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This essay examines the work of German-French photographer Gisèle Freund during the interwar years, with special focus on her volte-face from black-and-white depictions of the collective subject of political demonstrations in pre-exile Frankfurt to color portraits of individual French intellectuals after her arrival in Paris. Pivoting around the short period between 1938 and 1940, when using color became the standard rhetorical maneuver of Freund's portrait series, this essay will trace the photographer's change in practice as a response to the mounting crisis within France's Popular Front and its aesthetic strategies in the face of the rise of fascism. One of the essay's claims is that Freund turned color photography from a material and commercial commodity into the emblem of an alternative, mass-mediated culture—the culture of Americanism—that she, like many European intellectuals of the 1920s, imagined capable of competing with and ultimately countering the fascist mobilization of spectacle.

As an alternative to Cd-free quantum dots (QDs), graphene quantum dots (GQDs) have potential because of their various advantages, such as color tunability, nontoxicity, cost effectiveness, and phot...

Inorganic zero-dimensional perovskites, such as Cs4PbBr6, offer an underexplored opportunity to achieve efficient exciton formation and radiative recombination. In particular, the origin of sub-ban...

TiO2-based photocatalysis has been considered to be one of the most promising avenues for environmental remediation including water purification. However, several technical issues such as the limited surface area of bulk TiO2, the large band gap of TiO2, and rapid charge recombination still limit the practical application of TiO2 photocatalysts. Therefore, here we focus on two structural design strategies: (i) monolithic three-dimensional (3D) nanostructuring, and (ii) heterostructuring with graphitic carbon nanomaterials. A monolithic 3D nanostructure enables maximal surface area in a given volume and efficient reuse of the photocatalyst without recollection. Heterostructuring with carbon nanomaterials helps achieve maximal utilization of the solar spectrum and charge separation and provides efficient TiO2 photocatalysts. In this review, recent progress on TiO2 photocatalysts toward the abovementioned strategies will be summarized. Further discussion and direction will provide insights into the rational design of highly efficient TiO2 photocatalysts, and help develop advanced photocatalyst models.

The charge excitation and decay pathways of two-dimensional heteroatomic quantum dots (QDs) are affected by the quantum confinement effect, bandgap structure and strong exciton binding energy. Recently, semiconducting transition metal dichalcogenides (TMDs) have been intensively studied; however, the charge dynamics of metallic phase QDs (mQDs) of TMDs remain relatively unknown. Herein, we investigate the photophysical properties of TMD-mQDs of two sizes, where the TMD-mQDs show different charge excitation and decay pathways that are mainly ascribed to the defect states and valence band splitting, resulting in a large Stokes shift and two excitation bands for maximum photoluminescence (PL). Interestingly, the dominant excitation band redshifts as the size increases, and the time-resolved PL peak redshifts at an excitation wavelength of 266 nm in the smaller QDs. Additionally, the lifetime is shortened in the larger QDs. From the structural and theoretical analysis, we discuss that the charge decay pathway in the smaller QDs is predominantly affected by edge oxidation, whereas the vacancies play an important role in the larger QDs. Metallic phase transition metal dichalcogenides quantum dots show different pathways of optical charge excitation and decay according to the size and sort of defects, resulting into the large Stoke shift, two bands for charge excitation, and TRPL peak shift. This result is mainly ascribed to the valance band splitting and the emerging defect states originated from atomic vacancy of basal plane and edge oxidation.

