Tianlong Wen

High-speed and broadband terahertz wave modulators based on large-area graphene field-effect transistors

We present a broadband terahertz wave modulator with improved modulation depth and switch speed by cautiously selecting the gate dielectric materials in a large-area graphene-based field-effect transistor (GFET). An ultrathin Al2O3 film (∼60  nm) is deposited by an atomic-layer-deposition technique as a high-k gate dielectric layer, which reduces the Coulomb impurity scattering and cavity effect, and thus greatly improves the modulation performance. Our modulator has achieved a modulation depth of 22% and modulation speed of 170 kHz in a frequency range from 0.4 to 1.5 THz, which is a large improvement in comparison to its predecessor of SiO2-based GFET.

Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers

High quality VO2 thin films have been fabricated on silicon substrates using magnetron sputtering by introducing Al2O3 thin films as a buffer. The ultrathin Al2O3 deposited by plasma-assisted atomic layer deposition leads to a greatly improved crystallinity and textures in VO2 films. Dramatic change in electrical resistivity (4 orders of magnitude) and a small thermal hysteresis loop (~4 K) are obtained across the metal–insulator phase transition (MIT). Remarkably, by applying perpendicular voltage to a VO2/Al2O3 based metal/VO2/semiconductor device, electrically driven MIT switching characteristics have been observed with a tiny tunneling leakage current of ~10 μA. These results show that an electric field alone is sufficient to trigger the MIT, and the realization of VO2 based ultrafast electrical switching devices on a silicon substrate is possible.

Open-top TiO2 nanotube arrays with enhanced photovoltaic and photochemical performances via a micromechanical cleavage approach

Anodic growth of TiO2 nanotube (NT) arrays has been proved to be very promising for energy conversion applications, e.g. in photovoltaic devices and fuel cells. However, disordered “nano-grass” layers were always found on the top of the anodic TiO2 NT arrays. In this paper, we demonstrate a novel and simple method using a micromechanical cleavage technique to peel off the disordered nanograss layer. Using this method, ∼1 × 1.5 cm−2 of uncapped TiO2 NT arrays with a high-aspect ratio can be easily obtained. The results further indicate that the treatment can improve the photovoltaic and photochemical performances. After the treatment, the conversion efficiency (η) of the dye sensitized solar cells (DSSCs) increased by 29.3%. This work facilitates the growth and applications of high aspect-ratio anodic TiO2 NT arrays in related devices and systems.

Manipulate the magnetic anisotropy of nanoparticle assemblies in arrays

Tuning the magnetic anisotropy of nanoparticle assemblies is critical for their applications such as on-chip magnetic electronic components and electromagnetic wave absorption. In this work, we developed a facile hierarchical self-assembly method to separately control the magnetic shape and magnetocrystalline anistropy of individual nanoparticle assemblies in arrays. Since magnetic nanoparticle assemblies in the array have the same size, shape and alignment, we are able to study the magnetic properties of individual nanoparticle assembly by measuring the whole arrays. The interplay between the two magnetic anisotropies was systematically studied for disk- and bar-shaped nanoparticle assemblies. Maximum magnetic anisotropy was obtained when the easy axis of magnetic nanoparticles was aligned along the long axes of the bar-shaped nanoparticles assemblies.

A novel sol-gel method for preparing favorable TiO2 thin film

Nanocrystalline TiO2 thin films were synthesized by the sol-gel spin-coating method with different variables. Tetrabutyl titanate (TBOT) proportion and C5H8O2: TBOT molar ratio were confirmed to be influential on the gelation time. X-ray diffraction analysis indicated that the samples presented rutile TiO2 phases, which is a basis for subsequent experiments. Scanning electron microscope results revealed that TiO2 thin films with homogeneous and compact surfaces were synthesized successfully when adding moderate TBOT. It was found the thickness of films could reach about 60 nm when sintered at 750 °C, and the influence of sintering temperature was also investigated.

Low Temperature-Derived 3D Hexagonal Crystalline Fe3O4 Nanoplates for Water Purification

Fe3O4 nanoplates were fabricated by an anodic oxidation process and a subsequent water assisted crystallization process at low temperature, which was found to be very efficient and environmentally friendly. The as-prepared Fe3O4 nanoplates have hexagonal outlines with a thickness of about 20 nm. Tremendous grooves were distributed on the entire surfaces of the nanoplates, making the two-dimension nanoplates have a unique 3D morphology. Transmission electron microscopy results confirmed that the single-crystalline nature of the nanoplates was well maintained. Owing to the unique structures and porous morphologies, the as-prepared 3D nanoplates show excellent ability for absorbing solar energy and absorbing organic pollutants, which can be utilized for cleaning up water. Moreover, the Fe3O4 nanoplates show good magnetic properties that enable them to be easily collected and recycled. We believe this study will inspire the application of Fe3O4 nanoplates with 3D structures in energy and environmental areas.

Infrared and Terahertz Modulation Characteristics of n-GeBi/p-Si Photodiodes

In this paper, germanium-bismuth (Ge_1-xBi_x, x = 0-0.32) is grown by low-temperature molecular beam epitaxy. Because Bi is an element belonging to group V, GeBi films show inherent n-type doping properties compared with GeSn ones. Inherent n-type Ge_1-xBi_x films with a doping concentration of 2 × 10¹⁵-2 × 10¹⁶/cm³ are epitaxially deposited on p-type Si substrates to form p-n junctions. Current-voltage measurements show that the dark current density of the diodes can approach 0.32 A/cm². The influence of Bi concentration on the infrared (IR) and terahertz (THz) transmittance of the films is investigated. Near-IR (1-2 μm) and mid/far-IR (2.6-10 μm) responsivities of the films are 0.65 and 0.032 A/W, respectively. The THz wave transmittance is tuned by -6%-8% by tailoring the bias voltage. A modulation depth of -12% is obtained for a Ge_0.78Bi_0.22/p-Si diode. The dynamic modulation characteristics of n-Ge_1-xBi_x/ p-Si diodes are further investigated using a 340-GHz carrier. The experimental maximum THz wave modulation speed is up to 2 MHz. The present results demonstrate that n-GeBi/ p-Si diodes are promising for both mid/far-IR photodetectors and broadband high-speed THz wave modulators.

Magnonic waveguide based on exchange-spring magnetic structure

We propose to use a soft/hard exchange-spring coupling bilayer magnetic structure to introduce a narrow channel for spin-wave propagation. We show by micromagnetic simulations that broad-band Damon-Eshbach geometry spin waves can be strongly localized into the channel and propagate effectively with a proper high group velocity. The beamwidth of the bound mode spin waves is almost independent from the frequency and is smaller than 24nm. For a low-frequency excitation, we further investigate the appearance of two other spin beams in the lateral of the channel. In contrast to a domain wall, the channel formed by exchange-spring coupling can be easier to realize in experimental scenarios and holds stronger immunity to surroundings. This work is expected to open new possibilities for energy-efficient spin-wave guiding as well as to help shape the field of beam magnonics.

Infrared Properties and Terahertz Wave Modulation of Graphene/MnZn Ferrite/p-Si Heterojunctions

MnZn ferrite thin films were deposited on p-Si substrate and used as the dielectric layer in the graphene field effect transistor for infrared and terahertz device applications. The conditions for MnZn ferrite thin film deposition were optimized before device fabrication. The infrared properties and terahertz wave modulation were studied at different gate voltage. The resistive and magnetic MnZn ferrite thin films are highly transparent for THz wave, which make it possible to magnetically modulate the transmitted THz wave via the large magnetoresistance of graphene monolayer.

Preparation and Optical Properties of GeBi Films by Using Molecular Beam Epitaxy Method

Ge-based alloys have drawn great interest as promising materials for their superior visible to infrared photoelectric performances. In this study, we report the preparation and optical properties of germanium-bismuth (Ge1-xBix) thin films by using molecular beam epitaxy (MBE). GeBi thin films belong to the n-type conductivity semiconductors, which have been rarely reported. With the increasing Bi-doping content from 2 to 22.2%, a series of Ge1-xBix thin film samples were obtained and characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. With the increase of Bi content, the mismatch of lattice constants increases, and the GeBi film shifts from direct energy band-gaps to indirect band-gaps. The moderate increase of Bi content reduces optical reflectance and promotes the transmittance of extinction coefficient in infrared wavelengths. The absorption and transmittance of GeBi films in THz band increase with the increase of Bi contents.