Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing

Nanosized-laser-induced sub-20 nm homogenous alloy nanoparticles

Alloy nanoparticles (NPs) have great potential in nanosized 3D-printing, surface coating, plasmonic enhancement, information coding, and so forth. However, chemical-pollution-free and homogeneous sub-20 nm NPs maintain still a challenge in preparation. Here we present a smart nanosecond laser scan strategy of alloy-NPs preparation on a bilayer metal film by using a nanosized focused beam, successfully realizing controllable fabrication of the sub-20 nm homogeneous alloy NPs without pollution. As a demonstration, various sub-20 nm AgCu NPs with different volume ratios have been prepared, all NPs show narrow size distribution and uniform interparticle spacing. This simple and cost-effective method is stable and adaptable for other alloy-NPs such as AuAg NPs. In addition, such alloy NPs exhibit two-peak plasma resonance feature and information coding capacity. We believe that homogenous alloy sub-20 nm NPs will provide new application opportunities in many fields.

Thin-Film-Assisted Photothermal Deformation of Gold Nanoparticles: A Facile and In-Situ Strategy for Single-Plate-Based Devices

Optical-induced shape transformation of single nanoparticles on substrates has shown benefits of simplicity and regularity for single-particle device fabrication and on-chip integration. However, most of the existing strategies are based on wet chemical growth and etching, which could lead to surface contamination with limited local selectivity and device compatibility. Shape deformation via the photothermal effect can overcome these issues but has limited versatility and tunability largely due to the high surface tension of the molten droplet. Here we show gold nanoparticles (Au NPs) can drastically transform into nanoplates under the irradiation of a continuous wave laser (446 nm). We reveal the dielectric thin film underneath the molten Au is critical in deforming the NP into faceted nanoplate under the drive of photothermophoretic forces, which is sufficient to counteract the surface tension of the molten droplet. Both experimental evidence and simulations support this thin-film-assisted photothermal deformation mechanism, which is local selective and generally applicable to differently shaped Au NPs. It provides a facile and robust strategy for single-plate-based device applications.

Thermometries for Single Nanoparticles Heated with Light

The development of efficient nanoscale photon absorbers, such as plasmonic or high-index dielectric nanostructures, allows the remotely controlled release of heat on the nanoscale using light. These photothermal nanomaterials have found applications in various research and technological fields, ranging from materials science to biology. However, measuring the nanoscale thermal fields remains an open challenge, hindering full comprehension and control of nanoscale photothermal phenomena. Here, we review and discuss existent thermometries suitable for single nanoparticles heated under illumination. These methods are classified in four categories according to the region where they assess temperature: (1) the average temperature within a diffraction-limited volume, (2) the average temperature at the immediate vicinity of the nanoparticle surface, (3) the temperature of the nanoparticle itself, and (4) a map of the temperature around the nanoparticle with nanoscale spatial resolution. In the latter, because it is the most challenging and informative type of method, we also envisage new combinations of technologies that could be helpful in retrieving nanoscale temperature maps. Finally, we analyze and provide examples of strategies to validate the results obtained using different thermometry methods.

Optothermal Needle-Free Injection of Vaterite Nanocapsules

The propulsion and acceleration of nanoparticles with light have both fundamental and applied significance across many disciplines. Needle-free injection of biomedical nano cargoes into living tissues is among the examples. Here a new physical mechanism of laser-induced particle acceleration is explored, based on abnormal optothermal expansion of mesoporous vaterite cargoes. Vaterite nanoparticles, a metastable form of calcium carbonate, are placed on a substrate, underneath a target phantom, and accelerated toward it with the aid of a short femtosecond laser pulse. Light absorption followed by picosecond-scale thermal expansion is shown to elevate the particle's center of mass thus causing acceleration. It is shown that a 2 µm size vaterite particle, being illuminated with 0.5 W average power 100 fsec IR laser, is capable to overcome van der Waals attraction and acquire 15m sec-1 velocity. The demonstrated optothermal laser-driven needle-free injection into a phantom layer and Xenopus oocyte in vitro promotes the further development of light-responsive nanocapsules, which can be equipped with additional optical and biomedical functions for delivery, monitoring, and controllable biomedical dosage to name a few.

Thermal dynamics of gold nanoshell dimers under femtosecond laser pulse irradiation: A numerical approach

We present a numerical investigation of the photothermal response of gold nanoshell (AuNS) dimers when subjected to femtosecond laser pulse irradiation. The time-varying temperature fields for core-shell AuNS dimers are quantified by implementing finite element modeling, integrating the electromagnetic and thermal dual-physics simulations. Given the ultrafast nature of laser pulses, we employ a two-temperature model to accurately portray the energy transfer from excited electrons to the lattice system, a process typically completed post pulse-termination. The temporal analysis of the temperature in the AuNS and the surrounding medium, together with the spatial temperature distribution under different separation distances, elucidates the processes that drive the AuNS dimers' transient temperature distribution and heat dissipation. We report on the critical effects of geometrical parameters on the photothermal response, demonstrating that thinner shells maximize the total deposited energy per unit volume, resulting in increased temperature fields, while decreasing separation distances result in excessive field amplification due to plasmonic modes' production. Our robust numerical approach, enabling simulations with tunable material properties and configurations, may help design nanomaterials with desired features for photothermal cancer treatment and imaging.

Anapole-Assisted Low-Power Optical Trapping of Nanoscale Extracellular Vesicles and Particles

This study addresses the challenge of trapping nanoscale biological particles using optical tweezers without the photothermal heating effect and the limitation presented by the diffraction limit. Optical tweezers are effective for trapping microscopic biological objects but not for nanoscale specimens due to the diffraction limit. To overcome this, we present an approach that uses optical anapole states in all-dielectric nanoantenna systems on distributed Bragg reflector substrates to generate strong optical gradient force and potential on nanoscale biological objects with negligible temperature rise below 1 K. The anapole antenna condenses the accessible electromagnetic energy to scales as small as 30 nm. Using this approach, we successfully trapped nanosized extracellular vesicles and supermeres (approximately 25 nm in size) using low laser power of only 10.8 mW. This nanoscale optical trapping platform has great potential for single molecule analysis while precluding photothermal degradation.

Multimodal Optothermal Manipulations along Various Surfaces

Optical tweezers have provided tremendous opportunities for fundamental studies and applications in the life sciences, chemistry, and physics by offering contact-free manipulation of small objects. However, it requires sophisticated real-time imaging and feedback systems for conventional optical tweezers to achieve controlled motion of micro/nanoparticles along textured surfaces, which are required for such applications as high-resolution near-field characterizations of cell membranes with nanoparticles as probes. In addition, most optical tweezers systems are limited to single manipulation modes, restricting their broader applications. Herein, we develop an optothermal platform that enables the multimodal manipulation of micro/nanoparticles along various surfaces. Specifically, we achieve the manipulation of micro/nanoparticles through the synergy between the optical and thermal forces, which arise due to the temperature gradient self-generated by the particles absorbing the light. With a simple control of the laser beam, we achieve five switchable working modes [i.e., tweezing, rotating, rolling (toward), rolling (away), and shooting] for the versatile manipulation of both synthesized particles and biological cells along various substrates. More interestingly, we realize the manipulation of micro/nanoparticles on rough surfaces of live worms and their embryos for localized control of biological functions. By enabling the three-dimensional control of micro/nano-objects along various surfaces, including topologically uneven biological tissues, our multimodal optothermal platform will become a powerful tool in life sciences, nanotechnology, and colloidal sciences.

Single-Step Plasmonic Dimer Printing by Gold Nanorod Splitting with Light

Optical printing is a flexible strategy to precisely pattern plasmonic nanoparticles for the realization of nanophotonic devices. However, the generation of strongly coupled plasmonic dimers by sequential particle printing can be a challenge. Here, we report an approach to generate and pattern dimer nanoantennas in a single step by optical splitting of individual gold nanorods with laser light. We show that the two particles that constitute the dimer can be separated by sub-nanometer distances. The nanorod splitting process is explained by a combination of plasmonic heating, surface tension, optical forces, and inhomogeneous hydrodynamic pressure introduced by a focused laser beam. This realization of optical dimer formation and printing from a single nanorod provides a means for dimer patterning with high accuracy for nanophotonic applications.

Thermofluidic assembly of colloidal crystals

Colloidal crystals are interesting as functional structures due to their emergent photonic properties like photonic stop bands and bandgaps that can be used to redirect light. They are commonly formed by a drying process that is assisted by capillary forces at the drying fronts. In this manuscript, we demonstrate the optically induced dynamic thermofluidic assembly of 2D and 3D colloidal crystals. We quantify in experiment and simulation the structure formation and identify thermo-osmosis and temperature induced depletion interactions as the key contributors to the colloidal crystal formation. The non-equilibrium nature of the assembly of colloidal crystals and its dynamic control by laser-induced local heating promise new possibilities for a versatile formation of photonic structures inaccessible by equilibrium processes.

Light-Programmed Bistate Colloidal Actuation Based on Photothermal Active Plasmonic Substrate

Active particles have been regarded as the key models to mimic and understand the complex systems of nature. Although chemical and field-powered active particles have received wide attentions, light-programmed actuation with long-range interaction and high throughput remains elusive. Here, we utilize photothermal active plasmonic substrate made of porous anodic aluminum oxide filled with Au nanoparticles and poly( N -isopropylacrylamide) (PNIPAM) to optically oscillate silica beads with robust reversibility. The thermal gradient generated by the laser beam incurs the phase change of PNIPAM, producing gradient of surface forces and large volume changes within the complex system. The dynamic evolution of phase change and water diffusion in PNIPAM films result in bistate locomotion of silica beads, which can be programmed by modulating the laser beam. This light-programmed bistate colloidal actuation provides promising opportunity to control and mimic the natural complex systems.

From cavity optomechanics to cavity-less exciton optomechanics: a review

Cavity optomechanical coupling based on radiation pressure, photothermal forces and the photoelastic effect has been investigated widely over the past few decades, including optical measurements of mechanical vibration, dynamic backaction damping and amplification, nonlinear dynamics, quantum state transfer and so on. However, the delicate cavity operation, including cavity stabilization, fine detuning, tapered fibre access etc., limits the integration of cavity optomechanical devices. Dynamic backaction damping and amplification based on cavity-less exciton optomechanical coupling in III-V semiconductor nanomechanical systems, semiconductor nanoribbons and monolayer transition metal dichalcogenides have been demonstrated in recent years. The cavity-less exciton optomechanical systems interconnect photons, phonons and excitons in a highly integrable platform, opening up the development of integrable optomechanics. Furthermore, the highly tunable exciton resonance enables the exciton optomechanical coupling strength to be tuned. In this review, the mechanisms of cavity optomechanical coupling, the principles of exciton optomechanical coupling and the recent progress of cavity-less exciton optomechanics are reviewed. Moreover, the perspectives for exciton optomechanical devices are described.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Plasmonic nanoscale temperature shaping on a single titanium nitride nanostructure

Arbitrary shaping of temperature fields at the nanometre scale is an important goal in nanotechnology; however, this is challenging because of the diffusive nature of heat transfer. In the present work, we numerically demonstrated that spatial shaping of nanoscale temperature fields can be achieved by plasmonic heating of a single titanium nitride (TiN) nanostructure. A key feature of TiN is its low thermal conductivity (k_TiN = 29 [W m^-1 K^-1]) compared with ordinary plasmonic metals such as Au (k_Au = 314 [W m^-1 K^-1]). When the localised surface plasmon resonance of a metal nanostructure is excited, the light intensity is converted to heat power density in the nanostructure via the Joule heating effect. For a gold nanoparticle, non-uniform spatial distributions of the heat power density will disappear because of the high thermal conductivity of Au; the nanoparticle surface will be entirely isothermal. In contrast, the spatial distributions of the heat power density can be clearly transcribed into temperature fields on a TiN nanostructure because the heat dissipation is suppressed. In fact, we revealed that highly localised temperature distributions can be selectively controlled around the TiN nanostructure at a spatial resolution of several tens of nanometres depending on the excitation wavelength. The present results indicate that arbitrary temperature shaping at the nanometre scale can be achieved by designing the heat power density in TiN nanostructures for plasmonic heating, leading to unconventional thermofluidics and thermal chemical biology.

Programmable Multimodal Optothermal Manipulation of Synthetic Particles and Biological Cells

Optical manipulation of tiny objects has benefited many research areas ranging from physics to biology to micro/nanorobotics. However, limited manipulation modes, intense lasers with complex optics, and applicability to limited materials and geometries of objects restrict the broader uses of conventional optical tweezers. Herein, we develop an optothermal platform that enables the versatile manipulation of synthetic micro/nanoparticles and live cells using an ultralow-power laser beam and a simple optical setup. Five working modes (i.e., printing, tweezing, rotating, rolling, and shooting) have been achieved and can be switched on demand through computer programming. By incorporating a feedback control system into the platform, we realize programmable multimodal control of micro/nanoparticles, enabling autonomous micro/nanorobots in complex environments. Moreover, we demonstrate in situ three-dimensional single-cell surface characterizations through the multimodal optothermal manipulation of live cells. This programmable multimodal optothermal platform will contribute to diverse fundamental studies and applications in cellular biology, nanotechnology, robotics, and photonics.

Universal optothermal micro/nanoscale rotors

Rotation of micro/nano-objects is important for micro/nanorobotics, three-dimensional imaging, and lab-on-a-chip systems. Optical rotation techniques are especially attractive because of their fuel-free and remote operation. However, current techniques require laser beams with designed intensity profile and polarization or objects with sophisticated shapes or optical birefringence. These requirements make it challenging to use simple optical setups for light-driven rotation of many highly symmetric or isotropic objects, including biological cells. Here, we report a universal approach to the out-of-plane rotation of various objects, including spherically symmetric and isotropic particles, using an arbitrary low-power laser beam. Moreover, the laser beam is positioned away from the objects to reduce optical damage from direct illumination. The rotation mechanism based on opto-thermoelectrical coupling is elucidated by rigorous experiments combined with multiscale simulations. With its general applicability and excellent biocompatibility, our universal light-driven rotation platform is instrumental for various scientific research and engineering applications.

Centimeter-Scale Superlattices of Three-Dimensionally Orientated Plasmonic Dimers with Highly Tunable Collective Properties

The precise organization and orientation of plasmonic molecules on substrates is crucial to their application in functional devices but still remains a grand challenge. This article describes a bottom-up strategy to efficiently fabricate centimeter-scale superlattices of three-dimensionally oriented plasmonic dimers with highly tunable collective optical properties on substrates. The in-plane (i.e., X-Y plane) and out-of-plane (i.e., along Z-axis) orientation of the constituent plasmonic dimers can be precisely controlled by a combination of directional capillary force and supporting polymer film. Our experimental measurements and numerical simulations show that the macroscopic dimer superlattices exhibit polarization-dependent plasmon Fano resonances in air and multimodal surface lattice resonances with high quality factors in a homogeneous medium, owing to the high positional and orientational ordering of the subunits. Our strategy enables the fabrication of complex plasmonic nanostructures with precise configurations for advanced plasmonic devices such as plasmon nanolasing and metamaterials.

Gold nanodoughnut as an outstanding nanoheater for photothermal applications

Photoinduced hyperthermia is a cancer therapy technique that induces death to cancerous cells via heat generated by plasmonic nanoparticles. While previous studies have shown that some nanoparticles can be effective at killing cancer cells under certain conditions, there is still a necessity (or the need) to improve its heating efficiency. In this work, we perform a detailed theoretical study comparing the thermoplasmonic response of the most effective nanoparticle geometries up to now with a doughnut-shaped nanoparticle. We numerically demonstrate that the latter exhibits a superior tunable photothermal response in practical illumination conditions (unpolarized light). Furthermore, we show that nanoparticle heating in fluidic environments, i.e., nanoparticles undergoing Brownian rotations, strongly depends on the particle orientation with respect to the illumination source. We conclude that nanodoughnuts are the best nanoheaters in our set of structures, with an average temperature increment 40% higher than the second best nanoheater (nanodisk). Furthermore, nanodoughnuts feature a weak dependence on orientation, being therefore ideal candidates for photothermal therapy applications. Finally, we present a designing guide, covering a wide range of toroid designs, which can help on its experimental implementation.

Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer

Surface enhanced Raman scattering (SERS) is optically sensitive and chemically specific to detect single-molecule spectroscopic signatures. Facilitating this capability in optically trapped nanoparticles at low laser power remains a significant challenge. In this letter, we show single molecule SERS signatures in reversible assemblies of trapped plasmonic nanoparticles using a single laser excitation (633 nm). Importantly, this trap is facilitated by the thermoplasmonic field of a single gold nanoparticle dropcasted on a glass surface. We employ the bianalyte SERS technique to ascertain the single molecule statistical signatures and identify the critical parameters of the thermoplasmonic tweezer that provide this sensitivity. Furthermore, we show the utility of this low power (≈ 0.1 mW/μm²) tweezer platform to trap a single gold nanoparticle and transport assembly of nanoparticles. Given that our configuration is based on a dropcasted gold nanoparticle, we envisage its utility to create reconfigurable plasmonic metafluids in physiological and catalytic environments and to be potentially adapted as an in vivo plasmonic tweezer.

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO₃ solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag⁺ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Light-Driven Magnetic Encoding for Hybrid Magnetic Micromachines

Remote manipulation of a micromachine under an external magnetic field is significant in a variety of applications. However, magnetic manipulation requires that either the target objects or the fluids should be ferromagnetic or superparamagnetic. To extend the applicability, we propose a versatile optical printing technique termed femtosecond laser-directed bubble microprinting (FsLDBM) for on-demand magnetic encoding. Harnessing Marangoni convection, evaporation flow, and capillary force for long-distance delivery, near-field attraction, and printing, respectively, FsLDBM is capable of printing nanomaterials on the solid-state substrate made of arbitrary materials. As a proof-of-concept, we actuate a 3D polymer microturbine under a rotating magnetic field by implementing γ-Fe₂O₃ nanomagnets on its blade. Moreover, we demonstrate the magnetic encoding on a living daphnia and versatile manipulation of the hybrid daphnia. With its general applicability, the FsLDBM approach provides opportunities for magnetic control of general microstructures in a variety of applications, such as smart microbots and biological microsurgery.

In Situ Photothermal Response of Single Gold Nanoparticles through Hyperspectral Imaging Anti-Stokes Thermometry

Several fields of applications require a reliable characterization of the photothermal response and heat dissipation of nanoscopic systems, which remains a challenging task for both modeling and experimental measurements. Here, we present an implementation of anti-Stokes thermometry that enables the in situ photothermal characterization of individual nanoparticles (NPs) from a single hyperspectral photoluminescence confocal image. The method is label-free, potentially applicable to any NP with detectable anti-Stokes emission, and does not require any prior information about the NP itself or the surrounding media. With it, we first studied the photothermal response of spherical gold NPs of different sizes on glass substrates, immersed in water, and found that heat dissipation is mainly dominated by the water for NPs larger than 50 nm. Then, the role of the substrate was studied by comparing the photothermal response of 80 nm gold NPs on glass with sapphire and graphene, two materials with high thermal conductivity. For a given irradiance level, the NPs reach temperatures 18% lower on sapphire and 24% higher on graphene than on bare glass. The fact that the presence of a highly conductive material such as graphene leads to a poorer thermal dissipation demonstrates that interfacial thermal resistances play a very significant role in nanoscopic systems and emphasize the need for in situ experimental thermometry techniques. The developed method will allow addressing several open questions about the role of temperature in plasmon-assisted applications, especially ones where NPs of arbitrary shapes are present in complex matrixes and environments.

Liquid Optothermoelectrics: Fundamentals and Applications

Liquid thermoelectricity describes the redistribution of ions in an electrolytic solution under the influence of temperature gradients, which leads to the formation of electric fields. The thermoelectric field is effective in driving the thermophoretic migration of charged colloidal particles for versatile manipulation. However, traditional macroscopic thermoelectric fields are not suitable for particle manipulations at high spatial resolution. Inspired by optical tweezers and relevant optical manipulation techniques, we employ laser interaction with light-absorbing nanostructures to achieve subtle heat management on the micro- and nanoscales. The resulting thermoelectric fields are exploited to develop new optical technologies, leading to a research field known as liquid optothermoelectrics. This Invited Feature Article highlights our recent works on advancing fundamentals, technologies, and applications of optothermoelectrics in colloidal solutions. The effects of light irradiation, substrates, electrolytes, and particles on the optothermoelectric manipulations of colloidal particles along with their theoretical limitations are discussed in detail. Our optothermoelectric technologies with the versatile capabilities of trapping, manipulating, and pulling colloidal particles at low optical power are finding applications in microswimmers and nanoscience. With its intricate interfacial processes and tremendous technological promise, optothermoelectrics in colloidal solutions will remain relevant for the foreseeable future.

Atomistic modeling and rational design of optothermal tweezers for targeted applications

Optical manipulation of micro/nanoscale objects is of importance in life sciences, colloidal science, and nanotechnology. Optothermal tweezers exhibit superior manipulation capability at low optical intensity. However, our implicit understanding of the working mechanism has limited the further applications and innovations of optothermal tweezers. Herein, we present an atomistic view of opto-thermo-electro-mechanic coupling in optothermal tweezers, which enables us to rationally design the tweezers for optimum performance in targeted applications. Specifically, we have revealed that the non-uniform temperature distribution induces water polarization and charge separation, which creates the thermoelectric field dominating the optothermal trapping. We further design experiments to systematically verify our atomistic simulations. Guided by our new model, we develop new types of optothermal tweezers of high performance using low-concentrated electrolytes. Moreover, we demonstrate the use of new tweezers in opto-thermophoretic separation of colloidal particles of the same size based on the difference in their surface charge, which has been challenging for conventional optical tweezers. With the atomistic understanding that enables the performance optimization and function expansion, optothermal tweezers will further their impacts.

Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

Noble metal nanoparticles illuminated at their plasmonic resonance wavelength turn into heat nanosources. This phenomenon has prompted the development of numerous applications in science and technology. Simultaneous optical manipulation of such resonant nanoparticles could certainly extend the functionality and potential applications of optothermal tools. In this article, we experimentally demonstrate optical transport of single and multiple resonant nanoparticles (colloidal gold spheres of radius 200 nm) directed by tailored transverse phase-gradient forces propelling them around a 2D optical trap. We show how the phase-gradient force can be designed to efficiently change the speed of the nanoparticles. We have found that multiple hot nanoparticles assemble in the form of a quasi-stable group whose motion around the laser trap is also controlled by such optical propulsion forces. This assembly experiences a significant increase in the local temperature, which creates an optothermal convective fluid flow dragging tracer particles into the assembly. Thus, the created assembly is a moving heat source controlled by the propulsion force, enabling indirect control of fluid flows as a micro-optofluidic tool. The existence of these flows, probably caused by the temperature-induced Marangoni effect at the liquid water/superheated water interface, is confirmed by tracking free tracer particles migrating towards the assembly. We propose a straightforward method to control the assembly size, and therefore its temperature, by using a nonuniform optical propelling force that induces the splitting or merging of the group of nanoparticles. We envision further development of microscale optofluidic tools based on these achievements.

High-Rate Printing of Micro/Nanoscale Patterns Using Interfacial Convective Assembly

Printing of electronics has been receiving increasing attention from academia and industry over the recent years. However, commonly used printing techniques have limited resolution of micro- or sub-microscale. Here, a directed-assembly-based printing technique, interfacial convective assembly, is reported, which utilizes a substrate-heating-induced solutal Marangoni convective flow to drive particles toward patterned substrates and then uses van der Waals interactions as well as geometrical confinement to trap the particles in the pattern areas. The influence of various assembly parameters including type of mixing solvent, substrate temperature, particle concentration, and assembly time is investigated. The results show successful assembly of various nanoparticles in patterns of different shapes with a high resolution down to 25 nm. In addition, the assembly only takes a few minutes, which is two orders of magnitude faster than conventional convective assembly. Small-sized (diameter below 5 nm) nanoparticles tend to coalesce during the assembly process and form sintered structures. The fabricated silver nanorods show single-crystal structure with a low resistivity of 8.58 × 10^-5 Ω cm. With high versatility, high resolution, and high throughput, the interfacial convective assembly opens remarkable opportunities for printing next generation nanoelectronics and sensors.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Opto-thermophoretic separation and trapping of plasmonic nanoparticles

Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 μm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.

From Optical to Chemical Hot Spots in Plasmonics

In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. In fact, the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons, and molecular states. These degrees of freedom can affect the reaction rates, the product selectivity, or the spatial localization of a chemical reaction. In this Account, we discuss the oportunities to control chemical hot spots by tuning the cascade of events that follows the excitation and decay of plasmonic modes in nanostructures. We discuss a series of techniques to spatially map and image plasmonic nanoscale reactivity at the single photocatalyst level. We show how to optimize the reactivity of carriers by manipulating their excitation and decay mechanisms in plasmonic nanoparticles. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Understanding and controlling these processes is essential for the rational design of solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

Digital manufacturing of advanced materials: challenges and perspective

The rapid development in materials science and engineering requests the manufacturing of materials in a more rational and designable manner. Beyond traditional manufacturing techniques, such as casting and coating, digital control of material morphology, composition, and structure represents a highly integrated and versatile approach. Digital manufacturing systems enable users to fabricate freeform materials, which lead to new functionalities and applications. Digital additive manufacturing (AM), which is a layer-by-layer fabrication approach to create three-dimensional (3D) products with complex geometries, is changing the way materials manufacturing is approached in traditional industry. More recently, digital printing of chemically synthesized colloidal nanoparticles has paved the way towards manufacturing a class of designer nanomaterials with properties precisely tailored by the nanoparticles and their interactions down to atomic scales. Despite the tremendous progress being made so far, multiple challenges have prevented the broader applications and impacts of the digital manufacturing technologies. This review features cutting-edge research in the development of some of the most advanced digital manufacturing methods. We focus on outlining major challenges in the field and providing our perspectives on the future research and development directions.

Plasmonic Heating of Nanostructures

The absorption of light by plasmonic nanostructures and their associated temperature increase are exquisitely sensitive to the shape and composition of the structure and to the wavelength of light. Therefore, much effort is put into synthesizing novel nanostructures for optimized interaction with the incident light. The successful synthesis and characterization of high quality and biocompatible plasmonic colloidal nanoparticles has fostered numerous and expanding applications, especially in biomedical contexts, where such particles are highly promising for general drug delivery and for tomorrow's cancer treatment. We review the thermoplasmonic properties of the most commonly used plasmonic nanoparticles, including solid or composite metallic nanoparticles of various dimensions and geometries. Common methods for synthesizing plasmonic particles are presented with the overall goal of providing the reader with a guide for designing or choosing nanostructures with optimal thermoplasmonic properties for a given application. Finally, the biocompatibility and biological tolerance of structures are critically discussed along with novel applications of plasmonic nanoparticles in the life sciences.

Optical Nanoprinting of Colloidal Particles and Functional Structures

Recent advances in chemical sciences have enabled the tailorable synthesis of colloidal particles with variable composition, size, shape, and properties. Building superstructures with colloidal particles as building blocks is appealing for the fabrication of functional metamaterials and nanodevices. Optical nanoprinting provides a versatile platform to print various particles into arbitrary configurations with nanometric precision. In this review, we summarize recent progress in optical nanoprinting of colloidal particles and its related applications. Diverse techniques based on different physical mechanisms, including optical forces, light-controlled electric fields, optothermal effects, laser-directed thermocapillary flows, and photochemical reactions, are discussed in detail. With its flexible and versatile capabilities, optical nanoprinting will find promising applications in numerous fields such as nanophotonics, energy, microelectronics, and nanomedicine.

