Noncontact AFM

Interpretation of IC-AFM images is complicated by the fact that the tip-sample force is a nonlinear function of tip-sample separation. The tip-surface interactions in IC-AI have been modeled extensively and have been recently reviewed [109, 143]. Two important conclusions have come from the modeling. First, the nonlinear interaction of the dynamic tip with the surface can lead to two stable oscillation states one that follows a net attractive path and the other that follows a net repulsive path [147, 148]. A hint of this is seen in the phase versus frequency plot (see Fig. 3.32) where the cantilever initially oscillates along an adhesive path and then abruptly transitions to the repulsive path. Simulated amplitude and phase (z-sweep) curves can reproduce those determined experimentally. These have been interpreted in terms of force based interaction models that include the effect of capillary forces and adhesive forces when they are known or can be estimated. The transition between the bistable states depends on a number of factors including the cantilever Q, Ao, and r p, and the drive frequency as well as the surface properties [149]. In general high Q cantilevers or small Ao favor the net attractive path. 

This technique was first described in 1991 [153] and has since been reviewed [143,154]. It has been used to give molecular resolution images of electrically conductive polymers [155] and PS-h-PMMA block copolymers [156] in UHV. Because the technique is truly non-contact, it does not have the complexity of dual interaction paths (either repulsive or attractive) that are inherent to amplitude modulation IC-AFM. This makes the technique suitable for the study of metal-polymer bonding under controlled ambient conditions [157]. 

The second application of noncontact AFM is in near-field detection of force gradients at 5-50 nm above the surface. This is the approach used in electric force microscopy (EFM) [158] and magnetic force microscopy (MFM) [159]. 

Usually in these modes the topography of the sample is first determined using IC-AFM. This topography is stored in memory and a second noncontact scan of the same area is made, using the stored height information to make the probe stay a fixed distance above the sample (a lift height of typically 5-50nm). Feedback is disabled for this second scan and the direct 

Another interesting application of NC-AFM is in the detection of thermal events such as melting and glass transition temperature in polymers on heating. The technique uses FM detection with the tip fixed above the surface as the polymer temperature is ramped underneath [162]. The mechanism behind this technique is not well understood. 

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Sasaki N and Tsukada M 1999 Theory for the effect of the tip-surface interaction potentiai on atomic resoiution in forced vibration system of noncontact AFM Appl. Surf. Sc/. 140 339... 

Other noncontact AFM methods have also been used to study the structure of water films and droplets [27,28]. Each has its own merits and will not be discussed in detail here. Often, however, many noncontact methods involve an oscillation of the lever in or out of mechanical resonance, which brings the tip too close to the liquid surface to ensure a truly nonperturbative imaging, at least for low-viscosity liquids. A simple technique developed in 1994 in the authors laboratory not only solves most of these problems but in addition provides new information on surface properties. It has been named scanning polarization force microscopy (SPFM) [29-31]. SPFM not only provides the topographic stracture, but allows also the study of local dielectric properties and even molecular orientation of the liquid. The remainder of this paper is devoted to reviewing the use of SPFM for wetting studies. 

All the STM results from our group presented in this chapter employed the variable temperature STM, with tips made by electrochemical etching of tungsten wire. For noncontact AFM (NC-AFM), we employ commercial conducting silicon cantilevers with force constants of approximately 2-14 rn 1 and resonant frequencies of approximately 60-350kHz (Nanosensors and Mikromasch). The NC-AFM images we present here were recorded in collaboration with Professor Onishi at Kobe University and employed a UHV JEOL (JSPM-4500A) microscope. 

An alternative technique is noncontact AFM [18]. Figure 19 illustrates the concept. The tip oscillates above the surface, and the modulation in amplitude, phase, or frequency of the oscillating cantilever in response to force gradients from the sample can be measured to indicate the surface topography. Even without contact, the amplitude, phase, or frequency can be affected by the van der Waals forces of the sample within a nanometer range, which is the theoretical resolution however, this effect can be easily blocked by the fluid contaminant layer, which is substantially thicker than... 

Noncontact AFM [240] Insulating substrates, atomic resolution Molecular systems, atomic resolution Biocluster and biomolecular imaging Imaging and spectroscopic data in liquid environments Nanoscale charge measurement Nanoscale magnetic properties... 

Application of non-contact mode AFM in anodic oxidation based lithography is an important step forward in this field [76]. Noncontact AFM has been used to create feature sizes <30 nm on a 65 nm resist (polymer resist) and was transferred through reactive ion etching (RIE) to the Si substrate (P-doped (100)) on which the resist is coated [77]. A scan speed of 10 pm s has been achieved using a heavily B-doped Si cantilever with a large spring constant ( 340 N m ) and mechanical resonance frequency ( /) MHz) and with a tip of radius of curvature... 

