High-performance carbon nanotube light-emitting diodes with asymmetric contacts

Electrical nature of randomly oriented low-dimensional structural hybrids of carbon

Low-dimensional carbon materials are of great interest and have tremendous potential for application in flexible plastic electronics. However, the development of devices based on carbon structural hybrids is often hindered due to the high recombination rate of photoexcited charges, low absorbance, and other factors. This work discusses the emergence of multi-component structural forms of carbon from single-wall carbon nanotubes (SWCNTs) and demonstrates the electrical nature of the film containing these heterogeneous low-dimensional structural derivatives that are amalgamated in a polyurethane matrix. SWCNTs serve as a building block to give rise to multi-structural compounds, including multi-wall carbon nanotubes (MWCNTs), graphene sheets (GSs), carbon nanoscrolls (CNS), 'Y' and 'T' junctions, twisted CNTs and carbon nano-onion (CNO)-like structures, after performing oxidative purification and covalent functionalization processes. These one- and two-dimensional (1D and 2D) components with different individual electrical characteristics when integrated in a polyurethane binder and spin-coated on a SiO2/Si substrate exhibit an overall semiconducting behaviour. Current (I)-voltage (V) characteristics reveal thermally driven photo-excited charges that are mainly responsible for the observed current trend of the film. Herein, we explore a facile cost-effective strategy to fabricate stable thin film coatings comprising a random network of functionalized structural derivatives of carbon and polymer conjugates and investigate the overall electrical nature to envisage incorporating these nanomaterials in future plastic electronics.

Wafer-Scale Carbon Nanotubes Diodes Based on Dielectric-Induced Electrostatic Doping

Diodes based on p-n junctions are fundamental building blocks for numerous circuits, including rectifiers, photovoltaic cells, light-emitting diodes (LEDs), and photodetectors. However, conventional doping techniques to form p- or n-type semiconductors introduce impurities that lead to Coulomb scattering. When it comes to low-dimensional materials, controllable and stable doping is challenging due to the feature of atomic thickness. Here, by selectively depositing dielectric layers of Y2O3 and AlN, direct formation of wafer-scale carbon-nanotube (CNT) diodes are demonstrated with high yield and spatial controllability. It is found that the oxygen interstitials in Y2O3, and the oxygen vacancy together with Al-Al bond in AlN/Y2O3 electrostatically modulate the intrinsic CNTs channel, which leads to p- and n-type conductance, respectively. These CNTs diodes exhibit a high rectification ratio (>104) and gate-tunable rectification behavior. Based on these results, we demonstrate the applicability of the diodes in electrostatic discharge (ESD) protection and photodetection.

Infrared Light-Emitting Diodes Based on Chirality-Sorted Carbon Nanotube Films

An important goal in carbon nanotube optoelectronics is to achieve a high-performance near-infrared light source. But there are still many challenges such as the purity of single-walled carbon nanotube (SWCNT) chirality, nonradiative defects, thin-film quality, and device structure design. Here, we realize infrared light-emitting diodes (LEDs) based on chirality-sorted (10, 5) SWCNT network films, which operate at a low bias voltage and emit at a telecom O band of 1290 nm. Asymmetric palladium (Pd) and hafnium (Hf) contacts are used as electrodes for hole and electron injection, respectively. However, the large Schottky barrier at the interface of the SWCNTs and the Hf electrode, primarily resulting from the polymer wrapped on the nanotube surface during the sorting process, leads to inefficient electron injection and thus a low electroluminescence efficiency. We find that the efficiency of electron injection can be improved by the local doping of the nanotubes with dielectric layers of YOX-HfO2, which reduces the Schottky barrier at the SWCNT/Hf interface. Accordingly, the (10, 5) SWCNT film-based LED achieves an external quantum efficiency of larger than 0.05% without any optical coupling structure. With further improvement, we expect that such an infrared light source will have great application potential in the carbon nanotube monolithic optoelectronic integrated system and on-chip optical interconnection, especially in the field of short-distance optical fiber communications and data center.

Controlled Preparation of Single-Walled Carbon Nanotubes as Materials for Electronics

Single-walled carbon nanotubes (SWCNTs) are of particular interest as channel materials for field-effect transistors due to their unique structure and excellent properties. The controlled preparation of SWCNTs that meet the requirement of semiconducting and chiral purity, high density, and good alignment for high-performance electronics has become a key challenge in this field. In this Outlook, we outline the efforts in the preparation of SWCNTs for electronics from three main aspects, structure-controlled growth, selective sorting, and solution assembly, and discuss the remaining challenges and opportunities. We expect that this Outlook can provide some ideas for addressing the existing challenges and inspire the development of SWCNT-based high-performance electronics.

