Reliable fabrication of 3D freestanding nanostructures via all dry stacking of incompatible photoresist.

Three-dimensional (3D) free-standing nanostructures based on electron-beam lithography (EBL) have potential applications in many fields with extremely high patterning resolution and design flexibility with direct writing. In numerous EBL processes designed for the creation of 3D structures, the multilayer resist system is pivotal due to its adaptability in design. Nevertheless, the compatibility of solvents between different layers of resists often restricts the variety of feasible multilayer combinations. This paper introduces an innovative approach to address the bottleneck issue by presenting a novel concept of multilayer resist dry stacking, which is facilitated by a near-zero adhesion strategy. The poly(methyl methacrylate) (PMMA) film is stacked onto the hydrogen silsesquioxane (HSQ) resist using a dry peel and release technique, effectively circumventing the issue of HSQ solubilization by PMMA solvents typically encountered during conventional spin-coating procedures. Simultaneously, a dry lift-off technique can be implemented by eschewing the use of organic solvents during the wet process. This pioneering method enables the fabrication of high-resolution 3D free-standing plasmonic nanostructures and intricate 3D free-standing nanostructures. Finally, this study presents a compelling proof of concept, showcasing the integration of 3D free-standing nanostructures, fabricated via the described technique, into the realm of Fabry-Perot cavity resonators, thereby highlighting their potential for practical applications. This approach is a promising candidate for arbitrary 3D free-standing nanostructure fabrication, which has potential applications in nanoplasmonics, nanoelectronics, and nanophotonics.

Phase-Change Stamp with Highly Switchable Adhesion and Stiffness for Damage-Free Multiscale Transfer Printing.

Transfer printing is a technology widely used in the production of flexible electronics and vertically stacked devices, which involves the transfer of predefined electronic components from a rigid donor substrate to a receiver substrate with a stamp, potentially avoiding the limitations associated with lithographic processes. However, the stamps typically used in transfer printing have several limitations related to unwanted organic solvents, substantial loading, film damage, and inadequate adhesion switching ratios. This study introduces a thermally responsive phase-change stamp for efficient and damage-free transfer printing inspired by the adhesion properties observed during water freezing and ice melting. The stamp employs phase-change composites and simple fabrication protocols, providing robust initial adhesion strength and switchability. The underlying mechanism of switchable adhesion is investigated through experimental and numerical studies. Notably, the stamp eliminates the need for extra preload by spontaneously interlocking with the ink through in situ melting and crystallization. This minimizes ink damage and wrinkle formation during pickup while maintaining strong initial adhesion. During printing, the stamp exhibits a sufficiently weak adhesion state for reliable and consistent release, enabling multiscale, conformal, and damage-free transfer printing, ranging from nano- to wafer-scale. The fabrication of nanoscale short-channel transistors, epidermal electrodes, and human-machine interfaces highlights the potential of this technique in various emerging applications of nanoelectronics, nano optoelectronics, and soft bioelectronics.

The practice of reaction window in an electrocatalytic on-chip microcell.

To enhance the efficiency of catalysis, it is crucial to comprehend the behavior of individual nanowires/nanosheets. A developed on-chip microcell facilitates this study by creating a reaction window that exposes the catalyst region of interest. However, this technology's potential application is limited due to frequently-observed variations in data between different cells. In this study, we identify a conductance problem in the reaction windows of non-metallic catalysts as the cause of this issue. We investigate this problem using in-situ electronic/electrochemical measurements and atom-thin nanosheets as model catalysts. Our findings show that a full-open window, which exposes the entire catalyst channel, allows for efficient modulation of conductance, which is ten times higher than a half-open window. This often-overlooked factor has the potential to significantly improve the conductivity of non-metallic catalysts during the reaction process. After examining tens of cells, we develop a vertical microcell strategy to eliminate the conductance issue and enhance measurement reproducibility. Our study offers guidelines for conducting reliable microcell measurements on non-metallic single nanowire/nanosheet catalysts.

Wafer-Level Highly Dense Metallic Nanopillar-Enabled High-Performance SERS Substrates for Molecular Detection.

