15 research outputs found
Enhancing the Dispersibility of TiO<sub>2</sub> Nanorods and Gaining Control over Region-Selective Layer Formation
We
demonstrate that the dispersibility and reactivity of core–shell
TiO<sub>2</sub> nanorods (NRs) can be controlled significantly through
functionalization with a combination of ligands based on phosphonic
acid derivatives (PAs). Specifically, a glycol based PA allows dispersion
of the NRs in methanol (MeOH). On the other hand, incorporating an
alkyne terminated PA in the ligand shell of the NRs allows for a copper-catalyzed
alkyne–azide cycloaddition (CuAAC) reaction with an azide-patterned
aluminum oxide (AlO<sub><i>x</i></sub>) substrate and forms
a region-selectively deposited film of NRs. We clearly demonstrate
that the quality of the NR films correlates strongly with the stability
of the NR dispersions in the reaction medium. In particular, tuning
the concentration of alkyne PA in the ligand shell inhibits aggregation
of the NRs on the substrate, while reactivity for the CuAAC reaction
is maintained. The surface coverage with NRs fits the Langmuir model.
This study illustrates that surface functionalization of AlO<sub><i>x</i></sub> substrates can be effectively and conveniently controlled
through enhancing the dispersibility of the NRs using mixed ligand
shells
Self-Assembled Monolayer Exchange Reactions as a Tool for Channel Interface Engineering in Low-Voltage Organic Thin-Film Transistors
In this work, we compared the kinetics of monolayer self-assembly
long-chained carboxylic acids and phosphonic acids on thin aluminum
oxide surfaces and investigated their dielectric properties in capacitors
and low-voltage organic thin-film transistors. Phosphonic acid anchor
groups tend to substitute carboxylic acid molecules on aluminum oxide
surfaces and thus allow the formation of mixed or fully exchanged
monolayers. With different alkyl chain substituents (<i>n</i>-alkyl or fluorinated alkyl chains), the exchange reaction can be
monitored as a function of time by static contact angle measurements.
The threshold voltage in α,α′-dihexyl-sexithiophene
thin-film transistors composed of such mixed layer dielectrics correlates
with the exchange progress and can be tuned from negative to positive
values or vice versa depending on the dipole moment of the alkyl chain
substituents. The change in the dipole moment with increasing exchange
time also shifts the capacitance of these devices. The rate constants
for exchange reactions determined by the time-dependent shift of static
contact angle, threshold voltage, and capacitance exhibit virtually
the same value thus proving the exchange kinetics to be highly controllable.
In general, the exchange approach is a powerful tool in interface
engineering, displaying a great potential for tailoring of device
characteristics
Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X‑ray Reflectivity
ConspectusSelf-assembled monolayers (SAMs) have been established
as crucial
interlayers and electronically active layers in organic electronic
devices, such as organic light emitting diodes (OLEDs), organic photovoltaics
(OPVs), organic thin film transistors (OTFTs), and nonvolatile memories
(NVMs). The use of self-assembling functionalized organic molecules
is beneficial due to mainly three advantages compared with common
thin film deposition approaches. (1) Molecular self-assembly occurs
with surface selectivity, determined by the interaction between the
functional anchor group of the organic molecules and the target surface.
(2) The film thickness of the resulting layers is perfectly controllable
on the angstrom scale, due to the self-terminating film formation
to only a single molecular layer. And finally, (3) the wide variability
in the chemical structure of such molecules enables different SAM
functionalities for devices, ranging from electrical insulation to
charge storage to charge transport. The SAM approach can be further
expanded by employing several functionalized molecules to create mixed
SAMs with consequently mixed properties.The function of SAMs
in devices depends not only on the chemical
structure of the molecules but also on their final arrangement and
orientation on the surface. A reliable and nondestructive in-depth
characterization of SAMs on nonconductive oxide surfaces is still
challenging because of the very small thickness and the impracticality
of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron
spectroscopy (XPS).In this Account, we illustrate how X-ray
reflectivity (XRR) provides
analytical access to major questions of SAM composition, morphology,
and even formation by means of investigations of pure and mixed SAMs
based on phosphonic acids (PAs) of various chain structures on flat
alumina (AlO<sub><i>x</i></sub>) surfaces. XRR is an analytical
method that provides access to spatially averaged structural depth
profiles over a relatively large area of several square micrometers.
