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Raman excitation profiles of hybrid systems constituted by single-layer graphene and free base phthalocyanine: Manifestations of two mechanisms of graphene-enhanced Raman scattering: Raman excitation profiles of hybrid systems
Abstract
The ability of single-layer graphene (SLG) to enhance Raman scattering of planar aromatic molecules denoted graphene-enhanced Raman scattering (GERS) is currently the subject of focused interest. We report on manifestations of two mechanisms of GERS in Raman spectra of glass/SLG/free base phthalocyanine (H2Pc) monolayer (ML) hybrid systems: (i) photoinduced charge transfer from SLG Fermi level to LUMO of H2Pc excited at onset of the near IR region, and (ii) modification of resonance Raman scattering of H2Pc in the visible region by SLG-H2Pc interaction resulting into delocalization of the electronic transition over the benzene rings of H2Pc. Glass/SLG/H2Pc hybrid systems with either a bilayer or a monolayer of H2Pc molecules and a graphite/H2Pc (ML) reference system were prepared by a spectrally controlled adsorption-desorption of H2Pc from solution, followed by Raman mapping of samples at excitation wavelengths in the 532-830 nm range, construction of excitation profiles for H2Pc Raman bands of the glass/SLG/H2Pc samples and determination of GERS enhancement factors for the glass/SLG/H2Pc (ML) sample versus the graphite/H2Pc (ML) reference sample (3-24 at 633 nm and 3-19 at 647 nm excitations). Selectivity of the excitation profiles and of the GERS enhancement factors with respect to localization of the vibrational modes within the H2Pc molecule demonstrates involvement of a different resonant electronic transition in each of the two mechanisms of GERS.
... Noteworthy, the 633 nm excitation (Fig. 4b) allows a better discrimination of signature bands arising from the ZnPcB [77][78][79][80][81][82], an observation also extended to the other ZnPc@GO hybrid materials ( Figure S5, Table S1). This is due to a resonant Raman (RR) effect that gives rise to a strong enhancement of the Raman bands of molecules when the wavelength excitation coincides with the strong absorption of the molecule. ...
... The most intense Raman band of ZnPcB@GO upon excitation at 633 nm was observed at 1520 cm À1 . This band is sensitive to the presence of the Zn(II) metal ion, which affects the CANAC bridging bonds (namely C a = C b and C a = N b in pyrrole structures, Table S1 and Figure S6) [77][78][79][80][81]85]. Taking advantage of this signature band associated with ZnPcB (at 1520 cm À1 ) and the D-band from GO (at 1333 cm À1 ), the distribution of ZnPcB molecules on the GO sheets was mapped by Raman confocal microscopy (Fig. 5). ...
Hypothesis
The aggregation of phthalocyanines (Pcs) enfeebles their suitability as G-quadruplex (G4) ligands over time. It is hypothesized that the interfacial assembly of Pcs on graphene oxide (GO) influences intermolecular interactions, thereby affecting their physicochemical properties and inducing stabilization of Pcs in solution. Hence, the stacking of Pcs on GO could be tuned to create nanosystems with the ability to detect G4 for longer periods through a slow release of Pcs.
Experiments
Four cationic structurally-related zinc(II) phthalocyanines (ZnPc) were non-covalently assembled on GO by ultrasonic exfoliation. A comprehensive characterization of [email protected] was carried out by spectroscopic techniques and electron microscopy to understand the organization of ZnPcs on GO. The fluorescence of [email protected] was studied in the presence of G4 (T2G5T)4 and duplex ds26 through spectrofluorimetric titrations and monitored along time.
Findings
GO induced a re-organization of the ZnPcs mostly to J-aggregates and quenched their original fluorescence up to 98% (“turn-off”). In general, [email protected] recovered their fluorescence (“turn-on”) after the titrations and showed affinity to G4 (KD up to 1.92 μM). This is the first report that highlights the contribution of GO interfaces to assemble ZnPcs and allow their slow and controlled release to detect G4 over longer periods.
Metal macrocycles with well-defined molecular structures are ideal platforms for the in-depth study of electrochemical oxygen reduction reaction (ORR). Structural integrity of metal macrocycles is vital but remain challenging since the commonly used high-temperature pyrolysis would cause severe structure damage and unidentifiable active sites. Herein, we propose a pyrolysis-free strategy to precisely manipulate the exfoliated 2D iron polyphthalocyanine (FePPc) anchored on reduced graphene oxide (rGO) via π-π stacking using facile high-energy ball milling. A delocalized electron shift caused by π-π interaction is firstly found to be the mechanism of facilitating the remarkable ORR activity of this hybrid catalyst. The optimal FePPc@rGO-HE achieves superior half-wave potential (0.90 V) than 20 % Pt/C. This study offers a new insight in designing stable and high-performance metal macrocycle catalysts with well-defined active sites.
As a novel and potentially strong analytical technique, graphene-enhanced Raman scattering (GERS) has attracted since its first discovery in 2010 increasing attention among many scientists in recent years. GERS combines ultra-high sensitivity and spectral analysis of structural properties of many molecules found at surface-enhanced Raman scattering (SERS), and graphene simple sample processing and excellent uniformity. This facilitates a fast and very sensitive analysis of a wide range of analytes. Here we present new method for the analysis of amino acids based on the combination of a drop coating deposition Raman (DCDR) and GERS. The potential of the method was evaluated in the analysis of Trp, Leu, Phe, and DOPA. Limits of detection are in tens of ng.mL⁻¹.
Discrimination of enantiomers poses a scientific challenge as the chemical and physical properties of enantiomers are nearly identical. The chiral analysis is usually performed by separation techniques, including chromatography, electrophoresis, or optical instrumentation based on an interaction of the analyzed sample with a polarized beam of light. Here we present a novel method for a chiral screening based on a combination of the black [email protected] nanocomposite and Raman spectroscopy. The nanocomposite allows to enhance the Raman signal with factors higher than 100 asymmetrically and provide altered signals for mixtures containing varying enantiomeric ratios of target compounds. Tryptophan, Phenylalanine, DOPA, Isoleucine, and Leucine were selected as model compounds; the method allows us to discriminate between mixtures with 10, 25, 50, 75, and 100 % enantiomeric purity.
