Search

Scanning tunneling microscopy shows that copper deposited at room temperature onto a freshly exfoliated MoS2 surface forms Cu(111) clusters with periodic preferred heights of 5, 8, and 11 atomic layers. These height intervals correlate with Fermi nesting regions along the necks of the bulk Cu Fermi surface, indicating a connection between physical and electronic structures. Density functional theory calculations of freestanding Cu(111) films support this as well, predicting a lower density of states at the Fermi level for these preferred heights. This is consistent with other noble metals deposited on MoS2 that exhibit electronic growth, in which the metal films self-assemble as nanostructures minimizing quantum electronic energies. Here, we have discovered that it is critical for the metal deposition to begin on a clean MoS2 surface. If copper is deposited onto an already Cu coated surface, even if the original film displays electronic growth, the resulting Cu film lacks quantization. Instead, the preferred heights of the Cu clusters simply increase linearly with the amount of Cu deposited upon the surface. We believe this is due to different bonding conditions during the initial stages of growth. Newly deposited copper would bond strongly to the already present copper clusters, rather than the weak bonding, which exists to the van der Waals terminated surface of MoS2. The stronger bonding with previously deposited clusters hinders additional Cu atoms from reaching their lowest quantum energy state. The interface characteristics of the van der Waals surface enable surface engineering of self-assembled structures to achieve different applications. [ABSTRACT FROM AUTHOR]

An electronic growth mode has been reported to occur in several noble metals on MoS2 but has not been observed on other layered semiconductors. In this work, the experiments show that Au(111) islands initially follow an electronic growth mode on WS2, matching the quantization seen in Au/MoS2. However, while epitaxial nanostructures with similar features are observed on WSe2, there is no sign of electronic growth. Binding energy calculations show that multiple bonding sites have nearly the same energy on both WS2 and MoS2, while Au strongly prefers a single bonding site on WSe2. Having multiple sites with the same energy gives flexibility in interfacial bonding that can alleviate strain from the 9+% lattice mismatch in these systems, which would, otherwise, easily suppress quantum size effects from electronic growth modes. These results should be useful in predicting which systems undergo quantized electronic growth on layered semiconductors. [ABSTRACT FROM AUTHOR]

Near surface defects can significantly impact the quality of metallic interconnects and other interfaces necessary to create device structures incorporating two-dimensional materials. Furthermore, the impact of such defects can strongly depend on their organization. In this study, we present scanning tunneling microscopy images and tunneling spectroscopy of point and linear defects near the surface of natural MoS2. The point defects share similar structural and electronic characteristics and occur with comparable frequency as subsurface sulfur vacancies observed previously on natural MoS2. The linear defects observed here occur less frequently than the point defects but share the same depth profile and electronic structure. These data indicate that the linear defects are actually a one-dimensional organization of subsurface sulfur vacancies. Our density functional calculations agree with this assessment in that, for sufficient local defect concentrations, it is energetically more favorable for the defects to be organized in a linear fashion rather than as clusters or even isolated single point defects. Given these measurements were taken from naturally formed MoS2, this organization likely occurs during crystal formation. Considering the impact of one-dimensional organization on the local properties of layered materials, and the potential for them to be introduced purposefully during crystal formation, research into the formation mechanism and properties of these defects could enable new paths for defect engineering in MoS2-based systems. [ABSTRACT FROM AUTHOR]

Quantum confinement effects can induce the formation of discrete nanostructures with well-defined preferred heights in thin metallic films. In most systems, such electronic growth modes are weak and limited to cryogenic conditions. Recently, however, we have discovered that metals grown upon van der Waals surfaces can exhibit electronic growth at, or even above, room temperature to spontaneously form well-defined and highly stable nanostructures. Here, we explore the initial stages of room temperature deposition of Pd onto MoS2. We found that, even for minimal thicknesses, Pd spontaneously formed discrete islands with three atomic layers. The islands maintained this preferred height for nominal coverages below three atomic layers. At higher coverages, the preferred height switched abruptly to six atomic layers. Unlike previous studies using Au or Ag, the islands did not increase laterally with coverage but rather increased in number with lateral size remaining about the same. The preferred heights in Pd/MoS2 correlate to the Pd Fermi surface topography and are also consistent with thicknesses showing minima in the density of states at the Fermi level, which suggest that the electronic growth modes are the driving factors in these self-assembled Pd nanostructures. The Pd system shows a preference for island nucleation compared to Au and Ag which grow laterally with increasing coverage. This is likely related to differences in bonding at the interface as Pd is typically much more reactive than Ag or Au. [ABSTRACT FROM AUTHOR]

Half-metallic Heusler alloys have been intensively studied in recent years due to their potential applications in spin-based devices, e.g., in magnetic tunnel junctions. Yet, their properties may be very sensitive to the choice of the substrates, i.e., to the epitaxial strain and interface properties. Here, we report the results of our computational work on the half-Heusler compound CrMnSb(1−x)Px. In particular, we demonstrate that the parent compound CrMnSb is close to a half-metallic material at the optimized lattice parameter, with the onset of the half-metallic bandgap a few meV above the Fermi energy. Moreover, although it undergoes a half-metallic transition under a uniform compression of ∼1.5%, such a transition is absent under epitaxial strain. At the same time, we show that a half-metallic transition could be induced by a chemical substitution of Sb with P, which results in a volume reduction of the unit cell. In particular, 50% substitution of Sb with P leads to a robust half-metallicity in CrMnSb(1−x)Px, with 100% spin polarization being retained at a large range of epitaxial strain. Thus, our results indicate that CrMnSb0.5P0.5 could be grown on different types of substrates, e.g., GaAs, without its electronic properties being detrimentally affected by biaxial strain. In addition, CrMnSb0.5P0.5 exhibits a fully compensated ferrimagnetic alignment, which could be potentially useful in applications where stray magnetic fields are undesirable. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy shows that copper deposited at room temperature onto a freshly exfoliated MoS2 surface forms Cu(111) clusters with periodic preferred heights of 5, 8, and 11 atomic layers. These height intervals correlate with Fermi nesting regions along the necks of the bulk Cu Fermi surface, indicating a connection between physical and electronic structures. Density functional theory calculations of freestanding Cu(111) films support this as well, predicting a lower density of states at the Fermi level for these preferred heights. This is consistent with other noble metals deposited on MoS2 that exhibit electronic growth, in which the metal films self-assemble as nanostructures minimizing quantum electronic energies. Here, we have discovered that it is critical for the metal deposition to begin on a clean MoS2 surface. If copper is deposited onto an already Cu coated surface, even if the original film displays electronic growth, the resulting Cu film lacks quantization. Instead, the preferred heights of the Cu clusters simply increase linearly with the amount of Cu deposited upon the surface. We believe this is due to different bonding conditions during the initial stages of growth. Newly deposited copper would bond strongly to the already present copper clusters, rather than the weak bonding, which exists to the van der Waals terminated surface of MoS2. The stronger bonding with previously deposited clusters hinders additional Cu atoms from reaching their lowest quantum energy state. The interface characteristics of the van der Waals surface enable surface engineering of self-assembled structures to achieve different applications. [ABSTRACT FROM AUTHOR]

Near surface defects can significantly impact the quality of metallic interconnects and other interfaces necessary to create device structures incorporating two-dimensional materials. Furthermore, the impact of such defects can strongly depend on their organization. In this study, we present scanning tunneling microscopy images and tunneling spectroscopy of point and linear defects near the surface of natural MoS2. The point defects share similar structural and electronic characteristics and occur with comparable frequency as subsurface sulfur vacancies observed previously on natural MoS2. The linear defects observed here occur less frequently than the point defects but share the same depth profile and electronic structure. These data indicate that the linear defects are actually a one-dimensional organization of subsurface sulfur vacancies. Our density functional calculations agree with this assessment in that, for sufficient local defect concentrations, it is energetically more favorable for the defects to be organized in a linear fashion rather than as clusters or even isolated single point defects. Given these measurements were taken from naturally formed MoS2, this organization likely occurs during crystal formation. Considering the impact of one-dimensional organization on the local properties of layered materials, and the potential for them to be introduced purposefully during crystal formation, research into the formation mechanism and properties of these defects could enable new paths for defect engineering in MoS2-based systems. [ABSTRACT FROM AUTHOR]

