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    Title: Associate Professor Member: Full Time Faculty
    Name: Mario Hofmann Eduction: Ph.D.Massachusetts Institute of Technology( 2011)
    NTU AH: http://mylabntu.weebly.com/
    Room: R628
    Tel1(1): (02)3366-5177
    E-Mail(1): E-Mail(2):
    Since 2017-2 Professor in Physics, NTU. 2011-2012 Postdoctoral researcher, National Taiwan University
    Visiting Scientist,
     ●Cornell University, USA, 2011
     ●UFMG, Brazil, 2010
     ●NTU, Taiwan 2009
     ●Tokyo University, Japan, 2008
     ●Columbia University, USA, 2007
    2012-2017 Assistant Professor in Materials Science, National Cheng Kung University
    My research is focusing on novel nanostructured materials that can enrich our daily lives. Because the field is relatively new there are still many aspects that require our attention. Consequently, my work is divided into fundamental studies which reveal the novel properties that arise from dimensional confinement and combination of nanostructures and application of these unique phenomena. I subsequently describe selected aspects of fundamental studies into the characterization and improved synthesis of nanomaterials on the one hand and application of these materials on the other hand.
    Fundamental studies of graphene properties
    Carbon is an amazing model system for nanostructure-induced changes in properties. One important issue of understanding these novel properties is to find a suitable way to characterize them. Raman spectroscopy is an optical technique that reveals a wealth of information about the bonding state and crystallinity of carbon materials. Especially the energy dependence of the Raman features is an area that has not been fully explored due to the challenges in experiments. We have studied the resonance effect of carbon nanotubes as the electronic band structure interacts with the phononic structure.
    Recently, however, we found that nanometer sized openings in graphene significantly affect the behavior of the material but cannot be detected by Raman spectroscopy. Therefore I developed a new tool for the metrology of graphene and other 2D materials. The metrology of chemical vapor deposition (CVD) grown two-dimensional materials such as graphene, Boron Nitride or Molybdenum Disulfide, is critical for the optimization of their synthesis. We demonstrated the use of film-induced frustrated etching (FIFE) as a facile, scalable method to reveal and quantify structural defects in continuous thin sheets. The sensitivity of the analysis technique to intentionally induced latticed defects in graphene compares favorably to the sensitivity of Raman spectroscopy. A strong correlation between the measured defectiveness and the maximum carrier mobility in graphene emphasizes the importance of the technique for growth optimization. Due to its ease and widespread availability, we anticipate that FIFE will find wide application in the characterization of CVD synthesized 2D materials.
    Using angle-resolved X-ray photoelectron spectroscopy in conjunction with Raman spectroscopy we have recently investigated the interaction of metal surfaces with graphene and found that Aluminum exhibits a significant chemical interaction with graphene that results in a hybridized material with novel properties.
    1.Saito, R.; Hofmann, M.; Dresselhaus, G.; Jorio, A.; Dresselhaus, M. S., Raman spectroscopy of graphene and carbon nanotubes. Adv Phys 2011, 60 (3), 413-550;
    2.Hofmann, M.; Shin, Y. C.; Hsieh, Y. P.; Dresselhaus, M. S.; Kong, J., A facile tool for the characterization of two-dimensional materials grown by chemical vapor deposition. Nano Res 2012, 5 (7), 504-511.
    3.Hsieh, Y. P.; Hofmann, M.; Farhat, H.; Barros, E. B.; Kalbac, M.; Kong, J.; Liang, C. T.; Chen, Y. F.; Dresselhaus, M. S., Chiral angle dependence of resonance window widths in (2n+m) families of single-walled carbon nanotubes. Appl Phys Lett 2010, 96 (10);
    4.Hsu, A. L.; Koch, R. J.; Ong, M. T.; Fang, W.; Hofmann, M.; Kim, K. K.; Seyller, T.; Dresselhaus, M. S.; Reed, E. J.; Kong, J.; Palacios, T., Surface-Induced Hybridization between Graphene and Titanium. Acs Nano 2014, 8 (8), 7704-7713.

