Electrical and geometrical tuning of MoS2 field effect transistors via direct nanopatterning

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

MoS2 Defect Healing for High-Performance Chemical Sensing of Polycyclic Aromatic Hydrocarbons

The increasing population and industrial development are responsible for environmental pollution. Among toxic chemicals, polycyclic aromatic hydrocarbons (PAHs) are highly carcinogenic contaminants resulting from the incomplete combustion of organic materials. Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), are ideal sensory scaffolds, combining high surface-to-volume ratio with physical and chemical properties that are strongly susceptible to environmental changes. TMDCs can be integrated in field-effect transistors (FETs), which can operate as high-performance chemical detectors of (non)covalent interaction with small molecules. Here, we have developed MoS₂-based FETs as platforms for PAHs sensing, relying on the affinity of the planar polyaromatic molecules for the basal plane of MoS₂ and the structural defects in its lattice. X-ray photoelectron spectroscopy analysis, photoluminescence measurements, and transfer characteristics showed a notable reduction in the defectiveness of MoS₂ and a p-type doping upon exposure to PAHs solutions, with a magnitude determined by the correlation between the ionization energies (E_I) of the PAH and that of MoS₂. Naphthalene, endowed with the higher E_I among the studied PAHs, exhibited the highest output. We observed a log-log correlation between MoS₂ doping and naphthalene concentration in water in a wide range (10^-9-10^-6 M), as well as a reversible response to the analyte. Naphthalene concentrations as low as 0.128 ppb were detected, being below the limits imposed by health regulations for drinking water. Furthermore, our MoS₂ devices can reversibly detect vapors of naphthalene with both an electrical and optical readout, confirming that our architecture could operate as a dual sensing platform.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg²⁺) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS₂-based field-effect transistors (FETs) and exploited them as platforms for Hg²⁺ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS₂ lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS₂ when exposed to Hg²⁺ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg²⁺ exposure. Transfer characteristics in MoS₂ FETs provided further evidence that Hg²⁺ acts as a p-dopant of MoS₂. Interestingly, we observed a strict correlation of doping with the concentration of Hg²⁺, following a semi-log trend. Hg²⁺ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg²⁺ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Stable and high-performance piezoelectric sensor via CVD grown WS2

Piezoelectric materials are widely used as electromechanical couples for a variety of sensors and actuators in nanoscale electronic devices. The majority of piezoelectric devices display lateral patterning of counter electrodes beside active materials such as two-dimensional transition metal dichalcogenides (2D TMDs). As a result, their piezoelectric output response is strongly dependent on the lattice orientation of the 2D TMD crystal structure, limiting their piezoelectric properties. To overcome this issue, we fabricated a vertical sandwich design of a piezoelectric sensor with a conformal contact to enhance the overall piezoelectric performance. In addition, we enhanced the piezoelectric properties of 2D WS₂ by carrying out a unique solvent-vapor annealing process to produce a sulfur-deficient WS_2(1-x) structure that yielded a 3-fold higher piezoelectric response voltage (96.74 mV) than did pristine WS₂ to a 3 kPa compression. Our device was also found to be stable: it retained its piezoelectric performance even after a month in an ambient atmospheric condition. Our study has revealed a facile methodology for fabricating large-scale piezoelectric devices using an asymmetrically engineered 2D WS₂ structure.

High-Resolution Three-Dimensional Sculpting of Two-Dimensional Graphene Oxide by E-Beam Direct Write

On-demand switchable "additive/subtractive" patterning of two-dimensional (2D) nanomaterials is an essential capability for developing new concepts of functional nanomaterials and their device realizations. Traditionally, this is performed via a multistep process using photoresist coating and patterning by conventional photo or electron beam lithography, which is followed by bulk dry/wet etching or deposition. This limits the range of functionalities and structural topologies that can be achieved as well as increases the complexity, cost, and possibility of contamination, which are significant barriers to device fabrication from highly sensitive 2D materials. Focused electron beam-induced processing (FEBIP) enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for "direct-write", single-step surface patterning of 2D nanomaterials with an in situ imaging capability. It allows for realizing a rapid multiscale/multimode approach, ranging from an atomic scale manipulation (e.g., via targeted defect introduction as an active site) to a large-area surface modification on nano- and microscales, including patterned doping and material removal/deposition with 2D (in-plane)/three-dimensional (3D) (out-of-plane) control. In this work, we report on a new capability of FEBIP for nanoscale patterning of graphene oxide via removal of oxygenated carbon moieties with no use of reactive gas required for etching complemented by carbon atom deposition using a focused electron beam. The mechanism of experimentally observed phenomena is explored using the density functional theory (DFT) calculations, revealing that interactions of e-beam that liberated reactive oxygen radicals with carbon atoms on the graphene basal plane lead to the creation of atomic vacancies in the material. The reaction byproducts are volatile carbon dioxides, which are dissociated and volatilized from the graphene oxide surface functional groups by interactions with an energetic focused electron beam. Along with selective subtractive patterning of graphene oxide, the same electron beam with increased irradiation doses can deposit out-of-plane 3D carbon nanostructures on top of or around the 2D etched pattern, thus forming a hybrid 2D/3D nanocomposite with a feature control down to a few nanometers. This in operando dual nanofabrication capability of FEBIP is unmatched by any other nanopatterning techniques and opens a new design window for forming 2D/3D complex nanostructures and functional nanodevices.

Electronic properties and low lattice thermal conductivity (κl) of mono-layer (ML) MoS2: FP-LAPW incorporated with spin–orbit coupling (SOC)

This paper focuses on the electronic and thermoelectric properties of monolayer MoS₂.