pubs.acs.org/NanoLett Letter Anisotropic Thermal Conductivity of Crystalline Layered SnSe2 Peng Xiao,⊥ Emigdio Chavez-Angel,⊥ Stefanos Chaitoglou, Marianna Sledzinska, Athanasios Dimoulas, Clivia M. Sotomayor Torres, and Alexandros El Sachat* Cite This: Nano Lett. 2021, 21, 9172−9179 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The degree of thermal anisotropy affects critically See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe2 films of varying thickness (16−190 nm) and uncover a thickness-independent thermal conductivity Downloaded via 158.109.48.66 on November 29, 2022 at 12:16:26 (UTC). anisotropy ratio of about ∼8.4. Experimental data obtained using Raman thermometry and frequency domain thermoreflectance showed that the in-plane and cross-plane thermal conductivities monotonically decrease by a factor of 2.5 with decreasing film thickness compared to the bulk values. Moreover, we find that the temperature-dependence of the in-plane component gradually decreases as the film becomes thinner, and in the range from 300 to 473 K it drops by more than a factor of 2. Using the mean free path reconstruction method, we found that phonons with MFP ranging from ∼1 to 53 and from 1 to 30 nm contribute to 50% of the total in-plane and cross-plane thermal conductivity, respectively. KEYWORDS: Phonon transport, mean free path, SnSe2, thermal conductivity anisotropy, frequency-domain thermoreflectance, Raman thermometry U nderstanding the propagation of heat in two-dimensional (2D) materials is of paramount importance to establish its potential in, for example, energy harvesting in thermal In this work, we study the in-plane and cross-plane thermal conductivity of crystalline SnSe2 films with thickness between 16 and 190 nm using two different thermal characterization management in future nanoelectronic devices. SnSe2 is a techniques, namely, Raman thermometry and frequency promising 2D material with desirable properties such as domain thermoreflectance (FDTR). We focus on unraveling relatively high electron mobility of ∼85 cm2 V−1 s−1 at room the influence of film thickness on kr and kz in suspended and temperature,1 high bulk electron affinity of 5.2 eV,2 and low supported SnSe2, respectively, from which we obtain the thermal conductivity according to recent first principle thermal conductivity anisotropy ratio. In addition, we study the calculations.3−6 It has already shown excellent performance effect of film thickness on the temperature-dependent kr of in field-effect transistors (FETs),7−9 phase-change memory,10 suspended SnSe2 films. Finally, using the mean free path and optoelectronic devices,11 and recently it has been reconstruction method,20−22 we extract the in-plane and cross- suggested that it is an ideal layered material for thermoelectric plane phonon mean free path (MFP) distribution, showing applications.3,5,12 In particular, recent calculations have shown that the main contribution to the thermal transport is made by that at 800 K the thermoelectric figure of merit in a and c phonons with MFPs of tens to hundreds of nanometers. directions can reach values as high as 2.95 and 0.68, The SnSe2 films were mechanically exfoliated from single respectively.12 crystals (purchased from HQ Graphene) by the assistance of a While extensive electrical and optical characterization of viscoelastic stamp (GelPak)23 onto gold-coated silicon nitride SnSe2 is available,13−17 experimental studies on the intrinsic substrates with 20 and 30 μm holes. Schematics of the SnSe2 thermal properties of SnSe2 are few and far between.18,19 To crystal structure are shown in Figure 1a. The crystal and date, the thickness-dependence of the in-plane thermal conductivity (kr) has been investigated only in a few samples grown by chemical vapor deposition in a narrow range of Received: August 5, 2021 thickness using the traditional optothermal Raman techni- Revised: October 12, 2021 que.18 Moreover, there are no reports on measurements of the Published: October 28, 2021 cross-plane thermal conductivity (kz) in SnSe2; thus its anisotropic thermal properties have so far not been experimentally investigated. © 2021 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.nanolett.1c03018 9172 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter Figure 1. (a) Side and top views of the SnSe2 crystal structure. Each SnSe2 layer consists of three atomic sublayers, in which Sn atoms (blue spheres) are