Electrochromic graphene molecules

Engineering of graphene quantum dots by varying the properties of graphene oxide for fluorescence detection of picric acid

The study examines the effect of different forms of graphene oxide (GO) on the synthesis of graphene quantum dots (GQD). GO synthesized at various temperatures i.e. 30, 50, 110 °C possessed different structural and functional properties and was used as a substrate for GQD preparation. Thorough characterization of the GQDs in terms of their structural, morphological, functional, and optical properties was performed. The GQDs exhibited variation in their size and fluorescence properties depending upon the type of GO used. Hydrothermal reduction of GO, prepared at an oxidation temperature of 50 °C (GO-50), minimized the particle size (3.6 nm) and maximized the photoluminescence (PL) intensity and quantum yield (64.8%) of the GQD (GQD-50). GQD-50 was found to detect picric acid (PA) in an aqueous solution via 'turn-off' fluorescence quenching, unlike the other GQDs where the initial precursor is synthesized at 30, 110 °C. Experimental studies summarize that interaction between the fluorophore-quencher resulted in static quenching. The limit of detection was estimated to be 1.2 μM with a detection range of 0-200 μM. The work concludes that optimization of the substrate i.e. GO can result in the development of a simple, non-toxic, cost-effective GQD based sensor for PA detection. The study eliminates the need for doping/functionalization of GQDs as reported previously, and hence finds a promising impact on the development of sensors.

Exciton Coherence Length and Dynamics in Graphene Quantum Dot Assemblies

Exciton size and dynamics were studied in assemblies of two well-defined graphene quantum dots of varying size: hexabenzocoronene (HBC), where the aromatic core consists of 42 C atoms, and carbon quantum dot (CQD) with 78 C atoms. The synthesis of HBC and CQD were achieved using bottom-up chemical methods, while their assembly was studied using steady-state UV/vis spectroscopy, X-ray scattering, and electron microscopy. While HBC forms long ordered fibers, CQD was found not to assemble well. The exciton size and dynamics were studied using time-resolved laser spectroscopy. At early times (∼100 fs), the exciton was found to delocalize over ∼1-2 molecular units in both assemblies, which reflects the confined nature of excitons in carbon-based materials and is consistent with the calculated value of ∼2 molecular units. Exciton-exciton annihilation measurements provided the exciton diffusion lengths of 16 and 3 nm for HBC and CQD, respectively.

Tuning of 2D Nanographene Adlayers on Au(111) by Electrodeposition of Metal Halide Complexes

The electrodeposition of AuBr₄^- and PtBr₄^2- onto an adlayer of circobiphenyl-a structurally defined nanographene with low symmetry-on a Au(111) electrode was investigated via electrochemical scanning tunneling microscopy (EC-STM) to control and understand the formation of characteristic nanoclusters. By immersing a circobiphenyl-coated Au(111) substrate in a 0.1 mM aqueous AuBr₄^- solution, AuBr₄^- was spontaneously reduced, and a characteristic mixed adlayer consisting of circobiphenyl molecules and Br^- ions with monatomic Au islands was produced on the Au(111) surface. A similar electrodeposition process was performed in an aqueous solution of PtBr₄^2-, and an identical mixed adlayer was obtained with Pt nanoclusters. The electrodeposition of Au and Pt complexes was facilitated by the "negatively charged" reconstructed Au(111) surface, which is stabilized by the formation of a highly ordered circobiphenyl adlayer. EC-STM revealed the formation of characteristic dimers of Pt clusters ranging 2-4 nm in diameter on the circobiphenyl adlayer. Thus, Br^- metal complexes were found to play an important role in controlling the structure and size of a mixed adlayer containing Br^- and the shape of Pt clusters.

