Hydroxypropyl Cellulose Adhesives for Transfer Printing of Carbon Nanotubes and Metallic Nanostructures.

Transfer printing is one of the key nanofabrication techniques for the large-scale manufacturing of complex device architectures. It provides a cost-effective and high-throughput route for the integration of independently processed materials into spatially tailored architectures. Furthermore, this method enables the fabrication of flexible and curvilinear devices, paving the way for the fabrication of a new generation of technologies for optics, electronics, and biomedicine. In this work, hydroxypropyl cellulose (HPC) membranes are used as water soluble adhesives for transfer printing processes with improved performance and versatility compared to conventional silicone alternatives. The high-water solubility and excellent mechanical properties of HPC facilitate transfer printing with high yield for both metal and carbon nanotubes (CNTs) inks. In the case of metal inks, crack-free stripping of silver films and the simple fabrication of Moiré Plasmonic architectures of different geometries are demonstrated. Furthermore, HPC membranes are used to transfer print carbon nanotube films with different thicknesses and up to 77% transparency in the visible and near infrared region with potential applications as transparent conductive substrates. Finally, the use of prepatterned HPC membranes enables nanoscale patterning of CNT with feature resolution down to 1 µm.

Energy Transfer and Interference by Collective Electromagnetic Coupling.

The physics of collective optical response of molecular assemblies, pioneered by Dicke in 1954, has long been at the center of theoretical and experimental scrutiny. The influence of the environment on such phenomena is also of great interest due to various important applications in, e.g., energy conversion devices. In this Letter, we demonstrate both experimentally and theoretically the spatial modulations of the collective decay rates of molecules placed in proximity to a metal interface. We show in a very simple framework how the cooperative optical response can be analyzed in terms of intermolecular correlations causing interference between the response of different molecules and the polarization induced on a nearby metallic boundary and predict similar collective interference phenomena in excitation energy transfer between molecular aggregates.

Templated-Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission.

Perovskite nanocrystals (NCs) have revolutionized optoelectronic devices because of their versatile optical properties. However, controlling and extending these functionalities often requires a light-management strategy involving additional processing steps. Herein, we introduce a simple approach to shape perovskite nanocrystals (NC) into photonic architectures that provide light management by directly shaping the active material. Pre-patterned polydimethylsiloxane (PDMS) templates are used for the template-induced self-assembly of 10 nm CsPbBr3 perovskite NC colloids into large area (1 cm2 ) 2D photonic crystals with tunable lattice spacing, ranging from 400 nm up to several microns. The photonic crystal arrangement facilitates efficient light coupling to the nanocrystal layer, thereby increasing the electric field intensity within the perovskite film. As a result, CsPbBr3 2D photonic crystals show amplified spontaneous emission (ASE) under lower optical excitation fluences in the near-IR, in contrast to equivalent flat NC films prepared using the same colloidal ink. This improvement is attributed to the enhanced multi-photon absorption caused by light trapping in the photonic crystal.

Coherent phonon-grain boundary scattering in silicon inverse opals.

We report measurements and modeling of thermal conductivity in periodic three-dimensional dielectric nanostructures, silicon inverse opals. Such structures represent a three-dimensional "phononic crystal" but affect heat flow instead of acoustics. Employing the Stober method, we fabricate high quality silica opal templates that on filling with amorphous silicon, etching and recrystallizing produce silicon inverse opals. The periodicities and shell thicknesses are in the range 420-900 and 18-38 nm, respectively. The thermal conductivity of inverse opal films are relatively low, ~0.6-1.4 W/mK at 300 K and arise due to macroscopic bending of heat flow lines in the structure. The corresponding material thermal conductivity is in the range 5-12 W/mK and has an anomalous ~T(1.8) dependence at low temperatures, distinct from the typical ~T(3) behavior of bulk polycrystalline silicon. Using phonon scattering theory, we show such dependence arising from coherent phonon reflections in the intergrain region. This is consistent with an unconfirmed theory proposed in 1955. The low thermal conductivity is significant for applications in photonics where they imply significant temperature rise at relatively low absorption and in thermoelectrics, where they suggest the possibility of enhancement in the figure of merit for polysilicon with optimal doping.

