Individual Nanostructures Research Articles

Lattice Resonances for Thermoplasmonics

Thanks to their ability to support localized surface plasmons, metallic nanostructures have emerged as ideal tools to transduce light into heat at the nanoscale, giving birth to the field of thermoplasmonics. When arranged in a periodic array, the localized plasmons of metallic nanostructures can interact coherently to generate a collective mode known as a lattice resonance. This collective mode, whose wavelength is controlled by the periodicity of the array, produces a stronger and more spectrally narrow optical response than that of the localized plasmons supported by the individual nanostructures. Motivated by the exceptional properties of the lattice resonances of periodic arrays of metallic nanoparticles, here, we investigate their use for applications in thermoplasmonics. Through a comprehensive analysis based on a coupled dipole model, we show that arrays supporting a lattice resonance absorb more energy per nanoparticle, and thus achieve a much larger increase in temperature under pulsed illumination conditions, than those that do not support such a mode. On the contrary, for continuous wave illumination conditions, we find that the temperature increase is mostly independent of the array period for the systems under consideration. Furthermore, by analyzing arrays with two nanoparticles per unit cell, we show that it is possible to engineer their lattice resonances to selectively absorb light in one of the nanoparticles without exciting the other. The results of this work pave the way for the development of thermoplasmonics applications exploiting the exceptional optical response and tunability provided by lattice resonances.

Optical Response of Periodic Arrays of Graphene Nanodisks

Doped graphene nanostructures are a promising platform for photonics due to their exceptionally strong and tunable plasmonic resonances. When placed in a periodic array configuration, the plasmons supported by the individual nanostructures interact with each other and, under the appropriate conditions, can give rise to a collective mode known as a lattice resonance. Here, we perform a comprehensive analysis of the response of periodic arrays of graphene nanodisks and identify the conditions under which the system is able to support lattice resonances. We find that the ratio between the period of the array and the wavelength of the plasmon completely determines the behavior of the system. As a consequence, strong lattice resonances are achieved for micron-size nanodisks in the terahertz regime. We develop a theoretical model valid beyond the electrostatic approximation and use it to derive closed analytical expressions for the strength, the wavelength, and the width of the optical resonance of the arrays. The theoretical framework developed in this work paves the way for facile design and discovery of emerging properties of periodic arrays of graphene nanostructures that could enable applications in photonics and plasmonics.

Replica-mold nanopatterned PHEMA hydrogel surfaces for ophthalmic applications

Biomimicking native tissues and organs require the development of advanced hydrogels. The patterning of hydrogel surfaces may enhance the cellular functionality and therapeutic efficacy of implants. For example, nanopatterning of the intraocular lens (IOL) surface can suppress the upregulation of cytoskeleton proteins (actin and actinin) within the cells in contact with the IOL surface and, hence, prevent secondary cataracts causing blurry or opaque vision. Here we introduce a fast and efficient method for fabricating arrays consisting of millions of individual nanostructures on the hydrogel surface. In particular, we have prepared the randomly distributed nanopillars on poly(2-hydroxyethyl methacrylate) hydrogel using replica molding and show that the number, shape, and arrangement of nanostructures are fully adjustable. Characterization by atomic force microscopy revealed that all nanopillars were of similar shape, narrow size distribution, and without significant defects. In imprint lithography, choosing the appropriate hydrogel composition is critical. As hydrogels with imprinted nanostructures mimic the natural cell environment, they can find applications in fundamental cell biology research, e.g., they can tune cell attachment and inhibit or promote cell clustering by a specific arrangement of protrusive nanostructures on the hydrogel surface.

Lattice Resonances Excited by Finite-Width Light Beams.

Periodic arrays of metallic nanostructures support collective lattice resonances, which give rise to optical responses that are, at the same time, stronger and more spectrally narrow than those of the localized plasmons of the individual nanostructures. Despite the extensive research effort devoted to investigating the optical properties of lattice resonances, the majority of theoretical studies have analyzed them under plane-wave excitation conditions. Such analysis not only constitutes an approximation to realistic experimental conditions, which require the use of finite-width light beams, but also misses a rich variety of interesting behaviors. Here, we provide a comprehensive study of the response of periodic arrays of metallic nanostructures when excited by finite-width light beams under both paraxial and nonparaxial conditions. We show how as the width of the light beam increases, the response of the array becomes more collective and converges to the plane-wave limit. Furthermore, we analyze the spatial extent of the lattice resonance and identify the optimum values of the light beam width to achieve the strongest optical responses. We also investigate the impact that the combination of finite-size effects in the array and the finite width of the light beam has on the response of the system. Our results provide a solid theoretical framework to understand the excitation of lattice resonances by finite-width light beams and uncover a set of behaviors that do not take place under plane-wave excitation.

Chemical Energy‐Driven Lithiation Preparation of Defect‐Rich Transition Metal Nanostructures for Electrocatalytic Hydrogen Evolution

Transition metal nanostructures are widely regarded as important catalysts to replace the precious metal Pt for hydrogen evolution reaction (HER) in water splitting. However, it is difficult to obtain uniform-sized and ultrafine metal nanograins through general high-temperature reduction and sintering processes. Herein, a novel method of chemical energy-driven lithiation is introduced to synthesize transition metal nanostructures. By taking advantage of the slow crystallization kinetics at room temperature, more surface and boundary defects can be generated and remained, which reduce the atomic coordination number and tune the electronic structure and adsorption free energy of the metals. The obtained Ni nanostructures therein exhibit excellent HER performance. In addition, the bimetal of Co and Ni shows better electrocatalytic kinetics than individual Ni and Co nanostructures, reaching 100mAcm-2 at a low overpotential of 127mV. The high HER performance originates from well-formed synergistic effect between Ni and Co by tuning the electronic structures. Density functional theory simulations confirm that the bimetallic NiCo possesses a low Gibbs free energy of hydrogen adsorption, which are conducive to enhance its intrinsic activity. This work provides a general strategy that enables simultaneous defect engineering and electronic modulation of transition metal catalysts to achieve an enhancement in HER performance.

Effect of Nitrogen Atoms in the CNT Structure on the Gas Sensing Properties of PANI/CNT Composite

Herein we report the gas-sensitive properties to ammonia (at 2–10 ppm) of individual nanostructures of a polyaniline/nitrogen-doped carbon nanotube composite with a nitrogen content of 0 at.% (uCNTs), 2 at.% (N-CNTs) and 4 at.% (N+-CNTs). Doping of nanotubes with nitrogen was carried out in order to both reduce the electron work function, to form a potential barrier at the “PANI-CNTs” interface, and reduce the contribution of nanotubes to the composite conductivity. An increase in the nitrogen content in CNTs leads to an increase in conductivity, a decrease in the work function, and the formation of defects in the outer walls of CNTs. It was found that the structural and chemical state of the polymer layer of all composites is the same. However, polymer morphology on nanotubes changes dramatically with increasing nitrogen content in CNTs: a thin smooth layer on uCNTs, a globular layer on N-CNTs, and a thick layer with a sheet-like structure on N+-CNTs. All composites showed the same response time (~20 s) and recovery time (~120 s). Ammonia sensitivity was 10.5 ± 0.2, 15.3 ± 0.5 and 2.2 ± 0.1 ppm−1 for PANI/uCNTs, PANI/N-CNTs and PANI/N+-CNTs, respectively. Based on the results obtained here, we came to the conclusion that the morphological features of the polymer layer on CNTs with different nitrogen content have a dominant effect on the gas reaction than the change in the electronic properties of the polymer at the interface “PANI-CNT”.

Highly‐Packed Proximity‐Coupled DC‐Josephson Junction Arrays by a Direct‐Write Approach

AbstractFocused ion beam induced deposition (FIBID) is a direct‐write technique enabling the growth of individual nanostructures of any shape and dimension with high lateral resolution. Moreover, the fast and reliable writing of periodically arranged nanostructures can be used to fabricate devices for the investigation of collective phenomena and to design novel functional metamaterials. Here, FIBID is employed to prepare dc‐Josephson junction arrays (dc‐JJA) consisting of superconducting NbC dots coupled through the proximity effect via a granular metal layer. The fabrication is straightforward and allows the preparation of dc‐JJA within a few seconds. Microstructure and composition of the arrays are investigated by transmission electron microscopy and energy dispersive X‐ray spectroscopy. The superconductor‐to‐metal transition of the prepared dc‐JJA is studied in a direct way, by tuning the Josephson junction resistance in 70 nm‐spaced superconducting NbC dots. The observed magnetoresistance oscillations with a period determined by the flux quantum give evidence for the coherent charge transport by paired electrons. Moreover, the measured resistance minima correspond to two fundamental matching configurations of fluxons in the dc‐JJA, caused by magnetic frustration. The robust properties of the prepared dc‐JJA demonstrate the opportunities for a fast preparation of complex device configurations using direct‐write approaches.

Novel use of selectivity of resonance Raman spectroscopy to study polytypism and mixed to pure phase conversion in individual InAs NWs on laser irradiation

Conversion of Wurtzite to Zincblende phase of InAs NWs at high simulated temperature (>1300 K) is investigated using resonance Raman spectroscopy with specially designed two step heating–cooling experiment. For smaller nanowires (diameter < 700 nm), the certainty of determining the polytypism using polarized Raman spectroscopy, reduces significantly. Low signal to noise ratio for smaller diameter NWs is improved by employing the wavelength dependent unpolarized and polarized Raman spectroscopy to examine the sensitivity of resonance Raman spectroscopy as an alternate for probing polytypism. Innovative use of resonance Raman spectroscopy to probe phase purity of the III-V individual nanostructures, where, conventional methods may not be applicable is realized in this work.This work paves the way for online monitoring of the conversion of mixed phase to phase pure III-–V nanowires using laser irradiation for their fast electronic device applications. Further, this kind of surface modification study provides basis for the futuristic crystal structure engineering, which can facilitate the controlled tailoring of material properties in order to enhance their functionality.

Bending‐Insensitive Flexible SERS Sensor for Stable and Sensitive Detection on Curved Surfaces

AbstractSurface‐enhanced Raman scattering (SERS) based flexible sensing technology has been considered an essential candidate for future diagnostics in diverse fields. To obtain a reliable signal from the flexible SERS sensor, signal reproducibility even when the substrates are bent is highly desirable because there are various curvatures in the practical applications. However, this remains challenging because the “hot‐spots” were created between the individual nanostructures on the surface of the previous flexible SERS sensor. The distance between each nanostructure varied through the bending of the sensor, leading to an adverse effect on the unform intensity of the Raman signal. Herein, a bending‐insensitive flexible SERS substrate is reported for reliable and sensitive sensing on curved surfaces. It is based on silver nanodendrites on the surface of graphene‐modified PI film via electrodeposition. Numerous hot spots are created between the trunk‐branch and branch‐branch junctions inside individual silver dendrites. The inherent hot‐spots of silver nanodendrites exhibit a uniform and extremely enhanced Raman signal under the bending of the substrate, because the morphology of silver nanodendrites is insensitive to diverse degrees of bending. Thus, the flexible SERS sensor can detect 10−12 m (in buffer solution) and 10−8 m (serum‐containing condition) of 6‐thioguanine on arbitrary curved surfaces.

Magnetic Anisotropy of Individual Nanomagnets Embedded in Biological Systems Determined by Axi-asymmetric X-ray Transmission Microscopy.

Over the past few years, the use of nanomagnets in biomedical applications has increased. Among others, magnetic nanostructures can be used as diagnostic and therapeutic agents in cardiovascular diseases, to locally destroy cancer cells, to deliver drugs at specific positions, and to guide (and track) stem cells to damaged body locations in regenerative medicine and tissue engineering. All these applications rely on the magnetic properties of the nanomagnets which are mostly determined by their magnetic anisotropy. Despite its importance, the magnetic anisotropy of the individual magnetic nanostructures is unknown. Currently available magnetic sensitive microscopic methods are either limited in spatial resolution or in magnetic field strength or, more relevant, do not allow one to measure magnetic signals of nanomagnets embedded in biological systems. Hence, the use of nanomagnets in biomedical applications must rely on mean values obtained after averaging samples containing thousands of dissimilar entities. Here we present a hybrid experimental/theoretical method capable of working out the magnetic anisotropy constant and the magnetic easy axis of individual magnetic nanostructures embedded in biological systems. The method combines scanning transmission X-ray microscopy using an axi-asymmetric magnetic field with theoretical simulations based on the Stoner-Wohlfarth model. The validity of the method is demonstrated by determining the magnetic anisotropy constant and magnetic easy axis direction of 15 intracellular magnetite nanoparticles (50 nm in size) biosynthesized inside a magnetotactic bacterium.

Development of a composite structured surface for durable anti-hydrate and enhancing thermal conductivity

The durability and thermal conductivity of the superhydrophobic surfaces are of great challenge in the application. In this work, a composite structured (CS) surface was developed with a water contact angle of 150°. The sintered porous copper was designed to provide mechanical stability and interconnected heat-conducting backbones. The superhydrophobic nanoparticle was designed to provide water repellency. The adhesion force test indicated that the CS surface has predominant anti-hydrate ability. The hydrate adhesion force of CS surface was only 0.01 mN/m. After decomposition, the CS surface has 92% smaller retained water mass than bare surface, indicating that it can be circulated to prevent adhesion. Additionally, the CS surface with copper size < 50 µm achieved a high thermal conductivity of 81.23 W ⋅m−1⋅K−1. In the case of thick coating (100 µm), the CS with copper size < 50 µm is predicted to ensure an overall thermal conductivity decrease of < 20%. Under the same wear condition, CS surface has a longer life than individual microstructure or nanostructure. The research can provide a guideline for design of heat exchanger coatings to achieve a lasting prevention of increase in system energy consumption.

Long-wave infrared super-resolution wide-field microscopy using sum-frequency generation

Super-resolution microscopy in the visible is an established powerful tool in various disciplines. In the long-wave infrared (LWIR) spectral range, however, no comparable schemes have been demonstrated to date. In this work, we experimentally demonstrate super-resolution microscopy in the LWIR range (λIR≈10–12 μm) using IR-visible sum-frequency generation. We operate our microscope in a wide-field scheme and image localized surface phonon polaritons in 4H-SiC nanostructures as a proof-of-concept. With this technique, we demonstrate an enhanced spatial resolution of ∼λIR/9, enabling to resolve the polariton resonances in individual sub-diffractional nanostructures with sub-wavelength spacing. Furthermore, we show that this resolution allows us to differentiate between spatial patterns associated with different polariton modes within individual nanostructures.

Two-Dimensional Weak Antilocalization Signatures Due to Quantum Coherent Transport in Nanocrystalline SnTe.

Nanostructured topological crystalline insulators (TCIs) in the presence of exotic surface states with spin momentum locking reported in individual nanostructures are predicted to hold a great promise for spintronics and quantum computing applications. However, practical application demands a strategy with large-scale production and integration for device applications. In this work, we demonstrate through prominent signatures of weak antilocalization (WAL), arising predominantly from destructive quantum interference on robust surface states, that a correlated TCI phase is possible in the nanobulk assembly of carefully nanostructured quasi-two-dimensional SnTe (edge-to-edge length ∼ 382 nm) synthesized by a simple, rapid, and scalable microwave-assisted solvothermal method. Hikami-Larkin-Nagaoka analysis (T-0.71), as well as the temperature dependence of resistivity, illustrates an interplay of both conductions from 2D channels and 3D EEI effects as the precursor for the observed WAL at low temperatures (2-6 K). Interestingly, the enhanced thermoelectric power of the sample of ∼45 μV/K, with a p-type carrier concentration of ∼1018/cm3 at 300 K, makes this SnTe nanocrystalline assembly more attractive as a multifunctional material for large-scale technological applications.

Accurate determination of electrical conductance in carbon nanostructures

Electrical characterization of nanostructures, such as nanotubes and wires, is a demanding task that is vital for future applications of nanomaterials. The nanostructures should ideally be analyzed in a free-standing state and also allow for other material characterizations to be made of the same individual nanostructures. Several methods have been used for electrical characterizations of carbon nanotubes in the past. The results are widely spread, both between different characterizations methods and within the same materials. This raises questions regarding the reliability of different methods and their accuracy, and there is a need for a measurement standard and classification scheme for carbon nanotube materials. Here we examine a two-probe method performed inside a transmission electron microscope in detail, addressing specifically the accuracy by which the electrical conductivity of individual carbon nanotubes can be determined. We show that two-probe methods can be very reliable using a suitable thermal cleaning method of the contact points. The linear resistance of the outermost nanotube wall can thus be accurately determined even for the highest crystallinity materials, where the linear resistance is only a few kΩ/μm. The method can thereby by used as a valuable tool for future classification schemes of various nanotube material classes.

Unified treatment for photoluminescence and scattering of coupled metallic nanostructures: I. Two-body system

Photoluminescence (PL) and scattering phenomena from metallic nanostructures have been explained and understood by several point of views. One of them is based on the classic harmonic oscillator model, which describes PL of single mode. In this study, we continue to expand this classic model to a coupling case, which involves two oscillators that interact with each other together with the excitation electric field. The new generated modes due to the coupling are carefully analyzed, including their behaviors varying with the coupling coefficients in different cases. Furthermore, for practical purpose, PL spectra and white light scattering spectra of two individual metallic nanostructures are calculated as examples employing the model to verify its validity. This work would give a deeper understanding on coupling PL and scattering phenomena and is helpful to relative applications.

Preparation and Characterization of Nano-Selenium Decorated by Chondroitin Sulfate Derived from Shark Cartilage and Investigation on Its Antioxidant Activity.

In the present study, a selenium-chondroitin sulfate (SeCS) was synthesized by the sodium selenite (Na2SeO3) and ascorbic acid (Vc) redox reaction using chondroitin sulfate derived from shark cartilage as a template, and characterized by SEM, SEM-EDS, FTIR and XRD. Meanwhile, its stability was investigated at different conditions of pH and temperatures. Besides, its antioxidant activity was further determined by the DPPH and ABTS assays. The results showed the SeCS with the smallest particle size of 131.3 ± 4.4 nm and selenium content of 33.18% was obtained under the optimal condition (CS concentration of 0.1 mg/mL, mass ratio of Na2SeO3 to Vc of 1:8, the reaction time of 3 h, and the reaction temperature of 25 °C). SEM image showed the SeCS was an individual and spherical nanostructure and its structure was evidenced by FTIR and XRD. Meanwhile, SeCS remained stable at an alkaline pH and possessed good storage stability at 4 °C for 28 days. The results on scavenging free radical levels showed that SeCS exhibited significantly higher antioxidant activity than SeNPs and CS, indicating that SeCS had a potential antioxidant effect.

Nanostructured silver substrates produced by cluster-assisted gas jet deposition for surface-enhanced Raman spectroscopy

The gas jet deposition (GJD) technique is a very promising method for synthesis of nanostructured films allowing fabrication of uniform thin layers over large areas with high growth rates. In this work, thin silver films of various morphology with the thickness in the range 10–50 nm are produced by the GJD method using a supersonic jet of silver vapor with helium as a carrier gas. The film morphology transformation with increasing thickness is shown to occur from individual nanostructures to a continuous film. The jet is investigated by time-of-flight mass spectrometry and small silver clusters in the deposited flow are observed which are supposed to play a key role in the nanostrtucture formation. The GJD-produced films demonstrate excellent uniformity over a ∼100 cm2 deposited area and exhibit remarkable plasmonic properties. The surface-enhancement Raman scattering (SERS) activity of the films to R6G is investigated and the enhancement factor of about 105 is obtained with the detection limit of 10−8 M R6G. The influence of morphology on plasmonic properties of the films is discussed.

Transdermal Delivery of Therapeutic Compounds With Nanotechnological Approaches in Psoriasis.

Psoriasis is a chronic, immune-mediated skin disorder involving hyperproliferation of the keratinocytes in the epidermis. As complex as its pathophysiology, the optimal treatment for psoriasis remains unsatisfactorily addressed. Though systemic administration of biological agents has made an impressive stride in moderate-to-severe psoriasis, a considerable portion of psoriatic conditions were left unresolved, mainly due to adverse effects from systemic drug administration or insufficient drug delivery across a highly packed stratum corneum via topical therapies. Along with the advances in nanotechnologies, the incorporation of nanomaterials as topical drug carriers opens an obvious prospect for the development of antipsoriatic topicals. Hence, this review aims to distinguish the benefits and weaknesses of individual nanostructures when applied as topical antipsoriatics in preclinical psoriatic models. In view of specific features of each nanostructure, we propose that a proper combination of distinctive nanomaterials according to the physicochemical properties of loaded drugs and clinical features of psoriatic patients is becoming a promising option that potentially drives the translation of nanomaterials from bench to bedside with improved transdermal drug delivery and consequently therapeutic effects.

An Accessible Integrated Nanoparticle in a Metallic Hole Structure for Efficient Plasmonic Applications.

Addressing the severe deterioration of gap mode properties in spherical-shaped nanoparticles (NPs) becomes necessary due to their utilization in a wide range of multi-disciplinary applications. In this work, we report an integrated plasmonic nanostructure based on a spherical-shaped nanoparticle (NP) in a metallic hole as an alternative to a NP-only structure. With the help of three-dimensional (3D) electromagnetic simulations, we reveal that when a NP is positioned on the top of a metallic hole, it can exhibit superior gap-mode-based local-field intensity enhancement. The integrated nanostructure displayed a ~22-times increase in near-field enhancement characteristics, similar to cube- or disk-shaped nanostructure’s plasmonic properties. From an experimental perspective, the NP positioning on top of the metallic hole can be realized more easily, facilitating a simple fabrication meriting our design approach. In addition to the above advantages, a good geometrical tolerance (metallic hole-gap size error of ~20 nm) supported by gap mode characteristics enhances flexibility in fabrication. These combined advantages from an integrated plasmonic nanostructure can resolve spherical-shaped NP disadvantages as an individual nanostructure and enhance its utilization in multi-disciplinary applications.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy.

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.