P a g e 1 18 Imaging and Controlling Coherent Phonon Wave Packets in Single Graphene Nanoribbon s

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P a g e 1 | 18

Imaging and Controlling Coherent Phonon Wave

Packets in Single Graphene Nanoribbons

Yang Luo1, Alberto Martin-Jimenez1, Michele Pisarra2, Fernando Martin3,4,5, Manish Garg1,*,

Klaus Kern1,6

1 Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

2 INFN-LNF, Gruppo Collegato di Cosenza, Via P. Bucci, cubo 31C, 87036, Rende (CS), Italy

3 Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nano), Faraday 9,

Cantoblanco, 28049 Madrid, Spain

4 Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain

5 Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid,

Spain

6 Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

*Author to whom correspondence should be addressed. mgarg@fkf.mpg.de

P a g e 2 | 18

The motion of atoms is at the heart of any chemical or structural transformation in molecules and

materials. Upon activation of this motion by an external source, several (usually many) vibrational

modes can be coherently coupled, thus facilitating the chemical or structural phase transformation.

These coherent dynamics occur on the ultrafast time scale, as revealed, e.g., by nonlocal ultrafast

vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and

controlling vibrational coherences locally at the atomic and molecular scales is, however, much more

challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational

coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be

probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a

scanning tunnelling microscope (STM). In addition to determining dephasing (~ 440 fs) and

population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and

control the corresponding quantum coherences, which we show to evolve on time scales as short as ~

70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals

the quantum couplings between different phonon modes in the GNR.

The periodic collective motion of chemically bonded atoms around their equilibrium configuration is

associated with discrete frequencies that are characteristic of the molecule or material to which these atoms

belong. This is the key for chemical sensing and structural determination based on vibrational

spectroscopies, as, e.g., Raman spectroscopy. By generating and tracking vibrational wavepackets, coherent

Raman scattering (CRS) techniques, such as time-resolved coherent anti-Stokes Raman spectroscopy

(CARS) and impulsively stimulated Raman spectroscopy (ISRS), can track the dynamics of vibrational

motion in bulk molecular ensembles, and thereby unravel isomerization, charge-transfer and conical

intersection dynamics, with femtosecond time resolution1-10. Moreover, coherently manipulating phonon

modes has a unique potential to control the electronic phases of materials. For example, selective excitation

of specific phonon modes has been successfully used to drive complex solids into metastable states with

novel physical properties11,12. Owing to the nonlocal nature of the ultrafast vibrational excitation, however,

the degree of control achieved so far is limited. In femtochemistry, due to the low absorption cross-section

of molecules to optical excitation, the success to drive chemical transformations by selective excitation of

vibrational coherences is restricted to molecular ensembles in gas and liquid phases13. In quantum materials,

on the other hand, the direct correlation between optical excitation and changes in the electronic properties

has proven to be cumbersome.

An important advancement to overcome these limitations is the development of a local probe, allowing for

exciting and probing the selective vibrational modes and their dynamics simultaneously in space and time.

Ideally, this probe should also be able to capture the electronic signature of the molecule/material under

P a g e 3 | 18

investigation. The capability to effectively excite and probe atomic vibrations at the molecular scale can be

realized by enhancing the light-matter interaction with localized surface plasmons14. In particular, by

spatially localizing the incoming light below the apex of a plasmonic nanotip, tip-enhanced Raman

spectroscopy (TERS) has enabled the nanoscale chemical analysis of molecules directly in real space15-34,

i.e., with atomic resolution. The spatial distribution of the vibrational modes inside a molecule25,26, as well

as chemical and structural changes29,30 of a single molecule on top of a metallic surface have been studied

by TERS experiments performed in a scanning tunnelling microscope (STM). However, since vibrational

coherences evolve on a timescale from a few tens of femtoseconds to a few picoseconds35, their time

evolution cannot be tracked by common TERS experiments performed with continuous wave (CW) lasers.

Hence the need for the implementation of time-resolved TERS with the appropriate time resolution to image

chemical reaction dynamics and ultrafast phase transitions in materials36.

The realization of time-resolved coherent Raman spectroscopy in an STM would not only allow for the

observation of ultrafast dynamics in the time domain but would also provide unprecedented spatial and

energy resolutions37, thus allowing for single-molecule dynamical studies. However, despite significant

efforts exploiting the plasmonic enhancement of metallic nanoantennas for CRS in molecules38-43, achieving

such a capability in an STM is still challenging and has not been realized so far. Here we demonstrate that

such an approach is feasible and apply it to obtain temporal information on the motion of vibrational wave

packets in single graphene nanoribbons (GNRs).

