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Molecule-substrate interaction channels of metal-phthalocyanines on graphene on Ni(111) surface
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Molecule-substrate interaction channels of metal-phthalocyanines (MPcs, including NiPc, CuPc, ZnPc, FePc, and CoPc) on graphene on Ni(111) were investigated by employing high-resolution electron energy loss spectroscopy (HREELS). Except the expected IR-active modes, some Raman-active modes were also observed in all of MPcs, which are considered in this study. From the origination of the Raman-active features, it was deduced that MPcs are coupled with the substrate mainly through their central metal atom. The Raman-active modes appear as symmetric peaks in the HREELS in the case of MPcs with Ni, Cu, and Zn, whereas they are asymmetric and appear as a Fano line shape in the case of MPcs with Fe and Co. This spectroscopic difference indicates that the molecule-substrate coupling is completely different in the two cases mentioned above. The molecule-substrate interaction strength is considerably weak and comparable with the π-π interaction between molecules in the case of MPcs with Ni, Cu, and Zn, whereas it is much stronger in the case of MPcs with Fe and Co. From the HREELS observations, it can be suggested that the whole molecule can be effectively decoupled from the underneath Ni(111) by inserting a single layer of graphene between them in the case of MPcs with Ni, Cu, and Zn, whereas only benzene rings can be completely decoupled in the case of MPcs with Fe and Co.
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... The rise of graphene and related twodimensional materials guides discovering new hybrid structures formed by MN4 macrocyclic compounds and two-dimensional materials. For instance, graphene-MN4 and oxidized graphene-MN4 hybrid structures showed high structural and chemical stability by either - stacking, dispersion, or covalent bonding, even with improved dispersibility in solvents [25][26][27]. In this regard, hybrid structures formed by MN4 macrocyclic compounds have shown promising applications for catalysis; i.e., oxygen reduction reaction and oxidation of dopamine [28][29][30][31]; photoelectrochemical cells and photocatalysis [22,32]; optoelectronic devices and solar cells [26,33,34]; and nanoelectronic devices [35]. ...
... Interestingly, EDISP term is the second-highest contribution to stability with an average contribution of 34%. In this regard, phosphorene does not show an extended delocalized -electron system like graphene and carbon nanotubes, where non-functionalized macrocyclic molecules are bonded by strong - stacking[25,28,32,89]. Nevertheless, the pattern of dispersion interactions in the MP−Phos and MPs−Phos systems is similar to the - stacking of MN4 complexes on graphene as obtained by the reduced density gradient method(Fig. ...
Porphyrin and phthalocyanine metal complexes are organic semiconductor molecules, also known as MN4 macrocyclic compounds, widely implemented as photosensitizers and electrocatalysts in solution. In this regard, phosphorene, an emerging layered material entirely composed of phosphorous atoms, shows excellent potential features to act as a building block of novel hybrid nanostructures with organic molecules. Here, we perform a first-principles study of hybrid nanostructures formed by phosphorene nanosheets and deposition of MN4 compounds (where M: Fe, Co, Ni, Zn). It is found that the MN4-phosphorene hybrids have remarkable stability in the gas phase and aqueous solution, comparable to that achieved with carbonaceous and metallic-based substrates. In terms of structure, the hybrid structures display non-degenerated adsorption patterns at room temperature, denoting the formation of stable ordered flat MN4 layers (self-assembly). To understand the origin of the binding stability, we conducted further analysis of binding and energy decomposition. It is revealed that the stability is driven by the interplay between permanent electrostatic and dispersion physical effects, which stand for up to 90% of the stabilizing interaction energy. In terms of electronic properties, on the one hand, spin-polarized complexes do not show either switching in their spin state or substrate magnetization. On the other hand, all MN4 complexes act as mild n-dopants, slightly decreasing the phosphorene bandgap but retaining the topology of frontier orbitals concerning the free complexes for orbital-controlled reactions, e.g., electrochemical reactions in solution. In terms of optical properties, phosphorene induces shifting in the Q-band of phthalocyanines to longer wavelengths, which turns (metal)phthalocyanine-phosphorene hybrid materials into excellent candidates for the design of light-harvesting devices.