Assessment of the optical and electrical properties of light-emitting diodes containing carbon-based nanostructures and plasmonic nanoparticles: a review

Light-emitting diodes (LED) are widely employed in display applications and lighting systems. Further research on LED that incorporates carbon nanostructures and metal nanoparticles exhibiting surface plasmon resonance has demonstrated a significant improvement in device performance. These devices offer lower turn-on voltages, higher external quantum efficiencies, and luminance. De facto, plasmonic nanoparticles, such as Au and Ag have boosted the luminance of red, green, and blue emissions. When combined with carbon nanostructures they additionally offer new possibilities towards lightweight and flexible devices with better thermal management. This review surveys the diverse possibilities to combine various inorganic, organic, and carbon nanostructures along with plasmonic nanoparticles. Such combinations would allow an enhancement in the overall properties of LED.

Ambipolar 2D Semiconductors and Emerging Device Applications

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

Delayed Fluorescence from Carbon Nanotubes through Singlet Oxygen-Sensitized Triplet Excitons

Single-wall carbon nanotubes (SWCNTs) in liquid suspension have been observed to emit delayed, microsecond-scale fluorescence arising from upconverted triplet excitons that are directly created through energy transfer from singlet oxygen molecules (¹O₂). The singlet oxygen is produced through quenching of an optically excited organic sensitizer. The mechanism of this delayed fluorescence has been deduced from measurements of time-resolved emission kinetics, delayed emission spectra, and polarization-resolved excitation-emission spectra. The observed strong dependence of ¹O₂ sensitization efficiency on SWCNT structure suggests that (7,6) triplet excitons have an energy near 970 meV. The yields for E₁₁^T → E₁₁^S upconversion are found to be in the range of several percent. These yields increase with increasing temperature and decrease with increasing excitation intensities, reflecting thermal activation and triplet-triplet exciton annihilation processes.

Electronic tongue and cyclic voltammetric sensors based on carbon nanotube/polylactic composites fabricated by fused deposition modelling 3D printing

In this work, 3D printed electrodes fabricated by blending Polylactic acid (PLA) with carbon nanotube (CNT), CNT/copper (Cu), CNT/zinc oxide (ZnO) composites were applied as cyclic voltammetric sensors for electronic tongue analysis. Porous rectangular rod-shape electrodes were fabricated by fused-deposition-modelling 3D printing of the CNT-based composites produced by a solution blending method. The physical and chemical properties of 3D printed electrodes were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, four-point-probe electrical tests and thermoelectric measurements. The characterization results confirmed uniform distributions of CNTs, Cu particles and ZnO nanorods in the composites and high electrical conductivity of interconnected CNT networks. The additions of Cu and ZnO nanostructures slightly modified the electrical conductivity but significantly changed thermoelectric properties of the material. Cyclic voltammetric (CV) data demonstrated satisfactory stability of the composite materials under corrosive CV conditions. In addition, Cu and ZnO additives provided distinct electrochemical behaviors towards K₄Fe(CN)₆, H₂O₂ and nicotinamide adenine dinucleotide. Principal component analysis of CV features could effectively distinguish the three chemicals with various concentrations, illustrating the possibility to apply 3D printed CNT/PLA-based electrodes for electronic tongue applications.

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.

Auger Suppression of Incandescence in Individual Suspended Carbon Nanotube pn-Junctions

There are various mechanisms of light emission in carbon nanotubes (CNTs), which give rise to a wide range of spectral characteristics that provide important information. Here we report suppression of incandescence via Auger recombination in suspended carbon nanotube pn-junctions generated from dual-gate CNT field-effect transistor (FET) devices. By applying equal and opposite voltages to the gate electrodes (i.e., V_g1 = -V_g2), we create a pn-junction within the CNT. Under these gating conditions, we observe a sharp peak in the incandescence intensity around zero applied gate voltage, where the intrinsic region has the largest spatial extent. Here, the emission occurs under high electrical power densities of around 0.1 MW/cm² (or 6 μW) and arises from thermal emission at elevated temperatures above 800 K (i.e., incandescence). It is somewhat surprising that this thermal emission intensity is so sensitive to the gating conditions, and we observe a 1000-fold suppression of light emission between V_g1 = 0 and 15 V, over a range in which the electrical power dissipated in the nanotube is roughly constant. This behavior is understood on the basis of Auger recombination, which suppresses light emission by the excitation of free carriers. Based on the calculated carrier density and band profiles, the length of the intrinsic region drops by a factor of 7-25× over the range from |V_g| = 0 to 15 V. We, therefore, conclude that the light emission intensity is significantly dependent on the free carrier density profile and the size of the intrinsic region in these CNT devices.

