Staff directory Marc Botifoll Moral

Marc Botifoll Moral

Doctoral Student
2022 FI_B2 00062
Advanced Electron Nanoscopy



  • Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

    Martí-Sánchez S., Botifoll M., Oksenberg E., Koch C., Borja C., Spadaro M.C., Di Giulio V., Ramasse Q., García de Abajo F.J., Joselevich E., Arbiol J. Nature Communications; 13 (1, 4089) 2022. 10.1038/s41467-022-31778-3.

    Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks. © 2022, The Author(s).


  • A singlet-triplet hole spin qubit in planar Ge

    Jirovec D., Hofmann A., Ballabio A., Mutter P.M., Tavani G., Botifoll M., Crippa A., Kukucka J., Sagi O., Martins F., Saez-Mollejo J., Prieto I., Borovkov M., Arbiol J., Chrastina D., Isella G., Katsaros G. Nature Materials; 20 (8): 1106 - 1112. 2021. 10.1038/s41563-021-01022-2. IF: 43.841

    Spin qubits are considered to be among the most promising candidates for building a quantum processor. Group IV hole spin qubits are particularly interesting owing to their ease of operation and compatibility with Si technology. In addition, Ge offers the option for monolithic superconductor–semiconductor integration. Here, we demonstrate a hole spin qubit operating at fields below 10 mT, the critical field of Al, by exploiting the large out-of-plane hole g-factors in planar Ge and by encoding the qubit into the singlet-triplet states of a double quantum dot. We observe electrically controlled g-factor difference-driven and exchange-driven rotations with tunable frequencies exceeding 100 MHz and dephasing times of 1 μs, which we extend beyond 150 μs using echo techniques. These results demonstrate that Ge hole singlet-triplet qubits are competing with state-of-the-art GaAs and Si singlet-triplet qubits. In addition, their rotation frequencies and coherence are comparable with those of Ge single spin qubits, but singlet-triplet qubits can be operated at much lower fields, emphasizing their potential for on-chip integration with superconducting technologies. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.

  • Enhancement of proximity-induced superconductivity in a planar Ge hole gas

    Aggarwal K., Hofmann A., Jirovec D., Prieto I., Sammak A., Botifoll M., Martí-Sánchez S., Veldhorst M., Arbiol J., Scappucci G., Danon J., Katsaros G. Physical Review Research; 3 (2, L022005) 2021. 10.1103/PhysRevResearch.3.L022005. IF: 0.000

    Hole gases in planar germanium can have high mobilities in combination with strong spin-orbit interaction and electrically tunable g factors, and are therefore emerging as a promising platform for creating hybrid superconductor-semiconductor devices. A key challenge towards hybrid Ge-based quantum technologies is the design of high-quality interfaces and superconducting contacts that are robust against magnetic fields. In this work, by combining the assets of aluminum, which provides good contact to the Ge, and niobium, which has a significant superconducting gap, we demonstrate highly transparent low-disordered JoFETs with relatively large ICRN products that are capable of withstanding high magnetic fields. We furthermore demonstrate the ability of phase-biasing individual JoFETs, opening up an avenue to explore topological superconductivity in planar Ge. The persistence of superconductivity in the reported hybrid devices beyond 1.8 T paves the way towards integrating spin qubits and proximity-induced superconductivity on the same chip. © 2021 authors. Published by the American Physical Society.

  • NbSe2 Meets C2N: A 2D-2D Heterostructure Catalysts as Multifunctional Polysulfide Mediator in Ultra-Long-Life Lithium–Sulfur Batteries

    Yang D., Liang Z., Zhang C., Biendicho J.J., Botifoll M., Spadaro M.C., Chen Q., Li M., Ramon A., Moghaddam A.O., Llorca J., Wang J., Morante J.R., Arbiol J., Chou S.-L., Cabot A. Advanced Energy Materials; 11 (36, 2101250) 2021. 10.1002/aenm.202101250. IF: 29.368

    The shuttle effect and sluggish conversion kinetics of lithium polysulfides (LiPS) hamper the practical application of lithium–sulfur batteries (LSBs). Toward overcoming these limitations, herein an in situ grown C2N@NbSe2 heterostructure is presented with remarkable specific surface area, as a Li–S catalyst and LiPS absorber. Density functional theory (DFT) calculations and experimental results comprehensively demonstrate that C2N@NbSe2 is characterized by a suitable electronic structure and charge rearrangement that strongly accelerates the LiPS electrocatalytic conversion. In addition, heterostructured C2N@NbSe2 strongly interacts with LiPS species, confining them at the cathode. As a result, LSBs cathodes based on C2N@NbSe2/S exhibit a high initial capacity of 1545 mAh g−1 at 0.1 C. Even more excitingly, C2N@NbSe2/S cathodes are characterized by impressive cycling stability with only 0.012% capacity decay per cycle after 2000 cycles at 3 C. Even at a sulfur loading of 5.6 mg cm−2, a high areal capacity of 5.65 mAh cm−2 is delivered. These results demonstrate that C2N@NbSe2 heterostructures can act as multifunctional polysulfide mediators to chemically adsorb LiPS, accelerate Li-ion diffusion, chemically catalyze LiPS conversion, and lower the energy barrier for Li2S precipitation/decomposition, realizing the “adsorption-diffusion-conversion” of polysulfides. © 2021 Wiley-VCH GmbH


  • Ballistic InSb Nanowires and Networks via Metal-Sown Selective Area Growth

    Aseev P., Wang G., Binci L., Singh A., Martí-Sánchez S., Botifoll M., Stek L.J., Bordin A., Watson J.D., Boekhout F., Abel D., Gamble J., Van Hoogdalem K., Arbiol J., Kouwenhoven L.P., De Lange G., Caroff P. Nano Letters; 19 (12): 9102 - 9111. 2019. 10.1021/acs.nanolett.9b04265. IF: 12.279

    Selective area growth is a promising technique to realize semiconductor-superconductor hybrid nanowire networks, potentially hosting topologically protected Majorana-based qubits. In some cases, however, such as the molecular beam epitaxy of InSb on InP or GaAs substrates, nucleation and selective growth conditions do not necessarily overlap. To overcome this challenge, we propose a metal-sown selective area growth (MS SAG) technique, which allows decoupling selective deposition and nucleation growth conditions by temporarily isolating these stages. It consists of three steps: (i) selective deposition of In droplets only inside the mask openings at relatively high temperatures favoring selectivity, (ii) nucleation of InSb under Sb flux from In droplets, which act as a reservoir of group III adatoms, done at relatively low temperatures, favoring nucleation of InSb, and (iii) homoepitaxy of InSb on top of the formed nucleation layer under a simultaneous supply of In and Sb fluxes at conditions favoring selectivity and high crystal quality. We demonstrate that complex InSb nanowire networks of high crystal and electrical quality can be achieved this way. We extract mobility values of 10※000-25※000 cm2 V-1 s-1 consistently from field-effect and Hall mobility measurements across single nanowire segments as well as wires with junctions. Moreover, we demonstrate ballistic transport in a 440 nm long channel in a single nanowire under a magnetic field below 1 T. We also extract a phase-coherent length of ∼8 μm at 50 mK in mesoscopic rings. © 2019 American Chemical Society.