Growth of Antimony on Copper. A Scanning Tunneling Microscopy Study

Ndlovu, Gebhu

Growth of Antimony on Copper. A Scanning Tunneling Microscopy Study

Summary

This study investigates the Copper(111) – Antimony (Sb) system which is characterized by a complex interplay between adsorbate interactions and adsorbate substrate interactions which manifest through self-assembly processes. Surface sensitive techniques such as Low Energy Electron Diffraction and Auger Electron Spectroscopy were utilized to determine the substrate cleanliness prior to the growth of monolayer Sb coverage. The surface chemical reactivity on an atom-by-atom basis of the Cu sample surface was studied by current imaging tunneling spectroscopy.
The use of surface sensitive techniques in studying the surface alloy in question allows for more precise statements to be made about the surface structure of the system at various temperatures. Based on the experimental results, a comprehensive study of the adsorption and segregation behavior of Sb on Cu(111), including the mechanisms for phase formation at the atomic scale, is presented in this study.

Excerpt

finally rearrangement to more energetically stable configuration. The experimental results

illustrated the presence of a surface alloy after annealing at ~360°C. The Cu Cu spacing

increased from 0.257 ± 0.01 nm (atomically clean Cu(111)) to 0.587 ± 0.02 nm after

annealing at 360°C. At that temperature, the STM images showed the surface protrusions of

different sizes and contrast, attributed to Cu and Sb atoms.

In addition to the conventional

)

(

R30°Sb structural phase acquired at

~400°C, new metastable structural phases:

)

R30°Sb stable up to 250°C and

)

R30°Sb stable up to 350°C were obtained for the first time after annealing at

600°C and 700°C, respectively. The

)

(

R30°Sb phase at 400°C showed an increase

in atomic spacing between Cu atoms (0.626 ± 0.001 nm) as compared to the surface alloy at

300°C (Cu-Cu spacing 0.460 ± 0.002 nm) and the asymmetry at the sample surface was

clearly evident. High resolution LEED studies revealed a rich phase showing extra spots on

the commensurate

)

(

R30°Sb phase compared to the clean Cu(111) substrate. STM

data after annealing at 600°C and 700°C was best described by a structural model involving

an ordered p(2×2) and p(2×1) overlayer structures superimposed onto the

)

(

R30°Sb

surface, respectively. At elevated temperatures LEED showed ring shaped diffraction

patterns composed of twelve equidistant spots which are consistent with the growth of a

hexagonal film forming three equivalent rotational domains. All the superstructures were

found to favour a structural model based on Sb atoms occupying substitutional rather than

overlayer sites within the top Cu(111) layer.

Other than the dissolution of Sb onto Cu(111), the study report also on the

segregation of Sb on Cu together with STS measurements. The surface chemical reactivity

on an atombyatom basis of the Cu sample surface was studied by current imaging

tunneling spectroscopy (CITS). The local density of states (LDOS) were derived from dI/dV

maps at low tunneling voltages by a simultaneous measurement of high resolution

topographic micrographs. The onset of the atomically clean Cu(111) surface was determined

to be ~8 meV below E

which reflects the onset of tunneling from the surface state band as

determined by scanning tunneling spectroscopy. The use of surface sensitive techniques

(LEED, AES, STM, STS) in studying the surface alloy in question has enabled more precise

statements to be made about the surface structure of the system at various temperatures.

Based on the experimental results, a comprehensive study of the adsorption and segregation

behavior of Sb on Cu(111), including the mechanisms for phase formation at the atomic

scale is presented in this study.

KEY WORDS

Selfassembly, Antimony, Copper, Organized growth, Scanning tunneling microscopy and

spectroscopy, Adsorbates, Monolayers, Diffusion, Superstructure, Surface alloys, Surface

tension, Root three by root three, Surface sensitive, Nanostructures, Annealing, Surfactant,

Silicone, Highly oriented pyrolytic graphite, Segregation, Activation energy, Tip

fabrication, Surface reconstruction, Structural phases, Density of states.

