Akihiro Nagoya

Optically tunable amino-functionalized graphene quantum dots.

Amino-functionalized graphene quantum dots (af-GQDs) with discrete molecular weights and specific edges were self-limitedly extracted from oxidized graphene sheet. Their optical properties can be precisely controlled only by the selective and quantitative functionalization at the edge sites. The af-GQDS exhibit bright colorful fluorescence under a single-wavelength excitation.

First-principles study of Cu2ZnSnS4 and the related band offsets for photovoltaic applications

First-principles calculations of the band offsets between Cu(2)ZnSnS(4) (CZTS) and XS (X = Cd, Zn) are performed. While the interface dipole contribution for the band offsets is calculated using the Perdew-Burke-Ernzerhof functional, the Heyd-Scuseria-Ernzerhof hybrid functional is employed to introduce the quasiparticle corrections to the band offsets. The calculated conduction band offset between CZTS and CdS is 0.2 eV, validating CdS for the buffer layer of the CZTS solar cell. The small conduction band offset stems from the band gap narrowing of CdS under the interface strain caused by the lattice misfit with CZTS. A large valence band offset over 0.9 eV between CZTS and ZnS indicates that precipitated ZnS is regarded as an inactive insulator phase in CZTS absorbers.

Molecularly Designed, Nitrogen‐Functionalized Graphene Quantum Dots for Optoelectronic Devices

Nitrogen-functionalized graphene quantum dots (NGQDs) with tailorable optical properties are prepared by a versatile technique, which allows the highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels and energy gaps to be continuously varied. The integration of NGQD layers into the structures significantly improves the performance of optoelectronic devices.

Highly luminescent flexible amino-functionalized graphene quantum dots@cellulose nanofiber–clay hybrids for white-light emitting diodes

Amino-functionalized graphene quantum dots (afGQDs) made from graphene sheets glow bright with various colors under single-wavelength excitation, depending on their surface functional groups. These afGQDs were incorporated into a flexible and transparent clay host using electrostatic interactions between cellulose nanofiber (CNF) wrapped-afGQDs (CNF@afGQDs) and clay platelets. The resultant flexible afGQDs@CNF–clay films with a well-organized structure exhibited bright colorful photoluminescence, and were applied as a color converting material for blue light-emitting diodes (LEDs) to achieve white light emission. The af-GQDs@CNF–clay film based white LEDs showed pure white light emission with a CIE coordinate of (0.33, 0.37). The afGQDs@CNF–clay films with tunable emission are promising for use in future light emitting devices.

Graphene/nitrogen-functionalized graphene quantum dot hybrid broadband photodetectors with a buffer layer of boron nitride nanosheets

A high performance hybrid broadband photodetector with graphene/nitrogen-functionalized graphene quantum dots (NGQDs@GFET) is developed using boron nitride nanosheets (BN-NSs) as a buffer layer to facilitate the separation and transport of photoexcited carriers from the NGQD absorber. The NGQDs@GFET photodetector with the buffer layer of BN-NSs exhibits enhanced photoresponsivity and detectivity in the deep ultraviolet region of ca. 2.3 × 106 A W-1 and ca. 5.5 × 1013 Jones without the application of a backgate voltage. The high level of photoresponsivity persists into the near-infrared region (ca. 3.4 × 102 A W-1 and 8.0 × 109 Jones). In addition, application in flexible photodetectors is demonstrated by the construction of a structure on a polyethylene terephthalate (PET) substrate. We further show the feasibility of using our flexible photodetectors towards the practical application of infrared photoreflectors. Together with the potential application of flexible photodetectors and infrared photoreflectors, the proposed hybrid photodetectors have potential for use in future graphene-based optoelectronic devices.

Graphene Quantum Dots: Molecularly Designed, Nitrogen-Functionalized Graphene Quantum Dots for Optoelectronic Devices (Adv. Mater. 23/2016).

H. Tetsuka and co-workers develop a versatile technique to tune the energy levels and energy gaps of nitrogen-functionalized graphene quantum dots (NGQDs) continuously through molecular structure design, as described on page 4632. The incorporation of layers of NGQDs into the structures markedly improves the performance of optoelectronic devices.