Fully Printed Flexible Dual-Gate Carbon Nanotube Thin-Film Transistors with Tunable Ambipolar Characteristics for Complementary Logic Circuits.

Semiconducting single-wall carbon nanotubes (sSWCNTs) have been widely used as the channel material for high-performance printed flexible thin-film transistors (TFTs). Due to the absorption of moisture and oxygen in air, the printed sSWCNT TFTs generally exhibit p-type characteristics only. In this paper, we report fully printed dual-gate sSWCNT TFTs that exhibit almost symmetric ambipolar characteristics. With the applied control gate voltage varying from -60 to +60 V, a threshold voltage tuning range of 27 V is achieved, allowing the device to be effectively tuned into either predominantly p-type or predominantly n-type. The tunable ambipolar characteristics are found to be very stable over a long period of time (4 months). By integrating two printed dual-gate TFTs biased with different control gate voltages, a complementary metal oxide semiconductor inverter with close to rail-to-rail output voltage swing is demonstrated. The use of a dual-gate structure for achieving n-type printed carbon nanotube TFTs is much more controllable and repeatable compared to other methods such as chemical doping. Our work shows the feasibility of implementing more sophisticated complementary logic circuits using printed flexible carbon nanotube transistors.

Single Pixel Black Phosphorus Photodetector for Near-Infrared Imaging.

Infrared imaging systems have wide range of military or civil applications and 2D nanomaterials have recently emerged as potential sensing materials that may outperform conventional ones such as HgCdTe, InGaAs, and InSb. As an example, 2D black phosphorus (BP) thin film has a thickness-dependent direct bandgap with low shot noise and noncryogenic operation for visible to mid-infrared photodetection. In this paper, the use of a single-pixel photodetector made with few-layer BP thin film for near-infrared imaging applications is demonstrated. The imaging is achieved by combining the photodetector with a digital micromirror device to encode and subsequently reconstruct the image based on compressive sensing algorithm. Stationary images of a near-infrared laser spot (λ = 830 nm) with up to 64 × 64 pixels are captured using this single-pixel BP camera with 2000 times of measurements, which is only half of the total number of pixels. The imaging platform demonstrated in this work circumvents the grand challenges of scalable BP material growth for photodetector array fabrication and shows the efficacy of utilizing the outstanding performance of BP photodetector for future high-speed infrared camera applications.

Vertically Stacked and Self-Encapsulated van der Waals Heterojunction Diodes Using Two-Dimensional Layered Semiconductors.

van der Waals heterojunctions using 2D semiconducting materials could overcome the defect issues included by lattice mismatch in conventional epitaxially grown heterojunctions with bulk materials and could enable a much wider palette for choice of materials and more sophisticated device design. Such 2D heterojunction devices are of great interest for important functional devices such as diodes, bipolar junction transistors, light-emitting diodes, and photodetectors. In this paper, we demonstrate a truly vertical p-n heterojunction diode built from 2D semiconductors (MoS2 and BP) and compare its performance against conventional lateral 2D heterojunction devices (partially overlapped 2D heterostructures). Both vertical and lateral p-n heterostructure diodes exhibit a strong rectification ratio even with no gate voltage applied. More importantly, the results show that the vertical diode delivers 70 times higher current density under forward bias than a conventional lateral device design and the improved device performance can be attributed to the complete elimination of series resistance. Low-temperature measurements and TCAD simulations are used to determine the barrier height at the junctions. Moreover, the vertical device structure allows certain ambiently unstable 2D semiconductors to be fully encapsulated by the materials on top, preventing the material from degradation. This work demonstrates the potential of using the vertically stacked 2D semiconductors for future nanoelectronic and optoelectronic devices with optimal performance.

Photothermal Effect Induced Negative Photoconductivity and High Responsivity in Flexible Black Phosphorus Transistors.

