Recent progress on reactively sputtered metal oxide PV interlayers is presented. A strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to pronounced device improvements. A new crystalline MoOx system employed for efficient hole extraction is shown to lead to prolonged OPV lifetimes1, and a new crystalline TiOx layer is shown to lead to efficient electron extraction. In order to meet the requirements on scalable OPV development, the up-scaling of the metal oxides from Roll-to-Roll (R2R) vacuum processing is discussed.
1 Ahmadpour et al, ACS Appl. Energy Mater. 2, 420 (2019)
Record efficiencies of OPV devices nowadays reach well above 10%. However, their organic nature makes them strongly sensitive to oxygen, light, heat and humidity.
We report on long-term stabilization by ternary blending the active layers with small amounts of stabilizing compounds of different classes of antioxidants[1,3], radical scavengers[1] and light stabilizers[2]. Lifetime testing was conducted under ISOS3-degradation conditions on bulk-heterojunction cells containing a wide selection of stabilizers. Microscopic and spectroscopic methods were applied to monitor chemical degradation over time, and the observed differences are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological and structural changes.
Both antioxidants and UV absorbers yielded an increase of the accumulated power generation by over a factor of 3 compared to the reference devices without additive. In both cases, stability improvement was caused by significant reduction of radicals within the photoactive layer, which in turn stabilizes the performance by decreasing exciton recombination. However, stabilization mechanisms of these two classes are quite different, as reflected in the burn-in. While antioxidant-stabilized cells manifested a simultaneous increase of the burn-in period and decrease of decay magnitude, UV-absorber-stabilized cells retained the same burn-in period as the reference.
[1] Turkovic V. et al, ACS Applied Materials & Interfaces 6, p 18525 (2014); http://dx.doi.org/10.1021/am5024989
[2] Turkovic V. et al, Journal of Physics D: Applied Physics 49, p 125604 (2016); http://dx.doi.org/10.1088/0022-3727/49/12/125604
[3] Turkovic V. et al, Applied Physics A 122:255 (2016); http://dx.doi.org/10.1007/s00339-016-9758-7
Although organic solar cells show intriguing features such as low-cost, mechanical flexibility and light weight, their efficiency is still low compared to their inorganic counterparts. One way of improving their efficiency is by the use of light-trapping mechanisms from nano- or microstructures, which makes it possible to improve the light absorption and charge extraction in the device’s active layer. Here, periodically arranged colloidal gold nanoparticles are demonstrated experimentally and theoretically to improve light absorption and thus enhance the efficiency of organic solar cells. Surface-ordered gold nanoparticle arrangements are integrated at the bottom electrode of organic solar cells. The resulting optical interference and absorption effects are numerically investigated in bulk hetero-junction solar cells based on the Finite-Difference Time-Domain (FDTD) and Transfer Matrix Method (TMM) and as a function of size and periodicity of the plasmonic arrangements. In addition, light absorption enhancement in the organic active layer is investigated experimentally following integration of the nanoparticle arrangements. The latter are fabricated using a lithography-free stamping technique, creating a centimeter scaled area with nanoparticles having a defined inter-particle spacing. Our study reveals the light harvesting ability of template-assisted nanoparticle assemblies in organic solar cells. As the approach is easily scalable, it is an efficient and transferable method for large-scale, low cost device fabrication.
Recent research on hybrid plasmonic systems has shown the existence of a loss channel for energy transfer between
organic materials and plasmonic/metallic structured substrates. This work focuses on the exciton-plasmon coupling
between para-Hexaphenylene (p-6P) organic nanofibers (ONFs) and surface plasmon polaritons
(SPPs) in organic/dielectric/metal systems. We have transferred the organic p-6P nanofibers onto a thin silver film
covered with a dielectric (silicon dioxide) spacer layer with varying thicknesses. Coupling is investigated by two-photon
fluorescence-lifetime imaging microscopy (FLIM) and leakage radiation spectroscopy (LRS). Two-photon excitation
allows us to excite the ONFs with near-infrared light and simultaneously avoids direct SPP excitation on the metal layer.
We observe a strong dependence of fluorescence lifetime on the type of underlying substrate and on the morphology of
the fibers. The experimental findings are complemented via finite-difference time-domain (FDTD) modeling. The
presented results lead to a better understanding and control of hybrid-mode systems, which are crucial elements in future
low-loss energy transfer devices.
A promising method for improving light-absorption in thin-film devices is demonstrated via electrode structuring
using Anodic Alumina Oxide (AAO) templates. We present nano-scale concave Al structures of controlled dimensions,
formed after anodic oxidation of evaporated high purity aluminum (Al) films and alumina etching. We investigate both
experimentally and theoretically the field-enhancement supported by these concave nanostructures as a function of their
dimensions. For the experimental investigations, a thin layer of organic polymer coating allows the application of a nondestructive
laser ablation technique that reveals field-enhancement at the ridges of the Al nanostructures. The
experimental results are complemented by finite-difference time-domain (FDTD) simulations, to support and explain the
outcome of the laser ablation experiments. Our method is easily up-scalable and lithography-free and allows one to
generate nanostructured electrodes that potentially support field-enhancement in organic thin-film devices, e.g., for the
use in future energy harvesting applications.
