Jiuxiang Dai

Abstract

Van der Waals stacking of two-dimensional crystals with rotation or mismatch in lattice constants gives rise to rich physical phenomena that are closely related to the strong correlations and band topology. Twisted graphene and silicene heterobilayers have been theoretically predicted to host a tunable transport gap due to the mismatch of Dirac cones in the graphene and silicene layers. However, experimental realization of such twisted structure is challenging. Here, we report the formation of twisted graphene/silicene bilayers on Ru (0001) crystal via intercalation. Different moiré patterns form as single-crystalline graphene grows over different grains of the Ru surface. After silicon intercalation, graphene/silicene bilayers are observed with different twisting angles on top of different grains of the Ru substrate. Our work provides a new pathway towards construction of graphene based twisted heterobilayers.

Author(s): Harley D. Scammell and Mathias S. Scheurer

We theoretically study a moiré superlattice geometry consisting of mirror-symmetric twisted trilayer graphene surrounded by identical transition metal dichalcogenide layers. We show that this setup allows us to switch on or off and control the spin-orbit splitting of the Fermi surfaces via applicati…

[Phys. Rev. Lett. 130, 066001] Published Fri Feb 10, 2023

Nature Communications, Published online: 10 February 2023; doi:10.1038/s41467-023-36447-7

Projective representations of crystal symmetries are indispensable for understanding artificial crystals. Here, authors establish a unified theory of projective crystal symmetries with time-reversal invariance, and construct models for all 458 projective symmetry algebras for the 17 two-dimensional wallpaper groups.

High-quality Bi₂O₂Te nanosheets and continuous films are grown using low-pressure chemical vapor deposition method. The construction of top single-crystalline native oxide Bi₂TeO₆ and bottom high-mobility Bi₂O₂Te heterostructure is demonstrated via O intercalative oxidation at elevated temperatures in air, with an atomically sharp and low-stress interface, indicating the potential of Bi₂O₂Te with native oxide in planar integrated functional nanoelectronics.

Abstract

Following logic in the silicon semiconductor industry, the existence of native oxide and suitable fabrication technology is essential for 2D semiconductors in planar integronics, which are surface-sensitive to typical coating technologies. To date, very few types of integronics are found to possess this feature. Herein, the 2D Bi₂O₂Te developed recently is reported to possess large-area synthesis and controllable thermal oxidation behavior toward single-crystal native oxides. This shows that surface-adsorbed oxygen atoms are inclined to penetrate across [Bi₂O₂]_n ²ⁿ⁺ layers and bond with the underlying [Te]_n ²ⁿ⁻ at elevated temperatures, transforming directly into [TeO₄]_n ²ⁿ⁻ with the basic architecture remaining stable. The oxide can be adjusted to form in an accurate layer-by-layer manner with a low-stress sharp interface. The native oxide Bi₂TeO₆ layer (bandgap of ≈2.9 eV) exhibits visible-light transparency and is compatible with wet-chemical selective etching technology. These advances demonstrate the potential of Bi₂O₂Te in planar-integrated functional nanoelectronics such as tunnel junction devices, field-effect transistors, and memristors.

Applied Physics Letters, Volume 122, Issue 6, February 2023.
Monolayer group monochalcogenides (MX; M = Sn, Ge; X = S, Se) in the orthogonal α-phase are excellent piezoelectric materials. In this study, a configuration with bonding features similar to the α-phase is proposed (T-phase) for monolayer MX using the first-principles method. Based on the modern theory of polarization, as implemented in Vienna Ab initio Simulation Package, the T-phase is determined to be an excellent piezoelectric phase for monolayer MX. The in-plane piezoelectric coefficient d11 of T-SnS is 452.3 pm/V, which is larger than that reported for most two-dimensional binary compounds in the α-phase, including α-SnSe (∼250 pm/V). The large piezoelectric coefficients of T-MX mainly stem from its distinctive puckered configuration, which make it extraordinarily flexible along the polarization direction. The study results suggest a possibility for designing high piezoelectric coefficient materials with MX, and the potential application of T-MX in the fields of energy collection and nanoelectromechanical systems needs to be analyzed in future studies.

Nature Synthesis, Published online: 09 February 2023; doi:10.1038/s44160-023-00242-5

Manipulating the properties of polymeric thin films independent of their chemistry is challenging. Now, a vapour solvation strategy is introduced to achieve targeted properties, including molecular weight, mechanical strength and film morphology, without the need for chemical modification.

Funnel devices are developed using WS₂ and MoS₂ transferred onto a fork-shaped SU-8 microstructure. Atomic force microscope, Raman, and photoluminescence measurements indicate that asymmetric strains are introduced to the transferred transition metal dichalcogenides. The scanning photocurrent mapping images acquired from various funnel devices follow a fork-shaped pattern, demonstrating that funneled excitons can be converted to electrical currents.