This paper reports negative photoconductivity mechanism in flexible black phosphorus (BP) transistors built on freestanding polyimide film. Near-infrared laser (λ = 830 nm) excitation leads to significantly suppressed device on-state current with a very high responsivity of up to 53 A/W. The underlying mechanism of the negative photoconductivity is attributed to the strong photothermal effect induced by the low thermal conductivity of the polyimide substrate used. The heat generated by the infrared light illumination results in enhanced phonon scattering, reduced carrier mobility, and consequently negative photocurrent. Such a phenomenon was not observed in similar BP devices built on SiO2/Si substrates whose thermal conductivity is much higher. The above photothermal mechanism is also supported by temperature-dependent electrical characterization and device simulation. Such a flexible BP infrared photodetector with ultrahigh responsivity may find potential applications in future wearable and biointegrated imaging systems.

Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus.

As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, producing air-stable BP sensors is extremely challenging because BP atomic layers degrade rapidly in ambient conditions. In this study, we explored the humidity sensing properties of BP field-effect transistors fully encapsulated by a 6 nm-thick Al2O3 encapsulation layer deposited by atomic layer deposition. The encapsulated BP sensors exhibited superior ambient stability with no noticeable degradation in sensing response after being stored in air for more than a week. Compared with the bare BP devices, the encapsulated ones offered long-term stability with a trade-off in slightly reduced sensitivity. Capacitance-voltage measurement results further reveal that instead of direct charge transfer, the electrostatic gating effect on BP flakes arising from the dipole moment of adsorbed water molecules is the basic mechanism governing the humidity sensing behavior of both bare and encapsulated BP sensors. This work demonstrates a viable approach for achieving air-stable BP-based humidity or chemical sensors for practical applications.

Fully Printed Stretchable Thin-Film Transistors and Integrated Logic Circuits.

This paper reports intrinsically stretchable thin-film transistors (TFTs) and integrated logic circuits directly printed on elastomeric polydimethylsiloxane (PDMS) substrates. The printed devices utilize carbon nanotubes and a type of hybrid gate dielectric comprising PDMS and barium titanate (BaTiO3) nanoparticles. The BaTiO3/PDMS composite simultaneously provides high dielectric constant, superior stretchability, low leakage, as well as good printability and compatibility with the elastomeric substrate. Both TFTs and logic circuits can be stretched beyond 50% strain along either channel length or channel width directions for thousands of cycles while showing no significant degradation in electrical performance. This work may offer an entry into more sophisticated stretchable electronic systems with monolithically integrated sensors, actuators, and displays, fabricated by scalable and low-cost methods for real life applications.

Discovery of Novel Oral Protein Synthesis Inhibitors of Mycobacterium tuberculosis That Target Leucyl-tRNA Synthetase.

The recent development and spread of extensively drug-resistant and totally drug-resistant resistant (TDR) strains of Mycobacterium tuberculosis highlight the need for new antitubercular drugs. Protein synthesis inhibitors have played an important role in the treatment of tuberculosis (TB) starting with the inclusion of streptomycin in the first combination therapies. Although parenteral aminoglycosides are a key component of therapy for multidrug-resistant TB, the oxazolidinone linezolid is the only orally available protein synthesis inhibitor that is effective against TB. Here, we show that small-molecule inhibitors of aminoacyl-tRNA synthetases (AARSs), which are known to be excellent antibacterial protein synthesis targets, are orally bioavailable and effective against M. tuberculosis in TB mouse infection models. We applied the oxaborole tRNA-trapping (OBORT) mechanism, which was first developed to target fungal cytoplasmic leucyl-tRNA synthetase (LeuRS), to M. tuberculosis LeuRS. X-ray crystallography was used to guide the design of LeuRS inhibitors that have good biochemical potency and excellent whole-cell activity against M. tuberculosis Importantly, their good oral bioavailability translates into in vivo efficacy in both the acute and chronic mouse models of TB with potency comparable to that of the frontline drug isoniazid.

Bolometric-Effect-Based Wavelength-Selective Photodetectors Using Sorted Single Chirality Carbon Nanotubes.

