Photogenerated charges and surface potential variations investigated on single Si nanorods by electrostatic force microscopy combined with laser irradiation
© Wu et al.; licensee Springer. 2014
Received: 4 March 2014
Accepted: 7 May 2014
Published: 20 May 2014
Photogenerated charging properties of single Si nanorods (Si NRs) are investigated by electrostatic force microscopy (EFM) combined with laser irradiation. Under laser irradiation, Si NRs are positively charged. The amount of the charges trapped in single NRs as well as the contact potential difference between the tip and NRs' surface is achieved from an analytical fitting of the phase shift - voltage curve. Both of them significantly vary with the laser intensity and the NR's size and construction. The photogenerated charging and decharging rates are obtained at a timescale of seconds or slower, indicating that the Si NRs are promising candidates in photovoltaic applications.
KeywordsSi nanorods EFM Photogenerated charging Surface potential
One-dimensional silicon nanostructures, such as Si nanowires (NWs), nanorods (NRs), or nanopillar (NPs) have gained particular interests due to their special properties and potential applications in electronic and optoelectronic devices [1–4]. Theoretical and experimental studies have reported that when arranged in a highly ordered fashion, Si NRs or NWs can improve light absorption and charge collection, making it possible to achieve high efficiency in solar cells [5–8]. Therefore, periodic Si NRs (or NWs) arrays have attracted considerable attentions in the fields of solar cells. However, despite the huge efforts to control and understand the growth mechanisms underlying the formation of these nanostructures [9, 10], some fundamental properties and inside mechanisms are still not well understood.
To reveal their properties, the investigation on single NRs is preferred. Recently conductive scanning probe microscopy techniques have been attempted to investigate the electrical properties of single NWs/NRs. Among them, electrostatic force microscopy (EFM) can provide direct information of trapped carriers in single nanostructures and has been applied to investigate the charge trapping in single nanostructures, such as carbon nanotubes , pentacene monolayer islands , CdSe quantum dots (QDs) [13, 14], and etc. More recently, photoionization of QDs [15, 16] and photo-induced charging of photovoltaic films [17–19] have been studied by EFM combined with laser irradiation. But the photogenerated charging effects have not been concerned on Si NRs or NWs yet. In this letter, EFM measurements combined with laser irradiation are applied to investigate the photogenerated charging properties on single vertically aligned Si NRs in periodic arrays.
Results and discussions
where Q is the quality factor and k is the spring constant of the probe.
Fitting results obtained by fitting ΔΦ − V EFM curves of NR1 with Equation 3
Laser intensity (W/cm2)
Q s /S(e/μm2)
Fitting results obtained by fitting ΔΦ − V EFM curves of NR2 with Equation 3
Laser intensity (W/cm2)
Q s /S(e/μm2)
Fitting results obtained by fitting ΔΦ − V EFM curves of NR3 with Equation 3
Laser intensity (W/cm2)
Q s /S(e/μm2)
The tip shape factor, α, is about 1.5 for a standard conical tip [12, 21]. The NRs' shape factor, g, is about 1 if we approximate the NRs as cylindrical nanoparticles . Q s /S is the trapped charge density to be derived, and ϵ r is the dielectric constant of Si. Thus, the charge densities can be obtained by using Equation 5, which are listed in Tables 1, 2, and 3 and also plotted as a function of laser intensity in Figure 3b. The results show a similar tendency of increase with the laser intensity as the trapped charges as given in Figure 3a, except the increase of tapped charge density in NR3 is much larger than that of the trapped charges, which may be due to more localization of charges in NR3. Again, the obtained values are not accurate due to the uncertainty of z.
In addition, from the description of B in Equation 4, the polarity of Qs can be obtained from the sign of B. From the fitting results, it is obtained that B increases from zero to positive values with the laser intensity for all the three samples, indicating that positive charges are trapped in the three types of NRs under laser irradiation. The increase of trapped charges is relatively small for NR1, which should be again due to its low absorbance of light. The reason why the NR3 contains more trapped charges than NR2 is most probably due to the existence of the GeSi quantum well, which can act as additional trappers of holes.
On the other hand, the values of VCPD can also be obtained from the fitting results, and the change of VCPD with laser intensity is presented in Figure 3c. It can be observed that, under 2 W/cm2 laser irradiation, the VCPD values change slightly for all the three samples, but they increase obviously when the laser intensity increase up to 4 W/cm2 and above. Also, the increase magnitude is different for the three types of NRs. The increase of VCPD with laser intensity is most significant for NR3, similar to the increase of trapped charges. Similar surface potential variation by photogenerated charges has been obtained by Kelvin potential force microscopy (KPFM) [26, 27]; it was declared that the positive (negative) shift in surface potential with laser corresponds to an increase in hole (electron) density. Thus, the positive shift in VCPD with laser intensity in our experiments can also be attributed to the increase of trapped hole density, which is consistent with the above results of charge density. As VCPD equals to (ϕtip − ϕsample) / e, the results declare that the work function of Si NR decrease upon laser irradiation should be due to the photogenerated holes trapped in NRs.
The reason why positive charging measured on n-type Si NRs is not very clear, and further studies are required to get a clear mechanism. The possible mechanism may be suggested to the tunneling of photogenerated electrons to the substrate and trapping the holes in the NRs. In previous studies on the photoionization of an individual CdSe nanocrystals [16, 28], it was found that a significant fraction of nanocrystals was positively charged and it was attributed to the tunneling of the excited electrons into the substrate. They assumed that the hole tends to be localized in the nanocrystal, while the electron is much more delocalized, with a nonnegligible fraction of the electron density outside the nanocrystal. Another possibility arises from that the holes can be captured at Si-Si bonds according to the reaction ≡ Si-Si ≡ + h → ≡Si+ + · Si≡, as reported in reference . By adopting the above viewpoint, it can be suggested that when Si NRs are irradiated, free charges are photogenerated after dissociation of the excitons. Due to the tunneling of photoelectrons and/or capture of holes, the Si NRs would be positively charged.
In conclusion, the photogenerated charging and trapping phenomena are directly measured on single Si NRs without the deposition of electrodes by the means of EFM combined with laser irradiation. The amounts of photogenerated charges trapped in single NRs and the CPD values are obtained from the analytical fitting of ΔΦ − VEFM curves. The quantities of charges and CPD values are found to increase with the laser intensity and vary with the type of NRs. Though the exact mechanism for explaining the photogenerated effects of single Si NRs is not variable at present, it is clear that photoexcitation can lead to obvious charges trapped in Si NRs and hence reduce the work function of NRs. Therefore, EFM can provide an effective way to gain direct information on the trapped charges and surface potential of single nanostructures by combining with laser irradiation, which should be important for both basic understanding and potential applications of nanostructures in optoelectronics and photovoltaics.
This work was supported by the Major State Basic Research Project of China (No. 2011CB925601), National Natural Science Foundation of China (No. 11274072), and Natural Science Foundation of Shanghai (No.12ZR1401300).
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