The success of semiconductor electronics is built on the creation of compact, low-power switching elements that offer routing, logic, and memory functions. The availability of nanoscale optical switches could have a similarly transformative impact on the development of dynamic and programmable metasurfaces, optical neural networks, and quantum information processing. Phase change materials are uniquely suited to enable their creation as they offer high-speed electrical switching between amorphous and crystalline states with notably different optical properties. Their high refractive index has also been harnessed to fashion them into compact optical antennas. Here, we take the next important step by realizing electrically-switchable phase change antennas and metasurfaces that offer strong, reversible, non-volatile, multi-phase switching and spectral tuning of light scattering in the visible and near-infrared spectral ranges. Their successful implementation relies on a careful joint thermal and optical optimization of the antenna elements that comprise an Ag strip that simultaneously serves as a plasmonic resonator and a miniature heating stage.

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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Phase change memory (PCM) is an emerging data storage technology, however its programming is thermal in nature and typically not energy-efficient. Here we reduce the switching power of PCM through the combined approaches of filamentary contacts and thermal confinement. The filamentary contact is formed through an oxidized TiN layer on the bottom electrode, and thermal confinement is achieved using a monolayer semiconductor interface, three-atom thick MoS2. The former reduces the switching volume of the phase change material and yields a 70% reduction in reset current versus typical 150 nm diameter mushroom cells. The enhanced thermal confinement achieved with the ultra-thin (~6 Å) MoS2 yields an additional 30% reduction in switching current and power. We also use detailed simulations to show that further tailoring the electrical and thermal interfaces of such PCM cells toward their fundamental limits could lead up to a six-fold benefit in power efficiency.

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