Nanoscale Patterning of Carbon Nanotubes: Techniques, Applications, and Future

Carbon nanotube (CNT) devices and electronics are achieving maturity and directly competing or surpassing devices that use conventional materials. CNTs have demonstrated ballistic conduction, minimal scaling effects, high current capacity, low power requirements, and excellent optical/photonic properties; making them the ideal candidate for a new material to replace conventional materials in next-generation electronic and photonic systems. CNTs also demonstrate high stability and flexibility, allowing them to be used in flexible, printable, and/or biocompatible electronics. However, a major challenge to fully commercialize these devices is the scalable placement of CNTs into desired micro/nanopatterns and architectures to translate the superior properties of CNTs into macroscale devices. Precise and high throughput patterning becomes increasingly difficult at nanoscale resolution, but it is essential to fully realize the benefits of CNTs. The relatively long, high aspect ratio structures of CNTs must be preserved to maintain their functionalities, consequently making them more difficult to pattern than conventional materials like metals and polymers. This review comprehensively explores the recent development of innovative CNT patterning techniques with nanoscale lateral resolution. Each technique is critically analyzed and applications for the nanoscale-resolution approaches are demonstrated. Promising techniques and the challenges ahead for future devices and applications are discussed.

Tailoring carbon nanotubes optical properties through chirality-wise silicon ring resonators

Semiconducting single walled carbon nanotubes (s-SWNT) have an immense potential for the development of active optoelectronic functionalities in ultra-compact hybrid photonic circuits. Specifically, s-SWNT have been identified as a very promising solution to implement light sources in the silicon photonics platform. Still, two major challenges remain to fully exploit the potential of this hybrid technology: the limited interaction between s-SWNTs and Si waveguides and the low quantum efficiency of s-SWNTs emission. Silicon micro-ring resonators have the potential capability to overcome these limitations, by providing enhanced light s-SWNT interaction through resonant light recirculation. Here, we demonstrate that Si ring resonators provide SWNT chirality-wise photoluminescence resonance enhancement, releasing a new degree of freedom to tailor s-SWNT optical properties. Specifically, we show that judicious design of the micro-ring geometry allows selectively promoting the emission enhancement of either (8,6) or (8,7) SWNT chiralities present in a high-purity polymer-sorted s-SWNT solution. In addition, we present an analysis of nanometric-sized silicon-on-insulator waveguides that predicts stronger light s-SWNT interaction for transverse-magnetic (TM) modes than for conventionally used transverse-electric (TE) modes.

Correction Scheme for Comparison of Computed and Experimental Optical Transition Energies in Functionalized Single-Walled Carbon Nanotubes

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) introduces red-shifted emission features in the near-infrared spectral range due to exciton localization around the defect site. Such chemical modifications increase their potential use as near-infrared emitters and single-photon sources in telecommunications applications. Density functional theory (DFT) studies using finite-length tube models have been used to calculate their optical transition energies. Predicted energies are typically blue-shifted compared to experiment due to methodology errors including imprecise self-interaction corrections in the density functional and finite-size basis sets. Furthermore, artificial quantum confinement in finite models cannot be corrected by a constant-energy shift since they depend on the degree of exciton localization. Herein, we present a method that corrects the emission energies predicted by time-dependent DFT. Confinement and methodology errors are separately estimated using experimental data for unmodified tubes. Corrected emission energies are in remarkable agreement with experiment, suggesting the value of this straightforward method toward predicting and interpreting the optical features of functionalized SWCNTs.

Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate Electroluminescence Devices

Metal halide perovskite nanostructures hold great promises as nanoscale light sources for integrated photonics due to their excellent optoelectronic properties. However, it remains a great challenge to fabricate halide perovskite nanodevices using traditional lithographic methods because the halide perovskites can be dissolved in polar solvents that are required in the traditional device fabrication process. Herein, we report single CsPbBr₃ nanoplate electroluminescence (EL) devices fabricated by directly growing CsPbBr₃ nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition. Bright EL occurs in the region near the negatively biased contact, with a turn-on voltage of ∼3 V, a narrow full width at half-maximum of 22 nm, and an external quantum efficiency of ∼0.2%. Moreover, through scanning photocurrent microscopy and surface electrostatic potential measurements, we found that the formation of ITO/p-type CsPbBr₃ Schottky barriers with highly efficient carrier injection is essential in realizing the EL. The formation of the ITO/p-type CsPbBr₃ Schottky diode is also confirmed by the corresponding transistor characteristics. The achievement of EL nanodevices enabled by directly grown perovskite nanostructures could find applications in on-chip integrated photonics circuits and systems.

Electrically driven monolithic subwavelength plasmonic interconnect circuits

In the post-Moore era, an electrically driven monolithic optoelectronic integrated circuit (OEIC) fabricated from a single material is pursued globally to enable the construction of wafer-scale compact computing systems with powerful processing capabilities and low-power consumption. We report a monolithic plasmonic interconnect circuit (PIC) consisting of a photovoltaic (PV) cascading detector, Au-strip waveguides, and electrically driven surface plasmon polariton (SPP) sources. These components are fabricated from carbon nanotubes (CNTs) via a CMOS (complementary metal-oxide semiconductor)-compatible doping-free technique in the same feature size, which can be reduced to deep-subwavelength scale (~λ/7 to λ/95, λ = 1340 nm) compared with the 14-nm technique node. An OEIC could potentially be configured as a repeater for data transport because of its "photovoltaic" operation mode to transform SPP energy directly into electricity to drive subsequent electronic circuits. Moreover, chip-scale throughput capability has also been demonstrated by fabricating a 20 × 20 PIC array on a 10 mm × 10 mm wafer. Tailoring photonics for monolithic integration with electronics beyond the diffraction limit opens a new era of chip-level nanoscale electronic-photonic systems, introducing a new path to innovate toward much faster, smaller, and cheaper computing frameworks.

Carbon-Nanotube-Confined Vertical Heterostructures with Asymmetric Contacts

Van der Waals (vdW) heterostructures have received intense attention for their efficient stacking methodology with 2D nanomaterials in vertical dimension. However, it is still a challenge to scale down the lateral size of vdW heterostructures to the nanometer and make proper contacts to achieve optimized performances. Here, a carbon-nanotube-confined vertical heterostructure (CCVH) is employed to address this challenge, in which 2D semiconductors are asymmetrically sandwiched by an individual metallic single-walled carbon nanotube (SWCNT) and a metal electrode. By using WSe₂ and MoS₂ , the CCVH can be made into p-type and n-type field effect transistors with high on/off ratios even when the channel length is 3.3 nm. A complementary inverter was further built with them, indicating their potential in logic circuits with a high integration level. Furthermore, the Fermi level of SWCNTs can be efficiently modulated by the gate voltage, making it competent for both electron and hole injection in the CCVHs. This unique property is shown by the transition of WSe₂ CCVH from unipolar to bipolar, and the transition of WSe₂ /MoS₂ from p-n junction to n-n junction under proper source-drain biases and gate voltages. Therefore, the CCVH, as a member of 1D/2D mixed heterostructures, shows great potentials in future nanoelectronics and nano-optoelectronics.

Carbon nanotube-based three-dimensional monolithic optoelectronic integrated system

Single material-based monolithic optoelectronic integration with complementary metal oxide semiconductor-compatible signal processing circuits is one of the most pursued approaches in the post-Moore era to realize rapid data communication and functional diversification in a limited three-dimensional space. Here, we report an electrically driven carbon nanotube-based on-chip three-dimensional optoelectronic integrated circuit. We demonstrate that photovoltaic receivers, electrically driven transmitters and on-chip electronic circuits can all be fabricated using carbon nanotubes via a complementary metal oxide semiconductor-compatible low-temperature process, providing a seamless integration platform for realizing monolithic three-dimensional optoelectronic integrated circuits with diversified functionality such as the heterogeneous AND gates. These circuits can be vertically scaled down to sub-30 nm and operates in photovoltaic mode at room temperature. Parallel optical communication between functional layers, for example, bottom-layer digital circuits and top-layer memory, has been demonstrated by mapping data using a 2 × 2 transmitter/receiver array, which could be extended as the next generation energy-efficient signal processing paradigm.

