Formation of Hybrid 2D Polymer− Metal Microobjects

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Formation of Hybrid 2D Polymer-Metal Microobjects Jean E. Comrie and Wilhelm T. S. Huck* Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ReceiVed July 5, 2006. In Final Form: October 31, 2006 This paper describes a fabrication strategy based on polymer brushes (20-150 nm thick) and soft lithographic techniques, for creating hydrophobic, cross-linked, laterally patterned polymer films. The hydrophobicity of the resulting micrometer-scale “quasi-2D” objects is shown to allow the polymer to act as an etch resist. By adjusting the etching time, we demonstrate that underetching of the gold from underneath the edges of the laterally patterned films can be used to create free-standing polymer-gold hybrid structures. These structures retain their structural integrity when lifted wholly or partially from the substrate and can hence be imaged in suspension. Characterization of the quasi-2D objects was carried out using atomic force microscopy (AFM), ellipsometry, optical microscopy, and Fourier transform infrared spectroscopy (FTIR). A continuous film, containing embedded polymer-gold objects, can be lifted, folded, and re-deposited onto a substrate without damaging the conductivity of the embedded metallic objects.

1. Introduction Research into “quasi-2D” objects (i.e., objects with lateral dimensions orders of magnitude larger than the thickness) has been inspired by theoretical interest in potential conformational changes that such objects could undergo in response to changes in temperature and solvent.1-3 Controlled folding of quasi-2D objects on a suitable size scale into well-defined 3D objects could find applications in biomimetic and microelectromechanical systems (MEMS). Polymeric objects are particularly attractive candidates for components to be used in such systems because they have the potential to be biocompatible, as well as providing a range of material properties inaccessible to components made from silicon (e.g., increased elasticity and fracture toughness).4,5 Several methods have previously been investigated for the creation of mechanically robust quasi-2D objects of which the thickness is on the nanoscale and the lateral dimensions are on the micrometer scale. These include UV cross-linking of LangmuirBlodgett films,6-9 cross-linking of lipid bilayers and vesicles,10-18 * To whom correspondence should be addressed. E-mail: wtsh2@ cam.ac.uk. (1) Kantor, Y.; Kardar, M.; Nelson, D. R. Phys. ReV. Lett. 1986, 57, 791-794. (2) Kantor, Y.; Nelson, D. R. Phys. ReV. Lett. 1987, 58, 2774-2777. (3) Abraham, F. F.; Kardar, M. Science 1991, 252, 419-422. (4) Meng, E. F.; Tai, Y- C. Abstr. Pap.-Am. Chem. Soc. 2003, 226, U383U383. (5) Yang, X.; Grosjean, C.; Tai, Y.-C. J. Microelectromech. Syst. 1999, 8, 393-402. (6) Aoki, A.; Miyashita, T. AdV. Mater. 1997, 9, 361-364. (7) Aoki, A.; Nakaya, M.; Miyashita, T. Macromolecules 1998, 31, 73217327. (8) Kado, Y.; Mitsuishi, M.; Miyashita, T. AdV. Mater. 2005, 17, 1857-1861. (9) Johnston, D. S.; Sanghera, S.; Manjon-Rubio, A.; Chapman, D. Biochim. Biophys. Acta 1980, 602, 213-216. (10) Morigaki, K.; Baumgart, T.; Offenha¨usser, A.; Knoll, W. Angew. Chem., Int. Ed. 2001, 40, 172-174. (11) O’Brien, D. F.; Armitage, B.; Benedicto, A.; Bennett, D. E.; Lamparski, H. G.; Lee, Y.-S.; Srisiri, W.; Sisson, T. M. Acc. Chem. Res. 1998, 31, 861-868. (12) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113-158. (13) Kunitake, T.; Nakashima, N.; Takarabe, K.; Nagai, M.; Tsuge, A.; Yanagi, H. J. Am. Chem. Soc. 1981, 103, 5945-5947. (14) Sells, T. D.; O’Brien, D. F. Macromolecules 1994, 27, 226-233. (15) Wang, G.; Hollingsworth, R. I. AdV. Mater. 2000, 12, 871-874. (16) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788-796. (17) Fendler, J. H. Science 1984, 223, 888-894. (18) Stupp, S. I.; Son, S.; Lin, H. C.; Li, L. S. Science 1993, 259, 59-63. (19) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531-537. (20) Asakuma, S.; Okada, H.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 1749-1755.

