Surface Chemistry of Materials, Chem 5610
Spring 2013
Lecture I: Introduction to Surfaces
A. Why are surfaces different from the bulk?
B. Why we need a vacuum (no Hoover jokes, please)
C. Methods for probing surfaces
Reading: Somorjai, Chapt. 1
Why Surface Science?
(1) Many important chemical reactions occur at outermost atomic layers of
materials (typically, outermost 1-50 Å)
Langmuir, H2 reactions at a W surface (1913)
Haber, N2 + 3 H2  2 NH3 over iron catalyst (about the same time)
The main drivers of surface science today:
(A) Catalysis
(B) Micro/nanoelectronics
(C) Energy (photovoltaics, fuel cells…)
And Tomorrow(?)
Biological issues (tissue/prosethic compatibility, membrane chemistries…)
Neuronetworks, biological and not
2
Atoms at a surface are low-coordinate relative to the bulk
Surface atom, 5
bonds to
nearest
neighbors
vacuum
Surface
Bulk
Bulk atom, 6
bonds to
nearest
neighbors
Unused surface bonds can interact, causing change in surface structure
Surface dimerization
Reconstruction of Si(100)
A. Unreconstructed Si(100)-(1x1)
surface. The Si atoms of the
topmost layer are highlighted in
orange; these atoms are bonded to
only two other Si atoms, both of
which are in the second layer
(shaded grey).
B. Reconstructed Si(100)-(2x1)
surface. The Si atoms of the
topmost layer form a covalent bond
with an adjacent surface atom are
thus drawn together as pairs; they
are said to form "dimers".
From
5
http://www.chem.qmul.ac.uk/surfaces/scc/scat1_6a.htm
Surface is different electronically: Distribution of Surface Charge
1. Electron density trails off exponentially away from the surface into the vacuum
2. This partially depletes negative charge just below the surface
Ion cores partially
unshielded, net + charge
Charge
neutrality in
the bulk
Bulk
Lang and Kohn, PRB 1 (1970) 4555
Region above surface
negatively charged (several
angstroms)
Surface
6
Redistribution of Charge near surface sets up the Surface Dipole
+
+
+
-
-
+
+
Bulk
7
Work function is the extra energy needed to promote an electron from the HOMO (Fermi
level) into the vacuum different for different surfaces
e.g~ 4.3 eV, W
~ 5.3 eV, Pt
EVacuum
E
Work Function
EFermi
8
Why do we need a vacuum?
O2
hydrocarbons
H2O
CO2
Atoms at the surface directly interact with gases in the environment
Rxns occur at the surface that don’t occur in the bulk
We need to control this
Typical Atom Surface Density:
~ 1015 atoms/cm2
Flux of atoms of mass M to this surface from the gas phase (F) is given
by (at gas temperature T) :
F (atoms/cm2-sec) = 3.51 x 1022 P(Torr) x [M(g/mole) T]-1/2 (Somorjai)
Note: At P = 3 x 10-5 Torr, M = 28 gr/mole; T = 300 K
F ~ 1015 atoms/cm2-sec.
Thus, assuming a “sticking coefficient” of 1, the surface is
covered by a fresh monolayer every second under a mild vacuum
Sticking Coefficient = probability/collision that an atom coming from the
vacuum and colliding with the surface will stick!
Sticking coefficients are often small (e.g., N2 on Au) but can approach 1 for ,
e.g., N2 on clean W.
We need to keep surface contaminant concentrations low over the course of
an experiment (~ 1 hour, say). Therefore, pressures ~ 10-9 or lower are required.
This is known as ultra-high vacuum (UHV).
Important: in measuring surface concentrations of adsorbed atoms, it is NOT
pressure, but Pressure x Time [Exposure] that is important.
1 Langmuir = 10-6 Torr-sec is the standard unit of exposure
Methods for maintaining and measuring ultrahigh vacuum (see standard texts,
such as Briggs and Seah, Practical Surface Analysis:
Chamber Materials: 304 Stainless steel now almost universal
Pumps: (1) Turbomolecular pump with mechanical pump backing
 can go to ~ 10-10 Torr if careful. Typically ~ 10-9 Torr – 5 x 10-10 Torr
 advantage, can pump many different types of gases, rapid pump down
from relatively high gas loadings back to UHV
 disadvantage, expensive, can malfunction during power outages, etc.
