CuPc Molecules Research Articles

Organic Memristor Utilizing Copper Phthalocyanine Nanowires with Infrared Response and Cation Regulating Properties

AbstractCurrent organic memristive devices have been suffering from unstable performance, ambiguous mechanism, and poor NIR response, thus restricting their commercial translation. Here, a near‐infrared‐sensitive (NIR) organic memristive device with high stability based on solution‐processed copper phthalocyanine nanowires (N‐CuMe2Pc NWs) is first reported. Compared with uneven thermal evaporated N‐CuMe2Pc film, the N‐CuMe2Pc NWs film possesses a uniform 3D mesh structure, which attribute to the localized cationic migration, robust formation/rupture of conductive filament and subsequent improvement of reproducibility, thermal stability, and retention characteristics. Furthermore, operating voltage and OFF current can be readily regulated by NIR illumination due to strong NIR absorption of the well‐aligned edge‐to‐edge interconnected N‐CuMe2Pc NWs and tunable potential barrier formed between active layer and Ag electrode, which are further verified by absorption spectrum and Kelvin probe force microscope analysis, respectively. This study provides a generalized method for optimizing device performance and attaching phototunable properties of organic memristive memories. In addition, compared with pristine CuPc molecules with low solubility, limitation of thermal evaporation approach that is incompatible with scaling up is expected to overcome by the solution‐processed N‐CuMe2Pc NWs.

The role played by the molecular geometry on the electronic transport through nanometric organic films.

The electronic transport properties in molecular heterojunctions are intimately connected with the molecular conformation between the electrodes, and the electronic structure of the molecule/electrode interface. In this work, we perform an ab initio density-functional-theory investigation of the structural and transport properties through self-assembled CuPc molecules sandwiched between gold contacts with (111) surfaces. We demonstrated (i) a tunneling regime ruled by the π orbitals of the aromatic rings of CuPc molecules; and (ii) a high variation (up to two orders of magnitude) of the current density with the orientation of the CuPc molecules relative to the gold surface. The source of this variation is the geometrical dependence of the energy of the highest-occupied-molecular-orbital with respect to the chemical potential of the metal and the generation of intra-molecular transport channels for a configuration with CuPc molecules tilted with respect to the gold surface.

基于电荷转移的钙钛矿单晶和多晶材料表面增强拉曼散射研究

The charge transfer(CT) process plays a key role in the operation of the optoelectronic device system so a better understanding of the interfacial CT property is greatly important. In this paper, Surface Enhanced Raman Scattering(SERS) was utilized to study the interfacial CT property between CuPc and perovskites(single crystal and polycrystalline). The Raman spectra of CuPc adsorbed on the perovskite surface was enhanced. The laser wavelength dependent SERS study indicates that this phenomenon is mainly arising from the CT from the VB band of perovskite to the LUMO band of the CuPc molecules. In comparison, the SERS signal of CuPc molecules adsorbed on a single crystal is much stronger than that on the polycrystalline perovskite. This result indicates that the defect status affects the enhancement ability of the materials. Further study shows that, after the decoration of a thin silver film, the SERS spectra of CuPc on both single crystal and polycrystalline perovskites are further enhanced. The extreme enhancement is not only due to the electromagnetic property of the silver film but also the fact that the SPR of the silver enhances the charge separation of the perovskite, which further promotes the CT process between the substrate and adsorbed molecules. The CT based SERS study shows great potential application value in the field of optoelectronic research.

Mechanism of field-induced manipulation of Cu-phthalocyanines on a Bi surface using scanning tunneling microscope

We have revisited our hypothesis [K. Nagaoka et al., Jpn. J. Appl. Phys. 57 (2018) 020301] regarding the control of the motion and assembly of copper phthalocyanine (CuPc) molecules on a Bi(001) surface using a scanning tunneling microscope (STM) tip. The proposal that the observed diffusion and condensation are not due to the adsorption/desorption of molecules via the tip by applying a bias voltage was investigated for field-induced manipulation. Our experiments show that CuPc cannot move across steps or condense on different terraces on the Bi(001) surface. When confined to a nano-scale region, the CuPc molecules assemble into ordered single-layer islands. The island position can be manipulated using the STM tip, with an almost constant number of condensed molecules remaining inside the terrace. These results allowed us to rule out the possibility of picking-up and depositing molecules by the STM tip.

