A Universal Approach toward Light-Responsive Two-Dimensional Electronics: Chemically Tailored Hybrid van der Waals Heterostructures

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.

Engineering the Interfacing of Molecules with 2D Transition Metal Dichalcogenides: Enhanced Multifunctional Electronics

ConspectusEngineering all interfaces between different components in electronic devices is the key to control and optimize fundamental physical processes such as charge injection at metal-semiconductor interfaces, gate modulation at the dielectric-semiconductor interface, and carrier modulation at semiconductor-environment interfaces. The use of two-dimensional (2D) crystals as semiconductors, by virtue of their atomically flat dangling bond-free structures, can facilitate the tailoring of such interfaces effectively. In this context, 2D transition metal dichalcogenides (TMDs) have garnered tremendous attention over the past two decades owing to their exclusive and outstanding physical and chemical characteristics such as their strong light-matter interactions and high charge mobility. These properties position them as promising building blocks for next-generation semiconductor materials. The combination of their large specific surface area, unique electronic structure, and properties highly sensitive to environmental changes makes 2D TMDs appealing platforms for applications in optoelectronics and sensing. While a broad arsenal of TMDs has been made available that exhibit a variety of electronic properties, the latter are unfortunately hardly tunable. To overcome this problem, the controlled functionalization of TMDs with molecules and assemblies thereof represents a most powerful strategy to finely tune their surface characteristics for electronics. Such functionalization can be used not only to encapsulate the electronic material, therefore enhancing its stability in air, but also to impart dynamic, stimuli-responsive characteristics to TMDs and to selectively recognize the presence of a given analyte in the environment, demonstrating unprecedented application potential.In this Account, we highlight the most enlightening recent progress made on the interface engineering in 2D TMD-based electronic devices via covalent and noncovalent functionalization with suitably designed molecules, underlining the remarkable synergies achieved. While electrode functionalization allows modulating charge injection and extraction, the functionalization of the dielectric substrate enables tuning of the carrier concentration in the device channel, and the functionalization of the upper surface of 2D TMDs allows screening the interaction with the environment and imparts molecular functionality to the devices, making them versatile for various applications. The tailored interfaces enable enhanced device performance and open up avenues for practical applications. This Account specifically focuses on our recent endeavor in the unusual properties conferred to 2D TMDs through the functionalization of their interfaces with stimuli-responsive molecules or molecular assemblies. This includes electrode-functionalized devices with modulable performance and charge carriers, molecular-bridged TMD network devices with overall enhanced electrical properties, sensor devices that are highly responsive to changes in the external environment, in particular, electrochemically switchable transistors that react to external electrochemical signals, optically switchable transistors that are sensitive to external light inputs, and multiresponsive transistors that simultaneously respond to multiple external stimuli including optical, electrical, redox, thermal, and magnetic inputs and their application in the development of unprecedented memories, artificial synapses, and logic inverters. By presenting the current challenges, opportunities, and prospects in this blooming research field, we will discuss the powerful integration of such strategies for next-generation electronic digital devices and logic circuitries, outlining future directions and potential breakthroughs in interface engineering.

Defect Engineering of MoTe2 via Thiol Treatment for Type III van der Waals Heterojunction Phototransistor

Molybdenum ditelluride (MoTe2) nanosheets have displayed intriguing physicochemical properties and opto-electric characteristics as a result of their tunable and small band gap (Eg ∼ 1 eV), facilitating concurrent electron and hole transport. Despite the numerous efforts devoted to the development of p-type MoTe2 field-effect transistors (FETs), the presence of tellurium (Te) point vacancies has caused serious reliability issues. Here, we overcome this major limitation by treating the MoTe2 surface with thiolated molecules to heal Te vacancies. Comprehensive materials and electrical characterizations provided unambiguous evidence for the efficient chemisorption of butanethiol. Our thiol-treated MoTe2 FET exhibited a 10-fold increase in hole current and a positive threshold voltage shift of 25 V, indicative of efficient hole carrier doping. We demonstrated that our powerful molecular engineering strategy can be extended to the controlled formation of van der Waals heterostructures by developing an n-SnS2/thiol-MoTe2 junction FET (thiol-JFET). Notably, the thiol-JFET exhibited a significant negative photoresponse with a responsivity of 50 A W-1 and a fast response time of 80 ms based on band-to-band tunneling. More interestingly, the thiol-JFET displayed a gate tunable trimodal photodetection comprising two photoactive modes (positive and negative photoresponse) and one photoinactive mode. These findings underscore the potential of molecular engineering approaches in enhancing the performance and functionality of MoTe2-based nanodevices as key components in advanced 2D-based optoelectronics.

