eew508 ii. structure of surfaces surface structure rice terrace

EEW508II. Structure of Surfaces

Surface structure

Rice terrace

http://3.bp.blogspot.com/_V8Z4bVIBBsE/SZ3YMETbAXI/AAAAAAAAA4M/gSoJLa2sXb8/s1600-h/74761773_9d49fda6af.jpg


Surface structure revealed by SEM and STM

Using STM (Scanning tunneling microscopy) or other techniques such as field ion microscopy (FIM) or LEED (low energy electron diffraction), atomic model of surface structure can be determined.

Surface Chemistryand Catalysis, second editionG. A. Somorjai and Y. Li (2010)

Terrace-step-kink model


Steps and kinks are line defects to distinguish them from atomic vacancies or adatoms, which are called point defects.

Relative concentration of atoms in terraces, in line defects, or in point defects can be altered, depending the methods of sample preparation.


Terrace – flat surface

Stepped surface

Kinked surface

Dislocations creat surface defects such as steps and kinks


Surface Chemistryand Catalysis, second editionG. A. Somorjai and Y. Li (2010)

On heterogeneous solid surface, atoms in terraces are surrounded by the largest number of nearest neighbors. Atoms in steps have fewer, and atoms in kinks have even fewer.

In a rough surface, 10-20% of atoms are often step sites, with about 5% of kink sites.

Limitation of Terrace-step-kink model


Terrace-step-kink model has the assumption of a rigid lattice where every surface atom is located in its bulk-like equilibrium position and can be located by the projection of the bulk structure to that surface.

The vertical position of surface atoms is shifted from the atomic positions in the bulk– exhibiting a significant contraction or ‘relaxation’ of the interlayer distance between the first and the second layer.

As the surface structure with less packing density, the contraction perpendicular to the surface becomes larger.

Not only the vertical direction, but the relocation of surface atoms along the surface takes place. Also, the adsorption of molecules or atoms lead to relocation of surface atoms to optimize the strength of the adsorption-substrate bond.

Determination of surface structure – Low energy electron diffraction (LEED)


LEED produce the quantitative data on bond distance and angles as well as on location of surface atoms and of adsorbed molecules.

Surface Diffraction – LEED, X-ray diffraction, and atom diffraction


The de Broglie wavelength of a particle is given by

mE

h

p

h

2

Where h is Planck’s constant, m is the mass of the particle, and E is the kinetic energy of the particle

For electron, and He atoms

)(

150)(

eVEAo

e )(

02.0)(

eVEAo

He

For X-ray

hchE )(

1024.1)(

4

eVEAo

photon

Surface Diffraction – LEED, X-ray diffraction, and atom diffraction


Electrons with energies in the range of 10-200 eV and helium atoms with thermal energy (~0.026 eV at 300K) has the atomic diffraction condition ( < 1A)

Glazing angle X-ray diffraction is used for surface and interface structure studies

X-ray bombardment induced emission of electron photoelectron diffraction

Principle of Low energy electron diffraction (LEED)


The single crystal surfaces are used in LEED studies. After chemical or ion-bombardment cleaning in UHV, the crystal is heated to permit the ordering of surface atoms by diffusion to their equilibrium positions.

The electron beam (in the range of 10-200 eV) is backscattered. The elastic electrons that retain their incident kinetic energy are separated from the inelastically scattered electron by applying the reverse potential to the retarding grids. These elastic electrons are accelerated to strike a fluorescent screen and LEED pattern can be obtained.

Types of LEED

Video LEED : LEED patterns can be visualized on a fluorescent screen.

Dynamic LEED or called I-V curve: the intensity I of the diffracted beam is measured as a function of the kinetic energy.

LEED pattern of a Si(100) reconstructed surface. The underlying lattice is a square lattice while the surface reconstruction has a 2x1 periodicity. The diffraction spots are generated by acceleration of elastically scattered electrons onto a hemispherical fluorescent screen. Also seen is the electron gun which generates the primary electron beam. It covers up parts of the screen.


http://en.wikipedia.org/wiki/File:Si100Reconstructed.png


Example – Si(111)- (7x7)

DAS structure: dimer, adatom, and stacking fault


Scanning Tunneling Microscopy – brief description


Example – Si(111)- (7x7)

Gerd Binnig and Heinrich Rohrer Nobel prize in Physics (1986)

If the surface unit-cell vector and that are different from and obtained from the bulk projection, then the surface unit vector can be related to the bulk unit vectors

'a

'b

a

b

bmamb

bmama

2221

1211

'

'

mij defines a matrix

2221

1211

mm

mmM

On unreconstructed surface

10

01M


Unreconstructed surface of the face-centered crystal structure


Unreconstructed surface of the body-centered crystal structure


Unreconstructed surface of the diamond crystal structure



For example, fcc (100) – (2x2)


20

02MFor example, fcc (111) – (2x2)


20

02MFor example, fcc (110) – (2x2)


Abbreviated and Matrix Notation for a variety of superlattices


Notation of High-Miller-Index Stepped Surface


Notation of High-Miller-Index Stepped Surface

stepped surface

kinked surface

6(111) x (100) 4(111) x (100)


Bond-Length Contraction or Relaxation

close-packed less close-packed

Chemical bonds and surface reconstruction



Strong chemical bonds

Ionic bonds: Na+ (cation) - Cl-(anion)These oppositely charged cations and anions are attracted to one another because of their opposite charges. That attraction is called an ionic bond.



