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Abstract: The widely employed approach to self-assembly of fullerene derivatives on gold can be complicated due to multilayer formations and head-to-tail assemblies resulting from the strong fullerene-fullerene and fullerene-gold interactions. These anomalies were not examined in detail in previous studies on fullerene self-assembled monolayers (SAMs) but were clearly detected in the present work using surface characterization techniques including ellipsometry, cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). This is the first time that SAMs prepared from fullerene derivatives of thiols/thiol esters/disulfides have been analyzed in detail, and the complications due to multilayer formations and head-to-tail assemblies were revealed. Specifically, we designed and synthesized several fullerene derivatives based on thiols, thiol acetates, and disulfides to address the characterization requirements, and these are described and delineated. These studies specifically address the need to properly characterize and control fullerene-thiol assemblies on gold before evaluating subsequent device performances.

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Characterization of Self-Assembled Monolayers of Fullerene

Derivatives on Gold Surfaces: Implications for DeviceEvaluations

Yasuhiro Shirai, Long Cheng, Bo Chen, and James M. Tour*J. Am. Chem. Soc. 2006, 128, 13479-13489

演講者：江柏誼

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Properties of Fullerene

• Electrically Insulating• Highly Electronegative• Intercalated to Becomes Superconductor• Can be Polymerized• Optical Properties• Hole-punched to Produce Nanoporous Materials

http://www.fullereneinternational.com/fic/fullerenes.html

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What is SAM?

SAM reagents are used for electrochemical, optical and other detection systems. Self-Assembled Monolayers (SAMs) are uni-directional layers formed on a solid surface by spontaneous organization of molecules. Thiol compounds and gold is one of the well-established combinations. Research areas include electron transfer mechanisms of proteins, molecular layers and biosensors.

http://www.dojindo.com/sam/SAM.html

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SAMs of C60

Langmuir 1993, 9, 1945-1947.J. Am. Chem. Soc. 1993, 115, 1193-1194.

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Design of Fullerene Derivatives

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Two Approaches to Prepare Fullerene SAMs

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Li+

N-

Si Si

lithium bis(trimethylsilyl)amide

N+

F-

Tetra Butyl Ammonium Fluoride

Li+

N-

Si Si

lithium hexamethlidisilazane

Synthesis of the Compound 1-SH and1-SAc

LHMDS TBAF

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Synthesis of the Compound 2-SH and2-SAc

OHO

F

F

F

Trifluoroacetic acid Pd/Cu = PdCl2(PPh3)2, CuI.

Sonogashira

N

Triethylamine

TFA TEA

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Synthesis of the Compound 3

Sonogashira Sonogashira

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Synthesis of the Compound 4

Si

trimethylsilylacetylene

Sonogashira

TMSA

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Ellipseometer

http://140.116.176.21/www/technique/SOP/SOP%20Ellipsometer.pdf

Compenastor

Polarizer

Laser Source

Sample

Analyzer

Detector

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The Film Thickness of SAMs of Compound 1-SH

The film thickness was determined byellipsometry

Cl

Cl

o-dichlorobenzene( ODCB )

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Assembly of the Fullerene Derivatives in ODCB/EtOH (4:1)

Table 1.

    thickness (nm)

compd assembly condition time (h) exptl calcd

1-SH N2 24 1.7 1.8

1-SH - 24 1.9 1.8

1-SAc acid 24 1.7 1.8

2-SH N2 24 2.4 2.6

2-SH - 24 2.7 2.6

2-SAc acid 24 2.3 2.6

3 - 24 2.3 2.6

4 - 24 3.4 3.4

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The Film Thickness of SAMs for Compound 1-4 in ODCB/EtOH (4:1)

The film thickness was determined byellipsometry

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Working electrode： Au in 0.1 M TBAPF6 in ODCB/EtOH (4:1)