This paper exploits the chirality-dependent optical properties of single-wall carbon nanotubes for applications in wavelength-selective photodetectors. We demonstrate that thin-film transistors made with networks of carbon nanotubes work effectively as light sensors under laser illumination. Such photoresponse was attributed to photothermal effect instead of photogenerated carriers and the conclusion is further supported by temperature measurements. Additionally, by using different types of carbon nanotubes, including a single chirality (9,8) nanotube, the devices exhibit wavelength-selective response, which coincides well with the absorption spectra of the corresponding carbon nanotubes. This is one of the first reports of controllable and wavelength-selective bolometric photoresponse in macroscale assemblies of chirality-sorted carbon nanotubes. The results presented here provide a viable route for achieving bolometric-effect-based photodetectors with programmable response spanning from visible to near-infrared by using carbon nanotubes with pre-selected chiralities.

Ultrashort Channel Length Black Phosphorus Field-Effect Transistors.

This paper reports high-performance top-gated black phosphorus (BP) field-effect transistors with channel lengths down to 20 nm fabricated using a facile angle evaporation process. By controlling the evaporation angle, the channel length of the transistors can be reproducibly controlled to be anywhere between 20 and 70 nm. The as-fabricated 20 nm top-gated BP transistors exhibit respectable on-state current (174 µA/µm) and transconductance (70 µS/µm) at a VDS of 0.1 V. Due to the use of two-dimensional BP as the channel material, the transistors exhibit relatively small short channel effects, preserving a decent on-off current ratio of 10(2) even at an extremely small channel length of 20 nm. Additionally, unlike the unencapsulated BP devices, which are known to be chemically unstable in ambient conditions, the top-gated BP transistors passivated by the Al2O3 gate dielectric layer remain stable without noticeable degradation in device performance after being stored in ambient conditions for more than 1 week. This work demonstrates the great promise of atomically thin BP for applications in ultimately scaled transistors.

Capacitance-Voltage Characteristics of Thin-film Transistors Fabricated with Solution-Processed Semiconducting Carbon Nanotube Networks.

We report the capacitance-voltage (C-V) measurements on thin-film transistors (TFTs) using solution-processed semiconducting carbon nanotube networks with different densities and channel lengths. From the measured C-V characteristics, gate capacitance and field-effect mobility (up to ~50 cm(2) V(-1) s(-1)) of the TFTs were evaluated with better precision compared with the results obtained from calculated gate capacitance. The C-V characteristics measured under different frequencies further enabled the extraction and analysis of the interface trap density at the nanotube-dielectric layer interface, which was found to increase significantly as the network density increases. The results presented here indicate that C-V measurement is a powerful tool to assess the electrical performance and to investigate the carrier transport mechanism of TFTs based on carbon nanotubes.

Synthesis and antiviral activity of novel HCV NS3 protease inhibitors with P4 capping groups.

We have synthesized and evaluated a series of novel HCV NS3 protease inhibitors with various P4 capping groups, which include urea, carbamate, methoxy-carboxamide, cyclic carbamate and amide, pyruvic amide, oxamate, oxalamide and cyanoguanidine. Most of these compounds are remarkably potent, exhibiting single-digit to sub-nanomolar activity in the enzyme assay and cell-based replicon assay. Selected compounds were also evaluated in the protease-inhibitor-resistant mutant transient replicon assay, and they were found to show quite different potency profiles against a panel of HCV protease-inhibitor-resistant mutants.

Discovery of novel urea-based hepatitis C protease inhibitors with high potency against protease-inhibitor-resistant mutants.

The macrocyclic urea 2, a byproduct in the synthesis of benzoxaborole 1, was identified to be a novel and potent HCV protease inhibitor. We further explored this motif by synthesizing additional urea-based inhibitors and by characterizing them in replicase HCV protease-resistant mutants assay. Several compounds, exemplified by 12, were found to be more potent in HCV replicon assays than leading second generation inhibitors such as danoprevir and TMC-435350. Additionally, following oral administration, inhibitor 12 was found in rat liver in significantly higher concentrations than those reported for both danoprevir and TMC-435350, suggesting that inhibitor 12 has the combination of anti-HCV and pharmacokinetic properties that warrants further development of this series.

