Publications by authors named "Daichi Kozawa"

19 Publications

  • Page 1 of 1

Toward Covalent Organic Framework Metastructures.

J Am Chem Soc 2021 Apr 16;143(13):5003-5010. Epub 2021 Mar 16.

State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China.

The bottom-up assembly of periodically ordered structures provides a scalable way for producing metastructured materials with exotic optical and mechanical properties. However, direct self-assembly of small molecules into such metastructures beyond the nanoscale remains an unresolved issue. Here we demonstrate that metastructured assemblies of two-dimensional (2D) polymers, specifically 2D covalent organic frameworks (COFs), can be directly synthesized in solution. We applied 2D COF monomer polycondensation to prepare flower-shaped particles consisting of highly crystalline "petals" with sizes larger than 20 μm. The petal comprises periodically arranged COF nanoflake units with tunable lengths of 490-850 nm, thicknesses about 20 nm, interflake spacing around 14 nm, and Hermans orientation factors up to 0.998. Such a metastructure is mechanically robust and remains almost intact even after full pyrolysis at 900 °C. It also demonstrates unique birefringence and polarization-dependent resonances under visible-near-infrared light not observed in its constituents, 2D COF polycrystals, and with well-defined nanopores of 1.8 nm and the high surface area of 1576 m/g. Such metastructured particles with nanopores are well-suited as novel particulate optical devices for collecting and storing information about their surroundings that can be easily read out by polarization imaging with high sensitivity, as demonstrated by their explosive detection and anticounterfeiting applications. Self-assembly of 2D polymers into metastructures may become an important method for developing functional materials with unprecedented properties and extensive applications.
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http://dx.doi.org/10.1021/jacs.0c13090DOI Listing
April 2021

Diameter Dependence of Water Filling in Lithographically Segmented Isolated Carbon Nanotubes.

ACS Nano 2021 Feb 29;15(2):2778-2790. Epub 2021 Jan 29.

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.

Although the structure and properties of water under conditions of extreme confinement are fundamentally important for a variety of applications, they remain poorly understood, especially for dimensions less than 2 nm. This problem is confounded by the difficulty in controlling surface roughness and dimensionality in fabricated nanochannels, contributing to a dearth of experimental platforms capable of carrying out the necessary precision measurements. In this work, we utilize an experimental platform based on the interior of lithographically segmented, isolated single-walled carbon nanotubes to study water under extreme nanoscale confinement. This platform generates multiple copies of nanotubes with identical chirality, of diameters from 0.8 to 2.5 nm and lengths spanning 6 to 160 μm, that can be studied individually in real time before and after opening, exposure to water, and subsequent water filling. We demonstrate that, under controlled conditions, the diameter-dependent blue shift of the Raman radial breathing mode (RBM) between 1 and 8 cm measures an increase in the interior mechanical modulus associated with liquid water filling, with no response from exterior water exposure. The observed RBM shift with filling demonstrates a non-monotonic trend with diameter, supporting the assignment of a minimum of 1.81 ± 0.09 cm at 0.93 ± 0.08 nm with a nearly linear increase at larger diameters. We find that a simple hard-sphere model of water in the confined nanotube interior describes key features of the diameter-dependent modulus change of the carbon nanotube and supports previous observations in the literature. Longer segments of 160 μm show partial filling from their ends, consistent with pore clogging. These devices provide an opportunity to study fluid behavior under extreme confinement with high precision and repeatability.
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http://dx.doi.org/10.1021/acsnano.0c08634DOI Listing
February 2021

A Fiber Optic Interface Coupled to Nanosensors: Applications to Protein Aggregation and Organic Molecule Quantification.

ACS Nano 2020 08 27;14(8):10141-10152. Epub 2020 Jul 27.

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.

Fluorescent nanosensors hold promise to address analytical challenges in the biopharmaceutical industry. The monitoring of therapeutic protein critical quality attributes such as aggregation is a long-standing challenge requiring low detection limits and multiplexing of different product parameters. However, general approaches for interfacing nanosensors to the biopharmaceutical process remain minimally explored to date. Herein, we design and fabricate a integrated fiber optic nanosensor element, measuring sensitivity, response time, and stability for applications to the rapid process monitoring. The fiber optic-nanosensor interface, or optode, consists of label-free nIR fluorescent single-walled carbon nanotube transducers embedded within a protective yet porous hydrogel attached to the end of the fiber waveguide. The optode platform is shown to be capable of differentiating the aggregation status of human immunoglobulin G, reporting the relative fraction of monomers and dimer aggregates with sizes 5.6 and 9.6 nm, respectively, in under 5 min of analysis time. We introduce a lab-on-fiber design with potential for at-line monitoring with integration of 3D-printed miniaturized sensor tips having high mechanical flexibility. A parallel measurement of fluctuations in laser excitation allows for intensity normalization and significantly lower noise level (3.7 times improved) when using lower quality lasers, improving the cost effectiveness of the platform. As an application, we demonstrate the capability of the fully integrated lab-on-fiber system to rapidly monitor various bioanalytes including serotonin, norepinephrine, adrenaline, and hydrogen peroxide, in addition to proteins and their aggregation states. These results in total constitute an effective form factor for nanosensor-based transducers for applications in industrial process monitoring.
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http://dx.doi.org/10.1021/acsnano.0c03417DOI Listing
August 2020

Highly Ordered Two-Dimensional MoS Archimedean Scroll Bragg Reflectors as Chromatically Adaptive Fibers.

Nano Lett 2020 May 29;20(5):3067-3078. Epub 2020 Apr 29.

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02141, United States.

Nanostructured fibers provide a basis for a unique class of multifunctional textiles, composites, and membrane applications, including those capable of chromatic modulating because of their high aspect ratio, surface area, and processing capability. Here in, we utilize two-dimensional (2D) materials including molybdenum disulfide (MoS) and hexagonal boron nitride (hBN) to generate single layer Archimedean scroll fibers, possessing cross sections formed from a single 2D molecular layer. Chemical vapor deposited (CVD) monolayer MoS (0.29-0.33% in volume) and 226-259 nm-thick poly(methyl methacrylate) (PMMA) were used to create Bragg reflector fibers, exploiting the anisotropic function, exhibiting reflection at 630-709 nm, and verifying the highly ordered nanoinclusions. The Bragg reflectors show a memory response to heating and cooling, which switches the reflection wavelength from 629 to 698 nm. We simulate the reflection and transmission spectra of MoS/PMMA and MoS/polydimethylsiloxane layered composites to provide the design of scroll fiber composites using the transfer matrix methods. Moreover, we demonstrate the incorporation of a few-layer CVD hBN into the scroll fiber composite that emits photons at 576 nm. The highly oriented layered structures extend the capability of the fiber nanocomposites to take advantage of anisotropic optical, electrical, and thermal properties unique to 2D materials.
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http://dx.doi.org/10.1021/acs.nanolett.9b05004DOI Listing
May 2020

Controlling Photoluminescence Enhancement and Energy Transfer in WS :hBN:WS Vertical Stacks by Precise Interlayer Distances.

Small 2020 Jan 18;16(3):e1905985. Epub 2019 Dec 18.

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK.

2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS :hBN:WS are fabricated utilizing chemical vapor deposition (CVD)-grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS constituents. Such doping effects originate from the interfaces that WS monolayers reside on and interact with. The electron density in the WS is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back-gated voltages. Other influential factors include the strain in WS and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.
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http://dx.doi.org/10.1002/smll.201905985DOI Listing
January 2020

Autoperforation of 2D materials for generating two-terminal memristive Janus particles.

Nat Mater 2018 11 23;17(11):1005-1012. Epub 2018 Oct 23.

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.

Graphene and other two-dimensional materials possess desirable mechanical, electrical and chemical properties for incorporation into or onto colloidal particles, potentially granting them unique electronic functions. However, this application has not yet been realized, because conventional top-down lithography scales poorly for producing colloidal solutions. Here, we develop an 'autoperforation' technique that provides a means of spontaneous assembly for surfaces composed of two-dimensional molecular scaffolds. Chemical vapour deposited two-dimensional sheets can autoperforate into circular envelopes when sandwiching a microprinted polymer composite disk of nanoparticle ink, allowing liftoff into solution and simultaneous assembly. The resulting colloidal microparticles have two independently addressable, external Janus faces that we show can function as an intraparticle array of vertically aligned, two-terminal electronic devices. Such particles demonstrate remarkable chemical and mechanical stability and form the basis of particulate electronic devices capable of collecting and storing information about their surroundings, extending nanoelectronics into previously inaccessible environments.
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http://dx.doi.org/10.1038/s41563-018-0197-zDOI Listing
November 2018

Colloidal nanoelectronic state machines based on 2D materials for aerosolizable electronics.

