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.     

Fabrication of patterned gold microstructure by selective electroless plating

Patterned gold microstructures on glass or Si wafers have been fabricated by a novel method which is composed of selective electroless plating and microcontact printing. This process may be widely used for the production of fine metal patterns in printed circuits or as a substrate to form patterned SAMs. In addition, these patterned metal microstructures can be readily transferred to adhesive tape surface to fabricate flexible metal microstructure, which may be applied in all-plastic circuit.     

Photopatterning of gold and copper surfaces by using self-assembled monolayers

A photolithographic technique was successfully employed to generate micropatterns of gold and copper by using self-assembled monolayer (SAMs) as resist materials. Copper patterns were successfully prepared from SAMs of 11-mercaptoundecanoic acid (MUA) and dodecanethiol (DDT) on Cu after UV irradiation followed by etching but gold patterns were prepared only from the SAM of MUA and not from the SAM of DDT, which revealed the difference of photooxidation of the metal–sulfur bond on SAMs. However, the maximum resolution of the pattern was about 1.0 μm on gold and 5.0 μm on copper. This may be due to lower quality packing of SAM on copper than gold. Ellipsometric and cyclovoltammetric observation of SAMs during the UV irradiation indicated the gradual removal of SAMs on copper and gold. Photopatterning of gold and copper by using SAM is more compatible with the current microelectronics process and is complementary to the microcontact printing technique.     

Surface engineering with ion beams: from self-organized nanostructures to ultra-smooth surfaces

Low-energy ion-beam sputtering, i.e. the removal of atoms from a surface due to the impact of energetic ions or atoms, is an inherent part of numerous surface processing techniques. Besides the actual removal of material, this surface erosion process often results in a pronounced alteration of the surface topography. Under certain conditions, sputtering results in the formation of well-ordered patterns. This self-organized pattern formation is related to a surface instability between curvature-dependent sputtering that roughens the surface and smoothing by different surface relaxation mechanisms. If the evolution of surface topography is dominated by relaxation mechanisms, surface smoothing can occur. In this presentation the current status of self-organized pattern formation and surface smoothing by low-energy ion-beam erosion of Si and Ge is summarized. In detail it will be shown that a multitude of patterns as well as ultra-smooth surfaces can develop, particularly on Si surfaces. Additionally, the most important experimental parameters that control these processes are discussed. Finally, examples are given for the application of low-energy ion beams as a novel approach for passive optical device engineering for many advanced optical applications. PACS 81.16.Dn; 81.16.Rf; 81.65.Cf; 81.65.Ps; 68.35.Ct     

Fabricating tunable nanoparticle density gradients with the contact printing based approach

Recently, the generation of spatial gradient nanoparticle assemblies has attracted much attention. Such assemblies can be intriguing templates for building novel molecular architectures, and be employed as a combinatorial tool for quick determination of interaction selectivity for nanoparticles. In this communication, we report on convenient contact printing based techniques for generating lateral gradients containing nanoparticles with tunable geometry, scale and steepness. In the first method, octadecyltrichlorosilane (OTS) gradient surfaces were generated via the contact printing approach, and then spaces un-occupied by OTS molecules were back-filled with an amine-terminated silane, which allows the grafting of nanoparticles that were surface functionalized with carboxylic acid. By varying the size and geometry of the stamp, different geometrical gradients were generated. In addition, by changing the stamping procedures, either well-defined stepwise gradients or continuous gradients can be achieved. Furthermore, the contact printing based technique can be utilized in combination with diffusion of the aminosilane molecules to directly create its gradient, and consequently a nanoparticle density gradient.     

