Forskjell mellom versjoner av «TMT4320 - Nanomaterialer»

Revisjonen fra 15. des. 2008 kl. 18:52

Emnet skal gi en innføring i grunnleggende kjemisk prinsipper for å lage nanomaterialer. Stikkord: "Self-assembled" monolag (SAM) og hvordan disse kan formes ved myk litografi og "dip pen" nanolitografi, syntese av tredimensjonale multilag strukturer. Tynne filmer ved kjemisk gassfase deponering. Syntese av nanopartikler, nanostaver, nanorør og nanoledninger. Våtkjemiske syntese av oksidbaserte nanomaterialer. "Self-asembly" av kolloidale mikrokuler til fotoniske krystaller, porøse nanomaterialer, blokk-kopolymere som nanomaterialer. "Self assembly" av store byggeblokker til funksjonelle anordninger.

Oppsummering av pensum

Her vil det etterhvert vokse fram et lite kompendium i faget. Dette følger i utgangspunktet pensumlista som gjelder for høsten 2008.

Chapter 1: Nanochemistry Basics

Not terribly important.

Chapter 2: Soft Lithography

Self-assembled monolayers (SAMs)

• The typical example of a SAM is a layer of alkanethiols on a gold substrate.
• The S-H bond is cleaved by oxidation on the gold surface and a covalent Au-S covalent bond is formed.
• The alkanethiols are tilted off-axis from the normal. The angle depends on the surface. (30 ° for a {111} gold surface, 10 ° for a silver surface).
• The end group on the alkanethiols can be tailored to achieve different monolayer properties, thus modifying the surface properties of the structure.

PDMS stamp

• PDMS (PolyDiMethylSiloxane) is a soft elastic polymer.
• A master (casting) of the stamp, with the desired pattern, is made with electron or UV-lithography. The master is silanized and made hydrophobic so removing of the stamp becomes easier.
• Liquid PDMS is then poured into the master, after which it is cured and a finished PDMS stamp is removed from the master.
• The critical dimensions of the stamp are limited by the lithography techniques used, and for photolithography the wavelengths of the light used to expose the photoresist limits the dimensions. Typical CDs given are, for lateral dimensions within the range of 500nm-200µm, and for the height of patterns 200nm-20µm.
• The PDMS stamp can be dipped in alkanethiol solutions (or solutions of other molecules, collectively known as "chemical ink") and be stamped onto surfaces.
• PDMS stamps work on both planar and curved surfaces.
• For the stamp to properly print a pattern onto a surface, the molecules need to adhere to the stamp from the solution, but the affinity for binding to the surface has to be stronger.

Hydrophilic / Hydrophobic stamps

• The endgroup/terminal group on the alkanethiols (or other molecules used) determine the properties of the monolayer, f. ex. a OH-terminal group makes the monolayer hydrophilic, while a <math>CH_3</math>-group makes it hydrophobic.
• Wetability is determined by the polarity of the endgroups.
• By introducing a wetability gradient or abrupt changes in wetability, different effects can be obtained:
  • Square drops, by having checkerboard square patterns of hydrophilic monolayers with hydrophobic lines inbetween, and condensating water onto the surface. This is called condensation figures and results from the condensation on the hydrophilic areas, when the substrate is cooled below the dew point. The diffraction pattern of the structure can be studied for obtaining information on the kinetics and structure of the water droplets. This can be used in biological sensing.
  • Droplets "running uphill" by having wetability gradients. The droplets are moving towards the more hydrophilic areas, against the force of gravity.
  • Nanoring arrays can be synthesized using the condensation figures as templates for molding. A solvent precursor which wets the regions between the microdroplets is added and then evaporated. Deposition of precursor occurs around the perimeter of the droplets. Finally, the water droplets is evaporated, and the precursor remains on the substrate as nanorings.
  • Solid state patterning by dipping a SAM-patterned substrate in a precursor solution. This creates microdroplets with a predetermined precursor concentration, which on evaporation and vertical drying leaves behind an array of size-tunable solid precursor dots.

