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

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

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

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

6. Molecular resolution

7. Intramolecular features

8. Tip induced manipulation

9. Force spectroscopy

SPM Research on C60 molecules

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

Experimetal Setup

Variable Temperature STM – Room Temperature

Atomic Resolution ImagingRoom Temperature Measurements

Si (111) 7x7

Au(111)-(23x√3)Au (110) 2x1

200 nm 50 nm

100 nm

6 nm

8 nm

300 nm

Atomic Resolution Imaging and Spectroscopy

Room Temperature STM Measurements - Ag-Si (111) √3x√3 R 30o

8 nm

8 nm 25 nm 150 nm

Honeycomb network

Scanning Tunnelling Spectra – metallic surface

Molecular Resolution ImagingRoom Temperature Measurements – C60 Islands

15 nm

100 nm

C60 ManipulationSTM – Room Temperature

C60 ManipulationSTM – 100 K Temperature

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

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

Theoretical CalculationsHückel 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

C60/Si(111)7x7 – Imaging by d-STM

HMO simulations: Hands et al.,Phys. Rev. B 81, 205440 (2010)

5-fold symmetryPentagon down

Single bond down

2-fold symmetryDouble bond down

3-fold symmetryHexagon down

3-fold symmetryHexagon down

3-fold symmetryHexagon down

2-fold symmetryDouble bond down

3-fold symmetryHexagon 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

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 AFMChiutu et al.,Chem. Commun., (2011) 47, 10575–10577

• 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).

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

On-tip adsorbed C60

Si (111) 7x7 side view

Tip

C60

Rotation axisSi (111) 7x7 top view

Adsorption angle

Chiutu et al., Phys. Rev. Lett. 2012, 108(26), 268302

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)

Manipulation of C60 Molecule

1. Molecule pick-up by scanning with low feedback gains, high speed2. Vertical transfer to tip by force spectroscopy3. Vertical manipulation by reducing tip-sample separation4. Lateral Manipulation to remove a molecule

Chiutu et al., Phys. Rev. Lett. 2012, 108(26), 268302

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

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.

Imaging Molecular Orientation

Tip

C60

Rotation axis

C3 case – Hexagon downHexagon 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

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.

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

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

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

Conclusions and Outlook• dynamic STM and qPlus AFM imaging of on-tip C60 moleculechemical 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

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