Hybrid Metrology for Nanostructured Optical Metasurfaces.

Metasurfaces have garnered increasing research interest in recent years due to their remarkable advantages, such as efficient miniaturization and novel functionalities compared to traditional optical elements such as lenses and filters. These advantages have facilitated their rapid commercial deployment. Recent advancements in nanofabrication have enabled the reduction of optical metasurface dimensions to the nanometer scale, expanding their capabilities to cover visible wavelengths. However, the pursuit of large-scale manufacturing of metasurfaces with customizable functions presents challenges in controlling the dimensions and composition of the constituent dielectric materials. To address these challenges, the combination of block copolymer (BCP) self-assembly and sequential infiltration synthesis (SIS), offers an alternative for fabrication of high-resolution dielectric nanostructures with tailored composition and optical functionalities. However, the absence of metrological techniques capable of providing precise and reliable characterization of the refractive index of dielectric nanostructures persists. This study introduces a hybrid metrology strategy that integrates complementary synchrotron-based traceable X-ray techniques to achieve comprehensive material characterization for the determination of the refractive index on the nanoscale. To establish correlations between material functionality and their underlying chemical, compositional and dimensional properties, TiO2 nanostructures model systems were fabricated by SIS of BCPs. The results from synchrotron-based analyses were integrated into physical models, serving as a validation scheme for laboratory-scale measurements to determine effective refractive indices of the nanoscale dielectric materials.

Periodic Arrays of Dopants in Silicon by Ultralow Energy Implantation of Phosphorus Ions through a Block Copolymer Thin Film.

In this work, block copolymer lithography and ultralow energy ion implantation are combined to obtain nanovolumes with high concentrations of phosphorus atoms periodically disposed over a macroscopic area in a p-type silicon substrate. The high dose of implanted dopants grants a local amorphization of the silicon substrate. In this condition, phosphorus is activated by solid phase epitaxial regrowth (SPER) of the implanted region with a relatively low temperature thermal treatment preventing diffusion of phosphorus atoms and preserving their spatial localization. Surface morphology of the sample (AFM, SEM), crystallinity of the silicon substrate (UV Raman), and position of the phosphorus atoms (STEM- EDX, ToF-SIMS) are monitored during the process. Electrostatic potential (KPFM) and the conductivity (C-AFM) maps of the sample surface upon dopant activation are compatible with simulated I-V characteristics, suggesting the presence of an array of not ideal but working p-n nanojunctions. The proposed approach paves the way for further investigations on the possibility to modulate the dopant distribution within a silicon substrate at the nanoscale by changing the characteristic dimension of the self-assembled BCP film.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice.

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5-2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Thermal Imaging of Block Copolymers with Sub-10 nm Resolution.

Thermal silicon probes have demonstrated their potential to investigate the thermal properties of various materials at high resolution. However, a thorough assessment of the achievable resolution is missing. Here, we present a probe-based thermal-imaging technique capable of providing sub-10 nm lateral resolution at a sub-10 ms pixel rate. We demonstrate the resolution by resolving microphase-separated PS-b-PMMA block copolymers that self-assemble in 11 to 19 nm half-period lamellar structures. We resolve an asymmetry in the heat flux signal at submolecular dimensions and assess the ratio of heat flux into both polymers in various geometries. These observations are quantitatively compared with coarse-grained molecular simulations of energy transport that reveal an enhancement of transport along the macromolecular backbone and a Kapitza resistance at the internal interfaces of the self-assembled structure. This comparison discloses a tip-sample contact radius of a ≈ 4 nm and identifies combinations of enhanced intramolecular transport and Kapitza resistance.

Directed Self-Assembly of Block Copolymers for the Fabrication of Functional Devices.

Directed self-assembly of block copolymers is a bottom-up approach to nanofabrication that has attracted high interest in recent years due to its inherent simplicity, high throughput, low cost and potential for sub-10 nm resolution. In this paper, we review the main principles of directed self-assembly of block copolymers and give a brief overview of some of the most extended applications. We present a novel fabrication route based on the introduction of directed self-assembly of block copolymers as a patterning option for the fabrication of nanoelectromechanical systems. As a proof of concept, we demonstrate the fabrication of suspended silicon membranes clamped by dense arrays of single-crystal silicon nanowires of sub-10 nm diameter. Resulting devices can be further developed for building up high-sensitive mass sensors based on nanomechanical resonators.

Self-assembly of block copolymers under non-isothermal annealing conditions as revealed by grazing-incidence small-angle X-ray scattering.

