Novel Self-Heated Gas Sensors Using on-Chip Networked Nanowires

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Novel self-heated gas sensors using on-chip networked nanowires with ultralow power consumption Ha Minh Tan, Chu Manh Hung, Minh Ngoc Trinh, Hugo Nguyen, Nguyen Duc Hoa, Nguyen Van Duy, and Nguyen Van Hieu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14516 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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ACS Applied Materials & Interfaces

Novel self-heated gas sensors using on-chip networked nanowires with ultralow power consumption Ha Minh Tan1, Chu Manh Hung1,*, Minh Ngoc Trinh1, Hugo Nguyen2, Nguyen Duc Hoa1, Nguyen Van Duy1, Nguyen Van Hieu1,* 1

International Training Institute for Materials Science, Hanoi University of Science and Technology, No 1 Dai Co Viet Road, Hai Ba Trung, 10000, Hanoi, Vietnam 2

Department of Engineering Sciences, Division of Microsystem Technology, Uppsala University, Lägerhyddsvägen 1, 751 21 Uppsala, Sweden

Corresponding authors *Chu Manh Hung, PhD *Nguyen Van Hieu, Professor International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No.1, Dai Co Viet Road, Hai Ba Trung, 10000, Hanoi, Vietnam Phone:

84 4 38680787

Fax:

84 4 38692963

E-mail:

[email protected] (CMH) [email protected] (NVH)

Post address:

Dai Co Viet, Hanoi, Vietnam

ACS Paragon Plus Environment


Submitted to ACS Applied Materials and Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The length of single crystalline nanowires (NWs) offers a perfect pathway for electron transfer, while small diameter of the NWs hampers losses to environment, substrate and metal electrodes. Therefore, Joule self-heating effect is nearly ideal for operating NW gas sensors at ultralow power consumption, without additional heaters. The realization of the selfheated NW sensors using ‘pick and place’ approach is complex, hardly reproducible, low yield and not applicable for mass production. Here, we present the sensing capability of the self-heated networked SnO2 NWs effectively prepared by on-chip growth. Our developed self-heated sensors exhibit good response of 25.6 to 2.5 ppm NO2 gas, while the response to 500 ppm H2, 100 ppm NH3, 100 ppm H2S and 500 ppm C2H5OH is very low, indicating the good selectivity of the sensors to NO2 gas. Furthermore, the detection limits is very low, down to 82 parts per trillion. As-obtained sensing performance under self-heating mode is nearly identical to that under external heating mode. While the power consumption under selfheating mode is extremely low, around hundreds of microwatt as scaled-down the size of electrode below 10 µm. The selectivity of the sensors can be controlled simply by tuning the loading power that enables simple detection of NO2 in mixed gases. Remarkable performance together with a significantly facile fabrication process of the present sensors enhances the potential application of NW sensors in next generation technologies such as electronic noses, the Internet of Things, and smartphone sensing. Key words: Self-heating; networked nanowires; low power sensors; NO2 gas; SnO2 nanowires

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1. INTRODUCTION Among various nanomaterials, the nanowires (NWs) have unique properties such as very large surface-to-volume ratio, dimensions comparable to the extension of surface charge region, superior stability owing to the high degree of crystallinity and relatively simple preparation methods that are potential candidates for the development of simple, costeffective, power-efficient, reliable, and sensitive gas sensors 1. Such sensors can be applied in priority fields such as environmental pollution control, the fossil fuel industry, the automotive industry, medical research, and homeland security. Up to now, various important technologies such as (1) on-chip growth 2, (2) surface-functionalization 3, (3) self-powering 4,5 and (4) selfheating 6 have been developed for NW gas sensors. The first two techniques are to enhance the well-known “3S” namely sensitivity, selectivity and stability, while the second two techniques are to avoid the integration of heating elements and to reduce power consumption. The length of NWs severs as resistance pathway for electron transfer thereby their Joule self-heating that can be effectively employed to warm up the NWs’ surface enough for ensuring rapid and reversible response to analyst gases, but the power consumes only microwatt levels due to small thermal capacitance and hampered thermal losses from NWs to their surroundings 6. Furthermore, the self-heated sensors without the integration of heating element offer simple fabrication and miniaturization capabilities for such NW sensors, especially for NW sensor arrays. The investigation of the self-heating effect of NWs has been carried out by some research groups

