Observation of intermolecular N–I interaction during the growth of a 4-cyano-4′-iodobiphenyl molecular crystal on GeS(001)

The electronic and atomic structures of 4-cyano-4′-iodobiphenyl (CIB) during the growth of a molecular crystal on a GeS(001) substrate were studied by ultraviolet photoemission spectroscopy (UPS), atomic force microscopy (AFM), and extended X-ray absorption fine structure (EXAFS) spectroscopy. AFM images suggest that the CIB molecule grows as a microcrystal at a nominal thickness of 80 Å. The microcrystal grows with the crystal plane parallel to the surface and isotropic crystal axis orientation. EXAFS analysis suggests that a CIB crystal forms by strong N···I interaction, called halogen bonding. The formation of the intermolecular N···I bond was demonstrated by EXAFS analyses in which the N–I distance was determined to be 3.29 Å. An upward shift of the highest occupied molecular orbital level was observed by UPS and can be attributed to the aggregation of CIB molecules caused by halogen bonding.     

Exploring the facets of “soft crystals” using an Atomic Force Microscope

We obtained monocrystalline droplets in a thermotropic cubic phase, of approximate size 100μm, deposited on a flat surface. The facets of these soft crystals are explored using both an optical microscope and an AFM. The height of individual steps on the principal facets and the lateral distance between steps in vicinal facets are measured using AFM in imaging (tapping) mode. Moreover, the elastic modulus is measured locally, using the AFM tip (in contact mode) as a local rheological probe.     

Coarse and Fine Control of the EPR Frequency of a Loop-Gap Resonator Using a Single-Turn Coil with a Varactor Diode Attached

Coarse control and fine control of the resonant frequency of a loop-gap resonator (LGR) operating at an electron paramagnetic resonance (EPR) frequency of ca. 650 MHz were achieved using a single-turn coil with a varactor diode attached (a frequency shift coil). When the distance between the LGR and the frequency shift coil was changed from 15 to 10 mm under the condition of constant voltage to the varactor diode (0 V), a shift of the resonant frequency of the LGR of ca. 20 MHz was observed (coarse frequency control). When the voltage applied to the varactor diode was changed from 0 to 15 V at the same distance between the LGR and the frequency shift coil (10 mm), a shift of the resonant frequency of the LGR of ca. 200 kHz was observed (fine frequency control). There were no significant changes in sensitivity of EPR measurements of a phantom (comprised of agar with a nitroxide radical and physiological saline solution) without and with the frequency shift coil. The EPR sensitivity did not change discernibly when the resonant frequency was shifted by the frequency shift coil. Furthermore, radio-frequency phase adjustment for homodyne detection could be performed by using the frequency shift coil without applying frequency modulation to the carrier wave.     

Local spectroscopy and atomic imaging of tunneling current, forces, and dissipation on graphite

Theory predicts that the currents in scanning tunneling microscopy (STM) and the attractive forces measured in atomic force microscopy (AFM) are directly related. Atomic images obtained in an attractive AFM mode should therefore be redundant because they should be similar to STM. Here, we show that while the distance dependence of current and force is similar for graphite, constant-height AFM and STM images differ substantially depending on the distance and bias voltage. We perform spectroscopy of the tunneling current, the frequency shift, and the damping signal at high-symmetry lattice sites of the graphite (0001) surface. The dissipation signal is about twice as sensitive to distance as the frequency shift, explained by the Prandtl-Tomlinson model of atomic friction.     

Anodization of aluminium thin films on pSi and annihilation of strong luminescence from Al2O3

Photoluminescence (PL) of Al₂O₃ films obtained by anodization of thermally evaporated and annealed thin Al films on p⁺⁺Si in 0.3 M oxalic acid has been investigated. Thermal annealing at 200–950 °C under the dry nitrogen atmosphere was used for deactivation of luminescence centres. Luminescence from as grown films was broad and located at 425 nm. This luminescence reached to highest level after annealing at 600 °C. Maximum 10 min was required for full optical activation and prolonged annealing up to 4 h did not change the luminescence intensity. Because of deep levels, absorption band edge of as grown films was shifted to the lower energy which is 3.25 eV. Annealing above 800 °C reduced the PL intensity and this observation was correlated with the blue shift of band edge as the defects annealed out. Disappearing PL intensity and blue shift of band edge absorption after annealing at 950 °C was mainly attributed to the oxygen-related defects and partly to impurities that may be originated from oxalic acid. AFM results did not show any hexagonally ordered holes but uniformly distributed nanosized Al₂O₃ clusters that were clearly seen. XRD measurements on as grown Al₂O₃ showed only [1 1 0] direction of α phase. Debye–Scherer calculation for this line indicates that cluster size is 35.7 nm. XRD and AFM pictures suggest that nanocrystalline Al₂O₃ are embedded in amorphous Al₂O₃.     