The controllable design of single‐atom electrocatalysts with high‐site‐density and enhanced mass/volume specific activity holds tremendous potential for clean energy devices involving the oxygen reduction reaction (ORR). Herein, two‐dimensional conjugated aromatic networks (CAN) with ultra‐thin conjugated layers (ca. 3.5 nm) and high single‐metal‐atom‐site density (mass content of 10.7 wt.%, and 0.73 metal atoms per nm2) are prepared via a facile pyrolysis‐free route involving a one‐step ball milling of the solid‐phase‐synthesized polyphthalocyanine. These materials display outstanding ORR mass activity of 47 mA mgcat.–1 represents 1.3‐ and 6.4‐fold enhancements compared with Pt and Pt/C in benchmark Pt/C, respectively. Moreover, the primary Zn−air batteries constructed with CAN as an air electrode demonstrate a mass/volume power density of 880 W gcat.–1/615 W cmcat.–3 and stable long‐term operation for 100 h. Our strategy offers a new way to design high‐performance electrocatalysts with atomic precision for use in other energy storage and conversion applications.
Two‐dimensional conjugated aromatic networks (CAN) with ultra‐thin conjugated layers (ca. 3.5 nm) and high single‐metal‐atom‐site density (mass content of 10.7 wt %, and 0.73 metal atoms per nm²) are prepared via a facile pyrolysis‐free route involving a one‐step ball milling of the solid‐phase‐synthesized polyphthalocyanine. These materials display outstanding oxygen reduction reaction (ORR) mass activity of 47 mA mgcat.⁻¹ represents 1.3‐ and 6.4‐fold enhancements compared to Pt and Pt/C in benchmark Pt/C, respectively. Moreover, the primary Zn‐air batteries constructed with CAN as an air electrode demonstrate a mass/volume power density of 880 W gcat.⁻¹/615 W cmcat.⁻³ and stable long‐term operation for 100 h. This strategy offers a new way to design high‐performance electrocatalysts with atomic precision for use in other energy‐storage and conversion applications.
In this study we report on UV-vis, IR and Raman studies of non-covalent copper phthalocyanine (CuPc)– flake graphene oxide (GO)complex in water and in the solid phase. Experimental results were supported by molecular modeling of structure, electronic and vibrational parameters for free CuPc and its 1: 1 complexes with water, benzene, phenol, neutral and deprotonated benzoic acid. HOMO-LUMO gaps for these complexes were calculated and compared with data derived from the absorption edge of Q-band in the recorded UV-vis spectra for free CuPc and its adduct with GO in water. Small but non-negligible changes in position of spectral bands observed as result of CuPc interaction with GO were consistent with theoretical models predicted by density functional theory (DFT). Both the observed spectroscopic changes and theoretical shifts of spectral bands supported a direct interaction between CuPc and GO. The interaction site of CuPc was copper ion - small molecules were bound preferably to Cu ²⁺ and its vicinity as an additional axial ligand forming a kind of T-shaped structure. In case of benzene and phenol sandwich type complexes were formed.
This annual review is an overview of advances in the field of Raman spectroscopy as found in papers published in the previous calendar year in the Journal of Raman Spectroscopy (JRS), as well as in trends over the past decade across journals that have published papers important to the field of Raman spectroscopy. This information is gleaned from statistical data on article counts obtained from the Clarivate Analytic's Web of Science Core Collection by year and by subfield of Raman spectroscopy. Additional information is gleaned from presentations at the XXVI International Conference on Raman Spectroscopy (ICORS 2018) in Jeju Island, Korea in August 2018, and contributions on Raman scattering at SCIX 2018, organized by the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) in Atlanta, Georgia, USA in October 2018. Coverage is also provided for topics from the European Conference on Nonlinear Optical Spetroscopy (ECONOS 2018) held in April in Milan, Italy and GeoRaman 2018 in Catania, Italy in June. Finally, papers published in JRS in 2017 are highlighted and arranged by topics at the frontier of Raman spectroscopy. On the basis of this survey information, it is clear that Raman spectroscopy continues as in recent years as a rapidly expanding field of research across a wide range of novel disciplines and applications that provide sensitive photonic information of matter at the molecular level.
Hybrid systems constituted by plasmonic nanostructures and single layer graphene (SLG) as well as their employment as platforms for surface-enhanced Raman scattering (SERS) of molecular species has recently become a subject of interest. By contrast, only a few studies were targeted specifically on combination of SERS with graphene-enhanced Raman scattering (GERS) of aromatic molecules. In this paper, we have investigated the mechanisms of combined SERS+GERS by micro-Raman spectral mapping of hybrid system constituted by annealed Ag nanoparticles (NPs) on glass substrate overdeposited first by SLG and, subsequently, by a monolayer (ML) of free-base phthalocyanine (H2Pc) molecules, as well as of glass/SLG/H2Pc(ML) and of graphite/H2Pc(ML) reference systems. Raman mapping was performed at multiple excitation wavelengths spanning the 532-830 nm range, and was complemented by surface plasmon extinction and TEM images of Ag NPs platform. Observation of SERS+GERS in the aforementioned hybrid system was established by determination of GERS, SERS and SERS+GERS enhancement factors. By construction and the mutual comparison of GERS+SERS and GERS excitation profiles of H2Pc vibrational modes, operation of two mechanisms of GERS additively with the electromagnetic SERS enhancement in SERS+GERS of H2Pc in glass/AgNPs/SLG/H2Pc(ML) hybrid system has been ascertained. Finally, achievement of the same level of weak negative doping of SLG by Ag NPs in the probed hybrid system and by glass in the reference system has been established as necessary condition for a proper evaluation of SERS and GERS mechanisms combination, and evidence for fulfillment of this condition in the hybrid systems reported here was provided.