Demand for an effective strategy for exfoliating layered materials into flakes without perturbing their intrinsic structure is growing. Herein, we introduce an effective fabrication method of large-sized non-oxidized graphene flakes (NOGFs) as a representative example of a general strategy using spontaneous insertion of exfoliating medium into a layered material. We fabricated a ternary graphite intercalation compound (t-GIC) with stoichiometry of KC24(THF)2, and analyzed its morphology and electronic structure through experimental and computational approach. Interactions between the t-GIC and aprotic organic solvents with different polarities were investigated, where a unique swelling behavior was observed with dimethyl sulfoxide (DMSO). Based on the analysis of the phenomena, we demonstrate facile exfoliation of the t-GIC in polyvinyl pyrrolidone (PVP)-DMSO solution for fabrication of highly crystalline and large-sized NOGFs. The lateral size of the NOGFs ranges over 30 μm, while the 98% having thickness below 10 layers. The NOGF film exhibits supreme electrical conductivity of 3.36 × 105 S/m, which is, to our best knowledge, the highest value for a thin conductive film made of graphene flakes.

Titanium oxide (TiO2) exhibits intrinsically strong absorption of ultraviolet (UV) light, which has been utilized in a variety of applications, such as environmental purification/sterilization, hea...

Graphene quantum dots (GQDs) are promising luminescent materials for various light-emitting, biological, and energy applications due to their low toxicity compared to cadmium-based semiconductor quantum dots. The practical application and use of GQDs is driven by their luminescent characteristics. As such, a critical need exists to control and tailor the emission properties of these materials to suit the targeted optoelectronic applications. In this review, the fundamental properties, synthesis methods, and strategies for incorporating GQDs into optoelectronic devices are discussed. We present the fundamentals on luminescence mechanism of GQDs, including reflections on the defect-related properties (i.e., oxygen functional groups and odd-numbered carbon rings in the basal plane). We then offer new guidelines for the fabrication of GQDs aimed at increasing the luminescence efficiency. Finally, we introduce strategies for integrating GQDs into optoelectronic devices based on architectural design of the fundamental properties.

Enhancing the filler-to-matrix contact area by incorporating large graphene flakes (GFs) is considered a viable approach for improving the mechanical properties of polymer composites. Graphene oxide (GO) flakes have been initially pursued for this application owing to the advantage of producing large-sized GFs. However, the defective nature of GO makes it technically challenging to precisely assess their dimensional effect on resulting mechanical properties, limiting their versatility as reinforcing materials. Therefore, it is highly desired to fabricate GFs with minimum defects and large lateral size. In this paper, we report the fabrication of high-quality edge functionalized non-oxidized graphene flakes (f-NOGFs) by the liquid exfoliation of graphite intercalation compounds. As-prepared f-NOGFs are over 1 μm in lateral size and contain less than 6.25% of impurity oxygen. By using size-sorted f-NOGFs, we systematically studied the relationship between mechanical properties and lateral sizes in f-NOGFs-incorporated epoxies. We identify that epoxy composites containing 0.6 wt% of f-NOGFs with a lateral size of 1 μm present outstanding mechanical properties; elastic modulus of 3.65 GPa, ultimate tensile strength of 95.74 GPa and toughness of 2.52 MJ m−3. The study presented in this paper could provide better understanding for optimization of the mechanical reinforcement of graphene-polymer composites.

TiO2-based photocatalysis has been considered to be one of the most promising avenues for environmental remediation including water purification. However, several technical issues such as the limited surface area of bulk TiO2, the large band gap of TiO2, and rapid charge recombination still limit the practical application of TiO2 photocatalysts. Therefore, here we focus on two structural design strategies: (i) monolithic three-dimensional (3D) nanostructuring, and (ii) heterostructuring with graphitic carbon nanomaterials. A monolithic 3D nanostructure enables maximal surface area in a given volume and efficient reuse of the photocatalyst without recollection. Heterostructuring with carbon nanomaterials helps achieve maximal utilization of the solar spectrum and charge separation and provides efficient TiO2 photocatalysts. In this review, recent progress on TiO2 photocatalysts toward the abovementioned strategies will be summarized. Further discussion and direction will provide insights into the rational design of highly efficient TiO2 photocatalysts, and help develop advanced photocatalyst models.

The photoluminescence (PL) origin of bright blue emission arising from intrinsic states in graphene quantum dots (GQDs) is investigated. The bright PL of intercalatively acquired GQDs is attributed to favorably formed subdomains composed of four to seven carbon hexagons. Random and harsh oxidation which hinders the energetically favorable formation of subdomains causes weak and redshifted PL.

