Photoluminescence Recovery from Single-Walled Carbon Nanotubes on Substrates

Horizontal Single-Walled Carbon Nanotube Arrays: Controlled Synthesis, Characterizations, and Applications

Single-walled carbon nanotubes (SWNTs) emerge as a promising material to advance carbon nanoelectronics. However, synthesizing or assembling pure metallic/semiconducting SWNTs required for interconnects/integrated circuits, respectively, by a conventional chemical vapor deposition method or by an assembly technique remains challenging. Recent studies have shown significant scientific breakthroughs in controlled SWNT synthesis/assembly and applications in scaled field effect transistors, which are a critical component in functional nanodevices, thereby rendering the horizontal SWNT array an important candidate for innovating nanotechnology. This review provides a comprehensive analysis of the controlled synthesis, surface assembly, characterization techniques, and potential applications of horizontally aligned SWNT arrays. This review begins with the discussion of synthesis of horizontally aligned SWNTs with regulated direction, density, structure, and theoretical models applied to understand the growth results. Several traditional procedures applied for assembling SWNTs on target surface are also briefly discussed. It then discusses the techniques adopted to characterize SWNTs, ranging from electron/probe microscopy to various optical spectroscopy methods. Prototype applications based on the horizontally aligned SWNTs, such as interconnects, field effect transistors, integrated circuits, and even computers, are subsequently described. Finally, this review concludes with challenges and a brief outlook of the future development in this research field.

Photoluminescence enhancement of aligned arrays of single-walled carbon nanotubes by polymer transfer

The photoluminescence of as-grown, aligned single-walled carbon nanotubes (SWNTs) on quartz is strongly quenched and barely detectable. Here we show that transferring these SWNTs to another substrate such as clean quartz or glass increases their emission efficiency by up to two orders of magnitude. By statistical analysis of large nanotube arrays we show at what point of the transfer process the emission enhancement occurs and how it depends on the receiving substrate and the employed transfer polymer. We find that hydrophobic polystyrene (PS) as the transfer polymer results in higher photoluminescence enhancement than the more hydrophilic poly(methyl methacrylate) (PMMA). Possible mechanisms for this enhancement such as strain relief, disruption of the strong interaction of SWNTs with the substrate and localized emissive states are discussed.

A review of carbon nanotube- and graphene-based flexible thin-film transistors

Carbon nanotubes (CNTs) and graphene have attracted great attention for numerous applications for future flexible electronics, owing to their supreme properties including exceptionally high electronic conductivity and mechanical strength. Here, the progress of CNT- and graphene-based flexible thin-film transistors from material preparation, device fabrication techniques to transistor performance control is reviewed. State-of-the-art fabrication techniques of thin-film transistors are divided into three categories: solid-phase, liquid-phase, and gas-phase techniques, and possible scale-up approaches to achieve realistic production of flexible nanocarbon-based transistors are discussed. In particular, the recent progress in flexible all-carbon nanomaterial transistor research is highlighted, and this all-carbon strategy opens up a perspective to realize extremely flexible, stretchable, and transparent electronics with a relatively low-cost and fast fabrication technique, compared to traditional rigid silicon, metal and metal oxide electronics.

Supramolecular hetero-porphyrin SWNT complexes

The complexation of single walled carbon nanotubes (SWNTs) with neutral, anionic and cationic porphyrins has been investigated under identical complex forming conditions. The determination of the porphyrin loading reveals large differences depending on the nature of the porphyrin used. Combinations of different porphyrins to form mixed hetero-porphyrin complexes shows that the mixture of a cationic and anionic porphyrin results in loading which is an order of magnitude larger than in all other complexes. This complex also exhibits high adsorption and emission intensities and can be regarded as an extended co-operative binary ionic (CBI) solid. The complexes were further studied using Raman spectroscopy, elemental analysis, AFM and cyclic voltammetry.

Electroluminescence from electrolyte-gated carbon nanotube field-effect transistors

We demonstrate near-infrared electroluminescence from ambipolar, electrolyte-gated arrays of highly aligned single-walled carbon nanotubes (SWNT). Using electrolytes instead of traditional oxide dielectrics in carbon nanotube field-effect transistors (FET) facilitates injection and accumulation of high densities of holes and electrons at very low gate voltages with minimal current hysteresis. We observe numerous emission spots, each corresponding to individual nanotubes in the array. The positions of these spots indicate the meeting point of the electron and hole accumulation zones determined by the applied gate and source-drain voltages. The movement of emission spots with gate voltage yields information about relative band gaps, contact resistance, defects, and interaction between carbon nanotubes within the array. Introducing thin layers of HfO(2) and TiO(2) provides a means to modify exciton screening without fundamentally changing the current-voltage characteristics or electroluminescence yield of these devices.

Electroluminescence from suspended and on-substrate metallic single-walled carbon nanotubes

In this work, we carried out electroluminescence (EL) measurements on metallic single-walled carbon nanotubes (SWNTs) and compared the light emission from the suspended section with the on-substrate section along the same SWNT. In addition to the lowest excitonic emission for metallic SWNTs (M(11)), a side peak was observed at an energy of 0.17-0.20 eV lower than the M(11) peak, which is attributed to a phonon-assisted sideband. Interestingly, this side peak was only observed from on-substrate sections but not from suspended sections. This is likely due to the higher electric field used in the EL measurement of on-substrate sections and the asymmetric surroundings for on-substrate SWNT sections. When the drain voltage is increased, either a blue shift or a red shift of the M(11) emission (up to +/-20 meV) was observed in different suspended SWNTs. The red shift can be explained by the temperature-dependence of the M(11) transition energy, whereas the blue shift is surprising and has never been observed before. Some possible mechanisms for the blue shift are discussed.

Creation of nanostructures with poly(methyl methacrylate)-mediated nanotransfer printing

We demonstrate here a PMMA-mediated nanotransfer printing technique for reliably transferring nanoscale building blocks and sequentially building purpose-directed nanostructures. The utilization of PMMA film as a mediator introduced several features to this transfer approach, such as high efficiency, fidelity, universality, controllability, and multilevel transferability. Various nanostructures, such as an SWNTs-on-SAM structure, high-density crossbar array of SWNTs, a hybrid n-ZnO nanowire/p-SWNT cross-junction, a gold nanostructure-SAM-gold sandwich structure, a zigzag array of SWNTs, and gold nanobowl array were generated with this transfer approach. A metallic-semiconducting SWNT cross circuit was built to demonstrate its potential application in fabricating nanoelectronic devices. This technique paves the way to generate various structures with homo- or heterogeneous nanoscale building blocks, which facilitates exploring their fundamental properties and building novel devices.