Supporting Information

Detection and Discrimination of Volatile Organic Compounds Using a Single Multi-resonance Mode Piezotransduced Silicon Bulk Acoustic Wave Resonator (PSBAR) as Virtual Sensor Array

Yuan Zhao#, Qingrui Yang#, Ye Chang, Hemi Qu, Wei Pang*, Hao Zhang, Xuexin Duan*

Y. Zhao, Q. Yang, Y. Chang, Prof. X. DuanState Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072, China E-mail: xduan@tju.edu.cn

Dr. H. Qu, Prof. W. Pang, Prof. H. Zhang College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China

Supplementary Figure S1. Fabrication process flow of PSBARs. To fabricate PSBARs, 0.2 μm Mo, 1 μm AlN and 0.2 μm Mo were deposited and patterned sequentially on a blank SOI wafer with 20 μm device silicon layer. Afterwards, the handle Si beneath devices was removed by DRIE. Next, trenches on both sides of resonators were formed by etching relevant AlN and device Si layer through KOH and DRIE respectively. Finally, Au pads were formed by lift-off process followed by releasing buried oxide layers in BOE solution to suspend the resonant Si block.

Supplementary Figure S2. Surface Characterization by contact angle (CA) measurements. (a) DI water contact angle (CA) on AlN before coating OTES SAMs. (b) DI water contact angle (CA) on AlN after coating OTES SAMs. (c) DI water contact angle (CA) on low resistivity silicon substrate before coating OTES SAMs. (d) DI water contact angle (CA) on low resistivity silicon substrate after coating OTES SAMs. The CA values increase after the chemical treatment, indicating the hydrophobicity of OTES monolayers. On the other hand, the different CA values on the AlN and silicon substrate after the modification imply the dissimilar of interface properties, which impact the interaction of VOCs with these two kinds of substrates.

Supplementary Figure S3. The SEM image of side surface of the PSBAR. The rough side walls of the PSBAR are due to DRIE etching process, which can greatly increase the adsorption area of the device. Furthermore, the different roughness can lead to non-uniform adsorption of gas molecules on the top, side and bottom regions of the resonator to enhance the selectivity.

Supplementary Figure S4. The home-made VOCs detection setup. In the VOCs delivery system, the VOC line and the dilution line were employed to adjust the partial pressure of VOCs. A fixed amount of saturated VOC vapors was produced by bubbling carrier nitrogen (N2) gas into VOC liquid. Different gas partial pressures corresponding to different VOC concentrations were achieved by changing the flow velocity of pure N2 from the dilution line controlled by a mass flow controller through computer instructions; In the VOCs testing system, a functionalized PSBAR sensor was wire-bonded to evaluation boards, and was sealed in a plastic cavity with a couple of vents to allow the gas to pass through. Two-

port S-parameter data of the sensor were measured by a network analyzer.

Supplementary Figure S5. Real-time multi-mode responses of a PSBAR sensor functionalized with OTES SAM in exposure to (a) acetone, (b) IPA, (c) methanol, (d) hexane, (e) cyclohexane, and (f) heptane vapors respectively at eight different concentrations in terms of P/P0 from 0.1 to 0.8.

Supplementary Figure S6. The adsorption isotherms of (a) acetone, (b) IPA, (c) methanol, (d) hexane, (e) cyclohexane, and (f) heptane vapors on multiple resonant modes. Brunauer-Emmett-Teller (BET) formula with finite adsorbed layers was applied to fit the adsorption isotherms of different resonant modes.

Supplementary Figure S7. The fingerprints of vm for (a) ethanol, (b) acetone, (c) IPA, (d) methanol, (e) hexane, (f) cyclohexane, and (g) heptane vapors.

Supplementary Figure S8. Concentration-independent fingerprint based on the constant c for (a) ethanol, (b) acetone, (c) IPA, (d) methanol, (e) hexane, (f) cyclohexane, and (g) heptane vapors.

Table. S1 Adsorption Constants of seven VOCs Fitted by BET EquationVOC Parameters Mode1 Mode2 Mode3 Mode4 Mode5 Mode6 Mode7

Ethanol

vm (×10-4) 1.538 1.833 2.101 2.961 2.249 1.988 2.650

c 8.675 9.555 7.084 7.335 7.236 9.913 7.443

n 16

R2 0.9981 0.9931 0.9924 0.9937 0.9925 0.9924 0.9891

Acetone

vm (×10-4) 1.572 1.711 2.023 3.083 2.179 1.864 2.586

c 8.872 16.74 9.366 7.891 9.962 13.52 9.346

n 8

R2 0.9975 0.9909 0.9918 0.9865 0.992 0.9929 0.9892

IPA vm (×10-4) 1.436 1.646 1.872 2.584 2.117 1.795 2.467

c 10.640 17.340 11.380 12.310 9.228 12.620 7.279

n 40

R2 0.9817 0.9812 0.9785 0.9773 0.9803 0.9769 0.9742

Methanol

vm (×10-4) 1.214 1.570 1.615 2.310 1.768 1.500 1.976

c 16.870 39.410 18.090 20.470 15.560 30.770 16.350

n 5

R2 0.9943 0.9768 0.9966 0.9979 0.9981 0.9910 0.9906

n-Hexane

vm (×10-4) 5.086 5.336 7.446 10.180 7.032 4.100 10.670

c 0.811 0.975 0.715 0.731 0.877 1.525 0.594

n 4

R2 0.9969 0.9981 0.9972 0.9899 0.9913 0.9869 0.9973

Cyclohexane

vm (×10-4) 7.247 7.140 9.992 13.040 8.524 4.341 14.820

c 0.862 0.989 0.830 0.884 1.146 2.107 0.661

n 4

R2 0.9850 0.9818 0.9960 0.9951 0.9824 0.9842 0.9962

n-Heptane

vm (×10-4) 6.714 7.087 10.510 13.070 8.363 4.191 20.480

c 1.238 1.617 1.121 1.228 1.583 3.066 1.086

n 2

R2 0.9850 0.9950 0.9866 0.9835 0.9949 0.9608 0.9693

Table. S2 Normalized constant vm for each VOCMode1 Mode2 Mode3 Mode4 Mode5 Mode6 Mode7

Ethanol 0.5194 0.6190 0.7096 1 0.7595 0.6714 0.8950

Acetone 0.5099 0.5550 0.6562 1 0.7068 0.6046 0.8388

IPA 0.5557 0.6370 0.7245 1 0.8193 0.6947 0.9547

Methanol 0.5255 0.6797 0.6991 1 0.7654 0.6494 0.8554

n-Hexane 0.4767 0.5001 0.6978 0.9541 0.6590 0.3843 1

Cyclohexane 0.8799 0.4818 0.6742 0.8799 0.5752 0.2929 1

n-Heptane 0.3278 0.3460 0.5132 0.6382 0.4083 0.2046 1

Table. S3 Normalized constant c for each VOCMode1 Mode2 Mode3 Mode4 Mode5 Mode6 Mode7

Ethanol 0.8751 0.9639 0.7146 0.7399 0.7300 1 0.7508

Acetone 0.5300 1 0.5595 0.4714 0.5951 0.8076 0.5583

IPA 0.6136 1 0.6563 0.7099 0.5322 0.7278 0.4198

Methanol 0.4281 1 0.4590 0.5194 0.3948 0.7808 0.4149

n-Hexane 0.5318 0.6393 0.4691 0.4791 0.5748 1 0.3897

Cyclohexane 0.4196 0.4695 0.3940 0.4196 0.5439 1 0.3139

n-Heptane 0.4038 0.5274 0.3656 0.4005 0.5163 1 0.3542