Variable temperature scanning tunneling microscopy and spectroscopy: Electronic and physical properties of single and two component thin films

A newly acquired, commercial scanning tunneling microscope (STM) designed to work in ultra high vacuum at variable temperatures down to 50 K was used for scanning tunneling microscopy and spectroscopy (STM/STS) studies of organic adsorbates on metal surfaces. Au(111) substrates were prepared by evaporation of gold onto mica. Cobalt(II) phthalocyanine (CoPc), cobalt(II) tetraphenyl porphyrin (CoTPP), and vanadyl(II) phthalocyanine (VOPc) were deposited from vapor onto the substrates to form submonolayer, single and two component thin films. Ultraviolet photoelectron spectroscopy and metal-insulator-sample-metal ′ (MISM′) tunnel diode spectroscopy were used as complementary techniques to compare ionization potentials and electron affinities.; STM constant current images of CoPc/CoTPP mixtures show two compositionally disordered close-packed surface structures. CoPc can also be observed as 1-d chains and single, isolated molecules below 220 K, and these structures cannot be imaged at 294 K (the areas appear noisy) unless the coverage is very close to a full monolayer. This is attributed to surface diffusion of the molecules. Molecular identification is determined by the appearance of the molecules in high resolution images and by orbital mediated tunneling spectroscopy (OMTS). Occupied and unoccupied orbitals are identified by the typical broad peaks observed in dI/dV versus V, and by sharp steps in z versus V. The OMT dependent z(V) curves are unique and this is the first report of such observations. The transient oxidation of the Co d_z2 is identified in dI/dV(V) just negative of 0V bias for both molecules. Direct comparison of d_z2 OMT detected in constant current images for CoPc and CoTPP reveals a lower Co-Au electron transfer rate for CoTPP, which is attributed to the ∼0.15 nm greater Co-Au distance for CoTPP relative to CoPc.; STM/STS of pure VOPc films has previously shown some unusual properties which are not consistent with observations of CoPc or with UPS data. These observations consisted of extraordinarily strong π*-LUMO mediated tunneling, no observed π-HOMO mediated tunneling, and anomalous changes in apparent height. Codeposition of VOPc with CoPc for a reference also yielded very unusual results that were not consistent with previous two-component MPc thin film studies.