(Invited) Biomimetic Eyes with Perovskite Nanowire Array Retina
Authors: Gu Leilei , Fan Zhiyong 
Published: November 2022

Human eyes are masterpiece of nature, which can realize powerful image sensing with a very concise structure. Biomimetic eyes with characters comparable to human eyes are highly desirable in many technological applications, particularly in the fields of visual prosthesis and machine vision. The World Health Organization reported that globally there are over 252 million people suffering from visual impairment in the year of 2020. This number is exploding due to the prevalence of various mobile devices with small displays. Bionic eyes implantation is one the most effective strategy for vison restoration. However, today, even the most mature bionic eyes have only 300 clinical trials, which is merely 1 ppm of all the visual impaired patients, mainly due to their poor performance and high cost. In addition, industrial applications such as autonomous driving and humanoids also desire cameras with high performance. To achieve biomimetic eyes with imaging performance on a par with human eyes, innovative device structure design and material selection have to be carried out simultaneously. Among all the photosensing material candidates, the ordered one-dimensional (1D) semiconductor nanowire (NW) arrays exhibit unique advantages such as fast carrier transportation and strong light-material interaction due to their anisotropic structure in axial and radial direction, thereby leading to fast response and high responsivity. This presentation focuses on our work on the NW arrays image sensing and can be highlighted as:
1) Controlled growth of well aligned, high-density NW arrays
Despite the advantages abovementioned, the integration of NWs has always been a bottleneck challenge hurdling their practical applications. Direct growth of NW arrays can be an effective way to tackle this challenge. We reported for the first time a unique fabrication process to form large-scale, 3D, high-density arrays of lead halide perovskite NWs with well-engineered geometry through a template-assisted growth approach. The array demonstrated amazingly geometry dependent optoelectronic properties and thereby can be used to improve performance of various devices. Particularly, its high regularity leads to the possibility to electrically addressable individual NWs which makes it an ideal candidate for very-large-scale integrated (VLSI) electronics and optoelectronics. The interaction between nanotemplate and sensing materials can introduce unique device performance such as tunable photoresponse and improved stability of perovskites.
2) Assembly of integrated, NW arrays based planar image sensors
To demonstrate the technological potency of the NW arrays, we have developed new process to fabricate them into proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected onto it. The high density of this NW array and the addressability of individual NW enable this unique image sensor design a potential, extremely high resolution approaching optical diffraction limit. To realize the device assembly, we have developed a PDMS-assisted dry transfer process which can transfer the thin (µm scale) and large-area (cm scale) film onto any substrate without any bubbles and wrinkles. This unique NW manipulation strategy, together with NW geometrical and compositional tunability, may lead to new functionality and inspire novel device design in the future.
3) Invention of a 3D, biomimetic eye with a hemispherical NW array retina
The high performance of human eyes originates from the dome shape of retina and the high-density photoreceptors in it. However, the commercial image sensors are dominantly using planar device structures shaped by the mainstream planar microfabrication processes, making it hardly possible for hemispherical device fabrication. In our work, we have developed a novel artificial visual system using a spherical biomimetic electrochemical eye with hemispherical retina made of a high-density perovskite NW array. The device has high similarity to human eyes with part of its photodetection performance superior to human eyes. The hemispherical artificial retina has NW density much higher than that of photoreceptors in a human retina thus can potentially achieve a higher image resolution which is bolstered by implementation of a single-NW ultra-small photodetector. The work may lead to a new generation of photosensing devices based on a bioinspired design that can benefit a wide spectrum of technological applications.
In all, with the unique structural properties of NW arrays as the starting point, we realized their controlled growth, developed strategies to manipulate them and designed proper device structures to fully fulfill their advantages. In the future, we will combine them with brains, either biological or digital brains, to explore their applications in fields such as visual prothesis and artificial intelligence.

Figure 1