Table of contents

Here, we report the fabrication of aligned carbon nanotube (A-CNT)/conducting polymer (CP) heterostructures with both uniform conformal and periodic beaded polymer morphologies via oxidative chemical vapor deposition of poly(ethylenedioxythiophene). Periodic beaded CP morphologies are realized utilizing the Plateau–Rayleigh instability to transform the original uniform conformal film, yielding a beaded CP morphology with a >50% enhancement in specific surface area (SSA). Modeling indicates that this SSA increase originates from the internal volume of the A-CNTs becoming available for adsorption, and that these internal A-CNT surfaces, if they could be made accessible to electrolyte ions, could lead to >30% enhancement of specific gravimetric and volumetric capacitances of current state-of-the-art A-CNT/CP heterostructures.

A novel concept to obtain a ferroelectric material with enhanced piezoelectric properties is proposed. This approach is based on the combination of two pathways: (i) the evolution of a ferroelectric monoclinic phase and, (ii) the coexistence of different types of ferroelectric domains leading to polarization discontinuities at the domain walls. Each of these pathways enables polarization rotation in the material which is responsible for giant piezoelectricity. Targeted incorporation of anisotropic epitaxial lattice strain is used to implement this approach. The feasibility of our concept is demonstrated for K_0.9Na_0.1NbO₃ epitaxial layers grown on NdScO₃ substrates where the coexistence of (100)_pc and (001)_pc pseudocubic oriented monoclinic domains is experimentally verified. This coexistence results in a complex periodic domain pattern with alternating emergence of ferroelectric in-plane a₁a₂ and inclined M_C monoclinic phases, which differ in the direction of the electrical polarization vector. Our approach opens the possibility to exploit ferroelectric properties in both vertical and lateral directions and to achieve enhanced piezoelectric properties in lead-free material caused by singularities at the domains walls.

Nanoparticles, and more specifically gold nanoparticles (AuNPs), have attracted much scientific and technological interest in the last few decades. Their popularity is attributed to their unique optical, catalytic, electrical and magnetic properties when compared with the bulk. However, one of the main problems with AuNPs is their long-term stability. Two-dimensional materials like MoS₂ (WS₂) are semiconductors that exhibit a combination of properties which make them suitable for electronic, optical and (photo)catalytic devices. Few-layer MoS₂ (WS₂) nanoparticles (NPs), and in particular single-layer ones, show intriguing optical and electrical properties which are very different from those of the bulk compounds. Here we demonstrate the synthesis of AuNPs sheathed by a single layer of MoS₂ (WS₂), i.e. a core–shell nanostructure (AuNP@1L-MoS₂). The hybrid NPs exhibit optical properties that are different from those of either constituent and are amenable for modulation via their chemistry, offering a myriad of applications.

The oxygen evolution reaction (OER) is a critical reaction in energy storage and conversion systems such as metal–air batteries and water splitting. The current commercial OER catalysts are the noble metal based materials, which have a high cost and a limited supply. Xu et al have prepared the nanoporous N-Co₃O₄ materials as cost-effective and high efficient electrocatalysts for OER by N₂ plasma (Xu et al 2017 Nanotechnology28 165402). This work has successfully demonstrated the simple N₂ plasma treatment to be a powerful technique to introduce the nitrogen doping and nanoporous structure in the bulk materials, enhancing the performance of electrochemical catalysis. Based on this study, more future work on developing highly porous non-precious metal-based materials with good conductivity would be promising for energy storage and conversion.

