UHV atomic force microscopy and development of a new ... · UHV atomic force microscopy and...

UHV atomic force microscopy and development of a new understanding of the atomic resolution AFM

imaging of molecules

Adam Sweetman

University of Nottingham, Nottingham, U.K.

11Thursday, 15 October 2015 VS-6 2015

Thursday, 15 October 2015 2VS-6 2015

Overview

• Review of scanning probe techniques– Non-contact atomic force microscopy (NC-AFM)

– Intramolecular imaging by tip functionalization

• Intermolecular features, fact or artefact?– Intermolecular feature in NTCDI islands

– Flexible tip modelling

– Control experiments

• Conclusions


• Scanning tunnelling microscopy (STM) – conductors and semiconductors

• Tip follows a trajectory which maintains a predetermined current set-point 𝐼𝑠𝑝• z piezoelectric movement required to maintain current → topographic image

• Feedback used to maintain current at 𝐼𝑠𝑝 (monotonic dependence on 𝑧)

• Most commonly used form of SPM in UHV

SPM 101: STM

NC-AFM Fundamentals


SPM 102: NC-AFM

• Noncontact atomic force microscopy (NC-AFM) – insulators possible

• Tip follows a trajectory which maintains a constant frequency shift set-point Δ𝑓𝑠𝑝• 𝑧 piezoelectric movement required to maintain frequency → topographic image

• Feedback used to maintain current at Δ𝑓𝑠𝑝 (non-monotonic dependence on 𝑧)

Lantz et al., Science 291, 2580 (2001)

chemical

+ background

only

background

Individual Si-Si chemical bonds can be probed with an Si-terminated tip and an Si adatom

difference conversion

Measuring atomic forces


SPM 103: Constant height

• Scan tip at fixed height 𝑧 and measure frequency shift Δ𝑓 at each position

• Topographic features result in change in frequency shift → frequency shift image

• In general, repulsive interactions increase frequency shift, and attractive

interactions reduce frequency shift

10/15/2015 8

A Problem With AFM…solved(?)

AFMs are the tools for studying individual atomic and molecular interactions

However, the exact tip apex structure is often hard to define…

Functionalising with a known molecule/atom can remove this uncertainty… with

some caveats!

Pou et al., Nanotechnology 20, 264015 (2009)

Gross et al., Science 325, 1110 (2009)

10/15/2015 VS-6 2015 9

Intramolecular imaging in NC-AFM

Review article: Jarvis, S.P. Microscopy. Int. J. Mol. Sci. 2015, 16, 19936-19959.

One or two papers resulted…!

Equipment at Nottingham

Omicron Gmbh LT qPlus STM/NC-AFM

With Matrix control system

• Cantilever replaced by quartz tuning fork

• Small amplitudes (~ 0.1 - 0.5nm)

• Combined STM/NC-AFM

• All experiments performed at ~78K or ~4.7K

• No prior tip treatment

Thursday, 15 October 2015 10

Custom Atom Tracking (AT) system

• Feed forward drift compensation allows

true constant height and 3D force

mapping at 77K and 5K.

• Custom scripting for force mapping.

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Motivation: Intermolecular interactions

• Significant interest in molecular self

assembly both in UHV and liquids.

• H-Bonding governs assembly by

influencing directionality of

intermolecular interaction.

• However nature of H-bonding

interaction poorly understood.

• Can AFM be used to measure and

map H-bonding interactions?

J. Theobald et al. Nature 424, 1029-1031 (2003)

A. Stannard et al. Nature Chemistry 4, 112–117 (2012)

Thursday, 15 October 2015

Temirov, R., et al. NJP 10(5), 053012. (2008)

Gross, L., et al. Science, 325 1110–1114 (2009)

Weiss, C., et al. PRL, 105(8), 086103. (2010)

•Tip functionalization as a tool to obtain

intramolecular resolution in SHTM (2008) and

NC-AFM (2009)

Intra– and Intermolecular Contrast

Weiss, C. et al. JACS 132(34), 11864–11865. (2010)

Zhang, et al Science 342 (6158), 611-614. (2013)

•Intermolecular features observed using SHTM

(2010), NC-AFM (2013).

Some book keeping….

13

NTCDI deposited onto Si(111) and Ag:Si(111) at RT in UHV, imaged at 77K or 5K

Ag:Si(111) - Passivated – weak vdW interaction, diffusion and island formation.

True constant height imaging – AT used to compensate thermal drift

Vgap~ 0V (No It detected)

A0 ~ 0.1 – 0.25 nm

Forces extracted by

Df(z) measurement (not constant height slices)

On-off to recover SR Df

Sader-Jarvis to recover U, F

All data shown raw (no filtering)

NB: (Energy numerically differentiated using

Lanczos differentiator filter to obtain force)

A.Sweetman et al. Phys. Rev. B 87, 075310 (2013)

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NTCDI on Ag:Si(111)


At high coverages molecules form islands with structure influenced by

hydrogen bonding between molecules

Images acquired at 77K

Intermolecular observed between molecules ‘at the positions where we

would expect hydrogen bonding to occur’ - similar to previous STHM

and NC-AFM experiments.

STM AFM(Constant Df)

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A.Sweetman and S.P. Jarvis et al. Nature Comms. 5, 3931 (2014)

AFM(Constant

height)


NTCDI on Ag:Si(111)

• Clear intermolecular contrast in networks at expected H-bonding locations.

