Graphene quantum dots (GQDs) are an allotrope of carbon with a planar surface amenable to functionalization and nanoscale dimensions that confer photoluminescence. Collectively, these properties render GQDs an advantageous platform for nanobiotechnology applications, including optical biosensing and delivery. Towards this end, noncovalent functionalization offers a route to reversibly modify and preserve the pristine GQD substrate, however, a clear paradigm has yet to be realized. Herein, we demonstrate the feasibility of noncovalent polymer adsorption to GQD surfaces, with a specific focus on single-stranded DNA (ssDNA). We study how GQD oxidation level affects the propensity for polymer adsorption by synthesizing and characterizing four types of GQD substrates ranging ~60-fold in oxidation level, then investigating noncovalent polymer association to these substrates. Adsorption of ssDNA quenches intrinsic GQD fluorescence by 31.5% for low-oxidation GQDs and enables aqueous dispersion of otherwise insoluble no-oxidation GQDs. ssDNA-GQD complexation is confirmed by atomic force microscopy, by inducing ssDNA desorption, and with molecular dynamics simulations. ssDNA is determined to adsorb strongly to no-oxidation GQDs, weakly to low-oxidation GQDs, and not at all for heavily oxidized GQDs. Finally, we reveal the generality of the adsorption platform and assess how the GQD system is tunable by modifying polymer sequence and type. Graphene is a two-dimensional hexagonal carbon lattice that possesses a host of unique properties, including exceptional electronic conductivity, mechanical strength, and adsorptive capacity 1-3. However, graphene is a zero-bandgap material, and this lack of bandgap limits its use in semiconducting applications 4. To engineer a bandgap, the lateral dimensions of graphene must be restricted to the nanoscale, resulting in spatially confined structures such as graphene quantum dots (GQDs) 5. The bandgap of GQDs is attributed to quantum confinement 6,7 , edge effects 8 , and localized electron-hole pairs 9. Accordingly, this gives rise to tunable fluorescence properties based upon GQD size, shape, and exogenous atomic composition. In comparison to conventional semiconductor quantum dots, GQDs are an inexpensive and less environmentally harmful alternative 10,11. Moreover, for biological applications, GQDs are a low toxicity, biocompatible, and photostable material that offer a large surface-to-volume ratio for bioconjugation 11,12. Exploiting the distinct material properties of graphene often requires or benefits from exogenous functionalization. The predominant mechanism for graphene or graphene oxide (GO) functionalization is via covalent linkage to a polymer. However, noncovalent adsorption of polymers to carbon substrates is desirable in applications requiring reversibility for solution-based manipulation and tunable ligand exchange 13 , and preservation of the pristine atomic structure to maintain nanoscale graphene's fluorescence characteristics 14. Functionalization of graphene and GO ...

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Novel approaches to boost quantum dot solar cell (QDSC) efficiencies are in demand. Herein, three strategies are used: (i) a hydrothermally synthesized TiO-multiwalled carbon nanotube (MWCNT) composite instead of conventional TiO, (ii) a counter electrode (CE) that has not been applied to QDSCs until now, namely, tin sulfide (SnS) nanoparticles (NPs) coated over a conductive carbon (C)-fabric, and (iii) a quasi-solid-state gel electrolyte composed of S, an inert polymer and TiO nanoparticles as opposed to a polysulfide solution based hole transport layer. MWCNTs by virtue of their high electrical conductivity and suitably positioned Fermi level (below the conduction bands of TiO and PbS) allow fast photogenerated electron injection into the external circuit, and this is confirmed by a higher efficiency of 6.3% achieved for a TiO-MWCNT/PbS/ZnS based (champion) cell, compared to the corresponding TiO/PbS/ZnS based cell (4.45%). Nanoscale current map analysis of TiO and TiO-MWCNTs reveals the presence of narrowly spaced highly conducting domains in the latter, which equips it with an average current carrying capability greater by a few orders of magnitude. Electron transport and recombination resistances are lower and higher respectively for the TiO-MWCNT/PbS/ZnS cell relative to the TiO/PbS/ZnS cell, thus leading to a high performance cell. The efficacy of SnS/C-fabric as a CE is confirmed from the higher efficiency achieved in cells with this CE compared to the C-fabric based cells. Lower charge transfer and diffusional resistances, slower photovoltage decay, high electrical conductance and lower redox potential impart high catalytic activity to the SnS/C-fabric assembly for sulfide reduction and thus endow the TiO-MWCNT/PbS/ZnS cell with a high open circuit voltage (0.9 V) and a large short circuit current density (∼20 mA cm). This study attempts to unravel how simple strategies can amplify QDSC performances.

A novel assembly of a photocathode and a photoanode is investigated to explore their complementary effects in enhancing the photovoltaic performance of a quantum-dot solar cell (QDSC). While p-type nickel oxide (NiO) has been used previously, antimony selenide (SbSe) has not been used in a QDSC, especially as a component of a counter electrode (CE) architecture that doubles as the photocathode. Here, near-infrared (NIR) light-absorbing SbSe nanoparticles (NPs) coated over electrodeposited NiO nanofibers on a carbon (C) fabric substrate was employed as the highly efficient photocathode. Quasi-spherical SbSe NPs, with a band gap of 1.13 eV, upon illumination, release photoexcited electrons in addition to other charge carriers at the CE to further enhance the reduction of the oxidized polysulfide. The p-type conducting behavior of SbSe, coupled with a work function at 4.63 eV, also facilitates electron injection to polysulfide. The effect of graphene quantum dots (GQDs) as co-sensitizers as well as electron conduits is also investigated in which a TiO/CdS/GQDs photoanode structure in combination with a C-fabric CE delivered a power-conversion efficiency (PCE) of 5.28%, which is a vast improvement over the 4.23% that is obtained by using a TiO/CdS photoanode (without GQDs) with the same CE. GQDs, due to a superior conductance, impact efficiency more than SbSe NPs do. The best PCE of a TiO/CdS/GQDs-nS/S-SbSe/NiO/C-fabric cell is 5.96% (0.11 cm area), which, when replicated on a smaller area of 0.06 cm, is seen to increase dramatically to 7.19%. The cell is also tested for 6 h of continuous irradiance. The rationalization for the channelized photogenerated electron movement, which augments the cell performance, is furnished in detail in these studies.