H2Pc Molecules Research Articles

Charge-state lifetimes of single molecules on few monolayers of NaCl

In molecular tunnel junctions, where the molecule is decoupled from the electrodes by few-monolayers-thin insulating layers, resonant charge transport takes place by sequential charge transfer to and from the molecule which implies transient charging of the molecule. The corresponding charge state transitions, which involve tunneling through the insulating decoupling layers, are crucial for understanding electrically driven processes such as electroluminescence or photocurrent generation in such a geometry. Here, we use scanning tunneling microscopy to investigate the decharging of single ZnPc and H2Pc molecules through NaCl films of 3 to 5 monolayers thickness on Cu(111) and Au(111). To this end, we approach the tip to the molecule at resonant tunnel conditions up to a regime where charge transport is limited by tunneling through the NaCl film. The resulting saturation of the tunnel current is a direct measure of the lifetimes of the anionic and cationic states, i.e., the molecule’s charge-state lifetime, and thus provides a means to study charge dynamics and, thereby, exciton dynamics. Comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator’s conduction and valence band, and the metal-insulator interface state.

Bistable H2Pc Molecular Conductance Switch on Ag(100)

Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study the tautomerization reaction of an H2Pc molecule adsorbed on a Ag(100) surface. The presence of two hydrogen atoms in the cavity of the H2Pc molecule enforces the existence of two molecular tautomers. It causes a reduction from 4- to 2-fold symmetry in STM images that can be recorded as two current states over the H2Pc molecule with a high-to-low current state ratio of ∼1.2. These findings are confirmed by the spatial distributions of the all-atom electron charge density calculated by using DFT and transmission maps together with tunneling current ratios (∼1.2) determined from the nonequilibrium Green’s function transport calculations. Therefore, we demonstrate that an H2Pc molecule adsorbed on a Ag(100) surface is a good candidate for a bistable molecular conductance switch since neither the presence of the Ag(100) surface nor that of the STM tip alters the tautomerization.

Molecular Chains: Arranging and Programming Logic Gates.

One particularly fascinating vision for charge-operated devices is the controlled assembly of structures from single surface-deposited molecules. Here, we report on the assembly of linear clusters that consist of phthalocyanine (H2Pc) molecules on a Ag(111) surface. The molecules are imaged as well as manipulated with a low-temperature scanning tunneling microscope (STM). Upon deprotonation of every second H2Pc, the resulting HPc molecule exhibits an isomeric bistability which can be used as inputs in logic gates. Combining our STM measurements with density functional theory calculations we show that the HPc isomers exhibit a repulsive electrostatic interaction with adjacent H2Pc molecules which, due to the asymmetric charge distribution on HPc, results in a counterclockwise or clockwise molecule tilt of the latter, thereby defining the logic 0 and 1 of the output. It is shown that information can be relayed along molecule chains over distances equivalent to at least nine molecules.

Study of Spectroscopic and Vibrational Properties of the Phthalocyanine Molecule (C_32 H_18 N_8) and Magnesium Phthalocyanine (C_32 H_16 MgN_8) by Using Quantum Programs

In this paper calculation spectroscopic and vibrational properties using Semi-empirical method, through the results showed the IR spectra, the value of the lowest amount of total energy for H2Pc molecule was equal to for (C16-H41) and for (C27-H43), for MgPc moleculeis equal to for (C17-H42) and for (C28-H44), and after that equilibrium distance is ( ) for each molecule. The theoretical and experimental results were compared to the energy gap, for H2Pc (5.67 eV), (5.04 eV), for MgPc (5.043 eV), (2.73 eV), respectively.

