Space charge limited current measurements on conjugated polymer films using conductive atomic force microscopy

We describe local (~150 nm resolution), quantitative measurements of charge carrier mobility in conjugated polymer films that are commonly used in thin-film transistors and nanostructured solar cells. We measure space charge limited currents (SCLC) through these films using conductive atomic force microscopy (c-AFM) and in macroscopic diodes. The current densities we measure with c-AFM are substantially higher than those observed in planar devices at the same bias. This leads to an overestimation of carrier mobility by up to 3 orders of magnitude when using the standard Mott-Gurney law to fit the c-AFM data. We reconcile this apparent discrepancy between c-AFM and planar device measurements by accounting for the proper tip-sample geometry using finite element simulations of tip-sample currents. We show that a semiempirical scaling factor based on the ratio of the tip contact area diameter to the sample thickness can be used to correct c-AFM current-voltage curves and thus extract mobilities that are in good agreement with values measured in the conventional planar device geometry.     

The role of the tip in non-contact atomic force microscopy dissipation images of ionic surfaces

In this paper we use simulations to investigate the role of the tip in nc-AFM measurements of dissipated energy. Using a virtual AFM we simulate the experiment focusing on the atomic scale energy dissipation on an NaCl(100) flat surface. The non-conservative interaction was treated with the theory of dynamic response and all the calculations were carried out using an atomistic model; several sets of tips were tested using ionic crystals (NaCl, KBr, MgO), each in different configurations (ideal, vacant, divacant, doped). Using an MgO-doped tip we were able to calculate a dissipation signal comparable to what is typically measured in experiments. It was not possible to see any dissipation with ideal tips, although they still have a significant interaction with the surface and give atomic contrast in the frequency shift signal. The effect of the scanning speed on measured frequency shift and dissipation is also calculated and discussed.     

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (～1?aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.     

An atomic force microscopy tip model for investigating the mechanical properties of materials at the nanoscale

Investigation of the mechanical properties of materials at the nanoscale is often performed by atomic force microscopy nanoindentation. However, substrates with large surface roughness and heterogeneity demand careful data analysis. This requirement is even more stringent when surface indentations with a typical depth of a few nanometers are produced to test material hardness. Accordingly, we developed a geometrical model of the nanoindenter, which was first validated by measurements on a reference gold sample. Then we used this technique to investigate the mechanical properties of a coating layer made of Balinit C, a commercially available alloy with superior anti-wear features deposited on steel. The reported results support the feasibility of reliable hardness measurements with truly nanosized indents.     

Subsurface atomic force microscopy: towards a quantitative understanding

Recent experiments in the field of subsurface atomic force microscopy have demonstrated that it is possible to nondestructively image micro- and even nanoparticles that are embedded significantly deep within the bulk of a sample. In order to get insights into the contrast formation mechanism, we performed a finite element analysis and an analytical study, in which we calculated the amplitude and phase variation on the surface of an ultrasound wave that has traveled through the sample. Our calculations were performed as closely as possible to the situation in the experiments to enable a (future) comparison based on our predictions. We show that Rayleigh scattering of acoustic waves accounts for the measured contrast and we verify the characteristic Rayleigh dependences. The numerical results show that the contrast is independent of the depth at which a particle is buried, whereas the analytical study reveals a 1/depth dependence. In addition, we find a large deviation in the width of the particle in the contrast at the surface when applying the numerical or the analytical calculation respectively. These results indicate the importance of both the reflections of sound waves at the sample interfaces and bulk damping, as both are treated differently in our two models.     

Electrical transport properties of oligothiophene-based molecular films studied by current sensing atomic force microscopy

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.     

Probing CO on a rutile TiO2(110)surface using atomic force microscopy and Kelvin probe force microscopy

Probing CO at a specific site on a metal oxide surface is essential for characterizing various applications such as CO oxidation,hydrogenation,and water-gas shi...     

