Atomic Layer Deposition of Alumina-Coated Thin-Film Cathodes for Lithium Microbatteries.

This work shows the electrochemical performance of sputter-deposited, binder-free lithium cobalt oxide thin films with an alumina coating deposited via atomic layer deposition for use in lithium-metal-based microbatteries. The Al2O3 coating can improve the charge-discharge kinetics and suppress the phase transition that occurs at higher potential limits where the crystalline structure of the lithium cobalt oxide is damaged due to the formation of Co4+, causing irreversible capacity loss. The electrochemical performance of the thin film is analysed by imposing 4.2, 4.4 and 4.5 V upper potential limits, which deliver improved performances for 3 nm of Al2O3, while also highlighting evidence of Al doping. Al2O3-coated lithium cobalt oxide of 3 nm is cycled at 147 µA cm-2 (~2.7 C) to an upper potential limit of 4.4 V with an initial capacity of 132 mAh g-1 (65.7 µAh cm-2 µm-1) and a capacity retention of 87% and 70% at cycle 100 and 400, respectively. This shows the high-rate capability and cycling benefits of a 3 nm Al2O3 coating.

Reconfigurable horizontal-vertical carrier transport in graphene/HfZrO field-effect transistors.

We have fabricated at wafer level field-effect-transistors (FETs) having as channel graphene monolayers transferred on a HfZrO ferroelectric, grown by atomic layer deposition on a doped Si (100) substrate. These FETs display either horizontal or vertical carrier transport behavior, depending on the applied gate polarity. In one polarity, the FETs behave as a graphene FET where the transport is horizontal between two contacts (drain and grounded source) and is modulated by a back-gate. Changing the polarity, the transport is vertical between the drain and the back-gate and, irrespective of the metallic contact type, Ti/Au or Cr/Au, the source-drain bias modulates the height of the potential barrier between HfZrO and the doped Si substrate, the carrier transport being described by a Schottky mechanism at high gate voltages and by a space-charge limited mechanism at low gate voltages. Vertical transport is required by three-dimensional integration technologies to increase the density of transistors on chip.

MoS2 radio: detecting radio waves with a two-dimensional transition metal dichalcogenide semiconductor.

In this paper, we designed, fabricated and tested a microwave circuit based on a MoS2 self-switching diode. The MoS2 thin film (10-monolayers nominal thickness) was grown on a 4 inch Al2O3/high-resistivity silicon wafer by chemical vapor deposition process. The Raman measurements confirm the high quality of the MoS2 over the whole area of the 4 inch wafer. We show experimentally that a microwave circuit based on a few-layers MoS2 self-switching diode fabricated at the wafer level is able to detect the audio spectrum from amplitude-modulated microwave signals in the band 0.9-10 GHz, i.e. in the frequency range mostly used by current wireless communications. In particular, the 900 MHz band is widely exploited for GSM applications, whereas the 3.6 GHz band has been identified as the primary pioneer band for 5G in the European Union.

Electromagnetic energy harvesting based on HfZrO tunneling junctions.

HfZrO ferroelectrics with a thickness of 6 nm were grown directly on Si using atomic layer deposition, top and bottom metallic electrodes being subsequently deposited by electron-beam metallization techniques. Depending on the polarity of the ±10 V poling voltages, the current-voltage dependence of these tunneling diodes shows a rectifying behavior for different polarizations, the ON-OFF ratio being about 104. Because the currents are at mA level, the HfZrO tunneling diodes coupled to an antenna array can harvest electromagnetic energy at 26 GHz (a bandwidth designated for internet of things), with a responsivity of 63 V W-1 and a NEP of 4 nW/Hz0.5.

Wafer-scale very large memory windows in graphene monolayer/HfZrO ferroelectric capacitors.

We have fabricated and electrically characterized at the wafer scale tens of metal-ferroelectric (HfZrO)-semiconductor capacitors and metal-graphene monolayer-ferroelectric (HfZrO)-semiconductor capacitors with the same top electrode dimensions. We have found that the memory windows of the capacitors containing graphene are 3-4 times larger than the ferroelectric capacitors without graphene, and increase even more after annealing. This physical effect can be attributed to the additional electric field exerted by the graphene monolayer on the HfZrO ferroelectric semiconductor capacitor, and to the negative thermal extension coefficient of graphene, respectively.