sandwiched between Se atoms (green spheres). (b) HR-STEM image of a 30 nm thick exfoliated suspended SnSe2 film and (c) Fourier transform of the squared region depicted in (b), from which we extract the a and b lattice constants, a = b = 0.387 nm. (d) Representative optical image of a 60 nm thick SnSe2 film. The diameter of the suspended circular area is 30 μm. (e) AFM image of the region outlined by the rectangle box in (d) and the corresponding measured height profile along the dashed black line in the topography image. (f) Raman spectrum of a 60 nm thick SnSe2 film using an excitation wavelength of 532 nm and a 50× objective. In the inset to (f), red arrows indicate the direction of the movement of Sn and Se atoms. morphological characterization of the samples was studied by thermal conductivity since it yields directly maps of the high-resolution scanning transmission electron microscopy thermal field, both connected through the Fourier’s law, Q/A = (HR-STEM), atomic force microscopy (AFM), and Raman −krT, where Q is the heat flux, A is the area of the heat flux, spectroscopy measurements. Figure 1b and Figure 1c display and T is the temperature. the planar HR-STEM and the corresponding electron We point out that for the in-plane thermal conductivity diffraction image, respectively, of a 30 nm thick SnSe2 measurements it is important to ensure that the phonons with suspended film. The high-magnification HR-STEM image large MFP are not affected by the lateral dimensions. For the (Figure 1b) depicts a honeycomb arrangement of atoms, case of graphene, the in-plane MFP distribution is very broad indicating the single-crystalline structure. The simulated FFT (200 nm to 10 μm)30 and the impact of the lateral dimensions pattern (yellow spots in the inset of Figure 1c) displays on the in-plane thermal conductivity has been detected hexagonal symmetrical patterns, indicating the hexagonal experimentally.31 However, in our experiments the lateral lattice structure of the SnSe2. From the simulations, we size effect should not affect the measured thermal conductivity extracted the in-plane lattice constants a = b = 0.387 nm, in considering that SnSe2 has MFP distribution (1−1000 nm)6,12 very good agreement with the literature.24,25 Figure 1d displays much smaller than the size of the suspended films and the heat a representative optical image of a 60 nm thick suspended sink is located 10 or 15 μm away from the heating area. In the exfoliated SnSe2 film. The AFM topography image of the two-dimensional case, where no temperature gradients are interface between the substrate and the exfoliated film together present in the third dimension and for temperature ranges that with the corresponding height profile is shown in Figure 1e. All exhibit a purely diffusive heat transport regime, the radial the thicknesses of the SnSe2 films were measured in tapping symmetry of the temperature distribution upon a Gaussian mode by AFM (see Supporting Information). The chemical thermal excitation allows reduction of Fourier’s law to its one- structure of SnSe2 was confirmed by Raman spectroscopy as dimensional form depending only on the radial coordinate r shown in Figure 1f. The crystalline SnSe2 exhibits two Raman and the area A = 2πrt, where t is the thickness of the suspended active vibrational modes at 113 cm−1 (Eg) and 180 cm−1 (A1g), film. Thus, the Fourier law gives a simple expression for the in agreement with previous reports.19,26 thermal conductivity as follows: The kr of suspended SnSe2 films at room temperature was measured by using our steady-state two-laser Raman −P0 k r (T ) = thermometry technique.27−29 In this approach, a hot spot is 2πtξ(r ) (1) produced by a heating laser with wavelength λ1 = 405 nm fixed in the center of a suspended film, while a second laser with λ2 = where P0 is the absorbed power by the film and ξ(r) = dT/ 532 nm measures the temperature distribution based on a (d ln r). In the case of a temperature-independent thermal calibration of its Raman spectra with temperature (see Figure conductivity in the temperature range under study, the thermal 2a). In these measurements, we monitored the temperature field decays as T(r) ∝ ln r in the diffusive limit and kr is directly through the frequency of the longitudinal (LO) optical obtained from eq 1 after extracting ξ(r) from the slope of a phonons of SnSe2 at ∼180 cm−1. Both lasers were focused linear fit to the logarithmic temperature profile and by using a 50× NA = 0.55 objective. The main advantage of this measuring P0 and t. We note that this procedure is valid for approach, with respect to the conventional optothermal Raman a data range starting at a distance of a few micrometers from technique, is that no assumption has to be made to obtain the the central point r = 0, where diffusive transport dominates. 