Mapping Hot Spots at Heterogeneities of Few-Layer Ti3C2 MXene

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Mapping Hot-Spots at Heterogeneities of Few-Layer Ti3C2 MXene Sheets Poya Yasaei, Qing Tu, Yaobin Xu, Louisiane Verger, Jinsong Wu, Michel W. Barsoum, Gajendra Shekhawat, and Vinayak P. Dravid ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09103 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Mapping Hot-Spots at Heterogeneities of Few-Layer Ti3C2 MXene Sheets Poya Yasaei1,2, Qing Tu1,2, Yaobin Xu2, Louisiane Verger3, Jinsong Wu2, Michel W. Barsoum3, Gajendra S. Shekhawat2,[*], Vinayak P. Dravid1,2,[*] 1

Department of Materials Science & Engineering, Northwestern University, Evanston, IL 60208, USA 2

Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, Northwestern University, Evanston, IL 60208, USA 3

Department of Materials Science & Engineering, Drexel University, Philadelphia, PA, 19104, USA. [*]

Corresponding authors: Dr. Gajendra S. Shekhawat: [email protected] Prof. Vinayak P. Dravid: [email protected] Keywords: scanning thermal microscopy (SThM), temperature mapping, hot-spot identification, heterogeneities, defect, 2D materials, MXene Abstract: Structural defects and heterogeneities play an enormous role in the formation of localized hotspots in 2D materials used in a wide range of applications from electronics to energy systems. In this report, we employ scanning thermal microscopy (SThM) to spatially map the temperature rise across various defects and heterogeneities of titanium carbide (Ti3C2Tx - T stands for surface terminations) MXene nanostructures under high electrical bias with sub-50-mK temperature resolution and sub-100-nm spatial resolution. We investigated several Ti3C2Tx flakes having different thicknesses as well as heterogeneous MXene structures incorporating line defects or vertical heterojunctions. High-resolution temperature rise maps allow us to identify localized hotspots and to quantify the non-uniformity of the temperature fields across various morphological features. The results show that the local heating is most severe in vertical junctions of MXene flakes and is highly affected by non-uniform conduction due to the presence of line defects. These results provide a direct insight into the power dissipation of MXene-based devices and the roles of various heterogeneities that are inherent to the material synthesis process. This study provides a guideline that how a better understanding of the structure-property-processing correlations and further optimization of the synthesis routes could improve the lifetime, safety, and operation limits of the MXene-based devices. 1 ACS Paragon Plus Environment

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Transition metal carbides and nitrides (MXene) are a large family of two dimensional (2D) materials that exhibit a wide range of intriguing physical properties and potential in several applications.1–11 Titanium carbide (Ti3C2Tx - T stands for surface termination) is the most-studied member of the 2D MXene family and holds great promise for use in electronic devices as well as electrochemical energy storage systems such as supercapacitors and batteries.1–18 In such applications, it is reasonable to assume that system failure could cohort with localized temperature rise caused by internal energy losses (heat generation) in the electrodes at high currents. Due to the harsh wet chemistry processes used in MXene synthesis, the synthesized nanostructures are far from pristine and typically exhibit different types of irregularities, heterogeneities, and defects. 19 In a report by Lipatov et al. the effect of the synthesis process on quality and electronic properties of individual Ti3C2Tx flakes was extensively studied and optimized to produce large MXene flakes with a low concentration of defects.9 However, even the optimized synthesis routes (that is used in this study) result in various defects and morphological features (wrinkles, tears, vertical interfaces, etc.) that could lead to the formation of localized hot-spots. These features are the most vulnerable sites to heat-induced failure, but to our knowledge, the effect of these morphological features on self-heating of MXene-based structures have been overlooked. In addition, the continuous shrinkage in the characteristic length of the structural heterogeneities and the existence of various interfaces in such systems calls for an ever-rising demand in the development of thermometry techniques that are capable of mapping temperature fields and heat transport with nanoscale resolution.20,21 It is also crucial that the nanoscale thermometry to be accompanied with a registered high-resolution morphology map to establish structure-property correlations and to serve as a meaningful feedback for the design of the nextgeneration devices and systems with improved performance and functionality.20–22 Over the past

