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

Abstract

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp³ defects leads to additional narrow and tunable photoluminescence features in the near-infrared and even enables single-photon emission at room temperature, thus strongly expanding their application potential. However, the successful integration of sp³-functionalized SWCNTs in optoelectronic devices with efficient defect state electroluminescence not only requires control over their emission properties but also a detailed understanding of the impact of functionalization on their electrical performance, especially in dense networks. Here, we demonstrate ambipolar, light-emitting field-effect transistors based on networks of pristine and functionalized polymer-sorted (6,5) SWCNTs. We investigate the influence of sp³ defects on charge transport by employing electroluminescence and (charge-modulated) photoluminescence spectroscopy combined with temperature-dependent current-voltage measurements. We find that sp³-functionalized SWCNTs actively participate in charge transport within the network as mobile carriers efficiently sample the sp³ defects, which act as shallow trap states. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states that can be tuned by the defect density.

Keywords: charge modulation spectroscopy; covalent functionalization; electroluminescence; light-emitting field-effect transistors; single-walled carbon nanotubes; sp3 defects.

Figure 1

(a) Reaction scheme for the controlled sp³ functionalization of PFO-BPy-wrapped (6,5) SWCNTs with 4-bromobenzenediazonium tetrafluoroborate and schematic illustration of the PL properties of functionalized SWCNTs. After optical E₂₂ excitation, red-shifted E₁₁* and E₁₁*^–emission from defect-localized excitons is observed in addition to E₁₁ emission. (b) Normalized PL spectra of sp³-functionalized (6,5) SWCNT dispersions with different defect densities under pulsed excitation at the E₂₂ transition (575 nm, ∼0.02 mJ cm^–2). (c) Correlation between integrated PL ratio and Raman D/G⁺ ratio and linear fit to the data.

Figure 2

(a) Schematic illustration of charge transport through sp³-functionalized SWCNTs. Defects may act as scattering sites or trap states for charge carriers. (b) Schematic device architecture of bottom-contact, top-gate SWCNT network FETs. (c) Atomic force micrograph (scale bar, 1 μm) of a representative, dense network of sp³-functionalized (6,5) SWCNTs. (d) Ambipolar transfer characteristics (source–drain voltage, V_ds = −100 mV; L = 20 μm; W = 10 mm) of FETs with networks of pristine (blue) and sp³-functionalized (6,5) SWCNTs (red). Solid lines are drain currents, I_d; dashed lines are gate leakage currents, I_g. (e) Absolute and (f) normalized linear charge carrier mobilities (holes, blue squares; electrons, red circles) of pristine and sp³-functionalized (6,5) SWCNT FETs vs Raman D/G⁺ area ratio.

Figure 3

(a) Schematic illustration of electroluminescence from sp³-functionalized SWCNTs through ambipolar carrier recombination of thermalized holes and electrons. (b) Near-infrared EL image of the channel (L = 20 μm) of an sp³-functionalized SWCNT network FET, showing a homogeneous recombination and emission zone when the device is biased in the ambipolar regime (drain current, I_d = −100 μA). (c) Composite EL image for a full gate voltage sweep at a constant current (I_d = −100 μA), showing homogeneous EL emission from the entire channel area (scale bars, 20 μm). (d, e) Representative EL spectra of pristine and sp³-functionalized (high defect density) SWCNT network FETs in the ambipolar regime for different drain currents.

Figure 4

(a) Static, gate voltage-dependent PL spectra (V_ds = −10 mV) of sp³-functionalized SWCNT network transistors (L = 20 μm) with high defect density in hole accumulation. Spectra were acquired from the middle of the channel under nonresonant continuous wave excitation (785 nm, ∼320 W cm^–2). (b) Normalized PL spectra show that E₁₁* and E₁₁*^– defect PL is more efficiently quenched than the E₁₁ emission when gate voltages are applied. At high V_g, PL from positively charged trions can be observed at wavelengths very similar to the E₁₁* emission. Note that the peak at ∼985 nm marked with an asterisk corresponds to the Raman 2D mode of (6,5) SWCNTs.

Figure 5

(a) Voltage-dependent CMPL spectra of pristine (6,5) SWCNT network, showing an initial increase and subsequent decrease of the E₁₁ ΔPL signal with V_os. (b) Voltage-dependent CMPL spectra of a sp³-functionalized SWCNT network with high defect density, showing PL modulation of mobile (E₁₁) as well as defect-localized (E₁₁*, E₁₁*^–) excitons. (c) CMPL spectra normalized to the ΔPL signal of E₁₁. All spectra were acquired from the middle of the channel (L = 20 μm) at a modulation frequency f = 363 Hz and V_pp = 0.2 V.

Figure 6

(a) Temperature-dependent transfer characteristics (four-point probe geometry, L = 40 μm, W = 1 mm, V_ds = −100 mV) of pristine and sp³-functionalized SWCNT network FETs between 25 and 300 K (only every other curve is shown). (b) Temperature-dependent charge carrier mobilities for holes and electrons normalized to the values at 300 K. For better comparison, all mobilities were contact resistance-corrected and extracted at a fixed gate voltage overdrive of ±6 V for electrons and holes, respectively. Lines are guides to the eye.