Printing thin films

• As long as the adhesion between the chemical ink and the substrate is stronger than the adhesion between the ink and the stamp, printing thin films is no problem
• Metal thin films can be evaporated onto a PDMS stamp (f. ex. gold). Evaporation gives homogenous and directional coatings, and no covering of the side walls on the stamp. This pattern is printed onto a SAM-primed substrate with exposed thiol groups (gold adheres strongly to the metal layer).
• This is a very gentle technique for metal film depositing, good for making contacts on fragile layers. Also good for making 3D stuctures by printing multiple layers. Also, there is no need for photoresist because the pattern is printed directly.

Electrically contacting SAMs

• Molecular electronic devices need to make good electrical contact with SAMs.
• Making electrical contacts by vapor deposition on the SAMs may sometimes be more convenient than thin-film printing with a PDMS stamp.
• Other, less gentle methods of metal deposition than printing with PDMS stamps (sputtering, CVD, etc) can cause the metal layer to penetrate the SAM and deposit on the substrate, or even diffuse into the substrate, introducing defects to the structure.
• Morale: Use stamps to deposit metals on SAMs!

Patterning by photocatalysis

• Photocatalysis is used to remove parts of a SAM (making patterns)
• Titania (<math>TiO_2</math>) can photocatalytically decompose organic molecules.
• A quartz slide patterned with titanium dioxide in the required pattern using ALD is pressed against a wafer with the SAM on it.
• The assembly is exposed to UV radiation, triggering the degradation of the (organic) SAM. When titania is exposed to UV, radiation free radicals are created, which react with the organic molecues, removing the parts of the SAM that is in contact with the titania. Thus, the substrate in these areas is revealed.

Kapittel 3: Building layer-by-layer

Electrostatic superlattices

• LbL multilayer films formed by alternate immersion in suspensions of opposite charges.
• A primer layer with a charge adheres to the substrate. The substrate is then dipped in a solution of polyelectrolytes of opposite charge from the primer layer. This process can be repeated numerous times in order to get the desired thickness or functionality of the film.
• As the amount and identity of constituents of each layer can be controlled, a composition gradient can easily be constructed throughout the structure.
• Any species bearing multiple ionic charges can be layered.
• Can be applied to curved surfaces like microspheres, enables applications like hollow spheres with a semipermeable cap.

Some applications

• Electrochromic layers (change color when a potential is applied), used in "smart windows" for instance
• Construction of cantilevers for AFMs and similar equipment, using photolithography and LbL

Analysis, measuring film thickness

• Optical spectroscopy: If the substrate is transparent, and the film absorbs light at a certain wavelength, the film thickness can be found by monitoring the optical absorption as a function of number of layers. A dye can be introduced to ensure absorption. Easy to perform but hard to interpret - must know the observation area and extinction coefficient of the absorbing group.
• Ellipsometry: Film is probed by polarized light, and change in polarization in the reflected light is measured. This can be used to find the refractive index, thickness, roughness and orientation of a thin film. Ellipsometry works with films much thinner than the wavelength of light - down to atomic layers.
• Quartz crystal microbalance (QCM): Quartz (piezoelectric material) in an alternating electric field contracts/expands with a characteristic oscillation frequency. When mass is added to a QCM the frequency decreases, which correlates directly with the amount of mass added. This allows real-time thickness measurements when the density of the material is known. Works well for hard materials like metals and ceramics, but not for viscoelastic materials.
• Direct techniques: Label each layer with heavy metal atoms and image by TEM. Alternately, deposit a thin gold layer on top of the surface and image cross section by TEM.

Non-electrostatic lbl assembly

• LbL doesn't need electrostatic bridges - can use hydrogen bonding, ligand-receptor interactions or even covalent bonds.
• Example: DNA (adenine-thymine and guanine-cytosine bridges)
• Hydrogen bonds can be broken again by changing the pH, or can be strengthened by UV irradiation

Low-pressure layers

• Molecular beam epitaxy (MBE): Performed in ultrahigh vacuum, sources of constituents (elemental) are heated, and a thin film alloyed from the constituents is deposited. The result is a single crystal film with homogeneous thickness grown epitaxially on the substrate. The substrate should have a similar lattice constant to that of the layer deposited. If the lattice constant of the substrate is substantially different from that of the deposited material, there will be a dewetting effect where the material can form quantum dots.
• Chemical vapor deposition (CVD): Volatile precursors are introduced in gas phase in a low-pressure reactor chamber. Argon or nitrogen gas are usually used as carrier gas to dilute the precursor and achieve optimal pressure and concentration. The substrate is heated, and the precursor decomposes at the surface. There are several different types of CVD reactors, such as cold wall and hot wall reactors. There are also plasma enhanced reactors (PECVD) where the electric field in the plasma can force growth of nanowires in the direction of the electric field.

Lbl self-limiting reactions

• Atomic layer deposition: Similar to CVD, but usually carried out in solution.
• Iterative saturating reactions. ALD is a self-limiting process where only one layer at a time is deposited. When the first layer is deposited it needs to be reactivated in order to grow a second layer. It is therefore easy to kontroll thickness.

Kapittel 4: Nanocontact printing and writing

Soft lithography and microcontact printing

• Sub 100 nm Soft Lithography: Previous chapters has covered printing on 10.000-100 nm scale. Need for further miniaturization because of demand for more power, efficiency, and density. This can be done by manipulating PDMS stamp, Dip Pen Nanolithography (DPN), Whittling Nanostructures or by Nanoplotters

Manipulating PDMS stamp

• Manipulating PDMS stamp can be done in various ways, and seven of the basic ideas will now be explained. Illustrating pictures are in the book and in foils.

1. Compress the stamp, mold to get a new stamp with inverse pattern, peel off and repeat.
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.

• Overpressure printing
  • Defect-free contact printing is restricted to a certain range of height-to-width ratios. If ratio is outside 0,2-2, the roof of the grooves on stamp will touch the substrate. Too high perpendicular force on stamp has the same effect, but overpressure can also be used to form new patterns such as micron scale discs and rings of ferromagnetic core-shell nanoparticles. Nanoparticles are then transferred to PDMS stamp by Langmuir-Blodgett technique (chapter 6) and then into contact with Au-coated silicon substrate. Low pressure => discs, high pressure => rings.
• Limitations
  • Deformation can be a shortcoming if care is not taken with the dimensions of surface relief pattern in the stamp as this can give unwanted deformations. Quality of printed pattern will not be good.

Dip pen nanolithography

• Alkanethiols can be written on gold substrate with AFM tip. The alkanethiols are delivered to the tip via a water meniscus, and this can be adapted to suit other surface chemistries. The result is 10 nm fine patterns of molecules (biomolecules, polymers etc.) on metals, semiconductors and dielectrics.
• Sol-gel DPN: patterning of solid-state materials. Nanoscale patterns are written using a metal oxide sol-gel precursor in a solvent carrier. The sol-gel precursors are hydrolyzed to metal oxide by use of atmospheric moisture and water meniscus at the tip-substrate interface. pH, substrate temperature and post treatment can be varied.
• Enzyme DPN: A scanning microscope tip can be used to place an enzyme on a specific site on a biomolecule with nanometer presicion. This method leads to the possibility of bionanodegradable electronic and optical devices.
• Electrostatic DPN: Like thin films can be made of charged polyelectrolytes, an AFM tip can "draw" lines or structures of charged polymers with for example specific electrical properties to build nanoscale electronic devices.
• Electrochemical DPN: The meniscus that forms between surface and tip is used as a nanochemical reactor. Electrochemical deposition can be done by applying voltage between tip and substrate. Ex: making platinum lines can be done by reducing Pt salt at -4 V, and silica lines can be made by oxidation of a silicon surface at +10 V.

