Magnetism and Piezoelectricity in Stable Transition Metal Silicate Monolayers

Two-dimensional van der Waals (2D vdW) materials that display ferromagnetism and piezoelectricity have received increased attention. Despite numerous 2D materials have so far been reported as ferromagnetic, developing an air stable and transferable vdW material that is multiferroic has been challenging. To address this problem, we report our work on layered transition metal silicates that are derivatives of kaolinites and lizardites with transition metal substituting on Al$^{3+}$ and Mg$^{2+}$ sites using ab-initio calculations. Using Density Functional Theory (DFT), we show that these compounds are stable under varying O$_2$ partial pressure and can be synthesized using a surface assisted method. We show that these materials have finite out-of-plane piezoelectric response thanks to the lack of inversion symmetry and also they can be tailored to be ferrimagnetic with a non-zero net moment

Ordered Au Nanoparticle Array on Au(111) through Coverage Control of Precursor Metal−Organic Chains

Metal–organic overlayer structures formed by 1,4-phenylene-diisocyanide (PDI) and Au adatoms on Au(111) in UHV, their stability in air, and the tip-induced Au nanoparticle formation on PDI–Au(111) surfaces in air were investigated using scanning tunneling microscopy (STM) and vibrational spectroscopy. This study reveals that the distribution of Au nanoparticles created during tip-induced release of Au atoms from molecule-Au adatom complexes shows strong dependence on the PDI coverage. Ordered Au nanoparticle arrays form in the medium-coverage regime, while more disordered distributions are observed at low and saturation coverages. The different distributions of Au nanoparticles are a direct consequence of the coverage-dependent assembly of (PDI–Au)n chains, their different stability in air, and a templating effect of the Au(111) surface, which is most pronounced for medium coverage, where phases of densely packed (PDI–Au)n chains and disordered PDI–Au assemblies are confined, respectively, to the fcc and hcp regions of the (22 × √3) surface reconstruction of Au(111). The Au nanoparticles nucleate preferentially in the disordered or defective regions of the PDI–Au precursor overlayer, and their formation requires ambient air and high negative tip-bias, suggesting an electrochemical initiation of Au release from the molecule–Au adatom complexes.Fil: Ghalgaoui, Ahmed. University Of Graz. Institute Of Physics; AustriaFil: Doudin, Nassar. University Of Graz. Institute Of Physics; AustriaFil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina. Fritz-Haber-Institut der Max-Planck-Gesellschaft; AlemaniaFil: Surnev, Svetlozar. University Of Graz. Institute Of Physics; AustriaFil: Sterrer, Martin. University Of Graz. Institute Of Physics; Austri

Molybdenum Trioxide on Anatase TiO2(101) - Formation of Monodispersed (MoO3)1 Monomers from Oligomeric (MoO3)n Clusters

Complex oxide systems with hierarchical order are of critical importance in material science and catalysis. Despite their immense potential, their design and synthesis are rather difficult. In this study we demonstrate how the deposition of small oligomeric (MoO3)1-6 clusters, which can be formed by the sublimation of MoO3 powders, leads to the formation of locally ordered layers of (MoO3)1 monomers on anatase TiO2(101). Using both high-resolution imaging and theoretical calculations, we show that at room temperature, such oligomers undergo spontaneous dissociation to their monomeric units. In initial stages of the deposition, this is reflected by the observation of one to six neighboring (MoO3)1 monomers that parallel the size distribution of the oligomers. A transient mobility of such oligomers on both bare TiO2(101) and (MoO3)1 covered areas is key to the formation of a complete layer with a saturation coverage of one (MoO3)1 per two undercoordinated surface Ti sites. We further show that such layers are stable to 500 K, making them highly suitable for a broad range of applications. </p

Molecular Permeation in Freestanding Bilayer Silica

Naberezhnyi D, Mai L, Doudin N, et al. Molecular Permeation in Freestanding Bilayer Silica. Nano Letters . 2022.Graphene and other single-layer structures are pursued as high-flux separation membranes, although imparting porosity endangers their crystalline integrity. In contrast, bilayer silica composed of corner-sharing (SiO4) units is foreseen to be permeable for small molecules due to its intrinsic lattice openings. This study sheds light on the mass transport properties of freestanding 2D SiO2 upon using atomic layer deposition (ALD) to grow large-area films on Au/mica substrates followed by transfer onto Si3N4 windows. Permeation experiments with gaseous and vaporous substances reveal the suspended material to be porous, but the membrane selectivity appears to diverge from the size exclusion principle. Whereas the passage of inert gas molecules is hindered with a permeance below 10-7 mol·s-1·m-2·Pa-1, condensable species like water are found to cross vitreous bilayer silica a thousand times faster in accordance with their superficial affinity. This work paves the way for bilayer oxides to be addressed as inherent 2D membranes

Twisting and Rippling of a Two-Dimensional Kagome Lattice: Silica on Au(111)

Doudin N, Saritas K, Li M, et al. Twisting and Rippling of a Two-Dimensional Kagome Lattice: Silica on Au(111). ACS Materials Letters . 2022.The intrinsic properties of two-dimensional (2D) SiO2 were revealed by forming the material on inert Au(111). Growth by SiO deposition enabled formation of a crystalline phase consisting of two linked sheets of six-membered rings of tetrahedral [SiO4] building units. The bases of the tetrahedra form an isostatic 2D kagome lattice. The weak interaction with Au allowed a new corrugation of the 2D material to be detected: ripples with a similar to 4 nm periodicity that help stabilize the 2D crystalline layer. On the atomic scale, substantial distortions from ideal hexagonal rings were observed even though crystalline defects were extremely rare. The ring distortions were reproduced in theoretical models that showed that they were associated with a twisting of the 2D kagome lattice, which stabilizes the material when subjected to disturbances. The twisting and rippling impart 2D silica with the flexibility to adapt to strain and changes in the crystallographic direction without introducing defects

Understanding Heterolytic H2 Cleavage and Water-Assisted Hydrogen Spillover on Fe3O4(001)-Supported Single Palladium Atoms

The high specific activity and cost-effectiveness of single-atom catalysts (SACs) hold great promise for numerous catalytic chemistries. In hydrogenation reactions, the mechanisms of critical steps such as hydrogen activation and spillover are far from understood. Here, we employ a combination of scanning tunneling microscopy and density functional theory to demonstrate that on a model SAC comprised of single Pd atoms on Fe3O4(001), H2 dissociates heterolytically between Pd and surface oxygen. The efficient hydrogen spillover allows for continuous hydrogenation to high coverages, which ultimately leads to the lifting of Fe3O4 reconstruction and Pd reduction and destabilization. Water plays an important role in reducing the proton diffusion barrier, thereby facilitating the redistribution of hydroxyls away from Pd. Our study demonstrates a distinct H2 activation mechanism on single Pd atoms and corroborates the importance of charge transport on reducible support away from the active site

Scalable production of single 2D van der Waals layers through atomic layer deposition: bilayer silica on metal foils and films

Hutchings GS, Shen X, Zhou C, et al. Scalable production of single 2D van der Waals layers through atomic layer deposition: bilayer silica on metal foils and films. 2D Materials. 2022;9(2): 021003.The self-limiting nature of atomic layer deposition (ALD) makes it an appealing option for growing single layers of two-dimensional van der Waals (2D-VDW) materials. In this paper it is demonstrated that a single layer of a 2D-VDW form of SiO2 can be grown by ALD on Au and Pd polycrystalline foils and epitaxial films. The silica was deposited by two cycles of bis(diethylamino) silane and oxygen plasma exposure at 525 K. Initial deposition produced a three-dimensionally disordered silica layer; however, subsequent annealing above 950 K drove a structural rearrangement resulting in 2D-VDW. The annealing could be performed at ambient pressure. Surface spectra recorded after annealing indicated that the two ALD cycles yielded close to the silica coverage obtained for 2D-VDW silica prepared by precision SiO deposition in ultra-high vacuum (UHV). Analysis of ALD-grown 2D-VDW silica on a Pd(111) film revealed the co-existence of amorphous and incommensurate crystalline 2D phases. In contrast, ALD growth on Au(111) films produced predominantly the amorphous phase while SiO deposition in UHV led to only the crystalline phase, suggesting that the choice of Si source can enable phase control