Non-lithographic organization of nickel catalyst for carbon ... - CiteSeerX

IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 18 (2007) 335306 (7pp)

doi:10.1088/0957-4484/18/33/335306

Non-lithographic organization of nickel catalyst for carbon nanofiber synthesis on laser-induced periodic surface structures Y F Guan1 , A V Melechko2 , A J Pedraza1 , M L Simpson1,2 and P D Rack1,3 1

Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA 2 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA E-mail: [email protected]

Received 24 May 2007, in final form 26 June 2007 Published 25 July 2007 Online at stacks.iop.org/Nano/18/335306 Abstract We present a non-lithographic technique that produces organized nanoscale nickel catalyst for carbon nanofiber growth on a silicon substrate. This technique involves three consecutive steps: first, the substrate is laser-irradiated to produce a periodic nanorippled structure; second, a thin film of nickel is deposited using glancing-angle ion-beam sputter deposition, followed by heat treatment, and third, a catalytic dc plasma-enhanced chemical vapor deposition (PECVD) process is conducted to produce the vertically aligned carbon nanofibers (VACNF). The nickel catalyst is distributed along the laser-induced periodic surface structure (LIPSS) and the Ni particle dimension varies as a function of the location on the LIPSS and is correlated to the nanoripple dimensions. The glancing angle, the distance between the ion beam collimators and the total deposition time all play important roles in determining the final catalyst size and subsequent carbon nanofiber properties. Due to the gradual aspect ratio change of the nanoripples across the sample, Ni catalyst nanoparticles of different dimensions were obtained. After the prescribed three minute PECVD growth, it was observed that, in order for the carbon nanofibers to survive, the nickel catalyst dimension should be larger than a critical value of ∼19 nm, below which the Ni is insufficient to sustain carbon nanofiber growth. (Some figures in this article are in colour only in the electronic version)

standard photolithographic techniques, several recent advances have been reported to tackle the challenges of developing architectures for biological, electronic, photonic and sensor applications. The dip-pen lithography has been developed that allows writing organic structures on substrates [5]. Also, Si/SiOx and GaAs have been patterned with organic structures using this technique [6]. Laser-assisted growth of semiconductor nanowires [7] has been achieved using a metallic catalyst that also serves to control the wire diameter [8, 9]. Electron-beam-induced processing has also been explored as a direct write deposition and etching process: however, it is rather slow due to the serial nature of the process [10].

1. Introduction Lithographic techniques can be used to produce nanostructures and have the advantage that lithography is a very mature technology [1]. Advanced lithography tools, however, face important challenges such as cost of ownership and complicated processing [1]. Current limits of conventional lithography are at sub-50 nm, and it is envisioned that to push these limits further will require the use of electron beams [2], x-rays [3], extreme UV light [4] or advanced optical techniques such as immersion lithography. Aside from 3 Author to whom any correspondence should be addressed.

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Nanotechnology 18 (2007) 335306