Seeking sensitive, large-scale, and low-cost substrates is highly important for practical applications of surface-enhanced Raman scattering (SERS) technology. Noble metallic plasmonic nanostructures with dense hot spots are considered an effective construction to enable sensitive, uniform, and stable SERS performance and thus have attracted wide attention in recent years. In this work, we reported a simple fabrication method to achieve wafer-scale ultradense tilted and staggered plasmonic metallic nanopillars filled with numerous nanogaps (hot spots). By adjusting the etching time of the PMMA (polymethyl methacrylate) layer, the optimal SERS substrate with the densest metallic nanopillars was obtained, which possessed a detection limit down to 10-13 M by using crystal violet as the detected molecules and exhibited excellent reproducibility and long-term stability. Furthermore, the proposed fabrication approach was further used to prepare flexible substrates; for example, a SERS flexible substrate was proven to be an ideal platform for analyzing low-concentration pesticide residues on curved fruit surfaces with significantly enhanced sensitivity. This type of SERS substrate possesses potential in real-life applications as low-cost and high-performance sensors.

Dry-Transferable Photoresist Enabled Reliable Conformal Patterning for Ultrathin Flexible Electronics.

Photolithographic techniques, which are widely used in the silicon-based semiconductor industry, enable the manufacture of high-yield and high-resolution features at the micrometer and nanometer scales. However, conventional photolithographic processes cannot accommodate the micro/nanofabrication of flexible and stretchable electronics. In this study, a microfabrication approach that uses a synthesized, environmentally friendly, and dry-transferable photoresist to enable the reliable conformal manufacturing of thin-film electronics is reported, which is also compatible with the existing cleanroom processes. Photoresists with high-resolution, high-density, and multiscale patterns can be transferred onto various substrates in a defect-free and conformal-contact manner, thus enabling multiple wafer reuses. Theoretical studies are conducted to investigate the damage-free peel-off mechanism of the proposed approach. The in situ fabrication of various electrical components, including ultralight and ultrathin biopotential electrodes, has been demonstrated, which offer lower interfacial impedance, durability, and stability, and the components are applied to collect electromyography signals with superior signal-to-noise ratio (SNR) and quality. Additionally, an exemplary demonstration of a human-machine interface indicates the potential of these electrodes in many emerging applications, including healthcare, sensing, and artificial intelligence.

Programmable Nanoscale Crack Lithography for Multiscale PMMA Patterns.

Crack lithography is important for preparing microstructured materials. This strategic use of cracking breaks with the traditional idea that cracks are unwanted and has great potential for high-resolution and high-throughput production. However, the ability to control nanoscale crack patterning is still insufficient. Here, we present a nanoscale, programmable angle-dependent technique to control crack generation that relies on standard electron-beam lithography. Multiscale patterns of poly(methyl methacrylate) of arbitrary shape, geometric size, and large area were obtained, greatly expanding the processing capacity of electron-beam lithography. In addition, we observed the interaction between adjacent structures and cracks, which resulted in crack suppression or second-order cracks. We also demonstrated that angle-dependent nanoscale cracks can be used in physical unclonable functions and have great application prospects in the field of information security. We believe that our strategy for programmable nanoscale crack patterning provides new opportunities and perspectives for nanofabrication.

Porous Metal-Organic Framework/ReS2 Heterojunction Phototransistor for Polarization-Sensitive Visual Adaptation Emulation.

Visual adaptation allows organisms to accurately perceive the external world even in dramatically changing environments, from dim starlight to bright sunlight. In particular, polarization-sensitive visual adaptation can effectively process the polarized visual information that is ubiquitous in nature. However, such an intriguing characteristic still remains a great challenge in semiconductor devices. Herein, a novel porous metal-organic-framework phototransistor with anisotropic-ReS2 -based heterojunction is demonstrated for polarization-sensitive visual adaptation emulation. The device exhibits intriguing polarized sensitivity and an adaptive ability due to its strong anisotropic and trapping-detrapping characteristics, respectively. A series of polarization-sensitive neuromorphic behaviors like polarization-perceptual excitatory postsynaptic current, multimode adjustable dichroic ratio and reconfigurable sensory adaption, are experimentally demonstrated through this porous heterojunction phototransistor. More importantly, with the polarization-electricity cooperation strategy, advanced polarization-sensitive visual adaptation with strong bottom-gate control and environment dependence is successfully realized. These results represent a significant step toward the new generation of intelligent visual perception systems in autonomous navigation and human-machine interaction, etc.

