Detection of Light Emission from (S)-PTCDI Molecules Adsorbed on

Article pubs.acs.org/JPCC

Detection of Light Emission from (S)‑PTCDI Molecules Adsorbed on Au(111) and NiAl(110) Surfaces Induced by a Scanning Tunneling Microscope Pawel Krukowski,*,† Takuro Tsuzuki,‡ Yuto Minagawa,† Nami Yajima,† Songpol Chaunchaiyakul,† Megumi Akai-Kasaya,† Akira Saito,† Yusuke Miyake,† Mitsuhiro Katayama,‡ and Yuji Kuwahara† †

Department of Precision Science and Technology, ‡Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan ABSTRACT: We report the suppression and enhancement of light emission from (S)-PTCDI molecules (the S-type enantiomer of the chiral binaphthyleneperylenebiscarboxydiimide dimer) adsorbed on Au(111) and NiAl(110) surfaces, which were investigated by scanning tunneling microscopy-induced luminescence (STM-LE). We deposited (S)-PTCDI molecules on a Au(111) surface maintained at low (150 ± 20 K) and elevated (355 ± 20 K) temperatures. At the low temperature, we observed preferential adsorption at face-centered cubic elbows of the herringbone reconstruction and the formation of a quasi-ordered molecular layer with increasing coverage. At the elevated temperature, (S)PTCDI molecules formed highly ordered domains consisting of self-assembled molecules with positional and orientation order. We observed four different phases of ordered domains with different molecular arrangements and orientations on the Au(111) surface. In contrast to the Au(111) surface, the formation of small molecular clusters at random positions was observed on a NiAl(110) surface at the elevated temperature. We observed the strong enhancement of light emission when the STM tip was placed above the (S)-PTCDI clusters adsorbed on the NiAl(110) surface at bias voltages of 2.4 V and above. We discuss the molecular origin of the light detected above the molecular clusters grown on the NiAl(110) surface.

■

INTRODUCTION Optical activity associated with molecular chirality can be studied by different conventional macroscale techniques, such as circular dichroism (CD)1 and optical rotatory dispersion (ORD).2 Unfortunately, owing to the optical diffraction limit and low sensitivity of such macroscopic techniques, it is impossible to investigate optical activity at the single-molecule level. To overcome the disadvantages of these conventional techniques, we apply scanning tunneling microscopy-induced luminescence (STM-LE).3,4 Because we are highly interested in detecting the dissymmetry of circularly polarized luminescence (CPL) resulting from the difference between the emission intensity of right- and left-handed circularly polarized light from chiral molecules at the single-molecule scale using STM-LE, we initially investigated the adsorption and light emission properties of chiral molecules deposited on various surfaces.5 STM-LE is known to be a unique technique that combines high spatial resolution with the spectroscopic analysis of luminescence, allowing the investigation of light emission at the molecular scale. This is because single molecules adsorbed on a surface can emit light from their electronically excited states induced by highly localized inelastic tunneling electrons. Moreover, the light emission intensity from molecules adsorbed on metal surfaces may also be selectively enhanced by localized surface plasmons. This enhancement is because the tunneling electrons excite localized surface plasmons on metal surfaces. One notable study demonstrating the capabilities of STM-LE © XXXX American Chemical Society

revealed vibrationally resolved light emission with submolecular resolution from magnesium porphine (MgP) molecules adsorbed on an Al2O3 buffer layer grown on a NiAl(110) surface.6 Recently, it has been shown that molecular light emission from tetraphenyl-H2-porphyrin (TPP) molecules can be observed in the absence of electron transport through the molecules.7 This unexpected result was explained in terms of plasmon-mediated molecular excitation. However, strong light emission owing to the decay of localized surface plasmons is a major problem in the STM-LE configuration, which makes it difficult to identify weak molecular light emission. To suppress strong tip-induced plasmon light emission from a metal surface and decrease the strong interaction between adsorbed molecules and metal substrates, which leads to the quenching of electronically excited molecular states, decoupling buffer layers including Al2O3,6 NaCl,8 and even C609 are often utilized. Moreover, single molecules adsorbed on a surface can serve as spatially and energetically well-defined nanogates for STM tipinduced localized surface plasmon excitation, leading to enhanced light emission and making the interpretation of light emission much more complicated. For example, the study of single fac-tris(2-phenylpyridine)iridium(III) molecules (Ir(ppy)3) deposited on a C60 buffer layer grown on Ag(111) Received: December 16, 2015 Revised: January 25, 2016

A

DOI: 10.1021/acs.jpcc.5b12313 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Schematic illustration of the synthesis of chiral binaphthylene-perylenebiscarboxydiimide dimer including the molecular structures of the pair of enantiomers. R indicates the hexylheptyl group. Only the (S)-PTCDI enantiomer was considered in our study.

the molecule−substrate interaction is a key factor determining molecular adsorption, and thus the light emission efficiency, we decided to use two different substrates. We used a Au(111) surface as a substrate for molecular deposition owing to the existence of strong localized surface plasmons induced by inelastic tunneling electrons, which may be involved in enhancing molecular luminescence.22 We also used a NiAl(110) surface owing to the possibility of forming an aluminum oxide film, which may serve as a buffer layer in further experiments. An (S)-PTCDI molecule consists of a pair of chromophores based on a PTCDI segment that are linked to a 1,1′-binaphthalene bridge that induces axial chirality, as shown in Figure 1. Additionally, the investigated molecule contains a hexylheptyl group attached to PTCDI segments which increase its solubility in nonpolar solvents.23 Because chiral PTCDI molecules are not commercially available, there have been only few studies on them,21,23−25 despite their very interesting light emission properties. In particular, the detection of luminescence from both enantiomers of chiral PTCDI reveals an approximately 11-fold enhancement of enantioselectivity in the molecular excitation by superchiral light in comparison with that by conventional circularly polarized light.25 It should be emphasized that chiral PTCDI molecules have never been investigated using STM or STM-LE. We observed the formation of four different phases of ordered domains consisting of self-assembled (S)-PTCDI molecules with different molecular arrangements on the Au(111) surface. The STM tip-induced light emission from the highly ordered molecular domains formed on the Au(111) surface was strongly suppressed in comparison with that on bare Au(111). In contrast, the deposition of (S)-PTCDI molecules on the NiAl(110) surface led to the formation of molecular clusters. The enhancement of light emission from (S)-PTCDI clusters adsorbed on the NiAl(110) surface was

showed that the intensity and spatial distribution of the tipinduced plasmon light emission are strongly correlated with the spatial shape and the energy of the molecular orbital closest to the Fermi level, clearly indicating the plasmonic character of light emission.9 Perylenetetracarboxylic diimide (PTCDI) and its derivatives are promising candidates for investigations using STM-LE owing to their many desirable properties including high photosensitivity, electron mobility, thermal stability, photostability, and extremely high luminescence efficiency. PTCDI and its derivatives have been extensively studied in recent years, leading to the fabrication of high-performance optoelectronic devices, such as organic light-emitting diodes (OLEDs),10 thinfilm transistors (OTFTs),11 solar cells (OSCs),12,13 and photodetectors.14 In particular, considerable interest has been aroused in the investigation of the adsorption of PTCDI and its derivatives on various metal, semiconductor, and even insulator substrates including Au(111),15 Cu(100), 16 graphene,17 TiO2(110),18 and NaCl(001)19 using STM. The molecular light emission from single molecules of a PTCDI derivative adsorbed on silicon carbide, which is a wide bandgap semiconductor, has also been observed by STM-LE.20 Here, we present our preliminary investigations of the adsorption, self-assembly formation, and light emission properties of the S-type enantiomer of chiral binaphthyleneperylenebiscarboxydiimide dimer (referred to in this paper as (S)-PTCDI) molecules adsorbed on Au(111) and NiAl(110) surfaces. The (S)-PTCDI and (R)-PTCDI enantiomers of the compound can be seen in Figure 1. Because the (S)-PTCDI molecule is an excellent example of an organic dye exhibiting a large CPL dissymmetry accompanied by high fluorescence emission efficiency in solution,21 which is extremely rare among organic molecules, our choice of the molecular system is easily justified. Because B


