Ultrafast Charge Transfer Pathways Through A. Prototype Amino-Carboxylic Molecular Junction.

Transcription

1 Supporting Information Ultrafast Charge Transfer Pathways Through A Prototype Amino-Carboxylic Molecular Junction. Gregor Kladnik, ± Michele Puppin, ǂ, Marcello Coreno, Monica de Simone, Luca Floreano, Alberto Verdini, Alberto Morgante, ǂ, Dean Cvetko, ±, Albano Cossaro ± Faculty of mathematics and physics, University of Ljubljana, Jadranska ul. 19, 1000, Ljubljana, Slovenia; ǂ, via A. Valerio 2, I-34127, Trieste, Italy; CNR-IOM Laboratorio TASC, Basovizza SS-14, km 163.5, I-34012, Trieste, Italy; CNR-ISM, UOS Trieste, Area Science Park Basovizza, Trieste, Italy; Contents: Experimental details Chemistry and Morphology of the systems Details of the DFT calculations VB, NEXAFS Details of RPES and CHC method RPES of BA/Au(111) CT dynamics RPES of CA/Au(111). 1

2 Experimental details. The solid state measurements were performed at the ALOISA beamline 1 of the Elettra Synchrotron. The Au(111) surface was prepared by cycles of Ar+ sputtering (1 kev) and annealing up to 750 K. The photoemission signal was detected by a hemispherical electron analyzer in normal emission geometry, with 4 of photon beam incidence angle; the overall energy resolution was about 400 mev. The NEXAFS C K-edge spectra were acquired in partial electron yield by a wide acceptance angle channeltron. The BA/CA/Au system was prepared as described in 2. The measurements were performed at low sample temperature (Ts 180 K). The irradiated area was continuously displaced after each spectrum to minimize effects related to beam-induced damage. Gas phase spectra. Gas phase spectra have been acquired at the gas phase beamline 3, Elettra, Trieste. The absorption spectra (NEXAFS) were acquired by measuring the total ion yield with a channeltron multiplier placed near the ionization region. The photon resolution was set to 60 mev at the C K-edge. NEXAFS photon energy was calibrated to the 1s energy of CO 2. 4 The Valence Band binding energy has been calibrated by measuring the Ar VB as a reference. Chemistry and Morphology of the systems The growth of a ML of BA on CA/Au(111) and on Au(111) has been previously described. 2 We summarize here the description of the two systems in order to facilitate the reading of the present manuscript. In figure S1 the N1s XPS is reported for the CA/Au and for the BA/CA/Au systems. In the pristine CA layer, most of the amino-terminations of the SAM are in their neutral state, characterized by a N1s binding energy of ev. Upon the deposition of a ML of BA, a 2

3 transition occurs and the peak converts to a novel component at ev, which can be assigned to the ionic state of the amino group (NH3+). Figure S1: N1s XPS spectra of the CA/Au (bottom) and BA/CA/Au (top) systems. The amino group converts from its neutral state (Nn component) to it ionic state (Nzw) upon the formation of the amino-carboxylic hydrogen bond. Reprinted with permission from ref. (2). Copyright 2011 American Chemical Society. The ionic component can be assigned to the formation of the A-C hydrogen bond, as observed in the self assembly of amino-acids 5. To confirm this, the O1s XPS signal has been measured and is reported in left panels of Fig. S2, for the gas phase, BA/CA/Au and BA/Au systems. In gas 3

4 phase, two components are visible, corresponding to the chemically inequivalent oxygen atoms of the carboxylic group. On the contrary, the anchored BA/CA/Au system shows mainly a single component at ev, which is compatible with the carboxylic groups in their COO- state, where the two oxygens are equivalent, and confirms the formation of the A-C hydrogen bond between CA and BA molecules. Figure S2. O1s XPS and NEXAFS spectra taken on the BA gas phase (top panels), BA/CA/Au system (central panels) and BA/Au system (bottom panels) Reprinted with permission from ref. (2). Copyright 2011 American Chemical Society. In the right panels of Figure S2 the C1s NEXAFS is reported for the three systems. In particular, in the two condensed cases, the spectra are reported for both s and p pol (electric field parallel and perpendicular to the surface). In the BA/Au case, the complete dichroism between 4