Enhanced Raman Scattering of CuPc Films on Imperfect WSe2 Monolayer Correlated to Exciton and Charge‐Transfer Resonances

AbstractRecently, 2D transition‐metal dichalcogenides (2D TMDCs) are identified as ideal substrates for surface‐enhanced Raman scattering (SERS). However, the effect of enhancement factor (EF) on TMDCs is lower than metal‐based SERS substrates. Here, it is demonstrated that the SERS performance of WSe2 monolayer can be enhanced via tailoring the atomic ratio of WSe2; this correlates to the exciton and charge‐transfer resonances. CuPc molecules are adsorbed onto WSe2 monolayers as the probe molecules, and the atomic ratio (Se:W) of WSe2 is tailored from 2 to 1.92. For an atomic ratio of 1.96, the maximum EF on irradiated WSe2 is more than 120; this is enhanced by more than 40 times compared with pristine WSe2. The amplitude of exciton and charge‐transfer resonances is estimated by femtosecond optical pump‐probe measurement and a bipolar junction transistor (BJT) consisting of CuPc film and 2D materials. It is found that the intensity of resonances in the CuPc–WSe2 system is tailored by the atomic ratio of WSe2. This is closely correlated to the SERS performance of WSe2. This study shows that the SERS performance of WSe2 is enhanced by tuning the atomic ratio of WSe2 and provides qualitative theoretical explanations for the mechanism of enhanced SERS.

Emergent Properties of the Organic Molecule-Topological Insulator Hybrid Interface: Cu-Phthalocyanine on Bi2Se3

We unravel the bidirectional influence of the adsorbate–substrate hybrid interface in the case of CuPc doping of the topological insulator (TI) surface Bi2Se3. Using ultrahigh vacuum scanning tunneling microscopy at low temperature (77 K), we observe that for a dilute concentration, single CuPc molecules are dispersed on terraces of Bi2Se3 as individual entities or as clusters. The site-dependent submolecular resolution images of CuPc on Bi2Se3 reveal three different sites for CuPc adsorption. Scanning tunneling spectroscopy (STS) measurements show a rigid shift of the Dirac point toward negative voltage by 336 meV upon CuPc deposition. This is a clear signature that the topological surface state experiences a charge transfer because of CuPc. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of CuPc on Bi2Se3 is also measured using STS. It is found to be larger than that on the Au(111) substrate because of the reduced screening offered by the TI Bi2Se3 sur...

Probing Interaction between Individual Submonolayer Nanoislands and Bulk MoS2 Using Ambient TERS

Tip-enhanced Raman spectroscopy (TERS) has shown that detecting single molecules with a high spatial resolution is possible in ultrahigh vacuum (UHV) at low temperature with plasmonic metallic substrates. It is still challenging to probe interactions of molecules with semiconductors, which is important in biosensing, photovoltaics, and many other applications. Here we demonstrate that in ambient conditions it is possible to obtain Raman signals from submonolayer molecular islands on bulk MoS2 using TERS. Analysis of relative Raman signal intensity ratio and Raman spectral peak position from spatial TERS mapping showed differences in the adsorbate–adsorbate and adsorbate–substrate interactions on Au and MoS2 substrates. The Raman transition which involves the vibration of the metal center of the CuPc molecule experienced a change in the relative Raman signal intensity ratio due to the differences in the molecule–substrate charge transfer interaction. In comparison to the other vibrational modes, the vibrat...

Chirality switching of the self-assembled CuPc domains induced by electric field.

Chiral switching of the self-assembled domains of CuPc molecules on the Cd(0001) surface has been investigated by means of a low temperature scanning tunneling microscopy (STM). With the coverage increasing, the CuPc molecules show the structural evolutions from an initial gas-like state to a network phase, a square phase, and finally to a compact phase at full monolayer. In the network and square phases, the achiral CuPc molecules reveal both the point chirality and chiral domains. In particular, the chirality of network domain can be switched from one enantiomer to another driven by the electric filed from a STM tip, which can also lead to the lattice rotation of network phase. These results demonstrate that (i) there is strong interaction between the CuPc molecules and STM tip; (ii) the adsorbed CuPc molecules carry considerable net charge or polarizability due to the charge transfer; (iii) the network phase has a low barrier for the interconversion between right- and left-handed domains. Our findings are significant for the understanding and control of the domain's chirality in the self-assembled structures.