Graphene surface plasmons (GSPs) have shown great potential in biochemical sensing, thermal imaging, and optoelectronics. To excite GSPs, several methods based on the near-field optical microscope and graphene nanostructures have been developed in the past few years. However, these methods suffer from their bulky setups and low GSP-excitation efficiency due to the short interaction length between free-space vertical excitation light and the atomic layer of graphene. Here we present a CMOS-compatible design of graphene-on-silicon hybrid plasmonic-photonic integrated circuits that achieve the in-plane excitation of GSP polaritons as well as localized surface plasmon (SP) resonance. By employing a suspended membrane slot waveguide, our design is able to excite GSP polaritons on a chip. Moreover, by utilizing a graphene nanoribbon array, we engineer the transmission spectrum of the waveguide by excitation of localized SP resonance. Our theoretical and computational study paves a new avenue to enable, modulate, and monitor GSPs on a chip, potentially applicable for the development of on-chip electro-optic devices.

Graphene quantum dots (GQDs) have received much research attention, because of their useful structure and optical absorption/emission. We report the tunable amplified spontaneous emission (ASE) in GQD-doped cholesteric liquid crystal (CLC), which to the best of our knowledge has not been previously observed. The GQDs are uniformly dispersed with a weight ratio of 0.5 wt.% in CLC. Under optical excitation, typical ASE is triggered in the system at pump energies greater than 1.25 mJ cm⁻². The emission peak at the long wavelength edge of the photonic bandgap shifts from 662 to 669 nm, as the working temperature is increased from 50 to 90 °C. The preparation of the combined GQDs and CLC is simple and low-cost, and the resulting material is photostable and non-toxic. Combining the GQD gain material with the self-assembled CLC resonator has potential in the fabrication of ASE source and laser devices.

The size of ZnO nanorods (NRs) plays an important role in tuning the external quantum efficiency (EQE) and quality of light generated by white light emitting diodes (LEDs). In this work, we report on the enhancement of EQE and the quality of ZnO NR-based hetrojunction white LEDs fabricated on a p-GaN substrate using a low temperature solution method. Cathodoluminescence spectra demonstrate that ultraviolet (UV) emission decreases and visible deep band emission increases with an increase in the length of the ZnO NRs. The UV emission could be internally reabsorbed by the ZnO NR excitation, thus enhancing the emission intensity of the visible deep band. Photocurrent measurements validated the fact that the EQE depends on the size of ZnO NRs, increasing by 87% with an increase in the length of the ZnO NRs. Furthermore, the quality of white light was measured and clearly indicated an increase in the color rendering indices of the LEDs with an increase in the length of the ZnO NRs, confirming that the quality of light generated by LEDs can be tuned by varying the length of the ZnO NRs. These results suggest that the EQE and visible deep band emission from n-ZnONRs/p-GaN heterojunction LEDs can be effectively controlled by adjusting the length of the ZnO NRs, which can be useful for realizing tunable white LEDs.

The electrode/organic interface is one of the key factors in attaining superior device performance in organic electronics, and inserting a tailor-made layer can dramatically modify its properties. The use of nano-composite (NC) materials leads to many advantages by combining materials with the objective of obtaining a desirable combination of properties. In this context, zinc oxide/polyethyleneimine (ZnO:PEI) NC film was incorporated as an interfacial layer into inverted bottom-emission organic light emitting diodes (IBOLEDs) and fully optimized. For orange-red emissive MEH-PPV based IBOLEDs, a high power efficiency of 6.1 lm W⁻¹ at a luminance of 1000 cd m⁻² has been achieved. Notably, the external quantum efficiency (EQE) increased from 0.1 to 4.8% and the current efficiency (CE) increased from 0.2 to 8.7 cd A⁻¹ with rise in luminance (L) from 1000 to above 10 000 cd m⁻² levels when compared to that of pristine ZnO-based devices. An identical device architecture containing a ZnO:PEI NC layer has also been used to successfully fabricate green and blue emissive IBOLEDs. The significant enhancement in the inverted device performance, in terms of luminance and efficiency, is attributed to a good energy-level alignment between the cathode/organic interface which leads to effective carrier balance, resulting in efficient radiative-recombination.

