Anisotropic magnetoresistance as evidence of spin-momentum inter-locking in topological Kondo insulator SmB6 nanowires.

SmB6, which opens up an insulating bulk gap due to hybridization between itinerant d-electrons and localized f-electrons below a critical temperature, turns out to be a topological Kondo insulator possessing exotic conducting states on its surface. However, measurement of the surface-states in SmB6 draws controversial conclusions, depending on the growth methods and experimental techniques used. Herein, we report anisotropic magnetoresistance (AMR) observed in the Kondo energy gap of a single SmB6 nanowire that is immune to magnetic dopant pollution and features a square cross-section to show high-symmetry crystal facets. The AMR clearly shows a cosine function of included angle θ between magnetic field and measuring current with a period of π. The positive AMR is interpreted by anisotropically lifting the topological protection of spin-momentum inter-locking surface-states by rotating the in-plane magnetic field, which, therefore, provides the transport evidence that supports the topologically nontrivial nature of the SmB6 surface-states.

Interface Tuning between Two Connecting Bulk Heterojunctions in Small Molecule Bilayer Ternary Solar Cells.

Bilayer ternary solar cells are a kind of novel organic photovoltaic device with a triple-component active layer but are different from the ternary bulk heterojunction (BHJ) blend. Two binary BHJs with a common acceptor (or donor) are deposited sequentially in this kind of device. Here, we study the fabrication and optimization of bilayer ternary solar cells using metal phthalocyanine donors and fullerene acceptor. The device power conversion efficiency (PCE) shows a significant dependence on the interface between the two binary BHJs. The interface formed by stacking two BHJs directly demonstrates severe restrictions on the device efficiency. We find that the photovoltaic performance of bilayer ternary cells can be improved by inserting a C60 molecular monolayer between the two binary BHJs. The effect of the C60 interfacial layer on charge transport is analyzed based on their transport characteristics under negative bias. A relationship between the C60 interfacial layer and recombination under illumination is discussed. This work reveals a particular influence due to the interface facing three materials in organic solar cells.

Observation and Ultrafast Dynamics of Inter-Sub-Band Transition in InAs Twinning Superlattice Nanowires.

A variety of infrared applications rely on semiconductor superlattices, including, notably, the realization of high-power, compact quantum cascade lasers. Requirements for atomically smooth interface and limited lattice matching options set high technical standards for fabricating applicable heterostructure devices. The semiconductor twinning superlattice (TSL) forms in a single compound with periodically spaced twin boundaries and sharp interface junctions and can be grown with convenient synthesis methods. Therefore, employing semiconductor TSL may facilitate the development of optoelectronic applications related to superlattice structures. Here, it is shown that InAs TSL nanowires generate inter-sub-band transition channels due to the band projection and the Bragg-like electron reflection. The findings reveal the physical mechanisms of inter-sub-band transitions in TSL structure and suggest that TSL structures are promising candidates for mid-infrared optoelectronic applications.

Improved performance of small molecule organic solar cells by incorporation of a glancing angle deposited donor layer.

Improving the photovoltaic performance directly by innovative device architectures contributes much progress in the field of organic solar cells. Photovoltaic device using different kinds of heterojunction with the given set of organic semiconductors paves the way to a better understanding of the working mechanism of organic heterojunction. Here, we report on the fabrication of a new device structure without employing extra material. A thin film of the donor material (chloroaluminum phthalocyanine (ClAlPc)) is inserted between ClAlPc:C60 bulk heterojunction (BHJ) and C60 layer by glancing angle deposition. A ClAlPc/C60 planar heterojunction co-exists with ClAlPc:C60 BHJ simultaneously in this device. Higher efficiency is obtained with this novel device structure. The effects of this additional ClAlPc layer on open-circuit voltage and fill factor in photovoltaic cells are studied. This work provides a new route to improve the device performance of organic solar cells.

Growth Mechanism of SmB6 Nanowires Synthesized by Chemical Vapor Deposition: Catalyst-Assisted and Catalyst-Free.

SmB6 nanowires, as a prototype of nanostructured topological Kondo insulator, have shown rich novel physical phenomena relating to their surface. Catalyst-assisted chemical vapor deposition (CVD) is a common approach to prepare SmB6 nanowires and Ni is the most popular catalyst used to initiate the growth of SmB6 nanowires. Here, we study the effect of growth mechanism on the surface of SmB6 nanowires synthesized by CVD. Two types of SmB6 nanowires are obtained when using Ni as the catalyst. In addition to pure SmB6 nanowires without Ni impurity, a small amount of Ni is detected on the surface of some SmB6 nanowires by element analysis with transmission electron microscopy. In order to eliminate the possible distribution of Ni on nanowire surface, we synthesize single crystalline SmB6 nanowires by CVD without using catalyst. The difference between catalyst-assisted and catalyst-free growth mechanism is discussed.

3D Printed High-Performance Lithium Metal Microbatteries Enabled by Nanocellulose.

Batteries constructed via 3D printing techniques have inherent advantages including opportunities for miniaturization, autonomous shaping, and controllable structural prototyping. However, 3D-printed lithium metal batteries (LMBs) have not yet been reported due to the difficulties of printing lithium (Li) metal. Here, for the first time, high-performance LMBs are fabricated through a 3D printing technique using cellulose nanofiber (CNF), which is one of the most earth-abundant biopolymers. The unique shear thinning properties of CNF gel enables the printing of a LiFePO4 electrode and stable scaffold for Li. The printability of the CNF gel is also investigated theoretically. Moreover, the porous structure of the CNF scaffold also helps to improve ion accessibility and decreases the local current density of Li anode. Thus, dendrite formation due to uneven Li plating/stripping is suppressed. A multiscale computational approach integrating first-principle density function theory and a phase-field model is performed and reveals that the porous structures have more uniform Li deposition. Consequently, a full cell built with a 3D-printed Li anode and a LiFePO4 cathode exhibits a high capacity of 80 mA h g-1 at a charge/discharge rate of 10 C with capacity retention of 85% even after 3000 cycles.

Applied Stress-Assisted Growth of Single Crystal γ-Fe2O3 Nanowires.

It is difficult to obtain γ-Fe2O3 nanostructures by heating iron substrate in ambient conditions because γ-Fe2O3 is less thermodynamically stable than α-Fe2O3. In this work, we synthesize γ-Fe2O3 nanowires by heating iron particles under an external force. The stacking style of iron and oxygen ions under a strong shearing stress tends to adopt the γ-Fe2O3 structure regardless of the thermodynamic restriction. These γ-Fe2O3 nanowires exhibit a clear ferromagnetic property. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements confirm that γ-phase structure appears only under the applied external force during the heating period. A window of the magnitude of the external force is found to help the nanowire growth on iron particles. The growth mechanism of γ-Fe2O3 nanowires other than α-Fe2O3 under the external force is discussed and an applied stress-assisted growth model is proposed. This work presents an easy approach to produce ferromagnetic iron oxide nanowires on a large scale.

Tunable Low Loss 1D Surface Plasmons in InAs Nanowires.

Due to the ability to manipulate photons at nanoscale, plasmonics has become one of the most important branches in nanophotonics. The prerequisites for the technological application of plasmons include high confining ability (λ0 /λp ), low damping, and easy tunability. However, plasmons in typical plasmonic materials, i.e., noble metals, cannot satisfy these three requirements simultaneously and cause a disconnection to modern electronics. Here, the indium arsenide (InAs) nanowire is identified as a material that satisfies all the three prerequisites, providing a natural analogy with modern electronics. The dispersion relation of InAs plasmons is determined using the nanoinfrared imaging technique, and show that their associated wavelengths and damping ratio can be tuned by altering the nanowire diameter and dielectric environment. The InAs plasmons possess advantages such as high confining ability, low loss, and ease of fabrication. The observation of InAs plasmons could enable novel plasmonic circuits for future subwavelength applications.

Improved performance of small molecule solar cell by using oblique deposition technique and zinc phthalocyanine cathode buffer layer.

Short circuit current density (J sc) and open circuit voltage (V oc) are two important parameters to evaluate the performance of organic solar cells. How to increase these two parameters without using novel material still remains a challenge. Two small molecules, zinc phthalocyanine (ZnPc) and 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI), are used to fabricate ITO/ZnPc/ZnPc:PTCBI/PTCBI/ZnPc/Al photovoltaic device. We find that J sc and V oc are enhanced by using oblique deposition technique and ZnPc cathode buffer layer, respectively. Analysis of the active layer reveals phase segregation in obliquely deposited ZnPc:PTCBI bulk heterojunction layer. Field emission measurement is used to probe the band bending and internal field in ZnPc/PTCBI planar heterojunction. The effects of phase segregation and internal field are discussed. This work shows that careful assembly of donor and acceptor material is beneficial to small molecule photovoltaic device.

Ultralight, highly thermally insulating and fire resistant aerogel by encapsulating cellulose nanofibers with two-dimensional MoS2.

Thermally insulating materials, made from earth-abundant and sustainable resources, are highly desirable in the sustainable construction of energy efficient buildings. Cellulose from wood has long been recognized for these characteristics. However, cellulose can be a flammability hazard, and for construction this has been addressed via chemical treatment such as that with halogen and/or phosphorus, which leads to further environmental concerns. Fortunately, the structure of cellulose lends itself well to chemical modification, giving great potential to explore interaction with other compounds. Thus, in this study, cellulose nanofibers (CNFs) were nano-wrapped with ultrathin 1T phase molybdenum disulfide (MoS2) nanosheets via chemical crosslinking, to produce an aerogel. Thermal and combustion characterization revealed highly desirable properties (thermal conductivity k = 28.09 mW m-1 K-1, insulation R value = 5.2, limit oxygen index (LOI) = 34.7%, total heat release = 0.4 MJ m-2). Vertical burning tests also demonstrated excellent fire retardant and self-extinguishing capabilities. Raman spectra further revealed that MoS2 remained unscathed after 30 seconds of burning in a 1300 °C butane flame. Considering the inherently low density of this material, there is significant opportunity for its usage in a number of insulating applications demanding specific fire resistance properties.

Synthesis of Bi2Te3 Nanotubes Using Te Nanotubes as a Template.

Bi2Te3 nanotubes are synthesized by a facile two-step hydrothermal method. Te nanotubes are prepared firstly and then used as a template to produce Bi2Te3 nanotubes. The structure and morphology of the synthesized nanotubes are characterized by X-ray diffraction, field emission scanning electron microscope, and transmission electron microscope. The synthesized Bi2Te3 nanotubes are several microns in length and about 400 nm in diameter. The growth process is investigated in detail under different experimental conditions. The formation mechanism of Bi2Te3 nanotubes from the Te nanotube template is proposed and discussed. Electrical property of single Bi2Te3 nanotube is investigated. The synthesis of smooth Bi2Te3 nanotubes opens up the opportunities of investigating novel physical phenomena of topological insulators with two independent surfaces.

Growth of High Material Quality Group III-Antimonide Semiconductor Nanowires by a Naturally Cooling Process.

We report on a simple but powerful approach to grow high material quality InSb and GaSb nanowires in a commonly used tube furnace setup. The approach employs a process of stable heating at a high temperature and then cooling down naturally to room temperature with the nanowire growth occurred effectively during the naturally cooling step. As-grown nanowires are analyzed using a scanning electron microscope and a transmission electron microscope equipped with an energy-dispersive X-ray spectroscopy setup. It is shown that the grown nanowires are several micrometers in lengths and are zincblende InSb and GaSb crystals. The FET devices are also fabricated with the grown nanowires and investigated. It is shown that the grown nanowires show good, desired electrical properties and should have potential applications in the future nanoelectronics and infrared optoelectronics.

Bulk heterojunction organic solar cells fabricated by oblique angle deposition.

Bulk heterojunction organic solar cells based on small molecules are often fabricated by the co-deposition of donor and accepter materials on substrates placed horizontally. An intimate blend of donor and acceptor molecules is the common product in as-prepared samples. Using metal phthalocyanine as the donor and fullerene as the acceptor, we tilt the substrate to deposit the active layer. Both short circuit current density and open circuit voltage increase obviously in bulk heterojunction organic solar cells fabricated by oblique angle deposition. Analysis of the active layer reveals that phase segregation occurs in obliquely deposited bulk heterojunction layers. The mechanism of phase segregation is discussed. A change in the stacking style of fullerene molecules on metal phthalocyanine grains is proposed on the tilted substrate. This result provides a simple method to form phase segregation in bulk heterojunction layers.