Surface Charge Transfer Doping of Low-Dimensional Nanostructures toward High-Performance Nanodevices

Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

Realization of Extremely High-Gain and Low-Power in nMOS Inverter Based on Monolayer WS2 Transistor Operating in Subthreshold Regime

In this work, we report an n-type metal-oxide-semiconductor (nMOS) inverter using chemical vapor deposition (CVD)-grown monolayer WS2 field-effect transistors (FETs). Our large-area CVD-grown monolayer WS2 FETs exhibit outstanding electrical properties including a high on/off ratio, small subthreshold swing, and excellent drain-induced barrier lowering. These are achieved by n-type doping using AlOx/Al2O3 and a double-gate structure employing high-k dielectric HfO2. Due to the superior subthreshold characteristics, monolayer WS2 FETs show high transconductance and high output resistance in the subthreshold regime, resulting in significantly higher intrinsic gain compared to conventional Si MOSFETs. Therefore, we successfully realize subthreshold operating monolayer WS2 nMOS inverters with extremely high gains of 564 and 2056 at supply voltage (VDD) of 1 and 2 V, respectively, and low power consumption of ∼2.3 pW·μm-1 at VDD = 1 V. In addition, the monolayer WS2 nMOS inverter is further expanded to the demonstration of logic circuits such as AND, OR, NAND, NOR logic gates, and SRAM. These findings suggest the potential of monolayer WS2 for high-gain and low-power logic circuits and validate the practical application in large areas.

Influence of High-κ Dielectrics Integration on ALD-Based MoS2 Field-Effect Transistor Performance

The integration of high-κ dielectrics on MoS2 field-effect transistors (FETs) is essential for the realization of MoS2 in ultrascaled nanoelectronic devices and circuits. Most studies covering this topic are based on exfoliated MoS2 flakes or chemical vapor deposition (CVD) grown MoS2 films, whereas other techniques, such as atomic layer deposition (ALD), are also gaining attention for the growth of MoS2 in recent years. In this work, we grow large-area MoS2 by means of plasma-enhanced (PE-)ALD and evaluate the influence of high-κ dielectrics on the properties of ALD-based MoS2 FETs through electrical characterization combined with surface-chemical and high-resolution scanning transmission electron microscopy (HR-STEM) analyses. We grow HfO x , AlO x , or both by means of PE-ALD or thermal ALD on our fabricated devices and show that, in addition to the dielectric constant, three other major parameters related to the processing of the dielectrics can simultaneously affect the MoS2 FET electrical characteristics and govern its doping. These parameters are the stoichiometry of the dielectric, its carbon impurity content, and the degree to which the MoS2 surface oxidizes upon the dielectric growth. When grown at 100 °C, our HfO x films are oxygen-vacant whereas our AlO x films are oxygen-rich. In addition, carbon impurities are incorporated into the dielectrics at low deposition temperatures, being one of the likely causes of the MoS2 FET overall n-type performance in all of the studied cases. Our investigations also reveal that PE-ALD of HfO x or AlO x oxidizes the MoS2 surface, whereas thermal ALD AlO x leaves MoS2 almost intact. In this respect, if thermal ALD AlO x of proper thickness is grown between MoS2 and HfO x , it can reduce the degree to which the MoS2 surface oxidizes by HfO x and meanwhile improve the total dielectric constant, altogether leading to the most optimal electrical performance in ALD-based MoS2 FETs.

Photoluminescence of Chemically and Electrically Doped Two-Dimensional Monolayer Semiconductors

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers exhibit unique physical properties, such as self-terminating surfaces, a direct bandgap, and near-unity photoluminescence (PL) quantum yield (QY), which make them attractive for electronic and optoelectronic applications. Surface charge transfer has been widely used as a technique to control the concentration of free charge in 2D semiconductors, but its estimation and the impact on the optoelectronic properties of the material remain a challenge. In this work, we investigate the optical properties of a WS2 monolayer under three different doping approaches: benzyl viologen (BV), potassium (K), and electrostatic doping. Owing to the excitonic nature of 2D TMDC monolayers, the PL of the doped WS2 monolayer exhibits redshift and a decrease in intensity, which is evidenced by the increase in trion population. The electron concentrations of 3.79×1013 cm-2, 6.21×1013 cm-2, and 3.12×1012 cm-2 were measured for WS2 monolayers doped with BV, K, and electrostatic doping, respectively. PL offers a direct and versatile approach to probe the doping effect, allowing for the measurement of carrier concentration in 2D monolayer semiconductors.

Defective Diamane: A Superior Sensor for Toxic Gases Capture and Detection with Excellent Selectivity, Sensitivity, and Reversibility at Room Temperature

The toxic gases emitted from industrial production have caused significant damage to the environment and human health, necessitating efficient gas sensors for their detection and removal. In this work, first-principles calculations are employed to investigate the potential application of diamanes for high-performance toxic gas sensors. The results show that nine gas molecules (CO, CO2, NO, NO2, NH3, SO2, N2, O2, and H2O) are physisorbed on pristine diamane by weak van der Waals interactions. After introducing H/F defects, diamane can effectively capture specific toxic gases (CO, NO, NO2, and SO2) in the presence of interfering gases (N2, O2, and H2O), suggesting excellent selectivity and anti-interference ability. Orbital hybridization and significant charge redistribution between gas molecules and defective diamane dominate the enhanced adsorbate-substrate interactions. More importantly, the high sensitivity and good reversibility of defective diamane for detecting CO, NO, and SO2 molecules enable its reuse as a superior resistance-type gas sensor. Our calculations provide valuable insights into the potential of defective diamane for detecting toxic gases and shed light on the practical application of novel carbon-based materials in the gas-sensing field.