Self-assembly of silanated poly(ethylene glycol) on silicon and glass surfaces for improved haemocompatibility

Surface immobilization of poly(ethylene glycol) (PEG) is an effective method to produce a material surface with protein repulsive property. This property could be made permanent by using covalent grafting of the PEG molecules onto material surfaces. In this study, self-assembled monolayers (SAMs) of PEG on silicon-containing materials (silicon chip and glassplate) were obtained through a one-step coating procedure of one kind of silanated PEG molecules made through the reaction between monomethoxy PEG and 3-isocyanatopropyltriethoxysilane. Atomic force microscopy (AFM) and water static contact angle measurement were employed to investigate the surface topography and wettability of the PEGylated material surfaces. The changes in the topography and the water contact angle of the surfaces with time of incubation in PBS solution were also measured. The results revealed that stable and uniform self-assembled monolayers of PEG could be formed on silicon or glass surfaces by simply soaking the substrates in the solution of silanated PEGs. The covalent coupling of PEGs to the substrates was also confirmed. In order to evaluate the stability of the SAMs, blood compatibility of the modified glassplate surface was evaluated by measuring full blood activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT), as well as by scanning electron microscopy (SEM) analysis of the appearance of adherence and denaturation of blood platelets onto the glassplate. The silanated PEGs were shown to have good effect on the protein-repulsion as well as haemocompatibility of the substrates.     

Formation and characterization of long-chained alkylsiloxane self-assembled monolayers on atomic-layer-deposited aluminum oxide surfaces

We report the formation of highly robust long-chained alkylsiloxane self-assembled monolayers (SAMs) on aluminum oxide films prepared by atomic-layer deposition (ALD). The surface chemistry and the morphological characteristics of the SAMs were examined by X-ray photoelectron spectroscopy, infrared spectroscopy, atomic-force microscopy, and contact-angle goniometry. The octadecylsiloxane-derived SAMs initially hydrolyze and deposit on the alumina surface as ∼1.8 nm thick, monolayer-high islands ≤50 nm in diameter. The size of these islands increases with time, likely through a surface-diffusion aggregation process. Coalescence of neighboring islands leads to a densely packed and robust monolayer on the alumina surface. The SAMs on ALD alumina are expected to be useful in a number of nanostructure applications where the combination of conformal alumina deposition and conformal coverage of the alumina by an organic layer is critical. PACS 81.16.Dn; 81.65.Kn; 82.45.Mp; 81.65.-b     

GaAs(111)表面硅烯、锗烯的几何及电子性质研究

基于密度泛函理论的第一性原理计算方法, 系统研究了硅烯、锗烯在GaAs(111) 表面的几何及电子结构. 研究发现, 硅烯、锗烯均可在As-中断和Ga-中断的GaAs(111) 表面稳定存在, 并呈现蜂窝状六角几何构型. 形成能计算结果证明了其实验制备的可行性. 同时发现硅烯、锗烯与GaAs表面存在共价键作用, 这破坏了其Dirac电子性质. 进一步探索了利用氢插层恢复硅烯、锗烯Dirac电子性质的方法. 发现该方法可使As-中断面上硅烯、锗烯的Dirac电子性质得到很好恢复, 而在Ga-中断面上的效果不够理想. 此外, 基于原子轨道成键和杂化理论揭示了GaAs表面硅烯、锗烯能带变化的物理机理. 研究结果为硅烯、锗烯在半导体基底上的制备及应用奠定了理论基础.     

Unidirectional hexagonal rare-earth disilicide nanowires on vicinal Si(100)-2×1

Rare-earth disilicide nanowires grown on vicinal Si(100) with a miscut of 2–2.5° toward the [110] azimuth at 600 °C were studied by scanning tunneling microscopy and compared with those grown on flat Si(001). In contrast to rare-earth disilicide nanowires grown on flat Si(100) surfaces, the nanowires grow unidirectionally along the [01̄1] direction of the vicinal Si(100) surface. Rare-earth disilicide nanowires form bundles composed of single nanowire units on both flat and vicinal surfaces. Yet, on the vicinal surface, the bundle width is comparable to the width of the terrace. The average nanowire length on the vicinal substrate is longer than that on the flat substrate. Scanning tunneling spectroscopy shows that the rare-earth disilicide nanowires have metallic properties. PACS 81.07.Vb; 81.16.Dn; 68.65.La; 68.37.Ef     

Morphology of Si nanowires fabricated by laser ablation using gold catalysts

Si nanowires (NWs) were fabricated successfully by laser ablation using Au as catalyst. Si wafers were used as the collector. The diameters of Si NWs ranged from 20 to 150 nm. Different forms of Si NWs were observed at different local sites inside a furnace: Si NWs with a high aspect ratio of length to diameter, Si NWs with defects and Si NWs with Au-containing nanoparticles being embedded. Especially, a nano-particle embedded Si NW is a new nanostructure that is observed for the first time. PACS 81.07.-b; 81.07.Bc; 81.16.-c; 81.20.-n     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

An anchoring strategy for photoswitchable biosensor technology: azobenzene-modified SAMs on Si(111)

A mild and efficient procedure has been developed to obtain covalently attached self-assembled monolayers (SAMs) on Si(111) with photochromic azobenzene head-groups. Starting from neat or diluted carboxylic acid functionalized monolayers on-chip coupling reactions were applied to attach hydroxyl-functionalized azobenzene units to the SAMs by ester bond formation. The modified surfaces were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FT-IR), and contact angle measurements. Reversible cis ↔ trans isomerizations of photoswitchable SAMs were monitored by wettability measurements. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users. Dedicated with respect and compliments to Professor Helmut Schwarz on the occasion of his 65th birthday.     

Gold nanoparticle growth on self-assembled monolayers of ferrocenyl-substituted terpyridine on graphite

In this contribution we demonstrate that densely packed gold nanoparticles can be grown by Volmer–Weber mode on ferrocenyl functionalized terpyridine (FcTerp) on graphite. FcTerp forms highly ordered and dense self-assembled monolayers (SAMs) on graphite which significantly reduces the diffusion length of gold atoms and increases the sticking coefficient compared to bare graphite. Both effects lead to an increased nucleation and thus, to the growth of densely packed gold nanoparticles with diameters in the nanometer range. The optical properties of the nanoparticles as well as their morphology and the structure of the SAMs were characterized by optical extinction spectroscopy and scanning tunneling microscopy.     

Micropatterned ZnO rod arrays prepared by Au‐catalyzed electroless deposition

Micropatterned ZnO was synthesized by an electroless deposition process using Au stripes as catalytic surfaces. The Au‐patterned electrodes were prepared on SiO₂/Si wafers using photolithography. The site‐selective deposition of patterned ZnO hexagonal rod arrays is confirmed by scanning electron microscopy. The ZnO micropatterned surface revealed a conversion of wettability from hydrophilic to superhydrophobic depending on the deposition reaction param‐ eters. The electrical measurements carried out at room temperature before and after exposure to ammonia vapors of the patterned ZnO arrays show a resistance variation with exposure time. Highly reproducible, easy scalable and low‐cost, photolithography and electroless deposition techniques could provide a facile approach to fabricate functionalized micropatterns, for a wide range of applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A 14-ps full width at half maximum high-speed photoconductor fabricated with intersecting InP nanowires on an amorphous surface

We demonstrate a high-speed polarization-insensitive photoconductor based on intersecting InP nanowires synthesized between a pair of hydrogenated silicon electrodes deposited on amorphous SiO₂ surfaces prepared on silicon substrates. A 14-ps full width at half maximum de-embedded impulse response is measured, which is the fastest reported response for a photodetector fabricated using nanowires. The high-speed electrical signal measurements from the photoconductor are performed by an integrated coplanar waveguide transmission line. The demonstrated ability to grow intersecting InP nanowires on hydrogenated microcrystalline Si surfaces will facilitate the construction of ultra-fast photodetectors on a wide range of substrates. PACS 61.46.Km; 42.70.-a; 81.16.Hc; 81.05.Ea; 85.60.Dw     

