First-Principles Study on Formation and Electron-Transport Properties of Single Oligothiophene Molecular Junctions

In this work, the formation of single oligothiophene molecular junctions was studied using density functional theory. The elastic scattering Green’s function method was applied to investigate the electron-transport properties of the molecular junctions and their conductance switching properties caused by an electrochemical gate. Given four configurations, the optimized structures and breakdown forces of the molecular junctions were obtained. The breakdown of the oligothiophene molecular junctions is likely to occur at the Au–S bond as the electrodes are pulled. The simulated results show that the experimental findings that the four-repeating-unit oligothiophene is more conductive than the three-repeating-unit oligothiophene are due to their different configurations. The oligothiophenes’ electronic structures are sensitive to the gate field, and their conductance switching properties are explained when a gate field is applied

Dynamics of Excited States for Fluorescent Emitters with Hybridized Local and Charge-Transfer Excited State in Solid Phase: A QM/MM Study

The highly efficient organic light-emitting diodes (OLEDS) based on fluorescent emitters with hybridized local and charge-transfer (HLCT) excited state have attracted great attention recently. The excited-state dynamics of the fluorescent molecule with consideration of molecular interaction are studied using the hybrid quantum mechanics/molecular mechanics method. The results show that, in solid state, the internal conversion rate (<i>K</i>_IC) between the first singlet excited state (S1) and the ground state (S0) is smaller than the fluorescent rate (<i>K</i>_r), while in gas phase <i>K</i>_IC is much larger than <i>K</i>_r. By analyzing the Huang–Rhys (HR) factor and reorganization energy (λ), we find that these two parameters in solid state are much smaller than those in gas phase due to the suppression of the vibration modes in low-frequency regions (<200 cm^–1) related with dihedral angles between donor and acceptor groups. This is further demonstrated by the geometrical analysis that variation of the dihedral angle between geometries of S1 and S0 is smaller in solid state than that in gas phase. Moreover, combining the dynamics of the excited states and the adiabatic energy structures calculated in solid state, we illustrate the suggested “hot-exciton” mechanism of the HLCT emitters in OLEDs. Our work presents a rational explanation for the experimental results and demonstrates the importance of molecular interaction for theoretical simulation of the working principle of OLEDs

First-Principles Study of Gate-Tunable Reversible Rectifying Behavior in 2D WGe₂N₄–TaSi₂N₄ Heterojunction Diodes: Implications for Logic Devices

Since the diode is one of the common electronic components in modern semiconductor electronics, realizing diodes with superior and controllable rectifying behaviors based on two-dimensional materials is important for next-generation electronics. Herein, gate-tunable in-plane (IP) and out-of-plane (OP) heterojunction diodes composed of the semiconductive WGe2N4 and metallic TaSi2N4 are reported based on first-principles calculations. The interfacial properties and rectifying characteristics of the IP and OP heterojunction diodes are systematically investigated. The results demonstrate that the Schottky barrier in the IP diode is much larger than that in the OP diode, resulting in a smaller current of the IP diode. The IP diode exhibits a much higher rectification ratio of 107 than the OP diode of 104 under the zero gate voltage. Noticeably, the rectifying behaviors of both diodes can be effectively modulated by the gate voltages. The positive gate voltages increase the current of IP and OP Schottky diodes and improve the rectification ratio to 109 and 105, respectively. Moreover, the negative gate voltage makes the rectifying direction of the OP Schottky diode reverse with a rectification ratio larger than 106. Our results provide a reference for designing superior two-dimensional diodes with controllable rectifying behaviors and pave the way for the design of logic devices in the future

Bias Dependence of Rectifying Direction in a Diblock Co-oligomer Molecule with Graphene Nanoribbon Electrodes

By applying nonequilibrium Green’s function method in combination with density functional theory, we study the rectifying properties of dipyrimidinyl-diphenyl co-oligomer molecules embedded in a carbon atomic chain sandwiched between two graphene nanoribbon (GNR) electrodes. Both the length of the carbon atomic chains and the edge geometry of the graphene nanoribbon electrodes are shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular devices. As for GNRs with zigzag edges, the parallel (perpendicular) conformation between the principal plane of the molecule and the zigzag-edged GNR electrode is observed to be dependent on the odd (even) number of carbon atoms in the carbon chain, whereas for armchair-edged GNRs the parallel (perpendicular) case corresponds to an even (odd) number of carbon atoms. Taking an asymmetric arrangement of armchair and zigzag GNR electrodes, we demonstrate a molecular device having very interesting rectifying behaviors with marked rectification ratios at low bias voltages and inversion of rectifying direction when the bias voltage is large. Analysis of the transmission coefficients and molecular projected self-consistent Hamiltonian as well as band structures of the electrodes under various external bias voltages reveals an underlying mechanism of the observed results

Giant Rectification Ratios of Azulene-like Dipole Molecular Junctions Induced by Chemical Doping in Armchair-Edged Graphene Nanoribbon Electrodes

Electron transport properties of an azulene-like dipole molecule anchored with carbon atomic chains sandwiched between two graphene nanoribbon (GNR) electrodes are theoretically investigated at the <i>ab initio</i> level. The molecular junctions are constructed with a strategy of modulating symmetry of Bloch wave functions. The chemical doping in an armchair-edged GNR is shown to play a significant role in determining the conductance behavior and rectifying performance of the molecular junctions. Giant rectification ratios up to 10⁴ at low bias voltages are obtained for the molecular junctions with asymmetric arrangement of undoped zGNR and doped aGNR electrodes. The boron (aluminum) dopants in the aGNR electrode induce a better rectifying performance for the molecular junctions than the respective nitrogen (phosphorus) dopants. Moreover, the boron or nitrogen doping is more advantageous than the respective aluminum or phosphorus doping in view of improving rectifying behaviors of the molecular junctions. Taking double doping in the aGNR electrode, we just demonstrate that the double boron-doping displays an improvement of rectifying features in comparison with the single case. The observed results are understood in terms of the transmission spectrum and the molecular projected self-consistent Hamiltonian as well as band structures of the electrodes with applied bias combined with symmetry analyses of Bloch wave functions of the corresponding subbands

Predicting and researching adsorption configurations of pyridazine on Si(100) surface by means of X-ray spectroscopies in theory

The landscape of organic molecule on Si(100) surface has a great significance for organic functionalisation of Si semiconductor. Several possible adsorption configurations for pyridazine on Si(100) surface have been forecasted by systemic comparison and investigation. The C1s XPS and NEXAFS spectra of these adsorption systems based on density functional theory and full core-hole potential approximation have been calculated. Although the sensibility of XPS to these adsorption configurations is not very strong, these configurations can be absolutely distinguished by NEXAFS spectra, which will bring tremendous reference to the future experimental study. Mode II, III, V and VI have a significantly higher adsorption energy, which are most likely to be present in experiment. In addition, we have made the research on specific sources of the peaks in spectra by analysing their decomposed NEXAFS spectra, the results show that the Carbon atoms which do not bond to surface atoms, make the most contribute to the intensity of characteristic peaks in spectra.</p

Prediction of Semiconducting 2D Nanofilms of Janus WSi₂P₂As₂ for Applications in Sub‑5 nm Field-Effect Transistors

Searching for eligible two-dimensional (2D) semiconductors to fabricate high-performance (HP) short-channel field-effect transistors (FETs) at the nanoscale is essential toward the continuous miniaturization of devices. Herein, we predict the 2D Janus WSi2P2As2 semiconductor and propose it as a qualified channel material for sub-5 nm FETs by using first-principles calculations. The results demonstrate that the monolayer Janus WSi2P2As2 is a 2D semiconducting nanofilm with a band gap of 0.83 eV, a hole mobility of 490 cm2 V–1 s–1 in the armchair direction, and an out-of-plane polarization. Benefiting from these outstanding intrinsic characteristics, the performance of the 5 and 3 nm gate-length WSi2P2As2 FETs can fulfill the International Technology Roadmap for Semiconductors for HP standards after employing optimizing strategies, including underlap structure, dielectric project, and cold source. Our results promote the development of new 2D nanomaterials and device architectures for designing HP short-channel FETs

Structural Isomerization Effect on the Triplet Energy Consumption Process of Organic Room-Temperature Phosphorescence Molecules: A QM/MM Study

Organic room-temperature phosphorescence (RTP) materials with long lifetimes and high efficiency have attracted great attention in recent studies. Structural isomerism with ester substituents at different positions could intrinsically influence the luminescence efficiency and operational lifetime of RTP molecules. A theoretical study to reveal the intrinsic structure–property relationship is highly desired. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the geometric and electronic properties of three isomers (o-MCBA, m-MCBA, and p-MCBA proposed by Tang) are investigated. Furthermore, the Huang–Rhys factor and reorganization energy are analyzed, and exciton dynamic processes, such as the intersystem crossing (ISC) process and three decay channels for the energy consumption process of the first triplet excited state (T1) based on the thermal vibration correlation function (TVCF) method, are discussed in detail. The results show that intermolecular interactions can restrict the rotation motions of the dihedral angle and the vibration motions of the bond angle for o-MCBA and m-MCBA. In addition, decreased Huang–Rhys factor and reorganization energy are found and a hindered nonradiative consumption process is determined. For p-MCBA in the solid phase, the rotation motions are partly restricted by the solid-state effect and the vibration motions of the bond length are effectively promoted by intermolecular H-bond interactions. In addition, the spin–orbit coupling (SOC) effect is enhanced by the solid-state effect, which is helpful to facilitate the ISC process. Through this study, we pursue opportunities to detect the relationship between basic molecular structures and RTP properties, which could take advantage of the unique molecular design to develop high-performance emitting molecules