Scanning Force Microscope Tip Research Articles

Unidirectional Current in Layered Metal Hexacyanometallate Thin Films: Implication for Alternative Wet-Processed Electronic Materials.

Rectifying behavior of alternative electronic materials is demonstrated with layered structures of a crystalline coordination network whose mixed ionic and electronic conductivity can be manipulated by switching the redox state of coordinated transition-metal ions. The coordinated transition-metal ions can convey additional functionality such as (redox)catalysis or electrochromism. In order to obtain rectifying behavior and charge trapping, layered films of such materials are explored. Specifically, layered films of iron hexacyanoruthenate (Fe-HCR) and nickel hexacyanoferrate (Ni-HCF) were formed by the combination of different deposition procedures. They comprise electrodeposition during voltammetric cycles for Fe-HCR and Ni-HCF, layer-by-layer deposition of Ni-HCF without redox chemistry, and drop casting of presynthesized Ni-HCF nanoparticles. The obtained materials were structurally characterized by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy for nanoparticles, and scanning force microscopy (SFM). Voltammetry in 1 mol L-1 KCl and current-voltage curves (I-V curves) recorded between a conductive SFM tip and the back electrode outside of an electrolyte solution demonstrated charge trapping and rectifying behavior based on the different formal potentials of the redox centers in the films.

Probing the Internal Heterogeneity of Responsive Microgels Adsorbed to an Interface by a Sharp SFM Tip: Comparing Core-Shell and Hollow Microgels.

Microgels composed of thermoresponsive polymer poly( N-isopropylacrylamide) (PNIPAM) are interfacial active. Their adsorption leads to deformation, causing conformational changes that have profound effects on the macroscopic properties of these films. Yet, methods to quantitatively probe the local density are lacking. We introduced scanning force microscopy (SFM) to quantitatively probe the internal structure of microgels physically adsorbed on a solid (SiO2)/water interface. Using a sharp SFM tip, we investigated the two types of microgels: (i) core-shell microgels featuring a hard silica core and a PNIPAM shell and (ii) hollow microgels obtained by dissolution of the silica core. Thus, both systems have the same polymer network as the peripheral structure but a distinctly different internal structure, that is, a rigid core versus a void. By evaluating the force-distance curves, the force profile during insertion of the tip into the polymer network enables to determine a depth-dependent contact resistance, which closely correlates with the density profiles determined in solution by small-angle neutron scattering. We found that the cavity of the swollen hollow microgels is still present when adsorbed to the solid substrate. Remarkably, while currently used techniques such as colloidal probe or reflectometry only provide an average of the z-profile, the methodology introduced herein actually probes the real three-dimensional density profile, which is ultimately important to understand the macroscopic behavior of microgel films. This will bridge the gap between the colloidal probe experiments that deform the microgel globally and the insertion in which the disturbance is located near the tip.

Dependence of the ferroelectric domain shape on the electric field of the microscope tip

A theory of an equilibrium shape of the domain formed in an electric field of a scanning force microscope (SFM) tip is proposed. We do not assume a priori that the domain has a fixed form. The shape of the domain is defined by the minimum of the free energy of the ferroelectric. This energy includes the energy of the depolarization field, the energy of the domain wall, and the energy of the interaction between the domain and the electric field of the SFM tip. The contributions of the apex and conical part of the tip are examined. Moreover, in the proposed approach, any narrow tip can be considered. The surface energy is determined on the basis of the Ginzburg-Landau-Devonshire theory and takes into account the curvature of the domain wall. The variation of the free energy with respect to the domain shape leads to an integro-differential equation, which must be solved numerically. Model results are illustrated for lithium tantalate ceramics.

Quasi in situ scanning force microscope with an automatic operated reaction chamber

We describe the design and performance of a quasi in situ scanning force microscope with an automatic operated reaction chamber. The design provides a repetitive hermetically sealed sample environment for successive processing. The reaction chamber is based on a combination of a flexure-guided cover, a piezo-positioning system and a force applicator system. An axial force seals the cover against the reactor enabling flow-through applications at low pressure, ambient pressure, or elevated pressure. The position stability of the sample relative to the probe is characterized and a full automated operation of the instrument is explored by the alignment of an ABC terblock copolymer thin film undergoing solvent vapor annealing in the presence of a high electric field. Due to the high electric field strength and the sharp scanning force microscope tip it is impossible to perform in situ scanning in the presence of the electric field.

