Bundling dynamics regulates the active mechanics and transport in carbon nanotube networks and their nanocomposites.

Hahm, Myung Gwan,Wang, Hailong,Jung, Hyun Young,Hong, Sanghyun,Lee, Sung-Goo,Kim, Sung-Ryong,Upmanyu, Moneesh,Jung, Yung Joon RSC Pub 2012 Nanoscale Vol.4 No.11

<P>High-density carbon nanotube networks (CNNs) continue to attract interest as active elements in nanoelectronic devices, nanoelectromechanical systems (NEMS) and multifunctional nanocomposites. The interplay between the network nanostructure and its properties is crucial, yet current understanding remains limited to the passive response. Here, we employ a novel superstructure consisting of millimeter-long vertically aligned single walled carbon nanotubes (SWCNTs) sandwiched between polydimethylsiloxane (PDMS) layers to quantify the effect of two classes of mechanical stimuli, film densification and stretching, on the electronic and thermal transport across the network. The network deforms easily with an increase in the electrical and thermal conductivities, suggestive of a floppy yet highly reconfigurable network. Insight from atomistically informed coarse-grained simulations uncover an interplay between the extent of lateral assembly of the bundles, modulated by surface zipping/unzipping, and the elastic energy associated with the bent conformations of the nanotubes/bundles. During densification, the network becomes highly interconnected yet we observe a modest increase in bundling primarily due to the reduced spacing between the SWCNTs. The stretching, on the other hand, is characterized by an initial debundling regime as the strain accommodation occurs via unzipping of the branched interconnects, followed by rapid rebundling as the strain transfers to the increasingly aligned bundles. In both cases, the increase in the electrical and thermal conductivity is primarily due to the increase in bundle size; the changes in network connectivity have a minor effect on the transport. Our results have broad implications for filamentous networks of inorganic nanoassemblies composed of interacting tubes, wires and ribbons/belts.</P>

Carbon Nanotube Core Graphitic Shell Hybrid Fibers

Hahm, Myung Gwan,Lee, Jae-Hwang,Hart, Amelia H. C.,Song, Sung Moo,Nam, Jaewook,Jung, Hyun Young,Hashim, Daniel Paul,Li, Bo,Narayanan, Tharangattu N.,Park, Chi-Dong,Zhao, Yao,Vajtai, Robert,Kim, Yoong American Chemical Society 2013 ACS NANO Vol.7 No.12

<P>A carbon nanotube yarn core graphitic shell hybrid fiber was fabricated <I>via</I> facile heat treatment of epoxy-based negative photoresist (SU-8) on carbon nanotube yarn. The effective encapsulation of carbon nanotube yarn in carbon fiber and a glassy carbon outer shell determines their physical properties. The higher electrical conductivity (than carbon fiber) of the carbon nanotube yarn overcomes the drawbacks of carbon fiber/glassy carbon, and the better properties (than carbon nanotubes) of the carbon fiber/glassy carbon make up for the lower thermal and mechanical properties of the carbon nanotube yarn <I>via</I> synergistic hybridization without any chemical doping and additional processes.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2013/ancac3.2013.7.issue-12/nn4045276/production/images/medium/nn-2013-045276_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn4045276'>ACS Electronic Supporting Info</A></P>

A review: controlled synthesis of vertically aligned carbon nanotubes

Myung-Gwan Hahm,Daniel P. Hashim,Robert Vajtai,Pulickel M. Ajayan 한국탄소학회 2011 Carbon Letters Vol.12 No.4

Carbon nanotubes (CNTs) have developed into one of the most competitively researched nano-materials of this decade because of their structural uniqueness and excellent physical properties such as nanoscale one dimensionality, high aspect ratio, high mechanical strength, thermal conductivity and excellent electrical conductivity. Mass production and structure control of CNTs are key factors for a feasible CNT industry. Water and ethanol vapor enhance the catalytic activity for massive growth of vertically aligned CNTs. A shower system for gas flow improves the growth of vertically aligned single walled CNTs (SWCNTs) by controlling the gas flow direction. Delivery of gases from the top of the nanotubes enables direct and precise supply of carbon source and water vapor to the catalysts. High quality vertically aligned SWCNTs synthesized using plasma enhance the chemical vapor deposition technique on substrate with suitable metal catalyst particles. This review provides an introduction to the concept of the growth of vertically aligned SWCNTs and covers advanced topics on the controlled synthesis of vertically aligned SWCNTs.