9173 https://doi.org/10.1021/acs.nanolett.1c03018 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter Figure 2. Schematics of (a) the two-laser Raman thermometry and FDTR experimental configurations and (b) the multilayer structure (Au/SnSe2/ Au) with the two interface thermal resistances R1 and R2. (c) Representative temperature profile of a 16 nm thick suspended SnSe2 film with a maximum temperature rise at the heating spot at the center of the membrane of Tmax = 426 K. The inset shows the linear fit of the temperature profile in logarithmic scale from which we extract the logarithmic derivate ξ(r) ∼ dT/(d ln r). (d) Typical FDTR data measured in a 130 nm thick supported SnSe2 film (blue circles) and the corresponding best model fit (red curve) in the whole frequency range. The light red area indicates the fitting frequency range used to extract R1 and R2, and the inset to (d) shows the fitting of the experimental data in this range. A representative temperature profile of the 16 nm thick lag between the pump and probe beam as the observable suspended SnSe2 film is shown in Figure 2c. The temperature quantity. accuracy was ±2 K with a spatial resolution better than 600 nm Prior to the FDTR measurements 100 nm and 1.5 μm thick (given by λ2). The inset to Figure 2c shows the linear fit of the Au layers are deposited on the top and bottom of the measured temperature profile in logarithmic scale. For each suspended films, respectively. The top Au layers were used as sample we measured a line scan T(r) fulfilling the condition transducers, while the thick bottom Au layers become the that the maximum temperature is that at the heating laser spot substrates. Therefore, our multilayer system consists of Au/ at r = 0. The range r was chosen to observe as full relaxation as SnSe2/Au stacks (see Figure 2b). For each SnSe2 film we possible of the temperature field to the heat sink temperature, obtained frequency-domain measurements and extracted the kz Tsink = 300 K. However, preliminary measurements indicated following a multilayer three-dimensional heat diffusion that this depends on the thermal conductivity of each sample. model.34 This model requires material parameters such as Then, to improve the efficiency of the heat sink at the the volumetric heat capacity (C), thickness, interface thermal boundaries, we deposited 100 nm Au layer on the SiN4 resistance, and kz. The thermal conductivity and volumetric substrates and later the films were deposited. The measured heat capacity of Au as well as the volumetric specific heat of SnSe2 were taken from the literature.32,35 The thickness of the kr values are plotted in Figure 3a. All the measurements were Au transducer layer was measured by AFM (see Figure S1d,e conducted in vacuum at 10−3 mbar to avoid heat conduction of the Supporting Information). Thus, the remaining through the air.29 The experimental data of the thickness- parameters are the kz of SnSe2, the thermal resistance of the dependent absorbance at room temperature are shown in top layer Au/SnSe2 interface (R1), and the thermal resistance Figure S12 in the Supporting Information. of the SnSe2/Au (substrate) interface (R2). To quantify the To measure kz of the SnSe2 films, we performed pump-and- sensitivity of the measured phase signal to different parameters probe experiments using the FDTR technique.32,33 The in the case of Au/SnSe2/Au stacks, a sensitivity analysis is experimental setup is based on two lasers operating at 488 carried out in a similar manner to that of Schmidt et al.34 In nm (pump) and 532 nm (probe). The modulated pump beam Figure S2 of the Supporting Information we show the is focused on the SnSe2 samples using a 50× objective, creating calculated phase sensitivity (−Vin/Vout) to multiple parameters a periodic heat flux with a Gaussian spatial distribution on the (kz, C, anisotropy, R1, and R2) as a function of thickness and sample surface (see Figure 2a). The reflected low power probe modulation frequency. From this analysis we found that the beam is aligned with the