Microcontact printing of self-assembled monolayers to pattern the light-emission of polymeric light-emitting diodes

By patterning a self-assembled monolayer (SAM) of thiolated molecules with opposing dipole moments on a gold anode of a polymer light-emitting diode (PLED), the charge injection and, therefore, the light-emission of the device can be controlled with a micrometer-scale resolution. Gold surfaces were modified with SAMs based on alkanethiols and perfluorinated alkanethiols, applied by microcontact printing, and their work functions have been measured. The molecules form a chemisorbed monolayer of only ∼1.5 nm on the gold surface, thereby locally changing the work function of the metal. Kelvin probe measurements show that the local work function can be tuned from 4.3 to 5.5 eV, which implies that this anode can be used as a hole blocking electrode or as a hole injecting electrode, respectively, in PLEDs based on poly(p-phenylene vinylene) (PPV) derivatives. By microcontact printing of SAMs with opposing dipole moments, the work function was locally modified and the charge injection in the PLED could be controlled down to the micrometer length scale. Consequently, the local light-emission exhibits a high contrast. Microcontact printing of SAMs is a simple and inexpensive method to pattern, with micrometer resolution, the light-emission for low-end applications like static displays. Both authors (J.J. Brondijk and X. Li) contributed equally.     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

Polymer gate dielectrics with self-assembled monolayers for high-mobility organic thin-film transistors based on copper phthalocyanine

Organic thin-film transistors based on polycrystalline copper phthalocyanine (CuPc) were fabricated by using poly(vinyl alcohol) as gate dielectric. After treatment of the gate dielectric using an octadecyltrichlorosilane self-assembled monolayer, a mobility of up to 0.11 cm²/V s was achieved, which is comparable to that of single-crystal CuPc devices (0.1–1 cm²/V s). The surface morphology was analyzed and the possible reasons for the enhanced mobility are discussed.     

Template growth of vertically aligned carbon nanotubes using self-assembled monolayers of SiO2 particles by Langmuir–Blodgett technique

The thermal stability of nanocrystalline 3R-CuCrO₂ obtaining by hydrothermal method was investigated by annealing treatment, XRD, FT-IR, XPS and TG. The three temperature domains corresponding to thermal stability of 3R-CuCrO₂ nanocrystals (25–400 °C), destabilization of nanocrystalline 3R-CuCrO₂ phase (400–800 °C) and recrystallization of 3R-CuCrO₂ in microcrystalline state over 800 °C, were determined by the specific hydrothermal synthesis conditions. This study has indicated that nanocrystals with delafossite structure synthesized by hydrothermal method exhibit nanocrystalline state up to a reasonably high temperature, about 390 °C, which could be interesting for technical applications and the classical theory of the grain growth.     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

Effect of aspect ratio on the inter-surface plasmonic coupling of tubular gold nanoparticle

A theoretical study based on discrete dipole approximation (DDA) and coupling effect is presented on the tunable transverse surface plasmon resonance (SPR) in a gold nanotube with varying aspect ratio (AR). Because gold nanotube has the shape features from both rod and shell, both the AR and wall thickness can greatly affect the transverse SPR. It is observed that the maximum red shift can be obtained with small wall thickness and AR. By calculating the local field distribution, the physical mechanism of this multi shape factors controlled plasmon shifting has been illustrated by the coulombic interaction from the charges at the interfaces of gold nanotube. This study indicates that finding the surface charge distribution by calculating the local electric field can be used as an effective method to analyze the plasmonic characters in complicated metallic nanostructure.     

Influence of self-assembled monolayer chain length on modified gate dielectric pentacene thin-film transistors

Self-assembled monolayers are widely used to modify the gate dielectric/semiconductor interface in organic thin-film transistors. By modifying the interaction between the molecular semiconductor and the substrate, thin-film ordering and the electronic properties of the semiconducting channel can be controlled. The modified semiconductor/dielectric properties result in macroscopically observed changes in the charge-carrier mobilities, threshold voltages, subthreshold swing and transfer characteristic hysteresis. The latter two are determined by the density of charge-trapping states at the interface. Here, we investigate the influence of the thickness of the self-assembled monolayer, via the alkyl chain length in n-alkyl phosphonic acid-based monolayers on SiO₂, on the electronic properties of pentacene-based organic thin-film transistors. Rather than a monotonic increase or decrease in performance with increasing chain length, we have found that the optimum performance occurs with chains of 8–10 carbon atoms. Atomic force microscopy shows a correlation between pentacene crystalline grain size and transistor performance.     