Creating Multifunctional Optofluidic Potential Wells for Nanoparticle Manipulation

Optical forces have enabled various nanomanipulation in microfluidics such as optical trapping, sorting, and transporting of nanoparticles (NPs), but the manipulation is usually specific with a certain optical field. Tightly focused Gaussian beams can trap NPs but not sort them; moderately focused Gaussian beams allow sorting microparticles in a flow but not NPs; quasi-Bessel beams can sort NPs in a flow but cannot control their positions due to low trapping stiffness. All these methods rely on the axial variation of laser intensity. Here we show that multifunctional and tunable optofluidic potential wells can be created for nanomanipulation by synchronizing optical phase gradient force with fluid drag force. We demonstrate controlled trapping and transporting of 150 nm Ag NPs over 10 μm and sorting of 80 and 100 nm Au NPs using optical line traps with tunable phase gradients in experiments. Our simulations further predict that simultaneous sorting and trapping of sub-50 nm Au NPs can be achieved with a sorting resolution of 1 nm using optimized optical fields. Our method provides great freedom and flexibility for nanomanipulation in optofluidics with potential applications in nanophotonics and biomedicine.

Dissipative Self-Assembly of Anisotropic Nanoparticle Chains with Combined Electrodynamic and Electrostatic Interactions

Dissipative self-assembly of colloidal nanoparticles offers the prospect of creating reconfigurable artificial materials and systems, yet the phenomenon only occurs far from thermodynamic equilibrium. Therefore, it is usually difficult to predict and control. Here, a dissipative colloidal solution system, where anisotropic chains with different interparticle separations in two perpendicular directions transiently arise among largely disordered silver nanoparticles illuminated by a laser beam, is reported. The optical field creates a nonequilibrium dissipative state, where a disorder-to-order transition occurs driven by anisotropic electrodynamic interactions coupled with electrostatic interactions. Investigation of the temporal dynamics and spatial arrangements of the nanoparticle system shows that the optical binding strength and entropy of the system are two crucial parameters for the formation of the anisotropic chains and responsible for adaptive behaviors, such as self-replication of dimer units. Formation of anisotropic nanoparticle chains is also observed among colloidal nanoparticles made from other metal (e.g., Au), polymer (e.g., polystyrene), ceramic (e.g., CeO₂ ), and hybrid materials (e.g., SiO₂ @Au core-shell), suggesting that light-driven self-organization will provide a wide range of opportunities to discover new dissipative structures under thermal fluctuations and build novel anisotropic materials with nanoscale order.

Photothermal Convection Lithography for Rapid and Direct Assembly of Colloidal Plasmonic Nanoparticles on Generic Substrates

Controlled assembly of colloidal nanoparticles onto solid substrates generally needs to overcome their thermal diffusion in water. For this purpose, several techniques that are based on chemical bonding, capillary interactions with substrate patterning, optical force, and optofluidic heating of light-absorbing substrates are proposed. However, the direct assembly of colloidal nanoparticles on generic substrates without chemical linkers and substrate patterning still remains challenging. Here, photothermal convection lithography is proposed, which allows the rapid placement of colloidal nanoparticles onto the surface of diverse solid substrates. It is based on local photothermal heating of colloidal nanoparticles by resonant light focusing without substrate heating, which induces convective flow. The convective flow, then, forces the colloidal nanoparticles to assemble at the illumination point of light. The size of the assembly is increased by either increasing the light intensity or illumination time. It is shown that three types of colloidal gold nanoparticles with different shapes (rod, star, and sphere) can be uniformly assembled by the proposed method. Each assembly with a diameter of tens of micrometers can be completed within a minute and its patterned arrays can also be achieved rapidly.

Nanoradiator-Mediated Deterministic Opto-Thermoelectric Manipulation

Optical manipulation of colloidal nanoparticles and molecules is significant in numerous fields. Opto-thermoelectric nanotweezers exploiting multiple coupling among light, heat, and electric fields enables the low-power optical trapping of nanoparticles on a plasmonic substrate. However, the management of light-to-heat conversion for the versatile and precise manipulation of nanoparticles is still elusive. Herein, we explore the opto-thermoelectric trapping at plasmonic antennas that serve as optothermal nanoradiators to achieve the low-power (∼0.08 mW/μm2) and deterministic manipulation of nanoparticles. Specifically, precise optical manipulation of nanoparticles is achieved via optical control of the subwavelength thermal hot spots. We employ a femtosecond laser beam to further improve the heat localization and the precise trapping of single ∼30 nm semiconductor quantum dots at the antennas where the plasmon-exciton coupling can be tuned. With its low-power, precise, and versatile particle control, the opto-thermoelectric manipulation can have applications in photonics, life sciences, and colloidal sciences.