Article


Figure 2. STM topography images and corresponding photon integration maps of the Au(111) surface (150 × 75 nm2, 256 × 128 pixels, 7.8 ms/ pixel) obtained at bias voltages of (a) 3.0 V and (b) −3.0 V with the same tunneling current of 1 nA. The Au(111) surface before molecular deposition shows defect-free, atomically flat terraces and step edges with an apparent height of 0.24 ± 0.02 nm. The strong light emission observed can be ascribed to the radiative decay of localized surface plasmons induced by the STM tip. The suppression of light emission above the step edges of the surface is considered to be due to the difference in the local density of states close to the Fermi level.

9,10-imide (3) was prepared by the reaction between 1hexylheptylamine (2) and excess perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) in DMF. The crude product of the reaction between 1-hexylheptylamine and PTCDA included a symmetric dialkyled compound and an asymmetric monoalkyled compound. The monoalkyled compound (3) was purified by column chromatography on a silica gel (eluent: chloroform). The chiral binaphthylene-perylenebiscarboxydiimide dimer (4) was synthesized via a condensation reaction between binaphthyl diamine and 3. Compounds were identified by 1H NMR, IR, and elemental analysis. We utilized commercially available thick Au(111) epitaxial films evaporated on mica (Georg Albert PVD) and NiAl(110) crystal (MaTecK GmbH) as substrates for molecular deposition. The Au(111) surface was cleaned by repeated cycles of 0.7 keV Ar+ ion sputtering for 30 min and subsequent annealing at 800 K for 15 min. The cleanliness of the Au(111) surface was verified by observation of the clear herringbone reconstruction by STM. The NiAl(110) surface was cleaned by repeated cycles of 0.8 keV Ar+ sputtering for 15 min and subsequent annealing at 1270 K for 5 min. All STM images were obtained at 79 K under a vacuum better than 1.0 × 10−8 Pa in constant-current (topography) mode using electrochemically etched Pt/Ir (Unisoku Co.) or Ag (Unisoku Co.) tips and analyzed using WSxM software.26 We thermally deposited (S)PTCDI molecules on the substrates inside a preparation chamber by sublimation using a Knudsen-type organic deposition evaporator (Kitano Seiki Co.) containing a boron nitride crucible. The photoluminescence (PL) spectra of (S)PTCDI molecules dissolved in chloroform were obtained using a Raman spectrometer (Nanofinder FLEX, Tokyo Instruments Inc.).

observed at bias voltage higher than 2.4 V. We consider the molecular character of the light emission from the (S)-PTCDI clusters formed on the NiAl(110) surface.

■

EXPERIMENTAL METHODS We used a commercially available low-temperature STM instrument with an integrated lens system (Unisoku Co., USM1400) operating in ultrahigh vacuum (UHV) that was controlled by a Nanonis BP 4.5 system in combination with a high-voltage amplifier (RHK-SPM 100). Our UHV system consists of separate analysis and preparation chambers with laboratory-built surface-cleaning facilities. The light emitted from a tunneling junction induced by tunneling electrons was collected and parallelized by a plano-convex lens mounted as a part of the microscope head inside the UHV chamber. Because of the possibility of precise motion in three directions, we could align the focus of the lens very accurately. Then the light was transmitted through a viewport, refocused outside the UHV chamber by a second plano-convex lens, and finally transmitted through an optical fiber to high-sensitivity detectors. To obtain the photon integration map, we used a photomultiplier tube (Hamamatsu Photonics, R943-02; wavelength detection range, 160−930 nm) cooled to 240 K. To obtain light emission spectra, we used a grating spectrometer (Roper Scientific, SpectraPro-300i) equipped with a liquid-N2-cooled chargecoupled device camera (Roper Scientific, Spec-10:100B/LN; detection range, 200−1100 nm). Chiral PTCDI molecules were synthesized by modifying a method described previously.21,24 The synthesis was performed in accordance with the schematic illustration shown in Figure 1. Briefly, 1-hexylheptylamine (2) was obtained by the reaction between dihexyl ketone (1) and NH4OAc/NaBH3CN. N-(1Hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydrideC


Article


Figure 3. STM topography image and simultaneously acquired photon integration map of the herringbone reconstruction of the Au(111) surface (30 × 15 nm2, −3.0 V, 500 pA, 256 × 128 pixels, 9.8 ms/pixel). The light emission contrast is strongly correlated with the herringbone reconstruction of the Au(111) surface, indicating stronger light emission above the parallel corrugation lines.

Figure 4. STM topography images of (S)-PTCDI molecules adsorbed on Au(111) at a low temperature obtained at (a) (150 × 150 nm2, −1.0 V, 25 pA), (b) (85 × 85 nm2, −1.0 V, 50 pA), and (c) (20.4 × 20.4 nm2, −1.0 V, 50 pA). (a) The preferential adsorption of the molecules at fcc elbows of the herringbone reconstruction and on the upper part of the step edges was associated with the formation of amorphous molecular islands. Inset: single molecule adsorbed at fcc elbows of the herringbone reconstruction. (b,c) Higher molecular coverage led to the formation of ordered islands exhibiting a single molecular orientation along the ⟨112̅⟩ direction.

■

RESULTS AND DISCUSSION To obtain a spatial view of the light emission distributions from the Au(111) surface induced by the STM tip, we carried out STM imaging while simultaneously acquiring photon integration maps. Figure 2 shows STM topography images and the corresponding photon integration maps of the Au(111) surface before the deposition of (S)-PTCDI molecules imaged at bias voltages of (a) 3.0 V and (b) −3.0 V with the same tunneling current of 1 nA using a Pt/Ir tip. As can be seen from the topography images in Figure 2, the Au(111) surface is characterized by the presence of atomically flat triangular terraces mostly separated by monatomic steps with an apparent height of 0.24 ± 0.02 nm. The topography images show the well-known 22 × √3 surface reconstruction (known as the herringbone reconstruction) of the Au(111) surface indicating the formation of parallel corrugation lines along the ⟨112̅⟩ direction owing to the contraction of the topmost layer of gold atoms along the ⟨11̅0⟩ direction.27 Thus, the Au(111) reconstructed surface has alternating hexagonal close-packed (hcp) and face-centered cubic (fcc) regions separated by transitional regions. As can be clearly seen from the photon integration maps of the Au(111) surface presented in Figure 2, STM tip-induced light emission was detected. The occurrence of light emission can be ascribed to the radiative decay of localized surface

