Effect of tip size on force measurement in atomic force microscopy

An atomic force microscope (AFM) has been used to study solvation forces at the solid-liquid interface between highly oriented pyrolytic graphite (HOPG) and the liquids octamethylcyclotetrasiloxane (OMCTS), n-hexadecane (n-C16H34), and n-dodecanol (n-C11H23CH2OH). Oscillatory solvation forces (F) are observed for various measured tip radii (Rtip=15-100 nm). It is found that the normalized force data, F/Rtip, differ between AFM tips with a clear trend of decreasing F/Rtip with increasing Rtip.     

Freezing of fluids confined between mica surfaces

Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.     

Calorimetric study of solvation of alkanes with mixtures of acetone or 1,4-dioxane with 1-octanol at 25°C

Heats of solution of cyclohexane and squalane in mixtures of 1-octanol with acetone or 1,4-dioxane within the entire range of compositions were measured calorimetrically. The enthalpies of solution of cyclohexane, squalane, and hexadecane in the mixtures of acetone or 1,4-dioxane with 1-octanol show linear correlation. Enthalpies of solvation of propane, decane, and eicosane in these mixtures were calculated.     

Unraveling the properties of octamethylcyclotetrasiloxane under nanoscale confinement: atomistic view of the liquidlike state from molecular dynamics simulation

We developed an atomistic model of octamethylcyclotetrasiloxane (OMCTS) liquid confined within the nanospace between two flat mica surfaces. Molecular dynamics simulation was carried out for the liquidlike state where OMCTS liquid is not frozen, while forming molecular layers parallel to the surface. With the aid of a layer by layer analysis of the intra- and interlayer microscopic structures and the dynamics, it is found that the difference in the properties of the inner layers and the bulk liquid are relatively small in spite of the clear differences in the structure. This leads to the conclusion that the layered structure itself is an appearance of the microscopic structure that already exists in the bulk liquid. The most striking difference from the bulk liquid is mainly seen in the contact layer, where characteristic molecular orientations that are not seen in the crystalline phase appeared, and the dynamics of the liquid becomes 2-3 orders of magnitude slower than that of the bulk.     

Solvation and squeeze out of hexadecane on graphite

We have performed simultaneous force and conductivity measurement of hexadecane liquid confined between a conducting atomic force microscope tip and a graphite surface. Both the current and the force data reveal discrete solvation layering of the hexadecane near the surface. We typically observe that the current does not vary with load in a simple way as the layer closest to the surface is compressed, but increases markedly prior to the expulsion of material from the tip-sample gap. We infer that even for a nanoscale asperity there is conformation change of the confined hexadecane under the tip apex prior to squeeze out of the molecules.     

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.     

Surface-tuned assembly of porphyrin coordination oligomers

Two self-complementary phenanthroline-strapped porphyrins bearing imidazole arms and C 12 or C 18 alkyl chains were synthesized, and their surface self-assembly was investigated by atomic force microscopy (AFM) on mica and highly ordered pyrrolitic graphite (HOPG). Upon zinc(II) complexation, stable porphyrin dimers formed, as confirmed by DOSY (1)H NMR and UV-visible spectroscopy. In solution, the dimers formed J-aggregates. AFM studies of the solutions dip-coated onto mica or drop-casted onto HOPG revealed that the morphologies of the assemblies formed were surface-tuned. On mica, fiber-like assemblies of short stacks of J-aggregates were observed. The strong influence of the mica's epitaxy on the orientation of the fibers suggested a surface-assisted assembly process. On HOPG, interactions between the alkyl chains and the graphite surface resulted in the stabilization and trapping of monomer species followed by their subsequent association into coordination polymers on the surface. Interdigitation of the alkyl chains of separate polymer strands induced lateral association of wires to form islands that grew preferentially upon drop-casting and slow evaporation. Clusters of laterally assembled wires were observed for the more mobile functionalized porphyrins bearing C 12 chains.     

AFM imaging of adsorbed Nafion polymer on mica and graphite at molecular level

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.     

Solvation of the N-(1-Phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea chiral stationary phase in mixed alcohol/water solvents

A theoretical and experimental study of alcohol/water and alcohol/alcohol solvent mixtures near a surface of N-(1-phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea (PEPU), a Pirkle-type chiral stationary phase, is presented. Molecular dynamics simulations are performed at room temperature for water/methanol, water/1-propanol, water/2-propanol, and methanol/1-propanol solvent mixtures confined between two PEPU surfaces. The interface was also prepared experimentally by attaching the PEPU molecules to atomic force microscopy tips and oxidized Si(111) substrates. Chemical force spectrometric measurements between such PEPU-terminated tips and samples were taken in the solvent mixtures, and the results are compared to the molecular dynamics study. We find that the extent of hydrogen bonding at the surface is the dominant contributor to the measured forces.     

