# Presentation: 942061

## Plasmon Spectroscopy of Molecular Junctions

**Prof. Nick Melosh, Stanford University**

**DMR-0449385**

-  We have developed a new technique, ‘Surface plasmon spectroscopy**’** (SPS) to probe the optical absorption of molecular layers between metal electrodes. This method enables us to ‘see’ inside a 2-3 nm thick molecular film between two opaque metal electrodes. The optical absorption is measured from Fresnel fitting of reflectivity vs incident angle scans, similar to a surface plasmon resonance experiment.
-  Surface plasmons are evanescent electric fields arising from electron density oscillations on the surface of a metal. The wavevector necessary to excite surface plasmons is very sensitive to the optical properties at the metal surface, allowing accurate measurements of the index of refraction even for organic monolayer films on top of the metal.
-  We found the absorption of molecules on a single metal surface is almost identical to solution phase, however when placed between electrodes significant absorption maxima shifts (15-50 nm) are observed. We believe these shifts are due  to Stark effects within the junctions, and need to be included in electron transport calculations.
-  These measurements are a significant step towards understanding the switching mechanisms and electronic structure of metal-molecule-metal junctions. Further experiments will elucidate the optical behavior during voltage cycling, allowing correlation of electronic and optical characteristics.

- SPS absorption

- Absorption
- on glass

- Figure 1: Schematic of the (a) surface plasmon spectroscopy set-up, (b) plasmon excitations and (c) reflectivity curves. Surface plasmons arise from electron density oscillations in a metal (b). To excite these oscillations, the wavevector of the incident light, ksin, must match the surface plasmon wavevector. At the correct coupling angle, a dip in the sample reflectivity is seen (c), since some of the incident light converts into surface plasmons. Fitting reflectivity vs angle scans to a Fresnel model provides the optical constants of the molecular junction.

- Figure 2: The absorption spectra of a thin film of Rhodamine 6G either (a) on a 20 nm thick Au film, or (b) between two 20 nm-thick Au electrodes. The peak maximum shifted 15 nm when placed between electrodes, which corresponds to roughly a 60 meV change in HOMO-LUMO gap. These effects may be important for molecular electronic devices.

- 20 nm Au

- R6G film

- 20 nm Au

- R6G film

- 20 nm Au

- (a)

- (b)

- (b)

- Reflectivity

- Angle (deg)

- plasmon coupling

- _wavelength_

- (a)

- (c)

