Article
pubs.acs.org/acssensors

Nanoscale Thermal Management of Single SnO2 Nanowire: picoJoule Energy Consumed Molecule Sensor
Gang Meng,† Fuwei Zhuge,† Kazuki Nagashima,† Atsuo Nakao,‡ Masaki Kanai,† Yong He,†
Mickael Boudot,† Tsunaki Takahashi,§ Ken Uchida,§ and Takeshi Yanagida*,††Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan
Panasonic Corporation, 1006 Kadoma, Kadoma City, Osaka 571-8506, Japan§
Department of Electrical Engineering, Keio University, 3-14-1 Hiyoshi, Kouhokuku, Yokohama 223-8522, Japan‡S Supporting Information*

ABSTRACT: Here we report the thermal management of oxide nanowire sensor in both
spatial and time domains by utilizing unique thermal properties of nanowires, which are (1)the reduced thermal conductivity and (2) the short thermal relaxation time down to several
microseconds. Our method utilizes a pulsed self-Joule-heating of suspended SnO2 nanowire
device, which enables not only the gigantic reduction of energy consumption down to ∼102
pJ/s, but also enhancement of the sensitivity for electrical sensing of NO2 (100 ppb).
Furthermore, we demonstrate the applicability of the present method as sensors on ﬂexible
PEN substrate. Thus, this proposed thermal management concept of nanowires in both spatial
and time domains oﬀers a strategy for exploring novel functionalities of nanowire-based
devices.

KEYWORDS: nanoscale thermal management, single nanowire device, pulse measurement, thermal relaxation time,
thermal conductivity, ﬂexible deviceE

spatial temperature distribution on the membrane.17 The
fundamental design concepts include (1) to reduce the heatdissipation, (2) to decrease the sensor volume, and (3) todecrease the distance between the external heater and thesensing part. A suspended nanowire sensor with self-Jouleheating is an ideal sensing platform, which satisﬁes all the above
requirements. This is because (1) the volume of the nanowire is
extremely small (∼10−3 μm3), (2) the suspended nanowire
structure substantially suppresses the heat dissipation into thesurroundings, and (3) the self-Joule-heating method ultimately
decreases the distance between the heater and the sensor.
Comini et al. reported their pioneering work of nanowire gas
sensors using SnO2 nanobelts, which was a nonsuspended
device without self-Joule-heating.18 Prades et al.19,20 havedemonstrated the reduction of energy consumption down to
20 μJ/s using self-Joule-heating in nonsuspended SnO2
nanowire sensors. However, there seems to be a fundamental
limitation to further enhancing the sensing characteristics,
including the energy consumption and the sensitivity, evenusing these nanowire sensors.21−23 One of major reasons theseprevious studies of nanowire sensors have failed to further tailor
the sensing properties seems to be that they have focused on

lectrical sensing of volatile molecular species in environments and/or from our bodies by using mobile electronics
is an important issue for future electronic devices.1,2 This isbecause these sensor electronics will have an impact on ourdaily life and healthcare via collecting and analyzing big data.3
However, conventional gas sensors, including semiconductoroxide sensors, have been diﬃcult to be integrated into CMOS
electronics and wearable electronics due to the relatively highenergy consumption (∼J/s) to ensure a rapid and reversible
sensing operation via programming the temperature of thesensor between ∼470 and 670 K.4−6 For example, mostconventional metal oxide gas sensors are typically operated at
the temperature ∼510 K by using an external heater with 835
mJ/s.7 The major reason that the conventional semiconductor
gas sensors require relatively high energy consumption is thatthe external heater must heat up a large volume (typically ∼107μm3) of sensors via solid heat conduction by competing withthe heat dissipation into the surroundings.7 Thus, thermal
management is an important issue for designing sensor
electronics.
Various concepts have been proposed to manage the thermal
properties of sensors. Among them, microelectromechanicalsystems (MEMS) sensors with a microhotplate integrated on a
Si3N4 membrane have recently emerged utilizing advanced
lithographic technology.8−17 The state-of-the-art membrane
MEMS sensors have proven the feasibility to reduce the energy
consumption down to ∼20 mJ/s at 520 K via controlling the© 2016 American Chemical Society

