NitingZ

Quantum phase transition (QPT) is the keystone in understanding the quantum state of matter. Here we report observation of QPT and its evolution in nanosheets of Li x MoS 2 with various degrees of disorder introduced by progressive thermal annealing. The material exhibits onset of superconducting transition at temperatures ranging between 3 and 6 K. We show that the angular dependence of the upper critical field is well described by Tinkham’s (2D) model. Finite-size scaling analysis reveals that the pristine samples with predominantly T/T′ phases exhibit magnetic-field driven superconductor-to-metal QPT. We further show that annealed samples with a higher content of 2H phase exhibit QPT with diverging critical exponents similar to the recently reported behaviours of highly crystalline 2D superconductors.

The number of two-dimensional (2D) materials has grown steadily since the discovery of graphene. Each new 2D material demonstrated unusual physical properties offering a large flexibility in their tailoring for high-tech applications. Here, we report on the formation and characterization of an uncharted 2D material: ‘Cu 2 Te alloy monolayer on Cu(111) surface’. We have successfully grown a 2D binary Te-Cu alloy using a straightforward approach based on chemical deposition method. Low electron energy diffraction (LEED) and scanning tunneling microscopy (STM) results reveal the existence of a well-ordered alloy monolayer characterized by (√3 × √3)R30° superstructure, while the x-ray photoemission spectroscopy (XPS) measurements indicate the presence of single chemical environment of the Te atoms associated with the Te-Cu bonding. Analysis of the valence band properties by angle resolved photoemission spectroscopy (ARPES); in particular the electronic states close to the F...

Transition metal dichalcogenide (TMD) represented by molybdenum disulfide (MoS 2 ) is a promising platform for versatile applications in biosensing. The semiconducting properties of MoS 2 coupled with its large surface area allows for highly sensitive detection of external environments. Although chemical vapor deposition (CVD) is a promising method for synthesizing large and high quality MoS 2 , optical properties of CVD-grown MoS 2 under physiological solution is not clear. Furthermore, understanding of the pH effect on its optical properties is still limited. Here, we report their photoluminescence (PL) responses to solution pH under aqueous condition by means of PL spectroscopy and imaging. MoS 2 with different Mo sources and additive such as MoS 2 , MoO 2 and NaCl were synthesized. It was found that the PL response was highly affected by the Mo sources. While MoS 2 made from MoO 2 s...

The concept of two-dimensional (2D) materials was first proposed after the first successful separation of graphene, a monoatomic layered material. 2D materials are referred to materials in which electrons can only move freely on the nanoscale in two dimensions, and have expanded to also include the transition metal dichalcogenides (TMDs), black phosphorus (BP) and hexagonal boron nitride (hBN). Different 2D materials have unique electrical or optical characteristics due to the special properties of the crystal structure, and are widely used as field-effect transistors and optoelectronic devices. Moreover, based on the electrochemical and fluorescence quenching properties, many researchers have developed and fabricated light-switching and current-switching sensors for various medical diagnostics. In this paper, we summarize the characteristics of 2D materials and introduce their photoelectric properties, and review the recent applications of 2D materials based switching sensors f...

Carbon nanomaterials such as graphene exhibit unique material properties including high electrical conductivity, surface area, and biocompatibility that have the potential to significantly improve the performance of electrochemical sensors. Since in-field electrochemical sensors are typically disposable, they require materials that are amenable to low-cost, high-throughput, and scalable manufacturing. Conventional graphene devices based on low-yield chemical vapor deposition techniques are too expensive for such applications, while low-cost alternatives such as screen and inkjet printing do not possess sufficient control over electrode geometry to achieve favorable electrochemical sensor performance. In this work, aerosol jet printing (AJP) is used to create high-resolution (∼40 μ m line width) interdigitated electrodes (IDEs) on flexible substrates, which are then converted into histamine sensors by covalently linking monoclonal antibodies to oxygen moieties created on th...

2D materials support unique excitations of quasi-particles that consist of a material excitation and photons called polaritons. Especially interesting are in-plane propagating polaritons, which can be confined to a single monolayer and carry large momentum. In this work, we theoretically predict the existence of a new type of in-plane propagating polariton, supported on monolayer transition-metal-dicalcogonides (TMDs) in the visible spectrum. This 2D in-plane exciton-polariton (2DEP) is described by the coupling of an electromagnetic light field with the collective oscillations of the excitons supported by monolayer TMDs. We experimentally demonstrate the specific conditions required for the excitation of the 2DEP and show that these can be achieved if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and cooled to cryogenic temperatures. In addition, we compare the properties of the 2DEP with those of the surface-plasmon-polariton at the same spectral range, and find t...