TE-TM coupled mode dynamics in a semiconductor laser subject to feedback with variably rotated polarization

We report on the experimental observation of dynamical collapses which address selectively the TM-mode of a laser subject to optical feedback with variably rotated polarization. Simultaneously, the TE-mode which remains the dominant lasing mode exhibits power bursts. We analyze the relative phase shift between the feedback fields into the TE- and TM-mode and find that the dynamical collapses, observed in a specific range of polarization rotation angles, can be attributed to large phase shift feedback conditions applied on the TM-mode.     

Effects of ultrasonic-assisted pH shift treatment on physicochemical properties of electrospinning nanofibers made from rapeseed protein isolates

Electrospinning nanofibers (NFs) made from natural proteins have drawn increasing attention recently. Rapeseed meal is a by-product that rich in protein but not fully utilized due to poor properties. Therefore, modification of rapeseed protein isolates (RPI) is necessary to expand applications. In this study, pH shift alone or ultrasonic-assisted pH shift treatment was adopted, the solubility of RPI, along with the conductivity and viscosity of the electrospinning solution were detected. Moreover, the microstructure and functional characteristics of the electrospinning NFs, as well as the antibacterial activity of clove essential oil loaded-NFs were investigated. The tested parameters were remarkably improved after different treatments compared with the control, and synergistic effects were observed, especially under alkaline conditions. Hence, pH_12.5 + US showed the maximum value of solubility, conductivity, and viscosity, which was more than 7-fold, 3-fold, and almost 1-fold higher than the control respectively. Additionally, SEM and AFM images showed a finer and smoother surface of NFs after treatments, and the finest diameter of 216.7 nm was obtained after pH_12.5 + US treatment in comparison with 450.0 nm in control. FTIR spectroscopy of NFs demonstrated spatial structure changes of RPI, and improved thermal stability and mechanical strength of NFs were achieved after different treatments. Furthermore, an inhibition zone with a diameter of 22.8 mm was observed from the composite NFs. This study indicated the effectiveness of ultrasonic-assisted pH shift treatment on the physicochemical properties improvement and functional enhancement of NFs made from RPI, as well as the potential antibacterial application of the composite NFs in the future.     

How to measure energy dissipation in dynamic mode atomic force microscopy

When studying a mechanical system like an atomic force microscope (AFM) in dynamic mode it is intuitive and instructive to analyse the forces involved in tip–sample interaction. A different but complementary approach is based on analysing the energy that is dissipated when the tip periodically interacts with the sample surface. This method does not require solving the differential equation of motion for the oscillating cantilever, but is based entirely on the analysis of the energy flow in and out of the dynamic system. Therefore the problem of finding a realistic model to describe the tip–sample interaction in terms of non-linear force–distance dependencies and damping effects is omitted. Instead, it is possible to determine the energy dissipated by the tip–sample interaction directly by measuring such quantities as oscillation amplitude, frequency, phase shift and drive amplitude. The method proved to be important when interpreting phase data obtained in tapping mode, but is also applicable to a variety of scanning probe microscopes operating in different dynamic modes. Additional electronics were designed to allow a direct mapping of local energy dissipation while scanning a sample surface. By applying this technique to the cross-section of a polymer blend a material specific contrast was observed.     

原子力显微镜在轻敲模式下的动力学模型

基于Hamaker假设、Lennard-Jones势能定律及经典弹性理论建立了一种新型的球体与平面黏着接触的弹性模型，该模型显示黏着力在原子力显微镜(AFM)针尖趋近和撤离样品表面，即加载和卸载的两个过程中存在黏着滞后现象，表明了AFM在轻敲工作模式中存在能量耗散.同时，根据所建的黏着接触弹性模型，建立了AFM在轻敲工作模式下的动力学模型，研究了AFM在轻敲工作模式下的振动幅度、相位差及耗散功率随针尖与样品表面间距的变化规律，仿真结果与现有的实验结果相一致.     

MFM and AFM study of thin cobalt films modified by fluorosilane

Polycrystalline cobalt films 100 nm thick were thermally evaporated on oxidized Si(100) substrates. Then 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) films of various thicknesses, in the range of about 2 nm to 30 nm, were grown on cobalt surfaces by vapor phase deposition (VPD). The cobalt films modified by FDTS were investigated using magnetic force microscopy (MFM) and atomic force microscopy (AFM). MFM observation showed that the magnetic structure of the cobalt films modified by FDTS is composed of domains with a considerable component of magnetization perpendicular to the film surface. This in turn indicates that the cobalt films on oxidized Si(100) substrates crystallize in the hexagonal close-packed (HCP) phase and exhibit a texture with the hexagonal axis perpendicular to the film surface. The magnetic domains formed a maze structure. The domain width increased from typically 80–120 nm to 400–500 nm with increasing the thickness of FDTS films from about 2 nm to 30 nm. AFM imaging of the surfaces of FDTS films revealed the presence of an agglomerate morphology. The agglomerates varied in size from typically 30–70 nm to 150–300 nm as the film thickness was increased from about 2 nm to 30 nm.     

High-speed non-intrusive measurements of fuel velocity fields at high-pressure injectors

Using a single high-speed camera and a frequency modulated laser, a novel approach is presented for fast velocity field measurements in unsteady spray flows. The velocity range is from zero up to several 100 m/s, which requires a high measurement rate and a large dynamic. Typically, flow measurements require to seed tracer particles to the fluid. A paradigm shift to seeding-free measurements is presented. The light scattered at the phase boundaries of the fluid droplets is evaluated. In order to validate the high-speed measurement system, a detailed uncertainty analysis is performed by means of measurements as well as simulations. Thereby, variations of the scattered light intensity, which are based on the high temporal velocity gradients, are found to be the main contribution to the uncertainty. The eventually measurement results, obtained at a measurement rate of 500 kHz, exhibit spray velocities ranging from 0 m/s up to 400 m/s in less than 1 ms, and the detection of unsteady and irregular flow phenomena with a characteristic time of several μs is achieved. This demonstrates the high measurement rate, the high temporal resolution and the large measurement range of the proposed high-speed measurement system.     

Observation of angiotensin II-induced changes in fixed and live mesangial cells by atomic force microscopy

Glomerular mesangial cells (MCs) are centrally located in the glomerulus. MCs control not only glomerular filtration, but also the response to local injury, including cell proliferation and basement membrane remodeling. Angiotensin II (Ang II) plays an important role in kidney function regulation, and participates in the progression of renal damage, as well as mesangial injury. However, studies on Ang II effects on MCs have used indirect methods, such as gene and protein expression after MC injury. In this study, we visually observed structural and mechanical changes to MC after Ang II treatment using atomic force microscopy (AFM). We obtained AFM topography and deflection images of live MCs, as well as fixed MCs in liquid, before and after Ang II treatment. Real-time imaging showed the dynamic movement of live MCs induced by Ang II. Changes in MC elastic property after Ang II treatment were measured using force–distance curves. AFM images of fixed and live MCs showed that cells contracted after Ang II exposure, with the nucleus height increasing within 20 min of Ang II stimulation. Force–distance analysis showed that Ang II caused MCs to stiffen (p < 0.0001). In conclusion, we demonstrated that AFM is an effective tool for real-time monitoring of live cell responses to drugs and stimuli.     

Effect of optical feedback on a VCSEL TDLAS

This paper describes the effects of optical feedback on the sensitivity of VCSEL tunable-diode laser spectroscopy (TDLS). Three VCSELs, emitting at different wavelengths in the near-infrared, were used. A TDLS system, subjected to optical feedback, exhibited a common signal-to-noise ratio profile for all three lasers. A catastrophic degradation of TDLS sensitivity occurred when feedback exceeded a level which we associate with coherence collapse. The TDLS system had a CH₄ minimum detection limit of 7.5 ppmm without optical feedback. Optical feedback of less than ten percent reduced this sensitivity by two orders of magnitude. This reduction of system sensitivity was accompanied by a second-harmonic absorption signal baseline shift which degraded the system accuracy.     

Nitrogen nanobubbles and butane nanodroplets at Si(1 0 0)

Nanobubbles and nanodroplets were spontaneously formed at Si(1 0 0) in contact with nitrogen and butane saturated water, respectively. The topographic images obtained by tapping mode AFM were similar truncated nanospheres, but the phase images suggested that the nanobubbles were harder than the nanodroplets. The tip–sphere interactions showed the nanodroplets were much viscoelastic than the nanobubbles. The surface and three-phase contact line energies were estimated by analysis of the topographic images. The nanodroplet was stable, but the nanobubble was unstable in spite of the experimental long life. The two-dimensional spatial distribution indicated an attractive interaction between the nanodroplets, but no interaction was observed between the nanobubbles.     

Nano peel-off fabrication pattern with polymer overcoat layer on glassy carbon electrodes for Pt electrodeposition

This article describes a new technique for fabricating an electrocatalyst model in which the particle size and interparticle distance are controlled independently. We designed a uniform insulating polymer layer as a mask on an electroconductive glassy carbon substrate and then peeled off a part of the layer in nano-sized dots by scratching the overcoat layer using an atomic force microscope (AFM) cantilever. Pt particles electrodeposited only on the peeled off area of the glassy carbon. To peel-off a small area on the glassy carbon, a 29 ± 2 nm thick insulating polymer overcoat layer and a cantilever operating area of 10 nm × 10 nm were used, and the smallest peel-off area obtained was 30 nm × 30 nm. Thereafter, we performed the peel-off procedure on the polymer overcoat layer of the glassy carbon substrate having a cantilever operating area of 80 nm × 80 nm. Pt deposition of 100–150 nm in diameter was successfully achieved by adjusting the interparticle distance.     

