Page 7 - khu_ric_newsletter_vol_14

Q3
교수님의 주요 논문들 중 한 논문이
고분자 분야의 세계적인 학술지인
‘Soft Matter’ Cover Paper에 실렸다고
얘기를 들었는데, 어떤 논문인지,
자세히 알고 싶습니다.
디스플레이 산업의 지속적인 발전을 위해서는 끊임없이
새로운 성장 ‘Momentum Application’이 필요합니다. 저
는 미래 혁신 제품이 Cableless 전력 수송 제품이라 항상
생각해 오고 있었습니다. 소비전력을 획기적으로 낮출 수
있는 유일한 기술은 삼색구동 기술이라 보는데, 현재 이 기
술의 핵심적인 문제점은 느린 응답속도로 발생하는 색분
해입니다. 액정모드 관련하여, 20년간 연구해 오면서 소비
전력을 획기적으로 낮출 수 있는 초고속 액정모드에 집중
하였습니다. 현재 양산중인 Nematic 액정인 Uniaxial 액
정에서(이등방 굴절률) Biaxial 액정을(세개의 굴절률 축
액정) 적용하게 되면 응답속도를 획기적으로 개선할 수 있
고, 삼색구동을 적용함으로서 소비전력을 획기적으로 낮
출 수 있습니다.
본 논문의 핵심은 Bend-core Biaxial 액정을 적용하여 모드
화 했을 때 Defect 혹은 Domain walls에 의한 Disclination
을 제거할 수 있는 연구입니다. 이미 모바일 디스플레이는
Cableless 충전 기술이 개발되었고, TV Application 소비전
력 60W 이하 구현 시(현 150W 55“UD), 필요한 기술은 초
고속모드에 의한 삼색구동 구현으로 소비전력을 획기적으
로 낮춤으로 인해 새로운 미래를 맞이하게 될 것이라고 생
각하고 있습니다.
Q4
교수님은 주로 어떤 연구를 해오셨고,
앞으로는 어떤 연구를 하고 싶으신지
궁금합니다.
켄트주립대 액정연구소에서 액정연구로 학위를 받아 삼성
전자 입사 후 3년 액정모드 연구(PVA 발명자), 대학 10년간
신규 액정모드만 연구를 해왔으며, 삼성전자 LCD총괄 재입
사 후 OLED 개발팀장으로서 OLED 물질 포함 연구/개발,
그리고 차세대 디스플레이 개발 팀장으로서 액정 광배향 개
발, 차세대 디스플레이인 Green Display(PenTile Display),
삼색구동 디스플레이 등 새로운 디스플레이를 개발하였습
니다. 제가 개발을 주도한 기술 대부분 신규 디스플레이가
현재 삼성디스플레이에서 양산 중에 있습니다. 향후 연구
방향은 20년간 디스플레이 연구/개발 경험을 토대로 경희
대내에 기업체 R&D Center를 설립하여 아래와 같이 5가
지 연구에 집중하고 싶습니다.
•OLED 빛 축출
•Flexible Substrate Film
• 투명 디스플레이用 고투과 디스플레이
모드를 연구/개발
• 삼색구동 디스플레이
(Color Sequential Display)
•그린 디스플레이(Green Display)
Q5
저희 센터와 함께 일할 수 있게 되어
정말 기쁘게 생각하고 있으며,
마지막으로 교수님의 한 말씀
부탁드립니다.
경희대 국제캠퍼스 내에 설립되어 있는 RIC 일원으로서,
미래 디스플레이를 연구/개발을 할 수 있는 여건을 갖게 된
것은 행운이라 생각합니다. RIC는 국내 최고의 연구/개발
Fab 보유 그리고 측정/분석 장비를 보유 하고 있는 디스플
레이센터라고 생각합니다. RIC 연구원분들과 함께 노력하
여 국내·외 기업 R&D 센터 유치 등 RIC가 세계적인 디스플
레이 연구센터로 발돋움 하는데 일조하고자 합니다.