An accurate knowledge of the parameters governing the kinetics of block copolymer self-assembly is crucial to model the time- and temperature-dependent evolution of pattern formation during annealing as well as to predict the most efficient conditions for the formation of defect-free patterns. Here, the self-assembly kinetics of a lamellar PS-b-PMMA block copolymer under both isothermal and non-isothermal annealing conditions are investigated by combining grazing-incidence small-angle X-ray scattering (GISAXS) experiments with a novel modelling methodology that accounts for the annealing history of the block copolymer film before it reaches the isothermal regime. Such a model allows conventional studies in isothermal annealing conditions to be extended to the more realistic case of non-isothermal annealing and prediction of the accuracy in the determination of the relevant parameters, namely the correlation length and the growth exponent, which define the kinetics of the self-assembly.

Exploring Strategies to Contact 3D Nano-Pillars.

This contribution explores different strategies to electrically contact vertical pillars with diameters less than 100 nm. Two process strategies have been defined, the first based on Atomic Force Microscope (AFM) indentation and the second based on planarization and reactive ion etching (RIE). We have demonstrated that both proposals provide suitable contacts. The results help to conclude that the most feasible strategy to be implementable is the one using planarization and reactive ion etching since it is more suitable for parallel and/or high-volume manufacturing processing.

Grain-Boundary-Induced Alignment of Block Copolymer Thin Films.

We present and discuss the capability of grain boundaries to induce order in block copolymer thin films between horizontally and vertically assembled block copolymer grains. The system we use as a proof of principle is a thermally annealed 23.4 nm full-pitch lamellar Polystyrene-block-polymethylmetacrylate (PS-b-PMMA) di-block copolymer. In this paper, grain-boundary-induced alignment is achieved by the mechanical removal of the neutral brush layer via atomic force microscopy (AFM). The concept is also confirmed by a mask-less e-beam direct writing process. An elongated grain of vertically aligned lamellae is trapped between two grains of horizontally aligned lamellae. This configuration leads to the formation of 90° twist grain boundaries. The features maintain their orientation on a characteristic length scale, which is described by the material's correlation length ξ. As a result of an energy minimization process, the block copolymer domains in the vertically aligned grain orient perpendicularly to the grain boundary. The energy-minimizing feature is the grain boundary itself. The width of the manipulated area (e.g., the horizontally aligned grain) does not represent a critical process parameter.

Geometric frustration in ordered lattices of plasmonic nanoelements.

Inspired by geometrically frustrated magnetic systems, we present the optical response of three cases of hexagonal lattices of plasmonic nanoelements. All of them were designed using a metal-insulator-metal configuration to enhance absorption of light, with elements in close proximity to exploit near-field coupling, and with triangular symmetry to induce frustration of the dipolar polarization in the gaps between neighboring structures. Both simulations and experimental results demonstrate that these systems behave as perfect absorbers in the visible and/or the near infrared. Besides, the numerical study of the time evolution shows that they exhibit a relatively extended time response over which the system fluctuates between localized and collective modes. It is of particular interest the echoed excitation of surface lattice resonance modes, which are still present at long times because of the geometric frustration inherent to the triangular lattice. It is worth noting that the excitation of collective modes is also enhanced in other types of arrays where dipolar excitations of the nanoelements are hampered by the symmetry of the array. However, we would like to emphasize that the enhancement in triangular arrays can be significantly larger because of the inherent geometric incompatibility of dipolar excitations and three-fold symmetry axes.

Role of Penetrability into a Brush-Coated Surface in Directed Self-Assembly of Block Copolymers.

High-density and high-resolution line and space patterns on surfaces are obtained by directed self-assembly of lamella-forming block copolymers (BCPs) using wide-stripe chemical guiding patterns. When the width of the chemical pattern is larger than the half-pitch of the BCP, the interaction energy between each BCP domain and the surface is crucial to obtain the desired segregated film morphology. We investigate how the intermixing between BCPs and polymer brush molecules on the surface influences the optimal surface and interface free energies to obtain a proper BCP alignment. We have found that computational models successfully predict the experimentally obtained guided patterns if the penetrability of the brush layer is taken into account instead of a hard, impenetrable surface. Experiments on directed self-assembly of lamella-forming poly(styrene- block-methyl methacrylate) using chemical guiding patterns corroborate the models used in the simulations, where the values of the surface free energy between the BCP and the guiding and background stripes are accurately determined using an experimental method based on the characterization of contact angles in droplets formed after dewetting of homopolymer blends.