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Direct laser interference lithography has been used to induce localized melting/resolidification and precipitation in thin films of pulse-laser-deposited FeCr [11] and sputterdeposited CoC [12], respectively. A significant advance was achieved using 157 nm UV light lensless interference lithography at a large angle of incidence (60◦ ). This technique allowed patterning 45 nm wide well-defined lines with a period of 91 nm using a photoresist optimized for this wavelength [13]. Soft lithography methods, such as nanoimprint lithography, have also been actively pursued [14–19]. Essentially, a polymeric elastic stamp is inked with a solution containing alkanethiols and pressed against a gold substrate where the ink is transferred. The pattern built in the stamp is printed on the substrate with features as small as 50 nm [15, 16]. An alternative bottom-up approach to align nanoparticles into organized patterns is based on the generation of appropriate templates. One fast and convenient way to fabricate such templates is using pulsed excimer UV lasers, which have wavelengths ranging from 153–355 nm. In particular, periodic ripple structures with spacing equal to or larger than half the photon wavelength can be generated by irradiating a specimen with a few hundred nanosecond laser pulses [20]. Furthermore, we have employed a laser interference system using a Lloyd’s mirror configuration by putting a mirror and the substrate together orthogonally. Unlike conventional beamsplitter systems, which require very complicated and delicate optical settings, the Lloyd’s mirror system uses two laser beams incident at the same angle but in diametrically opposite directions. This promotes the formation of a grating nanostructure that can be centimeters long, covering most of the irradiated area [21]. The periodic structures produced under this two-beam illumination system are not only long but also remarkably straight. The polarized light is not required to form the gratings and the spacing between the periodic structures is controlled by the angle of incidence of the laser beam. As a universal phenomenon, nanostructured templates can be generated through this simple and controllable technique, and the templates can be used to generate aligned nanostructures, thus bypassing the need for conventional lithography. In this paper, we report the investigation of the production of nickel nanoparticles on silicon nanostructured substrates that are subsequently used as a catalyst for carbon nanofiber growth. The procedure involves three steps: (1) the development of a nanostructured template using laser irradiation, (2) film deposition using glancingangle ion beam sputtering followed by a heat treatment, and (3) catalytic PECVD growth, which leads to the formation of an organized array of vertically aligned carbon nanofibers over an extended area. In our previous papers [21, 22], the technical details on how to fabricate the template and the physical mechanisms involved in the process are discussed. This approach takes advantage of the two salient features of the silicon template that is generated using laser irradiation and a Lloyd’s mirror configuration: its regularity and extended coverage. In addition, it is compatible with a wide range of nanoparticle/substrate combinations. Periodic arrays of carbon nanostructures covering a large area (5 mm × 5 mm) have many applications including gene delivery arrays [23, 24],

Figure 1. (a) Schematic drawing of ion-beam deposition configuration. The collimator, which was made of two long, narrow pieces of smooth silicon, largely prevented the passage of nickel atoms away from the vertical direction. (b) Schematic drawing shows the location of the four different regions, where LIPSS of different dimensions were found. Note: nanoprotrusions were always located at region IV, and the height of nanoripples decreases gradually from region III to region I, where the shallowest ripples were found.

and active superhydrophobic surfaces [25]. Larger areas are possible with higher power lasers and extended areas can be accomplished by rastering the beam and/or substrate.

2. Experimental details Silicon nanostructured templates, containing nanoripples and/or nanoprotrusions, were produced by irradiating (001) substrates using a Lambda Physik LPX-305i, KrF excimer laser (248 nm wavelength), with pulse duration (FWHM) of 25 ns. The beam emerging from the laser cavity was directed to an optical bench using a MgF2 -coated fused silica mirror. The optical bench contained an aperture, a fused-silica lens array and the irradiation chamber. The rectangular aperture partially removed the low energy tails of the trapezoidal laser beam. The laser fluence was varied by adjusting the position of the fusedsilica lens array relative to the sample holder in the irradiation chamber and/or changing the laser operating voltage. The incident power employed was approximately 35 MW cm−2 per pulse. Equal contributions of the p- and s-components were measured from the laser pulses indicating that the light was not linearly polarized. The native silicon oxide layer was not removed prior to the experiments. The Lloyd’s mirror arrangement used to produce the surface structures consists of a sample holder with an attached mirror so that mirror and substrate are at a right angle to each other [21]. The template fabrication was done in air. No surface preparation was done prior to either nanostructuring or Ni deposition. Nickel films were ion-beam sputter-deposited onto the irradiated substrate areas from a 99.99% pure nickel target, using 8 kV Ar+ ions and an Ar pressure of 5 × 10−5 Torr. Prior to introducing the argon gas, the deposition chamber was evacuated to a base pressure of 1 × 10−7 Torr. The ion beam sputter deposition module uses a Paulus and Reverchon type ion gun [26]. In one set of experiments the beam of Ni atoms emerging from the target was collimated as indicated in figure 1(a). The film thickness was measured in films deposited for 20 and 30 min at normal incidence, in each case scanning several sharp film/substrate edges using an AFM. These measurements consistently yielded a ∼2 nm min−1 film deposition rate. However, during the glancing-angle deposition, the substrates were positioned in such a way that 2


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Figure 2. AFM images of the nanoripples and nanoprotrusions with different dimensions found at different regions, and the insets indicate the heights of the structures. (a) Ripples in region I have the shallowest feature of ∼8 nm; (b) the height of ripples found in region II is 15.5 nm; (c) tall ripples of ∼23 nm were located close to the edge of the sample; (d) nanoprotrusions of close to 80 nm were found exclusively at region IV, where the sample bottom is.