Silver ions adsorbed to self-assembled monolayers of alkanedithiols on gold surfaces form Ag–dithiol–Au multilayer structures

Double-ended alkanedithiols, 1,9-nonanedithiol and 1,5-pentanedithiol, formed self-assembled monolayers (SAMs) on Au(l11) substrates and were used to adsorb silver ions from an ethanolic solution of silver nitrate and formed Ag–dithiol–Au multilayer structures. Ellipsometry, contact angle measurement and X-ray photoelectron spectroscopy (XPS) confirmed that the alkanedithiol molecules formed SAMs with only one-ended thiol groups attached to the Au substrates, which was supported by molecular mechanics calculation. XPS and X-ray Auger electron spectroscopy (XAES) indicated that silver ions were deposited onto the SAMs from the solution by the chemical reaction of silver nitrate with another-ended thiol groups of the SAMs. Atomic force microscopy (AFM) was used to observe SAMs and multilayer structures. Received: 20 January 2000 / Accepted: 18 April 2000 / Published online: 9 August 2000     

In-situ formation and detailed analysis of imine bonds for the construction of conjugated aromatic monolayers on Au(111)

We present the synthesis of 4′-amino-4-mercaptobiphenyl (AMB) and its deposition from solution onto Au(111) substrates. The resulting organic thin films were characterized by contact angle, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) measurements. It is demonstrated that the majority of AMB molecules are coupled to the gold surface via S–Au covalent bonds, although only little orientational order of the AMB layer could be detected by NEXAFS. Furthermore, aromatic imine bonds between AMB and 4-hydroxybenzaldehyde (HB), 4-carboxybenzaldehyde (CB), 4-methylbenzaldehyde (MB), or 4-(trifluoromethyl) benzaldehyde (TMB) have been successfully formed. As a result of the limited order, this coupling reaction was incomplete. Nevertheless, the experimental results confirmed the formation of conjugated aromatic imine bonds.     

Self-Assembly of Gold Nanoparticles on Nanometre-Patterned Surface

The self-assembly processes of gold nanoparticles on nanometre-step-patterned Si surface and polished Si surface are investigated by the convective self-assembly method. The convective self-assembly method is used to deposit the colloids dispersed in benzene onto the substrates. The SEM results show that the configurations of the gold arrays depend on the surface morphology of the substrates. On the nanometre-step-patterned Si surface, the nanoparticles self assemble into parallel lines, and the distance between the neighbouring lines is around 35 nm. On the polished Si surface the nanoparticles form compact domains. In each domain the particles are closepacked in a two-dimensional hexagonal superlattice and are separated by uniform distances. The analysis shows that on the nanometre-step-patterned Si surface, the steps play critical roles in the self-assembly process of gold nanoparticles. The capillary force from the steps drives the particles to lines along the steps. Therefore, the particles tend to self-assemble into one-dimensional line structures when the solvent evaporates. For the polished Si substrate there is a little difference that the particles form two-dimensional hexagonal superlattices without the directional confinement.     

Gold and gold–silver core-shell nanoparticle constructs with defined size based on DNA hybridization

Nanoparticles represent versatile building blocks in material science and nanotechnology. Thereby, the defined assembly of nanostructures (13 and 56 nm in diameter, respectively) is of significant importance. Short DNA sequences can be bound to the nanoparticle surface thus enabling highly specific DNA hybridization-driven events that direct the formation of nanoparticle constructs. In this paper, examples for the defined formation of gold nanoparticle constructs are demonstrated. In addition, gold–silver core-shell nanoparticles are introduced as further building blocks for the hybridization-controlled formation of nanoparticle constructs.     

Key role of molecular kinetic energy in early stages of pentacene island growth

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO _x /Si. We show that the kinetic energy of the impinging molecule (E _k ) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing E _k , the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E _k ≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.     

Synthesis and characteristics of form-stable n-octadecane/expanded graphite composite phase change materials

N-octadecane/expanded graphite composite phase-change materials were prepared by absorbing liquid n-octadecane into the expanded graphite. The n-octadecane was used as the phase-change material for thermal energy storage, and the expanded graphite acted as the supporting material. Fourier transformation infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal diffusivity measurement were used to determine the chemical structure, crystalline phase, microstructure and thermal diffusivity of the composite phase-change materials, respectively. The thermal properties and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the composite phase-change materials exhibited the same phase-transition characteristics as the n-octadecane and their latent heat increased with the n-octadecane content in composite phase-change materials. The SEM results showed that the n-octadecane was well absorbed in the porous network of the expanded graphite, and there was no leakage of the n-octadecane from the composites even when it was in the molten state.     

