Dynamic Scanning Force Microscopy Research Articles

Low temperature scanning force microscopy using piezoresistive cantilevers

A low temperature dynamic scanning force microscope has been constructed using commercially available piezoresistive cantilevers that can be coated with a ferromagnetic material for MFM application. The setup is able to work in a temperature range from room temperature down to 1.5 K. The performance of the piezoresistive cantilevers has been investigated under different working conditions. Topographic as well as magnetic images of a magnetite thin film sample have been taken at 50 and 4.2 K confirming the proper operation of the microscope at cryogenic temperatures. Furthermore, force-distance-curves taken on thin lead films at 4.2 K demonstrate the levitation forces between the magnetized cantilever tip and the superconducting films. Flux lines were generated by the magnetized cantilever tip itself when approaching the sample. It has also been shown that the microscope is sensitive to the detection of single magnetic flux lines penetrating the lead films.

Photothermal excitation of microcantilevers in liquid: effect of the excitation laser position on temperature and vibrational amplitude

Demands to improve the sensitivity and measurement speed of dynamic scanning force microscopy and cantilever sensing applications necessitate the development of smaller cantilever sensors. As a result, methods to directly drive cantilevers, such as photothermal or magnetic excitation, are gaining in importance. Presented is a report on the effect of photothermal excitation of microcantilevers on the increase in steady-state temperature and the dynamics of higher mode vibrations. First, the local temperature increase upon continuous irradiation with laser light at different positions along the cantilever was measured and compared with finite element analysis data. The temperature increase was highest when the heating laser was positioned at the free end of the cantilever. Next, the laser intensity was modulated to drive higher flexural modes to resonance. The dependence of the cantilever dynamics on the excitation laser position was assessed and was in good agreement with the analytical expressions. An optimal position to simultaneously excite all flexural modes of vibration with negligible heating was found at the clamped end of the cantilever. The reports findings are essential for optimisation of the excitation efficiency to minimise the rise in temperature and avoid damaging delicate samples or functionalisation layers.

Calibration of oscillation amplitude in dynamic scanning force microscopy

A method to precisely calibrate the oscillation amplitude in dynamic scanning force microscopy is described. It is shown that the typical electronics used to process the dynamic motion of the cantilever can be adjusted to transfer the thermal noise of the cantilever motion from its resonance frequency to a much lower frequency within the typical bandwidth of the corresponding data acquisition electronics of a scanning force microscopy system. Based on this concept, two procedures for the calibration of the oscillation amplitude are proposed. One is based on a simple calculation of the root mean square deviation measured at the outputs of the electronics used to process the dynamic motion of the cantilever, and the second one is based on analysis of the corresponding spectrum and the calculation of the quality factor, the resonance frequency and the signal strength. We show that the proposed scheme for amplitude calibration using thermal noise is experimentally and theoretically robust, with soft as well as with hard cantilevers. Moreover, it is directly related to well-defined quantities such as the force constant and thermal energy, in contrast to the calibration using amplitude versus distance curves, which requires non-trivial a priori assumptions regarding the amplitude versus distance relation.

Molecular-resolution imaging of pentacene on KCl(001).

The growth of pentacene on KCl(001) at submonolayer coverage was studied by dynamic scanning force microscopy. At coverages below one monolayer pentacene was found to arrange in islands with an upright configuration. The molecular arrangement was resolved in high-resolution images. In these images two different types of patterns were observed, which switch repeatedly. In addition, defects were found, such as a molecular vacancy and domain boundaries.

Organic Transistors: Supramolecular Order of Solution‐Processed Perylenediimide Thin Films: High‐Performance Small‐Channel n‐Type Organic Transistors (Adv. Funct. Mater. 23/2011)

The non-contact dynamic scanning force microscopy image of a solution-processed perylenediimide thin film shows a layer-by-layer structure with closed molecular planes, upon annealing. Antonio Facchetti, Yueh-Lin Loo, Maria A. Loi, and co-workers show, on page 4479, that the strong intermolecular π–π stacking interactions induce long-range supramolecular order, comparable to that of highly crystalline vapor-deposited films. Correlations between molecular orientation and device performance are highlighted in high-performance small-channel n-type organic transistors.

