Morphology control of epitaxial monolayer transition metal dichalcogenides

Funding: AFM system (funded via an EPSRC equipment grant: EP/L017008/1) used in this work and experimental support. The Leverhulme Trust (Grant no. RL-2016-006); The Royal Society; the European Research Council (Grant No. ERC-714193-QUESTDO). K.U. acknowledges EPSRC for PhD studentship support through grant no. EP/L015110/1.To advance fundamental understanding and ultimate application of transition-metal dichalcogenide (TMD) monolayers, it is essential to develop capabilities for the synthesis of high-quality single-layer samples. Molecular beam epitaxy (MBE), a leading technique for the fabrication of the highest-quality epitaxial films of conventional semiconductors has, however, typically yielded only small grain sizes and suboptimal morphologies when applied to the van der Waals growth of monolayer TMDs. Here, we present a systematic study on the influence of adatom mobility, growth rate, and metal:chalcogen flux on the growth of NbSe2, VSe2, and TiSe2 using MBE. Through this, we identify the key drivers and influence of the adatom kinetics that control the epitaxial growth of TMDs, realizing four distinct morphologies of the as-grown compounds. We use this to determine optimized growth conditions for the fabrication of high-quality monolayers, ultimately realizing the largest grain sizes of monolayer TMDs that have been achieved to date via MBE growth.PostprintPeer reviewe

Weyl-like points from band inversions of spin-polarised surface states in NbGeSb

Funding: Leverhulme Trust (Grant No. PLP-2015-144), The Royal Society, and the Engineering and Physical Sciences Research Council, UK (Grant No. EP/R031924/1); CALIPSOplus project under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020; International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM) (I.M.); EPSRC for studentship support through grant nos. EP/K503162/1 and EP/L505079/1, and EP/L015110/1 (O.J.C., J.M.R., and K.U.).Band inversions are key to stabilising a variety of novel electronic states in solids, from topological surface states to the formation of symmetry-protected three-dimensional Dirac and Weyl points and nodal-line semimetals. Here, we create a band inversion not of bulk states, but rather between manifolds of surface states. We realise this by aliovalent substitution of Nb for Zr and Sb for S in the ZrSiS family of nonsymmorphic semimetals. Using angle-resolved photoemission and density-functional theory, we show how two pairs of surface states, known from ZrSiS, are driven to intersect each other near the Fermi level in NbGeSb, and to develop pronounced spin splittings. We demonstrate how mirror symmetry leads to protected crossing points in the resulting spin-orbital entangled surface band structure, thereby stabilising surface state analogues of three-dimensional Weyl points. More generally, our observations suggest new opportunities for engineering topologically and symmetry-protected states via band inversions of surface states.Publisher PDFPeer reviewe

Morphology and polymorph control of NbSe₂ monolayers grown via molecular beam epitaxy

Abstract redacted"This work was supported by the CM-CDT with EPSRC funding through grant no. EP/L015110/1 and The Leverhulme Trust grant no. RL-2016-006. The AFM system was funded via an EPSRC equipment grant no. EP/L017008/1."--Fundin

Morphology and polymorph control of NbSe2 monolayers grown via molecular beam epitaxy (thesis data)

The data files are embargoed until 26/10/202

SPM characterisation of nanomechanical proprieties of C60 monolayer formed by LB

C60 is a fascinating material due to its unusual and sometimes spectacular mechanical and optoelectronic properties, allowing a series of interesting applications in the fields of nanoscience, material science, optics and electrics [1,2,3,4]. However, since its discovery a large number of experiments have been carried out to study the intriguing proprieties of this exciting material but methods for preparing macroscopic quantities of monolayer C60 [5] in a facile and scalable way has proved challenging [6]. Here we report a potential method for achieving monolayer (ML) production of C60 by Langmuir-Blodgett(LB) technique. We have characterised large area films produced by assembly at the air-water interface and transferred by a modified Langmuir-Blodgett (LB) method onto cleaned SiO2 substrates using a variety of scanning probe methods. We have mapped the mechnaical, thermal and electrical properties with nanoscale resolution. We investigated optimisation of the deposition through monitoring of the mean molecular area (Π - MMA) as a function of surface pressure (isotherms), Brewster angle microscopy, Raman spectroscopy and optical microscopy. Results were obtained using deionised water on a commercial KVS-NIMA trough; optimal solvent for monolayer formation was found to be toluene and methanol in 5:1 ratio by volume. C60 solution was spread on the water surface using a custom built electrospray system, enabling the formation of stable C60 thin films with high stability. The transferred samples, on SiO2 substrates, were allowed to dry in controlled atmosphere and the amount of C60 on the substrate was monitored using a quartz crystal microbalance (QCM). Optical microscopy images of transferred samples showed large area coverage of the C60, additionally Raman spectroscopy confirmed the presence of C60 on the sample surface. The height of the obtained monolayer and its mechanical and thermal proprieties were measured by ultrasound force microscopy (UFM), quantitative nano-mechanics atomic force microscopy (AFM-QNM) and scanning thermal microscopy (SThM). Low noise measurements were made by TappingMode AFM in the new state-of-the-art ISOLAB facilities at Lancaster University. Investigations revealed a step height of about 0.76 nm ± 0.06 nm, which is in agreement with the expected molecular dimension of a single C60 layer. Furthermore, measurements conducted at the ISOLAB have shown a uniform and homogeneous C60 ML, suggesting the validity of this technique as a viable method for the deposition of large area ML of C60 and other fullerene moieties on a hydrophilic/hydrophobic substrate. QNM-AFM and UFM were used to study the mechanical proprieties of C60 ML, showing an high degree of layer stability under repeated scanning, resistance to mechanical deformation and a stiffness lower than that of the SiO2 substrate. Young's modulus of 7 GPa for the C60 ML were obtained through QNM-AFM, in a good agreement with other similar studies. Preliminary high resolution AFM measurements made in the ISOLAB have allowed us to observe the close packed molecular structure of the C60 ML, and confirm that this methodology is ideally suited to the deposition of such films. In conclusion, we present a straight forward, rapid and scalable way to produce large area ML of C60 using the Langmuir-Blodgett technique as an alternative to other methods such as evaporation or drop cast film. Analyzing the samples with a range of scanning probe microscopy techniques have afforded a wealth of vital information about the condition, topography and properties of C60 monolayers

Orbital-selective band hybridisation at the charge density wave transition in monolayer TiTe2TiTe_2

Reducing the thickness of a material to its two-dimensional (2D) limit can have dramatic consequences for its collective electronic states, including magnetism, superconductivity, and charge and spin ordering. An extreme case is $TiTe_2$, where a charge density wave (CDW) emerges in the single-layer, which is absent for the bulk compound, and whose origin is still poorly understood. Here, we investigate the electronic band structure evolution across this CDW transition using temperature-dependent angle-resolved photoemission spectroscopy. Our study reveals an orbital-selective band hybridisation between the backfolded conduction and valence bands occurring at the CDW phase transition, which in turn leads to a significant electronic energy gain, underpinning the CDW transition. For the bulk compound, we show how this energy gain is almost completely suppressed due to the three-dimensionality of the electronic band structure, including via a kz-dependent band inversion which switches the orbital character of the valence states. Our study thus sheds new light on how control of the electronic dimensionality can be used to trigger the emergence of new collective states in 2D materials