Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase

Bae, Sung-Soo

Blackstock, Jason J

Kim, Sehun

Lee, Sangyeob

Medeiros-Ribeiro, Gilberto

Ragan, Regina

Stewart, Duncan R

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleScanning Tunneling Microscopy of Template-Stripped Au Surfaces and Highly Ordered Self-Assembled MonolayersPermalinkhttps://escholarship.org/uc/item/5h1488mfJournalLangmuir, 24(12)ISSN0743-7463AuthorsLee, SangyeobBae, Sung-SooMedeiros-Ribeiro, Gilbertoet al.Publication Date2008-06-01DOI10.1021/la800265qCopyright InformationThis work is made available under the terms of a Creative Commons Attribution License, available at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaScanning Tunneling Microscopy of Template-Stripped Au Surfacesand Highly Ordered Self-Assembled MonolayersSangyeob Lee,† Sung-Soo Bae,‡ Gilberto Medeiros-Ribeiro,§ Jason J. Blackstock,§Sehun Kim,‡ Duncan R. Stewart,§ and Regina Ragan*,†Department of Chemical Engineering and Materials Science, UniVersity of California, IrVine, California 92697,Department of Chemistry, Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong,Yuseong-gu, Daejeon 305-701, Korea, and Quantum Science Research Group, Hewlett-Packard Laboratories,MS 1123, 1501 Page Mill Road, Palo Alto, California 94303ReceiVed January 25, 2008. ReVised Manuscript ReceiVed March 28, 2008Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serveas substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunnelingmicroscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat,predominantly 〈111〉 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Aushow a c(4 × 2) superlattice with (3 × 3)R30° basic molecular structure having an ordered domain size up to100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparationmethod for SAMs deposited from both vapor phase and solution phase.The self-assembly of organic molecules onto surfaces hasproduced rich science and technology, including new nanoscalelithography methods,1 receptor-ligand dynamics,2 and sub-strates for cell adhesion.3 More recently, the integration ofbiological molecules into powerful new chip-scale sensors1and medical diagnostic devices4 demands even more controlover these inorganic/organic interfaces. Although self-as-sembled monolayers (SAMs) have been shown to producelocally well-ordered molecular structures via both solutionphase5 and vapor phase6 deposition, in almost all cases, thelonger range structural order in the SAM is limited by thesurface roughness of the underlying substrate.7–11 Solution-deposited alkanethiol SAMs on the coinage metals Au, Ag, Pt,and Pd are among the best studied monolayer systems; it is wellestablished that these molecular monolayers of 1-3 nm thicknessare readily disordered by the 2-5 nm peak-to-valley roughnessand grain boundaries typical of the metal surfaces. Ultraflattemplate-stripped (TS) metal surfaces12–15 have been shown toserve as a substrate for ordered molecular structures of solution-deposited SAMs.16–19 Until now, however, no study has validatedthe TS surface preparation through in situ ultrahigh vacuum(UHV) investigations of the TS surface and the subsequentlyassembled molecular monolayer. In this paper, we report sucha UHV scanning tunneling microscopy (STM) investigation ofTS Au surfaces stripped in UHV, and octanethiol SAMs vapor-deposited in situ onto those surfaces. We find that the strippingprocess produces a flat 〈111〉 textured atomically clean surface.The octanethiol SAMs show a clear c(4 × 2) superlattice with(3 × 3)R30° molecular structure over large 100 nm regions.These data agree favorably with “gold-standard” SAMs on singlecrystal Au(111) also prepared under UHV conditions, and thusvalidate the TS Au as a much simpler, but clean and high-qualitysurface preparation method for many studies of vapor or solution-deposited SAMs.The thin film layer structure and fabrication process for theTS Au is described in Figure 1. This process has been improvedfrom our initial UHV study of TS metals14,15 to better controlsurface grain structure. In this improved process, Au thin filmswith a thickness of 500 nm were electron-beam deposited at 300K onto a Si(001) template that had a 5 nm dry, thermally grownoxide layer. A 5 nm thick Ta layer followed by a 100 nm thicktungsten nitride (WN) layer was then deposited on Au thinfilms. Ta acts as an adhesion layer between WN and Au.Incorporation of a WN layer was found to improve layertransfer from template to substrate in the case of TS Ag andtherefore was utilized for TS Au fabrication. STM images ofTS Ag with WN and without WN are included in the SupportingInformation. The Au/Ta/WN samples were annealed in air at473 K for 2 h to obtain low defect density Au surfaces at theAu/SiO2 interface. After annealing, a Si substrate was glued tothe WN surface by silver epoxy, and a clasp was attached to theSi substrate for stripping in UHV.