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Frequently Asked Questions

Graphene on Cu Graphene on Cu with PMMA coating Graphene on Substrate TEM Grids

Device Fabrication CVD Graphene Graphene Oxide Reduced Graphene Oxide

Graphene on Cu (CVD Graphene on Cu)

A: The backside of Copper is partially removed, but not completely, so an additional treatment like RIE is needed before transfer to eliminate the bottom layer totally.

A: The Cu foil is pollycrystalline. The main orientation is XXX determined by X-ray Diffraction.

A: We recommend to keep the samples in vacuum or inert atmosphere if you are not going to use them in a while, in order to avoid copper oxidation.

A: The Cu is not atomically flat. It has 100nm roughness

Graphene on Cu with PMMA coating (CVD Graphene on Cu with PMMA coating)

A: PMMA is used as a protective layer, and this layer can be removed using organic solvents (acetone and IPA) or with thermal treatment. Our standard process is to remove the PMMA with solvents. As an alternative a thermal treatment can be done. This thermal treatment leaves less amount of residues, but the Raman spectra is modified due to the strain created during the treatment. Although the electrical properties of the graphene remain intact thermal treatment is avoided for not modifying Raman signal.

Graphene is transferred via wet process. In order to etch the copper, the graphene is protected with a sacrificial polymethyl methacrylate (PMMA) layer. A ferric chloride solution is used for etching. Once the etching is complete, the graphene is washed and transferred onto the desired substrate. Finally, the PMMA layer is removed using organic solvents.

A: The thickness is up to 200 nm but typically we use a thinner PMMA layer of 60 nm.

A: No, we cannot. The PMMA is only a supportive layer for the transfer method.

Graphene on Substrate (CVD Graphene on SiO2/Si – Quartz - PET)

 A: Usually additional cleaning process is not needed to obtain good electronics results using our graphene directly. If additional cleaning is needed a "thermal annealing" can be applied, typically at 350-400C under inert atmosphere for 30mins but this recipe depends on the annealing oven to be used.

 

A: It has to be done in a dry way. Two alternatives:

1) Use a diamond pen to cleave it, but then you can have some debris on the graphene due to the Si wafer cleavage.

2) In order to protect it from the debris, you can cleave it using a protective layer such as the PMMA on top of graphene. After cut it with a diamond pen remove the PMMA with solvents. In this case, we can provide you the sample with the PMMA already on top.

We recommend to blow the substrate with a N2 gun.

TEM Grids (CVD Graphene on TEM Grids)

A: The TEM grid substrate is a standard Quantifoil TEM grid Au coated. It is 500k in vacuum and X-ray Resistive.

A: The total thickness is about 12 microns which is the thickness of the grid. The thickness of the carbon is only about 0.345 nm, i.e., it is negligible in comparison to the grid thickness.

A: Our standard Suspended Monolayer Graphene on TEM Grids is transferred on holes of 2microns. You can provide your own grids with your desire size and we can transfer on them. The maximum size that we can cover with good coverage is up to 7microns.

A: There is around 2-3% of hydrocarbon contamination in the surface. An annealing at 300C to clean areas can be done and to be measured with a TEM microscope.

Device Fabrication and Electrical Characterisation

A: Graphene can be contacted by metallisation. The most widely used contacts are Ti/Au, Cr/Au and Ni, because they result in relatively low contact resistances compared to other metals. The lowest contact resistances are obtained with side-contacts, due to the better coupling of the metal to the graphene film.

Once the contacts are fabricated, an annealing step can help to further reduce the contact resistance: either thermal annealing in vacuum or in an inert atmosphere, or a current annealing with a relatively large current (10 8 A/cm 2 ).

A: Graphene can be coated by a thin layer of Al (2-3 nm) prior to processing to avoid polymeric residues. This Al layer can be removed later by using some developers (for example, MF-319 developer).

A: -We highly recommend carrying out device fabrication (especially the developing step) in an environment with less than 40% of humidity to try to avoid detachment. Besides that, thermal annealing just before the fabrication of the device (300 ºC, 9h in inert atmosphere) can also be helpful.