Very large phase shift of microwave signals in a 6 nm Hf x Zr1-x O2 ferroelectric at ±3 V.

In this letter, we report for the first time very large phase shifts of microwaves in the 1-10 GHz range, in a 1 mm long gold coplanar interdigitated structure deposited over a 6 nm Hf x Zr1-x O2 ferroelectric grown directly on a high resistivity silicon substrate. The phase shift is larger than 60° at 1 GHz and 13° at 10 GHz at maximum applied DC voltages of ±3 V, which can be supplied by a simple commercial battery. In this way, we demonstrate experimentally that the new ferroelectrics based on HfO2 could play an important role in the future development of wireless communication systems for very low power applications.

Defect-promoted photo-electrochemical performance enhancement of orange-luminescent ZnO nanorod-arrays.

Intentionally defect-rich zinc oxide (ZnO) nanorod-arrays were grown from solution by carefully adjusting the concentration ratio of the growth-precursors used followed by various post-deposition thermal treatments. Post-deposition rapid thermal annealing (RTA) at moderate temperatures (350 °C-550 °C) and in various atmospheres was applied to vary the defect composition of the grown nanorod-arrays. It is demonstrated that, intense, defect-related orange emission occurs solely upon RTA around 450 °C and is essentially independent of the atmosphere used. Extensive materials characterization was carried out in order to evaluate the origin of the orange-luminescent defects and what influence they have on the ZnO material properties. It is concluded that the oxygen vacancy-zinc interstitial defect complex (VO-Zni) is responsible for the orange luminescence in the prepared materials. A kinetic formation mechanism of the VO-Zni complex dependent on the RTA temperature is proposed and shown to be in accordance with the experimental findings. Furthermore it is shown that this bulk deep-level defect could act as a trap state for photo-generated electrons prolonging the charge carrier lifetime of photo-generated holes and therefore improving the charge carrier separation in the material. As a result the photo-current density under simulated sunlight is found to increase by almost 150% over as-grown samples. The potential use of this defective material in applications such as solar water splitting is outlined.

Monolayer Capping Provides Close to Optimal Resistance to Laser Dewetting of Au Films.

Next-generation heat-assisted magnetic recording (HAMR) relies on fast, localized heating of the magnetic medium during the write process. Au plasmonic near-field transducers are an attractive solution to this challenge, but increased thermal stability of Au films is required to improve long-term reliability. This work compares the effect of nanoscale Al, AlOx, and Ta capping films on Au thin films with Ti or Ta adhesion layers for use in HAMR and other high-temperature plasmonic applications. Thermal stability is investigated using a bespoke laser dewetting system, and SEM and AFM are extensively used to interrogate the resulting dewet areas. The most effective capping layers are found to be 0.5-1 nm of Al or AlOx, which can eliminate dewetting under certain conditions. Even one monolayer of AlOx is shown to be highly effective in reducing dewetting. In the case of thicker capping layers of Ta and AlOx, the Au film can easily dewet underneath, leaving an intact capping layer. It is concluded that thinner capping layers are most effective against dewetting as the Au cannot dewet without breaking them and pulling them apart during the dewetting process. A simple model based on energetics considerations is developed, which explains how thinner capping layers can more effectively protect the metal from pore or fissure creation. The model provides some convenient guidelines for choosing both the substrate and capping layer, for a given metal, to maximize the resistance to laser-induced damage.

The effects of using ALD-grown ZnO buffer layers on the properties of indium tin oxide grown by chemical solution deposition.