Received: June 2, 2016
Accepted: July 4, 2016
Published: July 4, 2016
997

DOI: 10.1021/acssensors.6b00364
ACS Sens. 2016, 1, 997−1002

ArticleACS Sensors

measure the short transition time from low voltage (room temperatureT0) to high voltage (equilibrium temperature Tequiv). Sampling rate
was set to be 10 MHz; a spot mean measurement of 10 points ensuresmonitoring resistance variation with the time resolution of 1 μs. NO2
response was also carried out by operating under both self-Jouleheating and external heating (RT - 523 K) mode. Resistance of the
SnO2 nanowire under vacuum was recorded as baseline (R0). The
resistance upon exposure of NO2 gas with an equivalent concentrationof 100 ppb (by injecting 100 ppm of NO2 into the chamber to be 100Pa) was noted as R. Finally, the chamber was re-evacuated forrecovery. Sensitivity was deﬁned as R/R0 × 100%.
Simulation. Numerical simulation (COMSOL Multiphysics) was
performed to examine the nanoscale thermal transport of suspended
nanowire devices. In order to achieve a reliable simulation, several
important physical parameters, such as electrical conductivity (σ),
temperature dependence of resistivity, and thermal conductivity (κ) of
SnO2 nanowires, were experimentally extracted by 4-probe, temperature dependent 4-probe, and 3-omega (3ω) method, respectively.
Additional information on simulation can be seen in SI Figure S3−S8.

only controlling the spatial thermal properties of nanowires.19−23
Here we propose the thermal management of oxide nanowire
sensors in both spatial and time domains, which enables not
only the reduction of energy consumption of volatile molecule
sensors down to ∼102 pJ/s, but also the sensitivity higher thanconventional continuous heating method. Our method focuseson the short thermal relaxation time (∼μs) of suspended SnO2
nanowire device, which is deﬁned as the time required forheating up/or cooling down to a desired temperature. The
thermal relaxation time of the nanowire device is much shorter
than other existing semiconductor gas sensors mainly due tothe smaller volume of nanowire sensors and characteristics ofself-Joule-heating. Since the energy consumption of sensors
depends on not only the heating power but also the heatingtime, this short thermal relaxation time of nanowire sensors
essentially allows us to reduce the energy consumption of
sensors by heating up only during sensing using a pulse mode.
In addition, the reduced thermal conductivity of nanowires
plays an important role in the spatial thermal management of
nanowire sensors for increasing local temperature via self-Joule
heating. Thus, the present methodology based on the thermal
management of nanowire sensors in both spatial and time
domains oﬀers a new strategy to tailor the properties ofnanowire-based devices.

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RESULTS AND DISCUSSIONFirst, we numerically examine the eﬀect of self-Joule-heating on
the temperature distribution of suspended nanowire devices, inorder to obtain the spatial thermal information. The schematic
model is illustrated in Figure 1a, and the details of numerical

EXPERIMENTAL SECTION

SnO2 Nanowire Growth. Single-crystalline SnO2 nanowires weresynthesized by pulsed laser deposition (PLD) method, as described inprevious work.24,25 Al2O3(110) substrate with Au layer of 0.7 nm washeated up to 750 °C under O2 and Ar mixed gas (O2:Ar = 1:1000,
total pressure of 10 Pa). SnO2 target (purity 99.99%) was ablated by
ArF excimer laser (wavelength of 193 nm, energy of 40 mJ, andrepetition of 10 Hz). By carefully controlling supplied metal and O2
ﬂux, SnO2 nanowires were obtained via a vapor−liquid−solid (VLS)
process. Fabricated nanowires were characterized by ﬁeld emission
scanning electron microscope (FESEM), X-ray diﬀraction (XRD), andhigh-resolution transmission electron microscope (HRTEM). The
detailed results can be seen in the SI Figure S1.
Device Fabrication. SnO2 nanowire sensor devices were
fabricated by conventional lithography techniques using electron
beam lithography (EBL) and radiofrequency (RF) sputtering.25,26 Asgrown SnO2 nanowires were deposited onto SiO2/Si (100) substratewith predeﬁned Ti/Pt metal electrodes. Then EBL was employed todeﬁne contact electrodes. After Ti/Au layer (1 nm Ti/300 nm Au)
deposition and lift-oﬀ process, conventional nonsuspended devices
could be obtained. For the case of suspended devices, an MgO sacriﬁce
layer with thickness of 100 nm was deposited onto the SiO2/Si or
polyethylene naphthalate (PEN) substrates to create an air-gap
between nanowire and substrate. After the EBL and electrodedeposition processes, the MgO sacriﬁce layer was removed by adilute HCl solution (12 mM), resulting in a suspended nanowire
device. It should be noted that the suspended nanowires weresusceptible to attachment to the substrate in the ﬁnal drying process,owing to a strong meniscus force.27,28 To avoid this problem, insteadof distilled water (surface tension of 72.8 mN/m at 20 °C), low surface
tension solvent, such as ethanol (22.1 mN/m at 20 °C) or
hydroﬂuoroether (HFE, 13.6 mN/m at 20 °C), was utilized in theﬁnal rinsing and drying process. More detailed procedures can befound in SI Figure S2.
Electrical Measurements. Thermal relaxation time of suspended
SnO2 nanowires and NO2 response experiments were measured bysemiconductor parameter analyzer (Keithley 4200 SCS), connectedwith a probe station in which ambient pressure and stage temperature
can be controlled. To estimate the thermal relaxation time, the SnO2
nanowire devices were initially evacuated to a base pressure of 2 ×
10−2 Pa.29 Keithley pulse module (PMU 4225) was employed to

Figure 1. Simulated thermal properties for self-Joule-heating of
nanowire device. (a) Schematic model for numerical simulation. (b)Temperature distribution of self-Joule-heating nanowire device. Inset
shows the cross-sectional temperature distribution at the nanowire
center. (c) Eﬀect of nanowire diameter on temperature at the
nanowire center, Tcenter, when varying the Joule heat power. (d) Eﬀect
of nanowire length on temperature at the nanowire center, Tcenter,when varying the Joule heat power. (e) Eﬀect of thermal conductivityof the nanowire on temperature at the nanowire center, Tcenter, underﬁxed Joule heat power of 10 μW. (f) Thermal conductivity
measurement of the present SnO2 nanowires by the 3 omega method.

simulation can be seen in SI Figure S3. The temperature at thecenter of the nanowire (Tcenter) was highest due to the heatdissipation into electrodes,30 as shown in Figure 1b. We utilize
this temperature value at the center of nanowire toquantitatively examine the eﬀects of various device parameters,
including the values of length (Lnanowire), diameter (Dnanowire),and thermal conductivity of nanowires (κnanowire), on the Joule
heating under a ﬁxed input power. Figure 1c−e shows the998

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electrodes and SiO2/Si substrate. The details of the fabrication
processes of the suspended nanowire devices can be seen in SI
Figure S2. In order to estimate the actual thermal relaxation
time of the suspended nanowire device, ﬁrst we measured thetemperature dependence of nanowire resistivity by the 4-probe
method; the temperature is programmed by an external heater,
as shown in Figure 2b. Elevating the temperature results in thedecrease of resistivity, revealing the semiconducting behavior of
SnO2 nanowires with the activation energy of 79.9 meV.37
Next, we performed ultrafast current−voltage (IV) measurement to experimentally extract the thermal relaxation time.
Figure 2c shows the waveform of pulse voltage V, the resistivity
ρ, and estimated temperature of suspended nanowire device
Tequiv. It can be seen that the resistivity change occurs within amicrosecond, which is the time resolution of our experimental
systems. The elevated temperature value of the nanowire wasestimated to be ∼520 K from the temperature dependence data
in Figure 2b. To validate the thermal relaxation time insimulations, the time series data of Tcenter when performing selfJoule-heating have been calculated. As shown in the gray line in
Figure 2c, there is good agreement between experimental data
and the simulated values. In most device conﬁgurations
employed in our experiments, the thermal relaxation times
were found to be in a microsecond range. Thus, these results
highlight that the short thermal relaxation time of nanowire
devices allows us to manage the temperature time proﬁle in therange of microseconds. Since the thermal relaxation times of
conventional gas sensors and recent MEMS gas sensors areseveral tens of seconds and several tens of milliseconds,13,15,38,39 respectively, our method utilizing the microsecond thermal relaxation time of nanowire devices enables us
to further tailor the thermal properties in time domains beyond
the limitation of conventional methodologies.
We examine the feasibility of our thermally managed
nanowire devices for volatile molecule sensors. The device
conﬁguration (Lnanowire = 1850 nm, Dnanowire = 57 nm) is
designed based on numerical study in Figure 1. NO2 gas (100ppb) was employed for the present sensing experiments. Figure
3 shows the comparison between diﬀerent heating methods,

simulation results. We found that several device properties cansigniﬁcantly lower the input power for Joule heating of thesuspended nanowire devices. The smaller diameter of nanowires lowers the necessary input power to reach the equilibrium
temperature via Joule heating due to the smaller volume, as
shown in Figure 1c. However, decreasing the volume via
decreasing the nanowire length did not result in the
suppression of input power due to the signiﬁcant heatdissipation into electrodes.31 Thus, there was an optimal lengthof nanowires for eﬀective self-Joule-heating, as shown in Figure
1d. This is due to the competition between the heat dissipation
into electrodes and the heat dissipation into surrounding air.
Therefore, (i) the small volume of nanowires and (ii) the
optimal device conﬁguration to minimize the heat dissipation
into surroundings are important characteristics to lower the
input power for Joule heating in the suspended nanowire
devices. One more important parameter is the thermal
conductivity of nanowires, which critically determines theheat dissipation into surroundings though nanowire/electrodeand nanowire/surrounding-gas interfaces, as shown in Figure
1e. We measured the thermal conductivity of the present SnO2
nanowires via the 3ω method,32 as shown in Figure 1f. The
thermal conductivity of the present [100] orientated SnO2
nanowires was 2 W/mK (diameter of 79 nm), which is about
27 times lower than that of bulk single crystalline SnO2
(perpendicular to c-axis 55 W/mK).33 This lowered thermal
conductivity of nanowires34 signiﬁcantly suppresses Joule heatdissipation into the surroundings, resulting in the eﬃcienttemperature increase within the conﬁned spatial position of the
nanowire.35
Next, we focus on the thermal relaxation time τ, which isdeﬁned as a characteristic time during heating up or cooling
down.36 This is an important parameter for thermal management in a time domain. We have performed both experiments
and simulations to extract the thermal relaxation time of thesuspended nanowire device. Figure 2a shows the SEM image ofsuspended nanowire devices utilizing single crystalline SnO2
nanowires grown by a vapor−liquid−solid (VLS) method.24,25
As designed, SnO2 nanowire was fully suspended between

Figure 2. Evaluation of thermal relaxation time. (a) SEM image ofsuspended 4-terminal SnO2 nanowire device. (b) Temperaturedependence of electrical resistivity of fabricated suspended SnO2
nanowire device. Inset shows the current−voltage (IV) curves forvarious temperatures. (c) Time series data of applied voltage,
measured resistivity, and estimated temperatures during ultrafast IV
measurement, shown by dots. Time dependent simulation results havebeen shown by gray lines. Both experiment and simulation reveal the
short thermal relaxation time of ∼μs.

Figure 3. Sensing data of suspended nanowire device whenintroducing NO2 (100 ppb). (a) Sensing data of external heating
mode using an external heater. (b) Sensing data of continuous selfJoule-heating mode. (c) Sensing data of pulsed self-Joule-heating
mode, measured under a constant Joule power of 13 μW (during pulse
on).
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including (a) external heating, (b) self-Joule-heating, and (c)pulsed self-Joule-heating, on the resistance change of suspended
nanowire devices when introducing NO2 gas (100 ppb). For all
heating systems, introducing NO2 gas increased the resistanceof SnO2 nanowire, as reported elsewhere.18,19 In external
heating system, increasing the temperature from 300 to 520 K
signiﬁcantly enhanced the sensitivity via promoting thechemisorption on the oxide surface.4 A similar trend on thesensitivity was also observed when increasing the current value
for the self-Joule-heating system. For the pulsed self-Jouleheating system, there pulse width was dependent on thesensitivity. Increasing the pulse width enhanced the sensitivity.
However, the sensitivity change was not signiﬁcant when
considering the fact that the pulse width decreased down to lessthan 1/100. For example, in the case of the pulse width (1 ms),the sensitivity was almost 60% of the continuous heating data,
although the nanowire was heated up during just 0.03% of total
time. Thus, the present results highlight that the suspended
nanowire sensor does not require the continuous heating
condition for the electrical sensing. Table 1 shows the

Figure 4. Enhanced sensitivity of pulse measurement and theunderlying mechanism. (a) Transient response of the suspended
SnO2 nanowire device for sensing NO2 (100 ppb). Resistance values
read at high voltage (red dots) and low voltage (purple dots) afterNO2 injection are shown. (b) Mechanism to explain the enhancementof sensitivity of pulse measurement when reading the resistance value
at lower temperature during pulse-oﬀ. The diﬀerence between two
TCR curves of pristine SnO2 nanowire (blue line) and SnO2 nanowire
oxidized by NO2 molecules (red line) can explain the observed
enhancement of sensitivity of pulse measurements.

constant cumulative heating time, there is a diﬀerence between
the pulse heating method and the continuous heating methodon the sensitivity. Although it is diﬃcult to obtain the direct
experimental evidence, it is considered that the physisorption ofmolecules was promoted at lower temperatures during coolingtime,4,41 which increases the molecular concentration near the
nanowire surface for chemisorption of molecules during pulse
heating. The other interesting feature of the thermal management in both spatial and time domains is the enhancement ofsensitivity via reading the electrical resistance at lower
temperature during pulses. As shown in Figure 4, the sensitivity
was signiﬁcantly enhanced when decreasing the reading voltage
between pulse heating. This sensitivity enhancement can beinterpreted in terms of diﬀerent temperature coeﬃcient ofresistance (TCR),42,4 as illustrated in Figure 4b. In principle,
the temperature dependence of semiconductors tends to bestronger as carrier concentration decreases. When oxidizing for
n-type SnO2 nanowires by NO2, the carrier concentration of
SnO2 nanowires decreases. Thus, the sensitivity, deﬁned as theresistance ratio of the pristine SnO2 nanowire to SnO2
nanowire oxidized by NO2, can be enhanced by reading theresistance values at the lower temperature, as illustrated. To thebest of our knowledge, such a sensitivity enhancement concept
has not been reported, probably due to relatively long thermal
relaxation time of external heating for other existing
sensors.13,15,38,39 Thus, the thermal management in both spatial
and time domains signiﬁcantly enhances the sensing properties.
Finally, we demonstrate the feasibility of the present pulsed
heating method for wearable sensors, which can beconveniently attached onto human skin or plants.43 We havefabricated suspended SnO2 nanowires on the ﬂexible PEN
substrate. Figure 5a shows the device on PEN substrate. Metal
oxide gas sensors, which require high temperature operation,
have been diﬃcult to integrate into such polymers withrelatively low glass transition temperatures.44 As shown in
Figure 5b, conventional external heating up to 523 K damaged
the PEN substrate. On the other hand, the pulsed heating
method can be performed without any damage to the PEN
substrate. Figure 5c shows the NO2 sensing data of the SnO2
nanowire sensor devices on PEN substrate, under the pulsed
self-heating mode, with pulse width of 10 μs and period of 3 s.
Thus, the present methodology oﬀers tremendous opportunities for wearable sensors on ﬂexible sensors.

