Patterning and doping of transition metals in tungsten dichalcogenides.

Substitutional transition metal doping in two-dimensional (2D) layered dichalcogenides is of fundamental importance in manipulating their electrical, excitonic, magnetic, and catalytic properties through the variation of the d-electron population. Yet, most doping strategies are spatially global, with dopants embedded concurrently during the synthesis. Here, we report an area-selective doping scheme for W-based dichalcogenide single layers, in which pre-patterned graphene is used as a reaction mask in the high-temperature substitution of the W sublattice. The chemical inertness of the thin graphene layer can effectively differentiate the spatial doping reaction, allowing for local manipulation of the host 2D materials. Using graphene as a mask is also beneficial in the sense that it also acts as an insertion layer between the contact metal and the doped channel, capable of depinning the Fermi level for low contact resistivity. Tracing doping by means of chalcogen labelling, deliberate Cr embedment is found to become energetically favorable in the presence of chalcogen deficiency, assisting the substitution of the W sublattice in the devised chemical vapor doping scheme. Atomic characterization using scanning transmission electron microscopy (STEM) shows that the dopant concentration is controllable and varies linearly with the reaction time in the current doping approach. Using the same method, other transition metal atoms such as Mo, V, and Fe can also be doped in the patterned area.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention.

The preservation of two-dimensional WS2 in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS2 grown by NaCl-assisted chemical vapor deposition in its original state. The WS2 monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS2 grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS2. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS2 can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS2, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS2. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS2 under environmental conditions.

Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4-15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2 . Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Scanning Moiré Fringe Method: A Superior Approach to Perceive Defects, Interfaces, and Distortion in 2D Materials.

Scanning moiré fringe (SMF) is a widely utilized technique for the precise measurement of the strain field in semiconductor transistors and heterointerfaces. With the growing challenges of traditional chip scaling, two-dimensional (2D) materials turn out to be ideal candidates for incorporation into semiconductor devices. Therefore, a method to efficiently locate defects and grain boundaries in 2D materials is highly essential. Here, we present a demonstration of using the SMF method to locate the domain boundaries at the nearly coherent interfaces with sub-angstrom spatial resolution under submicron fields of views. The strain field of small angle grain boundary and lateral heterojunction are instantaneously found and precisely determined by a quick SMF method without any atomic resolution images.

Synthesis and properties of MoCl4 complexes with thio- and seleno-ethers and their use for chemical vapour deposition of MoSe2 and MoS2 films.

Treatment of trans-[MoCl4(MeCN)2] with L (L = Me2S, Me2Se, THT, ½MeSCH2CH2SMe) in CH2Cl2 solution, or reaction of MoCl5 with excess L' (L' = nBu2S, nBu2Se, ½MeSCH2CH2SMe, ½iPrSCH2CH2SiPr, ½MeSCH2CH2CH2SMe, ½MeSeCH2CH2CH2SeMe) in MeCN, produces the Mo(iv) complexes, [MoCl4(L)2] and [MoCl4(L')2], respectively, in good yield. The new complexes have been characterised by IR and UV-vis spectroscopy, elemental analysis and magnetic measurements, whilst crystal structure analyses of trans-[MoCl4(Me2S)2], cis-[MoCl4{RS(CH2)2SR}] (R = Me, iPr) and cis-[MoCl4{MeS(CH2)3SMe}] confirmed their identities and distorted octahedral geometries. The potential of [MoCl4(nBu2E)2] (E = S, Se) as the first examples of molybdenum halide derived single source CVD precursors for the growth of MoE2 thin films was first probed by TGA, which showed multi-step decomposition processes, with the masses of the final residues consistent with MoSe2 (E = Se) and MoCl4 (E = S), respectively. Low pressure CVD experiments conducted at 750 °C (E = S) and 525 °C (E = Se) gave silvery and golden yellow films, respectively. Grazing incidence and in plane XRD data confirmed these to be 2H-MoE2, whilst the texture of the MoSe2 was investigated using pole figure measurements. SEM and EDX data, optical and Raman data on the films are also reported.

Chalcogenoether complexes of Nb(v) thio- and seleno-halides as single source precursors for low pressure chemical vapour deposition of NbS2 and NbSe2 thin films.

NbSCl3 was obtained via reaction of NbCl5 with S(SiMe3)2 in anhydrous CH2Cl2, whilst in MeCN solution the same reaction gives [NbSCl3(MeCN)2]. [NbSeCl3(MeCN)2] was obtained similarly from NbCl5 with Se(SiMe3)2. The chalcogenoether complexes, [NbSCl3(ER2)] (E = S: R = Me, nBu; E = Se: R = nBu), were obtained from reaction of NbCl5, ER2 and S(SiMe3)2 in CH2Cl2. The structure of the [Nb2S2Cl6(SMe2)2] reveals a Cl-bridged dimer with the SMe2 ligands disposed syn. The Cl bridges are highly asymmetric, with the long Nb-Cl bond trans Nb[double bond, length as m-dash]S. The complexes, [NbSCl3(L-L)] (L-L = MeSCH2CH2SMe, MeS(CH2)3SMe, iPrSCH2CH2SiPr, MeSe(CH2)3SeMe and nBuS(CH2)3SnBu), were obtained from reaction of L-L with preformed [NbSCl3(MeCN)2]. The structures of the Me-substituted complexes reveal distorted octahedral monomers with the neutral ligands trans to S/Cl. Solution 1H and 77Se{1H} NMR data showed that the neutral ligands are partially dissociated and undergoing fast exchange at ambient temperatures in CH2Cl2 solution, consistent with weak Lewis acidity for NbSCl3. The complexes containing nBu-substituted ligands have been used as single source precursors for low pressure chemical vapour deposition (CVD) of 3R-NbS2 thin films. 2H-NbSe2 thin films were also obtained via low pressure CVD using [NbSe2Cl3(SenBu2)]. The thin films were characterised by grazing incidence and in-plane XRD, pole figure analysis, scanning electron microscopy and energy dispersive X-ray analysis.

Developments in the chemistry of the hard early metals (Groups 1-6) with thioether, selenoether and telluroether ligands.

The coordination chemistry of neutral thio-, seleno- and telluroether ligands towards the hard s-block, f-block and higher oxidation state early d-block metals has developed significantly over the last 15 or so years. This has revealed several hitherto unknown classes of complexes and new insights into the chemistries of these hard-soft metal-ligand combinations. This Perspective describes the synthetic routes used to access such complexes and draws out their key structural features and spectroscopic properties. Where appropriate, applications of these species are also highlighted, including their use as single source precursors for the chemical vapour deposition of semiconducting metal chalcogenide thin films and as pre-catalysts for olefin polymerisation reactions.

Niobium tetrachloride complexes with thio-, seleno- and telluro-ether coordination - synthesis and structures.

