Optical Properties Of 2D Materials Research Articles

Spatially selective p-type doping for constructing lateral WS2 p-n homojunction via low-energy nitrogen ion implantation

The construction of lateral p-n junctions is very important and challenging in two-dimensional (2D) semiconductor manufacturing process. Previous researches have demonstrated that vertical p-n junction can be prepared simply by vertical stacking of 2D materials. However, interface pollution and large area scalability are challenges that are difficult to overcome with vertical stacking technology. Constructing 2D lateral p-n homojunction is an effective strategy to address these issues. Spatially selective p-type doping of 2D semiconductors is expected to construct lateral p-n homojunction. In this work, we have developed a low-energy ion implantation system that reduces the implanted energy to 300 eV. Low-energy implantation can form a shallow implantation depth, which is more suitable for modulating the electrical and optical properties of 2D materials. Hence, we utilize low-energy ion implantation to directly dope nitrogen ions into few-layer WS2 and successfully realize a precise regulation for WS2 with its conductivity type transforming from n-type to bipolar or even p-type conduction. Furthermore, the universality of this method is demonstrated by extending it to other 2D semiconductors, including WSe2, SnS2 and MoS2. Based on this method, a lateral WS2 p-n homojunction is fabricated, which exhibits significant rectification characteristics. A photodetector based on p-n junction with photovoltaic effect is also prepared, and the open circuit voltage can reach to 0.39 V. This work provides an effective way for controllable doping of 2D semiconductors.

Elucidating Solvatochromic Shifts in Two-Dimensional Photocatalysts by Solving the Bethe-Salpeter Equation Coupled with Implicit Solvation Method.

Many studies have focused on tailoring the photophysical properties of two-dimensional (2D) materials for photocatalytic (PC) or photoelectrochemical (PEC) applications. To understand the optical properties of 2D materials in solution, we established a computational method that combined the Bethe-Salpeter equation (BSE) calculations with our GW-GPE method, allowing for GW/BSE-level calculations with implicit solvation described using the generalized Poisson equation (GPE). We applied this method to MoS2, phosphorene (PP), and g-C3N4 and found that when the solvent dielectric increased, it reduced the exciton binding energy and quasiparticle bandgap, resulting in almost no solvatochromic shift in the excitonic peaks of MoS2 and PP, which is consistent with previous experiments. However, our calculations predicted that the solvent dielectric had a significant impact on the excitonic properties of g-C3N4, exhibiting a large solvatochromic shift. We expect that our GW/BSE-GPE method will offer insights into the design of 2D materials for PC and PEC applications.

Nonlinear Optical Properties of 2D Materials and their Applications.

2D materials are a subject of intense research in recent years owing to their exclusive photoelectric properties. With giant nonlinear susceptibility and perfect phase matching, 2D materials have marvelous nonlinear light-matter interactions. The nonlinear optical properties of 2D materials are of great significance to the design and analysis of applied materials and functional devices. Here, the fundamental of nonlinear optics (NLO) for 2D materials is introduced, and the methods for characterizing and measuring second-order and third-order nonlinear susceptibility of 2D materials are reviewed. Furthermore, the theoretical and experimental values of second-order susceptibility χ(2) and third-order susceptibility χ(3) are tabulated. Several applications and possible future research directions of second-harmonic generation (SHG) and third-harmonic generation (THG) for 2D materials are presented.

Strain engineering of the mechanical properties of two-dimensional WS2.

Tuning the physical properties of two-dimensional (2D) materials is crucial for their successful integration into advanced applications. While strain engineering demonstrated an efficient means to modulate the electrical and optical properties of 2D materials, tuning their mechanical properties has not been carried out. Here we applied compressive strain through the buckling metrology to 2D tungsten disulfide (WS2), which demonstrated mechanical softening manifested by the reduction of its effective Young's modulus. Raman modes analysis of the strained WS2 also showed strain-dependent vibrational modes softening and revealed its Grüneisen parameter (γ E2g = 0.29) and its shear deformation potential (β E2g = 0.56) - both are similar to the values of other 2D materials. In parallel, we conducted a molecular dynamic simulation that confirmed the validity of continuum mechanics modeling in the nanoscale and revealed that due to sequential atomic-scale buckling events in compressed WS2, it shows a mechanical softening. Therefore, by tuning the mechanical properties of WS2 we shed light on its fundamental physics, thus making it an attractive candidate material for high-end applications, such as tunable sensors and flexible optoelectronic devices.