6–18

, including the present authors

previous work, various NW-device geometries such as single NW 14,19

, and networked NWs

12,19

19

. According to

6–10,16,17,19

, arrayed NWs

have been developed for self-heated gas sensors. In general,

there are two common approaches to preparing NW materials: top-down and bottom-up methods. The latter are more frequently used for producing single crystalline NWs that have outstanding stability with a self-heating effect 6. As far as we know, most self-heated gas

3




sensors using single crystalline NWs were fabricated by the off-chip method, which involves a series of tedious processes such as sonication, and dispersal of ensemble NWs on another substrate with prefabricated electrodes, the use of complicated equipment such as electronbeam lithography 20 to define or focused ion beam system 7 to deposit the electrical contacts, and specially low yield fabrication and hardly reproducible process. These are key reasons why this sensing platform of NWs has just remained an objective research, with limited applicability. As compared to individual NW or networked NWs sensors fabricated by off-chip methods, on-chip networked NW sensors exhibit great advantages such as higher sensitivity, reliability, reproducibility, perfect yield, and convenience for industrial-scale applications 19–23. Significant progress in on-chip networked SnO2 NWs gas sensors has recently been reported by some research groups

2,21–25

and the present authors

26,27

. However, the self-heating effect

of on-chip network NW gas sensors have been not yet investigated. The use of self-heating effect to operate on-chip networked NW gas sensors at microwatt power consumption level and without requirement of heating element can provide a new approach for avoiding the shortcomings of the self-heated NW sensors fabricated by off-chip methods7,12 as mentioned above. We believe that the self-heated on-chip networked NWs sensors with simple fabrication ensures not only the “3S” rule and but also power consumption at microwatt level that enables a wide range applications for such sensors in important technologies such as electronic noses, the Internet of Things, and smartphone sensing. The present work demonstrates, for the first time, the sensing capability towards NO2 gas of self-heated on-chip networked SnO2 NWs, which differ from the conventional self-heated NW gas sensors. Scaling down of the size of the on-chip networked SnO2 NW sensors is also performed in order to demonstrate the power consumption of such sensor at the microwatt level, similarly to conventionally self-heated NW sensors. This offers a new approach to self-

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heated NW gas sensors that is applicable to mass production due to the simplicity of the fabrication process. 2. EXPERIMENTAL The procedure of fabricating on-chip networked SnO2 NWs sensors studied in the present work is illustrated in Figure 1(a-f). First, prior to on-chip growth of SnO2 NWs, pairs of electrodes were fabricated on a glass substrate using conventional microelectronic technologies. These include depositing a photoresist (PR) layer by a spin-coating method, exposing the PR layer under UV light using a photomask with a pattern of the electrode pair, removing the exposed area, depositing four layers of ITO (20 nm)/Pt (100 nm)/ITO (10 nm)/Pt (5 nm) in sequence by a sputtering method, and finally patterning by a lift-off process. In particular, the purpose of the first ITO (20 nm) layer is to enhance the adhesion between the glass substrate and the Pt layer, while the purpose of the second ITO (10 nm) layer is to protect the Pt (100 nm) layer from damage caused by the on-chip NW growth process. The top Pt (5 nm) layer is the catalyst for growing SnO2 NWs by the well-known vapor–liquid– solid (VLS) mechanism. We have purposely designed three pairs of electrodes with the same electrode gap of 60 µm and different overlap lengths of 30, 90, and 270 µm on the same chip to systematically investigate their gas-sensing performance under the self-heating effect. It should be noted that the electrodes with these size can be fabricated via other techniques such screen-printing or metal showdown mask. These methods are frequently used for fabricating commercial gas sensors. As a general rule, smaller NW sensing areas need lower power to heat up to elevated temperature by self-heating. Therefore, we have also designed another pair of electrodes with a space of about 10 µm to operate the self-heated sensors in the microwatt range.

