WS2 ribbon arrays with defined chirality and coherent polarity.

One-dimensional transition metal dichalcogenides exhibiting an enhanced bulk photovoltaic effect have the potential to exceed the Shockley-Queisser limit efficiency in solar energy harvest within p-n junction architectures. However, the collective output of these prototype devices remains a challenge. We report on the synthesis of single-crystalline WS2 ribbon arrays with defined chirality and coherent polarity through an atomic manufacturing strategy. The chirality of WS2 ribbon was defined by substrate couplings into tunable armchair, zigzag, and chiral species, and the polarity direction was determined by the ribbon-precursor interfacial energy along a coherent direction. A single armchair ribbon showed strong bulk photovoltaic effect and the further integration of ~1000 aligned ribbons with coherent polarity enabled upscaling of the photocurrent.

Efficient Constant Envelope Precoding for Massive MU-MIMO Downlink via Majorization-Minimization Method.

The practical implementation of massive multi-user multi-input-multi-output (MU-MIMO) downlink communication systems power amplifiers that are energy efficient; otherwise, the power consumption of the base station (BS) will be prohibitive. Constant envelope (CE) precoding is gaining increasing interest for its capability to utilize low-cost, high-efficiency nonlinear radio frequency amplifiers. Our work focuses on the topic of CE precoding in massive MU-MIMO systems and presents an efficient CE precoding algorithm. This algorithm uses an alternating minimization (AltMin) framework to optimize the CE precoded signal and precoding factor, aiming to minimize the difference between the received signal and the transmit symbol. For the optimization of the CE precoded signal, we provide a powerful approach that integrates the majorization-minimization (MM) method and the fast iterative shrinkage-thresholding (FISTA) method. This algorithm combines the characteristics of the massive MU-MIMO channel with the second-order Taylor expansion to construct the surrogate function in the MM method, in which minimizing this surrogate function is the worst-case of the system. Specifically, we expand the suggested CE precoding algorithm to involve the discrete constant envelope (DCE) precoding case. In addition, we thoroughly examine the exact property, convergence, and computational complexity of the proposed algorithm. Simulation results demonstrate that the proposed CE precoding algorithm can achievean uncoded biterror rate (BER) performance gain of roughly 1dB compared to the existing CE precoding algorithm and has an acceptable computational complexity. This performance advantage also exists when it comes to DCE precoding.

Modularized batch production of 12-inch transition metal dichalcogenides by local element supply.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are regarded as pivotal semiconductor candidates for next-generation devices due to their atomic-scale thickness, high carrier mobility and ultrafast charge transfer. In analog to the traditional semiconductor industry, batch production of wafer-scale TMDs is the prerequisite to proceeding with their integrated circuits evolution. However, the production capacity of TMD wafers is typically constrained to a single and small piece per batch (mainly ranging from 2 to 4 inches), due to the stringent conditions required for effective mass transport of multiple precursors during growth. Here we developed a modularized growth strategy for batch production of wafer-scale TMDs, enabling the fabrication of 2-inch wafers (15 pieces per batch) up to a record-large size 12-inch wafers (3 pieces per batch). Each module, comprising a self-sufficient local precursor supply unit for robust individual TMD wafer growth, is vertically stacked with others to form an integrated array and thus a batch growth. Comprehensive characterization techniques, including optical spectroscopy, electron microscopy, and transport measurements unambiguously illustrate the high-crystallinity and the large-area uniformity of as-prepared monolayer films. Furthermore, these modularized units demonstrate versatility by enabling the conversion of as-produced wafer-scale MoS2 into various structures, such as Janus structures of MoSSe, alloy compounds of MoS2(1-x)Se2x, and in-plane heterostructures of MoS2-MoSe2. This methodology showcases high-quality and high-yield wafer output and potentially enables the seamless transition from lab-scale to industrial-scale 2D semiconductor complementary to silicon technology.

Asymmetric and zwitterionic Blatter diradicals.

