2D Nanopatterns Research Articles

Wafer‐scale single‐crystal two‐dimensional materials for integrated optoelectronics

AbstractSince the pioneering research on graphene, two‐dimensional (2D) materials have been considered as the most promising candidates to continue advancing Moore's Law, and an emerging material family, which has bred a lot of novel functional applications beyond the Si‐based integrated circuit. Unfortunately, abundant challenges in the synthesis of wafer‐scale single‐crystal (WSSC) 2D materials and their on‐chip integration technology severely hinder their commercialization road. Over the past few years, significant technique breakthroughs of WSSC 2D materials have been increasingly achieved, accordingly a comprehensive review and critical evaluation of these new advances are pressingly required. In this review article, the outstanding research progress on the synthesis of WSSC 2D materials and 2D material‐based on‐chip integration technology, including 2D materials integration, nanopatterning, electrode integration, and dielectric integration, are summarized in detail. Then, the major application prospect of different types of WSSC 2D materials in optoelectronics is discussed. Finally, a critical assessment of these advancements is given, as well as the potential challenges and opportunities in the foreseeable future.

The 3D Controllable Fabrication of Nanomaterials with FIB-SEM Synchronization Technology.

Nanomaterials with unique structures and functions have been widely used in the fields of microelectronics, biology, medicine, and aerospace, etc. With advantages of high resolution and multi functions (e.g., milling, deposition, and implantation), focused ion beam (FIB) technology has been widely developed due to urgent demands for the 3D fabrication of nanomaterials in recent years. In this paper, FIB technology is illustrated in detail, including ion optical systems, operating modes, and combining equipment with other systems. Together with the in situ and real-time monitoring of scanning electron microscopy (SEM) imaging, a FIB-SEM synchronization system achieved 3D controllable fabrication from conductive to semiconductive and insulative nanomaterials. The controllable FIB-SEM processing of conductive nanomaterials with a high precision is studied, especially for the FIB-induced deposition (FIBID) 3D nano-patterning and nano-origami. As for semiconductive nanomaterials, the realization of high resolution and controllability is focused on nano-origami and 3D milling with a high aspect ratio. The parameters of FIB-SEM and its working modes are analyzed and optimized to achieve the high aspect ratio fabrication and 3D reconstruction of insulative nanomaterials. Furthermore, the current challenges and future outlooks are prospected for the 3D controllable processing of flexible insulative materials with high resolution.

Chemical Strain Engineering of Copper Atoms on Continuous Three-Dimensional-Nanopatterned Nickel Nitride to Accelerate Alkaline Hydrogen Evolution

Maximizing electrocatalytic activity by incorporating heteroatoms into the microstructures of non-noble metal catalysts is vital for their application in industrial water-alkali electrolyzers, yet modulation of the catalytic properties by the synergistic effects of three-dimensional (3D) nanopatterning and the incorporation of heteroatoms remains challenging. Here, we fabricated a tunable Cu atom-incorporated continuous 3D-nanopatterned nickel nitride (Ni3N) and investigated its electrocatalytic activity for the alkaline hydrogen evolution reaction. By altering Cu atom incorporation and the synergistic effect between lattice strain and 3D nanopatterning, efficient electrocatalytic activity was achieved. The resulting catalyst delivered an overpotential of 48 mV at a current density of 10 mA/cm2 and a Tafel slope of 51 mV/dec, which is similar to those of the benchmark electrocatalyst Pt/C (overpotential = 27 mV and Tafel slope = 29 mV/dec). The excellent electrocatalytic activity of copper atom-incorporated 3D-nanopatterned nickel nitride (CuNi2N-3DNi) was attributed to manipulation of the electronic structure by inducing lattice strain, which reduces the energy barrier for the adsorption of H2O, prompting balanced adsorption–desorption of the intermediate hydrogen H* by lowering the Gibbs free energy (ΔGH* = −0.06 eV). This work provides an effective strategy to improve the electrocatalytic activity of synthesized 3D microstructure catalysts incorporating a large area by precise strain engineering and nanopatterning.

Template-directed 2D nanopatterning of S = 1/2 molecular spins.

Molecular spins are emerging platforms for quantum information processing. By chemically tuning their molecular structure, it is possible to prepare a robust environment for electron spins and drive the assembly of a large number of qubits in atomically precise spin-architectures. The main challenges in the integration of molecular qubits into solid-state devices are (i) minimizing the interaction with the supporting substrate to suppress quantum decoherence and (ii) controlling the spatial distribution of the spins at the nanometer scale to tailor the coupling among qubits. Herein, we provide a nanofabrication method for the realization of a 2D patterned array of individually addressable Vanadyl Phthalocyanine (VOPc) spin qubits. The molecular nanoarchitecture is crafted on top of a diamagnetic monolayer of Titanyl Phthalocyanine (TiOPc) that electronically decouples the electronic spin of VOPc from the underlying Ag(100) substrate. The isostructural TiOPc interlayer also serves as a template to regulate the spacing between VOPc spin qubits on a scale of a few nanometers, as demonstrated using scanning tunneling microscopy, X-ray circular dichroism, and density functional theory. The long-range molecular ordering is due to a combination of charge transfer from the metallic substrate and strain in the TiOPc interlayer, which is attained without altering the pristine VOPc spin characteristics. Our results pave a viable route towards the future integration of molecular spin qubits into solid-state devices.

Hybrid MoS2/polymer nanoarrays for large-scale photon harvesting and enhanced molecular photo-bleaching

Header Two-dimensional (2D) Transition Metal Dichalcogenide semiconductors (TMDs) are a promising platform in view of developing a novel generation of optoelectronics devices and renewable photon to energy conversion technologies. However new ultra-compact photon harvesting schemes are urgently required to mitigate their poor photon absorption properties. In this work we propose a novel flat-optic solution where a few layers MoS2 film is conformally deposited on a large area (cm2) nanogrooved template. The subwavelength reshaping of the TMDs film induces the excitation of photonic Rayleigh Anomalies (RA). The latter promote a strong in-plane electromagnetic confinement and can be easily tuned over a broadband spectrum by tailoring the illumination conditions. As a demonstration of the potential of this large-area surface functionalization we employed the hybrid 2D nanopatterns as a photocatalyst in a prototype photobleaching reaction of Methylene Blue (MB) molecules. We demonstrate a strong enhancement of the photochemical MB degradation, well above a factor 2, by effectively tuning the RA mode in resonance to the molecular absorption band. Therefore, these findings pave the way to flat-optics photon harvesting schemes for boosting photoconversion efficiency in large-scale hybrid 2D-TMD/polymer layers, with a strong impact in applications ranging from waste-water remediation to new-generation photonics and renewable energy storage

Self-Aligned Plasmonic Lithography for Maskless Fabrication of Large-Area Long-Range Ordered 2D Nanostructures.

This paper proposes a one-step maskless 2D nanopatterning approach named self-aligned plasmonic lithography (SPL) by line-shaped ultrafast laser ablation under atmospheric conditions for the first time. Through a theoretical calculation of electric field and experimental verification, we proved that homogeneous interference of laser-excited surface plasmon polaritons (SPPs) can be achieved and used to generate long-range ordered 2D nanostructures in a self-aligned way over a wafer-sized area within several minutes. Moreover, the self-aligned nanostructures can be freely transferred between embossed nanopillars and engraved nanoholes by modulating the excitation intensity of SPPs interference through altering the incident laser energy. The SPL technique exhibits further controllability in the shape, orientation, and period of achievable nanopatterns on a wide range of semiconductors and metals by tuning processing parameters. Nanopatterned films can further act as masks to transfer structures into other bulk materials, as demonstrated in silica.

Photolithographic realization of target nanostructures in 3D space by inverse design of phase modulation.

The mass production of precise three-dimensional (3D) nanopatterns has long been the ultimate goal of fabrication technology. While interference lithography and proximity-field nanopatterning (PnP) may provide partial solutions, their setup complexity and limited range of realizable structures, respectively, remain the main problems. Here, we tackle these challenges by applying an inverse design to the PnP process. Our inverse design platform based on the adjoint method can efficiently find optimal phase masks for diverse target lattices and motifs. We fabricate a 2D rectangular array of nanochannels, which has not been reported for conventional PnP with normally incident light, as a proof of concept. With further demonstration of material conversion, our work provides versatile platforms for nanomaterial fabrication.

Development and Proof of Concept of a Miniaturized MEMS Quantum Tunneling Accelerometer Based on PtC Tips by Focused Ion Beam 3D Nano-Patterning.

Most accelerometers today are based on the capacitive principle. However, further miniaturization for micro integration of those sensors leads to a poorer signal-to-noise ratio due to a small total area of the capacitor plates. Thus, other transducer principles should be taken into account to develop smaller sensors. This paper presents the development and realization of a miniaturized accelerometer based on the tunneling effect, whereas its highly sensitive effect regarding the tunneling distance is used to detect small deflections in the range of sub-nm. The spring-mass-system is manufactured by a surface micro-machining foundry process. The area of the shown polysilicon (PolySi) sensor structures has a size smaller than 100 µm × 50 µm (L × W). The tunneling electrodes are placed and patterned by a focused ion beam (FIB) and gas injection system (GIS) with MeCpPtMe3 as a precursor. A dual-beam system enables maximum flexibility for post-processing of the spring-mass-system and patterning of sharp tips with radii in the range of a few nm and initial distances between the electrodes of about 30–300 nm. The use of metal–organic precursor material platinum carbon (PtC) limits the tunneling currents to about 150 pA due to the high inherent resistance. The measuring range is set to 20 g. The sensitivity of the sensor signal, which depends exponentially on the electrode distance due to the tunneling effect, ranges from 0.4 pA/g at 0 g in the sensor operational point up to 20.9 pA/g at 20 g. The acceleration-equivalent thermal noise amplitude is calculated to be 2.4–3.4 mg/. Electrostatic actuators are used to lead the electrodes in distances where direct quantum tunneling occurs.

New perspectives on the roles of nanoscale surface topography in modulating intracellular signaling

The physical properties of biomaterials, such as elasticity, stiffness, and surface nanotopography, are mechanical cues that regulate a broad spectrum of cell behaviors, including migration, differentiation, proliferation, and reprogramming. Among them, nanoscale surface topography, i.e., nanotopography, defines the nanoscale shape and spatial arrangement of surface elements, which directly interact with the cell membranes and stimulate changes in the cell signaling pathways. In biological systems, the effects of nanotopography are often entangled with those of other mechanical and biochemical factors. Precise engineering of 2D nanopatterns and 3D nanostructures with well-defined features has provided a powerful means to study the cellular responses to specific topographic features. In this Review, we discuss efforts in the last three years to understand how nanotopography affects membrane receptor activation, curvature-induced cell signaling, and stem cell differentiation.

Artificial Double-Helix for Geometrical Control of Magnetic Chirality.

Chirality plays a major role in nature, from particle physics to DNA, and its control is much sought-after due to the scientific and technological opportunities it unlocks. For magnetic materials, chiral interactions between spins promote the formation of sophisticated swirling magnetic states such as skyrmions, with rich topological properties and great potential for future technologies. Currently, chiral magnetism requires either a restricted group of natural materials or synthetic thin-film systems that exploit interfacial effects. Here, using state-of-the-art nanofabrication and magnetic X-ray microscopy, we demonstrate the imprinting of complex chiral spin states via three-dimensional geometric effects at the nanoscale. By balancing dipolar and exchange interactions in an artificial ferromagnetic double-helix nanostructure, we create magnetic domains and domain walls with a well-defined spin chirality, determined solely by the chiral geometry. We further demonstrate the ability to create confined 3D spin textures and topological defects by locally interfacing geometries of opposite chirality. The ability to create chiral spin textures via 3D nanopatterning alone enables exquisite control over the properties and location of complex topological magnetic states, of great importance for the development of future metamaterials and devices in which chirality provides enhanced functionality.

Fabrication of Free-Standing Three-Dimensional Block Copolymer Patterns on Substrate

As the importance of three-dimensiona (3D) nano patterns and structures has recently emerged, interest in the study of 3D structures of block copolymers has increased. However, most existing studies on block copolymer 3D patterns on substrates are limited to simple 3D structures such as a multi-layered forms. In this study, we propose an experimental method for realizing free-standing 3D block copolymer patterns on substrates using an e-beam lithographic template and film transfer method. The block copolymer 3D structure formed in wide hole templates are similar to simple multi-layered structures; however, as the width of the hole template become narrower, more complex block copolymer 3D structures are formed in which the upper and lower layer structures are interconnected. Furthermore, we introduce a method to fabricate novel block copolymer structures in which the 2D planar structures are connected to 3D complex structures. Proposed 3D block copolymer fabrication method provides a framework for generation of unconventional 3D structures of block copolymer, which can be useful for next generation 3D devices.

Conformal 3D Nanopatterning by Block Copolymer Lithography with Vapor-Phase Deposited Neutral Adlayer.

Block copolymer (BCP) lithography is an effective nanopatterning methodology exploiting nanoscale self-assembled periodic patterns in BCP thin films. This approach has a critical limitation for nonplanar substrate geometry arising from the reflow and modification of BCP films upon the thermal or solvent annealing process, which is inevitable to induce the mobility of BCP chains for the self-assembly process. Herein, reflow-free, 3D BCP nanopatterning is demonstrated by introducing a conformally grown adlayer by the initiated chemical vapor deposition (iCVD) process. A highly cross-linked poly(divinylbenzene) layer was deposited directly onto the BCP thin film surface by iCVD, which effectively prevented the reflow of BCP thin film during an annealing process. BCP nanopatterns could be stabilized on various substrate geometry, including a nonplanar deformed polymer substrate, a pyramid shape substrate, and a graphene fiber surface. A fiber-type hydrogen evolution reaction (HER) catalyst is suggested by stabilizing lamellar Pt nanopatterns on severely rough graphene fiber surfaces.

