Frequently Asked Questions

Graphene on Cu Graphene on Cu with PMMA coating Graphene on Substrate TEM GridsDevice Fabrication Graphene Oxide Reduced Graphene Oxide

Graphene on Cu (CVD Graphene on Cu)

A: No. Graphene bottom layer has to be removed before transferring onto the desired substrate. Different methods can be used such as oxygen plasma, Reactive Ion Etching (RIE) or chemical reactants such as diluted HNO3.

A: The main orientation is (100).

A: We recommend to keep the samples in vacuum or inert atmosphere in order to avoid copper oxidation.

A: The Rms is around 100nm.

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

A: The PMMA can be removed using organic solvents or thermal treatment. Our standard process removes the PMMA with solvents. In order to improve adhesion before PMMA removal, we recommend a thermal treatment at 150ºC for 1h in inert atmosphere. After cooling down, the PMMA should be removed very shortly to maintain the effect of the treatment. Using acetone and IPA baths, the sample is submerged for 30min in each one, then dried with a N2 gun. Use of an ultrasonic bath should be avoided in order to prevent detachment of the graphene film.

Thermal treatment up to 450ºC in inert atmosphere leaves fewer residues. In this case, the Raman spectrum is modified due to some strain induced during the treatment and polymer polymerization. Also, graphene gets more p-doped and the mobility decreases compared to the solvent-treated samples, due to a stronger adhesion of graphene to the substrate after thermal treatment, and the activation of the ability of diatomic oxygen to accept electrons from graphene.

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 around 60nm.

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 - PEN)

 A: For most of the purposes, graphene can be directly used without any additional cleaning. However, in those cases where an extra cleaning is needed (STM experiments, for instance), a thermal annealing can be applied to the sample. We recommend annealing at temperatures between 250ºC-450ºC under inert atmosphere in order to reduce absorbents on the graphene surface. It is important to take into account that above 300ºC the electrical properties of graphene will be slightly deteriorated due to the interaction with the substrate and the increased reactivity of the surface. Also, this process is not compatible with flexible substrates.

A: It has to be done under dry conditions. When using wafers such as Si or quartz a diamond pen can be used to cleave it. In order to protect the graphene film from debris, we recommend doing it with the protective PMMA layer on top of graphene. In this case, we can provide you the sample with the PMMA on top.

When using thin substrates such as PEN or PEN you can easily cut them using scissors.

A: Our graphene on SiO2 is p-doped, with a charge carrier density of around 1013 cm-2. This intrinsic doping can be reduced by at least one order of magnitude by thermal treatments, which lower the Dirac voltage down to 40-80 V. Another alternative is using a passivation layer on top of the graphene, which prevents the presence of water between the substrate and the graphene film.

TEM Grids (CVD Graphene on TEM Grids)

A: Yes, it is.

A: Our standard product is a Quantifoil TEM grid of Au. The holes are 2microns and the total thickness is about 12 microns.

A: Yes. The customer can provide us with their own grids with the desired hole size. The maximum size that we can cover with good coverage is up to 7microns. We can transfer up to 40microns but the coating percentage decreases.

A: The standard film contains around 10% of residues. An annealing treatment can be performed in order to achieve a cleaner graphene film.

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). However, MF-319 and similar TMAH-based developers must be carefully used because they can
result on graphene detachment from the substrate (see next question).

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.

Regarding the fabrication process, using freshly coated resist is very important. The use of basic developers, such as TMAH-based ones, can result on graphene detachment from the substrate if the process is not properly controlled. For instance, the developing time is a crucial parameter on this process: in order to minimize the probability for detachment, the developing time must be reduced as much as possible. We strongly recommend a previous optimization on a bare substrate in order to know the exact developing time for each resist and developer.

When working with 1x1 cm2 samples, this issue becomes critical: due to the small area of the sample, the resist coating might not be uniform, ending up with slightly thicker resist at the edges of the substrate. During the developing in these samples, the resist on the edges might still be present once the structure on the central part of the sample are already well-developed. At this stage, if the sample is kept on the developer for a longer time, the probability for detachment increases. Therefore, we recommend to keep the developing time needed for the central part of the sample. In case the presence of resist at the edges of the sample is critical for the process, we recommend using an extra step before the structure patterning for removing the excess of resist at the edges.

This process works in a similar way for developers diluted in H2O: in this case, the developing time must be increased with respect to the non-diluted developer, which in some cases makes the developing process more controllable.

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.