In this work, enhancement of the second harmonic response of organic nanofibers deposited on encapsulated and robust
plasmonic active substrate is experimentally demonstrated. Organic nanofibers grown from functionalized paraquaterphenylene
(CNHP4) molecules have been transferred on lithographically defined regular arrays of gold
nanostructures, which subsequently have been coated with thin films of diamond-like carbon with 25, 55 and 100 nm
thickness. Femtosecond laser scanning microscopy enables us to identify enhancement of the second harmonic response
of the fibers. This is facilitated by a preservation of the field enhancement effects, which appear on the nanostructures
and remain significant on top of the coating layer.
Leakage radiation spectroscopy of organic para-Hexaphenylene (p-6P) molecules has been performed in the spectral
range 420-675 nm which overlaps with the p-6P photoluminescence band. The p-6P was deposited on 40 nm silver (Ag)
films on BK7 glass, covered with SiO2 layers. The SiO2 layer thickness was varied in the range 5-30 nm. Domains of
mutually parallelly oriented organic nanofibers were initially grown under high-vacuum conditions by molecular beam
epitaxy onto a cleaved muscovite mica substrate and afterwards transferred onto the sample by a soft transfer technique.
The sample placed on a flat side of a hemisphere fused silica prism with an index matching liquid was illuminated under
normal incidence by a He-Cd 325 nm laser. Two orthogonal linear polarizations were used both parallel and
perpendicular to the detection plane. Spectrally resolved leakage radiation was observed on the opposite side of the Ag
film (i.e. at the hemisphere prism) as a function of the scattering angle. Each spectrum contains a distinct peak at a
wavelength dependent angle above the critical angle. This way the dispersion curve was measured, originating from a
hybrid mode, i.e. the interaction between the p-6P excitons and surface plasmon polaritons (SPPs) of the metal/dielectric
boundary. The presence of the SiO2 layer considerably changes the dispersion curve in comparison to the one of the
Ag/p-6P/air system. However, the Ag/SiO2/p-6P/air stack forms a stable structure allowing construction of organic
plasmonic devices such as nano-lasers.
Second harmonic generation in nonlinearly optically active organic nanofibers, generated via self-assembled surface
growth from nonsymmetrically functionalized para-quarterphenylene (CNHP4) molecules, has been investigated. After
the growth on mica templates, nanofibers have been transferred onto lithographically defined regular arrays of metal and
dielectric nanostructures. Such hybrid systems were employed to correlate the second harmonic response to both
morphology of the fibers i.e. local field enhancement due to local changes in the fiber’s morphology and field
enhancement effects appearing on the nanostructures. With the help of femtosecond laser scanning microscopy two-dimensional
second-harmonic images of individual nanoaggregates were obtained and analyzed.
Functionalization of small, rod like organic molecules can be used to optimize organic devices. Here we report on nanofiber formation and thin film growth of a methoxy-functionalized para-quaterphenylene (1,4'''-Dimethoxy- 4,1':4',1'':4'',1'''-quaterphenylene, MOP4) on prototypical dielectric substrates such as muscovite mica, phlogopite mica, highly ordered pyrolytic graphite (HOPG), and on the alkali halide NaCl. The nanofibers consist of lying molecules, the films of upright standing ones. The grown samples are characterized by polarized optical microscopy (fluorescence, birefringence, bireflectance), by atomic force microscopy (AFM), and by Kelvin probe force microscopy (KPFM) to gain insight into their structure and epitaxial relation with the substrates.
We report on development of flexible PCPDTBT:PCBM solar cells with integrated diffraction gratings on the bottom electrodes. The presented results address PCPDTBT:PCBM solar cells in an inverted geometry, which contains implemented grating structures whose pitch is tuned to match the absorption spectra of the active layer. This optimized solar cell structure leads to an enhanced absorption in the active layer and thus improved short-circuit currents and power conversion efficiencies in the fabricated devices. Fabrication of the solar cells on thin polyimide substrates which are compatible with the lithographically processed grating structures are done in order to obtain the efficiency enhancement in thin, flexible devices.
A versatile method for integrating liquid waveguides into PDMS microfluidic flow-cytometer chips is presented. By using a one-step direct replication, PDMS chips are produced with both liquid and waveguide channels. Filling the waveguide channels with high refractive index media, a simple waveguide is created using the PDMS of the chip itself as cladding. Optical fibers are used to couple laser light and fluorescence into, and out of the chip. Experimental results and ray-tracing simulations show that the light intensity at distances above 5 mm from the source is more than four times higher when using gelatin or DMSO as compared to channels containing only air.
Leakage radiation spectroscopy of organic nanofibers composed of self-assembled organic molecules (para-Hexaphenylene,
p-6P) deposited on a thin (40-60 nm) Ag film has been performed in the spectral range 420-675
nm which overlaps with the nanofiber photoluminescence band. Using a soft transfer technqiue, domains of
mutually parallel oriented organic nanofibers were initially grown under high-vacuum conditions by molecularbeam
epitaxy onto a cleaved muscovite mica substrate and afterwards transferred onto a silver film prepared on
the glass carrier. The sample placed on a flat side of a hemisphere prism with an index matching liquid was
illuminated by either a He-Cd 325 nm laser or by white light from a bulb. In the case of laser excitation two
orthogonal linear polarizations and two different configurations of p-6P nanofibers were applied, both parallel and
perpendicular to the plane of detection. The leakage radiation was observed on the opposite side of the Ag film
at the phase matching angle. The spectrally resolved intensity of the scattered radiation has been measured as a
function of scattering angle at normally incident light. The spectrum contains a distinct peak at an wavelength
dependent angle above the critical angle. By analyzing this dispersion curve one can argue that it originates
from the interaction between the nanofiber excitons and surface plasmon polaritons of the metal film.