plasmons induced by inelastic tunneling electrons. It should be emphasized that the light emission intensity strongly depended on the condition of the tip apex. Depending on the tip condition we observed intense, weak, and even no light emission from Au(111) using the Pt/Ir tip. Intriguingly, the light emission from the Au(111) surface was not homogeneous, exhibiting spatial contrast between the flat areas and step edges. The suppression of light emission was observed when the STM tip was placed above the step edges. We consider the spatial variation in the light emission intensity result from the difference in the local density of states close to the Fermi level, which leads to different probabilities of the elastic and inelastic tunneling of electrons.27 We exclude the possibility that the light emission contrast can be interpreted in terms of local deviation of the tunneling current from the set-point value due to incorrect feedback parameters because both forward and backward images exhibited the same behavior, i.e., the suppression of light emission above the step edges. It should be note that we occasionally observed an effect of the herringbone reconstruction of the Au(111) surface on the photon integration map. Figure 3 shows an STM topography image of the surface and the corresponding photon integration map obtained at a bias voltage of −3.0 V and tunneling current of 500 pA using a Pt/Ir tip. It is clear that the light emission in the photon map is strongly correlated with the herringbone D


Article


Figure 5. STM topography images showing adsorption of (S)-PTCDI molecules on a continuous ordered layer adsorbed directly on Au(111) obtained at (a) (75 × 75 nm2, −1.0 V, 50 pA) and (b) (17 × 17 nm2, −1.0 V, 50 pA). Single molecules, clusters, and small islands adsorbed on the ordered layer can be easily recognized. STM topography images and simultaneously acquired photon integration maps obtained at bias voltages of (c) 4.0 V and (d) −4.0 V with the same parameters (35 × 35 nm2, 150 pA, 128 × 128 pixels, 35 ms/pixel). Weak homogeneous light emission without correlation with the corresponding topography images was observed.

the light emission contrast in the photon integration map was negatively correlated with the herringbone reconstruction of the Au(111) surface for positive and negative bias voltages. Parallel corrugation lines along the ⟨112̅⟩ direction decreased light emission. This is the main difference between previous results and our results. Moreover, for a negative bias voltage, the light contrast was reported to be strongly reduced. In

reconstruction of the Au(111) surface. The parallel corrugation lines along the ⟨112̅⟩ direction induce approximately two times stronger light emission. It should be emphasized that such an effect of the herringbone reconstruction on light emission has seldom been observed. A similar observation has been reported for a reconstructed Au(111) surface investigated using a Au tip.28 It was shown that E


Article


Figure 6. STM images of highly ordered domains consisting of self-assembled (S)-PTCDI molecules on a Au(111) surface preheated to an elevated temperature of 355 ± 20 K during molecular deposition. Four different phases, labeled as phases I, II, III, and IV, were formed on the Au(111) surface, each of which had a different molecular adsorption geometry, orientation, and structure. (a,b) Phase I is characterized by the presence of highly ordered islands consisting of molecular parallel rows oriented along the ⟨112⟩̅ direction. The rows consist of individually resolved elongated features aligned parallel to each other (1.0 V, 50 pA). (c,d) Phase II is characterized by highly ordered domains consisting of molecular rows tilted clockwise or anticlockwise from the ⟨112⟩̅ direction by an angle of 22 ± 2°. The molecules within the row are aligned parallel to each other. Phase II allowed us to achieve high-resolution imaging of single molecules, suggesting a molecular flat-lying adsorption geometry (1.5 V, 50 pA). (e,f) Phase III is characterized by the presence of highly ordered domains forming parallel rows with the ⟨110̅ ⟩ direction. The molecules within a row are parallel to each other (−1.0 V, 50 pA). (g,h) Phase IV is characterized by the presence of domains consisting of quasi-ordered clusters. The scanning area of the images is (a,c,e,g) 15 × 15 nm2, (b,d,f) 3.6 × 3.6 nm2, or (h) 7 × 7 nm2.

associated with the formation of amorphous molecular islands differing in size and shape. The molecular resolution on the amorphous islands could not be achieved. The average length of the long axis of the bright protrusions ascribed to the long molecular axis of single molecules was estimated to be 2.0 ± 0.1 nm. The adsorption geometry of the molecules adsorbed at the fcc elbows deduced from the STM image suggests a nonplanar adsorption geometry. With increasing molecular coverage, we observed the formation of quasi-ordered islands, as shown in Figure 4b,c. Surprisingly, the estimated average size of the bright protrusions ascribed to (S)-PTCDI single molecules within the quasi-ordered islands was 1.5 ± 0.1 nm. This suggests that the configurations and adsorption of the (S)PTCDI molecules within the quasi-ordered islands are different than those of the isolated molecules. To prevent molecular quenching leading to radiationless deexcitation of the (S)-PTCDI molecules owing to energy transfer from the excited molecules to the metal surface and to decrease the plasmon light emission from the Au(111) surface, we attempted to utilize a molecular layer as an electronically decoupling buffer layer. The formation of second and third molecular layers consisting of (S)-PTCDI molecules is shown in Figure 5a,b. The (S)-PTCDI molecules were deposited by evaporation from the evaporator crucible heated to 585 ± 5 K while the substrate was maintained at 150 ± 20 K. The second layer was grown on the first continuous quasi-ordered layer adsorbed directly on the Au(111). The single molecules, clusters, and small islands grown on the first layer can be seen easily in Figure 5a,b. Note that we have never observed the formation of second and further self-assembled layers. Panels c and d of Figure 5 show typical low-resolution STM topography images and corresponding photon integration maps obtained at positive (4.0 V) and negative (−4.0 V) bias

accordance with the explanation of the light emission contrast on a reconstructed Au(111) surface presented in ref 28, we also consider the light contrast caused by spatial variation of the local density of states close the Fermi level resulting in different probabilities of elastic and inelastic tunneling processes. In ref 28, the variation in the coupling of inelastic tunneling electrons with the tip-induced localized surface plasmon mode was excluded as the reason for the light emission contrast in the photon integration map. Figure 4 shows STM images of (S)-PTCDI molecules adsorbed on Au(111) at a low temperature. The (S)-PTCDI molecules were deposited by evaporation from an evaporator heated to 585 ± 5 K while the substrate was maintained at 150 ± 20 K. The evaporation of (S)-PTCDI molecules onto a reconstructed Au(111) surface with submonolayer coverage mainly led to the preferential adsorption at fcc elbow of the herringbone reconstruction and on the upper part of the surface step edges, as shown in Figure 4a. Single (S)-PTCDI molecules adsorbed at fcc elbows of the herringbone reconstruction are easily recognizable. A typical protrusion image of a single (S)-PTCDI molecule adsorbed at the fcc elbows sites is presented in the inset in Figure 4a. The adsorption of various molecules at fcc elbows of the herringbone reconstruction has often been reported29−31 and is explained in terms of higher potential of the elbow fcc sites than at the fcc and hcp regions.30 For aromatic molecules adsorbed on metal substrates, the preferential adsorption at the upper part of step edges is generally ascribed to the charge transfer at step edges known as the Smoluchowski electron smoothing effect, to perturbations owing to molecules at the step edge, or to the enhancement of the local density of states at the step edge caused by surface-state electron scattering.32 The adsorption of single (S)-PTCDI molecules at fcc elbows is F


