Adsorption Site Selectivity for Thiophene on Reconstructed Si(5 5 12)–2 × 1 Surface

Hahn, Jae Ryang,Bharath, Satyaveda C.,Kim, Gyu-Hyeong,Kim, Ki Wan,Jeong, Sukmin,Pearl, Thomas P. American Chemical Society 2013 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.117 No.21

<P>The covalent binding of thiophene molecules to a Si(5 5 12)–2 × 1 surface was investigated using scanning tunneling microscopy and density functional theory calculations. The molecular attachment occurred exclusively between the bonding of the 2,5 carbon atoms and the two silicon adatoms without the involvement of the sulfur atom (over 90%). The binding structure formed a di−σ bond with a planar butterfly-like configuration. Adsorption at other sites, including at the dimer, tetramer, or honeycomb rows, occurred much less frequently. Our calculations predicted that the adsorption energies of the thiophene molecules were 1.02–1.56 eV at the adatom, dimer, and tetramer sites. The molecules adsorbed on the honeycomb rows with a low adsorption energy (below 1 eV). The binding modes of the simple aromatic molecules onto the Si(5 5 12)–2 × 1 surface are compared and discussed.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2013/jpccck.2013.117.issue-21/jp401932r/production/images/medium/jp-2013-01932r_0005.gif'></P>

Electrical and Thermal Properties of Poly(p-phenylene sulfide) Reduced Graphite Oxide Nanocomposites

Byung-Jae Chae,Do Hwan Kim,In-Soo Jeong,Jae Ryang Hahn,Bon-Cheol Ku 한국탄소학회 2012 Carbon Letters Vol.13 No.4

Graphite oxide (GO) was produced using the modified Hummer's method. Poly(p-phenylene sulfide) (PPS)/reduced graphite oxide (RGO) composites were prepared by in situ polymerization method. The electrical conductivity of the PPS/RGO composites was no more than 82 S/m. It was found that as GO content increased in the PPS/RGO composites, the crystallization temperature and electrical conductivity of the composites increased and the percolation threshold value was at 5-8 wt% of GO content.

Electronic structure and photocatalytic band offset of few-layer GeP2

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok Royal Society of Chemistry 2017 Journal of Materials Chemistry A Vol.5 No.42

<P>Based on a sophisticated crystal structure prediction method, we propose two-dimensional (2D) GeP2in the tetragonal (T) phase never observed for other group IV-V compounds. The bulk of 2D GeP2is more stable than both 2D orthogonal (O) and three-dimensional pyrite (P) phases that have been experimentally observed for group IV-V compounds. According to our calculations of phonon dispersion relations and elastic constants, as well as<I>ab initio</I>molecular dynamics simulation, monolayers of both the T and O phases (penta-GeP2and O-GeP2, respectively) are dynamically, mechanically, and thermally stable. In addition, our HSE06 calculation shows that these monolayers are semiconductors with band gaps in the visible region. Among the various stacking patterns of their bilayers, specific ones are identified to be most stable, which are still semiconductors with band gaps redshifted in the visible region. Different from the case of their bulk, few-layers of O-GeP2are more stable than those of penta-GeP2up to a pentalayer. Furthermore, band offset with respect to the Fermi levels of appropriate half-reactions shows that both n-type few-layer penta-GeP2and O-GeP2can be useful in photocatalyzed CO2splitting to CO as well as in photocatalyzed water splitting, specifically under acidic conditions.</P>

Mechanical and Electronic Properties of π-Conjugated Metal Bis(dithiolene) Complex Sheets

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok American Chemical Society 2014 Chemistry of materials Vol.26 No.9

<P>Using first-principles calculations, we have investigated the mechanical properties and electronic structures of π<SUP>–</SUP>conjugated metal bis(dithiolene) complex sheets (MC<SUB>4</SUB>S<SUB>4</SUB>), where M = Ni and Pd. First, the sheets are much softer than graphene due to their large porosity. At zero strain, NiC<SUB>4</SUB>S<SUB>4</SUB> is a semiconductor with an indirect gap, while PdC<SUB>4</SUB>S<SUB>4</SUB> is a metal. Under either biaxial or uniaxial strain, our band structure analysis demonstrates that the band gap of the NiC<SUB>4</SUB>S<SUB>4</SUB> slowly decreases to zero with increased strain, which can be attributed to the gradual weakening of π-bonds of the sheet. However, the PdC<SUB>4</SUB>S<SUB>4</SUB> becomes a magnetic system beyond the deformation threshold that causes a plastic deformation along the <I>X</I>-axis. In addition, we also observe that both two-dimensional sheets undergo different types of nonreversible plastic changes under the uniaxial strains along the <I>X</I>- and <I>Y</I>-axes.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2014/cmatex.2014.26.issue-9/cm500767u/production/images/medium/cm-2014-00767u_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm500767u'>ACS Electronic Supporting Info</A></P>

Effect of Si–Si Bonds in Silicon-Doped α-Phosphorene Bilayers: Two-Dimensional Layers and One-Dimensional Nanoribbons

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.30

<P>We investigate the geometrical and electronic structures of various configurations of 2Si-doped two-dimensional (2D) bilayers of black phosphorene (alpha P)(2), in which two P atoms are substituted by Si atoms. Our first-principles calculations suggest that doping is cooperative, which is clearly manifested in the formation of Si-Si bonds in the two most stable configurations. As a result, both configurations become indirect-gap semiconductors, which differ from that of the pristine 2D bilayer. On the one hand, 2Si-doped armchair phosphorene nanoribbon (APNR) bilayers possess pseudodirect band gaps in the most stable configuration, which are one-dimensional materials cut with armchair edges saturated with hydrogen atoms. Comparisons of the deformation energy and the activation barrier suggest that Stone-Wales (SW) deformation can occur substantially more easily in the doped APNR than in carbon nanotubes, and molecular dynamics simulations show that the SW defect will be kinetically stable. This is because the deformation brings about shortening and strengthening of weak Si-Si bonds. As a result, the APNRs turn into real direct-gap materials.</P>