In comparison to ITO films prepared by chemical solution deposition on bare substrates, the use of a ZnO buffer layer and Al2O3 barrier layer has been shown to have a significant effect on morphology, measured sheet resistance and therefore resistivity. In the case of quartz substrates, ITO resistivity decreased from 9.6 x 10(-3) ohms cm to 4.3 x 10(-3) ohms cm on incorporation of a ZnO buffer layer and Al2O3 barrier layer, both grown by ALD. A change in surface morphology was observed, due to the presence of the buffer layer, however, the ZnO buffer layer was not found to influence the XRD pattern of the ITO films.

A Slot-Die Technique for the Preparation of Continuous, High-Area, Chitosan-Based Thin Films.

Chitosan-based films have a diverse range of potential applications but are currently limited in terms of commercial use due to a lack of methods specifically designed to produce thin films in high volumes. To address this limitation directly, hydrogels prepared from chitosan, chitosan-tetraethoxy silane, also known as tetraethyl orthosilicate (TEOS) and chitosan-glutaraldehyde have been used to prepare continuous thin films using a slot-die technique which is described in detail. By way of preliminary analysis of the resulting films for comparison purposes with films made by other methods, the mechanical strength of the films produced was assessed. It was found that as expected, the hybrid films made with TEOS and glutaraldehyde both show a higher yield strength than the films made with chitosan alone. In all cases, the mechanical properties of the films were found to compare very favorably with similar measurements reported in the literature. In order to assess the possible influence of the direction in which the hydrogel passes through the slot-die on the mechanical properties of the films, testing was performed on plain chitosan samples cut in a direction parallel to the direction of travel and perpendicular to this direction. It was found that there was no evidence of any mechanical anisotropy induced by the slot die process. The examples presented here serve to illustrate how the slot-die approach may be used to create high-volume, high-area chitosan-based films cheaply and rapidly. It is suggested that an approach of the type described here may facilitate the use of chitosan-based films for a wide range of important applications.

Effect of Surface and Defect Chemistry on the Photocatalytic Properties of Intentionally Defect-Rich ZnO Nanorod Arrays.

Due to the abundance of intrinsic defects in zinc oxide (ZnO), the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photocatalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photocatalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore, strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g., shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photocatalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which, it is suggested, should be employed when assessing the photocatalytic properties of defect-rich ZnO.

The langmuir-blodgett approach to making colloidal photonic crystals from silica spheres.

The area of colloidal photonic crystal research has attracted enormous attention in recent years as a result of the potential of such materials to provide the means of fabricating new or improved photonic devices. As an area where chemistry still predominates over engineering the field is still in its infancy in terms of finding real applications being limited by ease of fabrication, reproducibility and 'quality'- for example the extent to which ordered structures may be prepared over large areas. It is our contention that the Langmuir-Blodgett assembly method when applied to colloidal particles of silica and perhaps other materials, offers a way of overcoming these issues. To this end the assembly of silica and other particles into colloidal photonic crystals using the Langmuir-Blodgett (LB) method is described and some of the numerous papers on this topic, which have been published, are reviewed. It is shown that the layer-by-layer control of photonic crystal growth afforded by the LB method allows for the fabrication of a range of novel, layered photonic crystals that may not be easily assembled using any other approach. Some of the more interesting of these structures, including so-called heterostructured photonic crystals comprising of layers of spheres having different diameters are presented and their optical properties described. Finally, we offer our comments as to future applications of this interesting technology.

Langmuir-Blodgett assembly of colloidal photonic crystals using silica particles prepared without the use of surfactant molecules.

This short communication reports the observation that in contrast to most previously reported procedures, it is possible to prepare 3D photonic crystal structures from silica particles that have not been deliberately treated with surfactant molecules, using the Langmuir-Blodgett method. We find that colloidal particles prepared simply via the Stöber method with diameters in the range 180-360 nm and dispersed in ethanol, may be effectively floated at the air/water interface and compressed into close packed layers prior to depositing the layers on a substrate. We also find, by comparing structures made with both particles treated with the surfactants 3-(trimethoxysilyl) propyl methacrylate or (3-aminopropyl)triethoxysilane and particles which have not been treated with any surfactant species, that the position of the Bragg peak and the reflectivity of the sample does not appear to be influenced by the presence of the surfactant molecules.