Title: Surfaces and Interfaces
1Surfaces and Interfaces
- Nanotechnology
- Foothill DeAnza Colleges
2Surfaces to Ponder
Triply periodic minimal balance surfaces with
cubic symmetry New Geometries for New Materials
http//metalrg.iisc.ernet.in/lord/
3Overview
- Importance of surfaces
- What is a surface?
- Surface structure
- Surface processes
- Surface interfaces
- Surfaces in nature
- Measuring surfaces
- Modifying surfaces
4Importance 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
5Biosphere Our Surface
All important things happen at a surface and
almost all of life on earth!
6Surfaces 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)
7Surfaces 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
8Water Phase Diagram
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
9CO2 Phase Diagram
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
10HeterogeneousSurface 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
11Real 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
12Surface 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
13Silicon 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
14Surface Structure
http//w3.rz-berlin.mpg.de/reuter/highlights/2003
/highlight_karsten.html
15Si 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
16(No Transcript)
17Structure 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/
18Structure of Si Surface
STM image of the Si(1Â 1Â 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.
19Surfaces of Interest
- Silicon Si-OH, carbon
- Metal M-OH, carbon
- Polymer reconstruction / orientation
- Liquid liquid interface / SAMs
- Molecular
- Proteins, lipid walls, etc.
20Surface Processes
- Passivation
- Oxide formation
- Adventitious carbon
- Reconstruction
- Crystalline
- Polymer orientation
- Adsorption of gases and water vapor
- Both can lead to surface passivation
21Surface 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
22Surface 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
23Surface 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
24Surface 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
25Surface Tension in Action
http//www.chem.ufl.edu/itl/2045/lectures/lec_f.h
tml
26How 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/
27Adsorption / 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
28Chemi / 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
29Adsorption 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
30Structure 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
31Polymer 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
32Ozone 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
33Oxide 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
34Metal-oxide Interfaces in Magnetic Tunnel
Junctions
http//shell.cas.usf.edu/oleynik/research-project
s.html
35Surface 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
36Forces 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.
37Importance of Interfaces
- Chemical reactions occur at interfaces
- Particularly corrosion
- Scattering energy
- Electrons
- Light
- Phonons
- An interface is actually two surfaces
38Constant 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
39Defects at Interfaces
- Missing atoms
- Defects and holes
- Extra atoms
- Surface segregation
- Dangling bonds
- Disrupted electronic properties
- Dimensional issues
- Lattice mismatch / shelves
40Atomic 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
41Cohesive / 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
42Cohesive Failure
Material fails cohesively within B
43Adhesive Failure
Material A
Material fails adhesively between A and B
44Adhesive 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
45Surface Reactions
- Oxidation
- Surface diffusion
- Diffusion and oxidation
- Adventitious carbon bonding
- Hydrocarbons from the atmosphere
- Surface rearrangement
- Polymers may reorient to minimize energy
46A 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.
47Langmuir-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
48Langmuir-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"
49Langmuir-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.
50Self 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
51Molecular 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
52Self 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
53The 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.
54SAM 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
55SAM 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
56SAMs C10 Imaging with AFM
http//sibener-group.uchicago.edu/has/sam2.html
57Multilayer LB Film Process
Smart Materials for Biosensing Devices Cell
Mimicking Supramolecular Assemblies and
Colorimetric Detection of Pathogenic Agents
58Surface 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
59Measuring 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
60XPS/AES Analysis Volume
61Surface Analysis Tools
SSX-100 ESCA on the left, Auger Spectrometer on
the right
62XPS 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
63Surface 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)
64Surface Treatment of NiTi
Biomedical Devices and Biomedical Implants SJSU
Guna Selvaduray
65Surface Treatment of NiTi
Biomedical Devices and Biomedical Implants SJSU
Guna Selvaduray
66Surface 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
67Thermal Spray Coating PhotomicrographsPlasma
Spray Chromium Oxide Coatings
Plasma Sprayed Chromium Oxide Coatings with base
coatings of Hastelloy C for use in very
corrosive environments
68Thermal Spray Coating PhotomicrographsPlasma
Spray Chromium Oxide Coatings
Plasma Sprayed Chromium Oxide Coatings with base
coatings of Hastelloy C for use in very
corrosive environments
69Surface 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
70Snow Cleaning with CO2
http//www.co2clean.com/polymers.html
71Surfaces 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
72Molecular Self Assembly
73Cell Membranes
http//faculty.clintoncc.suny.edu/faculty/Michael.
Gregory/default.htm
74Summary
- 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!
75References
- 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