Pulsed Radiofrequency Glow Discharge Time-of-Flight Mass

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Pulsed Radiofrequency Glow Discharge Time-of-Flight Mass Spectrometry for Nanostructured Materials Characterization Marta Bustelo,† Beatriz Fernańdez,† Jorge Pisonero,‡ Rosario Pereiro,† Nerea Bordel,‡ Victor Vega,‡,§ Victor M. Prida,‡ and Alfredo Sanz-Medel*,† Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Claverıá, 8. 33006 Oviedo, Spain, Department of Physics, Faculty of Science, University of Oviedo, Calvo Sotelo. 33007 Oviedo, Spain, and Scientific-Technical Services, University of Oviedo, 33006 Oviedo, Spain Progress in the development of advanced materials strongly depends on continued efforts to miniaturizing their structures; thus, a great variety of nanostructured materials are being developed nowadays. Metallic nanowires are among the most attractive nanometer-sized materials because of their unique properties that may lead to applications as interconnectors in nanoelectronic, magnetic, chemical or biological sensors, and biotechnological labels among others. A simple method to develop selfordered arrays of metallic nanowires is based on the use of nanoporous anodic alumina (NAA) and self-assembled nanotubular titanium dioxide membranes as templates. The chemical characterization of nanostructured materials is a key aspect for the synthesis optimization and the quality control of the manufacturing process. In this work, the analytical potential of pulsed radiofrequency glow discharge with detection by time-of-flight mass spectrometry (pulsed rf-GD-TOFMS) is investigated for depth profile analysis of self-assembled metallic nanostructures. Two types of nanostructured materials were successfully studied: self-assembled NAA templates filled with arrays of single metallic nanowires of Ni as well as arrays of multilayered Au/FeNi/Au and Au/Ni nanowires and nanotubular titanium dioxide templates filled with Ni nanowires, proving that pulsed rf-GD-TOFMS allows for fast and reliable depth profile analysis as well as for the detection of contaminants introduced during the synthesis process. Moreover, ion signal ratios between elemental and molecular species (e.g., 27Al+/16O+ and 27Al+/32O2+) were utilized to obtain valuable information about the filling process and the presence of possible leaks in the system. The synthesis of structured solid-state materials with nanometer-sized geometries is currently receiving special interest due to the unique physical and chemical properties exhibited by many materials when nanostructured (e.g., improved mechanical, opti* To whom correspondence should be addressed. Phone/fax: +34 0.985103474. E-mail: [email protected]. † Department of Physical and Analytical Chemistry, Faculty of Chemistry. ‡ Department of Physics, Faculty of Science. § Scientific-Technical Services. 10.1021/ac102347v  2011 American Chemical Society Published on Web 12/02/2010

