Utilizing Nearest-Neighbor Interactions To Alter Charge Transport

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Utilizing Nearest-Neighbor Interactions to Alter Charge Transport Mechanisms in Molecular Assemblies of Porphyrins on Surfaces Amanda E. Schuckman, Bradley W. Ewers, Lam H. Yu, Joao P. C. Tome, Lisa M Perez, Charles Michael Drain, James G Kushmerick, and James D. Batteas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01223 • Publication Date (Web): 27 May 2015 Downloaded from http://pubs.acs.org on June 5, 2015

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The Journal of Physical Chemistry

Utilizing Nearest-Neighbor Interactions to Alter Charge Transport Mechanisms in Molecular Assemblies of Porphyrins on Surfaces Amanda E. Schuckman,1a Bradley W. Ewers, 1,b Lam H. Yu,3,c João P. C. Tomé,4,d Lisa M. Pérez,2 Charles M. Drain,*,4 James G. Kushmerick, *,3 James D. Batteas*,1 1

Department of Chemistry and 2Laboratory for Molecular Simulation, Texas A&M University, PO Box 30012, College Station, TX 77842 3

National Institute of Standards and Technology, Materials Measurement, 100 Bureau Drive, Gaithersburg, MD 20899

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Department of Chemistry and Biochemistry, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10021

ACS Paragon Plus Environment

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Keywords: molecular electronics, charge transport, scanning tunneling microscopy, single electron tunneling, self-assembled monolayers ABSTRACT. When tunneling is the dominant mechanism of charge transport in a molecular junction, the conductivity of the junction is largely insensitive to chemical and structural perturbations which do not impact the overall length of the junction. This severely hampers the seemingly limitless potential of molecules to modulate charge transport at interfaces and their application in a host of device designs. This is a particular challenge for molecules baring insulating features like saturated hydrocarbons which decouple functional groups from the surface. Such decoupling groups increase the energy required to isolate charge on the molecule, pushing transport into the tunneling regime in many cases. Herein, we demonstrate that, through enhancement of nearest neighbor interactions, lateral delocalization of charge states in molecular islands can be used to shift transport out of the tunneling regime to the more efficient, and more chemically tunable, charge hopping regime. In a previous study, it was found that through-bond tunneling was the dominant mechanism of charge transport through a hydrocarbon tethered freebase porphyrin thiol. With coordination of zinc(II), the formation of large molecular islands in an alkanethiol matrix on a Au(111) surface was facilitated. Bias induced switching and unphysical tunneling efficiencies observed by Scanning Tunneling Microscopy of these molecular islands, as well as Coulomb blockade observed in low temperature crossed-wire tunnel junction measurements, indicate charge hopping becomes the dominant mechanism of transport in the molecular islands, whereas transport in single molecules was consistent with through-bond tunneling. These results elucidate the basis for functional conductivity-structure and supramolecular relationships that may be employed in the design of molecular junctions in organic thin films.


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INTRODUCTION The use of molecules to modulate the flow of current at interfaces is not a new concept,1 but it has been fraught with many challenges. There are several advantages of molecules over solid state materials for the control of charge transport at interfaces, including improved thermal properties,2 virtually limitless options via chemical synthetic design, and the opportunity for “bottom-up” design and fabrication.3 Among the many challenges in implementing molecular/organic electronic devices, perhaps the most significant is understanding the correlation between chemical structure and the resulting conductive characteristics of a molecule assembled on a surface. Molecular electronic properties can be evaluated in terms of current density, or in device-like characteristics such as current rectification,1,

4-5

conductance

switching,6-10 and non-differential resistance.11-13 Rectification of carrier flow is of particular significance for devices and materials composed of molecules on a variety of interfaces, including solid state device interfaces,14-15 photocatalytic surfaces,16-18 and dye sensitized19-20 and bulk heterojunction solar cells,21 but has proven difficult to reliably and effectively achieve. The conductive properties of molecular wires such as highly conjugated oligo(p-phenylene ethynylene)s22-23 are tunable when directly bound to the electrode, thereby providing strong electronic coupling between molecular and electrode states. The strong electronic coupling; however, limits the functionality such that persistent switchable conductance9 must arise from structural6,

24

or environmental changes,25 while current rectification,26 persistent charge

storage27-29 and redox-based switching7-8, 30 are generally not feasible because the presence of the nearby coupled electrode inhibits lasting electronic changes. These limitations can be overcome by decoupling the conductive features of a molecule, such as aromatic rings or coordinated metal ions, from the electrode surface.31 Electronic decoupling


