Research Theme: Molecular Electronics and Plasmonics
Molecular electronics is concerned with the electronic, optical and mechanical properties of organic molecules sandwiched
between two metal electrodes. These investigations may enable sub-50 nm computer circuits, with the possibility of memories of
>1 Tbit/cm2 densities, ultra-low power computing and THz opto-electronic circuits. Molecules are especially
advantageous because of their structural variety, "bottom-up" assembly and atomic-scale uniformity. However, there is limited
understanding of the physical and electronic structure of molecules within metal-molecule-metal junctions, particularly larger
molecules with reactive functional groups that may undergo electronic or structural rearrangements in response to an applied
voltage.
One of the major challenges of molecular electronics is the difficulty in making reproducible electronic contact to the active molecular layer. We have recently demonstrated a technique to reproducibly fabricate large area molecular electronic junctions that do not suffer from many of the drawbacks of previous methods. The method involves using atomic layer deposition (ALD) to create a nanometer thick alumina layer on top of a self-assembled monolayer (SAM) to passivate defects and protect the molecules from subsequent processing.
Process flow for ALD passivated top contacts in metal-molecule-metal junctions. a) Monolayers of organic molecules with hydrophilic headgroups can be deposited on bottom metallic contacts through a variety of techniques. b) Atomic layer deposition of aluminum oxide on top of organic monolayer. c) Top metal electrodes can be deposited through electron-beam deposition or other conventional techniques without molecular damage that cause shorting.
We have also developed a highly robust and reproducible technique called polymer-assisted lift-off (PALO) process that allows for the transfer of a top metal contact onto molecular films without macroscopic distortion or damage. The key component is a hydrophobic polymer backing layer on preformed electrodes that provides mechanical stability and a thermodynamic driving force to eliminate wrinkling. Through this technique, high-quality metal-electrode devices can be fabricated in parallel over a wide range of electrode dimensions with lithographically defined spatial registry. Nonshorting molecular junctions were obtained for greater than 90% of the devices examined, with active areas ranging from 100 um2 to 9 mm2. The PALO method has many advantages including parallel device fabrication, nanoscale to centimeter device sizes, high-quality metal films, and non-damaging deposition.
Process for polymer-assisted, lift-off electrode deposition. a) Poly(methyl methacrylate) (PMMA) is spin-cast as a
hydrophobic backing layer onto metal electrodes patterned on a sacrificial substrate. b) After a brief KOH etch, the
metal–polymer layer cleanly lifts off onto the water surface upon immersion of the substrate. c) The PMMA–metal film floats on
the water surface without wrinkling owing to the surface tension of the water. d) A device is assembled by floating the
metal–polymer film onto a bottom substrate, often patterned with electrodes and/ or molecular layers. The polymer film can be
handled directly, allowing accurate placement and wrinkle-free deposition. e) A completed crossbar device. Metal electrodes
ranged from 10 lm to 3 mm wide, and 20 to 50 nm thick. f) Definition of surface energies during metal–polymer deposition.
Hydrophobic substrates and the top films result in wrinklefree lift-on due to expulsion of water.
In collaboration with Prof. Mark Brongersma's group at Stanford, we have used surface plasmon resonance spectroscopy (SPRS) to characterize the optical absorption spectra of nanometer-thick organic films and molecular monolayers sandwiched between two metal contacts. The electric field within metal-insulator (organic)-metal (MIM) cross-bar junctions created by surface plasmon-polaritons excited on the metal surface allows sensitive measurement of molecular optical properties. Specifically, this spectroscopic technique extracts the real and imaginary indices of the organic layer for each wavelength of interest. The SPRS sensitivity was calculated for several device architectures, metals, and layer thicknesses to optimize the organic film absorptivity measurements. Distinct optical absorption features were clearly observed for R6G layers as thin as a single molecular monolayer between two metal electrodes. This method also enables dynamic measurement of molecular conformation inside metallic junctions, as shown by following the optical switching of a thin spiropyran/polymer film upon exposure to UV light. Finally, optical and electrical measurements can be made simultaneously to study the effect of electrical bias and current on molecular conformation, which may have significant impact in areas such as molecular and organic electronics.
(a) Schematic of the Kretchmann configuration. Light is coupled into the surface plasmon-polariton (SPP) through an SF10 hemispherical prism. At a critical angle, the light is resonantly converted into SPP that evanescently decays into the metal and air layers. (b) Typical reflectance trace and model fit using the Fresnel equations. (c) Calculated electric field strength profile for surface plasmon modes of the metal-insulator-metal (MIM) structure. The MIM "slow" modes inside the organic layer are inaccessible coupling via the Kretchmann configuration because of the much higher wavevectors.
We have further optimized this technique using efficient optical coupling into metal-insulator-metal (MIM) plasmon modes. Subwavelength grating couplers are used to optically excite the MIM plasmon mode, which is observed with reflection spectroscopy. Coupling efficiencies of up to 28% are measured for insulator thicknesses of 12 nm. It is found that the MIM resonance has a significant shift in energy as a function of grating depth. This shift is much larger than that seen from traditional surface plasmon modes. MIM plasmons are promising tools for probing molecular junctions due to strong field confinement and high field intensities within the insulator.
Calculated reflectivity (R) spectrum for TM modes in a planar MIM structure. The solid black lines indicate light line with an incident angle of 32°, without a grating and with single and multiple scattering from a grating with Lambda=2pi/130 nm. The MIM geometry is 60 nm Au/12 nm Al2O3/20 nm Au and tabulated values were used for the frequency dielectric constants (Ref. 11). First order diffraction into plasmon modes occurs at points A and B, while second order coupling occurs at C and D. The inset shows a schematic of the simulated device and the field distribution for the MIM mode (surface modes are omitted for clarity).
Related Publications
Identification and Passivation of Defects in Self-Assembled Monolayers.
Michael J. Preiner and Nicholas A. Melosh. Langmuir. 25(5), 2585 (2009)
Interfacial effects in thin films of polymeric semiconductors.
Jonathan Rivnay, Leslie H. Jimison, Michael F. Toney, Michael Preiner, Nicholas A. Melosh and Alberto Salleo. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures. 26(4), 1454 (2008)
Creating Large Area Molecular Electronic Junctions Using
Atomic-Layer Deposition
Michael J. Preiner and Nicholas A. Melosh. Applied Physics Letters.
92, 213301 (2008)
A Nonvolatile Plasmonic Switch Employing
Photochromic Molecules.
Ragip A. Pala, Ken T. Shimizu, Nicholas A. Melosh, and Mark L. Brongersma. Nano
Letters. 8(5), 1506 (2008)
Efficient Optical Coupling into Metal-Insulator-Metal Plasmon Modes with
Subwavelength Diffracton Gratings.
Michael J. Preiner, Ken T. Shimizu, Justin S. White and Nicholas A. Melosh.
Applied Physics Letters. 92, 113109 (2008) PDF reprint.
Ken T. Shimizu, Ragip A. Pala, Jason D. Fabbri, Mark L. Brongersma, and Nicholas A. Melosh. Nano Letters 6, 2797 (2006)
Soft Deposition of Large-Area Metal Contacts for
Molecular Electronics.
Ken T. Shimizu, Jason D. Fabbri, Jim J. Jelincic and Nicholas A. Melosh. Advanced Materials.
18, 1499-1504 (2006).