Self-Assembled Size-Tunable Microlight-Emitting Diodes Using Multiple Sapphire Nanomembranes.

Microlight-emitting diode (Micro-LED) is the only display production technology capable of meeting the high-performance requirements of future screens. However, it has significant obstacles in commercialization due to etching loss and efficiency reduction caused by the singulation process, in addition to expensive costs and a significant amount of time spent on transfer. Herein, multiple-sapphire nanomembrane (MSNM) technology has been developed that enables the rapid transfer of arrays while producing micro-LEDs without the need for any singulation procedure. Individual micro-LEDs of tens of µm size were formed by the pendeo-epitaxy and coalescence of GaN grown on 2 µm width SNMs spaced with regular intervals. We have successfully fabricated micro-LEDs of different sizes including 20 × 20 µm2, 40 × 40 µm2, and 100 × 100 µm2, utilizing the membrane design. It was confirmed that the 100 × 100 µm2 micro-LED manufactured with MSNM technology not only relieved stress by 80.6% but also reduced threading dislocation density by 58.7% compared to the reference sample. It was proven that micro-LED arrays of varied chip sizes using MSNM were all transferred to the backplane. A vertical structure LED device could be fabricated using a 100 × 100 µm2 micro-LED chip, and it was confirmed to have a low operation voltage. Our work suggests that the development of the MSNM technology is promising for the commercialization of micro-LED technology.

Computational approaches for investigating interfacial adhesion phenomena of polyimide on silica glass.

This manuscript provides a comprehensive study of adhesion behavior and its governing mechanisms when polyimide undergoes various modes of detachment from silica glass. Within the framework of steered molecular dynamics, we develop three different adhesion measurement techniques: pulling, peeling, and sliding. Such computational methodologies can be applied to investigate heterogeneous materials with differing interfacial adhesion modes. Here, a novel hybrid potential involving a combination of the INTERFACE force field in conjunction with ReaxFF and including Coulombic and Lennard-Jones interactions is employed to study such interfaces. The studies indicate that the pulling test requires the largest force and the shortest distance to detachment as the interfacial area is separated instantaneously, while the peeling test is observed to exhibit the largest distance for detachment because it separates via line-by-line adhesion. Two kinds of polyimides, aromatic and aliphatic type, are considered to demonstrate the rigidity dependent adhesion properties. The aromatic polyimide, which is more rigid due to the stronger charge transfer complex between chains, requires a greater force but a smaller distance at detachment than the aliphatic polyimide for all of the three methodologies.

Atomic layer deposition of diisopropylaminosilane on WO3(001) and W(110): a density functional theory study.

The decomposition reactions of the Si precursor, diisopropylaminosilane (DIPAS), on W(110) and hydroxylated WO3(001) surfaces are investigated to elucidate the initial reaction mechanism of the atomic layer deposition (ALD) process using density functional theory (DFT) calculations combined with ab initio molecular dynamics (AIMD) simulations. The decomposition reaction of DIPAS on WO3(001) consists of two steps: Si-N dissociative chemisorption and decomposition of SiH3*. It is found that the Si-N bond cleavage of DIPAS is facile on WO3(001) due to hydrogen bonding between the surface OH group and the N atom of DIPAS. The rate-determining step of DIPAS decomposition on WO3(001) is found to be the Si-H dissociation reaction of the SiH3* reaction intermediate which has an activation barrier of 1.19 eV. On the contrary, sequential Si-H dissociation reactions first occur on W(110) and then the Si-N dissociation reaction of the C5H7NSi* reaction intermediate is found to be the rate-determining step, which has an activation barrier of 1.06 eV. As a result, the final products in the DIPAS decomposition reaction on WO3(001) are Si* and SiH*, whereas Si* atoms remain with carbon impurities on W(110), which imply that the hydroxylated WO3 surface is more efficient for the ALD process.

The prediction of hole mobility in organic semiconductors and its calibration based on the grain-boundary effect.

