Search

Two‐terminal memristor has emerged as one of the most promising neuromorphic artificial electronic devices for their structural resemblance to biological synapses and ability to emulate many synaptic functions. In this work, a memristor based on the back‐end‐of‐line (BEOL) material silicon carbide (SiC) is developed. The thin film memristors demonstrate excellent binary resistive switching with compliance‐free and self‐rectifying characteristics which are advantageous for the implementation of high‐density 3D crossbar memory architectures. The conductance of this SiC‐based memristor can be modulated gradually through the application of both DC and AC signals. This behavior is demonstrated to further emulate several vital synaptic functions including paired‐pulse facilitation (PPF), post‐tetanic potentiation (PTP), short‐term potentiation (STP), and spike‐rate‐dependent plasticity (SRDP). The synaptic function of learning‐forgetting‐relearning processes is successfully emulated and demonstrated using a 3 × 3 artificial synapse array. This work presents an important advance in SiC‐based memristor and its application in both memory and neuromorphic computing.

The “one-to-many” problem is a typical challenge that faced by many machine learning aided inverse nanophotonics designs where one target optical response can be achieved by many solutions (designs). Although novel training approaches, such as tandem network, and network architecture, such as the mixture density model, have been proposed, the critical problem of solution degeneracy still exists where some possible solutions or solution spaces are discarded or unreachable during the network training process. Here, we report a solution to the “one-to-many” problem by employing a conditional generative adversarial network (cGAN) that enables generating sets of multiple solution groups to a design problem. Using the inverse design of a transmissive Fabry–Pérot-cavity-based color filter as an example, our model demonstrates the capability of generating an average number of 3.58 solution groups for each color. These multiple solutions allow the selection of the best design for each color which results in a record high accuracy with an average index color difference ΔE of 0.44. The capability of identifying multiple solution groups can benefit the design manufacturing to allow more viable designs for fabrication. The capability of our cGAN is verified experimentally by inversely designing the RGB color filters. We envisage this cGAN-based design methodology can be applied to other nanophotonic structures or physical science domains where the identification of multi-solution across a vast parameter space is required.

The combination of lithographic methods and sol gel bottom-up techniques is a promising approach for nanopatterning substrates. The integration and scalable fabrication of such substrates are of great interest for the development of nanowire-based materials opening potentialities in new technologies. We demonstrate the deposition of ordered mesoporous silica into nanopatterned silica substrates by dip coating. Using scanning electron microscopy and grazing incidence small angle X-ray scattering, the effect of the sol composition on the pore ordering was probed. Optimising the sol composition using anodic alumina membranes as confined spaces, we showed how the pH controlled the transformation from circular to columnar mesophase. Vertical mesopores were obtained with very good repeatability. The effect of the sol chemistry on the surfactant curvature was then shown to be similar in nanopatterned substrates made by e-beam lithography.

The homologous series [GenBu3(EnBu)] (E = Te, Se, S; (1), (3) and (4)) and [GenBu2(TenBu)2] (2) have been synthesized as mobile oils in excellent yield (72-93%) and evaluated as single-source precursors for the low-pressure chemical vapor deposition (LPCVD) of GeE thin films on silica. Compositional and structural characterizations of the deposits have been performed by grazing-incidence X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and Raman spectroscopy, confirming the phase purity and stoichiometry. Electrical characterization via variable-temperature Hall effect measurements is also reported. Given the strong interest in GeTe and its alloys for thermoelectric applications, variable-temperature Seebeck data were also investigated for a series of p-type GeTe films. The data show that it is possible to tune the thermoelectric response through intrinsic Ge vacancy regulation by varying the deposition temperature, with the highest power factor (40 μW/K2cm@629 K) and effective ZT values observed for the films deposited at higher temperatures.

We report a simple process for the electrodeposition of tungsten disulfide thin films from a CH2Cl2-based electrolyte using a tailored single source precursor, [NEt4]2[WS2Cl4]. This new precursor incorporates the 1:2 W:S ratio required for formation of WS2, and eliminates the need for an additional proton source in the electrolyte to remove excess sulfide. The electrochemical behaviour of [NEt4]2[WS2Cl4] is studied by cyclic voltammetry and electrochemical quartz crystal microbalance techniques, and the WS2 thin films are grown by potentiostatic electrodeposition.

