Free-Standing Metal Halide Perovskite Nanowire Arrays with Blue-Green Heterostructures

Abstract

Vertically aligned metal halide perovskite (MHP) nanowires are promising for various optoelectronic applications, which can be further enhanced by heterostructures. However, present methods to obtain free-standing vertically aligned MHP nanowire arrays and heterostructures lack the scalability needed for applications. We use a low-temperature solution process to prepare free-standing vertically aligned green-emitting CsPbBr₃ nanowires from anodized aluminum oxide templates. The length is controlled from 1 to 20 μm by the precursor amount. The nanowires are single-crystalline and exhibit excellent photoluminescence, clear light guiding and high photoconductivity with a responsivity of 1.9 A/W. We demonstrate blue-green heterostructured nanowire arrays by converting the free-standing part of the nanowires to CsPbCl_1.1Br_1.9 in an anion exchange process. Our results demonstrate a scalable, self-aligned, and lithography-free approach to achieve high quality free-standing MHP nanowires arrays and heterostructures, offering new possibilities for optoelectronic applications.

Keywords: free-standing nanowires arrays; metal halide perovskite; nanowire heterostructure arrays; vapor anion exchange.

Figure 1

(a) Schematic diagram of the growth process. (b) Cross-sectional SEM images of the as-grown samples with free-standing nanowires on the back side of the AAO template.

Figure 2

Control of nanowire length. SEM images and statistical distribution of the length of the free-standing nanowire arrays, grown by using (a) 7 μL, (b) 15 μL, and (c) 25 μL precursor solution. The sample stage is tilted by by 30°, that is, the nanowires are twice as long as they appear in the SEM.

Figure 3

Structural characterization (a) Cross-sectional SEM images of free-standing nanowires grown from the AAO. (b) Tilted top view of the free-standing nanowires from the AAO pores. XRD patterns of nanowires inside AAO (c) and free-standing nanowires transferred onto another substrate (d). The insets show the diffraction geometries. (e) SAED pattern and low-magnification TEM image. (f) HRTEM image of a single CsPbBr₃ nanowire. (g) Schematic diagram of the growth mechanism of the free-standing nanowires from the AAO nanopores.

Figure 4

Optical and electrical properties. (a) Photoluminescence spectrum of a single nanowire. The left inset shows an optical image of the transferred single nanowire, while the right inset shows as-grown free-standing nanowires in a cross-sectional view under the excitation of a focused 378 nm laser, aligned onto the center of the images. (b) Time-resolved photoluminescence decay profile of the nanowires. (c) I−V curve of a single nanowire transistor in dark condition and under illumination of 25 and 40 mW/cm² 405 nm light. (The voltage scan is from 0 to −5 V, then to +5 V and finally back to 0 V in steps of 0.1 V.) The dark current is shown multiplied by a factor of 1000 for clarity. The inset shows a SEM image of the device. (d) Current generated by the device under periodical illumination of 40 mW/cm² 405 nm light at an electrical bias of 5 V.

Figure 5

Free-standing nanowire blue-green heterojunction nanowire array (a) Schematic diagram of the anion exchange process. (b) Cross sectional microscopy images of the same heterojunction nanowire array sample under excitation of focused UV laser at different positions. Position 1: laser excited at the AAO part (left); all of the nanowires show green emission. The green spot in the red square indicates that the free-standing nanowire has light guiding of the green light. Position 2: laser excited near the heterojunction (right). The nanowire segments inside the AAO have green emission and the free-standing segments have blue emission. (c) Photoluminescence spectra under excitation of UV laser at the different positions on the heterojunction array, as indicated in (b).