The nanostructuring of materials to create bactericidal and antibiofouling surfaces presents an exciting alternative to common methods of preventing bacterial adhesion. The fabrication of synthetic bactericidal surfaces has been inspired by the anti-wetting and anti-biofouling properties of insect wings, and other topologies found in nature. Black silicon is one such synthetic surfaces which has established bactericidal properties. In this study we show that time-dependent plasma etching of silicon wafers using 15, 30, and 45 min etching intervals, is able to produce different surface geometries with linearly increasing heights of approximately 280, 430, and 610 nm, respectively. After incubation on these surfaces with Gram-positive Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa bacterial cells it was established that smaller, more densely packed pillars exhibited the greatest bactericidal activity with 85% and 89% inactivation of bacterial cells, respectively. The decrease in the pillar heights, pillar cap diameter and inter-pillar spacing corresponded to a subsequent decrease in the number of attached cells for both bacterial species.

We rationally designed a facile two-step approach to synthesize ZnMn₂O₄@G composite anode material for lithium-ion batteries (LIBs), involving a template-free fabrication of ZnMn₂O₄ nanorings and subsequent coating of graphene sheets. Notably, it is the first time that ring-like ZnMn₂O₄ nanostructure is reported. Moreover, our system has been demonstrated to be quite powerful in producing ZnMn₂O₄ nanorings regardless of the types of Zn and Mn-containing metal salts reactants. The well-known inside-out Ostwald ripening process is tentatively proposed to clarify the formation mechanism of the hollow nanorings. When evaluated as anode material for LIBs, the resulting ZnMn₂O₄@G hybrid displays significantly improved lithium-storage performance with high specific capacity, good rate capability, and excellent cyclability. After 500 cycles, the ZnMn₂O₄@G hybrid can still deliver a reversible capacity of 958 mAh g⁻¹ at a current density of 200 mA g⁻¹, much higher than the theoretical capacity of 784 mAh g⁻¹ for pure ZnMn₂O₄. The outstanding electrochemical performance should be reasonably ascribed to the synergistic interaction between hollow and porous ZnMn₂O₄ nanorings and the three-dimensional interconnected graphene sheets.

In this work, the morphology of ZnO nanostructures is engineered to demonstrate enhanced supercapacitor characteristics of ZnO nanocones (NCs) compared to ZnO nanowires (NWs). ZnO NCs are obtained by chemically etching ZnO NWs. Electrochemical characteristics of ZnO NCs and NWs are extensively investigated to demonstrate morphology dependent capacitive performance of one dimensional ZnO nanostructures. Cyclic voltammetry measurements on these two kinds of electrodes in a three-electrode cell confirms that ZnO NCs exhibit a high specific capacitance of 378.5 F g⁻¹ at a scan rate of 20 mV s⁻¹, which is almost twice that of ZnO NWs (191.5 F g⁻¹). The charge–discharge and electrochemical impedance spectroscopy measurements also clearly result in enhanced capacitive performance of NCs as evidenced by higher specific capacitances and lower internal resistance. Asymmetric supercapacitors are fabricated using activated carbon (AC) as the negative electrode and ZnO NWs and NCs as positive electrodes. The ZnO NC⫽AC can deliver a maximum specific capacitance of 126 F g⁻¹ at a current density of 1.33 A g⁻¹ with an energy density of 25.2 W h kg⁻¹ at the power density of 896.44 W kg⁻¹. In contrast, ZnO NW⫽AC displays 63% of the capacitance obtained from the ZnO NC⫽AC supercapacitor. The enhanced performance of NCs is attributed to the higher surface area of ZnO nanostructures after the morphology is altered from NWs to NCs.