Electronic Devices Based on Heterostructures of 2D Materials and Self-Assembled Monolayers

2D materials (2DMs), known for their atomically ultrathin structure, exhibit remarkable electrical and optical properties. Similarly, molecular self-assembled monolayers (SAMs) with comparable atomic thickness show an abundance of designable structures and properties. The strategy of constructing electronic devices through unique heterostructures formed by van der Waals assembly between 2DMs and molecular SAMs not only enables device miniaturization, but also allows for convenient adjustment of their structures and functions. In this review, the fundamental structures and fabrication methods of three different types of electronic devices dominated by 2DM-SAM heterojunctions with varying architectures are timely elaborated. Based on these heterojunctions, their fundamental functionalities and characteristics, as well as the regulation of their performance by external stimuli, are further discussed.

A Molecular Coordination Strategy for Regulating the Interface of MoS2 Field Effect Transistors

Chemically modifying monolayer two-dimensional transition metal dichalcogenides (TMDs) with organic molecules provides a wide range of possibilities to regulate the electronic and optoelectronic performance of both materials and devices. However, it remains challenging to chemically attach organic molecules to monolayer TMDs without damaging their crystal structures. Herein, we show that the Mo atoms of monolayer MoS2 (1L-MoS2) in defect states can coordinate with both catechol and 1,10-phenanthroline (Phen) groups, affording a facile route to chemically modifying 1L-MoS2. Through the design of two isomeric molecules (LA2 and LA5) comprising catechol and Phen groups, we show that attaching organic molecules to Mo atoms via coordinative bonds has no negative effect on the crystal structure of 1L-MoS2. Both theoretical calculation and experiment results indicate that the coordinative strategy is beneficial for (i) repairing sulfur vacancies and passivating defects; (ii) achieving a long-term and stable n-doping effect; and (iii) facilitating the electron transfer. Field effect transistors (FETs) based on the coordinatively modified 1L-MoS2 show high electron mobilities up to 120.3 cm2 V-1 s-1 with impressive current on/off ratios over 109. Our results indicate that coordinatively attaching catechol- or Phen-bearing molecules may be a general method for the nondestructive modification of TMDs.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Highly Reversible Molecular Photoswitches with Transition Metal Dichalcogenides Electrodes

The molecule-electrode coupling plays an essential role in photoresponsive devices with photochromic molecules, and the strong coupling between the molecule and the conventional electrodes leads to/ the quenching effect and limits the reversibility of molecular photoswitches. In this work, we developed a strategy of using transition metal dichalcogenides (TMDCs) electrodes to fabricate the thiol azobenzene (TAB) self-assembled monolayers (SAMs) junctions with the eutectic gallium-indium (EGaIn) technique. The current-voltage characteristics of the EGaIn/GaOx //TAB/TMDCs photoswitches showed an almost 100% reversible photoswitching behavior, which increased by ∼28% compared to EGaIn/GaOx //TAB/AuTS photoswitches. Density functional theory (DFT) calculations showed the coupling strength of the TAB-TMDCs electrode decreased by 42% compared to that of the TAB-AuTS electrode, giving rise to improved reversibility. our work demonstrated the feasibility of 2D TMDCs for fabricating SAMs-based photoswitches with unprecedentedly high reversibility.

