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Owing to the recent developments of dielectric metasurfaces, their applications have been expanding from those pertaining to the thickness shrinkage of passive optical elements, such as lenses, polarizers, and quarter-wave plates, to applications pertaining to their integration with active optical devices, such as vertical-cavity surface-emitting lasers. Even though directional lasing and beam shaping of laser emission have been successfully demonstrated, the integration of metasurfaces with random light sources, such as light-emitting diodes, is limited because of function and efficiency issues attributed to the fact that metasurfaces are basically based on the resonance property of the nanostructure. To control the direction of emission from colloidal quantum dots, we present a dielectric metasurface deflector composed of two asymmetric TiO2 nanoposts. TiO2 deflector arrays were fabricated with a dry etching method that is adaptive to mass production and integrated with a colloidal quantum dot resonant cavity formed by sandwiching two distributed Bragg reflectors. To ensure the deflection ability of the fabricated sample, we measured the photoluminescence and far-field patterns of emission from the resonant cavity. From the obtained results, we demonstrated that the colloidal quantum dot emission transmitted through our deflector arrays was deflected by 18°, and the efficiency of deflection was 71% with respect to the emission from the resonant cavity. This integration of dielectric metasurfaces with a resonant cavity shows the possibility of expanding the application of visible metasurfaces in active devices and may help to develop next-generation active devices with novel functions.

The surface of colloidal‐type quantum dots (QDs) is protected with surfactants to maintain their nanoscale size and dispersion in various solvents. Such protection is advantageous to the processability of the dots, but the surfactants also act as insulators, thereby adversely affecting the electrical properties of the resulting devices. Anion‐type molecular metal chalcogenide complexes (MCCs) have been developed as an alternative to these surfactants. However, anionic ligands limit the charges of the resulting QDs to negative values; thus, additional processes are required to compensate for these charges. Herein, this work prepares cationic‐type dimetal diselenium perchlorate MCCs (M2Se2(ClO4)2, M = Mn, Zn, and Sn) and applies them to stabilize colloidal QDs. The resulting M2Se2‐PbS QDs are then used to fabricate near‐infrared photodetectors (NIR‐PDs). The devices exhibit remarkably suppressed dark current densities (JD) of as low as 4.92 × 10−10 A cm−2 when compared with PDs based on 1,2 ethandithiol‐PbS (EDT‐PbS) (JD = 1.41 × 10−8 A cm−2). In addition, the devices exhibit a high responsivity of 0.07 A W−1 in response to 950 nm NIR light signals, as well as an excellent specific detectivity of 5.66 × 1012 Jones at 950 nm. Finally, this work fabricates hybrid‐type QD films using cationic and anionic MCC‐PbS QDs and obtains a high external quantum efficiency (EQE) of 13.1%. [ABSTRACT FROM AUTHOR]

Amorphous oxide-based thin-film logic circuits have been fabricated using only an n-type amorphous silicon–zinc–tin–oxide (a-SZTO) channel layer with different source/drain electrodes. Enhancement-mode thin-film transistors (TFTs) were fabricated with oxide electrodes. Depletion-mode TFTs were fabricated with metal electrode. Work functions were measured by Kelvin probemicroscopy. The barrier heights ($\Phi _{\textsf {B}}$) between the a-SZTO channel layer and the electrode were measured to be 1.831, 2.341, and 2.339 eV for the Ti/Al electrode, Indium–silicon–oxide (ISO) electrode, and indium–tin–oxide (ITO) electrode, respectively. The physicalmechanism on the variation of sheet and contact resistances was investigated using a transmission line method, and the change in the resistances is closely related to $\Phi _{\textsf {B}}$. Inverters were fabricated, with different $V_{\textsf {TH}}$ values adopted simply by using different contact characteristics between various electrodes and the semiconductor channel. High values of voltage gains in inverters were obtained: 12.33 (ISO) and 11.75 (ITO) at $V_{\textsf {DD}}= 5$ V. Itwas also confirmed that more complicated n-type-based NAND and NOR thin-film circuits, implementedwith different electrodes, functioned as conventional logic circuits. This simple fabrication method of thin-film logic circuits opens the possibility of implementing next-generation stacked integrated circuit technology. [ABSTRACT FROM AUTHOR]