Single-material monolithic optoelectronic integrated circuits via CMOS compatible low-temperature approaches are crucial to the continued development of post-Moore electronics. Liu et al., report carbon nanotube based electrically driven 3D monolithic optoelectronic integrated circuits.

Contact-dominated transport in carbon nanotube thin films: toward large-scale fabrication of high performance photovoltaic devices

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been widely regarded as potential channel materials for not only replacing silicon to extend Moore's law but also for building high performance optoelectronic devices. To realize these goals, high quality s-SWCNTs and contacts are needed to outperform devices based on traditional materials such as silicon. For a high quality conducting or active channel, the ideal CNTs consist of a pure s-SWCNTs array with a confined pitch of less than 10 nm via e.g., chemical vapor deposition (CVD) methods, although this has not been realized experimentally. On the other hand, significant progress has been made on solution-processed CNTs. However, only network and low performance optoelectronic devices have been realized. In this study, we systematically studied the performance of devices using solution-processed CNT films with different s-SWCNT purity, with particular emphasis being placed on disentangling those metallic-CNTs (m-CNTs)-dominated low performance and contacts-dominated high performance devices. We demonstrated that using high purity s-SWCNTs allowed for the construction of high performance diodes via a doping-free method. These diodes behave similarly to those based on individual CVD-grown s-CNTs, resulting in 250 mV photovoltage for a typical single diode and more than 4.35 V for cascading cells using the virtual contact technique and thus paving the way for large scale fabrication of higher performance photovoltaic devices using readily available solution processed CNTs.

Essential Role of Viscosity of SWCNT Inks in Homogeneous Conducting Film Formation

Newly developed inorganic single-wall carbon nanotube (SWCNT) inks of the Zn/Al complex and colloidal silica give a quite homogeneous SWCNT film on the polyethylene terephthalate (PET) substrate by the bar-coating method, whereas the surfactant-based SWCNT inks of sodium dodecyl sulfonate (SDS) and sodium dodecyl benzene sulfonate (SDBS) cannot give a homogeneous film. The key properties of SWCNT inks were studied for the production of homogeneous SWCNT films. The contact angle and surface tension of the inorganic dispersant-based SWCNT inks were 70° and 72 mN m(-1), respectively, being close to those of water (71.5° and 71 mN m(-1)). The viscosity was significantly higher than that of water (0.90 mPa·s), consequently, providing sufficient wettability, spreadability, and slow drying of the ink on the substrate, leading to homogeneous film formation. On the other hand, the surfactant dispersant-aided SWCNT inks have the contact angle and surface tension twice lower than the inorganic dispersant-based SWCNT inks, guaranteeing better wettability and spreadability than the inorganic dispersant-based inks. However, the small viscosity close to that of water induces a heterogeneous flow of SWCNT ink on rapid drying, leading to inhomogeneous film formation.

Solid state carbon nanotube device for controllable trion electroluminescence emission

Semiconducting carbon nanotubes (CNTs) have a direct chirality-dependent bandgap and reduced dimensionality-related quantum confinement effects, which are closely related to the performance of optoelectronic devices. Here, taking advantage of the large energy separations between neutral singlet excitons and charged excitons, i.e. trions in CNTs, we have achieved for the first time all trion electroluminescence (EL) emission from chirality-sorted (8,3) and (8,4) CNT-based solid state devices. We showed that strong trion emission can be obtained as a result of localized impact excitation and electrically injected holes, with an estimated efficiency of ∼5 × 10(-4) photons per injected hole. The importance of contact-controlled carrier injection (including symmetric and asymmetric contact configurations) and EL spectral stability for gradually increasing bias were also investigated. The realization of electrically induced pure trion emission opens up a new opportunity for CNT film-based optoelectronic devices, providing a new degree of freedom in controlling the devices to extend potential applications in spin or magnetic optoelectronics fields.

Flexible conducting electrodes based on an embedded double-layer structure of gold ribbons and silver nanowires

A hierarchical double-layer structure of gold ribbons and silver nanowires is fabricated for a flexible electronic conductor that can sensitively detect various deformation modes.

