Blue Photons Research Articles

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

Solid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s‐1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

High‐Efficiency and Low Efficiency Roll‐Off Blue‐Emission Crystalline Organic Light‐Emitting Diodes by embedding Triplet‐Triplet Annihilation Nanoaggregates

AbstractCrystalline host matrix (CHM)‐nanoaggregates (NA) structure combines the advantages of a crystalline host matrix and high‐performance luminescent materials. This structure emerges as a highly effective strategy for achieving efficient blue organic light‐emitting diodes (OLEDs). This approach can tackle the current challenge of achieving low voltage and high brightness in amorphous OLEDs. In this work, triplet‐triplet annihilation (TTA) material NA are embedded into CHM and sensitize the fluorescent dopant (D) BD1. As a result, a CHM‐TTANA‐D structure device is created, demonstrating an outstanding external quantum efficiency (EQE) of 8.46%. Notably, the efficiency roll‐off is mitigated. The device maintains an EQE of ≈7.50% at 1000 cd m−2, which is the highest values among blue crystalline OLEDs. The device leverages the benefits of CHM and exhibits swift turn‐on and a rapid increase in brightness and current density. This leads to a significant improvement in blue photon output and a reduced series‐resistance Joule heat loss ratio. The findings underscore the promising nature of the CHM‐TTANA‐D structure as a favorable design paradigm for high‐performance crystalline thin‐film OLEDs (C‐OLEDs).

Visible/near-infrared luminescence and concentration effects of Pr3+-doped Sr2Al2GeO7 downconversion phosphors

The Pr3+ ion has been widely doped into various materials as a red and near-infrared (NIR) emitting center for applications in lighting and solar spectrum downconversion. Herein, the preparation of a new library of Pr3+-doped Sr2Al2GeO7 phosphors was proved by powder x-ray diffraction patterns and Rietveld refinements and characterized by a scanning electron microscope with energy-dispersive x-ray spectrometry. The Sr2Al2GeO7:Pr3+ sample strongly absorbs blue photons over 420–500 nm and yields intense visible emissions with dominant peaks around 490 nm from the Pr3+ 3P0 → 3H4 transition, as well as robust NIR emission bands over 800–1200 nm. In addition to the typical transitions of 1D2 → 3F2 at 880 nm, 1G4 → 3H4 at 1000 nm, and 1D2 → 3F3,4 at 1070 nm, the distinguishable NIR emission at 929 nm was demonstrated from the 3P0 → 1G4 transition via static and dynamic spectroscopic analysis. Most interestingly, for the 3P0 blue-excited state, a considerably elevated concentration of about 10%Pr3+ was optimal for the visible/NIR emissions, in stark contrast to the diluted optimal 1%Pr3+ for the 1D2 state. The relevant cross-relaxation from the 3P0 and 1D2 states between Pr3+ was comprehensively treated by theoretical speculations and experimental results. Such concentrated Pr3+ blue activators would significantly facilitate the blue-to-NIR downconversion through a desired two-step sequential transition from the 3P0 initial state to the 1G4 intermediate level for quantum efficiency exceeding unity. The current results would consolidate the basis of concentrated Pr3+ donors to promote the novel Pr3+/Yb3+ codoping downconversion for greatly increasing Si solar cell efficiency.

Blue light as an important factor increasing plant tolerance to acute photooxidative stress

