57 research outputs found
Growth methods for controlled large-area fabrication of high-quality graphene analogs
In some embodiments, the present disclosure pertains to methods of growing chalcogen-linked metallic films on a surface in a chamber. In some embodiments, the method comprises placing a metal source and a chalcogen source in the chamber, and gradually heating the chamber, where the heating leads to the chemical vapor deposition of the chalcogen source and the metal source onto the surface, and facilitates the growth of the chalcogen-linked metallic film from the chalcogen source and the metal source on the surface. In some embodiments, the chalcogen source comprises sulfur, and the metal source comprises molybdenum trioxide. In some embodiments, the growth of the chalcogen-linked metallic film occurs by formation of nucleation sites on the surface, where the nucleation sites merge to form the chalcogen-linked metallic film. In some embodiments, the formed chalcogen-linked metallic film includes MoS2
Second harmonic microscopy of monolayer MoS2
We show that the lack of inversion symmetry in monolayer MoS2 allows strong
optical second harmonic generation. Second harmonic of an 810-nm pulse is
generated in a mechanically exfoliated monolayer, with a nonlinear
susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor
of seven in trilayers, and by about two orders of magnitude in even layers. A
proof-of-principle second harmonic microscopy measurement is performed on
samples grown by chemical vapor deposition, which illustrates potential
applications of this effect in fast and non-invasive detection of crystalline
orientation, thickness uniformity, layer stacking, and single-crystal domain
size of atomically thin films of MoS2 and similar materials.Comment: 6 pages, 4 figure
Blue shifting of the A exciton peak in folded monolayer 1H-MoS2
The large family of layered transition-metal dichalcogenides is widely
believed to constitute a second family of two-dimensional (2D) semiconducting
materials that can be used to create novel devices that complement those based
on graphene. In many cases these materials have shown a transition from an
indirect bandgap in the bulk to a direct bandgap in monolayer systems. In this
work we experimentally show that folding a 1H molybdenum disulphide (MoS2)
layer results in a turbostratic stack with enhanced photoluminescence quantum
yield and a significant shift to the blue by 90 meV. This is in contrast to the
expected 2H-MoS2 band structure characteristics, which include an indirect gap
and quenched photoluminescence. We present a theoretical explanation to the
origin of this behavior in terms of exciton screening.Comment: 16 pages, 8 figure
Growth-substrate induced performance degradation in chemically synthesized monolayer MoS2ᅠfield effect transistors
We report on the electronic transport properties of single-layer thick chemical vapor deposition (CVD) grown molybdenum disulfide (MoS2) field-effect transistors (FETs) on Si/SiO2 substrates. MoS2 has been extensively investigated for the past two years as a potential semiconductor analogue to graphene. To date, MoS2 samples prepared via mechanical exfoliation have demonstrated field-effect mobility values which are significantly higher than that of CVD-grown MoS2. In this study, we will show that the intrinsic electronic performance of CVD-grown MoS2 is equal or superior to that of exfoliated material and has been possibly masked by a combination of interfacial contamination on the growth substrate and residual tensile strain resulting from the high-temperature growth process. We are able to quantify this strain in the as-grown material using pre- and post-transfer metrology and microscopy of the same crystals. Moreover, temperature-dependent electrical measurements made on as-grown and transferred MoS2 devices following an identical fabrication process demonstrate the improvement in field-effect mobility
Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition
Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth
large-area single layers grown by chemical vapor deposition. The layer qualities and physical
properties were characterized using high-resolution Raman and photoluminescence spectroscopy,
scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET
devices was measured using field effect mobility measurements as a function of temperature. The
back-gated devices had mobilities of 6.0 cm2/V s at 300K without a high-j dielectric overcoat and
increased to 16.1 cm2/V s with a high-j dielectric overcoat. In addition the devices show on/off
ratios ranging from 105 to 109
Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: an Insight into Current Flow across Schottky Barriers
In this article, we study the properties of metal contacts to single-layer
molybdenum disulfide (MoS2) crystals, revealing the nature of switching
mechanism in MoS2 transistors. On investigating transistor behavior as contact
length changes, we find that the contact resistivity for metal/MoS2 junctions
is defined by contact area instead of contact width. The minimum gate dependent
transfer length is ~0.63 {\mu}m in the on-state for metal (Ti) contacted
single-layer MoS2. These results reveal that MoS2 transistors are Schottky
barrier transistors, where the on/off states are switched by the tuning the
Schottky barriers at contacts. The effective barrier heights for source and
drain barriers are primarily controlled by gate and drain biases, respectively.
We discuss the drain induced barrier narrowing effect for short channel
devices, which may reduce the influence of large contact resistance for MoS2
Schottky barrier transistors at the channel length scaling limit.Comment: ACS Nano, ASAP (2013
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