polymerization of monomers within the hydrophobic cavity of lipid bilayers,19,20 cross-linking of vinylbenzene-modified nanoparticles at a fluid-fluid interface,21 and layer-by-layer deposition on patterned substrates.22-28 Approaches based on prepatterned surfaces offer potential control over the micrometerscale, lateral dimensions. Although multilayer deposition offers some control over composition and structure inside the film, the range of different architectures is limited; the use of patterned polymer brushes has therefore been explored as a general tool for the fabrication of 2D polymer films. Previous work within our group established that cross-linking of patterned polymer brushes is possible by exploiting reactive functional groups on the brush side chains and leads to quasi-2D polymer films which are mechanically strong enough to be lifted from the substrate upon electrical reduction of the gold-thiol bonds.29 Due to the synthetic flexibility of the cross-linking procedure, quasi-2D films with a range of chemical and mechanical properties are accessible. In future applications, the quasi-2D sheets should not only introduce shape but also carry functionality; polymer brushes show interesting responsive behavior based on swelling-collapse transitions, which allows the triggered buildup and release of in-plane stress in polymer brush films.30 It is therefore desirable to develop a fabrication procedure that incorporates polymeric and metallic elements into the quasi-2D sheet structure. In the present work, we exploit the incorporation of a hydrophobic functional group into the polymer brushes during the crosslinking step as a means of creating a robust, etch-resistant film. A combination of partial underetching and subsequent lift-off allows the formation of a new class of “hybrid” 2D objects, (21) Lin, Y.; Skaff, H.; Bo¨ker, A.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. J. Am. Chem.Soc. 2003, 125, 12690-12691. (22) Huck, W. T. S.; Stroock, A. D.; Whitesides, G. M. Angew. Chem., Int. Ed. 2000, 39, 1058-1061. (23) Stroock, A. D.; Kane, R. S.; Weck, M.; Metallo, S. J.; Whitesides, G. M. Langmuir 2003, 19, 2466-2472. (24) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485-1488 (25) Moya, S.; Donath, E.; Sukhorukov, G. B.; Auch, M.; Ba¨umler, H.; Lichtenfeld, H.; Mo¨hwald, H. Macromolecules 2000, 33, 4538-4544. (26) Decher, G. Science 1997, 277, 1232-1237. (27) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237-7244. (28) Hammond, P. T.; Whitesides, G. M. Macromolecules 1995, 28, 75697571. (29) Edmondson, S.; Huck, W. T. S. AdV. Mater. 2004, 16, 1327-1331. (30) Zhou, F.; Shu, W.; Welland, M. E.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 5326-5327.

10.1021/la0619372 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006

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Figure 1. Schematic diagram of the experimental method, incorporating microcontact printing, polymer brush growth, cross-linking, and etching.

containing both ultrathin polymer and metal films. These objects are of interest from a fundamental point of view, as they demonstrate a new level of control over the composition of components on this scale. The metal components of these hybrid objects are shown to be conductive after folding of the goldpolymer film, in accordance with several recent studies which have demonstrated that ultrathin films of a conductive material can retain their conductivity when supported on a flexible layer which is subject to some bending.31-32 The approach reported here to forming flexible, conductive quasi-2D microobjects, by making use of soft lithographic fabrication methods, allows for ease of fabrication and good control over the dimensions of the polymeric objects and the metallic layers to which they are attached. 2. Experimental Materials and Methods All reagents were purchased from the Aldrich Chemical Co. and used as received unless otherwise stated. Solvents were purchased from Fisher Scientific and were distilled prior to use. Silicon wafers were obtained from Compart Technology Ltd. (100 mm diameter, boron-doped, 〈100〉 orientation), plasma oxidized for 10 min at 100 W in an Emitech Plasma Asher and coated by deposition of chromium (15 nm) followed by gold (200 nm) in a BOC Edwards-Auto 500 thermal evaporation and RF sputter-coater. Gold-coated glass slides were prepared by the same method (5 nm of Cr, 20 nm of Au). Thiol initiator (11-mercaptoundecyl bromoisobutyrate) and trichlorosilane initiator (2-bromo-2-methylpropionic acid 3-(trichlorosilanyl)propyl ester) were synthesized according to published procedures.33,34 Deionized water was obtained using a Milli-Q water purification system. Initiator-coated silicon wafers were fabricated by plasma oxidation of a silicon wafer for 10 min before covering the wafer with a solution of 30 mL of anhydrous toluene, 10 µL of trichlorosilane initiator, and 50 µL of distilled triethylamine in a sealed container for 24 h. The wafers were sonicated in toluene for 1 min after being (31) Gleskova, H.; Cheng, I.-C.; Wagner, S.; Suo, Z. Appl. Phys. Lett. 2006, 88, 011905. (32) Gray, D. S.; Tien, J.; Chen, C. S. AdV. Mater. 2004, 16, 393-397. (33) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 12651269. (34) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424-1431.

