Transcript Pt/SrTiO3 Interface - Nc State University

The World of Atoms
Instructor:
Dr. Gerd Duscher
http://www4.ncsu.edu/~gjdusche
email: [email protected]
Office:
Office Hours:
2156 Burlington Nuclear Lab.
Tuesday: 10-12pm
Objective today: How do atoms arrange themselves ?
Why is symmetry important ?
Why do atoms break symmetry?
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What is an Atoms?
Bohr Model
that is too simple
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How do they bond?
Ionic Bonding Covalent Bonding
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H
CH4
+
H
C
H
shared electrons
from carbon atom
H
shared electrons
from hydrogen
atoms
Van Der Waals Bonding
arises from interaction between dipoles
asymmetric electron
clouds
+
-
+
van der Waals
bonding
-ex: liquid HCl
-
H Cl
van der Waals
bonding
H Cl
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What properties does that imply?
• bond length, r
F
• melting temperature, Tm
Energy (r)
F
r
• bond energy, Eo
ro
r
Energy (r)
smaller T m
unstretched length
ro
r
Eo =
“bond energy”
larger T m
Tm is larger if Eo is larger.
Summary: Primary Bonds
Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers
(Covalent & Secondary):
large bond energy
large Tm
large E
small a
variable bond energy
moderate Tm
moderate E
moderate a
directional Properties
van der Waals bonding dominates
small T
small E
large a
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• Non dense, random packing
energy
Energy And Packing
typical neighbor
bond length
typical neighbor
bond energy
energy
• Dense, regular packing
r
typical neighbor
bond length
typical neighbor
bond energy
Dense, regular-packed structures tend to have
lower energy.
r
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Materials And Packing
Crystalline materials...
• atoms pack in periodic, 3D arrays
• typical of:
-metals
-many ceramics
-some polymers
crystalline SiO2
Noncrystalline materials...
• atoms have no periodic packing
• occurs for: -complex structures
-rapid cooling
"Amorphous" = Noncrystalline
noncrystalline SiO2
Metallic Crystals
• tend to be densely packed.
• have several reasons for dense packing:
-Typically, only one element is present, so all atomic
radii are the same.
-Metallic bonding is not directional.
-Nearest neighbor distances tend to be small in
order to lower bond energy.
• have the simplest crystal structures.
We will look at three such structures...
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Simple Cubic Structure (sc)
• rare due to poor packing (only Po has this structure)
• close-packed directions are cube edges.
• Coordination # = 6
(# nearest neighbors)
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Body Centered Cubic Structure
(bcc)
• Close packed directions are cube diagonals.
--Note: All atoms are identical; the center atom is shaded
differently only for ease of viewing.
• Coordination # = 8
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Face Centered Cubic Structure
(fcc)
• Close packed directions are face diagonals.
--Note: All atoms are identical; the face-centered atoms are shaded
differently only for ease of viewing.
• Coordination # = 12
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fcc Stacking Sequence
• ABCABC... stacking sequence
• 2D projection
A
B
B
C
A
B
B
B
A sites
C
C
B sites
B
B
C sites
• fcc unit cell
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Hexagonal Close-Packed
Structure (hcp)
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• ABAB... Stacking Sequence
• 3D Projection
• 2D Projection
A sites
B sites
A sites
• Coordination # = 12
• APF = 0.74
12
Hexagonal Close-Packed
Structure (hcp)
graphite
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Diamond Structure
silicon, diamond
ZnS – type (GaAs)
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Structure Of Compounds: Nacl
• Compounds: Often have similar close-packed structures.
• Structure of NaCl
• Close-packed directions
--along cube edges.
Perovskite Strucutre
SrTiO3
Applications: non-linear resistors (PTC), SMD capacitors,
piezoelectric sensors and actuators, ferroelectric memory.
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Densities Of Material Classes
rmetals rceramics rpolymers
Why?
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Ceramics have...
r (g/cm3)
Metals have...
• close-packing
(metallic bonding)
• large atomic mass
• less dense packing
(covalent bonding)
• often lighter elements
Polymers have...
