Ph. Guaino et. al (NCSR), Dublin City University

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An STM investigation of the intera ction and ordering of pentacene mo lecules on the Ag/Si(111)-(√3x√3)R30° surface Ph. Guaino et. al (NCSR), Dublin City University 24 May 2003 Surface Science Lee Min Hee (Ph.D Course 2)

description

An STM investigation of the interaction and ordering of pentacene molecules on the Ag/Si(111)-(√3 x √3)R 30 ° surface. SS Journal Club. 24 May 2003 Surface Science. Ph. Guaino et. al (NCSR), Dublin City University. Lee Min Hee (Ph.D Course 2). The structure of pentacene(C 22 H 14 ). - PowerPoint PPT Presentation

Transcript of Ph. Guaino et. al (NCSR), Dublin City University

Page 1: Ph. Guaino et. al (NCSR), Dublin City University

An STM investigation of the interaction and ordering of pentacene molecules on the Ag/Si(111)-(√3x√3)R30° surface

Ph. Guaino et. al(NCSR), Dublin City University

24 May 2003 Surface Science

Lee Min Hee (Ph.D Course 2)

Page 2: Ph. Guaino et. al (NCSR), Dublin City University

FIG. 1.(a) Molecular structure and (b) and (c) Herringbone crystalline structure of pentacene. (d) Typical OTFT geometry (top contact).

The structure of pentacene(C22H14)

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Purpose

• Studying the ordering of pentacene (C22H14) molecules on the Ag/Si(111)-(√3x√3)R30° surface at RT.

• The role of competition between intermolecular and molecule-substrate interactions and the nature of the adsorption sites is discussed.

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A new generation of devices based on organic semiconductors <Aromatic hydrocarbon molecules (C22H14) >

Investigation of the interactions of pentacene molecules on surfaces:

molecule-substrate interactions on semiconductor surface.

chemisorbtion and none-diffusion at RT.molecule-molecule interactions on the metallic sur

faces weaker Van der Waals type interactions.

Introduction

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Pre-researches

C22H14/Cu(110) A long range ordering of the unidirectional

close packed side-by-side configuration molecular rows.

The formation of standing waves (Friedel oscillations) in the surface-state free-electron gas.

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Ag/Si(111)-(√3x√3)R30° substrate. C60 molecules are found to bond preferentially at defect s

ites and step edges and molecular domains are formed which are ordered in a double domain configuration.

Cobalt-phthalocyanine (CoPc) molecules form a close packed arrangement at a coverage close to a monolayer.

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• STM(Omicron Vakumphysik GmbH). RT. pressure of 5 x10-11 mbar.

• Si(111) substrate. p-type boron-doped with 0.1~1.0 Ωcm. current heating to 1250℃ after degassing (600℃ for 10 h).• Silver 5N (Goodfellow Metals) evaporated onto the Si substrate at 500 ℃ • Pentacene (Aldrich Chemicals) degassing( 12 h) to remove water vapor. evaporated at a rate of about 1 ML/min at 180 ℃.

Experimental details

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Results and discussion• The honeycomb-chained-trimer (HCT) model

Ag trimer

• Size =5x5 nm2, V sample = 1.0 V, It =2.0 nA

Si trimer

Unit average length : 6.8 ±0.5Ǻ

• Hexagonal structure in the STM empty state image

Ag/Si(111)-(√3x√3)R30°

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Fig 2. STM image of S1 phase.~0.35ML Pentacene. Packing Density = 3.4x10-3 mol Ǻ -2

Size= 40 x 40 nm2, V sample =1:5V,It= 100 pA.

Parallel shifts of the molecular rows

Perpendicular shifts of the molecular rows

Small ordered region

The symmetryOrientation(√3x√3)R30° Honeycomb geometry

120°

Molecule-Substrate interaction.

Molecular row

7.9±1.0Ǻ

18.4±2.0Ǻ

15.8±2.0Ǻ

Head to Head

orientation

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Results and discussion

Fig. 3. STM image of Ag/Si-√3 coverage after pentacene deposition of 0.2 ML. Image parameters: size =120 x120 nm2, V sample=1.0 V, It=100 pA.

short parallel rows of pentacene molecules.

