Organic Molecular Electronics Research Articles

Professor Boris Filippovich Minaev: More than Half a Century of Scientific Activity and the 80th Anniversary of His Birth

The article highlights the main periods of life, scientific and pedagogical activity of Boris Filippovich Minaev: Doctor of Chemical Sciences, Professor, Academician of the Academy of Sciences of the Higher School of Ukraine, Honored Worker of Science and Technology of Ukraine. The article reflects Minaev's great contribution to scientific research and informs about his achievements in the field of physical organic chemistry and molecular electronics. At the end of this article an overview of his most important scientific papers is presented.

Quantum interference effects elucidate triplet-pair formation dynamics in intramolecular singlet-fission molecules.

Quantum interference (QI)-the constructive or destructive interference of conduction pathways through molecular orbitals-plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation.

Potential Building Blocks for 1,4-Dihydro-N-heteroacenes.

Studies have been performed aimed at the synthesis of N‐heteroacenes via substitution reactions of 4,5‐difluoro‐1,2‐dinitrobenzene with a diamine. The fluorine atoms are displaced first, followed by an activated nitro group. Two intermediates have been characterised by X‐ray single‐crystal structure determinations. Their intermolecular interactions were examined by Hirshfeld surfaces to assess their suitability for organic molecular electronics. The high reactivity of the phenazine, which is prone to oxidise and rearrange, as are displacement products prepared from it, is explained by the formation of a cis‐aci‐nitro form from the secondary amine of the phenazine and a nitro group.

Adsorption and spin polarization of pyridine on Fe/W(1 1 0) interface: A DFT study

Organic molecules containing conjugated π electrons acquire a magnetic moment when adsorbed on ferromagnetic surfaces. Plane-wave DFT calculations show that pyridine molecule, which has no magnetism in the gas phase, acquires a magnetic moment of 0.10 μB magnitude after adsorption on the ferromagnetically ordered Fe/W (110) surface. The molecule is strongly chemisorbed onto the surface, and the induced moment anti-aligns with respect to the surface moment. Analysis of the interface electronic structure shows that the spin polarization results from the hybridization of the out-of-plane pz orbitals of carbon and nitrogen in pyridine molecule with the d orbitals of Fe atoms on the ferromagnetic surface. As a result of this interaction, the magnetization of individual atoms in the Fe/W (110) layers directly beneath the molecule change from their clean surface values. The electronegativity of nitrogen results in the induction of a more substantial magnetic moment in pyridine than benzene. Understanding this organic-ferromagnetic interface is important to developing research in the fields of organic spintronics and molecular electronics.

Carbon & boron substituted-Dibenzo[1,4]azaborinine as a quantum dot-based organic molecular single-electron device for high switching speed applications

The present work investigates the carbon & boron substituted-Dibenzo[1,4]azaborinine molecule as an island or quantum dot(QD) of an organic molecular single electron device(SED). The ‘carbon & boron substituted-Dibenzo[1,4]azaborinine(C13H9BN) as a QD’ is a modified variant of the acene group organic molecule ‘anthracene(C14H10)’, where boron atom are substitution of dangling hydrogens of middle honeycomb and nitrogen substitution of its parent molecule anthracene. Similar to anthracene, the carbon & boron substituted-Dibenzo[1,4]azaborinine is also made up of three linearly fused benzene rings, and has a planar structure with aromatic character. The carbon & boron substituted-Dibenzo[1,4]azaborinine molecule is considered as a QD owing to its impressive stability and conductance that originate from the aromaticity, making it ideal for low power electronic and high switching speed device applications. The calculations have been carried out using the tools AtomisticToolkit-Virtual Nanolab(ATK-VNL) and Gaussian09 packages, which is based on density functional theory (DFT), non-equilibrium green’s function (NEGF) formalisms and ab-initio approach. The charging energies in both isolated and SED conditions, total energy, and the charge stability diagram (CSD) as a function of bias potentials have been used to evaluate the device's efficiency. In comparison to anthracene based SED and its other acene organic molecular derivative SEDs, the proposed ‘carbon & boron substituted-Dibenzo[1,4]azaborinine as a quantum dot based organic molecular SED’ has high switching speed and power efficiency.

