Nanoscience Weeks 1 and 2
Nanoscience Weeks 1 and 2 CHEM 4200
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This 9 page Class Notes was uploaded by Juliana Herran on Tuesday August 16, 2016. The Class Notes belongs to CHEM 4200 at University of Northern Iowa taught by Dr. Weeks in Fall 2016. Since its upload, it has received 53 views. For similar materials see Into Nano Fall 2016 in Chemistry and Biochemistry at University of Northern Iowa.
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Date Created: 08/16/16
Introduction to Nanoscience and Nanotechnology Lecture notes 1 LECTURE 1: Nanoscience Introduction - SI Unit length and conversions are important. When referring to -6 something small is something about 1 mm. Something smalle-9is 10 m (micro circuitry scale). Teeny is a nanometer (10 m). Itty bity is one Angstrom (1Å=0.1nm=10 m). -10 - The length scale between atoms is 1Å. The diameter of an atom is ≈ -10 10 - Nanoscience studies the length scales from 1 to 100 nm. - The macroscopic and microscopic properties of materials are different. Size influences behavior → When the dimensions of the material are the nanoscale, it is considered a nanomaterial. - An example of nanotechnology used in commercial products is Zinc Oxide in sun screens (Which reflects UV through a 30 nm layer of this material). Nanotechnology is oftern used in cosmetics, paint and computer chips. - Surface Area (SA) becomes more important in nanoscale since the proportion of atom in the surface becomes a lot bigger (The smaller the size the more important the SA). With bigger sizes, volume becomes more important (The bigger the size the more important the volume). Volume∝¿¿ ¿1 SurfaceArea Example: In dust particles the number of atoms in the SA is higher and makes it more reactive (Explosion in sugar factory). - Number of atoms in an object of size L is proportional to L . The3 -10 number of atoms per side is L/10 3 L Numberof atoms∈acube=( 10−10) - If you add or subtract a few atoms, properties in bulk behavior may change. Changing the size of devices will not lead to devices with the same behavior. - The main properties that change when changing size are: Reactivity – Reaction Rates Melting temperature Conductivity – Hent Electrical conductivity Color Other optical properties - The main thing to remember when moving between the quantum, nano, micro and every day worlds is that Science does NOT Introduction to Nanoscience and Nanotechnology Lecture notes 2 change!!! It is the approach that is used to understand certain scenarios that changes and adjusts to the different scales found in nature. - When understanding tunneling, an electron colliding with a wall is seen but only a small fraction of that electron “tunnels” or passes through the wall and gets to the other side (this occurs because the energy of the electron was enough to tunnel it through the wall). Example (not seen in class): Look for the ball going up the hill analogy (mechanical and quantum explanations). - - It is because of the quantum nature that the e (electron) can tunnel through certain surfaces. h Lambda= p Where h is Planck’s constant and p is momentum (p=mv). Quantum tunneling is the basis for Scanning Tunneling. Only occurs at distances of 10 nm or less (and it and it only happens with very small objects like an electron). - Nanoscale systems are discrete in relating to number of atoms. LECTURE 2: Microscopy at the Nanoscale - In an optical microscope light can be either reflected or transmitted. The main limitation is light diffraction (The minimum resolution for a decent microscope is about 1 micro meter. - 2 objects at distance d will be Md apart in the image produced by the lens. - M 0.61Lambda Md= nsin∝ dmin= 0.61Lambda nsin∝ nsin∝ represents the properties of the lens. The formal name is numerical aperture. In this equation M is the magnification of the lens, Lambda is the wavelength, n is the refractive index of the material and alpha is the acceptance angle of the lens. - Changing the wavelength changes resolution. Reducing wavelength improves resolution. - Light in higher energies can also be used to improve resolution. However, in the x-ray and gamma ray region there is too much diffraction. - Diffraction can be used to measure size through the “aura” or circle of light that is seen. - Always remember wave-particle duality ~ We want to use a particle with enough wave properties that it can be focused. Introduction to Nanoscience and Nanotechnology Lecture notes 3 Particles need to be small and light to have good wave properties. - For the optical microscope the limitation is lambda. Lambda is greater than size of probe particle but it is much less than the size of object being image. This is all taking relativistic effects into account. - Scanning Electron Microscopy (SEM): Uses electrons instead of light. Similar to a normal microscope and has a Max Resolution of approx. 