Shi-Woo Rhee

This review covers recent progress in atomic doping and surface functionalization of carbon nanodots to improve their functionality for a variety of applications ranging from biomedicine to optoelectronics to energy.

Chemically derived graphene quantum dots (GQDs) to date have showed very broad emission linewidth due to many kinds of chemical bondings with different energy levels, which significantly degrades the color purity and color tunability. Here, we show that use of aniline derivatives to chemically functionalize GQDs generates new extrinsic energy levels that lead to photoluminescence of very narrow linewidths. We use transient absorption and time-resolved photoluminescence spectroscopies to study the electronic structures and related electronic transitions of our GQDs, which reveals that their underlying carrier dynamics is strongly related to the chemical properties of aniline derivatives. Using these functionalized GQDs as lumophores, we fabricate light-emitting didoes (LEDs) that exhibit green, orange, and red electroluminescence that has high color purity. The maximum current efficiency of 3.47 cd A⁻¹ and external quantum efficiency of 1.28% are recorded with our LEDs; these are the highest values ever reported for LEDs based on carbon-nanoparticle phosphors. This functionalization of GQDs with aniline derivatives represents a new method to fabricate LEDs that produce natural color.

In this work, nitrogen and sulfur‐doped carbon nanodots (CNDs) have been synthesized from ethylenediamine‐N,N′‐diacetic acid and 2,2′‐(ethylenedithio)diacetic acid, respectively. The method used in this work features the use of “single” molecular precursors that contain both carbon and dopant atoms, which allows examining the effects of doping in a molecular level. The effects of doping on the electronic structure of CNDs could be examined by a series of spectroscopic measurements including UV–vis absorption and photoluminescence. It is found that doping gives rise to new light absorption and photoluminescence bands at around 500 nm. Finally, light‐emitting diodes (LEDs) with the CNDs synthesized have been demonstrated to show that the electronic states induced by direct doping influence their electroluminescence. Such LEDs successfully exhibit broadband electroluminescence covering the visible light range from 500 to 700 nm, resulting in bright pure white light whose CIE (Commission Internationale d'Éclairage) coordinate is (0.2894, 0.3351).

Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.

Carbon nanodots (CND) have recently attracted tremendous attention for a variety of optical and optoelectronic applications. Although many attempts have been made to obtain high-quality CND in high yield, there have been few endeavors to regulate its energy structure. In this work, we have synthesized nitrogen-doped carbon nanodots (N-CND) through soft-template synthesis using citric acid and ethylenediamine (EDA) as carbon and nitrogen sources, respectively. The energy structure of N-CND has been modulated by regulating the concentration of EDA. A series of spectroscopic measurements showed that the absorption peak is developed near 370 nm and the photoluminescence peak is red-shifted with increasing nitrogen content. To utilize N-CND's strong near-UV absorption, broad-range visible-light emission, high quantum yield, and good organic compatibility, we have demonstrated phosphor-based light-emitting diodes employing N-CND as a phosphor.

Cauliflower-like tin oxide (SnO₂) hollow microspheres (HMS) sensitized with multilayer quantum dots (QDs) as photoanode and alternative stable, low-cost counter electrode are employed for the first time in QD-sensitized solar cells (QDSCs). Cauliflower-like SnO₂ hollow spheres mainly consist of 50 nm-sized agglomerated nanoparticles; they possess a high internal surface area and light scattering in between the microspheres and shell layers. This makes them promising photoanode material for both QDSCs and dye-sensitized solar cells (DSCs). Successive ionic layer adsorption and reaction (SILAR) method and chemical bath deposition (CBD) are used for QD-sensitizing the SnO₂ microspheres. Additionally, carbon-nanofiber (CNF) with a unique structure is used as an alternative counter electrode (CE) and compared with the standard platinum (Pt) CE. Their electrocatalytic properties are measured using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and Tafel-polarization. Under 1 sun illumination, solar cells made with hollow SnO₂ photoanode sandwiched with the stable CNF CE showed a power conversion efficiency of 2.5% in QDSCs and 3.0% for DSCs, which is quite promising with the standard Pt CE (QDSCs: 2.1%, and DSCs: 3.6%).

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.

Low-voltage operable organic field-effect transistors (OFETs) were fabricated using a low-temperature curable high-k polymer cyanoethyl pullulan (CEP) as a gate insulator with trimethylolpropane triglycidyl ether (TTE) as a crosslinking agent. With this crosslinking agent, the gate insulators showed low leakage current and a high dielectric constant of 14.8–16, and most importantly, could be cured at low temperatures around 100 °C, which is compatible with most plastic substrates for future plastic electronics. The favorable semiconductor–dielectric interface was obtained due to the surface alignment of well-packed pentacene domains, which enabled excellent transistor performance, with one of the highest mobilities around 6 cm² V⁻¹ s⁻¹, an on/off current ratio (I_on/I_off) ∼10⁵, and a steep subthreshold (SS) ∼0.095 V dec⁻¹. The high mobility was found to be closely correlated with the surface CN dipole density rather than other possibilities, such as dielectric roughness and surface energy. The initial growth of the semiconductors was also studied and correlated with the affecting variables.