Nature-Inspired Stalactite Nanopores for Biosensing and Energy Harvesting.

Nature provides a wide range of self-assembled structures from the nanoscale to the macroscale. Under the right thermodynamic conditions and with the appropriate material supply, structures like stalactites, icicles, and corals can grow. However, the natural growth process is time-consuming. This work demonstrates a fast, nature-inspired method for growing stalactite nanopores using heterogeneous atomic deposition of hafnium dioxide at the orifice of templated silicon nitride apertures. The stalactite nanostructures combine the benefits of reduced sensing region typically for 2-dimensional material nanopores with the asymmetric geometry of capillaries, resulting in ionic selectivity, stability, and scalability. The proposed growing method provides an adaptable nanopore platform for basic and applied nanofluidic research, including biosensing, energy science, and filtration technologies.

High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing.

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS2, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS2 nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS2 nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

A Universal Stamping Method of Graphene Transfer for Conducting Flexible and Transparent Polymers.

Transfer method of chemically vapor deposition graphene is an appealing issue to realize its application as flexible and transparent electrodes. A universal stamping method to transfer as grown graphene from copper onto different flexible and transparent polymers (FTPs) reported here ensures simple, robust, rapid, clean and low-cost. This method relies on coating ethylene vinyl acetate (EVA) onto the as grown graphene, binding EVA coated graphene/Cu with FTPs and delamination by hydrogen bubbling process, which is analogous to the method used by stamping process where ink carries the imprint of the object onto any materials. The fate of the stamping method depends on how strongly the adhesion of EVA coated graphene/Cu with target FTPs. Interestingly, we have found that the thin film of EVA/graphene/Cu can only bind strongly with the FTPs of less than 25 µm in thickness and lower glass transition temperature value to the EVA while wide range of other FTPs are considered upon surface engineering to enhance the binding strength between FTPs and EVA. What's more, the electrical performance was investigated with a demonstration of triboelectric nanogenerators which confirmed the reliability of graphene transfer onto the FTPs and prospect for the development of flexible and transparent electronics.

Oil boundary approach for sublimation enabled camphor mediated graphene transfer.

HYPOTHESIS: Transfer of chemical-vapor-deposition (CVD) grown monolayer graphene from one substrate to another requires a transfer agent. The transfer agent usually needs to be removed by washing with organic solvent such as acetone or high temperature annealing, which is harmful to the structure integrity and intrinsic property of a graphene film. Here, we propose the use of camphor as a transfer agent to transfer monolayer graphene onto a target dielectric substrate, which bypasses these demanding steps and only needs the common alcohol solvent rinsing. EXPERIMENTS: To facilitate a crack-free graphene transfer, the proposed approach allows the camphor supported polycrystalline graphene to be rationally fastened with a thickened and solidified edge bead (i.e. camphor oil-filled boundary). A layer of camphor was first deposited onto a graphene/copper surface. The backside copper substrate was then etched away, whilst the camphor/graphene bilayer was placed onto a SiO2/Si substrate. Finally, the camphor remaining on the camphor/graphene/SiO2/Si sublimed into a vapor. The graphene/SiO2 stack was then examined by microscopic, spectral and electrical characterization. FINDINGS: The results of our examination suggest that the proposed method can guarantee a clean and damage-free graphene transfer. This method is particularly attractive in the application area for nano/micro-electronics, where it provides CVD-grown graphene the ability to be used on wide varieties of substrates that are sensitive to organic solvents and high temperature.

Directly Probing Light Absorption Enhancement of Single Hierarchical Structures with Engineered Surface Roughness.

Hierarchical nanostructures are ideal architectures to harvest solar energy. The understanding of light absorption in single hierarchical structures is emergently important and greatly helpful in enhancing multiscale optical phenomena and light management. However, due to the geometrical complexity of hierarchical architectures, theoretical and experimental studies of light absorption have faced significant challenges. Here, we directly quantify light absorption in single hierarchical structures for the first time by utilizing VO2-based near field powermeter. It is found that light trapping is significantly enhanced in rough microwires when the roughness amplitude is comparable to the incident light wavelength. The roughness enhanced light absorption is verified as a general phenomenon on both VO2 and Si hierarchical structures. Therefore, our work not only provides a simple and quantitative method of measuring light absorption upon single geometrically complex structures in micro/nanoscale, but also contributes a general rule to rationally design of hierarchical structures for enhanced performance in photoelectric and photochemical applications.

Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites.

Morphology management for tailoring the properties of monolayer transition-metal dichalcogenides (TMDCs), that is, molybdenum disulfide (MoS2), has attracted great interest for promising applications such as in electrocatalysis and optoelectronics. Nevertheless, little progress has been made in engineering the shape of MoS2. Herein, we introduce a modified chemical vapor deposition method to grow monolayer MoS2 dendrites by pretreating substrates with adhesive tapes. The as-grown MoS2 crystals are featured with hexagonal backbones with fractal shapes and tunable degrees. By characterizing the atomic structure, it is found that these morphologies are mainly initiated from the twin defect derived growth and controlled by the S:Mo vapor ratio. Due to the accumulated sulfur vacancies in the cyclic twin regions, strong enhancement of photoluminescence emission is localized, which determines the shape dependency of optical property. This work not only enriches the understanding of the twin defects derived crystal growth mechanism and extends its applications from nanomaterials to two-dimensional crystals, but also offers a robust and controllable protocol for shape-engineered monolayer TMDCs in electrochemical and optoelectronic applications.