A method of wet‐transferring a densely packed colloidal quantum dot (CQD) film is reported herein. Layers of poly(vinyl alcohol) (sacrificial layer) and poly(methyl methacrylate) (temporary mechanical support) are spin‐coated below and above the CQD film, respectively; the two layers are eventually removed, resulting in a single CQD film on an alien substrate. CQD films as large as 1 cm × 1 cm are transferred to any substrate, including flexible and prepatterned ones, with very little mechanical or optical damage. The wet‐transfer method can be repeated to form a CQD heterostructure. A CQD distributed feedback (DFB) laser is also demonstrated by transferring a thick CdSe‐based CQD layer (for both optical gain and waveguiding) on top of a surface grating structure. The resultant CQD structure exhibits a single‐mode DFB lasing action, with a high spontaneous emission factor (β ≈ 10−3), owing to a strong vertical mode confinement and smooth surface morphologies on both sides of the CQD waveguide layer. [ABSTRACT FROM AUTHOR]

In this study, N-type amorphous indium zinc oxide thin-film transistors are fabricated and temperature-dependent electrical characteristics in the range of 170–295 K are analyzed through experimental measurements and using an equivalent-circuit model. In this model, thermionic field emission for reverse bias and a thermionic emission mechanism for forward bias are applied. The barrier height coefficient of a contact region between the channel and the Ti/Au electrode is 1.26 meV/K, and the resistance of the channel material decreases at a rate of - 0.39 \Omega \cdot \mathrm K^-1 at various temperatures. The obtained energy level is experimentally confirmed through a Kelvin probe measurement. In addition, the simulation results of the channel resistance successfully describe the Arrhenius behavior of the drain current and the Mott variable-range hopping conduction mechanism in a low-temperature regime below 230 K. [ABSTRACT FROM PUBLISHER]

In this study, large area mono-, bi-, and multi-layer graphenes (MLG) were synthesized using low pressure chemical vapor deposition (LPCVD) process and co-planar waveguides (CPW) with graphenes being fabricated using conventional photolithography. The intrinsic room temperature radio frequency (RF) characteristics were measured via direct contact to the graphene surface with a probe-tip in the range of 0.1 GHz to 40 GHz without using metal electrodes. The device characteristics were also evaluated by physics-based RLC equivalent circuit simulation. The proposed equivalent circuit model reproduced all of the measured characteristics within 3% of RMSE and we numerically extracted the set of component values which minimize the error between the measured data and the simulation results over the measured regime. Our findings demonstrate that MLG is an effective candidate in RF interconnect device applications. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Recently, achieving flexible and highly efficient light‐emitting elements is the most noticeable demand for lighting or displays. Here, fully flexible gallium nitride (GaN) light‐emitting diodes (LEDs) are demonstrated based on a unique transfer method. The LED structure consisting of GaN pyramid arrays are first fabricated on an amorphous glass‐based template with a low‐temperature gallium nitride/titanium (LT‐GaN/Ti) hetero‐interface, then released and embedded into a flexible or stretchable substrate using a specialized interface control. Nanovoids created during thermal annealing render the hetero‐interface weaker than the other interfaces. This interface is further weakened by a post‐mechanical treatment for gentle release of the GaN pyramid arrays from the interface during a transfer process. The LEDs typically have a total thickness of }70 lm and exhibit stable surface‐emitting electroluminescence even at a bending radius of }2 mm with exceptionally high luminance values of 595 and 175 cd/m2 at peak wavelengths of 514 and 483 nm, respectively. The results suggest a route to high brightness, large, flexible/stretchable blue or green lighting or displays. [ABSTRACT FROM AUTHOR]