Localized Surface Doping Induced Ultralow Contact Resistance between Metal and (Bi,Sb)2Te3 Thermoelectric Films

Micro thermoelectric devices are expected to further improve the cooling density for the temperature control of electronic devices; nevertheless, the high contact resistivity between metals and semiconductors critically limits their applications, especially in chip cooling with extremely high heat flux. Herein, based on the calculated results, a low specific contact resistivity of ∼10-7 Ω cm2 at the interface is required to guarantee a desirable cooling power density of micro devices. Thus, we developed a generally applicable interfacial modulation strategy via localized surface doping of thermoelectric films, and the feasibility of such a doping approach for both n/p-type (Bi,Sb)2Te3 films was demonstrated, which can effectively increase the surface-majority carrier concentration explained by the charge transfer mechanism. With a proper doping level, ultralow specific contact resistivities at the interfaces are obtained for n-type (6.71 × 10-8 Ω cm2) and p-type (3.70 × 10-7 Ω cm2) (Bi,Sb)2Te3 layers, respectively, which is mainly attributed to the carrier tunneling enhancement with a narrowed interfacial contact barrier width. This work provides an effective scheme to further reduce the internal resistance of micro thermoelectric coolers, which can also be extended as a kind of universal interfacial modification technique for micro semiconductor devices.

High-Performance 2D Ambipolar MoTe2 Lateral Memristors by Mild Oxidation

2D transition metal dichalcogenides (TMDCs) have been intensively explored in memristors for brain-inspired computing. Oxidation, which is usually unavoidable and harmful in 2D TMDCs, could also be used to enhance their memristive performances. However, it is still unclear how oxidation affects the resistive switching behaviors of 2D ambipolar TMDCs. In this work, a mild oxidation strategy is developed to greatly enhance the resistive switching ratio of ambipolar 2H-MoTe2 lateral memristors by more than 10 times. Such an enhancement results from the amplified doping due to O2 and H2O adsorption and the optimization of effective gate voltage distribution by mild oxidation. Moreover, the ambipolarity of 2H-MoTe2 also enables a change of resistive switching direction, which is uncommon in 2D memristors. Consequently, as an artificial synapse, the MoTe2 device exhibits a large dynamic range (≈200) and a good linearity (1.01) in long-term potentiation and depression, as well as a high-accuracy handwritten digit recognition (>96%). This work not only provides a feasible and effective way to enhance the memristive performance of 2D ambipolar materials, but also deepens the understanding of hidden mechanisms for RS behaviors in oxidized 2D materials.

Recent Advance in Synaptic Plasticity Modulation Techniques for Neuromorphic Applications

Manipulating the expression of synaptic plasticity of neuromorphic devices provides fascinating opportunities to develop hardware platforms for artificial intelligence. However, great efforts have been devoted to exploring biomimetic mechanisms of plasticity simulation in the last few years. Recent progress in various plasticity modulation techniques has pushed the research of synaptic electronics from static plasticity simulation to dynamic plasticity modulation, improving the accuracy of neuromorphic computing and providing strategies for implementing neuromorphic sensing functions. Herein, several fascinating strategies for synaptic plasticity modulation through chemical techniques, device structure design, and physical signal sensing are reviewed. For chemical techniques, the underlying mechanisms for the modification of functional materials were clarified and its effect on the expression of synaptic plasticity was also highlighted. Based on device structure design, the reconfigurable operation of neuromorphic devices was well demonstrated to achieve programmable neuromorphic functions. Besides, integrating the sensory units with neuromorphic processing circuits paved a new way to achieve human-like intelligent perception under the modulation of physical signals such as light, strain, and temperature. Finally, considering that the relevant technology is still in the basic exploration stage, some prospects or development suggestions are put forward to promote the development of neuromorphic devices.

Anti-Ambipolar Heterojunctions: Materials, Devices, and Circuits

Anti-ambipolar heterojunctions are vital in constructing high-frequency oscillators, fast switches, and multivalued logic (MVL) devices, which hold promising potential for next-generation integrated circuit chips and telecommunication technologies. Thanks to the strategic material design and device integration, anti-ambipolar heterojunctions have demonstrated unparalleled device and circuit performance that surpasses other semiconducting material systems. This review aims to provide a comprehensive summary of the achievements in the field of anti-ambipolar heterojunctions. First, the fundamental operating mechanisms of anti-ambipolar devices are discussed. After that, potential materials used in anti-ambipolar devices are discussed with particular attention to 2D-based, 1D-based, and organic-based heterojunctions. Next, the primary device applications employing anti-ambipolar heterojunctions, including anti-ambipolar transistors (AATs), photodetectors, frequency doublers, and synaptic devices, are summarized. Furthermore, alongside the advancements in individual devices, the practical integration of these devices at the circuit level, including topics such as MVL circuits, complex logic gates, and spiking neuron circuits, is also discussed. Lastly, the present key challenges and future research directions concerning anti-ambipolar heterojunctions and their applications are also emphasized.

