High-Resolution TEM and the Application of Direct and Indirect Aberration Correction

Hetherington, Chang, Haigh, Nellist, Gontard, Dunin-Borkowski, Kirkland. High-Resolution TEM and the Application of Direct and Indirect Aberration Correction. Microscopy and Microanalysis. 14 60-67(2008).

Purpose of the Study

The purpose of the study was to apply a combination of both direct and indirect aberration correction to the high-resolution mode of a transmission electron microscope and to study the resulting images.

Methods

Delocalization in images has traditionally led to a difficulty in attaining high resolution micrographs. The development of direct and indirect aberration correction has led to an improvement in the resolution of HRTEM images. Direct aberration correction involves the insertion of elements below the objective lens to correct for spherical aberration. Indirect correction involves taking a series of tilt images and post-processing to remove aberrations.

The instrument used was a JEOL 200 kV FEG-STEM with both probe and imaging aberration correctors. The correctors are hexapoles which enhance resolution from 0.19 nm to less than 0.12 nm. Resolution is then limited by chromatic aberration and energy spread. Diffractograms tilted through a range of angles spaced 18 mrads apart are collected to create a wave aberration correction function.

Indirect and direct methods may be combined. Optical correction is taken to the third order with post-processing aberration coefficients giving a 5^th order correction.

Key Findings

1. Restoration of the exit wavefunction reveals more structural detail

2. <111> Strontium Titanate showed a resolution improvement from 0.12 nm to 0.08 nm

3. Correction of pre- and post-field allows for large condenser and aperture sizes, which allows the instrument to be run as a scanning confocal electron microscope to produce depth of field measurements

Definitions

Confocal Imaging: An imaging technique where pinholes are used to eliminate out-of-focus light

Exit Wavefunction: Used to simulate images of the focal series and compare with experimental results in order to minimize delocalization

Oligonucleotide Loading Determines Cellular Uptake of DNA-Modified Gold Nanoparticles

Purpose
To examine the cellular uptake of gold nanoparticles (~13nm) densely functionalized with antisense oligonucleotide sequences.
-Previously shown that these NPs can be used as agents for transfection or gene regulation
-Do not require additives/other reagents to enter cells effectively
-Antisense oligonucleotides can bind their targets with high affinity

Methods
-Cofunctionalized Au NPs with OEG (oligoethylene glycol) and antisense DNA to form “ASNPs” with varying surface concentrations of single-stranded DNA (ssDNA)
-Incubated three cell lines (differing species, tissue types represented) with ASNPs, investigated uptake of particles by digesting cells and measuring Au content by ICP-MS
-Analyzed ASNP sizes before and after exposure to media by DLS & fluorescence-based protein quantification assay

Key Findings
-Cell uptake of ASNPs was dependent on surface concentrations of ssDNA on the ASNPs.
-Protein adsorption on ASNP surface dependent on surface concentration of ssDNA, as confirmed by DLS & the fluorescence-based protein quantification assay
-Even at high doses, OEG-functionalized NPs (no ssDNA) did not demonstrate significant uptake by cells, indicating that oligonucleotides provided uptake ability.’
-Mechanism of ASNP uptake remains to be studied, but protein adsorption is likely an important factor.

Definitions
-Antisense oligonucleotide: ssDNA or single-stranded RNA that can be targeted to a complementary strand of oligonucleotide.
-Oligoethylene glycol: a oligomer of ethylene glycol that, when immobilized on a surface, yields a hydrophilic, biologically inert, and neutrally charged monolayer.
-ICP-MS: inductively coupled plasma mass spectroscopy; very sensitive

Review: Homogeneously alloyed CdSxSe1-x nanocrystals: Synthesis, characterization, and composition/size-dependent band gap

Swafford, Laura A. ; Weigand, Lauren A.; Bowers II, Michael J.; McBride, James R.; Rapaport, Jason L.; Watt, Tony L.; Dixit, Sriram K.; Feldman, Leonard C.; Rosenthal, Sandra J.