Controlling molecular condensation/diffusion of copper phthalocyanine by local electric field induced with scanning tunneling microscope tip

We have discovered the condensation/diffusion phenomena of copper phthalocyanine (CuPc) molecules controlled with a pulsed electric field induced by the scanning tunneling microscope tip. This behavior is not explained by the conventional induced dipole model. In order to understand the mechanism, we have measured the electronic structure of the molecule by tunneling spectroscopy and also performed theoretical calculations on molecular orbitals. These data clearly indicate that the molecule is positively charged owing to charge transfer to the substrate, and that hydrogen bonding exists between CuPc molecules, which makes the molecular island stable.

Mechanism of Surface-Enhanced Raman Scattering Based on 3D Graphene-TiO2 Nanocomposites and Application to Real-Time Monitoring of Telomerase Activity in Differentiation of Stem Cells.

With a burst development of new nanomaterials for plasmon-free surface-enhanced Raman scattering (SERS), the understanding of chemical mechanism (CM) and further applications have become more and more attractive. Herein, a novel SERS platform was specially designed through electrochemical deposition of graphene onto TiO2 nanoarrays (EG-TiO2). The developed EG-TiO2 nanocomposite SERS platform possessed remarkable Raman activity using copper phthalocyanine (CuPc) as a probe molecule. X-ray photoelectron spectroscopy measurement revealed that the chemical bond Ti-O-C was formed at the interface between graphene and TiO2 in EG-TiO2 nanocomposites. Both experimental and theoretical results demonstrated that the obvious Raman enhancement was attributed to TiO2-induced Fermi level shift of graphene, resulting in effective charge transfer between EG-TiO2 nanocomposites and molecules. Taking advantage of a marked Raman response of the CuPc molecule on the EG-TiO2 nanocomposite surface as well as specific recognition of CuPc toward multiple telomeric G-quadruplex, EG-TiO2 nanocomposites were tactfully employed as the SERS substrate for selective and ultrasensitive determination of telomerase activity, with a low detection limit down to 2.07 × 10-16 IU. Interestingly, the self-cleaning characteristic of EG-TiO2 nanocomposites under visible light irradiation successfully provided a recycling ability for this plasmon-free EG-TiO2 substrate. The present SERS biosensor with high analytical performance, such as high selectivity and sensitivity, has been further explored to determine telomerase activity in stem cells as well as to count the cell numbers. More importantly, using this useful tool, it was discovered that telomerase activity plays an important role in the proliferation and differentiation from human mesenchymal stem cells to neural stem cells. This work has not only established an approach for gaining fundamental insights into the chemical mechanism (CM) of Raman enhancement but also has opened a new way in the investigation of long-term dynamics of stem cell differentiation and clinical drug screening.

Delayed Triplet-State Formation through Hybrid Charge Transfer Exciton at Copper Phthalocyanine/GaAs Heterojunction.

Light absorption in organic molecules on an inorganic substrate and subsequent electron transfer to the substrate create so-called hybrid charge transfer exciton (HCTE). The relaxation process of the HCTE states largely determines charge separation efficiency or optoelectronic device performance. Here, the study on energy and time-dispersive behavior of photoelectrons at the hybrid interface of copper phthalocyanine (CuPc)/p-GaAs(001) upon light excitation of GaAs reveals a clear pathway for HCTE relaxation and delayed triplet-state formation. According to the ground-state energy level alignment at the interface, CuPc/p-GaAs(001) shows initially fast hole injection from GaAs to CuPc. Thus, the electrons in GaAs and holes in CuPc form an unusual HCTE state manifold. Subsequent electron transfer from GaAs to CuPc generates the formation of the triplet state in CuPc with a few picoseconds delay. Such two-step charge transfer causes delayed triplet-state formation without singlet excitation and subsequent intersystem crossing within the CuPc molecules.

