Thermal co-evaporation of Sb2Te3 thin-films optimized for thermoelectric applications

Antimony telluride (Sb₂Te₃) is a chalcogenide material used in thermoelectric applications. The deposition of thin films of Sb₂Te₃ requires a precisely controlled process to achieve a desirable high thermoelectric figure-of-merit. The optimization of the thermal co-evaporation process for p-type Sb₂Te₃ thin-film onto plastic substrates (Kapton© polyimide) for thermoelectric applications is reported. The influence of deposition parameters and composition on thermoelectric properties was studied, seeking optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy all confirmed the formation of Sb₂Te₃ thin films. Seebeck coefficient (up to 190 μVK⁻¹), in-plane electrical resistivity (8-15 μΩm), carrier concentration (1 × 10¹⁹-7 × 10¹⁹ cm⁻³) and Hall mobility (120-180 cm²V⁻¹s⁻¹) were measured at room temperature for the best Sb₂Te₃ thin-films.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Microstructure and thermoelectric properties of Sn-doped Bi2Te2.7Se0.3 thin films deposited by flash evaporation method

(Bi_1 − xSn_x)₂Te_2.7Se_0.3 thermoelectric thin films with thickness of 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. The structures, morphology of the thin films were analyzed by X-ray diffraction and field emission scanning electron microscopy respectively. Effects of Sn-doping concentration on thermoelectric properties of the annealed thin films were investigated by room-temperature measurement of Seebeck coefficient and electrical resistivity. The thermoelectric power factor was enhanced to 12.8 μW/cmK² (x = 0.003). From x = 0.004 to 0.01 Sn doping concentration, the Seebeck coefficients are positive and show p-type conduction. The Seebeck coefficient and electrical resistivity gradually decrease with increasing Sn doping concentration.     

Studying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO₂ thin films (M_xTi_1−xO₂, M = Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Mn²⁺ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti⁴⁺ oxidation state of the Ti2p band in the doped p-TiO₂. The homogenous M_xTi_1−xO₂ was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current (I)–voltage (V) characteristics for the Ag/n-ZnO/M_xTi_1−xO₂/ITO/glass assembly showed rectifying behavior with small turn-on voltages (V₀) < 1 V. The ideality factor (η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.     

Molten salt synthesis and thermoelectric properties of Ca2Co2O5

A molten salt method was applied to synthesize Ca₂Co₂O₅ powder by using Co₃O₄ and CaCO₃ as raw materials and CaCO₃-CaCl₂ as eutectic salt. The formation process and molten salt mechanism of Ca₂Co₂O₅ were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that sheet-like particles can be obtained at 963 K. Moreover, the electrical conductivity and the Seebeck coefficient of the specimen were measured from 323 to 963 K. The properties increased with increasing temperature and the highest value of power factor (2.0 × 10^− 4 W m^− 1 K^− 2) was obtained at 963 K, which indicates that molten salt synthesis is a promising method to prepare high thermoelectric performance material.     

Synthesis and characterization of a new organic dihydrogen phosphate-arsenate: [H2(C4H10N2)][H2(As, P)O4]2

Chemical preparation, crystal structure, calorimetric and spectroscopic investigations (IR and RMN) are given for a new non-centrosymmetric organic-cation dihydrogen phosphate-arsenate [H₂(C₄H₁₀N₂)][H₂(As, P)O₄]₂. This compound is triclinic P1 with the following unit-cell parameters: a = 7.082(2) Å, b = 7.796(1) Å, c = 12.05(3) Å, α = 95.37(2)°, β = 98.38(3)°, γ = 62.98(1)°, Z = 2, V = 586.2(1) Å³ and D_x = 1.836 g cm⁻³. The crystal structure has been solved and refined to R = 0.03 using 2328 independent reflections. The structure can be described as infinite (H₂XO)_n chains spreading parallel to the b direction. These chains are themselves interconnected by a set of NH?O hydrogen bonds generated by the organic entities, alternating with the chains. Solid-state ¹³C, ¹⁵N and ³¹P MAS NMR spectroscopies are in agreement with the X-ray structure.     

