11 research outputs found
One-Dimensional MoS<sub>2</sub> Nanoscrolls as Miniaturized Memories
Dimensionality of materials is closely related to their
physical
properties. For two-dimensional (2D) semiconductors such as monolayer
molybdenum disulfide (MoS2), converting them from 2D nanosheets
to one-dimensional (1D) nanoscrolls could contribute to remarkable
electronic and optoelectronic properties, yet the rolling-up process
still lacks sufficient controllability, which limits the development
of their device applications. Herein we report a modified solvent
evaporation-induced rolling process that halts at intermediate states
and achieve MoS2 nanoscrolls with high yield and decent
axial uniformity. The accordingly fabricated nanoscroll memories exhibit
an on/off ratio of ∼104 and a retention time exceeding
103 s and can realize multilevel storage with pulsed gate
voltages. Such open-end, high-curvature, and hollow 1D nanostructures
provide new possibilities to manipulate the hysteresis windows and,
consequently, the charge storage characteristics of nanoscale field-effect
transistors, thereby holding great promise for the development of
miniaturized memories
Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process
We report the synthesis of large-scale uniform graphene films on high carbon solubility substrates of Rh foils for the first time using an ambient-pressure chemical vapor deposition method. We find that, by increasing the cooling rate in the growth process, the thickness of graphene can be tuned from multilayer to monolayer, resulting from the different segregation amount of carbon atoms from bulk to surface. The growth feature was characterized with scanning electron microscopy, Raman spectra, transmission electron microscopy, and scanning tunneling microscopy. We also find that bilayer or few-layer graphene prefers to stack deviating from the Bernal stacking geometry, with the formation of versatile moiré patterns. On the basis of these results, we put forward a segregation growth mechanism for graphene growth on Rh foils. Of particular importance, we propose that this randomly stacked few-layer graphene can be a model system for exploring some fantastic physical properties such as van Hove singularities
Unravelling Orientation Distribution and Merging Behavior of Monolayer MoS<sub>2</sub> Domains on Sapphire
Monolayer MoS<sub>2</sub> prepared
by chemical vapor deposition (CVD) has a highly polycrystalline nature
largely because of the coalescence of misoriented domains, which severely
hinders its future applications. Identifying and even controlling
the orientations of individual domains and understanding their merging
behavior therefore hold fundamental significance. In this work, by
using single-crystalline sapphire (0001) substrates, we designed the
CVD growth of monolayer MoS<sub>2</sub> triangles and their polycrystalline
aggregates for such purposes. The obtained triangular MoS<sub>2</sub> domains on sapphire were found to distributively align in two directions,
which, as supported by density functional theory calculations, should
be attributed to the relatively small fluctuations of the interface
binding energy around the two primary orientations. Using dark-field
transmission electron microscopy, we further imaged the grain boundaries
of the aggregating domains and determined their prevalent armchair
crystallographic orientations with respect to the adjacent MoS<sub>2</sub> lattice. The coalescence of individual triangular flakes
governed by unique kinetic processes is proposed for the polycrystal
formation. These findings are expected to shed light on the controlled
MoS<sub>2</sub> growth toward predefined domain orientation and large
domain size, thus enabling its versatile applications in next-generation
nanoelectronics and optoelectronics
Periodic Modulation of the Doping Level in Striped MoS<sub>2</sub> Superstructures
Although
the recently discovered monolayer transition metal dichalcogenides
exhibit novel electronic and optical properties, fundamental physical
issues such as the quasiparticle bandgap tunability and the substrate
effects remain undefined. Herein, we present the report of a quasi-one-dimensional
periodically striped superstructure for monolayer MoS<sub>2</sub> on
Au(100). The formation of the unique striped superstructure is found
to be mainly modulated by the symmetry difference between MoS<sub>2</sub> and Au(100) and their lattice mismatch. More intriguingly,
we find that the monolayer MoS<sub>2</sub> is heavily n-doped on the
Au(100) facet with a bandgap of 1.3 eV, and the Fermi level is upshifted
by ∼0.10 eV on the ridge (∼0.2 eV below the conduction
band) in contrast to the valley regions (∼0.3 eV below the
conduction band) of the striped patterns after high-temperature sample
annealing process. This tunable doping effect is considered to be
caused by the different defect densities over the ridge/valley regions
of the superstructure. Additionally, an obvious bandgap reduction
is observed in the vicinity of the domain boundary for monolayer MoS<sub>2</sub> on Au(100). This work should therefore inspire intensive
explorations of adlayer–substrate interactions, the defects,
and their effects on band-structure engineering of monolayer MoS<sub>2</sub>
Epitaxial Monolayer MoS<sub>2</sub> on Mica with Novel Photoluminescence
Molybdenum disulfide (MoS<sub>2</sub>) is back in the spotlight
because of the indirect-to-direct bandgap tunability and valley related
physics emerging in the monolayer regime. However, rigorous control
of the monolayer thickness is still a huge challenge for commonly
utilized physical exfoliation and chemical synthesis methods. Herein,
we have successfully grown predominantly monolayer MoS<sub>2</sub> on an inert and nearly lattice-matching mica substrate by using
a low-pressure chemical vapor deposition method. The growth is proposed
to be mediated by an epitaxial mechanism, and the epitaxial monolayer
MoS<sub>2</sub> is intrinsically strained on mica due to a small adlayer-substrate
lattice mismatch (∼2.7%). Photoluminescence (PL) measurements
indicate strong single-exciton emission in as-grown MoS<sub>2</sub> and room-temperature PL helicity (circular polarization ∼0.35)
on transferred samples, providing straightforward proof of the high
quality of the prepared monolayer crystals. The homogeneously strained
high-quality monolayer MoS<sub>2</sub> prepared in this study could
competitively be exploited for a variety of future applications
Dendritic, Transferable, Strictly Monolayer MoS<sub>2</sub> Flakes Synthesized on SrTiO<sub>3</sub> Single Crystals for Efficient Electrocatalytic Applications
Controllable synthesis of macroscopically uniform, high-quality monolayer MoS<sub>2</sub> is crucial for harnessing its great potential in optoelectronics, electrocatalysis, and energy storage. To date, triangular MoS<sub>2</sub> single crystals or their polycrystalline aggregates have been synthesized on insulating substrates of SiO<sub>2</sub>/Si, mica, sapphire, <i>etc.</i>, <i>via</i> portable chemical vapor deposition methods. Herein, we report a controllable synthesis of dendritic, strictly monolayer MoS<sub>2</sub> flakes possessing tunable degrees of fractal shape on a specific insulator, SrTiO<sub>3</sub>. Interestingly, the dendritic monolayer MoS<sub>2</sub>, characterized by abundant edges, can be transferred intact onto Au foil electrodes and serve as ideal electrocatalysts for hydrogen evolution reaction, reflected by a rather low Tafel slope of ∼73 mV/decade among CVD-grown two-dimensional MoS<sub>2</sub> flakes. In addition, we reveal that centimeter-scale uniform, strictly monolayer MoS<sub>2</sub> films consisting of relatively compact domains can also be obtained, offering insights into promising applications such as flexible energy conversion/harvesting and optoelectronics
Modulating the Electronic Properties of Monolayer Graphene Using a Periodic Quasi-One-Dimensional Potential Generated by Hex-Reconstructed Au(001)
The structural and
electronic properties of monolayer graphene
synthesized on a periodically reconstructed substrate can be widely
modulated by the generation of superstructure patterns, thereby producing
interesting physical properties, such as magnetism and superconductivity.
Herein, using a facile chemical vapor deposition method, we successfully
synthesized high-quality monolayer graphene with a uniform thickness
on Au foils. The hex-reconstruction of Au(001), which is characterized
by striped patterns with a periodicity of 1.44 nm, promoted the formation
of a quasi-one-dimensional (1D) graphene superlattice, which served
as a periodic quasi-1D modulator for the graphene overlayer, as evidenced
by scanning tunneling microscopy/spectroscopy. Intriguingly, two new
Dirac points were generated for the quasi-1D graphene superlattice
located at −1.73 ± 0.02 and 1.12 ± 0.12 eV. Briefly,
this work demonstrates that the periodic modulation effect of reconstructed
metal substrates can dramatically alter the electronic properties
of graphene and provides insight into the modulation of these properties
using 1D potentials
Controllable Growth and Transfer of Monolayer MoS<sub>2</sub> on Au Foils and Its Potential Application in Hydrogen Evolution Reaction
Controllable synthesis of monolayer MoS<sub>2</sub> is essential for fulfilling the application potentials of MoS<sub>2</sub> in optoelectronics and valleytronics, <i>etc.</i> Herein, we report the scalable growth of high quality, domain size tunable (edge length from ∼200 nm to 50 μm), strictly monolayer MoS<sub>2</sub> flakes or even complete films on commercially available Au foils, <i>via</i> low pressure chemical vapor deposition method. The as-grown MoS<sub>2</sub> samples can be transferred onto arbitrary substrates like SiO<sub>2</sub>/Si and quartz with a perfect preservation of the crystal quality, thus probably facilitating its versatile applications. Of particular interest, the nanosized triangular MoS<sub>2</sub> flakes on Au foils are proven to be excellent electrocatalysts for hydrogen evolution reaction, featured by a rather low Tafel slope (61 mV/decade) and a relative high exchange current density (38.1 μA/cm<sup>2</sup>). The excellent electron coupling between MoS<sub>2</sub> and Au foils is considered to account for the extraordinary hydrogen evolution reaction activity. Our work reports the synthesis of monolayer MoS<sub>2</sub> when introducing metal foils as substrates, and presents sound proof that monolayer MoS<sub>2</sub> assembled on a well selected electrode can manifest a hydrogen evolution reaction property comparable with that of nanoparticles or few-layer MoS<sub>2</sub> electrocatalysts
Substrate Facet Effect on the Growth of Monolayer MoS<sub>2</sub> on Au Foils
MoS<sub>2</sub> on polycrystalline metal substrates emerges as an intriguing growth system compared to that on insulating substrates due to its direct application as an electrocatalyst in hydrogen evolution. However, the growth is still indistinct with regard to the effects of the inevitably evolved facets. Herein, we demonstrate for the first time that the crystallography of Au foil substrates can mediate a strong effect on the growth of monolayer MoS<sub>2</sub>, where large-domain single-crystal MoS<sub>2</sub> triangles are more preferentially evolved on Au(100) and Au(110) facets than on Au(111) at relative high growth temperatures (>680 °C). Intriguingly, this substrate effect can be weakened at a low growth temperature (∼530 °C), reflected with uniform distributions of domain size and nucleation density among the different facets. The preferential nucleation and growth on some specific Au facets are explained from the facet-dependent binding energy of MoS<sub>2</sub> according to density functional theory calculations. In brief, this work should shed light on the effect of substrate crystallography on the synthesis of monolayer MoS<sub>2</sub>, thus paving the way for achieving batch-produced, large-domain or domain size-tunable growth through an appropriate selection of the growth substrate
Controlled Growth of High-Quality Monolayer WS<sub>2</sub> Layers on Sapphire and Imaging Its Grain Boundary
Atomically thin tungsten disulfide (WS<sub>2</sub>), a structural analogue to MoS<sub>2</sub>, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS<sub>2</sub> is underdeveloped (as compared with that of MoS<sub>2</sub>) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS<sub>2</sub> flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm<sup>2</sup>. Intriguingly, we show that, with adding minor H<sub>2</sub> carrier gas, the shape of monolayer WS<sub>2</sub> flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions