Showing papers by "Hoi Sing Kwok published in 2013"
TL;DR: Replacement of phenyl ring (s) in tetraphenylethene by naphthalene ring(s) generates a series of new luminogens with aggregation-induced emission (AIE) characteristics, demonstrating that bulky naphthaene rings can serve as a rotor to construct AIE luminogens.
117 citations
TL;DR: A new design of luminescent materials by decorating a tetraphenylethene core with four aromatic chromophores is proposed, showing aggregation-enhanced emission and excellent solid-state fluorescence efficiency.
82 citations
TL;DR: In this paper, photo-alignment of the sulfonic azo dye (SD1) layer has been used to achieve submicron-scale resolution for liquid crystals by using Lloyd's mirror.
Abstract: In this article, we disclose a method to achieve submicron-scale resolution for liquid crystals by photo-alignment of the sulfonic azo dye (SD1) layer. The interference pattern produced by Lloyd’s mirror has been used to produce two alignment domains. The easy axes in these alignment domains are mutually perpendicular to each other, in the plane of the substrate. The two-step alignment process, i.e. the first uniform alignment and the second alignment by interference pattern, provides resolution up to 105 nm for the liquid crystal alignment domain size that typically corresponds to the existing experimental limits. Optical methods such as optical microscopy and diffraction in the transmissive and reflective regimes have been used to analyze the fabricated gratings.
80 citations
TL;DR: In this article, two new iridium(III) cyclometallated complexes (1 and 2) based on the 2-(1-phenoxy-4-phenyl)-5-methylpyridine ligand have been developed.
Abstract: Two new iridium(III) cyclometallated complexes (1 and 2) based on the 2-(1-phenoxy-4-phenyl)-5-methylpyridine ligand have been developed. By attaching a flexible phenoxy group on the phenyl ring of 2-phenylpyridine (Hppy), the light-emitting properties of the resulting IrIII complexes have been improved, while the introduction of an electron-donating methyl group on the pyridyl ring of Hppy can keep the triplet emission in the green region by compensating for the reduced energy gap caused by the phenoxy group. Owing to the unique electronic structures induced by the ligand, the vacuum-evaporated organic light-emitting devices (OLEDs) of the type [ITO/NPB (40 nm)/(1 or 2):CBP (20 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm)] furnished peak OLED efficiencies at 10.0%, 31.1 cd A−1 and 14.5 lm W−1 for 1 and 11.7%, 38.1 cd A−1 and 31.8 lm W−1 for 2. By replacing the electron-injection/electron-transporting materials (BCP and Alq3) with TPBi, the green-emitting devices based on 1 gave outstanding peak efficiencies at 22.5%, 76.2 cd A−1 and 72.8 lm W−1. Extremely high peak efficiencies of 24.5%, 84.6 cd A−1 and 77.6 lm W−1 were even obtained for the 2-doped devices and both of them are superior in performance to the benchmark dopants Ir(ppy)3 and Ir(ppy)2(acac). Moreover, polymer light-emitting devices were also fabricated using 1 and 2via the spin-coating method, and their device performances are characterized by 14.4%, 39.5 cd A−1 and 12.4 lm W−1 for 1 and 12.6%, 29.6 cd A−1 and 18.1 lm W−1 for 2. When 2 was used to make three-color white-light OLEDs, respectable device efficiencies of 15.3 cd A−1, 7.5% and 9.1 lm W−1 were achieved and their white color CIE coordinates are improved relative to Ir(ppy)3.
64 citations
TL;DR: The key theoretical result is that the principal refractive indices of homogenized VADHFLC cells exhibit the quadratic nonlinearity and such behavior might be interpreted as an orientational Kerr effect caused by the electric-field-induced orientational distortions of the FLC helix.
