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We report on morphology-controlled remote epitaxy via hydrothermal growth of ZnO micro- and nanostructure crystals on graphene-coated GaN substrate. The morphology control is achieved to grow diverse morphologies of ZnO from nanowire to microdisk by changing additives of wet chemical solution at a fixed nutrient concentration. Although the growth of ZnO is carried out on poly-domain graphene-coated GaN substrate, the direction of hexagonal sidewall facet of ZnO is homogeneous over the whole ZnO-grown area on graphene/GaN because of strong remote epitaxial relation between ZnO and GaN across graphene. Atomic-resolution transmission electron microscopy corroborates the remote epitaxial relation. The non-covalent interface is applied to mechanically lift off the overlayer of ZnO crystals via a thermal release tape. The mechanism of facet-selective morphology control of ZnO is discussed in terms of electrostatic interaction between nutrient solution and facet surface passivated with functional groups derived from the chemical additives.

We report on the electroluminescent (EL) and electrical characteristics of graphene-inserted ZnO nanorods (NRs)/p-GaN heterojunction diode. In a comparative study, ZnO NRs/p-GaN and ZnO NRs/graphene/p-GaN heterojunctions exhibit white and yellow EL emissions, respectively, at reverse bias (rb) voltages. The different EL colors are results of different dichromatic EL peak intensity ratios between 2.25 and 2.8 eV light emissions which are originated from ZnO and p-GaN sides, respectively. The 2.25 eV EL is predominant in both the heterojunctions, because of recombination by numerous electrons tunneled from p-GaN to ZnO across the thin barriers of the staggered broken gap with a large band offset in ZnO/p-GaN and the van der Waals (vdW) gap formed by graphene insertion at ZnO NRs/p-GaN. However, as for the 2.8 eV EL intensity, ZnO NRs/graphene/p-GaN hardly shows the EL emission, whereas ZnO NRs/p-GaN exhibits the substantially strong EL peak. We discuss that the significantly reduced 2.8 eV EL emission of ZnO NRs/graphene/p-GaN is a result of decreased depletion layer thickness at p-GaN side where the recombination events occur for 2.8 eV EL before the reverse bias-driven tunneling because the insertion of graphene (or vdW gap barrier) inhibits the carrier diffusion whose amount determines the depletion thickness when forming the heterojunctions. This study opens a way of suppressing (or enhancing) the specific EL wavelength for the dichromatic EL-emitting heterojunctions simply by inserting atom-thick vdW layer.

Biocompatible materials are expected to play important roles in biomedical applications. In particular, dipeptide nanostructures have attracted considerable attention because of their biocompatibility, functional molecular recognition, and unique biological and electronic properties. Nevertheless, a lack of mass production and facile fabrication of the stable peptide nanostructure array is one of the major hurdles holding back the practical applications. This study developed a simple and wafer-scale process to produce a cyclo-diphenylalanine (Cyclo-FF) nanowire array via a thermal evaporation process and fabricated Cyclo-FF-based triboelectric energy generators (TEG) with high performance. This approach allowed control of the dimensions of the Cyclo-FF nanowires and the preparation of stable peptide nanostructures under ambient conditions and moisture. The Cyclo-FF nanowires were employed as triboelectric materials for biocompatible nanogenerators, which exhibited high-performance power outputs for future applications. The voltage and current output reached ~ 350 V and ~ 10 μA, respectively, with a surface treatment, which was sufficient to light up over a hundred LEDs. Even after dipping Cyclo-FF nanowires in water or TBS buffer solution, it still worked as a TEG material with minor degradation. This method is compatible for the synthesis of bionanomaterials over a large area, and thus can be a powerful strategy to fabricate high-performance biocompatible energy devices for a range of practical applications in the future.

Two-dimensional atomic layered materials (2d-ALMs) are emerging candidates for use as epitaxial seed substrates for transferrable epilayers. However, the micrometer-sized domains of 2d-ALMs preclude their practical use in epitaxy because they cause crystallographically in-plane disordering of the overlayer. Ultrathin graphene can penetrate the electric dipole momentum from an underlying crystal layer to the graphene surface, which then drives it to crystallize the overlayer during the initial growth stage, thus resulting in substantial energy saving. This study demonstrates the remote homoepitaxy of ZnO microrods (MRs) on ZnO substrates across graphene layers via a hydrothermal method. Despite the presence of poly-domain graphene in between the ZnO substrate and ZnO MRs, the MRs were epitaxially grown on a- and c-plane ZnO substrates, whose in-plane alignments were homogeneous within the wafer's size. Transmission electron microscopy revealed a homoepitaxial relationship between the overlayer MRs and the substrate. Density-functional theory calculations suggested that the charge redistribution occurring near graphene induces the electric dipole formation, so the attracted adatoms led to the formation of the remote homoepitaxial overlayer. Due to a strong potential field caused by long-range charge transfer given from the substrate, even the use of bi-layer and tri-layer graphene resulted in remote homoepitaxial ZnO MRs. The effects of substrate crystal planes were also theoretically and empirically investigated. The ability of graphene, which can be released from the mother substrate without covalent bonds, was utilized to transfer the overlayer MR arrays. This method opens a way for producing well aligned, transferrable epitaxial nano/microstructure arrays while regenerating the substrate for cost-saving device manufacturing.

