Stephan J. Stranick

Nanoscale Characterization of Gold Colloid Monolayers: A Comparison of Four Techniques

Atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and near-field scanning optical microscopy (NSOM) have been used to characterize the nanostructure of Au colloid-based surfaces. Because these substrates are composed of particles whose dimensions are known prior to assembly, they are well-suited for a critical comparison of the capabilities and limitations of each nanoscale imaging technique. The three criteria for this comparison, which are relevant to the field of nanoparticle assemblies in general, are (i) accuracy in establishing particle size, particle coverage, and interparticle spacing; (ii) accuracy in delineating surface topography; and (iii) ease of sample preparation, data acquisition, and image analysis. For colloidal Au arrays, TEM gives the most reliable size and spacing information but exhibits the greatest constraints with regard to sample preparation; in contrast, AFM is widely applicable but yields data that are the least straightforward to interpret. For accurate information regarding nanometer-scale architecture of particle-based surfaces, a combination of at least one scanning probe method (AFM, NSOM) and one accelerated-electron method (TEM, FE-SEM) is required.

Programmable vector point-spread function engineering

We use two nematic liquid crystal spatial light modulators (SLMs) to control the vector point spread function (VPSF) of a 1.3 numerical aperture (NA) microscope objective. This is achieved by controlling the polarization and relative phase of the electric field in the objectives pupil. We measure the resulting VPSFs for several different pupil field polarization states. By using single fluorescent molecules as local field probes, we are able to map out the focal field distributions and polarization purity of the synthesized fields. We report the achieved field purity and address the experimental issues that currently limit it.

Horizontal growth and in situ assembly of oriented zinc oxide nanowires

The positioning and directed assembly of semiconductor nanowires (NWs) is of considerable current interest for “bottom-up” approaches to the engineering of intricate structures from nanoscale building blocks. We report a horizontal growth mode for ZnO NWs on the (112¯0) sapphire surface in which NWs grow in the [11¯00]sap direction. This growth mode strictly depends on the size and spacing of the Au nanodroplet catalysts and competes with the vertical growth of the NWs. An approach is presented which promotes the horizontal growth, in situ alignment, and predictable positioning of ZnO NWs. This strategy allows for the large scale assembly of NWs, width control, and production of quantum wires.

Comparison of nanoscale measurements of strain and stress using electron back scattered diffraction and confocal Raman microscopy

Stresses in Si as small as 10 MPa have been measured using electron backscattered diffraction (EBSD) and confocal Raman microscopy (CRM) with spatial resolutions of 10 nm and 100 nm, respectively. In both techniques, data were collected across wedge indentations in (001) Si. EBSD measured the stress and strain tensors and CRM measured the uniaxial stress. The results agreed very well except close to the indentation, where the surface-sensitive EBSD results indicated larger stresses. Results converged when the CRM laser excitation wavelength was reduced, probing smaller depths. The stress profiles are consistent with the inverse-square power law predicted by Eshelby analysis.

Scanning near-field infrared microscopy and spectroscopy with a broadband laser source

A scanning near-field microscope that allows the fast acquisition of midinfrared absorption spectra is described. The microscope couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. Key design elements of the microscope include a tunable broadband infrared light source; an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth (200 cm−1); and a single mode, fluoride glass, near-field probe fabricated with a chemical etching protocol. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution conservatively estimated to be λ/7.5 at 3.4 μm, in the absence of optical artifacts due to topography. Constant height mode images of a polymer nanocomposite demonstrate instrumental sensitivity to fractional transmission changes of 1×10−3. Near-field absorption spectra (λ=3.4 μm) of a 2 μm thick polystyrene film are presented which de...

Superresolution four-wave mixing microscopy

We report on the development of a superresolution four-wave mixing microscope with spatial resolution approaching 130 nm which represents better than twice the diffraction limit at 800 nm while retaining the ability to acquire materials- and chemical- specific contrast. The resolution enhancement is achieved by narrowing the microscopes excitation volume in the focal plane through the combined use of a Toraldo-style pupil phase filter with the multiplicative nature of four-wave mixing.