Monitoring DNA immobilization and hybridization on surfaces by atomic force microscopy force measurements

DNA immobilization and hybridization was carried out on Au substrates that were modified with mercaptopropanoic acid and then treated with aluminum(III) solution. The positively charged AI(III) film can be used to immobilize both ds-DNA and ss-DNA. Atomic force microscopy (AFM) was used to monitor the process by force measurements between a negatively charged silica tip and the substrates while immersed in dilute electrolyte. Surface hybridization of ss-DNA produces an increase in the surface charge and surface potential of the substrates, which is reflected by the increasing repulsive force as determined from AFM force-separation curves. A single-base mismatch was detectable in surface hybridization. The AFM force measuring technique was also employed to investigate the interaction of Ru(phen)3(2+) with ss-DNA and ds-DNA. The force measurement results showed that there is a small interaction between Ru(phen)3(2+) and ss-DNA, which was ascribed to the electrostatic binding of Ru(phen)3(2+) to the ss-DNA surface. For ds-DNA, there is a strong interaction which is believed to be due to the association or intercalation of Ru(phen)3(2+) with ds-DNA.     

Potassium-selective atomic force microscopy on ion-releasing substrates and living cells

A new method for simultaneous mapping of cell topography and ion fluxes was developed. A highly sensitive ion sensor system was generated by coating atomic force microscopy tips with a PVC layer containing valinomycin, an ionophore for potassium. The activity of specific ions was traced on artificial ion-releasing PVC substrates. A boundary potential was generated owing to the selective exchange of a specific ion between coated tip and ion-releasing substrate. The boundary potential was detectable as a force induced by ion-selective electrostatic interactions. The selectivity coefficient of valinomycin for potassium against sodium (K(K,Na)f) was -2.5 +/- 0.5. Potassium efflux was measured on living MDCK-F1 cells expressing BK(Ca) channels. We could demonstrate localized areas of high potassium concentrations at the cell surface. The potassium efflux could be reversibly inhibited by thapsigargin, which is known to inhibit the efflux of potassium from BK(Ca) channels by suppression of calcium ATPase.     

Contact force identification using the subharmonic resonance of a contact-mode atomic force microscopy

We propose a step-by-step experimental procedure for characterization of the nonlinear contact stiffness on surfaces using contact-mode atomic force microscopy. Our approach directly estimates the first-, second-, and third-order coefficients of the contact stiffness. It neither uses nor requires the underlying assumptions of the Hertzian contact theory. We use a primary resonance excitation of the probe to estimate the linear coefficient of the contact stiffness. We use the method of multiple scales to obtain closed-form expressions approximating the response of the probe to a subharmonic resonance excitation of order one-half. We utilize these expressions and higher-order spectral measurements to independently estimate the quadratic and cubic coefficients of the contact stiffness.     

Nanometer-scale flow of molten polyethylene from a heated atomic force microscope tip

We investigate the nanometer-scale flow of molten polyethylene from a heated atomic force microscope (AFM) cantilever tip during thermal dip-pen nanolithography (tDPN). Polymer nanostructures were written for cantilever tip temperatures and substrate temperatures controlled over the range 100-260?°C and while the tip was either moving with speed 0.5-2.0 μm s(-1) or stationary and heated for 0.1-100 s. We find that polymer flow depends on surface capillary forces and not on shear between tip and substrate. The polymer mass flow rate is sensitive to the temperature-dependent polymer viscosity. The polymer flow is governed by thermal Marangoni forces and non-equilibrium wetting dynamics caused by a solidification front within the feature.     

