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In a recent paper, Kiran Kuman Manga et al. [1] claimed that ‘for the FLG/PbSe/TiO2 device at the applied bias of -1V, the calculated detectivities in the visible and IR regions are D*≈3×1013 jones and D*≈5.7×1012, respectively (Figure 3c)’ (ref.[1], page 1700, 8th line right-sided) and ‘under illumination at 1 mW cm-2, the calculated LDR is 90dB’ (ref.[1], page 1701, 13rd line left-sided). According to those results, the detector was found to significantly outperform many similar semiconductor nanocrystal devices that have been reported to date [2,3,4]. In this comment, the validity of those important conclusions is examined base on the data reported by the authors as well as the equations used in the same paper.
The detectivity is defined by ref.[1] as:
D*=(Jph/Llight)/(2qJd)1/2 (1)
where q is the absolute value of electronic charge (1.6×10-19 Coulombs), Llight is the light intensity and Jph and Jd are the corresponding light and dark current. By introducing the concept of responsivity , R= Jph/Llight, Eq. 1 also can be rewritten as:
D*=R/(2qJd)1/2 (2)
The authors pointed out that ‘FLG/PbSe/TiO2 device shows…a high UV to IR photocurrent reponsivity of 0.506 A W-1 and 0.13 A W-1, respectively, when illuminated by 350nm (0.0425 mW cm-2) and 1000nm (2.34 mW cm-2)’ (Ref.[1], page 1699, 1st line right-sided). According to Fig. 3a of ref.[1], dark current at bias of -1V for FLG/PbSe/TiO2 device is slight greater than 1×10-6A/cm2. When R350nm =0.506 A W-1, R1000nm =0.13 A W-1 and Jd≈1×10-6A/cm2 are plugged into Eq. 2, the calculated detectivities are D*≈8.9×1011 jones at 350nm and D*≈2.3×1011 jones at 1000nm, respectively, which are significantly lower than the authors’ report that ‘for the FLG/PbSe/TiO2 device at the applied bias of -1V, the calculated detectivities in the visible and IR regions are D*≈3×1013 jones and D*≈5.7×1012, respectively (Figure 3c)’. In fact, to reach the authors’ conclusion that the detectivities in visible and IR regions are D*≈3×1013 jones and D*≈5.7×1012, respectively, either the dark current should be around 8.89×10-10 A/cm2 and 1.62×10-9 A/cm2, respectively, or the responsivity should be Rvisible ≈16.9 A W-1, and RIR ≈3.2 A W-1, respectively. However, neither in ref. [1] nor in its supporting information can the readers find such data. Therefore, it is evident that the report of detectivity is significantly inconsistent with the data of dark current and responsivity.
The second inconsistence occurs in the report of the linear dynamic range (LDR). LDR is defined as:
LDR=20log(Jph*/Jd) (3)
Jph* is the photocurrent measured at the light intensity of 1 mW cm-2. When Jph*≈2×10-4A/cm2 (Fig. 3d of ref. [1]) and Jd≈1×10-6A/cm2 (Fig. 3a of ref.[1]) are plugged into Eq.3, the calculated LDR is 46 dB, which is, again, significantly lower than the reported value (90dB) by the authors.
In summary, it is found that there exists some inconsistence between the authors’ conclusion and the reported data in ref. [1]. The measurement results of ref[1] do not support the following key statements, i.e., (1) ‘for the FLG/PbSe/TiO2 device at the applied bias of -1V, the calculated detectivities in the visible and IR regions are D*≈3×1013 jones and D*≈5.7×1012, respectively (Figure 3c)’, and (2) ‘under illumination at 1 mW cm-2, the calculated LDR is 90dB’. We respectively suggest the authors to clarify those insistent statements.
Reference
[1] K.K. Manga, J. Wang, M. Lin, J. Zhang, M. Nesladek, V. Nalla,W. Ji and K.P. Loh, Adv. Mater. 2012, 24, 1697-1702
[2] G. Sarasqueta, K.R. Choudhury, F. So, Chem. Mater. 2010, 22, 3496.
[3] G. Sarasqueta, K.R. Choudhury, J. Subbiah, F. So, Adv. Funct. Mater. 2011, 21, 167-171.
[4] J.P. Clifford, G. Konstantatos, K.W. Johnston, S. Hoogland, L. Levina, E.H. Sargent, Nature Nanotech. 2009, 4, 40-44.
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