自组装硫醇分子膜电输运特性的导电原子力显微镜研究

在Au(111)表面自组装制备了不同链长的烷烃硫醇分子膜，并利用导电原子力显微镜研究了 自组装分子膜的输运特性随外加压力的变化.结果发现分子膜的电流随压力的增加而增大， 其变化特征可以较好地用Hertz模型描述.在相同压力和电压下，通过分子膜的电流随分子链 长的增加呈指数衰减，其衰减因子先随压力的增加而减小，后逐渐趋于稳定.此外，长链分 子自组装膜的电流随压力的变化比短链分子膜更为明显.分析表明，自组装硫醇分子膜输运 特征的压力依赖性主要源于电荷在分子膜中的链间隧穿过程. 关键词： 分子自组装 输运特性 原子力显微镜     

硫醇自组装分子膜末端基团对其电荷输运特性的影响

在金(111)表面组装了具有不同末端基团的硫醇单层分子膜，并利用导电原子力显微镜研究了分子膜的电输运性质，发现不同末端基团的分子自组装膜的导电能力有明显差别.结合X射线光电子能谱，研究了末端基团中碳原子的结合能与相应硫醇分子电导的关系.结果表明不同末端基团分子膜导电能力的差别可归结为末端基团碳原子电子结合能的差异.结合能越高，末端基团电子的局域化程度越强，导致电子有效注入分子主链的势垒越高，从而减弱了分子膜对电子的输运能力.此外，实验还发现不同末端基团的硫醇单层分子膜有不同的表面电势，导致分子膜电流电压特性曲线的零点产生偏离. 关键词： 分子自组装膜 输运特性 末端基团 导电原子力显微镜     

Atomic force microscopy evidence for K domains on freshly cleaved mica

The unit cell height in the c-direction of muscovite mica is well established at 10 Å. However, we have observed steps much lower than this whilst imaging freshly cleaved mica surfaces in an atomic force microscope. The steps, 1.0±0.05 Å high, are unstable and disappear in a period of minutes after cleavage. We propose that they are due to the presence of domains of residual K⁺ ions that form two matching patchworks on the cleaved faces. Upon cleavage, they relax inwards from the bulk equilibrium position 1.6 Å above the oxygen atoms of the tetrahedral silicate. Possible mechanisms for the disappearance of the steps are discussed.     

Visualization of human plasma fibrinogen adsorbed on highly oriented pyrolytic graphite by scanning probe microscopy

Human plasma fibrinogen (HPF) was observed by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) conducted in non-contact mode. The HPF was adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate as single molecules, as aggregated bundles, and as aggregated fibers. Topographic and phase images confirmed structural changes in the HPF after exposure to air, while topographic and KPFM images confirmed fibers with the width of a single HPF molecule. Additionally, KPFM confirmed the surface potential difference between the HPF and the HOPG, and periodical potential drop reflecting the E and D domains in the fiber.     

Needles and clusters of zinc porphyrin molecules on mica

Molecular monolayers of zinc porphyrins deposited from a liquid solution on mica substrates have been characterised using atomic force microscopy (AFM). At room temperature, needle-shaped single layer porphyrin islands are formed due to an anisotropic growth rate. The average island size depends on the concentration of the porphyrin solution, but the length to width aspect ratio is rather independent of the island size. Annealing the porphyrin monolayers to 450 K leads to the reduction of the size of the monolayer islands in two different ways depending on the heating rate. With slow heating rate, the island size was found to shrink by losing molecules from the edge of the island. With high heating rate, nanoscale molecular clusters were found to form.     