High Q-Factor Plasmonic Surface Lattice Resonances in Colloidal Nanoparticle Arrays.

Surface lattice resonances (SLRs) sustained by ordered metal arrays are characterized by their narrow spectral features, remarkable quality factors, and the ability to tune their spectral properties based on the periodicity of the array. However, the majority of these structures are fabricated using classical lithographic processes or require postannealing steps at high temperatures to enhance the quality of the metal. These limitations hinder the widespread utilization of these periodic metal arrays in various applications. In this work, we use the scalable technique of template-assisted assembly of metal colloids to produce plasmonic supercrystals over centimeter areas capable of sustaining SLRs with high Q factors reaching up to 270. Our approach obviates the need for any postprocessing, offering a streamlined and efficient fabrication route. Furthermore, our method enables extensive tunability across the entire visible and near-infrared spectral ranges, empowering the design of tailored plasmonic resonant structures for a wide range of applications.

Nanoimprinted 2D-Chiral Perovskite Nanocrystal Metasurfaces for Circularly Polarized Photoluminescence.

The versatile hybrid perovskite nanocrystals (NCs) are one of the most promising materials for optoelectronics by virtue of their tunable bandgaps and high photoluminescence (PL) quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry, or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral PL so far. Here, a general and scalable approach is introduced to produce chiral PL from arbitrary nanoemitters assembled into 2D-chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite NC inks: green-emissive CsPbBr3 and red-emissive CsPbBr1 I2 . The perovskite 2D-metasurfaces exhibit remarkable PL dissymmetry factors (glum ) of 0.16 that can be further improved up to glum of 0.3 by adding a high-refractive-index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications.

Tuning of Ultra-Thin Gold Films by Photoreduction.

Ultrathin metal films (UTMFs) are used in a wide range of applications, from transparent electrodes to infrared mirrors and metasurfaces. Due to their small thickness (<5 nm), the electrical and optical properties of UTMFs can be changed by external stimuli, for example, by applying an electric field through an ion gel. It is also known that oxidized thin films and nanostructures of Au can be reduced by irradiating with short-wavelength light. Here we show that the resistance, reflectance, and resonant optical response of Au UTMFs is changed significantly by ultraviolet light. More specifically, photoreduction and oxidation processes can be sequentially applied for continuous tuning, with observed modulation ranges for sheet resistance (Rs) and reflectance of more than 40% and 30%, respectively. The proposed method has the potential for achieving reconfigurable UTMF structures and trimming their response to specific working points, e.g., a predetermined resonance wavelength and amplitude. This is also important for large scale deployment of such surfaces as one can compensate material nonuniformity, morphological, and structural dimension errors occurring during fabrication.

Imbricate scales as a design construct for microsystem technologies.

Spatially overlapping plates in tiled configurations represent designs that are observed widely in nature (e.g., fish and snake scales) and man-made systems (e.g., shingled roofs) alike. This imbricate architecture offers fault-tolerant, multifunctional capabilities, in layouts that can provide mechanical flexibility even with full, 100% areal coverages of rigid plates. Here, the realization of such designs in microsystems technologies is presented, using a manufacturing approach that exploits strategies for deterministic materials assembly based on advanced forms of transfer printing. The architectures include heterogeneous combinations of silicon, photonic, and plasmonic scales, in imbricate layouts, anchored at their centers or edges to underlying substrates, ranging from elastomer sheets to silicon wafers. Analytical and computational mechanics modeling reveal distributions of stress and strain induced by deformation, and provide some useful design rules and scaling laws.

Surface Lattice Plasmon Resonances by Direct In Situ Substrate Growth of Gold Nanoparticles in Ordered Arrays.

Precise arrangements of plasmonic nanoparticles on substrates are important for designing optoelectronics, sensors and metamaterials with rational electronic, optical and magnetic properties. Bottom-up synthesis offers unmatched control over morphology and optical response of individual plasmonic building blocks. Usually, the incorporation of nanoparticles made by bottom-up wet chemistry starts from batch synthesis of colloids, which requires time-consuming and hard-to-scale steps like ligand exchange and self-assembly. Herein, an unconventional bottom-up wet-chemical synthetic approach for producing gold nanoparticle ordered arrays is developed. Water-processable hydroxypropyl cellulose stencils facilitate the patterning of a reductant chemical ink on which nanoparticle growth selectively occurs. Arrays exhibiting lattice plasmon resonances in the visible region and near infrared (quality factors of >20) are produced following a rapid synthetic step (<10 min), all without cleanroom fabrication, specialized equipment, or self-assembly, constituting a major step forward in establishing in situ growth approaches. Further, the technical capabilities of this method through modulation of the particle size, shape, and array spacings directly on the substrate are demonstrated. Ultimately, establishing a fundamental understanding of in situ growth has the potential to inform the fabrication of plasmonic materials; opening the door for in situ growth fabrication of waveguides, lasing platforms, and plasmonic sensors.

Spatial light interference tomography (SLIT).

We present spatial light interference tomography (SLIT), a label-free method for 3D imaging of transparent structures such as live cells. SLIT uses the principle of interferometric imaging with broadband fields and combines the optical gating due to the micron-scale coherence length with that of the high numerical aperture objective lens. Measuring the phase shift map associated with the object as it is translated through focus provides full information about the 3D distribution associated with the refractive index. Using a reconstruction algorithm based on the Born approximation, we show that the sample structure may be recovered via a 3D, complex field deconvolution. We illustrate the method with reconstructed tomographic refractive index distributions of microspheres, photonic crystals, and unstained living cells.

Mechanically Tunable Lattice-Plasmon Resonances by Templated Self-Assembled Superlattices for Multi-Wavelength Surface-Enhanced Raman Spectroscopy.

Lattice plasmons, i.e., diffractively coupled localized surface plasmon resonances, occur in long-range ordered plasmonic nanostructures such as 1D and 2D periodic lattices. Such far-field coupled resonances can be employed for ultrasensitive surface-enhanced Raman spectroscopy (SERS), provided they are spectrally matched to the excitation wavelength. The spectral positions of lattice plasmon modes critically depend on the lattice period and uniformity, owing to their pronounced sensitivity to structural disorder. We report the fabrication of superlattices by templated self-assembly of gold nanoparticles on a flexible support, with tunable lattice-plasmon resonances by means of macroscopic strain. We demonstrate that the highest SERS performance is achieved by matching the lattice plasmon mode to the excitation wavelength, by post-assembly fine-tuning of long-range structural parameters. Both asymmetric and symmetric lattice deformations can be used to adapt a single lattice structure to both red-shifted and blue-shifted excitation lines, as exemplified by lattice expansion and contraction, respectively. This proof-of-principle study represents a basis for alternative designs of adaptive functional nanostructures with mechanically tunable lattice resonances using strain as a macroscopic control parameter.

Large-Scale Soft-Lithographic Patterning of Plasmonic Nanoparticles.

Micro- and nanoscale patterned monolayers of plasmonic nanoparticles were fabricated by combining concepts from colloidal chemistry, self-assembly, and subtractive soft lithography. Leveraging chemical interactions between the capping ligands of pre-synthesized gold colloids and a polydimethylsiloxane stamp, we demonstrated patterning gold nanoparticles over centimeter-scale areas with a variety of micro- and nanoscale geometries, including islands, lines, and chiral structures (e.g., square spirals). By successfully achieving nanoscale manipulation over a wide range of substrates and patterns, we establish a powerful and straightforward strategy, nanoparticle chemical lift-off lithography (NP-CLL), for the economical and scalable fabrication of functional plasmonic materials with colloidal nanoparticles as building blocks, offering a transformative solution for designing next-generation plasmonic technologies.

