Kompendium i TMT4320 - Nanomaterialer

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Chapter 1: Nanochemistry Basics

Not terribly important.

Chapter 2: Soft Lithography

Self-assembled monolayers (SAMs)

PDMS stamp

Hydrophilic / Hydrophobic stamps

Printing thin films

Electrically contacting SAMs

Patterning by photocatalysis



Kapittel 3: Building layer-by-layer

Electrostatic superlattices

Some applications

Analysis, measuring film thickness

Non-electrostatic LbL assembly

Low-pressure layers

Lbl self-limiting reactions



Kapittel 4: Nanocontact printing and writing

Soft lithography and microcontact printing

Manipulating PDMS stamp

  1. Compress the stamp, mold to get a new stamp with inverse pattern, peel off and repeat. The new stamp has lower dimensions than the master.
  2. Apply force perpendicular onto stamp when on substrate. The areas in contact with substrate will then increase, and spaces in between gets smaller.
  3. Size reduction by reactive spreading of ink when in contact with substrate. The contact time + properties of the ink decide to which degree the ink spreads. The printed area is increased and the spacing between is reduced.
  4. Size reduction by extraction of inert filler (just like removing water from a sponge).
  5. Size reduction by swelling the stamp in toluene. The areas in contact with the surface are increased in size while the spacing between is reduced.
  6. Size reduction by stretching stamp so that dimensions get smaller in one direction and larger in another.
  7. Size reduction by double-printing.

Dip pen nanolithography

Whittling of nanostructures

Nanoplotters and nanoblotters

Combinatorial libraries



Kapittel 5: Nano-rod, nanotube, nanowire self-assembly

Templating nanowires and nanorods

Templates can be used for making solid nanorods and nanotubes of controlled size. Examples of templates are alumina, silicon, zeolites and lipid bilayers. If the holes are completely filled nanorods and nanowires result, while a partial filling with continuous coating gives rise to nanotubes.

Making modulated diameter silicon templates

A p-doped silicon wafer is put in aqueous HF and an oxidizing potential is applied. The result from this is nanoporous silicon with a random network of pores. The diameter of the pores can be tuned by controlling the voltage or current. The higher the current is, the wider the channels get. If the current is modulated during oxidation, the resulting structure is an array of modulated diameter nanochannels. If perfectly ordered pores are desired, the wafer can be lithographically patterned with regular array of nanowells in advance. The electric field will then be focused at the tip of these wells.

Making porous alumina membranes

Porous alumina membranes can be made by anodic oxidation of lithograpically embossed aluminum sheet in phosphoric or oxalic acid electrolyte (the almunium sheet functions as the anode).

2Al + 3PO_4^{3-} \rightarrow Al_2O_3 + 3PO_3^{3-}

The residual Al and Al2O3 is removed by mercuric chloride and phosphoric acid. The diameter is controlled and can be 20-500nm. Mechanisms that give ordered channels are the fact that electric fields created by applied voltage (which is concentrated at the tips of the growing tubes) repell each other, and that we have volume expansion when aluminum becomes alumina. Temperature is also a factor that affects the reaction. In this process oxygen diffuses through the alumina layer from the electrolyte and alumina grows at the alumina/aluminum interface, while alumina is slowly dissolved at the alumina/electrolyte interface. This growth/dissolution comes to an equilibrium at the bottom of the pore, giving a specific thickness for a certain current/voltage. The growth of alumina is still allowed to continue upwards (along the pore walls) where the electric field is weaker, giving longer pores. Growth continues until the electric field is quenced or there is no more aluminum left.

Modulated diameter gold nanorods

With use of silicon template. The back surface of the silicon membrane is subjected to a local thermal oxidation which formes silica. The silica is then removed by HF. By proceeding with a KOH anisotropic etch on the same area, and a dip in HF, the pores in the template are opened. A gold sputter deposition can then be done on the backside. This gold layer acts as a catalyst for continued electroless deposition of gold. Finally, the silicon membrane is etched away, and the gold nanorod dispersion can be collected.

