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Optimizing the conductance switching performance in photoswitchable dimethyldihydropyrene/cyclophanediene single-molecule junctions
摘要： Designing molecular switches with high stability and performance is still a great challenge in the field of molecular electronics. For this aim, key factors influencing the charge transport properties of molecular devices require to be carefully addressed. Here, by using the nonequilibrium Green’s function method in combination with the density functional theory, effect of bridging manner (i.e., linkage sites) for dimethyldihydropyrene (DHP)/cyclophanediene (CPD) photoswitchable molecule sandwiched between two Au(111) electrodes has been theoretically investigated. The computational results show that the fully conjugated DHP is more conductive than the less conjugated CPD, manifesting an evident switching effect. It is also found that the ON/OFF switching performance of DHP/CPD is dramatically modulated by the bridging manner. Further analysis attributes the switching feature to the different alignments of conducting HOMO with the Fermi energy for DHP and CPD. And it also reveals that local bond contributions to the electron transmission pathway are closely related to both the conjugation characteristics and bridging manner of the core molecule. This work suggests that the bridging manner of core molecule in single-molecule junction plays an essential role in determining the switching performance and should be carefully addressed in future design of molecular switches.
关键词： Single-molecule switch,Density functional theory,Nonequilibrium Green’s function method
更新于2025-09-23 15:23:52
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Local photo-excitation of shift current in noncentrosymmetric systems
摘要： Photocurrent in solids is an important phenomenon with many applications including the solar cells. In conventional photoconductors, the electrons and holes created by light irradiation are separated by the external electric field, resulting in a current flowing into electrodes. Shift current in noncentrosymmetric systems is distinct from this conventional photocurrent in the sense that no external electric field is needed and, more remarkably, is driven by the Berry phase inherent to the Bloch wavefunction. It is analogous to the polarization current in the ground state but is a dc current continuously supported by the nonequilibrium steady state under the pumping by light. Here we show theoretically, by employing Keldysh–Floquet formalism applied to a simple one-dimensional model, that the local photo excitation can induce the shift current which is independent of the position and width of the excited region and also the length of the system. This feature is in stark contrast to the conventional photocurrent, which is suppressed when the sample is excited locally at the middle and increases towards the electrodes. This finding reveals the unconventional nature of shift current and will pave a way to design a highly efficient photovoltaic effect in solids.
关键词： nonequilibrium Green’s function method,shift current,photovoltaic effect
更新于2025-09-23 15:21:21
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Effects of different tailoring graphene electrodes on the rectification and negative differential resistance of molecular devices
摘要： The electronic transport properties of oligo p-phenylenevinylene (OPV) molecule sandwiched with symmetrical or asymmetric tailoring graphene nanoribbons (GNRs) electrodes are investigated by nonequilibrium Green’s function in combination with density functional theory. The results show that different tailored GNRs electrodes can modulate the current–voltage characteristic of molecular devices. The rectifying behavior can be observed with respect to electrodes, and the maximum rectification ratio can reach to 14.2 in the asymmetric AC–ZZ GNRs and ZZ–AC–ZZ GNRs electrodes system. In addition, the obvious negative differential resistance can be observed in the symmetrical AC-ZZ GNRs system.
关键词： negative differential resistance,Graphene nanoribbons electrodes,nonequilibrium Green’s function method,rectifying behavior
更新于2025-09-23 15:21:01
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Bayesian Optimization of Terahertz Quantum Cascade Lasers
摘要： We use Bayesian optimization algorithms in combination with a nonequilibrium Green’s function transport model to increase the maximum operating temperature of terahertz quantum cascade lasers. This procedure lead to the recent temperature record of 210 K in terahertz quantum cascade lasers, and here we provide even-further-improved structures. The Bayesian optimization algorithm, which takes into account all the available history of the optimization, converges much faster and more securely than the commonly used genetic algorithm. Designs based on two and three wells per period are considered, and using the large amount of data generated, we systematically evaluate the studied schemes in terms of optimal extraction energy and relevance of electron-electron correlations. This analysis shows that the two-well scheme is superior for reaching high operating temperatures, while the three-well scheme is more robust to variations in layer thicknesses. Furthermore, we study the sensitivity to ?ux-rate ?uctuations during growth and simulation-model inaccuracies, showing the period thickness needs to be controlled to within a few percent, which is challenging but achievable with present-day molecular-beam epitaxy. These limits to the growth accuracy can be a guiding principle for experimentalists, along with the suggestion to fabricate devices across the wafer radius so as to ?nd the optimal period thickness.
关键词： Bayesian optimization,electron-electron correlations,nonequilibrium Green’s function,terahertz quantum cascade lasers,molecular-beam epitaxy
更新于2025-09-23 15:19:57
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Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations
摘要： We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green’s function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green’s functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited Bi1.5Sb0.5Te1.7Se1.3.
关键词： relaxation rates,nonequilibrium electron dynamics,excited phonon populations,nonequilibrium Green’s function formalism,pump-probe spectroscopy
更新于2025-09-09 09:28:46
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Alloy Engineered Nitride Tunneling Field-Effect Transistor: A Solution for the Challenge of Heterojunction TFETs
摘要： Being fundamentally limited to a current–voltage steepness of 60 mV/dec, MOSFETs struggle to operate below 0.6 V. Further reduction in VDD and, consequently, power consumption can be achieved with novel devices, such as tunneling transistors (TFETs) that can overcome this limitation. TFETs, however, face challenges with low ON-current leading to slow performance. TFETs made from III-nitride heterostructures are quite promising in this regard. The lattice mismatch induces a piezoelectric polarization field in a nitride heterojunction that can boost the ON-current. However, it is shown here that the carrier thermalization at the heterointerface degrades the subthreshold characteristics. Therefore, a good design should minimize the number of confined quantum well (QW) states at the heterointerface so as not to degrade the subthreshold characteristics while maintaining the lattice mismatch induced polarization to boost the ON-current. We show here that an InAlN QW on an InGaN substrate alloy engineered TFET design is promising to fulfill these requirements. Proper engineering of the alloy mole fractions and the width of the well can eliminate (or at least minimize) the undesired thermalization effects and, at the same time, provide a lattice mismatch to induce a piezoelectric field for boosting the ON-current. We have used a suitable atomistic quantum transport model to simulate these devices. The model accounts for the different mechanisms that are involved, and captures realistic scattering thermalization effects. This model has been benchmarked in our earlier work with experimental measurements of nitride tunneling heterojunction diodes and is used here to optimize the alloy engineered nitride TFET.
关键词： EQNEQ,band-to-band tunneling (BTBT),phenomenological scattering,nitrides,TFET,tight binding (TB),Atomistic,nonequilibrium Green’s function (NEGF),internal polarization,steep devices
更新于2025-09-09 09:28:46
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Physically based Diagonal Treatment of the Self-Energy of Polar Optical Phonons: Performance Assessment of III-V Double-Gate Transistors
摘要： We propose a diagonal approximation for the self-energy that describes the interaction between electrons and polar optical phonons in the framework of nonequilibrium Green’s function transport simulations. Our model is based on the definition of a scaling factor, which renormalizes the local electron-phonon coupling, to take into account the nonlocality of the interaction and provide the correct scattering rates. While previous studies relied on empirical values of this factor, we derive, from basic physical relationships, analytical expressions in the presence of the one- and two-dimensional confinement of phonons. We apply our model to the self-consistent simulation of double-gate p-type transistors made of technologically relevant III-V materials (InAs, InSb, and GaSb). Their performance is benchmarked, for different crystallographic orientations and strain constraints, against the corresponding Si and Ge devices. We find that the electron-polar optical phonon scattering plays a major role in degrading the performance of the III-V devices and typically results in a widening of the performance gap existing between III-V and Si or Ge devices in ballistic transport conditions.
关键词： nonequilibrium Green’s function,electron-phonon scattering,double-gate transistors,self-energy,III-V materials,polar optical phonons
更新于2025-09-04 15:30:14