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Figure 1: The scheme for on-chip fabrication of networked SnO2 nanowires: (a) Oxidized Si substrate; (b) the Si substrate with photoresist coating; (c) the Si substrate with patterned photoresist; (d) the Si substrate with patterned Pt electrodes; (e) the Si substrate with onchip growth of SnO2 nanowires; (f) thermal evaporation set-up for on-chip growth of SnO2 nanowires.

The SnO2 NWs were directly grown on the glass substrate with the pair of electrodes through thermal evaporation at 800 oC using Sn as the source and an O2 flow of 0.5 sccm (standard cubic centimeters per minute); more detail about the synthesis of SnO2 NWs can be found in our previous works

28,29

. SnO2 NWs selectively grown on the pair of electrodes became

tangled with each other in the varying gaps between the electrode pair as illustrated in Figure 1(e). The three sensors on the same chip were named S1, S2, and S2, corresponding with the overlap lengths of 30, 90, and 270 µm. The morphologies of the SnO2 NWs were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan), high resolution transmission electron

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microscopy (HRTEM, Tecnai G2 F20, FEI), and selective area electron diffraction (SAED). The gas-sensing response were measured by using the flow-through technique with a standard flow rate of 200 sccm for both reference (dry air) and target gases (NO2, H2, NH3, H2S, and C2H5OH). The NW sensors (S1, S2 and S3) were operated under self-heating mode by applying different powers loads (from 16 to 60 mW), while a bias voltage of 1 V was used to measure the NW sensors under external heating mode at different temperatures (150, 200, 250, or 300 oC). This bias voltage is selected to avoid the self-heating effect, because the loading power at 1 V is very small for the resistance sensors in the range of kΩ. The loading power was controlled by the applied voltage using a Keithley model K2602A interface with a personal computer via home-made software. The electrical circuit and algorithm scheme of measurement program for measuring the sensors is presented in Figure S1 (See Supporting Information for detail). To confirm the self-heating effect of the sensors, thermographic images of the sensors were obtained with an infrared camera (Thermovision A40). The self-heating and external heating effects were compared via the sensor resistance, gas response-recovery time, and thermal emission microscopy. The sensor response was calculated by the ratios of Ra/Rg or Rg/Ra, corresponding to reduced or oxidized gas, respectively, where, Ra and Rg are the resistances under air and target gas exposure, respectively. Response time (τresp) and recovery time (τrecov) were defined as the time taken to reach 90% of the steady-state response for NO2 gas and air exposure, respectively.

3. RESULTS AND DISCCSUION Optical microscopy images of the three sensors (S1, S2, and S2) on a chip after directly growing SnO2 NWs are shown in Figure 2(a). The FESEM images taken from the gap of each sensor are shown in Figures 2(b,c) and indicate that the SnO2 NWs were selectively grown on the electrodes thanks to the Pt catalytic layer deposited on the top of the electrodes. Although

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we observed the SnO2 NWs randomly grown on the other area, they are very low density compared to those on the electrodes. It can also be seen that the SnO2 NWs grown on the pair of electrodes are long enough to tangle together over the gap of 60 µm. One can clearly see that the number of NW–NW junctions increases as the overlap length of the electrode pairs increases. These junctions act as electrical conducting paths for self-heating. The high density of NW junctions allows the sensors to be robust under self-heating mode. It can be seen that the device structure in the present work is very simple and efficient for large-scale fabrication as compared with the self-heated sensors realized by previous works

6–19

, in which a single

NW is usually contacted by a pair of electrodes, which is very complicated in large-scale fabrication. The microstructure of SnO2 NWs was further characterized by TEM and HRTEM. Figure 2(e) shows a typical TEM image of a single SnO2 NW with a diameter of around 95 nm. Figure 2(f) and the inset show HRTEM and SAED images from the outline region marked in Figure 2(e) and indicate that the NWs are single crystals with a clear lattice structure with an interspace of 0.23 nm of the (111 ) planes 30. The temperature achieved by self-heating was first evaluated by infrared camera. Thermographic images of the sensors (S2) were obtained under different loading powers. Figure 3(a) shows the linear relation between the self-heating temperature and loading power. The temperatures were estimated from the thermographic images as typically shown in Figures 3(b,c) under loading powers of 16.9 and 34.5 mW. It should be noted that the temperature estimated by infrared camera represents a relatively large area but not for the tiny networked NWs sites with nanoscale area. Thus, we have further evaluated the self-heating temperatures by comparing the resistance of the sensors under self-heating and external heating modes.

