Investigation of the role of the interplay between water and temperature on the growth of alkylsiloxane submonolayers on silicon

We have investigated the influence of the interplay of the temperature and the water concentration in the adsorption solution on the growth of self-assembled monolayers on silicon using octadecyltrichlorosilane as the precursor. Toluene has been used as the solvent. The morphology of the submonolayer films has been investigated by atomic force microscopy (AFM). The surface coverages have been determined both with ellipsometry and through quantitative evaluation of AFM images. The size distribution of species in the precursor solution has been studied with dynamic light scattering. The influence of water concentrations between 8 and 18 mmol/L has been investigated in the temperature range from 2 to 35 degrees C. Dynamic light scattering revealed a unimodal size distribution of ordered aggregates in solution with a hydrodynamic radius of 200 nm regardless of the temperature and water concentration. However, formation of these features was faster at higher water contents and lower temperatures. Moreover, a characteristic temperature, which was higher for higher water concentrations, was found, above which such aggregates could not be detected anymore. Below this temperature an increase of the aggregate concentration has been observed until a plateau had been reached within a temperature range of approximately 5 degrees C. AFM measurements and ellipsometry on the corresponding submonolayer films showed that this temperature range is also associated with a transition from fast growth via characteristic fractally shaped islands to comparatively slow homogeneous growth via adsorption of individual molecules. The results are discussed in terms of diffusion and adsorption limitations.     

Atomically flat gold on elastomeric substrate

We present a procedure to fabricate extremely smooth Au films supported on thin elastomeric (PDMS) substrates. Minimum rms roughness and largest grain size are obtained using Si wafers, coated with native oxide and release layers, as templates for the growth of thermally evaporated Au films. The wafers are held at a temperature of 300 degrees C during deposition. The Au films, up to 200 nm thick, are then transferred onto poly(dimethylsiloxane) substrates which have been previously surface-functionalized with a (3-mercaptopropyl)trimethoxysilane adhesion layer. The resulting Au films have been found by AFM to be extremely smooth with rms-roughness 2.5-4 angstroms and to exhibit a crystalline morphology with flat grains >500 nm in size. Thinner films, down to 20 nm, are grown at lower temperature and are comparably smooth, but with a loss in crystalline morphology. We compare the results of this optimized procedure with other gold films grown on mica sheets as templates and to those produced using Ti-O-Si interfacial chemistry.     

Characterization of silicon wafers through deposition of self-assembled monolayers

AFM imaging of the adsorption of self-assembled octadecylsiloxane (ODS) monolayers has been utilized for probing surface properties of silicon wafers. It has been found that both growth rate of the organic films and island size of sub-monolayer films are influenced by the doping level of the wafers as well as by the surface finishing step during wafer production. Generally, higher doping levels led to lower adsorption rates and smaller islands. Variation of the sample pretreatment used for surface finishing of similarly doped wafers led only to significant changes of the island size, but not of the surface coverage. The results presented open up a valuable perspective for characterizing the surface homogeneity of silicon wafers which is an important parameter for monitoring-wafers in semiconductor industry. Received: 26 June 2000 / Revised: 26 July 2000 / Accepted: 1 August 2000     

Characterization of silicon wafers through deposition of self-assembled monolayers

AFM imaging of the adsorption of self-assembled octadecylsiloxane (ODS) monolayers has been utilized for probing surface properties of silicon wafers. It has been found that both growth rate of the organic films and island size of sub-monolayer films are influenced by the doping level of the wafers as well as by the surface finishing step during wafer production. Generally, higher doping levels led to lower adsorption rates and smaller islands. Variation of the sample pretreatment used for surface finishing of similarly doped wafers led only to significant changes of the island size, but not of the surface coverage. The results presented open up a valuable perspective for characterizing the surface homogeneity of silicon wafers which is an important parameter for monitoring-wafers in semiconductor industry. Received: 26 June 2000 / Revised: 26 July 2000 / Accepted: 1 August 2000     

