Electrically-Driven Vibronic Spectroscopy with Sub-Molecular Resolution
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Electrically-driven vibronic spectroscopy with sub-molecular resolution Benjamin Doppagne1, Michael C. Chong1, Etienne Lorchat 1, St´ephane Berciaud1, Michelangelo Romeo1, Herv´eBulou1, Alex Boeglin1, Fabrice Scheurer1, Guillaume Schull1∗ 1 Universit´ede Strasbourg, CNRS, IPCMS, UMR 7504, F-67000 Strasbourg, France, (Dated: August 9, 2018) A scanning tunneling microscope is used to generate the electroluminescence of phthalocyanine molecules deposited on NaCl/Ag(111). Photon spectra reveal an intense emission line at ≈ 1.9 eV that corresponds to the fluorescence of the molecules, and a series of weaker red-shifted lines. Based on a comparison with Raman spectra acquired on macroscopic molecular crystals, these spectroscopic features can be associated to the vibrational modes of the molecules and provide a detailed chemical fingerprint of the probed species. Maps of the vibronic features reveal sub- molecularly-resolved structures whose patterns are related to the symmetry of the probed vibrational modes. PACS numbers: 78.67.-n,78.60.Fi,68.37.Ef Near infrared [1], Raman [2] and low-temperature of the STM is lost. fluorescence [3] spectroscopies are powerful optical ap- With this work we demonstrate that spatial resolution proaches to gather detailed chemical, structural or en- and detailed vibronic spectroscopy can be combined vironmental information on organic systems. Extremely in a single experiment without the need for an optical sensitive to vibrational modes, it is generally considered excitation. Following an experimental approach reported that these techniques provide an unambiguous finger- recently [13{15], we address the optoelectronic proper- print of the probed species. Conversely, the scanning ties of zinc-phthalocyanine (ZnPc) molecules decoupled tunneling microscope (STM) enables imaging organic or from a Ag(111) surface by a thin insulating layer of salt inorganic objects with atomic resolution but lacks chem- (NaCl). STM-LE spectra of these molecules reveal a ical sensitivity. Pechenezhskiy et al. recently developed sharp and intense emission line around 1.9 eV which an infrared STM able to detect the vibronic signature corresponds to the fluorescence of the zinc phthalocya- of monolayer assemblies of molecules [4]. This method, nine. Our spectra also show several weaker emission however, could not yet reach single-molecule sensitivity. lines at lower energy. Extremely well reproduced by Nearly at the same time, Zhang et al. made an important Raman spectra acquired on bulk ZnPc aggregates and step towards combining the chemical sensitivity of Ra- fluorescence measurements on ZnPc trapped in frozen man spectroscopy with the spatial resolution of the STM matrices [16], these emission lines are associated to the [5]. They performed tip-enhanced Raman spectroscopy vibrational modes of the molecule and constitute an (TERS) of molecules with sub-nanometric resolution us- accurate spectroscopic fingerprint of the probed species. ing the tip of a STM as a plasmonic antenna to amplify Sub-molecular spatial variations of the vibronic peak a laser excitation. intensities were observed by scanning the molecule with Because they can be confined to atomic-scale pathways, the STM tip. In contrast to resonant TERS maps we propose here to use electrons rather than photons [5] that are insensitive to the probed vibronic mode as an excitation source of the vibronic signal. This ap- [17], the patterns in our STM-LE vibronic maps reflect proach is based on our recent observation of a large num- the symmetry of the considered modes, an effect that ber of faint vibronic peaks in the low-temperature flu- is interpreted in the framework of vibronic coupling orescence spectra of a single-molecule excited by STM theory[18]. [6, 7]. A comparison to calculated vibrational spectra led us to suggest that these faint peaks correspond to the The STM data were acquired with a low temperature different molecular vibration modes, in contrast to ear- (4.5 K) Omicron setup operating in ultrahigh vacuum arXiv:1612.04653v1 [cond-mat.mes-hall] 14 Dec 2016 lier STM-induced light emission (STM-LE) experiments adapted to detect the light emitted at the tip-sample where similar features pertained to the harmonic progres- junction. The optical detection setup is composed of a sion of a single mode [8{12]. However, because of the low grating spectrograph coupled to a cooled CCD camera intensity of the signal and the lack of comparable spectro- and provides a spectral resolution of ≈ 1 nm [6]. Tung- scopic data for this molecule in the literature, a definitive sten STM-tips were introduced in the sample to cover conclusion regarding the nature of the vibronic features them with silver and to tune their plasmonic response. could not be reached. Moreover, in this experiment, the The Ag(111) substrates were cleaned with successive molecular emitter is