The optical near-field of metal films can be modified in a straightforward manner by incorporating nanostructures on the surface. The corresponding field enhancement, which may be due to the lightning rod effect as well as the excitation of plasmon modes, results in a local change of the optical surface response. A transparent thin film on top of the nanostructures can be partially ablated via illumination with near-infrared light. Local variations of the ablation rate due to field enhancement are readily mapped with subdiffractional resolution, as confirmed by a direct comparison to theoretical calculations. Variation of the thickness of the transparent film enables discrimination between localized enhancements at the sharp corners of the structures and collective enhancements at locations between the structures due to surface plasmon polariton modes. In addition, applying the same method to study the effect of nanostructure morphology on localized second-harmonic generation using arrays of rectangular as well as triangular structures, we observed a second-harmonic (SH) signal from both centrosymmetric and noncentrosymmetric nanostructure arrays, indicating that the SH excitation is not due to a collective phenomenon but originates locally from the individual structures.
In-situ grown organic nanofibers have been prepared on metal electrodes patterned by electron beam lithography. A
systematic investigation shows that the light emission from these nanofibers driven by an AC gate voltage depends nonlinearly
on the amplitude of the AC gate voltage and linearly on the frequency of the gate voltage, which indicates that a
model involving thermally assisted charge-carrier tunneling can be applied. The photoluminescence spectra of parahexaphenylene
(p6P) and α-sexithiophene (6T) nanofibers illustrate that the emission color of the in-situ grown
nanofibers can be tuned by depositing two types of discontinuous organic layers on the same platform.
Electroluminescence from two nanofiber thin films suggests that the relative light emission contribution from the two
organic molecules can be varied by changing, e.g., the nominal thickness of the two materials.
We report an optimized inverted bulk-heterojunction (P3HT:PCBM) organic solar cell geometry in
order to both efficiently trap incident light within in the cell (increasing light absorption) and at the
same time provide efficient transport of the generated carriers to the electrodes (reducing the active
layer thicknesses). To address these issues, we have used two approaches. The first one consists of
including diffraction gratings that increase the light path length in the cell and thus enhance absorption
in wavelength intervals matching the absorption peak of the organic active layer on the bottomelectrode,
while the second approach includes Ag nanoparticles embedded on the solar cell topelectrode,
which scatter the incident light into the solar cell active layer.
The solar cells containing either gratings or nanoparticles exhibit a significant enhancement on the
power conversion efficiency. Furthermore, the solar cells do not contain the rare metal indium, but
employ a PEDOT:PSS based transparent electrode.
Surface plasmon polariton (SPP) excitation at a gold-vacuum interface via 800 nm light pulses mediated by a periodic
array of gold ridges is probed at high lateral resolution by means of photoemission electron microscopy (PEEM). We
directly monitor and quantify the coupling properties as a function of the number of grating ridges and compare the
PEEM results with analytic calculations. An increase in the coupling efficiency of ≈ 3 is observed when increasing the
number of ridges from 1 to 6. We observe, however, that a further addition of ridges is rather ineffective. This saturation
behavior is assigned to the grazing incidence excitation geometry intrinsic to a conventional PEEM scheme and the
limited propagation distance of the SPP modes at the gold-vacuum interface at the used wavelength.
Second-harmonic generation upon femto-second laser irradiation of nonlinearly optically active nanofibers grown from
nonsymmetrically functionalized para-quarterphenylene (CNHP4) molecules is investigated. Following growth on mica
templates, the nanofibers have been transferred onto lithography-defined regular arrays of gold square nanostructures.
These nanostructure arrays induce local field enhancement, which significantly lowers the threshold for second harmonic
generation in the nanofibers.
Light-emitting transistors based on organic semiconductors have a range of potential advantages incl. tunability,
flexibility, and high energy efficiency. A remaining challenge is the required high driving voltages that are caused by
energy barriers at the interfaces between the metal electrodes and the organic material that hinder efficient charge carrier
injection. In this work, we study the influence of two different self-assembled monolayers based on polar molecules
deposited on the metal electrodes in terms of electrical and light-emitting properties of such organic transistors. The
dipoles of the two monolayers are in opposite directions, so one monolayer is expected to lower the electrode work
function while the other is expected to increase the work function. From an energy barrier perspective, it is thus expected
that one monolayer should increase charge carrier injection while the other should reduce it. We find, however, that both
types of monolayers improve both the electrical conductance and the emitted light intensity significantly. This is
attributed to a change in interfacial microstructure due to the change in surface energy that results from the monolayer.
This strategy is therefore a promising route to achieve higher device efficiencies in organic light-emitting transistors.
Environmentally stable, non-toxic squarylium dyes with strong absorption maxima in the red and near infrared
spectral region are known for almost fifty years. Despite the fact that their optoelectronic properties distinguish
them as promising materials for organics based photovoltaic cells, they have regained attention only very
recently. For their application in heterojunction solar cells knowledge of their nanoscopic morphology as well
as nanoscopic electrical properties is paramount. In this paper thin films from two different squarylium dyes,
from squarylium (SQ) and from hydroxy-squarylium (SQOH) are investigated. The thin films are either solution
casted or vacuum sublimed onto substrates such as muscovite mica, which are known to promote self-assembly
into oriented, crystalline nanostructures such as nanofibers. Local characterization is performed via (polarized)
optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Kelvin probe
force microscopy (KPFM).
Organic crystalline nanofibers made from phenylene-based molecules exhibit a wide range of extraordinary optical
properties such as intense, anisotropic and polarized luminescence that can be stimulated either optically or
electrically, waveguiding and random lasing. For lighting and display purposes, the high quantum yield and the easy
tunability of the color by changing the molecular building blocks are especially important.
The application of such nanostructures as electrically driven light-emitters requires integration with suitable metal
electrodes for efficient carrier injection. Here, we demonstrate the implementation of a method for achieving such
nanostructure integration. The method relies on growing the nanostructures directly between metal electrodes on a
substrate that has been specially designed to guide the nanostructures growth. We present results in terms of
morphological characterization and demonstrate how appropriate biasing with an AC gate voltage enables
electroluminescence from these in-situ grown organic nanostructures.
KEYWORDS: Gold, Nanostructures, Near field, Laser ablation, Polymers, Polymethylmethacrylate, Near field optics, Scanning electron microscopy, Metals, Polarization
The optical near-field of lithography-defined gold nanostructures, arranged into regular arrays on a gold film, is
characterized via ablation of a polymer coating by laser illumination. The method utilizes femto-second laser pulses from
a laser scanning microscope which induces electrical field enhancements on and around the gold nanostructures. At the
positions of the enhancements, the ablation threshold of the polymer coating is significantly lowered creating subdiffractional
topographic modifications on the surface which are quantified via scanning electron microscopy and atomic force microscopy. The obtained experimental results for different polymer coating thicknesses and nanostructure geometries are in good agreement with theoretical calculations of the near field distribution for corresponding enhancement mechanisms. The developed method and its tunable experimental parameters show that the different stages
in the ablation process can be controlled and characterized making the technique suitable for characterizing optical near-fields
of metal nanostructures.
The morphological stability of organic nanowires over time and under thermal load is of major importance for
their use in any device. In this study the growth and stability of organic nanowires from a naphthyl end-capped
thiophene grown by organic molecular beam deposition is investigated via atomic force microscopy (AFM). Aging
experiments under ambient conditions already show substantial morphological changes. Nanoscopic organic
clusters, which initially coexist with the nanowires, vanish within hours. Thermal annealing of nanowire samples
leads to even more pronounced morphology changes, such as a strong decrease in nanowire number density, a
strong increase in nanowire height, and the formation of new types of crystallites. This happens even before
sublimation of organic material starts. These experiments also shine new light on the formation process of the
nanowires.
Specially designed surface micro- and nanostructures allow one to steer the bottom up self-organized growth of crystalline nanoaggregates from wide bandgap organic molecules, which possess extraordinary optoelectronic properties. Polarized light-emitting para-hexaphenylene nanofiber arrays exemplify such "self-growing" nanophotonic devices. The methodology behind this growth is an alternative to transfer of nanofiber arrays from specific growth substrates onto device platforms. We compared the optical properties of transferred and in situ grown nanofibers in terms of polarization function and emission homogeneity and also studied the temperature dependence of the emission spectra of transferred nanofiber arrays. Both types of nanofibers show the same spatial emission characteristics along their long axes and also the same polarization ratio. However, in nanofiber arrays, the polarization ratio decreases in the case of structured surface-grown nanofibers since the mutual orientation of the nanofibers is less perfect than for transferred fibers.
Para-hexaphenylene (p6P) molecules have the ability to self-assemble into organic nanofibers, which exhibit a
range of interesting optical and optoelectronic properties such as intense, polarized luminescence, waveguiding and
lasing. The nanofibers are typically grown on specific single-crystalline templates, such as muscovite mica, on which
mutually parallel nanofibers are self-assembled upon vapor deposition of the organic material under high vacuum
conditions. Besides such single-crystalline templates, the nanofibers can also be grown on non-crystalline gold
surfaces, on which the orientation of the nanofibers can be manipulated by structuring the gold surface prior to parahexaphenylene
(p6P) deposition. In this work it is demonstrated, how such organic nanofiber growth can be controlled
by modifying the design of the underlying gold structures prior to growth. Here, the investigated designs include
pinning lines and gratings. We demonstrate how gold gratings fabricated on an insulating substrate can enable
electrical contact to in-situ grown p6P nanofibers. Furthermore, the electrical characteristics of in-situ grown fibers are compared to that of transferred p6P nanofibers. The transferred nanofibers are initially grown on muscovite mica, and
subsequently transferred onto a target substrate by drop casting, and electrodes are applied on top by a special shadow
mask technique.
Nanofibers made from para-hexaphenylene (p6P) molecules hold unique optoelectronic properties, which make them
interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on
specific single-crystalline substrates, on which long, mutually parallel nanofibers are formed. However, the lack of
ability to further process these substrates restrains their use in devices. In this work, a novel method for in-situ growth of
p6P nanofibers on nano- and micro-structured gold surfaces is presented. The substrates are prepared by conventional
microfabrication techniques such as lithography, etching and metal deposition, which increase their potential as device
platforms. The results presented here demonstrate, that both the growth direction and the nanofiber length can be
controlled by placement of nano- and micro-structured lines on the substrate. It is shown that the preferred growth
direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and
placement of the structures. This work therefore demonstrates a new technique, which can be useful within future
organic nanofiber based applications.
Nanofibers from light-emitting organic molecules such as para-phenylenes have already demonstrated a promising
application potential in nanophotonic devices and can act as waveguides or nanolasers. Here, the basic mechanisms
for self-assembly of three different green- and green/blue-light emitting thiophene/phenylene co-oligomers
into nanofibers are investigated. Under well defined conditions in high vacuum the molecules are deposited on
cleaved mica surfaces. The effect of substrate surface energy as well as epitaxy on the overall film morphology
is studied and significant differences between different co-oligomers are found.
Forming structures similar to or smaller than the optical wavelength offers a wide range of possibilities to modify the
optical properties of materials. Tunable optical nanostructures can be applied as materials for surface-enhanced
spectroscopy, optical filters, plasmonic devices, and sensors. In this work we present experimental results on technology
and properties of periodical, polymer based optical structures modified by ordered adsorption of silver nanoparticles.
These structures were formed combining UV hardening and dip coating from colloidal solutions. We have investigated
the influence of silver nanoparticles assembly on the ambient conditions (deposition temperature and time) and surface
features (periodicities and shape) of the template micro structures. Optical absorbance as well as morphology of the
structures containing silver nanoparticles were investigated by UV-VIS spectroscopy, AFM, SEM and optical
microscopy. The influence of silver nanoparticles on the optical properties of the structures was investigated by polarized
light spectroscopy (Grating Light Reflection Spectroscopy - GLRS). From the results of this study we propose a low cost
procedure for fabricating structures that could be potentially new type of plasmonic sensors exploiting surface enhanced
plasmon resonance in silver nano structures.
Aligned ensembles of nanoscopic nanofibers from organic molecules such as para-phenylenes for photonic applications
can be fabricated by self-assembled molecular growth on a suited dielectric substrate. Epitaxy together
with alignment due to electric surface fields determines the growth directions. In this paper we demonstrate
how aligned growth along arbitrary directions can be realized by depositing the molecules on a micro-structured
and gold covered Silicon surface, consisting of channels and ridges. For the correct combination of ridge width
and deposition temperature fibers grow perpendicular to the ridge edge, emitting light polarized along the ridges
after UV excitation.
A new way of developing optical nanosensors is presented. Organic
nanofibers serve as key elements in these new types of devices,
which exploit both the smallness and brightness of the
nanoaggregates to make new compact and sensitive optical
nanosensors. On the basis of bottom up technology, we functionalize
individual molecules, which are then intrinsically sensitive to
specific agents. These molecules are used as building blocks for
controlled growth of larger nanoscaled aggregates. The aggregates in
turn can be used as sensing elements on the meso-scale in the size
range from hundred nanometers to a few hundred microns. The organic
nanofibers thereby might become a versatile tool within nanosensor
technology, allowing sensing on the basis of individual molecules
over small aggregates to large assemblies. First experiments of
Bovine Serum Albumin (BSA) coupling to para-hexaphenyl (p-6P)
nanofibers are presented, which could lead towards a new type of protein
sensors. Besides large versatility and sensitivity, the nanofibers
benefit from the fact that they can be integrated in devices,
either in liquids by the use of microfluidic cavities or all in
parallel.
Organic nanofibers from semiconducting conjugated molecules are well suited to meet refined demands for advanced
applications in future optoelectronics and nanophotonics. In contrast to their inorganic counterparts,
the properties of organic nanowires can be tailored at the molecular level by chemical synthesis. Recently we
have demonstrated the complete route from designing hyperpolarizabilities of individual molecules by chemically
functionalizing para-quaterphenylene building blocks to the growth and optical characterization of nonlinear,
optically active nanoaggregates. For that we have investigated nanofibers as grown via organic epitaxy. In the
present work we show how chemically changing the functionalizing end groups leads to a huge increase of second
order susceptibility, making the nanofibers technologically very interesting as efficient frequency doublers. For
that the nanofibers have to be transferred either as individual entities or as ordered arrays onto specific target
substrates. Here, we study the applicability of contact printing as a possible route to non-destructive nanofiber
transfer.
Nanofibers from symmetrically and unsymmetrically functionalized p-quaterphenylenes are fabricated by a bottom-up process on muscovite mica. The symmetrically functionalized p-quaterphenylenes emit intense, polarized blue light after unpolarized UV-excitation. Upon implementing electron push-pull functional groups like chlor and methoxy groups to the molecular building block new properties of the nanoaggregates have been generated: the nanofibers exhibit increased non-linear optical properties, acting, e.g., as frequency doublers after
excitation with NIR femtosecond laser pulses. Depending on the
growth conditions the chloro-methoxy-p-quaterphenylene forms
either parallel nanofibers or nano-branches on a muscovite mica
substrate, adding another degree of freedom for the design of, e.g.,
resonator structures.