Single-crystal PbSe nanowires are synthesized in solution through oriented attachment of nanocrystal building blocks. Reaction temperatures of 190-250 °C and multicomponent surlactant mixtures result in a nearly defect-free crystal lattice and high uniformity of nanowire diameter along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from ∼4 to ∼20 nm in diameter with wire lengths up to ∼30 μm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant molecules present in the reaction solution. While PbSe nanocrystals have the centrosymmetric rocksalt lattice, they can lack central symmetry due to a noncentrosymmetric arrangement of Pb- and Se-terminated {111} facets and possess dipole driving one-dimensional oriented attachment of nanocrystals to form nanowires. In addition to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepared in one-pot reactions by careful adjustment of the reaction conditions. [ABSTRACT FROM AUTHOR]

Photoinduced electron transport processes in hetero-Langmuir–Blodgett (LB) films composed of electron sensitizer (S), relay (R) and acceptor (A) molecules were investigated by femtosecond transient absorption technique. 7,8-Dimethyl-10-dodecyl isoalloxazine (flavin), N-allyl-N′-[3-propylamido-N″,N″-di(n-octadecyl)]-4,4′-bipyridium dibromide (viologen) and N-docosilquinolinium tetracyanoquinodimethane (TCNQ) were employed as S, R and A units, respectively. Based on the time-resolved transient absorption spectra of S/R/A series hetero-LB films with different numbers of R layers, the optimum number of R layers for efficient photoinduced charge transport from S to A layers via R was determined to be two. Over two layers of R molecules in the S/R/A series LB films, the charge transport rate became slower due to an increase in the back electron transport from R to A layers. S(30 layers)/A(31 layers) (S(30)/A(31)) hetero-LB film exhibits nearly the same charge transport rate as S(30)/R(2)/A(31) hetero-LB film. But the charge recombination from A to S occurred in S(30)/A(31) hetero-LB film. In the S(30)/A(31) hetero-LB film, charge separated species were directly monitored in the transient absorption spectra. At 660 nm, a long decay component become manifest due to the contribution of bulk (TCNQ)2− species. At 740 and 840 nm, on the other hand, a slow rise and long-time decay components were observed due to the contribution of TCNQ− monomer existing at the interface between S and A layers. [Copyright &y& Elsevier]

Presents information on a study which aims to find a model that allows the design of a linear quadratic Gaussian (LQG) compensator that is a robust in the sense of improved sensitivity and stability margin. Problem formulation; Spatial discretization; Control strategy.

Local microheating of amorphous LaB6 film could control the degree of crystallization as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. With full crystallization of the LaB6, we achieved micrometer-sized thermionic electron emission source with the maximum current density of 1.2 A/cm^2. The advantage of fabricating a micrometer-sized emitter with high current density enables versatile applications such as compact X-ray or vacuum type terahertz radiation sources. A new structure of micrometer-sized, lateral-type vacuum channel transistor is proposed based on the simulation. The calculated electron travelling time from emitter-to-collector was 8.3 ps at the traveling distance of \sim10~\mum, meaning that the maximum operation frequency is 120 GHz. [ABSTRACT FROM AUTHOR]

As a continuing effort to improve the performance of photonic crystal (PhC) phosphors—an engineered nanophotonic phosphor structure developed by the authors' group—hereby 2D PhC phosphor films are introduced. In particular, the excitation polarization dependence inherent to and suffered by the precedent 1D PhC phosphors can be eliminated using a square lattice PhC structure due to its fourfold rotational symmetry. In addition, the thickness of the high index PhC backbone layer is intentionally increased so that dual excitation resonances occur for both the transverse‐electric (TE) and transverse‐magnetic (TM) waveguide modes, thereby effectively doubling the phosphor efficiency. 2D PhC phosphor structures are implemented and examined using colloidal quantum dots (CQDs) as phosphor material. Unlike conventional impurity‐doped phosphor materials composed of micron‐sized grains, nanometer‐sized CQDs are structurally compatible with PhCs whose feature sizes are ≈100 nm. Excitation polarization–insensitive CQD fluorescence is observed approx. four times enhanced at dual resonance wavelengths for the TE‐ and TM‐guided modes. The validity of the observed results is confirmed through simulations. [ABSTRACT FROM AUTHOR]