Paper View Article Online
Soft Matter
Soft Matter phase transition. The SDs were associated with disclinations in with the polyimide PI2555 (HD Microsystems) serving as planar
the orientational order of the short molecular axes, i.e., with the
alignment layers.
secondary director (different from n ˆ) of the N b phase. 37
PAPER Domain walls and anchoring transitions mimicking namely, POM, LC PolScope, and fluorescence polarizing optical 3. Alignment, anchoring transition and
In this paper, we apply a battery of imaging techniques,
View Article Online
View Journal
microscopy (FCPM), to explore the C7 textures and their tem- domain walls
perature and electric field-induced behaviour in cells with
To re-enact the appearance of SDs, we fabricated cells with two
different surface alignment layers. Our studies demonstrate
polyimide aligning agents, SE1211 and SE5661 (both supplied by
that the SDs are caused by a surface anchoring transition, i.e.,
Nissan Chemical Industries) deposited onto indium-tin-oxide
liquid crystal C7
to the bounding plates to being tilted. The SDs thus represent
Cite this: DOI: 10.1039/c5sm00580a nematic biaxiality in the oxadiazole bent-core by the realignment of the uniaxial director n ˆ from being parallel (ITO) electrodes at glass substrates. The polyimide layers were
domain walls (DWs) that separate regions of the tilted director
spin-coated from solutions and then baked but not rubbed.
Young-Ki Kim, a Greta Cukrov, a Jie Xiang, a Sung-Tae Shin b and
with different azimuthal directions. The anchoring transition is
3.1
Polarizing optical microscopy
Oleg D. Lavrentovich* a
associated with the balance of two different mechanisms
In the SE1211 cells, one observes the classic Schlieren texture
responsible for the alignment of C7: alignment by the poly-
imide substrate and by the electric field within the surface
characteristics of the uniaxial order in the entire temperature
We investigate the origin of ‘‘secondary disclinations’’ that were recently described as new evidence of a
electric double layer formed by ionic impurities. The findings
range of the N phase, Fig. 2. 41 The Schlieren textures feature point
biaxial nematic phase in an oxadiazole bent-core thermotropic liquid crystal C7. Using an assortment of
defect-boojums (with four brushes of extinction emerging from
demonstrate a uniaxial nature of the nematic order in C7 and
optical techniques such as polarizing optical microscopy, LC PolScope, and fluorescence confocal
their centers) and vertical disclinations of semi-integer strength
are thus consistent with the previous claims of uniaxial char-
polarizing microscopy, we demonstrate that the secondary disclinations represent non-singular domain
(with two dark brushes emanating from their cores), Fig. 2. 42
acteristics of C7. 31,32 The DWs observed in C7 are similar to the
walls formed in a uniaxial nematic phase during the surface anchoring transition, in which surface
In the SE5661 cells, one observes two different textures as
molecules. 38
domain wall separates two regions with the director tilted in opposite azimuthal directions. At the centre
the textures are of the classic N u Schlieren type, Fig. 3a and c.
of the wall, the director remains parallel to the bounding plates. The domain walls can be easily removed
Below t*, secondary textures with SDs form, Fig. 3b and d.
by applying a moderate electric field. The anchoring transition is explained by the balance of (a) the
The SDs show a reversible behaviour, disappearing once the
2. Materials and techniques
Published on 23 March 2015. Downloaded by KENT STATE UNIVERSITY on 02/04/2015 23:11:02. Received 11th March 2015, orientation of the director changes from tangential (parallel to the bounding plates) to tilted. Each Published on 23 March 2015. Downloaded by KENT STATE UNIVERSITY on 02/04/2015 23:11:02. DWs observed in another uniaxial nematic LC, with the H-shaped the temperature is varied. Above
intrinsic perpendicular surface anchoring produced by the polyimide aligning layer and (b) tangential
alignment caused by ionic impurities forming electric double layers. The model is supported by the fact
Fig. 1a and b show the chemical structure of the oxadiazole
that the temperature of the tangentially tilted anchoring transition decreases as the cell thickness
bent-core LC, C7. The phase diagram confirmed by POM upon
Accepted 23rd March 2015
increases and as the concentration of ionic species (added salt) increases. We also demonstrate that the
surface alignment is strongly affected by thermal degradation of the samples. This study shows that
cooling is
DOI: 10.1039/c5sm00580a
C7 exhibits only a uniaxial nematic phase and demonstrates yet another mechanism (formation of
Cr (148 1C) Sm (173 1C) N (222 1C) I
www.rsc.org/softmatter ‘‘secondary disclinations’’) by which a uniaxial nematic phase can mimic a biaxial nematic behaviour. where Cr, Sm, N, and I are crystal, smectic, nematic, and and polarizer directions, respectively.
