Attachment of nanoparticles to the AFM tips for direct measurements of interaction between a single nanoparticle and surfaces

Here we report a universal method of attachment/functionalization of tips for atomic force microscope (AFM) with nanoparticles. The particles of interest are glued to the AFM tip with epoxy. While the gluing of micron size particles with epoxy has been known, attachment of nanoparticles was a problem. The suggested method can be used for attachment of virtually any solid nanoparticles. Approximately every other tip prepared with this method has a single nanoparticle terminated apex. We demonstrate the force measurements between a single approximately 50 nm ceria nanoparticle and flat silica surface in aqueous media of different acidity (pH 4-9). Comparing forces measured with larger ceria particles ( approximately 500 nm), we show that the interaction with nanoparticles is qualitatively different from the interaction with larger particles.     

Free energy of the solid C60 fullerene orientational order-disorder transition

The free energies of the orientationally ordered crystal phase of C60 at low temperatures and the disordered crystal phase at high temperatures are calculated to an accuracy of +/-0.05 kJ/mol using the expanded ensemble Monte Carlo method with the potential model of Sprik et al. [J. Phys. Chem. 96, 2027 (1992)]. The order-disorder transition temperature at zero pressure is determined directly from these free energies, and is found to be consistent with the abrupt changes in configurational energy and unit cell size also found in simulation. A modification of the potential results in predictions of the transition temperature of 257 K and the entropy change of 18.1 J/mol K at this transition, which are in good agreement with the experimental values of 260 K and 19 J/mol K, respectively. The orientational distinguishability in the ordered phase and the indistinguishability in the disordered phase lead to a contribution to the entropy difference of k ln 60, with 60 being the symmetry number of C60. This quantum mechanical correction is important for the accurate prediction of the phase transition properties of the C60 crystals.     

Potential-induced phase transition of low-index Au single crystal surfaces in propylene carbonate solution

In situ scanning tunneling microscopy (STM) was employed to examine the surface structures of Au(111), Au(100), and Au(110) single crystals in propylene carbonate (PC) containing tetrabutylammonium perchlorate (TBAP). All three electrodes exhibited potential-induced phase transition between the reconstructed and unreconstructed (1 × 1) structures at negative and positive potentials, respectively. The potential-induced phase transition of the Au electrode surfaces is attributed to the interaction of the TBA cation and the perchlorate anion at the electrode surface, which is similar to that which takes place in aqueous solutions. In addition to static atomic structures, dynamic processes of both the reconstruction and the lifting of the reconstruction were investigated by means of in situ STM. The lifting of reconstructed Au(111)-(√3 × 22) on Au(111) to the (1 × 1) structure is completed within 1 min at a positive potential. The diffusion of Au atoms on the Au(100) plane in the PC solution proceeds more rapidly than that in the aqueous solution, suggesting that the PC solvent plays an important role in accelerating the diffusion of Au atoms.     

The rotation and the phase transition in solid C60

The interaction between C60's in solid C60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C60 's were discussed. In order to describe the disordered degree of C60 rotation, an equivalent M is introduced. The phase transitions at the ~260 K and at the ~90 K are studied from the viewpoint of C60 rotation. The potential barriers of the ordered rotation below the ~260 K and the disordered rotation above the ~260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.     

Barotropic phase transition between the lamellar liquid crystal phase and the inverted hexagonal phase of dioleoylphosphatidylethanolamine

The phase transition between the lamellar liquid crystal (Lalpha) phase and the inverted hexagonal (H(II)) phase of dioleoylphosphatidylethanolamine (DOPE) in aqueous NaCl solutions was observed by means of differential scanning calorimetry (DSC) under ambient pressure and light-transmittance technique under high pressure. The pressure dependence of the transition temperature (dT/dp) and the thermodynamic quantities for the Lalpha/H(II) transition were compared with those of another phase transition found in the DOPE bilayer membrane, which is the transition from the lamellar crystal (Lc) phase to the Lalpha phase. The dT/dp value of the Lalpha/H(II) transition was about 3.5 times as large as that of the Lc/Lalpha transition while the thermodynamic quantities were significantly smaller than those of the latter to the contrary. Comparing the enthalpy and volume behavior of the Lalpha/H(II) transition with that of the Lc/Lalpha transition, we concluded that the Lalpha/H(II) transition can be regarded as the volume-controlled transition for the reconstruction of molecular packing.     

