Fundamental Understanding of the Formation Mechanism for Graphene Quantum Dots Fabricated by Pulsed Laser Fragmentation in Liquid: Experimental and Theoretical Insight

The pulsed laser fragmentation in liquid (PLFL) process is a promising technique for the synthesis of carbon‐based functional materials. In particular, there has been considerable attention on graphene quantum dots (GQDs) derived from multiwalled carbon nanotubes (MWCNTs) by the PLFL process, owing to the low cost and rapid processing time involved. However, a fundamental deep understanding of the formation of GQDs from MWCNTs by PLFL has still not been achieved despite the high demand. In this work, a mechanism for the formation of GQDs from MWCNTs by the PLFL process is reported, through the combination of experimental and theoretical studies. Both the experimental and computational results demonstrate that the formation of GQDs strongly depends on the pulse laser energy. Both methods demonstrate that the critical energy point, where a plasma plume is generated on the surface of the MWCNTs, should be precisely maintained to produce GQDs; otherwise, an amorphous carbon structure is favorably formed from the scattered carbons.     

Luminescent quantum dots fluorescence resonance energy transfer-based probes for enzymatic activity and enzyme inhibitors

The paper describes the development and characterization of analytical properties of quantum dot-based probes for enzymatic activity and for screening enzyme inhibitors. The luminescent probes are based on fluorescence resonance energy transfer (FRET) between luminescent quantum dots that serve as donors and rhodamine acceptors that are immobilized to the surface of the quantum dots through peptide linkers. Peptide-coated CdSe/ZnS quantum dots were prepared using a one-step ligand exchange process in which RGDC peptide molecules replace trioctylphosphine oxide (TOPO) molecules as the capping ligands of the quantum dots. The peptide molecules were bound to the surface of the CdSe/ZnS quantum dots through the thiol group of the peptide cysteine residue. The peptide-coated quantum dots were labeled with rhodamine to form the FRET probes. The emission quantum yield of the quantum dot FRET probes was 4-fold lower than the emission quantum yield of TOPO-capped quantum dots. However, the quantum dot FRET probes were sufficiently bright to enable quantitative enzyme and enzyme inhibition assays. The probes were used first to test the enzymatic activity of trypsin in solution based on FRET signal changes of the quantum dot-based enzymatic probes in the presence of proteolytic enzymes. For example, exposure of the quantum dot FRET probes to 500 microg/mL trypsin for 15 min resulted in 60% increase in the photoluminescence of the quantum dots and a corresponding decrease in the emission of the rhodamine molecules. These changes resulted from the release of rhodamine molecules from the surface of the quantum dots due to enzymatic cleavage of the peptide molecules. The quantum dot FRET-based probes were used to monitor the enzymatic activity of trypsin and to screen trypsin inhibitors for their inhibition efficiency.     

Size-selective carbon nanoclusters as precursors to the growth of epitaxial graphene

The nucleation and growth mechanisms of graphene on Rh(111) via temperature-programmed growth of C(2)H(4) are studied by scanning tunneling microscopy and spectroscopy, and by density functional theory calculations. By combining our experimental and first-principles approaches, we show that carbon nanoislands form in the initial stages of graphene growth, possessing an exclusive size of seven honeycomb carbon units (hereafter labeled as 7C(6)). These clusters adopt a domelike hexagonal shape indicating that bonding to the substrate is localized on the peripheral C atoms. Smoluchowski ripening is identified as the dominant mechanism leading to the formation of graphene, with the size-selective carbon islands as precursors. Control experiments and calculations, whereby coronene molecules, the hydrogenated analogues of 7C(6), are deposited on Rh(111), provide an unambiguous structural and chemical identification of the 7C(6) building blocks.     

Rupturing C60 Molecules into Graphene‐Oxide‐like Quantum Dots: Structure,Photoluminescence, and Catalytic Application

The large‐scale synthesis of graphene‐oxide‐like quantum dots (GOLQDs) is reported by oxidizing C₆₀ molecules using a modified Hummers method with a yield of ≈25 wt% readily achieved. The GOLQDs are highly soluble in water and in addition to hexagons have other carbon rings in the structure. They have an average height of ≈1.2 nm and a diameter distribution of 0.6–2.2 nm after drying on substrates. First‐principle calculations indicate that a possible rupturing route may include the insertion of oxygen atoms to C C bonds in the C₆₀ molecule, followed by rupture of that C C bonds. The GOLQD suspension has a strong photoluminescence (PL) with peak position dependent on excitation wavelength. The PL is related to the size and emissive traps caused by oxygen‐containing groups. The GOLQDs also catalyze the oxidation of benzyl alcohol with a high selectivity.     

