Creating the ultimate virtual reality display.

Scientists are exploring new material designs to make smaller and denser pixel displays.

Synthetic Topological Nodal Phase in Bilayer Resonant Gratings.

The notion of synthetic dimensions in artificial photonic systems has received considerable attention as it provides novel methods for exploring hypothetical topological phenomena as well as potential device applications. Here, we present nanophotonic manifestation of a two-dimensional topological nodal phase in bilayer resonant grating structures. Using the mathematical analogy between a topological semimetal and vertically asymmetric photonic lattices, we show that the interlayer shift simulates an extra momentum dimension for creating a two-dimensional topological nodal phase. We present a theoretical model and rigorous numerical analyses showing the two nodal points that produce a complex gapless band structure and localized edge states in the topologically nontrivial region. Therefore, our results provide a practical scheme for producing high-dimensional topological effects in simple low-dimensional photonic structures.

Metasurface-driven OLED displays beyond 10,000 pixels per inch.

Optical metasurfaces are starting to find their way into integrated devices, where they can enhance and control the emission, modulation, dynamic shaping, and detection of light waves. In this study, we show that the architecture of organic light-emitting diode (OLED) displays can be completely reenvisioned through the introduction of nanopatterned metasurface mirrors. In the resulting meta-OLED displays, different metasurface patterns define red, green, and blue pixels and ensure optimized extraction of these colors from organic, white light emitters. This new architecture facilitates the creation of devices at the ultrahigh pixel densities (>10,000 pixels per inch) required in emerging display applications (for instance, augmented reality) that use scalable nanoimprint lithography. The fabricated pixels also offer twice the luminescence efficiency and superior color purity relative to standard color-filtered white OLEDs.

Subwavelength pixelated CMOS color sensors based on anti-Hermitian metasurface.

The demand for essential pixel components with ever-decreasing size and enhanced performance is central to current optoelectronic applications, including imaging, sensing, photovoltaics and communications. The size of the pixels, however, are severely limited by the fundamental constraints of lightwave diffraction. Current development using transmissive filters and planar absorbing layers can shrink the pixel size, yet there are two major issues, optical and electrical crosstalk, that need to be addressed when the pixel dimension approaches wavelength scale. All these fundamental constraints preclude the continual reduction of pixel dimensions and enhanced performance. Here we demonstrate subwavelength scale color pixels in a CMOS compatible platform based on anti-Hermitian metasurfaces. In stark contrast to conventional pixels, spectral filtering is achieved through structural color rather than transmissive filters leading to simultaneously high color purity and quantum efficiency. As a result, this subwavelength anti-Hermitian metasurface sensor, over 28,000 pixels, is able to sort three colors over a 100 nm bandwidth in the visible regime, independently of the polarization of normally-incident light. Furthermore, the quantum yield approaches that of commercial silicon photodiodes, with a responsivity exceeding 0.25 A/W for each channel. Our demonstration opens a new door to sub-wavelength pixelated CMOS sensors and promises future high-performance optoelectronic systems.

Unraveling the Structural and Electronic Properties of Graphene/Ge(110).

The direct growth of graphene on a semiconducting substrate opens a new avenue for future graphene-based applications. Understanding the structural and electronic properties of the graphene on a semiconducting surface is key for realizing such structures; however, these properties are poorly understood thus far. Here, we provide insight into the structural and electronic properties of graphene grown directly on a Ge(110) substrate. Our scanning tunneling microscopy (STM) study reveals that overlaying graphene on Ge(110) promotes the formation of a new Ge surface reconstruction, i.e., a (6 × 2) superstructure, which has been never observed for a bare Ge(110) surface. The electronic properties of the system exhibit the characteristics of both graphene and Ge. The differential conductance (d I/d V) spectrum from a scanning tunneling spectroscopy (STS) study bears a parabolic structure, corresponding to a reduction in the graphene Fermi velocity, exhibiting additional peaks stemming from the p-orbitals of Ge. The density functional theory (DFT) calculations confirm the existence of surface states due to the p-orbitals of Ge.

Realization of continuous Zachariasen carbon monolayer.

Rapid progress in two-dimensional (2D) crystalline materials has recently enabled a range of device possibilities. These possibilities may be further expanded through the development of advanced 2D glass materials. Zachariasen carbon monolayer, a novel amorphous 2D carbon allotrope, was successfully synthesized on germanium surface. The one-atom-thick continuous amorphous layer, in which the in-plane carbon network was fully sp2-hybridized, was achieved at high temperatures (>900°C) and a controlled growth rate. We verified that the charge carriers within the Zachariasen carbon monolayer are strongly localized to display Anderson insulating behavior and a large negative magnetoresistance. This new 2D glass also exhibited a unique ability as an atom-thick interface layer, allowing the deposition of an atomically flat dielectric film. It can be adopted in conventional semiconductor and display processing or used in the fabrication of flexible devices consisting of thin inorganic layers.

