CH676 Physical Chemistry: Principles and Applications. CH676 Physical Chemistry: Principles and Applications

Transcription

1 CH676 Physical Chemistry: Principles and Applications

2 History of Nanotechnology: Time Line Democritus in ancient Greece: concept of atom 1900 : Rutherford : discovery of atomic nucleus The first TEM was built by Max Knoll and Ernst Ruska in 1931, with this group developing the first TEM with resolving power greater than that of light in 1933 The first commercial TEM in : Richard Feynman : speech at Caltech "There is plenty of room at the bottom" 1981 : Invention of the Scanning Tunneling Microscope (STM) by Rohrer and Binnig at IBM Zurich (Nobel Prize 1986) 1982 : First STM atomic resolution by Binnig on Si 7x : Fullerene " buckyballs" discovered at Rice University (Nobel prize awarded in 1996) 1986 : Invention of Atomic Force Microscope (AFM) by Binnig, Gerber, and Quate : Invention of Optical Tweezers, first commercially available microfabricated cantilevers for AFMs 1990 : First commercially available AFMs, Eigler, et al. spells out "IBM" with Xenon atoms 1992 : First single molecule force spectroscopy experiments (DNA, Bustamante) 2000 : President Clinton mentions Nanotechnology in his state of the Union address : US National Nanotechnology Initiative since 2000 (14 federal agencies) -$422 M in 01 (federal), $604 M in 02, $774 M in 03, $847 M in Federal agencies 2004 : Journals: Nanotechnology, Nano Letters, Journal of Nanoscience and Nanotechnology, IEEE Transactions on Nanotechnology, IEEE Transactions on Nanobioscience

3 Moore s Law The continued decrease in device dimensions has followed the well-known Moore s law predicted in1965 and illustrated here. Moore s Law plot of transistor size versus year. The trend line illustrates the fact that the transistor size has decreased by a factor of 2 every 18 months since Integration of nanostructures and nanomaterials into macroscopic systems 2. Building novel tools to study at the nanoscale 3. Doping of semiconductor nanomaterials 4. Uniform production 5. Stabilizations of nanostructures and nanomaterials 6. Overcoming and control of the enormous surface energy

4 Surface Energy Surface energy, γ, by definition, is the energy required to create a unit area of new surface. number of broken bonds, N b, multiplying by half of the bond strength, ε. where ρ a is the surface atomic density, the number of atoms per unit area on the new surface.

5 Reduce Surface Energy (i) (ii) (iii) Surface relaxation Surface restructuring through combining surface dangling bonds into strained new chemical bonds Surface adsorption through chemical or physical adsorption of terminal chemical species onto the surface by forming chemical bonds or weak attraction forces such as electrostatic or van der Waals forces Surface atoms shifting either inwardly or laterally The surface of diamond is covered with hydrogen and silicon is covered with hydroxyl groups. The (2 X 1) restructure of silicon {100} surface

6 Surface Energy of Crystals Surface energy, γ, by definition, is the energy required to create a unit area of new surface. number of broken bonds, N b, multiplying by half of the bond strength, ε. where ρ a is the surface atomic density, the number of atoms per unit area on the new surface. Schematic representing low index faces of a face-centered cubic (fcc) crystal structure:

7 Surface Area and Energy Nanomaterials possess a large fraction of surface atoms per unit volume. A cube of iron of 1cm 3, the percentage of surface atoms would be only 10-5 % and when the cube is 10 nm 3, the percentage of the surface atoms would increase to 10 %. The percentage of surface atoms changes with the palladium cluster diameter. Variation of surface energy with NaCl (1 g) particle size. The calculation was done based on the following assumptions: surface energy of 2X10-5 J/cm 2 and edge energy of 3 X 10-5 J/cm. Surface energy is around 10,000,000 (10 7 ) times larger!

