IV-VI monochalcogenide SnSe or SnS has recently been proposed as a promising two-dimensional (2D) material for valleytronics and thermoelectrics. We report the synthesis of SnSe nanoflakes and nanostructured thin films with chemical vapor deposition method and their thermoelectric properties. As grown SnSe nanostructures are found to be intrinsically p-type and the single SnSe nanoflake field effect transistor was fabricated. By Ag doping, the power factor of SnSe nanostructured thin films can be improved by up to one order of magnitude compared to the “intrinsic” as grown materials. Our work provides an initial step in the pursuit of IV-VI monochalcogenides as novel 2D semiconductors for electronics and thermoelectrics.
I. INTRODUCTION
Renewable energy has always been of strong interest to humankind, for it could solve the energy and environment crises by reducing fossil fuel consumption. Among the renewable energy technologies, thermoelectric materials attract a lot of attention due to their ability to provide direct conversion between thermal energy and electrical energy. SnSe, as one of the promising thermoelectric materials, recently made news that it reached a record-high figure of merit ZT of 2.6 ± 0.3 at 923 K.1 The dimensionless thermoelectric figure of merit ZT is defined as , where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. ZT reflects how efficient a material can convert thermal energy to electrical energy (or vice versa). In addition, σS2 is termed as the power factor, which indicates the ability of a material to convert the energy, regardless of the efficiency.
Power factor of the undoped SnSe single crystal is quite low (<10 μW cm−1 K−2 around 900 K) compared to other good thermoelectric materials even though the band structure of SnSe in which multiple band extrema are close enhances its Seebeck coefficient.1–3 The amazingly high ZT is primarily due to the ultra-low thermal conductivity (<0.4 W m−1 K−1 at 923 K along the b-axis in SnSe single crystal) from the anharmonic bondings in SnSe crystal structure.1–3 Hole doping by Na has been experimented to tune hole density and improve the power factor of SnSe. For SnSe single crystal, the hole doping gives a higher average ZT over a broad range of temperature, but the maximum ZT of hole doped SnSe single crystals still cannot rival that of the undoped SnSe single crystal because of higher thermal conductivity.3,4 At the same time, efforts have also been made to theoretically understand the characteristics of SnSe from the band structures.5–9 Due to the slow process of single crystal growth, many researchers have focused on the fabrication/doping and thermoelectric properties of polycrystalline SnSe. Sintering,10–18 arc melting,19 and pulsed laser deposition (PVD)20 are mostly used to produce the polycrystalline SnSe. Generally, polycrystalline SnSe has lower ZT compared to the SnSe single crystal because of the inferior power factor originating from random orientation of constituent crystals, grain boundaries, and defects. However, these factors also give rise to an increased phonon scattering rate, which reduces thermal conductivity of the sample. As summarized by Zhao et al.,3 the thermal conductivity of polycrystalline SnSe can be as low as 0.2–0.3 W m−1 K−1 at ∼800 K.15 The two-dimensional nature of SnSe also makes it an interesting material to be studied in the low-dimensional forms. The fabrication of SnSe nanoflake by solution-process method,21,22 mechanical exfoliation,23 vapor deposition,24,25 and plasma-assisted synthesis26 has been reported for the application of field effect transistor (FET) and photodetector. Recently, good thermoelectric properties of SnSe nano-flake/ribbon/sheet have been predicted by theory,27–29 but no thermoelectric properties of the nanostructured SnSe thin films formed by nanoflakes has been reported yet.