Abstract

The strain applied to transition metal dichalcogenides (TMDs) reduces their energy bandgap, and local strains result in a funnel-like band structure in which funneled excitons move toward the most strained region. Herein, a funnel device based on asymmetrically strained WS₂ and MoS₂ is reported. Asymmetric strains are induced by transferring the TMD flakes onto a fork-shaped SU-8 microstructure. Raman and photoluminescence spectra peaks are shifted according to the morphology of the SU-8 microstructure, indicating the application of asymmetric strains to the TMDs. To investigate whether funneled excitons can be converted to electrical currents, various devices are constructed by depositing symmetric and asymmetric electrodes onto the strained TMDs. The scanning photocurrent mapping images follow a fork-shaped pattern, indicating probable conversion of the funneled excitons into electrical currents. In the case of the funnel devices with asymmetric Au and Al electrodes, short-circuit current (I _SC) of WS₂ is enhanced by the strains, whereas I _SC of MoS₂ is suppressed because the Schottky barrier lowers with increasing strain for the MoS₂. These results demonstrate that the funnel devices can be implemented using asymmetrically strained TMDs and the effect of strains on the Schottky barrier is dependent on the TMD used.

A new approach for the bottom-up growth of free-standing 2D InSb nanostructures (“nanoflakes”) is presented, which employs colliding InSb nanowires. The study investigates the crystal structure and the formation dynamics of these nanoflakes and studies their transport properties. The growth method presented in this study can be exploited to fabricate complex hybrid semiconductor-superconductor devices to study novel quantum phenomena.

Abstract

Indium Antimonide (InSb) is a semiconductor material with unique properties, that are suitable for studying new quantum phenomena in hybrid semiconductor-superconductor devices. The realization of such devices with defect-free InSb thin films is challenging, since InSb has a large lattice mismatch with most common insulating substrates. Here, the controlled synthesis of free-standing 2D InSb nanostructures, termed as “nanoflakes”, on a highly mismatched substrate is presented. The nanoflakes originate from the merging of pairs of InSb nanowires grown in V-groove incisions, each from a slanted and opposing {111}B facet. The relative orientation of the two nanowires within a pair, governs the nanoflake morphologies, exhibiting three distinct ones related to different grain boundary arrangements: no boundary (type-I), Σ3- (type-II), and Σ9-boundary (type-III). Low-temperature transport measurements indicate that type-III nanoflakes are of a relatively lower quality compared to type-I and type-II, based on field-effect mobility. Moreover, type-III nanoflakes exhibit a conductance dip attributed to an energy barrier pertaining to the Σ9-boundary. Type-I and type-II nanoflakes exhibit promising transport properties, suitable for quantum devices. This platform hosting nanoflakes next to nanowires and nanowire networks can be used to selectively deposit the superconductor by inter-shadowing, yielding InSb-superconductor hybrid devices with minimal post-fabrication steps.

Applied Physics Letters, Volume 122, Issue 6, February 2023.
Cd3As2 provides an excellent platform for studying the physics of three-dimensional Dirac semimetals due to its stability as well as its compatibility with thin film growth. Crystals made using both bulk and thin film synthesis are unintentionally doped n-type, and other than introducing Zn to reduce the carrier concentration, no efforts have been reported to alter this intrinsic doping without major changes to the band structure. Here, group VI elements Te and Se are introduced during epitaxy to increase the electron concentration of the films. Starting from an unintentionally doped electron concentration of 1–2 × 1017 cm−3, concentrations of up to 3 × 1018 cm−3 are achieved. Analysis of Shubnikov–de Haas oscillations reveals good agreement in calculated effective mass and Fermi velocity of highly doped films with unintentionally doped single crystals with similar electron concentrations. The density functional theory is also performed to study the effects of group VI substitutions and confirms no strong perturbations in the electronic structure. This work ultimately demonstrates tunability in the carrier concentration using extrinsic dopants without substantial changes in the band structure, allowing for intentional design of Fermi-level position for device applications.

Applied Physics Letters, Volume 122, Issue 6, February 2023.
Temperature-dependent photoluminescence was used to investigate the impact of H on the optical properties of α-Ga2O3 films grown by halide vapor phase epitaxy. An additional UV luminescence line centered at 3.8 eV is observed at low temperatures, which strongly correlates with the concentration of H in the films. This luminescence line is assigned to donor–acceptor pair recombination involving an H-related shallow donor and H-decorated Ga vacancy (VGa-nH) as the acceptor, where n = 1, 2, 3. Previous reports have already suggested the impact of H on the electrical properties of Ga2O3, and the present study shows its clear impact on the optical properties of α-Ga2O3.