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

We developed the new intermolecular interaction model of C(60) with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (DeltaE) and the activation barrier (E(barrier)) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (DeltaE approximately +11 meV, E(barrier)=+235-+290 meV in experiments). The relaxation calculation for C(60) crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (p(H)) in crystals, when p(H)>0.83, DeltaE<0. From the molecular dynamics calculations for C(60) crystals using our model, it is found that the phase transition is induced at T(C)=200-260 K, which is consistent with the experimental value of 260 K. Immediately below T(C), we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10(-9) s due to the thermal activation. In the high temperature phase (>T(C)), all molecules rotate irregularly like in Brownian motion involving the rotational "slumber" for approximately 10(-12)-10(-11) s.     

The crystal structure of the 2D polymerized tetragonal phase of C60

Single crystals of the 2D polymerized tetragonal phase of C₆₀ without orientational domains were obtained under high pressure and high temperature. The crystal structure of this phase was resolved using single crystal X-ray diffraction data. The cell parameters are a=9.064(3) and c=15.039(8) Å with the space group P4₂/mmc and Z=2. A structural model of this phase proposed early was confirmed and refined to final R=0.075. The structure is slightly disordered. It consists of a random combination of the P4₂/mmc layers (84%) and of the Immm layers (16%), along the c-axis.     

Commensurate-incommensurate transition of 4He adsorbed on a single C60 molecule

Path-integral Monte Carlo calculations have been performed to study (4)He adsorption on a single C(60) molecule. Helium corrugations on the fullerene molecular surface are incorporated with the (4)He-C(60) interaction described by the sum of all (4)He-C interatomic pair potentials. Radial density distributions show a layer-by-layer growth of (4)He with the first adlayer being located at a distance of ~6.3 ? from the center of the C(60) molecule. The monolayer shows different quantum states as the number of (4)He adatoms N varies. For N = 32, we find a commensurate solid, with each of the 32 adsorption sites on the molecular surface being occupied by a single (4)He atom. Various domain-wall structures are observed as more (4)He atoms are added and the first layer crystallizes into an incommensurate solid when it is completely filled. This commensurate-incommensurate transition of the helium monolayer is found to be accompanied by re-entrant superfluid response at a low temperature of 0.31 K with the superfluidity being totally quenched at N = 32, 44, and 48. Finally, the different quantum states observed in the helium monolayer around C(60) are compared with phase diagrams proposed for the corresponding layer on a graphite surface.     

2H NMR study of phase transitions and orientational order in a side chain liquid crystal polymer: Evidence of a nematic-nematic transition

We have investigated the mesomorphic behaviour of a partially deuteriated side chain LC polymethacrylate, poly[4-[6-methacryloyloxyhexyl-oxy]-4'-methoxyazobenzene], by means of ²H NMR, DSC and optical microscopy. Evidence for a phase transition occurring in the middle of the nematic phase has been found.     