Contacts at the Nanoscale and for Nanomaterials

Contact scaling is a major challenge in nano complementary metal-oxide-semiconductor (CMOS) technology, as the surface roughness, contact size, film thicknesses, and undoped substrate become more problematic as the technology shrinks to the nanometer range. These factors increase the contact resistance and the nonlinearity of the current-voltage characteristics, which could limit the benefits of the further downsizing of CMOS devices. This review discusses issues related to the contact size reduction of nano CMOS technology and the validity of the Schottky junction model at the nanoscale. The difficulties, such as the limited doping level and choices of metal for band alignment, Fermi-level pinning, and van der Waals gap, in achieving transparent ohmic contacts with emerging two-dimensional materials are also examined. Finally, various methods for improving ohmic contacts' characteristics, such as two-dimensional/metal van der Waals contacts and hybrid contacts, junction doping technology, phase and bandgap modification effects, buffer layers, are highlighted.

Analysis of p-Type Doping in Graphene Induced by Monolayer-Oxidized TMDs

Doping is one of the most difficult technological challenges for realizing reliable two-dimensional (2D) material-based semiconductor devices, arising from their ultrathinness. Here, we systematically investigate the impact of different types of nonstoichiometric solid MOx (M are W or Mo) dopants obtained by oxidizing transition metal dichalcogenides (TMDs: WSe2 or MoS2) formed on graphene FETs, which results in p-type doping along with disorders. From the results obtained in this study, we were able to suggest an analytical technique to optimize the optimal UV-ozone (UVO) treatment to achieve high p-type doping concentration in graphene FETs (∼2.5 × 1013 cm-2 in this study) without generating defects, mainly by analyzing the time dependency of D and D' peaks measured by Raman spectroscopy. Furthermore, an analysis of the structure of graphene sheets using TEM indicates that WOx plays a better protective role in graphene, compared to MoOx, suggesting that WOx is more effective for preventing the degradation of graphene during UVO treatment. To enhance the practical application aspect of our work, we have fabricated a graphene photodetector by selectively doping the graphene through oxidized TMDs, creating a p-n junction, which resulted in improved photoresponsivity compared to the intrinsic graphene device. Our results offer a practical guideline for the utilization of surface charge transfer doping of graphene toward CMOS applications.

Engineering the Electrical and Optical Properties of WS2 Monolayers via Defect Control

Two-dimensional (2D) materials as tungsten disulphide (WS2 ) are rising as the ideal platform for the next generation of nanoscale devices due to the excellent electric-transport and optical properties. However, the presence of defects in the as grown samples represents one of the main limiting factors for commercial applications. At the same time, WS2 properties are frequently tailored by introducing impurities at specific sites. Aim of this review paper is to present a complete description and discussion of the effects of both intentional and unintentional defects in WS2 , by an in depth analysis of the recent experimental and theoretical investigations reported in the literature. First, the most frequent intrinsic defects in WS2 are presented and their effects in the readily synthetized material are discussed. Possible solutions to remove and heal unintentional defects are also analyzed. Following, different doping schemes are reported, including the traditional substitution approach and innovative techniques based on the surface charge transfer with adsorbed atoms or molecules. The plethora of WS2 monolayer modifications presented in this review and the systematic analysis of the corresponding optical and electronic properties, represent strategic degrees of freedom the researchers may exploit to tailor WS2 optical and electronic properties for specific device applications.

Synthesis, Properties, and Application of Ultrathin and Flexible Tellurium Nanorope Films: Beyond Conventional 2D Materials

Nanomaterials that can be easily processed into thin films are highly desirable for their wide range of applicability in electrical and optical devices. Currently, Te-based 2D materials are of interest because of their superior electrical properties compared to transition metal dichalcogenide materials. However, the large-scale manufacturing of these materials is challenging, impeding their commercialization. This paper reports on ultrathin, large-scale, and highly flexible Te and Te-metal nanorope films grown via low-power radiofrequency sputtering for a short period at 25 °C. Additionally, the feasibility of such films as transistor channels and flexible transparent conductive electrodes is discussed. A 20 nm thick Te-Ni-nanorope-channel-based transistor exhibits a high mobility (≈450 cm2 V-1 s-1 ) and on/off ratio (105 ), while 7 nm thick Te-W nanorope electrodes exhibit an extremely low haze (1.7%) and sheet resistance (30 Ω sq-1 ), and high transmittance (86.4%), work function (≈4.9 eV), and flexibility. Blue organic light-emitting diodes with 7 nm Te-W anodes exhibit significantly higher external quantum efficiencies (15.7%), lower turn-on voltages (3.2 V), and higher and more uniform viewing angles than indium-tin-oxide-based devices. The excellent mechanical flexibility and easy coating capability offered by Te nanoropes demonstrate their superiority over conventional nanomaterials and provide an effective outlet for multifunctional devices.

High-Performance Self-Powered WSe2/ReS2 Photodetector Enabled via Surface Charge Transfer Doping

Two-dimensional (2D) van der Waals heterostructures based on various 2D transition metal dichalcogenides are widely used in photodetection applications. However, their response time and photoresponsivity are limited, posing a challenge for their applications in high-sensitivity photodetection. Surface charge transfer doping (SCTD) has emerged as a novel doping approach for low-dimensional materials with high specific surface area and attracted considerable attention, as it is simple and effective, does not damage the lattice, and considers various types of dopants. Herein, we prepare p-i-n junction-based photodetectors via the SCTD of WSe2/ReS2 heterojunctions using p-type dopant F4-TCNQ molecules, where doped WSe2 serves as a p-type semiconductor, undoped WSe2 acts as an intrinsic layer, and ReS2 functions as an n-type semiconductor. The surface-charge-transfer-doped WSe2/ReS2 heterojunction leads to a reduction in the Schottky barrier and an increase in the built-in electric field compared with the as-fabricated heterojunction. In the photovoltaic mode and under 785 nm laser illumination, the photodiode exhibits an increase in responsivity from 0.08 to 0.29 A/W, specific detectivity from 1.89 × 1012 to 8.02 × 1012 Jones, and the external quantum efficiency from 12.67 to 46.29%. Additionally, the p-i-n structure expands the depletion region width, resulting in a photovoltaic response time of 7.56/6.48 μs and a -3 dB cutoff frequency of over 85 kHz, an order of magnitude faster than the pristine response time. Herein, we derive an effective and simple scheme for designing high-performance, low-power optoelectronic devices based on 2D van der Waals heterostructures.

