Introduction to Surface Chemistry

(1)

• Why is surface chemistry a separate field in chemistry?

• What makes surface atoms different from bulk ones?

• What are the significant properties of surface atoms that we can take into advantage?

• How “surface” is defined?

(2)

Surface chemistry is a separate field

Surface atoms behave chemically and physically different from bulk atoms

Surface atoms interact with the environment

Chemical reaction on surfaces are carried out differently Surfaces chemistry is analyzed differently

(3)

What makes surface atoms different

Bulk atoms experience equal forces (on average) around

:

isotopically

(4)

What makes surface atoms different

Surface atoms experience unsymmetrical forces

⇒ higher ^E

: surface ^E ^.

(5)

Surface atoms experience unsymmetrical forces and often relax and reconstruct in order to lower their energy

Surface Relaxation

Surface Reconstruction

B A A

(6)

What makes surface atoms different

Surface atoms experience unsymmetrical forces:

Unreconstructed and reconstructed Si 111 surface

←

(7)

Surface atoms contain unsaturated bonds (chemical and/or intermolecular)

Dangling bonds makes surface atoms reactive

Surface atoms are in contact with harsh environment, thus are susceptible to corrosion, oxidation, hydration, patination, etc.

(8)

The significant properties of surface atoms and their utilization

Heterogeneous Catalysis Surface tension

Adhesion and friction Electrochemistry

Refraction coatings

Anticorrosion and patination Sensors

(9)

How “surface” is defined?

1. how thick a surface is?

Depends on the application, analysis, significance of properties, etc.

For interior decorator, surface is the last 60 mm of paint For electrochemists its the last 2-4 monolayers

For the catalysis technologist it’s the last 1-2 monolayers

For refractive coatings its as thin as 10 nm

Surface analytical techniques are surface “sensitive” in various ways

(10)

How “surface” is defined?

2. how many atoms in a surface?

In one cubic cm of a metal ~ 10²³ atoms

On one plane of the 1cm² cube are ~ 10¹⁵ atoms The ratio of surface to bulk atoms is ~ 10^-6 %

For a typical analyzed area of 1mm², taking into account the first 10 monolayers, this means 10^-10 mole atoms.

For similar conditions, in XPS and a detectibility limit of 0.1% it comes to 10^-13 mole

(11)

How “surface” is defined?

3. surface analysis techniques

* it

→^er

"

✓& ^"

Thin

- film

→ TO

(12)

How “surface” is defined?

4. size wise

A 10mm thick sample, surface analyzed (by XPS, SIMS, AES, etc.) to depth of 50 A, is analogous to a layer as thick as a tennis ball on top of Azriely towers.

(13)

Surface Analysis

Chemical information

Qualitative chemical information Quantitative chemical information

Physical information

Conductivity

Electronic structure Hardness

Morphology/topography

(14)

The role of surface analysis in Nanotechnology

• In nanomaterials the surface to bulk atoms number is increasing dramatically.

• Thus, more of the surface properties of the materials is of importance

• In nanomaterials there might be a mix of bulk and surface analysis

(15)

A survey of surface Analysis

techniques: Qualitative Analysis

• XPS

• Sample is irradiated by X-rays

• Electrons are ejected due to the photoelectric effect

• The kinetic energy of the emitted electrons is analyzed

• Each element has unique set of emitted electrons

• Differences in oxidation state cause shift to the peaks

• XP Auger emissions also carry elemental information

• Valence band region is “kind of” fingerprint

• Surface sensitivity due to electron transport in solids

E ^-^-

Wy mi

^K ^E ^- ^" ^went

0

sample

( ^o ^- ³⁰ ^eV _region)

A

(16)

Cut ^Cu ^- ^- ^- ^. ^. ^- ^- ^- ^r ^. ^O ^: free ^e-

* * is

ftp.T

_by ^oxidation

⑦

^nucleus

Tefft

: effective nuclear charge ^.

(17)

• Auger Electron Spectroscopy

• Sample is irradiated by a beam of high energy electrons

• Auger electrons are ejected due to relaxation

• The kinetic energy of the Auger electrons is analyzed

• Each element has unique set of emitted electrons

• Differences in oxidation state causes peak shift

• Possible for chemical mapping

• Surface sensitivity due to electron transport in solids

(18)

A survey of surface Analysis

• Energy Dispersive Spectroscopy

• Electron beam hit sample

• Secondary electrons eject

• During relaxation X-ray emit

• Each element has characteristic emission spectrum

• Elemental analysis and chemical mapping.