Highly Organized Two- and Three-Dimensional Single-Walled Carbon Nanotube–Polymer Hybrid Architectures

Li, Bo,Hahm, Myung Gwan,Kim, Young Lae,Jung, Hyun Young,Kar, Swastik,Jung, Yung Joon American Chemical Society 2011 ACS NANO Vol.5 No.6

<P>Single-walled carbon nanotube (SWCNT) network architectures combined with flexible mediums (especially polymers) are strong candidates for functional flexible devices and composite structures requiring the combination of unique electronic, optical, and/or mechanical properties of SWCNTs and polymer materials. However, to build functional flexible devices with SWCNTs, it is required to have abilities to assemble and incorporate SWCNTs in desired locations, orientations, and dimensions on/inside polymer substrates. Here, we present unique two- and three-dimensional SWCNT network–polymer hybrid architectures by combining unprecedented control over growth, assembly, and transfer processes of SWCNTs. Several SWCNT architectures have been built on polymer materials ranging from two-dimensional suspended SWCNT microlines on PDMS microchannels to three-dimensional “PDMS-vertically aligned SWCNTs-PDMS” sandwich structures. Also a combined lateral SWCNT microline and vertically aligned SWCNT flexible device was demonstrated with good electrical conductivity and low junction resistance. The results reported here open the pathway for the development of SWCNT-based functional systems in various flexible device applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2011/ancac3.2011.5.issue-6/nn2008782/production/images/medium/nn-2011-008782_0003.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn2008782'>ACS Electronic Supporting Info</A></P>

Tunable Electrical Properties of Aligned Single-Walled Carbon Nanotube Network-based Devices: Metallization and Chemical Sensor Applications

Kim, Young Lae,Hahm, Myung Gwan The Korean Ceramic Society 2017 한국세라믹학회지 Vol.54 No.6

Here we report the tunable electrical properties and chemical sensor of single-walled carbon nanotubes (SWCNTs) network-based devices with a functionalization technique. Formation of highly aligned SWCNT structures is made on $SiO_2/Si$ substrates using a template-based fluidic assembly process. We present a Platinum (Pt)-nanocluster decoration technique that reduces the resistivity of SWCNT network-based devices. This indicates the conversion of the semiconducting SWCNTs into metallic ones. In addition, we present the Hydrogen Sulfide ($H_2S$) gas detection by a redox reaction based on SWCNT networks functionalized with 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) as a catalyst. We summarize current changes of devices resulting from the redox reactions in the presence of $H_2S$. The semiconducting (s)-SWCNT device functionalized with TEMPO shows high gas response of 420% at 60% humidity level compared to 140% gas response without TEMPO functionalization, which is about 3 times higher than bare s-SWCNT sensor at the same RH. These results reflect promising perspectives for real-time monitoring of $H_2S$ gases with high gas response and low power consumption.

In Situ Synthesis of ThermochemicallyReduced Graphene Oxide Conducting Nanocomposites

Park, Ok-Kyung,Hahm, Myung Gwan,Lee, Sungho,Joh, Han-Ik,Na, Seok-In,Vajtai, Robert,Lee, Joong Hee,Ku, Bon-Cheol,Ajayan, Pulickel M. American ChemicalSociety 2012 NANO LETTERS Vol.12 No.4

<P>Highly conductive reduced graphene oxide (GO) polymer nano composites are synthesized by a well-organized in situ thermochemical synthesis technique. The surface functionalization of GO was carried out with aryl diazonium salt including 4-iodoaniline to form phenyl functionalized GO (I-Ph-GO). The thermochemically developed reduced, GO (R-I-Ph-GO) has five times higher electrical conductivity (42 000 S/m) than typical reduced GO (R-GO). We also demonstrate a R-I-Ph-GO/polyimide (PI) composites having more than 10(4) times higher conductivity (similar to 1 S/m) compared to a R-GO/PI composites. The electrical resistances of PI composites with R-I-Ph-GO were dramatically dropped under similar to 3% tensile strain. The R-I-Ph-GO/PI composites with electrically sensitive response caused by mechanical strain are expected to have broad implications for nanoelectromechanical systems.</P>

On the development of multifunctional luminescent supramolecular hydrogel of gold and egg white

Patra, Sudeshna,Ravulapalli, Sathyavathi,Hahm, Myung Gwan,Tadi, Kiran Kumar,Narayanan, Tharangattu N IOP 2016 Nanotechnology Vol.27 No.41

