Surfaces and Interfaces Nanotechnology Foothill DeAnza Colleges

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

Structure of Silicon Surface Measured 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/

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.

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

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

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

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

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

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/SurfaceTension.html

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

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/

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

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

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

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

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

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

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

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

~99% of living organisms live in the top 1cm of the ocean 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

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.

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

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

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

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

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

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

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

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

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

A Typical Surface Hydrocarbons and water rapidly adsorb to a metal or 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.

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

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"

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.

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

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

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

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.

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

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

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

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

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

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

XPS/AES Analysis Volume

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

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

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)

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

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

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

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

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

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

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

Surfaces in Nature Cell membranes Skin (ectoderm) Lungs 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

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

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

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!

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