Scanning probe microscopy techniques were employed to investigate C60 molecules adsorbed on Si(111)-(7x7) and Ag-Si(111)-(√3x√3)R30o using imaging, spectroscopy, and manipulation methods. First, dynamic scanning tunnelling microscopy revealed the lowest unoccupied molecular orbital features of C60 molecules adsorbed on Si(111)-(7x7) with extremely high resolution at 77 K. Experimental data were compared with Hückel molecular orbital theory simulations to determine the orientation of the molecules on these surfaces. Second, C60 molecules were imaged with a qPlus atomic force microscope, in the attractive force regime and appeared as bright spherical protrusions. The potential energy of interaction between the AFM tip and C60 molecules adsorbed on Si(111)-(7x7) was quantified by force spectroscopy. Furthermore, a C60 molecule was transferred to the scanning probe microscope tip and used as molecular probe to image the Si(111)-(7x7) surface and other C60 molecules. The on-tip C60 molecule was imaged with high precision. Hückel molecular orbital theory calculations accurately predicted the shape and characteristics of molecular orbitals observed with dynamic scanning tunnelling microscopy, which were strongly dependent on molecular symmetry, orientation, and adsorption angle. Using qPlus atomic force microscopy, chemical reactivity was probed close to or at the carbon atom positions in the C60 cage. Density functional theory simulations showed that an (iono)covalent bond formed between a carbon atom and the underlying Si adatom was responsible for contrast formation. The pair potential for two C60 molecules was also determined experimentally and found to be in very good agreement with the Girifalco potential (Girifalco, L.A., J. Phys. Chem., 1992. 96(2): p. 858). Using Hückel molecular orbital theory, the mutual orientation of a C60 molecule adsorbed on the STM/AFM tip and a C60 molecule adsorbed on the Si(111)-(7x7) surface was determined via comparison of simulated images to the experimental data. Individual C60 molecules were also manipulated with qPlus atomic force microscopy. Manipulation of single C60 molecules was performed on the Ag-Si(111)-(√3x√3)R30o surface using scanning tunnelling microscopy at room temperature and at 100 K. The interaction was predominantly attractive. Due to weak molecule-substrate interaction, a short-range chemical force between the C60 molecule and the tip was considered to be responsible for the manipulation process.

1. Imaging, Spectroscopy and Manipulation of C60 Molecule on Semiconductor
Surfaces
Cristina Chiutu, Andrew Lakin, Andrew Stannard, Adam Sweetman, Sam Jarvis,
Lev Kantorovich, Janette Dunn, Philip Moriarty
School of Physics and Astronomy, University of Nottingham and Department of Physics, King’s College London
Background: http://3d-desktop-wallpaper.thundafunda.com/3D75219.php; http://funny.pho.to

2. Contents
1. Introduction
1.1. C60 Molecule and Brief SPM History on C60 molecule
1.2. SPM Techniques: Dynamic STM, qPlus sensor
1.3 VT Omicron STM system and C60 on Ag-Si(111) data
1.4. LT Omicron STM/qPlus AFM System in Nottingham
2. Experimental Results
2.1. C60 /Si(111)-(7x7)
2.2. On-tip C60
2.3. C60 on C60

3. Buckminsterfullerene
Rotation axes
• discovered in 1985
• football shape ~0.7 nm diametre
• 60 carbon atoms
• high symmetry
• 3D object at nanoscale
Applications
• electron acceptor
• molecular electronics
• dyads for solar cells
• optics
• biomedical sciences

4. Scanning Tunnelling Microscopy
1. Imaging of molecular orbitals of single molecule
2. Interaction with semiconductor or metallic surfaces
3. Scanning tunnelling spectroscopy
4. Manipulation
5. Self-assembly
Atomic Force Microscopy
1. Molecular resolution
2. Intramolecular features
3. Tip induced manipulation
4. Force spectroscopy
SPM Research on C60 molecules

5. Dynamic STM and qPlus AFM
qPlus Sensor
1. quartz crystal tuning fork
2. two prongs: one fixed and one free
3. transforms mechanical deformation
into electric charge
4. high force sensitivity
5. low noise
6. very small vibration amplitude
7. high stiffness
8. high quality factor
9. oscillation amplitude as
decisive parameter
Dynamic Mode
tip is vibrated at constant amplitude and at
its resonance frequency