Supramolecular Order of Solution‐Processed Perylenediimide Thin Films: High‐Performance Small‐Channel n‐Type Organic Transistors

AbstractN,N′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDIF‐CN2), a soluble and air stable n‐type molecule, undergoes significant reorganization upon thermal annealing after solution deposition on several substrates with different surface energies. Interestingly, this system exhibits an exceptional edge‐on orientation regardless of the substrate chemistry. This preferential orientation is rationalized in terms of strong intermolecular interactions between the PDIF‐CN2 molecules. The presence of a pronounced π–π stacking is confirmed by combining near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS), dynamic scanning force microscopy (SFM) and surface energy measurements. The remarkable charge carrier mobility measured in field‐effect transistors, using both bottom‐ and top‐contact (bottom‐gate) configurations, underlines the importance of strong intermolecular interactions for the realization of high performing devices.

Response to the “Comment on ‘Thermal frequency noise in dynamic scanning force microscopy’” [J. Appl. Phys. 110, 036107 (2011)

Response to the “Comment on ‘Thermal frequency noise in dynamic scanning force microscopy’” [J. Appl. Phys. 110, 036107 (2011)

Voltage preamplifier for extensional quartz sensors used in scanning force microscopy

Extensional-mode quartz resonators are being increasingly used as force sensors in dynamic scanning force microscopy or atomic force microscopy (AFM). We propose a voltage preamplifier in order to amplify the charge induced on quartz electrodes. The proposed solution has some advantages over the typically used current-to-voltage converters. First, the gain does not depend on the inner parameters of the quartz resonator, which are usually unknown for the specific resonator and may even vary during the measurement. Second, with such an amplifier a better signal-to-noise ratio can be achieved. Finally, we present AFM images of the Si(111) and the SiO(2) surfaces obtained by the voltage preamplifier with simultaneously recorded tunneling current.

Thermal frequency noise in dynamic scanning force microscopy

Thermal fluctuation of the cantilever position sets a fundamental limit for the precision of any scanning force microscope. In the present work we analyze how these fluctuations limit the determination of the resonance frequency of the tip-sample system. The basic principles of frequency detection in dynamic scanning force microscopy are revised and the precise response of a typical frequency detection unit to thermal fluctuation of the cantilever is analyzed in detail. A general relation for thermal frequency noise is found as a function of measurement bandwidth and cantilever oscillation. For large oscillation amplitude and low bandwidth, this relation converges to the result known from the literature, while for low oscillation amplitude and large bandwidth, we find that the thermal frequency noise is equal to the width of the resonance curve and, therefore, stays finite, contrary to what is predicted by the relation known so far. The results presented in this work fundamentally determine the ultimate limits of dynamic scanning force microscopy.

TEM-assisted dynamic scanning force microscope imaging of (001) antigorite: Surfaces and steps on a modulated silicate

Ultra-high vacuum dynamic scanning force microscopy (dynamic SFM) has been performed on in situ cleaved and as-grown (001) surfaces of low- T , m = 18 and m = 20, antigorite from the Kovdor Mine, Russia. The internal microstructure of the same crystal before cleavage has been checked by conventional TEM on FIB-cut cross-sections. The structural wave is imaged by dynamic SFM with a ~0.25 nm topographic amplitude (outcropping tetrahedral sheet) on cleaved and as-grown surfaces, and with a ~0.5 nm topographic amplitude (outcropping tetrahedral + octahedral sheets) mostly found on cleaved surfaces. Atomic resolution imaging was successfully applied on the cleavage surface through imaging individual atomic features on the outer hexagonal net of the emerging (Mg, O, OH) octahedra of the half-wave. The antigorite cleavage crack undulates through a single octahedral sheet, thereby avoiding rupture of strong Si-O bonds. The two tetrahedral reversals, which form the edges of the modulation repeat, are found to be strongly non-equivalent in structure: across , one reversal is sharp as expected from the standard models of the antigorite structure, whereas the other reversal is unexpectedly “extended.” The latter suggests some scheme of anti-polar positioning of silicate tetrahedra along at the 6-membered ring reversal. High-resolution transmission electron microscopy (HRTEM) structure imaging of antigorite viewed down to confirms spread out electron densities at this reversal. Numerous step height measurements on (001) surfaces show incremental results as integral multiples of 0.25 nm, the spacing between O,OH surfaces along c *. Many of them differ in height from integral multiples of the unit cell repeat along c * and could be explained from carving the bulk wave structure. For all surfaces and steps, local stoichiometry and global electro-neutrality of the surface are satisfied.

Contrast inversion in non-contact Dynamic Scanning Force Microscopy: What is high and what is low?

