Properties of Molecules
- Porphyrin-Metal Interface Properties
- Resonant Tunneling Simulator
- Defect Mediated Transport
- Preparation routes of self-assembled organic thin films
- Electronic Transport in Porphyrin Supermolecule-Gold Nanoparticle Assemblies
- Direct Probe of Moleculler Polarization in de novo Protein-based Devise
Deposition of a nominal monolayer of TET – H2 – TET porphyrin on HOPG results in islands with 2 step heights: 0.6nm and 1.55 nm, Fig. a. Layer thicknesses imply that in the islands with ~ 0.7 nm height the plane of the porphyrin ring is perpendicular to the substrate, while in the islands with ~ 1.6 nm height the ring is parallel to the substrate.
Both layers (A and B) assemble into laterally ordered structures. Fig. d and e compare molecular resolution nc-AFM contrast of both layers. Within the layer A the porphyrin ring is oriented perpendicular to the substrate, superposition of the molecular structure oriented to match the lattice dimensions and the topographic contrast is shown in d. The structure on island B is determined in similar fashion. Based on the lattice parameters of the monolayer and the fact that the porphyrin ring is parallel to the substrate from c, the arrangement of porphyrin molecules in e is proposed. The phenyl rings are situated at the highest contrast since the alkane chains extend above them.
In order to relate these structures to interface properties, KFM was performed consecutively to nc-AFM, Fig. b. Variations in surface potential are correlated with the locations of the porphyrin monolayers. The difference between the potential of step A and the graphite substrate was below the energy resolution, i.e. <5 mV. The potential on island A, Vstep A = 0.64 V, and island B, Vstep B = 0.59 V, differs by 50mV.
The difference in work function with orientation reflects a difference in the coupling between the molecule and the substrate. A graphite surface presents a satisfied p-orbital which is not chemically reactive. The fact that the porphyrin oriented perpendicular to the substrate does not alter the work function implies the absence of reaction with the surface. In this case self assembly would be dominated by van der Walls interactions. The decrease in work function that occurs when the molecule is oriented parallel is indicative of a substrate-molecule interaction. The orientation of the p-systems in the graphite and molecule in this configuration might be amenable to overlap, but the distance above the surface (0.8 nm) makes direct interaction unlikely. A time varying induced dipole across this separation would result in a work function difference.
As computer chips approach the nanoscale, current techniques in chip design will become inadequate due to fundamental physical reasons. More specifically, quantum physical properties become significant at the nanoscale and foil traditional device designs. However, a solution may lie in the field of nanoelectronics. By taking advantage of quantum physical properties instead of ignoring them at the nanoscale, it may be possible to circumvent the limitations of traditional chip designs. This Java 2 applet simulates the quantum mechanical effect of particle tunneling across two potential barriers: a fundamental problem related to the issue at hand.
Authors: Anand Jagota and Edward Peng
Uses code from Dean Zollman (Kansas State University)
Uses the Complex and SFun classes from Visual Numerics
Recent progress in nanoelectronic device fabrication necessitates measurements of transport properties on the nanometer scale. Defects in particular will play a dominant role in the behavior of molecular and nano devices. Here Scanning Probe Microscopy based techniques quantify dc and ac transport behavior in molecular electronic devices and characterize individual defects.
- Elucidate fundamental chemical and physical adsorbate-substrate interactions
- Isolate individual molecules/biomolecules for photo-induced charge transport measurements
|Figure from Shreiber, F. Prog. Surf. Sci. 65 (2000), 151-256.
Other key references: Ulman, A., Chem. Rev. 96 (1996), 1533-1554. Smith et al. , Prog. Surf. Sci. 75 (2004), 1-68.
Ralph Nuzzo1 and Dave Allara1 demonstrated that alkanethiolate molecules on a gold (Au) surface, formed a ordered monolayer.
1 Nuzzo, R.G., Allara, D.L., J. Am. Chem. Soc. 105 (1983), 4481.
These SAMs have become a ‘model organic surface’ for exploring the chemistries of organic surfaces.
