T. H. Li

Green light stimulates terahertz emission from mesocrystal microspheres

The discovery of efficient sources of terahertz radiation has been exploited in imaging applications, and developing a nanoscale terahertz source could lead to additional applications. High-frequency mechanical vibrations of charged nanostructures can lead to radiative emission, and vibrations at frequencies of hundreds of kilohertz have been observed from a ZnO nanobelt under the influence of an alternating electric field. Here, we observe mechanical resonance and radiative emission at ∼ 0.36 THz from core-shell ZnO mesocrystal microspheres excited by a continuous green-wavelength laser. We find that ∼ 0.016% of the incident power is converted into terahertz radiation, which corresponds to a quantum efficiency of ∼ 33%, making the ZnO microspheres competitive with existing terahertz-emitting materials. The mechanical resonance and radiation stem from the coherent photo-induced vibration of the hexagonal ZnO nanoplates that make up the microsphere shells. The ZnO microspheres are formed by means of a nonclassical, self-organized crystallization process, and represent a straightforward route to terahertz radiation at the nanoscale.

Glycerol-Bonded 3C-SiC Nanocrystal Solid Films Exhibiting Broad and Stable Violet to Blue-Green Emission

We have produced glycerol-bonded 3C-SiC nanocrystal (NC) films, which when excited by photons of different wavelengths, produce strong and tunable violet to blue-green (360-540 nm) emission as a result of the quantum confinement effects rendered by the 3C-SiC NCs. The emission is so intense that the emission spots are visible to the naked eyes. The light emission is very stable and even after storing in air for more than six months, no intensity degradation can be observed. X-ray photoelectron spectroscopy and absorption fine structure measurements indicate that the Si-terminated NC surfaces are completely bonded to glycerol molecules. Calculations of geometry optimization and electron structures based on the density functional theory for 3C-SiC NCs with attached glycerol molecules show that these molecules are bonded on the NCs causing strong surface structural change, while the isolated levels in the conduction band of the bare 3C-SiC NCs are replaced with quasi-continuous bands that provide continuous tunability of the emitted light by changing the frequencies of exciting laser. As an application, we demonstrate the potential of using 3C-SiC NCs to fabricate full-color emitting solid films by incorporating porous silicon.

Optical identification of oxygen vacancy types in SnO2 nanocrystals

The oxygen vacancies in spherical and cuboid SnO2 nanocrystals prepared by hydrothermal and laser ablation methods are investigated optically. Three oxygen-vacancy-related photoluminescence peaks at ∼430, ∼501, and ∼618u2009nm are observed, and Raman scattering and density functional calculation disclose that they originate from in-plane, sub-bridging, and bridging oxygen vacancies, respectively. This work reveals that the photoluminescence peaks together with the Raman modes can be used to identify the oxygen vacancy types in SnO2 nanostructures.

Enhanced Photocatalytic Oxygen Evolution by Crystal Cutting

Uniformly cut In2O3 truncated octahedrons are fabricated on a large scale by a simple chemical vapor deposition (CVD) technique. This theoretical analysis predicts that the emergence of {100} facets on the In2O3 truncated octahedrons enhances oxygen evolution significantly in photocatalysis and experimental photoelectrochemical measurements are consistent.

Oxygen-vacancy and depth-dependent violet double-peak photoluminescence from ultrathin cuboid SnO2 nanocrystals

A double peak in the violet region between 360 and 400u2009nm is observed from the photoluminescence spectra acquired from cuboid SnO2 nanocrystals and the energy separation between the two subpeaks increases with nanocrystal size. The phenomenon arises from band edge recombination caused by different in-depth distributions of oxygen vacancies (OVs). Density functional theory calculations disclose that variations in the oxygen vacancies with depth introduce valence-band peak splitting leading to the observed splitting and shift of the double peak.

Raman investigation of oxidation mechanism of silicon nanowires

Raman spectra are acquired from Si nanowires (NWs) with diameters of 2–15 nm oxidized for different time durations. The Si TO optical phonon peak downshifts asymmetrically finally becoming an amorphous Si peak after a long oxidation time. The spectral changes cannot be correlated using the phonon confinement model of cylindrical NWs. Microstructural observations disclose that the strain induced by oxidization breaks the NWs into small nanocrystals. By considering the morphological transformation, we adopt the phonon confinement models on wires and dots to explain very well the Raman spectra acquired from Si NWs with different diameters.

Investigation of activated oxygen molecules on the surface of Y2O3 nanocrystals by Raman scattering

Activation of surface oxygen molecules on cubic Y2O3 nanocrystals (NCs) is investigated. As the annealing temperature under O2 is increased, the strong Raman band at 965u2009cm−1 previously never assigned weakens gradually, while the intensity of the 378u2009cm−1 Raman band arising from Y3+-O2− vibration increases. X-ray diffraction reveals no structural change during annealing and energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and theoretical calculation suggest that the interstitial oxygen O22− connected to the F centers gives rise to the 965u2009cm−1 Raman band. The results provide direct evidence of the existence of activated oxygen ions on Y2O3 NCs.

Oxygen vacancy density-dependent transformation from infrared to Raman active vibration mode in SnO 2 nanostructures

Raman spectra acquired from spherical, cubic, and cuboid SnO2 nanocrystals (NCs) reveal a morphologically independent Raman mode at ∼302u2009cm(-1). The frequency of this mode is slightly affected by the NC size, but the intensity increases obviously with decreasing NC size. By considering the dipole changes induced by oxygen vacancies and derivation based on the density functional theory and phonon confinement model, an oxygen vacancy density larger than 6% is shown to be responsible for the transformation of the IR to Raman active vibration mode, and the intensity enhancement is due to strong phonon confinement.

Size-independent low-frequency Raman scattering in Ge-nanocrystal-embedded SiO2 films.

The peak position and linewidth of the low-frequency Raman mode observed from amorphous silica films embedded with Ge nanocrystals doped with Si show a size-independent behavior. Spectral analysis reveals the formation of a thin amorphous GeSi layer on the surface of the Ge nanocrystal. Theoretical calculation based on a modified three-region model discloses that the acoustic impedance of the interfacial GeSiO layer is responsible for the size-independent behavior. During high-temperature annealing, Ge atoms are segregated from the interface into the core, and the GeSiO interface layer is converted to SiO(2), leading to disappearance of the size-independent vibration mode.

Influence of GeSi interfacial layer on Ge–Ge optical phonon mode in SiO2 films embedded with Ge nanocrystals

The Ge–Ge optical phonon peak at 300u2002cm−1 acquired from amorphous SiO2 films embedded with Ge nanocrystals by Raman scattering is sensitive to the Si content. When the Si concentration is high, a thin GeSi interfacial layer forms around the Ge nanocrystals. A tensile stress is produced to partially offset the compressive stress imposed by the SiO2 matrix on the Ge nanocrystals, consequently downshifting the frequency of the optical phonon and increasing its linewidth. Theoretical calculation based on phonon confinement and compressive effects discloses that the interfacial layer plays a crucial role in the optical phonon behavior.