(2) Ion pump with Ti Sublimator
 can maintain vacuums better than 5 x 10-11 Torr
 Fussy about what it will pump (O2, H2O good, CO bad)
 Relatively cheap, long lasting, restarts after power outages
 low pumping speeds, needs turbo to rough down from high gas
loadings
Other pumps include oil diffusion pumps, but not much used anymore.
Measuring a vacuum: The “nude” (Bayard-Alpert) ion gauges
A+
e- + A  A+ + 2 ee-
Filament emits electrons accelerated by grid
Electrons ionize gas phase molecules
Ions collected a grid. Grid current proportional to pressure.
Ion gauges:
Practical upper limit ~ 10-3 Torr
Lower limit ~ 10-11 Torr
Very reliable: They only fail during important experiments
Methods for Probing Surfaces
How do we investigate surfaces?
Low energy electrons(Ekin < 1000 eV)
are surface sensitive:
penetration/escape depths < 100 Å
hvin
hvout
e-
Photons in/electrons out:
e-
Are surface sensitive
16
How do we investigate surfaces?
Photon penetration and escape
depths, typically > 0.1 microns, not
surface sensitive
hvin
hvout
Photons in/photons out:
Not surface sensitive
17
Since we are interested in the structures of (typically) the outermost 1-20 atomic
layers, we want surface probes with sampling depths of ~ 50 Å or less
What determines sampling depth
Typically, it is the escape depth of the detected photon/ion/electron
Photon escape depths typically ~ 10 nm or more (not good)
Ions, can be as little as one monolayer, but may present other problems
Electrons ~ escape depth determined by inelastic mean free path (IMFP = λ)
Typically, λ = λ(KE, electron density of medium)
From surf. Sci. Western (Univ. of W. Ontario)
hv
Surface, no
electrons
attenuated
e-
e-
e-
Some e- from bulk
or near/surface
suffer inelastic
collisions, change
kinetic energy, lose
chemical
information
λ and the continuum model for attenuation of electron intensity
Electron intensity out (Ix)
Electron intensity in
I0
dI = -(dx/λ) I
dI/I = -dx/λ
I(x) = I0 exp (-x/λ)
dx
λ-1 is the probability per unit length for the electron to undergo inelastic
collision
An overlayer thickness of λ will attenuate signal intensity by a factor of 1/e
At a thickness of 3λ, signal attenuated by 1/e3 ~ 98%
Since we want escape/sampling depths < 100 Å, we want to detect low energy electrons
coming from surfaces (EK < 1000 eV)
From surf. Sci. Western (Univ. of W. Ontario)
Surface Probes using low energy electrons
Technique
In
Out
XPS
hv ~ 1250 ev-1500ev
e-, Ek < 1000 eV
AES
e- ~ 3000 eV
e-, Ek < 1000 eV
LEED
e- ~ 50-300 eV
e-, Ek = Ein
UPS
hv ~ 21 – 40 ev
e-, EK < hv
Development of low energy electron-based surface probes:
1. LEED (low energy electron diffraction) –since 1927 (Davisson
and Germer and the birth of modern quantum mechanics)
2. AES (Auger electron spectroscopy) –1960’s, Palmberg, et al.
3. XPS (x-ray photoelectron spectroscopy )-1960’s Kai Sigbahn
and others at Uppsala University
All the above linked to technological developments:
LEED: Glass-based vacuum systems, fluorescent screens
AES, XPS: Development of accurant electron energy analyzers
All the above: improved vacuum technology
Improvements:
*Angle-Resolved photoemission, (band structure)
*spin polarized LEED (magnetic systems)
*spin-polarized photoemission (magnetic systems)
*time –resolved measurements (has not caught on)
*synchrotron-based photoemission, very popular for tuning
sampling depths.