cal, electrical, magnetic, or catalytic properties).1 For example, highly ordered and densely packed arrays of nanopores, nanotubes, and nanowires2 become promising candidates for attractive applications in materials scientific and technological areas (such as functionalized arrays for magnetic sensors,3 ultrahigh density data storage media,4 thermoelectrics,5 or spin-based electronic devices).6 In particular, self-organized magnetic nanowires, whose diameter ranges from a few atomic distances to a few hundreds of nanometers and their lengths can vary from a few nanometers up to micrometers, are known to offer exceptional characteristics of technological interest, due to the convergence of high spatial ordering degree with the intrinsic nature of the materials synthesized at the nanoscale.7 There are several methods to fabricate nanowire arrays,8 and a simple one is based on the use of anodized metal membranes as templates. These patterned membranes can be filled afterward with the desired metallic elements via electrodeposition.9 As a result, self-ordered arrays of dense, continuous, and highly crystalline metallic nanowires can be synthesized inside the template. Nanoporous anodic alumina (NAA) nanostructures, which are formed by dense rows of highly hexagonally ordered nanopores in a honeycomb like configuration, are nowadays among the most common templates. The reason to use these NAA templates as precursor lies in the relatively simple and inexpensive process of self-assembling nanopore formations by aluminum anodization and the fact that well controlled patterned nanostruc(1) Yang, X.-C.; Zou, X.; Liu, Y.; Li, X.-N.; Hou, J.-W. Mater. Lett. 2010, 64, 1451–1454. (2) Feng, X.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781–3786. (3) McGary, P. D.; Tan, L.; Zou, J.; Stadler, B. J. H.; Downey, P. R.; Flatau, A. B. J. Appl. Phys. 2006, 99, 08B310.108B310.6. (4) Ross, C. A. Annu. Rev. Mater. Res. 2001, 31, 203–235. (5) Trahey, L.; Becker, C. R.; Stacy, A. M. Nano Lett. 2007, 7, 2535–2539. (6) Crowley, T. A.; Daly, B.; Morris, M. A.; Erts, D.; Kazakova, O.; Boland, J. J.; Wu, B.; Holmes, J. D. J. Mater. Chem. 2005, 15, 2408–2413. Piraux, L.; Renard, K.; Guillemet, R.; Matefi-Tempfli, S.; Matefi-Tempfli, M.; Antohe, V. A.; Fusil, S.; Bouzehouane, K.; Cros, V. Nano Lett. 2007, 7, 2563–2567. (7) Gao, T. R.; Yin, L. F.; Tian, C. S.; Lu, M.; Sang, H.; Zhou, S. M. J. Magn. Magn. Mater. 2006, 300, 471–478. (8) Martıń, J. I.; Nogue´s, J.; Liu, K.; Vicent, J. L.; Schuller, I. K. J. Magn. Magn. Mater 2003, 256, 449–501. (9) Nielsch, K.; Mu ¨ ller, F.; Li, A.-P.; Go ¨sele, U. Adv. Mater. 2000, 12, 582– 586.

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tures can be formed over a large surface area.10 However, NAA membranes present some disadvantages, including insufficient chemical stability and low mechanical resistance. More recently, the synthesis of self-aligned titanium dioxide nanotube arrays has acquired special interest due to its high yield strength at low and high temperatures, low density, and excellent biocompatibility.11 Additionally, the technological applications of titania nanotube arrays is increasing in photovoltaics, photoelectrolysis, photocatalysis, and sensing, as titania is a nontoxic and noncorrosive material formed by a wide band gap semiconductor oxide.12,13 Fast and reliable depth chemical characterization of the nanostructured materials remains of critical importance to assist the optimization of the synthesis procedure and to evaluate their routine manufacturing quality. Therefore, analytical techniques capable of characterizing the new materials at the nanoscale are currently required. Surface analytical techniques with lateral and depth resolution on the nanometer range, such as secondary ion mass spectrometry, Auger electron spectroscopy, or X-ray photoelectron spectroscopy, provide interesting analytical capabilities for the analysis of nanostructured materials.14 However, those techniques require long analysis time and expensive and complex instrumentation (need of ultrahigh vacuum), while glow discharge (GD) spectroscopy may tackle most of such problems. In fact, GDs coupled to optical emission spectrometry (OES) and mass spectrometry (MS) are today widely used for direct solid analysis due to their advantages of fast sputtering rate (micrometers/ minute), high depth resolution (nanometer), multielement capability, high sample throughput, minimal matrix effects, good detection limits (microgram/gram to nanogram/gram) and experimental simplicity with no requirement for ultrahigh vacuum.15 However, for direct solid analysis a potential drawback of GDs is their poor lateral resolution (of the order of a few millimeters). Pulsed-GDs (PGDs), wherein each pulse generates a packet of sample atoms of the analyzed sample, are being shown to offer an attractive analytical option to the more common GD operation mode, based on continuous powering. PGDs allow different time domains (prepeak, plateau, and afterglow) along the pulse period, related to different dominant ionization processes.16 With the use of a gated detector it is possible to select temporal intervals within the power pulse, in which analyte ions have higher signal-to-noise ratios. Moreover, on average, the applied power along the pulse period is lower compared to the continuous mode, resulting in reduced thermal stress of the solid sample to be analyzed. GD-MS presents some significant advantages compared to GDOES, including isotopic information and lower limits of detection. The double-focusing and quadrupole have been the most common mass analyzers in GD-MS. However, time-of-flight (TOF) mass spectrometers are increasingly utilized due to their high sampling (10) Sulka, G. D.; Brzo´zka, A.; Zaraska, L.; Jaskula, M. Electrochim. Acta 2010, 55, 4368–4376. (11) Prida, V. M.; Hernańdez-Ve´lez, M.; Pirota, K. R.; Meneńdez, A.; Va´zquez, M. Nanotechnology 2005, 16, 2696–2702. (12) Vega, V.; Prida, V. M.; Hernańdez-Ve´lez, M.; Manova, E.; Aranda, P.; RuizHitzky, E.; Va´zquez, M. Nanoscale Res. Lett. 2007, 2, 355–363. (13) Kim, J. C.; Choi, J.; Lee, Y. B.; Hong, J. H.; Lee, J. I.; Yang, J. W.; Lee, W. I.; Hur, N. H. Chem. Commun. 2006, 5024–5026. (14) Fernańdez, B.; Costa, J. M.; Pereiro, R.; Sanz-Medel, A. Anal. Bioanal. Chem. 2010, 396, 15–29. (15) Pisonero, J.; Fernańdez, B.; Pereiro, R.; Bordel, N.; Sanz-Medel, A. Trends Anal. Chem. 2006, 25, 11–18. (16) Harrison, W. W.; Yang, C.; Oxley, E. Anal. Chem. 2001, 73, 480A–487A.

