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Yeo, Junyeob,Hong, Sukjoon,Manorotkul, Wanit,Suh, Young Duk,Lee, Jinhwan,Kwon, Jinhyeong,Ko, Seung Hwan American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.28
<P>We introduce laser growth of iron oxide micro and nanorods by the photothermal chemical liquid growth method at low temperature, ambient pressure, and solution environment. By focusing a 532 nm continuous-wave laser on a Pt substrate immersed in iron oxide precursor solution, vertically aligned iron oxide micro- and nanorods are successfully fabricated with the length up to >100 μm, whereas the length can be easily controlled by changing the laser power or the illumination time. It is also found that the direction of the laser ray determines the growth direction of the iron oxide micro- and nanorods, which is the property that makes this process suitable for the fabrication of complex 3D structures as confirmed by making an iron oxide junction and kinked iron oxide microrod structure. Moreover, the resultant iron oxide microrod is applied as a microtemplate for the growth of nanostructure to show that this process can be further integrated to other 3D structures to achieve trans-scale hierarchical structures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-28/jp501642j/production/images/medium/jp-2014-01642j_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp501642j'>ACS Electronic Supporting Info</A></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>
Digital selective growth of a ZnO nanowire array by large scale laser decomposition of zinc acetate.
Hong, Sukjoon,Yeo, Junyeob,Manorotkul, Wanit,Kang, Hyun Wook,Lee, Jinhwan,Han, Seungyong,Rho, Yoonsoo,Suh, Young Duk,Sung, Hyung Jin,Ko, Seung Hwan RSC Pub 2013 Nanoscale Vol.5 No.9
<P>We develop a digital direct writing method for ZnO NW micro-patterned growth on a large scale by selective laser decomposition of zinc acetate. For ZnO NW growth, by replacing the bulk heating with the scanning focused laser as a fully digital local heat source, zinc acetate crystallites can be selectively activated as a ZnO seed pattern to grow ZnO nanowires locally on a larger area. Together with the selective laser sintering process of metal nanoparticles, more than 10,000 UV sensors have been demonstrated on a 4 cm 4 cm glass substrate to develop all-solution processible, all-laser mask-less digital fabrication of electronic devices including active layer and metal electrodes without any conventional vacuum deposition, photolithographic process, premade mask, high temperature and vacuum environment.</P>
Hong, Sukjoon,Yeo, Junyeob,Manorotkul, Wanit,Kim, Gunho,Kwon, Jinhyeong,An, Kunsik,Ko, Seung Hwan Hindawi Limited 2013 Journal of nanomaterials Vol.2013 No.-
<P>We demonstrate ZnO nanowire based UV sensor by laser-induced hydrothermal growth of ZnO nanowire. By inducing a localized temperature rise using focused laser, ZnO nanowire array at<I>~</I>15 <I>μ</I>m size consists of individual nanowires with<I>~</I>8 <I>μ</I>m length and 200<I>~</I>400 nm diameter is readily synthesized on gold electrode within 30 min at the desired position. The laser-induced growth process is consecutively applied on two different points to bridge the micron gap between the electrodes. The resultant photoconductive ZnO NW interconnections display 2<I>~</I>3 orders increase in the current upon the UV exposure at a fixed voltage bias. It is also confirmed that the amount of photocurrent can be easily adjusted by changing the number of ZnO NW array junctions. The device exhibits clear response to the repeated UV illumination, suggesting that this process can be usefully applied for the facile fabrication of low-cost UV sensor array.</P>
Herman, Indria,Yeo, Junyeob,Hong, Sukjoon,Lee, Daeho,Nam, Koo Hyun,Choi, Jun-ho,Hong, Won-hwa,Lee, Dongjin,Grigoropoulos, Costas P,Ko, Seung Hwan IOP Pub 2012 Nanotechnology Vol.23 No.19
<P>In this paper we have demonstrated the simple, low cost, low temperature, hydrothermal growth of weeping willow ZnO nano-trees with very long branches to realize high efficiency dye-sensitized solar cells (DSSCs). We also discuss the effects of branching on solar cell efficiency. By introducing branched growth on the backbone ZnO nanowires (NWs), the short circuit current density and the overall light conversion efficiency of the branched ZnO NW DSSCs increased to almost four times that for vertically grown ZnO NWs. The efficiency increase is attributed to the increase in surface area for higher dye loading and light harvesting and also to reduced charge recombination through direct conduction along the crystalline ZnO branches. As the length of the branches increased, the branches became flaccid and the increase in solar cell efficiency slowed down because the effective surface area increase was hindered by branch bundling during the drying process and subsequent decrease in the dye loading.</P>
Hong, Sukjoon,Yeo, Junyeob,Lee, Jinhwan,Lee, Habeom,Lee, Phillip,Lee, Seung S,Ko, Seung Hwan American Scientific Publishers 2015 Journal of Nanoscience and Nanotechnology Vol.15 No.3
<P>We introduce a facile method to enhance the functionality of a patterned metallic transparent conductor through selective laser ablation of metal nanowire percolation network. By scanning focused nanosecond pulsed laser on silver nanowire percolation network, silver nanowires are selectively ablated and patterned without using any conventional chemical etching or photolithography steps. Various arbitrary patterns of silver nanowire transparent conductors are readily created on the percolation network by changing various laser parameters such as repetition rate and power. The macroscopic optical and electrical properties of the percolation network transparent conductor can be easily tuned by changing the conductor pattern design via digital selective laser ablation. Further investigation on the silver nanowire based electrode line prepared by the ablation process substantiates that the general relation for a conducting thin film fails at a narrow width, which should be considered for the applications that requires a high resolution patterns. Finally, as a proof of concept, a capacitive touch sensor with diamond patterns has been demonstrated by selective laser ablation of metal nanowire percolation network.</P>
Hong, Sukjoon,Yeo, Junyeob,Kim, Gunho,Kim, Dongkyu,Lee, Habeom,Kwon, Jinhyeong,Lee, Hyungman,Lee, Phillip,Ko, Seung Hwan American Chemical Society 2013 ACS NANO Vol.7 No.6
<P>We introduce a facile approach to fabricate a metallic grid transparent conductor on a flexible substrate using selective laser sintering of metal nanoparticle ink. The metallic grid transparent conductors with high transmittance (>85%) and low sheet resistance (30 Ω/sq) are readily produced on glass and polymer substrates at large scale without any vacuum or high-temperature environment. Being a maskless direct writing method, the shape and the parameters of the grid can be easily changed by CAD data. The resultant metallic grid also showed a superior stability in terms of adhesion and bending. This transparent conductor is further applied to the touch screen panel, and it is confirmed that the final device operates firmly under continuous mechanical stress.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2013/ancac3.2013.7.issue-6/nn400432z/production/images/medium/nn-2013-00432z_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn400432z'>ACS Electronic Supporting Info</A></P>