Welcome to the Atomic-Scale Science on Insulators Group!

In this work we found a way to access energies of charge exchange for ground and excited states of a single molecule. This allows probing many quantum transitions of different types individually, including radiative and non-radiative transitions and redox transitions, in which the charge state changes. The novel spectroscopy builds on the recently developed techniques to measure the tunneling of single electrons between a conductive tip of an atomic force microscope and a single molecule. Specifically, by slowly changing the energy of the electrons available in the tip and observing when the molecule undergoes charge-state transitions, the different excited states could be accessed, identified and their energies measured. We envision that this technique could be applied for the quantification of excitation energies that are difficult to access otherwise, for example those of triplet excitations.? Similarly, it can guide the understanding of STM-induced luminescence experiments on ultrathin insulating films, for which sequential and competing transitions are extremely challenging to characterize individually.

In this work we expanded lightwave-driven scanning tunneling microscopy combining atomic scale and sub-picosecond temporal resolution by its spectroscopy variant, now additionally providing energy resolution. Our results, thus, establish ultrafast spectroscopy on atomic length scales providing single-entity and real-space access to the ultrafast local density of states, thereby complementing techniques operating in momentum space, such as time-resolved photoemission orbital tomography. Specifically, this new combination of atomic, sub-picosecond and 10-meV-scale resolution in lightwave-driven scanning tunneling? spectroscopy allowed us to observe how the energy levels of a single Se vacancy in a tungsten diselenide monolayer evolve during dynamic atomic displacements. We revealed how the excitation by a pump pulse adiabatically shifts the first defect level on picosecond timescales.

Routinely, molecules are imaged using atomic force microscopy (AFM), giving a visual insight into their structure. Electron spin resonance (ESR) is commonly used to characterize compounds in chemistry providing complementary information, even giving access to the isotopic composition. But ESR typically relies on measuring a countless number of molecules. In this publication, we demonstrate that we combined ESR with AFM, detecting the ESR signal via the measurement of the triplet lifetime (see below). Thereby, we could measure ESR-AFM spectra with sub-nanoelectronvolt spectral resolution in a molecule-by-molecule fashion, allowing to determine the isotopic composition of each individual molecule being measured and imaged.
Importantly, ESR relies on the manipulation of electron spin states. Due to the weakly perturbing nature of the ESR-AFM technique, we could coherently manipulate the electron spins in pentacene over tens of microseconds. The combination of atomic-scale spatial information provided by AFM and access to long coherences by means of ESR-AFM could offer insight into atomic aspects of decoherence mechanisms, potentially relevant for future quantum information processing.

The excited triplet in organic molecules can be quite long lived because the transition to the singlet ground state is optically forbidden. Nearby oxygen is known to quench the triplet state. We now succeeded in tracking this transfer of energy between a pentacene and an oxygen molecule directly in space. In the junction of an atomic force microscope we applied a sequence of fast electrical pulses to the pentacene molecule, driving it into the magnetic triplet state in a controlled fashion. The energy transfer from this excited triplet state to oxygen molecules nearby was then tracked in time by measuring miniscule changes in the force acting on the tip. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet.

Visible light has a wavelength more than a thousand times larger than the size of an atom. Quite ironically, while the fundamental arrangement of the atomic world can nowadays be routinely imaged, at these atomistic length scales the behavior of light remains a mystery in many aspects. This applies in particular to the temporal behavior of light at these ultrasmall scales, where the well-known laws of classical physics lose their validity and quantum physics rules.

Together with Prof. Rupert Huber and his team as well as our theory collaborators at the MPSD in Hamburg we have developed a method to detect the dynamics of light on such a small scale with high temporal resolution. The key ingredient is a single-molecule switch acting as an atomic-scale voltage standard.

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Implementing functionality in molecular self-assembled monolayers

Nemanja?Koci?, Dominik?Blank, Paula?Abufager, Nicolas?Lorente, Silvio?Decurtins, Shi-Xia?Liu and Jascha?Repp

In any type of future molecular-based quantum cellular automata, the perfect placement and alignment of cells is of crucial importance, as the latter critically determines the interaction between neighboring cells. Self-assembly allow for such a precise atomic-scale alignment of zillions of mol