Enhancing the Carrier Transport in Monolayer MoS2 through Interlayer Coupling with 2D Covalent Organic Frameworks

The coupling of different 2D materials (2DMs) to form van der Waals heterostructures (vdWHs) is a powerful strategy for adjusting the electronic properties of 2D semiconductors, for applications in opto-electronics and quantum computing. 2D molybdenum disulfide (MoS2 ) represents an archetypical semiconducting, monolayer thick versatile platform for the generation of hybrid vdWH with tunable charge transport characteristics through its interfacing with molecules and assemblies thereof. However, the physisorption of (macro)molecules on 2D MoS2 yields hybrids possessing a limited thermal stability, thereby jeopardizing their technological applications. Herein, the rational design and optimized synthesis of 2D covalent organic frameworks (2D-COFs) for the generation of MoS2 /2D-COF vdWHs exhibiting strong interlayer coupling effects are reported. The high crystallinity of the 2D-COF films makes it possible to engineer an ultrastable periodic doping effect on MoS2 , boosting devices' field-effect mobility at room temperature. Such a performance increase can be attributed to the synergistic effect of the efficient interfacial electron transfer process and the pronounced suppression of MoS2 's lattice vibration. This proof-of-concept work validates an unprecedented approach for the efficient modulation of the electronic properties of 2D transition metal dichalcogenides toward high-performance (opto)electronics for CMOS digital circuits.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS₂-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS₂) is prepared via a sample solution process. Our results show that 4NTP-MoS₂ exhibits higher response (increased by 200 %) to ppb-level NO₂ with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS₂. Notably, the limit of detection (LOD) toward NO₂ of 4NTP-MoS₂ is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS₂ and the corresponding increment of surface absorption energy to NO₂. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe₂, WS₂, and WSe₂.

Molecular-Switch-Embedded Solution-Processed Semiconductors

Recent improvements in the performance of solution-processed semiconductor materials and optoelectronic devices have shifted research interest to the diversification/advancement of their functionality. Embedding a molecular switch capable of transition between two or more metastable isomers by light stimuli is one of the most straightforward and widely accepted methods to potentially realize the multifunctionality of optoelectronic devices. A molecular switch embedded in a semiconductor can effectively control various parameters such as trap-level, dielectric constant, electrical resistance, charge mobility, and charge polarity, which can be utilized in photoprogrammable devices including transistors, memory, and diodes. This review classifies the mechanism of each optoelectronic transition driven by molecular switches regardless of the type of semiconductor material or molecular switch or device. In addition, the basic characteristics of molecular switches and the persisting technical/scientific issues corresponding to each mechanism are discussed to help researchers. Finally, interesting yet infrequently reported applications of molecular switches and their mechanisms are also described.

Recent Progress in Contact Engineering of Field-Effect Transistor Based on Two-Dimensional Materials

Two-dimensional (2D) semiconductors have been considered as promising candidates to fabricate ultimately scaled field-effect transistors (FETs), due to the atomically thin thickness and high carrier mobility. However, the performance of FETs based on 2D semiconductors has been limited by extrinsic factors, including high contact resistance, strong interfacial scattering, and unintentional doping. Among these challenges, contact resistance is a dominant issue, and important progress has been made in recent years. In this review, the Schottky-Mott model is introduced to show the ideal Schottky barrier, and we further discuss the contribution of the Fermi-level pinning effect to the high contact resistance in 2D semiconductor devices. In 2D FETs, Fermi-level pinning is attributed to the high-energy metal deposition process, which would damage the lattice of atomically thin 2D semiconductors and induce the pinning of the metal Fermi level. Then, two contact structures and the strategies to fabricate low-contact-resistance short-channel 2D FETs are introduced. Finally, our review provides practical guidelines for the realization of high-performance 2D-semiconductors-based FETs with low contact resistance and discusses the outlook of this field.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

Series of High Magnetic Resonance-Guided Photoinduced Nanodelivery Systems for Precisely Improving the Efficiency of Cancer Therapy

Nanochemotherapy is recognized as one of the most promising cancer treatment options, and the design of the carrier has a crucial impact on the final efficacy. To precisely improve the efficacy and reduce the toxicity, we combined the clinical contrast agent (Gd-DTPA) with a stimulus-sensitive o-nitrobenzyl ester and then prepared a series of nNBGD lipids by varying the carbon chain length of the hydrophobic group. The self-assembled nNBGD liposomes can be tracked by MRI to localize the aggregation of drug carriers in vivo, so as to prompt the application of light stimulation at the optimal time to facilitate the precise release of carriers at the lesion site. And the application potential of this strategy was verified with 88% tumor suppression effect in the 12NBGD-DOX+UV group. In addition, this paper emphasizes that small differences in structure can affect the overall performance of the carriers. By exploration of the differences in stability, drug loading, stimulus responsiveness, MRI imaging effect, and toxicity of the series of nNBGD carriers, the relationship between the length of the hydrophobic group of nNBGD lipids and the overall performance of the carriers is given, which provides experimental support and design reference for other carriers.