Modulated composition nanorods/nanobarcodes

Modulated composition nanorods can be made by electrochemical deposition of different metal segments within the channels of an alumina template (electrodeposition will be better explained in the following section). Any type of material that can be electrodeposited can be used in the nanobarcodes. One synthesis route is to evaporate thin metal film to one side of an alumina membrane. This metal film function as the cathode, and metal deposition begins at the bottom. Bath can be switched between different metal salts to grow several segments. The lenght of the metal segments scales directly with the current. The alumina membrane is dissolved using sodium hydroxide, and the metal backing is dissolved using acid.

Nanobarcodes can be used to tag molecules in analytical chemistry and biology. Characteristic of metals are optical reflectivity, which means that different segments of the barcode nanorod can be distinguished in optical microscopy. Probe molecules must be anchored to different segments, and the rods must be dispersed in analyte containing target molecules which bear a luminescent label. By molecular recognition, the target molecules bind to the probe molecules (ex: ligand-receptor binding for biological applications). By looking at the segments that light up, it can be decided which molecules exist in the solution.

Electroplating/electrodeposition

The part to be plated is the cathode, while the anode is made of the material to be plated. Both components are immersed in electrolyte solution. The dissolved metal ions (cations) are reduced at the interface between the solution and the cathode when current is applied.

Electroless deposition

This is an auto-catalytic plating method that involves several simultaneous reactions in an aqueous solution. The reaction involves plating of a metal onto a conductive surface and occurs without the use of external electrical power. This is accomplished when hydrogen is released by a reducing agent and thus producing a negative charge on the surface of the metal. There is no direct control over length or thickness of the deposited layer. This needs to be calibrated with regards to concentration of precursor and amount of time that reaction is allowed to run.

Nanotubes

Nanotubes can be made by partial filling of the membranes radially. This means that a uniform coating must be deposited on the pore walls. One way to do this is by letting fluid spontaneously wet inside the template pores. Fluids that can be used are molten polymers, polymer solution or sol-gel preparation. These are coated onto template using capillary forces resulting from small diameter channels with a large available surface. Solidification of these fluids can be done by heating, cooling, waiting or using a catalyst. With this method it is difficult to control the wall thickness. Another way to make nanotubes is by using LbL growth procedure inside the pores. This can be done by CVD of gas phase species, solution phase ALD or LbL electrostatic assembly. Wall thickness is easier to control with these methods. Finally, the membrane is dissolved. It can also be deposited other material inside the remaining void to get coaxially coated rod or wire.

Nanotubes can also be made from LbL electrostatic coating of nanorods. The rods can be dissolved afterwards, and will leave a closed-ended tube. This method is applicable to any material that can be coated onto a nanorod and not be affected by the etching step.

Magnetic Nanorods

Magnetic metals such as iron, cobalt or nickel can easily be deposited into membranes. Magnetic properties are direction and size dependent. By applying a magnetic field, the segments become permanently magnetized and there will be attractions between the rods. If the thickness of the magnetic segments on a nanorod is smaller than the diameter, magnetization is perpendicular to the rod axis, and they will self assemble into 3D bundles. If the thickness is bigger than the diameter, magnetization is parallel to the rod axis, and they will align in chains of rods. If the thickness is the same as the diameter they will be in random aggregates.

Magnetic nanorods can be used for separation of molecules. A tri-segmented Au-Ni-Au nanorods can be used as affinity template for histidine- tagged proteins. Nickel selectively captures the labeled protein, and a magnetic field can be used to separate the rod with the captured protein from the rest of the solution of biomolecules. After this, the proteins can be chemically released from the magnetic nanorod. The gold segments must be in the rod to protect nickel from the etching during dissolution of alumina template after electrodeposition, and also to prevent aggregation.

Making Single Crystal Nanowires

Single crystal nanowires can be made by Vapor-Liquid-Solid (VLS) synthesis, Supercritical Fluid-Liquid-Solid (SFLS) synthesis or by Pulsed laser deposition.

A catalyst droplet first melts on a substrate, then becomes saturated with precursors. Elements extrude out of the catalyst droplet as a single crystal nanowire in a furnace where the temperature is controlled to maintain liquid state of the catalyst droplet. Micrometer length with diameter less than 10 nm can be done. The diameter is controlled by the diameter of the catalyst droplet, and growth stops when the nanowire pass out of the hot zone, if the precursor is depleted or the catalyst droplet no longer is in liquid state. One example is to use laser ablation of Fe-Si target to evaporate the precursors and to create a Fe-Si nanocluster catalyst droplet. The Si nanowire grow with the (111) lattice planes perpendicular to the growth axis due to epitaxy at the nanocluster-nanowire interface. Doping can be done by controlling stoichiometry of the target, or by introducing dopant into gas phase during growth.

