Variant Plateau's law in atomically thin transition metal dichalcogenide dome networks.

Since its fundamental inception from soap bubbles, Plateau's law has sparked extensive research in equilibrated states. However, most studies primarily relied on liquids, foams or cellular structures, whereas its applicability has yet to be explored in nano-scale solid films. Here, we observed a variant Plateau's law in networks of atomically thin domes made of solid two-dimensional (2D) transition metal dichalcogenides (TMDs). Discrete layer-dependent van der Waals (vdWs) interaction energies were experimentally and theoretically obtained for domes protruding in different TMD layers. Significant surface tension differences from layer-dependent vdWs interaction energies manifest in a variant of this fundamental law. The equivalent surface tension ranges from 2.4 to 3.6 N/m, around two orders of magnitude greater than conventional liquid films, enabling domes to sustain high gas pressure and exist in a fundamentally variant nature for several years. Our findings pave the way towards exploring variant discretised states with applications in opto-electro-mechanical devices.

Discovery of Type II Interlayer Trions.

In terms of interlayer trions, electronic excitations in van der Waals heterostructures (vdWHs) can be classified into Type I (i.e., two identical charges in the same layer) and Type II (i.e., two identical charges in the different layers). Type I interlayer trions are investigated theoretically and experimentally. By contrast, Type II interlayer trions remain elusive in vdWHs, due to inadequate free charges, unsuitable band alignment, reduced Coulomb interactions, poor interface quality, etc. Here, the first observation of Type II interlayer trions is reported by exploring band alignments and choosing an atomically thin organic-inorganic system-monolayer WSe2 /bilayer pentacene heterostructure (1L + 2L HS). Both positive and negative Type II interlayer trions are electrically tuned and observed via PL spectroscopy. In particular, Type II interlayer trions exhibit in-plane anisotropic emission, possibly caused by their unique spatial structure and anisotropic charge interactions, which is highly correlated with the transition dipole moment of pentacene. The results pave the way to develop excitonic devices and all-optical circuits using atomically thin organic-inorganic bilayers.

Enhanced Room Temperature Ferromagnetism in Highly Strained 2D Semiconductor Cr2Ge2Te6.

Emergent magnetism in van der Waals materials offers exciting opportunities in fabricating atomically thin spintronic devices. One pertinent obstacle has been the low transition temperatures (Tc) inherent to these materials, precluding room temperature applications. Here, we show that large structural gradients found in highly strained nanoscale wrinkles in Cr2Ge2Te6 (CGT) lead to significant increases of Tc. Magnetic force microscopy was utilized in characterizing multiple strained CGT nanostructures leading to experimental evidence of elevated Tc, depending on the strain percentage estimated from finite element analysis. Our findings are further supported by ab initio and DFT studies of the strained material, which indicates that strain directly augments the ferromagnetic coupling between Cr atoms in CGT, influenced by superexchange interaction; this provides strong insight into the mechanism of the enhanced magnetism and Tc.

Enhanced interactions of interlayer excitons in free-standing heterobilayers.

Strong, long-range dipole-dipole interactions between interlayer excitons (IXs) can lead to new multiparticle correlation regimes1,2, which drive the system into distinct quantum and classical phases2-5, including dipolar liquids, crystals and superfluids. Both repulsive and attractive dipole-dipole interactions have been theoretically predicted between IXs in a semiconductor bilayer2,6-8, but only repulsive interactions have been reported experimentally so far3,9-16. This study investigated free-standing, twisted (51°, 53°, 45°) tungsten diselenide/tungsten disulfide (WSe2/WS2) heterobilayers, in which we observed a transition in the nature of dipolar interactions among IXs, from repulsive to attractive. This was caused by quantum-exchange-correlation effects, leading to the appearance of a robust interlayer biexciton phase (formed by two IXs), which has been theoretically predicted6-8 but never observed before in experiments. The reduced dielectric screening in a free-standing heterobilayer not only resulted in a much higher formation efficiency of IXs, but also led to strongly enhanced dipole-dipole interactions, which enabled us to observe the many-body correlations of pristine IXs at the two-dimensional quantum limit. In addition, we firstly observed several emission peaks from moiré-trapped IXs at room temperature in a well-aligned, free-standing WSe2/WS2 heterobilayer. Our findings open avenues for exploring new quantum phases with potential for applications in non-linear optics.

Extraordinary Nonlinear Optical Interaction from Strained Nanostructures in van der Waals CuInP2S6.

