Mapping charge transport by electroluminescence in chirality-selected carbon nanotube networks

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

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.

Chirality effects on an electron transport in single-walled carbon nanotube

In our work, we investigate characteristics of conductivity for single-walled carbon nanotubes caused by spin–orbit interaction. In the case study of chirality indexes, we especially research on the three types of single-walled carbon nanotubes which are the zigzag, the chiral, and the armchair. The mathematical analysis employed for our works is the Green-Kubo Method. For the theoretical results of our work, we discover that the chirality of single-walled carbon nanotubes impacts the interaction leading to the spin polarization of conductivity. We acknowledge such asymmetry characteristics by calculating the longitudinal current–current correlation function difference between a positive and negative wave vector in which there is the typical chiral-dependent. We also find out that the temperature and the frequency of electrons affect the function producing the different characteristics of the conductivity. From particular simulations, we obtain that the correlation decrease when the temperature increase for a low frequency of electrons. For high frequency, the correlation is nonmonotonic temperature dependence. The results of the phenomena investigated from our study express different degrees of spin polarization in each chiral of single-walled carbon nanotube and significant effects on temperature-dependent charge transport according to carrier backscattering. By chiral-induced spin selectivity that produces different spin polarization, our work could be applied for intriguing optimization charge transport.

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

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.

Low-Temperature Electroluminescence Excitation Mapping of Excitons and Trions in Short-Channel Monochiral Carbon Nanotube Devices

Single-walled carbon nanotubes as emerging quantum-light sources may fill a technological gap in silicon photonics due to their potential use as near-infrared, electrically driven, classical or nonclassical emitters. Unlike in photoluminescence, where nanotubes are excited with light, electrical excitation of single tubes is challenging and heavily influenced by device fabrication, architecture, and biasing conditions. Here we present electroluminescence spectroscopy data of ultra-short-channel devices made from (9,8) carbon nanotubes emitting in the telecom band. Emissions are stable under current biasing, and no enhanced suppression is observed down to 10 nm gap size. Low-temperature electroluminescence spectroscopy data also reported exhibit cold emission and line widths down to 2 meV at 4 K. Electroluminescence excitation maps give evidence that carrier recombination is the mechanism for light generation in short channels. Excitonic and trionic emissions can be switched on and off by gate voltage, and corresponding emission efficiency maps were compiled. Insights are gained into the influence of acoustic phonons on the line width, absence of intensity saturation and exciton-exciton annihilation, environmental effects such as dielectric screening and strain on the emission wavelength, and conditions to suppress hysteresis and establish optimum operation conditions.

Probing Mobile Charge Carriers in Semiconducting Carbon Nanotube Networks by Charge Modulation Spectroscopy

Solution-processed networks of semiconducting, single-walled carbon nanotubes (SWCNTs) have attracted considerable attention as materials for next-generation electronic devices and circuits. However, the impact of the SWCNT network composition on charge transport on a microscopic level remains an open and complex question. Here, we use charge-modulated absorption and photoluminescence spectroscopy to probe exclusively the mobile charge carriers in monochiral (6,5) and mixed SWCNT network field-effect transistors. Ground-state bleaching and charge-induced trion absorption features as well as exciton quenching are observed depending on applied voltage and modulation frequency. Through correlation of the modulated mobile carrier density and the optical response of the nanotubes, we find that charge transport in mixed SWCNT networks depends strongly on the diameter and thus bandgap of the individual species. Mobile charges are preferentially transported by small bandgap SWCNTs especially at low gate voltages, whereas large bandgap species only start to participate at higher carrier concentrations. Our results demonstrate the excellent suitability of modulation spectroscopy to investigate charge transport in nanotube network transistors and highlight the importance of SWCNT network composition for their performance.

