15 research outputs found
Tunable Transport Gap in Phosphorene
In this article, we experimentally
demonstrate that the transport
gap of phosphorene can be tuned monotonically from âŒ0.3 to
âŒ1.0 eV when the flake thickness is scaled down from bulk to
a single layer. As a consequence, the ON current, the OFF current,
and the current ON/OFF ratios of phosphorene field effect transistors
(FETs) were found to be significantly impacted by the layer thickness.
The transport gap was determined from the transfer characteristics
of phosphorene FETs using a robust technique that has not been reported
before. The detailed mathematical model is also provided. By scaling
the thickness of the gate oxide, we were also able to demonstrate
enhanced ambipolar conduction in monolayer and few layer phosphorene
FETs. The asymmetry of the electron and the hole current was found
to be dependent on the layer thickness that can be explained by dynamic
changes of the metal Fermi level with the energy band of phosphorene
depending on the layer number. We also extracted the Schottky barrier
heights for both the electron and the hole injection as a function
of the layer thickness. Finally, we discuss the dependence of field
effect hole mobility of phosphorene on temperature and carrier concentration
Asymmetric Growth of Bilayer Graphene on Copper Enclosures Using Low-Pressure Chemical Vapor Deposition
In this work, we investigated the growth mechanisms of bilayer graphene on the outside surface of Cu enclosures at low pressures. We observed that the asymmetric growth environment of a Cu enclosure can yield a much higher (up to 100%) bilayer coverage on the outside surface as compared to the bilayer growth on a flat Cu foil, where both sides are exposed to the same growth environment. By simultaneously examining the graphene films grown on both the outside and inside surfaces of the Cu enclosure, we find that carbon can diffuse from the inside surface to the outside <i>via</i> exposed copper regions on the inside surface. The kinetics of this process are examined by coupling the asymmetric growth between the two surfaces through a carbon diffusion model. Finally, using these results, we show that the coverage of bilayer graphene can be tuned simply by changing the thickness of the Cu foil, further confirming our model of carbon delivery through the Cu foil
Black Phosphorus Radio-Frequency Transistors
Few-layer
and thin film forms of layered black phosphorus (BP) have recently
emerged as a promising material for applications in high performance nanoelectronics and infrared
optoelectronics. Layered BP thin films offer a moderate bandgap of
around 0.3 eV and high carrier mobility, which lead to transistors
with decent onâoff ratios and high on-state current densities.
Here, we demonstrate the gigahertz frequency operation of BP field-effect
transistors for the first time. The BP transistors demonstrated here
show respectable current saturation with an onâoff ratio that
exceeds 2 Ă 10<sup>3</sup>. We achieved a current density in
excess of 270 mA/mm and DC transconductance above 180 mS/mm for hole
conduction. Using standard high frequency characterization techniques,
we measured a short-circuit current-gain cutoff frequency <i>f</i><sub>T</sub> of 12 GHz and a maximum oscillation frequency <i>f</i><sub>max</sub> of 20 GHz in 300 nm channel length devices.
BP devices may offer advantages over graphene transistors for high
frequency electronics in terms of voltage and power gain due to the
good current saturation properties arising from their finite bandgap,
thus can be considered as a promising candidate for the future high
performance thin film electronics technology for operation in the
multi-GHz frequency range and beyond
Electrical Transport Properties of Polycrystalline Monolayer Molybdenum Disulfide
Semiconducting MoS<sub>2</sub> monolayers have shown many promising electrical properties, and the inevitable polycrystallinity in synthetic, large-area films renders understanding the effect of structural defects, such as grain boundaries (GBs, or line-defects in two-dimensional materials), essential. In this work, we first examine the role of GBs in the electrical-transport properties of MoS<sub>2</sub> monolayers with varying line-defect densities. We reveal a systematic degradation of electrical characteristics as the line-defect density increases. The two common MoS<sub>2</sub> GB types and their specific roles are further examined, and we find that only tilt GBs have a considerable effect on the MoS<sub>2</sub> electrical properties. By examining the electronic states and sources of disorder using temperature-dependent transport studies, we adopt the Anderson model for disordered systems to explain the observed transport behaviors in different temperature regimes. Our results elucidate the roles played by GBs in different scenarios and give insights into their underlying scattering mechanisms
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Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor
Transition
metal dichalcogenides (TMDs) are particularly sensitive
to mechanical strain because they are capable of experiencing high
atomic displacements without nucleating defects to release excess
energy. Being promising for photonic applications, it has been shown
that as certain phases of layered TMDs MX<sub>2</sub> (M = Mo or W;
X = S, Se, or Te) are scaled to a thickness of one monolayer, the
photoluminescence response is dramatically enhanced due to the emergence
of a direct electronic band gap compared with their multilayer or
bulk counterparts, which typically exhibit indirect band gaps. Recently,
mechanical strain has also been predicted to enable direct excitonic
recombination in these materials, in which large changes in the photoluminescence
response will occur during an indirect-to-direct band gap transition
brought on by elastic tensile strain. Here, we demonstrate an enhancement
of 2 orders of magnitude in the photoluminescence emission intensity
in uniaxially strained single crystalline WSe<sub>2</sub> bilayers.
Through a theoretical model that includes experimentally relevant
system conditions, we determine this amplification to arise from a
significant increase in direct excitonic recombination. Adding confidence
to the high levels of elastic strain achieved in this report, we observe
strain-independent, mode-dependent GruÌneisen parameters over
the entire range of tensile strain (1â3.59%), which were obtained
as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103
for the E<sub>2g</sub>, A<sub>1g</sub>, and A<sup>2</sup><sub>1g</sub> optical phonon modes, respectively. These results can inform the
predictive strain-engineered design of other atomically thin indirect
semiconductors, in which a decrease in out-of-plane bonding strength
may lead to an increase in the strength of strain-coupled optoelectronic
effects
Integrated Circuits Based on Bilayer MoS<sub>2</sub> Transistors
Two-dimensional (2D) materials, such as molybdenum disulfide
(MoS<sub>2</sub>), have been shown to exhibit excellent electrical
and optical
properties. The semiconducting nature of MoS<sub>2</sub> allows it
to overcome the shortcomings of zero-bandgap graphene, while still
sharing many of grapheneâs advantages for electronic and optoelectronic
applications. Discrete electronic and optoelectronic components, such
as field-effect transistors, sensors, and photodetectors made from
few-layer MoS<sub>2</sub> show promising performance as potential
substitute of Si in conventional electronics and of organic and amorphous
Si semiconductors in ubiquitous systems and display applications.
An important next step is the fabrication of fully integrated multistage
circuits and logic building blocks on MoS<sub>2</sub> to demonstrate
its capability for complex digital logic and high-frequency ac applications.
This paper demonstrates an inverter, a NAND gate, a static random
access memory, and a five-stage ring oscillator based on a direct-coupled
transistor logic technology. The circuits comprise between 2 to 12
transistors seamlessly integrated side-by-side on a single sheet of
bilayer MoS<sub>2</sub>. Both enhancement-mode and depletion-mode
transistors were fabricated thanks to the use of gate metals with
different work functions
A Dynamically Reconfigurable Ambipolar Black Phosphorus Memory Device
Nonvolatile
charge-trap memory plays an important role in many
modern electronics technologies, from portable electronic systems
to large-scale data centers. Conventional charge-trap memory devices
typically work with fixed channel carrier polarity and device characteristics.
However, many emerging applications in reconfigurable electronics
and neuromorphic computing require dynamically tunable properties
in their electronic device components that can lead to enhanced circuit
versatility and system functionalities. Here, we demonstrate an ambipolar
black phosphorus (BP) charge-trap memory device with dynamically reconfigurable
and polarity-reversible memory behavior. This BP memory device shows
versatile memory properties subject to electrostatic bias. Not only
the programmed/erased state current ratio can be continuously tuned
by the back-gate bias, but also the polarity of the carriers in the
BP channel can be reversibly switched between electron- and hole-dominated
conductions, resulting in the erased and programmed states exhibiting
interchangeable high and low current levels. The BP memory also shows
four different memory states and, hence, 2-bit per cell data storage
for both n-type and p-type channel conductions, demonstrating the
multilevel cell storage capability in a layered material based memory
device. The BP memory device with a high mobility and tunable programmed/erased
state current ratio and highly reconfigurable device characteristics
can offer adaptable memory device properties for many emerging applications
in electronics technology, such as neuromorphic computing, data-adaptive
energy efficient memory, and dynamically reconfigurable digital circuits
Intricate Resonant Raman Response in Anisotropic ReS<sub>2</sub>
The
strong in-plane anisotropy of rhenium disulfide (ReS<sub>2</sub>)
offers an additional physical parameter that can be tuned for advanced
applications such as logic circuits, thin-film polarizers, and polarization-sensitive
photodetectors. ReS<sub>2</sub> also presents advantages for optoelectronics,
as it is both a direct-gap semiconductor for few-layer thicknesses
(unlike MoS<sub>2</sub> or WS<sub>2</sub>) and stable in air (unlike
black phosphorus). Raman spectroscopy is one of the most powerful
characterization techniques to nondestructively and sensitively probe
the fundamental photophysics of a 2D material. Here, we perform a
thorough study of the resonant Raman response of the 18 first-order
phonons in ReS<sub>2</sub> at various layer thicknesses and crystal
orientations. Remarkably, we discover that, as opposed to a general
increase in intensity of all of the Raman modes at excitonic transitions,
each of the 18 modes behave <i>differently</i> relative
to each other as a function of laser excitation, layer thickness,
and orientation in a manner that highlights the importance of electronâphonon
coupling in ReS<sub>2</sub>. In addition, we correct an unrecognized
error in the calculation of the optical interference enhancement of
the Raman signal of transition metal dichalcogenides on SiO<sub>2</sub>/Si substrates that has propagated through various reports. For ReS<sub>2</sub>, this correction is critical to properly assessing the resonant
Raman behavior. We also implemented a perturbation approach to calculate
frequency-dependent Raman intensities based on first-principles and
demonstrate that, despite the neglect of excitonic effects, useful
trends in the Raman intensities of monolayer and bulk ReS<sub>2</sub> at different laser energies can be accurately captured. Finally,
the phonon dispersion calculated from first-principles is used to
address the possible origins of unexplained peaks observed in the
Raman spectra, such as infrared-active modes, defects, and second-order
processes
Graphene/MoS<sub>2</sub> Hybrid Technology for Large-Scale Two-Dimensional Electronics
Two-dimensional (2D) materials have
generated great interest in
the past few years as a new toolbox for electronics. This family of
materials includes, among others, metallic graphene, semiconducting
transition metal dichalcogenides (such as MoS<sub>2</sub>), and insulating
boron nitride. These materials and their heterostructures offer excellent
mechanical flexibility, optical transparency, and favorable transport
properties for realizing electronic, sensing, and optical systems
on arbitrary surfaces. In this paper, we demonstrate a novel technology
for constructing large-scale electronic systems based on graphene/molybdenum
disulfide (MoS<sub>2</sub>) heterostructures grown by chemical vapor
deposition. We have fabricated high-performance devices and circuits
based on this heterostructure, where MoS<sub>2</sub> is used as the
transistor channel and graphene as contact electrodes and circuit
interconnects. We provide a systematic comparison of the graphene/MoS<sub>2</sub> heterojunction contact to more traditional MoS<sub>2</sub>-metal junctions, as well as a theoretical investigation, using density
functional theory, of the origin of the Schottky barrier height. The
tunability of the graphene work function with electrostatic doping
significantly improves the ohmic contact to MoS<sub>2</sub>. These
high-performance large-scale devices and circuits based on this 2D
heterostructure pave the way for practical flexible transparent electronics
Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy
The growth of large-area bilayer
graphene has been of technological
importance for graphene electronics. The successful application of
graphene bilayers critically relies on the precise control of the
stacking orientation, which determines both electronic and vibrational
properties of the bilayer system. Toward this goal, an effective characterization
method is critically needed to allow researchers to easily distinguish
the bilayer stacking orientation (i.e., AB stacked or turbostratic).
In this work, we developed such a method to provide facile identification
of the stacking orientation by isotope labeling. Raman spectroscopy
of these isotopically labeled bilayer samples shows a clear signature
associated with AB stacking between layers, enabling rapid differentiation
between turbostratic and AB-stacked bilayer regions. Using this method,
we were able to characterize the stacking orientation in bilayer graphene
grown through Low Pressure Chemical Vapor Deposition (LPCVD) with
enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene.
Furthermore, by combining surface sensitive fluorination with such
hybrid <sup>12</sup>C/<sup>13</sup>C bilayer samples, we are able
to identify that the second layer grows underneath the first-grown
layer, which is similar to a recently reported observation