Previous studies have confirmed the stimulating effect of blue light on phenolic compound accumulation and emphasized that sufficient dose of blue light is essential for biosynthesis of B-dihydroxylated flavonoids with enhanced antioxidant properties (under UV-lacking conditions). This study investigates the importance of blue light and complex role of phenolic compounds in plant tolerance against acute photooxidative stress. Hordeum vulgare (L. Cv. Bojos) seedlings were acclimated to different light spectra (blue, green:red 1:1, and white composed of blue:green:red 1:1:1) at total irradiance 400 µmol.m−2.s−1. Subsequently, they were subjected to a 3-hour combined stress induced by high photosynthetically active (850–950 µmol.m−2.s−1) and UV-B (2.0–2.5 W.m−2) radiation. Content of flavonoids, expression of genes involved in their biosynthesis (phenylalanine ammonia-lyase, chalcone synthase, flavonoid 3′-hydroxylase), and antioxidant activity of plant extracts were significantly highest in plants acclimated to blue light. As an indicator of reactive oxygen species interaction with biomolecules, the content of lipid hydroperoxides was estimated. It was demonstrated that plants acclimated to blue light revealed significantly lower extent of lipid peroxidation compared to those acclimated to white or green:red light. Plants exposed to combined light-induced stress for 3 hours exhibited pronounced disruption of PSII function: FV/FM tended to decrease proportionally with decreasing amount of blue photons in the treatments. Additionally, stress exposure upregulated the expression of genes related to phenolic compounds but not genes encoding antioxidant enzymes. We confirmed higher resistance of plants acclimated to blue light and presume that phenolic compounds are significantly involved in protection during the acute phase of stress.

Photonic crystal-assisted sub-bandgap photocatalysis via triplet-triplet annihilation upconversion for the degradation of environmental organic pollutants

This study explores novel approaches to enhance photocatalysis efficiency by introducing a photonic crystal (PC)-enhanced, multi-layered sub-bandgap photocatalytic reactor. The design aims to effectively utilize sub-bandgap photons that might otherwise go unused. The device consists of three types of layers: (1) two polymeric triplet-triplet annihilation upconversion (TTA-UC) layers converting low-energy green photons (λEx = 532 nm, 2.33 eV) to high-energy blue photons (λEm = 425 nm, 2.92 eV), (2) a platinum-decorated WO3 layer (Eg = 2.8 eV) serving as a visible-light photocatalyst, and (3) a PC layer optimizing both TTA-UC and photocatalysis. The integration of the PC layer resulted in a 1.9-fold increase in UC emission and a 7.9-fold enhancement in hydroxyl radical (•OH) generation, achieved under low-intensity sub-bandgap irradiation (17.6 mW cm–2). Consequently, the combined layered structure of TTA/Pt-WO3/TTA/PC achieved a remarkable 38.8-fold improvement in •OH production, leading to outstanding degradation capability for various organic pollutants (e.g., 4-chlorophenol, bisphenol A, and methylene blue). This multi-layered sub-bandgap photocatalytic structure, which uniquely combines TTA-UC and PC layers, offers valuable insights into designing efficient photocatalytic systems for future solar-driven environmental remediation.

Temporal Separation of Red and Blue Photons Does Not Increase Photon Capture or Yield of Lettuce

Temporal separation of red (R) and blue (B) (alternating R/B) photons has been reported to increase leaf area, photon capture, and yield of lettuce compared with delivering both colors together (concurrent R+B). We grew three diverse lettuce cultivars (Grand Rapids, Rex, and Red Sails) under concurrent R+B photons (9/1 ratio) and alternating R/B photons (9/1 ratio) under an equal daily light integral (DLI) of either 8.6 or 23 mol⋅m−2⋅d−1. Contrary to five previous studies, we found no increase in either leaf area or fresh mass and dry mass in any of the alternating R/B photon treatments compared with concurrent R+B photons. In fact, at a DLI at 8.6 mol⋅m−2⋅d−1, alternating R/B photons decreased the dry mass of ‘Grand Rapids’ and ‘Rex’ lettuce by 38% and 17%, respectively. Two previous studies reported that photosynthetic rates increased with alternating R/B photons; however, we found that the net assimilation rate was generally decreased by alternating R/B photons. An analysis of images obtained from automated digital photography revealed that the relative expansion rate of leaves was 61% higher during intervals of pure B rather than intervals of pure R photons at the same photosynthetic photon flux density; however, this did not result in a higher leaf area compared with concurrent R+B photons. Overall, our studies do not indicate that alternating R/B photons increase lettuce leaf area or yield compared with concurrent R+B photons.

