A Day in the Life of a Nanoparticle

1. Or how I learned to not sunburn and still look good. A Day in the Life of a Nanoparticle http://media.photobucket.com/image/sunscreen%20and%20nanoparticles/vivawoman/badger-spf30-sunscreen.jpg http://www.rdecom.army.mil/rdemagazine/200402/images/itl_arl_particles.jpg http://www.wsu.edu/~jtd/physunder/physun2.jpg

2. Nanoparticle Uses • Sunscreens • Make-up • Automotive Paint • Sporting Goods • anti-bacterial Hong Dong FE-SEM: Zeiss(1550)-Clark This image shows electrospun nylon 6 nanofibers decorated with surface bound Ag nanoparticles. Immersing nylon 6 nanofibers into Ag colloidal solution with pH 5, Ag nanoparticles were assembled onto nylon 6 nanofibers via interaction between nylon 6 and protection groups of Ag nanoparticles. Future applications include antibacterial filtration. Fiber Science and Apparel Design Advisor        Juan Hinestroza

3. ☻ When physicists first began investigating the structure of atoms in the early 1900s, they uncovered a strange new world. The subatomic particles they found -- electrons, protons, and neutrons -- seemed to behave according to a completely different set of laws than those governing our everyday world. ☻Then, in the late 1920s, a team of young physicists led by Niels Bohr introduced a theory that explained the behavior of atoms and their particles. Not surprisingly, the theory, called quantum mechanics, was as bizarre as the world it attempted to explain.

4. ☻Rather than identifying precisely where an electron should be, for example, quantum mechanics predicts only the probability of finding that electron in one place or another. ☻This description of unpredictability at the atomic level -- indeed, at any level -- was completely unacceptable to Einstein; it flew in the face of everything he believed, and directly contradicted his orderly theories of the universe. ☻Despite Einstein's disapproval, quantum mechanics has only grown in acceptance as a theory.

5. The Quantum Café – Michael Greene http://www.pbs.org/wgbh/nova/programs/ht/qt/3012_qd_05.html

6. Opinions on quantum mechanics I think it is safe to say that no one understands quantum mechanics. Do not keep saying to yourself, if you can possibly avoid it, “But how can it be like that?” because you will get “down the drain” into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that. - Richard Feynman Those who are not shocked when they first come across quantum mechanics cannot possibly have understood it. - Niels Bohr Richard Feynman (1918-1988)‏

7. Important Questions # How did our understanding of the atom change in the 1920s? # How did quantum mechanics contradict Einstein's view of physics? What did Einstein mean when he said, "God does not throw dice"? # What are some of the "bizarre" things that quantum mechanics predicts?

8. The Birth of Modern Physics • Classical Physics of the 1890s • The Kinetic Theory of Gases • Waves and Particles • Conservation Laws and Fundamental Forces • The Atomic Theory of Matter • Outstanding Problems of 1895 and New Horizons James Clerk Maxwell The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote… Our future discoveries must be looked for in the sixth place of decimals. - Albert A. Michelson, 1894 There is nothing new to be discovered in physics now. All that remains is more and more precise measurement. - Lord Kelvin, 1900

9. Classical Physics of the 1890s • Mechanics → Electromagnetism → ← Thermodynamics

10. Electromagnetism culminated with Maxwell’s Equations • Gauss’s law: • (electric field)‏ • Gauss’s law: • (magnetic field)‏ • Faraday’s law: • Ampère’s law: James Clerk Maxwell (1831-1879)‏ in the presence of only stationary charges.

11. Faraday saw the World in a new way!

12. The Nature of Light • Newton promoted the corpuscular (particle) theory • Particles of light travel in straight lines or rays • Explained sharp shadows • Explained reflection and refraction Newton in action "I procured me a triangular glass prism to try therewith the celebrated phenomena of colours." (Newton, 1665)

13. Double refraction The Nature of Light • Huygens promoted the wave theory. He realized that light propagates as a wave from the point of origin. He realized that light slowed down on entering dense media. Christiaan Huygens (1629-1695)‏ He explained polarization, reflection, refraction, and double refraction.