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decade, many efforts have been focused on the development of non-contact optical thermometry techniques based on different physical phenomena such as thermoreflectance, Raman, infrared (IR) emission, or fluorescence.20,21,23–27 While these methods provide high-throughput thermal mapping capabilities, they collectively suffer from low spatial resolution due to optical diffraction limit which is in the order of several hundred nanometers for the state-of-the-art systems.28,29 Simultaneous structural mapping in these techniques is also limited to optical microscopy and is often impossible to carry-out during the thermal measurements due to exposure of the measurement sites to high-intensity lasers. Scanning thermal microscopy (SThM) is a versatile technique based on a scanning probe microscopy (SPM) platform that enables simultaneous morphological imaging, temperature mapping, and measurements of thermophysical properties in nanoscale devices with great temperature sensitivity and spatial resolution that is beyond optical diffraction limits.28,30–36 The sensing element of SThM systems is typically either a thermocouple30,33–36 or a thermistor.30–32 Other physical phenomena such as Schottky diode,37 bimetallic cantilevers,38 and Joule expansion39 have also been successfully utilized for temperature sensing. Recent advances in this field have provided for improved spatial and temperature resolutions as well as a better understanding of contact artifacts.28,30,33 SThM-based methods have widely been utilized to study various thermo-physical effects such as Joule heating, Peltier cooling and heating, and current crowding in nanoscale and 2D-material-based devices.39,40 In this work, we employ SThM to investigate the roles of various defects and heterogeneities on localized heating of a few layers of Ti3C2Tx under high electrical bias.41 The SThM system used provides sub-50-mK temperature resolution and sub-100-nm spatial resolution, thanks to a custom-designed thermometry probe consisting of a hollow silicon tip with an integrated


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thermocouple sensor in a vertical orientation, the apex of which interacts with the sample through a metallic nanowire (50 nm diameter). This architecture provides for a minute thermal mass (short transient time constants) together with a high lateral resolution.41 Ti3C2Tx is unusual in that conductance is metallic and independent of extrinsic factors such as gate voltage and contact doping.3,9,42 This is not the case in virtually all other 2D materials (except a few rare and air-sensitive members such as WTe2)43 in which the electrical conductance depends on gate voltage and is locally affected by contact doping near the metal electrodes.27,44 Such convolutions make it challenging to isolate the nonuniformities in heating caused by the defects and heterogeneities in almost all other 2D material systems besides Ti3C2Tx and a handful of other members of this 2D family. Results/Discussion: MXene multilayers, MLs, are formed by chemical etching of their parent MAX phase45 (Mn+1AXn - where M is an early transition metal, A is a group IIIA or IVA element such as aluminum, Al, and X is either C or N) to their corresponding Mn +1XnTz phase (where Tz represents surface termination such as O, OH, or F).9,45–47 The extraction of the A atom (e.g., Al) from the interlayers releases the MXene flakes and results in the formation of a 2D structure in which T depends on the aqueous exfoliation environment.9,12,46 The Ti3C2Tx atomic layers used in this study are produced using a similar procedure, as previously reported.12 In this approach, a mixture of lithium fluoride, LiF, and hydrochloric acid, HCl, were used to selectively etch out the Al layers from the parent Ti3AlC2 MAX phase powders and produce the Ti3C2Tx, where T is predominantly O-containing surface terminations (e.g., O, OH).9,48,49 Further details on material synthesis can be found in the Methods section.


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X-ray diffraction (XRD) is performed on Ti3C2Tx and Ti3AlC2 powders to evaluate the crystalline structure of the materials before and after the etching process (Figure 1a). Before the etching (Ti3AlC2 powder), the lowest angle XRD feature is a sharp 002 peak at 9.5° which corresponds to a c-lattice parameter of 18.6 Å.47 Several additional XRD peaks are observed which correspond to other crystalline directions and are labeled accordingly. After etching the lowest angle feature is a broader 002 peak at 6.6°, which is associated with the stacking of the layers with an average c-lattice parameter of 26.6 Å, or a dc/2 of 13.3 Å.12 We note that some of the Ti3AlC2 peaks are also observed in the XRD pattern of Ti3C2Tx multilayers which suggests that some unreacted MAX phase is still present after the etching process. These unreacted phases are typically removed in a centrifugation process.3 The Ti3C2Tx ML powder was dispersed in DI water, vigorously shook to separate the layers, and then centrifuged at high speeds to remove bulky and unreacted particles. We then separated the supernatant (top half of the vial) and drop-cast it on transmission electron microscopy (TEM) grids (for characterization) or on a silicon chip (for device fabrication and measurements). TEM characterization (Fig. 1b-d) shows that the synthesized flakes are atomically thin and are well ordered. Figure 1b shows a low-resolution transmission electron microscope (TEM) image of a Ti3C2Tx flake along the [0001] zone axis. The flake is uniform in thickness and single crystalline as shown in the electron diffraction pattern (inset of Fig. 1b). As the crystal has six-fold symmetry along the [0001] zone axis, the diffraction pattern shows also six-fold symmetry. A high-resolution TEM image of another flake is shown in Fig. 1c, which again illustrates the hexagonal symmetry of the structure.