Whittling of nanostructures (section 4.19)

• Only be able to explain basic principle
  • The spatial extent of SAMs can be reduced by so-called "whittling". Whittling is an electrochemical desorption process where a voltage applied will cause ligands to desorb. It has been found that the larger the accessibility of a molecule, the lower the desorbation voltage is (fig. 4.22)

Nanoplotters and nanoblotters

• What are these and what can they be used for?
  • Nanoplotter: Parallel cantilevers write SAM nanopatterns simultaneously.
  • Nanoblotters: An PDMS inkwell has been created to deliver ink to the nanoplotter cantilever tips (fig. 4.26)

Combinatorial libraries

• Be able to explain the basic principle and how it is used to find new and improved materials.
  • DPN can be used to put different materials together in the research of new material composition. With DPN, many different combinations can be made with small material amounts used.

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

Emily skriver på denne. Håper folk retter opp dersom de finner feil, og legg gjerne til flere ting:)

Templating nanowires and nanorods

Templates can be used for making solid nanorods and nanotubes of controlled sizes. Examples of templates are alumina, silicon, zeolites and lipid bilayers.

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).

<math> 2Al + 3PO_4^{3-} \rightarrow Al_2O_3 + 3PO_3^{3-}</math>

The residual Al and <math>Al_2O_3</math> 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 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 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 excist 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 controll 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. If the thickness of the magnetic segments on a nanorod is smaller than the diameter, they will self assemble into 3D bundles. If the thickness is bigger than the diameter, 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.

• VLS Synthesis

A catalyst droplet first melts, 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 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 stoichometry of the target, or by introducing dopant into gas phase during growth.

• SFLS Synthesis

Similar to VLS, but used for high-eutectic temperature combinations. 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.

• Pulsed laser deposition

A pulsed laser is used to ablate a target (pulsed laser ablation), 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 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 size 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 nanocluster emits plane-polarized light, while nanorod exhibits linearly polarized light.

Alignment methods

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

• Electric Field Based Alignment

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.

• Microfluidic Alignment

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.

• Langmuir-Blodgett Technique

A Langmuir film is first created when a small amount of insoluble liquid (amphiphile) is poured onto another liquid. 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.

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 [TMT4320_-_Nanomaterialer#Alignment_methods|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.

How can nanowire transistors 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 <math>Mg_3N_2</math> 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 <math>PbTiO_3</math> nanorods which is a ferroelectric material and potentially useful as building blocks in nanoelectrochemical systems. (Amorphous <math>PbTiO_{(3-X)}OH_{2X}</math> (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 <math>TiO_2</math> + PVP. To crystallize <math>TiO_2</math> and oxidate PVP, the tubes can be calcined in air at 500 degrees.

Dual electrospinning

A side by side spinneret can be used to make bicomponent fibers. Ex: two solutions containing <math>TiO_2</math>/<math>SnO_2</math> are simultaneously jetted. This is calcined. A heterojunction of <math>SnO_2</math>/<math>TiO_2</math> can create devices with extremely high quantum efficiency and photocatalytic activity for treatment of organic pollutants in water and air.

Dette mangler:

• Carbon nanotubes (sections 5.41, 5.42, 5.44, 5.45-5.48 and lecture notes)
  • What are carbon nanotubes? Be able to describe the three different structures they can have and how their properties are different.
  • Be able to describe briefly (basic principles) at least two of the three main methods used to synthesize carbon nanotubes
    • Arc discharge
    • Laser ablation
    • CVD
  • How can the different structure nanotubes be separated from each other and from other carbon particles.
  • Be able to say something about their properties
    • Mechanical
    • Electrical
    • Chemical
  • Know some about carbon nanotube chemistry (reactivity on the surface vs the ends etc.)
  • Aligning of carbon nanotubes
    • Evaporation induced self-assembly
    • Patterned hydrophilic SAM on substrate – carbon nanotubes will assemble only on the hydrophilic patches.
    • Alignment by pre-existing patterns
      • Perpendicular to substrate
      • Parallel to substrate
    • AC/DC electric fields
  • Applications of carbon nanotubes
    • Sensors
    • Strengthening of materials (composites)
    • 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:

• Change solubility properties
• Enable size-selective crystallization
• Surface functionalization
• Protect nanoclusters from luminescence or charge-carrier quenching

General principles for synthesis of capped nanoclusters (arrested nucleation and growth)

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:

• Desired precursors are added to a solution containing a proper capping agent, which is held at an intermediate temperature (200-400 °C depending on the materials. Temperature needs to be high enough to overcome the activation energy for the reaction.).
• Precursors need to be added at an amount that is over the saturation point for the materials in that specific solution.
• Materials will rapidly nucleate (precipitate) and start growing. Once the first molecules have reacted and created a small seed, the energy required for further growth is smaller than the initial activation energy. The nucleated seed can therefore continue to grow below the saturation concentration for the precursor materials.
• Once the nanoclusters reach a certain size range, which may vary from one material to the other, the capping agents will adsorb on the surface of the nanoclusters and prevent further growth. The nanoclusters that are formed will not all have the same diameter, but a range of different diameter clusters will be formed. This can be due to for example concentration gradients in the reactor or reaction medium.

Minimize size dispersity by confining the reaction space

The size of the capped nanoclusters can be controlled by growing them in nanowells made by the methode in figure x. The nanowells are obtained by patterning a silicon wafer with a layer of well-ordered microspheres. By pressing the microspheres against a 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.

Tuning properties through physical dimensions rather than chemical composition (QSE)

When electrons are confined in space the size invariant continuum of electronic states of bulk matter transformes 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 blue quantum size effect shift in the energy of the first exciton band or band gap that scales with the reciprocal of the square of the radius of the nanocluster. The wavelengths absorbed change, and the colors of the nanoclusters can be alterd from yellow to red, by changing the physical size of the clusters

How can 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 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 of nanoclusters can change just by exposing them to a different chemical environment at room temperature.

Makeing nanoclusters water soluble

Why? Water is cheap, widely available and use of it avoides the disposal o organic solvents, which can be quiet 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 polyers. The polymer coating is stabilized by partially cross linking the anhydride gropuos with bis(6-aminohexyl)amine. 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 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:

• Tri-layer solvent diffusion crystallization - Three immiscible solvents are arranged to form separate layers in a test tube. Bottom layer →capped CdSe nanoclusters dissolved in toluene. Middle layer →buffer layer of 2-propanol selected for poor solvent properties wrt the nanoclusters. Top layer →non-solvent for the nanoclusters such as methanol. The process involves slow diffusion of the nanoclusters from the toluene bottom layer and the methanol from the top layer into the buffer layer. The change in solvent properties causes a slow and controlled nucleation and growth of capped CdSe nanocluster crystals.
• Sedimentation –
• Evaporation induced self-assembly – Strong capillary forces in an evaporating water meniscus drives the nanocomponents into close-packing.
• Langmuir-Blodgett – A dilute monolayer of capped silver nanoclusters is spread on an air-water interface. Using Langmuir – Blodgett “equipment”, this monolayer can gradually be compressed until a compact monolayer is formed.

Gjenstår

• Why do we want to make superlattices? (change of properties, properties of superlattice does not necessarily equal the sum of the properties of the individual constituents)How can capping agents (different type and length) affect the properties of a superstructure? (section 6.15)Alloying core-shell nanoclusters

• Nanocluster-polymer composites
  • What is it?
  • How can it be used for down-conversion of light?
• Be able to give one or two examples of how different size nanoclusters labeled with different fluorescent molecules can be used in biology.
• What is a tetrapod and what is the main priciples of the synthesis behind the tetrapod?
  • Using a material that has two common crystal polymorphs where growth of one over the other can be controlled by synthesis temperature.
  • Use of a long chain molecule which selectively binds to specific facets of the structure and hinders growth in those directions. This confines the growth of the material to one spatial dimension.
• Photochromic metal nanoclusters (section 6.31)
  • Be able to explain what happens to silver nanoclusters embedded in a titania matrix when it is exposed to either UV-light or visible light.
• What is a buckyball and what can it be used for? What special properties does it exhibit? (Do not need to know specific details of synthesis or assembly techniques.)

Kapittel 7: Microspheres – Colors from the Beaker

Nå ferdig med så mye som forfatteren greide, men finn gjerne ut resten og del det med alle!

What is a photonic crystal (PC)?

• It is a crystal consisting of a material with high dielectric contrast and periodicity at the light scale
• Wavelengths of light that are allowed to travel are known as modes, and groups of allowed modes form bands. Disallowed bands of wavelengths are called photonic band gaps (PBG).
• Vullums definition: Natural gratings that diffract light are based on dielectric lattices with periodicity at optical wavelengths. 3D optical diffraction gratings have dielectric lattices that are geometrically complimentary.
• 1D PC (planes) is a crystal which only inhibit light to travel in one direction
• 2D PC (rods) inhibits light to travel in two directions
• 3D PC (spheres) inhibits litght to travel in any direction and has a full photonic band gap, whilst 1D and 2D only have so called stopgaps

Photonic Crystal defects

• Point defects: Holes, missing spheres, in a 3D PC can trap light inside the crystal
• Line defects: Many holes which make a line can guide light through a crystal
• Plane defects: A missing plane or a defect in a plane can make photons slip through to the other side. Planes consisting of another type of material can cause the perfect reflection curve of a PBG-crystal to drop at certain wavelengths depending on the size of the defect.

Making defects

• Writing defects: Multiphoton laser writing using a confocal optical microscope induced polymerization of an organic monomer in the colloidal crystal to create small line inside the photonic lattice. Then you treat the crystal and remove the polymer. In reversed opal structures you can use laser microwriting where you attach a laser to a scanning optical microscope which again changes the phase (which again changes the refractive index) of the inverse opal by annealing.
• Synthesizing planar defects: Introducing a dense layer or a layer with spheres of a different size than the surrounding colloidal crystal. Dense layers can be introduced by either CVD, electrolyte LbL, PDMS-stamps or maybe another deposition technique. The process consists of growing a photonic crystal, then using electrolyte LbL-deposition or PDMS-stamp make a thin film before making another photonic crystal. It's like a sandwich.

Manipulating photonic crystals usage

• Color of the structure is partially determined by the size of its spheres, where small spheres give blue/purple colors and larger spheres goes towards red (from yellow to green and then red).
• Non-close-packed polymerized colloidal crystalline arrays can be made to swell or shrink by external influence. As the diffraction colors of the crystal depend on the spacing between microspheres you can place a hydrogel between the spheres and this gel will swell or shrink depending on external environments. This will make the color change when the gel shrinks or swells as the pH, temperature, water concentration or ionic strength changes.
• The dielectric constant can be changed by changing the material, the structure of the crystal or something else that others edit in here
• An example: Removal of cation causes a hydrogel to shrink, which can be detected at even very small concentrations. The order of cation complexation determines how sensitive the sensor is. Cation selectively binds covalently to the polymer network, sol-gel or hydrogel.

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

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.

Growth synthesis

• One stage: Reagents are mixed and the microspheres are obtained in solution by a nucleation and growth
• Re-growth: First a sees is produced. The seed is then allowed to grow in several steps. Surface tension controls the shape, where low surface tension gives spherical particles.

Self assembly of photonic crystals

• Sedimentation (be able to explain in more detail): Use Stokes equation to make the radius as you want it by changing the viscosity very slowly.
• Electrophoresis – noen som veit?
• Hydrodynamic shear – noen som veit?
• Spin coating – noen som veit?
• Langmuir-Blodgett layer-by-layer (be able to explain in more detail) – noen som veit?
• Parallel plate confinement: Force spheres to assemble by placing them between two parallel plates and slowly moving one plate closer to the other. Important with slow movement to prevent defects. This can be done both dry and in fluid. It is necessary to increase density and viscosity of solvent so that settling occurs slowly in order to control structure and shape, and to avoid defects.
• Evaporation induced self-assembly, EISA (be able to explain in more detail) Capillary forces drive the assembly of spheres in a solution as you remove a wetting plate out of the solution. These the need to be dried and this can cause cracking. Vertical substrate is placed in a dispersion of microspheres. As solvent evaporates, the microspheres are driven by convective forces (forces from movement in solvent towards wall, surface, water meniscus) to the solvent-air meniscus. The layer thickness is determined by the diameter of the microspheres, their volume, concentration and the wetting properties of the solvent on the substrate.

Colloidal aggregates

• CA are made either by templated pattern in a surface or by aggregation in a homogeneous emulsion.

Emulsion-way:

• They are disperse microspheres in a solvent such as toulene.
• Add dispersion to solution of surfactant and water
• Stir or shake to get emulsion
• Toulene evapourates and as toulene droplets shrink, microspheres are pulled together in a stable cluster through capillary forces.

Photonic crystal marbles:

• Aqueous dispersion of microspheres is forced, under pressure, through a small syringe in the presence of an electric field. Surface charge on the liquid jet make it break into homogeneously sized spherical particles. Each droplet (sphere) contains a preset quantity of microspheres.
• Electrospraying - noen forslag?

Bragg-Snell law

• The reflected light has a wavelength depending on Bragg's and Snell's law. This then tells us that the wavelength of the first stop band is proportional to distance between the lattice plains. This gives that the longer the distance between the plains (bigger microspheres) gives longer wavelength.

<math>\lambda_{c(hkl)} = 2d_{hkl}\sqrt{\langle \epsilon \rangle - sin^2{\theta}} </math> der <math>\langle \epsilon \rangle</math> is the effective dielectric constant of the colloidal crystal.

Cracking

This happens when the thin hydration layers around the crystal spheres dry out. This creates capillary stress and thermal expansion. To prevent cracking you can dry the crystal slowly, use hydrophobic spheres. Methods for preventing this is:

• <math>SiCl_4</math> reacting within the hydration layer to create a <math>SiO_2</math> layer between the spheres. Rehydrate to form multiple layers. Advantages as good control of layer thickness as it can be controlled/monitores by optical diffraction as a thicker layer res-shifts the diffraction peak.
• Necking at room temperature using vapor phase alternating chemical reactions
• Heat treatment before assembly. This may require pretreatment before assembly to give desired surface charges. Redeisperse and crystallize without volume contraction

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.

• Themotropics are liquid crystals which consists of melted anisotropical shapes (rods or discs) where they ar partially alligned. The order of the components in the liquid crystal is determined and changed bu the temperature.
• Two groups of thermotropics are nematic, where the molecules have no positional order, but they have a long-range orientational order, and discotic, which consists of disc-shaped particles that can orient in a layer-like fashion.
• By applying electric- and/or magnetic fields the small crystals in the liquid will align after the applied fields and this can control the refractive index of the film or whatever you have made out of this liquid crystal. Electric/magnetic fields or temperature changes can make it go from nearly transparent to reflective. Eksample of usage is privacy/smart windows.
• By filling the voids in an inverse opal photonic crystal with liquid crystal we make what's called a Liquid Crystal Photonic Crystal. (LCPC) Applying a field or changing the temperature makes the refractive index of the liquid crystal inside the voids change. This means that other wavelengths will satisfy Bragg's criterion, which in practice means that the color of the LCPC changes (you alter the stop band frequency) See Bragg-Snell law.
• LCPC is thought to be used as tunable photonic crystal device and liquid crystal-colloidal crystal switch.

Reactions that you need to know:

• Reaction of alkane thiolate with gold. Important to know that alkane thiols have a specific affinity for gold (also keep in mind that silver and gold have very similar properties).
• Reaction that occurs when during anodic oxidation of Al to produce porous alumina membranes.
• Reaction that occurs when silica microspheres are formed from Si(OEt)4 and water (section 7.9): <math>Si(OEt)_4 + 2H_2O \rightarrow SiO_2 + 4EtOH</math>

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