Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced ...information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.
The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p‐n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy ...structures at the interface, limiting the development for high‐efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy‐free 2D monolayer WSe2‐MoS2 lateral p‐n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode‐spacing design can lead to environment‐independent PV properties. These robust PV properties deriving from the atomically sharp lateral p‐n interface can help develop the next‐generation photovoltaics.
By sequential growth of alloy‐free 2D monolayer WSe2‐MoS2 lateral p‐n heterojunction, photovoltaic devices show extraordinary power conversion efficiencies of 2.56%. The large surface active area of the devices enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic. Modeling studies demonstrate the devices comply with typical principles. The appropriate electrode‐spacing design leads to environment‐independent properties.
Using scanning tunneling microscopy and spectroscopy, we probe the electronic structures of single layer MoS2 on graphite. The apparent quasiparticle energy gap of single layer MoS2 is measured to be ...2.15 ± 0.06 eV at 77 K, albeit a higher second conduction band threshold at 0.2 eV above the apparent conduction band minimum is also observed. Combining it with photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV (or 0.42 eV if the second threshold is use), a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe metallic edge states of single layer MoS2. In the bulk region of MoS2, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS2 heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.
Ferroelectrics have been demonstrated as excellent building blocks for high‐performance nonvolatile memories, including memristors, which play critical roles in the hardware implementation of ...artificial synapses and in‐memory computing. Here, it is reported that the emerging van der Waals ferroelectric α‐In2Se3 can be used to successfully implement heterosynaptic plasticity (a fundamental but rarely emulated synaptic form) and achieve a resistance‐switching ratio of heterosynaptic memristors above 103, which is two orders of magnitude larger than that in other similar devices. The polarization change of ferroelectric α‐In2Se3 channel is responsible for the resistance switching at various paired terminals. The third terminal of α‐In2Se3 memristors exhibits nonvolatile control over channel current at a picoampere level, endowing the devices with picojoule read‐energy consumption to emulate the associative heterosynaptic learning. The simulation proves that both supervised and unsupervised learning manners can be implemented in α‐In2Se3 neutral networks with high image recognition accuracy. Moreover, these heterosynaptic devices can naturally realize Boolean logic without an additional circuit component. The results suggest that van der Waals ferroelectrics hold great potential for applications in complex, energy‐efficient, brain‐inspired computing systems and logic‐in‐memory computers.
Ferroelectric switching in the van der Waals ferroelectric α‐In2Se3 is used to achieve a resistance‐switching ratio of heterosynaptic memristors above 103. The α‐In2Se3 heterosynaptic memristors feature picojoule read‐energy consumption. The simulation proves that both supervised and unsupervised learning manners can be implemented in α‐In2Se3 neutral networks with high image‐recognition accuracy. Moreover, these devices can also realize in‐memory computing.
Abstract
Two-dimensional (2D) layered materials with a non-centrosymmetric structure exhibit great potential for nano-scale electromechanical systems and electronic devices. Piezoelectric and ...ferroelectric 2D materials draw growing interest for applications in energy harvesting, electronics, and optoelectronics. This article first reviews the preparation of these functional 2D layered materials, including exfoliation methods and vapor phase deposition growth, followed by a general introduction to various piezo/ferro-electric characterization methods. Typical 2D piezoelectric and ferroelectric materials and their electronic properties, together with their potential applications, are also introduced. Finally, future research directions for 2D piezoelectric and ferroelectric materials are discussed.
Doping control: The doping level (or Fermi energy) of single‐layer graphene (SLG) films can be modulated by aromatic compounds. Raman 2D‐ and G‐band frequency sampling is used to distinguish between ...n‐ and p‐doped SLG films (see picture), which result from electron‐donating and ‐withdrawing groups, respectively.
Ultrathin two-dimensional (2D) semiconducting layered materials offer great potential for extending Moore's law of the number of transistors in an integrated circuit
. One key challenge with 2D ...semiconductors is to avoid the formation of charge scattering and trap sites from adjacent dielectrics. An insulating van der Waals layer of hexagonal boron nitride (hBN) provides an excellent interface dielectric, efficiently reducing charge scattering
. Recent studies have shown the growth of single-crystal hBN films on molten gold surfaces
or bulk copper foils
. However, the use of molten gold is not favoured by industry, owing to its high cost, cross-contamination and potential issues of process control and scalability. Copper foils might be suitable for roll-to-roll processes, but are unlikely to be compatible with advanced microelectronic fabrication on wafers. Thus, a reliable way of growing single-crystal hBN films directly on wafers would contribute to the broad adoption of 2D layered materials in industry. Previous attempts to grow hBN monolayers on Cu (111) metals have failed to achieve mono-orientation, resulting in unwanted grain boundaries when the layers merge into films
. Growing single-crystal hBN on such high-symmetry surface planes as Cu (111)
is widely believed to be impossible, even in theory. Nonetheless, here we report the successful epitaxial growth of single-crystal hBN monolayers on a Cu (111) thin film across a two-inch c-plane sapphire wafer. This surprising result is corroborated by our first-principles calculations, suggesting that the epitaxial growth is enhanced by lateral docking of hBN to Cu (111) steps, ensuring the mono-orientation of hBN monolayers. The obtained single-crystal hBN, incorporated as an interface layer between molybdenum disulfide and hafnium dioxide in a bottom-gate configuration, enhanced the electrical performance of transistors. This reliable approach to producing wafer-scale single-crystal hBN paves the way to future 2D electronics.
Development of flexible and freestanding electrode is attracting great attention in lithium–sulfur (Li–S) batteries, but the severe capacity fading caused by the lithium polysulfides (PSs) shuttle ...effect remains challenging. Herein, a completely new polymeric binder of polyethersulfone is introduced. Not only it enables massive production of flexible/current‐free electrode by a novel concept of “phase‐inversion” approach but also the resultant polymeric networks can effectively trap the soluble polysulfides within the electrode, owing to the higher hydrophilicity and stronger affinity properties than the routine polyvinylidene fluoride. Coupling with polysulfide‐based electrolyte, the Li–S cell shows a higher capacity of 1141 mAh g−1, a lower polarization of 192 mV, and a more stable capacity retention with 100% Coulombic efficiency over 100 cycles at 0.25C. The advantages of favored binder and electrolyte are further demonstrated in lithium‐ion sulfur full battery with lithiated graphite anode, which demonstrates much improved performance than those previously reported. This work not only introduces a novel strategy for flexible freestanding electrodes but also enlightens the importance of coupling electrodes and electrolytes to higher performances for Li–S battery.
Synergetic coupling of polyethersulfone binder in flexible freestanding cathode that fabricated via phase inversion and polysulfides‐modified electrolyte results in high lithium‐sulfur batteries performances. The improvements result from better electrolyte absorption and sulfur immobilization by pores from phase‐inversion, sulfonyl functional group, and electrolyte buffer, leading to better polarization, higher capacity retention, and improved Coulombic efficiency than the conventional polyvinylidene fluoride binder.
Transition metal dichalcogenides (TMDCs) have attracted significant attention for their great potential in nano energy. TMDC layered materials represent a diverse and largely untapped source of 2D ...systems. High-quality TMDC layers with an appropriate size, variable thickness, superior electronic and optical properties can be produced by the exfoliation or vapor phase deposition method. Semiconducting TMDC monolayers have been demonstrated feasible for various energy related applications, where their electronic properties and uniquely high surface areas offer opportunities for various applications such as nano generators, green electronics, electrocatalytic hydrogen generation and energy storage. In this review, we start from the structure, properties and preparation, followed by detailed discussions on the development of TMDC-based nano energy applications.
The structure characterizations and preparative methods of 2D TMDCs have obtained significant progresses. Their recent advances for nano energy generation, solar harvesting, conversion and storage, and green electronics are reviewed.
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•The unique structure and properties of two-dimensional transition metal dichalcogenides (TMDC) are discussed for energy applications.•Energy applications such as piezoelectric-based energy generators, solar energy harvesting, conversion and storage, green electronics and catalysis for hydrogen generation are reviewed in details.•Perspectives on future development of TMDCs in energy applications are presented.
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