Homogeneous and stable dispersion of nanoparticles in polymer matrix remains a significant challenge. Here, we propose a simple and effective route for synthesizing functionalized TiO2 nanoparticles through oleic acid. The Oleic-TiO2 nanoparticles homogeneously dispersed in SBR matrix show remarkable improvement of mechanical properties such as modulus, tensile strength, and fatigue properties, even at low loadings, which are ascribed to the enhanced interfacial bonding with the polymer matrix. Moreover, the incorporation of the Oleic-TiO2 into SBR matrix significantly prevents photo and thermal degradation. Therefore, we confirm that our approach can suggest fascinating potentials to be scalable and commercial in polymer engineering. POLYM. COMPOS., 38:E174–E180, 2017. © 2016 Society of Plastics Engineers

The applicability of graphene quantum dots (GQDs) for the in vitro and in vivo live imaging and tracking of different types of human stem cells is investigated. GQDs synthesized by the modified graphite intercalated compound method show efficient cellular uptake with improved biocompatibility and highly sensitive optical properties, indicating their feasibility as a bio-imaging probe for stem cell therapy.

The photoluminescence of graphene quantum dots (GQDs) is rigorously investigated due to their potential applications. However, GQDs from graphene oxide are inherently embedded with non-negligible defects and oxygen grafted onto the edge and basal plane, which induce a change of innate electronic structure in the GQDs. Thus, graphene oxide based GQDs can misrepresent the characteristic properties of primitive GQDs. Here we report the size and pH dependent photophysical properties of GQDs that minimize the content of oxygen and defects. From auger electron spectroscopy, the oxygen content of the GQDs with two different lateral sizes (∼2 nm and ∼18 nm) was probed and found to be ∼5% and ∼8%, respectively. Two common photoluminescence (PL) peaks were observed at 436 nm (the intrinsic bandgap) and 487 nm (the extrinsic bandgap) for both GQDs. The characteristic PL properties in extrinsic and intrinsic bandgaps examined by optical spectroscopic methods show that the emission peak was red-shifted and that the peak width was widened as the size increased. Moreover, the PL lifetime and intensity were not only reversibly changed by pH but also depended on the excitation wavelength. This is in line with our previous report and is ascribed to the size variation of sp2 subdomains and edge functionalization.

Long-lived afterglow emissions, such as room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), are beneficial in the fields of displays, bioimaging, and data security. However, it is challenging to realize a single material that simultaneously exhibits both RTP and TADF properties with their relative strengths varied in a controlled manner. Herein, a new design approach is reported to control singlet-triplet energy splitting (∆EST ) in graphene quantum dots (GQD)/graphene oxide quantum dots (GOQDs) by varying the ratio of oxygenated carbon to sp2 carbon (γOC ). It is demonstrated that ∆EST decreases from 0.365 to 0.123 eV as γOC increases from 4.63% to 59.6%, which in turn induces a dramatic transition from RTP to TADF. Matrix-assisted stabilization of triplet excited states provides ultralong lifetimes to both RTP and TADF. Embedded in boron oxynitride, the low oxidized (4.63%) GQD exhibits an RTP lifetime (τTavg ) of 783 ms, and the highly oxidized (59.6%) GOQD exhibits a TADF lifetime (τDFavg ) of 125 ms. Furthermore, the long-lived RTP and TADF materials enable the first demonstration of anticounterfeiting and multilevel information security using GQD. These results will open up a new approach to the engineering of singlet-triplet splitting in GQD for controlled realization of smart multimodal afterglow materials.

