Academic Appointments


All Publications


  • 'Molecular anvlils' for stercially controlled mechanochemistry under hydrostatic pressure Yan, H., Schreiner, P., Mao, W., Shen, Z., Melosh, N. AMER CHEMICAL SOC. 2018
  • Electrically Conductive Copper Core-Shell Nanowires through Benzenethiol-Directed Assembly. Nano letters Xiao, Q., Burg, J. A., Zhou, Y., Yan, H., Wang, C., Ding, Y., Reed, E., Miller, R. D., Dauskardt, R. H. 2018

    Abstract

    Ultrathin nanowires with <3 nm diameter have long been sought for novel properties that emerge from dimensional constraint as well as for continued size reduction and performance improvement of nanoelectronic devices. Here, we report on a facile and large-scale synthesis of a new class of electrically conductive ultrathin core-shell nanowires using benzenethiols. Core-shell nanowires are atomically precise and have inorganic five-atom copper-sulfur cross-sectional cores encapsulated by organic shells encompassing aromatic substituents with ring planes oriented parallel. The exact nanowire atomic structures were revealed via a two-pronged approach combining computational methods coupled with experimental synthesis and advanced characterizations. Core-shell nanowires were determined to be indirect bandgap materials with a predicted room-temperature resistivity of 120 Omega·m. Nanowire morphology was found to be tunable by changing the interwire interactions imparted by the functional group on the benzenethiol molecular precursors, and the nanowire core diameter was determined by the steric bulkiness of the ligand. These discoveries help define our understanding of the fundamental constituents of atomically well-defined and electrically conductive core-shell nanowires, representing significant advances toward nanowire building blocks for smaller, faster, and more powerful nanoelectronics.

    View details for DOI 10.1021/acs.nanolett.8b01623

    View details for PubMedID 29985626

  • Sterically controlled mechanochemistry under hydrostatic pressure NATURE Yan, H., Yang, F., Pan, D., Lin, Y., Hohman, J., Solis-Ibarra, D., Li, F., Dahl, J. P., Carlson, R. K., Tkachenko, B. A., Fokin, A. A., Schreiner, P. R., Galli, G., Mao, W. L., Shen, Z., Melosh, N. A. 2018; 554 (7693): 505-+

    Abstract

    Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress using ring-opening and reorganization, polymer unzipping and disulfide reduction as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components-a compressible ('soft') mechanophore and incompressible ('hard') ligands. In these 'molecular anvils', isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal-organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.

    View details for DOI 10.1038/nature25765

    View details for Web of Science ID 000425597400042

    View details for PubMedID 29469090

  • Monochromatic Photocathodes from Graphene-Stabilized Diamondoids NANO LETTERS Yan, H., Narasimha, K. T., Denlinger, J., Li, F., Mo, S., Hohman, J., Dahl, J. P., Carlson, R. K., Tkachenko, B. A., Fokin, A. A., Schreiner, P. R., Hussain, Z., Shen, Z., Melosh, N. A. 2018; 18 (2): 1099–1103

    Abstract

    The monochromatic photoemission from diamondoid monolayers provides a new strategy to create electron sources with low energy dispersion and enables compact electron guns with high brightness and low beam emittance for aberration-free imaging, lithography, and accelerators. However, these potential applications are hindered by degradation of diamondoid monolayers under photon irradiation and electron bombardment. Here, we report a graphene-protected diamondoid monolayer photocathode with 4-fold enhancement of stability compared to the bare diamondoid counterpart. The single-layer graphene overcoating preserves the monochromaticity of the photoelectrons, showing 12.5 meV ful width at half-maximum distribution of kinetic energy. Importantly, the graphene coating effectively suppresses desorption of the diamondoid monolayer, enhancing its thermal stability by at least 100 K. Furthermore, by comparing the decay rate at different photon energies, we identify electron bombardment as the principle decay pathway for diamondoids under graphene protection. This provides a generic approach for stabilizing volatile species on photocathode surfaces, which could greatly improve performance of electron emitters.