... Two kinds of vacuum sublimation methods have been employed to grow organic molecules on 2D materials. One typical approach is directly thermal evaporating organic molecules from a evaporator to the 2D materials which are placed in a sample holder above the evaporator (Wang and Hersam, 2009;Dou et al., 2011;Emery et al., 2011;Mao et al., 2011;Lemaitre et al., 2012;Singha Roy et al., 2012;Kim et al., 2015a,b;Nguyen et al., 2015Nguyen et al., , 2020Zheng et al., 2016). Another technique is physical vapor transport, that is when the organic powder is placed in a vacuum tube where the substrate with the 2D materials is placed in the same tube with a distance of several inches away from the organic powder, during which carrier gas can be employed or not (He et al., 2014;Lee et al., 2014Lee et al., , 2017Yang et al., 2018). ...
... Mao et al. (2011) also found that the molecular orientation of chloroaluminum phthalocyanine (ClAlPc) changes from random arrangement on a bare indium tin oxide (ITO) substrate to the lying-down manner on the chemical vapor deposition (CVD) graphene modified ITO with improved charge transport efficiency along the direction perpendicular to the ITO surface ( Figure 2B). In addition, in situ ultraviolet photoelectron spectroscopy (UPS) spectra proved that ClAlPc molecules adopted identical packing on HOPG (Dou et al., 2011). Similar face-on orientations on graphene were also found in many other OSC molecules, such as copper phthalocyanine (CuPc) (Singha Roy et al., 2012), pentacene (Kim et al., 2015b;Nguyen et al., 2015), and 9,10-bis(phenylethynyl) anthracene (BPEA) (Zheng et al., 2016;Figures 2C,D). ...
In the past three decades, organic semiconductor field-effect transistors (OFETs) have drawn intense attention as promising candidates for drive circuits of flat panel display, radio frequency identifications, chemical/bio-sensors, and other devices. Generally, the key parameters of OFETs, carrier mobility, threshold voltage, and on/off current ratio are closely related to the degree of order and surface/interface electronic structure of organic semiconductor (OSC) films. The ordering of films is crucially determined by the molecule-substrate interactions. On inert substrates (such as SiO2) OSC films can hardly reach a high degree of ordering without growth templates, while traditional single crystal surfaces usually force the OSC molecules to deviate from their favorite assemble manner resulting in an unstable structure. Recently, the rise of two-dimensional materials (2D) provides a possible solution. The in-plane lattice of 2D materials can offer possible epitaxy templates for OSCs while the weak van der Waals (vdWs) interaction between OSC and 2D layers allows for more flexibility to realize the epitaxy growth of OSCs with their favored assemble manner. In addition, the various band structures tuned by the layer numbers of 2D materials encourage widely modified OSC electronic structures by interface doping between the OSC and 2D layers, which benefits the structure by obtaining high-performance OFETs. In this review, we emphasize and discuss the recent advances of OSC-2D hybrid OFETs. The OSC-2D heterostructures not only promote OFET device performances by film morphology/structure optimization and channel electronic structure modification, but also offer platforms for basic organic solids physics investigation and further functional optoelectronic devices.
... But since the bare Fe substrate is highly interactive, there is the need for a decoupling layer between the metal substrate and the organic molecules to be deposited. Graphene has been also proposed to act as a well-functioning decoupling layer, e.g. on Ni [13]. However, the orthohexagonal lattice structure of graphene is not compatible with the body centered cubic lattice structure of iron crystal. ...