pump beam and focused with the sensitivity of the recorded phase signal strongly depends on the pump spot to monitor temperature variation through changes modulation frequency; thus it is possible to extract different in optical reflectivity of the surface. We used a lock-in amplifier thermal properties directly from the model fit of the (Zurich Instruments HF2LI) to record the amplitude and experimental data in a specific frequency range. In Figure S3 phase response of the reflected probe beam to the thermal we also show the phase sensitivity as a function of the wave. The pump signal was used as reference while the phase modulation frequency for the case of a 16 nm thick SnSe2 film. 9174 https://doi.org/10.1021/acs.nanolett.1c03018 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter Figure 3. (a) In-plane (black squares) and cross-plane (red circles) thermal conductivities of exfoliated SnSe2 films with thickness between 16 and 190 nm measured with 2LRT and FDTR. The in-plane32,33 and cross-plane19,34 Fuchs−Sondheimer (FS) fits to the experimental data are indicated with black and red dashed lines, respectively. The light gray and red areas display the calculated in-plane and cross-plane bulk thermal conductivities taken from the literature.6,12 The errors in kz were estimated by the standard deviation of several measurements, including the numerical errors from the fits. The errors in kr were obtained from the error propagation of each parameter of eq 1. (b) Normalized accumulated thermal conductivity as a function of the phonon MFP (red and black solid curves). In dashed curves we plot the phonon MFP distribution in bulk SnSe2 calculated by Wang et al.6 and Ding et al.12 (c) Thermal conductivity anisotropy (kr/kz) as a function of SnSe2 film thickness. (d) Total interface thermal resistance, Rint = R1 + R2 (red squares), plotted versus film thickness extracted from the FDTR measurements. Previous total interface thermal resistance calculations on Al/MoS2/SiO2 (blue triangles) where similar interfacial contributions from bottom (2D material/ substrate) and top (metal/2D material) interfaces were taken into account. In particular, to extract the kz of each film from a single previously reported in-plane thermal conductivity data in FDTR measurement, we followed a fitting approach similar to CVD and exfoliated SnSe2 by Zou et al.18 and Liu et al.,19 that used in previous works34,36 and supported by our respectively, which are in good agreement with our measure- sensitivity analysis. First, we extract kz by fitting the ments. experimental data in a low frequency range (20 kHz to 1 The origin of the kr and kz decreasing with decreasing film MHz), where the phase signal sensitivity to R1, R2, and heat thickness can be understood considering phonon diffuse capacity of the films is negligible. Then, we fix kz and fit the (incoherent) boundary scattering at the surface of the films experimental data in a high frequency range (1−40 MHz) to as the main scattering mechanism. In particular, at room estimate simultaneously R1 and R2. For the first estimate to temperature the only characteristic dimension that dominates determine the value of the free parameters (kz, R1, and R2), we phonon scattering is the thickness of the SnSe2 films. Thus, t is used previous reported values of similar materials.37−39 A the dimension that sets a cutoff to the phonon-MFP due to typical example of the recorded phase signal and correspond- diffuse boundary scattering, i.e., phonons with MFP > t will not ing best model fit for a 130 nm thick SnSe2 film is shown in fully propagate, thus reducing their contribution to kr and kz. Figure 2d. The inset to Figure 2d shows the fitting of our Moreover, the similar rate of increase of kr and kz with experimental data in the high frequency range. increasing film thickness observed in Figure 3a suggests that Figure 3a displays the measured kr and kz values for all the phonons with MFP commensurate with the sample thickness SnSe2 films with thicknesses between 16 and 190 nm. We contribute to the thermal conductivity. observe that both kr and kz monotonically increase with To gain a further insight on how phonons contribute to the increasing thickness and reach the calculated bulk in-plane and thermal conductivity, we use the MFP reconstruction model to cross-plane thermal conductivity values, respectively.6,12 In extract the cross-plane and in-plane phonon MFP