Alignment Of Molecular Levels Research Articles

Probing Energy-Level Alignment in Molecular Multilayers by Frequency-Modulation Electrostatic Force Microscopy under Tapping-Mode-Combined Fowler-Nordheim Tunneling Spectroscopy.

The alignment of molecular electronic levels in a molecular multilayer is of crucial importance to realize desired functions for molecular devices. Amplitude-modulation-feedback frequency-modulation electrostatic force microscopy combined with Fowler-Nordheim tunneling spectroscopy is utilized as a probe for the energy-level alignment in an organic multilayer. Bias-dependent electrostatic force spectra were examined for bilayers including a Ru complex as a benchmark multilayer system. Electrostatic properties in the low-bias region were captured well by a single-capacitor model, which indicates weak coupling at the bilayer interface between the Ru complex and self-assembled monolayer. In contrast, in the high-bias region, significant disagreement with the expected electrostatic force was recognized for the bilayers and evaluated as the loss of electrostatic energy through the Fowler-Nordheim tunneling process. Alignment of the lowest unoccupied molecular orbital (LUMO) level of the Ru complex was determined by Fowler-Nordheim emission through resonant tunneling. These results indicate an effective method to probe level alignment at interfaces inside multilayers and to provide the partition factor β that depicts a divided electric field.

Epitaxial growth of crystalline thin-films of perfluorinated copper phthalocyanine on the single-crystal copper phthalocyanine

Understanding and controlling the growth structures of molecular heterojunctions are important for the pursuit of highly efficient organic semiconductor devices. In this study, a well-defined molecular heterojunction was constructed by the deposition of an n-type organic semiconductor molecule, perfluorinated copper phthalocyanine (F16CuPc), on single-crystals of a typical p-type material, copper phthalocyanine (CuPc). The heterojunction was characterized using synchrotron radiation surface X-ray diffraction techniques to unveil the epitaxial growth of crystalline thin-films of F16CuPc on the CuPc surface. The inter-lattice relationships between F16CuPc and CuPc deduced from the present results imply that the in-plane orientation of the F16CuPc crystal lattice was predominantly governed by the molecular-level alignments of the π-backbones of the F16CuPc and CuPc molecules at the nucleation center for crystal growth at the interface.

Electronic Decoupling and Single‐Molecule Charging of C60 on h‐BN/Rh(111)

AbstractA detailed understanding of the interaction between molecules and 2D materials is crucial to implement molecular films into next‐generation 2D material‐organic hybrid devices effectively. In this regard, energy level alignment and charge transfer processes are particularly relevant. This work investigates the interplay between a hexagonal boron nitride (h‐BN) monolayer on an Rh(111) single crystal and self‐assembled C60 thin films. The influence of the corrugated topography and electrostatic surface potential originating from the h‐BN/Rh(111) Moiré superstructure on the electronic level alignment and charging characteristics of C60 is being studied. A combination of scanning tunneling microscopy/spectroscopy (STM/STS) and a theoretical tight‐binding approach is used to gain insight into the C60 bandstructure formation and electronic decoupling of specific C60. This decoupling results from adsorption site‐dependent variations of the molecular energy level alignment, which controls the strength of intermolecular hybridization. The decoupling of specific C60 enables the direct observation of single‐electron charging processes via STS and Kelvin probe force microscopy. The charging of the C60 is enabled by combining two gating mechanisms: the electrostatic surface potential of the monolayer h‐BN/Rh(111) Moiré and the electric field of the STM tip.

A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet ( 3 CT) energy levels are gradually reduced, while the locally excited triplet ( 3 LE A ) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3 LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ( 3 LE A ) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3 CT and the 3 LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference ( ΔE ST ) of 0.02 eV, a fast RISC rate of 1.54 × 10 6 s −1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3 LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1 CT and 3 CT state to lower energies without affecting the 3 LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.

