Jinho An

University of Texas at Austin, Austin, TX, United States

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Publications (26)192.8 Total impact

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    ABSTRACT: Mechanical properties of nanometer-thick multilayer vermiculite, a layered silicate, were investigated by atomic force microscopy (AFM) contact mode imaging. Membranes suspended over circular holes were with exfoliated vermiculite platelets. The elastic modulus and pre-stress of each membrane were obtained using AFM combined with finite element analysis. The exfoliated multilayer vermiculite membranes had an average in-plane elastic modulus and average pre-stress of 175 ± 16 GPa and 55 ± 13 MPa, respectively.
    Thin Solid Films 01/2013; 527:205–209. · 1.87 Impact Factor
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    ABSTRACT: Macroscopic, freestanding graphene-based paper-like materials are of interest for use as mechanically strong, stiff, and flexible and electrically conductive materials. Chemically reduced graphene oxide paper shows promise for such applications. In this work, we studied the mechanical and electrical properties of a set of paper materials prepared by filtration of homogeneous colloidal suspensions of hydrazine-reduced graphene oxide with different concentrations. Young’s modulus, fracture strength, and fracture strain of each type of sample was determined by tensile tests. The paper sample prepared from the colloidal suspension with the lowest concentration of reduced graphene oxide platelets had the highest modulus and fracture strength and showed the smoothest surface morphology. The electrical conductivity measured by the four-probe measurement method increased as the concentration was increased.
    Carbon. 10/2012; 50(12):4573–4578.
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    ABSTRACT: The elastic modulus of ultra-thin amorphous carbon films was investigated by integrating atomic force microscopy (AFM) imaging in contact mode with finite element analysis (FEA). Carbon films with thicknesses of ∼10 nm and less were deposited on mica by electron beam evaporation and transferred onto perforated substrates for mechanical characterization. The deformation of these ultra-thin membranes was measured by recording topography images at different normal loads using contact mode AFM. The obtained force-distance relationship at the center of membranes was analyzed to evaluate both the Young’s modulus and pre-stress by FEA. From these measurements, Young’s moduli of 178.9 ± 32.3, 193.4 ± 20.0, and 211.1 ± 44.9 GPa were obtained for 3.7 ± 0.08, 6.8 ± 0.12, and 10.4 ± 0.17 nm thick membranes, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used for characterizing the chemical and structural properties of the films, including the content of sp2 and sp3 hybridized carbon atoms.
    Carbon. 05/2012; 50(6):2220–2225.
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    ABSTRACT: Reproducible dry and wet transfer techniques were developed to improve the transfer of large-area monolayer graphene grown on copper foils by chemical vapor deposition (CVD). The techniques reported here allow transfer onto three different classes of substrates: substrates covered with shallow depressions, perforated substrates, and flat substrates. A novel dry transfer technique was used to make graphene-sealed microchambers without trapping liquid inside. The dry transfer technique utilizes a polydimethylsiloxane frame that attaches to the poly(methyl methacrylate) spun over the graphene film, and the monolayer graphene was transferred onto shallow depressions with 300 nm depth. The improved wet transfer onto perforated substrates with 2.7 μm diameter holes yields 98% coverage of holes covered with continuous films, allowing the ready use of Raman spectroscopy and transmission electron microscopy to study the intrinsic properties of CVD-grown monolayer graphene. Additionally, monolayer graphene transferred onto flat substrates has fewer cracks and tears, as well as lower sheet resistance than previous transfer techniques. Monolayer graphene films transferred onto glass had a sheet resistance of ∼980 Ω/sq and a transmittance of 97.6%. These transfer techniques open up possibilities for the fabrication of various graphene devices with unique configurations and enhanced performance.
