Self-Assembly of the Nonplanar Fe(III) Phthalocyanine Small-Molecule: Unraveling the Impact on the Magnetic Properties of Organic Nanowires

Abstract

In this article we show for the first time the formation of magnetic supported organic nanowires (ONWs) driven by self-assembly of a nonplanar Fe(III) phthalocyanine chloride (FePcCl) molecule. The ONWs grow by a crystallization mechanism on roughness-tailored substrates. The growth methodology consists of a vapor deposition under low vacuum and mild temperature conditions. The structure, microstructure, and chemical composition of the FePcCl NWs are thoroughly elucidated and compared with those of Fe(II) phthalocyanine NWs by a consistent and complementary combination of advanced electron microscopies and X-ray spectroscopies. In a further step, we vertically align the NWs by conformal deposition of a SiO 2 shell. Such orientation is critical to analyze the magnetic properties of the FePcCl and FePc supported NWs. A ferromagnetic behavior below 30 K with an easy axis perpendicular to the phthalocyanine plane was observed in the two cases with the FePcCl nanowires presenting a wider hysteresis. These results open the path to the fabrication of nanostructured one-dimensional small-molecule spintronic devices. T ailoring material structure at the nanoscale has prompted research on new and exciting properties over the last two decades. This is particularly true for one-dimensional (1D) nanostructures (nanotubes, nanowires, nanorods, or nanobelts) with longitudinal sizes orders of magnitude larger than the cross-sectional sizes.1 Among them, organic nanowires (ONWs) form a family of nanostructures with remarkable optic, electronic, sensing, and wetting properties.2−12 In addition, the development of ONW devices offers potential for low cost and flexible devices and versatility through the molecular design of their building blocks. The methods for the synthesis of these 1D systems generally rely on three approaches including template procedures, solution-phase deposition, and vapor transport. [2][3][4]6,10,13 Condensation from the vapor phase at low pressure (or simply physical vapor deposition, PVD) counts within the last group and presents interesting features such as (i) direct formation of single crystal nanowires with controlled morphology on different substrates; (ii) high adaptability to a large amount of different π-conjugated small molecules; (iii) high growth rate and controlled density of nanowires with appropriated homogeneity; (iv) solventless and one-step synthesis; and (v) straightforward compatibility with other vacuum deposition and processing methods. 2,14−18 The method consists of the low-pressure sublimation of purely organic or metalorganic small-molecules from the bulk crystals counterpart and their condensation on the surface of diverse supports. These molecules self-assemble into supported single-crystal nanowires tens of nanometers in width and several micrometers in length.14,15,18 Such a heterogeneous crystallization process can be enhanced by including nucleation centers at the substrate to trigger the preferential condensation of molecules on those spots.14,15 Formation of ONWs...

Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires Built from Iron Phthalocyanine Chains

“…The difference in coercive fields for systems with identical structure but different textures could therefore also be attributed to a slower relaxation when the field is not applied along the easy plane of magnetization. An increase in coercivity (from 3 to 10 mT at 5 K) was also observed for short FePc standing nanowires when the applied field was rotated from parallel to perpendicular to the substrate, i.e., to a direction with a shorter projection along the molecular plane, which we define as the easy plane …”

“…They were also obtained for CoPc, and recent integration into field effect transistors has highlighted their exceptionally high on–off ratio, competitive mobility, and long lifetimes . FePc nanowires were also recently produced in a high vacuum system with high substrate temperature and argon partial pressure up to 0.02 mbar, but this resulted in shorter lengths, and has not been attributed to the η‐phase …”

“…Iron phthalocyanine (FePc) has already been shown to exhibit ferromagnetism at low temperature with some tunability of the magnetic properties achievable through altering the crystal structure or grain size, and is characterized by an unusually high orbital moment . Ferromagnetic FePc nanowires with crystal structure identical to that of thin films were recently highlighted, with patterned substrate and inorganic shells promoting vertical growth . However, the magnetic anisotropy, as deduced from differences in coercivity for hysteresis curves measured along perpendicular directions, was opposite to literature values obtained from films and more conclusive data are required.…”

“…The next step is to search for ferromagnetic analogues which are more susceptible to small external magnetic fields and for which electron gases in transport channels (e.g., between source and drain in a transistor) could be spin polarized. Iron phthalocyanine (FePc) has already been shown to exhibit ferromagnetism at low temperature with some tunability of the magnetic properties achievable through altering the crystal structure or grain size, and is characterized by an unusually high orbital moment . Ferromagnetic FePc nanowires with crystal structure identical to that of thin films were recently highlighted, with patterned substrate and inorganic shells promoting vertical growth .…”

Plasma Enabled Conformal and Damage Free Encapsulation of Fragile Molecular Matter: from Surface‐Supported to On‐Device Nanostructures

“…On the other hand, the selected molecular structures are very sensitive to damage. Indeed, the outstanding electronic photonic and magnetic properties of the organic nanowires owe to the integrity of their single‐crystalline structure . Similarly, graphene is a 2D π‐conjugated system extraordinarily sensitive to defect generation.…”