distribution particular, the kr values range from 6.45 ± 0.71 W m−1 K−1 for (see details in Supporting Information). A detailed description the thickest film (t = 190 nm) down to 2.54 ± 0.31 W m−1 K−1 of this model can be found elsewhere.20−22,41−43 Figure 3b for the thinnest film (t = 16 nm), which corresponds to about a displays the normalized accumulated thermal conductivity as a 2.5-fold reduction. A similar decrease was observed in the kz, function of the phonon MFP. It is seen that the MFP with the values dropping from 0.83 ± 0.12 W m−1 K−1 in the distribution shifts to longer values for in-plane transport thickest SnSe2 film down to 0.28 ± 0.05 in the thinnest film. compared to the cross-plane. In particular, we found that We note that the linear kz increase with increasing thickness phonons with MFP ranging from 1 to 53 nm and from 1 to 30 might indicate quasi-ballistic phonon propagation, similar to nm contribute to 50% of the kr and kz, respectively. Similar previous experiments in MoS2;40 however, more calculations observations were also predicted by Wang et al.6 and Ding et are necessary to conclude. In the same graph we plot al.12 in SnSe2 bulk using DFT calculations, while the same 9175 https://doi.org/10.1021/acs.nanolett.1c03018 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter Figure 4. Influence of ambient temperature and thickness on the in-plane thermal conductivity of SnSe2 films. (a) Temperature and (b) thickness dependence of kr for a series of suspended crystalline SnSe2 films with thickness between 16 and 75 nm, from 300 to 473 K. The y-axis in (a) and (b) shows the extracted thermal conductivities using single laser Raman thermometry measurements and FEM simulations (see details in Supporting Information). The x-axis in (a) and the temperatures indicated in (b) correspond to the ambient temperatures inside the cryostat. The rectangular dashed regions depicted in (b) show the gradual weaker temperature dependence of kr with decreasing thickness (c) The percentage of the kr decrease at 300 K with respect to the bulk SnSe2 value (black full circles) and the percentage of decrease of kr at 473 K with respect to the kr at 300 K (red full squares). The black line and red curve are a guide for the eye. The right image in (c) depicts the dominant scattering mechanisms with decreasing thickness and increasing temperature. methodology was used by Zhang et al.21 to estimate the MFP power at different ambient temperatures is shown in Figure S7 distribution in graphite. From the experimental kr and kz in the Supporting Information. The temperature coefficient values, we directly obtain the anisotropy ratio kr/kz for each (−0.0107 ± 0.0015 K/(cm−1)) was determined in advance film thickness, as is shown in Figure 3c. The average value of through a calibration procedure (see Figure S6 in Supporting kr/kz is 8.4 ± 1, which is in close agreement with theoretical Information). A deeper analysis of the power measurements calculations that showed a thermal anisotropy ratio of 7.1 in and the FEM simulations is given in the Supporting the SnSe2 bulk sample.6 Information (see Figures S9 and S10). Our result is also in agreement with a recent experimental Figure 4a shows the measured kr of four suspended SnSe2 study that showed a similar thickness-independent thermal films with t = 16, 25, 60, and 75 nm as a function of ambient anisotropy ratio (∼10) in InSe thin films using time-domain temperature. We observe a systematic decrease of the kr with thermoreflectance measurements.44 Moreover, in Figure 3d we increasing temperature for all the SnSe2 films and a maximum plot the calculated total interface thermal resistance, Rtot = R1 + reduction of ∼56% at 473 K for the thickest SnSe2 film R2, as a function of film thickness and compare our values with compared to the room temperature value. This trend originates those of supported MoS2 films also extracted using time- in the temperature dependence of the phonon MFP, since with domain thermoreflectance measurements.40 The observed increasing temperature the thermal phonon MFP becomes variation of Rint can be attributed to the sample processing shorter as higher energy phonon states are populated.47 (exfoliation and transfer conditions, storage before gold Therefore, the decrease of kr with increasing temperature can evaporation, etc.), which could result in different interfacial be attributed to the phonon MFP suppression due to