On chip growth of semiconductor metal oxide nanowires for gas sensors a review

Accepted Manuscript On-chip growth of semiconductor metal oxide nanowires for gas sensors: A review Chu Manh Hung, Dang Thi Thanh Le, Nguyen Van Hieu PII: S2468-2179(17)30130-2 DOI: 10.1016/j.jsamd.2017.07.009 Reference: JSAMD 113 To appear in: Journal of Science: Advanced Materials and Devices Received Date: 10 July 2017 Revised Date: 27 July 2017 Accepted Date: 31 July 2017 Please cite this article as: C.M Hung, D.T.T Le, N Van Hieu, On-chip growth of semiconductor metal oxide nanowires for gas sensors: A review, Journal of Science: Advanced Materials and Devices (2017), doi: 10.1016/j.jsamd.2017.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT On-chip growth of semiconductor metal oxide nanowires for gas RI PT sensors: A review Chu Manh Hung, Dang Thi Thanh Le, Nguyen Van Hieu* International Training Institute for Materials Science, Hanoi University of Science and M AN U Corresponding authors SC Technology, Hanoi, Viet Nam AC C EP TE D * Nguyen Van Hieu, Ph.D Professor International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No.1, Dai Co Viet Road, Hanoi, Vietnam Phone: 84 38680787 Fax: 84 38692963 E-mail: hieu@itims.edu.vn/hieu.nguyenvan@hust.edu.vn Post address: No.1 Dai Co Viet, Hanoi, Vietnam ACCEPTED MANUSCRIPT On-chip growth of semiconductor metal oxide nanowires for gas sensors: A review Chu Manh Hung, Dang Thi Thanh Le, Nguyen Van Hieu* RI PT International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet, Hanoi, Vietnam SC Abstract: A semiconductor metal oxide nanowires (SMO-NWs) shows potential for novel gas sensor applications because of its distinct properties, such as high ratio of surface area to M AN U volume, high crystallinity, and perfect pathway for electron transfer (length of NW) SMONW sensors can be configured as resistors or field-effect transistors for gas detection, and different configurations, such as single NW, multiple NWs, and networked NW films, have been established A surface-functionalizing NWs with catalyst elements and a self-heating TE D NWs provides other options for highly selective and low-power consumption gas sensors However, an appropriate design and integration of SMO-NWs should also be considered to enhance the gas-sensing performance of SMO-NW sensors The on-chip growth of SMO- EP NWs exhibits many advantages and thus can be effectively used for the large-scale fabrication of SMO-NW sensors with improved gas response and stability This review summarizes AC C relevant reports on the on-chip fabrication of SnO2, ZnO, WO3, CuO, and other SMO-NW sensors This review also discusses the promising approaches that help develop the on-chip fabrication of SMO-NWs-based gas sensors and other NWs-based devices Keywords: On-chip growth, Gas sensors, Nanowires, Nanosensors, Metal oxides * Corresponding author hieu@itims.edu.vn/hieu.nguyenvan@itims.edu.vn Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT Introduction Nanostructured materials are classified into three categories, namely, zero-dimensional structures (e.g., nanoparticles), one-dimensional (1D) structures (e.g., NW, nanobells [NB], and nanorods [NR]), and two-dimensional (2D) nanostructures (e.g., thin films) [1] Among RI PT these nanostructures, 1D nanostructures, such as NW, NB, and NR, have been extensively investigated since carbon nanotubes were found in 1991 [2] NWs, which belongs to the group of 1D nanostructures, is characterized by a diameter of 1–100 nm [3] An important SC property of NWs is that carriers are only conducted in one dimension; conversely, the two other dimensions are quantum confined because of their nanoscale size [4] In addition to a M AN U large specific surface area, a Debye thickness of 1–10 nm is observed in NWs because of its nanoscale size, which should be considered in comparison to its diameter when surface effects are examined Among NW materials, SMO-NWs has been widely explored because of its novel properties compared with those of its bulk counterparts [4–6] For example, Wang and TE D co-workers reported the probability of fabricating various metal oxide nanobelts through thermal evaporation [7] Various types of SMO-NWs, such as SnO2, ZnO, TiO2, WO3, In2O3, and NWs, have been prepared using the same method [5] Studies on many distinct EP physicochemical properties of SMO-NWs have revealed that SMO-NWs presents a strong potential for applications in electronic nanodevices, optoelectronic devices, and nanosensors AC C [2,4–6] The gas-sensing properties of SMO-NWs have