The water-soluble fluorescent carbon quantum dots (CQDs) are synthesized by utilizing lemon juice as carbon resource via a simple hydrothermal reaction. The obtained CQDs are with an average size of 3.1 nm. They reveal uniform morphology and well-crystalline and can generate bright blue-green light emission under UV or blue light irradiation. We find that the fluorescence from these CQDs is mainly induced by the presence of oxygen-containing groups on the surface and edge of the CQDs. Moreover, we demonstrate that the as-prepared CQDs can be applied to imaging plant cells. This study is related to the fabrication, investigation, and application of newly developed carbon nanostructures.

We present a detailed investigation on the effect of functional group modulation at the edges of carbon quantum dots (CQDs) on the fluorescence from the CQDs. The CQDs attached by N, S, and P elements are synthesized via pyrolysis of a mixture of citric acid and NH3H2O, H2SO4, and H3PO4, respectively. Thus, part of –COOH at the edges of CQDs can be converted into –C=O and functional groups such as –NH2, –SO2, –HSO3, and –H2PO4 can connect to the carbon bonds. We find that the formation of the N/S/P-CQDs can reduce the amount of –COOH that attaches to the edges of sp2-conjugated π-domains located at centers of these CQDs. This effect can result in the reduction of the non-radiative recombination for electronic transition in these CQDs. As a result, the quantum yield (QY) for fluorescence from the CQDs can be efficiently enhanced. We demonstrate experimentally that the QYs for N/S/P-CQDs can reach up to 18.7%, 29.7%, and 10.3%, respectively, in comparison to 9% for these without functional group modulation. This work can provide a practical experimental approach in improving the optical properties of fluorescent CQDs.

We present an investigation on carbon quantum dots (CQDs) synthesized from wastewater induced during the production of tofu. We find that tofu wastewater is a good source of raw material in making fluorescent CQDs. The corresponding CQDs can be fabricated simply via hydrothermal reaction to carbonize the organic matter in the yellow serofluid of tofu wastewater. Two sorts of CQDs can be obtained within the deionized water and NaOH solution, respectively, where the CQDs in water (NaOH solution) can emit blue (green) light under the UV irradiation. It is found from X-ray photoelectron spectroscopy (XPS) that the basic difference between these two sorts of CQDs is the contents of C–O and C=O bonds on the surface of the CQDs. This difference can cause different features of the photoluminescence (PL) spectra of the CQDs. On the basis of the obtained results from the XPS and PL measurements, we propose a mechanism in understanding and explaining the photon-induced light emission from CQDs. This study is relevant to the fabrication and application of fluorescent CQDs as, e.g., light display materials.

In this study, one-dimensional porous silicon nanowire (1D–PSiNW) arrays were fabricated by one-step metal-assisted chemical etching (MACE) to etch phosphorus-doped silicon wafers. The as-prepared mesoporous 1D–PSiNW arrays here had especially high specific surface areas of 323.47 m²·g⁻¹ and were applied as anodes to achieve fast charge–discharge performance for lithium ion batteries (LIBs). The 1D–PSiNWs anodes with feature size of ~7 nm exhibited reversible specific capacity of 2061.1 mAh·g⁻¹ after 1000 cycles at a high current density of 1.5 A·g⁻¹. Moreover, under the ultrafast charge–discharge current rate of 16.0 A·g⁻¹, the 1D–PSiNWs anodes still maintained 586.7 mAh·g⁻¹ capacity even after 5000 cycles. This nanoporous 1D–PSiNW with high surface area is a potential anode candidate for the ultrafast charge–discharge in LIBs with high specific capacity and superior cycling performance.

We present a theoretical study of the electrical generation of acoustic-phonon emission from graphene at room temperature. The drift velocity (v_x) and temperature of electrons driven by dc electric field (F_x) are determined by solving self-consistently the momentum- and energy-balance equations derived from the Boltzmann equation. We find that in the presence of impurity, acoustic- and optic-phonon scattering, v_x can be much larger than the longitudinal (v_l) and transverse (v_t) sound velocities in graphene even within the linear response regime. As a result, although the acoustic Cerenkov effect cannot be obviously seen in the analytical formulas, the enhanced acoustic-phonon emission can be observed with increasing F_x when $v_x>v_l$ and $v>v_t$ . The frequency of acoustic-phonon emission from graphene can be above 10 THz, which is much higher than that generated from conventional semiconductor systems. This study is pertinent to the application of graphene as hypersonic devices such as terahertz sound sources.

Monolayer (ML) hexagonal boron nitride (hBN) is an important material in making, e.g., deep ultraviolet optoelectronic and power devices and van der Waals heterojunctions in combination with other two-dimensional (2D) electronic systems such as graphene and ML MoS ${}_2$ . In this work, we present a comparative study of the basic optoelectronic properties of low resistance ML hBN placed on different substrates such as SiO ${}_2$ /Si, quartz, PET, and sapphire. The measurement is carried out by using terahertz (THz) time-domain spectroscopy (TDS) in a temperature regime from 80 to 280 K. We find that the real and imaginary parts of the optical conductivity obtained experimentally for low resistance ML hBN on different substrates can fit well to the Drude–Smith formula. Thus, we are able to determine optically the key sample and material parameters (e.g., the electronic relaxation time or mobility, the carrier density, the electronic localization factor, etc.) of ML hBN. The effect of temperature on these parameters is also examined and analyzed. The results obtained from this study enable us to suggest the appropriate substrate for ML hBN based electronic and optoelectronic devices. This work is relevant to the application to a newly developed 2D electronic system as advanced electronic and optoelectronic materials.