    Improved graphene synthesis
    Based on the gained understanding from improved characterization we are trying to enhance the quality and selectivity of current synthesis methods.
    Graphene edges are an interesting system since they will be of great importance for future graphene-based electronics. We have studied the kinetic stability of different edge configurations using atomic resolution transmission electron microscopy. The edge structure can be controlled by Joule heating which leads to a prevalence of one type.
    Furthermore, we showed that a process of catalyst-deactivation limit the achievable quality of current production methods of graphene. Through addition of a second catalytic material, such as Nickel or Molybdenum, as this issue can be overcome. Kinetic studies show that a decrease of the dehydrogenation energy barrier by a promoter can be achieved and an increase the graphene quality is found. These findings not only answer fundamental questions about graphene growth but also provide a route for improving the CVD synthesis of graphene.

    5.Hsieh, Y.-P.; Hofmann, M.; Kong, J., Promoter-assisted chemical vapor deposition of graphene. Carbon 2014, 67, 417-423.
    6.Jia, X. T.; Hofmann, M.; Meunier, V.; Sumpter, B. G.; Campos-Delgado, J.; Romo-Herrera, J. M.; Son, H. B.; Hsieh, Y. P.; Reina, A.; Kong, J.; Terrones, M.; Dresselhaus, M. S., Controlled Formation of Sharp Zigzag and Armchair Edges in Graphitic Nanoribbons. Science 2009, 323 (5922), 1701-1705.

    Applications of 2D materials
    A novel direction that we have been pursuing for the last few years is the focus on applications of nanostructured materials in line with the current proposal. This is a challenging route due to the novelty of the field. Consequently, a part of the effort is invested into developing superior nanomaterials for existing applications.
    We demonstrated that oxidative graphene treatment can improve the antibacterial properties of the material and enable applications in health care.
    Furthermore, we developed a new approach to directly generating patterned graphene from chemical vapor deposition(CVD that can improve the quality and commercial potential of current graphene devices and also enables a new class of graphene devices. By selective passivation of the catalyst, graphene growth on Cu-foil could be restricted to certain areas. High resolution and high quality graphene patterns were achieved by using aluminum oxide as the passivation layer. Several approaches for depositing the patterned passivation layer were demonstrated that can satisfy application demands for low cost, high resolution and scalability. The transfer of pre-patterned graphene allows the production of high resolution graphene circuits on non-planar substrates which opens up novel application areas for graphene based electronic devices.
    Finally, we study the use of graphene in corrosion protection barriers and demonstrate a fundamentally new fluidic phenomenon on the nanoscale that deteriorates the performance of graphene passivation barriers. By selectively passivation graphene defects using atomic layer deposition we can improve the performance to 99.8% which is amongst the highest reported values for corrosion protection barriers and enables the use of graphene in a multibillion dollar market.
    7.Liu, S. B.; Zeng, T. H.; Hofmann, M.; Burcombe, E.; Wei, J.; Jiang, R. R.; Kong, J.; Chen, Y., Antibacterial Activity of Graphite, Graphite Oxide, Graphene Oxide, and Reduced Graphene Oxide: Membrane and Oxidative Stress. Acs Nano 2011, 5 (9), 6971-6980.
    8.Hofmann, M.; Hsieh, Y.-P.; Hsu, A. L.; Kong, J., Scalable, flexible and high resolution patterning of CVD graphene. Nanoscale 2014, 6 (1), 289-292.
    9.Hsieh, Y.-P.; Hofmann, M.; Chang, K.-W.; Jhu, J. G.; Li, Y.-Y.; Chen, K. Y.; Yang, C. C.; Chang, W.-S.; Chen, L.-C., Complete Corrosion Inhibition through Graphene Defect Passivation. Acs Nano 2014, 8 (1), 443-448;

