Phase Transitions of Thermoelectric TAGS-85.

The alloys (GeTe)x(AgSbTe2)100-x, commonly known as TAGS-x, are among the best performing p-type thermoelectric materials for the composition range 80 ≤ x ≤ 90 and in the temperature range 200-500 °C. They adopt a rhombohedrally distorted rocksalt structure at room temperature and are reported to undergo a reversible phase transition to a cubic structure at ∼250 °C. However, we show that, for the optimal x = 85 composition (TAGS-85), both the structural and thermoelectric properties are highly sensitive to the initial synthesis method employed. Single-phase rhombohedral samples exhibit the best thermoelectric properties but can only be obtained after an annealing step at 600 °C during initial cooling from the melt. Under faster cooling conditions, the samples obtained are inhomogeneous, containing multiple rhombohedral phases with a range of lattice parameters and exhibiting inferior thermoelectric properties. We also find that when the room-temperature rhombohedral phase is heated, an intermediate trigonal structure containing ordered cation vacancy layers is formed at ∼200 °C, driven by the spontaneous precipitation of argyrodite-type Ag8GeTe6 which alters the stoichiometry of the TAGS-85 matrix. The rhombohedral and trigonal phases of TAGS-85 coexist up to 380 °C, above which a single cubic phase is obtained and the Ag8GeTe6 precipitates redissolve into the matrix. On subsequent cooling a mixture of rhombohedral, trigonal, and Ag8GeTe6 phases is again obtained. Initially single-phase samples exhibit thermoelectric power factors of up to 0.0035 W m-1 K-2 at 500 °C, a value that is maintained on subsequent thermal cycling and which represents the highest power factor yet reported for undoped TAGS-85. Therefore, control over the structural homogeneity of TAGS-85 as demonstrated here is essential in order to optimize the thermoelectric performance.

Reversible amorphous-crystalline phase changes in a wide range of Se(1-x)Te(x) alloys studied using ultrafast differential scanning calorimetry.

The reversible amorphous-crystalline phase change in a chalcogenide material, specifically the Se1-xTex alloy, has been investigated for the first time using ultrafast differential scanning calorimetry. Heating rates and cooling rates up to 5000 K/s were used. Repeated reversible amorphous-crystalline phase switching was achieved by consecutively melting, melt-quenching, and recrystallizing upon heating. Using a well-conditioned method, the composition of a single sample was allowed to shift slowly from 15 at. %Te to 60 at. %Te, eliminating sample-to-sample variability from the measurements. Using Energy Dispersive X-ray Spectroscopy composition analysis, the onset of melting for different Te-concentrations was confirmed to coincide with the literature solidus line, validating the use of the onset of melting Tm as a composition indicator. The glass transition Tg and crystallization temperature Tc could be determined accurately, allowing the construction of extended phase diagrams. It was found that Tm and Tg increase (but Tg/Tm decrease slightly) with increasing Te-concentration. Contrarily, the Tc decreases substantially, indicating that the amorphous phase becomes progressively unfavorable. This coincides well with the observation that the critical quench rate to prevent crystallization increases about three orders of magnitude with increasing Te concentration. Due to the employment of a large range of heating rates, non-Arrhenius behavior was detected, indicating that the undercooled liquid SeTe is a fragile liquid. The activation energy of crystallization was found to increase 0.5-0.6 eV when the Te concentration increases from 15 to 30 at. % Te, but it ceases to increase when approaching 50 at. % Te.

Strain Relaxation in "2D/2D and 2D/3D Systems": Highly Textured Mica/Bi2Te3, Sb2Te3/Bi2Te3, and Bi2Te3/GeTe Heterostructures.

