Capillary‐Driven Self‐Assembled Microclusters for Highly Performing UV Photodetectors

Abstract

Self‐assembled nanoparticle networks have emerged as multifunctional building blocks for a new generation of highly sensitive sensing technologies that offer large surface‐to‐volume ratios and a range of associated benefits. Unfortunately, with nanoparticle networks often being held together by weak van der Waals forces, the development of useful commercial devices is slowed by the relatively low robustness and poor carrier transport characteristics. This study shows how the application of a single droplet of ethanol can induce capillary forces capability of delivering significant changes to the morphological, structural, optical, and electronic properties of ZnO nanoclusters. It demonstrates how ZnO nanocluster “dendrites” and nanoparticles are forced together to form micro‐scale islands and larger nanoparticles, and thereby improve the robustness of the layers and the quality of the junctions between the nanoparticles without significantly reducing the overall porosity of the layer or degrading the structural or optical properties in any way. The commensurate improvement in the electronic transport within the layers is found to greatly improve the photoresponse of UV detectors. It seems likely that the application of ethanol and the exploitation of capillary force can provide a technique that can greatly benefit any nanostructured, ultra‐porous device where poor charge transport currently limits performance.

Facile Fabrication of UV Photodetectors Using Spin-Coating Flame-Synthesized ZnO Nanoparticles

“…The enhanced photodetection performance achieved by increasing the film thickness (from ZnO-2 to ZnO-10) can be explained, as shown in Figure b–e. As shown in Figure ai, ZnO nanoparticles exhibit discontinuous coverage in ZnO-2, forming random microislands, due to the capillary force of ethanol, during evaporation process. This lack of sufficient coverage hinders efficient interaction with incident UV light, leading to a weak UV photocurrent.…”

“…As the number of DSC cycles is increased to 5 (Figure c), complete substrate coverage with homogeneously stacked ZnO nanoparticles is achieved (Figure bi), resulting in a notable increase in photocurrent. Furthermore, the close proximity of adjacent ZnO nanoparticles facilitates efficient free electron transport within the thin film, leading to a significantly improved response speed, particularly in terms of rise time (Table ). These advantages are retained with further iterations of DSC cycles, especially when 10 cycles are performed (Figure d), resulting in an optimized film thickness of approximately 2 μm that enables an effective interaction between ZnO nanoparticles and UV light, yielding a significantly high responsivity for UV light detection (74 mA W – 1 ).…”

“…In our recent work, a similar phenomenon was reported, where ethanol allowed ZnO nanoparticles to "fuse" together or coalesce, enabling the growth of larger particles. 10 Another plausible mechanism is grain-rotation-induced grain coalescence (GRIGC), 41,42 a crystal growth mechanism occurring when two grains merge into a larger grain due to significant misorientation and high surface energy.…”

“…The resulting crack-free nanostructured ZnO thin films with an average film porosity of ∼80% depict a high photocurrent density of ∼300 μA cm –2 under 375 nm UV light illumination with a light density of 3 mW cm –2 at a low bias of 3.3 V, making it a promising candidate for wearable technologies and personalized devices. Furthermore, compared to flame-made UNN ,, and conventional SG-SC films, these nanostructured films feature a thin active layer and a reduced number of trapping states. This minimizes carrier transit times and results in photocurrent rise and decay times of approximately 1 and less than 4 s, respectively.…”

“…Among potential UV-sensitive materials, including silicon carbide (SiC), gallium nitride (GaN), titanium dioxide (TiO 2 ), and zinc oxide (ZnO), − the latter is a promising transparent metal oxide with an absorption edge of 3.37 eV (λ = 370 nm), which is conveniently located at the end of the visible light spectrum. The direct bandgap structure of ZnO enables a sharp photoresponse contrast between the visible and UV spectra, , facilitating efficient detection of UV light and a high selectivity against the visible spectrum ( R UV / R vis ). , Due to its nontoxic nature, high chemical and mechanical stability, excellent electrical properties, and relatively economical fabrication cost, ZnO emerges as a promising candidate for the development of cost-effective photodetectors across a diverse range of applications, including wearable and personalized UV monitoring devices .…”