Thin films of Au ranging from 7-24 nm were grown on MoS2 at room temperature using thermal evaporation and studied using scanning tunneling microscopy and ballistic electron emission spectroscopy. Topographic images show the surface morphology of Au transitions from terraced triangles to a mix of terraced hexagonal and irregular-shaped structures as film thickness exceeded 16 nm. Raman spectra reveal the presence of tensile strain in the MoS2 with thicker Au films and is likely the driving force behind this transition. All samples exhibit a Schottky barrier significantly lower than that predicted by the Schottky-Mott model due to Fermi-level pinning at the interface. The pinning mechanism is thought to be caused, in part, by the presence of gap states induced by a weakening of the interlayer Mo-S bonding in the presence of the Au film. Although relatively consistent in thinner films, the Schottky barrier increases concurrently with structural changes on the surface. At the same time, transmission through the interface begins to drop at an increased exponential rate with film thickness. These observations are consistent with a widening separation between the Au and MoS2 that would reduce the number of gap states and cause transmission through the interface to be more characteristic of quantum tunneling. An increased separation such as this could result from changes in equilibrium conditions at the interface with increasing strain. [ABSTRACT FROM AUTHOR]

Here, we present results of a computational and experimental study of adsorption of various metals on MoS 2 . In particular, we analyzed the binding mechanism of four metallic elements (Ag, Au, Cu, Ni) on MoS 2 . Among these elements, Ni exhibits the strongest binding and lowest mobility on the surface of MoS 2 . On the other hand, Au and Ag bond very weakly to the surface and have very high mobilities. Our calculations for Cu show that its bonding and surface mobility are between these two groups. Experimentally, Ni films exhibit a composition characterized by randomly oriented nanoscale clusters. This is consistent with the larger cohesive energy of Ni atoms as compared with their binding energy with MoS 2 , which is expected to result in 3D clusters. In contrast, Au and Ag tend to form atomically flat plateaued structures on MoS 2 , which is contrary to their larger cohesive energy as compared to their weak binding with MoS 2 . Cu displays a surface morphology somewhat similar to Ni, featuring larger nanoscale clusters. However, unlike Ni, in many cases Cu exhibits small plateaued surfaces on these clusters. This suggests that Cu likely has two competing mechanisms that cause it to span the behaviors seen in the Ni and Au/Ag film morphologies. These results indicate that calculations of the initial binding conditions could be useful for predicting film morphologies. In addition, out calculations show that the adsorption of adatoms with odd electron number like Ag, Au, and Cu results in 100% spin-polarization and integer magnetic moment of the system. Adsorption of Ni adatoms, with even electron number, does not induce a magnetic transition., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Metal/transition metal dichalcogenide interfaces are the subject of active research, in part because they provide various possibilities for interplay of electronic and magnetic properties with potential device applications. Here, we present results of our first principles calculations of nearly strain-free Ni/WSe 2 and Ni/MoS 2 interfaces in thin-film geometry. It is shown that while both the WSe 2 and MoS 2 layers adjacent to Ni undergo metallic transition, the layers farther from the interface remain semiconducting. In addition, a moderate value of spin-polarization is induced on interfacial WSe 2 and MoS 2 layers. At the same time, the electronic and magnetic properties of Ni are nearly unaffected by the presence of WSe 2 and MoS 2 , except a small reduction of magnetic moment at the interfacial Ni atoms. These results can be used as a reference for experimental efforts on epitaxial metal/transition metal dichalcogenide heterostructures, with potential application in modern magnetic storage devices., (© 2021 IOP Publishing Ltd.)

Despite an enormous amount of research on carbon based nanostructures, relatively little is known about the electronic structure of multi-walled carbon fullerenes, also known as carbon onions. In part, this is due to the very high computational expense involved in estimating electronic structure of large molecules. At the same time, experimentally, the exact crystal structure of the carbon onion is usually unknown, and therefore one relies on qualitative arguments only. In this work we present the results of a computational study on a series of multi-walled fullerenes and compare their electronic structures to experimental data. Experimentally, the carbon onions were fabricated using ultrasonic agitation of isopropanol alcohol and deposited onto the surface of highly ordered pyrolytic graphite using a drop cast method. Scanning tunneling microscopy images indicate that the carbon onions produced using this technique are ellipsoidal with dimensions on the order of 10 nm. The majority of differential tunneling spectra acquired on individual carbon onions are similar to that of graphite with the addition of molecular-like peaks, indicating that these particles span the transition between molecules and bulk crystals. A smaller, yet sizable number exhibited a semiconducting gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. These results are compared with the electronic structure of different carbon onion configurations calculated using first-principles. Similar to the experimental results, the majority of these configurations are metallic with a minority behaving as semiconductors. Analysis of the configurations investigated here reveals that each carbon onion exhibiting an energy band gap consisted only of non-metallic fullerene layers, indicating that the interlayer interaction is not significant enough to affect the total density of states in these structures.