Transmission electron microscopy study of Pb-depleted disks in PbTe-based alloys
- PDF / 460,698 Bytes
- 5 Pages / 584.957 x 782.986 pts Page_size
- 59 Downloads / 139 Views
Gang Chena) Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Zhifeng Renb) Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467 (Received 30 September 2010; accepted 8 December 2010)
Even though the crystal structure of lead telluride (PbTe) has been extensively studied for many years, we discovered that the structure has a strong tendency to form Pb-depleted disks on {001} planes. These disks are around 2–5 nm in diameter and less than 0.5 nm in thickness, with a volume density of around 9 1017 cm3, resulting in lattice strain fields (3–20 nm) on both sides of the disks along their normal directions. Moreover, such disks were also observed in Pb-rich Pb1.3Te, Pb-deficient PbTe1.3, and thallium (Tl)-doped Tl0.01Pb0.99Te and Tl0.02Pb0.98Te crystals. Because of the effects of diffraction contrast imaging by transmission electron microscopy and orientations of the crystals, these native lattice strain fields were incorrectly recognized as precipitates or nanoinclusions in PbTe-based materials. This discovery provides new insight into the formation mechanism of the precipitates or nanoinclusions in PbTe-based materials. I. INTRODUCTION
Lead telluride (PbTe, Fm3m, a 5 0.64 nm) has been recognized as one of the best thermoelectric materials since the 1960s.1,2 Normally, the maximum thermoelectric dimensionless figure-of-merit (ZT) of bulk PbTe is around 0.8–1.0 at ~650 K.3,4 Recently, by nanostructuring and alloying (elemental substitution), the ZT values of PbTebased thermoelectric materials have been dramatically increased to 1.5 at ~650 K.4–7 All those researchers4–7 reported the findings of various nanostructures: nanoinclusions,4 nanodots,5 nanoscale domains,6 or nanoscopic inhomogeneities,7 and they believed that these nanoinclusions (3–20 nm)4 led to stronger phonon scattering, which caused the reduction of the lattice thermal conductivity and ultimately the increase in the ZT value of the PbTe-based thermoelectric materials. However, as pointed out by Kanatzidis,4 considerably more characterizations are required to fully understand the nanoinclusions, which are endotaxially embedded in the PbTe matrix. More specifically, the origin of the nanoinclusions, which is a key feature of the bulk nanostructured PbTebased thermoelectric materials,4 remains unclear. To study the formation mechanism of the nanoinclusions, we have carried out extensive transmission electron microscopy (TEM) experiments on the nanostructure of the
PbTe and PbTe-related materials. To our surprise, we discovered that the nanoinclusions, as thought before, are nothing but strain fields (3–20 nm) caused by Pb-depleted disks (2–5 nm in diameter, less than 0.5 nm in thickness, and 9 1017 cm3 in volume density) lying on the {001} planes. These strain fields generate TEM bright-field contrast images closely resembling those of the reported nanoinclusions in size, shape, distribution, and orientation.4–7 Furthermore, it is known that both the Pb-depleted disks (coherent
Data Loading...
