Adsorption-induced surface stress

G.Y. Chen, T. Thundat, E.A. Wachter and R.J. Warmack, Adsorption-induced surface stress and its effects on resonance frequency of microcantUevers, Journal of Applied Physics 77 (1995) 3618-3622. 

FIGURE 7.4 (a) Deflection, Az, and change in surface stress, Ac, of a gold-coated AFM microcantilever are plotted as a function of time after exposure to vapors of alkanethiols with different chain lengths, (b) Adsorption-induced surface stress at saturation coverage (AOjai) is plotted as a function of alkyl chain length for = 4, 6, 8, 12, and 14. (From Berger, R., et al. 1997. Science 276 2021. Reprinted with permission from AAAS.)... 

S. Kim, K.D. Kihm, Effect of adsorption-induced surface stress change on the stiffness of a microcantilever used as a salinity detection sensor. Appl. Phys. Lett. 93(8), 081911 (2008)... 

A huge database has been established regarding the temperature coefficient of siuface tension for metals, alloys, and polymers. Tables 24.1a and 24.1b tabulate the data for some typical samples and includes information derived and discussed later in Sect. 24.4.2. The temperature dependence of surface tension provided an opportunity for one to derive information regarding atomic cohesive energy in the bulk and with possible mechanism for the adsorbate-induced surface stress. The latter could be a challenging topic of research on adsorption of various adsorbates to liquid surfaces of relatively low-Tn, metals. 

Fig. 17. O-induced surface stress on Si(lll) vs. O coverage. The slope of the solid line indicates an oxygen induced compressive surface stress of -5.7 N / m per monolayer O. The inset shows the resnlt for adsorption on Si(100). A slight tensile stress of order 0.2 N / m is indnced per monolayer O. Data from [91San], rescaled to account for one dimensional bending [97Ibal]...

Fig. 28. CO-induced surface stress on Ni(lll). The adsorption in bridge and three-fold coordinated hollow sites induces compressive stress for all coverages. Data from [94Qro]. (coverage 1.0 1.86x10 atoms / cm ).

Therefore, the deflection of the cantilever is directly proportional to the adsorption-induced differential surface stress. Surface stress has units of N/m or J/m2. Equation (12.3) shows a linear relation between cantilever bending and differential surface stress. 

Kefalas VA (1995) Solvent crazing as a stress-induced surface adsorption and bulk plasticization effect. J Appl Polym Sci 58(4) 711-717... 

The mechanism for cantilever bending is assumed to be adsorption-induced stress. The adsorption decreases the surface free energy and surface free energy is density is surface stress. It is speculated that the hydrogen bonding between the nitro groups of the explosives molecules and the hydroxyl group of 4-MBA is responsible for the easily reversible adsorption of explosive vapours on the SAM-coated top surface of the cantilever. This... 

The resistance of many metals and alloys to corrosion depends critically upon the presence of a thin (10-1000 A [168]) passive surface film [169]. In "aggressive environments, this film may become damaged locally via several processes, e.g. surface stress effects (either flow-induced [170,171] or as a result of anion adsorption [168]), the impingement of small particles on the surface [169], spontaneous depassivation [169]. Retention of the protective film by the metal only results if repassivation of the unprotected area is feasible compared with pit growth. 

Fig. 40. Changes in the surface stress of Ni(100) induced by oxygen adsorption at room temperature [92 San, 97Iba]. Reference state is the clean surface at Ax = 0.

Other factors affecting the signals in the dynamic mode are adsorption-induced effects, such as surface stress and position dependence, which can either stiffen or soften the cantilever, thereby varying the spring constant. The relationship between the surface stress and stiffness of a cantilever has been intensively discussed [20-22]. Lee et al. visually demonstrated the dependence of resonance frequency on a pattern of a gold layer on the surface of a cantilever [23]. In any case, we have to be careful about these effects when we analyze the signals obtained with the dynamic mode. 

To summarize, in order to determine the deformation of the gel, we have to solve the set of equations (2.1), (2.7), (2.9) and (2.10). We obtain 1) current density from the voltage of electrodes, 2) the adsorption rate from the current density, 3) surface stress and strain from the adsorption rate, respectively. Fig. 2.2 describes the deformation process of the beam of gel in uniform electric fields. Three arrows are current density vectors. Molecular density on the gel is large near the root and small near the tip of the gel. We can estimate that deformation speed near the root of the gel is larger than that near the tip due to the adsorption-induced deformation model. 

In the case of intensive repetitive actions, the facilitation of plastic deformation in the surface layer may at some point result in the opposite effect, namely, an additional strength increase due to the accelerated accumulation of distortions in the metal structure. Direct observations by electron microscopy, conducted by Kostetskiy et al., indicated a significant increase in the dislocation density in the surface layer. Under the appropriate conditions (temperature, stress, velocity, etc.), such a peculiar sample training may be used in the improvement of the structure and the mechanical properties of the surface layer. However, this already corresponds to the adsorption-induced fatigue region, studied in detail by Karpenko et al. These studies showed that at a certain level of stress the adsorption-caused acceleration of defect accumulation within the surface layer may lead to the premature development of cracks and partial failure after a certain number of cycles (cyclic fatigue). 

However, adsorbate could induce various kinds of stresses accompanied with versatile patterns of relaxation and reconstruction [59, 81, 82]. The spacing between the first and the second atomic layers expands if an adsorbate such as C, N, and O buckles into space between the atomic layers even if there is contraction of bonds between the adsorbates and the host atoms [81]. For example, H, C, N, O, S, and CO adsorbates on a metal surface could change the surface stress and cause surface reconstmction because of bond making and breaking. Surface adsorption of sodium ions also increases the stiffness of a microcantilever [83]. 

An example is HKUST-1, which, although small, shows adsorption-induced distortions when water, methanol, or ethanol are reversibly adsorbed. Allendorf et al. showed this property when a thin film of HKUST-1 was integrated with a microcantiveler surface [94]. The time-dependent responses to H O are shown in Fig. 3.12. It was also demonstrated that the sensor responds to CO2 when the MOF layer is dehydrated. The stress in this case was due to the coordination of COj with imsaturated copper sites in HUKST-1. Because the framework distortions were quite small, the group concluded that higher sensitivities can be achieved using more flexible MOFs. 

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