Nanoindentation in Crystal Engineering: Quantifying Mechanical Properties of Molecular Crystals

Abstract

Nanoindentation is a technique for measuring the elastic modulus and hardness of small amounts of materials. This method, which has been used extensively for characterizing metallic and inorganic solids, is now being applied to organic and metal-organic crystals, and has also become relevant to the subject of crystal engineering, which is concerned with the design of molecular solids with desired properties and functions. Through nanoindentation it is possible to correlate molecular-level properties such as crystal packing, interaction characteristics, and the inherent anisotropy with micro/macroscopic events such as desolvation, domain coexistence, layer migration, polymorphism, and solid-state reactivity. Recent developments and exciting opportunities in this area are highlighted in this Minireview.

Application of instrumented nanoindentation in preformulation studies of pharmaceutical active ingredients and excipients

“…Review articles and handbooks provide a good starting point for basic principles of the method (35,38,39,(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61). However, it is the author's experience that there is a common set of misconceptions and errors in the application of nanoindentation in pharmaceutical technology.…”

“…Plastic deformation due to twinning occurs in crystals with low symmetry and few slip systems or when plastic deformation is obstructed due to unfavorable alignment or under high loads. Unlike slip, twinning deformation is homogenous over the entire crystal lattice and is not restricted to multiples of the lattice parameter (45,46,50,58,(92)(93)(94)(95). Compounds with a disordered, corrugated layered structure do not show shearing; they bend or break under applied mechanical stress.…”

Enhancement of thermoelectric figure of merit in Bi2Se3crystals through a necking process

“…The presence of pop-ins is due to the plastic nature of the crystal. Hence, we can say pop-ins imply an elastic to plastic transition, also known as a displacement burst, on the load-displacement (Fd) curve (Varughese et al, 2013). An F-d curve with pop-ins also implies a weak bond, i.e.…”

“…the van der Waals interaction, between Se atoms of two different planes, along which the crystal can be easily cleaved (Varughese et al, 2011). These pop-ins are also attributed to homogeneous or heterogeneous nucleation of dislocations under the indenter tip (Varughese et al, 2013). In the case of bismuth selenide single crystals, these pop-ins tend to be heterogeneous.…”

Bimodal nanoindentation response of the (001) face in crystalline sodium saccharin dihydrate

“…Nanoindentation is a technique that can be employed to quantitatively measure the mechanical properties of relatively small volumes of materials [1][2][3][4][5][6][7][8][9][10][11][12]. In the crystal engineering context [13][14][15][16], this particular feature has been utilized in the recent past to establish structure-property correlations for a variety of molecular solids; one such aspect is in the study of intergrowth polymorphism in active pharmaceutical ingredients (APIs) such as aspirin [3] and felodipine [8].…”