Surfaces and Interfaces

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Surfaces and Interfaces

• Nanotechnology
• Foothill DeAnza Colleges

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

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

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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

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Biosphere Our Surface
All important things happen at a surface and
almost all of life on earth!
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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)

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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

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Water Phase Diagram
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
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CO2 Phase Diagram
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
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HeterogeneousSurface 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/projectSPP1095s
olidification.html
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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

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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
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Silicon Surface Planes

• Model of the ideal surface for Si1111x1.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
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Surface Structure
http//w3.rz-berlin.mpg.de/reuter/highlights/2003
/highlight_karsten.html
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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_dar
iniai/sol_st_phys/defects.htm
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Structure of Silicon SurfaceMeasured using STM
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).
http//www.chm.ulaval.ca/chm10139/
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Structure of Si Surface
STM image of the Si(1 1 1)(77) 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.
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Surfaces of Interest

• Silicon Si-OH, carbon
• Metal M-OH, carbon
• Polymer reconstruction / orientation
• Liquid liquid interface / SAMs
• Molecular
• Proteins, lipid walls, etc.

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Surface Processes

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

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Surface Free Energy
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
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Surface Energetics

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

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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
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Surface Tension

• 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.

http//scienceworld.wolfram.com/physics/SurfaceTen
sion.html
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Surface Tension in Action
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
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How do MoleculesBond 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/
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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

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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.ht
m
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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.d
oc
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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.htm
l
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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/screenSho
ts.htm
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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
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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/su
rfaces/SURF2.html
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Metal-oxide Interfaces in Magnetic Tunnel
Junctions
http//shell.cas.usf.edu/oleynik/research-project
s.html
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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
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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.

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Importance of Interfaces

• Chemical reactions occur at interfaces
• Particularly corrosion
• Scattering energy
• Electrons
• Light
• Phonons
• An interface is actually two surfaces

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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_se
m_images/Omicron
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Defects at Interfaces

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

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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_se
m_images/Omicron
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Cohesive / AdhesiveFailure 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

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Cohesive Failure
Material fails cohesively within B
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Adhesive Failure
Material A
Material fails adhesively between A and B
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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?ArticleID2089
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Surface Reactions

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

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A Typical Surface
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.
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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

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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_fi
  lm"

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

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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

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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

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The Self-Assembly Process
A schematic of SAM (n-alkanethiol CH3(CH2)nSH
molecules) formation on a Au(111) sample.
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.
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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
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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.d
oc
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SAMs C10 Imaging with AFM
http//sibener-group.uchicago.edu/has/sam2.html
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Multilayer LB Film Process
Smart Materials for Biosensing Devices Cell
Mimicking Supramolecular Assemblies and
Colorimetric Detection of Pathogenic Agents
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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

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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

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XPS/AES Analysis Volume
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Surface Analysis Tools
SSX-100 ESCA on the left, Auger Spectrometer on
the right
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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-CO

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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)

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Surface Treatment of NiTi
Biomedical Devices and Biomedical Implants SJSU
Guna Selvaduray
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Surface Treatment of NiTi
Biomedical Devices and Biomedical Implants SJSU
Guna Selvaduray
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Surface Treatment of NiTi

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

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Molecular Self Assembly
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Cell Membranes
http//faculty.clintoncc.suny.edu/faculty/Michael.
Gregory/default.htm
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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