Luminescent Quantum Dot Films Increase the Radiation Capture and Yield of Lettuce and Sweet Basil Compared to a Traditional/Neutral-density Greenhouse Glazing

Utilizing quantum dot (QD) luminescent films as a greenhouse covering material is an innovative method of modifying the greenhouse light spectrum. The QD films convert a portion of high-energy ultraviolet and blue photons to lower-energy photons. Previous research has shown that the application of QD films in greenhouses led to improved crop yields of red lettuce and tomatoes. However, the underlying mechanism of the yield increases has not been fully explored. We quantified the effects of solar spectral shifts attributable to QD films on plant morphology, radiation capture, and, subsequently, crop yield. Green and red leaf lettuces and basil were grown in a greenhouse under four treatments: regular-concentration QD film (reg QD film); high-concentration QD film (high QD film); color-neutral polyethylene (PE) film; and control treatment without any films. Compared to the reg QD film, the high QD film converted a higher fraction of blue photons into longer-wavelength photons, resulting in enhanced leaf expansion, stem elongation, and shoot fresh weight of red lettuce and basil compared with those grown under the PE film without spectral modifications. No significant growth differences were observed between the control and high QD film treatments of red lettuce and basil despite a 23% reduction in the average daily light integral (DLI) under the high QD film treatment. Compared to that grown under the control treatment, green lettuce grown under the high QD film treatment had a similar total leaf area but reduced shoot biomass; this was likely associated with reductions in leaf thickness and chlorophyll content. In contrast, the red lettuce showed more pronounced leaf expansion and reduced leaf anthocyanin content under the high QD film, which likely helped to offset the reduction in DLI. Overall, our results indicated that modifying the solar spectrum with QD films as greenhouse covering material could result in improved crop radiation capture and yield in greenhouse production of lettuce and basil. However, the spectral shifts caused by the QD films may affect crop quality attributes, such as anthocyanin levels and the production of other beneficial secondary metabolites. This effect on crop quality should be carefully considered and requires further study.

A Comprehensive Study of Light Quality Acclimation in Synechocystis Sp. PCC 6803.

Cyanobacteria play a key role in primary production in both oceans and fresh waters and hold great potential for sustainable production of a large number of commodities. During their life, cyanobacteria cells need to acclimate to a multitude of challenges, including shifts in intensity and quality of incident light. Despite our increasing understanding of metabolic regulation under various light regimes, detailed insight into fitness advantages and limitations under shifting light quality remains underexplored. Here, we study photo-physiological acclimation in the cyanobacterium Synechocystis sp. PCC 6803 throughout the photosynthetically active radiation (PAR) range. Using light emitting diodes (LEDs) with qualitatively different narrow spectra, we describe wavelength dependence of light capture, electron transport and energy transduction to main cellular pools. In addition, we describe processes that fine-tune light capture, such as state transitions, or the efficiency of energy transfer from phycobilisomes to photosystems (PS). We show that growth was the most limited under blue light due to inefficient light harvesting, and that many cellular processes are tightly linked to the redox state of the plastoquinone (PQ) pool, which was the most reduced under red light. The PSI-to-PSII ratio was low under blue photons, however, it was not the main growth-limiting factor, since it was even more reduced under violet and near far-red lights, where Synechocystis grew faster compared to blue light. Our results provide insight into the spectral dependence of phototrophic growth and can provide the foundation for future studies of molecular mechanisms underlying light acclimation in cyanobacteria, leading to light optimization in controlled cultivations.

A Colloidal-Quantum-Dot Integrated U-Shape Micro-Light-Emitting-Diode and Its Photonic Characteristics.

A special micro LED whose light emitting area is laid out in a U-like shape is fabricated and integrated with colloidal quantum dots (CQDs). An inkjet-type machine directly dispenses the CQD layer to the central courtyard-like area of this U-shape micro LED. The blue photons emitted by the U-shape mesa with InGaN/GaN quantum wells can excite the CQDs at the central courtyard area and be converted into green or red ones. The U-shape micro LEDs are coated with Al2O3 by an atomic layer deposition system and exhibit moderate external quantum efficiency (6.51% max.) and high surface recombination because of their long peripheries. Low-temperature measurement also confirms the recovery of the external quantum efficiency due to lower non-radiative recombination from the exposed surfaces. The color conversion efficiency brought by the CQD layer can be as high as 33.90%. A further continuous CQD aging test, which was evaluated by the strength of the CQD emission, under current densities of 100 A/cm2 and 200 A/cm2 injected into the micro LED, showed a lifetime extension of the unprotected CQD emission up to 1321 min in the U-shape device compared to a 39 min lifetime in the traditional case, where the same CQD layer was placed on the top surface of a squared LED.