Controlling the conjugation extension inside acceptors for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

Increasing the reverse intersystem crossing (RISC) rates remains a challenge for red thermally activated delayed fluorescence (TADF) emitters, and energy level alignments play an important role. In this work, we demonstrate that locally excited triplet energy levels from acceptor segments (3LEA) can be well modulated by changing the conjugation extension inside acceptors. Our newly designed compounds pDPAPQ-PXZ and pDPBPZ-PXZ show nearly identical charge-transfer (CT) properties because of similar donor and acceptor strengths. On the other hand, the 3LEA state of pDPBPZ-PXZ was significantly lower than that of pDPAPQ-PXZ only because of its greater π-delocalization. In the case of pDPBPZ-PXZ, ideal molecular energy level alignments are obtained with near-degenerate CT featuring singlet and triplet states (1CT, 3CT) and 3LEA. As a result, pDPBPZ-PXZ simultaneously achieves an approximately 10-fold improved kRISC and a dramatically improved photoluminescence quantum yield of 85.6% compared with pDPAPQ-PXZ. The corresponding OLED successfully doubles the external quantum efficiency to 20.3%. This work demonstrates that the conjugation extensions of acceptors play a fatal role in determining 3LEA levels and that they can be isolated from controlling the acceptor strengths. This provides a promising approach toward enhancing RISC efficiency and developing highly efficient red TADF emitters.

Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor.

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Wet process feasible novel fluorene-based molecular hole transporting layer for phosphorescent organic light emitting diodes

Solution processable hole transporting materials (HTMs) are indispensable component for the cost-effective and large-area roll-to-roll fabrication of organic light emitting diodes (OLEDs). Hole transport layers should display high triplet energy, good hole injection properties along with potential electron blocking capability. In this paper, we report a series of novel fluorene-based HTMs, 9,9-diethyl-2,7-bis(2-(trifluoromethyl)phenyl)-9H-fluorene (3), 9,9-diethyl-2,7-bis(3-(trifluoromethyl)phenyl)-9H-fluorene (4) and 9,9- diethyl-2,7-bis(4-(trifluoromethyl)phenyl)-9H-fluorene (5) containing symmetrical and asymmetrical substitution of trifluoromethyl pendants at different positions. The synthesized HTMs have an exceptional solubility in common organic solvents such as THF (tetrahydrofuran), o-Xylene, etc. and possess good thermal stability to form morphologically stable films. The suitable molecular energy level alignment (HOMO-LUMO) for barrier free injection, adequate ionization potential (6.12, 6.20, and 6.23 eV) and high triplet energies (3.12,3.17, and 3.13 eV) for compound 3, 4, and 5, these suitable characteristics promote materials performance as a good hole transporting layer (HTL). Solution-processed yellow phosphorescent OLEDs fabricated by utilizing these HTMs with a conventional yellow emitter Iridium (III)bis (4-phenylthieno [3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01) doped in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The best device with molecule 4 showed an improvement of 54 % in current efficiency (CE) from 23.3 to 35.8 cd/A and 14 % in external quantum efficiency (EQE) from 11.3 to 12.9 %, compared to reference device containing N, N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) as HTL. Compounds 3 and 5 based devices also showed a competitive performance with the reference device. The effect of solvent was also studied. These outcomes recommend that this type of solution processable HTMs will be promising contender for high-efficiency OLED devices.

Double‐Sided Graphene‐Enhanced Raman Scattering and Fluorescence Quenching in Hybrid Biological Structures

AbstractDue to their large contact and loading surfaces as well as high sensitivities to chemical changes, graphene‐based materials (GBMs) are increasingly being employed into novel nanomedicine technologies. Here biomolecule—monolayer graphene—kidney tissue hybrid structures are studied using mapping micro‐Raman and fluorescence spectroscopies. Because in this configuration graphene interacts with molecules on both sides, a double‐sided graphene‐enhanced Raman scattering (GERS) effect up to ≈10.1 is found for biomolecules adsorbed on graphene and amino acids in the kidney tissue below graphene. Moreover, graphene causes an efficient autofluorescence quenching (FLQ) up to ≈20% emitted by the kidney tissue. Despite the complexity of such layered materials, the intriguing simultaneous occurrence of double‐sided GERS (a new development of GERS) and FLQ phenomena can be well explained by suitable molecular structure and energy level alignment between molecules and graphene. These result in effective charge transfer mediated by non‐covalent interactions as indicated by correlative strain, doping, and defect analyses in graphene based on the Raman data and energy level calculations. Last, the advantages of using graphene over standard photobleaching are demonstrated. This work can be extended to other macromolecular entities toward integrating GBMs in versatile drug delivery, imaging, and sensing devices.