1nm. Is looking at the surface of an object and can measure up to several nanometers. It uses a W filament. Electrons are ejected, accelerated by attraction to very charged anode (go through hole in anode). Lenses are electromagnets meaning coils. Direct electrons beam back and forth across sample. Secondary electrons are low in energy and weakly bonded. Valence electrons of sample bonded off. Elemental composition can be found using the emitted x-rays characteristic of the element. Limitations: o Must be in vacuum. o Electrons are very reactive. o Electron source – W filament o Electrons do not penetrate very far. o Electron mean free path is only a few nm. o Electrons have 1-30 keV energy and 1-30 KV potential. o It cannot look at samples that do not conduct electricity (Although the sample can be coated with a conductive material). o Relies on tunneling current. o Sensitive to electrical noise. - Scanning Probe Microscopy (SPM): Needle scans across surface and records as surface structure. The tip is an electrically conductive material. Electrons tunnel from tip to surface (there is a very small distance separating the tip and the surface). ~ Electrons are hopping from to surface by tunneling. Very sensitive to tip-sample distance. The voltage used in approx. 1V. The tip scans the surface in parallel lines from top to bottom, bottom to top. It has two usage modes. In the first one the tip is at constant height which can cause troubles like hitting the surface, breaking the tip or not measuring anything. The second mode uses constant current (will take better measurements). In this one, tip moves up and down to keep current constant. Resolution is affected by: Introduction to Nanoscience and Nanotechnology Lecture notes 4 o How sharp tip is o Corrugation of surface Large variation in electron density with height above the surface gives good vertical resolution. Probes electron density not atomic density. Limitations: o The ultimate sharpness of the tip is about 1 atomic diameter. o Vertical atomic resolution is pretty good but it has no lateral atomic resolution. o Exponentially sensitive to tip-sample distance. o Sensitive to vibrations (Since tip is so close to sample). o Sensitive to contamination (Since it only sees the surface). o Small Scan area (less than 100 micro meters). o Sensitive to quality/sharpness of tip. - Atomic Force Microscopy (AFM): Very similar to STM (Tip not as sharp). Cantilever flexes with surface. The tip is mounted on a cantilever and dragged across surface. The angle of refraction from the laser reflected in the cantilever is measured and converted to height in order to study the surface of the material. The limitations are similar to those of SPM. - AFM Vs. STM: AFM STM + Any sample can be measured. → Lateral ~ 0.1 nm + Tips are commercial (expensive but → Vertical ~ 0.1 nm good). → Atomic spacing ~ 0.3 nm + Not as sensitive to electrical noise. -Poorer resolution than STM. -The bigger the tip, the smaller the resolution. -Only rarely can attain true atomic resolution. → Lateral ~ 1nm → Vertical ~ 0.1 nm → Atomic resolution, vertical. - Noise is usually sinusoidal, does not scale. Rotating scan, changing its speed, size or removing it from the image are important to ensure is real and take it out of the measurements. It is best to get noise disappear from instrument. - Thermal drift ~ Expands from center of object. Every one degree warmer, gets larger by a factor of 10×10 K (expansion coefficient). It changes by 0.1 degrees/min and move up and down, never truly constant. - Good temperature control is very important for good resolution. The instrument needs to be run for a while before data collection (warm up). The sample temperature will not initially be the same as the STM so the first Introduction to Nanoscience and Nanotechnology Lecture notes 5 measurements will not be really good. It is necessary to wait for the system to come to equilibrium with its surroundings. - Tunneling does not see nucleus, only the electron cloud. - Thermal drift is a very possible explanation for the funny hexagon. - In the transmission electron microscope (TEM), the image appears darker when it is -6nser. -7e TEM uses very thin samples (100 nm or less), it uses vacuum (10 to 10 ). The resolution is about 0.1 nm. In order to increase contrast, the sample can be stained with heavy elements (Pb, Os, U). - The STM simulator program page is: www.nanonis.com/en/stm- downloads.html LECTURE 3: Applications of STM, AFM, SEM and TEM - AFM Applications: The tip is held just above the surface, oscillates fast. When it gets close to the surface Van der Waals interactions dump the amplitude of the oscillation. Lateral resolution is about 10 nm (things appear broader than they really are because of how wide the tip is). Vertical resolution is about 1 nm. Imaging resolution can also be controlled with the size of each quantum dot. o