    View details for DOI 10.1021/acs.nanolett.7b04645

    View details for Web of Science ID 000425559700063

    View details for PubMedID 29286670

  • Hybrid metal-organic chalcogenide nanowires with electrically conductive inorganic core through diamondoid-directed assembly. Nature materials Yan, H., Hohman, J. N., Li, F. H., Jia, C., Solis-Ibarra, D., Wu, B., Dahl, J. E., Carlson, R. M., Tkachenko, B. A., Fokin, A. A., Schreiner, P. R., Vailionis, A., Kim, T. R., Devereaux, T. P., Shen, Z., Melosh, N. A. 2017; 16 (3): 349-355

    Abstract

    Controlling inorganic structure and dimensionality through structure-directing agents is a versatile approach for new materials synthesis that has been used extensively for metal-organic frameworks and coordination polymers. However, the lack of 'solid' inorganic cores requires charge transport through single-atom chains and/or organic groups, limiting their electronic properties. Here, we report that strongly interacting diamondoid structure-directing agents guide the growth of hybrid metal-organic chalcogenide nanowires with solid inorganic cores having three-atom cross-sections, representing the smallest possible nanowires. The strong van der Waals attraction between diamondoids overcomes steric repulsion leading to a cis configuration at the active growth front, enabling face-on addition of precursors for nanowire elongation. These nanowires have band-like electronic properties, low effective carrier masses and three orders-of-magnitude conductivity modulation by hole doping. This discovery highlights a previously unexplored regime of structure-directing agents compared with traditional surfactant, block copolymer or metal-organic framework linkers.

    View details for DOI 10.1038/nmat4823

    View details for PubMedID 28024157

  • Vertical-Substrate MPCVD Epitaxial Nanodiamond Growth. Nano letters Tzeng, Y., Zhang, J. L., Lu, H., Ishiwata, H., Dahl, J., Carlson, R. M., Yan, H., Schreiner, P. R., Vuckovic, J., Shen, Z., Melosh, N., Chu, S. 2017

    Abstract

    Color center-containing nanodiamonds have many applications in quantum technologies and biology. Diamondoids, molecular-sized diamonds have been used as seeds in chemical vapor deposition (CVD) growth. However, optimizing growth conditions to produce high crystal quality nanodiamonds with color centers requires varying growth conditions that often leads to ad-hoc and time-consuming, one-at-a-time testing of reaction conditions. In order to rapidly explore parameter space, we developed a microwave plasma CVD technique using a vertical, rather than horizontally oriented stage-substrate geometry. With this configuration, temperature, plasma density, and atomic hydrogen density vary continuously along the vertical axis of the substrate. This variation allowed rapid identification of growth parameters that yield single crystal diamonds down to 10 nm in size and 75 nm diameter optically active center silicon-vacancy (Si-V) nanoparticles. Furthermore, this method may provide a means of incorporating a wide variety of dopants in nanodiamonds without ion irradiation damage.

    View details for DOI 10.1021/acs.nanolett.6b04543

    View details for PubMedID 28182433

  • Ubiquitous strong electron-phonon coupling at the interface of FeSe/SrTiO3 NATURE COMMUNICATIONS Zhang, C., Liu, Z., Chen, Z., Xie, Y., He, R., Tang, S., He, J., Li, W., Jia, T., Rebec, S. N., Ma, E. Y., Yan, H., Hashimoto, M., Lu, D., Mo, S., Hikita, Y., Moore, R. G., Hwang, H. Y., Lee, D., Shen, Z. 2017; 8

    Abstract

    The observation of replica bands in single-unit-cell FeSe on SrTiO3 (STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

    View details for DOI 10.1038/ncomms14468

    View details for Web of Science ID 000393739700001

    View details for PubMedCentralID PMC5311057

  • . Nature communications Zhang, C., Liu, Z., Chen, Z., Xie, Y., He, R., Tang, S., He, J., Li, W., Jia, T., Rebec, S. N., Ma, E. Y., Yan, H., Hashimoto, M., Lu, D., Mo, S., Hikita, Y., Moore, R. G., Hwang, H. Y., Lee, D., Shen, Z. 2017; 8: 14468-?