6. Experimetal Setup
Variable Temperature STM – Room Temperature

7. Atomic Resolution Imaging
Room Temperature Measurements
Si (111) 7x7
Au(111)-(23x√3)
Au (110) 2x1
200 nm 50 nm
100 nm
6 nm
8 nm
300 nm

8. Atomic Resolution Imaging and Spectroscopy
Room Temperature STM Measurements - Ag-Si (111) √3x√3 R 30o
8 nm
8 nm
25 nm150 nm
Honeycomb network
Scanning Tunnelling Spectra – metallic surface

9. Molecular Resolution Imaging
Room Temperature Measurements – C60 Islands
15 nm
100 nm

10. C60 Manipulation
STM – Room Temperature

11. C60 Manipulation
STM – 100 K Temperature

12. Experimental details:
• phase-locked loop circuit
• twin regulator
• low temperature 77 K
• Q ~ 5000 - 20000
• f = 20 – 25 kHz
• k = 2600 N/m (± 400 N/m)
• p = low 10-11 mbar
Omicron LT STM/qPlus AFM

13. Contents
2. Experimental Results
2.1. C60 adsorbed on Si(111)-(7x7):
- imaging of molecular orbitals by dynamic STM
- imaging individual molecules by qPlus AFM
- force and potential interaction between a silicon-terminated tip and C60
- manipulation of individual molecules by qPlus AFM
2.2. On-tip adsorbed C60
- high sub-molecular resolution by dynamic STM and qPlus AFM
- orientation dependent force spectroscopy
2.3. C60 on C60
- new intramolecular features observed in dynamic STM
- energy potential determined by qPlus AFM for C60 - C60 pair

14. Theoretical Calculations
Hückel molecular orbital (HMO) theory
• simple analytical method
• very quick method of simulating STM images (constant-current)
• Bardeen approach, plots the molecular/atomic orbitals
•constructs the overlap integral for the interaction between tip and sample
• theory can take account of different molecular orientations, orbital splitting

15. C60/Si(111)7x7 – Imaging by d-STM
HMO simulations: Hands et al.,Phys.Rev. B 81, 205440 (2010)
5-fold symmetry
Pentagon down
Single bond down
2-fold symmetry
Double bond down
3-fold symmetry
Hexagon down
3-fold symmetry
Hexagon down
3-fold symmetry
Hexagon down
2-fold symmetry
Double bond down
3-fold symmetry
Hexagon down
• d-STM reveals a rich variety of shapes for molecular orbitals
• positive bias imaging – LUMO of C60
• intramolecular features dependent on adsorption site and tip
apex structure – the molecule orientation can be interpreted

16. C60/Si(111)-(7x7) – Imaging by qPlus AFM
• constant frequency shift mode
• negative frequency-shift setpoint : attractive regime
• C60 imaged as bright spheres
• apparent diametre 1-1.5 nm
• 0 V bias voltage
• amplitude = 0.5-7 nm peak –to-peak
d-STM
qPlus AFM
Chiutu et al.,Chem. Commun., (2011) 47, 10575–10577

17. • interaction between a silicon-terminated tip and
C60 molecules
• force spectra converted from df-vs-z raw data
• short-range chemical force obtained after
removing long-range contribution
• measurements performed at 0 V bias to eliminate
crosstalk
C60/Si(111)-(7x7) – Force Spectroscopy
Chiutu et al.,Chem. Commun., (2011) 47, 10575–10577 (retracted)
.

18. C60/Si(111)-(7x7) – Manipulation with qPlus AFM

19. On-tip adsorbed C60
Si (111) 7x7 side view
Tip
C60
Rotation axis
Si (111) 7x7 top view
Adsorption
angle
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

20. Atomic Orbitals Revealed by Dynamic STM and AFM
Si (111) 7x7
Sm tip
Franz Giessibl et al.: Science 289, 422 (2000); Phys. Rev. B 68,045301 (2003); Science 305, 380 (2004)

21. Manipulation of C60 Molecule
1. Molecule pick-up by scanning with low feedback gains, high speed
2. Vertical transfer to tip by force spectroscopy
3. Vertical manipulation by reducing tip-sample separation
4. Lateral Manipulation to remove a molecule
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

