Atomically Thin Optical Lenses and Gratings

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides and phosphorene have emerged as brand new photonic materials with huge potential in a variety of optical applications in recent years, due to their unique properties and multiple functions. Owning to the significantly increased surface to volume ratio in the atomic thin layered structure, the interactions between 2D materials with light become remarkably strong in either elastic or inelastic interactions. Most of previous studies were focused on the inelastic interactions between 2D materials with light, namely Raman and photoluminescence properties. However, we found the elastic interactions between them and light are also very attractive. Based on the properties discovered, we demonstrated several optical applications, such as atomically thin optical lenses and gratings. On the other hand, we found the inelastic interactions with light in the 2D limitation affected a lot by the external environment, which could serve as a unique method to control the working status of the optical devices to meet multiple application needs. Several progresses were demonstrated such as control of excitons and trions in few-layer MoS2 via the co-modulations by both temperature and electric field and the defect engineering by oxygen plasma etching to stimulate new photon emission.

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 have brought a spectacular revolution in fundamental research and industrial applications due to their unique physical properties of atomically thin thickness, strong light-matter interaction, unity valley polarization and enhanced many-body interactions. To fully explore their exotic physical properties and facilitate potential applications in electronics and optoelectronics, an effective and versatile characterization method is highly demanded. Among the many methods of characterization, optical second harmonic generation (SHG) has attracted broad attention because of its sensitivity, versatility and simplicity. The SHG technique is sufficiently sensitive at the atomic scale and therefore suitable for studies on 2D materials. More importantly, it has the capacity to acquire abundant information ranging from crystallographic, and electronic, to magnetic properties in various 2D materials due to its sensitivity to both spatial-inversion symmetry and time-reversal symmetry. These advantages accompanied by its characteristics of non-invasion and high throughput make SHG a powerful tool for 2D materials. This review summarizes recent experimental developments of SHG applications in 2D materials and also provides an outlook of potential prospects based on SHG.

It is highly desirable for exploring and discovering new materials and outcome‐based approaches to exceed the Shockley–Queisser limit for single‐junction photovoltaic cells. Low‐dimensional piezoelectric materials have the potential to generate the optoelectronic phenomenon called the bulk photovoltaic effect, which is not limited by the theoretical limit for solar radiation into electricity conversion. The recent development of 2D materials has demonstrated that by using the bulk photovoltaic effect (BPVE) for crystals lacking inversion symmetry, it is possible to overcome this limit. So far, the exploration of p–n junction designs has been addressed in several review articles. However, the mechanism of BPVE differs from traditional p–n junction‐based photovoltaics in 2D materials. In this focused review, various concepts regarding the shift‐current response are explored, both from theoretical and experimental points of view, which are generated in the framework of deformed 2D materials. Finally, prospective approaches for building BPVE‐based next‐generation solar cells using ultrathin 2D materials are presented. These materials are expected to work better than current methods of turning energy into electricity.

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.

Current state-of-the-art ultraviolet (UV) photodetectors are extensively used in applications ranging from industrial to biomedical science. As a result, there have been consistent efforts to improve their efficiency and to address challenges related to sensitivity and spectral selectivity. Two-dimensional (2D) materials, with their distinct properties, offer a promising solution due to their excellent UV detection characteristics. Additionally, their atomic scalability and CMOS compatibility enable easy integration with flexible platforms and current technologies. The exceptional optoelectronic characteristics, strong light-matter interaction, and quantum confinement in certain 2D materials have led to extensive investigations into 2D material-based UV photodetectors. This review provides a broad overview of the mechanisms of photodetection, various device configurations, and the current progress in both layered and non-layered 2D UV photodetectors. Finally, strategies for improving UV photodetection properties are discussed, along with a future outlook on the integration of 2D UV detectors into commercial platforms.

Two-dimensional (2D) materials with dangling-bond-free surfaces have attracted considerable attention in next-generation electronic applications due to their distinctive properties, such as high carrier mobility, short-channel immunity, and strong light-matter interactions. Recently, various methods have been developed for the preparation of 2D materials, including mechanical exfoliation, chemical vapor deposition, and chemical solution-based processes. Among these, solution processing provides a cost-effective and low-temperature approach for large-scale production. This review highlights recent advancements in solution-processed 2D materials, focusing on preparation methods and their applications in electronic and optoelectronic devices. Additionally, future perspectives are proposed to advance the breakthrough and commercialization of solution technologies for value-added chemical production.

With strong light-matter interaction in their atomically thin layered structures, two-dimensional (2D) materials have been widely investigated for optoelectronic applications such as photodetectors and photovoltaic devices. Depending on the aim of optoelectronic applications, different device structures have been employed. Lateral phototransistor structures have been employed for high optical gain, while vertical photodiode structures have been employed for fast response and low power operation. Herein, we demonstrate a multimodal photodetector platform based on 2D materials, combining both a phototransistor and a photodiode and taking the corresponding desirable characteristics from each structure within a single device. In this platform, a multilayered transition-metal dichalcogenide flake is transferred on top of metal electrodes, and a transparent gate electrode is employed. The channel region of the flake between electrodes operates as a phototransistor providing a high gain mode, while the electrode region in the same flake operates as a vertical Schottky photodiode providing a fast response mode. These modes can be dynamically selected by controlling the drain voltage and gate voltage.

Two-dimensional (2D) materials provide a platform for strong light-matter interactions, creating wide-ranging design opportunities via new-material discoveries and new methods for geometrical structuring. We derive general upper bounds to the strength of such light-matter interactions, given only the optical conductivity of the material, including spatial nonlocality, and otherwise independent of shape and configuration. Our material figure-of-merit shows that highly doped graphene is an optimal material at infrared frequencies, whereas single-atomic-layer silver is optimal in the visible. For quantities ranging from absorption and scattering to near-field spontaneous-emission enhancements and radiative heat transfer, we consider canonical geometrical structures and show that in certain cases the bounds can be approached, while in others there may be significant opportunity for design improvement. The bounds can encourage systematic improvements in the design of ultrathin broadband absorbers, 2D antennas, and near-field energy harvesters.

ABSTRACTThe emergence of 2-dimensional (2D) materials could herald numerous advanced scientific methodologies for both fundamental and applied research. These ultrathin materials can be functionalized and, thus, have the potential to make new devices and sensors that are both highly efficient and sensitive. In addition to being mechanically robust, the 2D materials can be engineered to provide sensor architectures that further increase their inherent high surface area by creating 3D geometries using layer by layer assembly to make stacked devices that could potentially be transparent. The increased sensor surface area would deliver increased signal-to-noise and sensitivity. Here highly sensitive and selective electrochemical detection of bio-analytes using some of engineered 2D materials such as graphene nano-ribbons, fluorinated graphene, and molybdenum disulfide is presented. It is found that surface moieties, defects and surface charges in these ultra-thin layers result in enhanced electron transfer kinetics between the electrodes and biomolecules. This in turn results in an oxidation or reduction of biomolecules with a high peak current, indicating the possible uses of 2D materials for various point-of-care devices. A novel stable 3D electrode geometry has been found to have enhanced heterogeneous electron transfer properties compared to 2D electrodes and provides evidence that electrode geometry and surface area could significantly impact the performance of biosensors.

Strain engineering is a promising method to manipulate the electronic and optical properties of two-dimensional (2D) materials. However, with weak van der Waals interaction, severe slippage between 2D material and substrate could dominate the bending or stretching processes, leading to inefficiency strain transfer. To overcome this limitation, we report a simple strain engineering method by encapsulating the monolayer 2D material in the flexible PVA substrate through spin-coating approach. The strong interaction force between spin-coated PVA and 2D material ensures the mechanical strain can be effectively transferred with negligible slippage or decoupling. By applying uniaxial strain to monolayer MoS2, we observe a higher bandgap modulation up to ~300 meV and a highest modulation rate of ~136 meV/%, which is approximate two times improvement compared to previous results achieved. Moreover, this simple strategy could be well extended to other 2D materials such as WS2 or WSe2, leading to enhanced bandgap modulation.

Racetrack resonator based integrated phase shifters on silicon nitride platform

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are recent members of the 2D materials family with wide tunability, highly dynamic structural features, and excellent physical properties. Ultrathin 2D HOIPs and their heterostructures with other 2D materials have been exploited for study of physical phenomena and device applications. The in-plane mechanical properties of 2D ultrathin HOIPs are critical for understanding the coupling between mechanical and other physical fields and for integrated devices applications. Here we report the in-plane mechanical properties of ultrathin freestanding 2D lead iodide perovskite membranes and their dependence on the membrane thickness. The in-plane Young's moduli of 2D HOIPs are smaller than that of conventional covalently bonded 2D materials. As the thickness increases from monolayer to three-layer, both the Young's modulus and breaking strength decrease, while three-layer and four-layer 2D HOIPs have almost identical in-plane mechanical properties. These thickness-dependent mechanical properties can be attributed to interlayer slippage during deformation. Our results show that ultrathin 2D HOIPs exhibit outstanding breaking strength/Young's modulus ratio compared to many other widely used engineering materials and polymeric flexible substrates, which renders them suitable for application into flexible electronic devices.

The design of spin-orbit torque properties in two-dimensional (2D) materials presents one of the challenges of modern spintronics. In this context, 2D layers involving rare-earth ions − which give rise to robust magnetism, exhibit pronounced orbital polarization of the states, and carry strong spin-orbit interaction—hold particular promise. Here, we investigate ferromagnetic Janus H-phase monolayers of 4f-Eu rare-earth dichalcogenides EuSP, EuSSe, and EuSCl using first-principles calculations. We demonstrate that all compounds exhibit significant spin-orbit torques which originate predominantly in the colossal current-induced orbital response on the Eu f-electrons. Moreover, we demonstrate that the corresponding orbital torques can be used to drive strong in-plane currents of orbital angular momentum with non-trivial direction of orbital polarization, constituting the effect of in-plane orbital pumping. We provide an interpretation of this effect in terms of orbital-to-orbital-curent conversion, and draw a simple qualitative picture of orbital pumping by magnetization dynamics in two dimensional systems. Our findings promote f-orbital-based 2D materials as a promising platform for in-plane orbital pumping and spin-orbit torque applications, and motivate further research on educated design of orbital properties for orbitronics with 2D materials.