Figure 3. AFM images showing different regions in the laser-irradiated template. (a) 2D AFM image showing the straight nanoripples, which can be found throughout regions I–III (far away from the sample edge); (b) 3D AFM image of the same location; (c) and (d) are the 2D and 3D AFM images of the nanoprotrusions found at the edge/bottom of the sample (region IV).

so the laser energy density is inhomogeneous throughout the laser-irradiated area. This is why LIPSS with different aspect ratios were formed as demonstrated below. However, by using more coherent lasers and applying different laser interference configurations, more uniform nanostructures can be achieved [27]. Nevertheless, it will be demonstrated that the inhomogeneous energy density distribution is useful in creating a graded nanorippled structure valuable for determining the catalyst size threshold for carbon fiber growth for the prescribed 3 min growth time. In our study, specimens with ripple spacing in the range of 300–600 nm and height between 8–22 nm were fabricated using the Lloyd’s mirror configuration. Additionally, nanoprotrusions were found near the edge (bottom) of the sample, and their height is ∼80 nm with diameters ranging typically from 50–100 nm. Figure 1(b) is the schematic picture showing the location of the four regions, where LIPSS of different dimensions were found. Nanoprotrusions were always located at region IV, and the height of nanoripples increases gradually from region I to region III, where the tallest ripples were found. Figure 2 shows AFM images of the nanoripples and nanoprotrusions with different dimensions, representative of different regions. The insets are line profiles of the nanoripples which indicate the height of each structure. Figure 2(a) illustrates that ripples in region I have the shallowest amplitude of ∼8 nm; figure 2(b) shows that the height of ripples in region II is 15.5 nm; figure 2(c) shows the tallest ripples of ∼23 nm were located closest to the edge of the sample; and finally in figure 4(d) nanoprotrusions ∼80 nm tall were found exclusively at region IV, near the sample bottom. Figure 3 includes 2D and 3D AFM images of the typical periodic ripple structure and protrusions used as template; the

their surfaces are parallel to the incoming ion beam. Due to the shallow periodic gratings on the substrate, the Ni can be selectively deposited onto those features at a much slower deposition rate than normal incidence. The angle between the substrate surface and the incident ion beam was varied from 0◦ to 3◦ and the distance between the collimator and the substrate varied in the range from 1–3 mm. Thermal annealing treatments following Ni film deposition were performed by heating the substrate in ammonia (5 Torr, 180 sccm) to 700 ◦ C for 5 min in a PECVD chamber. Normally, an ammonia plasma is used to provide an environment necessary for reducing the nickel oxide film that forms due to exposure to air and hinders the dewetting process and nanoparticle formation. However, because the nickel films were so thin, we just annealed the nickel films in ammonia and found that the nanoparticles were formed. After formation of catalyst nanoparticles the vertically aligned carbon nanofibers were grown by the catalytic PECVD process. Acetylene and ammonia gas mixture was introduced at 117 and 185 sccm, respectively, and dc plasma was started at 5 Torr total pressure and under 600 mA constant current mode. The nanofiber growth was stopped after 3 min. Characterization of the samples before and after growth of carbon nanofibers was evaluated ex situ after each of the processing stages using a Hitachi 4700 scanning electron microscope (SEM) and a Digital Instruments 3100 atomic force microscope (AFM).

3. Results and discussion The laser used in this study has a very narrow coherence and the laser beam profile follows a Gaussian distribution, 3


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Table 1. LIPSS feature sizes and the dimensions of the Ni catalysts located at different regions on the substrate (glancing angle during deposition: 3◦ ; collimator–substrate distance: 3 mm). The standard deviation (SD) data are also included. Average Average catalyst Average catalyst LIPSS height diameter in the valley diameter at the crest Region (nm) (nm)/SD (nm) (nm)/SD (nm) I II III IV

