11

Qiang Li

Department of Chemistry

Surface Chemistry of Hexacyclic Surface Chemistry of Hexacyclic Aromatic Hydrocarbons on 2Aromatic Hydrocarbons on 21 1 and Modified Surfaces of Si(100)and Modified Surfaces of Si(100)

22

Introduction• Purpose• Si(100) surface• Aromatic hydrocarbons• Experimental

Present work• Organic functionalization of Si with aromatic hydrocarbons

• Kinetics of surface processes in hydrocarbon/Si(100) systems

Summary & Outlook

OutlineOutline

33

Introduction – Organic SemiconductorsIntroduction – Organic Semiconductors

• The first organic FETTsumura et al., Appl. Phys. (1986)

S( )

X

• Organic switches

Lopinski et al., Nature (2000)

• Self-directed growth of molecular lines

• Growth of a polymer film with molecular layer deposition

T. Bitzer et al. Appl. Phys. Lett. (1997)

Joachim et al., Nature (2000)

44

• Diamond structure of Si

Introduction – Si(100) substrateIntroduction – Si(100) substrate

• Si(100)2x1 surface

Reconstruction

1x1 2x1

• Silicon is the predominant material in the semiconductor industry

• The Si-Si dimer on Si(100)2x1 mimics a double-bond organic reagent

• Ideal platform for building hybrid devices by seeding unsaturated hydrocarbons on Si(100) template

• Comparison with our previous work on the Si(111) surface

55

Introduction – Aromatic hydrocarbonsIntroduction – Aromatic hydrocarbons

• Adsorbates of interest • Functional units

phenyl methyl vinyl Heteroatom H

benzene 1 6

toluene 1 1 5+3

p-xylene 1 2 4+6

m-xylene 1 2 4+6

o-xylene 1 2 4+6

styrene 1 1 5+3

pyridine 1 1 1

CH3 H2C CH

• Organic molecules comprise of over 95% of all known chemical compounds

• Organic functional units take effect in organic-semiconductor interactions

• Aromatic hydrocarbons are relative stable during surface processes

• Study on prototypical aromatic hydrocarbons helps analysis and synthesis of oligomers and/or polymers

NN

66

Organic functionalization Organic functionalization of semiconductorsof semiconductors

Organic moleculesOrganic molecules

Organic Organic functional functional

unitsunits

Si-SiSi-Sidimerdimer

Si(100) surfaceSi(100) surface

SemiconductorsSemiconductors

InterfaceInterface

Investigated characteristic functions of phenyl, methyl, vinyl, heteroatom, H in the surface chemistry on Si(100) under different surface conditions

Derived the Kinetics of chemisorption, dissociation, desorption, and condensation polymerization

Developed a new kinetics theory for reactions in 2D-diffusion system

77

IntroductionIntroduction Experimental & theoretical methods Experimental & theoretical methods

Experimental• UHV chamber (P<10-10 Torr)

• Thermal desorption spectrometry (TDS)

• Low energy electron diffraction (LEED)

• Auger electron spectroscopy (AES)

Computational• Density functional theory

(DFT)

• Gaussian 98 package

• Geometry and energy of chemisorption

88

Cracking pattern for a specific molecule

Experimental: TDS setupExperimental: TDS setup

Temperature ramps as a linear function of time

• Intensities for different masses Amount of desorbed species

• Desorption temperature bonding energy & adsorption geometry

• Shape of TDS profiles reaction order

99

Cracking pattern for a specific molecule

Experimental: TDSExperimental: TDS

Temperature ramps as a linear function of time

• Intensities for different masses Amount of desorbed species

• Desorption temperature bonding energy & History of surface reactions

• Shape of TDS profiles reaction order

Zeroth order

First order

halfth order

second order

1010

Chemisorption at RT• Adsorption kinetics (by AES)• Surface structure (by LEED)

Thermal desorption• Adsorption Energy & geometry (by TDS, DFT)• Surface chemistry (by TDS)

Surface condition• Ar+ sputtered a-Si• Oxidized and hydrogenated Si

Post-exposure treatments• Oxidization and hydrogenation• Electron and UV light irradiation

Study of surface kinetics

Present work - ProcedurePresent work - Procedure

1111

Present work - ChemisorptionPresent work - Chemisorption

Si(100)2x1

pyridine/Si(100)2x1

AES relative intensity Adsorption coverageThe shape of coverage vs. exposure

• 1st order – molecular adsorption (benzene, toluene, xylene isomers, styrene)

