Scalable Fabrication of Metallic Nanogaps at the Sub-10 nm Level

Small Feature-Size Transistors Based on Low-Dimensional Materials: From Structure Design to Nanofabrication Techniques

For several decades after Moore's Law is proposed, there is a continuous effort to reduce the feature-size of transistors. However, as the size of transistors continues to decrease, numerous challenges and obstacles including severe short channel effects (SCEs) are emerging. Recently, low-dimensional materials have provided new opportunities for constructing small feature-size transistors due to their superior electrical properties compared to silicon. Here, state-of-the-art low-dimensional materials-based transistors with small feature-sizes are reviewed. Different from other works that mainly focus on material characteristics of a specific device structure, the discussed topics are utilizing device structure design including vertical structure and nano-gate structure, and nanofabrication techniques to achieve small feature-sizes of transistors. A comprehensive summary of these small feature-size transistors is presented by illustrating their operation mechanism, relevant fabrication processes, and corresponding performance parameters. Besides, the role of small feature-size transistors based on low-dimensional materials in further reducing the small footprint is also clarified and their cutting-edge applications are highlighted. Finally, a comparison and analysis between state-of-art transistors is made, as well as a glimpse into the future research trajectory of low dimensional materials-based small feature-size transistors is briefly outlined.

Bridging Nanomanufacturing and Artificial Intelligence-A Comprehensive Review

Nanomanufacturing and digital manufacturing (DM) are defining the forefront of the fourth industrial revolution-Industry 4.0-as enabling technologies for the processing of materials spanning several length scales. This review delineates the evolution of nanomaterials and nanomanufacturing in the digital age for applications in medicine, robotics, sensory technology, semiconductors, and consumer electronics. The incorporation of artificial intelligence (AI) tools to explore nanomaterial synthesis, optimize nanomanufacturing processes, and aid high-fidelity nanoscale characterization is discussed. This paper elaborates on different machine-learning and deep-learning algorithms for analyzing nanoscale images, designing nanomaterials, and nano quality assurance. The challenges associated with the application of machine- and deep-learning models to achieve robust and accurate predictions are outlined. The prospects of incorporating sophisticated AI algorithms such as reinforced learning, explainable artificial intelligence (XAI), big data analytics for material synthesis, manufacturing process innovation, and nanosystem integration are discussed.

Periodic Folded Gold Nanostructures with a Sub-10 nm Nanogap for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy has emerged as a powerful spectroscopy technique for detection with its capacity for label-free, nondestructive analysis, and ultrasensitive characterization. High-performance surface-enhanced Raman scattering (SERS) substrates with homogeneity and low cost are the key factors in chemical and biomedical analysis. In this study, we propose the technique of atomic force microscopy (AFM) scratching and nanoskiving to prepare periodic folded gold (Au) nanostructures as SERS substrates. Initially, folded Au nanostructures with tunable nanogaps and periodic structures are created through the scratching of Au films by AFM, the deposition of Ag/Au films, and the cutting of epoxy resin, reducing fabrication cost and operational complexity. Periodic folded Au nanostructures show the three-dimensional nanofocusing effect, hotspot effect, and standing wave effect to generate an extremely high electromagnetic field. As a typical molecule to be tested, p-aminothiophenol has the lowest detection limit of up to 10-9 M, owing to the balance between the electromagnetic field energy concentration and the transmission loss in periodic folded Au nanostructures. Finally, by precisely controlling the periods and nanogap widths of the folded Au nanostructures, the synergistic effect of surface plasmon resonance is optimized and shows good SERS properties, providing a new strategy for the preparation of plasmonic nanostructures.

Nanorod reassembling on a sprayed SERS substrate under confined evaporation inducing ultrasensitive TPhT detection

Triphenyltin is an estrogen like pollutant that poses significant environmental threats due to its highly accumulative toxicity. To improve regulation, a fast and sensitive detection method is urgently needed. SERS can capture fingerprint information and is capable of trace detection, making it an ideal solution. Here, we present a sprayed substrate comprised of lightconfining structures and gold nanorod assemblies that are easy to prepare, low-cost, and can form dense hotspots under confined evaporation. The substrates are three-layered: initially, a gold nanorod layer is sprayed as a support, then sputter Ag film on the surface to form a lightconfining structure, followed by another gold nanorod layer sprayed on the Ag film. The coupling of nanorod assembly with lightconfining Ag films leads to 10-fold sensitivity. In addition, sample droplet evaporation in a limited area called confined evaporation contributes to nanorod migration and reassembly on the corner of the substrate, enhancing analytes absorption, and substantially lowered the detection limits. By systematically evaluating the substrate performance, we were able to obtain an average enhancement factor of 3.31 × 106. After confined evaporation, the detection limit reached 10-18 M for R6G and for triphenyltin, it achieved 10-9 M. This novel method represents a significant advancement toward SERS application in detecting trace pollutants.

