L. Z. Liu

Quantum confinement effects across two-dimensional planes in MoS2 quantum dots

The low quantum yield (∼10−5) has restricted practical use of photoluminescence (PL) from MoS2 composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS2 QDs is also found to be sensitive to aggregation due to the size effect.

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 ∼618 nm 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.

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

A double peak in the violet region between 360 and 400 nm 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.

Growth of tin oxide nanorods induced by nanocube-oriented coalescence mechanism

SnO2 nanocrystals (NCs) with spherical, cubic, and cuboid nanorod morphologies are obtained at different stages in hydrothermal synthesis using a SnCl4⋅5H2O to CO(NH2)2 ratio of 1 to 10. Microstructural examination and theoretical derivation reveal that small spherical NCs are formed initially and some of them morph into cylindrical NCs because of the low surface free energy. These NCs transform into bigger cubic NCs with time finally evolving into cuboid nanorods due to Brownian motion. The cuboid nanorods have a lower surface free energy than the cubic NCs and constitute a stable nanostructure.

Resonant Raman scattering from CdS nanocrystals enhanced by interstitial Mn

Different Raman scattering effects are observed from CdS and Mn-doped CdS nanocrystals (NCs) with an average size of 5.1 nm synthesized by the reverse-micelle method. The intensity of the longitudinal optical (LO) phonon spectrum acquired from the Mn-doped CdS NCs is more than 20 times larger than that from the undoped CdS NCs. Spectroscopic and theoretical analyses reveal that the enhancement is caused by the interstitial Mn dopants, which decrease the NC surface deformation potential due to the small dielectric constant of the metal resulting in enhanced coupling between the LO phonon and surface plasmon.

Twinning Ge0.54Si0.46 nanocrystal growth mechanism in amorphous SiO2 films

Ge0.54Si0.46 alloy nanocrystals (NCs) with different twinning structures are synthesized by magnetron sputtering followed by high temperature (>1100 °C) annealing and rapid cooling. The local strain induced by rapid cooling enables neighboring NCs to coalesce quickly. Because of insufficient time to form individual structures, a leading twinning interface forms inevitably in the interior of the NCs. The twinning NCs with large surface free energies reconstruct for energy optimization at high temperature. Consequently, the twinning layer thickness shrinks slowly, finally transforming into untwined stable NCs with the lowest surface free energy. Our experimental observations are corroborated by theoretical calculation.

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.

Electronic states and photoluminescence of TiO2 nanotubes with adsorbed surface oxygen

The electronic states associated with enhanced photocatalytic activity of anodic anatase TiO2 nanotubes (NTs) annealed in N2 and O2 are investigated by photoluminescence (PL). The NTs annealed in N2 show a green peak related to oxygen vacancies and its position blueshifts with deceasing temperature, whereas those annealed in O2 show a double peak at 475–600 nm and the energy separation increases with decreasing temperature. Spectral analysis and density function theory calculation disclose that the double peak results from residual oxygen vacancies and oxygen atoms on the NT wall and the increased energy separation arises from the larger difference between the inner and outer NT stress at low temperature.

Monolayer borophene electrode for effective elimination of both the Schottky barrier and strong electric field effect

The formation of Schottky barriers between 2D semiconductors and traditional metallic electrodes has greatly limited the application of 2D semiconductors in nanoelectronic and optoelectronic devices. In this study, metallic borophene was used as a substitute for the traditional noble metal electrode to contact with the 2D semiconductor. Theoretical calculations demonstrated that no Schottky barrier exists in the borophene/2D semiconductor heterostructure. The contact remains ohmic even with a strong electric field applied. This finding provides a way to construct 2D electronic devices and sensors with greatly enhanced performance.

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 ∼302 cm(-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.