Resist nanokirigami for multipurpose patterning.

Resist-based patterning solutions play essential roles in modern micro- and nanoscale science and technology. The commonly used 'resist' patterning strategy depends on selective-area scission or cross-linking of resist molecules under the action of an energy beam. In this work, we propose and demonstrate a different resist-patterning strategy, termed 'resist nanokirigami', in which the resist structures are defined by their outlines and revealed by selective mechanical peeling of the unwanted resist film. Unlike conventional resist-based patterning processes, the final resist-nanokirigami structures do not undergo exposure and the exposure area is dramatically reduced. With these two advantages, a variety of functional structures that are difficult or impossible to fabricate by conventional processes, such as inverse nanostructures and their oligomers, multi-scale electrodes and freestanding plasmonic nanogaps, can be easily achieved with much higher efficiency. Thus, with its unique and complementary capabilities, the resist-nanokirigami process provides a new patterning solution that expands the family of lithography techniques and will play a significant role in fabricating multi-scale functional structures.

Suspended 3D metallic dimers with sub-10 nm gap for high-sensitive SERS detection.

The suspended metallic nanostructures with tiny gaps have certain advantages in surface-enhanced Raman scattering (SERS) due to the coaction of the tiny metallic nanogaps and the substrate-decoupled electromagnetism resonant modes. In this study, we used the lithographic HSQ/PMMA electron-beam bilayer resist exposure combined with a deposition-induced nanogap-narrowing process to define elevated suspended metallic nanodimers with tiny gaps for surface-enhanced Raman spectroscopy detection. By adjusting the deposited metal thickness, the metallic dimers with sub-10 nm gaps can be reliably obtained. These dimers with tunable nanogaps successfully served as excellent SERS substrates, exhibiting remarkable high-sensitivity detection ability for crystal violet molecules. Systematic experiments and simulations were conducted to explain the origin of the improved SERS performance. The results showed that the 3D elevated suspended metallic dimers could achieve a higher SERS enhancement factor than the metallic dimers on HSQ pillars and a common Si substrate, demonstrating that this kind of suspended metallic dimer is a promising route for high-sensitive SERS detection and other plasmonic applications.

Polarization-perceptual anisotropic two-dimensional ReS2 neuro-transistor with reconfigurable neuromorphic vision.

Polarization is a common and unique phenomenon in nature, which reveals more camouflage features of objects. However, current polarization-perceptual devices based on conventional physical architectures face enormous challenges for high-performance computation due to the traditional von Neumann bottleneck. In this work, a novel polarization-perceptual neuro-transistor with reconfigurable anisotropic vision is proposed based on a two-dimensional ReS2 phototransistor. The device exhibits excellent photodetection ability and superior polarization sensitivity due to its direct band gap semiconductor property and strong anisotropic crystal structure, respectively. The fascinating polarization-sensitive neuromorphic behavior, such as polarization memory consolidation and reconfigurable visual imaging, are successfully realized. In particular, the regulated polarization responsivity and dichroic ratio are successfully emulated through our artificial compound eyes. More importantly, two intriguing polarization-perceptual applications for polarized navigation with reconfigurable adaptive learning abilities and three-dimensional visual polarization imaging are also experimentally demonstrated. The proposed device may provide a promising opportunity for future polarization perception systems in intelligent humanoid robots and autonomous vehicles.

Asymmetric Nanofractures Determined the Nonreciprocal Peeling for Self-Aligned Heterostructure Nanogaps and Devices.