H2Pc and pentacene on Cu(110)-(2×1)O: A combined STM and nc-AFM study

Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) with a CuO tip is used to investigate adsorption of metal-free phthalocyanine (H2Pc) and pentacene on the Cu(110)-(2×1)O striped-phase reconstructed surface. We show that the combination of STM and nc-AFM is necessary to reveal the detailed adsorption geometry, necessary for interpreting the observed STM contrast. Comparison of the nc-AFM images with simulations shows that some of the H2Pc molecules are deformed out-of-plane on this surface, while pentacene retains its molecular geometry regardless of adsorption site. Our work highlights how the stiffness of CuO tips makes it possible to probe the structure of organic semiconductor / metal interfaces.

Imprinting Directionality into Proton Transfer Reactions of an Achiral Molecule.

Directionality is key for the functionality of molecular machines, which is often achieved by built-in structural chiralities. Here, we present a scanning tunneling microscopy study of achiral H2Pc and HPc molecules that acquire chirality by adsorption onto a Ag(100) surface. The adsorption-geometry-induced chirality is caused by a -29° (+29°) rotation of the molecules with respect to the [011] substrate direction, resulting in tautomerization processes that preferentially occur in a clockwise (counterclockwise) direction. The directionality is found to be independent of the particular energy and location of charge carrier injection. In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate.

Imaging Intermolecular Exciton Coupling in Metal-Free Phthalocyanine Nanofilms Using Tip-Enhanced Near-Field Optical Microscopy

Spatially correlated topography, fluorescence, and Raman scattering signals of metal-free phthalocyanine (H2Pc) nanofilms were imaged using confocal and tip-enhanced (fluorescence and Raman) near-field optical microscopy at lateral spatial resolutions of 500 nm and ∼25 nm, respectively. The far-field fluorescence intensity was found to vary inversely with film thickness up to 10 nm, with the strongest emission coming from isolated H2Pc molecules on samples with submonolayer coverage. Changes in the fluorescence energy distribution were imaged and were consistent with exciton coupling predictions for H2Pc molecules lying edge-to-edge on the surface. Thicker H2Pc films (>10 nm) were found to consist entirely of nonfluorescing aggregates and crystal domains. Tip-enhanced near-field measurements were carried out with W and Au (plasmonic) tips to investigate the local optical properties of H2Pc films and nanostructures. In general, fluorescence increased by a factor of 2–3× compared to the far-field when the t...

Barrierless On-Surface Metal Incorporation in Phthalocyanine-Based Molecules

On-surface metalation of metal-free phthalocyanine derivatives is a simple and solvent-free way to fabricate MPc compounds. Using phthalocyanine (H2Pc) molecules on Ag(111) as an example, we investigated the atomic-scale mechanisms of on-surface metalation processes using first-principles calculations based on density functional theory. When the molecules are deposited on a substrate first, we find that transition-metal atoms, except for Zn, drop directly from the vacuum into the molecule’s cavity without an energy barrier and bond with the inner four nitrogen atoms, with the two pyrrolic H atoms still in place. Subsequently, the two H atoms transfer to the substrate by overcoming small energy barriers and diffuse away. The substrate participates in the reaction by hybridization. In the alternative process, when metal atoms are adsorbed first on the surface and the H2Pc molecules are then added, the metal atoms diffuse into the cavity of the molecule via the molecule–surface interface by overcoming finite...

Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization

We present a detailed analysis of the energetic landscape of phthalocyanine (H2Pc) tautomerization on Ag(111), i.e., the switching of protons between different sites in the molecular frame, which is induced and measured by a low-temperature scanning tunneling microscope (STM). We demonstrate that tautomerization of this molecule is preferentially triggered by the excitation of the N–H stretching mode. Interestingly, a step-like increase of the tautomerization rate is observed at a bias voltage that corresponds to the second harmonic of this vibrational mode, which we attribute to the crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. This hypothesis is supported by the analysis of four modified versions of H2Pc, produced by single silver atom manipulation and/or deprotonation of the pristine H2Pc molecule. Depending on the particular modification, the step-like increase varies strongly, spanning the entire range from further enhancement to almost comple...