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can be achieved with solid state dielectric materials like oxides or salts,32 or by inserting a saturated hydrocarbon chain between the electronically active part of the molecule and the surface.27, 33 Decoupling the electronically active parts of the molecule from the electrode affords the possibility to develop persistent charges via reduction and oxidation,5,

30, 34

thus vastly

increasing the potential functionality. Unfortunately in single molecule junctions, isolating the redox active molecular component of the molecule from the electrode decreases its capacitance,28 inhibiting redox activity such that charge transport occurs solely via tunneling. The most determinative factor in the magnitude of a tunnel current is the junction length,35-37 and variation in barrier height through chemical variation has limited effect. Control of current flow through spatial variations of the barrier height via chemical functionalization is also ineffective. Rectification ratios between positive and negative bias for example, are generally limited to ~20 in the tunneling transport regime when rectification is achieved only through variations in barrier height,4,

26, 38

which is several orders of magnitude smaller than what can be achieved with

semiconductors. Our previous studies of transport in a hydrocarbon tethered porphyrin thiol on Au (111) reached the same conclusions.39 Porphyrins are of significant interest for modulating transport properties at interfaces, due to their small HOMO/LUMO gap and accessible redox chemistry,40 their prevalence in natural light harvesting systems and applications in photovoltaic devices,41 and their ability to facilitate efficient charge transfer.42-43 For our molecule or interest, however, the transport efficiency was dominated by off-resonant tunneling, suggesting that chemical variation of the highly chemically and redox modifiable porphyrin macrocycle would have limited impact on the transport properties. Our focus is to design molecular systems that shift away from tunneling as the dominant


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mechanism of charge transport so that greater sensitivity to the chemical structure can be achieved, thereby enabling a range of device properties. Transport via charge-hopping, for example, is purported to increase the functional relationship between molecular structure and transport characteristics, bringing the rich chemical changes that can be achieved synthetically back into play as a means to tune charge transport at interfaces. This provides a robust means of enhancing CMOS technologies through advantageous chemical modifications. In systems dominated by charge hopping, conduction greatly depends on the chemical potential of the molecular states within the junction,28, 44 which allows a better understanding of the relationship between chemical structure and chemical potential on device properties. The correlation of molecular properties with performance affords design principles for devices with sharper responses to applied bias or gate bias. For molecular systems the challenge of stabilizing high charging energies on a molecule must be overcome, which generally institutes a minimum molecular size.28 Examples of large, multifunctional molecules exhibiting charge transfer via charge hopping are reported,7-8,

33, 44-45

but this approach substantially increases synthetic

complexity, and negatively impacts facile tuning of chemical properties. The synthetic challenges can be ameliorated by a molecular design facilitating nearest-neighbor interactions to drive molecular self-assembly resulting in the controlled formation of aggregates. Control of aggregate domain size proffers an additional mechanism by which charge transport behavior may be tuned. Herein we report lateral delocalization of charged states within a self-assembled metalloporphyrin aggregate as a promising and versatile avenue to achieve charge hopping transport in molecular junctions.46,47 We now report the charge transport properties of a hydrocarbon tethered Zn-porphyrin thiol, the Zn(II) complex of the free-base molecule we studied previously.39 Single molecules and


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aggregates are embedded in an alkanethiol matrix on a Au(111) surface that enforces a standing geometry for the thiol tethered porphyrin molecules. The chelation of Zn(II) in the macrocycle enhances intermolecular interactions by increasing the pi-stacking energetics, and provides a pathway for axial coordination of the Zn center by pyridyl groups on neighboring porphyrins to aid in driving supramolecular assembly on the surface.48-49 At low insertion density of the porphyrins in the alkanethiol matrix, the isolated single molecules exhibit simple tunneling behavior. Upon increasing the porphyrin coverage, islands of molecular aggregates form that exhibit a clear change in transport efficiency which we attribute to a transition of mechanism to charge hopping. Our hypothesis is that charge hopping arises from the ability of the aggregates to form stable charge states through lateral, intermolecular delocalization. This conclusion is supported by the observation of unrealistic tunneling efficiencies, given the physical parameters of the molecule, and conductance switching of the large molecular aggregates. Additionally, Coulomb blockade was observed in low temperature crossed-wire tunnel junction measurements, indicated by a sharp increase in conductivity once sufficient energy is available to inject charge into the molecular island. These results indicate that the challenge of achieving charge confinement in molecular junctions may be overcome by employing small aggregates of molecules self-organized into specific architectures. This supramolecular approach enables rapid modifications of promising candidate molecules to control intermolecular interactions and optimize charge transport at interfaces to achieve desired functional devices.