Article


Figure 7. STM topography images and corresponding photon integration maps of (S)-PTCDI self-assembled molecules on the Au(111) surface adsorbed in phase I imaged at (a) positive (3.0 V, 150 pA) and (b) negative (−3.0 V, 250 pA) bias voltages with the same parameters (30 × 30 nm2, 256 × 256 pixels, 9.8 ms/pixel). The suppression of light emission above molecules suggests the molecular quenching of excited molecules.

rows on the Au(111) surface. We observed three preferential orientations with a rotation angle of 120 ± 3° reflecting the 3fold symmetry of the Au(111) surface. The preferential orientation of the self-assembled rows of phase I is parallel to the ⟨112̅⟩ highly symmetric direction of the underlying Au(111) surface. The highly ordered domains do not have a perfectly continuous structure and have some missing molecules. As shown in Figure 6a, the (S)-PTCDI molecules are arranged in a unit cell (indicated by black arrows) with lattice constants of 2.7 ± 0.2 nm and 1.4 ± 0.2 nm and an angle between the two lattice vectors of 75 ± 2°. The average dimensions of the bright protrusions ascribed to single molecules of (S)-PTCDI were estimated to be 1.8 ± 0.2 and 1.25 ± 0.2 nm. Figure 6b shows a high-resolution image of (S)PTCDI molecules obtained on a highly ordered island of phase I. It can be clearly seen that each of the protrusions consists of two lobes, which can be attributed to the two PTCDI segments of the same molecule. The adsorption of (S)-PTCDI molecules on the Au(111) surface in phase I led to the disappearance of the herringbone reconstruction underneath the molecular selfassembled layer, suggesting a strong molecule−surface interaction. Phase II is characterized by the presence of highly ordered domains adsorbed directly on the Au(111) surface and consisting of self-assembled (S)-PTCDI molecules with positional and orientation order, as can be seen in Figure 6c,d. We observed three preferential orientations with a rotation angle of 120 ± 3° reflecting the 3-fold symmetry of the Au(111) surface. Surprisingly, the preferential orientation of self-assembled rows of phase II is not parallel to the highly symmetric directions of the Au(111) surface but is tilted clockwise or anticlockwise from the ⟨112̅⟩ direction by an angle of 22 ± 2°. The highly ordered domains do not have a perfectly continuous structure and have some missing molecules. As can

voltages, respectively, with the same tunneling current of 150 pA. It should be emphasized that a tunneling current of 150 pA or higher always decreased the STM image resolution owing to the strong tip−sample interaction. The photon integration maps show weak homogeneous light emission and no correlation with the corresponding topography images. From the photon integration maps we conclude that the first quasiordered layer did not significantly improve the light emission properties of the adsorbed molecules. Because the evaporation of (S)-PTCDI molecules on a Au(111) surface maintained at a low temperature of 150 ± 20 K led to the formation of a quasi-ordered layer, we changed the molecular evaporation parameters to investigate the possibility of forming a highly ordered layer on an entire surface. We decided to evaporate the molecules onto a Au(111) substrate preheated to an elevated temperature of 355 ± 20 K to increase molecular diffusion on the surface and intramolecular interactions. It is well-known that the substrate temperature strongly affects molecular adsorption and layer formation. Kinetically controlled adsorption through the substrate temperature can be used to control the structure and size of the ordered layer and minimize the number of defects.33 The evaporator crucible was maintained at the same temperature as before, i.e., 585 ± 5 K. Surprisingly, we observed four different phases with highly ordered structures covering a wide area of the Au(111) surface, labeled as phases I, II, III, and IV, as shown in Figure 6. Figure 6a,b shows STM images of phase I, which was the most frequently observed phase. Phase I is characterized by the presence of highly ordered domains (islands) adsorbed directly on the Au(111) surface and consisting of self-assembled (S)PTCDI molecules with positional and orientation order. The islands, which have dimensions in the range of tens to hundreds of nanometers, show preferentially oriented self-assembled G


Article


Figure 8. STM topography images and corresponding photon integration maps of the NiAl(110) surface (50 × 25 nm2, 256 × 128 pixels, 11.7 ms/ pixel) obtained at (a) positive (3.0 V) and (b) negative (−3.0 V) bias voltages with the same tunneling current of 500 pA. NiAl(110) surface before molecular deposition showing defect-free, atomically flat terraces and step edges with an apparent height of 0.2 ± 0.1 nm. The strong light emission observed can be ascribed to the radiative decay of localized surface plasmons induced by the STM tip. The enhancement of light emission above the step edges of the surface is considered in terms of the difference in the local density of states close to the Fermi level.

be seen in Figure 6c, the (S)-PTCDI molecules are arranged in a unit cell (indicated by black arrows) with lattice constants of 2.5 ± 0.1 and 1.3 ± 0.1 nm, and the angle between the two lattice vectors is 95 ± 1°. The average dimensions of the bright protrusions ascribed to single molecules of (S)-PTCDI were estimated to be 2.0 ± 0.1 and 1.3 ± 0.1 nm. Surprisingly, despite the stereoscopic structure of individual (S)-PTCDI molecules in the gas phase, the molecules adopt a flat-lying configuration on the Au(111) surface. A detailed inspection of Figure 6d reveals the submolecular resolution of (S)-PTCDI molecules adsorbed on the Au(111) surface. We consider that the (S)-PTCDI molecules adsorb with the planes of the two chromophores oriented nearly parallel to the surface. Phase III is characterized by the presence of highly ordered domains consisting of self-assembled (S)-PTCDI molecules surrounded by molecular disordered domains or clean Au(111). Single molecules can be clearly recognized as elongated protrusions with an internal structure that is very different from that of the molecules observed in phases I and II, as shown in Figures 6e,f. The molecules within the islands are arranged parallel to each other, forming parallel rows with the ⟨11̅0⟩ direction. The unit cell with lattice vectors a and b having lengths of 2.7 ± 0.2 and 1.3 ± 0.2 nm, respectively, is indicated by the black arrows in Figure 6e. The angle between the lattice vectors was measured to be 82 ± 1°. Phase IV is characterized by the presence of large domains consisting of quasi-ordered clusters, as can be seen in Figures 6g,h. The quasi-ordered domains coexist with amorphous molecular domains. The clusters within the domains are ordered with a preferential adsorption direction, as shown in Figure 6g. The average lateral size of the clusters was estimated to be 3.5 ± 0.3 nm, as a result of which we consider a single cluster to consist of two (S)PTCDI molecules. The formation of quasi-ordered clusters is

very intriguing. However, we are not able to give a concrete explanation why such a dimer molecular structure occurs. Figure 7 shows typical STM topography images and the corresponding photon integration maps of self-assembled (S)PTCDI molecules adsorbed in phase I on the Au(111) surface imaged at (a) positive (3.0 V) and (b) negative (−3.0 V) bias voltages using a Ag tip. Panels a and b of Figure 7 were obtained with tunneling currents of 150 and 250 pA, respectively. For positive and negative bias voltages, strong STM tip-induced light emission was detected, as can be seen from photon light emission histograms in Figure 7. The strong correlation between the topography images and the corresponding photon integration maps can be easily observed. Unfortunately, we observed the suppression of light emission above the (S)-PTCDI molecules, suggesting the molecular quenching of excited molecules. In this study, we obtained STM topography images and the corresponding photon integration maps over a wide range of bias voltages from 1.8 to 4.0 V and always observed the suppression of light emission above the molecules adsorbed on the Au(111) surface. Moreover, owing to the strong interaction between the STM tip and the molecules, we observed strong tip modification, resulting in changes in the topography image and photon emission intensity, as can be seen in the lower part of the STM topography image and photon integration map in Figure 7b. Because we observed the suppression of light emission above the (S)-PTCDI molecules adsorbed on the Au(111) surface, we decided to use a NiAl(110) surface as a substrate for molecular deposition. To obtain a spatial view of the light emission distributions from a NiAl(110) surface induced by an STM tip, we carried out STM imaging while simultaneously acquiring photon integration maps. Figure 8 shows STM topography images and the corresponding photon integration maps of the NiAl(110) H


