Frances I. Allen

Chemical mapping of a block copolymer electrolyte by low-loss EFTEM spectrum-imaging and principal component analysis.

Energy-filtered transmission electron microscopy spectrum-imaging (EFTEM SI) in the low electron energy-loss range is a valuable technique for probing the chemical structure of a material with nanoscale spatial resolution using a reduced electron dose. By analyzing EFTEM SI datasets using principal component analysis (PCA), the constituent chemical phases of the material can be identified in an efficient manner without prior knowledge of the specimen. We implement low-loss EFTEM SI together with PCA to investigate thin films of the block copolymer electrolyte poly(styrene-block-ethylene oxide) (PS-b-PEO) blended with a sodium salt. PCA identifies three main phases, the first and second phases corresponding to the two blocks of the copolymer and a third phase corresponding to the salt. The low-loss spectra for these phases are extracted from a noise-reduced EFTEM SI dataset and used to generate a chemical map of the material by multiple linear least square fitting. We validate the results of the low-loss EFTEM SI/PCA technique by applying the method to a control PS-b-PEO sample that does not contain the sodium salt, and by conducting spatially resolved X-ray energy-dispersive spectrometry on the salt-containing PS-b-PEO thin film.

High refractive index Fresnel lens on a fiber fabricated by nanoimprint lithography for immersion applications

In this Letter, we present a Fresnel lens fabricated on the end of an optical fiber. The lens is fabricated using nanoimprint lithography of a functional high refractive index material, which is suitable for mass production. The main advantage of the presented Fresnel lens compared to a conventional fiber lens is its high refractive index (n=1.68), which enables efficient light focusing even inside other media, such as water or an adhesive. Measurement of the lens performance in an immersion liquid (n=1.51) shows a near diffraction limited focal spot of 810 nm in diameter at the 1/e2 intensity level for a wavelength of 660 nm. Applications of such fiber lenses include integrated optics, optical trapping, and fiber probes.

Nanoimprint of a 3D structure on an optical fiber for light wavefront manipulation

Integration of complex photonic structures onto optical fiber facets enables powerful platforms with unprecedented optical functionalities. Conventional nanofabrication technologies, however, do not permit viable integration of complex photonic devices onto optical fibers owing to their low throughput and high cost. In this paper we report the fabrication of a three-dimensional structure achieved by direct nanoimprint lithography on the facet of an optical fiber. Nanoimprint processes and tools were specifically developed to enable a high lithographic accuracy and coaxial alignment of the optical device with respect to the fiber core. To demonstrate the capability of this new approach, a 3D beam splitter has been designed, imprinted and optically characterized. Scanning electron microscopy and optical measurements confirmed the good lithographic capabilities of the proposed approach as well as the desired optical performance of the imprinted structure. The inexpensive solution presented here should enable advancements in areas such as integrated optics and sensing, achieving enhanced portability and versatility of fiber optic components.

In situ TEM Raman spectroscopy and laser-based materials modification

We present a modular assembly that enables both in situ Raman spectroscopy and laser-based materials processing to be performed in a transmission electron microscope. The system comprises a lensed Raman probe mounted inside the microscope column in the specimen plane and a custom specimen holder with a vacuum feedthrough for a tapered optical fiber. The Raman probe incorporates both excitation and collection optics, and localized laser processing is performed using pulsed laser light delivered to the specimen via the tapered optical fiber. Precise positioning of the fiber is achieved using a nanomanipulation stage in combination with simultaneous electron-beam imaging of the tip-to-sample distance. Materials modification is monitored in real time by transmission electron microscopy. First results obtained using the assembly are presented for in situ pulsed laser ablation of MoS2 combined with Raman spectroscopy, complimented by electron-beam diffraction and electron energy-loss spectroscopy.

Evaluation of neon focused ion beam milling for TEM sample preparation.

Gallium‐based focused ion beams generated from liquid–metal sources are widely used in micromachining and sample preparation for transmission electron microscopy, with well‐known drawbacks such as sample damage and contamination. In this work, an alternative (neon) focused ion beam generated by a gas field‐ionization source is evaluated for the preparation of electron‐transparent specimens. To do so, electron‐transparent sections of Si and an Al alloy are prepared with both Ga and Ne ion beams for direct comparison. Diffraction‐contrast imaging and energy dispersive x‐ray spectroscopy are used to evaluate the relative damage induced by the two beams, and cross‐sections of milled trenches are examined to compare the implantation depth with theoretical predictions from Monte Carlo simulations. Our results show that for the beam voltages and materials systems investigated, Ne ion beam milling does not significantly reduce the focused ion beam induced artefacts. However, the Ne ion beam does enable more precise milling and may be of interest in cases where Ga contamination cannot be tolerated.

