Tingting Yin

Accurate Modeling of Dark-Field Scattering Spectra of Plasmonic Nanostructures

Dark-field microscopy is a widely used tool for measuring the optical resonance of plasmonic nanostructures. However, current numerical methods for simulating the dark-field scattering spectra were carried out with plane wave illumination either at normal incidence or at an oblique angle from one direction. In actual experiments, light is focused onto the sample through an annular ring within a range of glancing angles. In this paper, we present a theoretical model capable of accurately simulating the dark-field light source with an annular ring. Simulations correctly reproduce a counterintuitive blue shift in the scattering spectra from gold nanodisks with a diameter beyond 140 nm. We believe that our proposed simulation method can be potentially applied as a general tool capable of simulating the dark-field scattering spectra of plasmonic nanostructures as well as other dielectric nanostructures with sizes beyond the quasi-static limit.

High‐Pressure‐Induced Comminution and Recrystallization of CH3NH3PbBr3 Nanocrystals as Large Thin Nanoplates

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

Enhanced light-matter interaction in atomically thin MoS2 coupled with 1D photonic crystal nanocavity

Engineering the surrounding electromagnetic environment of light emitters by photonic engineering, e.g. photonic crystal cavity, can dramatically enhance its spontaneous emission rate through the Purcell effect. Here we report an enhanced spontaneous emission rate of monolayer molybdenum disulfide (MoS2) by coupling it to a 1D silicon nitride photonic crystal. A four times stronger photoluminescence (PL) intensity of MoS2 in a 1D photonic crystal cavity than un-coupled emission is observed. Considering the relative ease of fabrication and the natural integration with a silicon-based system, the high Purcell factor renders this device as a highly promising platform for applications such as visible solid-state cavity quantum electrodynamics (QED).

Growth and characterization of highly tensile strained Ge1−xSnx formed on relaxed InyGa1−yP buffer layers

Ge0.94Sn0.06 films with high tensile strain were grown on strain-relaxed InyGa1−yP virtual substrates using solid-source molecular beam epitaxy. The in-plane tensile strain in the Ge0.94Sn0.06 film was varied by changing the In mole fraction in InxGa1−xP buffer layer. The tensile strained Ge0.94Sn0.06 films were investigated by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. An in-plane tensile strain of up to 1% in the Ge0.94Sn0.06 was measured, which is much higher than that achieved using other buffer systems. Controlled thermal anneal experiment demonstrated that the strain was not relaxed for temperatures up to 500 °C. The band alignment of the tensile strained Ge0.94Sn0.06 on In0.77Ga0.23P was obtained by high resolution x-ray photoelectron spectroscopy. The Ge0.94Sn0.06/In0.77Ga0.23P interface was found to be of the type I band alignment, with a valence band offset of 0.31 ± 0.12 eV and a conduction band offset of 0.74 ± 0.12 eV.

Towards understanding and engineering the properties of hybrid perovskites (Conference Presentation)

Hybrid lead halide perovskites APbX3 for 3D structures, and A2PbX4 for 2D structures, comprise fully corner-sharing (for 3D) and corner-sharing sheets of Pb-X octahedra for 2D structures. These organic semiconductors are great interests for applications in solar cells and LEDs, due to their high carrier mobility, tunable spectral absorption range and easy processing. In this presentation, the novel optical properties of 3D bulk CH3NH3PbBr3 under high pressure will be discussed. At ~2.3 GPa, photoluminescence intensity is enhanced by ~400 times, and broad emission appear at 4.2 GPa. All structural phases and physical properties are reversible after release. For the CH3NH3PbBr3 nanocrystals (NCs), pressure-induced sintering of 10 nm into nanoplate of 100 nm with different optical and electrical properties is reported. For 2D layered perovskite, the structure-property relationship is resolved and established via a comprehensive pressure study, where the decrease of bond angle and Pb-I bond length exhibit an opposite influence on the band gap, i.e., smaller bond angle results a widened band gap, while smaller bond length results a narrowed band gap. In addition, the evolution of hydrogen-bonding and CH3NH3+ (MA) cation orientations in CH3NH3PbBr3 perovskite are investigated at temperature. The H atoms in NH3+ groups form H-bonds in all three polymorphs below room temperature, but with different Br atoms in different phases. However, the H atoms in CH3 groups form H-bonds with Br atoms only in the low-temperature orthorhombic phase. Extensive characterization techniques, including time-resolved spectroscopy, Raman micro-spectroscopy and imaging, photoluminescence and absorption spectroscopy, in-situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) as well as ab initio calculations, are introduced to study the pressure-/temperature-induced structural evolution and physical properties change from hybrid perovskites.

Stability of CH 3 NH 3 PbBr 3 and evolution of H-bonding during its polymorphic transformations

The structural stability of the organometal halide perovskites (CH3NH3PbX3, where X = Br, I, Cl) is highly correlated with the rotation of the methylammonium (MA) cations and the tilting of the PbX6 octahedra, leading to a series of structural phase transitions upon cooling. However, the hydrogen-bonding status and the relationship with the electronic properties remain unclear and need to be addressed. In this study, the various status of the hydrogen-bonding between the H atoms of the MA cation and the halide ions is resolved by combining ab initio calculations with temperature-dependent Raman scattering and powder X-ray diffraction (XRD) measurements on MAPbBr3 hybrid perovskites. The consequent dynamical change in the electronic band structures, triggers indirect bandgap to direct bandgap along with ∼60-fold photoluminescence emission enhancement upon cooling.

Photoluminescence studies on the Dolmen-like plasmonic nanoantennas(Conference Presentation)

Localized surface plasmon resonance (LSPR) on metallic nanostructures is able to enhance photoluminescence (PL) emission significantly. However, the mechanism for anomalous blue-shifted peak of PL emission from metallic nanostructures, relative to the corresponding scattering spectra, is still unclear so far. In this paper, we presented the detailed investigations on both the Lorentz-like PL profile with blueshifted peak and Fano-like one with almost unshifted dip, as observed on dolmen-like metallic nanostructures. Such anomalous PL emission profile is the product of the density of plasmon states (DoPS) with Lorentz-/Fano-like profile and the population distribution of the relaxed collective free electrons during relaxation. To be more specific, the fast relaxation process of these collective free electrons contributes to the PL shifting characteristics of both Lorentz-like and Fano-like emission profiles. We believed that our results provide a general solid foundation and guidance for analyzing and manipulating the physical processes of the PL emission from various plasmonic nanostructures.