The Great Electron Escape: Measuring Through-Space Charge Transfer in Metal-Molecule Interfaces

Arun Batra

Prof. Latha Venkataraman Lab

Motivation

•  Through-space charge transport is important in: – Organic electronics/photovoltaics

– Biocomplexes (eg. Chromophores), Molecular wires

– Multilayer Graphene devices, molecular electronics, nonlinear optical phenomena…

[2,2]Paracyclophane (22PCP) // Au(111)

[4,4]Paracyclophane (44PCP) //Au(111)

3Å 4Å

Molecular systems

Experimental Techniques

•  X-Ray Photoemission Spectroscopy (XPS) •  Near-Edge X-Ray Absorption Fine Structure

Spectroscopy (NEXAFS) •  Resonant Photoemission Spectroscopy

(ResPES)

X-Ray Photoemission Spectroscopy (XPS)

Occupied Levels

Unoccupied Levels •  Measure Ekinetic of

photoelectrons •  Convert to Ebinding via

hν – Ekinetic = Ebinding hν

LIGHT IN ELECTRONS OUT

Film Morphology (XPS)

Film Morphology (XPS)

•  XPS for Monolayer looks like multilayer, except for ‘screening’ shift. Both molecules adsorb weakly

•  Similar Ebinding, similar

shape. Both molecules have similar adsorption energies

Au(111)

Au(111)

Near Edge X-Ray Absorption Fine Structure (NEXAFS)

LUMO •  Scan photon energy

across C1sà LUMO transition

•  Count all photoelectrons

coming out

•  Multiple decay processes, no Ekinetic information

HOMO

hν

LIGHT IN EXCITATION/DECAY ELECTRONS OUT

Photon Energy (eV)

LIGHT IN EXCITATION/DECAY ELECTRONS OUT

Near Edge X-Ray Absorption Fine Structure (NEXAFS)

LIGHT IN EXCITATION/DECAY ELECTRONS OUT

Near Edge X-Ray Absorption Fine Structure (NEXAFS)

S-polarization

P-polarization

•  Ratio of peak heights gives us tilt angle

•  22PCP ~ 47o •  44PCP ~ 45o

Photon Energy (eV)

Experimental Techniques

•  X-Ray Photoemission Spectroscopy (XPS) •  Near-Edge X-Ray Absorption Fine Structure

Spectroscopy (NEXAFS) •  Resonant Photoemission Spectroscopy

(ResPES)

•  22PCP and 44PCP adsorb very similarly on Au(111) and therefore can be compared.

•  Such ‘tunable’ systems are difficult to find!

Measuring Charge Transfer time

•  Excite electron to LUMO (just like NEXAFS)

•  Measure photoelectron intensity from LUMO electron decay

•  Intensity from

monolayer is lower, due to electrons escaping to substrate

•  Ratio of mono/multi intensities gives CT time

Au(111)

Au(111) Multilayer

Au(111)

Au(111)

Au(111)

Au(111)

Au(111)

Au(111)

Monolayer Au(111)

Au(111)

Au(111)

Au(111)

Resonant Photoemission (ResPES)

•  Can we use NEXAFS signal for lifetime? •  Problem: NEXAFS loses all Ekinetic information

(multiple processes contribute to electron count)

•  Solution: Take XPS scans (Ekinetic preserving) across

the C1sàLUMO NEXAFS Peak

Photon Energy (eV)

286.0

285.5

285.0

284.5

284.0

Phot

on E

nerg

y (e

V)

20 15 10 5 0

286.0

285.5

285.0

284.5

284.020 15 10 5 0

286.0

285.5

285.0

284.5

284.0

Phot

on E

nerg

y (e

V)

20 15 10 5 0Electron Binding Energy (eV)

286.0

285.5

285.0

284.5

284.020 15 10 5 0

Electron Binding Energy (eV)

Resonant Photoemission (ResPES) 22PCP 44PCP

Multi Multi

Mono Mono

From these intensity ratios, and the lifetime of the C1s core-hole, we can calculate:

<τ22> = 1.4 fs and <τ44> = 6.0 fs

286.0

285.5

285.0

284.5

284.0

Phot

on E

nerg

y (e

V)

20 15 10 5 0

1.00.50.0

Beyond averaged charge transfer times

•  We excite all carbons, at different distances from surface, does it make sense to quote averages?