... 12,13 More recently, there has been a growing interest for integrating graphene into organic electronic devices in order to exploit its combination of flexibility, excellent conductivity, and high optical transparency. 14−16 Hence, various studies investigated the molecule−substrate interactions and the self-assembly of the MPc structure deposited on graphene epitaxially grown on SiC(0001), 17,18 Ru(0001), 19 Ni(111), 20,21 Pt(111), 22 and so forth. While these studies achieved important milestones in better understanding the interfacial coupling between MPc molecules and graphene, they mostly focused on selfassembled ultrathin (i.e., submonolayer) MPc films deposited in ultrahigh vacuum equipment. ...
Graphene shows great promise not only as a highly conductive flexible and transparent electrode for fabricating novel device architectures but also as an ideal synthesis platform for studying fundamental growth mechanisms of various materials. In particular, directly depositing metal phthalocyanines (MPc’s) on graphene is viewed as a compelling approach to improve the performance of organic photovoltaics and light-emitting diodes. In this work, we systematically investigate the ZnPc physical vapor deposition (PVD) on graphene either as-grown on Cu or as-transferred on various substrates including Si(100), C-plane sapphire, SiO2/Si, and h-BN. To better understand the effect of the substrate on the ZnPc structure and morphology, we also compare the ZnPc growth on highly crystalline single- and multilayer graphene. The experiments show that, for identical deposition conditions, ZnPc exhibits various morphologies such as high-aspect-ratio nanowires or a continuous film when changing the substrate supporting graphene. ZnPc morphology is also found to transition from a thin film to a nanowire structure when increasing the number of graphene layers. Our observations suggest that substrate-induced changes in graphene affect the adsorption, surface diffusion, and arrangement of ZnPc molecules. This study provides clear guidelines to control MPc crystallinity, morphology, and molecular orientations which drastically influence the (opto)electronic properties.
Magnetic molecules on ferromagnetic metallic substrates have been widely explored to exploit the potential for molecular magnetic storage and spintronics applications. Recent advances in these hybrid interfaces integrated with two-dimensional materials have been proposed as a flexible platform for realizing new spin-related effects. Herein, the impact of inserting graphene on the electronic and magnetic properties of a family of transition metal phthalocyanines (TMPcs, TM = Cr, Mn, Fe, Co, and Cu) deposited on ferromagnetic Ni(111) surfaces have been systematically rationalized by density functional theory analysis. Our calculations reveal that the magnetic exchange interaction across the molecule-substrate interfaces can be significantly mediated by the introduction of a graphene interlayer. Interestingly, these TMPcs exhibit ferromagnetic coupling with the Ni substrate. However, the strength of this coupling is reduced in the presence of a graphene decoupling layer, with the exception of CoPc. In the case of CoPc, the original ferromagnetic coupling with Ni(111) can be altered to antiferromagnetic when a graphene interlayer is introduced. By analyzing the different channels of communication involved in the spin interaction between the molecule and the magnetic substrate, we attribute these significant differences to the varied influences on the exchange interaction caused by the intermediary graphene layer. The presence of the inserted graphene layer may block the direct exchange interaction between the TMPc molecule and substrate, while the indirect superexchange interaction facilitated by the nitrogen atoms of the organic ligands is only reduced. Our study thus demonstrates that the inserted graphene can serve as an optimal intermediary layer for mediating the magnetic couplings across the molecule-substrate interfaces while allowing effective spin communication between them. These findings provide important insights into relevant experiments and offer a promising strategy to control the magnetic exchange interactions via utilizing graphene at metal-molecule interfaces.