Transformation of a hydrophilic membrane into semi-super-hydrophobic based on self-assembly of stearic acid monolayer over induced nanostructures on the membrane surface

Both the chemical and structural properties of a surface determine the contact angle. For the formation of super-hydrophobic surfaces, modification of surface chemistry must be always combined with surface roughness enhancement. The used methods to make a super-hydrophobic surface are expensive and need very complicated equipments and cannot be scale up easily. In this study a simple and less expensive method was developed to transform a hydrophilic membrane into a semi-super-hydrophobic. In order to modify the membrane surface geometrically, the required needle-like rugosities were created by boiling the membrane in the water. The chemical modification of the roughened surface was created by the chemical adsorption and controlling the reaction time of stearic acid (STA) on the polymer of the membrane surface. Finally, by controlling the surface roughness, the concentration of the STA solution and duration of reaction time, a semi-super-hydrophobic membrane with the contact angle of 120° was prepared.     

Binding studies of costunolide and dehydrocostuslactone with HSA by spectroscopy and atomic force microscopy

Human serum albumin (HSA), a major plasma protein and plasma-derived therapeutic, interacts with a wide variety of drugs and native plasma metabolites. In this study the interactions of costunolide (CE) and dehydrocostuslactone (DE) with HSA were investigated by molecule modeling, atomic force microscopy (AFM), and different optical techniques. In the mechanism discussion, it was proved that fluorescence quenching of HSA by both of the drugs is a result of the formation of drug-HSA complexes. Binding parameters for the reactions were determined according to the Stern-Volmer equation and static quenching. The results of thermodynamic parameters ΔG⁰, ΔH⁰, and ΔS⁰ at different temperatures indicated that hydrogen bonding interactions play a major role in the drug-HSA associations process. The binding properties were further studied by quantitative analysis of CD, FTIR, and Raman spectra. Furthermore, AFM results showed that the dimension of HSA molecules became more swollen after binding with the drugs.     

Improved aging performance of vapor phase deposited hydrophobic self-assembled monolayers

A hydrophobic self-assembled monolayer (SAM) of fluoro-octyl-trichloro-silane (FOTS) was deposited on silicon using a vapor phase technique. The aging of the hydrophobic layer was examined using water contact angle measurements. It has been found that while such monolayer films suffer from a loss of hydrophobicity with time, pre-immersion nitrogen annealing can significantly improve the aging characteristics of these monolayers. The effect of nitrogen annealing on the improved aging properties of SAM coatings has been investigated by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The hydrolytic stability and the effect of nitrogen annealing were studied by morphological evolution during immersion. A spontaneous formation of silane mounds on the surface of the monolayers was found by AFM. These mounds have been irreversibly transformed from initially uniform hydrophobic surface layers. It is highly probable that the compliance of these mounds can reasonably allow hydrophilic sites to be located around the mounds. Interestingly, the density of these mounds formation is very less on the annealed samples. XPS reveals a higher level of coverage by the N₂-annealed film due to agglomeration. A relative abundance of CF₃ and CF₂ moieties in the annealed film may explain the enhancement of the hydrophobicity as revealed by higher level of water contact angle. This hydrophobicity was found to be significantly stable in water. This novel finding explains the improved hydrophobic stability of FOTS monolayers as primarily a morpho-chemical effect that originates from the densification of the monolayers upon annealing.     

Effects of flow ratios on surface morphology and structure of hydrogenated amorphous carbon films prepared by microwave plasma chemical vapor deposition

A series of hydrogenated amorphous carbon (a-C:H) films were deposited on silicon substrates by microwave plasma chemical vapor deposition technique with a mixture of hydrogen and acetylene. The effects of flow ratio of hydrogen to acetylene on surface morphology and structure of a-C:H films were investigated using surface-enhanced Raman spectroscopy and scanning probe microscope (SPM) in the tapping AFM mode. Raman data imply a transition from graphite-like phase to diamond-like bonding configurations when the flow ratio increases. AFM measurements show that the increase in hydrogen content, to some extent, can smoothen the surface morphology and decrease the RMS roughness. Excessive hydrogen is found to cause the formation of polymeric hydrocarbon clusters in the films and reduce deposition rate.     

