Ultralong Copper Phthalocyanine Nanowires with New Crystal Structure and Broad Optical Absorption

Abstract

The development of molecular nanostructures plays a major role in emerging organic electronic applications, as it leads to improved performance and is compatible with our increasing need for miniaturisation. In particular, nanowires have been obtained from solution or vapour phase and have displayed high conductivity 1, 2 or large interfacial areas in solar cells. 3 In all cases however, the crystal structure remains as in films or bulk, and the exploitation of wires requires extensive post-growth manipulation as their orientations are random. Here we report copper phthalocyanine (CuPc) nanowires with diameters of 10-100 nm, high directionality and unprecedented aspect ratios. We demonstrate that they adopt a new crystal phase, designated η-CuPc, where the molecules stack along the long axis. The resulting high electronic overlap along the centimetre length stacks achieved in our wires mediates antiferromagnetic couplings and broadens the optical absorption spectrum. The ability to fabricate ultralong, flexible metal phthalocyanine nanowires opens new possibilities for applications of these simple molecules.Keywords: nanowire, phthalocyanine, organic vapour phase deposition, polymorph, molecular magnetism, molecular optoelectronics.Organic vapour phase deposition (OVPD) has been very successful at generating a wide range of film morphologies with a number of functional molecular materials, including smooth amorphous films, 4 textured islands, 5 and, more recently, nanowires . 6 The latter have been investigated for a variety of applications including sensors, 7 field effect transistors 1-2, 8 and photovoltaic cells. 3 So far, with the exception of 2 some very short nanobrushes, 9 only wires randomly oriented on a substrate have been produced. Their implementation on a device therefore requires micromanipulation, which is severely limiting for future applications where texture and length are desirable.Metal phthalocyanines (MPcs) are planar aromatic macrocyles which crystallise as a range of at least fifteen distinct polymorphs. 10,11 Typically, thin films exist as either α or β-phases, which have also been observed in relatively thick ( > 100 nm in diameter) wires. 6 The optoelectronic 12 and magnetic 13 properties of Pc materials are strongly dependent on their structure, and the creation of a new polymorph can have important consequences both for devices and in industry where α, β and ε-CuPc forms are routinely used as pigments.Through the optimisation of growth parameters in the OVPD we have obtained CuPc nanowires with diameters of 10-100 nm for lengths up to 1.4 cm. They adopt a new structure, with novel electronic absorption and magnetic properties. Their high directionality could significantly facilitate parallel integration into devices, and leads to confined antiferromagnetically coupled spin chains along the long wire axis. Figure 1.a shows a photograph of the CuPc branches after a growth time of typically 120 minutes. Those branches nucleate in a region of the sample tube which is ~ 10 cm outside the...

Theoretical modeling of the electronic structure and exchange interactions in a Cu(II)Pc one-dimensional chain

“…In previous work we have reported the electronic structure of the Cu(II)Pc molecule and the variation of exchange interactions as a function of the molecular stacking angle in a molecular dimer [10,12]. The calculations of the electronic structure of particular crystalline polymorphs of Cu(II)Pc have previously been reported by several groups [13][14][15][16].…”

“…Magnetic organic materials [1][2][3][4][5] combining optical and magnetic functionality are promising candidates for novel spintronical devices [5][6][7][8] and quantum information processing (QIP) [9,10]. The electronic structure of magnetic organic materials is also of fundamental interest and underlies both their magnetic and optical properties.…”

Exchange interaction between the triplet exciton and the localized spin in copper-phthalocyanine

“…Electronic structure The Kohn-Sham energy level alignments near the HOMO-LUMO gap for the FM configuration (spin- 3 2 ) with the corresponding molecular orbitals are shown in Fig.1. The functionals chosen include further developed PBE functional, namely, PBEh1PBE, hybrid-exchange functionals O3LYP, B3LYP, PBE0, HSE06, and the long-range corrected hybrid-exchange functional CAM-B3LYP.…”

“…They can have very long spin-lattice relaxation times [11], which may pave the way towards robust electron-spin qubit. Moreover, as organic semiconductors, due to their plasticity, they can be prepared in different forms such as bulk crystalline, thin films [2,8], and nanowires [3]. In conjugation with their magnetic tunability realised by the state-of-the-art molecular growth techniques [2,8], these advanced materials properties are highly promising in the applications including quantum information processing (QIP) [11] and organic spintronics [12].…”

“…Therefore, the spin-bearing MPc, having a broad optical absorption spectrum [3], is an ideal platform for studying optically induced exchange interaction. The experiments on the triplet state of CuPc have indicated its lifetime T is very short (∼ 35 ns in Ref.…”