1. 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)

2. FIG. 1.(a) Molecular structure and (b) and (c) Herringbone crystalline structure of pentacene. (d) Typical OTFT geometry (top contact). The structure of pentacene(C 22 H 14 )

3. Purpose Studying the ordering of pentacene (C 22 H 14 ) 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.

4.  A new generation of devices based on organic semiconductors  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. Introduction

5. Pre-researches C 22 H 14 /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.

6. Ag/Si(111)-(√3x√3)R30° substrate.  C 60 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.

7. 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

8. Results and discussion The honeycomb-chained-trimer (HCT) model Ag trimer Size =5x5 nm 2, V sample = 1.0 V, I t =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°

9. Fig 2. STM image of S1 phase. ~0.35ML Pentacene. Packing Density = 3.4x10 -3 molǺ -2 Size= 40 x 40 nm 2, V sample =1:5V, I t = 100 pA. Parallel shifts of the molecular rows Perpendicular shifts of the molecular rows Small ordered region The symmetry Orientation (√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

10. Results and discussion Fig. 3. STM image of Ag/Si-√3 coverage after pentacene deposition of 0.2 ML. Image parameters: size =120 x120 nm 2, V sample =1.0 V, I t =100 pA. short parallel rows of pentacene molecules. A shadowing of the sample by the sample holder during deposition.

11. Histogram of the statistical distribution of the inter-rows spacings. d 1 =16.2±2 Ǻ(2.5 x unit cell length) d 2 =33.5±2 Ǻ(5 x unit cell length) d 3 =44.1±2 Ǻ (6.5 x unit cell length) The adsorbed layer is in registry with the structure of the substrate. Fig. 4.

12. High resolution image of Ag/Si-√3 coverage after pentacene deposition of 0.2 ML. Molecule row Honeycomb structure of the substrate Perpendicular shifts of the molecular rows 40nm V sample=+1.0 V, It=100 pA. Fig. 5.

13. Molecular row align direction 15.8±2 Ǻ 18.4±2 Ǻ 20.0±2 Ǻ 113°±3° 6-fold hollow site(CB-site) 3-fold hollow site(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 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). Fig. 6.

14. STM image of solid phase S 2 Major axis 6.5±0.5Ǻ inter row distance AB=15.6±2Ǻ molecular pair CD=20.85±2Ǻ neighbouring pair D = (9.6± 0.2)x 10 -3 mol Ǻ -2. V sample =1.1 V, I t =200 pA. Fig. 7. STM image of solid phase S 2 formed after deposition of 1 ML of pentacene. Close-packed side-by-side arrangement domains 21.5nm

15. STM images of solid phase S 2 recorded in dual scan mode. 7.8 nm high bias in forward scan direction V sample =-1.9 V, I t =50 pA. low bias in the backward scan direction V sample =+0.3 V, I t =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.

16. Proposed model of the Ag/Si(111)-(2x3)-pentacene structure, S 2 phase. 19.5 Ǻ 15.6 Ǻ 118±2° Unit Cell Inter-row separation correspond to Van der Waals interaction radius of 6.5Ǻ. 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.

17. Crystallographic data for pentacene superstructures on the Ag/Si-√3 substrate High ~x3

18. 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 S 1 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 S 2 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.

19. 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)- 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.