Local strain engineering and structural modification of 2D materials furnish benevolent control over their optoelectronic properties and provide an exciting approach to tune light-matter interaction in layered materials. Application of strain at the nanoscale is typically obtained through permanently deformed nanostructures such as nanowrinkles, which yield large band gap modulation, photoluminescence enhancement, and surface potential. Ultrathin transition metal dichalcogenides (TMDs) have been greatly analyzed for such purposes. Herein, we extend strain-induced nanoengineering to an emerging 2D material, CuInP2S6 (CIPS), and visualize extraordinary control over nonlinear light-matter interaction. Wrinkle nanostructures exhibit ∼160-fold enhancement in second harmonic generation (SHG) compared to unstrained regions, which is additionally influenced by a change in the dielectric environment. The SHG enhancement was significantly modulated by the percentage of applied strain which was numerically estimated. Furthermore, polarization-dependent SHG revealed quenching and enhancement in the parallel and perpendicular directions, respectively, due to the direction of the compressive vector. Our work provides an important advancement in controlling optoelectronic properties beyond TMDs for imminent applications in flexible electronics.

Direct observation of contact resistivity for monolayer TMD based junctions via PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

Vibrational Properties in Highly Strained Hexagonal Boron Nitride Bubbles.

Hexagonal boron nitride (hBN) is widely used as a protective layer for few-atom-thick crystals and heterostructures (HSs), and it hosts quantum emitters working up to room temperature. In both instances, strain is expected to play an important role, either as an unavoidable presence in the HS fabrication or as a tool to tune the quantum emitter electronic properties. Addressing the role of strain and exploiting its tuning potentiality require the development of efficient methods to control it and of reliable tools to quantify it. Here we present a technique based on hydrogen irradiation to induce the formation of wrinkles and bubbles in hBN, resulting in remarkably high strains of ∼2%. By combining infrared (IR) near-field scanning optical microscopy and micro-Raman measurements with numerical calculations, we characterize the response to strain for both IR-active and Raman-active modes, revealing the potential of the vibrational properties of hBN as highly sensitive strain probes.

Experimental Adhesion Energy in van der Waals Crystals and Heterostructures from Atomically Thin Bubbles.

The formation of gas-filled bubbles on the surface of van der Waals crystals provides an ideal platform whereby the interplay of the elastic parameters and interlayer forces can be suitably investigated. Here, we combine experimental and numerical efforts to study the morphology of the bubbles at equilibrium and highlight unexpected behaviors that contrast with the common assumptions. We exploit such observations to develop an accurate analytical model to describe the shape and strain of the bubbles and exploit it to measure the adhesion energy between a variety of van der Waals crystals, showing sizable material-dependent trends.

2D Materials and Heterostructures at Extreme Pressure.

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Towards future physics and applications via two-dimensional material NEMS resonators.

Two-dimensional materials (2Dm) offer a unique insight into the world of quantum mechanics including van der Waals (vdWs) interactions, exciton dynamics and various other nanoscale phenomena. 2Dm are a growing family consisting of graphene, hexagonal-Boron Nitride (h-BN), transition metal dichalcogenides (TMDs), monochalcogenides (MNs), black phosphorus (BP), MXenes and 2D organic crystals such as small molecules (e.g., pentacene, C8 BTBT, perylene derivatives, etc.) and polymers (e.g., COF and MOF, etc.). They exhibit unique mechanical, electrical, optical and optoelectronic properties that are highly enhanced as the surface to volume ratio increases, resulting from the transition of bulk to the few- to mono- layer limit. Such unique attributes include the manifestation of highly tuneable bandgap semiconductors, reduced dielectric screening, highly enhanced many body interactions, the ability to withstand high strains, ferromagnetism, piezoelectric and flexoelectric effects. Using 2Dm for mechanical resonators has become a promising field in nanoelectromechanical systems (NEMS) for applications involving sensors and condensed matter physics investigations. 2Dm NEMS resonators react with their environment, exhibit highly nonlinear behaviour from tension induced stiffening effects and couple different physics domains. The small size and high stiffness of these devices possess the potential of highly enhanced force sensitivities for measuring a wide variety of un-investigated physical forces. This review highlights current research in 2Dm NEMS resonators from fundamental physics and an applications standpoint, as well as presenting future possibilities using these devices.

In-Plane Isotropic/Anisotropic 2D van der Waals Heterostructures for Future Devices.