Abstract: We study both theoretically and experimentally the electro-optical properties of vertically aligned deformed helix ferroelectric liquid crystals (VADHFLC) with subwavelength pitch that are governed by the electrically induced optical biaxiality of the smectic helical structure. The key theoretical result is that the principal refractive indices of homogenized VADHFLC cells exhibit the quadratic nonlinearity and such behavior might be interpreted as an orientational Kerr effect caused by the electric-field-induced orientational distortions of the FLC helix. In our experiments, it has been observed that, for sufficiently weak electric fields, the magnitude of biaxiality is proportional to the square of electric field in good agreement with our theoretical results for the effective dielectric tensor of VADHFLCs. Under certain conditions, the 2$\ensuremath{\pi}$ phase modulation of light, which is caused by one of the induced refractive indices, is observed without changes in ellipticity of incident light.
57 citations
TL;DR: The LCFZL, based on alternate twisted nematic and planar aligned regions, has been prepared by means of a two-step photo-alignment process and offers double the efficiency of existing FZLs.
Abstract: In this Letter we disclose a method to fabricate a liquid crystal (LC) Fresnel zone lens (FZL) with higher efficiency The LCFZL, based on alternate twisted nematic (TN) and planar aligned (PA) regions, has been prepared by means of a two-step photo-alignment process The FZL profile for both optical regimes, ie, in TN and PA alignment domains, generates the same focal length (f) Thus, the proposed LCFZL manifests double light intensity at the focal point and therefore offers double the efficiency of existing FZLs Moreover, because of lower driving voltage and fast response, these elements could find application in many modern devices
53 citations
TL;DR: The proposed grating is optically tunable for diffractive and nondiffractive states with a small response time that depends on the exposure energy and LC parameters and can be erased and rewritten for different diffraction characteristics.
Abstract: An optically tunable and rewritable liquid crystal (LC) diffraction grating cell has been revealed that consists of an optically active and an optically passive alignment layer. The grating profile is created by confining the LC director distribution in alternate planar and twisted alignment domains by means of photoalignment of the LCs. The proposed grating is optically tunable for diffractive and nondiffractive states with a small response time that depends on the exposure energy and LC parameters. In addition, the grating can be erased and rewritten for different diffracting characteristics. These optically tunable diffractive elements could find application in various photonic devices.
45 citations
TL;DR: In this article, a self-aligned top-gate amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors with source/drain (S/D) regions doped by implanted arsenic are developed.
Abstract: Self-aligned top-gate amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with source/drain (S/D) regions doped by implanted arsenic are developed in this letter. The resulting a-IGZO TFTs exhibit much better thermal stability than those with S/D regions doped by hydrogen or argon plasma. They also show good electrical performance, including field-effect mobility of 12 cm2/V·s, threshold voltage of 3.5 V, subthreshold swing of 0.5 V/dec, and on/off current ratio of 9 ×107.
40 citations
TL;DR: This Letter reports a method to prepare a liquid crystal (LC) lens based on a variable pretilt angle of a photoalignment (PA) layer that provides good control on thepretilt angle that can be tuned in the range of 1°-89°.
Abstract: In this Letter, we report a method to prepare a liquid crystal (LC) lens based on a variable pretilt angle of a photoalignment (PA) layer. With precise control of the irradiance, such PA provides good control on the pretilt angle that can be tuned in the range of 1°–89°. Therefore, the precise control of the irradiance generates a spatially varying pretilt angle with any desired pretilt angle profile. Thus, the tunable LC lenses have been fabricated and characterized. With low voltage operations, easy fabrication, and relatively high switching speed, such lenses can be applied in many modern optical and photonic devices.
35 citations
TL;DR: An alignment free mask patterning method has been proposed for fabricating the side-by-side color tunable organic light-emitting diodes (OLEDs) as discussed by the authors.
34 citations
TL;DR: In this paper, the fabrication procedure and optical characterization of ferroelectric liquid-crystal (FLC) gratings based on photo-alignment is described, which can be operated with high frequency of around 2 kHz at the electric field of 6.67 V/μm.