phonon− energy coupling. However, we cannot exclude the possibility phonon scattering processes, as has previously been reported in that Rint is increased due to reduced mechanical coupling of the thin film semiconductors at high temperatures.29,48,49 Fur- films to the underlying Au resulting from a potential increase in thermore, by plotting kr as a function of thickness at different sample stiffness or changes in the vibrational spectra as the ambient temperatures (Figure 4b), we observe that the thickness is increasing. Similar thickness-dependent Rint has dependence of kr with temperature becomes weaker for been found in MoS2, InSe, and graphene stacks.40,44,45 thinner samples. In other words, as the temperature increases, To examine the dependence of kr of suspended SnSe2 films kr becomes less sensitive to the sample thickness since these on temperature, we conducted single-laser Raman thermom- phonons responsible for most of the heat transport have a etry measurements at different ambient conditions. The kr of MFP shorter than the film thickness. The weaker temperature the SnSe2 films was extracted by measuring the absorbed dependence in thinner films seems to be reasonable given that power and the temperature rise of the films at the laser spot at phonon boundary scattering becomes dominant. different ambient temperatures (298, 373, and 473 K) and In Figure 4c we show the importance of different scattering solving the 2D heat equation, as described in detail in our mechanism by plotting the percentage of the kr decrease at 300 previous work.46 The temperature of each suspended K with respect to the bulk SnSe2 value (black points) and the membrane at the laser spot as a function of the absorbed percentage of the kr decrease at 473 K with respect to the kr at 9176 https://doi.org/10.1021/acs.nanolett.1c03018 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter 300 K (red points). The nearly linear behavior of the black Optical images and atomic force microscopy measure- data points is analogous to the observed linear thickness- ments of the exfoliated SnSe2 films; mean free path dependent behavior of kr (see Figure 3a) and suggests that at reconstruction method; FDTR sensitivity analysis; spot room-temperature phonon boundary scattering becomes size measurements; single-laser Raman thermometry dominant with decreasing thickness. On the other hand, measurements and FEM calculations; power absorption from the red data points we observe the dominant role of measurements; Raman thermometry calibration; effect phonon−phonon scattering processes at high temperatures as of strain, defects, and nonuniform Raman signal on the film thickness increases. The competing role of the two suspended samples (PDF) ■ different scattering mechanisms is illustrated in Figure 4c (right image). AUTHOR INFORMATION We point out that phonons with MFP longer than the film thickness still contribute to the thermal conductivity provided Corresponding Author that phonons wavevector directions are close to the direction Alexandros El Sachat − Catalan Institute of Nanoscience and of the temperature gradient. However, at high temperatures, Nanotechnology (ICN2), CSIC and BIST, Bellaterra 08193 the wavelength of phonons dominating the heat transport Barcelona, Spain; orcid.org/0000-0003-3798-9724; becomes shorter; thus boundary scattering becomes less Email: alexandros.elsachat@icn2.cat probable unless the thickness of the film is close to the MFP Authors of the phonons responsible for most of the heat transport. For Peng Xiao − Catalan Institute of Nanoscience and instance, in the case of the 16 nm thick film, the kr at room Nanotechnology (ICN2), CSIC and BIST, Bellaterra 08193 temperature decreased by 45% compared to the bulk value, Barcelona, Spain; Departamento de Física, Universidad whereas the influence of thickness on its temperature Autónoma de Barcelona, Bellaterra 08193 Barcelona, Spain; dependence is relatively small. Thus, phonons with MFP > orcid.org/0000-0002-4711-2566 16 nm contribute substantially to the thermal conductivity with Emigdio Chavez-Angel − Catalan Institute of Nanoscience a rather small influence on its temperature dependence. and Nanotechnology (ICN2), CSIC and BIST, Bellaterra In conclusion, we investigated the thickness-dependence of 08193 Barcelona, Spain; orcid.org/0000-0002-9783- the in-plane and cross-plane thermal conductivity of crystalline 0806 SnSe2 films. Both show a