A Thermoelectric Performance Study of Layered Bi 2 TeI Weak Topological Insulator Materials

Topological insulators have been considered as promising thermoelectric materials because of their high electrical transport properties and low thermal conductivity. In this work, the crystal structure, chemical composition, and thermoelectric transport properties of a weak topological insulator, Bi2TeI, were studied. Bi2TeI possesses the lowest lattice thermal conductivity compared with the analogously layered compounds Bi2Te3 and BiTeI. Cu and Zn were used as dopants with the aim of optimizing the thermoelectric performance. It was found that doping Bi2TeI with Cu led to decreased carrier concentration and power factor while doping with Zn resulted in increased carrier concentration and improved power factor. Accompanied with reduced lattice thermal conductivity, Zn-doped samples showed a largely improved dimensionless figure of merit, ZT. The largest ZT was 0.077 for Zn0.01Bi2TeI, increased by 70% as compared with the undoped Bi2TeI.


Introduction
Thermoelectric (TE) technology can realize direct energy conversion between heat and electricity, having played an important role in the field of waste heat recovering, power generation, and efficient cooling [1].The performance of a TE material is characterized by the dimensionless figure of merit ZT = α 2 σT/κ, where α, σ, and T are the Seebeck coefficient, electrical conductivity, and absolute temperature.κ is the thermal conductivity, which is comprised of electronic (κ E ) and lattice (κ L ) parts.High-performance TE materials should have high power factor (α 2 σ) and poor thermal conduction.However, the optimization of TE performance is challenging because these transport properties are strongly interdependent, and it is necessary to synergistically regulate the electrical and thermal transport properties [2,3].In the past decade, electronic band engineering and phonon engineering approaches have been developed for the improvement of TE properties [4][5][6].Recently, magnetic nanoparticles embedded in TE materials were reported as a new strategy to optimize the electron and phonon transport [7,8].It is thus of great interest to explore new materials in which unconventional effects would be beneficial to the TE performance.
As a kind of novel condensed matter, topological insulators (TIs) exhibit gapped states in bulk and nontrivial metallic states on the surface [9,10].It was suggested that the unique electron transport behavior of the surface states might provide new insights into the TE community [11][12][13][14].Theoretical calculations have shown that the TE performance of strong TIs such as Bi 2 Te 3 and Bi 2 Se 3 can be enhanced by the surface state hybridization with bulk states [13], or by the largely varied electron scattering rates spanning from the surface states to the bulk states [14].These studies have provided more opportunities to explore potential TE candidates among the TI systems.As many researchers have focused on the TE performance of the strong TIs [11][12][13][14], the weak TIs, which are distinguished from the strong ones by an even number of Dirac cones on the surface, are less studied as TE materials.Structurally, some weak TIs can be regarded as an alternate stack of quantum spin Hall (QSH) layers and normal insulator (NI) layers [10,15].The superlattice-like stacked structure should have lower intrinsic lattice thermal conductivity [16,17].Experimental work has showed that the weak TI Bi 14 Rh 3 I 9 manifested extremely low thermal conductivity (as low as ~0.3 W m −1 K −1 ) benefitting from low sound velocity, low Debye temperature, and large lattice anharmonicity [18].Meanwhile, the first-principles calculations also predicted that low thermal conductivity could be found in another weak TI Bi 2 TeI due to the similar stacking structure and lattice dynamics [18].
In this work, the weak TI Bi 2 TeI samples were synthesized using a combination of melting, annealing, and spark plasma sintering (SPS) techniques.The microstructure and TE transport properties of Bi 2 TeI were studied.The TE properties of Bi 2 TeI were compared with two other important layered compounds, Bi 2 Te 3 and BiTeI, since Bi 2 Te 3 represents one of the most important TE candidates and is meanwhile a strong TI [13,14], while BiTeI has the same structure as the NI layers of Bi 2 TeI (Figure 1a) [19,20].It was found that Bi 2 TeI possessed the lowest lattice thermal conductivity.Cu and Zn were used as dopants to further improve the TE performance of Bi 2 TeI.It was revealed that doping Bi 2 TeI with Zn could improve the TE properties with enhanced electrical conductivity and Seebeck coefficient and depressed thermal conductivity.

