Mladen Barbic

Shape effects in plasmon resonance of individual colloidal silver nanoparticles

We present a systematic study of the effect of size and shape on the spectral response of individual silver nanoparticles. An experimental method has been developed that begins with the detection and characterization of isolated nanoparticles in the optical far field. The plasmon resonance optical spectrum of many individual nanoparticles are then correlated to their size and shape using high-resolution transmission electron microscopy. We find that specific geometrical shapes give distinct spectral responses. In addition, inducing subtle changes in the particles’ morphology by heating causes a shift in the individual particle spectrum and provides a simple means of tuning the spectral response to a desired optical wavelength. Improved colloidal preparation methods could potentially lead to homogeneous populations of identical particle shapes and colors. These multicolor colloids could be used as biological labels, surface enhanced Raman scattering substrates, or near field optical microscopy sources cove...

Nanowire-based very-high-frequency electromechanical resonator

Fabrication and readout of devices with progressively smaller size, ultimately down to the molecular scale, is critical for the development of very-high-frequency nanoelectromechanical systems (NEMS). Nanomaterials, such as carbon nanotubes or nanowires, offer immense prospects as active elements for these applications. We report the fabrication and measurement of a platinum nanowire resonator, 43 nm in diameter and 1.3 μm in length. This device, among the smallest NEMS reported, has a fundamental vibration frequency of 105.3 MHz, with a quality factor of 8500 at 4 K. Its resonant motion is transduced by a technique that is well suited to ultrasmall mechanical structures.

Single crystal silver nanowires prepared by the metal amplification method

We present a method of fabricating single crystal silver nanowires based on the electroless deposition of silver into the pores of the polycarbonate membranes by the metal amplification process. A gold film on one side of the nanoporous membrane is used as the initiation layer for the silver crystal growth, while the pores of the membrane are used for guiding the growth of the silver crystal into a cylindrical nanostructure. Optical microscopy and spectroscopy of individual nanowires, transmission electron microscopy (TEM), and TEM diffraction crystallography were used to characterize the silver nanostructures. The metal amplification technique presents an electroless, simple, and inexpensive solution to the challenge of fabricating silver nanowires for electronic, optical, and biological applications.

Electromagnetic micromotor for microfluidics applications

An electromagnetic micromotor has been developed that combines microcoils and magnetic microtips as stator elements and individual permanent magnet single domain particles as rotors. The three-phase stator microcoils (with poles separated by 100 μm) are positioned outside the fluid, and the rotor (a 40 μm long, 1 μm diam permanent magnet) spins in aqueous solution 50 μm from the stator. The small size and essentially disposable rotors make the magnetic micromotor attractive for use in microfluidics systems for various physical, chemical, and biological applications.

Scanning probe electromagnetic tweezers

We present a micromanipulation technique that utilizes integrated microcoils and magnetic microtips for localized positioning of micron-sized magnetic objects. Forces of 10 pN, and submicron positioning control are demonstrated on the 2.8 μm diameter superparamagnetic beads. The technique also implements an optical illumination scheme that provides a clear viewing of the magnetically trapped objects without including the scattering background from the magnetic manipulator tip. This simple instrument provides a noninvasive, low cost alternative to the optical trapping techniques normally used in micromanipulation. Among the possible advantages are the negligible heating of the manipulated sample, effective decoupling of the manipulation component of the experiment from the optical studies of the systems of interest, and the ability to perform studies in a variety of fluids.

Magnetic wires in MEMS and bio-medical applications

Magnetic wires of appropriate design have special features making them useful to micro-electromechanical systems and bio-medical applications. Several applications that exploit the properties of magnetic wires are reviewed including: (a) a magnetic micro-manipulation technique that utilizes integrated micro-coils and magnetic micro-wires for localized positioning of micron-sized magnetic objects, (b) integrated micro-coil/micro-wire system operating as a micro-fluidic micro-motor, (c) mechanical tweezers using magneto-static interaction between two magnetic micro-wires, and (d) ultra-high gradient magnetic separation system based on porous membranes partially filled with magnetic wires.

Recording processes in perpendicular patterned media using longitudinal magnetic recording heads

An experimental study of the recording processes in patterned magnetic media is presented. The reading of patterned media using spin-valve elements is compared to the signal levels from magneto-resistive sensors. Writing and reading of patterned columnar media at high areal densities is demonstrated. A new experimental technique has been developed that allows precise determination of the location of the write gap poles with respect to the patterned media column during the write process. Implications for patterned media write synchronization and the write head field requirements are discussed.

Magnetic resonance diffraction using the magnetic field from a ferromagnetic sphere

The theory of magnetic resonance diffraction is developed for the case of a crystal in close proximity of a ferromagnetic sphere. Distinct spectral peaks in the magnetic resonance signal are discovered for the specific ferromagnetic sphere and magnetic field configurations, and the appearance of the peaks is a direct signature of the presence of discrete atomic sites in the crystal lattice. The positions of the spectral peaks are sensitive to the crystal unit-cell size, thereby providing a method for determination of the basic parameters of the crystal at the atomic scale. The appearance of the spectral peaks is explained, and the dependence of the magnetic resonance spectra on the sphere size and the angle of the sphere magnetization with respect to the sample surface is analyzed. Applications to the studies of crystals, thin films, and crystallites are reviewed, and potential measurement methods for the confirmation of the diffraction theory are proposed. The analysis suggests that the long-desired goal of detecting atomic resolution magnetic resonance diffraction is well within reach of current experimental techniques.

Sensitive measurement of reversible parallel and transverse susceptibility by alternating gradient magnetometry

Reversible susceptibility tensor measurements reveal important information about the switching fields and anisotropies of magnetic materials. We show that a simple reconfiguration of an alternating gradient magnetometer can be used to measure both reversible parallel and transverse susceptibilities with high sensitivity. It is demonstrated that positioning the sample off axis with respect to the magnetometer gradient field coils results in a signal at twice the frequency of the gradient field that is directly proportional to the reversible susceptibility. Offsetting the sample along the x axis results in a sensor signal proportional to the reversible parallel susceptibility, while rotating the sample holder by 90° and offsetting it along the y axis results in a sensor signal proportional to the reversible transverse susceptibility. Examples of reversible parallel and transverse susceptibility measurements of aligned nanoparticle systems are demonstrated.

Perpendicular Patterned Media in an (Al0.9Ga0.1)2O3/GaAs Substrate for Magnetic Storage

By using electron beam lithography, chemically assisted ion beam etching, and electroplating, we have fabricated high aspect ratio magnetic columns, 60–170 nm in diameter, embedded in an aluminum–gallium–oxide/gallium–arsenide [(Al0.9Ga0.1)2O3/GaAs] substrate. In our previous work, we demonstrated storage of data in individual columns spaced 2 µm apart. Here the electroplated Ni columns are in the form of tracks (0.5 and 0.25 µm in the down-track direction, and 1 µm in the cross-track direction), corresponding to areal densities of 1.3 and 2.6 Gbits/in.2, respectively. In this report we describe in more detail the issues in the fabrication of patterned media samples, such as dry etching and oxidation of AlGaAs, and electrodeposition of Ni into GaAs substrate. Initial characterization of the resulting magnets using magnetic force microscopy are also presented.