Raman spectra and phase transition of the phenothiazine crystal

Polarized Raman spectra of single crystals of phenothiazine which undergoes a phase transition around 250 K and is ferroelastic in the low-temperature phase were measured in the lattice-vibrational region for temperatures ranging from 89 to 300 K. The spectra of the high- and low-temperature phases obey the selection rules required for the orthorhombic and monoclinic structures, respectively. Anomalous temperature dependences are observed in the frequency, intensity and linewidth of a band appearing in the lowest-frequency region. This band is attributed mainly to a librational motion of the molecule, and is important in the evaluation of this phase transition. A gradual change of the molecular orientation is inferred to occur over a wide temperature interval in the low-temperature phase along the vibrational coordinate of the above libration. Another strong, low-frequency band, which is characteristic of the phenothiazine spectra, is suggested to arise from mixing between intramolecular and lattice vibrations. This mode is considered to play some role in the phase transition too. The interaction between the low-frequency optical modes and the acoustic modes is briefly discussed in connection with the ferroelasticity of the low-temperature phase. The transition temperature depends on the quality of the specimen; the correct transition temperature is found to be 248.8 K and slightly lower than the previously reported value.     

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.     

Friction and forces between cellulose model surfaces: a comparison

Four different cellulose model surfaces, and one silica surface, have been studied by means of atomic force microscopy (AFM). The normal interactions have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion. Studies of friction reveal that surface roughness is the determining factor for the friction coefficient, with which it increases monotonically. The absolute value, however, is determined by the surface chemistry. All studied cellulose surfaces show similar behavior in response to xyloglucan addition.     

Quantitative measurement of the coverage on single crystal surfaces

Summary Two methods for the quantitative determination of the surface coverage of adsorbates on single crystal surfaces are discussed. In the first case a quartz crystal microbalance is used to calibrate an Auger spectrometer. The frequency change of the quartz and the Auger signal of the adsorbate are measured simultaneously during adsorption. With the calibrated Auger spectrometer the absolute coverage of oxygen on Ni (110) and Ni (111) is determined. The second method is used for adsorbates which can be thermally desorbed. The desorption spectra of the sample are compared with the calibration spectrum. This test spectrum can be obtained by introducing a well defined amount of gas into the vacuum chamber. The gas introduced produces a pressure increase comparable to a flash desorption spectrum. With this method the saturation coverages of H₂-Ni (111), H₂-Ni (100) and H₂-W (100) are determined.

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Absolutbestimmung adsorbierter Gasmengen an Einkristalloberflächen

Zusammenfassung Für die quantitative Bestimmung der Oberflächenbedeckung an Einkristallen werden zwei Methoden diskutiert. Im ersten Fall wird eine Schwingquarzmikrowaage zur Eichung des Augerspektrometers verwendet. Auf den Schwingquarz wird ein Adsorbens aufgedampft und anschließend das Augersignal des Adsorbats und die Frequenzänderung des Schwingquarzes während der Gasadsorption gleichzeitig gemessen. Mit diesem geeichten Augersignal werden absolute Bedeckungsbestimmungen an den Systemen O₂-Ni (110) und O₂-Ni (111) durchgeführt. Die zweite Methode eignet sich für thermisch desorbierbare Adsorbate. Die Desorptionsspektren der Probe werden mit einem Eichspektrum verglichen. Dieses erhält man, indem eine definierte Gasmenge in den Rezipienten eingelassen und damit ein Desorptionsspektrum erzeugt wird. Mit dieser Methode werden die Sättigungsbedeckungen der Adsorptionssysteme H₂-Ni (111), H₂-Ni (100) und H₂-W (100) bestimmt.

     

Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: a photoemission study

We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells.     

Recent advances in the use of curved single crystal surfaces

In surface science, research traditionally employs macroscopically flat surfaces of single crystals. Curved surfaces have been applied more sporadically, but their history stretches back for many decades. Realization of the potential benefits and practical applications in surface physics and surface chemistry research progressed slowly in the 20th century. In more recent decades, research employing partial cylinders and dome-shaped crystals have found renewed interest. Modern surface sensitive techniques are being employed allowing the inherent large range of surface structures to reveal new insights. We briefly review the history, describe several types of surfaces and the range of structures they contain, suggest a notation for common types of curved surfaces, and discuss recent studies in more detail. We mainly focus on metal samples. We close with a short outlook.     