Asymmetric Chemical Functionalization of Top-Contact Electrodes: Tuning the Charge Injection for High-Performance MoS2 Field-Effect Transistors and Schottky Diodes

The fabrication of high-performance (opto-)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode-semiconductor interfaces. While chemical functionalization with chemisorbed self-assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom-contact devices, such a strategy has not been demonstrated for the top-contact configuration, despite the latter being known to offer enhanced charge-injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top-contact field-effect transistors (FETs) via the transfer of chemically pre-modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS₂ ) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron-donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high-performance Schottky diode with a rectification ratio of ≈10³ . This unprecedented strategy to tune the charge injection in top-contact MoS₂ FETs is of general applicability for the fabrication of high-performance (opto-)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Janus 2D materials via asymmetric molecular functionalization

Janus two-dimensional materials (2DMs) are a novel class of 2DMs in which the two faces of the material are either asymmetrically functionalized or are exposed to a different local environment. The diversity of the properties imparted to the two opposing sides enables the design of new multifunctional materials for applications in a broad variety of fields including opto-electronics, energy storage, and catalysis. In this perspective, we summarize the most enlightening experimental methods for the asymmetric chemical functionalization of 2DMs with tailored made (macro)molecules by means of a supratopic binding (one side) or antaratopic binding (two sides) process. We describe the emergence of unique electrical and optical characteristics resulting from the asymmetric dressing of the two surfaces. Representative examples of Janus 2DMs towards bandgap engineering, enhanced photoresponse and photoluminescence are provided. In addition, examples of Janus 2DMs for real applications such as energy storage (batteries and supercapacitors) and generation (photovoltaics), opto-electronics (field-effect transistors and photodetectors), catalysis, drug delivery, self-healing materials, chemical sensors and selective capture and separation of small molecules are also described. Finally, we discuss the future directions, challenges, and opportunities to expand the frontiers of Janus 2DMs towards technologies with potential impact in environmental science and biomedical applications.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Spin-crossover nanoparticles anchored on MoS2 layers for heterostructures with tunable strain driven by thermal or light-induced spin switching

In the past few years, the effect of strain on the optical and electronic properties of MoS₂ layers has attracted particular attention as it can improve the performance of optoelectronic and spintronic devices. Although several approaches have been explored, strain is typically externally applied on the two-dimensional material. In this work, we describe the preparation of a reversible 'self-strainable' system in which the strain is generated at the molecular level by one component of a MoS₂-based composite material. Spin-crossover nanoparticles were covalently grafted onto functionalized layers of semiconducting MoS₂ to form a hybrid heterostructure. Their ability to switch between two spin states on applying an external stimulus (light irradiation or temperature change) serves to generate strain over the MoS₂ layer. A volume change accompanies this spin crossover, and the created strain induces a substantial and reversible change of the electrical and optical properties of the heterostructure.

Light-Programmable Logic-in-Memory in 2D Semiconductors Enabled by Supramolecular Functionalization: Photoresponsive Collective Effect of Aligned Molecular Dipoles

Nowadays, the unrelenting growth of the digital universe calls for radically novel strategies for data processing and storage. An extremely promising and powerful approach relies on the development of logic-in-memory (LiM) devices through the use of floating gate and ferroelectric technologies to write and erase data in a memory operating as a logic gate driven by electrical bias. In this work, we report an alternative approach to realize the logic-in-memory based on two-dimensional (2D) transition metal dichalcogenides (TMDs) where multiple memorized logic output states have been established via the interface with responsive molecular dipoles arranged in supramolecular arrays. The collective dynamic molecular dipole changes of the axial ligand coordinated onto self-assembled metal phthalocyanine nanostructures on the surface of 2D TMD enables large reversible modulation of the Fermi level of both n-type molybdenum disulfide (MoS₂) and p-type tungsten diselenide (WSe₂) field-effect transistors (FETs), to achieve multiple memory states by programming and erasing with ultraviolet (UV) and with visible light, respectively. As a result, logic-in-memory devices were built up with our supramolecular layer/2D TMD architecture where the output logic is encoded by the motion of the molecular dipoles. Our strategy relying on the dynamic control of the 2D electronics by harnessing the functions of molecular-dipole-induced memory in a supramolecular hybrid layer represents a versatile way to integrate the functional programmability of molecular science into the next generation nanoelectronics.

