Thermoelectric Performance and Optoelectronic Properties of ZrXY(X=O,S)(Y=S,Se) Janus Monolayer (2023)


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Computational Materials Science

Band 218,

February 5, 2023

, 111993

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Low-dimensional TMDC materials are suitable for thermoelectric applications because of their specific quantum confinement and different density of states (DOS). Here we studied the thermoelectric properties of the 2D TMDC monolayer of ZrXY ((X=O,S,)(Y=S,Se)) using density functional theory (DFT) in combination with the Boltzmann transport equation (BTE). We obtained a standard thermoelectric figure of merit (ZT) of ZrOS, ZrOSe, and ZrSSe at 900 ˚C. As a ZrSSe monolayer, aZTValue of 0.76 and 0.69 forn-type or p-type materials at 900˚C, so we probed the new Janus monolayers of ZrOS and ZrOSe and found a maximumZTof 0.82 and 0.72 forn-type or p-type materials at 900˚C, which is quite satisfactory. Upon examining the electronic property, we found that ZrOS, ZrOSe, and ZrSSe all have an indirect band gap (BG) of 1.88 eV, 1.02 eV, and 0.74 eV, respectively. ZrOS and ZrOSe showed very high absorption in the ultraviolet (UV) and blue regions. However, ZrSSe showed strong absorption in the green region of the visible region, suggesting that these materials are useful in optoelectronic devices along with thermoelectric applications.


The search for materials to produce high-efficiency thermoelectric generators is one of the most recent research. The thermoelectric material regime has been populated mainly by metal chalcogenides, alloys [1], [2], [3], [4], [5], [6] and their oxides [7], [8], [9]. Among them bi2That3[10], SnSe [11] and PbTe [12] have very highZTValue. After the discovery of graphene, 2D materials (TMDC) attracted our attention because of their exceptional optical [13], electronic [14] and thermal [14] properties. Devices based on TMDC monolayers showed excellent electrical properties with lower power consumption; they can even be used to generate electricity from waste heat. In this modern era, renewable energy sources are essential as the non-renewable energy sources are exhausted very quickly. In this work, we studied low-dimensional TMDC materials using Boltzman transport theory in conjunction with DFT, which showed a very high Seebeck coefficient (S). The Seebeck coefficient is the measure of the potential created when a temperature difference is introduced between the two ends of the material. For good efficiency or a highZTvalue, both the electrical conductivity (p) and the Seebeck coefficient (S) should be high, along with lower thermal conductivity (k=kDie+kPh). The thermal conductivitykhas two componentskDie, andkPh, which signify the contribution of electrons and phonons, respectively. In general, TMDC materials have very good propertieskDie, andkPhalong with bandgap tunability [15], [16], [17]. The layered 2D materials also showed excellent tunable optoelectronic properties compared to the bulk [18], prompting researchers to use them in electronic [19], optoelectronic [20] applications, as well as thermoelectric [21] and piezoelectric [22] energy generators. TMDCs are the types of materials that can be represented by X-M-X, where X is the chalcogenide atom (S, Se, Te) and M is the transition metal atom (Mo, Sn, W, Zr, Pt, etc.) in hexagonal planes lying between the arranged are chalcogenide by weak van der Waal interaction [23]. They can be obtained by peeling off the crystal using Scotch tape or synthesized by chemical vapor deposition (CVD) and thermal deposition methods. Mei Zhang et al. synthesized hexagonal ZrS2monolayer on Borniride (BN) and reported a maximum mobility of 2300 cm2v−1s−1[24]. Huang et al. reported a lowZTValue < 0.2 with MoSe2Monolayer along with a high thermal conductivity of about 60 W/m K. SD Guo and co-workers studied the thermoelectric properties of ZrS2and ZrSSe Janus monolayer and reported on aZTValue greater than 0.9 at 600 K. The ZrS2and ZrSSe monolayer were reported to have thermal conductivity of 47.8 W/K and 33.6 W/K, respectively. As a result, the ZrSSe Janus monolayer showed higherZTValue compared to ZrS2monolayer [25]. Similarly, it was also reported that the Janus monolayer of MoSSe has a lower thermal conductivity of 13.90 W/mK compared to that of MoS2(23.15 W/mk), resulting in a higherZTValue of MoSSe compared to MoS2[26]. Janus monolayer HfSSe showed a thermal conductivity of 2.23 W/mK, which is lower than that of HfS2(2.83W/mK), aberZThas not changed accordingly (i.e.ZTHfS2=0,90andZTHfSSe=0,81) as S of HfS2significantly larger than that of the HfSSe monolayer[27]. In 2020, Wang-Li Tao and his collaborators studied the thermoelectric properties of all possible monolayers along with Janus structures MXY (M=Pt,Pd:X,Y=S,Se,Te), they reported a very high value ofZTabout 2.54 with PtSeTe Janus monolayer and a maximumZTabout 1.12 among the rest of the materials and in this also Janus monolayers showed better performance compared to other structures.