14. Diffraction confirmed light to be a wave. While scientists of Newton’s time thought shadows were sharp, Young’s two-slit experiment could only be explained by light behaving as a wave. Fresnel developed an accurate theory of diffraction in the early 19th century. • Diffraction patterns One slit Two slits Augustin Fresnel

16. Waves can interfere.

17. Maxwell strove to prove his Mentor correct

18. visible microwave infrared UV X-ray 106 105 radio gamma-ray wavelength (nm)‏ Light waves were found to be solutions to Maxwell’s Equations. • All electromagnetic waves travel in a vacuum with a speed c given by: The electromagnetic spectrum is vast. where μ0 and ε0 are the permeability and permittivity of free space

19. Light is an electromagnetic wave. • The electric (E) and magnetic (B) fields are in phase. The electric field, the magnetic field, and the propagation direction are all perpendicular.

20. Triumph of Classical Physics: The Conservation Laws • Conservation of energy: The sum of energy (in all its forms) is conserved (does not change) in all interactions. • Conservation of linear momentum: In the absence of external forces, linear momentum is conserved in all interactions. • Conservation of angular momentum: In the absence of external torque, angular momentum is conserved in all interactions. • Conservation of charge: Electric charge is conserved in all interactions. These laws remain the key to interpreting even particle physics experiments today.

21. Problems in 19th-century physics • In a speech to the Royal Institution in 1900, Lord Kelvin himself described two “dark clouds on the horizon” of physics: The question of the existence of an electro-magnetic medium—referred to as “ether” or “aether.” The failure of classical physics to explain blackbody radiation.

22. The Ultraviolet Catastrophe • Lord Rayleigh used the classical theories of electromagnetism and thermodynamics to show that the blackbody spectrum should be: Rayleigh-Jeans Formula This worked at longer wavelengths but deviates badly at short ones. This problem became known as the ultraviolet catastrophe and was one of the many effects classical physics couldn’t explain.

24. More problems: discrete spectral lines For reasons then unknown, atomic gases emitted only certain narrow frequencies, unique to each atomic species. Emission spectra from gases of hot atoms. Wavelength

25. Additional discoveries in 1895-7 contributed to the complications. • X-rays (Roentgen)‏ • Radioactivity (Becquerel)‏ • Electron (Thomson)‏ • Zeeman effect Roentgen’s x-ray image of his wife’s hand (with her wedding ring)‏

26. c Special relativity Speed General relativity Quantum mechanics 19th-century physics 0 Log (size)‏ The Beginnings of Modern Physics • These new discoveries and the many resulting complications required a massive revision of fundamental physical assumptions. • The introduction (~1900) of the modern theories of special relativity and quantum mechanics became the starting point of this most fascinating revision. General relativity (~1915) continued it.

27. Triumph of Classical Physics: The Conservation Laws • Conservation of energy: The sum of energy (in all its forms) is conserved (does not change) in all interactions. • Conservation of linear momentum: In the absence of external forces, linear momentum is conserved in all interactions. • Conservation of angular momentum: In the absence of external torque, angular momentum is conserved in all interactions. • Conservation of charge: Electric charge is conserved in all interactions. These laws remain the key to interpreting even particle physics experiments today.

28. For our sunscreen to work we will need to look at an experiment designed to determine how tightly bound electrons are to a surface. • This requires coming up with Planck's Constant. • This also requires the determination of the work Function.

29. Work function experiment. • http://www.walter-fendt.de/ph11e/photoeffect.htm Workfunction for ZnO is ~4.5

30. What is Quantum Physics? Quantum physics is a branch of Science that deals with discrete, indivisible units of energy called quanta as described by Quantum Theory. There are five main ideas represented in Quantum Theory which are: 1. Energy is not continuous, but comes in small, but discrete units. 2. The elementary particles behave both like particles and like waves. 3. The movement of these particles is inherently random. 4. It is physically impossible to know both the position and momentum of a particle at any instant in time so that the more accurate the measurement of one is, the more inaccurate the measure of the other is. 5. The atomic world is NOTHING like the world we live in.

31. Structure of the Atom • The Atomic Models of Thomson and Rutherford • Rutherford Scattering • The Classic Atomic Model • The Bohr Model of the Hydrogen Atom • Successes & Failures of the Bohr Model • Characteristic X-Ray Spectra and Atomic Number • Atomic Excitation by Electrons Niels Bohr (1885-1962) The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth. An expert is a person who has made all the mistakes that can be made in a very narrow field. Never express yourself more clearly than you are able to think. Prediction is very difficult, especially about the future. - Niels Bohr

32. Structure of the Atom Evidence in 1900 indicated that the atom was not a fundamental unit: There seemed to be too many kinds of atoms, each belonging to a distinct chemical element (way more than earth, air, water, and fire!). Atoms and electromagnetic phenomena were intimately related (magnetic materials; insulators vs. conductors; different emission spectra). Elements combine with some elements but not with others, a characteristic that hinted at an internal atomic structure (valence). The discoveries of radioactivity, x rays, and the electron (all seemed to involve atoms breaking apart in some way).

