rganic thin films Robin Ras Soft Matter and Wetting group Dept. Applied Physics Aalto University http://physics.aalto.fi/groups/smw/
rganic thin films as stabilizer for colloids (not discussed here) solid-liquid interface nanoparticles liquid-liquid interface emulsions vesicles liquid-gas interface foams bubbles Lipid bi-layers (vesicles, membranes) P P P P P P P CdS Pe P P P P nanoparticles foam emulsion (e.g. milk) rganic thin films: deposition techniques Langmuir-Blodgett films self-assembled monolayers spin coating layer-by-layer assembly polymer grafting molecular layer deposition
Langmuir-Blodgett technique to make thin films based surfactant-like molecules Langmuir-Blodgett technique is based on formation of floating film on water, its compaction by squeezing to make densified 2- dim solid-like film on water surface, and its transfer on a substrate To form the film on the water surface, the method works usually required to make molecules surface active, i.e. they have both hydrophobic and hydropilic parts For example hydrophobic polymers to have hydrophilic side chains etc. Prototypical examples are surfactants (and phospholipids) hydrophilic hydrophobic hydrophobic hydrophilic d Kontturi et al
Langmuir-Blodgett: Pressurization curve
Coating of substrates Dippings of the substrates (glass, silica wafer etc) ne dipping: one layer More dippings: more layers Two-dimensional nanometer thickness films This is one of the standard methods to make eg. semiconducting polymer layers, nanoparticles assemblies, optical elements etc Hall: The New Chemistry
Examples for Langmuir-Blodgett applications Any molecules that float on water can be used in Langmuir- Blodgett method For example, nanoparticles that have surface brushes Gold nanoparticles with C12 brushes n substrate hydrophobic r eg. Conjugated electro-active polymers that are made surfactant like by side chains allow preparation of semiconducting layers hydrophilic
Self-Assembled Monolayers (SAMs) Liquid phase coating of a surface (funtionalized with a suitable reactive or interacting groups) with a one molecule thick layer (monolayer) self-limiting growth The most common example is given by thiols ( SH) reacting very easily with gold-surfaces Example: C n H 2n+1 -SH forms a dense surface layer licon wafers can be cold-plated first to allow SAM Au X SH X S Au zin: Nanochemistry
Self-Assembled Monolayers (SAM) The preparation of SAMs typically involves immersing a gold-coated substrate in a dilute solution of the alkanethiol in ethanol (Figure 3). A monolayer spontaneously assembles at the surface of the substrate over the next one to twenty four hours. Initially, within a few seconds to minutes, a disordered monolayer is formed. Within this early time frame, the thickness reaches 80-90% of its final value. As the layer continues to form, van der Waals forces between the hydrocarbon chains help pack the molecules into a well-ordered, crystalline layer. During this ordering phase, contaminants are displaced (for example, adventitious hydrocarbons on the gold), solvents are expelled from the monolayer, and defects are reduced while packing is enhanced by increased packing of the alkanethiols.
Surface coatings and functionalizations with organosilanes: First activation of the surface to achieve lots of hydroxyl groups H. Then liquid phase addition of molecules containing triethoxysilane end group The triethoxysilane endgroups react with the surface H groups and other H-groups due to hydrolysis and allow reactive bonding This procedure can be repeated if there is another H group at the other end of the added molecule to make multiple layers r if there is another functional group, this allows to connect other chemically active groups on the surfaces t R + H0H Hydrolysis H + R0H Water condensation H + H + H0H H R H 5 C 2 C 2 H 5 C 2 H 5 Some spacer group X R H 5 C 2 C 2 H 5 C 2 H 5 R + Alcohol condensation H + R0H
Example for the use of Self-assembled Monolayers: Microcontact Printing I Polydimethylsiloxane (PDMS) Amorphous Glass transition T g = -123 o C Soft Cross-link: soft rubber Preparation of a master licon wafer Prepolymer (linear) Crosslinked rubber (network) Electron beam lithography slow But each master allows tens of stamps photoresist spin coated patterning using eg. electron beam lithography Expose with radiation Develop (remove the radiation degraded photoresist) ch to prepare patterns Use the master as a mold to fill with liquid PDMS uncured polymer Cure (cross-link) PDMS to make the stamp Remove the stamp zin: Nanochemistry
Example for the use of Self-assembled Monolayers: Microcontact Printing II Use stamps to replicate patterns Use alkanethiols as ink for the stamp Alkanethiols stamped on substrate patterns to protect gold in etching Chemical treatment of the unproctected patterns to remove Au Leads to patterns without photolithography mple and cheap zin: Nanochemistry
Dip Pen Lithography (DPN) Use Atomic Force Microscope (AFM) tip to draw lines Ink: alkanethiols Less than 10 nm linewidths zin: Nanochemistry
There is plenty of room at the bottom Richard Feynman, 1959 written at the nanoscale Dip-Pen lithography Developed by Chad Mirkin (1999) an atomic force microscope tip is used to transfer molecules to a surface via a solvent meniscus. DPN is the nanotechnology analog of the dip pen (also called the quill pen), where the tip of an atomic force microscope cantilever acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper."
A dip-pen nanolithography that has an array of 55,000 pens that can create 55,000 identical molecular patterns The background shows some of the 55,000 miniature images of a 2005 US nickel made with dip-pen lithography. (Each circle is only twice the diameter of a red blood cell.) Each nickel image with Thomas Jefferson's profile (in red) is made of a series of 80 nm dots. The inset (right) is an electron microscope image of a portion of the 55,000-pen array (Angewandte Chemie 45 1-4, 2006 )
Various other schemes to make SAM Require simple surface reactive group
Reminder: Polyelectrolyte multilayers (layer-by-layer deposition) First make the substrate charged See the next page Use oppositely charged polymers i.e. polyelectrolytes in solutions Not all of the ionic groups of the polymer are used for compensation of the surface charge, therefore the charge is overcompensated Layer-by-layer growth of organic polyelectrolytes
How to make a surface charged Example 1: Self-assembled monolayers (SAM) on gold Aminoalkanethiols Thiol SH makes a covalent bond to Au Amino NH 2 becomes positively charged NH 3 + under acidic conditions NH 2 NH 2 Note: SAM is a general and very important surface functionalization to functionalize gold surfaces by generic X by using HS-(CH 2 ) n -X Au SH Au S Au Example 2: lane chemistry on silica, glass, -wafer H 2 N H 2 N H 2 N H 3 C CH 3 CH 3 H H
Examples of polyelectrolytes for layer-by-layer multilayers Polyanions: Negatively charged Polycations: Positively charged
Layer edges are not sharp The layers do not have well defined interfaces They do not have Bragg scattering (X-ray) Like layers of differently colored spagetties layered
The thickness of the coating behaves smoothly as a function of number of layers linear or exponential Layer thickness increases as a function of the cycles
How Thick is Thin Ellipsometry Polarized incident light Polarization of reflected light studied Measures how the parallel-to-surface polarization vs the perpendicular-to-the surface polarization components change Allows to determine the film thickness Quartz crystal microbalance Piezoelectric quartz crystal oscillates at a resonance frequency f If more mass is provided to the oscillation, the resonance frequency goes down More mass is provided by adsorbed surface layers Allows to deduce on adsorption on the surface Pictures from Wikipedia Also Ultraviolet and visible spectroscopy (absorption) Neutron scattering X-ray reflectivity
Alternating hard and soft matter imitate nacre which has expectional combination of high stiffnes, strength, and toughness Biomineralized nacre in abalone shells Alternating CaC 3 (hard) and protein (soft) layer Stiff, tough, and strong CaC3 stiff and strong Now Layer-by-layer concepts allow one example of biomimetics, i.e. to imitate the biological structures Real nacre Scheme for nacre mimic: Alternating negatively charged CaC 3 nanosheets and positively charged polymers made by layerby-layer deposition
Spin coating liquid dispensing spinning thinning + evaporation short coating time reliable cheap uniform film thickness in case of Newtonian liquids (=viscosity remains constant with shear rate) Film thickness is largely a balance between the force applied to shear the fluid towards the edge of the substrate and the drying rate which affects the solids content and thus viscosity. As the film dries, the viscosity increases until the radial force of the spin process can no longer appreciably move the liquid over the surface. Annu. Rep. Prog. Chem., Sect. C, 2005, 101, 174
Spin coating Factors affecting the film thickness: polymer concentration (viscosity) boiling point of solvent spinning speed (larger speeds, thinner films) spinning acceleration atmosphere no dust evaporation is slower in presence of solvent vapor, leading to thinner films substrate clean appropriate surface groups for adhesion of the films Possible problems: striations ( comets ) due to dust particles (better substrate cleaning, or filtering of solution) due to air bubbles (degassing of solution) incomplete filling of substrate (dispensing of larger volume) inhomogeneities due to dust (better substrate cleaning, or filtering of solution due to inhomogeneous evaporation of solvent (try with higher boiling point solvent, solvent vapor) due to insufficient levelling of liquid films (try higher spinning speed) Applications: photoresist (lithography) organic field-effect transistors active matrix displays LEDs photovoltaics sensors protective coating in CDs and DVDs optical coating anti-reflection
Grafting to H H H H H H H H H 3 C H 3 C C H 3 GPS H H H H H H H C H 3 H H HC H H H H ooc H H H H H HC H H H H ooc H H H H ooc
Grafting from H H H H H H H C H 3 H H H2 N NH 2 R NH 2 R NH CN N N Me ACP Me CN H R NH CN Me N N Me CN H N
Grafting density affects conformation
Ultralow-Fouling, Functionalizable, and Hydrolyzable Zwitterionic Materials At present, there are few materials that can effectively resist nonspecific protein adsorption from real-world complex media and meet the challenges of practical applications, such as medical implants, drug-delivery carriers, and biosensors. grafted polymers Adv. Mater. 2010, 22, 920 932 Due to electrostatically induced hydration, surfaces coated with zwitterionic groups are highly resistant to nonspecific protein adsorption, bacterial adhesion, and biofilm formation.
Artificial cartilage: reduced friction of Klein et al. Science 323, 1698 (2009) grafted polymers molecular brushes prostheses lubricating layers of "molecular brushes" can outperform nature under the highest pressures encountered within joints, with potentially important implications for joint replacement surgery. By introducing charges to the brushes you get a hydration layer of water molecules around the charges. These water molecules are tightly bound, in the sense that it's hard to remove them all at once, but individual molecules are able to rapidly exchange with water in the surrounding solvent or the hydration layer of another charge. This gives them the properties of molecular ball-bearings - you can press hard on them and they won't release their water, but when you come to shear them [move the surfaces over each other] they behave in a fluid way, which gives the excellent lubrication properties.'
Molecular Layer Deposition (MLD) Analogous to ALD, but for organic films. Another type of grafting, but this is in principle extremely well controlled: monomer by monomer. Based on sequential, self-limiting surface reactions. limitation: precursors have to be volatile Acc. Chem. Res., 2009, 42 (4), 498