75 research outputs found
Electronic Transport in Two-Dimensional Materials
Two-dimensional (2D) materials have captured the attention of the scientific
community due to the wide range of unique properties at nanometer-scale
thicknesses. While significant exploratory research in 2D materials has been
achieved, the understanding of 2D electronic transport and carrier dynamics
remains in a nascent stage. Furthermore, since prior review articles have
provided general overviews of 2D materials or specifically focused on charge
transport in graphene, here we instead highlight charge transport mechanisms in
post-graphene 2D materials with particular emphasis on transition metal
dichalcogenides and black phosphorus. For these systems, we delineate the
intricacies of electronic transport including bandstructure control with
thickness and external fields, valley polarization, scattering mechanisms,
electrical contacts, and doping. In addition, electronic interactions between
2D materials are considered in the form of van der Waals heterojunctions and
composite films. This review concludes with a perspective on the most promising
future directions in this fast-evolving field.Comment: 48 pages, 8 figures, Annual Reviews of Physical Chemistr
Characterizing Voltage Contrast in Photoelectron Emission Microscopy
A non-destructive technique for obtaining voltage contrast information with
photoelectron emission microscopy (PEEM) is described. Samples consisting of
electrically isolated metal lines were used to quantify voltage contrast in
PEEM. The voltage contrast behavior is characterized by comparing measured
voltage contrast with calculated voltage contrast from two electrostatic
models. Measured voltage contrast was found to agree closely with the
calculated voltage contrast, demonstrating that voltage contrast in PEEM can be
used to probe local voltage information in microelectronic devices in a
non-intrusive fashion.Comment: 26 pages, 8 figure
Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing
In the last three decades, zero-dimensional, one-dimensional, and
two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and
graphene, respectively) have attracted significant attention from the
scientific community due to their unique electronic, optical, thermal,
mechanical, and chemical properties. While early work showed that these
properties could enable high performance in selected applications, issues
surrounding structural inhomogeneity and imprecise assembly have impeded robust
and reliable implementation of carbon nanomaterials in widespread technologies.
However, with recent advances in synthesis, sorting, and assembly techniques,
carbon nanomaterials are experiencing renewed interest as the basis of numerous
scalable technologies. Here, we present an extensive review of carbon
nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices
with a particular focus on the latest examples based on the highest purity
samples. Specific attention is devoted to each class of carbon nanomaterial,
thereby allowing comparative analysis of the suitability of fullerenes, carbon
nanotubes, and graphene for each application area. In this manner, this article
will provide guidance to future application developers and also articulate the
remaining research challenges confronting this field.Comment: Review article, 15 figure
Gate-tunable memristors from monolayer MoS2
We report here gate-tunable memristors based on monolayer MoS2 grown by
chemical vapor deposition (CVD). These memristors are fabricated in a
field-effect geometry with the channel consisting of polycrystalline MoS2 films
with grain sizes of 3-5 um. The device characteristics show switching ratios up
to 500, with the resistance in individual states being continuously
gate-tunable by over three orders of magnitude. The resistive switching results
from dynamically varying threshold voltage and Schottky barrier heights, whose
underlying physical mechanism appears to be vacancy migration and/or charge
trapping. Top-gated devices achieve reversible tuning of the threshold voltage,
with potential utility in non-volatile memory or neuromorphic architectures
Elucidating the photoresponse of ultrathin MoS2 field-effect transistors by scanning photocurrent microscopy
The mechanisms underlying the intrinsic photoresponse of few-layer (FL)
molybdenum disulphide (MoS2) 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 MoS2/Au interface. The wavelength-dependent photocurrents of few layer MoS2
transistors qualitatively follow the optical absorption spectra of MoS2,
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.Comment: 4 figure letter + supporting information. Journal of Physical
Chemistry Letters (2013
Low Frequency Electronic Noise in Single-Layer MoS2 Transistors
Ubiquitous low frequency 1/f noise can be a limiting factor in the
performance and application of nanoscale devices. Here, we quantitatively
investigate low frequency electronic noise in single-layer transition metal
dichalcogenide MoS2 field-effect transistors. The measured 1/f noise can be
explained by an empirical formulation of mobility fluctuations with the Hooge
parameter ranging between 0.005 and 2.0 in vacuum (< 10-5 Torr). The
field-effect mobility decreased and the noise amplitude increased by an order
of magnitude in ambient conditions, revealing the significant influence of
atmospheric adsorbates on charge transport. In addition, single Lorentzian
generation-recombination noise was observed to increase by an order of
magnitude as the devices were cooled from 300 K to 6.5 K.Comment: Nano Letters (2013
Low Frequency Carrier Kinetics in Perovskite Solar Cells
Hybrid organic-inorganic halide perovskite solar cells have emerged as
leading candidates for third-generation photovoltaic technology. Despite the
rapid improvement in power conversion efficiency (PCE) for perovskite solar
cells in recent years, the low-frequency carrier kinetics that underlie
practical roadblocks such as hysteresis and degradation remain relatively
poorly understood. In an effort to bridge this knowledge gap, we perform here
correlated low-frequency noise (LFN) and impedance spectroscopy (IS)
characterization that elucidates carrier kinetics in operating perovskite solar
cells. Specifically, we focus on planar cell geometries with a SnO2 electron
transport layer and two different hole transport layers, namely,
poly(triarylamine) (PTAA) and Spiro-OMeTAD. PTAA and Sprio-OMeTAD cells with
moderate PCEs of 5 to 12 percent possess a Lorentzian feature at 200 Hz in LFN
measurements that corresponds to a crossover from electrode to dielectric
polarization. In comparison, Spiro-OMeTAD cells with high PCEs (15 percent)
show four orders of magnitude lower LFN amplitude and are accompanied by a
cyclostationary process. Through a systematic study of more than a dozen solar
cells, we establish a correlation with noise amplitude, power conversion
efficiency, and fill factor. Overall, this work establishes correlated LFN and
IS as an effective methodology for quantifying low frequency carrier kinetics
in perovskite solar cells, thereby providing new physical insights that can
rationally guide ongoing efforts to improve device performance,
reproducibility, and stability
Correlated In-Situ Low-Frequency Noise and Impedance Spectroscopy Reveal Recombination Dynamics in Organic Solar Cells using Fullerene and Non-Fullerene Acceptors
Non-fullerene acceptors based on perylenediimides (PDIs) have garnered
significant interest as an alternative to fullerene acceptors in organic
photovoltaics (OPVs), but their charge transport phenomena are not well
understood, especially in bulk heterojunctions (BHJs). Here, we investigate
charge transport and current fluctuations by performing correlated
low-frequency noise and impedance spectroscopy measurements on two BHJ OPV
systems, one employing a fullerene acceptor and the other employing a dimeric
PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a
greater degree of recombination in comparison to fullerene-based OPVs.
Furthermore, for the first time in organic solar cells, 1/f noise data are fit
to the Kleinpenning model to reveal underlying current fluctuations in
different transport regimes. Under illumination, 1/f noise increases by
approximately four orders of magnitude for the fullerene-based OPVs and three
orders of magnitude for the PDI-based OPVs. An inverse correlation is also
observed between noise spectral density and power conversion efficiency.
Overall, these results show that low-frequency noise spectroscopy is an
effective in-situ diagnostic tool to assess charge transport in emerging
photovoltaic materials, thereby providing quantitative guidance for the design
of next-generation solar cell materials and technologies.Comment: 37 pages, 7 figure
Extrinsic and Intrinsic Photoresponse in Monodisperse Carbon Nanotube Thin Film Transistors
Spectroscopic, time-resolved scanning photocurrent microscopy is shown to
distinguish the intrinsic photoresponse of monodisperse semiconducting (99%)
single-walled carbon nanotubes (SWCNTs) from the extrinsic photoresponse of the
substrate. A persistent positive photocurrent induced by near-IR excitation is
attributed to the generation of free carriers by inter-band excitation in
SWCNTs. For shorter excitation wavelengths, absorption by the Si substrate
generates two types of photocurrent: a transient positive photoresponse,
identified as a displacement current, and a persistent negative photocurrent
that arises from photogating of the SWCNT thin film
Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides
With advances in exfoliation and synthetic techniques, atomically thin films
of semiconducting transition metal dichalcogenides have recently been isolated
and characterized. Their two-dimensional structure, coupled with a direct band
gap in the visible portion of the electromagnetic spectrum, suggests
suitability for digital electronics and optoelectronics. Towards that end,
several classes of high-performance devices have been reported along with
significant progress in understanding their physical properties. Here, we
present a review of the architecture, operating principles, and physics of
electronic and optoelectronic devices based on ultrathin transition metal
dichalcogenide semiconductors. By critically assessing and comparing the
performance of these devices with competing technologies, the merits and
shortcomings of this emerging class of electronic materials are identified,
thereby providing a roadmap for future development.Comment: Review article, 10 figures. ACS Nano, 2014, Article ASA
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