been extensively studied to detect toxic gases SMO-NWs exhibits novel properties, which are much greater than those of its bulk or thin film counterparts; therefore, SMO-NWs may be used for new gas sensor generations [3,8,9] However, gas sensors based on SMO-NWs have yet to be successfully developed and commercialized To our best knowledge, SMO-NW applications are limited by the lack of an efficient method to integrate SMO-NWs on functional substrates for mass production with good Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT repeatability and reasonable cost In early research stage, SMO-NWs-based devices are commonly fabricated using pick and place [10] In this fabrication method, NWs grown on a substrate or in a solution are initially dispersed in a volatile solvent and then deposited on functional substrates through spin-coating or dip-coating techniques Interdigitated electrodes RI PT are finally fabricated on the NWs through microelectronic technologies, such as lithography, sputtering, and lift-off Although this process is frequently used for fundamental research on a laboratory scale, researchers generally experience difficulties in applying this process to SC fabricate gas sensors for mass production with good repeatability and acceptable price Technologies integrating NWs on functional substrates have been widely investigated, and M AN U relevant results have been obtained [11–13] The proposed techniques for the mass production of SMO-NW gas sensors are divided into two groups: on-chip or direct methods and off-chip or indirect approaches [11] This paper aims to present an overview of the recent progress on the on-chip fabrication of SMO-NW gas sensors Advanced gas-sensing properties of SMO-NW sensors TE D SMO-NWs have been widely investigated as gas sensors because of its typical characteristics, such as high surface-to-volume ratio, high crystallinity, and quantum size in two EP dimensions and other dimensions for electron conduction Thus, SMO-NWs are optimum platforms for the development of novel chemical sensors [2,6,14] Studies on gas sensor AC C applications of SMO-NWs are summarized in Table Table Outstanding works performed on SMO-NWs gas sensors science 2002 NWs materials Sensor types Target gases Years 2002 Cited times (Scopus, 2017) 1156 SnO2 Resistive sensor SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 Resistive sensor Optical sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor C2H5OH CO, NO2 CO, O2 NO2 H2 CO, C2H5OH, NO2 CO C2H5OH LPG Ref [15] 2003 2002 2008 2007 2008 2007 2010 2010 830 698 180 135 134 92 20 19 [16] [17] [18] [19] [20] [21] [22] [23] Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT Resistive sensor Resistive sensor Resistive sensor FET transistor Resistive sensor Resistive sensor FET transistor Resistive sensor Resistive sensor Resistive sensor Schottky diode Schottky Crystal balance Schottky NO2 C2H5OH NO2 H2, O2 C2H5OH NO2 O2 C2H5OH H2 H2S O2 NO2 NH3 C2H5OH 2012 2005 2005 2005 2004 2008 2004 2007 2010 2007 2010 2009 2012 2007 18 122 115 851 1459 350 295 236 239 103 97 130 25 112 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] WO3 WO3 WO3 W18O49 WO3-Pt WO3-Pd In2O3 In2O3 In2O3 In2O3 In2O3 Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor FET transistor FET transistor FET transistor Resistive sensor FET transistor NO2 NO2 NO2 NO2 H2 H2 NO2 NO2, NH3 NH3 NO2 H2S 2006 2009 2012 2010 2010 2013 2004 2003 2003 2008 2009 290 111 37 60 62 42 599 417 96 84 39 [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] Zn-In2O3 In2O3-Au FET transistor FET transistor CO CO 2010 2011 32 29 [49] [50] Mg-In2O3-Au, -Ag, -Pt TiO2 TiO2 TiO2 TiO2 TiO2-Pd TiO2-Pt SnO2, TiO2, In2O3 FET transistor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor Resistive sensor CO C2H5OH C2H5OH H2 NH3, C2H5OH VOCs CO, NO2 H2,CO 2013 2008 2013 2008 2014 2014 2014 2006 38 109 48 10 135 [51] [52] [53] [54] [55] [56] [57] [58] CuO CuO CuO CuO CuO CuO CuO CuO V2O5 V2O5 FTE transistor Resistive sensor Resistive sensor Resistive sensor Resistive sensor FET transistor Resistive sensor Resistive sensor FET Transistor Resistive sensor CO CO, NO2 H2S H2 C2H5OH CO2 CO H2S H2 H2 2009 2008 2008 2010 2009 2010 2009 2012 2009 2007 178 162 145 57 48 37 26 21 119 12 [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] SC M AN U TE D EP AC C RI PT SnO2 Sb-SnO2 Ru-SnO2 SnO2-Pd ZnO ZnO ZnO ZnO ZnO ZnO ZnO ZnO ZnO ZnO-Pd The advanced gas-sensing properties of 1D nanostructured SMO have been described [69,15] Comini and coworkers [69] demonstrated single-crystalline SnO2 nanobelts as stable gas sensors for C2H5OH and NO2 gases These sensors yield high response values (Ra/Rg or Rg/Ra) of 41.26 and 15.5 for 250 ppm C2H5OH and 0.5 ppm NO2, respectively, at 400 °C [15] The thermal evaporation method for SnO2 nanobelt preparation has been utilized to synthesize Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT ZnO, In2O3, CdO Ga2O3, and PdO2 nanobelts Wan and coworkers prepared ZnO NW via thermal evaporation for C2H5OH gas sensors [28] Sensing devices have also been developed by coating dispersed ZnO NWs on electrodes, which are fabricated by applying microelectromechanical system (MEMS) technology The sensor response to ppm C2H5OH RI PT gas is as high as 47 at 300 °C, and this response has yet to be obtained with ZnO-based gas sensors [28] Zhang and coworkers [44–46] conducted a series of studies on In2O3 NW gas sensors In the gas-sensing mechanism of SMO-NWs, its Debye length is the main factor that SC determines the gas-sensing performance of NWs Thus, the gas-sensing properties of NWs have shown remarkable improvements because the Debye length is comparable to the diameter of M AN U NWs For example, the diameter of an In2O3 NWs prepared through laser ablation is about 10 nm, and this material has been used to fabricate single NW field-effect transistors (FET) as gas sensors This type of sensors can detect NO2 gas at ppb level at room temperature [44] Nevertheless, Zhang and coworker also demonstrated that NW–NW junctions importantly TE D contribute to the gas-sensing properties of multiple NW sensors For instance, the detection limit of multiple In2O3 NW sensors is about ppb NO2, which is lower than that single In2O3 EP NW sensors (20 ppb NO2) as demonstrated in Figure In general, SMO-NW generates a high gas response, but other types of nanostructured AC C SMO exhibit a higher gas response than SMO-NW does Xu and coworkers systematically investigated the C2H5OH gas-sensing properties of SnO2 NP assembled in a porous film and a SnO2 NW film [70] [Figures 2(a–c)] and found that the gas response of SnO2 NP with a porous film is better than that of SnO2 NW sensors However, the comparative gas-sensing properties of SnO2 NPs- and NWs-based sensors indicate that the long-term stability of NW gas sensors is greater than that of NP gas sensors The gas-sensing mechanism of NWs- and NPs-based sensors [Figures 2(d and e)] revealed that NW–NW and NP–NP junctions determine their gas- Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT sensing performance Gas sensors under long-term operation at increased temperatures result in the aggregation and encapsulation of NPs into large agglomerates because of this aging process, which involves sensing at increased temperatures Consequently, the number of junctions and the porosity of the NPs-based gas sensors decrease, thereby reducing the gas-sensing RI PT performance Sysoev and coworkers systematically compared the gas-sensing performance of SnO2 NWs and NPs and revealed that the stability of SnO2 NW sensors is better than that of SnO2 NP sensors [71] SC SMO-NWs exhibits size confinement in two coordinates and length as an ideal channel for M AN U electrical conduction carriers This property is an advantage for FET and self-heating sensor development FET as sensor uses SMO-NW as conduction channel between source and drain contacts, which can be modulated by the Fermi level shift during interaction with surrounding gases [72] Wang and coworkers first demonstrated single SnO2 NW-FET as a gas sensor [73] TE D Single SnO2 NW FET is then prepared by depositing dispersed SnO2 NW on a SiO2/Si substrate and electrodes are fabricated through electron-beam lithography Electrical characterization shows that SnO2 NW conductivity increases, and the gate threshold voltage decreases when the EP SnO2 NW-FET sensors are transferred from an ambient condition to a vacuum setting Sensors with reduced size and power are essential for applications in the Internet of Things AC C (IoT), and the development of these sensors involves NW materials as optimum platforms Joule heating of NWs can be used to manufacture gas sensors with power consumption at a microwatt level without requirement of external heaters Strelcov and coworkers developed single SnO2 NW sensors by using membrane electrodes to fabricate self-heating gas sensors [74] Under pulse H2 gas exposure, the conductivity of single NW varies at different applied voltages that heats the NW surface to the activated temperatures of reactions between adsorbed Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT oxygen and H2 gas molecules Furthermore, the transient response is strongly dependent on applied voltages, and these sensors can detect H2 gas at a microwatt power level Despite the high sensitivity and stability of SMO-NW sensors, the selectivity of this sensing platform should be improved for practical applications Several approaches to enhance RI PT the selectivity of SMO-NW sensors include the modulation of operating temperatures, use of relevant SMO-NW materials, and functionalization of SMO-NW with relevant catalyst The third approach possibly enhances the selectivity of SMO-NW sensors Kolmakov and coworker SC demonstrated an enhanced selectivity of SnO2 NW sensors toward H2 gas by decorating Pd NPs sensor arrays and multiple sensors M AN U on the NW surface [27] This selectivity provides a wide range of SMO-NW applications for Gas-sensing mechanisms of SMO-NW sensors 3.1 Sensing mechanism of single SMO-NW A schematic of single SMO-NW gas sensor is presented on the top of Figure NW is TE D sufficiently long to spread over the micrometer-sized trench between two electrodes If the contact resistance between NW and electrodes is disregarded, an electron depletion layer for n-type semiconductors or a hole accumulation layer for p-type semiconductors on the surface EP of SMO-NW as a core–shell model is responsible for the performance of single NW gas sensors The core and the shell could regulate the conductivity of n-type and p-type SMO- AC C NW, respectively Therefore, the gas-sensing mechanism of n-type and p-type SMO-NW in reducing and oxidizing gases is rather different Gas molecules interact with preadsorbed oxygen on the surface of NW when the n-type SMO-NW is exposed to reducing gases and consequently release free electrons Hence, the thickness of the electron depletion layer is therefore reduced in comparison to the one in air and sensor conductance increases [Figure 3(a)] Gas molecules become adsorbed on the surface of NW when the n-type SMO-NW is exposed to oxidizing gases, such as oxygen molecules, thereby capturing free electrons from Submitted to Journal of Science: Advanced Materials and Devices ACCEPTED MANUSCRIPT the surface of NW Thus, the electron depletion layer extends and sensor conductance decreases [Figure 3(b)] If p-type SMO-NW exposed to reducing gases is considered, gas molecules interact with preadsorbed oxygen and consequently release free electrons, thereby reducing the RI PT thickness of the hole accumulation layer compared with that of the one in air Thus, sensor conductance decreases [Figure 3(c)] Conversely, gas molecules become adsorbed on the NW surface when oxidizing gases come in contact with a p-type NW surface and capture free SC electrons Consequently, the thickness of the hole accumulation layer increases compared with that of the one in air Sensor conductance also increases [Figure 3(d)] The conductivity ߪ݀ ‫ܦ‬ ߨሺ‫ ܦ‬− 2‫ ݊ܦܮ‬ሻ2 ߨ ൬ − ‫ ݊ܦܮ‬൰ = ܰ݀ ‫݊ߤݍ‬ , ℓ 4ℓ (1) ߨ൫‫ ݌ܦܮܦ‬− ‫ܮ‬2‫ ݌ܦ‬൯ ߪܽ ‫ ܦ‬2 ‫ܦ‬ ߨ ቈ൬ ൰ − ൬ − ‫ ݌ܦܮ‬൰ ቉ = ܰܽ ‫݌ߤݍ‬ , ℓ 2 ℓ (2) ‫= ݊ܩ‬ TE D ‫= ݌ܩ‬ M AN U of n- and p-type SMO-NW is calculated by using the following equations [75]: where σ = qµN is the conductivity of a semiconductor NW; D is the diameter of NW; l is the length of NW; N is the carrier concentration; µ is the carrier mobility; q is the carrier charge; EP Vs is the surface bending potential in the reducing gas environment; and LDn and LDp are the 3.2 AC C thickness of depletion and accumulation layers, respectively Sensing mechanism of multiple-junction NW Figure illustrates the gas-sensing mechanism of multiple-junction NW sensors, which are frequently applied to explain the gas-sensing mechanism of the on-chip growth of SMONWs In this case, NW–NW junctions play a role as a conduction channel Therefore, the gassensing characteristics of multiple-junction NW sensors depend on the thickness of electron depletion and accumulation layers on a NW surface and the potential barrier heights of NW– ... Response Ref o 3.2 (Ra/Rg) [18] o 300 C Thermal evaporstion Thermal evaporation Thernal evaporation Thermal evaporation Thermal evaporation Thermal oxidation Thermal evaporation Thermal evaporation... evaporation Thermal evaporation Thermal evaporation Thermal evaporation Thermal evaporation Thermal evaporation Wet chemical Wet chemical Hydorthermal Thermal evaporation Wet chemical Wet chemical... Bridged NWs Bridged NWs Thermal oxidation Thermal evaporation Thermal evaporation Thermal evaporation Thermal evaporation Thermal evaporation Thermal evaporation Target gas NO2 (100 ppm) NO2 (5 ppm)