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Figure 1. Structural and electrical characterization of MXene flakes (a) X-ray diffractograms (XRD) from Ti3C2Tx (top) and Ti3AlC2 (bottom) MAX phase powders. (b) Low-resolution TEM image of a thin MXene flake. The inset shows an electron diffraction pattern obtained from the same image, showing hexagonal lattice symmetry. (c) High-resolution TEM image of a Ti3C2Tx flake. (d) X-ray energy dispersive spectroscopy (XEDS) composition mapping a Ti3C2Tx flake showing Ti, C and O maps. (e) Room-temperature current-voltage (I-VSD) characteristics of several MXene FET devices at zero gate voltage (VG). Different lines correspond to different devices, having different lateral and vertical geometrical dimensions. The inset shows an optical image of a representative device (f) Current vs. gate voltage (I-VG) characteristics of the same devices shown in (e). Figure 1d presents X-ray energy dispersive spectroscopy (XEDS) elemental maps of a typical Ti3C2Tx flake. The XEDS spectrum is presented in section S1 in SI and shows that the flake mainly consists of Ti, C, and O, with minor amounts of F and Cl. These elements are all expected based on the material chemistry, where the Ti and C form the MXene layers and the O, F and Cl are


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surface terminations. It is noteworthy that the Al signal at 1.486 KeV is absent (see section S1 in SI), confirming that any unreacted Ti3AlC2 is removed in the centrifugation process. Next, we drop-cast the MXene flakes on a silicon, Si, substrate having a 300 nm thermallygrown oxide (SiO2/Si). This substrate enables us to optically identify atomically-thin and uniform flakes and select them for device fabrication and subsequent electrical and thermal mapping tests. We used standard lithography and fabrication processes to pattern metal contacts across the flakes in a two-probe configuration. An aluminum oxide (AlOx) layer was deposited on the device using atomic layer deposition (ALD) to prevent ambient degradation of the flakes during subsequent measurements. Further details of the device fabrication can be found in the Methods section. The underlying Si chip is highly doped which enables us to apply a back-gate voltage and probe and field-effect transistor (FET) behavior of the flakes. Figure 1e shows the current-voltage, I-VSD, characteristics of several Ti3C2Tx few-layer FETs at zero gate voltage, VG. Depending on the lateral dimensions and thickness of the films, the device resistances varied in the range of 0.251.62 kΩ. The I-VSD curves are linear near zero VSD which indicates that the contacts are Ohmic. Figure 1f shows the I vs. VG dependency which is found to be negligible (< 0.4%) across all tested devices and the entire tested range (±60 VG) and is consistent with the expected metallic behavior for single Ti3C2Tx layers.42 Since the gate dependency is negligible, unless otherwise noted, all subsequent experiments were carried out without applying a gate voltage. In general, the average temperature rise of a uniform 2D material dissipating power depends on its thermal boundary conductance (TBC) with the substrate and the thermal resistance of the substrate.3,21,50–54 These factors have been carefully studied for Ti3C2Tx in a recent report by some of us where the room temperature TBC of the Ti3C2-SiO2 interface were found to be 10.8±3 MW.m-2.K-1 for a bare flake and 19.5±6 MW.m-2.K-1 for a flake encapsulated with aluminum oxide


