2D Material-based Devices Research Articles

Monolithic 3D integration of back-end compatible 2D material FET on Si FinFET

The performance enhancement of integrated circuits relying on dimension scaling (i.e., following Moore’s Law) is more and more challenging owing to the physical limit of Si materials. Monolithic three-dimensional (M3D) integration has been considered as a powerful scheme to further boost up the system performance. Two-dimensional (2D) materials such as MoS2 are potential building blocks for constructing upper-tier transistors owing to their high mobility, atomic thickness, and back-end-of-line (BEOL) compatible processes. The concept to integrate 2D material-based devices with Si field-effect transistor (FET) is technologically important but the compatibility is yet to be experimentally demonstrated. Here, we successfully integrated an n-type monolayer MoS2 FET on a p-type Si fin-shaped FET with 20 nm fin width via an M3D integration technique to form a complementary inverter. The integration was enabled by deliberately adopting industrially matured techniques, such as chemical mechanical planarization and e-beam evaporation, to ensure its compatibility with the existing 3D integrated circuit process and the semiconductor industry in general. The 2D FET is fabricated using low-temperature sequential processes to avoid the degradation of lower-tier Si devices. The MoS2 n-FETs and Si p-FinFETs display symmetrical transfer characteristics and the resulting 3D complementary metal-oxide-semiconductor inverter show a voltage transfer characteristic with a maximum gain of ~38. This work clearly proves the integration compatibility of 2D materials with Si-based devices, encouraging the further development of monolithic 3D integrated circuits.

Atomic-Scale Surface Engineering for Giant Thermal Transport Enhancement Across 2D/3D van der Waals Interfaces.

Heat dissipation in two-dimensional (2D) material-based electronic devices is a critical issue for their applications. The bottleneck for this thermal issue is inefficient for heat removal across the van der Waals (vdW) interface between the 2D material and its supporting three-dimensional (3D) substrate. In this work, we demonstrate that an atomic-scale thin amorphous layer atop the substrate surface can remarkably enhance the interfacial thermal conductance (ITC) of the 2D-MoS2/3D-GaN vdW interface by a factor of 4 compared to that of the untreated crystalline substrate surface. Meanwhile, the ITC can be broadly manipulated through adjusting substrate surface roughness. Phonon dynamic and heat flux spectrum analyses show that this giant enhancement is attributed to the increased phonon densities and channels at the interfaces and enhanced phonon coupling. The slight surface fluctuation in MoS2 and the increased diffuse interfacial scattering facilitate energy transfer from MoS2's in-plane phonons to its out-of-plane phonons and then to the substrate. In addition, it is further found that the substrate and its surface topology can dramatically influence the thermal conductivity of MoS2 due to the reduction of phonon relaxation time, especially for low-frequency acoustic phonons. This study elucidates the effects of the amorphous surface of the substrate on thermal transport across 2D/3D vdW interfaces and provides a new dimension to aid in the heat dissipation of 2D-based electronic devices via atomic-scale surface engineering.

High-Temperature Annealing Effects on Atomically Thin Tungsten Diselenide Field-Effect Transistor

Two-dimensional (2D) material-based devices are expected to operate under high temperatures induced by Joule heating and environmental conditions when integrated into compact integrated circuits for practical applications. However, the behavior of these materials at high operating temperatures is obscure as most studies emphasize only room temperature or low-temperature operation. Here, the high-temperature electrical response of the tungsten diselenide (WSe2) field-effect transistor was studied. It is revealed that 350 K is the optimal annealing temperature for the WSe2 transistor, and annealing at this temperature improves on-current, field-effect mobility and on/off ratio around three times. Annealing beyond this temperature (360 K to 670 K) adversely affects the device performance attributed to the partial oxidation of WSe2 at higher temperatures. An increase in hysteresis also confirms the formation of new traps as the device is annealed beyond 350 K. These findings explicate the thermal stability of WSe2 and can help design 2D materials-based durable devices for high-temperature practical applications.

Graphene Via Contact Architecture for Vertical Integration of vdW Heterostructure Devices.

