is an order magnitude larger than previously thought, yet near the low end of known solidsolid interfaces. Our study also reveals unexpected insight into non-uniformities of the MoS2 transistors (small bilayer regions), which do not cause significant self-heating, suggesting that such semiconductors are less sensitive to inhomogeneity than expected. These results provide key insights into energy dissipation of 2D semiconductors and pave the way for the future design of energy-efficient 2D electronics. Keywords: Energy dissipation, 2D semiconductors, thermal boundary conductance, Raman thermometry, MoS2 2The performance of nanoelectronics is most often constrained by thermal challenges, 1, 2 memory bottlenecks, 3 and nanoscale contacts. 4 The former have become particular acute, with high integration densities leading to high power density, and numerous interfaces (e.g. between silicon, copper, SiO2) leading to high thermal resistance. New applications and new form-factors call for dense vertical integration into multi-layer "high-rise" processors for high-performance computing, 3 or integration with poor thermal substrates like flexible plastics (of thermal conductivity 5xlower than SiO2 and nearly 500x lower than silicon) for wearable computing. 5 These are the two most likely platforms for incorporating 2D semiconductors into electronics, yet very little is known about fundamental limits or practical implications of energy dissipation in these contexts.At its most basic level, energy dissipation begins in the ultra-thin transistor channel and is immediately limited by the insulating regions and thermal resistance with the interfaces surrounding it. Herbert Kroemer's observation 6 that "the interface is the device" is remarkably aptfor 2D semiconductors such as monolayer MoS2. These have no bulk, and are thus strongly limited by their interfaces. For instance, even some of the best electrical contacts known today add >50% parasitic resistance to MoS2 transistors when these are scaled to sub-100 nm dimensions. Similarly, thermal interfaces may be expected to limit energy dissipation from 2D electronics, and their understanding is essential. Nevertheless, a key challenge is the need to differentiate heating of the sub-nanometer thin 2D material from its environment. Here, Raman spectroscopy holds a unique advantage, 8, 9 as the temperature of even a monolayer semiconductor can be distinguished from the material directly under (or above) it, if the Raman signatures are distinct. 10Figure 1a shows our typical device structure and measurement setup. We utilize high-qual- Minor, randomly distributed non-uniformities in the temperature seen in Figure 2 are within the uncertainty of the measurement and are also visible in the reference map taken at VDS = 0 (on a hot stage), for which the temperature is known to be uniform, as shown in Supporting Information Figure S4. The uniform self-heating of transistors from CVD-grown MoS2 suggests that any change in energy dissipation around the 2L spots or other non-uniformit...

Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomicallythin limit. Here we introduce a facile growth mechanism for semimetallic WTe 2 crystals, then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 µm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe 2 devices studied to date (3 to 20 layers thick). High-field electrical measurements and electro-thermal modeling demonstrate that ultra-thin WTe 2 can carry remarkably high current density (approaching 50 MA/cm 2 , higher than most common interconnect metals) despite a very low thermal conductivity (of the order ~3 Wm -1 K -1 ). These results suggest several pathways for air-stable technological viability of this layered semimetal.Keywords: two-dimensional (2D) atomic layers; semimetals; transition metal dichalcogenides; current density; thermal conductivity; environmental stability * Contact: epop@stanford.edu 2 The preceding decade has seen much interest in two-dimensional (2D) nanomaterials, often exhibiting distinct evolution of chemical and physical properties as material thickness is scaled from layered bulk to individual atomic or molecular monolayers. [1][2][3] While semiconducting 2D materials have received much attention, layered 2D semimetals other than graphene have been relatively underexplored in the atomically thin limit. Materials like β-MoTe 2 and WTe 2 stabilize as semimetals in a distortion of the octahedral 1T (CdI 2 structure) geometry, with in-plane buckled chains formed by pairs of Mo/W atoms dimerizing in intermetallic charge-exchange, 4-6 while van der Waals bonding dominates interlayer interaction. Whereas MoTe 2 may be synthesized in both 2H and 1T' polytypes, or reversibly switched between the two as a function of temperature or strain, 7, 8 WTe 2 has been known since the 1960s to adopt an orthorhombic structure with space group Pmn2 1 (sometimes called "Td"), irrespective of growth conditions 4, 5, 6, 9, 10 or conventional strain, 8 as the heaviest of the Group VI dichalcogenides.Despite the inaccessibility of a semiconducting phase, semimetallic WTe 2 has received renewed attention from the experimental observation of non-saturating magnetoresistance in bulk samples, in excess of 13,000,000% at 60 T. 11 This behavior was attributed to perfect compensation between balanced electron and hole populations at the Fermi surface below 150 K, projected to persist down to individual monolayers. 12,13 Recent studies have also identified WTe 2 as a potential contact for 2D semiconductors, with a relatively low workfunction (Φ < 4.4 eV) amongst 2D metals, 14 recently applied in realizing unipolar n-type transport in the typically ambipolar semiconductor WSe 2 . 15 Layer-dependent experiments of any kind are nonetheless limited, [16][17][18][19] owing to a lack of geological sources, challenges in precursor purifica...

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Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. Here we measure the thermal conductivity of crystalline AlN by the 3ω method, finding it ranges from 674 ± 56 Wm -1 K -1 at 100 K to 186 ± 7 Wm -1 K -1 at 400 K, with a value of 237 ± 6 Wm -1 K -1 at room temperature. We compare these data with analytical models and first principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 10% of heat conduction is contributed by phonons with long mean free paths (MFPs), over ~7 μm at room temperature, and 50% by phonons with MFPs over ~0.3 μm. Consequently, the effective thermal conductivity of AlN is strongly reduced in sub-micron thin films or devices due to phonon-boundary scattering.

The two-phase flow characterization (CO 2 /water) of a Triassic sandstone core from the Paris Basin, France, is reported in this paper. Absolute properties (porosity and water permeability), capillary pressure, relative permeability with hysteresis between drainage and imbibition, and residual trapping capacities have been assessed at 9 MPa pore pressure and 28 C (CO 2 in liquid state) using a single core-flooding apparatus associated with magnetic resonance imaging. Different methodologies have been followed to obtain a data set of flow properties to be upscaled and used in large-scale CO 2 geological storage evolution modeling tools. The measurements are consistent with the properties of well-sorted water-wet porous systems. As the mineralogical investigations showed a nonnegligible proportion of carbonates in the core, the experimental protocol was designed to observe potential impacts on flow properties of mineralogical changes. The magnetic resonance scanning and mineralogical observations indicate mineral dissolution during the experimental campaign, and the core-flooding results show an increase in porosity and water absolute permeability. The changes in two-phase flow properties appear coherent with the pore structure modifications induced by the carbonates dissolution but the changes in relative permeability could also be explained by a potential increase of the water-wet character of the core. Further investigations on the impacts of mineral changes are required with other reactive formation rocks, especially carbonate-rich ones, because the implications can be significant both for the validity of laboratory measurements and for the outcomes of in situ operations modeling.