The synthesis of molecules consisting of various combinations of phenylene and thiophene groups and the subsequent
vacuum growth of needle-shaped nanoaggregates on specific surfaces allows us a systematic investigation of
the transition between single parallel and multiple aligned needle ('nanofiber') growth. The former growth mode
is observed for blue light emitting phenylene fibers, whereas the latter growth mode appears for single crystalline
fibers made from green- and orange-light emitting oligo-thiophenes and thiophene/phenylene co-oligomers. In all
cases the tailored bottom-up growth results in strongly polarized light emission along specific surface directions.
The results are compared to those found for nanoaggregates made from less rod-like organic molecules, namely
rubrene and POPOP.
We report waveguide amplification of spontaneous emission and coherent random laser action in individual self-assembled organic nanofibers grown by high-vacuum deposition. The interpretation of the experimental results is given on the basis of simple models, including transfer matrix calculations in one-dimensionally disordered structures. We present also the numerical results for light scattering from a nanofiber which can be used as a basis for further experiments.
It has been shown recently, that organic nanofibers grown from para-hexaphenyl and from α-sexithiophene molecules can be used as a new type of nanoscopic waveguides. Their growth is due to a self-assembly process, thus large quantities of aligned nanofibers can be fabricated simultaneously. Because of the growth mechanism of the nanofibers, their widths and heights are limited to a few 100 nm and a few 10 nm, respectively. In this paper we show how this kind of control has been obtained via modification of the bare muscovite surface before organic molecule deposition. Introducing e.g. a thin layer of Au islands before nanofiber growth results in an up to 15-fold increase in height, whereas the mean width and the optical properties of the fibers remain almost unchanged. Au films of varying thickness lead to tailor-made height profiles along the fiber. Using atomic force microscopy the details of these Au/organic heterostructures are examined and the growth is compared to growth on untreated mica. By scratching the fibers with an AFM tip grating structures have been written into the fibers.
Systematic investigations of luminescence lifetimes of organic phenylene nanofibers are presented as a function of intrinsic parameters such as morphology or bleaching factor as well as extrinsic parameters such as substrate material, coating or excitation intensity. By varying either one of these parameters, the decay times of the electronic excitation can be varied. This should have a strong influence on the efficiency of nanolasing, which is observed by increasing the excitation intensity of a femtosecond pump laser. Lasing action starts at pump fluences as low as a few μJ/cm2 per pulse. In ensemble measurements, the number of lasing modes depends strongly on the density of contributing nanofibers. In spatially resolved measurements, the nonlinear optical response of individual nanofibers is investigated. This enables us to make a correlation between the morphological features of the nanofibers, as deduced from atomic-force microscopy, and their lasing properties.
Single crystalline organic nanoaggregates from organic semiconductors such as para-hexaphenyl and sexithiophene might become building blocks for a new type of organic electronic and optoelectronic devices. For the performance of such devices detailed knowledge about the mechanisms responsible for formation and for alignment of the aggregates on the growth substrate is important. On muscovite mica long, mutually parallel fibers of para-hexaphenyl grow, whereas on alkali halides mainly two different orientations, on phlogopite mica three different orientations exist. For sexithiophene on muscovite mica depending on the growth temperature either three equivalent aggregate orientations exist, or a single one dominates. The interplay between epitaxy and dipole assisted alignment on different growth substrates favors either unidirectional or multidirectional growth.
Light-emitting nanofibers grown from organic molecules such as para-hexaphenyl or substituted para-quaterphenyl have extraordinary morphological, optical and electrical properties that make them interesting candidates as key elements in future electronics and photonics. These fibers are generated in a self assembly fashion on template substrates. In order to integrate them into more complex structures, a transfer from the growth substrate is necessary. In this paper we show results from optical and morphological measurements on nanofibers transferred onto semiconductors, kept freely floating in solution and frozen in gel. The former investigations allow us to study with nanometric resolution via an atomic force microscope the deformability of nanofibers. The latter studies, based on single photon as well as confocal two-photon microscopy, provide three-dimensional optical images and also the angular distribution of light emitted from individual aggregates. It is observed that waveguiding affects the spatial emission characteristics.
The growth of nanoscopic oligophenylene and oligothiophene aggregates on muscovite mica by vacuum deposition has been investigated. In the case of para-phenylenes a dipole assisted self assembly generation of needle-like aggregates is observed on mica. At optimum fiber growth temperature phenylene aggregates grow in most cases without a layer of upright oriented molecules. In contrast, vacuum deposition of oligothiophenes results simultaneously in fibers of laying molecules as well as islands of upright molecules. Since both phenylenes and thiophenes are strongly polarizable but differ in the lattice parameters of the resulting crystalline overlayers a direct comparison between the two classes of molecules allows us to study the role of epitaxy on the growth of nanoaggregates. Besides straight
aggregates we also observe thiophene rings on water and methanol treated mica surfaces, which consist of radially oriented, laying molecules.
