Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

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Engineering metallic nanostructures for plasmonics and nanophotonics

The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrow's electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

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Metal oxide semiconductor thin-film transistors for flexible electronics

When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.

Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering

This review examines characterization challenges inherently associated with understanding nanomaterials and the roles surface and interface characterization methods can play in meeting some of the challenges. In parts of the research community, there is growing recognition that studies and published reports on the properties and behaviors of nanomaterials often have reported inadequate or incomplete characterization. As a consequence, the true value of the data in these reports is, at best, uncertain. With the increasing importance of nanomaterials in fundamental research and technological applications, it is desirable that researchers from the wide variety of disciplines involved recognize the nature of these often unexpected challenges associated with reproducible synthesis and characterization of nanomaterials, including the difficulties of maintaining desired materials properties during handling and processing due to their dynamic nature. It is equally valuable for researchers to understand how characterization approaches (surface and otherwise) can help to minimize synthesis surprises and to determine how (and how quickly) materials and properties change in different environments. Appropriate application of traditional surface sensitive analysis methods (including x-ray photoelectron and Auger electron spectroscopies, scanning probe microscopy, and secondary ion mass spectroscopy) can provide information that helps address several of the analysis needs. In many circumstances, extensions of traditional data analysis can provide considerably more information than normally obtained from the data collected. Less common or evolving methods with surface selectivity (e.g., some variations of nuclear magnetic resonance, sum frequency generation, and low and medium energy ion scattering) can provide information about surfaces or interfaces in working environments (operando or in situ) or information not provided by more traditional methods. Although these methods may require instrumentation or expertise not generally available, they can be particularly useful in addressing specific questions, and examples of their use in nanomaterial research are presented.

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Surface characterization of nanomaterials and nanoparticles: Important needs and challenging opportunities

The introduction of high-throughput and high-resolution nanofabrication techniques operating at low cost and low complexity is essential for the advancement of nanoplasmonic and nanophotonic fields. In this paper, we demonstrate a novel fabrication approach based on nanostencil lithography for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays. The technique relying on deposition of materials through a shadow mask enables plasmonic substrates supporting spectrally sharp collective resonances. We show that reflectance spectra of these antenna arrays are comparable to that of arrays fabricated by electron beam lithography. We also show that nanostencils can be reused multiple times to fabricate a series of infrared nanoantenna arrays with identical optical responses. Finally, we demonstrate fabrication of plasmonic nanostructures in a variety of shapes with a single metal deposition step on different substrates, including nonconducting ones. Our approach, by enabling the reu...

High-Throughput Nanofabrication of Infrared Plasmonic Nanoantenna Arrays for Vibrational Nanospectroscopy

We describe a flexible technique for fabricating 10-nm-scale devices for use as high-resolution scanning sensors and functional probes. Metallic structures are deposited directly onto atomic force microscope tips by evaporation through nanoscale holes fabricated in a stencil mask. We report on the lithographic capabilities of the technique and discuss progress in one initial application, to make high-spatial-resolution magnetic force sensors.

https://www.lassp.cornell.edu/lassp_data/RALPH/Pubs/ChampagneStencil.pdf

Nanometer-scale scanning sensors fabricated using stencil lithography

A technique is provided for forming a multi-layer radiation detector. The technique includes a charge-integrating photodetector layer provided in conjunction with a photon-counting photodetector layer. In one embodiment, a plurality of photon-counting photosensor elements are disposed adjacent to a plurality of charge-integrating photosensor elements of the respective layers. Both sets of elements are connected to readout circuitry and a data acquisition system. The detector arrangement may be used for energy discriminating computed tomography imaging and similar computed tomography systems.

Multi-layer radiation detector assembly

A detector module for a CT imaging system includes a scintillator to convert x-rays to optical photons. The scintillator is optically coupled to a solid-state photomultiplier with internal gain to receive the optical photons and convert them into a corresponding electrical signal output.

Photon counting CT detector using solid-state photomultiplier and scintillator

An imaging system includes a gantry having a bore therethrough designed to receive a patient being translated through the bore an x-ray source disposed in the gantry and configured to emit x-rays toward the patient, and a detector module disposed in the gantry to receive x-rays attenuated by the patient. The detector module includes a scintillator configured to absorb the x-rays and to convert the x-rays into optical photons, a device configured to receive the optical photons and to convert the optical photons to electrical signals, and an adaptive data acquisition system (DAS) configured to switch an operating mode of the device from a charge integrating mode to a photon counting mode, and vice versa.

Hybrid energy discriminating charge integrating CT detector

The present approach involves a radiation detector module with increased quantum efficiency and methods of fabricating the radiation detector module. The module includes a scintillator substrate and a photodetector fabricated on the scintillator substrate. The photodetector includes an anode, active organic elements, and a cathode. The module also includes a pixel element array disposed over the photodetector. During imaging, radiation attenuated by an object to be imaged may propagate through the pixel element array and through the layers of the photodetector to be absorbed by the scintillator which in response emits optical photons. The photodetector may absorb the photons and generate charge with improved quantum efficiency, as the photons may not be obscured by the cathode or other layers of the module. Further, the module may include reflective materials in the cathode and at the pixel element array to direct optical photons towards the active organic elements.

Aaron Judy Couture

Papers

Nanometer-scale scanning sensors fabricated using stencil lithography

Multi-layer radiation detector assembly

Photon counting CT detector using solid-state photomultiplier and scintillator

Hybrid energy discriminating charge integrating CT detector

Photodetector having improved quantum efficiency