Mapping behavioral specifications to model parameters in synthetic biology

With recent improvements of protocols for the assembly of transcriptional parts, synthetic biological devices can now more reliably be assembled according to a given design. The standardization of parts open up the way for in silico design tools that improve the construct and optimize devices with respect to given formal design specifications. The simplest such optimization is the selection of kinetic parameters and protein abundances such that the specified design constraints are robustly satisfied. In this work we address the problem of determining parameter values that fulfill specifications expressed in terms of a functional on the trajectories of a dynamical model. We solve this inverse problem by linearizing the forward operator that maps parameter sets to specifications, and then inverting it locally. This approach has two advantages over brute-force random sampling. First, the linearization approach allows us to map back intervals instead of points and second, every obtained value in the parameter region is satisfying the specifications by construction. The method is general and can hence be incorporated in a pipeline for the rational forward design of arbitrary devices in synthetic biology

Half-Metallicity in Co-Doped WSe₂ Nanoribbons

The recent development of two-dimensional transition-metal dichalcogenides in electronics and optoelelectronics has triggered the exploration in spintronics, with high demand in search for half-metallicity in these systems. Here, through density functional theory (DFT) calculations, we predict robust half-metallic behaviors in Co-edge-doped WSe₂ nanoribbons (NRs). With electrons partially occupying the antibonding state consisting of Co 3d_yz and Se 4p_z orbitals, the system becomes spin-polarized due to the defect-state-induced Stoner effect and the strong exchange splitting eventually gives rise to the half-metallicity. The half-metal gap reaches 0.15 eV on the DFT generalized gradient approximation level and increases significantly to 0.67 eV using hybrid functional. Furthermore, we find that the half-metallicity sustains even under large external strain and relatively low edge doping concentration, which promises the potential of such Co-edge-doped WSe₂ NRs in spintronics applications

Chirality-Dependent Vapor-Phase Epitaxial Growth and Termination of Single-Wall Carbon Nanotubes

Structurally uniform and chirality-pure single-wall carbon nanotubes are highly desired for both fundamental study and many of their technological applications, such as electronics, optoelectronics, and biomedical imaging. Considerable efforts have been invested in the synthesis of nanotubes with defined chiralities by tuning the growth recipes but the approach has only limited success. Recently, we have shown that chirality-pure short nanotubes can be used as seeds for vapor-phase epitaxial cloning growth, opening up a new route toward chirality-controlled carbon nanotube synthesis. Nevertheless, the yield of vapor-phase epitaxial growth is rather limited at the present stage, due in large part to the lack of mechanistic understanding of the process. Here we report chirality-dependent growth kinetics and termination mechanism for the vapor-phase epitaxial growth of seven single-chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7), covering near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Our results reveal that the growth rates of nanotubes increase with their chiral angles while the active lifetimes of the growth hold opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly determined by both growth rates and lifetimes. These results correlate nanotube structures and properties with their growth behaviors and deepen our understanding of chirality-controlled growth of nanotubes

Chemical Vapor Deposition Growth of Monolayer WSe₂ with Tunable Device Characteristics and Growth Mechanism Study

Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe₂) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe₂ and their growth mechanism studies are important steps toward applications of WSe₂. Here, we report systematical studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer and few layer WSe₂ flakes directly on silica substrates. The WSe₂ flakes were characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. We investigated how growth parameters, with emphases on growth temperatures and durations, affect the sizes, layer numbers, and shapes of as-grown WSe₂ flakes. We also demonstrated that transport properties of CVD-grown monolayer WSe₂, similar to mechanically exfoliated samples, can be tuned into either <i>p</i>-type or ambipolar electrical behavior, depending on the types of metal contacts. These results deepen our understandings on the vapor phase growth mechanism of WSe₂, and may benefit the uses of these CVD-grown monolayer materials in electronic and optoelectronics

A La₂O₃/MXene composite electrode for supercapacitors with improved capacitance and cycling performance

Developing efficient electrode materials is a key towards high power electrochemical energy storage devices. Two-dimensional (2D) MXene shows excellent conductivity and electrochemical performance among other materials. However, the restacking of MXene layers may degrade their specific capacity and cycling performance. Considering this challenge, here we have designed a composite made of 2D MXene nanosheets and lanthanum oxide (La2O3) nanoparticles to overcome the limitations. The bifunctionality of La2O3 nanoparticle prevents the restacking of MXene layers and enhances the electrochemical properties of the electrode due to its good Faradic characteristics. The specific capacitance of the La2O3/MXene composite electrode is 366 F/g at 1 A/g, which is 4.5 and 3 times higher than those of the individual La2O3 and MXene. The composite electrode displays a capacitance retention of 96% after 1,000 cycles, which is due to synergistic effect between the two components and indicates the potential of La2O3/MXene composite for supercapacitors.</p

Step-Edge-Guided Nucleation and Growth of Aligned WSe₂ on Sapphire <i>via</i> a Layer-over-Layer Growth Mode

Two-dimensional (2D) materials beyond graphene have drawn a lot of attention recently. Among the large family of 2D materials, transitional metal dichalcogenides (TMDCs), for example, molybdenum disulfides (MoS₂) and tungsten diselenides (WSe₂), have been demonstrated to be good candidates for advanced electronics, optoelectronics, and other applications. Growth of large single-crystalline domains and continuous films of monolayer TMDCs has been achieved recently. Usually, these TMDC flakes nucleate randomly on substrates, and their orientation cannot be controlled. Nucleation control and orientation control are important steps in 2D material growth, because randomly nucleated and orientated flakes will form grain boundaries when adjacent flakes merge together, and the formation of grain boundaries may degrade mechanical and electrical properties of as-grown materials. The use of single crystalline substrates enables the alignment of as-grown TMDC flakes via a substrate–flake epitaxial interaction, as demonstrated recently. Here we report a step-edge-guided nucleation and growth approach for the aligned growth of 2D WSe₂ by a chemical vapor deposition method using C-plane sapphire as substrates. We found that at temperatures above 950 °C the growth is strongly guided by the atomic steps on the sapphire surface, which leads to the aligned growth of WSe₂ along the step edges on the sapphire substrate. In addition, such atomic steps facilitate a layer-over-layer overlapping process to form few-layer WSe₂ structures, which is different from the classical layer-by-layer mode for thin-film growth. This work introduces an efficient way to achieve oriented growth of 2D WSe₂ and adds fresh knowledge on the growth mechanism of WSe₂ and potentially other 2D materials

Crystallographic Tailoring of Graphene by Nonmetal SiO_<i>x</i> Nanoparticles

Crystallographic Tailoring of Graphene by Nonmetal SiOx Nanoparticle

High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS₂ Transistors

Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS₂ as high-performance room temperature chemical sensors. The Schottky-contacted MoS₂ transistors show current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO₂ and NH₃. Specifically, the MoS₂ sensors show clear detection of NO₂ and NH₃ down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS₂ sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors

Sandwiching h‑BN Monolayer Films between Sulfonated Poly(ether ether ketone) and Nafion for Proton Exchange Membranes with Improved Ion Selectivity

Two-dimensional (2D) hexagonal boron nitride (h-BN) has attracted great interest due to its excellent chemical and thermal stability, electrical insulating property, high proton conductivity, and good flexibility. Integration of 2D h-BN into commercial proton exchange membranes (PEMs) has the potential to improve ion selectivity while maintaining the proton conductivity of PEMs simultaneously, which has been a longstanding challenge in membrane separation technology. Until now, such attempts are only limited in mechanically exfoliated small area h-BN and in proof-of-concept devices, due to the difficulty of growing and transferring large area uniform h-BN monolayers. Here, we develop a space-confined chemical vapor deposition approach and achieve the growth of wafer-scale uniform h-BN monolayer films on Cu rolls. We further develop a Nafion functional layer assisted transfer method which effectively transfers as-grown h-BN monolayer films from the Cu roll to sulfonated poly­(ether ether ketone) (SPEEK) membrane. The as-fabricated Nafion/h-BN/SPEEK sandwich structure is used as the membrane and compared with the pure SPEEK membrane for flow batteries. Results show that the sandwich membrane exhibits ion selectivity 3-fold greater than that of a pure SPEEK membrane (i.e., 32.1 × 104 vs 9.7 × 104 S min cm–3). In addition, we fabricate vanadium flow batteries using the Nafion/h-BN/SPEEK sandwich membrane and find that the sandwich structure does not affect the proton transport but inhibits vanadium crossover at low current density (–2) due to the selective blocking of vanadium ions by 2D h-BN. As a result, the sandwich membrane exhibits a significantly improved Coulombic efficiency and energy efficiency, ∼95% and ∼91%, respectively. Our results suggest that a functional layer/2D film/target substrate-based sandwich structure shows clear potential for future 2D material-based membranes in separation technologies

High-Energy MnO₂ Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors

In order to achieve high energy and power densities, we developed a high-voltage asymmetric electrochemical capacitor (EC) based on graphene as negative electrode and a MnO2 nanowire/graphene composite (MGC) as positive electrode in a neutral aqueous Na2SO4 solution as electrolyte. MGC was prepared by solution-phase assembly of graphene sheets and α-MnO2 nanowires. Such aqueous electrolyte-based asymmetric ECs can be cycled reversibly in the high-voltage region of 0−2.0 V and exhibit a superior energy density of 30.4 Wh kg−1, which is much higher than those of symmetric ECs based on graphene//graphene (2.8 Wh kg−1) and MGC//MGC (5.2 Wh kg−1). Moreover, they present a high power density (5000 W kg−1 at 7.0 Wh kg−1) and acceptable cycling performance of ∼79% retention after 1000 cycles. These findings open up the possibility of graphene-based composites for applications in safe aqueous electrolyte-based high-voltage asymmetric ECs with high energy and power densities