Design and Construction of an Optical TEM Specimen Holder
Joel Martis,1 * Ze Zhang,1 Hao-Kun Li,1 Ann Marshall,2 Roy Kim,2
1Department of Mechanical Engineering, Stanford University, Stanford, CA 94305 2Stanford Nano Shared Facilities, Stanford University, Stanford, CA 94305
3Department of Photon Science, SLAC National Accelerator Laboratory, Menlo Park, CA 94025 *
martis@stanford.edu
Abstract: Electron microscopy has enabled atomic resolution
imaging of matter. However, unlike optical spectroscopic imaging, traditional electron microscopes provide limited spectroscopic information in terms of their energy resolution. Only recently, owing to advances in monochromated STEM-EELS, have transmission electron microscopes (TEMs) been able to attain a high energy resolution. We recently proposed combining spectrally selective photoexcitation with HRTEM to achieve sub-nanometer scale optical imaging, a technique we called photoabsorption microscopy using electron analysis (PAMELA). To realize PAMELA-TEM experimentally, we constructed a TEM holder with an optical feedthrough, capable of photoexciting materials with different wavelengths. In this article, we describe our process for designing and fabricating an optical TEM specimen holder, highlighting important aspects of the design.
Keywords: TEM specimen holder, optical spectroscopy, in situ TEM, optical fiber, photoabsorption microscopy
Introduction Optical imaging and spectroscopy provide rich
information about the energy states of materials. However, their spatial resolution is limited by the wavelength of (visible) light to about 0.5 μm. Electron microscopy can achieve sub- Angstrom resolution but typically provides limited spectral information. Only due to recent developments of electron monochromators, has scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) become a powerful tool for probing energy states of materials [1]. We recently demonstrated ∼10 nm optical imaging in scanning electron microscopy (SEM) via a new imaging technique called photoabsorption microscopy using electron analysis (PAMELA), where spectrally selective photoabsorption modulates secondary electron emission [2]. We also proposed combining spectrally selective photoexcitation with high- resolution transmission electron microscopy (HR)TEM (PAMELA-TEM) to achieve sub-nm scale optical imaging [3]. Te experimental
realization of PAMELA-TEM, however,
requires introduction of light inside a TEM with a sufficiently high photon flux to produce a detectable signal. Light can be introduced in two ways: through a port
on the TEM (like the objective aperture) [4,5], or through a TEM specimen holder [6–8]. Te former approach requires modification of a TEM column, which can be expensive and potentially detrimental to the instrument, but has the advantage of combining other multifunctional specimen holders (heating, biasing, cryo, etc.) with light input. Te latter is advantageous since a TEM holder can be used in any compatible TEM, and the holder can be modified without affecting the TEM column itself. Since the basic principle of PAMELA-TEM requires only a light input, we chose the latter approach and constructed a
40 doi:10.1017/S1551929521001103
TEM holder with light input capabilities. In the following, we explain our approach to designing such a holder, bearing in mind our experimental requirements, vacuum considerations, X-ray safety, and ease of operation. Several researchers have designed and fabricated TEM
holders with light input capabilities in the past [6–8]. Tese designs have used both free-space optics and fiber-optics- based approaches. Free-space
optics designs offer the
possibility of high optical fluxes but require the light source to be rigidly connected to the specimen holder, thereby limiting the light sources that can be used. Fiber-based designs offer the flexibility of using a variety of light sources but are typically limited in the largest optical flux they can achieve owing to their damage threshold. In our case, we chose the fiber-based approach because it met our optical flux requirements and offered other advantages as outlined above.
Methods and Materials Prior to designing and fabrication, we outlined the target
specifications of our optical TEM specimen holder. Tese specifications determined the design approach and materials used in the construction of the holder. We designed a holder to fit into a Termo Fisher Scientific (Portland, OR) TEM based on the following requirements:
• Optical flux of ∼ 107 W/m2
• Laser spot size ∼ between 25–100 μm • Laser wavelengths ∼ visible light (400–700 nm) • Motion control ∼ 2 mm range of motion • Te optical fiber should be easily replaceable
In addition, any TEM holder needs to be vacuum-
flux limit of about 1010 requirement of 107 flux of 107
W/m2
compatible, X-ray safe, and ideally not affect the resolution and performance of the microscope. Our process flowchart is shown in Figure 1. In the following sections, we discuss each of these design aspects in detail. Optics. Optical fibers typically have an upper optical W/m2
W/m2
, which is much higher than our . However, even to achieve an optical
at the sample, a focusing optic needs to be used
between the fiber and the sample. Given the space constraints due to the objective pole pieces in the TEM, we decided to use a microlens fused to the optical fiber with a working distance to the sample of about 1 mm. Te microlens was fabricated with a reflector built in (Figure 2) to help mount the fiber such that it stayed within the space constraints imposed by the objective lens. Te microlens assembly was fabricated by WT&T Inc., Montreal, Canada. Te assembly was then inserted into a metal tube and connectorized and polished on the other
www.microscopy-today.com • 2021 September and Arun Majumdar1,3
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