TABLE OF CONTENTS

CHAPTER 1

INTRODUCTION ... 1

1.1

BACKGROUND ... 1

1.2

OBJECTIVES OF PRESENT WORK ... 8

1.3

THESIS OUTLINE ... 9

1.4

REFERENCES ... 11

CHAPTER 2

THEORY ... 19

2.1

THE FCC(111) SURFACE STRUCTURE ... 20

2.2

RELAXATION AND RECONSTRUCTION OF SURFACES ... 21

2.2.1

RELAXATION ... 22

2.2.2

RECONSTRUCTION OF CLEAN SURFACES ... 22

2.2.3

ADSORBATEINDUCED RECONSTRUCTION ... 23

2.3

ADSORPTION ... 24

2.4

KINETICS OF ADSORPTION ... 25

2.5

THE 3 STRUCTURES ON (111) FCC METAL SURFACES ... 26

2.6

DYNAMICS OF ADATOMS DIFFUSION ... 27

2.7

INTERACTION AT THE SURFACE ... 29

2.7.1

INFLUENCE OF ADSORBATES ON SURFACES ... 29

2.8

TYPES OF GROWTH ... 31

2.8.1

FRANKVAN DER MERWE (FM) OR LAYERBY LAYER ... 32

2.8.2

VOLMERWEBER (VW) OR 3D ISLAND ... 33

2.8.3

STRANSKIKRASTANOW (SK) OR 3D ISLANDON WETTINGLAYER

GROWTH ... 33

2.9

INTRODUCTION TO SEGREGATION ... 33

2.10 THE MODIFIED DARKEN MODEL ... 34

2.11 REFERENCES ... 36

vii

CHAPTER 3

SCANNING TUNNELING MICROSCOPY (STM) ... 40

3.1

INTRODUCTION ... 40

3.2

ELECTRON TUNNELING ... 40

3.3

STM PRINCIPLE OF OPERATION ... 42

3.4

TUNNELING CURRENT ... 44

3.5

BARDEEN THEORY OF TUNNELING ... 46

3.6

MODES OF OPERATION ... 48

3.6.1

CONSTANT CURRENT IMAGING MODE ... 48

3.6.2

CONSTANT HEIGHT IMAGING MODE ... 50

3.7

SCANNING TUNNELING SPECTROSCOPY (STS) ... 51

3.7.1

CURRENT IMAGING TUNNELING SPECTROSCOPY (CITS) ... 52

3.7.2

CONSTANT CURRENT SPECTROSCOPY ... 53

3.7.3

CONSTANT VOLTAGE SPECTROSCOPY ... 53

3.8

STM MAJOR COMPONENTS ... 54

3.8.1

VIBRATION ISOLATION ... 54

3.8.2

ELECTRONIC FEEDBACK CONTROL SYSTEM ... 55

3.8.3

THE STM TIP ... 55

3.8.4

IMAGES AND FILTERING ... 56

3.9

ATOMIC RESOLUTION ON WELL KNOWN SUBSTRATES ... 57

3.10 HIGHLY ORIENTED PYROLYTIC GRAPHITE (HOPG) ... 58

3.10.1

EXPERIMENTAL ... 59

3.10.2

RESULTS AND DISCUSSION ... 60

3.11 SI(111) 7×7 RECONSTRUCTION ... 64

3.11.1

EXPERIMENTAL ... 65

3.11.2

RESULTS AND DISCUSSION ... 66

3.12 REFERENCES ... 69

CHAPTER 4

EXPERIMENTAL FACILITIES AND PROCEDURES ... 73

4.1

TIP FABRICATION ... 73

4.2

CHARACTERIZATION TECHNIQUES ... 76

4.2.1

THE UHV VARIABLE TEMPERATURE STM SYSTEM ... 76

viii

4.2.2

LOWENERGY ELECTRON DIFFRACTION (LEED) ... 79

4.2.3

AUGER ELECTRON SPECTROSCOPY (AES) ... 81

4.3

REFERENCES ... 84

CHAPTER 5

DISSOLUTION OF SB ON CU(111) ... 85

5.1

INTRODUCTION ... 85

5.2

EXPERIMENTAL PROCEDURE ... 87

5.3

RESULTS AND DISCUSSION ... 89

5.3.1

ATOMICALLY CLEAN CU(111) ... 89

5.4

GROWTH OF SB ON CLEAN CU(111) SURFACES ... 91

5.5

STAGES OF DISSOLUTION ... 95

5.5.1

ANNEALING AT 360°C ... 95

5.5.2

ANNEALING AT 400°C ... 97

5.5.3

ANNEALING AT 600°C ... 103

5.5.4

ANNEALING AT 700°C ... 109

5.6

SEGREGATION OF SB ON CU(111) ... 112

5.7

SUMMARY ... 114

5.8

REFERENCES ... 116

CHAPTER 6

SCANNING TUNNELING SPECTROSCOPY (STS) ... 118

6.1

INTRODUCTION ... 118

6.2

STS OF CU(111) AT ROOM TEMPERATURE ... 120

6.3

SUMMARY ... 126

6.4

REFERENCES ... 127

CHAPTER 7

SUMMARY AND CONCLUSION ... 128

FUTURE PROSPECTS ... 134

PUBLICATIONS ... 135

INTERNATIONAL MEETINGS AND CONFERENCES ... 136

LOCAL MEETINGS AND CONFERENCES ... 136

AWARDS ... 138

ACCRONYMS