Effects of cis-trans Conformation between Thiophene Rings on Conductance of Oligothiophenes.

Oligothiophenes have been established as important π-conjugated frameworks in organic electronics and molecular electronics. Although oligothiophenes possess the rotational flexibility of thiophene rings, the effects of cis-trans conformations on their electrical conductance have not been investigated yet. To investigate the effects of cis-trans conformations between thiophene rings on the conductance of oligothiophenes, we performed first-principles transport calculations. The conductance of the cis-oligothiophene was calculated to be higher than that of trans-oligothiophene, because the highest occupied molecular orbital was closer to the Fermi level of the gold electrode in the cis isomer than the trans isomer. This prediction was confirmed through mechanically controllable break junction measurements and fitting of the current-voltage characteristics for the newly synthesized, insulated oligothiophenes with controlled cis-trans conformations. This study demonstrates that cis- and trans-conformations can affect the electrical properties of oligothiophene frameworks and can potentially be used to control the electronic structure of long oligothiophene molecular wires.

Improved performance of quantum dot light-emitting diodes by hybrid electron transport layer comprised of ZnO nanoparticles doped organic small molecule

Abstract ZnO nanoparticles (NPs) are widely employed as the electron transport layer (ETL) material in quantum dot light-emitting diodes (QLEDs) due to their excellent electron transport properties and inherent stability. However, the unbalanced carrier injection caused by the redundant electron injection is a common phenomenon existing in QLEDs with ZnO ETL. Here, based on the lower electron mobility and shallower lowest unoccupied molecular orbit (LUMO) of most organic electron transport ·materials than ZnO NPs, we report a strategy to modulate the electron transporting properties of ZnO NPs by doping organic small molecular electron transport materials. Several kinds of small molecular dopants including LiQ (lithium 8-quinolate), TPBi (1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene), and BPhen (4,7-diphenyl-1,10-phenanthroline) are separately introduced to modify ZnO. The resulting device with LiQ-doped ZnO ETL at a doping ratio of 7 wt% exhibits the optimal performance at peak current efficiency (CE) and external quantum efficiency (EQE) up to 8.07 cd/A and 7.74%, which are about 1.93- and 1.94-fold higher than 4.19 cd/A and 3.98% of the device with pristine ZnO ETL, respectively. The improvement of performances can be mainly attributed to the inhibition of electron injection and interfacial exciton quenching by the use of LiQ-doped ZnO ETLs. This work may offer a promising method for fabricating high performance QLEDs and promote their application development in the display fields.

Through-Space Conjugation: A Thriving Alternative for Optoelectronic Materials

Efficient electronic coupling is the key to constructing optoelectronic functional π systems. Generally, the delocalization of π electrons must comply with the framework constructed by covalent bonds (typically σ bonds), representing classic through-bond conjugation. However, through-space conjugation offers an alternative that achieves spatial electron communication with closely stacked π systems instead of covalent bonds thus enabling multidimensional energy and charge transport. Because of the ever-accelerating advances of through-space conjugation studies, researchers are inspired greatly by the beauty of through-space conjugated systems and their potential in high-tech applications. In this mini review, we introduce some representative and newly developed π systems having the through-space conjugation feature. In addition to discussing the profound impacts of through-space conjugation on the luminescence properties and charge transport, we will review some impressive findings of distinctive molecules with attractive characteristics, such as aggregation-induced emission, thermally activated delayed fluorescence, bipolar charge transport, and multichannel. These achievements may bring about new breakthroughs of theory, materials, and devices in the fields of organic electronics and molecular electronics.

