Browsing by Author "Dimitrov, Dimitre"

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Atomic Layer-Deposited Al-Doped ZnO Thin Films for Display Applications
(2020-06-01)
Dimitrov, Dimitre
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Tsai, Che-Liang
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Petrov, Stefan
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Marinova, Vera
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Petrova, Dimitrina
The integration of high uniformity, conformal and compact transparent conductive layers into next generation indium tin oxide (ITO)-free optoelectronics, including wearable and bendable structures, is a huge challenge. In this study, we demonstrate the transparent and conductive functionality of aluminum-doped zinc oxide (AZO) thin films deposited on glass as well as on polyethylene terephthalate (PET) flexible substrates by using an atomic layer deposition (ALD) technique. AZO thin films possess high optical transmittance at visible and near-infrared spectral range and electrical properties competitive to commercial ITO layers. AZO layers deposited on flexible PET substrates demonstrate stable sheet resistance over 1000 bending cycles. Based on the performed optical and electrical characterizations, several applications of ALD AZO as transparent conductive layers are shown-AZO/glass-supported liquid crystal (LC) display and AZO/PET-based flexible polymer-dispersed liquid crystal (PDLC) devices.
Flexible light valves using polymer-dispersed liquid crystals and TiO2/Ag/TiO2 multilayers
(2019-06-10)
Chiou, Chung Chin
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Hsu, Fan Hsi
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Petrov, Stefan
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Marinova, Vera
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Dikov, Hristosko
The era of flexible optoelectronics demands development of wearable and bendable structures, foldable touch screens, paper-like displays, and curved and flexible solid-state lighting devices. Here, we demonstrate the fabrication of highly flexible light valves using polymer-dispersed liquid crystal (PDLC) and TiO2/Ag/TiO2 transparent conductive films. TiO2/Ag/TiO2 multilayers were prepared by magnetron sputtering technique on polyethylene terephthalate (PET) substrates at room temperature. By keeping the equivalent TiO2 layers and varying the deposition time of the Ag layer, proper metal nanograins on TiO2 planar plane were formed, providing the best tradeoff between the transmittance, sheet resistance and bending ability. The results are validated by numerical simulations that suggest the best match between the deposition time and individual layer thickness. Based on the performed characteristics of TiO2/Ag/TiO2/PET structures, several flexible light valves are fabricated and characterized. The sheet resistance values of TiO2/Ag/TiO2/PET remain unchanged over 1000 bending cycles. The measured driving voltage and response time values open great potential of TiO2/Ag/TiO2/PET for integration into next-generation ITO-free flexible and stretchable devices. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Polymer Dispersed Liquid Crystals Devices on Rigid and Flexible Substrates Using Graphene Electrodes
(2019-01-01)
Angelova, Ina
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Chiou, Chong Chin
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Marinova, Vera
;
Lin, Shiuan Huei
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Petrova, Dimitrina
Polymer Dispersed Liquid Crystal (PDLC) devices on rigid and flexible substrates using graphene as transparent conductive electrodes are fabricated and demonstrated. Graphene is growth by using low pressure Chemical Vapor Deposition (CVD) method on Cu foil and transferred on polyethylene terephthalate (PET) substrates. Electro-optical modulation characteristics of several PDLC devices are measured and discussed. It is found that the transmittance dependence on the driving voltage demonstrates high potential of graphene for next generation ITO-free optoelectronic and display technology.
Single and Multilayer Graphene Grown by CVD Technique: Characterization for Electro-optical Applications
(2019-01-01)
Fidanova, Tsvetelina
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Petrov, Stefan
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Napoleonov, Blagovest
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Marinova, Vera
;
Petrova, Dimitrina
In this paper we demonstrate graphene growth by using Chemical Vapor Deposition (CVD) method. Single layer graphene was produced by a low pressure CVD method and multilayer graphtme by an atmospheric pressure CVD method. The graphene quality and number of layers were evaluated by Raman analysis and optical transmittance measurements.
Symmetry of the Optical Phonons in LuVO4: A Raman Study
(2020-05-01)
Rafailov, Peter
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Dimitrov, Dimitre
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Chen, Yen-Fu
;
Lee, Chi-Shen
;
Juang, Jenh-Yih
A thorough analysis of the first-order vibrational spectrum of LuVO4 is presented by using polarized micro-Raman spectroscopy with special focus on the phonon modes with the weakest intensity and occasional controversial assignment. Group-theory analysis is carried out to demonstrate the determination of numbers and symmetries of the Raman active modes. Crystal- and correlation-field splitting effects in the vibrational spectrum of LuVO4 are discussed. Under conditions adjusted to minimize the birefringence effects we recorded, in each main scattering configuration, a series of Raman spectra in different sample orientations achieved by rotating the sample around the incident laser beam. The dependence of the Raman intensity on the rotational angle allowed us to identify the correct symmetry of the phonons with exceptionally weak scattering cross-section. A complete assignment of all twelve first-order Raman active phonons of LuVO4 is thus obtained.