Multiphoton Photoemission Electron Microscopy of Porphyrin Films on

Article pubs.acs.org/JPCC

Multiphoton Photoemission Electron Microscopy of Porphyrin Films on Nanostructured Ag: Molecular Resonances and Plasmonic Field Enhancement Klaus Stallberg, Gerhard Lilienkamp, and Winfried Daum*

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpcc.5b05828

Institut für Energieforschung und Physikalische Technologien, TU Clausthal, Leibnizstraße 4, 38678 Clausthal-Zellerfeld, Germany ABSTRACT: Dye molecules like porphyrins alter their optical properties upon condensation to a solid film. Besides intermolecular interactions, specific substrates can influence their optical characteristics, especially if the molecular film is in contact to plasmonic nanostructures. We apply multiphoton photoemission electron microscopy (nP-PEEM) with tunable laser excitation to the laterally resolved spectroscopy of magnesium-tetraphenylporphyrin (MgTPP) films deposited on nanostructured silver substrates. The high molecular specificity of nP-PEEM is demonstrated by the observation of a strong resonance at about 425 nm caused by optical excitation of the S0 → S2 transition (Soret band) of the MgTPP molecules. This molecular excitation gives rise to remarkably strong threephoton photoemission. The spectral position of the Soret resonance in our nP-PEEM laser spectra points to reduced excitonic coupling between the surface molecules of the MgTPP film. By comparison of the photoemission intensities from MgTPP on nanostructured and unstructured silver regions, we conclude noticeable plasmon-mediated enhancement of photoemission at resonant Soret excitation even under off-resonance conditions for the localized surface plasmons (LSPs). Combining molecular specificity with high sensitivity to field enhancements, we demonstrate that multiphoton PEEM is an excellent tool for the investigation of solid dye films and their interaction with plasmonic substrates.

■

INTRODUCTION Porphyrin molecules combine versatile geometries and chemical functionalities with interesting optoelectronic and physicochemical properties and have attracted much interest for their potential as energy harvesters. The self-assembly of tailored porphyrin molecules offers potentials for the fabrication of functional nanostructures at a molecular level.1,2 Therefore, intermolecular and molecule−substrate interactions of porphyrins have been extensively studied within the past decade under well-defined conditions in ultrahigh vacuum3−6 as well as at solid−liquid interfaces.7−9 Promising applications of porphyrins include photovoltaic10−12 or photocatalytic13 devices as well as substance-specific sensors.14−16 For sensor applications, adsorbate-induced changes of the absorption characteristics of porphyrin molecular films can be utilized for the detection of chemical species.14,16 For photovoltaic and photocatalytic applications, molecular excitons generated by light absorption provide a source of electrons and holes to initiate a photocurrent or a photocatalytic reaction. Devices with molecular light harvesters have been realized with porphyrin films as light-converting active layers.11,12 The optical and electronic properties of porphyrin thin films are of fundamental interest, as they determine the optical excitation characteristics, relaxation pathways, as well as charge and energy transport to the substrate or the reaction interface. The © XXXX American Chemical Society

excitation and relaxation dynamics of porphyrin molecules in solution have been investigated in detail by time-resolved fluorescence upconversion or transient absorption measurements.17−20 So far, only few studies have addressed the optical properties of the condensed phase of porphyrins in a solid molecular film.21,22 Time-resolved studies of porphyrin films have not been reported to our knowledge. Two-photon photoemission (2PPE) has proven to be particularly useful for the investigation of thin films in the energy and time domain and has successfully been applied to films of phthalocyanines23−25 which are closely related to porphyrins. Combining 2PPE with photoemission electron microscopy (PEEM) opens the way to perform spectroscopic and time-resolved investigations of organic films with a lateral resolution down to 50 nm. Buckanie et al. utilized the high lateral resolution of PEEM and the molecular sensitivity of 2PPE for the investigation of excitons localized on individual anthracene islands.26,27 In their work, photoelectrons were excited at a fixed wavelength of 400 nm. Owing to the high sensitivity of multiphoton processes to local electric fields, PEEM in combination with femtosecond Received: June 18, 2015 Revised: August 10, 2015

A

DOI: 10.1021/acs.jpcc.5b05828 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article


