Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices.

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to the memory cell dimensions. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in interfacial thermal resistance as GST transitions from cubic to hexagonal crystal structure, resulting in a factor of 4 reduction in the effective thermal conductivity. Simulations reveal that interfacial resistance between PCM and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~ 40% and ~ 50%, respectively. These thermal insights present a new opportunity to reduce power and operating currents in PCMs.

Constructing metallic nanoroads on a MoS2 monolayer via hydrogenation.

Monolayer transition metal dichalcogenides recently emerged as a new family of two-dimensional materials potentially suitable for numerous applications in electronic and optoelectronic devices due to the presence of a finite band gap. Many proposed applications require efficient transport of charge carriers within these semiconducting monolayers. However, constructing a stable conducting nanoroad on these atomically thin semiconductors is still a challenge. Here we demonstrate that hydrogenation on the surface of a MoS2 monolayer induces a semiconductor-metal transition, and strip-patterned hydrogenation is able to generate a conducting nanoroad. The band-gap closing arises from the formation of in-gap hybridized states mainly consisting of Mo 4d orbitals, as well as the electron donation from hydrogen to the lattice host. Ballistic conductance calculations reveal that such a nanoroad on the MoS2 surface exhibits an integer conductance, indicating small carrier scattering, and thus is ideal for serving as a conducting channel or an interconnect without compromising the mechanical and structural integrity of the monolayer.

Transition metal atoms pathways on rutile TiO2 (110) surface: distribution of Ti3+ states and evidence of enhanced peripheral charge accumulation.

Charge transfer between metal nanoparticles and the supported TiO2 surface is primarily important for catalytic applications as it greatly affects the catalytic activity and the thermal stability of the deposited nanoparticles on the surface. Herein, systematic spin-polarized density functional and HSE06 calculations are performed to evaluate the adsorption, diffusion, and charge state of several transition metal monomers on both stoichiometric and reduced rutile TiO2 (110) surface. Although the presence of oxygen vacancy (Ov) increases the binding of Au, Pt and Pd on the surface, it weakens the interaction thus enhancing the diffusion for Fe, Co, Ni, Ag, and Cu adatoms on the surface. For pristine reduced surface, only a small portion (around 5%) of the excess electrons occupy the topmost surface, which are mainly delocalized at the second nearest and third nearest fivefold coordinated Ti (Ti5c) atoms. Excess electrons populating at the Ti5c atoms on the basal plane can be transferred to strongly electronegative adsorbates like Au and Pt thus enabling a moderate adsorption at this site, whereas no stable adsorption is found for other less electronegative transition metal adatoms (Ag, Cu, Fe, Co, Ni, and Pd) on the reduced surface and for all the adatoms on stoichiometric surface. This result clarifies the origin of the experimental observation of the adsorption of O2 and CO molecules at Ti5c sites in connection with charge transfer. In addition, the spatial redistribution of the excess electrons around the Ov upon the adsorption of the monomers is thoroughly examined. Our finding of an accumulation of excess electrons at the Ti5c sites around the monomers explains the critical role of the perimeter interface of the deposited nanoparticles in promoting the adsorption and activation of reactants observed in experiments.

Strain-engineered surface transport in Si(001): complete isolation of the surface state via tensile strain.

By combining density functional theory, nonequilibrium Green's function formulism and effective-Hamiltonian approaches, we demonstrate strain-engineered surface transport in Si(001), with the complete isolation of the Si surface states from the bulk bands. Our results show that sufficient tensile strain can effectively remove the overlap between the surface valence state and the bulk valence band, because of the drastically different deformation potentials. Isolation of the surface valence state is possible with a tensile strain of ∼1.5%, a value that is accessible experimentally. Quantum transport simulations of a chemical sensing device based on strained Si(001) surface confirm the dominating surface conductance, giving rise to an enhanced molecular sensitivity. Our results show promise for using strain engineering to further our ability to manipulate surface states for quantum information processing and surface state-based devices.