P-type doping effect of few-layer MoS₂ and WSe₂ by selective Ag photo-reduction

이윤경,김은파,고창현,박윤정,Costas P. Grigoropoulos,김경훈 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.5

Direct Micro/Nano Patterning of Multiple Colored Quantum Dots by Large Area and Multilayer Imprinting

Oh, Yong Suk,Lee, Kyung Heon,Kim, Hyunki,Jeon, Duk Young,Ko, Seung Hwan,Grigoropoulos, Costas P.,Sung, Hyung Jin American Chemical Society 2012 The Journal of Physical Chemistry Part C Vol.116 No.21

<P>A novel direct method of micro/nano quantum dot (QD) patterning via one-step imprinting over a large area at low temperatures and low pressures was demonstrated as an alternative to conventional vacuum deposition and photolithography methods. More complex QD patterning could be demonstrated by expanding the QD direct imprinting process for multiple colored QD and patterning on the multiple layers. Additionally, a self-alignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. Our approach may be useful for QD-based optoelectronic device fabrication and patterning on large flexible substrates due to the low-temperature requirements of this process.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-21/jp301397t/production/images/medium/jp-2012-01397t_0001.gif'></P>

Hierarchical weeping willow nano-tree growth and effect of branching on dye-sensitized solar cell efficiency

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>

Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowires for a High Efficiency Dye-Sensitized Solar Cell

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>

Metal nanoparticle direct inkjet printing for low-temperature 3D micro metal structure fabrication

Ko, Seung Hwan,Chung, Jaewon,Hotz, Nico,Nam, Koo Hyun,Grigoropoulos, Costas P IOP 2010 JOURNAL OF MICROMECHANICS AND MICROENGINEERING - Vol.20 No.12

<P>Inkjet printing of functional materials is a key technology toward ultra-low-cost, large-area electronics. We demonstrate low-temperature 3D micro metal structure fabrication by direct inkjet printing of metal nanoparticles (NPs) as a versatile, direct 3D metal structuring approach representing an alternative to conventional vacuum deposition and photolithographic methods. Metal NP ink was inkjet-printed to exploit the large melting temperature drop of the nanomaterial and the ease of the NP ink formulation. Parametric studies on the basic conditions for stable 3D inkjet printing of NP ink were carried out. Furthermore, diverse 3D metal microstructures, including micro metal pillar arrays, helices, zigzag and micro bridges were demonstrated and electrical characterization was performed. Since the process requires low temperature, it carries substantial potential for fabrication of electronics on a plastic substrate.</P>

Laser-Induced Hydrothermal Growth of Heterogeneous Metal-Oxide Nanowire on Flexible Substrate by Laser Absorption Layer Design

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>

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>

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>

Rapid, One‐Step, Digital Selective Growth of ZnO Nanowires on 3D Structures Using Laser Induced Hydrothermal Growth

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>

Laser direct writing and inkjet printing for a sub-2 <i>μ</i>m channel length MoS₂ transistor with high-resolution electrodes

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>