An STM investigation of the interaction and ordering of pentacene molecules on the AgSi111v3xv3R30 s

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An STM investigation of the interaction and
ordering of pentacene molecules on the
Ag/Si(111)-(v3xv3)R30 surface
SS Journal Club
24 May 2003 Surface Science
Ph. Guaino et. al (NCSR), Dublin City University
Lee Min Hee (Ph.D Course 2)
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The structure of pentacene(C22H14)
FIG. 1.(a) Molecular structure and (b) and (c)
Herringbone crystalline structure of pentacene.
(d) Typical OTFT geometry (top contact).
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Purpose

• Studying the ordering of pentacene (C22H14)
  molecules on the Ag/Si(111)-(v3xv3)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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Introduction

• A new generation of devices based on organic
  semiconductors ltAromatic hydrocarbon molecules
  (C22H14) gt
• 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
  surfaces
• weaker Van der Waals type interactions.

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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)-(v3xv3)R30 substrate.
• C60 molecules are found to bond preferentially at
  defect sites 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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Experimental details

• STM(Omicron Vakumphysik GmbH).
• RT. pressure of 5 x10-11 mbar.
• Si(111) substrate.
• p-type boron-doped with 0.11.0 Ocm.
• 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 ?.

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Results and discussion

• The honeycomb-chained-trimer (HCT) model

Ag trimer
Unit average length 6.8 0.5?
Si trimer

• Hexagonal structure in the STM
• empty state image

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

Ag/Si(111)-(v3xv3)R30
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Perpendicular shifts of the molecular rows
120
Parallel shifts of the molecular rows
Small ordered region
The symmetry Orientation (v3xv3)R30 Honeycomb
geometry
Fig 2. STM image of S1 phase. 0.35ML Pentacene.
Packing Density 3.4x10-3 mol? -2 Size 40 x 40
nm2, V sample 15V, It 100 pA.
Molecule-Substrate interaction.
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Results and discussion
short parallel rows of pentacene molecules.
A shadowing of the sample by the sample holder
during deposition.
Fig. 3. STM image of Ag/Si-v3 coverage after
pentacene deposition of 0.2 ML. Image
parameters size 120 x120 nm2, V sample1.0 V,
It100 pA.
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Histogram of the statistical distribution of the
inter-rows spacings.
Fig. 4.
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High resolution image of Ag/Si-v3 coverage after
pentacene deposition of 0.2 ML.
Perpendicular shifts of the molecular rows
40nm
Molecule row
Fig. 5.
Honeycomb structure of the substrate
V sample1.0 V, It100 pA.
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Proposed model of the Ag/Si(111)-(25X25)-pentacene
structure, S1 phase.

• Pentacene molecules On the Ag/Si-v3 surface
• no preferentially decorating the steps
• no alignment occurs along step edges
• The S1 phase growth of the film is governed by
  molecule-substrate interactions and
  molecule-molecule interactions .
• Adsorption takes place (CA-site) and (CB-site).

15.82 ?
Molecular row align direction
6-fold hollow site(CB-site)
20.02 ?
18.42 ?
3-fold hollow site(CA-site)
Fig. 6.
1133
The high-order (25 X 25) commensurate cell
containing 75 molecules.
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STM image of solid phase S2
Major axis
Close-packed side-by-side arrangement domains
21.5nm
6.50.5? inter row distance
AB15.62? molecular pair
CD20.852? neighbouring pair
D (9.6 0.2)x 10-3 mol ?-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.
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STM images of solid phase S2 recorded in dual
scan mode.
Resonant tunneling through the energy levels of
the molecule.
Tunneling occurs within the HOMO-LUMO gap
7.8 nm
Fig. 8.
high bias in forward scan direction V sample
-1.9 V, It 50 pA.
low bias in the backward scan direction V sample
0.3 V, It 190 pA.
Charge transfer between Ag/Si(111)-(v3xv3)R30
surface and the p electron of the pentacene
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Inter-row separation correspond to Van der Waals
interaction radius of 6.5?.
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.
Proposed model of the Ag/Si(111)-(2x3)-pent
acene structure, S2 phase.
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Crystallographic data for pentacene
superstructures on the Ag/Si-v3 substrate
High
x3
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Conclusion

• The formation of well-ordered pentacene layers
  on the Ag/Si-v3 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 sites (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 pentacene A
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 pentacene A molecular reorganization and a
new commensurate structure Ag/Si(111)-(2x3)-pentac
ene,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.