22. Sub-molecular Resolution of On-tip C60
• Transfer molecule to tip either by (attempted) lateral or vertical manipulation.
• Zoom on clean silicon area
• Can observe sub-molecular contrast arising from C60 orbital structure for each
adatom of the (7x7) surface
• Each silicon adatom plays the role of a “mini-tip”.
• Imaging HOMO – positive bias voltage
Si (111) 7x7 – C60-free tip, d-STM
Si (111) 7x7 – C60 functionalized tip, d-STM
2V,380pA, A=0.5 nm -2.25V,760pA, A=0.5 nm
1.6V,360pA, A=0.5 nm 2.7V,500pA, A=1.5 nm
2.8V,1.8nA, A=1.5 nm2.7V,560pA, A=1.5 nm
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

23. Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302
Dynamic STM images of on-tip C60
White lines mark the (7x7) unit cell. (A): Single bond down A = 0.22 nm,V = 1 V,
I =100 pA. (B): C2-Double bond down, white arrow: tilt in molecule position A =
2.8 nm,V = 2.3 V, I =1.8 nA, df = -32 Hz. (C): C3-Hexagon down, white arrow: tilt in
molecule position A = 2.7 nm,V = 2.3 V, I = 0.4 nA, df = -70 Hz. (D): C5-Pentagon
down, A = 3 nm,V = 2.7 V, I = 1.33 nA, df = -29 Hz.

24. Imaging Molecular Orientation
Tip
C60
Rotation axis
C3 case – Hexagon down
Hexagon down 5 degrees 10 degrees 15 degrees 20 degrees
• can engineer particular tip state
• symmetry of lobes depends on molecular
orientation
Adsorption
angle
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

25. Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302
qPlus AFM images of on-tip C60
White lines mark the (7x7) unit cell.
(A): five maxima per silicon adatom , A = 0.5 nm,V = 0 V, df = -22.3 Hz.
(B): two maxima per silicon adatom, A = 0.5 nm,V = 0 V, df = -46 Hz
(C): three maxima per silicon adatom, A = 0.6 nm,V = 0 V, df = -20 Hz.

26. Orientation - Dependent Chemical Force
C5 – Pentagon Down
• short-range chemical force responsible for contrast formation
• interaction between the closest C atom to the surface and the silicon adatom
• “jump-to-contact” effect pointed by the blue and the black arrows
• DFT: the SIESTA code was run for C60 as a probe and a cluster of silicon atoms
approximating the local configuration of a silicon adatom
• theoretical models of the C-Si bond formation, (iono)covalent bond
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

27. C60 on C60 – Energy Potential
• on-tip C60 molecule facing with a pentagon down
• three df(z) spectra were measured with qPlus AFM at different points on the molecule
• potential energy was calculated from df(z) curves
• good agreement with the analytical Girifalco potential for the C60-C60 interaction (solid
green line)
• arises exclusively from the short-range dispersion forces
Chiutu et al.,Phys.Rev.Lett.2012, 108(26), 268302

28. C60 on C60 – Imaging by d-STM
Surface molecule:
double bond down (c-f)
single bond down (h-k)
On-tip molecule:
double bond down,
slightly tilted
Surface molecule:
double bond down, slightly tilted
On-tip molecule:
single bond down, slightly tilted
A. J. Lakin, C. Chiutu, A. M. Sweetman, P. Moriarty, and J. L. Dunn, Phys. Rev. B, 2013, 83(3), 035447

29. Conclusions and Outlook
• dynamic STM and qPlus AFM imaging of on-tip C60 molecule
chemical reactivity was responsible for contrast formation
• high molecular ‘orbital’ resolution with 2-fold, 3-fold and
5-fold symmetry
• theoretical data accurately predict the d-STM experimental results
• C60-C60 energy potential and Si-C60 force spectroscopy with qPlus AFM
• outlook: imaging LUMO of on-tip C60 and
sub-molecular resolution of surface-adsorbed C60 using qPlus AFM

30. Acknowledgements
Force conversion and experiment:
Prof. Philip Moriarty
Dr. Andrew Stannard
Dr. Adam Sweetman
HMO theoretical calculations:
Dr. Janette Dunn
Dr. Andrew Lakin
DFT AFM simulations:
Prof. Lev Kantorovich (King’s College)
Dr. Sam Jarvis
Nanoscience Group Nottingham