# The Attachment of Carbon Nanotubes to Atomic Force Microscopy Tips Using the Pick-Up Method
## 1 Introduction  
Atomic force microscopy (AFM) has now been commercially available for over 30 years and the technique has been applied to analysing a diverse range of samples including, for example, living cells [1, 2], hair and wool fibres [3, 4], nanomaterials [5, 6], DNA [7, 8] and polymers [9, 10]. AFM has also been applied to measuring the strength of chemical bonds [11, 12] and surface forces [13, 14] as well as determining the mechanical properties of a variety materials [15, 16, 17]. Over the years more and more functionality has been developed and built-in to commercial systems and there are now a large number of imaging modes beyond the standard contact mode, tapping mode and force distance spectroscopy that have increased the power of this technique. Indeed with the development of high-speed AFM [18, 19], imaging modes such as TREC [20, 21, 22], Peakforce tapping mode [23, 24], calibration procedures to precisely quantify force [25, 26, 27, 28] and tip shape [29, 30] as well as combining AFM with other types of microscopy [31, 32, 33, 34] this technique will continue to develop and demonstrates considerable promise into the future. However, what remains consistent in the acquisition of AFMdata is that the resolution and measurement accuracy of that data is strongly dependant on the AFM tip diameter, shape and durability. AFM tips are typically composed of silicon or silicon nitride and have a conical or pyramidal shape. One method that has been explored to optimise tip shape parameters and increase tip lifetime is attaching carbon nanotubes (CNTs) to AFM tips. CNTs are good candidates for this purpose due to their structural, chemical and mechanical properties [35]. These properties have seen CNTs applied to a large number of research areas including, for example, solar cells [36; 37], enhancing the performance of materials [38; 39], sensor applications [40; 41] and desalination [42; 43]. The advantages of using CNTs as AFM tips include nanometre-scale diameter, high aspect (length-to-diameter) ratio, excellent thermal and electrical conductivity and significantly greater resistance to wear than conventional silicon AFM tips [44]. Methods exist to produce CNT-AFM probes including chemical vapour deposition (CVD) [45; 46], dielectrophoresis [47; 48] and manual attachment using electron microscopy [49; 50]. While the development of CNT-AFM probes has potential benefits it is worth noting there now exist a range of AFM tips available that satisfy many of the advantages of CNT-AFM probes. Examples of these types of probes are summarized in Table 1.  
Table 1 shows the nominal parameters for AFM probes with parameters similar to CNT-AFM probes. Despite the potential advantages of CNT-AFM probes their application in AFM has not been as dramatic as early indications would suggest. Difficulty in attaching CNTs to AFM tips, scaling up the attachment process to multiple probes and the parallel technological development of silicon probes have seen their use limited. It should also be noted that CNT-AFM probes experience scanning artefacts unique to their usage. [PERSON] et al. [51] demonstrated that adhesion between the sidewall of the CNT and surface structures can produce a \"ringing\" artefact in AFM images. However, recent research has demonstrated that CNT-AFM probes are effective in determining the thickness and layer number of 2D nanomaterials such as graphene [24]. Two other areas that CNT-AFM probes can be beneficial is in their wear resistance and as AFM probes to perform conductive AFM imaging and measurements. While the technology for producing sharper AFM tips made from silicon has advanced considerably (refer to the nominal tip diameter for Scan Asyst probes) the smaller the diameter the tip is the higher the pressure on the tip while imaging and the greater the probability of blunting and wearing the silicon tip. [PERSON] et al. [52] demonstrated that CNT-AFM probes have at least 20 times the lifetime of silicon probes and sample wear is also minimized using CNT-AFM probes.