Electrostatic screening mediated by interfacial charge transfer in molecular assemblies on semiconductor substrates

Abstract

Using scanning tunneling microscopy and spectroscopy (STM/STS), we report the electronic structures of self-assembled zinc phthalocyanine (ZnPc) and hexadecafluorinated zinc phthalocyanine (F 16 ZnPc) monolayers on the Si(111)-B surface. We show that interfacial charge transfer occurs in the F 16 ZnPc monolayer, which gives rise to a pronounced spatial variation of the occupied molecular state across the molecular assembly, a feature not observed in the molecular states of the ZnPc overlayer without the presence of interfacial charge transfer. We attribute this observation to the inhomogeneous electrostatic screening of the intra-orbital Coulomb interaction in molecular adsorbates arising from the substrate boron distribution. This study highlights the impact of the substrate electrostatic environment on molecular electronic structures, an essential aspect in the applications of organic and molecular electronic devices.

Tailoring the growth and electronic structures of organic molecular thin films

“…Scanning probe techniques like STM offer the ability to identify the local ordering of the molecular thin film, allowing for the precise determination of the nature of defects within the film. Furthermore, STM, when used in conjunction with STS, enables the simultaneous measurement of the geometric and electronic structure of the thin film, which will be discussed in more detail later [36,[106][107][108][109].…”

“…Since charge injection barrier and contact resistance in electronic devices are most directly related to the electronic structures of organic molecules near the contact interface, we discuss the effects of substrate electrostatic screening quanti tatively. As shown in figure 4(c), the substrate screening effect can be estimated by the classical image-charge model qq 4π o •2(d−zo) , where d is the molecule-substrate distance, q = q ( − 1) / ( + 1) is the effective image charge, and z o the effective position of the image plane [41,106,[169][170][171]]. Metals, for example, typically have more protruding image planes, and when considered in conjunction with the enormous collection of free charge carriers, results in a very large screening capability.…”

“…Naturally, this image charge effect will decay with increasing film thickness, which often leads to the widening of the band gap in thicker molecular films [164,175,176]. It should be noted that the image charge plane position can also be adjusted by variations in the surface or even the sub-surface features of the same substrate [106,177].…”

“…ICT implies a reversible and dynamic charging of molecules where the charge state of each individual molecule is strongly dependent on the arrangement and charge states of neighboring molecules at any given moment in time. While ensemble averaging techniques such as photoemission spectroscopy have been able to detect the coexistence of neutral and charged molecules in organized molecular assemblies, STM/ STS has primarily verified the electronic effects of the charged states of individual molecules [106,163,174,[206][207][208]. STM/STS has shown compelling evidence on the SOMO-SUMO splitting, for instance, of individual pentacene molecules adsorbed on top of NaCl/Cu(1 1 1) with the charge state of molecules controlled through side attachment of Au atoms (figure 9(a)) [210].…”

“…Despite the evidence of SOMO-SUMO splitting of individual charged molecules, STS's lack of temporal resolution and the strong electric field between the tip and sample makes the measurement of the reversible and dynamic charging behavior in molecular assemblies extremely difficult [207]. To the best of our knowledge, scanning probe experiments performed on organic molecule assemblies on weakly interacting substrates have either revealed the presence of neutral molecular systems or fully charged assemblies, in which all the molecules appear similar regardless of their initial charge state [41,106,163,174,204,207,214]. As an example, spatially resolved mapping of apparent barrier height, determined by the local work function of the sample, is performed using dI/ dS (S: tip-sample distance) STM/STS technique on the TBP (tetra[1,3-di(tert-butyl)phenyl]pyrene) assemblies on Au(1 1 1) and Cu(1 1 1), respectively [207].…”