* Corresponding author. Telephone: (949)824-6830; fax: (949)824-2541;e-mail: rragan@uci.edu.† University of California, Irvine.‡ Korea Advanced Institute of Science and Technology.§ Hewlett-Packard Laboratories.(1) Tinazli, A.; Piehler, J.; Beuttler, M.; Guckenberger, R.; Tampe, R. Nat.Nanotechnol. 2007, 2, 220.(2) Hinterdorfer, P.; Dufrene, Y. F. Nat. Methods 2006, 3, 347.(3) Mrksich, M.; Whitesides, G. M. Annu. ReV. Biophys. Biomol. Struct. 1996,25, 55.(4) Cooper, M. A. Nat. ReV. Drug DiscoVery 2002, 1, 515.(5) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.J. Am. Chem. Soc. 1991, 113, 7152.(6) Poirier, G. E. Langmuir 1999, 15, 1167.(7) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151.(8) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17, 3307.(9) Leopold, M. C.; Black, J. A.; Bowden, E. F. Langmuir 2002, 18, 978.(10) Leopold, M. C.; Bowden, E. F. Langmuir 2002, 18, 2239.(11) Yang, J.; Han, J. M.; Isaacson, K.; Kwok, D. Y. Langmuir 2003, 19, 9231.(12) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39.(13) Blackstock, J. J.; Li, J. Y.; Freeman, M. R.; Stewart, D. R. Surf. Sci. 2003,546, 87.(14) Ragan, R.; Ohlberg, D.; Blackstock, J. J.; Kim, S.; Williams, R. S. J.Phys. Chem. B 2004, 108, 20187.(15) Ohlberg, D.; Blackstock, J. J.; Ragan, R.; Kim, S.; Williams, R. S. Appl.Phys. A: Mater. Sci. Process. 2005, 80, 1327.(16) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995,11, 3867.(17) Samori, P.; Diebel, J.; Lowe, H.; Rabe, J. P. Langmuir 1999, 15, 2592.(18) Gupta, P.; Loos, K.; Korniakov, A.; Spagnoli, C.; Cowman, M.; Ulman,A. Angew. Chem., Int. Ed. 2004, 43, 520.(19) Weiss, E. A.; Kaufman, G. K.; Kriebel, J. K.; Li, Z.; Schalek, R.; Whitesides,G. M. Langmuir 2007, 23, 9686.5984 Langmuir 2008, 24, 5984-598710.1021/la800265q CCC: $40.75  2008 American Chemical SocietyPublished on Web 05/10/2008Figure 2a shows a 200 nm × 200 nm STM image of a TS Ausurface that was stripped in a UHV chamber with an OmicronVT STM/AFM system attached. Atomically flat terraces with ameasured average terrace size 28 nm ( 8.3 nm (1σ) are clearlyresolved in the STM image. The average step height was measuredas 0.284 nm ( 0.053 nm (1σ), which corresponds to a singleatomic step height on Au. The line profile of the white line inthe Figure 2a, having a height range of 0.4 nm, shows only singleatomic steps between terraces. Pits were observed in terraces ofTS Au surfaces. Atomically resolved STM image analysisrevealed that these pits have one atomic height difference fromthe nearby terrace. These atomic-scale pits can be considered asa characteristic of the TS Au surface, since similar pits wereobserved on a TS Au surface when mica was used as a template.16Figure 2b shows a 10 nm × 10 nm atomic resolution STM imageof the TS Au surface. A Fourier transform (FT) pattern of Figure2b, shown as an inset in the figure, was used to determine thenearest-neighbor spacing (nns) of Au atoms as 0.284 nm( 0.011nm (1σ), which is in good agreement with the nns (0.288 nm)of an unrelaxed single crystal Au(111) surface. From this image,we confirm that the TS Au surface has a 〈111〉 textured structure.An alternating bright and dark striped pattern is also observedin Figure 2b. The periodicity of this feature was measured as 6.9nm along the [j110] direction from the image in Figure S1 in theSupporting Information. Considering that the piezodrift of theSTM scanner was calculated as approximately 0.04 Å/s alongthis direction, thereby leading to distortion of the measurementof up to 0.4 nm under the data acquisition conditions, the measuredperiodicity is in agreement with the periodicity of the herringbonereconstruction on Au(111) that is generally accepted as 23 ×3.20 The herringbone reconstruction is characteristic of cleansingle