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

On-chip growth of semiconductor metal oxide nanowires for gas sensors: A review International Training Institute for Materials Science ITIMS, Hanoi University of Science and Technology

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

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Chu Manh Hung, Dang Thi Thanh Le, Nguyen Van Hieu*

International Training Institute for Materials Science, Hanoi University of Science and

Technology, Hanoi, Viet Nam

Corresponding authors

* 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

On-chip growth of semiconductor metal oxide nanowires for

gas sensors: A review

International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet, Hanoi, Vietnam

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 volume, high crystallinity, and perfect pathway for electron transfer (length of NW) SMO-

NW 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 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- 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 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

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

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

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 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 applications of SMO-NWs are summarized in Table 1

Table 1 Outstanding works performed on SMO-NWs gas sensors science 2002

NWs materials Sensor types Target gases Years Cited times

ZnO

Resistive sensor FET transistor

In2O3-Au FET transistor CO 2011 29 [50] Mg-In2O3-Au, -Ag, -Pt FET transistor CO 2013 38 [51] TiO2 Resistive sensor C2H5OH 2008 109 [52]

TiO 2 Resistive sensor C2H5OH 2013 48 [53] TiO2 Resistive sensor H2 2008 10 [54] TiO2 Resistive sensor NH3,C2H5OH 2014 4 [55] TiO2-Pd Resistive sensor VOCs 2014 3 [56] TiO2-Pt Resistive sensor CO, NO2 2014 2 [57] SnO2, TiO2, In2O3 Resistive sensor H2,CO 2006 135 [58] CuO FTE transistor CO 2009 178 [59] CuO Resistive sensor CO, NO2 2008 162 [60] CuO Resistive sensor H2S 2008 145 [61] CuO Resistive sensor H2 2010 57 [62] CuO Resistive sensor C2H5OH 2009 48 [63] CuO FET transistor CO2 2010 37 [64] CuO Resistive sensor CO 2009 26 [65] CuO Resistive sensor H2S 2012 21 [66]

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

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 contribute to the gas-sensing properties of multiple NW sensors For instance, the detection limit of multiple In2O3 NW sensors is about 5 ppb NO2, which is lower than that single In2O3

NW sensors (20 ppb NO2) as demonstrated in Figure 1

<Figure 1>

In general, SMO-NW generates a high gas response, but other types of nanostructured 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-

<Figure 2>

SMO-NWs exhibits size confinement in two coordinates and length as an ideal channel for 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

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 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 (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

on the NW surface [27] This selectivity provides a wide range of SMO-NW applications for sensor arrays and multiple sensors

A schematic of single SMO-NW gas sensor is presented on the top of Figure 3 NW is 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

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-

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

of n- and p-type SMO-NW is calculated by using the following equations [75]:

Figure 4 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 SMO- NWs In this case, NW–NW junctions play a role as a conduction channel Therefore, the gas- sensing 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–

The width of electron depletion layers is reduced when an n-type multiple-junction NW

is exposed to reducing gases and the potential barrier heights of NW–NW junctions are consequently decreased Thus, the conductance of multiple-junction NW increases [Figure 4(a)] Conversely, the thickness of depletion layers and the potential barrier height increase when NW is exposed to oxidizing gases Sensor conductance consequently decreases [Figure 4(b)] The thickness of a hole accumulation layer or the main conduction layer and the height

of the potential barrier decrease when a p-type multiple-junction NW is exposed to reducing gases Likewise, the density of the hole at the NW–NW junction [Figure 4(c)] and the sensor conductance decrease Conversely, sensor conductance increases upon exposure to oxidizing gases because the width of the hole accumulation, height of potential barrier, and hole density

at the NW–NW junction increase [Figure 4(d)]

Hubert and co-workers [76] indicated that conductivity mainly depends on electron

concentration (ns), which displays sufficient energy to overcome the potential energy barrier

qVs of NW–NW junctions in n-type semiconductors This relationship can be expressed through Boltzmann distribution:

݌෤ݏ= ݌ܾ݁ݔ ݌ ൬ 2݇ܶ൰ ݍܸݏ (5)

p-type SMO-NW sensors can be given as follows:

In Equations Error! Reference source not found and Error! Reference source not found.,

the relationship between the responses of n-type and p-type SMO-NW is given by the

following [77]:

<Figure 4>

Different methods have been developed to integrate NW into functional substrates These methods can be classified into the following groups: indirect and direct NW integration methods (on-chip growth) [11] In this section, some important indirect and direct methods are briefly introduced

A large number of mediation methods are used to integrate NW into various functional substrates for nanoelectronic and nanosensor device applications [11,13,78] However, we only summarize several important methods, which are potential for the effective integration of SMO-NW on a large area of substrates to fabricate SMO-NW gas sensors through mass production at a low cost

4.1.1 Integration of NW by contact printing

Contact printing has been extensively applied to integrate NWs on functional

substrates For instance, Javey et al developed a simple method to fabricate NWs-based

electronic and sensor devices [79–86] In this method, a growth substrate consisting of randomly distributed NWs is slid into a functional substrate, and NWs are detached from the growth substrate because of the sliding step NWs are simultaneously transferred to the surface of a functional substrate through van der Waals interaction The alignment direction

of NW distributed on the functional substrate is the same as the sliding direction The integration of NW into the functional substrate is as follows First, NWs are grown on a substrate and a new functional substrate is patterned through photolithography for the selective integration of NWs on the desired area Next, the NW growth substrate is slid on a prepatterned substrate, and NWs are formed on the desired area through a lift-off process In addition to contact printing, roll printing is also utilized to integrate NW into a functional substrate In this technique, NWs are grown on a cylindrical substrate with a random orientation NWs are subsequently transferred to the functional substrate by rolling the cylindrical substrate on the surface of a receiver substrate With roll printing, the contact area between the NW growth substrate and the functional substrate is relatively small This small contact area is an advantage to integrate NWs on a large surface without a high demand for the smooth and planar properties of functional substrates Another advantage of this method is the ability to integrate multilayer NWs into receiver substrates to fabricate 3D electronic devices [79] Contact/roll printing can be utilized to integrate either inorganic NWss [87–90]

or organic NWs [91] into functional solid and plastic substrates [87,91]

4.1.2 Integration of NWs by chemical binding or electrostatic interactions

This method relies on the direct interaction between decorated NWs and patterned surfaces through hydrogen bonding, van der Waals interactions, and electrostatic interactions

Myung et al [92] reported the integrations of highly aligned V2O5 NWs on the desired area of

NW, which is negatively charged Therefore, V2O5 NWs become attracted and aligned on positively charged regions through van der Waals interactions [Figure 5(c)] Finally, multiple electrodes are deposited onto the aligned NWs [Figure 5(d)] With simple experimental procedures, this method is widely utilized to integrate various 1D nanostructures, such as ZnO NWs [93], carbon nanotubes [94], and Au/Ni/Au NWs [95], on the functional substrates

<Figure 5>

4.1.3 Integration of NWs by a microfluidic channel

The microfluidic channel method is an alternative for the alignment of NWs on functional substrates This technique was developed by Lieber and co-workers [96] to fabricate functional nanoscale electronic devices relying on individual GaP and InP NW (Figure 6) NWs are integrated on the surface of a substrate by passing a NWs-dispersed solution through

a microfluidic channel, which is fabricated using poly-(dimethylsiloxane) The width and length of this channel are 50–500 µm and 6–20 mm, respectively [96] The alignment of NWs

on the substrate is controlled by changing the position and flow direction of a microfluidic channel With microfluidic channel method, any of the NW structures of ZnO, GaP, InP, CdS [97,98], or SWCNT [99] can be integrated

<Figure 6>

4.1.4 Integration of NW by Langmuir–Blodgett (L–B) method

The L–B technique is normally utilized to integrate structures with large aspect ratios, such as NPs, NRs, NWs, and 2D nanosheets [100–102], into a receiver substrate This method allows the transfer of monolayers of nanostructured materials in solvents on a substrate to

successfully integrated into various functional substrates by using the L–B technique

<Figure 7>

4.1.5 Integration of NWs by bubble-blowing method

Bubble-blowing technique is a common method to integrate well-aligned NWs and carbon nanotubes over large areas onto solid substrates and plastic, curved surfaces This method involves three simple processes First, the NWs are dispersed in a polymer to form a stable suspension This liquid solution is then blown through a circular die with a controlled pressure and blowing rate to form stable bubbles of aligned NWs Finally, the NW-blown bubble film is transferred to various substrates For example, a Si NWs-blown bubble is transferred to several substrates, such as Si wafer, curved substrate, and an open frame [109]

