Single-atom electron energy loss spectroscopy of light elements

Abstract

Light elements such as alkali metal (lithium, sodium) or halogen (fluorine, chlorine) are present in various substances and indeed play significant roles in our life. Although atomic behaviours of these elements are often a key to resolve chemical or biological activities, they are hardly visible in transmission electron microscope because of their smaller scattering power and higher knock-on probability. Here we propose a concept for detecting light atoms encaged in a nanospace by means of electron energy loss spectroscopy using inelastically scattered electrons. In this method, we demonstrate the single-atom detection of lithium, fluorine, sodium and chlorine with near-atomic precision, which is limited by the incident probe size, signal delocalization and atomic movement in nanospace. Moreover, chemical shifts of lithium K-edge have been successfully identified with various atomic configurations in one-dimensional lithium compounds.

Figure 1. A scheme of EELS chemical map.

EELS profile generally involves the electron probe shape, the atomic movement and the signal delocalization. EELS contrast reflects the inelastic cross-section, which is high enough to discriminate a single light atom such as Li. Here we employ the cage effect to trap mobile atoms since the light atom has higher knock-on probability.

Figure 2. Na atom detection in a 1D NaI crystal.

(a,b) A schematic model and an ADF image of a NaI atomic chain in a DWNT, respectively. In the ADF image, Na atoms are invisible while I atoms present as bright spots. (c) An elemental map (upper panel) of a NaI atomic chain shown in a ADF image (bottom panel). (d) The EELS spectrum (red line in d) taken from the NaI atomic chain shown in c. A reference spectrum taken from the 3 × 3 NaI nanowire is also shown (black line in d). The Na L-edge is clearly visible at 33 eV, even for a single atomic chain of NaI. The EELS chemical map for the Na L-edge displays the positions of Na atoms (green spots in the upper panel of c) and proves that Na and I are alternatively aligned in the 1D configuration. The Na map is smoothed by a convolution of a 3 × 3 pixel matrix and overlapped with a simultaneously recorded ADF image, which reflects the positions of I atoms (blue spots in the upper panel of c). Note that there was a slight specimen drift during the EELS chemical map acquisition. Scale bars, 0.5 nm.

Figure 3. Detection of single Cl atoms.

(a) Atomic model of a CsCl atomic chain inside a DWNT. (b) An ADF image of a CsCl atomic chain. (c,d) EELS chemical maps for the Cs M-edge and Cl L-edge corresponding to b, respectively. (e) An EELS spectrum of the CsCl atomic chain in b showing a trace of Cl and Cs, as well as the carbon K-edge which corresponds to the DWNT. The ADF image b only shows the Cs atomic positions as bright spots which are consistent with the red spots in the EELS chemical map of the Cs M-edge c. The EELS map for the Cl L-edge d clearly shows the existence of Cl atoms in between Cs atoms despite of hardly visible ADF contrast in b. Scale bar, 0.5 nm.

Figure 4. Detection of single Li atoms in a peapod.

(a) Atomic model of a Li@C₆₀ peapod. (b) A typical ADF image, which shows fullerene molecules but no visible contrast for the Li atom inside. (c) An EELS chemical map of the Li K-edge smoothed by a convolution of 3 × 3 pixel matrix. Note that only two of the molecules contain the Li atoms. (d) A typical EELS spectrum showing the trace of the Li K-edge used for the Li map c. (e) A schematic model of a Li@C₆₀ under the experimental condition. The yellow region in d presents a possible mobile space for the Li atom inside the C₆₀ molecule (∼0.4 nm), which is roughly estimated from the cage size taking account of van der Waals distance. The EELS detectable region is indicated by the blue circle in d, which involves the effect of the atomic movement as well as the delocalization distance of the Li K-edge (∼1.1 nm at 30 kV) and is eventually larger than the fullerene cage. Scale bars, 0.5 nm.

Figure 5. EELS chemical map of a LiI atomic chain.

(a) Atomic model of the double LiI atomic chain inside a DWNT. (b) An ADF image, which only shows the I contrast. (c) An EELS chemical map of the Li K-edge smoothed by a convolution of 3 × 3 pixel matrix corresponding to b. Li atoms are clearly aligned in a zigzag pattern between I atoms displayed as bright spots in b. (d) An EELS spectrum, confirming the trace of the Li K-edge used for the Li map c, collected from within a white square in the inset in d. The confidence level of the single Li atom detection using Li K-edge is sufficiently high (Supplementary Fig. 4 and Supplementary Note 3). (e) Fine structure analysis of the Li K-edge of various LiI 1D structures. Each spectrum corresponds to the right-side ADF images and models as follows: (from the top) Li@C₆₀ inside the SWNT, the 1 × 2 structure, the 2 × 2 structure and the 3 × 3 structure of LiI, respectively. There is a systematic variation for the Li K-edge with those atomic configurations. The peaks apparently shift to the higher energy as the coordination number decreases for Li from 4 to 6 (the 3 × 3 structure) down to 2 (the 1 × 2 structure) or 0 (the Li@C₆₀). Scale bars, 0.3 nm.