Palladium‐Decorated Silicon Nanomesh Fabricated by Nanosphere Lithography for High Performance, Room Temperature Hydrogen Sensing

DOI: 10.1002/smll.201703691 in electrical properties in the presence of H2 allows Pd or Pd hybrid materials to be utilized as H2 sensors.[9–12] Among all such materials, Pd-Si-based devices have attracted considerable attention because of their compatibility with conventional complementary metal–oxide–semiconductor (CMOS) process technology.[3,11] The sensing mechanism in Pd-gated Si devices has been well developed. When exposed to H2 gas, H atoms dissociated from the H2 molecules are dissolved into the Pd metal, which becomes polarized in the vicinity of the boundary with an underlying insulation layer, and this modulates the electrical carrier concentration in the semiconductor layer underneath.[13] Unlike the metal oxide or other semiconductor materials, Si itself is inert to most gas components. Also, Si can be easily decorated with various materials to create sensitivity to particular target gases. This enables the functionalized Si device to have excellent selectivity against introduced chemical or biological species.[14–16] Si nanowire-based devices have been employed as efficient components in high performance sensors for detecting gases and other chemical and biological components.[17–21] Since the nanowires have a high surface-to-volume ratio, they respond more sensitively to the surrounding environment. The techniques used to fabricate Si nanowires can be categorized into bottom-up[22–24] or top-down[25–27] approaches. The bottom-up fabrication routine can produce high yields of nanoscale structures at low cost. However, it suffers from poor reproducibility and difficult device integration.[28,29] The standard top-down routine requires high-resolution lithography techniques such as electron beam (E-beam) lithography[26,27] to achieve nanoscale patterns. Although nanostructures with dimensions down to a few nanometers can be achieved using the top-down techniques, their high cost and low yields make them impractical for low-cost mass production.[30,31] The nanosphere lithography (NSL) method, also known as colloidal lithography or natural lithography,[32,33] utilizes the self-assembly of a nanosphere monolayer and could be an alternative choice for achieving uniform and well-ordered nanopatterns with minimum sub-10 nm dimensions.[34,35] At the same time, the NSL method is scalable up to wafer-level fabrication with high yield and is compatible with the conventional CMOS process.[36] Various nanostructures and applications based on this method have been reported.[37–39] For example, well-ordered and high-aspect-ratio Si nanowires have been fabricated using A hydrogen (H2) gas sensor based on a silicon (Si) nanomesh structure decorated with palladium (Pd) nanoparticles is fabricated via polystyrene nanosphere lithography and top-down fabrication processes. The gas sensor shows dramatically improved H2 gas sensitivity compared with an Si thin film sensor without nanopatterns. Furthermore, a buffered oxide etchant treatment of the Si nanomesh structure results in an additional performance improvement. The final sensor device shows fast H2 response and high selectivity to H2 gas among other gases. The sensing performance is stable and shows repeatable responses in both dry and high humidity ambient environments. The sensor also shows high stability without noticeable performance degradation after one month. This approach allows the facile fabrication of high performance H2 sensors via a cost-effective, complementary metal–oxide–semiconductor (CMOS) compatible, and scalable nanopatterning method. Hydrogen Sensors