Pursuit of hidden rules behind the irregularity of nano capillary lithography by hybrid intelligence

Nature finds a way to leverage nanotextures to achieve desired functions. Recent advances in nanotechnologies endow fascinating multi-functionalities to nanotextures by modulating the nanopixel's height. But nanoscale height control is a daunting task involving chemical and/or physical processes. As a facile, cost-effective, and potentially scalable remedy, the nanoscale capillary force lithography (CFL) receives notable attention. The key enabler is optical pre-modification of photopolymer's characteristics via ultraviolet (UV) exposure. Still, the underlying physics of the nanoscale CFL is not well understood, and unexplained phenomena such as the "forbidden gap" in the nano capillary rise (unreachable height) abound. Due to the lack of large data, small length scales, and the absence of first principles, direct adoptions of machine learning or analytical approaches have been difficult. This paper proposes a hybrid intelligence approach in which both artificial and human intelligence coherently work together to unravel the hidden rules with small data. Our results show promising performance in identifying transparent, physics-retained rules of air diffusivity, dynamic viscosity, and surface tension, which collectively appear to explain the forbidden gap in the nanoscale CFL. This paper promotes synergistic collaborations of humans and AI for advancing nanotechnology and beyond.

Chemically Amplified Molecular Glass Photoresist Regulated by 2-Aminoanthracene Additive for Electron Beam Lithography and Extreme Ultraviolet Lithography

2-Aminoanthracene was used as a nucleophilic additive in a molecular glass photoresist, bisphenol A derivative (BPA-6-epoxy), to improve advanced lithography performance. The effect of 2-aminoanthracene on BPA-6-epoxy was studied by electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL). The result indicates that the additive can optimize the pattern outline by regulating epoxy cross-linking reaction, avoiding photoresist footing effectively in EBL. The EUVL result demonstrates that 2-aminoanthracene can significantly reduce line width roughness (LWR) for HP (Half-Pitch) 25 nm (from 4.9 to 3.8 nm) and HP 22 nm (from 6.9 to 3.0 nm). The power spectrum density (PSD) curve further confirms the reduction of roughness at medium and high frequency for HP 25 nm and the whole range of frequency for HP 22 nm, respectively. The study offers useful guidelines to improve the roughness of a chemically amplified molecular glass photoresist with epoxy groups for electron beam lithography and extreme ultraviolet lithography.

Atomic layer deposition assisted fabrication of large-scale metal nanogaps for surface enhanced Raman scattering

Metal nanogaps can confine electromagnetic field into extremely small volumes, exhibiting strong surface plasmon resonance effect. Therefore, metal nanogaps show great prospects in enhancing light-matter interaction. However, it is still challenging to fabricate large-scale (centimeter scale) nanogaps with precise control of gap size at nanoscale, limiting the practical applications of metal nanogaps. In this work, we proposed a facile and economic strategy to fabricate large-scale sub-10 nm Ag nanogaps by the combination of atomic layer deposition (ALD) and mechanical rolling. The plasmonic nanogaps can be formed in the compacted Ag film by the sacrificial Al₂O₃deposited via ALD. The size of nanogaps are determined by the twice thickness of Al₂O₃with nanometric control. Raman results show that SERS activity depends closely on the nanogap size, and 4 nm Ag nanogaps exhibit the best SERS activity. By combining with other porous metal substrates, various sub-10 nm metal nanogaps can be fabricated over large scale. Therefore, this strategy will have significant implications for the preparation of nanogaps and enhanced spectroscopy.

A Single-Component Molecular Glass Resist Based on Tetraphenylsilane Derivatives for Electron Beam Lithography

A novel molecular glass (TPSiS) with photoacid generator (sulfonium salt group) binding to tetraphenylsilane derivatives was synthesized and characterized. The physical properties such as solubility, film-forming ability, and thermal stability of TPSiS were examined to assess the suitability for application as a candidate for photoresist materials. The sulfonium salt unit underwent photolysis to effectively generate photoacid on UV irradiation, which catalyzed the deprotection of the t-butyloxycarbonyl groups. It demonstrates that the TPSiS can be used as a 'single-component' molecular resist without any additives. The lithographic performance of the TPSiS resist was evaluated by electron beam lithography. The TPSiS resist can resolve 25 nm dense line/space patterns and 16 nm L/4S semidense line/space patterns at a dose of 45 and 85 μC/cm² for negative-tone development (NTD). The etching selectivity of the TPSiS resist to Si substrate is 8.6 under SF₆/O₂ plasma, indicating a potential application. Contrast analysis suggests that the significant solubility switch within a narrow exposure dose range (18-47 μC/cm²) by NTD is favorable for high-resolution patterns. This study supplies useful guidelines for the optimization and development of single-component molecular glass resists with high lithographic performance.