Article

The Journal of Physical Chemistry C surface before the deposition of (S)-PTCDI molecules imaged at (a) positive (3.0 V) and (b) negative (−3.0 V) bias voltages with the same tunneling current of 500 pA using a Ag tip. As can be seen from the topography images in Figure 8, the NiAl(110) surface is characterized by the presence of defectfree atomically flat terraces with sizes ranging from a few tenths of a nanometer and step edges. The apparent height of the step edges is 0.20 ± 0.01 nm. The observed well-defined step height is consistent with the height of one atomic layer of the NiAl(110) surface. It is believed that increases in the number of step edges on the NiAl(110) surface are caused by a small miscut of the crystal surface or by an inhomogeneous local sputtering ratio during the cleaning treatment.34 As can be clearly seen from the photon integration maps of the NiAl(110) surface presented in Figure 8, strong light emission was observed. The occurrence of light emission can be ascribed to the radiative decay of localized surface plasmons induced by inelastic tunneling electrons.35 Note that the damping of surface plasmon resonance owing to the presence of relatively localized d electrons and their large contribution to the density of states close to the Fermi level in NiAl crystal35 considerably decreases photon light emission in comparison with that from a gold surface. Intriguingly, the light emission from the NiAl(110) surface was not homogeneous, showing spatial contrast between the flat areas and step edges. Considerably more intense light emission was observed when the STM tip was placed above the step edges. The average light emission intensity observed for the negative bias voltage was approximately 3.5 times as large as that for the positive bias voltage, as shown in the histograms presented in Figure 8. For the positive bias voltage of 3.0 V, the average light emission intensities observed above the flat terraces and step edges were estimated to be 0.9 and 4 kcps, respectively. For the negative bias voltage of −3.0 V, the average light emission intensities observed above the flat terraces and step edges were estimated to be 3.5 and 6 kcps, respectively. We consider the spatial variation in the light emission intensity to result from the difference in the local density of states close to Fermi level, which leads to different probabilities of elastic and inelastic tunneling processes of electrons.27 We exclude the possibility that the light emission contrast can be interpreted in terms of local deviation of the tunneling current from the set-point value due to incorrect feedback parameters because both the forward and backward images showed the same behavior, i.e., an increase in light emission above the step edges. To gain insight toward understanding the behavior of light emission from the NiAl(110) surface, we obtained STM-LE spectra. Figure 9 shows a series of STM-LE spectra taken from the NiAl(110) surface at (a) positive and (b) negative bias voltages. The STM-LE spectra were observed over bias voltages ranging from 2.0 to 3.5 V and −2.0 to −3.5 V, with the same tunneling current of 500 pA using a Ag tip. Acquisition time for each spectrum was 5 min. Strong changes in the maximum peak position, integrated peak intensity, and cutoff energy of light emission as a function of bias voltage were observed. For positive bias voltages, we observed that the integral peak intensity of light emission decreases with increasing bias voltage, as shown in Figure 9a. At a bias voltage of 2.0 V, an intense broad peak at 690 nm appears in the spectrum that gradually shifts to lower wavelengths with increasing bias voltage. For negative bias voltages, we observed different spectral behavior. We found that the integral intensity of light emission increases with increasing bias voltage, achieving a

Figure 9. STM-LE spectra taken from the NiAl(110) surface at (a) positive (2.0 to 3.5 V) and (b) negative (−2.0 V to −3.5 V) bias voltages with the same tunneling current of 500 pA and an acquisition time of 5 min for each spectrum. For positive bias voltages, with increasing bias voltage the integral peak intensity of light emission decreases and gradually shifts to lower wavelengths. For negative bias voltages, with increasing bias voltage the integral peak intensity of light emission increases, achieving a maximum at −3.0 V, then also gradually shifts to lower wavelengths. The two-peak structure of the STM-LE spectra observed for positive and negative bias voltages can be interpreted in terms of low- and high-energy plasmon modes induced by a truncated STM tip.

maximum at −3.0 V, as shown in Figure 9b. At a bias voltage of −2.0 V, a weak peak at 700 nm appears in the STM-LE spectrum that gradually shifts to lower wavelengths with increasing bias voltage. Additionally, we consider that the twopeak structure of the STM-LE spectra for positive and negative bias voltages can be interpreted in terms of the resonant property of localized surface plasmons and ascribed to low- and high-energy plasmon modes, which strongly depend on the shape of the tip apex.36 The multiple-peak structure of the STM-LE spectra results from a truncated tip−flat substrate surface geometry, and the number and energies of the peaks depend on the tip shape, as was shown for a Au(111) surface imaged using a Au tip.36 Figure 10 shows STM topography images of (S)-PTCDI molecules adsorbed on the NiAl(110) surface with different molecular coverages. The (S)-PTCDI molecules were deposited by evaporation from the evaporator crucible heated to 585 ± 5 K while the substrate was maintained at 355 ± 20 K. The evaporation of (S)-PTCDI molecules onto the NiAl(110) surface with submonolayer coverage mainly led to the formation of single- and double-height small clusters. The clusters were distributed on the surface at random positions, as shown in Figure 10a, suggesting the low mobility of the molecules on the surface as a result of strong molecule−surface interactions. It is not easy to unambiguously recognize a single molecule adsorbed on the NiAl(110) surface. This is because (S)-PTCDI molecules on the NiAl(110) surface do not adopt a planar adsorption geometry owing to their stereoscopic structure. The single molecules exhibit different lateral protrusions depending on their configuration and orientation on the NiAl(110) surface. The submolecular resolution of (S)PTCDI molecules adsorbed on the NiAl(110) surface could not be achieved. We observed energetically preferential adsorption sites at the upper step edges of the NiAl(110) surface, as can be seen in Figure 10a. At a higher surface coverage, the growth of single- and double-height small clusters was observed, as can be seen in Figure 10b. The formation of molecular domains with a self-assembled arrangement has never been observed on a NiAl(110) surface. Figure 10c shows I


Article


Figure 10. STM topography images of (S)-PTCDI molecules adsorbed on the NiAl(110) surface with different molecular coverages obtained at (a) (150 × 150 nm2, 2.0 V, 50 pA), (b) (100 × 100 nm2, 1.0 V, 50 pA), and (c) (100 × 100 nm2, 1.0 V, 25 pA). (a,b) The formation of randomly distributed single- and double-height small molecular clusters suggests the low mobility of the molecules on the substrate. The molecules do not adopt a planar adsorption geometry owing to their stereoscopic structure. (c) Formation of an inhomogeneous rough molecular multilayer without an ordered molecular arrangement.