suspended between the tip and the sputtering and annealing cycles. Approximately 0.5 sample of the STM junction, and the spatial resolution monolayer of NaCl was sublimed on Ag(111) kept at 2 room temperature, forming square bi- and tri-layers. (a) (b) Eventually, zinc-phthalocyanine (ZnPc) molecules were evaporated on the cold (≈ 5 K) NaCl/Ag(111) sample in the STM chamber. DFT calculations of single ZnPc were carried out at the B3LYP/6-13G level in the full D4h geometry to determine the vibrational modes and their symmetries [19]. The calculated vibrational frequencies 2 nm were scaled by 0.9613 as recommended [20]. Figure 1(a) is an illustration of the experiment where an STM tip is used to excite the luminescence of ZnPc (c) molecules adsorbed on a 2ML-NaCl/Ag(111) substrate. Figure 1(b) is an STM image of a single molecule, a dimer and a tetramer of ZnPC molecules assembled on this surface. The molecular arrangements are formed by STM-tip manipulation following the procedure estab- lished in ref. [13]. The optical spectrum in Fig. 1(c) is obtained by locating the STM-tip on the extremity of the tetramer (see the arrow in Fig. 1(b)) and applying a sample voltage of V = - 2.5 V with a current setpoint I = 0.75 nA. The electroluminescence spectrum shows an intense emission line at 663 nm { the 0-0 transition { and several red-shifted peaks of lower intensity (magnified by a factor 50 in the top part of the panel). The main emission line is assigned to the emission of an excitonic state delocalized over several molecules [13]. The weaker emission lines (which were not reported in Ref.13) are (d) STM-LE Raman Ar matrix DFT represented as a function of their energy shift from 475 474 490 471 B2g the 0-0 line chosen as origin of the abscissa. They are 579 580 598 579 A1g 679 669 689 664 A compared in Fig. 1c to an experimental Raman spectrum 1g 752 738 751 734 B of a bulk ZnPc crystal acquired for close-to resonance 1g 837 817 845 823 B excitation. The number, the energies and the relative 1g 942 938 945 923 B intensities of the different peaks are nearly identical 2g 1144 1130 1150 1165 B1g in these two spectra. These data are also in excellent 1209 1210 1221 1195 B agreement with photoluminescence spectra acquired 2g 1335 1330 1346 1333 A1g on ZnPc molecules isolated in cryogenic matrices [16]. 1438 1420 1448 1429 B Based on a comparison with DFT calculations, the 1435 1g 1509 1497 1525 1528 B emission lines can be precisely assigned to distinct 1g vibrational modes of the ZnPc molecule (Fig. 1(d)). Our STM-induced light emission spectra therefore constitute FIG. 1: (a) Sketch of the STM-induced emission experi- ment. (b) STM image (I = 30 pA, V = - 2.5 V) of a sin- a precise chemical fingerprint of the probed species. gle ZnPc molecule (lower left), a dimer (lower right) and a linear tetramer (top right) of ZnPc molecules deposited on In Fig. 2(b) we display the STM-LE spectra recorded NaCl/Ag(111). (c) STM-induced light emission spectrum for linear arrangements made of 1 to 4 ZnPc molecules (black line) of the ZnPc linear tetramer (I = 0.75 nA, V = - (Fig. 2(a)). The 0-0 peak sharpens and shifts to lower en- 2.5 V, t = 300 s, tip located at the white arrow in (b)). The ergies with the number of molecules in the chain. These right part of the spectrum is represented magnified by a factor of 50 and compared to a resonance Raman spectra acquired effects are the direct consequence of the coherent coupling on a ZnPc crystal at room temperature and in air (excitation of the molecular dipoles [13]. While the vibronic peaks wavelength 632.8 nm). The raw (smoothed) data appear in appear at different wavelengths (Fig. 2(c)) depending on grey (black) in the magnified spectrum. (d) Frequency shifts the length of the linear arrangement, Fig. 2(d) reveals an (in cm−1) of the vibronic peaks in the STM-LE and Raman invariant energy shift of the vibronic modes with respect spectra displayed in (c), in the fluorescence spectra of ZnPc to the 0-0 line. Moreover, the widths of the vibronic molecules trapped in frozen Argon matrices taken from Ref.16 peaks decrease when the number of molecules in the lin- and assignment to vibrational modes of the ZnPc with their respective symmetry based on DFT calculations. ear cluster increases, following the same trend observed for the 0-0 line. These observations suggest that the 0-0 and the vibronic emissions correspond to radiative tran- 3 Photon energy (eV) (a) (b) Wavelength (nm) 1.92 1.9 1.88 1.86 1.84 a 650 660 680 700 720 740 4 FWHM 20 (meV) A 1g B A A 1g 19 1g B 1g 1.0 75 1g B B 15 10 B2g 1g 1g 3 A 2 nm 5 1g ) counts/nC/nm) B 2g ) -1 2 10 3.6 -1 0.8 5 2 1 50 0 0.6 Intensity (10 650 660 670 Wavelength (nm) (c) (d) 0.4 10 15 ) 25 -1 0 0 Intensity (counts/nC/cm 0.2 15 10 Intensity (counts/nC/cm 0 0 counts/nC/nm) 1 10 15 0 0.0 0 0 -200 0 500 750 1000 1250 1500 4 -1 5 Energy shift (cm ) Intensity (counts/nC/cm Intensity (10 0 0 650 675 700 725 0 500 1000 1500 bCurrent map c 0-0 line Wavelength (nm) Energy shift (cm-1) FIG.