• Very small and controlled sampling point

EDAX ^. ^EDS ^. =XRF EDK

X ^- ray fluorescence #

2-

&

(19)

A survey of surface Analysis

• FTIR/ATR

• Sample is irradiated by iR

• iR radiation is absorbed to vibrational levels

• Absorption is measured

• The low energy region is “kind of” fingerprint

• Due to its high sensitivity it is capable of measuring monolayers

• Can be made more surface sensitive by ATR and by angle resolved spectra

bulk ^e

✓ ^seu

"

Curet

→ attenuated ^total reflectance

FT ^- ^IR

{ #

transmitting

HVaw

'

¥

ATR

tearful

fire

Sen^"

(20)

A survey of surface Analysis

• Surface Enhanced Raman

• Sample is irradiated by a laser beam of iR to UV

• A fraction of the scattered photons loose or gain energy in to vibrational states

• This technique is not inherently surface sensitive due to the low efficiency of the Raman phenomenon. Its surface sensitivity is enhanced by special surface

preparation ^SERS ^: ^Surface ^- ^enhanced ⁱ

usual ^.

= Raman spectroscopy

- W

}

(21)

A survey of surface Analysis

• Surface Second Harmonic Generation

• Sample is irradiated by a laser beam

• A fraction of the reflected photons nonlinearly combine to give frequency doubled photons

• Surface sensitivity is due to the broken symmetry (a fundamental requirement for SHG) at the interface of the reflecting surface

hr

hEy ^went

GHG

asymmetry

-

→%uple

-

(22)

A survey of surface Analysis

• Electron Energy Loss Spectroscopy

• Sample irradiated by monochromatic electron beam

• Some of the reflected electrons loose energy for electronic transitions (and vibrational transitions in HREELS)

• The reflected electrons are energy analyzed to give elemental, oxidation state and surface vibrational spectra

EELS

inelastic scattering^.

(23)

A survey of surface Analysis

• Secondary Ion Mass Spectroscopy

• Sample is irradiated by a beam of ions or high energy atoms

• Fragments of surface molecules are shattered and ejected

• These secondary ions are mass analyzed to obtain surface elemental and molecular information

• Surface sensitivity and depth resolution is tuned by beam energy and dose level.

• Extremely sensitive. Chemical mapping

(24)

A survey of surface Analysis

techniques: Quantitative Analysis

• XPS

• Integration of peak areas, and corrected for relative sensitivity factor and machine parameters

• quantification is only in the form of relative concentration ratio within single sample

• Accuracy of 1-300%

• Detection limit: 0.1 atomic % submonolayer (depends on sample and element)

mainly qualitative

Mg ^20%

{ Cu ^80% }

(25)

A survey of surface Analysis

• AES

• Integration of peak areas, and corrected for relative sensitivity factor and machine parameters

• quantification is only in the form of relative concentration ratio within single sample

• Very problematic and highly inaccurate

• Detection limit: 0.1 atomic % submonolayer (depends on sample and element)

• Better detection limit for light elements

⇒ mainly qualitative ^.

(26)

A survey of surface Analysis

• FTIR

• Comparison of peak areas or heights

• Can be made better by using known standard

• quantification is mainly in the form of relative concentration ratio within single sample

• Detection limit: 0.1 - 1 w%. Can detect submonolayer (depends on sample and functional groups)

Good mainly for comparison and trends

(27)

A survey of surface Analysis

• Energy Dispersive Spectroscopy

• Concentration calculated from peak areas and sensitivity factor

• Must be calibrated with standards

• quantification is in the form of relative concentration ratio within single sample

• Detection limit: 0.1 - 1 mole%.

• Accuracy varies. Inherently prone to gross errors

• Chemical mapping ^quite ^accurate ^.

= .

not surface ^sensitive ^.

(28)

A survey of surface Analysis

• Rutherford Back Scattering

• Sample is hit by high energy He⁺⁺ particles

• Few backscatter due to collision with sample’s nuclei

• The energy losses is used for elemental analysis

• Concentration calculated from peak heights

• Principally utilized for depth profiling

• Detection limit and mass resolution varies with molecular weight: 0.001 - 1 mole%.