A transistor device relying on Si NWs can be fabricated through mass production with a good performance [109] Wu and co-workers also successfully integrated Te NWs into a Si substrate to develop electronic devices and sensors [110]

be conducted through different methods, such as vertical and horizontal NW growth and bridging NW growth

3.2.1 Direct growth of vertical NWs

Figures 8(a and b) present the schematic of the direct growth of a vertical NWs on a solid substrate The orientation of a NW growth axis is vertical to the substrate This aligned NWs are achieved through either vapor–liquid–solid (VLS) or vapor–solid mechanisms With this method, a substrate with a good lattice mismatch and growing NWs is initially deposited with nanosized seeds NWs are subsequently grown vertically on the substrate from these seeds

For instance, Yang et al conducted a successful experiment on the growth of vertically aligned ZnO NW on silicon and sapphire substrates via the VLS mechanism [111] Wei et al

also developed an alternative method combining a laser interference patterning technique and

a hydrothermal method to grow vertical ZnO NWs on a large surface area [112] A hole array

is fabricated on the surface of substrate through laser patterning NWs are then formed from these holes in a mixed growth solution containing Zn(NO3)2 and HMTA by using a hydrothermal method [Figure 8(c)] The top view of a SEM image [Figure 8(d)] shows the uniformity of vertically aligned ZnO NW with a height of 5 µm [112] In addition to these two methods, other techniques have been developed to grow vertically aligned NW on the substrate [113] A number of vertical SMO-NW types, such as WO3 [114,115], TiO2 [116,117], Fe2O3 [118,119], and SnO2 [120,121], have also been successfully formed on functional substrates

3.2.2 Direct growth of horizontal NW

Figures 9(a and b) illustrate the horizontal NW growth on the plane of a substrate surface The axis of a growing NWs is parallel to the substrate surface The specific growth conditions

of NWs should be considered For example, the crystallographic property of a substrate

should be suitable for the orientation of NWs along the substrate surface Nikoobakht et al successfully fabricated a horizontally aligned ZnO NWs on an α-plane sapphire (11 0) [122,123] ZnO NWs are grown along [1 00] direction, in which the lattice parameters of c- plane ZnO NWs and α-plane sapphire are similar This method is also utilized to grow the

horizontally aligned NWs on the selected positions of a substrate for electronic devices and sensors fabrication Figures 9(c and d) show the horizontal growth of NWs on Au films fabricated through photolithography via the VLS mechanism The growing NW is in contact

with another growing NW (Figure 9(e)) Xu et al used other growth methods and fabricated a

horizontal ZnO NW onto a single-crystal ZnO substrate by applying a hydrothermal process

[124] Fortuna et al successfully grew horizontally aligned GaA NWs on a GaA substrate by

utilizing the MOCVD method However, the choice of growth substrates is limited For

example, substrates should exhibit a good lattice mismatch with the growing NW Wang et al

developed a method to fabricate horizontally aligned ZnO NWs on various substrates, such as organic, inorganic, single crystal, polycrystal, and amorphous structures, by applying the

combined effect of a ZnO seed layer and a catalytically inactive layer [125] Wang et al also

application, and the sensitivity of this sensor is higher than that of conventional sensors based

on SnO2 NWs [126]

<Figure 9>

3.2.3 Direct growth of bridging NWs

The direct growth of bridging NWs is simpler than the two aforementioned methods, and the orientation and alignment of growing NWs are not required However, NWs should be sufficiently long to create a connection between two electrodes fabricated on a substrate This method is utilized widely to fabricate electronic and sensor devices based on semiconductor NWs [11,13,78] Figures 10(a and b) illustrate the direct growth of bridging NWs Haraguchi

et al successfully utilized this technique to fabricate a bridging GaN NW device, which is

shown in SEM images in Figures 10(c and d) [127] Long and short GaN NWs are directly bridged over a trench between two electrodes, respectively A critical step to grow NWs is the deposition of a catalytic film on the sidewall of electrodes This step allows NWs to grow initially on the catalytic film and then bridge across the trench to the opposite electrode side

[128] Kim et al used this method and successfully fabricated bridging NWs-based logic

gates for electronic and photonic device applications [129]

<Figure 10>

In the context of SMO-NWs-based gas sensor applications, the direct growth of bridging NWs is also utilized widely because of the simple experimental method and ability to fabricate NWs-based sensors through mass production and high reproduction This method commonly forms a NW–NW junction between two electrodes, which are responsible for the sensing characteristic of sensors with large-diameter NW (50–100 nm) [35] Therefore, the sensitivity and stability of a bridged NW gas sensor are higher than those of a sensor based on NWs grown by indirect methods [35] A comprehensive review on the direct growth of NWs

or on-chip growth for gas sensors is presented in the following sections

Tin oxide (SnO2) is a frequently used material for gas sensor applications [130] Most of commercial semiconductor gas sensors have been prepared using SnO2 materials as gas-

sensitive elements Different nanostructures of SnO2 materials have also been developed for

for gas nanosensor development Comini and coworkers initially explored the gas-sensing

post-synthesis techniques, but these techniques are limited by various factors Thus, SnO2 NW gas

been extensively investigated, and different SnO2 NW gas sensors have been developed through on-chip growth (Table 2)

Table 2 A survey of SnO2 NW gas sensors fabricated by on-chip growth

Sensor types Growth method Target gas T ( o C) Response Ref

Bridged NWs Thermal evaporation H2 (1000 ppm) 300oC 3.2 (R a /R g) [18] Bridged NWs Thermal evaporstion NO2 (1 ppm) 200oC 39.8 (R a /R g) [20] Bridged NWs Thermal evaporation NO2 (1 ppm) 200oC 98.5 (R a /R g) [132] Bridged NWs

Bridged NWs

Thernal evaporation Thermal evaporation

NWs thin film

Thermal evaporation Thermal evaporation

SnO2 NWs-based H2 gas sensors are the first ones to be fabricated through on-chip growth

Au electrodes at 950 °C, and a gap between two electrodes is bridged The sensing performance of this sensor to H2 gas is better than that of SnO2 NWs-based sensors fabricated

by off-chip methods SnO2 NWs are directly grown on Au electrode at 950 °C, but this process can decrease the conductivity of Au electrode Choi and coauthors proposed a

remarkable process for the on-chip growth of SnO2 NWs on Pt electrodes for NO2 gas sensors

films as a catalyst layer, and the thickness of the Au film can determine the morphological characteristics of SnO2 NWs [146] Thus, a considerably thin Au catalyst layer (~10 nm) is deposited and patterned on Pt electrodes, and SnO2 NW is grown at 750 °C by using Sn

grown on Pt electrodes and used to detect NO2 gas with high sensitivity However, the

electrodes Le and coworkers developed an alternative process that deposits an indium tin oxide thin layer in between Au and Pt layers before the on-chip growth of SnO2 NW is performed [142] Figure 11 presents the results of the fabrication and gas-sensing characteristics of on-chip-grown SnO2 NW sensors

<Figure 11>

The formation of bridged NW junction is key to enhancing the gas-sensing performance of

an on-chip grown NWs The on-chip growth of SnO2 NW thin films has yielded relatively

buffer layer between SnO2 NWs and electrodes, and this layer contributes to the resistance of sensors, although this layer is partially sensitized to any gases To overcome this problem,

masks are used to develop heaters and electrodes because lithography cannot be applied to fabricate heaters and electrodes SnO2 NWs grown on this substrate still possesses a buffer layer between electrodes and SnO2 NWs Nevertheless, the buffer layer is not a continuous

on a rough Al2O3 substrate still exhibit a high gas response [137] To address this problem

Gas sensors based on ZnO materials have been widely compared with those based on SnO2 materials [147] Hence, the convenient synthesis of high-quality single-crystalline ZnO NWs has facilitated intensive investigations on gas sensor applications ZnO NWs can be used for other important applications, such as UV sensors, biosensors, and nanopiezotronics [148,149] The on-chip growth of ZnO NWs can be used for gas sensors and other

ZnO NW gas sensors fabricated via on-chip growth are listed in Table 3

Table 3 A survey of ZnO NW gas sensors fabricated by on-chip growth

Sensor types Growth method Target gas T ( o C) Response Ref

Bridged NWs Thermal evaporation CO (500 ppm) 320oC 0,3 (∆R/R a) [150]Bridged NWs Thermal evaporation C2H5OH (100 ppm) 300oC 0,26 (∆R/R a) [31]Bridged NWs Thermal evaporation CO (100 ppm) @RT 900 (R a /R g) [151]Bridged NWs Thermal evaporation NO2 (5 ppm) 225oC 65 (R a /R g) [29,35]NWs thin film

NWs thin film

Thermal evaporation Thermal evaporation

N(CH3)3 (5 ppm) HCHO (5 ppm)

than those of other types of ZnO NW sensors [35,29] Youn and coworkers fabricated and compared the gas-sensing properties of ZnO NW sensors with different types of bridged NWs

by developing ZnO NWs in multiple and parallel junctions [151] They showed that ZnO NW sensors with parallel junctions exhibit an improved gas-sensing response and can detect CO at room temperature The formation of grown ZnO NWs can affect the gas sensing performance

of NW sensors The proper engineering design of ZnO NW sensors is necessary to develop novel gas sensors Hugo and coworkers presented a novel design for ZnO NW sensors, and discrete islands of Au catalyst are deposited on and between Pt electrodes for the controllable growth of ZnO NWs [159] Consequently, NW–NW junctions on devices are enhanced, current leakage through seed layers is eliminated Thus, the gas-sensing performance is significantly enhanced Figures 12(a–c) illustrate the fabrication of ZnO NW gas sensors by using discrete islands of Au catalyst The density of ZnO NWs can be controlled further by varying growth times [Figures 12(e–h)] The gas-sensing characteristics shown in Figures 12(i and k) reveal that a moderate NW density should be explored for maximum gas-sensing responses

<Figure 12>

ZnO NW thin films contain a large number of NW–NW junctions because of the formation of networked NW in films A continuous Au thin film (~10 nm) is used as a

ZnO NWs can selectively grow on substrates containing a ZnO seed layer through various wet chemical methods [148] ZnO NWs formed through wet chemical methods are commonly grown at low temperature Thus, ZnO NWs are effectively used for wafer-scale fabrication and compatible with a conventional microelectronic process Wang and coworkers demonstrated the on-chip fabrication of ZnO NW gas sensors [162] via a hydrothermal method, and as-prepared sensors elicit a good response to CO, H2, and NH3 gases at 250 °C Khoang and coworkers applied a wet chemical method for the wafer-scale fabrication of ZnO NW sensors through on- chip growth [155] Figure 13 demonstrates that ZnO NWs are grown on a 4-inch silicon wafer with a predeposited Pt electrode and a ZnO seed layer (Figure 13(b)) A dense ZnO NWs are selectively grown on a ZnO seed layer [Figures 13 (c and d)] The gas-sensing characteristics indicate that the length of ZnO NWs should be optimized for gas sensing This work also reveals that wet chemical methods are applied to form three types of NW–NW junctions, namely, point-junctions, cross-junctions, and block-junctions [163] These junctions are utilized

to explain the gas-sensing properties of ZnO NW thin film prepared by wet chemical methods Moreover, wet chemical method can be employed to develop a multiple-junction ZnO NWs by growing on discrete catalyst islands [156,160].

<Figure 13>

<Figure 14>

Nanostructured tungsten oxide shows potential for a wide range of applications, such as electrochromic devices, dye-sensitized solar cells, photocatalysts, field emission devices, and gas sensors [166] For gas-sensing applications, tungsten oxide NW has been extensively investigated because of their distinct properties similar to those of SnO2 and ZnO NWs [167] Nevertheless, most of gas sensors based on tungsten oxide NWs are fabricated by off-chip

significantly enhances the gas-sensing response Thus, on-chip growth technique can be applied to develop high-performance tungsten oxide NW gas sensors However, studies on the on-chip growth of tungsten oxide NWs for gas sensors have been rarely performed Wang and

system [173] The length, diameter, geometry, and position of NWs are precisely controlled

the equipment used in this fabrication is relatively expensive, the growth of WO3 NWs requires no high-temperature process, which can be completely compatible with current integrated circuit technology Tungsten oxide NWs can be synthesized through thermal evaporation, and the implementation of this method for the on-chip fabrication of tungsten oxide NWs on prefabricated electrodes similar to that of SnO2 and ZnO NWs remains challenging Hence, the on-chip growth of tungsten oxide NWs for gas sensors should be further explored (Table 4)

Table 4 A survey of tungsten oxide NW gas sensors fabricated by on-chip growth

Sensor type Growth method Target gas T ( o C) Response Ref

Bridged NWs Thermal oxidation NO2 (100 ppm) 200oC 9,3 (R a /R g) [172]Bridged NWs Thermal evaporation NO2 (5 ppm) 180oC 34,1 (R a /R g) [39]Singe NW Thermal evaporation H2 (100 ppm) @RT 1,9 (∆R/Ra) [173] NWs thin film Thermal evaporation NO2 (1 ppm) 300oC 2.0 (Ra/Rg) [174]*Bridged NWs Thermal evaporation Cl2 (5ppm) 250oC 3.0 (R a /R g) [144]*Bridged NWs Thermal evaporation NO2 (1 ppm) 250oC 19.9 (R a /R g) [175]*Bridged NWs Thermal evaporation NO2 (2.5 ppm) 150oC 10.2 (R a /R g) [176]

Our research group developed an effective process for the on-chip growth of tungsten oxide NWs for gas sensor applications [174] through simple thermal evaporation In this

layer via thermal evaporation The on-chip growth of tungsten oxide NW on a solid substrate

is difficult because the adhesion between W film and the substrate is largely weak at growth temperatures The use of unpolished Al2O3 substrates enhances the adhesion between the W film and substrate, and this phenomenon is a key issue for the successful on-chip growth of tungsten oxide NWs via thermal evaporation [174] The use of unpolished substrates cannot

be applied to lithography for device fabrication Therefore, the on-chip fabrication of tungsten oxide NWs on a Si substrate has been investigated [144,175] We fabricated discrete islands

of bilayer Cr/W on Si substrate for the on-chip growth of tungsten oxide NWs via thermal evaporation [Figures 15(a–d)] and found that tungsten oxide NWs are selectively grown on the island [Figures 15(d and e)] The gas-sensing performance of this sensor is better than that

of the sensor fabricated on an Al2O3 unpolished substrate [Figures 15(e and f)] The growth of tungsten oxide NWs through thermal evaporation is commonly conducted at relatively high temperatures, which remain a problem in microelectronic fabrication Qin and coworkers carried out the on-chip fabrication of tungsten oxide NWs (W19O48) through the thermal oxidation of W film at 650 °C [176] The length of tungsten oxide NWs increases by prolonging growth time This fabrication process shows potential for the development of tungsten oxide NW sensors on Si substrate A comparative NO2 gas-sensing performance reveals that the performance of tungsten oxide NW sensors prepared by annealing a continuous W film is not significantly higher than those of other on-chip-grown tungsten oxide NW sensors through thermal evaporation [175] (Table 4) because of the presence of a buffer layer between electrodes and NW film Thus, the sensing performance of this sensor may be improved further by growing tungsten oxide NWs from discontinued W layers

<Figure 15>

eV), and this semiconductor can be used for gas sensors and other applications [177,178] Furthermore, p-type SMO gas sensors exhibit more promising gas sensing materials for various applications than n-type SMO gas sensors do Huber and coworkers demonstrated that the response of p-type SMO gas sensors is equal to the square root of that of n-type SMO gas sensors [179] Hence, CuO NW has been considerably investigated because of their simple growth [178] Different methods are used to grow CuO NWs However, thermal oxidation is frequently applied to grow CuO NWs because of its simplicity [180,181] This method is convenient for the on-chip growth of CuO NWs from Cu film for gas sensor applications Studies on the on-chip fabrication of CuO NW sensors are summarized in Table 5

Table 5 A survey of CuO NW gas sensors fabricated by on-chip growth

Sensor type Growth method Target gas T (oC) Response Ref

Bridged NWs Thermal oxidation H2S (50 ppm) 160oC 1.1 (∆I/I a) [61] NWs network

C2H5OH (500 ppm) Hơi ẩm (92%)

NWs are grown on a Cu foil through thermal oxidation, and electrodes are subsequently deposited on the top of the aligned NWs to characterize their gas-sensing performance Raksa [63], Hsueh [182], and their coworkers developed similar CuO NW sensor structures to detect ethanol and humidity Steinhauer and coworkers fabricated novel bridged CuO NW sensors through the oxidation of a Cu film site-selectively deposited on interdigitated electrodes by electroplating method [183,184] (Figure 16) An electroplated Cu film is prepared using an

structures and used site-selectively deposited Cu films as a source to grow CuO NWs through sputtering or e-beam evaporation methods Cu films are patterned through lithography and lift-off techniques Therefore, ion-milling process is unnecessary

