Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

Plasmon enhanced luminescence

from fullerene molecules excited by

local electron tunneling

Frederic Rossel, Marina Pivetta, Francois Patthey andWolf-Dieter Schneider

Ecole Polytechnique Federale de Lausanne (EPFL), Institut de Physique desNanostructures,

CH-1015 Lausanne, Switzerland

[email protected]

Abstract: Tunneling electrons from a scanning tunneling mi-croscope (STM) induce luminescence from C60 and C70 moleculesforming fullerene nanocrystals grown on ultrathin NaCl films onAu(111). Intramolecular fluorescence and phosphorescence associatedwith the transitions between the lowest electronic excited state andground state of C70 molecules are identified, leading to unambiguouschemical recognition on the nanoscale. Moreover we demonstratethat the molecular luminescence is selectively enhanced by localizedsurface plasmons in the STM tip-sample gap.

© 2009 Optical Society of America

OCIS codes: (240.7040) Tunneling; (240.6680) Surface plasmons; (300.6390)Spectroscopy, molecular; (300.6280) Spectroscopy, fluorescence and luminescence

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#106070 - $15.00 USD Received 6 Jan 2009; revised 5 Feb 2009; accepted 5 Feb 2009; published 10 Feb 2009

(C) 2009 OSA 16 February 2009 / Vol. 17, No. 4 / OPTICS EXPRESS 2714

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1. Introduction

Light emission from pristine metal surfaces stimulated by scanning tunneling microscopy(STM) has been investigated in the past 20 years [1, 2]. It is now clearly establishedthat the spectral characteristic of the light depends on the substrate as well as on thetip, and that its origin is the radiative decay of a localized surface plasmon (LSP) thatis excited in the tip-sample gap region by inelastic electron tunneling.

Recently, STM-induced light emission (STM-LE) from adsorbed molecules which wereelectronically decoupled from the metal substrate by a thin spacer [3, 4] or severalmolecular layers [5, 6] has been observed. However, the exact role of the LSP in themolecular luminescence mechanism was not clear. Ho et al. [3] reported on an influenceof the plasmon modes on the molecular light emission spectra, but it is only veryrecently that a plasmon enhancement phenomenon was proposed [6, 7]. On the otherhand, the decisive role of plasmons is well known in surface- and tip-enhanced Ramanspectroscopy [8,9], as well as in surface- or nanoparticle-enhanced fluorescence [10,11].In particular, the tip-sample junction or the nanoparticle act as a nanoantenna whichamplifies the local electromagnetic field [12].



Here we demonstrate, on the basis of measurements performed on C60 molecules, thatthe molecular light emission is enhanced in the STM tip-sample gap by efficient couplingto localized surface plasmon modes and report the first observation of fluorescence andphosphorescence from C70 molecules excited by tunneling electrons.

2. Experimental details

C60 and C70 nanocrystals were grown on thin insulating NaCl layers deposited onto anatomically flat Au(111) substrate. NaCl was deposited from a resistively heated evap-orator onto a clean Au(111) surface at room temperature. Subsequently, the fullereneswere sublimated onto the NaCl covered substrate. Experiments have been performedwith a homebuilt ultrahigh vacuum (UHV) STM operating at a temperature of 50 K,using etched W tips. Photons emitted from the tunnel junction were collected by aplano-convex lens (NA= 0.34) near the tip-sample gap along the direction 60◦ withrespect to the surface normal. The collected beam was then transmitted through a viewport outside the UHV chamber and guided simultaneously to (i) a grating spectrometer(50 l/mm) coupled to a liquid-nitrogen-cooled CCD camera for spectral analysis (90%of the signal) and to (ii) an avalanche photodiode to record the total light intensity andto optimize the alignment of the lens with the tunnel junction (10% of the signal). Forthe LE measurements, the tip was positioned over a target location with a fixed tunnelresistance. Spectra were not corrected for the wavelength dependent sensitivity of thedetection system.

3. Results and discussion

A

B

C

D70 nm

(a)

A B

CD40 nm

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Fig. 1. STM topographic images of a C60 (a) and C70 (b) nanocrystal formed on athin NaCl film grown on Au(111) (V = −3 V, I= 0.02 nA). (A) fullerene moleculeson Au(111). (B) fullerene molecules on NaCl/Au(111). (C) NaCl/Au(111). (D)Au(111).

Figure 1 shows STM topographic images of the investigated two types of samples. Thefullerene molecules, C60 in (a) and C70 in (b), aggregate into extended monolayer islandson the bare metal surface (A) and form truncated triangular nanocrystals with a heightof several molecular layers on NaCl (B). NaCl forms large (100)-terminated ultrathinislands on Au(111) of thickness between 2 and 3 monolayers (C).

The next step in the investigation of these complex samples is the characterization oftheir plasmonic properties related to the gold substrate. Figure 2(a) shows the plasmon-



mediated light emission spectrum acquired on the bare Au surface (curve 1). Thislocalized plasmon is excited through an inelastic electron tunneling mechanism andthen decays radiatively [1, 2]. Similar spectra are obtained with a reduced intensity onthe ultrathin NaCl films (curve 2) and with very low intensity on the fullerene monolayerislands grown on the metal surface, as shown for C70 (curve 3). No intrisic luminescenceis observed for the fullerene monolayers on Au(111), as radiationless relaxation takesplace owing to the direct contact with the metal substrate, a process known in both,photoluminescence [13] and STM-LE experiments [5, 14].