Experimental
A combination of melting, annealing, and SPS techniques were used to prepare Bi 2 Te 3 , BiTeI, and undoped and doped Bi 2 TeI samples.Highly pure Bi (99.999%, powder), Te (99.999%, powder), and BiI 3 (99.99%,powder) were used as starting materials.The mixtures of these materials were weighted according to the compositions and sealed into silica tubes under vacuum.The tubes were transferred into a programmable furnace for heat treatment.For the synthesis of undoped and doped Bi 2 TeI samples, the tubes were heated to 723 K and held for 8 h followed by an annealing treatment at 403 K for 48 h.The annealed products were ground into fine powders then sintered to pellets by SPS method under vacuum.The sintering temperature was 653 K and the applied pressure was 40 MPa.The synthesis process of Bi 2 Te 3 and BiTeI followed the same procedures.For BiTeI, the melting and annealing treatments were carried out at 893 K for 8 h and at 403 K for 48 h, respectively.The SPS conditions were sintering temperature 653 K and a pressure of 40 MPa.For Bi 2 Te 3 , the melting and annealing were carried out at 893 K for 8 h and at 723 K for 48 h, respectively.The SPS conditions were sintering temperature 723 K and a pressure of 40 MPa.Since iodide is volatile, excessive iodide (1 at%) was added in the starting materials to compensate the iodide loss during the annealing and sintering processes of Bi 2 TeI and BiTeI.
Phase constituents were determined by X-ray diffraction (XRD, PANalytical X'Pert PRO, PANalytical, Almelo, The Netherlands) using Cu Kα radiation.The microstructure and chemical compositions were analyzed by an electron probe micro-analyzer (EPMA, JXA-8230, JEOL, Tokyo, Japan).Selected area electron diffraction (SAED) and elemental mapping of Bi 2 TeI were performed with transmission electron microscopy (TEM, Talos F200S, Thermo Fisher Scientific, Waltham, MA, USA) equipped with energy dispersion X-ray spectroscopy (EDX).Scanning transmission electron microscopy-energy dispersion X-ray spectroscopy (STEM-EDX) mode was used to capture the elemental distribution in the nanometre scale.The TEM sample for SAED and STEM-EDX analysis was made by a focused ion beam (FIB, Helios NanoLab G3 UC, Thermo Fisher Scientific).σ and α were measured with the standard four-probe method (ZEM-3, ULVAC Technologies, Inc., Kanagawa, Japan) in a He atmosphere.Hall effect measurements were performed on a commercial testing system (Thermoelectric and magnetic performance of multilayer film materials, NYMS-1, Shanghai, China) [7].The measurements were performed in a He atmosphere with a maximum applied magnetic field of 1.5 T. κ was calculated using the equation κ = C p λρ 0 , where C p is the specific heat capacity, ρ 0 is the bulk density, and λ is the thermal diffusion coefficient.λ was measured by a laser flash technique (Netzsch LFA 457, Selb, Germany) in a flowing Ar atmosphere.ρ 0 was measured by the Archimedes method.Uncertainties for σ and κ were ±5-7%, and for α was ±5%.Since these layered compounds sintered by the SPS technique exhibited strong c-axis preferred orientation along the direction of pressure, the TE transport properties along the in-plane (perpendicular to the direction of pressure) and cross-plane (parallel to the direction of pressure) directions were separately measured.

Synthesis and TE Properties of Bi 2 TeI
As shown in Figure 1a, Bi 2 TeI is crystallized in a monoclinic lattice (space group C2/m) [19].Resembling the weak TI Bi 14 Rh 3 I 9 , Bi 2 TeI has a layered structure stacked by bismuth QSH bilayers [Bi] 2 and BiTeI NI bilayers [BiTeI] 2 .The formation mechanism of topological insulator states could be connected with the strong interaction between QSH and NI layers [20] and the crystal mirror symmetry [21].
Differently to Bi 2 Te 3 and BiTeI, experimental study performed on Bi 2 TeI was rare since the preparation of Bi 2 TeI was a challenge [22][23][24].It has been reported that phase-pure Bi 2 TeI could not be obtained by long-term annealing, gas phase reaction, or slow cooling [19,[22][23][24].As guided with the solid-state equilibria of Bi-Te-I systems [25][26][27], polycrystalline Bi 2 TeI bulk samples were synthesized using a combination of melting, annealing, and SPS techniques in this work.This combined approach has been widely used for preparing varioustable TE materials [2,7].Figure 1b shows the powder XRD patterns of Bi 2 Te 3 , BiTeI, and Bi 2 TeI.All the diffraction peaks can be indexed to the standard files of the Joint Committee on Powder Diffraction Standards (JCPDS) 89-2009 of Bi 2 Te 3 and the JCPDS 82-0484 of BiTeI, respectively.The XRD pattern of Bi 2 TeI was calculated with the reported monoclinic structure [16].All the diffraction peaks can be indexed to the calculated pattern, indicating that the synthesized Bi 2 TeI was single-phase.Figure 2a   Figure 3 shows the temperature dependence of transport properties of Bi 2 Te 3 , BiTeI, and Bi 2 TeI, including electrical conductivity, absolute Seebeck coefficient, and thermal conductivities measured along the in-plane and cross-plane directions.The charge transport properties at 300 K including carrier concentration and Hall mobility along the two directions are listed in Table 1.Along the two mutually perpendicular directions, the anisotropy in σ was large for Bi 2 Te 3 and BiTeI but in α was small.The anisotropy in layered compounds is determined by the highly anisotropic mobility.σ of Bi 2 Te 3 decreased with the increase in temperature, indicating a heavily doped semiconducting transport behavior.BiTeI and Bi 2 TeI had σ increased with the increase in temperature, showing a semiconducting transport behavior.Bi 2 Te 3 and BiTeI were n-type while Bi 2 TeI was p-type.The absolute α of BiTeI was around 100 µV K −1 at 300 K and increased with the increase temperature from 300 to 500 K, which is consistent with the reported work [28].Bi 2 TeI had lower α values as compared with BiTeI, which might be related to the higher carrier concentration and much smaller band gap of Bi 2 TeI (0.1 eV for Bi 2 TeI [20] and 0.38 eV for BiTeI [29]).
Table 1.Charge transport properties of Bi 2 Te 3 , BiTeI, and Bi 2 TeI along the in-plane and cross-plane directions at 300 K.The comparison of thermal conductivity and lattice thermal conductivity of Bi 2 Te 3 , BiTeI, and Bi 2 TeI, along the in-plane and cross-plane directions, is shown in Figure 3c,d.κ L was obtained by subtracting κ E from κ. κ E was calculated by Wiedemann-Franz law κ E = L 0 Tσ, where the Lorenz number L 0 had a numerical value of 2.0 × 10 −8 V 2 K −2 , by considering their high carrier concentration.Compared with binary Bi 2 Te 3 , ternary BiTeI and Bi 2 TeI exhibited much lower κ and relatively weak temperature dependence of κ.At 300 K, κ L of Bi 2 TeI was about 0.44 W m −1 K −1 along the cross-plane direction, which was the lowest as compared with those of Bi 2 Te 3 and BiTeI.The measured κ L of Bi 2 TeI was higher than those predicted by the first-principles calculations [18], which might be attributed to the fact that (1) the sound velocity and Debye temperature were overestimated in the calculations and (2) the contribution of bipolar thermal conductivity as caused by the small band gap was neglected in theoretical calculations.

