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Adsorption-Induced Strains of a Nanoscale Silicon Honeycomb
Porous Silicon obtained from highly doped Si can be thought of as a nanoscale random honeycomb with pores parallel to the [001] axis. For about 10 years, this system has received renewed attention as a model system for adsorption studies. One of the intriguing features of this system is that the emptying of liquid-filled pores occurs in a collective-like process. This is usually attributed to the connectivity of the pore network but here pores in Porous Si are non-connected. It has been proposed that the adsorption induced mechanical deformation of the pore walls could lead to a coupling between pores and could trigger the collective emptying. In order to test this idea, we have performed systematic measurements of both adsorption and anisotropic mechanical deformations of porous Si, using heptane at room temperature. We show that strains measured along and transverse to the pore axis exhibit a hysteretic behavior as a function of the fluid pressure, which is due to the hysteresis in fluid adsorption. The pressure dependence of the strains together with the independent measurement of the transverse stress, allows us to determine the biaxial transverse modulus and to estimate the longitudinal Young’s modulus of porous Si. We argue that the value of these constants implies that the Young’s modulus of the 6 nm thick walls of the honeycomb is about 5 times smaller than that of bulk silicon, striking evidence of finite-size effects.