In Situ Monitored Self-Assembly of Three-Dimensional Polyhedral Nanostructures

Ultra-small mode area V-groove waveguide design for on-chip single-photon emission

We numerically investigate the figures of merit for single-photon emission in a planar GaAs-on-insulator waveguide featuring a V-groove geometry. Thanks to a field enhancement effect arising due to boundary conditions of this waveguide, the structure features an ultra-small mode area enabling a factor of a maximum 2.8 times enhancement of the Purcell factor for quantum dot and a more significant 7 times enhancement for the atomic-size solid-state emitters with the aligned dipole orientation. In addition, the coupling efficiency to the fundamental quasi-TE mode is also improved. To take into account potential on-chip integration, we further show that the V-groove mode profile can be converted using a tapering section to the mode profile of a standard ridge waveguide while maintaining both the high Purcell factor and the good fundamental mode coupling efficiency.

Electron Beam Maneuvering of a Single Polymer Layer for Reversible 3D Self-Assembly

Reversible self-assembly that allows materials to switch between structural configurations has triggered innovation in various applications, especially for reconfigurable devices and robotics. However, reversible motion with nanoscale controllability remains challenging. This paper introduces a reversible self-assembly using stress generated by electron irradiation triggered degradation (shrinkage) of a single polymer layer. The peak position of the absorbed energy along the depth of a polymer layer can be modified by tuning the electron energy; the peak absorption location controls the position of the shrinkage generating stress along the depth of the polymer layer. The stress gradient can shift between the top and bottom surface of the polymer by repeatedly tuning the irradiation location at the nanoscale and the electron beam voltage, resulting in reversible motion. This reversible self-assembly process paves the path for the innovation of small-scale machines and reconfigurable functional devices.

Recent advances in focused ion beam nanofabrication for nanostructures and devices: fundamentals and applications

The past few decades have witnessed growing research interest in developing powerful nanofabrication technologies for three-dimensional (3D) structures and devices to achieve nano-scale and nano-precision manufacturing. Among the various fabrication techniques, focused ion beam (FIB) nanofabrication has been established as a well-suited and promising technique in nearly all fields of nanotechnology for the fabrication of 3D nanostructures and devices because of increasing demands from industry and research. In this article, a series of FIB nanofabrication factors related to the fabrication of 3D nanostructures and devices, including mechanisms, instruments, processes, and typical applications of FIB nanofabrication, are systematically summarized and analyzed in detail. Additionally, current challenges and future development trends of FIB nanofabrication in this field are also given. This work intends to provide guidance for practitioners, researchers, or engineers who wish to learn more about the FIB nanofabrication technology that is driving the revolution in 3D nanostructures and devices.

Electron Irradiation Driven Nanohands for Sequential Origami

Sequence plays an important role in self-assembly of 3D complex structures, particularly for those with overlap, intersection, and asymmetry. However, it remains challenging to program the sequence of self-assembly, resulting in geometric and topological constrains. In this work, a nanoscale, programmable, self-assembly technique is reported, which uses electron irradiation as "hands" to manipulate the motion of nanostructures with the desired order. By assigning each single assembly step in a particular order, localized motion can be selectively triggered with perfect timing, making a component accurately integrate into the complex 3D structure without disturbing other parts of the assembly process. The features of localized motion, real-time monitoring, and surface patterning open the possibility for the further innovation of nanomachines, nanoscale test platforms, and advanced optical devices.

3D Self-Assembled Microelectronic Devices: Concepts, Materials, Applications

Modern microelectronic systems and their components are essentially 3D devices that have become smaller and lighter in order to improve performance and reduce costs. To maintain this trend, novel materials and technologies are required that provide more structural freedom in 3D over conventional microelectronics, as well as easier parallel fabrication routes while maintaining compatability with existing manufacturing methods. Self-assembly of initially planar membranes into complex 3D architectures offers a wealth of opportunities to accommodate thin-film microelectronic functionalities in devices and systems possessing improved performance and higher integration density. Existing work in this field, with a focus on components constructed from 3D self-assembly, is reviewed, and an outlook on their application potential in tomorrow's microelectronics world is provided.