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Figure 2: Optical images of typical on-chip networked SnO2 NWs sensor (a) and FESEM images of the sensors with overlap lengths of 30 (b), 90 (c) and 270 µm (d). TEM (e) and HRTEM (f) image of a single SnO2 nanowire.

We first measured the resistance of the sensors versus temperature using a temperaturecontrollable external heater. Then resistances under self-heating mode were measured with different loading powers. The resistance as a function of external temperature and loading power are presented in Figure 3(d), in which the linear fits of resistance with either temperature or loading power have been normalized. The fitting has a certain error, but the normalization can be used to build up the relation between the self-heating temperature and 9




the loading power. It can be seen that the resistances are analogous for the external heating temperature range of 150‒350 oC and the loading power range of 20‒35 mW. This suggests that self-heating with such a loading power range can generate such a temperature range for the tiny networked NWs sites.

Figure 3: (a) Self-heating temperature versus power loads, (b and c) Typical thermographic images with power loads of 16.9 mW and 34.5 mW and (d) the normalized resistances under self-heating and external heating modes.

To compare the gas-sensing performance of the sensor under self-heating and external heating modes, we have used typical sensor sample S2 to measure their transient response under different external heating temperatures (150, 200, 250, and 300oC) and different loading powers (20, 25, 30, and 35 mW). Figures 4(a–d) and (e–h) show the transient response under external heating and self-heating modes, respectively. The sensors exhibited stable response and recovery characteristics in both cases. It is apparently observed that the resistance of the

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sensors was increased as exposed to NO2 gas. This can be attributed to the modification of potential barriers at the NW-NW junction as well as depletion layers surrounded the NWs. The depletion layers are slightly influenced by the density and mobility of the electron in the NWs, while they have strong influence on the potential barriers in case of the NWs with diameter relatively larger than Debye length 31. Thus the modulation of the contact resistance derived from the potential barriers at NW-NW junction can be dominated gas-sensing mechanism. When the sensors are exposed to NO2 gas, the NO2 molecules adsorb on the NWs’ surface and form adsorbed ion such as NO2-, NO3-, NO+, and NO- by extracting electron from the conduction band of SnO2 NWs

32

. The capture of electron of the NWs’

surface results in the extension of the depletion layers and increases the potential barriers (see Figure S2 in Supporting Information), leading to an increase of the sensor resistance. A very low response–recovery speed was obtained at either the lower temperature of 150oC or the lower power of 16 mW, which cannot be used for practical applications. The response (Rg/Ra) to NO2 gas (2.5‒20 ppm) is plotted as a function of temperature and power in Figure 4(i) and (k), respectively. In both cases, the response exhibited a bell-shaped relation with either temperature or power. The maximum response values of the sensor to 2.5 ppm were about 2.1 and 2.2 under the operating temperature of 250 oC and power of 18 mW, respectively. This indicates that nearly equivalent responses were observed in both cases. To further confirm the analogous sensing performance of the on-chip networked NWs sensors, the response and recovery times operated by the two modes were also compared. The response and recovery times are plotted as a function of temperature and power in Figures 4(j) and (l), respectively. It can be seen that the response and recovery times increase equivalently with increases in either temperature or power. This is consistent with a dynamic point of view: NO2 sensing is a thermally active process, and the higher the temperature, the shorter the residence time of the gas molecules on the SnO2 NWs surface 7. According to Figure 4(j, l),

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Submitted to ACS Applied Materials and Interfaces the variations of response and recovery times are in a similar range in the operating temperature range of 150‒300 oC and the loading power range of 16‒22 mW. These experimental results indicate that the on-chip networked SnO2 NWs sensors operated under self-heating mode with very low power consumption can obtain a similar performance under

12 9 6

o

(a)