Raman excitation profiles of hybrid systems constituted by single‐layer graphene and free base phthalocyanine: Manifestations of two mechanisms of graphene‐enhanced Raman scattering

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. Copyright © 2017 John Wiley & Sons, Ltd.

Remote Single-Molecule Switching: Identification and Nanoengineering of Hot Electron-Induced Tautomerization.

Molecular electronics where single molecules perform basic functionalities of digital circuits is a fascinating concept that one day may augment or even replace nowadays semiconductor technologies. The tautomerization of molecules, that is, the bistable functional position of hydrogen protons within an organic frame, has recently been intensively discussed as a potential avenue toward nanoscale switches. It has been shown that tautomerization can be triggered locally or nonlocally, that is, by a scanning tunneling microscope (STM) tip positioned directly above or in close vicinity to the molecule. Whereas consensus exists that local switching is caused by inelastic electrons that excite vibrational molecular modes, the detailed processes responsible for nonlocal tautomerization switching and, even more important in the context of this work, methods to control, engineer, and potentially utilize this process are largely unknown. Here, we demonstrate for dehydrogenated H2Pc molecules on Ag(111) how to controllably decrease or increase the probability of nonlocal, hot electron-induced tautomerization by atom-by-atom designed Ag nanostructures. We show that Ag atom walls act as potential barriers that exponentially damp the hot electron current between the injection point and the molecule, reducing the switching probability by up to 83% for a four-atom wide wall. By placing the molecule in one and the STM tip in the other focal point of an elliptical nanostructure, we could coherently focus hot electrons onto the molecule that led to an almost tripled switching probability. Furthermore, single and double slit experiment based on silver atom structures were used to characterize the spatial extension of hot electron packets. The absence of any detectable interference pattern suggests that the coherence length of the hot electrons that trigger tautomerization processes is rather short. Our results demonstrate that the tautomerization switching of single molecules can remotely be controlled by utilizing suitable nanostructures and may pave the way for designing new tautomerization-based switches.

Experimental and theoretical study of X-ray K edges absorption spectra of carbon and nitrogen in the phthalocyanine H2Pc molecule

The study of the electronic structure of H2Pc was carried out to examine the structure of the lowest unoccupied molecular orbitals (LUMO) of molecule phthalocyanine by X-ray absorption spectroscopy using quantum-chemical calculations. The theoretical calculations were performed on the stationary theory (frozen orbital approximation, Z+1 model) and time-dependent density functional theory (TDDFT). A consideration of K edges absorption spectra of carbon and nitrogen in the common scale of binding energies allows estimating the contributions of AO of all phthalocyanine atoms to the LUMO, defining the sequence of levels, the binding energies of the corresponding levels, and also the character of electronic interactions between individual atoms. It was shown that the best agreement between the experimental and theoretical pre-edge structures of the absorption spectra of nitrogen and carbon for H2Pc is observed in the case of the application of stationary density functional theory in Z+1 model to account for an X-ray hole. In this case the 2p π AO of the Nα(1,2) and Сα atoms make a predominant contribution to the LUMO. The 2p π AO of the Nα(1,2) atoms mainly contribute to the boundary LUMO with the energy ~–2.3 eV.

Real-space investigation of energy transfer in heterogeneous molecular dimers.

Given its central role in photosynthesis and artificial energy-harvesting devices, energy transfer has been widely studied using optical spectroscopy to monitor excitation dynamics and probe the molecular-level control of energy transfer between coupled molecules. However, the spatial resolution of conventional optical spectroscopy is limited to a few hundred nanometres and thus cannot reveal the nanoscale spatial features associated with such processes. In contrast, scanning tunnelling luminescence spectroscopy has revealed the energy dynamics associated with phenomena ranging from single-molecule electroluminescence, absorption of localized plasmons and quantum interference effects to energy delocalization and intervalley electron scattering with submolecular spatial resolution in real space. Here we apply this technique to individual molecular dimers that comprise a magnesium phthalocyanine and a free-base phthalocyanine (MgPc and H2Pc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby H2Pc molecule as a result of resonance energy transfer from the former to the latter. A reciprocating resonance energy transfer is observed when exciting the second singlet state (S2) of H2Pc, which results in energy transfer to the first singlet state (S1) of MgPc and final funnelling to the S1 state of H2Pc. We also show that tautomerization of H2Pc changes the energy transfer characteristics within the dimer system, which essentially makes H2Pc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.