Similar to VLS, but used for materials with a higher eutectic temperature. This technique increases the variety of available source materials. The solvent is pressurized above its critical point to reach higher temperatures. Can be applied to semiconductor/metal combinations (Ga/GaAs, In/InN) with eutectic temperature below 600 degrees. Au is used as catalytic seed, and diameter depends on this.

A high-power pulsed laser is used to ablate a target (pulsed laser ablation) in a vacuum chamber, meaning that the pulsed laser vaporizes small parts of the target for each pulse. This creates a plume of vaporized precursor material which is allowed to deposit as a thin film onto a substrate that is placed in the reaction chamber. When small catalyst particles are placed on the substrate, small single crystal nanowires can be grown. The diameter of the nanowires are determined by the diameter of the catalyst particles.

Nanowires branch out

Can create branched nanowires by VLS growth. The catalytic nanoclusters from solution placed on specific point on the body of a parent nanowire before growth. The process can be repeated for a hyper-branched construction. This could be the future development of nanowire electronics in 3D.

Quantum Size Effects (QSE)

QSE appear when the particle size becomes smaller than the exciton size for the material (about 5 nm for silicon). Exciton is a bound state of an electron and an electron hole in an insulator or semiconductor, which is defined by the energy gap between the valence band and the conduction band. Color of the emitted light is determined by the size of gap energy. Gap energy increases with decreasing nanowire diameter. This can be used for LEDs and lasers. Both quantum confined nanoclusters and nanowires show QSE, but anisotropy make them different. Luminescent nanoclusters emits plane-polarized light, while nanorods exhibits linearly polarized light.

Alignment methods

Alignment methods include electric field based alignment, microfluidic alignment and Langmuir-Blodgett technique.

Apply voltage between two micropatterned electrodes to produce electric field. Charges within a nanowire in solution become polarized, creating an attraction between the electrodes and the nanowire. The electric field is quenched when the gap between the electrodes are bridged by a nanowire. This eliminates absorption of a second nanowire at the same electrodes. Metal spots can be evaporated onto insulator surface to focus the electric field.

A PDMS stamp with a series of parallel rectangular grooves is used for this purpose. The channels are aligned under a microscope with electrodes that have been previously patterned on a substrate (these will function as metal contacts for the conducting or semiconducting lines made by this method). A drop of nanowire suspension is flowed into the microchannels by capillary forces, and solvent evaporation aligns the wires at the edges of the channels.

A Langmuir film is created when hydrophobic molecules float on a water-air surface, and an aligned monolayer is formed at the interface when external film pressure is applied. The balance of surface tension forces determines the profile of the meniscus formed when a substrate is pushed into this liquid. If the substrate is hydrophobic it will experience deposition of the amphiphiles during immersion. If it is hydrophilic it will experience deposition during retraction. A nanowire array can be made by firstly compressing the interface to increase the surface density of nanowires (so they align parallel to each other), and then do a double dip. The second dip must be done so that the wires align normal to the previous once. It is important that the film pressure is mantained at a constant magnitude during the immersion.

Applications

Application areas for these methods are in LED’s, transistors and in nanowire UV photodetectors.

LED

A LED can be made by assembling an n-doped and a p-doped semiconductor nanowire perpendicular to each other. This is done by electric field based alignment with two electrode pairs aligned perpendicular to each other where voltage is applied to one pair at a time. They can also be assembled by using the microfluidic approach. When a potential is applied across the junction, light is emitted when electrons recombine with holes at the junction between the differently doped wires. Color of the emitted light depends on composition and condition of semiconducting material used. The LED can only conduct current in one direction. With positive voltage current flows. With negative voltage current is inhibited. The key for success is to achieve abrupt and uncontaminated junction between n- and p-doped wire. Efficiency can be improved by using core-shell-shell nanowire axial heterostructure. The greatest challenge is to make arrays of closely spaced junctions because the nanowires are so thin. This leads to the pitch problem, how to pack light sources into smallest possible area.