33. Knowledge of atoms in 1900 Electrons (discovered in 1897) carried the negative charge. Electrons were very light, even compared to the atom. Protons had not yet been discovered, but clearly positive charge had to be present to achieve charge neutrality.

34. Thomson’s Atomic Model Thomson’s “plum-pudding” model of the atom had the positive charges spread uniformly throughout a sphere the size of the atom, with electrons embedded in the uniform background. In Thomson’s view, when the atom was heated, the electrons could vibrate about their equilibrium positions, thus producing electromagnetic radiation. Unfortunately, Thomson couldn’t explain spectra with this model.

35. Experiments of Rutherford, Geiger and Marsden Rutherford, Geiger, and Marsden conceived a new technique for investigating the structure of matter by scattering a particles from atoms.

36. Experiments of Rutherford, Geiger and Marsden 2 Geiger showed that many a particles were scattered from thin gold-leaf targets at backward angles greater than 90°.

37. Rutherford’s Atomic Model Ernest Rutherford (1871-1937) Experimental results were not consistent with Thomson’s atomic model. Rutherford proposed that an atom has a positively charged core (nucleus) surrounded by the negative electrons. Geiger and Marsden confirmed the idea in 1913.

38. The Classical Atomic Model • Consider an atom as a planetary system. • The Newton’s 2nd Law force of attraction on the electron by the nucleus is: where v is the tangential velocity of the electron: The total energy is then: This is negative, so the system is bound, which is good.

39. The Planetary Model is Doomed • From classical E&M theory, an accelerated electric charge radiates energy (electromagnetic radiation), which means the total energy must decrease. So theradius r must decrease!! Electron crashes into the nucleus!? Physics had reached a turning point in 1900 with Planck’s hypothesis of the quantum behavior of radiation, so a radical solution would be considered possible.

40. n = 1 n = 2 n = 3 The Bohr Model of the Hydrogen Atom • Bohr’s general assumptions: • 1. Stationary states, in which orbiting electrons do not radiate energy, exist in atoms and have well-defined energies, En. Transitions can occur between them, yielding light of energy: • E = En− En’ = hn • 2. Classical laws of physics do not apply to transitions between stationary states, but they do apply elsewhere. Angular momentum is quantized! 3. The angular momentum of the nth state is: where n is called the Principal Quantum Number.

41. a0 Consequences of the Bohr Model • The angular momentum is: So the velocity is: But: So: Solving for rn: where: a0 is called the Bohr radius. It’s the diameter of the Hydrogen atom (in its lowest-energy, or “ground,” state).

42. Bohr Radius • The Bohr radius, • is the radius of the unexcited hydrogen atom and is equal to: • The “ground” state Hydrogen atom diameter is: /

43. The Hydrogen Atom Energies Use the classical result for the energy: and: • So the energies of the stationary states are: En = - E0/n2 or: where E0 = 13.6 eV.

44. The Hydrogen Atom Emission of light occurs when the atom is in an excited state and decays to a lower energy state (nu→ nℓ). where n is the frequency of a photon. R∞is theRydberg constant.

45. Transitions in the Hydrogen Atom The atom will remain in the excited state for a short time before emitting a photon and returning to a lower stationary state. In equilibrium, all hydrogen atoms exist in n = 1.

46. Characteristic X-Ray Spectra and Atomic Number • Shells have letter names: • K shell for n = 1 • L shell for n = 2 • The atom is most stable in its ground state. • When it occurs in a heavy atom, the radiation emitted is an x-ray. • It has the energy E (x-ray) = Eu− Eℓ. An electron from higher shells will fill the inner-shell vacancy at lower energy.

47. The Correspondence Principle Bohr’s correspondence principle is rather obvious: In the limits where classical and quantum theories should agree, the quantum theory must reduce the classical result.

48. Successes and Failures of the Bohr Model Success: • The electron and hydrogen nucleus actually revolve about their mutual center of mass. • The electron mass is replaced by its reduced mass: • The Rydberg constant for infinite nuclear mass, R∞, is replaced by R.

49. Limitations of the Bohr Model • Works only for single-electron (“hydrogenic”) atoms. • Could not account for the intensities or the fine structure of the spectral lines (for example, in magnetic fields). • Could not explain the binding of atoms into molecules. The Bohr model was a great step in the new quantum theory, but it had its limitations. Failures: