Two-dimensional graphitic metal carbides: structure, stability and electronic properties.

Via first-principles computational modeling and calculations, we propose a new class of two-dimensional (2D) atomically thin crystals that contain metal-C3(MC3) moieties periodically distributed in a graphenic lattice, which we refer to as 2D graphitic metal carbides (g-MCs). Most g-MCs are dynamically stable as verified by the calculated phonon spectra. Our detailed chemical bonding analyzes reveal that the high stability of g-MCs can be attributed to a unique bonding feature, which manifests as the carbon-backbone-mediated metal-metal interactions. These analyzes provide new insights for understanding the stability of 2D materials. It is found that the calculated electronic band gaps and magnetic moments (per unit cell) of g-MCs can range from 0 to 1.30 eV and 0 to 4.40µB, respectively. Highly tunable electronic properties imply great potential of 2D g-MCs in various applications. As an example, we show that 2D g-MnC can be an excellent electrocatalyst towards CO2reductive reaction for the formation of formic acid with an exceptionally high loading of Mn atoms (∼43 wt%). We expect this work to simulate new experiments for fabrication and applications of g-MCs.

Efficient Fabrication of Stable Graphene-Molecule-Graphene Single-Molecule Junctions at Room Temperature.

We present a robust approach to fabricate stable single-molecule junctions at room temperature using single-layer graphene as nanoelectrodes. Molecular scale nano-gaps in graphene were generated using an optimized fast-speed feedback-controlled electroburning process. This process shortened the time for creating a single nano-gap to be less than one minute while keeping a yield higher than 97 %. To precisely control the gap position and minimize the effects of edge defects and the quantum confinement, extra-narrow grooves were pre-patterned in the graphene structures with oxygen plasma etching. Molecular junctions were formed by bridging the nano-gaps with amino-functionalized hexaphenyl molecules by taking advantage of chemical reactions between the amino groups at the two ends of the molecules and the carboxyl groups at the edges of graphene electrodes. Electronic transport measurements and transition voltage spectroscopy analysis verified the formation of single-molecule devices. First-principles quantum transport calculations show that the highest occupied molecular orbital of hexaphenyl is closer to the Fermi level of the graphene electrodes and thus the devices exhibit a hole-type transport characteristics. Some of these molecular devices remained stable up to four weeks, highlighting the potential of graphene nano-electrodes in the fabrication of stable single-molecule devices at room temperature.

Quantitative Interpretation of the Low-Bias Conductance of Au-Mesitylene-Au Molecular Junctions Formed from Mesitylene Monolayers.

The atomic structure and electronic transport properties of Au-mesitylene-Au molecular junctions formed from a mesitylene monolayer without any anchoring groups are investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. The intermolecular and adsorbate-substrate interactions are described by the non-local optB88 van der Waals functional. Two types of Au-mesitylene-Au molecular junctions are constructed, in which either an isolated mesitylene molecule or a mesitylene molecule embedded into a monolayer lying flat on one electrode surface is in contact with an atomic protrusion of the other electrode surface. The calculated low-bias conductance values of these two junctions are both in quantitative agreement with the reported experimental values [S. Afsari, Z. Li, and E. Borguet, Angew. Chem. Int. Ed. 2014, 53, 9771; Angew. Chem. 2014, 126, 9929]. This indicates that the measured conductance is intrinsic at the single-molecule Au-mesitylene-Au junction and that the intermolecular interactions in the mesitylene monolayer have little effect.

Effects of the molecule-electrode interface on the low-bias conductance of Cu-H2-Cu single-molecule junctions.

The atomic structure and electronic transport properties of a single hydrogen molecule connected to both symmetric and asymmetric Cu electrodes are investigated by using the non-equilibrium Green's function formalism combined with the density functional theory. Our calculations show that in symmetric Cu-H2-Cu junctions, the low-bias conductance drops rapidly upon stretching, while asymmetric ones present a low-bias conductance spanning the 0.2-0.3 G0 interval for a wide range of electrode separations. This is in good agreement with experiments on Cu atomic contacts in a hydrogen environment. Furthermore, the distribution of the calculated vibrational energies of the two hydrogen atoms in the asymmetric Cu-H2-Cu junction is also consistent with experiments. These findings provide clear evidence for the formation of asymmetric Cu-H2-Cu molecular junctions in breaking Cu atomic contacts in the presence of hydrogen and are also helpful for the design of molecular devices with Cu electrodes.

Revisiting the inelastic electron tunneling spectroscopy of single hydrogen atom adsorbed on the Cu(100) surface.

Inelastic electron tunneling spectroscopy (IETS) of a single hydrogen atom on the Cu(100) surface in a scanning tunneling microscopy (STM) configuration has been investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. The electron-vibration interaction is treated at the level of lowest order expansion. Our calculations show that the single peak observed in the previous STM-IETS experiments is dominated by the perpendicular mode of the adsorbed H atom, while the parallel one only makes a negligible contribution even when the STM tip is laterally displaced from the top position of the H atom. This propensity of the IETS is deeply rooted in the symmetry of the vibrational modes and the characteristics of the conduction channel of the Cu-H-Cu tunneling junction, which is mainly composed of the 4s and 4pz atomic orbitals of the Cu apex atom and the 1s orbital of the adsorbed H atom. These findings are helpful for deepening our understanding of the propensity rules for IETS and promoting IETS as a more popular spectroscopic tool for molecular devices.

Liquid-Metal-Printed Ultrathin Oxides for Atomically Smooth 2D Material Heterostructures.

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOTs), excellent gate control, and low leakage currents. Here, large-area liquid-metal-printed ultrathin Ga2O3 dielectrics for 2D electronics and optoelectronics are reported. The atomically smooth Ga2O3/WS2 interfaces enabled by the conformal nature of liquid metal printing are directly visualized. Atomic layer deposition compatibility with high-k Ga2O3/HfO2 top-gate dielectric stacks on a chemical-vapor-deposition-grown monolayer WS2 is demonstrated, achieving EOTs of ∼1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultrascaled low-power logic circuits. These results show that liquid-metal-printed oxides can bridge a crucial gap in dielectric integration of 2D materials for next-generation nanoelectronics.

Prominent nonequilibrium effects beyond the standard first-principles approach in nanoscale electronic devices.

The standard density functional theory (DFT) based first-principles approach has been widely used for modeling nanoscale electronic devices. A recent experiment, however, reported surprising transport properties of thiol-terminated silane junctions that cannot be understood using the standard DFT approach, presenting a severe challenge for the current computational understanding of electron transport at the nanoscale. Using the recently proposed steady-state DFT (SS-DFT) for nonequilibrium quantum systems, we found that in silane junctions, underlying the puzzling experimental observations is a novel type of intriguing nonequilibrium effect that is beyond the framework of the standard DFT approach. Our calculations show that the standard DFT approach is a good approximation of SS-DFT when silane junctions are near equilibrium, but the aforementioned nonequilibrium effects could drive the thiol-terminated silanes far away from equilibrium even at low biases of around 0.2 V. Further analysis suggests that these nonequilibrium effects could generally exist in nanoscale devices in which there are conducting channels mainly residing at the source contact and close to the bias window. These findings significantly broaden our fundamental understanding of electron transport at the nanoscale.