In layered magnetic materials, the magnetic coupling between neighboring van der Waals layers is challenging to understand and anticipate, although the exchange interaction inside a layer can be well rationalized for example by the superexchange mechanism. In this work, we elucidate the interlayer exchange mechanism and propose an electron-counting rule to determine the interlayer magnetic order between van der Waals layers, based on counting the d -orbital occupation ( d n , where n is the number of d -electrons at the magnetic cation). With this rule, we classify magnetic monolayers into two groups, type-I ( ##IMG## [http://ej.iop.org/images/2053-1583/7/4/045010/tdmab9ea4ieqn1.gif] {$n\lt5$} ) and type-II ( n  ≥ 5), and derive three types of interlayer magnetic coupling for both insulators and metals. The coupling between two type-II layers prefers the antiferromagnetic (AFM) order, while type-I and type-II i...

Following the graphene isolation, strong interest in two dimensional (2D) materials has been driven by their outstanding properties. Their typical intrinsic structure, including strong in-plane covalent bonding and weak out-of-plane Van der Waals interaction, makes them highly promising in diverse areas such as electronics, catalysis, and environment. Growth of 2D materials requires a synthesis approach able to control the deposition onto a support at the atomic scale. Thanks to their simplicity, versatility and ability to control thickness at the angstrom level, atomic layer deposition (ALD) and its variant atomic layer etching (ALET) appear as ones of the most suited techniques to synthesize 2D materials. The development of ALD technique for fabricating 2D materials in the last ten years justifies reviewing its most recent groundbreaking discoveries and progresses. Particular attention will be paid to stable 2D materials especially graphene, h-BN, Mo and W dichalcogenides and ...

Precision and chip contamination-free placement of two-dimensional (2D) materials is expected to accelerate both the study of fundamental properties and novel device functionality. Current deterministic transfer methods of 2D materials onto an arbitrary substrate deploy viscoelastic stamping. However, these methods produce (a) significant cross-contamination of the substrate inherent from typical dense sources of 2D material flakes and (b) are challenged with respect to spatial alignment, and (c) multi-transfer at a single step. Here, we demonstrate a novel method of transferring 2D materials resembling the functionality known from printing; utilizing a combination of a sharp micro-stamper and viscoelastic polymer, we show precise placement of individual 2D materials resulting in vanishing cross-contamination to the substrate. Our 2D printer-method results in an aerial cross-contamination improvement of two to three orders of magnitude relative to state-of-the-art transfer metho...

Black phosphorus (bP) has emerged as an interesting addition to the category of two-dimensional materials. Surface-science studies on this material are of great interest, but they are hampered by bP’s high reactivity to oxygen and water, a major challenge to scanning tunneling microscopy (STM) experiments. As a consequence, the large majority of these studies were performed by cleaving a bulk crystal in situ . Here we present a study of surface modifications on exfoliated bP flakes upon consecutive annealing steps, up to 550 °C, well above the sublimation temperature of bP. In particular, our attention is focused on the temperature range 375 °C–400 °C, when sublimation starts, and a controlled desorption from the surface occurs alongside with the formation of characteristic well-aligned craters. There is an open debate in the literature about the crystallographic orientation of these craters, whether they align along the zigzag or the armchair direction. Thanks to the atom...

While it has been demonstrated that large scale liquid exfoliation of graphene is possible using high-shear exfoliation, it has not yet been shown to be applicable to a broader range of layered materials. In addition, it would be useful to determine whether the mechanisms reported for shear exfoliation of graphene also apply to other 2D materials. In this work we show that previous models describing high-shear exfoliation of graphene apply to MoS 2 and WS 2 . However, we find the minimum shear rate required to exfoliate MoS 2 and WS 2 to be ~3  ×  10 4 s −1 , somewhat higher than the value for graphene. We also demonstrate the scalability of shear exfoliation of WS 2 . By measuring and then optimising the scaling parameters, shear exfoliation of WS 2 is shown to be capable of reaching concentrations of 1.82 g l −1 in 6 h and demonstrating a maximum production rate of 0.95 g h −1 .

Orthorhombic MoO 3 ( α -MoO 3 ) is a typical layered n-type semiconductor with optical band gap over 2.7 eV, which have been widely studied in catalysis, gas sensing, lithium-ion batteries, field-emission, photoelectrical, photochromic and electrochromic devices, supercapacitors and organic solar cells. However, the bottleneck of generation large size atomic thin two-dimensional (2D) α -MoO 3 crystals remain challenging this field (normally several micrometers size). Herein, we developed a facile vapor–solid (VS) process for controllable growth of large-size 2D α -MoO 3 single crystals with a few nanometers thick and over 300 μ m in lateral size. High-performance solar-blind photodetectors were fabricated based on individual 2D α -MoO 3 single crystal. The detectors demonstrate outstanding optoelectronic properties under solar-blind UV light (254 nm), with a photoresponsivity of 67.9 A W −1 , ...

Transition-metal dichalcogenides (TMDCs) have recently open a new perspective in electronics and optoelectronics due to their unique planar crystal structures and incredible physical characteristics. Strong in-plane piezoelectricity is their unique property owing to non-centrosymmetric structure, differing from other two dimension (2D) materials, such as graphene and black phosphorus. In this work, we develop a flexible photodiode based on monolayer MoS 2 lateral p-n homojunction with significant enhancement in photoresponsivity and detectivity. Piezo-phototronic effect is used to achieve this enhancement by adjusting the barrier height and broadening depletion zone at p-n junction interface under external strain. The wider depletion zone benefits the separation and transport of photogenerated carriers, thus enhancing the photocurrent. When a 0.51% external static tensile strain was applied, the photoresponsivity and detectivity are improved up to 1162 A W −1 ...

Recently, the fabricated MoS 2 field effect transistors (FETs) with 1T-MoS 2 electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS 2 (Kappera et al 2014 Nat. Mater . 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS 2 interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS 2 interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS 2 interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS 2. In addition, as compared to the n-type Au/MoS 2 and Pd/MoS 2 contacts, the p-type ...

A controlled and eco-friendly, scalable CVD method for the production of single and few layer MoS 2 crystals is proposed. The MoS 2 crystals are fabricated at atmospheric pressure through the reaction of pre-deposited sodium molybdate (Na 2 MoO 4 ) in solution and elemental sulfur at 800 °C, offering the flexibility to achieve two growth regimes -either homogeneously distributed single layer MoS 2 crystals or continuous MoS 2 films- by varying the Na 2 MoO 4 solution concentration. In particular, for low precursor concentrations, isolated single layer MoS 2 crystals with controllable mean lateral size were produced. Higher concentrations resulted in continuous single layer films grown in tandem with highly oriented few layer epitaxial domains. The area of the monolayer relative to the few-layer domains can be adjusted. The significant impact on the optical properties of single layer MoS 2

The 2D graphdiyne has attracted much attention in spin-dependent physical properties recently due to its superior intrinsic semiconductor properties and greater paramagnetic moment compared with graphene. However, to date, no ferromagnetic ordering has been successfully induced into graphdiyne (GDY), which restricts its further application in electronics involving spin. Herein, we for the first time obtain the ferromagnetic graphdiyne based on transition metal ferrum (Fe) doping through a simple and affordable synthetic route. In particular, by means of chemical reduction with heat-treatment in the special carbon matrix of graphdiyne, a typical ferromagnetic curve with T c above room temperature is observed in Fe/GDY hybrid, which can be attributed to the presence of low valence Fe ions. After annealing, the remanent magnetization of Fe/GDY increases and the coercive field reduces by an order of magnitude compared with the raw Fe/GDY hybrid even at 300...

Monolayer transition metal dichalcogenides (TMDCs) are promising candidates for quantum technologies, such as spin qubits in quantum dots, because they are truly two-dimensional semiconductors with a direct band gap. In this work, we analyse theoretically the behaviour of a double quantum dot (DQD) system created in the conduction band of these materials, with two electrons in the (1,1) charge configuration. Motivated by recent experimental progress, we consider several scenarios, including different spin–orbit splittings in the two dots and including the case when the valley degeneracy is lifted due to an insulating ferromagnetic substrate. Finally, we discuss in which cases it is possible to reduce the low energy subspace to the lowest Kramers pairs. We find that in this case the low energy model is formally identical to the Heisenberg exchange Hamiltonian, indicating that such Kramers pairs may serve as qubit implementations.