When using TMAH-based solvents, diluting them in water strongly reduces the detachment problem. In the particular case of AZ developer, we recommend a dilution of 1:3 AZ:H 2 O, with a previously optimised developing time.

CVD Graphene (CVD Graphene on SiO2/Si – Quartz - PET)

A: p type doping, charge carrier density can vary depending on the device fabrication method and type used to calculate the mobility but we have measured values around 2 x 10^12.

A: We can do HMDS treatment of the SiO2 surface but it is not part of our standard process. If you need HMDS treated Si/SiO2 wafer please contact us. *

A: The electron mobility values depend heavily on the method used to determine them but typical Hall mobility values for our graphene samples are around 2000-2700 cm^2/Vs with very good uniformity. These mobility values apply to your samples as well since they were manufactured using the same methods (recipe, transfer, etc.).*

A:

A: The grain size distribution is random, the smallest grains you can find are about 2 microns length and the biggest grain about 10 microns length.

A: Our graphene is doped due to the transfer method we use and the Dirac point is usually quite high (40-80V) in our samples, since the doping is also high (n= 8x10E12cm2). In order to decrease it we recommend applying a thermal annealing at 250ºC for 2h in N2 before measurements. Another alternative is using a passivation layer

A: The work function of the graphene is 4.6eV. This value can be modified applying external doping such as different oxides layers (MoO3, Va2O5,...)

Graphene Oxide

A: GO is extensively washed and some metal traces can be found in ppb.

A: The GO is a polydisperse material where most of the particles have a size around 15 μm, as shows the next results measured by laser diffraction:

- D90 29.05 - 32.9 μm

- D50 14.30 - 16.6 μm

- D10 5.90 - 6.63 μm

A: The surface area in dispersion form is not possible to measure since most techniques require the dry form and GO tends to agglomerate when water is removed. All GO surface is exposed to the water when it is in dispersion form so the surface area should be very high. The GO flakes are monolayer thick (2nm measured via AFM) in dilute dispersions.

A: In general terms, the less you sonicate the dispersion the bigger the flakes are.

A: Our standard dispersion has a 4 mg/mL concentration. At this concentration flakes tend to stack so in order to get monolayer flakes it would be recommendable to dilute it to 0.5 mg/mL and sonicate further. After this process 95% monolayer content can be achieved*

A: The GO is inherently acidic due to the presence of acidic functional groups so the acidity is not an indication of purity or lack of it. The acidity is more related to the concentration of the GO in the water. The higher the concentration the greater the acidity.

A: Our GO is dispersed in water because it is very stable and no surfactant is needed. GO is functionalized with epoxy, alcohol and COOH groups on its edges. These functional groups make GO highly hydrophilic and also it loses part of its sp2 hybridization, making GO insulating. In order to recover conductivity is has to be reduced. We can work with other solvents, please contact us to know more about it.*

A: The average flake size is from nm up to 10 microns

A: Graphene Oxide contains oxygen functional groups (hydroxyl, carboxyl and epoxy groups) that make water dispersions very stable and no surfactants are needed.

A- It has to be taken into account that the dried GO is formed with GO flakes that contain oxygen functionalities as well as intercalated water. When the material is heated with a fast ramp (>5º/min), at T between 150-180ºC the gases are released very fast and that is what makes the volume of the material to increase dramatically and to have what it seems an "explosion". However, this phenomena is not observed when heating the material at slow ramps (2º/min) as the gases are slowly released and the increase of the volume is not observed.

Reduced Graphene Oxide

A: rGO is obtained afer a chemical reduction of GO. As a consequence of this reduction part of the sp2 hybridization is recovered and also its conductivity. In the reduction process also part of the functional groups are lost, making rGO hydrophobic. In order to dispersed it in water surfactants have to be added. Also rGO can be dispersed in low concentrations in NMP or DMF.

A: In the rGO most of the functional groups have been reduced. Although there have been several research works about it there is still a lack of information about the structure of both GO and rGO. As in can be observed in XPS data, the C-C bonding quantity increases considerably whereas the C-O bonding decreases. At the same time, a reduction in the carbonyl groups is observed. Unfortunately there is no quantitative data about the functional groups present in the rGO, some carboxyl, epoxy and hydroxyl groups remains can be present.

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