Scalable and low-cost doping of graphene could improve technologies in a wide range of fields such as microelectronics, optoelectronics, and energy storage. While achieving strong p-doping is relatively straightforward, non-electrostatic approaches to n-dope graphene, such as chemical doping, have yielded electron densities of 9.5 × 1012 e/cm2 or below. Furthermore, chemical doping is susceptible to degradation and can adversely affect intrinsic graphene’s properties. Here we demonstrate strong (1.33 × 1013 e/cm2), robust, and spontaneous graphene n-doping on a soda-lime-glass substrate via surface-transfer doping from Na without any external chemical, high-temperature, or vacuum processes. Remarkably, the n-doping reaches 2.11 × 1013 e/cm2 when graphene is transferred onto a p-type copper indium gallium diselenide (CIGS) semiconductor that itself has been deposited onto soda-lime-glass, via surface-transfer doping from Na atoms that diffuse to the CIGS surface. Using this effect, we demonstrate an n-graphene/p-semiconductor Schottky junction with ideality factor of 1.21 and strong photo-response. The ability to achieve strong and persistent graphene n-doping on low-cost, industry-standard materials paves the way toward an entirely new class of graphene-based devices such as photodetectors, photovoltaics, sensors, batteries, and supercapacitors.
Nonresonance (or normal) Raman scattering (NRS), resonance Raman scattering (RRS), surface-enhanced Raman scattering (SERS), and surface-enhanced RRS (SERRS) spectra of [Fe(tpy)2]2+ complex dication (tpy = 2,2':6',2''-terpyridine) are reported. The comparison of RRS/NRS and SERRS/SERS excitation profiles of [Fe(tpy)2]2+ spectral bands in the range of 445–780 nm is supported by density functional theory (DFT) calculations, Raman depolarization measurements, comparison of the solid [Fe(tpy)2](SO4)2 and solution RRS spectra, and characterization of the Ag nanoparticle (NP) hydrosol/[Fe(tpy)2]2+ SERS/SERRS active system by surface plasmon extinction spectrum and transmission electron microscopy image of the fractal aggregates (D = 1.82). By DFT calculations, both the Raman active modes and the electronic states of the complex have been assigned to the symmetry species of the D2d point group. It has been demonstrated that upon the electrostatic bonding of the complex dication to the chloride-modified Ag NPs, the geometric and ground state electronic structure of the complex and the identity of the three different metal-to-ligand charge transfer (1MLCT) electronic transitions remain preserved. On the other hand, the effect of ion pairing manifests itself by a slight change in localization of one of the electronic transitions (with max. at 552 nm) as well as by promotion of the Herzberg–Teller activation of E modes resulting from coupling of E and B2 excited electronic states. Finally, the very low, 1 × 10−11 M SERRS spectral detection limit of [Fe(tpy)2]2+ at 532-nm excitation is attributed to a concerted action of the electromagnetic and molecular resonance mechanism, in conjunction to the electrostatic bonding of the complex dication to the chloride-modified Ag NP surface. Copyright © 2014 John Wiley & Sons, Ltd.
Using low-temperature scanning tunneling microscopy (STM), we observed the bonding configuration of the metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface. A local lattice formation started from a quasi-square lattice aligned to the close-packed directions of the Au(111) surface. Although we expected the lattice alignment to be equally distributed along the three crystallographically equivalent directions, the domain aligned normal to the ridge of the herringbone structure was missing in the STM images. We attribute this effect to the uniaxial contraction of the reconstructed Au(111) surface that can account for the formation of a large lattice domain along a single crystallographical direction.
Standard Reference Materials SRMs 2241 through 2243 are certified spectroscopic standards intended for the correction of the relative intensity of Raman spectra obtained with instruments employing laser excitation wavelengths of 785 nm, 532 nm, or 488 nm/514.5 nm. These SRMs each consist of an optical glass that emits a broadband luminescence spectrum when illuminated with the Raman excitation laser. The shape of the luminescence spectrum is described by a polynomial expression that relates the relative spectral intensity to the Raman shift with units in wavenumber (cm(-1)). This polynomial, together with a measurement of the luminescence spectrum of the standard, can be used to determine the spectral intensity-response correction, which is unique to each Raman system. The resulting instrument intensity-response correction may then be used to obtain Raman spectra that are corrected for a number of, but not all, instrument-dependent artifacts. Peak area ratios of the intensity-corrected Raman spectrum of cyclohexane are presented as an example of a methodology to validate the spectral intensity calibration process and to illustrate variations that can occur in this measurement.
Metal-free phthalocyanines(H2Pc) and its derivatives are an important organic electro and photo active material with remarkable chemical stability and flexibility offering considerable interest for many intensive applications. The complexity of the molecule makes the vibrational study of the Raman spectrum difficult. A vibrational assignment of the frequencies in the F.T Raman spectrum of H2Pc has been made in comparison with its metal derivatives, other related porphyrin, pyrrole, indole and ortho-subsituted benzene structures.
This book gives an overview of recent developments in RS and SERS for sensing and biosensing considering also limitations, possibilities and prospects of this technique. Raman scattering (RS) is a widely used vibrational technique providing highly specific molecular spectral patterns. A severe limitation for the application of this spectroscopic technique lies in the low cross section of RS. Surface-enhanced Raman scattering (SERS) spectroscopy overcomes this problem by 6-11 orders of magnitude enhancement compared with the standard RS for molecules in the close vicinity of certain rough metal surfaces. Thus, SERS combines molecular fingerprint specificity with potential single-molecule sensitivity. Due to the recent development of new SERS-active substrates, labeling and derivatization chemistry as well as new instrumentations, SERS became a very promising tool for many varied applications, including bioanalytical studies and sensing. Both intrinsic and extrinsic SERS biosensing schemes have been employed to detect and identify small molecules, nucleic acids and proteins, and also for cellular and in vivo sensing.
Surface Enhanced Vibrational Spectroscopy (SEVS) has reached maturity as an analytical technique, but until now there has been no single work that describes the theory and experiments of SEVS. This book combines the two important techniques of surface-enhanced Raman scattering (SERS) and surface-enhanced infrared (SEIR) into one text that serves as the definitive resource on SEVS. Discusses both the theory and the applications of SEVS and provides an up-to-date study of the state of the art. Offers interpretations of SEVS spectra for practicing analysts. Discusses interpretation of SEVS spectra, which can often be very different to the non-enhanced spectrum - aids the practicing analyst.
The structure of free base phthalocyanine (H2Pc) has been predicted using density functional theory (DFT) at the B3LYP/6-31G* level and time-dependent density functional theory (TDDFT) has been applied to the excited states of H2Pc. The effects of tetraaza substitutions and tetrabenzo annulations on the molecular orbitals and excited states of H2Pc were examined by the comparison with H2P, H2Pz and H2TBP. These substitutions lead to the inversion of HOMO-1 (132 b1u,) and HOMO-3 (130 b1u) and as a result the spectra of H2Pc could not be interpreted by using the Gouterman four-orbital model. The effects of tetraaza subsitution are especially obvious which cause not only the B band but also the Q bands of H2Pz and H2Pc unable to be interpreted by using the Gouterman four-orbital model. Both the tetraaza substitutions and tetrabenzo annulations cause the oscillator strengths of Q band stronger, so the oscillator strengths of H2Pc are the largest among these compounds. The TDDFT energies are in good agreement with the experimental spectra.
Surface enhanced Raman scattering (SERS) is a popular technique to detect the molecules with high selectivity and sensitivity. It has been developed for 40 years, and many reviews have been published to summarize the progress in SERS. Nevertheless, how to make the SERS signals repeatable and quantitative and how to have deeper understanding of the chemical enhancement mechanism are two big challenges. A strategy to target these issues is to develop a Raman enhancement substrate that is flat and nonmetal to replace the conventional rough and metal SERS substrate. At the same time, the newly developed substrate should have a strong interaction with the adsorbate molecules to guarantee strong chemical enhancement. The flatness of the surface allows better control of the molecular distribution and configuration, while the nonmetal surface avoids disturbance of the electromagnetic mechanism.
Graphene-enhanced Raman scattering (GERS) is a recently discovered Raman enhancement phenomenon that uses graphene as the substrate for Raman enhancement, and can produce clean and reproducible Raman signals of molecules with increased signal intensity. Compared to conventional Raman enhancement techniques, such as surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), in which the Raman enhancement is essentially due to the electromagnetic mechanism, GERS mainly relies on a chemical mechanism, and therefore shows unique molecular selectivity. In this paper, we report graphene-enhanced Raman scattering of a variety of different molecules with different molecular properties. We report a strong molecular selectivity for the GERS effect with enhancement factors varying by as much as two orders of magnitude for different molecules. Selection rules are discussed with reference to two main features of the molecule, namely its molecular energy levels and molecular structures. In particular, the enhancement factor involving molecular energy levels requires the molecular highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energies to be within a suitable range with respect to graphene's Fermi level, and this enhancement effect can be explained by the time-dependent perturbation theory of Raman scattering. The enhancement factor involving the choice of molecular structures indicates that molecular symmetry and substituents similar to that of the graphene structure are found to be favorable for GERS enhancement. The effectiveness of these factors can be explained by group theory and the charge-transfer interaction between molecules and graphene. Both factors, involving the molecular energy levels and structural symmetry of the molecules, suggest that a remarkable GERS enhancement requires strong molecule-graphene coupling and thus effective charge transfer between the molecules and graphene. These conclusions are further experimentally supported by the change of the UV-visible absorption spectra of molecules when in contact with graphene and these conclusions are theoretically corroborated by first-principles calculations. These research findings are important for gaining fundamental insights into the graphene-molecule interaction and the chemical mechanism in Raman enhancement, as well as for advancing the role of such understanding both in guiding chemical and molecule detection applications, and in medical and biological technology developments.
In the chemical enhancement mechanism for Raman scattering, the two types of charge-transfer models, the excited-state and the ground-state charge-transfer mechanisms, present the different dependence of the enhanced Raman signals on the excitation wavelength. To investigate the type of charge-transfer mechanism in graphene-enhanced Raman scattering (GERS), the Raman excitation profiles of the copper phthalocyanine (CuPc) molecule were obtained in the range of 545—660 nm. The profiles in the GERS system were fitted well with the function of the ordinary resonant Raman scattering expression, where the incident and scattered resonance peaks were well-distinguished with the energy difference equaling the energy of the molecular vibrations. This result meets the prediction of ground-state charge transfer, in which model the dependence of the enhanced Raman signals on the excitation wavelength is the same as that of the ordinary Raman scattering, and rules out the prediction of the excited-state charge transfer because of no the possible charge-transfer resonance peak observed. Therefore, the GERS was proved to be a ground-state charge-transfer mechanism. Meanwhile, because the Raman excitation profiles of molecule can be obtained in the GERS system easily, which is usually difficult to obtain due to the self-absorption of the molecules, GERS opens up a new way to suppress this effect. This work contributes the deeper understanding of the graphene-enhanced Raman scattering.
For many applications, in particular in electronics, where chemical vapor deposited (CVD) graphene is used, it needs to be transferred from the growth substrate to the device substrate, generally employing a polymer film as a support layer. This process step is crucial for the overall integrity and electronic performance of the graphene. In this work we will investigate the effects of the transfer using various polymers with atomic force, Raman spectroscopy and X-ray photoelectron spectroscopy combined with field dependent transport measurements to build up a complete picture regarding the morphology, structural integrity and electrical performance of CVD graphene. Further, we introduce nitrocellulose based polymer alternatives to the most commonly used poly(methyl methacrylate) resist, outperforming the latter in most respects.
