9 research outputs found
Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure
Interfaces
between organic semiconductors and dielectrics may exhibit
interfacial electronic polarization, which is equivalently quantified
as a contact potential difference (CPD), an interface dipole, or a
vacuum level shift. Here we report quantitative measurements by scanning
Kelvin probe microscopy (SKPM) of surface potentials and CPDs across
ultrathin (1ā2 monolayer) crystalline islands of the benchmark
semiconductor pentacene thermally deposited on a variety of polymer
dielectrics (e.g., polyĀ(methyl methacrylate), polystyrene). The CPDs
between the pentacene islands and the polymer substrates are in the
range of ā10 to +50 mV, they depend strongly on the polymer
type and deposition temperature, and the CPD magnitude is correlated
with the dipole moment of the characteristic monomers. Surface potential
variations within 2 monolayer (3 nm) thick pentacene islands are ā¼15
mV and may be ascribed to microstructure (epitaxial) differences.
Overall, the microscopy results reveal both strong variations in interfacial
polarization and lateral electrostatic heterogeneity; these factors
ultimately should affect the performance of these interfaces in devices
Measuring the Thickness and Potential Profiles of the Space-Charge Layer at Organic/Organic Interfaces under Illumination and in the Dark by Scanning Kelvin Probe Microscopy
Scanning
Kelvin probe microscopy was used to measure band-bending
at the model donor/acceptor heterojunction polyĀ(3-hexylthiophene)
(P3HT)/fullerene (C<sub>60</sub>). Specifically, we measured the variation
in the surface potential of C<sub>60</sub> films with increasing thicknesses
grown on P3HT to produce a surface potential profile normal to the
substrate both in the dark and under illumination. The results confirm
a space-charge carrier region with a thickness of 10 nm, consistent
with previous observations. We discuss the possibility that the domain
size in bulk heterojunction organic solar cells, which is comparable
to the space-charge layer thickness, is actually partly responsible
for less than expected electron/hole recombination rates
Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function
We report the results of an extensive investigation of metalāmoleculeāmetal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M<sub>1</sub>āSā(C<sub>6</sub>H<sub>4</sub>)<i><sub>n</sub></i>āSāM<sub>2</sub>, with 1 ā¤ <i>n</i> ā¤ 4 and M<sub>1,2</sub> = Ag, Au, Pt) to examine the impact of molecular length (<i>n</i>) and metal work function (Ī¦) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ĪĪ¦ = Ī¦<sub>SAM</sub> ā Ī¦) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction currentāvoltage (<i>I</i>ā<i>V</i>) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full <i>I</i>ā<i>V</i> curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment Īµ<sub>h</sub> = <i>E</i><sub>F</sub> ā <i>E</i><sub>HOMO</sub> of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i><sub>F</sub> of the junction. Where photoelectron spectroscopy data are available, the Īµ<sub>h</sub> values agree very well with those determined by TVS. Using a single-level model, which we justify <i>via ab initio</i> quantum chemical calculations at post-density functional theory level and additional UVāvisible absorption measurements, we are able to quantitatively reproduce the <i>I</i>ā<i>V</i> measurements in the whole bias range investigated (ā¼1.0ā1.5 V) and to understand the behavior of Īµ<sub>h</sub> and Ī (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metalāS bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in Īµ<sub>h</sub> exhibiting a weak dependence with the work function Ī¦. Both of these parameters play a key role in determining the tunneling attenuation factor (Ī²) and junction resistance (<i>R</i>). Correlation among Ī¦, ĪĪ¦, <i>R</i>, transition voltage (<i>V</i><sub>t</sub>), and Īµ<sub>h</sub> and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions
Exceptionally Small Statistical Variations in the Transport Properties of MetalāMoleculeāMetal Junctions Composed of 80 OligoĀphenylene Dithiol Molecules
Strong stochastic
fluctuations witnessed as very broad resistance
(<i>R</i>) histograms with widths comparable to or even
larger than the most probable values characterize many measurements
in the field of molecular electronics, particularly those measurements
based on single molecule junctions at room temperature. Here we show
that molecular junctions containing 80 oligophenylene dithiol molecules
(OPDn, 1 ā¤ <i>n</i> ā¤ 4) connected in parallel
display small relative statistical deviationsīøĪ“<i>R</i>/<i>R</i> ā 25% after only ā¼200
independent measurementsīøand we analyze the sources of these
deviations quantitatively. The junctions are made by conducting probe
atomic force microscopy (CP-AFM) in which an Au-coated tip contacts
a self-assembled monolayer (SAM) of OPDs on Au. Using contact mechanics
and direct measurements of the molecular surface coverage, the tip
radius, tip-SAM adhesion force (<i>F</i>), and sample elastic
modulus (<i>E</i>), we find that the tip-SAM contact area
is approximately 25 nm<sup>2</sup>, corresponding to about 80 molecules
in the junction. Supplementing this information with <i>IāV</i> data and an analytic transport model, we are able to quantitatively
describe the sources of deviations <i>Ī“R</i> in <i>R</i>: namely, <i>Ī“N</i> (deviations in the
number of molecules in the junction), <i>Ī“Īµ</i> (deviations in energetic position of the dominant molecular orbital),
and <i>Ī“Ī</i> (deviations in molecule-electrode
coupling). Our main results are (1) direct determination of <i>N</i>; (2) demonstration that <i>Ī“N</i>/<i>N</i> for CP-AFM junctions is remarkably small (ā¤2%)
and that the largest contributions to <i>Ī“R</i> are <i>Ī“Īµ</i> and <i>Ī“Ī</i>; (3)
demonstration that Ī“<i>R</i>/<i>R</i> after
only ā¼200 measurements is substantially smaller than most reports
based on >1000 measurements for single molecule break junctions.
Overall,
these results highlight the excellent reproducibility of junctions
composed of tens of parallel molecules, which may be important for
continued efforts to build robust molecular devices
High-Mobility Transistors Based on Single Crystals of Isotopically Substituted Rubreneā<i>d</i><sub>28</sub>
We have performed a comprehensive
study of chemical synthesis,
crystal growth, crystal quality, and electrical transport properties
of isotopically substituted rubrene-<i>d</i><sub>28</sub> single crystals (D-rubrene, C<sub>42</sub>D<sub>28</sub>). Using
a modified synthetic route for protonated-rubrene (H-rubrene, C<sub>42</sub>H<sub>28</sub>), we have obtained multigram quantities of
rubrene with deuterium incorporation approaching 100%. We found that
the vapor-grown D-rubrene single crystals, whose high qualities were
confirmed by X-ray diffraction and atomic force microscopy, maintained
the remarkable transport properties originally manifested by H-rubrene
crystals. Specifically, field-effect hole mobility above 10 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> was consistently
achieved in the vacuum-gap transistor architecture at room temperature,
with an intrinsic band-like transport behavior observed over a broad
temperature range; maximum hole mobility reached 45 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> near 100 K. Theoretical
analysis provided estimates of the density and characteristic energy
of shallow and deep traps presented in D-rubrene crystals. Overall,
the successful synthesis and characterization of rubrene-<i>d</i><sub>28</sub> paves an important pathway for future spin-transport
experiments in which the H/D isotope effect on spin lifetime can be
examined in the testbed of rubrene
An ADMET Route to Low-Band-Gap Poly(3-hexadecylthienylene vinylene): A Systematic Study of Molecular Weight on Photovoltaic Performance
The effect of molecular weight on organic photovoltaic
device performance
is investigated for a series of low-band-gap (ca. 1.65 eV) polyĀ(3-hexadecylthienylene
vinylene)Ās (C16-PTVs) prepared by acyclic diene metathesis (ADMET)
polymerization. By utilizing monomers of varying cis:trans (<i>Z</i>:<i>E</i>) content, seven C16-PTVs were prepared
with a number-average molecular weight range of 6ā30 kg/mol.
Polymers were characterized by size-exclusion chromatography, <sup>1</sup>H NMR spectroscopy, ultravioletāvisible spectroscopy,
thermogravimetric analysis, wide-angle X-ray scattering, and differential
scanning calorimetry. C16-PTVs were integrated into bulk-heterojunction
(BHJ) solar cells with [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl
ester (PCBM), and conversion efficiency was found to increase with
increasing molecular weight. This observation is attributable to an
increase in polymer aggregation in the solid state and a corresponding
increase in hole mobility. Finally, phase behavior and morphology
of the C16-PTV:PCBM active layers were investigated by differential
scanning calorimetry and atomic force microscopy, respectively
Rubrene-Based Single-Crystal Organic Semiconductors: Synthesis, Electronic Structure, and Charge-Transport Properties
Correlations among the molecular
structure, crystal structure,
electronic structure, and charge-carrier transport phenomena have
been derived from six congeners (<b>2</b>ā<b>7</b>) of rubrene (<b>1</b>). The congeners were synthesized via
a three-step route from known 6,11-dichloro-5,12-tetracenedione. After
crystallization, their packing structures were solved using single-crystal
X-ray diffraction. Rubrenes <b>5</b>ā<b>7</b> maintain
the orthorhombic features of the parent rubrene (<b>1</b>) in
their solid-state packing structures. Control of the packing structure
in <b>5</b>ā<b>7</b> provided the first series
of systematically manipulated rubrenes that preserve the Ļ-stacking
motif of <b>1</b>. Density functional theory calculations were
performed at the B3LYP/6-31GĀ(d,p) level of theory to evaluate the
geometric and electronic structure of each derivative and reveal that
key properties of rubrene (<b>1</b>) have been maintained. Intermolecular
electronic couplings (transfer integrals) were calculated for each
derivative to determine the propensity for charge-carrier transport.