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(AlOx).3 However, very little is known about the spatial distribution of temperature fields in operating devices, given the potential presence of various heterogeneities and defects in MXene nanostructures. Herein we employed an SThM system to study the self-heating of Ti3C2Tx devices as a function of applied power. Details of the SThM setup and experimental procedure can be found in the Methods section as well as in section S2 of SI file. Figure 2 shows typical SThM results obtained from two individual MXene devices subjected to different electrical power. The power is calculated as resistance multiplied by square root of current (P = R.I2) in which the bias is applied in 2-probe measurement configuration. Figures 2a and 2g are the height profile images of the two devices, with thicknesses of 5.7 nm and 26 nm. Given that the thickness of a single layer is ≈ 1 nm,42 these devices are comprised of ~5 and ~26 layers, respectively. The SThM results show that the temperature increases monotonically as the applied power is increased. It is worth noting that the temperature profiles of the typical MXene devices tested in this study have a weak dependence on the in-plane thermal conductivity of MXene because the heat dissipation mostly takes place in the through-plane (vertical) direction into the SiO2/Si substrate.3,39 The heat generated in the metal electrodes dissipates partly through the metal electrode themselves and partly through the underlying substrate. In the thinner flake (5.7 nm thick), the temperature rise is uniform throughout the flake, and the contacts are cooler than the flake itself (see Fig. 2b-e). Fig. 2f shows a line profile obtained from Figs 2b-e on the red dashed line shown in Fig. 2e. The spikes in the unbiased (blue) curve at the edge of the metal electrodes originate from the contact artifacts and is due to changes in the contact force in the SPM scans. However, comparative study of the biased and unbiased scans allows us to rule out such artifacts and obtain the true heating caused by electrical power


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dissipation. More specifically, the heating at the center of the flake is ~140% of that in the contact regions with the metal electrodes. This implies that the flake resistance at this thickness dominates the overall resistance and that the contact resistances are not the bottle-neck for electrical transport. This contrasts with the results obtained from 26 nm thick flake where the heating is only 63% of that of the metal contacts (Figure 2h-k). This observation can be explained by the thickness dependency of the different resistance contributions in the tested devices. Specifically, we expect the intrinsic flake resistance to inversely scale with thickness (due to cross-section area scaling), while the electrical contact resistance is not expected to scale with the flake thickness.

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Figure 2. Scanning thermal microscopy (SThM) characterization of individual MXene flakes. (a) height profile image of a 5.7-nm-thick MXene multilayer, ML. Inset height profile is obtained on the red dashed line. (b-e) Temperature rise maps of the few MXene flakes at different applied electrical powers. (f) Temperature line profiles obtained from Figs. 2b-e on the red dashed line shown in Fig. 1e. (g) Height profile a 26-nm-thick MXene ML. Inset height profile is obtained on the red dashed line. (h-j) Temperature maps of the thick MXene stack at different applied electrical powers. (k) Temperature line profiles obtained from Figs. 2h-j on the red dashed line shown in Fig. 1j. In general, different physical phenomena can contribute to the heating at dissimilar currentcarrying junctions of 2D materials.28,39,40,55,56 Joule heating originates from the dissipation of energy from charge carriers to the lattice and contributes to the heating of junctions having a finite electrical contact resistance. Joule heating is proportional to the resistance and square of the 9 ACS Paragon Plus Environment

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electrical current passing through the material. In semiconducting and semimetallic materials, the resistivity may spatially vary along the device due to carrier concentration inhomogeneities near the junctions (contact doping effect). This phenomena (i.e., current crowding effect) causes a nonuniform temperature profile in the vicinity of the junctions.27 This effect is most dominant in metalsemiconductor junctions at large applied biases and typically leads to heating and failure of devices near the drain electrode. For graphene devices with a long channel, it is shown that by changing the applied gate voltage, the hot-spot moves from one junction to the other.27 Additionally, if the thermopower of the adjacent media across the junction are not similar, there could be a Peltier effect contribution which either heats or cools the junction, depending on the direction of charge flow.28,39,40,55,56 In the case of Ti3C2Tx, since the dependency of the drain current to gate voltage is negligible, we believe that the charge carrier density is quite uniform within the flakes, thus ruling out the current crowding effect. To investigate the Peltier effect, we mapped and compared the temperature rise profile of MXene devices at forward and reverse applied biases. The results shown in section S3 of SI - showed almost identical temperature profiles for both forward and reverse bias. This measurement rules out the Peltier contribution to the device heating and leaves the Joule effect as the most dominant phenomenon defining the temperature profile of Ti3C2Tx devices. It is previously shown that for the typical length-scales and flake thicknesses used in this study, the in-plane contributions of thermal transport through the Ti3C2Tx MXene flake and the topcoating AlOx layer are negligible compared to the through-plane heat transport contribution.3,21 Hence, the average device temperature rise, at a given applied power in steady-state transport, is primarily defined by the thermal dissipation resistance in the through-plane direction which is