A shadowing of the sample by thesample holder during deposition.

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Histogram of the statistical distribution of the inter-rows spacings.

d1=16.2±2 Ǻ(2.5 x unit cell length)

d2=33.5±2 Ǻ(5 x unit cell length)

d3=44.1±2 Ǻ(6.5 x unit cell length)

The adsorbed layer is in registry with the structure of the substrate.

Fig. 4.

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High resolution image of Ag/Si-√3 coverage after pentacene deposition of 0.2 ML.

Molecule rowHoneycomb structure of the substrate

Perpendicular shifts of the molecular rows

40nm

V sample=+1.0 V, It=100 pA.

Fig. 5.

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Molecular rowalign direction

15.8±2 Ǻ

18.4±2 Ǻ20.0±2 Ǻ

113°±3°

6-fold hollowsite(CB-site)

3-fold hollowsite(CA-site)

The high-order (25 X 25) commensurate cell containing 75 molecules.

Proposed model of the Ag/Si(111)-(25X25)-pentacene structure, S1 phase.Pentacene molecules On the Ag/Si-√3 surface

no preferentially decorating the steps no alignment occurs along step edgesThe S1 phase growth of the film is governed

by molecule-substrate interactions and molecule-molecule interactions .

Adsorption takes place (CA-site) and (CB-site).

Fig. 6.

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STM image of solid phase S2

Major axis

6.5±0.5Ǻ inter row distanceAB=15.6±2Ǻ molecular pair

CD=20.85±2 Ǻ neighbouring pair

D = (9.6± 0.2)x 10-3 mol Ǻ-2 . V sample =1.1 V, It =200 pA.Fig. 7. STM image of solid phase S2 formed after deposition of 1 ML of pentacene.

Close-packed side-by-side arrangement domains

21.5nm

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STM images of solid phase S2 recorded in dual scan mode.

7.8 nm

high bias in forward scan directionV sample =-1.9 V, It =50 pA.

low bias in the backward scan directionV sample =+0.3 V, It =190 pA.

Resonant tunneling through the energy levels of the molecule.

Tunneling occurs within the HOMO-LUMO gap

Charge transfer between Ag/Si(111)-(√3x√3)R30° surface and the π electron of the pentacene

Fig. 8.

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Proposed model of the Ag/Si(111)-(2x3)-pentacene structure, S2

phase.

19.5 Ǻ

15.6 Ǻ

118±2°Unit Cell

Inter-row separation correspond toVan der Waals interaction radius of 6.5 .Ǻ

An attractive force between the molecules leading to a lateral rearrangement

Relatively large quadrupole moment may provide a significant portion of the lateral molecule-molecule interaction

Fig. 9.

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Crystallographic data for pentacene superstructures on the Ag/Si-√3 substrate

High~x3

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Conclusion

The formation of well-ordered pentacene layers on the Ag/Si-√3 substrate depends not only on competition between intermolecular and molecule-substrate interactions but also on the nature of the adsorption site.

For the S1 phase, adsorption takes place a three-fold hollow site at the centre of a Ag-trimer (CA-site) and a six-fold hollow site at the centre of the hexagonal arrangement of silver atoms (CB-site).

No adsorption takes place on six-fold hollow site (CB-site) for the S2 phase, in which the short- range interactions are stronger.

Adsorption takes place exclusively on the Ag-trimer (CA) site The strength of the molecule-substrate interaction at the Ag trimer si

tes (CA-site) is sufficient to produces a new commensurate structure, even in the presence of short-range lateral interactions.

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Abstract

S1 solid phase :coverage of 0.35 ML pentaceneA high-order commensurate lattice, Ag/Si(111)-(25x25)-pentacene, containing 75 molecules per unit cell.Adsorption takes place a three-fold hollow site at the centre of a Ag-trimer (CA-site) and a six-fold hollow site at the centre of the hexagonal arrangement of silver atoms (CB-site).

S2 solid phase :coverage of 1.0 ML pentaceneA molecular reorganization and a new commensurate structure Ag/Si(111)-(2x3)-pentacene,containing two molecules per unit cell.Adsorption takes place exclusively on the Ag-trimer (CA) site and the CB-site is lost because of strong lateral molecule-molecule interactions.