Abstract: This paper describes the fabrication procedure and optical characterization of ferroelectric liquid-crystal (FLC) gratings based on photo-alignment. The fabrication procedure includes only one side photo-alignment substrate, while the other substrate does not have any alignment layer. Both 1D and 2D gratings have been fabricated. The proposed diffraction element shows high diffraction efficiency ~ 65% and fast response time of 50 μs, which is much faster than the existing technologies. Such gratings can be operated with high frequency of around 2 kHz at the electric field of 6.67 V/μm. Moreover, the proposed grating can be erased and rewritten optically for different grating vector in simple steps. Therefore, with these advance features, such gratings have high potential to be applied in verity of devices and for the improvement of some important existing devices.
TL;DR: A series of triarylamine-containing rhenium(I) diimine alkynyl complexes has been synthesized and some of their X-ray crystal structures have been determined.
Abstract: A series of triarylamine-containing rhenium(I) diimine alkynyl complexes has been synthesized and some of their X-ray crystal structures have been determined. Low-energy transition bands at 402–444 nm were observed in the electronic absorption spectra and were tentatively assigned as an admixture of [dπ(Re) → π*(diimine)] metal-to-ligand charge-transfer (MLCT) and [π(CC−C6H4–R) → π* (diimine)] ligand-to-ligand charge-transfer (LLCT) transitions. They were found to give emission upon photo-excitation in the solid state and in fluid solution at room temperature and their emissive origins were assigned as derived predominantly from the triplet 3MLCT excited state, mixed with 3LLCT character as well as the 3IL state. Electrochemical studies revealed that the first oxidation process was mainly alkynyl ligand-based with mixing of the metal-centred contribution, whereas the first reduction was attributed to the ligand-centred reduction of the diimine ligand. Computational studies have been performed to provide further insight into the electrochemical and photophysical properties. The photochemical properties with triphenylamine were also studied by quenching experiments and time-resolved transient absorption spectroscopy. Some of these complexes were able to act as emitters in OLED devices and their electroluminescence (EL) behaviour was investigated.
TL;DR: Enlarged tetrasubstituted alkenes with high fluorescent efficiency and good thermal stability are constructed and exhibit luminance up to 35,160 cd m(-2) and efficiency in OLEDs.
TL;DR: In this paper, a bridged-grain structure for low-temperature polycrystalline silicon thin-film transistors (TFTs) is introduced, which can reduce the threshold voltage and sub-threshold swing, increase the on-off ratio, and suppress leakage current and kink effect of TFTs significantly.
Abstract: We introduce a new structure for low-temperature polycrystalline silicon thin-film transistors (TFTs). This bridged-grain structure can reduce the threshold voltage and the subthreshold swing, increase the on-off ratio, and suppress leakage current and kink effect of TFTs significantly. This technique can be applied to all polycrystalline silicon TFTs, including those made by solid-phase crystallization, metal-induced crystallization, metal-induced lateral crystallization, and excimer laser annealing.
TL;DR: In this article, a self-aligned top-gate amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) utilizing SiO2/Al2O3 stack thin films as gate dielectric are developed.
TL;DR: In this paper, the authors proposed a new approach to fiber optic voltage sensors via voltage-controlled liquid crystal (LC) which would allow direct measurement of up to 400 kV/m electric fields at multiple points.
