Nanoscale Conductance in Lead Phthalocyanine Thin Films: Influence of Molecular Packing and Humidity

Abstract

The influence of crystalline phase and disorder on vertical charge transport in nonplanar metallophthalocyanines is studied by depositing thin films of shuttlecock molecules, lead phthalocyanine (PbPc), and SnPc on various substrates such as highly oriented pyrolytic graphite (HOPG), silicon, and gold and investigating the structural and electrical characteristics of the films. The films represent three cases of crystalline packing such as monoclinic, triclinic, and disordered phases, directed by the substrate−molecule interactions and intermolecular interactions. Theoretical calculations are performed to understand the molecular orientations on the substrate and give insights into the possible origin of ordered and disordered packing. Vertical charge transport in the films is studied by employing conducting atomic force microscopy and compared with bulk electrical measurements. It is revealed that the monoclinic phase of PbPc on HOPG exhibits two orders of magnitude higher conductance than silicon or gold and is related to the ordered stacking of the macrocycles through which the charge transport is facilitated. Local conductance mapping reveals inhomogeneity among conducting domains and is correlated with the grain structure. The charge transport is best fitted with tunneling mechanism for nanoscale and space-charge-limited current for bulk measurements. The presence of humidity is also shown to influence the nanoscale charge transport with enhanced conductance. Under saturating humidity conditions, the conductance is found to decrease. This study emphasizes the implication of graphite or graphite-like platforms that promote the ordered stacking of delocalized molecular systems for enhanced vertical charge transport.

Metallophthalocyanine-nanofibre-based electrodes for electrochemical sensing of biomolecules

“…Metal phthalocyanines (MPcs) are highly delocalized aromatic systems possessing metal ions at the centre of the macrocycle. They are structural analogues of porphyrins and exhibit unique physicochemical properties, which depend on their molecular packing [1]. Apart from being thermally and chemically stable, they also possess rich redox chemistry, which enables them to be useful candidates for electrochemical applications including electrocatalysis, supercapacitance, fuel cells, etc.…”

Templating effect of single-layer graphene supported by an insulating substrate on the molecular orientation of lead phthalocyanine

“…In our earlier studies, we have reproducibly obtained different crystallites of PbPc on substrates such as HOPG, Au(111), Si and SiO 2 . Detailed studies using Raman spectroscopy and 2D-GIXRD show ordered monoclinic and triclinic moieties on HOPG and Si substrates, respectively, while the Au(111) surface gives rise to disordered fractions due to the absence of long-range ordering [9,26]. In the present study, the presence of single-layer graphene on SiO 2 has a templating effect and induces a monoclinic packing of PbPc in the face-on configuration.…”

“…The molecular orientation of both planar and nonplanar MPc molecules on oxide substrates such as SiO 2 is well known. The molecules were shown to preferably have an edge-on orientation [3,7,9,[27][28][29]. In our earlier studies, we have reproducibly obtained different crystallites of PbPc on substrates such as HOPG, Au(111), Si and SiO 2 .…”

“…The signature of monoclinic moieties along the q z direction indicates that the PbPc molecules exhibit ordered stacking, normal to the substrate plane. This type of vertical ordering known as cofacial arrangement can promote charge transport in the vertical direction [6,9,26]. Further, a small fraction of the (320) reflection is measured at an azimuthal angle ( Figure 3c).…”

“…Pristine substrate surfaces of HOPG and Si themselves can induce orientation control over the growth of MPc structures without the aid of additional templating layers. In our earlier work, we have observed that orientation and molecular packing of nonplanar PbPc molecules are influenced by the nature of the substrate, which has been attributed to different substrate-molecule interactions [9]. With the application of 2D materials, such as graphene in device configurations, it is important to understand the orientation of MPc molecules on these atomically thin materials [10,11].…”