The remote epitaxy of GaN p–n homojunction microcrystals (μCs) is demonstrated for fabricating transferable, flexible white light-emitting diodes (WLEDs). The GaN p–n junction μCs are randomly grown on graphene-coated Al2O3(0001), which are then delaminated for mass-transfer onto conducting copper tape. The μCs-LED shows electrical rectification and white electroluminescence (EL) emission. The μCs-WLED exhibits reliable LED performances after repetitive bending deformations and cycling temperature environments. Based on the transferability, the μCs-WLEDs are patterned and assembled to matrix arrays, which exhibit homogeneous, reliable performances even at a bent form. We discuss that the origin of white EL emission is mixing of blue and yellow–red EL emissions from p-GaN and n-GaN sides, respectively, based on photoluminescence spectroscopic measurements. This study opens a way of fabricating the transferable, flexible, and modular light panels through remote epitaxy.

Remote epitaxy enables to fabricate flexible GaN micro-LED sticker releasable from wafer., There have been rapidly increasing demands for flexible lighting apparatus, and micrometer-scale light-emitting diodes (LEDs) are regarded as one of the promising lighting sources for deformable device applications. Herein, we demonstrate a method of creating a deformable LED, based on remote heteroepitaxy of GaN microrod (MR) p-n junction arrays on c-Al2O3 wafer across graphene. The use of graphene allows the transfer of MR LED arrays onto a copper plate, and spatially separate MR arrays offer ideal device geometry suitable for deformable LED in various shapes without serious device performance degradation. Moreover, remote heteroepitaxy also allows the wafer to be reused, allowing reproducible production of MR LEDs using a single substrate without noticeable device degradation. The remote heteroepitaxial relation is determined by high-resolution scanning transmission electron microscopy, and the density functional theory simulations clarify how the remote heteroepitaxy is made possible through graphene.

Two-dimensional nanomaterials are emerging for use as seed substrates toward transferrable, transparent, or flexible device applications.[1,2] In the van der Waals heteroepitaxy, graphene acted as seed for epitaxial overlayers of III–V or II–VI nanostructures or thin films.[3-5] Because the hydrophobic surface of graphene, given from lack of dangling bonds, substantially hinders the high-density nucleation in heteroepitaxy,[3] the use of graphene substrates was precluded for practical use as a substrate in the semiconductor epitaxy. Such surface properties of graphene have been known to be changeable by contacting other substances underneath graphene layer, because of the wetting transparency of graphene. More recently, the chemical property transparency was applied to the epitaxy technique to obtain single crystalline thin film overlayer whose crystal structure and orientation can be copied from underlying substrate across graphene. This is the so-called remote epitaxy, which was firstly demonstrated by Kim et al.[6,7] Here, we report on remote homoepitaxy of ZnO microrods on graphene-coated ZnO substrates with different a- and c-planes. The remote epitaxy was carried out via a hydrothermal growth using a nutrient solution with zinc nitrate hexahydrate and hexamethylenetetramine at 95 °C. For preparation of a- and c-plane ZnO substrate, ZnO thin films were heteroepitaxially coated on r-plane Al2O3 and c-plane GaN, respectively. Then, monolayer graphene, which was synthesized via a chemical vapor deposition (CVD) on copper foil, was transferred onto these substrates using a poly(methyl-methacrylate)-supported etching-and-transfer technique. ZnO epitaxial overlayers grew to be horizontally and vertically aligned microrod arrays on a-plane and c-plane ZnO substrates, respectively, across graphene layers, which were also found to show a well-defined long-range in-plane orientation over the entire surface of graphene-coated ZnO substrate, despite the presence of CVD-grown graphene with typical domain size of several micrometers. Cross-sectional transmission electron microscopic and selected area electron diffraction analyses revealed the homoepitaxial relationship with a remote gap of graphene. We further studied the successful remote homoepitaxy of ZnO across bilayer- and trilayer graphene, on which lower density of horizontal- and vertical ZnO microrods were obtained in comparison with that on monolayer graphene-coated substrate. This result strongly suggests that ZnO substrates played a crucial role in determining the crystallographic orientation and lattice arrangement of ZnO microrod overlayer across graphene layer, but as increasing the thickness of graphene, the impact of substrate is lesser for nucleation and growth of ZnO overlayer in the remote homoepitaxy. Such finding that electric field given from the substrate can be penetrated across graphene was simulated via density functional theory (DFT) calculations. The DFT calculations also indicated that the field penetration strength decreases with the increase of graphene thickness. The ability of graphene, which can be released from the host substrate without covalent bonds, was exploited to transfer the microrod arrays via a thermal release tape exfoliation technique. After the exfoliation, the host substrate was reused for repeating the remote epitaxy over again. This study opens a way for producing well aligned, transferrable epitaxial semiconductor microrod arrays while regenerating the substrate for cost-saving device manufacturing process. [1] K. Chung, C. H. Lee and G. C. Yi, Science, 2010, 330, 655–657. [2] C. H. Lee, Y. J. Kim, Y. J. Hong, S. R. Jeon, S. Bae, B. H. Hong, G. C. Yi, Adv. Mater., 2011, 23, 4614–4619. [3] Y. J. Hong, T. Fukui, ACS Nano, 2011, 5, 7576–7584. [4] A. M. Munshi, D. L. Dheeraj, V. T. Fauske, D. C. Kim, A. T. J. van Helvoort, B. O. Fimland, H. Weman, Nano Lett., 2012, 12, 4570–4576. [5] Y. J. Hong, J. W. Yang, W. H. Lee, R. S. Ruoff, K. S. Kim, T. Fukui, Adv. Mater., 2013, 25, 6847–6853. [6] Y. Kim, S. S. Cruz, K. Lee et al., Nature, 2017, 544, 340–343. [7] W. Kong, H. S. Li, K. Qiao et al., Nat. Mater., 2018, 17, 999–1004.