Antimony

Silver

Copper

2DEG

Twodimensional electron gas

AES

Auger electron spectroscopy

HOPG

Highly oriented pyrolytic graphite

LEED

Low energy electron diffraction

LDOS

Local density of states

STM

Scanning tunneling microscopy

STS

Scanning tunnelling spectroscopy

UHV

Ultrahigh vacuum

VTSTM

Variable temperature scanning tunneling microscopy

Fermi energy

FCC

Face centered cubic

HCP

Hexagonal closed packed

Three dimensional

Room temperature

Brillouin zone

FHUC

Faulted half unit cell

UHUC

Unfaulted half unit cell

CHAPTER 1

INTRODUCTION

1.1

Background

Materials and their properties have fascinated the human race since the past centuries

when fabrication was regarded more as an art than a science [13]. Today's way of life

depends heavily on the ability of a few million transistors to process data and a few million

more to store it, whether temporarily or permanently [410]. Until now, investigative

research is still carried out in the field of materials and surface science which is primarily

intended to expand the range of knowledge and properties of materials of various types for

both technological and fundamental applications [412]. Given that most material products

come in the form of alloys, some of the ongoing questions in the field of materials science

includes, whether different elemental materials are immiscible when combined under

controlled conditions, whether or not the materials dissolve in one another, and, if they do,

to what extent. Alternatively will they react to form a compound? If so, in what atomic

ratios (or stoichiometry)? And how does processing conditions influence the product of the

combined materials?

The field of modern surface science is driven by the ever increasingly specialized

intricate set of characterization tools that are drawn from a wide range of scientific

disciplines utilized to give answers to some, if not all of the abovementioned questions

with a high degree of accuracy [1117]. Due to the demand for faster processing speeds and

huge storage capacity of electronic devices, the study of materials has ushered in the field of

nanoscience and nanotechnology where materials are investigated in dimensions less than

100 nm [4,5,1820]. In particular, the control of the shape and size in nanostructural growth

has been a very hot research field topic [1921], since the properties and applications of

nanostructures mainly depend on their shape, size and surface morphology [9, 2226].

It can be argued with considerable justification that the field of nanotechnology is

driven by the possibility of fabricating tailordesigned nanostructures with unique

properties. Two different approaches are generally used in the fabrication of nanostructures,

namely, topdown and bottomup. The topdown method mainly includes photolithography

and etching techniques which permit the creation of nanostructures over large sample areas

[4,5,18]. Other examples of topdown include ball milling and arc discharge. The

photolithography process has some disadvantages, such as the sizes of nanostructures are

limited by wavelength of the photons used in the photolithography and mask sizes [4,5,20].

