Spatial Mapping of Thermal Boundary Conductance at Metal

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Surfaces, Interfaces, and Applications

Spatial mapping of thermal boundary conductance at metal-molybdenum diselenide interfaces David B Brown, Wenqing Shen, Xufan Li, Kai Xiao, David B Geohegan, and Satish Kumar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22702 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

Spatial Mapping of Thermal Boundary Conductance at Metal-Molybdenum Diselenide Interfaces David B. Brown1, Wenqing Shen1, Xufan Li2, Kai Xiao2, David B Geohegan2, Satish Kumar1,* 1G.

W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, GA 30332, USA 2Center

for Nanophase Materials Sciences

Oak Ridge National Laboratory Oak Ridge, TN 37831, USA Keywords: two-dimensional materials, transition metal dichalcogenides, molybdenum diselenide, metal

contacts,

interfacial

bonding,

thermal

boundary

conductance,

time-domain

thermoreflectance Abstract Improving the thermal transport across interfaces is a necessary consideration for micro- and nanoelectronic devices and necessitates accurate measurement of the thermal boundary conductance (TBC) and understanding of transport mechanisms. Two-dimensional transition metal dichalcogenides (TMDs) have been studied extensively for their electrical properties, including the metal-TMD electrical contact resistance, but the thermal properties of these interfaces are significantly less explored irrespective of their high importance in their electronic devices. We isolate individual islands of MoSe2 grown by chemical vapor deposition using photolithography and correlate the 2D variation of TBC with optical microscope images of the MoSe2 islands. We measure the 2D spatial variation of the TBC at metal-MoSe2-SiO2 interfaces using a modified time-domain thermoreflectance (TDTR) technique which requires much less time than full TDTR scans. The thermoreflectance signal at a single probe delay time is compared with a correlation curve which enables us to estimate the change in the signal with respect to the TBC at metalMoSe2-SiO2 interface as opposed to recording the decay of the thermoreflectance signal over delay times of several nanoseconds. The results show higher TBC across Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2. An image clustering method is developed to differentiate the TBC for different number of MoSe2 layers, which reveals the TBC in single-layer regions is higher than bilayer. We perform traditional TDTR measurements over a range of delay times and verify TBC

*Corresponding author. E-mail: [email protected] (Satish Kumar) ACS Paragon Plus Environment

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is higher at Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2 highlighting the importance of the choice of metal to heat dissipation at electrical contacts in TMD devices. Introduction Transition metal dichalcogenides (TMDs) with the chemical formula MX2 (M = transition metal atom, X = chalcogen)1 and semiconducting properties, e.g., MoS2, WSe2, and MoSe2, are promising for many applications including field-effect transistors2-5 and photodiodes6-8. The large Seebeck coefficient of TMDs has also created interest in thermoelectric applications9-11. Thermal management of such devices must be addressed to ensure optimal performance and reliability. Specifically, the ultrathin thickness of monolayer or few layer TMDs in its electronic devices dictates that the thermal transport across the interfaces (e.g., substrate, metal contacts) will limit heat removal12 but is still not well understood. The metal contacts can be an important heat transfer pathway for flexible devices on polymer or low conductivity substrates13. A fundamental understanding of phonon transport and estimation of thermal boundary conductance (TBC), or Kapitza conductance14, at the interfaces of 2D materials and metal contacts and supporting substrates is of great importance for improved heat dissipation and energy efficiency. As expected, the electrical properties of metal-TMD interfaces have been studied extensively15-19, but much less attention has been given to the thermal properties. Previous studies have looked at the effect of different work function metal contacts on electrical properties of the metal-MoSe2 interface20-21. The choice of metal contact can affect binding energy, charge redistribution, and doping resulting in varying Schottky barrier heights for carrier injection from metal to MoSe2. In addition, the interfacial properties may change for face- and edge-contacted devices similar to graphene22. Theoretical and experimental studies have reported TBC at metalMoS223-25 or substrate-MoS2 interfaces26-29. Mao et al.23 and Yan et al.25 reported enhanced TBC for chemisorbed (i.e., strongly adsorbed) metal-MoS2 interfaces caused by orbital hybridization and electronic charge redistribution enhancing the phonon transmission at metal-MoS2 interface. Very few studies have investigated the thermal properties of MoSe230-32, despite its technological importance, and the emphasis has been on thermal conductivity. Zhang et al.33 used Raman spectroscopy to measure the TBC across MoSe2-metal-SiO2 interface and reported an extremely low value of 0.1 MW/m2-K; however, we note this interface structure is quite different than the metal-MoSe2-SiO2 interfaces reported here. To the author’s knowledge, the thermal properties of the metal-MoSe2 interface have not been studied beyond this.

