Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides.

Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (E) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of E pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure.

NbO, LiNbO, and (Na, K)NbO Thin Films from High-Concentration Aqueous Nb-Polyoxometalates.

Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of NbO, LiNbO, and (Na, K)NbO (KNN) from high-concentration solutions, up to 2.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La Sr MnO Heterostructures.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La Sr MnO (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface.

Enhanced Visible-Light-Driven Hydrogen Production through MOF/MOF Heterojunctions.

A strategy for enhancing the photocatalytic performance of MOF-based systems (MOF: metal-organic framework) is developed through the construction of MOF/MOF heterojunctions. The combination of MIL-167 with MIL-125-NH leads to the formation of MIL-167/MIL-125-NH heterojunctions with improved optoelectronic properties and efficient charge separation. MIL-167/MIL-125-NH outperforms its single components MIL-167 and MIL-125-NH, in terms of photocatalytic H production (455 versus 0.

Surface chemistry of 2-propanol and O mixtures on SnO(110) studied with ambient-pressure x-ray photoelectron spectroscopy.

Tin dioxide (SnO) has various applications due to its unique surface and electronic properties. These properties are strongly influenced by Sn oxidation states and associated defect chemistries. Recently, the oxidation of volatile organic compounds (VOCs) into less harmful molecules has been demonstrated using SnO catalysts.

Impact of Etch Processes on the Chemistry and Surface States of the Topological Insulator BiSe.

The unique properties of topological insulators such as BiSe are intriguing for their potential implementation in novel device architectures for low power and defect-tolerant logic and memory devices. Recent improvements in the synthesis of BiSe have positioned researchers to fabricate new devices to probe the limits of these materials. The fabrication of such devices, of course, requires etching of the topological insulator, in addition to other materials including gate oxides and contacts which may impact the topologically protected surface states.

Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature.

The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer.

High-κ Dielectric on ReS₂: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al₂O₃.

We report an excellent growth behavior of a high-κ dielectric on ReS₂, a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al₂O₃ thin film on the UV-Ozone pretreated surface of ReS₂ yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology.

Dislocation driven spiral and non-spiral growth in layered chalcogenides.

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal.

High-Mobility Helical Tellurium Field-Effect Transistors Enabled by Transfer-Free, Low-Temperature Direct Growth.

The transfer-free direct growth of high-performance materials and devices can enable transformative new technologies. Here, room-temperature field-effect hole mobilities as high as 707 cm V s are reported, achieved using transfer-free, low-temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO /Si. The Te nanostructures exhibit significantly higher device performance than other low-temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high-performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO /Si and HfO .

Fermi Level Manipulation through Native Doping in the Topological Insulator BiSe.

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in BiSe and achieving Fermi levels ( E) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.

Defects and Surface Structural Stability of MoTe Under Vacuum Annealing.

Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe.

Schottky Barrier Height of Pd/MoS Contact by Large Area Photoemission Spectroscopy.

MoS, as a model transition metal dichalcogenide, is viewed as a potential channel material in future nanoelectronic and optoelectronic devices. Minimizing the contact resistance of the metal/MoS junction is critical to realizing the potential of MoS-based devices. In this work, the Schottky barrier height (SBH) and the band structure of high work function Pd metal on MoS have been studied by in situ X-ray photoelectron spectroscopy (XPS).

New Mo Te Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe.

A novel phase transition, from multilayered 2H-MoTe to a parallel bundle of sub-nanometer-diameter metallic Mo Te nanowires (NWs) driven by catalyzer-free thermal-activation (400-500 °C) under vacuum, is demonstrated. The NWs form along the 〈11-20〉 2H-MoTe crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe due to favorable band-alignment.

Electronic properties of MoS/MoO interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS/MoO) using density functional theory (DFT).

Surface Analysis of WSe Crystals: Spatial and Electronic Variability.

Layered semiconductor compounds represent alternative electronic materials beyond graphene. WSe is one of the two-dimensional materials with wide potential in opto- and nanoelectronics and is often used to construct novel three-dimensional architectures with new functionalities. Here, we report the topography and the electronic property of the WSe characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry.

Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure.

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism.

Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures.

Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact.

Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides.

Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%.

Partially Fluorinated Graphene: Structural and Electrical Characterization.

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices.

Near-unity photoluminescence quantum yield in MoS₂.

Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density.

Manganese Doping of Monolayer MoS2: The Substrate Is Critical.

Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate.

Impurities and Electronic Property Variations of Natural MoS2 Crystal Surfaces.

Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS2 crystals. Our findings reveal that the semiconductor 2H-MoS2 exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.