The alkanethiolate SAMs assemble into its lowest energy configuration on Au
General formula: T(CH2)n H : Head – substrate specificity: T – surface property : n – chain length
Conductance measurements of multi-chromophoric molecular assemblies
Efforts to scale down electronics to the nanometer regime requires a comprehensive understanding of charge transport through individual molecules.
To achieve this goal, scanning tunneling microscopy can be used to image directly and measure charge transport through individual surface-bound molecules.
Here we utilize alkathiolate SAMs to isolate individual molecules for measurements.
Scanning tunneling microscopy images of (1,2) an octanethiolate SAM; (3) individual porphyrin complexes isolated within the SAM matrix. These images were recorded under high vacuum conditions and at room temperature.
Nanayakkara and Bonnell
We examine the transport properties of unique supermolecule/nanoparticle assemblies and take a different approach to the transport analysis, though based on familiar fundamental principles. We experimentally investigate the electronic properties of random arrays of two-dimensional gold nanoparticles (AuNPs) consisting of metal junctions linked by optically active dithiol-PZnn supermolecules. The conductance of the assemblies was determined as a function of bias voltage, particle size, particle distribution, and the dithiol-PZnn supermolecule. Using normalized differential conduction analysis, we find that the mechanism is thermally assisted tunneling (TAT), where the response is independent of the particle size and distribution.
Fig. a shows a general device configuration, in which Au NPs disordered bimodal array is deposited on the substrate, interconnected with dithiol-PZnn linkers. The temperature dependence of conductance of different samples were shown in Fig. b and fit well to an Arrhenius model or a variable-range hopping model. Fig. c compares the apparent activation energies obtained from Arrhenius analysis.
Differential analysis of transport of functionalized AuNPs shows temperature dependence of d ln(I)/dV for a variety of AuNP arrays and supermolecule linkers and led us to propose that thermally assisted tunneling is the mechanism controlling transport. This transport process is illustrated in Figure d-g, which shows an idealized band diagram as a function of molecular length. The energy at which the majority of tunneling occurs is above the Fermi energy (at temperatures above 0 K) but below the LUMO and, therefore, likely associated with an energetic metal or molecular state, as expected for a thermally assisted tunneling mechanism.
By combining peptide design, monolayer patterning, and a new probe of bio-optoelectronic function to characterize the dielectric and optoelectronic properties of an ambient protein-electrode system, we report the simultaneous detection of electron transport and the effect of optical absorption on dielectric polarizabilityin oriented peptide single molecular layers.
66 Å amphiphilic alpha helices self assemble in detergent solution to form 4-helix bundles, as illustrated in Fig a and b. 5 ZnPP (red) are added per 4-helix bundle on the histidine site (dark blue). Self-assembled protein layers were patterned on atomically smooth HOPG using microcontact printing. The helices are oriented perpendicular to the underlying substrate. A 425 nm LED (blue) is directed at the tip-sample junction (Fig. c).
Torsionally stabilized nano-impedance microscopy was developed to simultaneously probe transport and impedance of single molecule layers at interfaces, and was used to investigate the sample impedance. Protein patterns are shown in the topography image, Fig. d. Cross-sectional line profiles are taken and red arrow indicates regions of one monolayer in height in Fig. e. Fig. f and g gave X and Y components of impedance, which can be converted to resistance and capacitance of the protein molecular layer.
Resistance (R) and capacitance (C) of monolayer ZnPP maquettes at 70 kHz in the presence and absence of optical illumination are compared in Fig. h and i. The 2D/3D histograms show the clear reduction of resistance and increase in capacitance under light. Optical absorption causes an increase of charge carriers that reside in delocalized states in ZnPP, thus the decrease of the resistance. Most interesting is the optical dependence of capacitance, which is related to molecular polarizability. The photo induced increase of 117%-267% in dielectric constant corresponds to an excited state polarization volume increase and agrees quantitatively with that found in porphyrin molecules in liquid.