ACS Paragon Plus Environment

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This work investigates the thermal transport across Al-MoSe2-SiO2 and Ti-MoSe2-SiO2 interfaces using time-domain thermoreflectance (TDTR). The metal-MoSe2-SiO2 interface represents a contact geometry in a typical TMD based device which is an important heat transfer pathway especially in short channel devices34, thus this interface must be engineered to maximize heat removal. Al and Ti are two low work function, transition metals which form weakly and strongly bonded interfaces, respectively, with MoSe221. We use a direct-write photolithography technique to isolate individual islands of MoSe2 grown by chemical vapor deposition (CVD) and deposit metal transducer to enable TBC measurement. We map the TBC at Al-MoSe2-SiO2 and Ti-MoSe2-SiO2 interfaces to show its spatial variation and compare the effect of metals with different chemical interactions on the TBC. We develop an image clustering technique to reveal the difference in TBC in single-layer regions compared to bilayer. TBC increase at Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2 interface showing the selection of metal contacts can impact the thermal dissipation from electrical contacts in future TMD devices. Methodology for TBC Measurement TDTR has become a widely used technique to measure the thermal conductivity of thin films and substrates as well as TBC. Briefly, TDTR is a pump-probe optical technique whereby a modulated laser beam (pump) heats the surface of a sample and an unmodulated beam (probe) measures the change in optical reflectivity of the surface. Modulation of the pump beam allows the signal to be measured using a lock-in amplifier, and the experimental data is fit to a thermal model35 to extract the thermal properties of interest (e.g. TBC). The TBC is given by the relationship, 𝑇𝐵𝐶 = 𝑄" ∆𝑇, where 𝑄" and ∆𝑇 are the heat flux and temperature drop, respectively, at the interface of different materials in the sample. The two-color TDTR36 setup used here was described previously37. Briefly, the output of a Spectra Physics Ti:Sapphire (λ=800 nm, 40 nJ/pulse) laser with ~150 fs pulse width and a repetition rate of ~80 MHz was split into two beam paths (pump and probe). The pump beam was modulated at a frequency of 8.8 MHz and frequency doubled using a BiBO crystal. The pump and probe focused, concentrically, to 1/e2 radii of ~5 and ~3 µm (measured using a DataRay Inc. Beam’R2 beam profiler), respectively, using pump and probe powers of 7 and 3 mW, respectively. In traditional TDTR experiments, the arrival time of the probe is delayed relative to the pump beam (in contrast, the pump beam can be advanced relative to the probe35) to map the decay of the thermoreflectance signal. This typically requires several minutes per measurement, but the



time to scan a 2D area can be decreased substantially by measuring the thermoreflectance signal, ― 𝑉in 𝑉 , at a single delay time where 𝑉 and 𝑉 are the in-phase and out-of-phase signal, out in out respectively, of the lock-in amplifier. This approach was initially used to map the thermal conductivity of Nb-Ti-Cr-Si diffusion couples38 and was extended recently to show thermal conductivity suppression near grain boundaries in polycrystalline diamond39. The method is analogous to a technique used by Yang et al.40 to map TBC at metal-graphene interfaces. The 2D map of TBC is created by raster scanning the sample over a 120x120 µm2 area. Precise control of the sample position is achieved using a motorized translation stage and DC servo motor controller (Thorlabs MTS25-Z8 and KDC101, respectively). The resolution of the resulting 2D image is determined by the step size of sample translation (3 µm in this study). The TBC is calculated by comparing the measured ― 𝑉in 𝑉out value to a correlation curve which predicts ― 𝑉in 𝑉out as a function of TBC at metal-MoSe2-SiO2. Sample Fabrication and Characterization Single-layer MoSe2 flakes were grown on a 310 nm thermally grown SiO2 through a lowpressure chemical vapor deposition (LPCVD) process, described previously41-42, by reacting 0.2 g MoO3 with 1.2 g Se powder under 40 sccm Ar + 6 sccm H2 carrier gas resulting in triangular MoSe2 islands. The largest single-layer islands found on the sample were equilateral triangles with 80 µm side length. Individual MoSe2 islands were patterned using Futurrex Inc. NR9-1500PY negative photoresist exposed using a Microtech LaserWriter LW405 system. The LaserWriter allows for low-throughput, direct-write, photolithographic patterning using a focused laser beam (λ=375 nm) with 0.7 µm minimum resolution. After identifying candidate islands, those with approximately 80 µm side length, 100 µm square windows were drawn above them along with 500 µm square windows at each corner (Figure 1a). The larger windows were used to locate the 100 µm windows for site-specific TDTR measurements. The optical microscope image (10x objective) in Figure 1b shows the exposure pattern of MoSe2 islands prior to metal deposition for TDTR measurements. Following patterning, the samples were coated with Al, with and without a Ti adhesion layer, using electron-beam evaporation to form Al-MoSe2-SiO2 and Ti-MoSe2-SiO2 interfaces. The Al-Ti interface can be ignored since the TBC at metal-metal interfaces is an order of magnitude higher than metal-semiconductor or metal-dielectric interfaces43-45. Cheaito et al.46 showed that inclusion of 2 nm Ti adhesion layer was enough to effectively change the TBC at Au-


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Si interfaces, with little change up to 40 nm Ti. Therefore, with Ti adhesion layer we can consider the interface to be Ti-MoSe2-SiO2. The TBC values reported here are equivalent to the total thermal conductance of the metal-MoSe2-SiO2 interface. In both geometries, the Al layer acts as an ideal transducer to absorb incident laser energy during thermal measurements because of its relatively large thermoreflectance coefficient at the wavelength of our probe laser (λ=800 nm)47. The transducer thicknesses (Figure 1c and 1d) were measured on co-deposited glass slides using a Veeco Dimension 3100 Atomic Force Microscope (AFM) in tapping mode and confirmed using picosecond acoustics48. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific K-Alpha+ spectrometer with an Al Kα monochromatic X-ray source (1486.6 eV). Raman spectroscopy data was acquired using a Renishaw InVia Raman microscope with 180 backscattering geometry and 488 nm Ar+ laser focused using a 50x objective lens (NA=0.5). Results and Discussion The optical contrast of the MoSe2 regions in Figure 1b can be used to identify single-layer regions49. As stated previously, the largest islands on the sample were equilateral triangles with ~80 µm side length. The AFM image in Figure 1e shows the sample surface and the height of an isolated island was measured to be ~0.7 nm along the line in Figure 1f, confirming the MoSe2 is single-layer50. The Mo 3d (Fig. 2a) and Se 3d (Fig. 2b) high resolution spectra shows the characteristic Mo 3d5/2 and Mo 3d3/2 doublet with peak binding energies of 230 and 233 eV, respectively, with slight MoOx peak, along with Se 3d5/2 and Se 3d3/2 doublet peaks at 54.8 and 55.7 eV, respectively, in agreement with the previous studies51. The Raman spectrum (Fig. 2c) shows two distinct peaks at ~244 and ~291 cm-1 corresponding to A1g (out-of-plane) and E12g (inplane) modes in MoSe2, respectively52. Less pronounced peaks corresponding to B12g and E1g modes can also be identified. Prior to 2D TDTR mapping, we created correlation curves from theoretically calculated values of ― 𝑉in 𝑉out as a function of TBC at metal-MoSe2-SiO2 interface38. To accomplish this, we must first know the geometry and thermal properties of each layer of our sample (Figure 1c and 1d). As stated earlier, metal transducer thickness (𝑑) was measured using atomic force microscopy and confirmed using picosecond acoustics48. Thermal conductivity (𝑘) of metal films were measured using the Wiedemann-Franz law, 𝐿0 = 𝑘 𝜎𝑇, where 𝑘 is thermal conductivity, 𝜎 is electrical conductivity, 𝑇 is absolute temperature, and 𝐿0 is the Lorenz number (taken from



