First-Principles Insights into the Interface Chemistry between 4-Aminothiophenol and Zinc Phosphide (Zn3P2) Nanoparticles

Parallel Plate Capacitor Model at the Nanoscale for Stable and Gigantic SERS Activity of the 4-MBA@R-AuNP-4-MBA@R-AuNP System

Selective use of ingredients out of a specific natural product (e.g., fruit, leaf, flower, or honey extract) or their mixture (e.g., bacteria, viruses, fungi, plants, etc.) by smart manipulation of precursors and reaction conditions to synthesize nanoparticles can provide us a low-cost, environmentally friendly route for their industrial-scale production. The presence of more than one active ligand (sourced natural product extract) on the surface not only makes them the most stable (electrostatically) and monodispersed (controlled kinetics) but also devoid of any external ligand-assisted aggregation. This empowered us to modify the surface of the nanoparticles in a monolayered fashion or to couple between nanoparticles through a ligand-assisted chemical coupling pathway to avoid their aggregation and hence to keep their nanoscale property intact. A metal-to-ligand charge transfer (MLCT) trajectory combined with electromagnetic field-induced coherent capacitive coupling between two nanoparticles was introduced to explain the gigantic Raman enhancement observed from these nanoparticles. As a model system, we have synthesized the nanoparticles from rose extract as the active ligand ingredient source for 2-phenyl ethanol, linalool, citronellol, nerol, geraniol, pyrogallol (C6H3(OH)3), and quercetin (3,3',4',5,7-pentahydroxyflavone) and the surface of the synthesized nanoparticles has been modified by 4-mercaptobenzoic acid (4-MBA) acting as a Raman tag. The obtained structural and spectroscopic data correlate well between our numerical and density functional theory (DFT)-based calculations to justify their gigantic SERS activity, which may lead us to propose an unexplored coherent capacitive coupling-based Raman enhancement mechanism.

Nanoscale Growth Initiation as a Pathway to Improve the Earth-Abundant Absorber Zinc Phosphide

Growth approaches that limit the interface area between layers to nanoscale regions are emerging as a promising pathway to limit the interface defect formation due to mismatching lattice parameters or thermal expansion coefficient. Interfacial defect mitigation is of great interest in photovoltaics as it opens up more material combinations for use in devices. Herein, an overview of the vapor-liquid-solid and selective area epitaxy growth approaches applied to zinc phosphide (Zn3P2), an earth-abundant absorber material, is presented. First, we show how different morphologies, including nanowires, nanopyramids, and thin films, can be achieved by tuning the growth conditions and growth mechanisms. The growth conditions are also shown to greatly impact the defect structure and composition of the grown material, which can vary considerably from the ideal stoichiometry (Zn3P2). Finally, the functional properties are characterized. The direct band gap could accurately be determined at 1.50 ± 0.1 eV, and through complementary density functional theory calculations, we can identify a range of higher-order band gap transitions observed through valence electron energy loss spectroscopy and cathodoluminescence. Furthermore, we outline the formation of rotated domains inside of the material, which are a potential origin of defect transitions that have been long observed in zinc phosphide but not yet explained. The basic understanding provided reinvigorates the potential use of earth-abundant II-V semiconductors in photovoltaic technology. Moreover, the transferrable nanoscale growth approaches have the potential to be applied to other material systems, as they mitigate the constraints of substrate-material combinations causing interface defects.

Insights from density functional theory calculations into the effects of the adsorption and dissociation of water on the surface properties of zinc diphosphide (ZnP2) nanocrystals

Zinc phosphides (ZnP₂ and Zn₃P₂) are emerging absorber materials for photovoltaic applications owing to their abundancy and non-toxic nature. Herein, we provide a comprehensive characterisation of the surface structure, composition, stabilities, morphology, and electronic properties of both bare and hydrated/hydroxylated low-Miller index surfaces of β-ZnP₂ by means of density functional theory (DFT) calculations. Mechanistic insights into the fundamental aspects of water adsorption and dissociation, including the adsorption geometries, energetics, and structural parameters along the reaction path are systematically characterised. The stabilities of the surfaces under dry and wet conditions are discussed in detail and the predicted phase diagrams for the water adsorption are presented. Using calculated surface energies, we have derived the equilibrium morphology of the β-ZnP₂ nanocrystals under vacuum and upon hydration or hydroxylation. Atomic-level insights into the origin of the incipient oxidation of β-ZnP₂ surfaces are provided through analysis of Bader charges, which reveal that the Zn sites to which H₂O and OH species are bound undergo oxidation due to the transfer of charge to the adsorbed species. Adsorption-induced changes to the electronic properties before and after hydration/hydroxylation were characterised by the work function and partial density of states. The results highlight the need for protection of β-ZnP₂ nanocrystals against possible oxidation in the presence of water through post-synthesis organic functionalisation.

Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (Zn3P2) nanowires

Earth-abundant and low-cost semiconductors, such as zinc phosphide (Zn₃P₂), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of Zn₃P₂, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of Zn₃P₂, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured Zn₃P₂ (space group P4₂/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A_1g, B_1g and E_g Raman modes, while B_2g modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8 A_1g + 9 B_1g + 3 B_2g + 14 E_g) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (<210 cm^-1) which is dominated by Zn-related vibrations, (ii) intermediate region (210-225 cm^-1) which represents a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm^-1) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal Zn₃P₂ phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.

Electronic Structure and Surface Properties of Copper Thiocyanate: A Promising Hole Transport Material for Organic Photovoltaic Cells

Considering the significance of hexagonal copper thiocyanate (β-CuSCN) in several optoelectronic technologies and applications, it is essential to investigate its electronic structure and surface properties. Herein, we have employed density functional theory (DFT) calculations to characterise the band structure, density of states, and the energy-dependent X-ray photoelectron (XPS) valence band spectra at variable excitation energies of β-CuSCN. The surface properties in the absence and presence of dimethyl sulfoxide (DMSO), a solvent additive for improving perovskite solar cells' power conversion efficiency, have also been systematically characterised. β-CuSCN is shown to be an indirect band gap material (Eg = 3.68 eV) with the valence band edge demonstrated to change from being dominated by Cu-3d at soft X-ray ionisation photon energies to Cu-3p at hard X-ray ionisation photon energies. The adsorption energy of dimethyl sulfoxide (DMSO) on the (100) and (110) β-CuSCN surfaces is calculated at -1.12 and -0.91 eV, respectively. The presence of DMSO on the surface is shown to have a stabilisation effect, lowering the surface energy and tuning the work function of the β-CuSCN surfaces, which is desirable for organic solar cells to achieve high power conversion efficiencies.

Unravelling the early oxidation mechanism of zinc phosphide (Zn3P2) surfaces by adsorbed oxygen and water: a first-principles DFT-D3 investigation

Zinc phosphide (Zn₃P₂) is a novel earth-abundant photovoltaic material with a direct band gap of 1.5 eV. Herein, the incipient oxidation mechanism of the (001), (101), and (110) Zn₃P₂ surfaces in the presence of oxygen and water, which severely limits the fabrication of efficient Zn₃P₂-based photovoltaics, has been investigated in detail by means of dispersion-corrected density functional theory (DFT-D3) calculations. The fundamental aspects of the oxygen and water adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are presented and discussed. A chemical picture and origin of the initial steps of Zn₃P₂ surface oxidation are proposed through analyses of Bader charges, partial density of states, and differential charge density isosurface contours. The results presented show that while water interacts weakly with the Zn ions on the Zn₃P₂ surfaces, molecular and dissociative oxygen species interact strongly with the (001), (101), and (110) surface species. The adsorption of oxygen is demonstrated to be characterized by a significant charge transfer from the interacting surface species, causing them to be oxidized from Zn²⁺ to Zn³⁺ formal oxidation states. Preadsorbed oxygen species are shown to facilitate the O-H bond activation of water towards its dissociation, with the adsorbed hydroxide species (OH^-) demonstrated to draw a significant amount of charges from the interacting surface sites. Despite the fact that the semiconducting nature of the different Zn₃P₂ surfaces is preserved, we observe noticeable adsorption induced changes in their electronic structures, with the covered surface exhibiting smaller band gaps than the naked surfaces. The present study demonstrates the importance of the oxygen-water/solid interface to understand the oxidation mechanism of Zn₃P₂ in the presence of oxygen and water at the molecular level. The study also highlights the need for Zn₃P₂ nanoparticles to be protected against possible oxidation in the presence of oxygen and moisture via in situ functionalization, wherein the Zn₃P₂ nanoparticles are exposed to a vapour of organic functional molecules immediately after synthesis.