Optical Gain in MoS2 via Coupling with Nanostructured Substrate: Fabry-Perot Interference and Plasmonic Excitation.

Despite the direct band gap of monolayer transition metal dichalcogenides (TMDs), their optical gain remains limited because of the poor light absorption in atomically thin, layered materials. Most approaches to improve the optical gain of TMDs mainly involve modulation of the active materials or multilayer stacking. Here, we report a method to enhance the optical absorption and emission in MoS2 simply through the design of a nanostructured substrate. The substrate consisted of a dielectric nanofilm spacer (TiO2) and metal film. The overall photoluminescence intensity from monolayer MoS2 on the nanostructured substrate was engineered based on the TiO2 thickness and amplified by Fabry-Perot interference. In addition, the neutral exciton emission was selectively amplified by plasmonic excitations from the local field originating from the surface roughness of the metal film with spacer thicknesses of less than 10 nm. We further demonstrate that the quality factor of the device can also be engineered by selecting a spacer material with a different refractive index.

Selective exfoliation of single-layer graphene from non-uniform graphene grown on Cu.

Graphene growth on a copper surface via metal-catalyzed chemical vapor deposition has several advantages in terms of providing high-quality graphene with the potential for scale-up, but the product is usually inhomogeneous due to the inability to control the graphene layer growth. The non-uniform regions strongly affect the reliability of the graphene in practical electronic applications. Herein, we report a novel graphene transfer method that allows for the selective exfoliation of single-layer graphene from non-uniform graphene grown on a Cu foil. Differences in the interlayer bonding energy are exploited to mechanically separate only the top single-layer graphene and transfer this to an arbitrary substrate. The dry-transferred single-layer grapheme showed electrical characteristics that were more uniform than those of graphene transferred using conventional wet-etching transfer steps.

Wafer-scale growth of single-crystal monolayer graphene on reusable hydrogen-terminated germanium.

The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.

Reliability enhancement of germanium nanowires using graphene as a protective layer: aspect of thermal stability.

We synthesized thermally stable graphene-covered Ge (Ge@G) nanowires and applied them in field emission devices. Vertically aligned Ge@G nanowires were prepared by sequential growth of the Ge nanowires and graphene shells in a single chamber. As a result of the thermal treatment experiments, Ge@G nanowires were much more stable than pure Ge nanowires, maintaining their shape at high temperatures up to 850 °C. In addition, field emission devices based on the Ge@G nanowires clearly exhibited enhanced thermal reliability. Moreover, field emission characteristics yielded the highest field enhancement factor (∼2298) yet reported for this type of device, and also had low turn-on voltage. Our proposed approach for the application of graphene as a protective layer for a semiconductor nanowire is an efficient way to enhance the thermal reliability of nanomaterials.

Large-scale fabrication of 2-D nanoporous graphene using a thin anodic aluminum oxide etching mask.

A large-scale nanoporous graphene (NPG) fabrication method via a thin anodic aluminum oxide (AAO) etching mask is presented in this paper. A thin AAO film is successfully transferred onto a hydrophobic graphene surface under no external force. The AAO film is completely stacked on the graphene due to the van der Waals force. The neck width of the NPG can be controlled ranging from 10 nm to 30 nm with different AAO pore widening times. Extension of the NPG structure is demonstrated on a centimeter scale up to 2 cm2. AAO and NPG structures are characterized using optical microscopy (OM), Raman spectroscopy and field-emission scanning electron microscopy (FE-SEM). A field effect transistor (FET) is realized by using NPG. Its electrical characteristics turn out to be different from that of pristine graphene, which is due to the periodic nanostructures. The proposed fabrication method could be adapted to a future graphene-based nano device.

Fabrication and RF characterization of a single nickel silicide nanowire for an interconnect.

We fabricated a nickel silicide nanowire (NiSi NW) device with a low thermal budget and characterized it by measuring the S-parameters in the radio-frequency (RF) regime. A single silicon nanowire (Si NW) was assembled on a substrate with a two-port coplanar waveguide structure using the dielectrophoresis method. Then, the Si NW on the device was perfectly transformed into a NiSi NW. The NiSi NW device was characterized by performing measurements in the DC and RF regimes. The transformation into the NiSi NW resulted in reducing about three-order more the resistance than before the transformation. Hence, the transmission of the NiSi NW device was 25 dB higher than that of the Si NW device up to gigahertz. We also discussed extracting the intrinsic properties of the NiSi NW by using de-embedding, circuit modeling, and simulation.