literature as 2.44  10-8 WK-2 53). Electrical conductivity measurements were performed at RT on reference glass slides using a four-point probe technique. SiO2 thickness was measured using reflectometry (Nanometrics Nanospec 3000 reflectometer). The TBC at SiO2-Si interface was taken from literature54 along with Si thermal conductivity55; however, the sensitivity to these two parameters were very small and did not significantly affect the measured TBC at metal-MoSe2-SiO2 interface. Bulk values for volumetric heat capacity were used for each layer. The SiO2 thermal conductivity is the most important parameter which remains to be determined. We showed previously the importance of the properties of the underlying layer immediately adjacent to the interface of interest for metal-graphene-SiO2 interfaces45 which also applies here for metal-MoSe2-SiO2. The sensitivity of our model to SiO2 thermal conductivity makes its measurement critical to accurately determining TBC. Performing full TDTR scans (from 100 – 6000 ps) on the larger (500 µm) windows, the SiO2 thermal conductivity was determined to be 1.38 (+0.68/-0.46) W/m-K in agreement with bulk56. The upper and lower bounds were calculated using Monte Carlo (MC) simulations37, 45. With all properties of the sample now known, a correlation curve (Figure 2d) was created by calculating ― 𝑉in 𝑉out while varying the metal-MoSe2-SiO2 TBC from 1 to 150 MW/m2-K at a delay time of 100 ps. We chose 100 ps because the sensitivity to TBC is higher at short delay times. The solid line in Figure 2d is used to create TBC maps and corresponds to SiO2 thermal conductivity, 𝑘, of 1.38 W/m-K from TDTR measurements, while the dashed lines represent the upper and lower limits from MC simulations. For each curve, ― 𝑉in 𝑉out increases with TBC then plateaus which can be explained by the decreased sensitivity compared to SiO2 thermal conductivity as TBC increases45. Spatial mapping of TBC was performed by raster scanning of the sample over 120x120 µm2 area using 3 µm increments (i.e., the 1/e2 radius of the probe beam), much larger than the 100 µm square area of metal coated MoSe2 islands. By analyzing the thermoreflectance signal (i.e., ― 𝑉in 𝑉 ), we can easily distinguish between areas with and without metal present. Figure 3a out (Al) and 3b (Ti) show the optical images (100x) of individual MoSe2 islands which were probed during this study. From this figure, we can see that the growth process produced mostly singlelayer islands and some bilayer regions where smaller islands have grown above larger regions. A map of ― 𝑉in 𝑉out signal across the 120x120 µm2 area in Figure 3c and 3d shows the alignment