碳材料在钙钛矿太阳能电池中的应用

钙钛矿太阳能电池具有材料成本低廉、生产工艺简单、光电转换效率高等优点,发展前景十分光明。碳材料因其价格低廉、高导电性、疏水性和化学稳定性等特点,被应用在钙钛矿太阳能电池的各个组成部分,用于提高电池性能和降低成本。本文根据应用在钙钛矿太阳能电池中的碳材料的维数进行分类,分别介绍了零维的C60、碳量子点和石墨烯量子点,一维的碳纳米管,二维的石墨烯及其衍生物、石墨炔和三维的石墨等在钙钛矿太阳能电池中的应用,对于将来实现钙钛矿太阳能电池的低成本商业化和大规模制造具有重要意义。     

Carbon Dots,Unconventional Preparation Strategies,and Applications Beyond Photoluminescence

Carbon dots (C‐dots) are generally separated into graphene quantum dots (GQDs) and carbon nanodots (CNDs) based on their respective top‐down and bottom‐up preparation processes. However, GQDs can be prepared by carbonization of small‐molecule precursors as revealed with unconventional preparation strategies. Thus, it is their structures rather than their precursors and preparation strategy that govern whether C‐dots are GQDs or CNDs. Here, the composites, structure, and electronic properties of C‐dots are discussed. C‐dots generally consist of a graphite‐like core and amorphous oxygen‐containing shell. When graphite becomes C‐dots, its conduction and valence bands are separated, and the quantum confinement effect appears. Combined with the light‐harvesting ability inherited from graphite, electrons in the core of C‐dots are transferred from conduction to valence bands, leading to electron–hole pair formation upon light excitation. The photoexcitation activities, such as photovoltaic conversion, photocatalysis, and photodynamic therapy, are influenced by the electronic properties of the core. Different to the semiconductor properties of core, the C‐dot shell is electrochemically active, leading to electrochemiluminescence (ECL). The oxygen‐containing groups in shell can conjugate to functional species for use in imaging and therapy. The applications of C‐dots beyond photoluminescence, including ECL, solar photovoltaics, photocatalysis, and theranostics, are reviewed.     

Polymorphic Architectures of Graphene Quantum Dots

A systematic strategy for designing structured nanomaterials is demonstrated through self‐assembly of graphene quantum dots. The approach reveals that graphene derivatives at the nanoscale assemble into various architectures of nanocrystals in a binary solution system. The shapes of the nanocrystals continue to evolve in terms of the intimate association of organic molecules with the dispersion medium, obtaining a high index faceted superlattice. This facile synthetic process provides a versatile strategy for designing particles to new structured materials systems, exploiting the crystallization of layered graphitic carbon structures within single crystals.     

Study of nanoscale photopolymerized fullerene clusters in solution droplets using an ultrasonic nebulizer unit

In a designed and developed ultrasonic nebulizer system for obtaining macroscopic-quantity photopolymerized fullerene (C60) clusters, a C60 solution was vaporized to several micro-sized droplets in vacuum, resulting in the formation of C60 aggregates by evaporating the solvent (toluene). The system was invented to produce nanoscale photopolymerized carbon clusters through the irradiation of ultraviolet (UV) light on the C60 aggregates in vacuum. The products, photopolymerized C60 clusters obtained from the system using UV-visible (UV-Vis) absorption and high-performance (or high-pressure) liquid chromatography (HPLC) spectra, were characterized. Compared with the non-irradiating C60 solution, the UV-Vis absorption spectrum of the irradiated C60 solution was drastically decreased, especially at lambda = 335 nm and in the visible region from lambda = 450-650 nm. As such, the UV-Vis absorption spectra provide information about the polymerization of C60 molecules. These photopolymerized C60 clusters can be detected as having a heavy molecular mass order through the HPLC system, and the C60 and photopolymerized C60 cluster can be extracted from the trapped solution on the molecular mass. Although there is a possibility that the products include various forms of C60 clusters, the results suggest that the products obtained from the system using a vaporizer establish a new method of obtaining macroscopic-quantity C60 clusters.     

Crystallization behavior of silicon quantum dots in a silicon nitride matrix

Silicon quantum dot superlattice was fabricated by alternating deposition of silicon rich nitride (SRN) and Si3N4 layers using RF magnetron co-sputtering. Samples were then annealed at temperatures between 800 and 1,100 degrees C and characterized by grazing incident X-ray diffraction (GIXRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). GIXRD and Raman analyses show that the formation of silicon quantum dots occurs with annealing above 1,100 degrees C for at least 60 minutes. As the annealing time increased the crystallization of silicon quantum dots was also increased. TEM images clearly showed SRN/Si3N4 superlattice structure and silicon quantum dots formation in SRN layers after annealing at 1,100 degrees C for more than 60 minutes. The changes in FTIR transmission spectra observed with annealing condition corresponded to the configuration of Si-N bonds. Crystallization of silicon quantum dots in a silicon nitride matrix started stabilizing after 60 minutes' annealing and approached completion after 120 minutes'. The systematic investigation of silicon quantum dots in a silicon nitride matrix and their properties for solar cell application are presented.     

