A Semiconducting Layered Metal-Organic Framework Magnet

Semiconductors that display spontaneous magnetization are attractive for spintronic applications. Historically, the research of ferromagnetic semiconductor materials has been primarily focused on inorganic dilute magnetic semiconductors (DMSs) and layered inorganic two-dimensional (2D) materials. As major drawbacks, their low magnetic ordering temperature as well as poor chemical tunability greatly hindered their practical application. As a new kind of promising alternatives, metal-organic frameworks (MOFs) have been reported to show promise in tailoring the magnetic ordering temperature and carrier motilities respectively through rational design and synthesis of MOFs materials. However, the simultaneous realization of room-temperature magnetic ordering and semiconducting behavior in a MOF has not been experimentally demonstrated to date. 

To address this situation, a group of scientists from the Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry at TU Dresden, together with researchers from HZDR/MPIP/IKTS/IMDEA/SJTU/Leipzig have developed a semiconducting layered metal-organic framework (MOF, K3Fe2[PcFe-O8]) with spontaneous magnetization. This layered MOF features in-plane full π-d conjugation and exhibits semiconducting behavior with a room temperature carrier mobility of ~15 cm2/Vs as determined by time-resolved Terahertz spectroscopy. Magnetization experiments and 57Fe Mössbauer spectroscopy demonstrate the presence of long-range magnetic correlations in obtained K3Fe2[PcFe-O8] arising from the magnetic coupling between iron centers via delocalized π electrons. The sample exhibits superparamagnetic features due to a distribution of crystal size and possesses magnetic hysteresis up to 350 K. This work sets the stage for the development of spintronic materials exploiting magnetic MOF semiconductors.

Figure (a) Schematic illustration for the synthesis of K3Fe2[PcFe-O8] framework with iron ions and organic PcFe-OH8 linkers connected by coordination bonds (light cyan: C; blue: N; light pink: O; orange: Fe3+ in the phthalocyanine ring; green: Fe2+ in the linkage; H atoms and K+ counter-ions omitted for clarity). (b) Real (black dots) and imaginary (red dots) components of the frequency-resolved complex conductivity; solid lines represent a Drude-Smith description of the data. (c) Magnetic hysteresis loops obtained at different temperatures. 

This work was financially supported by ERC Grant on 2DMATER, EU Graphene Flagship, Coordination Networks: Building Blocks for Functional Systems (SPP 1928, COORNET) as well as the German Science Council, Center of Advancing Electronics Dresden, EXC1056, (cfaed) and OR 349/1 and the Max Planck Society. E.C. acknowledges financial support from the Max Planck Graduate Center and the regional government of Comunidad de Madrid under project (2017-T1/AMB-5207). We also acknowledge Dresden Center for Nanoanalysis (DCN) at TUD, Dr. Petr Formanek and Dr. Konrad Schneider (Leibniz Institute for Polymer Research, IPF, Dresden), Shanghai Synchrotron Radiation Facility (SSRF, China) for the use of facilities. We appreciate Prof. Bernd Büchner, Dr. Vladislav Kataev, Dr. Yulia Krupskaya (IFW Dresden) for the helpful discussion. Prof. T. Heine and Dr. P. Petkov aknowledge the Centre for Information Services and HighPerformance Computing (ZIH) in Dresden, Germany for the provided computational resources. 

Reference: 
Chongqing Yang, Renhao Dong*, Mao Wang, Petko St. Petkov, Zhitao Zhang, Mingchao Wang, Peng Han, Marco Ballabio, Sascha A. Bräuninger, Zhongquan Liao, Jichao Zhang, Friedrich Schwotzer, Ehrenfried Zschech, Hans-Henning Klauss, Enrique Cánovas, Stefan Kaskel, Mischa Bonn, Shengqiang Zhou, Thomas Heine, Xinliang Feng*. Nature Communications 2019, DOI: 10.1038/s41467-019-11267.