Modelling strategies for the covalent functionalization of 2D phosphorene

2D Atomic-Molecular Heterojunctions toward Brainoid Applications

Brainoid computing using 2D atomic crystals and their heterostructures, by emulating the human brain's remarkable efficiency and minimal energy consumption in information processing, poses a formidable solution to the energy-efficiency and processing speed constraints inherent in the von Neumann architecture. However, conventional 2D material based heterostructures employed in brainoid devices are beset with limitations, performance uniformity, fabrication intricacies, and weak interfacial adhesion, which restrain their broader application. The introduction of novel 2D atomic-molecular heterojunctions (2DAMH), achieved through covalent functionalization of 2D materials with functional molecules, ushers in a new era for brain-like devices by providing both stability and tunability of functionalities. This review chiefly delves into the electronic attributes of 2DAMH derived from the synergy of polymer materials with 2D materials, emphasizing the most recent advancements in their utilization within memristive devices, particularly their potential in replicating the functionality of biological synapses. Despite ongoing challenges pertaining to precision in modification, scalability in production, and the refinement of underlying theories, the proliferation of innovative research is actively pursuing solutions. These endeavors illuminate the vast potential for incorporating 2DAMH within brain-inspired intelligent systems, highlighting the prospect of achieving a more efficient and energy-conserving computing paradigm.

Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Halogen-Bonding-Mediated Radical Reactions: The Unexpected Behavior of Piperazine-Based Dithiooxamide Ligands in the Presence of Diiodine

N,N'-Dialkylpiperazine-2,3-dithiones (R2pipdt) were recognized as a class of hexa-atomic cyclic dithiooxamide ligands with peculiar charge-transfer donor properties toward soft electron-acceptors such as noble metal cations and diiodine. The latter interaction is nowadays better described as halogen bonding. In the reaction with diiodine, R2pipdt unexpectedly provides the corresponding triiodide salts, differently from the other dithiooxamides, which instead typically achieve ligand·nI2 halogen-bonded adducts. In this paper, we report a combined experimental and theoretical study that allows elucidation of the nature of the cited products and the reasons behind the unpredictable behavior of these ligands. Specifically, low-temperature single-crystal X-ray diffraction measurements on a series of synthetically obtained R2pipdt (R = Me, iPr, Bz)/I3 salts, complemented by neutron diffraction experiments, were able to experimentally highlight the formation of [R2pipdtH]+ cations with a -S-H bond on the dithionic moiety. Differently, with R = Ph, a benzothiazolylium cation, resulting from an intramolecular condensation reaction of the ligand, is obtained. Based on density functional theory (DFT) calculations, a reasonable reaction mechanism where diiodine plays the fundamental role of promoting a halogen-bonding-mediated radical reaction has been proposed. In addition, the comparison of combined experimental and computational results with the corresponding reactions of N,N'-dialkylperhydrodiazepine-2,3-dithione (R2dazdt, a hepta-atomic cyclic dithiooxamide), which provide neutral halogen-bonded adducts, pointed out that the difference in the torsion angle of the free ligands represents the structural key factor in determining the different reactivities of the two systems.

Functionalization of antimonene and bismuthene with Lewis acids

Elemental 2D pnictogens (group 15) are an interesting class of materials with tunable band structures and high carrier mobilities. Heavier pnictogens (Sb and Bi) are stable under ambient conditions compared to lighter members (P and As) and are emerging as interesting candidates for various electronic and optoelectronic applications. The reactivity of these materials is due to the presence of a lone pair which can be effectively utilized to tune material properties via different functionalization strategies. In this work, we have synthesized antimonene and bismuthene nanosheets by liquid exfoliation which are emissive in the visible range and functionalized these nanosheets with group 12 and 13 Lewis acids (ZnCl₂, CdCl₂, BCl₃, GaCl₃, AlCl₃, and InCl₃). Interaction of these Lewis acids with the lone pairs on Sb/Bi leads to the formation of Lewis acid-base adducts with the corresponding changes in the bonding environment along with lattice distortion and rehybridization of the band structure. Interestingly, the changes in band structure upon functionalization were realized as a blue shift in the emission of few-layered Sb and Bi. This is the first report on the functionalization of heavier pnictogens by the formation of Lewis acid-base adducts and opens a path for tuning their properties for integration in electronic and optoelectronic devices.

Pd Speciation on Black Phosphorene in CO and C2H4 Atmosphere: A First-principles Investigation

Deposited transition metal clusters and nanoparticles are widely used as catalysts and have long been thought stable in reaction conditions. We investigated the electronic structure and stability of freestanding and...

Effect of metal decoration on sulfur-based gas molecules adsorption on phosphorene

Based on first-principles calculation, the adsorption of sulfur-based gas molecules (H2S, SO2, SO3) on various metal-decorated phosphorenes is researched systematically. Eleven metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl) which can avoid the formation of clusters on the phosphorene are considered. Noticeably, all metal decorations can enhance the adsorption strength of phosphorene to sulfur-based gas molecules except for H2S on Tl-decorated phosphorene. Meanwhile, the adsorption energy (Eads) shows the trend of Eads(H2S) < Eads(SO2) < Eads(SO3) for the same metal decoration case. In addition, some metal-decorated phosphorene systems exhibit intriguing magnetic and electrical variation after sulfur-based gas molecule adsorptions, indicating that these systems are promising to be candidates for the detection and removal of sulfur-based gas molecules.

Easy and fast in situ functionalization of exfoliated 2D black phosphorus with gold nanoparticles

Heterostructures of single- and few-layer black phosphorus (2D bP) functionalized with gold nanoparticles (Au NPs) have been recently reported in the literature, exploiting their intriguing properties and biocompatibility for catalytic, therapeutical and diagnostic applications. However, a deeper insight on the structural and electronic properties at the interface of the 2D bP/Au NP heterostructure is still lacking. In this work, 2D bP is functionalized with Au nanoparticles (NPs) through in situ deposition-precipitation heterogeneous reaction. The smallest realized Au NPs have a diameter around 10 nm as revealed by atomic-force and scanning electron microscopy, and are partially positively charged as revealed by X-ray Photoelectron Spectroscopy (XPS). XPS, UV-vis and Raman spectroscopy, supported by density functional theory (DFT) calculations, confirmed that while the structural and electronic properties of 2D bP are overall preserved, a soft-pairing between P atoms at the surface of 2D bP and Au atoms at the surface of Au NPs occurs, leading to a partial charge transfer at the 2D bP/Au interface, with a positive charge being localized on the Au atoms directly bonded to 2D bP. DFT calculations also predicted a band gap lowering, by 0.8 eV, for phosphorene functionalized with a tetranuclear Au cluster. Larger effects are expected as the Au cluster nuclearity (and coverage) increases.

Interlayer Coordination of Pd-Pd Units in Exfoliated Black Phosphorus

The chemical functionalization of 2D exfoliated black phosphorus (2D BP) continues to attract great interest, although a satisfactory structural characterization of the functionalized material has seldom been achieved. Herein, we provide the first complete structural characterization of 2D BP functionalized with rare discrete Pd₂ units, obtained through a mild decomposition of the organometallic dimeric precursor [Pd(η³-C₃H₅)Cl]₂. A multitechnique approach, including HAADF-STEM, solid-state NMR, XPS, and XAS, was used to study in detail the morphology of the palladated nanosheets (Pd₂/BP) and to unravel the coordination of Pd₂ units to phosphorus atoms of 2D BP. In particular, XAS, backed up by DFT modeling, revealed the existence of unprecedented interlayer Pd–Pd units, sandwiched between stacked BP layers. The preliminary application of Pd₂/BP as a catalyst for the hydrogen evolution reaction (HER) in acidic medium highlighted an activity increase due to the presence of Pd₂ units.

Surface Modification of Black Phosphorus with Group 13 Lewis Acids for Ambient Protection and Electronic Tuning

Herein we introduce a facile, solution-phase protocol to modify the Lewis basic surface of few-layer black phosphorus (bP) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft-ness are evaluated. The nature of the interaction between the Lewis acids and the bP lattice is investigated using a range of microscopic (optical, atomic force, scanning electron) and spectroscopic (energy dispersive, X-ray photoelectron) methods. Al and Ga halides are most effective at preventing ambient degradation of bP (>84 h for AlBr₃ ), and the resulting field-effect transistors show excellent IV characteristics, photocurrent, and current stability, and are significantly p-doped. This protocol, chemically matched to bP and compatible with device fabrication, opens a path for deterministic and persistent tuning of the electronic properties in bP.

Interfacial chemistry and electroactivity of black phosphorus decorated with transition metals

Black phosphorus (BP) exhibits a significant chemical reactivity toward transition metals at room temperature, forming metal–BP nanohybrids that have much higher catalytic activity in the hydrogen evolution reaction with respect to the bare BP.

Prospects for Functionalizing Elemental 2D Pnictogens: A Study of Molecular Models

Despite the intense amount of attention and huge potential of 2D-layered pnictogens for applications in chemistry, physics, and materials science, there has yet to be a robust strategy developed to systematically functionalize them to tailor their properties. This is due to a number of factors, including practical instability toward ambient conditions, difficulty in characterizing modified materials, and also more inherent reactivity issues. Here, avenues for functionalization are discussed using examples of molecular models from the wider literature, along with their possible advantages and likely pitfalls. Finally, a critical appraisal of the current field and its future is offered.

Catalysis Mediated by 2D Black Phosphorus Either Pristine or Decorated with Transition Metals Species

Among the novel class of mono-elemental two-dimensional (2D) materials, termed Xenes, phosphorene is emerging as a great promise for its peculiar chemical and physical properties. This review collects a selection of the recent breakthroughs that are related to the application of phosphorene in catalysis and electrocatalysis. Noteworthy, thanks to its intrinsic Lewis basic character, pristine phosphorene turned out to be more efficient and more selective than other non-metal catalysts, in chemical processes as the electroreduction of nitrogen to ammonia or the alkylation of nucleophiles with esters. Once functionalized with transition metals nanoparticles (Co, Ni, Pd, Pt, Ag, Au), its catalytic activity has been evaluated in several processes, mainly hydrogen and oxygen evolution reactions. Under visible light irradiation, it has shown a great improvement of the activity, demonstrating high potential as a photocatalyst.

