Ze Xiong

Programmable artificial phototactic microswimmer

Phototaxis is commonly observed in motile photosynthetic microorganisms. For example, green algae are capable of swimming towards a light source (positive phototaxis) to receive more energy for photosynthesis, or away from a light source (negative phototaxis) to avoid radiation damage or to hide from predators. Recently, with the aim of applying nanoscale machinery to biomedical applications, various inorganic nanomotors based on different propulsion mechanisms have been demonstrated. The only method to control the direction of motion of these self-propelled micro/nanomotors is to incorporate a ferromagnetic material into their structure and use an external magnetic field for steering. Here, we show an artificial microswimmer that can sense and orient to the illumination direction of an external light source. Our microswimmer is a Janus nanotree containing a nanostructured photocathode and photoanode at opposite ends that release cations and anions, respectively, propelling the microswimmer by self-electrophoresis. Using chemical modifications, we can control the zeta potential of the photoanode and program the microswimmer to exhibit either positive or negative phototaxis. Finally, we show that a school of microswimmers mimics the collective phototactic behaviour of green algae in solution.

High-Performance Flexible Transparent Electrode with an Embedded Metal Mesh Fabricated by Cost-Effective Solution Process

A new structure of flexible transparent electrodes is reported, featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, and a cost-effective solution-based fabrication strategy for this new transparent electrode. The embedded nature of the metal-mesh electrodes provides a series of advantages, including surface smoothness that is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. The novel fabrication process replaces vacuum-based metal deposition with an electrodeposition process and is potentially suitable for high-throughput, large-volume, and low-cost production. In particular, this strategy enables fabrication of a high-aspect-ratio (thickness to linewidth) metal mesh, substantially improving conductivity without considerably sacrificing transparency. Various prototype flexible transparent electrodes are demonstrated with transmittance higher than 90% and sheet resistance below 1 ohm sq(-1) , as well as extremely high figures of merit up to 1.5 × 10(4) , which are among the highest reported values in recent studies. Finally using our embedded metal-mesh electrode, a flexible transparent thin-film heater is demonstrated with a low power density requirement, rapid response time, and a low operating voltage.

A Silicon Nanowire as a Spectrally Tunable Light-Driven Nanomotor

Over the last decades, scientists have endeavored to develop nanoscopic machines and envisioned that these tiny machines could be exploited in biomedical applications and novel material fabrication. Here, a visible-/near-infrared light-driven nanomotor based on a single silicon nanowire is reported. The silicon nanomotor harvests energy from light and propels itself by the self-electrophoresis mechanism. Due to the high efficiency, the silicon nanowire can be readily driven by visible and near-infrared illumination at ultralow light intensity (≈3 mW cm-2 ). The experimental study and numerical simulation also show that the detailed structure around the concentrated reaction center determines the migration behavior of the nanomotor. Importantly, due to the optical resonance inside the silicon nanowire, the spectral response of the nanowire-based nanomotor can be readily modulated by the nanowires diameter. Compared to other methods, light controlling potentially offers more freedom and flexibility, as light can be modulated not only with its intensity and direction, but also with the frequency and polarities. This nanowire motor demonstrates a step forward to harness the advantages of light, which opens up new opportunities for the realization of many novel functions such as multiple channels communication to nanorobots and controllable self-assembly.

Orthogonal navigation of multiple visible-light-driven artificial microswimmers

Nano/microswimmers represent the persistent endeavors of generations of scientists towards the ultimate tiny machinery for device manufacturing, targeted drug delivery, and noninvasive surgery. In many of these envisioned applications, multiple microswimmers need to be controlled independently and work cooperatively to perform a complex task. However, this multiple channel actuation remains a challenge as the controlling signal, usually a magnetic or electric field, is applied globally over all microswimmers, which makes it difficult to decouple the responses of multiple microswimmers. Here, we demonstrate that a photoelectrochemically driven nanotree microswimmer can be easily coded with a distinct spectral response by loading it with dyes. By using different dyes, an individual microswimmer can be controlled and navigated independently of other microswimmers in a group. This development demonstrates the excellent flexibility of the light navigation method and paves the way for the development of more functional nanobots for applications that require high-level controllability.The ability to direct microwimmers along a particular trajectory is advantageous for targeted drug delivery and manufacturing at the microscale. Here Zheng et al. use different types of light-absorbing dyes to enable selective activation of different microswimmers independently from each other.

C=C π Bond Modified Graphitic Carbon Nitride Films for Enhanced Photoelectrochemical Cell Performance

Applications of graphitic carbon nitride (g-CN) in photoelectrochemical and optoelectronic devices are still hindered due to the difficulties in synthesis of g-CN films with tunable chemical, physical and catalytic properties. Herein we present a general method to alter the electronic and photoelectrochemical properties of g-CN films by annealing. We found that N atoms can be removed from the g-CN networks after annealing treatment. Assisted by theoretical calculations, we confirm that upon appropriate N removal, the adjacent C atoms will form new C=C π bonds. Detailed calculations demonstrate that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located at the structure unit with C=C π bonds and the electrons are more delocalized. Valence band X-ray photoelectron spectroscopy spectra together with optical absorption spectra unveil that the structure changes result in the alteration of the g-CN energy levels and position of band edges. Our results show that the photocurrent density of the annealed g-CN film is doubled compared with the pristine one, thanks to the better charge separation and transport within the film induced by the new C=C π bonds. An ultrathin TiO2 film (2.2 nm) is further deposited on the g-CN film as stabilizer and the photocurrent density is kept at 0.05 mA cm-2 at 1.23 V versus reversible hydrogen electrode after two-cycle stability assessment. This work enables the applications of g-CN films in many electronic and optoelectronic devices.

Thermal Transport in Supported Graphene Nanomesh

Graphene is considered as a promising candidate material to replace silicon for the next-generation nanoelectronics because of its superb carrier mobility. To evaluate its thermal dissipation capability as electronic materials, the thermal transport in monolayer graphene was extensively explored over the past decade. However, the supported chemical vapor deposition (CVD) grown monolayer graphene with submicron structures were seldom studied, which is important for practical nanoelectronics. Here we investigate the thermal transport properties in a series of CVD graphene nanomeshes patterned by a hard-template-assisted etching method. The experimental and numerical results uncovered the phonon backscattering at hole boundary (<100 nm neck width) and its substantial contribution to the thermal conductivity reduction.

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 Ω/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) - up to 1.5 x 104 - which are amongst the best values in the published literature.

Solution-Processed CdS/Cu2S Superlattice Nanowire with Enhanced Thermoelectric Property

Previously, the solution-based cation exchange reaction has been extensively studied for the synthesis of the complex heteroepitaxial nanocolloidals. Here, we demonstrated that the strain induced selective phase segregation technique can also be applied to large size nanowires in a well-studied CdS/Cu2S system, leading to the formation of superlattice nanowire structure with a simple solution-based cation exchange reaction. This structural evolution is driven by the distinct interface formation energy at different CdS facets as indicated by ab initio calculation. Because of the energy filtering effect, the superlattice nanowire shows an enhanced thermopower without significant decrease of the electrical conductivity. This study provides a promising low-cost solution process to produce superlattice nanostructures for practical thermoelectric applications.