Thermochemical nanolithography (TCNL) Patterning

Figure A: Micro and nano-patterns of proteins, DNA, and C₆₀: (a) Epi-fluorescence images of micro triangle patterns of Cy5-streptavidin (red), Alexa350-antiCD3 (blue), and atto488 fibronectin (green). (b), Left: AFM tapping mode topography and phase images of a triangular pattern of DNA single strands. Right: AFM tapping mode topography and electrostatic force microscopy (EFM) phase images of a triangular pattern of C60 covalently bound to amines. (c), Fibronectin nanopatterns. Scale bars: (a) 5 µm, (b) 1 µm, (c) 500 nm.

Patterning with nanoscale resolution is crucial for fundamental studies and applications such as protein chips, biological sensors, and electronics. Scanning probe microscopy based techniques have been successful in generating patterns on various substrates. However, several challenges still exist in terms of resolution, writing speed, cost, substrate choice, bioactivity, and multi-component patterning. Recently, the Riedo and Marder groups in collaboration with the Curtis group (GT Physics) reported the use of thermochemical nanolithography (TCNL) to produce, at speeds of mm/s, nanopatterns of different orthogonal chemical functionalities on a polymer surface. These tailored chemical nanopatterns are then used to direct the assembly of different nano-objects, (e.g. proteins, C₆₀ and DNA). In particular, they produced nano-assemblies, as small as 40 nm, of two different species of bioactive proteins coexisting.

Systematic study has been carried out to answer fundamental and technical questions that define the full potential of this new nanolithography technique developed by the groups of Marder and Riedo in 2007 (R. Szoszkiewicz, T. Okada, S. C. Jones, T.-D. Li, W. P. King, S. R. Marder, E. Riedo, Nano Letters, 2007, 7, 1064; D. B. Wang, R. Szoszkiewicz, M. Lucas, E. Riedo, T. Okada, S. C. Jones, S. R. Marder, J. Lee, W. P. King; Appl. Phys. Lett., 2007, 91, 243104). Several strategies have been investigated to apply TCNL to fabrication of one dimensional and two dimensional nanostructures that are appealing for applications in nanofluidics, organic nanoelectronics and nanophotonics, and biosensors.

Curtis, Marder and Riedo's groups investigated TCNL's ability to thermally activate the deprotection of amine groups on a polymer surface in arbitrary nano- and micro-patterns at linear speeds of up to millimeters per second (D. Wang, R. Szoszkiewicz, J. Curtis, E. Riedo, Applied Scanning Probe Methods, NanoScience and Technology (Springer, 2010); D. Wang, Kodali V.K., Underwood W.D., Jarvholm J.E., Okada T., Jones S.C., Rumi M., Dai Z.T., King W.P., Marder S.R., Curtis J.E., and Riedo E., Adv. Funct. Mat.,2009, 19, 3696). This speed is faster than the speed of other AFM-based patterning approaches, for example TCNL is over 106 times faster than dip-pen nanolithography (performed with a single tip) and 103 times faster than thermal dip-pen nanolithography. The unmasked amine nanotemplates are then selectively and covalently functionalized to create patterns of thiols, maleimides, aldehydes or biotins in distinct areas of the polymer surface. Multiple functionalities of these patterns can be exploited to create nanoarrays, with features as small as 40 nm, of nano-objects such as bioactive proteins and DNA. By repeating the TCNL steps followed by different protocols for chemically converting the unprotected amines, it is possible to generate surfaces with orthogonal functionalities in distinct areas that can be further functionalized at a later date. The bioactivity of the TCNL-designed protein arrays was confirmed by using fluorescent antibodies and a cell signaling experiment (Figure A).

Figure B: Fluorescence AFM topography images of PPV nanostructures made by TCNL at a range of temperatures, 240-360 ºC. The average profile of the PPV trench shows that the width of the line is as narrow as 70 nm. B. Raman spectra of untreated precursor, PPV reference, TCNL PPV pattern. Scale bars: (A) 5 µm.

Marder and Riedo's groups also investigated TCNL's feasibility of direct writing of poly(p-phenylene vinylene) (PPV) nanostructures, a widely studied electroluminescent conjugated polymer, by local heating a sulfonium salt precursor, poly(p-xylene tetrahydrothiophenium chloride) (D. B. Wang., S. Kim, W.D. Underwood, A. J. Giordano, C. L. Henderson, Z. T. Dai, W. P. King, S. R. Marder, and E. Riedo, Appl. Phys. Lett. 2009, 95, 233108). Such a thermochemical local conversion route is realized by locally heating in ambient conditions with a resistively heated AFM probe at 240ºC. The excellent quality of the TCNL-produced PPV nanostructures was verified by in-situ Raman spectroscopy (Figure B).

The Marder and Riedo groups studied the time and temperature evolution of the nanometer-scale surface undulations (ripples) produced by a heated atomic force microscope (AFM) tip scanning across surfaces of several amorphous polymers (W. P. King, S. Marder, E. Gnecco, E. Riedo and R. Szoszkiewicz, Phys.Rev. B 2009, 79, 235421). During linear zig-zag scanning we obtain pseudo-linear ripples approximately perpendicular to the fast scan direction in a range of scan rates and probe temperatures. As expected, the size of the ripples increases massively in vicinity of the glass temperature for each polymer. Also, a novel case in which the AFM tip follows a circular path was explored. Contrary to the "steady" linear ripples we obtain circular ripples which rotate along the scanning path during consecutive scans. The group velocity of the circular ripples is two orders of magnitude lower than the scan speed. The experimental data were interpreted using a phenomenological model accounting for erosion and smoothing effects caused by the probing tip (Figure C).

Figure C: Linear and circular ripples generated by sliding a hot AFM tip on a polymer surface.