Topological transitions in carbon nanotube networks via nanoscale confinement Sivasubramanian Somu, 1 Hailong Wang, 2, 1 Younglae Kim, 1 Laila Jaberansari, 1 Myung Gwan Hahm, 1 Bo Li, 1 Taehoon Kim, 1 Xugang Xiong, 1 Yung Joon Jung, 1 Moneesh Upmanyu, 1, 2 and Ahmed Busnaina 1 $^{1}$NSF Nanoscale Science and Engineering Center for High-rate Nanomanufacturing, Department of Mechanical and Industrial Engineering, Northeastern University, Boston MA 02115 $^{2}$Group for Simulation and Theory of Atomic-Scale Material Phenomena (stAMP), Department of Mechanical and Industrial Engineering, Northeastern University, Boston MA 02115 Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior elec- tronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative ap- proaches based on ultra-thin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to i) an often uncontrollable interplay between network cov- erage and its topology and ii) the inherent electrical heterogeneity of the constituent SWNTs. In this letter, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto sub-micron wide ultra-thin polymer channels exhibit a topology that becomes increasingly aligned with decreasing channel width and thickness. Experimental scale coarse-grained computa- tions of interacting SWCNTs show that the effect is a reflection of an aligned topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effec- tiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices. PACS numbers: 62.25.+g, 85.85+j, 85.35.Kt, 81.07.De, 62.20.Dc Keywords: carbon nanotube networks; fluidic assembly; topology; electronic transport Harnessing the highly efficient electronic transport in carbon nanotubes continues to be a challenge due to difficulties in manipulating and packaging individual CNTs [1–3]. In instances where devices have been suc- cessfully integrated, issues related to device reliability and scalability remain largely unresolved. In contrast, top-down integration efforts centered around SWCNTs networks (CNNs) as electronic material of choice have enjoyed considerable success. Several device applications have been realized [4–6], from gate-modulated thin film transistors (TFTs) [7–9] and RF oscillators [10] to wafer- scale integrated circuits on flexible substrates [11, 12]. A key limitation of this approach is that current CNT syn- thesis routes yield a mixture of metallic and semiconduct- ing nanotubes that cannot be easily separated [13–15]. This intrinsic electrical heterogeneity leads to networks with drastically different effective transport characteris- tics: as-synthesized networks can be metallic (m-CNNs) or semiconducting (s-CNNs). Since the latter response is desirable for gate modulation, it is the presence of active (metallic) elements that continues to be an issue. Purification efforts targeted at increasing the fraction of semiconducting nanotubes hold promise yet they in- variably compromise the scalability. Alternative strate- gies have revolved around suppressing the transport through active elements by eliminating the off-state con- duction. Efforts to this end involve patterning hetero- geneous networks within channels with controlled topol- In this article, we use a solution-based approach to demonstrate a facile yet scalable route for synthesizing CNNs with prescribed transport characteristics. A no- table aspect of this approach is the robust control over the CNN topology, which we accomplish via template- directed fluidic self-assembly of dispersions of functional- ized SWCNTs onto suitably patterned channels with pre- scribed dimensions ( W$_{c}$ × L$_{c}$ ). Plasma treated ∼ 100 nm silicon dioxide channels are grown thermally on a sil- icon wafer and 150 − 500 nm polymethylmethacrylate (PMMA) thick photoresists are used to etch nano- to micron-scale channels using e-beam lithography. The channels are then dip-coated in a dispersion solution of commercially acquired SWCNTs ( ∼ 0 . 25%, mean ogy, by i) catalytic growth of CNTs along morpholog- ically pattered substrates [16], or ii) via solution-based approaches [17, 18]. The former rely on chemical va- por deposition (CVD) to directly grow CNNs on pre- scribed substrates. Network topology can be tuned by controlling the density or distribution of the catalytic particles and the underlying substrate morphology, re- spectively. Random, partially aligned and fully aligned networks have been realized via these routes [8]. How- ever, the lack of control over the density and distribution of catalytic particles and the substrate morphology, espe- cially at synthesis temperatures, limits the technique to low temperatures and results in network topologies which are coupled to the coverage. arXiv:1001.0819v1 [cond-mat.mtrl-sci] 6 Jan 2010