Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films

Film thickness and microstructure critically affect the spin crossover transition of a 2D coordination polymer.

Mastering the nanostructuration of molecular materials onto solid surfaces and understanding how this process affects their properties are of utmost importance for their integration into solid-state electronic devices. This is even more important for spin crossover (SCO) systems, in which the spin transition is extremely sensitive to size reduction effects. These bi-stable materials have great potential for the development of nanotechnological applications provided their intrinsic properties can be successfully implemented in nanometric films, amenable to the fabrication of functional nanodevices. Here we report the fabrication of crystalline ultrathin films (<1–43 nm) of two-dimensional Hofmann-type coordination polymers by using an improved layer-by-layer strategy and a close examination of their SCO properties at the nanoscale. X-ray absorption spectroscopy data in combination with extensive atomic force microscopy analysis reveal critical dependence of the SCO transition on the number of layers and the microstructure of the films. This originates from the formation of segregated nanocrystals in early stages of the growth process that coalesce into a continuous film with an increasing number of growth cycles for an overall behaviour reminiscent of the bulk. As a result, the completeness of the high spin/low spin transition is dramatically hindered for films of less than 15 layers revealing serious limitations to the ultimate thickness that might be representative of the performance of the bulk when processing SCO materials as ultrathin films. This unprecedented exploration of the particularities of the growth of SCO thin films at the nanoscale should encourage researchers to put a spotlight on these issues when contemplating their integration into devices.

Correction: Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films

Correction for ‘Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films’ by Víctor Rubio-Giménez et al., Chem. Sci., 2019, DOI: 10.1039/c8sc04935a.

Spontaneous growth of 2D coordination polymers on functionalized ferromagnetic surfaces

2D coordination polymers grow spontaneously on reactive surfaces due to surface oxidation. The growth process is observed in real time.

The spontaneous growth of lamellar metal-alkanethiolates (LMAs) on reactive ferromagnetic surfaces as a result of surface oxidation has been observed. When alkanethiol self-assembled monolayers (SAMs) grown under an inert atmosphere over cobalt or permalloy (Ni₈₀Fe₂₀) are exposed to air, oxygen diffuses through the molecular layer. This induces an oxidation of metal atoms at the metal surface and a release of the resulting metal cations that migrate coordinated by the alkanethiol molecules to form lamellar structures over the SAMs. This process has been imaged in real-time, under ambient conditions, by means of different microscopy techniques. The influence of the alkyl chain length, the nature of the ferromagnet, the temperature and the atmospheric moisture on the number, area and height of the resulting features has been systematically evaluated. Remarkably, the possibility to follow the migration in real-time makes it a promising model system for the study of surface/molecule interface processes. Most importantly, the composition and crystallinity of these LMAs have been studied, evidencing that real 2D coordination polymers are formed on the surface. Hence, one could envision this strategy as a new method for the assembly of more complex low-dimensional (2D) magnetic materials based on coordination polymers.

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4 paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science & Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of Cu^II ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Bottom-Up Fabrication of Semiconductive Metal-Organic Framework Ultrathin Films

Though generally considered insulating, recent progress on the discovery of conductive porous metal-organic frameworks (MOFs) offers new opportunities for their integration as electroactive components in electronic devices. Compared to classical semiconductors, these metal-organic hybrids combine the crystallinity of inorganic materials with easier chemical functionalization and processability. Still, future development depends on the ability to produce high-quality films with fine control over their orientation, crystallinity, homogeneity, and thickness. Here self-assembled monolayer substrate modification and bottom-up techniques are used to produce preferentially oriented, ultrathin, conductive films of Cu-CAT-1. The approach permits to fabricate and study the electrical response of MOF-based devices incorporating the thinnest MOF film reported thus far (10 nm thick).

Surface Functionalization of Metal-Organic Framework Crystals with Catechol Coatings for Enhanced Moisture Tolerance

Robust catechol coatings for enhanced moisture tolerance were produced in one step by direct reaction of Hong Kong University of Science and Technology (HKUST) with synthetic catechols. We ascribe the rapid formation of homogeneous coatings around the metal-organic framework particles to the biomimetic catalytic activity of Cu(II) dimers in the external surface of the crystals. Use of fluorinated catechols results in hydrophobic, permeable coatings that protect HKUST from water degradation while retaining close to 100% of its original sorption capacity.

Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport

Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La2/3Sr1/3MnO3, a ferromagnetic oxide, with self-assembled monolayers for spintronics. La2/3Sr1/3MnO3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La2/3Sr1/3MnO3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics.