Figure 11. STM topography images and corresponding photon integration maps of (S)-PTCDI clusters formed on the NiAl(110) surface imaged at bias voltages of (a) 1.9 V, (b) 2.4 V, (c) 3.0 V, and (d) 3.5 V with the same tunneling current of 1.5 nA. The scanning area of the images is (a,b) 10 × 10 nm2, (c) 25 × 25 nm2, or (d) 75 × 75 nm2. The topography images are strongly correlated with the corresponding photon integration maps, revealing light emission contrast between the areas uncovered and covered by molecular clusters. (a) Suppression of light emission above the molecular clusters acting as a dielectric molecular spacer. (b−d) The enhancement of light emission above the molecular clusters suggests the molecular character of the detected light. Owing to the strong suppression of localized surface plasmon light emission at higher bias voltages, the identification of molecular light emission from superimposed plasmon light emission is possible.

the formation of an inhomogeneous rough molecular multilayer with a coverage of more than 3 ML. Figure 11 shows STM topography images and the corresponding photon integration maps of (S)-PTCDI molecules adsorbed on the NiAl(110) surface imaged at bias voltages of (a) 1.9 V, (b) 2.4 V, (c) 3.0 V, and (d) 3.5 V with

the same tunneling current of 1.5 nA using a Ag tip. It is easy to see that all topography images are strongly correlated with the corresponding photon integration maps, revealing clear contrast between the light emission of the NiAl(110) surface and (S)-PTCDI clusters. The enhancement of light emission when the STM tip was placed above the (S)-PTCDI clusters J


Article

The Journal of Physical Chemistry C can be observed, except at the bias voltage of 1.9 V. Figure 11a shows the suppression of light emission owing to the presence of (S)-PTCDI clusters acting as a dielectric molecular spacer, by increasing the tip−sample distance. We consider that an increase in the tip−sample distance decreases the electromagnetic coupling between the tip and the metal substrate, leading to a decrease in plasmon light emission intensity.37 The average light intensities observed from the NiAl(110) surface and above the (S)-PTCDI clusters were estimated to be 1.5 and 0.6 kcps, respectively, as can be seen from the histogram in Figure 11a. At the bias voltage of 2.4 V we found a threshold for the suppression of light emission above the molecular clusters, as shown in Figure 11b. This value is consistent with the optical HOMO−LUMO energy gap deduced from the ultraviolet−visible (UV−vis) absorption spectrum of (S)PTCDI molecules dissolved in CHCl3, which has a value of 2.3 eV (spectrum not shown). At this bias voltage, some molecular clusters suppress light emission whereas others enhance light emission. Despite considerable effort to correlate the suppression and enhancement of light emission with the size, shape, and height of the molecular clusters, we did not succeed. With increasing bias voltage, the enhancement of light emission becomes clearer, as can be seen in Figure 11c. At the bias voltage of 3.5 V the light emission above the molecular clusters is approximately three times as large as that on the NiAl(110) surface, as shown in Figure 11d. The average light emission intensities observed from the NiAl(110) surface and above the (S)-PTCDI clusters were estimated to be 0.7 and 2 kcps, respectively, as can be seen from the histogram in Figure 11d. Interestingly, the bright protrusions ascribed to molecular clusters that can be seen in the topography images are smaller than the corresponding bright protrusions in the photon integration maps, suggesting molecular excitation not only above the molecular clusters but also near them. This conclusion is consistent with a previous study of light emission from tetraphenyl-H2-porphyrine molecules adsorbed on a Ag(111) surface using a Ag tip. It has been shown that molecules in the vicinity of a tunneling junction are excited through tip-induced plasmons followed by radiative decay7 despite the absence of electron transport through the molecules. Unfortunately, for negative bias voltages we were unable to obtain any reliable and reproducible photon integration maps. The most important issue is clarifying the origin of enhanced light emission above the molecular clusters. Bearing in mind that molecular light may be superimposed on strong plasmon light emission,3 the identification of the molecular light above (S)-PTCDI clusters appears to be a complicated issue. To gain insight into the origin of the enhanced light emission from the molecular clusters observed in the STM-LE experiment, we carried out a conventional laser-induced PL investigation. PL measurement was performed using the 532 nm line of green laser with a power of several milliwatts. The main motivation of the PL investigation was to identify the vibronic progressions of (S)-PTCDI molecules, which may serve as spectroscopic fingerprint of the molecules during STM-LE spectral analysis. Figure 12a shows the PL spectrum of (S)-PTCDI molecules dissolved in chloroform at a concentration of 1.0 × 10−4 M obtained at room temperature. Strong light emission was detected despite the strong π−π stacking interaction between the two chromophores, as expected from the structure of the molecules. The spectrum shows typical Franck−Condon vibronic progressions with two strong

Figure 12. (a) Conventional PL spectrum of (S)-PTCDI molecules dissolved in chloroform obtained using the 532 nm line of green laser with power of several milliwatts. The spectrum shows typical Franck− Condon vibronic progressions with two strong emission peaks at 545.5 and 586.2 nm and a weak peak at 639.8 nm corresponding to the perylene units. (b) STM-LE spectra obtained from the NiAl(110) surface and an (S)-PTCDI cluster. The spectra were observed with the same bias voltage of 3.5 V and tunneling current of 800 pA. Acquisition time for each spectrum was 5 min. The enhancement of light emission above the molecular cluster suggests the molecular character of the emitted light.

emission peaks at 545.5 and 586.2 nm and a weak peak at 639.8 nm corresponding to the emission from the perylene units.38 To clarify the origin of the strong enhancement of light emission from above the molecular clusters, we obtained STMLE spectra from the NiAl(110) surface and a molecular cluster, as shown in Figure 12b. The STM-LE spectra were obtained consecutively with the same parameters (bias voltage of 3.5 V and tunneling current of 2.5 nA) using a Ag tip. Acquisition time for each spectrum was 5 min. The STM-LE spectrum taken on the NiAl(110) surface has a typical wide tip-induced plasmon emission peak. It is easy to see the enhancement of light emission above the molecular cluster. This is consistent with the enhancement observed above the molecular clusters presented in the photon integration maps in Figure 11. The comparison between the STM-LE spectrum taken from the (S)-PTCDI cluster adsorbed on the NiAl(110) surface and the PL spectrum taken from the molecules dissolved in chloroform allows us to suggest that the STM-LE spectra from the clusters exhibit strong plasmon-mediated molecular excitation accompanied by red-shifted molecular light emission. Molecular light emission is enhanced by localized surface plasmons. Unfortunately, we could not identify the vibronic progressions of (S)PTCDI molecules in the STM-LE spectrum taken from the molecular cluster, which was probably due to the strong interaction between the (S)-PTCDI molecules and the Au(111) surface. We next discuss the enhancement of light emission above the molecular clusters while bearing in mind the above observations. From the STM-LE spectra, we consider that we K