For rubrenes <b>5</b>ā<b>7</b>, evaluations of
the transfer integrals and periodic electronic structures suggest
these derivatives should exhibit transport characteristics equivalent
to, or in some cases improved on, those of the parent rubrene (<b>1</b>), as well as the potential for ambipolar behavior. Single-crystal
field-effect transistors were fabricated for <b>5</b>ā<b>7</b>, and these derivatives show ambipolar transport as predicted.
Although device architecture has yet to be fully optimized, maximum
hole (electron) mobilities of 1.54 (0.28) cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> were measured for rubrene <b>5</b>.
This work lays a foundation to improve our understanding of charge-carrier
transport phenomena in organic single-crystal semiconductors through
the correlation of designed molecular and crystallographic changes
to electronic and transport properties
Rubrene-Based Single-Crystal Organic Semiconductors: Synthesis, Electronic Structure, and Charge-Transport Properties
Correlations among the molecular
structure, crystal structure,
electronic structure, and charge-carrier transport phenomena have
been derived from six congeners (<b>2</b>ā<b>7</b>) of rubrene (<b>1</b>). The congeners were synthesized via
a three-step route from known 6,11-dichloro-5,12-tetracenedione. After
crystallization, their packing structures were solved using single-crystal
X-ray diffraction. Rubrenes <b>5</b>ā<b>7</b> maintain
the orthorhombic features of the parent rubrene (<b>1</b>) in
their solid-state packing structures. Control of the packing structure
in <b>5</b>ā<b>7</b> provided the first series
of systematically manipulated rubrenes that preserve the Ļ-stacking
motif of <b>1</b>. Density functional theory calculations were
performed at the B3LYP/6-31GĀ(d,p) level of theory to evaluate the
geometric and electronic structure of each derivative and reveal that
key properties of rubrene (<b>1</b>) have been maintained. Intermolecular
electronic couplings (transfer integrals) were calculated for each
derivative to determine the propensity for charge-carrier transport.
For rubrenes <b>5</b>ā<b>7</b>, evaluations of
the transfer integrals and periodic electronic structures suggest
these derivatives should exhibit transport characteristics equivalent
to, or in some cases improved on, those of the parent rubrene (<b>1</b>), as well as the potential for ambipolar behavior. Single-crystal
field-effect transistors were fabricated for <b>5</b>ā<b>7</b>, and these derivatives show ambipolar transport as predicted.
Although device architecture has yet to be fully optimized, maximum
hole (electron) mobilities of 1.54 (0.28) cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> were measured for rubrene <b>5</b>.
This work lays a foundation to improve our understanding of charge-carrier
transport phenomena in organic single-crystal semiconductors through
the correlation of designed molecular and crystallographic changes
to electronic and transport properties
Controllable SpināOrbit Torque Induced by Interfacial Ion Absorption in Ta/CoFeB/MgO Multilayers with Canted Magnetizations
Electrically generated spināorbit torque (SOT)
has emerged
as a powerful pathway to control magnetization for spintronic applications
including memory, logic, and neurocomputing. However, the requirement
of external magnetic fields, together with the ultrahigh current density,
is the main obstacle for practical SOT devices. In this paper, we
report that the field-free SOT-driven magnetization switching can
be successfully realized by interfacial ion absorption in perpendicular
Ta/CoFeB/MgO multilayers. Besides, the tunable SOT efficiency exhibits
a strong dependence on interfacial Ti insertion thicknesses. Polarized
neutron reflection measurements demonstrate the existence of canted
magnetization with Ti inserted, which leads to deterministic magnetization
switching. In addition, interfacial characterization and first-principles
calculations reveal that B absorption by the Ti layer is the main
cause behind the enhanced interfacial transparency, which determines
the tunable SOT efficiency. Our findings highlight an attractive scheme
to a purely electric control spin configuration, enabling innovative
designs for SOT-based spintronics via interfacial engineering