Microstructure and thermoelectric properties of n-type 95%Bi2Te3-5%Bi2Se3 alloy produced by rapid solidification and hot extrusion

n-type SbI₃-doped 95%Bi₂Te₃+5%Bi₂Se₃ compounds were prepared by a rapid solidification and extrusion at the temperature range 420-480 °C using an extrusion ratio of 25:1. The microstructure and thermoelectric properties of the compounds were investigated as a function of extrusion temperature. The fabricated powder consists of homogeneous Bi₂Te₃+Bi₂Se₃ solid solution and the relative density of over 99% was obtained by hot extrusion. The values of Seebeck coefficient for the compounds hot extruded at 420, 450, and 480 °C were −160.8, −170.2, and −165.7 μV K⁻¹, respectively. The values of electrical resistivity (ρ) for the compounds hot extruded at 420, 450, and 480 °C were 0.49, 0.57, and 0.51×10⁻⁵ Ω m, respectively. The maximum power factor value of the compounds hot extruded at 480 °C was 53.8×10⁶ μW cm⁻¹ K⁻².     

Copper phthalocyanine thin-film field-effect transistor with SiO2/Ta2O5/SiO2 multilayer insulator

Top-contact Copper phthalocyanine (CuPc) thin-film field-effect transistor (TFT) with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer as the dielectric was fabricated and investigated. With the multi-layer dielectric, drive voltage was remarkably reduced. A relatively large on-current of 1.1 × 10⁻⁷ A at a V_GS of −15 V was obtained due to the strong coupling capability provided by the STS multilayer gate insulator. The device shows a moderate performance: saturation mobility of μ_sat = 6.12 × 10⁻⁴ cm²/V s, on-current to off-current ratio of I_on/I_off = 1.1 × 10³, threshold voltage of V_TH = −3.2 V and sub-threshold swing SS = 1.6 V/dec. Atomic force microscope images show that the STS multilayer has a relative smooth surface. Experiment results indicate that STS multilayer is a promising insulator for the low drive voltage CuPc-based TFTs.     

Synthesis and characterization of a new monophosphate (5-Cl-2,4-(OCH3)2C6H2NH3)H2PO4

Chemical preparation, crystal structure and NMR spectroscopy of a new organic cation 5-chloro(2,4-dimethoxy)anilinium monophosphate H₂PO₄ are given. This new compound crystallizes in the monoclinic system, with the space group P2₁/c and the following parameters: a = 5.524(2) Å, b = 9.303(2) Å, c = 23.388(2) Å, β = 90.66(4), V = 1201.8(2) Å³, Z = 4 and D_x = 1.573 g cm⁻³. Crystal structure has been determined and refined to R = 0.031 and R_w = 0.080 using 1702 independent reflections. Structure can be described as an infinite (H₂PO₄)_nⁿ⁻ corrugated chains in the a-direction. The organic groups (5-Cl-2,4-(OCH₃)₂C₆H₂NH₃)⁺ are anchored between adjacent polyanions through multiple hydrogen bonds. This compound is also investigated by IR, thermal, and solid-state, ¹³C, ³¹P MAS NMR spectroscopies.     

Growth and spectroscopic properties of Er-doped Li3Ba2Y3(MoO4)8 crystal

Er³⁺:Li₃Ba₂Y₃(MoO₄)₈ crystal has been grown by the top seeded solution growth method (TSSG) from a flux of Li₂MoO₄ and its morphology was analyzed. The polarized absorption spectra, fluorescence spectra and fluorescence decay curves of the crystal were measured. Based on the Judd-Ofelt (J-O) theory, spectroscopic parameters of Er³⁺:Li₃Ba₂Y₃(MoO₄)₈ crystal, including the oscillator intensity parameters Ω_t (t = 2, 4, 6), spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes were calculated and analyzed. Stimulated emission cross-sections of the ⁴I_13/2 → ⁴I_15/2 transition were estimated by the reciprocity method (RM) and the Fuchtbauer-Ladenburg (F-L) formula. Five up-conversion fluorescence bands around 490, 530, 550, 660 and 800 nm were observed with 977 nm excitation, and the possible up-conversion mechanisms were proposed.     

Effect of the cooling rate on the thermoelectric properties of p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 after melting in the bismuth-telluride system

The p-type (Bi_0.25Sb_0.75)₂Te₃ and n-type Bi₂(Te_0.94Se_0.06)₃ ingots were prepared by cooling at various cooling rates C after melting so that they have an intermediate state between the polycrystalline and Bridgman ingots which lowers their thermal conductivity κ, where C was changed from 0.10 to 2375 K/min in an evacuated glass tube. When the ingots were cooled at C = 0.50 K/min under the uniaxial temperature gradient of 5 K/cm, it was observed that the c axis of some grains points to the freezing direction. The electrical resistivity ρ, Seebeck coefficient α and κ of ingots were measured at 298 K along the freezing direction, so that ρ and κ at C = 0.50 K/min were lower by 20-30% and 9% than those of the corresponding Bridgman ingots. The thermoelectric figure of merits ZT(=α²T/ρκ) estimated for the p- and n-type ingots then reached high values of 1.27 and 1.25 at 298 K, respectively.     

