Predictive Synthesis of Transition Metal Carbide via Thermochemical Oxocarbon Equilibrium.

Fabricating nanoscale metal carbides is a great challenge due to them having higher Gibbs free energy of formation (ΔG°) values than other metal compounds; additionally, these carbides have harsh calcination conditions, in which metal oxidation is preferred in the atmosphere. Herein, we report oxocarbon-mediated calcination for the predictive synthesis of nanoscale metal carbides. The thermochemical oxocarbon equilibrium of CO-CO2 reactions was utilized to control the selective redox reactions in multiatomic systems of Mo-C-O, contributing to the phase-forming and structuring of Mo compounds. By harnessing the thermodynamically predicted processing window, we controlled a wide range of Mo phases (MoO2, α-MoC1-x, and ß-Mo2C) and nanostructures (nanoparticle, spike, stain, and core/shell) in the Mo compounds/C nanofibers. By inducing simultaneous reactions of C-O (selective C combustion) and Mo-C (Mo carbide formation) in the nanofibers, Mo diffusion was controlled in C nanofibers, acting as a template for the nucleation and growth of Mo carbides and resulting in precise control of the phases and structures of Mo compounds. The formation mechanism of nanostructured Mo carbides was elucidated according to the CO fractions of CO-CO2 calcination. Moreover, tungsten (W) and niobium (Nb) carbides/C nanofibers have been successfully synthesized by CO-CO2 calcination. We constructed the thermodynamic map for the predictive synthesis of transition metal carbides to provide universal guideline via thermochemical oxocarbon equilibrium. We revealed that our thermochemical oxocarbon-mediated gas-solid reaction enabled the structure and phase control of nanoscale transition metal compounds to optimize the material-property relationship accordingly.

Robust Co alloy design for Co interconnects using a self-forming barrier layer.

With recent rapid increases in Cu resistivity, RC delay has become an important issue again. Co, which has a low electron mean free path, is being studied as beyond Cu metal and is expected to minimize this increase in resistivity. However, extrinsic time-dependent dielectric breakdown has been reported for Co interconnects. Therefore, it is necessary to apply a diffusion barrier, such as the Ta/TaN system, to increase interconnect lifetimes. In addition, an ultrathin diffusion barrier should be formed to occupy as little area as possible. This study provides a thermodynamic design for a self-forming barrier that provides reliability with Co interconnects. Since Cr, Mn, Sn, and Zn dopants exhibited surface diffusion or interfacial stable phases, the model constituted an effective alloy design. In the Co-Cr alloy, Cr diffused into the dielectric interface and reacted with oxygen to provide a self-forming diffusion barrier comprising Cr2O3. In a breakdown voltage test, the Co-Cr alloy showed a breakdown voltage more than 200% higher than that of pure Co. The 1.2 nm ultrathin Cr2O3 self-forming barrier will replace the current bilayer barrier system and contribute greatly to lowering the RC delay. It will realize high-performance Co interconnects with robust reliability in the future.

Phase Engineering of Transition Metal Dichalcogenides via a Thermodynamically Designed Gas-Solid Reaction.

Polymorph conversion of transition metal dichalcogenides (TMDs) offers intriguing material phenomena that can be applied for tuning the intrinsic properties of 2D materials. In general, group VIB TMDs can have thermodynamically stable 2H phases and metastable 1T/T' phases. Herein, we report key principles to apply carbon monoxide (CO)-based gas-solid reactions for a universal polymorph conversion of group VIB TMDs without forming undesirable compounds. We found that the process conditions are strongly dependent on the reaction chemical potential of cations in the TMDs, which can be predicted by thermodynamic calculations, and that polymorphic conversion is triggered by S vacancy (VS) formation. Furthermore, we conducted DFT calculations for the reaction barriers of VS formation and S diffusion to reveal the polymorph conversion mechanism of WS2 and compared it with that of MoS2. We believe that phase engineering 2D materials via thermodynamically designed gas-solid reactions could be functionally used to achieve defect-related nanomaterials.