Modulation of the electronic, optical and transport properties of graphene via chemical doping: Towards application as a transparent conductive electrodes

Motivation:
​Graphene has emerged as a strong candidate as a transparent conductive electrodes (TCE) as a potential replacement for the popular Indium Tin oxide (ITO). This is due to the unique fundamental electronic, optical, mechanical and chemical properties of graphene. However, large scale production routes of graphene (currently most popular it the Chemical Vapor Deposition "CVD") is  poly-crystalline and defective, resulting in a lower conductivity as compared to higher quality graphene. 
Hence to capture the advantage of the large-scale processability of CVD graphene, chemical doping can significantly reduce its sheet resistance, in addition to largely modifying its work function which enables universality of using graphene either as a low work function cathode or a high work function anode in optoelectronic applications. 
Approach: 
  • In my research I focus on non-covalent chemical doping routs, which would interact with the graphene surface without inducing damage to it basal plane. This way, the expected reduction in the carrier mobility would only result from charged scatterer rather a damaged structure, which would be overcome by the increase in the a carriers density and generally result in an increased conductivity.
  • My research specially focus on optically transparent CVD few layers graphene (FLG), due to its higher resilience and robustness as compared to single layer graphene (SLG), in addition to the possibility of intercalating dopants in between the sheets in a similar manner to graphite intercalation compounds (GIC). The intercalation of FLG can be imagined as a bulk doping route to graphene, which allows to higher uptake of the dopant and major reduction in the sheet resistances as compared to the minor drops in the optical transmittance.  
  • Large, high molecular weight molecules with a large redox potential are strong and stable dopant to graphene. I use these molecules to act as surface dopant that modulate the transport properties of the materials in addition to largely modulating the work function of graphene.

Chemical Vapor Deposition (CVD) of Graphene - optimization and clean transfer

Motivation:
Chemical vapor deposition (CVD) is currently the most promising route towards large-scale production of graphene, especially with very recent reports on the roll-to-roll adaptation of both the synthesis and transfer process. The properties and uniformity of CVD graphene are largely influence by the conditions using during it preparation and to a large extent by the transfer process. A proper optimization of both of these processes is an essential step for any laboratory working on graphene. 
Approach:
  • Proper cleaning and characterization of the copper catalyst foil (Solvent cleaning, Electro-polishing, vacuum annealing or sputtering)
  • Control the evaporation of the copper foil during the annealing and the growth steps of graphene by encapsulation of the foils in a small closure. 
  • Optimization of the polymer support layer during the transfer process, via minimizing the thickness and controlling the adhesion. 
  • Proper design of CVD furnace to avoid leakage and contamination. 

Structural and Morphological changes to CVD graphene upon exposure to Oxygen plasma

Motivation:
Graphene oxide has emerged as a solution-processed route to produce graphene thin films. However, to be the conductivity of graphene oxide films are significantly higher than those of synthesized from CVD, and hence a reduction process is required to restore the properties of pristine graphene. However, it has been challenging to restore the structure of pristine graphene and achieve full removal of the oxide species. A better understanding of the structural, morphological and chemical evolution of the oxide species on graphene can aid in developing better strategies for reducing graphene oxide towards pristine graphene.
Approach:
  • Controllable oxidation of an initially high quality pristine graphene (CVD Single Layer Graphene).
  • Chemical and Structural analysis at each oxidation stage via XPS and Raman spectroscopy
  • Monitoring the morphological changes via AFM.