EXPERIMENTAL METHODS Materials. Self-assembled monolayers used for Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) analysis were prepared on 150 nm (Agilent, Phoenix, AZ) and


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200 nm (Phasis, Geneva, Switzerland) thick annealed Au(111) films on mica. Dodecanethiol used for self-assembled monolayer (SAM) preparation was purchased from Aldrich (98% purity) and used as received. Certain commercial equipment or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for this purpose. Synthesis of Zinc Porphyrin Thiol Compound.

The 5,10,15-tri(4-pyridyl)-20-(4-(1’,5’-

dithiopentyl)-2,3,5,6-tetrafluorophenyl) porphyrin (TPy3PF4-SC5SH) was synthesized in two steps and the complete details and associated spectral characterizations are provided in a previous paper.39 Metalation of the porphyrin thiol with Zn(II), as well as characterization of the compound, are provided in the Supporting Information. Preparation of Mixed Monolayers. Prior to SAM preparation, Au substrates were treated with UV/ozone for 20 min, followed by rinsing the gold film in sequence with high purity (18.2 MΩ•cm) water (NANOpure Diamond, Barnstead), ethanol, and then dried with streaming nitrogen. The Au films were then immersed in 1 mM dodecanethiol for 24 hours to form the alkanethiol SAM, and rinsed with ethanol. Mixed monolayers of single or few molecule clusters were prepared by soaking the SAM in 0.1 mM zinc porphyrin thiol in dichloromethane (DCM), while mixed monolayers containing large clusters were prepared by soaking the SAM in 0.5 mM zinc porphyrin thiol in DCM/pyridine (1 mmol) to promote dissolution. Samples were soaked in the zinc porphyrin thiol solutions for periods of one to five days, allowing for insertion of the molecule into the SAM matrix. Upon removal, the substrates were rinsed with DCM and dried under streaming nitrogen. Atomic Force Microscopy. AFM images were acquired with an Agilent 4500 Pico SPM


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(Agilent, Phoenix, AZ) with a deflection-type detection scanning head interfaced with an SPM1000 control electronics Revision 8 (RHK Technology Inc., Troy, MI) as well as an Agilent 5500 AFM. All AFM images were acquired in contact mode under ethanol using commercially available silicon nitride AFM tips (Bruker AFM Probes, MSCT, Sunnyvale, CA) with nominal tip radii of ~10 nm and nominal spring constants ranging from 0.03 - 0.1 N/m. Forces applied during imaging did not exceed 250 pN so as to minimize depression of the porphyrin molecules into the SAM, resulting in decreased image resolution due to minimal interactions between the tip and the surface and decreased tip deflection. Scanning Tunneling Microscopy. STM was performed using an Omicron UHV-XA STM system with a typical base pressure < 3 x 10-10 Torr. Mechanically cut Pt/Ir (80/20) tips were used, with typical imaging currents of 5-100 pA with tip bias from -2.0 to +2.0 V. Currentvoltage (I-V) spectra were collected over a bias range of -2.0 V to +2.0 V. The STM images were analyzed with commercially available Scanning Probe Image Processor software (Image Metrology, Lyngby, Denmark). Apparent height and width distributions of porphyrin molecules and clusters on the surface were determined using the grain analysis feature, in each case using 5-10 representative images. Geometry and Electronic Structure Calculations. Density functional theory (DFT)50 was used to calculate the molecular and electronic structure of the zinc porphyrin thiol. Geometry optimization and single point energy calculations of the zinc porphyrin thiol were performed with the Gaussian 03 computational suite.51 The Tao, Perdew, Staroverov, and Scuseria (TPSS) DFT functional52 was used for geometry optimization, single point energy calculations, and vibrational spectra. Calculations were performed using the 6-31+G(d’) basis set53 plus the D95 full double zeta basis set54 for the zinc cation, and orbital population analysis was performed


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using the AOMix software package.55 Additional calculations of the optimized geometry and binding energy of pi-stacked zinc porphyrin dimers were performed using the Amsterdam Density Functional suite,56 with computational details and results provided in the Supporting Information. Inelastic Tunneling and Conductance Spectroscopy.

