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Ko, Seung Hwan,Lee, Daeho,Kang, Hyun Wook,Nam, Koo Hyun,Yeo, Joon Yeob,Hong, Suk Joon,Grigoropoulos, Costas P.,Sung, Hyung Jin American Chemical Society 2011 Nano letters Vol.11 No.2
<P>In this paper, in order to increase the power conversion efficiency we demonstrated the selective growth of “nanoforest” composed of high density, long branched “treelike” multigeneration hierarchical ZnO nanowire photoanodes. The overall light-conversion efficiency of the branched ZnO nanowire DSSCs was almost 5 times higher than the efficiency of DSSCs constructed by upstanding ZnO nanowires. The efficiency increase is due to greatly enhanced surface area for higher dye loading and light harvesting, and also due to reduced charge recombination by providing direct conduction pathways along the crystalline ZnO “nanotree” multi generation branches. We performed a parametric study to determine optimum hierarchical ZnO nanowire photoanodes through the combination of both length-wise growth and branched growth processes. The novel selective hierarchical growth approach represents a low cost, all solution processed hydrothermal method that yields complex hierarchical ZnO nanowire photoanodes by utilizing a simple engineering of seed particles and capping polymer.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-2/nl1037962/production/images/medium/nl-2010-037962_0002.gif'></P>
Synthesis of hierarchical TiO2 nanowires with densely-packed and omnidirectional branches
Lee, Daeho,Rho, Yoonsoo,Allen, Frances I.,Minor, Andrew M.,Ko, Seung Hwan,Grigoropoulos, Costas P. The Royal Society of Chemistry 2013 Nanoscale Vol.5 No.22
<P>In this study, a hierarchical TiO2 nanostructure with densely-packed and omnidirectional branches grown by a hydrothermal method is introduced. This morphology is achieved via high-concentration TiCl4 treatment of upright backbone nanowires (NWs) followed by hydrothermal growth. Secondary nanobranches grow in all directions from densely distributed, needle-like seeds on the jagged round surface of the backbone NWs. In addition, hierarchical, flower-like branches grow on the top surface of each NW, greatly increasing the surface area. For dye-sensitized solar cell (DSSC) applications, the TiO2 nanostructure demonstrated a photoconversion efficiency of up to 6.2%. A parametric study of the DSSC efficiency showed that branched TiO2 DSSCs can achieve nearly four times the efficiency of non-branched TiO2 nanowire DSSCs, and up to 170% the efficiency of previously-reported sparsely-branched TiO2 NW DSSCs.</P>
Yeo, Junyeob,Hong, Sukjoon,Wanit, Manorotkul,Kang, Hyun Wook,Lee, Daeho,Grigoropoulos, Costas P.,Sung, Hyung Jin,Ko, Seung Hwan WILEY‐VCH Verlag 2013 Advanced functional materials Vol.23 No.26
<P><B>Abstract</B></P><P>For functional nanowire based electronics fabrication, conventionally, combination of complex multiple steps, such as (1) chemical vapor deposition (CVD) growth of nanowire, (2) harvesting of nanowire, (3) manipulation and placement of individual nanowires, and (4) integration of nanowire to circuit are necessary. Each step is very time consuming, expensive, and environmentally unfriendly, and only a very low yield is achieved through the multiple steps. As an alternative to conventional complex multistep approach, original findings are presented on the first demonstration of rapid, one step, digital selective growth of nanowires directly on 3D micro/nanostructures by developing a novel approach; laser induced hydrothermal growth (LIHG) without any complex integration of series of multiple process steps such as using any conventional photolithography process or CVD. The LIHG process can grow nanowires by scanning a focused laser beam as a local heat source in a fully digital manner to grow nanowires on arbitrary patterns and even on the non‐flat, 3D micro/nano structures in a safer liquid environment, as opposed to a gas environment. The LIHG process can greatly reduce the processing lead time and simplify the nanowire‐based nanofabrication process by removing multiple steps for growth, harvest, manipulation/placement, and integration of the nanowires. LIHG process can grow nanowire directly on 3D micro/nano structures, which will be extremely challenging even for the conventional nanowire integration processes. LIHG does not need a vacuum environment to grow nanowires but can be performed in a solution environment which is safer and cheaper. LIHG can also be used for flexible substrates such as temperature‐sensitive polymers due to the low processing temperature. Most of all, the LIHG process is a digital process that does not require conventional vacuum deposition or a photolithography mask.</P>
Yeo, Junyeob,Hong, Sukjoon,Kim, Gunho,Lee, Habeom,Suh, Young Duk,Park, Inkyu,Grigoropoulos, Costas P.,Ko, Seung Hwan American Chemical Society 2015 ACS NANO Vol.9 No.6
<P>Recent development of laser-induced hydrothermal growth enabled direct digital growth of ZnO nanowire array at an arbitrary position even on 3D structures by creating a localized temperature field through a photothermal reaction in liquid environment. However, its spatial size was generally limited by the size of the focused laser spot and the thermal diffusion, and the target material has been limited to ZnO. In this paper, we demonstrated a next generation laser-induced hydrothermal growth method to grow nanowire on a selected area that is even smaller than the laser focus size by designing laser absorption layer. The control of laser-induced temperature field was achieved through adjusting the physical properties of the substrate (dimension and thermal conductivity), and it enabled a successful synthesis of smaller nanowire array without changing any complex optics. Through precise localized temperature control with laser, this approach could be extended to various nanowires including ZnO and TiO<SUB>2</SUB> nanowires even on heat sensitive polymer substrate.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-6/acsnano.5b01125/production/images/medium/nn-2015-01125s_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b01125'>ACS Electronic Supporting Info</A></P>
