A WOx/MoOx hybrid oxide based SERS FET and investigation on its tunable SERS performance.

Active control of the surface-enhanced Raman scattering (SERS) enhancement shows great potential for realizing smart detection of different molecules. However, conventional methods usually involve time-consuming structural design or a sophisticated fabrication process. Herein, we reported an electrically tunable field effect transistor (FET) comprising a WOx/MoOx hybrid as the SERS active layer. In the experiment, WOx/MoOx hybrids were first prepared by mixing different molar ratios of WOx and MoOx oxides. Then, R6G molecules were used as Raman reporters, showing that the intensity of the SERS signal observed on the most optimal hybrids (molar ratio = 1 : 3) could be increased by two times as high as that observed on a single WOx or MoOx based substrate, which was ascribed to enhanced charge transfer efficiency by the constructed nano-heterojunction between the WOx and MoOx oxides. Thereafter, a back-gate FET was fabricated on a SiO2/Si substrate, and the most optimal WOx/MoOx hybrid was deposited as the gate channel and the SERS active layer. After that, a series of gate biases (from -15 V to 15 V) were implemented to actively tune the SERS performance of the FET. It is evident that the SERS EF can be further tuned from 2.39 × 107 (-15 V) to 6.55 × 107 (+10 V), which is ∼7.4/4.1 times higher than that observed on the pure WOx device (8.81 × 106) or pure MoOx (1.61 × 107) device, respectively. Finally, the mechanism behind the electrical tuning strategy was investigated. It is revealed that a positive voltage would bend the conduction band down, which increased the electron density near the Fermi level. Consequently, it triggered the resonance charge transfer and significantly improved the SERS performance. In contrast, a negative gate voltage attracted the holes to the Fermi level, which deferred the charge transfer process, and caused the reduction of the SERS enhancement.

Lattice Expansion in Rb-Doped Hybrid Organic-Inorganic Perovskite Crystals Resulting in Smaller Band-Gap and Higher Light-Yield Scintillators.

Two-dimensional hybrid-organic-inorganic perovskite (2D-HOIP) lead bromide perovskite crystals have demonstrated great potential as scintillators with high light yields and fast decay times while also being low cost with solution-processable materials for wide energy radiation detection. Ion doping has been also shown to be a very promising avenue for improvements of the scintillation properties of 2D-HOIP crystals. In this paper, we discuss the effect of rubidium (Rb) doping on two previously reported 2D-HOIP single crystals, BA2PbBr4 and PEA2PbBr4. We observe that doping the perovskite crystals with Rb ions leads to an expansion of the crystal lattices of the materials, which also leads to narrowing of band gaps down to 84% of the pure compounds. Rb doping of BA2PbBr4 and PEA2PbBr4 shows a broadening in the photoluminescence and scintillation emissions of both perovskite crystals. Rb doping also leads to faster γ-ray scintillation decay times, as fast as 4.4 ns, with average decay time decreases of 15% and 8% for Rb-doped BA2PbBr4 and PEA2PbBr4, respectively, compared to those of undoped crystals. The inclusion of Rb ions also leads to a slightly longer afterglow, with residual scintillation still being below 1% after 5 s at 10 K, for both undoped and Rb-doped perovskite crystals. The light yield of both perovskites is significantly increased by Rb doping with improvements of 58% and 25% for BA2PbBr4 and PEA2PbBr4, respectively. This work shows that Rb doping leads to a significant enhancement of the 2D-HOIP crystal performance, which is of particular significance for high light yield and fast timing applications, such as photon counting or positron emission tomography.

Interfacial interactions of doped-Ti3C2 MXene/MAPbI3 heterostructures: surfaces and the theoretical approach.

The work function (WF) of perovskite materials is essential for developing optoelectronic devices enabling efficient charge transfer at their interfaces. Perovskite's WF can be tuned by MXenes, a new class of two-dimensional (2D) early transition metal carbides, nitrides, and carbonitrides. Their variable surface terminations or the possibility of introducing elemental dopants could advance perovskites. However, the influence of doped-MXenes on perovskite materials is still not fully understood and elaborated. This study provides mechanistic insight into verifying the tunability of MAPbI3 WF by hybridizing with fluorine-terminated Ti3C2Tx (F-MXene) and nitrogen-doped Ti3C2Tx (N-MXene). We first reveal the interfacial interaction between MAPbI3 and MXenes via X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). UPS supported by density functional theory (DFT) calculations allowed the description of the influence of F and N on MXene's WF. Furthermore, we developed MAPbI3/MXene heterostructures using F- and N-MXenes. The F-MXenes extended the most WF of MAPbI3 from 4.50 eV up to 3.00 eV, compared to only a small shift for N-MXene. The underlying mechanism was charge transfer from low WF F-MXene to MAPbI3, as demonstrated by PL quenching in MAPbI3/F-MXene heterostructures. Altogether, this work showcases the potential of fluorine-doped MXenes over nitrogen-doped MXenes in advancing perovskite heterostructures, thus opening a door for efficient optoelectronic devices.