In situ Raman characterization of nanoparticle aerosols during flame synthesis

Raman spectroscopy is applied to diagnose nanoparticle presence and characteristics in a gaseous flow field. Specifically, in situ monitoring of the Raman-active modes of TiO₂ and Al₂O₃ nanoparticles in aerosol form is demonstrated in high-temperature flame environments. This technique serves as a sensitive and reliable way to characterize particle composition and crystallinity (e.g. anatase versus rutile) and delineate the phase conversion of nanoparticles as they evolve in the flow field. The effect of temperature on the solid-particle Raman spectra is investigated by seeding nanoparticles into a co-flow jet diffusion flame, where local gas-phase temperatures are correlated by shape-fitting the N₂ vibrational Stokes Q-branch Raman spectra. Applying the technique to a flame synthesis environment, the results demonstrate that in situ Raman of as-formed nanoparticles can be readily applied to other gas-phase synthesis systems, especially as an on-line diagnostic.     

Low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigation of epitaxial growth of F<Subscript>16</Subscript>CuPc thin films on graphite

Low-temperature scanning tunneling microscopy (LT-STM) and near-edge X-ray absorption fine structure (NEXAFS) measurements are used to study the epitaxial growth and molecular orientation of organic thin films of copper hexadecafluorophthalocyanine (F₁₆CuPc) on highly oriented pyrolytic graphite (HOPG). Our results show that F₁₆CuPc molecules lie flat on HOPG up to 5 nm thickness, stabilized by interfacial and interlayer π–π interactions. LT-STM experiments reveal the coexistence of two different in-plane orientations of the F₁₆CuPc monolayer on HOPG. On the second layer of F₁₆CuPc on HOPG, however, all F₁₆CuPc molecules possess the same in-plane orientation.     

The effect of pre-annealing of sputtered ZnO seed layers on growth of ZnO nanorods through a hydrothermal method

Oriented ZnO nanorods were grown on ion-beam-sputtered ZnO seed layers through a hydrothermal approach without any metal catalyst. The sputtered ZnO seed layers were pre-annealed at different temperatures before the growth of ZnO nanorods. The effects of pre-annealing of the ZnO seed layers on the growth rate, crystallinity and optical properties of ZnO nanorods thereon were studied. The obtained ZnO nanorods had a wurtzite structure and grew along the preferential [0001] orientation with a normal direction to the substrates. Results show that the growth rate and density of the ZnO nanorods strongly depend on the pre-treatment conditions of the ZnO seed layer. With higher pre-treatment temperature, the crystallinity and surface characteristics of the ZnO seed layer were improved and thereafter the growth rate of ZnO nanorods thereon increased. Photoluminescence spectroscopy results show that the UV emission also becomes stronger and sharper with increasing annealing temperature of the ZnO seed layer.     

Controlling SERS intensity by tuning the size and height of a silver nanoparticle array

It is demonstrated that the surface-enhanced Raman scattering (SERS) intensity of R6G molecules adsorbed on a Ag nanoparticle array can be controlled by tuning the size and height of the nanoparticles. A firm Ag nanoparticle array was fabricated on glass substrate by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). Different sizes of Ag nanoparticles were fabricated with seed polystyrene nanospheres ranging from 430 nm to 820 nm in diameter. By depositing different thicknesses of Ag film and lifting off nanospheres from the surface of the substrate, the height of the Ag nanoparticles can be tuned. It is observed that the SERS enhancement factor will increase when the size of the Ag nanoparticles decreases and the deposition thickness of the Ag film increases. An enhancement factor as high as 2×10⁶ can be achieved when the size of the polystyrene nanospheres is 430 nm in diameter and the height of the Ag nanoparticles is 96 nm. By using a confocal Raman mapping technique, we also demonstrate that the intensity of Raman scattering is enhanced due to the local surface plasmon resonance (LSPR) occurring in the Ag nanoparticle array.     

Information-Geometric Indicators of Chaos in Gaussian Models on Statistical Manifolds of Negative Ricci Curvature

A new information-geometric approach to chaotic dynamics on curved statistical manifolds based on Entropic Dynamics (ED) is proposed. It is shown that the hyperbolicity of a non-maximally symmetric 6N-dimensional statistical manifold ℳ _s underlying an ED Gaussian model describing an arbitrary system of 3N degrees of freedom leads to linear information-geometric entropy growth and to exponential divergence of the Jacobi vector field intensity, quantum and classical features of chaos respectively.     

Condensation of the Roots of Real Random Polynomials on the Real Axis

We introduce a family of real random polynomials of degree n whose coefficients a _k are symmetric independent Gaussian variables with variance , indexed by a real α≥0. We compute exactly the mean number of real roots 〈N _n 〉 for large n. As α is varied, one finds three different phases. First, for 0≤α<1, one finds that . For 1<α<2, there is an intermediate phase where 〈N _n 〉 grows algebraically with a continuously varying exponent, . And finally for α>2, one finds a third phase where 〈N _n 〉∼n. This family of real random polynomials thus exhibits a condensation of their roots on the real line in the sense that, for large n, a finite fraction of their roots 〈N _n 〉/n are real. This condensation occurs via a localization of the real roots around the values , 1≪k≤n.