Geometric frustration in a hexagonal lattice of plasmonic nanoelements.

We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems.

Nano-confinement of block copolymers in high accuracy topographical guiding patterns: modelling the emergence of defectivity due to incommensurability.

Extreme ultraviolet interference lithography (EUV-IL) is used to manufacture topographical guiding patterns to direct the self-assembly of block copolymers. High-accuracy silicon oxide-like patterns with trenches ranging from 68 nm to 117 nm width are fabricated by exposing a hydrogen silsesquioxane (HSQ) resist layer using EUV-IL. We investigate how the accuracy, the low line width roughness and the low line edge roughness of the resulting patterns allow achieving DSA line/space patterns of a PS-b-PMMA (polystyrene-block-poly methyl methacrylate) block copolymer of 11 nm half-pitch with low defectivity. We conduct an in-depth study of the dependence of the DSA pattern morphology on the trench width and on how the neutral brush covers the guiding pattern. We identify the relation between trench width and the emergence of defects with nanometer precision. Based on these studies, we develop a model that extends available free energy models, which allows us to predict the patterning process window.

Towards molecular electronic devices based on 'all-carbon' wires.

Nascent molecular electronic devices based on linear 'all-carbon' wires attached to gold electrodes through robust and reliable C-Au contacts are prepared via efficient in situ sequential cleavage of trimethylsilyl end groups from an oligoyne, Me3Si-(C[triple bond, length as m-dash]C)4-SiMe3 (1). In the first stage of the fabrication process, removal of one trimethylsilyl (TMS) group in the presence of a gold substrate, which ultimately serves as the bottom electrode, using a stoichiometric fluoride-driven process gives a highly-ordered monolayer, Au|C[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CSiMe3 (Au|C8SiMe3). In the second stage, treatment of Au|C8SiMe3 with excess fluoride results in removal of the remaining TMS protecting group to give a modified monolayer Au|C[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CH (Au|C8H). The reactive terminal C[triple bond, length as m-dash]C-H moiety in Au|C8H can be modified by 'click' reactions with (azidomethyl)ferrocene (N3CH2Fc) to introduce a redox probe, to give Au|C6C2N3HCH2Fc. Alternatively, incubation of the modified gold substrate supported monolayer Au|C8H in a solution of gold nanoparticles (GNPs), results in covalent attachment of GNPs on top of the film via a second alkynyl carbon-Au σ-bond, to give structures Au|C8|GNP in which the monolayer of linear, 'all-carbon' C8 chains is sandwiched between two macroscopic gold contacts. The covalent carbon-surface bond as well as the covalent attachment of the metal particles to the monolayer by cleavage of the alkyne C-H bond is confirmed by surface-enhanced Raman scattering (SERS). The integrity of the carbon chain in both Au|C6C2N3HCH2Fc systems and after formation of the gold top-contact electrode in Au|C8|GNP is demonstrated through electrochemical methods. The electrical properties of these nascent metal-monolayer-metal devices Au|C8|GNP featuring 'all-carbon' molecular wires were characterised by sigmoidal I-V curves, indicative of well-behaved junctions free of short circuits.

Quantification of nanomechanical properties of surfaces by higher harmonic monitoring in amplitude modulated AFM imaging.

The determination of nanomechanical properties is an intensive topic of study in several fields of nanophysics, from surface and materials science to biology. At the same time, amplitude modulation force microscopy is one of the most established techniques for nanoscale characterization. In this work, we combine these two topics and propose a method able to extract quantitative nanomechanical information from higher harmonic amplitude imaging in atomic force microscopy. With this method it is possible to discriminate between different materials in the stiffness range of 1-3 GPa, in our case thin films of PS-PMMA based block copolymers. We were able to obtain a critical lateral resolution of less than 20 nm and discriminate between materials with less than a 1 GPa difference in modulus. We show that within this stiffness range, reliable values of the Young's moduli can be obtained under usual imaging conditions and with standard dynamic AFM probes.

Identifying the nature of surface chemical modification for directed self-assembly of block copolymers.

In recent years, block copolymer lithography has emerged as a viable alternative technology for advanced lithography. In chemical-epitaxy-directed self-assembly, the interfacial energy between the substrate and each block copolymer domain plays a key role on the final ordering. Here, we focus on the experimental characterization of the chemical interactions that occur at the interface built between different chemical guiding patterns and the domains of the block copolymers. We have chosen hard X-ray high kinetic energy photoelectron spectroscopy as an exploration technique because it provides information on the electronic structure of buried interfaces. The outcome of the characterization sheds light onto key aspects of directed self-assembly: grafted brush layer, chemical pattern creation and brush/block co-polymer interface.

