Nanoscale Thermal Management of Single SnO2 Nanowire: pico-Joule
Energy Consumed Molecule Sensor
Gang Meng,† Fuwei Zhuge,† Kazuki Nagashima,† Atsuo Nakao,‡ Masaki Kanai,† YongHe,† 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, JapanS1Outline
Part I Nanowire properties and procedures to fabricate suspended device
Figure S1. Structural and transport properties of fabricated SnO2 nanowires.
Figure S2. Procedures to fabricate suspended SnO2 nanowire devices.
Part II Analysis of self-Joule-heating in nanowire device
Figure S3. Detailed information of simulation model.
Figure S4. Heat dissipation of self-heated nanowires.
Figure S5. Temperature dependence on the electrical conductivity of SnO2 nanowires.
Figure S6. Importance of suspended configuration on the self-Joule-heating.
Figure S7. Optimization of nanowire diameter and length for an efficient thermal
management in spatial domain.
Part III Continuous operation of self-heated SnO2 nanowire sensor
Figure S8. Response to NO2 molecules with increased concentrations.

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Part I Nanowire properties and procedures to fabricate suspended device

Figure S1. Structural and electrical properties of SnO2 nanowires. (a) SEM image andXRD data of SnO2 nanowires grown on Al2O3(110) substrate. (b) TEM, HRTEM andSAED data, demonstrating a single-crystallite structure of nanowire. (c) Typical 2-probeand 4-probe current-voltage (I-V) data of Ti/Au contacted nanowire devices. The device
image is shown in the inset. (d) The resistivity distribution data.

SnO2 nanowires were grown on on Al2O3(110) substrate by Au catalyzed vapor-liquidsolid (VLS) method.1,2 Fig.S1 (a) shows the SEM image and XRD data of SnO2nanowires. Fig. S1(b) presents the TEM characterizations. Fig. S1(c) gives the typical 2probe and 4-probe current-voltage (I-V) results of SnO2 nanowires with Ti/Au electrodes,
revealing the relatively low contact resistance (Rcontact/R4-probe < 2%), which is anS3

important prerequisite for an efficient self-Joule-heating.3 SnO2 nanowires with resistivity
range of 0.4 ± 0.3 Ω·cm were prescreened for the following self-Joule-heating
experiments, shown in Fig.S1(d).

Figure S2. Schematic procedure to fabricate suspended SnO2 nanowire devices. (a)SiO2/Si (or PEN) substrate with predefined Ti/Pt electrodes. (b) Depositing MgOsacrificial layer (100 nm) and dispersing SnO2 nanowires. (c) Spin coating of ZEP resist,EBL drawing, development and etching uncovered MgO layer. (d) Depositing Ti/Au(1nm/300nm) electrode. (e) Lift-off ZEP resist. (f) Dissolving MgO sacrificial layer bydilute HCl solution.

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Part II Analysis of self-Joule-heating in nanowire device

Figure S3. Detailed information of the simulation model. (a) Geometrical image ofemployed simulation model. (b) Cross-sectional view of the device area. Domain
information, electrical boundary condition (voltage bias V and ground electrode) andtemperature boundary condition (the temperature of device backside is set to be theambient temperature of 293 K) are marked. (c) Typical dimensions of domains.

Evaluating and programing the temperature of self-heated nanowire are important issues
for the practical sensor operation, as investigations of micro-hotplates have done in thepast two decades.4-7 Because of fundamental difficulties to probe the temperature atnanoscale,8 a comprehensive study of device configurations (dimension, physicalS5

parameters of nanowire) dependent self-Joule-heating has been difficult. In this work, wehave performed COMSOL simulations, a model is shown in Figure S3. Applying avoltage bias on the nanowire device would generate the Joule heat. By considering thelow power of heat source (< 100 µW), the temperature increase of substrate is negligible.
Therefore, we assign the temperature of device backside to be 293 K. Based on
fundamental knowledge of heat transfer, three heat transfer modes (conduction,convection and radiation) may occur in a heated nanowire. The net heat power (∆P)
determines the temperature (T) at equilibrium.
∆ P = PJoule − Psolid − cond − Pgas − cond − Pconv − Prad(Eq-S1)

where, PJoule, Psolid-cond, Pgas-cond, Pconv and Prad represent the power of Joule heat source,
the power of conduction from nanowire to solid electrodes and/or substrate, the power ofconduction from nanowire to gas surrounding, the power of convection and the power ofradiation, respectively.

∆P ⋅ t = c ⋅ m ⋅ (T − Tamb )(Eq-S2)where, t is a time, c is the specific heat capacity of SnO2 (c= 52.6 J/mol·K),9 m is themass of SnO2 nanowire and Tamb is the ambient temperature.

S6Radiation power (W)

10-7
773 K
10-9

523 K

10-11
300 K
10-13

00.2

0.4
0.8
0.6
Emissivity (-)

1Figure S4. Radiation power from a self-heated SnO2 nanowire as a function of emissivity
and nanowire temperature. Radiation power is less than 10-8 W for a nanowire heated to773 K, even overestimating the nanowire as black-body (ε=1).

Among all the possible dissipation routes listed in Eq. S1, the convection process isnegligible in our experimental configurations due to the absence of gas flow in ourexperiments. Radiation can be estimated by the Stefan-Boltzmann law:104

Prad = εσ A (T 4 − Tamb )(Eq-S3)where, ε is emissivity (ε<1), σ is Stefan-Boltzmann constant (σ=5.67×10-8 W/m2K4),11 A
is the surface area of heat source. We estimated the radiation power by assuming ahomogeneous temperature field. The radiation power is negligible (<10-8 W for T=773 K),
even overestimating nanowire as a black-body (ε=1), as shown in Figure S4. Therefore,
the heat conduction (nanowire to electrode/substrate and nanowire to gas surrounding)should be crucial for heat dissipation of self-heated nanowires.
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Fourier conduction law is given as∆Q
∆T= −κA∆t∆x(Eq-S4)

The heat flow rate (∆Q/∆t) is proportional to the thermal conductivity (κ), the interface
area ((A) and the temperature gradient (∆T/∆x). As can be seen, thermal conductivity ofnanowire (κSnO2) plays a crucial role on the equilibrium temperature. Thus, thedetermination of κ is important for a semi-quantitative analysis of self-Joule-heating.
Unfortunately, the bulk conductivity (55 W/mK)12 is inappropriate for simulation due toincreased phonon-boundary scattering in quasi-one-dimensional nanowire.13 In this work,
we measured κ of SnO2 nanowire by 3-omega method,14 The generated V3ω as a functionof cubic 1ω current (I1ω) was plotted in Figure 1f. Thermal conductivity of as-synthesized
SnO2 nanowires at room temperature (RT) can be estimated by fitting Figure 1f, based on
the following equation:144RR,l 3
V3ω = 4 I1ω
π κS(Eq-S5)

where, R is 4-probe resistance of nanowire at RT, R' is temperature coefficient ofresistance at RT, deduced by linear fitting of Figure S5 (from 293 to 393 K). l is thelength of nanowire between two inner electrodes, S is the cross section area of nanowire.
Thermal conductivity of present SnO2 nanowire (diameter of 79 nm) is ~2 W/mK, which
is much lower than their bulk counterparts (55 W/mK)12 and thus offers great benefits foran efficient self-Joule-heating, as seen in Figure 1e. It should be noted that we utilized aS8

fixed κ (2 W/mK) for the simulations for the sake of simplicity. This can lead to anunderestimation of the simulated temperature in the case of (1) high input Joule power (κis reduced at high temperature 12) and (2) thinner nanowires (κ is reduced 15).

Conductivity (S/m)

1000
8006004002000250350

450550650

Temperature (K)Figure S5. Temperature dependence of the electrical conductivity of SnO2 nanowires.
Experimental points (violet squares) and the fitting curve used for calculating thermal
conductivity and temperature mapping of nanowires.

S9Figure S6. Importance of suspended configuration for an efficient self-Joule-heating. (a)Joule heat power dependent temperature for both suspended and non-suspended nanowiredevices. (The dimension and physical parameters of nanowires are same). (b)Temperature field mapping of a suspended nanowire (P=25 µW) and non-suspended(P=50 µW), inferring noticeable heat conduction into the underlying substrate for nonsuspended device. (c) NO2 response of non-suspended SnO2 device. We failed to observe
the enhancement of sensitivity by increasing Joule power up to 53 µW, implying a lowtemperature of non-suspended nanowire.

Extracting these physical parameters by experiments allows to approach a relatively
accurate estimation of self-Joule-heating. Good agreements between numericalsimulations (Figure 1, Figure S6) and NO2 response experiments (Figure2, Figure S6)
have been achieved. The successful implementation of numerical simulation on
experiments stimulates us to establish the correlation between dimension of device andequilibrium temperature under certain Joule power. By fixing the resistivity of nanowires,
assuming the same κSnO2, center temperature as a function of nanowire length anddiameter under constant Joule power of 1 µW (Figure S7((a)) and 10 µW (Figure S7((b))
is calculated. Below Joule power of 1 µW, the temperature of self-heated nanowires(diameter of 40-80 nm and length of < 6000 nm) is lower than 323 K. Increasing theJoule power up to ~10 µW enables an apparent self-Joule-heating up to 400-530 K. We
have added the dimension and power consumption of tested self-Joule-heating devices inFigure S7((b). Thinner nanowires clearly show good performance in terms of low power

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or energy consumption. Figure S7 could provide a useful guideline for managing thermal
properties of nanodevices in future.

Figure S7. Numerical simulation of center temperature as a function of nanowire
diameter and length, under a fixed Joule power of (a) 1 µW and (b) 10 µW. The diameter,
length of tested suspended devices and the power consumption are marked by open
circles in Figure (b). Size of circle is proportional with power consumption.