Nanofibers made from organic molecules such as para-hexaphenyl allow guiding of electromagnetic waves. Since they possess nanometric widths and heights but macroscopic lengths they represent the smallest possible optical waveguides. Recently, gain enhancement has been observed, pointing to possible applications as nanolasers. Owing to their self assembly growth mode on mica substrates the nanofibers posses well-defined morphology. In order to implement these aggregates into new optical devices or to enhance feedback and thus build up a resonator structure a defined cutting of the end faces is necessary. This article presents results from the first experimental studies in this direction. Irradiation with UV laser pulses of 193 nm at a fluence of 100 mJ/cm2 removes the fibers completely without damaging the substrate. In addition, the fibers can be cut in any orientation relative to their long axes. The quality of the ablation process in terms of readsorbed debris and steepness of cutting is investigated by atomic force and scanning electron as well as fluorescence microscopy.
In this work we exploit growth as well as linear and nonlinear
optical properties of long, parallel single-crystalline
hexaphenylene (p-6P) nanofibers grown on mica surfaces. Typical
widths and heights of these needle-like structures are a few 10 -
400 nanometers, whereas lengths of up to one millimeter can be
achieved. The nanofibers allow us to perform experiments at either
densely packed, well-aligned bunches of aggregates or at isolated
entities. Linear optical properties are probed by local
spectroscopy using a fiber-optic spectrometer and by guiding
UV-light through individual fibres and relating the waveguiding
efficiency to their morphology using atomic force and fluorescence
microscopy. Results are compared with an analytical theory. As
nonlinear optical probes we use two-photon luminescence as well as
optical second harmonic generation induced by ultrashort
laserpulses in the near-infrared spectral range.
KEYWORDS: Nanofibers, Luminescence, Spectroscopy, Near field optics, Atomic force microscopy, Fluorescence spectroscopy, Near field scanning optical microscopy, Scanning tunneling microscopy, Molecules, Mica
Nanoscaled photonic devices rely on a thorough understanding of
the influence of microscopic morphological changes on the
optoelectronic properties. Here, we investigate as a model system
organic nanofibers from para-phenylene molecules, which provide
high flexibility in terms of controlled growth manipulation, while
on the other hand showing self assembled multiplication of
individual entities. Examples on selective spectroscopy, scanning
fluorescence optical microscopy and waveguiding of individual
nanofibers as well as arrays of nanofibers are given. Both the
linear optical properties as well as the waveguiding efficiency
are strongly related to the nanofibers morphology, which turn out
to be an interesting benchmark system for the investigation of the
applicability of a variety of optical methods in the nanodomain.
Blue light-emitting aggregates of para-hexaphenyl molecules
('needles') are generated via dipole-assisted self assembly on
single crystalline mica substrates. By applying atomic force
microscopy and fluorescence microscopy we deduce size
distributions as well as height-width correlations of individual
aggregates. We find growth of densely-packed, needle-like
structures even in the initial growth stage at low surface
temperatures. However, long (up to one millimeter) and sparsely
distributed, individually addressable needles grow only at high
substrate temperatures and low adsorption rates. For a given
sample at constant deposition conditions the height of the needles
seems to be independent of width. This opens up the possibility to
control the morphology of individual nanostructured aggregates.
In this work we investigate with the help of low energy electron scattering, force microscopy and optical spectroscopy the growth of ultrathin p-nP (n=4,5,6) films on mica surfaces. We find conditions under which we obtain on a nanometric scale spatially localized emission and conditions under which the emission is spatially non-localized. The latter case results from continuous films of upright molecules (i.e. with transition dipole moments oriented parallel to the surface normal) whereas the former case is obtained if - under certain growth conditions - films consisting of high-aspect ratio 'needles' of laying molecules with well-oriented transition dipole moments along the surface have been grown. Because of a strong interaction between substrate surface dipoles and induced dipoles along the long molecular axes, the needles form macroscopic domains of almost perfectly mutually parallel aligned entities. The quality of the alignment depends on the length of the molecules.
Crystallographic unit cells of vacuum grown ultrathin films of blue-light emitting para-phenylene oligomers on alkali halides, on mica and on Au(111) have been determined via low energy electron diffraction (LEED). On the alkali halides the growth of continuous single crystalline films with either standing or laying molecules dominates. On mica, single-crystalline aggregates (needles) of laying molecules are grown. As the chain length of the molecules increases the mutual order of the needles increases. For p-6P the parallel orientation of the needles is strictly determined by the orientation of surface dipole fields in large dipole domains. A combination of LEED structural results with optical and morphological information from fluorescence microscopy and from atomic force microscopy allows us to deduce subtle details of the organic film aggregates. E.g., bright fluorescence spots could be assigned to nanoscaled gaps in the needles.
A systematic investigation has been performed in order to determine the unit cells of ultrathin p-nP films grown under surface science conditions on dielectrics. We have employed low energy electron diffraction (LEED) as well as polarization dependent linear absorption spectroscopy on films deposited onto alkali halide (NaCl, KCl, NaF, LiF, KBr) and mica(0001) single crystals at various substrate temperatures. Surface unit cells of the films are determined as a function of chain length n(n=3-6) and deposition parameters. Linear optical spectroscopy reveals a strong dichroism and allows us to distinguish between laying and standing molecules on the substrate. In contrast to the alkali halides, we observe on mica at elevated surface temperatures the growth of single crystalline needles, the orientation of which is controlled by the presence of surface dipoles on the cleaved mica surface.
Non-intrusive laser-spectroscopical methods for the diagnostics of the gas-surface interaction in the near-field of a dielectric surface have been developed both experimentally and theoretically. The techniques base on either two-photon evanescent wave excitation or combined evanescent-volume wave excitation. Spectra obtained for sodium atoms at a glass-vacuum interface are quantitatively reproduced by a rigorous theoretical approach. In the case of pure evanescent wave excitation the excitation conditions can be chosen such that either the velocity distribution of desorbing atoms is determined or that atom-surface interaction parameters (polarizability of the adsorbed atoms, desorption rate, sticking coefficient) are obtained from the Autler- Townes splitting of the two-photon lines. By the use of combined evanescent-volume wave excitation one is able to distinguish optically between different groups of atoms interacting with the surface and one can extract their two- dimensional velocity distributions. In addition, information is obtained about superelastic scattering of the atoms interacting with the dielectric substrate.
Ablation yields and thresholds for 193 nm UV laser ablation of ultrathin HfO2 are presented. The single shot threshold fluence increases approximately linearly with HfO2 thickness form 28 nm to 120 nm. Due to the logarithmic dependence of ablation depth on fluence this result with increasing layer thickness in an exponential increase of fluence necessary for clean ablation of the whole layer. The observed ablation depth for fixed HfO2 thickness can be reproduced phenomenologically by taking ablation from the HfO2 film as well as the quartz substrate into account. As a first approach to a quantitative understanding we calculate numerically the heat evolution in the layered system and identificate ablation with the onset of melting of the absorbing layer. Whereas the ablation curve for a 74 nm thick film can be reproduced that way, this is not the case for the case for the overall thickness dependence of the ablation threshold. This points to possible finite size effects for the phonon-phonon scattering rate in the thin dielectric layers.
In this work we study metal surface-induced changes of lifetime and transition frequency of alkali atoms and clusters, deposited onto nanoscaled insulator-metal systems. The systems are made of rough metallic surfaces (characterized by atomic force microscopy), onto which ultrathin organic films as spacer layers (characterized by LEED) are epitaxially grown. We observe an unusually small red shift of the transition frequency of Na atoms of a few hundred Megahertz, which is due to the interaction with the metal surface. This is explained by the nonlocal response of the surface, i.e., the excitation of multipole surface plasmons (MSPs) in the selvedge region of the metal surface, which is influenced by surface roughness. The MSPs should become observable also via linear optical methods such as attenuated total reflection spectroscopy. As a first step in this direction, we present linear extinction spectra of alkali cluster films that are grown on top of organic spectra layers of different length. Due to the interaction with the gold films a red shift of the dipole plasmon resonance is observed, which increases with decreasing chain length.
We investigate via optical (viz. linear extinction and nonlinear second harmonic generation) and non-optical methods (viz. helium atom scattering) rough alkali cluster films, which are formed on dielectric substrates. The application of nonlinear optical methods to these systems allows us to obtain real-time dynamical information on the time-constants for laser-induced desorption and for the decay of the elementary optical surface plasmon excitation into single electron excitations and finally into lattice oscillations of the clusters and the substrate. The combination of linear and nonlinear optics also enables us to deduce structural information about the morphology of the cluster films, which--in the submonolayer regime--is complementary to information deduced from atom scattering data.
In this work we present experimental data that show that the roughness of a metal surface strongly influences the metal induced optical transition frequency shift of alkali atoms that are adsorbed close (a few tens of Angstroms) to the metal. The metal induced changes of electronic lifetime depend on distance (alpha) d4, suggesting surface electron hole pair excitation to be the dominant relaxation mechanism for electronically excited Na atoms at distances between 24 and 32 angstroms from a rough Au surface. The nonlinear response of metal surfaces is also well known to be enhanced by surface roughness. It has been anticipated that this enhancement should be most pronounced for a third order nonlinear optical process. Here, we present data of strong enhancement of (chi) (3)eff for rough metal surfaces. The surfaces consist of large alkali metal clusters, adsorbed on dielectrics. By changing the cluster size distribution we are able to study the third order nonlinearity as a function of shape of all the alkali protrusions.
The photodesorption of Na atoms from Na clusters deposited on dielectric surfaces is investigated via pulsed laser excitation and cw two-photon laser-induced fluorescence detection. The combination of pulsed excitation and cw detection within the focii of two counterpropagating lasers provides high spatial resolution (allowing one to obtain accurate angular and kinetic energy distributions) while at the same time preserving the high temporal resolution given by the pulsed laser. Moreover, spatial overlap of desorption and detection lasers on the surface facilitates to follow directly the conversion of initial surface plasmon excitation into bond-breaking in the clusters or phonon excitation in the substrate. It is found that the final step of photodesorption of Na from large Na clusters bound to mica surfaces can be described via multiphonon scattering of the desorbing particles from the surface of the clusters (with nearly zero initial kinetic energy). By use of lithium fluoride as the supporting substrate it is observed that the activation energy for photodesorption from the clusters depends on the Debye temperature of the supporting substrate. This emphasizes the need to include coupling between clusters and substrate in order to understand the overall desorption process.
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