Industrial Outreach Programs: Holiday and Museum Lectures, Project Teach, and Undergraduate Advising
H.A. Stone (DEAS)

The IOP Workshop is directed by Associate Dean Fawwaz Habbal and is aimed at strengthening external collaborations by facilitating mutually beneficial relationships between outside groups and DEAS interdisciplinary research groups.

CMOS / Microfluidic Chip for the Manipulation of Biological Systems
Donhee Ham and R.M. Westervelt

By combining the sophistication of CMOS integrated circuits with the biocompatibility of microfluidic systems, powerful new systems can be created to manipulate biological systems. To construct the chip above, a microfluidic system was fabricated at Harvard on top of a custom CMOS integrated circuit made in a foundry. The chip contains a microcoil array with current and control circuits. By energizing microcoils, the chip can trap and move magnetic beads and attached cells through the fluid above, to perform cell sorting or to assemble artificial tissues.

CMOS / Microfluidic Chip for Tissue Assembly
Kit Parker, Donhee Ham, and R.M. Westervelt

A difficulty in the use of engineered tissues in the treatment of traumatic injury is the inability to make the time needed for surgical reconstruction of the injured tissue (hours) comparable to that needed for in vitro assembly of engineered replacement tissues (weeks).

Our new technology to address this problem consists of a custom-designed microfluidic chamber built on top of a CMOS chip (Fig. 1). The CMOS chip contains an array of microcoils that creates spatially patterned magnetic fields in inside the microfluidic system above. These magnetic fields interact with ligand-coated magnetic beads that are bound to the membrane receptors on cells salvaged, and sorted, from the damaged tissues. Using the chip, these cells can be rapidly moved by synchronized current flow in the CMOS microcoil array (Figure 2). This synchrony allows the cells to be assembed on, extracellular matrix targets created by microcontact printing.

Cell Assembly by CMOS/Microfluidic Chip
Kit Parker, Donhee Ham, and R.M. Westervelt

Schematic diagrams illustrating rapid 2D tissue assembly from multiple, salvaged, cell types. A) Cell type A is directed to designated locations on an ECM-micropatterned surface within the microfluidic chamber; B) The homogeneous tissue is complete, with type A cells adhering to, and spreading on, the micropatterned surface. Many cell types will spontaneously form cell-cell junctions when in contact adjacent cells; C) Cell type B is directed into previously designated locations within the tissue completed in B; D) The engineered tissue is completed as these cell type B adheres to the micrppatterned surface, spread, and form cell-cell junctions amongst themselves and with cell type A.

Controlled Motion of Magneto-Elastic Filaments
J. Bibette (Paris) and H.A. Stone (DEAS)

Spherical paramagnetic particles, with diameters from tens to hundreds of nanometers, can be linked together to form a chain, which can be aligned with and controlled by, a magnetic field. In the experimental images on the left the chain begins perpendicular to the horizontal magnetic field and then deforms and unfolds progressively until it eventually is aligned with the field. Intermediate shapes of the filament include rather long-lived hairpin structures. Numerical simulations shown on the right capture the hairpin shapes (top) and the time-dependent evolution from the hairpin shape (bottom) to the final aligned equilibrium orientation (the details of the evolution depend on several dimensionless parameters). The present state of comparison of experiments and simulations is qualitative and on-going studies are aimed at providing a quantitative comparison. These magnetically controlled filaments offer new possibilities for transport and manipulation on the nano- and micro-scales.

Artificial Flagella from Magneto-Elastic Filaments
J. Bibette (Paris) and H.A. Stone (DEAS)

Spherical paramagnetic particles, with diameters from tens to hundreds of nanometers, can be linked together to form a chain, which can be aligned with a uniform magnetic field, and further manipulated to swim using a time-varying orthogonal magnetic field. Attaching the chain to a cell thus enables swimming of the cell as shown in the series of images at the top. Numerical simulations, shown in the second figure above, allow detailed exploration of the swimming behavior and identification of optimum swimming conditions.