Article


We observed four different phases of ordered domains with different molecular arrangements. The photon integration maps obtained on the highly ordered domains of phase I showed suppression of light emission above (S)-PTCDI molecules. We also investigated light emission distributions over a NiAl(110) surface induced by an STM tip. The photon integration maps obtained over the NiAl(110) surface showed strong light emission ascribed to the radiative decay of localized surface plasmons induced by the inelastic tunneling electrons. More intense light emission was observed when the STM tip was placed above step edges. To clarify the light emission from the NiAl(110) surface, we obtained STM-LE spectra. For the STM-LE spectra obtained at positive bias voltages, the integral peak intensity of light emission decreased with increasing bias voltage, whereas for negative bias voltages, the integral intensity of light emission increased with increasing bias voltage up to −3.0 V and then decreased. The two-peak structure of the STM-LE spectra observed for both positive and negative bias voltages was ascribed to low- and high-energy plasmon modes, whose excitation strongly depends on the shape of the tip apex. The formation of small clusters at random positions on the surface was observed instead of the self-assembled layer observed on the Au(111) surface. The single molecules adsorbed on the NiAl(110) surface exhibited different lateral protrusions owing to the nonplanar adsorption geometry resulting from the stereoscopic structure of the molecules. The enhancement of light emission when the STM tip was placed above the (S)-PTCDI clusters was observed for bias voltages higher than 2.4 V. To clarify the origin of the strong enhancement of light emission above the molecular clusters, we obtained STM-LE spectra, which also indicated the strong enhancement of light emission above a molecular cluster. We consider that strong plasmon-mediated molecular excitation accompanied by red-shifted molecular light emission occurred above the molecular clusters. The advantages of a molecular system consisting of (S)-PTCDI molecules adsorbed on a NiAl(110) surface will be further demonstrated in a future experiment on detecting the dissymmetry of circularly polarized light.

are dealing with molecular light emission above the molecular clusters, which is superimposed on the plasmon light emission. The weak molecular light emission is hidden by the strong plasmon light emission at low bias voltages. However, the weak molecular light is observed when the strong plasmon light emission is suppressed. Because for positive bias voltages the plasmon light emission rapidly decreases with increasing voltage, as was shown in the STM-LE spectra presented in Figure 9a, the weak molecular light was able to be detected. In general, the formation of clusters with ordered or random structures leads to the disappearance of light emission owing to molecular quenching. This phenomenon is known as aggregation-caused quenching (ACQ).39 However, occasionally the opposite effect, called aggregation-induced emission (AIE), is observed.39−42 For example, silole molecules in solution exhibit a nonemissive character in the solvent, whereas strong emission is observed after the formation of molecular aggregates as a thin film or as a nanoparticle suspension in a poor solvent.40 Because enhancement of light emission above molecular clusters was much stronger than that above single molecules, we consider that the AIE effect may have also played an important role in enhancing molecular light emission in our study. This is because the perylene-based PTCDI molecules tend to form clusters owing to the attractive dipole−dipole interactions and the strong stacking interaction of perylene units. In the near future we plan to observe the molecular optical dissymmetry resulting from the difference between the emission intensity of right- and left-handed circularly polarized light from (S)-PTCDI molecules adsorbed on a NiAl(110) surface at the molecular scale. This may provide additional proof for the molecular character of the light detected above the molecular clusters. We believe that the molecular system consisting of (S)-PTCDI molecules adsorbed on a NiAl(110) surface meets the main requirements for such a breakthrough observation for the following reasons. First, (S)-PTCDI molecules exhibit extremely large molecular optical dissymmetry, as was deduced in solution by conventional CD spectroscopy. The quenching of electronically excited molecular states of (S)-PTCDI molecules adsorbed on a NiAl(110) surface was not observed. On the other hand, strong plasmonmediated enhancement of the light emission above the molecular clusters was observed. Additionally, the enhancement of the light emission resulting from the AIE effect owing to the attractive dipole−dipole interactions and the strong π−π stacking interaction among neighboring molecules may have also played an important role.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-6-68797299. Fax: +81-6-6879-7299. Notes

The authors declare no competing financial interest.

■

■

CONCLUSIONS We have investigated light emission distributions over a Au(111) surface induced by an STM tip. The photon integration maps obtained on the Au(111) surface showed strong light emission, which was ascribed to the radiative decay of localized surface plasmons induced by inelastic tunneling electrons. Occasionally, we observed an effect of the herringbone reconstruction of the Au(111) surface on the light emission distribution, indicating enhancement of light emission above the parallel corrugation lines along the ⟨112̅⟩ direction. We deposited (S)-PTCDI molecules on a Au(111) surface maintained at low (150 ± 20 K) and elevated (355 ± 20 K) temperatures. At the elevated temperature, (S)-PTCDI molecules formed highly ordered domains consisting of a selfassembled arrangement with positional and orientation order.

ACKNOWLEDGMENTS This work is supported by a Grant-in-Aid for Scientific Research (S) (24221009) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

■

REFERENCES

(1) Ranjbar, B.; Gill, P. Circular Dichroism Techniques: Biomolecular and Nanostructural Analyses - A Review. Chem. Biol. Drug Des. 2009, 74, 101−120. (2) Charney, E. The Molecular Basis of Optical Activity: Optical Rotatory Dispersion and Circular Dichroism; John Wiley & Sons Inc.: New York, 1985. (3) Rossel, F.; Pivetta, M.; Schneider, W.-D. Luminescence Experiments on Supported Molecules with the Scanning Tunneling Microscope. Surf. Sci. Rep. 2010, 65, 129−144.