Planar heterostructures composed of two or more adjacent structures with different materials are a kind of building blocks for various applications in surface plasmon resonance sensors, rectifiers, photovoltaic devices, and ambipolar devices, but their reliable fabrication with controllable shape, size, and positioning accuracy remains challenging. In this work, we propose a concept for fabricating planar heterostructures via directional stripping and controlled nanofractures of metallic films, with which self-aligned, multimaterial, multiscale heterostructures with arbitrary geometries and sub-20 nm gaps can be obtained. By using a split ring as the template, the asymmetric nanofracture of the deposited film at the split position results in nonreciprocal peeling of the film in the split ring. Compared to the conventional processes, the final heterostructures are defined only by their outlines, thus providing the ability to fabricate complex heterostructures with higher resolutions. We demonstrate that this method can be used to fabricate heterodimers, multimaterial oligomers, and multiscale asymmetrical electrodes. An Ag-MoS2-Au photodiode with a strong rectification effect is fabricated based on the nanogap heterostructures prepared by this method. This technology provides a unique and reliable approach to define nanogap heterostructures, which are supposed to have potential applications in nanoelectronics, nanoplasmonics, nano-optoelectronics, and electrochemistry.

Fabrication of single-nanometer metallic gaps via spontaneous nanoscale dewetting.

Ultrasmall metallic nanogaps are of great significance for wide applications in various nanodevices. However, it is challenging to fabricate ultrasmall metallic nanogaps by using common lithographic methods due to the limited resolution. In this work, we establish an effective approach for successful formation of ultrasmall metallic nanogaps based on the spontaneous nanoscale dewetting effect during metal deposition. By varying the initial opening size of the exposed resist template, the influence of dewetting behavior could be adjusted and tiny metallic nanogaps can be obtained. We demonstrate that this method is effective to fabricate diverse sub-10 nm gaps in silver nanostructures. Based on this fabrication concept, even sub-5 nm metallic gaps were obtained. SERS measurements were performed to show the molecular detection capability of the fabricated Ag nanogaps. This approach is a promising candidate for sub-10 nm metallic gaps fabrication, thus possessing potential applications in nanoelectronics, nanoplasmonics, and nano-optoelectronics.

Enhancing Plasmonic Spectral Tunability with Anomalous Material Dispersion.

The field confinement of plasmonic systems enables spectral tunability under structural variations or environmental perturbations, which is the principle for various applications including nanorulers, sensors, and color displays. Here, we propose and demonstrate that materials with anomalous dispersion, such as Ge in the visible, improve spectral tunability. We introduce our proposal with a semianalytical guided mode picture. Using Ge-based film (Ag/Au)-coupled gap plasmon resonators, we implement two architectures and demonstrate the improved tunability with single-particle dark-field scattering, ensemble reflection, and color generation. We observe three-fold enhancement of tunability with Ge nanodisks compared with that of Si, a normal-dispersion material in the visible. The structural color generation of large array systems, made of inversely fabricated Ge-Ag resonators, exhibits a wide gamut. Our results introduce anomalous material dispersion as an extra degree of freedom to engineer the spectral tunability of plasmonic systems, especially relevant for actively tunable plasmonics and metasurfaces.

Operando Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co3O4 for Oxygen Evolution Reaction.

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure-activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO-Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co-OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

Growth of Large-Area Homogeneous Monolayer Transition-Metal Disulfides via a Molten Liquid Intermediate Process.

Growth of large-area, uniform, and high-quality monolayer transition-metal dichalcogenides (TMDs) for practical and industrial applications remains a long-standing challenge. The present study demonstrates a modified predeposited chemical vapor deposition (CVD) process by employing an annealing procedure before sulfurization, which helps in achieving large-area, highly uniform, and high-quality TMDs on various substrates. The annealing procedure resulted in a molten liquid state of the precursors in the CVD process, which not only facilitated a uniform redistribution of the precursor on the substrate (avoid the aggregation) because of the uniform redistribution of the liquid precursor on the substrate but more importantly avoided the undesired multilayer growth via the self-limited lateral supply precursors mechanism. A 2 in. uniform and continuous monolayer WS2 film has been synthesized on the SiO2/Si substrate. Moreover, uniform monolayer WS2 single crystals can be prepared on more general and various substrates including sapphire, mica, quartz, and Si3N4 using the same growth procedure. Besides, this growth mechanism can be generalized to synthesize other monolayer TMDs such as MoS2 and MoS2/WS2 heterostructures. Hence, the present method provides a generalized attractive strategy to grow large-area, uniform, single-layer two-dimensional (2D) materials. This study has significant implications in the advancement of batch production of various 2D-material-based devices for industrial and commercial applications.

