Hyperspectral stochastic optical reconstruction raman microscopy for label-free super-resolution imaging using surface enhanced raman spectroscopy

Super-resolution imaging is an emerging field that has attracted attention in the recent years due to far the reaching impact in biology. All super-resolution techniques use fluorescent labels to image nanoscale biomolecular structures. In contrast, label-free nanoscopic imaging of the chemical environment of biological specimens would readily bridge the supramolecular and the cellular scales, if a chemical fingerprint technique such as Raman scattering can be coupled with superresolution imaging, overcoming the diffraction limit. In order to achieve this goal, we propose to develop a super-resolved stochastic hyperspectral Raman microscopy technique for imaging of biological architectures. The surface enhanced Raman spectroscopy (SERS) signal contains information about the presence of various Raman bands, allowing for the discrimination of families of biomolecules such as lipids, proteins, DNA. The rich, fluctuating spectral information contained in the single molecule SERS signal possesses a great potential in label-free imaging, using stochastic optical reconstruction microscopy (STORM) methods. In a recently published work, we demonstrated 20 nm spatial resolution using the spectrally integrated Raman signal on highly uniform SERS substrates. A mature version of our method would require development of spectrally resolved nanoscale Raman imaging. Development of stochastic Raman imaging addresses the issue by design and construction of a Raman microscope with hyperspectral imaging capability that will allow imaging of different Raman bands of the SERS signal. Novel computational techniques must also be developed that will enable extraction of hyperspectral STORM images corresponding to different Raman bands, while simultaneously allowing conventional STORM data to be collected using the wellestablished labelling techniques. The resulting technique (Hyperspectral Raman STORM or HyperSTORRM) has the potential to complement the available labeled stochastic imaging methods and enable chemically resolved nanoscopy. © Springer Science+Business Media Dordrecht 2015

Lineshapes, shifts and broadenings in dynamical X-ray photoelectron spectroscopy

We describe in detail a model that can be used to estimate the X-ray photoelectron spectroscopic data of surfaces when a time varying bias or a modulation of the electrical properties of the surface is applied by external stimulation, in the presence of a neutralizing electron beam. Using the model and spectra recorded under periodic sample bias modulation, certain electronic properties related to charging dynamics of the surface can be estimated. The resulting technique is a non-contact impedance measurement technique with chemical specificity. Typical behavior of spectra under a square wave bias is given. Alternative modulation schemes are investigated, including small-signal square wave modulation, sinusoidal modulation and modulation of sample resistivity under fixed bias. © 2009 Elsevier B.V. All rights reserved

X-ray photoemission for probing charging/discharging dynamics

A novel technique is introduced for probing charging/discharging dynamics of dielectric materials in which X-ray photoemission data is recorded while the sample rod is subjected to ± 10.0 V square-wave pulses with varying frequencies in the range of 10-3 to 103 Hz. For a clean silicon sample, the Si2p(Si0) peak appears at correspondingly -10.0 eV and +10.0 eV binding energy positions (20.0 eV difference) with no frequency dependence. However, the corresponding peak of the oxide (Si4+) appears with less than 20.0 eV difference and exhibits a strong frequency dependence due to charging of the oxide layer, which is faithfully reproduced by a theoretical model. In the simplest application of this technique, we show that the two O1s components can be assigned to SiOx and TiO y moeties by correlating their dynamical shifts to those of the Si2p and Ti2p peaks in a composite sample. Our pulsing technique turns the powerful X-ray photoemission into an even more powerful impedance spectrometer with an added advantage of chemical resolution and specificity. © 2006 American Chemical Society

Two-dimensional X-ray photoelectron spectroscopy for composite surface analysis

We describe a method for obtaining two-dimensional X-ray photoelectron spectroscopic data derived from the frequency dependence of the XPS peaks recorded under electrical square-wave pulses, which control and affect the binding energy positions via the electrical potentials developed as a result of charging. By using cross-correlations between various peaks, our technique enables us to elucidate electrical characteristics of surface structures of composite samples and bring out various correlations between hidden/overlapping peaks. © 2008 American Chemical Society

Differentiation of domains in composite surface structures by charge-contrast x-ray photoelectron spectroscopy

An external bias is applied to two samples containing composite surface structures, while recording an XPS spectrum. Altering the polarity of the bias affects the extent of differential charging in domains that are chemically or electronically different to create a charge contrast. By utilizing this charge contrast, we show that two distinct silicon nitride and silicon oxynitride domains are present in one of the composite samples. Similarly, we use this technique to show that titanium oxide and silicon oxide domains exist as separate chemical entities in another composite sample. © 2007 American Chemical Society

Chemically specific dynamic characterization of photovoltaic and photoconductivity effects of surface nanostructures

We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates. © 2010 American Chemical Society

A figure of merit for optimization of nanocrystal flash memory design

Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media. Copyright © 2008 American Scientific Publishers All rights reserved

Parametrically coupled multiharmonic force imaging

We report use of nonlinear tip-sample interactions to parametrically convert the frequency components of periodic tip-sample interaction forces to frequencies where they can be resonantly detected. One flexural mode of a cantilever is used for tapping-mode imaging and another flexural mode is used for detection of forces converted in presence of an externally injected mechanical oscillation at the difference frequency of the detecting mode and a harmonic of the tapping mode. Material contrast in attractive and repulsive regimes are demonstrated on samples with polymethyl methacrylate patterns and with deoxyribonucleic acid strands on silicon. © 2008 American Institute of Physics

Spectroscopic ellipsometric study of Ge nanocrystals embedded in SiO 2 using parametric models

Ge-rich SiO2 layers on top of Si substrates were deposited using plasma enhanced chemical vapour deposition. Ge nanocrystals embedded in the SiO2 layers were formed by high temperature annealing. The samples were measured and evaluated by spectroscopic ellipsometry. Effective medium theory (EMT) and parametric semiconductor models have been used to model the dielectric function of the layers. Systematic dependences of the layer thickness and the oscillator parameters have been found on the annealing temperature (nanocrystal size). © 2008 WILEY-VCH Verlag GmbH & Co. KGaA

Raman enhancement on a broadband meta-surface

Plasmonic metamaterials allow confinement of light to deep subwavelength dimensions, while allowing for the tailoring of dispersion and electromagnetic mode density to enhance specific photonic properties. Optical resonances of plasmonic molecules have been extensively investigated; however, benefits of strong coupling of dimers have been overlooked. Here, we construct a plasmonic meta-surface through coupling of diatomic plasmonic molecules which contain a heavy and light meta-atom. Presence and coupling of two distinct types of localized modes in the plasmonic molecule allow formation and engineering of a rich band structure in a seemingly simple and common geometry, resulting in a broadband and quasi-omni-directional meta-surface. Surface-enhanced Raman scattering benefits from the simultaneous presence of plasmonic resonances at the excitation and scattering frequencies, and by proper design of the band structure to satisfy this condition, highly repeatable and spatially uniform Raman enhancement is demonstrated. On the basis of calculations of the field enhancement distribution within a unit cell, spatial uniformity of the enhancement at the nanoscale is discussed. Raman scattering constitutes an example of nonlinear optical processes, where the wavelength conversion during scattering may be viewed as a photonic transition between the bands of the meta-material. © 2012 American Chemical Society