Engineered 2D Materials for Efficient Biosensors

Young's modulus plays a crucial role in determining the suitability of materials for various applications, including two-dimensional (2D) materials like graphene and transition metal dichalcogenides. Traditional indentation methods struggle with ultrathin 2D materials due to substrate effects. To overcome this, we propose using polydimethylsiloxane (PDMS) as a compliant substrate for atomic force microscopy force curves. This method, building on a 1950s analytical model, allows for accurate Young's modulus estimates by measuring flake thickness, applied force, and deformation. The results from our approach aligns well with existing literature for various 2D materials. PDMS, commonly used for mechanical exfoliation and transfer, offers an easily measurable Young's modulus, facilitating more efficient determinations. As the range of ultrathin materials grows, this platform enhances accessibility and efficiency in measuring Young's modulus, significantly contributing to the advancement of applications for these innovative materials.

Two-dimensional (2D) materials are considered for numerous applications in microelectronics, although several challenges remain when integrating them into functional devices. Weak adhesion is one of them, caused by their chemical inertness. Quantifying the adhesion of 2D materials on three-dimensional surfaces is, therefore, an essential step toward reliable 2D device integration. To this end, button shear testing is proposed and demonstrated as a method for evaluating the adhesion of 2D materials with the examples of graphene, hexagonal boron nitride (hBN), molybdenum disulfide, and tungsten diselenide on silicon dioxide and silicon nitride substrates. We propose a fabrication process flow for polymer buttons on the 2D materials and establish suitable button dimensions and testing shear speeds. We show with our quantitative data that low substrate roughness and oxygen plasma treatments on the substrates before 2D material transfer result in higher shear strengths. Thermal annealing increases the adhesion of hBN on silicon dioxide and correlates with the thermal interface resistance between these materials. This establishes button shear testing as a reliable and repeatable method for quantifying the adhesion of 2D materials.

The performance of two-dimensional (2D) materials ispromisingfor electronic, photonic, and sensing devices since they possess largesurface-to-volume ratios, high mechanical strength, and broadbandlight sensitivity. While significant advances have been made in synthesizingand transferring 2D materials onto different substrates, there isstill the need for scalable patterning of 2D materials with nanoscaleprecision. Conventional lithography methods require protective layerssuch as resist or metals that can contaminate or degrade the 2D materialsand deteriorate the final device performance. Current resist-freepatterning methods are limited in throughput and typically requirecustom-made equipment. To address these limitations, we demonstratethe noncontact and resist-free patterning of platinum diselenide (PtSe2), molybdenum disulfide (MoS2), and graphene layerswith nanoscale precision at high processing speed while preservingthe integrity of the surrounding material. We use a commercial, off-the-shelftwo-photon 3D printer to directly write patterns in the 2D materialswith features down to 100 nm at a maximum writing speed of 50 mm/s.We successfully remove a continuous film of 2D material from a 200μm × 200 μm substrate area in less than 3 s. Sincetwo-photon 3D printers are becoming increasingly available in researchlaboratories and industrial facilities, we expect this method to enablefast prototyping of devices based on 2D materials across various researchareas.

Devices based on two-dimensional (2D) materials such as graphene and molybdenum disulfide have shown extraordinary potential in physics, nanotechnology, and electronics. The performances of these applications are heavily affected by defects in utilized materials. Although great efforts have been spent in studying the formation and property of various defects in 2D materials, the long-term evolution of vacancies is still unclear. Here, using a designed program based on the kinetic Monte Carlo method, we systematically investigate the vacancy evolution in monolayer graphene on a long-time and large spatial scale, focusing on the variation of the distribution of different vacancy types. In most cases, the vacancy distribution remains nearly unchanged during the whole evolution, and most of the evolution events are vacancy migrations with a few being coalescences, while it is extremely difficult for multiple vacancies to dissolve. The probabilities of different categories of vacancy evolutions are determined by their reaction rates, which, in turn, depend on corresponding energy barriers. We further study the influences of different factors such as the energy barrier for vacancy migration, coalescence, and dissociation on the evolution, and the coalescence energy barrier is found to be dominant. These findings indicate that vacancies (also subnanopores) in graphene are thermodynamically stable for a long period of time, conducive to subsequent characterizations or applications. Besides, this work provides hints to tune the ultimate vacancy distribution by changing related factors and suggests ways to study the evolution of other defects in various 2D materials.

A perpendicularly magnetized spin injector with a high Curie temperature is a prerequisite for developing spin optoelectronic devices on two-dimensional (2D) materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropy (PMA) on full-coverage monolayer (ML) molybdenum disulfide (MoS2). A large perpendicular interface anisotropy energy of 0.975 mJ/m2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic (FM) metal and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical role in establishing the large PMA. First-principles calculations on a similar Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band structure, from a direct band gap with 3ML-MgO to an indirect band gap with 7 ML-MgO. The proximity effect induced by Fe results in splitting of 10 meV in the valence band at the Γ point for the 3ML-MgO structure, while it is negligible for the 7 ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices based on 2D transition-metal dichalcogenide materials.