Various experimental and theoretical studies showed that a unique combination of the low value ofkand high value ofpof 2D TMDC materials makes them suitable materials for thermoelectric applications to convert waste heat into electricity [28], [29], [30]. However, more research is needed on thermoelectric materials to achieve efficient heat-to-electricity conversion at near room temperature. As a Janus structure, ZrSSe showed better thermoelectric performance compared to ZrS2[31] we were motivated to work on Janus monolayers. Here we have systematically and in great detail studied the electronic and thermoelectric properties along with the optical properties of Janus monolayers of ZrOS and ZrOSe using DFT and BTE. In ZrOS monolayer, theZTProduct reaches a maximum of 0.82 forn-type (0.72 for p-type) at a temperature of 900 K. Similarly, ZrOSe and ZrSSe Janus monolayers also give a maximumZTfrom 0.81 and 0.82 forn-type (0.72 or 0.73 for p-type). Therefore, to understand the physical and chemical properties that enhance the thermoelectric efficiency of these materials, a theoretical study of thermoelectric and optical properties is required. ZrSSe showed the absorption peak in the visible region (553 nm); whereas ZrOS and ZrOSe showed the absorption peak in the UV region (393 nm and 436 nm). Thus, these materials can also be used in optoelectronic applications.

section cutouts


We replaced sulfur (S) and selenium (Se) with oxygen (O) in a ZrS monolayer2and ZrSe2and the other sulfur(S) with selenium(Se) atom in ZrS2Monolayer to create the Janus monolayer structures [32]. We performed the first principal calculations using DFT with Vanderbilt ultrasoft pseudopotential [33] and Perdew-Burke-Ernzerhof (PBE) as Generalized Gradient Approximation (GGA) [34] in the Quantum Espresso (QE) package. To avoid the interaction of two layers, we kept a vacuum of 20 Å

properties and stability

ZrXY ((X=O,S,)(Y=S,Se)) monolayer has hexagonal honeycomb structure and belongs to space group 164 (P-3m1) [42]. This Janus structure has a slightly reduced symmetry due to the relaxed symmetry along the central Zr atom. In a monolayer, the Zr atom is sandwiched between two layers of X or Y atoms and the unit cell consists of 3 atoms. Each Zr atom is surrounded by 3 X atoms and 3 Y atoms in upper and lower planes. Figure 12 is the top and side view of the ZrXY monolayer


We calculated the electronic, optical and thermoelectric properties using DFT, HSE and BTE. Dynamic stability is verified by phonon dispersion curves with no imaginary frequency, implying that these structures are stable. Although theZTandkPhfor ZrOS and ZrOSe are almost the same, but for ZrSSe theZTalong withkPhis less. Although thekPhof ZrOS and ZrOSe is comparatively high, but theSandpcompensate the power factor andZTProduct. The maximumZTProduct is 0.82 forn-Typ

CRediT Authorship Contribution Statement

Chayan Das:Conceptualization, Methodology, Software, Formal Analysis, Writing - Original Draft, Writing - Original Draft, Formal Analysis, Software, Methodology, Conceptualization.Atanu Betal:Formal analysis, software, methodology.Mahfouz Alam:Formal analysis, software, methodology.Jayanta Bera:Formal analysis, software, methodology.Appala Naidu Gandhi:Formal analysis, software, methodology.Satyajit Sahu:Supervision, conceptualization, methodology, formal analysis, writing – review &

Declaration of Competing Interests

The authors declare that they are not aware of any competing financial interests or personal relationships that may have influenced the work described in this document.


We are thankful for thatMinistry of Science and Technology (DST), Indiafor support through the INSPIRE program. We also thank the Indian Institute of Technology Jodhpur for providing the infrastructure to conduct the research.


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