Measurement of electronic structure and surface reconstruction in the superionic Cu2−xTe

Abstract

Recently, layered copper chalcogenides Cu2X family (X=S, Se, Te) has attracted tremendous research interests due to their high thermoelectric (TE) performance, which is partly due to the superionic behavior of mobile Cu ions, making these compounds "phonon liquids". Here, we systematically investigate the electronic structure and its temperature evolution of the less studied single crystal Cu2-xTe by the combination of angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscope/spectroscopy (STM/STS) experiments. While the band structure of the Cu2-xTe shows agreement with the calculations, we clearly observe a 2 × 2 surface reconstruction from both our low temperature ARPES and STM/STS experiments which survives up to room temperature. Interestingly, our low temperature STM experiments further reveal multiple types of reconstruction patterns, which suggests the origin of the surface reconstruction being the distributed deficiency of liquid-like Cu ions. Our findings reveal the electronic structure and impurity level of Cu2Te, which provides knowledge about its thermoelectric properties from the electronic degree of freedom.

Observation of Charge Density Wave in Layered Hexagonal Cu1.89Te Single Crystal

“…structure with respect to different substrates, [17][18][19] but two lattice-matched distinct phases were also reported in Cu2Te monolayer growth via the vapor-liquid-solid (VLS) method. [20] Chen et al observed a 2×2 surface reconstruction on Cu2−𝑥Te single crystal through spectroscopy measurements, but multiple other types of reconstruction patterns were detected meanwhile.…”

“…1(b), the lattice parameters are identified with 𝑎0 = 𝑏0 = 4.186 Å, 𝑐0 = 7.232 Å and 𝛼 = 𝛽 = 90 ∘ , 𝛾 = 120 ∘ , which are consistent with the previous reports. [15,[18][19][20]22] The estimated layer spacing revealed by atomic force microscopy (AFM) is about 7 ± 0.5 Å as shown in Figs. S1(b) and S1(c) in the Supplementary Materials (SM).…”

“…Although there was no consensus on the fundamental structure of room-temperature Cu2−𝑥Te, recent studies have focused more on the stable Nowotny hexagonal structure. [15,[18][19][20]22] In this layered structure, the Cu cations lie in the middle of the Te anions layers, the adjacent Te-Te layers stack along the 𝑐 axis and bond with weak van der Waals force, while the Cu-Te and Cu-Cu bonds are mainly strong covalent, as shown in Fig. 1(a).…”

“…[20] Chen et al observed a 2×2 surface reconstruction on Cu2−𝑥Te single crystal through spectroscopy measurements, but multiple other types of reconstruction patterns were detected meanwhile. [19] Therefore, the reconstructed structure of Cu2−𝑥Te seems to perform strongly samples dependence perhaps due to the existence of liquidlike Cu ions. However, the structure reconstruction may play an important role on the electronic states close to the Fermi level, and then bring new physical properties.…”

“…However, the structure reconstruction may play an important role on the electronic states close to the Fermi level, and then bring new physical properties. [21] Many studies tended to recognize Cu2−𝑥Te as a stable ptype semiconductor due to the cation vacancies, [16,19,20] but some band structure calculations argued that Cu2−𝑥Te was a metallic stable conductor. [22] More interestingly, it was recently shown to have characteristics of topologically nontrivial band feature and may exhibit topological insulator or topological semimetal state.…”