AFM Contact Mode Research Articles

Determination of the radii of coated and uncoated silicon AFM sharp tips using a height calibration standard grating and a nonlinear regression function.

AFM sharp tips are used to characterize nanostructures and quantify the mechanical properties of the materials in several areas of research. The analytical results can show unpredicted errors if we do not know the exact values of the AFM tip radius. There are many techniques of in situ measurements for determining the actual AFM tip radius, but they are limited to uncoated tips. This paper presents an alternative and simple method to determine the radii of coated tips and an uncoated tip. Pt-coated, Cr/Au-coated, and uncoated Si tips were used to scan a calibration standard grating in AFM contact mode with sub-nanonewton load to obtain the curved scan profile of the edge corner of the grating structure. The data points of the curved profile of each tip were fitted with a nonlinear regression function to estimate the curvature radius of the tip. The results show that the estimated radius of the coated tips is in the range of nominal values provided by the tip manufacturer, while the estimated radius of the uncoated Si tip is bigger than the nominal radius because of tip blunting during the scan. However, this method yields an accurate estimate of the tip radius with a low root mean squared error of the curve fitting results.

Mid-infrared photocurrent nano-spectroscopy exploiting the thermoelectric effect in graphene

We develop a conductive-atomic force microscope coupled to a mid-infrared (IR) quantum cascade laser that allows to measure mid-IR photocurrent maps and spectra with nanometric spatial resolution. Here, we exploit the photo-induced thermoelectric effect of graphene to obtain the mid-IR local absorption spectra of nanoscale devices and materials. By scanning the probe in AFM-contact mode, the thermoelectric photocurrent can be mapped throughout the graphene layer, which acts as a semitransparent transducer. At zero voltage bias applied to the probe tip, we observe very low noise (about 25 pA/Hz), which makes our technique competitive with others in measuring the local IR absorption in nanomaterials with insufficient thermal expansion coefficient, or to characterize nanoscale devices that significantly absorb IR radiation, such as mid-infrared photodetectors.

Methodology for estimating the modulus of elasticity of bitumen under different aging conditions by AFM

This paper presents a micro and nano mechanical analysis method to measure the elastic modulus and properties of bitumen. Tests were performed using the AFM in pulsed force mode to determine the adhesion and stiffness of bitumen. In AFM contact mode, a methodology was developed to determine the modulus of elasticity of the bitumen using the nanoindentation technique for small-volume specimens using a Berkovich tip. Two different sources of bitumen were studied with a performance grade of PG 64 S (22) and PG 64 H (25), in its neat, RTFO, and RTFO + PAV aged conditions. The mechanical properties of the bitumen were analysed from the responses of the measured materials. The AFM images and mechanical results reveal that the properties of the bitumen are considerably affected by aging, associated with oxidation and variations in its morphology. Modulus measured by AFM was also compared to traditional G* measurements from a rheometer, and their relationship analysed.

Electrical and mechanical characterization of two-phase alloys by means of scanning probe microscopy in dynamic impedance spectroscopy mode

Dynamic impedance spectroscopy, evaluated for measuring non-stationary systems, was used in combination with scanning probe microscope. Using this approach, localized impedance measurements in the AFM contact mode could be carried out. Additionally, impedance–force curves were made at each phase of investigated materials to illustrate the relation between impedance and the force applied to a probe. Therefore, correlation of electrical and mechanical properties with particular phases of investigated two-phase alloys was made possible. The materials used in this study were spheroidal graphite cast iron and 2205 duplex stainless steel, both materials with clearly defined phases having significantly different properties.

Electrical anisotropy properties of ZnO nanorods analyzed by conductive atomic force microscopy

In this study, we have prepared ZnO nanorods on cracked GaN substrates using aqueous solution method. Unique electrical characterization of one individual lying ZnO nanorod is analyzed by conductive atomic force microscopy (C-AFM). Effect of anisotropy properties on the conductivity of a single nanorod has been investigated. The current maps of ZnO nanorods have been simultaneously recorded with the topography which is gained by AFM-contact mode. The C-AFM measurement present local current–voltage (I–V) characteristics of the side facets of one individual lying nanorod, however, no current is detected on the top facets of ZnO nanorods. Measurement results indicate that the side facets are more electrically active than the top facets of ZnO nanorods due to lower Schottky barrier height of the side facets.

