Size-controlled soft-template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue-shifted photoluminescence (PL) or so-called an inverse PL shift. Using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND-based optoelectronic devices, including multicolored light-emitting diodes and air-stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.

Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthetic routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Highly luminescent graphitic carbon quantum dots (GQDs) are synthesized employing reverse micelles as nanoreactors. This method offers size tunability and narrow size distribution without any unpractical size separation process. Also, high quantum yields of maximum 35% at the 360 nm excitation wavelength are achieved.

The atomic layer deposition (ALD) of silicon dioxide (SiO 2 ) was initially explored using a variety of silicon precursors with H 2 O as the oxidant. The silicon precursors were (N,N-dimethylamino)trimethylsilane) (CH 3 ) 3 SiN(CH 3 ) 2 , vinyltrimethoxysilane CH 2 dCHSi(OCH 3 ) 3 , trivinylmethoxysilane (CH 2 dCH) 3 SiOCH 3 , tetrakis(dimethylamino)silane Si(N(CH 3 ) 2 ) 4 , and tris(dimethylamino)silane (TDMAS) SiH(N(CH 3 ) 2 ) 3 . TDMAS was determined to be the most effective of these precursors. However, additional studies determined that SiH* surface species from TDMAS were difficult to remove using only H 2 O. Subsequent studies utilized TDMAS and H 2 O 2 as the oxidant and explored SiO 2 ALD in the temperature range of 150-550 °C. The exposures required for the TDMAS and H 2 O 2 surface reactions to reach completion were monitored using in situ FTIR spectroscopy. The FTIR vibrational spectra following the TDMAS exposures showed a loss of absorbance for O-H stretching vibrations and a gain of absorbance for C-H x and Si-H stretching vibrations. The FTIR vibrational spectra following the H 2 O 2 exposures displayed a loss of absorbance for C-H x and Si-H stretching vibrations and an increase of absorbance for the O-H stretching vibrations. The SiH* surface species were completely removed only at temperatures >450 °C. The bulk vibrational modes of SiO 2 were observed between 1000-1250 cm -1 and grew progressively with number of TDMAS and H 2 O 2 reaction cycles. Transmission electron microscopy (TEM) was performed after 50 TDMAS and H 2 O 2 reaction cycles on ZrO 2 nanoparticles at temperatures between 150-550 °C. The film thickness determined by TEM at each temperature was used to obtain the SiO 2 ALD growth rate. The growth per cycle varied from 0.8 Å/cycle at 150 °C to 1.8 Å/cycle at 550 °C and was correlated with the removal of the SiH* surface species. SiO 2 ALD using TDMAS and H 2 O 2 should be valuable for SiO 2 ALD at temperatures >450 °C.

Sub-micrometer-sized colloidal graphite (CG) was tested as a conducting electrode to replace transparent conducting oxide (TCO) electrodes and as a catalytic material to replace platinum (Pt) for I(3)(-) reduction in dye-sensitized solar cell (DSSC). CG paste was used to make a film via the doctor-blade process. The 9 μm thick CG film showed a lower resistivity (7 Ω/◻) than the widely used fluorine-doped tin oxide TCO (8-15 Ω/◻). The catalytic activity of this graphite film was measured and compared with the corresponding properties of Pt. Cyclic voltammetry and electrochemical impedance spectroscopy studies clearly showed a decrease in the charge transfer resistance with the increase in the thickness of the graphite layer from 3 to 9 μm. Under 1 sun illumination (100 mW cm(-2), AM 1.5), DSSCs with submicrometer-sized graphite as a catalyst on fluorine-doped tin oxide TCO showed an energy conversion efficiency greater than 6.0%, comparable to the conversion efficiency of Pt. DSSCs with a graphite counter electrode (CE) on TCO-free bare glass showed an energy conversion efficiency greater than 5.0%, which demonstrated that the graphite layer could be used both as a conducting layer and as a catalytic layer.

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In this work, nitrogen-rich carbon nanodots (CNDs) are prepared by the emulsion-templated carbonization of polyacrylamide. The formation mechanism and chemical structure are investigated by infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopies. Transmission electron microscopy also reveals that the obtained CNDs have well-developed graphitic structure and narrow size distribution without any size selection procedure. We vary the molecular weight of the polymer to control the size of the CNDs and finally obtain the CNDs rendering bright visible light under UV illumination with a high quantum yield of 40%. Given that the CNDs are worth utilizing in phosphor applications, we fabricate large-scale (20 × 20 cm) freestanding luminescent films of the CNDs based on a poly(methyl methacrylate) matrix. The polymer matrix can not only provide mechanical support but also disperse the CNDs to prevent solid-state quenching. For practical application, we demonstrate white LEDs consisting of the films as color-converting phosphors and InGaN blue LEDs as illuminators. Such white LEDs exhibit no temporal degradation in the emission spectrum under practical operation conditions. This study would suggest a promising way to exploit the luminescence from solid-state CNDs and offer strong potential for future CND-based solid-state lighting systems.