Raman spectroscopy of bottom-up synthesized graphene quantum dots: size and structure dependence

Graphene quantum dots (GQDs) have attracted significant interest as synthetically tunable optoelectronic and photonic materials that can also serve as model systems for understanding size-dependent behaviors of related graphene structures such as nanoribbons. We present a Raman spectroscopy study of bottom-up synthesized GQDs with lateral dimensions between 0.97 to 1.62 nm, well-defined (armchair) edge type, and fully benzenoid structures. For a better understanding of observed size-dependent trends, the study is extended to larger graphene structures including nano-graphene platelets (>25 nm) and large-area graphene. Raman spectra of GQDs reveal the presence of D and G bands, as well as higher order modes (2D, D + G, and 2G). The D and G band frequencies and intensity were found to increase as GQD size increases, while higher order modes (2D, D + G, and 2G) also increased in intensity and became more well-defined. The integrated intensity ratios of D and G bands (I_D/I_G) increase as the size of the GQDs approaches 2 nm and rapidly decrease for larger graphene structures. We present a quantitative comparison of I_D/I_G ratios for the GQDs and for defects introduced into large area graphenes through ion bombardment, for which inter-defect distances are comparable to the sizes of GQDs studied here. Close agreement suggests the I_D/I_G ratio as a size diagnostic for other nanographenes. Finally, we show that Raman spectroscopy is also a good diagnostic tool for monitoring the formation of bottom-up synthesized GQDs.

Size-Dependent Electronic Properties of Uniform Ensembles of Strongly Confined Graphene Quantum Dots

The electronic structure of a series of bottom-up synthesized graphene quantum dots (GQDs) smaller than 2 nm was investigated by spectroelectrochemistry, yielding insights not previously available from ensemble-level studies. The results show that for the strongly confined GQDs the dependence of the band gap on the GQD size deviates from the prediction of the standard Dirac Fermion model but agrees well with the models explicitly accounting for the electron-electron and electron-hole interactions. The HOMO/LUMO energy levels are found to be distributed nearly symmetrically around the 0 V value versus normal hydrogen electrode (NHE), becoming more positive/negative, respectively, with increasing GQD size. The exciton binding energies are found to follow power dependence on the number of carbon atoms per GQD, with the experimental values falling within the range of ∼0.1 to ∼0.6 eV. Given the broad accessibility of the described experimental tools and methods, our work opens a path to a more systematic examination of quantum confinement effects in GQDs.

Phenalenyl based neutral radical as a novel electrochromic material modulating visible to short-wave infrared light

Applications for energy saving smart windows require materials which can switch from transmissive to black states in both the visible and short-wave IR range. We introduce an electrochromic phenalenyl based neutral radical small molecule and design devices capable of modulating light in both of these ranges.

A Supramolecular Approach to the Preparation of Nanographene Adlayers Using Water-Soluble Molecular Capsules

Polycyclic aromatic hydrocarbons (PAHs) are excellent building blocks for the creation of two-dimensional (2D) nanosheets. However, large PAHs tend to exhibit poor or no solubility in organic solvents and water. To overcome this issue, we employed water-soluble micellar capsules consisting of V-shaped amphiphilic molecules. Characteristic electrochemical behavior was observed in 0.1 m H₂ SO₄ in the presence of the water-soluble capsules containing PAHs, such as ovalene, circobiphenyl, and dicoronylene. Furthermore, under these conditions, PAHs were released from the capsules, resulting in the formation of a 2D adlayer of PAHs at the electrochemical interface. Finally, using electrochemical scanning tunneling microscopy, we demonstrate that our molecular containers based on water-soluble molecular capsules allow the facile preparation of 2D PAH adlayers in addition to structurally controlling nanostructure formation on Au surfaces.

Multicolor Electrochromic Devices Based on Molecular Plasmonics

Polycyclic aromatic hydrocarbon (PAH) molecules, the hydrogen-terminated, sub-nanometer-scale version of graphene, support plasmon resonances with the addition or removal of a single electron. Typically colorless when neutral, they are transformed into vivid optical absorbers in either their positively or negatively charged states. Here, we demonstrate a low-voltage, multistate electrochromic device based on PAH plasmon resonances that can be reversibly switched between nearly colorless (0 V), olive (+4 V), and royal blue (-3.5 V). The device exhibits highly efficient color change compared to electrochromic polymers and metal oxides, lower power consumption than liquid crystals, and is shown to reversibly switch for at least 100 cycles. We also demonstrate the additive property of molecular plasmon resonances in a single-layer device to display a reversible, transmissive-to-black device. This work illuminates the potential of PAH molecular plasmonics for the development of color displays and large-area color-changing applications due to their processability and ultralow power consumption.

Methyl-viologen modified ZnO nanotubes for use in electrochromic devices

Efficiently changing optical properties (reflectance, transmittance, and absorbance) through reversible color changes of electrochromic (EC) materials is challenging and critical in achieving high-performance EC devices.