Teemu Hynninen

Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.

AFM tip characterization by Kelvin probe force microscopy

Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.

Hydrophobicity within the three-dimensional Mercedes-Benz model : potential of mean force

We use the three-dimensional Mercedes-Benz model for water and Monte Carlo simulations to study the structure and thermodynamics of the hydrophobic interaction. Radial distribution functions are used to classify different cases of the interaction, namely, contact configurations, solvent separated configurations, and desolvation configurations. The temperature dependence of these cases is shown to be in qualitative agreement with atomistic models of water. In particular, while the energy for the formation of contact configurations is favored by entropy, its strengthening with increasing temperature is accounted for by enthalpy. This is consistent with our simulated heat capacity. An important feature of the model is that it can be used to account for well-converged thermodynamics quantities, e.g., the heat capacity of transfer. Microscopic mechanisms for the temperature dependence of the hydrophobic interaction are discussed at the molecular level based on the conceptual simplicity of the model.

Two-dimensional nanostructured growth of nanoclusters and molecules on insulating surfaces.

Noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM) and first principle calculations show that the nanostructured (001) Suzuki surface of Cd(2+) doped NaCl can be used to confine the growth of palladium clusters and functionalized brominated pentahelicene molecules into only the Suzuki regions, which contain the impurities. The Suzuki surface is an ideal model surface for nanostructuring metal clusters and molecules.

High Curie temperatures in "Ga,Mn…N from Mn clustering

The effect of microscopic Mn cluster distribution on the Curie temperature (TC) of (Ga,Mn)N is studied using density-functional calculations together with the mean field approximation. We find that the calculated TC depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental TC values from a few K to well above room temperature. The partially dimerized Mn2-Mn1 distribution is found to give the highest TC>500K, and in general, the presence of the Mn2 dimer has a tendency to enhance TC. The lowest TC values close to zero are obtained for the Mn4-Mn1 and Mn4-Mn3 distributions.

Faster simulations of step bunching during anisotropic etching: formation of zigzag structures on Si(1 1 0)

We propose that the formation of zigzag structures on Si(1 1 0) during anisotropic etching is mainly a result of the formation of inhomogeneous regions in the etchant due to diffusion phenomena. In the same way as the presence of these etchant inhomogeneities results in step bunches on miscut (1 1 1) surfaces, it results in zigzags on the (1 1 0) surface. To support this proposal, we present an incremental activity monitoring (IAM) method for the simulation of step bunching using a kinetic Monte Carlo scheme. For stepped (1 1 1) surfaces, comparison with a previous step density monitoring (SDM) method shows that IAM is typically faster by one order of magnitude and is well suited for the simulation of step bunching. By applying IAM to (1 1 0), the formation of zigzag structures can be simulated, strongly suggesting that the morphology of this surface is dominated by the formation of inhomogeneous regions close to the surface in the etchant phase. (Some figures in this article are in colour only in the electronic version)

Defect mediated manipulation of nanoclusters on an insulator

With modern scanning probe microscopes, it is possible to manipulate surface structures even at the atomic level. However, manipulation of nanoscale objects such as clusters is often more relevant and also more challenging due to the complicated interactions between the surface, cluster and apparatus. We demonstrate the manipulation of nanometer scale gold clusters on the NaCl(001) surface with a non-contact atomic force microscope, and show that the movement of clusters is in certain cases constrained to specific crystallographic directions. First principles calculations explain this kinetic anisotropy as the result of the cluster attaching to surface defects: cation vacancies allow the clusters to bond in such a way that they only move in one direction. Constraining the movement of clusters could be exploited in the construction of nanostructures or nanomechanical devices, and the manipulation signatures may also be used for identifying cluster-defect complexes.

Effect of Cu impurities on wet etching of Si(110): formation of trapezoidal hillocks

We simulate the formation of experimentally observed trapezoidal hillocks on etched Si(110) surfaces, describing their generic geometrical shape and analyzing the relative stability and/or reactivity of the key surface sites. In our model, the hillocks are stabilized by Cu impurities in the etchant adsorbing on the surface and acting as pinning agents. A model of random adsorptions will not result in hillock formation since a single impurity is easily removed from the surface. Instead a whole cluster of Cu atoms is needed as a mask to stabilize a hillock. Therefore we propose and analyze mechanisms that drive correlated adsorptions and lead to stable Cu clusters.

Adsorption of metal impurities on H-terminated Si surfaces and their influence on the wet chemical etching of Si

We use first-principles methods to investigate the adsorption of Cu, Pb, Ag, and Mg onto a H-terminated Si surface. We show that Cu and Pb can adsorb strongly while Ag and Mg are fairly inert. In addition, adsorption states of two types are seen to exist for Pb. We also study the clustering energetics of Cu and Pb on the surface and find that while Cu clusters eagerly, Pb may prefer to form only small clusters of a few atoms. This kind of behavior of impurities is incorporated in kinetic Monte Carlo simulations of wet etching of Si. The simulation results agree with experiments supporting the idea that micromasking by Cu clusters and Pb atoms is the mechanism through which these impurities affect the etching process.

A multiscale study of ferromagnetism in clustered (Ga,Mn)N

Magnetic interactions in (Ga,Mn)N are studied on the microscopic intercluster and intra-cluster scales using first-principles calculations. Ferromagnetic transition temperatures are calculated using Monte Carlo methods. Randomness in Mn substitution is found to reduce Curie temperatures by about 10–20% fro mt hose of the corresponding regular (Ga,Mn)N lattice due to clustering. Nevertheless, high Curie temperatures reaching above room temperature are obtained even for completely random Mn distribution in (Ga,Mn)N for the Mn concentration of x 13.5%. Increasing clustering is always found to decrease the Curie temperature—especially when tetramer clusters are formed. The active search for diluted magnetic semiconductors (DMSs) with high Curie temperatures (TCs) exceeding room temperature started with the discovery of ferromagnetism in (In,Mn)As [1]. The calculations by Dietl et al predicted that (Ga,Mn)N should have the highest TC (� 400 K) among the prospective III–V DMS materials suggested fo rs pintronics applications [2]. However, the measured TC values for (Ga,Mn)N range from 10 to 940 K [3–6] and also paramagnetic behaviour is reported at lower Mn concentrations (typically x < 0.02) [7]. The reasons for the wide range of the observed TC values and especially for the high values are poorly understood. One of the suggestions for this anomalous behaviour of TC is that the increase is due to clustering (or precipitation) of Mn atoms and the formation of giant magnetic moments at the Mn clusters [6, 8–10]. However, Mn clustering is usually found to decrease calculated TC s[ 11–15 ]a lthough in some very special situations an increase may also be obtained [9–11]. In this paper we consider small clusters and define a Mnn cluster as a collection of n substitutional Mn atoms (n = 2, 3, and 4 for dimers, trimers, and tetramers, respectively) which have a common N neighbour. At high Mn concentrations x ac onsiderable amount of Mn clusters are present even in the case of a completely random distribution of substitutional