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.
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).
- Zhang Z, Zou R, Yu L, Hu J: Recent research on one-dimensional silicon-based semiconductor nanomaterials: synthesis. Crit Rev Solid State 2011, 36: 148–173. 10.1080/10408436.2011.589233View ArticleGoogle Scholar
- Barth S, Hernandez-Ramirez F, Holmes JD, Romano-Rodriguez A: Synthesis and applications of one-dimensional semiconductors. Prog Mater Sci 2010, 55: 563–627. 10.1016/j.pmatsci.2010.02.001View ArticleGoogle Scholar
- Kenry , Lim CT: Synthesis, optical properties, and chemical-biological sensing applications of one-dimensional inorganic semiconductor nanowires. Prog Mater Sci 2013, 58: 705–748. 10.1016/j.pmatsci.2013.01.001View ArticleGoogle Scholar
- Hu L, Chen G: Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications. Nano Lett 2007, 7: 3249–3252. 10.1021/nl071018bView ArticleGoogle Scholar
- Yoo J, Dayeh SA, Tang W, Picraux ST: Epitaxial growth of radial Si p-i-n junctions for photovoltaic applications. Appl Phys Lett 2013, 102: 093113. 10.1063/1.4794541View ArticleGoogle Scholar
- Perraud S, Poncet S, Noël S, Levis M, Faucherand P, Rouvière E, Thony P, Jaussaud C, Delsol R: Full process for integrating silicon nanowire arrays into solar cells. Sol Energ Mater Sol C 2009, 93: 1568–71. 10.1016/j.solmat.2009.04.009View ArticleGoogle Scholar
- Tsakalakos L, Balch J, Fronheiser J, Korevaar BA, Sulima O, Rand J: Silicon nanowire solar cells. Appl Phys Lett 2007, 91: 233117. 10.1063/1.2821113View ArticleGoogle Scholar
- Tang H, Zhu L-G, Zhao L, Zhang X, Shan J, Lee S-T: Carrier dynamics in Si nanowires fabricated by metal-assisted chemical etching. Acs Nano 2012, 6: 7814–7819. 10.1021/nn301891sView ArticleGoogle Scholar
- Kim J, Rhu H, Lee W: A continuous process for Si nanowires with prescribed lengths. J Mater Chem 2011, 21: 15889. 10.1039/c1jm13831fView ArticleGoogle Scholar
- Kiraly B, Yang S, Huang TJ: Multifunctional porous silicon nanopillar arrays: antireflection, superhydrophobicity, photoluminescence, and surface-enhanced Raman scattering. Nanotechnology 2013, 24: 245704. 10.1088/0957-4484/24/24/245704View ArticleGoogle Scholar
- Jespersen TS, Nygard J: Charge trapping in carbon nanotube loops demonstrated by electrostatic force microscopy. Nano Lett 2005, 5: 1838–1841. 10.1021/nl0505997View ArticleGoogle Scholar
- Heim T, Lmimouni K, Vuillaume D: Ambipolar charge injection and transport in a single pentacene monolayer island. Nano Lett 2004, 4: 2145–2150. 10.1021/nl0487673View ArticleGoogle Scholar
- Yalcin SE, Labastide JA, Sowle DL, Barnes MD: Spectral properties of multiply charged semiconductor quantum dots. Nano Lett 2011, 11: 4425–4430. 10.1021/nl2026103View ArticleGoogle Scholar
- Yalcin SE, Yang B, Labastide JA, Barnes MD: Electrostatic force microscopy and spectral studies of electron attachment to single quantum dots on indium tin oxide substrates. J Phys. Chem C 2012, 116: 15847–53. 10.1021/jp305857dView ArticleGoogle Scholar
- Li S, Steigerwald ML, Brus LE: Surface states in the photoionization of high-quality CdSe core/shell nanocrystals. Acs Nano 2009, 3: 1267–1273. 10.1021/nn900189fView ArticleGoogle Scholar
- Cherniavskaya O, Chen LW, Islam MA, Brus L: Photoionization of individual CdSe/CdS core/shell nanocrystals on silicon with 2-nm oxide depends on surface band bending. Nano Lett 2003, 3: 497–501. 10.1021/nl0340529View ArticleGoogle Scholar
