Synthesis And Oxidation Of Silver Nano-particles
10-1226-2542Synthesis and Oxidation of Silver Nano-particlesHua Qi*, D. A. Alexson, O.J. Glembocki and S. M. Prokes*Electronics Science and Technology Division,Naval Research Laboratory, Washington, DC 20375huaqi@ccs.nrl.navy.mil; sharka.prokes@nrl.navy.milABSTRACTWe demonstrated a fast and easy way to synthesize Ag nanoparticles (NPs) on ZnOnanowires (NWs) and silicon substrates by an electroless (EL) plating approach. ZnO NWsused here were grown via vapor-solid (VS) mechanism at 560 ºC for 30 min. The stability tooxidation of these EL-produced homogeneous Ag NPs on ZnO nanowires was investigated bysurface enhanced Raman spectroscopy (SERS), showing that the attachment of thiol to the Agsurface can slow down the oxidation process, and the SERS signal remains strong for morethan ten days. Furthermore, we examined the surface oxidation kinetics of the Ag NPs as afunction of NPs size and size distribution by monitoring the oxygen amount in the compositesusing energy dispersive x-ray (EDX). Results indicate that the EL plated Ag NPs show fasteroxidation rates than those produced by e-beam (EB) evaporation in air. We attribute this tothe fact that the EL produced silver particles are very small, in the 20 nm range, and thus havehigh surface energy, thus enhancing the oxidation. These studies provide extensiveinformation related to the Ag NP oxidation rates, which can help in extending the Ag lifetimefor various applications.Keywords: silver nanoparticles (NPs), oxidation kinetics, electroless (EL) silver plating,surface enhanced Raman spectroscopy (SERS), energy dispersive x-ray (EDX)1. INTRODUCTIONNano-particles (NPs) have attracted much attention due to their potential applications incatalysis, biology, computing, solar cells and optoelectronic devices [1-5]. For example, silver(Ag) NPs have been widely used to enhance the surface sensitivity of some spectroscopicmeasurements, such as fluorescence, Raman scattering, and second harmonic generation.Researchers have reported many different ways to produce silver NPs, such asphotochemistry, electrochemistry, chemical reduction, microwave processing, ultra-soundprocessing, gamma irradiation, ion irradiation, plasma processing, electron irradiation etc. [614], to meet the requirement of various applications. However, the oxidation process isseldom studied in detail on the nanoscale. Here, we employed an easy and reproducibleelectroless approach to synthesize silver NPs on ZnO NWs and on a bare silicon surface, andwe have investigated the silver oxidation rates by monitoring surface enhanced Ramanspectroscopy (SERS) signal and by energy dispersive x-ray (EDX) techniques. The resultsQuantum Dots and Nanostructures: Synthesis, Characterization, and Modeling VIII,edited by Kurt G. Eyink, Frank Szmulowicz, Diana L. Huffaker, Proc. of SPIE Vol. 7947,79470Y · 2011 SPIE · CCC code: 0277-786X/11/ 18 · doi: 10.1117/12.871122Proc. of SPIE Vol. 7947 79470Y-1Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
Form ApprovedOMB No. 0704-0188Report Documentation PagePublic reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.1. REPORT DATE3. DATES COVERED2. REPORT TYPE201100-00-2011 to 00-00-20114. TITLE AND SUBTITLE5a. CONTRACT NUMBERSynthesis and Oxidation of Silver Nano-particles5b. GRANT NUMBER5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S)5d. PROJECT NUMBER5e. TASK NUMBER5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Naval Research Laboratory,Electronics Science and TechnologyDivision,Washington,DC,203759. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)8. PERFORMING ORGANIZATIONREPORT NUMBER10. SPONSOR/MONITOR’S ACRONYM(S)11. SPONSOR/MONITOR’S REPORTNUMBER(S)12. DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release; distribution unlimited13. SUPPLEMENTARY NOTES14. ABSTRACTWe demonstrated a fast and easy way to synthesize Ag nanoparticles (NPs) on ZnO nanowires (NWs) andsilicon substrates by an electroless (EL) plating approach. ZnO NWs used here were grown via vapor-solid(VS) mechanism at 560 ?C for 30 min. The stability to oxidation of these EL-produced homogeneous AgNPs on ZnO nanowires was investigated by surface enhanced Raman spectroscopy (SERS), showing thatthe attachment of thiol to the Ag surface can slow down the oxidation process, and the SERS signalremains strong for more than ten days. Furthermore, we examined the surface oxidation kinetics of the AgNPs as a function of NPs size and size distribution by monitoring the oxygen amount in the compositesusing energy dispersive x-ray (EDX). Results indicate that the EL plated Ag NPs show faster oxidationrates than those produced by e-beam (EB) evaporation in air. We attribute this to the fact that the ELproduced silver particles are very small, in the 20 nm range, and thus have high surface energy, thusenhancing the oxidation. These studies provide extensive information related to the Ag NP oxidation rates,which can help in extending the Ag lifetime for various applications.15. SUBJECT TERMSsilver nanoparticles (NPs), oxidation kinetics, electroless (EL) silver plating, surface enhanced Ramanspectroscopy (SERS), energy dispersive x-ray (EDX)16. SECURITY CLASSIFICATION OF:a. REPORTb. ABSTRACTc. THIS PAGEunclassifiedunclassifiedunclassified17. LIMITATION OFABSTRACT18. NUMBEROF PAGESSame asReport (SAR)1119a. NAME OFRESPONSIBLE PERSONStandard Form 298 (Rev. 8-98)Prescribed by ANSI Std Z39-18
indicate that a self assembled monolayer of benzene thiol on the silver NPs can slow down theoxidation of silver to a certain extent. Further quantitative investigation demonstrated the sizeand size distribution of the silver NPs also play important roles in the oxidation rate.2. EXPERIMENTAL DETAILES2.1 Growth of ZnO NWsZnO nanowires were grown in a horizontal furnace under flowing Ar atmosphere at 560ºC for 30 minutes. No catalyst was used and growth was performed on a Si (100) substrate[15]. In this process, Zn powder was placed at one end of a quartz boat, and the substrateswere placed a set distance from the powder, as shown in scheme 1.(a)Ar flowingSilicon(b)SourceNanowires (NWs)(c)NW as grownSensitizing of NW(d)Silver plating of NWSEM, EDX and SERS investigationsScheme 1. Experimental Scheme of the ZnO nanowire growth, electroless deposition, andcharacterization of the NWs/Ag shealth composites.2.2 EL and EB silver deposition on ZnO NWs and on a silicon surfaceThe concentrated solutions for the silver plating were received from Peacoak Lab andfurther diluted with de-ionized water. The dilution factors were 1:100 ratio by volume forsilver solution “A” (25%-30% wt silver diammine and 10-15% wt ammonium hydroxide inProc. of SPIE Vol. 7947 79470Y-2Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
water), activator solution “B” (sodium hydroxide 10%wt and ammonium hydroxide 5%wt inwater), reducer solution “C” (1%wt Formaldehyde in water) and sensitizer solution (20%wtpropyl alcohol, 5%wt hydrochloric acid and 5%wt stannous chloride in water). Scheme 1band c illustrate the sensitization and silver plating process [16]. Firstly, the substrate with theNWs was immersed into the diluted stannous chloride solution, and then extra Sn2 ions wereremoved by de-ionized water rinsing and keeping the surface wet. Hence Sn2 ions wereabsorbed on the surface of the ZnO NWs via electrostatic interaction. Secondly the mixture ofequal amounts of diluted silver, activator, and reducer solutions was poured onto the substrateand kept there. During this process, the surface Sn2 ions were oxidized to Sn4 , while the Ag ions were reduced to neutral Ag and deposited on the ZnO nanowire surface uniformly.Finally, the surface was rinsed with DI-water and dried under flowing nitrogen.Experimentally, 10 to 20 seconds of reaction time would usually give a homogeneouscoverage with a high density of