On the other hand, in bottomup approach the building block materials for fabricating self

assembled nanostructures are atoms, molecules or clusters [47,18,20]. Bottomup

examples include chemical vapour deposition (CVD) [27,28], molecular beam epitaxy

(MBE) [29-31], sputtering, liquid to solid nucleation, pulsed laser deposition (PLD) [32

34], solgel techniques [3537], to mention but a few. The selfassembled nanostructures

can be formed in a growth environment taking advantages of some energetic, geometric and

kinetic effects of overlayer materials and substrates where molecules diffuse from atomic

to the mesosopic scale [18,20,38]. In this study, the latter approach is utilized to grow thin

film layers by controlling the flux of the deposited material, the kinetics and

thermodynamics of the underlying substrate.

Crystalline surfaces of noble metals (Cu, Ag, Au) have broken translational bulk

symmetry [39]. The lack of translational symmetry along the surface normal results in

electronic states localized perpendicular to the surface, the socalled surface states [4042].

These surface states are localized to the first few atomic layers at the surface and form a

quasi twodimensional free electron gas (2DEG) which is confined to the first few atomic

layers at the crystal surface [6,43,44]. Therefore, the electrons strongly influence the

interaction between the surface and its environment and thereby contribute to the chemistry

of surfaces, such as adsorption processes, equilibrium surface structures, or catalytic

reactions [10,54,46]. Furthermore, surface states are responsible for longrange substrate

mediated adsorbate interactions, which dominate the bulkstate mediated contribution for

large adsorbateadsorbate separation [47,48]. In addition, the contribution from surface

states is relevant for the total energy balance of surface reconstructions [6,40].

Thin films are of special interest especially when they have a thickness of only a few

monolayers. Properties of such films may drastically differ from their bulk due to the

surface restriction to two dimensions and due to the interaction with the substrate [4951].

The surfaces of bulk alloys are of practical interest for their chemical properties be it novel

activity or selectivity to certain reactions [5254] in a way which differs from the

constituent elements in isolation or novel passiveness to corrosion. Some of the basic

thermodynamics of segregation in alloys is far from new. Nevertheless, our understanding

of these chemical and physical phenomena is far from complete, and the application of

surface science methods to investigate these phenomena is a manifestation of a general trend

to the study of surfaces of increasing complexity.

Copper (Cu) has proved to be a favoured substrate for studies of ultrathin metallic

film growth for many reasons, including the relative ease of cleaning and maintaining

surface cleanliness, the high level of crystalline quality and the advantages of a full dband

electronic configuration allowing high resolution studies of the surface and bulk electronic

structure [55]. Some surface adsorbate species such as antimony (Sb) and indium (In) are

known to act as surfactants in both homo and hetero epitaxy because of their low surface

energy [56,57]. These adsorbates appear to induce layerbylayer growth [58] in systems

which otherwise tend to island growth, whilst remaining at the surface as the growth

proceeds rather than being incorporated into the growing film [59,60]. Despite increased

interest in the applications of surfactants on metal growth, the microscopic mechanisms of

dissolution and segregation of Sb is not yet fully understood [61]. Established examples of

this phenomenon is the role of Sb as a surfactant on the growth of Co on Cu(111) [62] and

the growth of Ag on Ag(111) [63,64].

The phenomenon of surface segregation is defined as the preferential enrichment of

one component of a multicomponent system at a boundary or interface [6568]. The extent

of segregation is influenced by strain energy due to the atomic size mismatch between the

solute and the solvent, as well as the differences in their surface energies [52,69,70]. During

surface segregation the surface enthalpy is different from the bulk enthalpy and occurs at

finite temperatures (or in the materials growth process) when energy activation barriers to

diffusion are overcome.

