Home

Search

Collections

Journals

About

Contact us

My IOPscience

Fullerene-porphyrin supramolecular nanocables

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 235201 (http://iopscience.iop.org/0957-4484/25/23/235201) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 130.92.9.55 This content was downloaded on 02/09/2014 at 09:40

Please note that terms and conditions apply.

Nanotechnology Nanotechnology 25 (2014) 235201 (4pp)

doi:10.1088/0957-4484/25/23/235201

Fullerene-porphyrin supramolecular nanocables A Buldum1 and D H Reneker2 1 2

Department of Physics, The University of Akron, Akron, Ohio 44325, USA Department of Polymer Science, The University of Akron, Akron, Ohio 44325, USA

E-mail: [email protected] Received 26 December 2013, revised 26 March 2014 Accepted for publication 31 March 2014 Published 21 May 2014 Abstract

Novel fullerene-porphyrin supramolecular nanocables were designed and their electronic properties were studied using computational modeling and first-principles density functional theory. It is demonstrated that two well-defined fullerene-porphyrin nanocables have clear channels for charge transport by electrons and holes. These two interesting nanocables have zigzag or helical chains of C60 molecules around a π-stacked porphyrin core. They also have frontier electronic states which are spatially extended along the axes of the nanocables. Ballistic electronic transport is possible for ultrafast transfer of electrons along C60 chains. We believe these supramolecular nanocables can play important roles in molecular electronics, optoelectronics and photovoltaics. Keywords: photovoltaics, nanocables, nanowires, charge transport, simulations, fullerenes, molecular electronics (Some figures may appear in colour only in the online journal) 1. Introduction

nanocables. The electronic structures of the C60 -ZnTPP supramolecular nanocables were calculated. It was found that these nanocables have clear electron and hole pathways for charge transport. They contain frontier electronic states that are localized on electron donating porphyrins and electron accepting C60 ʼs and are spatially extended along the axes of the nanocables. Different atomic structures of the nanocables with different electronic structures were found. These nanocables can be engineered to possess particular electronic properties appropriate for photovoltaic or electronic devices.

Molecular level synthesis and design of functional materials have attracted great interest due to their possible applications in cataysis, energy storage and energy conversion [1, 2]. Porphyrins are attractive molecular building blocks for such functional materials due to their electronic and photonic properties [3–5]. C60 -porphyrin dyads act as molecular donoracceptor systems for charge separation in photovoltaic devices [6, 7]. Synthesis of porphyrin based materials possessing hierarchical arthitectures was demonstrated [1, 8–10]. Porphyrins self-assembled into macroscopic columns. Wang et al synthesized supramolecular ‘Double-Cable’ structures using C60 -porphyrin dyads as building blocks [11]. They showed that such dyads formed precise and stable structures. In this article, we report computational design and electronic properties of C60 -Zinc tetraphenylporphyrin (ZnTPP) supramolecular nanocables. Atomic models of a variety of C60 -ZnTPP structures were created and studied using first-principles density functional theory. The most interesting structures contain π-stacked porphyrin cores with C60 chains attached to the porphyrin cores creating supramolecular 0957-4484/14/235201+04$33.00

2. Methods Our electronic structure and geometry optimization calculations employ the ab initio density funtional theory (DFT) method. The Quickstep module [12] in the CP2K package was used. Perodic structures can be investigated using this module, but only Γ -point calculations can be performed. The exchange-correlation term in the Kohn–Sham equations were approximated by the Perdew–Burke–Erznerhof (PBE) [13] generalized gradient approximation (GGA). The Van der Waals interactions were included using the semi-empirical 1

© 2014 IOP Publishing Ltd

Nanotechnology 25 (2014) 235201

A Buldum and D H Reneker

minimum energy ZnTPP core nanowire structure contained ZnTPPs rotated by 28.3° with respect to each other and there were 4.2 angstrom separation between the centers of the ZnTPPs. ZnTPP core nanowires were decorated with C60 molecules by choosing binding sites of C60 molecules in such a way that C60 –C60 interactions were maximized. Periodic boundary conditions were applied in all three directions and the supramolecular nanocables containing both ZnTPPs and C60 molecules were geometrically optimized. The atomic structures of the supramolecular nanocables were continuous and the nanocables were infinitely long due to the periodic boundary condition in the perpendicular direction. There was at least 23 angstrom spacing between the nanocables in the lateral directions.