8 Reducing Surface Energy of Nanomaterials (a) (b) (c) (d) Shaping Agglomeration Sintering is to combine individual particles to a bulk with solid interfaces to connect each other. Ostwald ripening is to merge smaller particles into a larger particle. (a) (c) (b) (d)

9 Reducing Surface Energy of Nanomaterials Wulff plot is often used to determine the shape of an equilibrium crystal: (1) Given a set of surface energies for the various crystal faces, draw a set of vectors from a common point of length proportional to the surface energy and direction normal to that the crystal face. (2) Construct the set of faces normal to each vectors and positioned at its end. Conformation for a hypothetical two-dimensional crystal. (a) (1 0) plane, (b) (1 1) plane, (c) shape given by the Wulff construction, and (d) Wulff construction considering only (10) and (1 1) planes. (3) Find a geometric figure whose sides are made up entirely from a particular set of such faces that do not interest any of the other planes.

10 Reducing Surface Energy of Nanomaterials Wulff plot is often used to determine the shape of an equilibrium crystal: Chad Mirkin et al. Peidong Yang et al. Murphy et al.

11 Reducing Surface Energy of Nanomaterials (a) (b) (c) (d) Shaping Agglomeration Sintering is to combine individual particles to a bulk with solid interfaces to connect each other. Ostwald ripening is to merge smaller particles into a larger particle. (a) (c) (b) (d)

12 Reducing Surface Energy of Nanomaterials (a) (b) (c) (d) Shaping Agglomeration Sintering is to combine individual particles to a bulk with solid interfaces to connect each other. Ostwald ripening is to merge smaller particles into a larger particle. (a) (c) (b) (d)

13 Reducing Surface Energy of Nanomaterials R2 R1 R2 R1 2 -

14 Reducing Surface Energy of Nanomaterials (a) (b) (c) (d) Shaping Agglomeration Sintering is to combine individual particles to a bulk with solid interfaces to connect each other. Ostwald ripening is to merge smaller particles into a larger particle. (a) (c) (b) (d)

15 van der Waals Attraction Potential When particles are small, van der Waals attraction force and Brownian motion play important roles, whereas the influence of gravity becomes negligible. A is a constant termed the Hamaker constant

16 Van der Waals attraction potential Hamaker constants for some common materials S/ r << 1 A is a constant termed the Hamaker constant

17 Interactions Between Two Particles: DLVO Theory The electrostatic stabilization of particles in a suspension is successfully described by the DLVO theory, named after Derjaguin, Landau, Venvey and Overbeek. The interaction between two particles in a suspension is considered as the combination of van der Waals attraction potential and the electric repulsion potential. The maximum is also known as repulsive barrier. If the barrier is greater than ~ 10kT, where k is Boltzmann constant, the collisions of two particles produced by Brownian motion will not overcome the barrier and agglomeration will not occur.

18 Electrostatic Stabilization (1) Coulombic force or electrostatic force (2) Entropic force or dispersion (3) Brownian motion Electrical double layer structure and the electric potential near the solid surface with both Stern and diffuse double layers indicated. Surface charge is assumed to be positive. 1/k is known as the Debye--Huckel screening strength and is also used to describe the thickness of double layer. F is Faraday's constant, ε 0, is the permittivity of vacuum, ε r, is the dielectric constant of the solvent, and C i and Z i are the concentration and valence of the counter ions.

19 Electrostatic Stabilization E, is the standard electrode potential, n i is the valence state of ions, a i is the activity of ions, R g is the gas constant and T is temperature, and F is the Faraday s constant. In the oxide systems, typical charge determining ions are protons and hydroxyl groups. The concentrations are described by ph (ph = -log [H + ]). As the concentration of charge determining ions varies, the surface charge density changes from positive to negative or vice versa. The concentration of charge determining ions corresponding to a neutral or zero-charged surface is defined as a point of zero charge (p.z.c.) or zero-point charge (z.p.c.).

20 Steric Stabilization Not all capping agents are dissolvable into solvents. When a solvable capping agent dissolves into a solvent, the agent interacts with the solvent. Such interaction varies with the system as well as temperature. When capping agent in a solvent tends to expand to reduce the overall Gibbs free energy of the system, such a solvent is called a good solvent. When agent in a solvent tends to coil up or collapse to reduce the Gibbs free energy, the solvent is considered to be a poor solvent.

21 Mixed Steric and Electric Interactions Electrosteric stabilization: (a) charged particles with nonionic polymers and (b) polyelectrolytes attached to uncharged particles.