• Not inherently surface sensitive, but can have

(29)

"

destructive ^" nondestructive

depth profiling depth profiling

: XPS ^on AES

: RBS

Hett ^C ^a ⁾

w/ ^sputtering^. ^wi ^deafer

't: !

^"

¥4.44

^.

*if _V If

(30)

A survey of surface Analysis

• Total Reflection X-Ray Fluorescence

• Sample is irradiated by X-ray at critical angle

• Atoms in the first few monolayers fluoresce X-rays

• These are energy analyzed to obtain elemental analysis

• Good quantification for High Z elements with appropriate standard and sample preparation Detection limit 10⁹-10¹² atoms/cm²

⇒ thin film ^.

(31)

A survey of surface Analysis

• SIMS (Dynamic, Static, TOF)

• Quantification by peak height (in principle) and integration with time.

• Good quantification is very complicated and requires special calibration samples.

• Depends on type of SIMS sensitivity is extremely high: 10

⁷

– 10

¹³

at/cm

²

• Depth resolution: monolayers to several 10s A

(32)

techniques: Quantitative Analysis

• Glow Discharge Spectroscopy

• Sample eroded by sputtering (RF).

• Recombination of electrons with atoms in plasma emits photons characteristic to the element.

• Peak height proportional to concentration.

• Calibration is crucial and sometimes complicated.

• Used primarily for depth profiling of thin to thick samples with depth resolution in the nm scale.

⇒ surfajenuitire ^.

(33)

A survey of surface Analysis

techniques: Physical Properties

• Contact Angle

• A drop of liquid is placed on the sample.

• The angle between the sample and the drop is measured.

• Qualitative data on

hydrophobicity/hydrophility.

• Sample cleanliness, surface termination.

(34)

• Scanning Probe Microscopy: STM, AFM, magnetic force, etc.

• A sharp tip scans the surface with feedback based on electronic, magnetic, electrostatic and other

interactions

• Mapping the samples surface for hardness, magnetic domains, friction or other physical properties

• In some cases it reaches atomic resolution Can be used also to manipulate the surface

morphology ⁱⁿ

atomic= reef

:&

(35)

A survey of surface Analysis

• XPS

• In semiconductors and insulators sample’ s

surface charge due to the loss of the ejected electrons

• Extent of charging can be used to learn about

the sample's surface electronic conductivity i

_t

(36)

• Low Energy Electron Diffraction

• Low energy electrons hit the sample

• Reflected electrons show diffraction pattern in case that long-range order of atoms on the surface

• Information similar to XRD for bulk samples: surface crystalline order

• Very stringent sample preparation and vacuum requirements

Surface ^structure

(37)

A survey of surface Analysis

• Low Energy He Diffraction

• Very similar to LEED, but much more surface sensitive

• Especially applicable for organic and other sensitive samples

.

(38)

A survey of surface Analysis techniques: Microscopy

• (HR) Scanning Electron Microscopy

• Focused electron beam scans the sample’s surface

• Reflected electrons are registered

• The mapping forms a surface morphology picture

• Can be used in conjunction to EDAX, AES and other chemistry sensing techniques

• Usually simple to interpret

(39)

A survey of surface Analysis techniques: Microscopy

• STM, AFM, etc

• Surface morphology down to atomic scale

• Based on sensing mode (electronic, force, etc) can reflect different surfaces

• In STM the sample must be conductive

• Can be acquired in situ

• Sometimes interpretation is just seemingly

simple

(40)

A survey of surface Analysis techniques: Summary

• Before selecting your surface analysis

• What EXACTLY is the question you want to solve

• Qualitative/quantitative information

• Accuracy tolerance

• Detection limit

• Spectral, lateral and depth resolution

• Sample preparation and requirements

• Difficulty in interpretation

(41)

A survey of surface Analysis techniques: Summary

• Before selecting your surface analysis

• What EXACTLY is the question you want to solve

• Qualitative/quantitative information

• Accuracy tolerance

• Detection limit

• Spectral, lateral and depth resolution

• Sample preparation and requirements

• Difficulty in interpretation

• Analysis expenses

• Possible interferences