Understanding the properties and behavior of groups of interacting atoms more than simple molecules

Transcription

1 Condensed Matter Physics Scratching the Surface Understanding the properties and behavior of groups of interacting atoms more than simple molecules Solids and fluids in ordinary and exotic states low energy physics Structure of materials: Crystalline vs. Amorphous Electrical properties: Metals Semiconductors Insulators Extremes: low temperature, high pressures Special states & Phase Transitions: Magnetism Superconductivity Superfluidity Surfaces vs. bulk Evolution from quantum mechanical world of atoms to classical macroscopic stuff p. 1/29

2 Some length scales m =.01 m = 1cm Diameter of a dime 10-3m =.001 m = 1mm Thickness of a CD 10-4 m =.1 mm = 100 µ m Diameter of a hair 10-5 m =.01 mm = 10 µ m Red blood cell 10-6 m =.001 mm = 1 µ m 10-7 m =.1 µ m = 100nm 10-8 m =.01 µ m = 10nm 10-9 m =.001 µ m = 1 nm m = 0.1 nm = 1 A Bit of data on CD Transistor Smallest features in integrated circuits Pitch of DNA helix Spacing of atoms in a solid To nuclear physics <10-14 m p. 2/29

3 Atoms at Surfaces Macroscopic objects Surface atoms are in a tiny minority in everyday objects. Most atoms are unaware that the surfaces exist and are unaffected. 1 cm 3 holds up to 125 x atoms with 1 cm 0.2 nm = 50 x 10 6 atoms along an edge and 25 x atoms on a face Roughly 1 atom in 10,000,000 is at the surface. 1 cm 10,000 X p. 3/29

4 Atoms at Surfaces Micro-scale 1 µm 3 holds up to 125 x 10 9 atoms with 1 µm 0.2 nm = 5 x 10 3 atoms along an edge and 25 x 10 6 atoms on a face Roughly 1 atom in 1000 is at surface. xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx xxxxxxxxxxxxxx 1 µm 1000X p. 4/29

5 Atoms at Surfaces Nano-scale 1 nm 3 holds up to 125 atoms with 1 nm 0.2 nm = 5 atoms along an edge and 25 atoms on a face Roughly 4 atoms in 5 are at the surface! 1 nm As structures become smaller, surface atoms become dominant in determining properties. Nano-physics and nano-technology understanding and making small things. p. 5/29

6 Scanned Probe Microscopes Two tools for studying surfaces Scanning Tunneling Microscope (STM) * Measures tiny electric current between sharp probe and nearby surface * Sample surface must be electrical conductor (metals, semiconductors) * Relies on quantum mechanical effect Atomic Force Microscope (AFM) Senses forces between surface and sharp probe near the surface. Very sensitive record player! p. 6/29

7 Quantum vs Classical The behavior of atomic-sized systems is governed by quantum mechanics. Matter (protons, electrons, and neutrons) at this scale exhibits a dual personality Particle-like aspects familiar from every-day life becomes less obvious Wave-like aspects dominate object no longer has sharply defined position cannot know everything simultaneously (e.g. position and velocity) wave nature described by probability density (e.g. electron clouds) some classical rules can be broken for short times p. 7/29

8 Classical mechanics RED marble starts with enough energy to travel over hill into right valley and back. BLUE marble is confined to the left valley forever - insufficent energy to climb over the hill. p. 8/29

9 Quantum mechanics BLUE marble initially on left... QUANTUM TUNNELING...can appear on the right even though its energy is insufficient to carry it over the hill! Marble is not localized in quantum mechanics - it behaves as if smeared out and can pass thru or tunnel thru barriers For significant tunneling probability need very light marbles (electrons are good) very thin hills ( < 1 nanometer = m) p. 9/29

10 Electron tunneling - metals Electrons inside metal: move freely within, reflected at the surfaces. 1 cm Metals have lots of electrons > per cm 3 Barrier <1 nm Tunneling barrier is formed by thin space between 2 pieces of metal. p. 10/29

11 Tunneling as microscopic probe Tunnel between sharp wire and sample of interest. WIRE TIP SAMPLE Tunneling occurs only through atom at very tip of wire. p. 11/29

12 Tunneling as microscopic probe Tunnel between sharp wire and sample of interest WIRE TIP SAMPLE - Battery + Hook up battery to sustain net tunneling from tip to sample. p. 12/29

13 Scanning Tunneling Microscope Piezoelectric Scanner Tip Constant z Current varies z y x xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xx z y x x xx xx x x x I ~ e -cz z ~ 1 nm I I(x,y) 1 cm Sample Tunnel Current Tip Position p. 13/29

14 STM constant current imaging Adjust tip height to keep current constant Z-piezo voltage provides image xx xx xx x xx x xx Tip I Sample Tip Height Z Tunnel Current Tip X Position p. 14/29

15 STM raster imaging Adjust tip height to keep current constant Z-piezo voltage provides image xx xx xx x xx x xx Tip I Sample y Tip Height Z Tip Position x p. 15/29

16 STM p. 16/29

17 STM image Silver film 100 nm 100 nm scan p. 17/29

18 STM image - Graphite 2 nm 2 nm scan p. 18/29

19 STM image Graphite 1 nm 1 nm scan Atomic resolution of electron clouds around each atom. p. 19/29

20 IBM STM atom manipulator Fe atoms arranged in a ring on Cu surface by STM tip (Eigler et al., IBM - San Jose) Note electron waves on Cu surface inside corral. p. 20/29

21 Current efforts Electromigration - the movement of atoms as a result of electrons flowing through a material as an electric current. Caused rapid failure in the early days of integrated circuits STM may provide a way to make a dent in the surface and watch at the atomic level the dent move or change shape as a result of electromigration. STM Tip Dent surface Electric Current? Gold film Mica substrate p. 21/29

22 Atomic Force Microscope (AFM) Conventional AFM senses forces between tip and surface by deflection of a cantilever, measured by bouncing laser light off the back. Cantilever Probe tip SAMPLE Tuning Fork Scanner Element PROBE f SAMPLE x Dynamic AFM senses forces by change in frequency of tuning fork. p. 22/29

23 Forces between Atoms Force Repulsive (+) Attractive (-) Distance f > 0 f < 0 p. 23/29

24 Quartz tuning forks p. 24/29

25 AFM p. 25/29

26 AFM Graphite Image 7000 nm nm scan Z, Phase/Error, Amplitude p. 26/29

27 AFM TPS Image 2000 nm 2000 nm scan Space Shuttle Thermal Insulation Material p. 27/29

28 Beyond physics Protein molecules in crystalline form. (Hörber & Miles Science, vol. 302, p. 1002) p. 28/29

29 Further reading For some links to related topics on the web, see jmaps/phys1021/ p. 29/29