8 15.5 23 80

N/A 7/0.8 9/0.9 20/1.2

10/0.9 13/1.0 19/1.2 40/1.5

of ∼19 nm in diameter were found along the ripple crest, while very small particles (∼9 nm) were distributed in the ripple valley. Figure 4(d) shows that nanoprotrusions were found exclusively in region IV and the largest catalyst particles (∼40 nm) were observed among the four regions. Additionally, smaller particles ∼20 nm were found between the protrusion lines. Table 1 summarizes all the LIPSS feature sizes and the dimensions of the Ni catalysts located at different regions. The arrows in the inset point to the silicon nanoprotrusions and the nickel catalysts, respectively. Based on the location of the catalyst and the Ni ‘shadow’ that extends from the protrusion, we can tell that the ion source was to the right of the sample as oriented in the SEM image. PECVD synthesis of carbon nanofibers is a complex process that requires careful selection of parameters [28, 29]. Specifically, the ratio of ammonia to acetylene and plasma power is chosen so that carbon is supplied to the catalyst while carbon film formation on the surface is prevented. In addition to the processing parameters, this balance depends on the substrate type, conditioning of the reactor and even catalyst particle density [30]. Another complication is that catalyst size reduces during synthesis [31]. This effect essentially creates a limit on the nanofiber length for a given size of the initial nanoparticle. In simple terms, if there is a distribution of catalyst size as in our experiments described here, the nanofibers catalyzed by the smallest nanoparticles start to grow, then when the nickel particle is exhausted, the fiber begins etching away, which, depending on the deposition time, leaves only fibers with a critical nickel particle size. Thus optimum conditions (including growth time) can be obtained for some area of the sample with large variation of nanoparticle size and density. The same sample shown in figure 4 was then used to grow carbon fibers under the optimum PECVD conditions mentioned above. Figure 5 shows the SEM images showing the same sample after the growth. Figure 5(a) shows region I after carbon nanofiber deposition. The carbon nanofibers were actually grown and have begun to be etched away because of the smaller amount of Ni catalyst. Carbon nanofibers in region II are shown in figure 5(b), which have relatively short and fat conical fibers. The diameter of the Ni catalyst found in this area (region II) is ∼15 nm and grew longer and appeared to just start experiencing some etching. Figure 5(c) are images of fibers in region III, where carbon fibers with much higher aspect ratios were obtained along the ripples as a result of larger Ni catalyst particles. Although tiny Ni particles were located between the ripples according to figure 4(c), the size is less than the critical catalyst size (19 nm), below

Figure 4. SEM images showing the distribution of nickel clusters after heat treatment at different regions on the laser-irradiated area. The glancing angle is ∼3◦ and the distance between the collimators is 3 mm. The total deposition time is 18 min. The insets illustrate the same region with higher magnifications. (a) Region I: shallowest ripples collect the smallest amount of nickel; (b) region II: larger catalysts were observed along the taller ripples; (c) region III: Ni catalysts of ∼19 nm were found along the ripples, while much thinner ones (∼5–10 nm) were distributed between the ripples due to the deposition scattering and non-zero glancing angle; (d) region IV: nanoprotrusions were found exclusively in this region and the largest catalyst (∼40 nm) was observed among the four regions. The arrows in the inset point to the silicon nanoprotrusions and the nickel catalysts, respectively.

line spacing between both the ripples and protrusions is given by the formula = λ/ (1.0762 − sin2 θ ), where λ is the laser wavelength (248 nm) and θi (normally, ranging from 30◦ to 65◦ ) is the angle of incidence of the laser light. In most cases, θi was determined to be 65◦ for this experiment, which yielded the 428 nm ripple/protrusion spacing. Nickel catalysts of different dimensions were obtained under the same deposition conditions in the various regions after the heat treatment, due to the different aspect ratios of the LIPSS found at different locations throughout the laserirradiated area. Figure 4 shows SEM images of the nickel cluster distributions after the heat treatment at different regions on the laser-irradiated area. The glancing angle during the ion-beam deposition was ∼3◦ , and the distance between the collimator and the substrate was 3 mm. The three insets in figure 4 illustrate the same region at higher magnifications. In order to systematically study the relationship between the Ni catalyst dimension and the carbon fibers’ properties, we measured the average particle size along the ripples/protrusions crest and valley, which deposits because of scattering during the deposition and the non-zero glancing angle. We first divided the LIPSS regions into several 100 nm wide strips, with the longitude parallel to the LIPSS orientation. In each strip, we randomly chose 50 nanoparticles and manually measured their dimensions and calculated the average particle size. The standard deviation for the measurement is ∼1 nm. Figure 4(a) shows that, at region I, the shallowest ripples collected the smallest amount of nickel and yielded particles ∼10 nm on the ripple crest, and very little was collected in the ripple valley; and at region II (figure 4(b)), larger catalysts were observed along the taller ripples. At the ripple crest, ∼13 nm particles were observed, and particles of only ∼7 nm in diameter were observed in the ripple valley. In figure 4(c), Ni catalysts 4


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Figure 5. SEM images showing the same sample as in figure 4 after carbon fiber growth through the PECVD process for 5 min. (a) Region I: the columns along the ripples are not fully developed, due to the insufficient amount of catalyst; (b) region II: taller carbon columns were observed along the ripples; (c) region III: carbon fibers with much higher aspect ratios were obtained along the ripples as a result of larger Ni catalyst; (d) region IV: well-developed VACNFs were observed along the protrusions, as well as shorter ones found in between, which may be due to the catalysts scattering during the deposition.

which fibers are etched away. Finally, figure 5(d) shows the only region—region IV, where well-developed VACNFs were observed along the protrusions (tall fibers). In addition, some shorter fibers were found between the protrusions which are from small catalyst particles observed in figure 4(d) which are above the catalyst particle size threshold for carbon nanofiber growth. In the first sample, the glancing angle during the deposition is ∼3◦ and the collimator to substrate distance is large enough (∼3 mm), so scattering of the catalyst atoms during deposition was observed. According to our previous work [32], by decreasing the collimator–substrate distance, one can significantly reduce the amount of films collected during deposition to undetectable levels even for depositions that generate a significant film thickness at the ripple ridge. Similarly, the amount of deposition can be controlled by changing the glancing angle [32]. In order to confine the catalyst distribution to one side of the LIPSS edge, a 0◦ glancing angle and a much shorter collimator–substrate distance of 1 mm were used on a second sample, which drastically decreased the amount of Ni deposited in the ripple valleys. Figure 6 shows the SEM images taken at different regions on the second sample before and after fiber growth. The nickel catalyst’s deposition time was 20 min. Figures 6(a)– (d) were taken at region I to region IV, respectively. Single lines of nickel catalyst with a diameter of 10 nm was observed in region I as an indication of the very small amount of Ni deposition as well as narrow Ni distribution (figure 6(a)). In region II, as the height of the ripples increases, more catalyst was deposited, resulting in larger particles (diameter: ∼13 nm). Thus multiple particle lines were formed along the ripples (figure 6(b)). In figure 6(c), even larger Ni particles (diameter: ∼15 nm) and more lines were observed due to the larger feature size of the ripples. Unlike the sample shown in figure 5(d), where Ni catalysts were found almost everywhere,

Figure 6. SEM images showing the samples before and after fiber growth. The nickel catalysts were deposited for 20 min at a zero glancing angle, and the distance between the collimators is 1 mm, which can drastically decrease the amount of scattering during deposition. (a) Region I: single line of nickel catalyst was achieved; (b) region II: as the height of ripples increases, more catalyst was collected, thus the multiple lines along the ripples; (c) region III: even larger Ni particles and more lines were observed due to the larger feature size; (d) region IV: Ni catalysts were only observed along the protrusions as a result of drastically decreased scattering during the deposition; (e) carbon fibers were observed only along the silicon nanoprotrusions due to the catalyst size control through decreased scattering.

in figure 6(d) the Ni catalysts (diameter: ∼45 nm) were only observed along the protrusions as a result of drastically decreased Ni collection during the deposition. Because of the new deposition configuration, the catalysts collected along the LIPSS decreased drastically. After heat treatment, the size of most of the clustered Ni particles is less than the 19 nm critical threshold for the three minute carbon nanofiber growth, which explains why the growth of carbon fibers was observed only along the nanoprotrusions, where the deposited catalyst size (45 nm) is larger than the critical value (figure 6(e)). More detailed analysis of the VACNFs is presented in figure 7. The SEM image in figure 7(a) was acquired from region III of the same sample as shown in figure 5 at a larger (45◦ ) viewing angle to better display the actual aspect ratio (15 nm tip diameter to 989 nm height for a nanofiber at the center). Out of approximately 60 nanofibers in the image 12 retained the catalyst nanoparticle as indicated in the inset. The conical shape of the nanofibers has been studied [33, 34] before and can be a result of sidewall carbon or silicon nitride [35, 36] deposition depending on the C2 H2 /NH3 ratio, substrate and catalyst density. In a very narrow operational regime it is possible to obtain cylindrical nanofibers [30]. The absence of nitrogen peak in the EDX spectra (figure 7(b)) obtained at 5


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Figure 7. (a) SEM image taken at region III of the same sample as shown in figure 5. The viewing angle was increased to 45◦ to better display the actual aspect ratio (15 nm tip diameter to 989 nm height for a nanofiber at the center). Out of approximately 60 nanofibers in the image 12 retained the catalyst nanoparticle as indicated in the inset; (b) EDX spectra obtained at points A and B indicated by arrows (corresponding to a cylindrical region right under the nanoparticle and in the middle of the conical part). Note: the absence of a nitrogen peak leads us to the conclusion that in this particular case the sidewall deposition material is carbon. For silicon nitride-coated fibers the nitrogen peak exceeds carbon’s significantly. Figure 8. A series of SEM images taken at the same location but at different magnifications. The images illustrate that the technique introduced in this paper can produce a large amount of vertically aligned carbon nanofibers in an extended area in a matter of seconds. The regions with carbon fibers are widely spread and the LIPSS are very regular throughout the whole region.

points A and B (corresponding to a cylindrical region right under the nanoparticle and in the middle of the conical part) indicated by arrows lead us to the conclusion that in this particular case the sidewall deposition material is carbon as discussed in [33]. For silicon nitride-coated fibers the nitrogen peak exceeds carbon’s significantly. This non-lithographic technique can produce nanoscale elements organized on the nanoscale over an extended area, thus bridging the nano- to macroscale gap necessary for many multiscale applications. Figure 8 shows a series of SEM images taken at the same location but under different magnifications, to illustrate the extent and organization of the process.

further synthesis of vertically aligned carbon nanofiber arrays has been demonstrated. This technique involves three successive steps: first, treatment of the silicon substrate by laser irradiation under a Lloyd’s mirror configuration to form LIPSS, which includes nanoripples and nanoprotrusions; second, sputter a nickel thin-film catalyst onto the LIPSS by glancing angle deposition, followed by a heat treatment to cluster the thin film into nanoparticles; and third, deposit vertically aligned carbon nanofibers using a PECVD process. Nickel thin films were selectively deposited by ionbeam glancing-angle deposition. Therefore, deposition took place preferentially at the undulated elevations. Furthermore, because of the different aspect ratios of the nanostructures found in the template, thin film catalysts of different

4. Summary and conclusion A non-lithographic technique that produces organized nickel catalyst particles on a nanostructured silicon substrate for 6


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thicknesses were achieved in different regions of the substrate. In this manner, nickel nanoparticles with varied dimensions were obtained after the subsequent thermal treatment, which involves clustering of the Ni thin films. Other than the different template feature size, the angle of incidence, configuration of the collimator and the total deposition time are all important parameters for determining the catalyst dimensions. In this work, the optimum catalyst dimension for the three minute carbon fiber growth is 19 nm. As a bottom-up process, this technique utilizes tools that are already available in the electronics industry, making it appealing to implement nanotechnologies in fabrication procedures. In principle, the size of the nanoparticles produced by this method can be controlled quite easily. Due to the general mechanism on which the method is based, this effort has the potential to impact a variety of applications because a wide range of substrate–nanoparticle combinations are possible using this method. Production of sensors, lasing lines, wave guides and large surface area electrodes may be enabled using this technique.

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Acknowledgments YFG and AJP acknowledge funding by the National Science Foundation (NSF) DMI-NER-0403444. PDR, AVM and MLS acknowledge support from the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, US Department of Energy.

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