Latest Innovations in 2D Flexible Nanoelectronics.

2D materials with dangling-bond-free surfaces and atomically thin layers have been shown to be capable of being incorporated into flexible electronic devices. The electronic and optical properties of 2D materials can be tuned or controlled in other ways by using the intriguing strain engineering method. The latest and encouraging techniques in regard to creating flexible 2D nanoelectronics are condensed in this review. These techniques have the potential to be used in a wider range of applications in the near and long term. It is possible to use ultrathin 2D materials (graphene, BP, WTe2 , VSe2 etc.) and 2D transition metal dichalcogenides (2D TMDs) in order to enable the electrical behavior of the devices to be studied. A category of materials is produced on smaller scales by exfoliating bulk materials, whereas chemical vapor deposition (CVD) and epitaxial growth are employed on larger scales. This overview highlights two distinct requirements, which include from a single semiconductor or with van der Waals heterostructures of various nanomaterials. They include where strain must be avoided and where it is required, such as solutions to produce strain-insensitive devices, and such as pressure-sensitive outcomes, respectively. Finally, points-of-view about the current difficulties and possibilities in regard to using 2D materials in flexible electronics are provided.

Hybrid Metasurfaces of Plasmonic Lattices and 2D Materials

AbstractPlasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high‐quality fabrication, all‐dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light‐matter interactions and contribute to the revolutions on nanophotonic devices.

The effect of different strain on the structural and optical properties of multilayer γ-InSe

Strain engineering is one promising approach to tune electronic and optical properties of 2D materials. But which type mechanical strain is most efficient is unclear. Here we systematically investigate and compare the tuning effect from four different type strains for multilayer γ-InSe, including in-plane uniaxial tensile strain, in-plane biaxial tensile strain, hydrostatic compressive strain and out-of-plane axial compressive strain. The evolution of its phonon modes and luminescent properties upon strain were investigated using in situ Raman and photoluminescence (PL) spectroscopy. All phonon modes exhibit blue shift upon compression strain, while red shift upon tensile strain. Band gap reduction was observed for both compression strain and tensile strain. Strain type has great effect on the shift rate of Raman mode and band gap. Among four types strain, biaxial tensile strain exhibits the largest shift rates, being 8.49, 9.24 and 8.65 cm-1/% for three phonon modes and 85 meV/% for band gap. It suggests that biaxial tensile strain is more efficient for γ-InSe. This result is helpful for practical strain-based manipulation for 2D materials.

Advances in the Field of Two-Dimensional Crystal-Based Photodetectors.

Two-dimensional (2D) materials have sparked intense interest among the scientific community owing to their extraordinary mechanical, optical, electronic, and thermal properties. In particular, the outstanding electronic and optical properties of 2D materials make them show great application potential in high-performance photodetectors (PDs), which can be applied in many fields such as high-frequency communication, novel biomedical imaging, national security, and so on. Here, the recent research progress of PDs based on 2D materials including graphene, transition metal carbides, transition-metal dichalcogenides, black phosphorus, and hexagonal boron nitride is comprehensively and systematically reviewed. First, the primary detection mechanism of 2D material-based PDs is introduced. Second, the structure and optical properties of 2D materials, as well as their applications in PDs, are heavily discussed. Finally, the opportunities and challenges of 2D material-based PDs are summarized and prospected. This review will provide a reference for the further application of 2D crystal-based PDs.

