The intrinsic activity of in-plane chalcogen atoms plays a significant role in the catalytic performance of transition metal dichalcogenides (TMDs). A rational modulation of the local configurations ...is essential to activating the in-plane chalcogen atoms but restricted by the high energy barrier to break the in-plane TM-X (X = chalcogen) bonds. Here, we theoretically design and experimentally realize the tuning of local configurations. The electron transfer capacity of local configurations is used to screen suitable TMDs materials for hydrogen evolution reaction (HER). Among various configurations, the triangular-shape cobalt atom cluster with a central sulfur vacancy (3Co
-V
) renders the distinct electrocatalytic performance of MoS
with much reduced overpotential and Tafel slope. The present study sheds light on deeper understanding of atomic-scale local configuration in TMDs and a methodology to boost the intrinsic activity of chalcogen atoms.
Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge ...recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.
The limited memory retention for a ferroelectric field-effect transistor has prevented the commercialization of its nonvolatile memory potential using the commercially available ferroelectrics. Here, ...we show a long-retention ferroelectric transistor memory cell featuring a metal-ferroelectric-metal-insulator-semiconductor architecture built from all van der Waals single crystals. Our device exhibits 17 mV dec
operation, a memory window larger than 3.8 V, and program/erase ratio greater than 10
. Thanks to the trap-free interfaces and the minimized depolarization effects via van der Waals engineering, more than 10
cycles endurance, a 10-year memory retention and sub-5 μs program/erase speed are achieved. A single pulse as short as 100 ns is enough for polarization reversal, and a 4-bit/cell operation of a van der Waals ferroelectric transistor is demonstrated under a 100 ns pulse train. These device characteristics suggest that van der Waals engineering is a promising direction to improve ferroelectronic memory performance and reliability for future applications.
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π-π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing ...efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π-π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π-π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.
Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet–triplet annihilation (TTA) materials, basically harness triplet excitons from the ...lowest triplet excited state (T1) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high‐lying excited state (T2), referred to here as a “hot exciton” path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S1 and T2 and rather large T2–T1 energy gap, which are expected to impede the internal conversion (IC) from T2 to T1 and facilitate the reverse intersystem crossing from the high‐lying triplet state (hRISC). Through sensitizing the T2 state with ketones, the existence of the hRISC process with an ns‐scale delayed lifetime is confirmed. Benefiting from this fast triplet–singlet conversion, the nondoped device based on this “hot exciton” material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll‐off and CIE coordinates of (0.15, 0.13). These results reveal that the “hot exciton” path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure‐blue and deep‐blue fluorescent organic light‐emitting devices.
A pure‐blue fluorescent organic light‐emitting device (OLED) based on phenanthroimidazole−anthracene derivative obtains a maximum external quantum efficiency of 10.5% with excellent stability. Experimental investigations reveal that the high efficiency is attributed to triplet exciton harvesting by reverse intersystem crossing from the high‐lying triplet state. The results demonstrates that “hot exciton” channels represent a promising way to construct high‐performance fluorescent OLEDs.
Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronic devices. However, the direct growth of their crystals is in ...its infancy. Here we report a chemical vapor deposition approach to controllably grow layered tetragonal and non-layered hexagonal FeTe nanoplates with their thicknesses down to 3.6 and 2.8 nm, respectively. Moreover, transport measurements reveal these obtained FeTe nanoflakes show a thickness-dependent magnetic transition. Antiferromagnetic tetragonal FeTe with the Néel temperature (T
) gradually decreases from 70 to 45 K as the thickness declines from 32 to 5 nm. And ferromagnetic hexagonal FeTe is accompanied by a drop of the Curie temperature (T
) from 220 K (30 nm) to 170 K (4 nm). Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from its concomitant lattice distortion and Stoner instability. This study highlights its potential applications in future spintronic devices.
Two‐dimensional (2D) materials have been an emerging platform for future device applications. Among them, 2D transition metal dichalcogenides (TMDs) have attracted substantial interest because of ...their fascinating properties ranging from semiconductor, semimetal, metal, to superconductor. The electric controllability over their physical properties is extremely important for their device application. This feature article presents recent progress in the electrical manipulation of the optical, electric, and spin/valley dependent properties of 2D TMDs. Also, some of the outstanding challenges and opportunities in this promising research field are highlighted.
The outstanding electric, optical, and valleytronic properties of 2D transition metal dichalcogenides (TDMDs) make them promising for device applications, while the electric controllability is extremely important for demonstrating their device application. In this review, we highlight the recent progress on the electric controllability of 2D TMDs and provide the opportunities that need to be investigated.
Semiconductors are the basis of many vital technologies such as electronics, computing, communications, optoelectronics, and sensing. Modern semiconductor technology can trace its origins to the ...invention of the point contact transistor in 1947. This demonstration paved the way for the development of discrete and integrated semiconductor devices and circuits that has helped to build a modern society where semiconductors are ubiquitous components of everyday life. A key property that determines the semiconductor electrical and optical properties is the bandgap. Beyond graphene, recently discovered two-dimensional (2D) materials possess semiconducting bandgaps ranging from the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides, to the ultraviolet in hexagonal boron nitride. In particular, these 2D materials were demonstrated to exhibit highly tunable bandgaps, achieved via the control of layers number, heterostructuring, strain engineering, chemical doping, alloying, intercalation, substrate engineering, as well as an external electric field. We provide a review of the basic physical principles of these various techniques on the engineering of quasi-particle and optical bandgaps, their bandgap tunability, potentials and limitations in practical realization in future 2D device technologies.
Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect
, valley polarization
and ...two-dimensional superconductivity
, suggesting potential applications for functional devices
. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization
, selenization
and tellurization
of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature
and this approach
was recently employed to facilitate the growth of monolayer WS
and WSe
. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of superconductivity in our monolayer NbSe
and MoTe
samples
and of high mobilities in MoS
and ReS
. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.
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