Kinetic effects in the self-assembly of pure and mixed tetradecyl and octadecylamine molecules on mica

The self-assembly of tetradecylamine (C14) and of mixtures of tetradecyl and octadecylamine (C18) molecules from chloroform solutions on mica has been studied using atomic force microscopy (AFM). For pure components self-assembly proceeds more slowly for C14 than for C18. In both cases after equilibrium is reached islands of tilted molecules cover a similar fraction of the surface. Images of films formed by mixtures of molecules acquired before equilibrium is reached (short ripening time at room temperature) show only islands with the height corresponding to C18 with many pores. After a long ripening time, when equilibrium is reached, islands of segregated pure components are formed.     

High-temperature pretreatment of Ni nanoparticles enhances the growth of high-density carbon fiber bundles during microwave plasma chemical vapor deposition

This paper presents an experimental study on the effect of the pretreatment procedure of Ni nanoparticles (NPs) on the growth of multiwalled carbon fiber (CNF) bundles by means of microwave plasma chemical vapor deposition (MPCVD). We used atomic force microscopy to investigate a series of pretreated Ni films. The structures and compositions of the CNFs on the via were investigated using scanning electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy.The geometric shape of the Ni NPs was identified in terms of their roughness, which decreased upon increasing the pretreatment temperature, resulting subsequently in the synthesis of high-density CNFs. The diameter and shape of the Ni NPs were the dominant factors affecting the size and density of the CNFs bundles. We obtained CNFs that fully filled the via effectively; they might serve as potential interconnects in future nanodevices.     

Surface morphology, chemical contrast, and ferroelectric domains in TGS bulk single crystals differentiated with UHV non-contact force microscopy

Ferroelectric bulk single crystals of tri-glycine sulphate (TGS) have been investigated in ultra-high vacuum (UHV) using dynamic force microscopy (DFM) in the non-contact (nc) mode. Both chemical contrast from different sub-unit cell cleavage steps, and ferroelectric domains were differentiated by recording the variation in interaction force affecting the excitation amplitude A_exc applied to the piezo shaker in constant amplitude DFM. No chemical difference was found for steps measuring half the unit cell height b, in contrast to b/4—steps where sulphate ions change the local short-range chemical forces. By varying the bias voltage applied to the TGS counter electrode, the sign of bound surface charge within each ferroelectric domain was determined. Domain walls separating regions with antiparallel polarisation vectors are resolved down to a 9 nm domain wall width. Furthermore, we achieved atomic resolution with nc-DFM on cleaved TGS samples indicating the monoclinic unit cell at the ferroelectric sample surface with a=1.0±0.05 nm, c=0.55±0.05 nm, and β=107±3°.     

Cu ions induced reorientation of crystallite in ZnO nano/micro rod arrays thin films

Undoped and Cu doped ZnO nano/microrod arrays (N/MRAs) films were grown on seeded glass substrates by chemical bath deposition technique. The structural and morphological characterizations (X-ray diffraction and scanning electron microscopy) clearly illustrate that the pure ZnO N/MRAs has well-defined hexagonal prismatic cross-sectional crystallographic facets and its crystallographic plane is more vertically grown along (002) crystallographic direction. The horizontally aligned (100) crystallographic plane is induced at 20% Cu doping level. Morphological structure of ZnO N/MRAs changed into nanoplates/microballs with increasing of Cu doping level. Hexagonal crystal structure of undoped and Cu doped ZnO N/MRAs is then confirmed by µ-Raman scattering. Energy band gaps are found to be decreased with Cu doping and reached a minimum of 3.04 eV when doping level is 20%. The defect related peak intensity is suppressed in ZnO N/MRAs by Cu doping.     

Thermal stability of alkoxy monolayers grafted on Si(1 1 1)

Direct grafting of organic monolayers on Si is of prime interest in order to give specific properties to a silicon surface. However, for microelectronics applications, this possibility is hampered by the limited stability of the grafted layers. It has been previously established that alkyl layers attached to Si surfaces through Si-C bonds become unstable at 250-300 °C, by desorption of alkenes. Changing the nature of the bonding to the surface might allow one to circumvent this desorption pathway and increase the layer stability. In our work, decanol and decyl aldehyde are reacted with the Si(1 1 1)-H surface at ∼100 °C during 20 h in order to obtain alkoxy monolayers. FTIR measurements performed in ATR geometry show that the grafted molecule surface coverage is on the order of 33% after reaction with decanol and 50% after reaction with decyl aldehyde. Characterization by AFM essentially reveals that the morphology of the grafted surfaces is unaffected as compared to that of Si-H surfaces. However, the edges of the terraces at alcohol-grafted surfaces exhibit some pitting, probably due to the presence of water in the grafting liquid. Thermal stability studies show that alkoxy chains progressively disappear from the Si surface between 200 and 400 °C. From the CH₂/CH₃ ratio in the CH region (2760-3070 cm⁻¹), it appears that the chains undergo progressive dissociation by C-C bond breaking before their complete disappearance from the surface. Therefore, the thermal behaviour of alkoxy monolayers appears quite distinct from that of alkyl monolayers that tend to leave the surface in a much narrower temperature range (250-350 °C), essentially via breaking of the Si-C bonds.     

Nanoindentation characterization of GaN epilayers on A-plane sapphire substrates

Gallium nitride (GaN) epilayers was deposited on a-axis sapphire substrate by means of metal-organic chemical vapor deposition (MOCVD) method. The GaN epilayers has been investigated in their repetition pressure-induced impairment events from nanoindentation technique and, the relative deformation effect was observed from atomic force microscopy (AFM). From the morphological studies, it is revealed that none of crack and particle was found even after the indentation beyond the critical depth on the residual indentation impression. The ‘pop-in’ event was explained by the interaction of the deformed region, produced by the indenter tip, with the inner threading dislocations in the GaN films. Pop-in events indicate the generation and motion of individual dislocation, which is measured under critical depth and, no residual deformation of the GaN films is observed.     

Effects of Sm substitution on ferroelectric domains and conductivity in bismuth ferrite ceramics

The large piezoelectric coefficient and multiferroicity of bismuth ferrite (BFO) make it an attractive candidate for lead-free ferroelectric devices. However, large leakage currents have limited broader applications. Rare-earth substitutions in BFO have been shown to improve ferroelectric and magnetic properties. In this work, we employed piezoresponse and conductive atomic force microscopy to study ferroelectric domains in Bi_1-xSm_xFeO₃ (x = 0–0.150) grown by the co-precipitation method. The combined piezoresponse and conductivity measurements can directly visualize the local ferroelectric domains under different sample bias. At Sm mol% > 7.5, Sm-substitution effectively lowers defect-generated conductivity. At Sm mol% < 7.5, conductivity increases due to conductive domain walls inside sample grains. The surfaces of these conductive samples exhibit a p-type rectifying behavior while the bulk is n-type. Our work details how the local piezoelectric properties and transport behaviors of BFO ceramics change as a function of Sm-substitution.     

Investigations of C60 molecules deposited on Si(111) by noncontact atomic force microscopy

We demonstrated the high resolution imaging of the organic molecules using noncontact atomic force microscopy in ultrahigh vacuum. The sample was C₆₀ molecules deposited on the Si(111)-7×7 reconstructed surface. When the thickness of the C₆₀ film was submonolayer, we could image some isolated C₆₀ molecules and the reconstructed Si surface simultaneously. However, the imaging was highly unstable not only because of the large structure but also due to the large difference between the interaction forces on the molecules and on the Si surface. On the other hand, when the thickness of the C₆₀ molecules was almost monolayer, individual molecules could be stably imaged.     

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator layer

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator (SOI) layer has been studied. A square-shaped 12 nm thick SOI layer was patterned by lithography and by selective etching with a KOH solution. The structural change by ultrahigh vacuum annealing in a temperature range of 900–1100 °C was observed by atomic force microscopy. The agglomeration takes place preferentially from the pattern edges at a lower annealing temperature than that for the unpatterned layer, indicating enhanced diffusion of Si atoms at the edges. Additionally, the patterning causes formation of smaller islands than those for the unpatterned layer, reflecting that the patterning limits the amount of Si atoms supplied for the island formation.