Supporting Information:
Influence of in-plane and bridging oxygen vacancies of SnO2
nanostructures on CH4 sensing at low operating temperatures
Venkataramana Bonu,1* A. Das,1* Arun K Prasad,1 Nanda Gopala Krishna,2
Sandip Dhara,1 and A. K.Tyagi.1
1Surface
2Corrosion
and Nanoscience Division, Indira Gandhi Center for Atomic Research, Kalpakkam-603102, India.
Science & Technology Group, Indira Gandhi Center for Atomic Research, Kalpakkam-603102, India.
FIG. S1. Gaussian deconvolution of PL spectra in Fig. 3a
FIG. S2. Temperature dependent PL of SnO2 NPs of size 25 nm.
FIG. S3 (a), (b) and (c) are Gaussian deconvolution of PL spectra in Fig. S2 for temperatures 300, 200, 80 K
respectively. Table S3(d). Area under each peak of deconvoluted of PL spectra (FIG. S3 (a), (b) and (c)).
In general, the PL peak intensity was found to increase with decreasing temperature (Fig. S2). This
observation indicates existence of shallow donors (SD) in the SnO2 NPs.1 Two different decays of carriers using
time resolved X-ray induced PL were reported, a very fast decay lifetime (below 10 nsec) for PL transitions
observed above 2.6 eV, and slow decay (570 nsec) for PL emission below 2.6 eV. The latter transition with E < 2.6
eV, was assigned to initial trapped state of the electrons at some vacancies, slightly below the conduction band edge
(shallow donor).2 There is a systematic change for the intensities of the peaks at 1.97, 2.12, 2.28 and 2.45 eV with
respect to temperature (Fig. S2 and S3). Low energy peak at 1.96 eV showed the highest PL intensity among all four
peaks at 300 K (Table. S3(d)) and it continued the same until obtaining a temperature of 240 K. Below the
temperature of 240 K, the peak at 2.12 eV dominated with a maximum intensity until a temperature of 160 K
(Table. S3(d)). Further lowering the temperature to 80 K, peak at 2.28 eV started showing high intensity (Table.
S3(d)). Similarly, the PL peak intensity at 2.45 eV was also affected by the decrease in temperature. This
observation of change in PL intensity with temperature can be explained on the basis of presence of SD defects at
various energy levels. Relatively deep SD levels did not have sufficient thermal energy to participate in ionization
and it ceased to contribute in PL intensity.1 It indicates that, all the four defect states belong near to the conduction
band minimum following a sequence for shallow donors SD1 (2.45) > SD2 (2.28) > SD3 (2.12) > SD4 (1.97) (Fig.
3(b)).
FIG. S4 Normalized resistance of NPs and NWs CH4 sensor at their lowest possible operating temperatures.
CH4 gas response at the lowest measurement temperatures for all four samples is displayed (Fig. S4). NPs
of 4 nm size could detect 400 ppm of CH 4 at its best performance at 150 °C (Fig. S4 (a)). Intriguingly 9 nm NPs
with enhanced in-plane oxygen vacancies detected even 200 ppm of CH4 at much lower temperature of 100 °C.
With maximum in-plane ‘O’ vacancies among the NPs, 25 nm SnO2 was found to respond the thermodynamically
stable CH4 at a low temperature of 50 °C, however, for 400 ppm gas. In the case of SnO 2 NWs prepared from the
same pristine nanoparticle as precursor had nearly twice the amount of the in-plane ‘O’ vacancies in comparison to
the bridging ‘O’ vacancies. It extraordinarily detected 300 ppm of CH 4 even at an operating temperature of 50 °C. It
is noteworthy that these NWs showed relatively weak PL signal at ~ 2 eV with respect to the observed PL intensity
for 9 nm SnO2 NPs. Low PL signal signified presence of low amount of defects in the system. On the other hand, the
observed sensor response remained very high for the NWs. It indicated that the total amount of defects was not sole
reason for the low temperature gas sensing. It also revealed that the in-plane ‘O’ vacancies plays a crucial role in
detecting the gas at low temperature.
FIG. S5 Response curves of NPs and NWs for different concentration of CH4 with respect to temperature.
References
1
S. Luoa, and P. K. Chub Appl. Phys. Lett. 88, 183112 (2006).
2
X. T. Zhou, F. Heigl, M. W. Murphy, T. K. Shama, T. Regier, I. Coulthard, and R. I. R. Blyth, Appl. Phys. Lett.
89, 213109 (2006).