Surface Chemistry of Materials, Chem 5610Spring 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

Surface

vacuum

Bulk

Bulk atom, 6 bonds to nearest neighbors

Surface atom, 5 bonds to nearest neighbors

Atoms at a surface are low-coordinate relative to the bulk

Unused surface bonds can interact, causing change in surface structure

Surface dimerization

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".

Reconstruction of Si(100)

From http://www.chem.qmul.ac.uk/surfaces/scc/scat1_6a.htm5

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

Bulk Surface

Ion cores partially unshielded, net + charge

Region above surface negatively charged (several angstroms)

Charge neutrality in the bulk

Lang and Kohn, PRB 1 (1970) 4555 6

Bulk

Redistribution of Charge near surface sets up the Surface Dipole

++ + + +

- - - - -

7

EFermi

EVacuum

Work Function

Work function is the extra energy needed to promote an electron from the HOMO (Fermi level) into the vacuum different for different surfacese.g~ 4.3 eV, W ~ 5.3 eV, Pt

E

8

Why do we need a vacuum?

O2H2O CO2hydrocarbons

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

e-e- + A A+ + 2 e-

A+

Filament emits electrons accelerated by gridElectrons ionize gas phase moleculesIons 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?

hvin

hvout

Low energy electrons(Ekin < 1000 eV) are surface sensitive: penetration/escape depths < 100 Å

Photons in/electrons out:

Are surface sensitive e-

e-

16

How do we investigate surfaces?

hvin

hvout

Photon penetration and escape depths, typically > 0.1 microns, not surface sensitive

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

e- e-

Surface, no electrons attenuated

Some e- from bulk or near/surface suffer inelastic collisions, change kinetic energy, lose chemical information

e-

λ and the continuum model for attenuation of electron intensity

dx

Electron intensity in

I0

Electron intensity out (Ix)

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/eAt a thickness of 3λ, signal attenuated by 1/e3 ~ 98%

dI = -(dx/λ) I

dI/I = -dx/λ

I(x) = I0 exp (-x/λ)

From surf. Sci. Western (Univ. of W. Ontario)

Since we want escape/sampling depths < 100 Å, we want to detect low energy electrons coming from surfaces (EK < 1000 eV)

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

Surface Probes using low energy electrons

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.