# The Attachment of Carbon Nanotubes to Atomic Force Microscopy Tips Using the Pick-Up Method
## 1 Introduction
The method involves simply scanning the AFM tip in tapping mode across a CNT covered surface. Van der Waals forces are sufficiently strong that occasionally a CNT will detach from the surface and adhere to the AFM tip. This typically causes a change in image resolution as the diameter of the attached CNT will typically be smaller than the diameter of the AFM tip. This method has been used by research groups to attach CNTs to AFM tips and have been used to analyse a variety of sample surfaces [59; 60]. However, one of the drawbacks of the method is that only vertically aligned CNTs will adhere to the AFM tip and these make up a small proportion of CNTs on a typical CVD grown CNT surface. [PERSON] et al. [60] also suggested that after some time has elapsed the vertically aligned CNTs may collapse onto the surface and, therefore, CVD produced CNT surfaces may have a limited lifetime for this time of attachment method. [PERSON] et al. [61] demonstrated how high-force tapping mode improves the rate and stability of CNT attachment to AFM tips using the pick-up method. This has the advantage of not only potentially adhering CNTs that are vertically aligned but also CNTs that are laying down on the substrate. [PERSON] et al. [47] also used high-force tapping mode imaging to demonstrate how this technique could be used to stabilize images produced using CNT-AFM tips. SEM images indicated that high-force tapping mode improved CNT-AFM performance by straightening and/or shortening the attached CNTs. There are still challenges associated with the fabrication and implementation of CNT-AFM probes. The strength of attachment of the CNT to the AFM tip and the imaging stability of the CNT-AFM probe are crucial to their performance. [PERSON] et al. [62] demonstrated that certain imaging modes, such as peak-force tapping mode, may provide advantages to using CNT-AFM probes. The other major challenge is that the techniques to produce CNT-AFM probes tend to only attach CNTs to AFM tips individually. Some research has been done using CVD to fabricate multiple CNT-AFM probes in a single process, but they have limitations including non-uniformity of growing conditions. As yet, no AFM probe manufacturer offers CNT-AFM tips on the scale of a wafer and indeed it is becoming increasingly difficult to purchase these types of probes. In this work we demonstrate, for the first time, the pick-up method can also be used to attach CNTs to two AFM probes simultaneously. This proof of concept is significant, and we will discuss how the pick-up technique could be further improved to produce larger numbers of CNT-AFM tips in a single process. This has potential applications in not only AFM imaging but AFM-based data storage devices such as the \"Millipede\" [63; 64; 65] or \"Snomipede\" [66] which require large arrays of cantilevers. These types of instruments have the potential for considerable data storage densities. [PERSON] et al. [67] demonstrated that 500 Gbit/in2 can be achieved using thermo-mechanical writing and readout in thin polymer films with bit sizes of 30-40 nm each. The performance of such devices could be improved by using CNT-AFM probes due to their wear characteristics, chemical stability and small diameter. The pick-up method has been primarily used to attach CNTs to tapping mode AFM probes and possess relatively large spring constants (e.g., greater than 5 N/m). For contact mode cantilevers with spring constants typically less than 0.5 N/m the pick-up method is less effective. This is because capillary forces on the sample surface can cause the tip to adhere to the surface and prevent or disrupt the tip from disengaging from the surface and therefore makes stable CNT attachment difficult to achieve. We demonstrate that high-force tapping mode can be used with contact mode probes to achieve stable CNT attachment. These low spring constant CNT-AFM tips can then be used in tapping mode which will reduce imaging force compared to standard tapping mode probes.