Electrochemical nanoimprinting of silicon

Scalable nanomanufacturing enables the commercialization of nanotechnology, particularly in applications such as nanophotonics, silicon photonics, photovoltaics, and biosensing. Nanoimprinting lithography (NIL) was the first scalable process to introduce 3D nanopatterning of polymeric films. Despite efforts to extend NIL's library of patternable media, imprinting of inorganic semiconductors has been plagued by concomitant generation of crystallography defects during imprinting. Here, we use an electrochemical nanoimprinting process-called Mac-Imprint-for directly patterning electronic-grade silicon with 3D microscale features. It is shown that stamps made of mesoporous metal catalysts allow for imprinting electronic-grade silicon without the concomitant generation of porous silicon damage while introducing mesoscale roughness. Unlike most NIL processes, Mac-Imprint does not rely on plastic deformation, and thus, it allows for replicating hard and brittle materials, such as silicon, from a reusable polymeric mold, which can be manufactured by almost any existing microfabrication technique.

Semianalytical modeling of arbitrarily distributed quantum emitters embedded in nanopatterned hyperbolic metamaterials

Nanopatterned hyperbolic metamaterials (NPHMs) have proven to be efficient structures for enhancing the spontaneous emission rate (Γ) and quantum efficiency (η) of quantum emitters (QEs). However, many of the NPHM designs still rely on computationally costly 3D numerical simulations. In this context, we propose a fast, semianalytical method capable of calculating both Γ and η of QEs placed inside a medium bounded by nanopatterned structures. The low computational cost of our approach makes it attractive for optimizing the NPHMs’ geometrical parameters that maximize η for a desired Γ. Furthermore, we suggest a more realistic procedure to calculate the decay behavior of multiple QEs arbitrarily distributed in the NPHM. This calculation is only feasible with the knowledge of Γ and η mapped for all possible positions of the QEs, which is easily achieved with the proposed model. For the validation procedure, we compare the model results with those obtained by the FDTD method. We apply the proposed model to an NPHM composed of nine Ag/SiO2 layers, with the polymer host layer embedded with rhodamine 6G, to maximize η for a specified tenfold increase of Γ. This procedure allowed η to be increased by 69% and 170% for 1D and 2D nanopatterning, respectively. The time required to build the Γ and η maps (used in the calculation of the decay behavior) is reduced by approximately 96% when compared with those numerically calculated via FDTD.

Ultra-violet nanoimprint lithography-compatible positive-tone electron beam resist for 3D hybrid nanostructures

A fabrication technique to produce a 3D hybrid structure having different scale complex patterns is studied. A fabrication method is proposed and an ultra-violet nanoimprint lithography (UV-NIL) compatible positive-tone electron beam (EB) resist is prepared. The resist can be shown to have microstructures and nanostructures on the same surface by UV-NIL and positive-tone EB lithography (EBL). The resist can behave as a positive-tone EB resist after the UV-NIL process, exhibits a high UV-NIL and EBL resolution, and a sensitivity sufficient to fabricate 3D hybrid nanopatterns with a high aspect ratio. The fabrication of a micro lens array with nano anti-reflection structure is then performed. The resist can accurately duplicate a microscale concave lens array with a diameter of 3.8 μm. Nano anti-reflection structures are then directly fabricated by EBL on the lens array. The positive-tone dot pattern, as anti-reflection structures with diameters of 220 nm, are fabricated with an EB dose of 1000 μC/cm2. UV-NIL is used to duplicate the fabricated sample. The resist products can successfully be used as the master mold for UV-NIL. A convex micro lens array, with a 200 nm anti-reflection structure, is obtained. These methods using the resist can provide complex 3D hybrid nanostructures. The usefulness of this method and resist are discussed.

Maskless laser nano-lithography of glass through sequential activation of multi-threshold ablation

Controllable nanofabrication is at the very foundation of nano-science and nano-technology. Today, ultrafast laser writing has been broadly adopted for micro-fabrication because of its ability to make precise and rapid processing of almost all types of materials in an ambient environment. However, direct laser writing is typically unsuitable for high-quality 2D nano-patterning. In this work, we introduce a maskless laser nano-lithographic technique that allows us to create regular 2D periodic nanopatterns on glass. Glass is a particularly challenging material since it does not absorb light readily. Our strategy starts with a glass sample being coated with a thin layer of metal, and then irradiated with a series of pulse bursts at progressively increasing fluence levels. This process allows us to sequentially activate a series of tailored physical processes that lead to the formation of regular 2D periodic nanopatterns on glass. The formation mechanism of this nano-patterning is also simulated numerically and further corroborated by a series of control experiments. We also show controllability in forming various shapes and sizes of nanopatterns through tailored fluence doses. Our technique provides a high-speed and low-cost method for glass nanofabrication.