High surface coverage of a self-assembled monolayer by in situ synthesis of palladium nanodeposits.

Nascent metal|monolayer|metal devices have been fabricated by depositing palladium, produced through a CO-confined growth method, onto a self-assembled monolayer of an amine-terminated oligo(phenylene ethynylene) derivative on a gold bottom electrode. The high surface area coverage (85%) of the organic monolayer by densely packed palladium particles was confirmed by X-ray photoemission spectroscopy (XPS) and atomic force microscopy (AFM). The electrical properties of these nascent Au|monolayer|Pd assemblies were determined from the I-V curves recorded with a conductive-AFM using the Peak Force Tunneling AFM (PF-TUNA™) mode. The I-V curves together with the electrochemical experiments performed rule out the formation of short-circuits due to palladium penetration through the monolayer, suggesting that the palladium deposition strategy is an effective method for the fabrication of molecular junctions without damaging the organic layer.

Suspended tungsten-based nanowires with enhanced mechanical properties grown by focused ion beam induced deposition.

The implementation of three-dimensional (3D) nano-objects as building blocks for the next generation of electro-mechanical, memory and sensing nano-devices is at the forefront of technology. The direct writing of functional 3D nanostructures is made feasible by using a method based on focused ion beam induced deposition (FIBID). We use this technique to grow horizontally suspended tungsten nanowires and then study their nano-mechanical properties by three-point bending method with atomic force microscopy. These measurements reveal that these nanowires exhibit a yield strength up to 12 times higher than that of the bulk tungsten, and near the theoretical value of 0.1 times the Young's modulus (E). We find a size dependence of E that is adequately described by a core-shell model, which has been confirmed by transmission electron microscopy and compositional analysis at the nanoscale. Additionally, we show that experimental resonance frequencies of suspended nanowires (in the MHz range) are in good agreement with theoretical values. These extraordinary mechanical properties are key to designing electro-mechanically robust nanodevices based on FIBID tungsten nanowires.

Functional dependence of resonant harmonics on nanomechanical parameters in dynamic mode atomic force microscopy.

We present a combined theoretical and experimental study of the dependence of resonant higher harmonics of rectangular cantilevers of an atomic force microscope (AFM) as a function of relevant parameters such as the cantilever force constant, tip radius and free oscillation amplitude as well as the stiffness of the sample's surface. The simulations reveal a universal functional dependence of the amplitude of the 6th harmonic (in resonance with the 2nd flexural mode) on these parameters, which can be expressed in terms of a gun-shaped function. This analytical expression can be regarded as a practical tool for extracting qualitative information from AFM measurements and it can be extended to any resonant harmonics. The experiments confirm the predicted dependence in the explored 3-45 N/m force constant range and 2-345 GPa sample's stiffness range. For force constants around 25 N/m, the amplitude of the 6th harmonic exhibits the largest sensitivity for ultrasharp tips (tip radius below 10 nm) and polymers (Young's modulus below 20 GPa).

Design and Synthesis of Aviram-Ratner-Type Dyads and Rectification Studies in Langmuir-Blodgett (LB) Films.

The design and synthesis of Aviram-Ratner-type molecular rectifiers, featuring an anilino-substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the σ-spacer bicyclo[2.2.2]octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X-ray analysis. The photophysical properties of the TTF-BCO-exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir-Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current-voltage (I-V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I-V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor.

Nanoparticles with tunable shape and composition fabricated by nanoimprint lithography.

Cone-like and empty cup-shaped nanoparticles of noble metals have been demonstrated to provide extraordinary optical properties for use as optical nanoanntenas or nanoresonators. However, their large-scale production is difficult via standard nanofabrication methods. We present a fabrication approach to achieve arrays of nanoparticles with tunable shape and composition by a combination of nanoimprint lithography, hard-mask definition and various forms of metal deposition. In particular, we have obtained arrays of empty cup-shaped Au nanoparticles showing an optical response with distinguishable features associated with the excitations of localized surface plasmons. Finally, this route avoids the most common drawbacks found in the fabrication of nanoparticles by conventional top-down methods, such as aspect ratio limitation, blurring, and low throughput, and it can be used to fabricate nanoparticles with heterogeneous composition.