In this letter, we present a facile chemical vapor deposition (CVD) technique for growing/doping SnSe nanoflakes and nanostructured thin films and report the nanoflakes' and nanostructured thin films' electrical and thermoelectric properties. Single SnSe nanoflake FET device was first fabricated to study the electrical transport property. Subsequently, the conductivity and the Seebeck coefficient of SnSe nanostructured thin films were obtained to calculate the power factor in comparison with those of SnSe single crystal. We focused our measurements on the low temperature range (300 K–150 K), different from the high temperature range (up to 1000 K) reported in most publications. The low dimension and low temperature range we studied here show potential applications of SnSe nanostructured thin films in miniaturized thermoelectric device and cooling. The low dimensional nature and boundary scatterings in individual SnSe grain or between SnSe grains are expected to induce strong phonon scatterings and suppress the sample's thermal conductivity to values even lower than those in bulk single crystals, providing another route to further optimize the ZT.2,3
II. EXPERIMENTS
Figure 1(a) shows the crystal structure of SnSe at room temperature. It belongs to Pnma space group and has a layered structure along the a-axis, which makes the CVD growth of planar nanostructures favorable. Heating above ∼800 K, SnSe will undergo a phase transition to higher symmetry space group Cmcm.3 Figure 1(b) shows the schematic of CVD growth setup, similar to the growth method for the layered Bi2Se3 film mentioned in Ref. 30. 99.999% SnSe powder (Se or Ag may be added for stoichiometry adjustment or doping) was placed in an alumina boat in the center of a tube furnace as the source. To grow SnSe nanoflakes for FET fabrication, a rectangular piece of Si wafer with 300 nm thick silicon oxide on surface was used as the growth substrate to allow direct FET fabrication without the need of transferring SnSe nanoflakes, while freshly cleaved mica (∼5 cm long and 1 cm wide) served as the growth substrate to grow continuous SnSe nanostructured thin films. The substrate was placed downstream in the quartz tube, close to the end of furnace area, where the temperature gradient was the largest (substrate temperature varies from ∼750 K to ∼550 K over the 5 cm length of mica substrate). During growth, the furnace temperature was set to be 923 K, 1000 sccm flow of Ar (10% H2) was used as the carrier gas, and the pressure in the quartz tube was maintained at ∼100 Torr. A typical growth time was around 20 min. As shown in Figs. 1(c) and 1(d), in the high-temperature area on the substrate, SnSe deposits as individual nanoflakes that are separated from each other. But near the cold end of the substrate, the SnSe nanoflake growth becomes dense, and the SnSe nanostructured thin films are formed continuously. The nanostructured thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and energy-dispersive X-ray spectroscopy (EDX).
SnSe structure and CVD growth. (a) Crystal structure of SnSe at room temperature (gray: Sn, yellow: Se), layers form perpendicular to the a-axis. (b) Schematic for the CVD growth. (c) SnSe nanoflakes grown in the high-temperature area on substrate, and (d) nanostructured thin films formed in the low-temperature area on substrate.
SnSe structure and CVD growth. (a) Crystal structure of SnSe at room temperature (gray: Sn, yellow: Se), layers form perpendicular to the a-axis. (b) Schematic for the CVD growth. (c) SnSe nanoflakes grown in the high-temperature area on substrate, and (d) nanostructured thin films formed in the low-temperature area on substrate.
A two-step process was used to fabricate the single SnSe nanoflake FET device. First, a copper TEM grid was fixed on the substrate as a shadow mask to cover the nanoflake and leaving four square shaped open areas around the nanoflake, followed by the first metal deposition to define the four square shaped Ni metal electrodes. Then, with electron beam lithography and a second metal deposition, four Ni leads were patterned and fabricated to connect the nanoflake to the square electrodes.
Thermoelectrical characterization and Hall measurement were conducted inside the vacuum chamber of a Physical Property Measurement System (PPMS) made by Quantum Design Inc.