Synthesis of TaS2 Nanobelts
Hongkun Park

Left: High resolution TEM image of a represent-ative TaS2 nanobelt, with selected area electron diffraction pattern from the same nanobelt as inset. Right: Layered crystal structure of TaS2.

Magnetic and Fluorescent Silica Nanospheres
M. Bawendi and R.M. Westervelt

Fluorescent (CdSe/CdZnS) and magnetic (Fe2O3) nanocrystals have been incorporated into silica core/shell nanospheres, imaged using fluorescence microscopy, and manipulated using magnetic field gradients generated by passing current through specified wires within an array. These fluorescent/magnetic composite demonstrate the ability to create multifunctional nanospheres with potential applications in the magnetically driven assembly of nanostructures.

Exchange Biasing in Co/CoO Ferromagnetic / Antiferromagnetic Core/Shell Nanoparticles
M. Bawendi

Magnetic nanoparticles have potential as the magnetic medium for high density information storage. The observation of anomolous magnetic behavior in Co nanoparticles at low temperature led us to design Co/CoO core/shell nanoparticles. We found that the magnetic behavior of these particles at low temperature is dominated by paramagnetic and superparamagnetic defects within the CoO shell and at the interface between the core and the shell.

Skin-Depth Effect on the Casimir Force
Mariangela Lisanti, Davide Iannuzzi, and Federico Capasso

According to the theory of Quantum Electrodynamics, vacuum is filled with virtual photons that give rise to fluctuations of the electric and magnetic fields. These particles are responsible for the Casimir force, i.e. the attraction between two electrically neutral metallic surfaces. Studying the Casimir force between a metallized micromachined torsional device and a dielectric sphere coated with a thin metallic layer, we were able to demonstrate that, if the metallic layer is thin enough (~ 10 nm), the Casimir force is smaller compared to the case of a thick (250 nm), bulk-like metallic film. This is due to the finite penetration depth of the electromagnetic waves in a metal (skin depth), which leads to enhanced transparency in films.

Frictionless Bearing for the Measurement of the Quantum Electrodynamical Torque
Davide Iannuzzi, Jeremy N. Munday, Y. Barash, and Federico Capasso

We have calculated that quantum fluctuations of the electromagnetic field should induce on a micromachined calcite (or quartz) disk kept parallel to a barium titanate plate, a torque of ~10-19 Nm. This torque should be measurable by immersing the two slabs in liquid ethanol. In this case, the Casimir force between them is known from theory to be repulsive. The disk should float on top of the plate at a distance where its weight is counterbalanced by the Casimir force. The static friction between the slabs should be virtually zero, and the disk should be free to rotate suspended in close proximity (~100 nm) with the plate. In the experiment a polarized laser beam is used to impart a torque to the disk and is then blocked; the disk under the action of the QED torque will return to a configuration of optical axis aligned to that of the plate, corresponding to zero torque. The inset shows the calculated angular position as a function of time after the laser has been blocked. Note that calcite and quartz experience torques of opposite sign.

Mirrors Switch, the Force Does Not
Davide Iannuzzi, Mariangela Lisanti, and Federico Capasso

The Casimir force is the attraction between two uncharged surfaces arising from quantum fluctuations of the electromagnetic field. We have measured the Casimir force using Hydrogen Switchable Mirrors (HSM), shiny metals that become transparent when exposed to hydrogen. On intuitive grounds, the force in air should be larger than that in hydrogen due to the reduced reflectivity of the mirror. On the contrary, we did not observe any change upon hydrogenation. This counterintuitive finding can be explained by an elusive property of the theory (D. Iannuzzi, M. Lisanti, and F. Capasso, Proc. Nat. Ac. Sci. USA 101, 419 (2004))

Silica Nanowires
Liming Tong, Rafael R. Gattas, and Eric Mazur

To use silica nanometer-diameter wires as building blocks for integrated microphotonics devices, the wires must be mounted on a substrate without losing their optical wave guiding properties. We address this problem by using silica aerogel as a substrate. Because the refractive index of an aerogel is very close to that of air, the optical guiding properties of aerogel-supported wires are virtually identical to those of air-clad ones. (a) is a scanning electron image of a 450-nm diameter silica wire supported on a substrate of silica aerogel. (b) is an optical microscopy image of a silica wire guiding light on the surface of silica aerogel.