Poly (thienylene methine) grafted nanocrystalline TiO2 based hybrid solar cells

A novel hybrid solar cell fabricated from TiO2 and in situ polymerized poly (thienylene methane) (PTBBQ) onto the surface of TiO2 was described. 1HNMR, FTIR, UV–Vis absorption, thermal and electrochemical properties of PTBBQ were investigated, and the surface morphology of TiO2/in situ polymerized PTBBQ film was studied via AFM contact mode. Device I/V characterizations with and without in situ polymerized PTBBQ were measured. Efficiency improvements were observed in the in situ polymerized PTBBQ/TiO2 system, and such device showed a short-circuit current density of 1.15 mA/cm2, an open-circuit voltage of 0.50 V, a fill factor of 0.50, and a energy conversion efficiency of 0.29% at A.M. 1.5 solar illumination (100 mW/cm2).

Atomic force microscopy based approach to local impedance measurements of grain interiors and grain boundaries of sensitized AISI 304 stainless steel

The paper presents the results of the AFM based approach to local impedance spectroscopy (LIS) measurements performed within the region of austenite grain interiors and the austenite grain boundaries for both sensitized and non-sensitized AISI 304 stainless steel (SS). The local impedance investigations in the AFM contact mode were carried out after standard tests for susceptibility to intergranular corrosion (IGC). Local impedance spectra recorded in the AFM contact mode for both sensitized and non-sensitized AISI 304 stainless steel within the region of the austenite grain interiors and the austenite grain boundaries demonstrated different shapes and characters. Performed researches revealed significant differences in the charge transfer resistance values recorded for the austenite grain interiors and the grain boundaries depending on the sensitization state. Moreover, proposed AFM based approach allows identification of the IGC process even when a double-loop electrochemical potentiokinetic reactivation (DL-EPR) method does not provide evidence of it.

Surface characterizations of poly(ethylene terephthalate) film modified by a carbohydrate-bearing photoreactive azide group

Grafting of a carbohydrate UV-reactive molecule, the β- d-galactopyranosyl-(1–4)-1- N-[2-(4-azidophenyl amino)-ethylamino]-1-deoxy- d-glucitol (AzPhLac), has been achieved on poly(ethylene terephthalate) film. The dependence of surface density and yield of grafted AzPhLac have been studied versus the number of moles of UV-treated AzPhLac via the deposit of a drop of solution with a known volume and concentration. A nearly complete grafting of initial AzPhLac molecules was reached for the lowest concentration and lowest volume of solution deposit conditions. Grafting density values in the range of 29–181 nmol/cm 2 confirm the polymeric nature of the grafted layer. FTIR–ATR demonstrated the heterogeneity in thickness of the grafted surface due to the drop-deposit method and solvent evaporation process. AFM (imaging) allowed us to find a correlation between grafting density and rms roughness. Water-contact angle and AFM (contact mode) gave further evidence of the hydrophilic nature of the extreme surface.

Hybrid polishing mechanism of single crystal SiC using mixed abrasive slurry (MAS)

Single crystal SiC is a mechanically hard and chemically inert material used in optical and power devices. This work proposes the development of a hybrid polishing technique using a mixed abrasive slurry (MAS) with colloidal silica and nano-diamond. Hybrid removal mechanism of the MAS on the SiC is investigated by polishing results, chemical analyses and AFM studies. Each role of two abrasives is distinguished by scratching tests with AFM contact mode on the chemically reacted SiC surface. Finally, this paper provides an optimum MAS condition to realize highly efficient material removal rate (MRR) keeping defect-free surface.

On the use of SPM to probe the interplay between polymer surface chemistry and polymer surface mechanics

Adhesive properties of a polymer surface results from the complex contribution of surface chemistry and activation of sliding and dissipating mechanisms within the polymer surface layer. The purpose of this study is to dissociate the different contributions (chemical and mechanical) included in an AFM force-distance curve in order to establish relationships between the surface viscoelastic properties of the polymer, the surface chemistry of functionalized polymer surfaces and the adhesive forces, as determined by C-AFM experiments. Indeed we are interested in the measurements of local attractive or adhesive forces in AFM contact mode, of controlled chemical and mechanical model substrates. In order to investigate the interplay between mechanical or viscoelastic mechanisms and surface chemistry during the tip - polymer contact, we achieved force measurements on model PDMS polymer networks, whose surfaces are chemically controlled with the same functional groups as before (silicon substrates). On the basis of AFM nano-indentation experiments, surface Young moduli have been determined. The results show that the viscoelastic contribution is dominating in the adhesion force measurement. We propose an original model, which express the local adhesion force to the energy dissipated within the contact and the surface properties of the material (thermodynamic work of adhesion). Moreover we show that the dissipation function is related to Mc, the mass between crosslinks of the network.