Dielectric response of branched copper phthalocyanine

The dielectric constant of pressed pellets and thin films of branched copper phthalocyanine (CuPc) was investigated as a function of frequency from 0.1 kHz to 1 MHz and temperature from 20 °C to 100 °C. Surface morphology was studied using a scanning electron microscope. The high-frequency values of the dielectric constant of pellets and thin films are ~3.5 and ~5.8, respectively. The response was only weakly dependent on frequency and temperature. The branched structure of the CuPc molecules helped to cancel out the effects of low-frequency polarization mechanisms. A planar delocalized charge system with two-dimensional localization was found using time-resolved photoluminescence measurements.

Nanorod growth of copper phthalocyanine on fluorinated phosphonic acid SAM-modified indium tin oxide substrate for organic photovoltaic devices

ABSTRACTCuPc nanorod growth was investigated on ITO substrates with and without 1H,1H,2H,2H-perfluoro-noctylphosphonic acid (FOPA) SAM modification, using vapor phase deposition, through the control of the temperature and surface energy of ITO substrates. At a substrate temperature of 200°C, copper phthalocyanine (CuPc) thin films that composed nanorods were prepared on both substrates. On FOPA-ITO, it was suggested that CuPc molecules were stacked with an edge-on orientation because of the dominant intermolecular π–π interaction through the decrease of surface energy of ITO with FOPA modification. Organic photovoltaic cells containing CuPc nanorods as a p-type semiconductor were fabricated and photovoltaic characteristics were observed.

Study of copper-phthalocyanine and pentacene film growth on transferred graphene: The influence of polymer residues

Wet transferring of chemical vapor deposition (CVD) graphene with the assistance of poly(methyl methacrylate) (PMMA) usually leaves polymer contaminations onto graphene surface. Knowledge about the influence of PMMA residues on organic film growth properties is crucial for graphene-related organic electronic devices. In this study, we investigate growth properties of coper-phthalocyanine (CuPc) and pentacene on PMMA contaminated graphene substrate. Grain size and morphologies of CuPc and pentacene films were found to be governed by the density of PMMA residues on graphene. Nevertheless, the molecular packing structures of CuPc and pentacene respond differently with the density with PMMA residues. CuPc molecules tend to nucleated into grains with molecular π-π stacking parallel to graphene surface, which is irrespective of the PMMA residues. Whereas the packing behavior of pentacene on PMMA-transferred CVD graphene is sensitive to the density of PMMA residues. Molecular π-π stacking perpendicular to graphene is observed when pentacene was deposited onto as-transferred graphene. Parallel mode was observed only on the substrates without PMMA residues.

Fluffy and Ordered Graphene Multilayer Films with Improved Electromagnetic Interference Shielding over X-Band.

Highly ordered nitrogen-doped graphene multilayer films with large interlayer void are successfully fabricated by thermal annealing of the compact stacking graphene oxide/copper phthalocyanine (GO/CuPc) multilayer films. Scanning electron microscopic (SEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopic (XPS), and electrical conductivity measurements indicate that the breakaway of oxygen functional groups on/in the GO sheets at high temperature and the in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets synergistically facilitate the restoration of GO in graphitization, the effective nitrogen doping by replacing carbon atoms in graphene frameworks, the retention of layer-by-layer stacking structure of graphene sheets in plane, and the formation of interlayer voids, leading to the enhancement in the electrical conductivity (3.64 × 103 S/m). In addition, due to the formation of a Fabry-Pérot resonance cavity in the unique layer-by-layer stacking structure with larger interlayer voids, constructive interference of internal reflections aligned between parallel reflecting planes endows the fluffy graphene multilayer films with excellent electromagnetic interference (EMI) shielding effectiveness (exceeds 25 dB in all X-bands). The optimal shielding effectiveness is up to 55.2 dB with a smaller thickness of 0.47 mm, which makes it possible to become a practical EMI shielding material with a distinct competitive advantage.