Imaging Electron Flow
M. Topinka, R.M. Westervelt, and E.J. Heller

Cover of Physics Today, Dec. 2003 - simulation of the flow of electron waves by E.J. Heller. This issue contains a review Imaging Electron Flow by Topinka, Heller and Westervelt about images of two-dimensional electron gases, including their collaborative research.

Laboratory for Integrated Science
Images of the Quantum Hall Liquid
Ray Ashoori

(a) An SPM charging image of a 2DEG in the quantum Hall regime at filling factor í = 0.89. The image shows bright arcs on a dark background over the 10x10m² field of view. (b) At this magnetic field, the positive tip bias induces a bubble of electrons in the next Landau level. A ring of incompressible states at í = 1 separates the interior of the bubble from the bulk. (c) Arcs arise in the image when the ring intersects a "hotspot" in the 2DEG, causing its resistance to drop.

Image of a Single Electron Quantum Dot
R.M. Westervelt, E.J. Heller, and A.C. Gossard

Image of a GaAs quantum dot that contains just one electron, taken with a liquid-He cooled scanning probe microscope. The color scale shows the change in dot conductance in the Coulomb blockade regime as the SPM tip scanned above. When the charged tip moves inside the ring, it pushes the last electron off the dot.

Focal Point "Cusp" Seen in Electron Imaging Data
Eric J. Heller

Simulations undertaken to understand the branched flow discovered experimentally in semiconductor two degree of freedom electron gases have shown that electrons flowing through them are subject to focusing by "lenses" which are accumulations of relative positive charge, to which they are attracted. Simulations based on ray tracing the electrons through such devices show focal cusps; these showed up in the experimental data as well. The images below show the theory and the experiment revealing a focal cusp on the submicron scale:

On the right we see a focal cusp as determined by ray tracing in a simulation of electron paths in a two degree of freedom electron gas. Darker regions are where more electrons have traveled. On the left is experimental data with an arrow pointing to the focus of a cusp seen in an electron imaging experiment. Quantum fringing is also seen, and a theoretical wave simulation of what that fringing should look like is given in the upper left as a yellow contour map. The match is very good, showing that an accumulation of relative positive charge must exist in the device in the right hand region of the panel, causing the electron focusing.

Connections discovered between electron flow in a 2DEG and freak waves in the ocean
Eric J. Heller

We have discovered that applying microwave light to the transistor can restore the Kondo effect for a quantum dot with a voltage applied between the leads. Figure A shows the conductance as a function of the microwave voltage (horizontal scale) applied to a quantum dot transistor. The bright red peak for zero microwaves corresponds to the weak bond, and increasing the voltage between the leads (vertical scale) destroys it. However, at increasing microwave voltages two new peaks appear. Figure B shows that the voltage difference between the central peak and one of the new ones is hf/e, where f is the microwave frequency. This means that microwave photons restore the Kondo bond when the photon energy hf equals the electrical energy eV.

Figure showing wave energy focussing (left) due to current eddies (second from left, in color), locations of dangerous extreme wave events in a long simulation shown in red in the next panel, and on the right the location of less devastating but still dangerous freak waves events in the same simulation. Far right: view from the bridge of a ship of an approaching freak wave.

Kondo Effect
Marc A. Kastner

We have discovered that applying microwave light to the transistor can restore the Kondo effect for a quantum dot with a voltage applied between the leads. Figure A shows the conductance as a function of the microwave voltage (horizontal scale) applied to a quantum dot transistor. The bright red peak for zero microwaves corresponds to the weak bond, and increasing the voltage between the leads (vertical scale) destroys it. However, at increasing microwave voltages two new peaks appear. Figure B shows that the voltage difference between the central peak and one of the new ones is hf/e, where f is the microwave frequency. This means that microwave photons restore the Kondo bond when the photon energy hf equals the electrical energy eV.