Infrared frequencies and intensities for the free-base phthalocyanine H2Pc and its N,N-dideuterio-derivative D2Pc have been calculated at density functional B3LYP, RBLYP and SVWN levels using the 6-31G∗ basis set to investigate the isotope effect in the IR spectra. The calculated vibrational frequencies were scaled by the factors 0.9613, 0.9940 and 0.9833 for 6-31G∗ basis set at B3LYP, RBLYP and SVWN levels, respectively. Detailed assignments of N–H related vibrational bands in the IR spectra have been made on the basis of comparison between the calculated data of H2Pc and D2Pc as well as the available experimental results of corresponding β form compounds. The normal mode descriptions were obtained with assistance of animated pictures produced on the basis of the normal coordinates. In the IR spectrum of H2Pc, the typical N–H stretching, in-plane bending (IPB) and out-of-plane bending (OPB) modes are assigned to the bands observed at 3273, 1250 and 752 cm−1, respectively. The bands observed at 1539, 1441, 1043 and 735 cm−1 that show isotope effects are also interpreted. The characteristic band of H2Pc at 1005 cm−1 is re-interpreted as a C–N (pyrrole) in-plane bending mode instead of the N–H in-plane or out-of-plane bending mode assigned by traditional empirical method. Calculations at B3LYP/6-31G∗ and RBLYP/6-31G∗ levels are found more suitable for further vibrational spectroscopic investigation of phthalocyanine and its derivatives. Animated pictures of the vibrational modes are useful for exact assignment of the correspondence between the vibrational modes of H2Pc and D2Pc.
A simple model of the charge distribution in a π-system is used to explain the strong geometrical requirements for interactions between aromatic molecules. The key feature of the model is that it considers the σ-framework and the π-electrons separately and demonstrates that net favorable π-π interactions are actually the result of π-σ attractions that overcome π-π repulsions. The calculations correlate with observations made on porphyrin π-π interactions both in solution and in the crystalline state. By using an idealized π-atom, some general rules for predicting the geometry of favorable π-π interactions are derived. In particular a favorable offset or slipped geometry is predicted. These rules successfully predict the geometry of intermolecular interactions in the crystal structures of aromatic molecules and rationalize a range of host-guest phenomena. The theory demonstrates that the electron donor-acceptor (EDA) concept can be misleading: it is the properties of the atoms at the points of intermolecular contact rather than the overall molecular properties which are important.
We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.
The structure, optical properties and surface morphology of thin films of metal free phthalocyanine (H2Pc) deposited in an ultra-high vacuum environment by organic molecular beam deposition have been studied using a variety of ex-situ techniques. The growth conditions have a strong influence on the properties of the films. H2Pc undergoes a phase transition (α→β) at a deposition temperature of ∽330°C, or upon post annealing a film grown at room temperature. Both the structure and optical properties of the films change and powder X-ray diffraction, electronic absorption spectroscopy, Raman and photoluminescence spectroscopies are used to characterise the differences between the two phases. Atomic force microscopy and Nomarski interference microscopy show that the lower temperature α-phase is characterised by a smooth morphology with spherical islands that show no apparent long-range order. By contrast, the β-phase has a much greater root mean square roughness and long thin needle-like crystals are observed on the surface of the films. The morphology of the β-phase depends on the method of preparation and there are two distinct types, β1 and β2. The crystallites show a preferential orientation and alignment with respect to each other for growth at room temperature followed by annealing (β1), but are randomly oriented for films grown at elevated substrate temperatures (β2).
Resonance Raman spectra are obtained when the wave number of the exciting radiation is close to, or coincident with, that of an electronic transition of the scattering species. Such spectra are usually characterized by a very large enhancement of the intensities of particular Raman bands, sometimes with the appearance of intense overtone and combination tone progressions. The technique provides detailed information about excited electronic states because it is only the vibrational modes associated with the chromophore that are resonance-Raman active. Additionally, the high sensitivity is such that compounds at concentrations as low as 10−6 mol/L may be detected, enabling resonance Raman spectroscopy to be used as an analytical tool and for the study of chromophores in molecules of biological interest.
The preresonance and resonance Raman spectra of solid films of metal-free phthalocyanine have been studied. A vibrational description of prominent spectral features is given in terms of the macrocycle (inner ring) and isoindole moieties. The observed changes in relative intensities of Raman lines with different excitation frequencies are also discussed.
Raman
scattering for a range of metal phthalocyanines using excitation frequencies from 457.9 to 1064 nm have
been interpreted in the light of the recent DFT calculation on zinc phthalocyanine. The intention was to determine
the features of the spectra that might be used for in situ analysis of specific phthalocyanines. B1
bands
were found to be the most intense. The frequency of one band with a large C–N–C ring displacement is particularly
sensitive to the metal ion size. The effect is determined not only by the size of the ion but also by the effect
it has on ring shape. First and second order overtone bands of zinc and copper phthalocyanines are broadly
similar, with some coupling differences at about 2000 cm−1. The region between 1350 and 1550 cm−1 has been
little studied previously. It shows a remarkable sensitivity to the metal ion present and provides a specific signature
for each phthalocyanine studied. In contrast, a study of α-, β-, γ- and ε-copper phthalocyanines using
514.5 nm excitation showed very few differences, suggesting that intra- rather than intermolecular
markers are most efficiently determined by Raman scattering. The study enables the interpretation of
the Raman spectra of the phthalocyanines in terms of molecular structure and due to the resonant enhancement involved
will enable the
in situ
identification
of specific phthalocyanines in matrices.
The optical absorption spectra of small particles of α-, β-, and x-form of metal-free phthalocyanine (H2Pc) and vanadyl phthalocyanine (VOPc) surrounded by a dielectric (polymers) have been studied. The true bulk absorption can be obtained by measuring the difference in transmittance of two films of identical composition but different thicknesses. This approach eliminates effectively light reflection losses. Comparison between the absorption spectra measured by direct vs. difference in transmittance revealed a number of differences: shift in the absorption frequencies, change in absorption intensities, and change in peak-to-valley ratio. These differences are due to light scattering which can be significantly minimized by using the differential transmittance method. The true peak absorption of x-H2Pc particles located at 770 and 615 nm had an absorption coefficient of 35.3 and 34.2 μm−1, respectively. The absorption minimum occurs at 477 nm. The peak-to-valley ratio was 27.The absorption spectra of VOPc particles did not correspond to any one polymorph, but are due to a mixture of two known polymorphs. Some discrepancies between the particle absorption of α-H2Pc or β-H2Pc and the corresponding evaporated film absorption were observed. These discrepancies are attributed to the spread in particle size and also to local agglomeration of the particles.
