NitingZ

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS 2 via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS 2 strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN x based stressors implemented in industrial complementary metal-oxide-semiconductor (CMOS) processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS 2 strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS 2 with atomistic simulations, we can explore layer-by-layer strain tran...

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State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS 2 and MoS 2 /hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS 2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm −1 K −1 and 24 Wm −1 K −1 . Additionally, after determining the thermal conductivity of the latter MoS 2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS 2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the ...

The Computational 2D Materials Database (C2DB) is a highly curated open database organising a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman spectra and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as signifi...

The dynamics of suspended two-dimensional (2D) materials has received increasing attention during the last decade, yielding new techniques to study and interpret the physics that governs the motion of atomically thin layers. This has led to insights into the role of thermodynamic and nonlinear effects as well as the mechanisms that govern dissipation and stiffness in these resonators. In this review, we present the current state-of-the-art in the experimental study of the dynamics of 2D membranes. The focus will be both on the experimental measurement techniques and on the interpretation of the physical phenomena exhibited by atomically thin membranes in the linear and nonlinear regimes. We will show that resonant 2D membranes have emerged both as sensitive probes of condensed matter physics in ultrathin layers, and as sensitive elements to monitor small external forces or other changes in the environment. New directions for utilizing suspended 2D membranes for material characte...

2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron–hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm 2 range based on MoS 2 /WS 2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5–6 orders of magnitude with respect to reference devices based on either MoS 2 or WS 2 monolayers only. The large gain enhan...

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe 2 monolayers (ML) encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS 2 ML at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening attributed to a much less pronounced halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as therma...

Vertical stacking of two-dimensional materials with weak van der Waals (vdW) interactions has laid the ground for breakthroughs in physics as well as in technological applications. Although vdW interactions dominate interlayer binding, interlayer electronic coupling may not be negligible and can lead to properties beyond the superposition of constituent monolayers. Here, studying heterobilayers of transition-metal dichalcogenides ( MQ 2 ; M = Mo, Ni, Pt; Q = S, Se) by means of density functional theory calculations, we show two mechanisms that influence the band gaps of vdW heterostructures beyond the Anderson rule: (1) interfacial hybridization (mainly involving out-of-plane states, such as chalcogen p z -states), which leads to an upshift in the valence band maxima and accordingly a decrease in the band gap. (2) Formation of an interfacial electric dipole, resulting in an effective gap increase in type-II junctions. While...

The folding of Bernal-stacked bilayer graphene leads to electronic devices that can be understood as combinations of a twisted double-bilayer graphene and a fold. In magnetotransport experiments contributions of the two different parts can be identified. For the twisted double-bilayer graphene Landau fan diagrams with satellite fans depending on twist angle are observed. The fold gives rise to a local minimum in conductance which does not shift with applied perpendicular magnetic field. Regardless of twist angle the fold favors electron doping attributed to compressive strain at the kink geometry. The curvature of the folded structure provides for a systematic explanation, which is also in agreement with the observed correlation between twist angle and interlayer distance. Finally, the appearance of the topological zero line mode formed at the fold is discussed.

The interface between two different semiconductors is crucial in determining the electronic properties at the heterojunction, therefore novel techniques that can probe these regions are of particular interest. Recently it has been shown that heterojunctions of two-dimensional transition metal dichalcogenides have sharp and epitaxial interfaces that can be used to the next generation of flexible and on chip optoelectronic devices. Here, we show that second harmonic generation (SHG) can be used as an optical tool to reveal these atomically sharp interfaces in different lateral heterostructures. We observed an enhancement of the SH intensity at the heterojunctions, and showed that is due to a coherent superposition of the SH emission from each material. This constructive interference pattern reveals a phase difference arising from the distinct second-order susceptibilities of both materials at the interface. Our results demonstrate that SHG microscopy is a sensitive characterizatio...

The decoration of atomically thin metal flakes on two-dimensional (2D) material membranes to impart intriguing properties for applications such as sensors and catalysis has attracted tremendous interest. Here, we report the formation of atomically thin Mo nanoflakes on a molybdenum disulfide monolayer (ML-MoS 2 ) via a ‘self-feeding’ process using in situ transmission electron microscopy. Driven by energetic e-beam irradiation and thermal excitation, metallic Mo atoms preferentially segregate out and aggregate around mirror twin boundaries in the host MoS 2 ML, which then assemble into metallic nanoflakes: the associated dynamic process captured at the atomic scale. The Mo atoms constituting the nanoflakes tend to sit on the Mo-top and S-top sites if they are viewed as absorbed atoms with respect to the ML-MoS 2 substrate, which is further confirmed by theoretical calculations. Density functional theory calculations reveal that the entire syst...

Atomically thin monoisotopic hexagonal boron nitride (BN) which is electrically insulating and has a high thermal conductivity could be utilized as fillers in electronic packaging materials for thermal dissipation in integrated and miniaturized modern devices. Thermal expansion mismatch in electronic packaging could cause strain and ultimately device failure, so it is valuable to measure and understand the thermal expansion coefficient (TEC) of atomically thin isotopically pure BN. In this work, we studied the TECs of mono-, bi-, and tri-layer isotope-purified BN using Raman spectroscopy and density functional theory calculations including van der Waals dispersion forces. Monolayer (1L) 10 BN had a slightly larger experimental TEC than 1L 11 BN at close to room temperature: (−5.1 ± 0.8) × 10 −6 K −1 and (−4.6 ± 0.8) × 10 −6 K −1 , respectively. The negative TECs up to 700 K were attributed to the competition between the ...

Two-dimensional (2D) materials offer opportunities to explore both fundamental science and applications in the limit of atomic thickness. Beyond the prototypical case of graphene, other 2D materials have recently come to the fore. Of particular technological interest are 2D semiconductors, of which the family of materials known as the group-VI transition metal dichalcogenides (TMDs) has attracted much attention. The presence of a bandgap allows for the fabrication of high on–off ratio transistors and optoelectronic devices, as well as valley/spin polarized transport. The technique of chemical vapor deposition (CVD) has produced high-quality and contiguous wafer-scale 2D films, however, they often need to be transferred to arbitrary substrates for further investigation. In this review, the various transfer techniques developed for transferring 2D films will be outlined and compared, with particular emphasis given to CVD-grown TMDs. Each technique suffers undesirable process-relat...

Transmission electron microscopy (TEM) and scanning TEM (STEM) are indispensable tools for materials characterization. However, during a typical (S)TEM experiment, the sample is subject to a number of effects that can change its atomic structure. Of these, perhaps the least discussed are chemical modifications due to the non-ideal vacuum around the sample. With single-layer graphene, we show that even at relatively low pressures typical for many instruments, these processes can have a significant impact on the sample structure. For example, pore growth becomes up to two orders of magnitude faster at a pressure of ca. 10 −6 mbar as compared to ultra-high vacuum (UHV; 10 −10 mbar). Even more remarkably, the presence of oxygen at the sample also changes the observed atomic structure: When imaged in UHV, nearly 90% of the identifiable graphene edge configurations have the armchair structure, whereas armchair and zigzag structures are nearly equally likely to occu...

Structural defects play an important role in the optimization of material structures and properties, especially in low-dimensional systems such as two-dimensional (2D) materials. In this work, we investigated the formation, aggregation, and diffusion of vacancy defects in atomically thin black phosphorus (BP) via in situ high-resolution transmission electron microscopy. Vacancy defects including di-vacancies (DVs), vacancy clusters (e.g. tetra-vacancy and TV), and vacancy lines were confirmed as the primary forms of structural defects in BP. DV and TV defects were found to be highly mobile. The defects preferentially diffused and migrated along the diagonal and in a zigzag pattern (rather than an armchair pattern). After prolonged thermal excitation and electron-beam irradiation, all these as-formed vacancies tended to aggregate and line up parallel to the zigzag pattern direction to form extended vacancy lines with a total length reaching hundreds of nanometers or even th...