L


Article

The Journal of Physical Chemistry C (4) Shamai, T.; Selzer, Y. Spectroscopy of Molecular Junctions. Chem. Soc. Rev. 2011, 40, 2293−2305. (5) Chaunchaiyakul, S.; Krukowski, P.; Tsuzuki, T.; Minagawa, Y.; Akai-Kasaya, M.; Saito, A.; Osuga, H.; Kuwahara, Y. Self-assembly Formation of M-type Enantiomer of 2,13-Bis(hydroxymethyl)[7]thiaheterohelicene Molecules on Au(111) Surface Investigated by STM/CITS. J. Phys. Chem. C 2015, 119, 21434−21442. (6) Wu, S. W.; Nazin, G. V.; Ho, W. Intramolecular Photon Emission from a Single Molecule in a Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 205430. (7) Schneider, N. L.; Berndt, R. Plasmonic Excitation of Light Emission and Absorption by Porphyrine Molecules in a Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 035445. (8) Ć avar, E.; Blüm, M.-C.; Pivetta, M.; Patthey, F.; Chergui, M.; Schneider, W.-D. Fluorescence and Phosphorescence from Individual C60 Molecules Excited by Local Electron Tunneling. Phys. Rev. Lett. 2005, 95, 196102. (9) Lutz, T.; Große, C.; Dette, C.; Kabakchiev, A.; Schramm, F.; Ruben, M.; Gutzler, R.; Kuhnke, K.; Schlickum, U.; Kern, K. Molecular Orbital Gates for Plasmon Excitation. Nano Lett. 2013, 13, 2846− 2850. (10) Yun, W. M.; Jang, J.; Nam, S.; Park, C. E.; Kim, S. H.; Chung, D. S. Organic Light-Emitting Diodes with Low Turn-On Voltages and Improved Stability Featuring a PTCDI-C13:CuPc Mixed Hole Injection Layer. Sci. Adv. Mater. 2014, 6, 1676−1680. (11) Yu, S. H.; Kang, B.; An, G.; Kim, B.; Lee, M. H.; Kang, M. S.; Kim, H.; Lee, J. H.; Lee, S.; Cho, K.; et al. pn-Heterojunction Effects of Perylene Tetracarboxylic Diimide Derivatives on Pentacene FieldEffect Transistor. ACS Appl. Mater. Interfaces 2015, 7, 2025−2031. (12) Hardin, B. E.; Hoke, E. T.; Armstrong, P. B.; Yum, J.-H.; Comte, P.; Torres, T.; Frechet, J. M. J.; Nazeeruddin, M. K.; Gratzel, M.; McGehee, M. D. Increased Light Harvesting in Dye-sensitized Solar Cells with Energy Relay Dyes. Nat. Photonics 2009, 3, 406−411. (13) Rahimi, R.; Narang, V.; Korakakis, D. Optical and Morphological Studies of Thermally Evaporated PTCDI-C8 Thin Films for Organic Solar Cell Applications. Int. J. Photoenergy 2013, 2013, 205105. (14) Wang, F.-X.; Wang, K.-K.; Yang, B.; Liu, Y.-Q.; Pan, G.-B. Facile Fabrication of Ultralong Nanobelts of Positively Charged PTCDI for High-performance Photodetectors. Mater. Res. Express 2014, 1, 035032. (15) O’Shea, J. N.; Saywell, A.; Magnano, G.; Perdigão, L. M.; Satterley, C. J.; Beton, P. H.; Dhanak, V. R. Adsorption of PTCDI on Au(111): Photoemission and Scanning Tunnelling Microscopy. Surf. Sci. 2009, 603, 3094−3098. (16) Moreno-López, J. C.; Grizzi, O.; Martiarena, M. L.; Sánchez, E. A. Initial Growth of N,N′-Bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxdiimide Films on Cu(100). J. Phys. Chem. C 2013, 117, 11679− 11685. (17) Yang, H.; Mayne, A. J.; Comtet, G.; Dujardin, G.; Kuk, Y.; Sonnet, P.; Stauffer, L.; Nagarajan, S.; Gourdon, A. STM Imaging, Spectroscopy and Manipulation of a Self-assembled PTCDI Monolayer on Epitaxial Graphene. Phys. Chem. Chem. Phys. 2013, 15, 4939−4946. (18) Lanzilotto, V.; Lovat, G.; Otero, G.; Sanchez, L.; López, M. F.; Mén dez, J.; Martín-Gago, J. A.; Bavdek, G.; Floreano, L. Commensurate Growth of Densely Packed PTCDI Islands on the Rutile TiO2(110) Surface. J. Phys. Chem. C 2013, 117, 12639−12647. (19) Topple, J. M.; Burke, S. A.; Fostner, S.; Grütter, P. Thin Film Evolution: Dewetting Dynamics of a Bimodal Molecular System. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205414. (20) Yang, H.; Mayne, A. J.; Comtet, G.; Dujardin, G.; Kuk, Y.; Nagarajan, S.; Gourdon, A. Single-molecule Light Emission at Room Temperature on a Wide-band-gap Semiconductor. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 125427. (21) Kawai, T.; Kawamura, K.; Tsumatori, H.; Ishikawa, M.; Naito, M.; Fujiki, M.; Nakashima, T. Circularly Polarized Luminescence of a

Fluorescent Chiral Binaphtylene−Perylenebiscarboxydiimide Dimer. ChemPhysChem 2007, 8, 1465−1468. (22) Rossel, F.; Pivetta, M.; Patthey, F.; Schneider, W.-D. Plasmon Enhanced Luminescence from Fullerene Molecules Excited by Local Electron Tunneling. Opt. Express 2009, 17, 2714−2721. (23) Langhals, H. Control of the Interactions in Multichromophores: Novel Concepts. Perylene Bis-imides as Components for Larger Functional Units. Helv. Chim. Acta 2005, 88, 1309−1343. (24) Langhals, H.; Gold, J. Chiral Bifluorophoric Perylene Dyes with Unusually High CD Effects − a Simple Model for the Photosynthesis Reaction Center. Liebigs Ann. 1997, 1997, 1151−1153. (25) Tang, Y.; Cohen, A. E. Enhanced Enantioselectivity in Excitation of Chiral Molecules by Superchiral Light. Science 2011, 332, 333−336. (26) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (27) Hoffmann, G.; Maroutian, T.; Berndt, R. Color View of Atomic Highs and Lows in Tunneling Induced Light Emission. Phys. Rev. Lett. 2004, 93, 076102. (28) Perronet, K.; Barbier, L.; Charra, F. Influence of the Au(111) Reconstruction on the Light Emission Induced by a Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 201405. (29) Müllegger, S.; Rashidi, M.; Fattinger, M.; Koch, R. Interactions and Self-Assembly of Stable Hydrocarbon Radicals on a Metal Support. J. Phys. Chem. C 2012, 116, 22587−22594. (30) Liu, L. W.; Yang, K.; Xiao, W. D.; Jiang, Y. H.; Song, B. Q.; Du, S. X.; Gao, H.-J. Selective Adsorption of Metal-phthalocyanine on Au(111) Surface with Hydrogen Atoms. Appl. Phys. Lett. 2013, 103, 023110. (31) Mielke, J.; Hanke, F.; Peters, M. V.; Hecht, S.; Persson, M.; Grill, L. Adatoms underneath Single Porphyrin Molecules on Au(111). J. Am. Chem. Soc. 2015, 137, 1844−1849. (32) Pascual, J. I.; Jackiw, J. J.; Kelly, K. F.; Conrad, H.; Rust, H.-P.; Weiss, P. S. Local Electronic Structural Effects and Measurements on the Adsorption of Benzene on Ag(110). Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 12632−12635. (33) Uemura, S.; Tanoue, R.; Yilmaz, N.; Ohira, A.; Kunitake, M. Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM. Materials 2010, 3, 4252−4276. (34) Heinke, L.; Lichtenstein, L.; Simon, G. H.; König, T.; Heyde, M.; Freund, H.-J. Structure and Electronic Properties of Step Edges in the Aluminum Oxide Film on NiAl(110). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 075430. (35) Nilius, N.; Ernst, N.; Freund, H.-J.; Johansson, P. Photon Emission Spectroscopy of NiAl(110) in the Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 12682−12685. (36) Meguro, K.; Sakamoto, K.; Arafune, R.; Satoh, M.; Ushioda, S. Origin of Multiple Peaks in the Light Emission Spectra of a Au(111) Surface Induced by the Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 165405. (37) Tao, X.; Dong, Z. C.; Yang, J. L.; Luo, Y.; Hou, J. G.; Aizpurua, J. Influence of a Dielectric Layer on Photon Emission Induced by a Scanning Tunneling Microscope. J. Chem. Phys. 2009, 130, 084706. (38) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. Effect of Side-Chain Substituents on SelfAssembly of Perylene Diimide Molecules: Morphology Control. J. Am. Chem. Soc. 2006, 128, 7390−7398. (39) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 11718−11940. (40) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332−4353. (41) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced Emission. Chem. Soc. Rev. 2011, 40, 5361−5388. M


Article

The Journal of Physical Chemistry C (42) Li, H.; Guo, Y.; Li, G.; Xiao, H.; Lei, Y.; Huang, X.; Chen, J.; Wu, H.; Ding, J.; Cheng, Y. Aggregation-Induced Fluorescence Emission Properties of Dicyanomethylene-1,4-dihydropyridine Derivatives. J. Phys. Chem. C 2015, 119, 6737−6748.

N


Detection of Light Emission from (S)-PTCDI Molecules Adsorbed on

Recommend Documents