Composite Nanowire-Based Probes for Magnetic Resonance Force

Microwave synthesis of Cr nanowires on polymeric substrate. Daeseob Shim , Seung-Ho Jung , Eun-Ha Kim , Dong-Myung Yoon , Kun-Hong Lee , Soo-Hwan ...
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NANO LETTERS

Composite Nanowire-Based Probes for Magnetic Resonance Force Microscopy

2005 Vol. 5, No. 1 187-190

Mladen Barbic* Department of Physics and Astronomy, California State UniVersity, Long Beach, 1250 Bellflower BlVd., Long Beach, California 90840

Axel Scherer Departments of Physics, Applied Physics, and Electrical Engineering, California Institute of Technology, M/C 200-36, Pasadena, California 91125 Received October 1, 2004; Revised Manuscript Received October 28, 2004

ABSTRACT We present a nanowire-based methodology for the fabrication of ultrahigh sensitivity and resolution probes for atomic resolution magnetic resonance force microscopy (MRFM). The fabrication technique combines electrochemical deposition of multifunctional metals into nanoporous polycarbonate membranes and chemically selective electroless deposition of optical nanoreflector onto the nanowire. The completed composite nanowire structure contains all the required elements for an ultrahigh sensitivity and resolution MRFM sensor with (a) a magnetic nanowire segment providing atomic resolution magnetic field imaging gradients as well as large force gradients for high sensitivity, (b) a noble metal enhanced nanowire segment providing efficient scattering cross-section from a sub-wavelength source for optical readout of nanowire vibration, and (c) a nonmagnetic/nonplasmonic nanowire segment providing the cantilever structure for mechanical detection of magnetic resonance.

Achievement of atomic resolution magnetic resonance microscopy with three-dimensional (3D) imaging capability remains one of the main challenges in bio- and nanomaterial characterization. The challenge stems from the exceptionally weak signals typical in the magnetic resonance process,1 whether they originate from electronic or nuclear spins. Following the original 1973 reports of applying magnetic field gradients to samples in order to demonstrate magnetic resonance imaging of spatial spin distribution by Lauterbur2 and Mansfield and Grannell,3,4 magnetic resonance imaging (MRI) has become a well-established three-dimensional visualization technology. Although improvements in imaging resolution through conventional inductive detection5,6 have steadily progressed in the past three decades, current spatial resolution is limited to approximately 1 µm in nuclear and electron spin magnetic resonance microscopy.7-10 Despite the challenges, the attraction and intense research interest toward 3D atomic resolution MRI remains due to the wellknown advantages of MRI as a noninvasive, three-dimensional, multicontrast, and chemically specific imaging tool.11,12 Motivated by the potential of combining the 3D imaging capability of conventional magnetic resonance and the atomic resolution of scanning probe techniques that utilize mechanical cantilevers, Sidles proposed a unique atomic resolution 3D magnetic resonance imaging technique.13 This method, magnetic resonance force microscopy (MRFM), uses a * Corresponding author. E-mail: [email protected]. 10.1021/nl048373b CCC: $30.25 Published on Web 11/13/2004

© 2005 American Chemical Society

microscopic magnetic particle as the source of an atomic scale imaging gradient field and a mechanical resonator as a sensitive detector of magnetic resonance.14 Proof-of-concept demonstrations of the technique were carried out for various magnetic resonance systems including electron spin resonance,15 nuclear magnetic resonance,16 and ferromagnetic resonance.17 The technique has been rapidly progressing by the incorporation of smaller magnetic particles,18,19 more sensitive mechanical resonators,20,21 experimental implementation at progressively lower temperatures,22 and better understanding of spin relaxation processes for both probe23 and sample magnetizations.24 This approximately decadelong progress has recently culminated in the detection of a single electron spin by MRFM.25 Further progress of the technique, however, still places challenging demands on the technical requirements, both in the sensitivity and resolution arenas. Mechanical detection of a single electron magnetic resonance was performed on a sample with spatially well-isolated spins with significant averaging time of 13 h per point.25 To improve the sensitivity and therefore reduce the averaging time, further miniaturization of the mechanical detector and the magnetic field gradient source is required, while at the same time still allowing efficient optical detection of cantilever vibration. The reduction of the magnetic field gradient source will, additionally, improve the imaging resolution, although careful attention has to be paid to the appropriate magnetic resonance