3. Results and discussions Two different well-defined supramolecular nanocables in which the C60 molecules formed molecular chains attached to the prophyrin cores were found. In the first supramolecular nanocable, the C60 molecules formed a zigzag chain attached to the prophyrin core (figure 1(b)). In the second supramolecular nanocable, the C60 molecules formed a chiral chain attached to the prophyrin core (figures 1(c), (d)). The supramolecular nanocable that contained both the prophyrin core and the zigzag C60 chain is called a zigzag nanocable and the supramolecular nanocable that contained both the prophyrin core and the helical C60 chain is called a helical nanocable in the rest of this paper. In the zigzag nanocable, the supercell was 9.0 angstroms long along the wire axis and the cell contained two fullerenes with a total of 288 atoms. In the helical wire the supercell was 50.40 angstroms long and it contained 12 fullerenes with a total of 1728 atoms. In both the zigzag and the helical nanocables, the separation between ZnTPPʼs were 4.5 and 4.2 angstroms. The separation between the fullerenes was 3.28 and 2.95 angstroms. The fullerenes in the helical nanocable were closer to each other than those in the zigzag nanocable. The calculated total energy of the helical nanocable was lower than the total energy of the zigzag nanocable by 0.011 eV per atom. The stability of one dimensional nanostructures such as nanowires can be investigated by studying their energetics in different geometries [18, 19]. We investigated the stability of zigzag and helical nanocables by calculating total energies as a function of ZnTPP–ZnTPP separation in different geometries. Figure 2 presents the results for the zigzag and helical nanocables that contain C60 fullerenes. We found that the nanocable in helical form is more stable than the nanocable in zigzag form, which is consistent with recent experimental results [11]. Total density of states (DOS) were calculated and the occupied and unoccupied electronic states close to Fermi level (energy of the highest occupied state of the entire system) were analyzed. Our electronic structure calculations give ground state electronic properties of the nanocables. Although

Figure 1. Atomic structure of the C60 -ZnTPP dyad and the atomic structures of the molecular nanocables. (a) Top view of the structure of the C60 -ZnTPP dyad. (b) Side view of the nanocable containing πstacked porphyrin core and zigzag C60 chain. (c) Side view of the nanocable containing helical C60 chain. (d) Top view of the nanocable containing helical C60 chain. Periodic boundary conditions are used and supramolecular nanocables are continuous along the nanocable axes.

‘DFT+D3’ term [14]. The basis sets DZVP-MOLOPT [15] were used and pseudopotentials of the Goedecker–Teter–Hutter type [16] were employed with a cut-off energy of 900 Ry for the density grid. The same computational approach was succesfully used to investigate C60 -Ce (TPP)2 dyads and good agreement with the experiments was obtained [17]. The atomic structure of the C60 -ZnTPP dyad and the atomic structures of the two promising supramolecular nanocables are presented in figure 1 . The geometrically optimized structure of a single C60 -ZnTPP dyad is shown in figure 1(a). The molecular structure was minimized and the fullerene was in the same plane with the prophyrin in the dyad. The structural design of the supramolecular nanocables was started with the investigation of π-stacking of the ZnTPP units and creating π-stacked porphyrin core nanowires. It is believed that the main contribution to the interaction between the C60 -ZnTPP dyads are from the ZnTPP–ZnTPP interactions [5]. After geometrical optimization of individual ZnTPP molecules, dimers were created by placing one ZnTPP on top of another. Different dimer geometries were studied by rotating or translating one ZnTPP with respect to the other and the dimer with the minimum energy was found. Next, by placing the dimers on top of each other and using periodic boundary conditions, infinitely long ZnTPP nanowire structures were created. The ZnTPP nanowires were geometrically optimized for different ZnTPP–ZnTPP separations. The 2

Nanotechnology 25 (2014) 235201

A Buldum and D H Reneker

Figure 2. The variation of total energy as a function of

ZnTPP–ZnTPP separation. The hollow circles correspond to the zigzag nanocables and the hollow squares correspond to the helical nanocables. One atomic unit of energy is 27.211 eV.