    ACS Nano 09/2011; 5(9):6916-24. · 12.03 Impact Factor
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    ABSTRACT: Understanding and engineering the domain boundaries in chemically vapor deposited monolayer graphene will be critical for improving its properties. In this study, a combination of transmission electron microscopy (TEM) techniques including selected area electron diffraction, high resolution transmission electron microscopy (HR-TEM), and dark field (DF) TEM was used to study the boundary orientation angle distribution and the nature of the carbon bonds at the domain boundaries. This report provides an important first step toward a fundamental understanding of these domain boundaries. The results show that, for the graphene grown in this study, the 46 measured misorientation angles are all between 11° and 30° (with the exception of one at 7°). HR-TEM images show the presence of adsorbates in almost all of the boundary areas. When a boundary was imaged, defects were seen (dangling bonds) at the boundaries that likely contribute to adsorbates binding at these boundaries. DF-TEM images also showed the presence of a "twinlike" boundary.
    ACS Nano 03/2011; 5(4):2433-9. · 12.03 Impact Factor
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    ABSTRACT: Using micro-Raman spectroscopy, the thermal conductivity of a graphene monolayer grown by chemical vapor deposition and suspended over holes with different diameters ranging from 2.9 to 9.7 μm was measured in vacuum, thereby eliminating errors caused by heat loss to the surrounding gas. The obtained thermal conductivity values of the suspended graphene range from (2.6 ± 0.9) to (3.1 ± 1.0) × 10(3) Wm(-1)K(-1) near 350 K without showing the sample size dependence predicted for suspended, clean, and flat graphene crystal. The lack of sample size dependence is attributed to the relatively large measurement uncertainty as well as grain boundaries, wrinkles, defects, or polymeric residue that are possibly present in the measured samples. Moreover, from Raman measurements performed in air and CO(2) gas environments near atmospheric pressure, the heat transfer coefficient for air and CO(2) was determined and found to be (2.9 +5.1/-2.9) and (1.5 +4.2/-1.5) × 10(4) Wm(-2)K(-1), respectively, when the graphene temperature was heated by the Raman laser to about 510 K.
    ACS Nano 01/2011; 5(1):321-8. · 12.03 Impact Factor
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    ABSTRACT: We prepared hydrazine-reduced materials from both graphite oxide (GO) particles, which were not exfoliated, and completely exfoliated individual graphene oxide platelets, and then analyzed their chemical and structural properties by elemental analysis, XPS, TGA, XRD, and SEM. Both reduced materials showed distinctly different chemical and structural properties from one another. While hydrazine reduction of graphene oxide platelets produced agglomerates of exfoliated platelets, the reduction of GO particles produced particles that were not exfoliated. The degree of chemical reduction of reduced GO particles was lower than that of reduced graphene oxide and the BET surface area of reduced GO was much lower than that of reduced graphene oxide.
    Carbon. 01/2011; 49(9):3019-3023.
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    ABSTRACT: The morphology and thermomechanical properties of composites of poly(methyl methacrylate) (PMMA) and chemically modified graphene (CMG) fillers were investigated. For composites made by in situ polymerization, large shifts in the glass transition temperature were observed with loadings as low as 0.05wt.% for both chemically-reduced graphene oxide (RG-O) and graphene oxide (G-O)-filled composites. The elastic modulus of the composites improved by as much as 28% at just 1wt.% loading. Mori–Tanaka theory was used to quantify dispersion, suggesting platelet aspect ratios greater than 100 at low loadings and a lower quality of dispersion at higher loadings. Fracture strength increased for G-O/PMMA composites but decreased for RG-O/PMMA composites. Wide angle X-ray scattering suggested an exfoliated morphology of both types of CMG fillers dispersed in the PMMA matrix, while transmission electron microscopy revealed that the platelets adopt a wrinkled morphology when dispersed in the matrix. Both techniques suggested similar exfoliation and dispersion of both types of CMG filler. Structural characterization of the resulting composites using gel permeation chromatography and solid state nuclear magnetic resonance showed no change in the polymer structure with increased loading of CMG filler.
    Carbon. 01/2011; 49(8):2615-2623.
