Deterministic assembly of metamolecules by atomic force microscope-enabled manipulation of ultra-smooth, super-spherical gold nanoparticles

Degradation phenomena of quantum dot light-emitting diodes induced by high electric field

Quantum dots possess exceptional optoelectronic properties, such as narrow bandwidth, controllable wavelength, and compatibility with solution-based processing. However, for efficient and stable operation in electroluminescence mode, several issues require resolution. Particularly, as device dimensions decrease, a higher electric field may be applied through next-generation quantum dot light-emitting diode (QLED) devices, which could further degrade the device. In this study, we conduct a systematic analysis of the degradation phenomena of a QLED device induced by a high electric field, using scanning probe microscopy (SPM) and transmission electron microscopy (TEM). We apply a local high electric field to the surface of a QLED device using an atomic force microscopy (AFM) tip, and we investigate changes in morphology and work function in the Kelvin probe force microscopy mode. After the SPM experiments, we perform TEM measurements on the same degraded sample area affected by the electric field of the AFM tip. The results indicate that a QLED device could be mechanically degraded by a high electric field, and work function changes significantly in degraded areas. In addition, the TEM measurements reveal that In ions migrate from the indium tin oxide (ITO) bottom electrode to the top of the QLED device. The ITO bottom electrode also deforms significantly, which could induce work function variation. The systematic approach adopted in this study can provide a suitable methodology for investigating the degradation phenomena of various optoelectronic devices.

The Development of Microscopic Imaging Technology and its Application in Micro- and Nanotechnology

As a typical microscopic imaging technology, the emergence of the microscope has accelerated the pace of human exploration of the micro world. With the development of science and technology, microscopes have developed from the optical microscopes at the time of their invention to electron microscopes and even atomic force microscopes. The resolution has steadily improved, allowing humans to expand the field of research from the initial animal and plant tissues to microorganisms such as bacteria, and even down to the nanolevel. The microscope is now widely used in life science, material science, geological research, and other fields. It can be said that the development of microscopes also promotes the development of micro- and nanotechnology. It is foreseeable that microscopes will play a significant part in the exploration of the microworld for a long time to come. The development of microscope technology is the focus of this study, which summarized the properties of numerous microscopes and discussed their applications in micro and nanotechnology. At the same time, the application of microscopic imaging technology in micro- and nanofields was investigated based on the properties of various microscopes.

Morphologically homogeneous, pH-responsive gold nanoparticles for non-invasive imaging of HeLa cancer

Gold nanoparticles (AuNPs) have been widely used as nanocarriers in drug delivery to improve the efficiency of chemotherapy treatment and enhance early disease detection. The advantages of AuNPs include their excellent biocompatibility, easy modification and functionalization, facile synthesis, low toxicity, and controllable particle size. This study aimed to synthesize a conjugated citraconic anhydride link between morphologically homogeneous AuNPs and doxorubicin (DOX) (DOX-AuNP). The carrier was radiolabeled for tumor diagnosis using positron emission tomography (PET). The systemically designed DOX-AuNP was cleaved at the citraconic anhydride linker site under the mild acidic conditions of a cancer cell, thereby releasing DOX. Subsequently, the AuNPs aggregated via electrostatic attraction. HeLa cancer cells exhibited a high uptake of the radiolabeled DOX-AuNP. Moreover, PET tumor images were obtained using radiolabeled DOX-AuNP in cancer xenograft mouse models. Therefore, DOX-AuNP is expected to provide a valuable insight into the use of radioligands to detect tumors using PET.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

The scaling down of meta-atoms or metamolecules (collectively denoted as metaunits) is a long-lasting issue from the time when the concept of metamaterials was first suggested. According to the effective medium theory, which is the foundational concept of metamaterials, the structural sizes of meta-units should be much smaller than the working wavelengths (e.g., << 1/5 wavelength). At relatively low frequency regimes (e.g., microwave and terahertz), the conventional monolithic lithography can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as the visible and near-infrared regimes) through the lithographic approaches. This serves as the rationale for using colloidal self-assembly as a strategy for the realization of optical metamaterials. Colloidal self-assembly can address various critical issues associated with the materialization of optical metamaterials, such as achieving nanogaps over a large area, increasing true 3D structural complexities, and cost-effective processing, which all are difficult to attain through monolithic lithography. Nevertheless, colloidal self-assembly is still a toolset underutilized by optical engineers. Here, the design principle of the colloidally self-assembled optical metamaterials exhibiting unnatural refractions, the practical challenge of relevant experiments, and the future opportunities are critically reviewed.

Soft Plasmonic Assemblies Exhibiting Unnaturally High Refractive Index

The increases in refractive indices (n) of materials are crucial for transformative optical technologies. With the progress of monolithic lithography, large advances have been achieved with several semiconductors, including silicon, germanium, and gallium arsenide, which generally provide higher n of ∼4.0 compared to those of other elements. Nevertheless, above this upper limit of naturally available n, the range of light-matter interactions could be unprecedentedly expanded, which in turn enriches the possible applications. Here, we present a soft self-assembly of polyhedral Au colloids as a promising method to achieve unnaturally high n values. The interfacial assembly of Au nanocubes provides n of 6.4 at the resonant wavelength (near-infrared) and 4.5 in the off-resonant regimes (mid-infrared), which have not been previously reached. The soft self-assembly of polyhedral Au colloids can be a versatile and highly effective route for the fabrication of optical metamaterials with unnaturally high n values.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Chiral metamaterials via Moiré stacking

Chiral metamaterials have attracted strong interest due to their versatile capabilities in spin-dependent light manipulation. Benefiting from advancements in nanofabrication and mechanistic understanding of chiroptical effects, chiral metamaterials have shown potential in a variety of applications including circular polarizers, chiral sensors, and chiroptical detectors. Recently, chiral metamaterials made by moiré stacking, superimposing two or more periodic patterns with different lattice constants or relative spatial displacement, have shown promise for chiroptical applications. The moiré chiral metamaterials (MCMs) take advantage of lattice-dependent chirality, giving cost-effective fabrication, flexible tunability, and reconfigurability superior to conventional chiral metamaterials. This feature article focuses on recent progress of MCMs. We discuss optical mechanisms, structural design, fabrication, and applications of the MCMs. We conclude with our perspectives on the future opportunities for the MCMs.

Progress and Opportunities in Soft Photonics and Biologically Inspired Optics

Optical components made fully or partially from reconfigurable, stimuli-responsive, soft solids or fluids-collectively referred to as soft photonics-are poised to form the platform for tunable optical devices with unprecedented functionality and performance characteristics. Currently, however, soft solid and fluid material systems still represent an underutilized class of materials in the optical engineers' toolbox. This is in part due to challenges in fabrication, integration, and structural control on the nano- and microscale associated with the application of soft components in optics. These challenges might be addressed with the help of a resourceful ally: nature. Organisms from many different phyla have evolved an impressive arsenal of light manipulation strategies that rely on the ability to generate and dynamically reconfigure hierarchically structured, complex optical material designs, often involving soft or fluid components. A comprehensive understanding of design concepts, structure formation principles, material integration, and control mechanisms employed in biological photonic systems will allow this study to challenge current paradigms in optical technology. This review provides an overview of recent developments in the fields of soft photonics and biologically inspired optics, emphasizes the ties between the two fields, and outlines future opportunities that result from advancements in soft and bioinspired photonics.

Photofluidic Near-Field Mapping of Electric-Field Resonance in Plasmonic Metasurface Assembled with Gold Nanoparticles

We present a near-field mapping of electric fields from the individual superspherical and ultrasmooth gold nanoparticles (AuNPs) and artificially assembled AuNP nanostructures by measuring the reconfiguration of an azobenzene-containing polymer(azo-polymer) film. Various configurations of AuNPs and the azo-polymer were studied with atomic force microscopy measurements and calculations. The interference was systematically studied with AuNP dimers of various gap distances and different embedding depth in the polymer film. Finally, we successfully demonstrated the interference of standing waves in artificially assembled plasmonic metasurface.

