Modulating the Optoelectronic Properties of MoS2 by Highly Oriented Dipole-Generating Monolayers

Design of stimuli-responsive transition metal dichalcogenides

Stimuli-responsive systems are an emerging class of materials in fields as diverse as electronics, optoelectronics, cancer detection, drug delivery, or sensing. Especially focusing on nanomaterials, 2D transition metal dichalcogenides have recently attracted the scientific community's attention due to their remarkable intrinsic stimuli-responsive behaviour upon external stimuli such as pH, light, voltage, or certain pathogens. This significant response can be further enhanced by forming mixed-dimensional heterostructures and by molecular functionalization, capitalizing on chemistry to manipulate and boost their intrinsic stimuli-responsive properties. Furthermore, thanks to the endless possibilities of chemistry, a new class of smart materials based on the combination of stimuli-responsive molecular systems with transition metal dichalcogenides has recently been synthesized. In these materials, the physical properties of the 2D layers are reversibly modified by the switchable molecules, not only enhancing their stimuli-responsive behaviour but also providing memory to the hybrid. Therefore, this review explores the recent breakthroughs in the chemical design of smart transition metal dichalcogenides with built-in responsiveness.

Light-Modulated Humidity Sensing in Spiropyran Functionalized MoS2 Transistors

The optically tuneable nature of hybrid organic/inorganic heterostructures tailored by interfacing photochromic molecules with 2D semiconductors (2DSs) can be exploited to endow multi-responsiveness to the exceptional physical properties of 2DSs. In this study, a spiropyran-molybdenum disulfide (MoS2) light-switchable bi-functional field-effect transistor is realized. The spiropyran-merocyanine reversible photo-isomerization has been employed to remotely control both the electron transport and wettability of the hybrid structure. This manipulation is instrumental for tuning the sensitivity in humidity sensing. The hybrid organic/inorganic heterostructure is subjected to humidity testing, demonstrating its ability to accurately monitor relative humidity (RH) across a range of 10%-75%. The electrical output shows good sensitivity of 1.0% · (%) RH-1. The light-controlled modulation of the sensitivity in chemical sensors can significantly improve their selectivity, versatility, and overall performance in chemical sensing.

Edge-Based Two-Dimensional α-In2Se3-MoS2 Ferroelectric Field Effect Device

Heterostructures based on two-dimensional materials offer the possibility to achieve synergistic functionalities, which otherwise remain secluded by their individual counterparts. Herein, ferroelectric polarization switching in α-In₂Se₃ has been utilized to engineer multilevel nonvolatile conduction states in a partially overlapping α-In₂Se₃-MoS₂-based ferroelectric semiconducting field effect device. In particular, we demonstrate how the intercoupled ferroelectric nature of α-In₂Se₃ allows to nonvolatilely switch between n-i and n-i-n type junction configurations based on a novel edge state actuation mechanism, paving the way for subnanometric scale nonvolatile device miniaturization. Furthermore, the induced asymmetric polarization enables enhanced photogenerated carriers' separation, resulting in an extremely high photoresponse of ∼1275 A/W in the visible range and strong nonvolatile modulation of the bright A- and B- excitonic emission channels in the overlaying MoS₂ monolayer. Our results show significant potential to harness the switchable polarization in partially overlapping α-In₂Se₃-MoS₂ based FeFETs to engineer multimodal, nonvolatile nanoscale electronic and optoelectronic devices.

Ni and O co-modified MoS2 as universal SERS substrate for the detection of different kinds of substances

Surface-enhanced Raman scattering (SERS) has attracted extensive attention as an ultrasensitive detection method. However, the poor biocompatibility and expensive synthesis cost of noble metal SERS substrates have become non-negligible factors that limit the development of SERS technology. Metal chalcogenide semiconductors as an alternative to noble metal SERS substrates can avoid these disadvantages, but the enhancement effect is lower than that of noble metal substrates. Here, we report a method to co-modify MoS₂ by Ni and O, which improves the carrier concentration and mobility of MoS₂. The SERS effect of the modified MoS₂ is comparable to that of noble metals. We found that the improved SERS performance of MoS₂ can be attributed to the following two factors: strong interfacial dipole-dipole interaction and efficient charge transfer effect. During the doping process, the incorporation of Ni and O enhances the polarity and carrier concentration of MoS₂, enhances the interfacial interaction of MoS₂, and provides a basis for charge transfer. During the annealing process, the introduction of O atoms into the S defects reduces the internal defects of doped MoS₂, improves the carrier mobility, and promotes the efficient charge transfer effect of MoS₂. The final modified MoS₂ as a SERS substrate realizes low-concentration detection of bilirubin, cytochrome C, and trichlorfon. This provides promising guidance for the practical inspection of metal chalcogenide semiconductor substrates.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

Dynamical Behavior of Two Interacting Double Quantum Dots in 2D Materials for Feasibility of Controlled-NOT Operation

Two interacting double quantum dots (DQDs) can be suitable candidates for operation in the applications of quantum information processing and computation. In this work, DQDs are modeled by the heterostructure of two-dimensional (2D) MoS₂ having 1T-phase embedded in 2H-phase with the aim to investigate the feasibility of controlled-NOT (CNOT) gate operation with the Coulomb interaction. The Hamiltonian of the system is constructed by two models, namely the 2D electronic potential model and the 4×4 matrix model whose matrix elements are computed from the approximated two-level systems interaction. The dynamics of states are carried out by the Crank-Nicolson method in the potential model and by the fourth order Runge-Kutta method in the matrix model. Model parameters are analyzed to optimize the CNOT operation feasibility and fidelity, and investigate the behaviors of DQDs in different regimes. Results from both models are in excellent agreement, indicating that the constructed matrix model can be used to simulate dynamical behaviors of two interacting DQDs with lower computational resources. For CNOT operation, the two DQD systems with the Coulomb interaction are feasible, though optimization of engineering parameters is needed to achieve optimal fidelity.

First-principles investigation of in-plane anisotropies in XYTe4 monolayers with X = Hf, Zr, Ti and Y = Si, Ge

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe₄ (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe₄ reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe₄ monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe₄ differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe₄ monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.

Bright excitonic multiplexing mediated by dark exciton transition in two-dimensional TMDCs at room temperature

2D-semiconductors with strong light-matter interaction are attractive materials for integrated and tunable optical devices. Here, we demonstrate room-temperature wavelength multiplexing of the two-primary bright excitonic channels (A_b-, B_b-) in monolayer transition metal dichalcogenides (TMDs) arising from a dark exciton mediated transition. We present how tuning dark excitons via an out-of-plane electric field cedes the system equilibrium from one excitonic channel to the other, encoding the field polarization into wavelength information. In addition, we demonstrate how such exciton multiplexing is dictated by thermal-scattering by performing temperature dependent photoluminescence measurements. Finally, we demonstrate experimentally and theoretically how excitonic mixing can explain preferable decay through dark states in MoX₂ in comparison with WX₂ monolayers. Such field polarization-based manipulation of excitonic transitions can pave the way for novel photonic device architectures.

Nanoscale self-assembly: concepts, applications and challenges

Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.