Transistors

A transistor can switch or amplify signals, and has three terminals (n-p-n). The n-type region attached to the negative end of the battery sends electrons into p-region, and the n-type region attached to the positive end slows the electrons down. The p-type region in the middle does both. Because of this, a depletion layer develops between the base and the emitter, and the base and the collector. The thickness of the layer is varied by the potential in each region. Active bipolar n-p-n transistor can be built from heavy and lightly n-doped nanowires crossing a common p-type wire base.

Nanowire transistors can be used as sensors. Si nanowires are naturally coated with silica through VLS synthesis. This makes it easy for surface silanol groups to attach to the wire. If probe molecules are anchored to the surface silanols, highly sensitive real time electrically based sensors can be made. Low levels of chemical and biological species can be detected. Boron doped silicon nanowire is used as a FET. The wire is self assembled across electrodes (source and drain), and aminoethylsilane anchored to SiOH surface groups. The conductance of the wire changes with pH linearly due to protonation or deprotonation of the amine. An increase of the surface negative charge (deprotonation) attracts additional holes into the p-channel and the conductance is enhanced. The reverse action at low pH, an increase of surface positive charge causes protonation which repell holes from the channel. The conductance is decreased. Almost any type of molecule can be anchored to silica, so sensors can be designed to detect almost anything. For example, a biotin could be strapped to the surface amine groups to detect streptavidin.

Nanowire UV photodetector

The conductivity of ZnO nanowires is extremely sensitive to ultraviolet light exposure, which means that UV light can switch the nanowires between ON and OFF states. ZnO nanowires are highly insulating in the dark, but UV light with wavelength less than 380 nm decreases resistivity by 4 to 6 orders of magnitude. These nanowire photoconductors exhibit excellent wavelength selectivity. Green light (532nm) gives no response, while less intense UV light increases conductivity 4 orders. The response cut-off wavelength is at about 370 nm.

Simplifying complex nanowires

Complex oxides with superconducting, ferroelectric and ferromagnetic properties can not easily be made as nanowires by conventional methods. MgO nanowires must be used as templates. Firstly, single crystal orthogonal MgO nanowires are grown on single crystal MgO substrate. Oxygen is flowed over Mg3N2 at 900 degrees as precursor for VLS, using Au catalyst. After the MgO nanowires have been made, the complex metal oxide is deposited by pulsed laser deposition to create a shell on the surface of MgO wires. Another approach to simplify complex nanowires is to use hydrothermal synthesis. This can be used to make PbTiO3 nanorods which is a ferroelectric material and potentially useful as building blocks in nanoelectrochemical systems. (Amorphous PbTiO(3 - X)OH2X (mulig jeg rettet feil/misforstod?) precursor is mixed with sodium dodecyl benzene sulfonate surfactant and reacted at 48 h at 180 degrees at alkaline conditions in the presence of a substrate.) The nanorods obtained have a squared cross section 35-400 nm, and up to 5 um long. The rods grow in the (001) direction by self-assembly of nanocubes to anisotropic mesocrystals, which is ripened into nanorods.

Electrospinning

Electrospinning is nanofiber extrusion in a capillary jet. A polymer solution or polymer sol-gel pass through a high voltage metal capillary to create a thin charged stream. The stream undergoes stretching, bending and solvent evaporation. The charged nanofibers are driven to ground electrodes. The dimensions of the fibers depend on solvent viscosity, conductivity, surface tension and precursor concentration. The collector electrodes can be patterned to make organized arrays between them by electrostatic self assembly. The electrodes can be grounded simultaneously or sequentially. This can be used to make single layer or multilayer nanowire architectures.

Hollow nanofibers by electrospinning

Hollow nanofibers can be made by co-axial double capillary electrospinning that creates heavy mineral oil core with inorganic polymer around (Ti and PVP). The core-shell nanofibers are collected on an aluminum or silicon substrate and hydrolyzed. The oily core can be extracted with octane, which creates nanotubes with amorphous TiO2 + PVP. To crystallize TiO2 and oxidate PVP, the tubes can be calcined (varmebehandlet) in air at 500 degrees.

