Gas Release as an Efficient Strategy to Tune Mechanical Properties and Thermoresponsiveness of Dynamic Molecular Crystals.

Dynamic molecular crystals combining multiple and finely tunable functionalities are attracting and an increasing attention due to their potential applications in a broad range of fields as efficient energy transducers and stimuli-responsive materials. In this context, a multicomponent organic salt, piperazinium trifluoroacetate (PZTFA), endowed with an unusual multidimensional responsive landscape is reported. Crystals of the salt undergo smooth plastic deformation under mechanical stress and thermo-induced jumping. Furthermore, via controlled crystal bending and release of trifluoroacetic acid from the lattice, which is anticipated from the design of the material, both the mechanical response and the thermoresponsive behavior are efficiently tuned while partially preserving the crystallinity of the system. In particular, mechanical deformation hampers guest release and hence the macroscopic jumping effect, while trifluoroacetic acid release stiffens the crystals. These complex adaptive responses establish a new crystal engineering strategy to gain further control over dynamic organic crystals.

Broadband miniaturized spectrometers with a van der Waals tunnel diode.

Miniaturized spectrometers are of immense interest for various on-chip and implantable photonic and optoelectronic applications. State-of-the-art conventional spectrometer designs rely heavily on bulky dispersive components (such as gratings, photodetector arrays, and interferometric optics) to capture different input spectral components that increase their integration complexity. Here, we report a high-performance broadband spectrometer based on a simple and compact van der Waals heterostructure diode, leveraging a careful selection of active van der Waals materials- molybdenum disulfide and black phosphorus, their electrically tunable photoresponse, and advanced computational algorithms for spectral reconstruction. We achieve remarkably high peak wavelength accuracy of ~2 nanometers, and broad operation bandwidth spanning from ~500 to 1600 nanometers in a device with a ~ 30×20 µm2 footprint. This diode-based spectrometer scheme with broadband operation offers an attractive pathway for various applications, such as sensing, surveillance and spectral imaging.

Perceptual learning deficits mediated by somatostatin releasing inhibitory interneurons of olfactory bulb in an early life stress mouse model.

Early life adversity (ELA) causes aberrant functioning of neural circuits affecting the health of an individual. While ELA-induced behavioural disorders resulting from sensory and cognitive disabilities can be assessed clinically, the neural mechanisms need to be probed using animal models by employing multi-pronged experimental approaches. As ELA can alter sensory perception, we investigated the effect of early weaning on murine olfaction. By implementing go/no-go odour discrimination paradigm, we observed olfactory learning and memory impairments in early life stressed (ELS) male mice. As olfactory bulb (OB) circuitry plays a critical role in odour learning, we studied the plausible changes in the OB of ELS mice. Lowered c-Fos activity in the external plexiform layer and a reduction in the number of dendritic processes of somatostatin-releasing, GABAergic interneurons (SOM-INs) in the ELS mice led us to hypothesise the underlying circuit. We recorded reduced synaptic inhibitory feedback on mitral/tufted (M/T) cells, in the OB slices from ELS mice, explaining the learning deficiency caused by compromised refinement of OB output. The reduction in synaptic inhibition was nullified by the photo-activation of ChR2-expressing SOM-INs in ELS mice. The role of SOM-INs was revealed by learning-dependent refinement of Ca2+dynamics quantified by GCaMP6f signals, which was absent in ELS mice. Further, the causal role of SOM-INs involving circuitry was investigated by optogenetic modulation during the odour discrimination learning. Photo-activating these neurons rescued the ELA-induced learning deficits. Conversely, photo-inhibition caused learning deficiency in control animals, while it completely abolished the learning in ELS mice, confirming the adverse effects mediated by SOM-INs. Our results thus establish the role of specific inhibitory circuit in pre-cortical sensory area in orchestrating ELA-dependent changes.

Strain Engineering for Enhancing Carrier Mobility in MoTe2 Field-Effect Transistors.