Two-dimensional (2D) devices and their van der Waals (vdW) heterostructures attract considerable attention owing to their potential for next-generation logic and memory applications. In addition, 2D devices are projected to have high integration capabilities, while maintaining nanoscale thickness. However, the fabrication of 2D devices and their circuits is challenging because of the high precision required to etch and pattern ultrathin 2D materials for integration. Here, the fabrication of a graphene via contact architecture to electrically connect graphene electrodes (or leads) embedded in vdW heterostructures is demonstrated. Graphene via contacts comprising of edge and fluorinated graphene (FG) electrodes are fabricated by successive fluorination and plasma etching processes. A one-step fabrication process that utilizes the graphene contacts is developed for a vertically integrated complementary inverter based on n- and p-type 2D field-effect transistors (FETs). This study provides a promising method to fabricate vertically integrated 2D devices, which are essential in 2D material-based devices and circuits.

The influence of laser-induced alignment on Z-scan properties of 2D carbon nanomaterials suspension dependent on polarization

The Z-scan technique uses a single beam that can be used for observing the nonlinear or optical limiting properties of materials. For the first time, the Z-scan properties dependent on the polarization of 2D carbon nanomaterial suspension were experimentally investigated using optical Z-scan technology. The Z-scan curves of graphene and graphene oxide (GO) in N-methyl-2-pyrrolidinone suspensions exhibited strong polarization-dependent characteristics. In paper, a reverse saturated absorption (RSA) dip surrounded the lens focus when the horizontal polarized beam was focused in the suspension, and two saturated absorption (SA) peaks appeared adjacent to the dip. However, for the vertical polarized beam, only one RSA dip surrounded the lens focus, and the threshold was higher than the SA for a horizontally polarized beam. The transmission of RSA for the GO suspension was evidently lower than that of the graphene suspension. The polarization-dependent characteristic can be ascribed to the laser-induced alignment in case the suspension is moved in or out of the beam focal point. Furthermore, the polarization-dependent 2D carbon nanomaterial suspension can be applied in several practical purposes such as 2D material-based optical and opto-fludic devices.

Probing Nanoscale Exciton Funneling at Wrinkles of Twisted Bilayer MoS2 Using Tip-Enhanced Photoluminescence Microscopy.

In twisted bilayer (t2L) two-dimensional (2D) transition metal dichalcogenides, local strain at wrinkles strongly modulates the local exciton density and PL energy resulting in an exciton funneling effect. Probing such exciton behaviors especially at nanometer length scales is beyond the limit of conventional analytical tools due to the limited spatial resolution and low sensitivity. To address this challenge, herein we applied high-resolution tip-enhanced photoluminescence (TEPL) microscopy to investigate exciton funneling at a wrinkle in a t2L MoS2 sample with a small twist angle of 0.5°. Owing to a spatial resolution of <10 nm, excitonic behavior at nanoscale sized wrinkles could be visualized using TEPL imaging. Detailed investigation of nanoscale exciton funneling at the wrinkles revealed a deformation potential of -54 meV/%. The obtained results provide novel insights into the inhomogeneities of excitonic behaviors at nanoscale and would be helpful in facilitating the rational design of 2D material-based twistronic devices.

Phase-controlled epitaxial growth of MoTe2: Approaching high-quality 2D materials for electronic devices with low contact resistance

Because silicon transistors are approaching the limit of device miniaturization, 2D semiconductors show great promise in electronic devices as post-silicon alternatives. However, critical bottlenecks that impede applications remain in 2D material-based devices, such as the lack of scalable fabrication techniques of highly crystalline samples and the challenge of contact resistance. In this Perspective, we review the recently developed 2D MoTe2 as an excellent material in phase-controlled epitaxial growth and phase transition. The high flexibility in phase engineering of MoTe2 enables (1) wafer-scale fabrication of semiconducting MoTe2 single crystals and (2) intrinsically ideal contact geometry for high-performance electronic devices.

Phase Modulation of Self-Gating in Ionic Liquid-Functionalized InSe Field-Effect Transistors.