5 the two spectra indicates that the BA molecules are flat on the surface. In the BA/CA/Au system the dichroism is less evident and from the ratio of the peak intensities in the two polarizations, an adsorption angle of about 47 deg between the BA axis and the surface is calculated. Details of DFT calculations The optimized geometry of the isolated BA molecule was calculated using density functional theory (DFT) with the B3LYP functional and the double-zeta correlation-consistent basis set (ccpvdz) of Duning et al. as implemented in the NWChem computational chemistry package 6. The optimized geometry then served as input to GPAW, a DFT Python code based on the projectoraugmented wave (PAW) method to calculate the core electron binding energies (CEBEs) as well as the X-ray absorption spectra (XAS) 7 9. First we calculated the CEBEs of all inequivalent C1s and O1s sites with the BLYP functional with a cell size of 25.6 Å and grid spacing of 0.2 Å using the delta Kohn-Sham scheme. The full core-hole PAW setups were created with GPAW and default parameter values. Figure S3 shows a comparison of the calculated C1s (blue) and O1s (red) CEBEs with the gas-phase measured XPS (black markers). The simulated peak shapes were obtained by gaussian convolution with FWHM = 0.3 ev. We find a mismatch of 0.25 ev at the main C1s (phenyl) peak and 0.75 ev at the carboxyl peak. These shifts were later used as an empirical correction in the simulation of the X-ray absorption spectra. For the O1s peaks a shift of 0.50 ev was found. 5

6 Figure S3: Comparison of the calculated C1s (blue) core electron binding energies (CEBEs) with the gas-phase measured XPS (black markers). Inset: O1s (red) CEBEs and measured XPS are shown. The simulated peak shapes were obtained by Gaussian convolution of 0.3 ev. The indicated offsets between the calculated and measured C1s peak positions were used as an empirical correction for the X-ray absorption spectra simulation. Next, X-ray absorption spectra at the C K-edge were simulated for each inequivalent C atom separately using the half core-hole transition potential method as implemented in GPAW. The 6

7 calculation parameters were the same as for the CEBE calculations described before. The absolute excitation energies were calculated by adding the delta Kohn-Sham calculated energy of the first (lowest energy) excitation (core -> LUMO) to account for the initial state effect of the different inequivalent C atoms. The absolute energy position was further refined by adding an empirical correction term found from the offsets between the CEBEs and the experimental XPS (Fig. S3) 10. The spatial distribution of the complete set of the calculated Kohn-Sham occupied MOs (isosurface plots) of the BA molecule is reported in Fig. S4 together with the comparison of the calculated density of states (DOS) and the gas-phase measured valence band (VB). The VB features are very well reproduced after shifting the calculated DOS to match the experimental HOMO peak position. 7

8 Figure S4 Representation of the spatial distribution of the calculated occupied MOs, together with the comparison between the calculated DOS and experimental VB spectra. Resonant photoemission Resonant photoemission (RPES) at the carbon K-edge was conducted by taking a series of XPS scans with incident photon energies between 280 ev and 310 ev, in steps of ev. For each photon energy, XPS spectrum covering ~60 ev kinetic energy range was measured to construct a photoemission map as a function of photon energy and electron kinetic energy. The non-resonant spectrum was measured in the pre-edge region at a photon energy of 283 ev; I(hv=283eV, E K ), and was subtracted from each XPS spectrum in the RPES map, I(E K, hv). RPES maps may be shown as a function of binding energy, I(E b, hv) or kinetic electron energy, I(E k, hv,). In the former the photoemission features appear at constant E b, whereas in the latter the Auger like features show up at constant E k. For all RPES measurements, incident light was polarized at 54.7 with respect to the surface normal, which provides the absorption signal independent of molecular orientation. The electron analyzer for RPES was placed at 54.7 from the surface normal and along the photon electric field. Details of the CHC method 8

