Final Report Summary - CAMGRAPH (Catalysis driven Manufacture and patterning of Graphene at the nanoscale using probe technologies.)
There have been many successful attempts to grow high-quality large-area graphene on flat substrates. However, the direct growth of graphene on a continuous basis on high-aspect ratio features has not been widely reported. Doing so at the nanoscale has thus far been plagued by significant scalability problems, particularly because of the need for delicate transfer processes onto predefined features, which are necessarily low-yield processes and which can introduce undesirable residues. Our main objective has been therefore to obtain highly functional graphene nanometric 3d surfaces by using thin film deposition techniques for the catalytic graphitization of solid carbon sources as a scalable, clean, inexpensive and less hazardous reproducible technique.
Our one-step methodology consists on sputter deposition process following by annealing to achieve graphene coatings on nanoscale 3d surfaces in situ: a solid carbon film of finite thickness is deposited below a catalyst film and then, by thermal annealing, it is possible to obtain few-layer graphene at the catalyst surface.
On a first stage, we optimised the process so that we were able to create continuous multi-layer-graphene on uneven nanoscale surfaces. That consisted on the identification of the best metal catalyst, optimal thicknesses of the solid source of carbon and the catalyst, the best film deposition methodology and the best annealing methodology and deposition and annealing conditions for our particular system.
We used RF sputtering to deposit carbon and catalyst films directly onto a SiO2/Si flat substrate as well as on atomic force microscopy (AFM) cantilevers at room temperature. The structure of the deposited films starting from the outermost layer was Pt (100nm)/C (30nm)/SiO2 (300nm)/Si. The sample was subsequently inserted in a quartz tube furnace with an argon flow at 800ºC for 30 min. Finally, the sample was rapidly cooled to room temperature under Ar atmosphere by removing it from the “hot-zone” of the furnace. Advanced characterisation techniques such as Scanning Electron Microscopy, Raman were essential for the verification of the graphitic layers on the nanometric features. We demonstrated that this is a clean reproducible technique and is possible on a high-aspect ratio model system such as atomic force probe tips of various radii. And most importantly, that has the potentiality of being a scalable process.
On a second stage of the project we have focused on the fine characterisation of the system: growth characteristics of this technique as well as the film’s superior conduction and lower adhesion forces in air at these scales. In order to do that, we used sophisticated characterisation techniques such as Transmission Electron Microscopy or Atomic Force Microscopy. This sets the stage for such a process to allow the use of highly functional graphene in high-aspect-ratio nanoscale components.
Therefore, this procedure could facilitate the direct integration of electrical and mechanical functionality on nanoscale features. For instance, MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale as well as moving parts such as micromotors, where both stiction and contact reliability are major issues. In the field of AFM probes, low tip-sample interactions would be a benefit for those applications that require high stability or long-term operation. Low tip- sample forces are enabling in applications requiring reduction of the damage of the sample and probe and to avoid creating unwanted artifacts while imaging. Furthermore, because of the aromatic characteristic of graphene, such tips could be used as basis for biological engineering (i.e. Bio-MEMS). For instance, the ability to chemically modify small surfaces may open up new possibilities to study molecular electronic devices or biofunctionalized surfaces down to the nanoscale.
Figure 1 Multi layer graphene-3D High aspect ratio atomic force microscopy probe (AFM probe) characterisation: a) AFM current-voltage curves showing conduction of ML-Graphene vs. Pt and amorphous carbon coated tips. b) Scanning electron microscopy and c) transmission electron microscopy image of the tip covered with ML graphene.
Our one-step methodology consists on sputter deposition process following by annealing to achieve graphene coatings on nanoscale 3d surfaces in situ: a solid carbon film of finite thickness is deposited below a catalyst film and then, by thermal annealing, it is possible to obtain few-layer graphene at the catalyst surface.
On a first stage, we optimised the process so that we were able to create continuous multi-layer-graphene on uneven nanoscale surfaces. That consisted on the identification of the best metal catalyst, optimal thicknesses of the solid source of carbon and the catalyst, the best film deposition methodology and the best annealing methodology and deposition and annealing conditions for our particular system.
We used RF sputtering to deposit carbon and catalyst films directly onto a SiO2/Si flat substrate as well as on atomic force microscopy (AFM) cantilevers at room temperature. The structure of the deposited films starting from the outermost layer was Pt (100nm)/C (30nm)/SiO2 (300nm)/Si. The sample was subsequently inserted in a quartz tube furnace with an argon flow at 800ºC for 30 min. Finally, the sample was rapidly cooled to room temperature under Ar atmosphere by removing it from the “hot-zone” of the furnace. Advanced characterisation techniques such as Scanning Electron Microscopy, Raman were essential for the verification of the graphitic layers on the nanometric features. We demonstrated that this is a clean reproducible technique and is possible on a high-aspect ratio model system such as atomic force probe tips of various radii. And most importantly, that has the potentiality of being a scalable process.
On a second stage of the project we have focused on the fine characterisation of the system: growth characteristics of this technique as well as the film’s superior conduction and lower adhesion forces in air at these scales. In order to do that, we used sophisticated characterisation techniques such as Transmission Electron Microscopy or Atomic Force Microscopy. This sets the stage for such a process to allow the use of highly functional graphene in high-aspect-ratio nanoscale components.
Therefore, this procedure could facilitate the direct integration of electrical and mechanical functionality on nanoscale features. For instance, MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale as well as moving parts such as micromotors, where both stiction and contact reliability are major issues. In the field of AFM probes, low tip-sample interactions would be a benefit for those applications that require high stability or long-term operation. Low tip- sample forces are enabling in applications requiring reduction of the damage of the sample and probe and to avoid creating unwanted artifacts while imaging. Furthermore, because of the aromatic characteristic of graphene, such tips could be used as basis for biological engineering (i.e. Bio-MEMS). For instance, the ability to chemically modify small surfaces may open up new possibilities to study molecular electronic devices or biofunctionalized surfaces down to the nanoscale.
Figure 1 Multi layer graphene-3D High aspect ratio atomic force microscopy probe (AFM probe) characterisation: a) AFM current-voltage curves showing conduction of ML-Graphene vs. Pt and amorphous carbon coated tips. b) Scanning electron microscopy and c) transmission electron microscopy image of the tip covered with ML graphene.