Growth and spectral properties of Cr:KAl(MoO4)2 crystal

The Cr³⁺:KAl(MoO₄)₂ single crystal was grown by top seeding solution growth method (TSSG). Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ?ω and the Huang-Rhys factor S were calculated: Dq = 1494.8 cm^− 1, B = 585.5 cm^− 1 and C = 3049 cm^− 1, ?ω = 373.8 cm^− 1 and the Huang-Rhys factor S = 3.74, respectively. The value Dq/B = 2.55 indicates that Cr³⁺ ion occupies the strong crystal field site in KAl(MoO₄)₂ crystal. A comparison of crystal field parameters for Cr³⁺:KAl(MoO₄)₂ with other Cr³⁺-doped crystals was presented. The results of spectral measurement show that Cr³⁺:KAl(MoO₄)₂ may be a potential candidate for broadband laser applications.     

Crystal structure and magnetic properties of double perovskite Mn2FeSbO6

The double perovskite Mn₂FeSbO₆ has been synthesized under pressure 6 GPa and temperature 1000 °C. The crystal structure refinement of Mn₂FeSbO₆ was carried out with the GSAS program suite using X-ray diffraction data. XRD pattern of Mn₂FeSbO₆ was indexed with a monoclinic unit cell (space group P2₁/n) with parameters: a = 5.2431(3) Å, b = 5.3935(3) Å, c = 7.6358(5) Å, β = 89.693(2)°, V = 215.927 Å³, Z = 2. It found that Fe and Sb atoms are completely ordered in 2d and 2c positions of double perovskite structure respectively. According to XPS measurements, manganese in this compound is present as Mn²⁺, whiles the iron - as Fe³⁺. Magnetization measurements revealed the presence about 3 mass% of ferromagnetic impurity in the sample. Dependence of AC susceptibility χ″ from temperature showed that magnetic properties compound are determined probably by transformation in antiferromagnetic state below 19.5 K.     

Structural and electrical properties of HfO2/Dy2O3 gate stacks on Ge substrates

In the present work we report on the structural and electrical properties of metal-oxide-semiconductor (MOS) devices with HfO₂/Dy₂O₃ gate stack dielectrics, deposited by molecular beam deposition on p-type germanium (Ge) substrates. Structural characterization by means of high-resolution Transmission Electron Microscopy (TEM) and X-ray diffraction measurements demonstrate the nanocrystalline nature of the films. Moreover, the interpretation of the X-ray reflectivity measurements reveals the spontaneous growth of an ultrathin germanium oxide interfacial layer which was also confirmed by TEM. Subsequent electrical characterization measurements on Pt/HfO₂/Dy₂O₃/p-Ge MOS diodes show that a combination of a thin Dy₂O₃ buffer layer with a thicker HfO₂ on top can give very good results, such as equivalent oxide thickness values as low as 1.9 nm, low density of interfacial defects (2-5 × 10¹² eV^− 1 cm^− 2) and leakage currents with typical current density values around 15 nA/cm² at V_g = V_FB − 1V.     

New arsenates (V) NaKAl2O[AsO4]2 and Na2KAl3[AsO4]4

The title compounds have been synthesized by a solid state reaction route using a salt flux. Their crystal structures were determined from single crystal X-ray data. NaKAl₂O[AsO₄]₂ crystallizes with the orthorhombic K₂Fe₂O[AsO₄]₂-type, Pnma, a = 8.2368(6) Å, b = 5.5228(3) Å, c = 17.0160(13) Å and Z = 4, whereas Na₂KAl₃[AsO₄]₄ crystallizes with the orthorhombic K₃Fe₃[AsO₄]₄-type, Cmce, a = 10.5049(9), b = 20.482(2), c = 6.3574(6) Å and Z = 4. The NaKAl₂O[AsO₄]₂ structure is built up of [Al₂As₂O₉]²⁻ layers perpendicular to the c-axis which are separated by A⁺ alkali layers. The [Al₂As₂O₉]²⁻ layers consist of ribbons of edge-sharing AlO₆ octahedra, running along the a direction and which are connected through AsO₄ tetrahedra by sharing corners. The Na₂KAl₃[AsO₄]₄ structure contains [Al₃As₄O₁₆]³⁻ layers perpendicular to the b-axis separated by A⁺ alkali layers. The [Al₃As₄O₁₆]³⁻ layer consists of a layer of corner-sharing AlO₆ octahedra which are also connected to the AsO₄ tetrahedra by sharing corners.     