HBN Encapsulation Effects on the Phonon Modes of MoS2 with a Thickness of 1 to 10 Layers

AbstractInterfaces with surrounding materials, where charged impurities and surface roughness are present, have a significant impact on the electrical and optical properties of 2D materials. In the change of the phonon modes of MoS2 accompanied by thickness variation, the portion caused by intrinsic factors and the portion caused by the interface effect are separated by examining the result of encapsulation with hexagonal boron nitride (hBN). For instance, the frequency of the A1g peak of MoS2 supported by SiO2 decreases by ≈4 cm−1 in air for a thickness reduction from ten layers to monolayer. Of this decrease, roughly 2 cm−1 is attributable to the weakening of the van der Waals interlayer interaction, while the remaining 2 cm−1 is due to the interface effect. The interface state, that is, the types and concentrations of impurities at the interface, between MoS2 and SiO2 is estimated to be similar to that between MoS2 and air because the Raman properties when one surface of MoS2 is in contact with SiO2 and with air are identical within the measurement error. When entirely encapsulated with hBN, the width of the A1g peak of few‐layer MoS2 is significantly reduced, becoming comparable or equal to that of bulk MoS2.

Emerging Trends in 2D Flexible Electronics

AbstractPotential for integrated flexible electronics has been shown for 2D materials with atomically thin layers and dangling‐bond‐free surfaces. Strain engineering is a fascinating technique for tuning or controlling the electronic and optical properties of 2D materials. In the review article, an effort to condense the most recent and promising approaches that can be used to create flexible nanoelectronic and optoelectronic devices in the future and expand their range of useful applications is made. In order to investigate their electrical behavior, the majority of the devices are created using chemical vapor deposition (CVD) or mechanical exfoliation of ultrathin 2D TMD materials. This makes it possible to develop flexible piezo‐phototronic photodetectors, self‐powered sensors, and wearable and implantable devices with higher strain tolerance. On the basis of 2D TMDs materials, a comparison of the performance and characteristics of 2D flexible electronic devices is also carried out. In order to advance the development of wearable and implantable electronics with greater strain tolerance for health monitoring and usher in a new era of flexible technologies, this overview of recent research on 2D flexible electronics based on nanomaterials is intended to aid in the advancement of the field. Finally, it summarizes the current challenges and opportunities.

Recent Progress in Strain Engineering on Van der Waals 2D Materials: Tunable Electrical, Electrochemical, Magnetic, and Optical Properties.

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2Dmaterials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

Built-in tensile strain dependence on the lateral size of monolayer MoS2 synthesized by liquid precursor chemical vapor deposition.

Strain engineering is an efficient tool to tune and tailor the electrical and optical properties of 2D materials. The built-in strain can be tuned during the synthesis process of a two-dimensional semiconductor, such as molybdenum disulfide, by employing different growth substrates with peculiar thermal properties. In this work, we demonstrate that the built-in strain of MoS2 monolayers, grown on a SiO2/Si substrate by liquid precursor chemical vapor deposition, is mainly dependent on the size of the monolayer. In fact, we identify a critical size equal to 20 μm, from which the built-in strain increases drastically. The built-in strain is the maximum for a 60 μm sized monolayer, leading to 1.2% tensile strain with a partial release of strain close to the monolayer triangular vertexes due to the formation of nanocracks. These findings also imply that the standard method for evaluation of the number of layers based on the Raman mode separation can become unreliable for highly strained monolayers with a lateral size above 20 μm.

Two-dimensional optoelectronic devices for silicon photonic integration

With the unprecedented increasing demand for extremely fast processing speed and huge data capacity, traditional silicon-based information technology is becoming saturated due to the encountered bottlenecks of Moore's Law. New material systems and new device architectures are considered promising strategies for this challenge. Two-dimensional (2D) materials are layered materials and garnered persistent attention in recent years owing to their advantages in ultrathin body, strong light-matter interaction, flexible integration, and ultrabroad operation wavelength range. To this end, the integration of 2D materials into silicon-based platforms opens a new path for silicon photonic integration. In this work, a comprehensive review is given of the recent signs of progress related to 2D material integrated optoelectronic devices and their potential applications in silicon photonics. Firstly, the basic optical properties of 2D materials and heterostructures are summarized in the first part. Then, the state-of-the-art three typical 2D optoelectronic devices for silicon photonic applications are reviewed in detail. Finally, the perspective and challenges for the aim of 3D monolithic heterogeneous integration of these 2D optoelectronic devices are discussed.

