Low-temperature, plasma assisted, cyclic synthesis of MoS2

Two-dimensional transition metal dichalcogenides (TMDs) such as MoS2 and WS2 hold great promise for many novel applications. Recent years have therefore witnessed tremendous efforts on large scale manufacturing of these 2D crystals. A long-standing puzzle in the field is the effect of different types of defects in their electronic, magnetic, catalytic and optical properties. In this presentation an overview of different defects in transmission metal di-chalcogenides (TMDs) will be presented [1,2]. We will first focus on: 1) defining the dimensionalities and atomic structures of defects; 2) pathways to generating structural defects during and after synthesis and, 3) the effects of having defects on the physico-chemical properties and applications. We will also emphasize doping and allowing monolayers of MoS2 and WS2, and their implications in electronic and thermal transport. We will also describe the catalytic effects of edges, vacancies and local strain observed in MoxW(1-x)S2 monolayers by correlating the hydrogen evolution reaction (HER) with aberration corrected scanning transmission electron microscopy (AC-HRSTEM) [3]. Our findings demonstrates that it is now possible to use chalcogenide layers for the fabrication of more effective catalytic substrates, however, defect control is required to tailor their performance. By studying photoluminescence spectra, atomic structure imaging, and band structure calculations, we also demonstrate that the most dominating synthetic defect—sulfur monovacancies in TMDs, is responsible for a new low temperature excitonic transition peak in photoluminescence 300 meV away from the neutral exciton emission [4]. We further show that these neutral excitons bind to sulfur mono-vacancies at low temperature, and the recombination of bound excitons provides a unique spectroscopic signature of sulfur mono-vacancies [4]. However, at room temperature, this unique spectroscopic signature completely disappears due to thermal dissociation of bound excitons [4]. Finally, hetero-interfaces in TMDs, will be studied and discussed by AC-HRSTEM and optical emission.

Two-dimensional (2D) Janus transition metal dichalcogenide (TMDC) layers with broken mirror symmetry exhibit giant Rashba splitting and unique excitonic behavior. For their one-dimensional (1D) counterparts, the Janus nanotubes possess curvature, which introduces an additional degree of freedom to break the structural symmetry. This can potentially enhance these effects or even give rise to novel properties. Moreover, Janus MSSe nanotubes (M = W, Mo), with diameters surpassing 40 Å and Se positioned externally consistently demonstrate lower energy states compared to their Janus monolayer counterparts. However, there are limited studies on the preparation of Janus nanotubes, due to the synthesis challenge and limited sample quality. In this study, we first synthesized MoS2 nanotubes on single-walled carbon nanotube (SWCNT) and boron nitride nanotube (BNNT) heterostructuresand then explored the growth of Janus MoSSe nanotubes from MoS2 nanotubes at room temperature with the assistance of H2 plasma. The successful formation of the Janus structure is confirmed by Raman spectroscopy, and atomic structure and elemental distribution of the grown samples are further characterized by advanced electronic microscopy. The synthesis of Janus MoSSe nanotubes based on SWCNT-BNNT heterostructures paves the way for further exploration of novel properties in Janus TMDC nanotubes.

Structural changes of Y2O3 films and La2O3 films deposited on some oxidized silicon substrates were studied using X-ray photoelectron spectroscopy (XPS), Secondary ion mass spectrometry (SIMS), and Fourier transform infrared spectroscopy attenuated total reflection method (FT-IR ATR). Y2O3 and La2O3 films on chemical oxide and NH3 annealed oxy-nitride were prepared by the Low-pressure chemical vapor deposition (LPCVD) method using an lanthanide–dipivaloyl-methanate (Ln–DPM) complex. The Y2O3 film and the La2O3 film on the both kinds of substrate already contained a partly silicate structure at the interface side as a result of an interface reaction during the deposition process. During post deposition annealing, the whole film structure of the Y2O3 and the La2O3 on the chemical oxide changed to a silicate structure due to silicon diffusion with interface reaction. In the case of the Y2O3 film, this interface reaction can be suppressed using thermal oxy-nitride as the interfacial layer. In the case of the La2O3 film, the suppression effect using oxy-nitride was smaller than the case with the Y2O3 film. Also, it was found that there was a strong correlation between the structural change of the films and the change of flat-band-voltage of both Y2O3 and La2O3 MIS diodes during post-deposition-annealing.

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.

Here we present recent work on the synthesis and characterization of two-dimensional transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE). Bulk TMD crystals consist of two-dimensional layers bonded together by van der Waal interactions; the lack of primary bonds at the TMD interface allow for the growth of TMD heterostructures on a variety of substrates without the need to consider lattice constant mismatch or crystal structure of the substrate which we aim to demonstrate. TMDs are grown and characterized in an all in-vacuo ultra-high vacuum system which enables the simultaneous co-deposition of multiple transition metals and chalcogens allowing for the study of TMD ternary alloys and layered heterostructures followed by their compositional and intrinsic electronic characterization prior to atmospheric exposure. Presented will be a review of our work studying the interface chemistry between TMD and conventional semiconducting substrates, as well as our investigations of 2D-2D heterostructure grown by MBE. Figure 1 shows an example of in-vacuo x-ray photoelectron spectroscopy (XPS) carried out on GaAs(001) before an after the growth of monolayer MoSe2. The sample was initial passivated by a sulfur based treatment, and the presence of sulfur bonded to the surface can be observed in the As 2p and Ga 2p spectra. The asymmetry observed in the As 2p and Ga 2p core-levels suggest that some reactions with Se take place during the growth. The thermal stability of this interface will be investigated using in-situ UHV heating and XPS. In separate work we evaluated the valence band offsets using in-vacuo XPS and UPS (ultraviolet photoelectron spectroscopy). This is achieved by measuring core-level to valence band maximum separation for control samples as well as the core-level shifts in heterostructure. In-vacuo XPS analysis can also be used to reveal any interface reactors that may occur between the TMDs. It is found that for chalcogen deficient growths, metallic signatures can be detected. Of particular interest is the process structure relationships. Presented will be parametric studies showing impact of growth temperature and relative metal-chalcogen flux ratios on the electronic structure and interface chemistry in both the 2D-2D and 2D-3D systems. The all in-vacuo synthesis and analysis cluster tool allows these investigations to be carried out without air exposure which can induce substantial changes in the chemistry of these materials. Figure 1

Two-dimensional transition metal dichalcogenides (TMDCs) have the extensive application prospect in multifunctional electronics and photonics due to their unique electro-optical properties. In order to further expand their application scope in micro-nano optoelectronic devices and improve the performance of devices, the band-gap and defective engineering have been studied to tune the band-gap, morphology and structure of two-dimensional semiconductor materials. The tunning of the bandgap of MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy has been typically achieved by controlling the Se concentration. Theoretical calculations revealed that layered stacked two-dimensional alloy materials with a larger aspect ratio, exposed edges and obvious edge dangling bonds show enhanced HER activity as compared with TMDCs. In this paper, the properties of stacked MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy grown by the chemical vapor deposition method in a quartz tube furnace are investigated by using optical microscopy (OM), atomic force microscopy (AFM), scanning tunneling microscopy (SEM), Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The OM and SEM images of the as-synthesized stacked MoS<sub>2(1-<i>x</i>)</sub>Se<sub>2<i>x</i></sub> alloy show apparent interface between layers and their thickness is further acquired by AFM. Unlike most of single-layer or few-layer MoS<sub>2(1-<i>x</i>)</sub>Se<sub>2<i>x</i></sub> alloys, stack-grown stepped MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy materials all present the strong luminescence properties despite the thickness increasing from 2.2 nm (~3 layers) to 5.6 nm (~7 layers). And even till 100 nm, the emission spectrum with two luminescence peaks can still be observed. The two exciton luminescence peaks A and B are derived from the valence band splitting caused by the spin-orbit coupling, respectively. As the thickness increases, the two luminescence peaks are red-shifted and exhibit a band-bending effect that is only present when the alloy doping concentration is changed. As the sample thickness is 5.6 nm, a C-peak at 650 nm at the high energy end of the PL spectrum is observed, which may be attributed to the transition luminescence from the defect energy level introduced by Se (S) substitution, interstice or cluster. When the number of layers is small, the number of defects is small, so that the luminescence is not observed. As the number of layers increases, the defects increase to form a defect energy level. However, when the material thickness continuously increases until the bulk material is formed, the luminescence disappears in the PL spectrum because the band gap is reduced and the band gap is made smaller than the defect energy level. Raman spectroscopy gives two sets of vibration modes:like-MoS<sub>2</sub> and like-MoSe<sub>2</sub>. The Raman peak is almost unchanged as the thickness increases, but the two vibration modes E<sub>2g (Mo-Se)</sub> and E<sub>2 g (Mo-S)</sub> in the plane gradually appear and increase. At the same time, the intensity ratio and line width of Mo-Se related vibration mode E<sub>2g</sub>/A<sub>1g</sub> increase with thickness increasing, which indicates the enhancement of the Mo-Se in-plane vibration mode and the incorporation of randomness of Se into the lattice. Obviously, the defects and stress are the main factors affecting the electronic structure of stacked MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.

Whether grown as clusters or ultrathin films, extremely small quantities of active elements exhibit changes in catalytic activity that arise from both size effects and electron-transfer effects. These size and transfer effects can be related to increased propensity for oxidation of the metallic deposit, as well as to various changes in electrochemical performance such as durability or required overpotential for a given reaction. A review of our recent work on layer-by-layer near-surface effects of atomic and electronic structure on catalytically relevant properties will be presented. We use electrochemical atomic layer deposition for layer-by-layer growth of platinum group metals (PGM), and measure the evolution of electronic and atomic structure using synchrotron-based X-ray Photoelectron Spectroscopy (XPS) and X-ray absorption spectroscopy. XPS using a tunable energy synchrotron X-ray source allows us to profile the transitions in the electronic structure from the surface down to the adlayer/support interface and beyond. In addition to depth profile studies of these layered metal architectures, the effects of thermally and electrochemically activated near-surface alloying in the PMG systems will be presented here. We observe undulations of near-surface electronic structure brought on by the low dimensionality of the PMG adlayer as well as the adlayer-support interactions, effects that can explain the resulting surface.

The electron spectroscopy techniques (X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), High Resolution Electron Energy Losses Spectroscopy (HREELS), Extended Energy Loss Fine Structure (EELFS) were used for characterisation of semiconductor nanocrystals (Si, PbS, CdS) formed by various technologies: Si ion implantation in Al2O3 matrix and annealing; Si-rich multilayered SiNx Low Pressure Chemical Vapour Deposition (LPCVD); PbS and ZnS nanocrystals on Polydiacetylene (PDA) Langmuir-Blodget polymer films. The chemical, phase and atomic structure non uniformity were determined using depth profiling techniques. The peculiarities of the 'matrix nanocrystal' interface atomic and electronic structure, especially for nanocrystals were determined from analysis of XPS chemical shifts and vibrational spectra. The accommodation strains in the Si-nc:Al2O3 system were determined by EELFS. The comparison of electron spectroscopy methods and other techniques for nanocrystals investigations (PL, HR-TEM, ellipsometry) is made.