Improved thermoelectric properties by adding Al for Zn in (ZnO)mIn2O3

The grain size and density of the sintered (Zn_1 − xAl_xO)_mIn₂O₃ bodies decreased with the small Al₂O₃ content (≤ 0.012), and then increased gradually by further increasing the Al₂O₃ content. The addition of Al for Zn in the (ZnO)_mIn₂O₃ led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient. This indicates that the power factor was significantly enhanced by adding Al for Zn. The thermoelectric power factor was maximized to 1.67 × 10^− 3 W m^− 1 K^− 2 at 1073 K for the (Zn_0.992Al_0.008O)_mIn₂O₃ sample.     

Structural characterization, thermal, spectroscopic and magnetic studies of the (C3H12N2)0.75[Mn1.50Fe1.50(AsO4)F6] and (C3H12N2)0.75[Co1.50Fe1.50(AsO4)F6] compounds

The (C₃H₁₂N₂)_0.94[Mn_1.50Fe_1.50^III(AsO₄)F₆] and (C₃H₁₂N₂)_0.75[Co_1.50Fe_1.50^III(AsO₄)F₆] compounds 1 and 2 have been synthesized using mild hydrothermal conditions. These phases are isostructural with (C₃H₁₂N₂)_0.75[Fe_1.5^IIFe_1.5^III(AsO₄)F₆]. The compounds crystallize in the orthorhombic Imam space group. The unit cell parameters calculated by using the patterns matching routine of the FULPROOF program, starting from the cell parameters of the iron(II),(III) phase, are: a = 7.727(1) Å, b = 11.047(1) Å, c = 13.412(1) Å for 1 and a = 7.560(1) Å, b = 11.012(1) Å, c = 13.206(1) Å for 2, being Z = 8 in both compounds. The crystal structure consists of a three-dimensional framework constructed from edge-sharing [M^II(1)₂O₂F₈] (M = Mn, Co) dimeric octahedra linked to [Fe^III(2)O₂F₄] octahedra through the F(1) anions and to the [AsO₄] tetrahedra by the O(1) vertex. This network gives rise two kinds of chains, which are extended in perpendicular directions. Chain 1 is extended along the a-axis and chain 2 runs along the c-axis. These chains are linked by the F(1) and O(1) atoms and establish cavities delimited by eight or six polyhedra along the [1 0 0] and [0 0 1] directions, respectively. The propanediammonium cations are located inside these cavities. The thermal study indicates that the structures collapse with the calcination of the organic dication at 255 and 285 °C for 1 and 2, respectively. The Mössbauer spectra in the paramagnetic state indicate the existence of two crystallographically independent positions for the iron(III) cations and a small proportion of this cation in the positions of the divalent Mn(II) and Co(II) ones. The IR spectrum shows the protonated bands of the H₂N- groups of the propanediamine molecule and the characteristic bands of the [AsO₄]³⁻ arsenate oxoanions. In the diffuse reflectance spectra, it can be observed the bands characteristic of trivalent iron(III) cation and divalent Mn(II) and Co(II) ones in a distorted octahedral symmetry. The calculated Dq and B-Racah parameters for the cobalt(II) phase are 710 and 925 cm⁻¹, respectively. The ESR spectra of compound 1 maintain isotropic with variation in temperature, being g = 1.99. Magnetic measurements for both compounds indicate that the main magnetic interactions are antiferromagnetic in nature. However, at low temperatures small ferromagnetic components are detected, which are probably due to a spin decompensation of the two different metallic cations. The hysteresis loops give values of the remnant magnetization and coercive field of 84.5, 255 emu/mol and 0.01, 0.225 T for phases 1 and 2, respectively.     

Enhanced thermoelectric properties of NaCo2O4 by adding ZnO

The primary phase present in the as-sintered Na(Co_1 − xZn_x)₂O₄ (0 ≤ x ≤ 0.1) bodies was the solid solution of the constituent oxides with a bronze-type layered structure. The electrical conductivity of the Na(Co_1 − xZn_x)₂O₄ samples significantly increased with an increase in ZnO content. The sign of the Seebeck coefficient for all samples was positive over the whole temperature range (723-1073 K), i.e., p-type conduction. The power factor of Na(Co_0.95Zn_0.05)₂O₄ showed an outstanding power factor (1.7 × 10^− 3Wm^− 1 K^− 2) at 1073 K. The power factor was above four times superior to that of ZnO-free NaCo₂O₄ (0.4 × 10^− 3Wm^− 1 K^− 2). This originates from an unusually large Seebeck coefficient (415 μVK^− 1) accompanied with high conductivity (127Ω^− 1 cm^− 1) at 1073 K.