The Journal of Physical Chemistry C laser excitation allows for direct imaging of the electric field enhancement caused by nanoscopic metallic structures. Multiphoton PEEM (nP-PEEM) has proven to be a valuable tool for the investigation of plasmonic excitations with high lateral resolution.28−32 With its sensitivity to both molecular resonances and plasmonic field enhancement nP-PEEM is a distinguished method for studying the influence of plasmonic excitations on the photophysics of porphyrin thin films. In the present work we extend nP-PEEM to the laterally resolved laser spectroscopy of molecular films by variation of the exciting laser wavelength. While laser spectroscopic nPPEEM has strong potentials for the spectromicroscopy of molecular layers, it is quite demanding on the instrumental side: variations of the laser wavelength change the characteristics of the laser pulses, especially intensity and pulse duration, and thus strongly influence the photoemission yield. Hence, for a quantitative comparison of nP-PEEM spectra and images obtained for different excitation wavelengths, a proper normalization of photoemission intensities is indispensable. Munzinger et al. performed wavelength scans of the photoemission from individual Ag clusters and used the background photoemission from areas between the Ag clusters as an in situ reference signal.28 Such an in situ reference measurement excels in accuracy as it accounts for the laser pulse characteristics directly at the sample surface. Unfortunately, this method is only applicable if the reference area on the surface gives rise to a smooth wavelength dependence of photoemission without pronounced spectral features in the respective tuning range. This condition is certainly not met if the sample surface is covered with dye molecules with strong absorption bands in the relevant tuning range. Accordingly, we address the problem of a suitable normalization of nP-PEEM and introduce a new normalization method by using the two-photon photoemission signal from a gold sample as a reference signal. We apply nP-PEEM to thin films of magnesium-tetraphenylporphyrin (MgTPP) (Figure 1) on structured silver

emission and optical absorption spectra are related to the spatial extent of excitonic coupling in MgTPP films. Moreover, by comparison of the excitation spectra of the nanostructured and unstructured silver regions, we are able to assess the influence of plasmonic enhancement on multiphoton photoemission from MgTPP.

■

EXPERIMENTAL DETAILS Sample Preparation. Nanostructured silver samples were produced by a nanosphere lithography process on Si(100) with a native surface oxide. After cleaning the substrate with acetone and isopropanol, it was immersed in a hot mixture of H2SO4 and H2O2 for about 20 min and washed several times in boiling ultrapure water afterward. This treatment efficiently removes organic contaminations and additionally creates a hydrophilic surface, which is advantageous for the consecutive deposition of nanospheres from aqueous solution. A drop of polystyrene (PS) nanosphere solution (sphere diameter: 112 nm, concentration: 1012 mL−1) was deposited onto the substrate and subsequently dried in air for several hours. In some regions of the surface the PS spheres arranged in a closed-packed structure providing a suitable mask for the lithographic process. The silicon substrate with the PS sphere mask was then transferred into a vacuum chamber, and a 20 nm thick silver layer was deposited onto the substrate by thermal evaporation from a Knudsen-type evaporator at a pressure of 5 × 10−7 mbar. After removal of the PS sphere mask by ultrasonication in tetrahydrofuran for 30 s the sample was transferred into the PEEM chamber (base pressure 50 nm) can be found as well. The regions which have not been covered with PS spheres during the evaporation step exhibit a flat, polycrystalline silver film (upper left region in Figure 2(a)). This sample allows a direct comparison of the photoemission from Ag nanoparticles with that from a smooth Ag film under identical illumination. PEEM images of the same sample as in Figure 2(a) are presented in Figures 2(b) and (c). In 1PPE (Figure 2(b)) the flat silver area appears bright in the PEEM image indicating a strong photoemission, while due to the reduced silver density, the nanostructured region exhibits much weaker emission resulting in a darker appearance. Most of the silver clusters are C