When monolayers of Sb are grown on Cu(111) or Ag(111) there are generally two

proposed atomic structural models [71,72]. On the first model, Sb atoms are located above the

(111) surface in face centered stacking sites as adatoms, while the second structure, involves Sb

atoms sitting substitutionally within the (111) surface layer of the substrate [7174]. Previous

experimental [7173] and theoretical [74,75] studies have suggested that the energetics of this

systems are such that in the ordered 0.33 ML Ag(111)

)

(

R30°Sb or Cu(111)

)

(

R30°Sb phase, the Sb atoms substitute onethird of the outermost substrate (Ag or Cu) atoms to

produce an ordered surface alloy which shows outward relaxation of the Sb atoms. Thus these

previously reported studies rules out the first proposition of Sb sitting as adatoms on the substrate.

The formation of the

)

(

R30° superstructure, is well understood in the case of Sb

Ag system since Ag and Sb form bulk intermetallic compounds and their atomic radii mismatch is

small (~1%), however, it is rather difficult to understand how such a phase can form in the SbCu

system because of the large atomic mismatch (~ 15%) between Cu and Sb [76,77]. Therefore, the

observation of almost identical twodimensional surface alloys in both cases reveals that the main

driving forces for formation is the tendency to chemical ordering, almost independently of size

mismatch between deposit and substrate atoms. The Cu Sb system has been widely studied for

(100) and (111) Cu surface orientations [76,77], considering both segregation and dissolution of

deposited Sb layers. Previous in situ STM, Auger electron spectroscopy (AES), and low energy

electron diffraction (LEED) studies of the dissolution and segregation of ~ 1 ML of Sb on Cu(111)

has revealed that at ~ 400°C, the dissolution stops, due to formation of a surface alloy which

exhibits a p

)

(

R30° superstructure which is fully consistent with one Sb atom per unit cell

in the p

)

(

R30° structure [78]. The dissolution and segregation kinetics are thus closely

linked to the equilibrium surface segregation.

At higher Sb coverages, a p(2×2) reconstruction has been observed on Ag(111), and

explained as an ordered p(2×2)Sb overlayer superimposed on the

)

(

R30°Sb surface

[72,73]. A similar structure at higher Sb coverages has not been reported on the CuSb system.

Theoretical calculations have shown that the creation of

)

(

R30°Sb surface alloy is

energetically favoured [75]. The mechanism underlying the dissolution and segregation of the

large Sb (atomic radius = 0.159 nm) atoms deposited on the (111) closepacked Cu (atomic radius

= 0.128 nm) surface is still an open topic for investigation.

From an experimental point of view, the first available methods capable of investigating

surfaces at the atomic level were diffraction methods in the late 20s [17,79] followed by scanning

tunneling microscopes in the late 70s [8082]. Quantifying and understanding the structure of

surfaces, and particularly of adsorbates on surfaces, is a key step to understanding many aspects of

the behavior of surfaces including the electronic structure and the associated chemical properties.

Both lowenergy electron diffraction (LEED) and reflection highenergy electron diffraction

(RHEED) has for example brought many successes in understanding atomic surface

reconstruction [8385]. One of the major disadvantages of these methods is that the surface is

represented in the reciprocal space, making interpretation of measured physical data more

intricate.

The first real space experimental technique capable of imaging atoms at a surface is the

field ion microscope (FIM) [8688]. While this method has been widely and successfully used to

study the diffusion of adatoms and clusters at a surface, it is not suited to investigate large areas of

nanostructurecovered singular surfaces.

The cornerstone of modern growth investigation was laid by the invention of the scanning

tunneling microscope (STM) in 1982 by Binnig and Rohrer (Nobel laureates in 1986) [80,81]

which will be the main experimental tool for this study. The invention of the STM solved one of

the most intriguing problems which fascinated surface scientist for quite some time, the Si(111)

(7×7) reconstruction [81,89]. Other than providing images of surfaces and adsorbate atoms and

molecules with unprecedented resolution, the STM has also been used previously to modify

surfaces, for example by locally pinning molecules to a surface [90] and by transfer of an atom

from the STM tip to the surface [91]. Recent developments in nanoscience make it possible to

engineer artificial structures at surfaces [9299]. The STM tip can be used as an engineering or

analytical tool, to fabricate artificial atomicscale structures where novel quantum phenomena can

be probed and properties of single atoms and molecules can be studied at an atomic level

[82,100,101]. These structures include quantum corrals [100,101] and atomic chains [102].