We report a simple process for the electrodeposition of tungsten disulfide thin films from a CH

Optical Solar Reflectors (OSRs) play a crucial role in thermal management of spacecraft as the outer-surface to the space environment. To achieve the thermal control, an ideal OSR should be able to reflect all the UV/Vis/NIR spectrum radiation to avoid heating up by the son and also emit blackbody radiation in infrared (IR) spectrum for thermal dissipation. Conventionally, OSRs are made of aluminium plated quartz tiles and therefore have serious issues for their heavy weight and fragile format, significantly adding the launch and assembly cost, respectively. Recently, we have proposed a meta-OSR solution based on thin film technology using Al-doped ZnO (AZO) [1] and the meta-OSRs can be fabricated on flexible substrates, making them light in weight and easy for surface mounting. In addition, we have also proposed a smart meta-OSR solution based on VO 2 , which has a tunable emissivity depending on the operational temperature [2] . In this work, we report a novel meta-OSR with a planar AZO metasurface unlike island-like metasurfaces formed by etch. The planar metasurface is fabricated through our patented plasma patterning technique [3] [4] , which can locally modulate carrier density in AZO.

This work has demonstrated that the single source precursor [nBu3Sn(TenBu)], bearing n-butyl groups and containing the necessary 1 : 1 Sn : Te ratio, facilitates growth of continuous, stoichiometric SnTe thin films. This single source CVD precursor allows film growth at significantly lower temperatures (355-434 °C at 0.01-0.05 Torr) than required for CVD from SnTe powder. This could be advantageous for controlling the surface states in topological insulators. The temperature-dependent thermoelectric performance of these films has been determined, revealing them to be p-type semiconductors with peak Seebeck coefficient and power factor values of 78 μV K-1 and 8.3 μW K-2 cm-1, respectively, at 615 K; comparing favourably with data from bulk SnTe. Further, we have demonstrated that the precursor facilitates area selective growth of SnTe onto the TiN regions of SiO2/TiN patterned substrates, which is expected to be beneficial for the fabrication of micro-thermoelectric generators.

Heterostructures involving two-dimensional (2D) transition metal dichalcogenides and other materials such as graphene have a strong potential to be the fundamental building block of many electronic and optoelectronic applications. The integration and scalable fabrication of such heterostructures are of the essence in unleashing the potential of these materials in new technologies. For the first time, we demonstrate the growth of few-layer MoS

The ever-increasing demand for renewable energy and zero carbon dioxide emission have been the driving force for the development of thermoelectric generators with better power generation performance. Alongside with the effort to discover thermoelectric materials with higher figure-of-merit, the geometrical and structural optimisation of thermoelectric generators are also essential for maximized power generation and efficiency. This work demonstrates for the first time the application of artificial neural network, a deep learning technique, in forward modelling the maximum power generation and efficiency of a thermoelectric generator and its application in the generator design and optimisation. After training using a dataset containing 5000 3-D finite element method based simulations, the artificial neural networks with 5 layers and 400 neurons per layer demonstrate extremely high prediction accuracy over 98% and are able to operate under both constant temperature difference and heat flux conditions while taking into account of the contact electrical resistance, surface heat transfer and other thermoelectric effects. Coupling with genetic algorithm, the trained artificial neural networks can optimise the leg height, leg width, fill factor and interconnect height of the thermoelectric generator for different operating and contact resistance conditions. With almost identical optimised values obtained, our neural networks can realise geometrical optimisation within 40 s for each operating condition, which is averagely over 1,000 times faster than the optimisation performed by finite element method. The up-front computational time for the neural network can be recovered when more than 2 optimisations are needed. The successful application of this data-driven approach in this work clearly represents a new and cost-effective avenue for conducting system level design and optimisation of thermoelectric generators and other energy harvesting technologies.

Micro-thermoelectric generators are attractive for autonomous sensor systems. This work reports an approach for fabricating thin film micro-thermoelectric generators via selective chemical vapor deposition. The approach utilizes our single-source precursors, which enable the production of high-quality thermoelectric materials, as exemplified by Sb2Te3 in this work. It also allows great control over the thermoelectric properties of the as-deposited Sb2Te3 by simply varying the deposition temperature. A competitive power factor of 16.5 μW/(cm·K2) can be obtained for a Sb2Te3 film deposited at 364 °C, with the highest estimated ZT value of 0.65. The selective deposition over the conductive TiN surface enables the fabrication of a prototype single-leg thermoelectric generator, which produces a maximum power of 0.27 nW, with a temperature gradient of only 0.11 K while reducing 87% of tellurium consumption.

Heterostructures involving two-dimensional (2D) transition metal dichalcogenides and other materials such as graphene have a strong potential to be the fundamental building block of many electronic and opto-electronic applications. The integration and scalable fabrication of such heterostructures is of essence in unleashing the potential of these materials in new technologies. For the first time, we demonstrate the growth of few-layer MoS2 films on graphene via non-aqueous electrodeposition. Through methods such as scanning and transmission electron microscopy, atomic force microscopy, Raman spectroscopy, energy and wavelength dispersive X-ray spectroscopies and X-ray photoelectron spectroscopy, we show that this deposition method can produce large-area MoS2 films with high quality and uniformity over graphene. We reveal the potential of these heterostructures by measuring the photo-induced current through the film. These results pave the way towards developing the electrodeposition method for the large-scale growth of heterostructures consisting of varying 2D materials for many applications., 11 pages and 6 figures