Stable solvatochromic light-emitting diodes and their potential for color temperature adjustment of white LEDs

A viable technology for liquid phosphor-based light-emitting diodes (LEDs) with emission color tuneability is described. These devices are based on Tris(bipyridine)ruthenium(II) chloride but can be constructed from other similar metal coordination charge transfer complexes. The Frank-Condon excited state generated upon the absorption of blue (450 nm) pump photons has a large electric dipole moment due to intramolecular metal-to-ligand charge transfer from the ruthenium atom to one specific bipyridyl ligand. Polar solvents respond to the dipole field creating a solvatochromic shift that is solvent dependent. The devices were fully sealed in either a long path length or a short path length configuration. The latter encapsulating the luminescing medium in hollow silica spheres that can be easily applied like a slurry on top of InGaN/GaN LED dies. The devices were optically, thermally and environmentally stable. Also discussed is the possibility of using solvatochromic emitters for achieving color temperature tuneability in white LEDs.

Transparent, low dark current, fast response ultraviolet and short-wavelength blue photons photodetector based on Graphene/SrTiO3 single crystal

Ultraviolet light and short-wavelength blue photons (UV-SWB) can cause damage to the human body under excessive exposure. Strontium titanate (SrTiO3), which is a crucial perovskite oxide with a direct energy band gap of ∼3.2 eV, has great potential for UV-SWB photodetection. In this work, UV-SWB photodetector with high transparency based on Graphene/SrTiO3 heterojunction was fabricated by graphene transfer method. The Graphene/SrTiO3 photodetector exhibits an ultra-low dark current of 6 pA at a 30 V bias. The response spectrum of the Graphene/SrTiO3 photodetector was 220–410 nm, covering the UV-SWB band. Under 385 nm light irradiation, the photodetector exhibits a response of 3.4 mA/W, a detectivity of 1.5 × 1010 Jones, a response time of 186 ms, and a light-to-dark current ratio of 103 magnitude at a 30 V bias. In addition, the transmittance of the photodetector in the visible light band reaches 80 %, which has excellent visualization performance. The excellent performance of Graphene/SrTiO3 photodetector is attributed to the high crystal quality of SrTiO3 wafer and the van der Waals interface constructed by Graphene/SrTiO3 reduces the bulk defect density and interfacial state density, which could be beneficial the separation and transport of photogenerated carriers in the active layer and heterojunction. This study has important reference value for the design and manufacture of UV-SWB photodetectors.

B-Box transcription factor BBX28 requires CONSTITUTIVE PHOTOMORPHOGENESIS1 to induce shade-avoidance response in Arabidopsis thaliana.

Shade avoidance syndrome is an important adaptive strategy. Under shade, major transcriptional rearrangements underlie the reallocation of resources to elongate vegetative structures and redefine the plant architecture to compete for photosynthesis. BBX28 is a B-box transcription factor involved in seedling de-etiolation and flowering in Arabidopsis (Arabidopsis thaliana), but its function in shade-avoidance response is completely unknown. Here, we studied the function of BBX28 using two mutant and two transgenic lines of Arabidopsis exposed to white light and simulated shade conditions. We found that BBX28 promotes hypocotyl growth under shade through the phytochrome system by perceiving the reduction of red photons but not the reduction of photosynthetically active radiation or blue photons. We demonstrated that hypocotyl growth under shade is sustained by the protein accumulation of BBX28 in the nuclei in a CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1)-dependent manner at the end of the photoperiod. BBX28 up-regulates the expression of transcription factor- and auxin-related genes, thereby promoting hypocotyl growth under prolonged shade. Overall, our results suggest the role of BBX28 in COP1 signaling to sustain the shade-avoidance response and extend the well-known participation of other members of BBX transcription factors for fine-tuning plant growth under shade.

