In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2.

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

DDoS Flood and Destination Service Changing Sensor.

In this paper, we aim to detect distributed denial of service (DDoS) attacks, and receive a notification of destination service, changing immediately, without the additional efforts of other modules. We designed a kernel-based mechanism to build a new Transmission Control Protocol/Internet Protocol (TCP/IP) connection smartly by the host while the users or clients not knowing the location of the next host. Moreover, we built a lightweight flooding attack detection mechanism in the user mode of an operating system. Given that reinstalling a modified operating system on each client is not realistic, we managed to replace the entry of the system call table with a customized sys_connect. An effective defense depends on fine detection and defensive procedures. In according with our experiments, this novel mechanism can detect flooding DDoS successfully, including SYN flood and ICMP flood. Furthermore, through cooperating with a specific low cost network architecture, the mechanism can help to defend DDoS attacks effectively.

Deep Learning Enabled Strain Mapping of Single-Atom Defects in Two-Dimensional Transition Metal Dichalcogenides with Sub-Picometer Precision.

Two-dimensional (2D) materials offer an ideal platform to study the strain fields induced by individual atomic defects, yet challenges associated with radiation damage have so far limited electron microscopy methods to probe these atomic-scale strain fields. Here, we demonstrate an approach to probe single-atom defects with sub-picometer precision in a monolayer 2D transition metal dichalcogenide, WSe2-2xTe2x. We utilize deep learning to mine large data sets of aberration-corrected scanning transmission electron microscopy images to locate and classify point defects. By combining hundreds of images of nominally identical defects, we generate high signal-to-noise class averages which allow us to measure 2D atomic spacings with up to 0.2 pm precision. Our methods reveal that Se vacancies introduce complex, oscillating strain fields in the WSe2-2xTe2x lattice that correspond to alternating rings of lattice expansion and contraction. These results indicate the potential impact of computer vision for the development of high-precision electron microscopy methods for beam-sensitive materials.

Atom-by-atom imaging of moiré transformations in 2D transition metal dichalcogenides.

Understanding the atomic-scale mechanisms that govern the structure of interfaces is critical across materials systems but particularly so for two-dimensional (2D) moiré materials. Here, we image, atom-by-atom, the thermally induced structural evolution of twisted bilayer transition metal dichalcogenides using in situ transmission electron microscopy. We observe low-temperature, local conversion of moiré superlattice into nanoscale aligned domains. Unexpectedly, this process occurs by nucleating a new grain within one monolayer, whose crystal orientation is templated by the other. The aligned domains grow through collective rotation of moiré supercells and hopping of 5|7 defect pairs at moiré boundaries. This provides mechanistic insight into the atomic-scale interactions controlling moiré structures and illustrates the potential to pattern interfacial structure and properties of 2D materials at the nanoscale.

Achieving sub-0.5-angstrom-resolution ptychography in an uncorrected electron microscope.

Subangstrom resolution has long been limited to aberration-corrected electron microscopy, where it is a powerful tool for understanding the atomic structure and properties of matter. Here, we demonstrate electron ptychography in an uncorrected scanning transmission electron microscope (STEM) with deep subangstrom spatial resolution down to 0.44 angstroms, exceeding the conventional resolution of aberration-corrected tools and rivaling their highest ptychographic resolutions​. Our approach, which we demonstrate on twisted two-dimensional materials in a widely available commercial microscope, far surpasses prior ptychographic resolutions (1 to 5 angstroms) of uncorrected STEMs. We further show how geometric aberrations can create optimized, structured beams for dose-efficient electron ptychography. Our results demonstrate that expensive aberration correctors are no longer required for deep subangstrom resolution.

Spin-Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet.

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin-orbit torque induced switching in the Heusler alloy Mn2 Ru1- x Ga, it is found that efficient current-induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy-efficient spintronic devices.

Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving Improved Voc and Jsc of Nonfullerene Organic Solar Cells via End Group Manipulation.

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor-donor-acceptor type nonfullerene acceptors (A-D-A type NFAs). The extension of the end-group conjugation of nonfullerene acceptors (NFAs) is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based end-group acceptor, 2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)malononitrile, denoted as α-BC was designed and synthesized. The Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the α-position of the thiophene unit, leading to the isomerically pure benzothiophene-fused α-BC. The well-defined α-BC with extended conjugation was condensed with three different ladder-type diformylated donors to form three new A-D-A NFAs named BDCPDT-BC, DTCC-BC, and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC, and ITIC model compounds were also synthesized for comparison. The incorporation of the electron-rich benzothiophene unit in the end-group upshifts the lowest unoccupied molecular orbital energy levels of the NFAs, which beneficially enlarges the Voc values. On the other hand, the benzothiophene unit in α-BC not also imparts an optical transition in the shorter wavelengths around 340-400 nm for a better light harvesting ability but also promotes the antiparallel π-π stacking of the end-groups for efficient electron transport. The organic photovoltaic cell devices using a PBDB-T polymer and BC-based NFAs all showed the improved Voc and Jsc values. The BDCPDT-BC- and DTCC-BC-based devices exhibited a power conversion efficiency (PCE) of 10.82 and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33 and 9.25%). More importantly, the ITBC-based device delivered the highest PCE of 12.07% with a Jsc of 19.90 mA/cm2, a Voc of 0.94 V, and an fill factor of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.05%).

Side-chain modulation of dithienofluorene-based copolymers to achieve high field-effect mobilities.

A ladder-type dithieno[3,2-b:6,7-b']fluorene (DTF), where the central fluorene is fused with two outer thiophene rings at its 2,3- and 6,7-junctions, is developed. The pentacyclic DTF monomers were polymerized with dithienodiketopyrrolopyrrole (DPP) acceptors to afford three alternating donor-acceptor copolymers PDTFDPP16, PDTFDPP20, and PDTFDPP32 incorporating different aliphatic side chains (R1 group at DTF; R2 group at the DPP moieties). The side-chain variations in the polymers play a significant role in determining not only the intrinsic molecular properties but also the intermolecular packing. As evidenced by the 2-dimensional GIXS measurements, PDTFDPP16 with octyl (R1) and 2-ethylhexyl (R2) side chains tends to align in an edge-on π-stacking orientation, whereas PDTFDPP20 using 2-butyloctyl (R1) and 2-ethylhexyl (R2) adopts a predominately face-on orientation. PDTFDPP32 with the bulkiest 2-butyloctyl (R1) and 2-octyldodecyl (R2) side chains shows a less ordered amorphous character. The OFET device using PDTFDPP20 with a face-on orientation determined by GIXS measurements achieved a high hole-mobility of up to 5 cm2 V-1 s-1. The high rigidity and coplanarity of the DTF motifs play an important role in facilitating intramolecular 1-dimensional charge transport within the polymer backbones. The implementation of main-chain coplanarity and side-chain engineering strategies in this research provides in-depth insights into structure-property relationships for guiding development of high-mobility OFET polymers.

Effects of surface oxidation of Cu substrates on the growth kinetics of graphene by chemical vapor deposition.

Although the success of graphene research has opened up a new route for wearable electronic and optoelectronic devices, producing graphene with controllable quality and cost-effective growth on a large scale remains challenging due to the lack of understanding about its growth kinetics. Domain boundaries interrupt lattice continuity of graphene; therefore, lowering the nucleation density at the initial stage of graphene growth in the chemical vapor deposition (CVD) process is beneficial for improving the quality of graphene for applications. Herein, we show that by forming an oxide passivation layer on Cu substrates before CVD graphene growth, graphene nucleation density can be effectively decreased. The nucleation mechanism in the presence of an oxide passivation layer is of interest. The analysis of graphene growth kinetics suggests that the thickness of the boundary layer for mass transfer on the substrate surface plays an important role in controlling the reduction rate of the oxide passivation layer. A thick boundary layer created under slow gas flow causes slow reduction of the oxide passivation layer, making finite sites for graphene nucleation. The domain density in a graphene layer is therefore significantly reduced. Graphene sheets of various domain densities (ranging from 104 to 1 mm-2) can be fabricated by suitably choosing the growth parameters. The graphene sheet with a lower density of domain boundaries exhibits better electrical conductivities.

Dual targeting of solid lipid nanoparticles grafted with 83-14 MAb and anti-EGF receptor for malignant brain tumor therapy.

AIMS: Solid lipid nanoparticles (SLNs) conjugated with surface 83-14 monoclonal antibody (8314MAb) and anti-epithelial growth factor receptor (AEGFR) were synthesized and applied to permeate the blood-brain barrier (BBB) and inhibit the growth of glioblastoma multiforme (GBM). MAIN METHODS: 8314MAb and AEGFR were crosslinked on SLNs to carry etoposide (ETP) across human brain-microvascular endothelial cells (HBMECs) and to treat U87MG cells. KEY FINDINGS: An increase in the 8314MAb concentration increased the permeability for propidium iodide (PI) and ETP across the BBB, however, decreased the 8314MAb grafting efficiency and transendothelial electrical resistance of the monolayer of HBMECs. In addition, an increase in the AEGFR concentration enhanced the viability of HBMECs and human astrocytes (HAs), however, reduced the AEGFR grafting efficiency and ETP release rate. An incorporation of both 8314MAb and AEGFR increased the particle size, however, decreased the zeta potential, ETP release rate, and viability of HBMECs and HAs. SIGNIFICANCE: The conjugation of 8314MAb and AEGFR on ETP-loaded SLNs can be a promising strategy to deliver antitumor ETP to the brain and restrain the propagation of GBM.

Inhibition against growth of glioblastoma multiforme in vitro using etoposide-loaded solid lipid nanoparticles with p-aminophenyl-α-D-manno-pyranoside and folic acid.

Solid lipid nanoparticles (SLNs) grafted with p-aminophenyl-α-D-manno-pyranoside (APMP) and folic acid (FA) (APMP-FA-SLNs) were applied to encapsulate 4'-demethylepipodophyllotoxin 9-(4,6-O-ethylidene-ß-D-glucopyranoside) (etoposide) (ETP) for promoting the antiproliferation of malignant glioblastoma multiforme. ETP-loaded APMP-FA-SLNs (APMP-FA-ETP-SLNs) were used to penetrate the blood-brain barrier (BBB) and retard the propagation of U87MG cells. An incorporation of APMP and FA increased the particle size, the cytotoxicity to U87MG cells, and the permeability coefficient for propidium iodide and ETP across the BBB. In addition, an increase in the APMP and FA concentration reduced the zeta potential, the grafting efficiency of APMP and FA, the dissolution rate of ETP, and the transendothelial electrical resistance. Immunochemical staining images evidenced that APMP-FA-ETP-SLNs could infiltrate the BBB via glucose transporter 1 and recognize U87MG cells via folate receptor. APMP-FA-ETP-SLNs can be an effective pharmacotherapeutic formulation in targeting delivery to the brain and in inhibitory efficacy against tumorous cells for cancer therapy.

Synthesis and supramolecular assembly of pentacyclic dithienofluorene and diselenophenofluorene derivatives.

2,7-Diiodo-3,6-dibromofluorene and 2,7-dichloro-3,6-dibromofluorene have been successfully synthesized. The two key intermediates enable us to implement a regioselective Sonogashira reaction followed by intramolecular thiolate/acetylene cyclization, forming two regiospecific pentacyclic dithieno[2,3-b:7,6-b']fluorene (2,7-DTF) and dithieno[3,2-b:6,7-b']fluorene (3,6-DTF) isomeric molecules, respectively. By using a similar strategy, selenophene-based diselenopheno[2,3-b:7,6-b']fluorene (2,7-DSF) as well as diselenopheno[3,2-b:6,7-b']fluorene (3,6-DSF) were also prepared. The isomeric and sulfur/selenium effects determine the optical, electrochemical, and orbital properties. X-ray crystallography revealed that 2,7-DTF and 3,6-DTF molecules assemble into supramolecular helical structures.

Synthesis and molecular properties of four isomeric dialkylated angular-shaped naphthodithiophenes.

A new strategy to synthesize 4,9- and 5,10-dialkylated α-aNDTs as well as 4,9- and 5,10-dialkylated ß-aNDTs is described. Four isomeric precursors with different dithienyl-ene-diyne arrangements undergo base-induced double 6π-cyclization to construct the central naphthalene cores, leading to the formation of the regiospecific products. These 2,7-distannylated dialkylated aNDT-based monomers can be used for Stille cross-coupling to produce promising conjugated materials for various optoelectronic applications.

Combination of molecular, morphological, and interfacial engineering to achieve highly efficient and stable plastic solar cells.

A flexible solar device showing exceptional air and mechanical stability is produced by simultaneously optimizing molecular structure, active layer morphology, and interface characteristics. The PFDCTBT-C8-based devices with inverted architecture exhibited excellent power conversion efficiencies of 7.0% and 6.0% on glass and flexible substrates, respectively.