Large-Area Flexible Memory Arrays of Oriented Molecular Ferroelectric Single Crystals with Nearly Saturated Polarization.

Molecular ferroelectrics (MFs) have been proven to demonstrate excellent properties even comparable to those of inorganic counterparts usually with heavy metals. However, the validation of their device applications is still at the infant stage. The polycrystalline feature of conventionally obtained MF films, the patterning challenges for microelectronics and the brittleness of crystalline films significantly hinder their development for organic integrated circuits, as well as emerging flexible electronics. Here, a large-area flexible memory array is demonstrated of oriented molecular ferroelectric single crystals (MFSCs) with nearly saturated polarization. Highly-uniform MFSC arrays are  prepared on large-scale substrates including Si wafers and flexible substrates using an asymmetric-wetting and microgroove-assisted coating (AWMAC) strategy. Resultant flexible memory arrays exhibit excellent nonvolatile memory properties with a low-operating voltage of <5 V, i.e., nearly saturated ferroelectric polarization (6.5 µC cm-2 ), and long bending endurance (>103 ) under various bending radii. These results may open an avenue for scalable flexible MF electronics with high performance.

Roadmap on emerging hardware and technology for machine learning.

Recent progress in artificial intelligence is largely attributed to the rapid development of machine learning, especially in the algorithm and neural network models. However, it is the performance of the hardware, in particular the energy efficiency of a computing system that sets the fundamental limit of the capability of machine learning. Data-centric computing requires a revolution in hardware systems, since traditional digital computers based on transistors and the von Neumann architecture were not purposely designed for neuromorphic computing. A hardware platform based on emerging devices and new architecture is the hope for future computing with dramatically improved throughput and energy efficiency. Building such a system, nevertheless, faces a number of challenges, ranging from materials selection, device optimization, circuit fabrication and system integration, to name a few. The aim of this Roadmap is to present a snapshot of emerging hardware technologies that are potentially beneficial for machine learning, providing the Nanotechnology readers with a perspective of challenges and opportunities in this burgeoning field.

Enhanced microphase separation of thin films of low molecular weight block copolymer by the addition of an ionic liquid.

We report on the use of a selective, non-volatile ionic liquid (IL) to enhance the self-assembly via solvent annealing of a low molecular weight block copolymer (BCP) of styrene and 2-vinylpyridine (2VP) suitable for generating sub-10 nm features. Diblock and triblock copolymers of different molecular weights of styrene and 2VP are individually blended with the IL and then solvent annealed in acetone, a non-preferential solvent for the BCPs. Differential scanning calorimetry indicates that the IL selectively resides in the 2VP block of the BCP, resulting in a decrease of the block's Tg and an increase of the effective Flory-Huggins parameter (χeff) of the BCP. The influence of the IL on the non-preferential window of a random copolymer brush used to treat the substrate for self-assembly of the BCPs is also analyzed. Well-defined lamellar patterns form when the optimal weight ratio of IL (∼1%) is added to the BCPs. A detailed analysis of the orientational correlation length and pitch size of the BCPs quantitatively shows that the addition of the IL enhanced the microphase separation of the low molecular weight version of the BCP. Subsequent treatment of the self-assembled BCP with sequential infiltration synthesis yields sub-10 nm AlOx lines.

Combining double patterning with self-assembled block copolymer lamellae to fabricate 10.5 nm full-pitch line/space patterns.

Directed self-assembly of block copolymers and self-aligned double patterning are two commonly used pitch scaling techniques to increase the density of lithographic features. In this work, both of these pitch scaling techniques were combined, enabling patterning at even higher densities. In this process, directed self-assembly of a high-χ block copolymer was used to form a line/space pattern, which served as a template for mandrels. Via these mandrels, atomic layer deposition was used to deposit a thin aluminium oxide spacer. By this method, a total pitch scaling factor of 8, equivalent to a 10.5 nm full pitch, was reached. The types of defects and the line roughness at the different steps of the process were discussed.

Directed Self-Assembly of Polystyrene-b-poly(propylene carbonate) on Chemical Patterns via Thermal Annealing for Next Generation Lithography.

