We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p–n junction core/shell nitride nanowires. GaN nanowires containing seven radial In0.2Ga0.8N/GaN quantum wells were grown by metal–organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p–n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

The fabrication technologies and the performance of flexible nanowire light emitting diodes (LEDs) are reviewed with a special focus on nitride materials.

Single nitride nanowire core/shell n-p photodetectors are fabricated and analyzed. Nanowires consisting of an n-doped GaN stem, a radial InGaN/GaN multiple quantum well system and a p-doped GaN external shell were grown by catalyst-free metal–organic vapour phase epitaxy on sapphire substrates. Single nanowires were dispersed and the core and the shell regions were contacted with a metal and an ITO deposition, respectively, defined using electron beam lithography. The single wire photodiodes present a response in the visible to UV spectral range under zero external bias. The detector operation speed has been analyzed under different bias conditions. Under zero bias, the  −3 dB cut-off frequency is ~200 Hz for small light modulations. The current generation was modeled using non-equilibrium Green function formalism, which evidenced the importance of phonon scattering for carrier extraction from the quantum wells.

We demonstrate green emission from InGaN/GaN multiple quantum wells (MQWs) grown on m-plane sidewalls of GaN wires. To tune the emission wavelength, InGaN radial wells were grown by metal–organic vapor phase epitaxy (MOVPE) at decreasing temperatures ranging from 710 down to 620 °C to increase the In incorporation. A comprehensive investigation combining structural and optical analyses demonstrates that the green emission from the nonpolar m-plane wire sidewalls is achieved for the wells grown at 650 °C (namely, for 2.7 nm thick wells sandwiched by 11 nm thick GaN barriers). The observed emission wavelength of 500–550 nm is consistent with an average In-content of MQWs measured in the range of 24 ± 4% by energy dispersive X-ray (EDX) and atom probe tomography (APT). Single wires were electrically contacted and the green electroluminescence from m-plane facets was established on single wire-LED devices. This demonstrates the possibility to produce green emitters with core–shell wire LEDs elaborated by industrial and scalable MOVPE technique.

We present a new approach to achieve selective area growth of GaN nanowires by plasma-assisted molecular beam epitaxy. The nanowires are grown on graphene nanodots, which are patterned by electron beam lithography from polycrystalline graphene patches transferred to SiO2 substrates. The GaN nanowires grow on these graphene nanodomains with a perfect selectivity with respect to the SiO2 surrounding surface. The results demonstrate that a single monolayer of graphene can withstand the lithography process without losing its ability to induce epitaxial growth. The nanowire length distribution and patterns’ fill factor are analyzed in the framework of a theoretical model, which takes into account an incubation time dependent on the graphene dot size. Overall, these results represent the first demonstration of selective area nanowire growth on a regular array of graphene nanodomains.

Optical properties of GaN nanowires grown on chemical vapor deposited graphene transferred on an amorphous support are reported. The growth temperature was optimized to achieve a high nanowire density with a perfect selectivity with respect to a SiO 2 surface. The growth temperature window was found to be rather narrow (815 ± 5) °C. Steady-state and time-resolved photoluminescence from GaN nanowires grown on graphene was compared with the results for GaN nanowires grown on conventional substrates within the same molecular beam epitaxy (MBE)reactor showing a comparable optical quality for different substrates. Growth at temperatures above 820°C led to a strong NW density reduction accompanied with a diameter narrowing. This morphology change leads to a spectral blueshift of the donor bound exciton emission line due to either surface stress or dielectric confinement. Graphene multi-layered microdomains were explored as a way to arrange GaN nanowires in a hollow hexagonal pattern. The nanowires grown on these domains show a luminescence spectral linewidth as low as 0.28 meV (close to the setup resolution limit).

We demonstrate flexible nanowire white light-emitting-diodes (LEDs) with an optimized colour quality. The devices consist of flexible InGaN/GaN nanowire LEDs acting as pumps, capped with removable phosphor-doped polydimethylsiloxane membranes. Five different phosphors with tens of microns in grain size emitting from green to orange are investigated using both violet-blue and a bluegreen nanowire-based LED pumps. In addition, a flexible nanowire white LED with a warm white emission is demonstrated using two layers of different phosphors. Compared to the previous realizations of flexible nanowire white LEDs, these novel LEDs improve the colour rendering index from 54 to 86 and show a colour tuneable from a bluish cool white colour to natural white and finally to warm white. The flexibility tests show that the LEDs can be bent down to 1.5 cm curvature radius without significant degradation. Therefore, the replacement of the nano-phosphors used in the previous realization by relatively inexpensive micro-phosphors does not degrade the good mechanical flexibility of the white nanowire LEDs.