Negative as well as positive co-stimulation appears to play an important role in controlling T cell activation. CTLA-4 has been proposed to negatively regulate T cell responses. CTLA-4-deficient mice develop a lymphoproliferative disorder, initiated by the activation and expansion of CD4+ T cells. To assess the function of CTLA-4 on CD8+ T cells, CTLA-4(-/-) animals were crossed to an MHC class I-restricted 2C TCR transgenic mouse line. We demonstrate that although the primary T cell responses were similar, the CTLA-4-deficient 2C TCR+ CD8+ T cells displayed a greater proliferative response upon secondary stimulation than the 2C TCR+ CD8+ T cells from CTLA-4 wild-type mice. These results suggest that CTLA-4 regulates antigen-specific memory CD8+ T cell responses.

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Two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors exhibit many important structural and optoelectronic properties, such as strong light− matter interactions, direct bandgaps tunable from visible to near-infrared regions, flexibility and atomic thickness, quantum-confinement effects, valley polarization possibilities, and so on. Therefore, they are regarded as a very promising class of materials for next-generation state-of-the-art nano/micro optoelectronic devices. To explore different applications and device structures based on 2D TMDs, intrinsic material properties, their relationships, and evolutions with fabrication parameters need to be deeply understood, very often through a combination of various characterization techniques. Among them, steady-state photoluminescence (PL) spectroscopy has been extensively employed. This class of techniques is fast, contactless, and nondestructive and can provide very high spatial resolution. Therefore, it can be used to obtain optoelectronic properties from samples of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. In this article, the mechanism and applications of steady-state PL spectroscopy in 2D TMDs are reviewed. The first part of this review details the physics of PL phenomena in semiconductors and common techniques to acquire and analyze PL spectra. The second part introduces various applications of PL spectroscopy in 2D TMDs. Finally, a broader perspective is discussed to highlight some limitations and untapped opportunities of PL spectroscopy in characterizing 2D TMDs.

We characterize and discuss the impact of hydrogenation on the performance of phosphorus-doped polycrystalline silicon (poly-Si) films for passivating contact solar cells. Combining various characterization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi-steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation process of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 μs to 2.7 ms and from 668 to 722 mV.

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One of the most fundamental parameters of any photovoltaic material is its quasi‐Fermi level splitting (∆µ) under illumination. This quantity represents the maximum open‐circuit voltage (Voc) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of Voc that can possibly be achieved from monolayer WS2, MoS2, WSe2, and MoSe2‐based solar cells, and they are compared with state‐of‐the‐art perovskites. These results show that Voc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS2, MoS2, WSe2, and MoSe2 monolayers at 1 Sun illumination, respectively. It is also observed that ∆µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ∆µ heterogeneity by electrically gating the TMD monolayers in a metal‐oxide‐semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ∆µ heterogeneity. The values of ∆µ determined from this work reveal the potential of atomically thin TMDs for high‐voltage, ultralight, flexible, and eye‐transparent future solar cells.

Jumping insects such as fleas, froghoppers, grasshoppers, and locusts take off from the ground using a catapult mechanism to push their legs against the surface of the ground while using their pairs of flapping wings to propel them into the air. Such combination of jumping and flapping is expected as an efficient way to overcome unspecified terrain or avoid large obstacles. In this work, we present the conceptual design and verification of a bio-inspired flapping-wing-assisted jumping robot, named Jump-flapper, which mimics jumping insects' locomotion strategy. The robot, which is powered by only one miniature DC motor to implement the functions of jumping and flapping, is an integration of an inverted slider-crank mechanism for the structure of the legs, a dog-clutch mechanism for the winching system, and a rack-pinion mechanism for the flapping-wing system. A prototype of the robot is fabricated and experimentally tested to evaluate the integration and performance of the Jump-flapper. This 23 g robot with assisted flapping wings operating at approximately 19 Hz is capable of jumping to a height of approximately 0.9 m, showing about 30% improvement in the jumping height compared to that of the robot without assistance of the flapping wings. The benefits of the flapping-wing-assisted jumping system are also discussed throughout the study.

Organic–inorganic (O–I) heterostructures, consisting of atomically thin inorganic semiconductors and organic molecules, present synergistic and enhanced optoelectronic properties with a high tunability. Here, we develop a class of air-stable vertical O–I heterostructures comprising a monolayer of transition-metal dichalcogenides (TMDs), including WS2, WSe2, and MoSe2, on top of tetraphenylethylene (TPE) core-based aggregation-induced emission (AIE) molecular rotors. The created O–I heterostructures yields a photoluminescence (PL) enhancement of up to ∼950%, ∼500%, and ∼330% in the top monolayer WS2, MoSe2, and WSe2 as compared to PL in their pristine monolayers, respectively. The strong PL enhancement is mainly attributed to the efficient photogenerated carrier process in the AIE luminogens (courtesy of their restricted intermolecular motions in the solid state) and the charge-transfer process in the created type I O–I heterostructures. Moreover, we observe an improvement in photovoltaic properties of the TMDs in the heterostructures including the quasi-Fermi level splitting, minority carrier lifetime, and light absorption. This work presents an inspiring example of combining stable, highly luminescent AIE-based molecules, with rich photochemistry and versatile applications, with atomically thin inorganic semiconductors for multifunctional and efficient optoelectronic devices.