For the characterization of electrical and thermoelectric properties of ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series layer crystals, temperature-dependent resistivity and thermoelectric measurements from 20 to 300 K were carried out. Since many researchers have claimed that ZrX

_{2} (X = S, Se) series displayed good thermoelectric properties, we measured the experimental result and compared with previous theoretical results [

44,

45] herein. The layered samples of ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) crystals with dimension about 0.45 × 0.12 × 0.01 cm

^{3} were prepared. Because the existence of defects, surface states, surface oxide (i.e., ZrX

_{2−x}O

_{x}, x = 0 to 2), etc., room-temperature resistivity and Hall-effect measurements of ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) were also done to identify their electrical property. The results are listed in

Table S2, for comparison. The carrier type of all the ZrX

_{2} series was n type, and carrier concentrations were determined to be 4.05 × 10

^{17} cm

^{−3} for ZrS

_{2}, 1.6 × 10

^{18} cm

^{−3} for ZrSSe, and 2.9 × 10

^{18} cm

^{−3} for ZrSe

_{2}, respectively. The high carrier concentration also renders lower resistivity of 0.25 Ω-cm for ZrS

_{2}, 0.0211 Ω-cm for ZrSSe, and 0.0058 Ω-cm for ZrSe

_{2}, respectively.

Figure 4a shows temperature dependent resistivity of ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series as a function of reciprocal temperature. The series shows metallic carrier-conduction behavior due to the increasing resistivity as the temperature increased. The lower resistivity of the ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series may come from heavily doping from defects, like chalcogen vacancies or native impurities, i.e., surface oxidation states in air (or from environment water vapor) [

46], which cause the material to behave like a degenerate semiconductor, as the band scheme shown in the inset of

Figure 4d. Dissimilar to the other 2D materials, like MoS

_{2} and MoSe

_{2}, ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series exhibits relatively low resistivity owing to the surface states. The sign of Seebeck coefficient (S = −ΔV/ΔT) indicates all three ZrX

_{2} compounds are n-type semiconductor as shown in

Figure 4b. The carrier type is similar to that determined by Hall-effect measurement. The resistivity values of ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) in

Table S2 reveal reduction with the increase of Se content owing to the increased mobility and carrier concentration obtained by Hall measurement. This property may result in reduction of Seebeck coefficient of the high Se compound as displayed in

Figure 4b. The Seebeck coefficient of ZrSe

_{2} is about −50 μV/K at 300 K. The magnitude of S value is close to a previous ZrS

_{3−3x}Se

_{3x} compound [

47]. The efficiency of a thermoelectric material is determined by a dimensionless figure of merit (ZT) defined as ZT = S

^{2}⋅T/(ρ⋅κ), where ρ, T, and κ, are electrical resistivity, absolute temperature, and thermal conductivity. Thermal conductivity κ can be calculated from the heating power and temperature difference ΔT from the sample geometry as P = κ⋅(w⋅t/l)⋅ΔT, where w is width, t is thickness, and l is length, respectively. The ZT is an index of efficient thermoelectric material, but ZT value is inversely proportional to thermal conductivity κ [

48,

49].

Figure 4d shows the ZT values of the ZrS

_{2−x}Se

_{x} series from 20 to 300 K. The ZT values of all samples increase when the temperature is increased, while thermal conductivity κ decreases when the temperature is raised (see

Figure 4c). At 300 K, the highest ZT value of 0.085 was achieved by ZrSe

_{2}, followed by ZrSSe of ZT = 0.06 and ZrS

_{2} of ZT = 0.02, respectively. When the Se content increases, the mobility and electron concentration are also increased to promote thermoelectric performance in the ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series. The carrier concentration of ZrSe

_{2} is close to ~10

^{19} cm

^{−3} in

Table S2. The concentration value was claimed to be an optimum carrier density for achieving optimal ZT in the thermoelectric materials [

50]. ZrSe

_{2} is therefore shows the best thermoelectric performance in the ZrS

_{2−x}Se

_{x} (x = 0, 1, and 2) series layered TMDCs.