Study of intrinsic defect states of FeSe with scanning tunneling microscopy

We apply high-resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects, including the type-I dumbbell, type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, are extensively analyzed by scanning tunneling spectroscopy. Through characterized depression and enhancement of density of states measured in a large energy range, the type-I dumbbell and type-II dumbbell are determined to be the Fe vacancy and ${\mathrm{Se}}_{\mathrm{Fe}}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects is compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of the defect states in our experiment lays a foundation for its comparison with the result of complex theoretical calculations in the future.

Figure 1

(a), (b) Atomic resolved large area topographies $\left(50\mathrm{nm}\times 50\mathrm{nm}\right)$ under positive and negative bias voltages, respectively. The red, black, green, and blue frames label the type-I dumbbell, type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, respectively. The inset in (a) is an enlarged image with a schematic top-view lattice superimposed on it. The left part of the inset is the schematic front view of FeSe structure. The tunneling condition is $V_{\mathrm{b}}=100\mathrm{mV},I_{\mathrm{s}}=20\mathrm{pA}$ for (a) and $V_{\mathrm{b}}=-100\mathrm{mV},I_{\mathrm{s}}=20\mathrm{pA}$ for (b). (c), (e), (g), (i) Zoom-in images of the type-I dumbbell, type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, respectively. The tunneling condition is $V_{\mathrm{b}}=100\mathrm{mV},I_{\mathrm{s}}=20\mathrm{pA}$. (d), (f), (h), (j) Zoom-in images of the same area in (c), (e), (g), (i). The tunneling condition is $V_{\mathrm{b}}=-100\mathrm{mV},I_{\mathrm{s}}=20\mathrm{pA}$.

Figure 2

The bias voltage dependent STM images of the type-I dumbbell (a)-(j) and the type-II dumbbell (l)-(u). (k) The height profiles along the lobe structure of the type-I dumbbell, as indicated by the white arrow in (a). (v) The height profiles along the lobe structure of the type-II dumbbell, as indicated by the white arrow in (l). The neighboring curves in (k) and (v) are shifted by an interval of 15 pm for clarity.

Figure 3

The $dI/dV$ spectra at the defect site (red curves) and far away from the defect site (black curves). (a)–(d) Spectra for the type-I dumbbell, type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, respectively. The tunneling condition is $V_{\mathrm{b}}=100\mathrm{mV}$ and $I_{\mathrm{s}}=100\mathrm{pA}$. (e), (f) The $dI/dV$ spectra of the type-I dumbbell and the top-layer Se vacancy with a large voltage range. The tunneling condition is $V_{\mathrm{b}}=100\mathrm{mV}$ and $I_{\mathrm{s}}=20\mathrm{pA}$. (g), (h) The $dI/dV$ spectra of the type-I dumbbell and the type-II dumbbell with a large voltage range. The black dashed boxes highlight the different protruding features in the $dI/dV$ spectra. The tunneling condition is $V_{\mathrm{b}}=-600\mathrm{mV}$ and $I_{\mathrm{s}}=600\mathrm{pA}$.

Figure 4

(a) Topography of the type-I dumbbell. Two white arrows indicate line cut 1 and line cut 2, respectively. Black and red dots are positions where the spectra were taken. (b) Left and right panels show series of spectra along line cut 1 and line cut 2, respectively. The lower panel of (b) is the conductance map of 200 mV around the type-I dumbbell. (c)–(h) The same as (a) and (b) but with topographies and spectra for the type-II dumbbell (c), (d), the top-layer Se vacancy (e), (f), and the inner-layer Se-site defect (g), (h). All the topographies and spectra were taken under $V_{\mathrm{b}}=-100\mathrm{mV}$ and $I_{\mathrm{s}}=100\mathrm{pA}$. The lower panels of (d), (f), and (h) are conductance maps at the energy of 104, 200, and $-200\mathrm{meV}$ around the type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, respectively. The conductance maps in (f) and (h) are selected with different signs to present the defect states clearly.

Figure 5

(a) An overview of the exfoliated FeSe on silicon nitride grid. A three-layer FeSe is selected for the ADF-STEM measurement. (b) A $11.9\mathrm{nm}\times 11.9\mathrm{nm}$ ADF-STEM image of FeSe. (c) A $5.5\mathrm{nm}\times 5.5\mathrm{nm}$ zoom-in image. The ${\mathrm{Se}}_{\mathrm{Fe}}$ defect, Fe vacancy, and possible Se vacancy are labeled by the black, red, and green arrows, respectively. (d) An intensity plot along atoms in the yellow dashed frame in (c). The corresponding atoms and defects are labeled. The bottom of (d) is the zoom-in image of the yellow dashed frame in (c).