Ion-Induced Localized Nanoscale Polymer Reflow for Three-Dimensional Self-Assembly

Thermal reflow of polymers is a well-established phenomenon that has been used in various microfabrication processes. However, present techniques have critical limitations in controlling the various attributes of polymer reflow, such as the position and extent of reflow, especially at the nanoscale. These challenges primarily result from the reflow heat source supplying heat energy to the entire substrate rather than a specific area. In this work, a focused ion beam (FIB) microscope is used to achieve controllable localized heat generation, leading to precise control over the nanoscale polymer reflow. Through the use of the patterning capability of FIB microscopy, dramatically different reflow performances within nanoscale distances of each other are demonstrated in both discrete periodic and continuous polymer structures. Further, we utilize a self-assembly process induced by nanoscale polymer reflow to realize 3D optical devices, specifically, vertically aligned nanoresonators and graphene-based nanocubes. HFSS and Comsol simulations have been carried out to analyze the advantages of the polymer-based 3D metamaterials as opposed to those fabricated with a metallic hinge. The simulation results clearly demonstrate that the polymer hinges have a dual advantage; first, the removal of any interference from the transmission spectrum leading to strong and distinct resonance peaks and, second, the elimination of parasitic leeching of the enhanced field by the metallic hinge resulting in stronger volumetric enhancement. Thus, the 2-fold advantages existing in 3D polymer-hinge optical metamaterials can open pathways for applications in 3D optoelectronic devices and sensors, vibrational molecular spectroscopy, and other nanoscale 3D plasmonic devices.

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent properties has been of great interest for the development of novel device applications. Here, fabrication of 3D, microscale, hollow polyhedrons (cubes) consisting of a few layers of 2D graphene or graphene oxide sheets via an origami-like self-folding process is described. This method involves the use of polymer frames and hinges, and aluminum oxide/chromium protection layers that reduce tensile, spatial, and surface tension stresses on the graphene-based membranes when the 2D nets are transformed into 3D cubes. The process offers control of the size and shape of the structures as well as parallel production. In addition, this approach allows the creation of surface modifications by metal patterning on each face of the 3D cubes. Raman spectroscopy studies show the method allows the preservation of the intrinsic properties of the graphene-based membranes, demonstrating the robustness of our method.

Small-Scale Biological and Artificial Multidimensional Sensors for 3D Sensing

A vast majority of existing sub-millimeter-scale sensors have a planar, 2D geometry as a result of conventional top-down lithographic procedures. However, 2D sensors often suffer from restricted sensing capability, allowing only partial measurements of 3D quantities. Here, nano/microscale sensors with different geometric (1D, 2D, and 3D) configurations are reviewed to introduce their advantages and limitations when sensing changes in quantities in 3D space. This Review categorizes sensors based on their geometric configuration and sensing capabilities. Among the sensors reviewed here, the 3D configuration sensors defined on polyhedral structures are especially advantageous when sensing spatially distributed 3D quantities. The nano- and microscale vertex configuration forming polyhedral structures enable full 3D spatial sensing due to orthogonally aligned sensing elements. Particularly, the cubic configuration leveraged in 3D sensors offers an array of diverse applications in the field of biosensing for micro-organisms and proteins, optical metamaterials for invisibility cloaking, 3D imaging, and low-power remote sensing of position and angular momentum for use in microbots. Here, various 3D sensors are compared to assess the advantages of their geometry and its impact on sensing mechanisms. 3D biosensors in nature are also explored to provide vital clues for the development of novel 3D sensors.

Nano-beam and nano-target effects in ion radiation

Full three dimensional (3D) simulations of ion implantation are necessary in a wide range of nanoscience and nanotechnology applications to capture the increasing effect of ion leakage out of surfaces. Using a recently developed 3D Monte Carlo simulation code IM3D, we first quantify the relative error of the 1D approach in three applications of nano-scale ion implantation: (1) nano-beam for nitrogen-vacancy (NV) center creation, (2) implantation of nanowires to fabricate p-n junctions, and (3) irradiation of nano-pillars for small-scale mechanical testing of irradiated materials. Because the 1D approach fails to consider the exchange and leakage of ions from boundaries, its relative error increases dramatically as the beam/target size shrinks. Lastly, the "Bragg peak" phenomenon, where the maximum radiation dose occurs at a finite depth away from the surface, relies on the assumption of broad beams. We discovered a topological transition of the point-defect or defect-cluster distribution isosurface when one varies the beam width, in agreement with a previous focused helium ion beam irradiation experiment. We conclude that full 3D simulations are necessary if either the beam or the target size is comparable or below the SRIM longitudinal ion range.