(e)

o

30 mW

(b) 200 C 2.5 ppm

5 ppm

10 ppm

(f) 25 mW

20 ppm

(c)

2.5 ppm

o

5 4 3

10 ppm

(g)

(d)

(h)

5 4 3

Time (s)

2.5 ppm 5 ppm 10 ppm 20 ppm

2.5 ppm 5 ppm 10 ppm 20 ppm

(i)

(k) Response time Recovery time @ 2.5 ppm NO2

Response time Recovery time @ 2.5 ppm NO2

(l)

(j)

3.5 3.0 2.5 2.0 1.5 800 600 400 200

τ

Response (Rg/Ra)

(s) (resp., recov.)

15 10 5 15 10 5 15 10 5

700 1400 2100 2800 3500

Time (s)

800 600 400 200

20 ppm

5 ppm

20 mW

150 C

700 1400 2100 2800 3500 3.5 3.0 2.5 2.0 1.5

Self heating

R (kΩ)

12 9 6

250 C

35 mW

External heating

Response (Rg/Ra)

300 C

(s)

o

12 9 6

(resp., recov.)

R(kΩ)

conventional operating mode.

τ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150

200 o 250 T ( C)

300

20

25 30 P (mW)

35

Figure 4: Transient responses of on-chip networked NWs gas sensors under external heating (a-d) and self-heating (e-h) modes. The response and response-recovery time plotted as a function of temperature (i, j) and power load (k, l).

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Submitted to ACS Applied Materials and Interfaces To demonstrate the reproducibility of the on-chip networked SnO2 NWs sensor under self-heating mode as well as to investigate the effect of the electrode gap on the gas response and power consumption, we characterized the NO2 gas response of the sensor samples S1 and S3 (the electrode gaps are different from that of sensor sample S2). We measured the transient responses of samples S1 and S3 under self-heated loading power ranges of 16–22 mW and 30–60 mW, respectively. These loading powers were selected to obtain a response-recovery time similar to that operating under external heating mode at 150, 200, 250, and 300oC. The transient response at different loading powers of sensors S1 and S3 was also investigated, and good response-recovery characteristics to NO2 were obtained under external heating and self-heating modes (Figures S3 and S4 in the Supporting Information). Figures 5(a,b) and 5(c,d) shows the response of sensors S1 and S3 as a function of loading power and operating temperature, respectively.

10 ppm NO2 gas

2.4

5 ppm 2.5 ppm

2.0 1.6 3.6

16

18

20

1.6

NO2 gas

2.0 1.6

(d) 20 ppm

150

3.2 2.4 2.0 1.6

Sensor S3

30 35 40 45 50 55 60

3.6 2.8

10 ppm 5 ppm 2.5 ppm

Sensor S3

3.2

2.4

5 ppm 2.5 ppm

22

20 ppm 10 ppm 5 ppm 2.5 ppm

3.6

2.8

10 ppm

Sensor S1

(b)

2.8 2.0

20 ppm

Sensor S1

3.2 2.4

(c)

NO2 gas

2.8

NO2 gas

3.2

(a)

20 ppm

Response (Rg/Ra)

3.6

Response (Rg/Ra)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200

250

300

o

T ( C)

P (mW)

Figure 5. The response to NO2 gas of sensors (S1 & S3) as a function of loading power (a, b) and operating temperature (c, d).

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It can be seen that the sensors S1 and S3 exhibited peak responses at 18 and 40 mW, respectively. This means that the increase in the NW density can result in an increase of the optimum loading power. In the case of operating sensors under external heating mode, sensor S1, S2, and S3 all have the same optimum operating temperature of 250 oC. This indicates that the effects of self-heating and external heating on the gas response are not analogous. It should be noted that the gas response of junction-networked NWs is contributed by the surface-depleted layer of NWs and the depleted layer around the intersection of NW–NW junctions