Recent Progress in Obtaining Semiconducting Single-Walled Carbon Nanotubes for Transistor Applications

High purity semiconducting single-walled carbon nanotubes (s-SWCNTs) with a narrow diameter distribution are required for high-performance transistors. Achieving this goal is extremely challenging because the as-grown material contains mixtures of s-SWCNTs and metallic- (m-) SWCNTs with wide diameter distributions, typically inadequate for integrated circuits. Since 2000, numerous ex situ methods have been proposed to improve the purity of the s-SWCNTs. The majority of these techniques fail to maintain the quality and integrity of the s-SWCNTs with a few notable exceptions. Here, the progress in realizing high purity s-SWCNTs in as-grown and post-processed materials is highlighted. A comparison of transistor parameters (such as on/off ratio and field-effect mobility) obtained from test structures establishes the effectiveness of various methods and suggests opportunities for future improvements.

Guided Photoluminescence from Integrated Carbon-Nanotube-Based Optical Waveguides

Thin films and ridge waveguides based on large-diameter semiconducting single-wall carbon nanotubes (s-SWCNTs) dispersed in a polyfluorene derivative are fabricated and optically characterized. Ridge waveguides are designed with appropriate dimensions for single-mode propagation at 1550 nm. Using multimode ridge waveguides, guided s-SWCNT photoluminescence is demonstrated for the first time in the near-infrared telecommunications window.

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.

Length scaling of carbon nanotube electric and photo diodes down to sub-50 nm

Carbon nanotubes (CNTs) are promising candidates for future optoelectronics and logic circuits.1-3 Sub-10 nm channel length CNT transistors have been demonstrated with superb performance.4 Yet, the scaling of CNT p-n diodes or photodiodes, basic elements for most optoelectronic devices, is held back on a scale of micrometers.5-8 Here, we demonstrate that CNT diodes fabricated via a dopant-free technique show good rectifying characteristics and photovoltaic response even when the channel length is scaled to sub-50 nm. By making a trade-off between performance and size, a diode with both channel length and contact width around 100 nm, fabricated on a CNT with a small diameter (d ∼ 1.2 nm), shows a photovoltage of 0.24 V and a fill factor of up to 60%. Study on the dependence of turn-on voltage on scaled channel length reveals transferred charges induced potential barrier at the contact in long channel diodes and the effect of self-adjusting charge distribution. This effect could be utilized for realizing stable and high performance sub-100 nm pitch CNT diodes. As elementary building blocks, such tiny electric and photodiodes could be used in nanoscale rectifiers, photodetectors, light sources, and high-efficiency photovoltaic devices.

Antenna-enhanced optoelectronic probing of carbon nanotubes

We report on the first antenna-enhanced optoelectronic microscopy studies on nanoscale devices. By coupling the emission and excitation to a scanning optical antenna, we are able to locally enhance the electroluminescence and photocurrent along a carbon nanotube device. We show that the emission source of the electroluminescence can be pointlike with a spatial extension below 20 nm. Topographic and antenna-enhanced photocurrent measurements reveal that the emission takes place at the location of highest local electric field indicating that the mechanism behind the emission is the radiative decay of excitons created via impact excitation.

Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells

There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future.

Electroluminescence from serpentine carbon nanotube based light-emitting diodes on quartz

A light-emitting diode is fabricated and characterized on a semiconducting serpentine CNT which has many parallel segments with identical chirality. Compared with the individual CNT and CNT-film devices, the device with parallel segments shows improvement of an order of magnitude in current, significantly larger electroluminescent intensity, and narrower emission bands. Serpentine nanotubes are an ideal choice for practical applications of CNT-based light sources.

Intrinsic region length scaling of heavily doped carbon nanotube p-i-n junctions

We investigated the dependence of the transport properties of heavily doped intratube single-walled carbon nanotube (SWCNT) p-i-n junctions on the length of the intrinsic region by using empirical self-consistent quantum transport simulations. When the length of the intrinsic region is scaled from a few angstroms to over 10 nanometers, the SWCNT p-i-n junction evolves from a tunneling diode with a large negative rectification and large negative differential resistance to one with a large positive rectification (like a conventional positive rectifying diode). The critical length of the intrinsic length is about 8.0 nm. Therefore, one can obtain nanoscale diodes of different performance types by changing the intrinsic region length.