Mixed monolayers formed by

immersion in DCM/pyridine zinc porphyrin thiol solutions were employed for conductance spectroscopy and inelastic electron tunneling spectroscopy (IETS). Crossed-wire junctions of the mixed molecular monolayer were formed at liquid helium temperature.57-60 Transport measurements were performed with a standard AC modulation technique in which the dI/dV and d2I/dV2 signals are recorded simultaneously with the I-V characteristics. To remove any junction area dependency from the data, the IET spectra are presented as the amplitude (d2I/dV2)/(dI/dV).

RESULTS AND DISCUSSION The drive to integrate molecules as active components of optoelectronic devices requires a detailed understanding of how they assemble on surfaces and how the assembled architecture influences charge transport properties. The surface assembly of the mixed monolayers of the zinc porphyrin thiol inserted into a pre-assembled n-dodecanethiol monolayer on Au(111) surfaces was characterized utilizing STM and AFM, and conductance spectroscopy and IETS were conducted on low temperature crossed-wire tunnel junctions. During the insertion process, the pre-formed SAM is subjected to a solution of the porphyrin molecules, in which SAM molecules at defect sites preferentially desorb and exchange with porphyrin molecules.61-62 In addition, Xray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRAS) characterization of the mixed monolayers is presented in the


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Supporting Information. To aid in understanding the detailed molecular and electronic structure of the zinc porphyrin thiol, Density Functional Theory (DFT) calculations were also employed. Electronic Structure Calculations. The calculated gas phase optimized structure of the zinc porphyrin thiol molecule, and a model of its insertion geometry into the alkanethiol matrix, are shown in Figure 1A and 1B, respectively. Though the presence of the surface is likely to alter the electronic structure of the molecule, because the molecule is separated from the surface in a standing geometry by the alkane tether, the variations caused by the surface are likely to be most dominant at the sulfur tail,63 which is present in both the background matrix and the zinc porphyrin, with more mitigated impact on the porphyrin macrocycle. The optimized structure of the zinc porphyrin thiol is similar to that of the free base analog,39 in which the tetrafluorophenyl

Figure 1. (A)The optimized structure of the thiol-tethered zinc porphyrin molecule as determined from DFT calculations, where sky blue atoms are carbon, white are hydrogen, blue are nitrogen, pink are fluorine, yellow are sulfur, and gray is the zinc(II) ion. In (B), a model of


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the insertion geometry of the porphyrins in the alkanethiol is depicted. The rotational angle φ is such that the porphyrin would lie flat on the surface if tilted all the way over, and α is determined from AFM microscopy to be ~ 30-45°, corresponding to a height difference of ~ 3-5 Å relative to the SAM matrix.

Figure 2. The partial density of states for the freebase (A) and zinc substituted (B) porphyrin are depicted. The colors correspond to the coloration of the chemical components shown (C), and the HOMO and LUMO orbitals of each molecule are also depicted. The frontier density of states are nearly identical, with the majority of the state density existing only on the porphyrin macrocycle.

ring is canted nearly perpendicular at ~84 degrees with respect to the macrocycle, while the less sterically hindered pyridyl substituents are canted by 65 degrees. The tetrafluorophenyl ring is canted slightly more than is observed in crystal structures,64 likely due to the lack of nearest neighbor interactions in the calculated structure, while the cant of the pyridyl groups is consistent with structures of meso tetrapyridylporphyrins.65 The HOMO-LUMO gap for the zinc porphyrin thiol was calculated to be 1.99 eV which is slightly larger than 1.89 eV calculated for the free base analog. The HOMO energy level of -5.37 eV is close to the reported Fermi level of the Au(111) surface, suggesting a small charge


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injection barrier, though the absolute energy scale of DFT calculations can be unreliable. As shown in Figure 2, for both the zinc metalated and freebase porphyrin, there is no frontier state density present on the tetrafluorophenyl linker group because of the nearly perpendicular dihedral angle of this ring with respect to the macrocycle. The lack of extension of the pi system beyond the porphyrin macrocycle and the hydrocarbon tether effectively decouples the porphyrin macrocycles from the metal surface when adsorbed in the standing geometry,66-67 thus providing an ideal configuration of a double-barrier tunnel junction expected to support charge confinement. Analysis of Molecular Orientation by AFM. Because observations of the electronic properties of these molecules by STM are intrinsically coupled with the structural configuration of the molecules on the surface, and to confirm that the molecules are in a standing geometry, it is necessary to determine the structure of the porphyrin molecules in the mixed monolayer. AFM was used to determine that the porphyrins physically protrude 5 ± 2 Å above the dodecanethiol matrix (Figure 3). This value agrees with physical height measurements of the freebase,39 consistent with their nearly identical geometric structure.