strong thickness dependence in the Stefanos Chaitoglou − National Center for Scientific Research range of 16−190 nm, with a 2.5-fold reduction in their values “Demokritos”, 15310 Athens, Greece with respect to the bulk values. From this result we directly Marianna Sledzinska − Catalan Institute of Nanoscience and obtained the thermal conductivity anisotropy ratio, which was Nanotechnology (ICN2), CSIC and BIST, Bellaterra 08193 found to be independent of the film thickness and Barcelona, Spain; orcid.org/0000-0001-8592-1121 approximately 8.4. Importantly, we showed that at high Athanasios Dimoulas − National Center for Scientific temperatures the in-plane thermal conductivity of suspended Research “Demokritos”, 15310 Athens, Greece; orcid.org/ SnSe2 films can be reduced by more than ∼50% with respect to 0000-0003-3199-1356 the room temperature value. The strong dependence of the Clivia M. Sotomayor Torres − Catalan Institute of thermal conductivity on temperature becomes gradually Nanoscience and Nanotechnology (ICN2), CSIC and BIST, weaker with decreasing thickness due to the dominant role Bellaterra 08193 Barcelona, Spain; ICREA, 08010 of phonon boundary scattering. Finally, by using the MFP Barcelona, Spain; orcid.org/0000-0001-9986-2716 reconstruction method, we showed that at room temperature the main heat-carrying phonons that contribute to in-plane and Complete contact information is available at: cross-plane thermal conductivity have mean free paths of https://pubs.acs.org/10.1021/acs.nanolett.1c03018 several tens of nanometers. Our results provide valuable information to understand Author Contributions ⊥ thermal transport in two-dimensional materials and realize P.X and E.C.-A. contributed equally. The work was conceived functional devices that require highly anisotropic properties, by A.E.S. and E.C.-A. The sample fabrication was done by P.X. i.e., thermoelectrics, transistors, and optical sensors or and S.C., and the Raman characterization was performed by operating at high temperatures. Highly anisotropic 2D layered P.X. and A.E.S. The AFM and FDTR characterization was materials like SnSe2 can provide a through-thickness thermal carried out by A.E.S. The MFP reconstruction algorithm and insulation and heat flow propagation mainly in the in-plane TEM analysis were performed by E.C.-A. P.X., E.C.-A., S.C., direction. This can be advantageous, for example, in heat- M.S., A.D., C.M.S.T., and A.E.S. reviewed and edited the sensitive electronic components, such as microchips, where manuscript. C.M.S.T. and A.E.S. supervised the work. All directional heat flow is necessary to prevent hot spots or for authors have read and agreed to the published version of the cooling circuits.50,51 Finally, the ability to manipulate heat manuscript. All authors have given approval to the final version propagation through either dimensionality confinement or of the manuscript. The manuscript was written by A.E.S. and temperature modifications and the quantitative knowledge of E.C.-A. the thermal phonon MFPs give important insights for the Notes design of 2D thermoelectric devices with improved heat The authors declare no competing financial interest. management. ■ ASSOCIATED CONTENT ■ ACKNOWLEDGMENTS This work has been supported by the Severo Ochoa program, * sı Supporting Information the Spanish Research Agency (AEI, Grant SEV-2017-0706), The Supporting Information is available free of charge at and the CERCA Programme/Generalitat de Catalunya. The https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03018. authors acknowledge support from Spanish MICINN Project 9177 https://doi.org/10.1021/acs.nanolett.1c03018 Nano Lett. 2021, 21, 9172−9179 Nano Letters pubs.acs.org/NanoLett Letter SIP (Grant PGC2018-101743-B-I00) and the EU Project (17) Bertrand, Y.; Leveque, G.; Raisin, C.; Levy, F. Optical NANOPOLY (Grant GA 289061). S.C. and A.D. acknowledge Properties of SnSe2and SnS2. J. Phys. C: Solid State Phys. 1979, 12 financial support from the Flag-Era JTC 2017 Project (14), 2907−2916. MELODICA. P.X. acknowledges support by a Ph.D. fellowship (18) Zou, B.; Zhou, Y.; Zhang, X.; Zhang, M.; Liu, K.; Gong, M.; Sun, H. Thickness-Dependent Ultralow In-Plane Thermal Con- from the EU Marie Skłodowska-Curie COFUND PREBIST ductivity of Chemical Vapor-Deposited SnSe2 Nanofilms: Implica- (Grant Agreement 754558). tions for Thermoelectrics. ACS Appl. 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