Guiding Charge Transport in Semiconducting Carbon Nanotube Networks by Local Optical Switching

Abstract

Photoswitchable, ambipolar field-effect transistors (FETs) are fabricated with dense networks of polymer-sorted, semiconducting single-walled carbon nanotubes (SWCNTs) in top-gate geometry with photochromic molecules mixed in the polymer matrix of the gate dielectric. Both hole and electron transport are strongly affected by the presence of spiropyran and its photoisomer merocyanine. A strong and persistent reduction of charge carrier mobilities and thus drain currents upon UV illumination (photoisomerization) and its recovery by annealing give these SWCNT transistors the basic properties of optical memory devices. Temperature-dependent mobility measurements and density functional theory calculations indicate scattering of charge carriers by the large dipoles of the merocyanine molecules and electron trapping by protonated merocyanine as the underlying mechanism. The direct dependence of carrier mobility on UV exposure is employed to pattern high- and low-resistance areas within the FET channel and thus to guide charge transport through the nanotube network along predefined paths with micrometer resolution. Near-infrared electroluminescence imaging enables the direct visualization of such patterned current pathways with good contrast. Elaborate mobility and thus current density patterns can be created by local optical switching, visualized and erased again by reverse isomerization through heating.

Charge Transport in and Electroluminescence from sp3-Functionalized Carbon Nanotube Networks

“…Adding up a large number of such images for a complete gate voltage sweep yields a composite EL image (see Figure 3c) that reflects the current density distribution within the channel. 41,42 No preferential transport paths were observed on the length scale of this diffraction-limited imaging method. We then resolved the EL spectrally with the emission zone always positioned in the center of the channel (as in Figure 3b) for different drain currents.…”

“…Adding up a large number of such images for a complete gate voltage sweep yields a composite EL image (see Figure 3c) that reflects the current density distribution within the channel. 41,42 No preferential transport paths were observed on the length scale of this diffraction-limited imaging method.…”

“…This narrow emission zone (width 1–2 μm) extended without interruption along the entire channel width and could be arbitrarily positioned between source and drain electrodes by adjusting the gate voltage. Adding up a large number of such images for a complete gate voltage sweep yields a composite EL image (see Figure c) that reflects the current density distribution within the channel. , No preferential transport paths were observed on the length scale of this diffraction-limited imaging method.…”

“…EL is observed from a narrow recombination zone that is formed where the hole and electron accumulation layers meet in the channel. Since EL correlates directly with current density, its spatial and spectral resolution has been previously utilized to analyze charge transport through mixed-chirality SWCNT networks via the band gap-dependent current shares, , and to visualize patterned current pathways in photoswitchable SWCNT network FETs . Here, we use the spectral signatures of sp 3 defects (E 11 *) and mobile excitons (E 11 ) to study the contribution of functionalized nanotubes to charge transport in the networks.…”

Luminescent Defects in Single‐Walled Carbon Nanotubes for Applications

“…[127,153] Defect electroluminescence resulting from electron-hole recombination in ambipolar light-emitting field-effect transistors was shown by Zorn et al [154] Top-gate/bottom-contact transistors with dense networks of semiconducting nanotubes and a poly(methyl methacrylate)/HfO x gate dielectric (see Figure 4a-c) had been previously shown to enable highly reproducible ambipolar charge transport [155][156][157] and the formation of a very controlled recombination and emission zone within the channel. [158][159][160] Networks of polymer-sorted (6,5) SWCNTs with covalent bromoaryl defects showed decreasing hole and electron mobilities with increasing defect density (quantified by the D/G Raman ratios), indicating their role as charge traps. However, even at relatively high degrees of functionalization (when the total quantum yield drops again), the transistors remained fully functional with reasonable carrier mobilities (about one third to a quarter of the values for pristine nanotubes, see Figure 4d,e).…”