Self-Assembly Evolution of Metal-Free Naphthalocyanine Molecules on Ag(111) at the Submonolayer Coverage

Structures of self-assembled films play essential roles in the performance of potential organic molecular electronics; therefore, detailed knowledge of molecular adsorption and structural evolution is fundamental for the implementation of molecular electronics. Here, we systematically investigated the initial adsorption and structure evolution of metal-free naphthalocyanine (H2Nc) on the Ag(111) surface from dimers to ordered self-assembled structures in the submonolayer range. H2Nc molecules deposited on Ag(111) at 100 K accumulate into clusters dominated by dimers. Subsequent sample annealing induces the emergence of two ordered self-assembled structures, denoted by Sl and Sll. Molecule-resolved scanning tunneling microscopy images confirm that structure Sl is more stable with six rotation domains lying in two chiralities. Deviations of the lattice parameters from a threefold symmetry indicate non-neglectable intermolecular interactions in self-assembled patterns. Combined with density functional theory...

Acridinium Based Organic Molecular Single Electron Transistor for High Performance Switching Applications

The conductance and switching behaviour of acene series derivative `Acridinium' based organic molecular single electron transistor have been assessed in the vicinity of Density Functional Theory (DFT) and Non-equilibrium Green's Function (NEGF) formalisms based ab-initio approach. Computationally, acridinium has been obtained by nitrogen substitution of its parent molecule anthracene. Our analysis of structural and chemical properties predicts relatively high electrical conductivity for acridinium than anthracene owing to the high degree of delocalization associated with the former. The performance of acridinium as a quantum dot of SET has been studied by calculating charging energies and charge stability diagram. It has been observed that acridinium SET is power efficient and possesses better switching behaviour than anthracene and other acene derivative SETs reported so far. Moreover, the negative gate bias operation of acridinium SET is observed to be energetically more favourable than the positive bias operation.

Enabling single qubit addressability in a molecular semiconductor comprising gold-supported organic radicals.

A bis(dithiolene)gold complex is presented as a model for an organic molecular electron spin qubit attached to a metallic surface that acts as a conduit to electrically address the qubit. A two-membered electron transfer series is developed of the formula [AuIII(adt)2]1-/0, where adt is a redox-active dithiolene ligand that is sequentially oxidized as the series is traversed while the central metal ion remains AuIII and steadfastly square planar. One-electron oxidation of diamagnetic [AuIII(adt)2]1- (1) produces an S = 1/2 charge-neutral complex, [AuIII(adt2 3-˙)] (2) which is spectroscopically and theoretically characterized with a near negligible Au contribution to the ground state. A phase memory time (T M) of 21 μs is recorded in 4 : 1 CS2/CCl4 at 10 K, which is the longest ever reported for a coordination complex possessing a third-row transition metal ion. With increasing temperature, T M dramatically decreases becoming unmeasurable above 80 K as a consequence of the diminishing spin-lattice (T 1) relaxation time fueled by spin-orbit coupling. These relaxation times are 1-2 orders of magnitude shorter for the solid dilution of 2 in isoelectronic [Ni(adt)2] because this material is a molecular semiconductor. Although the conducting properties of this material provide efficient pathways to dissipate the energy through the lattice, it can also be used to electrically address the paramagnetic dopant by tapping into the mild reduction potential to switch magnetism "on" and "off" in the gold complex without compromising the integrity of its structure. These results serve to highlight the need to consider all components of these spintronic assemblies.