Effects of Impurities on the Spin Hall Effect
Bertrand I. Halperin

In the presence of "spin-orbit coupling," an electric field E_x in the plane of a two-dimensional electron gas (2DEG) can lead to a "spin current" j ^Z/_y where electrons have a velocity in the y direction with opposite sign for spins in the +z and -z directions. Sinova et al. predicted that in the absence of impurity scattering, the "spin Hall conductivity" -j ^Z_y /E_x should take on a universal value e/8p. We have shown that in the simplest case, an arbitrarily small concentration of impurities will cause the spin Hall conductivity to vanish far from the boundaries of the 2DEG. However, an electrical current can induce spin currents close to the contacts, and produce spin polarization, as illustrated in the figure.

Dynamic Behavior of Au Surfaces
Cynthia M. Friend

The Au-S interaction is very important in several technologies such as Au ore formation and controls the structure and stability self-assembled-monolayers (SAMs) used in soft lithography to write small features on a surface. We have demonstrated that the Au(111) surface is very dynamic when it interacts with sulfur. Sulfur induces lateral expansion of the surface which ultimately leads to ejection of Au from the surface and incorporation of Au atoms into a growing two-dimensional AuS phase. These results suggest that the smallest size scale and the definition of features made with soft lithography, which relies on S binding to Au, will be limited by the dynamic behavior of Au. Dynamics of the formation of a 2D AuS overlayer captured by real-time imaging using scanning tunnelling microscopy during continuous SO2 exposure at room temperature to make a Au-S layer. (A) Au(111) partly covered with sulfur (B) Early stage of S-induced corrosion/etching of the Au(111) surface. Dark areas (emphasized in the close-up) correspond to pits in the surface that are one atom deep. Simultaneously small AuS clusters appear on flat regions of the surface. (C) The number of both pits and AuS clusters increases as the amount of S on the surface increases. (D) The reaction is completed once the surface is covered with a sponge-like gold sulfide overlayer.

Raman Spectroscopy of Carbon Nanotube Sensors
Michael Tinkham

Raman spectroscopy is used for the first time to measure individual carbon nanotubes (SWNTs), strained by manipulation with an atomic force microscope (AFM) tip. Metallic nanotubes are found to shift on and off resonance with strain, implying a strain-induced shifting of the electronic band energies of the nanotube. We find that individual nanotubes are far more sensitive to strain than their bulk counterpart (graphite). We can observe changes in the Raman spectra for strains as small as 0.1%, which corresponds to a displacement of ~10 nm. The extreme sensitivity of nanotubes to strain and other perturbations make them suitable for many potential applications.

Improved Room Temperature Metallic Spin Valves
Sergio O. Valenzuela and M. Tinkham

Spintronics devices, which simultaneously exploit the spin and charge of the electron, are being explored due to promising new electronic applications. Our devices consist of an aluminum strip contacted with two different ferromagnets (FM) with different coercive fields, as shown in the SEM image. Tunnel barriers between the FMs and the Al are grown in situ using a stencil mask and angle evaporation techniques. The Al is 100 nm wide while the FMs are 50nm wide and 150 nm apart, dimensions well below the spin relaxation length of ~300 nm. The measurements show the spin-valve effect at room temperature for positive and negative H-field sweep directions, with ~100 times larger signal than in previous work, largely because of our smaller size.

A computer image of the Laboratory for Integrated Science and Engineering (LISE) that will house shared facilities for Harvard's Center for Imaging and Mesoscale Structures and NSEC, and will provide space for interdisciplinary research. LISE will contain an Imaging Laboratory for electron, scanning probe and optical microscopy, a cleanroom for nanofabrication and soft lithography, and an Advanced Materials Science Laboratory.

National Nanotechnology Infrastructure Network
Harvard University and UC Santa Barbara

Harvard and UC Santa Barbara are two of an integrated partnership of thirteen user facilities, led by Cornell and Stanford, that provide opportunities for nanoscience and nanotechnology research. At Harvard, the NNIN provides expertise in soft lithography and assembly, and computation through the Center for Imaging and Mesoscale Structures. At UCSB, the NNIN provides expertise in optics and electronic materials. The NNIN was funded by the NSF in January 2004.