Silver ions have been used to sterilize many products, however, it has recently been demonstrated that silver ions can be toxic. This toxicity has been studied over many years with the lethal concentration at 10 μM. Silver ions can accumulate through the food chain, causing serious health problems in many species. Hence, there is a need for a commercially available silver ion sensor, with high detection sensitivity. In this work, we develop an ultra-sensitive silver ion sensor platform, using cytosine based DNA and gold nanoparticles as the mass amplifier. We achieve a lower detection limit for silver ions of 10 pM; this detection limit is one million times lower than the toxic concentration. Using our sensor platform we examine highly selective characteristics of other typical ions in water from natural sources. Furthermore, our sensor platform is able to detect silver ions in a real practical sample of commercially available drinking water. Our sensor platform, which we have termed a 'MAIS' (mass amplifier ion sensor), with a simple detection procedure, high sensitivity, selectivity and real practical applicability has shown potential as an early toxicity assessment of silver ions in the environment.

Diabetic individuals need simple, accurate, and cost effective means by which to independently assess their glucose levels in a non-invasive way. In this work, a sensor based on randomly oriented CuO nanowire networks supported by a polyethylene terephthalate thin film is evaluated as a flexible, transparent, non-enzymatic glucose sensing system analogous to those envisioned for future wearable diagnostic devices. The amperometric sensing characteristics of this type of device architecture are evaluated both before and after bending, with the system's glucose response, sensitivity, lower limit of detection, and effect of applied bias being experimentally determined. The obtained data shows that the sensor is capable of measuring changes in glucose levels within a physiologically relevant range (0–12 mM glucose) and at lower limits of detection (0.05 mM glucose at +0.6 V bias) consistent with patient tears and saliva. Unlike existing studies utilizing a conductive backing layer or macroscopic electrode setup, this sensor demonstrates a percolation network-like trend of current versus glucose concentration. In this implementation, controlling the architectural details of the CuO nanowire network could conceivably allow the sensor's sensitivity and optimal sensing range to be tuned. Overall, this work shows that integrating CuO nanowires into a sensor architecture compatible with transparent, flexible electronics is a promising avenue to realizing next generation wearable non-enzymatic glucose diagnostic devices.

Novel tin oxide field-effect-transistors (SnO₂ NW-FET) for pH and protein detection applicable in the healthcare sector are reported. With a SnO₂ NW-FET the proof-of-concept of a bio-sensing device is demonstrated using the carrier transport control of the FET channel by a (bio-) liquid modulated gate. Ultra-thin Al₂O₃ fabricated by a low temperature atomic layer deposition (ALD) process represents a sensitive layer to H⁺ ions safeguarding the nanowire at the same time. Successful pH sensitivity is demonstrated for pH ranging from 3 to 10. For protein detection, the SnO₂ NW-FET is functionalized with a receptor molecule which specifically interacts with the protein of interest to be detected. The feasibility of this approach is demonstrated via the detection of a biotinylated protein using a NW-FET functionalized with streptavidin. An immediate label-free electronic read-out of the signal is shown. The well-established Enzyme-Linked Immunosorbent Assay (ELISA) method is used to determine the optimal experimental procedure which would enable molecular binding events to occur while being compatible with a final label-free electronic read-out on a NW-FET. Integration of the bottom-up fabricated SnO₂ NW-FET pH- and biosensor into a microfluidic system (lab-on-a-chip) allows the automated analysis of small volumes in the 400 μl range as would be desired in portable on-site point-of-care (POC) devices for medical diagnosis.

Graphene sheets, a flexible 2D material with excellent absorptive capacity, have great potential as absorbing materials. However, this material has always suffered from irreversible aggregation and thus loses the abundant active sites and large surface area. In this paper, large-scale graphene oxide (GO) sheets were cut and reduced to tiny reduced graphene oxide (RGO) sheets by a cell-break sonicator, for producing numerous defects, which are the center of chemisorption. Furthermore, sodium titanate nanowires functioned as a framework to help to disperse the tiny RGO sheets uniformly. And, in turn, the flexible tiny RGO sheets glued robust nanowires into a free-standing membrane. This novel composite membrane exhibited an ultra-high decoloration efficiency of 99.8% of rhodamine B in a continuous flow mode, and an outstanding absorptive capability of 1.30 × 10⁻² mol g⁻¹ correlated to RGO content in batch reaction, which is about two orders of magnitude higher than other reported graphene-based absorbents. In addition, an efficient and feasible method without any heat treatment for regenerating the membrane is illustrated, and the recycled membrane retains superior decoloration efficiency. The excellent absorptive performance indicates the framework-based disperse strategy has great potential for the construction and application of defect-rich graphene.