Directed self-assembly (DSA) of block copolymers (BCPs) combines advantages of conventional photolithography and polymeric materials and shows competence in semiconductors and data storage applications. Driven by the more integrated, much smaller and higher performance of the electronics, however, the industry standard polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) in DSA strategy cannot meet the rapid development of lithography technology because its intrinsic limited Flory-Huggins interaction parameter (χ). Despite hundreds of block copolymers have been developed, these BCPs systems are usually subject to a trade-off between high χ and thermal treatment, resulting in incompatibility with the current nanomanufacturing fab processes. Here we discover that polystyrene-b-poly(propylene carbonate) (PS-b-PPC) is well qualified to fill key positions on DSA strategy for the next-generation lithography. The estimated χ-value for PS-b-PPC is 0.079, that is, two times greater than PS-b-PMMA (χ = 0.029 at 150 °C), while processing the ability to form perpendicular sub-10 nm morphologies (cylinder and lamellae) via the industry preferred thermal-treatment. DSA of lamellae forming PS-b-PPC on chemoepitaxial density multiplication demonstrates successful sub-10 nm long-range order features on large-area patterning for nanofabrication. Pattern transfer to the silicon substrate through industrial sequential infiltration synthesis is also implemented successfully. Compared with the previously reported methods to orientation control BCPs with high χ-value (including solvent annealing, neutral top-coats, and chemical modification), the easy preparation, high χ value, and etch selectivity while enduring thermal treatment demonstrates PS-b-PPC as a rare and valuable candidate for advancing the field of nanolithography.

The One-Pot Directed Assembly of Cylinder-Forming Block Copolymer on Adjacent Chemical Patterns for Bimodal Patterning.

The direct self-assembly of cylinder-forming poly(styrene-block-methyl-methacrylate) (PS-b-PMMA) block copolymer is successfully assembled into two orientations, according to the underlying guiding pattern in different areas. Lying-down and perpendicular cylinders are formed, respectively, depending on the design of chemical pattern: sparse line/space pattern or hexagonal dot array. The first chemical pattern composed of prepatterned cross-linked polystyrene (XPS) line/space structure has a period (LS ) equal to twice the intercylinder period of the block copolymer (L0 ). The PS-b-PMMA thin film on the prepared chemical template after thermal annealing forms a lying-down cylinder morphology when the width of the PS strips is less than the width of PS block in the PS-b-PMMA block copolymer. The morphology is only applicable at the discrete thickness of the PS-b-PMMA film. In addition to forming the lying-down cylinders directly on the XPS guiding pattern, the cylinder-forming block copolymer can also be assembled in a perpendicular way on the second guiding pattern (the hexagonal dot array). The block copolymer films are registered into two orientations in a single directed self-assembly process. The features of the assembled patterns are successfully transferred down to the silicon oxide substrate.

Directed self-assembly of high-chi block copolymer for nano fabrication of bit patterned media via solvent annealing.

We report the formation of nanoimprint master templates that can be used for the fabrication of bit patterned media (BPM). The template was formed by directed self-assembly, with solvent annealing, of a symmetric ABA triblock copolymer to form perpendicularly oriented lamellae on chemical patterns. We used a high-χ block copolymer, poly(2-vinyl pyridine)-block-polystyrene-block-poly(2-vinyl pyridine) to achieve smaller feature sizes than are possible with polystyrene-block-poly(methyl methacrylate). The work shows that triblock copolymers can provide a large processing window in terms of pitch commensurability. Using block-selective infiltration (atomic layer deposition with sequential long soaking/purge cycles), an alumina composite with high etch resistance was specifically incorporated into the polar and hydrophilic P2VP domains. Subsequently, the surface pattern was successfully transferred into underlying Si substrates by etching with a fluorine-containing plasma to create a nanoimprint master. The line/space pattern of the nanoimprint master met the BPM fabrication requirement of defectivity <10(-3). For demonstration purposes, the nanoimprint master was used to imprint a replica pattern of photoresist on a quartz wafer.

Revealing the interfacial self-assembly pathway of large-scale, highly-ordered, nanoparticle/polymer monolayer arrays at an air/water interface.

The pathway of interfacial self-assembly of large-scale, highly ordered 2D nanoparticle/polymer monolayer or bilayer arrays from a toluene solution at an air/water interface was investigated using grazing-incidence small-angle scattering at a synchrotron source. Interfacial-assembly of the ordered nanoparticle/polymer array was found to occur through two stages: formation of an incipient randomly close-packed interfacial monolayer followed by compression of the monolayer to form a close-packed lattice driven by solvent evaporation from the polymer. Because the nanoparticles are hydrophobic, they localize exclusively to the polymer-air interface during self-assembly, creating a through thickness asymmetric film as confirmed by X-ray reflectivity. The interfacial self-assembly approach can be extended to form binary NP/polymer arrays. It is anticipated that by understanding the interfacial self-assembly pathway, this simple evaporative procedure could be conducted as a continuous process amenable to large area nanoparticle-based manufacturing needed for emerging energy technologies.

InAs nanowires grown by metal-organic vapor-phase epitaxy (MOVPE) employing PS/PMMA diblock copolymer nanopatterning.

Dense arrays of indium arsenide (InAs) nanowire materials have been grown by selective-area metal-organic vapor-phase epitaxy (SA-MOVPE) using polystyrene-b-poly(methyl methacrylate) (PS/PMMA) diblock copolymer (DBC) nanopatterning technique, which is a catalyst-free approach. Nanoscale openings were defined in a thin (~10 nm) SiNx layer deposited on a (111)B-oriented GaAs substrate using the DBC process and CF4 reactive ion etching (RIE), which served as a hard mask for the nanowire growth. InAs nanowires with diameters down to ~ 20 nm and micrometer-scale lengths were achieved with a density of ~ 5 × 10(10) cm(2). The nanowire structures were characterized by scanning electron microscopy and transmission electron microscopy, which indicate twin defects in a primary zincblende crystal structure and the absence of threading dislocation within the imaged regions.

Focused solar annealing for block copolymer fast self-assembly.

Block copolymer (BCP) self-assembly has tremendous potential applications in next-generation nanolithography. It offers significant advantages, including high resolution and cost-effectiveness, effectively overcoming the limitations associated with conventional optical lithography. In this work, we demonstrate a focused solar annealing (FSA) technique that is facile, eco-friendly, and energy-efficient for fast self-assembly of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films. The FSA principle involves utilizing a common biconvex lens to converge incident solar radiation into a high-temperature spot, which is directly used to drive the microphase separation of PS-b-PMMA thin films. As a result, PS-b-PMMA undergoes self-assembly, forming ordered nanostructures in a vertical orientation at seconds timescales on silicon substrates with a neutral layer. In addition, the FSA technique can be employed for grafting neutral polymer brushes onto the silicon substrate. Furthermore, the FSA's compatibility with graphoepitaxy-directed self-assembly (DSA) of BCP is also demonstrated in the patterning of contact holes. The results of contact hole shrinking show that contact hole prepatterns of ∼60.4 nm could be uniformly shrunk to ∼20.5 nm DSA hole patterns with a hole open yield (HOY) of 100 %. For contact hole multiplication, doublet DSA holes were successfully generated on elliptical templates, revealing an average DSA hole size of ∼21.3 nm. Most importantly, due to the direct use of solar energy, the FSA technique provides many significant advantages such as simplicity, environmental friendliness, solvent-free, low cost, and net-zero carbon emissions, and will open up a new direction for BCP lithography that is sustainable, pollution-free, and carbon-neutral.

Manipulating the processing window of directed self-assembly in contact hole shrinking with binary block copolymer/homopolymer blending.

Directed self-assembly (DSA) lithography has demonstrated significant potential in fabricating integrated circuits. However, DSA encounters limited processing windows due to the requirement for precise matching between the period of block copolymers (BCPs) and graphoepitaxy templates. We propose a binary BCP/homopolymer blending strategy to manipulate the self-assembly behavior and the processing window of graphoepitaxy DSA in contact hole shrinking. By carefully tailoring the blending rates of poly(methyl methacrylate) (PMMA) with different molecular weights in cylindrical polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA), we manipulate the period and morphology of BCP/homopolymer self-assembly. Specifically, we employ BCP/homopolymer blending to fine-tune the critical dimension (CD) of contact holes with PS-affined topographical templates. Subsequent pattern transferring is achieved by selectively etching defect-free shrinkable cylinders as hard masks. Furthermore, self-consistent field theory (SCFT) simulation was employed to explore the self-assembly of BCP/homopolymer blending in confined cylindrical space and the results were in good consistency with the experimental results.

Deep learning-based activity recognition and fine motor identification using 2D skeletons of cynomolgus monkeys.

Video-based action recognition is becoming a vital tool in clinical research and neuroscientific study for disorder detection and prediction. However, action recognition currently used in non-human primate (NHP) research relies heavily on intense manual labor and lacks standardized assessment. In this work, we established two standard benchmark datasets of NHPs in the laboratory: MonkeyinLab (MiL), which includes 13 categories of actions and postures, and MiL2D, which includes sequences of two-dimensional (2D) skeleton features. Furthermore, based on recent methodological advances in deep learning and skeleton visualization, we introduced the MonkeyMonitorKit (MonKit) toolbox for automatic action recognition, posture estimation, and identification of fine motor activity in monkeys. Using the datasets and MonKit, we evaluated the daily behaviors of wild-type cynomolgus monkeys within their home cages and experimental environments and compared these observations with the behaviors exhibited by cynomolgus monkeys possessing mutations in the MECP2 gene as a disease model of Rett syndrome (RTT). MonKit was used to assess motor function, stereotyped behaviors, and depressive phenotypes, with the outcomes compared with human manual detection. MonKit established consistent criteria for identifying behavior in NHPs with high accuracy and efficiency, thus providing a novel and comprehensive tool for assessing phenotypic behavior in monkeys.

Concentrated Solar Induced Graphene.

Graphene is one of the most promising nanomaterials with many extraordinary properties and numerous exciting applications. In this work, a green, facile, and rapid method was developed to prepare graphene directly from common biomass materials such as banana peels, cantaloupe peels, coconut peels, and orange peels by using concentrated solar radiation. The basic principle of this method is photothermal conversion. On a sunny day, the sunlight was concentrated by a biconvex lens to form a focused light spot with a high temperature above 1000 °C, which can directly convert fruit peels into graphene nanosheets within 2-3 s. The product is named concentrated-solar-induced graphene (CSIG) based on the process employed to generate it. The resulting CSIG was characterized using a range of analytical techniques. The Raman spectrum of the CSIG displayed two distinct peaks corresponding to the D and G bands at ∼1343 and ∼1568 cm-1, respectively. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to confirm that the CSIG consists of a few layers of turbostratic graphene nanosheets. Atomic force microscopy characterization revealed that the CSIG nanosheets have a thickness of ∼4 nm. The antibacterial potential of the CSIG was also explored. The CSIG had a strong inhibitory effect on the growth of Escherichia coli. This simple, green, and straightforward method for producing graphene may open a new route for turning waste into useful materials: an inexhaustible and pollution-free natural resource can be readily exploited by using a solar tracker-lens system for the large-scale production of graphene materials directly from low-cost biomass materials.

Dewetting-Assisted Patterning of Organic Semiconductors for Micro-OLED Arrays with a Pixel Size of 1 µm.

The emergence of near-eye displays, such as head-mounted displays, is triggering a requirement for highly enhanced display resolution. High-resolution micro-displays with micro-organic light-emitting diodes (micro-OLEDs) can be a preferential candidate, owing to the mature industrialization of OLEDs along with the advantages of flexibility, light weight, and ease of processing. However, micro-OLEDs with pixel sizes down to micrometers are difficult to be achieved using conventional techniques such as fine metal mask evaporation and lithography. Here, a solution-processing approach to pattern organic semiconductors (OSCs) for micro-OLED arrays with the assistance of templated dewetting is demonstrated. Solvents containing organic functional materials are dewetted on the surface with hydrophobic/hydrophilic patterns to form ordered droplet arrays using dip-coating. Subsequently, patterned OSC films are produced by effectively controlling solvent evaporation. Micro-OLED arrays with a pixel size down to 1 µm are successfully fabricated by further deposition of emitting/electron transport layers and top electrodes. This approach can open an avenue for low-cost manufacturing of flexible and high-resolution micro-displays.

Transformation of a close-packed Au nanoparticle/polymer monolayer into a large area array of oriented Au nanowires via E-beam promoted uniaxial deformation and room temperature sintering.

Transformation of 2D Au nanoparticle (NP) arrays into large scale, ordered, and oriented nanorod/nanowire arrays supported on a transferrable polymer film has been accomplished. E-beam irradiation followed by room temperature aging of a suspended Au NP/polymethylmethacrylate (PMMA) polymer close packed monolayer results in one-dimensional nanoparticle aggregation, reorientation, and sintering into a high density array of oriented Au nanowires with coherent single-crystal-like interfaces. Molecular dynamics simulations of alkane-thiol capped Au NPs, interacting through the Vincent potential and undergoing 2D Poisson compression, account semiquantitatively for the qualitative features of the transformation. This fabrication approach should be extendable to directing 1D aggregation of highly anisotropic nanostructures in arbitrary NP systems.

Self-Aligned Assembly of a Poly(2-vinylpyridine)-b-Polystyrene-b-Poly(2-vinylpyridine) Triblock Copolymer on Graphene Nanoribbons.

Directed self-assembly (DSA) of block copolymers is one of the most promising patterning techniques for patterning sub-10 nm features. However, at such small feature sizes, it is becoming increasingly difficult to fabricate the guiding pattern for the DSA process, and it is necessary to explore alternative guiding methods for DSA to achieve long-range ordered alignment. Here, we report the self-aligned assembly of a triblock copolymer, poly(2-vinylpyridine)-b-polystyrene-b-poly(2-vinylpyridine) (P2VP-b-PS-b-P2VP) on neutral graphene nanoribbons with the gap consisting of a P2VP-preferential silicon oxide (SiO2) substrate via solvent vapor annealing. The assembled P2VP-b-PS-b-P2VP demonstrated long-range, one-dimensional alignment on the graphene substrate in a direction perpendicular to the boundary of the graphene and substrate with a half-pitch size of 8 nm, which greatly alleviates the lithography resolution required for traditional chemoepitaxy DSA. A wide processing window is demonstrated with the gap between graphene stripes varying from 10 to 100 nm, overcoming the restriction on widths of guiding patterns to have commensurate domain spacing. When the gap was reduced to 10 nm, P2VP-b-PS-b-P2VP formed a straight-line pattern on both the graphene and the substrate. Monte Carlo simulations showed that the self-aligned assembly of the triblock copolymer on the graphene nanoribbons is guided at the boundary of parallel and perpendicular lamellae on graphene and SiO2, respectively. Simulations also indicate that the swelling of a system allows for rapid rearrangement of chains and quickly anneal any misaligned grains and defects. The effect of the interaction strength between SiO2 and P2VP on the self-assembly is systematically investigated in simulations.

Three-Dimensional PrGO-Based Sandwich Composites With MoS2 Flowers as Stuffings for Superior Lithium Storage.

Graphene-based MoS2 nanocomposites are expected to be promising anode materials for lithium ion batteries because of their large specific capacity and high conductivity. However, the aggregation of graphene and the weak interaction between the two components hinder their practical application. Inspired by the sandwich structure, novel three-dimensional flower-like MoS2-PrGO sandwich composites were proposed as an advanced anode material for lithium-ion batteries. The separated 2D ultrathin rGO nano-sheets were connected by PEO chains and assembled into a well-organized 3D layered spatial structure, which not only avoids the aggregation of graphene but also accommodates a high mass loading of the micro-scale MoS2 nano-flowers. MoS2 nano-flowers with open architecture deliver large specific area. The rGO interlayers act as a conductive framework, making all flower-like MoS2 nano-stuffing electrochemically active. The ultra-thin 2D nano-sheets provide excellent cycle stability due to their neglectable volume changes during cycling. The 3D flower-like MoS2-PrGO sandwich composites deliver high energy density, excellent conductivity and stable cyclic performance during charge-discharge process. With a nearly 100% coulombic efficiency, their reversible capacity is retained at 1,036 mA h g-1 even after 500 cycles at current densities of 100 mA g-1. This novel design strategy provides a broad prospect for the development of advanced anode materials for superior lithium storage.

Boundary-directed epitaxy of block copolymers.

Directed self-assembly of block copolymers (BCPs) enables nanofabrication at sub-10 nm dimensions, beyond the resolution of conventional lithography. However, directing the position, orientation, and long-range lateral order of BCP domains to produce technologically-useful patterns is a challenge. Here, we present a promising approach to direct assembly using spatial boundaries between planar, low-resolution regions on a surface with different composition. Pairs of boundaries are formed at the edges of isolated stripes on a background substrate. Vertical lamellae nucleate at and are pinned by chemical contrast at each stripe/substrate boundary, align parallel to boundaries, selectively propagate from boundaries into stripe interiors (whereas horizontal lamellae form on the background), and register to wide stripes to multiply the feature density. Ordered BCP line arrays with half-pitch of 6.4 nm are demonstrated on stripes >80 nm wide. Boundary-directed epitaxy provides an attractive path towards assembling, creating, and lithographically defining materials on sub-10 nm scales.

Pathways to Mesoporous Resin/Carbon Thin Films with Alternating Gyroid Morphology.

Three-dimensional (3D) mesoporous thin films with sub-100 nm periodic lattices are of increasing interest as templates for a number of nanotechnology applications, yet are hard to achieve with conventional top-down fabrication methods. Block copolymer self-assembly derived mesoscale structures provide a toolbox for such 3D template formation. In this work, single (alternating) gyroidal and double gyroidal mesoporous thin-film structures are achieved via solvent vapor annealing assisted co-assembly of poly(isoprene-block-styrene-block-ethylene oxide) (PI-b-PS-b-PEO, ISO) and resorcinol/phenol formaldehyde resols. In particular, the alternating gyroid thin-film morphology is highly desirable for potential template backfilling processes as a result of the large pore volume fraction. In situ grazing-incidence small-angle X-ray scattering during solvent annealing is employed as a tool to elucidate and navigate the pathway complexity of the structure formation processes. The resulting network structures are resistant to high temperatures provided an inert atmosphere. The thin films have tunable hydrophilicity from pyrolysis at different temperatures, while pore sizes can be tailored by varying ISO molar mass. A transfer technique between substrates is demonstrated for alternating gyroidal mesoporous thin films, circumventing the need to re-optimize film formation protocols for different substrates. Increased conductivity after pyrolysis at high temperatures demonstrates that these gyroidal mesoporous resin/carbon thin films have potential as functional 3D templates for a number of nanomaterials applications.

Sub-10-nm patterning via directed self-assembly of block copolymer films with a vapour-phase deposited topcoat.

Directed self-assembly (DSA) of the domain structure in block copolymer (BCP) thin films is a promising approach for sub-10-nm surface patterning. DSA requires the control of interfacial properties on both interfaces of a BCP film to induce the formation of domains that traverse the entire film with a perpendicular orientation. Here we show a methodology to control the interfacial properties of BCP films that uses a polymer topcoat deposited by initiated chemical vapour deposition (iCVD). The iCVD topcoat forms a crosslinked network that grafts to and immobilizes BCP chains to create an interface that is equally attractive to both blocks of the underlying copolymer. The topcoat, in conjunction with a chemically patterned substrate, directs the assembly of the grating structures in BCP films with a half-pitch dimension of 9.3 nm. As the iCVD topcoat can be as thin as 7 nm, it is amenable to pattern transfer without removal. The ease of vapour-phase deposition, applicability to high-resolution BCP systems and integration with pattern-transfer schemes are attractive properties of iCVD topcoats for industrial applications.