Plasma immersion ion implantation of Pebax polymer

Nitrogen plasma immersion ion implantation (PIII) was applied to Pebax thin films and plates using doses ranging from 5 × 10 14 to 10 17 ions/cm 2 at applied voltages of 5, 10, 20 and 30 kV. The analysis of the Pebax structure after implantation was performed using FTIR ATR, Raman, UV–vis transmission spectra, tensile and AFM contact mode data. The carbonization and depolymerisation processes were observed in the surface layer of Pebax. It was found, that graphitic- and diamond-like structures in Pebax are formed at PIII treatment of 30 kV applied voltage. AFM measurement data showed that the hardness of the Pebax surface layer increased sharply at PIII treatment with a dose higher then 10 16 ions/cm 2. The bulk mechanical properties of the Pebax film after PIII remained unchanged.

Evaluation of active semiconductor structures by combined scanning thermo-elastic microscopy and finite element simulations

In this contribution we report on combined investigations of hot areas in a high power high electron mobility transistor (HEMT) using a scanning thermo-elastic microscope and finite element simulations of the problem. The sample was a AlGaN/GaN-HEMT grown on sapphire substrate, with a gold coating for improved thermal management. The FE simulations were performed based on the ANSYS program version 5.7. The thermo-elastic response was detected with an Explorer AFM-head of Topometrix. To allow simultaneous detection of the topology and of the thermo-elastic expansion images, the explorer had been modified for AFM measurements in the DC mode and at the double frequency of the thermal sinus in AFM contact mode. The thermo-elastic image of the hot area of the HEMT recorded at 2f shows a bright line as the hot area which is located along the gate, between gate and drain. The absolute value of the vertical expansion has been calibrated from the measured diode signal by use of the microscope's force-distance calibration curve. In order to obtain a reliable estimate of the maximum temperature on the hot line, the temperature image obtained by FE simulation is calibrated using the thermal expansion of the gold film of known thermal expansion coefficient.

InP patterning using contact mode and non-contact AFM lithography for quantum dot localization

AFM contact and non-contact mode have been used to oxidize InP(001) surface at the nanoscale in order to create, after oxide etching, nano-holes used as nucleation sites for self-assembled quantum dots (QDs). Results obtained in contact mode are similar to those obtained with the local anodization of silicon, and principally explained by the effect of space charges that occurs during the initial stages of the oxide growth. Using a negative pulse permits the experimenter to reduce the space charge effects in the oxide and thus to enhance both homogeneity and resolution. Non-contact mode coupled with a modulated voltage brings even more specific results. For more than two a decade variation of probe velocity (0.01–5 μm s −1), the AFM oxidation introduces no significant changes in the features. Studies on the influence of oxidation time have identified two regimes. With the first one, a high growth rate was found for oxidation times shorter than 100 ms. Second, for oxidation times longer than 100 ms, we observe an oxide height saturation and a lower lateral growth rate. These results provide a way to control separately both depth and diameter of holes. The achievement of non-contact oxidation confirms the compatibility of this technique in both resolution and homogeneity for QD nucleation sites.

The evaluation of four conditioners for glass ionomer cements using field-emission scanning electron microscopy

Objectives: The purpose of this study was to evaluate the GIC-dentine interface morphology using FE-SEM after four different conditioners (Ketac Conditioner, Dentin Conditioner, Cavity Conditioner, and an experimental conditioner K-930), used with two RM-GICs and one self-cured GIC, and to observe the effect with an AFM of the four different conditioners on the surface of polished human dentine. Materials and methods: sample preparation for FE-SEM: Twenty-four 1-mm thick dentine discs were obtained from superficial occlusal dentine of extracted human third molars, and finished with wet 600-grit SiC paper. The discs were treated for each of the GICs using the conditioners according to the manufacturers’ instructions. The specimens were kept in tap water for 24 h at 37°C, and then assigned for one of two observational techniques; a fractured technique and an acid-base technique. Specimens were mounted on aluminium stubs, gold sputter-coated and observed using a FE-SEM. Sample preparation for AFM: Four rectangular dentine blocks approximately 2 mm 3 were prepared from two extracted human third molars, polished and finished with diamond paste down to 0.25 μm particle size. One half of each sample was treated with one of the conditioners according to the manufacturers’ instructions, and the other half was not conditioned. The samples were stored in distilled water prior to AFM contact mode observation. Results: FE-SEM: All specimens of all materials demonstrated good adaptation to the underlying dentine. However, the specimens conditioned with K-930 showed increased demineralization and a thicker acid-base resistant layer (2.8–3.4 μm) compared with the others (1–2 μm). AFM: The images of conditioned specimens showed demineralization of peritubular dentine. Funneling of dentinal tubule orifices of specimens conditioned with K-930 was observed, and was also seen for the FE-SEM specimens. Significance: The use of surface conditioners resulted in similar adaptation to the non-conditioned specimens at the GIC-dentine interface. All of the GICs demonstrated intimate adaptation to the dentine surface whether it was conditioned or not.