Interfacial Molecular Doping and Energy Level Alignment Regulation for Perovskite Solar Cells with Efficiency Exceeding 23%

Interfacial Molecular Doping and Energy Level Alignment Regulation for Perovskite Solar Cells with Efficiency Exceeding 23%

Self-assembly of ClAlPc molecules on moiré-patterned graphene grown on Pt(111)

Phthalocyanines are promising molecules for the development of organic electronic devices, for instance, molecular heterojunctions in organic solar cells or organic field-effect transistors. For an optimum performance of these devices, the molecular ordering on the substrate and the molecular electronic level alignment have been shown as crucial factors. In this work, the self-assembled structure and the electronic structure of chloroaluminum phthalocyanines (ClAlPc) on graphene grown on Pt(111) surfaces have been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) and low-temperature conditions. Graphene grown on Pt(111) exhibits multiple moiré patterns with different periodicities, offering a benchmark to investigate the influence of the graphene and the moiré patterns in the ClAlPc ordering. This surface allows to extend previous works performed on graphite and graphene on Cu(100), where no moiré patterns are found. Well-ordered molecular islands exhibiting rotational domains have been observed in the submonolayer regime. The orientation of individual ClAlPc molecules within the structure unit cell has been characterized pointing out to a Cl-Up configuration adopted by the molecules. Our measurements show a correlation between the molecular lattice orientation and the graphene directions, whereas no influence of the underlying moiré patterns has been found. Finally, the ClAlPc electronic structure has been characterized indicating a weak graphene-molecule interaction.

All-polymer indoor photovoltaics based on polymer acceptors with various bandgap

Abstract Indoor photovoltaics (IPVs) are receiving great attention because they can power the wireless microelectronic devices needed for Internet of Things (IoTs) applications. Many polymer acceptors have been designed and synthesized for solar cell applications. In this manuscript, we study the application of three polymer acceptors in all-polymer indoor photovoltaic devices. The polymer acceptors containing boron-nitrogen coordination bond (B←N) have medium bandgap of 1.8–1.9 eV and proper the lowest unoccupied molecular orbital energy level (ELUMO) and the highest occupied molecular orbital energy level (EHOMO) alignment and consequently are very suitable for IPV application. Under fluorescent lamp illumination at the brightness of 2000 lux, the devices based on the polymer acceptors containing B←N unit show the open-circuit voltage of 1.08 V and a power conversion efficiency (PCE) of 23.2%. This performance is much higher than that of other imide-based polymer acceptors with small bandgap and wide absorption spectrum.

Charge State Control of F16CoPc on h‐BN/Cu(111)

AbstractThe use of molecular materials in solar cells and nano‐electronics demands a fundamental understanding and control of their electronic properties. Particularly relevant is the molecular response to the environment, that is, the interaction with the support and adjacent molecules, as well as the influence of electrostatic gating. Here, the control of molecular level alignment and charge states of fluorinated cobalt phthalocyanines (F16CoPc) on atomically thin hexagonal boron nitride (h‐BN) sheets on Cu(111) is reported using scanning tunneling microscopy (STM) and spectroscopy (STS), as well as atomic force microscopy (AFM) and complementary density functional theory (DFT) calculations. Three parameters that govern the electronic level alignment of F16CoPc orbitals are investigated: i) template‐induced gating by the work function variation of the h‐BN/Cu(111) substrate, ii) gating by the STM tip, and iii) screening by neighboring molecules. The interplay of these parameters influences the charge distribution in the studied molecular arrangements and thus provides the possibility to tune their physicochemical behavior, for instance, the response toward electronic or optical excitation, charge transport, or binding of axial adducts.

The emergence of the local moment molecular spin transistor

Local moment molecular systems have now been used as the conduction channel in gated spintronics devices, and some of these three terminal devices might even be considered molecular spin transistors. In these systems, the gate voltage can be used to tune the molecular level alignment, while applied magnetic fields have an influence on the spin state, altering the magnetic properties, and providing insights to the magnetic anisotropy. More recently, the use of molecular spin crossover complexes, as the conduction channel, has led to devices that are both nonvolatile and have functionality at higher temperatures. Indeed, some devices have now been demonstrated to work at room temperature. Here, several molecular transistors, including those claiming to use single molecule magnets (SMM), are reviewed.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray.

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Rational Tuning of Molecular Interaction and Energy Level Alignment Enables High-Performance Organic Photovoltaics.

The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near-infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both-fluorinated or both-nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB-T-F/Y1-4F blend and PBDB-T-F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.

Effect of Asymmetric Anchoring Groups on Electronic Transport in Hybrid Metal/Molecule/Graphene Single Molecule Junctions.

A combined experimental and theoretical study on molecular junctions with asymmetry in both the electrode type and in the anchoring group type is presented. A scanning tunnelling microscope is used to create the "asymmetric" Au-S-(CH2 )n-COOH-graphene molecular junctions and determine their conductance. The measurements are combined with electron transport calculations based on density functional theory (DFT) to analyze the electrical conductance and its length attenuation factor from a series of junctions of different molecular length (n). These results show an unexpected trend with a rather high conductance and a smaller attenuation factor for the Au-S-(CH2 )n -COOH-graphene configuration compared to the equivalent junction with the "symmetrical" COOH contacting using the HOOC-(CH2 )n -COOH series. Owing to the effect of the graphene electrode, the attenuation factor is also smaller than the one obtained for Au/Au electrodes. These results are interpreted through the relative molecule/electrode couplings and molecular level alignments as determined with DFT calculations. In an asymmetric junction, the electrical current flows through the less resistive conductance channel, similarly to what is observed in the macroscopic regime.

Transparent, Anisotropic Biofilm with Aligned Bacterial Cellulose Nanofibers

AbstractCellulose nanofibrils are attractive as building blocks for advanced photonic, optoelectronic, microfluidic, and bio‐based devices ranging from transistors and solar cells to fluidic and biocompatible injectable devices. For the first time, an ultrastrong and ultratough cellulose film, which is composed of densely packed bacterial cellulose (BC) nanofibrils with hierarchical fibril alignments, is successfully demonstrated. The molecular level alignment stems from the intrinsic parallel orientation of crystalline cellulose molecules produced by Acetobacter xylinum. These aligned long‐chain cellulose molecules form subfibrils with a diameter of 2–4 nm, which are further aligned to form nanofibril bundles. The BC film yields a record‐high tensile strength (≈1.0 GPa) and toughness (≈25 MJ m−3). Being ultrastrong and ultratough, yet the BC film is also highly flexible and can be folded into desirable shapes. The BC film exhibits a controllable manner of alignment and is highly transparent with modulated optical properties, paving the way to enabling new functionalities in mechanical, electrical, fluidic, photonics, and biocompatible applications.

Probing charge transfer between molecular semiconductors and graphene

The unique density of states and exceptionally low electrical noise allow graphene-based field effect devices to be utilized as extremely sensitive potentiometers for probing charge transfer with adsorbed species. On the other hand, molecular level alignment at the interface with electrodes can strongly influence the performance of organic-based devices. For this reason, interfacial band engineering is crucial for potential applications of graphene/organic semiconductor heterostructures. Here, we demonstrate charge transfer between graphene and two molecular semiconductors, parahexaphenyl and buckminsterfullerene C60. Through in-situ measurements, we directly probe the charge transfer as the interfacial dipoles are formed. It is found that the adsorbed molecules do not affect electron scattering rates in graphene, indicating that charge transfer is the main mechanism governing the level alignment. From the amount of transferred charge and the molecular coverage of the grown films, the amount of charge transferred per adsorbed molecule is estimated, indicating very weak interaction.

Voltage-Induced Switching Dynamics of a Coupled Spin Pair in a Molecular Junction.

Molecular spintronics is made possible by the coupling between electronic configuration and magnetic polarization of the molecules. For control and application of the individual molecular states, it is necessary to both read and write their spin states. Conventionally, this is achieved by means of external magnetic fields or ferromagnetic contacts, which may change the intentional spin state and may present additional challenges when downsizing devices. Here, we predict that coupling magnetic molecules together opens up possibilities for all electrical control of both the molecular spin states as well as the current flow through the system. By tuning between the regimes of ferromagnetic and antiferromagnetic exchange interaction, the current can be at least an order of magnitude enhanced or reduced. The effect is susceptible to the tunnel coupling and molecular level alignment that can be used to achieve current rectification.

Controlling single-molecule junction conductance by molecular interactions.

For the rational design of single-molecular electronic devices, it is essential to understand environmental effects on the electronic properties of a working molecule. Here we investigate the impact of molecular interactions on the single-molecule conductance by accurately positioning individual molecules on the electrode. To achieve reproducible and precise conductivity measurements, we utilize relatively weak π-bonding between a phenoxy molecule and a STM-tip to form and cleave one contact to the molecule. The anchoring to the other electrode is kept stable using a chalcogen atom with strong bonding to a Cu(110) substrate. These non-destructive measurements permit us to investigate the variation in single-molecule conductance under different but controlled environmental conditions. Combined with density functional theory calculations, we clarify the role of the electrostatic field in the environmental effect that influences the molecular level alignment.