The STM can also be operated in the spectroscopy mode (STS) to study the local

electronic structure of a sample's surface. This is usually done by sweeping the bias voltage V and

measuring the tunneling current I while maintaining constant tipsample separation z which

results in current vs. voltage (IV) curves characteristic of the electronic structure at a specific x,y

location on the sample surface [103-105]. By numerically differentiating IV, the conductance

dI/dV can be obtained. The interpretation of dI/dV spectra can be complex but it can be shown

that, under ideal conditions, dI/dV is a good measure of the sample density of states (DOS) [103-

105]. This method makes it possible to get energy spectra of very small objects at the surface. This

can be used in the microelectronics industry, for example, to control electrical properties of

transistors, especially those of a very small size.

1.2

Objectives of present work

The main aim of this project is to utilize an UltraHigh Vacuum Variable Temperature

Scanning Tunneling Microscopy (UHV VTSTM) to study the dissolution and segregation

of fractional monolayer of antimony grown on copper (111) surfaces insitu.

The main objectives are:

(1)

Optimize the VTSTM system in order to obtain morphology of Cu(111), HOPG

and Si(111) with atomic scale resolution in order to calibrate the STM especially

using the well-known Si surfaces

(2)

Systematic study the nature of surface reconstruction of conducting and

semiconducting material in UHV

(3)

Investigate the atomic interaction of the CuSb system and its various structural

phases as a function of annealing temperature

(4)

Obtain local Auger spectral data at various temperatures on the segregation of Sb

to the Cu(111) surface in order to calculate diffusivity, segregation energy and

activation energy in such a system at the nanoscale and compare their bulk

values available in literature

(5)

Perform currentvoltage measurements to obtain valuable information regarding

the electronic structure of the CuSb system

1.3

Thesis outline

The thesis is structured as follows:

Chapter 2, introduces the theoretical background on surfaces and various processes

such as reconstructions at surfaces, the kinetics of thin film growth and types of growth. The

chapter also introduces the basic concepts of segregation and the modified Darken model

which describes segregation phenomenon for binary systems.

Chapter 3 focuses on the main experimental method of the thesis, the scanning

tunneling microscopy (STM). The chapter contains the necessary background information to

understand the experimental work presented later in the thesis. These include the theory of

electron tunneling and the operation principles of the STM system. The theoretical

background on scanning tunneling spectroscopy which forms part of the STM is also

explained in detail in this chapter. The chapter also explores the unprecedented resolution of

the STM by studying the surfaces of well-known substrates such as Si(111) and HOPG in

detail.

Chapter 4 comprises the experimental techniques and procedures. These include the

theory of surface sensitive techniques (AES, LEED). The UHV VTSTM system and its

components together with tip fabrication processes are also discussed.

Chapter 5 looks at the adsorption and dissolution studies of submonolayer Sb grown

on Cu(111) surface to get better insight into the atomic structure/s of the surface alloy and

the influence of temperature at the sample surface. In addition, the chapter reports on the

experimental segregation studies of Sb on the copper sample.

Chapter 6 looks at the added advantage of being able to acquire electronic

information simultaneously with topographical data utilizing STS which allow direct

comparisons of the topography and the electrical characteristics of the sample surface.

Chapter 7 focuses on discussions and conclusions. Key points regarding experimental

results from dissolution, segregation and the conductance measurements are summarized to

give a better insight with a complete conclusion of the study.

1.4

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CHAPTER 2

THEORY

The inception of modern surface science dates back to the early 1960s. The

breakthrough in the field resulted from a combination of factors, including progress in ultra

high vacuum technology, the development of experimental methods sensitive to the surface

atomic structure and the appearance of highspeed digital computers.

A surface is created by cleaving a solid and breaking of bonds between the atoms of

the cleavage planes [15]. The work done in forming a unit area of a new surface is called

the surface energy and is denoted by . The surface free energy of noble metals (Cu, Ag,

Au) is more than three times higher than that of van der Waals substrates like graphite. It is

this surface free energy which drives adsorption and catalysis and explains why metals are

very active materials in such processes [4].

