Understanding the lignin pyrolysis mechanism holds significant importance for enhancing thermochemical conversion processes and advancing the refinement of biofuels. In this paper, a mechanism study was conducted on the pyrolysis of eugenol within the temperature range of 450 to 750 C-o, utilizing synchrotron photoionization mass spectrometry coupled with density functional theory. 5-allyl-2-hydroxyphenoxyl radical (m/z 149) and other major radicals were detected experimentally during eugenol pyrolysis for the first time. The cleavage of the O-CH3 bond was confirmed as the main initial unimolecular pyrolysis pathway compared with other hemolysis and isomerization reactions. Radical-assisted bimolecular reactions initiated by CH(3 )and H atoms, such as ipso substitution and abstraction reactions, were well discussed. Radical-assisted abstractions made the most contribution to the formation of main radicals and products at high temperatures. Furthermore, quinone-type products were determined and their formation pathways were discussed. This research provides insights into eugenol pyrolysis, shedding light on the fundamental mechanisms of thermochemical conversion of lignin.

Two-dimensional p-type/intrinsic/n-type (p-i-n) homojunction opens up exciting opportunities for the advancement of next-generation electronic and optoelectronic devices. However, it is urgent to explore superior two-dimensional materials for p-i-n homojunction to enhance optoelectronic performance. Herein, the electronic structures, optical properties of monolayer XYN3 (X=V, Nb, Ta; Y=Si, Ge), and the related p-i-n homojunctions are constructed and investigated systematically based on the framework of density function theory and non-equilibrium Green's function calculations. The electronic structures and optical absorption of these stable monolayers reveal that they show semiconductor characteristic with indirect bandgaps of 1.23 similar to 2.13 eV, which possess strong absorption of visible light. The simulations of the p-i-n homojunctions based on these monolayers highlight that VSiN3 and TaSiN3-based p-i-n junctions possess maximum photocurrent densities of 21.43 and 18.48 A/m(2), respectively. Moreover, the photoresponses of VSiN3 and TaSiN3-based p-i-n junctions can reach up to 0.61 and 0.55 A/W, respectively, demonstrating that VSiN3 and TaSiN3-based p-i-n junctions could be the ideal candidates for optoelectronic devices. Our work is expected to pave the way for the realization of 2D p-i-n homojunction optoelectronic devices.

The chemistry of oxygenated polycyclic aromatic hydrocarbons (OPAHs) has received growing interest in recent years due to their significant role in the oxidation and degradation of polycyclic aromatic hydrocarbons (PAHs). This study focuses on the decomposition kinetics of 1-indanone, which is a ketonic OPAH compound significantly produced in the combustion of transportation fuels and biofuels, by utilizing a chemical microreactor coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry. This approach enables the identification of primary intermediates and minimizes interference from secondary reactions. Experimental results reveal that styrene, phenylacetylene, and indenyl are major decomposition products of 1-indanone. A considerable amount of reactive radical intermediates including 1-indanonyl, benzyl, fulvenallenyl, phenyl and cyclopentadienyl were also identified. Moreover, potential energy surfaces (PES) for both the unimolecular and Hassisted decomposition of 1-indanone were comprehensively explored using highly accurate quantum chemical calculations at the CCSD(T)-F12/cc-pVDZ-f12//B3LYP/6-311G(d,p) level. The preferred pathway for the unimolecular decomposition of 1-indanone involves the cleavage of a C-H bond on the cyclopentane-fused ring, leading to 1-indenyl radical and active H atoms. The thermal decomposition of 1-indanone is greatly facilitated by H atoms and exhibits considerably lower barrier heights compared to the unimolecular pathways. Two C8H9 8 H 9 isomers, styrene and benzyl are identified as the primary ring-opening products on the C9H8O 9 H 8 O + H PES, with the formation of C8H9 8 H 9 isomers being most energetically favored. The lowest-barrier pathways for the formation of styrene and indenyl, which are two major products of 1-indanone decomposition, were extensively analyzed in this study. Meanwhile, the reaction scheme of secondary reactions was elucidated to interpret the observed secondary aromatic and aliphatic products. The present study is expected to motivate further investigations into the chemistry of more complex OPAHs, thereby contributing to a deeper understanding of their role in the formation, growth, and oxidation chemistry of PAHs.

The pyrolysis of N-methylethylamine (MEA, C3H9N) was studied in a flow tube reactor at 30 Torr within 1073-1348 K. 12 intermediates and products were detected by the time-of-flight molecular beam mass spectrometry with the light source of tunable synchrotron vacuum ultraviolet. The photoionization cross section (PICS) of MEA was obtained over the photon energy range from 8.0 to 11.5 eV and used to calculate the mole fractions of the detected species. Based on the previous work of other amines, a detailed kinetic model consisting of 883 species and 4750 reactions was developed. Reasonable agreements between the predicted mole fractions and the measured values were obtained for MEA and main nitrogenous intermediates and products. To deeply understand the nitrogen conversion route of MEA pyrolysis, rate-of-production and sensitivity analyses were performed. MEA is dominantly consumed via H-abstraction reactions by H radical at C1 or C3 site. The sensitivity analysis indicates that the unimolecular decomposition reactions of MEA via the cleavage of two kinds of C-N bonds play important roles in promoting MEA consumption, especially at higher temperatures, because the subsequent H atom produced from the fragments decomposition enhances the radical pool. Three C2H5N isomers were observed in this work, with an interconversion between vinylamine and ethanimine. N-methyl-1-ethanimine (C3H7N) is the only identified C3 unsaturated amine with limited concentrations. This can be attributed to the fact that the chemical bonds between the C and N atoms are weaker than C-H or N-H in primary radicals. These results will enrich the knowledge of C3 amine chemistry and lay a solid foundation for exploring the kinetic behavior of more complicated amines.

Self-assembled AlGaN nanowires (NWs) are regarded as promising structures in the pursuit of ultraviolet photodetectors (UV PDs). However, AlGaN NW-based PDs currently suffer from degraded performance partially due to the existence of outstanding surface-related defects/traps as a result of their large surface-to-volume-ratio. Here, we propose an effective passivation approach to suppress such surface states via tetramethyl ammonium hydroxide (TMAH) solution treatment. We successfully demonstrate the fabrication of UV PDs using TMAH-passivated AlGaN quantum-disk NWs and investigate their optical and electrical properties. In particular, the dark current can be significantly reduced by an order of magnitude after surface passivation, thus leading to the improvement of photoresponsivity and detectivity. The underlying mechanism for such a boost can be ascribed to the effective elimination of oxygen-related surface states on the NW surface. Consequently, an AlGaN NW UV PD with a low dark current of 6.22 × 10 −9 A, a large responsivity of 0.95 A W −1 , and a high detectivity of 6.4 × 10 11 Jones has been achieved.

In this Letter, we perform a comprehensive investigation on the optical characterization of micro-sized deep-ultraviolet (DUV) LEDs (micro-LEDs) emitting below 280 nm, highlighting the light extraction behavior in relation to the design of chip sidewall angle. We found that the micro-LEDs with a smaller inclined chip sidewall angle ( ∼ 33 ∘ ) have improved external quantum efficiency (EQE) performance 19% more than that of the micro-LEDs with a larger angle ( ∼ 75 ∘ ). Most importantly, the EQE improvement by adopting an inclined sidewall can be more outstanding as the diameter of the LED chip reduces from 40 to 20 μ m . The enhanced EQE of the micro-LEDs with smaller inclined chip sidewall angles can be attributed to the stronger reflection of the inclined sidewall, leading to enhanced light extraction efficiency (LEE). In the end, the numerical optical modeling further reveals and verifies the impact of the sidewall angles on the LEE of the micro-LEDs, corroborating our experiment results. This Letter provides a fundamental understanding of the light extraction behavior with optimized chip geometry to design and fabricate highly efficient micro-LEDs in a DUV spectrum of the future.

The investigation of electrical and optical properties of micro-scale AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at ∼ 275 n m was carried out, with an emphasis on fabricated devices having a diameter of 300, 200, 100, 50, and 20 µm, respectively. It was revealed that the LED chips with smaller mesa areas deliver considerably higher light output power density; meanwhile, they can sustain a higher current density, which is mainly attributed to the enhanced current spreading uniformity in micro-scale chips. Importantly, when the diameter of LED chips decreases from 300 µm to 20 µm, the peak external quantum efficiency (EQE) increases by 20%, and the EQE peak current density can be boosted from 8.85 A / c m 2 and 99.52 A / c m 2 . Moreover, we observed a longer wavelength emission with enlarged full-width at half-maximum (FWHM) in the LEDs with smaller chip sizes because of the self-heating effect at high current injection. These experimental observations provide insights into the design and fabrication of high-efficiency micro-LEDs emitting in the DUV regime with different device geometries for various future applications.

We have fabricated the novel ultraviolet-photodetectors using AlGaN quantum-disks nanowires. The device responses to 254 nm light sensitively and exhibits a high on/off ratio, a large responsivity, and a fast recovery speed under 0 V.

Searching for power-independent, compact, and highly environment-sensitive photodetectors is a critical step towards the realization of next-generation energy-efficient and sustainable integrated optoelectronic systems. Particularly, the deep ultraviolet (UV) band, which has large photon energy, is extremely suitable for environment monitoring and invisible light communication application. Herein, the demonstration of self-powered deep UV solar-blind photodetectors in a photoelectrochemical (PEC) cell configuration is reported, adopting wide bandgap n-type aluminum gallium nitride (AlGaN) nanowires as photoelectrode. After decorating nanowires with noble metal ruthenium (Ru), the constructed solar-blind PEC photodetectors exhibited excellent responsivity of 48.8 mA W-1, fast response speed (rise time of 83 ms and decay time of 19 ms) with large photocurrent density of 55 mu A cm(-2) at 254 nm illumination. Such superior performance can be attributed to, firstly and foremost, the successful synthesis of highly uniform and defect-free n-type AlGaN nanowires which ensures efficient photogeneration via effective light-harvesting, and secondly, the boosted carrier separation and collection efficiency through Ru decoration. This novel nanoarchitecture enables deep UV photodetection to work stably with low energy consumption, intriguingly, opening the possibility for the development of high-performance PEC photodetectors based on group III-nitride semiconductors covering the entire spectral range from infrared to deep UV.

Wide bandgap aluminum gallium nitride (AlGaN) semiconductor alloys have established themselves as the key materials for building ultraviolet (UV) optoelectronic and power electronic devices. However, further improvements to device performance are lagging, largely due to the difficulties in precisely controlling carrier behavior, both carrier generation and carrier transport, within AlGaN-based devices. Fortunately, it has been discovered that instead of using AlGaN layers with fixed Al compositions, by grading the Al composition along the growth direction, it is possible to (1) generate high-density electrons and holes via polarization-induced doping; (2) manipulate carrier transport behavior via energy band modulation, also known as 'band engineering'. Consequently, such compositionally graded AlGaN alloys have attracted extensive interest as promising building blocks for efficient AlGaN-based UV light emitters and power electronic devices. In this review, we focus on the unique physical properties of graded AlGaN alloys and highlight the key roles that such graded structures play in device exploration. Firstly, we elaborate on the underlying mechanisms of efficient carrier generation and transport manipulation enabled by graded AlGaN alloys. Thereafter, we comprehensively summarize and discuss the recent progress in UV light emitters and power electronic devices incorporating graded AlGaN structures. Finally, we outline the prospects associated with the implementation of graded AlGaN alloys in the pursuit of high-performance optoelectronic and power electronic devices.

In this work, we demonstrate a high-performance ultraviolet phototransistor (UVPT) based on the AlGaN/GaN high-electron mobility transistor (HEMT) configuration. When the device is biased at off state, the peak photoresponsivity (R) of 3.6 x 10(7) A/W under 265 nm illumination and 1.0 x 10(6) A/W under 365 nm illumination can be obtained. Those two R values are one of the highest among the reported UVPTs at the same detection wavelength under off-state conditions. In addition, we investigate the gate-bias (V-GS) dependent photoresponse of the fabricated device with the assistance of band structure analysis. It was found that a more negative V-GS can significantly reduce the rise/decay time for 265 nm detection, especially under weak illumination. This can be attributed to a largely enhanced electric field in the absorptive AlGaN barrier that pushes the photo-generated carriers rapidly into the GaN channel. In contrast, the V-GS has little impact on the switching time for 365 nm photodetection, since the GaN channel has a larger absorption depth and the entire UVPT simply acts as a photoconductive-type device. In short, the proposed AlGaN/GaN HEMT structure with the superior photodetection performance paves the way for the development of next generation UVPTs.

III-nitride deep ultraviolet (DUV) light-emitting diodes (LEDs) have been identified as promising candidates for energy-efficient, environment-friendly and robust UV lighting sources with potential applications in water/air purification, sterilization, and bio-sensing. However, the performance of state-of-art DUV LEDs is far from satisfactory for commercialization due to their low internal quantum efficiency, large current leakage and efficiency droop at high current injection, etc. Extensive efforts have been devoted to properly designing the band structures of such luminescent devices to enhance their output power. In this review, we summarize the recent progress of various energy band designs and of the engineering of DUV LEDs, with particular attention paid to the various approaches in band engineering of electron-blocking layers, quantum wells, quantum barriers and the implementation of many novel structures such as tunnel junctions and ultrathin quantum heterostructures utilized to enhance their efficiency. These inspirational approaches pave the way towards the next generation of greener and more efficient UV sources suitable for practical applications.

Rapid advancement of wide-bandgap AlGaN semiconductor materials offers tremendous opportunities in the field of ultraviolet (UV) optoelectronics for a wide range of advanced applications. Additionally, SiC has large bandgap and excellent material properties, also making itself a suitable material for UV photodetection. More importantly, high-quality AlGaN alloys can be epitaxially grown on SiC substrates because of the very small lattice mismatch between them (less than 1%), which enables a possible monolithic integration of those two materials and allows us to take advantage of their material and physical properties to realize high-performance UV optoelectronics and eventually the integrated UV photonics systems. Herein, we review the recent progress in the development of UV optoelectronics based on AlGaN-SiC platform, mainly focusing on: (1) the growth strategies and material characterizations of AlGaN epilayers on SiC; (2) the fabrication and performance evaluation of UV optoelectronic devices built on the platform, including UV LEDs/lasers and UV photodetectors. Thereafter, we briefly discuss the initial efforts in the pursuit of monolithic integration of those UV optoelectronic devices. Finally, the challenges and potential advances associated with individual UV optoelectronic components as well as UV integrated photonics system on the prosperous AlGaN-SiC platform are outlined, providing insights and perspectives for possible deviceand system-level innovation in future.

Energy-saving photodetectors are the key components in future photonic systems. Particularly, self-powered photoelectrochemical-type photodetectors (PEC-PDs), which depart completely from the classical solid-state junction device, have lately intrigued intensive interest to meet next-generation power-independent and environment-sensitive photodetection. Herein, we construct, for the first time, solar-blind PEC PDs based on self-assembled AlGaN nanostructures on silicon. Importantly, with the proper surface platinum (Pt) decoration, a significant boost of photon responsivity by more than an order of magnitude was achieved in the newly built Pt/AlGaN nanoarchitectures, demonstrating strikingly high responsivity of 45 mA/W and record fast response/recovery time of 47/20 ms without external power source. Such high solar-blind photodetection originates from the unparalleled material quality, fast interfacial kinetics, as well as high carrier separation efficiency which suggests that embracement of defect-free wide-bandgap semiconductor nanostructures with appropriate surface decoration offers an unprecedented opportunity for designing future energy-efficient and large-scale optoelectronic systems on a silicon platform.

The non-centrosymmetric crystal structures of polar-semiconductors comprising GaN, InN, AlN, and ZnO intrigued the scientific community in investigating their potential for a strain-induced nano-energy generation. The coupled semiconducting and piezoelectric properties produce a piezo-potential that modulates the charge transport across their heterostructure interfaces. By using conductive-atomic force microscopy, we investigate the mechanism that gives rise to the piezotronic effect in AlGaN nanowires (NWs) grown on a molybdenum (Mo) substrate. By applying external bias and force on the NWs/Mo structure using a Pt–Ir probe, the charge transport across the two adjoining Schottky junctions is modulated due to the change in the apparent Schottky barrier heights (SBHs) that result from the strain-induced piezo-potential. We measured an increase in the SBH of 98.12 meV with respect to the background force, which corresponds to an SBH variation ∂ϕ∂F of 6.24 meV/nN for the semiconductor/Ti/Mo interface. The SBH modulation, which is responsible for the piezotronic effect, is further studied by measuring the temperature-dependent I–V curves from room temperature to 398 K. The insights gained from the unique structure of AlGaN NWs/Mo shed light on the electronic properties of the metal-semiconductor interfaces, as well as on the potential application of AlGaN NW piezoelectric nanomaterials in optoelectronics, sensors, and energy generation applications.

Enhanced dielectric constant of PDMS incorporating chain-ball structured CCTO@MWCNT nanoparticles.