The field of graphene research is quickly developing due to graphene’s unique physical, electrical and thermal properties.  Its performance in the lab has garnered it a lot of attention from the scientific community. Therefore, having the ability to cut graphene sheets precisely became an important goal in developing the correct shapes needed for both researchers and industry as graphene is introduced into new applications and processes from photovoltaics to chemical sensors.  Currently, no techniques are available which allow for such an action. This research, using silver nanoparticles to catalyze and etch trenches, see the video, across the surface of nanometer thick graphene sheets, illustrates a new reaction which yields reproducible results.
MATERIALS & METHODS
This catalytic reaction experiment began by producing silver nanoparticles via an aqueous solution of water and silver nitride being dropped on a highly oriented pyrolytic graphene (HOPG) surface.  The reactions were carried out at an elevated temperature of ~650 °C.  The elevated temperature allows for the degradation silver nitride into these silver particles and other side products.  The development of catalytic nanoparticles by contact between the aqueous solution and the surface defects of the graphene is assumed from previous literature. 
Varying timeframes were implemented for determination of the correct period in which the silver nanoparticles can successfully begin contacting and reacting with the graphene surface. Scanning Force Microscopy (SFM) and High Resolution Scanning Tunneling Microscopy (STM) were the methods used to visualize and determine if the silver nanoparticles were able to react with the graphene layers and therefore produce the etching, or trenching, effect.  Annealing of nanoparticles which were adhering to step edge deformities in the graphene layers was done at the 650 °C temperature mark in a quartz tube. The preheated quartz tube had a diameter of 3.0cm. 
The instrument used for temperature control during the reaction and annealing process was “a model 6100 PID West Instruments controller equipped with Pt/Ir thermocouple” which was placed under the holder containing the graphene sample.  A fluctuation of the temperature down to ~630 °C following each addition of the silver nitride solution was noted for each iteration, and lasted approximately 40-50s.  The control for this experiment was a graphene layer annealed at 650°C without the addition of silver nitride, which showed no abnormalities in surface morphology after a 60 second process. An overview of the carbon oxidation reaction process of the silver nanoparticles reacting with atmospheric oxygen and graphene to produce CO and CO2, is depicted in Figure 2.
The differing timeframes yielded different results, with SFM showing annealing from 30 seconds to yield zero change in surface morphology, while longer annealing times showed drastic changes and a variation of trench types were seen.
Three main types of graphene etches were categorized: straight segmented tranches, spiral trenches, and zig-zag trenches. SFM imaging showed that larger particles produced tranches which were significantly longer and deeper than their smaller nanoparticle counterparts with the longest being upwards of 9.0 μm in length  Rates of tunneling were calculated by the assumption that the particles start to form and react after the initial 30 second interval. In essence, the temperature has to get high enough to break down the silver nitride into the catalytic particles, which then become active after ~30s. This yielded a maximum speed of ~250nm s-1.
At this point the high resolution STM was used to see the texture and overall structure of the channels etched into the HOPG surface (figure 3), and this instrument subsequently revealed the trenches to be smooth in nature and the surface morphology of each sample was shown to yield a large variation in trench types between each iteration as well as varying aspects, such as trench depth and width. 
The formation of spiral and zigzag trenches, formed by the oscillations of rod-shaped silver nanoparticles as they move across the graphene surface, is proposed to be due to “poisoning of the metallic silver by air pollutants like silver dioxide”. These pollutants effectively cause the nanoparticle to cease catalyzing the graphene edge, which when it occurs at one edge of the rod particle causes the particle mass to rotate, see Figure 4. The annealing in presence of sulfur, and ventilated sulfur, showed the catalytic activities to cease in comparison to when sulfurs presence was lacking. 
Different shaped nanoparticles can affect the outcome of trenching as well.  The reaction rate of the carbon oxidation was studied and it was determined that the adsorption of molecular oxygen onto the silver nanoparticle surface was the limiting factor.  I am not going to get deep into kinetics here though.
|Trench Length||9.0 uM|
|Trench Width||6-100 nm|
|Max Speed||250 nm s-1|
CONCLUSIONS & CURRENT RESEARCH
In conclusion, researchers have demonstrated trench channeling in layered graphene by silver nanoparticles at speeds of up to 250 nm/s. While most trenches appear to be segment-wise straight, some particles channel spiral trenches, which we attributed to inhomogeneous catalytic activity of the particles due to poisoning by pollutions like sulfur dioxide. High-resolution STM imaging of the trenches reveals that most of the trench edges are very smooth; this and the high speed of particle channeling motivated us to discuss application of this phenomenon for the development of a new high precision lithography on graphenes.
Further research since this publication in 2009 has occurred, and involves the application of silver nanoparticles to a probe tip as discussed, as well as research focusing on the graphene cutting speed dependent on the various shapes and sizes of the silver nanoparticles.
Some of the shapes inspired by the art of cutting paper, or kirigami, can be seen in the following figures.
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- HOPG Detailed Description, http://nanoprobes.aist-nt.com/apps/HOPG%20info.htm, 2014, accessed: 8 October 2015.
- Blees, M.; Nature-Video Research-Graphene Kirigami. http://blogs.nature.com/aviewfromthebridge/2015/07/29/graphene-structures-at-the-cutting-edge/, 2015, accessed: 14 September 2015.
PowerPoint Presentation: PPT file