A new reliable computational model to predict the hole mobility of poly-crystalline organic semiconductors in thin films was developed. Site energy differences and transfer integrals in crystalline morphologies of organic molecules were obtained from quantum chemical calculations, in which periodic boundary conditions were efficiently applied to capture the interactions with the surrounding molecules in the crystalline organic layer. Then the parameters were employed in kinetic Monte Carlo (kMC) simulations to estimate the carrier mobility. Carrier transport in multiple directions has been considered in the kMC simulation to mimic poly-crystalline characteristics under thin-film conditions. Furthermore, the calculated mobility was corrected using a calibration equation based on microscopy images of the thin films to take the effect of grain boundaries into account. As a result, good agreement was observed between the predicted and measured hole mobility values for 21 molecular species: the coefficient of determination (R(2)) was estimated to be 0.83 and the mean absolute error was 1.32 cm(2) V(-1) s(-1). This numerical approach can be applied to any molecules for which crystal structures are available and will provide a rapid and precise way of predicting device performance.

Exciton Recombination, Energy-, and Charge Transfer in Single- and Multilayer Quantum-Dot Films on Silver Plasmonic Resonators.

We examine exciton recombination, energy-, and charge transfer in multilayer CdS/ZnS quantum dots (QDs) on silver plasmonic resonators using photoluminescence (PL) and excitation spectroscopy along with kinetic modeling and simulations. The exciton dynamics including all the processes are strongly affected by the separation distance between QDs and silver resonators, excitation wavelength, and QD film thickness. For a direct contact or very small distance, interfacial charge transfer and tunneling dominate over intrinsic radiative recombination and exciton energy transfer to surface plasmons (SPs), resulting in PL suppression. With increasing distance, however, tunneling diminishes dramatically, while long-range exciton-SP coupling takes place much faster (>6.5 ns) than intrinsic recombination (~200 ns) causing considerable PL enhancement. The exciton-SP coupling strength shows a strong dependence on excitation wavelengths, suggesting the state-specific dynamics of excitons and the down-conversion of surface plasmons involved. The overlayers as well as the bottom monolayer of QD multilayers exhibit significant PL enhancement mainly through long-range exciton-SP coupling. The overall emission behaviors from single- and multilayer QD films on silver resonators are described quantitatively by a photophysical kinetic model and simulations. The present experimental and simulation results provide important and useful design rules for QD-based light harvesting applications using the exciton-surface plasmon coupling.

Current driven magnetic damping in dipolar-coupled spin system.

Magnetic damping of the spin, the decay rate from the initial spin state to the final state, can be controlled by the spin transfer torque. Such an active control of damping has given birth to novel phenomena like the current-driven magnetization reversal and the steady spin precession. The spintronic devices based on such phenomena generally consist of two separate spin layers, i.e., free and pinned layers. Here we report that the dipolar coupling between the two layers, which has been considered to give only marginal effects on the current driven spin dynamics, actually has a serious impact on it. The damping of the coupled spin system was greatly enhanced at a specific field, which could not be understood if the spin dynamics in each layer was considered separately. Our results give a way to control the magnetic damping of the dipolar coupled spin system through the external magnetic field.

Surface ferromagnetic p-type ZnO nanowires through charge transfer doping.

We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.

Band gap opening by two-dimensional manifestation of peierls instability in graphene.

Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been usually attributed to quantum confinement effects, can be understood with the chiral symmetry breaking.

Passivation of metal surface states: microscopic origin for uniform monolayer graphene by low temperature chemical vapor deposition.

Scanning tunneling microscopy (STM) and density functional theory (DFT) calculations were used to investigate the surface morphology and electronic structure of graphene synthesized on Cu by low temperature chemical vapor deposition (CVD). Periodic line patterns originating from the arrangements of carbon atoms on the Cu surface passivate the interaction between metal substrate and graphene, resulting in flawless inherent graphene band structure in pristine graphene/Cu. The effective elimination of metal surface states by the passivation is expected to contribute to the growth of monolayer graphene on Cu, which yields highly enhanced uniformity on the wafer scale, making progress toward the commercial application of graphene.

Asymmetric doping in silicon nanostructures: the impact of surface dangling bonds.

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.