Abstract: In this paper we propose a new approach to fiber optic voltage sensors via voltage-controlled Liquid Crystals (LC), which would allow direct measurement of up to 400 kV/m electric fields at multiple points. In addition, the electro-optical behavior of deformed helix ferroelectric (DHF) liquid crystal in reflective mode is described and tested. The electrically controlled reflectance has been measured at sub-kilohertz driving voltage frequency for different polarizations of the incident light. Full Text: PDF References Y.V. Izdebskaya, V.G. Shvedov, A.S. Desyatnikov, W. Krolikowski and Y.S. Kivshar, "Soliton bending and routing induced by interaction with curved surfaces in nematic liquid crystals", Opt. Lett. 35, 1692-1694 (2010). CrossRef Z. Ge, S. Gauza, M. Jiao, H. Xianyu and S.T. Wu, "Electro-optics of polymer-stabilized blue phase liquid crystal displays", Appl. Phys. Lett. 94, 101104 (2009). CrossRef Z. Brodzeli, F. Ladouceur, L. Silvestri, A. Michie, V. Chigrinov, G. Q. Guo, E. P. Pozhidaev and A. D. Kiselev, "Liquid crystal-based hydrophone arrays", APOS (2012). CrossRef T. Tanigawa, Y. Sakakibara, S. Fang, T. Sekikawa and M. Yamashita, "Spatial light modulator of 648 pixels with liquid crystal transparent from ultraviolet to near-infrared and its chirp compensation application", Opt. Lett. 34, 1696-1698 (2009). CrossRef K. H. Yang and T. C. Chieu, "Transport Properties of Ions in Ferroelectric Liquid Crystal Cells", Jpn. J. Appl. Phys. 28, 2240-2246 (1989) CrossRef I.C. Khoo and S.T. Wu, "Optics and Nonlinear Optics of Liquid Crystals", World Scientific, Singapore (1993). V.G. Chigrinov, "Liquid Crystal Devices: Physics and Applications", Artech House, Boston: London, (1999). A.D. Kiselev, E.P. Pozhidaev, V.G. Chigrinov and H.S. Kwok, "Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch", Phys. Rev. E 83, 31703 (2011). CrossRef N. A. Clark and S. T. Lagerwall, "Submicrosecond bistable electro-optic switching in liquid crystals", Appl. Phys. Lett. 36, 899-901 (1980). CrossRef L.A. Beresnev, V.G. Chigrinov, D.I. Dergachev, E.P. Pozhidaev, J. Funfshilling and M. Schadt, "Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals", Liq. Cryst. 5, 1171-1177 (1989). CrossRef Z. Brodzeli, H.K. Bal, V.G. Chigrinov, A. Murauski, and F. Ladouceur, "Electrical energy harvesting device for current/voltage fibre-based sensors", Optical Fibre Technology (ACOFT), 2010 35th Australian Conference on Fibre Technology, 1-2, (2010). DirectLink X. Zhao, F. Boussaid, A. Bermak and V.G. Chigrinov, "High-resolution thin ?guest-host? micropolarizer arrays for visible imaging polarimetry", Opt. Exp. 19, 5565-5573 (2011). CrossRef X.H. Li, A. Murauski, A. Muravsky, P.Z. Xu, H.L. Cheung, and V. Chigrinov, "Grayscale Generation and Stabilization in Ferroelectric Liquid Crystal Display," J. Display Technol. 3, 273-279 (2007). CrossRef E.P. Pozhidaev, S. Torgova, M. Minchenko, C. Yednak, A. Strigazzi and E. Miraldi, "Phase modulation and ellipticity of the light transmitted through a smectic C* layer with short helix pitch", Liq. Cryst. 37, 1067-1081, (2010). CrossRef E.P. Pozhidaev and V.G. Chigrinov, "Bistable and multistable states in ferroelectric liquid crystals", Crystallography Reports. 51, 1030-1040 (2006). CrossRef Q. Guo, F. Fan, T. Du, and V.G. Chigrinov, "Electrooptic Response of FLC: Effect of Alignment Materials", SID'11 Conference, P-144, Los Angeles, USA, May, (2011). CrossRef G. Hegde, P. Xu, E.P. Pozhidaev, V.G. Chigrinov, and H.S. Kwok, "Electrically controlled birefringence colours in deformed helix ferroelectric liquid crystals", Liq. Cryst. 35, 1137-1144 (2008). CrossRef
01 May 2013
TL;DR: In this article, the authors provide a summary of selected important areas in LC science, technology and applications, written by key researchers in those areas, at the graduate and researchers level in electronic engineering, computer engineering, physics, chemistry and LCD engineers.
Abstract: The book consists of 2 volumes; each volume consists of about 12 chapters, to provide a summary of selected important areas in LC science, technology and applications, written by key researchers in those areas. It is at the graduate and researchers level in electronic engineering, computer engineering, physics, chemistry, and LCD engineers.
