Approaching the quantum limit in two-dimensional semiconductor contacts.

The development of next-generation electronics requires scaling of channel material thickness down to the two-dimensional limit while maintaining ultralow contact resistance1,2. Transition-metal dichalcogenides can sustain transistor scaling to the end of roadmap, but despite a myriad of efforts, the device performance remains contact-limited3-12. In particular, the contact resistance has not surpassed that of covalently bonded metal-semiconductor junctions owing to the intrinsic van der Waals gap, and the best contact technologies are facing stability issues3,7. Here we push the electrical contact of monolayer molybdenum disulfide close to the quantum limit by hybridization of energy bands with semi-metallic antimony ([Formula: see text]) through strong van der Waals interactions. The contacts exhibit a low contact resistance of 42 ohm micrometres and excellent stability at 125 degrees Celsius. Owing to improved contacts, short-channel molybdenum disulfide transistors show current saturation under one-volt drain bias with an on-state current of 1.23 milliamperes per micrometre, an on/off ratio over 108 and an intrinsic delay of 74 femtoseconds. These performances outperformed equivalent silicon complementary metal-oxide-semiconductor technologies and satisfied the 2028 roadmap target. We further fabricate large-area device arrays and demonstrate low variability in contact resistance, threshold voltage, subthreshold swing, on/off ratio, on-state current and transconductance13. The excellent electrical performance, stability and variability make antimony ([Formula: see text]) a promising contact technology for transition-metal-dichalcogenide-based electronics beyond silicon.

Mechanisms of Controllable Growth and Ohmic Contact of Two-Dimensional Molybdenum Disulfide: Insight from Atomistic Simulations.

ConspectusTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), in particular molybdenum disulfide (MoS2), have recently attracted huge interest due to their proper bandgap, high mobility at 2D limit, and easy-to-integrate planar structure, which are very promising for extending Moore's law in postsilicon electronics technology. Great effort has been devoted toward such a goal since the demonstration of protype MoS2 devices with high room-temperature on/off current ratios, ultralow standby power consumption, and atomic level scaling capacity down to sub-1-nm technology node. However, there are still several key challenges that need to be addressed prior to the real application of MoS2-based electronics technology. The controllable growth of wafer-scale single-crystal MoS2 on industry-compatible insulating substrates is the prerequisite of application while the currently synthesized MoS2 films mostly are polycrystalline with limited sizes of single-crystal domains and may involve metal substrates. The precise layer-control is also very important for MoS2 growth since its electronic properties are layer-dependent, whereas the layer-by-layer growth of multilayer MoS2 dominated by the van der Waals (vdW) epitaxy leads to poor thickness uniformity and noncontinuously distributed domains. High density up to 1013 cm-2 of sulfur vacancies (SVs) in grown MoS2 can cause unfavorable carrier scatting and electronic properties variations and will inevitably disturb the device performance. The dangling-bond-free surface of MoS2 gives rise to an inherent vdW gap at metal-semiconductor (M-S) contact, which leads to high electrical resistance and poor current-delivery capability at the contact interface and thereby substantially limits the performances of MoS2 devices.In this Account, we briefly review recent experimental and theoretical attempts for addressing the aforementioned challenges and present our own insights from atomistic simulations. We theoretically revealed the vital role of substrate steps for guiding unidirectional nucleation of monolayer MoS2 and uniform nucleation and edge-aligned growth of bilayer MoS2 by advanced simulations. The established thermodynamic mechanisms have successfully directed the experimental works on the controllable growth of 2 in. single-crystal monolayer and centimeter-scale uniform bilayer MoS2. The postgrowth repair mechanism of SV defect in MoS2 via thiol chemistry treatment has been theoretically explored with the consideration of side reaction of surface functionalization to help experimentally reduce SV defect density by 75%. Beyond the atomic level understanding, theoretical simulations proposed the electronic states hybridization mechanism across the semimetal-MoS2 vdW interface, thereby guiding experimental effort for realizing Ohmic contact at the MoS2-Sb(0112) vdW interface with record-low contact resistance.These advances provide a sound basis with an atomic-level understanding for addressing the related issues. However, there are still notable gaps in terms of system size and time scale of dynamics between atomistic simulations and experimental observations for the studies of MoS2 growth and interfaces. The combination of multiscale simulations and artificial intelligence technology is expected to narrow these gaps and provide a more insightful understanding of the controllable growth and interfacial properties modulation of MoS2. We conclude the Account with the standing challenges and outlook on future research directions from the theoretical perspective.

The formation and stability of junctions in single-wall carbon nanotubes.

The structure and stability of molecular junctions, which connect two single-wall carbon nanotubes (SWCNTs) of different diameters and chiral angles, (n 1, m 1)-(n 2, m 2), are systematically investigated by density functional tight binding calculations. More than 100 junctions, which connect well-aligned SWCNTs, were constructed and calculated. For a highly stable junction between two chiral (n 1, m 1) and (n 2, m 2) SWCNTs with opposite handedness, the number of pentagon-heptagon (5/7) pairs required to build the junction can be denoted as ∣∣n 2 - n 1∣ - âˆ£m 2 - m 1∣∣+min{∣n 2 - n 1∣, ∣m 2 - m 1∣} with (n 2, m 2) rotating π/3 angle or not. While for a junction connected by two zigzag, armchair or two chiral SWCNTs with the same handedness, the number of 5/7 pairs is equal to ∣n 1 - n 2∣ + âˆ£m 1 - m 2∣. Similar to the formation energies of grain boundaries in graphene, the curve of the formation energies vs. chiral angle difference present an 'M' shape indicating the preference of ∼30 degree junctions. Moreover, the formation energies of the zigzag-type and armchair-type junctions with zero misorientation angles are largely sensitive to the diameter difference of two sub-SWCNTs.

Towards the selective growth of two-dimensional ordered CxNy compounds via epitaxial substrate mediation.

Two-dimensional (2D) ordered carbon-nitrogen binary compounds (CxNy) show great potential in many fields owing to their diverse structures and outstanding properties. However, the scalable and selective synthesis of 2D CxNy compounds remain a challenge due to the variable C/N stoichiometry induced coexistence of graphitic, pyridinic, and pyrrolic N species and the competitive growth of graphene. Here, this work systematically explored the mechanism of selective growth of a series of 2D ordered CxNy compounds, namely, the g-C3N4, C2N, C3N, and C5N, on various epitaxial substrates via first-principles calculations. By establishing the thermodynamic phase diagram, it is revealed that the individualized surface interaction and symmetry match between 2D CxNy compounds and substrates together enable the selective epitaxial growth of single crystal 2D CxNy compounds within distinct chemical potential windows of feedstock. The kinetics behaviors of the diffusion and attachment of the decomposed feedstock C/N atoms to the growing CxNy clusters further confirmed the feasibility of the substrate mediated selective growth of 2D CxNy compounds. Moreover, the optimal experimental conditions, including the temperature and partial pressure of feedstock, are suggested for the selective growth of targeted 2D CxNy compound on individual epitaxial substrates by carefully considering the chemical potential of carbon/nitrogen as the functional of experimental parameters based on the standard thermochemical tables. This work provides an insightful understanding on the mechanism of selective epitaxial growth of 2D ordered CxNy compounds for guiding the future experimental design.

Mechanism of Thermodynamically Rationalized Selective Growth of a Two-Dimensional Ternary Ferromagnet on Insulating Substrates.

Two-dimensional (2D) semiconducting ferromagnet Fe3GeTe2 holds great promise for advanced spintronic applications because of its gate-tunable ferromagnetic ordering at room temperature, whereas the controllable growth of large-area single crystals remains very challenging due to its ternary nature and variable stoichiometry inducing many competitive phases. Here, we theoretically probe the mechanism of selective growth of monolayer Fe3GeTe2 on various epitaxial substrates. Thermodynamic analysis shows that the corresponding phase-pure chemical potential windows for the selective growth of Fe3GeTe2 can be reasonably attained in ternary phase space on insulating and chemically inert c-plane sapphire and Ga2O3(0001) substrates by properly modulating the interfacial interaction and employing suitable feedstocks to avoid competitive growth of possible impurity phases with different stoichiometry ratios. It is also revealed that both the weak edge-substrate interaction and interlayer coupling of Fe3GeTe2 together lead to a surface-dominated nucleation behavior and, thereby, energetically favor lateral growth of the monolayer rather than vertical growth of the multilayer. Importantly, straight protocols for the experimentally selective growth of phase-pure ternary Fe3GeTe2 are also provided by establishing the relationship between the feedstock chemical potential and growth parameters on a thermochemical basis. Our insightful study can also be reasonably extended to guide future experimental design for the selective growth of other multicomponent 2D materials.

Thermodynamic mechanism of controllable growth of two-dimensional uniformly ordered boron-doped graphene.