23,33,34

. These areas are at the same ambient temperature under the external heater,

while the temperature at the NW–NW junctions is higher than at the NWs surface due to current crowding under self-heating mode because of the current density through the smaller area of NW-NW junction as compared with the cross-section of the NWs. The responses of the sensors S1, S2, and S2 are plotted as a function of gas concentration at their own optimum loading powers in Figure 6 (a–c). It can be seen that the response increases as the NO2 gas concentration increases. It should be noted that the relation between ܰ

response and gas concentration can be expressed as ܴܽ/ܴ݃ ∝ ‫ ]ܥ[ܣ‬in the range of detectable NO2 concentrations, where C is the gas concentration, A and N are constant. N may have some rational fraction value (usually 1 or 0.5) depending the NWs’ diameter

35

. A simple linear

regression fit was applied for the relation between the common logarithm of gas concentration [Log(C)] and response. According to the IUPAC definition 36, the definition of sensitivity as

∆R/∆C. This means that a higher ∆R at a given increase in gas concentration, ∆C, yields a higher sensitivity. Consequently, the sensitivities of the sensors can be compared from the slopes of the plots of Rg/Ra versus the common logarithm of gas concentration. The fitting results showed that the sensor S2 has the highest slope value, indicating that it has the highest sensitivity compared to sensors S1 and S3. To evaluate the theoretical detection limit of the

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Submitted to ACS Applied Materials and Interfaces sensors S1, S2, and S2, we used the IUPAC definition of the detection limit (DL), which is calculated as DL = 3(rmsnoise/slope), where rmsnoise is the root-mean-square deviation and

slope is the slope value of the linear fit of the gas response versus the common logarithm of gas concentration. The rmsnoise can be calculated using the variation in the gas response at baseline using the root-mean-square deviation (see the Supporting Information for details). We took 10 experimental data points at the baseline of the transient response of each sensor for the fifth polynomial fitting as shown in the insets of Figure 6. The obtained detection limit values of S1, S2, and S3 are about 161, 82, and 658 ppt (parts per trillion). The ppt levels of the detection limits for NO2 gas suggest potential applications of self-heating NW sensors such as environmental monitoring and breathe analysis, especially for diagnosing asthma.

(a)

Sensor S2 @25 mW

(b)

slope=2.4 slope=1.5 1.0016

Experimental points The fifth-order polynomial fit

1.0008 1.0000 0.9992

rmsnoise=8.07x10-5

0.9984 96

98 100 102 104 106

1.0008 1.0000

10 1520

5

4


rmsnoise=15.3x10-5


1.0016

3

98 100 102 104 106 Time (s)

slope=0.7

2

0.9992

rmsnoise=6.5x10-5

0.9984 96

Time (s)

5

1.0010 1.0005 1.0000 0.9995 0.9990 96

Resp. (Rg/Ra)

Resp. (Rg/Ra)

2

(c)

DL (ppt) ~ 658

DL (ppt) ~ 82

DL (ppt) ~ 161

3

Sensor S3 @40 mW

Response(Rg/Ra)

4

Sensor S1 @18 mW

Resp. (Rg/Ra)

5

Response(Rg/Ra)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

98 100 102 104 106 Time (s)

5

10 1520

5

10 1520

NO2 conc. (ppm) Figure 6. The Linear fit responses as function of NO2 gas concentration for sensors S1 (a) S2 (b) and S3 (c). The inset of (a, b, c) is a fifth-order polynomial of ten points of base line for calculating the root-mean-square deviation (rmsd) noise.

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Submitted to ACS Applied Materials and Interfaces The gas-sensing performance of the present self-heating sensors to reducing gases has been also investigated, the sensor S2 was measured with different concentrations of H2, NH3, H2S, and C2H5OH at loading powers of 40 and 50 mW. The obtained dynamic sensing transient response to those gases (see Figure S5 in Supporting Information) exhibited good reversible characteristics. The responses versus gas concentration at loading powers of 40 and 50 mW are shown in Figure 7. It can be seen that the response to reducing gases is relatively low compared to that of NO2 gas (see Table S1 in Supporting Information). Various previous works have shown that the response of pristine SnO2 NWs sensors to reducing gases (H2, NH3, H2S, and C2H5OH) was significantly enhanced by decorating suitable catalyst nanoparticles (for H2 37–39, H2S

29,40

, and C2H5OH

41–43

). In addition, the temperatures at the

NW–NW junctions are higher than that at the NWs’ surfaces under self-heating mode.

