| Abstract |
Two-dimensional materials with an atomic scale thickness, including graphene and other related materials, have attracted considerable research interest due to their excellent physical properties. In this thesis we introduce new laser-based methods for the synthesis of 1D and 2D nanomaterials based on graphene and transition metal dichalcogenides (TMDs).
In particular we demonstrate the laser induced photochemical modification of graphene oxide (GO) for organic photovoltaic (OPV) devices and electronic applications. We report on a rapid and facile method, for the laser-based reduction, doping and patterning of GO. The photochemical method we developed for the simultaneous reduction and doping of GO layers is based on ultraviolet laser irradiation in the presence of a dopant precursor gases, including Cl2 and NH3. It is shown that a few seconds of irradiation is sufficient to dope the GO lattice, while the doping and reduction levels can be readily controlled upon variation of the irradiation time. This method was successfully employed for the in situ laser induced modification of prefabricated GO field effect transistors. Furthermore, this fast, non-destructive and roll-to-roll compatible photochemical method can be use for the reduction and doping of GO nanosheets as well. By tuning the laser exposure time, it is possible to control the doping and reduction levels and therefore to tailor the work function (WF) of the GO-Cl nanosheets to a maximum value of 5.23 eV, which is a value that matches the HOMO level of most polymer donors employed in OPV devices. Furthermore, a laser-based patterning technique—compatible with flexible, temperature sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. As a proof-of-concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution-processed graphene based electrodes.
The study of inorganic nanometer-scale materials with hollow closed-cage structures, such as inorganic fullerene-like nanostructures (IF) and nanotubes (INT), is a rapidly growing field. To date, numerous kinds of IF and INTs TMDs were synthesized for various applications, particularly for lubrication. In this thesis we demonstrate new simple, room temperature and environmentally friendly approaches for the synthesis of IF and INTs via a) ultrashort pulse laser ablation of TMDs in bulk form in ambient air as well as b) via pulsed laser deposition (PLD) of bulk TMD targets. These techniques can be further developed for the formation of nanostructures at any preselected location, such as microsized asperities, which are destined to numerous applications, including for electron emission cathodes and scanning probe microscopy field emitters.
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