Assembly of "3D" plasmonic clusters by "2D" AFM nanomanipulation of highly uniform and smooth gold nanospheres

Atomic force microscopy (AFM) nanomanipulation has been viewed as a deterministic method for the assembly of plasmonic metamolecules because it enables unprecedented engineering of clusters with exquisite control over particle number and geometry. Nevertheless, the dimensionality of plasmonic metamolecules via AFM nanomanipulation is limited to 2D, so as to restrict the design space of available artificial electromagnetisms. Here, we show that “2D” nanomanipulation of the AFM tip can be used to assemble “3D” plasmonic metamolecules in a versatile and deterministic way by dribbling highly spherical and smooth gold nanospheres (NSs) on a nanohole template rather than on a flat surface. Various 3D plasmonic clusters with controlled symmetry were successfully assembled with nanometer precision; the relevant 3D plasmonic modes (i.e., artificial magnetism and magnetic-based Fano resonance) were fully rationalized by both numerical calculation and dark-field spectroscopy. This templating strategy for advancing AFM nanomanipulation can be generalized to exploit the fundamental understanding of various electromagnetic 3D couplings and can serve as the basis for the design of metamolecules, metafluids, and metamaterials.

Spatial Manipulation and Assembly of Nanoparticles by Atomic Force Microscopy Tip-Induced Dielectrophoresis

In this article, we present a novel method of spatial manipulation and assembly of nanoparticles via atomic force microscopy tip-induced dielectrophoresis (AFM-DEP). This method combines the high-accuracy positioning of AFM with the parallel manipulation of DEP. A spatially nonuniform electric field is induced by applying an alternating current (AC) voltage between the conductive AFM probe and an indium tin oxide glass substrate. The AFM probe acted as a movable DEP tweezer for nanomanipulation and assembly of nanoparticles. The mechanism of AFM-DEP was analyzed by numerical simulation. The effects of solution depth, gap distance, AC voltage, solution concentration, and duration time were experimentally studied and optimized. Arrays of 200 nm polystyrene nanoparticles were assembled into various nanostructures, including lines, ellipsoids, and arrays of dots. The sizes and shapes of the assembled structures were controllable. It was thus demonstrated that AFM-DEP is a flexible and powerful tool for nanomanipulation.

Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules

The smallness of natural molecules and atoms with respect to the wavelength of light imposes severe limits on the nature of their optical response. For example, the well-known argument of Landau and Lifshitz and its recent extensions that include chiral molecules show that the electric dipole response dominates over the magneto-electric (bianisotropic) and an even smaller magnetic dipole optical response for all natural materials. Here, we experimentally demonstrate that both these responses can be greatly enhanced in plasmonic nanoclusters. Using atomic force microscopy nanomanipulation technique, we assemble a plasmonic metamolecule that is designed for strong and simultaneous optical magnetic and magneto-electric excitation. Angle-dependent scattering spectroscopy is used to disentangle the two responses and to demonstrate that their constructive/destructive interplay causes strong directional scattering asymmetry. This asymmetry is used to extract both magneto-electric and magnetic dipole responses and to demonstrate their enhancement in comparison to ordinary atomistic materials.

Colloidal superlattices for unnaturally high-index metamaterials at broadband optical frequencies

The recent advance in the assembly of metallic nanoparticles (NPs) has enabled sophisticated engineering of unprecedented light-matter interaction at the optical domain. In this work, I expand the design flexibility of NP optical metamaterial to push the upper limit of accessible refractive index to the unnaturally high regime. The precise control over the geometrical parameters of NP superlattice monolayer conferred the dramatic increase in electric resonance and related effective permittivity far beyond the naturally accessible regime. Simultaneously, effective permeability change, another key factor to achieving high refractive index, was effectively suppressed by reducing the thickness of NPs. By establishing this design rule, I have achieved unnaturally high refractive index (15.7 at the electric resonance and 7.3 at the quasi-static limit) at broadband optical frequencies (100 THz ~300 THz). I also combined this NP metamaterial with graphene to electrically control the high refractive index over the broad optical frequencies.

Design of optical metamaterial mirror with metallic nanoparticles for floating-gate graphene optoelectronic devices

The purpose of this work is to conceive the idea for using the gate dielectrics of floating-gate memory device (i.e., Au nanoparticle (AuNP) monolayer embedded within polymeric matrix) as a magnetic mirror, so as to harness the broadband light absorption of thin film optoelectronics. In particular, we systematically examined whether the versatile assembly of spherical AuNP monolayer can be indeed treated as the effective magnetic mirror for floating-gate graphene optoelectronic device. High amenability of the AuNP assembly with the large-area device fabrication procedures may make this strategy widely applicable to various thin film optoelectronic devices. Our study thereby advances the design of mirror for thin film optoelectronics.