Electroluminescence Generation in PbS Quantum Dot Light-Emitting Field-Effect Transistors with Solid-State Gating

The application of light-emitting field-effect transistors (LEFET) is an elegant way of combining electrical switching and light emission in a single device architecture instead of two. This allows for a higher degree of miniaturization and integration in future optoelectronic applications. Here, we report on a LEFET based on lead sulfide quantum dots processed from solution. Our device shows state-of-the-art electronic behavior and emits near-infrared photons with a quantum yield exceeding 1% when cooled. We furthermore show how LEFETs can be used to simultaneously characterize the optical and electrical material properties on the same device and use this benefit to investigate the charge transport through the quantum dot film.

Endeavor of Iontronics: From Fundamentals to Applications of Ion-Controlled Electronics

Iontronics is a newly emerging interdisciplinary concept which bridges electronics and ionics, covering electrochemistry, solid-state physics, electronic engineering, and biological sciences. The recent developments of electronic devices are highlighted, based on electric double layers formed at the interface between ionic conductors (but electronically insulators) and various electronic conductors including organics and inorganics (oxides, chalcogenide, and carbon-based materials). Particular attention is devoted to electric-double-layer transistors (EDLTs), which are producing a significant impact, particularly in electrical control of phase transitions, including superconductivity, which has been difficult or impossible in conventional all-solid-state electronic devices. Besides that, the current state of the art and the future challenges of iontronics are also reviewed for many applications, including flexible electronics, healthcare-related devices, and energy harvesting.

On-Chip Chemical Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes (SWNTs): Toward Robust and Scale Invariant SWNTs Transistors

In this paper, the fabrication of carbon nanotubes field effect transistors by chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on prepatterned substrates is demonstrated. Polyfluorenes derivatives have been demonstrated to be effective in selecting s-SWNTs from raw mixtures. In this work the authors functionalized the polymer with side chains containing thiols, to obtain chemical self-assembly of the selected s-SWNTs on substrates with prepatterned gold electrodes. The authors show that the full side functionalization of the conjugated polymer with thiol groups partially disrupts the s-SWNTs selection, with the presence of metallic tubes in the dispersion. However, the authors determine that the selectivity can be recovered either by tuning the number of thiol groups in the polymer, or by modulating the polymer/SWNTs proportions. As demonstrated by optical and electrical measurements, the polymer containing 2.5% of thiol groups gives the best s-SWNT purity. Field-effect transistors with various channel lengths, using networks of SWNTs and individual tubes, are fabricated by direct chemical self-assembly of the SWNTs/thiolated-polyfluorenes on substrates with lithographically defined electrodes. The network devices show superior performance (mobility up to 24 cm² V^-1 s^-1 ), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices. Importantly, the SWNTs assembled by mean of the thiol groups are stably anchored to the substrate and are resistant to external perturbation as sonication in organic solvents.

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes

Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3^'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN-SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities

Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix and a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (>110 meV), efficient polariton relaxation and narrow band emission (<15 meV). Given their high charge carrier mobility and excellent photostability, SWCNTs represent a promising new avenue towards practical exciton-polariton devices operating at telecommunication wavelengths.

Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks

The current percolation in polymer-sorted semiconducting (7,5) single-walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field-effect measurements, atomic force microscopy (AFM), and conductive AFM (C-AFM) techniques. From AFM measurements, the nanotube length in the as-processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field-effect charge transport measurements in the SWNT network using a 2D homogeneous random-network stick-percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C-AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as-spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field-effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.

Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.

Broadband Tunable, Polarization-Selective and Directional Emission of (6,5) Carbon Nanotubes Coupled to Plasmonic Crystals

We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth emission at specific near-infrared wavelengths. Emission from these hybrid films is spectrally narrow (20-40 meV) yet broadly tunable (∼1000-1500 nm) and highly directional (divergence <1.5°). In addition, SLR scattering renders the emission highly polarized, even though the SWNTs are randomly distributed. Numerical simulations are applied to correlate the increased local electric fields around the nanodisks with the observed enhancement of directional emission. The ability to control the emission properties of a single type of near-infrared emitting SWNTs over a wide range of wavelengths will enable application of carbon nanotubes in multifunctional photonic devices.