Journal of the American Chemical Society, v 128, n 37, Sep 20, 2006, p 12299-12306

Purpose: The puprose of this experiment was to synthesize homogenous CdS_xSe_1-x nanocrystals and characterize nanocrystals utilizing a variety of techniques that would precisely determine the size, structure and exact composition of the nanocrystals. The composition of the nanocrystals were also varied by changing the reaction conditions.

Method: After the nanocrystals were synthesized using previous methods with minor modifications, the nanocrystals were characterized using UV-visable absorption spectroscopy, TEM, X-Ray diffractrometry, and Rutherford backscattering spectometry.

Vary reaction method and conditions in order to tailor the composition of the nanocrystals.

Key Findings: The nanocrystals synthesized using this method were very homogeneous.  The crystals were very homogeneous thoughout, with a possible small CdS core, which scarcely contributed to the total volume of the nanocrystal.  By varying the amounts of Se and S added into the solution, the compostion of the nanocrystals could be controlled.  The rates in which the Se and S solutions were added to the Cd solution also effected the homogeneity.  The amount of tributylphosphine that was used in the reaction also has an effect on the compostion of the nanocrystals, from allowing the nanocrystals to grow into spherical shapes or into rods.

Definitions: TEM: Transmission Electron Microscopy

Key Note: This was the first time that CdS_xSe_1-x nanocrystals were homogeneously synthesized in for all values of x.

SYNTHESIS OF MONODISPERSE SUB-3 NM RE2 O3 AND GD2 O3 :RE3 +

SYNTHESIS OF MONODISPERSE SUB-3 NM RE2 O3 AND GD2 O3 :RE3 +
NANOCRYSTALS
S.V. MAHAJAN AND J.H. DICKERSON
Nanotechnology 18 (2007) 325605 (6pp)

Purpose
The purpose of this paper was to investigate synthetic methods that would yield sub-3nm monodispersed europium sequioxide, terbium sequioxide, RE3 +-doped gadolinium or yttrium sequioxides.
Methods
Several characterization techniques were used to investigate the particles. FTIR was
used to determine the molecular binding energy of the organic ligand to the surface of the material. XRD was used to determine the cubic structure of the material. TEM was used to determine the size and size distribution of the material. Spectrophotofluorometer was used to determine the absorption and emission spectra of the material. Thermogravimetric analysis was used to determine the decomposition temperature of the material.
Key Findings
• The thermal stabilities of all the particles studied are similar.
• The cubic structure for all the particles studied is body centered cubic.
• Luminescence studies show that this property is size-dependent.
• Beginning oleic acid is related to nanocrystal size.
Definitions
• RE : rare earth metals. In this paper, that would mean europium, terbium and
gadolinium.
• Sesquioxides : molecule that has three oxygens and two of another element. In this
paper, that would mean Eu2 O3 . Other examples include aluminum oxide, Al2 O3 .
• FTIR : Fourier transform infrared spectrophotometer
• XRD : X-ray Diffractometer
• TEM : Tunneling Electron Microscope

Laser Pulse Induced Gold Nanoparticle Gratings

Hung, Wen-Chi. Laser Pulse Induced Gold Nanoparticle Gratings. Applied Physics Letters 93, 2008.

Purpose

Synthesis of gold nanoparticle gratings using polarized laser pulses and characterization of their optical diffraction properties.

Methods

A pair Nd-YAG laser was used to produce the gold nanoparticle gratings via spatially periodic laser ablation on a gold thinfilm. The morphology of the gold nanoparticles was then examined using SEM and dark-filed optical microscopy.

Key Findings

1. Morphology is dependant on the fluence of the laser pulse.

2. In the SEM images, a fringe pattern occurs due to localized temperature differences on the gold surface. The higher temperature regions (and brighter bands) result in the formation of the gold nanoparticles. Some nanoparticle formation was observed on the dark stries, though this is probably due to sputtering of adjacent nanoparticles.

3. When a higher fluence laser pulse was used, nanoparticle formation was observed in both light and dark regions of the SEM image. However, the light and dark bands represented areas of high and low concentration, respectively.