Infrared spectra of metal-free phthalocyanine, chlorinated and unchlorinated copper phthalocyanines, and three new metal derivatives of phthalocyanine: MoPc, Na2Pc, and AlOH—Pc (Pc = C32H16N8), have been recorded. Assignments have been suggested for many of the peaks in the infrared spectra of the phthalocyanines.
Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.
The SAC (symmetry adapted cluster)/SAC-CI method was used to calculate the ground and excited states of free base phthalocyanine (FBPc). This is the first accurate ab initio study of the excited states of FBPc. The calculated electronic spectrum agrees reasonably well with experimental results with regard to both energy and intensity. The relationships among the molecular structure, excitation energy, and spectral intensity are discussed, and the present results are compared with those for free base porphine (FBP) and free base tetrazaporphine (FBTAP) studied previously. Two important effects of skeletal changes are clarified; meso-tetraaza substitution and tetrabenzo substitution cause a large splitting between the HOMO and next HOMO levels and lead to a breakdown of the quasi-degeneracy of the two main configurations of the Q band, resulting in strong visible absorption due to the incomplete cancellation of the individual contributions to the transition dipole moment. This explains why Pc's are so useful as pigments. Further, a new assignment of the B (Soret) band is proposed. The broad experimental B band is composed of at least four states. The main peak of the B band is due to transitions from the orbital lower than the so-called “four orbitals”, and the transitions arising from the “four orbitals” constitute the shoulder in the lower energy side of the band.
The vibrational properties of single-layer PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) and H2Pc (metal-free phthalocyanine) thin films and PTCDA-H2Pc double-layer heterostructures are studied by Raman scattering. The evidence of crystallinity of the single-layer films can be supported by the existence of phonons, as well as polarization dependence. Resonance enhancement Raman scattering has been used to selectively measure the individual layer properties in double-layer H2Pc/PTCDA and PTCDA/H2Pc heterostructures. When H2Pc is grown on PTCDA, its structure departs from the herringbone arrangement characteristic of unstrained H2Pc films, as demonstrated by the different peak intensities in the Raman spectra. Well-defined phonons and the polarization dependence in the H2Pc top layer are characteristic of long-range order, indicating that the H2Pc molecular planes in the templated structure are crystalline. No evidence for a new structure was observed for PTCDA deposited on top of a H2Pc first layer. Here, the relative intensity of the phonons in the PTCDA top layer demonstrates that the PTCDA forms microcrystallites due to strain at the molecular heterojunction, before relaxing to its bulk crystalline form.
We have used measurements of Raman scattering-excitation profile spectra for totally symmetric and non-totally symmetric vibration modes of free-base phthalocyanine molecules adsorbed on Ag, as a modulation spectroscopic probe, to obtain information about the energies, widths and vibronic character of the singlet (π to π*) valence electron excitations and about the character of the bonding of the adsorbate by the substrate. The data show that the Qx and Qy excitations are red-shifted relative to the corresponding excitations of isolated free-base phthalocyanine molecules, and broadened, and that the Qx - Qy energy separation is reduced by about a factor of two. On the other hand the frequencies of the various vibration modes are unshifted. We find that the resonant Raman scattering at the Qx and Qy excitations is due primarily to a Herzberg-Teller mechanism that involves vibronic coupling of the nearly degenerate Qx and Qy states with one another and with higher singlet states. These results together with the absence of any sizable xz, yz and zz components of the contribution to the Raman tensor from the Qx and Qy excitations suggest that the π electrons are not primarily involved in the bonding of the phthalocyanine to the Ag substrate. The decrease in the Qx - Qy separation is tentatively attributed to a weakening of the hydrogen bonds at the central nitrogen atoms that may result from the bonding of the molecules by the substrate via the lone pair electrons of the central nitrogens atoms.
The growth of graphene during Cu-catalyzed chemical vapor deposition was
studied using 12CH4 and 13CH4 precursor gasses. We suggest that the growth
begins by the formation of a multilayer cluster. This seed increases its size
but the growth speed of a particular layer depends on its proximity to the
copper surface. The layer closest to the substrate grows fastest and thus
further limits the growth rate of the upper layers. Nevertheless, the growth of
the upper layers continues until the copper surface is completely blocked. It
is shown that the upper layers can be removed by modification of the conditions
of the growth by hydrogen etching.
Polarized optical absorption spectra of a rare Fe2+, Fe3+-bearing silicate mineral, tuhualite,
(Na,K)2Fe22+Fe23+Si12O30, were measured at room temperature in the range 350–4000 nm (ca. 28500–2500 cm–1). The spectra display a number of strongly pleochroic absorption bands in the visible
and NIR range, which are attributed to ferric and ferrous ions distributed over octahedral and tetrahedral
sites in the structure. No absorption bands caused by H2O or OH stretching vibrations have
been observed in the 3000 to 4000 cm–1 range. A narrow, weak absorption line at ~422 nm (γ ≈ β) is
attributed to the 6A1g → 4A1g,4Eg spin-forbidden transition of six-coordinated Fe3+ ions that occupy
the octahedral Fe2 position of the tuhualite structure. A broad, intense band at ~573 nm (γ > β >> α) is assigned to a Fe2+/Fe3+ intervalence charge transfer transition (IVCT) between tetrahedral Fe2+ and octahedral Fe3+. Together with the high-energy absorption edge, the band at 573 nm causes the intense violet color and spectacular pleochroism (γ > β >> α) of tuhualite. An intense band at ~1040 nm (γ > β >> α) and a much weaker band ~ 2150 nm (α) are attributed to spin-allowed dd transitions of Fe2+ in the strongly distorted tetrahedral Fe1 sites. On the basis of the data obtained for tuhualite, optical spectra of the structurally related minerals beryl, cordierite, and osumilite are discussed and re-assigned. The spectra are re-interpreted on the basis of the distribution of Fe2+ in these minerals over structural octahedral and tetrahedral positions.