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is quite good compared to the optical images. The recorded ― 𝑉in 𝑉out signal in areas where no metal transducer coated the sample resulted in negative numbers which, when used as input to the correlation curve, produced negative TBC. These values were ultimately set to zero. We also excluded TBC values less than ~5 MW/m2-K which were only observed near the edge of the metal covered regions of the sample. Analyzing the TBC maps in Figures 3e and 3f, some single-layer MoSe2 regions can be distinguished from bilayer regions. Higher TBC values were located on areas not covered by MoSe2 (Al-SiO2 or Ti-SiO2 interfaces). The color contours across the MoSe2 islands show TBC is higher for the Ti sample compared with Al. We observed edge effects, as evidenced by differences in the color contours in the center and near the edge of MoSe2 islands in Figures 3c-3d, which could be due to different metal-MoSe2 interaction at the a-axis of MoSe2 or near the edge of metal coated area of the sample. The variation in TBC across the probed area could also result from the change in the interface structure near bilayer MoSe2 regions. TBC depends on chemical (i.e., bonding)57 and mechanical (i.e., roughness)58 properties of the interface. A systematic study correlating interfacial bonding59 and/or surface roughness60 to TBC using XPS or AFM, respectively, could provide more insight to the nature of thermal interaction at electrical contacts in TMD devices. As previously mentioned, the growth process for the MoSe2 samples result in single-layer and bilayer regions. While performing the TBC mapping, we probe single-layer and bilayer regions together without distinguishing between the two during the measurement. The results suggest there is a distinction between regions with different layer number, but finding a sharp boundary is arduous due to low resolution of TBC maps and difficulty in alignment with corresponding optical image. The distribution of TBC in single-layer and bilayer regions is compared using the contrast of the optical images in Figure 3a and 3b. Single-layer regions appear lighter than bilayer regions, allowing the pixels of the optical images to be clustered by applying k-means algorithm61. The k-means algorithm partitions all pixels into k clusters with each cluster having a mean value, and pixels in one cluster are closest to the corresponding mean value among all cluster means. Regions with no MoSe2 (i.e., 0-layer) can also be identified. The TBC maps in Figure 3e and 3f are also clustered based on the TBC values, enabling a sharp boundary to be established between different regions. Key points, mostly corners, of the optical images and TBC maps are selected, and the locations of these key points are used to



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calculate the perspective transformation matrix between the optical images and TBC maps which can be utilized to align optical images and TBC maps. The original TBC maps have much lower resolution than the optical images thus they are resized/scaled for alignment and better clustering results. The clustered optical images are shown in Figure 4a and 4b and the different colors in the figure correspond to single-layer (blue), bilayer (green) and 0-layer (red) regions of the sample. Following alignment, the corresponding TBC values for each cluster in the optical images were extracted from the aligned TBC maps. The histogram plots for each cluster are shown in Figures 4c and 4d for Al and Ti samples, respectively. The results reveal the TBC in single-layer regions are in fact higher than bilayer regions. Because the sensitivity of model parameters varies over a range of delay times, we perform full TDTR scans (100 – 6000 ps) at different positions across the areas shown in Figure 3a and 3b to obtain a more accurate estimation of TBC. The results of full TDTR scans shown in Figure 5 were, for the most part, in agreement with the TBC maps where we observed two, clearly-defined regions corresponding to metal-MoSe2-SiO2 and metal-SiO2 interfaces. In Figure 5 we see that TBC at Ti-MoSe2-SiO2 interface was consistently higher than Al-MoSe2-SiO2 interface in agreement with the contour plots in Figure 3e and 3f. On average, TBC increases of 4 and 24 MW/m2-K were observed at Ti-MoSe2-SiO2 and Ti-SiO2 interfaces compared to Al-MoSe2-SiO2 and Al-SiO2 interfaces, respectively, showing the choice of metal can significantly impact the TBC and thus heat dissipation from electrical contacts in TMD based devices. The error bars in Figure 5 were calculated using MC simulations37, 45. We must point out that the TBC from full TDTR scans differs from the 2D TBC maps. However, the general trend of increased TBC at Ti-MoSe2 and Ti-SiO2 interfaces can still be observed, thus the TBC mapping results are useful for comparative purposes. In addition, the TBC at Al-SiO2 and Ti-SiO2 interfaces is less than previously reported values (85 – 120 MW/m2-K) for metal-SiO2 interfaces45,

62-63

which we

attribute to residual photoresist or other residue from the CVD growth process. The difference in TBC for Al and Ti interfaces with MoSe2 could be caused by strong interaction between Ti 3d and Se 4p orbitals21. As mentioned previously, Al and Ti are two low work function, transition metals which form weakly and strongly bonded interfaces, respectively, with MoSe2 which is the motivation for studying these two metals in the current work. This is evidenced by decreased Schottky barrier height (0.29 eV) and equilibrium interfacial distance (2.24 Å) for Ti compared to Al (0.56 eV and 3.24 Å, respectively) both of which are an indication