Two-dimensional (2D) boron-doped graphene (B-G) exhibits remarkable properties for advanced applications in electronics, sensing and catalysis. However, the synthesis of large-area uniformly ordered 2D B-G remains a grand challenge due to the low doping level and uncontrolled distribution of dopants or even the phase separation from the competitive growth of boron polymorphs and graphene. Here, we theoretically explored the mechanism of the epitaxial growth of 2D uniformly ordered B-G on a metal substrate via ab initio calculations. We show that, by establishing the substrate-mediated thermodynamic phase diagrams, the controllable growth of 2D ordered B-G with different B/C stoichiometry can be achieved on appropriate substrates within distinct chemical potential windows of the feedstock by beating the competitive growth of graphene and other impurity phases. It is suggested that a suitable substrate for the controllable epitaxial growth of 2D ordered B-G can be efficiently screened based on the symmetry match and interaction between 2D B-G and the surfaces. Importantly, by carefully considering the chemical potential of boron/carbon as a function of temperature and partial pressure of the feedstock with the aid of the standard thermochemical tables, the optimal experimental parameters for the controllable growth of 2D ordered B-G are also suggested accordingly. This work provides a comprehensive and insightful understanding of the mechanism of controllable growth of 2D B-G, which will guide future experimental design.

Mechanism of Controllable Growth of Large-Area Single-Crystal Hexagonal Boron Nitride on Preoxidized Copper Substrate.

Two-dimensional (2D) hexagonal boron nitride (h-BN) exhibits promising properties for electronic and photoelectric devices, while the growth of high-quality h-BN remains challenging. Here we theoretically explored the mechanism of epitaxial growth of high-quality h-BN by using the preoxidized and hydrogen-annealed copper substrate, i.e., Cu2O. It is revealed thermodynamically that the unidirectional nucleation of h-BN can be rationalized on the symmetry-matched Cu2O(111) surface rather than the antiparallel nucleation on the Cu(111) surface. Kinetically, the dehydrogenation of feedstock of h-BN on the Cu2O(111) surface is also much easier than that on the Cu(111) surface. Both the B and N atoms are energetically more preferred to stay on the surface rather than inside the body of Cu2O, which leads to a surface-diffusion-based growth behavior on the Cu2O(111) surface instead of the precipitation-diffusion mixed case on the Cu(111) surface. Our work may guide future experimental design for the controllable growth of wafer-scale single-crystal h-BN.

The Intrinsic Thermodynamic Difficulty and a Step-Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness.

Multilayer MoS2 shows superior performance over the monolayer MoS2 for electronic devices while the growth of multilayer MoS2 with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS2 domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS2 domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS2 domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS2 on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS2 with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS2 films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Hormone Replacement Versus Natural Cycle Protocols of Endometrial Preparation for Frozen Embryo Transfer.

Research question: Endometrial preparation is one of the most important steps for ensuring frozen embryo transfer success. However, there is no clear evidence that identifies an optimal endometrial preparation protocol for frozen embryo transfer. In addition, in studies that assessed which were the optimal endometrial preparation protocols, few analyzed the stage and the number of embryos. This study compared the pregnancy outcomes and perinatal obstetric complications of patients who were transferred two cleavage-stage (day 2 or day 3) frozen embryos with the natural cycle and those with the hormone replacement therapy cycle. Design: This study was a secondary analysis of data from a multicentre randomized controlled trial designed to compare the pregnancy and perinatal outcomes after frozen versus fresh embryo transfer. In this study, a total of 908 patients who were transferred two cleavage-stage (day 2 or day 3) embryos in the original trial were analyzed. Pregnancy outcomes and perinatal obstetric complications after the natural cycle and the hormone replacement therapy cycle were compared. Result: We found the endometrium in the natural group was significantly thicker than the hormone replacement therapy cycle group (p<0.01). The implantation rate (42.6% vs 37.3% p=0.049) showed a significant difference between the natural cycle group and the hormone replacement therapy cycle group. Compared to the natural cycle group, the hormone replacement therapy cycle group was associated with an increased risk of caesarean section (72.3% vs 84.5, p=0.009). Conclusion: The natural cycle protocol yielded thicker endometria, a higher implantation rate and a lower risk of caesarean section than the hormone replacement therapy protocol in the transfer of two cleavage-stage frozen embryos. The natural cycle protocol was the better endometrial preparation protocol for frozen embryo transfer.