The adsorptions of iron(II) phthalocyanine (FePc) on graphene and defective graphene were investigated systematically using density functional theory. Three types of graphene defects covering stone-wales (SW), single vacancy (SV), and double vacancy (DV) were taken into account, in which DV defects included DV(5-8-5), DV(555-777), and DV(5555-6-7777). The calculations of formation energies of defects showed that the SW defect has the lowest formation energy, and it was easier for DV defects to form compared with the SV defect. It is more difficult to rotate or move FePc on the surface of defective graphenes than on the surface of graphene due to bigger energy differences at different sites. Although the charge analysis indicated the charge transfers from graphene or defective graphene to FePc for all studied systems, the electron distributions of FePc on various defective graphenes were different. Especially for FePc@SV, the d xy orbital of Fe in the conduction band moved toward the Fermi level about 1 eV, and the d xz of Fe in the valence band for FePc@SV also moved toward the Fermi level compared with FePc@graphene and other FePc@defective graphenes. Between the planes of FePc and defective graphene, the electron accumulation occurs majorly in the position of the FePc molecular plane for FePc@SW, FePc@DV(5-8-5), and FePc@DV(5555-6-7777) as well as FePc@graphene. However, electrons were accumulated on the upper and lower surfaces of the FePc molecular plane for FePc@SV and FePc@DV(555-777). Thus, the electron distribution of FePc can be modulated by introducing the interfaces of different defective graphenes.
The interest in graphene (a carbon monolayer) adsorbed on metal surfaces goes back to the 60's, long before isolated graphene was produced in the laboratory. Owing to the carbon-metal interaction and the lattice mismatch between the carbon monolayer and the metal surface, graphene usually adopts a rippled structure, known as moiré, that confers it interesting electronic properties not present in isolated graphene. These moiré structures can be used as versatile templates where to adsorb, isolate and assemble organic-molecule structures with some desired geometric and electronic properties. In this review, we first describe the main experimental techniques and the theoretical methods currently available to produce and characterize these complex systems. Then, we review the diversity of moiré structures that have been reported in the literature and the consequences for the electronic properties of graphene, attending to the magnitude of the lattice mismatch and the type of interaction, chemical or physical, between graphene and the metal surface. Subsequently, we address the problem of the adsorption of single organic molecules and then of several ones, from dimers to complete monolayers, describing both the different arrangements that these molecules can adopt as well as their physical and chemical properties. We pay a special attention to graphene/Ru(0001) due to its exceptional electronic properties, which have been used to induce long-range magnetic order in tetracyanoquinodimethane (TCNQ) monolayers, to catalyze the (reversible) reaction between acetonitrile and TCNQ molecules and to efficiently photogenerate large acenes.
We studied the influence of topological defects on the bonding strength, geometries and electronic parameters for free-base H2Pc and two 3d-metal derivatives (CuPc and ZnPc) interacting with graphene sheet models. The DFT calculation results obtained for the cluster models containing isolated pentagon (5), pyracylene unit (5665) and Stone-Wales (SW) defect were compared to those for a defect-free graphene model (G). The topology of graphene surface has a strong influence on the shape of usually planar phthalocyanine molecules. While on the models of pristine and SW defect-containing graphene macrocycles remain generally flat, the presence of isolated pentagonal defects and pyracylene units in most cases induces strong distortion of phthalocyanine system, in order to increase the area of its contact with graphene sheet, and thus to provide a more efficient π-π stacking interaction between the two components. In the case of cone-shaped 5 and 5665 nanoclusters, the macrocycle distortion is more pronounced in the case of ‘endo’ as compared to ‘exo’ adsorption; in turn, it is stronger for 5665 model, which has a sharper curvature. The noncovalent bonding is strong for all the complexes studied, increasing in the order of 5665_exo <5_exo <5665_endo <5_endo < G < SW.
The copper phthalocyanine and 2,3,5,6‐tetrafluoro‐tetracyano‐quino‐dimethane molecules adsorbed on the copper and single‐layer graphene‐modified copper surfaces have been investigated using Raman and X‐ray photoelectron spectroscopy. The adsorbed amounts for both molecules on the two different substrates are almost comparable as confirmed by the C 1s and N 1s core level X‐ray photoelectron spectra. Raman signal enhancement has been observed for both molecules when they are adsorbed on the graphene surface. The strongest chemical interaction is observed between the 2,3,5,6‐tetrafluoro‐tetracyano‐quino‐dimethane and copper substrate. Based on the Raman and X‐ray photoelectron spectroscopic data, the mechanism of the graphene‐enhanced Raman scattering is discussed, which might greatly rely on the unique electronic properties of graphene.
