# Presentation: 627062

## Tunable Semi-Transparent Conductive Filmsfrom Sorted Single-Walled Carbon NanotubesMark Hersam, Northwestern University, DMR-0706067

- Semi-transparent conductive films are used in many applications including flat-panel displays, organic light-emitting diodes, and solar cells.  In this project, single-walled carbon nanotubes (SWNTs) processed by density gradient ultracentrifugation (DGU) are incorporated into semi-transparent conductive films.  Since DGU allows SWNTs to be sorted by their electronic and optical properties, the resulting films have tailorable transparency throughout the visible and infrared portions of the electromagnetic spectrum.

**Tunable Semi-Transparent Conductive Films****from Sorted Single-Walled Carbon Nanotubes****Mark Hersam, Northwestern University, ****DMR-0706067**

**Notes:**
Aim of the project:
	Semi-transparent electrical conductors are critical components in many electronic devices, including light-emitting diodes, photovoltaics, and flat-panel displays.  Consequently, there is growing demand for low-cost semi-transparent conductors that offer not only high conductivity and transparency over a large wavelength range, but an array of other properties such as good mechanical flexibility, environmental stability, and desirable surface morphology.  Currently, the most widely used transparent conductor for electronic devices is indium tin oxide (ITO).  However, ITO is hindered by its relative brittleness, which degrades its performance on flexible substrates, and the limited availability of indium, a rare and expensive element obtained as a byproduct of mining for other elements.
	Single-walled carbon nanotubes (SWNTs) have recently emerged as a promising alternative to ITO for transparent electrical conduction.  These nanomaterials consist entirely of carbon, one of the most abundant elements on Earth, and exhibit both remarkably high conductivities and exceptional mechanical properties (e.g., tensile strength, resilience, etc.).  However, current synthetic methods for producing SWNTs lack control over the physical structure and properties of these nanomaterials.  This polydispersity problem limits their performance in many applications including semi-transparent conductive films.
	The goal of this project is to exploit the recently developed technique of density gradient ultracentrifugation (DGU) to produce SWNTs with homogeneous physical and electronic structure.  These homogeneous SWNTs raw materials will then be incorporated into semi-transparent conductive films with tunable properties such as low sheet resistance and tailorable transparency throughout the visible and infrared portions of the electromagnetic spectrum.
Research results:
	Semi-transparent electrical conductors comprising sorted SWNTs demonstrate enhanced electrical conductivity and optical transmissivity compared to the prior art produced using unsorted SWNTs.  In order to demonstrate the improved performance of films derived from such sorted nanotubes, as-produced HiPco SWNTs (Carbon Nanotechnologies, Inc.) were run through a density gradient optimized to sort by both electronic type and diameters from roughly 10 to 12 Å. After centrifugation, tight colored bands of sorted nanotubes were observed at different densities in the centrifuge tube and subsequently fractionated for characterization through optical absorbance.  The more buoyant fractions consisted predominantly of metallic nanotubes, as evidenced by the suppression of absorbance peaks for wavelengths longer than 700 nm.  Furthermore, these fractions exhibited sharp diameter distributions with strongly peaked optical absorbance spectra, with distributions centered at 553 nm corresponding to ~9.8 Å diameter SWNTs and those centered at 605 nm corresponding to ~11.4 Å diameter SWNTs.  The metal purity of both these solutions was > 94%.
	Following characterization, the nanotube solutions were processed into films using vacuum filtration and transferred to glass and quartz substrates.  Optical measurements on these sorted nanotube films showed significantly higher transmittance in the near-infrared.  This effect comes as a result of both the removal of semiconducting nanotubes and amorphous carbon impurities.  Moreover, the transmittance is also increased over much of the visible spectrum at the expense of low transmittance regions centered about the wavelengths associated with metallic SWNT optical transitions.
	To determine the effect of nanotube sorting on transparent conductor performance, the sheet resistance and transmittance of a series of films generated from sorted and unsorted HiPco solutions were measured.  For visible transmittance, the films derived from sorted metallic SWNTs displayed a roughly 20-fold decrease in sheet resistance compared to unsorted material, while the sheet resistance decreases further by a factor of roughly 30 in the near-infrared range of 2000 to 2600 nm wavelengths.  At select visible wavelengths corresponding to minima in the optical absorbance of the sorted SWNTs, the decreases in sheet resistance are further enhanced.  Increasing the proportion of metallic SWNTs in semi-transparent conductive films results in a greater proportion of nanotubes in the network contributing to conduction, leading to decreases in sheet resistance.  Furthermore, the removal of semiconducting SWNTs and additional carbonaceous impurities during the sorting process produces increases in the transparency of the resulting films.  This two-pronged strategy for optimizing transparent conductor performance produces significant benefits over prior studies.
Significance of the results:
	Applications based on semi-transparent conductive films are likely to benefit from the tunability enabled by SWNTs sorted by DGU.  For example, metallic SWNTs possess low sheet resistances, which is desirable for applications that require high electrical conductivity.  Similarly, diameter sorting of the SWNTs allows for selective optical absorption or transmission over a specified region of the electromagnetic spectrum.  In this manner, conductive “stained glass” with virtually any color can be produced from DGU-sorted SWNTs.  This feature is particularly useful for devices that operate over a narrow wavelength range, such as optical amplifiers, saturable absorbers, and light-emitting diodes.
Future plans:
	Future work will continue in three major directions:
 Further optimization of the DGU process by exploring other surfactants, combinations of surfactants, centrifugation conditions, and SWNT raw materials.
 Exploring other carbon-based nanomaterials such as double-walled carbon nanotubes and graphene.
 Utilization of the SWNT semi-transparent conductive films in device structures such as organic light-emitting diodes and solar cells.

## Nanoscape, The Journal of UndergraduateResearch in Nanoscience, Now Available OnlineMark Hersam, Northwestern University, DMR-0706067

- Mark Hersam has served as the Editor-in-Chief of *Nanoscape, the Journal of Undergraduate Research in Nanoscience*, for the past 4 years.  In addition to authoring original research papers, undergraduate students participate in the peer review process of *Nanoscape*, thus exposing them to all aspects of scientific publishing.  Recently, *Nanoscape* launched an interactive website that has streamlined the submission and reviewing of manuscripts.  Importantly, the website also provides global dissemination as all past and current *Nanoscape* articles can be downloaded in pdf format.

***Nanoscape*****, *****The Journal of Undergraduate******Research in Nanoscience*****, Now Available Online****Mark Hersam, Northwestern University, ****DMR-0706067**

**http://www.nanoscape.northwestern.edu/**