crystal Au(111) in UHV after a surface cleaning process,for example, flame annealing or sputtering. The atomic resolutionimage of TS Au in Figure 2b was obtained without any surfacecleaning process. Thus the template-stripping method was usedto obtain a pristine Au 〈111〉 textured surface that can be producedvia a simple and fast procedure.Au(111) is one of the prevalent substrates for the self-assemblyof (bio)molecular layers. In order to characterize the domain sizeand molecular packing structure of SAMs on TS Au, 500Langmuir (L) of octanethiol was deposited from the vapor-phaseonto TS Au. The deposition was performed in a chamber witha base pressure of 10-9 Torr that was connected to the UHVSTM chamber. Dosing pressures were typically 1 × 10-7 Torr.After deposition, the sample was transferred to the STM chamberand annealed at approximately 310 K for 1 h. Figure 3a is a 200nm × 200 nm STM image of octanethiol on a TS Au surface.The packing structure of the octanethiol SAMs is composed oflarge ordered domains and comparably smaller disordered regions.Figure 3b shows this structure more clearly. The ordered domain(20) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. Phys. ReV. B 1990, 42, 9307.Figure 1. Schematic of a thin film layer structure and the in situ UHV template-stripping procedure. (a) 500 nm Au + 5 nm Ta + 100 nm WN thinfilms were deposited on a SiOx template and then annealed at 473 K for 2 h. After, a Si substrate was attached to the WN surface by Ag epoxy.(b) A template clasp was attached to the sample for stripping in the UHV chamber. (c) The template was pulled off the substrate to expose the Ausurface in UHV.Letters Langmuir, Vol. 24, No. 12, 2008 5985size of the SAMs ranges from 40 to 100 nm. In comparison,SAMs on single crystal Au(111) show a complex network ofordered domains that are separated by disordered domainboundaries and vacancy islands along the boundary.21 The ordereddomain size of SAMs on single-crystal Au(111) ranges from 5to 15 nm.22,23 Thus, SAMs on TS Au surfaces have a remarkable1 order of magnitude larger domain size than SAM domains onsingle-crystal Au(111). The large domain size on TS Au may beattributed to the presence of a large number of single atomicsteps that can act as attachment sites for Au adatoms and sinksfor vacancies on the surface that result as the more densely packedherringbone surface reconstruction relaxes to the bulk latticestructure during SAM assembly.24 On single-crystal Au(111),vacancy islands form during SAM assembly as Au atoms areejected from the surface due to relaxation of the herringbonereconstruction.25 Vacancy island defects pin SAM domainboundaries, thereby limiting SAM domain size on single-crystalAu(111).21 Yet the formation of vacancy islands is not correlatedwith SAM assembly on TS Au. Approximately the same numbersof atomic-scale pits, which have the same appearance as a vacancyisland, are observed on the TS Au surface before (Figure 2a) andafter SAM deposition (Figure 3a). Furthermore, in contrast tosingle-crystal Au(111), the pits are found in the middle of SAMdomains on TS Au rather than on domain boundaries as onAu(111), indicating that pits do not limit domain size on TS Au.Step edges also appear less meandering after SAM deposition,(21) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853.(22) Poirier, G. E. Chem. ReV. 1997, 97, 1117.(23) Torrelles, X.; Vericat, C.; Vela, M. E.; Fonticelli, M. H.; Millone, M. A. D.;Felici, R.; Lee, T. L.; Zegenhagen, J.; Munoz, G.; Martin-Gago, J. A.; Salvarezza,R. C. J. Phys. Chem. B 2006, 110, 5586.(24) Camillone, N.; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys.1993, 98, 4234.(25) Poirier, G. E. Langmuir 1997, 13, 2019.Figure 2. STM images of a TS Au surface having (a) 200 nm × 200 nm and (b) 10 nm × 10 nm image sizes. (c) A line profile of the white linein panel a. The inset in panel b shows an FT of the image. Both STM images were taken at room temperature. (Sample bias: (a) -2.0 V, (b) -0.658V; feedback current: (a) 0.658 nA, (b) 1.16 nA.)Figure 3. (a) 200 nm × 200 nm (b) 35 nm × 35 nm (c) 15 nm × 15nm and (d) 5 nm × 5 nm STM images of TS Au dosed with 500 L ofoctanethiol. Inset in panel c shows an FT pattern of the image. (Samplebias: -2.0 V; feedback current: 50 pA.)5986 Langmuir, Vol. 24, No. 12, 2008 Lettersthus Au adatoms may prefer kink sites for attachment on stepedges. The presence of single atomic steps on TS surfaces, asobserved in the line profile of Figure 2c, versus step bunchingmay also contribute to the larger SAM domain size; terracesexpand as Au adatoms attach to step edges. The root-mean-square (rms) roughness of the TS Au surface was measured as0.149 nm over a 1 µm × 1 µm region. Since the value of therms roughness is dependent on the number of steps betweenterraces, the low rms roughness value indicates a low probabilityof finding multiple atomic steps between terraces (step bunching)on TS Au surfaces. For example, a previous STM study of TSPt versus single-crystal Pt(111) surfaces also showed that, whilethe terraces on single-crystal Pt were larger, there was a higherfrequency in the appearance of step bunching on Pt(111) thanon TS Pt. The rms roughness was 0.27 nm for TS Pt, whereas2.97 nm for single-crystal Pt.14 The lack of step bunching exhibitedby a low rms roughness value may prove to be very importantin regard to SAM assembly on metal surfaces.Molecular resolution images of the octanethiol SAMs areshown in Figure 3c,d. The inset of Figure 3c shows the FT patterncalculated from the image. The hexagonal symmetry of theoutmost shell of the FT pattern shows that the SAM adopts thesymmetry of the TS Au surface. The molecular nns wasdetermined as 0.502 nm ( 0.022 nm (1σ), which is 3 timesthe nns of Au atoms on the TS surface and corresponds to a (3× 3)R30° basic molecular packing structure. In the center ofthe FT pattern, two additional periodicities are present, whichare rotated 90° with respect to each other and represent aperiodicity of 4 nns and 2 nns of the TS Au surface. Thus, thec(4 × 2)superlattice structure observed on SAMs on single-crystal Au(111)21 is also present on SAMs on TS Au. The basicpacking unit and the superlattice are indicated in Figure 3d withdashed lines and solid lines, respectively.In conclusion, we have successfully fabricated atomically flatand clean TS Au surfaces and used this TS Au as a substrate forvapor phase assembly of octanethiol SAMs. STM images of TSAu showed atomic resolution of 28 ( 8.3 nm terraces with anultraflat rms roughness of 0.149 nm over 1 µm2. OctanethiolSAMs on TS Au show molecular resolution with 40-100 nmhighly ordered domains of (3 × 3)R30° structure with a c(4× 2) superstructure. While this local structure is consistent withthe best SAMs on single-crystal Au, most significant assemblyon TS Au on average yields much larger ordered moleculardomains. The template-stripping method is a simple and fastprocedure, well suited for both vapor and solution SAM growth.The UHV control study presented here demonstrates that templatestripping produces high quality Au surfaces that can be utilizedfor highly ordered molecular assembly as a superior alternativeto both single crystal and thin-film deposited Au surfaces.Acknowledgment. The authors thank D. A. A. Ohlberg andZ. Li (QSR, HPL) for their help in sample preparation andgratefully acknowledge the assistance of H. Ajormandi (Dept.of Chemical Engineering & Material Science, UCI) with imageprocessing. We thank R. S. Williams for use of laboratoryfacilities. J.J.B., G.M.R., and D.R.S. acknowledge DARPA forpartial funding of this research. S.B. and S.K. thank the BrainKorea 21 Project, the SRC program (Center for Nanotubes andNanostructured Composites) of MOST/KOSEF, the NationalR&D Project for Nano Science and Technology, and the KoreaResearch Foundation, Grant No. KRF-2005-070-C00063, forpartial funding. S.L. and R.R. acknowledge both an UCI AcademicSenate CORCLR Faculty Research Grant and the NSF NanoscaleExploratory Research program Grant No. ECCS-0709481 forfunding.Supporting Information Available: STM images of TS Ausurface that exhibit herringbone reconstruction. STM images of TS Agsurfaces showing the effect of WN layer between Ag and Si substrate.Thismaterial isavailablefreeofchargeviatheInternetathttp://pubs.acs.org.LA800265QLetters Langmuir, Vol. 24, No. 12, 2008 5987

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Scanning Tunneling Microscopy of Template-Stripped Au Surfaces and Highly Ordered Self-Assembled Monolayers

Scanning Tunneling Microscopy of Template-Stripped Au Surfaces and Highly Ordered Self-Assembled Monolayers

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