Systematic study of thresholdless oscillation in high-ß buried multiple-quantum-well photonic crystal nanocavity lasers.

Buried multiple-quantum-well (MQW) 2D photonic crystal cavities (PhC) achieve low non-radiative recombination and high carrier confinement thus making them highly efficient emitters. In this study, we have investigated the lasing characteristics of high-ß(spontaneous emission coupling factor) buried MQW photonic crystal nanocavity lasers to clarify the theoretically-predicted thresholdless operation in high-ß nanolasers. The strong light and carrier confinement and low non-radiative recombination in our nanolasers have enabled us to clearly demonstrate very smooth lasing transition in terms of the light-in vs light-out curve and cavity linewidth. To clarify the thresholdless lasing behavior, we carried out a lifetime measurement and a photon correlation measurement, which also confirmed the predicted behavior. In addition, we systematically investigated the dependence of ß on the detuning frequency, which was in good agreement with a numerical simulation based on the finite-difference time-domain method. This is the first convincing systematic study of nanolasers based on an MQW close to the thresholdless regime.

Movable high-Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform.

Subwavelength semiconductor nanowires have recently attracted interest for photonic applications because they possess various unique optical properties and offer great potential for miniaturizing devices. However, realizing tight light confinement or efficient coupling with photonic circuits is not straightforward and remains a challenge. Here we show that a high-Q nanocavity can be created by placing a single III­V semiconductor nanowire with a diameter of under 100 nm in a grooved waveguide in a Si photonic crystal, by means of nanoprobe manipulation. We observe very fast spontaneous emission (91 ps) from nanowires accelerated by the strong Purcell enhancement in nanocavities, which proves that very strong light confinement can be achieved. Furthermore, this system enables us to move the nanocavity anywhere along the waveguide. This configuration provides a significant degree of flexibility in integrated photonics and permits the addition and displacement of various functionalities of III­V nanocavity devices in Si photonic circuits.

Controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires.

We report controlled 1.1-1.6 µm luminescence in gold-free multi-stacked InAs/InP heterostructure nanowires (NWs). We realized the NWs by using an indium-particle-assisted vapor-liquid-solid synthesis approach. The growth temperature, as low as 320 °C, enables the formation of an atomically abrupt InP/InAs interface by supressing the diffusion and weakening the reservoir effect in the indium droplet. The low growth temperature also enables us to grow multi-stacked InAs/InP NWs in the axial direction without any growth on the NW side face. The high controllability of the growth technology ensures that the luminescence can be tailored by the thickness of InAs segment in InP NWs and cover the 1.3-1.5 µm telecommunication window range. By using the nanoscale-spatial-resolution technology combing cathodoluminescence with scanning electron microscopy, we directly correlated the site of different-thickness InAs segments with its luminescence property in a single NW and demonstrate the InAs-thickness-controlled energy of optical emission in 1.1-1.6 µm.

The Nanoplasmonic Purcell Effect in Ultrafast and High-Light-Yield Perovskite Scintillators.

The development of X-ray scintillators with ultrahigh light yields and ultrafast response times is a long sought-after goal. In this work, a fundamental mechanism that pushes the frontiers of ultrafast X-ray scintillator performance is theoretically predicted and experimentally demonstrated: the use of nanoscale-confined surface plasmon polariton modes to tailor the scintillator response time via the Purcell effect. By incorporating nanoplasmonic materials in scintillator devices, this work predicts over tenfold enhancement in decay rate and 38% reduction in time resolution even with only a simple planar design. The nanoplasmonic Purcell effect is experimentally demonstrated using perovskite scintillators, enhancing the light yield by over 120% to 88 ± 11 ph/keV, and the decay rate by over 60% to 2.0 ± 0.2 ns for the average decay time, and 0.7 ± 0.1 ns for the ultrafast decay component, in good agreement with the predictions of our theoretical framework. Proof-of-concept X-ray imaging experiments are performed using nanoplasmonic scintillators, demonstrating 182% enhancement in the modulation transfer function at four line pairs per millimeter spatial frequency. This work highlights the enormous potential of nanoplasmonics in optimizing ultrafast scintillator devices for applications including time-of-flight X-ray imaging and photon-counting computed tomography.