In Situ Adjustable Nanogaps and In-Plane Break Junctions

The ability to precisely regulate the size of a nanogap is essential for establishing high-yield molecular junctions, and it is crucial for the control of optical signals in extreme optics. Although remarkable strategies for the fabrication of nanogaps are proposed, wafer-compatible nanogaps with freely adjustable gap sizes are not yet available. Herein, two approaches for constructing in situ adjustable metal gaps are proposed which allow Ångstrom modulation resolution by employing either a lateral expandable piezoelectric sheet or a stretchable membrane. These in situ adjustable nanogaps are further developed into in-plane molecular break junctions, in which the gaps can be repeatedly closed and opened thousands of times with self-assembled molecules. The conductance of the single 1,4-benzenediamine (BDA) and the BDA molecular dimer is successfully determined using the proposed strategy. The measured conductance agreeing well with the data by employing another well-established scanning tunneling microscopy break junction technique provides insight into the formation of molecule dimer via hydrogen bond at single molecule level. The wafer-compatible nanogaps and in-plane dynamical break-junctions provide a potential approach to fabricate highly compacted devices using a single molecule as a building block and supply a promising in-plane technique to address the dynamical properties of single molecules.

A Reflectivity Enhanced 3D Optical Storage Nanostructure Application Based on Direct Laser Writing Lithography

To enable high-density optical storage, better storage media structures, diversified recording methods, and improved accuracy of readout schemes should be considered. In this study, we propose a novel three-dimensional (3D) sloppy nanostructure as the optical storage device, and this nanostructure can be fabricated using the 3D laser direct writing technology. It is a 900 nm high, 1 × 2 µm wide Si slope on a 200 nm SiO2 layer with 200 nm Si₃N₄ deposited on top to enhance reflectivity. In this study, we propose a reflected spectrum-based method as the readout recording strategy to stabilize information readout more stable. The corresponding reflected spectrum varied when the side wall angle of the slope and the azimuth angle of the nanostructure were tuned. In addition, an artificial neural network was applied to readout the stored information from the reflected spectrum. To simulate the realistic fabrication error and measurement error, a 20% noise level was added to the study. Our findings showed that the readout accuracy was 99.86% for all 120 data sequences when the slope and azimuth angle were varied. We investigated the possibility of a higher storage density to fully demonstrate the storage superiority of this designed structure. Our findings also showed that the readout accuracy can reach its highest level at 97.25% when the storage step of the encoded structure becomes 7.5 times smaller. The study provides the possibility to further explore different nanostructures to achieve high-density optical storage.

Large Area Patterning of Highly Reproducible and Sensitive SERS Sensors Based on 10-nm Annular Gap Arrays

Applicable surface-enhanced Raman scattering (SERS) active substrates typically require low-cost patterning methodology, high reproducibility, and a high enhancement factor (EF) over a large area. However, the lack of reproducible, reliable fabrication for large area SERS substrates in a low-cost manner remains a challenge. Here, a patterning method based on nanosphere lithography and adhesion lithography is reported that allows massively parallel fabrication of 10-nm annular gap arrays on large areas. The arrays exhibit excellent reproducibility and high SERS performance, with an EF of up to 10⁷. An effective wearable SERS contact lens for glucose detection is further demonstrated. The technique described here extends the range of SERS-active substrates that can be fabricated over large areas, and holds exciting potential for SERS-based chemical and biomedical detection.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Molecular Glass Resists Based on Tetraphenylsilane Derivatives: Effect of Protecting Ratios on Advanced Lithography

A series of t-butyloxycarbonyl (t-Boc) protected tetraphenylsilane derivatives (TPSi-Boc _x , x = 60, 70, 85, 100%) were synthesized and used as resist materials to investigate the effect of t-Boc protecting ratio on advanced lithography. The physical properties such as solubility, film-forming ability, and thermal stability of TPSi-Boc _x were examined to assess the suitability for application as candidates for positive-tone molecular glass resist materials. The effects of t-Boc protecting ratio had been studied in detail by electron beam lithography. The results suggest that the TPSi-Boc _x resist with different t-Boc protecting ratios exhibit a significant change in contrast, pattern blur, and the density of bridge defect. The TPSi-Boc_70% resist achieves the most excellent patterning capability. The extreme ultraviolet (EUV) lithography performance on TPSi-Boc_70% was evaluated by using the soft X-ray interference lithography. The results demonstrate that the TPSi-Boc_70% resist can achieve excellent patterning capability down to 20 nm isolated lines at 8.7 mJ/cm² and 25 nm dense lines at 14.5 mJ/cm². This study will help us to understand the relationship between the t-Boc protecting ratio and the patterning ability and supply useful guidelines for designing molecular resists.

Conductive Materials with Elaborate Micro/Nanostructures for Bioelectronics

Bioelectronics, an emerging field with the mutual penetration of biological systems and electronic sciences, allows the quantitative analysis of complicated biosignals together with the dynamic regulation of fateful biological functions. In this area, the development of conductive materials with elaborate micro/nanostructures has been of great significance to the improvement of high-performance bioelectronic devices. Thus, here, a comprehensive and up-to-date summary of relevant research studies on the fabrication and properties of conductive materials with micro/nanostructures and their promising applications and future opportunities in bioelectronic applications is presented. In addition, a critical analysis of the current opportunities and challenges regarding the future developments of conductive materials with elaborate micro/nanostructures for bioelectronic applications is also presented.

High-Throughput Fabrication of Triangular Nanogap Arrays for Surface-Enhanced Raman Spectroscopy

Squeezing light into nanometer-sized metallic nanogaps can generate extremely high near-field intensities, resulting in dramatically enhanced absorption, emission, and Raman scattering of target molecules embedded within the gaps. However, the scarcity of low-cost, high-throughput, and reproducible nanogap fabrication methods offering precise control over the gap size is a continuing obstacle to practical applications. Using a combination of molecular self-assembly, colloidal nanosphere lithography, and physical peeling, we report here a high-throughput method for fabricating large-area arrays of triangular nanogaps that allow the gap width to be tuned from ∼10 to ∼3 nm. The nanogap arrays function as high-performance substrates for surface-enhanced Raman spectroscopy (SERS), with measured enhancement factors as high as 10⁸ relative to a thin gold film. Using the nanogap arrays, methylene blue dye molecules can be detected at concentrations as low as 1 pM, while adenine biomolecules can be detected down to 100 pM. We further show that it is possible to achieve sensitive SERS detection on binary-metal nanogap arrays containing gold and platinum, potentially extending SERS detection to the investigation of reactive species at platinum-based catalytic and electrochemical surfaces.

A Parabola-like Gold Nanobowtie on a Sapphire Substrate as a Nano-Cavity

Plasmonic, metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for confining light waves in a nanoscale gap region. The near-field optical properties of the innovative PGNB structure were studied comprehensively, taking advantage of the time-resolved field calculation based on a finite-difference time-domain algorithm (FDTD). The calculation result showed that the resonance wavelength of the nano-cavity was quite sensitive to the geometry of the PGNB. The values that related to the scattering and absorption properties of the PGNB, such as the scattering cross section, absorption cross section, extinction cross section, scattering ratio, and also the absorption ratio, were strongly dependent on the geometrical parameters which affected the surface area of the nanobowtie. Increased sharpness of the gold tips on the parabola-like nano-wings benefited the concentration of high-density charges with opposite electric properties in the narrow gold tips with limited volume, thus, resulting in a highly enhanced electric field in the nano-cavity under illumination of the light wave. Reduction of the gap size between the two gold nano-tips, namely, the size of the nano-cavity, decreased the distance that the electric potential produced by the highly concentrated charges on the surface of each gold nano-tip had to jump across, therefore, causing a significantly enhanced field in the nano-cavity. Further, alignment of the linearly polarized electric field of the incident light wave with the symmetric axis of the PGNB efficiently enabled the free electrons in the PGNB to concentrate on the surface of the sharp gold tips with a high density, thus, strongly improving the field across the nano-cavity. The research provides a new insight for future design, nanofabrication, and characterization of PGNBs for applications in devices that relate to enhancing photons emission, improving efficiency for energy harvesting, and improving sensitivity for infrared detection.

Nanodevices for Biological and Medical Applications: Development of Single-Molecule Electrical Measurement Method

A comprehensive detection of a wide variety of diagnostic markers is required for the realization of personalized medicine. As a sensor to realize such personalized medicine, a single molecule electrical measurement method using nanodevices is currently attracting interest for its comprehensive simultaneous detection of various target markers for use in biological and medical application. Single-molecule electrical measurement using nanodevices, such as nanopore, nanogap, or nanopipette devices, has the following features:; high sensitivity, low-cost, high-throughput detection, easy-portability, low-cost availability by mass production technologies, and the possibility of integration of various functions and multiple sensors. In this review, I focus on the medical applications of single- molecule electrical measurement using nanodevices. This review provides information on the current status and future prospects of nanodevice-based single-molecule electrical measurement technology, which is making a full-scale contribution to realizing personalized medicine in the future. Future prospects include some discussion on of the current issues on the expansion of the application requirements for single-mole-cule measurement.