Graphene Nanoflakes: Thermal Stability, Infrared Signatures, and Potential Applications in the Field of Spintronics and Optical Nanodevices

Abstract

We investigate the structural, electronic, and vibrational properties of graphene nanoflakes (GNFs) with a small number of atoms (<250) and distinct shapes (triangular, rectangular, and hexagonal) through classical molecular dynamics (CMD) and density functional theory (DFT) calculations. We show that these graphene nanostructures are able to retain their planarity for simulated temperatures up to 1500 K, starting to degrade into amorphous nanocarbon for temperatures above 3000 K. The shapes and types of border of the GNFs have a strong influence on their electronic properties and spin. The HOMO−LUMO energy gap of the studied nanoflakes spans the full range of the visible spectrum, suggesting potential applications in the fabrication of optical emission nanodevices, which is confirmed by TDDFT calculations to obtain the UV−vis absorption spectra of triangular armchair GNFs. In particular, the UV−vis maximum absorption energies and intensities scale linearly with the linear size of the GNF. In the special case of zigzag-edged triangular nanoflakes, a nonzero net spin which increases linearly with the edge size was found, pointing toward possible spintronic applications by tuning the spin distribution. The DFT calculations of the infrared spectra allowed the identification of shape- and border-related fingerprints.

Optical properties of graphene nanoflakes: Shape matters

“…In general, the calculated DOS are consistent with previous calculations using a simple nearest-neighbor, oneorbital Hückel model 22,29,30,39 and DFT 28,33 . For structures comparable to the ones analyzed here, usually a wide range of energies is considered in the literature, which have very similar DOS profiles among the different structures 53 .…”

“…The quantum confinement caused by reduced dimensionality of these structures opens a tunable bandgap 7,17 . Several studies have characterized the electronic properties of GNF 5,18-39 and some have shed light on their optical features [20][21][22][23][24]27,33,34,[40][41][42][43][44] . Yamijala et al 42 performed a systematic study of the linear and nonlinear optical properties for ∼400 graphene quantum dots at the ZINDO/S semiempirical level.…”

Discerning Site Selectivity on Graphene Nanoflakes Using Conceptual Density Functional Theory Based Reactivity Descriptors

“…This observation is corroborated by the fact that zigzag edges are thermally more stable than armchair edges. 57 Site selectivity using relative reactivity descriptors for these GNFs is discussed in following subsections. 4 42 Armchair, Arm-Zig, and Zigzag GNFs a a The asterisk (*) indicates the sites with the highest relative reactivity indices, and "ac" indicates armchair pouches.…”

Equiangular Hexagon‐Shape‐Controlled Synthesis of Graphene on Copper Surface

“…The formation mechanism for well-defi ned graphene fl akes from atomic point of view is still lacking and seems complicated, but is possibly related to both the crystalline structure of Cu surface and the intrinsic stability of graphene shapes, [ 13 ] i.e., hexagonal shaped graphene shows higher temperature-stability among other shapes. Considering two polycrystalline Cu materials used, the surfaces likely have different crystal plane orientation at growth condition, inconsistent with single type of epitaxial graphene grown.…”