Selecting the tip electron orbital for scanning tunneling microscopy imaging with sub-ångström lateral resolution

We report on scanning tunneling microscopy (STM) studies performed with single crystalline W[001] tips on a graphite(0001) surface. Results of distance-dependent STM experiments with sub-̊angström lateral resolution and density functional theory electronic structure calculations show how to controllably select one of the tip electron orbitals for highresolution STM imaging. This is confirmed by experimental images reproducing the shape of the 5dxz,yz and 5dx2−y2 tungsten atomic orbitals. The presented data demonstrate that the application of oriented single crystalline probes can provide further control of spatial resolution and expand the capabilities of STM. Copyright c © EPLA, 2010 In quantum mechanics, a single-electron state in an atom is described by a set of wave functions associated with a particular electron energy, orbital momentum, spin and momentum projections on the quantization axis. The spatial distribution of these wave functions (atomic orbitals) determines the probability for an electron with quantum numbers n, l, ml, ms to be detected in a particular volume of space. This theoretical formalism allows to explain the behavior of the electron systems on the atomic level. However, for a long time the electron orbitals could not be directly observed in experiments. This only became possible with invention of scanning probe microscopy (SPM) methods [1–3]. These use sharp tips to image [4] and manipulate [5] atomic structures, to identify the chemical nature of atomic species [6,7] and to reveal magnetic contrast [8] on surfaces. The ultimate vertical resolution of scanning tunneling microscopy (STM) is determined by exponential dependence of the tunneling current between a sample and a tip [9]. The lateral resolution can be limited by the technical characteristics of a microscope. One of the first theoretical explanations of the atomic resolution was proposed by Tersoff and Hamann [10,11] who considered a spherically symmetric s-wave tip. The theory was (a)E-mail: [email protected] further advanced by Chen [12–14] who showed that the tunneling matrix elements are proportional to the derivatives of the wave functions of electrons involved in the tunneling process. Therefore, a variety of electron states can contribute to experimental images and, consequently, the limit of lateral resolution is physically restricted by spatial distribution of the tip and surface wave functions. However, it took two decades after the invention of STM to measure the asymmetric charge distribution related to apex atomic orbitals in atomic force microscopy (AFM) [15,16] and STM [17–19] experiments. The observed subatomic features [15–19] still could not be unambiguously identified with single-electron orbitals. Recently, two legs of the MnNi tip dxz-orbital (l= 2, ml = 1) were resolved in STM experiments on a Cu(014)-O surface [20,21]. Essential distance dependence of the atomically resolved Cu(014)-O STM images [21,22], however, did not allow to reveal the conditions where this tip orbital channels most of the tunneling current. The observed distance dependence, leading to chemical selective imaging either copper or oxygen atomic rows of the Cu(014)-O surface [7,21,22], was attributed to change of relative contribution of different electron orbitals of the surface atoms. However, the distance-dependent contribution of different d-orbitals could be proved experimentally only by direct experiments demonstrating