4. The diffraction studies revealed that maximum diffraction energies occur at 680 nm and 620 nm for the low and high fluence laser pulses, respectively.

Definition

Nd-YAG laser – yttrium aluminum garnet laser

fluence – number of particles intersecting a unit area

SEM – Scanning Electron Microscope

Networked Gold-Nanoparticle Coatings on Polyethylene: Charge Transport and Strain Sensitivity

Tobias Vossmeyer,* Carsten Stolte, Michael Ijeh, Andreas Kornowski, and Horst Weller

 

Adv. Funct. Mater.  2008, 18, 1611-1616.

 

Purpose of Study

 

Gold nanoparticles assembled into a matrix using dithiol chemistry have potential applications in strain sensing.  The authors want to determine the effects of mechanical strain on the electrical response of layered gold nanoparticles tethered using a dithiol ligand using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrical conductivity measurements.

 

Methods

 

Gold nanoparticles modified with 1,9 nonanedithiol ligands were deposited onto a functionalized low density polyethylene (LDPE) substrate.  Charge transport properties were measured at varying temperatures of the LDPE/Au samples.  The thermal expansion of the polyethylene film induced a mechanical strain on the gold nanoparticles.  Topography of the LDPE/Au layers were characterized using SEM and TEM images.

 

Key Findings

 

• Gold nanoparticles can be deposited layer-by-layer via self-assembly onto oxidized LDPE.
• Results are consistent with previously reported values for gold nanoparticles on other substrates.
• The relaxation and expansion response of the gold nanoparticles to strain cycling is fully reversible and reproducible under the conditions of the experiment with a guage sensitivity factor of 11-20.

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.

Manipulation and light-induced agglomeration of carbon nanotubes through optical trapping of attached silver nanoparticles

Chao Shi, Yi Zhang, Claire Gu, Leo Seballos and Jin Z Zhang

Nanotechnology 19 (2008).  p. 215304-1-4.

 Purpose of the study

 Evaluate a method of using lasers to control carbon nanotube position and agglomeration through the use of silver nanoparticles.

 Methods

 Silver nanoparticles with an average diameter of 25nm were fabricated by reducing silver nitrate with sodium citrate.  Size was determined using transmission electron microscopy (TEM).  Silver was chosen as the base metal because it has a surface plasmon resonance (SPR) in the visible region thereby simplifying the technique.  Multi-walled carbon nanotubes (MWCNTs) were synthesized by thermal enchanced chemical vapor deposition with Al₂O₃ -coated (200nm) silicon wafers as the substrate.  MWCNTs were subsequently scraped off the silicon substrate for mixture with SNPs.  High resolution TEM and Raman scattering were used to measure dimensions (30micron length; 20nm diameter) and multi-walled structure.  To verify adhesion of SNPs to MWCNTs, a TEM image was taken before and after adding SNPs to MWCNTs in an isopropyl alcohol solvent.  Optical trapping was achieved by applying a TEM₀₀ mode laser (532nm wavelength) to a sample (water-based solution of SNP-attached MWCNTs) placed on a glass slide.  Trapping was confirmed by taking TEM images before and after moving the slide.  An un-trapped MWCNT bundle is used as a reference point on the slide demonstrating relative movement of the trapped bundle to the un-trapped bundle. 

 Key Findings

1. SNPs appear on or at junctions of MWCNT implying binding; most likely due to electrostatic and/or van der Waals forces.
2. Optical trapping was not observed with MWCNTs when SNPs were not present.
3. Two-dimensional optical trapping was observed when SNPs were bound.
4. The minimum volume ratio of SNPs/CNTs to achieve optical trapping is 1:4.
5. Hydrodynamic drag force determines the maximum speed at which a CNT bundle can remain optically trapped:     F_drag = hdu    (h = viscosity; d = longest dimension of the moving object; u = object’s velocity relative to the fluid)
6. MWCNT cannot exceed 24.8micron s-1 and remain optically trapped all else remaining equal (water solution, size/shape of the MWCNT, fixed laser power).
7. MWCNT agglomeration was observed under the laser beam in a donut-shaped disk with the outer diameter reaching 100micron and center hole diameter reaching 20micron.  Size of the microstructure as well as formation rate increased with increased laser power.