The alpha and Beta units look like bumps in the ring of a protein. The shape and size is seen as a ring with lighter color in the phospholipid bilayer. ~ The lipid bilayer is very soft; thus a contact technique would ruin the sample. o If there is a uniformity seen in the membrane (no color change), molecules will have the same orientation. - STM Applications: Delocalized Pi bonds around the surface are the lighter part of the image. In multiple layers more electron density is seen. It looks like bumps, darker but also almost interconnected. Lateral resolution is at least 1Å. o In the STM image of mercapto succinic acid protected gold nanoparticles deposited on graphite at a graphite/water interface. Superlattice ~ Need to add HCl to protonate the mercapto succinic acid in order to form the superlattice. o The surface needs to be protonated. o Facets on individual particles. o Some limitations are: Streaking blurs image and the super lattice must be conductive. Another example is the circle of iron atoms, where the electron probability wave resonates inside the circle of Fe atoms or “electron coral”. - SEM Applications: Rough surface increases the electrochemistry → Electrolyte penetration. Introduction to Nanoscience and Nanotechnology Lecture notes 6 Regular crystallinity is lost in the second image of the composites for by ball milling Co doped FeF .3Also, there are clumps of material. As [Co] increases, crystals get smaller and smaller. In the next picture, deposited clusters of Ni metal were grown in carbon nanotubes by chemical vapor deposition (CPD). The carbon nanotubes are pretty clean, without bumps of other material and they stay attached to the fibers consistently after sonication. For the silicon nanopillars, the separation between pillars is 91 nm, 83 nm and 77 nm from left to right. For the gold nanoparticles incorporated into poli(vinylpyrolidone) microfibers and EDX spectrum of PVP fiber/gold nanoparticle there are things to notice: Stacking and depth factors. And the energy difference for every orbital is characteristic of the element so we can get elemental identity and quantity by that electron that moves to the unoccupied orbital. Energy dispersive X-rays (EDX) are used in order to find quantum dots. K = 1s electron replaced. L = 2 Shell electron replaced. It is also important to know what type of mixing technique is used. - TEM Applications: The ring that appears with several rings inside is a multi-walled carbon nanotube (CNT), the one that is only one ring is the single-walled CNT. There are 2 mirror planes and a 2-fold rotational symmetry in the electron diffraction patterns from individual CNTs. In the Samarium oxide inside multi-walled CNT, the image is presented sideways. The Samarium atom brings extra electrons to the oxide. The heavier the element, the better the contrast (because of electron scattering). Since Sm is much heavier than anything else in the sample it provides a much better contrast. For the Ni/MWCNT electrodes, pulsed laser ablation (PLA) was used to deposit on CNT. Essentially, vaporized Ni atoms deposit on CNT. In the Teflon surface coated with poly alkyne, embedded in epoxyresin irradiate electrons at 600˚C. The progression from A to D shows the metal progressively moving out of the carbon nanotubes as temperature increases. It is possible to do reactions and check material while it is still inside the TEM. Material has got to come out the ends of the tubes. In the TEM micrograph of Pt nanoparticle, heating makes the Pt particles much more crystalline. When samples get too thick (get really good at scattering electrons), the images are harder to see. In the WS s2mple, defects are found at the atomic level (Missing atoms, misalignment of layers, dislocations). LECTURE 4: Metal Nanoparticles and UV-visible Spectroscopy Introduction to Nanoscience and Nanotechnology Lecture notes 7 - Some limitations of the high energy ball milling are uneven shapes, wide range of sizes, contamination and components of the ball the break off (It is not the preferred method in Nano particles). - For the bottom up synthesis: combine atoms to make nanoparticles less than 100 nm in size. By dissolving the sample in water and clumping the particles together (not too much), a high control of size and uniformity is obtained (Control in size and shape is big). - Nucleation and size is important. - Monodisperse only has one size of particles (in practice means a very narrow size range). - Same chemical composition and size is important to retain the same properties. - The purity of the sample is very important (For example, quality control in pharmaceuticals). - Discrete behavior, a few more or less atoms make a difference to the properties. - In some applications, you may want to range of sizes. - For size and uniformity, it is important to know how much do you start with? How long will you have them nucleate? (Let