今年早些时候读到Advanced Materials新发的一篇文章( “High-performance Broadband Photodetector Using Solution-Processible PbSe-TiO2-Graphene Hybrids” [Adv. Mater. 2012, 24, 1697-1702])，讲到用PbSe-TiO2-Graphene的混合物，做出来的photoconductor器件的Detectivity在可见光达到了3*10^13 Jones, 而在红外区也达到了5.7*10^12 Jones。这个结果着实惊人。因为在较低偏压(-1V以内)下报道的可见到红外探测器只有诺奖获得者Heeger的组在Science(Science, (2009), 325, 1665.)上报道了一篇polymer材料的探测器达到了13次方量级的Detectivity(当然，我按照文章图中fig.3A给的数据一算，结果是12次方，也许是图画错了，总之只是读文章时的疑点，给Heeger发过一电子邮件问了下，人家大牛也没回复，那也没办法。虽然三年过去了，没人能follow这个工作，那也没办法。)。Detectivity是从入射光强，光电流和暗电流中推导出来的。可是仔细看过作者的数据，通篇只给出了一张暗电流的图，按照图上取暗电流的数值一算，Detectivity实际上只有11次方量级，比文中号称的13次方和12次方相去甚远。(大家可以按照作者给的公式算一次便知)。这是神马道理？带着这样的疑问，我写了一篇comment投了过去，全文如下：

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, 8^th line right-sided) and ‘under illumination at 1 mW cm^-2, the calculated LDR is 90dB’ (ref.[1], page 1701, 13^rd 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*=(J_ph/L_light)/(2qJ_d)^1/2              (1)

where q is the absolute value of electronic charge (1.6×10^-19 Coulombs), L_light is the light intensity and J_ph and J_d are the corresponding light and dark current. By introducing the concept of responsivity , R= J_ph/L_light, Eq. 1 also can be rewritten as:

D*=R/(2qJ_d)^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, 1^st 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 R_350nm =0.506 A W^-1, R_1000nm =0.13 A W^-1 and J_d≈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 R_visible ≈16.9 A W^-1, and R_IR ≈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(J_ph*/J_d)             (3)

J_ph* is the photocurrent measured at the light intensity of 1 mW cm^-2. When J_ph*≈2×10^-4A/cm2 (Fig. 3d of ref. [1]) and J_d≈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.

未完待续。。。。。。




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