Figure 2. (A). Percentage increase in PGMA film thickness with cross-linking time on three different samples, comprising PGMA brush layers of initial film thicknesses 25, 55, and 90 nm on a silicon substrate. Cross-linking was carried out in ethanolic 1,4-phenylenediamine solution at 60 °C. (B) FTIR spectrum of a 90 nm PGMA film (on a silicon substrate), obtained in transmission before crosslinking. (C) FTIR spectrum of the 90 nm PGMA film after crosslinking in 1,4-phenylenediamine (0.5 M) for 120 min at 60 °C.

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Figure 3. Optical micrographs of hydrophobic polymer films showing partial underetching of the gold substrate. (A, B) Square and line objects, where the thickness of the polymer brush is 50 nm and etching was carried out over 2 min. (C, D) Pentagonal and hexagonal objects of thickness 90 nm, where etching was carried out over 5 min. (E, F) Pentagonal objects of thickness 30 nm, with etching times of 4 and 8 min, respectively, showing that thinner films can fold during washing of the sample, though the region of polymer where gold has not been removed from the surface remains tethered to the substrate. In A-E the gold object underneath the polymer clearly reflects the shape of the polymer object used as a resist; in F etching has been allowed to continue until almost all of the gold has been removed. removed from the initiator solution, rinsed with distilled acetone then ethanol, and dried under a stream of nitrogen.35 Gold-coated wafers were patterned with thiol initiator by microcontact printing using a patterned poly(dimethylsiloxane) (PDMS) stamp with features of size 2-150 µm;36 after immersion in an ethanolic solution of the initiator (5 mM) and drying under a stream of nitrogen, the stamp was placed in conformal contact with the wafer for 15 s, after which the sample was immersed in an ethanolic solution of undecanethiol (5 mM) for 30 s. PGMA brushes were grown from the initiator-patterned substrate by immersion of the samples in a CuCl/CuBr2-catalyzed ATRP system in a MeOH/H2O solvent (molar ratio, 100:1:0.05:2.5 glycidyl methacrylate:CuCl:CuBr2:2′,2-dipyridyl), as described previously.35 Samples were then washed thoroughly with dichloromethane, water, and methanol. Cross-linking was carried out by immersion of each sample in an ethanolic solution of 1,4-phenylenediamine (0.5 M) for 30 min at 60 °C. Etching of the gold layer on the wafer was carried out by placing a drop of gold etchant (ratio by mass, 4:1:40 KI: (35) Edmondson, S.; Huck, W. T. S. J. Mater. Chem. 2004, 14, 730-734. (36) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153-184.

I2:H2O) on the patterned wafer for varying times. The drop was then washed off the surface using Milli-Q water. A chromium etchant was made by dissolving cerium ammonium nitrate (20 g) in an aqueous acetic acid solution (100 mL, 0.6 M), and chromium etching was carried out by covering each sample with this etchant for varying times. Ellipsometric measurements were carried out using a DRE ELX02C ellipsometer with a 632.8 nm laser at a 70° angle of incidence. Atomic force microscopy (AFM) images were obtained using a Digital Instruments Nanoscope III, in tapping mode. Optical micrographs were obtained using a Nikon Eclipse ME600L microscope using the DN100 capture system. I-V characteristics were determined using an Agilent 4155C semiconductor parameter analyzer. Fourier transform infrared (FTIR) spectra were recorded using a Bio-Rad FTS 6000 spectrometer in transmission mode.

3. Results and Discussion Fabricating Quasi-2D Objects. This paper describes an efficient procedure for the fabrication of quasi-2D sheets containing ultrathin metal and polymer films. Figure 1 describes the key steps in our approach.