TE Properties Optimization of Doped Bi 2 TeI
Since Bi 2 TeI possesses very low lattice thermal conductivity, it is expected to optimize the electrical properties and thus the overall TE performance.A corporation of Cu has been proven as an effective way to improve the TE performance of Bi 2 Te 3 and BiTeI [28,[30][31][32].Here, Cu was used as a dopant to optimize the TE performance of Bi 2 TeI.Meanwhile, Zn was selected as another dopant because Zn has larger electronegativity.Cu-doped and Zn-doped Bi 2 TeI samples with nominal compositions Cu x Bi 2 TeI and Zn y Bi 2 TeI were synthesized.Figure 4 shows the powder XRD patterns of Cu-doped and Zn-doped Bi 2 TeI.It can be seen that all the diffraction peaks can be indexed to the calculated diffraction patterns of Bi 2 TeI, suggesting that Cu and Zn dopants were incorporated into the lattice of Bi 2 TeI.Broadened diffraction peaks can be found for the (003), (205), and (020) planes.It is possible that the high concentration of dopants could bring about large lattice distortion.Figure 5 shows the elemental mapping of Cu on Cu x Bi 2 TeI and Zn on Zn y Bi 2 TeI samples measured by EPMA.For Cu x Bi 2 TeI, the distribution of Cu was uniform with increasing the content of dopant.A slight Zn richness can be found from y = 1.5% and 2% samples.Figure 6 shows the chemical composition of a large Zn-rich area on Zn 0.015 Bi 2 TeI sample determined by EPMA.Further analysis indicated that the Zn-rich area was composed of Bi 2 TeI and ZnI 2 .Combined with the distribution of Zn for y = 1.5% and 2% samples as shown in Figure 5e,f, the secondary phase ZnI 2 should be precipitated on the grain boundaries of the matrix, which will impact the electrical and thermal properties.These results suggest that Cu and Zn were incorporated into the lattice of Bi 2 TeI and a high concentration of Zn led to the formation of ZnI 2 impurity phase.2. σ of the doped samples was increased slowly while increasing the temperature, indicating a semiconducting transport behavior.Doping Bi 2 TeI with 1.5% and 2% Cu led to decreased σ.With increasing the Zn content, σ of Zn y Bi 2 TeI was first increased then largely decreased.The decrease in σ upon doping with large amount of Cu should be attributed to the decreased carrier concentration.After doping with Zn, the carrier concentration was greatly increased, from 8.1 × 10 20 cm −3 for the pristine Bi 2 TeI to 1.48 × 10 21 cm −3 for Zn 0.01 Bi 2 TeI, and kept almost unchanged after doping with 2% Zn.Therefore, the increase in σ for Zn 0.01 Bi 2 TeI was due to the increased carrier concentration, and the decrease in σ for Zn 0.015 Bi 2 TeI and Zn 0.02 Bi 2 TeI might be related to the formation of nonconductive ZnI 2 impurity phase.Since the carrier concentration was reduced after Cu doping, it can be inferred that Cu as an intercalated dopant has entered into the interlayer space of Bi 2 TeI.The electron donor behavior of the intercalated Cu has been reported in other layered compounds [29,30].For Zn doped samples, a substitution of Zn 2+ on Bi 3+ sites might be accounted for the increased carrier concentration of the p-type materials.The positive α values of Cu x Bi 2 TeI and Zn y Bi 2 TeI were indicative of p-type conduction behavior, which was consistent with Hall effect measurements.Doping Cu led to not only decreased σ but also decreased α, which was mainly related to the strengthened bipolar effect as the carrier concentration was decreased.Interestingly, simultaneously increased σ and α were obtained for Zn 0.01 Bi 2 TeI sample.It was highly possible that the dopant Zn introduced an additional resonant state near the Fermi level, which was beneficial for enhancing the Seebeck coefficient.However, a further theoretical calculation should be carried out to verify the electronic structure of Zn-doped Bi 2 TeI.
Due to the decreased σ and α, the α 2 σ values of the Cu-doped samples decreased with increasing x in the range of 0-2%.This result originated from the strengthened bipolar effect as caused by the lowered carrier concentration.For Zn-doped samples, the α 2 σ values first increased and then decreased.The highest value reached 0.14 mW m −1 K −2 at 500 K for Zn 0.01 Bi 2 TeI sample, which showed an approximately 50% improvement as compared with that of the undoped sample.At high temperatures, all Zn doped samples showed enhanced power factors.
Theoretical predictions have suggested that the gapless surface states on the surface could bring about positive contribution to the Seebeck coefficient [13,14], which needs to adjust the Fermi level into a relatively narrow energy range [12,21].The undoped Bi 2 TeI had a high carrier concentration of 8 × 10 20 cm −3 .By inserting Cu, the carrier concentration of Bi 2 TeI can be reduced to 4 × 10 20 cm −3 .However, an increase in Cu content to further lower the carrier concentration failed and finally led to the formation of impurities.Therefore, the TE properties of the undoped and doped Bi 2 TeI in this work should be contributed from the bulk states of this weak TI.
Figure 8 shows the temperature dependence of in-plane thermal conductivity, carrier thermal conductivity, and lattice thermal conductivity of Cu-doped and Zn-doped Bi 2 TeI.With an increase in temperature from 300 to 500 K, κ first decreased slightly and then increased gradually.The first decrease in κ was related to the phonon-phonon scattering and the increase at higher temperatures should be mainly due to the contribution of κ E .The κ L values for all the samples decreased with the increase in temperature, reflecting strong phonon-phonon scattering in the layered Bi 2 TeI.With the increase in the doping content of Cu and Zn, the κ L values first decreased and then increased.The first decrease in κ L values with doping Cu might originate from the point defect scattering and disorder scattering of the intercalated Cu.The increase in κ L values after doping more Cu should be related to the enhanced interlayer coupling between QSH and NI layers in these kinds of layered materials.It was inferred that the intercalation of dopants could lead to increased sound velocity and Debye temperature along the cross-plane directions, which leads to higher κ L .The first decrease in κ L values after doping with Zn should originate from the improved point defect scattering when the dopant Zn substituted for the Bi sites.The increase in κ L values after doping more Zn should be attributed to the presence of ZnI 2 impurity.Zn 0.01 Bi 2 TeI had the lowest in-plane lattice thermal conductivity with 0.69 W m −1 K −1 at 300 K and 0.58 W m −1 K −1 at 500 K.