Porous carbon film/WO3-x nanosheets based SERS substrate combined with deep learning technique for molecule detection.

The Surface-enhanced Raman scattering (SERS) is an attractive optical detecting method with high sensitivity and detectivity, however challenges on large-area signal uniformity and complex spectra analysis methods always retards its wide application. Herein, a highly sensitive and uniform SERS detection strategy supported by porous carbon film/WO3-x nanosheets (PorC/WO3-x) based noble-metal-free SERS substrate and deep learning algorithm are reported. Experimentally, the PorC/WO3-x substrate was prepared by high-temperature annealing the PorC/WO3 films under the argon atmosphere. The defect density of the WO3 was controlled by tuning the reducing reaction time during the annealing process. The SERS performance was evaluated by using R6G as the Raman reporter, it showed that the SERS intensity obtained on the substrate with the optimal annealing time of 3 h was about 8 times as high as that obtained on the PorC/WO3 substrate without annealing treatment. And detection limit of 10-7 M and Raman enhancement factor of 106 could be achieved. Moreover, the above optimal SERS substrate was utilized to detect flavonoids of quercetin, 3-hydroxyflavone and flavone, and a deep learning algorithms was incorporated to identify the quercetin. It revealed that quercetin can be accurately detected within the above flavonoids, and lowest detectable concentration of 10-5 M can be achieved.

Effect of Dual-Organic Cations on the Structure and Properties of 2D Hybrid Perovskites as Scintillators.

Two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) crystals show promise as scintillating materials for wide-energy radiation detection, outperforming their three-dimensional counterparts. In this study, we synthesized single crystals of (PEA2-xBZAx)PbBr4 (x ranging from 0.1 to 2), utilizing phenethylammonium (C6H5CH2CH2NH3+) and benzylammonium (C6H5CH2NH3+) cations. These materials exhibit favorable optical and scintillation properties, rendering them suitable for high light yield (LY) and fast-response scintillators. Our investigation, employing various techniques such as X-ray diffraction (XRD), photoluminescence (PL), time-resolved (TR) PL, Raman spectroscopy, radioluminescence (RL), thermoluminescence (TL), and scintillation measurements, unveiled lattice strain induced by dual-organic cations in powder X-ray diffraction. Density functional theory analysis demonstrated a maximal 0.13 eV increase in the band gap with the addition of BZA cation addition. Notably, the largest Stokes shift of 0.06 eV was observed in (BZA)2PbBr4. The dual-organic cation crystals displayed >80% fast component scintillation decay time, which is advantageous for the scintillating process. Furthermore, we observed a dual-organic cations-induced enhancement of electron-hole transfer efficiency by up to 60%, with a contribution of >70% to the fast component of scintillation decay. The crystal with the lowest BZA concentration, (PEA1.9BZA0.1)PbBr4, demonstrated the highest LYs of 14.9 ± 1.5 ph/keV at room temperature. Despite a 55-70% decrease in LY for BZA concentrations >5%, simultaneous reductions in scintillation decay time (12-32%) may work for time-of-flight positron emission tomography and photon-counting computed tomography. Our work underscores the crucial role of dual-organic cations in advancing our understanding of 2D-HOIP crystals for materials science and radiation detection applications.

Improvement of Light Output of MAPbBr3 Single Crystal for Ultrafast and Bright Cryogenic Scintillator.

The remarkable brightness and rapid scintillation observed in perovskite single crystals (SCs) become even more striking when they are operated at cryogenic temperatures. In this study, we present advancements in enhancing the scintillation properties of methylammonium lead bromide (MAPbBr3) SCs by optimizing the synthesis process. We successfully synthesized millimeter-sized MAPbBr3 SCs with bright green luminescence under UV light. However, both MAPbBr3 (Control-1M and THF-0.4M) SCs display notable radioluminescence exclusively at low temperatures due to their phase transitions. Notably, the THF-0.4M SCs exhibit a remarkable improvement in radioluminescence light yield, surpassing Control-1M SCs more than 2-fold. Further, THF-0.4M SCs demonstrate an ultrafast decay component of 0.52 ns (82.2%) and a slower component of 1.80 ns (17.8%), contributing to a rapid scintillation response at low temperatures. Therefore, the amalgamation of ultrafast decay components and improved radioluminescence light yield equips THF-0.4M SCs to emerge as a top choice for perovskite scintillators for X-ray timing applications.