On the comparison of oxygen and sulfur transfer reactivities in phosphine and phosphorene: the case of R3Sb(X) carriers (X = O or S)

Functionalization is one of the most powerful tools in materials science for the development of new and innovative materials with tailored properties purposefully designed to enhance the overall stability of the system. This is particularly true for exfoliated black phosphorus, which suffers from easy decomposition by air and moisture, hampering its highly desirable applications, especially in electronics. The present work suggests an innovative approach to the functionalization process of this 2D-material based on the selective introduction of chalcogen atoms on the material surface through a reaction with suitable molecular precursors such as stibine chalcogenides (R3Sb(X), X = O or S; R = organyl group). These molecules may readily act as chalcogen-transfer agents and, upon releasing the chalcogen atom atop the bP surface, leave stable stibines (R3Sb) as byproducts, which may be easily removed from the functionalized bP surface. The work provides an overview of all the possible structural, electronic and energy aspects associated with the chalcogen-atom transfer from the stibine to phosphorus based compounds, exemplified by trialkyl phosphines and single layer exfoliated black phosphorus, i.e. phosphorene, Pn. In both cases the oxygen transfer is more exergonic than the sulfur transfer, with the associated free energy barrier for the phosphine process being higher. Although the sulfur transfer for the Pn is found to be endergonic (ca. +3.6 kcal mol-1), the process may surely occur at high temperature. The evolution of the band structure upon the chalcogen transfer has been depicted in detail.

Nitrogen-based gas molecule adsorption of monolayer phosphorene under metal functionalization

Using the first-principles calculation based on density functional theory (DFT), the adsorption properties of nitrogen-based gases molecules (NH3, NO, NO2) on various metal (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl) decorated phosphorene systems have been studied systematically. The results show that all metal decorations can improve the adsorption strength of phosphorene to nitrogen-based gases molecules except for Tl decoration. Especially, the adsorption energy of NH3 molecule on Ni decorated phosphorene is 1.305 eV, and the adsorption energies of NO and NO2 on La decorated phosphorene can be up to 2.475 and 3.734 eV, respectively. In addition, after NO and NO2 adsorptions, the electronic and magnetic properties of some metal decorated phosphorenes change, indicating that the metal decorated phosphorenes have great potential in NO and NO2 detection.

Noncovalent Functionalization of 2D Black Phosphorus with Fluorescent Boronic Derivatives of Pyrene for Probing and Modulating the Interaction with Molecular Oxygen

We studied the chemical-physical nature of interactions involved in the formation of adducts of two-dimensional black phosphorus (2D BP) with organoboron derivatives of a conjugated fluorescent molecule (pyrene). Time-resolved fluorescence spectroscopy showed a stabilization effect of 2D BP on all derivatives, in particular for the adducts endowed with the boronic functionalities. Also, a stronger modulation of the fluorescence decay with oxygen was registered for one of the adducts compared to the corresponding organoboron derivative alone. Nuclear magnetic resonance experiments in suspension and density functional theory simulations confirmed that only noncovalent interactions were involved in the formation of the adducts. The energetic gain in their formation arises from the interaction of P atoms with both C atoms of the pyrene core and the B atom of the boronic functionalities, with a stronger contribution from the ester with respect to the acid one. The interaction results in the lowering of the band gap of 2D BP by around 0.10 eV. Furthermore, we demonstrated through Raman spectroscopy an increased stability toward oxidation in air of 2D BP in the adducts in the solid state (more than 6 months). The modification of the electronic structure at the interface between 2D BP and a conjugated organic molecule through noncovalent stabilizing interactions mediated by the B atom is particularly appealing in view of creating heterojunctions for optoelectronic, photonic, and chemical sensing applications.

A Perspective on Recent Advances in Phosphorene Functionalization and Its Applications in Devices

Phosphorene, the 2D material derived from black phosphorus, has recently attracted a lot of interest for its properties, suitable for applications in materials science. The physical features and the prominent chemical reactivity on its surface render this nanolayered substrate particularly promising for electrical and optoelectronic applications. In addition, being a new potential ligand for metals, it opens the way for a new role of the inorganic chemistry in the 2D world, with special reference to the field of catalysis. The aim of this review is to summarize the state of the art in this subject and to present our most recent results in the preparation, functionalization, and use of phosphorene and its decorated derivatives. We discuss several key points, which are currently under investigation: the synthesis, the characterization by theoretical calculations, the high pressure behavior of black phosphorus, as well as its decoration with nanoparticles and encapsulation in polymers. Finally, device fabrication and electrical transport measurements are overviewed on the basis of recent literature and the new results collected in our laboratories.