Recent Advances in 2D Lateral Heterostructures

Nano-Micro Letters

HIGHLIGHTS • The controllable fabrication methods, the unique properties, and relative applications of 2D heterostructures were summarized. • The generation and detection of interlayer excitons in 2D heterostructures with type II band alignment indicate a longer lifetime and larger binding energy than intralayer excitons. • The advances in magnetic tunneling junctions based on 2D heterostructures can be applied in spintronic devices to realize spin filtering. ABSTRACT With a large number of researches being conducted on two-dimensional (2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that provides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high-quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of different types of 2D heterostructures, followed by the discussions on present challenges and perspectives of further investigations.

iScience, 2022

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and their heterojunctions are prospective materials for future electronics, optoelectronics, and quantum technologies. Assembling different 2D layers offers unique ways to control optical, electrical, thermal, magnetic, and topological phenomena. Controlled fabrications of electronic grade 2D heterojunctions are of paramount importance. Here, we enlist novel and scalable strategies to fabricate 2D vertical and lateral heterojunctions, consisting of semiconductors, metals, and/or semimetals. Critical issues that need to be addressed are the device-to-device variations, reliability, stability, and performances of 2D heterostructures in electronic and optoelectronic applications. Also, stacking orderdependent formation of moiré excitons in 2D heterostructures are emerging with exotic physics and new opportunities. Furthermore, the realization of 2D heterojunction-based novel devices, including excitonic and valleytronic transistors, demands more extensive research efforts for real-world applications. We also outline emergent phenomena in 2D heterojunctions central to nanoelectronics, optoelectronics, spintronics, and energy applications.

Cornell University - arXiv, 2022

Ultrathin lateral heterostructures of monolayer MoS2 and WS2 have successfully been realized with the metal-organic chemical vapor deposition method. Atomic-resolution HAADF-STEM observations have revealed that the junction widths of lateral heterostructures range from several nanometers to single-atom thickness, the thinnest heterojunction in theory. The interfaces are atomically flat with minimal mixing between MoS2 and WS2, originating from rapid and abrupt switching of the source supply. Due to one-dimensional interfaces and broken rotational symmetry, the resulting ultrathin lateral heterostructures, 1~2 mixed-dimensional structures, can show emergent optical/electronic properties. The MOCVD growth developed in this work allows us to access various ultrathin lateral heterostructures, leading to future exploration of their emergent properties absent in each component alone.

Chem. Rev., 2022

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro−nano−pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, lowpower neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

The development of two-dimensional (2D) layered materials is driven by fundamental interest and their potential applications. Atomically thin 2D materials provide a wide range of basic building blocks with unique electrical, optical, and thermal properties which do not exist in their bulk counterparts. The van der Waals interlayer interaction enables the possibility to exfoliate and reassemble different 2D materials into arbitrarily and vertically stacked heterostructures. Recently developed vapor phase growth of 2D materials further paves the way of directly synthesizing vertical and lateral heterojunctions. This review provides insights into the layered 2D heterostructures, with a concise introduction to preparative approaches for 2D materials and heterostructures. These unique 2D heterostructures have abundant implications for many potential applications.

2021

A dvancement of digital microelectronics relied for decades on the classical scaling philosophy guided by the famous evolutionary trend known as "Moore's law". However, a slowdown of this relentless device scaling is becoming inevitable due to fundamental physical limits. This led to the rise of a new strategy called "more than Moore (MtM)" that targets on integrated circuit functionality diversification by promoting novel non-digital (analog) applications such as radio-frequency (RF) electronics, power management systems, optoelectronics, sensors, micro/nano electromechanical systems, next generation computing systems, etc. This strategy requires new device concepts and novel materials outperforming conventional ones. Novel two-dimensional (2D) materials such as graphene and molybdenum disulfide (MoS 2) are suitable for MtM applications due to their unique structural, electrical and optical properties. In line with the MtM goals, this thesis investigates vertical hybrid devices based on graphene, MoS 2 and their heterostructures integrated into conventional 3D silicon (Si) for applications toward RF electronics, optoelectronics and neuromorphic computing. The fabrication schemes used here are scalable and semiconductor process technology compatible. The primary milestone in this thesis has been the investigation of the potential of MoS 2 as the emitter diode of graphene-base hot electron transistors (GBTs). GBTs are promising devices for high-speed analog electronics and they have a vertical architecture comprising a Si-emitter, a graphene-base and a metal-collector, each isolated by a thin barrier. Maximizing the performance of GBTs requires efficient hot-electron injection across the emitter-base-barrier. Theory suggests that this can be realized by using semiconducting barrier materials that form low energy barriers to promote thermionic emission. MoS 2 is a good candidate in this regard owing to its semiconducting behavior with a bandgap and electron affinity values enabling band alignments providing a small barrier with respect to the Si-emitter. Hence, "Si/MoS 2 /Metal" vertical heterojunction devices were investigated usiii iv 0. ABSTRACT ing capacitance-voltage (C-V) and conductance-voltage (G-V) techniques. The static dielectric constant of MoS 2 is obtained from the measured C-V data. Measurements under electric-field stress, verified by analytical simulations, have indicated the presence of interface states and mobile negative ions in MoS 2. This observation was further supported by time-of-flight secondary ion mass spectroscopy analysis that showed hydroxyl ions (OH −) possibly originating from catalytic water splitting by MoS 2. Furthermore, transmission electron microscopy studies reveal the structural properties of the film including its polycystallinity with vertically aligned layers. Next, charge carrier transport properties were investigated across "n +-Si/MoS 2 /Graphene" vertical heterojunction diodes analogous to the emitter diodes of GBTs. Analyses of the measured temperature dependent I-V data in corroboration with analytical models confirmed that the electron transport across the n +-Si/MoS 2 heterojunction barrier is dominated by thermionic emission. This fulfils the prerequisites for using MoS 2 as the emission barrier of GBTs. The thesis also includes experiments on the "Si/MoS 2 /Metal" vertical heterojunctions for memristive switching. Static (DC) currentvoltage (I-V) and resistive switching (RS) characterizations including endurance and state-retention tests demonstrate the memristive functionality of the devices. The switching tests exhibit a bipolar and nonvolatile RS behavior with encouraging endurance and state retention for at least 140 DC switching cycles and 2500 seconds, respectively. Controlled C-V, G-V and switching measurements in ambient and vacuum conditions, elucidated by analytical simulations, suggest that the observed RS behavior is due to electric field-driven movements of the mobile OH − ions along the vertical MoS 2 layers and their influence on the potential barrier at the Si/MoS 2 interface. In addition, electro-optical characterizations, in particular I-V measurements with and without white light illumination and spectral responsivity (SR) measurements, were carried out on vertical "n +-Si/MoS 2 / Graphene" heterojunction diodes, which exhibit broadband optical sensitivity. The SR data feature multiple peaks in the ultraviolet and visible regions indicating that the measured photocurrent is mainly due to excitations in the MoS 2. In addition, an infrared response is observed for energies below the Si and MoS 2 bandgaps. This may be attributed to absorption in the graphene and/or inter-layer transitions in a staggered band alignment or absorptions via midgap states in the MoS 2 ix Elektro-optische Charakterisierungen, wie beispielsweise I-V-Messungen mit und ohne Weißlichtbeleuchtung und Messungen der spektralen Empfindlichkeit (SR), weisen eine breitbandige optische Empfindlichkeit von vertikalen "n +-Si/MoS 2 /Graphen" Heteroübergangsdioden nach. Die SR-Daten weisen mehrere Peaks im ultravioletten und sichtbaren Bereich auf, was darauf hindeutet, dass der gemessene Photostrom hauptsächlich auf Anregungen im MoS 2 zurückzuführen ist. Darüber hinaus wird bei Energien unterhalb der Si-und MoS 2-Bandlücken eine Reaktion im Infrarotbereich beobachtet. Dies kann auf Absorption im Graphen und/oder Zwischenschichtübergänge in einer gestaffelten Bandanordnung oder auf Absorption über Zustände in der MoS 2-Bandlücke zurückgeführt werden. Zusammenfassend lässt sich sagen, dass die Arbeiten und Ergebnisse dieser Dissertation als Leitfaden für die Integration von 2D-Materialien und deren Heterostrukturen in die bestehende Si-Plattform dienen können, um hybride Heteroübergangs-Bauelemente für potenzielle elektronische, optoelektronische und neuromorphe Anwendungen zu schaffen.

Nature Materials, 2014

Nature Electronics, 2019

Two-dimensional van der Waals heterostructures are of considerable interest for the next generation nanoelectronics because of their unique interlayer coupling and optoelectronic properties. Here, we report a modified Langmuir–Blodgett method to organize two-dimensional molecular charge transfer crystals into arbitrarily and vertically stacked het-erostructures, consisting of bis(ethylenedithio)tetrathiafulvalene (BEDT–TTF)/C 60 and poly (3-dodecylthiophene-2,5-diyl) (P3DDT)/C 60 nanosheets. A strong and anisotropic interfacial coupling between the charge transfer pairs is demonstrated. The van der Waals hetero-structures exhibit pressure dependent sensitivity with a high piezoresistance coefficient of −4.4 × 10 −6 Pa −1 , and conductance and capacitance tunable by external stimuli (ferroelectric field and magnetic field). Density functional theory calculations confirm charge transfer between the n-orbitals of the S atoms in BEDT–TTF of the BEDT–TTF/C 60 layer and the π* orbitals of C atoms in C 60 of the P3DDT/C 60 layer contribute to the inter-complex CT. The two-dimensional molecular van der Waals heterostructures with tunable optical–electronic–magnetic coupling properties are promising for flexible electronic applications.

Scientific Reports

Scalable heterojunctions based on two-dimensional transitional metal dichalcogenides are of great importance for their applications in the next generation of electronic and optoelectronic devices. However, reliable techniques for the fabrication of such heterojunctions are still at its infancy. Here we demonstrate a simple technique for the scalable fabrication of lateral heterojunctions via selective chemical doping of TMD thin films. We demonstrate that the resistance of large area MoS2 and MoSe2 thin film, prepared via low pressure chalcogenation of molybdenum film, decreases by up to two orders of magnitude upon doping using benzyl viologen (BV) molecule. X-ray photoelectron spectroscopy (XPS) measurements confirms n-doping of the films by BV molecules. Since thin films of MoS2 and MoSe2 are typically more resistive than their exfoliated and co-evaporation based CVD counterparts, the decrease in resistance by BV doping represents a significant step in the utilization of these sa...