Counter electrode： Pt wire

Reference electrode ： Ag/AgCl

Scan rates： 100 mV/s

Cyclic Voltammograms of SAMs for Compound 1-SAc

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Cyclic Voltammograms of SAMs for Compound 4

Working electrode： Au in 0.1 M TBAPF6 in ODCB/EtOH (4:1)

Counter electrode： Pt wire

Reference electrode ： Ag/AgCl

Scan rates： 100 mV/s

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Cyclic Voltammograms of SAM of Compound 3

The scan rate was raised from 0.1 to 1.0 V/s with 0.1 V/s increments

Working electrode： Au in 0.1 M TBAPF6 in ODCB/EtOH (4:1)

Counter electrode： Pt wire

Reference electrode ： Ag/AgCl

Scan rates： 100 mV/s

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Randles-Sevcik EquationPeak current, ip = (9.39×105) n2 A Γ vwhere n： number of electrons A： electrode surface area determined to be 5.85×10-3 [cm2]Γ： surface coverage…[mol/cm2] v： scan rate [V/s]

Peak Currents at The First Redox WavesVS Scan Rate

ip= 2.072 ×10-7 = ( 9.39×105 ) × 12 × 0.1 [V/s] × ( 5.85×10-3 [cm2] )ΓΓ =3.77×10-11 [ mol/cm2 ] = 0.227 [ molecules/nm2 ] → 4.41 [ nm2/molecule ] of compound 3

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Schematics for The Density of Compound 3 on The Surface

Γ ： 4.41 [ nm2/molecule ]

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• The head-to-tail assembly of fullerene-derivatized on gold surfaces.

• In addition, the XPS analysis on this class of compounds has not been reported.

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X-Ray Photoelectron Spectroscopy (XPS)

Photo-emitted electrons

X-Ray Source

Sample Energy analyzer

Detector Data treatment

http://www.chemicool.com/definition/x_ray_photoelectron_spectroscopy_xps.html

hν= EB + Ek （ hν: energy of X-ray EB: Binding energy ; Ek: Kinetic energy of electron）

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XPS S2p Spectra of Assembling Compound 1-SH

in pure ODCB

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in ODCB/EtOH (4:1)

XPS S2p Spectra of Assembling Compound 1-SAc,2-SAc

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XPS S2p Spectra of Assembling Compound 3,4

in ODCB/EtOH (4:1)

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Sulfur Atom Binding Energies and Atomic Ratios

Table 2. binding energy (eV) estimated atomic ratio

SAMbound S 2p3/2

unbound S 2p3/2

Oxidized S 2p3/2

unbound/ bound

oxidized/ bound

1-SH 161.9 163.5 168.6 1.1 1.2

1-SH 161.6 163.5 168.4 0.3 0.8

1-SAc 161.6 163.5   0.2 0

2-SAc 161.6 163.4   0.3 0

3 161.6 163.5   0.4 0

4 161.9 163.4 169.3 1.0 0.8

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Schematics of Ideal Dense Normal and Head-to-Tail SAMs on Gold Surfaces

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Conclusions

• Ellipsometry analysis showed that the use of a combination of ODCB solvent and protected thiols was necessary in some cases due to the low solubility of fullerene derivatives and the insolubility of byproduct generated by in situ disulfide formation.

• From this observation, and based on literature indicating the existence of strong fullerene-Au surface adhesion, we have proposed the head-to-tail assembly of fullerene SAMs, in which molecules are assembled on Au via fullerene-Au bonding, not the usual thiolate-Au bonding.

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• However, the assembly of disulfide 4, in which the possibility of deposition of fullerene derivatives on top of SAMs is greatly reduced because of good solubility and an inability to generate insoluble disulfides in situ, further supports the head-to-tail assembly.

• Electrochemistry on these fullerene SAMs confirmed the effect of packing density on electrochemical responses that has been reported previously. Only the tripod 3, that is designed to isolate the electroactive moiety in SAMs, showed ideal CV responses.