Xiaolei Peng

Bubble-Pen Lithography

Current lithography techniques, which employ photon, electron, or ion beams to induce chemical or physical reactions for micro/nano-fabrication, have remained challenging in patterning chemically synthesized colloidal particles, which are emerging as building blocks for functional devices. Herein, we develop a new technique - bubble-pen lithography (BPL) - to pattern colloidal particles on substrates using optically controlled microbubbles. Briefly, a single laser beam generates a microbubble at the interface of colloidal suspension and a plasmonic substrate via plasmon-enhanced photothermal effects. The microbubble captures and immobilizes the colloidal particles on the substrate through coordinated actions of Marangoni convection, surface tension, gas pressure, and substrate adhesion. Through directing the laser beam to move the microbubble, we create arbitrary single-particle patterns and particle assemblies with different resolutions and architectures. Furthermore, we have applied BPL to pattern CdSe/ZnS quantum dots on plasmonic substrates and polystyrene (PS) microparticles on two-dimensional (2D) atomic-layer materials. With the low-power operation, arbitrary patterning and applicability to general colloidal particles, BPL will find a wide range of applications in microelectronics, nanophotonics, and nanomedicine.

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10-100 mW/μm2) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm2. Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery.

Thermophoretic Tweezers for Low-Power and Versatile Manipulation of Biological Cells

Optical manipulation of biological cells and nanoparticles is significantly important in life sciences, early disease diagnosis, and nanomanufacturing. However, low-power and versatile all-optical manipulation has remained elusive. Herein, we have achieved light-directed versatile thermophoretic manipulation of biological cells at an optical power 100-1000 times lower than that of optical tweezers. By harnessing the permittivity gradient in the electric double layer of the charged surface of the cell membrane, we succeed at the low-power trapping of suspended biological cells within a light-controlled temperature gradient field. Furthermore, through dynamic control of optothermal potentials using a digital micromirror device, we have achieved arbitrary spatial arrangements of cells at a resolution of ∼100 nm and precise rotation of both single and assemblies of cells. Our thermophoretic tweezers will find applications in cellular biology, nanomedicine, and tissue engineering.

Opto-thermophoretic assembly of colloidal matter

Colloidal matter with a wide range of materials, sizes, and configurations was built with opto-thermophoretic assembly. Colloidal matter exhibits unique collective behaviors beyond what occurs at single-nanoparticle and atomic scales. Treating colloidal particles as building blocks, researchers are exploiting new strategies to rationally organize colloidal particles into complex structures for new functions and devices. Despite tremendous progress in directed assembly and self-assembly, a truly versatile assembly technique without specific functionalization of the colloidal particles remains elusive. We develop a new strategy to assemble colloidal matter under a light-controlled temperature field, which can solve challenges in the existing assembly techniques. By adding an anionic surfactant (that is, cetyltrimethylammonium chloride), which serves as a surface charge source, a macro ion, and a micellar depletant, we generate a light-controlled thermoelectric field to manipulate colloidal atoms and a depletion attraction force to assemble the colloidal atoms into two-dimensional (2D) colloidal matter. The general applicability of this opto-thermophoretic assembly (OTA) strategy allows us to build colloidal matter of diverse colloidal sizes (from subwavelength scale to micrometer scale) and materials (polymeric, dielectric, and metallic colloids) with versatile configurations and tunable bonding strengths and lengths. We further demonstrate that the incorporation of the thermoelectric field into the optical radiation force can achieve 3D reconfiguration of the colloidal matter. The OTA strategy releases the rigorous design rules required in the existing assembly techniques and enriches the structural complexity in colloidal matter, which will open a new window of opportunities for basic research on matter organization, advanced material design, and applications.

Opto-thermoelectric nanotweezers

Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers. By optically heating a thermoplasmonic substrate, a light-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles and tunable working wavelength, opto-thermoelectric nanotweezers will become a powerful tool in colloid science and nanotechnology.Heating due to optical losses in metal nanoparticles, which is usually an unwanted side effect, is harnessed to realize low-power opto-thermoelectric nanotweezers.

High-Resolution Bubble Printing of Quantum Dots

Semiconductor quantum dots (QDs) feature excellent properties, such as high quantum efficiency, tunable emission frequency, and good fluorescence stability. Incorporation of QDs into new devices relies upon high-resolution and high-throughput patterning techniques. Herein, we report a new printing technique known as bubble printing (BP), which exploits a light-generated microbubble at the interface of colloidal QD solution and a substrate to directly write QDs into arbitrary patterns. With the uniform plasmonic hot spot distribution for high bubble stability and the optimum light-scanning parameters, we have achieved full-color QD printing with submicron resolution (650 nm), high throughput (scanning rate of ∼10-2 m/s), and high adhesion of the QDs to the substrates. The printing parameters can be optimized to further control the fluorescence properties of the patterned QDs, such as emission wavelength and lifetime. The patterning of QDs on flexible substrates further demonstrates the wide applicability of this new technique. Thus, BP technique addresses the barrier of achieving a widely applicable, high-throughput and user-friendly patterning technique in the submicrometer regime, along with simultaneous fluorescence modification capability.