Tuning the Optical Properties of 2D Materials with Defects and Strain

Tuning the Optical Properties of 2D Materials with Defects and Strain

Quasiparticle, optical, and excitonic properties of layer dependent GaSe

The two-dimensional (2D) optical absorbance and complex optical conductivity can well define the optical properties of 2D materials and they are fundamental for the design and manufacture of optoelectronic devices. In this work, quasiparticle electronic structure, optical absorbance and complex optical conductivity of 1-layer to 3-layer GaSe are studied by the self-energy GW0 method and the Bethe–Salpeter equation (BSE). Relative to bulk GaSe with a direct bandgap, our calculation results show that three kinds of two-dimensional GaSe are all quasi-direct band gap semiconductors. Due to the exciton effect, their optical absorbance spectrums are red-shifted, and the absorption intensity increases rapidly near the band gap. The exciton binding energies of 1-layer to 3-layer GaSe are increased significantly with respect to bulk GaSe.

The Influence of Ionic Liquids Adsorption on the Electronic and Optical Properties of Phosphorene and Arsenene with Different Phases: A Computational Study.

Density functional theory (DFT) calculations have been performed to investigate the interfacial interactions of ionic liquids (ILs) on the α- and β-phases of phosphorene (P) and arsenene (As). Nine representative ILs based on the combinations of 1-ethyl-3-methylimidazolium ([EMIM]+), N-methylpyridinium ([MPI]+), and trimethylamine ([TMA]+) cations paired to tetrafluoroborate ([BF4]−), trifluoromethanesulfonate ([TFO]−), and chloridion (Cl−) anions were used as adsorbates on the 2D P and As nanosheets with different phases to explore the effect of IL adsorption on the electronic and optical properties of 2D materials. The calculated structure, adsorption energy, and charge transfer suggest that the interaction between ILs and P and As nanosheets is dominated by noncovalent forces, and the most stable adsorption structures are characterized by the simultaneous interaction of the cation and anion with the surface, irrespective of the types of ILs and surfaces. Furthermore, the IL adsorption leads to the larger change in the electronic properties of β-phase P and As than those of their α-phase counterparts, which demonstrates that the adsorption properties are not only related to the chemical elements, but also closely related to the phase structures. The present results provide insight into the further applications of ILs and phosphorene (arsenene) hybrid materials.

Tunable Electronic Properties of Few-Layer Tellurene under In-Plane and Out-of-Plane Uniaxial Strain.

Strain engineering is a promising and fascinating approach to tailoring the electrical and optical properties of 2D materials, which is of great importance for fabricating excellent nano-devices. Although previous theoretical works have proved that the monolayer tellurene has desirable mechanical properties with the capability of withstanding large deformation and the tunable band gap and mobility conductance induced by in-plane strain, the effects of in-plane and out-of-plane strains on the properties of few-layer tellurene in different phases should be explored deeply. In this paper, calculations based on first-principles density functional theory were performed to predict the variation in crystal structures and electronic properties of few-layer tellurene, including the α and β phases. The analyses of mechanical properties show that few-layer α-Te can be more easily deformed in the armchair direction than β-Te owing to its lower Young’s modulus and Poisson’s ratio. The α-Te can be converted to β-Te by in-plane compressive strain. The variations in band structures indicate that the uniaxial strain can tune the band structures and even induce the semiconductor-to-metal transition in both few-layer α-Te and β-Te. Moreover, the compressive strain in the zigzag direction is the most feasible scheme due to the lower transition strain. In addition, few-layer β-Te is more easily converted to metal especially for the thicker flakes considering its smaller band gap. Hence, the strain-induced tunable electronic properties and semiconductor-to-metal transition of tellurene provide a theoretical foundation for fabricating metal–semiconductor junctions and corresponding nano-devices.

Two-dimensional Material based Printed Photonics: A Review

Abstract Functional inks based on two-dimensional (2D) materials have potential application in building new and commercially viable photonic devices via different printing techniques. Printed photonics using 2D material-based inks brings together the unique optical properties of 2D materials via different printing techniques for fabricating photonic devices that can revolutionize various sectors in telecommunication, information technology, sensors and computing. Understanding the need for a comprehensive guide for researchers in the field of printed photonics based on 2D material inks, we have brought together the essential concepts in this field. The review begins with discussion on optical properties of commonly used 2D materials in photonic applications. Since different printing techniques demand different ink rheological properties for efficient printing, we have discussed it for different printing techniques. The substrates compatible for printed photonics application are also listed. Mechanisms of common printing methods in device fabrication are explained with more focus on the most commonly demonstrated method in printed photonics, i.e., inkjet printing. We have discussed a few examples of photonic devices where printed photonics is already demonstrated with 2D material functional inks. Finally, our perspective on future prospects of 2D materials, with excellent optical properties, that are yet to be formulated into inks for new and efficient photonic device fabrication as well as devices with potentially new functionalities are listed.

All-Optical Modulation Technology Based on 2D Layered Materials.

In the advancement of photonics technologies, all-optical systems are highly demanded in ultrafast photonics, signal processing, optical sensing and optical communication systems. All-optical devices are the core elements to realize the next generation of photonics integration system and optical interconnection. Thus, the exploration of new optoelectronics materials that exhibit different optical properties is a highlighted research direction. The emerging two-dimensional (2D) materials such as graphene, black phosphorus (BP), transition metal dichalcogenides (TMDs) and MXene have proved great potential in the evolution of photonics technologies. The optical properties of 2D materials comprising the energy bandgap, third-order nonlinearity, nonlinear absorption and thermo-optics coefficient can be tailored for different optical applications. Over the past decade, the explorations of 2D materials in photonics applications have extended to all-optical modulators, all-optical switches, an all-optical wavelength converter, covering the visible, near-infrared and Terahertz wavelength range. Herein, we review different types of 2D materials, their fabrication processes and optical properties. In addition, we also summarize the recent advances of all-optical modulation based on 2D materials. Finally, we conclude on the perspectives on and challenges of the future development of the 2D material-based all-optical devices.

Interface modulation and physical properties of heterostructure of metal nanoparticles and two-dimensional materials

<sec>Two-dimensional (2D) material has atomic smooth surface, nano-scale thickness and ultra-high specific surface area, which is an important platform for studying the interface interaction between metal nanoparticles (NPs) and 2D materials, and also for observing the surface atomic migration, structural evolution and aggregation of metal NPs in real time and <i>in situ</i>. By rationally designing and constructing the interfaces of metal NPs and 2D materials, the characterization of the interface structure on an atomic scale is very important in revealing the structure-property relationship. It is expected that the investigation is helpful in understanding the mechanism of interaction between metal and 2D materials and optimizing the performance of the devices based on metal-2D material heterojunctions.</sec><sec>In this review, the recent progress of interface modulation and physical properties of the heterostructure of metal NPs and 2D materials are summarized. The nucleation, growth, structural evolution and characterization of metal NPs on the surface of 2D materials are reviewed. The effects of metal NPs on the crystal structure, electronic state and energy band of 2D materials are analyzed. The possible interfacial strain and interfacial reaction are also included. Because of the modulation of electrical and optical properties of 2D materials, the performance of metal NPs-2D material based field effect transistor devices and optoelectronic devices are improved. This review is helpful in clarifying the physical mechanism of microstructure affecting the properties of metal NPs-2D material heterostructures on an atomic scale, and also in developing the metal-2D material heterostructures and their applications in the fields of electronic devices, photoelectric devices, energy devices, etc.</sec>