NbCl4 reacts with the dithioethers MeS(CH2)nSMe (n = 2 or 3), iPrS(CH2)2SiPr or o-C6H4(CH2SEt)2 in a 1 : 1 molar ratio in CH2Cl2 or toluene over several days, to give red or orange, paramagnetic complexes, [NbCl4(dithioether)]. Their X-ray crystal structures confirm distorted octahedral geometries with chelating dithioether. MeS(CH2)2SMe, alone, also forms an [NbCl4{MeS(CH2)2SMe}2] complex based upon eight-coordinate Nb(iv) in a square antiprismatic geometry. Similar six-coordinate cis-[NbCl4(diselenoether)] are formed by MeSe(CH2)nSeMe (n = 2 or 3) and nBuSe(CH2)3SenBu. The monodentate ligands Me2S, Me2Se and nBu2Se form [NbCl4(R2E)2] (E = S, Se) which are unstable in solution, losing R2E to form complexes with a 1 : 1 stoichiometry, which are shown to be diamagnetic dimers, [(Me2E)Cl3Nb(µ-Cl)2NbCl3(Me2E)], with single Nb-Nb σ-bonds. Tellurium ligands tend to decompose in these reactions, but reaction at room temperature of Me2Te and NbCl4 formed [NbCl4(Me2Te)2], the X-ray structure of which revealed an eight-coordinate dimer, [Nb2Cl4(µ-Cl)4(Me2Te)4]. The new complexes have been characterised by microanalysis, IR, UV-visible spectroscopy and magnetic measurements. X-ray crystal structures are reported for [NbCl4(L-L)] (L-L = MeS(CH2)nSMe, n = 2 or 3, iPrS(CH2)2SiPr, o-C6H4(CH2SEt)2, MeSe(CH2)3SeMe), [Nb2Cl8(Me2Te)4], [Nb2Cl8(Me2S)2] and [Nb2Cl8(Me2Se)2]. The complexes are very sensitive to moisture and dioxygen, and some also readily undergo C-E bond cleavage. Two ligand fragmentation products were identified -[Nb2Cl4(µ-S)2{MeS(CH2)3SMe}2] and [Nb2Cl4(µ-Se)2{o-C6H4(CH2)2Se}4]. X-ray crystal structures are also reported for the oxidation/hydrolysis products [NbOCl3(Me2S)] and [NbOCl3{MeS(CH2)2SMe}].

Niobium tetrahalide complexes with neutral diphosphine ligands.

The reactions of NbCl4 with diphosphine ligands o-C6H4(PMe2)2, Me2PCH2CH2PMe2 or Et2PCH2CH2PEt2 in a 1 : 2 molar ratio in MeCN solution produced eight-coordinate [NbCl4(diphosphine)2]. [NbBr4(diphosphine)2] (diphosphine = o-C6H4(PMe2)2 or Me2PCH2CH2PMe2) were made similarly from NbBr4. X-ray crystal structures show that [NbCl4{o-C6H4(PMe2)2}2] has a dodecahedral geometry, but the complexes with dimethylene-backboned diphosphines are distorted square antiprisms. The Nb-P distances and

Niobium(V) and tantalum(V) halide chalcogenoether complexes--towards single source CVD precursors for ME2 thin films.

A series of pentavalent niobium and tantalum halide complexes with thio-, seleno- and telluro-ether ligands, [MCl5(E(n)Bu2)] (M = Nb, Ta; E = S, Se, Te), [TaX5(TeMe2)] (X = Cl, Br, F) and the dinuclear [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta), has been prepared and characterised by IR, (1)H, (13)C{(1)H}, (77)Se, (93)Nb and (125)Te NMR spectroscopy, as appropriate, and microanalyses. Confirmation of the tantalum(V)-telluroether coordination follows from the crystal structure of [TaCl5(TeMe2)], which represents the highest oxidation state transition metal complex with telluroether coordination structurally authenticated. The Ta(V) monotelluroether complexes are much more stable than the Nb(V) analogues. In the presence of TaCl5 the ditelluroether, CH2(CH2Te(t)Bu)2, is decomposed; one of the products is the dealkylated [(t)BuTe(CH2)3Te][TaCl6], whose structure was determined crystallographically. Crystal structures of [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta) show ligand-bridged species. The complexes bearing ß-hydrogen atoms on the terminal alkyl substituents have also been investigated as single source reagents for the deposition of ME2 thin films via low pressure chemical vapour deposition. While the tantalum complexes proved to be unsuitable, the [NbCl5(S(n)Bu2)] and [NbCl5(Se(n)Bu2)] deposit NbS2 and NbSe2 as hexagonal platelets onto SiO2 substrates at 750 °C and 650 °C, respectively. Grazing incidence and in-plane X-ray diffraction confirm both materials adopt the 3R-polytype (R3mh), and the sulfide shows preferred orientation with the crystallites aligned predominantly with the c axis perpendicular to the substrate. Scanning electron microscopy and Raman spectra are consistent with the X-ray data.