Conclusions
Single-phase Bi 2 TeI was prepared by a combination of melt-annealing and SPS techniques.The TE performance of Bi 2 TeI was compared with Bi 2 Te 3 and BiTeI.It was found that Bi 2 TeI possessed high electrical conductivity and the lowest lattice thermal conductivity.A performance optimization study was performed by doping Bi 2 TeI with Cu and Zn.Structural analysis indicated that the dopants Cu and Zn were incorporated into the lattice of Bi 2 TeI.The doping of Zn could improve the power factor due to the enhanced electrical conductivity and Seebeck coefficient.The maximal power factor reached 0.14 mW m −1 K −2 at 500 K for the Zn 0.01 Bi 2 TeI sample.The phonon transport of Bi 2 TeI was suppressed due to the intercalation of Cu into the interlayer space and the substitution of Zn on Bi sites.The phonon scattering mechanisms included enhanced point defect scattering due to the Zn substitution and disorder scattering as a result of Cu intercalation.The largest ZT was 0.077 for the Zn 0.01 Bi 2 TeI sample, increased by 70% as compared with the undoped Bi 2 TeI.This work demonstrated that doping a proper amount of Zn can optimize the TE performance of Bi 2 TeI materials.
,b shows the SAED patterns taken on Bi 2 TeI sample.The SAED patterns of Bi 2 TeI can be indexed as [103] and [010] zone axis of the monoclinic crystal structure.The layered structure can be reflected from the SAED pattern along the [010] zone axis, which shows good agreement between the experiment and simulation results.Some diffraction points that were not observed indicated the existence of intrinsic defects in the sample.Figure 2d-f shows the elemental mapping of Bi, Te, and I on Bi 2 TeI FIB sample.It can be seen that the elements were uniformly distributed in the nanometre scale, indicating the good homogeneity of Bi 2 TeI samples.

