Reliable fabrication of micro/nanostructures with sub-10 nm features is of great significance for advancing nanoscience and nanotechnology. While the capability of current complementary metal-oxide semiconductor (CMOS) chip manufacturing can produce structures on the sub-10 nm scale, many emerging applications, such as nano-optics, biosensing, and quantum devices, also require ultrasmall features down to single digital nanometers. In these emerging applications, CMOS-based manufacturing methods are currently not feasible or appropriate due to the considerations of usage cost, material compatibility, and exotic features. Therefore, several specific methods have been developed in the past decades for different applications. In this review, we attempt to give a systematic summary on sub-10 nm fabrication methods and their related applications. In the first and second parts, we give a brief introduction of the background of this research topic and explain why sub-10 nm fabrication is interesting from both scientific and technological perspectives. In the third part, we comprehensively summarize the fabrication methods and classify them into three main approaches, including lithographic, mechanics-enabled, and post-trimming processes. The fourth part discusses the applications of these processes in quantum devices, nano-optics, and high-performance sensing. Finally, a perspective is given to discuss the challenges and opportunities associated with this research topic.

Single metallic nanostructures supporting strong Fano resonances allow more compact nanophotonics integration and easier geometrical control in practical applications such as enhanced spectroscopy and sensing. In this work, we designed a class of plasmonic split nanodisks that show pronounced Fano resonance comparable to that observed in widely studied plasmonic oligomer clusters. Using our recently developed "sketch and peel" electron-beam lithography, split nanodisks with varied diameter and split length were fabricated over a large area with high uniformity. Transmission spectroscopy measurements demonstrated that the fabricated structures with 15 nm split gap exhibit disk diameter and split length controlled Fano resonances in the near-infrared region, showing excellent agreement with simulation results. Together with the plasmon hybridization theory, in-depth full-wave analyses elucidated that the Fano resonances observed in the split nanodisks were induced by mode interference between the bright antibonding dipole mode of split disks and the subradiant mode supported by the narrow split gap. With the giant near-field enhancement enabled by the intensive Fano resonance at the tiny split gap, strong wavelength-dependent second harmonic generation was observed under near-infrared excitation. Our work demonstrated that single split nanodisks could serve as important building blocks for plasmonic and nanophotonic applications including sensing and nonlinear optics.

The {001}-faceted anatase TiO micro-/nanocrystals have been widely investigated for enhancing the photocatalysis and photoelectrochemical performance of TiO nanostructures, but their practical applications still require improved energy conversion efficiency under solar-light and enhanced cycling stability. In this work, we demonstrate the controlled growth of ultrathin {001}-faceted anatase TiO nanosheets on flexible carbon cloth for enhancing the cycling stability, and the solar-light photocatalytic performance of the synthesized TiO nanosheets can be significantly improved by decorating with vapor-phase-deposited uniformly distributed plasmonic gold nanoparticles. The fabricated Au-TiO hybrid system shows an 8-fold solar-light photocatalysis enhancement factor in photodegrading Rhodamine B, a high photocurrent density of 300 μA cm under the illumination of AM 1.5G, and 100% recyclability under a consecutive long-term cycling measurement. Combined with electromagnetic simulations and systematic control experiments, it is believed that the tandem-type separation and transition of plasmon-induced hot electrons from Au nanoparticles to the {001} facet of anatase TiO, and then to the neighboring {101} facet, is responsible for the enhanced solar-light photochemical performance of the hybrid system. The Au-TiO nanosheet system addresses well the problems of the limited solar-light response of anatase TiO and fast recombination of photogenerated electron-hole pairs, representing a promising high-performance recyclable solar-light-responding system for practical photocatalytic reactions.

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The phenomenon of extraordinary optical transmission (EOT) caused by light through metallic nanohole arrays has attracted significant attention due to its potential applications for monolithic color filters and ultrasensitive label-free biosensing. However, the EOT spectra of these nanohole arrays have multiple resonance peaks that are spectrally close to each other due to the multiple resonance modes generated by different media on the upper and lower surfaces of metal. In addition, owing to the absorption loss of metal and the scattering of holes, the EOT resonance peaks have low transmission coefficient for practical applications. In this work, utilizing a tapered nanohole arrays structure which is stacked by multiple cylindrical holes with the same depth but different radii, we show that tapered nanohole arrays can effectively suppress the excitation of multiple resonance peaks, and a single EOT peak emerges in the transmission spectrum and simultaneously exhibits significantly enhanced transmission (∼7 times) and narrow linewidth (∼15 nm). The enhanced EOT of tapered nanohole arrays can be also found in other wavelength regions and plasmonic materials. Benefiting from isolated transmission peak, high transmission efficiency and extremely narrow linewidth, a highly sensitive plasmonic nanosensor with sensitivity of 1580 nm/RIU and figure of merit of 105 can be attained. We believe that the tapered nanohole structure would enable applications for ultrasensitive sensors, switches and efficient filters.