    The novel properties of nanostructured materials can not only improve existing applications as detailed above but can be employed in novel devices with innovative working mechanisms.
    We show that the atomic thickness of graphene and its low carrier concentration can be exploited in novel plasmonic sensors that allow mapping of the plasmonic field with unprecedented resolution.
    Furthermore, we introduced a novel class of strain sensors based on percolative networks of 2D materials. The high sensitivity of the percolative carrier transport to strain induced morphology changes was exploited in strain sensors that can be produced from a wide variety of materials. Highly reliable and sensitive graphene based thin film strain gauges were produced from solution processed graphene flakes by spray deposition. Control of the gauge sensitivity could be exerted through deposition induced changes to the film morphology. The demonstrated scalable fabrication, superior sensitivity over conventional sensors and unique properties of the described strain gauges have the potential to improve existing technology and open up new fields of applications for strain sensors.
    Finally, we used graphene/dielectric interfaces to sense illumination with extremely high sensitivity. Due to the unique tunability of the Graphene Fermi level, carrier injection is very sensitive to illumination and broad band sensitivities en par with current devices could be demonstrated.

    10.Tan, W. C.; Hofmann, M.; Hsieh, Y. P.; Lu, M. L.; Chen, Y. F., A graphene-based surface plasmon sensor. Nano Res 2012, 5 (10), 695-702;
    11.Hempel, M.; Nezich, D.; Kong, J.; Hofmann, M., A Novel Class of Strain Gauges Based on Layered Percolative Films of 2D Materials. Nano Lett 2012, 12 (11), 5714-5718; Hsieh, Y.-P.; Yen, C.-H.; Lin, P.-S.; Ma, S.-W.; Ting, C.-C.; Wu, C.-I.; Hofmann, M., Ultra-high sensitivity graphene photosensors. Appl Phys Lett 2014, 104 (4);

    Fabrication and optoelectronic application of graphene heterojunctions
    Graphene is a single-atom thin carbon material that has captured the attention of researchers in fundamental and applied sciences for the last decade. Graphene’s unique properties, such as strong light-matter interaction and high carrier mobility, show great promise for optoelectronic devices. However, the absence of a bandgap and fast carrier relaxation make light emission and conversion in graphene very challenging. We here demonstrate the potential of graphene heterojunctions for such applications. First, challenges in the synthesis of graphene will be described and we will introduce our latest advances in optimizing the graphene quality through novel synthesis and transfer processes. We will then show the tunability of graphene’s carrier concentration by doping and the impact this process has on carrier transport. The demonstrated ability to control graphene’s fundamental properties makes it ideally suited for the integration in heterojunctions and we will present several examples of graphene enabled optoelectronic heterojunction devices.
    First, we produce graphene-based ultrathin solid-state dye-sensitized solar cells with thicknesses between 5 nm and 100 nm, which represents some of the thinnest solar cells produced to date and allows us to extract the scaling limits of solar cells.
    Furthermore, light emission in graphene/dielectric/p-GaN heterojunctions is discussed. Efficient and stable light emission was achieved through carrier tunneling from the graphene injector into prominent states of a luminescent material. Graphene’s unique properties enable fine control of the band alignment in the heterojunction and vertical tunneling-injection light-emitting transistors (VtiLET) were produced where electrostatic gating allows adjustment of the light emission process. This advance enables arbitrary color light emission from one single emitter.

    12.Tan, W.-C., Chen, Y.-C., Liou, Y.-R., Hu, H.-W., Hofmann, M. and Chen, Y.-F. (2016), An Arbitrary Color Light Emitter. Adv. Mater.. doi:10.1002/adma.201604076
    13.Tan WC, Chiang CW, Hofmann M, Chen YF. Tunneling-injection in vertical quasi-2D heterojunctions enabled efficient and adjustable optoelectronic conversion. Scientific Reports. 2016;6.
    14.Hsieh YP, Hong BJ, Ting CC, Hofmann M. Ultrathin graphene-based solar cells. Rsc Advances. 2015;5(121):99627-31.
    15.Hofmann M, Hsieh YP, Chang KW, Tsai HG, Chen TT. Dopant morphology as the factor limiting graphene conductivity. Scientific Reports. 2015;5.
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