silver 3D islands on the NW surface.For comparison, a bare silicon substrate was covered with 6 nm of silver using an FC2000 Temescal E-Beam Metals Evaporation system, at an evaporation rate of 1 Å/s.2.3 SEM and SERS investigations of silver coated silicon and NWs surfaceThe topography and chemical composition of EB and EL deposited silver nano structuresdeposited on the ZnO NWs and on the silicon surfaces were investigated using a LEOSUPRA 55 scanning electron microscope (SEM) with energy dispersive x-ray (EDX)capabilities. The dielectric ZnO NW core/Ag sheath composites as prepared above were thenimmersed in a solution of benzenthiol (10-4M) for 6 h, and dried for SERS studies toinvestigate the silver stabilization properties with laser scanning and exposure time in the air.The SERS measurements were carried out utilizing a confocal μ-Raman system whichconsisted of a Mitutoyo Microscope and an Ocean Optics QE65000 spectrometer equippedwith a thermoelectrically cooled CCD. The 514.5 nm line of an Ar ion laser was used as theexcitation source. The microscope utilized a 100X 0.7 NA objective for focusing the laserlight and was coupled to the spectrometer through a fiber optic cable.3. RESULTS AND DISCUSSION3.1 SEM characterization of ZnO NWs as grownRepresentative SEM images of the ZnO NWs as grown on Si (100) are shown in Figure 1and Figure 2.ZnO NWs can be grown with or without a gold catalyst and in our experiment, weemployed bare silicon substrate without any metal catalyst. It is clear that the tip is free of ametal cap, confirming the catalyst free growth process via the vapor-solid (VS) growthmechanism [15]. As shown in Figure 1, different density, geometry, length and diameter ofthe NWs can be obtained by controlling the growth conditions, such as the furnace tube size,gas flow rate, growth time, and the distance between the substrate and the source material.Proc. of SPIE Vol. 7947 79470Y-3Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
Figure 1 SEM images of ZnO growth. Representative SEM images of ZnO nanowiresgrown on Si with different geometries;Figure 2 shows high resolution SEM images of the individual nanostructures as grown.Different NW shapes, including single, angled crossing, “Y” shapes, “star” shapes and “nanosaw” shapes formed. The diameters varied from 50 nm to 300 nm, and the length was on theProc. of SPIE Vol. 7947 79470Y-4Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
order of several microns. Energy dispersive x-ray (EDX) analysis indicated that the chemicalcomposition of the NWs was stoichiometric ZnO.Figure 2 Representative SEM images of ZnO nanowires grown on Si with differentgeometries;Proc. of SPIE Vol. 7947 79470Y-5Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
3.2 SEM characterization of silver coated 81216202.53.54.55.5KeVKeVFigure 3 SEM images of EL (a) and EB (b) silver coated ZnO NWs, and energy diffraction xray (EDX) analysis of EL coated NWs.Figure 3a shows the tip of an electroless silver coated ZnO NW, which displays 3Dislands with diameters on the order of 20nm. In addition electroless silver islands weredistributed more uniformly on the NWs than those on bare silicon, which indicates themorphology of silver not only depends on the deposition approach, but also is related to thesubstrate or substrate geometry. The most likely explanation for these results is that thesurface structure, faceting and curvature or surface energy affects the nucleation of the metalislands. As can be seen in Fig. 1a, the advantages of EL silver plating overcomes theshortcomings of the EB evaporation technique, which cannot deposit the metal around thewhole NW, since it is a line-of- sight process. This also becomes a problem for closelyaligned vertical NWs or other oriented structures, as shown in Figure 3b, in which e-beamevaporation was used. Also note the high metal island coverage on the Si substrate in Fig. 1b,which is not the case in the EL process (Fig. 1a), supporting the idea that the geometry andsurface morphology is also important in the case of nucleation in the EL process. In addition,EL plating is a very inexpensive technique with potentially large throughput, which is notpossible in the EB deposition case.Proc. of SPIE Vol. 7947 79470Y-6Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
