Applications of a Novel CEOS Energy Filtering and Imaging Device for the Investigation of Electronic Structures of Delafossite CuFeO2 Single Crystals Growth by Optical Floating Zone Method

Electron energy loss spectroscopy (EELS) not only has been used to investigate electron excitations between the valence and conduction bands to obtain the information, including the plasmons and the inter- and intra-band transitions, but also to measure the core-loss energy range for excitations of orbital electrons from a few hundred to a few thousand eV [1, 2]. Furthermore, EELS has demonstrated its capability to analyze phonon vibrations in the energy loss region of less than 300 meV by improving the energy resolution with an electron monochromator [3]. Although Gatan Image Filters (GIFs) have become the most popular tool for EELS spectroscopy in the past decades, a newly developed post-column CEOS energy filtering and imaging device (CEFID) [4] has been recently successfully installed on various electron microscopes [2, 4, 5] providing an alternative to GIFs. The CEFID with its small non-isochromaticity allows to use larger entrance aperture size of 5 mm, thus improving the collection efficiency, especially for high energy core-loss spectra without compromising energy resolution. Combined with a hybrid-pixel ELA detector from Dectris Ltd [6], the CEFID-ELA is a high-sensitivity instrument that already demonstrated its powerful performance in exploring the electronic structures of various materials [2, 5]. Here, we report on the investigation of CuFeO2 (CFO) single crystal aimed at revisiting the CFO electronic structures using the CEFID-ELA recently installed on a JEOL-ARM 200CF STEM at Lehigh University [2]. TEM lamella specimens were prepared by thinning CFO grown by the optical floating zone method [7] using a Thermo Fisher Scios dual-beam focused ion beam (FIB/SEM) supplied with an in-situ EasyLift lift-out setup. Selected-area electron diffraction (SAED) pattern was performed using a JEOL-2100 TEM operated at 200 keV accelerating voltage. Atomically resolved HAADF imaging and EELS elemental mapping were performed using a JEOL-ARM 200CF Cs-STEM, equipped with a cold field emission gun (CFEG), an ASCOR probe aberration corrector (CEOS GmbH), and CEFID-ELA, and operated at 200 keV accelerating voltage. Both Cs-STEM HAADF images and EELS spectra were collected using a newly developed JEOL FEMTUS platform for data acquisition. The SAED pattern acquired along the [112¯0] zone axis of CFO in Figure 1(a) shows no additional Bragg spots except expected, thus indicating the high crystalline and phase purity of the CFO sample. The top panel in Figure 1(b) presents an atomically resolved HAADF image recorded along the same [112¯0] zone axis of CFO, which reveals slight contrast differences between odd columns (marked by pink arrows) and even columns (marked by green arrows). For illustration, the intensity profile from the red rectangle area in the top HAADF image is plotted in the bottom panel in Figure 1(b). The intensity of even columns appears somewhat higher than that of odd columns, indicating the even columns are Cu (atomic numbers Z= 29) column positions, whereas the odd columns are Fe (Z= 26) column positions. The top panel in Figure 1(c) presents the original two-dimensional (2D) dispersive image of the low-loss EELS region. The black curve in the bottom panel in Figure 1(c) shows the spectrum extracted from the top 2D dispersive image which displays several features, i.e., a sharp zero-loss peak (ZLP), two weak spectral features at 5.1 eV and 7.7eV energy loss marked by black dashed arrows, an intense and broadened feature at 23.7 eV, and a weak spectral feature around 60 eV energy loss. These spectral features were assigned to volume plasmons at 5.1 and 23.7 eV, surface exciton polariton at 7.7 eV, and the Fe M2,3 edges at 60 eV energy loss, respectively [7]. The performance of the CEFID-ELA in the study of CFO single crystals was compared with that of the GIF (Tridiem 863) equipped with an indirect electron detector (ID), which is charge-coupled devices an US1000FT model. Intriguingly, the volume plasmon at 5.1 eV energy loss was enhanced by CEFID-ELA while only the broadened hump was recorded by the GIF-ID at the same energy resolution of 0.7 eV. The top panel in Figure 2(a) shows the 2D dispersive image collected in the core-loss range from 500 eV to 1000 eV to cover the O K-edge, the Fe L2,3-edge, and the Cu L2,3-edge. The bottom panel in Figure 2(a) presents the EEL spectrum extracted from the top 2D dispersive image with the subtracted background. To further confirm the atomic arrangements shown in Figure 1(b), atomically resolved EELS elemental mapping was performed [see in Figure 2(b)]. The atomically resolved EELS elemental maps of Cu, Fe, O maps (from top to bottom) extracted from the Fe L2,3-edge, the Cu L2,3-edge, and the O K-edge in Figure 2(a) are displayed in green, red, and yellow color, respectively. The positions occupied by Fe and Cu atoms ambiguously match the atomic arrangements in the HAADF image. Furthermore, the positions occupied by Fe and O atoms overlap indicating the formation of FeO6 octahedra. The 2D dispersive image in the top panel in Figure 2(c) and spectra in the bottom panel in Figure 2(c) show the ELNES of the Cu L2,3-edge after the background subtraction. In comparison to the ELNES of the Cu L2,3-edge recorded by the GIF-ID [7], the signal-to-noise ratio of the Cu L2,3-edge recorded by CEFID-ELA was significantly improved and spectral features at around 940 and 960 eV energy loss were enhanced [8]. (a) The SAED pattern recorded along the [112¯0] zone axis of a CFO single crystal. (b) Corresponding atomically resolved Cs-STEM HAADF image acquired from the same [112¯0] zone axis of CFO single crystal (top). Bottom is the HAADF intensity line profile was measured from the red rectangle area. (c) 2D dispersive image recorded from a CFO single crystal in the low-loss region (top). The black curve shows the EELS spectrum extracted from the top 2D dispersive image. The red curve was recorded from the same single crystal using the GIF-ID detector. (a) (Top) 2D dispersive image recorded from the CFO single crystal in the energy loss range from 500 to 1000 eV. (Bottom) the EELS spectrum extracted from the top 2D dispersive image. (b) From top to bottom: HAADF image, atomically resolved mapping of Cu (green), Fe (red), and O (yellow). (c) (Top) 2D dispersive image in energy loss range of the Cu L2,3-edge recorded from a CFO single crystal. The black curve shows the ELNES spectrum of the Cu L2,3-edge extracted from the top 2D dispersive image after the background subtraction. The red curve shows the ELNES spectrum of the Cu L2,3-edge recorded from the same single crystal using the GIF-ID detector.