聚苯乙烯/聚二甲基硅氧烷嵌段共聚物的表面形态研究

采用原子力显微镜(AFM)和透射电镜(TEM)研究了聚苯乙烯/聚二甲基硅氧烷嵌段共聚物(PS-b-PDMS)薄膜的相形态.结果表明,当采用甲苯作为溶剂,旋转涂膜的薄膜样品呈现网络状的形态分布在表面,而样品所对应的透射电镜照片中,PDMS相作为球状分布在PS的连续相中.退火温度对共聚物表面形态有一定的影响,当退火温度高于PDMS的玻璃化温度,表面中PDMS相增多.PS-b-PDMS嵌段共聚物的表面形态随着所用溶剂的变化而有所不同,当采用甲苯作为溶剂时,样品的PS相形成凹坑分布在PDMS的相区之中,而采用环己烷作为溶剂时,PS相作为突起分布在PDMS相区之中.另外,基底对共聚物薄膜表面形态的有较大的影响,当采用硅晶片作为基底时,样品中的PDMS相和PS相呈现近似平行于表面的层状结构.     

An atomic force microscope study of thermal behavior of phospholipid monolayers on mica

We observed by using atomic force microscope (AFM) phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers on mica being annealed and cooled to a selection of temperatures through steps of 2-4 degrees C/min. The annealed phospholipid monolayers started to disappear at 45-50 degrees C and disappeared completely above 60-63 degrees C under AFM observation. The phospholipid monolayers reformed when the samples were cooled below 60 degrees C and developed from fractal into compact monolayer films with decreasing temperatures. Simultaneously the height of the reformed phospholipid films also increased with decreasing temperatures from 0.4 nm to the value before annealing. The observed thermal features are attributed to a phase-transition process that upon heating to above 45-50 degrees C, the lipids condensed in the monolayers transform into a low-density expanded phase in which the lipids are invisible to AFM, and the transformation continues and completes at 60-63 degrees C. The lipid densities of the expanded phase inferred from the dissociated area of the condensed phase are observed to be a function of the temperature. The behavior contrasts with a conventional first-order phase transition commonly seen in the Langmuir films. The temperature-dependent height and shape of the reformed phospholipid films during cooling are argued to arise from the adjustment of the packing and molecular tilting (with respect to the mica surface) of the phospholipids in order to accommodate more condensed phospholipids.     

Atomic force microscopy study of beta-substituted-T7 oligothiophene films on mica: mechanical properties and humidity-dependent phases

The structural and mechanical properties of Langmuir-Blodgett monolayer and multilayer films of 3",4"-didecyl-5,2'; 5',2"; 5",2'; 5',2"; 5",2''; 5'',2"-heptathiophene-4'-acetic acid on mica have been studied by atomic force microscopy (AFM) as a function of humidity, temperature, and applied force. The molecules orient with the carboxylic acid group pointing toward the mica surface and expose the alkyl side chains to the air interface. As the load applied by the AFM tip increases, the film is compressed easily from an initial height of 2 to 1.2 nm. After compression the films can support much higher loads without loss of height. The state of aggregation of the molecules was found to be sensitive to the environmental humidity, which induced reversible changes. Annealing the samples with monolayer or multilayer films resulted in irreversible changes when the temperature exceeded approximately 100 degrees C.     

Self-assembly of acridine orange dye at a mica/solution interface: formation of nanostripe supramolecular architectures

Optical waveguide spectroscopy and atomic force microscopy (AFM) have been used to characterize the supramolecular architectures of acridine orange (AO) dye self-assembled at a mica/aqueous solution interface. Under the saturated adsorption conditions, optical waveguide spectroscopy revealed that the dye formed H-type aggregates at the interface. In situ AFM visualized interesting morphology of the dye aggregates showing nanosized meandering stripes with the width of approximately 1.5 nm (or brightness periodicity of approximately 3 nm). Electrostatic adsorption of the dye cations onto a mica surface as well as the intermolecular pi-pi stacking brought about the ordered nanostructures. We propose an interfacial aggregation model that shows a meandering staircase structure with the intermolecular slip angle of 60 degrees. According to the model, the AO molecule occupies a surface area of about 1.0 nm2.     

