30 Besides metal ALD processes, ALD of oxides has demonstrated a great potential in preventing catalysts from sintering. 26–29 By adjusting different ALD processes, it is possible to fabricate core–shell structured or alloys nanoparticles with well-defined compositions ratio. 21–25 Recently, ALD has shown its potential in synthesizing single atom catalysts as well. 19,20 For instance, ALD was utilized to fabricate highly dispersed, size controllable metal nanoparticles, such as Ru, Pd, Pt, Ir, and Ni, during the nucleation stage. The use of ALD for synthesis of heterogeneous catalysts has been developed rapidly in the past few years. Thus, it enables direct modification of the surfaces and structures, 18 as well as adjustment of the shape and size of materials deposited on complex substrates. 15–17 Taking the advantage of self-limiting surface adsorption nature of ALD, the target materials can be deposited with controllability and uniformity in atomic level. So far, significant number of elements and their oxides can be synthesized via ALD. 8–14 ALD is based on successive and alternative surface reactions from gas phase to fabricate thin films and overlayers in the nanometer range. Among various synthesis methods, atomic layer deposition (ALD) has been recently developed as an effective method to synthesize composite catalysts. 5–7 To design and obtain catalysts with the desired activity, selectivity, and stability, synthesis strategies to build precise configurations with direct modulation of reactive sites are of great importance. 1–4 The catalytic performance of composite catalysts strongly depends on their size, heterointerfaces, active sites, etc. Composite catalysts based on metal-oxide with designed structures perform an irreplaceable role for most applications. This study represents an important advance in fabricating core–shell heterojunction NWs for high-performance optoelectronic applications.Catalysts are widely utilized to accelerate chemical reactions by decreasing reaction barriers in various industrial syntheses, environment pollution control, energy conversion, and so on. The complementary bandgaps of Ge and CdS also ensured that the device had a capability for broadband detection from visible to infrared light. Significantly, owing to the existence of a built-in electric field, the heterojunctions could serve as self-driven photodetectors, with a high photosensitivity of 18 000%, which is remarkably much better than previous reports on Ge NW photodetectors. The heterojunction NWs exhibited excellent diode characteristics, with a pronounced photoresponse under light illumination. The ALD technique allowed the conformal deposition of polycrystalline CdS shells on single-crystalline Ge NWs with high uniformity and controllable thickness. Herein, we demonstrated the fabrication of Ge–CdS core–shell heterojunction NWs via a facile atomic layer deposition (ALD) technique. However, the rational design and synthesis of core–shell heterojunction NWs are hindered by the complex fabrication process and the potential damage to core NWs during shell deposition. Core–shell p–n heterojunction nanowires (NWs) hold great promise for optoelectronic applications due to the large effective junction area and minimized carrier recombination.
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