Figure 1. (a) Electrodeposition of gold segment of the nanowires followed by the plated nickel section of the nanowire results in the filled nanoporous membrane. Ultrasonication in ethanol removes the gold plating layer. (b) Dissolution of the nanoporous polycarbonate membrane in chloroform results in the free-floating composite nanowires. The nanowires are deposited from solution onto the substrate in (c). Silver developer solution is placed onto the substrate in (d) to induce the growth of the nanoreflector onto the gold segment of the nanowire. Termination of the silver growth and drying results in the final nanoreflector-tipped nanowire in (e).

imaging contours,26 relative sample-probe positioning,27 and image reconstruction process.28 Here, we offer an alternative nanowire-based approach to MRFM sensor design and demonstrate the implementation of all of the abovementioned sensor requirements. We first demonstrate the fabrication procedure for the integration of an efficient sub-wavelength optical nanoreflector into the nanowire cantilever structure, shown in Figure 1. Commercially available nanoporous polycarbonate membrane (SPI Supplies) 7 µm in thickness with a predetermined pore diameter range of 10-50 nm is initially coated on one side with a 150 nm Au layer by thermal evaporation. This conductive Au layer serves as a starting point in the nanowire growth and as an electrode layer in the custom Teflon electrodeposition setup with a platinum counter electrode. The pores of the membrane serve to restrict the metal growth inside the one-dimensional cylindrical nanostructures.29 A short Au nanowire segment is first electroplated into the pores (Gold RTU, Technic, Inc.) and forms the initiation layer for the ex-situ silver growth. The short Au nanowire segment growth is followed by the electrodeposition of the nickel nanowire segment (Nickel RTU solution, Technic Inc.) that can serve as the cantilever structure. The gold electrode layer is then removed by ultrasonication of the nanoporous polycarbonate membrane in ethanol, and the resulting membrane structure is shown in Figure 1a. The polycarbonate membrane is then dissolved in chloroform, as shown in Figure 1b. A drop of the chloroform solution with free composite nanowires is then placed on the substrate, rinsed in chloroform and ethanol, and then dried, as shown in Figure 1c. The choice of the substrate structure will depend on the eventual procedure for nanowire sensor suspension,30 as well as on the consideration for the sensor implementation into the MRFM arrangement. Although the gold segment of the nanowire provides modest light scattering efficiency31 we use the welldeveloped chemical methods used in photography32 and the 188

immunogold-silver staining procedures in biology33 to fabricate a sub-wavelength silver nanoreflector with high efficiency as part of the nanowire structure. This is motivated by the optical characteristics of silver,34 which show superior properties compared to that of any other metals with respect to the losses in the localized surface plasmon resonance.35,36 Silver nanoparticles, although sub-wavelength in size, have a very large elastic scattering cross-section due to the plasmon resonance condition that occurs at optical wavelengths and show a strong size, shape, and polarization scattering dependence.37,38 Commercially available silver developer solution (Ted Pella) containing silver initiator and enhancer is placed onto the substrate, as shown in Figure 1d, as the source of silver ions to be deposited onto the gold segment of the nanowire. The reduction of the free silver ions from the developer solution is initiated spontaneously at the gold segment of the nanowire only. The silver growth is terminated after approximately 15 s by an ethanol rinse and drying, resulting in the final structure of Figure 1e. A scanning electron microscopy (SEM) micrograph of the fabricated nanowire structure is shown in Figure 2a. The nickel nanowire segment is 3.5 µm long and 50 nm in diameter, while the silver spherical nanostructure grown on the gold nanowire segment is 100 nm in diameter. The attractive part of the composite nanostructure with respect to the optical detection of nanoresonator vibration is revealed by investigating its optical scattering properties. Figure 2b shows the diffraction limited optical micrograph of the same nanoreflector-tipped nanowire illuminated by white light in the dark field illumination arrangement, and detected in the optical far field. The silver nanoparticle does not add significant mass to the nanowire and therefore does not compromise the mechanical properties of the resonator structure with respect to its sensitivity, but it is easily observed by standard far field optical microscopy. Therefore, it performs the task of an extremely efficient nanoreflector that could be implemented in the most common optical Nano Lett., Vol. 5, No. 1, 2005

Figure 2. (a) An SEM micrograph of the 100 nm diameter silver nanoreflector on the end of the 50 nm diameter nickel nanowire. (b) Far-field dark-field optical microscopy image of the nanostructure. Efficient optical scattering property of the sub-wavelength silver nanoparticle is clearly evident due to the optical plasmon resonance effect. Tuning the length of the gold segment of the nanowire could tune the scattering peak wavelength.