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Bulk metallic glassy surface native oxide: Its atomic structure, growth rate and electrical properties

Because of suitable band gap and high mobility, two-dimensional transition metal dichalcogenide (TMD) materials are promising in future microelectronic devices. However, controllable p-type and n-type doping of TMDs is still a challenge. Herein, we develop a soft plasma doping concept and demonstrate both n-type and p-type doping for TMDs including MoS2 and WS2 through adjusting the plasma working parameters. In particular, p-type doping of MoS2 can be realized when the radio frequency (RF) power is relatively small and the processing time is short: the off-state current increases from ∼10-10 A to ∼10-8 A, the threshold voltage is positively shifted from -26.2 V to 8.3 V, and the mobility increases from 7.05 cm2 V-1 s-1 to 16.52 cm2 V-1 s-1. Under a relatively large RF power and long processing time, n-type doping was realized for MoS2: the threshold voltage was negatively shifted from 6.8 V to -13.3 V and the mobility is reduced from 10.32 cm2 V-1 s-1 to 3.2 cm2 V-1 s-1. For the former, suitable plasma treatment can promote the substitution of N elements for S vacancies and lead to p-type doping, thus reducing the defect density and increasing the mobility value. For the latter, due to excessive plasma treatment, more S vacancies will be produced, leading to heavier n-type doping as well as a decrease in mobility. We confirm the results by systematically analyzing the optical, compositional, thickness and structural characteristics of the samples before and after such soft plasma treatments via Raman, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Due to its nondestructive and expandable nature and compatibility with the current microelectronics industry, this potentially generic method may be used as a reliable technology for the development of diverse and functional TMD-based devices.

The integration of two-dimensional transition metal dichalcogenide (TMD) layers into van der Waals (vdW) heterostructures offers substantial opportunities for both materials synthesis and device design. Interlayer interactions enable desirable functionalities, and manipulating these interactions is essential for optimizing device performance. In this work, we introduce ozone intercalation into vdW heterostructures and adopt laser irradiation as a manipulation tool, creating a photoluminescence (PL)-based modulation type using interface engineering. Interlayer engineering can be quantitatively achieved by precisely controlling the laser modification time, enabling a controllable PL intensity from no quenching to quenching. The mechanism behind this regulation can be attributed to the interlayer exciton suppression introduced by ozone intercalation being repaired by ozone molecule reduction during the laser treatment. This effective regulatory technique is universal and can be achieved in various type II band-aligned TMD heterostructures, providing an intriguing strategy for the design of two-dimensional vdW TMD device systems.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 have been widely studied due to their unique physical and electronic properties even at one atomic layer thickness. Since the first report on the MoS2 field-effect transistor (FET) device [1], great efforts have been made in the past decade to extend the intriguing features of TMDs to practical applications. However, such integration has been severely bottlenecked by the lack of effective approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.Till now, it is still prevalent to fabricate TMD-based electronic devices using mechanical exfoliation methods. Although single-crystal flakes with least defects can be obtained, the size, thickness and location of the exfoliated TMD flakes are largely uncontrollable, making it unsuitable for large-scale device integration and massive production. So far, various synthetic approaches have been proposed to grow large-area TMD thin films. Chemical vapor deposition (CVD)-based method is one of the major routines to synthesize high-quality monolayer and few-layer TMD flakes [2-4]. For example, in typical CVD processes to grow MoS2, the films can be achieved either by the reduction reaction between the S and MoO3 source powders or by sulfurizing the Mo (or MoOx) films pre-deposited by using physical vapor deposition. However, the nucleation sites in the CVD process are uncontrollable which may lead to the vertical stacking of the MoS2 flakes in triangle shape deteriorating the crystalline properties of the film. On the other hand, the pre-deposited Mo or MoOx film by using e-beam evaporation or sputtering usually suffer from rough surface morphology and unsatisfactory film uniformity which further limit the homogeneity of the large-scale film and device integration.Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a possible route towards the synthesis of TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism [5,6]. As compared to the conventional CVD method, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. In this work, we employed the ALD-based synthesis method to grow wafer-scale MoS2 and WS2 thin films. Non-toxic MoCl5, WCl5 and hexamethyldisilathiane (HMDST) precursors have been used which are different and superior to those used in previous work. Film characterizations have confirmed the wafer-level uniformity, crystallinity and stoichiometry of the synthesized films. Homogeneous electrical behaviors have also been obtained from the fabricated FET device arrays. In addition, optoelectronic devices such as photodetectors and simple logic gates such as n-type inverter arrays have been demonstrated using the wafer-scale MoS2 FET arrays. Furthermore, we have used the substitutional doping in the ALD process to achieve controllable p-type doping of MoS2 films, which can provide solid basis for the complimentary metal-oxide-semiconductor (CMOS) integrated circuit applications. This work shows that the ALD-based synthesis of the 2D TMD thin films is an attractive approach and has provided a promising platform to further push forward the circuit and system level applications of the TMD materials.