Asymmetric diradical molecular systems with different resonance mechanisms are largely unexplored. Herein, two conjugated asymmetric diradicals with Blatter and phenoxyl moieties (pBP and mBP) have been synthesized and studied in depth. A complete set of spectroscopic, X-ray crystallographic and magnetic techniques, together with quantum chemical calculations, have been used. The para-isomer (pBP) bears diradical and zwitterionic resonant forms, the latter by a electron delocalization mechanism, which are synergistically integrated by a sequence of nitrogen, provided by the Blatter moiety imine and amine (of different acceptor nature). In the meta-isomer (mBP), the zwitterionic form promoted in pBP by the lone-pair electron of the amine nitrogen is not available, yet it possesses a pseudo-hyperconjugation effect where the N lone pair mediates in a bonding coupling in a counter homolytic bond scission mechanism. Both electronic effects converge to promote medium diradical characters and narrow singlet-triplet gaps to the two electronic isomers. All these aspects delineate the subtle balance that shapes the electronic structure of open-shell molecules, which is even more challenging in the case of asymmetric systems, such as those described here with asymmetric phenoxyl-Blatter diradicals.

Hydrogen-bond-induced quantum interference in single-molecule junctions of regioisomers.

Solvents can play a significant role in tuning the electrical conductance of single-molecule junctions. In this respect, protic solvents offer the potential to form hydrogen bonds with molecular backbones and induce electrostatic gating via their dipole moments. Here we demonstrate that the effect of hydrogen bond formation on conductance depends on whether transport through the junction is controlled by destructive quantum interference (DQI) or constructive quantum interference (CQI). Furthermore, we show that a protic solvent can be used to switch the conductance of single-molecule junctions between the two forms of quantum interference. To explore this possibility, two regioisomers (BIT-Zwitterion and BIT-Neutral) were synthesized and their single-molecule conductances in aprotic and protic solvents were investigated using a scanning-tunneling-microscope-based break junction technique, combined with density functional theory and quantum transport theory. We find that the protic solvent twists the geometry of BIT-Zwitterion by introducing intermolecular hydrogen bonds between the solvent and target molecule. Moreover, it increases the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule by imposing different electrostatic gating on the delocalized HOMO and localized LUMO, leading to a lower conductance compared to that in aprotic solvent. In contrast, the conductance of BIT-Neutral increases due to a transformation from DQI to CQI originating from a change from a planar to a folded conformation in the protic solvent. In addition, the stacking between the two folded moieties produces an extra through-space transport path, which further contributes to conductance. This study demonstrates that combinations of protic solvents and regioisomers present a versatile route to controlling quantum interference and therefore single-molecule conductance, by enabling control of hydrogen bond formation, electrostatic gating and through-space transport.

Controlling the Isomerization of Photoresponsive Molecules through a Limiting Tautomerization Strategy.

Controlling the multistage photoresponsivity remains a challenge, in part, due to the spontaneous tautomerization between isomers. Herein, we present a strategy to access three independent states (linear, cyclic keto, and cyclic enolate) of crown ether (CE)-substituted donor-acceptor Stenhouse adducts (DASAs) by limiting the tautomerization of the closed isomers. The linear-cyclic keto isomerization is reversibly triggered by treatment with metal ions (Na+ or K+) and CE, while the linear-cyclic enolate isomerization is induced by green light and heat. Density functional theory and molecular dynamics calculation results suggest that the steric effect and supramolecular interaction between the electron-donating and electron-withdrawing moieties play an important role in hindering the tautomerization between cyclic keto and cyclic enolate DASA-CE. The strategy to influence key steps in the photoswitching process inspires well-controlled multistage isomerization of photoresponsive molecules.

Robust growth of two-dimensional metal dichalcogenides and their alloys by active chalcogen monomer supply.

The precise precursor supply is a precondition for controllable growth of two-dimensional (2D) transition metal dichalcogenides (TMDs). Although great efforts have been devoted to modulating the transition metal supply, few effective methods of chalcogen feeding control were developed. Here we report a strategy of using active chalcogen monomer supply to grow high-quality TMDs in a robust and controllable manner, e.g., MoS2 monolayers perform representative photoluminescent circular helicity of ~92% and electronic mobility of ~42 cm2V-1s-1. Meanwhile, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation, reaction, vacancy healing and alloy formation during the growth. Our work offers a degree of freedom for the controllable synthesis of 2D compounds and their alloys, benefiting the development of high-end devices with desired 2D materials.

Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS2 monolayer on vicinal a-plane sapphire.

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications1-4. Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported5-8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts9-20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS2 monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS2 interaction leads to two preferred antiparallel orientations of the WS2 crystal, and sapphire step edge-WS2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS2 islands. The unidirectional alignment and seamless stitching of WS2 islands are illustrated via multiscale characterization techniques; the high quality of WS2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS2 flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.

Normal & reversed spin mobility in a diradical by electron-vibration coupling.

π-conjugated radicals have great promise for use in organic spintronics, however, the mechanisms of spin relaxation and mobility related to radical structural flexibility remain unexplored. Here, we describe a dumbbell shape azobenzene diradical and correlate its solid-state flexibility with spin relaxation and mobility. We employ a combination of X-ray diffraction and Raman spectroscopy to determine the molecular changes with temperature. Heating leads to: i) a modulation of the spin distribution; and ii) a "normal" quinoidal → aromatic transformation at low temperatures driven by the intramolecular rotational vibrations of the azobenzene core and a "reversed" aromatic → quinoidal change at high temperatures activated by an azobenzene bicycle pedal motion amplified by anisotropic intermolecular interactions. Thermal excitation of these vibrational states modulates the diradical electronic and spin structures featuring vibronic coupling mechanisms that might be relevant for future design of high spin organic molecules with tunable magnetic properties for solid state spintronics.

A nitrogen-doped asymmetric phenalenyl with a zwitterionic ground state.

Herein, an asymmetric N-doped phenalenyl (BTAP) compound has been synthesized and carefully studied for the first time. The synthesis of BTAP reveals a planar configuration and an unexpected zwitterionic ground state with the negative charge delocalized around the circumjacent part and the positive charge mainly localized on the center.

Carbon Spacer Strategy: Control the Photoswitching Behavior of Donor-Acceptor Stenhouse Adducts.

Understanding the relationship between chemical structure and photoswitching property of donor-acceptor Stenhouse adducts (DASAs) is necessary for developments and applications of the novel photoresponsive molecule. In the current work, we demonstrated a close relationship between the length of carbon spacer and photoswitching property of DASAs. A series of DASAs with barbituric acid substituted electron-withdrawing part and N-methylaniline substituted electron-donating part were synthesized. With shortening the carbon spacer between the phenyl and amine groups in the electron-donating part, the efficiency and rate of the light-induced linear-to-cyclic isomerization are improved in all the test solvents. The molecular energy variation during the isomerization process was investigated by density functional theory calculation to further understand the mechanism. This work provides a reliable carbon spacer strategy to control the photoswitching behavior of DASAs using chemical methods.

Understanding the nature of quinoidal and zwitterionic states in carbazole-based diradicals.

We report two carbazole-based diradicals, out of which the m-isomer shows a large diradical character y0 (0.89) and a small singlet-triplet energy gap ΔES-T (-0.98 kcal mol-1), whereas the p-isomer exhibits smaller y0 (0.79) but a much larger ΔES-T (-6.16 kcal mol-1). DFT calculations reveal that this tendency is also suitable for nitrogen and carbon-centered diradicals.

Fine-tuning the diradical character of molecular systems via the heteroatom effect.

Heteroatoms were introduced as a novel modification strategy for fine-tuning the diradical character of molecular systems. Both the diradical character and the singlet-triplet energy gaps of diketopyrrolopyrrole based phenoxyl diradicaloids decreased as the size of the substituted heteroatoms (from O, S to Se atom) increased.