Preparation of Poly(acrylic acid) Nano-films by In-situ Polymerization of Acrylic Acid Macroclusters on Silicon Oxide Surfaces

A new method for preparing poly(acrylic acid) (PAA) films on silicon oxide surfaces with smooth morphology has been developed. Acrylic acid (AA) was preferably adsorbed on silicon oxide surfaces in AA/ chloroform binary liquids and formed a hydrogen-bonded organized structure, which was called molecular macrocluster. AA macroclusters on silicon oxide surfaces were in-situ polymerized to obtain molecularly flat polymer films with thickness up to 10 nm. In-situ polymerizations were conducted by photo-irradiation in the presence of a photo initiator, 2,2-dimethoxy-2-phenylacetophenone (DPA). As a reference, the adsorption of PAA polymerized in the bulk solution was examined on silicon oxide surfaces. A series of techniques such as attenuated total reflection–FTIR (ATR-FTIR) spectroscopy, ellipsometry and atomic force microscopy (AFM) was utilized for characterizing two types of films. It was found that flat PAA films with linear hydrogen-bonded COOH could only be obtained by in-situ polymerization, which demonstrated this method was an effective way for preparing molecularly uniform polymer films. The surface morphology and thickness of obtained PAA films were found to be dependent on the monomer concentration, initiator amount and photoirradiation time. Molecularly uniform and flat PAA films were obtained after 5 min irradiation at 0.8 mol% AA in the presence of 5 wt% DPA.     

Self-patterned mixed phospholipid monolayers for the spatially selective deposition of metals

Metal-reactive organosulfur groups were patterned onto mica and silicon surfaces by dewetting instabilities during the Langmuir-Blodgett (LB) deposition of phase-separated mixed phospholipid monolayers. Monolayers were formed from binary mixtures of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and their ω-methyldisulfide-dialkylphosphatidylcholine analogues, DSDPPC and DSDLPC. Patterns of highly parallel stripes of condensed DPPC or DSDPPC, protruding by 0.7-0.9 nm from a fluid matrix of DLPC or DSDLPC, were observed over areas extending at least 30 × 30 μm(2) in the LB films. The average stripe width varied from ca. 150 to 500 nm, depending on the lipid composition and deposition pressure. X-ray photoelectron spectroscopy confirmed that the phospholipid-monolayer-bound methydisulfides react with Au vapor to form a gold-thiolate species. The adsorption of thermally evaporated Au, Ag, and Cu onto DSDPPC/DLPC and DPPC/DSDLPC patterns was investigated by field emission gun scanning electron microscopy (FEGSEM) and atomic force microscopy (AFM). A change in phase contrast is observed in FEGSEM and AFM over the methyldisulfide-functionalized areas following metal deposition due to metal-thiolate bond formation. An increase in step height between the DSDPPC stripes and nonfunctionalized DLPC background following metal deposition, as well as the resistance of the metal-coated DSDPPC or DSDLPC regions to detergent extraction from the surface, attest to a selective metallization of the pattern. Our results indicate that the preferential adsorption of vapor-deposited metal onto the ω-methyldisulfide-terminated phase occurs at submonolayer coverages. The chemical reactivity exhibited by the organosulfur-modified phospholipid LB films make these templates potentially interesting for the fabrication of solid-supported patterns of metal nanostructures.     