High-Throughput Computational Screening of All-MXene Metal-Semiconductor Junctions for Schottky-Barrier-Free Contacts with Weak Fermi-Level Pinning

Van der Waals (vdW) metal-semiconductor junctions (MSJs) exhibit huge potential to reduce the contact resistance and suppress the Fermi-level pinning (FLP) for improving the device performance, but they are limited by optional (2D) metals with a wide range of work functions. Here a new class of vdW MSJs entirely composed of atomically thin MXenes is reported. Using high-throughput first-principles calculations, highly stable 80 metals and 13 semiconductors are screened from 2256 MXene structures. The selected MXenes cover a broad range of work functions (1.8-7.4 eV) and bandgaps (0.8-3 eV), providing a versatile material platform for constructing all-MXene vdW MSJs. The contact type of 1040 all-MXene vdW MSJs based on Schottky barrier heights (SBHs) is identified. Unlike conventional 2D vdW MSJs, the formation of all-MXene vdW MSJs leads to interfacial polarization, which is responsible for the FLP and deviation of SBHs from the prediction of Schottky-Mott rule. Based on a set of screening criteria, six Schottky-barrier-free MSJs with weak FLP and high carrier tunneling probability (>50%) are identified. This work offers a new way to realize vdW contacts for the development of high-performance electronic and optoelectronic devices.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Nucleation and Growth of Monolayer MoS2 at Multisteps of MoO2 Crystals by Sulfurization

Two-dimensional (2D) materials and their heterostructures are promising for next-generation optoelectronics, spintronics, valleytronics, and electronics. Despite recent progress in various growth studies of 2D materials, mechanical exfoliation of flakes is still the most common method to obtain high-quality 2D materials because precisely controlling material growth and synthesizing a single domain during the growth process of 2D materials, for the desired shape and quality, is challenging. Here, we report the nucleation and growth behaviors of monolayer MoS₂ by sulfurizing a faceted monoclinic MoO₂ crystal. The MoS₂ layers nucleated at the thickness steps of the MoO₂ crystal and grew epitaxially with crystalline correlation to the MoO₂ surface. The epitaxially grown MoS₂ layer expands outwardly on the SiO₂ substrate, resulting in a monolayer single-crystal film, despite multiple nucleations of MoS₂ layers on the MoO₂ surface owing to several thickness steps. Although the photoluminescence of MoS₂ is quenched owing to efficient charge transfer between MoS₂ and metallic MoO₂, the MoS₂ stretched out to the SiO₂ substrate shows a high carrier mobility of (15 cm² V^-1 s^-1), indicating that a high-quality monolayer MoS₂ film can be grown using the MoO₂ crystal as a seed and precursor. Our work shows a method to grow high-quality MoS₂ using a faceted MoO₂ crystal and provides a deeper understanding of the nucleation and growth of 2D materials on a step-like surface.

Large-Area and Visible-Light-Driven Heterojunctions of In2O3/Graphene Built for ppb-Level Formaldehyde Detection at Room Temperature

Achieving convenient and accurate detection of indoor ppb-level formaldehyde is an urgent requirement to ensure a healthy working and living environment for people. Herein, ultrasmall In₂O₃ nanorods and supramolecularly functionalized reduced graphene oxide are selected as hybrid components of visible-light-driven (VLD) heterojunctions to fabricate ppb-level formaldehyde (HCHO) gas sensors (named InAG sensors). Under 405 nm visible light illumination, the sensor exhibits an outstanding response toward ppb-level HCHO at room temperature, including the ultralow practical limit of detection (pLOD) of 5 ppb, high response (R_a/R_g = 2.4, 500 ppb), relatively short response/recovery time (119 s/179 s, 500 ppb), high selectivity, and long-term stability. The ultrasensitive room temperature HCHO-sensing property is derived from visible-light-driven and large-area heterojunctions between ultrasmall In₂O₃ nanorods and supramolecularly functionalized graphene nanosheets. The performance of the actual detection toward HCHO is evaluated in a 3 m³ test chamber, confirming the practicability and reliability of the InAG sensor. This work provides an effective strategy for the development of low-power-consumption ppb-level gas sensors.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

A polymer electrolyte design enables ultralow-work-function electrode for high-performance optoelectronics

Ambient solution-processed conductive materials with a sufficient low work function are essential to facilitate electron injection in electronic and optoelectronic devices but are challenging. Here, we design an electrically conducting and ambient-stable polymer electrolyte with an ultralow work function down to 2.2 eV, which arises from heavy n-doping of dissolved salts to polymer matrix. Such materials can be solution processed into uniform and smooth films on various conductors including graphene, conductive metal oxides, conducting polymers and metals to substantially improve their electron injection, enabling high-performance blue light-emitting diodes and transparent light-emitting diodes. This work provides a universal strategy to design a wide range of stable charge injection materials with tunable work function. As an example, we also synthesize a high-work-function polymer electrolyte material for high-performance solar cells.

