Thermochemical nanolithography (TCNL) employs heated nano-size tips to induce thermally activated chemical reactions to change the chemical functionality of surfaces, or obtain new nanostructures. Examples of thermally activated reactions possible with TCNL are deprotection of functional groups at surfaces, reduction of oxides and crystallization of amorphous materials. TCNL is based on a scanning probe microscopy (SPM) approach, where nano-size tips can be moved in three dimensions with a spatial resolution which can be as low as 1 nm. This technique offers advantages in terms of the combination of high speed, high resolution, material flexibility, potential towards parallelization, and the versatility of working under ambient conditions.
TCNL was first achieved at the Georgia Institute of Technology in 2007 by Riedo and Szoszkiewicz.
Thermochemical nanolithography (TCNL) is a scanning probe microscopy-based nanolithography technique, which can pattern a wide range of materials by using a heated atomic force microscope (AFM) tip to thermally activate chemical transformations.
Resistively heated AFM tips have been shown to thermally activate a chemical reaction at the nanometer scale at the surface of a material (Fig. 1). Similarly to thermal probe lithography, TCNL uses thermal cantilevers where a current through doped silicon wings heats a nanoscopic tip standing below an undoped area. The temperature of the tip and the tip’s distance from the surface can be controlled finely enough to drive heat-triggered chemical reactions with lithographic precision. TCNL can produce local chemical changes with feature sizes down to 12 nm at scan speeds up to 1 mm/s. The tips can be cycled between ambient temperature and 1100 °C at up to 10 MHz and single-tip writing speeds in excess of 1 mm/s are achievable. TCNL was invented at Georgia Tech in 2007 by Riedo, Szoszkiewicz, and Marder.
The distance of the tip from the surface and the temperature of the tip can be modulated independently, without the need for the tip to indent into the material in consideration. In addition, chemical changes can be written very quickly through rapid scanning of the substrate or the tip, since no mass is transferred from the tip to the surface, writing speed is limited only by the heat transfer rate. The use of a material that can undergo multiple chemical reactions at significantly different temperatures renders the possibility of a multi-state system, wherein different functionalities can be addressed at different temperatures.
The AFM thermal cantilever is generally made from a silicon wafer using traditional bulk and surface micromachining processes. The cantilever is heavily doped in the cantilever arms, and lightly in the tip to produce a resistive heater where the largest fraction of heat is dissipated in the tip. Such a small tip can heat and cool very fast; an average tip in contact with polycarbonate has a time constant of 0.35 ms.
A wealth of thermally activated chemistries can feasibly be employed to change the subsequent reactivity, surface energy, solubility, conductivity, etc., of the material, and to initiate the crystallization of nano-materials. TCNL can be used for fabrication of functional nanostructures that are appealing for various applications in nanofluidics, nanoelectronics, nanophotonics, and biosensing devices.
Application to polymers
It has been demonstrated that TCNL can be employed to modify the wettability of a polymer surface at the nanoscale,. TCNL can also create nanostructures of poly(p-phenylene vinylene), a typical electroluminescence conjugated polymer, from the organic semiconductor precursors poly(p-xylene tetrahydrothiophenium chloride. The result is a clear “turn-on” of luminescence.
Activation of functional groups at surfaces to direct assembly of nano-objects
It was demonstrated that TCNL can induce a change in the chemical functionality of acid and amine patterns on the surface of copolymers containing thermally labile groups. They change from ester to acid, or carbamate to amine, with speeds in excess of 1 mm/s in 80% relative humidity and with features as small as 12 nm.
Reduction of oxides
TCNL has shown to control the extent of reduction of GO and pattern nano-scale regions of rGO within a GO sheet at speeds of several µm/s by using a recently developed nanofabrication technique called thermochemical nanolithography (TCNL). Variably conductive nanoribbons of rGO with dimensions down to 12 nm can be produced by TCNL in oxidized epitaxial graphene films in a single step that is clean, rapid and reliable. GO can be converted to rGO with a 100% yield in dozens of structures patterned on random locations in the GO film. The relative increase in conductivity is as high as four orders of magnitude. No sign of tip wear or sample tearing is observed, indicating that the "carbon skeleton" is continuous across the GO/rGO junction.
Crystallization of amorphous materials
TCNL was used to fabricate arbitrary-shaped Pb(Zr0:52Ti0:48/O3 and PbTiO3 ferroelectric nanostructures through local crystallization of sol-gel derived glasses on a variety of substrates including plastic (Kapton), silicon and soda-lime glass. TCNL can make ferroelectric lines with widths >30 nm, spheres with diameter greater than 10 nm and densities up to 213 Gb/in2.
In 2013, the Georgia Institute of Technology group used TCNL to create a nano-scale replica of the Mona Lisa. Untitled the Mini Lisa, it measured was just 30 micrometres (0.0012 in) wide, about 1/25,000th the size of the original.
More Information on Thermochemical Nanolithography can be found at this link.
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