III. RESULTS AND DISCUSSION
Structural and morphological characterizations of SnSe nanostructured thin films are summarized in Fig. 2. Because nanoflakes were more densely deposited in the low-temperature area of the growth substrate, compared to the scattered nanoflake deposition in the high-temperature area [Figs. 1(c) and 1(d)], a continuous thin film of interconnected nanoflakes is formed in the low-temperature area of the growth substrate. This study is primarily focused on studying the nanostructured continuous thin films grown in the low-temperature area. Figure 2(a) shows a typical AFM image of the nanostructured thin films, in which the layered stacking of nanoflakes can be clearly seen. The total thickness of the nanostructured thin films was measured to be around 300 nm. With the help of EDX, we analyzed the stoichiometry or the ratio between Sn and Se in the grown nanostructured thin films. With SnSe being the only source, we found Sn:Se ∼ 1:0.75. To compensate the Se deficiency, Se powder (12% by total weight) was added to the SnSe source, while the other conditions remained the same. It turned out Sn: Se changed to 1:1.13. Due to the high Se vapor pressure and difficulty in controlling the Se deposition, the stoichiometry of 1:1 between Sn and Se was not achieved. In the following discussion, we mark these samples with different Sn: Se ratios as SnSe0.75 and SnSe1.13. The XRD data on SnSe nanostructured thin films are presented in Fig. 2(c). Compared to SnSe0.75 and SnSe1.13, Ag doped nanostrucutred thin films (here, the concentration of Ag in doped films was within the error of EDX, so it was not precisely determined. To distinguish the films with different Ag doping levels, we simply label the SnSe nanostructured films using the molar ratio of Ag in the source during growth.) give lower peaks, possibly due to the formation of AgSnSe2 phase and the lower density of the SnSe phase. The difference in XRD peaks are attributed to the varied preferred growth orientations due to the difference in composition between these samples.
Structural and morphological characterizations on SnSe thin films. (a) AFM on SnSe nanostructured thin films grown in the low-temperature area, which are formed by layered SnSe nanoflakes and a line-scan (b), as marked by the dashed line in (a), showing surface roughness and nanoflake thickness. The scanned area in (a) is 70 μm ×70 μm. (c) XRD on undoped and Ag doped SnSe nanostructured thin films.
Structural and morphological characterizations on SnSe thin films. (a) AFM on SnSe nanostructured thin films grown in the low-temperature area, which are formed by layered SnSe nanoflakes and a line-scan (b), as marked by the dashed line in (a), showing surface roughness and nanoflake thickness. The scanned area in (a) is 70 μm ×70 μm. (c) XRD on undoped and Ag doped SnSe nanostructured thin films.
For electrical and thermoelectrical characterizations, we first demonstrate the FET devices made from individual nanoflakes grown on Si/silicon oxide substrates. An example of the SnSe nanoflake device for FET characterization is shown in the inset of Fig. 3(a). In the gating measurement, the heavily doped Si substrate was used as the gate electrode and the 300 nm silicon oxide worked as the gate dielectric. A typical gating behavior of our SnSe nanoflakes is shown in Fig. 3(a). As the gate voltage increases, the sheet conductance decreases, indicating that the SnSe nanoflake here is p-type. Using a standard FET performance analysis for planar 2D semiconductors,31 the field effect hole mobility μFET and sheet hole density psheet are extracted to be separately 2.6 cm2 V−1 s−1 and 7 × 1012 cm−2. The μFET acquired here is comparable to the field effect hole mobility of the SnSe nanoplates reported by Zhao et al.24 and the SnS nanoflakes exfoliated from the bulk.31 The gating performance is quite limited here, possibly because of the large thickness of the nanoflake.31
FET device and measurements on undoped SnSe nanostructured thin films with different stoichiometry. (a) Optical image of SnSe nanoflake FET device (inset, scale bar is 10 μm) and typical gating behavior of sheet conductance vs. gate voltage, which exhibits p-type semiconductor property. (b) Schematic setup for thermoelectric measurements. (c) Electrical conductivity vs. temperature of SnSe nanostructured thin films. Two perpendicular transport directions along the sides of the square were measured. (d) Hole density and hole Hall mobility vs. temperature. (e) Seebeck coefficient vs. temperature. (f) Calculated power factor vs. temperature. In (c), (e) and (f), the 300 K a-axis and c-axis data for undoped SnSe bulk single crystal (SC) are included for comparison, and SC in (d) indicates the hole density by Hall measurement for a SnSe single crystal oriented along the a-axis (Ref. 1).
