Commercially available FIB systems are usually provided with spot-size specifications defined via imaging resolution. Hence, it is practically impossible to extract information on the beam tails in this way. With the rise of graphene  and technological developments of creating suspended graphene it becomes possible to study its interaction with FIBs . The key property of freestanding graphene in this case is its atomic thickness. FIBs interaction with such a self-suspending layer brings no collision cascade. Instead, atoms of the 2D membrane material are sputtered only by direct binary collision events with incident ions.
Recent Developments in Focused Ion Beam and its Application in Nanotechnology
In fact, it was shown that FIB interaction with 2D materials contains information about beam profiles [14,17,18]. Here we show that it is possible to fabricate pores in graphene membranes smaller than the ion beam diameter by carefully tailoring the exposure dose. The pore diameters directly depend on the time for which individual pixels are exposed to ion beams, and in return this dependency reflects information of the ion beam profile. We determine a Ga-FIB point spread function and verify its Gaussian profile for different beam current values. The volume under the Gaussian profile is used to extract the graphene sputtering yield in good agreement with previously reported values .
Helium focused ion beams are characterized in a similar way to determine the previously unknown profile and possible asymmetries in it. Under certain conditions triangular He beam shapes are observed, possibly attributable to the trimer nature of the beam source. All methods give similar results regarding the measured focused ion beam profiles. Finally, we discuss technical limitations and critical steps towards ion beam profiling using this method. One of the most crucial aspects of ion beam profiling via the direct interaction with suspended graphene is the preparation of the ultraclean graphene membranes.
First, graphene was grown on a copper foil using chemical vapor deposition CVD according to a previously developed method . This method is optimized for maximal grain connectivity resulting in uniform graphene films. A PMMA-based graphene transfer method with copper foil etching in ammonium persulfate was used.
As a result clean freestanding graphene membranes are obtained [14,15]. The smallest beam aperture gives a beam current value of 1. A suspended graphene layer is then exposed in a single-pixel exposure mode.
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A clear dependence of the pore size on the exposure dose is observed. It is also seen that pores can have slightly irregular shapes. In order to precisely extract pore diameters from such experiments, we perforated sets of pores with identical exposure parameters. ImageJ  is exploited for automatic contrast detection and extraction of the pore area indicated in red. The diameters are calculated from the pore areas, assuming a round shape of the pores. Mean values and standard deviations are extracted from these histograms. This method includes control over statistical variations and allows for precise pore-diameter measurements.
Throughout this paper we use this method to extract pore-diameter values for each parameter set of the ion beam exposure.go to link
Defect Control and n-Doping of Encapsulated Graphene by Helium-Ion-Beam Irradiation | Nano Letters
Pores diameter increases for longer dwell time. Scale bar is 50 nm. The red color indicates the pore areas used to calculate the diameters. Dots represent extracted mean values from the corresponding dose histograms, as shown in c. Lines are drawn for visual guidance. Pores within the same row are created with the same dwell time, indicated to the left of the image. Scale bar is nm. Pores diameter increases Data for two instrument apertures corresponding to two different beam currents are presented: squares for 1.
Pore diameters steeply increase for the low exposure doses and then gradually increase for larger doses. For 1. Dwell times of 0. For such small pores, STEM resolution comes to its limits. Carbon atoms of the graphene membrane will be sputtered only to the direction of the incident ion momentum. Ga-FIBs are known to have Gaussian beam profiles to a large extent . First, the rather flat top part of the beam profile will create enough collision events with graphene to open the initial pore.
This process is unstable and very sensitive to the dwell time. Thus, initial pore formation can be seen as a stochastic process. Once the initial pore is created the graphene material around it will be further removed by the bombarding ions within the Gaussian bell.
The growth of the pore diameter with dwell time is eventually slowed down, thereby reflecting the tail of the beam profile. The pore growth dynamics is defined very precisely by the incident ion beam distribution. Thus, the point spread function PSF of the incoming FIB can be extracted from the dependence of the pore diameter on the dwell time. The main difference is that in case of interaction between ion beam and resist there is a finite volume involved, resulting in a less precise transformation from the shape of the exposed resist to the PSF of the beam.
In the case of the suspended 2D layer the finite interaction volume is minimized to only one atomic layer, which ultimately sets the closest relation between the patterns created and the beam shapes. When focused ion beams with a certain PSF irradiate suspended graphene for a time period dwell time , a part of the graphene layer where the exposure dose is higher than a critical dose value defined by sputtering yield will be removed to yield a pore of a certain radius.
Therefore, a plot of inverted dwell time vs pore radius would represent the PSF of the beam. For fitting Gaussian functions to the experimental data the following relations are used:. The FWHM for the 1. Specifications of the instruments give 7 nm at 1.
Actually, to get this low beam current the smallest aperture has to be used. In our case the aperture was slightly worn out resulting in a higher current. This would explain the small deviation of measured FWHM from the specifications of the instrument. Another important parameter of the Gaussian curve is its bell volume, V 2D. For the 1. The solid line belongs to the reference report The dotted line is the SLG strain-stress plot at the strain rate of 0.
In all next strain-stress tests, the strain rate was set to 0.
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As shown in Fig. The reason for choosing a higher rate of the strain was increasing the speed of simulations, and reducing the time of the test process.
By changing the strain rate from 0. As previously stated, by changing the irradiation parameters, states are created in which the planes have no connection with each other, or at least there is no bond between the two graphene layers. In this study, all states with at least two non-bonded sheets lacked the necessary requirements of the strain test and therefore no mechanical test was performed on them.
These states are depicted in Fig. Strain-stress plots of graphene stacks. The top axis represents the diameter of the beam used, and the left axis represents the distance between the graphene layers in the stack. Each of the graphs results from the average of five strain-stress tests on the five configurations created under equal conditions.
The reason for this increase, as seen in the image of the structures formed after irradiation Fig. In fact, as the diameter of the beam increases, more carbon atoms are separated from the higher layers and transmitted through the interlayer thereby providing the necessary material to create higher density bonds with a longer chain length.
The other parameter, ion beam energy, does not have a significant effect on the results, so that in some cases it is seen that the energy of the collision increases the strength and in some cases, in reverse, it reduces the strength. This difference, more than it is related to the beam energy, seems to be due to system fluctuations and collisions and randomized events due to ion collisions on graphene sheets. Although the results were averaging of five separate tests with identical conditions, it should be noted that in the case of Fig. Other configurations have been excluded from the mechanical test due to the lack of links at least between two layers of graphene.
As is observable in Fig.