Synthesis and Optimization of Chalcogenides Quantum Dots Thermoelectric Materials by Chong Xiao

Author:Chong Xiao
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

3.2 Results and Discussion

AgBiSe2, as a typical member of I–V–VI2 compounds with phase transition behavior, its temperature-dependent structural evolution is well known as shown in Fig. 3.1a. At room temperature, AgBiSe2 is a p-type semiconductor and crystallizes in the hexagonal phase with parameters a = 4.18 Å and c = 19.67 Å (space group P-3m1). As temperature increases, AgBiSe2 is observed to undergo continuous phase transition to rhombohedral phase around 410 K and then to cubic phase around 580 K. This reversible phase transition will undergo from a high-temperature cubic phase to an intermediate-temperature rhombohedral structure around 560 K and then to a low-temperature hexagonal phase around 393 K as temperature decreases [21, 22]. In the intermediate-temperature rhombohedral phase (space group R-3 m with lattice constants a = 7.022 Å and α = 34.5°), ordering of Ag and Bi atoms takes place in quite distinguishable positions, while in the high-temperature cubic phase (space group Fm-3 m with lattice constants a = 5.832 Å), the Ag and Bi atoms are fully disordering. Inspired by the structural analysis, AgBiSe2 nanocrystals were synthesized through a simple colloidal method for the first time. X-ray diffractograms from synthetic AgBiSe2 nanocrystals (Fig. 3.1b) exhibit peaks corresponding to the hexagonal structure with space group of P-3m1 (see JCPDS files #74-0842). No extraneous peaks are observed, indicating that the samples consist of pure phase. The calculated lattice parameters a = 4.16 Å and c = 19.68 Å agree with JCPDS values. Temperature-dependent XRD patterns (Fig. 3.1b) clearly show the structural transition from the rhombohedral to the cubic phase. While the hexagonal–rhombohedral phase transition was not differentiated in the temperature-dependent XRD patterns, which prove that atoms slightly shift rather than rearrange during this phase transition. This is consistent with the DSC result (Fig. 3.1c). The appearance of endothermic and exothermal peaks in DSC curves during the heating and cooling process clearly confirms the fully reversible phase transition of AgBiSe2. Thermal analysis (Fig. 3.1c) externalizes the hexagonal–rhombohedral phase transition without showing drastic volume changes, which is responsive to the slight atoms shift in the lattice during this phase transition. Meanwhile, the reversible rhombohedral–cubic phase transition is observed around 580 K featuring a broad endothermic response over the temperature range 555 K < T < 595 K, consistent with temperature-dependent XRD results. In fact, since hexagonal–rhombohedral phase transition process only involves the slight movement of atoms, hexagonal and rhombohedral phase of AgBiSe2 exhibit almost the same XRD diffraction patterns. However, the clear-cut evidence in temperature-dependent Raman spectra (Fig. 3.1d) also confirmed that our originally synthetic sample is hexagonal AgBiSe2: apparent Raman spectra variations were observed during the phase transition, and the details will be discussed later.

Fig. 3.1 a Crystal structural evolution among hexagonal, rhombohedral, and cubic phase. b Temperature-dependent XRD patterns for AgBiSe2 nanocrystals. c TGA and DSC curves for AgBiSe2 nanocrystals. No obvious mass loss was detected in the whole temperature, while a broad endothermic was observed between 555 and 595 K. d Temperature-dependent Raman spectra for AgBiSe2 nanocrystals

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