Semiconductor Carbon Research Articles

Three-dimensional electromagnetic breathers in carbon nanotubes with the field inhomogeneity along their axes

We study the propagation of extremely short electromagnetic three-dimensional bipolar pulses in an array of semiconductor carbon nanotubes. The heterogeneity of the pulse field along the axis of the nanotubes is accounted for the first time. The evolution of the electromagnetic field and the charge density of the sample are described by Maxwell's equations supplemented by the continuity equation. Our analysis reveals for the first time the possibility of propagation of three-dimensional electromagnetic breathers in CNTs arrays. Specifically, we found that the propagation of short electromagnetic pulse induces a redistribution of the electron density in the sample.

Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C60 increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz...

Photocatalytically Prepared Metal Nanocluster–Oxide Semiconductor–Carbon Nanocomposite Electrodes for Driving Multielectron Transfer

Heterogeneous photocatalysis can be used to generate metal-nanocluster-decorated oxide semiconductor–carbon nanocomposite matrixes for driving multielectron processes of practical import. The oxide semiconductor nanoparticles in such assemblies not only facilitate heterogeneous photocatalytic deposition of the metal nanoclusters but have several important functions that are highlighted in this Perspective. This Perspective additionally describes structure–property relationships of various mono-, bi-, and trimetallic electrocatalysts and the roles of the carbon support and the oxide semiconductor in the performance and durability of the overall architectures. Further applicability of such nanocomposites in value-added environmental remediation, such as the conversion of carbon dioxide to alcohol fuels, is discussed.

Large Scale Assembly of Pristine Semiconducting Carbon Nanotube Network-Based Devices Exhibiting Intrinsic Characteristics

We report the large-scale fabrication of high-performance field effect transistors (FETs) based on pristine semiconducting single-walled carbon nanotube (s-SWCNT) networks without bundles or organic impurities, thus exhibiting its intrinsic characteristics. Here, the solution of pristine s-SWCNTs without bundles or organic impurities was prepared in nonpolar solvent via filtration and centrifugation processes, and the s-SWCNTs in the solution were selectively assembled onto specific regions on the solid substrates via a directed assembly strategy. The fabricated FET devices based on such pristine s-SWCNT networks could exhibit a rather large on–off ratio up to over ∼106 and a subthreshold swing as small as ∼490 mV/dec, which are comparable to those of a single s-SWCNT-based device with the same device structure. Importantly, the s-SWCNT devices exhibited anomalous gating behaviors such as an on-current saturation at a large gate bias and unconventional scaling behaviors, which are quite different from those of previous SWCNT network-based devices. Such anomalous behaviors can be explained by a simple model based on the networks of contact and CNT resistances, which implies that the anomalous behaviors are, in fact, the intrinsic characteristics of pristine s-SWCNT network-based devices. Our work provides a new insight about the intrinsic characteristics of pristine s-SWCNT network-based devices and, thus, should be an important guideline for the future research and applications of high-performance s-SWCNT network-based devices.

A highly selective non-radical diazo coupling provides low cost semi-conducting carbon nanotubes

Despite their recognized potential for low cost, flexible and high frequency electronics and photovoltaic, semi-conducting carbon nanotubes (sc-CNTs) did not lead to industrial development yet because they are synthesized as a mixture with metallic carbon nanotubes (m-CNTs). Sorting has been widely studied but cost remains high. Here, the proposed CNT chemical treatment uses diazoether with unprecedented selectivity, avoiding m-CNT and sc-CNT separation. Indeed like conventional diazonium coupling, partly selective for m-CNTs, diazoether coupling induces extinction of nanotube conductivity. The diazoether proved 50times more selective than diazonium as shown by luminescence, Raman spectroscopy and electrical conductivity. Treated CNT solutions containing inactive m-CNTs and highly preserved sc-CNTs were used in transistors with highly increased ON/OFF ratio. Furthermore, the diazoether/CNT reaction was found to follow an unexpected coupling mechanism: a stoichiometric, radical-free reaction instead of a radical chain reaction for diazonium. This drastic change in mechanism, together with a complete description of the reaction kinetics and thermodynamics, brought easy tuning and fine control on the reaction. As a versatile treatment diazoether provides a cheap sc-CNT material for large scale use in printed electronics.

A betavoltaic microcell based on Au/s-SWCNTs/Ti Schottky junction

This paper reports a new type betavoltaic (BV) microcell based on intrinsic p-type semiconductive single-walled carbon nanotubes (s-SWCNTs). Our device composes of Au and Ti asymmetrical electrode pairs with s-SWCNTs laid over them forming Schottky junction with Ti and ohmic contact with Au. SWCNT bundles were self-assembled between Au and Ti electrodes by Dieletrophoretic (DEP) technique, and metallic SWCNTs (m-SWCNTs) were selectively eliminated by current-induced electrical breakdown method. 3.3mCi/cm2 thin-film radioisotope 63Ni serves as beta particles source, with 100-year half-time and low toxicity. Test results show that SWCNTs BV microcell has the open circuit voltage (VOC) of 26mV, short circuit current density (JSC) of 2.19μA/cm2 and energy conversion efficiency (η) of 5.20%.

Brightening of carbon nanotube photoluminescence through the incorporation of sp3 defects

Semiconducting carbon nanotubes promise a broad range of potential applications in optoelectronics and imaging, but their photon-conversion efficiency is relatively low. Quantum theory suggests that nanotube photoluminescence is intrinsically inefficient because of low-lying 'dark' exciton states. Here we demonstrate the significant brightening of nanotube photoluminescence (up to 28-fold) through the creation of an optically allowed defect state that resides below the predicted energy level of the dark excitons. Emission from this new state generates a photoluminescence peak that is red-shifted by as much as 254 meV from the nanotube's original excitonic transition. We also found that the attachment of electron-withdrawing substituents to carbon nanotubes systematically drives this defect state further down the energy ladder. Our experiments show that the material's photoluminescence quantum yield increases exponentially as a function of the shifted emission energy. This work lays the foundation for chemical control of defect quantum states in low-dimensional carbon materials.

Silver Phosphate/Carbon Nanotube-Stabilized Pickering Emulsion for Highly Efficient Photocatalysis

This study demonstrates a first exploitation of the unique properties inherent in Pickering emulsions to develop a new kind of photocatalytic system. We engineered the system by using silver phosphate (Ag3PO4) as a photocatalytic active metal oxide semiconductor and multiwalled carbon nanotubes (MWNTs) as a hydrophobic conducting nanostructure to form the Pickering emulsions. The photocatalytic activity of the as-formed Ag3PO4/MWNT-stabilized Pickering emulsion-based system is studied toward dye decomposition and oxygen evolution. Results imply that the Pickering emulsion-based photocatalytic system exhibits a much higher efficiency, as compared with traditional solution-dispersed photocatalytic system. This high efficiency is elucidated in terms of the unique properties inherent in Pickering emulsions including (i) the self-assembled Ag3PO4/MWNT nanohybrid at water/oil interface, well ensuring a large surface area of the photocatalyst, (ii) the use of MWNTs to facilitate the formation of amphiphilic nanostructures self-assembled at water/oil interface, promoting the charge separation of the semiconductor through the π–π network of MWNTs by shuttling and storing photogenerated electrons from the visible light irradiated Ag3PO4, and (iii) the separation of the product (e.g., O2 evolved from water oxidation) from the reactants during the photocatalytic process, well accelerating the photocatalytic reactions. In addition to the high efficiency, the fast and simple procedures employed for demulsifying (e.g., sonication or centrifugation) and re-emulsifying (e.g., shaking) essentially make our Pickering emulsion-based photocatalytic system technically simple and thus practically applicable. This study opens a new way to developing novel photocatalytic systems with high efficiency and good practical applicability based on Pickering emulsion science and technology.

Semi-conducting carbon nanotube as variable capacitor

This paper proposes a novel, one-part, variable capacitor, using semi-conducting carbon nanotube (CNT). This variable capacitor works based on the change in the electronic structure of CNTs under applied voltage and deformations. Positive and negative charges are stored at both ends of a non-zero band gap nanotube which works as metallic electrodes in parallel plate capacitors. Also the neutral strip in the middle acts as the dielectric part of a conventional capacitor under the influence of an external electric field. Mechanical strains on carbon nanotube change its band gap energy and thus the length of neutral strip and charged regions. The lengths of these parts are primarily dependent on the nanotube chirality, deformation mode and applied voltage. This way, different parts of a conventional cantilever, parallel plate or bridge capacitor are reduced to a one part semi-conducting CNT capacitor. Analytical calculations based on classical electrostatics and density of states (DOS) relations are employed to investigate the effect of CNTs geometry, applied voltage and deformations on capacitive features. The proposed CNT-variable-capacitor can be useful for nano-electromechanical systems (NEMS), including displacement measurement sensors and tunable capacitor in integrated circuits.

Extremely short electromagnetic pulses in an array of carbon nanotubes with a longitudinal field inhomogeneity

The propagation of bipolar electromagnetic pulses in an array of semiconductor carbon nanotubes has been investigated. The inhomogeneity of the pulse field along the axis of the nanotubes has been taken into account for the first time. The evolution of the electromagnetic field and charge density in the sample has been described by the set of Maxwell's equations and the continuity equation. The possibility of stable propagation of bipolar electromagnetic pulses occurring in an array of nanotubes has been demonstrated by numerical simulation. It has been shown that the propagation of the electromagnetic pulses induces the redis� tribution of the electron density in the sample.