Optothermal Manipulations of Colloidal Particles and Living Cells

Optical manipulation techniques are important in many fields. For instance, they enable bottom-up assembly of nanomaterials and high-resolution and in situ analysis of biological cells and molecules, providing opportunities for discovery of new materials, medical diagnostics, and nanomedicines. Traditional optical tweezers have their applications limited due to the use of rigorous optics and high optical power. New strategies have been established for low-power optical manipulation techniques. Optothermal manipulation, which exploits photon-phonon conversion and matter migration under a light-controlled temperature gradient, is one such emerging technique. Elucidation of the underlying physics of optothermo-matter interaction and rational engineering of optical environments are required to realize diverse optothermal manipulation functionalities. This Account covers the working principles, design concepts, and applications of a series of newly developed optothermal manipulation techniques, including bubble-pen lithography, opto-thermophoretic tweezers, opto-thermoelectric tweezers, optothermal assembly, and opto-thermoelectric printing. In bubble-pen lithography, optical heating of a plasmonic substrate generates microbubbles at the solid-liquid interface to print diverse colloidal particles on the substrates. Programmable bubble printing of semiconductor quantum dots on different substrates and haptic control of printing have also been achieved. The key to optothermal tweezers is the ability to deliver colloidal particles from cold to hot regions of a temperature gradient or a negative Soret effect. We explore different driving forces for the two types of optothermal tweezers. Opto-thermophoretic tweezers rely on an abnormal permittivity gradient built by structured solvent molecules in the electric double layer of colloidal particles and living cells in response to heat-induced entropy, and opto-thermoelectric tweezers exploit a thermophoresis-induced thermoelectric field for the low-power manipulation of small nanoparticles with minimum diameter around 20 nm. Furthermore, by incorporating depletion attraction into the optothermal tweezers system as particle-particle or particle-substrate binding force, we have achieved bottom-up assembly and reconfigurable optical printing of artificial colloidal matter. Beyond optothermal manipulation techniques in liquid environments, we also review recent progress of gas-phase optothermal manipulation based on photophoresis. Photophoretic trapping and transport of light-absorbing materials have been achieved through optical engineering to tune particle-molecule interactions during optical heating, and a novel optical trap display has been demonstrated. An improved understanding of the colloidal response to temperature gradients will surely facilitate further innovations in optothermal manipulation. With their low-power operation, simple optics, and diverse functionalities, optothermal manipulation techniques will find a wide range of applications in life sciences, colloidal science, materials science, and nanoscience, as well as in the developments of colloidal functional devices and nanomedicine.

Plasmofluidics for Biosensing and Medical Diagnostics

Plasmofluidics, an extension of optofluidics into the nanoscale regime, merges plasmonics and micro-/nanofluidics for highly integrated and multifunctional lab on a chip. In this chapter, we focus on the applications of plasmofluidics in the versatile manipulation and sensing of biological cell, organelles, molecules, and nanoparticles, which underpin advanced biomedical diagnostics.

Optothermophoretic Manipulation of Colloidal Particles in Nonionic Liquids

The response of colloidal particles to a light-controlled external temperature field can be harnessed for opto-thermophoretic manipulation of the particles. The thermoelectric effect is regarded as the driving force for thermophoretic trapping of particles at the light-irradiated hot region, which is thus limited to ionic liquids. Herein, we achieve opto-thermophoretic manipulation of colloidal particles in various non-ionic liquids, including water, ethanol, isopropyl alcohol and 1-butanol, and establish the physical mechanism of the manipulation at the molecular level. We reveal that the non-ionic driving force originates from a layered structure of solvent molecules at the particle-solvent interface, which is supported by molecular dynamics simulations. Furthermore, the effects of hydrophilicity, solvent type, and ionic strength on the layered interfacial structures and thus the trapping stability of particles are investigated, providing molecular-level insight into thermophoresis and guidance on interfacial engineering for optothermal manipulation.

Nanoradiator-Mediated Deterministic Opto-Thermoelectric Manipulation

We explore the opto-thermoelectric trapping at plasmonic antennas that serve as optothermal nano-radiators to achieve the low-power and deterministic manipulation of nanoparticles.