Strain engineering as a method to control functional properties has seen in the last decades a surge of interest. Heterostructures comprising 2D-materials and containing van der Waals(-like) gaps were considered unsuitable for strain engineering. However, recent work on heterostructures based on Bi2Te3, Sb2Te3, and GeTe showed the potential of a different type of strain engineering due to long-range mutual straining. Still, a comprehensive understanding of the strain relaxation mechanism in these telluride heterostructures is lacking due to limitations of the earlier analyses performed. Here, we present a detailed study of strain in two-dimensional (2D/2D) and mixed dimensional (2D/3D) systems derived from mica/Bi2Te3, Sb2Te3/Bi2Te3, and Bi2Te3/GeTe heterostructures, respectively. We first clearly show the fast relaxation process in the mica/Bi2Te3 system where the strain was generally transferred and confined up to the second or third van der Waals block and then abruptly relaxed. Then we show, using three independent techniques, that the long-range exponentially decaying strain in GeTe and Sb2Te3 grown on the relaxed Bi2Te3 and Bi2Te3 on relaxed Sb2Te3 as directly observed at the growth surface is still present within these three different top layers a long time after growth. The observed behavior points at immediate strain relaxation by plastic deformation without any later relaxation and rules out an elastic (energy minimization) model as was proposed recently. Our work advances the understanding of strain tuning in textured heterostructures or superlattices governed by anisotropic bonding.

Strain engineering of van der Waals heterostructures.

Modifying the strain state of solids allows control over a plethora of functional properties. The weak interlayer bonding in van der Waals (vdWaals) materials such as graphene, hBN, MoS2, and Bi2Te3 might seem to exclude strain engineering, since strain would immediately relax at the vdWaals interfaces. Here we present direct observations of the contrary by showing growth of vdWaals heterostructures with persistent in-plane strains up to 5% and we show that strain relaxation follows a not yet reported process distinctly different from strain relaxation in three-dimensionally bonded (3D) materials. For this, 2D bonded Bi2Te3-Sb2Te3 and 2D/3D bonded Bi2Te3-GeTe multilayered films are grown using Pulsed Laser Deposition (PLD) and their structure is monitored in situ using Reflective High Energy Electron Diffraction (RHEED) and post situ analysis is performed using Transmission Electron Microscopy (TEM). Strain relaxation is modeled and found to solely depend on the layer being grown and its initial strain. This insight demonstrates that strain engineering of 2D bonded heterostructures obeys different rules than hold for epitaxial 3D materials and opens the door to precise tuning of the strain state of the individual layers to optimize functional performance of vdWaals heterostructures.

Combining Ultrafast Calorimetry and Electron Microscopy: Reversible Phase Transformations in SeTeAs Alloys.

Reversible amorphous-crystalline phase transitions are studied using complementary ultrafast differential scanning calorimetry and transmission electron microscopy techniques, which together allow a wealth of thermal and structural properties to be determined. The SeTe(As) system is investigated because these chalcogenide based materials have favorable properties as a phase-change memory material and in optical systems. Using calorimetry, we find that the addition of 10 at. % As to SeTe alloys strongly increases their glass forming ability, increasing both glass transition and crystallization temperatures while reducing critical quench rate. Ex situ investigation of Se x Te90-x As10 using electron microscopy and elemental mapping reveals a two-phase lamellar segregation mechanism, where a trigonal SeTe-phase and an amorphous As-rich phase are formed. These findings demonstrate the power of combining thermal and structural analysis techniques.

A cubic room temperature polymorph of thermoelectric TAGS-85.

The alloy (GeTe)85(AgSbTe2)15, commonly known as TAGS-85, is one of the best performing p-type thermoelectric materials in the temperature range 200-500 °C. In all reports thus far, TAGS-85 adopts a rhombohedral crystal structure at room temperature and undergoes a reversible transition to a cubic phase in the middle of the operating temperature range. Here, we report on a novel, metrically cubic polymorph of TAGS-85 that can be obtained at room temperature using a particular cooling protocol during initial synthesis. This polymorph transforms irreversibly on initial heating to a 21-layer trigonal structure containing ordered cation vacancy layers, driven by the spontaneous precipitation of argyrodite-type Ag8GeTe6. We show that the precipitation of Ag8GeTe6 is detrimental to the thermoelectric performance of TAGS-85 due to an increase in the vacancy concentration, which makes the samples more metallic in character and significantly reduces the Seebeck coefficient. The precipitation of Ag8GeTe6 can be suppressed by careful control of the synthesis conditions.