Disentangling Surface Energy and Surface/Interface Stress Effects in Spin Crossover Nanomaterials

Abstract

Understanding phase transformation processes at the nanoscale is an important step for the integration of phase change materials into functional nanodevices. Here, a numerical method to assess surface energy and surface stress contributions to finite size effects in spin crossover nanomaterials is proposed. Starting from their formal definitions, molecular dynamic simulations combined with continuum mechanics and thermodynamic considerations on model thin films are performed. The surface energy values extracted from the simulations are 71 and 79 mJ m−2 for the high spin (HS) and low spin (LS) states, respectively. In the limit of a weak lattice parameter misfit, the calculated isotropic stresses of a HS/LS interface are 94 and 45 mJ m−2 for the LS and HS states, respectively. From these results, a ≈10 K downshift of the spin transition temperature is predicted with the size reduction. Whereas surface energy favors systematically the HS state, it is not the case for the interface stress. Indeed, it is demonstrated that the anisotropy of mechanical stresses can lead to the stabilization of the LS state. These results confirm the possible control of the phase stability in these smart nanomaterials through interface engineering.

Effects of the surface energy and surface stress on the phase stability of spin crossover nano-objects: a thermodynamic approach

“…In the interim, we used molecular dynamics (MD) simulations, based on a recently built force field, to calculate the surface energies of the compound [Fe(pyrazine)][Ni(CN) 4 ] in the two spin states. 33,52 (see the ESI† for more details of the MD calculation.) Through these calculations, the obtained surface energies in the LS and HS states are E LS sur = 49 mJ m −2 and E HS sur = 21 mJ m −2 , respectively.…”

“… 29 Owing to the large volume misfit between HS and LS molecules, one can expect that this parameter also plays a crucial role in SCO nanometerials. 33 …”

Realm of Spin State Switching Materials: Toward Realization of Molecular and Nanoscale Devices

“…−8 mJ m −2 ) due to dangling bonds led to a reduction of 4−5 K in the transition temperature and accommodating the interfacial stress led to a reduction of ∼10 K in transition temperature in the thin-film slab. 70 The fabrication of robust thin films on various substrates remains crucial from the perspective of device applications. Various methods, such as spin coating, drop casting, and Langmuir−Blodgett techniques, have been used to deposit thin films.…”

“…Further, by utilizing molecular dynamics simulations with thermodynamic considerations on model thin films, it was found that the surface energy difference between HS and LS states ( ca . −8 mJ m –2 ) due to dangling bonds led to a reduction of 4–5 K in the transition temperature and accommodating the interfacial stress led to a reduction of ∼10 K in transition temperature in the thin-film slab …”