• graphene
• Raman spectroscopy
• strain
• strain engineering

Since its discovery in 2004 by Novoselov and Geim, graphene, a two-dimentional allotrope of carbon, has drawn the attention of the scientific community due to its surprising properties. The spatial arrangement of its carbon atoms in a honeycomb lattice creates in fact a unique electronic band structure, which causes conducting electrons to behave like massless Dirac fermions. Furthermore, the lattice configuration and the chemical composition of graphene leads to the formation of strong in-plane bonds, which grants this material an exceptional stability and mechanical strength.

One of graphene's most interesting features is the possibility to tailor its electronic properties through the application of strain, and to generate a variety of phenomena such as -- to cite one of the most intriguing possibility -- the creation of pseudospin-dependent gauge magnetic fields, or pseudomagnetic field, when the lattice is subject to triaxial strain. Moderate deformations are expected to lead to magnetic fields of various Teslas and to significantly impact the quantum states of electrons in graphene. These perspectives, along with its two-dimensional nature, make graphene appealing for the realization of new devices both in the context of opto- and electro-mechanics.

The control of strain in graphene, though, remains today a challenging objective. The aim of this thesis has been the creation and the investigation of different strain profiles on free-standing graphene monolayers. This target has been achieved by applying a differential pressure on silicon nitride (SiN) membranes patterned with pass-through holes of different sizes and shapes, onto which monolayer graphene has been deposited.

A first part of the work has been focused on the preparation of the samples, which were obtained by micro-fabricating the SiN patterned membranes in a cleanroom, thanks to a combination of e-beam lithography, UV lithography, wet and dry etching, and by depositing on top of them monolayer graphene grown by chemical vapor deposition. Subsequently, the induced strain has been studied by micro-Raman spectroscopy, as a function of the hole geometry and of the applied differential pressure.

The analysis of the Raman maps acquired over the suspended strained graphene indicates first of all a clear shift of both the G and 2D peaks: this is consistent with what expected in the presence of a hydrostatic deformation of graphene, due to the strain-induced modification of the energy of the phonons. The magnitude of the Raman peak shift can in fact be used as a direct measure of the local average strain. As a novel result, during my thesis I could also demonstrate the presence of an anisotropic component of the strain, in the case of devices obtained on elliptical holes in the SiN. Anysotropy was detected in terms of a non trivial evolution of the width of the doubly-degenerate peak of the G phononic mode and of its splitting in its two G+ and G- components. All the observed peak shifts were found to be in good agreement with the results reported in the literature for the value of the applied strain, which has been numerically estimated through simulations of the studied devices.

The results here described indicate thus a new strategy for the creation of a broad range of custom strain profiles, which can be controlled by the applied pressure and by the geometry of the supporting SiN membrane.