Electronic and magnetic properties of selected two-dimensional materials
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Abstract
Electronic devices, such as field-effect transistors (FETs), based on low-dimensional materials attract an immense interest as a potential inexpensive, flexible and transparent next generation of electronics. There have been considerable improvements in the accessibility of various low-dimensional materials and even the first ferromagnetic monolayers have been experimentally realized. Nevertheless, many open questions remain concerning basic physical properties of such materials. This work focusses on the electronic and magnetic properties of three carefully selected and especially interesting low-dimensional materials: Bottom-up chemically synthesized graphene nanoribbons (GNRs), nitrogen-doped graphene films and ferromagnetic chromium trihalides.
First, we investigate charge transport in bottom-up synthesized GNRs with various edge morphologies and ribbon widths. Although prototypes of FET devices based on such GNRs have recently been demonstrated, fundamental questions as for example on the dominant charge transport mechanism, or on the structure- and width-dependence of charge transport signatures in GNR devices, remain unanswered and need to be addressed experimentally. Therefore, we present the development of a reliable fabrication of GNR network FETs and their measurement. In devices with gold electrodes at micron and submicron channel lengths, we study length-dependent charge transport as a function of gate voltage and at a wide range of temperatures. First, we show that the contact resistance is low and behaves Ohmic-like. The channel current follows power laws for both temperature and drain voltage, which is explained by nuclear tunneling-mediated hopping as the dominant charge transport mechanism. In addition, we observe a large positive magnetoresistance of up to 14 % at magnetic fields of 8 T at low temperatures. We find this magnetoresistance only in GNRs which have a width of five carbon dimers across the ribbon and which are expected to exhibit a particularly low band gap. With our results we provide a better understanding of the nature of charge transport and the engineering of contacts, which both is evidently crucial to bolster any further development of GNR-based devices.
Besides geometrical confinement in GNRs, we explore heteroatomic nitrogen doping as a second route of tailoring charge transport properties of graphene. Here, extended two-dimensional graphene films with substitutional nitrogen dopants are studied and compared to pristine graphene fabricated under identical conditions. By combining structural and electrical characterization methods, we elucidate the role of structural disorder and electron localization for the electronic properties of this material induced by the nitrogen dopants. We quantify the transition from weak to strong localization with doping level based on the change of the length scales for phase coherent transport. This transition is accompanied by a conspicuous sign change from positive ordinary Kohler magnetoresistance in undoped graphene to large negative magnetoresistance in doped graphene.
In addition to charge carrier properties, also spin properties depend heavily on the dimensionality, where an important ingredient for stable magnetic order in lower dimensions is magnetic anisotropy. Therefore, we investigate chromium trihalides, which are layered and exfoliable semiconductors and exhibit unusual magnetic properties. In particular, we focus on the understanding of magnetocrystalline anisotropy by quantifying the anisotropy constant of chromium iodide (CrI3), where we find a strong change from 5 K towards the Curie temperature. We draw a direct comparison to chromium bromide (CrBr3), which serves as a reference, and where we find results consistent with literature. In particular, we show that the anisotropy change in the iodide compound is more than three times larger than in the bromide. We analyze this temperature dependence using a classical model for the behavior of spins and spin clusters showing that the anisotropy constant scales with the magnetization at any given temperature below the Curie temperature. Hence, the temperature dependence can be explained by a dominant uniaxial anisotropy where this scaling results from local spin clusters having thermally induced magnetization directions that deviate from the overall magnetization.