Solvation force, structure and thermodynamics of fluids confined in geometrically rough pores

The effect of periodic surface roughness on the behavior of confined soft sphere fluids is investigated using grand canonical Monte Carlo simulations. Rough pores are constructed by taking the prototypical slit-shaped pore and introducing unidirectional sinusoidal undulations on one wall. For the above geometry our study reveals that the solvation force response can be phase shifted in a controlled manner by varying the amplitude of roughness. At a fixed amplitude of roughness, a, the solvation force for pores with structured walls was relatively insensitive to the wavelength of the undulation, lambda for 2.3/=0.5. The predictions of the superposition approximation, where the solvation force response for the rough pores is deduced from the solvation force response of the slit-shaped pores, was in excellent agreement with simulation results for the structured pores and for lambda/sigma(ff)>/=7 in the case of smooth walled pores. Grand potential computations illustrate that interactions between the walls of the pore can alter the pore width corresponding to the thermodynamically stable state, with wall-wall interactions playing an important role at smaller pore widths and higher amplitudes of roughness.     

Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.     

Effect of substrate surface on dewetting behavior and chain orientation of semicrystalline block copolymer thin films

Three symmetrical semicrystalline oxyethylene/oxybutylene block copolymers (EmBn) were spin-coated on different substrates including silicon, hydrophobically modified silicon, and mica. The effects of surface property on the dewetting behavior of EmBn thin films and the chain orientation of the crystalline block were investigated with atomic force microscopy and grazing incidence X-ray diffraction . The EmBn thin films on silicon exhibit an autophobic dewetting behavior, while ordinary dewetting occurs for the thin films on modified silicon. It was observed that the stems of the E crystals in the first half-polymer layer contacting the mica surface were parallel to the surface, in contrast to the perpendicular chain orientation of the other polymer layers and of the first half-polymer layer on silicon. This is attributed to the strong interaction between the E block and mica, verified by infrared spectra.     

Forces between mica surfaces, prepared in different ways, across aqueous and nonaqueous liquids confined to molecularly thin films

We have measured normal and lateral interactions across a range of different liquids between mica surfaces using a surface force balance (SFB). The mica surfaces were prepared either by melt cutting using Pt wire and standard procedures in our laboratories or by tearing sheets (that had not been exposed to Pt) off from a freshly cleaved sheet of mica. AFM micrographs revealed the substantial absence of Pt nanoparticles on the melt cut and torn-off mica surfaces. Normal-force versus surface-separation (D) profiles and shear force versus D measurements for purified water (no added salt), for concentrated aqueous NaCl solutions, and for cyclohexane revealed that in all cases the behavior of the highly confined liquids between melt-cut and between torn-off mica sheets was identical within experimental scatter. These results demonstrate directly that interactions measured between melt-cut mica surfaces as routinely prepared using established procedures in our laboratories and in other laboratories are free of the effect of any Pt contamination.     

Nanoscale morphology of melt crystallized polyethylene/graphite (HOPG) interphase

High density polyethylene (PE) was crystallised from the melt on freshly cleaved surface of highly oriented pyrolitic graphite (HOPG) or mica. Atomic force microscopy (AFM) studies of structure of the polymer surface adjacent to the graphite or mica were performed after peeling of from the substrate. Significant differences of crystalline structure on the interface were found between PE crystallised on graphite and mica. The surface of polyethylene crystallised on graphite shows large areas with regularly arranged rectangular structures. These objects (ca 20‐80 nm big) probably represent the nucleation centres of the lamellar growth. The surface of polyethylene crystallised at mica surface shows some dot‐like structures showing no particular arrangement.     

Direct observation of layered structures at ionic liquid/solid interfaces by using frequency-modulation atomic force microscopy

Presence of inhomogeneous layered structures of ionic liquid (IL) molecules at IL/HOPG and IL/mica interfaces was directly detected and imaged by using frequency-modulation atomic force microscopy. High stability of the layered structures may disturb their interface applications to catalysis and electrochemistry.     

Shear dynamics of hydration layers

Molecular dynamics (MD) simulations have been performed to investigate the shear dynamics of hydration layers of the thickness of D=0.61-2.44 nm confined between two mica surfaces. Emphases are placed on the external shear response and internal relaxation properties of aqueous films. For D=0.92-2.44 nm liquid phase, the shear responses are fluidic and similar to those observed in surface force balance experiments [U. Raviv and J. Klein, Science 297, 1540 (2002)]. However, for the bilayer ice (D=0.61 nm) [Y. S. Leng and P. T. Cummings, J. Chem. Phys. 124, 74711 (2006)] significant shear enhancement and shear thinning over a wide range of shear rates in MD regime are observed. The rotational relaxation time of water molecules in this bilayer ice is found to be as high as 0.017 ms (10(-5) s). Extrapolating the shear rate to the inverse of this longest relaxation time, we obtain a very high shear viscosity for the bilayer ice, which is also observed quite recently for D< or =0.6+/-0.3 nm hydration layers [H. Sakuma et al., Phys. Rev. Lett. 96, 46104 (2006)]. We further investigate the boundary slip of water molecules and hydrated K(+) ions and concluded that no-slip boundary condition should hold for aqueous salt solution under extreme confinement between hydrophilic mica surfaces, provided that the confined film is of Newtonian fluid.     

Thermochemical investigations of nearly idial binary solvents. 6. Solubilities of iodine and benzil in systems of nonspecific interactions

Solubilities have been determined at 25°C for iodine in binary mixtures of carbon tetrachloride with cyclohexane, n-hexane, n-heptane, and octamethylcyclotetrasiloxane (OMCTS) and in mixtures of cyclohexane with n-hexane and OMCTS; and for benzil in binary mixtures of carbon tetrachloride with cyclohexane, n-hexane, and n-heptane, mixtures of n-hexane with cyclohexane and n-heptane, and mixtures of benzene with cyclohexane and toluene. With the exception of the benzene+cyclohexane system, the nearly ideal binary solvent model predicts these solubilities with a maximum deviation of 6% and an overall standard deviation of 2.4%. The model correctly predicts minima for solubility (mole fraction) of iodine in the OMCTS systems, and predicts solubilities within 4% for benzil in the carbon tetrachloride+n-hexane system, in which the solubility changes by a factor of 14. The failure of the model for predicting solubilities of benzil in mixtures of benzene and cyclohexane (maximum error of 25% for and 18-fold range of solubilities) is possibly due to specific interactions between benzil and benzene.     

Trapping Nanostructures on Surfaces through Weak Interactions

The assembly of imidazole‐functionalized phenanthroline‐strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl₃, THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three‐dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface–molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.     

石墨烯的纳米摩擦与磨损性质

采用乙醇溶剂剥离的方法制备石墨烯. 通过对溶剂温度、超声时间、超声功率和溶剂离心速度及时间的控制, 从高定向热解石墨(HOPG)制备得到少层石墨烯. 用原子力显微镜(AFM)研究了云母基底上不同层数石墨烯在真空中的纳米摩擦过程, 发现从约4 个原子单层(4 ML)起, 摩擦系数基本不再变化, 但摩擦力仍随着厚度的增加而显著减小, 7 ML之后, 其摩擦系数基本接近于零. 在磨损实验中, 少层石墨烯表面存在刮坏的现象,且不同厚度的石墨烯的磨损现象明显不同, 其中2 ML石墨烯相比4 ML石墨烯表现出较好的耐磨损性能, 且不具有摩擦方向依赖性. 测试了真空下少层石墨烯和云母表面的粘附力, 发现不同层厚的石墨烯相差不大, 因此认为基底效应并不是磨损性质差异的主要原因. 相对于单层石墨烯, 少层石墨烯在抗磨损涂层等领域有着很大的潜在应用价值.     

高氯酸根阴离子在HOPG中嵌入行为的EC-STM研究

运用电化学扫描隧道显微镜（EC-STM）和循环伏安（CV）技术对高氯酸根阴离子ClO₄^-在高序热解石墨（HOPG）中的电化学嵌入行为进行了研究. 通过观察嵌入前后石墨台阶处高度的变化,比较了不同高度的台阶对嵌入的影响,讨论了ClO₄^-离子嵌入石墨的可行性、可逆性和嵌入速率. 研究表明,3层以上的台阶位才有可能观察到由四阶和三阶嵌入引起的台阶高度变化,4 ~ 8个原子层高度的石墨台阶可以实现ClO₄^-在台阶处较为可逆的四阶嵌入,但1 ~ 2层台阶处无法观察到嵌入引起的台阶高度变化,嵌入反应通常会伴随台阶的剥离和脱落现象. 四阶的嵌入反应较三阶可逆,二阶和一阶时,嵌入所需反应电势较高,此时氧化反应较为剧烈,嵌入反应被掩盖,很难观察到台阶高度的变化,更多的形貌变化是台面和台阶处不可逆的损坏如剥落、断层、黑坑等.