Here, we study the plasmonic metal-enhanced fluorescence properties of blue-emitting graphene quantum dots (GQDs) and green-emitting graphene oxide quantum dots (GOQDs) using fluorescence lifetime imaging microscopy. Reactive ion sputtered silver (Ag) on zinc oxide (ZnO) thin films deposited on silicon (Si) wafers are used as the substrates. The morphology of the sputtered Ag gradually changes from nanoislands, via and elongated network and a continuous film with nanoholes, to a continuous film with increasing sputtering time. The fluorescence properties of GQD and GOQD on the Ag are modulated in terms of the intensities and lifetimes as the morphology of the Ag layers changes. Although both GQD and GOQD show similar fluorescence modulation on the Ag nanofilms, the fluorescence of GQD is enhanced, whereas that of GOQD is quenched due to the charge transfer process from GOQD to ZnO. Moreover, the GQD and GOQD exhibit different fluorescence lifetimes due to the effect of their electronic configurations. The theoretical calculation explains that the fluorescence amplification on the Ag nanofilms can largely be attributed to the enhanced absorption mechanism arising from accumulated optical fields around nanogaps and nanovoids in the Ag nanofilms.

Nano-fillers have provided a big advantage for enhancing the performance of rubber composites through leading the synergy effects in the physical and chemical properties. However, despite various approaches having been explored, the process to make a homogenous and stable dispersion of nano-filler in the rubber matrix remains a major challenge in this field. Herein, we propose a simple and effective route for synthesizing nanocomposites of rubber with homogenous and stable dispersed low defect graphene flakes (l-GFs), which are prepared using l-GFs/SBR composites via aqueous-phase mixing of exfoliated l-GFs with SBR latex. The l-GFs embedded SBR matrix shows a remarkable improvement in the modulus and tensile strength even at the low loading rate, which is ascribed to the efficient dispersion of the l-GFs enhancing interfacial interaction with the rubber matrix. The integration of l-GFs into the SBR matrix significantly improves the thermal and electrical conductivities, as well as the gas barrier property of the rubber composites. This method is water-mediated, green and scalable, showing great potential for the production of various l-GFs-based rubber composites at an industrial level.

H.Y. and K.L. contributed equally to this work. This study was supported by the Center for Advanced Soft Electronics as Global Frontier Project (CASE-2013M3A6A5073173) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning. This study was also supported by Multi-Ministry Collaborative R&D Program (Development of Techniques for Identification and Analysis of Gas Molecules to Protect Against Toxic Substances) through the National Research Foundation of Korea (NRF) funded by KNPA, MSIT, MOTIE, ME, NFA (Grant No. 2017M3D9A1073501).

Water soluble GQDs were systematically characterized as a multiphoton fluorophore and a cell imaging probe. When mouse primary hepatocytes were incubated with GQDs, no significant cytotoxicity was observed up to the treatment concentration of 100 μg ml(-1). Using these GQDs, mouse primary hepatocytes were successfully imaged by multiphoton fluorescence.

Emerging graphene quantum dots (GQDs) have received much attention for use as next-generation light-emitting diodes. However, in the solid-state, π-interaction-induced aggregation-caused photoluminescence (PL) quenching (ACQ) in GQDs makes it challenging to realize high-performance devices. Herein, GQDs incorporated with boron oxynitride (GQD@BNO) are prepared from a mixture of GQDs, boric acid, and urea in water via one-step microwave heating. Due to the effective dispersion in the BNO matrix, ACQ is significantly suppressed, resulting in high PL quantum yields (PL-QYs) of up to 36.4%, eightfold higher than that of pristine GQD in water. The PL-QY enhancement results from an increase in the spontaneous emission rate of GQDs due to the surrounding BNO matrix, which provides a high-refractive-index material and fluorescence energy transfer from the larger-gap BNO donor to the smaller-gap GQD acceptor. A high solid-state PL-QY makes the GQD@BNO an ideal active material for use in AC powder electroluminescent (ACPEL) devices, with the luminance of the first working GQD-based ACPEL device exceeding 283 cd m-2 . This successful demonstration shows promise for the use of GQDs in the field of low-cost, ecofriendly electroluminescent devices.