<Figure 16>

Lupan and coworkers demonstrated an effective on-chip fabrication of CuO NW sensors [187,188] (Figure 17) In these studies, a networked CuO NWs are grown from the microparticle of Cu dropped on a substrate equipped with two electrode pads The fabrication

of these studies is considerably simple, but sensors exhibit an ultrahigh response to reducing gases [Figures 17(d–g)] Sensors display good responses to C2H5OH, H2, CO, and CH4 gases,

ppm C2H5OH gas is as high as 313at 250 °C The structure of this sensor is similar to that of sensors with NW grown on discrete islands This high response is attributed to large numbers

of NW–NW junctions because of the formation of a long networked NW between two

temperatures at 450 °C, 400 °C, and 425 °C

<Figure 17>

CuO NWs are commonly grown from a Cu film at 400 °C–500 °C Thus, CuO NWs can be on-chip grown by using a gas sensor microheater Steinhauer and coworkers fabricated a CuO

NW gas sensor by using a microheater to grow CuO NWs from a Cu film deposited on the top

of the microheater [191] This fabrication platform can be utilized for the on-chip fabrication

of NW gas sensors with other metal oxides, such ZnO, SnO2, and WO3, in which NWs are

grown through thermal evaporation

In2O3 and TiO2 NW materials have been utilized as gas sensors Nevertheless, the on-chip fabrication of NW sensors has yet to be extensively investigated Single-crystal In2O3 and TiO2NWs can be prepared through thermal evaporation [192–196] and thermal oxidation of In and

Ti films [197–204], which are suitable for the on-chip fabrication of NW sensors Vomiero and coworkers [140] developed an on-chip fabrication process for In2O3 NW sensors In this process, an In2O3 NWs are grown on an Al2O3 substrate by using an In thin film as catalyst layers via thermal evaporation and patterned by using sacrificial SiO2 layers Afterward, Pt contacts are sputter deposited on sacrificial areas by utilizing a shadow mask In2O3 NW sensors can detect 25 ppm acetone gas However, gas response is relatively low because of the presence

of a buffer layer between the Al2O3 substrate and the In2O3 NW film Lee and coworkers demonstrated an on-chip fabrication of networked TiO2 NW sensor [205] TiO2 NWs are selectively grown from a Ti film (~1 µm) through thermal oxidation In this process, a Ti film is deposited and patterned on the top of Pt electrodes through e-beam evaporation and lithography techniques TiO2 NWs form a networked NWs between electrode fingers The on-chip-grown TiO2 NW sensors exhibit the highest response to CO gas among the tested gases (C6H6, C7H7, and NO2), and their response value [(Rg-Ra)/Ra × 100%] to 1 ppm is approximately 11% at 400

Our group investigated the on-chip growth of semiconductor ternary oxide (Zn2SnO4) for

NO2 gas sensors [208] Zn2SnO4 NWs are directly grown on Pt electrodes by using Au catalyst layer through the thermal evaporation of a mixture of ZnO, Sn, and graphite powders Zn2SnO4

NW sensors exhibit a superior response to NO2 gas compared with their counterparts The

response (Ra/Rg) to 10 ppm NO2 is as high as 35 at a working temperature of 200 °C

Several SMO-NW materials, such as V2O5, [67,68], Fe2O3 [209–211], NiO [212,213], and

Co3O4 [214,215], for gas sensors have been examined Nonetheless, the on-chip growth of these

NW sensors has yet to be reported As such, future research should address these challenges for the practical applications of NW sensors

This review summarizes relevant studies on the on-chip fabrication of SMO-NW sensors

We focused on the on-chip growth of single-crystal SMO-NWs for gas sensors, which can be synthesized through thermal evaporation, thermal oxidation, and wet-chemical methods The discovery of the on-chip growth of SMO-NWs provides a new paradigm in manufacturing novel gas sensors with improved sensing performance, stability, and massive production However, the on-chip growth of single-crystal SMO-NWs are frequently conducted at relatively high temperatures, which are incompatible with conventional microfabrication processes The site-selective growth of SMO-NWs has shown remarkable advancements Therefore, this process can be applied to solve relevant problems

Acknowledgment

This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2017.25

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