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Mass sensing with nanoelectromechanical systems has advanced significantly during the last decade. With nanoelectromechanical systems sensors it is now possible to carry out ultrasensitive detection of gaseous analytes, to achieve atomic-scale mass resolution and to perform mass spectrometry on single proteins. Here, we demonstrate that the spatial distribution of mass within an individual analyte can be imaged - in real time and at the molecular scale - when it adsorbs onto a nanomechanical resonator. Each single-molecule adsorption event induces discrete, time-correlated perturbations to all modal frequencies of the device. We show that by continuously monitoring a multiplicity of vibrational modes, the spatial moments of mass distribution can be deduced for individual analytes, one-by-one, as they adsorb. We validate this method for inertial imaging, using both experimental measurements of multimode frequency shifts and numerical simulations, to analyse the inertial mass, position of adsorption and the size and shape of individual analytes. Unlike conventional imaging, the minimum analyte size detectable through nanomechanical inertial imaging is not limited by wavelength-dependent diffraction phenomena. Instead, frequency fluctuation processes determine the ultimate attainable resolution. Advanced nanoelectromechanical devices appear capable of resolving molecular-scale analytes.

Current approaches to mass spectrometry (MS) require ionization of the analytes of interest. For high-mass species, the resulting charge state distribution can be complex and difficult to interpret correctly. Here, using a setup comprising both conventional time-of-flight MS (TOF-MS) and nano-electromechanical systems-based MS (NEMS-MS) in situ, we show directly that NEMS-MS analysis is insensitive to charge state: the spectrum consists of a single peak whatever the species’ charge state, making it significantly clearer than existing MS analysis. In subsequent tests, all the charged particles are electrostatically removed from the beam, and unlike TOF-MS, NEMS-MS can still measure masses. This demonstrates the possibility to measure mass spectra for neutral particles. Thus, it is possible to envisage MS-based studies of analytes that are incompatible with current ionization techniques and the way is now open for the development of cutting-edge system architectures with unique analytical capability., Mass spectrometry (MS) involves ionization of analytes with spectra dependent upon the mass-to-charge ratio. Here, the authors demonstrate that MS based on nanoelectromechanical systems gives results that are independent of the charge state and allow the mass spectrum of neutral species to be obtained.

The coupled dynamics of a nanomechanical resonator and superconducting quantum circuits are studied in three experiments, in the context of studying the quantum limit for force detection and quantum physics of macroscopic objects. In the first experiment, the dispersive mechanical resonance shift from the interaction with a Cooper-pair box qubit is studied. The measured coupling strength is large enough to satisfy one of the conditions required to perform many of the proposed quantum nanomechanical measurements. The resonance shift also probes the microwave-driven response of the qubit, showing Rabi oscillation and Landau-Zener tunneling, proving the coherent dynamics of the qubit. Second, the parametric excitation of nanomechanical motion is studied via experiments with a driven qubit. Degenerate parametric amplification and oscillation are demonstrated, with a new observation of nonlinear dissipation. The squeezing of the back-action noise from the detection amplifier is also observed, up to 4dB. It is the first demonstration using a qubit as an auxiliary system to modify the nanomechanical dynamics, showing a possible route for generation of nanomechanical quantum states. Finally, back-action cooling of nanomechanical motion has been investigated, which is implemented by capacitively coupling a high-Q coplanar waveguide microwave resonator to a nanomechanical resonator. The thermal state with 7.5 mechanical quanta on average is reached, a result that is ultimately limited due to increased bath heating with microwave power. The heating is consistent with a model based on two-level systems resonantly coupled to the nanomechanical mode. This additional heating suggests future efforts to improve coupling and for reducing two-level system density in materials employed to reach the motional ground state via back-action cooling.

Fullerene peapods, that is carbon nanotubes encapsulating fullerene molecules, can offer enhanced functionality with respect to empty nanotubes. However, the present incomplete understanding of how a nanotube is affected by entrapped fullerenes is an obstacle for peapods to reach their full potential in nanoscale electronic applications. Here, we investigate the effect of C60 fullerenes on electron transport via peapod quantum dots. Compared to empty nanotubes, we find an abnormal temperature dependence of Coulomb blockade oscillations, indicating the presence of a nanoelectromechanical coupling between electronic states of the nanotube and mechanical vibrations of the fullerenes. This provides a method to detect the C60 presence and to probe the interplay between electrical and mechanical excitations in peapods, which thus emerge as a new class of nanoelectromechanical systems., Comment: 7 pages, 3 figures. Published in Nature Communications. Free online access to the published version until Sept 30th, 2010, see http://www.nature.com/ncomms/journal/v1/n4/abs/ncomms1034.html

