3 P627_S13_L01 Jan 22
description
Transcript of 3 P627_S13_L01 Jan 22
1
Surface Properties
• Structure
• Chemical composition
• Bonding properties
• Kinetics (adsorption, diffusion, desorption, catalysis)
• Dynamics of surface processes
• Statistical mechanics of 2-D systems
Applications of Surfaces
• Catalysis
• Corrosion
• Friction, lubrication
• Semiconductor devices
• Electrochemistry hu
(IR X-rays)
e- ions heat
E-field
atoms, molecules
hu
atoms, molecules
ions
e-
Probing Surface Properties
Surface and Interface Science
Physics 627; Chemistry 542
Usually, strongly interacting probes!!
2
The first half of this course will focus on:
• Atomistic properties of surfaces
• Electronic properties
• Chemical composition
• Adsorption properties
The second half of this course will deal with applications:
• Biomedical applications
• Surface magnetism
• Calculation procedures
• Devices
Examination:
• Homework problem set (?)
• 3 labs
• Term paper
Primary text: Zangwill “Physics at Surfaces” Also: Woodruff and Delchar: Modern Techniques of Surface Science
3
Lectures 1 and 2
Surface crystallography
References:
1) Zangwill, Chapter 1
2) A.W. Andersen, Physical Chemistry of Surfaces, Fifth Edition (J. Wiley, New
York, 1990) Chapter VII
3) J.M. Blakely and M Eizenberg in Vol. 1, Clean Solid Surfaces, “The Chemical
Physics of Solid Surfaces and Heterogeneous Catalysis,” ed. By D.A. King
and D.P. Woodruff (Elsevier, Amsterdam, 1981) p. 1
4) G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, Chapter 3.
5) A. Keijna and K.F. Wojcieckowski, Metal Surface Electron Physics, Chapter 3;
Chapter 8.
4
Simple cubic (sc)
Surface structure – starting with the bulk
5
hcp vs fcc
6
Miller Indices
For simple cubic lattice,
consider plane that is shaded.
Vector from origin intercepts
that plane at x, y, z = 1, ,
The Miller indices of this plane are:
0,0,11
,1
,1
1
x
y
In two dimensions (2D crystals)
intercept (, 1) (0, 1)
intercept (1, 1) (1, 1)
7
A. Bulk truncation structures
Miller Indices (more)
• For plane with intersections bx, by, bz write:
• If all quotients intergers or 0, this is a Miller Index.
e.g., 1, 1, 0.5 (112)
For cubic 1 (100)
In general: (i j k) =
where cd = smallest common denominator
Here (i j k) =
In fcc and bcc
x, y, z, -x, -y, -z all equivalent (100), (010), , etc.
all equivalent.
NOTE: (i j k) identifies plane;
[i,jk] identifies vector ^ plane defines direction.
zyx bbb
1,
1,
1
zyx b
cd
b
cd
b
cd,,
)364(3
12,
2
12,
4
12
)001(
x
z
y
bz
bx
by
x
z
y [100]
x
z
y
3
4
2
(100)
]034[
]204[
8
Very different surfaces:
Close packed: fcc(111) bcc(110)
Very rough: fcc(210) bcc(111)
A. Bulk truncation structures
Note: Cross product of two vectors in a plane defines direction perp. to plane:
[i j k] = [s t w] x [p q r] where latter vectors lie in (i j k).
Angle between two planes:
222222
cosnmlkji
lmnijk
47.1963
112cos
e.g., for [1 1 1] and [2 1 1] :
bcc: 8 n.n. fcc: 12 n.n.
[l m n]
[i j k]
9
A. Bulk truncation structures fcc(100) fcc(110) fcc(111)
bcc(100) bcc(110) bcc(111)
hcp(100) hcp(110) hcp(111)
10
11
But i + j = -k
So often use 3-digit notation
Basal plane (b): (0 0 0 1) = (0 0 1)
Side (c)
Side (d)
In hcp, (1 0 0) (0 0 1)
NOTE: fcc(111) and hcp (0001) have same top layer structure,
but stacking is different: hcp: ABAB…; fcc: ABCABC…
A. Bulk truncation structures
zwyx b
cd
b
cd
b
cd
b
cdijkl
01101
1
11
1
1
For HCP surfaces:
11011
1
1
1
1
11
12
Wurtzite = hcp + basis
Example: GaN
Zinkblende = two
interpenetrating fcc
lattices
Example: GaAs, ~ Si (all
atoms the same)
More than one kind of atoms
13
A B C
Side view of 4-H silicon carbide. A,
B and C are three inequivalent
rows of atoms.
Stacking: a-b-c-b-a …
Red = Si, Black = C
Note the two different
terminations
Polar surfaces
14
A. Bulk truncation structures
15
B. Relaxations and reconstructions
Relaxation: no lateral motion
Usually vertically inwards,
sometime outwards
Crystal termination often not bulk-like
Shifts in atomic positions may be perpendicular and/or parallel to surface
Selvedge region extends several atomic layers deep
Rationale for metals: Smoluchowski smoothing of surface electronic charge
dipole formation
For semiconductors: heal “dangling bonds;
often lateral motion. Relax. Often oscillatory
dbulk
d12 d34
d56
d45
d23 Surface d12(%)
Ag(110) -8
Al(110) -10
Au(100) 0
Cu(110) -10
Cu(310) -5
Mo(100) -12.5
surf
ace
bulk
16
Periodic Lattice: repeat unit is unit cell
C. Classification of 2-D periodic structures
Unit cell is not unique.
Propagate lattice: n, m integers
Primitive cell: unit cell w/smallest
area, shortest lattice vectors,
smallest number of atoms
(if possible: |a1| = |a2|; g = 60, 90, 120;
1 atom)
Symmetry: translational symmetry // surface;
rotational symmetry: 1, 2, 3, 4, 6
mirror planes; glide planes.
All 2-D structures w/1 atom/unit cell have
at least one two-fold axis.
21 amanT
17
For 1 atom/cell and 2-D periodic structure, only 5 symmetrically different lattices
Bravais Lattices
C. Classification of 2-D periodic structures
When more than 1 atom/cell, more
complicated
•5 Bravais lattices
•10 2-D point symmetry groups
•17 types of surface structure
D. Substrate and Surface Structures Suppose overlayer of substrate surface layer has lattice different from bulk:
Substrate:
Overlayer:
21 amanTa
21 bmbnTb
18
Wood’s Notation: Simplest, most descriptive notation method
(NOTE: fails if a a’ or bi/ai irrational)
Ra
b
a
b
2
2
1
1
D. Substrate and Surface Structures
a1
a2
b1
b2
ai
aj
bi
bj
a
a’
Determine relative magnitude of respective a’s and b’s.
Identify angle of rotation ( = 0˚ here).
Notation: for above overlayer, (2 x 2) [often called p(2 x 2)]
3033 R(2 x 2)
¹
Hexa
gonal
19
D. Substrate and Surface Structures
Classification of Lattices
20
Examples of
coincidence lattice
Note that symmetry
does not identify
adsorption sites
all (2 X 2)
= 1/4
Domain structures:
(1 X 2) = (2 X 1)
D. Substrate and Surface Structures
21
D. Substrate and Surface Structures
Another complication:
indexing of stepped surfaces
Ambiguity: fcc(110) reconstruction models for (2 X 1) periodicity
Missing Row Paired Row Saw Tooth