Advances in MoS2-Based Field Effect Transistors (FETs)

Biomedical Uses for 2D Materials Beyond Graphene: Current Advances and Challenges Ahead

Currently, a broad interdisciplinary research effort is pursued on biomedical applications of 2D materials (2DMs) beyond graphene, due to their unique physicochemical and electronic properties. The discovery of new 2DMs is driven by the diverse chemical compositions and tuneable characteristics offered. Researchers are increasingly attracted to exploit those as drug delivery systems, highly efficient photothermal modalities, multimodal therapeutics with non-invasive diagnostic capabilities, biosensing, and tissue engineering. A crucial limitation of some of the 2DMs is their moderate colloidal stability in aqueous media. In addition, the lack of suitable functionalisation strategies should encourage the exploration of novel chemical methodologies with that purpose. Moreover, the clinical translation of these emerging materials will require undertaking of fundamental research on biocompatibility, toxicology and biopersistence in the living body as well as in the environment. Here, a thorough account of the biomedical applications using 2DMs explored today is given.

Environmental Effects on Hysteresis of Transfer Characteristics in Molybdenum Disulfide Field-Effect Transistors

Molybdenum disulfide (MoS2) has recently received much attention for nanoscale electronic and photonic applications. To explore the intrinsic properties and enhance the performance of MoS2-based field-effect transistors, thorough understanding of extrinsic effects such as environmental gas and contact resistance of the electrodes is required. Here, we report the effects of environmental gases on the transport properties of back-gated multilayered MoS2 field-effect transistors. Comparisons between different gases (oxygen, nitrogen, and air and nitrogen with varying relative humidities) revealed that water molecules acting as charge-trapping centers are the main cause of hysteresis in the transfer characteristics. While the hysteresis persisted even after pumping out the environmental gas for longer than 10 h at room temperature, it disappeared when the device was cooled to 240 K, suggesting a considerable increase in the time constant of the charge trapping/detrapping at these modestly low temperatures. The suppression of the hysteresis or instability in the easily attainable temperature range without surface passivation is highly advantageous for the device application of this system. The humidity dependence of the threshold voltages in the transfer curves indicates that the water molecules dominantly act as hole-trapping centers. A strong dependence of the on-state current on oxygen pressure was also observed.

Large-Area Deposition of MoS2 by Pulsed Laser Deposition with In Situ Thickness Control

A scalable and catalyst-free method to deposit stoichiometric molybdenum disulfide (MoS2) films over large areas is reported, with the maximum area limited by the size of the substrate holder. The method allows deposition of MoS2 layers on a wide range of substrates without any additional surface preparation, including single-crystal (sapphire and quartz), polycrystalline (HfO2), and amorphous (SiO2) substrates. The films are deposited using carefully designed MoS2 targets fabricated with excess sulfur and variable MoS2 and sulfur particle size. Uniform and layered MoS2 films as thin as two monolayers, with an electrical resistivity of 1.54 × 10(4) Ω cm(-1), were achieved. The MoS2 stoichiometry was confirmed by high-resolution Rutherford backscattering spectrometry. With the method reported here, in situ graded MoS2 films ranging from ∼1 to 10 monolayers can be deposited.

Unusual reactivity of MoS2 nanosheets

The reactivity of the 2D nanosheets of MoS2 with silver ions in solution, leading to their spontaneous morphological and chemical transformations, is reported. This unique reactivity of the nanoscale form of MoS2 was in stark contrast to its bulk counterpart. While the gradual morphological transformation involving several steps has been captured with an electron microscope, precise chemical identification of the species involved was achieved by electron spectroscopy and mass spectrometry. The energetics of the system investigated supports the observed chemical transformation. The reaction with mercury and gold ions shows similar and dissimilar reaction products, respectively and points to the stability of the metal-sulphur bond in determining the chemical compositions of the final products.

A Bi-layer Composite Film Based on TiO2 Hollow Spheres, P25, and Multi-walled Carbon Nanotubes for Efficient Photoanode of Dye-sensitized Solar Cell

ABSTRACT

A bi-layer photoanode for dye-sensitized solar cell (DSSC) was fabricated, in which TiO2 hollow spheres (THSs) were designed as a scattering layer and P25/multi-walled carbon nanotubes (MWNTs) as an under-layer. The THSs were synthesized by a sacrifice template method and showed good light scattering ability as an over-layer of the photoanode. MWNTs were mixed with P25 to form an under-layer of the photoanode to improve the electron transmission ability of the photoanode. The power conversion efficiency of this kind of DSSC with bi-layer was enhanced to 5.13 %, which is 14.25 % higher than that of pure P25 DSSC.

GRAPHICAL ABSTRACT

A bi-layer composite photoanode based on P25/MWNTs-THSs with improved light scattering and electron transmission, which will provide a new insight into fabrication and structure design of highly efficient dye-sensitized solar cells.

Label-free fluorescent catalytic biosensor for highly sensitive and selective detection of the ferrous ion in water samples using a layered molybdenum disulfide nanozyme coupled with an advanced chemometric model

The synergistically enhanced catalytic effect of a Fe²⁺/molybdenum disulfide (MoS₂) nanosheet was exploited to construct a nanozyme biosensor for Fe²⁺.

Tunneling in Systems of Coupled Dopant-Atoms in Silicon Nano-devices

Following the rapid development of the electronics industry and technology, it is expected that future electronic devices will operate based on functional units at the level of electrically active molecules or even atoms. One pathway to observe and characterize such fundamental operation is to focus on identifying isolated or coupled dopants in nanoscale silicon transistors, the building blocks of present electronics. Here, we review some of the recent progress in the research along this direction, with a focus on devices fabricated with simple and CMOS-compatible-processing technology. We present results from a scanning probe method (Kelvin probe force microscopy) which show direct observation of dopant-induced potential modulations. We also discuss tunneling transport behavior based on the analysis of low-temperature I-V characteristics for devices representative for different regimes of doping concentration, i.e., different inter-dopant coupling strengths. This overview outlines the present status of the field, opening also directions toward practical implementation of dopant-atom devices.

Nanometre-scale identification of grain boundaries in MoS2 through molecular decoration

In this paper, we address the challenge of identifying grain boundaries on the molybdenum disulphide (MoS2) surface at the nanometre scale using a simple self-assembled monolayer (SAM) decoration method. Combined with atomic force microscopy, octadecylphosphonic acid monolayers readily reveal grain boundaries in MoS2 at ambient conditions, without the need of atomic resolution measurements under vacuum. Additional ab initio calculations allow us to obtain the preferred orientation of the SAM structure relative to the MoS2 beneath, and therefore, together with the experiments, the MoS2 crystalline orientations at the grain boundaries. The proposed method enables the visualization of grain boundaries with sub-micrometer resolution for nanodevice investigation and failure analysis.

Investigation of Single-Wall MoS2 Monolayer Flakes Grown by Chemical Vapor Deposition

Recently, two-dimensional monolayer molybdenum disulfide (MoS2), a transition metal dichalcogenide, has received considerable attention due to its direct bandgap, which does not exist in its bulk form, enabling applications in optoelectronics and also thanks to its enhanced catalytic activity which allows it to be used for energy harvesting. However, growth of controllable and high-quality monolayers is still a matter of research and the parameters determining growth mechanism are not completely clear. In this work, chemical vapor deposition is utilized to grow monolayer MoS2 flakes while deposition duration and temperature effect have been systematically varied to develop a better understanding of the MoS2 film formation and the influence of these parameters on the quality of the monolayer flakes. Different from previous studies, SEM results show that single-layer MoS2 flakes do not necessarily grow flat on the surface, but rather they can stay erect and inclined at different angles on the surface, indicating possible gas-phase reactions allowing for monolayer film formation. We have also revealed that process duration influences the amount of MoO3/MoO2 within the film network. The homogeneity and the number of layers depend on the change in the desorption-adsorption of radicals together with sulfurization rates, and, inasmuch, a careful optimization of parameters is crucial. Therefore, distinct from the general trend of MoS2 monolayer formation, our films are rough and heterogeneous with monolayer MoS2 nanowalls. Despite this roughness and the heterogeneity, we observe a strong photoluminescence located around 675 nm.