Kirigami-inspired multiscale patterning of metallic structures via predefined nanotrench templates.

Reliable fabrication of multiscale metallic patterns with precise geometry and size at both the nanoscale and macroscale is of importance for various applications in electronic and optical devices. The existing fabrication processes, which usually involve film deposition in combination with electron-beam patterning, are either time-consuming or offer limited precision. Inspired by the kirigami, an ancient handicraft art of paper cutting, this work demonstrates an electron-beam patterning process for multiscale metallic structures with significantly enhanced efficiency and precision. Similar to the kirigami, in which the final pattern is defined by cutting its contour in a paper and then removing the unwanted parts, we define the target multiscale structures by first creating nanotrench contours in a metallic film via an electron-beam-based process and then selectively peeling the separated film outside the contours. Compared with the conventional approach, which requires the exposure of the whole pattern, much less exposure area is needed for nanotrench contours, thus enabling reduced exposure time and enhanced geometric precision due to the mitigated proximity effect. A theoretical model based on interface mechanics allows a clear understanding of the nanotrench-assisted selective debonding behaviour in the peeling process. By using this fabrication process, multiscale metallic structures with sub-10-nm up to submillimetre features can be reliably achieved, having potential applications for anti-counterfeiting and gap-plasmon-enhanced spectroscopy.

Ultrafast nonlinear absorption enhancement of monolayer MoS2 with plasmonic Au nanoantennas.

In this work, we experimentally study the nonlinear absorption enhancement of saturable absorption and two-photon absorption on a hybrid structure comprising a monolayer MoS2 and Au nanoantennas via femtosecond I-scan measurement. Specifically, a 13-fold increment in the linear absorption coefficient is attained at 1.85 eV, along with an 8-fold enhancement of the two-photon absorption coefficient at 1.65 eV, which is attributed to exciton-plasmon coupling resonance and plasmonic hot electron transfer. The exciton-plasmon coupling effect is characterized by stable photoluminescence experiments. Furthermore, the exciton recombination time is extracted from the pump-probe measurement, whose value in the hybrid structure is shortened from 18.5 ps (pure MoS2) to 1.84 ps. Our findings facilitate a new perspective to modulate the nonlinear optical response and to promote the performance of nonlinear photonic devices.

Short channel monolayer MoS2 field-effect transistors defined by SiO x nanofins down to 20 nm.

Layered semiconductors such as transition metal dichalcogenides (TMDs) with proper bandgaps complement the zero-bandgap drawback of graphene, demonstrating great potential for post-silicon complementary metal-oxide-semiconductor technology. Among the TMD family, molybdenum disulfide (MoS2) is highly attractive for its atomically thin body, large bandgap and decent mechanical and chemical stability. However, current nanofabrication techniques hardly satisfy the requirements of short channel and convenient preparation simultaneously. Here, we demonstrate a simple and effective approach to fabricate short channel chemical vapor deposition (CVD) monolayer MoS2 field-effect transistors (FET) with channel length down to 20 nm. Electron-beam lithography based on high-resolution negative-tone hydrogen silsesquioxane electron resists were applied to create 20 nm wide SiO x lines, defining the short channel length. The 20 nm MoS2 FET displays ON-sate current in excess of 100 µA µm-1. The corresponding current ON/OFF ratio at room temperature reaches 105. We carefully studied the short channel effect of as-fabricated MoS2 FETs. Combining with the large-scale growth of CVD method, our results will pave a way for short channel device applications based on atomically thin two-dimensional semiconductors.