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the polymerization to continue for 2, 4 and 6 h. A cross-linking reaction was then carried out by immersing each of these samples in ethanolic 1,4-phenylenediamine solution (0.5 M) at 60 °C. At intervals, each sample was taken out of the cross-linking solution and rinsed with methanol. Measuring the thickness of the PGMA film on the substrate by ellipsometry at each of these intervals allows us to track the cross-linking reaction with time; these data are summarized in Figure 2A. Over time, the total thickness of the film increases as more cross-linker is incorporated into the film (with the polymer film having to accommodate more material). The thickness increase is substantial, with the 25 nm film thickness increasing by over 100%; however, the percentage increase of the 90 nm film is lower than that of the 25 and 55 nm films, which is possibly indicative of the increased difficulty of penetration of the thicker brush by the cross-linker. Over time, the rate of thickness increase becomes lower, as with increased cross-linking fewer reactive sites are available for reaction with the cross-linker. Fourier transform infrared spectra of unpatterned PGMA films on silicon substrates were recorded in transmission mode (see Figure 2B,C). The peaks at ∼1730 cm-1 are assigned to the ester functionalities in the PGMA. The FTIR spectrum of PGMA contains a peak at ∼908 cm-1 before cross-linking (Figure 2B), which is assigned to the epoxide groups in the polymer side chains.37 After cross-linking for 120 min (see Figure 2C), the spectrum clearly shows that the height of this peak has diminished greatly, indicating that the epoxide rings have been opened by the nucleophilic cross-linker. There are two ways in which crosslinking within this structure can occur: a diamine molecule can link between two epoxide groups, possibly on adjacent polymer chains, or the alkoxide anion generated after epoxide ring-opening Figure 4. (A) AFM micrograph of a 20 µm underetched square. (B) can act as a nucleophile in a ring-opening chain reaction. After cross-linking, we see a broad peak centered at ∼3400 cm-1, Cross-section through this topographical image. This cross-section clearly shows the sharp “step” in the center of the hybrid object, attributed to either hydroxy groups formed by the opening of the which can be compared to the schematic diagrams shown in Figure epoxide ring or to N-H stretches due to the amine cross-linker. 1, containing a gold object on a Si/Cr substrate and a polymer film We also note the appearance of a sharp absorption at 1518 cm-1 “draped” over the gold. in the FTIR spectrum after cross-linking; this could be due to either an N-H bend or a stretching mode associated with the (i) A laterally patterned self-assembled monolayer (SAM) is aromatic ring. produced on a gold-coated silicon wafer by microcontact printing Although the FTIR spectra show only the opening of the (µCP) using an elastomeric stamp, which is patterned with 5-200 epoxide rings by the amine, and not the formation of cross-links, µm features and “inked” with an ethanolic solution of a thiol this may be deduced from the increase in mechanical robustness species containing a bromoisobutyrate functionality on the tail of the films. If one attempts to lift the films from the substrate group. The inked stamp is placed in conformal contact with the straight after polymerization, then the polymer disintegrates; surface for 15 s, followed by backfilling with an inert thiol. however, with the addition of a cross-linking step the films become (ii) The bromoisobutyrate functional group acts as an initiator strong enough to be moved around the surface (see the following for the surface-confined polymerization of glycidyl methacrylate section, and Figures 3, 5, and 6). Discussion of PGMA cross(GMA) by atom-transfer radical polymerization (ATRP) aclinking using hydroxide29 and octylamine35 has been published cording to procedures previously described, giving rise to a previously. polymer brush with attached epoxide groups which can be used The unpatterned PGMA films also show a change in the in a cross-linking step.35 advancing contact angle of the surface after cross-linking; from (iii) Cross-linking of the polymer brushes using ethanolic 1,4∼70° on the original PGMA film to ∼90° after 120 min crossphenylenediamine leads to hydrophobic films. linking in 1,4-phenylenediamine. Considering that films cross(iv) The cross-linked brushes provide an effective etch-resist linked in methanolic NaOH show decreases in contact angle to against aqueous KI/I2 gold etchant, and after prolonged etching ∼60°, these results indicate that postfunctionalization of the times an undercut develops in the gold underneath the polymer, polymer film produces a significant change in the hydrophobicity allowing for fine-tuning of the eventual hybrid quasi-2D sheets. of the surface. (v) Finally, a chromium etchant (cerium ammonium nitrate Etch Resist Properties. The incorporation of a hydrophobic dissolved in aqueous acetic acid) is used to lift the features off cross-linker into the film leads to changes in the etch-resist the surface, allowing their study in suspension. properties of the cross-linked films. A drop of aqueous gold Cross-linking Reaction. To characterize the effect of the crossetchant (KI:I2:H2O, 4:1:40) was pipetted onto the surface of each linker on the polymer brush, PGMA brushes were grown on sample and left for varying periods of time. The substrates were unpatterned silicon wafers coated with a trichlorosilane initiator. Samples of polymer films at different thicknesses (25, 55, and (37) Bower, D. I.; Maddams, W. F. The Vibrational Spectroscopy of Polymers; 90 nm as measured by ellipsometry) were created by allowing Cambridge University Press: Cambridge, U.K., 1989.

