Epitaxial Clusters Research Articles

The search for manganese incorporation in MoSe 2 monolayer epitaxially grown on graphene

The introduction of magnetism in two-dimensional (2D) materials represents an intense field of research nowadays and the quest to reach above-room-temperature ordering temperatures is still underway. Intrinsic ferromagnetism was discovered in 2017 in CrI 3 and Cr 2 Ge 2 Te 6 in the monolayer form with low Curie temperatures. An alternative method to introduce magnetism into conventional 2D materials is substitutional doping with magnetic impurities similarly to three-dimensional diluted magnetic semiconductors. The case of Mn-doped transition metal dichalcogenide (MoS 2 , MoSe 2 , WS 2 , WSe 2 ) monolayers is very interesting because combining out-of-plane ferromagnetism and valley contrast leads to ferrovalley materials. In this work, we focus on the incorporation of Mn in MoSe 2 by molecular beam epitaxy on graphene which has been rarely addressed up to now. By using a multiscale characterization approach, we demonstrate that Mn atoms are incorporated into the MoSe 2 monolayer up to 5 atomic percent. However, when incorporated into the film, Mn atoms tend to diffuse to the grain edges forming undefined Mo x Mn y Se z phase at grain boundaries after completion of the MoSe 2 monolayer. This segregation leaves the crystalline and electronic structure of MoSe 2 unmodified. Above 5%, the saturation of Mn content in MoSe 2 leads to the formation of epitaxial MnSe clusters.

X-ray magnetic dichroism and tunnel magneto-resistance study of the magnetic phase in epitaxial CrVO x nanoclusters

Epitaxial clusters of chromium and chromium–vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr2O3-like phase, whereas usual Cr2O3 is known to be anti-ferromagnetic in the bulk. For mixed clusters, vanadium ions also contribute to the total magnetization and they are coupled to the chromium ion spins. By measuring the dichroic signal at different temperatures, we get insight into the possible spin configurations of vanadium and chromium ions: we propose that the magnetic dipoles observed in the clusters assembly could be related to ionic spins that couple at a very short range, as for instance in short one-dimensional spins chains.

Diffusion of two-dimensional epitaxial clusters on metal (100) surfaces: Facile versus nucleation-mediated behavior and their merging for larger sizes

For diffusion of two-dimensional homoepitaxial clusters of $N$ atoms on metal (100) surfaces mediated by edge atom hopping, macroscale continuum theory suggests that the diffusion coefficient scales like ${D}_{N}\ensuremath{\sim}{N}^{\ensuremath{-}\ensuremath{\beta}}$ with $\ensuremath{\beta}=3/2$. However, we find quite different and diverse behavior in multiple size regimes. These include: (i) facile diffusion for small sizes $N<9$; (ii) slow nucleation-mediated diffusion with small $\ensuremath{\beta}<1$ for ``perfect'' sizes $N={N}_{p}={L}^{2}$ or $L(L+1)$, for $L=3,4, ...$ having unique ground-state shapes, for moderate sizes $9\ensuremath{\le}N\ensuremath{\le}\mathrm{O}({10}^{2})$; the same also applies for $N={N}_{p}+3, {N}_{p}+4, ...$ (iii) facile diffusion but with large $\ensuremath{\beta}>2$ for $N={N}_{p}+1$ and ${N}_{p}+2$ also for moderate sizes $9\ensuremath{\le}N\ensuremath{\le}\mathrm{O}({10}^{2})$; (iv) merging of the above distinct branches and subsequent anomalous scaling with $1\ensuremath{\lesssim}\ensuremath{\beta}<3/2$, reflecting the quasifacetted structure of clusters, for larger $N=\mathrm{O}({10}^{2})$ to $N=\mathrm{O}({10}^{3})$; (v) classic scaling with $\ensuremath{\beta}=3/2$ for very large $N=\mathrm{O}({10}^{3})$ and above. The specified size ranges apply for typical model parameters. We focus on the moderate size regime where we show that diffusivity cycles quasiperiodically from the slowest branch for ${N}_{p}+3$ (not ${N}_{p}$) to the fastest branch for ${N}_{p}+1$. Behavior is quantified by kinetic Monte Carlo simulation of an appropriate stochastic lattice-gas model. However, precise analysis must account for a strong enhancement of diffusivity for short time increments due to back correlation in the cluster motion. Further understanding of this enhancement, of anomalous size scaling behavior, and of the merging of various branches, is facilitated by combinatorial analysis of the number of the ground-state and low-lying excited state cluster configurations, and also of kink populations.

Evidence of a permanent electric polarisation in highly strained Cr2O3 clusters measured by a second harmonic generation technique

We investigate the second harmonic generation (SHG) signal in strained Cr2O3 clusters. We show that the SHG signal generated by nanometric Cr2O3 clusters embedded in MgO varies under an applied electric field, at room temperature. The variation of the intensity follows a Langevin law as a function of the electric field, which is consistent with a super-paraelectric clusters assembly. This reveals the presence of a weak spontaneous electric dipole in Cr2O3 when in the shape of highly strained epitaxial clusters, whereas this material does not posses any permanent electric dipole in the bulk phase. These results indicate that the multiferroic state recently observed at low temperature in those clusters, which was associated to a giant magneto-electric effect, might still exist at room temperature: this opens the way to new applications based on chromium oxide strained nanoparticles.

Cleaved thin-film probes for scanning tunneling microscopy

We introduce an alternative type of probe for scanning tunneling microscopy (STM). Instead of using a needle-like tip made from a piece of metallic wire, a sharp-edged cleaved insulating substrate, which is initially covered by a thin conductive film, is used. The sharp tip is formed at the intersection of the two cleaved sides. Using this approach a variety of materials for STM probes can be used, and functionalization of STM probes is possible. The working principle of different probes made of metallic (Pt, Co, and CoB), indium-tin oxide, as well as Cu/Pt and Co/Pt multilayer films are demonstrated by STM imaging of clean Cu(001) and Cu(111) surfaces as well as the epitaxial Co clusters on Cu(111).

Controlling the magnetic anisotropy in epitaxialCr2O3clusters by an electric field

Magnetic properties of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ epitaxial clusters inserted in an Fe/MgO/Fe tunnel barrier are revealed by their tunnel magnetoresistance signature. The cluster assembly has been shown in a previous work to behave as a superparamagnet when a magnetic field was applied in the plane of the tunnel junction. We here demonstrate that an external large out-of plane electric field (in the order of 0.5 GV/m) favors in-plane magnetization orientation. This is due to an electric-field-induced magnetic anisotropy along the normal to the plane, corresponding to large anisotropy fields reaching up to 2 T. The assembly of clusters is thus strictly speaking not superparamagnetic and its magnetization cannot be exactly described by a Langevin law. This is attributed either to a strain-induced enhanced magnetoelectric effect or to a voltage-induced change of the magnetic anisotropy at interfaces with MgO.

RHEED structural study of the novel tin-cerium oxide catalyst

We present our study of Sn interaction with CeO2 (111) thin film supported by a Cu (111) single crystal. After deposition of metallic tin, tin-cerium mixed oxide is formed in the subsurface region by substitution of cerium atoms by tin in the cerium oxide crystal lattice. This process leads to transformation of the original fluorite structure of cerium dioxide to a simple cubic structure. The excessive metallic tin remaining on the surface forms three populations of small epitaxial clusters with tetragonal structure and the (010) crystallographic plane parallel to the surface. Two of these populations are favored over the third one, due to their better accommodation to the substrate lattice. After annealing the sample at 400°C in oxygen atmosphere the metallic Sn clusters disappear and a very thin film of polycrystalline tin dioxide is formed.

Size-induced enhanced magnetoelectric effect and multiferroicity in chromium oxide nanoclusters

The control of the magnetization of a material with an electric field would make the design and the integration of novel electronic devices possible. This explains the renewed interest in multiferroic materials. Progress in this field is currently hampered by the scarcity of the materials available and the smallness of the magnetoelectric effects. Here we present a proof-of-principle experiment showing that engineering large strains through nanoscale size reduction is an efficient route for increasing magnetoelectric coefficients by orders of magnitude. The archetype magnetoelectric material, Cr2O3, in the form of epitaxial clusters, exhibits an unprecedented 600% change in magnetization magnitude under 1 V. Furthermore, a multiferroic phase, with both magnetic and electric spontaneous polarizations, is found in the clusters, while absent in the bulk.

Photoemission and RHEED study of the supported Pt and Au epitaxial alloy clusters

In this paper, we investigated formation and structure of gold, platinum and Pt–Au bimetallic clusters on tungsten oxide surface. Crystallographic structure and orientation were investigated by reflection high energy electron diffraction, chemical composition and cluster's electronic state by synchrotron radiation photoelectron spectroscopy. Small amount of Au and Pt was deposited on WO3 (111) surface prepared by RF plasma oxidation and thermal annealing of (110) tungsten single crystal surface. Formation of 3D Pt or Au clusters with (111) epitaxial plane was observed. On the contrary to Au, strong metal substrate interaction between Pt and oxide was found, leading to the encapsulation of Pt clusters. Formation of Au–Pt bimetallic clusters was investigated at the temperature range of 160–870K. It was found that cluster structure depends on the substrate temperature and order of materials deposition. At the temperatures around 570K, metals formed bimetallic clusters with core–shell structure. Annealing to the temperatures over 770K led to the decomposition of Pt–Au clusters and formation of Pt–W alloy.