Low-temperature investigation of the growth mechanism of alkylsiloxane self-assembled monolayers

The growth behavior of self-assembled monolayer films strongly depends on parameters such as solvent, water concentration in the solvent, substrate type, and deposition method. A further parameter, the temperature, is of particular importance. It has been found that growth kinetics, size, and shape of the structures obtained strongly depend on the deposition temperature. Thus, exact adjustment and control of the solution temperature is of crucial importance for investigation of deposition mechanisms. The development of a temperature control unit has been the basis for a series of experiments on deposition of octadecyltrichlorosilane (OTS) on silicon wafers to study the influence of temperature on growth kinetics and film structure. Characterization of the films was performed with ellipsometry and atomic-force microscopy. It has been found that octadecylsiloxane (ODS) island sizes decrease with increasing temperature. Furthermore, a characteristic temperature exists above which increasingly disordered deposition occurs. At low temperatures (5–10 °C) smaller dot-like features are observed besides larger fractally shaped islands characteristic for self-assembly growth of ODS films. Our results indicate that these small dot-like features originate from ordered aggregates in the adsorption solution and that they are the precursors of the formation of larger islands. However, they can only be observed at low temperatures, because at room temperature they coalesce quickly to form larger units, due to the high surface mobility.     

Initial stages of growth of poly(p-xylylene) coatings: AFM study

The morphology of poly(p-xylylene) ultrathin films prepared by vapor deposition polymerization on the surface of single-crystal silicon (100) and on the cleaved surface of mica at a substrate temperature of 20°C has been studied by atomic force microscopy. At the initial stage, the growth of the poly(p-xylylene) coating follows the island mechanism. Within the framework of pyramidal model of island growth, the mean diffusion length for monomer p-xylylene is calculated: For the single-crystal silicon, this parameter is 15 ± 3 nm; for the cleaved surface of mica, 9 ± 2 nm. The nature of the substrate and defects on its surface show a peculiar effect on the structure of the poly(p-xylylene) coating. Thus, at a low monomer flow, nucleation of polymer islands on the surface of silicon is predominantly homogeneous, whereas on the cleaved surface of mica, it is heterogeneous. A change in the monomer flow significantly affects the rate of nucleation of polymer islands.     

Dewetting pattern and stability of thin xyloglucan films adsorbed on silicon and mica

Thin polysaccharide films prepared with xyloglucan (XG), a neutral polysaccharide extracted from the seeds of Guibourtia hymenifolia were prepared by spin-coating and drop deposition under pH3, pH5 and pH12, on silicon and mica substrates. Atomic force microscopy (AFM) images show flat nanoporous matrices with additional grain-like structures on both mica and silicon for pH 3 and pH 5. However, X-ray photoelectron spectroscopy (XPS) and Auger spectra of these adsorbed biopolymers prepared under alkaline condition (pH 12) reveal that Na⁺ ions from the solution interact with the mica substrate surface and with XG forming chemical bonds. Both XPS and Auger results suggest XG depolymerisation during adsorption, caused by an alkaline ß-base catalyzed degradation mechanism, which is consistent with the more basic character of the mica surface under these conditions. Thus, the polysaccharide diffusion is inhibited during dewetting due to the surface bonding. On the other hand, the interaction of Na⁺ in solution with the silicon surface is weaker, favoring its interaction with the polysaccharide, conserving the overall polymer structure of XG and allowing the biopolymer to slip and diffuse during dewetting, forming the final branched fractal structure.     

The effect of surface ions on water adsorption to mica

We have measured the adsorption isotherms of water on a single surface of freshly cleaved mica with K+ on the surface, and on mica where the K+ has been exchanged for H+. Using a very sensitive interferometric technique, we have found a significant difference between the two isotherms at submonolayer coverage, for relative vapor pressures p/p0 < 0.5. The K+-mica isotherm shows a pronounced convexity, suggesting distinct adsorption sites, whereas the H+-mica isotherm is flatter. The two isotherms converge above monolayer coverage. The results give a graphic demonstration of the importance of nanoscale surface heterogeneities for vapor adsorption at submonolayer coverage.     

Adsorption of poly(N-isopropylacrylamide) on glass substrata

The adsorption of poly(N-isopropylacrylamide) (PNIPAAM), a well known thermosensitive polymer, on glass was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The polymer was dissolved in water at low (0.02 g/L) and high (2 g/L) concentration and the tested temperatures were below (25 degrees C) and above (50 degrees C) the lower critical solubility temperature (LCST). Whatever the conditions, a smooth layer of adsorbed molecules spread along the surface was observed. The thickness was about twice higher for high concentration compared to low concentration. The cohesion in the adsorbed layer, as revealed by scraping tests performed by AFM, was higher above the LCST than below the LCST. On top of this adsorbed layer, single-chain coils, globules, or aggregates were present, depending on concentration and temperature. The observation of these additional adsorbed entities was poorly reproducible, presumably due to the lack of shear control upon rinsing. These results emphasize the importance of the characterization of surface morphology to interpret amounts of adsorbed polymers.     

银纳米粒子在云母表面的二维组装及其表面增强拉曼效应

随着纳米技术的迅速发展 ,利用共价或非共价键作用将金属纳米粒子组装到固体基片上 ,因其方法简单、重复性好而成为研究热点 .目前 ,人们已经成功地利用带有— SH[1,2 ] ,—CN,— NH2 [3 ] 等基团的单层或多层膜作为偶联剂将 Au和 Ag等金属纳米粒子固定在玻璃、石英、硅、金等固体基片上 .但在许多情况下 ,偶联剂却成为一种干扰物质 .云母是一种重要的电子工业材料 ,并具有廉价、较易获得新鲜表面等特点 ,研究金属纳米粒子在云母表面的组装和排列无疑具有重要意义 .但是 ,迄今为止 ,在表面没有偶联剂修饰的条件下 ,以云母为基底的金属纳…     

Effect of protein solution components in the adsorption of Herbaspirillum seropedicae GlnB protein on mica

The adsorption of proteins and its buffer solution on mica surfaces was investigated by atomic force microscopy (AFM). Different salt concentration of the Herbaspirillum seropedicae GlnB protein (GlnB-Hs) solution deposited on mica was investigated. This protein is a globular, soluble homotrimer (36 kDa), member of PII-like proteins family involved in signal transducing in prokaryote. Supramolecular structures were formed when this protein was deposited onto bare mica surface. The topographic AFM images of the GlnB-Hs films showed that at high salt concentration the supramolecular structures are spherical-like, instead of the typical doughnut-like shape for low salt concentration. AFM images of NaCl and Tris from the buffer solution showed structures with the same pattern as those observed for high salt protein solution, misleading the image interpretation. XPS experiments showed that GlnB protein film covers the mica surface without chemical reaction.     

Nanoscale aggregate structures of trisiloxane surfactants at the solid-liquid interface

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.     

聚ε-己内酯结晶形态的研究

采用DSC测试了聚ε-己内酯(PCL)的结晶温度(TC)和熔融温度(Tm)。同时采用偏光显微镜(POM)探讨了结晶时间的影响,发现结晶时间的改变只能够改变其晶体的尺寸,对其结晶形态并没有太大的影响。最后采用原子力显微镜(AFM)讨论了基底材料、溶剂和过冷度对PCL结晶形态的影响。结果表明:基底材料对PCL结晶形态的影响是比较显著的,PCL在硅片上呈棒状,在云母和涂有碳膜的云母上呈树枝状。溶剂对PCL结晶形态的影响明显,其结晶形态的差别与蒸汽压有关。在不同的温度下PCL结晶形态都成树枝状晶体,且分枝宽度随着过冷度的降低而增加。     

AFM and XRD investigation of crystalline vapor-deposited C60 films

The morphological and structural properties of C(60) films deposited on quartz substrates by sublimation at 320-500 degrees C under high vacuum have been investigated using atomic force microscopy (AFM) and reflection X-ray diffraction (RXRD). The thickness of the films varied between 0.2 microm and 10 microm. AFM showed that the films consist predominantly of cubic crystals of a few micrometer in size with well-developed (111) and (100) faces. The crystallographic investigation revealed a strongly preferred [111] growth direction which is very sensitive to the deposition rate and substrate temperature. The influence of the experimental parameters on the morphology of the crystals and on the preferred orientation of the films is discussed in view of the AFM and RXRD results.