Deciphering the three-dimensional morphology of free-standing block copolymer thin films by transmission electron microscopy.

Block copolymer thin films with distinct morphologies are prepared by spin casting a nominally lamellar assay of poly(styrene-block-ethylene oxide) from a variety of solvents with and without salt doping. The 3-D morphologies of free-standing thin-film regions, which are obtained by casting directly onto holey substrates, are investigated in detail using various energy-filtering transmission electron microscopy techniques and by electron tomography. Surface characterization is achieved by atomic force microscopy. Our results demonstrate that in order to fully characterize the unique 3-D morphologies of the block copolymer thin films, a multi-method approach is required. When casting from a binary solvent, an unexpected layered honeycomb-type morphology is revealed, which likely results from an expansion of the poly(ethylene oxide) phase. A dramatic effect of selective cation coordination on the morphology of the as-cast block copolymer films is also directly observed.

Campanile Near-Field Probes Fabricated by Nanoimprint Lithography on the Facet of an Optical Fiber

One of the major challenges to the widespread adoption of plasmonic and nano-optical devices in real-life applications is the difficulty to mass-fabricate nano-optical antennas in parallel and reproducible fashion, and the capability to precisely place nanoantennas into devices with nanometer-scale precision. In this study, we present a solution to this challenge using the state-of-the-art ultraviolet nanoimprint lithography (UV-NIL) to fabricate functional optical transformers onto the core of an optical fiber in a single step, mimicking the ‘campanile’ near-field probes. Imprinted probes were fabricated using a custom-built imprinter tool with co-axial alignment capability with sub <100 nm position accuracy, followed by a metallization step. Scanning electron micrographs confirm high imprint fidelity and precision with a thin residual layer to facilitate efficient optical coupling between the fiber and the imprinted optical transformer. The imprinted optical transformer probe was used in an actual NSOM measurement performing hyperspectral photoluminescence mapping of standard fluorescent beads. The calibration scans confirmed that imprinted probes enable sub-diffraction limited imaging with a spatial resolution consistent with the gap size. This novel nano-fabrication approach promises a low-cost, high-throughput, and reproducible manufacturing of advanced nano-optical devices.

The Physics of Spin-Transfer Torque Switching in Magnetic Tunneling Junctions in Sub-10 nm Size Range

The spin-transfer torque magnetic tunneling junction (MTJ) technology may pave a way to a universal memory paradigm. MTJ devices with perpendicular magnetic anisotropy have the potential to have high thermal stability, high tunneling magnetoresistance, and low critical current for energy-efficient current-induced magnetization switching. Using devices fabricated through focused ion beam etching with Ga- and Ne-ion beams, this paper aimed to understand the size dependence of the current/voltage characteristics in the sub-10 nm range. The switching current density drastically dropped around 1 MA/cm2 as the device size was reduced below 10 nm. A stability of over 22 kT measured for a 5 nm device indicated a significantly reduced spin relaxation time.

In-situ Raman Spectroscopy in a TEM

Here we report the development of a unique tool to enable in-situ electro-optical characterization and processing of materials at the nanoscale in a dedicated analytical transmission electron microscope (TEM). This work builds upon previous experiments in which an optical fiber was coupled to a TEM using near-field optics to enable the investigation of the microstructural response of materials (imaged using the electron beam) as a result of laser-based nanofabrication processes [1]. Utilizing the near-field approach, optical illumination with a spatial resolution beyond the far-field diffraction limit is achieved. In our new setup, we combine near-field optics with Raman probing to extend the technique to enable photo-excitation and optical characterization, both at high spatial resolution. Furthermore, by installing the assembly onto an analytical TEM incorporating an energy filter, electron energy-loss spectroscopic imaging can be implemented during the in-situ experiments to compliment the photon-based Raman results.

Development of Quantitative In Situ TEM Nanomechanical Testing for Polymers

The recent development of in situ Transmission Electron Microscopy (TEM) nanomechanical testing techniques has mostly benefitted our understanding of fundamental deformation mechanisms in hard materials such as metals and ceramics [1]. Here, we report on recent progress extending these techniques to study polymeric materials using an in situ TEM nanoindenter with a Push-to-Pull (PTP) device to perform quantitative tensile tests on thin polymer sheets.