Fabrication and characterization of silicon quantum dots in Si-rich silicon carbide films

Amorphous Si-rich silicon carbide films were prepared by magnetron co-sputtering and subsequently annealed at 900-1100 degrees C. After annealing at 1100 degrees C, this configuration of silicon quantum dots embedded in amorphous silicon carbide formed. X-ray photoelectron spectroscopy was used to study the chemical modulation of the films. The formation and orientation of silicon quantum dots were characterized by glancing angle X-ray diffraction, which shows that the ratio of silicon and carbon significantly influences the species of quantum dots. High-resolution transmission electron microscopy investigations directly demonstrated that the formation of silicon quantum dots is heavily dependent on the annealing temperatures and the ratio of silicon and carbide. Only the temperature of about 1100 degrees C is enough for the formation of high-density and small-size silicon quantum dots due to phase separation and thermal crystallization. Deconvolution of the first order Raman spectra shows the existence of a lower frequency peak in the range 500-505 cm(-1) corresponding to silicon quantum dots with different atom ratio of silicon and carbon.     

Recent Advances in Functionalized Carbon Dots toward the Design of Efficient Materials for Sensing and Catalysis Applications

Since the past decade, enormous research efforts have been devoted to the detection/degradation and quantification of environmental toxic pollutants and biologically important molecules due to their ubiquitous necessity in the fields of environmental protection and human health. These fields of sensor and catalysis are advanced to a new era after emerging of nanomaterials, especially, carbon nanomaterials including graphene, carbon nanotube, carbon dots (C‐dots), etc. Among them, the C‐dots in the carbon family are rapidly boosted in the aspect of synthesis and application due to their superior properties of chemical and photostability, highly fluorescent with tunable, non/low‐toxicity, and biocompatibility. The C‐dot‐based functional materials have shown great potential in sensor and catalysis fields for the detection/degradation of environmental pollutants. The major advantage of C‐dots is that they can be easily prepared from numerous biomass/waste materials which are inexpensive and environment‐friendly and are suitable for a developing trend of sustainable materials. This review is devoted to the recent development (since 2017) in the synthesis of biomass‐ and chemical‐derived C‐dots as well as diverse functionalization of C‐dots. Their capability as a sensor and catalyst and respective mechanism are summarized. The future perspectives of C‐dots are also discussed.     

Glowing Graphene Quantum Dots and Carbon Dots: Properties,Syntheses, and Biological Applications

The emerging graphene quantum dots (GQDs) and carbon dots (C‐dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications.     

荧光碳点的制备和性质及其应用研究进展

荧光碳点是继碳纳米管、纳米金刚石和石墨烯之后,最受关注的碳纳米材料之一。与传统半导体量子点相比,碳点具有优异的荧光性能、小尺寸特性、良好的生物相容性、低毒性以及表面易于化学修饰等特点,在环境检测、生物成像、药物载体、光催化及电催化技术等领域具有很好的潜在应用价值。总结了碳点合成方法、结构与性能及应用面进展,剖析了目前制约碳点应用发展的瓶颈问题,并展望了其未来的研究发展重点方向。      

Patterning nanoroads and quantum dots on fluorinated graphene

Using ab initio methods we have investigated the fluorination of graphene and find that different stoichiometric phases can be formed without a nucleation barrier, with the complete “2D-Teflon” CF phase being thermodynamically most stable. The fluorinated graphene is an insulator and turns out to be a perfect matrix-host for patterning nanoroads and quantum dots of pristine graphene. The electronic and magnetic properties of the nanoroads can be tuned by varying the edge orientation and width. The energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) of quantum dots are size-dependent and show a confinement typical of Dirac fermions. Furthermore, we study the effect of different basic coverage of F on graphene (with stoichiometries CF and C₄F) on the band gaps, and show the suitability of these materials to host quantum dots of graphene with unique electronic properties.     