Mono- to few-layers of 2D semiconducting materials have uniquely inherent optical, electronic, and magnetic properties that make them ideal for probing fundamental scientific phenomena up to the 2D quantum limit and exploring their emerging technological applications. This Review focuses on the fundamental optoelectronic studies and potential applications of in-plane isotropic/anisotropic 2D semiconducting heterostructures. Strong light-matter interaction, reduced dimensionality, and dielectric screening in mono- to few-layers of 2D semiconducting materials result in strong many-body interactions, leading to the formation of robust quasiparticles such as excitons, trions, and biexcitons. An in-plane isotropic nature leads to the quasi-2D particles, whereas, an anisotropic nature leads to quasi-1D particles. Hence, in-plane isotropic/anisotropic 2D heterostructures lead to the formation of quasi-1D/2D particle systems allowing for the manipulation of high binding energy quasi-1D particle populations for use in a wide variety of applications. This Review emphasizes an exciting 1D-2D particles dynamic in such heterostructures and their potential for high-performance photoemitters and exciton-polariton lasers. Moreover, their scopes are also broadened in thermoelectricity, piezoelectricity, photostriction, energy storage, hydrogen evolution reactions, and chemical sensor fields. The unique in-plane isotropic/anisotropic 2D heterostructures may open the possibility of engineering smart devices in the nanodomain with complex opto-electromechanical functions.

Many-Body Complexes in 2D Semiconductors.

2D semiconductors such as transition metal dichalcogenides (TMDs) and black phosphorus (BP) are currently attracting great attention due to their intrinsic bandgaps and strong excitonic emissions, making them potential candidates for novel optoelectronic applications. Optoelectronic devices fabricated from 2D semiconductors exhibit many-body complexes (exciton, trion, biexciton, etc.) which determine the materials optical and electrical properties. Characterization and manipulation of these complexes have become a reality due to their enhanced binding energies as a direct result from reduced dielectric screening and enhanced Coulomb interactions in the 2D regime. Furthermore, the atomic thickness and extremely large surface-to-volume ratio of 2D semiconductors allow the possibility of modulating their inherent optical, electrical, and optoelectronic properties using a variety of different environmental stimuli. To fully realize the potential functionalities of these many-body complexes in optoelectronics, a comprehensive understanding of their formation mechanism is essential. A topical and concise summary of the recent frontier research progress related to many-body complexes in 2D semiconductors is provided here. Moreover, detailed discussions covering the aspects of fundamental theory, experimental investigations, modulation of properties, and optoelectronic applications are given. Lastly, personal insights into the current challenges and future outlook of many-body complexes in 2D semiconducting materials are presented.

Modulated interlayer charge transfer dynamics in a monolayer TMD/metal junction.

The performance of optoelectronic devices based on monolayer transition-metal dichalcogenide (mTMD) semiconductors is significantly affected by the contact at the mTMD-metal interface, which is dependent on interlayer interactions and coupling. Here, we report a systematic optical method to investigate the interlayer charge transfer and coupling in a mTMD-metal heterojunction. Giant photoluminescence (PL) quenching was observed in a monolayer MoS2/Pd (1L MoS2/Pd) junction which is mainly due to the efficient interlayer charge transfer between Pd and MoS2. 1L MoS2/Pd also exhibits an increase in the PL quenching factor (η) as the temperature decreases, due to a reduction of the interlayer spacing. Annealing experiments were also performed which supported interlayer charge transfer as the main mechanism for the increase of η. Moreover, a monolayer MoS2/Au (1L MoS2/Au) junction was fabricated for engineering the interlayer charge transfer. Interestingly, a narrowing effect of the full width at half maximum (FWHM) was encountered as the junctions changed from 1L MoS2/SiO2 → 1L MoS2/Au → 1L MoS2/Pd, possibly originating from a change of the doping level induced weakening of exciton-carrier scattering. Our results deepen the understanding of metal-semiconductor junctions for further exploring fundamental phenomena and enabling high-performance devices using mTMD-metal junctions.

Probing the chaotic boundary of a membrane resonator with nanowire arrays.

Mechanically induced nonlinearities in nano-electromechanical systems (NEMSs) are typically avoided in design due to their unpredictable nature; however, by incorporating these normally unwanted nonlinear and chaotic phenomena, the performance of NEMS devices displays substantially different characteristics opening a broad new range of potential applications for their use. In this work, experiments have been conducted for probing the chaotic boundary of a circular membrane mechanical resonator with and without a silicone nanowire array (Si NWA). The NWA resonator can transition from linear to nonlinear quasi-periodic behaviour, and further transition into a chaotic state at resonance. Moreover, the NWA resonator demonstrated a high level of complex nonlinear behaviours, as the device expands the power spectral response from a single frequency at a linear regime to a wideband continuous frequency spectrum when chaotic behaviour was initiated; the threshold power of this transition decreased with a smaller NWA diameter. It was also observed that the NWA resonator had higher damping compared to the resonator without a NWA; however, as the vibration velocity of the NWA resonator increased, complex air damping and thin squeeze film damping lowered the threshold for probing the chaotic boundary condition of the NWA resonator.