In this thesis single molecules, metal-organic complexes and single adatoms are characterized on a metal surface by means of scanning tunneling microscopy (STM). Specifically the organic molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) is primarily in the focus. The molecule has two functional groups, a pyridyl and a carboxylic group connected by a vinyl group giving rise to versatile electronic properties and interaction mechanisms. The PVBA molecules are deposited by molecular beam epitaxy on a Cu(111) substrate. Furthermore, Co is deposited on Cu(111) to study the properties of a single magnetic atom on the surface and in contact with the tip. The experimental studies are supported by calculations performed in collaboration with theoretical groups and are reported here to strengthen the interpretation of the experimental results allowing an enhanced view of the investigated structures from a different perspective.In Chapter 2 an overview of the theoretical background on scanning tunneling microscopy and spectroscopy is given. Other important spectroscopic techniques employed in this study, such as time resolved and point-contact spectroscopy are introduced. Besides an explanation of the experimental setup, a short introduction to density functional theory is given.In Chapter 3 the adsorption and electronic properties of single PVBA molecules and selfassembled supra-molecular structures are investigated by scanning tunneling microscopy and spectroscopy. Specific emphasis is given to the chiral nature of PVBA on the surface.In Chapter 4 the molecule-surface interface is investigated in detail. Measuring the current vs. tip-sample distance allows to unravel information about the potential barrier on and surrounding a molecule. Another aspect of this interface is the direct interaction of the molecules with the substrate. The adsorption geometry of PVBA serves to reveal information about the interaction with the substrate and the role of sub-surface atomic layers is explored. On surfaces such as Cu(111), a surface state exists, which lets electrons behave as a nearly free two-dimensional electron gas. Such electrons can scatter at adsorbates, such as single atoms or molecules. Due to the particle-wave duality this results in a characteristic interference pattern surrounding the adsorbate. The correlation between these patterns and the internal structure of adsorbed scatterers, such as molecules and metalorganic complexes, is presented, whereas specific emphasis is devoted to chiral scatterers.In Chapter 5 the use of the scanning tunneling microscope as a nanotool to manipulate adsorbates on the surface is reported. Two main techniques are employed to generate a displacement and modification of the adsorbates. Whereas in one case the adsorbate, such as a single adatom, is moved mechanically with the apex of the tip, in the other cases controlled voltage pulses are applied to manipulate and chemically modify single atoms and molecular structures. The formation of a metal-ligand bond of a single Cu atom and a PVBA molecule is explored. Furthermore, a transitory bond, where the bonding state varies as a function of time, is found for a self-assembled metal complex consisting of two PVBA molecules and one Cu atom. This is characterized spatially and temporally and the results are compared to the electronic structure measurements of the complex given in Chapter 3. By applying large voltages the molecule is subject to chemical changes. Several processes will be identified ranging from deprotonation, dehydrogentaion to a decomposition of PVBA. At the end of this chapter the formation of Co dimers is investigated by applying an electric field leading to an enhanced lateral movement of single atoms on the surface, which drives the system towards the formation of dimers.In the last Chapter investigations of operating the microscope in the point contact regime are reported. This regime is accessed by bringing the tip much closer to the surface than in the tunneling regime. As an example for single atom contacts, individual Co adatoms on Cu(111) are studied. Because Co atoms are single magnetic impurities with a localized spin, the itinerant electrons of the supporting non-magnetic metal surface can lead to a new many-body ground state in which the localized impurity spin is screened. This so called Kondo effect is sensitive to the geometry of the surface and serves to investigate the role of the atomic arrangement in the contact. The point contact experiments are expanded to single molecules and molecular structures. The contact of an STM tip to the adsorbates and the resulting conductance behavior is explored. Furthermore, a procedure is developed that allows to pick-up a molecule onto the tip and to measure the transport characteristics through the entire molecule.

The elastic properties of metallic and semiconducting nanowires were analyzed by different techniques employing static and dynamic loads. The reliability of the methods is verified by analyzing well defined microstructures and a good agreement for the values of the Young's modulus determined by the different methods was achieved. For the investigated materials systems (Au, W, Si, InN), basically no differences in the Young's moduli were observed between microstructures, bulk material, and nanowires with radii of 20-300 nm. Microstructure, morphological undulation, defects, and contaminations,however, can drastically change the apparent Young's moduli of nanowires. Examples are given, where an apparent increasing or decreasing of the Young's modulus with decreasing diameter is caused by such effects. The same effects have also influence on the fracture strength in nanowires. While perfect Au nanowires exhibit fracture strengths exceeding the bulk values up to two orders of magnitude, any anomaly causes earlier failure. In addition, failure mechanisms are observed to be dependent on themicrostructure. While single crystallineAu nanowires have shown a pure elastic deformation upon load, polycrystalline nanowires show a remarkable plastic deformation before breaking.