XPS and SERS Study of Silicon Phthalocyanine Monolayers:  Umbrella vs Octopus Design Strategies for Formation of Oriented SAMs

Abstract

Two strategies are compared for the formation of self-assembled monolayers (SAMs) of silicon phthalocyanines on gold. Silicon phthalocyanines were synthesized with thiol anchoring groups in either eight peripheral side chains (the "octopus") or with one short thiol in an axial position (the "umbrella"). Both approaches gave phthalocyanines capable of forming SAMs on gold surfaces. The orientation and coverage of the phthalocyanines were compared using ellipsometry, X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman scattering (SERS). The octopus silicon phthalocyanines form poorly organized SAMs in which the phthalocyanine (Pc) rings are strongly tilted with respect to the gold surface. On average, between 3 and 4 of the thiol "arms" fail to bind to the gold surface, even when limiting coverage is achieved after 7 days of soaking. The film thickness is 22 ( 5 Å. In contrast, the umbrella silicon phthalocyanine produces close-packed SAMs within 1 h in which the Pc rings lie parallel to the gold surface. The average thickness of the later SAMs is 11 ( 3 Å, and each phthalocyanine ring occupies an average area of 284 Å 2 .

Insights into the surface and redox properties of single-walled carbon nanotube—cobalt(II) tetra-aminophthalocyanine self-assembled on gold electrode

“…1 shows the comparative survey X-ray photoelectron spectra of the (i) bare; (ii) Au-AET; (iii) Au-AET-SWCNT; and (iv) Au-AET-SWCNT-CoTAPc. The S (2p) peak at 162.5 eV is assigned to the normal gold-sulphur bond [16], [55], [56], [57], [58] and [59]. The presence of the N atoms (in the BE region of 395-405 eV) in the SAMs are evident in Fig.…”

“…The sharpness and high intensity of this peak openUP possibly indicates the presence of free or unbound amino groups of the CoTAPc. It is well known [16] that oxygen and carbon are almost always found on putatively bare surfaces due to contaminants (e.g., "adventitious carbon"). Despite this disadvantage, however, the C (1s) and O (1s) peaks observed in the SAMs studied in this work provide some useful information on the carbon and oxygen moieties.…”

“…Self-assembly represents a more efficient technique for fabricating electrodes based on CNT [8], [12] and [13] or MPc [14], [15], [16], [17], [18], [19] and [20] thin films compared to such other thin film formation methodologies such as drop-dry and spin coating.…”

Synthesis and electrochemical properties of benzyl-mercapto and dodecyl-mercapto tetrasubstituted manganese phthalocyanine complexes

“…They have also been found to be useful in electrode modification for use as electrocatalysts and electrochemical sensors. Thiol-derivatised MPc complexes are employed for the modification of gold electrodes by the self-assembled monolayer (SAM) technique [7], [8] and [9]. However, the number of MPc complexes substituted with thiols is still limited and the electrochemistry of these molecules has not been fully explored, this is due to the fact that the synthesis of thiolderivatised metallophthalocyanines is tedious and requires toxic chemical reagents.…”

Voltammetric characterisation of the self-assembled monolayers (SAMs) of benzyl- and dodecyl-mercapto tetra substituted metallophthalocyanines complexes

“…Notable advantages of SAMs over other techniques for electrode modification are the high order and stability of the molecules on electrodes [19], [20] and [21]. The work on the use of thiol derivatised metallophthalocyanines to form SAMs on gold electrodes is still rather limited [15], [18], [22], [23] and [24] mainly due to the fact that openUP the synthesis of thiol-derivatised metallophthalocyanines is tedious and requires toxic chemical reagents.…”