Enhancement of Photoswitchable Dielectric Property by Conducting Electron Donors on Plasmonic Core–Shell Gold-Fluorenyl C60 Nanoparticles

A new nanomaterial design was made by the construction of a tetralayered core–shell configuration, containing a magnetic core of nanospherical γ-FeOx particle (NP) encapsulated by one plasmonic gold subshell, one middle layer of electron-polarizable C60(>DPAF-Cn)x-derived fullerosome membrane, and an outer shell of organic electron donors. This class of multilayered core–shell nanospherical materials was recently reported to be excellent microwave absorbers. We found that this analogous of multilayered NPs was capable of inducing photoswitchable dielectric property (permittivity) amplification at the microwave frequency range of 1.0–4.0 GHz. The enhanced phenomena were further investigated by the use of six variable organic molecular electron donors and two conducting conjugated polymers, serving as the supply of photoinduced transferrable electrons at the outer shell layer, for comparison. The design largely increased the number of polarizable charges, leading to a maximum of 387% amplification of the re...

12th International Conference on Nanomolecular Electronics (ICNME-2016)

Preface12th International Conference on Nano-Molecular Electronics (ICNME2016) was held on December 14-16, 2016. This international conference is a series conference and has been held at Kobe, Japan, every two year. This conference has made a great contribution to the development of organic electronics materials & molecular electronics since its inception in 1994 (see special issues, Japanese Journal of Applied Physics 34, No.7B, 1995, Thin Solid Films 331, 1998, Thin Solid Films 393, 2001, Thin Solid Films 438-439, 2003, Thin Solid Films 499, 2006, Thin Solid Films 516, Issue 9, 2008, Thin Solid Films 518, Issue 2, 2009, Physics Procedia 14, 2012, Thin Solid Films 554, Issue 3, 2014, Journal of Nanoscience and Nanotechnology 16, no.4, 2016). This conference focuses mainly on the most attractive and interesting research subjects concerning the electrical and photonic properties and functions of organic materials. Main topics of the ICNME2016 were Fundamentals of organic and molecular devices, Characterization and device physics, Energy harvesting materials and Devices, Molecular photonics and nanophotonics, Organic semiconductor devices and applications, Soft material physics and applications in electronics, and Bioelectronics and nanointerface fabrication. Since its inception of this conference, ICNME has been paying attention to “interface issues” in molecular systems and in devices. In this ICNME2016, recent progress in the science and technology aspect on electronics, photonics and molecular electronics using organic molecules, polymers, and bio-related materials were intensively discussed in terms of “interfaces”, from viewpoints of physics, chemistry and device applications. About 130 papers have been submitted from universities, research laboratories and industries from Japan and overseas, and these were presented as Plenary Talks, Invited Talks, and Oral and Poster presentations. Selective papers were accepted for publication after strict peer review process by experts in the world.

Spintronics: The molecular way.

Molecular spintronics is an interdisciplinary field at the interface between organic spintronics, molecular magnetism, molecular electronics and quantum computing, which is advancing fast and promises large technological payoffs.

Printed Organic and Inorganic Electronics: Devices To Systems

Affordable and versatile printed electronics can play a critical role for large area applications, such as for displays, sensors, energy harvesting, and storage. Significant advances including commercialization in the general area of printed electronics have been based on organic molecular electronics. Still some fundamental challenges remain: thermal instability, modest charge transport characteristics, and limited lithographic resolution. In the last decade, one-dimensional nanotubes and nanowires, like carbon nanotubes and silicon nanowires, followed by two-dimensional materials, like graphene and transitional dichalcogenide materials, have shown interesting promise as next-generation printed electronic materials. Challenges, such as non-uniformity in growth, limited scalability, and integration issues, need to be resolved for the viable application of these materials to technology. Recently, the concept of printed high-performance complementary metal–oxide semiconductor electronics has also emerged and been proven successful for application to electronics. Here, we review progress in CMOS technology and applications, including challenges faced and opportunities revealed.