Two-photon coherence in a DROP-FWM medium

In this work, we report experimental studies on coherence in a medium exhibiting DROP (Double Resonance Optical Pumping)-FWM (Four Wave Mixing). Here 5S1/2(F) → 5P3/2(F/) → 5D (F//) two photon transition of hot 87Rb atoms is used. The 5S→5P connection is modified by introducing an additional beam phase coupled to the original beam linking F = 2 → F/ transition. The frequency of the additional beam is offset from that of the original beam by ≈ +10Γ (Γ is natural linewidth). Such a two-beam configuration in F→F/ manifold effectively satisfies conditions of Vee (V) linkage or degenerate two-level connection (DTLC), depending on the detuning of the 780nm laser. This transformation profoundly affects the behavior of the ensuing Ladder (Ξ) system. While the (I) Ξ +V condition is favorable for Electromagnetically Induced Transparency (EIT), the (ii) Ξ + DTLC brings in the effect of Electromagnetically Induced Absorption (EIA). The EIT-dominated situation is helpful for FWM to take place, and the EIA effect augments the stronger presence of DROP. This condition is verified by monitoring the blue fluorescence emanating from the 5D→6P3/2→5S1/2 decay route. The DROP effect follows the Amplified Spontaneous Emission (ASE) pattern in the media. The origin of blue photons is also due to FWM under EIT conditions. In the case of EIA, the dominant condition increment in blue fluorescence is due to increased stimulated emission. The blue photons mainly contributed by (i) FWM and (ii) increased participation of stimulated emission are directional in nature and phase coherent.

Thermally activated sensitization of organics by lanthanide complexes for near-infrared photochemical upconversion

A series of solid-state triplet-triplet annihilation upconversion systems have been developed utilizing a penta-nuclear Yb complex as the sensitizer, converting near-infrared light into blue, green and red photons. The endothermic...

Female Mosquitoes Track to the Hosts by the Blue

Mosquitoes are the most harmful animals. Annually, they cause illnesses and deaths equivalent to the entire global COVID-19 pandemic over three years[1][2][3]. They are not naturally born with disease-causing pathogens but acquire them from infected individuals and then transmit them to the healthy population through their bites. Their ultimate hosts are those who carry blood, a water solution beneath their skin, a natural membrane that allows water vapor to leak out and their needle to access the liquid. Water vapors are theoretically visible because water molecules are known to naturally reflect blue[4] and absorb others, making it appear blue. This study investigates the reactions of female mosquitoes when blueness variable is added to the mosquito luring equation by amplifying the reflective the blueness variable of water vapor. This is done by adding more blue photons for them to reflect, increasing their visibility. The search for visible signs of water vapor is the top priority for these insects when they identify and target their hosts from afar, as they selectively respond to this over other mosquito attractants.

Visible Light Driven CO2 Photoreduction Using TiO2 Nanotube Arrays Embedded with Low Bandgap Carbon Nitride Nanoparticles

The extraordinary thermal and photochemical stability, superior charge transport, and tunable band positions of graphitic carbon nitride (g-CN), which is constituted of elements that are plentiful on Earth, renders g-CN an important semiconductor photocatalyst for heterogeneous catalysis [1,2]. Despite these advantages, carbon nitride-based semiconductors do not function effectively as freestanding photocatalysts or photoelectrodes due to a rapid carrier recombination rate and a slightly wide bandgap that only enables them to capture blue and UV photons [3,4]. Anodically formed TiO2 nanotube arrays (TNTAs) are semiconducting wide bandgap scaffolds with excellent photocatalytic properties due to the intrinsic orthogonalization of charge generation/transport and charge transfer processes. Herein, we use a novel in situ electrophoretic anodization to embed low bandgap carbon nitride nanoparticles (CNNPs) in the walls of titania nanotubes. The likelihood of the CNNPs leaching off the TNTA photoanode during photoelectrochemical processes was eliminated by encapsulating CN inside a TiO2 matrix. CNNPs were formed by the thermal condensation polymerization of carbon nitride utilizing citric acid and urea as the precursors, and exhibited some unusual properties, including a lower bandgap of 2.1 eV, a highly redshifted fluorescence emission maximum at 2.35 eV, surface carboxylate groups, and the emergence of unique structural characteristics corresponding to amorphous yet graphitic carbon [5]. In contrast to bulk g-CN, which has a C:N ratio of 0.75, the CNNPs possessed an elevated C:N ratio as high as 1.87 at the surface. The additional carbon was found to be both amorphous and graphitic, although the structural characteristics of g-CN were mostly unaffected, as validated by diffractometric and spectroscopic data. Even in the absence of a sacrificial agent, the CNNP@TNT nanocomposite demonstrated enhanced performance in sunlight-driven CO2 photoreduction. When compared to the freestanding TNT photocatalyst, the CO yield of photoreduction for the CN@TNT hybrid was more than three times higher. UV-filtered illumination of the CNNP@TNT heterojunction photocatalyst generated appreciable quantities of methane and CO (3.41 and 8.78 μmolg–1h–1 respectively). In situ electrophoretic anodization is an innovative approach to incorporate semiconductor quantum dots into TiO2 nanotubes or other electrochemically grown nanostructures.REFERENCES1. Kessler, F. K. et al., Nature Reviews Materials (2017) 2 (6), 1.2. Chaulagain, N. et al., ACS Applied Materials & Interfaces (2022) 14 (21), pp.24309-24320.3. Kumar, P. et al., Advanced Optical Materials (2020) 8 (4), Art. No. 1901275.4. Fu, J. et al., Advanced Energy Materials (2018) 8 (3), Art. No. 1701503.5. Alam, K.M. et al., Chemical Engineering Journal (2023) 456, Art. No. 141067.

Photons at the ultraviolet-visible interface: Effects on leaf expansion and photoinhibition

High energy ultraviolet-A (UV-A) and violet photons are at the edge of photosynthetically active radiation and may induce damage to sensitive metabolic pathways. The increasing use of light emitting diodes (LEDs) that provide these photons makes it important to better understand their effects on horticultural crops. We compared the effects of UV-A/violet photons to blue photons on short-term photosynthetic efficiency and long-term growth and morphological responses of lettuce ‘Rex’ and cucumber ‘Straight Eight’ under high and low photon flux densities (PFD). All plants received 67 % of the total PFD from white LEDs with the remaining 33 % of the photons from either blue (peak at 442 nm) or UV-A/violet LEDs (peak at 404 nm). Under a high PFD (500 μmol m−2 s−1; 16 hr photoperiod), the UV-A/violet photon treatment increased leaf area (by 24 %) and dry mass (by 17 %) in cucumber. In contrast, ‘Rex’ lettuce plants grown with UV-A/violet photons had significantly reduced chlorophyll content, leaf area, leaf photosynthetic rate, Fv/Fm ratio (indication of photoinhibition) and dry mass. The photoinhibition induced by UV-A/violet photons was reversible. Chlorophyll concentration of ‘Rex’ lettuce dramatically increased 2 days after being transferred from the UV-A/violet + white treatment to the blue + white light treatment, and plants regained full photosynthetic capacity after one week. The photoinhibition was also dose-dependent and did not occur when the photon flux density of UV-A/violet photons was reduced by 50 % at a lower daily light integral. We further evaluated the effects of UV-A/violet photons on three additional lettuce cultivars. Contrary to the detrimental effects in ‘Rex’ lettuce, UV-A/violet photons significantly increased leaf area and dry mass of ‘Marshall’ and tended to improve growth of ‘Parris Island’ and ‘Red Salad Bowl’ (although not statistically significant). Our analysis revealed that ‘Rex’ lettuce contained much lower levels of photoprotective pigments carotenoids and anthocyanins than other cultivars. We conclude that differences in photoprotective pigments are likely responsible for the contrasting responses to UV-A/violet photons.

Far-red Photons Increase Light Capture but Have Lower Photosynthetic Capacity Than Red Photons

Far-red photons (700–750 nm) can accelerate crop growth during indoor production through both physiological and morphological processes. A previous study showed that far-red photons can drive photosynthesis with efficiency similar to that of traditionally defined photosynthetically active photons (400–700 nm) if they are provided together with shorter-wavelength photons. Far-red photons also promote leaf and canopy expansion, which can increase light interception and growth. This study aimed to distinguish the contribution of morphological and physiological changes to crop growth induced by substituting red photons with far-red photons. We studied the long-term effects of substituting red photons with far-red photons on canopy light interception and whole-plant photosynthesis. ‘Little Gem’ lettuce (Lactuca sativa) seedlings were grown under four light spectrums of the same total photon flux density (400–750 nm). In addition to a background of a mixture of white and blue photons of 150 μmol⋅m−2⋅s−1, we provided 51 μmol⋅m−2⋅s−1 red photons, far-red photons, or mixtures of red and far-red photons. In the first run, plants were harvested twice. The first harvest was at canopy closure, and the second harvest was when plants reached full size. In the second run, we harvested lettuce plants more frequently to minimize leaf overlap and interplant competition. We found that far-red photon substitution promoted leaf and canopy expansion and increased light interception. The effect of far-red photon substitution on leaf and canopy expansion was stronger in the second run than in the first run, likely because of lower plant density in the second run when plants were harvested more frequently. Far-red photon substitution of red photons decreased the amount of extended photosynthetically active radiation (ePAR) photons (400–750 nm) absorbed by leaves because of the lower leaf absorptance of far-red photons. The greater effect on canopy expansion in the second run of far-red photons substitution was able to exceed the reduction of ePAR photon absorption by leaves; therefore, we observed an increased crop gross photosynthetic rate (Pg) between the second and third harvests during the second run. However, during the first run, lower absorptance of ePAR completely offset the effect of the greater canopy size and light interception, and crop Pg was decreased in the first run before the first harvest. The changes in light interception and crop Pg resulting from far-red photon substitution did not affect dry weight. Far-red photons had photosynthetic activity when applied with a blue and white light mixture, but their efficiency was approximately half that of red photons, potentially because of the lower absorptance of far-red photons. In conclusion, far-red photon substitution of red photons increased canopy size but decreased ePAR photons absorbed by leaves and did not increase the final dry weight. Because far-red light-emitting diodes (LEDs) have higher efficacy for converting electricity into photons, including far-red LEDs in fixtures for sole-source lighting can reduce energy costs without decreasing lettuce yields.

Negative Photoresponse Switching via Electron-Hole Recombination at The Type III Junction of MoTe2 Channel/SnS2 Top Layer.

Extensive study on 2Dvan der Waals (vdW) heterojunctions has primarily focused on PN diodes for fast-switching photodetection, while achieving the same from 2D channel phototransistors is rare despite their other advantages. Here, a high-speed phototransistor featuring a type III junction between p-MoTe2 channel and n-SnS2 top layer is designed. The photodetecting device operates with a basis of negative photoresponse (NPR), which originates from the recombination of photoexcited electrons in n-SnS2 and accumulated holes in the p-MoTe2 channel. For the NPR to occur, high-energy photons capable of exciting SnS2 (band gap ≈2.2eV) are found to be effective because lower-energy photons simply penetrate the SnS2 top layer only to excite MoTe2 , leading to normal positive photoresponse (PPR) which is known to be slow due to the photogating effects. The NPR transistor showcases 0.5ms fast photoresponses and a high responsivity over 5000 A W-1 . More essentially, such carrier recombination mechanism is clarified with three experimental evidences. The phototransistor is finally modified with Au contact on n-SnS2 , to be a more practical device displaying voltage output. Three different photo-logic states under blue, near infrared (NIR), and blue-NIR mixed photons are demonstrated using the voltage signals.