Understanding the Coulomb interactions between two-dimensional (2D) materials and adjacent ions/impurities is essential to realizing 2D material-based hybrid devices. Electrostatic gating via ionic liquids (ILs) has been employed to study the properties of 2D materials. However, the intrinsic interactions between 2D materials and ILs are rarely addressed. This work studies the intersystem Coulomb interactions in IL-functionalized InSe field-effect transistors by displacement current measurements. We uncover a strong self-gating effect that yields a 50-fold enhancement in interfacial capacitance, reaching 550 nF/cm2 in the maximum. Moreover, we reveal the IL-phase-dependent transport characteristics, including the channel current, carrier mobility, and density, substantiating the self-gating at the InSe/IL interface. The dominance of self-gating in the rubber phase is attributed to the correlation between the intra- and intersystem Coulomb interactions, further confirmed by Raman spectroscopy. This study provides insights into the capacitive coupling at the InSe/IL interface, paving the way to developing liquid/2D material hybrid devices.

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

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

Graphene–Silicon Diode for 2-D Heterostructure Electrical Failure Protection

Two-dimensional materials have modernized a broad interest in electronic devices. Along with many advantages, their atomic-level thickness makes them sensitive under high electrical stress. This work proposes a protection design using a Graphene/Silicon (Gr/Si) Schottky diode as the protective device, which helps to improve the endurance for unwanted fluctuations in operating voltage of 2D heterostructure-based devices. In this scheme, the 2D heterostructure was configured parallel with the protective device (Gr/Si diode) for electrical measurements. It was found that Gr/Si diode handles a large portion of initial surge current peaks, which significantly increases the durability and lifetime of 2D material-based heterostructure devices. This scheme potentially bridges mature CMOS technology and novel 2D-based heterostructure applications for robust futuristic devices.

Spin-Dependent Transport at 2D Solids: From Nonmagnetic Layers to Ferromagnetic van der Waals Structures

Spintronics is a promising alternative to the conventional silicon transistor-based electronics that are gradually approaching their physical limitations. Ultrathin two-dimensional van der Waals (vdW) materials (2D materials) with controllable spin degrees of freedom are recognized as extremely promising spintronic materials in architectures for the post-Moore era. In this Perspective, we review recent progress on spin-dependent transport behaviors (SDTBs) confined at the 2D scale, which are the mainstream paradigms for spintronic devices. We first present the mechanism and the key factors of SDTBs in 2D nonmagnetic materials-based hybrid devices. Then, some chemical modulation strategies for inducing short-range magnetic order and magneto-electric performance into 2D nonmagnetic materials are discussed. Furthermore, we concentrate on introducing intriguing SDTBs in 2D long-range ferromagnetic materials-based vdW devices. Finally, we highlight the current challenges in the study of spin-dependent transport of 2D modified materials and 2D material-based spintronic devices, in the hope of accelerating their applications.

Thermodynamic Perspective on the Oxidation of Layered Materials and Surface Oxide Amelioration in 2D Devices.

Surface oxidation is an unneglectable problem for 2D semiconductors because it hinders the practical application of 2D material-based devices. In this research, the oxidation of layered materials is investigated by a thermodynamic approach to verify their oxidation tendency. It was found that almost all 2D materials are thermodynamically unstable in the presence of oxygen at room temperature. Two potential solutions for surface oxidation are proposed in this work: (i) the conversion of the surface oxides to functional oxides through the deposition of active metals and (ii) the recovery of original 2D materials from the surface oxides by 2D material heterostructure formation with the same chalcogen group. Supported by thermodynamic calculations, both approaches are feasible to ameliorate the surface oxides of 2D materials by the appropriate selection of metals for deposition or 2D materials for heterostructure formation. Thermodynamic data of 64 elements and 75 2D materials are included and compared in this research, which can improve gate insulator or electrode contact material selection in 2D devices to solve the surface oxidation issue. For instance, yttrium and titanium are good candidates for surface oxide conversion, while zirconium and hafnium chalcogenide can trigger the recovery of original 2D materials from their surface oxides. The systematic diagrams presented in this work can serve as a guideline for considering surface oxidation in future device fabrication from various 2D materials.

Method of lines for the analysis of tunable plasmonic devices composed of graphene-dielectric stack arrays.