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rate and ability to collect complete mass spectra with the same precision, sensitivity, and resolution regardless of the total number of isotopes being measured.17 These characteristics make TOFMS especially suitable to be coupled as time gated detector to PGDs. The analytical capabilities of PGDs coupled to TOFMS are being lately studied for varied samples. For instance, GD-TOFMS have been successfully applied to the elemental and molecular analysis of organic compounds in gas phase,18 to the analysis of small volumes of molecular inorganic gases,19 and to direct analysis of solid materials.20,21 In this work, radiofrequency (rf) pulsed GD-TOFMS is investigated as a new analytical tool for the fast and accurate characterization of nanostructures, in particular single (Ni) and multilayered (Au/Ni(Fe)/Au) metallic nanowires, which are grown by templated-assisted electrochemical deposition procedures in self-ordered nanoporous alumina and titania nanotubes synthesized through potentiostatic anodization techniques. Highquality characterization of these specimens becomes necessary for a better understanding and control of their physical properties in order to tailor their enhanced soft magnetic properties, such as high magnetic permeability, uniaxial anisotropy, peculiar domain pattern, and magnetoresistance, which play a key role in the development of future magnetoelectronic devices based on well-controlled static and dynamic domain wall displacements in these low-dimensional systems.22-24 Therefore, depth profile analysis of nanostructures as well as synthesis of contaminant impurities are investigated. Moreover, the analytical potential of this MS technique to allow the simultaneous acquisition of elemental and molecular information is exploited here measuring molecular and elemental oxygen in order to discriminate between the oxygen coming from the solid material (e.g., oxides) and the molecular oxygen that comes from air, contained in the nanostructure or coming from possible leaks within the instrument. EXPERIMENTAL SECTION Samples Preparation. Samples were produced by using a template assisted filling method. Arrays of Ni nanowires with different lengths (1.2, 2.2, and 3.8 µm) and multilayered nanowires consisting of alternating layers (of Au/Ni and Au/FeNi/Au) with different thicknesses (from 0.8 µm up to 1.9 µm) were electrodeposited inside the nanopores of the alumina membrane. Moreover, arrays of Ni (550 nm length) were also deposited inside a titania (17) Pisonero, J.; Costa, J. M.; Pereiro, R.; Bordel, N.; Sanz-Medel, A. Anal. Bioanal. Chem. 2004, 379, 658–667. (18) Sola`-Va´quez, A.; Martıń, A.; Costa, J. M.; Pereiro, R.; Sanz-Medel, A. Anal. Chem. 2009, 81, 2591–2599. (19) Gago, C. G.; Pereiro, R.; Bordel, N.; Ramos, P. M.; Tempez, A.; Sanz-Medel, A. Anal. Chim. Acta 2009, 652, 272–277. (20) Lobo, L.; Pisonero, J.; Bordel, N.; Pereiro, R.; Tempez, A.; Chapon, P.; Michler, J.; Hohl, M.; Sanz-Medel, A. J. Anal. At. Spectrom. 2009, 24, 1373– 1381. (21) Mun ˜iz, A. C.; Pisonero, J.; Lobo, L.; Gonza´lez, C.; Bordel, N.; Pereiro, R.; Tempez, A.; Chapon, P.; Tuccitto, N.; Licciardello, A.; Sanz-Medel, A. J. Anal. At. Spectrom 2008, 23, 1239–1246. (22) Nguyen, T. M.; Cottam, M. G.; Liu, H. Y.; Wang, Z. K.; Ng, S. C.; Kuok, M. H.; Lockwood, D. J.; Nielsch, K.; Go¨sele, U. Phys. Rev. B 2006, 73, 140402_1–4. (23) Pardavi-Hovath, M.; Si, P. E.; Vazquez, M.; Rosa, W. O.; Badini, G. J. Appl. Phys. 2008, 103, 07D517. (24) Boone, C. T.; Katine, J. A.; Carey, M.; Childress, J. R.; Cheng, X.; Krivorotov, I. N. Phys. Rev. Lett. 2010, 104, 097203_1–4.

