B. Gotsmann

How to measure energy dissipation in dynamic mode atomic force microscopy

Abstract When studying a mechanical system like an atomic force microscope (AFM) in dynamic mode it is intuitive and instructive to analyse the forces involved in tip–sample interaction. A different but complementary approach is based on analysing the energy that is dissipated when the tip periodically interacts with the sample surface. This method does not require solving the differential equation of motion for the oscillating cantilever, but is based entirely on the analysis of the energy flow in and out of the dynamic system. Therefore the problem of finding a realistic model to describe the tip–sample interaction in terms of non-linear force–distance dependencies and damping effects is omitted. Instead, it is possible to determine the energy dissipated by the tip–sample interaction directly by measuring such quantities as oscillation amplitude, frequency, phase shift and drive amplitude. The method proved to be important when interpreting phase data obtained in tapping mode, but is also applicable to a variety of scanning probe microscopes operating in different dynamic modes. Additional electronics were designed to allow a direct mapping of local energy dissipation while scanning a sample surface. By applying this technique to the cross-section of a polymer blend a material specific contrast was observed.

Determination of tip–sample interaction forces from measured dynamic force spectroscopy curves

Abstract The forces between a sharp tip and a sample are characteristic for different sample materials. A new method for quantifying the elastic tip–sample interaction forces from measured frequency vs. distance curves is presented. The dynamic force–spectroscopy curves investigated were obtained by dynamic force microscopy under ultrahigh vacuum (UHV) conditions for large vibration amplitudes with commercial levers/tips. The full non-linear force–distance relationship is deduced via a numerical algorithm, where the equation of motion describing the oscillation of the tip is solved explicitly. The elastic force distance dependence can be determined by fitting the results of a computer simulation to experimental frequency vs. distance data. The obtained force–distance curves can be compared quantitatively with theoretical models.

Ar plasma treated and Al metallised polycarbonate: a XPS, mass spectroscopy and SFM study

Abstract Ar plasma etched and Al metallised bisphenol A carbonate was analysed by mass spectroscopy, photoelectron spectroscopy (XPS), and scanning force microscopy (SFM). We mainly used a technical polymer (Makrolon 2808, Bayer) made by injection-moulding, as well as spin coated bisphenol A carbonate ( n =1) and polycarbonate (PC) ( n =115). The mass spectroscopy during the etching process shows the degradation of the PC in the form of carbon monoxide, carbon dioxide and methyl groups. The photoelectron spectroscopy shows in detail the surface modification after Ar plasma treatment and metallisation. The plasma induces a reduction of the carboxylic carbon (C 1s), a strong reduction of singly bonded oxygen (O 1s) and also a slight reduction of doubly bonded oxygen. After Al metallisation, a reaction of Al with the oxygen groups and an interaction with the aromatic system is documented. Ar plasma etching increases the chemical interaction of Al mainly with the aromatic carbon. The X-ray photoelectron spectroscopy of metallised PC under different initial conditions shows a strong influence of incorporated water in the PC bulk that cannot be seen by XPS on uncoated PC. The O 1s signal increases during metallisation and results in an oxidation of Al probably caused by the fact that the hydrophobic surfaces becomes hydrophillic. Temperature-dependent XPS was done on technical PC samples and on spin coated samples ( n =1, n =115) and supports the influence of the bulk state for the Al–PC interface. For n =1 carbonate, a diffusion of Al into the PC volume was observed. The SFM measurements showed a roughening effect on the nanometer scale even after short treatment times. Al can be seen as a weakly bound cluster on the virgin PC, and if a pre-etching is done, Al seems to grow as a good wetting film. The adhesion force of Al films on PC without any influence of the volume can be explained by the chemical bonding of Al to the carboxylic and aromatic systems. The adhesion can be increased by plasma pre-treatment. A breakdown of the adhesion on technical PC is probably induced by a reaction of Al with mobile intercalated gas, that is enriched near the surface after Al coating.

Aluminium deposition on SF6 plasma‐treated polycarbonate: an AFM, XPS and mass spectroscopy study

A systematic investigation was made of the chemical and morphological influences of SF 6 plasma on polycarbonate and the influence of plasma treatment on Al metallization. Mass and ion spectroscopy were used for characterization of the plasma and the etching process. X-ray photoelectron spectroscopy (XPS) measurements were applied for the chemical characterization, while atomic force microscopy (AFM) (static and dynamic mode) served to inspect the surface morphology. All analytical techniques were performed in an ultrahigh vacuum system, in order to prevent the polycarbonate sample from being exposed to ambient air after the plasma treatment. During the etching process we used mass difference spectra to demonstrate the removal of masses 19, 28 and 32 corresponding to HF, CO (N 2 ) and CF. Additionally, the inclusion of fluorine was also observed by this technique. The XPS spectra of polycarbonate surfaces show a significant inclusion of fluorine (C-F, C-F 2 ) and a reduction of the oxygen content after the plasma treatment. Aluminium metallization leads to the formation of an A<F interlayer; metallic growth of Al is only observed when the metallic layers become thicker than a few nanometres. The AFM investigations have shown that even a short plasma treatment causes changes in morphology (structures with an extension of 20-40 nm). After extended plasma exposure the surface becomes very rough, resulting in poor Al adhesion. On untreated polycarbonate, Al grows in the form of weakly bound clusters, which can only be imaged in the dynamic AFM mode. After plasma treatment, Al grows in the form of well-adhering flat layers without clustering.

Dynamic AFM using the FM technique with constant excitation amplitude

Dynamic atomic force microscopy using the frequency modulation technique is investigated for the case that the excitation amplitude is kept constant. This mode of operation has very unique properties. A computer simulation is used to investigate the distance dependence of the measurement signals frequency and amplitude using various conservative and non-conservative interaction forces. It is shown how the two measurement channels are interlinked and influenced by both conservative and dissipative interactions. Further, discontinuous frequency shift versus distance curves as reported in the literature were not obtained.