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primarily comprised of the thermal boundary resistance (TBR) of the Ti3C2Tx-SiO2 interface and the spreading thermal resistance of the underlying substrate (SiO2 thin-film and bulk Si).21,44,51,57 However, one should note that this analysis is only effective when the power dissipation is uniform throughout the flake. In cases that the devices have morphological non-uniformities and defects, direct mapping of the temperature rise, with nanoscale resolution, is vital to understand hot-spot formation. We evaluated the effect of morphological defects and irregularities on the temperature rise profile of the Ti3C2Tx devices. These features are implicit to the fabrication process of the MXene flakes, even in cases where optimized synthesis methods are employed (as in this report).9 Figure 3 shows the height and temperature rise profiles of three MXene devices having morphological line defects at different applied electrical biases. Comparing the temperature maps at high applied electrical powers to the height profile images suggests that these morphological features are less conductive than the flat regions of the flakes. As a result, these defects significantly modify the current density in their vicinity. The non-uniform current conduction leads to the formation of localized hot-spots at locations where the current density is maximum (shown in Fig. 3a, 3b, and 3c with a yellow cross). In the device shown in the top row of Fig. 3, the maximum temperature on the flake is 3.15 °C at 0.93 mW applied power, which is more than twofold of the average flake temperature (1.5 °C). The flake shown in the second row exhibits a maximum temperature rise of 7.6 °C on the marked hot-spot (yellow cross in Fig. 3b) at 1.77 mW applied power which is almost double than the average temperature rise of the flake. For the device shown on the third row, three distinct hot-spots were identified, but the temperature rise at the most significant hot-spot (marked by a yellow cross in Fig. 3c) at 0.31 mW applied power is 2 °C, which is roughly thrice the average flake temperature.


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Our results can be compared to a paper by Beechem et al. that reported on self-heating induced failure of graphene devices synthesized from both chemical vapor deposited (CVD) and epitaxial means using a combination of infrared thermography and Raman imaging.58 This paper reports that despite a larger thermal resistance, CVD devices dissipate > 3x the amount of power before failure than their epitaxial counterparts which was attributed to more heat localization at morphological irregularities in the epitaxial graphene.58 For instance, it is reported that in bare epitaxial graphene, less than 20% of the device area reaches half of the maximum temperature at the time of failure.58 In another paper, Grosse et al., used scanning Joule expansion microscopy (SJEM) to map the resistive heating at wrinkles and grain boundaries (GBs) of monolayer CVD graphene.59 This paper reported a small temperature rise at wrinkles and a larger temperature rise at GBs (150%–300% greater than the surrounding graphene) due to the finite GB resistivity and to non-uniform current flow across GBs.59 These values are in the same range as our results obtained herein on Ti3C2Tx in which morphological defects caused the heat localization (Fig. 3). g

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Figure 3. SThM characterization of individual MXene flakes with morphological defects. (ac) Topography images of three Ti3C2Tx MLs having line defects formed during sample preparation. Flakes a, b, and c have thicknesses of 8 nm, 17 nm, and 7 nm, respectively. The yellow crosses show the position of the most dominant hot-spot on the devices. (d-l) Temperature maps of the ML flakes, shown in a-c, at different applied electrical powers. Hot-spots are formed in the proximity of the line defects. Next, we investigated vertical interfaces of Ti3C2Tx flakes under electrical bias. This is particularly important in the understanding of the failure modes in systems based on heterogeneous MXene structures and stacked MLs such as supercapacitors and battery electrodes. In this regard, it was previously shown that the resistivity of ML Ti3C2Tx films are just one order of magnitude higher than individual flakes, which suggests that the electrical transport across vertical interfaces is surprisingly better than most other 2D materials.9 This can be due to the presence of surface terminating functional groups which results in improved Van der Waals (VdW) interactions normal to the flakes. However, a microscopic investigation of high-field electrical transport, power dissipation, and localized heating at MXene interfaces is lacking. a