Definitions

Surface plasmon resonance:  Light-induced excitation of surface electron gas surrounding lattice metallic structures.

Optical trapping:  method of stabilizing and manipulating position of nanoparticles through use of laser-induced gradient forces.

Transverse ElectroMagnetic Mode (TEM₀₀ mode laser): type of laser in which there are no electric or magnetic fields in the direction of propagation.

Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity

Na Tian, et al. Science 316, 732 (2007);

Purpose:

A high yield method for fabricating noble metal nanocrystals with high-index facets is demonstrated.  Specifically, tetrahexahedral (THH) platinum nanocrystals are fabricated and shown to have enhanced catalytic properties which arise from an increased surface density of atomic steps and dangling bonds.

Method:

THH Pt NCs were fabricated via an eletrochemical treatment of Pt nanospheres electrodeposited on a glassy carbon substrate.  A 10Hz square-wave potential was applied using a standard three-electrode electrochemical cell and sulfuric and ascorbic acids at room temperature.  When starting with a typical dispersion of 750nm nanospheres, an example 60 minute run demonstrates the generation of 217nm diameter THH Pt NCs with greater than a 90% yield.  Tuning of experimental conditions allows for the production of THH Pt NCs down to 20nm in diameter.  The size and shape of the THH Pt NCs are characterized by scanning electron microscopy and transmission electron microscopy.

Key Findings:

• The catalytic activity of THH Pt NCs is shown to be 160 – 400% enhanced over that of Pt nanospheres and 200-310% over commercial Pt/C catalysts.
• The enhanced catalytic activity is shown to arise from high-index facets which exhibit a greater density of atomic steps and dangling bonds.
• Despite the large surface energy of the high-index planes, THH Pt NCs are demonstrated to be stable up to 800C.
• Lastly, the THH Pt NCs are applied in promoting the electro-oxidation of both formic acid and ethanol (which are promising alternative fuels for direct fuel cells) and are shown to yield similar improvements to output current density.

Definitions:

THH: tetrahexahedral, NC: nanocrystal, Catalyst: substance that increases the rate of a reaction

Thermal Conductivity of Polyethylene Chains Using Molecular Dynamics Simulations

Thermal Conductivity of Polyethylene Chains Using Molecular Dynamics Simulations

Asegun Henry and Gang Chen

Proceedings of 3rd Energy Nanotechnology International Conference
ENIC2008
August 10-14, 2008, Jacksonville, Florida USA

Purpose and Motivation

Polyethylene (and polymers in general) have low thermal conductivity. However, when mechanically stretched, polyethylene exhibits an order of magnitude increase in thermal conductivity. Since polyethylene is cheap, this motivates the study for its use in thermal applications.

The purpose of the study was to find the heat conductivity limit along a single chain of polyethylene.

Method

A modified molecular dynamics code, LAMMPS, was used to infer thermal conductivity. The polyethylene atoms were modeled as a chain with boundary conditions. The hydrogen and carbon atoms had their own degrees of freedom using the AIREBO potential.

The Green-Kubo relations were used to find thermal conductivity from the molecular trajectories.

Results

Infinite (theoretical) thermal conductivity was found for large systems. Faster divergence occurred with increased system size.

A finite thermal conductivity was calculated for small systems.

Conclusions

The resultant thermal conductivity figures arise from a physical aspect from the model (as opposed to numerical artifacts). In addition, the application of boundary conditions increases scattering of phonons; hence, smaller systems experienced slower divergence.

So, heat conduction in bulk polyethylene limited by disordered entanglement.

Notes:

LAMMPS: Large-scale Atomic/Molecular Massively Parallel Simulator developed by Sandia Labs.

AIREBO: Adaptive Intermolecular Reactive Empirical Bond Order potential is an empirical classical many-body potential.

Green-Kubo relations: give exact mathematical expression for transport coefficients in terms of integrals of time correlation functions.