9 Figure S5 Schematics of the RPES process. X-ray excitation of the core electron into unoccupied molecular orbital LUMO (a). Subsequent core-hole decay with participation of the excited electron, i.e. participator Auger decay (b). Here the final state has a single hole in the valence band, and the electron emission process is degenerate with the direct photoemission. When coupled to the empty continuum of the substrate the excited electron can delocalize prior to the core hole decay (c) which then occurs with a normal Auger emission (d) leaving 2 holes in the molecular VB. Figure S5 reports on the basic processes in the RPES experiment. In the core-hole-clock analysis we compared the resonant photoemission spectra at hv=285 ev and 288 ev for the 9

10 electronically coupled BA in the BA/CA/Au complex, and for the electronically isolated molecules in the BA gas phase. All spectra have been normalized to the integrated Auger intensity. The CT time has been evaluated as = CH Ip/(I p0 -I p ), where CH stands for the corehole lifetime (6 fs for C1s 11 ) and I p0 and I p are participator intensities of the resonant core-hole decay of the BA gas phase and of the BA/CA/Au complex, respectively. Participator intensities have been evaluated from the integrated intensities of the upper valence band peaks (HOMO at 6 ev for the ~285 ev spectrum, and HOMO-2 for the ~288 ev one). The Auger intensity contribution from the CA/Au layer beneath has been evaluated by comparing NEXAFS of the BA, CA/Au and BA/CA/Au spectra (see fig. S8). A simple linear superposition of the pristine BA (multilayer) and CA (monolayer) NEXAFS scans was used to model the NEXAFS signal of the BA/CA/Au complex. In this model we neglected the fact that the photon flux reaching the CA layer is attenuated due to the absorption in the BA overlayer. If considered, this would further reduce the contribution of CA absorption signal in the BA/CA/Au NEXAFS. Therefore the model gives the upper (lower) bound for the calculated CA (BA) signal contribution. We also notice a qualitatively very good agreement between the measured BA/CA/Au NEXAFS and the model fit, which further justifies the use of the simple linear superposition model. Whereas there is almost no CA signal in the absorption resonances from the phenyl group (at ~285 ev), we estimate an upper value for the CA contribution to the BA/CA/Au absorption signal at the carboxyl group (at ~288 ev) not to exceed 10%. This value has been then used to correct the BA Auger normalization in the BA/CA/Au system to yield an upper bound for the electron delocalization time over carboxyl orbitals in the BA/CA/Au complex as 20f. 10

11 Fig.S6 RPES map across the Carbon K- absorption edge for the BA monolayer on Au(111). Upper panel: 2D like color map of the photoemission intensity I(E k, hv). NEXAFS signal across C K-edge is shown in the right hand panel aside. Single resonant photoemission spectra at the excitation from the C of the carboxyl group (hv=288 ev, red arrow) and form the C atoms of the phenyl ring (hv=285 ev; white arrow) are shown in the middle and lower panel respectively, together with the corresponding resonant spectra of the BA gas phase. Participator intensity quenching is indicated and converted into CT times as indicated in the text. 11

12 Fig. S7 CA/Au RPES map across Carbon K- absorption edge. The corresponding NEXAFS spectrum is shown aside in the right hand panel. The absence of any appreciable participator resonances confirms that CA is well coupled to Au with ultrafast delocalization of core excited electrons in the CA empty orbitals. 12

13 Fig.S8 C K-edge NEXAFS comparison for BA multilayer (red markers), CA/Au (dark blue markers) and BA/CA/Au complex (black markers). Fit with linear superposition of BA and CA/Au to the BA/CA/Au is also shown (light blue line). The BA and CA/Au spectra have been scaled to match the fitted intensities of both components in the BA/CA/Au signal. Upper panel residual from the fit. We note that CA/Au contribution to NEXAFS signal at carboxylic excitation (288 ev) in the complex does not exceed 10%. This has been accounted for in the CHC analysis for the CT time estimation. 13

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