Design length scales for carbon nanotube photoabsorber based photovoltaic materials and devices

Semiconducting carbon nanotubes are attractive materials for harvesting light in photovoltaic solar cells and photodetectors. A crucial aspect of designing efficient photovoltaic devices using nanotubes is minimizing the length scale for the absorption of light (LA) and maximizing the length scale across which excitons diffuse (LD) in fibers and films of these materials. In order to facilitate the optimization of these parameters, here we model how LA and LD are affected by nanotube bandgap polydispersity, inter-nanotube coupling, film disorder, orientation, and defects. Our models are guided by previous experimental measurements of optical absorption spectra and exciton inter-nanotube transfer rates made on isolated and bundled nanotubes in conjunction with kinetic Monte Carlo simulations. Our results provide criteria for materials selection and the design of efficient carbon nanotube-based light harvesting devices, in various architectures.

Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes

We show that excitons are not the unique outcome of photoexcitation in single-walled carbon nanotubes (SWNTs). Our experiments of transient photoinduced absorption suggest that charge carriers are formed with quantum yield of a few percent and that such species strongly affect the long-lived transient spectrum. Photogenerated charge carriers induce strong local electric fields that shift by the Stark effect the second subband exciton absorption in SWNTs, resulting in a characteristic derivative shape of the transient absorption spectra.

Electronic Structure of Semiconducting and Metallic Tubes in TiO2/Carbon Nanotube Heterojunctions: Density Functional Theory Calculations.

The electronic structure of the TiO2(110) surface interfaced with both a semiconducting and metallic carbon nanotube (CNT) was investigated by density functional theory. Our simulations rationalized visible light photocatalytic activity of CNT/TiO2 hybrid materials higher than that under ultraviolent irradiation and showed that the photoactivity of a semiconducting CNT decorating TiO2 is better than that of the metallic CNT/TiO2 system due to efficient charge separation across the interface. This suggests that semiconducting CNT/TiO2 could be a potential photovoltaic material. In contrast, strong interaction between a metallic CNT and TiO2 leads to large charge transfer. Such charge transfer reduces the built-in potential, in turn resulting in inefficient charge separation. Functionalizing the metallic CNT with a small platinum cluster can increase the built-in potential and drive charge separation. These observations indicate that the CNT/TiO2 interface can be a potential photovoltaic material by a metal cluster decorating a CNT despite a real tube being composed of the mixture of metallic and semiconducting CNTs.

Our recent study on nanomaeterials for gas sensing applictaion

Recently, novel materials such as semiconductor metal oxide (SMO) nanowires (NWs), carbon nanotubes (CNTs), and hybrid materials SMO/CNTs have been attractively received attention for gas sensing applications. These materials are potential candidates for improving the well known “3S”: Sensitivity, Selectivity and Stability. In this article, we describe our recent studies on synthesis and characterizations of nanomaterials for gas-sensing applications. The focused topics include are: (i) various system of hybrid materials made CNTs and SMO; and (ii) quasi-one-dimension (Q1D) nanostructure of SMO materials. The synthesis, characterizations and gas-sensing properties are deal thoroughly. Gas-sensing mechanism of those materials, possibility producing new novel materials and other novel applications are also discussed

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.

PROPAGATION OF LASER BEAMS IN AN ARRAY OF SEMICONDUCTOR CARBON NANOTUBES

In this paper, we consider propagation of a monochromatic laser beam in an array of semiconductor carbon nanotubes. Initial distribution of the beam intensity is taken in the form of a Gaussian profile in the plane perpendicular to the wave vector. The electromagnetic field in an array of nanotubes is described by Maxwell equations, reduced to a multidimensional wave equation. With an approximation of the slowly varying amplitudes and phases, we derive the effective equation describing the time-averaged field intensity distribution of the laser beam in a medium. Numerical solution of the derived equations allows us to analyze the dependence of the diffractive spreading of the beam on its frequency and initial amplitude. Furthermore, the influence of the nanotube radius on the diffractive spreading of the laser beam is investigated.

Photoexcitation Dynamics of Coupled Semiconducting Carbon Nanotube Thin Films

Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5-10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ~10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices.

(Invited) Exciton Transport and Manipulation in Colloidal Semiconducting Carbon Nanotubes

not Available.

In Situ Real-Time TEM Reveals Growth, Transformation and Function in One-Dimensional Nanoscale Materials: From a Nanotechnology Perspective

This paper summarises recent developments in in situ TEM instrumentation and operation conditions. The focus of the discussion is on demonstrating how improved understanding of fundamental physical phenomena associated with nanowire or nanotube materials, revealed by following transformations in real time and high resolution, can assist the engineering of emerging electronic and optoelectronic devices. Special attention is given to Si, Ge, and compound semiconductor nanowires and carbon nanotubes (CNTs) as one of the most promising building blocks for devices inspired by nanotechnology.

Octagonal defects at carbon nanotube junctions.

We investigate knee-shaped junctions of semiconductor zigzag carbon nanotubes. Two dissimilar octagons appear at such junctions; one of them can reconstruct into a pair of pentagons. The junction with two octagons presents two degenerate localized states at Fermi energy (E F). The reconstructed junction has only one state near E F, indicating that these localized states are related to the octagonal defects. The inclusion of Coulomb interaction splits the localized states in the junction with two octagons, yielding an antiferromagnetic system.