Light generation at the anomalous dispersion high energy range of a nonlinear opal film.

We study experimentally and theoretically light propagation and generation at the high energy range of a close-packed fcc photonic crystal of polystyrene spheres coated with a nonlinear material. We observe an enhancement of the second harmonic generation of light that may be explained on the basis of amplification effects arising from propagation at anomalous group velocities. Theoretical calculations are performed to support this assumption. The vector KKR method we use allows us to determine, from the linear response of the crystal, the behavior of the group velocity in our finite photonic structures when losses introduced by absorption or scattering by defects are taken into account assuming a nonzero imaginary part for the dielectric constant. In such structures, we predict large variations of the group velocity for wavelengths on the order or smaller than the lattice constant of the structure, where an anomalous group velocity behavior is associated with the flat bands of the photonic band structure. We find that a direct relation may be established between the group velocity reduction and the enhancement of a light generation processes such as the second harmonic generation we consider. However, frequencies for which the enhancement is found, in the finite photonic crystals we use, do not necessarily coincide with the frequencies of flat high energy bands.

Tunable index metamaterials made by bottom-up approaches.

Despite the exciting optical properties metamaterials exhibit, their implementation in technology is being hampered nowadays by the inherent losses of their metal constituents and the expensive and low-throughput procedures used. As an alternative, we present a new design of double fishnet metamaterials that can be easily realized combining two inexpensive and up-scalable techniques: nanosphere lithography and metallic electrodeposition. A monolayer of polystyrene spheres is used as a template for the infiltration of two symmetric gold layers separated by an air gap. The effective refractive index of the metamaterial can be easily tuned by the appropriate choice of the diameter of the spheres and the gap width between the metallic layers, varying its value from positive to negative. The good agreement between optical measurements and finite-difference time-domain simulations confirms the success of our process. Fishnet metamaterials with refractive index going from 1.5 until -1.0 in the near infrared range are demonstrated and the key parameters for these architectures provided.

Solvent-Assisted Self-Assembly of Gold Nanorods into Hierarchically Organized Plasmonic Mesostructures.

Plasmonic supercrystals and periodically structured arrays comprise a class of materials with unique optical properties that result from the interplay of plasmon resonances, as well as near- and far-field coupling. Controlled synthesis of such hierarchical structures remains a fundamental challenge, as it demands strict control over the assembly morphology, array size, lateral spacing, and macroscale homogeneity. Current fabrication approaches involve complicated multistep procedures lacking scalability and reproducibility, which has hindered the practical application of plasmonic supercrystal arrays. Herein, these challenges are addressed by adding an organic solvent to achieve kinetic control over the template-assisted colloidal assembly of nanoparticles from aqueous dispersion. This method yields highly regular periodic arrays, with feature sizes ranging from less than 200 nm up to tens of microns. A combined experimental/computational approach reveals that the underlying mechanism is a combination of the removal of interfacial surfactant micelles from the particle interface and altered capillary flows. Assessing the efficacy of such square arrays for surface-enhanced Raman scattering spectroscopy, we find that a decrease of the lattice periodicity from 750 nm down to 400 nm boosts the signal by more than an order of magnitude, thereby enabling sensitive detection of analytes, such as the bacterial quorum sensing molecule pyocyanin, even in complex biological media.

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.

Nanostructured Back Reflectors for Efficient Colloidal Quantum-Dot Infrared Optoelectronics.

Colloidal quantum dots (CQDs) can be used to extend the response of solar cells, enabling the utilization of solar power that lies to the red of the bandgap of c-Si and perovskites. To achieve largely complete absorption of infrared (IR) photons in CQD solids requires thicknesses on the micrometer range; however, this exceeds the typical diffusion lengths (≈300 nm) of photoexcited charges in these materials. Nanostructured metal back electrodes that grant the cell efficient IR light trapping in thin active layers with no deterioration of the electrical properties are demonstrated. Specifically, a new hole-transport layer (HTL) is developed and directly nanostructured. Firstly, a material set to replace conventional rigid HTLs in CQD devices is developed with a moldable HTL that combines the mechanical and chemical requisites for nanoimprint lithography with the optoelectronic properties necessary to retain efficient charge extraction through an optically thick layer. The new HTL is nanostructured in a 2D lattice and conformally coated with MoO3 /Ag. The photonic structure in the back electrode provides a record photoelectric conversion efficiency of 86%, beyond the Si bandgap, and a 22% higher IR power conversion efficiency compared to the best previous reports.

A water-processable cellulose-based resist for advanced nanofabrication.

The ideal nanofabrication technique is one that allows the mass production of high resolution submicrometric features in a cost efficient and environmentally friendly fashion. A great step towards achieving this goal has been the development of nanoimprinting lithography, a procedure with tenths of nanometres resolution while being compatible with roll-to-roll manufacturing. However, an ecofriendly resist that can be efficiently combined with this process is still missing. In this work, we demonstrate the use of hydroxypropyl cellulose (HPC) as a biocompatible, biodegradable, and water processable resist for temperature assisted nanoimprint lithography (tNIL) by fabricating different photonic architectures. The cellulose derivative is easily patterned with submicrometric features with aspect ratios greater than 1 using an elastomeric stamp and a hot plate. Silicon photonic crystals and metal nanoparticle arrays are fabricated combining cellulose with traditional nanofrabrication processes such as spincasting, reactive ion etching and metal lift off. Furthermore, advanced nanofabrication possibilities are within reach by combining the HPC with traditional resists. In particular, poly(methyl methacrylate) and HPC stacks are easily produced by liquid phase processing, where one of the two materials can be selectively removed by developing in orthogonal solvents. This capability becomes even more interesting by including nanoimprinted layers in the stack, leading to the encapsulation of arrays of air features in the resist.

Gold Nanoparticle Plasmonic Superlattices as Surface-Enhanced Raman Spectroscopy Substrates.

Metal colloids are of great interest in the field of nanophotonics, mainly due to their morphology-dependent optical properties, but also because they are high-quality building blocks for complex plasmonic architectures. Close-packed colloidal supercrystals not only serve for investigating the rich plasmonic resonances arising in strongly coupled arrangements but also enable tailoring the optical response, on both the nano- and the macroscale. Bridging these vastly different length scales at reasonable fabrication costs has remained fundamentally challenging, but is essential for applications in sensing, photovoltaics or optoelectronics, among other fields. We present here a scalable approach to engineer plasmonic supercrystal arrays, based on the template-assisted assembly of gold nanospheres with topographically patterned polydimethylsiloxane molds. Regular square arrays of hexagonally packed supercrystals were achieved, reaching periodicities down to 400 nm and feature sizes around 200 nm, over areas up to 0.5 cm2. These two-dimensional supercrystals exhibit well-defined collective plasmon modes that can be tuned from the visible through the near-infrared by simple variation of the lattice parameter. We present electromagnetic modeling of the physical origin of the underlying hybrid modes and demonstrate the application of superlattice arrays as surface-enhanced Raman scattering (SERS) spectroscopy substrates which can be tailored for a specific probe laser. We therefore investigated the influence of the lattice parameter, local degree of order, and cluster architecture to identify the optimal configuration for highly efficient SERS of a nonresonant Raman probe with 785 nm excitation.

Ultrathin Semiconductor Superabsorbers from the Visible to the Near-Infrared.

The design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong-but narrowband-or broader-but moderate-light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. Broadband near-unity absorption in Ge ultrathin films is demonstrated, which extends from the visible to the Ge bandgap in the near-infrared and is robust against angle of incidence variation. The strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures.