AVS2011 Session NS-ThM: Molecular Assembly and Devices

As part of a bottom-up strategy, molecular self assembly can be a promising technique to create surface patterns with ultimately small feature sizes in an economic efficient fashion. Understanding of the factors which guide molecules into different patterns thus become an important goal for prediction and control of molecular patterns structures.

Here we present the formation of interlocked arrays (‘gear chains’) of pinwheels through self-assembly of 3-phenyl-propynenitrile (PPN) molecules on a Cu(111) surface. Variable temperature scanning tunneling microscopy (STM) reveals upon molecular deposition a pattern of small hexagonal features, which coalesce into sequences of larger, interlocking pinwheel-shaped structures. The pinwheels have an outer diameter as large as ~4nm. The driving force of this entropically disfavored pinwheel formation is discussed.

Understanding and controlling electron transfer across metal/organic interfaces is of critical importance to the field of organic electronics and photovoltaics. Single molecule devices offer an ideal test bed for probing charge transfer details at these interfaces. Results from these single-molecule measurements can be directly related directly theoretical models, unlike measurements at the ensemble level. The ability to fabricate single molecule devices and probe electron transfer reliably and reproducibly has enabled us to study and model transport through them.

In this talk, I will review the scanning tunneling microscope break-junction technique we use to measure electronic transport through single molecule junctions. I will discuss our measurements using novel metal-molecule link chemistries, including amines, phosphines[1] and results from recent work using tri-methyl tin linkers, which yield direct Au-C coupled single molecule junctions[2]. I will show how the intrinsic molecular properties influence measured single molecule conductance and bond rupture forces[3]. Finally, I will show how a mechanically controlled binary single molecule switch can be created using bipyridine molecules[4].

[1] Y. S. Park et al., J. Am. Chem. Soc. 129, 15768 (2007).

[2] Z.-L. Cheng et al., Nat. Nano. In Press (2011).

[3] M. Frei et al., Nano Lett. 11, 1518 (2011).

[4] S. Y. Quek et al., Nat. Nano. 4, 230 (2009).

Spin control provides new and interesting opportunities for control and study of the factors governing electron transport. Recent work by Naaman and co-workers has shown that self-assembled monolayers of double stranded DNA (ds-DNA) can act as a spin filter for electrons photoemitted from a gold substrate.[1,2] This phenomenon depends on the helicity of the ds-DNA, which leads to spin polarization and consequent capture of filtered electrons that tunnel back to the substrate. In this work, this effect is investigated for electron flow between two electrodes, a bottom Ni electrode to which one strand of the DNA is bound, and a top gold nanoparticle electrode which serves to identify the ds-DNA and provide good electrical contact through binding to the complementary strand. The current characteristics are measured by conductive scanning probe microscopy, as applied in a previous study of electron transport in DNA monolayers.[3] A magnetic field of approximately 0.3 T at the surface is provided by a permanent magnet placed below the sample. The results are consistent with the photoemission work, namely marked differences in the current flow depending on magnetic field alignment. Furthermore, the effect depends on length of DNA chain, with longer chains providing a more significant effect relative to shorter ones.These experimental findings, together with a physical model will be presented.

[1] Goehler, et al, Science 331, p. 894 (2011).

[2] Ray, et al, Phys. Rev. Lett., 96, 03101 (2006).

[3] Nogues et al, J. Phys. Chem. B 110, 8910 (2006).

Novel nanocarbons such as fullerenes, nanotubes, graphene, and nanodiamond reside at the cutting edge of nanoscience and technology. This paper presents a fundamental study of how size, shape, chemical functionalization, and bond strain affect electronic structure in several benchmark series of chemically pure, novel carbon-cage compounds ranging from diamondoids (a fully sp³ form of nanodiamond) to cubane. Size and shape are studied with the diamondoid series from adamantane to hexamantane, where the observed gap changes are primarily due to evolution in occupied states, as measured with photoelectron spectroscopy (XPS & UPS). Bond strain is studied with dodecahedrane, octahedrane, and cubane, where increasing bond strain leads to two major changes in the near-edge x-ray absorption fine structure (NEXAFS) spectra. First, a broad C-C σ* resonance in the absorption splits into two more narrow and intense resonances with increasing strain. Second, the first manifold of states previously associated with tertiary C-H σ* in the diamondoid series appears to broaden and shift to lower energy. This feature is more than twice as intense in cubane as octadedrane, even though these two molecules have similar stoichiometries (C₁₂H₁₂ vs. C₈H₈). We attribute the additional intensity to π* states, indicating a high degree of p interaction between parallel C-C bonds in the cubane.

Spin-capable multi-walled carbon nanotube (MWCNT) forests that can form webs, sheets, and yarns provide a promising means for advancing various technologies [1-4]. The important factors enabling the growth of the spin-capable forests are still not well understood. Growing spin-capable CNT forests depends on several growth factors such as the catalyst film thickness, the growth temperature, and the carrier and reactant gases [5-9]. Other factors still remain to be investigated more thoroughly. These include the flow rate (or ratio) of the reactant gas, the reactant gas species, and the pressure.

Herein we show how both the spinning capability and morphology of MWCNT forests are changed significantly by controlling the acetylene (C₂H₂) concentration and the thermal ramp rate. The acetylene gas flow is varied in the range of 0.25 ~ 7.5 % in volume. The MWCNTs grown at C₂H₂ concentrations between 1.5 ~ 3.5 % are well-aligned and are spin-capable. The well-aligned forests have higher areal density and shorter distances between the CNTs caused by strong Van der Waals interactions. CNTs grown at C₂H₂ concentrations under 1.5 % or over 3.5 % are curled and have random orientation. The resulting forests have reduced areal density and have poor spinnability. The thermal ramp rate is varied from 30 ^oC/min to 70 ^oC/min. Only the CNT forests grown with 50 ^oC/min condition are well-aligned and spinnable due to high areal density and closer spacing between adjacent CNTs. This condition alone results in Fe nanoparticles which have the proper size and density to produce spin-capable CNT forests.

Figure 1 shows SEM images and picture of spin-capable CNTs grown at 1.5 vol.% of acetylene and 50 ^oC/min on 70kΩ/sq Fe film at 780^oC for 5min with mixture of He, H₂, and C₂H₂. The spinnable CNTs of 330 µm have good alignment which is dependent on the ability to form ribbons. From a 1 × 1 cm substrate, the CNTs can form a 4 m length sheet.

[1] M. Zhang et al., Science 306, 1358 (2004).

[2] X. Zhang et al., Adv. Mater. 18, 1505 (2006).

[3] M. Zhang et al., Science 309, 1215 (2005).

[4] A. E. Aliev AE et al., Science 323, 1575 (2009).

[5] M. Zhang et al., Science, 306, 1358 (2004).

[6] X. Zhang et al., Adv. Mater., 18, 1505 (2006).

[7] X. Zhang et al., Small, 3, 244 (2007).

[8] K. Liu et al., Nano Lett., 8, 700 (2008).

[9] J-H. Kim et al., Carbon, 48, 538 (2010).