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Fig. 2. (a) Plasmon-emission spectra acquired on Au(111), NaCl/Au(111) andC70/Au(111) (I= 4 nA, V = −3 V, t= 60 s). Spectra are vertically shiftedfor clarity. (b) Bias dependance of STM-LE intensity for bare Au(111) andNaCl/Au(111). Spectra have been smoothed and are vertically shifted for clarity.

In Fig. 2(b) the total plasmon-mediated light intensity measured on Au (curve 1)and on a NaCl layer (curve 2) as a function of the applied bias voltage is displayed. Asexpected, the LSP is excited for both polarities [2]. However, the excitation parametersfor bare Au and for the NaCl layer are not the same in the positive voltage range, incontrast to the negative one, owing to different image potentials [15]. Considering theplasmon enhancement effect, the bias-dependent emission indicates that, if the NaCllayer is used as spacer to decouple the molecules from the metal substrate, a detectionof molecular luminescence is only expected for negative voltages larger than −1.8 V andfor positive ones between +1.8 V and +3.2 V.

In order to verify the effect of the plasmon amplification on the intrinsic molecularluminescence, different series of STM-LE spectra have been acquired on the fullerenenanocrystals. Figure 3 shows STM-induced light emission spectra acquired over twoC60 nanocrystals (a) and over their respective NaCl spacer (b), for two different tipconditions (runs 1 and 2). The origin and nature of the molecular emission will bediscussed later. The wavelength of the LSP resonance was changed between the twoexperimental runs by modifying the tip shape with a series of high-voltage pulses acrossthe tunnel junction. Even though the two main peaks are found at similar positions inboth molecular spectra, the spectral characteristics of the LSP influence their relativeintensity. To substantiate this statement, we divide each molecular spectrum by itscorresponding plasmon-emission spectrum. The resulting two normalized curves, shownin Fig. 3(c), identical in both spectral features and relative intensities, demonstrate anenhancement of the molecular luminescence in the STM tip-sample gap by coupling



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Fig. 3. STM-LE spectra for two different experimental runs (1 and 2). (a) Spectraacquired over C60 nanocrystals grown on NaCl/Au(111). Spectra 1 (V = −3 V,I= 2 nA, t= 300 s) and 2 (V = −3 V, I= 3 nA, t= 300 s) are vertically shiftedfor clarity. (b) Spectra acquired on NaCl at the same conditions as in (a). (c)Molecular spectra from (a), divided by the corresponding smoothed plasmon-mediated spectra from (b) and normalized to fit the same scale. Spectra arevertically shifted for clarity.



with the plasmon modes. This finding indicates that the localized surface plasmons areessential for the detection of molecular light emission.

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Fig. 4. (a) STM-LE spectra acquired over a C60 nanocrystal (spectrum 2, V =−3 V, I= 4 nA, t= 60 s) and over the underlying NaCl film (spectrum 1, V =−3 V, I= 2 nA, t= 60 s). (b) STM-LE spectra acquired over a C70 nanocrystal(spectra 2 and 3) and over the NaCl film (spectrum 1), (V = −3 V, I= 1 nA, t=60 s). Spectra are vertically shifted for clarity. The wavelength corresponding tothe pure electronic (0–0) transitions is indicated.

Figure 4 shows STM-induced light emission spectra acquired over a C60 (a) and aC70 (b) nanocrystal, as well as over the NaCl spacer layers. Luminescence from thesupported fullerenes, clearly distinguishable from the LSP emission, is observed fornegative excitation voltages larger than −2.3 V for C60 and −2.5 V for C70. For positivevoltages up to +5 V, no photon emission was detected for both fullerenes.

The observed bias dependence of the STM-induced light emission from the fullerenemolecules is characteristic of a hot electron injection mechanism [3,5] followed by a ra-diative decay associated with the highest occupied molecular orbital-lowest unoccupiedmolecular orbital (HOMO–LUMO) gap of the molecules. This interpretation implies



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Fig. 5. (a) Schematic energy diagram of a double-barrier tunnel junction at neg-ative bias voltage, corresponding to the conditions for luminescence. The simul-taneous population and depopulation of the LUMO and HOMO by tunnelingelectrons creates an electronically and vibrationally excited state of the molecule.(b) Fluorescence channel: the molecule relaxes to the ground vibrational state ofthe lowest electronic excited singlet state S1 followed by a radiative transition tothe singlet ground state S0. Phosphorescence channel: a radiationless transitionoccurs between the singlet state S1 and triplet state T1 with subsequent radiativemolecular transition to the ground state S0.