The scalable preparation of high‐quality and low‐cost two‐dimensional (2D) materials is critical to achieving their potential applications in various fields. Chemical vapor deposition (CVD) method is considered the most promising method for producing ultrathin 2D materials and has continued to develop in recent years. First‐principles calculations have provided important theoretical guidance for the CVD synthesis of 2D materials, and have played an increasingly important role in the field of material synthesis in recent years. In this review, we present recent advances in the growth mechanism of 2D materials, focusing on the theoretical research progress of four typical 2D materials: graphene, hexagonal boron nitride (hBN), transition metal dichalcogenide (TMDC), and phosphorene. Several aspects of the growth process are discussed in detail, including the decomposition of precursors, nucleation, growth kinetics, domain shape, and epitaxial and alignment of 2D crystals. Based on the understanding of these atomic‐scale growth processes, strategies toward the wafer‐scale growth of continuous and homogeneous 2D thin films are proposed and confirmed by experiments. In the final section, we summarize future challenges and opportunities in the computational studies of the growth mechanism of 2D materials.This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

Two-dimensional (2D) materials hold great promise for future nanoelectronics as conventional semiconductor technologies face serious limitations in performance and power dissipation for future technology nodes. The atomic thinness of 2D materials enables highly scaled field-effect transistors (FETs) with reduced short-channel effects while maintaining high carrier mobility, essential for high-performance, low-voltage device operations. The richness of their electronic band structure opens up the possibility of using these materials in novel electronic and optoelectronic devices. These applications are strongly dependent on the electrical properties of 2D materials-based FETs. Thus, accurate characterization of important properties such as conductivity, carrier density, mobility, contact resistance, interface trap density, etc is vital for progress in the field. However, electrical characterization methods for 2D devices, particularly FET-related measurement techniques, must be revisited since conventional characterization methods for bulk semiconductor materials often fail in the limit of ultrathin 2D materials. In this paper, we review the common electrical characterization techniques for 2D FETs and the related issues arising from adapting the techniques for use on 2D materials.

CONSPECTUS: The ultimate goal of solid state chemistry is to gain a clear correlation between atomic, defect, and electronic structure and intrinsic properties of solid state materials. Solid materials can generally be classified as amorphous, quasicrystalline, and crystalline based on their atomic arrangement, in which crystalline materials can be further divided into single crystals, microcrystals, and nanocrystals. Conventional solid state chemistry mainly focuses on studying single crystals and microcrystals, while recently nanocrystals have become a hot research topic in the field of solid state chemistry. As more and more nanocrystalline materials have been artificially fabricated, the solid state chemistry for studying those nanosolids has become a new subdiscipline: solid state nanochemistry. However, solid state nanochemistry, usually called "nanochemistry" for short, primarily studies the microstructures and macroscopic properties of a nanomaterial's aggregation states. Due to abundant microstructures in the aggregation states, it is only possible to build a simple but imprecise correlation between the microscopic morphology and the macroscopic properties of the nanostructures. Notably, atomically thin two-dimensional inorganic materials provide an ideal platform to establish clear structure-property relationships in the field of solid state nanochemistry, thanks to their homogeneous dispersion without the assistance of a capping ligand. In addition, their atomic structures including coordination number, bond length, and disorder degree of the examined atoms can be clearly disclosed by X-ray absorption fine structure spectroscopy. Also, their more exposed interior atoms would inevitably induce the formation of various defects, which would have a non-negligible effect on their physicochemical properties. Based on the obtained atomic and defect structural characteristics, density-functional calculations are performed to study their electronic structures. Then, after the properties of the individual ultrathin two-dimensional materials or their assembled highly oriented thin film-based nanodevices are measured, the explicit relationship between atomic, defect, and electronic structure and intrinsic properties could be established. In this Account, we focus on our recent advances in the field of solid state nanochemistry, including atomic structure characterization of ultrathin two-dimensional inorganic materials by X-ray absorption fine structure spectroscopy, characterization of their different types of structural defects by positron annihilation spectra and electron spin resonance, and investigation of their electronic structure by density-functional calculations. In addition, we summarize the close correlation between atomic, defect, and electronic structure variations and the optoelectronic, electrical, magnetic, and thermal properties of ultrathin two-dimensional materials. Finally, we also propose the major challenges and opportunities that face solid state nanochemistry. We believe that all the past achievements in ultrathin two-dimensional materials could bring new opportunities for solid state nanochemistry.