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Figure 4. SThM characterization of vertical interfaces of Ti3C2Tx flakes. (a) Topography maps and height profiles of three Ti3C2Tx MXene flakes. (b) Temperature map of the device before 13 ACS Paragon Plus Environment

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applying any current. (c-d) Temperature maps of the device under electrical biasing applied to different electrodes. (e-f) Temperature profiles of the black dashed lines shown in c and d, respectively. Figure 4 shows the SThM results obtained from three overlapping Ti3C2Tx MLs forming vertical interfaces. As shown in Fig. 4a, the MLs have thicknesses of 7, 11, and 26 nm and exhibit clean surfaces, as evidenced by the low surface roughness on the flakes in the topography image. We deposited three metal electrodes (at the corners) to independently bias these flakes and investigate power dissipation across the Ti3C2Tx vertical interfaces. Figure 4b shows the temperature distribution before applying a bias. Figures 4c and d show the temperature rise profiles of the device when different sets of electrodes (labeled as source and drain) were used to apply the bias. Fig. 4e and f show the temperature rise line profiles obtained from the black dashed lines in Fig. 4c and d, respectively. The blue and red curves correspond to the temperature line profiles without and with applied power. The spikes in the unbiased (blue) curves at the edge of the metal electrodes come from the contact artifacts. The maps and corresponding line profiles obtained from this particular device reveal that the Ti3C2Tx vertical interfaces exhibit localized heating that is comparable in magnitude to the heating at Ti3C2Tx–metal contacts. It is worth noting that the relative contributions of heating at Ti3C2Tx vertical interfaces and Ti3C2Tx–metal contacts depend on the geometry and surface cleanness. We tested multiple devices with overlapping Ti3C2Tx ML, and in several of them, a dominant localized heating was observed at the Ti3C2Tx vertical interface (see section S4 in SI). This observation can be due to presence of visible residue from the liquid-phase processing of the material, even using an optimized processing scheme.9 It is also partly due to smaller contact area in the Ti3C2Tx vertical interfaces compared to the Ti3C2Tx–metal junction. These results suggest that the vertical interfaces could impose an extrinsic limitation on the performance of Ti3C2Tx thin films, and clays used in


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electronics and electrochemical energy storage devices. Further optimization of the synthesis routes and engineering of the surface terminations aimed at strengthening vertical interactions could lead to higher reliability and improved system-level performance of such devices. Conclusions: In summary, we studied the self-heating of Ti3C2Tx devices by mapping the steady-state temperature rise of several operating devices using SThM. Our findings suggest that Joule heating is the most prominent effect defining the temperature profiles of these devices, with currentcrowing and Peltier effects having negligible roles. Our results also show that in thin and uniform Ti3C2Tx flakes (i.e., < 10 layers), the intrinsic flake resistance is dominant, and the majority of the heat dissipation takes place in the flake itself, leading to the uniform heating of the device. In thicker flakes (i.e., > 20 layers), the electrical contact resistances at the Ti3C2Tx–metal interfaces govern the hot-spot formation. Spatial temperature rise mapping of Ti3C2Tx devices with line defects and vertical interfaces reveals that these heterogeneities lead to localized heating that could, in turn, lead to pre-mature heat-induced failures which would limit the devices. The tested individual heterogeneities in Ti3C2Tx nanostructures are the building blocks of more complex structures such as thick electrodes, so can be used to better predict the behavior of such material systems and relevant devices. It follows that better understanding of the structure-propertyprocessing correlations and further optimization of material processing routes are crucial in the design of functional energy and electronic devices made of this family of materials. In particular, films and clays made of flakes with larger lateral sizes, cleaner surfaces, and less density of line defects and irregularities are expected to exhibit less internal heat losses at interfaces. An interesting extension of this work is to utilize SThM to investigate the evolution of hot-spot formation in uncoated Ti3C2Tx devices exposed to different controlled environmental conditions