Covalent bonds: H –Fboth atoms are trying to attract electrons that are shared tightly between the atoms. The force of attraction that each atom exerts on the shared electrons is what holds the two atoms together.



Metallic bonds : Metal consists of metal ions floating in a sea of electrons. The mutual attraction between all these positive and negative charges bonds them all together.

the sharing of "free" electrons among a lattice of positively-charged ions (cations),


Dangling bonds


Reconstruction – (2x1) Reconstruction of Si(100)

The (2x1) reconstruction of Si (100) crystal structure as obtained by LEED crystallography. Note that the surface relaxation extends to three atomic layer into the bulk


(7x7) Reconstruction of Si (111)

LEED and STM image of (7x7) reconstructed structure of Si (111)The total number of dangling bonds is reduced from 49 to 19 through this reconstruction.

DAS structure: dimer, adatom, and stacking fault


(7x7) Reconstruction of Si (111)

19 dangling bonds of (7x7) reconstructed surface (12 adatom, 6 rest atom, 1 corner hole)


Reconstruction on metallic surface – Ir(100)

Bulk structure:the square latticeSurface structure: hexagonally close packed layer

(5x1) reconstruction


Reconstruction on metallic surface –Ir (110) missing dimer row

(2x1) reconstruction structure


Reconstruction – Ionic crystal

Ionic crystal consists of charged spheres stacked in a lattice.

Surfaces with strong chemical bonds exhibits more drastic rearrangement of surface atoms

Generally speaking, surfaces with weak chemical bonds (van der Waals, hydrogen, dipole-dipole and ion-dipole) exhibits less pronounced reconstructed structure -- for example, Graphite (0001) surface



Reconstruction of high-Miller-index surfaces

Roughening transition: If the surface is heated near the melting temperature, the steps become curved and break up into small islands

Reconstruction at Cu(410) stepped surface. Atoms in the first row at the each step become adatoms which are pointed out in the side view of the reconstructed surface.

III. Molecular and Atomic Process on Surfaces

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Structure of ordered monolayer

When atoms or molecules adsorb on ordered crystal surface, they usually form ordered surface structure over a wide range of temperature and surface coverages.

Two factors which decide the surface ordering of adsorbates areAdsorbate-adsorbate(AA) interaction and adsorbate-substrate(AS) interaction

Chemisorption – adsorbate-substrate interaction is stronger than adsorbate-adsorbate interaction, so the adsorbate locations are determined by the optimum adsorbate-substrate bonding, while adsorbate-adsorbate interaction decides the long-range ordering of the overlayer.

Physisorption or physical adsorption – AA interaction dominates the AS interaction –the surface could exhibit incommensurate structures.


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Coverage of adsorbate molecules

Definition of coverage: one monolayer corresponds to one adsorbate atom or molecules for each unit cell of the clean, unreconstructed substrate surface.For example, the surface coverage of atom on fcc(100) is one-half a monolayer.


Atomic oxygen on Ni (100)Up to one quarter of the coverage: Ni(100)-(2x2)-OBetween one quarter and one half Ni(100)-c(2x2)-O

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Ordering of adsorbate molecules


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Epitaxial Growth

With metallic adsorbates, very close packed overlayers can form because of attractive force among adsorbed metal atoms.When the atomic sizes of the overlayer and substrate metals are nearly the same, we can observe a one-monolayer (1x1) surface. This is called epitaxial growth.


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Adsorbate induced restructuring – Ni (100) – c(2x2) - C

Carbon chemisorption induced restructuring of the Ni (100) surface. Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate.


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Adsorbate induced restructuring – Fe (110) – (2x2)-S

S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface.


Hydrogen: 1.7 atm.73 nm × 70 nm

Oxygen: 1 atm.90 nm × 78 nm

Carbon Monoxide: 1 atm.77 nm × 74 nm

“nested” missing-row reconstructions

fcc (111) microfacets

Unreconstructed (111) terraces separated by multiple height steps

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Adsorbate induced restructuring of steps to multiple-height step – terrace configuration


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Sulfur-chemisorption-induced restructuring of the Ir (110) surface

Open and rough surfaces reconstruct more readily upon chemisorption. For example, fcc(111) surface restructure more frequently upon chemisorption than do the closer-packed crystal faces.


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Penetration of atoms through or below the first layer


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Surface structure of alloy, AlCu

Cu84Al16 alloy (111) structure exhibiting 3 x 3 R30o

The surface composition is 50%


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Growth modes of metal surfaces

Auger signal of adsorbate

Aug

er s

igna

l of

subs

t rat

e


eew508 ii. structure of surfaces surface structure rice terrace

Documents

relocation of surface

ordering of surface

rough surface

bulk structure

structure of surfacesleed

structure of surfacessteps

structure of surfaceselectrons

heterogeneous solid