Nat Nanotechnol 2018 09 23;13(9):819-827. Epub 2018 Jul 23.

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.

A previously unexplored property of two-dimensional electronic materials is their ability to graft electronic functionality onto colloidal particles to access local hydrodynamics in fluids to impart mobility and enter spaces inaccessible to larger electronic systems. Here, we demonstrate the design and fabrication of fully autonomous state machines built onto SU-8 particles powered by a two-dimensional material-based photodiode. The on-board circuit connects a chemiresistor circuit element and a memristor element, enabling the detection and storage of information after aerosolization, hydrodynamic propulsion to targets over 0.6 m away, and large-area surface sensing of triethylamine, ammonia and aerosolized soot in inaccessible locations. An incorporated retroreflector design allows for facile position location using laser-scanning optical detection. Such state machines may find widespread application as probes in confined environments, such as the human digestive tract, oil and gas conduits, chemical and biosynthetic reactors, and autonomous environmental sensors.
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http://dx.doi.org/10.1038/s41565-018-0194-zDOI Listing
September 2018

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS :hBN:MoS Heterostructures for Exciton Energy Transfer.

Small 2018 03 7;14(13):e1703727. Epub 2018 Feb 7.

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK.

The 2D semiconductor monolayer transition metal dichalcogenides, WS and MoS , are grown by chemical vapor deposition (CVD) and assembled by sequential transfer into vertical layered heterostructures (VLHs). Insulating hBN, also produced by CVD, is utilized to control the separation between WS and MoS by adjusting the layer number, leading to fine-scale tuning of the interlayer interactions within the VLHs. The interlayer interactions are studied by photoluminescence (PL) spectroscopy and are demonstrated to be highly sensitive to the input excitation power. For thin hBN separators (one to two layers), the total PL emission switches from quenching to enhancement by increasing the laser power. Femtosecond broadband transient absorption measurements demonstrate that the increase in PL quantum yield results from Förster energy transfer from MoS to WS . The PL signal is further enhanced at cryogenic temperatures due to the suppressed nonradiative decay channels. It is shown that (4 ± 1) layers of hBN are optimum for obtaining PL enhancement in the VLHs. Increasing thickness beyond this causes the enhancement factor to diminish, with the WS and MoS then behaving as isolated noninteracting monolayers. These results indicate how controlling the exciton generation rate influences energy transfer and plays an important role in the properties of VLHs.
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http://dx.doi.org/10.1002/smll.201703727DOI Listing
March 2018

Observation of the Marcus Inverted Region of Electron Transfer from Asymmetric Chemical Doping of Pristine (n,m) Single-Walled Carbon Nanotubes.

J Am Chem Soc 2017 11 20;139(43):15328-15336. Epub 2017 Oct 20.

Department of Chemical Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

The concept of electrical energy generation based on asymmetric chemical doping of single-walled carbon nanotube (SWNT) papers is presented. We explore 27 small, organic, electron-acceptor molecules that are shown to tune the output open-circuit voltage (V) across three types of pristine SWNT papers with varying (n,m) chirality distributions. A considerable enhancement in the observed V, from 80 to 440 mV, is observed for SWNT/molecule acceptor pairs that have molecular volume below 120 Å and lowest unoccupied molecular orbital (LUMO) energies centered around -0.8 eV. The electron transfer (ET) rate constants driving the V generation are shown to vary with the chirality-associated Marcus theory, suggesting that the energy gaps between SWNT and the LUMO of acceptor molecules dictate the ET process. When the ET rate constants and the maximum V are plotted versus the LUMO energy of the acceptor organic molecule, volcano-shaped dependencies, characteristic of the Marcus inverted region, are apparent for three distinct sources of SWNT papers with modes in diameter distributions of 0.95, 0.83, and 0.75 nm. This observation, where the ET driving force exceeds reorganization energies, allows for an estimation of the outer-sphere reorganization energies with values as low as 100 meV for the (8,7) SWNT, consistent with a proposed image-charge modified Born energy model. These results expand the fundamental understanding of ET transfer processes in SWNT and allow for an accurate calculation of energy generation through asymmetric doping for device applications.
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http://dx.doi.org/10.1021/jacs.7b04314DOI Listing
November 2017

Correction: Thermal dissociation of inter-layer excitons in MoS/MoSe hetero-bilayers.

Nanoscale 2017 06;9(22):7686

Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan.

Correction for 'Thermal dissociation of inter-layer excitons in MoS/MoSe hetero-bilayers' by Shinichiro Mouri et al., Nanoscale, 2017, DOI: 10.1039/c7nr01598d.
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http://dx.doi.org/10.1039/c7nr90102jDOI Listing
June 2017

Thermal dissociation of inter-layer excitons in MoS/MoSe hetero-bilayers.

Nanoscale 2017 05;9(20):6674-6679

Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan.

We describe photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy of hetero-bilayers comprising monolayers (1L) of MoS and MoSe at cryogenic temperatures. A PL peak showing a decay time of 2.5 ns was observed below 100 K, which can be attributed to an inter-layer exciton emission in the 1L-MoS/1L-MoSe hetero-bilayers. An inter-layer exciton binding energy of ∼90 meV is determined from its thermal dissociation behavior; the band offset of each layer obtained from this value is consistent with previously reported first-principles calculations. Moreover, generation of inter-layer charged excitons (trions) is implied from the gate modulation of the inter-layer exciton PL spectra.
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http://dx.doi.org/10.1039/c7nr01598dDOI Listing
May 2017

Evidence for Fast Interlayer Energy Transfer in MoSe2/WS2 Heterostructures.

Nano Lett 2016 07 27;16(7):4087-93. Epub 2016 Jun 27.

Department of Physics, National University of Singapore , 2 Science Drive 3, 117551, Singapore.

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering.
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http://dx.doi.org/10.1021/acs.nanolett.6b00801DOI Listing
July 2016

Exciton-Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles.

Adv Mater 2016 Apr 2;28(14):2709-15. Epub 2016 Feb 2.

Department of Chemistry, Centre for Advanced 2D Materials and Graphene Research Centre, 3 Science Drive 3, 117543, Singapore.

Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.
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http://dx.doi.org/10.1002/adma.201504478DOI Listing
April 2016

Enhanced photovoltaic performances of graphene/Si solar cells by insertion of a MoS₂ thin film.

Nanoscale 2015 Sep;7(34):14476-82

Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

Transition-metal dichalcogenides exhibit great potential as active materials in optoelectronic devices because of their characteristic band structure. Here, we demonstrated that the photovoltaic performances of graphene/Si Schottky junction solar cells were significantly improved by inserting a chemical vapor deposition (CVD)-grown, large MoS2 thin-film layer. This layer functions as an effective electron-blocking/hole-transporting layer. We also demonstrated that the photovoltaic properties are enhanced with the increasing number of graphene layers and the decreasing thickness of the MoS2 layer. A high photovoltaic conversion efficiency of 11.1% was achieved with the optimized trilayer-graphene/MoS2/n-Si solar cell.
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http://dx.doi.org/10.1039/c5nr03046cDOI Listing
September 2015

Considerably improved photovoltaic performance of carbon nanotube-based solar cells using metal oxide layers.

Nat Commun 2015 Feb 18;6:6305. Epub 2015 Feb 18.

Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan.

Carbon nanotube-based solar cells have been extensively studied from the perspective of potential application. Here we demonstrated a significant improvement of the carbon nanotube solar cells by the use of metal oxide layers for efficient carrier transport. The metal oxides also serve as an antireflection layer and an efficient carrier dopant, leading to a reduction in the loss of the incident solar light and an increase in the photocurrent, respectively. As a consequence, the photovoltaic performance of both p-single-walled carbon nanotube (SWNT)/n-Si and n-SWNT/p-Si heterojunction solar cells using MoOx and ZnO layers is improved, resulting in very high photovoltaic conversion efficiencies of 17.0 and 4.0%, respectively. These findings regarding the use of metal oxides as multifunctional layers suggest that metal oxide layers could improve the performance of various electronic devices based on carbon nanotubes.
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http://dx.doi.org/10.1038/ncomms7305DOI Listing
February 2015

Photocarrier relaxation pathway in two-dimensional semiconducting transition metal dichalcogenides.

Nat Commun 2014 Jul 29;5:4543. Epub 2014 Jul 29.

1] Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore [2] Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore [3] Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

Two-dimensional crystals of semiconducting transition metal dichalcogenides absorb a large fraction of incident photons in the visible frequencies despite being atomically thin. It has been suggested that the strong absorption is due to the parallel band or 'band nesting' effect and corresponding divergence in the joint density of states. Here, we use photoluminescence excitation spectroscopy to show that the band nesting in mono- and bilayer MX2 (M=Mo, W and X=S, Se) results in excitation-dependent characteristic relaxation pathways of the photoexcited carriers. Our experimental and simulation results reveal that photoexcited electron-hole pairs in the nesting region spontaneously separate in k-space, relaxing towards immediate band extrema with opposite momentum. These effects imply that the loss of photocarriers due to direct exciton recombination is temporarily suppressed for excitation in resonance with band nesting. Our findings highlight the potential for efficient hot carrier collection using these materials as the absorbers in optoelectronic devices.
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http://dx.doi.org/10.1038/ncomms5543DOI Listing
July 2014

Two-dimensional array of particles originating from dipole-dipole interaction as evidenced by potential curve measurements at vertical oil/water interfaces.

Phys Chem Chem Phys 2014 Aug;16(32):16976-84

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.

We propose a new method to evaluate the interaction potential energy between the particles adsorbed at an oil/water interface as a function of interparticle distance. The method is based on the measurement of the interparticle distance at a vertical oil/water interface, at which the gravitational force is naturally applied to compress the particle monolayer in the in-plane direction. We verified the method by examining whether we obtained the same potential curve upon varying the gravitational acceleration by tilting the interface. The present method is applicable in the force range from ∼0.1 to ∼100 pN, determined by the effective weight of the particles at the interface. The method gives a rather simple procedure to estimate a long range interaction among the particles adsorbed at oil/water interfaces. We applied this method to polystyrene particles at the decane/aqueous surfactant solution interface, and obtained the interparticle potential curves. All the potential curves obtained by the present method indicated that the interparticle repulsion is due to the electrical dipole-dipole interaction based on the negative charge of the particles. The mechanism of the dipole-dipole interaction is further discussed on the basis of the effects of surfactants.
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http://dx.doi.org/10.1039/c4cp01710bDOI Listing
August 2014

Excitonic Photoluminescence from Nanodisc States in Graphene Oxides.

J Phys Chem Lett 2014 May 7;5(10):1754-9. Epub 2014 May 7.

†Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan.

The origin of near-infrared (NIR) luminescence from graphene oxide (GO) is investigated by photoluminescence (PL) excitation spectroscopy, time-resolved PL spectroscopy, and density functional theory based many body perturbation theories. The energy of experimentally observed NIR PL peak depends on the excitation energy, and the peak broadens with increasing excitation energy. It is found that the PL decay curves in time-resolved spectroscopy show build-up behavior at lower emission energies due to energy transfer between smaller to larger graphene nanodisc (GND) states embedded in GO. We demonstrate that the NIR PL originates from ensemble emission of GND states with a few nanometers in size. The theoretical calculations reveal the electronic and excitonic properties of individual GND states with various sizes, which accounts for the inhomogeneously broadened NIR PL. We further demonstrate that the electronic properties are highly sensitive to the protonation and deprotonation processes of GND states using both the experimental and theoretical approaches.
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http://dx.doi.org/10.1021/jz500516uDOI Listing
May 2014

Exploring the Origin of Blue and Ultraviolet Fluorescence in Graphene Oxide.

J Phys Chem Lett 2013 Jun 5;4(12):2035-40. Epub 2013 Jun 5.

†Institute of Advanced Energy, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan.

We studied the fluorescence (FL) properties of highly exfoliated graphene oxide (GO) in aqueous solution using continuous-wave and time-resolved FL spectroscopy. The FL spectra of highly exfoliated GO showed two distinct peaks at ∼440 (blue) and ∼300 nm [ultraviolet (UV)]. The FL of GO in the UV region at ∼300 nm was observed for the first time. The average FL lifetimes of the emission peaks at ∼440 and ∼300 nm are 8-13 and 6-8 ns, respectively. The experimentally observed peak wavelengths of pH-dependent FL, FL excitation spectra, and the FL lifetimes are nearly coincident with those of aromatic compounds bound with oxygen functional groups, which suggests that the FL comes from sp(2) fragments consisting of small numbers of aromatic rings with oxygen functional groups acting as FL centers in the GO.
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http://dx.doi.org/10.1021/jz400930fDOI Listing
June 2013