Fig. 2
Schlieren textures of C7 in the SE1211 cell (d = 4.5 mm) at (a) t = 1.0,
isotropic phases, respectively; the phase diagram is consistent
(b) 20.0, and (c) 45.0 1C observed using POM. A and P are crossed analyzer
1. Introduction
with the previous studies, 7,8,31 if one does not discriminate
cybotactic groups, 29,30 and surface anchoring transitions in which
The search for biaxial nematic (N b ) liquid crystals (LCs) repre- the uniaxial director n ˆ (depicting the average direction of long between the two nematic (N u and N b ) phases.
molecular axes) tilts as the sample’s temperature is varied. 31,32
The cells were assembled from flat glass substrates with
sents an active and fascinating area of study with arguments Recently, new evidence has been presented in favor of the N b alignment layers. The cell thickness d was set by glass spacers
presented both in favor 1–25 and against 26–36 their existence. phase of C7, based on the polarizing optical microscopy (POM) mixed with UV glue NOA 65 (Norland Products, INC.) that was
In the case of lyotropic surfactant systems, the discussion issue textures exhibiting the so-called ‘‘secondary disclinations’’ (SDs). 37 also used to seal the cells. Because C7 shows signs of aging at
the oxadiazole bent-core material 4,4 0 (1,3,4-oxadiazole-2,5-diyl) texture upon lowering the temperature and disappearing upon formed the experiments with freshly prepared cells within
POM showed a secondary Schlieren texture growing into a N u
elevated temperatures in the presence of oxygen, 31 we per-
is stability. 1,26,27 Among thermotropic LCs, the most studied is
di-p-heptylbenzoate derived from 2,5-bis-( p-hydroxyphenyl)-
5 hours or less. In order to prevent possible surface memory
heating. 37 The appearance of SDs was attributed to the N u –N b
1,3,4-oxadiazole, abbreviated as ODBP-Ph-C 7 , or simply C7
effects, 31 we injected C7 into the cells at temperatures above the
(Fig. 1a and b). The N b phase of C7 has been suggested by N–I transition temperature T NI and carried out the experiments
X-ray diffraction (XRD), 6,7 NMR, 8 and electro-optical studies. 9–12 within the temperature range of the N phase. The temperature
These suggestions were challenged 28–32 on the grounds that the was controlled using a LTS420 hot stage with a T95-HS con-
observed features represent the mimicking behaviour of the
uniaxial nematic (N u ) phase rather than a true biaxial order in rate of temperature change x = 0.4 1C min 1 was deliberately
troller (both Linkam Instruments), with 0.01 1C accuracy. The
the bulk of the specimen. For example, the N u phase of C7 might
slow to minimize the flows caused by thermal expansions that
mimic a biaxial behaviour through the appearance of smectic
might represent yet another facet of a uniaxial behaviour that
mimics the behaviour of N b . 35,39,40 Fig. 3 Temperature dependence of POM textures in the SE5661 cells of
In the analysis of optical textures, it is important to know the
a Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, different thicknesses: (a and b) d = 13 mm(t*= 5.5 1C) and (c and d) 4.5 mm
Kent State University, Kent, OH 44242, USA. E-mail: olavrent@kent.edu Fig. 1 (a) Chemical structure and (b) space filling model of the oxadiazole temperature dependence of birefringence Dn. It is presented in (t* = 0.9 1C). The left column shows standard Schlieren textures at elevated
b Department of Chemical Engineering, Kyung Hee University, Youngin, bent-core liquid crystal C7. (c) Birefringence Dn of C7 as a function of the Fig. 1c as a function of the relative temperature t = T T NI ; the temperatures t 4 t*, and the right column shows well-defined SDs at t o t*.
dependence was measured in planar cells of thickness d = 1.1 mm
Red arrows point towards the SDs.
This journal is © The Royal Society of Chemistry 2015 Soft Matter Soft Matter This journal is © The Royal Society of Chemistry 2015
Gyunggi-do 446-701, Republic of Korea
relative temperature t = T � T NI .
★ Soft Matter Cover Paper ★
“Defect domain walls induced by surface anchoring
transition in bent-core mesogen C7” Young-ki Kim,
rd
Sung-tae Shin et. al., Soft Matter, (March 23 , 2015)
2015년 제14호 _ 7015년 제14호 _ 7
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