FET device and measurements on undoped SnSe nanostructured thin films with different stoichiometry. (a) Optical image of SnSe nanoflake FET device (inset, scale bar is 10 μm) and typical gating behavior of sheet conductance vs. gate voltage, which exhibits p-type semiconductor property. (b) Schematic setup for thermoelectric measurements. (c) Electrical conductivity vs. temperature of SnSe nanostructured thin films. Two perpendicular transport directions along the sides of the square were measured. (d) Hole density and hole Hall mobility vs. temperature. (e) Seebeck coefficient vs. temperature. (f) Calculated power factor vs. temperature. In (c), (e) and (f), the 300 K a-axis and c-axis data for undoped SnSe bulk single crystal (SC) are included for comparison, and SC in (d) indicates the hole density by Hall measurement for a SnSe single crystal oriented along the a-axis (Ref. 1).
Next, we turn to the thermoelectric properties of SnSe nanostructured thin films formed in the low-temperature area. Figure 3(b) shows the schematic for measurement, similar to the setup mentioned in Ref. 32. The mica substrate with grown SnSe nanostructured thin films was cut into a square piece with the dimension of ∼2 mm × 2 mm or ∼1 mm × 1 mm, after which indium soldering was done at the corners of the square shaped sample for the standard van der Pauw electrical transport characterization and the thermoelectric power characterization. For thermoelectric power characterization, a rectangular plate of sapphire was used to hold the sample. To induce a temperature gradient along the sample, one end of the sapphire holder had a resistive heater attached to allow heating from one side of the sample, while the other end of the sapphire holder was attached to the sample stage that had a fixed bath temperature as the cryostat. The sapphire heating stage was pre-calibrated by characterizing the relation between the applied voltage on the heating resistor Vheat and the temperature gradient ΔT on the sapphire plate recorded by a thermocouple.32 Next, the mica substrate with the SnSe nanostructured thin film was fixed onto the sapphire plate using a small amount of vacuum grease. In the thermoelectric measurements, the sapphire sample holder with SnSe nanostructured thin films on the mica substrate was placed inside the vacuum chamber of PPMS. The thermoelectric voltage generated over the SnSe nanostructured thin films VTE due to the temperature gradient from the heating resistor at different bath temperatures was recorded, and the original data were converted to the Seebeck coefficient via S = VTE/ΔT and plotted as a function of temperature T. The electrical conductivity vs. temperature can be measured by the same configuration, while the Hall measurement was performed without the sapphire plate, and the mica substrate was directly secured to the sample stage.
Figures 3(c) and 3(d) present the electrical measurement results for SnSe1.13 and SnSe0.75 nanostructured thin films in both directions. The SnSe thin films are semiconducting as the electrical conductivity decreases with the lowering temperature. All nanostructured thin films grown are p-type according to Hall measurement, which agrees with the gating behavior of the SnSe nanoflake FET. In Fig. 3(d), it can be seen that the Se deficiency in SnSe0.75 gives rise to a much higher hole density than SnSe1.13, but the lower hole mobility in SnSe0.75 compared to the hole mobility in SnSe1.13 does not favor good thermoelectric performance. The Seebeck coefficient and the calculated power factor vs. temperature are presented in Figs. 3(e) and 3(f). Compared to the SnSe single crystal [Figs. 3(c)–3(f)], both samples have higher hole density at room temperature, which possibly results from more grain boundaries and defects. However, the electrical conductivity and the Seebeck coefficient do not benefit from this defect induced higher carrier density; particularly, the Seebeck coefficient of our nanostructured thin films is only ∼60% of that in SnSe single crystal. Power factor at room temperature for these “intrinsic” SnSe nanostructured thin films can be as high as ∼0.16 μW cm−1 K−2 in SnSe1.13. It is close to the power factor of a-axis, and ∼1/20 of that in c-axis in SnSe single crystal.1
