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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - PHOSFUN (Phosphorene functionalization: a new platform for advanced multifunctional materials)

Teaser

Summary

In the last 2 centuries, chemists were great builders of 0- and 1-D architectures, molecules first and polymers then. Nowadays they started to obtain a lot of control over 3-D objects, even in case of exceptional topological complexity, like metal-organic frameworks or some supramolecular architectures built in the laboratory. The last frontier is represented by 2D materials. These have attracted a great interest due to their exceptional electronic and optical properties, with a plethora of potential applications from micro- to optoelectronics, from innovative medicine to energy harvesting and production. Until a few years ago, this field was monopolized by graphene, although its use in electronics was hampered by the lack of a bandgap in its electronic structure. Such a drawback was eventually overcome by a new surprising and unexpected 2D-material, i.e. phosphorene. This all-phosphorus counterpart of graphene is just the monolayer of exfoliated black phosphorus and features a direct bandgap that can be largely tuned depending on the number of stacked layers.
PHOSFUN was imagined as a voyage in the reactivity and functionalization of this new 2D-material with small molecules and metal fragments up to the final assemblage and implementation of devices. The project requested the combination of different expertise asking to chemists, material scientists and physicists to work together. The overall objective of PHOSFUN was the demonstration and build-up of a chain-of-value based on phosphorene as a useful platform for a wide range of applications.

Work performed

The leitmotif was the investigation of phosphorene imagined as a 2D-extended atomic platform for chemical and physical functionalizion in view of its use for devices. The starting point was the synthesis of high quality crystalline black phosphorus (bP). These crystals provided few-layers bP after exfoliation, both in solution or by micromechanical methods. An efficient liquid exfoliation method by bP sonication in dimethylsulfoxide was estabished.
We contributed to a deeper knowledge of bP either as the bulk material or as its exfoliated product: (i) a new phase in the high-pressure phase diagram of bP was described; (ii) exfoliated bP could be decorated by nanoparticles of different metals with significant applications in catalysis and higher stability. The performance of the material in the catalytic hydrogenation of paradigmatic refractory substrates (i.e. the reduction of chloronitroarenes to chloroanilines) was proved to be quite effective and comparable with other state-of-the-art Pd heterogenised catalysts. In particular, once decorated with Pd nanoparticles, we were able to get a confirmatory evidence about the existence of genuine covalent P-Pd bonding interactions.
The phosphorene functionalization with a variety of metal fragments was carefully modelled in silico and evaluated in comparison with the highly reactive P4 allotrope. Physical measurements about weak-localization in bP flakes showed a behavior similar to that previously observed in quasi-one-dimensional systems (nanotubes or metallic nanowires). This was attributed to the strong crystalline anisotropy of bP. By scanning tunnelling microscopy (STM), we were able to directly observe the thermal desorption under ultra-high vacuum conditions and to determine the anisotropic crater alignment with respect to the crystallographic axes of the bP flake, as along the zigzag direction, solving a literature controversy.
The embedding of exfoliated bP in a polystyrene matrix by a solvent blending or direct exfoliation of bP in a polymeric solution, followed by in situ radical polymerization in the liquid monomer (methyl methacrylate, MMA), conferred an improved air and moisture stability to the new assembled hybrid materials. From the resultant heterostructures, the preparation of bP-based devices was anticipated, and effective devices were fabricated from PMMA/bP hybrid materials without the need of a protective atmosphere. Transport measurements demonstrated the electronic-grade quality of the resulting bP flakes.
During the project, 20 papers were published in international journals, and other will be submitted in due time. The results were regularly presented in both national and international scientific meetings, workshops and congresses. Three project workshops were organized. The first was held in Florence at the beginning of the project, while the second was organized in Rome at the headquarters of the Italian National Research Council in 2017. The last one took place as Symposium T â€œ2D Semiconductors: Applications and Perspectivesâ€ of the E-MRS, European Materials Research Society Spring Meeting held in Nice, France in May 2019. The first meeting was intended as a sort of â€œcall to armsâ€ for the Italian community active in the area of 2D-materials, paving the way to the birth of several sounds collaborations between chemists and physicists. This idea was later consolidated in the second â€œPhosphorene dayâ€ in Rome and finally opened to the international community of 2D-materials in the closing symposium, which was purposefully organized within the framework of the E-MRS convention just before the closing of the project.

Final results

Several important results were obtained. In particular:
- A new protocol for the liquid exfoliation of bP, resulting in the formation of few-layer phosphorene flakes, was established;
- A new series of composite materials, obtained either by dispersion of phosphorene in a polymer matrix or following the direct exfoliation of bP, were prepared;
- Once incorporated into the polymer an enhanced air stability of the exfoliated material was observed. This finding helped to build a phosphorene-based device without the need of a protective environment.
- The anisotropic crystalline structure of bP was correlated with weak localization effects measured in a few-layer bP field-effect transistor device.
- Non-conventional wet processing and deposition techniques, useful for the realization of electronic bP devices, were identified and tested.
- A new high pressure layered structure of elemental phosphorus was intercepted, significantly rising the high pressure limit for the layered structure of Phosphorus up to 30 GPa, possibly accounting for the anomalous pressure evolution of superconducting Tc in phosphorus.
- The possibility to use phosphorene as a suitable platform to host metal nanoparticles was firstly developed. Remarkably, evidence for a genuine covalent Pd/P interaction in these nanostructured materials was confirmed by the EXAFS methods.
- Once decorated with metal nanoparticles, the nanohybrid derivatives served as efficient catalysts to hydrogenate recalcitrant substrates like chloronitroarenes.
- New heterostructures based on few-layer bP were built up by exploiting weak interactions with organic compounds aiming at engineering active layers in optoelectronic devices. Of notice, (i) the decoration of bP by fluorescent organic semiconductors (i.e. pyrene derivatives) and (ii) the epitaxial formation of an insulating and protective layer of linear alkanes onto few-layer bP.
As a general consideration regarding the PHOSFUN project, we can recall that if chemistry is about control at molecular level, a fine control of the structure of phosphorene and 2D materials in general might open new pathways for the achievement of new devices as sensors, actuators and smart materials.