Table 1. Samples Classified by Type of Membrane, Nanopore or Nanotube Length, and Nanowire Compositiona alumina template nanopore length (µm)

nanowire

1.2 2.2 3.8 3.5 3.5

Ni (filled and overflowed) Ni (filled and overflowed) and empty Ni (filled and overflowed) Au(0.8 µm)/FeNi(0.9 µm)/Au(0.9 µm) Au(0.9 µm)/Ni(1.9 µm)/Au(- -)

titania template nanotube length (nm)

nanowire

550

Ni (filled and overflowed) and empty

a The term overflowed is used to designate the nanopores or nanotubes which are totally filled up to overflowing the whole sample surface.

nanotubes membrane. Table 1 collects the samples under investigation. The description of the method used for the sample preparation of filled nanoporuous alumina and titania nanotube membranes is included as Supporting Information. Instrumentation. Glow Discharge Time-of-Flight Mass Spectrometer. The rf-GD-TOFMS instrument consists of a rf-GD bay unit (rf generator, matching box, rf connector, refrigerator disk, and sample mounting system with a pneumatic piston to press the sample against the source) from Horiba Jobin Yvon (Longjumeau, France), coupled to a fast orthogonal time-of-flight mass spectrometer (TOFWERK, Switzerland) with a microchannel plate detector (Burle Industries Inc., Lancaster, PA).25 Additionally, an interface, consisting of two extraction cones (sampler and skimmer cones), connects the GD source to the TOFMS. This interface allows one to extract and focus the ions as well as to reduce the pressure between the GD source and the mass analyzer. Further details of the GD-TOFMS instrument are included as Supporting Information. In this work, the GD was operated in the pulsed mode. Experimental conditions (650 Pa, 55 W forward power, 2 ms pulse width, and 4 ms pulse period) were chosen as a compromise between high sensitivity and good depth resolution through the analysis of homogeneous materials with aluminum and titanium matrixes. The module and phase of the applied rf power were adapted to keep the reflected power to a minimum value. Analyte ion signals showed their maximum intensity in the afterglow region of the pulse. Therefore, pulsed rf-GD-TOFMS depth profiles of the nanostructured materials were always measured by integrating the ion signals in the afterglow region (each isotope at its maximum position). To achieve reliable depth profiling of thin layers using GDs, it is crucial to provide conditions for stable plasma generation at the beginning of the sputtering. This can be done through minimizing contaminations from the sample and anode surfaces, and thus, nanostructured materials were slightly pretreated before GD analysis with the following procedure: (i) since the fabrication process of alumina and titania templates uses liquid solutions, samples were dried with hot air for 5 min to remove residual (25) Hohl, M.; Kanzari, A.; Michler, J.; Nelis, T.; Fuhrer, K.; Gonin, M. Surf. Interface Anal. 2006, 38, 292–295.

humidity, (ii) high-purity silicon were presputtered in the GD source for 5 min to create a Si coating on the source that could reduce water desorption,26 and (iii) to reduce the amount of gas species that could be occluded in the sample and chamber surfaces, a flushing step with Ar during 3-4 min was applied to the GD source and the sample. Profilometer and Scanning Electron Microscopy. The depth of the craters produced on the samples after GD sputtering was measured with a profilometer (Perth-o-meter S5P, Mahr Perthen, Germany). Two profile traces at different direction across the center of the crater were measured. Additionally, the length of the nanowires inside the alumina and titania membranes was measured by scanning electron microscopy (SEM) (MEB JEOL6100, Japan). RESULTS AND DISCUSSION Qualitative Depth Profiles of Alumina Nanopores Filled with Single Metal Nanowires. Figure 1 shows the qualitative depth profiles (analyte ion intensity versus sputtering time) obtained by pulsed-rf-GD-TOFMS at the optimized experimental conditions for three nanoporous alumina substrates synthesized with different nanopore lengths (1.2, 2.2, and 3.8 µm) and then filled with electrodeposited Ni. As can be observed, 27Al+ ion signal always showed a fast increase at the interface between the nanoporous alumina and the aluminum substrate, indicating a good depth resolution. The 58Ni+ signal observed in the depth profiles demonstrates the presence of this metal within the alumina nanopores. Besides, a high 58Ni+ ion signal was always observed at the beginning of the analysis when the 27Al+ ion signal increased from zero, indicating the presence of a very thin layer of Ni on the aluminum membrane. Although the presence of Ni was clearly observed inside the nanopores for the three different lengths, the depth profiles showed that the Ni electrodeposition process was more uniform for the sample with 2.2 µm length since a homogeneous 58Ni+ signal was obtained throughout the nanopore length. As an example, Figure 1b collects also the SEM image of this alumina membrane filled with Ni. The presence of small knolls of Ni is clearly observed on the sample surface and it could be attributed to the overfilling of the nanopores during the Ni electrodeposition process, in agreement with our results obtained from pulsed-rfGD-TOFMS profiles. The sputtering time necessary to reach the interface between the Al2O3 nanopores filled with Ni nanowires arrays and the aluminum substrate itself was proportional to the length of the nanopores. The linear relationship observed for the three selected lengths with the sputtering time is included as Supporting Information as Figure S1a. In contrast to Ni filled nanopores, NAA with empty nanopores did not show a linear relationship between the length and the observed sputtering time. The qualitative depth profile obtained by pulsed-rf-GD-TOFMS for the empty NAA templates, synthesized with a nanopore length of 2.2 µm, is shown in Figure S1b. It can be noticed that the presence of air in the nanopores significantly increased the (26) Molchan, I. S.; Thompson, G. E.; Skeldon, P.; Trigoulet, N.; Chapon, P.; Tempez, A.; Malherbe, J.; Lobo-Revilla, L.; Bordel, N.; Belenguer, Ph.; Nelis, Th.; Zahri, A.; Therese, L.; Guillot, Ph.; Ganciu, M.; Michler, J.; Hohl, M. J. Anal. At. Spectrom. 2009, 24, 734–741.


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Figure 1. Pulsed-rf-GD-TOFMS qualitative depth profiles for NAA templates filled with single Ni nanowires: (a) 1.2 µm nanopore length, (b) 2.2 µm nanopore length, (c) 3.8 µm nanopore length. Figure 1b shows also the SEM cross-sectional view of the NAA template filled with Ni.

sputtering time required to reach the interface compared to filled nanopores (e.g., 400 and 150 s for the 2.2 µm length, respectively) and this could be attributed to the humidity and air inside the nanopores, which produces quenching effects in the GD plasma. 332


As will be further discussed, the capabilities of pulsed rf-GDTOFMS for the measurement of molecular 32O2+ ion signals could help up to identify the presence of leaks or residual air inside the nanopores of the NAA templates.

Figure 2. Depth profile characterization for the NAA template of 3.5 µm nanopore length filled with trilayer nanowires (Au/FeNi/Au) made of Au (0.8 µm), FeNi (0.9 µm), and Au (0.9 µm): (a) qualitative in-depth profile obtained by pulsed rf-GD-TOFMS and (b) SEM cross-sectional view using backscattered electrons to enhance the compositional contrast.

Qualitative Depth Profiles of Alumina Nanopores Filled with Multilayered Metal Nanowires. Concerning the analysis of multilayered metallic nanowires, two samples were investigated. Figure 2a shows the qualitative depth profile obtained by pulsedrf-GD-TOFMS for the NAA template of 3.5 µm nanopore length filled with three electrodeposited metal layers of Au (0.8 µm), FeNi alloy (0.9 µm), and Au (0.9 µm). As can be seen, the layers deposited inside the nanopore can be discriminated with a good depth resolution. In particular, the first deposited Au (inner layer) had a narrower distribution compared to the outer Au layer, while 56 Fe+ and 58Ni+ ion signals showed a perfect match along the depth profile. As reported above for the Ni filled nanopores, a high 197Au+ ion signal was observed at the beginning of the analysis, followed in this case by a drop in the ion signal, indicating a nonhomogeneous distribution of Au through the external film of the nanopore. Figure 2b shows the SEM image of this nanostructured material being in agreement with the experimental results obtained from pulsed rf-GD-TOFMS profiles. Thus, GD and SEM measurements were allowed to obtain complementary information, confirming that the distribution of Au in the external layer of the nanopores was not homogeneous and, also, that the thickness of the two Au layers was not the

same. Additionally, it should be highlighted that the analysis time required to sputter the nanowires reaching the aluminum substrate was only about 4 min, so pulsed rf-GD-TOFMS can be employed as a powerful tool for the fast and reliable characterization of nanostructured materials. The qualitative depth profile obtained by pulsed-rf-GD-TOFMS for the same NAA template (3.5 µm nanopore length) filled with a different configuration of metallic electrodeposited layers (0.9 µm Au, 1.9 µm Ni and Au up to covering the sample surface) can be observed in Figure 3a. In this case, the sputtering time required to reach the aluminum substrate was also about 4 min. However, although the intermediate Ni layer and the internal Au film were discriminated with a good depth resolution, the external electrodeposited Au layer could not be appropriately distinguished: a high 197Au+ ion signal was observed at the beginning of the analysis, indicating a very thin film of Au at the sample surface, but the Au layer (which during the synthesis process was thought to be of 0.9 µm length) was not found in this qualitative profile. This fact was validated by SEM measurements. Figure 3b shows the SEM image obtained and, as can be seen, only two clear Au and Ni layers were deposited inside the alumina nanopores. Analytical Chemistry, Vol. 83, No. 1, January 1, 2011

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Figure 3. Depth profile characterization of the NAA template of 3.5 µm nanopore length filled with trilayer nanowire arrays of Au (0.9 µm), Ni (1.9 µm), and Au: (a) qualitative in-depth profile obtained by pulsed rf-GD-TOFMS and (b) SEM cross-sectional views using backscattered electrons to enhance the compositional contrast.

The failure in the expected electrodeposition of the external Au layer could be related to the different metal deposition processes within the nanoporous membrane. As can be observed in Figure 3b, with the work with electroplated Au/Ni layers, loosening problems and cracked membrane pieces were observed after the Ni deposition. The enlargement of sample surface image in Figure 3b shows several zones where the membrane was broken and the aluminum substrate appeared at the sample surface. In such a way, during the deposition of the second and external Au layer, the electrolyte could be preferably deposited in zones where the alumina membrane has been removed, because the electric field will be locally more intense at the aluminum substrate than at those zones where the NAA membrane was still present. In other words, it seems clear that a fast and reliable depth characterization of nanopores quality, filled with single or multilayered metals, is of critical importance to assist the optimization of the electrodeposition procedures, as well as 334


to evaluate their routine manufacturing quality. Of course, results shown here provide evidence that pulsed rf-GD-TOFMS constitutes a promising technique for fast quality assurance tests. Qualitative Depth Profiles of Titania Nanotubes Filled with Single Metal Nanowires. Alternatively, titania nanotube membranes filled with single Ni nanowires in the nanometer range were also investigated. Figure 4 shows the qualitative depth profile obtained by pulsed rf-GD-TOFMS for a titania nanotube of 550 nm nanopore length filled with Ni. As can be observed, 58Ni+ and 48 + Ti ion signals showed a sharp interface, indicating a good depth resolution between the nanotubes and the titanium substrate. Moreover, the Gaussian distribution observed for the 58Ni+ profile suggests a good electrodeposition process and a homogeneous distribution of Ni inside the nanotube. Although not shown here, SEM images obtained for this sample also showed that the nanostructured membrane was full and overflowed with Ni. Interestingly, the time necessary to sputter

Figure 4. Qualitative in-depth profile obtained by pulsed rf-GDTOFMS for a titania membrane filled with single Ni nanowires of 550 nm length.

the titania nanotubes filled with Ni nanowires array (indicated by the remarkable increase of 48Ti+ signal and the decrease of the 58Ni+ signal) was only about 15 s. Again, the sputtering time required to reach the sample substrate for the nonfilled nanopores increased compared to the Ni filled. However, in this case the sputtering time required to reach the Ti substrate was just 20 s for the empty nanotube. In other words, with Ti instead of Al the sputtering rates were not so different for the Ni-filled and empty titania membranes. This could be ascribed to the shorter length of the nanotubes (nanometer) and, therefore, to the comparatively lower air presence in comparison with the micrometric sized nanopores in Al substrates (NAA templates). Analysis of Metal Impurities by Pulsed rf-GD-TOFMS. One of the recognized advantages that GD-TOFMS could offer compared to GDs coupled to OES are lower limits of detection for some specific elements as well as its ability to simultaneously collect complete mass spectra. On the other hand, analysis of possible impurities in nanostructures is a critical aspect for the optimization of the sample preparation and electrodeposition processes. Therefore, quality control of impurities is mandatory for their subsequent applications. Figure 5 shows the mass spectra obtained at the optimized pulsed rf-GD conditions for two nanostructured samples based on alumina template with 2.2 µm long nanopores (filled with Ni nanowires and empty) at two different positions along the GD-TOFMS profile: inside the aluminum substrate (Figure 5a) and at the sample surface (Figure 5b,c). As can be observed in Figure 5a, at the selected mass interval, the ion signals from 206Pb+, 207Pb+, and 208Pb+ were clearly observed, indicating that the aluminum used for the preparation of NAA membranes contained also Pb (the nominal composition of Pb in the Al foil was about 25 µg/g). On the other hand, significant differences can be found in the mass spectra at the sample surface region for the empty and Ni filled nanopores (parts b and c of Figure 5, respectively). As can be seen in Figure 5b, 206Pb+, 207Pb+, and 208Pb+ signals together with a small 197Au+ signal were observed at the sample surface for the empty nanopores, which could be attributed to the presence of Pb impurities in the aluminum substrate and the possible contamination of the Pt electrode with Au at low trace

concentration levels (i.e., during the fabrication step of NAA membranes by anodization using the Pt electrode, Au was also deposited on the sample surface). Nevertheless, for the Ni filled nanopores, not only 206Pb+, 207Pb+, and 208Pb+ ion signals where observed but also ion signals from 194Pt+, 195Pt+, 196Pt+, 197Au+, 198 Hg+,199Hg+, 200Hg+, 201Hg+, 202Hg+, and 204Hg+ were clearly identified in the mass spectrum (see Figure 5c). In this case, the presence of Pt, Au, and Hg at the sample surface and not in the aluminum substrate could be attributed not only to anodization but also to electrodeposition procedures employed in the sample preparation as well as to trace impurities on the electrodes and liquid solutions used. Therefore, pulsed rf-GD-TOFMS proves again to be a powerful tool for the depth characterization of nanostructured materials, offering information of the possible contamination sources (even at very low concentration levels) that could affect the final properties and quality of fabricated nanostructured devices. Different types of impurities were identified at the aluminum substrate and at the Ni filled nanowires. Therefore, two possible contamination sources were identified to be checked for final quality assurance: the purity of the sample substrate and reagents and process of the membrane synthesis, respectively. Small Molecules Information Potential: Analysis of Atomic and Molecular Oxygen Ions. Besides the good sensitivity achieved by pulsed rf-GD-TOFMS for the analysis of elemental impurities at trace concentration levels, the use of pulsed rf-GDs combined to a TOF mass spectrometer with time-gated detection offers the possibility of obtaining both elemental and molecular information using different pulse regions (i.e., prepeak, plateau, and afterpeak).18 A preliminary study was then carried out here, aiming to identify possible sources of 16O+ and 32O2+ ion signals: the alumina membrane (i.e., Al2O3) and the air (molecular oxygen) existing inside the nanopores or as a leak of the experimental system. Although several limitations were found to identify the origin of measured 16O+ and 32O2+ signals, due to fragmentation as well as recombination processes that may take place at the different pulse regions (especially at the afterglow where 16O+ from alumina could be recombined to 32 O2+, while the 32O2+ from air could be partially fragmented as16O+), promising preliminary results obtained are reported here. Figure 6 shows the observed 27Al+/16O+ and 27Al+/32O2+ ratios (logarithmic scale) obtained along the pulse (2 ms) at an intermediate depth for a Ni filled NAA membrane and for an empty NAA membrane of 2.2 µm length. As can be observed in the figure, the 27Al+/16O+ and 27Al+/32O2+ ratios in the plateau were similar for the Ni filled template, whereas the 27Al+/16O+ ratio was significantly higher than the 27Al+/32O2+ ratio for the empty nanopores thus indicating a higher 32O2+ content (as expected) in this latter case. Therefore, the efficiency of the Ni electrodeposition processes could be investigated through the 27Al+/16O+ and 27Al+/32O2+ ratios since the lower the 27Al+/ 32 O2+ ratio, the higher the presence of air (e.g., due to a nonhomogeneous metals distribution inside the nanopore or to the failure in the electrodeposition process, as was previously observed in Figure 3a for the multilayer nanostructure). CONCLUSIONS The interesting capabilities of pulsed-rf-GD-TOFMS for the chemical depth profile characterization of highly ordered and selfAnalytical Chemistry, Vol. 83, No. 1, January 1, 2011

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Figure 5. Mass spectra between m/z 190 and 215 of an alumina membrane with 2.2 µm nanopore length. Discharge conditions: 650 Pa, 55 W, 250 Hz pulse frequency and 2 ms pulse width: (a) mass spectrum for a Ni filled NAA template at the aluminum substrate; (b) mass spectrum for an empty NAA template at the sample surface position; (c) mass spectrum for a Ni filled NAA template at the sample surface position.

assembled nanostructured oxide templates with metallic nanowires deposited inside were here successfully demonstrated. Advantages of this fast direct solid analysis technique (not yet in 336


the market) for its use in the analytical characterization of those nanostructured samples include simple sample preparation stages before the measurement, good depth profile resolution, and high

Figure 6. 27Al+/16O+ and 27Al+/32O2 + ratios obtained along the pulse profile at two different positions of the qualitative in-depth profile at the center of the nanopores, both for Ni filled and empty nanopores of a NAA template with 2.2 µm pores length.

sensitivity to reveal the presence of contaminants during the synthesis process. Besides, ion signal ratios between elemental and molecular species allowed one to obtain valuable information about the homogeneity of the filling process and the presence of possible leaks in the system. Therefore, this new analytical tool offers a tremendous interest to assist the synthesis optimization process as well as for the quality control of nanowires growth in patterned nanostructured membranes. This work warrants further research to uncover further potentialities of the pulsed-rf-GD-TOFMS for its application in nanotechnology or in the “nanoworld”.

65097-C02 as well as from Consejerıá de Educacioń y Ciencia del Principado de Asturias (Ref COF08-10 and Ref FC09-IB09-131) is acknowledged. B. Fernańdez and J. Pisonero acknowledge financial support from the “Juan de la Cierva” and “Ramon y Cajal” Programs of the Ministry of Science and Innovation of Spain, respectively. Finally, we especially thank the contract with Horiba Jobin Yvon for the loan of the GD-TOFMS instrument.

ACKNOWLEDGMENT Financial support from Spanish Ministry of Science and Innovation and FEDER Programme through Grant MAT2007-

Received for review September 14, 2010. Accepted November 18, 2010.

SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AC102347V


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Pulsed Radiofrequency Glow Discharge Time-of-Flight Mass

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