Gradient nanoplasmonic imaging metasurface for rapid and label-free detection of SARS-CoV-2 sequences.

Compact and user-friendly nucleic acid biosensors play a crucial role in advancing infectious disease research, particularly for coronavirus (COVID-19). While nanophotonic metasurface sensors hold promise for high-performance sensing, they face challenges due to their complexity and bulky readout instruments. In this study, we propose a gradient nanoplasmonic imaging (GNI) metasurface that incorporates the concept of an optical potential well, enabling label-free single-step detection of SARS-CoV-2 sequences. The metasurface sensor consists of nanopillars with continuous variations, forming an optical potential well that results in a centimeter-scale dark ring. This dynamic well exhibits high sensitivity to refractive index changes, recorded by a CCD. To further enhance the visualized sensing performance, plasmonic coupling of gold nanoparticles with the gold nanostructure is employed. Our metasurface-based biosensor achieves rapid single-step detection of SARS-CoV-2 sequences, with a low detection limit of 77.2 pM and a detection range of 0.1-100 nM. This biosensor not only demonstrates exceptional reproducibility and outstanding detection performance, but also maintains remarkable specificity in differentiating SARS-CoV-2 from other diseases with similar symptoms. This simple and spectrometer-free refractometric sensing scheme enables the construction of a compact and cost-efficient prototype. Our imaging-based metasurface biosensing strategy demonstrates valuable merits for rapid, sensitive, and quantitative detection, showcasing its potential as a valuable on-site nucleic acid diagnostic tool.

Nanoimprint-induced strain engineering of two-dimensional materials.

The high stretchability of two-dimensional (2D) materials has facilitated the possibility of using external strain to manipulate their properties. Hence, strain engineering has emerged as a promising technique for tailoring the performance of 2D materials by controlling the applied elastic strain field. Although various types of strain engineering methods have been proposed, deterministic and controllable generation of the strain in 2D materials remains a challenging task. Here, we report a nanoimprint-induced strain engineering (NISE) strategy for introducing controllable periodic strain profiles on 2D materials. A three-dimensional (3D) tunable strain is generated in a molybdenum disulfide (MoS2) sheet by pressing and conforming to the topography of an imprint mold. Different strain profiles generated in MoS2 are demonstrated and verified by Raman and photoluminescence (PL) spectroscopy. The strain modulation capability of NISE is investigated by changing the imprint pressure and the patterns of the imprint molds, which enables precise control of the strain magnitudes and distributions in MoS2. Furthermore, a finite element model is developed to simulate the NISE process and reveal the straining behavior of MoS2. This deterministic and effective strain engineering technique can be easily extended to other materials and is also compatible with common semiconductor fabrication processes; therefore, it provides prospects for advances in broad nanoelectronic and optoelectronic devices.

High-fidelity and clean nanotransfer lithography using structure-embedded and electrostatic-adhesive carriers.

Metallic nanostructures are becoming increasingly important for both fundamental research and practical devices. Many emerging applications employing metallic nanostructures often involve unconventional substrates that are flexible or nonplanar, making direct lithographic fabrication very difficult. An alternative approach is to transfer prefabricated structures from a conventional substrate; however, it is still challenging to maintain high fidelity and a high yield in the transfer process. In this paper, we propose a high-fidelity, clean nanotransfer lithography method that addresses the above challenges by employing a polyvinyl acetate (PVA) film as the transferring carrier and promoting electrostatic adhesion through triboelectric charging. The PVA film embeds the transferred metallic nanostructures and maintains their spacing with a remarkably low variation of <1%. When separating the PVA film from the donor substrate, electrostatic charges are generated due to triboelectric charging and facilitate adhesion to the receiver substrate, resulting in a high large-area transfer yield of up to 99.93%. We successfully transferred the metallic structures of a variety of materials (Au, Cu, Pd, etc.) with different geometries with a <50-nm spacing, high aspect ratio (>2), and complex 3D structures. Moreover, the thin and flexible carrier film enables transfer on highly curved surfaces, such as a single-mode optical fiber with a curvature radius of 62.5 µm. With this strategy, we demonstrate the transfer of metallic nanostructures for a compact spectrometer with Cu nanogratings transferred on a convex lens and for surface-enhanced Raman spectroscopy (SERS) characterization on graphene with reliable responsiveness.