Removing optical artifacts in near-field scanning optical microscopy by using a three-dimensional scanning mode

We demonstrate a method of acquiring near-field scanning optical microscopy data that allow for the construction of three different types of images from one data set: topographic, constantgap, and constant-height. This data set includes the topographic features of the surface and the optical response at various heights above the sample surface. Comparisons are made between the images recorded in this format and both conventional, constant-gap mode images, and pseudoconstant-height mode images constructed using a single retraction curve. Zmotion artifacts are identified by analyzing the optical intensity for a given image as a function of the sample topography. Using this procedure it is shown that significant z-motion artifacts exist in the constant-gap images of gold particles immobilized on a glass slide. These artifacts are avoided by constructing constant-height images.

Micro-scale measurement and modeling of stress in silicon surrounding a tungsten-filled through-silicon via

The stress in silicon surrounding a tungsten-filled through-silicon via (TSV) is measured using confocal Raman microscopy line scans across the TSV both before and after etch removal of an oxide stack used as a mask to define the TSV during fabrication. Stress in the silicon arose in response to both athermal deposition and thermal expansion mismatch effects. The complex three-dimensional stress and strain field in silicon surrounding the TSV is modeled using finite element analysis, taking into account both athermal and thermal effects and the elastic anisotropy of silicon. Comparison of the measurements and model results shows that no one component of the stress tensor correlates with the Raman peak shift generated by the deformed silicon. An analysis is developed to predict the Raman shift in deformed silicon that takes into account all the components of the stress or strain tensor; the results of the model are then used as inputs to the analysis for direct comparison with measured peak shifts as a fun...

Advantages and artifacts of higher order modes in nanoparticle-enhanced backscattering Raman imaging.

In order to facilitate nanoparticle-enhanced Raman imaging of complicated biological specimens, we have examined the use of higher order modes with radial and azimuthal polarizations focused onto a Au nanoparticle atomic force microscope (AFM) tip utilizing a backscattering reflection configuration. When comparing the Raman intensity profiles with the observed sample topography, the radial-polarized configuration demonstrates enhanced spatial resolution. This enhanced resolution results from the direction of the induced electron oscillation in the metal nanoparticle oriented by the electromagnetic field at the laser focus. The electric field component along the direction of laser propagation, attendant to the radial polarization, creates an enhanced field along the z-axis and normal to the sample. Substantial enhancement is observed utilizing an intermediate numerical aperture objective (NA = 0.7), necessary for backscattering measurements. The azimuthal polarization, similar to linear polarization, results in an enhanced field predominantly parallel to the sample, resulting in imaging artifacts. The Raman intensity profiles observed as the exciting laser polarization is switched between either a radially polarized or an azimuthally polarized state illustrate these imaging artifacts. Because azimuthal polarization arises readily from changes in the incident polarization onto the mode converter, the results presented here aid in identifying such artifacts when analyzing nanoparticle-enhanced Raman spectroscopic images. Due to the power law decay of the enhanced field, an enhancement orientation normal to the sample enables contrast between structures smaller than the tip dimensions as the apex of the nanoparticle tip, where the enhancement is strongest, passes over the sample. These effects are demonstrated using both carbon nanotube and fixed biological samples.

Near-Field Infrared Imaging and Spectroscopy of a Thin Film Polystyrene/Poly(ethyl acrylate) Blend

The application of broadband, near-field infrared microscopy to the characterization of the mesoscale structure of a thin film polymer blend is described. Key features of this instrument, which couples the nanoscale spatial resolution of scanning probe microscopy with the chemical specificity of vibrational spectroscopy, include broad tunability and bandwidth, parallel spectral detection for high image acquisition rates, and infrared-transparent aperture probes. Near-field spectral transmission images of a thin film of polystyrene/poly(ethyl acrylate) acquired in the C–H stretching region are reported. An assessment of the relative importance of transmission image contrast mechanisms is a significant aim of this work. Analysis of the near-field infrared spectra indicates that the image contrast in the C–H stretching region is largely due to near-field coupling and/or scattering effects. Identification and differentiation of the operative contrast mechanisms on the basis of their relative dependence on wavelength is discussed. Analysis of the contrast attributed to absorption is consistent with the chemical morphology of this sample derived from previous chemical modification/atomic force microscopy studies.