Hybrid Approach to Fabricate Uniform and Active Molecular Junctions

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.

Nanoparticle contact printing with interfacial engineering for deterministic integration into functional structures

Deterministic, pristine, and scalable integration of individual nanoparticles onto arbitrary surfaces is an ongoing challenge, yet essential for harnessing their unique properties for functional nanoscale devices. To address this challenge, we present a versatile technique where spatially arranged nanoparticles assembled in a topographical template are printed onto diverse surfaces, through a single contact-and-release step, with >95% transfer yield and <50-nanometer placement accuracy. Through engineering of interfacial interactions, our approach uniquely promotes high-yield transfer of individual particles without needing solvents, surface treatments, and polymer sacrificial layers, which are conventionally inevitable. By avoiding these mediation steps, surfaces can remain damage and contamination free and accessible to integrate into functional structures. We demonstrate this in a particle-on-mirror model system, where >2000 precisely defined nanocavities display a consistent plasmonic response with minimized interstructure variability. Through fabricating arrays of emitter-coupled nanocavities, we further highlight the integration opportunities offered by our contact printing.

Molecular Platform for Fast Low-Voltage Nanoelectromechanical Switching

The use of molecules as active components to build nanometer-scale devices inspires emerging device concepts that employ the intrinsic functionality of molecules to address longstanding challenges facing nanoelectronics. Using molecules as controllable-length nanosprings, here we report the design and operation of a nanoelectromechanical (NEM) switch which overcomes the typical challenges of high actuation voltages and slow switching speeds for previous NEM technologies. Our NEM switches are hierarchically assembled using a molecular spacer layer sandwiched between atomically smooth electrodes, which defines a nanometer-scale electrode gap and can be electrostatically compressed to repeatedly modulate the tunneling current. The molecular layer and the top electrode structure serve as two degrees of design freedom with which to independently tailor static and dynamic device characteristics, enabling simultaneous low turn-on voltages (sub-3 V) and short switching delays (2 ns). This molecular platform with inherent nanoscale modularity provides a versatile strategy for engineering diverse high-performance and energy-efficient electromechanical devices.

Polymer dependent acoustic mode coupling and Hooke's law spring constants in stacked gold nanoplates

Metal nanoparticles are excellent acoustic resonators and their vibrational spectroscopy has been widely investigated. However, the coupling between vibrational modes of different nanoparticles is less explored. For example, how the intervening medium affects the coupling strength is not known. Here, we investigate how different polymers affect coupling in Au nanoplate-polymer-Au nanoplate sandwich structures. The coupling between the breathing modes of the Au nanoplates was measured using single-particle pump-probe spectroscopy, and the polymer dependent coupling strength was determined experimentally. Analysis of the acoustic mode coupling gives the effective spring constant for the polymers. A relative motion mode was also observed for the stacked Au nanoplates. The frequency of this mode is strongly correlated with the coupling constant for the breathing modes. The breathing mode coupling and relative motion mode were analyzed using a coupled oscillator model. This model shows that both these effects can be described using the same spring constant for the polymer. Finally, we present a new type of mass balance using the strongly coupled resonators. We show that the resonators have a mass detection limit of a few femtograms. We envision that further understanding of the vibrational coupling in acoustic resonators will improve the coupling strength and expand their potential applications.