Figure 3 .
Figure 3. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) thermal conductivity, and (d) comparison of the thermal conductivity and lattice thermal conductivity of Bi 2 Te 3 , BiTeI, and Bi 2 TeI at 300 K.

Figure 6 .
Figure 6.(a) Secondary electron image and (b) backscattered electron image of a large Zn-rich area on Zn 0.015 Bi 2 TeI sample.The elemental distribution along the line marked in (a) is also presented.

Figure 7
Figure7shows the temperature dependence of in-plane electrical conductivity, Seebeck coefficient, and power factor of Cu-and Zn-doped Bi 2 TeI.The in-plane charge transport properties of Cu-doped Cu x Bi 2 TeI and Zn-doped Zn y Bi 2 TeI at 300 K are listed in Table2.σ of the doped samples was increased slowly while increasing the temperature, indicating a semiconducting transport behavior.Doping Bi 2 TeI with 1.5% and 2% Cu led to decreased σ.With increasing the Zn content, σ of Zn y Bi 2 TeI was first increased then largely decreased.The decrease in σ upon doping with large amount of Cu should be attributed to the decreased carrier concentration.After doping with Zn, the carrier concentration was greatly increased, from 8.1 × 10 20 cm −3 for the pristine Bi 2 TeI to 1.48 × 10 21 cm −3 for Zn 0.01 Bi 2 TeI, and kept almost unchanged after doping with 2% Zn.Therefore, the increase in σ for Zn 0.01 Bi 2 TeI was due to the increased carrier concentration, and the decrease in σ for Zn 0.015 Bi 2 TeI and Zn 0.02 Bi 2 TeI might be related to the formation of nonconductive ZnI 2 impurity phase.Since the carrier concentration was reduced after Cu doping, it can be inferred that Cu as an intercalated dopant has entered into the interlayer space of Bi 2 TeI.The electron donor behavior of the intercalated Cu has been reported in other layered compounds[29,30].For Zn doped samples, a substitution of Zn 2+ on Bi 3+ sites might be accounted for the increased carrier concentration of the p-type materials.

Figure 7 .
Figure 7. Temperature dependence of in-plane (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor of Cu-doped Cu x Bi 2 TeI and Zn-doped Zn y Bi 2 TeI.

Figure 8 .
Figure 8. Temperature dependence of in-plane (a) thermal conductivity, (b) carrier thermal conductivity, (c) lattice thermal conductivity, and (d) ZT of Cu-doped Cu x Bi 2 TeI and Zn-doped Zn y Bi 2 TeI.

Figure
Figure 8d displays the ZT values of the doped Bi 2 TeI in the temperature range of 300-500 K.With the increase in temperature, the ZT values of all the samples increased gradually.With increasing the content of Cu, the ZT values of Cu-doped samples first slightly increased and then decreased.However, Zn y Bi 2 TeI showed increased ZT with increasing y in the range of 0-1%.At high temperatures, all Zn-doped samples showed increased ZT values.The largest ZT reached 0.077 at 500 K for the Zn 0.01 Bi 2 TeI sample, increased by 70% as compared with that of the undoped Bi 2 TeI.The reason behind the enhancement of ZT was twofold: (1) lowered lattice thermal conductivity due to the substitution of Zn on Bi sites and (2) simultaneously improved electrical conductivity and Seebeck coefficient.

Table 2 .
Charge transport properties of Cu-doped Cu x Bi 2 TeI and Zn-doped Zn y Bi 2 TeI at 300 K.