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Electrically Tunable Soft-Solid Block Copolymer Structural Color
Park, Tae Joon,Hwang, Sun Kak,Park, Sungmin,Cho, Sung Hwan,Park, Tae Hyun,Jeong, Beomjin,Kang, Han Sol,Ryu, Du Yeol,Huh, June,Thomas, Edwin L.,Park, Cheolmin American Chemical Society 2015 ACS NANO Vol.9 No.12
<P>One-dimensional photonic crystals based on the periodic stacking of two different dielectric layers have been widely studied, but the fabrication of mechanically flexible polymer structural color (SC) films, with electro-active color switching, remains challenging. Here, we demonstrate free-standing electric field tunable ionic liquid (IL) swollen block copolymer (BCP) films. Placement of a polymer/ionic liquid film-reservoir adjacent to a self-assembled poly(styrene-<I>block</I>-quaternized 2-vinylpyridine) (PS-<I>b</I>-QP2VP) copolymer SC film allowed the development of red (R), green (G), and blue (B) full-color SC block copolymer films by swelling of the QP2VP domains by the ionic liquid associated with water molecules. The IL–polymer/BCP SC film is mechanically flexible with excellent color stability over several days at ambient conditions. The selective swelling of the QP2VP domains could be controlled by both the ratio of the IL to a polymer in the gel-like IL reservoir layer and by an applied voltage in the range of −3 to +6 V using a metal/IL reservoir/SC film/IL reservoir/metal capacitor type device.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-12/acsnano.5b05234/production/images/medium/nn-2015-052345_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b05234'>ACS Electronic Supporting Info</A></P>
Park, Beomjin,Kim, Kyunghun,Park, Jaesung,Lim, Heeseon,Lanh, Phung Thi,Jang, A-Rang,Hyun, Chohee,Myung, Chang Woo,Park, Seungkyoo,Kim, Jeong Won,Kim, Kwang S.,Shin, Hyeon Suk,Lee, Geunsik,Kim, Se Hyun American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.33
<P>Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a 'lying-down' pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional 'standing-up' crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.</P>
Park, Beomjin,Park, Jaesung,Son, Jin Gyeong,Kim, Yong-Jin,Yu, Seong Uk,Park, Hyo Ju,Chae, Dong-Hun,Byun, Jinseok,Jeon, Gumhye,Huh, Sung,Lee, Seoung-Ki,Mishchenko, Artem,Hyun, Seung,Lee, Tae Geol,Han, American Chemical Society 2015 ACS NANO Vol.9 No.8
<P>Precise graphene patterning is of critical importance for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been grown by delicate multistep chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report one-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene <I>via</I> selective photo-cross-linking process. Graphene is successfully grown from neat polystyrene regions, while patterned cross-linked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least 2 orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene region. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The direct synthesis of patterned graphene from polymer pattern could be further exploited to prepare versatile heterostructures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-8/acsnano.5b03037/production/images/medium/nn-2015-03037u_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b03037'>ACS Electronic Supporting Info</A></P>
Park, Youngil,Kim, Beomjin,Lee, Ki-Ho,Lee, Ji-Hoon,Oh, Se-Young,Park, Jongwook American Scientific Publishers 2012 Journal of Nanoscience and Nanotechnology Vol.12 No.5
<P>DPBP-EPY has the same structure with DPBP-EIF, one of blue-light-emitting materials, except their cores, in which the former has two imine groups, but the latter has only carbon-containing groups. The electro-optical properties modulated by the adoption of the different core structures were systematically examined. It was confirmed that the maximum values in the UV-visible and PL spectra of DPBP-EPY were about 19-46 nm red-shifted from those of DPBP-EIF due to the electron-withdrawing effect of the imine groups whether in solution or in solid. In addition, in case of DPBP-EPY where imine group is substituted, LUMO level of DPBP-EPY decreased while HOME level did not show any significant change. Furthermore, the results of the non-doped OLED device built with these two materials for an emitting layer indicated that DPBP-EPY needed about 2 V lower operation-voltage, and produced higher quantum yield than DPBP-EIF. In particular, it was shown that DPBP-EPY emitted purer and deeper blue-light with CIE coordinate (0.157,0.131) than DPBP-EIF with CIE coordinate is (0.179, 0.191).</P>
High-Performance Stable <i>n</i>-Type Indenofluorenedione Field-Effect Transistors
Park, Young-Il,Lee, Joong Suk,Kim, Beom Joon,Kim, Beomjin,Lee, Jaehyun,Kim, Do Hwan,Oh, Se-Young,Cho, Jeong Ho,Park, Jong-Wook American Chemical Society 2011 Chemistry of materials Vol.23 No.17
<P>We developed high-performance stable <I>n</I>-type organic field-effect transistors (OFETs) using indenofluorenediones with different numbers of fluorine substituents (MonoF-IF-dione, DiF-IF-dione, and TriF-IF-dione). Top-contact OFETs were fabricated via the vacuum deposition of indenofluorenediones as the semiconducting channel material on polystyrene-treated SiO<SUB>2</SUB>/Si substrates. TriF-IF-dione FETs with Au source/drain contacts exhibited good device performances, with a field-effect mobility of 0.16 cm<SUP>2</SUP>/(V s), an on/off current ratio of 10<SUP>6</SUP>, and a threshold voltage of 9.2 V. We found that the electrical stability for OFETs based on indenofluorenedione improved with the number of fluorine substituents, which was attributed to higher activation energies for charge trap creation. Moreover, the TriF-IF-dione FETs yielded excellent environmental stability properties, because the LUMO energy levels were relatively low, compared with those of the MonoF-IF-dione FETs.</P><P>Indenofluorenediones with different numbers of fluorine substituents were synthesized for use in high-performance stable <I>n</I>-type FETs. TriF-IF-dione FETs with Au contacts showed excellent device performance, with a field-effect mobility of 0.16 cm<SUP>2</SUP>/(V s), and an on/off current ratio of 10<SUP>6</SUP>. Moreover, the TriF-IF-dione FETs yielded excellent electrical and environmental stabilities.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2011/cmatex.2011.23.issue-17/cm2016824/production/images/medium/cm-2011-016824_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm2016824'>ACS Electronic Supporting Info</A></P>
Park, Youngil,Kim, Beomjin,Lee, Changjun,Lee, Jaehyun,Lee, Ji-Hoon,Park, Jongwook American Scientific Publishers 2012 Journal of Nanoscience and Nanotechnology Vol.12 No.5
<P>New hole injection materials for organic light emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of synthesized materials were examined by through UV-visible (UV-vis), photoluminescence (PL) spectrum and cyclic voltammetry (CV). 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX showed T(g) of 127 and 200 degrees C, which are higher than that (110 degrees C) of 2-TNATA, a commercial hole injection layer (HIL) material. The highest occupied molecular orbital (HOMO) level of the synthesized materials of 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX were 4.97 and 4.91 eV, indicating values well-matched between HOMO (4.8 eV) of ITO and HOMO (5.4 eV) of NPB, hole transporting layer (HTL) material. As a result of using the synthesized materials in OLED device as HIL, 1-BPNA-t-BPBPOX of 2.43 Im/W was higher than 2-TNATA of 1.98 Im/W and 1-BPNA-t-BPBP of 1.39 Im/W in power efficiency. These results indicated that 1-BPNA-t-BPBPOX shows higher excellent power efficiency which is about 18% improved over 2-TNATA a commercial HIL material.</P>