marrying organic molecules onto silicon surface

Marrying Marrying organic moleculesorganic molecules onto onto Silicon surfacesSilicon surfacesXiaojing Zhou, K.T. Leung

Watlabs, Department of ChemistryUniversity of WaterlooWaterloo, Ontario N2L 3G1Canada

•• MotivationMotivation•• Silicon surfacesSilicon surfaces•• How to design the manipulation How to design the manipulation

through different organic moleculesthrough different organic molecules•• How to characterize their propertiesHow to characterize their properties•• My experimental setup, results and My experimental setup, results and

discussiondiscussion•• AcknowledgementsAcknowledgements

Outline

27 percent of the Earth's crust

The starting material for nearly all the microelectronic "chips

(Circuits per chip) = 2 (year-1975)/1.5

We are seeing the end of traditional microelectronic technology“By 2017, device features will have shrunk past a critical lengthwhere the device will no longer work under its current design”

Microelectronics miniaturization will depend on functionality of individual molecules and atoms.

Logic gates Si dimers

Using silicon dimers as memory bits

Ian Appelbaum, Tairan Wang, Shanhui Fan, J. D. Joannopoulos and V. Narayanamurti, Nanotechnology 12 (2001) 391

The modification of organic molecules on silicon surface provides variety of new capabilities in optical, electronic, mechanical , chemical and biological activities.

S.F. Bent / Surface Science 500(2002) 879

1300-1400 K

Si(100)

Si(100)2x1Si(111)7x7

A puzzle for Si(100)2x1: titled or flat dimer structure

+ -2.28 A1.7o tilted75+5 kcal/mol

[2+2] cycloaddition

[2+4] cycloaddition(Diels-Alder addition)

C2H4 C2H2 C4H6 (1,3-butadiene)

HOMO

LUMO

1.57 A

-43.2 kcal/mol

1.35 A

-60.2 kcal/mol -67.5 kcal/mol -41.9 kcal/mol

Woodward-Hoffmann selection rules for cycloaddition reaction:

[2+2] cycloaddition [2+4] cycloadditionSuprafacial bond formation

Antarafacial bond formation

ethylene

p T C

(b) benzene

C-F=132 kcal/mol C-Cl= 95 kcal/mol C-Br=67 kcal/mol C-H=105 kcal/mol

Br BrSi

Ca Ca

Si

HH

BrCa

Cb

Si

Br

Si

H

H

(a) Vinylene (b) Vinyl Bromide

Insertion reaction:

Si-Si breakage

X.J Zhou, Q. Li, Z.H. He, X. Yang and K.T. Leung, Surface Science Letter 543 (2003) L668.

Surface characterization for adsorbates:

o Scanning tunnelling microscopy

o Scanning tunnelling spectroscopy

o Atomic force microscopy

o Low-energy electron diffraction

o X-ray photoelectron spectroscopy

o Auger electron spectroscopy

o Infrared spectroscopy

o Electron energy loss spectroscopy

o other surface analytical techniques

My experiments

Desorption: Temperature programmed desorption (TPD)

XPS

polymethylmethacrylateKE = hv – BE

NOTE - the binding energies (BE) of energy levels in solids are conventionally measured with respect to the Fermi-level of the solid, rather than the vacuum level. This involves a small correction to the

equation given above in order to account for the work function (φ) of the solid.

TPD

Rdes = ν . Nx exp ( - Eades / R T )

Eades - activation energy for

desorption

x - kinetic order of desorption (typically 0,1 or 2)

Rdes - desorption rate ( = -dN/dt )

TPD provides information about the desorption kinetics such as desorption order, desorption activation energy etc.

Intensity ratio: 4:5:1

(a) Trichlorovinyl, ΔE=−63.3 kcal/mol (b) Cross-dimer-bridge, ΔE=−77.8 kcal/mol

(e) H-bridge, ΔE=−194.5 kcal/mol

(d) Out-of-plane dimer-bridge, ΔE=−97.7 kcal/mol(c) In-plane dimer-bridge, ΔE=−114.1 kcal/mol

B3LYP/6-31G(d) with fixing the bottom two layers of silicon and hydrogen atoms

3.2 A

3.75 A

C2H2

HCl

SiCl

TPD profiles indicates chlorine atoms leave surface above 800 K as either HCl or SiCl2

Summary

• Halogenated ethylenes react with Si(100)2x1 dissociatively at RT, following predominately insertion mechanism.

• Tetrachloroethylene molecules modify Si(100) surface with C2 dimer bridge, protected by four chlorine atoms.

• Chlorine atoms can be used in the further development in term of making devices.

Acknowledgements

Delaware surface theoretician Barriocanal (left) and Doren.

Stanford cycloaddition group

CNRC researchers

Wisconsin chemists University of Minnesota

self-assembled monolayers

North Carolina, surface chemists