Semiconductor epitaxy on two-dimensional materials is beneficial for transferrable and flexible device applications. Graphene, due to the absence of permanent electric dipoles, cannot screen the electric field coming from the opposite side surface, allowing remote epitaxy for heteroepitaxy. This study demonstrates remote heteroepitaxy of ZnO microrods (MRs) on the GaN substrate across graphene layers via hydrothermal growth. Even the use of tri-layer graphene yields the remote heteroepitaxial MR arrays. Transmission electron microscopy reveals the remote heteroepitaxial relation between ZnO MRs and the GaN substrate despite the existence of graphene interlayers in between them. Density-functional theory calculations show that charge transfer along the z-direction at graphene/c-GaN possibly attract adatoms leading to remote heteroepitaxy, implying the field permeability of graphene. The ability of graphene to be released from the host substrate is exploited to exfoliate the overlayer MRs and regenerate the substrate.

Integration of nanostructure lighting source arrays with well-defined emission wavelengths is of great importance for optoelectronic integrated monolithic circuitry. We report on the fabrication and optical properties of GaN-based p–n junction multishell nanotube microarrays with composition-modulated nonpolar m-plane InxGa1–xN/GaN multiple quantum wells (MQWs) integrated on c-sapphire or Si substrates. The emission wavelengths were controlled in the visible spectral range of green to violet by varying the indium mole fraction of the InxGa1–xN MQWs in the range 0.13 ≤ x ≤ 0.36. Homogeneous emission from the entire area of the nanotube LED arrays was achieved via the formation of MQWs with uniform QW widths and composition by heteroepitaxy on the well-ordered nanotube arrays. Importantly, the wavelength-invariant electroluminescence emission was observed above a turn-on of 3.0 V because both the quantum-confinement Stark effect and band filling were suppressed due to the lack of spontaneous inherent electric field in the m-plane nanotube nonpolar MQWs. The method of fabricating the multishell nanotube LED microarrays with controlled emission colors has potential applications in monolithic nonpolar photonic and optoelectronic devices on commonly used c-sapphire and Si substrates.

This study investigates the influence of voids on the electroluminescence (EL) emission color of ZnO microdisk/p-GaN heterojunction light-emitting diodes (LEDs). For this study, position-controlled microdisk arrays were fabricated on patterned p-GaN via wet chemical epitaxy of ZnO, and specifically, the use of trisodium citrate dihydrate (TCD) yielded high-density voids at the bottom of the microdisk. Greenish yellow or whitish blue EL was emitted from the microdisk LEDs formed with or without TCD, respectively, at reverse-bias voltages. Such different EL colors were found to be responsible for the relative EL intensity ratio between indigo and yellow emission peaks, which were originated from radiative recombination at p-GaN and ZnO, respectively. The relative EL intensity between dichromatic emissions is discussed in terms of (i) junction edge effect provoked by interfacial voids and (ii) electron tunneling probability depending on the depletion layer geometry.

Position-controlled n-ZnO microwire (MW) and nanowire-bundle (NW-B) arrays were fabricated using hydrothermal growth of ZnO on a patterned p-GaN film. Both the wire/film p–n heterojunctions showed electrical rectification features at reverse-bias (rb) voltages, analogous to backward diodes. Dichromatic electroluminescence (EL) emissions with 445- and 560-nm-wavelength peaks displayed whitish-blue and greenish-yellow light from MW- and NW-B-based heterojunctions at rb voltages, respectively. The different dichromatic EL emission colors were studied based on photoluminescence spectra and the dichromatic EL peak intensity ratios as a function of the rb voltage. The different EL colors are discussed with respect to depletion thickness and electron tunneling probability determined by wire/film junction geometry and size.

Selective-area remote epitaxy (SA-REpi) is demonstrated for fabricating the mechanically releasable position-controlled ZnO microrod (MR) arrays from the donor wafer in the arrayed form. Intaglio-p...