Article


The Journal of Physical Chemistry C too small to be resolved with PEEM. Several larger clusters, however, can be imaged within the structured area. The PEEM contrast between the flat silver film and the nanostructured region completely inverts upon changing the emission mechanism from 1PPE to 2PPE by using 390 nm laser pulses for illumination (Figure 2(c)). The flat region appears dark, while the structured region shows a much higher 2PPE yield due to field enhancement mediated by localized surface plasmons (LSPs) in the Ag nanoparticles. For a quantitative examination of LSP-enhanced 2PPE from the nanostructured part of the sample we performed wavelength scans and generated the photoemission excitation spectra using the gold reference sample for normalization as described in the preceding section. Figure 3 shows two normalized PEEM images of another area on the same sample obtained upon illumination with two different laser wavelengths. As in Figure 2, the field of view (FOV) contains a structured (left) and a flat (right) region. While virtually no photoemission is observed from the flat silver film for all laser wavelengths, except for some particles presumably located on top of the silver film, the photoemission intensity from the structured area strongly increases with the photon energy. This can also be seen from the excitation spectra (Figure 3(c)) of the regions of interest indicated by the red frames in Figure 3. The spectra were obtained by extracting the averaged intensities from the marked areas of the PEEM images for laser wavelengths between 390 and 450 nm. Although limitations in our laser system did not allow us to measure the photoemission yield for excitation wavelengths shorter than 390 nm, a strong increase of photoemission intensity from the nanostructured area for decreasing wavelengths is obvious. This result is consistent with the expectation that the majority of silver clusters has a localized plasmon resonance at wavelengths below 390 nm: the resonance condition ϵ = −2 for a small spherical (free-standing) particle35 (r ≪ λ) and for the dielectric function of silver36 is met at a resonance wavelength of 354 nm. The wavelength dependence of photoemission from Ag nanoclusters in Figure 3(c) is consistent with the result of Munzinger et al., who observed an increase of photoemission from silver clusters for decreasing excitation wavelengths down to 375 nm.28 Multiphoton Photoemission from MgTPP. Before we discuss the photoemission from MgTPP films we briefly summarize relevant optical properties of MgTPP. Optical spectra of MgTPP solved in methanol show a strong absorption band at 421 nm (Figure 4), the so-called Soret band. This absorption band is caused by an electronic transition from the molecular ground state S0 to the second excited singlet state S2.37 The much less pronounced Q-bands around 580 nm correspond to Franck−Condon transitions from the ground state S0 to the first excited singlet state S1.37 Direct transitions from the ground state to triplet states cannot be excited optically because of the condition of spin conservation. Upon condensation into a solid film, the coupling between the aromatic π-electron systems of the molecules gives rise to the formation of excitonic states wich are lowered in energy compared to the Soret band of the free molecule (Jaggregation).21,22 In a UV−vis absorption spectrum of a 20 nm thick MgTPP film on fused silica the maximum of the Soret band is shifted to 440 nm, while the Q-bands are only slightly red-shifted (Figure 4). Figure 5 shows multiphoton photoemission images after deposition of a few monolayers of MgTPP molecules.

Figure 4. UV−vis absorption spectra of MgTPP in solution (dotted line) and of a condensed MgTPP film (solid line). Spectra are scaled and shifted for clarity. Relevant electronic transitions are depicted in the inset.

Compared to the photoemission from the bare Ag substrate, a drastic increase of photoemission upon adsorption of MgTPP is observed. This increase is not obvious from Figure 5 because the detector sensitivity had to be decreased during MgTPP deposition to avoid saturation. At long-wavelength excitation (435 nm) the PEEM contrast between the structured and the unstructured area almost vanishes (Figure 5(a)). Likewise, at shorter wavelengths (410 nm, Figure 5(b)) the contrast is reduced compared to the MgTPP-free PEEM images in Figure 3. These results indicate that the MgTPP molecules, which homogeneously cover the whole surface, are now the prevailing source of photoelectrons, which is also confirmed by a measurement of the intensity dependence discussed below. In Figure 5(a) a pronounced wave-like modulation of the photoemission yield is apparent at the border between the structured and the unstructured area in the field of view. This pattern stems from the interference of the incident laser light with the near field produced by propagating surface plasmon polaritons launched at the edge of the continuous silver film.30,32 The pattern is present also in the 2PPE images of the MgTPP-free sample (Figure 3) but much less pronounced as the emission from the smooth silver film is much weaker without MgTPP coverage. Wavelength scans of the photoemission from the MgTPPcovered sample are presented in Figure 5(c). Comparing these photoemission spectra to the absorption spectra in Figure 4, the S0 → S2 excitation of MgTPP is readily observed as a Soret resonance also in the photoemission experiment. Assuming that the ionization potential of MgTPP is comparable to that of ZnTPP (about 6 eV6), at least a three-photon process is nessecary to generate photoelectrons from MgTPP at excitation wavelengths near the Soret resonance, while for excitation at shorter wavelengths the more effective two-photon photoemission process is possible. The change of the order n of nphoton photoemission from MgTPP for wavelengths close to the Soret band is documented in Figure 6 which displays the laser power dependence of the photoemission yield together with schemes of possible photoexcitation pathways. For 430 nm excitation the photoemission intensity follows a mostly cubic power law with respect to the exciting laser power, indicating 3PPE (Figure 6(a)). The dominating photoemission process is D


