Oxide-organic heterostructures: a case study of charge transfer disturbance at a SnO2-copper phthalocyanine buried interface.

Maciej Krzywiecki,Julian Rechmann,Monika Kwoka,Lucyna Grządziel,Paulina Powroźnik,Andreas Erbe

doi:10.1039/c8cp01976b

Abstract

Reduced tin dioxide/copper phthalocyanine (SnOx/CuPc) heterojunctions recently gained much attention in hybrid electronics due to their defect structure, allowing tuning of the electronic properties at the interface towards particular needs. In this work, we focus on the creation and analysis of the interface between the oxide and organic layer. The inorganic/organic heterojunction was created by depositing CuPc on SnOx layers prepared with the rheotaxial growth and vacuum oxidation (RGVO) method. Exploiting surface sensitive photoelectron spectroscopy techniques, angle dependent X-ray and UV photoelectron spectroscopy (ADXPS and UPS, respectively), supported by semi-empirical simulations, the role of carbon from adventitious organic adsorbates directly at the SnOx/CuPc interface was investigated. The adventitious organic adsorbates were blocking electronic interactions between the environment and surface, hence pinning energy levels. A significant interface dipole of 0.4 eV was detected, compensating for the difference in work functions of the materials in contact, however, without full alignment of the energy levels. From the ADXPS and UPS results, a detailed diagram of the interfacial electronic structure was constructed, giving insight into how to tailor SnOx/CuPc heterojunctions towards specific applications. On the one hand, parasitic surface contamination could be utilized in technology for passivation-like processes. On the other hand, if one needs to keep the oxide's surficial interactions fully accessible, like in the case of stacked electronic systems or gas sensor applications, carbon contamination must be carefully avoided at each processing step.

Highlights

We present photoemission-based studies on the interface between SnO1oxo[2] layers obtained by the rheotaxial growth and vacuum oxidation (RGVO) technique, hereafter called RGVO–SnOx, and CuPc ultrathin films deposited by physical vapor deposition (PVD)
The required la and inelastic mean free path (IMFP) parameters were determined from the NIST electron inelastic mean free path database and electron attenuation length calculating software,[39] which utilizes algorithms based on the analysis proposed by Werner,[40] and applying the TPP-2M (Tanuma–Powell–Penn)[41] algorithm
The chemical composition of the prepared RGVO–SnOx and their CuPc-covered analogues was monitored on the basis of the Sn 3d, O 1s and C 1s energy regions with XPS

Summary

Introduction

Hybrid heterojunctions created out of inorganic–organic material stacks are currently attracting more and more attention due to the number of technological applications.[1,2,3] Especially low-dimensional heterostructures with applications varying from photovoltaics[4,5] through other optoelectronic and thermoelectronic devices[6,7,8] to inorganic–organic transistors[9] are at the center of interest.Characterization of a number of systems has been lately presented in the field of hybrid materials,[13,14] highlighting the need to carefully control several aspects of the interface structure in order to obtain junctions of desired properties. Angle dependent X-ray photoelectron spectroscopy (ADXPS) was used in our former studies on ultra-thin tin oxides.[32] In the current work, we present photoemission-based studies on the interface between SnO1oxo[2] layers obtained by the rheotaxial growth and vacuum oxidation (RGVO) technique, hereafter called RGVO–SnOx, and CuPc ultrathin films deposited by physical vapor deposition (PVD).

Results

Conclusion