Shirui Cao

Chemical Regulation of Carbon Quantum Dots from Synthesis to Photocatalytic Activity

Carbon quantum dots (CQDs) were synthesized by heating various carbon sources in HNO3 solution at reflux, and the effects of HNO3 concentration on the size of the CQDs were investigated. Furthermore, the oxygen-containing surface groups of as-prepared CQDs were selectively reduced by NaBH4 , leading to new surface states. The experimental results show that the sizes of CQDs can be tuned by HNO3 concentration and then influence their photoluminescent behaviors; the photoluminescent properties are related to both the size and surface state of the CQDs, but the photocatalytic activities are determined by surface states alone. The different oxygen-containing groups on the surface of the CQDs can induce different degrees of the band bending upward, which determine the separation and combination of the electron-hole pairs. The high upward band bending, which is induced by C=O and COOH groups, facilitates separation of the electron-hole pairs and then enhances high photocatalytic activity. In contrast, the low upward band bending induced by C-OH groups hardly prevents the electron-hole pairs from surface recombination and then exhibits strong photoluminescence. Therefore, both the photocatalytic activities and optical properties of CQDs can be tuned by their surface states.

Simultaneous synthesis of luminescent carbon nanoparticles and carbon nanocages by laser ablation of carbon black suspension and their optical limiting properties

Both luminescent carbon nanoparticles (CNPs) and carbon nanocages (CNCs) were simultaneously synthesized using laser ablation of carbon black suspension in poly(ethylene glycol). The formation of luminescent CNPs and CNCs depends on the size of carbon black and the laser power density. The optical limiting (OL) measurements of luminescent CNPs, CNCs and their mixture were performed using the open Z-scan technique at the laser wavelength of 532 nm. The mixture of luminescent CNPs and CNCs shows much stronger OL response than carbon black, luminescent CNPs and CNCs in the PEG500N solution. The luminescent CNPs can help to enhance the OL effects of CNC suspensions.

Understanding the effects of the structures on the energy gaps in carbon nanoparticles from laser synthesis

The effects of the structures on the energy gaps in luminescent carbon nanoparticles (CNPs) were investigated. On the one hand, we fabricated CNPs with the different structures and a uniform size and then analyzed their photoluminescence (PL) behaviors. On the other hand, we calculated the dependence of the structures on the energy gaps in CNPs by a simple quantitative model. Both the experimental and calculated results show that the luminescent CNPs contain a mixture of sp2 and sp3 bonding and hence their PL behaviors and energy gaps are determined by the fraction of sp2-hybridized carbon atoms.

One-Step Synthesis of Graphitic Nanoplatelets that are Decorated with Luminescent Carbon Nanoparticles as New Optical-Limiting Materials

Graphitic nanoplatelets (GNPs) and luminescent carbon nanoparticles (CNPs) are simultaneously synthesized by controlling the laser parameters and the size of the graphite flakes. Because luminescent CNPs are attached onto GNPs, a new carbon nanostructure is obtained. Compared with carbon black, GNPs, and luminescent CNPs alone, this nanostructure shows better optical-limiting (OL) effects. The OL mechanism of GNPs that are decorated with luminescent CNPs can mainly be attributed to nonlinear scattering. The role of luminescent CNPs is to promote the formation and growth of nonlinear scattering bubbles, thereby enhancing their optical-limiting effects.

Thermodynamics of face-centered-cubic silicon nucleation at the nanoscale from laser ablation

The thermodynamic nucleation and the phase transition of the face-centered-cubic structure of Si (fcc-Si) on the nanoscale are performed by taking the effect of nanosize-induced additional pressure on the fcc-Si formation under the conditions generated by laser ablation in liquid into account. The thermodynamic analyses showed that the formation of fcc-Si nanocrystals with sizes of 2-6 nm would take place prior to that of large fcc-Si nanocrystals, and the phase transition probability from diamond-like structure Si (d-Si) to fcc-Si is rather high, up to 10(-3)-10(-2), under the conditions created by laser ablation of an Si target in water. These theoretical results suggest that laser ablation in liquid would be an effective industrial route to prepare ultrasmall fcc-Si nanocrystals.

Preparation and optical properties of phthalocyanine–carbon dot blends

To develop better light-capturing nanostructures, we plan to connect an excellent light-harvesting molecule with rich redox chemistry (phthalocyanine, Pc) to a fascinating luminescent material (carbon dots, CDs). Firstly, CDs with diameters of 3.0–5.5 nm were synthesized using a chemical oxidation, and were then blended with Pc phosphoric acid solution under ultrasonication. UV-vis and infrared spectroscopy were used to determine the interactions between CDs and phthalocyanine. The photoluminescent behavior of the blends depends on the proportion of CDs and phthalocyanine used. This can be attributed to the formation of two different structures; sandwich and plum pudding. The Pc–carbon dot blends exhibit broader UV-vis absorption and better visible light emission. Therefore, these findings pave the way for the development of new antenna systems that can capture solar light.

Formation and nonlinear optical properties of carbon nanospindles from laser ablation

Carbon nanospindles are synthesized by laser ablation of carbon black suspension. Their formation could be originated from the coalescence of small carbon nanocrystals because enough time and energy are provided at a long pulse width. At the same level of linear transmittance, carbon nanospindle suspension exhibits better nonlinear optical effects with a 532-nm laser beam than carbon black suspension. The possible mechanism for optical limiting is attributed to the nonlinear scattering from the micro-bubbles.

Theoretical analysis of the formation of face-centered cubic Si nanocrystals by magnetron sputtering

To have a clear insight into the physical origin of the nucleation of face-centered cubic Si in the magnetron sputtering condition, a theoretical model was proposed by taking the capillary effect of the nanosized curvatures of the critical nucleus and the concave surface on the substrate into account. The calculation shows that the size and energy of the critical nucleation depend on the radius of the concave surface on the substrate and the temperature. Both the smaller radius of concave surface and the higher temperature are propitious to the formation of face-centered cubic Si nanocrystals.