High-Quality Metal-Organic Framework Ultrathin Films for Electronically Active Interfaces

Currently available methodologies arguably lack the exquisite control required for producing metal-organic framework (MOF) thin films of sufficient quality for electronic applications. By directing MOF transfer with self-assembled monolayers (SAMs), we achieve very smooth, homogeneous, highly oriented, ultrathin films across millimeter-scale areas that display moderate conductivity likely due to electron hopping. Here, the SAM is key for directing the transfer thereby enlarging the number and nature of the substrates of choice. We have exploited this versatility to evolve from deposition onto standard Si and Au to nonconventional substrates such as ferromagnetic Permalloy. We believe that this strategy might be useful for the integration of MOFs as active interfaces in electronic devices.

Self-assembled monolayers on a ferromagnetic permalloy surface

Self-assembled monolayers (SAMs) are nowadays broadly used as surface protectors or modifiers and play a key role in many technological applications. This has motivated the study of their formation in all kind of materials; however, and despite the current interest in molecular spintronics, the study of SAMs on ferromagnetic surfaces remains almost unexplored. In this paper, we report for the first time a methodology for the formation of SAMs of n-alkylphosphonic acids on permalloy in ambient conditions. The formed monolayers have been fully characterized by means of contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, matrix assisted laser desorption ionization time-of-flight mass spectrometry, infrared reflection absorption spectroscopy, and X-ray reflectometry. Additionally, the magnetic stability of the modified permalloy after the solution process required for the SAM formation has been confirmed by magneto-optical Kerr effect magnetometry. Moreover, by means of microcontact printing lithography, very accurate SAM patterns have been transferred onto permalloy surfaces and used as resist mask in a chemical etching process giving rise to submicrometric permalloy surface patterns with potential interest in nanomagnetism, spintronics, and storage technologies.

Bioinspired catechol-terminated self-assembled monolayers with enhanced adhesion properties

The role of the catechol moiety in the adhesive properties of mussel proteins and related synthetic materials has been extensively studied in the last years but still remains elusive. Here, a simplified model approach is presented based on a self-assembled monolayer (SAM) of upward-facing catechols thiol-bound to epitaxial gold substrates. The orientation of the catechol moieties is confirmed by spectroscopy, which also showed lack of significant amounts of interfering o-quinones. Local force-distance curves on the SAM measured by atomic force microscopy (AFM) shows an average adhesion force of 45 nN, stronger than that of a reference polydopamine coating, along with higher reproducibility and less statistical dispersion. This is attributed to the superior chemical and topographical homogeneity of the SAM coating. Catechol-terminated SAMs are also obtained on high-roughness gold substrates that show the ability to assemble magnetic nanoparticles, despite their lack of enhanced adhesion at the molecular level. Finally, the influence of the catechol group on the formation and quality of the SAM is explored both theoretically (molecular dynamics simulations) and experimentally using direct-write AFM lithography.

Fast redox-triggered shuttling motions in a copper rotaxane based on a phenanthroline–terpyridine conjugate

Fast shuttling motions in solution have been observed for a Cu-complexed [2] rotaxane, whose thread contains a bidentate 1,10-phenanthroline chelating unit directly connected through its 3-position to the 5-position of a tridentate 2,2′:6′,2′′-terpyridine.

Unveiling self-assembled monolayers' potential for molecular spintronics: spin transport at high voltage

Molecular magnetic tunnel junctions using self-assembled monolayers (SAMs) as tunnel barriers show stable and efficient spin transport properties. Large tunnel magnetoresistance with a flat bias voltage dependence of the magnetoresistance is observed in La(2/3) Sr(1/3) MnO(3) /dodecylphosphonic acid SAM/Co nanocontacts. This opens the door to spintronic tailoring though SAM engineering and could also lead to new venues for spin injection in organic devices.

Self-assembled monolayer-functionalized half-metallic manganite for molecular spintronics

(La,Sr)MnO(3) manganite (LSMO) has emerged as the standard ferromagnetic electrode in organic spintronic devices due to its highly spin-polarized character and air stability. Whereas organic semiconductors and polymers have been mainly envisaged to propagate spin information, self-assembled monolayers (SAMs) have been overlooked and should be considered as promising materials for molecular engineering of spintronic devices. Surprisingly, up to now the first key step of SAM grafting protocols over LSMO surface thin films is still missing. We report the grafting of dodecyl (C12P) and octadecyl (C18P) phosphonic acids over the LSMO half-metallic oxide. Alkylphosphonic acids form ordered self-assembled monolayers, with the phosphonic group coordinated to the surface and alkyl chains tilted from the surface vertical by 43° (C12P) and 27° (C18P). We have electrically characterized these SAMs in nanodevices and found that they act as tunnel barriers, opening the door toward the integration of alkylphosphonic acid//LSMO SAMs into future molecular/organic spintronic devices such as spin OLEDs.