Controlled nanodot fabrication by rippling polycarbonate surface using an AFM diamond tip
© Yan et al.; licensee Springer. 2014
Received: 16 June 2014
Accepted: 24 July 2014
Published: 30 July 2014
The single scratching test of polymer polycarbonate (PC) sample surface using an atomic force microscope (AFM) diamond tip for fabricating ripple patterns has been studied with the focus on the evaluation of the effect of the tip scratching angle on the pattern formation. The experimental results indicated that the different oriented ripples can be easily machined by controlling the scratching angles of the AFM. And, the effects of the normal load and the feed on the ripples formation and their periods were also studied. Based on the ripple pattern formation, we firstly proposed a two-step scratching method to fabricate controllable and oriented complex three-dimensional (3D) nanodot arrays. These typical ripple formations can be described via a stick-slip and crack formation process.
KeywordsAtomic force microscope (AFM) Polycarbonate (PC) Scratching Ripples
Nowadays, organic polymers have replaced many traditional engineering materials because of their superior performance and low cost . The polymer-based nanoscale structures are used in a wide range of important applications, such as bioengineering, grating sensors, binary optics, and so on [2, 3]. One of the essential technologies used to fabricate nanoscale structures is atomic force microscopy (AFM), which is a tip-based nanomechanical machining method that possesses the advantages of precise spatial resolution, in situ imaging, and other unique features, including the inexpensive device, relatively easy control and operation . Especially, the AFM-based friction-induced nanomechanical method, which belongs to one of the AFM-based nanofabrication methods, is looked on as a new way for forming complex nanostructures [5, 6].
Ripple patterns can exist over a range of length scales including macroscopic linear ripples on sea and desert sands created by wind , microsized ripples on surfaces of metal substrates produced by ion sputtering , and nanoscale ripples on the surfaces of thermoplastic polymers obtained by an atomic force microscope (AFM) tip’s reciprocal scanning . In particular, it has been found that ripples can be formed on polymer surfaces by single scanning with an AFM tip. Acunto et al. [10, 11] reported that ripple patterns could be formed with a small applied load and single scanning on the surfaces of solvent-containing polyethylene terephthalate (PET) films. Gnecco et al.  reported that linear ripples with the period of 100 to several hundreds of nanometers can be produced by a heated AFM tip on the surfaces of polycarbonate (PC), poly (methyl methacrylate) (PMMA), and PSul films, and the ripples could also be obtained with circular scanning. The main mechanisms for the tip-induced ripple formation including Schallamach waves, stick-slip, and fracture-based deformation [9, 13, 14] have been proposed. The Schallamach waves are reviewed as the inability of the rubber surface under high shear forces . The stick-slip mechanism is the competition between the tangential force and the critical tangential force . And, the fracture-based deformation is perceived as the existence of the cracks in the deformed materials . All of the mechanisms are just the proposed model. They cannot be clearly conformed and came to an agreement for explaining the ripples’ formation. So, the mechanism for the process of such ripple formation is still controversial.
As mentioned above, just simple ripple-based structures had been formed by AFM tip’s scanning. And, for the novel friction-induced mechanical nanofabrication method, only the protrusive nanostructures including nanodots, nanolines, surface mesas, and nanowards have been produced by the mechanical interaction on the material surface. Until now, complex, ordered nanostructures on polymer surfaces using the friction-induced direct nanofabrication method are not reported [5, 6]. In previous work, we produced nanoscale ripples by scratching a PC surface with an AFM tip with a hard cantilever once . Such ripple patterns were actually quasi-sinusoidal structures that could be considered as typical three-dimensional (3D) nanostructures. To date, the formation of more complex polymer nanostructures by AFM scanning has not been reported.
Therefore, in the present paper, we use an AFM diamond tip with different scanning angles to trace a traditional zigzag pattern onto PC surfaces to study the effects of different scanning parameters including normal load and feed on the period of the resulting ripples. Based on these results, a novel two-step scanning method is then developed to realize controlled and oriented complex 3D nanodot arrays on PC surfaces. This permanent ripple structure appears to be caused by a stick-slip and crack formation process.
Results and discussion
Effect of scratching angle on ripple formation
Effect of the machining parameters on the ripple formation
Figure 3b,c,d shows the relationships between scratching parameters and the periods of the ripples. For feeds from 20 to 40 nm, the range of the normal load changes from 6.4 μN to 21 μN, 5.2 μN to 15 μN, and 1.5 μN to 14 μN for scratching angles of 0°, 45°, and 90°, respectively. Meanwhile, the period changes from 250 nm to 580 nm, 270 nm to 450 nm, and 230 nm to 500 nm for scratching angles of 0°, 45°, and 90°, respectively. For different scratching directions, the tip scratch face, the scratch edge, and the cantilever deformation are all different. The tip scratch face and the scratch edge affect the contact area, and the cantilever deformation affects the actual normal load acting on the sample surface in scratching test, which has been discussed in detail in our previous work . The contact area and the actual normal force will directly affect the contact press, which is the important factor for forming the ripple structures . For the three scratching angle, the contact area is the same due to the scan-scratch trace. So, the tip edge and faces have no effects on the different scratching angles. But, the actual normal load follows the order 0° < 45° < 90°, which means that in order to get the same contact press, the normal load follows the order 0° > 45° > 90°. For the change of the period scope in different scratching directions, it may be due to the change of the actual normal load under each scan-scratching direction. Therefore, for the three scratching angles, the normal load for ripple formation follows the order 0° > 45° > 90°, and the period scope for the ripples formed is 0° > 90° > 45°.
3D complex nanodot array formation based on ripples formed with different scanning angles
The above experimental results reveal that the length and orientation of nanodots can be regulated by manipulating the period of the ripples for a selected scratching direction. Using our two-step scratching method, by changing the period of the ripples formed using different scratching angles, complex, controllable 3D nanodot arrays can be fabricated easily.
Mechanism of ripples formation
Directional ripple patterns with perfect periodicity can be formed on PC surfaces by scratching zigzag patterns with an AFM tip. The range of normal load and feed used for ripple formation can be obtained to modulate the period of the ripples. By combining scratching angles of 90° and 0°, 90° and 45°, and 0° and 45° in two-step machining, we fabricated nanoscale dot and diamond-dot structures with controlled size and orientation. The typical rippling of the polymer surface can be presumed as a stick-slip and crack formation process. This study reveals that AFM-based nanomachining can be used to fabricate controllable complex 3D nanoripples and nanodot arrays on PC surfaces.
atomic force microscope
The authors gratefully acknowledge the financial supports of National Science Foundation of China (51275114, 51222504), Program for New Century Excellent Talents in University (NCET-11-0812), Heilongjiang Postdoctoral Foundation of China (LBH-Q12079), and the Fundamental Research Funds for the Central Universities (HIT.BRETIV.2013.08).
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