Fabrication of ring structures by anodization lithography on self-assembled OTS monolayers

A new and rational fabrication concept to obtain ring structures with nanometer dimensions is presented that utilizes a combination of anodization and electrochemical oxidation lithography on a self-assembled monolayer of n-octadecyltrichlorosilane performed with an electrically biased scanning force microscopy tip. During the oxidation process two different chemical areas are created; a silicon oxide core which is surrounded by a chemically active, acid functionalized rim feature. The oxidation conditions, as well as the scaling options of this lithographic process have been investigated and revealed good controllability of the ring dimensions. A straightforward electroless metal deposition process was used to convert the ring structures into metal features.

Low-voltage nanodomain writing in He-implanted lithium niobate crystals

A scanning force microscope tip is used to write ferroelectric domains in He-implanted single-crystal lithium niobate and subsequently probe them by piezoresponse force microscopy. Investigation of cross-sections of the samples showed that the buried implanted layer, ∼1 μm below the surface, is nonferroelectric and can thus act as a barrier to domain growth. This barrier enabled stable surface domains of <1 μm size to be written in 500 μm thick crystal substrates with voltage pulses of only 10 V applied to the tip.

Synthesis with Single Macromolecules: Covalent Connection between a Neutral Dendronized Polymer and Polyelectrolyte Chains as well as Graphene Edges

Single chains of a neutral, dendronized polymer with peripheral azide groups (PG3A) are co-deposited onto molecularly modified graphite substrates with a positively charged dendronized polymer (PG2) as well as with negatively charged plasmid dsDNA. PG3A is also prepared near graphite step-edges. Single PG3A chains are moved with a scanning force microscope tip, into close contact with either of the two polyelectrolytes, as well as the step-edge at predetermined positions. Treating these structures in situ with UV light leads to photochemically induced cross-linking between the PG3A chains carrying azide groups and PG2, dsDNA, and graphite step-edges, respectively, which is proven by mechanically challenging the "welding" points by pulling on the molecules with the SFM-tip.

Control of number density and swelling/shrinking behavior of P(NIPAM–AAc) particles at solid surfaces

Temperature and pH sensitive microgel particles of poly(N-isopropyl acrylamide) (PNIPAM) and acrylic acid (AAc) comonomers were adsorbed on silicon wafers pre-coated with PEI. The amount of adsorbed microgel particles was controlled by pH and concentration of microgel dispersions and the speed of rotation during the preparation. The swelling/deswelling behaviour of adsorbed P(NIPAM-co-AAc) microgel particles was studied by scanning force microscopy (SFM) and was compared to the temperature-induced volume phase transition in the respective bulk conditions investigated by dynamic light scattering (DLS) and electrophoretic mobility measurements. Also, after the adsorption, the swelling and shrinking is still reversible over several heating and cooling cycles, although the microgel particles are strongly compressed. The particles stay fixed at low ionic strength, up to 10−3M, and intermediate pH. At higher ionic strength or in the acidic or basic regime, they can be moved by the SFM tip or even leave the surface. At low ionic strength the particles swell and shrink in the lateral and vertical directions. At higher ionic strength the particles react more pronouncedly in the vertical direction, and almost not at all in the lateral direction. In contrast to volume measurements, the particles are more strongly swollen at high ionic strength than at low salt concentrations.

Mapping of ion beam induced current changes in FinFETs

We report on progress in ion placement into silicon devices with scanning probe alignment. The device is imaged with a scanning force microscope (SFM) and an aligned argon beam (20keV, 36keV) is scanned over the transistor surface. Holes in the lever of the SFM tip collimate the argon beam to sizes of 1.6μm and 100nm in diameter. Ion impacts upset the channel current due to formation of positive charges in the oxide areas. The induced changes in the source–drain current are recorded in dependence of the ion beam position with respect to the FinFET. Maps of local areas responding to the ion beam are obtained.

Single atom doping for quantum device development in diamond and silicon

The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species, and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. The authors describe a single atom injector, in which the imaging and alignment capabilities of a scanning force microscope (SFM) are integrated with ion beams from a series of ion sources and with sensitive detection of current transients induced by incident ions. Ion beams are collimated by a small hole in the SFM tip and current changes induced by single ion impacts in transistor channels enable reliable detection of single ion hits. They discuss resolution limiting factors in ion placement and processing and paths to single atom (and color center) array formation for systematic testing of quantum computer architectures in silicon and diamond.

Morphology of step structures on CeO2(111)

The morphology of step structures on the CeO2(111) surface is studied by dynamic scanning force microscopy (SFM) operated in the noncontact mode. The surface exhibits hexagonal islands and pits of O–Ce–O triple layer height with steps mostly enclosing an angle of 120°. Atomically resolved images reveal that the (111) surface almost exclusively exhibits alternating steps having (110) and (001) facets. Kink sites and missing oxygen atoms at step edges are identified to be the dominating defective sites at step edges. It is demonstrated that low coordinated oxygen atoms at step edges can be removed by the scanning SFM tip.