Carbon nanotube photoelectronic and photovoltaic devices and their applications in infrared detection

Semiconducting carbon nanotubes (CNTs) are direct bandgap materials with outstanding electronic and optoelectronic properties and have been investigated for various electronic and optoelectronic device applications, such as light-emitting diodes, photodetectors and photovoltaic cells. Here, a brief review of the various types of CNT diodes is presented, with a focus on one particular type of CNT diodes fabricated via a doping-free process. Their application for constructing high-performance optoelectronic and photovoltaic devices is also discussed, as well as the newly discovered photovoltage multiplication effect in CNTs and its application in improving the efficiency of CNT-based infrared detector.

Electroluminescence in aligned arrays of single-wall carbon nanotubes with asymmetric contacts

High quantum efficiencies and low current thresholds are important properties for all classes of semiconductor light emitting devices (LEDs), including nanoscale emitters based on single wall carbon nanotubes (SWNTs). Among the various configurations that can be considered in SWNT LEDs, two terminal geometries with asymmetric metal contacts offer the simplest solution. In this paper, we study, experimentally and theoretically, the mechanisms of electroluminescence in devices that adopt this design and incorporate perfectly aligned, horizontal arrays of individual SWNTs. The results suggest that exciton mediated electron-hole recombination near the lower work-function contact is the dominant source of photon emission. High current thresholds for electroluminescence in these devices result from diffusion and quenching of excitons near the metal contact.

Spatially resolved electrostatic potential and photocurrent generation in carbon nanotube array devices

We have used laser-excited photocurrent microscopy to map the internal electrostatic potential profile of semiconducting single-walled carbon nanotube (S-SWCNT) array devices with a spatial resolution of 250 nm. The measurements of S-SWCNTs on optically transparent samples provide new insights into the physical principles of device operation and reveal performance-limiting local heterogeneities in the electrostatic potential profile not observable with other imaging techniques. The experiments deliver photocurrent images from the underside of the S-SWCNT-metal contacts and thus enable the direct measurement of the charge carrier transfer lengths at the palladium-S-SWCNT and aluminum-S-SWCNT interfaces. We use the experimental results to formulate design rules for optimized layouts of S-SWCNT-based photovoltaic devices. Furthermore, we demonstrate the external control of the electrostatic potential profile in S-SWCNT array devices equipped with local metal gates.

Channel-length-dependent transport and photovoltaic characteristics of carbon-nanotube-based, barrier-free bipolar diode

Carbon nanotube (CNT) diodes with different channel length between L = 0.6 μm to 3.5 μm are fabricated on the same tube, and the electric and photovoltaic characteristics are investigated. It is found that although the open voltage of the diode increases rapidly for channel length L less than 1.0 μm, it saturates for longer channel devices. On the other hand, the short circuit current of the diode exhibites a clear peak at intermediate channel length of about 1.5 μm, a large leakage current via tunneling for short channel device and significantly decreased current for long channel device due to the increased recombination and channel resistance. The optimal channel length for a CNT diode in photovoltaic application is thus determined to be about 1.5 μm.

Electroluminescence from chirality-sorted (9,7)-semiconducting carbon nanotube devices

We have measured the electroluminescence and photoluminescence of (9,7)-semiconducting carbon nanotube devices and demonstrate that the electroluminescence wavelength is determined by the nanotube's chiral index (n,m). The devices were fabricated on Si₃N₄-membranes by dielectrophoretic assembly of tubes from monochiral dispersion. Electrically driven (9,7)-devices exhibit a single Lorentzian-shaped emission peak at 825 nm in the visible part of the spectrum. The emission could be assigned to the excitonic E22 interband-transition by comparison of the electroluminescence spectra with corresponding photoluminescence excitation maps. We show a linear dependence of the EL peak width on the electrical current, and provide evidence for the inertness of Si₃N₄ surfaces with respect to the nanotubes optical properties.

Self-aligned sub-10-nm nanogap electrode array for large-scale integration

A novel approach to creating a gap on the nanometer scale between two adjacent electrodes of the same or different metals is described. The gap size can be well controlled through sidewall coverage in a self-aligned manner and it can be tuned from 60 nm down to 5 nm with high reproducibility. This technique is fully compatible with traditional microfabrication technology and it is easily implemented to fabricate a nanogap electrode array for integration purposes. An array of short-channel single-walled carbon nanotube field-effect transistors is demonstrated.