GNRs have fascinating electronic and optical properties, which makes them ideal candidates for future

quantum electronic devices44. Thus, understanding vibrational and electronic dynamics in GNRs at the

single-molecule level is of great importance. We show that vibrational (phonon) coherences, i.e., the

vibrational wave packet, induced in a single graphene nanoribbon (GNR) grown on top of a Au(111) surface

can be tracked by three-pulse broadband CARS in an STM. By performing time-resolved measurements,

we determine the phonon dephasing (~ 440 fs) and population decay times (~1.8 ps), and demonstrate that

the beatings between different phonon modes associated with the vibrational wave packets can be

selectively generated by controlling the delay between two ultrashort pulses. Our approach reveals that the

vibrational coherences of a single GNR evolve on time scales as short as ~ 70 fs. By analysing a two-

dimensional frequency correlation spectrum, we are able to assign deterministically the quantum couplings

between various phonon modes of the GNR.

Stokes and Anti-Stokes Raman Spectroscopy of a GNR

In our experiments, the basic signatures (frequencies and their amplitudes) of the Raman modes of a single

GNR were studied by tip-enhanced Raman spectroscopy (TERS) excited both with a CW laser and with

P a g e 4 | 18

ultrashort laser pulses (Fig. 1a) 37. An electrochemically etched Au tip was used for plasmonic enhancement

of the TERS signal. 7-armchair graphene nanoribbons (7-AGNRs) on Au(111) surface were fabricated by

a surface-supported bottom-up synthesis from 10,10′-dibromo-9,9′-bianthryl (DBBA) precursor molecules

45. All measurements were conducted at a temperature of 90 K. An STM image of GNRs is shown in Fig.

1b. When the GNRs lie flat on the surface in the absence of the Au nanotip, we did not detect an appreciable

TERS signal, due to their weak Raman scattering cross-section. Nevertheless, a strong TERS signal appears

when the Au tip is placed on top of the GNR extremity and then approached until atomic point contact

forms. A pronounced enhancement of the TERS signal in molecular point contacts has been earlier reported

in the single C60 molecular junction in STM 46. In this work, all the TERS spectra were measured with the

GNR in atomic point contact with the Au nanotip (see Supplementary Information, Section I for details).

A TERS spectrum measured with CW laser excitation is shown in Fig. 1c. The measured peaks can be

assigned to the various phonon modes of the GNR. The spectral peak located around 1580 cm-1 can be

assigned to the G mode, corresponding to a carbon-carbon stretching mode, whereas the D-like modes at

1230 cm-1 and 1330 cm-1 are related to the edge termination of GNRs 18,47. The general features of the

Raman spectrum are well reproduced by the density functional theory (DFT) simulated spectrum shown by

the purple curve in Fig.1c (see also Supplementary Information, section VI). Apart from the D-like and G

modes, a few additional weaker Raman peaks due to other vibrations can be observed 48, which are also

present in the calculated spectrum. In the theoretical simulations, we have considered finite size 7-AGNR

in slightly bent geometries to account for the formation of the atomic point contact between the GNR and

the Au nanotip. The calculated Raman spectrum barely changes with the bending angle in a range from 2o

to 10o and remains qualitatively the same irrespective of the GNR length (Fig. S8 and Fig. S9 in

Supplementary Information). A detailed description of the theoretical calculations of the Raman active

modes and associated displacements of the atoms is discussed in Section VI of the Supplementary

Information.

In the TERS spectra measured with ultrashort laser pulses, the spectral resolution is mainly determined by

the spectral width of the exciting ~500 fs long laser pulses, hereafter referred to as the ‘probe’ pulses, which

is ~ 30 cm-1 for pulses centered at ~728 nm with a bandwidth of ~1.5 nm 37. Fig. 1d shows the Stokes Raman

spectrum measured with the probe pulses. The Raman peaks for both the D-like mode at 1330 cm-1 and the

G mode at 1580 cm-1 can be clearly identified, with a reduced spectral resolution compared to the CW

TERS. The contribution of the Raman mode at 1230 cm-1 is also visible as a shoulder. A linear dependence

of the intensity of the Stokes Raman signal with respect to the power of the incident probe pulses was

measured, indicating that the TERS signal purely arises from a spontaneous Raman scattering process (see

Fig. S3 in Supplementary Information).

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Page1|18ImagingandControllingCoherentPhononWavePacketsinSingleGrapheneNanoribbonsYangLuo1,AlbertoMartin-Jimenez1,MichelePisarra2,FernandoMartin3,4,5,ManishGarg1,*,KlausKern1,61MaxPlanckInstituteforSolidStateResearch,Heisenbergstr.1,70569Stuttgart,Germany2INFN-LNF,GruppoCollegatodiCosenza,ViaP.Bucci,...

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P a g e 1 18 Imaging and Controlling Coherent Phonon Wave Packets in Single Graphene Nanoribbon s

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