We report the thermoelectric properties of Bi2Te3 thin films electrodeposited from the weakly coordinating solvent dichloromethane (CH2Cl2). It was found that the oxidation of porous films is significant, causing the degradation of its thermoelectric properties. We show that the morphology of the film can be improved drastically by applying a short initial nucleation pulse, which generates a large number of nuclei, and then growing the nuclei by pulsed electrodeposition at a much lower overpotential. This significantly reduces the oxidation of the films as smooth films have a smaller surface-to-volume ratio and are less prone to oxidation. X-ray photoelectron spectroscopy (XPS) shows that those films with Te(O) termination show a complete absence of oxygen below the surface layer. A thin film transfer process was developed using polystyrene as a carrier polymer to transfer the films from the conductive TiN to an insulating layer for thermoelectrical characterization. Temperature-dependent Seebeck measurements revealed a room-temperature coefficient of −51.7 μV/K growing to nearly −100 μV/K at 520 °C. The corresponding power factor reaches a value of 88.2 μW/mK2 at that temperature.

This paper presents a novel continuously beam-steering dual-band, dual circularly polarized (CP) reflectarray. The dual band operation of the reflectarray is obtained by using the interleaved circularly polarized triangular patches as the radiating elements. Each of the patches has two proximity feed lines that are electromagnetically coupled to a branch line coupler, where the phase shifters are integrated. The phase shifter is realized by using a new type of high-frequency tunable dielectric materials, Lead Niobate Pyrochlores $Pb _{n} Nb _{2}O_{5+ {n}}\,($ PN). By controlling the applied voltage, the dielectric permittivity of the PN material can be changed. This characteristic is utilized to the design of tunable dielectric capacitors, which is placed between the ground and the signal line of a Co-planar waveguide (CPW) transmission line. To validate the design concept, X-band passive antenna prototype was designed, fabricated and measured. Preliminary results on the designed phase shifter and tunable dielectric materials are also presented in this paper.

Reaction of activated germanium with nBu2Te2 in THF solution was shown to be more effective for the preparation of the germanium(iv) tellurolate compound, [Ge(TenBu)4], than reaction of GeCl4 with LiTenBu in a 1 : 4 molar ratio in THF. The product was characterised by 1H, 13C{1H} NMR spectroscopy and microanalysis and evaluated as a single source precursor for the low pressure chemical vapour deposition of GeTe thin films. Depending upon deposition conditions, either dull grey films (predominantly elemental Te) or highly reflective (GeTe) films were obtained from the pure precursor. Grazing incidence X-ray diffraction shows that the highly reflective films are comprised of the rhombohedral α-GeTe phase, while scanning electron microscopy and energy dispersive X-ray analysis reveal rhomb-shaped crystallites with a 49(1) : 51(1)% Ge : Te ratio. This structure is also confirmed from Raman spectra. Van der Pauw measurements show ρ = 3.2(1) × 10-4 Ω cm and Hall electrical measurements indicate that the GeTe thin films are p-type, with a mobility of 8.4(7) cm2 V-1 s-1 and carrier concentration of 2.5(2) × 1021 cm-3. The high p-type concentration is most likely a result of the substantial Ge vacancies in its sub-lattice, in line with the EDX elemental ratios.

The identification and optimization of tunable dielectric materials exhibiting low loss characteristics in the GHz frequencies are essential in the development of low power devices for microwave applications. We have applied a combinatorial synthetic methodology employing multiple atomic evaporative sources to produce compositional gradient thin film (Ba,Sr)(Ti,Mn)O3 perovskites. High throughput screening chips of both capacitive and waveguide structures are used to measure the compositional dependence of the dielectric properties of the fully characterised thin film materials. The co-planar waveguides are shown to allow measurements at frequencies exceeding 10 GHz. Using this methodology, we haveidentified (Ba,Sr)(Ti,Mn)O3 compositions which exhibit good tunability with low losses at microwave frequencies: for a single device with a 10 V DC bias one can achieve a 12 phase shift with excellent transmission characteristics, and an insertion loss of ~3.2 dB We show that small changes in composition can result in a significant change in dielectric characteristics and device performance, and that the experimental protocol developed provides a powerful methodology for the development of materials and microwave devices.

We report the properties of a series of electrodeposited Ge-Sb-Te alloys with various compositions. It is shown that the Sb/Ge ratio can be varied in a controlled way by changing the electrodeposition potential. This method opens up the prospect of depositing Ge-Sb-Te super-lattice structures by electrodeposition. Material and electrical characteristics of various compositions have been investigated in detail, showing up to three orders of magnitude resistance ratio between the amorphous and crystalline states and endurance up to 1000 cycles.