<P>Highly stable, luminescent, and printable/paintable supramolecular egg white hydrogel-based surface enhanced Raman scattering (SERS) matrix is created by an <I>in situ</I> synthesis of gold clusters inside a luminescent egg white hydrogel (Au-Gel). The synthesis of stable luminescent egg-white-based hydrogel, where the hydrogel can act as a three dimensional (3D) matrix, using a simple cross-linking chemistry, has promising application in the biomedical field including in 3D cell culturing. Furthermore, this functional hydrogel is demonstrated for micromolar-level detection of Rhodamine 6G using the SERS technique, where Au-Gel is painted over a flexible cellulose pad.</P>

Influence of Gas Adsorption and Gold Nanoparticles on the Electrical Properties of CVD-Grown MoS₂ Thin Films

Cho, Yunae,Sohn, Ahrum,Kim, Sujung,Hahm, Myung Gwan,Kim, Dong-Ho,Cho, Byungjin,Kim, Dong-Wook American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.33

<P>Molybdenum disulfide (MoS2) has increasingly attracted attention from researchers and is now one of the most intensively explored atomic-layered two-dimensional semiconductors. Control of the carrier concentration and doping type of MoS2 is crucial for its application in electronic and optoelectronic devices. Because the MoS2 layers are atomically thin, their transport characteristics may be very sensitive to ambient gas adsorption and the resulting charge transfer. We investigated the influence of the ambient gas (N-2, H-2/N-2, and O-2) choice on the resistance (R) and surface work function (WF) of trilayer MoS2 thin films grown via chemical vapor deposition. We also studied the electrical properties of gold (Au)-nanoparticle (NP)-coated MoS, thin films; their R value was found to be 2 orders of magnitude smaller than that for bare samples. While the WF largely varied for each gas, R was almost invariant for both the bare and Au-NP-coated samples regardless of which gas was used. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction for bare and Au-NP-coated MoS2 thin films. The charges transferred from the gas adsorbates might be insufficient to induce measurable R change and/or be trapped in the defect states. The smaller WF and larger localization length of the Au-NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced conduction in MoS2.</P>

Topological Transitions in Carbon Nanotube Networks <i>via</i> Nanoscale Confinement

Somu, Sivasubramanian,Wang, Hailong,Kim, Younglae,Jaberansari, Laila,Hahm, Myung Gwan,Li, Bo,Kim, Taehoon,Xiong, Xugang,Jung, Yung Joon,Upmanyu, Moneesh,Busnaina, Ahmed American Chemical Society 2010 ACS NANO Vol.4 No.7

<P>Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative approaches based on ultrathin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to (i) an often uncontrollable interplay between network coverage and its detailed topology and (ii) the inherent electrical heterogeneity of the constituent SWNTs. In this article, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto submicrometer wide ultrathin polymer channels become increasingly aligned with decreasing channel width and thickness. Experimental-scale coarse-grained computations of interacting SWCNTs show that the effect is a reflection of a topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effectiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2010/ancac3.2010.4.issue-7/nn100714v/production/images/medium/nn-2010-00714v_0003.gif'></P>

Ultrathin SWNT Films with Tunable, Anisotropic Transport Properties

Li, Bo,Jung, Hyun Young,Wang, Hailong,Kim, Young Lae,Kim, Taehoon,Hahm, Myung Gwan,Busnaina, Ahmed,Upmanyu, Moneesh,Jung, Yung Joon WILEY‐VCH Verlag 2011 Advanced functional materials Vol.21 No.10

<P><B>Abstract</B></P><P>Directional transport properties at the nanoscale remain a challenge, primarily due to issues pertaining to control over the underlying anisotropy and scalability to macroscopic scales. Here, we develop a facile approach based on template‐guided fluidic assembly of high mobility building blocks – single walled carbon nanotubes (SWNTs) – to fabricate ultrathin and anisotropic SWNTs films. A major advancement is the complete control over the anisotropy in the assembled nanostructure, realized by three‐dimensional engineering of the dip‐coated SWNTs ultrathin film into alternating hydrophilic and hydrophobic microline patterns with prescribed intra/inter‐line widths and line thicknesses. Variations in the contact line profile results in an evaporation‐controlled assembly mechanism that leads to alternating, and more importantly, contiguous SWNTs networks. Evidently, the nanoscopic thickness modulations are direct reflections of the substrate geometry and chemistry. The nanostructured film exhibits significant anisotropy in electrical and thermal transport properties as well as an optically transparent nature, as revealed by characterization studies. The direct interplay between the anisotropy and the 3D microline patterns of the substrate combined with the wafer‐level scalability of the fluidic assembly allows us to tune the transport properties for a host of nanoelectronic applications.</P>