1.5

H2 gas

1.4

@40 mW @50 mW

1.5

(a) (c)

1.4

NH3 gas 1.2

@40 mW @50 mW

1.5

H2S gas @40 mW @50 mW

1.4

(b) (d)

1.2

1.8 1.6

1.3 1.2

50

100 150 200 H2 or H2S conc. (ppm)

C2H5OH gas

1.4

@40 mW @50 mW

1.2

Response (Ra/Rg)

1.3

1.3

Response (Ra/Rg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100 150 200 250 300 350 400 NH3 or C2H5OH conc. (ppm)

Figure 7. The response measured by self-heating mode with loading power of 40 and 50 mW as a function of reduced gas concentration H2 (a), H2S (b), NH3 (c) and C2H5OH (d).

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Consequently, only the NW–NW junction can reach the optimum working temperature and the reducing gases can react with oxygen ions at the junction but not at the NWs’ surface. This suggests that the self-heated network NWs sensors should be improved further for the detection of reducing gases. To study the selectivity of the self-heating network NWs sensor, we measured the transient response to 500 ppm H2, 100 ppm NH3, 100 ppm H2S, 500 ppm C2H5OH, and 2.5 ppm NO2 under a loading power of 20 mW; the results are shown in Figure 8(a). It can be seen that the responses to the reducing gases are negligible compared with the response to NO2 gas. This can be explained by the fact that the loading power of 20 mW can heat the NWs sensor up to approximately the optimum working temperature for NO2 gas but not for such reducing gases. This result suggests the idea that the loading power can be used to tune the selectivity of the self-heating sensors. We measured the transient response to mixed gases of 2.5 ppm NO2 and 100 ppm NH3 under varying loading powers (50–80 mW), as shown in Figure 8(b). As can be seen, the sensor shows a good response and recovery for NO2 gas for loading power ranging from 50 to 75 mW. When the power increased up to 80 mW, the sensor tends to response to NH3 gas. This aspect is similar to the use of working temperature to control the selectivity of gas sensors

44,45

. However, the control of loading power is more

effective than that of the temperature.

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2.2 @25 mW

(Rg/Ra)

(a)

2.0

1.0

2.5 ppm NO2

1.2

100 ppm H2S

1.4

100 ppm NH3

1.6

500 ppm C2H5OH

1.8 500 ppm H2

Response (Ra/Rg or Rg/Ra)

Submitted to ACS Applied Materials and Interfaces

200 400 200 400 200 400 200 400 200 400 Air

59 mW

63 mW 67 mW 71 mW 75 mW 80 mW

Air Air

(b)

Air

1.6

Air

1.4

Air

Air

(NO2& NH3)

50 mW 54 mW

500 2.4

1000

1500

2000

(NO2& NH3)

(NO2& NH3)

(NO2&NH3)

(NO2& NH3)

0.8

(NO2& NH3)

1.0

(NO2& NH3)

1.2

(NO2& NH3)

Response (Ra/Rg)

1.8

Response (Rg/Ra)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2500

@25 mW & 2.5 ppm NO2

Air

3000

(c)

2.0 1.6 1.2

500 1000 1500 2000 2500 3000 3500 4000

Time (s) Figure 8. The selectivity and stability characteristics of self-heating sensors: (a) The transient response of the sensors to H2 (500 ppm), NH3 (100 ppm), H2S (100 ppm) and C2H5OH (500 ppm) and NO2 (2.5 ppm) at 25 mW; (b) the transient response to mixed gas (2.5 ppm NO2 & 100 ppm NH3) at different loading power (50-79); (c) the transient response to 2.5 ppm NO2 at 25 mW.