溶液酸碱性对金纳米粒子在端基功能化单分子膜表面组装影响的SERS光谱研究

在4,4’-二硫联吡啶在Au表面形成自组装单分子层膜的基础上,采用表面增强拉曼散射光谱(SERS)研究了在不同pH值条件下金纳米粒子在4,4’-二硫联吡啶自组装单分子膜/Au体系表面的组装。研究结果表明,由于处于单分子膜表面的吡啶环中氮原子的质子化程度随溶液环境中pH值的变化而变化,使得金纳米粒子与单分子膜表面间的结合作用程度不同,由此会引起金纳米粒子在单分子膜表面的覆盖度存在差异,并最终导致所观测到的4-巯基吡啶自组装单分子膜的SERS光谱强度存在明显的差异。而且,令人感兴趣的是,所观测到的SERS谱峰强度随金纳米粒子组装时pH值的变化呈现出明显的规律性。结合分子结构特征的分析,初步阐明了SERS谱峰强度随pH值这一组装条件的改变而发生规律性变化的内在原因。     

Tip-induced nano-writing/machining of Si and DLC surfaces – “anodic” versus thermal oxidation?

Tip-induced oxidative manipulation of conducting surfaces, e.g., Si and some metals, has conventionally been described by a field-induced anodic mechanism. Likewise, in the case of electrically conducting graphitic and diamond-like carbon (DLC) films, tip-induced conversion of carbon to CO₂ was initially thought to be due to an ionisation process. There is now mounting evidence for thermal activation playing an important role. The state of the tip is a critical, but largely disregarded, factor in such experiments. The present project has been prepared and characterized by I–V analysis, tips with different initial characteristics (e.g., H-termination , Au-coating, native oxide). Likewise, several surfaces have been prepared (e.g., Si plus termination by either native or thermal oxide, or plus H-termination, DLC and Au), and also subjected to I–V analysis. The resultant point-contact characteristics were found to range from ohmic to non-ohmic (the latter due to either direct or Fowler–Nordheim tunnelling). The various combinations were tested with respect to oxidative yield and tip durability. It was found that the presence of a tunnelling barrier at the point of contact is essential for enhancing yield. Tip durability, on the other hand, is promoted by the barrier being located in the surface thus localizing thermal deposition in the surface rather than in the tip. PACS 81.16.Rf; 81.16.Pr; 07.79.Lh     

Chemisorption of Si on Al(111) surfaces: A local-chemical-bond analysis from Auger transition density of states

Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L_2,3VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.     

Synthesis of a composite consisting of carbon nanotubes and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition

We report a simple and effective one-step synthesis route for synthesizing a composite consisted of carbon nanotubes (CNTs) and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition on Si (1 0 0) substrate covered with catalyst Co particles, discharging a mixture of H₂ and CH₄ gas, and characterize the obtained composite by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscope, and X-ray photoelectron spectroscopy. The results show that CNTs align perpendicularly to the substrate and graphite shell-encapsulated Co nanoparticles clung to the external surfaces of aligned CNTs. The diameter of the graphite shell-encapsulated Co nanoparticles increases with increasing the H₂ content in H₂ and CH₄ carbonaceous gas. A possible growth mechanism of the CNTs and graphite shell-encapsulated cobalt nanoparticles composite has been explored.     

Formation and characterization of polyperinaphthalenic organic semiconductor nanoparticles by laser ablation of mixture targets of a perylene derivative with cobalt powder at 355 nm

Laser ablation of mixed targets of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) with cobalt powder (PTCDA-Co) was carried out using the third harmonic of a Nd:YAG laser (355 nm) in order to prepare nanoparticles consisting basically of polyperinaphthalene (PPN), a member of the low dimensional graphite family. The optimized content ratio of Co to PTCDA (Co:PTCDA) for effective elimination reaction of anhydride groups of PTCDA was determined. We found that the ablation at Co:PTCDA of 4:1 and 8:1 enabled us to eliminate the side groups effectively. Furthermore, surface enhanced Raman spectroscopy with a tip of an atomic force microscope was carried out to investigate the molecular structure of each nanoparticle. Amplification of the Raman intensity by more than 100 times compared with conventional micro Raman spectroscopy was achieved by tip enhancement. A nanoparticle having a diameter of approximately 70 nm prepared by this method was found to possess PPN structure. PACS 81.05.Uw; 81.16.Mk; 07.79.-v