**Notes:**
Motivation: Molecular and nanoscale electronics is an exciting new area at the crossroads of quantum mechanics, conventional electronics, surface chemistry and molecular structure. Devices just a few nanometers thick using non-conventional materials like organic molecules can exhibit impressive properties like negative differential resistance (NDR) and conductance switching, yet the physical explanations for these effects are difficult to identify. Typically these devices are made by sandwiching a few molecular monolayers between metal or silicon contacts, and measuring the current as a function of voltage. Unfortunately, it has not been possible to obtain additional information about the molecules during voltage cycling, such as optical absorption or vibrational spectra due to the presence of the metal contacts. With the physical mechanism for switching and NDR effects hotly in debate, there is an urgent need for quantitative information about the molecular configuration and electronic structure within metal-organic-metal (MOM) junctions. To this end, our NSF-funded research has developed and demonstrated a new technique to probe the optical absorption spectra of ultra-thin molecular layers within MOM junctions using surface plasmons to locally enhance the field intensity near the electrode surface.  Aim of the Project: Our goal is to develop a new technique, surface plasmon spectroscopy (SPS), to analyze the optical absorption of monolayer-thick metal-molecule-metal junctions and correlate the absorption behavior with electrical I-V behavior. Through these studies, we hope to understand the molecular features necessary to create reproducible electronic switches, and guide future synthetic efforts with quantitative information about the behavior of molecular species between electrodes.
Research Accomplishments: We have successfully been able to measure the optical absorption features of molecular monolayers between metallic electrodes, a key step to understanding the behavior of organic species for electronic applications. To achieve the extreme sensitivity needed to measure the absorption of a single monolayer of molecules, we turned to another emerging research area: plasmonics. Surface plasmons are electron density oscillations on a metal surface, which in turn creates an evanescent electromagnetic field (ie light) that propagates along the surface. Excitation of surface plasmons requires matching the wavevector of the incident light to the surface plasmon wavevector, which depends sensitively on the refractive index of the metal-dielectric interfaces.  In the Kretschmann configuration, the incident wavevector is varied by changing the incident angle of light striking a prism with the metal sample on top. By accurately measuring the amount of incident  light that couples into surface plasmons as a function of incident angle, the optical properties of even a monolayer of material on top of the metal surface can be detected. We constructed a surface plasmon resonance apparatus that can record the reflectance from a metallic sample as a function of incident angle (much as a conventional surface plasmon resonance (SPR) apparatus), coupled with a white light source and monochrometer to measure wavelength-dependent properties. These reflectivity curves (as shown in figure 1), contain information about the real and imaginary components of the refractive index for the molecular layer based upon the depth, width, and location of the reflection minima. We have written a software analysis routine based upon a n-layer matrix reflectivity scheme (W. Hansen, 1968) to fit the shape of the entire reflectivity curve, which provides better signal to noise ratios and lower error than fitting only the position of the minima. The results of these fits as a function of wavelength give the absorption spectra of the molecular layer, as shown in figure 2. We discovered that it makes a significant difference whether the molecular layer is just on top of a metal, or whether it is sandwiched between two layers. In the latter case we have observed repeatable absorption shifts of 15-50 nm, or 60-500 meV, which we believe are due to Stark shifts. Further research will determine the mechanism for these shifts. If these are due to Stark effects, this technique may be a new and sensitive way to measure the stark effect in oriented molecular systems.
Significance: Presently there are only various forms of current-voltage measurements to characterize molecular monolayer junctions, which leads to a large degree of ambiguity in explaining conductance switching events and the role of molecular configuration in the junction. Our measurements are the first to obtain optical absorption of a molecular monolayer between metal electrodes, and have highlighted that there is an appreciable difference between the electronic states of molecules merely on top of a metal versus within a metal junction. We hypothesize these differences are due to Stark shifts from an electric field that builds within the junction, as the molecules we have tested are not chemically or physically bonded to the metal. Further experiments with thiol-bonded molecules will elucidate the energy band-line up of the molecules within the junction as well as molecular reconfiguration due to direct interaction with the metal. This technique opens a whole new why to investigate molecular behavior at interfaces.

## Polymer Assisted Lift-Off Float-On Electrodes (PA-LOFO)

- Figure 3: Sequence to deposit top metal contacts using the PA-LOFO technique. (1) Patterned metal electrodes (as thin as 10 nm) are deposited onto a Si or mica substrate and coated with a polymer backing layer. This film is dipped into a dilute KOH solution to weaken the polymer-substrate adhesion. (2) the sample is transferred into a water bath, where the polymer and metal peel away from the substrate. (3) The hydrophobic polymer causes the electrodes to float on top of the water. (4) The bottom electrodes with the molecular film is dipped beneath the floating polymer/metal film, and lifted into contact with the floating electrodes. (5) UV/ozone or O2 plasma can be used to remove the polymer layer, leaving the completed molecular crossbar device.

- Figure 4: (a) Float-on electrode transferred onto a glass substrate from a water bath. Each electrode is 2 mm wide and 2 cm long. (b) Optical microscope image of an  evaporated Al bottom electrode with a fatty acid LB film with a 20 nm thick PA-LOFO Au electrode deposited on top. Note the lack or wrinkles or asperities of the Au.

**(a)**

**(b)**

**(1)**

**(2)**

**(3)**

**(4)**

**(5)**

-  We developed a new way to softly deposit the top metal contacts onto an organic film. Depositing the top metal contact for molecular electronics is always difficult, especially to achieve near-atomically flat films. Sputter or e-beam deposited metals always damage the organic sample to some extent, while other ‘soft’ methods like lift-off float-on electrodes wrinkled excessively and stamped contacts were always very rough.
-  We used a thin polymer as a backing to support evaporated metal electrodes, which gave them mechanical strength and hydrophobicity.
-  These contacts had the same optical and electrical properties as directly evaporated metal on a smooth substrate, thus the PA-LOFO procedure did not appear to harm the metal in any way, such as cracking or buckling.
-  Electrode arrays with wire widths as small as 1 micron to as large as 3 mm and up to 3 cm in length could be fabricated without wrinkling.
-  Studies of electron tunneling through carboxy alkanes with a PA-LOFO top contact show the same tunneling behavior as literature STM and nanopore measurements. Even with 4 mm2 contact areas, no shorting is observed.