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This indicates that the self-heated NW sensors can detect multiple gases without using external heaters. Consequently, the fabrication process of the present sensors is much simpler than in previous work

45

. To evaluate the short-term stability of the sensors sample S2, we

measured the transient response in ten continuous cycles of response–recovery to a fixed concentration of 2.5 ppm NO2 under a loading power of 25 mW. An identical dynamic response–recovery was obtained, as shown in Figure 8(c), indicating the good reversibility and stability of the networked NWs sensors under self-heating mode. The NW–NW junctions play an important role in the networked self-heating gas sensors, and the supposed in-situ formation of these junctions during the growth of NWs is more stable than the sensors prepared by a post-process. This result also reveals that the present sensors have comparable stability to the SnO2 NWs under an external heater 46,32. As mentioned above, the power consumption can be reduced by scaling down the size of the sensors. We designed and fabricated a pair of Pt electrodes with a gap and width of 10 µm, as shown in Figure 9(a). The SnO2 NWs were selectively grown on the electrode pair to form the NWs network via area-selective deposition of catalyst layers. The as-grown SnO2 NWs sensor was imaged by optical microscopy (Figure 9(b)) and SEM (Figure 9(c)). The SEM image at higher magnification, as presented in Figure 9(d), indicates that a dense NW network was formed between the pair of electrodes. We measured the NO2 gas-sensing response of this sensor at loading powers of 100 and 300 µW. The recorded transient response of the current and voltage are shown in Figure 9(e) to evaluate the power consumption. The calculated gas responses to various NO2 gas concentrations (2.5–20 ppm) are presented in Figure 9(f). As the gas concentration increased from 2.5 to 20 ppm, it can be seen that the response increased from 2.4 to 4.6 at a loading power of 100 µW. These responses are comparable with those of the sensor S2, but the loading power of sensor S2 is much higher.

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This power consumption is compared with conventional individual NW gas sensors as previously reported 19. This result confirms that the power consumption for the network NWs sensor can be reduced further by reducing the gap size of the sensor. This shows the potential for developing multiple gas sensors or sensor arrays with low power consumption for electronic nose applications.

Figure 9: Typical size-scaling down of self-heating gas sensors: (a) FESEM image of Ptelectrode; (b and c) optical and FESEM images of Pt-electrode after on-chip growth of SnO2 NWs; (d) Higher magnification of FESEM image of SnO2 NWs grown on the electrode; (e) the stressed current and voltage transient response at loading power of 100 and 300 µW; and (f) the transient response to various NO2 gas concentration (2.5-20 ppm) at 100 and 300 µW .

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4. CONCLUSION We have developed self-heated NO2 gas sensors based on on-chip growth of SnO2 network NWs with a facile fabrication process, good performance, and low power consumption without an external heater. As-developed SnO2 NW sensors were investigated under self-heating mode and external heater mode and the bell-shaped relation between the response and loading power have been found to be similar to that between the response and operating temperature. The loading power can be used to tune the selectivity of the sensors and consequent detection of not only NO2 gas but also other gases such H2, NH3, H2S, and C2H5OH. In addition, it has been shown that the developed sensors, scaled down in size (to a few micrometers) can detect NO2 gas at a power consumption in the microwatt range. We believe that the sensing platform with such low power consumption achieved with a facile fabrication process shows significant potential for the next generation of gas sensors based on metal oxide NWs. ASSOCIATED CONTENT Supporting Information. Electrical circuit for sensors measurement via V-A method used source/monitor (SMU) of Keithly K260A (Figure S1(a)). Algorithm scheme of measurement program controlled by personal computer (Figure S1(b)). The gas sensing mechanism described the modulation of potential barriers of NW-NW junction (Figure S2). Transient response of sensors sample S1 (Figure S3) and S2 (Figure S4) operated under external heating and self-heating modes. Transient response of sensor sample S2 to H2, NH3, H2S and C2H5OH gases operated under self-heating mode (Figure S5). The response values measured under self-heating mode to H2, NH3, H2S and C2H5OH gases (Table S1). The procedure of detection limit calculation is described. These materials are available free of charge via the internet at http://pubs.acs.org/.

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AUTHOR INFORMATION *E-mail: [email protected] (C M Hung) & [email protected] (N V Hieu) Phone: 84-38692963; Fax: 84-4-38692963 Notes The authors declare no competing financial interest 5. ACKNOELEDGEMENT This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2015.88. 5. REFERENCES (1)

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Novel Self-Heated Gas Sensors Using on-Chip Networked Nanowires

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