Atomic scale details of surface structure play a crucial role for solid-liquid interfaces. While macroscopic characterization techniques provide averaged information about bulk and interfaces, high resolution real space imaging reveals unique insights into the role of defects that are believed to dominate many aspects of surface chemistry and physics. Here, we use high resolution dynamic Atomic Force Microscopy (AFM) to visualize and characterize in ambient water the morphology and atomic scale structure of a variety of nanoparticles of common clay minerals adsorbed to flat solid surfaces. Atomically resolved images of the (001) basal planes are obtained on all materials investigated, namely gibbsite, kaolinite, illite, and Na-montmorillonite of both natural and synthetic origin. Next to regions of perfect crystallinity, we routinely observe extended regions of various types of defects on the surfaces, including vacancies of one or few atoms, vacancy islands, atomic steps, apparently disordered regions, as well as strongly adsorbed seemingly organic and inorganic species. While their exact nature is frequently difficult to identify, our observations clearly highlight the ubiquity of such defects and their relevance for the overall physical and chemical properties of clay nanoparticle-water interfaces.

The fundamental origins of the stability of the (Pd-Ni){sub 80}P{sub 20} bulk metallic glasses (BMGs), a prototype for a whole class of BMG formers, were explored. While much of the properties of their BMGs have been characterized, their glass-stability have not been explained in terms of the atomic and electronic structure. The local structure around all three constituent atoms was obtained, in a complementary way, using extended X-ray absorption fine structure (EXAFS), to probe the nearest neighbor environment of the metals, and extended energy loss fine structure (EXELFS), to investigate the environment around P. The occupied electronic structure was investigated using X-ray photoelectron spectroscopy (XPS). The (Pd-Ni){sub 80}P{sub 20} BMGs receive their stability from cumulative, and interrelated, effects of both atomic and electronic origin. The stability of the (Pd-Ni){sub 80}P{sub 20} BMGs can be explained in terms of the stability of Pd{sub 60}Ni{sub 20}P{sub 20} and Pd{sub 30}Ni{sub 50}P{sub 20}, glasses at the end of BMG formation. The atomic structure in these alloys is very similar to those of the binary phosphide crystals near x=0 and x=80, which are trigonal prisms of Pd or Ni atoms surrounding P atoms. Such structures are known to exist in dense, randomly-packed systems. The structure of the best glass former in this series, Pd{sub 40}Ni{sub 40}P{sub 20} is further described by a weighted average of those of Pd{sub 30}Ni{sub 50}P{sub 20} and Pd{sub 60}Ni{sub 20}P{sub 20}. Bonding states present only in the ternary alloys were found and point to a further stabilization of the system through a negative heat of mixing between Pd and Ni atoms. The Nagel and Tauc criterion, correlating a decrease in the density of states at the Fermi level with an increase in the glass stability, was consistent with greater stability of the Pd{sub x}Ni{sub 80-x}P{sub 20} glasses with respect to the binary alloys of P. A valence electron concentration of 1.8 e/a, which ensures the superpositioning of the first peak in the structure factor with twice the Fermi momentum, was used to calculate the interatomic potential of these alloys. The importance of Pd to the stability of the alloys is evidenced by the fact that replacing Ni and Pd places the nearest neighbor distances at more attractive positions in this potential.