Being able to precisely control the morphologies of noble metallic nanostructures is of essential significance for promoting the surface-enhanced Raman scattering (SERS) effect. Herein, we demonstrate an overgrowth strategy for synthesizing Au @ M (M = Au, Ag, Pd, Pt) core–shell heterogeneous nanocrystals with an orientated structural evolution and highly improved properties by using Au nanorods as seeds. With the same reaction condition system applied, we obtain four well-designed heterostructures with diverse shapes, including Au concave nanocuboids (Au CNs), Au @ Ag crystalizing face central cube nanopeanuts, Au @ Pd porous nanocuboids and Au @ Pt nanotrepangs. Subsequently, the exact overgrowth mechanism of the above heterostructural building blocks is further analysed via the systematic optimiziation of a series of fabrications. Remarkably, the well-defined Au CNs and Au @ Ag nanopeanuts both exhibit highly promoted SERS activity. We expect to be able to supply a facile strategy for the fabrication of multimetallic heterogeneous nanostructures, exploring the high SERS effect and catalytic activities.

Monolithic dye-sensitized solar cell (DSC) architectures hold great potential for building-integrated photovoltaics applications. They indeed benefit from lower weight and manufacturing costs as they avoid the use of a transparent conductive oxide (TCO)-coated glass counter electrode. In this work, a transparent monolithic DSC comprising a hierarchical 1D nanostructure stack is fabricated by physical vapor deposition techniques. The proof of concept device comprises hyperbranched TiO₂ nanostructures, sensitized by the prototypical N719, as photoanode, a hierarchical nanoporous Al₂O₃ spacer, and a microporous indium tin oxide (ITO) top electrode. An overall 3.12% power conversion efficiency with 60% transmittance outside the dye absorption spectral window is demonstrated. The introduction of a porous TCO layer allows an efficient trade-off between transparency and power conversion. The porous ITO exhibits submicrometer voids and supports annealing temperatures above 400 °C without compromising its optoelectronical properties. After thermal annealing at 500 °C, the resistivity, mobility, and carrier concentration of the 800 nm-thick porous ITO layer are found to be respectively 2.3 × 10⁻³ Ω cm⁻¹, 11 cm² V⁻¹ s⁻¹, and 1.62 × 10²⁰ cm⁻³, resulting in a series resistance in the complete device architecture of 45 Ω. Electrochemical impedance and intensity-modulated photocurrent/photovoltage spectroscopy give insight into the electronic charge dynamic within the hierarchical monolithic DSCs, paving the way for potential device architecture improvements.

Graphene nanomeshes (GNMs), new graphene nanostructures with tunable bandgaps, are potential building blocks for future electronic or photonic devices, and energy storage and conversion materials. In previous works, GNMs have been successfully prepared on Cu foils by the H₂ etching effect. In this paper, we investigated the effect of Ar on the preparation of GNMs, and how the mean density and shape of them vary with growth time. In addition, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM) revealed the typical hexagonal structure of GNM. Atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) indicated that large copper oxide nanoparticles produced by oxidization in purified Ar can play an essential catalytic role in preparing GNMs. Then, we exhibited the key reaction details for each growth process and proposed a growth mechanism of GNMs in purified Ar.