To explore the possibility of further improving the power factor, we made an effort to grow and characterize doped SnSe nanostructured thin films. According to Snyder and Toberer, to achieve a high power factor in semiconductors, the carrier density is typically around 1020 cm−3.33 As mentioned earlier, via doping of Na in the SnSe single crystal, an increase in hole density by around 100 times at room temperature was achieved by Zhao et al.4 Other elements such as Zn, Ag, In and Sm have also been reported to be successful in increasing the power factor of SnSe single crystal or polycrystalline SnSe.10,12–18 In this work, Ag was chosen to be the dopant and the doping was done by adding silver (99.99% purity) with different amounts to the SnSe source. We experimented adding up to 10% Ag in the SnSe source. However, it turned out that above 1% Ag, the hole density went down as more Ag was added in the source. Those cases with 1% and 1.5% Ag in the SnSe source (referred to as the 1% or 1.5% Ag sample) are only discussed here, and the results are shown in Fig. 4, in comparison to the results from SnSe1.13 and the SnSe single crystal.1 Ag doped samples remained p-type for all the growths conducted. The most important result in Fig. 4 is that for 1% Ag sample, the electrical conductivity and the Seebeck coefficient are clearly higher than those in the undoped and more heavily doped sample (the 1.5% Ag sample). Figure 4(b) also shows that although the 1% Ag sample does not have a higher hole density than the SnSe1.13 sample without Ag doping, it has an improved hole mobility by at least 70%. Increasing Ag concentration in the source to 1.5% significantly reduces the hole density and the mobility, as shown in Fig. 4(b). Therefore, the much better power factor in 1% Ag sample compared with the undoped or more heavily doped samples originates mostly from the higher hole mobility and thus the electrical conductivity although there is also about 20% improvement in the Seebeck coefficient [Fig. 4(c)]. After Ag doping, the highest power factor at room temperature is 0.66 μW cm−1 K−2 for the sample with 1% Ag in the source, which is higher than the power factor of a-axis and is already ∼20% of the power factor of c-axis in the SnSe single crystal.1 The lower power factor of our nanostructured thin films is mainly due to the deteriorated hole mobility caused by the grain boundaries and the possible defects during growth (along b-axis in SnSe single crystal, hole mobility by Hall measurement is >200 cm2 V−1 s−1 at room temperature).1 Similar results have also been seen in other low-dimension or polycrystalline thermoelectric materials.2,34–37 However, a much lower thermal conductivity might be achieved through the increased phonon scattering from the boundary and the two-dimensional nature of the SnSe nanoflakes.2,3,35–37 Due to the limit of our experimental setup, we could not control the Ag doping more precisely to less than 1%. However, these results suggest that further optimization of the stoichiometry and doping level in the SnSe nanostructured thin films may lead to even higher power factors. In combination with the greatly suppressed thermal conductivity in nanostructures, the doped SnSe nanostructured thin films appear to be a promising choice to obtain high thermoelectric figure of merit.
Measurements on Ag doped SnSe nanostructured thin films. Like Figs. 3(c)–3(f), (a)–(d) present the electrical conductivity, hole density/Hall mobility, Seebeck coefficient, and calculated power factor of Ag doped SnSe nanostructured thin films vs. temperature, respectively. Data for SnSe1.13 and 300 K data for SnSe single crystal from Ref. 1 are also plotted for comparison.
Measurements on Ag doped SnSe nanostructured thin films. Like Figs. 3(c)–3(f), (a)–(d) present the electrical conductivity, hole density/Hall mobility, Seebeck coefficient, and calculated power factor of Ag doped SnSe nanostructured thin films vs. temperature, respectively. Data for SnSe1.13 and 300 K data for SnSe single crystal from Ref. 1 are also plotted for comparison.
IV. CONCLUSION
In conclusion, a facile CVD method for growing and doping p-type SnSe nanoflakes and nanostructured thin films on the same substrate is presented. At room temperature, the undoped SnSe nanostructured thin films have a power factor up to 1/20 of that in undoped SnSe single crystal, and it can be improved by three times via doping proper amount of Ag. The decent power factor and possibly even a lower thermal conductivity make the SnSe nanostructured thin films a great candidate for miniaturized, environment friendly, and low-cost thermoelectrics.
ACKNOWLEDGMENTS
The work at CWRU was supported by the NSF under Grant # DMR-1151534. N. Sun thanks the National Natural Science Foundation of China (No. 51771197) for its financial support. M. Liu thanks the National Natural Science Foundation of China (Nos. 11774208 and 61307120) and Shandong Provincial Natural Science Foundation, China (No. ZR201709190343) for financial support.