Near‐Intrinsic Photo‐ and Electroluminescence from Single‐Walled Carbon Nanotube Thin Films on BCB‐Passivated Surfaces

Abstract

Their outstanding electrical and optical properties make semiconducting single‐walled carbon nanotubes (SWCNTs) highly suitable for charge transport and emissive layers in near‐infrared optoelectronic devices. However, the luminescence spectra of SWCNT thin films on commonly used glass and Si/SiO2 substrates are often compromised by broadening of the main excitonic emission and unwanted low‐energy sidebands. Surface passivation with a commercially available, low dielectric constant, cross‐linked bis‐benzocyclobutene‐based polymer (BCB) enhances the emission properties of SWCNTs to the same level as hexagonal boron nitride (h‐BN) flakes do. The presence of BCB suppresses sideband emission, especially from the Y1 band, which is attributed to defects introduced by the interaction of the nanotube lattice with oxygen‐containing terminal groups of the substrate surface. The facile and reproducible deposition of homogeneous BCB films over large areas combined with their resistance against common solvents and chemicals employed during photolithography make them compatible with standard semiconductor device fabrication. Utilizing this approach, light‐emitting (6,5) SWCNT network field‐effect transistors are fabricated on BCB‐treated glass substrates with excellent electrical characteristics and near‐intrinsic electroluminescence. Hence, passivation with BCB is proposed as a standard treatment for substrates used for spectroscopic investigations of and optoelectronic devices with SWCNTs and other low‐dimensional emitters.

Photo-Activated, Solid-State Introduction of Luminescent Oxygen Defects into Semiconducting Single-Walled Carbon Nanotubes

“…Before illumination, all as-deposited SWCNT networks (see Figure a) exhibited PL spectra with characteristically narrow excitonic E 11 emission at 1.232 eV and weak, red-shifted sideband features as commonly observed in networks of (6,5) SWCNTs. The sideband contributions to the PL signal increased from nonpolar BCB to polar substrates and from nonreactive to reactive oxide surfaces, as expected. , These sidebands have been assigned to momentum-forbidden dark excitons coupling to phonons and to shallow extrinsic defects of the (6,5) SWCNT lattice due to interaction with reactive groups on mostly polar surfaces. − The additional feature observed at 1.051 eV for the TiO x sample most likely corresponds to SWCNT trion (charged exciton) emission due to slight p-doping by the oxide . The relatively strong sideband for (6,5) SWCNTs even on BCB-passivated samples probably results from the low network density.…”

“…Note that the lifetimes measured for these sparse networks on substrates were very short compared to defect lifetimes of functionalized nanotubes in dispersion due to increased nonradiative decay paths, as observed previously. For example, Zorn et al reported amplitude-averaged E 11 * defect lifetimes of 222 ps in dispersion versus 32–60 ps when deposited on substrates …”

“…For example, Zorn et al reported amplitudeaveraged E 11 * defect lifetimes of 222 ps in dispersion versus 32−60 ps when deposited on substrates. 29 Furthermore, temperature-dependent PL measurements and defect/E 11 intensity ratios (see Figure S8a,b, Supporting Information) can help establish the thermal trap depth. The data collected from (6,5) SWCNT networks between 25 and 300 K on TiO x and ZnO x samples after 30 min of UV exposure in dry nitrogen showed the expected increase of defect emission with temperature due to increasing thermal energy to overcome potential barriers for mobile excitons around the defect sites, as reported by Kim et al 60 A further increase in temperature should result in a reduction of the relative defect intensity as thermal detrapping of excitons from the defect states starts to play a role.…”

“…However, unintentional overfunctionalization and degradation of nanotubes during sample processing and storage under ambient conditions might become an issue. Although nanotubes are overall much more stable than typical organic fluorophores, as evidenced by their photostability under laser excitation and high thermal decomposition temperatures (>400 °C), there is significant evidence that (photo)­oxidation of SWCNTs can alter their properties in undesired ways. , Furthermore, Zorn et al observed that SWCNTs strongly interact with polar silicon dioxide or glass surfaces, which are frequently used as dielectrics or substrates in optoelectronic devices, leading to defect-related broadening and emission sidebands, especially after high-temperature annealing . However, these undesired effects could be avoided by substrate passivation with hexagonal boron nitride ( h -BN) or cross-linked nonpolar polymers (e.g., BCB).…”

Tuning Electroluminescence from Functionalized SWCNT Networks Further into the Near-Infrared

“…Pristine and functionalized dispersions were used to create bottom-contact, top-gate field-effect transistors (FETs) with a bilayer gate dielectric of PMMA and HfO x , as introduced previously and schematically shown in Figure b. An additional surface passivation layer of BCB (a cross-linked benzocyclobutene-based polymer, see Experimental Section) on the glass substrate prevented undesired sideband emission and enabled nearly intrinsic PL and EL spectra . The SWCNT network density (see inset in Figure b and Figure b) for all samples was chosen to be well above the percolation limit and above the mobility saturation for nanotube networks to ensure comparability of the extracted mobility values.…”