Figures 3c and 3d show the EDX results for the EL Ag covered ZnO NWs shown in fig.1a, and a close-up of the curve of the silver EDX peak region. The peaks at 1.03 KeV, 8.62KeV and 9.59 KeV can be assigned to Zn [17, 18], and the weak features at 2.98 KeV and3.14 KeV are attributed to Ag [19, 20]. This result confirms the depositions of silver particleson the NWs.3.3 Ag stability investigation via SERSThe SERS investigations were performed with a laser power of 0.75 mW and a laserdiameter of 600 nm on the sample. A large number of SERS measurements were performedon the newly prepared samples in order to investigate the time dependence behavior. Asshown in Figure 4a, the main line of BZT at 1576 cm-1 remained steady on the electrolesssilver covered NWs, indicating good stability of the freshly prepared silver sheath NWs underrelatively short laser irradiation, and the power of 0.75 mW did not result in damage to thebenzenethiol or the EL coated Ag layer.1576 cm-1 line1 day8 days11 days(b)Intensity (a.u.)Time(a)6001200180024003000-1Raman shift [cm ]Figure 4. (a) Time dependent behavior of Ag on NW produced by electroless (EL)depositions. (b) Surface enhanced Raman spectroscopy (SERS) on ZnO NW core/Ag shealthcomposites with the process of time in air.To examine the stability to oxidation of the silver nanoparticles, SERS measurementswere carried out on the same single dielectric ZnO nanowire core/silver sheath compositeproduced by EL plating, and benzenethiol (BZT) served as the SERS-active molecule. Asingle NW was randomly selected for SERS measurements and monitored for the signalstrength from the moment that the composites were fabricated. Figure 4(b) showsrepresentative spectra which were obtained on the EL generated nanowire/silver structuresafter the samples were exposed in air for several days. The major Raman peaks at 1002, 1071,and 1576 cm-1 can be assigned to symmetric ring breathing modes, in plane C-H bendingmodes, and in plane C-C stretching modes of the phenyl ring of benzenthiol [21-26],respectively. In comparison with the spectrum obtained on the first day, the intensity of themain SERS signal at 1576 cm-1 does not show a significant decrease as a function of time inProc. of SPIE Vol. 7947 79470Y-7Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
the atmosphere, which suggests that the thiol attachment to the Ag surface protects the silverfrom oxidation for this time period.However, once these structures were left in the atmosphere for longer times afterfabrication, SERS mapping of these structures suggested that the composites did graduallydegenerate. Figure 5(a) shows the SERS mapping results of the dielectric ZnO nanowirecore/silver sheath composite at 1576 cm-1, and its corresponding microscope images includingbefore and after laser scanning (Figure 5(b) and 5(c)). This sample was produced by the ELplating process, and the sample was kept in the air for eight days. The mapped region was a4 4 micron square. As shown in the map, strong SERS signal was obtained on the nanowirecore/silver sheath composite region, while no SERS signal was observed on the bare siliconwafer (blue regions). However, exposure to the laser beam led to the near disappearance ofthe NW in the optical microscope (Figure 5(b) and 5(c)), which may be attributed to thedeterioration of the Ag coating, leading to a loss of contrast and visibility in the opticalmicroscope. In fact, the NWs are just barely visible if viewed through the microscopeeyepiece. When this sample was examined with AFM/TERS after the