Modelling of the phase transition of nanoscale confined liquid crystal

By dividing the bulk melting entropy, a simple thermodynamic model without any adjustable parameter for the size-dependent melting transition temperature has been extended to interpret the melting and freezing transitions of liquid crystals (LCs) confined in nanopores. The results show that as the size of the nanopore decreases, the melting, clearing and freezing transition temperatures of LCs drop. The transition temperatures directly depend on the density of hydrogen bond at the interface between inner pore wall and LC molecules. The model predictions agree well with the corresponding experimental results of LCs p-azoxyanisole and 4-pentyl-4′-cyanobiphenyl confined in nanopores.     

High pressure investigations of the photo-stimulated orientational ordering transition in a liquid crystal with photoactive dimeric molecules

High pressure investigations have been carried out on the photo-stimulated phase transition exhibited by a guest–host system, where the host is a non-photoactive liquid crystal. As guest molecules we chose successive homologues of a dimeric photoactive azobenzene liquid crystalline molecule to understand the influence of the length and parity of the alkylene spacer on such a transition. Our studies show that application of pressure as well as the parity of the spacer has a drastic influence both on the static and dynamic characters of the photo-stimulated phase transition.     

Friction and adhesion forces of Bacillus thuringiensis spores on planar surfaces in atmospheric systems

The kinetic friction force and the adhesion force of Bacillus thuringiensis spores on planar surfaces in atmospheric systems were studied using atomic force microscopy. The influence of relative humidity (RH) on these forces varied for different surface properties including hydrophobicity, roughness, and surface charge. The friction force of the spore was greater on a rougher surface than on mica, which is atomically flat. As RH increases, the friction force of the spores decreases on mica whereas it increases on rough surfaces. The influence of RH on the interaction forces between hydrophobic surfaces is not as strong as for hydrophilic surfaces. The friction force of the spore is linear to the sum of the adhesion force and normal load on the hydrophobic surface. The poorly defined surface structure of the spore and the adsorption of contaminants from the surrounding atmosphere are believed to cause a discrepancy between the calculated and measured adhesion forces.     

Interaction of hydrogen with surfaces of silicates: single crystal vs. amorphous

We have studied how the formation of molecular hydrogen on silicates at low temperature is influenced by surface morphology. At low temperature (<30 K), the formation of molecular hydrogen occurs chiefly through weak physical adsorption processes. Morphology then plays a role in facilitating or hindering the formation of molecular hydrogen. We studied the formation of molecular hydrogen on a single crystal forsterite and on thin films of amorphous silicate of general composition (Fe(x)Mg((x-1)))(2)SiO(4), 0 < x < 1. The samples were studied ex situ by Atom Force Microscopy (AFM), and in situ using Thermal Programmed Desorption (TPD). The data were analysed using a rate equation model. The main outcome of the experiments is that TPD features of HD desorbing from an amorphous silicate after its formation are much wider than the ones from a single crystal; correspondingly typical energy barriers for diffusion and desorption of H, H(2) are larger as well. The results of our model can be used in chemical evolution codes of space environments, where both amorphous and crystalline silicates have been detected.     

Homophilic interactions between cadherin fragments at the single molecule level: an AFM study

We report measurements of the adhesion forces between single E-cadherin fragments anchored on solid surfaces. These fragments consist of the two outermost extracellular domains of the protein. The specificity of the measured rupture forces was demonstrated by Ca2+ exchange experiments. Two series of experiments were performed using two linkers of different rigidity and length. We find that the pull-off force is distributed with a maximum value independent of the linker and logarithmically dependent on the velocity of separation of the two surfaces. Our dynamical results are compatible with previous flow chamber experiments performed with the same fragments and can be compared from a different perspective with previously reported AFM experiments on the full-length extracellular domain of the VE-cadherin. Interestingly, using a rigid linker, we have been able for the first time to evidence the deformation of the cadherin molecule under mechanical stress, a piece of information not accessible with more classical grafting strategies.