1,215 research outputs found
Threshold voltage and space charge in organic transistors
We investigate rubrene single-crystal field-effect transistors, whose
stability and reproducibility are sufficient to measure systematically the
shift in threshold voltage as a function of channel length and source-drain
voltage. The shift is due to space-charge transferred from the contacts, and
can be modeled quantitatively without free fitting parameters, using Poisson's
equation, and by assuming that the density of states in rubrene is that of a
conventional inorganic semiconductor. Our results demonstrate the consistency,
at the quantitative level, of a variety of recent experiments on rubrene
crystals, and show how the use of FET measurements can enable the determination
of microscopic parameters (e.g., the effective mass of charge carriers).Comment: 4 pages, 4 figure
Ambipolar Light-Emitting Transistors on Chemical Vapor Deposited Monolayer MoS2
We realize and investigate ionic liquid gated field-effect transistors (FETs)
on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under
electron accumulation, the performance of these devices is comparable to that
of FETs based on exfoliated flakes. FETs on CVD-grown material, however,
exhibit clear ambipolar transport, which for MoS2 monolayers had not been
reported previously. We exploit this property to estimate the bandgap {\Delta}
of monolayer MoS2 directly from the device transfer curves and find {\Delta}
2.4-2.7 eV. In the ambipolar injection regime, we observe
electroluminescence due to exciton recombination in MoS2, originating from the
region close to the hole-injecting contact. Both the observed transport
properties and the behavior of the electroluminescence can be consistently
understood as due to the presence of defect states at an energy of 250-300 meV
above the top of the valence band, acting as deep traps for holes. Our results
are of technological relevance, as they show that devices with useful
optoelectronic functionality can be realized on large-area MoS2 monolayers
produced by controllable and scalable techniques
Storage and retrieval of light pulses in atomic media with "slow" and "fast" light
We present experimental evidence that light storage, i.e. the controlled
release of a light pulse by an atomic sample dependent on the past presence of
a writing pulse, is not restricted to small group velocity media but can also
occur in a negative group velocity medium. A simple physical picture applicable
to both cases and previous light storage experiments is discussed.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Inhibition of electromagnetically induced absorption due to excited state decoherence in Rb vapor
The explanation presented in [Taichenachev et al, Phys. Rev. A {\bf 61},
011802 (2000)] according to which the electromagnetically induced absorption
(EIA) resonances observed in degenerate two level systems are due to coherence
transfer from the excited to the ground state is experimentally tested in a
Hanle type experiment observing the parametric resonance on the line of
Rb. While EIA occurs in the transition in a cell
containing only vapor, collisions with a buffer gas ( of )
cause the sign reversal of this resonance as a consequence of collisional
decoherence of the excited state. A theoretical model in good qualitative
agreement with the experimental results is presented.Comment: 8 pages, 7 figures, submitted to Physical Review
Progress in organic single-crystal field-effect transistors
Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic semiconductors and corresponding devices and have also led to unforeseen high values for carrier mobility in these materials. Here, we discuss developments in this area and present a brief outlook on future goals that have come into experimental reac
Temporal build-up of electromagnetically induced transparency and absorption resonances in degenerate two-level transitions
The temporal evolution of electromagnetically induced transparency (EIT) and
absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate
two-level atomic transition is studied for light intensities below saturation.
Analytical expression for the transient absorption spectra are given for simple
model systems and a model for the calculation of the time dependent response of
realistic atomic transitions, where the Zeeman degeneracy is fully accounted
for, is presented. EIT and EIA resonances have a similar (opposite sign) time
dependent lineshape, however, the EIA evolution is slower and thus narrower
lines are observed for long interaction time. Qualitative agreement with the
theoretical predictions is obtained for the transient probe absorption on the
line in an atomic beam experiment.Comment: 10 pages, 9 figures. Submitted to Phys. Rev.
Single-Crystal Organic Charge-Transfer Interfaces probed using Schottky-Gated Heterostructures
Organic semiconductors based on small conjugated molecules generally behave
as insulators when undoped, but the hetero-interfaces of two such materials can
show electrical conductivity as large as in a metal. Although charge transfer
is commonly invoked to explain the phenomenon, the details of the process and
the nature of the interfacial charge carriers remain largely unexplored. Here
we use Schottky-gated heterostructures to probe the conducting layer at the
interface between rubrene and PDIF-CN2 single crystals. Gate-modulated
conductivity measurements demonstrate that interfacial transport is due to
electrons, whose mobility exhibits band-like behavior from room temperature to
~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The
electron density decreases linearly with decreasing temperature, an observation
that can be explained quantitatively based on the heterostructure band diagram.
These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces
and show the potential of Schottky-gated organic heterostructures for the
investigation of transport in molecular semiconductors.Comment: 37 pages, 9 Figures (including supplementary information
Tuning the charge transfer in Fx-TCNQ/rubrene single-crystal interfaces
Interfaces formed by two different organic semiconductors often exhibit a
large conductivity, originating from transfer of charge between the constituent
materials. The precise mechanisms driving charge transfer and determining its
magnitude remain vastly unexplored, and are not understood microscopically. To
start addressing this issue, we have performed a systematic study of highly
reproducible single-crystal interfaces based on rubrene and Fx-TCNQ, a family
of molecules whose electron affinity can be tuned by increasing the fluorine
content. The combined analysis of transport and scanning Kelvin probe
measurements reveals that the interfacial charge carrier density, resistivity,
and activation energy correlate with the electron affinity of Fx-TCNQ crystals,
with a higher affinity resulting in larger charge transfer. Although the
transport properties can be described consistently and quantitatively using a
mobility-edge model, we find that a quantitative analysis of charge transfer in
terms of single-particle band diagrams reveals a discrepancy ~ 100 meV in the
interfacial energy level alignment. We attribute the discrepancy to phenomena
known to affect the energetics of organic semiconductors, which are neglected
by a single-particle description, such as molecular relaxation and band-gap
renormalization due to screening. The systematic behavior of the
Fx-TCNQ/rubrene interfaces opens the possibility to investigate these phenomena
experimentally, under controlled conditions
Very low bias stress in n-type organic single crystal transistors
Bias stress effects in n-channel organic field-effect transistors (OFETs) are
investigated using PDIF-CN2 single-crystal devices with Cytop gate dielectric,
both under vacuum and in ambient. We find that the amount of bias stress is
very small as compared to all (p-channel) OFETs reported in the literature.
Stressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week
results in a decrease of the source drain current of only ~1% under vacuum and
~10% in air. This remarkable stability of the devices leads to characteristic
time constants, extracted by fitting the data with a stretched exponential -
that are \tau ~ 2\cdot10^9 s in air and \tau ~ 5\cdot10^9 s in vacuum -
approximately two orders of magnitude larger than the best values reported
previously for p-channel OFETs.Comment: Submitted to Applied Physics Letters; 14 pages, 3 figure
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