Chemical Analysis of the Interface in Bulk-Heterojunction Solar Cells by X-ray Photoelectron Spectroscopy Depth Profiling

Despite the wide use of blends combining an organic p-type polymer and molecular fullerene-based electron acceptor, the proper characterization of such bulk heterojunction materials is still challenging. To highlight structure-to-function relations and improve the device performance, advanced tools and strategies need to be developed to characterize composition and interfaces with sufficient accuracy. In this work, high-resolution X-ray photoelectron spectroscopy (XPS) is combined with very low energy argon ion beam sputtering to perform a nondestructive depth profile chemical analysis on full Al/P3HT:PCBM/PEDOT:PSS/ITO (P3HT, poly(3-hexylthiophene); PCBM, [6,6]-phenyl-C61-butyric acid methyl ester; PEDOT, poly(3,4-ethylenedioxythiophene; PSS, polystyrenesulfonate; ITO, indium tin oxide) bulk-heterojunction solar cell device stacks. Key information, such as P3HT and PCBM composition profiles and Al-PCBM chemical bonding, are deduced in this basic device structure. The interface chemical analysis allows us to evidence, with unprecedented accuracy, the inhomogeneous distribution of PCBM, characterized by a strong segregation toward the top metal electrode. The chemical analysis high-resolution spectra allows us to reconstruct P3HT/PCBM ratio through the active layer depth and correlate with the device deposition protocol and performance. Results evidence an inhomogeneous P3HT/PCBM ratio and poorly controllable PCBM migration, which possibly explains the limited light-to-power conversion efficiency in this basic device structure. The work illustrates the high potential of XPS depth profile analysis for studying such organic/inorganic device stacks.

The interaction between fullerene-porphyrin dyad and graphene

Recently, we showed that novel fullerene-porphyrin supramolecular nanowires are very promising structures to be used in organic photovoltaics and molecular electronics. These nanowires have clear channels for charge transport for electrons and holes, however, transparent conducting electrodes such as graphene are necessary for charge collection for solar cell operation. Here, we present theoretical investigations on fullerene-porphyrin dyad-graphene interactions for organic photovoltaics and molecular electronics. Atomic models of fullerene (C60), zinc-tetraphenylporphyrin (ZnTPP) and their dyad on a single layer graphene are created and their structural and electronic properties are studied using classical molecular mechanics and quantum ab-initio density functional theory calculations. Equilibrium structures are determined. Our studies show that the C60ZnTPP dyad and graphene interact more strongly than the individual C60 or ZNTPP molecules with graphene. It is also found that C60ZnTPP-graphene combined structure has metallic character with it has half-filled mixed electronic states at the Fermi level. Our investigations show that graphene is a promising electrode for organic photovoltaics and electronics involving C60ZnTPP.

Spatially resolved micro-photoluminescence imaging of porphyrin single crystals

We describe the collection of both time-resolved and steady-state micro-photoluminescence data from solution-grown single crystals of 5,15-bis(4-carbomethoxyphenyl)porphyrin (BCM2PP). Linking molecular orientation and structure with excited-state dynamics is crucial for engineering efficient organic solar cells, light-emitting diodes, and related molecular electronics. Photoluminescence features of single porphyrin crystals were imaged using a laser scanning confocal microscope equipped with time-correlated single photon counting (TCSPC). We show enhanced exciton lifetimes (τs1=2.6ns) and stronger steady-state emission in crystalline BCM2PP samples relative to semicrystalline thin films (τs1=1.8ns).

Fulleretic Well-Defined Scaffolds: Donor-Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics.

Herein, we report the first example of a crystalline metal–donor–fullerene framework, in which control of the donor–fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The 13C cross‐polarization magic‐angle spinning NMR spectroscopy, X‐ray diffraction, and time‐resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy‐transfer (ET) studies of the fulleretic donor–acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well‐defined fulleretic donor–acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.

Fulleretic Well‐Defined Scaffolds: Donor–Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics

AbstractHerein, we report the first example of a crystalline metal–donor–fullerene framework, in which control of the donor–fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The 13C cross‐polarization magic‐angle spinning NMR spectroscopy, X‐ray diffraction, and time‐resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy‐transfer (ET) studies of the fulleretic donor–acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well‐defined fulleretic donor–acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.