Figure 4. (a) The highest occupied electronic state of the zigzag

nanocable; (b) the lowest unoccupied electronic state of the zigzag nanocable; (c) the twelfth occupied electronic state (from the Fermi level) of the helical nanocable; (d) the lowest unoccupied electronic state of the helical nanocable.

Our analysis of the occupied and unoccupied electronic states close to the Fermi level of the nanocables showed that the electronic states are spatially extended along the axes of the wires. The zigzag nanocableʼs highest occupied electronic state and lowest unoccupied electronic state are shown in figures 4 (a) and (b). The two highest occupied states are localized on the porphyrin core and the two lowest unoccupied states are localized on the fullerenes. In addition, these states are extended along the axis of the wire. In the case of the helical nanocable, there are twelve states below the Fermi level that have the same character and are more localized on the porphyrin core of the helical nanocable. The state with the lowest energy of the twelve states is shown in figure 4(c). This state is the Γ -point state and other eleven states have the same character due to zone folding along the k z direction. The lowest unccupied state of the helical nanocable is shown in figure 4(d). There are also twelve electronic states with the same character that extended along the helical nanocable, but were localized on the C60 chains. Figure 4 clearly shows that both nanocables contain frontier electronic states which are localized on the electron donating porphyrins and the electron accepting C60 ʼs and are spatially extended along the axes of the nanocables. The presence of spatially extended electronic states along the nanocables led us to investigate quantum electronic transport along the C60 chains in the nanocables. If ballistic electronic transport in the conduction band is possible, then very fast transfer of photoinduced electrons may occur. Quantum electronic transport calculations of the C60 chains were based on Landauer–Buttiker formalism and the Greenʼs function technique was used [22–24]. In these calculations,

Figure 3. The total density of states (DOS) of the zigzag and helical

nanocables are shown in black and red curves, respectively. The Fermi levels are taken as zero.

the DFT calculations contain only the Γ -point, the supercells are quite large. In the helical nanocable, due to zone folding, higher k z states appeared. The DOS of both zigzag and helical nanocables are presented in figure 3 for comparison. The Fermi levels of both nanocables were shifted to zero. The valence band DOS curves of both the zigzag and the helical nanocables are similar, but the conduction bands DOS curves are different due to the different C60 chain structures. The band gaps of the zigzag and the helical wires were calculated as 0.58 eV and 0.69 eV, respectively. A single C60 -ZnTPP dyadʼs gap was 0.85 eV. The band gaps of both the nanocables are less than the gap of a single dyad. Note that Kohn–Sham band gaps are usually small from DFT-GGA calculations [7]. Use of hybrid functionals can significantly improve the results, however, they are computationaly expensive for large structures such as these supramolecular nanocable structures. Meanwhile, PBE/DZP level calculations were shown to be very successful for prediciting the structures of fullerene-porphyrin complexes and had very good agreement with the experimental results [20, 21]. 3

Nanotechnology 25 (2014) 235201

A Buldum and D H Reneker

theory. Two different well-defined supramolecular nanocables were found. In the first supramolecular nanocable, the C60 molecules formed a zigzag chain and in the second supramolecular nanocable, the C60 molecules formed a chiral chain attached to the prophyrin core. It is demonstrated that these supramolecular nanocables have clear electron and hole channels for charge transport. They contain frontier electronic states which are localized on the electron donating porphyrins and electron accepting C60 ʼs and spatially extended along the axes of the nanocables. Quantum electronic transport calculations showed that ballistic electronic transport is possible along the fullerene chains. We believe, these supramolecular nanocables can play important roles in molecular electronics, optoelectronics and photovoltaics.