It is conventional to use the zaxis for the surface normal, leaving x and y for

directions in the surface. The interest in surface properties of a material usually

encompasses the bulk properties of the material. Therefore, understanding of the surface

properties requires a good knowledge of bulk properties. Almost all surface science studies

are concerned with addition of controlled amount of foreign atoms (adsorbates) on

atomically clean surfaces. There are various ways in which adsorbates or molecules can be

introduced onto the host material surface (substrate) [6].

The adatoms can condense onto the surface from the vapour phase through a process

called adsorption or alternatively segregate from the sample bulk, or diffuse along the

surface. Knowledge of the structure of surfaces and particularly of adsorbates on surfaces is

a key step to understanding many aspects of the behavior of surfaces including the

electronic structure and the associated chemical properties.

2.1 The FCC(111) surface structure

Surface structures of metals, being the most studied of all surface structures illustrate

the degree of reproducibility and accuracy achievable by modern surface science techniques.

Many metal elements crystallize in the facecentered cubic (FCC) structure. Among them

are the noble metals copper (Cu), silver (Ag), and gold (Au) [7]. In the atomic

configuration, noble metals are characterized by a completely filled dlevel subshell and

one single stype valence electron. In the solid state, these electrons are strongly delocalized

to form an spderived band. Close to the Fermilevel, the electronic properties are found to

be in very good agreement with that of a freeelectron model [8]. Even though the surface

region is in principle a threedimensional (3D) entity having a certain thickness, all

symmetry properties of the surface are 2D meaning the surface structure is periodic only

in two directions. Atoms in the bulk of FCC (111) materials have coordination number 12

while those at the surface have 9 nearest neighbours. This means that the meansquare

amplitude of the surface atoms is much larger than in the bulk. The surface energies per

atom increase with decreasing coordination of the surface atoms. For low index surfaces of

FCC metals (figure 2.1) the stability decreases in the trend (111) (100) (110).

trivial

hexag

densit

surfac

2.2

surfac

the b

proce

energ

severa

which

The surfac

l mirror pl

gonal close

ty is the hig

ces.

Figure 2.1

adsorption

Relaxatio

The subseq

ce) atoms to

bulk crystal

esses are the

gy by increa

al ways, as

h prevent

FCC hollow

(a)

ce layer of

lanes. The

packed (H

ghest for th

1. The (a) (1

n sites for ada

on and re

quent proce

o form a str

are called

e energetics

asing the c

shown in f

or hinder

HCP

the {111}

surface latt

HCP) site an

he {111} sur

111), (b) (10

atoms on the

econstruct

esses which

ructure with

relaxation

s of the sys

coordination

figure 2.2).

these rearr

Bridge

surface has

tice has tw

nd the face

rfaces, follo

00), and (c)

(111) FCC s

tion of sur

h involves t

h a different

and recons

stem [4,5] (

n number o

As with al

rangements

On top

(b)

s a sixfold

wo different

e centred cu

owed by the

(110) phases

surface phase

rfaces

the rearrang

periodicity

struction. T

(i.e. the des

of surface a

ll processes

at low te

d rotation ax

t three fold

ubic (FCC)

e {100} and

s of a perfec

e [9].

gements of

y and/or sym

The driving

sire to reduc

atoms and t

, there are

emperatures

(c)

xis and thre

d hollow si

site. The p

d finally the

ct FCC cryst

f surface (an

mmetry than

force behi

ce the surfa

this is achi

kinetic limi

. Relaxatio

ee non

tes, the

packing

e {110}

tals and the

nd near

n that of

ind this

ace free

eved in

itations,

ons and

reconstructions often occur for atomically clean surfaces in vacuum, in which the interaction

with any another medium is not at play.

2.2.1 Relaxation

Atoms at the surface of a crystal have fewer neighbours than atom in the bulk. The

reduction of nearest neighbour gives rise to redistribution in the selvedge. This changed

force field results in the decrease in the layer spacings d perpendicular to the surface (figure

2.2 (a)) with no change either in the periodicity parallel to the surface or to the symmetry of

the surface unit cell [5].

2.2.2 Reconstruction of clean surfaces

Reconstruction of surfaces is a much more readily observable effect, involving larger

(yet still atomic scale) displacements of the surface atoms. Unlike relaxation, the

phenomenon of reconstruction (figure 2.2 (b)) involves a change in the periodicity of the

surface structure. Surface reconstruction can affect one or more layers at the surface, and

can either conserve the total number of atoms in a layer (a conservative reconstruction) or

have a greater or lesser number than in the bulk (a nonconservative reconstruction). The

reconstructive surface phase transition can either occur spontaneously or be activated by

temperature or by addition of adsorbates [10].

2.2.3

chang

obser

the cl

surfac

type o

The t

adsor

substr

Figure 2.

(a) relaxe

reconstru

3 Adsorb

At the low

ges in atom

rved by diffr

lean state w

ces of trans

of reconstru

thermodynam

rbatesubstr

rate atoms i

.2. Schemat

ed surface (d

uction.

ateinduc

west level o

mic layer sp

raction tech

while other s

ition metals

uction (figu

mic driving

rate bonds

n the clean

tic side view

bulk

ced recon

of surface m

pacings tha

hniques. Cer

urface recon

s Ag, Pt and

ure 2.2 (c))

g force for t

that are co

substrate su

w of characte

(b) Surface

struction

modificatio

at occur clo

rtain surface

nstruct in th

d Au transf

when oxyg

this kind of

omparable t

urface.

eristic rearr

reconstruct

n, adsorbat

ose to the

es (bare (110

he presence

forms to (2

gen is adsorb

f reconstruct

to or stron

angements o

tion (c) mis

tes often in

surface of

0) Cu, Pd an

of adsorbed

1) pattern o

rbed onto th

tion is the f

nger than th

of surface a

sing row typ

nfluence the

f a clean su

nd Ni) are s

d atoms. Th

of the missi

he surface [

formation of

he bonds b

atoms

pe of

e subtle

ubstrate

stable in

he (110)

ing row

1113].

f strong

between

2.3 Adsorption

The process of adsorption occurs when a gas or liquid solute accumulates on the

surface of a solid forming a molecular or atomic film [14,15]. A qualitative distinction can

be made between chemisorption and physisorption in terms of their relative binding

strengths and mechanisms [11,16]. In the case of physisorption, the adsorbatesubstrate

interaction is weak due to van der Waals forces and the binding energies are within the

ranges of 10 100 meV. The overlap of the wave functions of the molecule and the

substrate is rather small, thus, the perturbations of the structural environment near the

adsorption site is negligible [5,17]. When the adsorbatesubstrate interactions forms strong

chemical bonds that are either covalent or ionic with binding energies of about 1 10 eV,

the process is often denoted chemisorption [11]. In the later process, the strong bonds alters

the adsorbate chemical state and alternatively changes the structure of the substrate either by

relaxation or reconstruction of the few top atomic layers of the substrate. This process is the

result of the long range order observed on most single crystal surfaces with adsorbates

which possess twodimensional phase characterised by its own electronic, chemical, and

mechanical properties [17,18].

Details

Pages

Type of Edition

Erstauflage

Year

2017

ISBN (PDF)

9783960676621

ISBN (Softcover)

9783960671626

File size

12.7 MB

Language

English

Publication date

2017 (June)

Grade

PhD

Keywords

Antimony Copper Organized growth Scanning tunneling microscopy and spectroscopy Adsorbates Monolayers Diffusion Superstructure Surface alloys Surface tension Root three by root three Surface sensitive Nanostructures Annealing Surfactant Silicone Highly oriented pyrolytic graphite Segregation Activation energy Tip fabrication Surface reconstruction Structural phases Density of states Self-assembly

Author

Dr. Gebhu Ndlovu (Author)

Growth of Antimony on Copper. A Scanning Tunneling Microscopy Study

Summary

Excerpt

Table Of Contents

Details

Author

Dr. Gebhu Ndlovu (Author)