- Groves C, Reid OG, Ginger DS: Heterogeneity in polymer solar cells: local morphology and performance in organic photovoltaics studied with scanning probe microscopy. Acc Chem Res 2010, 43: 612–620. 10.1021/ar900231qView ArticleGoogle Scholar
- Giridharagopal R, Shao G, Groves C, Ginger DS: New SPM techniques for analyzing OPV materials. Mater Today 2010, 13: 50–56.View ArticleGoogle Scholar
- Coffey DC, Ginger DS: Time-resolved electrostatic force microscopy of polymer solar cells. Nat Mater 2006, 5: 735–740. 10.1038/nmat1712View ArticleGoogle Scholar
- Wu Z, Lei H, Zhou T, Fan Y, Zhong Z: Fabrication and characterization of SiGe coaxial quantum wells on ordered Si nanopillars. Nanotechnology 2014, 25: 055204. 10.1088/0957-4484/25/5/055204View ArticleGoogle Scholar
- Mélin T, Diesinger H, Deresmes D, Stiévenard D: Electric force microscopy of individually charged nanoparticles on conductors: an analytical model for quantitative charge imaging. Phys Rev B 2004, 69: 035321.View ArticleGoogle Scholar
- Terris B, Stern J, Rugar D, Mamin H: Contact electrification using force microscopy. Phys Rev Lett 1989, 63: 2669–2672. 10.1103/PhysRevLett.63.2669View ArticleGoogle Scholar
- Mélin T, Diesinger H, Deresmes D, Stiévenard D: Probing nanoscale dipole-dipole interactions by electric force microscopy. Phys Rev Lett 2004, 92: 166101.View ArticleGoogle Scholar
- Lei CH, Das A, Elliott M, Macdonald JE: Quantitative electrostatic force microscopy-phase measurements. Nanotechnology 2004, 15: 627–634. 10.1088/0957-4484/15/5/038View ArticleGoogle Scholar
- Dokukin M, Olac-Vaw R, Guz N, Mitin V, Sokolov I: Addressable photocharging of single quantum dots assisted with atomic force microscopy probe. Appl Phys Lett 2009, 95: 173105. 10.1063/1.3254895View ArticleGoogle Scholar
- Chiesa M, Burgi L, Kim JS, Shikler R, Friend RH, Sirringhaus H: Correlation between surface photovoltage and blend morphology in polyfluorene-based photodiodes. Nano Lett 2005, 5: 559–563. 10.1021/nl047929sView ArticleGoogle Scholar
- Liscio A, Palermo V, Samori P: Nanoscale quantitative measurement of the potential of charged nanostructures by electrostatic and Kelvin probe force microscopy: unraveling electronic processes in complex materials. Acc Chem Res 2010, 43: 541–550. 10.1021/ar900247pView ArticleGoogle Scholar
- Krauss TD, Brus LE: Electronic properties of single semiconductor nanocrystals: optical and electrostatic force microscopy measurements. Mat Sci Eng B 2000, 69–70: 289–294.View ArticleGoogle Scholar
- Kim J-H, Noh H, Khim ZG, Jeon KS, Park YJ, Yoo H, Choi E, Om J: Electrostatic force microscopy study about the hole trap in thin nitride/oxide/semiconductor structure. Appl Phys Lett 2008, 92: 132901–3. 10.1063/1.2904646View ArticleGoogle Scholar
- Liu R: Imaging of photoinduced interfacial charge separation in conjugated polymer/semiconductor nanocomposites. J Phys Chem C 2009, 113: 9368–9374. 10.1021/jp810732nView ArticleGoogle Scholar
- Diesinger H, Mélin T, Deresmes D, Stiévenard D, Baron T: Hysteretic behavior of the charge injection in single silicon nanoparticles. Appl Phys Lett 2004, 85: 3546–3548. 10.1063/1.1808889View ArticleGoogle Scholar
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