The demand for new and more efficient energy technologies has motivated the need to design and construct materials at the nanoscale. Because it allows for the synthesis of materials with atomic-level control over nanostructure and composition, atomic layer deposition (ALD) will play an important role in the realization of precise nanoscale design. In this Perspective, we highlight successful uses of ALD toward improving the performance of materials for energy capture, storage, and consumption. We identify five roles that ALD can play in the synthesis and study of energy technologies: fabrication of active materials, modification of surface properties, deposition of barrier and encapsulation layers, synthesis of intricate nanostructures, and elucidation of fundamental material–property relations. Moreover, we offer our perspective on how ALD will fit into the future study, design, and fabrication of nanomaterials for sustainable energy technologies.

Opportunities for Atomic Layer Deposition in Emerging Energy Technologies

https://iopscience.iop.org/article/10.1149/2.0292001JSS/pdf

Review—Recent Advances and Challenges in Indium Gallium Nitride (InxGa1-xN) Materials for Solid State Lighting

Cathodoluminescence (CL) imaging spectroscopy provides two-dimensional optical excitation images of photonic nanostructures with a deep-subwavelength spatial resolution. So far, CL imaging was unable to provide a direct measurement of the excitation and emission probabilities of photonic nanostructures in a spatially resolved manner. Here, we demonstrate that by mapping the cathodoluminescence autocorrelation function g(2) together with the CL spectral distribution the excitation and emission rates can be disentangled at every excitation position. We use InGaN/GaN quantum wells in GaN nanowires with diameters in the range 200–500 nm as a model system to test our new g(2) mapping methodology and find characteristic differences in excitation and emission rates both between wires and within wires. Strong differences in the average CL intensity between the wires are the result of differences in the emission efficiencies. At the highest spatial resolution, intensity variations observed within wires are the res...

/pdf/nanoscale-relative-emission-efficiency-mapping-using-39zk91o49d.pdf

Nanoscale Relative Emission Efficiency Mapping Using Cathodoluminescence g(2) Imaging

Cathodoluminescence spectroscopy is a key analysis technique in nanophotonics research and technology, yet many aspects of its fundamental excitation mechanisms are not well understood on the single-electron and single-photon level. Here, we determine the cathodoluminescence emission statistics of InGaN quantum wells embedded in GaN under 6--30-keV electron excitation and find that the light emission rate varies strongly from electron to electron. Strong photon bunching is observed for the InGaN quantum well emission at 2.77 eV due to the generation of multiple quantum well excitations by a single primary electron. The bunching effect, measured by the ${g}^{(2)}(t)$ autocorrelation function, decreases with increasing beam current in the range 3--350 pA. Under pulsed excitation ($p=2--100\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$; 0.13--6 electrons per pulse), the bunching effect strongly increases. A model based on Monte Carlo simulations is developed that assumes a fraction $\ensuremath{\gamma}$ of the primary electrons generates electron-hole pairs that create multiple photons in the quantum wells. At a fixed primary electron energy (10 keV) the model explains all ${g}^{(2)}$ measurements for different beam currents and pulse durations using a single value for $\ensuremath{\gamma}=0.5$. At lower energies, when electrons cause mostly near-surface excitations, $\ensuremath{\gamma}$ is reduced ($\ensuremath{\gamma}=0.01$ at 6 keV), which is explained by the presence of a AlGaN barrier layer that inhibits carrier diffusion to the buried quantum wells. The combination of ${g}^{(2)}$ measurements in pulsed and continuous mode with spectral analysis provides a powerful tool to study optoelectronic properties and may find application in many other optically active systems and devices.

/pdf/photon-bunching-reveals-single-electron-cathodoluminescence-1ricggqvfg.pdf

Photon bunching reveals single-electron cathodoluminescence excitation efficiency in InGaN quantum wells

Surface passivation of light emitting diodes: From nano-size to conventional mesa-etched devices

Nanotextured surfaces provide an ideal platform for efficiently capturing and emitting light. However, the increased surface area in combination with surface defects induced by nanostructuring e.g. using reactive ion etching (RIE) negatively affects the device's active region and, thus, drastically decreases device performance. In this work, the influence of structural defects and surface states on the optical and electrical performance of InGaN/GaN nanorod (NR) light emitting diodes (LEDs) fabricated by top-down RIE of c-plane GaN with InGaN quantum wells was investigated. After proper surface treatment a significantly improved device performance could be shown. Therefore, wet chemical removal of damaged material in KOH solution followed by atomic layer deposition of only 10 alumina as wide bandgap oxide for passivation were successfully applied. Raman spectroscopy revealed that the initially compressively strained InGaN/GaN LED layer stack turned into a virtually completely relaxed GaN and partially relaxed InGaN combination after RIE etching of NRs. Time-correlated single photon counting provides evidence that both treatments—chemical etching and alumina deposition—reduce the number of pathways for non-radiative recombination. Steady-state photoluminescence revealed that the luminescent performance of the NR LEDs is increased by about 50% after KOH and 80% after additional alumina passivation. Finally, complete NR LED devices with a suspended graphene contact were fabricated, for which the effectiveness of the alumina passivation was successfully demonstrated by electroluminescence measurements.

https://iopscience.iop.org/article/10.1088/1361-6528/28/5/055201/pdf

Significant performance enhancement of InGaN/GaN nanorod LEDs with multi-layer graphene transparent electrodes by alumina surface passivation

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Significant performance enhancement of InGaN/GaN nanorod LEDs with multi-layer graphene transparent electrodes by alumina surface passivation