Two-dimensional materials (2DMs) with excellent mechanical, thermal, optical, and catalytic properties have attracted a great deal of attention in recent years. Chemical vapor deposition (CVD) is an important method to realize the synthesis of high-quality 2DMs. In the growth of 2DMs through the CVD method, the substrates play an important role and can greatly affect the lateral size, composition, thickness, orientation, and crystal quality of 2DMs. In this review, we first introduce the growth mechanism and the key parameters in the CVD system for the synthesis of 2DMs. Then, the unique physical and chemical properties, advantages, and disadvantages of various substrates used in the CVD technique are summarized. Finally, the opportunities and challenges about the use of the substrate in the CVD process in the future are discussed.
The geometrical, electronic, and vibrational properties of one monolayer of Zinc-phthalocyanine (ZnPc) adsorbed on Ag(110) are studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and high-resolution electron energy-loss spectroscopy (HREELS). STM and LEED revealed that the molecules lie flat on the surface, ordered in a compact arrangement with a supercell defining a coincidence mesh with the substrate lattice. By comparing the HREELS spectra of one monolayer to those of a multilayer film, in which the molecules are weakly interacting, it was found that the electronic and vibrational properties of the molecular film are sensibly perturbed at the interface. The Q and B bands corresponding to optical interband excitations measured for the multilayer are not detected for the monolayer film and an intense low-energy Drude-like plasmon loss in the infrared region is observed. The vibrational features are also modified: several Raman modes of the isolated molecule were found to become infrared active for the monolayer because of the lowering of the molecular symmetry induced by the interaction with the substrate. Moreover a sizeable vibrational softening was measured for the selected modes indicating a charge transfer from the substrate to the molecules. Finally the strong asymmetric line shape observed for one of the Raman modes is discussed in terms of interfacial dynamical charge transfer and electron-phonon coupling.
Quasi-two-dimensional intercalationlike systems, synthesized by surface intercalation of gold atoms underneath a monolayer of graphite formed on Ni(111), have been investigated by angle-resolved photoemission spectroscopy (PES) and high-resolution electron-energy-loss spectroscopy (HREELS). Modifications of both electronic structure and vibrational properties due to the presence of the intercalated Au monolayer between the graphite monolayer and Ni(111) are reported. The surface intercalation of the gold atoms underneath the graphite monolayer leads to a “stiffening” of the graphite-derived phonon modes and to an energetic shift of the graphite-derived π, σ electronic states in the valence band toward lower binding energies. The observed changes of the PE and HREEL spectra are explained by the saturation of the active Ni(d) bonds by the intercalated gold atoms and by the weakening of the interaction between graphite monolayer and substrate, due to the blockage of the graphite C(π)-Ni(d) hybrid bonds.
Despite the importance of interface dipole on the charge carrier injection at metal/organic contacts, there is yet no estimation of the various contributions to the overall dipole. We propose a simple approach to delineate and estimate the contribution of metal-induced interface states (MISs) toward the overall dipole. The relative contribution of the MIS was found to increase as the slope parameter decreases. By using published results, we estimate the relative MIS contributions in organic-silver contacts for various organic semiconductors to be −30%–80% of the overall dipole.
The infrared spectra of phthalocyanine and seven divalent metal derivatives have been recorded in the region 400–4000 cm−1. The metal-ligand vibration and other metal dependent bands were found and the anomalies of Cu2+ and Zn2+-phthalocyanines were interpreted on the basis of electronic configurations of outer orbitals of the metals.
The interfaces formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were examined using photoemission and inverse photoemission spectroscopy. It is observed that in F16CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F16CuPc the band bending is confined in BP2T only. The combination of the band bending and finite Debye lengths provides an explanation to the observed ambipolar behavior of the organic thin film transistors based on such heterojunctions.