Influence of dielectric substrate modification and deposition temperature on structure and morphology of CuPc thin films: X-ray reflectivity and angle dependent NEXAFS study

The performances of organic thin film transistor devices are significantly linked with the structural properties at organic semiconductor/dielectric interface. The changes in internal structure, molecular ordering and morphology of 20nm thick CuPc thin films have been investigated by modifying surface of the dielectric substrate with various organic buffer layers at different deposition temperatures. CuPc films are prepared on bare and modified SiO2 substrates at three deposition temperatures. Dielectric surface modification and deposition temperature modify the CuPc /dielectric interfaces accordingly and growth of subsequent CuPc layer. The internal structure, ordering and morphology of CuPc film strongly depends on the behavior of the dielectric layers at various temperatures as well as the diffusion of CuPc molecules. The XRR results reveal that the thickness and ordering of periodic part of CuPc film is varied with dielectric substrate modification as well as deposition temperature. The periodicity of CuPc molecules in the film is always obtained in its α-form. In addition, the angle dependent NEXAFS data determine the angle of CuPc molecular orientation in the range 64° to 71° in the range of 40° to 120°C deposition temperature, independent of surface modification. The results pave the way for the design and realization of CuPc based thin film transistor devices.

Off-Center Rotation of CuPc Molecular Rotor on a Bi(111) Surface and the Chiral Feature.

Molecular rotors with an off-center axis and the chiral feature of achiral CuPc molecules on a semi-metallic Bi(111) surface have been investigated by means of a scanning tunneling microscopy (STM) at liquid nitrogen (LN2) temperature. The rotation axis of each CuPc molecular rotor is located at the end of a phthalocyanine group. As molecular coverage increases, the CuPc molecules are self-assembled into various nanoclusters and finally into two-dimensional (2D) domains, in which each CuPc molecule exhibits an apparent chiral feature. Such chiral features of the CuPc molecules can be attributed to the combined effect of asymmetric charge transfer between the CuPc and Bi(111) substrate, and the intermolecular van der Waals interactions.

Two-Dimensional Heterostructure as a Platform for Surface-Enhanced Raman Scattering

Raman enhancement on a flat nonmetallic surface has attracted increasing attention, ever since the discovery of graphene enhanced Raman scattering. Recently, diverse two-dimensional layered materials have been applied as a flat surface for the Raman enhancement, attributed to different mechanisms. Looking beyond these isolated materials, atomic layers can be reassembled to design a heterostructure stacked layer by layer with an arbitrary chosen sequence, which allows the flow of charge carriers between neighboring layers and offers novel functionalities. Here, we demonstrate the heterostructure as a novel Raman enhancement platform. The WSe2 (W) monolayer and graphene (G) were stacked together to form a heterostructure with an area of 10 mm × 10 mm. Heterostructures with different stacked structuress are used as platforms for the enhanced Raman scattering, including G/W, W/G, G/W/G/W, and W/G/G/W. On the surface of the heterostructure, the intensity of the Raman scattering is much stronger compared with isolated layers, using the copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman enhancement effect on heterostructures depends on stacked methods. Phonon modes of CuPc have the strongest enhancement on G/W. W/G and W/G/G/W have a stronger enhancement than that on the isolated WSe2 monolayer, while lower than the graphene monolayer. The G/W/G/W/substrate demonstrated a comparable Raman enhancement effect than the G/W/substrate. These differences are due to the different interlayer couplings in heterostructures related to electron transition probability rates, which are further proved by first-principle calculations and probe-pump measurements.

Study on mobile hole generation in blend MoO3:CuPc by capacitance-voltage method

Systematic C-V measurements were carried out on specially designed capacitance-like devices to study the generation of mobile carriers in 1:1 v% MoO3:CuPc blend layer. In these devices, it was experimentally found that only mobile holes were generated by the external electrical field. These mobile holes were generated from CuPc molecules and could only transport among them. Based on this hole generation mechanism, all the enhancements of hole transport in CuPc layer reported previously could be explained by the extra mobile holes generated in the blend, increasing conductivity. Meanwhile a model of capacitance with mobile carriers has been proposed, and the relation between the amount of mobile holes generated and the applied bias voltage has been calculated accordingly. Quite reasonable results were obtained.

Si(100)上に吸着した銅フタロシアニン薄膜の表面構造解析

Si(100)上に吸着した銅フタロシアニン薄膜の表面構造解析