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(temperature and humidity) for different periods of time. That could shed light on oxidation mechanisms of Ti3C2Tx and to come up with more effective approaches to protect MXene-based functional systems against ambient degradation. Methods: Details of Material Synthesis: We produced Ti3AlC2 MAX phase powder similar to previous reports.3,47 To selectively etch the Al and produce Ti3C2Tx MLs, the sieved Ti3AlC2 powder reacted with a mixture of LiF (99% purity, Alfa Aesar) and HCl (12M, Fisher, technical grade) at a 1:7.5:11.7 molar ratio. We then stirred the mixture for 24 h at 35°C and washed with distilled water using a centrifugation and decantation process until a pH of ~6 was achieved. To produce atomically-thin MXene flakes, we re-dispersed the dried powder in distilled water (4 mg/ml) and vigorously shook the dispersion for ~5 mins to delaminate the layers.9 A centrifuge process was then used for 1 h at 3500 rpm to remove bulky particles and to achieve a supernatant of atomicallythin flakes. Our previous XRD results show that the centrifugation process is very effective in the removal of unreacted species and achieving pure Ti3C2Tx flakes.3 X-ray diffraction (XRD): We used a Rigaku SmartLab X-ray diffractometer to get XRD patterns in the 3–65° 2θ range using a step size of 0.02-0.07° and dwell time of 0.5-1.5 s per step. Electron Microscopy: A JEOL Grand ARM-300CF TEM equipped with high-angle annular darkﬁeld (HAADF) detector, bright-field (BF) detector, annular bright-field (BF) detector, X-ray energy-dispersive spectrometer (EDS) systems operated at 300 kV, was used for electron diffraction patterns (EDPs), high-resolution images and EDS analysis. Device Fabrication Process: To make the Individual Ti3C2Tx devices, we drop-cast the supernatant of the centrifugation process on a Si/SiO2 substrate (300 nm oxide thickness) and dried the device on a hot-plate at 50 ºC. Once dried, the chip was immediately coated with a Poly(methyl methacrylate) (PMMA) layer to protect it against ambient degradation during the optical microscopy process for flake identification and selection. The PMMA was removed right before coating the sample with photoresist to minimize ambient exposure. Next, we used photolithography process (using a Heidelburg maskless aligner - MLA150) and nanofabrication techniques to define and deposit the metal electrodes (5/50 nm Ti/Au by electron beam evaporation and lift-off). We then deposited an AlOx layer using atomic layer deposition (ALD) at 150 ºC for 150 cycles (Ultratech/Cambridge Savannah ALD System) which approximately corresponds to 16 ACS Paragon Plus Environment

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~15 nm thickness. The ALD process uses trimethylaluminum (TMA) and water vapor as precursors. We used pulse times of 0.025 s and dwell times of 10 s to deposit the films. Another lithography process followed by a chlorine-based plasma etching was carried out to enable access to the electrical pads. We tried to minimize the exposure of the MXene flakes to ambient conditions to prevent degradation. Finally, the devices were wire bonded to establish electrical contacts for the subsequent electrical measurements and SThM tests. Scanning Thermal Microscopy (SThM): For temperature mapping, we used a thermal module by AppNano which was interfaced with a Bruker ICON AFM system. Details of the measurement setup are reported in our previous paper.41 The probes were individually calibrated and have thermal sensitivities in the range of 5.5-10 μV/°C. A sample calibration curve is presented in SI. Acknowledgments: The work made use of the SPID and EPIC facilities of Northwestern University’s NUANCE center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation, and the State of Illinois, through the IIN. This work was supported by the National Science Foundation IDBR-1256188 and partially supported by Air Force Research Laboratory grant FA8650-15-25518. This work also made use of the Pritzker Nanofabrication Facility of the Institute for Molecular Engineering at the University of Chicago, which receives support from SHyNE Resource (NSF ECCS-1542205). Author contributions: P.Y., G.S.S., and V.P.D. conceived the idea and designed the experiments. P.Y. fabricated all devices and performed all the experiments and thermal analysis. V.P.D. and G.S.S. supervised the experiments. V.P.D. and J.W. supervised electron microscopy. Q.T. assisted in performing SThM experiments. L.V. synthesized the material under the supervision of M.W.B. Y.X. performed the TEM and XEDS characterization. All authors contributed to the writing of the manuscript. Additional Information: The authors declare no competing financial interests. Supporting Information: Supporting Information, including XEDS spectrum, details of SThM setup and measurements, temperature profile maps of MXene devices at forward and reverse bias, and additional SThM results of vertical MXene-MXene junctions is available online from the ACS website. REFERENCES (1)

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Table of Content (TOC) Figure: 26 nm

Height

Tem perature Rise Map

11 nm

Drain

Source 7 nm

0


2 °C

Mapping Hot Spots at Heterogeneities of Few-Layer Ti3C2 MXene

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