TL;DR: In this paper, Brided-grain polycrystalline silicon thin film transistors were fabricated containing twelve submicron sized doped conductive regions formed along the channel using a grating-shaped implantation mask, patterned using an ultraviolet curable nanoimprint lithography technique.
TL;DR: This article discloses a method to fabricate the three‐dimensional (3D) image on an optically rewritable (ORW) electronic paper (e‐paper) by deploying the lenticular lenses array on the top of the ORW e‐paper.
Abstract: In this article, we disclose a method to fabricate the three-dimensional (3D) image on an optically rewritable (ORW) electronic paper (e-paper) by deploying the lenticular lenses array on the top of the ORW e-paper. Unlike other autostereoscopic 3D liquid crystal displays, our autostereoscopic 3D ORW e-paper is actually a green technology, which is without electrode and voltage. No electricity is needed to maintain the 3D image on the ORW e-paper. We can easily change a 3D image by erasing and rewriting with photoalignment technology.
TL;DR: In this paper, polycrystalline silicon thin-film transistors (TFTs) integrating sputtered Al2O3?gate dielectric with bridged-grain active channel are demonstrated.
Abstract: Polycrystalline silicon thin-film transistors (TFTs) integrating sputtered Al2O3?gate dielectric with bridged-grain active channel are demonstrated. The proposed TFTs exhibit excellent device performance in terms of smaller threshold voltage, steeper subthreshold swing and higher on-current/off-current ratio. More importantly, the mobility of the proposed TFT is 5.5?times that of conventional TFTs with SiO2?gate dielectric. All of these results suggest that the proposed TFT is a good choice for low-power and high-speed driving circuits in display application.
TL;DR: In this paper, GaN thin films with wurtzite structure were deposited by the reactive dc magnetron sputtering technique using liquid gallium target and the resulting GaN TFTs exhibit good electrical performance, including a field effect mobility of 5 cm2/V·s, a threshold voltage of 115 V, an on/off current ratio of 6 × 106, and a sub-threshold swing of 4 V/dec.
Abstract: GaN thin films were utilized as an active channel layer to produce bottom-gate n-type thin-film transistors (TFTs) The GaN thin films with wurtzite structure were deposited by the reactive dc magnetron sputtering technique using liquid gallium target The resulting GaN TFTs exhibit good electrical performance, including a field-effect mobility of 5 cm2/V·s, a threshold voltage of 115 V, an on/off current ratio of 6 × 106, and a subthreshold swing of 04 V/dec The reported GaN TFTs have great potential in the application of next-generation flat-panel display
Patent•
01 Nov 2013TL;DR: In this article, a field sequential color (FSC) display cell, part of an FSC display, is provided, which includes two polarizers, an FLC layer comprising FLCs with helix pitch less than the thickness of the FLC, and a voltage source, configured to apply an electrical driving voltage to the FSC layer, the voltage applied to the LCL layer having an amplitude greater than a threshold voltage for helix unwinding, and wherein the voltage source is further configured to applying electrical driving voltages to light emitting diodes (LEDs
Abstract: A field sequential color (FSC) ferroelectric liquid crystal (FLC) display cell, part of an FSC display, is provided. The FSC FLC display cell includes: two polarizers; an FLC layer, positioned between the two polarizers, the FLC layer comprising FLCs with helix pitch less than the thickness of the FLC layer; and a voltage source, configured to apply an electrical driving voltage to the FLC layer, the electrical driving voltage applied to the FLC layer having an amplitude greater than a threshold voltage for helix unwinding, and wherein the voltage source is further configured to apply electrical driving voltages to light emitting diodes (LEDs) of the FSC display to illuminate pixels of the FSC display. The pixels are illuminated in an FSC manner The FLC layer is configured to provide a defect-free layer of FLC under the electrical driving voltage applied to the FLC layer.
01 Jan 2013
TL;DR: In this paper, a new type of nanostructured alignment surface is proposed and demonstrated, and a large pretilt angle of near 45° can be produced reliably with no randomness involved.
