Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

https://espace.library.uq.edu.au/view/UQ:df1181f/UQdf1181f_OA.pdf

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting.

Two-dimensional materials and single-atom catalysts are two frontier research fields in catalysis. A new category of catalysts with the integration of both aspects has been rapidly developed in recent years, and significant advantages were established to make it an independent research field. In this Review, we will focus on the concept of two-dimensional materials confining single atoms for catalysis. The new electronic states via the integration lead to their mutual benefits in activity, that is, two-dimensional materials with unique geometric and electronic structures can modulate the catalytic performance of the confined single atoms, and in other cases the confined single atoms can in turn affect the intrinsic activity of two-dimensional materials. Three typical two-dimensional materials are mainly involved here, i.e., graphene, g-C3N4, and MoS2, and the confined single atoms include both metal and nonmetal atoms. First, we systematically introduce and discuss the classic synthesis methods, advanced ...

Catalysis with Two-Dimensional Materials Confining Single Atoms: Concept, Design, and Applications.

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and −0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk Mo...

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics. Because of their atomically-thin structure and high surface to volume ratio, the interfaces involved in TMDC-based devices play a predominant role in determining the device performance, such as charge injection/collection at the metal/TMDC interface, and charge carrier trapping at the dielectric/TMDC interface. On the other hand, the crystalline structures of TMDCs are enriched by a variety of intrinsic defects, including vacancies, adatoms, grain boundaries, and substitutional impurities. Customized design and engineering of the interfaces and defects provides an effective way to modulate the properties of TMDCs and finally enhance the device performance. Herein, we summarize and highlight recent advances and state-of-the-art investigations on the interface and defect engineering of TMDCs and their corresponding applications in electronic and optoelectronic devices. Various interface engineering approaches for TMDCs are overviewed, including surface charge transfer doping, TMDC/metal contact engineering, and TMDC/dielectric interface engineering. Subsequently, different types of structural defects in TMDCs are introduced. Defect engineering strategies utilized to modulate the optical and electronic properties of TMDCs, as well as the developed high-performance and functional devices are summarized. Finally, we highlight the challenges and opportunities for interface and defect engineering in TMDC materials for electronics and optoelectronics.

Two-dimensional transition metal dichalcogenides: interface and defect engineering

Over the past decade, the field of two-dimensional (2D) layered materials has surged, promising a new platform for studying diverse physical phenomena that are scientifically intriguing and technologically relevant. Contacts are the communication links between these 2D materials and the three-dimensional world for probing and harnessing their exquisite electronic properties. However, fundamental challenges related to contacts often limit the ultimate performance and potential of 2D materials and devices. This article provides a comprehensive overview of the basic understanding and importance of contacts to 2D materials and various strategies for engineering and improving them. In particular, we elucidate the phenomenon of Fermi level pinning at the metal/2D contact interface, the Schottky versus Ohmic nature of the contacts and various contact engineering approaches including interlayer contacts, phase engineered contacts, and basal versus edge plane contacts, among others. Finally, we also discuss some of the relatively under-addressed and unresolved issues, such as contact scaling, and conclude with a future outlook.

https://par.nsf.gov/servlets/purl/10066926

Contact engineering for 2D materials and devices

P-type doping of MoS2 has proved to be a significant bottleneck in the realization of fundamental devices such as p-n junction diodes and p-type transistors due to its intrinsic n-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for p-type doping of MoS2. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS2. A wide range of nondegenerate to degenerate p-type doping is demonstrated in MoS2 field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous p-n junction diodes with a gate-tunable rectification ratio as high as 2 × 104 are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS2 layers shows a shift of 0.67 eV toward lower binding energie...

Few-Layer MoS2 p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation

We demonstrate a low and constant effective Schottky barrier height (ΦB ∼ 40 meV) irrespective of the metal work function by introducing an ultrathin TiO2 ALD interfacial layer between various metals (Ti, Ni, Au, and Pd) and MoS2. Transmission line method devices with and without the contact TiO2 interfacial layer on the same MoS2 flake demonstrate reduced (24×) contact resistance (RC) in the presence of TiO2. The insertion of TiO2 at the source-drain contact interface results in significant improvement in the on-current and field effect mobility (up to 10×). The reduction in RC and ΦB has been explained through interfacial doping of MoS2 and validated by first-principles calculations, which indicate metallic behavior of the TiO2-MoS2 interface. Consistent with DFT results of interfacial doping, X-ray photoelectron spectroscopy (XPS) data also exhibit a 0.5 eV shift toward higher binding energies for Mo 3d and S 2p peaks in the presence of TiO2, indicating Fermi level movement toward the conduction band (...

Interfacial n-Doping Using an Ultrathin TiO2 Layer for Contact Resistance Reduction in MoS2

We report the field effect transistor characteristics of exfoliated transition metal dichalcogenide alloy tungsten sulphoselenide. WSSe is a layered material of strongly bonded S-W-Se atoms having weak interlayer van der Waals forces with a significant potential for spintronic and valleytronic applications due to its polar nature. The X-ray photoelectron spectroscopy measurements on crystals grown by the chemical vapor transport method indicate a stoichiometry of the form WSSe. We report flake thickness tunable transport mechanism with n-type behavior in thin flakes ( ≤11 nm) and ambipolarity in thicker flakes. The devices with flake thicknesses of 2.4 nm–54.8 nm exhibit a maximum electron mobility of ∼50 cm2/V s along with an ION/IOFF ratio >106. The electron Schottky barrier height values of 35 meV and 52 meV extracted from low temperature I–V measurements for 3.9 nm and 25.5 nm thick flakes, respectively, indicate that an increase in hole current with thickness is likely due to lowering of the bandgap ...

Thickness tunable transport in alloyed WSSe field effect transistors

A controllable and area selective doping process, especially for p-type Molybdenum disulphide (MoS 2 ), is essential for the realization of various p/n junction-based devices. In this work, we demonstrate p-type doping of multilayer MoS 2 with phosphorus (P) as a dopant using a CMOS-compatible plasma immersion ion implantation (PIII) technique. Detailed physical characterization including XPS and SIMS, backed with ab-initio DFT calculations, confirms p-type doping in P-implanted MoS 2 that could be due to a combination of surface charge transfer from physisorbed phosphine ions and/or substitutional phosphorus present in the top few (∼5) layers. Controlled reduction in current levels and positive V T shifts were observed in channel-doped MoS 2 transistors. Further, selectively doped gated p/n-junction diodes (rectification ∼50X) have been demonstrated.

P-type doping of MoS 2 with phosphorus using a plasma immersion ion implantation (PIII) process

High contact resistance (R C ) at metal-Molybdenum disulfide (MoS 2 ) interface obscures the intrinsic transport properties of MoS 2 [1] and limits its potential as a channel material for future CMOS technology. In this work, we report reduction in the effective Schottky barrier height (SBH) and contact resistance at the metal-MoS 2 interface using an ultra-thin TiO 2 interfacial layer (IL) resulting in higher (upto 11X) field-effect mobility. Our results show, not only a reduction (upto 10X) but also a nearly constant (∼40 meV) SBH irrespective of the metal work function, unlike Ge/Si where TiO 2 unpins the Fermi-level [2]. XPS and UPS measurements suggest that low RC and constant effective SBH can be attributed to charge transfer at the TiO 2 -MoS 2 interface such that the MoS 2 layer near the interface is doped n-type. Additional improvement in MoS 2 FET performance is demonstrated by using TiO 2 as a dielectric on top of the MoS 2 channel.

Shruti Karande

Papers

Few-Layer MoS2 p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation

Interfacial n-Doping Using an Ultrathin TiO2 Layer for Contact Resistance Reduction in MoS2

Thickness tunable transport in alloyed WSSe field effect transistors

P-type doping of MoS 2 with phosphorus using a plasma immersion ion implantation (PIII) process

Contact resistance reduction in MoS 2 FETs using ultra-thin TiO 2 interfacial layers