We report on van der Waals epitaxy of two-dimensional (2D) molybdenum trioxide (MoO 3− x ) with monolayer thickness directly grown on highly oriented pyrolytic graphite by thermal evaporation under ultrahigh vacuum. The chemical composition, electronic and crystalline lattice structures of the mono-and few-layer MoO 3− x sheets are analysed. Using scanning tunnelling microscopy and spectroscopy, we investigate the electronic properties of MoO 3− x as a function of the number of layers and measure the apparent energy gap to be 0.4 eV for the first three layers of MoO 3− x on graphite. We carried out density functional theory calculations to shed light on the mechanism underlying the observed narrow bandgap with oxygen deficiency. Moreover, the air exposure effect on monolayer MoO 3− x is investigated confirming that the apparent bandgap closes, and additionally we show the reduction of the work function...

When complex mechanisms are involved, pinpointing high-performance materials within large databases is a major challenge in materials discovery. We focus here on phonon-limited conductivities, and study 2D semiconductors doped by field effects. Using state-of-the-art density-functional perturbation theory and Boltzmann transport equation, we discuss 11 monolayers with outstanding transport properties. These materials are selected from a computational database of exfoliable materials providing monolayers that are dynamically stable and that do not have more than six atoms per unit cell. We first analyze electron-phonon scattering in two well-known systems: electron-doped InSe and hole-doped phosphorene. Both are single-valley systems with weak electron-phonon interactions, but they represent two distinct pathways to fast transport: a steep and deep isotropic valley for the former and strongly anisotropic electron-phonon physics for the latter. We identify similar features in the d...

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin–valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo 1 −  x W x Se 2 /WSe 2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transit...

Our examination of the interplay of ultrafast charge dynamics and electron–phonon interaction in the AA′ stacked bilayer MoS 2 provides a microscopic basis for understanding the features (two peaks) in the emission spectrum. We demonstrate that while the initial accumulation of excited charge occurs at and near the Q point of the two-dimensional Brillioun zone, emission takes place predominantly through two pathways: direct charge recombination at the K point and indirect phonon-assisted recombination of electrons at the ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025018/tdmabd6b5ieqn1.gif] {${\text{K}}$} valley and holes at the ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025018/tdmabd6b5ieqn2.gif] {${{\Gamma }}$} hill of the Brillouin zone. Analysis of the wave vector dependencies of the electron–phonon interaction traces the higher energy peak to phonon-assisted relaxation of the excited electrons from the Q to the ...

Precise control of phase transitions in polymorphic 2D transition metal dichalcogenides (TMDs) is expected to play a key role in modern intelligent devices. However, an atomic-scale understanding and thus control of the phase transitions in the atomically-thin TMDs have not been reached, especially in some metastable phases. Here, in metastable monolayer 1T′ WS 2 , we demonstrate the dynamics of a phase transition nucleated from atomic defects by the means of time-resolved annular dark-field imaging and atomic-resolution electron energy-loss spectroscopy. It is found that the atomic and electronic structure of the 1T′ phase is inhomogeneous, which is decided by zone-dependent W–S bond strengths due to a Peierls-like structure distortion. Meanwhile, the W–S bonding is flexible to allow large nonequilibrium atom shifts for phase transition. Thus, just a few atomic defects can stabilize the atomic-scale nucleus of the new phase to initialize the phase transition from 1T′ t...

Optical memory unit with the ability to detect and store optical signals is increasingly becoming a crucial part of advanced data communication and image sensing technology. Despite great efforts devoted to develop high-performance optical memory devices based on two-dimensional (2D) material, the photoelectric conversion is still limited to defect-dominant photo-generated carrier trapping/de-trapping process at the interface of 2D materials. Here, a reconfigurable optical memory implanted with photonic programming/electric erasing operation is demonstrated based on MoS 2 /quantum dots (QDs) mixed-dimensional heterostructure. Unique photoelectric coupling effect between MoS 2 and QDs leads to a continuous n-doping on MoS 2 channel after light exposure removed, resulting in the generation of persistent photocurrent. Excellent optical memory characteristics such as high programming/erasing ratio, long retention time and stable operation cycles have bee...

Black phosphorous (BP) is a layered semiconductor with high carrier mobility, anisotropic optical response and wide bandgap tunability. In view of its application in optoelectronic devices, understanding transient photo-induced effects is crucial. Here, we investigate by time- and angle-resolved photoemission spectroscopy BP in its pristine state and in the presence of Stark splitting, chemically induced by Cs ad-sorption. We show that photo-injected carriers trigger bandgap renormalization, and a concurrent valence band flattening caused by Pauli blocking. In biased samples, photo-excitation leads to a long-lived (ns) surface photovoltage of few hundreds mV that counterbalances the Cs-induced surface band bending. This allows us to disentangle bulk from surface electronic states, and to clarify the mechanism underlying the band inversion observed in bulk samples.

Twistronic van der Waals heterostrutures offer exciting opportunities for engineering optoelectronic properties of nanomaterials, in particular, due to the formation of moiré superlattice structures. In twisted bilayers of transition metal dichalcogenides moiré superlattice effects are additionally enriched by the lack of inversion symmetry in each monolayer unit cell. Here, we use multiscale modelling to establish a rich variety of confinement conditions for electrons, holes and layer-indirect excitons in twistronic WX 2 /MoX 2 bilayers (X = S,Se). Such trapping of charge carriers and excitons is caused by ferroelectric (interlayer) polarisation and piezoelectric effects generated by the reconstruction of twistronic bilayers into preferential stacking domains separated by domain wall networks. For almost aligned bilayers with anti-parallel (AP) orientation of WX 2 and MoX 2 unit cells, we find that upon lattice relaxation piezoelectric pot...

Fabricating van der Waals bilayer heterostructures (BL-HS) by stacking the same or different two-dimensional layers, offers a unique physical system with rich electronic and optical properties. Twist-angle between component layers has emerged as a remarkable parameter that can control the period of lateral confinement, and nature of the exciton (Coulomb bound electron–hole pair) in reciprocal space thus creating exotic physical states including moiré excitons (MXs). In this review article, we focus on opto-electronic properties of excitons in transition metal dichalcogenide semiconductor twisted BL-HS. We look at existing evidence of MXs in localized and strongly correlated states, and at nanoscale mapping of moiré superlattice and lattice-reconstruction. This review will be helpful in guiding the community as well as motivating work in areas such as near-field optical measurements and controlling the creation of novel physical states.

Two-dimensional (2D) perovskites have recently received increasing attention due to their enhanced environmental stability, layered nature and excellent optoelectronic characteristics. Particularly, the layered nature of 2D perovskites allows to conveniently integrate with other materials to form heterostructures, which can extend device functionalities and improve the performance of the corresponding optoelectronic devices. In this review, we aim to provide a comprehensive presentation of the recent advances of 2D perovskite based heterostructures. We will first give a brief overview on the structure and basic optical properties of 2D perovskites. Then we will focus on 2D perovskite based heterostructures including 2D/2D perovskites, 2D/three dimensional perovskites, 2D perovskites with 2D layered materials as well as 2D perovskites with other optoelectronic materials. Finally, we will outline the current challenges and future research directions in the field of 2D perovskite b...

The remarkable properties of graphene and related materials (GRMs) promise substantial benefits for a wide range of technologies. Their industrial utilization, however, critically depends on the availability of suitable GRM supply in terms of both quality and quantity at a competitive price. All factors remain highly specific to the each application scenario, while the emerging GRM supply industry still struggles to engage prospective clients in active development of GRM-enhanced products. The present issue of Graphene Roadmap Briefs compiles results on the status and prospects of GRM industrialization gathered by several innovation interface investigations (3I) that we conducted throughout the past 3 years. Each 3I study focussed a unique prospective value chain that GRM innovation promises to enable or advance in the future. While individual roadmaps mainly cover the peculiarities of the specific GRM supply chain under investigation, we combine all results regarding the primar...

The emergence of various exciton-related effects in transition metal dichalcogenides (TMDC) and their heterostructures has inspired a significant number of studies and brought forth several possible applications. Often, standard photoluminescence (PL) with microscale lateral resolution is utilized to identify and characterize these excitonic phenomena, including interlayer excitons (IEXs). We studied the local PL signatures of van der Waals heterobilayers composed of exfoliated monolayers of the (Mo, W)(S, Se) 2 TMDC family with high spatial resolution (down to 30 nm) using tip-enhanced photoluminescence (TEPL) with different orders (top/bottom) and on different substrates. We evidence that in MoS 2 –WSe 2 heterobilayers, other PL signals may appear near the reported energy of the IEX transitions, possibly interfering in the interpretation of the results. The extra signals are only observed locally in small areas where the topography looks distorted....