This height implies a porphyrin

molecular tilt, α, of 30° - 45° from the surface normal, which is up to 15° more canted than the typical alkanethiol film, which is likely due to the steric interactions of the porphyrin68 and that the molecules insert predominantly at defect sites within the film.

By the surface dipole

selection rule,69 this tilt angle agrees with FTIR spectra of films with much greater insertion density (see Supporting Information).


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Figure 3. An AFM topographic image (A) and statistical distribution of physical heights (B) measured by AFM for the zinc porphyrin thiol clusters. The porphyrin protrusions appear as bright spots in the topograph, and statistical analysis of observed heights produce an average height of 5±2 Å above the dodecanethiol matrix. The sample shown was soaked in a 0.1 mM zinc porphyrin thiol solution for 3 days, and was imaged with an applied load of 25 pN submerged in ethanol, scale bar is 20 nm.


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Figure 4. (A) STM topographic image showing zinc porphyrin thiols inserted into the dodecanethiol matrix along with distributions of the apparent height (B) and width (C) of the embedded zinc porphyrin thiols collected over a number of images. The sample shown was immersed in a 0.1 mM zinc porphyrin thiol solution for 3 days. (bias = 1.4 V, I = 20 pA, scale bar 20 nm).

Conductance of Single Molecules and Small Clusters. Prior mixed monolayer studies of the freebase porphyrin thiol molecule39 in a dodecanethiol matrix showed a preference for insertions of single molecules to clusters of only a few molecules (typically 3 – 4) as determined by the width distributions. To compare the transport properties of the single metalloporphyrin or clusters of only a few metalloporphryins, the dodecanethiol matrix was immersed in dilute solutions of the zinc poryphyrin thiol in dichloromethane (DCM). A representative STM topographic image of the imbedded metalloporphyrins is shown in Figure 4A, with the height and width statistics from several images in Figures 4B and 4C, respectively. The widths of the clusters are consistent with insertions of one to a few molecules, and the primary population of the apparent heights of 0.5±0.2 nm is also comparable to the apparent height measured for the


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freebase analog.39 The secondary population with apparent height of 1.5 nm indicates much more conductive species on the surface, and this population can be amplified by increasing the concentration of the porphyrin solution (vida infra). The double-layer tunnel junction model70 was used to determine the overall tunneling efficiency of the molecules by the equation:  ZnPn    DDT hDDT    hSTM   h   hZnPn 1

(1)

where βDDT is the tunneling efficiency of the dodecanethiol matrix (1.2 Å-1)70, hDDT is the thickness of the dodecanethiol film (14 Å), α is the tunneling efficiency of vacuum (2.3 Å-1), δhSTM and δh are the protrusion heights of the zinc porphyrin thiol measured by STM and AFM respectively, and hZnPn is the height of the molecule determined by summing hDDT and δh. From this, the tunneling efficiency for the single metalloporphyrin was found to be 0.9 ± 0.1 Å-1. Assuming the hydrocarbon tether exhibits the same tunneling efficiency as the alkanethiol matrix, and adjusting the tunneling efficiency equation to determine only the macrocycle (MC) tunneling efficiency:71  MC    DDT  hDDT  htether     hSTM   h   hMC 1

(2)

where hMC is 17 Å projected along the surface normal, corresponding to the length from the 10(4-pyridyl) nitrogen to the sulfur atom linking the 20-(4-thiophenyl) ring to the hydrocarbon tether, this yields a tunneling efficiency of 0.7 ± 0.2 Å-1 for the macrocycle, similar to that observed for other aromatic, highly conjugated molecules72 but much greater (less efficient) than that observed for zinc(II) porphyrin nanowires.73 Though the total tunneling efficiency of the molecule does differ from the freebase, the apparent height measured by STM was nearly identical for the two molecules. The primary difference is the measured physical height, for which there is a significant amount of uncertainty, and in fact the 7 Å physical height measured for the freebase is within the ± 2 Å distribution of physical heights measured for the zinc


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complex. The error in the physical height arises from the dependence of the measurement on how the AFM tip interacts with the porphyrin thiol and the SAM, which can vary with tip shape and applied load. Though similar tips were used to measure the physical heights of the freebase and zinc analogues, there are variations in tip sharpness, and the forces used during imaging were the smallest achievable for the instruments (

Utilizing Nearest-Neighbor Interactions To Alter Charge Transport

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