Son, Yong,Yeo, Junyeob,Moon, Hanul,Lim, Tae Woo,Hong, Sukjoon,Nam, Koo Hyun,Yoo, Seunghyup,Grigoropoulos, Costas P.,Yang, Dong‐,Yol,Ko, Seung Hwan WILEY‐VCH Verlag 2011 ADVANCED MATERIALS Vol.23 No.28
<P><B>A digital, direct nanoscale metal patterning method</B> is developed using femtosecond laser digital processing of metal nanoparticles for nanoscale electronics fabrication without using conventional vacuum deposition or a photolithography mask. This method is expected to be a potential alternative to the conventional electron beam lithography method for arbitrary nanoscale direct patterning in single‐step, low‐temperature, and non‐vacuum environments.</P>
Laser welding of vertically aligned carbon nanotube arrays on polymer workpieces
In, Jung Bin,Kwon, Hyuk-Jun,Yoo, Jae-Hyuck,Allen, Frances I.,Minor, Andrew M.,Grigoropoulos, Costas P. Elsevier 2017 Carbon Vol.115 No.-
<P>Here we demonstrate laser transmission welding of vertically aligned carbon nanotube (VACNT) arrays for joining polymer sheets. The unique characteristics of VACNTs make them suitable for use in laser welding. First, the excellent light absorption of the VACNTs induces selective heating at the contact plane with a polymer sheet, minimizing thermal damage to the polymer. Second, the porous and compliant structure of the VACNTs prevents the formation of air pockets inside the contact space. Successful welding is obtained when the laser irradiation power is at an optimal level, below which the adhesion is too weak and above which the excessive heat causes periodic damage along the scanning path. The optimized laser welding technique is expected to become a new method for implementing carbon nanotubes as mechanical linkers for various thermoplastic polymers. (C) 2017 Elsevier Ltd. All rights reserved.</P>
Kwon, Hyuk-Jun,Chung, Seungjun,Jang, Jaewon,Grigoropoulos, Costas P IOP 2016 Nanotechnology Vol.27 No.40
<P>Patterns formed by the laser direct writing (LDW) lithography process are used either as channels or barriers for MoS<SUB>2</SUB> transistors fabricated via inkjet printing. Silver (Ag) nanoparticle ink is printed over patterns formed on top of the MoS<SUB>2</SUB> flakes in order to construct high-resolution source/drain (S/D) electrodes. When positive photoresist is used, the produced grooves are filled with inkjetted Ag ink by capillary forces. On the other hand, in the case of negative photoresist, convex barrier-like patterns are written on the MoS<SUB>2</SUB> flakes and patterns, dividing the printed Ag ink into the S/D electrodes by self-alignment. LDW lithography combined with inkjet printing is applied to?MoS<SUB>2</SUB>?thin-film transistors that exhibit moderate electrical performance such as mobility and subthreshold swing. However, especially in the linear operation regime, their features are limited by the contact effect. The?Y-function method can exclude the contact effect and allow proper evaluation of the maximum available mobility and contact resistance. The presented fabrication methods may facilitate the development of cost-effective fabrication processes.</P>
Lee, Habeom,Manorotkul, Wanit,Lee, Jinhwan,Kwon, Jinhyeong,Suh, Young Duk,Paeng, Dongwoo,Grigoropoulos, Costas P.,Han, Seungyong,Hong, Sukjoon,Yeo, Junyeob,Ko, Seung Hwan American Chemical Society 2017 ACS NANO Vol.11 No.12
<P>Exploration of the electronics solely composed of bottom-up synthesized nanowires has been largely limited due to the complex multistep integration of diverse nanowires. We report a single-step, selective, direct, and on-demand laser synthesis of a hierarchical heterogeneous nanowire-on-nanowire structure (secondary nanowire on the primary backbone nanowire) without using any conventional photolithography or vacuum deposition. The highly confined temperature rise by laser irradiation on the primary backbone metallic nanowire generates a highly localized nanoscale temperature field and photothermal reaction to selectively grow secondary branch nanowires along the backbone nanowire. As a proof-of-concept for an all-nanowire electronics demonstration, an all-nanowire UV sensor was successfully fabricated without using conventional fabrication processes.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2017/ancac3.2017.11.issue-12/acsnano.7b06098/production/images/medium/nn-2017-06098z_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn7b06098'>ACS Electronic Supporting Info</A></P>
Ko, Seung Hwan,Lee, Daeho,Hotz, Nico,Yeo, Junyeob,Hong, Sukjoon,Nam, Koo Hyun,Grigoropoulos, Costas P. American Chemical Society 2012 Langmuir Vol.28 No.10
<P>In this article, we introduce fully digital selective ZnO nanowire array growth on inkjet-printed seed patterning. Through proper natural convection suppression during hydrothermal growth, successful ZnO nanowire local growth can be achieved. Without any need for photolithographic processing or stamp preparation, the nanowire growth location can be easily modified when the inkjet printing process is integrated with a CAD (computer-aided design) system to allow a high degree of freedom when the design needs to be changed. The current proposed process is very fast, low-cost, environmentally benign, and low-temperature. Therefore, it can be applied to a flexible plastic substrate and scaled up for larger substrates for mass production or roll-to-roll processing.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2012/langd5.2012.28.issue-10/la203781x/production/images/medium/la-2011-03781x_0001.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/la203781x'>ACS Electronic Supporting Info</A></P>