Dual electrospinning

A side by side spinneret can be used to make bicomponent fibers. Ex: two solutions containing TiO2/SnO2 are simultaneously jetted. This is calcined. A heterojunction of SnO2/TiO2 can create devices with extremely high quantum efficiency and photocatalytic activity for treatment of organic pollutants in water and air.

Carbon nanotubes

Carbon nanotubes (CNT) was discovered in 1991 by Iijima, and have had a great impact on nanotechnology. The CNTs are made of rolled up graphite sheets to create a hollow tube. Both single-walled (SWNT) and layered multi-walled (MWNT) nanotubes exist.

Structure

Carbon nanotubes exist in three different structures, depending on the angle at which the graphite sheet is rolled up. These are characterized by their different properties in electron transport. The achiral tubes, which are the "zig-zag" and "armchair" tubes, are metallic. The metallic tubes have two mini-bands between the valence and conduction band. Quantum mechanical tunneling leads to electrical conductivity. For these, ballistic electron transport have been observed, which means that there is electrical conductivity with no phonon or surface scattering. The chiral tubes are semiconducting, and is the most common found of the CNTs.

Synthesis methods

Separation of nanotubes

Carbonaceous impurities an metal catalysts can be removed by a high temperature treatment in oxygen, followed by boiling in a diluted mineral acid. The carbon nanotubes can then be sorted by length by precipitation from non-solvent followed by centrifugation. Also, the metallic tubes can be separated from the semiconducting by electrophoresis or precipitation by evaporation of an octadecylamine solution.

Properties

Mechanical

CNTs are a extremely strong material compared to other known high-strength materials (high-carbon steel, kevlar). It has the highest specific strength value (strength-to-mass-ratio) of the currently discovered materials in the world. It also has a very high Young's modulus (E-modulus) and tensile strength. When the tubes is bended they deform reversibly. It's excellent mechanical properties makes it useful for lightweight fibers for strengthening of plastic, ceramic and metals. The properties were demonstrated creating a rotational actuator.

Electrical

As mentioned earlier, the achiral tubes, which are the "zig-zag" and "armchair" tubes, are metallic because they have two mini-bands between the valence and conduction band that leads to quantum mechanical tunneling, and electrical conductivity. For these, ballistic electron transport have been observed, which means that there is electrical conductivity with no phonon or surface scattering. The chiral tubes are semiconducting, and is the most common found of the CNTs.

Chemical

Carbon nanotubes are made of Carbon (C) and are by default chemically inert. They can be made chemically active by opening their ends by oxidation with a strong acid such as nitric acids, which also introduces carboxylate functionalities.

Carbon nanotube chemistry

Carbon nanotubes have strong van der Waals interactions between the walls, which cause them to precipitate when dispersed in a solution. Chemical modification of the nanotubes has been used to make them soluble. Oxidation with nitric acid opens the ends of the CNTs and introduces polar carboxylate groups, which makes them water soluble. Another method is to expose the CNTs to a starch solution, the big starch molecules wraps around the nanotubes by van der Waals interactions. Re-precipitation is possible by adding amylase (breaks down the starch). This method disrupts the properties of the CNTs to a lesser degree than the former method.

The nanotubes is reactive with many species due to dangling π-bonds on the inside and outside of the tube. The versatility in chemical species than can be anchored to the tubes, makes it possible to create a chemical force microscopy by using carbon nanotubes at the end of an AFM tip.

CNTs have also been used as a sensor. A FET CNT device is made by placing a tube between two electrodes (source and drain) on a Si-substrate (gate). Because CNTs have a conjugated pi-electron system, they can bind to benzene-derivatives. The electron donating ability of the benzene-derivatives depend on the substituents on the benzene rings, and affect the electron density of the tubes. This change in electron density is detected as a change in conductivity.

Aligning of carbon nanotubes

Applications

As mentioned earlier in this section, CNTs can be used as sensors, fiber-strengthening of composite materials and added to materials to improve conductivity.