Synthesis of Highly Dispersed and Conductive Graphene Sheets by Exfoliation of Preheated Graphite in a Sealed Bath and its Applications to Polyimide Nanocomposites

Hossain, Muhammad Mohsin,Hahn, Jae Ryang,Ku, Bon-Cheol Korean Chemical Society 2014 Bulletin of the Korean Chemical Society Vol.35 No.7

A simple method for exfoliating pristine graphite to yield mono-, bi-, and multi-layers of graphene sheets as a highly concentrated (5.25 mg/mL) and yielded solution in an organic solvent was developed. Pre-thermal treatment of pristine graphite at $900^{\circ}C$ in a sealed stainless steel bath under high pressures, followed by sonication in 1-methyl-2-pyrrolidinone solvent at elevated temperatures, produced a homogeneous, well-dispersed, and non-oxidized graphene solution with a low defect density. The electrical conductivities of the graphene sheets were very high, up to 848 S/cm. These graphene sheets were used to fabricate graphene-polyimide nanocomposites, which displayed a higher electrical conductivity (1.37 S/m) with an improved tensile strength (95 MPa). The synthesized graphene sheets and nanocomposites were characterized by transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy.

Effect of Carbon Nanofiber Structure on Crystallization Kinetics of Polypropylene/Carbon Nanofiber Composites

Lee, Sung-Ho,Hahn, Jae-Ryang,Ku, Bon-Cheol,Kim, Jun-Kyung Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.7

Effect of heat treatment of carbon nanofibers (CNF) on electrical properties and crystallization behavior of polypropylene was reported. Two types of CNFs (untreated and heat treated at 2300 $^{\circ}C$) were incorporated into polypropylene (PP) using intensive mixing. A significant drop in volume resistivity was observed with composites containing untreated 5 wt % and heat treated 3 wt % CNF. In non-isothermal crystallization studies, both untreated and heat treated CNFs acted as nucleating agents. Composites with heat treated CNFs showed a higher crystallization temperature than composites with untreated CNFs did. TEM results of CNF revealed that an irregular structure of CNFs can be converted into the continuous graphitic structure after heat treatment. Furthermore, STM showed that the higher carbonization temperature leads to the higher graphite degree which presents the larger carbon network size, suggesting that a more graphitic structure of CNFs led to a higher crystallization temperature of PP.

Effects of the Superlattices on STM Imaging of Self-organized Substituted Alkyl Chain Monolayers on a Graphite Surface

Son, Seung Bae,Hahn, Jae Ryang Korean Chemical Society 2012 Bulletin of the Korean Chemical Society Vol.33 No.12

We characterized the physisorption of p-iodo-phenyl octadecyl ether molecules (I-POE) onto superlattice regions of graphite surfaces using scanning tunneling microscopy (STM). The formation of self-organized I-POE monolayers does not affect the overall structures of moir$\acute{e}$ patterns and their modulation periods. However, the packing density of the I-POE monolayer and the orientations of lamella structures were sensitive to the underlying superlattice structure. Depending on the bias voltage, the STM images selectively showed moir$\acute{e}$ pattern, I-POE layer, or both. Reflecting the local density of states at a certain energy level, the STM images thereby revealed the relative energy level scale of the superlattice with respect to the molecular orbitals of I-POE.

Effects of Tunneling Current on STM Imaging Mechanism for Alkanethiol Self-assembled Monolayers on Au(111)

Mamun, Abdulla Hel Al,Son, Seung-Bae,Hahn, Jae-Ryang Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.1

We investigated the effects of tunneling current on scanning tunneling microscopy (STM) images of 1-octanethiol (OT) and 1-decanethiol (DT) self-assembled monolayers (SAMs). At a low tunneling current, the domain boundaries and ordered alkanethiol molecules were clearly resolved. As the tunneling current was increased at a constant bias voltage, however, the STM images showed disordered structures of the OT and DT SAMs. As the tunneling current was reduced back to low values, the ordered structures of the alkanethiol molecules reappeared. The reversibility of the process suggests that the sulfur head groups did not rearrange under any of the tunneling current conditions. On the basis of our observations, which are inconsistent with the standard model for STM imaging of molecules on metal surfaces, we consider the STM imaging mechanism in terms of a two-region tunneling junction model.

High yield and high concentration few-layer graphene sheets using solvent exfoliation of graphite with pre-thermal treatment in a sealed bath

Hossain, Muhammad Mohsin,Park, Ok-Kyung,Hahn, Jae Ryang,Ku, Bon-Cheol Elsevier 2014 Materials letters Vol.123 No.-

We developed an efficient method for exfoliating pristine graphite in an organic solvent to produce a high yield (14%) and high concentration (5.25 mg/mL) of well-dispersed mono-, bi-, and multi-layered graphene sheets. Pre-thermal activation of pristine graphite at 900 degrees C in a sealed stainless steel bath under high pressure, followed by sonication in a solvent of 1-methyl-2-pyrrolidinone at elevated temperatures produced a solution of homogeneous, well-dispersed, defect-free, and non-oxidized graphene. The resulting electrical conductivities of the graphene sheets were high, up to 849 S/cm. (c) 2014 Elsevier B.V. All rights reserved.