12 research outputs found
Mechanisms of Ultrafast Charge Separation in a PTB7/Monolayer MoS<sub>2</sub> van der Waals Heterojunction
Mixed-dimensional
van der Waals heterojunctions comprising polymer
and two-dimensional (2D) semiconductors have many characteristics
of an ideal charge separation interface for optoelectronic and photonic
applications. However, the photoelectron dynamics at polymerā2D
semiconductor heterojunction interfaces are currently not sufficiently
understood to guide the optimization of devices for these applications.
This Letter reports a systematic exploration of the time-dependent
photophysical processes that occur upon photoexcitation of a type-II
heterojunction between the polymer PTB7 and monolayer MoS<sub>2</sub>. In particular, photoinduced electron transfer from PTB7 to electronically
hot states of MoS<sub>2</sub> occurs in less than 250 fs. This process
is followed by a 1ā5 ps exciton diffusion-limited electron
transfer from PTB7 to MoS<sub>2</sub> and a sub-3 ps photoinduced
hole transfer from MoS<sub>2</sub> to PTB7. The equilibrium between
excitons and polaron pairs in PTB7 determines the charge separation
yield, whereas the 3ā4 ns lifetime of photogenerated carriers
is probably limited by MoS<sub>2</sub> defects
Ultrafast Exciton Dissociation and Long-Lived Charge Separation in a Photovoltaic PentaceneāMoS<sub>2</sub> van der Waals Heterojunction
van
der Waals heterojunctions between two-dimensional (2D) layered materials
and nanomaterials of different dimensions present unique opportunities
for gate-tunable optoelectronic devices. Mixed-dimensional pān
heterojunction diodes, such as p-type pentacene (0D) and n-type monolayer
MoS<sub>2</sub> (2D), are especially interesting for photovoltaic
applications where the absorption cross-section and charge transfer
processes can be tailored by rational selection from the vast library
of organic molecules and 2D materials. Here, we study the kinetics
of excited carriers in pentaceneāMoS<sub>2</sub> pān
type-II heterojunctions by transient absorption spectroscopy. These
measurements show that the dissociation of MoS<sub>2</sub> excitons
occurs by hole transfer to pentacene on the time scale of 6.7 ps.
In addition, the charge-separated state lives for 5.1 ns, up to an
order of magnitude longer than the recombination lifetimes from previously
reported 2D material heterojunctions. By studying the fractional amplitudes
of the MoS<sub>2</sub> decay processes, the hole transfer yield from
MoS<sub>2</sub> to pentacene is found to be ā¼50%, with the
remaining holes undergoing trapping due to surface defects. Overall,
the ultrafast charge transfer and long-lived charge-separated state
in pentaceneāMoS<sub>2</sub> pān heterojunctions suggest
significant promise for mixed-dimensional van der Waals heterostructures
in photovoltaics, photodetectors, and related optoelectronic technologies
Elucidating the Photoresponse of Ultrathin MoS<sub>2</sub> Field-Effect Transistors by Scanning Photocurrent Microscopy
The mechanisms underlying the intrinsic
photoresponse of few-layer
(FL) molybdenum disulfide (MoS<sub>2</sub>) field-effect transistors
are investigated via scanning photocurrent microscopy. We attribute
the locally enhanced photocurrent to band-bending-assisted separation
of photoexcited carriers at the MoS<sub>2</sub>/Au interface. The
wavelength-dependent photocurrents of FL MoS<sub>2</sub> transistors
qualitatively follow the optical absorption spectra of MoS<sub>2</sub>, providing direct evidence of interband photoexcitation. Time and
spectrally resolved photocurrent measurements at varying external
electric fields and carrier concentrations establish that drift-diffusion
currents dominate photothermoelectric currents in devices under bias
Charge Separation at Mixed-Dimensional Single and Multilayer MoS<sub>2</sub>/Silicon Nanowire Heterojunctions
Layered
two-dimensional (2-D) semiconductors can be combined with
other low-dimensional semiconductors to form nonplanar mixed-dimensional
van der Waals (vdW) heterojunctions whose charge transport behavior
is influenced by the heterojunction geometry, providing a new degree
of freedom to engineer device functions. Toward that end, we investigated
the photoresponse of Si nanowire/MoS<sub>2</sub> heterojunction diodes
with scanning photocurrent microscopy and time-resolved photocurrent
measurements. Comparison of n-Si/MoS<sub>2</sub> isotype heterojunctions
with p-Si/MoS<sub>2</sub> heterojunction diodes under varying biases
shows that the depletion region in the pān heterojunction promotes
exciton dissociation and carrier collection. We measure an instrument-limited
response time of 1 Ī¼s, which is 10 times faster than the previously
reported response times for planar Si/MoS<sub>2</sub> devices, highlighting
the advantages of the 1-D/2-D heterojunction. Finite element simulations
of device models provide a detailed understanding of how the electrostatics
affect charge transport in nanowire/vdW heterojunctions and inform
the design of future vdW heterojunction photodetectors and transistors
Solution-Processed Self-Assembled Nanodielectrics on Template-Stripped Metal Substrates
The coupling of hybrid organicāinorganic
gate dielectrics with emergent unconventional semiconductors has yielded
transistor devices exhibiting record-setting transport properties.
However, extensive electronic transport measurements on these high-capacitance
systems are often convoluted with the electronic response of the semiconducting
silicon substrate. In this report, we demonstrate the growth of solution-processed
zirconia self-assembled nanodielectrics (Zr-SAND) on template-stripped
aluminum substrates. The resulting Zr-SAND on Al structures leverage
the ultrasmooth (r.m.s. roughness <0.4 nm), chemically uniform
nature of template-stripped metal substrates to demonstrate the same
exceptional electronic uniformity (capacitance ā¼700 nF cm<sup>ā2</sup>, leakage current <1 Ī¼A cm<sup>ā2</sup> at ā2 MV cm<sup>ā1</sup>) and multilayer growth of
Zr-SAND on Si, while exhibiting superior temperature and voltage capacitance
responses. These results are important to conduct detailed transport
measurements in emergent transistor technologies featuring SAND as
well as for future applications in integrated circuits or flexible
electronics
Fundamental Performance Limits of Carbon Nanotube Thin-Film Transistors Achieved Using Hybrid Molecular Dielectrics
In the past decade, semiconducting carbon nanotube thin films have been recognized as contending materials for wide-ranging applications in electronics, energy, and sensing. In particular, improvements in large-area flexible electronics have been achieved through independent advances in postgrowth processing to resolve metallic <i>versus</i> semiconducting carbon nanotube heterogeneity, in improved gate dielectrics, and in self-assembly processes. Moreover, controlled tuning of specific device components has afforded fundamental probes of the trade-offs between materials properties and device performance metrics. Nevertheless, carbon nanotube transistor performance suitable for real-world applications awaits understanding-based progress in the integration of independently pioneered device components. We achieve this here by integrating high-purity semiconducting carbon nanotube films with a custom-designed hybrid inorganicāorganic gate dielectric. This synergistic combination of materials circumvents conventional design trade-offs, resulting in concurrent advances in several transistor performance metrics such as transconductance (6.5 Ī¼S/Ī¼m), intrinsic field-effect mobility (147 cm<sup>2</sup>/(V s)), subthreshold swing (150 mV/decade), and on/off ratio (5 Ć 10<sup>5</sup>), while also achieving hysteresis-free operation in ambient conditions
Large-Area, Low-Voltage, Antiambipolar Heterojunctions from Solution-Processed Semiconductors
The emergence of semiconducting materials
with inert or dangling bond-free surfaces has created opportunities
to form van der Waals heterostructures without the constraints of
traditional epitaxial growth. For example, layered two-dimensional
(2D) semiconductors have been incorporated into heterostructure devices
with gate-tunable electronic and optical functionalities. However,
2D materials present processing challenges that have prevented these
heterostructures from being produced with sufficient scalability and/or
homogeneity to enable their incorporation into large-area integrated
circuits. Here, we extend the concept of van der Waals heterojunctions
to semiconducting p-type single-walled carbon nanotube (s-SWCNT) and
n-type amorphous indium gallium zinc oxide (a-IGZO) thin films that
can be solution-processed or sputtered with high spatial uniformity
at the wafer scale. The resulting large-area, low-voltage pān
heterojunctions exhibit antiambipolar transfer characteristics with
high on/off ratios that are well-suited for electronic, optoelectronic,
and telecommunication technologies
Hybrid, Gate-Tunable, van der Waals pān Heterojunctions from Pentacene and MoS<sub>2</sub>
The
recent emergence of a wide variety of two-dimensional (2D)
materials has created new opportunities for device concepts and applications.
In particular, the availability of semiconducting transition metal
dichalcogenides, in addition to semimetallic graphene and insulating
boron nitride, has enabled the fabrication of āall 2Dā
van der Waals heterostructure devices. Furthermore, the concept of
van der Waals heterostructures has the potential to be significantly
broadened beyond layered solids. For example, molecular and polymeric
organic solids, whose surface atoms possess saturated bonds, are also
known to interact via van der Waals forces and thus offer an alternative
for scalable integration with 2D materials. Here, we demonstrate the
integration of an organic small molecule p-type semiconductor, pentacene,
with a 2D n-type semiconductor, MoS<sub>2</sub>. The resulting pān
heterojunction is gate-tunable and shows asymmetric control over the
antiambipolar transfer characteristic. In addition, the pentacene/MoS<sub>2</sub> heterojunction exhibits a photovoltaic effect attributable
to type II band alignment, which suggests that MoS<sub>2</sub> can
function as an acceptor in hybrid solar cells
High-Field Transport and Thermal Reliability of Sorted Carbon Nanotube Network Devices
We examine the high-field operation, power dissipation, and thermal reliability of sorted carbon nanotube network (CNN) devices, with <1% to >99% semiconducting nanotubes. We combine systematic electrical measurements with infrared (IR) thermal imaging and detailed Monte Carlo simulations to study high-field transport up to CNN failure by unzipping-like breakdown. We find that metallic CNNs carry peak current densities up to an order of magnitude greater than semiconducting CNNs at comparable nanotube densities. Metallic CNNs also appear to have a factor of 2 lower intrinsic thermal resistance, suggesting a lower thermal resistance at metallic nanotube junctions. The performance limits and reliability of CNNs depend on their makeup, and could be improved by carefully engineered heat dissipation through the substrate, contacts, and nanotube junctions. These results are essential for optimization of CNN devices on transparent or flexible substrates which typically have very low thermal conductivity
High-Field Transport and Thermal Reliability of Sorted Carbon Nanotube Network Devices
We examine the high-field operation, power dissipation, and thermal reliability of sorted carbon nanotube network (CNN) devices, with <1% to >99% semiconducting nanotubes. We combine systematic electrical measurements with infrared (IR) thermal imaging and detailed Monte Carlo simulations to study high-field transport up to CNN failure by unzipping-like breakdown. We find that metallic CNNs carry peak current densities up to an order of magnitude greater than semiconducting CNNs at comparable nanotube densities. Metallic CNNs also appear to have a factor of 2 lower intrinsic thermal resistance, suggesting a lower thermal resistance at metallic nanotube junctions. The performance limits and reliability of CNNs depend on their makeup, and could be improved by carefully engineered heat dissipation through the substrate, contacts, and nanotube junctions. These results are essential for optimization of CNN devices on transparent or flexible substrates which typically have very low thermal conductivity