Research on two-dimensional (2D) materials, such as graphene and molybdenum disulfide (MoS2), now involves researchers worldwide, implementing cutting edge technology to study them. However, when considering using 2D materials in such promising applications, one of the major concerns is the mechanical failure, which remains heavily underexplored. In this work, we demonstrate the use of sequential and concurrent multiscale modeling to study the effect of various fracture modes on graphene and molybdenum disulfide (MoS2). Some of the fracture modes explored are pure tensile loading, shear loading, and a combination of tensile and shear loading and cyclic loading to investigate fatigue.

Transistors based on two-dimensional (2D) materials have been considered one of the most promising techniques after the silicon-based technique. However, unlike the diversity and popularity of the study on the channel materials for 2D transistors, the research on dielectric materials received insufficient attention in the past decade. So far, the main candidates used as dielectric materials for 2D transistors are limited to native oxides such as SiO2, deposited high-κ oxides such as HfO2 by atomic layer deposition (ALD), and van der Waals crystals such as hexagonal boron nitride (h-BN). However, these materials have their respective problems. The native oxides, SiO2 as the representative, have relatively low dielectric constant and are limited by their parent materials, which can hardly be used for other semiconductors. The mechanism of deposition requires nucleation on the surface of the semiconductors, where the dangling-bond-free high-quality surface of 2D materials does not favor the uniform growth and leads to low-quality film and interface. The van der Waals crystals like h-BN are restricted by their crystal size and the dielectric constants are relatively low. Therefore, new candidates and techniques that can possibly satisfy the ultimate standards for dielectric materials in 2D transistors, i.e., scalable ultrathin high-κ materials with high quality, are urgently needed. It was found that the native oxides of liquid metals, such as gallium oxide (Ga2O3) from liquid gallium, could be prepared as ultrathin free-standing materials for applications. These ultrathin free-standing oxides might have potentials properties that meet the requirements as dielectric materials in 2D transistors. In this thesis, the potential properties of the free-standing Ga2O3 and its integration for high-performance 2D transistors are demonstrated. First, the morphology and dielectric properties, such as dielectric constant, breakdown field, optical band gap, etc. are characterized. Ultrathin thickness of less than 3 nm and an ultra-flat surface are measured. A high dielectric constant about 30 and high breakdown field about 11 MV/cm are extracted. A low equivalent oxide thickness (EOT) about 0.4 nm is calculated. These properties therefore indicate the free-standing Ga2O3 as an ultrathin and uniform high-κ dielectric material, which meets the requirements for high-performance 2D transistors. A surface-tension-driven method of wafer-scale preparation is introduced, providing the scalability for device fabrication. Second, the free-standing Ga2O3 is integrated in 2D MoS2 field-effect transistors (FETs) as dielectric layer. Subthreshold swing down to 60 mV/dec is obtained, close to the thermal limit at room temperature. Low leakage current density about 4×10–7 A cm–2 and low density of interfacial state about 4×1011 eV–1 cm–2 are extracted. Immunity to the short-channel effect is revealed with short-channel FETs with channel lengths down to about 28 nm. Scalable device fabrication is demonstrated with FET arrays and logic gates, including NOT, NAND, NOR, AND, XOR. The high performance is attributed to the outstanding dielectric properties of the Ga2O3 and the van der Waals integration. Third, the passivation ability of the ultrathin Ga2O3 as an encapsulation layer is evaluated with black phosphorus (BP) and 1T' MoTe2, which are representatives of air-sensitive 2D materials. Dramatic increases in retention time of both optical and electrical properties are obtained, due to the self-limiting oxidation that forms dense native oxide against O2, H2O, etc. in an ambient environment. Owing to the facile selective etching of the ultrathin Ga2O3, direct device fabrication without removal of the encapsulation layer. Dual-gate FETs can be directly fabricated with encapsulated BP, using the Ga2O3 simultaneously as encapsulation and dielectric material. This dissertation, therefore, exhibits that the free-standing native oxide from liquid metal has desired properties as dielectric material and encapsulation for the fabrication of high-performance 2D transistors.

In this chapter, recent research progress in engineered 2D material electrocatalysts for improved OER performance is summarized. Initially, we have outlined the sequential events that took place in an alkaline and an acidic medium in order to provide knowledge about the evolution of oxygen. The key points related to these mechanisms were also addressed. Next, various material designing strategies for improving OER performance, including defects, doping, alloying, strain, edge engineering, morphological tuning, and heterostructure generation used for enhancing OER performance are described. These approaches will contribute to increasing the availability of active sites and their exposure to catalytic reactions, which will help to improve the performance of 2D materials. Finally, we highlight a number of challenges and possibilities that may present design opportunities for 2D material electrocatalysts of the future.

This chapter discusses giant optical path length (OPL) from thin-layer molybdenum disulphide originated from their high refractive indices. Refractive optical components rely on the OPL to modify the phase front of an optical beam. It shows that high-index two-dimensional (2D) materials have extraordinary elastic interactions with light, enabled uniquely by the combination of their ultra-thin nature and high refractive indices. 2D layered materials have emerged as promising candidates for miniaturisation of optoelectronic devices, such as photo-detectors, light emitters, absorbers, edge non-linear optics and valleytronic devices, due to their strong inelastic interactions with light. The giant scattering efficiency in high-index 2D materials makes it possible to achieve sophisticated light manipulation based on collective scattering by nanostructured patterns. 2D optical components represents a significant advantage in manufacturing compared to conventional 3D optical components, because different functionalities can all be achieved in a 2D platform sharing the same fabrication processes and this will greatly facilitate large-scale manufacturing and integration.

Two-dimensional (2D) materials are promising candidates for future electronics due to their excellent electrical and photonic properties. Although promising results on the wafer-scale synthesis (≤150 mm diameter) of monolayer molybdenum disulfide (MoS2) have already been reported, the high-quality synthesis of 2D materials on wafers of 200 mm or larger, which are typically used in commercial silicon foundries, remains difficult. The back-end-of-line (BEOL) integration of directly grown 2D materials on silicon complementary metal-oxide-semiconductor (CMOS) circuits is also unavailable due to the high thermal budget required, which far exceeds the limits of silicon BEOL integration (<400 °C). This high temperature forces the use of challenging transfer processes, which tend to introduce defects and contamination to both the 2D materials and the BEOL circuits. Here we report a low-thermal-budget synthesis method (growth temperature < 300 °C, growth time ≤ 60 min) for monolayer MoS2 films, which enables the 2D material to be synthesized at a temperature below the precursor decomposition temperature and grown directly on silicon CMOS circuits without requiring any transfer process. We designed a metal-organic chemical vapour deposition reactor to separate the low-temperature growth region from the high-temperature chalcogenide-precursor-decomposition region. We obtain monolayer MoS2 with electrical uniformity on 200 mm wafers, as well as a high material quality with an electron mobility of ~35.9 cm2 V-1 s-1. Finally, we demonstrate a silicon-CMOS-compatible BEOL fabrication process flow for MoS2 transistors; the performance of these silicon devices shows negligible degradation (current variation < 0.5%, threshold voltage shift < 20 mV). We believe that this is an important step towards monolithic 3D integration for future electronics.

The formation and control of a room-temperature magnetic order in two-dimensional (2D) materials is a challenging quest for the advent of innovative magnetic- and spintronic-based technologies. To date, edge magnetism in 2D materials has been experimentally observed in hydrogen (H)-terminated graphene nanoribbons (GNRs) and graphene nanomeshes (GNMs), but the measured magnetization remains far too small to allow envisioning practical applications. Herein, we report experimental evidences of large room-temperature edge ferromagnetism (FM) obtained from oxygen (O)-terminated zigzag pore edges of few-layer black phosphorus (P) nanomeshes (BPNMs). The magnetization values per unit area are ~100 times larger than those reported for H-terminated GNMs, while the magnetism is absent for H-terminated BPNMs. The magnetization measurements and the first-principles simulations suggest that the origin of such a magnetic order could stem from ferromagnetic spin coupling between edge P with O atoms, resulting in a strong spin localization at the edge valence band, and from uniform oxidation of full pore edges over a large area and interlayer spin interaction. Our findings pave the way for realizing high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements.

We propose surface plasmon resonance biosensors based on crumpled graphene and molybdenum disulphide (MoS2) flakes supported on stretchable polydimethylsiloxane (PDMS) or silicon substrates. Accumulation of specific biomarkers resulting in measurable shifts in the resonance wavelength of the plasmon modes of two-dimensional (2D) material structures, with crumpled structures demonstrating large refractive index shifts. Using theoretical calculations based on the semiclassical Drude model, combined with the finite element method, we demonstrate that the interaction between the surface plasmons of crumpled graphene/MoS2 layers and the surrounding analyte results in high sensitivity to biomarker driven refractive index shifts, up to 7499 nm/RIU for structures supported on silicon substrates. We can achieve a high figure of merit (FOM), defined as the ratio of the refractive index sensitivity to the full width at half maximum of the resonant peak, of approximately 62.5 RIU-1. Furthermore, the sensing properties of the device can be tuned by varying crumple period and aspect ratio through simple stretching and integrating material interlayers. By stacking multiple 2D materials in heterostructures supported on the PDMS layer, we produced hybrid plasmon resonances detuned from the PDMS absorbance region allowing higher sensitivity and FOM compared to pure crumpled graphene structures on the PDMS substrates. The high sensitivity and broad mechanical tunability of these crumpled 2D material biosensors considerable advantages over traditional refractive index sensors, providing a new platform for ultrasensitive biosensing.