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of stronger interfacial interaction. Strong adsorption of Ti to MoSe2 is demonstrated by increased binding energy of Ti (0.71 eV) compared to Al (0.04 eV). As a result of this strong adsorption, the band structure of MoSe2 is strongly perturbed when contacted with Ti suggesting orbital hybridization between Ti 3d-orbitals and Se 4p-orbitals but is preserved when contacted by weakly absorbed metal Al with only slight hybridization. A similar effect was reported for Au-MoSe2 and Sc-MoSe2 interfaces20, and a previous study has shown the effect on TBC at Au-MoS2 and ScMoS225. Though MoS2 and MoSe2 are different materials altogether, the structure and vibrational properties are similar and are only used for comparative purposes because of the scarcity of literature pertaining to MoSe2. On the other hand, the difference in Debye temperatures of Al and Ti metals (Ti =420 K, Al=428 K64) and oxides (TiO2=750 K65, Al2O3=1100 K66) formed during high vacuum deposition could give rise to the observed differences in TBC. From this perspective, we would expect the Ti-MoSe2-SiO2 TBC to be higher since the Debye temperature of MoSe2 (200 K67) is much closer to TiO2. TBC may also be increased for both Al and Ti films, deposited in ultra-high vacuum environment, as shown previously for Ti-graphene-SiO268 and Ti-Al2O3 and Ti-MgO interfaces69 either due to lower Debye temperature or increased reactivity of metal films with less oxidation. A previous study by Ong et al.70 reported increased TBC in graphene encased in SiO2 compared to supported graphene due to increased transmission caused by interaction of low-frequency phonons and surface modes in SiO2. The results may vary based on superstrate-substrate combination and may also explain the TBC difference in the Al-MoSe2-SiO2 and Ti-MoSe2-SiO2 samples with disparate metal transducer (i.e., superstrate). Our study corresponds to encased MoSe2 and measured TBC results may be different from supported MoSe2. Finally, we compare our TBC results to values reported in literature for MoS2 and MoSe2 interfaces. Both are TMDs with similar semiconducting properties and structure; however, MoS2 has been more widely studied until recently thus we use MoS2 results for comparative purposes. The TBC at MoS2 and MoSe2 interfaces are not expected to be same as they are two different materials and the surface structure depends on the sample fabrication method, but the comparison aims to highlight the similarities and differences from other studies. The values presented here are slightly less than the previously reported values (20 – 26 MW/m2-K) at metal-MoS224 interface. Previous studies used exfoliated MoS2 and MoSe2 flakes which can have different interfacial properties compared to CVD grown. More recent studies of MoS2-SiO2 interfaces27, 29, reported



small differences between exfoliated and CVD-grown MoS2 with and without an AlOx capping layer which forms interface structure (AlOx-MoS2-SiO2) similar to the metal-MoSe2-SiO2 interfaces used here. It is important to note that in the present study the metal-MoSe2-SiO2 interfaces were treated as a single, diffuse interface. The results of Yasaei et al.28 for Au-Ti-MoS2SiO2 interfaces are more relevant to the current study. Reported TBC values were 20 and 33 MW/m2-K for exfoliated and CVD grown MoS2, respectively, in similar range of the values presented here. Conclusion We have investigated spatial variation of the TBC at interfaces of CVD grown MoSe2 with metal using a modified TDTR technique. The thermoreflectance signal at a single probe delay time is compared with a correlation curve which describes the change in the signal with respect the TBC at metal-MoSe2-SiO2 interface. Using this technique, we created a 2D map of TBC in reduced time than would be required to do full TDTR scans over delay times of several ns. The results show higher TBC at Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2. Clustered optical images were aligned with the TBC maps and revealed the TBC in single-layer regions was higher than bilayer regions of MoSe2. Our measurements using full TDTR scans confirm an increase in TBC at metal-MoSe2-SiO2 and metal-SiO2 for Ti compared to Al highlighting the importance of choice of metal contact. This technique and analysis can be conveniently extended to many other TMDs such as MoS2 and WSe2. Our study will provide insights into the energy efficient design and thermal management of such devices. Acknowledgements The authors would like to thank Ben Hollerbach at the Georgia Tech Institute of Electronics and Nanotechnology (IEN) and Luke Yates for assistance with sample patterning and TDTR mapping, respectively. Synthesis of the 2D materials was supported by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. References 1. Frindt, R. F.; Yoffe, A. D., Physical Properties of Layer Structures - Optical Properties and Photoconductivity of Thin Crystals of Molybdenum Disulphide. Proc R Soc Lon Ser-A 1963, 273 (1352), 69-+. 2. Podzorov, V.; Gershenson, M. E.; Kloc, C.; Zeis, R.; Bucher, E., High-mobility field-effect transistors based on transition metal dichalcogenides. Appl Phys Lett 2004, 84 (17), 3301-3303. 3. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 transistors. Nat Nanotechnol 2011, 6 (3), 147-150.


