AmericanChemicalSociety.com

Project Overview

Graphene is the building block of graphite and most carbon nanomaterials (such as carbon nanotubes). This project studies the fundamental materials properties of single and few layer graphene, and how molecular hydrogen may interact with graphene, motivated by the potential of hydrogen adsorption and storage by graphene-based nanomaterial and nanostructures. Contrasting most previous studies on hydrogen adsorption (using a bulk amount of nanomaterials), our study focuses on experimentally investigating H₂ adsorption on a microscopic level (isolated, single atomic layer of graphene) to achieve a fundamental understanding of interaction of hydrogen with graphene (2D carbon) and characterize/optimize the storage of hydrogen on or between graphene surfaces.

Research Accomplishments Up to Date

Graphene Material Preparation

In our first grant year, we have successfully fabricated graphene using two methods.

• We have mastered the popular technique of producing high quality single layer and few layer graphene by mechanical exfoliation (using scotch tapes) from bulk graphite. 
• We have also obtained high quality and large-size graphene layers using CVD-based synthesis on Ni and Cu.  Initial work was in collaboration with Dr. Qingkai Yu (University of Houston). More recently, we have set up our in-house CVD furnaces and started growing CVD graphene on Cu locally at Purdue.

Characterization of Graphene Materials Properties

We have characterized the materials properties of the graphene we fabricated using a combination of experimental methods.

• Electronic transport --- both electric field effect (tuning carrier density via a back gate, typically the doped Si substrate) and magnetotransport. Carrier density and mobility are characterized. Other electronic transport properties or phenomena, such as quantum Hall effect, weak localization (probing phase coherence and carrier scattering), have also been observed. 
• Optical characterization, particularly Raman spectroscopy. Thickness (number of graphene layers), uniformity, quality (amount of defects) of graphene samples are routinely characterized by Raman measurements, which probe lattice vibration modes of graphene.  A new scanning Raman microscope with pressure/temperature controlled sample stage has been setup in PI's lab.
• Scanning probe microscopy of graphene surface, such as AFM (atomic force microscopy) and STM (scanning tunneling microscopy). We have used STM to observe atomic resolution images of single layer graphene lattice. 

Interaction of Hydrogen with Graphene and Effects on Graphene Material Properties

We have started investigating how prolong exposure to H₂ or hydrogenation can affect various material properties of graphene. Some examples are given here. 

Electronic Transport Properties

We have measured how prolonged exposure to pressurized H₂ affects the electrical transport properties of graphene. The experiments were performed with graphene field effect transistor devices placed in a vacuum chamber connected with a H₂ source of controlled pressure.  We found that prolonged exposure to H₂ can shift the charge neutral point (Dirac point) of graphene to become smaller, and even negative. The findings may indicate that H₂ molecules are adsorbed on graphene and displace other surface adsorbates (eg. water molecules, which hole-dope graphene) and they themselves electron-dope graphene. 

Other studies

In one experiment, we exposed graphene in H plasma (by dissociating in a microwave plasma cleaner H₂) and performed Raman and magnetotransport measurements on graphene treated by such a process.  The findings suggest that short range disorder is created in graphene. Possible relation to hydrogenation (“graphane” formation) is being investigated.  In another experiment (in collaboration with Drs. Qingkai Yu and Jiming Bao in University of Houston), a 1nm-thick Pd metal layer is deposited on graphene and it was found that such the resistance of such Pd-decorated graphene is highly sensitive (in a reversible way) to H₂ exposure.  Finally, we have performed molecular dynamics simulations (based on ab-initial C-C and C-H potentials) on simple graphene and graphene-hydrogen nanostructures. For example, we investigated how hydrogen passivation of edges of graphene nanoribbons (GNR) may affect the thermal conductivity. We found that H-passivation significantly reduces the thermal conductivity. 

Further studies and plans for next year

We have initiated a number of studies that we plan to continue in the next year to systematically investigate hydrogen-graphene interaction:

• FTIR (Fourier transform infrared) and Raman spectroscopy of graphene after subject to pressurized H₂. Studying the spectra and their dependence on the temperature and H₂ pressure.
• A collaboration has been initiated with Argonne National Laboratory's Center of Nanoscale Materials (CNM) to use their high resolution STM user facilities to study graphene. We plan to image graphene surface and H₂ molecules/H atoms adsorbed on graphene under various temperatures and pressures.
• Analyzing our experimental results for in-depth understanding of the basic mechanisms of hydrogen-graphene interactions. Collaboration with several theorists working on ab-initio simulations is underway.
• Building on our fundamental studies on hydrogen-graphene interaction, we will start investigating, by experiments and simulations, the potential of H adsorption in more complicated graphene based nanostructures and nanomaterial architectures.