Abstract: A new type of nanostructured alignment surface is proposed and demonstrated. As in previous nanostructured alignment surfaces, a large pretilt angle of near 45° can be produced reliably. Moreover, this new structure is fabricated with a mask, and there is no randomness involved. The uniformity and anchoring properties of this new surface have been measured to be of excellent quality.
01 Jun 2013
TL;DR: In this article, photoaligning layers in FLC cells are used to achieve fast response time, high contrast ratio various FLC electrooptical modes through the application of photo aligning layers.
Abstract: Fast response time, high contrast ratio various FLC electrooptical modes are achieved through the application of photoaligning layers in FLC cells. Fast FLCD with a high resolution can be used in the screens field sequential color (FSC) FLC microdisplays, which are now one of the most advanced technologies for pico-projectors.
01 Jun 2013
TL;DR: In this article, a photo-alignment layer for ferroelectric liquid crystal (FLC) displays is explored, where a functional liquid crystal polymer is incorporated into a rewritable azo-dye material with proper mixing.
Abstract: A novel composite photo-alignment layer for ferroelectric liquid crystal (FLC) displays is explored. The key technique is to introduce a functional liquid crystal polymer into a rewritable azo-dye material with proper mixing. Bearing good alignment quality derived from the azo-dye, the composite layer provides strong anchoring energy comparable to that of rubbed polyimide layers. Furthermore, by using a two-step exposure process, the composite layer shows much better stability than that with a pure azo-dye material.
Patent•
06 May 2013TL;DR: In this paper, a patterned polarization grating with left and right hand polarizing domains is configured to diffract incident non-polarized light into beams having left or right circular polarization states.
Abstract: A polarization converter and polarization conversion systems are provided. The polarization converter and polarization conversion systems include a patterned polarization grating with left hand and right hand polarization grating domains. The polarization grating domains are configured to diffract incident non-polarized light into beams having left and right circular polarization states.
TL;DR: In this article, a stereoscopic three-dimensional (3D) system using a polarization rotator is proposed, which converts the polarizations of images from different eyes in temporal multiplexing.
Abstract: A stereoscopic three-dimensional (3D) system using a polarization rotator is proposed. The polarization rotator converts the polarizations of images from different eyes in temporal multiplexing. A novel fast nematic liquid crystal mode is also proposed. The new liquid crystal mode has a fast response time and a high contrast, and is thus a good candidate as a polarization rotator. The proposed 3D system uses only passive polarized glasses, which are more convenient, lightweight, and low-cost than what? In addition, the polarization rotator can be applied to current two-dimensional displays and can become 3D-capable displays.
08 Nov 2013
TL;DR: In this article, a new structure for thin-film transistors, exhibiting the benefits but not the drawbacks of both short-channel and multijunction effects, is proposed and demonstrated.
Abstract: A new structure for thin-film transistors, exhibiting the benefits but not the drawbacks of both short-channel and multijunction effects, is proposed and demonstrated. Theoretical modeling is performed to understand the physics of this device structure. All characteristics such as threshold voltage, pseudo subthreshold slope, on–off current ratio, and field-effect mobility are improved.
TL;DR: In this paper, the mechanism of hydrogen plasma passivation for poly-crystalline silicon (poly-Si) thin films is investigated by optical emission spectroscopy (OES) combined with Hall mobility, Raman spectra, absorption coefficient spectra and so on.
Abstract: The mechanism of hydrogen plasma passivation for poly-crystalline silicon (poly-Si) thin films is investigated by optical emission spectroscopy (OES) combined with Hall mobility, Raman spectra, absorption coefficient spectra, and so on. It is found that different kinds of hydrogen plasma radicals are responsible for passivating different defects in poly-Si. The H? with lower energy is mainly responsible for passivating the solid phase crystallization (SPC) poly-Si whose crystallization precursor is deposited by plasma-enhanced chemical vapor deposition (PECVD). The H* with higher energy may passivate the defects related to teh Ni impurity around the grain boundaries more effectively. In addition, H? and H? with the highest energy are required to passivate intra-grain defects in the poly-Si crystallized by SPC but whose precursor is deposited by low pressure chemical vapor deposition (LPCVD).