STM study of organic molecules on a hexagonal SiC surface

Tamara OvramenkoPhD supervisors: Andrew Mayne

Gérald DujardinGroupe de Nanosciences Moléculaires

Bât. 210, Université Paris-Sud, 91405 Orsay, Francehttp://voyager.ppm.u-psud.fr/nanophysics.html

Objectives

Molecular SiC “MOLSIC”

To functionalize SiC surface with organic molecules

We study interaction of different kinds of molecules withwide band gap 6H-SiC (0001) 33 surface

Ultra High Vacuum Scanning Tunneling Microscope

STM (P310-11 to 810-11 Torr)

STM head

6H-SiC (0001) 33

G. Baffou, A.J. Mayne et al Phys. Rev. B 77, 165320 (2008)

Surface states diagram

Molecular SiC “MOLSIC”

Si adatom, with non

compensated dangling bonds

Surface state inside band gapS2=-1.5eV, 0.03 GS1=-0.5eV, 1.0 G U1=0.5eV, 1.0 G

Top: top view of unit cell Bottom: bulk cross-section

Dangling bonds minimisation

STM images of 6H-SiC (0001) 33

1.2nm

8.0nm

4.7nm

a)

c)

b)

Molecular SiC “MOLSIC”

Fullerene C60

•C60 mean diameter 0.7nm• band gap size 1.7eV• well studied on other surfaces• fluorescent

STM images of C60 on 6H-SiC (0001) 33

What are the adsorption positions of C60 on SiC?

Si-3Between three adatoms

Eads=-0,28 eV

Si-1On top of Si adatom

Eads=-0,67 eV

Theoretical calculation

2.6nm

Single C60 molecule adsorption positions

2.5nm

Position “2”

Chemisorbed: E=-0,21 eV

Position “3”

Chemisorbed: E=-0,28 eV

Top view

Position “Top”

Chemisorbed: E=-0,67 eV

What are the adsorption positions of C60 on SiC?

Si-3Between three adatoms

Eads=-0,28 eV20%

Si-1On top of Si adatom

Eads=-0,67 eV40%

Experimental resalts

Si-2Between two adatoms

Eads=-0,21 eV40%

Position “Top”

Position “2” Position “3”

Statistical analysis and 2D Poisson distribution analysis- Analyze surfaces with 3 different deposition times (10s-60s)- Define molecular coverage corresponding to the each deposition time (1.8%-12.8%)- Determine percentage for each molecular cluster

Time, s Coverage M1 M2 M3 M4 M5

10 1/15 ML 63% 21% 16% 0 0

25 1/8 ML 44.6% 35.5% 11.7% 1.7% 0

60 1/4 ML 37% 22% 18% 6% 17%4.1nm

M3

M5

M6

M2

4.1nm

M3

M5

M6

M2

4.1nm

M3

M5

M6

M2

6.0nm6.0nm6.0nm6.0nm

2D Poisson distribution used to analyse the distribution of C60 for possible clustering

Probability to find k molecules in a particular square

•We have more empty (k=0) squares than expected

•2D Poisson distribution clearly indicates clusteringM. Cranney et al, Appl. Phys. A 94, 767(2009)

2D Poisson distribution analysis

Do they “like” to form clusters?

Creating a monolayer of C60 on SiC

3.3nm4.1nm 4.0nm

2.5V, 1/8ML -4.5V, 1/4ML -4.0V, 1ML

Objectives:• distribution of C60 molecular clusters• packing of monolayer

Monolayer of C60 on SiC

• not complete

• less than one molecule per SiC unit cell

• no long range packing in monolayer

just locally ordered packing

Work in progress and Perspectives

Caltrop molecule - is complex fluorescent molecule consisting of four linked PTCDI molecules.

Idea: To chemisorb it on the SiC surface with help of 3 PTCDI “legs”, leaving one “leg” free for luminescence.

Results:1.Deposited Caltrop on SiC. Complex: images of molecule convoluted with tip. Fragile: decomposed into fragments.2. STM images of caltrop deposited on Si (100) were obtained.

Perspective: to deposit new synthetized Caltrop with reinforced center on Si (100) and SiC

Terphenyl acid molecule

Idea: To chemisorb molecule next-nearest neighbor Si adatoms creatinga “molecular bridge” between two adatoms.

Results:1. Deposited TAM on Si(100) dissociate into small components2. Si surface can not be recuperated after deposition of Terphenyl

1.7nm

SiC nanowire flowers

Ghim Wei Ho et al, Nanotechnology 15, (2004) 996-999

Thank you for your attention

RT STM image of C60 on SiC at +2.5V

Internal structure and orientation

G. Schull et al, Phys. Rev. Lett. 99, 226105 (2007)N. Néel et al, Phys. Rev. B 77, 125431 (2008)

LT STM image of C60 on Cu (100) at 8K LT STM image of C60 on Au (111)

4.2Å

(one lobe)

5.6Å

(two lobe)

5.0Å

(three lobe)

C60 position analyses procedure

C60 position analyses procedure

1. Cut a piece of clean SiC surface 2. Overlay on C60 molecules

3. Make piece 30-40% transparent and match to Si adatoms of original picture with Si adatoms of transparent piece.