Surface enhanced Raman scattering has been observed for phthalocyanine films deposited on silver. Surface roughness or 'metal islands' seems to be crucial for large enhancement factors. Metal enhanced Raman spectra appear to show specific changes in the relative intensities of vibrations associated with the inner part of the macrocycle.
Raman spectra have been obtained for solid iron phthalocyanine and a 10−6 M solution of its tetrasulfonated derivative with Kr+ laser excitation using the 6471 Å line. A normal coordinate analysis was carried out to determine assignments for the observed frequencies in terms of the various Raman active planar modes of vibration of the complex. Good agreement was obtained between calculated and observed frequencies using an adjusted valence force field (AVFF) and incorporating empirical correlations for related types of force constants. The assignments of the normal modes of vibration appear to be generally consistent with other avilable experimental and theoretical information for phthalocyanine complexes and the related metalloporphins; however, the extensive mixing of the internal coordinates within the calculated normal modes shows clearly that the assignment of specific vibrational frequencies to individual internal coordinates is not justified. Rather, the vibrational kinematics appear to involve concerted motions of the various interior and exterior ring systems.
The absorption, emission and excitation spectra of ZnPc and H(2)Pc trapped in Ne, N(2), Ar, Kr and Xe matrices have been recorded in the region of the Q states. A comparison of the matrix fluorescence spectra with Raman spectra recorded in KBr pellets reveals very strong similarities. This is entirely consistent with the selection rules and points to the occurrence of only fundamental vibrational transitions in the emission spectra. Based on this behaviour, the vibronic modes in emission have been assigned using results obtained recently on the ground state with large basis-set DFT calculations [Murray et al. PCCP, 12, 10406 (2010)]. Furthermore, the very strong mirror symmetry between excitation and emission has allowed these assignments to be extended to the excitation (absorption) bands. While this approach works well for ZnPc, coupling between the band origin of the S(2)(Q(Y)) state and vibrationally excited levels of S(1)(Q(X)), limits the range of its application in H(2)Pc. The Q(X)/Q(Y) state coupling is analysed from data obtained from site-selective excitation spectra, revealing pronounced matrix and site effects. From this analysis, the splitting of the Q(X) and Q(Y) states has been determined more accurately than in any previous attempts.
Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of the second-order overtone and combination Raman modes in a mostly unexplored frequency range of 1690-2150 cm(-1) in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. On the basis of the double resonance theory, four dominant modes in this range have been assigned to (i) the second order out-of-plane transverse mode (2oTO or M band), (ii) the combinational modes of in-plane transverse acoustic mode and longitudinal optical mode (iTA+LO), (iii) in-plane transverse optical mode and longitudinal acoustic mode (iTO+LA), and (iv) longitudinal optical mode and longitudinal acoustic mode (LO+LA). Differing from AB-stacked bilayer graphene or few layer graphene, single layer graphene shows the disappearance of the M band. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of the M band, whereas the substrate has no effect on the presence (or absence) of the M band. Dispersive behaviors of these "new" Raman modes in graphene have been probed by laser excitation energy-dependent Raman spectroscopy. It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene. This work expands upon the unique and powerful abilities of Raman spectroscopy to study graphene and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit the electronic band structure of graphene layers.
The field of organic photovoltaics (OPV) represents one of the most promising technological areas. Porphyrins and phthalocyanines are perfectly suited for their integration in light energy conversion systems. These colored macrocycles exhibit very attractive physical properties, particularly very high extinction coefficients in the visible and near IR regions, where the maximum of the solar photon flux occurs, that is necessary for efficient photon harvesting, besides a rich redox chemistry, as well as photoinduced electron transfer and semiconducting capabilities.
The infrared absorption spectra of matrix-isolated zinc phthalocyanine (ZnPc) and free-base phthalocyanine (H(2)Pc) have been recorded in the region from 400 to 4000 cm(-1) in solid N(2), Ar, Kr and Xe. Raman spectra have been recorded in doped KBr pellets. The isotopomers HDPc and D(2)Pc have been synthesised in an attempt to resolve the conflicting assignments that currently exist in the literature for the N-H bending modes in H(2)Pc spectra. A complete correlation between the vibrational modes of the three free-base isotopomers and ZnPc has been achieved. Comparison of the IR and Raman spectroscopic results, obtained with isotopic substitution and with predictions from large basis set ab initio calculations, allows identification of the in-plane (IP) and out-of-plane (OP) N-H bending modes. The largest IP isotope shift is observed in the IR at 1046 cm(-1) and at 1026 cm(-1) in Raman spectra while the largest effect in the OP bending modes is at 764 cm(-1). OP bending modes are too weak to be observed in the experimental Raman data. The antisymmetric N-H stretching mode is observed at approximately 3310 cm(-1) in low temperature solids slightly blue shifted from, but entirely consistent with the literature KBr data. With the exception of the N-H stretches, the recorded H/D isotope shifts in all the N-H vibrations are complex, with the IP bending modes exhibiting small nu(H)/nu(D) ratios (the largest value is 1.089) while one of the observed OP modes has a ratio < 1. DFT results reveal that the small ratios arise in particular from strong coupling of the N-H IP bending modes with IP stretching modes of C-N bonds. The unexpected finding of a nu(H)/nu(D) ratio smaller than one was analysed theoretically by examining the evolution of the frequencies of the free base by increasing the mass from H to D in a continuous manner. A consequence of this frequency increase in the heavier isotopomer is that the direction of the N-D OP bend is reversed from the N-H OP bend.