Ambient-stable solution-processed conductive materials with a low work function are essential to facilitate electron injection. Here, the authors design and synthesise polymer electrolyte with work function down to 2.2 eV for applications in high-performance light-emitting diodes and solar cells.

Enhanced Thermoelectric Power Factor in Carrier-Type-Controlled Platinum Diselenide Nanosheets by Molecular Charge-Transfer Doping

2D transition metal dichalcogenides (TMDCs) have revealed great promise for realizing electronics at the nanoscale. Despite significant interests that have emerged for their thermoelectric applications due to their predicted high thermoelectric figure of merit, suitable doping methods to improve and optimize the thermoelectric power factor of TMDCs have not been studied extensively. In this respect, molecular charge-transfer doping is utilized effectively in TMDC-based nanoelectronic devices due to its facile and controllable nature owing to a diverse range of molecular designs available for modulating the degree of charge transfer. In this study, the power of molecular charge-transfer doping is demonstrated in controlling the carrier-type (n- and p-type) and thermoelectric power factor in platinum diselenide (PtSe₂ ) nanosheets. This, combined with the tunability in the band overlap by changing the thickness of the nanosheets, allows a significant increase in the thermoelectric power factor of the n- and p-doped PtSe₂ nanosheets to values as high as 160 and 250 µW mK^-2 , respectively. The methodology employed in this study provides a simple and effective route for the molecular doping of TMDCs that can be used for the design and development of highly efficient thermoelectric energy conversion systems.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Toward Reduced Interface Contact Resistance: Controllable Surface Energy of Sb2Te3 Films via Tuning the Crystallization and Orientation

The electrical contact resistance between a metal and semiconductor is one of the keys to improving the output performance of thin-film thermoelectric devices. Herein, we reduced the interface contact resistance by controlling the surface energy of a Sb₂Te₃ semiconductor via tuning of the crystallization and orientation, preparing an intrinsically compact and flat Sb₂Te₃ film with high surface energy and low roughness, which can give rise to a low average specific contact resistivity (8.2 × 10^-6 Ω cm²) with a Ni/Cu metal. The improvement in interface electrical properties is due to the increase in the surface energy and decrease in the surface roughness of the semiconductor surface, which lead to a transformation from three-dimensional island-shaped nucleation to two-dimensional layered nucleation for surface-attached metal films, forming a longitudinally tight connection contact with a low resistance. This approach allows the resistivity to become close to the fundamental theoretically calculated limit. Our work provides a new idea for reducing the contact resistivity of thin-film thermoelectric devices, which is conducive to supporting the development of thermoelectric semiconductor planarization.

Damage-Free Charge Transfer Doping of 2D Transition Metal Dichalcogenide Channels by van der Waals Stamping of MoO3 and LiF

To dope 2D semiconductor channels, charge-transfer doping has generally been done by thermal deposition of inorganic or organic thin-film layers on top of the 2D channel in bottom-gate field-effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge-transfer doping process is suggested. A damage-free charge-transfer doping is conducted for 2D MoTe₂ (or MoS₂ ) channels using a polydimethylsiloxane stamp. MoO₃ or LiF is initially deposited on the stamp as a doping medium. Hysteresis-minimized transfer characteristics are achieved from stamp-doped FETs, while other devices with direct thermal deposition-doped channels show large hysteresis. The stamping method seems to induce a van der Waals-like damage-free interface between the channel and doping media. The stamp-induced doping is also well applied for a MoTe₂ -based complementary inverter because MoO₃ - and LiF-doping by separate stamps effectively modifies two ambipolar MoTe₂ channels to p- and n-type, respectively.

Metal oxide charge transfer complex for effective energy band tailoring in multilayer optoelectronics

Metal oxides are intensively used for multilayered optoelectronic devices such as organic light-emitting diodes (OLEDs). Many approaches have been explored to improve device performance by engineering electrical properties. However, conventional methods cannot enable both energy level manipulation and conductivity enhancement for achieving optimum energy band configurations. Here, we introduce a metal oxide charge transfer complex (NiO:MoO₃-complex), which is composed of few-nm-size MoO₃ domains embedded in NiO matrices, as a highly tunable carrier injection material. Charge transfer at the finely dispersed interfaces of NiO and MoO₃ throughout the entire film enables effective energy level modulation over a wide work function range of 4.47 – 6.34 eV along with enhanced electrical conductivity. The high performance of NiO:MoO₃-complex is confirmed by achieving 189% improved current efficiency compared to that of MoO₃-based green OLEDs and also an external quantum efficiency of 17% when applied to blue OLEDs, which is superior to 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile-based conventional devices.

One pathway for improving the performance of optoelectronics is the tailoring energy bands of the charge transport layer. Here, Kim et al present a charge transfer complex composed out of nanodomains of MoO₃ embedded within an NiO matrix, significantly improving green and blue OLED performance.