A straightforward method for the synthesis of CoFe_2.7/CoFe₂O₄ core/shell nanowires is described. The proposed method starts with a conventional pulsed electrodeposition procedure on alumina nanoporous template. The obtained CoFe_2.7 nanowires are released from the template and allowed to oxidize at room conditions over several weeks. The effects of partial oxidation on the structural and magnetic properties were studied by x-ray spectrometry, magnetometry, and scanning and transmission electron microscopy. The results indicate that the final nanowires are composed of 5 nm iron-cobalt alloy nanoparticles. Releasing the nanowires at room conditions promoted surface oxidation of the nanoparticles and created a CoFe₂O₄ shell spinel-like structure. The shell avoids internal oxidation and promotes the formation of bi-magnetic soft/hard magnetic core/shell nanowires. The magnetic properties of both the initial single-phase CoFe_2.7 nanowires and the final core/shell nanowires, reveal that the changes in the properties from the array are due to the oxidation more than effects associated with released processes (disorder and agglomeration).

Recent advancement in liquid-environment atomic force microscopy (AFM) has enabled us to visualize three-dimensional (3D) hydration structures as well as two-dimensional (2D) surface structures with subnanometer-scale resolution at solid–water interfaces. However, the influence of ions present in solution on the 2D- and 3D-AFM measurements has not been well understood. In this study, we perform atomic-scale 2D- and 3D-AFM measurements at fluorite–water interfaces in pure water and a supersaturated solution of fluorite. The images obtained in these two environments are compared to understand the influence of the ions in solution on these measurements. In the 2D images, we found clear difference in the nanoscale structures but no significant difference in the atomic-scale contrasts. However, the 3D force images show clear difference in the subnanometer-scale contrasts. The force contrasts measured in pure water largely agree with those expected from the molecular dynamics simulation and the solvent tip approximation model. In the supersaturated solution, an additional force peak is observed over the negatively charged fluorine ion site. This location suggests that the observed force peak may originate from cations adsorbed on the fluorite surface. These results demonstrate that the ions can significantly alter the subnanometer-scale force contrasts in the 3D-AFM images.

Silicon–germanium is an important alloy mainly used in thermoelectricity and electronics. However, its thermal and optical properties still need further investigation at the nanoscale. That is why in this study, the size and shape effect on the silicon–germanium phase diagram is investigated through the nano-thermodynamics methodology. As expected, the phase diagram undergoes a shift down in temperature when the size decreases. However, it is demonstrated and explained why the size effect on the solidus–liquidus curves is much stronger than the one on the miscibility gap. Moreover, the shape effect is investigated for various faceted polyhedral nanoparticles as well as for the sphere. Phase maps are then provided as a function of the number of facets, at 4 and 10 nm, in order to determine the structure of the alloy. Furthermore, the size and shape effects on the energy bandgap are also studied. The energy bandgap increases when the size is reduced. The cube and tetrahedral shapes exhibit the largest size effect on the thermal and optical properties of the silicon–germanium alloy. Finally, this paper provides a useful roadmap for experimentalists willing to tune the properties of this alloy.

Nanoscale friction on two-dimensional (2D) materials is closely associated with their mechanical, electronic and photonic properties, which can be modulated through changing thickness. Here, we investigated the thickness dependent friction on few-layer MoS₂, WS₂, and WSe₂ using atomic force microscope at ambient condition and found two different behavior. When a sharp tip was used, the regular behavior of decreasing friction with increasing thickness was reproduced. However, when a pre-worn and flat-ended tip was used, we observed an abnormal trend: on WS₂ and WSe₂, friction increased monotonically with thickness, while for MoS₂, friction decreased from monolayer to bilayer and then subsequently increased with thickness. As suggested by the density functional theory calculation, we hypothesize that the overall frictional behavior is a competition between the puckering effect and the intrinsic energy corrugation within the compressive region. By varying the relative strength of the puckering effect via changing the tip shape, the dependence of friction on sample thickness can be tuned. Our results also suggest a potential means to measure intrinsic frictional properties of 2D materials with minimum impact from puckering.