Three-Dimensional Nanomolds Fabrication for Nanoimprint Lithography

Abstract Nanoimprint lithography is a fabrication method by applying nanomolds on resists to form inversed patterns. It can be utilized to fabricate high-resolution nanopatterns in a low cost and rapid fashion on both flat and curved surfaces. The fidelity of fabricated master nanopatterns and elastomer nanomolds are essential to the quality of the nanoimprinted features. Although two-dimensional nanoimprint lithography has been widely investigated, there are limited researches about three-dimensional master nanopatterns and nanomolds. Even less of them discussed the fidelity of entire procedures throughout the initial master fabrication to the final nanoimprinting. In this paper, we demonstrated the effectiveness of our approach, atomic force microscope (AFM) based ultrasonic vibration assisted nanomachining, in fabricating three-dimensional master nanopatterns with complex contours and continuously height changes. Nanomolds for nanoimprint lithography were fabricated subsequently based on the master nanopatterns with a positive draft, which is naturally generated from the AFM tip. We investigated the fidelity of the nanomachining results and the residual effect of reusing mater patterns. Besides, we demonstrated the 3D nanoimprinting capability of the solvent-assisted microcontact molding process, discovering the possibility of tuning the layer-thickness of imprinted 3D nanopatterns.

2D Nanopatterning: 2D Metal Chalcogenide Nanopatterns by Block Copolymer Lithography (Adv. Funct. Mater. 50/2018)

In article number 1804508, Sang Ouk Kim and co-workers present a reliable and effective nanopatterning strategy for 2D transition metal dichalcogenides via block copolymer (BCP) lithography. By exploiting various BCP templates, diverse nanostructures, including nanomeshes, nanodots, and nanowire arrays, are demonstrated with wafer-scale scalability. Edge-exposed MoS2 nanomeshes exhibit superior sensitivity and reversibility for NO2 gas sensing.

Combination of thermal scanning probe lithography and ion etching to fabricate 3D silicon nanopatterns with extremely smooth surface

An emerging and extremely enabling route towards rapid prototyping of complex topographies with nanoscale precision and with highly controlled surface position is thermal scanning probe lithography. This technique, coupled to the recent advances in dry etching, allows pattern transfer into the target material at high vertical amplification and could therefore provide access to novel devices. However, the dry etch processing used to transfer the resist pattern into the structural material results in pronounced surface roughening, ultimately diminishing the functional quality of the nanostructures. To access the area of applications such as nano-photonics, nanoelectronics, nano-mechanics and nano-fluidics to name a few, a better control of surface morphology down to the sub-nm scale is a key request. Here, we explore different ion-beam polishing techniques for the surface roughness reduction of thermal scanning probe lithography patterns in silicon. Direct ion beam etch was optimized to smoothen thermal scanning probe lithography features and further extended to a polymer mask technique. The critical step of introducing the polymer mask for the ion beam etching enables extremely smooth three-dimensional surface contours at a sub-nanometer surface roughness.

Increased Efficiency of Solar Cells Protected by Hydrophobic and Hydrophilic Anti-Reflecting Nanostructured Glasses.

We investigated the fabrication of large-area (cm2) nanostructured glasses for solar cell modules with hydrophobic and hydrophilic properties using soft lithography and colloidal lithography. Both of these techniques entail low-cost and ease of nanofabrication. We explored the use of simple 1D and 2D nanopatterns (nanowires and nanocones) and the effect of introducing disorder in the nanostructures. We observed an increase in the transmitted light for ordered nanostructures with a maximum value of 99% for wavelengths >600 nm when ordered nanocones are fabricated on the two sides of the solar glass. They produced an increment in the efficiency of the packaged solar cell with respect to the glass without nanostructures. On the one hand, the wettability properties showed that the ordering of the nanostructures improved the hydrophobicity of the solar glasses and increased their self-cleaning capacity. On the other hand, the disordered nanostructures improved the hydrophilic properties of solar glasses, increasing their anti-fogging capacity. The results show that by selecting the appropriate nanopattern, the wettability properties (hydrophobic or hydrophilic) can be easily improved without decreasing the efficiency of the solar cell underneath.