•  The excitation changes the LUMO wavefunction

X-ray

absorption

X-ray

absorption

22PCP

44PCP

CT from each Benzene ring

•  Assumption: Bottom rings couple equally for 22PCP and 44PCP

•  Assumption: CT from bottom rings must be at least as fast as from top rings

τ bottom

τ 22top τ 44

top

τ bottom

CT from each Benzene ring

fstop 6.03.222 ±=τ fstop 5044 >τ

fsbottom 3.07.0 ±=τ fsbottom 3.07.0 ±=τ

22PCP is >20X faster

τ bottom

τ 22top τ 44

top

τ bottom

Do these CT times make sense?

•  Compare to the theoretical inter-ring coupling:

Figure S2: Bonding and Antibonding combinations of the xylene LUMO in a) [2,2]paracyclophane and b) [4,4]paracyclophane

References

1. Cumpson, P. J. & Seah, M. P. Elastic Scattering Corrections in AES and XPS. II. Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments. Surface and Interface Analysis 25, 430-446 (1997).

2. Fohlisch, A. et al. Direct observation of electron dynamics in the attosecond domain. Nature 436, 373-376 (2005).

3. Shao, Y. et al. Advances in methods and algorithms in a modern quantum chemistry program package. Physical Chemistry Chemical Physics 8 (2006).

4. Schneebeli, S. T. et al. Single-molecule conductance through multiple pi-pi-stacked benzene rings determined with direct electrode-to-benzene ring connections. J Am Chem Soc 133, 2136-9 (2011).

5. Mortensen, J. J., Hansen, L. B. & Jacobsen, K. W. Real-space grid implementation of the projector augmented wave method. Physical Review B 71, 035109 (2005).

6. Nilsson, A. & Pettersson, L. G. M. Chemical bonding on surfaces probed by X-ray emission spectroscopy and density functional theory. Surface Science Reports 55, 49-167 (2004).

LUMO+3

LUMO+1

�E= 0.21 eV

LUMO+6

LUMO

�E= 1.31 eV a)

b)

Figure S2: Bonding and Antibonding combinations of the xylene LUMO in a) [2,2]paracyclophane and b) [4,4]paracyclophane

References

1. Cumpson, P. J. & Seah, M. P. Elastic Scattering Corrections in AES and XPS. II. Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments. Surface and Interface Analysis 25, 430-446 (1997).

2. Fohlisch, A. et al. Direct observation of electron dynamics in the attosecond domain. Nature 436, 373-376 (2005).

3. Shao, Y. et al. Advances in methods and algorithms in a modern quantum chemistry program package. Physical Chemistry Chemical Physics 8 (2006).

4. Schneebeli, S. T. et al. Single-molecule conductance through multiple pi-pi-stacked benzene rings determined with direct electrode-to-benzene ring connections. J Am Chem Soc 133, 2136-9 (2011).

5. Mortensen, J. J., Hansen, L. B. & Jacobsen, K. W. Real-space grid implementation of the projector augmented wave method. Physical Review B 71, 035109 (2005).

6. Nilsson, A. & Pettersson, L. G. M. Chemical bonding on surfaces probed by X-ray emission spectroscopy and density functional theory. Surface Science Reports 55, 49-167 (2004).

LUMO+3

LUMO+1

�E= 0.21 eV

LUMO+6

LUMO

�E= 1.31 eV a)

b)

ΔE22 = 1.3 eV

ΔE44 = 0.21 eV

4044

22 ≅kk

Conclusion •  We go beyond average CT times for molecule-

metal junctions – First measurement of through-space CT time as a

function of coupling – 20X faster CT from the top ring of 22PCP than

44PCP

•  Generalizable to other monolayer or few-layer physisorbed molecular films

A. Batra, G. Kladnik, H. Vázquez, J.S. Meisner, L. Floreano, C. Nuckolls, D. Cvetko, A. Morgante, L. Venkataraman, Quantifying Through-Space Charge Transfer Dynamics in π-Coupled Molecular Systems, in review.

Acknowledgements

•  ALOISA Beamline –  Gregor Kladnik –  Dr. Luca Floreano –  Prof. Dean Cvetko –  Prof. Alberto Morgante

•  Columbia University/BNL –  Dr. Mark Hybertsen –  Dr. Hector Vázquez –  Prof. Latha Venkataraman

Elettra Synchrotron, Trieste, Italy