Stefan Martin Wippermann

Optically excited structural transition in atomic wires on surfaces at the quantum limit

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In–In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

Novel silicon phases and nanostructures for solar energy conversion

Silicon exhibits a large variety of different bulk phases, allotropes, and composite structures, such as, e.g., clathrates or nanostructures, at both higher and lower densities compared with diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, which are non-toxic and earth-abundant, with properties tailored precisely towards specific applications. We illustrate how such novel Si based materials either in the bulk or as nanostructures may be used to significantly improve the efficiency of solar energy conversion devices.

Atomic-Scale Imaging of the Surface Dipole Distribution of Stepped Surfaces.

Stepped well-ordered semiconductor surfaces are important as nanotemplates for the fabrication of 1D nanostructures. Therefore, a detailed understanding of the underlying stepped substrates is crucial for advances in this field. Although measurements of step edges are challenging for scanning force microscopy (SFM), here we present simultaneous atomically resolved SFM and Kelvin probe force microscopy (KPFM) images of a silicon vicinal surface. We find that the local contact potential difference is large at the bottom of the steps and at the restatoms on the terraces, whereas it drops at the upper part of the steps and at the adatoms on the terraces. For the interpretation of the data we performed density functional theory (DFT) calculations of the surface dipole distribution. The DFT images accurately reproduce the experiments even without including the tip in the calculations. This underlines that the high-resolution KPFM images are closely related to intrinsic properties of the surface and not only to tip-surface interactions.

Surface chemistry and buried interfaces in all-inorganic nanocrystalline solids

Semiconducting nanomaterials synthesized using wet chemical techniques play an important role in emerging optoelectronic and photonic technologies. Controlling the surface chemistry of the nano building blocks and their interfaces with ligands is one of the outstanding challenges for the rational design of these systems. We present an integrated theoretical and experimental approach to characterize, at the atomistic level, buried interfaces in solids of InAs nanoparticles capped with Sn2S64– ligands. These prototypical nanocomposites are known for their promising transport properties and unusual negative photoconductivity. We found that inorganic ligands dissociate on InAs to form a surface passivation layer. A nanocomposite with unique electronic and transport properties is formed, that exhibits type II heterojunctions favourable for exciton dissociation. We identified how the matrix density, sulfur content and specific defects may be designed to attain desirable electronic and transport properties, and we explain the origin of the measured negative photoconductivity of the nanocrystalline solids.An integrated theoretical and experimental approach characterizes buried interfaces in nanocrystal-in-glass systems at the atomistic level, revealing how matrix density and specific defects may be designed to attain desirable electronic and transport properties.

Impurity-Mediated Early Condensation of a Charge Density Wave in an Atomic Wire Array

We directly show how impurity atoms induce the condensation of a representative electronic phase, the charge density wave (CDW) phase, in atomic scale with scanning tunneling microscopy. Oxygen impurity atoms on the self-assembled metallic atomic wire array on a silicon crystal condense the CDW locally above the pristine transition temperature. More interestingly, the CDW along the wires is induced not by a single atomic impurity but by the cooperation of multiple impurities. First-principles calculations disclose the mechanism of the cooperation as the coherent superposition of the local lattice strain induced by impurities, stressing the coupled electronic and lattice degrees of freedom for the CDW. This opens the possibility of the strain engineering over electronic phases of atomic-scale systems.

Copper Substrate Catalyzes Tetraazaperopyrene Polymerization

The polymerization of tetraazaperopyrene (TAPP) molecules on a Cu(111) substrate, as observed in recent STM experiments, has been investigated in detail by first principles calculations. Tautomerization is the first step required for the formation of molecular dimers and polymers. The substrate is found to catalyze this tautomerization.

Ensemble teleportation under suboptimal conditions

The possibility of teleportation is certainly the most interesting consequence of quantum non-separability. In the present paper, the feasibility of teleportation is examined on the basis of the rigorous ensemble interpretation of quantum mechanics if non-ideal constraints are imposed on the teleportation scheme. Importance is attached both to the case of noisy Einstein–Podolsky–Rosen (EPR) ensembles and to the conditions under which automatic teleportation is still possible. The success of teleportation is discussed using a new fidelity measure which avoids the weaknesses of previous proposals.

Atomically resolved scanning force studies of vicinal Si(111)

Well-ordered stepped semiconductor surfaces attract intense attention owing to the regular arrangements of their atomic steps that makes them perfect templates for the growth of one- dimensional systems, e.g. nanowires. Here, we report on the atomic structure of the vicinal Si(111) surface with 10 degree miscut investigated by a joint frequency-modulation scanning force microscopy (FM-SFM) and ab initio approach. This popular stepped surface contains 7 x 7-reconstructed terraces oriented along the Si(111) direction, separated by a stepped region. Recently, the atomic structure of this triple step based on scanning tunneling microscopy (STM) images has been subject of debate. Unlike STM, SFM atomic resolution capability arises from chemical bonding of the tip apex with the surface atoms. Thus, for surfaces with a corrugated density of states such as semiconductors, SFM provides complementary information to STM and partially removes the dependency of the topography on the electronic structure. Our FM-SFM images with unprecedented spatial resolution on steps confirm the model based on a (7 7 10) orientation of the surface and reveal structural details of this surface. Two different FM-SFM contrasts together with density functional theory calculations explain the presence of defects, buckling and filling asymmetries on the surface. Our results evidence the important role of charge transfers between adatoms, restatoms, and dimers in the stabilisation of the structure of the vicinal surface.

Exotic phase Si nanoparticles and Si-ZnS nanocomposites: New paradigms to improve the efficiency of MEG solar cells

The efficiency of nanoparticle (NP) solar cells may substantially exceed the Shockley-Queisser limit by exploiting quantum confinement enhanced multi-exciton generation (MEG). However, (i) quantum confinement tends to increase the electronic gap and thus the MEG threshold beyond the solar spectrum and (ii) charge extraction through NP networks may be hindered by facile recombination. Using ab initio calculations we found that (i) Si NPs with exotic core structures such as BC8 exhibit significantly lower gaps and MEG thresholds than particles with diamond cores, and an order of magnitude higher MEG rates. (ii) We also investigated Si NPs embedded in a ZnS host matrix and observed complementary charge transport networks, where electron transport occurs by hopping between NPs and hole transport through the ZnS-matrix. Such complementary pathways may substantially reduce recombination, as was indeed observed in recent experiments. We employed several levels of theory, including DFT with hybrid functionals and GW calculations.

Comparative study of Si and Ge nanoparticles with exotic core phases for solar energy conversion

Third generation photovoltaic cells promise to overcome the Schockley-Quessier limit of solar cell energy conversion. In the Multiple Exciton Generation (MEG) pathway quantum confined highly energetic electron-hole pairs relax by emitting additional electron-hole pairs. The overall utility of this process is undermined, however, by the very fact that quantum confinement pushes the gap of nanoparticles (NPs) out of the solar spectrum. Here we propose that Si and Ge NPs with core structures made out of exotic high-pressure phases of bulk Si and Ge have lower gaps, more intense absorption and higher MEG rates than those of made out of the cubic diamond phase. Some of these exotic phases have already been proven to exist in colloidal NPs or on laser treated surfaces, therefore, our findings may open the door for promising solar applications of such exotic nanoparticle systems.