# Na-Doping Effects on Thermoelectric Properties of Cu\({}_{2-x}\)Se Nanoplates
## 3 Results and Discussion
Figure 2c,d show SEM images of Cu\({}_{2-x}\)Se nanowire with lengths larger than \(\sim\)30 \(\upmu\)m and diameters of \(\sim\)200 nm. Figure 3a shows the SEM images of Na-doped Cu\({}_{2-x}\)Se nanoplates. The size and shape of Na-doped nanoplate are similar to those of undoped Cu\({}_{2-x}\)Se nanoplate. The TEM image of a typical Na-doped Cu\({}_{2-x}\)Se nanoplate is shown in Figure 3b, where a hexagonal shaped nanoplate can be clearly identified. The inset of Figure 3b is a selected-area electron diffraction pattern taken along the [111] direction of the nanoplate, showing the single crystallinity of the nanoplate. To check the chemical composition of the Cu\({}_{2-x}\)Se samples, energy-dispersive spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) are performed. The result of elemental-mapping  
Figure 2: Scanning electron microscope (SEM) images of (**a**,**b**) nanoplate; and (**c**,**d**) nanowire of Cu\({}_{2-x}\)Se powder. using the TEM-EDS analysis is shown in Figure 3c. The distributions of the elements Cu, Se, and Na are homogeneous in Na-doped Cu\({}_{2-x}\)Se nanoplate. About \(\sim\)16% of Na was detected in Na-doped Cu\({}_{2-x}\)Se nanoplate. Similar results were observed by inductively coupled plasma optical emission spectroscopy (ICP-OES), shown in Table 1. The atomic ratio of Cu and Se for Cu\({}_{2-x}\)Se nanoplate calculated from the ICP-OES results is around 63.7:36.3, whereas the Na-doped Cu\({}_{2-x}\)Se nanoplate has atomic ratio between Cu:Se of around 63.6:31.3, which means that the Na-doped Cu\({}_{2-x}\)Se nanoplate we got through the two-step procedure is the Cu-rich Cu\({}_{2-x}\)Se phase. The SPS pressed Na-doped Cu\({}_{2-x}\)Se bulk sample was also characterized by X-ray photoelectron spectroscopy (XPS). The XPS spectra provide useful information for understanding the chemical bonding states of the constituent atoms. The binding energies obtained in the XPS analysis were corrected for specimen charging by a setting C 1s to 284.6 eV. Representative XPS spectra of Na, Cu, and Se are shown in Figure 4. As shown in Figure 4b, one strong peak located at 1071.7 eV, indicative of Na 1s, suggested the existence of Na\({}^{+}\) species in Na-doped Cu\({}_{2-x}\)Se nanoplates. As shown in Figure 4c,d, the resulted binding energies of Se 3d and Cu 2p\({}_{3/2}\) are given as 54.0 and 933.4 eV, respectively. All of the observed binding energy values for Cu 2p and Se 3d are nearly in agreement with the reported data in the literature [30]. In the literature, the binding energy of Cu 2p\({}_{3/2}\) has been reported to be \(\sim\)933 eV regardless of its oxidation state (Cu\({}^{0}\), Cu\({}^{1+}\) and Cu\({}^{2+}\)) [31; 32]. Generally, Cu\({}^{2+}\) is best distinguished from other Cu oxidation states by observing low-intensity satellite peaks in the region of 940-950 eV. According to Figure 4c, the satellite peaks indicate that the presence of \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Na** & **Cu** & **Se** & **Composition** \\ \hline Cu\({}_{2-x}\)Se-wire & - & 0.613 & 0.397 & Cu\({}_{1.59}\)Se \\ Cu\({}_{2-x}\)Se-plate & - & 0.637 & 0.363 & Cu\({}_{1.75}\)Se \\ Na-Cu\({}_{2-x}\)Se-plate & 0.051 & 0.636 & 0.313 & Na\({}_{0.162}\)Cu\({}_{2.03}\)Se \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Atomic ratio of Cu, Se, and Na for Cu\({}_{2-x}\)Se calculated from ICP-AES. Figure 3: **(a)** SEM image; **(b)** Transmission electron microscope (TEM) image with selected area electron diffraction (SAED) patterns (inset) for Na-doped Cu\({}_{2-x}\)Se nanoplates; **(c)** the corresponding mapping on compositions by an energy-dispersive spectrometer (EDS) for Na-doped Cu\({}_{2-x}\)Se nanoplate. Cu\({}^{2+}\) ion in bulk Na-doped Cu\({}_{2-x}\)Se nanoplates. A significant broadening of the Se 3d spectra is observed, as shown in Figure 4d. Similar features were previously found in InSe compounds upon Na deposition [33].