Electrochemical Replication and Transfer for Low-Cost, Sub-100 nm Patterning of Materials on Flexible Substrates.

The fabrication of high-resolution patterns on flexible substrates is an essential step in the development of flexible electronics. However, the patterning process on flexible substrates often requires expensive equipment and tedious lithographic processing. Here, a bottom-up patterning technique, termed electrochemical replication and transfer (ERT) is reported, which fabricates multiscale patterns of a wide variety of materials by selective electrodeposition of target materials on a predefined template, and subsequent transfer of the electrodeposited materials to a flexible substrate, while leaving the undamaged template for reuse for over 100 times. The additive and parallel patterning attribute of ERT allows the fabrication of multiscale patterns with resolutions spanning from sub-100 nm to many centimeters simultaneously, which overcomes the trade-off between resolution and throughput of conventional patterning techniques. ERT is suitable for fabricating a wide variety of materials including metals, semiconductors, metal oxides, and polymers into arbitrary shapes on flexible substrates at a very low cost.

Hybrid modeling of perovskite light-emitting diodes with nanostructured emissive layers.

Perovskite light-emitting diodes (PeLEDs) have attracted much attention due to their superior performance. When a bottleneck of energy conversion efficiency is achieved with materials engineering, nanostructure incorporation proves to be a feasible approach to further improve device efficiencies via light extraction enhancement. The finite-difference time-domain simulation is widely used for optical analysis of nanostructured optoelectronic devices, but reliable modeling of PeLEDs with nanostructured emissive layers remains unmet due to the difficulty of locating dipole light sources. Herein we established a hybrid process for modeling light emission behaviors of such nanostructured PeLEDs by calibrating light source distribution through electrical simulations. This hybrid modeling method serves as a universal tool for structure optimization of light-emitting diodes with nanostructured emissive layers.

Spatial modulation of nanopattern dimensions by combining interference lithography and grayscale-patterned secondary exposure.

Functional nanostructures are exploited for a variety of cutting-edge fields including plasmonics, metasurfaces, and biosensors, just to name a few. Some applications require nanostructures with uniform feature sizes while others rely on spatially varying morphologies. However, fine manipulation of the feature size over a large area remains a substantial challenge because mainstream approaches to precise nanopatterning are based on low-throughput pixel-by-pixel processing, such as those utilizing focused beams of photons, electrons, or ions. In this work, we provide a solution toward wafer-scale, arbitrary modulation of feature size distribution by introducing a lithographic portfolio combining interference lithography (IL) and grayscale-patterned secondary exposure (SE). Employed after the high-throughput IL, a SE with patterned intensity distribution spatially modulates the dimensions of photoresist nanostructures. Based on this approach, we successfully fabricated 4-inch wafer-scale nanogratings with uniform linewidths of <5% variation, using grayscale-patterned SE to compensate for the linewidth difference caused by the Gaussian distribution of the laser beams in the IL. Besides, we also demonstrated a wafer-scale structural color painting by spatially modulating the filling ratio to achieve gradient grayscale color using SE.

On-Demand 3D Printing of Nanowire Probes for High-Aspect-Ratio Atomic Force Microscopy Imaging.

With the growing importance of three-dimensional (3D) nanomaterials and devices, there has been a great demand for high-fidelity, full profile topographic characterizations in a nondestructive manner. A promising route is to employ a high-aspect-ratio (HAR) probe in atomic force microscopy (AFM) imaging. However, the fabrication of HAR-AFM probes continues to suffer from extravagant cost, limited material choice, and complicated manufacturing steps. Here, we report one-step, on-demand electrohydrodynamic 3D printing of metallic HAR-AFM probes with tailored dimensions. Our additive fabrication approach yields a freestanding metallic nanowire with an aspect ratio over 30 directly on a cantilever within tens of seconds, producing a HAR-AFM probe. Furthermore, the benefits associated with unprecedented simplicity in the probe's dimension control, material selection, and regeneration are provided. The 3D-printed HAR-AFM probe exhibits a better fidelity in deep trench AFM imaging than a standard pyramidal probe. We expect this approach to find facile, material-saving manufacturing routes in particular for customizing functional nanoprobes.