
• A Langmuir-Blodgett film contains of one or more
monolayers of an organic material, deposited from
the surface of a liquid onto a solid by immersing (or
emersing) the solid substrate into (or from) the liquid.
A monolayer is added with each immersion or
emersion step, thus films with very accurate
thickness can be formed. Langmuir Blodgett films are
named after Irving Langmuir and Katherine Blodgett,
who invented this technique. An alternative technique
of creating single monolayers on surfaces is that of
self-assembled monolayers. Retrieved from
"http://en.wikipedia.org/wiki/Langmuir-Blodgett_film"

49. Langmuir-Blodgett Films
http://www.ksvltd.com/pix/keywords_html_m4b17b42d.jpg
Deposition of Langmuir-Blodgett molecular assemblies of lipids on solid substrates.
http://www.bio21.bas.bg/ibf/PhysChem_dept.html

50. Self Assembly
• Self-assembly is the fundamental principle
which generates structural organization on all
scales from molecules to galaxies. It is
defined as reversible processes in which pre-
existing parts or disordered components of a
preexisting system form structures of
patterns. Self-assembly can be classified as
either static or dynamic.
• http://en.wikipedia.org/wiki/Self-assembly

51. Molecular Self-Assembly
• Molecular self-assembly is the assembly of molecules without
guidance or management from an outside source. There are two
types of self-assembly, intramolecular self-assembly and
intermolecular self-assembly, although in some books and
articles the term self-assembly refers only to intermolecular self-
assembly. Intramolecular self-assembling molecules are often
complex polymers with the ability to assemble from the random
coil conformation into a well-defined stable structure (secondary
and tertiary structure). An example of intramolecular self-
assembly is protein folding. Intermolecular self-assembly is the
ability of molecules to form supramolecular assemblies
(quarternary structure). A simple example is the formation of a
micelle by surfactant molecules in solution.
• http://en.wikipedia.org/wiki/Self-assembly

52. Self Assembled Monolayers
• SAMs – Self Assembled Monolayers
• Alkane Thiol complexes on gold
– C10 or longer, functionalized end groups
• Can build multilayer / complex structures
• Used for creating biosensors
– Link bioactive molecules into a scaffold
• The first cells on earth formed from SAMs

53. The self-assembly process. An n-alkane thiol is added to an ethanol solution (0.001 M). A
gold (111) surface is immersed in the solution and the self-assembled structure rapidly
evolves. A properly assembled monolayer on gold (111) typically exhibits a lattice.
The Self-Assembly Process
A schematic of SAM (n-
alkanethiol CH3(CH2)nSH
molecules) formation on
a Au(111) sample.

54. SAM Technology Platform
• SAM reagents are used for
electrochemical, optical and
other detection systems.
Self-Assembled Monolayers
(SAMs) are unidirectional
layers formed on a solid
surface by spontaneous
organization of molecules.
• Using functionally
derivatized C10 monolayer,
surfaces can be prepared
with active chemistry for
binding analytes.
http://www.dojindo.com/sam/SAM.html

55. SAM Surface Derivatization
• Biomolecules (green)
functionalized with
biotin groups (red) can
be selectively
immobilized onto a gold
surface using a
streptavidin linker (blue)
bound to a mixed
biotinylated thiol /
ethylene glycol thiol
self-assembled
monolayer.
http://www.chm.ulaval.ca/chm10139/peter/figures4.doc

56. SAMs C10 Imaging with AFM
http://sibener-group.uchicago.edu/has/sam2.html

57. Multilayer LB Film Process
Smart Materials for Biosensing Devices – Cell Mimicking Supramolecular
Assemblies and Colorimetric Detection of Pathogenic Agents

58. Surface Contamination
• All surfaces become contaminated!
• It is a form of ‘passivation’
– Oxidation of metals
– Adventitious hydrocarbons
– Chemisorption of ions
• It can happen very rapidly
• And be very difficult to remove

59. Measuring Surfaces
• AFM – Atomic Force Microscopy
• SEM – Scanning Electron Microscopy
• XPS (ESCA) – X-Ray Photoelectron
Spectroscopy
• AES – Auger Electron Spectroscopy
• SSIMS – Static Secondary Ion Mass
Spectroscopy
• Laser interferometry / Profilometry

60. XPS/AES Analysis Volume

61. Surface Analysis Tools
SSX-100 ESCA on the left, Auger Spectrometer on the right

62. XPS Spectrum of Carbon
• XPS can determine
the types of carbon
present by shifts in
the binding energy
of the C(1s) peak.
These data show
three primary types
of carbon present in
PET. These are C-C,
C-O, and O-C=O

63. Surface Treatments
• Control friction, lubrication, and wear
• Improve corrosion resistance (passivation)
• Change physical property, e.g., conductivity,
resistivity, and reflection
• Alter dimension (flatten, smooth, etc.)
• Vary appearance, e.g., color and roughness
• Reduce cost (replace bulk material)

64. Surface Treatment of NiTi
Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray

65. Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray
Surface Treatment of NiTi

66. • XPS spectra of the
Ni(2p) and Ti(2p)
signals from Nitinol
undergoing surface
treatments show
removal of surface Ni
from electropolish, and
oxidation of Ni from
chemical and plasma
etch. Mechanical etch
enhances surface Ni.
Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray
Surface Treatment of NiTi

67. Thermal Spray Coating Photomicrographs
Plasma Spray Chromium Oxide Coatings
Plasma Sprayed Chromium Oxide Coatings with base coatings of Hastelloy C for use in very corrosive environments

68. Thermal Spray Coating Photomicrographs
Plasma Spray Chromium Oxide Coatings
Plasma Sprayed Chromium Oxide Coatings with base coatings of Hastelloy C for use in very corrosive environments

69. Surface Derivatization
• A functionalized gold
surface contains a
polar amino tail,
imparting a
hydrophilic character
compared to the
straight chain alkane
thiol. This is an
example of a SAM
http://www.dojindo.com/sam/SAM.html

70. Snow Cleaning with CO2
http://www.co2clean.com/polymers.html

71. Surfaces in Nature
• Cell membranes
– Self-assembled phospholipid bilayers
– Proteins add functionality to the membrane
• Skin (ectoderm)
• Lungs
– Exchange of O2, CO2, and water vapor
• Cell surface recognition (m-proteins)
– Major histocompatibility complex

72. Molecular Self Assembly
3D diagram of a lipid bilayer membrane - water molecules not represented for clarity
http://www.shu.ac.uk/schools/research/mri/model/micelles/micelles.htm
Different lipid model
-top : multi-particles lipid molecule
-bottom: single-particle lipid molecule

73. Cell Membranes
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/default.htm

74. Summary
• Surfaces are discontinuities
• Surface area creates energy
• Dangling bonds lead to passivation
• Interfaces are critical to ‘bonding’
• Surfaces can be modified / derivatized
• Surfaces are critical to life
– All important things happen at a surface!

75. References
• http://www.eaglabs.com/
• http://www.ksvinc.com/LB.htm
• http://www.dojindo.com/sam/SAM.html
• http://www.co2clean.com/clnmech.htm
• http://en.wikipedia.org/wiki/Self-assembly
• http://www.azom.com/default.asp
• SJSU Biomedical Materials Program