Numerical study of the hydrodynamic drag force in atomic force microscopy measurements undertaken in fluids

When atomic force microscopy (AFM) is employed for in vivo study of immersed biological samples, the fluid medium presents additional complexities, not least of which is the hydrodynamic drag force due to viscous friction of the cantilever with the liquid. This force should be considered when interpreting experimental results and any calculated material properties. In this paper, a numerical model is presented to study the influence of the drag force on experimental data obtained from AFM measurements using computational fluid dynamics (CFD) simulation. The model provides quantification of the drag force in AFM measurements of soft specimens in fluids.The numerical predictions were compared with experimental data obtained using AFM with a V-shaped cantilever fitted with a pyramidal tip. Tip velocities ranging from 1.05 to 105 μm/s were employed in water, polyethylene glycol and glycerol with the platform approaching from a distance of 6000 nm. The model was also compared with an existing analytical model. Good agreement was observed between numerical results, experiments and analytical predictions. Accurate predictions were obtained without the need for extrapolation of experimental data. In addition, the model can be employed over the range of tip geometries and velocities typically utilized in AFM measurements.     

Carrier-envelope phase shift caused by variation of grating separation

The effects of variation of the grating separation in a stretcher on the carrier-envelope (CE) phase of amplified pulses are investigated. By translating one of the telescope mirrors in the stretcher with a piezoelectric transducer, it is found that a 1 mum change of the distance causes a 3.7+/-1.2 rad shift of the CE phase, which is consistent with theoretical estimations. The results indicate that optical mounts used for gratings and telescope mirrors must be interferometrically stable; otherwise their vibration and thermal drift will cause significant phase error. The CE phase drift was corrected by feedback controlling the grating separation.     

AFM measurement of atomic-scale Si surface etching by active oxidation

Atomic-scale etching of a clean Si surface by active oxidation with oxygen molecules was examined using ex-situ atomic force microscopy (AFM). The etch rate was directly determined by measuring the etch depth with AFM. A SiO₂ anti-etching mask was used on a H-terminated Si(001)-2 × 1:H surface prepared by low pH HF treatment followed by annealing in H₂. The etch rate under active oxidation conditions was almost proportional to the O₂ pressure, which was consistent with previous reports. The etch rate exhibited a weak temperature dependence with an apparent activation energy of 0.2 eV. A distinct transition of the reaction mode from etching to oxide formation was observed in detail as an abrupt decrease of the etch rate by lowering the temperature near the boundary condition between the etching and oxide formation conditions.     

Size effect on the structure and magnetic properties of SmMn2O5 nanorods

Three sizes of SmMn₂O₅ nanorods that are labeled with (<L_C>) × axial lengths of 58(17) nm × 25(6) nm, 92(21) nm × 32(8) nm, and 126(25) nm × 52(13) nm were fabricated by the hydrothermal method. All the samples exhibited an antiferroicmagnetic (AFM) peak at approximately 6 K, which was associated with Sm magnetic ordering and no size independence. Another AFM magnetic ordering that belongs to the Mn ion was found with <L_C> = 58 nm, 92 nm, and 126 nm at 26 K, 28 K, and 30 K, respectively. The spin-orbit interaction increases with size in the magnetic susceptibility experiment. All the samples displayed a hysteresis loop at 2 K. The coercivity decreases as the size increases. The effects of the size on the crystal structure were elucidated from the Raman spectra of the <L_C> = 92 and 126 nm samples at various temperatures. The 126 nm sample displayed a red-shift for the Ag mode with warming, revealing that the Mn–O bonds are more sensitive to temperature in larger SmMn₂O₅ nanorods. These results demonstrate that the size effect importantly affects the structure and magnetic properties in SmMn₂O₅ multiferroic nanorods.     

Frequency shifts of cantilever in AFM

The frequency shift and frequency shift image of cantilever in AFM have been studied by numerical integration of the equation of motion of cantilever for silicon tip with rutile TiO₂(0 0 1) surface in UHV conditions and by the Hamaker summation method for the tip-surface interaction forces. The effects of the excitation frequency at the cantilever base and the equilibrium position of the tip on the frequency shift have been calculated and the results showed the same phenomena as those measured, e.g., the frequency shift increased dramatically or rapidly before the contact point and was then almost level off after the contact point. The effects of scanning speed and the initial closest distance of tip to the contact point have been calculated at different excitation frequencies at the cantilever base and the results showed that proper frequency shift image could be obtained either by noncontact mode at the excitation frequency slightly less than the resonance frequency of free cantilever, or by tapping mode at the excitation frequency a few times smaller than the resonance frequency of free cantilever.