Novel Strategy for Diameter-Selective Separation and Functionalization of Single-Wall Carbon Nanotubes

By: R. M. Tromp, A. Afzali, M. Freitag, D. B. Mitzi, and Zh. Chen

Published in: Nano Lett. 2008, 8, 469-472

Purpose of Study

The electronic properties of carbon nanotubes (CNTs) are related to diameter and chirality. Chirality determines whether the CNT is semiconducting or metallic. Efforts to separate CNTs based on chirality and diameter have been met with varying degrees of success. The paper describes a diameter-selective method to separate CNTs.

Methods Used

Size matching between anchor molecules and the diameter of the carbon nanotube was used. This technique makes use of the arene-arene (π- π) interaction between condensed aromatic compounds and carbon nanotubes. This results in the formation of a host-guest pair. The anchor molecule used was C₂₆H₁₇NO₂. C₂₆H₁₇NO₂ has the shape of a folded ribbon with a folding angle of 126.5 °. The arms of the ribbons possess two conjugated carbon rings. These are expected to interact with the CNTs.

To determine the degree of interaction between the anchor and the CNT, the anchor was functionalized to make it soluble in toluene. A mixture of CNT, toluene and C₂₆H₁₇NO₂ was sonicated, then centrifuged. The Raman spectra of the liquid and solid portions were measured.

Key Findings

1. CNTs are insoluble in toluene. However in the presence of the functionalized anchor molecule some of the CNTs went into suspension.

2. Raman spectra of the solid and solution after centrifugation showed different peaks. The precipitate spectra showed peaks for CNTs about 1.6 nm in diameter. While the solution spectra showed peaks for CNTs with diameters of about 1.15 nm. This indicates that the anchor molecule interacted with CNTs with diameters smaller than 1.3 nm.

3. The separation of CNTs is sensitive to diameter but, not to the electronic characteristics of the nanotubes. So both metallic and semiconducting nanotubes below 1.3 nm in diameter were present in solution.

4. Carbon nanotube field effect transistors (CNTFETs) were prepared using larger diameter CNTs and mixed diameter (untreated) CNTs. CNTFETs with larger diameter nanotubes showed better performance over those made with untreated CNTs.

Sulfur as a Novel Nanopatterning Material: An Ultrathin Resist and a Chemically Addressable Template for Nanocrystal Self-Assembly

By: Jonathan Germain, Marco Rolandi, Scott A. Backer, and Jean M. J. Fre´chet

Published in: Adv. Mater. 2008, 20, 1–4

Purpose of Study

The paper describes the development of a high-field patterning lithographic technique in which an AFM was used to induce the localized polymerization of a thin sulfur film. This technique was used to prepare a negative tone resist which may be used as a mask for substrate etching or as a chemical template for the self assembly of nanocrystals of gold.

Methods Used

A sulfur film (2.5 to 4 nm in thickness) was spin-coated onto a Si chip. A conducting AFM tip operated in tapping mode was used to apply a positive bias to the material and thereby induce chemical modifications of the sulfur. This was done in a water-free environment. After rinsing in toluene and ethanol or isopropanol, the unpatterned regions of sulfur were removed and the nanometer scale patterns in the resist were revealed.

Time of flight-secondary ion mass spectroscopy (TOF-SIMS) was used to characterize the products formed on the sulfur resist.

Key Findings

1. TOF-SIMS and etching studies proved that the patterns consist of high molecular weight sulfur. The AFM probe converts the starting material, S₈, into a polymeric sulfur material. This occurs because the high energy field causes S₈ to fragment. These highly reactive fragments subsequently polymerize.

1. The lithographic technique yielded patterns which were as narrow as 25 nm. The patterns exhibited resistance to fluorinated etchants and therefore maybe used for negative tone transfer.

1. The resist was successfully employed as a template for the directed self-assembly of gold nanocrystals. This was achieved by modifying the functional groups present on the patterns and then anchoring Au nanocrystals onto the free thiol groups in specific regions of the resist.

Scanning Photoemission Microscopy of Graphene Sheets on Silicon Dioxide

By: Ki-jeong Kim, Hangil Lee, Jae-Hyun Choi, Young-Sang Youn, Junghun Choi, Hankoo Lee, Tai-Hee Kang, M. C. Jung, H. J. Shin, Hu-Jong Lee, Sehun Kim, and Bongsoo Kim

Published in: Adv. Mater. 2008, 20, 3589–3591

Purpose of Study

1. To use Scanning Photoemission Microscopy (SPEM) to investigate the properties of graphene flakes on SiO₂

2. To determine if there are any differences between monolayer and multilayer graphite in the Carbon 1s core-level spectra and if the layer thickness may be determined using these differences.

Methods Used

Graphene flakes (prepared by the exfoliation of bulk graphite) of monolayer thickness were selected with the aid of optical microscopy. The flakes were deposited onto the surface of a Si wafer covered with a layer of SiO₂ of 300 nm thickness. AFM was used to measure step heights. SPEM measurements were conducted under ultra high vacuum conditions. The SPEM images were obtained using binding energies between 288.2 and 281 eV of the C 1s core-level spectra. The spectra were obtained using emission and incident angles of 55° and 0° respectively. The incident photon beam size on the sample was 1µm.

Key Findings

1. The authors were able to more accurately determine layer thickness using AFM than SPEM images.
2. SPEM was used to create a chemical contrast image which was used to distinguish between single-layer and multi-layer graphene.
3. As the binding energies used were changed different regions of the graphite film became more visible. This indicated the presence of core-level-shifted features. These features represent different chemical states which depend on the thickness of the graphene flakes.
4. The core-level shift in monolayer graphene originates from graphene-SiO₂ interactions.

Definitions

SPEM- A surface is irradiated with UV photons focused to a narrow spot and the total yield (I) of electrons emitted from the surface is measured. I depends on work function and photon energy and is measured as the spot is rastered across the sample. Intensity differences are converted into an image which reflects spatial variations of the work function.

Surfactant-Assisted Orientation of Thin Diblock Copolymer Films

By: Jeong Gon Son, Xavier Bulliard, Huiman Kang, Paul F. Nealey, and Kookheon Char

Published in: Advanced Materials 2008 (preprint)

Published online: August 19, 2008

Purpose of Study

Exact control of the morphology and orientation of block copolymers (BCPs) is difficult to achieve because BCP nanodomains spontaneously assemble into the configuration that minimizes the total free energy of the system. Surfactants modify the surface properties of materials. The researchers set out to induce perpendicular orientation from the top of a polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) diblock copolymer thin film toward the bottom of the substrate by the use of the surfactant oleic acid (OA).

Methods Used

Thermal Gravimetric Analysis (TGA) was used to indirectly study the energetic interaction between OA and the polymer blocks. Neutron Reflectivity was used to determine the position of OA in the film. Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Grazing Incidence Small Angle X-ray Scattering (GISAXS) were used to study film morphology. Ellipsometry was used to study film thickness.

Key Findings

1. OA changed the surface morphology to perpendicular growth on the PS-b-PMMA films.
2. There is preferred interaction between the OA and the PMMA block domains because of the affinity between the acid groups of OA and the hydrophilic part of PMMA. This reduces the surface energy difference between PS and PMMA toward the surface energy neutrality.
3. OA rapidly segregates to the upper part of the film resulting in surface neutral conditions at the top of the film and inducing perpendicular orientation of BCP domains.
4. The perpendicular orientation of BCP thin films extends from the top surface to the inner film but not to the bottom interface. Consequently BCP domains near the substrate adopt a parallel orientation and the BCP film exhibits mixed morphology.