The present work proves that when non-contact Dynamic Scanning Force Microscopy (DSFM) is performed in ambient conditions wrong height measurements of heterogeneous samples can be obtained. In some extreme cases even contrast inversion can be observed. Alkanethiol islands on Au (1 1 1) have been used as model system, where contrast inversion is observed with different DSFM modes and various data acquisition parameters. To understand this effect, spectroscopy measurements have been used to show that contrast inversion is really a consequence of the differences in the interaction measured between tip and sample on the bare Au substrate and on the alkanethiol islands. We propose that this interaction is mainly induced by liquid necks forming between tip and sample, which is much stronger on the hydrophilic Au substrate than on the hydrophobic alkanethiol islands.

Unravelling the atomic structure of cross-linked (1 × 2) TiO2(110)

The cross-linked (1 × 2) reconstruction of TiO(2)(110) is a frequently observed phase reflecting the surface structure of titania in a significantly reduced state. Here we resolve the atomic scale structure of the cross-linked (1 × 2) phase with dynamic scanning force microscopy operated in the non-contact mode (NC-AFM). From an analysis of the atomic-scale contrast patterns of the titanium and oxygen sub-structures obtained by imaging the surface with AFM tips having different tip apex termination, we infer the hitherto most accurate model of the atomic structure of the cross-linked (1 × 2) phase. Our findings suggest that the reconstruction is based on added rows in [001] direction built up of Ti(3)O(6) units with an uninterrupted central string of oxygen atoms accompanied by a regular sequence of cross-links consisting of linear triples of additional oxygen atoms in between the rows. The new insight obtained from NC-AFM solves previous controversy about the cross-linked TiO(2)(110) surface structure, since previously proposed models based on cross-links with a lower O content do not appear to be consistent with the atom-resolved data presented here. Instead, our measurements strongly support the Ti(3)O(6) motif to be the structural base of the cross-linked (1 × 2) reconstruction of TiO(2)(110).

NaCl(001) surfaces nanostructured by Suzuki precipitates: a scanning force microscopy study

Suzuki precipitates on the (001) surfaces of as-cleaved and additionally annealed NaCl:Cd2+ crystals were studied by means of dynamic scanning force microscopy and Kelvin probe force microscopy in ultra-high vacuum. Rectangular precipitates with edge lengths of up to several hundred nanometres can be found on the surfaces of as-cleaved crystals. The precipitates exhibit steps, which are a result of the cleavage of dislocations at the Suzuki–NaCl interfaces. Additional annealing of the crystals below 260 °C leads to surface diffusion and to a restructuring of the whole surface producing atomically flat precipitates at steps, but also on flat terraces in regions of low step density. On surfaces evaporated at 350 °C, the main surface characteristics of evaporated (001) surfaces of pure NaCl can be found. The step edges are merely changed in shape by nanometre-size Suzuki precipitates.

Enhancing dynamic scanning force microscopy in air: as close as possible

Frequency modulation dynamic scanning force microscopy has beenimplemented in ambient conditions using low oscillation amplitudes(<1 nm) to simultaneously record not only topographic but also additional channels ofinformation, in particular contact potential images. The performance of this mode ascompared to the conventional amplitude modulation mode is analyzed in detail using abiological molecule, turning yellow mosaic virus RNA, as the model sample. On the basis ofscanning force microscopy imaging as well as spectroscopy experiments, we find that forsuch very small samples the frequency modulation mode is superior since it can beoperated with smaller tip–sample interaction, smaller effective tip–sample distance andlower forces. Combined with Kelvin probe microscopy it results not only in considerablyhigher electrostatic resolution, but also in correct quantitative values for the contactpotential as compared to traditional amplitude modulation scanning force microscopy.

Ferroelectric Lithography: Bottom-up Assembly and Electrical Performance of a Single Metallic Nanowire

We report on both the assembly of noble-metal nanowires by means of the nanotechnological and large-scale integrable approach of ferroelectric lithography and their performance testing upon electrical transport. Our results on LiNbO(3) single crystal templates show that the deposition of different elemental metals from ionic solutions by photochemical reduction is confined to the ferroelectric 180 degrees domain walls. Current-voltage-characteristics recorded from such nanowires of typically 30-300 microm in length revealed an Ohmic behavior that even improved with time. Additionally, we also examined the local topographic and potentiostatic properties of such wires using dynamic scanning force microscopy in combination with Kelvin probe force microscopy.

Design and synthesis of new trehalose‐conjugated pentapeptides as inhibitors of Aβ(1–42) fibrillogenesis and toxicity

Aggregation of the amyloid Abeta peptide and its accumulation into insoluble deposits (plaques) are believed to be the main cause of neuronal dysfunction associated with Alzheimer's disease (AD); small molecules that can interfere with the Abeta amyloid fibril formation are therefore of interest for a potential therapeutic strategy. Three new trehalose-conjugated peptides of the well known beta-sheet breaker peptide iAbeta5p, were synthesized. The disaccharide was covalently attached to different sites of the LPFFD peptide chain, i.e. at the N-terminus, C-terminus or at the Asp side chain. CD spectroscopy in different solvents was used to assess changes in the peptide conformation of these compounds. The effects of these glycopeptides on the self-assembly and morphology of Abeta aggregates were investigated by ThT fluorescence assay and dynamic Scanning Force Microscopy, respectively. All the synthesized compounds were tested as inhibitors of Abeta toxicity toward pure cultures of rat cortical neurons.

Nanostructured molecular surfaces: advances in investigation and patterning tools

This feature article is aimed to showcase advanced soft and radiationless nanotools for the morphological characterization and for the preparation/modification of molecular surfaces, namely solid supported ultrathin films not exceeding 1–2 molecular layers. As to the characterization, the development of dynamic scanning force microscopy in attractive regime is presented as an important progress at least as far as it concerns imaging of nanoscale features of molecular surfaces with minimal probe–sample physical interaction. To date, this tool has been applied only by a few groups in spite of its larger resolution and image quality than the conventional scanning probe methods. As to the preparation/modification, the realization of ultrathin nanoscale patterns is reviewed by highlighting at first some recent approaches in the field of molecular self-assembly on solid substrates including the possibility to induce order under both equilibrium and non-equilibrium conditions. Secondly, the area of nanoscale man-made fabrication is featured by choosing some of those techniques of urgent need (high resolution, low cost, potentially of simple use) like scanning probe lithographies and nanodispensing tools.

Morphology, Dispersion, and Stability of Cu Nanoclusters on Clean and Hydroxylated α-Al2O3(0001) Substrates

Using high-resolution dynamic scanning force microscopy (SFM) operated in the noncontact mode, we investigate here the detailed morphology, dispersion, and thermal stability of nanometer-sized three-dimensional Cu clusters on an α-Al2O3(0001) substrate. We systematically study the effect of surface hydroxylation by depositing metallic Cu either on an ultrahigh vacuum prepared Al-terminated alumina surface or on surfaces prehydroxylated/hydrated by exposure to varying doses of H2O. Three-dimensional growth of Cu nanoclusters dispersed evenly on the surface is observed independent of surface preparation. As the top facet of the nanoclusters appears hexagonal in SFM images, we conclude that they are regular Cu crystallites in the equilibrium form. For strongly hydroxylated surfaces, however, a reduced aspect ratio (height-to-width) of the nanoclusters indicates an increased wetting of the Cu. We propose that Cu bonding to the hydroxylated surface is enhanced by the formation of Cu−O−Al bonds. Surprisingly when heating the surface, Cu remains dispersed on the surface, but it is observed that the total coverage of Cu on the surface is rapidly reduced when the sample is heated to temperatures above 450 °C for clean and 550 °C for hydroxylated surfaces. Our results unexpectedly show that Cu is desorbed from the ∼3 nm wide Cu nanoclusters at temperatures well-below the melting temperature of bulk Cu, and the sublimation is completed before the onset of sintering or ripening of the Cu nanoclusters under the conditions of our experiment. Our observations thus reveal significant changes in the cohesive energies of nanometer-sized Cu clusters, which is an effect that should be taken into account when modeling the stability of Cu crystallites, for example, with regard to sintering of Cu-based catalysts.

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.

Improvement of a dynamic scanning force microscope for highest resolution imaging in ultrahigh vacuum

We report on a modification of a commercial scanning force microscope (Omicron UHV AFM/STM) operated in noncontact mode (NC-AFM) at room temperature in ultrahigh vacuum yielding a decrease in the spectral noise density from 2757 to 272 fm/Hz. The major part of the noise reduction is achieved by an exchange of the originally installed light emitting diode by a laser diode placed outside the vacuum, where the light is coupled into the ultrahigh vacuum chamber via an optical fiber. The setup is further improved by the use of preamplifiers having a bandpass characteristics tailored to the cantilever resonance frequency. The enhanced signal to noise ratio is demonstrated by a comparison of atomic resolution images on CeO(2)(111) obtained before and after the modification.