Kapittel 6: Nanocluster Self-Assembly

Capped nanoclusters

A capped nanocluster is a nanometer scale particle with well-defined positions of the constituent atoms. They nucleate from atoms and enter a size range where they behave electronically as molecular nanoclusters. As the number of atoms increases further, they cross over into the nanoscale size domain where quantum size effects dominate, they become quantum dots. A capped nanocluster has a monolayer of a capping ligand on the surface, which can be a polymer or an alkane thiol (if the surface is silver or gold) or some other molecule with an end group that will bind to the surface of the nanocluster. The capping molecules will prevent further growth of the nanocluster. Capping groups serve multiple purposes:

General principles for synthesis of capped nanoclusters

One general synthesis method is the arrested nucleation and growth synthesis. The basic idea is to rapidly create a large number of nucleated seeds (of desired materials) and then allow these to grow at the same rate below supersaturation conditions. This method can be described by the following steps:

Minimize size dispersity by confining the reaction space

An illustration of how to make a confined reaction space

The size of the capped nanoclusters can be controlled by growing them in nanowells made by the methode in figure below. The nanowells are obtained by patterning a silicon wafer with a layer of well-ordered microspheres. By pressing the microspheres against the wafer and at the same time melt the surface of the wafer with a pulsed laser, molten silicon will flow into the voids between the spheres. The size of the nanowells depend on the size of the spheres, the energy density of the laser pulse and applied mechanical pressure, while the size of the crystals depend on the well volume and concentration of the reactants. The crystals can be removed by ultrasound. The downside of the approach is that the amount of nanocrystals obtained will be quiet small.

Quantum size effects in nanoclusters

When electrons are confined in space, the size invariant continuum of electronic states of bulk matter transforms into size-dependent discrete electronic states in a quantum dot. At the 1-5 nm length scale, which is the CdSe nanocluster size range, the parent continuous electron bands of the bulk semiconductor becomes discrete. The nanoclusters then belong to the quantum size regime, and the properties begin to scale in a predictable fashion with size. By looking at the Schrödinger wave equation it can be seen that there is a wavelength shift towards the blue spectrum in the energy of the first exciton band. Band gap scales with the reciprocal of the square of the radius of the nanocluster(E_g \propto r^{-2}). The wavelengths absorbed change, and the colors of the nanoclusters can be altered from yellow to red, by changing the physical size of the clusters.

Different phases occur for smaller size particles

Similar to temperature and pressure, phase transformations in bulk materials are dependent on size. Phase transitions that are prohibited or slowed down by activation energies in the bulk, can occur much more readily in nanocrystals of the same material. Because of the small size of the crystal, the influence of bulk and surface-free energies are different from in a bulk matter. Phase transformations show a distinct dependence on nanocrystal size. It can be shown that phase transformation for nanoclusters can occur just by exposing them to a different chemical environment at room temperature.

Making nanoclusters water soluble

Why? Water is cheap, widely available and use of it avoids the disposal of organic solvents, which can be quite harmful for the environment (green chemistry). You can use the same principles as for the SAM surface chemistry. A hydrophilic SAM is made by choosing a hydrophilic group such as a carboxylate, ammonium or oligo ethylene glycol. In the case of a gold nanocluster, a thiol with a terminal carboxyl group gives an ionized, water loving carboxylate when in aqueous solution. Hydrophobic nanoclusters can be wrapped by amphiphilic polymers. The polymer coating is stabilized by partially cross linking the anhydride groups with bis(6-aminohexyl)amine. The key physical properties of the nanocluster is mantained. Can also coat with silica. Often, the resulting crystals bear a surface charge, which allows their use in electrostatic layer-by-layer deposition.

Separation of nanoclusters by size

Separation of nanoclusters by size using using a non-solvent and centrifugation. Nanoclusters can be dissolved in toluene and by gradually adding a non-solvent (e.g. acetone) the nanoclusters will precipitate. The largest clusters precipitate first. Every time a bit of acetone is added the solution is centrifuged and the precipitate collected. The result is highly monodisperse nanoclusters collected in each fraction.

Superlattice

A superlattice is a material with periodically alternating layers of several substances. Such structures possess periodicity both on the scale of each layer's crystal lattice and on the scale of the alternating layers.

Assembling of superlattices

A superlattice can be assembled by means of these techniques:

Why do we want to make superlattices?

Making superlattices can give you a material with unique properties. Heterocrystals is ordered assemblies of more than one component. The properties of the superlattice does not necessarily equal the sum of the properties of the individual constituents. “The ability to assemble different nanoclusters with size-tunable optical, electronic and magnetic properties into well-defined structures gives us the opportunity to examine new effects due to electronic and magnetic coupling between constituent units” – Nanochemistry, a chemical approach to nanomaterials by Ozon and Arsenault.