Opening the way to molecular cycloaddition of large molecules on supported silicene.

Within density functional theory, the adsorption of the H2Pc molecule on the (3 × 3) silicene/(4 × 4) Ag(111) surface has been investigated. We observe an electronic redistribution in the central macrocycle of the H2Pc molecule and the formation of two Si - N covalent bonds between the molecule and the silicene, in agreement with a cycloaddition reaction. However, while on SiC(0001)(3 × 3) or Si(111)(√3×√3)R30°-boron, the H2Pc molecule remains planar, and the H2Pc molecule takes a butterfly conformation on the silicene/Ag substrate due to an electrostatic or a polarization repulsion between the molecule and the silicene. Our study opens a way to the experimental adsorption of large organic molecules on supported silicene.

Positioning and switching phthalocyanine molecules on a Cu(100) surface at room temperature.

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual phthalocyanine (H2Pc) molecules adsorbed on a Cu(100) surface at room temperature. Scanning tunneling microscopy (STM) imaging directly demonstrates that H2Pc molecules can be controlled to move along the [011] or [011̅] surface direction of the Cu(100) surface, and the orientation of H2Pc molecules can also be switched between two angles of ±28° with respect to the [011] surface direction by a lateral manipulation. Owing to the highly efficient control over the adsorption site and orientation of H2Pc adsorbed on the Cu(100) surface by lateral manipulation, a pyramidal array formed by 10 H2Pc molecules has been constructed on the Cu surface as a prototype of binary memory, and every molecule within such a molecular array can be individually and reversibly controlled by a STM tip.

A new assisted molecular cycloaddition on boron doped silicon surfaces: a predictive DFT-D study.

In the framework of the Density Functional Theory (DFT-D), we investigate the phthalocyanine (H2Pc) molecule adsorption on SiC(0001)3 × 3 and Si(111)√3 × √3R30°-B (SiB) surfaces, and particularly compare the involved molecular adsorptions. In the H2Pc-SiC(0001)3 × 3 system, the molecular adsorption can be ascribed to a [10+2] cycloaddition. The H2Pc-SiB system is considered in three cases: defectless SiB surface (denoted SiB-0D) and SiB surfaces presenting one or two boron defects (denoted SiB-1D and SiB-2D respectively). The SiB-0D surface is passivated by a charge transfer from the Si adatoms to the boron atoms and therefore no chemical bond between the molecule and the substrate is observed. A similar molecular adsorption as already evidenced in the H2Pc-SiC(0001)3 × 3 system is involved in the SiB-2D case. In the case of the SiB-1D surface, two Si-N bonds (Si1-N1 and Si2-N2) are observed. One of them, Si1-N1, is nearly similar to that found in the H2Pc-SiB-2D system, but the Si2-N2 bond is unexpected. The Bader charge analysis suggests that, in the presence of the H2Pc molecule, the boron atoms behave like an electron reservoir whose availability varies following the involved molecular adsorption process. In the SiB-1D case, charges are transferred from the substrate to the molecule, allowing the Si2-N2 bond formation. Such a kind of molecular adsorption, not yet observed, could be designed by "assisted pseudo cycloaddition".

How Contacting Electrodes Affect Single π-Conjugated Molecular Electronic States: Local Density of States of Phthalocyanine Nanomolecules on MgO(001), Cu(111), Ag(001), Fe(001), and Mn(001)

Single molecules have attracted much interest as new materials for future spin electronic devices; however, many open questions still remain. One of them is how the electronic local density of states (LDOS) of single molecules is affected when they are in contact with electrodes. We show a systematic study of the LDOS of π-conjugated phthalocyanine (H2Pc) nanomolecules adsorbed on various electrodes, namely, (1) MgO(001) thin films grown on Ag(001), (2) noble metals of Cu(111) and Ag(001), and (3) 3d magnetic metals of Fe(001) and Mn(001), adupting scanning tunneling spectroscopy techniques with an ultrahigh-vacuum scanning tunneling microscopy setup at room temperature. Since MgO thin films cut the electronic coupling from the substrate Ag(001), we could observe H2Pc molecular states at -1.5 and +1.0 eV. H2Pc molecules on the noble metal substrates form a pattern with a square unit cell of about 1.5×1.5 nm2 and have similar LDOS peaks near the Fermi energy. Strong hybridizations between the substrate 3d spin-polarized states and the molecular π orbitals produce new molecular states of H2Pc molecules adsorbed on Fe(001) and Mn(001) near the energy positions of the Fe(001) minority spin state and the Mn(001) majority spin state, respectively.

Mechanical properties of H2Pc self-assembled monolayers at the single molecule level by noncontact atomic force microscopy

The mechanical properties of molecular self-assembled monolayers (SAMs) play an important role in understanding the interactions between molecules in the self-assembly, the interactions between molecules and substrate, and thus the formation mechanism of SAMs. Using a high-resolution noncontact atomic force microscope (NC-AFM) combined with a scanning tunneling microscope (STM), we have successfully obtained the sub-molecular resolution of a H2Pc self-assembled monolayer grown on a Pb(111) surface. A 2 × 2 superstructure was observed in both AFM and STM topographic images. The lateral critical force of removing a H2Pcmolecule from its SAM and moving a single H2Pc molecule on Pb(111) were measured. An oscillation of the critical force along the edge of the H2Pc SAM with a period of two molecular sites was observed, which can be attributed to the 2 × 2 superstructure. The lateral critical force caused by intermolecular interaction was found to be 25 pN on average and is typically two times larger than the molecule–substrate interaction.

Controlling vertical alignment of phthalocyanine nanofibers on transparent graphene-coated ITO electrodes for organic field emitters

In this study, we developed a simple and efficient approach—using graphene coatings on ITO as transparent electrodes—for inducing the growth of 29H,31H-phthalocyanine (H2Pc) nanofiber arrays through a thermal evaporation process, with potential use in organic field emitters (FEs). By controlling the surface energy and temperature of the electrodes during evaporation, H2Pc molecules readily self-assemble, forming an out-of-plane morphology on reduced graphene oxide surfaces (rGO; surface energy: ca. 50 mJ m−2). The devices fabricated on rGO/indium tin oxide (FE-rGO) exhibited not only excellent FE performance but also outstanding anti-degradation capability during stability tests. This facile approach toward rGO coatings opens a new avenue for the transformation of small organic molecule films into vertically standing nanostructures on transparent electrodes, with various organic electronics applications.

Magneto-Optical Kerr Effect Spectroscopy—A Sensitive Tool for Investigating the Molecular Orientation in Organic Semiconductor Films

The detection and control of the molecular growth mode is a key prerequisite for fabricating opto-electronic devices. In this work we present the magneto-optical Kerr effect (MOKE) spectroscopy to be a highly sensitive method for the detection of the molecular orientation. On the example of metal free phthalocyanine (H(2)Pc) in thin films, it will be shown that also for diamagnetic molecules a strong magneto-optical response can be expected. The growth mode and thus the intensity of the MOKE signal of H2Pc is strongly influenced by a templating effect using ultrathin layers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). From the MOKE spectra in the energy range from 1.5 to 5.0 eV and the optical constants, the Voigt constant of thin organic films was determined. From the strong in-plane/out-of-plane anisotropy of the optical constants and the value of the Voigt constant the average molecular tilt angle of H2Pc molecules with respect to the substrate plane can be obtained.