Resonant Raman Scattering Study of Strain and Defects in Chemical Vapor Deposition Grown MoS2 Monolayers.

The development of bottom-up synthesis routes for semiconducting transition metal dichalcogenides (TMDs) and the assessment of their defects are of paramount importance to achieve their applications. TMD monolayers grown by chemical vapor deposition (CVD) can be subjected to significant strain and, here, Raman and photoluminescence spectroscopies are combined to characterize strain in over one hundred MoS2 monolayer samples grown by CVD. The frequency changes of phonons as a function of strain are analyzed, and used to extract the Grüneisen parameter of both zone-center and edge phonons. Additionally, the intensity of the defect-induced longitudinal acoustic (LA) and transverse acoustic (TA) Raman bands are discussed in relation to strain and electronic doping. The experimental mode-Grüneisen parameters obtained are compared with those calculated by density functional theory (DFT), to better characterize the type of strain and its resulting effects on Grüneisen parameters. The findings indicate that the use of Raman spectra to determine defect densities in 2D MoS2 must be always conducted considering strain effects. To the best of the authors' knowledge, this work constitutes the first report on double resonance Raman processes studied as a function of strain in 2D-MoS2. The new approach to obtain the Grüneisen parameter from zone-edge phonons in MoS2 can also be extended to other 2D semiconducting TMDs.

Spatial Control of Substitutional Dopants in Hexagonal Monolayer WS2 : The Effect of Edge Termination.

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants. Particularly, in chemical vapor deposition (CVD)-grown monolayer WS2 , it is found that a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples demonstrate this spatial distribution control, including hexagonal iron- and vanadium-doped WS2 monolayers. Density functional theory (DFT) calculations are further performed, indicating that the edge-dependent dopant distribution is due to a strong binding of tungsten atoms at tungsten-zigzag edges, resulting in the formation of open sites at sulfur-zigzag edges that enable preferential dopant incorporation. Based on these results, it is envisioned that edge termination in crystalline TMD monolayers can be utilized as a novel and effective knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that display fascinating electronic, optoelectronic, and magnetic properties.

Superconductivity enhancement in phase-engineered molybdenum carbide/disulfide vertical heterostructures.

Stacking layers of atomically thin transition-metal carbides and two-dimensional (2D) semiconducting transition-metal dichalcogenides, could lead to nontrivial superconductivity and other unprecedented phenomena yet to be studied. In this work, superconducting α-phase thin molybdenum carbide flakes were first synthesized, and a subsequent sulfurization treatment induced the formation of vertical heterolayer systems consisting of different phases of molybdenum carbide-ranging from α to γ' and γ phases-in conjunction with molybdenum sulfide layers. These transition-metal carbide/disulfide heterostructures exhibited critical superconducting temperatures as high as 6 K, higher than that of the starting single-phased α-Mo2C (4 K). We analyzed possible interface configurations to explain the observed moiré patterns resulting from the vertical heterostacks. Our density-functional theory (DFT) calculations indicate that epitaxial strain and moiré patterns lead to a higher interfacial density of states, which favors superconductivity. Such engineered heterostructures might allow the coupling of superconductivity to the topologically nontrivial surface states featured by transition-metal carbide phases composing these heterostructures potentially leading to unconventional superconductivity. Moreover, we envisage that our approach could also be generalized to other metal carbide and nitride systems that could exhibit high-temperature superconductivity.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Defect Coupling and Sub-Angstrom Structural Distortions in W1-xMoxS2 Monolayers.

Two-dimensional materials offer a remarkably rich materials platform to study the origin of different material behaviors at the atomic level, and doping provides a key means of tailoring such materials' functional properties. The local atomic structure around such dopants can be critically important in determining the material's behavior as it could modulate scattering, catalytic activity, electronic and magnetic properties, and so forth. Here, using aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom resolution in conjunction with density functional theory calculations, we demonstrate a strong coupling between Mo dopants and two types of defects in WS2 monolayers: sulfur monovacancies and grain boundaries. Although Mo does occupy a transition metal lattice site, it is not an ideal substitutional dopant: ∼80% of the S vacancies identified by STEM colocalize with Mo dopants, an affinity that appears to be enhanced by symmetry breaking of a partially occupied midgap defect state. Although a Mo dopant by itself does not considerably distort the WS2 lattice, it induces substantial lattice deformation by apparently facilitating the charging of a sulfur monovacancy paired with it, which is consistent with the results of first-principles calculations. This coupling of foreign substitutional dopants with vacancies could potentially be exploited to control the distribution and location of chalcogenide vacancies within transition metal dichalcogenides (TMD), by segregating vacancies into regions of high Mo concentration that are purposely placed away from active regions of TMD-based devices.

Spontaneous Formation of Atomically Thin Stripes in Transition Metal Dichalcogenide Monolayers.

Whether an alloy is random or ordered can have profound effects on its properties. The close chemical similarity of W and Mo in the two-dimensional semiconductors MoS2 and WS2 has led to the expectation that WxMo1-xS2 is a random alloy. Here we report that triangular monolayer flakes of WxMo1-xS2 produced by sulfurization of MoO3/WO3 are not only nonrandom, but also anisotropic: W and Mo form atomically thin chains oriented parallel to the edges of the triangle, especially around x ∼ 0.5, as resolved by aberration-corrected transmission electron microscopy. First-principles calculations reveal that the binding energies of striped and random alloys are nearly identical but that phase segregation at the growth edge favors one metal over another depending on the local sulfur availability, independent of the composition deeper inside the monolayer. Thus, atomically thin striping is kinetically driven and controlled by fluctuations that couple the local chemical potentials of metals and chalcogenide. Considering the nearly identical electronic properties but very different atomic masses of Mo and W, the resulting striped alloy is electronically isotropic, but vibrationally anisotropic. Phonon anomalies associated with the stripe ordering are predicted, as is an anisotropic thermal conductivity. More generally, fluctuation-driven striping provides a mechanism to produce in-plane subnanometer superlattices within two-dimensional crystals, with broad implications for controlling the electronic, optical, and structural properties of these systems.

Band gap engineering and layer-by-layer mapping of selenium-doped molybdenum disulfide.

Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure, and hence, control of dopant concentration within each individual layer of these compounds provides a powerful tool to efficiently modify their physical and chemical properties. The main challenge arises when quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of molybdenum disulfide substitutionally doped with a broad range of selenium concentrations, resulting in over 10% optical band gap modulations in atomic layers. Chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local optical band gaps. Furthermore, in a bilayer structure, the dopant distribution is imaged layer-by-layer. This work demonstrates that each layer in the bilayer system contains similar local Se concentrations, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.

Ultrashort optical pulse characterization using WS2 monolayers.

We demonstrate the application of two-dimensional materials for ultrashort optical pulse characterization. Monolayer transition metal dichalcogenides, such as tungsten disulfide (WS2), possess extraordinarily large second-order nonlinear susceptibility, and due to their atomic thickness, have relaxed phase-matching requirements and, hence, an inherently wide bandwidth. Synthesized monolayer WS2 triangular islands were used to characterize ultrashort optical pulses at the focal point of an objective lens through second-harmonic generation collinear frequency-resolved optical gating.

Extraordinary room-temperature photoluminescence in triangular WS2 monolayers.

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from those of their bulk counterparts. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL weakens with increasing number of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet's center. The structure and chemical composition of the platelet edges appear to be critical for PL enhancement.

Three-dimensional nitrogen-doped multiwall carbon nanotube sponges with tunable properties.

A three-dimensional (3D) nitrogen-doped multiwall carbon nanotube (N-MWCNT) sponge possessing junctions induced by both nitrogen and sulfur was synthesized by chemical vapor deposition (CVD). The formation of "elbow" junctions as well as "welded" junctions, which are attributed to the synergistic effect of the nitrogen dopant and the sulfur promoter, plays a critically important role in the formation of 3D nanotube sponges. To the best of our knowledge, this is the first report showing the synthesis of macroscale 3D N-MWCNT sponges. Most importantly, the diameter of N-MWCNT can be simply controlled by varying the concentration of sulfur, which in turn controls both the sponge's mechanical and its electrical properties. It was experimentally shown that, with increasing diameter of N-MWCNT, the elastic modulus of the sponge increased while the electrical conductivity decreased. The mechanical behaviors of the sponges have also been quantitatively analyzed by employing strain energy function modeling.

Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures.

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.

Longitudinal cutting of pure and doped carbon nanotubes to form graphitic nanoribbons using metal clusters as nanoscalpels.

We report the use of transition metal nanoparticles (Ni or Co) to longitudinally cut open multiwalled carbon nanotubes in order to create graphitic nanoribbons. The process consists of catalytic hydrogenation of carbon, in which the metal particles cut sp(2) hybridized carbon atoms along nanotubes that results in the liberation of hydrocarbon species. Observations reveal the presence of unzipped nanotubes that were cut by the nanoparticles. We also report the presence of partially open carbon nanotubes, which have been predicted to have novel magnetoresistance properties.(1) The nanoribbons produced are typically 15-40 nm wide and 100-500 nm long. This method offers an alternative approach for making graphene nanoribbons, compared to the chemical methods reported recently in the literature.

Quantification and Healing of Defects in Atomically Thin Molybdenum Disulfide: Beyond the Controlled Creation of Atomic Defects.

Atomically thin 2D materials provide an opportunity to investigate the atomic-scale details of defects introduced by particle irradiation. Once the atomic configuration of defects and their spatial distribution are revealed, the details of the mesoscopic phenomena can be unveiled. In this work, we created atomically small defects by controlled irradiation of gallium ions with doses ranging from 4.94 × 1012 to 4.00 × 1014 ions/cm2 on monolayer molybdenum disulfide (MoS2) crystals. The optical signatures of defects, such as the evolution of defect-activated LA-bands and a broadening of the first-order (E' and A'1) modes, can be studied by Raman spectroscopy. High-resolution scanning transmission electron microscopy (HR-STEM) analysis revealed that most defects are vacancies of few-molybdenum atoms with surrounding sulfur atoms (VxMo+yS) at a low ion dose. When increasing the ion dose, the atomic vacancies merge and form nanometer-sized holes. Utilizing HR-STEM and image analysis, we propose the estimation of the finite crystal length (Lfc) via the careful quantification of 0D defects in 2D systems through the formula Lfc = 4.41/ηion, where ηion corresponds to the ion dose. Combining HR-STEM and Raman spectroscopy, the formula to calculate Lfc from Raman features, I(LA)/I(A'1) = 5.09/Lfc2, is obtained. We have also demonstrated an effective route to healing the ion irradiation-induced atomic vacancies by annealing defective MoS2 in a hydrogen disulfide (H2S) atmosphere. The H2S annealing improved the crystal quality of MoS2 with Lfc greater than the calculated size of the A exciton wave function, which leads to a partial recovery of the photoluminescence signal after its quenching by ion irradiation.

Universal In Situ Substitutional Doping of Transition Metal Dichalcogenides by Liquid-Phase Precursor-Assisted Synthesis.

Doping lies at the heart of modern semiconductor technologies. Therefore, for two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), the significance of controlled doping is no exception. Recent studies have indicated that, by substitutionally doping 2D TMDs with a judicious selection of dopants, their electrical, optical, magnetic, and catalytic properties can be effectively tuned, endowing them with great potential for various practical applications. Herein, and inspired by the sol-gel process, we report a liquid-phase precursor-assisted approach for in situ substitutional doping of monolayered TMDs and their in-plane heterostructures with tunable doping concentration. This highly reproducible route is based on the high-temperature chalcogenation of spin-coated aqueous solutions containing host and dopant precursors. The precursors are mixed homogeneously at the atomic level in the liquid phase prior to the synthesis process, thus allowing for an improved doping uniformity and controllability. We further demonstrate the incorporation of various transition metal atoms, such as iron (Fe), rhenium (Re), and vanadium (V), into the lattice of TMD monolayers to form Fe-doped WS2, Re-doped MoS2, and more complex material systems such as V-doped in-plane WxMo1-xS2-MoxW1-xS2 heterostructures, among others. We envisage that our developed approach is universal and could be extended to incorporate a variety of other elements into 2D TMDs and create in-plane heterointerfaces in a single step, which may enable applications such as electronics and spintronics at the 2D limit.

Selective Synthesis of Bi2Te3/WS2 Heterostructures with Strong Interlayer Coupling.

The vertical integration of atomically thin-layered materials to create van der Waals heterostructures (vdWHs) has been proposed as a method to design nanostructures with emergent properties. In this work, epitaxial Bi2Te3/WS2 vdWHs are synthesized via a two-step vapor deposition process. It is calculated that the vdWH has an indirect band gap with a valence band edge that bridges the vdW gap, resulting in a quenched photoluminescence (PL) from the WS2 monolayer, reduced intensity of its resonance Raman vibrational peaks, improved vertical charge transport, and a decrease in the intensity of second harmonic generation (SHG). Furthermore, it is observed that induced defects strongly influence the nucleation and growth of vdWHs. By creating point defects in WS2 monolayers, it is shown that the growth of Bi2Te3 platelets can be patterned. This work offers important insights into the synthesis, defect engineering, and moiré engineering of an emerging class of two-dimensional (2D) heterostructures.

Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2.

Surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites, which can diminish the potential applications of the decorated 2D materials, as structural defects can have substantial drawbacks on the electronic and optoelectronic characteristics. Here, we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS2(1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping and enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single-atom catalysis, quantum information devices, optoelectronics, and enhanced sensing.

Monolayer Vanadium-Doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor.

Dilute magnetic semiconductors (DMS), achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional (2D) systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here, room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation, is described. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of ~2 at% and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry, and first-principles calculations. Room-temperature 2D-DMS provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures into the realm of practical application.

Carbon doping of WS2 monolayers: Bandgap reduction and p-type doping transport.

Chemical doping constitutes an effective route to alter the electronic, chemical, and optical properties of two-dimensional transition metal dichalcogenides (2D-TMDs). We used a plasma-assisted method to introduce carbon-hydrogen (CH) units into WS2 monolayers. We found CH-groups to be the most stable dopant to introduce carbon into WS2, which led to a reduction of the optical bandgap from 1.98 to 1.83 eV, as revealed by photoluminescence spectroscopy. Aberration corrected high-resolution scanning transmission electron microscopy (AC-HRSTEM) observations in conjunction with first-principle calculations confirm that CH-groups incorporate into S vacancies within WS2. According to our electronic transport measurements, undoped WS2 exhibits a unipolar n-type conduction. Nevertheless, the CH-WS2 monolayers show the emergence of a p-branch and gradually become entirely p-type, as the carbon doping level increases. Therefore, CH-groups embedded into the WS2 lattice tailor its electronic and optical characteristics. This route could be used to dope other 2D-TMDs for more efficient electronic devices.

Light-Emitting Transition Metal Dichalcogenide Monolayers under Cellular Digestion.

2D materials cover a wide spectrum of electronic properties. Their applications are extended from electronic, optical, and chemical to biological. In terms of biomedical uses of 2D materials, the interactions between living cells and 2D materials are of paramount importance. However, biointerfacial studies are still in their infancy. This work studies how living organisms interact with transition metal dichalcogenide monolayers. For the first time, cellular digestion of tungsten disulfide (WS2 ) monolayers is observed. After digestion, cells intake WS2 and become fluorescent. In addition, these light-emitting cells are not only viable, but also able to pass fluorescent signals to their progeny cells after cell division. By combining synthesis of 2D materials and a cell culturing technique, a procedure for monitoring the interactions between WS2 monolayers and cells is developed. These observations open up new avenues for developing novel cellular labeling and imaging approaches, thus triggering further studies on interactions between 2D materials and living organisms.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition.

We show that hexagonal domains of monolayer tungsten disulfide (WS2) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS2 hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS2 domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.