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    ABSTRACT: Multi-walled carbon nanotubes (MWCNTs) underwent failure during crack opening in a MWCNT/alumina composite. Transmission electron microscope observations and single nanotube pullout tests revealed that the MWCNTs, rather than pulling out from the alumina matrix, broke in the outer shells and then the inner core was pulled away, leaving fragments of the outer shells in the matrix (i.e., they underwent failure in a “sword-in-sheath” fracture mode, as observed for MWCNTs under tensile loading). Some MWCNTs failed leaving either a very short sword-in-sheath failure or a clean break. Theoretical predictions based on the MWCNT failure and pullout models suggested that the use of MWCNTs having a much higher load carrying capacity may lead to composites with a higher fracture toughness. These results may provide new insight into the fracture mechanisms and suggest a new design methodology for MWCNT-based ceramic composites, leading to improved fracture toughness.
    Carbon. 01/2011; 49(12):3709-3716.
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    ABSTRACT: Understanding and engineering the domain boundaries in chemically vapor deposited (CVD) monolayer graphene will be critical for improving its properties. In this study, a combination of transmission electron microscopy (TEM) techniques including selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM), and dark field (DF) TEM was used to study the boundary orientation angle distribution and the nature of the carbon bonds at the domain boundaries. This report provides an important first step towards a fundamental understanding of these domain boundaries. The results show that, for the graphene grown in this study, the 46 measured misorientation angles are all between 11-30 degrees (with the exception of one at 7 degrees). HRTEM images show the presence of adsorbates in almost all of the boundary areas. When a boundary was imaged, defects were seen (dangling bonds) at the boundaries that likely contribute to adsorbates binding at these boundaries. DFTEM images also showed the presence of a 'twin like' boundary.
    10/2010;
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    ABSTRACT: Mechanical properties of ultrathin membranes consisting of one layer, two overlapped layers, and three overlapped layers of graphene oxide platelets were investigated by atomic force microscopy (AFM) imaging in contact mode. In order to evaluate both the elastic modulus and prestress of thin membranes, the AFM measurement was combined with the finite element method (FEM) in a new approach for evaluating the mechanics of ultrathin membranes. Monolayer graphene oxide was found to have a lower effective Young's modulus (207.6 ± 23.4 GPa when a thickness of 0.7 nm is used) as compared to the value reported for "pristine" graphene. The prestress (39.7-76.8 MPa) of the graphene oxide membranes obtained by solution-based deposition was found to be 1 order of magnitude lower than that obtained by others for mechanically cleaved graphene. The novel AFM imaging and FEM-based mapping methods presented here are of general utility for obtaining the elastic modulus and prestress of thin membranes.
    ACS Nano 10/2010; 4(11):6557-64. · 12.03 Impact Factor
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    ABSTRACT: The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.
    Nano Letters 10/2010; 10(11):4328-34. · 13.03 Impact Factor
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    ABSTRACT: There has been no attempt to date to specifically modify the nodes in carbon nanotube (CNT) networks. If the nodes can be modified in favorable ways, the electrical and/or thermal and/or mechanical properties of the CNT networks could be improved. In an attempt to influence the performance as a transparent conductive film, gold nanoparticles capped with the amino acid cysteine (Au-CysNP) have been selectively attached at the nodes of multiwalled carbon nanotubes (MWCNTs) networks. These nanoparticles have an average diameter of 5 nm as observed by TEM. FTIR and XPS were used to characterize each step of the MWCNT chemical functionalization process. The chemical process was designed to favor selective attachment at the nodes and not the segments in the CNT networks. The chemical processing was designed to direct formation of nodes where the gold nanoparticles are. The nanoparticles which were loosely held in the CNT network could be easily washed away by solvents, while those bound chemically remained. TEM results show that the Cys-AuNPs are preferentially located at the nodes of the CNT networks when compared to the segments. These nanoparticles at the nodes were also characterized by a novel technique called diffraction scanning transmission electron microscopy (D-STEM) confirming their identity. Four-probe measurements found that the sheet resistance of the modified CNT networks was half that of similarly transparent pristine multiwalled CNT networks.
