Evgeniya Sheremet

Enhancement of the thermoelectric properties of PEDOT:PSS thin films by post-treatment

In this work, the thermoelectric (TE) properties of poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films at room temperature are studied. Different methods have been applied for tuning the TE properties: 1st addition of polar solvent, dimethyl sulfoxide (DMSO), into the PEDOT:PSS solution; 2nd post-treatment of thin films with a mixture of DMSO and ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). It is verified that DMSO post-treatment is more efficient than DMSO addition in improving the electrical conductivity with a trivial change in the Seebeck coefficient. The power factor is increased up to 30.1 μW mK−2 for the film with DMSO post-treatment, while the optimized power factor by DMSO addition is 18.2 μW mK−2. It is shown that both DMSO addition and post-treatment induce morphological changes: an interconnected network of elongated PEDOT grains is generated, leading to higher electrical conductivity. In contrast, for those films post-treated in the presence of EMIMBF4, an interconnected network of short and circular PEDOT grains with increased polaron density is created, resulting in the improvement in the Seebeck coefficient and a concomitant compromise in the electrical conductivity. An optimized power factor of 38.46 μW mK−2 is achieved at 50 vol% of EMIMBF4, which is the highest reported so far for PEDOT:PSS thin films to our knowledge. Assuming a thermal conductivity of 0.17 W mK−1, the corresponding ZT is 0.068 at 300 K. These results demonstrate that post-treatment is a promising approach to enhance the TE properties of PEDOT:PSS thin films. Furthermore, ionic liquid, EMIMBF4, shows the potential for tuning the TE properties of PEDOT:PSS thin films via a more environmentally benign process.

Chemical post-treatment and thermoelectric properties of poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) thin films

We report on the modification of the thermoelectric properties of poly(3,4-ethylenedioxylthiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films by means of a simple post treatment of the solid thin films realized by drop-coating. We show that the organic polar solvents, dimethyl sulfoxide and ethylene glycol as secondary dopants for PEDOT:PSS, only affect the film morphology for which a high electrical conductivity is observed. In contrast, ethanolamine (MEA) and ammonia solutions are reduction agents that improve the density of PEDOT chains in the reduced forms (polaron and neutral states), resulting in the trade-off between Seebeck coefficient and electrical conductivity. Furthermore, we show that the nature of amines determines the reduction degree: the nitrogen lone pair electrons in MEA are easier to be donated than those in ammonia solution and will therefore neutralize the PEDOT chains.

Compact metal probes: a solution for atomic force microscopy based tip-enhanced Raman spectroscopy.

There are many challenges in accomplishing tip-enhanced Raman spectroscopy (TERS) and obtaining a proper tip is probably the greatest one. Since tip size, composition, and geometry are the ultimate parameters that determine enhancement of intensity and lateral resolution, the tip becomes the most critical component in a TERS experiment. However, since the discovery of TERS the cantilevers used in atomic force microscopy (AFM) have remained basically the same: commercial silicon (or silicon nitride) tips covered by a metallic coating. The main issues of using metal-coated silicon cantilevers, such as wearing off of the metal layer or increased tip radius, can be completely overcome by using all-metal cantilevers. Until now in TERS experiments such probes have only been used in a scanning tunneling microscope or in a tuning fork-based shear force microscope but not in AFM. In this work for the first time, we show the use of compact silver cantilevers that are fully compatible with contact and tapping modes in AFM demonstrating their superb performance in TERS experiments.

The substrate matters in the Raman spectroscopy analysis of cells

Raman spectroscopy is a powerful analytical method that allows deposited and/or immobilized cells to be evaluated without complex sample preparation or labeling. However, a main limitation of Raman spectroscopy in cell analysis is the extremely weak Raman intensity that results in low signal to noise ratios. Therefore, it is important to seize any opportunity that increases the intensity of the Raman signal and to understand whether and how the signal enhancement changes with respect to the substrate used. Our experimental results show clear differences in the spectroscopic response from cells on different surfaces. This result is partly due to the difference in spatial distribution of electric field at the substrate/cell interface as shown by numerical simulations. We found that the substrate also changes the spatial location of maximum field enhancement around the cells. Moreover, beyond conventional flat surfaces, we introduce an efficient nanostructured silver substrate that largely enhances the Raman signal intensity from a single yeast cell. This work contributes to the field of vibrational spectroscopy analysis by providing a fresh look at the significance of the substrate for Raman investigations in cell research.

Combination of surface- and interference-enhanced Raman scattering by CuS nanocrystals on nanopatterned Au structures

Summary We present the results of a Raman study of optical phonons in CuS nanocrystals (NCs) with a low areal density fabricated through the Langmuir–Blodgett technology on nanopatterned Au nanocluster arrays using a combination of surface- and interference-enhanced Raman scattering (SERS and IERS, respectively). Micro-Raman spectra of one monolayer of CuS NCs deposited on a bare Si substrate reveal only features corresponding to crystalline Si. However, a new relatively strong peak occurs in the Raman spectrum of CuS NCs on Au nanocluster arrays at 474 cm−1. This feature is related to the optical phonon mode in CuS NCs and manifests the SERS effect. For CuS NCs deposited on a SiO2 layer this phonon mode is also observed due to the IERS effect. Its intensity changes periodically with increasing SiO2 layer thickness for different laser excitation lines and is enhanced by a factor of about 30. CuS NCs formed on Au nanocluster arrays fabricated on IERS substrates combine the advantages of SERS and IERS and demonstrate stronger SERS enhancement allowing for the observation of Raman signals from CuS NCs with an ultra-low areal density.