References [1] Chen Z, Lohr A, Saha-Moller C R and Wurthner F 2009 Chem. Soc. Rev. 38 564 [2] Busseron E, Ruff Y, Moluin E and Giuseppone N 2013 Nanoscale 5 7089 [3] Kadish K M, Smith K M and Guilard R 2000 The Porphyrin Handbook (New York: Academic Press) [4] Guldi D M 2002 Chem. Soc. Rev. 31 22 [5] Boyd P D W and Reed C A 2005 Acc. Chem. Res. 38 235 [6] Guldi D M 2003 Pure Appl. Chem. 75 1069 [7] Ciammaichella A, Dral P O, Clark T, Tagliatesta P, Sekita M and Guldi D M 2012 Chem. Eur. J. 18 14008 [8] Li M, Ishihara S, Ji Q, Akada M, Hill J P and Ariga K 2012 Sci. Technol. Adv. Mater. 13 053001 [9] Imahori H and Fukuzumi S 2004 Adv. Funct. Mater. 14 525 [10] Hasobe T 2010 Phys. Chem. Chem. Phys. 12 44 [11] Wang C, Zhang W, Sun H, van Horn R M, Kulkarni R R, Tsai C, Hsu C, Lotz B, Gong X and Cheng S Z D 2012 Adv. Energy Mater. 2 1375 [12] VandeVondele J, Krack M, Mohamed F, Parrinello M, Chassaing T and Hutter J 2005 J. Comput. Phys. Comm. 167 103 [13] Perdew J P, Burke K and Ernzerhof M 1996 Phys. Rev. Lett. 77 3895 [14] Grimme S, Antony J, Ehrlich S and Krieg H 2010 J. Chem. Phys. 132 154104 [15] VandeVondele J and Hutter J 2007 J. Chem. Phys. 127 114105 [16] Goedecker S, Teter M and Hutter J 1996 Phys. Rev. B 54 1703 [17] Vijayaraghavan S, Ecija D, Auwarter W, Joshi S, Seufert K, Seitsonen A P, Tashiro K and Barth J V 2012 Nano Lett. 12 4077 [18] Ponomareva I, Menon M, Srivastava D and Andriotis A N 2005 Phys. Rev. Lett. 95 265502 [19] Akiyama T, Nakamura K and Ito T 2006 Phys. Rev. B 73 235308 [20] Wang Y B and Lin Z 2003 J. Am. Chem. Soc. 125 6072 [21] Zandler M E and DʼSouza F 2006 C. R. Chimie 9 960 [22] Datta S M 1995 Electronic Transport in Mesoscopic Systems (Cambridge: Cambridge University Press) [23] Nardelli M B 1999 Phys. Rev. B 60 7878 [24] Buldum A and Lu J 2001 Phys. Rev. B 63 161403

Figure 5. The variation of conductance as a function of energy at the conduction bands of the (a) zigzag (b) helical C60 chains. The conduction band energies in the quantum transport calculations are aligned with conduction bands in the DFT calculations

porphyrin cores were removed and only the C60 chains remained. A π-orbital tight-binding Hamiltonian [24] was employed. Figure 5 shows the conductance values of the zigzag and helical C60 chains. The conduction band energies in the quantum transport calculations were aligned with the conduction band energies in the DFT calculations. In both zigzag and helical nanocables, ballistic transport is possible for certain energy values. In the zigzag nanocable, the maximum value of conductance is one unit of quantum conductance 2e 2 h . In the helical nanocable, the maximum

(

)

value is 2 units, which is due to the reduced C60 –C60 distance in the helical wire. The overall physical picture of the electronic structure and electronic transport properties of these supramolecular nanocables is that the stacked C60 -ZnTPP dyads can form two electronically continuous channels. The electrons and holes have clear pathways and they can move to terminals anywhere along the channels, most conveniently at the ends if finite lengths are considered.

4. Conclusions Atomic models of a variety of C60 -ZnTPP structures were created and studied using first-principles density functional

4

Fullerene-porphyrin supramolecular nanocables.

Novel fullerene-porphyrin supramolecular nanocables were designed and their electronic properties were studied using computational modeling and first-...
664KB Sizes 2 Downloads 3 Views