detection technique used in MRFM, fiber optic interferometry.39 The plasmon resonance wavelength of the silver nanoparticle is generally dependent on the particle size, shape, and illumination polarization,37,38 and our technique of controlled growth of the Au nanowire segment length can be utilized to tune the center wavelength of the silver plasmon resonance, typically between 400-700 nm,38 to match the laser wavelength in the interferometric vibration detection setup. To prepare the sensor suitable for magnetic resonance force microscopy, we also introduce a nonmagnetic/nonplasmonic segment into the nanowire structure. The nickel segment is electroplated first, followed by the gold segment electrodeposition, and finally the platinum (Platinum RTU, Technic, Inc.) nanowire electrodeposition, as shown in Figure 3a. The rest of the procedure is the same as described in Figure 1, with release from nanoporous membrane, deposition onto substrate, and selective silver enhancement of the gold segment of the sensor structure, resulting in the final form shown in Nano Lett., Vol. 5, No. 1, 2005

Figure 3. Sequential electrodeposition of nickel, gold, and platinum sections of the nanowires results in the filled nanoporous membrane shown in (a). Membrane dissolution, deposition onto the substrate, and silver enhancement of the gold segment of the nanowire only results in the structure shown in (b). Scanning electron microscopy image of the resulting composite nanowire-based magnetic resonance force microscopy sensor is shown in (c). Magnetic nickel section is on the left, followed by the silver nanoreflector, and completed by the platinum nanowire-based cantilever structure.

Figure 3b. An SEM micrograph of the final structure is shown in Figure 3c, consisting of the 50 nm diameter, 500 nm long nickel magnetic field gradient section of the sensor on the left, followed by the spherical silver enhanced gold nanoreflector section, and completed by the platinum nanowire section acting as the cantilever part of the MRFM sensor, shown on the right. It is evident that the location of the silver nanoreflector along the nanowire can be easily tuned by the electrodeposition process, and, if desired, moved away from the magnetic segment. Suspension of a similar nanowire structure on a mechanical support has been previously demonstrated,30 and similar fabrication techniques can be utilized for the MRFM implementation of the nanowire-based sensor described in this article. When considering the potential performance of the composite nanowire-based sensor in a particular MRFM setting, 189

many factors determine the ultimate sensitivity and resolution, such as operating frequency, temperature, spring constant of the nanowire-based cantilever, gradient magnetic field generated by the magnetic segment of the sensor, measurement bandwidth, distance between the sensor and the spins of the sample, and quality factor Q of the resonator. For our composite nanowire-based MRFM sensor, as deduced from previous nanowire-based resonator measurements,30 it is expected that the resonant frequency would be in the range of 100 MHz, which is particularly convenient as it closely couples to the proton nuclear magnetic resonance, while the quality factor Q could be in the range of 10 000.30 The magnetic field gradient from the 50 nm diameter magnetic segment of the sensor is expected to be in the range of 10-100 (Gauss/Angstrom), providing significantly larger imaging gradients and magnetic forces at the sample spin location than currently utilized in the stateof-the-art MRFM instruments. In conclusion, the general rule-of-thumb in MRFM advancements14 has been that the overall miniaturization of the MRFM sensor is required for eventually achieving single nuclear spin sensitivity and resolution, a feat that would be significant in molecular imaging applications. By utilizing a nanowire-based approach to the sensor design for creating an ultrasmall magnetic gradient field source and implementing a plasmon resonant nanoreflector into the nanowire structure, all of the crucial components of the MRFM sensor are miniaturized simultaneously. We believe that this development will provide a further impetus toward reaching single nuclear spin detection by MRFM. Acknowledgment. This work was supported by the Caltech Grubstake Fund and by the NSF-CAREER award (DMR-0349319). The authors thank Dr. Joyce Wong for helpful suggestions and careful reading of the manuscript. References (1) Abragam, A. Principles of Nuclear Magnetism; Oxford University Press: New York, 1983. (2) Lauterbur, P. C. Nature 1973, 242, 190. (3) Mansfield, P.; Grannell, P. K. J. Phys. C 1973, 6, L422. (4) Mansfield, P.; Grannell, P. K. Phys. ReV. B 1977, 12, 3618. (5) Hoult, D. I.; Richards, R. E. J. Magn. Reson. 1976, 24, 71. (6) Hoult, D. I.; Lauterbur, P. C. J. Magn. Reson. 1979, 34, 425. (7) Aguayo, J.; Blackband, S.; Schoeniger, J.; Mattingly, M.; Hintermann, M. Nature (London) 1986, 322, 190. (8) Lee, S. C.; Kim, K.; Kim, J.; Lee, S.; Yi, J. H.; Kim, S. W.; Ha, K. S.; Cheong, C. J. Magn. Reson. 2001, 150, 207.

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Nano Lett., Vol. 5, No. 1, 2005