- Figure 5: (a) Schematic of a metal-molecule-metal junction with a Langmuir-Blodgett fatty acid films and a PA-LOFO top electrode. (b) Tunneling current though the molecular monolayer. A fit to a Simmons tunneling model gives the decay constant  = 0.85 Å-1, similar to literature reports, indicating no shorting and good metal electrodes.

**(a)**

**(b)**

**Notes:**
Aim of the Project:
	The approach of this project was to develop a method to create large-area, pin-hole free metal electrodes to contact thin organic films. Direct evaporation on organic or molecular films almost always leads to electrical shorts, thus a new procedure was needed. 
Research Results:
	We developed a new way to softly deposit top metal contacts onto an organic film. Depositing the top metal contact for molecular electronics is always difficult, especially to achieve near-atomically flat films. Sputter or e-beam deposited metals always damage the organic sample to some extent, while other ‘soft’ methods like lift-off float-on electrodes wrinkled excessively and stamped contacts were always very rough. To solve this issue, we used a thin polymer backing to support the a flat evaporated metal film, which gave it mechanical strength and hydrophobicity. This film could be peeled away from its initial substrate and transferred to an organic device as the top contact. These contacts had the same optical and electrical properties as directly evaporated metal on a smooth substrate, thus the lift-off float-on procedure did not appear to harm the metal in any way, such as cracking or buckling. 
	Electrical measurements with these junctions showed the same tunneling behavior as other measurement techniques (STM, Hg electrodes, nanopores, crossed wires), leading us to believe they are forming high-quality interfaces with the molecular layer, even over very large contact areas. Since optical measurements such as plasmon spectroscopy are very sensitive to surface roughness and wrinkles in the metal film, this technique is a break-through as it provides a large scale, smooth and non-shorting electrodes with which to investigate both optical and electrical properties of the device simultaneously.
Significance:
	These electrodes will allow researchers to easily create metal-molecule-metal junctions without shorts in order to study the basic electrical properties of the molecules. In addition, this technique requires very little in terms of equipment, such that labs without extensive fabrication equipment or even demonstrations in schools could use this method. Thus it is enabling technology for a wide range of users, and is of particular interest to the molecular electronics/optics community because of the large area and near-atomic flatness of the electrodes. 

## Molecular Electronics: Rotaxane Switching

- Remnant Current Measurements

- Figure 6. (a) UV-vis absorption spectra of [2]-rotaxane molecules in CHCl3 solution with the cyclophane ring either on the TTF group (green line), or the napthalene group (red line). Adapted from Ballardini et al, *New J. Chem*. 2001 (b) Surface plasmon spectrum of Al-Au junction (white) and Al- rotaxane-Au junction (green). This rotaxane exhibited mixed conformations (red and green from (a)) in solution, which is also observed in the solid state device.

- Figure 7. Remnant current measurement of a [2]-rotaxane Langmuir Blodgett monolayer between Al and Au electrodes. We observed voltage-induced switching very similar to that of Luo, *ChemPhysChem*, 2002. The red trace is the remnant current measured at a constant voltage of 100 mV, and the black trace is the applied ‘write’ voltage. Repeated cycling (b) shows an initial decay in remnant current, after which the switching magnitude stabilizes.

-  Rotaxanes exhibit two different absorption spectra in solution depending on where the cyclophane ring is located. When the ring is on the napthalene group, the rotaxane is a red color, while on the tetrathiafulvalene (TTF) group it is green. In solution, this particular rotaxane was a mixture both red and green conformations.
-  Using our surface plasmon spectroscopy (SPS)  technique, we have been able for the first time to directly measure the absorption spectrum of a rotaxane monolayer between two metal electrodes (Figure 6b). We find that just as in solution, the rotaxane monolayer between electrodes has a distribution between both red and green conformations.
-  We have been able to reproduce the rotaxane monolayer conductance switching others have reported using our PA-LOFO technique to create large area electrodes (4 mm2) without electrical shorting. The remnant current switching behavior was reproducible and could be cycled over 10 times (figure 7)
-  Upcoming research will determine whether the “high” and “low” conductance state correlate to a particular location of the cyclophane ring as determined by SPS.

**O**ne of the most controversial molecular electronic systems is the conductance switching behavior of [2]-rotaxanes within a molecular junction. Solution-phase studies have shown that the cyclophane ring encircling the rotaxane may be shuttled back and forth between two electron rich moieties along the dumbbell-like core, however whether this behavior persists when the molecules are placed between metal electrodes is unknown. In particular, the question is whether or not changes in the position of the molecular cyclophane ring are correlated with the conductance switching observed in the remnant current-voltage scans.

- TTF group

- napthalene

- cyclophane ring

- “high” state

- “low” state

**Notes:**
Aim of the Project:
	One of the most controversial molecular electronic systems is the conductance switching behavior of [2]-rotaxanes within a molecular junction. Solution-phase studies have shown that the cyclophane ring encircling the rotaxane may be shuttled back and forth between two electron rich moieties along the dumbbell-like core, however whether this behavior persists when the molecules are placed between metal electrodes is subject to debate. In particular, the question is whether or not changes in the position of the molecular cyclophane ring are correlated with the conductance switching observed in the remnant current-voltage scans. In solution phase, the rotaxane has two distinct absorption spectra depending on where the cyclophane ring is located and the oxidation state of the tetrathiafulvalene (TTF) group. Our goal is to use the surface plasmon spectroscopy (SPS) technique we have developed to determine the location of the cyclophane ring for a rotaxane monolayer between two metal electrodes. We will then electrically cycle the monolayer to achieve conductance switching and determine whether some fraction of the cyclophane rings concurrently change position with the conductance change. We needed to achieve two critical milestones in order to make these measurements: (1) Develop a way to make electrically switching samples large enough to use with the SPS method, and (2) Demonstrate sufficient sensitivity to measure the absorption spectra of a single rotaxane monolayer. Here we show we have achieved these intermediate steps towards understanding rotaxane switching.
Research Results:
	The mechanism for switching behavior in [2]-rotaxane monolayer devices is hotly debated, with ideas ranging from binary position electrochemical motion of the cyclophane ring to metal filament formation. We have made significant headway towards understanding these devices. First, we have been able to replicate the electrical switching behavior reported by Luo et al. in [2]-rotaxane monolayers using our PA-LOFO electrodes (figure 7). The reproducible conductance switching observed gives us confidence we are measuring the same phenomenon as reported in the literature. These large-area devices allow us to probe the optical absorption of the metal-rotaxane-metal junction using our surface plasmon spectrometer, as shown in Figure 6. We observe that just as in solution, the cyclophane ring for rotaxanes between metal eelctrodes is located at both the red and green sites when initially deposited. We are now in the process of measuring the absorption spectrum after applied voltage. Control samples without the rotaxane monolayer showed flat absorption in the visible, as seen from the white symbols in Figure 6. This is the first time anyone has been able to make a measurement of the actual conformation of [2]-rotaxanes within a metal-molecule-metal junction. Further experiments are underway to correlate the electrical switching with the optical absorption, which will provide the first concrete evidence for or against ring motion as the mechanism for electrical conductivity changes.
Significance:	
	This work is an important first step to understanding whether the conductance switching of [2]-rotaxanes is due to molecular reconfiguration, or another, non-molecular based mechanism. It is also an important milestone towards understanding how molecular structure changes and reacts to electric fields within molecular junctions, and how new molecules may be engineered for new molecular electronic functions.

## Teaching Others about Science

*Teaching Others about Science*

- Over the past year, the Melosh group has hosted three undergraduates (Adam Freund, Jim Jelincic, and Vimalier Reyes-Ortiz) and one high school student (Dan Vinson, from Palo Alto High) to order to give younger students access to the excitement of real hands-on research. One student, Vimalier, came from Puerto Rico for the summer to see what working in a large laboratory was like. After the summer research, he defended his work in a public presentation, and has since decided to go to graduate school and to seek a position in the high-tech industry.

- In addition, in June 2005 Dr. Melosh visited Homestead High, a local under-privileged high school, on several occasions** **in an NSF-supported effort to invigorate young people with an interest in science. In co-ordination with local science teacher Jillian Fackrell, these visits sought not just to excite students about our work and materials science, but encourage an interest in a scientific career by presenting work and anecdotes from a wide variety of researchers. Students got to test the modulus of crystalline metals vs metallic glasses, demonstrate the principle of ‘sacrificial bonds’ within living systems, and discover the amazing phenomena of shape-memory alloys. These visual and hands-on demonstrations gave students a concrete idea of what types of exciting careers might spring from a degree in science. At the end of the school year, more than a quarter of the students mentioned that they had really liked the opportunity to see ‘real science’ first hand. Future visits over the coming year will feature demonstrations integrated into their curriculum throughout the school year, and a chance to intern in our labs over the summer.