The key outcome of XRR, the surface-normal electron density profile
of the SAMs, leads to precise information on the SAM thickness with
subangstrom resolution and allows for the determination of molecular
tilt angles and packing densities.We have systematically increased
the chemical complexity of PA
molecules and the resulting SAMs, utilizing XRR to provide insight
into the SAM structures. In SAMs composed of functionalized molecules
or complex chain structures, the distribution of electron rich and
electron poor signatures is detected and thus the molecular order
within the SAM is determined.In mixed SAMs of two different
molecular species, electron density
profiles reveal the morphology and how the surface-normal structure
changes if one component of the mixed SAM is altered. Furthermore,
XRR was applied to investigate in situ the self-assembly of functionalized
PA molecules from solution by tracking the monolayer growth over time.
Even though the results provided by XRR on in-plane molecular arrangement
are limited, it presents excellent information on the molecular scale
along the surface normal and in addition allows for drawing conclusions
on the intermolecular interactions within the SAM
Low-Voltage Organic Field Effect Transistors with a 2‑Tridecyl[1]benzothieno[3,2‑<i>b</i>][1]benzothiophene Semiconductor Layer
An asymmetric <i>n</i>-alkyl substitution pattern
was
realized in 2-tridecyl[1]ÂbenzothienoÂ[3,2-<i>b</i>]Â[1]Âbenzothiophene
(C<sub>13</sub>-BTBT) in order to improve the charge transport properties
in organic thin-film transistors. We obtained large hole mobilities
up to 17.2 cm<sup>2</sup>/(V·s) in low-voltage operating devices.
The large mobility is related to densely packed layers of the BTBT
Ï€-systems at the channel interface dedicated to the substitution
motif and confirmed by X-ray reflectivity measurements. The devices
exhibit promising stability in continuous operation for several hours
in ambient air
Impact of Oxygen Plasma Treatment on the Device Performance of Zinc Oxide Nanoparticle-Based Thin-Film Transistors
Thin-films of zinc oxide nanoparticles were investigated
by photoluminescence
spectroscopy and a broad defect-related yellow-green emission was
observed. Oxygen plasma treatment was applied in order to reduce the
number of defects, and the emission intensity was quenched to 4% of
the initial value. Thin-film transistors that incorporate the nanoparticles
as active semiconducting layers show an improved device performance
after oxygen plasma treatment. The maximum drain current and the charge
carrier mobility increased more than 1 order of magnitude up to a
nominal value of 23 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and the threshold voltage was lowered
Phosphonate- and Carboxylate-Based Self-Assembled Monolayers for Organic Devices: A Theoretical Study of Surface Binding on Aluminum Oxide with Experimental Support
We report a computational study on
the chemical bonding of phosphonates and carboxylates to aluminum
oxide surfaces and how the binding properties are related to the amount
of water in the experimental environment. Two different surface structures
were used in the calculations in order to model representative adsorption
sites for the phosphonates and carboxylates and to account for the
amorphous nature of the hydroxylated AlO<sub><i>x</i></sub> films in experiment. For the phosphonates, we find that the thermodynamically
preferred binding mode changes between mono-, bi-, and tridentate
depending on the surface structure and the amount of residual water.
For the carboxylates, on the other hand, monodentate adsorption is
always lower in energy at all experimental conditions. Phosphonates
are more strongly bound to aluminum oxide than carboxylates, so that
carboxylates can be replaced easily by phosphonates. The theoretical
findings are consistent with those obtained in adsorption, desorption,
and exchange reactions of <i>n</i>-alkyl phosphonic and
carboxylic acids on AlO<sub><i>x</i></sub> surfaces. The
results provide an atomistic understanding of the adsorption and help
to optimize experimental conditions for self-assembly of organic films
on aluminum oxide surfaces
High-Mobility ZnO Nanorod Field-Effect Transistors by Self-Alignment and Electrolyte-Gating
High mobility, solution-processed
field-effect transistors are important building blocks for flexible
electronics. Here we demonstrate the alignment of semiconducting,
colloidal ZnO nanorods by a simple solvent evaporation technique and
achieve high electron mobilities in field-effect transistors at low
operating voltages by electrolyte-gating with ionic liquids. The degree
of alignment varies with nanorod length, concentration and solvent
evaporation rate. We find a strong dependence of electron mobility
on the degree of alignment but less on the length of the nanorods.