Molybdenum ditelluride (MoTe2 ) exhibits immense potential in post-silicon electronics due to its bandgap comparable to silicon. Unlike other 2D materials, MoTe2 allows easy phase modulation and efficient carrier type control in electrical transport. However, its unstable nature and low-carrier mobility limit practical implementation in devices. Here, a deterministic method is proposed to improve the performance of MoTe2 devices by inducing local tensile strain through substrate engineering and encapsulation processes. The approach involves creating hole arrays in the substrate and using atomic layer deposition grown Al2 O3 as an additional back-gate dielectric layer on SiO2 . The MoTe2 channel is passivated with a thick layer of Al2 O3 post-fabrication. This structure significantly improves hole and electron mobilities in MoTe2 field-effect transistors (FETs), approaching theoretical limits. Hole mobility up to 130 cm-2  V-1 s-1 and electron mobility up to 160 cm-2  V-1 s-1 are achieved. Introducing local tensile strain through the hole array enhances electron mobility by up to 6 times compared to the unstrained devices. Remarkably, the devices exhibit metal-insulator transition in MoTe2 FETs, with a well-defined critical point. This study presents a novel technique to enhance carrier mobility in MoTe2 FETs, offering promising prospects for improving 2D material performance in electronic applications.

Non-stoichiometric carbamazepine cocrystal hydrates of 3,4-/3,5-dihydroxybenzoic acids: coformer-water exchange.

The cocrystallisation of carbamazepine (CBZ) with 3,4-/3,5-dihydroxybenzoic acids (34/35DHBA) with different stoichiometries formed molecular alloys, exchanging a water molecule, in their isostructural CBZ dihydrate form. Furthermore, we show a correlation between the mechanical properties of the CBZ-DHBA cocrystals with the amount of coformer present.

Heparin-Induced Dual Mode Luminescence Modulation of Organic Nanoparticles and Efficient Energy Transfer.

Amphiphilic di-cationic fumaronitrile derivatives with peripheral n-alkyl chains formed organic nanoparticles (NPs) through self-assembly in aqueous media. The NPs exhibited enhanced luminescence in both steady-state and delayed mode in comparison to their molecularly dissolved state. Due to the presence of the multivalent array of positive charges on their surface, they were found to bind heparin, a bio-polyanion which is routinely employed during surgery as an anticoagulant. The electrostatically driven co-assemblies resulted in a significant enhancement in the steady-state and delayed luminescence of the NPs. This provided a highly sensitive detection of the polyanion in aqueous buffer as well as in highly competitive serum and plasma media. Furthermore, and most notably, the heparin based co-assemblies were found to act as efficient donors exhibiting fluorescence resonance energy transfer (FRET) to acceptor dyes with the energy transfer efficiency reaching up to 88%.

Elastic organic semiconducting single crystals for durable all-flexible field-effect transistors: insights into the bending mechanism.

Although many examples of mechanically flexible crystals are currently known, their utility in all-flexible devices is not yet adequately demonstrated, despite their immense potential for fabricating high performance flexible devices. Here, we report two alkylated diketopyrrolopyrrole (DPP) semiconducting single crystals, one of which displays impressive elastic mechanical flexibility whilst the other is brittle. Using the single crystal structures and density functional theory (DFT) calculations, we show that the methylated diketopyrrolopyrrole (DPP-diMe) crystals, with dominant π-stacking interactions and large contributions from dispersive interactions, are superior in terms of their stress tolerance and field-effect mobility (µ FET) when compared to the brittle crystals of the ethylated diketopyrrolopyrrole derivative (DPP-diEt). Periodic dispersion-corrected DFT calculations revealed that upon the application of 3% uniaxial strain along the crystal growth (a)-axis, the elastically flexible DPP-diMe crystal displays a soft energy barrier of only 0.23 kJ mol-1 while the brittle DPP-diEt crystal displays a significantly larger energy barrier of 3.42 kJ mol-1, in both cases relative to the energy of the strain-free crystal. Such energy-structure-function correlations are currently lacking in the growing literature on mechanically compliant molecular crystals and have the potential to support a deeper understanding of the mechanism of mechanical bending. The field effect transistors (FETs) made of flexible substrates using elastic microcrystals of DPP-diMe retained µ FET (from 0.019 cm2 V-1 s-1 to 0.014 cm2 V-1 s-1) more efficiently even after 40 bending cycles when compared to the brittle microcrystals of DPP-diEt which showed a significant drop in µ FET just after 10 bending cycles. Our results not only provide valuable insights into the bending mechanism, but also demonstrate the untapped potential of mechanically flexible semiconducting crystals for designing all flexible durable field-effect transistor devices.

Optical Control of High-Harmonic Generation at the Atomic Thickness.

High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light-matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.

Viscoelastic Covalent Organic Nanotube Fabric via Macroscopic Entanglement.

Covalent organic nanotubes (CONTs) are one-dimensional porous frameworks constructed from organic building blocks via dynamic covalent chemistry. CONTs are synthesized as insoluble powder that restricts their potential applications. The judicious selection of 2,2'-bipyridine-5,5'-dicarbaldehyde and tetraaminotriptycene as building blocks for TAT-BPy CONTs has led to constructing flexible yet robust and self-standing fabric up to 3 µm thickness. The TAT-BPy CONTs and TAT-BPy CONT fabric have been characterized by solid-state one-dimensional (1D) 13C CP-MAS, two-dimensional (2D) 13C-1H correlation NMR, 2D 1H-1H DQ-SQ NMR, and 2D 14N-1H correlation NMR spectroscopy. The mechanism of fabric formation has been established by using high-resolution transmission electron microscopy and scanning electron microscopy techniques. The as-synthesized viscoelastic TAT-BPy CONT fabric exhibits high mechanical strength with a reduced modulus (Er) of 8 (±3) GPa and hardness (H) of 0.6 (±0.3) GPa. Interestingly, the viscoelastic fabric shows time-dependent elastic depth recovery up to 50-70%.

Coherent modulation of chiral nonlinear optics with crystal symmetry.

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS2) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).

Inducing Strong Light-Matter Coupling and Optical Anisotropy in Monolayer MoS2 with High Refractive Index Nanowire.

Mixed-dimensional heterostructures combine the merits of materials of different dimensions; therefore, they represent an advantageous scenario for numerous technological advances. Such an approach can be exploited to tune the physical properties of two-dimensional (2D) layered materials to create unprecedented possibilities for anisotropic and high-performance photonic and optoelectronic devices. Here, we report a new strategy to engineer the light-matter interaction and symmetry of monolayer MoS2 by integrating it with one-dimensional (1D) AlGaAs nanowire (NW). Our results show that the photoluminescence (PL) intensity of MoS2 increases strongly in the mixed-dimensional structure because of electromagnetic field confinement in the 1D high refractive index semiconducting NW. Interestingly, the 1D NW breaks the 3-fold rotational symmetry of MoS2, which leads to a strong optical anisotropy of up to ∼60%. Our mixed-dimensional heterostructure-based phototransistors benefit from this and exhibit an improved optoelectronic device performance with marked anisotropic photoresponse behavior. Compared with bare MoS2 devices, our MoS2/NW devices show ∼5 times enhanced detectivity and ∼3 times higher photoresponsivity. Our results of engineering light-matter interaction and symmetry breaking provide a simple route to induce enhanced and anisotropic functionalities in 2D materials.

On-chip photonics and optoelectronics with a van der Waals material dielectric platform.

During the last few decades, photonic integrated circuits have increased dramatically, facilitating many high-performance applications, such as on-chip sensing, data processing, and inter-chip communications. The currently dominating material platforms (i.e., silicon, silicon nitride, lithium niobate, and indium phosphide), which have exhibited great application successes, however, suffer from their own disadvantages, such as the indirect bandgap of silicon for efficient light emission, and the compatibility challenges of indium phosphide with the silicon industry. Here, we report a new dielectric platform using nanostructured bulk van der Waals materials. On-chip light propagation, emission, and detection are demonstrated by taking advantage of different van der Waals materials. Low-loss passive waveguides with MoS2 and on-chip light sources and photodetectors with InSe have been realised. Our proof-of-concept demonstration of passive and active on-chip photonic components endorses van der Waals materials for offering a new dielectric platform with a large material-selection degree of freedom and unique properties toward close-to-atomic scale manufacture of on-chip photonic and optoelectronic devices.

Engineering the Dipole Orientation and Symmetry Breaking with Mixed-Dimensional Heterostructures.

Engineering of the dipole and the symmetry of materials plays an important role in fundamental research and technical applications. Here, a novel morphological manipulation strategy to engineer the dipole orientation and symmetry of 2D layered materials by integrating them with 1D nanowires (NWs) is reported. This 2D InSe -1D AlGaAs NW heterostructure example shows that the in-plane dipole moments in InSe can be engineered in the mixed-dimensional heterostructure to significantly enhance linear and nonlinear optical responses (e.g., photoluminescence, Raman, and second harmonic generation) with an enhancement factor of up to ≈12. Further, the 1D NW can break the threefold rotational symmetry of 2D InSe, leading to a strong optical anisotropy of up to ≈65%. These results of engineering dipole orientation and symmetry breaking with the mixed-dimensional heterostructures open a new path for photonic and optoelectronic applications.

Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure.

van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity χInSe and work function ΦNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.

Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies.

Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2 , such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier-exciton and carrier-phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.

Temperature-Reliant Dynamic Properties and Elasto-Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt.

Although, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid-state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two-face elasticity (within ≈1 % strain), followed by elasto-plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X-ray diffraction, µ-Raman and polarized light microscopy experiments reveal that the elasto-plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi-functional dynamic crystals.

Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons.

All-optical control of nonlinear photonic processes in nanomaterials is of significant interest from a fundamental viewpoint and with regard to applications ranging from ultrafast data processing to spectroscopy and quantum technology. However, these applications rely on a high degree of control over the nonlinear response, which still remains elusive. Here, we demonstrate giant and broadband all-optical ultrafast modulation of second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides mediated by the modified excitonic oscillation strength produced upon optical pumping. We reveal a dominant role of dark excitons to enhance SHG by up to a factor of ∼386 at room temperature, 2 orders of magnitude larger than the current state-of-the-art all-optical modulation results. The amplitude and sign of the observed SHG modulation can be adjusted over a broad spectral range spanning a few electronvolts with ultrafast response down to the sub-picosecond scale via different carrier dynamics. Our results not only introduce an efficient method to study intriguing exciton dynamics, but also reveal a new mechanism involving dark excitons to regulate all-optical nonlinear photonics.

Autonomous self-repair in piezoelectric molecular crystals.

Living tissue uses stress-accumulated electrical charge to close wounds. Self-repairing synthetic materials, which are typically soft and amorphous, usually require external stimuli, prolonged physical contact, and long healing times. We overcome many of these limitations in piezoelectric bipyrazole organic crystals, which recombine following mechanical fracture without any external direction, autonomously self-healing in milliseconds with crystallographic precision. Kelvin probe force microscopy, birefringence experiments, and atomic-resolution structural studies reveal that these noncentrosymmetric crystals, with a combination of hydrogen bonds and dispersive interactions, develop large stress-induced opposite electrical charges on fracture surfaces, prompting an electrostatically driven precise recombination of the pieces via diffusionless self-healing.

Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2.

Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light-matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to ∼150 nm) enhancement of optical nonlinearity in monolayer MoS2 with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS2-plasmonic structure is in the order of 10-17 m2/V2, far superior (∼10-100-times larger) to the widely used conventional bulk materials (e.g., LiNbO3, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.