• 2nd order – dissociative adsorption (pyridine)

Adsorption rate Adsorption order

LEED patternLEED pattern

1212

Assignment of adsorption states

TDS

DFT computation• Chemisorption geometry• Bonding energy

Present work - Thermal desorptionPresent work - Thermal desorption

• Desorption species• Desorption

temperature

A B

A

B

1313

Cycloaddition and dative adsorptionCycloaddition and dative adsorption

• [4+2] cycloaddition exists for all the hydrocarbons; it may convert to tight-bridge at low coverages

• [2+2] cycloaddition found in styrene/Si(100) involving the vinyl group

• Dative adsorption involving the heteroatom (N) for pyridine/Si(100)

tight-bridge

Summary of chemisorption geometries found in the present work:

1414

Present work - Surface conditionPresent work - Surface condition

Ar+ sputtered a-SiA TDS feature with a lower

adsorption energy (at a lower temperature)

B Desorption of smaller hydrocarbon fragments at a higher temperature

Oxidized and hydrogenated SiC Making the surface inert to

molecular adsorption D Except in the case of pyridine

that undergo dissociation

AA

N

BB

CC

DD

1515

Present work - Post-exposure treatmentsPresent work - Post-exposure treatments• First observation of RT

oligomerization of pyridine stimulated by low energy electron irradiation

• Unexpected elevation of D2 desorption

• The surprising recurrence of molecular desorption in the second-run TDS

• Room-temperature condensation oligomerization of pyridine on Si(100)

D2

C5D5N

N

1616

( V )

( V ) Si

H

)

( XI )

( X )

( IX ))

)

( VIII )

C2D3H (g)

D DD

SiSi

H

H

HSi

H

Si

HH

Si

( VII )

(c)

HH

( V )

( VI )

(b)

(a)

( IV )( III )

( II )( I )

(c)

(b)

(a)

+

D

H2 (g)

DD

SiSiH H

HSi

H

D

D

D

D DD

D DD

HSi

H

D

D

D

HSi

H

D

D

DDDD

+ H2 (g)

750 KH

Si SiH

+ +

+

+

H2 (g)

750 K

H

Si SiH

700 K H

Si SiH

C2D3H (g)

HH

Si Si

Si700 K

680 K

SiSi

H

H

SiHSiH+ 2H

SiSiH H

+ 2H

SiSiH H

~ 560 K+Si SiSiSi

DDD

( V )

12

12

Present work: Post-exposure hydrogenationPresent work: Post-exposure hydrogenation

D

M

First observation of surface-mediated organic chemistry driven by thermal diffusion and desorption of hydrogen

C

DD-M

D-M

D-M

( )( )

( )( )

M

1717

R H(ad)

Si H

Adsorbate

MonohydrideSi SiH H

Hydrogen abstraction

H pairing

H2 (g)

Thermal desorption

Condensation

Present work: kinetics of hydrogen evolutionPresent work: kinetics of hydrogen evolution

DD

DD

D

H

HH

Si SiSi Si

HH

+ H2 (g)

DOD

Model II

Model III (g)HPPP 2nmnm

Model I

Broader feature with multi-states

• Near first-order desorption kinetics• H diffusion independent of co-adsorbates• Exothermic formation of monohydride

I

• Near second-order desorption kinetics• H diffusion influenced by co-adsorbates• Endothermic formation of monohydride

II

800 K

DD

DD

D

D

DD

H2

Hn lnln 2

or

RT

EnH

RT

EH

aa

eedt

d

νν

θ2

n – Reaction order of desorption

Hydrogen evolution – Desorption orderHydrogen evolution – Desorption order

nHθθ2

Model I

1919

Hydrogen evolution – Model IIIHydrogen evolution – Model III

Mobile monomers Mobile dimers

Ea 23 kcal/molEd 4 kcal/molEd 8 kcal/mol

Ed(1) 3.5 kcal/molEd(2) 7 kcal/mol

Model Model IIII

(adsorbate)H

H

Si Si

R

+R CH

HH

Si Si

H

Si SiSi Si

HH

+ Si Si

H

2

(DOD) (NOD) (SOD)

Hydrogen evolutionHydrogen evolution

Si Si+Si SiR H

2 Si Si+Si SiR

2H H

(R-DOD-H) (R-SOD)

Model Model IIIIIICondensation polymerizationCondensation polymerization

• Development of the Collision Theory for the Diffusion System on surface

(g)HPPP 2nmnm

RTEnm

aekTNNdt

dH /2/12 8

• Traditional gas-phase collision theory:

• Our collision theory for diffusion system:

RTEEnm

adedt

dH /2

The activation energy of the reaction in such type of systems consists of contributions from both collision and diffusion.

