Electronic properties of graphene encapsulated with different two-dimensional atomic crystals.

The research of two-dimensional (2D) atomic crystals has progressed rapidly since the isolation of graphene in 2004 [1, 2]. The family of 2D crystals now include many different types of materials, including metals (e.g. graphene, NbSe2), semiconductors (e.g. phosphorene, MoS2, WSe2), insulators (e.g. BN), superconductors and charge-density-wave materials (e.g. NbSe2 and TiSe2). Alongside with the rapid development of individual 2D materials, the research frontier has also advanced to explore their hybrid systems [3, 4]. In particular, the flat and inert surfaces of 2D materials enable the construction of heterogeneous stacks of different 2D crystals with atomically sharp interfaces, coupled vertically only by van der Waals forces. These van der Waals heterostructures exhibit many unique properties that cannot be realized in individual 2D crystals [3, 4]. For instance, graphene on hexagonal boron nitride (BN) can exhibit the Hofstadter’s butterfly phenomenon because of the nanoscale periodic interaction between the graphene and BN lattices [5–7]. Transition metal dichalcogenide (TMD) heterostructures can host long-lived interlayer excitons due to the staggered band alignment between different TMD layers [8]. Electronic and optoelectronic devices made from van der Waals heterostructures can exhibit performance superior to that of traditional devices with lateral 2D junctions [9–11]. More generally, the 2D building blocks can be combined to form more complex structures. By incorporating the unique properties of each class of 2D crystal (e.g., semiconducting TMDs, insulating BN and metallic graphene), integrated circuits can in principle be constructed entirely with 2D materials. Such 2D systems of electronics, once realized, could open a route to post-silicon technology.

While boron nitride (BN) substrates have been utilized to achieve high electronic mobilities in graphene field effect transistors, it is unclear how other layered two dimensional (2D) crystals influence the electronic performance of graphene. In this Letter, we study the surface morphology of 2D BN, gallium selenide (GaSe), and transition metal dichalcogenides (tungsten disulfide (WS2) and molybdenum disulfide (MoS2)) crystals and their influence on graphene's electronic quality. Atomic force microscopy analysis shows that these crystals have improved surface roughness (root mean square value of only ∼0.1 nm) compared to conventional SiO2 substrate. While our results confirm that graphene devices exhibit very high electronic mobility (μ) on BN substrates, graphene devices on WS2 substrates (G/WS2) are equally promising for high quality electronic transport (μ ∼ 38 000 cm2/V s at room temperature), followed by G/MoS2 (μ ∼ 10 000 cm2/V s) and G/GaSe (μ ∼ 2200 cm2/V s). However, we observe a significant asymmetry in electron and hole conduction in G/WS2 and G/MoS2 heterostructures, most likely due to the presence of sulphur vacancies in the substrate crystals. GaSe crystals are observed to degrade over time even under ambient conditions, leading to a large hysteresis in graphene transport making it a less suitable substrate.

Stabilizing Solid Electrolyte-Anode Interface in Li-Metal Batteries by Boron Nitride-Based Nanocomposite Coating

From two-dimensional materials to heterostructures

The polaron is the archetypal example of a quasiparticle emerging from the interaction between fermionic and bosonic fields in quantum field theory. In crystalline solids, polarons are formed when electrons and holes become dressed by the quanta of lattice vibrations. While experimental signatures of polarons in bulk three-dimensional materials abound, only rarely have polarons been observed in two-dimensional atomic crystals. Here, we shed light on this asymmetry by developing a quantitative ab initio theory of polarons in atomically-thin crystals. Using this conceptual framework, we unravel the real-space structure of the recently-observed hole polaron in hexagonal boron nitride, we discover an unexpected critical condition for the existence of polarons in two-dimensional crystals, and we establish the key materials descriptors and the universal laws that underpin polaron physics in two dimensions.

Electronic and photonic devices based on two-dimensional (2D) atomic crystals, such as graphene and layered transition-metal dichalcogenides (TMDCs), are perceived as potential candidates to complement, or even replace, conventional semiconductor devices in various applications. 2D crystals are of high material quality and stability, even so, they can be produced with large-area dimensions and at low cost. Moreover, the possibility of stacking different atomically-thin 2D layers on top of each other provides the opportunity of creating "artificial" designer materials, so-called van der Waals heterostructures. In this paper, optoelectronic devices based on 2D materials will be presented. We will discuss photodetection, light emission and photovoltaic energy conversion in 2D monolayers and van der Waals heterojunctions.

High mobility is a crucial requirement for a large variety of electronic device applications. The state of the art for high-quality graphene devices is based on heterostructures made with graphene encapsulated in >40 nm-thick flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer hBN while precisely controlling the number of layers remains an outstanding challenge, resulting in a rough material unable to enhance the mobility of graphene. This leads to the pursuit of alternative, scalable materials, which can be used as substrates and encapsulants for graphene. Tungsten disulfide (WS2) is a transition metal dichalcogenide, which was grown in large (∼mm-size) multi-layers by chemical vapor deposition. However, the resistance vs gate voltage characteristics when gating graphene through WS2 exhibit largely hysteretic shifts of the charge neutrality point on the order of Δn∼ 3 × 1011 cm−2, hindering the use of WS2 as a reliable encapsulant. The hysteresis originates due to the charge traps from sulfur vacancies present in WS2. In this work, we report the use of WS2 as a substrate and overcome the hysteresis issues by chemically treating WS2 with a super-acid, which passivates these vacancies and strips the surface from contaminants. The hysteresis is significantly reduced by about two orders of magnitude, down to values as low as Δn∼ 2 × 109 cm−2, while the room-temperature mobility of WS2-encapsulated graphene is as high as ∼62 × 103 cm2 V−1 s−1 at a carrier density of n ∼ 1 ×1012 cm−2. Our results promote WS2 as a valid alternative to hBN as an encapsulant for high-performance graphene devices.

Abstract

A comprehensive review on synthesis and applications of molybdenum disulfide (MoS2) material: Past and recent developments

The emergence of two-dimensional (2D) materials has attracted a great deal of attention due to their fascinating physical properties and potential applications for future nano-electronic devices. Since the first isolation of graphene, a Dirac material, a large family of new functional 2D materials have been discovered and characterized, including insulating 2D boron nitride, semiconducting 2D transition metal dichalcogenides and black phosphorus, and superconducting 2D bismuth strontium calcium copper oxide, molybdenum disulphide and niobium selenide, etc. Here, we report the identification of ferromagnetic thin flakes of Cr2Ge2Te6 (CGT) with thickness down to a few nanometers, which provides a very important piece to the van der Waals structures consisting of various 2D materials. We further demonstrate the giant modulation of the channel resistance of 2D CGT devices via electric field effect. Our results illustrate the gate voltage tunability of 2D CGT and the potential of CGT, a ferromagnetic 2D material, as a new functional quantum material for applications in future nanoelectronics and spintronics.

A strategy of forming sandwich electrodes based on graphene and two-dimensional transition metal dichalcogenides as supercapacitors to avoid gas evolution

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

Monolayer transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2), tungsten diselenide (WSe2) and tungsten disulfide (WS2), have shown in the last years remarkable physical properties. These direct badgap three-atoms thick monolayers, with a broken inversion symmetry, present a unique coupling between the spin and valley degrees of freedom originated from the relativistic spin¿orbit interaction. Together with their mechanical flexibility, result in materials promising for flexible, transparent and low power electronic, optoelectronic and spin/valleytronic applications. In this thesis, we investigate optoelectronic and spin/valleytronic devices based on monolayer MoS2, the most studied monolayer from the TMD familly, with 1.9 eV direct band gap and 6.5 Å thick. We show a monolayer MoS2 photodetector with 100,000-fold improved photoresponsivity from previous monolayer MoS2 phototransistors, first time reported entire junction light emitting diodes (LED) and solar cells based on monolayer MoS2/p-type silicon heterojunctions, which can work as an avalanche photodiodes too, and first spin-valley tunable light emitting diode made with monolayer MoS2/monolayer WSe2. Utilizing high-quality monolayer MoS2, we achieved a broad spectral range phototransistor with photoresponsivity of 880 A/W and low noise equivalent power (NEP) of 1.8 x 10^-15 W/Hz^1/2. Afterwards, we used the two-dimensional (2D) n-type monolayer MoS2 combined with three-dimensional (3D) p-type silicon to build vertical p-n junctions. The entire junction area of our 2D/3D heterostructures emitted light with a low emission threshold power density of 3.2 W/cm^2 and spectrum related to the direct band gap of monolayer MoS2. The heterojunction diode could operate as a solar cell with an external quantum efficiency (EQE) of 4.4% and a broad spectral response. With the use of large area chemical vapor deposition (CVD) grown monolayer MoS2, we scaled up the manufacturing process and showed the capability of these heterostructures to work as avalanche photodiodes (APD) with a multiplication exceeding 1000 for -10 V. Finally, we combined monolayer MoS2 with WSe2 or WS2 and made 2D/2D heterojunctions able to emit light with different characteristic spectrums related to the type of heterojunction. Monolayer WS2/monolayer MoS2 showed one of the emission peaks in the green region of the visible spectrum while all heterojunctions showed peaks in the red region. Spin-polarized charge carriers were injected across the Schottky barrier between a ferromagnetic electrode and the monolayer WSe2 of a monolayer WSe2/monolayer MoS2 heterostructure, resulting in valley polarization due to spin-valley locking. The degree of spin/valley polarization was controlled by a magnetic field between a polarization of ± 20%. A slope of 0.47 ± 0.08 meV/T was also seen related to the valley Zeeman effect.

The superconducting magnet energy storage (SMES) has become an increasingly popular device with the development of renewable energy sources. The power fluctuations they produce in energy systems must be compensated with the help of storage devices. A toroidal SMES magnet with large capacity is a tendency for storage energy because it has great energy density and low stray field. A key component in the creation of these superconducting magnets is the material from which they are made. The present work describes a comparative numerical analysis with finite element method, of energy storage in a toroidal modular superconducting coil using two types of superconducting material with different properties bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). Regarding the design of the modular torus, it was obtained that for a 1.25 times increase of the critical current for the BSCCO superconducting material compared with YBCO, the dimensions of the BSCCO torus were reduced by 7% considering the same stored energy. Also, following a numerical parametric analysis, it resulted that, in order to maximize the amount of energy stored, the thickness of the torus modules must be as small as possible, without exceeding the critical current. Another numerical analysis showed that the energy stored is maximum when the major radius of the torus is minimum, i.e., for a torus as compact as possible.

Two-dimensional (2D) semiconducting layered materials of transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulphide (WS2) with typical energy band gap of 1-2 eV have emerged as novel materials and have presented various fascinating physical properties. Recently, worldwide researchers have made significant efforts on building flash memories with MoS2 channel based field-effect transistor. They have showed 28-40% charge loss after 10 years retention by using few-layered graphene, MoS2, metallic nanocrystal, hafnium oxide, and guanine as a charge trapping layer. However, limited works built their nonvolatile memories using entirely of assembled 2D crystals. Moreover, few-layered graphene with more than seven layers of graphene is attractable for use in floating-gates, because the high capacity of the charge storage as the density of state of few-layered graphene is five folds larger than that of graphene. WS2 has an indirect band gap of 1.3 eV (in bulk) and an electron affinity of 4.0-4.4 eV. Theoretical studies have predicted that the electron/hole effective mass of WS2 is smaller than that of other semiconducting 2D crystals. Along with high thermal stability and stable chemicals make it suitable for use as a channel of memory cells. To our knowledge, WS2 based memory is still undeveloped to date. Here, we report a novel concept of nonvolatile memories utilizing functional van der Waals heterostructure in which multi-layered graphene is encapsulated between SiO2 and hexagonal boron nitride. We observed that a large memory window (20 V) allows to reveal high on-/off-state ratio (>103). The devices with high charge storage capacity of few-layered graphene and high quality of WS2 manifest perfect retention of 13% charge loss after 10 years, which was recorded as 2 to 3 folds smaller than that of reported MoS2 memories. This study offers new device architecture for achieving future nanoelectronic building blocks by utilizing atomically thin van der Waals stacks. Figure 1