Article

The Journal of Physical Chemistry C


Figure 5. PEEM images of the same sample area as in Figure 3 after deposition of MgTPP. Illumination with (a) 435 nm and (b) 400 nm laser light. (c) Excitation spectra of regions of interest ROI1 and ROI2 in (a) and (b).

pulses, photoemission from the incessantly populated T1 occurs as an effective 2PPE process. Comparing the strengths of 2PPE and 3PPE at 430 nm excitation, the dominating 3PPE process points to the MgTPP molecules as the prevailing source of photoelectrons as already noted above. For a wavelength of 400 nm the laser power dependence is purely quadratic as expected for a 2PPE process (Figure 6(b)). Besides direct, nonresonant 2PPE from S0, emission from T1 and from the silver film could also contribute to the 2PPE yield. Assuming an ionization potential of 6 eV for MgTPP, the photon order n of n-photon photoemission should change from n = 2 to n = 3 at 412 nm, which is consistent with our results. Notwithstanding, the excitation spectrum in Figure 5(c) is normalized for all wavelengths by means of 2PPE from the gold reference. It is a remarkable result that despite “overnormalization” at resonant Soret excitation the normalized 3PPE yield from flat Ag is still six times stronger than the yield from the off-resonant 2PPE process at shorter wavelengths. Accordingly, with a photon-order-consistent but experimentally unfeasible normalization the molecular Soret resonance would arise even stronger. This exceptionally strong three-photon photoemission even should allow for the detection of submonolayers of MgTPP in PEEM. Another notable observation is the lack of a pronounced redshift due to excitonic coupling typically observed for the molecular Soret resonance of MgTPP solid films21,22 (Jaggregation). On the flat silver area (ROI1 in Figures 5(a) and (b)), where the molecular film is not exposed to LSP-enhanced optical fields, MgTPP exhibits a photoemission resonance at 425 nm (Figure 5(c)). Compared to the marked red-shift of the maximum absorption at 440 nm for the solid MgTPP film in the UV−vis absorption experiment (Figure 4), the Soret resonance in the PEEM experiment appears much less redshifted. The different spectral positions of the Soret bands can be related to the difference between a photoemission experiment, in which only the outermost molecular layers contribute to the measured signal, and an optical absorption experiment, where the bulk of buried molecules inside the molecular film dominates the spectrum. Due to the lower coordination of the outermost molecules at the surface the excitonic coupling is reduced, and the red-shift of the Soret band in the photoemission spectra in Figure 5(c) is much less pronounced compared to the absorption spectrum. The dramatic effect of localized surface plasmons on 2PPE from porphyrin-free Ag nanoclusters, documented in Figure 3(c), has been discussed above. Multiphoton photoemission

Figure 6. Photoemission yield from MgTPP on a flat Ag film area illuminated with laser light of (a) 430 nm and (b) 400 nm wavelength. Schemes on the right illustrate the possible photoexcitation pathways.

obviously 3PPE from the molecular ground state S0 via nearresonant excitation into S2 as an intermediate state. Electrons which are emitted from the S1 state after relaxation from S2 with a relaxation time much shorter than the period between two consecutive laser pulses also contribute to the 3PPE yield (S2 lifetime < 2 ps for ZnTPP solved in benzene19 versus a 12.5 ns laser pulse period). A comparatively small quadratic contribution to the power dependence in Figure 6(a) indicates an additional, weak 2PPE process. 2PPE can occur via two pathways: first, with a two-photon energy of 5.76 eV for excitation at 430 nm, direct 2PPE from the silver Fermi level EF is possible for sufficiently thin MgTPP films. The second pathway involves intersystem crossing from the S2 singlet state to the triplet system. As the lifetime of T1 is orders of magnitude longer38 than the period between consecutive laser E


Article


The Journal of Physical Chemistry C from porphyrin films is also significantly enhanced for the nanostructured substrate in comparison to the flat Ag surface: for wavelengths shorter than 405 nm the excitation spectra in Figure 5(c) reveal more than six times stronger 2PPE intensity from the nanostructured area compared to the unstructured region. As for the porphyrin-free samples, this ratio decreases with increasing wavelength due to the increasing spectral distance to the LSP resonance (

Multiphoton Photoemission Electron Microscopy of Porphyrin Films on

Recommend Documents