Self-Assembled Three-Dimensional Graphene-Based Polyhedrons Inducing Volumetric Light Confinement

The ability to transform two-dimensional (2D) materials into a three-dimensional (3D) structure while preserving their unique inherent properties might offer great enticing opportunities in the development of diverse applications for next generation micro/nanodevices. Here, a self-assembly process is introduced for building free-standing 3D, micro/nanoscale, hollow, polyhedral structures configured with a few layers of graphene-based materials: graphene and graphene oxide. The 3D structures have been further modified with surface patterning, realized through the inclusion of metal patterns on their 3D surfaces. The 3D geometry leads to a nontrivial spatial distribution of strong electric fields (volumetric light confinement) induced by 3D plasmon hybridization on the surface of the graphene forming the 3D structures. Due to coupling in all directions, resulting in 3D plasmon hybridization, the 3D closed box graphene generates a highly confined electric field within as well as outside of the cubes. Moreover, since the uniform coupling reduces the decay of the field enhancement away from the surface, the confined electric field inside of the 3D structure shows two orders of magnitude higher than that of 2D graphene before transformation into the 3D structure. Therefore, these structures might be used for detection of target substances (not limited to only the graphene surfaces, but using the entire volume formed by the 3D graphene-based structure) in sensor applications.

Folding Sheets with Ion Beams

Focused ion beams (FIBs) are versatile tools with cross-disciplinary applications from the physical and life sciences to archeology. Nevertheless, the nanoscale patterning precision of FIBs is often accompanied by defect formation and sample deformation. In this study, the fundamental mechanisms governing the large-scale plastic deformation of nanostructures undergoing FIB processes are revealed by a series of molecular dynamic simulations. A surprisingly simple linear correlation between atomic volume removed from the film bulk and film deflection angle, regardless of incident ion energy and current, is revealed, demonstrating that the mass transport to the surface of material caused by energetic ion bombardment is the primary cause leading to nanostructure deformation. Hence, by controlling mass transport by manipulation of the incident ion energy and flux, it is possible to control the plastic deformation of nanostructures, thereby fabricating nanostructures with complex three-dimensional geometries.

Plasma Triggered Grain Coalescence for Self-Assembly of 3D Nanostructures

Grain coalescence has been applied in many areas of nanofabrication technology, including modification of thin-film properties, nanowelding, and self-assembly of nanostructures. However, very few systematic studies of self-assembly using the grain coalescence, especially for three-dimensional (3D) nanostructures, exist at present. Here, we investigate the mechanism of plasma triggered grain coalescence to achieve the precise control of nanoscale phase and morphology of the grain coalescence induced by exothermic energy. Exothermic energy is generated through etching a silicon substrate via application of plasma. By tuning the plasma power and the flow rates of reactive gases, different etching rates and profiles can be achieved, resulting in various morphologies of grain coalescence. Balancing the isotropic/anisotropic substrate etching profile and the etching rate makes it possible to simultaneously release 2D nanostructures from the substrate and induce enough surface tension force, generated by grain coalescence, to form 3D nanostructures. Diverse morphologies of 3D nanostructures have been obtained by the grain coalescence, and a strategy to achieve self-assembly, resulting in desired 3D nanostructures, has been proposed and demonstrated.

Atomic layer deposition ultrathin film origami using focused ion beams

Focused ion beam (FIB) micromachining is a powerful tool for maskless lithography and in recent years FIB has been explored as a tool for strain engineering. Ion beam induced deformation can be utilized as a means for folding freestanding thin films into complex 3D structures. FIB of high energy gallium (Ga⁺) ions induces stress by generation of dislocations and ion implantation within material layers, which create creases or folds upon mechanical relaxation enabled by motion of the material layers. One limitation on such processing is the ability to fabricate flat freestanding thin film structures. This capability is limited by the residual stresses formed during processing and fabrication of the films, which can result in initial curvature and deformation of films upon release from a sacrificial fabrication layer. This paper demonstrates folding in freestanding ultrathin films (<40 nm thin) of heterogeneous composition (metal, insulator, semiconductor, etc) with large lateral dimension structures (aspect ratio >1:1000) by ion-induced stress relaxation. The ultrathin flat structures are fabricated using atomic layer deposition on sacrificial polyimide. We have demonstrated vertical folding with 30 keV Ga⁺ ions in structures with lateral dimensions varying from 10 to 50 μm.

Tunable Optical Transparency in Self-Assembled Three-Dimensional Polyhedral Graphene Oxide

The origami-like self-folding process is an intellectually stimulating technique for realizing three-dimensional (3D) polyhedral free-standing graphene oxide (GO) structures. This technique allows for easy control of size, shape, and thickness of GO membranes, which in turn permits fabrication of free-standing 3D microscale polyhedral GO structures. Unlike 2D GO sheets, the 3D polyhedral free-standing GO shows a distinct optical switching behavior, resulting from a combination of the geometrical effect of the 3D hollow structure and the water-permeable multilayered GO membrane that affects the optical paths.