them grow). It is also important to have size controls. Number of nuclei controls size, many nuclei, smaller particles. Particle uniformity is important since you do not want nucleation at lots of different times. o Start, grow, stop. How big they get depend on the growth; thus, in the chemistry of the reactions and amounts of reagents (for reaction control ~ some are hard to stop). - Bottom-Up Synthesis: Some general solvent classes are: aqueous, organic. Begin with water (Dissolves polar, ionic, good for pH control, not toxic ~ good at an industrial scale). Aqua regia is used as solvent → Nitric acid/ hydrochloric acid Concentrated HNO Oxi3izing agent (oxidizes Au to Au 3+ and Cl- stabilizes the Ag ion in solution by forming a complex ion. Au nanoparticles: Citrate is both the reducing agent and the ligand that coats the particles to prevent a conglomeration into large particles. o In the gold nanoparticles, the surface has a partial positive charge. The positive charges on the surface repel each other which generates an electrostatic repulsion. o Citrate binds to surface. Nanoparticles of Au neutral ~ Van de Waals + Dispersion (London forces). o Clusters remain in solution, Au cluster + citrates = neutral o Synthesis web: http://mrsec.wisc.edu/Edetc/nanolab/gold/index.html - Van der Waal Forces: Forces between two dipoles Instantaneous dipole that induces a dipole in a neighboring molecule (London). Introduction to Nanoscience and Nanotechnology Lecture notes 8 Metals are good conductors; no net charge can accumulate in Au nanoparticles – Not permanent dipoles. When two close, electrons repel each other, nuclei exposed which further increases the repulsive force. Objects oscillate about some minimum energy position. - Clusters remain in solution when they are very small because of the following reasons = Viscous forces dominate for small particles in a fluid. - In a nanoparticle observation, all colors are brought up in eyes by a combination of red, green and blue. All of visible light arrives to the rods (when there is no light, we see black and white). In the cones, a different wavelength of light is processed (ratio of signals from different cells). Eyes detect a combination of wavelength NOT absorbed. Color observed is the complement of absorbed. - Light emission color depends on wavelength emitted. Electrons excited to higher E shell drop back down to produce light. The released energy is hat is detected. Color depends on emission vs. absorption. - Measurement of solution starts with measuring the absorbance using a spectrometer. Need light source, sample, spectrometer. I Transmittance: T= I0 Make two measurements → One of solvent with NPs, one without. I =I 0with NPs). - Basic Absorption Spectroscopy Used in physics to study the semi-conductive band gap. The wavenumber is directly proportional to energy (used in vibrational spectroscopy UV-vis). Some of the challenges are that white light sources are NOT smooth, the “peaks and dips” (molecules in thin air or something in the apparatus) can correlate to absorption features and ultimately, interesting behaviors can be hidden by the light source. I0 Quantitative aspect ~ A=log (I) =εlc Where c = concentration, ε = molar absorptivity and l = pathlength. Different particles have different molar extinction coefficients (can be worked out). - Optical properties depend on surface plasmon. It indicates that we have plasma (diffused electron cloud ~ in the metal conductor). The plasmon is quantum of plasma oscillation (just like a photon is a quanta of electromagnetic oscillation). It is produced by incident light. Light cannot penetrate metal nanoparticles. Electron ripples, resonant with particular energy of light (electron ripples are on surface). Introduction to Nanoscience and Nanotechnology Lecture notes 9 Nanoparticles are heavy on surface; light does not enter metals! Electron ripples on surface: o Absorb certain energy of light. o Energy defined by surface. o Surface defined by size. o Energy defined by size ~ The size of electron oscillation will depend on particle size. - An electromagnetic field attenuates to an intensity of 1/e of its surface value within the skin depth of the material. With thicker surface, there is negligible electric field. - Plasma resonance oscillates in time with the light ~ Absorption of light takes place. - Au NP spectra: Optical properties based on size ~ Only certain energies of light set off resonance. Bigger particles resonate with the E of longer wavelength. As it shifts to the right, the range is wider. Probably because of how much dispersion in sizes of samples are in bigger samples. With really small samples different techniques can be used. The particle diameter calculation for Au NP from 35-110 nm. λ spr 0 L1 ¿ ln?¿ d=¿ - Mechanical chameleon ~ Indium tin oxide electrode (transparent).
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