1. 1 Qiang Li Department of Chemistry Surface Chemistry of Hexacyclic Aromatic Hydrocarbons on 2  1 and Modified Surfaces of Si(100)

2. 2 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 Outline

3. 3 Introduction – Organic Semiconductors The first organic FET Tsumura et al., Appl. Phys. (1986) 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)

4. 4 Diamond structure of Si Introduction – 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

5. 5 Introduction – Aromatic hydrocarbons Adsorbates of interest Functional units phenylmethylvinylHetero atom H benzene16 toluene115+35+3 p-xylene124+64+6 m-xylene124+64+6 o-xylene124+64+6 styrene115+35+3 pyridine111 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 N

6. 6 Organic functionalization of semiconductors Organic molecules Organicfunctionalunits Si-Sidimer Si(100) surface Semiconductors Interface Interface 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

7. 7 Introduction 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

8. 8 Cracking pattern for a specific molecule Experimental: 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

9. 9 Cracking pattern for a specific molecule Experimental: 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

10. 10 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 - Procedure

11. 11 Present work - Chemisorption Si(100)2x1 pyridine/Si(100)2x1 AES relative intensity  Adsorption coverage The shape of coverage vs. exposure 1st order – molecular adsorption (benzene, toluene, xylene isomers, styrene) 2nd order – dissociative adsorption (pyridine) Adsorption rate Adsorption order LEED pattern

12. 12 Assignment of adsorption states TDS DFT computation Chemisorption geometry Bonding energy Present work - Thermal desorption Desorption species Desorption temperature AB A B

13. 13 Cycloaddition 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:

14. 14 Present work - Surface condition Ar+ sputtered a-Si A A TDS feature with a lower adsorption energy (at a lower temperature) B B Desorption of smaller hydrocarbon fragments at a higher temperature Oxidized and hydrogenated Si C C Making the surface inert to molecular adsorption D D Except in the case of pyridine that undergo dissociation A B C D

15. 15 Present work - Post-exposure treatments First observation of RT oligomerization of pyridine stimulated by low energy electron irradiation Unexpected elevation of D 2 desorption The surprising recurrence of molecular desorption in the second-run TDS Room-temperature condensation oligomerization of pyridine on Si(100) D2D2 C5D5NC5D5N

16. 16 Present work: Post-exposure hydrogenation D M First observation of surface- mediated organic chemistry driven by thermal diffusion and desorption of hydrogen C D D-M ( ) M

17. 17 Present work: kinetics of hydrogen evolution Model II Model III 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 H2H2

18. or n – Reaction order of desorption Hydrogen evolution – Desorption order Model I

19. 19 Hydrogen evolution – Model III Mobile monomersMobile dimers E a  23 kcal/mol E d  4 kcal/mol E d  8 kcal/mol E d (1)  3.5 kcal/mol E d (2)  7 kcal/mol

20. Model II Hydrogen evolution Model III Condensation polymerization Development of the Collision Theory for the Diffusion System on surface Traditional gas-phase collision theory: Our collision theory for diffusion system: The activation energy of the reaction in such type of systems consists of contributions from both collision and diffusion. D2D2

21. 21 Hydrogen evolution – Model III Mobile monomersMobile dimers E a  23 kcal/mol E d  4 kcal/mol E d  8 kcal/mol E d (1)  3.5 kcal/mol E d (2)  7 kcal/mol

22. 22 Functional units in different surface processes PhenylMethyl  CH 3 Vinyl  CH=CH 2 Heteroatom N H Cycloaddition  Dative bonding  Hydrogen abstraction  Desorption  Diffusion  Dissociation  Condensation polymerization  Benzene Toluene p-xylene m-xylene o-xylene styrene pyridine

23. 23 [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

24. 24 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.

25. 25 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 you

26. 26 Model I  H = +6.0 kcal/mol Dehydrogenate d adsorbate SODDODUOD aa 11 22 00

27. 27 Carbon concentration after annealing to different temperatures 100% = 1/4 monolayer 90% 30% Molecular adsorption 350-600 K 10% Molecular desorption Dissociative desorption plus... 800-1000 K 60%

28. 28 Our collision theory for adsorbates diffusing on surface: Our collision theory for adsorbates diffusing on surface: