Multicolor single InGaN/GaN dot-in-nanowire light emitting diodes (LEDs) were fabricated on the same substrate using selective area epitaxy. It is observed that the structural and optical properties of InGaN/GaN quantum dots depend critically on nanowire diameters. Photoluminescence emission of single InGaN/GaN dot-in-nanowire structures exhibits a consistent blueshift with increasing nanowire diameter. This is explained by the significantly enhanced indium (In) incorporation for nanowires with small diameters, due to the more dominant contribution for In incorporation from the lateral diffusion of In adatoms. Single InGaN/GaN nanowire LEDs with emission wavelengths across nearly the entire visible spectral were demonstrated on a single chip by varying the nanowire diameters. Such nanowire LEDs also exhibit superior electrical performance, with a turn-on voltage ∼2 V and negligible leakage current under reverse bias. The monolithic integration of full-color LEDs on a single chip, coupled with the capacity to tune light emission characteristics at the single nanowire level, provides an unprecedented approach to realize ultrasmall and efficient projection display, smart lighting, and on-chip spectrometer.

To date, there have been no efficient semiconductor light emitters operating in the green and amber wavelengths. This study reports on the synthesis of InGaN nanowire photonic crystals, including dot-in-nanowires, nanotriangles, and nanorectangles with precisely controlled size, spacing, and morphology, and further demonstrates that bottom-up InGaN photonic crystals can exhibit highly efficient and stable emission. The formation of stable and scalable band edge modes in defect-free InGaN nanowire photonic crystals is directly measured by cathodoluminescence studies. The luminescence emission, in terms of both the peak position (λ ≈ 505 nm) and spectral linewidths (full-width-half-maximum ≈ 12 nm), remains virtually invariant in the temperature range of 5-300 K and under excitation densities of 29 W cm −2 to 17.5 kW cm −2 . To the best of our knowledge, this is the first demonstration of the absence of Varshni and quantum-confined Stark effects in wurtzite InGaN light emitters-factors that contribute significantly to the efficiency droop and device instability under high-power operation. Such distinct emission properties of InGaN photonic crystals stem directly from the strong Purcell effect, due to efficient coupling of the spontaneous emission to the highly stable and scalable band-edge modes of InGaN photonic crystals, and are ideally suited for uncooled, high-efficiency light-emitting-diode operation.

Nanophotonics capable of directing radiation or enhancing quantum-emitter transition rates rely on plasmonic nanoantennas. We present here a novel Babinet-inverted magnetic-dipole-fed multislot optical Yagi-Uda antenna that exhibits highly unidirectional radiation to free space, achieved by engineering the relative phase of the interacting surface plasmon polaritons between the slot elements. The unique features of this nanoantenna can be harnessed for realizing energy transfer from one waveguide to another by working as a future "optical via".

The photoresponse in graphene has drawn significant attention for potential applications owing to its gapless linear electronic band structure. To enhance both the spectral selectivity and responsivity in graphene, we demonstrate a novel but versatile and simple method of introducing surface plasmons. We utilize block copolymers to fabricate different nanostructured metal nanoparticle arrays on a single graphene surface. The plasmonic resonances could be tuned using Ag, Au, and Cu metal nanoparticles. By extending the synthetic route for the metallic particles, dual surface plasmonic bands from a single material were also successfully realized. Furthermore, enhanced photoresponsivity through the entire visible spectra could be achieved by mixing metallic nanoparticles and by controlling their shapes. Owing to its all-band transition characteristics, the ultrabroad band photocurrent generation in graphene can be tailored for an arbitrary photoresponse, which could be utilized in flexible CMOS image sensors (CIS) or other optoelectronic devices in the future. G raphene, a two-dimensional network of carbon atoms in a honeycomb lattice, has been intensively studied for the past decade owing to its unique optical and electronic properties in both fundamental science and applications. 1−6 Numerous studies have proven that the linear, gapless band structure of graphene surpasses the performance of currently available semiconductor-based electronic and optical devices on the market; in other words, unlike other superconductors, the ultrahigh mobility 1,7 and ultrabroad-band photoresponse originated by the all-band transition in graphene can be substantially modified through electrical gating. 8,9 Photocurrent generation in graphene has been observed at the graphene/metallic electrode junction, 10−12 at the p-/njunction formed on graphene by top gating, 12−14 and at the interface of graphene and various materials in the heterostructure. 15−24 Graphene-based interband photodetectors have been demonstrated from the ultraviolet, visible to mid-infrared (mid-IR) range, covering all optical communication bands. 16 Photocurrent at the metal−graphene junction and graphene p− n junctions has been described as either photovoltaic 10,12 or thermoelectric. 25 However, it is challenging to identify photovoltaic and thermoelectric currents in metal−graphene or graphene p−n junctions due to its identical polarity. Recently, in biased graphene, the thermoelectric effects are insignificant, but the photovoltaic and photoinduced bolometric effects dominate the photoresponse. 12 The property of the linearly dispersive and zero band gap, however, leads to a low density of states and no spectral selectivity. Therefore, there have been efforts to improve the low responsivity of graphene-based photodetectors by enhancing the light absorption with the aid of localized surface plasmon enhancement 26−28 or by near-field coupling with guided waveguides. 20−22 Recently, the responsivity was significantly increased by 3 orders of magnitude over that...

We investigated systematic modulation of the Dirac point voltage of graphene transistors by changing the type of ionic liquid used as a main gate dielectric component. Ion gels were formed from ionic liquids and a non-triblock-copolymer-based binder involving UV irradiation. With a fixed cation (anion), the Dirac point voltage shifted to a higher voltage as the size of anion (cation) increased. Mechanisms for modulation of the Dirac point voltage of graphene transistors by designing ionic liquids were fully understood using molecular dynamics simulations, which excellently matched our experimental results. It was found that the ion sizes and molecular structures play an essential role in the modulation of the Dirac point voltage of the graphene. Through control of the position of their Dirac point voltages on the basis of our findings, complementary metal-oxide-semiconductor (CMOS)-like graphene-based inverters using two different ionic liquids worked perfectly even at a very low source voltage (V(DD) = 1 mV), which was not possible for previous works. These results can be broadly applied in the development of low-power-consumption, flexible/stretchable, CMOS-like graphene-based electronic devices in the future.