Table 1. Comparison between Three Methods (External
Heating, Self-Joule-Heating, and Pulsed Self-Joule-Heating)on the Energy Consumption to Reach the Sensitivity of
200% for NO2 (100 ppb)

energyconsumption

external
heatingself-Jouleheating

pulsed self-Jouleheating

0.6 J/s

1.6 μJ/s

0.6 nJ/s

comparison between the three heating methods on the
consumed energy value to reach the sensitivity of 200%. As
clearly seen, the present pulsed self-Joule-heating method insuspended nanowire device can lower the energy consumption
down to 0.6 nJ/s (adjusting the pulse width to be 140 μs for a
Joule power of 13 μW), which is much lower than the othertwo methods. The substantial reduction of energy consumption
can be ascribed to the thermal management of suspended
nanowire in both spatial and time domains based on the uniquethermal properties of nanowires, such as the low thermal
conductivity34,35 and the short thermal relaxation time.
Suspended nanowire can be heated up to a higher temperature
within a short electrical pulse to activate the charge exchange
between absorbed molecules and nanowire surface even
without continuous heating.40 Thus, the thermal management
in both spatial and time domains substantially reduces theenergy consumption of the nanowire sensor.
Next, we question why the thermally managed nanowire
sensor using a pulse heating exhibited relatively good sensitivity
in spite of the fact that the cumulative heating time for pulse
heating is less than 1% of the continuous heating method. To
answer this, we have performed transient measurements when
applying a pulse heating, as shown in Figure 4a. As can be seen,
the resistance increased when the nanowire was heated up
during the pulse. The modulated resistance change wasmaintained until the next pulse heating. This indicates thatthe molecules chemisorbed onto the nanowire surface might befrozen between two pulses at lower temperature, as illustrated
in Figure 4b. This can be understood by considering the factthat an energy barrier exists for desorption from thechemisorbed state.41 These “frozen” chemisorbed moleculeson the nanowire surface are mainly responsible for the relatively
high sensitivity of the pulse heating method. However, even for
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CONCLUSIONSIn summary, we demonstrated the impact of thermal
management of oxide nanowire sensors on the sensing
characteristics. The thermal management of nanowires was
performed in both spatial and time domains by utilizing theunique thermal properties of nanowires, which are (1) the
reduced thermal conductivity and (2) the short thermal
relaxation time down to several microseconds. This method
enables not only the reduction of energy consumption down to∼102 pJ/s, but also the enhancement of sensitivity for electricalsensing of NO2 (100 ppb). Furthermore, we demonstrate theapplicability of the present methodology as gas sensors on
ﬂexible substrates.
ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssensors.6b00364.
Nanowire properties and procedures to fabricate
suspended device; analysis of self-Joule-heating in
nanowire device; continuous operation of self-heated
SnO2 nanowire sensor (PDF)■

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Figure 5. Applications of the present methodology to ﬂexible
substrate. (a) Photograph of suspended SnO2 nanowire device formedon the ﬂexible PEN substrate. (b) Photographs of PEN devices
operated under external heating (left) and pulsed self-Joule-heating(right). SEM images of nanowire device taken before and after sensing
experiments are shown at the bottom. (c) Sensing data of suspended
SnO2 nanowire device on PEN substrate to 100 ppb NO2 molecules.

■

ArticleAUTHOR INFORMATION

Corresponding Author*E-mail: yanagida@cm.kyushu-u.ac.jp.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by CREST of Japan Science andTechnology Corporation (JST). This work was performed
under the Cooperative Research Program of “Network Joint
Research Center for Materials and Devices”. We thank Dr.
Yuhui He and Dr. Keju Sun for fruitful discussions.
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DOI: 10.1021/acssensors.6b00364
ACS Sens. 2016, 1, 997−1002