Effects of capping agents

The length and size of the capping agents determine the separation between nanoclusters and the packing in a superstructure. The superlattice period is thus altered by varying capping agents.

Alloying core-shell nanoclusters

Thermally driven inter-diffusion of core and shell elements to form solid-solution nanocrystals:

Can be used to tune optical absorbtion and luminescence properties. It this process is utilised for core-shell metal nanocrystals, a precise command over their magnetic properties may be possible.

Nanocluster-polymer composites

A nanocluster-polymer composite is a nanocluster stabilized in a polymer. A polymer which prevents nanocluster phase separation and agglomeration, and which does not cause quenching of luminescence, can be used to tune the colors of capped nanoclusters.

How can it be used for down-conversion of light?

One example is down conversion of light made by encapsulating a GaN LED in a sheath of capped semiconductor nanoclusters in a polymer. A 425 nm wavelenght emitted from the encapsulated GaN LED evokes a 590 nm light emission from the nanocluster-polymer sheath. This process is responsible for the down conversion of light energy.

Nanoclusters in biology

Example:

Tetrapods and principles of the synthesis

Photochromic metal nanoclusters

After embedding silver nanoclusters in a titania matrix, one can use UV-light to photo reduce the the metal salt. This makes the the silver nanoclusters absorb most light. Later, one can use light with different wavelength to induce local oxidation of silver nanoclusters of specific size. For example will red light cause cause oxidation of nanoclusters in the red light band which will cause them to reflect red light and therefore appear red, and white light (all wavelengths) will cause oxidation of all nanoclusters which makes the substrate will appear white.

Buckyballs

Molecules that are composed of 60 carbon atoms, in the form of a hollow sphere, with 20 hexagons and 12 pentagons. Buckyballs are stable, but not totally unreactive. In a buckyball all the carbons are conjugated through a huge circular π-cloud, which can be easily reduced and loaded with up to 4 electrons. The anionic buckyball can function as a good reducing agent and reduce nitrogen to ammonia with high yield. Other atoms can be trapped inside buckyballs to form inclusion compounds. Buckyballs are potentially the smallest building blocks that can be used to improve computing power in the near future.

Kapittel 7: Microspheres – Colors from the Beaker

Photonic crystals (PC)

Photonic Crystal defects

Making defects

Manipulating photonic crystals usage

Core-corona, core-shell-corona and multi-shell microspheres

Corona is the term always used for the outermost layer of multi-shell microspheres. A corona usually consist of a monolayer with functional end groups, that can either passivate or impart different surface functionalities to the structure. Core-corona and core-shell-corona can be made by both re-growth and one stage growth as multishell microspheres probably is better off being made by the re-growth process. The purpose of making these spheres is to put a lot more functionalities into just one sphere. The shells can be fluorescent, magnetic, photoactive, semiconductive, sacrificial or something else pulled out of a hat.

Microsphere growth synthesis

Self assembly of photonic crystals

Colloidal aggregates

Emulsion-way:

Photonic crystal marbles by electrospraying:

Bragg-Snell law

\lambda_{c(hkl)} = 2d_{hkl}\sqrt{\langle \epsilon \rangle - sin^2{\theta}} where \langle \epsilon \rangle = \sum_i V_i \epsilon_i is the effective dielectric constant of the colloidal crystal, given by the volume fractions Vi of each of the materials with dielectric constants εi.

Cracking

Colloidal crystals are found to crack after crystallization, thermal and chemical anneling. This happens when the thin hydration layers around the crystal spheres dry out, which cause lattice contraction. This creates capillary stress and thermal expansion. To prevent cracking you can dry the crystal slowly or use hydrophobic spheres to eliminate the hydration layer. Confining spheres in templates can also reduce cracking by localizing the cracks at the crystal edges. Methods for preventing this is:

Liquid crystal photonic crystal

A liquid crystal is neither a liquid nor a crystal, but an intermediate state of matter, so called mesophase. Lacks the long range order of the crystalline state and does not exhibit the randomness of the liquid state.

Reactions that you need to know

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