Discovery of novel P3-oxo inhibitor of hepatitis C virus NS3/4A serine protease.

A novel series of P3 oxo-modified macrocyclic hepatitis C virus NS3/4A serine protease inhibitor was designed, synthesized and biologically evaluated. The hydroxy-substituted inhibitor 10 demonstrated high potency in genotype 1a and 1b replicon and in the panel of HCV protease mutants. Interestingly, the t-butyl carbonate analog 9c, while not the most potent one in this series, exhibited a virtually flat potency profile in the panel of HCV protease mutants, thus providing opportunity for further optimization.

Synthesis and SAR of acyclic HCV NS3 protease inhibitors with novel P4-benzoxaborole moieties.

We have synthesized and evaluated a new series of acyclic P4-benzoxaborole-based HCV NS3 protease inhibitors. Structure-activity relationships were investigated, leading to the identification of compounds 5g and 17 with low nanomolar potency in the enzymatic and cell-based replicon assay. The linker-truncated compound 5j was found to exhibit improved absorption and oral bioavailability in rats, suggesting that further reduction of molecular weight and polar surface area could result in improved drug-like properties of this novel series.

Synthesis of new acylsulfamoyl benzoxaboroles as potent inhibitors of HCV NS3 protease.

HCV NS3/4A serine protease is essential for the replication of the HCV virus and has been a clinically validated target. A series of HCV NS3/4A protease inhibitors containing a novel acylsulfamoyl benzoxaborole moiety at the P1' region was synthesized and evaluated. The resulting P1-P3 and P2-P4 macrocyclic inhibitors exhibited sub-nanomolar potency in the enzymatic assay and low nanomolar activity in the cell-based replicon assay. The in vivo PK evaluations of selected compounds are also described.

Synthesis and biological evaluations of P4-benzoxaborole-substituted macrocyclic inhibitors of HCV NS3 protease.

We disclose here a series of P4-benzoxaborole-substituted macrocyclic HCV protease inhibitors. These inhibitors are potent against HCV NS3 protease, their anti-HCV replicon potencies are largely impacted by substitutions on benzoxaborole ring system and P2∗ groups. P2∗ 2-thiazole-isoquinoline provides best replicon potency. The in vitro SAR studies and in vivo PK evaluations of selected compounds are described herein.

Novel macrocyclic HCV NS3 protease inhibitors derived from α-amino cyclic boronates.

A novel series of P2-P4 macrocyclic HCV NS3/4A protease inhibitors with α-amino cyclic boronates as warheads at the P1 site was designed and synthesized. When compared to their linear analogs, these macrocyclic inhibitors exhibited a remarkable improvement in cell-based replicon activities, with compounds 9a and 9e reaching sub-micromolar potency in replicon assay. The SAR around α-amino cyclic boronates clearly established the influence of ring size, chirality and of the substitution pattern. Furthermore, X-ray structure of the co-crystal of inhibitor 9a and NS3 protease revealed that Ser-139 in the enzyme active site traps boron in the warhead region of 9a, thus establishing its mode of action.

1H-Pyrazolo-[3,4-c]cyclophepta[1,2-c]thiophenes: a unique structural class of dopamine D4 selective ligands.

A series of novel 1H-pyrazolo-[3,4-c]cyclophepta[1,2-c]thiophenes was prepared and screened at selected dopamine receptor subtypes. Compound 4 (NGB 4420) displayed high affinity and selectivity (>100-fold) for the D(4) over D(2) and other CNS receptors. This compound was identified as a D(4) antagonist via its attenuation of dopamine agonist-induced GTPgamma(35)S binding at D(4) receptor.

Regioselective synthesis of bifunctional macrolides for probing ribosomal binding.

[structure: see text] Bifunctional macrolides projecting an anchor group to the right or left spatial position of the lactone ring were synthesized. The regioselectivity of the key [3 + 2] cycloaddition process was controlled by the remote cladinose group attached to the C-3 position. These conformationally constrained molecules were employed as molecular probes to study the ribosomal binding sites of bifunctional macrolide antibiotics.