High-resolution vibrational spectroscopy of fullerene thin films is achieved by sum-frequency generation and infrared absorption. The Ag\(2\) mode of C60 gains strong infrared activity when the molecule is adsorbed on Ag(111). K doping allows us to demonstrate the process of adsorbate/substrate dynamical charge transfer, by showing, for the first time, the quenching of the mode softening and infrared activity upon tuning of the admolecule electronic properties. These observations enable the quantitative evaluation of the coupling strength of the Ag\(2\) vibration to the t1u orbital of C60.
We present a mechanism that explains the energy-level alignment at organic-organic (OO) semiconductor heterojunctions. Following our work on metal/organic interfaces, we extend the concepts of charge neutrality level (CNL) and induced density of interface states to OO interfaces, and propose that the energy-level alignment is driven by the alignment of the CNLs of the two organic semiconductors. The initial offset between the CNLs gives rise to a charge transfer across the interface, which induces an interface dipole and tends to align the CNLs. The initial CNL difference is reduced according to the screening factor S, a quantity related to the dielectric functions of the organic materials. Good quantitative agreement with experiment is found. Our model thus provides a simple and intuitive, yet general, explanation of the energy-level alignment at organic semiconductor heterojunctions.
In this paper, the formation of ordered structures at the pentacene/Ag(111) interface is investigated. Pentacene molecules are thermally evaporated and deposited onto a Ag(111) substrate held at room temperature. The interface is studied with a combination of x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). To begin with, pentacene molecules form a loosely bound, disordered contact layer at the interface in which the molecules are oriented parallel to the surface. Remarkably, the molecular layer which grows on top of the contact layer exhibits order, with molecules arranged in rows and tilted around their long axis out of the surface plane. The in-plane orientation of molecular rows in this second molecular layer is templated by the steps of the substrate. A few further ordered layers with similar in-plane structure can be grown on top of the bilayer. The results presented here illustrate a mechanism by which ordered structures of molecular materials can form in spite of an initial lack of order in the contact layer at the interface.
After prolonged annealing of a Ru(0001) sample in ultrahigh vacuum a superstructure with a periodicity of ∼30 Å was observed by scanning tunneling microscopy (STM). Using x-ray photoelectron spectroscopy it was found that the surface is covered by graphitic carbon. Auger electron spectroscopy shows that between 1000 and 1400 K carbon segregates to the surface. STM images recorded after annealing to increasing temperatures display islands of the superstructure, until, after annealing to T⩾1400 K, it covers the entire surface. The morphology of the superstructure shows that it consists of a single graphene layer. Atomically resolved STM images and low-energy electron diffraction reveal an (11×11) structure or incommensurate structure close to this periodicity superimposed by 12×12 graphene cells. The lattice mismatch causes a moiré pattern. Unlike the common orientational disorder of adsorbed graphene, the graphene layer on Ru(0001) shows a single phase and very good rotational alignment. Misorientations near defects in the overlayer only amount to ∼1°, and the periodicity of ∼30 Å is unaffected. In contrast to bulk graphite both carbon atoms in the graphene unit cell were resolved by STM, with varying contrast depending on the position above the Ru atoms. The filled and empty state images of the moiré structure differ massively, and electronic states at −0.4 and +0.2 V were detected by scanning tunneling spectroscopy. The data indicate a significantly stronger chemical interaction between graphene and the metal surface than between neighboring layers in bulk graphite. The uniformity of the structure and its stability at high temperatures and in air suggest an application as template for nanostructures.
The visible and near ultraviolet absorption spectra have been measured for thin films of H2Pc [C32H18N8], MgPc, FePc, CoPc, CuPc, and ZnPc. Magnesium and iron phthalocyanine have very different absorption spectra. For most of the phthalocyanines, structure of magnitude 0.21 eV is seen on the visible and Soret bands. A d band is indicated in the region 4.0–4.7 eV. This is the spectral region most sensitive to substitutions of the central metal atom.