Maximum field-effect mobilities reach up to 9 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for optimal alignment. Because
of the low process temperature (150 °C), ZnO nanorod thin films
are suitable for application on flexible polymer substrates
Effect of Structure and Disorder on the Charge Transport in Defined Self-Assembled Monolayers of Organic Semiconductors
Self-assembled
monolayer field-effect transistors (SAMFETs) are
not only a promising type of organic electronic device but also allow
detailed analyses of structure–property correlations. The influence
of the morphology on the charge transport is particularly pronounced,
due to the confined monolayer of 2D-Ï€-stacked organic semiconductor
molecules. The morphology, in turn, is governed by relatively weak
van-der-Waals interactions and is thus prone to dynamic structural
fluctuations. Accordingly, combining electronic and physical characterization
and time-averaged X-ray analyses with the dynamic information available
at atomic resolution from simulations allows us to characterize self-assembled
monolayer (SAM) based devices in great detail. For this purpose, we
have constructed transistors based on SAMs of two molecules that consist
of the organic p-type semiconductor benzothienoÂ[3,2-<i>b</i>]Â[1]Âbenzothiophene (BTBT), linked to a C<sub>11</sub> or C<sub>12</sub> alkylphosphonic acid. Both molecules form ordered SAMs; however,
our experiments show that the size of the crystalline domains and
the charge-transport properties vary considerably in the two systems.
These findings were confirmed by molecular dynamics (MD) simulations
and semiempirical molecular-orbital electronic-structure calculations,
performed on snapshots from the MD simulations at different times,
revealing, in atomistic detail, how the charge transport in organic
semiconductors is influenced and limited by dynamic disorder
The Relationship between Threshold Voltage and Dipolar Character of Self-Assembled Monolayers in Organic Thin-Film Transistors
We report a quantitative study that describes and correlates
the threshold voltage of low-voltage organic field-effect transistors
with the molecular structure of self-assembled monolayer dielectrics.
We have observed that the component of the dipole moment of such self-assembled
molecules perpendicular to the surface correlates linearly with the
threshold voltage shift in devices. The model was validated using
three different organic semiconductors (pentacene, α,α′-dihexylsexithiophene,
and fullerene–C<sub>60</sub>) on six different self-assembled monolayers.
The correlation found can help optimize future devices, by tuning
the dipole moments of the molecules that constitute the self-assembled
monolayer
Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer
Current–voltage
hysteresis is a major issue for normal architecture organo-halide
perovskite solar cells. In this manuscript we reveal a several-angstrom
thick methylammonium iodide-rich interface between the perovskite
and the metal oxide. Surface functionalization via self-assembled
monolayers allowed us to control the composition of the interface
monolayer from Pb poor to Pb rich, which, in parallel, suppresses
hysteresis in perovskite solar cells. The bulk of the perovskite films
is not affected by the interface engineering and remains highly crystalline
in the surface-normal direction over the whole film thickness. The
subnanometer structural modifications of the buried interface were
revealed by X-ray reflectivity, which is most sensitive to monitor
changes in the mass density of only several-angstrom thin interfacial
layers as a function of substrate functionalization. From Kelvin probe
force microscopy study on a solar cell cross section, we further demonstrate
local variations of the potential on different electron-transporting
layers within a solar cell. On the basis of these findings, we present
a unifying model explaining hysteresis in perovskite solar cells,
giving an insight into one crucial aspect of hysteresis for the first
time and paving way for new strategies in the field of perovskite-based
opto-electronic devices