Low‐Temperature Conversion of Alcohols into Bulky Nanoporous Graphene and Pure Hydrogen with Robust Selectivity on CaO

The direct conversion of biorenewable alcohols into value‐added graphene and pure hydrogen (H₂) at benign conditions is an important challenge, especially, considering the open carbon‐reduced cycle. In this study, it is demonstrated that inexpensive calcium oxide (CaO, from eggshells) can transform alcohols into bulky nanoporous graphene and pure hydrogen (≈99%) with robust selectivity at the temperature as low as 500 °C. Consequently, the growth of graphene can follow the direction of alcohol flow and uniformly penetrate into bulky nanoporous CaO platelets longer than 1 m without clogging. The experimental results and density functional theory calculations demonstrate that alcohol molecules can be catalytically carbonized on the surface of CaO at low temperature. The concept of the comprehensive utilization of biomass‐derived alcohols offers a carbon‐negative cycle for mitigating global warming and the energy demand.     

Performance Evaluation of Composite Electrolyte with GQD for All-Solid-State Lithium Batteries

The use a stabilized lithium structure as cathode material for batteries could be a fundamental alternative in the development of next-generation energy storage devices. However, the lithium structure severely limits battery life causes safety concerns due to the growth of lithium (Li) dendrites during rapid charge/discharge cycles. Solid electrolytes, which are used in high-density energy storage devices and avoid the instability of liquid electrolytes, can be a promising alternative for next-generation batteries. Nevertheless, poor lithium ion conductivity and structural defects at room temperature have been pointed out as limitations. In this study, through the application of a low-dimensional graphene quantum dot (GQD) layer structure, stable operation characteristics were demonstrated based on Li⁺ ion conductivity and excellent electrochemical performance. Moreover, the device based on the modified graphene quantum dots (GQDs) in solid state exhibited retention properties of 95.3% for 100 cycles at 0.5 C and room temperature (RT). Transmission electron microscopy analysis was performed to elucidate the Li⁺ ion action mechanism in the modified GQD/electrolyte heterostructure. The low-dimensional structure of the GQD-based solid electrolyte has provided an important strategy for stably-scalable solid-state lithium battery applications at room temperature. It was demonstrated that lithiated graphene quantum dots (Li-GQDs) inhibit the growth of Li dendrites by regulating the modified Li⁺ ion flux during charge/discharge cycling at current densities of 2.2–5.5 mA cm, acting as a modified Li diffusion heterointerface. A full Li GQD-based device was fabricated to demonstrate the practicality of the modified Li structure using the Li–GQD hetero-interface. This study indicates that the low-dimensional carbon structure in Li–GQDs can be an effective approach for stabilization of solid-state Li matrix architecture.     

Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells.     

Electronic structure and self-assembly of cross-linked semiconductor nanocrystal arrays

We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.     

Spinodal decomposition of mono- to few-layer graphene on Ni substrates at low temperature

Mono to few-layer graphene were prepared on pre-annealed polycrystalline nickel substrates by chemical vapor deposition at a relatively low temperature of 800 degrees C using fast cooling rate. It was observed that the reduced solubility of Carbon in Ni at low temperature and an optimum gas mixing ratio (CH4:H2 = 60/80 (sccm)) can be used to synthesize mano-layer graphene that covers about 100 microm2 area. The number of graphene layers strongly depends upon the hydrogen and methane flow rates. An increase in the methane flow is found to increase the growth density of the single-layer graphene. The number of graphene layers was identified from micro-Raman spectra. The thinnest areas containing mono-layer graphene formed at small Ni grains surrounded by large Ni Grains can be explained in terms of Spinodal decomposition. Scanning tunneling microscopy observations of the graphene samples indicate that the graphene structure exhibits no defects, and extremely symmetry hexagon carbon at flat graphene surface is observed.     

Surface charge induced tuning of electrical properties of CVD assisted graphene and functionalized graphene sheets

We demonstrate a new approach to tune the electrical properties of graphene and functionalized graphene. Graphene was synthesized using thermal chemical vapour deposition(TCVD) method on copper foil using precursor gas acetylene and co-catalyst H2 gas. TCVD assisted graphene was successfully transferred onto a silicon wafer. Transferred graphene sheet was then functionalized to prepare graphene oxide(GO) and reduced graphene oxide(rGO). Different surface charge carbon nanoparticles, e.g. carbon nanoparticle with net positive charge and carbon nanoparticle with net negative charge were then immobilized on transferred graphene and functionalized graphene sheets. The functionalized graphene and charge mobilized functionalized graphene were characterized by Uv–vis spectroscopy,Fourier transformed infrared spectroscopy, scanning electron microscopy, and Raman spectroscopy. After immobilization of carbon nanomaterials, the ac electrical conductivity was found to increase due to enhancement of the surface charge, electron density, and mobility. It was observed that negative surface charge immobilized graphene and functionalized graphene show higher conductivity. Thus, the electrical property of graphene and functionalized graphene can be tuned by surface modification with different surface charge carbon nanomaterials.