mapping was finished, acomplete loss of detectable SERS signal was noted. This observation indicates that althoughinitially, the Ag is protected by the thiol layer, gradual oxidation in air does 0000Figure 5 (a) Surface enhanced Raman spectroscopy (SERS) mapping of ZnO NWs core/silversheath composites at the line of 1576 cm-1; (b) and (c) microscope images before and afterperforming the mapping.3.4 Ag NPs stabilization investigation via EDXIt is well known that properties, such as the melting temperature, depend on size, shapeand dimensionality of the nano structures. The small size usually means a large surface tovolume ratio and a higher surface energy than the bulk, as well as a larger surface energy.Here we demonstrate that the silver oxidation process also depends on the size of thenanostructures. In order to investigate the stability of a silver layer with a broad particle sizeProc. of SPIE Vol. 7947 79470Y-8Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
(a)200 nmAmount of oxygen (%)distribution, Ag NPs were deposited on bare silicon via the EL chemistry process. This flatsubstrate exhibited a much broader size distribution in the range of 5-100 nm than on thecurved NWs, as shown in Figure 6(a). A piece of the flat silicon covered with the EL platedsilver NPs was stored in air after the silver plating. Energy diffraction X-ray (EDX) wasperformed in order to obtain the silver oxidation rate. Although the oxidation rate is irregular,as shown in Figure 6(b), it is obvious that the oxygen amount in Ag NPs is increasing with theprocess of time.To further examine the dependence of the oxidation process on the silver NP sizedistribution, a layer of 6 nm of silver was deposited on a bare silicon piece by E-beamevaporation, at an evaporation rate of 1 Å s-1. As shown in Figure 6(c), the EB evaporatedsilver particle size was in the range of 15-55 nm with a mean diameter of 35 nm. As can beseen, the oxidation rate of the EB produced silver is slower than that formed by the ELchemical plating approach (Figure 6d). These results suggest that the particle size, as well asthe particle size distribution, can have a significant effect on the oxidation rate.(b)2.11.4EL-Ag-Air0.7048Time (days)12200 nmAmount of oxygen (%)1.6(c)(d)1.2EB-Ag-Air0.804812Time (days)Figure 6. SEM images of silver NPs on silicon surface produced by EL plating (a) and EBcoating(c); (b) and (d) plot of oxygen amount vs time.4. SUMMARYThe stability of silver nano structures has been studied by investigating the change of theamount of oxygen as a function of time in the atmosphere, using the Si/Ag system, producedvia electroless (EL) plating and e-beam (EB) deposition. SERS measurements on NW/Ag NPsProc. of SPIE Vol. 7947 79470Y-9Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
composites were also performed, indicating that a monolayer of benzene thiol can protect thesilver surface for more than ten days. Our results also demonstrate that the sharp distributionof very small Ag NPs produced by EL show significantly enhanced oxidation rates comparedto those formed by EB approach. These results provide useful information on the silver nanostructure oxidation process which should be helpful in extending the silver lifetime for variousapplications.5. ACKOWNLEGEMENTThis work was supported by the Office of Naval Research (ONR) and Nanoscience Institute(NSI) of the US Naval Research Laboratory.6. REFERENCES[1][2][3][4][5]K. M. Manesh; A. I. Gopalan; K. P. Lee and S. Komathi Catal. Commun. 11, 913-918, (2010).Wang, G. F.; Stender, A. S.; Sun, W.; Fang, N.; Analyst 135, 215-221, (2010).Wong, V.; Ratner, M. A. J. Phys. Chem. B 110, 19243-19253, (2006).Akimov, Y. A.; Koh, W. S. Nanotechnology 21, 235201 (2010).Nelayah, J.; Kociak, M.; Stephan, O.; de Abajo F. J. G.; Tence, M.; Henrard, L.; Taverna, D.;Pastoriza-Santos, I.; Liz-Marzan, L. M.; Colliex, C.; Nat. Phys. 3, 348-353, (2007).[6] Arnim, H. Chem. Mater., 10, 444–450, (1998).[7] Reetz, M. T.; Helbig, W.; J. Am. Chem. Soc., 116, 7401–7402, (1994).[8] N. Leopold; and B. Lendl J. Phys. Chem. B, 107, 5723–5727, (2003).[9] W. Tu and H. Liu Chem. Mater., 564–567, 12, (2000).[10] Y. Nagata; Y. Watananabe; S. Fujita; T. Dohmaru; and S. Taniguchi; J. Chem. Soc., Chem.Commun., 114, 1620-1622, 1992,[11] W. Wu; Y. Wang; L. Shi; Q. Zhu; W. Pang; G. Xu; and F. Lu Nanotechnology 16, 3017,(2005)[12] A. L. Stepanov; D. E. Hole and P. D. Townsend J. Non-Cryst. Solids 260, 65-74,(1999).[13] C. Balasubramanian; V. P. Godbole; V. K. Rohatgi; A. K. Das and S. V. BhoraskarNanotechnology 15, 370, (2004).[14] K. A. Bogle; S. D. Dhole and V. N. Bhoraskar, Nanotechnology 17, 3204-3208 (2006).[15] Prokes S. M.; Glembocki, O. J., Rendell, R. W. and Ancona, M. G. Appl. Phys. Lett. 90,093105, (2007).[16] Qi, H.; Alexson, D.; Glembocki, O.; and Prokes, S. M. Nanotechnology 21, 085705,(2010).[17] Liu, S.; Zhang, H. and Swihart, M. T. Nanotechnology 20, 235603, (2009).[18] Büsgen, T.; Hilgendorff, M.; Irsen, S.; Wilhelm, F.; Rogalev, A.; Goll, D.; and Giersig,M. J. Phys. Chem. C 112, 2412, (2008).[19] Gao, Y.; Shan, D.; Cao, F.; Gong, J.; Li, X.;, Ma, H.; Su, Z. and Qu, L. J. Phys. Chem. C113, 15175, (2009).Proc. of SPIE Vol. 7947 79470Y-10Downloaded from SPIE Digital Library on 12 May 2011 to 132.250.22.10. Terms of Use: http://spiedl.org/terms
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2.2 EL and EB silver deposition on ZnO NWs and on a silicon surface The concentrated solutions for the silver plating were received from Peacoak Lab and further diluted with de-ionized water. The dilution factors were 1:100 ratio by volume for silver solution “A” (25%-30% wt silver dia
Nano? Silver containing substance (SCAS) CAS No. Elemental silver a) Particulate silver 7440-22-4 b) Silver ionisation systems 7440-22-4 Silver adsorbed to silica dioxide Not yet allocated Silver chloride adsorbed to titan dioxide Not yet allocated Silver nitrate 7761-88-8 Silver sodium hydrogen zirconium phosphate 265647-11-8 Silver zinc .
Oxidation-reduction reactions are those which take place with a change in the oxidation state of atoms through the redistribution of electrons between them. or Biological oxidation is catalysed by enzymes which function in combination with coenzymes and/or electron carrier proteins. The differences between biological oxidation and combustion
Oxidation Numbers Worksheet . Directions: Use the . Rules for Assigning Oxidation Numbers. to determine the oxidation number assigned to each element in each of the given chemical formulas. Formula Element and Oxidation Number Formula Element and Oxidation Number . 1. Cl. 2. Cl 16. Na. 2. O. 2. Na O 2. Cl-Cl 17. SiO. 2. Si O 3. Na Na 18. CaCl .
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Oxidation states were defined in Chapter 7. The oxidation number assigned to an element in a molecule is based on the distribution of elec-trons in that molecule.The rules by which oxidation numbers are assigned were given in Chapter 7.These rules are summarized in Table 19-1. OXIDATION-REDUCTION REACTIONS 591 SECTION 19-1 O BJECTIVES Assign .
The consequence is a silver boom. Silver in cleaning agents keeps your households free from germs. Silver in cosmetic articles serves as a preservative. Silver in textiles promises to prevent odor caused by sweating. Silver in refrigerators protects your food. And even waste bags containing silver are available on the market.
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Your sheet-pile program FADSPABW (B-9) is a special case of this method. It was separately written, although several subroutines are the same, because there are special features involved in sheet-pile design. These additional considerations would in-troduce unnecessary complexity into a program for lateral piles so that it would be a little more difficult to use. Many consider it difficult in .