Understanding Charge Transport in Mixed Networks of Semiconducting Carbon Nanotubes

The ability to select and enrich semiconducting single-walled carbon nanotubes (SWNT) with high purity has led to a fast rise of solution-processed nanotube network field-effect transistors (FETs) with high carrier mobilities and on/off current ratios. However, it remains an open question whether it is best to use a network of only one nanotube species (monochiral) or whether a mix of purely semiconducting nanotubes but with different bandgaps is sufficient for high performance FETs. For a range of different polymer-sorted semiconducting SWNT networks, we demonstrate that a very small amount of narrow bandgap nanotubes within a dense network of large bandgap nanotubes can dominate the transport and thus severely limit on-currents and effective carrier mobility. Using gate-voltage-dependent electroluminescence, we spatially and spectrally reveal preferential charge transport that does not depend on nominal network density but on the energy level distribution within the network and carrier density. On the basis of these results, we outline rational guidelines for the use of mixed SWNT networks to obtain high performance FETs while reducing the cost for purification.

On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical source of excitons coupled to surface plasmons based on a polymer light-emitting field-effect transistor (LEFET). Gold nanorods that are integrated into the channel support localized surface plasmons and serve as nanoantennas for enhanced electroluminescence. By precise spatial control of the near-infrared emission zone in the LEFET via the applied voltages the near-field coupling between electrically generated excitons and the nanorods can be turned on or off as visualized by a change of electroluminescence intensity. Numerical calculations and spectroscopic measurements corroborate significant local electroluminescence enhancement due to the high local density of photonic states in the vicinity of the gold nanorods. Importantly, the integration of plasmonic nanostructures hardly influences the electrical performance of the LEFETs, thus, highlighting their mutual compatibility in novel active plasmonic devices.

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9-dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led to significantly lower contact resistance (R_c) and increased mobilities for both holes and electrons. The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced ambipolar transport properties with a hole mobility of 0.5 cm²V^-1S^-1 and an electron mobility of 0.5 cm²V^-1S^-1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated polymers are not critical to improving charge-injection properties. We found that the improvements caused by the interlayer were due to the better charge injection at the metal/organic semiconductor contact interface and the increase in the charge concentration through a detailed examination of charge transport with low-temperature measurements. Finally, we successfully demonstrated complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Light-emitting quantum dot transistors: emission at high charge carrier densities

For the application of colloidal semiconductor quantum dots in optoelectronic devices, for example, solar cells and light-emitting diodes, it is crucial to understand and control their charge transport and recombination dynamics at high carrier densities. Both can be studied in ambipolar, light-emitting field-effect transistors (LEFETs). Here, we report the first quantum dot light-emitting transistor. Electrolyte-gated PbS quantum dot LEFETs exhibit near-infrared electroluminescence from a confined region within the channel, which proves true ambipolar transport in ligand-exchanged quantum dot solids. Unexpectedly, the external quantum efficiencies improve significantly with current density. This effect correlates with the unusual increase of photoluminescence quantum yield and longer average lifetimes at higher electron and hole concentrations in PbS quantum dot thin films. We attribute the initially low emission efficiencies to nonradiative losses through trap states. At higher carrier densities, these trap states are deactivated and emission is dominated by trions.

Flexible light-emitting devices based on chirality-sorted semiconducting carbon nanotube films

Near-infrared light-emitting devices based on chirality-sorted (8,3), (8,4) enriched carbon nanotubes (CNTs) are fabricated on transparent and flexible substrate. The devices emit near-infrared light with well-defined wavelength, narrow peak width and high intensity. 500 times bending test also shows that the electric properties and electroluminescence (EL) spectra of devices do not decay apparently. This work demonstrates that chirality-sorted CNTs have large advantages in transparent and flexible infrared light source applications.

Polymer-sorted semiconducting carbon nanotube networks for high-performance ambipolar field-effect transistors

Efficient selection of semiconducting single-walled carbon nanotubes (SWNTs) from as-grown nanotube samples is crucial for their application as printable and flexible semiconductors in field-effect transistors (FETs). In this study, we use atactic poly(9-dodecyl-9-methyl-fluorene) (a-PF-1-12), a polyfluorene derivative with asymmetric side-chains, for the selective dispersion of semiconducting SWNTs with large diameters (>1 nm) from plasma torch-grown SWNTs. Lowering the molecular weight of the dispersing polymer leads to a significant improvement of selectivity. Combining dense semiconducting SWNT networks deposited from an enriched SWNT dispersion with a polymer/metal-oxide hybrid dielectric enables transistors with balanced ambipolar, contact resistance-corrected mobilities of up to 50 cm(2)·V(-1)·s(-1), low ohmic contact resistance, steep subthreshold swings (0.12-0.14 V/dec) and high on/off ratios (10(6)) even for short channel lengths (<10 μm). These FETs operate at low voltages (<3 V) and show almost no current hysteresis. The resulting ambipolar complementary-like inverters exhibit gains up to 61.

Mapping charge-carrier density across the p-n junction in ambipolar carbon-nanotube networks by Raman microscopy

In situ confocal Raman microscopy is used to map the recombination zone (induced p-n junction) in an ambipolar carbon-nanotube-network transistor with high spatial resolution. The shift of the 2D mode (G' mode) depending on hole and electron accumulation serves as a measure for the local charge-carrier density and provides complementary information about charge transport and recombination in ambipolar transistors.

Tuning of the thermoelectric properties of one-dimensional material networks by electric double layer techniques using ionic liquids

We report across-bandgap p-type and n-type control over the Seebeck coefficients of semiconducting single-wall carbon nanotube networks through an electric double layer transistor setup using an ionic liquid as the electrolyte. All-around gating characteristics by electric double layer formation upon the surface of the nanotubes enabled the tuning of the Seebeck coefficient of the nanotube networks by the shift in gate voltage, which opened the path to Fermi-level-controlled three-dimensional thermoelectric devices composed of one-dimensional nanomaterials.

Experimental Measurement of the Binding Configuration and Coverage of Chirality-Sorting Polyfluorenes on Carbon Nanotubes

Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) exhibits exceptional (n,m) chirality and electronic-type selectivity for near-armchair semiconducting carbon nanotubes. To better understand and control the factors governing this behavior, we experimentally determine the surface coverage and binding configuration of PFO on nanotubes in solution using photoluminescence energy transfer and anisotropy measurements. The coverage increases with PFO concentration in solution, following Langmuir-isotherm adsorption behavior with cooperativity. The equilibrium binding constant (PFO concentration in solution at half coverage), KA, depends on (n,m) and is 1.16 ± 0.30, 0.93 ± 0.12, and 1.13 ± 0.26 mg mL(-1) for the highly selected (7,5), (8,6), and (8,7) species, respectively, and the corresponding PFO wrapping angle at low coverage is 12, 17, and 14 ± 2°, respectively. In contrast, the inferred KA for metallic nanotubes is nearly an order of magnitude greater, indicating that the semiconducting selectivity increases with decreasing PFO concentration. This understanding will quantitatively guide future experimental and computational efforts on electronic type-sorting carbon nanotubes.

Trion electroluminescence from semiconducting carbon nanotubes

Near-infrared emission from semiconducting single-walled carbon nanotubes (SWNTs) usually results from radiative relaxation of excitons. By binding an additional electron or hole through chemical or electrochemical doping, charged three-body excitons, so-called trions, are created that emit light at lower energies. The energy difference is large enough to observe weak trion photoluminescence from doped SWNTs even at room temperature. Here, we demonstrate strong trion electroluminescence from electrolyte-gated, light-emitting SWNT transistors with three different polymer-sorted carbon nanotube species, namely, (6,5), (7,5) and (10,5). The red-shifted trion emission is equal to or even stronger than the exciton emission, which is attributed to the high charge carrier density in the transistor channel. The possibility of trions as a radiative relaxation pathway for triplets and dark excitons that are formed in large numbers by electron-hole recombination is discussed. The ratio of trion to exciton emission can be tuned by the applied voltages, enabling voltage-controlled near-infrared light sources with narrow line widths that are solution-processable and operate at low voltages (<3 V).