Publications by authors named "Vanya Darakchieva"

9 Publications

  • Page 1 of 1

Tunable Structural Color Images by UV-Patterned Conducting Polymer Nanofilms on Metal Surfaces.

Adv Mater 2021 Aug 5;33(33):e2102451. Epub 2021 Jul 5.

Laboratory of Organic Electronics, Department of Science and Technology (ITN), Linköping University, Norrköping, SE-601 74, Sweden.

Precise manipulation of light-matter interactions has enabled a wide variety of approaches to create bright and vivid structural colors. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive-index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability of these approaches impede their further development toward flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Here, a novel method is presented to generate structural color images based on monochromic conducting polymer films prepared on metallic surfaces via vapor phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose enables synergistic control of both nanoscale film thickness and polymer permittivity, which generates controllable structural colors from violet to red. Together with grayscale photomasks this enables facile fabrication of high-resolution structural color images. Dynamic tuning of colored surfaces and images via electrochemical modulation of the polymer redox state is further demonstrated. The simple structure, facile fabrication, wide color gamut, and dynamic color tuning make this concept competitive for applications like multifunctional displays.
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http://dx.doi.org/10.1002/adma.202102451DOI Listing
August 2021

Tunable cavity-enhanced terahertz frequency-domain optical Hall effect.

Rev Sci Instrum 2020 Aug;91(8):083903

Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA.

Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry-Pérot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.
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http://dx.doi.org/10.1063/5.0010267DOI Listing
August 2020

Conductive polymer nanoantennas for dynamic organic plasmonics.

Nat Nanotechnol 2020 Jan 9;15(1):35-40. Epub 2019 Dec 9.

Laboratory of Organic Electronics, Department of Science and Technology (ITN), Linköping University, Norrköping, Sweden.

Being able to dynamically shape light at the nanoscale is one of the ultimate goals in nano-optics. Resonant light-matter interaction can be achieved using conventional plasmonics based on metal nanostructures, but their tunability is highly limited due to a fixed permittivity. Materials with switchable states and methods for dynamic control of light-matter interaction at the nanoscale are therefore desired. Here we show that nanodisks of a conductive polymer can support localized surface plasmon resonances in the near-infrared and function as dynamic nano-optical antennas, with their resonance behaviour tunable by chemical redox reactions. These plasmons originate from the mobile polaronic charge carriers of a poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) polymer network. We demonstrate complete and reversible switching of the optical response of the nanoantennas by chemical tuning of their redox state, which modulates the material permittivity between plasmonic and dielectric regimes via non-volatile changes in the mobile charge carrier density. Further research may study different conductive polymers and nanostructures and explore their use in various applications, such as dynamic meta-optics and reflective displays.
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http://dx.doi.org/10.1038/s41565-019-0583-yDOI Listing
January 2020

Electronic and optical characterization of 2D TiC and NbC (MXene) thin films.

J Phys Condens Matter 2019 Apr 22;31(16):165301. Epub 2019 Jan 22.

Department of Physics, Chemistry and Biology (IFM), Thin Film Physics Division, Linköping University, SE-58183 Linköping, Sweden.

Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) are a new class of 2D materials, with extensive opportunities for property tailoring due to the numerous possibilities for varying chemistries and surface terminations. Here, TiAlC and NbAlC MAX phase epitaxial thin films were deposited on sapphire substrates by physical vapor deposition. The films were then etched in LiF/HCl solutions, yielding Li-intercalated, 2D TiCT and NbCT films, whose terminations, transport and optical properties were characterized. The former exhibits metallic conductivity, with weak localization below 50 K. In contrast, the Nb-based film exhibits an increase in resistivity with decreasing temperature from RT down to 40 K consistent with variable range hopping transport. The optical properties of both films were determined from spectroscopic ellipsometry in the 0.75 to 3.50 eV range. The results for TiCT films confirm the metallic behavior. In contrast, no evidence of metallic behavior is observed for the NbCT film. The present work therefore demonstrates that one fruitful approach to alter the electronic and optical properties of MXenes is to change the nature of the transition metal.
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http://dx.doi.org/10.1088/1361-648X/ab00a2DOI Listing
April 2019

2D Transition Metal Carbides (MXenes) for Carbon Capture.

Adv Mater 2019 Jan 4;31(2):e1805472. Epub 2018 Nov 4.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever-increasing CO emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO . However, challenges remain because of limited CO /N selectivity and long-term stability. The effective adsorption of CO gas (≈12 mol kg ) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO /N selectivity, together with a chemical and thermal stability, identifies the archetype Ti C MXene as a new material for carbon capture (CC) applications.
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http://dx.doi.org/10.1002/adma.201805472DOI Listing
January 2019

Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases.

Small 2018 Apr 3;14(17):e1703676. Epub 2018 Apr 3.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo Y ) AlC, the resulting MXene can be either: i) (Mo Y ) C with in-plane elemental order through selective removal of Al atoms or ii) Mo C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo Y ) C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm is obtained, which is 40% higher than that of its Mo C counterpart. With H SO , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.
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http://dx.doi.org/10.1002/smll.201703676DOI Listing
April 2018

In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene.

Sci Rep 2017 07 11;7(1):5151. Epub 2017 Jul 11.

Terahertz Materials Analysis Center, Department of Physics, Chemistry and Biology, Linköping University, SE 581 83, Linköping, Sweden.

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.
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http://dx.doi.org/10.1038/s41598-017-05333-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5506066PMC
July 2017

Optical Hall effect-model description: tutorial.

J Opt Soc Am A Opt Image Sci Vis 2016 Aug;33(8):1553-68

The optical Hall effect is a physical phenomenon that describes the occurrence of magnetic-field-induced dielectric displacement at optical wavelengths, transverse and longitudinal to the incident electric field, and analogous to the static electrical Hall effect. The electrical Hall effect and certain cases of the optical Hall effect observations can be explained by extensions of the classic Drude model for the transport of electrons in metals. The optical Hall effect is most useful for characterization of electrical properties in semiconductors. Among many advantages, while the optical Hall effect dispenses with the need of electrical contacts, electrical material properties such as effective mass and mobility parameters, including their anisotropy as well as carrier type and density, can be determined from the optical Hall effect. Measurement of the optical Hall effect can be performed within the concept of generalized ellipsometry at an oblique angle of incidence. In this paper, we review and discuss physical model equations, which can be used to calculate the optical Hall effect in single- and multiple-layered structures of semiconductor materials. We define the optical Hall effect dielectric function tensor, demonstrate diagonalization approaches, and show requirements for the optical Hall effect tensor from energy conservation. We discuss both continuum and quantum approaches, and we provide a brief description of the generalized ellipsometry concept, the Mueller matrix calculus, and a 4×4 matrix algebra to calculate data accessible by experiment. In a follow-up paper, we will discuss strategies and approaches for experimental data acquisition and analysis.
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http://dx.doi.org/10.1364/JOSAA.33.001553DOI Listing
August 2016

Direct graphene growth on Co3O4(111) by molecular beam epitaxy.

J Phys Condens Matter 2012 Feb 6;24(7):072201. Epub 2012 Jan 6.

Department of Chemistry, University of North Texas, Denton, TX 76203-5017, USA.

Direct growth of graphene on Co(3)O(4)(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp(2) carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp(2) carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ~1800 Å, and show an incommensurate interface with the Co(3)O(4)(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO(2) or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.
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http://dx.doi.org/10.1088/0953-8984/24/7/072201DOI Listing
February 2012
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