Temperature dependence on the grating formation in a low-Tg polymeric photorefractive composite.

We investigated a dependence of the grating formation on the temperature in polymeric photorefractive (PR) composite, in terms of magnitude and buildup speed of the PR grating. For polymeric PR materials, the temperature is one of the most important factors together with the external electric field because it is closely related on photocharge generation efficiency, mobility of generated carrier, electro-optic coefficient tensor, and so on. Above the glass transition temperature, the diffraction efficiency of degenerate four-wave mixing decreased with increasing the temperature; it can be explained with the magnitude of space-charge field and the electro-optic behavior at different temperatures. The space-charge field decreased linearly with increasing temperature due to a decrease in the photocharge generation efficiency and an increase in the hole detrapping by the high dark conductivity. Also as we expected, the PR grating buildup speed, which is strongly dependent on the photoconductivity, steeply decreased with increasing the temperature, and its tendency was similar to the temperature dependence of the phase shift.

Hierarchical organization of Au nanoparticles in a poly(vinyl carbazole) matrix for hybrid electronic devices.

We report a novel one-step method for the preparation of hierarchically patterned Au nanoparticles in a conducting polymer matrix by controlling the interface properties between Au nanoparticles and the conducting polymer matrix. The terminal group of capping molecules for the Au nanoparticles was modified to change the interface properties, not to change the size of the Au nanoparticles which affects their intrinsic properties. By modulating the interface properties, it is possible to construct Au nanoparticle-conducting polymer composites with two different structures: one presents a triple layer in which the conducting polymer layer is sandwiched between Au nanoparticle layers at the top and bottom; the other exhibits a form like a raisin cake in which Au nanoparticles are homogeneously organized in the conducting polymer matrix. High-resolution transmission electron microscopy was used to study the morphology and patterning of Au nanoparticles in the conducting polymer matrix.

Study on threshold behavior of operation voltage in metal filament-based polymer memory.

In the metal filament formation-based organic memory, the positive voltage application over the threshold electric field strength (170 MV/m) is necessary for the filament formation in Cu/P3HT/Al device. By the positive voltage application, the copper ions are generated and drifted into polymer layer, which is clearly confirmed by the secondary ion mass spectroscopy. Also, the field strength (100 MV/m) required for the drift process could be independently determined with a new pulse operation method. We could conclude that the threshold field strength of 170 MV/m was determined by the ionization process of copper. Furthermore, the dependence of the positive field strength and the temperature on the memory behavior was studied.

Metal filament growth in electrically conductive polymers for nonvolatile memory application.

Solution processable polymers that can reproducibly form metal filament by applying voltage are investigated for nonvolatile memory application. Up to present, the understanding of materials enabling to make the metal filament has not been well-documented and the vacuum deposition methods were dominantly used in device fabrication. After screening various polymers, we found that only the polymers having two functionalities, the presence of strongly coordinating heteroatom (S or N) with metal ions and the electrical conductivity, showed the reproducible filament formation behavior. Among the polymers screened, the regiorandom poly(3-hexylthiophene) showed the best switching endurance over 30,000 write-read-erase-read cycles without any switching failure.

Simple method for determining the gain coefficient of a photorefractive polymer film.

We propose a simple method for measuring the gain coefficient of two-beam coupling for photorefractive polymer film. To measure the gain coefficient, we attached a phase grating onto the photorefractive polymer film and made an interference pattern between the transmitted and diffracted beams in the photorefractive film by using an illuminating single beam. The gain coefficient values measured at various external fields by our method showed good agreement with those measured by a standard two-beam coupling method. Compared to standard two-beam coupling, this method exhibited fairly improved stability of the output signal to external factors such as human sounds and airstreams.

Dependence of the Bragg condition on an external electric field for a polymeric photorefractive material.

We investigated the effect of an applied electric field on the Bragg condition of degenerate four-wave mixing a polymeric photorefractive material with a low glass-transition temperatue. For a polymeric photorefractive material the application of an external electric field is necessary for photorefractivity leads to birefringence of the material by poling of the nonlinear optical chromophore. Because the propagation vectors of the pumping and reading beams inside the material are influenced by the refractive index of the material, the Bragg condition depends on the magnitude of the external field. Using an oriented gas model and the-coupled-mode theory, we numerically analyzed the Bragg-mismatch effect that causes a reduction in diffraction efficiency as a function of an external field. We present the boundary conditions for sample thickness and grating spacing for which the Bragg-mismatch effect must be taken into account.