MECHANICAL BUCKLING OF INDIVIDUAL SILICON NANORIBBONS ON A COMPLIANT SUBSTRATE
Abstract
About a decade ago, mechanical buckling evolves as a promising metrology to rapidly measure the elastic moduli of thin films and one-dimensional (1D) nanostructures. This form of metrology translates the geometrical measurements of the buckling profile into material property value. The strategy applied in the mechanical buckling-based metrology consists of three steps: i) applying pre-strain to the compliant substrate, ii) depositing or transferring 1D nanostructures onto the surface of the pre-strained compliant substrate, and iii) releasing the pre-strain of the compliant substrate so as to allow it to return to its original dimensions. The release of the pre-strain on the compliant substrate induces a compressive stress on the strain-free 1D nanostructures which later buckle spontaneously into sinusoidal shape in order to relieve the compressive stress. Using the Newtonian mechanics model, previous researchers developed a relation to identify the elastic properties of the 1D nanostructures by correlating the buckling wavelength with materials and geometric parameters of the system. However, previous work mainly emphasized on application of small strain (less than 40%), and single material system of relatively long and having equal moment of inertia (e.g. circular or hexagon). Without acknowledging the existence of the native sheath layer surrounding the nanostructures could lead to measurement error of ~15% or more. In this dissertation work, silicon nanoribbons of variable aspect ratios (width B ranging 40, 60, 80, 100 and 200 nm, and constant thickness H of 30 nm) were individually deposited on a 100% pre-strained elastomeric (polydimethylsiloxane, PDMS) substrate. Since the as-fabricated Si nanoribbons consist of native oxide layer of ~5 nm, the core-sheath structure had to be accounted for the property measurement where the effective modulus was being considered. Upon relaxing the 100% pre-strain on the pristine substrate, Si nanoribbons showed increased buckling wavelength as a function of aspect ratio, owing to the greater effective Young’s modulus in larger ribbons. On the other hand, the standard deviation of the measured buckling wavelength was considerably high ~20%, possibly resulted from the increased of area moment of inertia as the ribbons were not placed perfectly flat on the substrate surface during the transfer process. In terms of buckling mode, three forms of buckling mode were observed in this work, in general. In-plane buckling mode was observed on ribbons having aspect ratio B/H ˂ 1.20, whilst ribbons with aspect ratio greater than 1.20 exhibited out-of-plane buckling mode. The findings was in good agreement with the adopted analytical solution which suggested that B/H ≈ 1.14 is transition point of the buckling mode. Meanwhile, out-of-plane buckling mode observed in this work can be divided into two variants, i.e. incline-to-plane and normal-to-plane. Ribbons with 1.68 ≤ B/H ≤ 2.70 buckled incline-to-plane with excessive incline angle with respect to the normal direction of substrate surface, in which the asymmetrical feature of the ribbons and application of 100% pre-strain were accounted for this observation. In contrast, normal-to-plane buckling mode was predominant in ribbons having larger aspect ratio i.e. B/H = 5.12, not forgetting that half of the widest ribbons failed to buckle.In the existing models, perfect bonding is assumed between the two constituents of the buckled system. However, in practical, the adhesion between nanostructures and substrate is ambiguous. Thus, ultra-violet/ozone (UVO) surface treatment was employed to improve the interfacial adhesion and its effect on the ribbon buckling profile was investigated. UVO treatment increased hydrophilicity of the substrate surface, which helped to enhance chemical bonding at the interface between ribbon and substrate. Though so, the treatment had insignificant effect on the buckling profile since UVO treatment only altered the surface modulus of substrate within tens of nanometer. Whilst the overall bulk modulus of the substrate remained unaffected, no substantial variations in the buckling wavelength was observed as compared to that of untreated system. Nevertheless, the treatment seemed to improve the interfacial adhesion marginally for ribbons having aspect ratios 2.70 and 5.12. Besides, it was also evidenced from the AFM topographic images that substrate surface suffered from depression at regions where peaks or valleys of the buckled ribbons formed, in the event of treatment duration lasted 150 s. This further affirmed that UVO treatment is an effective technique to enhance ribbon-substrate interfacial properties. Additionally, transition of buckling mode as a function of treatment duration was not observed in this work owing to the unequal area moment of inertia of the ribbon structure.