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Figure 5. Optical micrographs of gold-polymer hybrid objects. (A, B) Pentagon and line shaped objects after washing of the sample has caused movement across the silicon wafer. These objects are robust enough to withstand being moved out of the original pattern. (C) A gold-polymer square after transferral to a glass slide. (D) A gold-polymer hexagon in suspension (viewed in transmission mode).

then rinsed in Milli-Q H2O. Brushes cross-linked in methanolic NaOH solution (2 M) allowed the aqueous etchant to be transported through the film, and all of the gold substrate was removed from behind the polymer objects in less than 2 min (these results are consistent with the observation made by Zhou et al.38 that hydrophilic polymer brushes allow the transport of etchant through the material). However, polymer objects crosslinked in, e.g., 1,4-phenylenediamine are more etch-resistant and etching first removes gold from that part of the substrate not covered by the polymer brush (see Figure 1). Continued etching then leads to slow removal of the metal from underneath the edges of each polymer film until all of the gold is removed. This process is sufficiently slow that, even for polymer films with lateral dimensions as small as 2 µm, the etching can be stopped before all of the gold has been removed from underneath the polymer. This opens up interesting possibilities for the creation of novel gold-polymer “hybrid objects”. Polymer brushes provide the most general method possible for the creation of these objects; lateral patterning of the organic layer is achieved easily and inexpensively with soft lithography (as opposed to conventional photoresists which require more sophisticated patterning techniques). The thickness of the polymer brush layer is also easily tunable by varying the length of time for which the polymerization is allowed to occur. The use of polymer brushes is further advantageous in that the properties of the system may be tuned, either by incorporating different functional monomers or by postfunctionalization of the brushes after polymerization (in this case, by ring-opening of the epoxide group using a base in a protic solvent). Examples of such gold-polymer hybrids can be seen in Figure 3. These images demonstrate objects that may be created from differently patterned PDMS stamps during the µCP stage of the fabrication process. Parts A and B of Figure 3 show patterns consisting of square and line-shaped polymer brushes, of width 6 µm and thickness ∼50 nm; gold square and line objects remain underneath this polymer pattern. Parts C and (38) Zhou, F.; Liu, W.; Hao, J.; Xu, T.; Chen, M.; Xue, Q. AdV. Funct. Mater. 2003, 13, 938-942.

D of Figure 3 show larger pentagon and hexagon patterns (widths, 14 and 150 µm, respectively) of polymer brushes, at a thickness of ∼90 nm. Interestingly, some buckling of the polymer material around the edge of the gold object is seen in Figure 3D and appears to be typical of larger pattern features; we hypothesize that this is due to expansion of the polymer after release from the surface, followed by draping back down to the Si wafer.39 Parts E and F of Figure 3 show pentagonal patterns of polymer brushes with a thickness of ∼30 nm; the center of each polymer film is still attached to the central gold object, but the edges of each polymer film have been released from the surface and redeposited. The films retained their shape integrity, and redeposition led to folding of the released polymer. Figure 3E suggests that it might be possible to exploit the folding of the 2D polymer sheets in the design of surface structures that can “wrap” around surface-immobilized objects. That the functional groups present in the polymer remain so after polymerization is confirmed by FTIR; the FTIR spectrum of a 90 nm thick GMA film shows the same peaks after 5 min of immersion in a solution of the etchant, with little change in the height of the peaks (see Supporting Information for these FTIR spectra). This conclusion is also supported by a recent study in which gold nanoparticles were etched using an iodine/potassium iodide system from within an organic matrix.40 This is therefore a convenient system for the etching of gold, as the etching conditions are mild enough to be tolerant of the functional groups on the organic material. Figure 4 shows an AFM image of a 20 µm square hybrid object after partial underetching. AFM analysis of the underetched features clearly show the “step” under the polymer object where there is gold underneath the polymer. The cross-section of this AFM scan shows that around the edge of the object, where gold has been removed, the thickness of the polymer is 182 nm, whereas the height of the “step” in the center of the object above the (39) Edmondson, S.; Frieda, K.; Comrie, J. E.; Onck. P. R.; Huck, W. T. S. AdV. Mater. 2006, 18, 724-728. (40) Koenig, S.; Chechik, V. Chem. Commun. (Cambridge) 2005, 41104112.

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Figure 6. Optical micrographs showing various arrays of gold objects created within polymer films. These are hypothetically due to a thin layer of material physisorbed in the spaces between polymer objects during brush growth. Micrographs A-E were recorded in reflection mode; micrograph (F) was recorded in transmission mode after transferral of the film to a glass slide. (A-D) Square, dot, and line objects of thickness ∼80-90 nm incorporated into a film of thickness ∼10 nm. (E, F) Square objects of thickness ∼120 nm incorporated into a film of thickness ∼15 nm.

substrate is 176 nm. Since the original thickness of gold deposited on the wafer was 200 nm, we conclude that even after crosslinking in 1,4-phenylenediamine the polymer film is not entirely impermeable to the etchant solution as the gold object left under the film is thinner than the initial gold film deposited on the wafer. However, the lateral underetching rate is ∼100 times faster than the etching rate in the direction normal to the plane of the silicon wafer. Images of polymer objects on a gold-coated glass substrate being underetched were captured at numerous points during the etching procedure (see Supporting Information). In this manner it is possible to observe the gold being removed from underneath the edges of the gold objects over time. The underetching process proceeds at a lateral rate of the order of 1 µm/min, giving sufficient control to produce well-defined metallic polymer objects, although the etch front develops curvatures after prolonged etching. Lift-off of Gold-Polymer Hybrids from the Substrate. Gold-polymer objects created by our fabrication process can be lifted from the surface by etching the chromium layer away from underneath the gold. Lift-off can be achieved by pipetting a

small quantity of chromium etchant (a solution of cerium ammonium nitrate in aqueous acetic acid) onto the surface for a period of ∼20 s followed by washing the surface with Milli-Q H2O. After the etchant is washed away, several gold-polymer objects are seen to have moved out of the pattern and across the surface (see Figure 5A,B). If, instead of being washed away, the etchant is washed onto a glass slide, hybrid objects can be transferred onto the slide (Figure 5C) or viewed in suspension (Figure 5D). It is clear from these images that the hybrid goldpolymer objects are mechanically robust. The growth of thick polymer brushes (>100 nm) is accompanied by small amounts of polymer formed in bulk, presumably due to chain-transfer reactions. Some of this physisorbs onto the surface, and during the cross-linking step the adsorbed polymer will be incorporated into a continuous polymer sheet (although the physisorbed layer may be removed before cross-linking by a short plasma etch). The physisorbed regions of this film are considerably thinner than the patterned polymer brush objects (of the order of 10 nm when the polymer brushes are 90 nm thick; see Supporting Information for a typical AFM

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Figure 7. (A, B) Optical micrographs, showing a sample incorporating gold line-shaped objects underneath a cross-linked PGMA film, after a measurement of their I-V characteristics by touching electrical probes to the gold objects on the substrate and applying a voltage between the probes. After the measurement, the number of gold line-shaped objects touched by either probe can be estimated from the damage done to the gold objects by the probe, shown by the circled areas on the micrographs. In experiment A the probes were positioned on a “flat” section of the gold-polymer objects, whereas in experiment B current must flow around a bend in the film. The micrographs indicate that in each case only one to two of the gold lines touch both of the areas damaged by the probes (the circled areas on the micrographs). Black lines have been added as a guide to the eye, showing the direction of the gold wires; gold wires which are inside the two black lines on each micrograph are touching at least one of the probes. The measured I-V characteristics are also shown, as well as a 3D AFM image illustrating the curvature of the gold line-shaped objects around the bend in the film (inset).

image), so that when gold etchant is applied to the surface, the etch will penetrate this ultrathin film and remove gold from the areas of the sample not covered by the patterned polymer brushes. The thin regions are sufficiently strong to maintain their integrity during the lift-off process, leading to continuous sheets with micropatterned arrays of gold features. Examples of such embedded gold arrays in polymer films are shown in Figure 6. Figure 6A shows a sample of square-patterned polymer brushes; the stamp used for this pattern consists of 9 µm wide squares separated by 1 µm. As can be seen, part of the continuous film produced has remained on the substrate during the lift-off procedure, but part has lifted from the substrate and folded onto the section remaining on the surface. Parts B and C of Figure 6 show similar results for 4 µm dot patterns. Note that in Figure 6A most of the film is taken up by the 90 nm thick brushes, so that there is a distinct difference in the polymer thin film color in the region where two polymer layers are present as opposed to the region where only one layer is present; this is in contrast to the smaller features (with a larger spacing) in Figure 6B,C, where most of the polymer film consists of the thin, “physisorbed” layer, so that the change in thin film color is not so easily visible. Figure 6D depicts a series of polymer lines which have lifted as a continuous sheet; the gold “wires” which have remained under the polymer after etching are of width ∼1.5 µm. Figure 6E depicts larger polymer brush features, of width 20 µm and thickness ∼90 nm. Figure 6F shows that these films containing isolated gold areas can easily be transferred to another surface by being floated off the original Si wafer onto a glass slide.

To test the electrical conductivity of the gold objects embedded in the polymer, electrical probes were positioned on the line objects shown in Figure 6D such that current would flow along the lines. The number of lines across which the probes were positioned could be estimated after the measurement, from the area of the sample which had been damaged (see Figure 7A,B). In experiment A the probes were positioned on a “flat” section of the gold-polymer objects, whereas in experiment B current must flow around a bend in the film. The I-V characteristics shown in Figure 7A,B give resistance values (R) of 43.1 and 147.1 Ω, respectively. From the micrographs we can determine the approximate distance (L) along the gold wires between the probes (226 and 550 µm, respectively), and from AFM scans of the objects we estimate the area of the gold cross-section for each line to be around 3 × 10-13 m2. From the optical micrographs in Figure 7 it appears that only one to two of the gold lines touch both probes in either case; multiplying the estimated cross-sectional area of each line by this therefore gives a total cross-sectional area (A) of 3 - 6 × 10-13 m2. Therefore, calculating a value of resistivity (F) from the standard formula F ) RA/l gives a value between 5 × 10-8 and 13 × 10-8 Ωm. This value is of the same order of magnitude as the expected value for the resistivity of bulk gold, i.e., 1.67 × 10-8 Ω m,41 indicating that lifting the gold objects underneath the polymer from the substrate has not significantly damaged them. These measurements demonstrate that the gold strips are fairly intact (41) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; ButterworthHeinemann: Oxford, U.K., 1997.

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and lifting them from the surface does not induce fractures such that conductivity is lost, even when the line objects are subjected to bending. An AFM image of the bent lines (see Figure 7, inset), indicates that, at the point where the gold lines fold over, the radius of curvature of the hybrid objects is ∼1.5 µm, i.e., significantly larger than their thickness.

4. Conclusions We have demonstrated that the physical properties of a polymer brush film containing cross-linkable functional groups can vary substantially with the reagent used for cross-linking the film. This can have striking consequences for the use of such films as etch resists; a sufficiently hydrophobic cross-linker can render the film less permeable to aqueous etchants. It was demonstrated that a patterned polymer film on a gold-coated silicon wafer will behave as an etch resist toward an aqueous iodine/KI solution and that once the gold has been stripped away from the areas between polymer objects on such a surface, etching will occur from underneath the edges of each polymer object. This process can be terminated before all of the metal has been removed from the substrate, yielding interesting gold-polymer hybrid objects

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where the thickness and lateral dimensions of both the gold and the polymer components are easily controllable. When physisorbed material deposited on the substrate is allowed to remain between the polymer objects after cross-linking, the material can be lifted from the surface as a continuous film containing embedded gold objects in a regular array. Acknowledgment. The authors thank Steve Edmondson for useful discussions and advice; Jason Pinto and Saghar Khodabaksh for measurements using the Agilent 4155C semiconductor parameter analyzer; Andrew Brown, Vicky Osborne, and Greg Whiting for initiator synthesis; and Piers Andrew for PDMS stamps. Financial support from the EPSRC is gratefully acknowledged. Supporting Information Available: Supporting optical micrographs of underetched hydrophilic/hydrophobic objects, optical micrographs showing how etching proceeds with time, and further AFM images. This material is available free of charge via the internet at http://pubs.acs.org. LA0619372