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

The noncovalent functionalization of two-dimensional materials (2DMs) with bespoke organic molecules is of central importance for future nanoscale electronic devices. Of particular interest is the incorporation of molecular functionalities that can modulate the physicochemical properties of the 2DMs via noninvasive external stimuli. In this study, we present the reversible modulation of the photoluminescence, spectroscopic properties (Raman), and charge transport characteristics of molybdenum disulfide (MoS₂)-based devices via photoisomerization of a self-assembled monolayer of azobenzene-modified triazatriangulene molecules. The observed (opto)electronic modulations are explained by the n-type doping of the MoS₂ lattice induced by the photoisomerization of the highly ordered azobenzene monolayer. This novel behavior could have profound effects on future composite 2DM-based (opto)electronics.

Challenges for Incorporating Optical Switchability in Organic-Based Electronic Devices

Transistors operate by controlling the current flowing from a source to a drain electrode via a third electrode (gate), thus giving access to a binary treatment (ON/OFF or 0/1) of the signal currently exploited in microelectronics. Introducing a second independent lever to modulate the current would allow for more complex logic functions amenable to a single electronic component and hence to new opportunities for advanced electrical signal processing. One avenue is to add this second dimension with light by incorporating photochromic molecules in current organic-based electronic devices. In this Spotlight, we describe different concepts that have been implemented in organic thin films and in molecular junctions as well as some pitfalls that have been highlighted thanks to theoretical modeling.

Asymmetric Dressing of WSe2 with (Macro)molecular Switches: Fabrication of Quaternary-Responsive Transistors

The forthcoming saturation of Moore's law has led to a strong demand for integrating analogue functionalities within semiconductor-based devices. As a step toward this goal, we fabricate quaternary-responsive WSe₂-based field-effect transistors (FETs) whose output current can be remotely and reversibly controlled by light, heat, and electric field. A photochromic silane-terminated spiropyran (SP) is chemisorbed on SiO₂ forming a self-assembled monolayer (SAM) that can switch from the SP to the merocyanine (MC) form in response to UV illumination and switch back by either heat or visible illumination. Such a SAM is incorporated at the dielectric-semiconductor interface in WSe₂-based FETs. Upon UV irradiation, a drastic decrease in the output current of 82% is observed and ascribed to the zwitterionic MC isomer acting as charge scattering site. To provide an additional functionality, the WSe₂ top surface is coated with a ferroelectric co-polymer layer based on poly(vinylidene fluoride-co-trifluoroethylene). Because of its switchable inherent electrical polarization, it can promote either the accumulation or depletion of charge carriers in the WSe₂ channel, thereby inducing a current modulation with 99% efficiency. Thanks to the efficient tuning induced by the two components and their synergistic effects, the device polarity could be modulated from n-type to p-type. Such a control over the carrier concentration and device polarity is key to develop 2D advanced electronics. Moreover, the integration strategy of multiple stimuli-responsive elements into a single FET allows us to greatly enrich its functionality, thereby promoting the development for More-than-Moore technology.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

Light-Triggered Switching of Quantum Dot Photoluminescence through Excited-State Electron Transfer to Surface-Bound Photochromic Molecules

This paper describes reversible "on-off" switching of the photoluminescence (PL) intensity of CdSe quantum dots (QDs), mediated by photochromic furylfulgide carboxylate (FFC) molecules chemisorbed to the surfaces of the QDs. Repeated cycles of UV and visible illumination switch the FFC between "closed" and "open" isomers. Reversible switching of the QDs' PL intensity by >80% is enabled by different rates and yields of PL-quenching photoinduced electron transfer (PET) from the QDs to the respective isomers. This difference is consistent with cyclic voltammetry measurements and density functional calculations of the isomers' frontier orbital energies. This work demonstrates fatigue-resistant modulation of the PL of a QD-molecule complex through remote control of PET. Such control potentially enables applications, such as all-optical memory, sensing, and imaging, that benefit from a fast, tunable, and reversible response to light stimuli.