    Abstract

    The observation of replica bands in single-unit-cell FeSe on SrTiO3 (STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

    View details for DOI 10.1038/ncomms14468

    View details for PubMedID 28186084

    View details for PubMedCentralID PMC5311057

  • Electronic devices: Nanoparticles make salty circuits. Nature nanotechnology Yan, H., Melosh, N. 2016; 11 (7): 579-580

    View details for DOI 10.1038/nnano.2016.46

    View details for PubMedID 26974955

  • Large-Scale Production of Graphene Nanoribbons from Electrospun Polymers JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, N., Kim, K., Hsu, P., Sokolov, A. N., Yap, F. L., Yuan, H., Xie, Y., Yan, H., Cui, Y., Hwang, H. Y., Bao, Z. 2014; 136 (49): 17284-17291

    Abstract

    Graphene nanoribbons (GNRs) are promising building blocks for high-performance electronics due to their high electron mobility and dimensionality-induced bandgap. Despite many past efforts, direct synthesis of GNRs with controlled dimensions and scalability remains challenging. Here we report the scalable synthesis of GNRs using electrospun polymer nanofiber templates. Palladium-incorporated poly(4-vinylphenol) nanofibers were prepared by electrospinning with controlled diameter and orientation. Highly graphitized GNRs as narrow as 10 nm were then synthesized from these templates by chemical vapor deposition. A transport gap can be observed in 30 nm-wide GNRs, enabling them to function as field-effect transistors at room temperature. Our results represent the first success on the scalable synthesis of highly graphitized GNRs from polymer templates. Furthermore, the generality of this method allows various polymers to be explored, which will lead to understanding of growth mechanism and rational control over crystallinity, feature size and bandgap to enable a new pathway for graphene electronics.

    View details for DOI 10.1021/ja509871n

    View details for Web of Science ID 000346544200045

    View details for PubMedID 25407608

  • Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2. Nature nanotechnology Zhang, Y., Chang, T., Zhou, B., Cui, Y., Yan, H., Liu, Z., Schmitt, F., Lee, J., Moore, R., Chen, Y., Lin, H., Jeng, H., Mo, S., Hussain, Z., Bansil, A., Shen, Z. 2014; 9 (2): 111-115

    Abstract

    Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of ∼180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

    View details for DOI 10.1038/nnano.2013.277

    View details for PubMedID 24362235

  • Covalent attachment of diamondoid phosphonic Acid dichlorides to tungsten oxide surfaces. Langmuir Li, F. H., Fabbri, J. D., Yurchenko, R. I., Mileshkin, A. N., Hohman, J. N., Yan, H., Yuan, H., Tran, I. C., Willey, T. M., Bagge-Hansen, M., Dahl, J. E., Carlson, R. M., Fokin, A. A., Schreiner, P. R., Shen, Z., Melosh, N. A. 2013; 29 (31): 9790-9797

    Abstract

    Diamondoids (nanometer-sized diamond-like hydrocarbons) are a novel class of carbon nanomaterials that exhibit negative electron affinity (NEA) and strong electron-phonon scattering. Surface-bound diamondoid monolayers exhibit monochromatic photoemission, a unique property that makes them ideal electron sources for electron-beam lithography and high-resolution electron microscopy. However, these applications are limited by the stability of the chemical bonding of diamondoids on surfaces. Here we demonstrate the stable covalent attachment of diamantane phosphonic dichloride on tungsten/tungsten oxide surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy revealed that diamondoid-functionalized tungsten oxide films were stable up to 300-350 °C, a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100-200 °C. Extreme ultraviolet (EUV) light stimulated photoemission from these diamondoid phosphonate monolayers exhibited a characteristic monochromatic NEA peak with 0.2 eV full width at half-maximum (fwhm) at room temperature, showing that the unique monochromatization property of diamondoids remained intact after attachment. Our results demonstrate that phosphonic dichloride functionality is a promising approach for forming stable diamondoid monolayers for elevated temperature and high-current applications such as electron emission and coatings in micro/nano electromechanical systems (MEMS/NEMS).

    View details for DOI 10.1021/la401781e

    View details for PubMedID 23855923