• poor packing
(often amorphous)
• lighter elements (C,H,O)
Composites have...
• intermediate values
Metals/
Alloys
20
Platinum
Gold, W
Tantalum
10
Silver, Mo
Cu,Ni
Steels
Tin, Zinc
5
4
3
2
1
0.5
0.4
0.3
Titanium
Aluminum
Magnesium
Graphite/
Ceramics/ Polymers
Semicond
Composites/
fibers
Based on data in Table B1, Callister
*GFRE, CFRE, & AFRE are Glass,
Carbon, & Aramid Fiber-Reinforced
Epoxy composites (values based on
60% volume fraction of aligned fibers
in an epoxy matrix).
Zirconia
Al oxide
Diamond
Si nitride
Glass-soda
Concrete
Silicon
Graphite
PTFE
Silicone
PVC
PET
PC
HDPE, PS
PP, LDPE
Glass fibers
GFRE*
Carbon fibers
CFRE*
Aramid fibers
AFRE*
Wood
Crystals as Building Blocks
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• Some engineering applications require single crystals:
--diamond single
crystals for abrasives
--turbine blades
• Crystal properties reveal features
of atomic structure.
--Ex: Certain crystal planes in quartz
fracture more easily than others.
POLYCRYSTALS
• Most engineering materials are polycrystals.
1 mm
• Nb-Hf-W plate with an electron beam weld.
• Each "grain" is a single crystal.
• If crystals are randomly oriented,
overall component properties are not directional.
• Crystal sizes typ. range from 1 nm to 2 cm
(i.e., from a few to millions of atomic layers).
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Single vs Polycrystals
• Single Crystals
E (diagonal) = 273 GPa
-properties vary with
direction: anisotropic.
-example: the modulus
of elasticity (E) in bcc iron:
• Polycrystals
-properties may/may not
vary with direction.
-if grains are randomly
oriented: isotropic.
(Epoly iron = 210 GPa)
-if grains are textured,
anisotropic.
E (edge) = 125 GPa
200 mm
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TEMs at NCSU
The NEW JEOL 2010F
This is a TEM/STEM,
which can do everything
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TEMs at NCSU
TEM Lab Course at the OLD TEM: Topcon
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STEM at
ORNL
This STEM provides the
smallest beam in the world.
It uses the brightest source
in the universe,
1000 times brighter than
a supernova.
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Why?
That is what happens when pulling wires.
• before deformation
• after tensile elongation
slip steps
Dislocation move, more dislocation get generated and
entangle (interact) with themselfs, and other defects.
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Incremental Slip
• Dislocations slip planes incrementally...
• The dislocation line (the moving red dot)...
...separates slipped material on the left
from unslipped material on the right.
push
Simulation of dislocation
motion from left to right
as a crystal is sheared.
fixed
Bond Breaking And Remaking
• Dislocation motion requires the successive bumping
of a half plane of atoms (from left to right here).
• Bonds across the slipping planes are broken and
remade in succession.
push
Atomic view of edge
dislocation motion from
left to right as a crystal
is sheared.
fixed
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Point Defects
• Vacancies:
-vacant atomic sites in a structure.
distortion
of planes
Vacancy
• Self-Interstitials:
-"extra" atoms positioned between atomic sites.
distortion
of planes
selfinterstitial
Vacancy in Silicon
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Point Defects In Alloys
Two outcomes if impurity (B) added to host (A):
• Solid solution of B in A (i.e., random dist. of point defects)
OR
Substitutional alloy
(e.g., Cu in Ni)
Interstitial alloy
(e.g., C in Fe)
• Solid solution of B in A plus particles of a new
phase (usually for a larger amount of B)
Second phase particle
--different composition
--often different structure.
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Imaging of Single Bi Atoms in Si(110)
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A. Lupini, VG HB501UX with Nion Aberration Corrector, 100 kV
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Types of Imperfections
• Vacancy atoms
• Interstitial atoms
• Substitutional atoms
• Anti-site defects
Point defects
(0 dimensinal)
• Dislocations
Line defects
(1 dimensional)
• Grain Boundaries
Area defects
(2dimensional)
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