Multimodal atomic force microscopy: Biological imaging using atomic force microscopy combined with light fluorescence and confocal microscopies and electrophysiologic recording

Because the atomic force microscope (AFM) allows molecular resolution imaging of hydrated specimens, it provides a unique window to the microscopic biological world. A high signal-to-noise ratio in AFM images sets them apart from the images obtained from other techniques: One does not need extensive image analyses often required by other techniques to obtain high-resolution information. AFM can provide molecular details on crystalline as well as amorphous materials. However, it is often limited in providing identity of the imaged structures, especially in a complex system such as a cellular membrane. AFM's application for biological imaging will rely on an unambiguous identification of imaged structures. For mixed macromolecules, it may be essential to make critical comparisons of the same structural features imaged with AFM and other techniques such as light fluorescence and confocal microscopies, electron microscopy and X-ray diffraction, and biochemical, immunologic, and pharmacologic techniques and electrophysiologic recordings. Significantly, the simple design of AFM allows it to be integrated with other techniques for simultaneous multimodal imaging. Recent combined multimodal imaging include light fluorescence, confocal, and near-field optical imaging as well as electrophysiologic recordings. Preliminary studies from such multimodal imaging include 1) an independent identification of macromolecules in a complex specimen using appropriately labeled markers such as fluorescent-dye labeled antibodies or dark-field microscopy; 2) imaging real-time reorganization of surface features using laser confocal and AFM; 3) a direct correlation of structural features and ion transfer via pores in a membrane; and 4) macromolecular complexes such as receptor-ligand and antigen-antibody. These features of a multimodal imaging system will provide new and significant avenues for a direct real-time structure-function correlation studies of biological macromolecules. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 293–300, 1997     

原子力声显微镜的原理及应用

原子力声显微镜结合了超声检测技术的三维成像能力与原子力显微镜的纳米尺度成像的近场显微技术。它在商用的原子力显微镜设备的基础上加以压电超声传感器产生声激励,并使用锁相放大器对数据进行收集分析,既可得到三维的纳米级的清晰形貌图,又能通过建模分析样品表面的接触刚度及样品的弹性模量。目前,原子力显微镜被广泛应用于材料领域,用于检测样品的机械性能,比如样品的接触刚度、薄膜高分子材料的弹性模量,同时还运用于医学生物领域,用于观察细胞的超微结构及其表面和亚表面的弹性模量等。     

Nanoindentation and atomic force microscopy measurements on reactively sputtered TiN coatings

Titanium nitride (TiN) coatings were deposited by d.c. reactive magnetron sputtering process. The films were deposited on silicon (111) substrates at various process conditions, e.g. substrate bias voltage (V_B) and nitrogen partial pressure. Mechanical properties of the coatings were investigated by a nanoindentation technique. Force vs displacement curves generated during loading and unloading of a Berkovich diamond indenter were used to determine the hardness (H) and Young’s modulus (Y) of the films. Detailed investigations on the role of substrate bias and nitrogen partial pressure on the mechanical properties of the coatings are presented in this paper. Considerable improvement in the hardness was observed when negative bias voltage was increased from 100–250 V. Films deposited at |V _B| = 250 V exhibited hardness as high as 3300 kg/mm². This increase in hardness has been attributed to ion bombardment during the deposition. The ion bombardment considerably affects the microstructure of the coatings. Atomic force microscopy (AFM) of the coatings revealed fine-grained morphology for the films prepared at higher substrate bias voltage. The hardness of the coatings was found to increase with a decrease in nitrogen partial pressure.     

Improved performances in a top-emitting green organic light-emitting device with light magnification

A top-emitting organic light-emitting device (TOLED) with an architecture of Si/SiO₂/Ag (100 nm)/Ag₂O (UV ozone treatment for 30 s)/ 4′,4?-tris(3-methylphenylphenylamino)triphenylamine (45 nm)/4,4′-bis [N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl (5 nm)/tris-(8-hydroxyquinoline) aluminum (Alq₃):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one (C545T) (1: 0.5 weight %, 20 nm)/Alq₃ (30 nm)/LiF(1 nm)/Al (0.5 nm)/Ag(30 nm) is designed with a resonance wavelength in the TOLED corresponding to the peak wavelength of C545T. With this enhanced cavity structure, light magnification with a coefficient of ∼ 19 (forward direction) is observed, leading to significantly improved performances with brightness of 80215 cd/m² at 9 V, luminous efficiency of 32.7 cd/A at 6 V, external quantum efficiency of 8.9% at 7.5 V, and low turn-on voltage of 2.5 V.