Combination of a pulsed bias and noncontact AFM has been found to improve the control of the writing process [78]. This method reduces the tip-substrate interaction time and thus improves the reliability and lithographic resolution. The frequency of oscillation and the field pulsing frequencies need to be adjusted to create a definite phase relation between the two and it was found that the minimum line width is obtained when the applied field is on during the time the cantilever tip is furthest from the substrate. The process also needs adjustment of the duty cycle. 

Sensing of evanescent waves with an optical tip has been proposed for use as an optical device to sense AFM forces by means of an optical microlever which is illuminated by a laser under conditions of total internal reflection and which is connected to an atomic force tip [77], Thus tunneling photons from the microlever to the optical tip at the evanescent light coupling may be used for the feedback loop. This instrument combines noncontact AFM and PSTM techniques. 

Figure 39. Simultaneous SNOM and noncontact AFM topography images of a thin section of eye tissue (retina) showing receptor cells (i.e., rods and cones). Images obtained with TopoMetrix Aurora SNOM, courtesy of TopoMetrix, Santa Clara. 

Much progress has been made in understanding atomistic properties of surfaces by noncontact AFM [216]. In noncontact mode true atomic resolution was first obtained on Si(l 11)7 x 7 [217], on lnP(l 10) [218], and on NaCl [219]. Meanwhile, even subatomic features are observable by noncontact AFM [220]. In contact mode, atomic resolution is achievable but unlike with STM and noncontact AFM it is inconclusive if this resolution is real. True atomic resolution can be recognized by the correct imaging of lattice defects, for example, vacancies as depressions. Otherwise, apparent atomic resolution can arise from the corrugations of the tip s surface and the sample s surface being in phase. The image is then a superposition of many patches of the surface and vacancies cannot be seen. 

Noncontact AFM is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. Tip-sample spacing is in the order of tens to hundreds of angstroms (Fig. 5). 

Noncontact AFM is desirable where little or no contact between the tip and sample is required. The total force between the tip and sample is relatively low at about 10 N which makes it advantageous for studying soft or elastic samples. 

Intermittent contact AFM is similar to the noncontact AFM mode except that the vibrating cantilever is brought closer to the sample so that at the maximum amplitude it makes slight contact with the surface (i.e., taps the surface). The intermittent contact region is shown on Fig. 2. As with noncontact measurement, the change in cantilever oscillation amplitude is in response to variation in tip-to-sample spacing. 

For providing a proof-of-concept for covalent linking on a bulk insulator surface, we present the results of a systematic noncontact AFM study, investigating the reactions of four different halide-substituted benzoic acids. The molecules used were 4-iodo benzoic acid (IBA), 2,5-diiodo benzoic acid (DIBA), 2,5-dichloro benzoic acid (DCBA), and 3,5-diiodo salicylic acid (DISA), as shown in Fig. 67b. Within this series of systematically varied benzoic acid derivatives, IBA can be regarded as the conceptually simplest molecule. As only one halide atom is available for the linking reaction, dimer structures are expected. Based on the pKx... 

Noncontact AFM overcomes the frictional and adhesive forces between the tip and sample by hovering the tip a few Angstroms above the surface. In this mode, the attractive van der Waal forces between the tip and surface are monitored. As you might expect, these attractive forces are much weaker than those generated in... 

In 2009, long-time AFM manufacturers Park Systems (Suwon, South Korea) developed an SICM instrument geared specifically toward live cell imaging. This system integrates SICM, noncontact AFM, and optical microscopy into one instrument. The system also allows for electrophysiological patch-clamp measurements and can function in approach retract scan mode, which is similar to hopping mode. A specialized environmental chamber for temperature, pH, and humidity control is available, which can help to keep cells viable for longer than 20 h. 

It is known that for an asymmetric back-and-forth motion of an oscillating tip, dissipation occurs during an oscillation cycle. Such an asymmetric motion can be due to the adhesion force when the tip touches the surface that gives a backward oscillating path different from the forward path. Therefore the adhesion leads to a loss of energy either in tapping or noncontact AFM mode. When operated in an ambient condition, a nanomeniscus may form between the tip and the surface at each oscillation and modify the imaging process and mechanical information, especially with soft objects. This is more detrimental than in contact mode, for which the presence of the meniscus is less problematic. 

For information on properties of cantilevers, the second edition of Sarid s book is an excellent choice [102]. Garcia and Perez [103] have reviewed the theory behind dynamic imaging modes of intermittent contact AFM (IC-AFM) [104] and noncontact AFM (NC-AFM) [105]. The influence of the tip shape in imaging has been reviewed by Villarrubia [106]. The application of SPM to polymer ultrastructure and crystallinity has been reviewed by many including the work of Reneker et al. [107], Lotz et al. [108,109], Hobbs et al. [110], and Magonov and Yerina [111]. Some of the ultrastructural studies have been conducted using hot/cold stages to study in situ crystallization [112-114]. 

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