Opto-Thermophoretic Manipulation and Construction of Colloidal Superstructures in Photocurable Hydrogels

Light-based manipulation of colloidal particles holds great promise in fabrication of functional devices. Construction of complex colloidal superstructures using traditional optical tweezers is limited by high operation power and strong heating effect. Herein, we demonstrate low-power opto-thermophoretic manipulation and construction of colloidal superstructures in photocurable hydrogels. By introducing cationic surfactants into a hydrogel solution under a light-directed temperature field, we create both thermoelectric fields and depletion attraction forces to control the suspended colloidal particles. The particles of various sizes and compositions are thus trapped and organized into various superstructures. Furthermore, the colloidal superstructures are immobilized and patterned onto solid-state substrates through UV-induced photopolymerization of the hydrogel. Our opto-thermophoretic technique will open up avenues for bottom-up assembly of colloidal materials and devices.

Trapping and Deposition of Dye-Molecule Nanoparticles in the Nanogap of a Plasmonic Antenna

Plasmonic nanostructures, which allow light focusing at the deep subwavelength scale, and colloidal nanoparticles with unique optoelectronic properties are nowadays fabricated with nanometer precision. However, to fully control and exploit nanoscale light-matter interactions in hybrid plasmonic-nanophotonic devices, both materials must be assembled in heterostructures with similar precision. Near-field optical forces have recently attracted much attention, as they can precisely trap and position nanoparticles at plasmonic hotspots. However, long-range attraction and the surface bonding of nanoparticles usually require other specific techniques, such as electrothermal heating and surface chemical treatments. This Letter reports on the optical trapping and deposition of dye-molecule nanoparticles in the nanogap of a gold antenna. The nanoparticles are captured by focusing a near-infrared laser beam on a targeted plasmonic antenna. This single-step deposition process requires only a few seconds under 1.4-1.8 MW·cm^-2 continuous-wave illumination and shows a polarization dependence smaller than expected. Fluorescence and electronic microscopy observations suggest that nanoparticle deposition arises from a trade-off between optical and thermal effects.

Nondestructive Approach for Additive Nanomanufacturing of Metallic Nanostructures in the Air

In this article, a mechanism for quick release and transfer of gold nanoparticles (GNPs) from a soft substrate to another substrate under laser illumination is investigated. The heating of GNPs on a soft substrate with a continuous-wave laser causes a rapid thermal expansion of the substrate, which can be used to selectively release and place GNPs onto another surface. In-plane and out-of-plane nanostructures are successfully fabricated using this method. This rapid release-and-place process can be used for additive nonmanufacturing of metallic nanostructures under ambient conditions, which paves a way for affordable nanomanufacturing and enables a wide variety of applications in nanophotonics, ultrasensitive sensing, and nonlinear plasmonics.

Raman photostability of off-resonant gap-enhanced Raman tags

Surface-enhanced Raman scattering (SERS) nanoprobes show promising potential for biosensing and bioimaging applications due to advantageous features of ultrahigh sensitivity and specificity. However, very limited research has been reported on the SERS photostability of nanoprobes upon continuous laser irradiation, which is critical for high-speed and time-lapse microscopy. The core–shell off-resonant gap-enhanced Raman tags (GERTs) with built-in Raman reporters, excited at near-infrared (NIR) region but with a plasmon resonance at visible region, allow decoupling the plasmon resonance behaviors with the SERS performance and therefore show ultrahigh Raman photostability during continuous laser irradiation. In this work, we have synthesized five types of off-resonant GERTs with different embedded Raman reporters, numbers of shell layer, or nanoparticle shapes. Via thorough examination of time-resolved SERS trajectories and quantitative analysis of photobleaching behaviors, we have demonstrated that double metallic-shell GERTs embedded with 1,4-benzenedithiol molecules show the best photostability performance, to the best of our knowledge, among all SERS nanoprobes reported before, with a photobleaching time constant up to 4.8 × 10⁵ under a laser power density of 4.7 × 10⁵ W cm⁻². Numerical calculations additionally support that the local plasmonic heating effect in fact can be greatly minimized using the off-resonance strategy. Moreover, double-shell BDT-GERTs are highly potential for high-speed and high-resolution Raman-based cell bioimaging.

Off-resonant gap-enhanced Raman tags (GERTs) show ultrahigh Raman enhancement and photostabilities and therefore can be used as ideal highly photostable nanoprobes for high-speed and high-resolution Raman bioimaging.