D2

Si Si

2121

Hydrogen evolution – Model Hydrogen evolution – Model IIIIII

Mobile monomers Mobile dimers

Ea 23 kcal/molEd 4 kcal/molEd 8 kcal/mol

Ed(1) 3.5 kcal/molEd(2) 7 kcal/mol

2222

Functional units in Functional units in different surface processesdifferent surface processes

Phenyl Methyl

CH3

Vinyl

CH=CH2

Heteroatom

N

H

Cycloaddition

Dative bonding

Hydrogen abstraction

Desorption

Diffusion

Dissociation

Condensation polymerization

Benzene Toluene p-xylene m-xylene o-xylene styrene pyridineBenzene Toluene p-xylene m-xylene o-xylene styrene pyridine

2323

• [4+2] cycloaddition occurs for all the hydrocarbons; [2+2] cycloaddition found in styrene/Si(100); Dative adsorption involving the heteroatom for pyridine/Si(100)

• First observation of surface-mediated organic chemistry driven by thermal diffusion and desorption of hydrogen in styrene/Si(100)

• First observation of electron-induced RT oligomerization of pyridine

• A new collision theory for the 2-D diffusion system has been developed to clearly describe the nature of the reaction kinetics in lattice-diffusion systems

• Hydrogen abstraction is found to play an important role in stabilizing the adsorbed hydrocarbons for further surface processes at higher temperatures

• Three kinetics models have been developed to successfully describe all hydrogen evolution processes in the present work

Summary Summary

2424

Outlook Outlook

• Further studies by using more structural-sensitive techniques (e.g. STM and FTIR)

• Larger aromatic molecules (e.g. naphthalene and biphenyl), smaller aromatic heterocyclic molecules (e.g. pyrrole, thiophene and furan), and halogen-substituted aromatic hydrocarbons

• Monolayer system multiple organic layers • Complete and Extend the collision theory to

surface chemistry on metals (catalysis study!) and 3D-diffusion system.

2525

• Dr. K.T. Leung

• Dr. Dan Thomas, Dr. Jean Duhamel and Dr. Bruce Torrie

• Dr. Shihong Xu and Xiaojin Zhou • Zhenhua He and Sergey Mitlin• Xiang Yang, Dr. Nina Heinig , Qiang Gao• Dr. Hui Yu and Xiang He

• Entire staff of the Science Shops

• Chunling Yang

• Friends: Lili Zheng, Jingying Yin, Grace Yin, Ben Yang, Benda Liu, Yan Wu and their families

Thank youThank you

2626

Model IModel I

Si Si+SiSi

HHH

+ Si SiH H

SiSi

H

Si Si 2 Si Si

H

+ Si Si

HH

H = +6.0 kcal/mol

Dehydrogenated adsorbate

SOD DOD UOD

a 1 2 0

)1(2

)1()1(4'

2

2 x

xxxx HHH

RTEH aedt

d /2 ν

SiSi

HSi Si

HSi Si

H H Si Si

2727

Carbon concentration after annealing Carbon concentration after annealing to different temperaturesto different temperatures

100% = 1/4 monolayer

90%

30%Molecularadsorption

350-600 K

10%

Moleculardesorption

Dissociativedesorption plus...

800-1000 K

200 400 600 800 1000 1200 1400

20

40

60

80

100

100% = 100 L toluene

AES C(KLL)/Si(LVV)

Ca

rb

on

Co

nce

ntr

atio

n (

%)

Temperature (K)

60%

2828

Our collision theory for adsorbates diffusing on surface:Our collision theory for adsorbates diffusing on surface:

/RT,E/RTEnm

ad ee)(θθr nmmnm m

/RTEnmnm

de)(θθθθ meffnm mZ

m n

mmdt

dH /RTnm,En

/RTEm

nm,nm

2 ad eθe)(θr

m

m

m

m

mE

n

E

mRm

mdt

dHda

/RTEEm

/RT)(mn

/RT2

adeθ

e)(θθe

pppp rrrrrr

RRdt

dH

22222212121121211111

2 )2()1(

221221

2221221112

11221111

2

2

rrrdt

d

rrrrrdt

d

rrrrdt

d

p

p

p