that the energy levels of the emitting molecules are not pinned to the gold substrate,but shift with the applied bias, as illustrated in Fig. 5(a). When the substrate is neg-atively biased with a voltage above the observed threshold value, the LUMO falls atan energy level lower than the Fermi energy (EF) of the sample and the HOMO atan energy level higher than the Fermi energy of the tip. Electrons tunnel elasticallyfrom the gold substrate into the LUMO through the NaCl barrier (hot electron injec-tion) and simultaneously from the HOMO into the tip through the vacuum barrier tocreate electronically and vibrationally excited states of molecules. The transitions tothe ground state, via the relaxation pathways illustrated in Fig. 5(b), give rise to theobserved molecular luminescence. The fact that the latter is not observed for positiveexcitation voltages, in spite of the presence of the plasmon resonance between +1.8 Vand +3.2 V, see Fig. 2(b), may be due to an asymmetry of the HOMO and LUMOposition with respect to EF and to different characteristics for the tip-molecule andmolecule-substrate double barrier [4, 16]. As a consequence, an energy level alignmentleading to luminescence can not be realized in the range of positive bias correspondingto the plasmon enhancement.

The luminescence spectrum from a C60 nanocrystal shown in Fig. 4(a) (curve 2) ischaracterized by a peak at 738 nm followed by two shoulders, and a second peak at828 nm. On the other hand, the excitation of a C70 molecular nanocrystal gives riseto two substantial different types of optical spectra shown in (b): (i) one characterizedby an intense onset at 690 nm and small broad peaks up to 800 nm (type I) and (ii)a second one covering the spectral range between 700 and 950 nm with an intense lineat 800 nm (type II). It is important to mention that the plasmon resonance, centeredat 620 nm for C60 measurements (curve 1 in a) and at 650 nm (curve 1 in b) in thecase of C70, was unchanged during each set of measurements. Thus, the existence ofthe two types of spectra observed for C70 can not be attributed to a different plasmonenhancement. For measurements on both fullerenes, a rigid shift of the entire spectrumhas been observed, independently of the excitation voltages, covering to the followingwavelength range for the main peaks: between 718 and 745 nm for C60; between 682and 709 nm for C70 spectra of type I; between 800 and 813 nm for C70 spectra of type



II. These shifts are independent of the corresponding plasmon-mediated emission.In order to identify the electronic transitions giving rise to the observed molecu-

lar light emission spectra shown in Fig. 4, we compare our results with laser-inducedphotoluminescence data from fullerene molecules in different media.

Photoluminescence spectra acquired for C60 in rare gas matrices [17, 18], thin films[19, 20], and single crystals [21, 22] have been considered. Although extensively investi-gated, the importance of solid state effects in the description of the luminescence processfor C60 solids has not been completely clarified. Nevertheless, this comparison permitsto attribute the observed spectra to fluorescence associated with the transition fromthe first excited singlet state to the ground state (S1 → S0). In the light of the presentresults on the plasmon-mediated amplification, the previously reported STM-inducedluminescence from C60 molecules [4] has to be reinterpreted. Two different types ofmolecular spectra, assigned to fluorescence and phosphorescence, were claimed to beobserved. The dissimilarity between both spectra is now understood as arising mainlyfrom differently structured LSP spectra, originating from cut PtIr tips with unstableand ill-defined shape used in those measurements. We interpret the observed lumines-cence spectra as fluorescence from C60, although a contribution from a phosphorescencechannel, corresponding to the transition from the first excited triplet state (T1 → S0),cannot be excluded in the low energy part of the spectra.

Intriguingly, the observation of both, fluorescence and phosphorescence is realizedin the case of C70, as deduced from the comparison of our results with laser-inducedphotoluminescence data from dispersed C70 molecules in different media [23–25] andfrom C70 solids [26–28]. The observed spectra of type I are attributed to fluorescence(S1 → S0 transition) and the spectra of type II to phosphorescence (T1 → S0 transition).Because of a relatively low spectral resolution in the present experiment (8 nm), onlythe pure electronic origin of the triplet-to-singlet ground state transition is identified atabout 800 nm. The other spectral features correspond to unresolved multiplet vibronicstructures. Note that the broad peak at 800 nm in the spectrum of type I shown inFig. 4(b) is assigned to the phosphorescence channel rather than to the fluorescenceone. We indeed observed several times emission spectra consisting of a superposition ofcomponents from both radiative transitions.

The spectral shifts discussed above for both C60 and C70 which have been alsoobserved in laser-induced photoluminescence spectra are attributed to a site depen-dence [21, 25]. The local environment or the presence of defects may also be at theorigin of the two types of relaxation channels, fluorescence and phosphorescence, ob-served for C70.

4. Conclusion

In summary, light emission induced by tunneling electrons was observed from C60 andC70 molecules that were electronically decoupled from the metal substrate by a thinNaCl film. The molecular transitions giving rise to light emission were identified bycomparison with laser-induced photoluminescence data from dispersed molecules ormolecular crystals reported in the literature. The key role of localized surface plasmonsin the enhancement of molecular emission is unambiguously demonstrated. The presentobservation of local fluorescence and phosphorescence represents a crucial step towardschemical recognition on the single molecule scale.

Acknowledgments

Financial support of the Swiss National Science Foundation is acknowledged.



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Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling