Comparison of carrier doping in ZnSnO3 and ZnTiO3 from first principles.

Ferroelectric materials have attracted increasing attention due to their rich properties. Unlike perovskite ferroelectric oxides, in the LiNbO3-type ferroelectric oxides of ABO3, ferroelectrically active cations are not necessary. While the effects of carrier doping on perovskite ferroelectric oxides have been extensively studied, the studies on LiNbO3-type ferroelectric oxides are rare. We consider two LiNbO3-type ferroelectric oxides ZnSnO3 and ZnTiO3, where the former has no ferroelectrically active cation and the latter has ferroelectrically active cation Ti4+, and study the effect of carrier doping by performing first-principles calculations. Comparison results indicate that the B-site cation has significant effects on the polar distortion in LN-type ferroelectrics. Our studies show that LN-type materials can maintain the coexistence of ferroelectricity and conductance over a very wide range of concentrations. The polar displacement is even enhanced under hole doping. More importantly, ZnSnO3 can be doped by electrons up to a high level to realize the conducting ferroelectrics of high mobility due to its isolated s conduction band.

Improving the multi-functionality of optical tweezers with FPGA integration.

The development of optical tweezers aims to extend their operating function and pattern. However, excessive programming can lead to a decrease in the system's operating speed and introduce bugs or data transmission delays. In this study, we present a time-shared optical tweezers system that allows for parallel operation of multiple functions. To enable efficient data transmission, we employ a queue structure and a buffer. To assess the system's performance, we utilize a biological sample in conjunction with the optical tweezers system and scanning imaging technique. We quantify the trapping parameter while concurrently running power stabilization programs. As a result, the standard deviation of the measured stiffness is reduced by 60% in the x and y directions and 30% in the z direction, indicating a significant improvement in calibration precision. Throughout the program execution, the system maintains an operating rate of 110 kHz, and the data are continuously updated in real time on the host. The system's performance demonstrates its potential for quantification and morphological reconstruction of biological samples.

Stability of oxygen vacancies at metal/oxide interfaces.

We investigated the influence of work function and charge doping on the formation of oxygen vacancies in metal/oxide heterojunctions by first-principles calculations. SrTiO3 is used as a typical oxide. Simple metals Pt, Au and Ag are used as electrodes. We show that electron doping could improve the formation energy of oxygen vacancies. In such a heterojunction, we found that the work function of the metal electrode affects the stability of oxygen vacancies in SrTiO3. For an electrode with a smaller work function, more electrons are induced and accumulated in the oxides near the interface and improve the formation energy of oxygen vacancies. We also studied the effect of ferroelectric polarization in a heterojunction of metal/BaTiO3 and found similar properties. We hope that our work could help in the design of complex-oxide-based electronic devices.

Continuous tuning of unidirectional emission wavelength by bending a notched-elliptical polymer microdisk.

An approach of continuously tunable unidirectional emission through bending a notched-elliptical polymer microdisk is proposed. The characteristics of the bending-dependent action are carefully analyzed, and the resonance wavelength for unidirectional emission can be tuned continuously through bending the device. Such a whispering-gallery-mode microresonator enables unidirectional emission with ultra-low divergence, of which the emission efficiency and Q factor are stabilized, demonstrating the whole structure is robust and relatively insensitive within a certain bending angle range. A maximum resonance wavelength shift of ∼100 nm and Q factor of 1500 can be achieved with the total size of the microdisk less than 10 µm. This kind of microresonator is promising for applications in multilevel integrated photonics circuits and may open the door to new functionalities of resonator devices, from sensing to optical amplification.

Rational design of [e]-fusion induced high-performance DHP/CPD based photoswitches.

We report an effective strategy for improving the electronic transport and switching behaviors of dimethyldihydropyrene/cyclophanediene (DHP/CPD)-based molecular devices, an intriguing photoswitch that can be triggered by ultraviolet/visible (UV-vis) light irradiation. Aiming to obtain molecular devices with high on-off ratios, we assess a series of molecular designs formed by [e]-fusing different arenes on a conjugated macrocycle to modulate the photochemical and electronic properties. Here, the switching mechanism and transport properties of [e]-fused DHP/CPD-based nanojunctions are theoretically investigated by first-principles calculations. As a result, the large diversity in electrical conductance between the closed and open forms certifies the substantial switching behavior observed in these sandwich structures. The maximum on-off ratios in all designed photoswitches are greater than 102. Further analysis confirms the improvement of switching performance caused by [e]-fusion. Notably, in the benzo-fused molecular junctions, the maximum on-off ratio is up to 103, which is 55 times larger than that of the un-fused one. We also find that the position of the switch core can remarkably affect the performance of photoswichable nanodevices.

Calculation of Free-Energy Barriers with TD-DFT: A Case Study on Excited-State Proton Transfer in Indigo.

The performance of time-dependent density functional theory (TD-DFT) for the calculation of excited states of molecular systems has been the subject of many benchmark studies. Often, these studies focus on excitation energies or, more recently, excited-state equilibrium geometries. In this work, we take a different angle by instead exploring how well TD-DFT reproduces experimental free-energy barriers of a well-known photochemical reaction: the excited-state proton transfer (ESPT) in indigo. Specifically, by exploiting the possibility of using TD-DFT to locate and compute free energies of first-order saddle points in excited states, we test the performance of several popular density functionals in reproducing recently determined experimental free-energy barriers for ESPT in indigo and in an N-hexyl substituted derivative thereof. Through the calculations, it is found that all of the tested functionals perform quite well, uniformly overestimating the experimental values by 1.4-3.5 (mean error) and 2.5-5.5 kcal mol-1 (maximum error) only. Given that these errors are not larger than those typically observed when barriers for ground-state proton transfer reactions are calculated in ground-state DFT, the results highlight the potential of TD-DFT to enable accurate modeling of ESPT reactions based on free energies and explicit localization of transition states.

Spin-state energies of heme-related models from spin-flip TDDFT calculations.

Spin-state energies of heme-related models are of vital importance in biochemistry. To compute the energies of different spin states, the traditional ΔSCF method based on the density functional theory (DFT) is usually employed. In this work, the spin-flip TDDFT (SF-TDDFT) approach is investigated to compute the spin-state energies, with six different exchange-correlation (XC) functionals. With the present protocol, the spin contamination is fully avoided by choosing appropriate reference states. Additionally, multiple excited states can be obtained with SF-TDDFT. Compared with the CCSD(T) results, it is shown that the SF-TDDFT calculations with the BHandHLYP functional provide better accuracy than ΔSCF for D-Q (doublet-quartet) and Q-S (quartet-sextet) gaps and agree well with the experimental results. A possible solution for the precise calculation of spin-state energies is proposed to improve the performance of SF-TDDFT, on account of that the excitation energies show highly linear dependence on the amount of Hartree-Fock (HF) exchange in the XC functionals.

The electronic transport properties of zigzag silicene nanoribbon slices with edge hydrogenation and oxidation.

First principles calculations are performed to study the transport properties of H or H2 edge-hydrogenated zigzag silicene nanoribbon slices with 6 zigzag chains (6ZSiNR) as well as OH or O edge-oxidized 6ZSiNR slices connected with H-terminated 6ZSiNR electrodes. We mainly focus on two configurations: symmetric edge modification and asymmetric edge modification. It is found that these configurations show distinctly different transport behaviours under bias voltages, depending on whether their structures satisfy c2 symmetry operation along the central axis. In addition, the effects of various functional groups on the electronic transport are investigated; comparison of the current magnitudes indicates that the H group has the strongest effect, followed by the OH group, the O group, and the H2 group. This difference is revealed to be related to the coupling interaction between the edge groups of the ZSiNR slices and the H groups of the ZSiNRs electrodes, as well as the transmission channels around the Fermi level.

Computational design of faster rotating second-generation light-driven molecular motors by control of steric effects.

We report a systematic computational investigation of the possibility to accelerate the rate-limiting thermal isomerizations of the rotary cycles of synthetic light-driven overcrowded alkene-based molecular motors through modulation of steric interactions. Choosing as a reference system a second-generation motor known to accomplish rotary motion in the MHz regime and using density functional theory methods, we propose a three-step mechanism for the thermal isomerizations of this motor and show that variation of the steric bulkiness of the substituent at the stereocenter can reduce the (already small) free-energy barrier of the rate-determining step by a further 15-17 kJ mol(-1). This finding holds promise for future motors of this kind to reach beyond the MHz regime. Furthermore, we demonstrate and explain why one particular step is kinetically favored by decreasing and another step is kinetically favored by increasing the steric bulkiness of this substituent, and identify a possible back reaction capable of impeding the rotary rate.

Rectification inversion in oxygen substituted graphyne-graphene-based heterojunctions.

Current rectification is found in oxygen-substituted zigzag graphyne nanoribbon/hydrogen-terminated zigzag graphene nanoribbon heterostructure junctions, from the application of nonequilibrium Green's function formalism combined with density functional theory. This behavior could be tuned by varying the number and location of oxygen atoms in the zigzag graphyne nanoribbon parts, and the rectification direction could be reversed due to the parity limitation tunneling effect. Moreover, an obvious negative differential resistance behavior is found and may be explained by two different mechanisms.

How method-dependent are calculated differences between vertical, adiabatic, and 0-0 excitation energies?

Through a large number of benchmark studies, the performance of different quantum chemical methods in calculating vertical excitation energies is today quite well established. Furthermore, these efforts have in recent years been complemented by a few benchmarks focusing instead on adiabatic excitation energies. However, it is much less well established how calculated differences between vertical, adiabatic and 0-0 excitation energies vary between methods, which may be due to the cost of evaluating zero-point vibrational energy corrections for excited states. To fill this gap, we have calculated vertical, adiabatic, and 0-0 excitation energies for a benchmark set of molecules covering both organic and inorganic systems. Considering in total 96 excited states and using both TD-DFT with a variety of exchange-correlation functionals and the ab initio CIS and CC2 methods, it is found that while the vertical excitation energies obtained with the various methods show an average (over the 96 states) standard deviation of 0.39 eV, the corresponding standard deviations for the differences between vertical, adiabatic, and 0-0 excitation energies are much smaller: 0.10 (difference between adiabatic and vertical) and 0.02 eV (difference between 0-0 and adiabatic). These results provide a quantitative measure showing that the calculation of such quantities in photochemical modeling is well amenable to low-level methods. In addition, we also report on how these energy differences vary between chemical systems and assess the performance of TD-DFT, CIS, and CC2 in reproducing experimental 0-0 excitation energies.

Electrostatic current switching and negative differential resistance behavior in a molecular device based on carbon nanotubes.

The electronic transport properties of an all-carbon mechanically controlled molecular device based on carbon nanotubes are studied using non-equilibrium Green's function in combination with density functional theory. A segment of (10,0) single-walled carbon nanutube (SWCNT) is placed concentrically outside a (5,0) SWCNT, namely, a (5,0)@(10,0) double-walled carbon nanotube (DWCNT). It is found that the position, orientation and length scaling of the (10,0) SWCNT have crucial effects on the electronic transport properties of the system. When the (10,0) SWCNT is mechanically pushed forward along the axial direction, alternation of on/off switching behavior under low bias and negative differential resistance behavior under high bias are observed. Significant changes in the electronic transport properties arise when rotating the (10,0) SWCNT around the common axis or adding carbon atom layers in the transport direction. Theoretical explanations are proposed for these phenomena.

Lateral scaling and positioning effects of top-gate electrodes on single-molecule field-effect transistors.

Molecular electronics aims at integrating controllable molecular devices into circuits or machines to realize certain functions. According to device configuration, molecular field-effect transistors with top-gate electrodes have great advantages for integration. Nevertheless, from technical aspects, it is difficult to control lateral scale and position of a top-gate electrode precisely. Therefore, one problem arises in how lateral scaling and positioning effects of a top-gate electrode affect device performance. To solve this problem, the electronic transport properties of single-molecule field-effect transistor configurations modulated by a series of partial-scale top-gate electrodes with different lateral scales and positions are studied by using non-equilibrium Green's function in combination with density functional theory, and compared with those of the full gate electrode (can be considered as a bottom gate electrode). The results show that lateral scaling and positioning effects indeed have a great impact on electronic transport properties of single-molecule field-effect transistor configurations. For [Formula: see text]-saturated 1,12-dodecanedithiol devices, larger lateral scale of a partial-scale top-gate electrode obtains larger amplification coefficient [Formula: see text] (ratio of device conductances with/without a gate electrode), and even larger [Formula: see text] than that of the full gate electrode. While lateral positioning effect has little influence on this device. For [Formula: see text]-conjugated 1,3,5,7,9,11-dodehexaene-1,12-dithiol devices, performance of a partial-scale top-gate electrode mainly depends on locations of its two edges, i.e. the number of [Formula: see text] bonds that it breaks. These results will provide theoretical directions in device designing and manufacturing in the future.