RHEED and XPS study of cerium interaction with SnO2 (110) surface

Abstract Interaction of cerium with thin epitaxial film of tin dioxide with (110) surface orientation was investigated by methods of X-ray Photoelectron Spectroscopy (XPS) and Reflection High Energy Electron Diffraction (RHEED). Strong interaction of cerium and tin leads to cerium diffusion into the tin dioxide bulk and formation of tin–cerium mixed oxide indicated by highly non-stoichiometric cerium oxide Ce 3d core level spectrum. This process is more pronounced with higher temperature during the deposition of cerium. With higher amount of deposited cerium, the tin dioxide substrate is gradually saturated with cerium atoms and the cerium atoms start to create 3D epitaxial clusters of cerium oxide with the (100) and (111) crystallographic planes parallel to the surface. The clusters show a higher oxidation state than the cerium atoms located in the tin oxide bulk. Oxygen required for the creation of these clusters comes from the tin dioxide substrate, which is partially reduced to metallic tin in this process. At the end of the experiment, we obtained a multilayer system consisting of layers of partially reduced tin dioxide, tin–cerium mixed oxide and top layer of partially reduced epitaxial grains of cerium oxide.

Molecular dynamics simulation of the influence of Cu(010) substrate on the melting of supported Co-Cu bimetallic clusters

Structural evolution and thermal stability of nanoclusters supported on the substrate play a key role in their applications. In this paper, we study the melting of the mixed Co281Cu280 bimetallic cluster with the icosahedral configuration by using molecular dynamics simulation with a general embedded atom method. The influence of the free or fixed Cu(010) substrate on the melting of the supported cluster is explored. It is found that the melting is strongly related to the substrate condition. There is a sharp increase in the temperature-energy curve for the cluster on the free substrate. The melting point (1320 K) is much lower than that (1630 K) of the cluster on the fixed substrate. The icosahedral configuration is converted into epitaxial cluster along the (010) of the substrate. Premelting occurs for the epitaxial cluster with the increase of temperature. The premelted atoms diffuse to the surface of the substrate and form surface layer until the cluster melt. The variation of the atomic spreading for the cluster on the free substrate is different from the case on the fixed substrate due to the atomic embedding into the substrate.

Influence of Energy and Temperature in Cluster Coalescence Induced by Deposition

Coalescence induced by deposition of different Cu clusters on an epitaxial Co cluster supported on a Cu(001) substrate is studied by constant-temperature molecular dynamics simulations. The degree of epitaxy of the final system increases with increasing separation between the centres of mass of the projectile and target clusters during the collision. Structure, roughness, and epitaxial order of the supported cluster also influence the degree of epitaxy. The effect of energy and temperature is determinant on the epitaxial condition of the coalesced cluster, especially both factors modify the generation, growth and interaction among grains. A higher temperature favours the epitaxial growth for low impact parameters. A higher energy contributes to the epitaxial coalescence for any initial separation between the projectile and target clusters. The influence of projectile energy is notably greater than the influence of temperature since higher energies allow greater and instantaneous atomic reorganizations, so that the number of arisen grains just after the collision becomes smaller. The appearance of grain boundary dislocations is, therefore, a decisive factor in the epitaxial growth of the coalesced cluster.

Structural and electronic studies of supported Pt and Au epitaxial clusters on tungsten oxide surface

In the present paper we investigated the growth and electronic properties of Pt or Au clusters and formation of Pt–Au bimetallic clusters prepared “in-situ” on tungsten oxide surface by physical deposition under vacuum. The epitaxial tungsten oxide thin films were prepared by oxidation of W(110) single-crystal surface using a RF oxygen plasma source followed by thermal annealing. The chemical state of the system, the interaction between deposit and substrate and formation of Pt–Al alloy were investigated by photoelectron spectroscopy excited by synchrotron radiation (SRPES) and K;α X-ray source (XPS). We found that in contrary to Au clusters the Pt ones strongly interacts with the substrate. Deposition of both Pt and Au on the surface at the substrate temperature of 300 °C gave rise to the formation of bimetallic core–shell clusters. The detail structure of the bimetallic system depends on the order of deposited metals. These findings can explain some properties of Pt/WOx and Au/WOx as well as Pt–Au/WOx bimetallic catalysts and gas sensors.

Ferromagnetic resonance behavior of spark plasma sintered Ni–Zn ferrite nanoparticles produced by a chemical route

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 were prepared by forced hydrolysis in a polyol (polyol process) from the corresponding iron, nickel, and zinc acetates. Synthesis conditions allowed for obtaining polycrystalline epitaxial clusters of about 22 nm in size with an average crystal size of about 5 nm. These powders were subsequently consolidated by spark plasma sintering (SPS) technique under a pressure of 80 MPa and temperatures in the 350 to 500 °C range for short periods (5 to 10 min). Densities reached 92% to 94% of the theoretical density. Particle size remained smaller than 61 nm, even for the highest temperature. Ferromagnetic resonance experiments at 77 K showed broadened resonance lines. In contrast with powdered nanoparticles, these high-density bodies can be used in many high-frequency applications.

Nanoparticle heterocoalescence induced by deposition

AbstractCoalescence induced by collision between a projectile Cu cluster and an epitaxial Co cluster supported on a Cu (001) substrate is studied by constant‐temperature molecular dynamics simulations. Results are compared with Co‐Co and Cu‐Cu coalescences. The degree of epitaxy of the Cu cluster decreases with the impact parameter and is related to the evolution of temperature with time in both clusters after the collision, especially, with the maximum. Firstly, the type of material, and secondly, the initial structure of the projectile cluster, icosahedral or cuboctahedral, determine the evolution of grains and their boundaries, and therefore, the degree of matching between both clusters. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A versatile fabrication method for cluster superlattices

On the graphene moiré on Ir(111) a variety of highly perfect cluster superlattices can be grown as shown for Ir, Pt, W and Re. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low-temperature deposition or the application of cluster seeding through Ir as shown for Au, AuIr and FeIr. Criteria for the suitability of a material to form a superlattice are given and largely confirmed. It is proven that at least Pt and Ir form epitaxial cluster superlattices. The temperature stability of the cluster superlattices is investigated and understood on the basis of positional fluctuations of the clusters around their sites of minimum potential energy. The binding sites of Ir, Pt, W and Re cluster superlattices are determined and the ability to cover samples macroscopically with a variety of superlattices is demonstrated.

Epitaxial growth of hierarchical PbS nanowires

Growth of extensively aligned hierarchical lead sulfide (PbS) nanowires with hyperbranched morphology has been achieved by synthesizing nanowires epitaxially on single crystal NaCl, rutile TiO2 (001), and muscovite mica in a chemical vapor deposition process. The morphology of as-grown PbS nanowires has been examined using scanning electron microscopy. Epitaxial match with the (100) plane of PbS has been observed on all substrates, and epitaxial match with PbS (111) was also observed on mica. In addition, the preferred orientation of nanowires led to particularly strong (200) reflections from PbS in powder X-ray diffraction. The potential epitaxial relationship and lattice match are proposed and discussed. PbS nanowires of the pine tree morphology can only be formed non-epitaxially in the presence of epitaxial hyperbranched clusters. The difficulty of forming epitaxial nanowire pine trees suggested that epitaxial growth might not be conducive to the creation of dislocations that drive the formation of pine tree nanowires. Electrical properties of PbS nanowires have also been investigated.

Structural transformation of Ge dimers on Ge(001) surfaces induced by bias voltage

Scanning tunnelling microscopy is utilized to investigate the local bias voltage tunnelling dependent transformation between (2×1) and c(4×2) structures on Ge(001) surfaces, which is reversibly observed at room temperature and a critical bias voltage of -0.80V. Similar transformation is also found on an epitaxial Ge islands but at a slightly different critical bias voltage of -1.00 V. It is found that the interaction between the topmost atoms on the STM tip and the atoms of the dimers, and the pinning effect induced by Sb atoms, the vacancies or the epitaxial clusters, can drive the structural transformation at the critical bias voltage.

Silicon in functional epitaxial oxides: A new group of nanostructures

The ability to integrate low-dimensional crystalline silicon into crystalline insulators with high dielectric constant (high-k) can open the way for a variety of novel applications ranging from high-k replacement in future nonvolatile memory devices to insulator/Si/insulator structures for nanoelectronic applications. We will present an approach for nanostructure fabrication by incorporation of crystalline silicon into epitaxial oxide that is based on a solid-phase epitaxy of Si. In dependence on the preparation conditions we obtained nanostructures containing an either ultra-thin single-crystalline Si quantum-well buried in single-crystalline oxide matrix with sharp interfaces or Si-nanocrystals (ncs) embedded into single-crystalline oxide layer. As an example, we demonstrate the growth of Si buried in Gd2O3 and the incorporation of epitaxial Si clusters into single-crystalline Gd2O3 on silicon as well as silicon carbide substrates using molecular beam epitaxy. The leakage current of the obtained nanostructures exhibited negative differential resistance at lower temperatures. For structures containing Si-ncs a large hysteresis in capacitance–voltage measurements due to charging and discharging of the Si-ncs was obtained.

The Adsorption and Growth of Copper on As-Terminated GaAs(001): Physical Vapour versus Electrochemical Deposition

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.