Graphene Electrodes: Universal architecture for 2D electronics

The production of graphene and the other 2D materials is presented in the beginning of this thesis. Micromechanical exfoliation is the best method for obtaining relatively small and top quality samples. The invention of Graphene Finder simplifies the procedure of finding the exfoliated flakes. In this work, largearea films are exclusively obtained by CVD, mostly on copper films and foils. Several forms of copper are used for CVD growth. A new substrate named ultrafoil is invented to overcome the roughness and contamination of commercially available copper foils. The formation of copper silicide in copper thin films is studied and found to be detrimental for the growth of graphene. The modified synthesis of rGO is introduced, as rGO represents a cheap alternative to CVD for large scale production of graphene.
The transfer of flakes is performed by several methods, such as with PVA/PMMA support, CAB wedging and the pick-up technique with hBN. Several important improvements of the pick-up technique are introduced. These allowed us to transfer any 2D crystals and patterned graphene flakes with PMMA residues. We also developed the drop-down technique, which is used to release any crystal on the surface of the PPC/PDMS. CVD MoS2 and MoSe2 crystals are transferred from oxide with hBN protection by wedging. Ultra clean suspended crystals are obtained by further adaptation of the pick-up technique.
CVD graphene is commonly transferred by etching of the growth substrate or by the bubbling method. Cleaner samples are transferred by combining the drop-down technique and ultrafoil, with hBN flakes protecting the graphene from contacting the polymer support. A new electrochemical transfer method is invented, named ODT. Graphene transferred by ODT shows high coverage compared to other conventional transfer mothods. Another important aspect of the ODT is the preservation of the growth catalyst.
THz-TDS is used to generate sheet conductance maps and to characterize the transferred graphene films. Ultrabroad band THz analysis showed a perfect Drude response for graphene grown on copper single crystal and transferred by ODT, while graphene grown on copper foil presented a Drude-Smith response typical of films with extended line defects. M4PP allowed to investigate the continuity of graphene films in the micrometer range. We showed that graphene from Cu single crystal behaved as a perfect 2D conductor, differently from what was previously reported for graphene from Cu foils. An extension of Graphene Finder allowed the generation of high resolution coverage maps that helped characterizing the transferred graphene films.
The pick-up transfer method is used to fabricate structures sandwiched in hBN, in which the electrical connection is obtained by one dimensional edge contact. High quality trilayer encapsulated device is presented, with measured mobility more than 5 times higher than any published result. A new architecture for TMDCs based devices is introduced. The crystals are encapsulated in hBN and graphene parts intermediate the (edge) contact between gold and the TMDC. In this way, the MoS2 FET with the highest reported mobility to date has been fabricated. The protection by hBN of CVD graphene grown on ultrafoil allowed to fabricate for the first time encapsulated stacks with CVD graphene.
Novel ways of patterning 2D materials are presented. In particular the catalytic etching of graphene by metal nanoparticles is studied. Ag particles in contact with graphene at high temperatures in oxygen are able to form channels aligned along the ZZ direction of graphene. We monitored this phenomenon in-situ with an ETEM. The motion of the particles etching the suspended membrane is discrete, consequence of the interaction with the carbon atoms. DFT calculations supported the hypothesis that it is energetically unfavourable to etch ZZ atoms. The surprisingly strong interaction between the particle and the graphene edge is able to dictate the 3D morphology of the particle. Crystallographic patterning of graphene and hBN is achieved without the catalytic action of metallic particles. Holes are first induced by knock-on damage with high intensity e-beam and enlarged by oxygen at high temperatures.