Surface Charge Transfer Doping of MoS2 Monolayer by Molecules with Aggregation-Induced Emission Effect

Surface charge transfer doping has attracted much attention in modulating the optical and electrical behavior of 2D transition metal dichalcogenides (TMDCs), where finding controllable and efficient dopants is crucial. Here, 1,1,2,2-tetraphenylethylene (TPE) derivative molecules with aggregation-induced emission (AIE) effect were selected as adjustable dopants. By designing nitro and methoxyl functional groups and surface coating, controlled p/n-type doping can be achieved on a chemical vapor deposition (CVD) grown monolayer, MoS2. We investigated the electron transfer behavior between these two dopants and MoS2 with fluorescence, Raman, X-ray photoelectron spectra and transient absorption spectra. 1,1,2,2-Tetrakis(4-nitrophenyl)ethane (TPE-4NO2) with a negative charge aggregation can be a donor to transfer electrons to MoS2, while 1,1,2,2-Tetrakis(4-methoxyphenyl)ethane (TPE-4OCH3) is the opposite and electron-accepting. Density functional theory calculations further explain and confirm these experimental results. This work shows a new way to select suitable dopants for TMDCs, which is beneficial for a wide range of applications in optoelectronic devices.

Atomic Layer MoTe2 Field-Effect Transistors and Monolithic Logic Circuits Configured by Scanning Laser Annealing

Atomically thin semiconductors such as transition metal dichalcogenides have recently enabled diverse devices in the emerging two-dimensional (2D) electronics. While scalable 2D electronics demand monolithic integrated circuits consisting of complementary p-type and n-type transistors, conventional p-type and n-type doping in desired regions, monolithically in the same semiconducting atomic layers, remains elusive or impractical. Here, we report on an agile, high-precision scanning laser annealing approach to realizing 2D monolithic complementary logic circuits on atomically thin MoTe₂, by reliably designating p-type and n-type transport polarity in the constituent transistors via localized laser annealing and modification of their Schottky contacts. Pristine p-type field-effect transistors (FETs) transform into n-type ones upon controlled laser annealing on their source/drain gold electrodes, exhibiting a mobility of 96.5 cm² V^-1 s^-1 (the highest known to date) and an On/Off ratio of 10⁶. Elucidation and validation of such an on-demand configuration of polarity in MoTe₂ FETs further enable the construction and demonstration of essential logic circuits, including both inverter and NOR gates. This dopant-free, spatially precise scanning laser annealing approach to configuring monolithic complementary logic integrated circuits may enable programmable functions in 2D semiconductors, exhibiting potential for additively manufactured, scalable 2D electronics.

Tuning the carrier type and density of monolayer tin selenide via organic molecular doping

Utilizing first-principles calculations, charge transfer doping process of single layer tin selenide (SL-SnSe) via the surface adsorption of various organic molecules was investigated. Effective p-type SnSe, with carrier concentration exceeding 3.59 × 10¹³ cm^-2, was obtained upon adsorption of tetracyanoquinodimethane or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane on SL-SnSe due to their lowest unoccupied molecular orbitals acting as shallow acceptor states. While we could not obtain effective n-type SnSe through adsorption of tetrathiafulvalene (TTF) or 1,4,5,8-tetrathianaphthalene on pristine SnSe due to their highest occupied molecular orbitals (HOMO) being far from the conduction band edge of SnSe, this disadvantageous situation can be amended by the introduction of an external electric field perpendicular to the monolayer surface. It is found that Sn_vacwill facilitate charge transfer from TTF to SnSe through introducing an unoccupied gap state just above the HOMO of TTF, thereby partially compensating for the p-type doping effect of Sn_vac. Our results show that both effective p-type and n-type SnSe can be obtained and tuned by charge transfer doping, which is necessary to promote its applications in nanoelectronics, thermoelectrics and optoelectronics.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane

Low-energy (0-15 eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ^- are detected in a wide electron energy range (0-3 eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ^- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6 eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

The Undoped Polycrystalline Diamond Film-Electrical Transport Properties

The polycrystalline diamonds were synthesized on n-type single crystalline Si wafer by Hot Filament CVD method. The structural properties of the obtained diamond films were checked by X-ray diffraction and Raman spectroscopy. The conductivity of n-Si/p-diamond, sandwiched between two electrodes, was measured in the temperature range of 90–300 K in a closed cycle cryostat under vacuum. In the temperature range of (200–300 K), the experimental data of the conductivity were used to obtain the activation energies Ea which comes out to be in the range of 60–228 meV. In the low temperature region i.e., below 200 K, the conductivity increases very slowly with temperature, which indicates that the conduction occurs via Mott variable range hopping in the localized states near Fermi level. The densities of localized states in diamond films were calculated using Mott’s model and were found to be in the range of 9×1013 to 5×1014eV−1cm−3 depending on the diamond’s surface hydrogenation level. The Mott’s model allowed estimating primal parameters like average hopping range and hopping energy. It has been shown that the surface hydrogenation may play a crucial role in tuning transport properties.

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

Improved Contact Resistance by a Single Atomic Layer Tunneling Effect in WS2 /MoTe2 Heterostructures

Manipulation of Ohmic contacts in 2D transition metal dichalcogenides for enhancing the transport properties and enabling its application as a practical device has been a long-sought goal. In this study, n-type tungsten disulfide (WS2 ) single atomic layer to improve the Ohmic contacts of the p-type molybdenum ditelluride (MoTe2 ) material is covered. The Ohmic properties, based on the lowering of Schottky barrier height (SBH) owing to the tunneling barrier effect of the WS2 monolayer, are found to be unexpectedly excellent at room temperature and even at 100 K. The improved SBH and contact resistances are 3 meV and 1 MΩ µm, respectively. The reduction in SBH and contact resistance is confirmed with temperature-dependent transport measurements. This study further demonstrates the selective carrier transport across the MoTe2 and WS2 layers by modulating the applied gate voltage. This WS2 /MoTe2 heterostructure exhibits excellent gate control over the currents of both channels (n-type and p-type). The on/off ratios for both the electron and hole channels are calculated as 107 and 106 , respectively, indicating good carrier type modulation by the electric field of the gate electrode. The Ohmic contact resistance using the tunneling of the atomic layer can be applied to heterojunction combinations of various materials.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Enhanced photocatalytic properties of a chemically modified blue phosphorene

It is high time to placate the peak demand for an efficient, economic and green fuel in the form of H2 through photocatalytic water splitting. Several low dimensional materials have been explored for their photocatalytic properties on account of their surface to volume ratio. The present study illustrates the excellent photocatalytic potential of a two-dimensional material, viz. a chemically tempered blue-phosphorene sheet, with single atom thickness and high carrier mobility. Metal-free element, sulphur, is explored as a dopant in a 32-atom blue-phosphorene sheet. The dopant is inserted at three locations viz. central, edge and central edge positions with varying concentrations from 3.125% to 18.75% (corresponding to n = 1 to 6 sulphur atoms within a 32-atom blue-phosphorene sheet, P32-n S n ). The cohesive energy studies predict the higher stability of even number S doped sheets as compared to their odd counterparts. Photocatalytic activity is studied in terms of band gap and band alignment for different concentrations of the former. Studies reveal that edge doping demonstrates better water molecule activation independent of S atom concentration. The edge doped systems not only provide the chemical activity to activate water, but also show feasible HER overpotentials of 1.24-1.29 eV at neutral medium. Finally, this work opens up a driving lead of non-corrosive catalysts for water molecule splitting.

Mechanical design of brush coating technology for the alignment of one-dimension nanomaterials

Widespread approaches to fabricate surfaces with aligned nanostructured topographies have been stimulated by opportunities to enhance interface performance by combing physical and chemical effects, in which brush-coating technology (BCT) is a cost-effective and feasible method for aligned film and large-scale production. Here, we reported a BCT process to realize the alignment of various 1D nanostructures through mechanical design that provides a more precise and higher shear force. By regulating the viscosity of dispersion, shear force is proved to be 24 and 20.3 times larger (when the volume ratio of water and glycerol is 1:3) according to the theoretical calculation and ANSYS simulating calculation results respectively, which plays a vital role in brush coating process. The universality was demonstrated by the alignment of one-dimension nanomaterials with different diameters, including silver nanowires (~80 nm), molybdenum trioxide nanobelts (~150 nm), vanadium pentoxide nanobelts (~150 nm) and bismuth sulfide nanobelts (~200 nm), et al., which in consequence have different alignment ratios. Meanwhile, anisotropic and flexible electrical conductors (the resistance anisotropic ratio was 2) and thermoelectric films (Seebeck coefficient was calculated to be 56.7 µV/K) were demonstrated.

Selective p-Doping of 2D WSe2 via UV/Ozone Treatments and Its Application in Field-Effect Transistors

Development of two-dimensional (2D) semiconductor devices with good Ohmic contact is essential to utilize their full potential for nanoelectronics applications. Among the methods that have been introduced to reduce the Schottky barrier in 2D material-based electronic devices, charge transfer doping has attracted significant interest because of its efficiency, simplicity, and compatibility with the microelectronic fabrication process. In this study, 2D WSe₂-based field-effect transistors (FETs) were subjected to selective UV/ozone treatment to improve the Ohmic contact by forming WO_X with a high work function, which induced hole doping in the neighboring WSe₂ via electron transfer. The atomic force microscopy, cross-sectional transmission electron microscopy, and micro-Raman spectroscopy analyses confirmed the self-limiting formation of WO_X while maintaining the crystallinity of the underlying WSe₂. The channel layer of the back-gated 2D WSe₂ FETs was encapsulated using 2D hexagonal boron nitride to prevent the UV/ozone-induced oxidation. By contrast, the regions that were in contact with the underlying metal electrodes were open, which allowed area-selective p-doping in the 2D WSe₂. Our study demonstrated that the Ohmic-like behaviors obtained after area-selective UV/ozone treatment improved the electrical properties of the 2D WSe₂-based FETs such as the field-effect mobility (improvement of 3-4 orders of magnitude) and current on/off ratio (improvement of five orders of magnitude), while maintaining the p-type normally-off characteristics. These results provide useful insights into an effective and facile method to reduce contact resistance in 2D semiconductor materials, thereby enhancing the electrical performances of 2D material-based electronic devices.

Enhanced Photoluminescence of Multiple Two-Dimensional van der Waals Heterostructures Fabricated by Layer-by-Layer Oxidation of MoS2

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS₂ and MoO_x with enhanced photoluminescence (PL). We directly oxidized multilayer MoS₂ to a MoO_x/1 L-MoS₂ heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS₂ covered by MoO_x showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS₂ because of preserved crystallinity and compensated dedoping by MoO_x. By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoO_x/MoS₂/MoO_x/MoS₂, where the MoS₂ monolayers are separated by MoO_x. The heterostructures showed the multiplied PL intensity as the number of embedded MoS₂ layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS₂ layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.

Semiconductivity Transition in Silicon Nanowires by Hole Transport Layer

The surface of nanowires is a source of interest mainly for electrical prospects. Thus, different surface chemical treatments were carried out to develop recipes to control the surface effect. In this work, we succeed in shifting and tuning the semiconductivity of a Si nanowire-based device from n- to p-type. This was accomplished by generating a hole transport layer at the surface by using an electrochemical reaction-based nonequilibrium position to enhance the impact of the surface charge transfer. This was completed by applying different annealing pulses at low temperature (below 400 °C) to reserve the hydrogen bonds at the surface. After each annealing pulse, the surface was characterized by XPS, Kelvin probe measurements, and conductivity measured by FET based on a single Si NW. The mechanism and conclusion were supported experimentally and theoretically. To this end, this strategy has been demonstrated as an essential tool which could pave a new road for regulating semiconductivity and for other low-dimensional nanomaterials.

Surface Modification of Monolayer MoS2 by Baking for Biomedical Applications

Molybdenum disulfide (MoS₂), a transition metal dichalcogenide material, possesses great potential in biomedical applications such as chemical/biological sensing, drug/gene delivery, bioimaging, phototherapy, and so on. In particular, monolayer MoS₂ has more extensive applications because of its superior physical and chemical properties; for example, it has an ultra-high surface area, is easily modified, and has high biodegradability. It is important to prepare advanced monolayer MoS₂ with enhanced energy exchange efficiency (EEE) for the development of MoS₂-based nanodevices and therapeutic strategies. In this work, a monolayer MoS₂ film was first synthesized through a chemical vapor deposition method, and the surface of MoS₂ was further modified via a baking process to develop p-type doping of monolayer MoS₂ with high EEE, followed by confirmation by X-ray photoelectron spectroscopy and Raman spectroscopy analysis. The morphology, surface roughness, and layer thickness of monolayer MoS₂ before and after baking were thoroughly investigated using atomic force microscopy. The results showed that the surface roughness and layer thickness of monolayer MoS₂ modified by baking were obviously increased in comparison with MoS₂ without baking, indicating that the surface topography of the monolayer MoS₂ film was obviously influenced. Moreover, a photoluminescence spectrum study revealed that p-type doping of monolayer MoS₂ displayed much greater photoluminescence ability, which was taken as evidence of higher photothermal conversion efficiency. This study not only developed a novel MoS₂ with high EEE for future biomedical applications but also demonstrated that a baking process is a promising way to modify the surface of monolayer MoS₂.

Thickness Trends of Electron and Hole Conduction and Contact Carrier Injection in Surface Charge Transfer Doped 2D Field Effect Transistors

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O₂ plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS₂, MoSe₂, MoTe₂, WS₂, and WSe₂ flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the doping-induced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

Pushing the conductance and transparency limit of monolayer graphene electrodes for flexible organic light-emitting diodes

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti-reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light-emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A-1, 124.9 lm W-1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics.

Modulating the Electronic, Optical, and Transport Properties of CdTe and ZnTe Nanostructures with Organic Molecules: A Theoretical Investigation

In this paper, we systematically investigated the electronic, optical, and transport properties of CdTe and ZnTe nanostructures before and after adsorption with benzyl viologen (BV) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) organic molecules based on the first principles calculation. First, the band gaps of CdTe and ZnTe nanostructures obviously decrease after BV and F4-TCNQ adsorptions. Interestingly, the electronic property calculation shows that BV and F4-TCNQ can donate/accept electrons to/from the surface of CdTe and ZnTe nanostructures, leading to an effective n-/p-type doping, respectively. Second, the optical absorption in a broad spectral range (from visible to near-infrared) of CdTe and ZnTe is significantly improved by adsorption of BV and F4-TCNQ molecules, offering great opportunities for the use of CdTe and ZnTe nanostructures in renewable energy fields. Lastly, the electrical transfer characteristics on CdTe and ZnTe nanostructure-based field-effect transistors clearly showed that the conduction of the nanostructures can be rationally tuned into n- and p-type conductivity with BV and F4-TCNQ adsorptions, respectively. Our work clearly demonstrates that the electronic, optical, and transport properties of CdTe and ZnTe nanostructures are effectively modulated by adsorption of BV and F4-TCNQ, which can be used to construct high-performance electronic and optoelectronic devices.

Binding and electronic level alignment of π-conjugated systems on metals

We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

Highly Efficient n-Type Doping of Graphene by Vacuum Annealed Amine-Rich Macromolecules

Flexible transparent conducting electrodes (FTCE) are an essential component of next-generation flexible optoelectronic devices. Graphene is expected to be a promising material for the FTCE, because of its high transparency, large charge carrier mobilities, and outstanding chemical and mechanical stability. However, the electrical conductivity of graphene is still not good enough to be used as the electrode of an FTCE, which hinders its practical application. In this study, graphene was heavily n-type doped while maintaining high transmittance by adsorbing amine-rich macromolecules to graphene. The n-type charge-transfer doping of graphene was maximized by increasing the density of free amine in the macromolecule through a vacuum annealing process. The graphene adsorbed with the n-type dopants was stacked twice, resulting in a graphene FTCE with a sheet resistance of 38 ohm/sq and optical transmittance of 94.1%. The figure of merit (FoM) of the graphene electrode is as high as 158, which is significantly higher than the minimum standard for commercially available transparent electrodes (FoM = 35) as well as graphene electrodes doped with previously reported chemical doping methods. Furthermore, the n-doped graphene electrodes not only show outstanding flexibility but also maintain the doping effect even in high temperature (500 K) and high vacuum (~10⁻⁶ torr) conditions. These results show that the graphene doping proposed in this study is a promising approach for graphene-based next-generation FTCEs.