    ACS Nano 08/2010; 4(8):4962. · 12.03 Impact Factor
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    Advanced Materials 04/2010; 22(15):1736-40. · 14.83 Impact Factor
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    ABSTRACT: We report the deposition of films composed of overlapped and stacked platelets of graphene oxide (G-O) reduced by an electrophoretic deposition (EPD) process. The oxygen functional groups of G-O were significantly removed by the EPD process, and the as-deposited G-O film by EPD showed improved electrical conductivity (1.43 × 104 S·m−1) over G-O papers made by the filtration method (0.53 × 10−3 S·m−1). This method for reducing G-O without added reducing agents has the potential for high-yield, large-area, low-cost, and environmentally friendly production of films composed of reduced G-O platelets.Keywords (keywords): graphene; reduced graphene oxide; electrophoretic deposition (EPD); electrochemical reduction; reduced graphene oxide film
    Journal of Physical Chemistry Letters - J PHYS CHEM LETT. 03/2010; 1(8).
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    ABSTRACT: We report the mechanical behavior of a unique type of multi-walled carbon nanotube (MWCNT) and an acid-treated version of this MWCNT type that have nanoscale defects on their surfaces from the acid treatment. These defects, from scanning electron microscope (SEM) and transmission electron microscope (TEM) imaging have a 'channel-like' appearance, as if a ring of material was cut away from the MWCNT around the circumference. The mechanical properties of individual MWCNTs have been experimentally shown to strongly depend on their structure and structural disorder can drastically reduce the mechanical properties. Tensile-loading experiments using a nanomanipulator tool operated inside a SEM revealed that the tensile strengths of 10 pristine MWCNTs ranged from ∼ 2 to ∼ 48 GPa (mean 20 GPa). For 10 acid-treated MWCNTs with channel-like defects, tensile strengths ranged from ∼ 1 to ∼ 18 GPa (mean 6 GPa, thus roughly 70% lower than those of the pristine MWCNTs). Microstructural observations revealed that the fracture of the acid-treated MWCNTs occurred at a channel-like defect region in 8 of the 10 samples. This indicates that the channel-like defects associated with the acid etching are typically going to be the weakest points in the acid-treated MWCNT structure and that stress concentration is present at the defect region. © 2010 Published by Elsevier B.V.
    Diamond and Related Materials 03/2010; 19(7). · 1.71 Impact Factor
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    ABSTRACT: A method for growing polymers directly from the surface of graphene oxide is demonstrated. The technique involves the covalent attachment of an initiator followed by the polymerization of styrene, methyl methacrylate, or butyl acrylate using atom transfer radical polymerization (ATRP). The resulting materials were characterized using a range of techniques and were found to significantly improve the solubility properties of graphene oxide. The surface-grown polymers were saponified from the surface and also characterized. Based on these results, the ATRP reactions were determined to proceed in a controlled manner and were found to leave the structure of the graphene oxide largely intact.
    Macromolecular Rapid Communications 02/2010; 31(3):281-8. · 4.93 Impact Factor
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    Small 11/2009; 6(2):210-2. · 7.82 Impact Factor
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    ABSTRACT: Transmission electron microscope (TEM) is used in conjunction with an Automated Crystallography for TEM (ACT) to index the crystal orientation of 180 nm wide Cu interconnects using the Nano Beam Diffraction (NBD) mode in the TEM. An FEM software, OOF2, was used to simulate the local quasi‐hydrostatic stresses in the interconnect lines based on the local orientation data, and was compared results obtained from stress induced void (SIV) formation in 180 nm Cu interconnects studied through in‐situ TEM (Transmission Electron Microscope) heating. SIV were induced at temperatures of around 230° C. Correlation between the stress simulations and the experimental results show that point of high local stresses and high stress gradients seem to influence the formation of the SIV in the Cu interconnect lines. A description of a new technique, called D‐STEM that allows for obtaining diffraction patterns from crystals only a few nanometers in size is also given, that will allow for characterizing even smaller width Cu interconnects for future generation microelectronic devices.
    AIP Conference Proceedings. 06/2009; 1143(1):166-171.
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    ABSTRACT: Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room temperature.
    Science 05/2009; 324(5932):1312-4. · 31.20 Impact Factor