We investigated the band gap of SiZnSnO (SZTO) with different Si contents. Band gap engineering of SZTO is explained by the evolution of the electronic structure, such as changes in the band edge states and band gap. Using ultraviolet photoelectron spectroscopy (UPS), it was verified that Si atoms can modify the band gap of SZTO thin films. Carrier generation originating from oxygen vacancies can modify the band-gap states of oxide films with the addition of Si. Since it is not easy to directly derive changes in the band gap states of amorphous oxide semiconductors, no reports of the relationship between the Fermi energy level of oxide semiconductor and the device stability of oxide thin film transistors (TFTs) have been presented. The addition of Si can reduce the total density of trap states and change the band-gap properties. When 0.5 wt% Si was used to fabricate SZTO TFTs, they showed superior stability under negative bias temperature stress. We derived the band gap and Fermi energy level directly using data from UPS, Kelvin probe, and high-resolution electron energy loss spectroscopy analyses. [ABSTRACT FROM AUTHOR]

The patterning of colloidal quantum dots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing and microcontact printing of quantum dots have been recently developed. Herein, we present a simple method of patterning colloidal quantum dots for photonic nanostructures such as straight lines, rings and dot patterns either on transparent or metallic substrates. Sub-10 nm width of the patterned line could be achieved with a well-defined sidewall profile. Using this method, we demonstrate a surface plasmon launcher from a quantum dot cluster in the visible spectrum. [ABSTRACT FROM AUTHOR]

By use of electrostatic force microscopy, we measure the charge and polarizability of 12-nm PbSe nanocrystals on n- and p-type silicon with a 2-nm thermal oxide layer. Individual nanocrystals show a dielectric constant of >100. In ambient light, the nanocrystals generate static electric fields of magnitudes too weak to be caused by a full elementary charge. These nanocrystals are statically polarized by surface electric fields generated by fixed charges in the oxide substrate. We model this effect quantitatively and assign charge locations in the oxide. Upon 442-nm photoexcitation, we observe some of the nanocrystals (∼35%) photoionize and slowly relax overnight back to their initial states. Just above a charge in the oxide, the surface electric field can approach 108 V/cm. [ABSTRACT FROM AUTHOR]

The continuous and concerted development of colloidal quantum dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect that persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and nonradiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired nonradiative decay, potentially in a variety of luminescent materials for future device applications.

The band gap properties of amorphous SiInZnO (a-SIZO) thin-film transistors (TFTs) with different Si concentrations have been studied. The electronic structures of the films, engineered by controlling the Si content, have been investigated through the changes of the band gap and band edge states. Carrier generation at oxygen vacancies can modify the band gap states of oxide thin films. Si suppresses the number of oxygen vacancies-which are carrier generation sites-so shifts the Fermi energy level away from the conduction band. It is difficult to derive the electronic structures of amorphous oxide semiconductors by electrical measurements. Thus, we used a combination of ultraviolet photoelectron spectroscopy, Kelvin probe measurements, and electron energy loss spectroscopy to measure the band gap and electrical performance variations of SIZO TFTs with Si doping. To verify the versatility of Si doping in modulating electronic properties, high-performance depletion-mode inverter circuits consisting of 0.1 to 0.3 wt% Si-doped a-SIZO TFTs were fabricated. These inverter models operate through the threshold voltage difference that arises from the different Si contents. High voltage gains of ~20.62 at a supply voltage of 15 V were obtained with the two TFTs, with a strong dependence on the subthreshold swing.

The ability of metasurfaces to manipulate light at the subwavelength scale offers unprecedented functionalities for passive and active lasing devices. However, applications of metasurfaces to optical devices are rare due to fabrication difficulties. Here, we present quantum dot light emitting diodes (QDLEDs) with a metasurface-integrated metal electrode and demonstrate microscopically controlled LED emission. By incorporating slot-groove antennas into the metal electrode, we show that LED emission from randomly polarized QD sources can be polarized and directed at will. Utilizing the relation between polarization and emission direction, we also demonstrate microscopic LED beam splitting through the selective choice of polarization.