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Figure 1: (a) Schematic of exposure pattern for MoSe2 islands for metal deposition and thermal measurements. (b) Optical microscope image showing exposed region of sample. (c), (d) Sample geometry used in TDTR measurements. (e) Atomic force microscope image, and (f) Profile of single-layer MoSe2 (~0.7 nm) along the line shown in (e) corresponding to single-layer MoSe2 50.



Figure 2: (a) High resolution Mo 3d XPS spectrum with Mo 3d5/2 and Mo 3d3/2 doublet with peak binding energies of 230 and 233 eV, respectively, with slight MoOx peak. (b) High resolution Se 3d XPS spectrum with Se 3d5/2 and Se 3d3/2 doublet with peak binding energies of 54.8 and 55.7 eV, respectively. (c) Single-layer MoSe2 Raman spectrum with two peaks corresponding to A1g (out-of plane) and E12g (in-plane) modes at ~244 and ~291 cm-1, respectively. (d) Correlation curve used to calculate TBC from the ― 𝑉in 𝑉out created by varying the metal-MoSe2-SiO2 TBC from 1 to 150 MW/m2-K at a delay time of 100 ps. The solid line is used to create TBC maps and corresponds to SiO2 thermal conductivity, 𝑘, of 1.38 W/m-K from TDTR measurements. The dashed lines are calculated using the upper and lower bounds from MC simulations 37, 45.


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Figure 3: Optical microscope images (100x) showing probed area of 2D TDTR mapping experiments at (a) Al-MoSe2-SiO2 and (b) Ti-MoSe2-SiO2 interfaces. (c), (d) Spatial variation of ― 𝑉in 𝑉 signal showing good alignment with the optical images. 2D TBC map across probed out 120x120 µm2 area of (e) Al and (f) Ti covered regions of sample. Some single-layer MoSe2 regions can be distinguished from bilayer regions in both images. The color contours suggest higher TBC across Ti-MoSe2-SiO2 interfaces compared to Al-MoSe2-SiO2. Also, higher TBC values were located on areas not covered by MoSe2 (Al-SiO2 or Ti-SiO2 interfaces). Edge effects are apparent from differences in color contours at the center and near the edges of MoSe2 islands in (c)-(f).



Figure 4: Clustered optical images for (a) Al and (b) Ti samples with single-layer (blue), bilayer (green), and 0-layer (red) regions highlighted. Histograms showing the distribution of TBC values across (c) Al and (d) Ti samples confirm TBC in single-layer regions is higher than bilayer regions.


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Figure 5: Summary of TBC values from full TDTR scans at several positions across MoSe2 islands from 2D mapping experiments. Error bars were calculated using MC simulations. We observed two, clearly-defined regions corresponding to metal-MoSe2-SiO2 and metal-SiO2 interfaces. Also, TBC at Ti-MoSe2-SiO2 interface was consistently higher than Al-MoSe2-SiO2 interface. TBC increases of 4 and 24 MW/m2-K, on average, at Ti-MoSe2-SiO2 and Ti-SiO2 interfaces, respectively, compared to Al-MoSe2-SiO2 and Al-SiO2 interfaces show the impact of metal contact on TBC.



Table of Contents/Abstract Graphic


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Spatial Mapping of Thermal Boundary Conductance at Metal

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