Research Article www.acsami.org
Ultrasensitive Silicon Nanowire Sensor Developed by a Special Ag Modification Process for Rapid NH3 Detection Yuxiang Qin,*,†,‡,§,∥ Diao Liu,†,‡ Tianyi Zhang,†,‡ and Zhen Cui†,‡ †
School of Microelectronics, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Imaging and Sensing Microelectronic Technology, Tianjin University, Tianjin 300072, China § Key Laboratory for Advanced Ceramics and Machining Technology, Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China ‡
S Supporting Information *
ABSTRACT: Surface functionalization is very effective in enhancing sensing properties of a chemiresistive gas sensor. In this work, we develop a novel and cost-effective process to prepare Ag-modified silicon nanowire (SiNW) sensors and further suggest a resistance effect model to clarify the enhanced sensing mechanism of Ag-modified SiNWs. The SiNWs were formed via metal-assisted chemical etching (MACE), and the Ag nanoparticle (NP) modification was achieved in situ based on the MACE-produced Ag dendrites by involving a crucial anisotropic postetching of TMAH. The TMAH etching induces a loose array of needle-like, rough SiNWs (RNWs) with firm attachment of tiny Ag NPs. Comparative investigations for NH3-sensing properties indicate that the RNWs modified by discrete Ag NPs (Ag@RNWs) display an ∼3-fold enhancement in gas response at room temperature compared with pristine SiNWs. Meanwhile, transient response and ultrafast recovery are observed for the Ag@RNW sensor (tres ≤ 2 s and trec ≤ 9 s to 0.33−10 ppm of NH3). The study demonstrates the considerable effect and potential of the Ag modification process developed in this work. A resistance effect model was further suggested to clarify the underlying mechanism of the enhanced response and the response saturation characteristic of the Ag@RNWs. The promotion of TMAH etchinginduced microstructure modulation to sensing properties was also demonstrated. KEYWORDS: silicon nanowires, gas sensor, modification, metal-assisted chemical etching, mechanism resistance with 10 ppb NO2 exposure.6,7 These hint toward the great potential of vertical SiNW arrays in highly sensitive sensors operating at room temperature. However, the involved nanosphere lithography process for fabrication of SiNWs and top PTE is too complicated and hardly controlled. Most crucially, the response harvested from the pristine SiNW sensor to rarefied NH3 or NO2 was extremely weak; stable response is hardly achieved at room temperature especially to the ultrararefied gas exposure. Further improvements in the fabrication process and response properties is therefore highly required for high performance SiNW sensor development. So far elemental doping and catalyst functionalization have been developed to enhance the sensing abilities of semiconductor sensors effectively, and especially, the surface modification with nanoparticles of noble metals, such as Pt, Pd, Ag, and Au, has been used most widely for sensor improvement in response magnitude or response time thus far.8−10 To achieve an effective modification, the uniform distribution of tiny
1. INTRODUCTION Silicon nanowires (SiNWs) have aroused tremendous attention in recent years due to their unique physical and chemical properties and potential applications in nano/optoelectronics, solar cells, and chemo/biosensors.1−3 Especially, the high chemical activity and large specific surface area of the featured one-dimensional nanostructure make them an attractive candidate for gas sensor applications to detect various toxic and hazardous gases such as H2, NOx, and NH3.4−6 Another pronounced advantage is that the SiNW sensors are easily integrated with other Si-based nanodevices. SiNW is a promising gas-sensing material capable of operating at room temperature. For instance, ultrasensitive response of SiNW array sensors was reported by In et al.6,7 They revealed that SiNWs were capable of response to rarefied gases of NO2 and NH3 with ppb level when operating at room temperature, in spite of very slow response speed.6,7 Through fabricating a porous top electrode (PTE) on the vertical SiNW arrays using a nanosphere-enabled method, the sensor exhibited a weak increase in resistance upon 500 ppb NH3 exposure at ∼30%RH. When the humidity level was reduced to