• No deliberate functionalization of tip.

• Striking sub-molecular contrast similar to CO/Xe/H2 terminated tips.

• Images acquired at 5K and 77K.

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3D Force mapping

• Approx. 1000 individual

measurements

• Approx. 29 hours measurement time.

• Drift corrected by atomic tracking/feed

forward.

• Force profile over C=C bond and H-

Bond very similar.

• Contrast arises in the repulsive

regime. (No sub-molecular contrast in

attractive regime).

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Understanding the contrast

• Simulated spectroscopy performed

using CP2K DFT code.

• Local vdW interactions included via

Grimme approximation.

• NTCDI tip with Oxygen down

provides best agreement with

experimental measurements.

• Good quantitative agreement with

experiment – critically the ordering of

curves in repulsive regime matches

experiment.

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Apparent Inter-molecular contrast?

P. Hapala et al., PRB. B 90, 085421 (2014)

S.. Hämäläinen et al PRL 113, 186102 (2014)

• Simulations of CO tip interacting with NTCDI network using DFT geometries

• Simple LJ force field model: no C-C or H-bonding taken into account.

• Fixed tip does not show strong intermolecular contrast...

• Allowing tip to relax produces apparent intermolecular contrast.

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• Intermolecular contrast can also occur in locations where no

Hydrogen bonding is expected to occur

• It is clear relaxation at the tip must be taken into account for

imaging interpretation


NTCDI on Ag:Si(111)

We also observe intermolecular contrast between molecules arranged in non-

equilibrium configurations where we do not expect hydrogen bonding to be

present…


Full relaxation vs. fixed geometry/electronic density

(DFT NTCDI tip on NTCDI island)

Relaxed vs. Fixed geometries

• NTCDI tip with relaxation (DFT), CO tip with relaxation (Force field)

• Fixed geometries in both instances only produce small quantitative effect on forces in

repulsive regime versus calculations with relaxation, qualitatively similar F(z) curves.

• Relaxation effect difficult to detect from single F(z) despite strong effect on x-y images.

Fz

(eV

/A)

Z(A)

Fixed tip vs. Flexible tip

(Force field CO tip on NTCDI island)

Force field results courtesy Prokop Hapala et al. (unpublished results)


Experimental 3D force field Simulated data for z = 2.8 Å

Quantitative comparison

C-C Bond

H Bond

- Remarkably still have reasonable

quantitative agreement.

- z=2.8A: point before we start to see

significant deflection in relaxed NTCDI

tip simulations.

- Close approach, but without significant

deformation.


What about simple convolution?

- Even in the absence of relaxation a sufficiently broad tip and/or surface

wavefunction/density must lead to some broadening by simple convolution.

Simulated data courtesy of Prokop Hapala and Pavel Jelinek Nanosurf Lab, FZU, Prague (unpublished results)


- From left to right:

- Experimental results

- DFT with fixed geometries

- Simple LJ (12-6) model with fixed geometries

- XY line profiles through 3D force fields show relative magnitude of C=C bond and H-bond

regions agrees within 10-20% between experiment and different simulations even without

tip relaxation.

- Simple charge density arguments do not reproduce this similarity with experiment.

3D force fields: experiment vs. simulation


Control experiment 1 – junction without H-bond

• Hydrogen bonded assembly on metal surface, however design of network also brings non-

hydrogen bonding parts of the molecule in close proximity

• Hydrogen bonding between C=N and C=H

• No Hydrogen bonding between C=N and C=N

• Similar contrast observed in both junctions….

• Contrast reproduced by flexible tip L-J simulations with no consideration of electronic

density…

S.. Hämäläinen et al PRL 113, 186102 (2014)


Control experiment 2 – C60 islands with C60 tip

• C60 closed packed islands – intermolecular interaction is pure dispersion (no charge

density between molecules)

• Probable C60 terminated tip (contrast observed on several surfaces with adsorbed C60 )

• Scaled up system – replace atoms with C60 and increase intermolecular spacing by

approximately the same ratio

• Same type of intermolecular features observed

• Contrast only reproduced in simulations assuming a flexible C60 terminated tip

• Intermolecular contrast can arise in the absence of intermolecular bonding

S.P. Jarvis et al. PRB (R). Accepted (2015)

Conclusions

• NC-AFM offers unparalleled resolution of planar organic molecules

– Constant height operation

– Well defined passivated tips

– Pauli repulsion regime

• Intramolecular contrast:

– Surprisingly good qualitative agreement with simple L-J modelling + tip flexibility

• Intermolecular contrast:

– Features cannot be explained by charge density alone

– Reproduced experimentally where no charge density is present

– Reproduced in simulation without consideration of bond charge density

• Do we ‘see’ Hydrogen bonds in NC-AFM?

– Probably not, but the picture is complicated…

2626Thursday, 15 October 2015 VS-6 2015

Acknowledgements

Nottingham Nanoscience

Sam Jarvis

Philip Moriarty

Ioannis Lekkas

Philipp Rahe

Kings College London

Lev Kantorovich

Hongqian Sang

Funding

The Leverhulme trust

EPSRC


Thank you (and questions)!

Thursday, 15 October 2015 DREAMS Warwick 2013 28


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