Graphene is a monolayer of carbon atoms packed into a two-dimensional (2D) honeycomb crystal structure, which is a special material with many excellent properties. In the present study, we will discuss the possibility that graphene can be used as a substrate for enhancing Raman signals of adsorbed molecules. Here, phthalocyanine (Pc), rhodamine 6G (R6G), protoporphyin IX (PPP), and crystal violet (CV), which are popular molecules widely used as a Raman probe, are deposited equally on graphene and a SiO(2)/Si substrate using vacuum evaporation or solution soaking. By comparing the Raman signals of molecules on monolayer graphene and on a SiO(2)/Si substrate, we observed that the intensities of the Raman signals on monolayer graphene are much stronger than on a SiO(2)/Si substrate, indicating a clear Raman enhancement effect on the surface of monolayer graphene. For solution soaking, the Raman signals of the molecules are visible even though the concentration is low to 10(-8) mol/L or less. What's more interesting, the enhanced efficiencies are quite different on monolayer, few-layer, multilayer graphene, graphite, and highly ordered pyrolytic graphite (HOPG). The Raman signals of molecules on multilayer graphene are even weaker than on a SiO(2)/Si substrate, and the signals are even invisible on graphite and HOPG. Taking the Raman signals on the SiO(2)/Si substrate as a reference, Raman enhancement factors on the surface of monolayer graphene can be obtained using Raman intensity ratios. The Raman enhancement factors are quite different for different peaks, changing from 2 to 17. Furthermore, we found that the Raman enhancement factors can be distinguished through three classes that correspond to the symmetry of vibrations of the molecule. We attribute this enhancement to the charge transfer between graphene and the molecules, which result in a chemical enhancement. This is a new phenomenon for graphene that will expand the application of graphene to microanalysis and is good for studying the basic properties of both graphene and SERS.
The optical absorption and photoluminescence of metal-free phtalocyanine (H2Pc) in a glassy matrix were discussed regarding the preparation method used. We show that fluorescent monodisperse free metal phtalocyanine can be doped in polyphenylsiloxane glass films using a non-aqueous sol–gel derived method. No fluorescence study of metal-free phtalocyanine doped in polysiloxane sol–gel materials has been reported so far due to the miscibility problem of the dye and its aggregation behavior in aqueous sols.
Photodynamic therapy (PDT) is an innovative and attractive modality for the treatment of small and superficial tumours. PDT, as a multimodality treatment procedure, requires both a selective photosensitizer and a powerful light source which matches the absorption spectrum of the photosensitizer. Quadra Logic's Photofrin, a purified haematoporphyrin derivative, is so far the only sensitizer approved for phase III and IV clinical trials. The major drawbacks of this product are the lack of chemical homogeneity and stability, skin phototoxicity, unfavourable physicochemical properties and low selectivity with regard to uptake and retention by tumour vs. normal cells. Second-generation photosensitizers, including the phthalocyanines, show an increased photodynamic efficiency in the treatment of animal tumours and reduced phototoxic side effects. At the time of writing of this article, there were more than half a dozen new sensitizers in or about to start clinical trials. Most available data suggest a common mechanism of action. Following excitation of photosensitizers to long-lived excited singlet and/ or triplet states, the tumour is destroyed either by reactive singlet oxygen species (type II mechanism) and/or radical products (type I mechanism) generated in an energy transfer reaction. The major biological targets of the radicals produced and of singlet oxygen are well known today. Nucleic acids, enzymes and cellular membranes are rapidly attacked and cause the release of a wide variety of pathophysiologically highly reactive products, such as prostaglandins, thromboxanes and leukotrienes. Activation of the complement system and infiltration of immunologically active blood cells into the tumorous region enhance the damaging effect of these aggressive intermediates and ultimately initiate tumour necrosis. The purpose of this review article is to summarize the up-to-date knowledge on the mechanisms responsible for the induction of tumour necrotic reactions.
Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)3]2+ on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)3] Cl2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at approximately 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)3]2+ upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)3]2+) and in off-resonance (458 and 1064 nm for Ag-bpy, 1064 nm for [Ru(bpy)3]2+). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)3]2+, the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.
The covalent attachment of organic films and of biological molecules to fused silica and glass substrates is important for many applications. For applications such as biosensor development, it is desired that the immobilised molecules be assembled in a uniform layer on the surface so as to provide for reproducibility and speed of surface interactions. For optimal derivatisation the surface must be appropriately cleaned to remove contamination, to create surface attachment sites such as hydroxyl groups, and to control surface roughness. The irregularity of the surface can be significant in defining the integrity and density of immobilised films. Numerous cleaning methods exist for fused silica and glass substrates and these include gas plasmas, and combinations of acids, bases and organic solvents that are allowed to react at varying temperatures. For many years, we have used a well established method based on a combination of washing with basic peroxide followed by acidic peroxide to clean and hydroxylate the surface of fused silica and glass substrates before oligonucleotide immobilisation. Atomic force microscopy (AFM) has been used to evaluate the effect of cleaning on surface roughness for various fused silica and glass samples. The results indicate that surface roughness remains substantial after use of this common cleaning routine, and can provide a surface area that is more than 10% but less than 30% larger than anticipated from geometric considerations of a planar surface.
Low temperature scanning tunneling microscopy (STM) studies of metal-free phthalocyanine (H2Pc) adsorbed on highly oriented pyrolytic graphite (HOPG) have shown ordered arrangement of molecules for low coverages up to 1 ML. Evaporation of H2Pc onto HOPG and annealing of the sample to 670 K result in a densely packed structure of the molecules. Arrangements of submonolayer, monolayer, and monolayer with additional adsorbed molecules have been investigated. The high resolution of our investigations has permitted us to image single molecule orientation. The molecular plane is found to be oriented parallel to the substrate surface and a square adsorption unit cell of the molecules is reported. In addition, depending on the bias voltage, different electronic states of the molecules have been probed. The characterized molecular states are in excellent agreement with density functional theory ground state simulations of a single molecule. Additional molecules adsorbed on the monolayer structures have been observed, and it is found that the second layer molecules adsorb flat and on top of the molecules in the first layer. All STM measurements presented here have been performed at a sample temperature of 70 K.
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