A complementary metal oxide semiconductor inverter array with 3D integrated architecture has been fabricated with high device yield, based on large-scale n-MoS₂ and p-MoTe₂ grown by chemical vapor deposition. The transfer curves of n-MoS₂ and p-MoTe₂ field-effect transistors are balanced by proper processing conditions. The typical voltage gain and power consumption are 4.2 and 0.11 nW, respectively, at V _DD of 1 V.

Abstract

In this work, monolithic three-dimensional complementary metal oxide semiconductor (CMOS) inverter array has been fabricated, based on large-scale n-MoS₂ and p-MoTe₂ grown by the chemical vapor deposition method. In the CMOS device, the n- and p-channel field-effect transistors (FETs) stack vertically and share the same gate electrode. High k HfO₂ is used as the gate dielectric. An Al₂O₃ seed layer is used to protect the MoS₂ from heavily n-doping in the later-on atomic layer deposition process. P-MoTe₂ FET is intentionally designed as the upper layer. Because p-doping of MoTe₂ results from oxygen and water in the air, this design can guarantee a higher hole density of MoTe₂. An HfO₂ capping layer is employed to further balance the transfer curves of n- and p-channel FETs and improve the performance of the inverter. The typical gain and power consumption of the CMOS devices are about 4.2 and 0.11 nW, respectively, at V _DD of 1 V. The statistical results show that the CMOS array is with high device yield (60%) and an average voltage gain value of about 3.6 at V _DD of 1 V. This work demonstrates the advantage of two-dimensional semi-conductive transition metal dichalcogenides in fabricating high-density integrated circuits.

This work reports on ultra-high interfacial thermal conductance in encapsulated van der Waals heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂, WSe₂, WS₂) sandwiched between two hexagonal boron nitride (hBN) layers is reported. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂/hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.

Abstract

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂, WSe₂, WS₂) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX₂/hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂ and hBN reaches 74 ± 25 MW m⁻² K⁻¹, which is at least ten times higher than the interfacial thermal conductance of MX₂ in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂/hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.

Topological metal molybdenum phosphide (MoP) nanowires are demonstrated as a new class of interconnect material for on-chip applications. Owing to its high carrier density, low resistivity, and short electron mean free path, MoP nanowires exhibit dimensional scaling of resistivity far superior to effective copper (Cu) and ruthenium (Ru). MoP is also oxidation resistant and has a high cohesive energy, potentially enabling a barrier-free interconnect design. Thus, this work demonstrates MoP as a breakthrough material for interconnect technologies for continued downscaling of integrated circuits and future energy-efficient computing.

Abstract

The increasing resistance of copper (Cu) interconnects for decreasing dimensions is a major challenge in continued downscaling of integrated circuits beyond the 7 nm technology node as it leads to unacceptable signal delays and power consumption in computing. The resistivity of Cu increases due to electron scattering at surfaces and grain boundaries at the nanoscale. Topological semimetals, owing to their topologically protected surface states and suppressed electron backscattering, are promising candidates to potentially replace current Cu interconnects. Here, we report the unprecedented resistivity scaling of topological metal molybdenum phosphide (MoP) nanowires, and it is shown that the resistivity values are superior to those of nanoscale Cu interconnects <500 nm² cross-section areas. The cohesive energy of MoP suggests better stability against electromigration, enabling a barrier-free design . MoP nanowires are more resistant to surface oxidation than the 20 nm thick Cu. The thermal conductivity of MoP is comparable to those of Ru and Co. Most importantly, it is demonstrated that the dimensional scaling of MoP, in terms of line resistance versus total cross-sectional area, is competitive to those of effective Cu with barrier/liner and barrier-less Ru, suggesting MoP is an attractive alternative for the scaling challenge of Cu interconnects.

npj 2D Materials and Applications, Published online: 07 February 2023; doi:10.1038/s41699-023-00371-7

Monolithic 3D integration of back-end compatible 2D material FET on Si FinFET

Nature Electronics, Published online: 06 February 2023; doi:10.1038/s41928-022-00911-x

Multilayers of hexagonal boron nitride can be grown using a chemical vapour deposition process on iron–nickel foil and integrated into a large array of graphene devices that exhibit room-temperature carrier mobilities of up to around 10,000 cm2 V−1 s−1.

Nature Electronics, Published online: 06 February 2023; doi:10.1038/s41928-023-00917-z

Multilayer hexagonal boron nitride can be synthesized over large areas and used to enhance mobility in graphene heterostructures, illustrating the potential of the material as an insulator in commercial two-dimensional electronics.

Nature Physics, Published online: 06 February 2023; doi:10.1038/s41567-022-01900-9

Majorana zero modes are emergent excitations in topological superconductors. This Perspective introduces the physics of these modes, recaps the recent history of the experimental search for them and discusses the future prognosis for success.