Deterministic down-sampling of an unordered point cloud in a deep neural network has not been rigorously studied so far. Existing methods down-sample the points regardless of their importance for the network output and often address down-sampling the raw point cloud before processing. As a result, some important points in the point cloud may be removed, while less valuable points may be passed to next layers. In contrast, the proposed adaptive down-sampling method samples the points by taking into account the importance of each point, which varies according to application, task and training data. In this paper, we propose a novel deterministic, adaptive, permutation-invariant down-sampling layer, called Critical Points Layer (CPL), which learns to reduce the number of points in an unordered point cloud while retaining the important (critical) ones. Unlike most graph-based point cloud down-sampling methods that use k-NN to find the neighboring points, CPL is a global down-sampling method, rendering it computationally very efficient. The proposed layer can be used along with a graph-based point cloud convolution layer to form a convolutional neural network, dubbed CP-Net in this paper. We introduce a CP-Net for 3D object classification that achieves high accuracy for the ModelNet 40 dataset among point cloud-based methods, which validates the effectiveness of the CPL.
Recently, the high pressure study on the TiO2 nanomaterials has attracted considerable attention due to the typical crystal structure and the fascinating properties of TiO2 with nanoscale sizes. In this paper, we briefly review the recent progress in the high pressure phase transitions of TiO2 nanomaterials. We discuss the size effects and morphology effects on the high pressure phase transitions of TiO2 nanomaterials with different particle sizes, morphologies, and microstructures. Several typical pressure-induced structural phase transitions in TiO2 nanomaterials are presented, including size-dependent phase transition selectivity in nanoparticles, morphology-tuned phase transition in nanowires, nanosheets, and nanoporous materials, and pressure-induced amorphization (PIA) and polyamorphism in ultrafine nanoparticles and TiO2-B nanoribbons. Various TiO2 nanostructural materials with high pressure structures are prepared successfully by high pressure treatment of the corresponding crystal nanomaterials, such as amorphous TiO2 nanoribbons, α-PbO2-type TiO2 nanowires, nanosheets, and nanoporous materials. These studies suggest that the high pressure phase transitions of TiO2 nanomaterials depend on the nanosize, morphology, interface energy, and microstructure. The diversity of high pressure behaviors of TiO2 nanomaterials provides a new insight into the properties of nanomaterials, and paves a way for preparing new nanomaterials with novel high pressure structures and properties for various applications.
The traditional concept of the synthesis of semiconductor nanocrystals (NCs) by solvent routes usually performed under high temperatures, causes the semiconductor materials to nucleate and grow into various shaped NCs in solution. Therefore, these methods are named as "solvent-thermal approachs". In this work, we describe a simple and reproducible strategy for the synthesis of PbS NCs at temperatures even as low as −20 °C by using frozen and solidified precursors. With the aid of alkylamines, nano-sized PbS could also nucleate and grow at such low temperatures within a short time (a few seconds). The experimental results not only break people's traditional thinking but also provide a significant and novel direction in the engineering of the synthesis of NCs. In addition, we further systematically investigated the effect of two types of temperatures (the mixing temperature of the precursors and the ripening temperature of the PbS NCs). Combining this with different alkylamines, we found an obvious competition between a growth kinetic process caused by the alkylamines and a thermodynamic process induced by the temperature, which formed variously shaped monodispersed PbS NCs, including flower-, star-, sphere-, truncated octahedron-, cuboctahedron-, quasi cube-, cube-shaped and some hollow PbS NCs. Furthermore, this competition process could also provide a facile and cost-effective route to synthesize size-tunable but shape-permanent PbS NCs and their self-assembly superlattices in the same reaction systems, which is still a major challenge at present. Afterward, both the formation mechanisms of the PbS nanostructures synthesized below room temperature and the shape transformation depending on two types of temperature and alkylamines are systematically discussed.
A facile and reproducible approach for the synthesis of magic-sized CdSe and CdTe nanocrystals is established. The as-synthesized CdSe nanocrystals exhibit strong and fixed absorption features with unusually narrow emission spectra. White-light emission can be achieved by two different routes. One is to mix colours emitted from both the magic-sized nanocrystals and the subsequently transformed regular-sized nanocrystals; the other is to expose the magic-sized nanocrystals to ambient conditions. A systematic study of the nanocrystal formation process shows that monomer activity and injection/growth temperatures are important parameters to the growth kinetics of magic-sized nanocrystals. Variation of these parameters provides tunable existence periods in the hot solution.
The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ∼14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ∼17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P1 to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.
In this work, we present a facile and reproducible one-pot approach to synthesize a series of nanosized metal oxides (CdO, PbO, ZnO, SnO and Ga2O3) using their corresponding bulk materials as precursors. These nanosized metal oxides were synthesized by heating the mixture of the metal oxide, oleic acid and oleylamine directly, and retained the structure of the corresponding bulk material. For lead oxide, both the original orthorhombic and the tetragonal PbO were successfully synthesized by varying the reaction temperature. The optical properties of nanosized CdO, PbO and ZnO show a significant quantum confinement effect compared with their behavior in the bulk phase. Moreover, we proposed the possible mechanism of the reaction in which the dissolution–recrystallization process provided the dominant contribution.
Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) represents a new frontier in the research field of SERS. However, relatively few studies have focused on PI-SERS due to many difficulties, such as easy aggregation of nanoparticles, and difficulty in understanding the interaction mechanisms between probe molecules and the SERS substrate at high pressure. Here we developed an efficient semiconductor-metal SERS substrate (MoS2/Au) to study PI-SERS. Different from the previously reported monotonous decrease in Raman intensities upon compression, an anomalous Raman enhancement of R6G molecules adsorbed on the MoS2/Au substrate was observed up to 2.39 GPa, at which the degree of charge transfer (ρCT) between the R6G molecules and the MoS2/Au substrate reaches a maximum. By comparison, it is proposed that the decoration of Au on the SERS system could bring about a two-step charge transfer (CT) process, introduce localized surface plasmon resonance (LSPR), and thus favor the PI-SERS enhancement. Moreover, this charge transfer also causes obvious changes in the optical behaviors of R6G molecules upon compression. This brings new insights into the SERS study and also offers new ideas for the development of SERS application in high pressure studies.
A nontoxic, simple, cheap, and reproducible strategy, which meets the standard of green chemistry, is introduced for the synthesis of ZnSe nanoparticles and nanoflowers. The production of these green nanomaterials can be readily scaled up and performed directly at ambient condition without affecting their qualities. The experimental results show that the as-synthesized ZnSe nanoparticles and nanoflowers with a zinc blende structure have a narrow size distribution without resorting to any postsynthetic size-selective procedure. A systematic study of the nanocrystal formation process indicates the following properties. (i) The amount of precursors plays a greater role in the determination of the nanoparticle size than other reaction parameters. Variation of this parameter allows us to tune the nanoparticle size in the high-temperature annealing process. This tunability is interpreted well by the growth kinetics. (ii) The limited ligand protection mechanism cannot be employed to explain the formation of our nanoflowers. Instead, a new growth mechanism is proposed. Upon heating at high temperature, a mononuclear Zn complex converts to a polynuclear Zn complex with multiple Zn atoms. Each Zn atom grows into one ZnSe nanoparticle after the injection of Se solution. These nanoparticles closely connect and thus look like nanoflowers.
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