ME 4875/MTE 575 - C16 Introduction to Nanomaterials and Nanotechnology Lecture 3 - Atomic Structure and Bonding 1
Atomic Structure and Bonding • It’s important to know about atomic structure and bonding to understand how properties change at the nanoscale • What are materials composed of? • Where are these components in a materials? • How are they held together? • What accounts for the different properties of materials? • Goals: – Qualitative picture of electrons, atoms, binding between atoms, behavior of atoms and electrons in materials (mostly modern physics and chemistry) – Give you enough background to read papers about nanomaterials 2
Basic Structure of Atoms • Discovery of the electron (1896, J. J. Thompson) • Discovery of atomic nucleus (1911, Rutherford) Alpha particles are He 2+ (two protons and two neutrons) Uniform distribution of charge - + • Positively charged nucleus containing positively charged protons and neutral neutrons (both very heavy) • Negatively charged electrons moving around the nucleus 3
Wave-Particle Duality of Electrons mass-energy equivalence, 𝐹 = 𝑛𝑑 2 𝑛𝑑 2 = ℎ𝑑 • 𝜇 (1905, Einstein) ℎ𝑑 𝜇 = ℎ photoelectric effect 𝐹 = ℎ𝑔 = 𝜇 • 𝑛𝑑 (1905, Einstein) ℎ de Broglie relationship 𝜇 = 𝑛 𝑓 𝑤 • (1924) • wavelength of the electrons in a 200 kV TEM is 2.5 pm (2.5 x 10 -12 m) 4
Electron Diffraction • Typical electron wavelength is comparable to atomic spacing in crystal, leading to diffraction constructive interference 2𝑒 𝑡𝑗𝑜𝜄 = 𝑜𝜇 destructive interference 5
Isolated Atoms • Bohr Model - electron ‘orbits’ (1913) 𝑜ℎ 𝑜ℎ • 2𝜌𝑠 = 𝑜𝜇 = 𝑛 𝑓 𝑤 → 𝑠 = 2𝜌𝑛 𝑓 𝑤 (each orbit has integer number of wavelengths) electron (-) 𝑛 𝑓 𝑤 2 𝑎𝑙𝑓 2 𝑎𝑙𝑓 2 nucleus = 𝑠 2 → 𝑤 = • 𝑠𝑛 𝑓 (+) 𝑠 (centripetal force = electrostatic force) 𝑎𝑙𝑓 2 𝑎𝑙𝑓 2 E vac = 0 2 𝑛 𝑓 𝑤 2 − 1 • 𝐹 = = − 2𝑠 𝑠 n=3 (total energy = kinetic energy + electrical potential energy) E n=2 4𝜌 2 𝑎 2 𝑙𝑓 2 2 𝑛 𝑓 • 𝐹 = − 2ℎ 2 𝑜 2 n=1 (Final result: energies of orbits are quantized) 6
Isolated Atoms • Schrödinger equation (1926) • Solutions are ‘orbitals’ with quantum numbers n, l, m, s – n is principal quantum number (which shell) – l is angular momentum quantum number l = 0, 1, 2,…. (n -1) (which shape) – m is magnetic quantum number m = 0, ±1, ± 2,… ±l (which orientation) – s is spin quantum number ± ½ (two electrons can occupy each orbital) • Tells us how many electrons can be in each shell (1s 2 , 2s 2 , 2p 6 , etc.) p (l=1) d (l=2) f (l=3) s (l=0) 7
Bonds between Atoms Ionic Bond (transferred electrons) to make “complete” shells 1s 2 2s 2 2p 6 3s 2 3p 5 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6 1s 2 2s 2 2p 6 3s 1 Gives two oppositely charged ions, which then have an electrostatic attraction (a bond) 8
Bonds between Atoms • Covalent Bond (shared electrons ) to make “complete” shells • Electrons are localized between the ion cores 1s 2 2s 2 2p 4 O + O O 2 1s 2 2s 2 2p 6 The positive ion cores of each atom are attracted electrostatically to the shared electrons, resulting in a bond. 9
Bonds between Atoms Metallic Bonding (shared delocalized electrons) 10
Bonds between Atoms Van der Waals Bond (attraction between dipoles) - - + + permanent dipoles (Keesom Force) - - + + permanent/induced dipoles (Debye Force) - - + + 11 instantaneous induced dipoles (London Force)
Bonds between Atoms • What happens to the atomic orbitals when bonds are formed between atoms? Atomic Orbitals: p (l=1) d (l=2) f (l=3) s (l=0) 12
Molecular Orbitals Here, sign means phase, not charge Bonding Antibonding • When two atomic orbitals combine (by overlapping in space), they form two molecular orbitals with different energies • Linear combination of atomic orbitals (approximate) 13
Molecular Orbitals Bonding Antibonding 14
Solids (Giant Molecules) • What happens when more than two atoms combine into a molecule? • The final number of molecular orbitals is equal to the number of atoms Single Two Four Many Atom Atoms Atoms Atoms (Solid) Bonding Electronic Energy Band Antibonding E Outer Electrons Band Gap Core Electrons 15 (approximate picture)
Metals, Semiconductors and Insulators 0 K > 0 K E thermal energy Metal Semiconductor Insulator 16
Next Class • Next class, we’ll look at crystal structures (how atoms are arranged in a solid) • For students who are waiting for the registrar to add them to the course and do not have access to myWPI, please go to Prof. Rao’s website nanoenergy.wpi.edu , then click on “ME 4875/MTE 575 Content” tab to access the course materials. 17
Project Topics (Reminder) Due tonight by 11:59 pm on myWPI 1. Solar Cells 9. Chemical Sensing 2. Batteries 10. Catalysis 3. Structural Materials 11. Energetic Materials 4. Thermoelectrics 12. Piezoelectrics 5. Computing (transistors) 13. Robotics 6. Memory (magnetic, flash, etc) 14. Photonics 15. Coatings 7. Drug delivery/Nanomedicine 8. Biological Sensing 18
How to Rank Topics • Fill out your preferences on myWPI - – ‘Course Materials’ > ’Project Resources’ • Rank topics from 1-15, with your most preferred topic as # 1 (rate all) • Be adventurous • Preferences due by midnight today • TA will match preferences to form the groups • Groups and presentation day will be announced on Friday, Jan 22 19
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