Universal Emission Characteristics of Upconverting Nanoparticles Revealed by Single-Particle Spectroscopy

Upconverting nanoparticles (UCNPs) have been extensively investigated for nanophotonics and biomedical applications. However, establishing a unified view of their emission characteristics to elucidate the underlying photophysics and expand the application fields of these materials is a great challenge due to their sophisticated internal energy transfer and lack of standardized single-particle spectroscopy (SPS) platform. Here, we present an SPS technique called multiband single-particle irradiance-dependent imaging (multiband SPIDI). We demonstrate that the emission characteristics of Yb³⁺,Tm³⁺-doped UCNPs are universal for three emission bands over a wide range of irradiance and dependent on the Tm³⁺ doping concentration, indicating that the number of emitted photons of each band is proportional to the number of activator ions and is dependent on the number of absorbed photons and the activator interionic distance. We also suggest a cooperative energy transfer upconversion (CETU) mechanism for transition to a higher-energy state through photon accumulation. For a single UCNP, the emission at 800 nm is detectable at an ultralow irradiance of 4.9 W cm^-2; moreover, that at 450 nm is measurable at 98 W cm^-2, based on the optimal concentration. These findings based on the multiband SPIDI platform can provide insights into the interionic energy transfer by studying irradiance-dependent steady-state dynamics to achieve brighter UCNPs and their broader applications.

Interface effects of Schottky devices built from MoS2and high work function metals

Schottky junctions, formed by high work function metals and semiconductors, are important devices in electronics and optoelectronics. The metal deposition in traditional Schottky interfaces usually damages the semiconductor surface and causes defect states, which reduces the Schottky barrier height and device performance. This can be avoided in the atomically smooth interface formed by two-dimensional (2D) metals and semiconductors. For better interface tailoring engineering, it is particularly important to understand various interface effects in such 2D Schottky devices under critical or boundary conditions. Here we report the fabrication and testing of three types of MoS₂devices, i.e., using PtTe₂, Cr and Au as contact materials. While the Cr/MoS₂contact is an ohmic contact, the other two are Schottky contacts. The van-der-Waals interface of PtTe₂-MoS₂results in a well-defined OFF state and a significant rectification ratio of 10⁴. This parameter, together with an ideality factor 2.1, outperforms the device based on evaporated Au. Moreover, a device in the intermediate condition is also presented. An abrupt increase in the reverse current is observed and understood based on the enhanced tunneling current. Our work manifests the essential role of doping concentration and provides another example for 2D Schottky interface design.

The Investigation of the Seebeck Effect of the Poly(3,4-Ethylenedioxythiophene)-Tosylate with the Various Concentrations of an Oxidant

Poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) can be synthesized through an in situ polymerization and doping process with iron(III) p-toluenesulfonate hexahydrate as an oxidant. Both the Seebeck coefficient and the electrical conductivity were modified by varying the concentration of the oxidant. We investigated the effects of varying the concentration of the oxidant on the particle sizes and doping (oxidation) levels of PEDOT-Tos for thermoelectric applications. We demonstrated that an increase in the oxidant enabled an expansion of the particle sizes and the doping levels of the PEDOT-Tos. The modification of the doping levels by the concentration of the oxidant can provide another approach for having an optimal power factor for thermoelectric applications. De-doping of PEDOTs by reduction agents has been generally investigated for changing its oxidation levels. In this study, we investigated the effect of the concentration of the oxidant of PEDOT-Tos on the oxidation levels, the electrical conductivities and the Seebeck coefficients. As loading the oxidant of PEDOT-Tos, the Seebeck coefficient was compromised, while the electrical conductivity increased.

The Effect of Alloying Elements on the Structural Stability, Mechanical Properties, and Debye Temperature of Al₃Li: A First-Principles Study

The structural stability, mechanical properties, and Debye temperature of alloying elements X (X = Sc, Ti, Co, Cu, Zn, Zr, Nb, and Mo) doped Al₃Li were systematically investigated by first-principles methods. A negative enthalpy of formation ΔH_f is predicted for all Al₃Li doped species which has consequences for its structural stability. The Sc, Ti, Zr, Nb, and Mo are preferentially occupying the Li sites in Al₃Li while the Co, Cu, and Zn prefer to occupy the Al sites. The Al–Li–X systems are mechanically stable at 0 K as elastic constants C_ij has satisfied the stability criteria. The values of bulk modulus B for Al–Li–X (X = Sc, Ti, Co, Cu, Zr, Nb, and Mo) alloys (excluding Al–Li–Zn) increase with the increase of doping concentration and are larger than that for pure Al₃Li. The Al₆LiSc has the highest shear modulus G and Young’s modulus E which indicates that it has stronger shear deformation resistance and stiffness. The predicted universal anisotropy index A^U for pure and doped Al₃Li is higher than 0, implying the anisotropy of Al–Li–X alloy. The Debye temperature Θ_D of Al₁₂Li₃Ti is highest among the Al–Li–X system which predicts the existence of strong covalent bonds and thermal conductivity compared to that of other systems.

Analysis of Indium Oxidation State on the Electronic Structure and Optical Properties of TiO₂

Due to the high formation energy of Indium interstitial defect in the TiO₂ lattice, the most probable location for Indium dopant is substitutional sites. Replacing Ti by In atom in the anatase TiO₂ shifted the absorption edge of TiO₂ towards visible regime. Indium doping tuned the band structure of TiO₂ via creating In 5p states. The In 5p states are successfully coupled with the O 2p states reducing the band gap. Increasing In doping level in TiO₂ improved the visible light absorption. Compensating the charge imbalance by oxygen vacancy provided compensated Indium doped TiO₂ model. The creation of oxygen vacancy widened the band gap, blue shifted the absorption edge of TiO₂ and declined the UV light absorption. The 2.08% In in TiO₂ is the optimal Indium doping concentration, providing suitable band structure for the photoelectrochemical applications and stable geometrical configuration among the simulated models. Our results provide a reasonable explanation for the improved photoactivity of Indium doped TiO₂.

High efficiency Cu2ZnSn(S,Se)4 solar cells by applying a double In2S3/CdS emitter

High-efficiency Cu2ZnSn(S,Se)4 solar cells are reported by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in a substantial enhancement in open-circuit voltage. The 12.4% device is obtained with a record open-circuit voltage deficit of 593 mV.