Imaging micro-cracks in gold films: a comparative study of scanning tunneling and atomic force microscopies

In this work, three different scanning probe microscopy (SPM) modes, scanning tunneling microscopy, atomic force microscopy (AFM) -intermittent contact mode; and AFM-contact mode, are compared by imaging micro-cracks on a thin Au film. The excellent capability of SPM techniques to image prominences are confirmed by easily imaging Au particles. Their capabilities to image surface cavities are also analyzed, with an indication that the AFM-intermittent contact mode, using etched Silicon tips, is close to its resolution limit. These results are discussed in terms of spatial extension of the interaction and tip radius of curvature. It is also found that the AFM-contact images undergo an evolution with time producing sharper images. Several possibilities to this effect are reviewed, resulting in an indication that AFM-intermittent contact and AFM-contact images, in this case, are generated by different tip–sample interaction processes in the nanometric scale.

Ultra-high-vacuum magnetic force microscopy of the domain structure of ultra-thin Co films

Two scan modes were developed for an Omicron atomic force microscope/scanning tunneling microscope (AFM/STM) operating in ultra-high vacuum (UHV). These modes allow scanning at a constant tip–sample distance in order to measure forces or force gradients. Samples and tip probes were prepared and used in the same UHV system. Co films were deposited on Si substrates by electron beam evaporation. The thickness of these films was 2–120 monolayers (ML). Si tips, coated with 20 ML Fe, were used as probes. The Co films showed a magnetic structure with in-plane magnetization. Cross-tie domain walls were also observed. Furthermore, the Co film was sensitive to forces exerted by the tip on the sample in contact AFM mode. Operating a magnetic force microscope (MFM) in ultra-high vacuum (UHV) has some fundamental advantages over using it in ambient conditions. The MFM tips are entirely prepared in UHV, and used in situ. Ultra-thin magnetic films are prepared without any protecting cap layer. Lack of tip protection reduces the effective tip–sample distance, which has an essential effect on the resolution and the sensitivity. Furthermore, any possible effect of the sample cap layer on domain structure is excluded. UHV–MFM therefore provides the best conditions for investigating the domain structures of as-prepared magnetic films. We adapted the software of a commercial Omicron UHV AFM/STM system (Omicron Vacuumphysik GmbH, D65232 Taunusstein) to enable MFM operation. This includes software-controlled determination of the sample slope to ensure measurement of force or force gradient at a welldefined height above the sample surface. The system was initially tested on Co/Pt multilayers, garnet films and barium ferrite [1]. We describe the application of UHV–MFM to magnetic domain and domain wall investigations of Co thin films prepared in situ. 1 Experimental 1.1 Experimental set-up The UHV system consists of two chambers, both at a base pressure below 109 Pa (10−10 torr). One chamber, equipped with an argon ion gun, was used entirely for tip and sample preparation. The tips were ion-sputtered before iron was evaporated on to them using an electron beam evaporation technique. Another evaporator was mounted to deposit ultrathin Co films on Si(100) substrates. The second chamber (the analysis chamber) was equipped with a LEED/AES unit and an Omicron AFM/STM mounted on a damping stage. Cantilevers and samples could be changed in situ. The samples were on a holder attached to a piezo element. The maximum range of scanning was 6 μm along the x and the y axes. The cantilevers were mounted on a piezo element which in turn could be excited at the cantilever resonance frequency. Owing to force gradients acting on the probe tip, the resonance frequency varied as ω =√k− (∂F/∂z)/meff. The shift of the frequency was measured by an FM detector. The frequency shift could be kept constant by a feedback loop to measure topography in noncontact mode. The height data obtained in this manner represent a surface of constant force gradient. Using a magnetic tip over a magnetic sample the force gradient at low frequency shifts (i.e. relatively far apart from the sample surface) is dominated by magnetic interaction. Thus information about the local domain structure is obtained. A different approach is applied to measure forces or force gradients at a constant tip to sample distance. Two scan modes were built into the system to realize this approach. The first mode works similarly to the “lift mode” of the Nanoscope (Digital Instruments GmbH, D-68199 Mannheim). The topography of the sample is measured along a single line. This can be done either in STM, AFM contact or AFM noncontact mode. Subsequently, the topographic information is used to scan the tip at a given height above the same scan