Photomodulation of Charge Transport in All-Semiconducting 2D-1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom-up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS₂ are fabricated. Field-effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS₂ , resulting from the interfacial charge transfer process. The MoS₂ -GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS₂ -GNR VDWHs. The physisorption of photochromic molecules onto the MoS₂ -GNR VDWHs enables reversible light-driven control over charge transport. In particular, the drain current of the MoS₂ -GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS₂ -GNR VDWHs for multilevel memory devices.

Optically Triggered Control of the Charge Carrier Density in Chemically Functionalized Graphene Field Effect Transistors

Field effect transistors (FETs) based on 2D materials are of great interest for applications in ultrathin electronic and sensing devices. Here we demonstrate the possibility to add optical switchability to graphene FETs (GFET) by functionalizing the graphene channel with optically switchable azobenzene molecules. The azobenzene molecules were incorporated to the GFET channel by building a van der Waals heterostructure with a carbon nanomembrane (CNM), which is used as a molecular interposer to attach the azobenzene molecules. Under exposure with 365 nm and 455 nm light, azobenzene molecules transition between cis and trans molecular conformations, respectively, resulting in a switching of the molecular dipole moment. Thus, the effective electric field acting on the GFET channel is tuned by optical stimulation and the carrier density is modulated.

Molecular Approach to Electrochemically Switchable Monolayer MoS2 Transistors

As Moore's law is running to its physical limit, tomorrow's electronic systems can be leveraged to a higher value by integrating "More than Moore" technologies into CMOS digital circuits. The hybrid heterostructure composed of two-dimensional (2D) semiconductors and molecular materials represents a powerful strategy to confer new properties to the former components, realize stimuli-responsive functional devices, and enable diversification in "More than Moore" technologies. Here, an ionic liquid (IL) gated 2D MoS₂ field-effect transistor (FET) with molecular functionalization is fabricated. The suitably designed ferrocene-substituted alkanethiol molecules not only improve the FET performance, but also show reversible electrochemical switching on the surface of MoS₂ . Field-effect mobility of monolayer MoS₂ reaches values as high as ≈116 cm² V^-1 s^-1 with I_on /I_off ratio exceeding 10⁵ . Molecules in their neutral or charged state impose distinct doping effect, efficiently tuning the electron density in monolayer MoS₂ . It is noteworthy that the joint doping effect from IL and switchable molecules results in the steep subthreshold swing of MoS₂ FET in the backward sweep. These results demonstrate that the device architecture represents an unprecedented and powerful strategy to fabricate switchable 2D FET with a chemically programmed electrochemical signal as a remote control, paving the road toward novel functional devices.

Advanced Photoresponsive Materials Using the Metal-Organic Framework Approach

When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coordinate to metal or metal-oxo centers so as to yield stable, crystalline frameworks. Photoexcitations in such chromophore-based MOFs are surveyed, with a special emphasis on light-switchable MOFs from photochromic molecules. The conventional powder form of MOFs obtained using solvothermal approaches carries certain disadvantages for optical applications, such as limited efficiency resulting from absorption and light scattering caused by the (micrometer-sized) powder particles. How these problems can be avoided by using MOF thin films is demonstrated.

Simultaneous Optical Tuning of Hole and Electron Transport in Ambipolar WSe2 Interfaced with a Bicomponent Photochromic Layer: From High-Mobility Transistors to Flexible Multilevel Memories

The interfacing of 2D materials (2DMs) with photochromic molecules provides an efficient solution to reversibly modulate their outstanding electronic properties and offers a versatile platform for the development of multifunctional field-effect transistors (FETs). Herein, optically switchable multilevel high-mobility FETs based on few-layer ambipolar WSe₂ are realized by applying on its surface a suitably designed bicomponent diarylethene (DAE) blend, in which both hole and electron transport can be simultaneously modulated for over 20 cycles. The high output current modulation efficiency (97% for holes and 52% for electrons) ensures 128 distinct current levels, corresponding to a data storage capacity of 7 bit. The device is also implemented on a flexible and transparent poly(ethylene terephthalate) substrate, rendering 2DM/DAE hybrid structures promising candidates for flexible multilevel nonvolatile memories.

Recent Advances in Chemical Functionalization of 2D Black Phosphorous Nanosheets

Owing to their tunable direct bandgap, high charge carrier mobility, and unique in-plane anisotropic structure, black phosphorus nanosheets (BPNSs) have emerged as one of the most important candidates among the 2D materials beyond graphene. However, the poor ambient stability of black phosphorus limits its practical application, due to the chemical degradation of phosphorus atoms to phosphorus oxides in the presence of oxygen and/or water. Chemical functionalization is demonstrated as an efficient approach to enhance the ambient stability of BPNSs. Herein, various covalent strategies including radical addition, nitrene addition, nucleophilic substitution, and metal coordination are summarized. In addition, efficient noncovalent functionalization methods such as van der Waals interactions, electrostatic interactions, and cation-π interactions are described in detail. Furthermore, the preparations, characterization, and diverse applications of functionalized BPNSs in various fields are recapped. The challenges faced and future directions for the chemical functionalization of BPNSs are also highlighted.

Simultaneous non-covalent bi-functionalization of 1T-MoS2 ruled by electrostatic interactions: towards multi-responsive materials

Dual functionalization of chemically exfoliated MoS₂ has been achieved by exploiting coulombic interactions among positively charged molecules and the negatively charged 2D flakes.

High-performance ultra-violet phototransistors based on CVT-grown high quality SnS2 flakes

van der Waals layered two-dimensional (2D) metal dichalcogenides, such as SnS₂, have garnered great interest owing to their new physics in the ultrathin limit, and become potential candidates for the next-generation electronics and/or optoelectronics fields. Herein, we report high-performance UV photodetectors established on high quality SnS₂ flakes and address the relatively lower photodetection capability of the thinner flakes via a compatible gate-controlling strategy. SnS₂ flakes with different thicknesses were mechanically exfoliated from CVT-grown high-quality 2H-SnS₂ single crystals. The photodetectors fabricated using SnS₂ flakes reveal a desired response performance (R _λ ≈ 112 A W^-1, EQE ≈ 3.7 × 10⁴%, and D* ≈ 1.18 × 10¹¹ Jones) under UV light with a very low power density (0.2 mW cm^-2 @ 365 nm). Specifically, SnS₂ flakes present a positive thickness-dependent photodetection behavior caused by the enhanced light absorption capacity of thicker samples. Fortunately, the responsivity of thin SnS₂ flakes (e.g. ∼15 nm) could be indeed enhanced to ∼140 A W^-1 under a gate bias of +20 V, reaching the performance level of thicker samples without gate bias (e.g. ∼144 A W^-1 for a ∼60 nm flake). Our results offer an efficient way to choose 2D crystals with controllable thicknesses as optimal candidates for desirable optoelectronic devices.

Two-Dimensional WSe2/Organic Acceptor Hybrid Nonvolatile Memory Devices Based on Interface Charge Trapping

Two-dimensional (2D) materials, with atomic thickness and unique electronic structure, hold great potentials in electronic device applications. Charge transfer at the interface of 2D materials further provides a versatile platform for applications in electronics. Here, we report nonvolatile memory devices based on interface charge trapping between 2D WSe₂ and organic electron acceptors. The 2D WSe₂-organic acceptor hybrid structure exhibits a high storage performance, such as large gate memory windows, high on/off ratios (>10³), and long retention time (>1000 s). Further analysis revealed that organic acceptors with a stronger electron affinity (i.e., higher redox potential) have a larger electron-trapping ability and hence a better memory performance.

Modulating the Charge Transport in 2D Semiconductors via Energy-Level Phototuning

The controlled functionalization of semiconducting 2D materials (2DMs) with photoresponsive molecules enables the generation of novel hybrid structures as active components for the fabrication of high-performance multifunctional field-effect transistors (FETs) and memories. This study reports the realization of optically switchable FETs by decorating the surface of the semiconducting 2DMs such as WSe₂ and black phosphorus with suitably designed diarylethene (DAE) molecules to modulate their electron and hole transport, respectively, without sacrificing their pristine electrical performance. The efficient and reversible photochemical isomerization of the DAEs between the open and the closed isomer, featuring different energy levels, makes it possible to generate photoswitchable charge trapping levels, resulting in the tuning of charge transport through the 2DMs by alternating illumination with UV and visible light. The device reveals excellent data-retention capacity combined with multiple and well-distinguished accessible current levels, paving the way for its use as an active element in multilevel memories.