Time dependent self-Joule-heating is studied based on the following equation:

ρC P∂T+ ∇ ⋅ ( − k∇ T ) = Q∂t(Eq-S6)

where, ρ represents density, CP is heat capacity, Q is heat sink. Dimension, resistivity of
SnO2 nanowire and applied voltage bias are utilized based on Figure 3. Time scale ofthermal relaxation is found to be around ~ µs range, as seen in Figure 2c.

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Part III Continuous operation of self-heated SnO2 nanowire sensor

400
300

05000
10000
Time (sec)

100 ppm

10 ppm01 ppm100

100 ppb

20010 ppb

Sensitivity (%)

50015000

Figure S8. Response of self-heated SnO2 nanowires to NO2 with increasing
concentrations from 10 ppb to 100 ppm. Gradual increase of the sensitivity upon highNO2 concentrations and steady recovery of device can be found, indicating a goodstability.

Supplementary References(1) Meng, G.; Yanagida, T.; Nagashima, K.; Yoshida, H.; Kanai, M.; Klamchuen, A.;
Zhuge, F. W.; He, Y.; Rahong, S.; Fang, X. D.; Takeda, S.; Kawai, T. Impact ofPreferential Indium Nucleation on Electrical Conductivity of Vapor-Liquid-Solid Grown
Indium-Tin Oxide Nanowires. J. Am. Chem. Soc. 2013, 135, 7033-7038.
(2) Klamchuen, A.; Yanagida, T.; Kanai, M.; Nagashima, K.; Oka, K.; Kawai, T.; Suzuki,M.; Hidaka, Y.; Kai, S. Role of surrounding oxygen on oxide nanowire growth. Appl.
Phys. Lett. 2010, 97, 073114.
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(3) Prades, J. D.; Jimenez-Diaz, R.; Hernandez-Ramirez, F.; Barth, S.; Cirera, A.;
Romano-Rodriguez, A.; Mathur, S.; Morante, J. R. Ultralow power consumption gassensors based on self-heated individual nanowires. Appl. Phys. Lett. 2008, 93, 123110.
(4) Suehle, J. S.; Cavicchi, R. E.; Gaitan, M.; Semancik, S. Tin oxide gas sensor
fabricated using CMOS micro-hotplates and in-situ processing. IEEE Electron Device
Lett. 1993, 14, 118-120.
(5) Sheng, L. Y.; Tang, Z. N.; Wu, J.; Chan, P. C. H.; Sin, J. K. O. A low-power CMOS
compatible integrated gas sensor using maskless tin oxide sputtering. Sensor Actuat. BChem. 1998, 49, 81-87.
(6) Afridi, M. Y.; Suehle, J. S.; Zaghloul, M. E.; Berning, D. W.; Hefner, A. R.; Cavicchi,R. E.; Semancik, S.; Montgomery, C. B.; Taylor, C. J. A Monolithic CMOS
Microhotplate-Based Gas Sensor System. IEEE Sens. J. 2002, 2, 644-655.
(7) Graf, M.; Barrettino, D.; Kirstein, K. U.; Hierlemann, A. CMOS microhotplate sensor
system for operating temperatures up to 500 degrees C. Sensor Actuat. B-Chem. 2006,117, 346-352.
(8) Mecklenburg, M.; Hubbard, W. A.; White, E. R.; Dhall, R.; Cronin, S. B.; Aloni, S.;
Regan, B. C. Nanoscale temperature mapping in operating microelectronic devices.
Science 2015, 347, 629-632.
(9) Haynes, W. M. CRC Handbook of Chemistry and Physics, 93rd Edition; Taylor &Francis, 2012.
(10) Surhone, L. M.; Timpledon, M. T.; Marseken, S. F. Stefan-Boltzmann Law: StefanBoltzmann Law, Black Body, Irradiance, Thermodynamic Temperature, Ultraviolet

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Catastrophe, History of Quantum Mechanics, Thermodynamics; Betascript Publishing,
2010.
(11) Rybicki, G. B.; Lightman, A. P. Radiative processes in astrophysics; Wiley: NewYork, 1979.
(12) Turkes, P.; Pluntke, C.; Helbig, R. Thermal-Conductivity of Sno2 Single-Crystals. J.
Phys. C Solid State 1980, 13, 4941-4951.
(13) Pop, E. Energy dissipation and transport in nanoscale devices. Nano Research 2010,3, 147-169.
(14) Lu, L.; Yi, W.; Zhang, D. L. 3 omega method for specific heat and thermal
conductivity measurements. Rev. Sci. Instrum. 2001, 72, 2996-3003.
(15) Li, D. Y.; Wu, Y. Y.; Kim, P.; Shi, L.; Yang, P. D. Thermal conductivity ofindividual silicon nanowires. Appl. Phys. Lett. 2003, 83, 2934-2936.

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