Anomalous polarization inversion in ferroelectrics via scanning force microscopy

Local poling of ferroelectrics by the sharp conducting tip of a scanning force microscope(SFM) is studied experimentally and theoretically. The formation of the inverse domainunder the SFM tip, where the polarization is oriented in the direction opposite to that ofthe poling field, is reported for bulk ferroelectrics (single crystals of solid solutionsPbZn1/3Nb2/3O3–PbTiO3). This finding confirms earlier results on ferroelectric thick films, thus proving theuniversality of the anomalous polarization inversion in ferroelectric media. Itis shown that the inverse domain grows with the increase of the poling voltageand duration and remains stable for a long time after the removal of electricfield. The growth process is described by a dynamic model assuming that theappearance of inverse domains is due to a local internal electric field directedagainst the poling one. This field is attributed to the space charge formed beneaththe SFM tip due to injection of charge carriers and their subsequent drift andtrapping. Poling voltage and poling time dependences of the domain size are correctlydescribed by the model. Implications of the anomalous polarization inversion forthe domain engineering and dense data storage in ferroelectrics are discussed.

Blowing DNA Bubbles

We report here experimental observations which indicate that topologically or covalently formed polymer loops embedded in an ultrathin liquid film on a solid substrate can be "blown" into circular "bubbles" during scanning force microscopy (SFM) imaging. In particular, supercoiled vector DNA has been unraveled, moved, stretched, and overstretched to two times its B-form length and then torn apart. We attribute the blowing of the DNA bubbles to the interaction of the tapping SFM tip with the ultrathin liquid film.

Adhesion on the Nano‐ and Macroscale: Interaction between Copper and SAN/SMAh Copolymers

Adhesion-promoting linkers: Block copolymers hold promise for their application in improving the properties of substrates. Copper-coated scanning force microscopy tips (see figure) can be used to investigate the adhesion differences of two model copolymers: poly(styrene-co-acrylonitrile) and poly(styrene-alt-maleic anhydride) (SAN and SMAh). Adhesion properties are analyzed by force spectroscopy and macroscopic pull-off tests.

Covalent Connection of Individualized, Neutral, Dendronized Polymers on a Solid Substrate Using a Scanning Force Microscope

The synthesis of a neutral, high-molar-mass, acrylamide-based, third-generation dendronized polymer (denpol) with a defined number of azide groups at its periphery is reported. An attach-to route is used in which a first-generation denpol is reacted with second-generation (G2) dendrons. The degree of structure perfection of the resulting denpol is quantified as 99.8 %. This value was obtained after the introduction of a fluorescence label at the sites that remained unaffected by the dendronization. The high coverage was independently confirmed for the dendronization of another first-generation polymer and a closely related G2 dendron. The third-generation denpol resulting from the first dendronization experiment was spin-coated as a sub-monolayer onto highly oriented graphite precoated with an ultrathin layer of C12H25NH2, which was introduced to provide a well-defined substrate for denpol adsorption and manipulation. Scanning force microscopy revealed single denpols, which could be moved across the surface and "welded" by covalent cross-linking induced by photochemical decomposition of the azides into highly reactive nitrenes. The successful formation of covalent bonds between two denpols was confirmed by mechanically challenging the link with the scanning force microscope (SFM) tip. This is the second reported case of a move-connect-prove sequence using polymers and the SFM, which for the first time employs noncharged denpols, thus widening the applicability of this method significantly.

Surface potential measurements on Ni–(Al)GaN lateral Schottky junction using scanning Kelvin probe microscopy

The surface potential distribution across lateral Ni–(Al)GaN Schottky junctions was measured by scanning Kelvin probe microscopy. The bare surface barrier heights of unintentionally doped Al0.22Ga0.78N and n-GaN in air were estimated to be ∼1.15 and 0.7 eV, respectively. Upon 364.5 nm (band edge for GaN) illumination, the surface barriers of both n-GaN and AlGaN∕GaN were observed to decrease. The minority carrier diffusion length in n-GaN(Si∼3.5×1017cm−3) was extracted from the surface photovoltage profile near the Schottky junction, ∼1.8±0.4μm. The scanning Kelvin probe surface photovoltage technique for measuring minority carrier diffusion length, while similar to the electron beam induced current technique, offers greater accuracy and higher spatial resolution due to separation of the minority carrier excitation source (relatively large area, above band gap light beam) from the nanometer-size probe (scanning force microscope tip).

Investigating native coronary artery endothelium in situ and in cell culture by scanning force microscopy

The purpose of our studies is to better understand the morphology and functioning of the arteries and their changes in pathogenesis. The most frequently used imaging techniques are intravascular ultrasound, magnetic resonance imaging, and optical coherence tomography. These methods do not image cell-level structural details and only provide biomechanical properties indirectly. We present a new protocol for imaging the endothelial surface and measuring elastic properties of vascular tissue by scanning force microscopy. Full-thickness sections of native pig coronary arteries were prepared. In addition, cultured human umbilical vein endothelial cells were studied as an in vitro model system and for comparison. We encountered a variety of difficulties mostly due to the softness of vascular tissue which required significant adaptations of standard equipment: (i) a new specimen holder designed to stably immobilize the coronary arteries; (ii) a phase-contrast microscope incorporated for assessing the status of the cultured endothelial cells and positioning the scanning force microscope (SFM) tip at a site of interest; and (iii) a continuous exchange of the culture medium at 37 °C to assure viability of the cells in the SFM over extended times. We were thus able to investigate both fresh arterial tissue and living endothelial cells in a near-physiological environment. We present initial SFM images of vascular tissue at a spatial resolution similar to scanning electron microscopy, but which also provide a closer view of the bona fide structure of native tissue. Novel morphological features such as distinct granular particles were observed. Moreover, we report initial measurements of vascular tissue surface stiffness, obtained by indentation-type SFM.

Nonlinear local piezoelectric deformation in ferroelectric thin films studied by scanning force microscopy

Local piezoelectric deformation is investigated in (Pb,La)TiO3 (PLT) and Pb(Zr,Ti)O3 (PZT) thin films via scanning force microscopy (SFM) as a function of the ac voltage Vac applied between the conducting tip and the bottom electrode. Thus obtained voltage dependence of the effective piezoelectric coefficient (local piezoelectric nonlinearity) is compared with the corresponding macroscopic piezoelectric behavior determined by laser interferometry. As expected, the local piezoresponse of PLT films measured inside uniformly polarized areas (ferroelectric domains) remains almost linear with increasing Vac until the driving voltage becomes comparable with the coercive one. The corresponding macroscopic response is substantially nonlinear, suggesting significant contribution from the motion of 90° domain walls. On the contrary, in PZT films the local piezoelectric behavior is strongly nonlinear, whereas the macroscopic piezoelectric coefficient is almost field independent. Moreover, depending on the polarity of probed as-grown domains, the local piezocoefficient of PZT films is found to display either enhancement or reduction with increasing ac voltage. The “positive” domains (i.e., domains having polarization vector pointing to the film-free surface) are often unstable with increasing Vac and switch into the opposite polarization state under an ac voltage several times smaller than that required for global polarization reversal. This effect is explained by the presence of charged domain boundaries below the surface and their local depinning induced by external field. It is envisaged that SFM can be used not only for polarization mapping of ferroelectric surfaces but also as a probe for studying local polarization profiles beneath the SFM tip.

Calculations of the threshold force and threshold power to move adsorbed nanoparticles

We propose and analyze a simple model for the calculation of the power ${P}^{*}$ necessary to depin an essentially rigid cluster or nanoparticle on a surface with a scanning force microscope tip in tapping mode. The model contains the coupling between the particle's lateral and normal motion. We show that there are two important limiting regimes. (i) If momentum transfer occurs gradually between tip and particle, ${P}^{*}$ depends mainly on the viscous-type drag between particle and surface. (ii) If momentum transfer occurs instantaneously once per oscillation, ${P}^{*}$ is dominated by the minimum energy barrier necessary to move the cluster by one lattice constant. In the quasistatic driving mode (i), a critical impact angle ${\ensuremath{\alpha}}_{\mathrm{t}}^{*}$ is identified below which depinning cannot be achieved due to lateral-normal coupling.

Porphyrinphosphonate fibers on mica and molecular rows on graphite.

meso-Tetra(phenyl-p-phosphonate) porphyrin forms rigid and well-separated fibers of monomolecular thickness (2.8 nm) and lengths of several micrometers on mica at pH 13 (octasodium salt). The formation of these fibers could be observed directly by tapping mode scanning force microscopy (SFM) and was induced by capillary forces. Normal height images or images with a topographical inversion were observed depending on the distance of the SFM tip. Amplitude-distance curves indicated that a stable meniscus was formed on hydrophilic surface areas below a tip-sample separation of 20 nm. The meniscus let the original nanorods appear as ditches in the mica surface and enabled rearrangements. A partly protonated form of the same porphyrin (pH 11.5) gave rows of flat-lying porphyrins on graphite, which appear with molecular resolution in SFM images as well as two-dimensional platelets of monomolecular thickness.