Dihalogenated benzene on Si surface

Silicon surfacesSelected moleculesMotivationExperiment setupAdsorption model of benzeneExperimental resultsSummaryAcknowledgement

Si surface

dimers

Ideal structure 2x1 reconstruction

Selected Molecules

C-Cl 1.75 ÅC-F 1.35 Å C-Br 1.90 Å

1,2-difluorobenzene 1,2-dichlorobenzene 1,2-dibromobenzene

Bond Strength:

C-F=552 kJ/mol C-Cl= 397kJ/mol C-Br=280 kJ/mol C-H=338 kJ/mol

Motivations

Methyl or ethyl substituted benzene molecules show Methyl or ethyl substituted benzene molecules show different reaction mechanism from benzene on Si(100) different reaction mechanism from benzene on Si(100) 2x1. 2x1. R.J. Hamer, J.V.S.T,18 (2000) 1965, Q. Li, K.T. Leung,R.J. Hamer, J.V.S.T,18 (2000) 1965, Q. Li, K.T. Leung,Surf. Sci, 479 (2001) 69.Surf. Sci, 479 (2001) 69.

Halogenated unsaturated hydrocarbons such as Halogenated unsaturated hydrocarbons such as halogenated ethylene and benzene are potential halogenated ethylene and benzene are potential molecules to functionalize the ordered Si(100)2x1 molecules to functionalize the ordered Si(100)2x1 surface for applications of molecular devices and surface for applications of molecular devices and chemical sensors. chemical sensors. M. Bollani, et al., Applied surface science M. Bollani, et al., Applied surface science 175(2001) 379.175(2001) 379.

Halogenated benzenes are fundamentally interesting for Halogenated benzenes are fundamentally interesting for the study of organic silicon chemistry as a benchmark the study of organic silicon chemistry as a benchmark comparison to halogenated ethylenes on Si(100)2x1 in comparison to halogenated ethylenes on Si(100)2x1 in our previous study. our previous study. X.J. Zhou et al., Surface science letter, X.J. Zhou et al., Surface science letter, 543 (2003) L668543 (2003) L668.

Techniques used for the present experiments:Techniques used for the present experiments:

XX--ray photoelectron spectroscopy (XPS), ray photoelectron spectroscopy (XPS), Temperature programmed desorption (TPD), heating rate= 2 K/S.Temperature programmed desorption (TPD), heating rate= 2 K/S.

TPD

XPS

Taking a look at benzene on Si(100)2x1

0.48 eV 1.14 eV (420 K) 1.68 eV (620 K)

For a first desorption, Ed=RTp[ln υ Tp/β-3.64] , Tp=300K, Ed=0.81 eVSo the first adsorption model do not exist at RT. (Consider =10 13 S -1 )υ

Calculated by Gaussian 98 from a model of Si15H16 cluster at B3LYP/6-31G(d) level

B.Borovsky, M. Krueger, E. Ganz, PRB 57(1998),4270.

C 284.6/ C 286.7=2/1Note: Molecular adsorption

1.02 eV

1.06 eV

1.59 eV

1.68 eV

1.66 eV

1.65 eV

C 284.6/C 286.0=3:1

Note: 28% dissociation

Dissociation ratio isin the range of 8-28%

0.99 eV

1.17 eV

1.49 eV

1.63 eV

1.67 eV

1.81 eV

Dissociative adsorption models

F.Y. Naumkin et al, Surface Science 547 (2003) 335

Dissociative adsorption is thermodynamicallyfavorable, but kinetically there is 0.8 eV activation barrier.

Note: 96 % dissociation

Summary

A transition from molecular adsorption to dissociative adsorption of dihalogenated benzene on Si has been observed.The criteria to govern the adsorption transition is the bonding strength of C-X ( X=F, Cl, Br). Cl and Br atoms are easily attacked by atomic H atoms and replaced, which causes the dissociated ratio not reproducible.Adsorption interface can be used to applications such as chemical sensors.

Acknowledgement

Thanks NSERC for funding the present research.

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Binding Energy (eV)

30 min atomic H induction