Due to the increasing interest in emerging applications of graphene or other 2D material-based devices in photonics, a powerful, fast and accurate tool for the analysis of such structures is really in need. In this paper, the semi-analytical method of lines (MoL) is generalized for the diffraction analysis of tunable graphene-based plasmonic devices possessing three dimensional periodicity. We employ Floquet's theorem to handle analytically propagation of waves in the periodicity of the graphene-dielectric arrays in the direction of the layers stacking. This makes the method very effective in terms of computational time and memory consumption. To validate its efficiency and accuracy, the method is applied to plasmonic devices formed by alternating patterned graphene sheets and dielectric layers. Direct comparison with results available in literature and those obtained by a commercial software exhibits their full consistency.

Negative Differential Resistance in Carbon-Based Nanostructures

Nonlinear electrical properties, such as negative differential resistance (NDR), are essential in numerous electrical circuits, including memristors. Several physical origins have been proposed to lead to the NDR phenomena in semiconductor devices in the last more than half a century. Here, we report NDR behavior formation in randomly oriented graphene-like nanostructures up to 37 K and high on-current density up to 10^5 A/cm^2. Our modeling of the current-voltage characteristics, including the self-heating effects, suggests that strong temperature dependence of the low-bias resistance is responsible for the nonlinear electrical behavior. Our findings open opportunities for the practical realization of the on-demand NDR behavior in nanostructures of 2D and 3D material-based devices via heat management in the conducting films and the underlying substrates.

Upconversion Photovoltaic Effect of WS2/2D Perovskite Heterostructures by Two-Photon Absorption.

Photovoltaic devices work by converting sunlight energy into electric energy. The efficiency of current photovoltaic devices, however, is significantly limited by the transmission loss of photons with energies below the bandgap of channel semiconductors, which can be circumvented by photon energy upconversion. Energy upconversion has been widely employed to improve the efficiency of traditional solar cells. However, the employment of energy upconversion in two-dimensional (2D) heterostructure photovoltaic devices has not been investigated yet. Here, we report the upconversion photovoltaic effect of WS2 monolayer/(C6H5C2H4NH3)2PbI4 (PEPI) 2D perovskite heterostructures by below-bandgap two-photon absorption via a virtual intermediate state. An open circuit voltage of 0.37 V and short circuit current of 7.4 pA are obtained with a photoresponsivity of 771 pA/W and current on/off ratio of 130:1. This work demonstrates that upconversion by two-photon absorption may potentially be a strategy for boosting the efficiency of 2D material-based photovoltaic devices by virtue of the absorption of photons below the bandgap energy of channel semiconductors.

Damage-free transfer mechanics of 2-dimensional materials: competition between adhesion instability and tensile strain

The transfer of two-dimensional (2D) materials is crucial to the realization of 2D material-based devices for practical applications. The thinness of 2D materials renders them prone to mechanical damage during the transfer process and to degradation of their superior electrical and mechanical properties. Herein, the mechanisms involved in the damage of chemical vapor deposition-grown graphene (Gr) and MoS2 are investigated during a roll-based transfer process. We identify two different damage mechanisms, i.e., instability-induced damage and tensile strain-induced damage. The two mechanisms compete, depending on the thickness of the transfer medium, and induce dissimilar damage. By minimizing these two mechanisms, we realize and demonstrate the damage-free transfer of 2D materials. The sheet resistance and mobility of transferred Gr are 235 ± 29 Ω sq–1 and 2250 cm2 V–1 s–1, respectively, with no microscopic cracks or tear-out damage. We observe instability-induced damage to be ubiquitous in monolayer MoS2, thin metals, and thin oxide films. By understanding the instability-induced damage mechanism, a broad range of 2D materials and thin films can be transferred without mechanical damage. Damage-free transfer will contribute to the high-yield fabrication of 2D material-based electronic devices.

2D Materials Enabled Next-Generation Integrated Optoelectronics: from Fabrication to Applications.

2D materials, such as graphene, black phosphorous and transition metal dichalcogenides, have gained persistent attention in the past few years thanks to their unique properties for optoelectronics. More importantly, introducing 2D materials into silicon photonic devices will greatly promote the performance of optoelectronic devices, including improvement of response speed, reduction of energy consumption, and simplification of fabrication process. Moreover, 2D materials meet the requirements of complementary metal‐oxide‐semiconductor compatible silicon photonic manufacturing. A comprehensive overview and evaluation of state‐of‐the‐art 2D photonic integrated devices for telecommunication applications is provided, including light sources, optical modulators, and photodetectors. Optimized by unique structures such as photonic crystal waveguide, slot waveguide, and microring resonator, these 2D material‐based photonic devices can be further improved in light‐matter interactions, providing a powerful design for silicon photonic integrated circuits.

Recent progress in solution assembly of 2D materials for wearable energy storage applications

Wearable energy storage devices are desirable to boost the rapid development of flexible and stretchable electronics. Two-dimensional (2D) materials, e.g., graphene, transition metal dichalcogenides and oxides, and MXenes, have attracted intensive attention for flexible energy storage applications because of their ultrathin 2D structures, high surface-to-volume ratio, and unique physical/chemical properties. To achieve commercialization of 2D material-based wearable energy storage devices (2DM-WESDs), scalable and cost-efficient manufacturing is a critical challenge. Among existing manufacturing technologies, solution-based assembly strategies show strong potential to achieve low-cost and scalable production. A timely review of the recent progress in solution-based assembly strategies and the resultant 2DM-WESDs will be meaningful to guide the future development of 2DM-WESDs. In this review, first, a brief introduction of exfoliation and solution preparation of 2D material species from bulk materials is discussed. Then, the solution-based assembly strategies are summarized, and the advantages and disadvantages of each method are compared. After that, two major categories of 2DM-WESDs, supercapacitor and battery, are discussed, emphasizing their state-of-the-art energy storage performances and flexibilities. Finally, insights and perspectives on current challenges and future opportunities regarding the solution assembly of 2DM-WESDs are discussed.

Engineering Schottky-to-Ohmic contact transition for 2D metal–semiconductor junctions

An Ohmic contact is critical for achieving 2D material-based high performance electronic devices. Unfortunately, the formation of an intrinsic Ohmic contact for 2D materials is difficult; thus, current studies mostly stay in the Schottky regime. In this work, density functional calculations are performed for work function engineering for metal–semiconductor junctions involving 2D H-WSe2 and 2D metals of MX2 (M = Ti, V, Nb, Ta, Mo, and W and X = S and Se). We unambiguously identify a Schottky-to-Ohmic contact transition boundary, beyond which p-type Ohmic contacts are demonstrated to be stable. We show that the Fermi level pinning effect is relatively weak in the Schottky region, while similar pinning-like behavior is strong in the Ohmic region, creating a discontinuity near the contact transition boundary. The observed deviation from the ideal Schottky–Mott limit is directly related to the charge redistribution and interface dipole-induced potential step, reflected by metal work function modification due to contact formation. Our work not only provides a strategy to identify effective Ohmic contacts but also offers insights for prospection into the fundamental electronic properties of van der Waals-based heterojunctions.

Electrochemical Functionalization of Selectively Addressed MoS2 Nanoribbons for Sensor Device Fabrication

Tailoring the surface properties of 2D materials, such as transition metal dichalcogenides (TMDCs), at the nanoscale is becoming essential in the fabrication of various 2D material-based nanoelectronic devices. Due to the chemical inertness of their basal plane, the surface modification of 2D TMDCs is limited to their defective sites, often requiring special treatments, such as the conversion of the TMDC from its semiconducting into its metallic phase. In this work, we show that the basal plane of a semiconducting 2D TMDC, molybdenum disulfide (MoS2) can be modified electrochemically by electrografting of aryl-diazonium salt. To demonstrate the advantages of this method at the nanoscale, we perform electrografting of 3,5-bis(trifluoromethyl)benzenediazonium tetrafluoroborate on predefined MoS2 nanoribbons by addressing them individually via a different electrode. The ability to selectively address individually contacted 2D layers opens the possibility for specific surface modification of neighboring 2D nanostructures by different functional groups. This method could be extended to other aryl-diazonium compounds, and other 2D semiconducting materials.