Enhanced field emission from cerium hexaboride coated multiwalled carbon nanotube composite films: A potential material for next generation electron sources

Intensified field emission (FE) current from temporally stable cerium hexaboride (CeB6) coated carbon nanotubes (CNTs) on Si substrate is reported aiming to propose the new composite material as a potential candidate for future generation electron sources. The film was synthesized by a combination of chemical and physical deposition processes. A remarkable increase in maximum current density, field enhancement factor, and a reduction in turn-on field and threshold field with comparable temporal current stability are observed in CeB6-coated CNT film when compared to pristine CeB6 film. The elemental composition and surface morphology of the films, as examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray measurements, show decoration of CeB6 nanoparticles on top and walls of CNTs. Chemical functionalization of CNTs by the incorporation of CeB6 nanoparticles is evident by a remarkable increase in intensity of the 2D band in Raman spectrum of coated films as c...

Enhanced field emission from lanthanum hexaboride coated multiwalled carbon nanotubes: Correlation with physical properties

Detailed results from field emission studies of lanthanum hexaboride (LaB6) coated multiwalled carbon nanotube (MWCNT) films, pristine LaB6 films, and pristine MWCNT films are reported. The films have been synthesized by a combination of chemical and physical deposition processes. An impressive increase in field enhancement factor and temporal stability as well as a reduction in turn-on field and threshold field are observed in LaB6-coated MWCNTs compared to pristine MWCNT and pristine LaB6 films. Surface morphology of the films has been examined by scanning electron microscopy. Introduction of LaB6 nanoparticles on the outer walls of CNTs LaB6-coated MWCNTs films is confirmed by transmission electron microscopy. The presence of LaB6 was confirmed by X-ray photoelectron spectroscopy results and further validated by the Raman spectra. Raman spectroscopy also shows 67% increase in defect concentration in MWCNTs upon coating with LaB6 and an upshift in the 2D band that could be attributed to p-type doping. U...

Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.

Surface-enhanced Raman scattering and gap-mode tip-enhanced Raman scattering investigations of phthalocyanine molecules on gold nanostructured substrates

This study reports the use of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), both independent and in combination, to investigate Raman enhancement of films under different confinement geometries. The experiments are performed on ultrathin cobalt phthalocyanine (CoPc) films deposited on specially designed SERS structures. The SERS structures are fabricated by electron-beam lithography and contain nanostructured gold films and gold dimer arrays with controlled size and internanocluster distance. Such structures allow investigation of the effects of nanocluster size and internanocluster distance, excitation wavelength, and polarization of light upon the electromagnetic SERS enhancement. Significant enhancement of the Raman scattering by CoPc is observed under 632.8 nm excitation because of the double resonance originating from the energy match between the laser excitation and the localized surface plasmon and electronic transitions in CoPc. The SERS signal of CoPc is further enhanced by decreasing the internanocluster distance. Maximum SERS enhancement occurs when the polarization of the incident light is perpendicular to the dimer axis. Under 514.5 nm excitation, nanostructured gold films give greater enhancement than any of the nanocluster arrays, with the highest enhancement realized using the so-called “gap-mode TERS” wherein the SERS structures are probed in the TERS condition. The TERS experiment is performed using a customized TERS setup and all-metal atomic force microscopy tips custom fabricated. In terms of obtaining the ultimate sensitivity in Raman spectroscopy, further enhancement is achieved by confining the electromagnetic field in a gap between two metallic nanostructures either by using SERS or by combining SERS and TERS.This study reports the use of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), both independent and in combination, to investigate Raman enhancement of films under different confinement geometries. The experiments are performed on ultrathin cobalt phthalocyanine (CoPc) films deposited on specially designed SERS structures. The SERS structures are fabricated by electron-beam lithography and contain nanostructured gold films and gold dimer arrays with controlled size and internanocluster distance. Such structures allow investigation of the effects of nanocluster size and internanocluster distance, excitation wavelength, and polarization of light upon the electromagnetic SERS enhancement. Significant enhancement of the Raman scattering by CoPc is observed under 632.8 nm excitation because of the double resonance originating from the energy match between the laser excitation and the localized surface plasmon and electronic transitions in CoPc. The SERS signal of CoPc is furth...

Selective Raman modes and strong photoluminescence of gallium selenide flakes on sp2 carbon

Two-dimensional materials awakened a strong interest in the scientific and technological communities due to their exceptional properties that can be tuned by the material thickness and chemistry. In order to correlate optical properties with crystallographic structure and morphology, in this work, the authors aim at studying GaSe nanoflakes deposited on highly ordered pyrolytic graphite by means of atomic force microscopy, Raman, and photoluminescence (PL) spectroscopies. The authors found that the basal plane of the flakes can be attributed to the e-phase expected for bulk samples grown by the Bridgman method. However, a strong difference in the Raman spectra was systematically found at the edge of our GaSe flakes. Forbidden Raman modes located around 250 cm−1 were selectively observed at specific locations. These modes could not be directly attributed to the e-phase observed in the basal plane or in the bulk. The atomic force microscopy investigations show that high topographical features characterize t...