AFM probing

Figure 1 shows the results obtained by Qian et al. [1 ] in a process when AFM probe approaches and then separates from a SiQ2 substrate. The normal force required for separating the probe-substrate contact reads 33 nN. From a thermodynamic point of view, adhesion is in fact a state of the system at the energy minimum when the contact pairs interact with each other through interface, and additional work has to be applied to change the state of the system. 

Fig. 1—Results from an experiment of approach/separation between AFM probe and Si02 substrate (from Ref. [1]).

A change in the amphtude of oscillating AFM probe is chosen for a control of tip-sample force interactions in tapping mode. At the beginning, an operator adjusts the piezo-drive of the probe to its resonant frequency and chooses initial amphtude (Aq) and set-point amplitude (Ajp). The latter is... 

AFM probes with tip having rounded apex whose diameter is in the 50-150 nm are available from two probe manufacturers (Team Nanotec, http //www.team-nanotec.de/) and Nanoworld (http //www. nanoworld.com). 

Sagvolden et al. [86] also combined the use of colloids with AFM force sensors to study adhesion. In their case, instead of attaching the colloid to the end of the AFM probe and applying a normal force, they approached the free colloids from side on, with the AFM cantilevered at an angle of approximately 30° to the surface normal. Thus, they applied a predominantly lateral force to the colloid particles. The colloids were coated with protein molecules, and their adhesion was studied against three nonbiological surfaces, consisting... 

Many years have passed since the early days of AFM, when adhesion was seen as a hindrance, and it is now regarded as a useful parameter for identification of material as well as a key to understanding many important processes in biological function. In this area, the ability of AFM to map spatial variations of adhesion has not yet been fully exploited but in future could prove to be particularly useful. At present, the chemical nature and interaction area of the AFM probe are still rarely characterized to a desirable level. This may be improved dramatically by the use of nanotubes, carbon or otherwise, with functionalized end groups. However, reliance on other measurement techniques, such as transmission electron microscopy and field ion microscopy, will probably be essential in order to fully evaluate the tip-sample systems under investigation. 

Bhushan, B., Kwak, K. J., and Palacio, M. (2008). Nanotribology and nanomechanics of AFM probe-based data recording technology. /. Phys. Condens. Matter 20, Article Number 365207. 

Preformed gold nanoparticles have been used as the top electrode for interrogating SAMs of dithiols diluted within an alkylthiol matrix. The alkyl tails of the alkylthiols do not coordinate to the nanoparticles, but a dithiol s second SH group, sticking above the alkyl level, can bond to the nanoparticle, which is then contacted with an Au-coated AFM probe [42]. 

Frisbie et al. [120] and Findsay et al. [28] introduced CP-AFM for conductance measurements in (single) molecular junctions. CP-AFM allows precise control of the AFM probe, employing the feedback of the force signal and the simultaneous acquisition of the molecular current response. In order to achieve a better electric contact between the conducting AFM tip and the target molecules of interest, Cui et al. proposed contacting bifunctionalized target molecules embedded in inert... 

Lee SY, Nakaya K, Hayashi T, Hara M (2009) Quantitative study of the gold-enhanced fluorescence of CdSe/ZnS nanocrystals as a function of distance using an AFM probe. Phys Chem Chem Phys 11 4403-9... 

Hafner JH, Cheung CL, Woolley AT, Lieber CM (2001). Structural and functional imaging with carbon nanotube AFM probes. Progress in Biophysics Molecular Biology 77 73-110. 

TappingMode scanning techniques, (3) improved AFM probes (such as standard silicon nitride probes modified by electron beam deposition and... 

In principle, there are two methods for the AFM probing of amorphous layers, namely (i) an active probing, which probes the surface chemistry, and (ii) a passive probing, which probes the surface physics. The active AFM probing, which is essentially an extension of the AFM-based single molecule studies, relies on the measurement of LF by an AFM tip that is fimctionahsed with molecular recognition-active molecules (e.g., a potentially complementary DNA). 

Fig. 20. Illustration of the actual AFM probe in tapping mode above and in contact with the surface. A laser beam is used to help amplify the change in oscillation. (Courtesy of Digital Instruments, Veeco Metrology Group, Santa Barbara, CA.)...

Poborchii, V., Tada, T., and Kanayama, T. 2005. Subwavelength-resolution Raman microscopy of Si structures using metal-particle-topped AFM probe. Jpn. J. Appl. Phys. 44 L202-5. 

FIGURE 5.6. The dithiol-containing OPEs 5 and 6 that were inserted into dodecanethiol SAMs. Au nanoparticles were attached to the top unreacted thiol terminus of the molecule and a Au-coated AFM probe was used to make contact with the molecule and measure its conductance. 

Woolley, A.T., C.L. Cheung, J.H. Hafner, and C.M. Lieber. 2000. Structural biology with carbon nanotube AFM probes. Chem. Biol. 7 R193-R204. 

Carboxylic groups positioned at the open ends of SWCNTs were coupled to amines to form AFM probes with basic or hydrophobic functionalities by Wong et al. [117] (Scheme 1.8). Force titrations recorded between the ends of the SWCNT-AFM tips and hydroxy-terminated SAMs confirmed the chemical sensitivity and robustness of the AFM tips. Images recorded on patterned SAM allowed real molecular-resolution imaging [117]. 

Obtaining an image of the first layer deposited on a foreign substrate can be rightly considered to be not completely representative of bulk crystallization. However, this is only the first layer of a thin polymer film, the structure of which can be investigated by electron diffraction. The two techniques are indeed very complementary. AFM probes the first layer, whereas electron diffraction determines the structure of the thin film as a whole - the structure of the film interior. 

Electron diffraction used in combination with AFM probes the thickness of the layer (and therefore the true bulk crystallization). Its results are consistent with those of AFM about the organization of helical hands. Both techniques indicate that for iPBul and sPP, selection processes are at play for helical hand, that involve the immediate neighborhood of the depositing stem substrate topography on which the helix is deposited and/or the neighboring helix in the growth front. 

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