Nanomechanical sensing using spins in diamond

Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy. For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively-charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step towards combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nano-spin-mechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to, not only detect the mass of a single macromolecule, but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale.Comment: Errors in the stress susceptibility parameters present in the original arXiv version have been correcte

Charakterisierung von Nanostrukturen mittels Raster-Tunnel-Mikroskopie und Raster-Tunnel-Spektroskopie

In die Aktivitäten der Abteilung Festkörperphysik sollte die Raster-Sonden-Mikroskopie mit ihrem breiten Potential als Analyse-Methode eingebettet und grundlagenorientiert das Verständnis und die Möglichkeiten insbesondere der Raster-Tunnel-Mikroskopie und -Spektroskopie (STS) vorangetrieben werden. Hierzu waren Instrumente zu entwickeln und aufzubauen, die höchsten Ansprüchen insbesondere in Bezug auf die Stabilität und damit die Messgenauigkeit genügen. Ein erklärtes Ziel dieser Arbeit wird es folglich sein, mithilfe der neuen Instrumente aus der STS weitere Informationen über die untersuchten Systeme zu extrahieren. In einem zweiten Bereich sollten Untersuchungen durchgeführten werden, die einen Beitrag zu den seit Jahren sehr intensiven Bemühungen leisten, Quantum-size-Effekte einerseits zu messen und zu verstehen und andererseits auszunützen, um Materialien bzw. Systeme mit neuen Eigenschaften herzustellen. Einer dieser Quantum-size-Effekte ist die Coulomb-Blockade, bei der einzelne Elektronen die Transporteigenschaften des Systems definieren. Auf dieser Basis lassen sich Ein-Elektronen-Transistoren realisieren, die wegen ihrer Kleinheit die Elektronik der kommenden Generationen gravierend beeinflussen dürften. Die Kleinheit bietet nicht nur einen schier unvorstellbaren Integrationsgrad, sie bietet auch durch den Übergang in den Quantenbereich die Möglichkeit zu nichtklassischen Eigenschaften und damit zum Quantum-computing. Während auf Halbleitern basierende SETs wegen der zur Verfügung stehenden, ausgefeilten Technologie im Verständnis bereits sehr weit fortgeschritten sind, ist ihre Anwendbarkeit wegen noch zu großer Strukturen auf tiefe Temperaturen eingeschränkt. Metallische Systeme, die bei Zimmertemperatur betrieben werden könnten, bedürfen wegen der notwendigen kleinen Abmessungen von wenigen Nanometern, bei denen die eben besagten Quantum-size-Effekte mehr und mehr zum Tragen kommen, noch einer sehr intensiven Forschung

Deconvolution of the density of states of tip and sample through constant-current tunneling spectroscopy

We introduce a scheme to obtain the deconvolved density of states (DOS) of the tip and sample, from scanning tunneling spectra determined in the constant-current mode (z–V spectroscopy). The scheme is based on the validity of the Wentzel–Kramers–Brillouin (WKB) approximation and the trapezoidal approximation of the electron potential within the tunneling barrier. In a numerical treatment of z–V spectroscopy, we first analyze how the position and amplitude of characteristic DOS features change depending on parameters such as the energy position, width, barrier height, and the tip–sample separation. Then it is shown that the deconvolution scheme is capable of recovering the original DOS of tip and sample with an accuracy of better than 97% within the one-dimensional WKB approximation. Application of the deconvolution scheme to experimental data obtained on Nb(110) reveals a convergent behavior, providing separately the DOS of both sample and tip. In detail, however, there are systematic quantitative deviations between the DOS results based on z–V data and those based on I–V data. This points to an inconsistency between the assumed and the actual transmission probability function. Indeed, the experimentally determined differential barrier height still clearly deviates from that derived from the deconvolved DOS. Thus, the present progress in developing a reliable deconvolution scheme shifts the focus towards how to access the actual transmission probability function

Detection of surface plasmons by scanning tunneling microscopy

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced dignal reveals structures on a nanometer scale. Some of them are correlated to the surface topography

Terthiophene on Au(111): A scanning tunneling microscopy and spectroscopy study

Terthiophene (3T) molecules adsorbed on herringbone (HB) reconstructed Au(111) surfaces in the low coverage regime were investigated by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra-high vacuum conditions. The 3T molecules adsorb preferentially in fcc regions of the HB reconstruction with their longer axis oriented perpendicular to the soliton walls of the HB and at maximum mutual separation. The latter observation points to a repulsive interaction between molecules probably due to parallel electrical dipoles formed during adsorption. Constant-separation (I-V) and constant-current (z-V) STS clearly reveal the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, which are found at −1.2 eV and +2.3 eV, respectively. The HOMO–LUMO gap corresponds to that of a free molecule, indicating a rather weak interaction between 3T and Au(111). According to conductivity maps, the HOMO and LUMO are inhomogeneously distributed over the adsorbed 3T, with the HOMO being located at the ends of the linear molecule, and the LUMO symmetrically with respect to the longer axis of the molecule at the center of its flanks. Analysis of spectroscopic data reveals details of the contrast mechanism of 3T/Au(111) in STM. For that, the Shockley-like surface state of Au(111) plays an essential role and appears shifted outwards from the surface in the presence of the molecule. As a consequence, the molecule can be imaged even at a tunneling bias within its HOMO–LUMO gap. A more quantitative analysis of this detail resolves a previous discrepancy between the fairly small apparent STM height of 3T molecules (1.4–2.0 nm, depending on tunneling bias) and a corresponding larger value of 3.5 nm based on X-ray standing wave analysis. An additionally observed linear decrease of the differential tunneling barrier at positive bias when determined on top of a 3T molecule is compared to the bias independent barrier obtained on bare Au(111) surfaces. This striking difference of the barrier behavior with and without adsorbed molecules is interpreted as indicating an adsorption-induced dimensionality transition of the involved tunneling processes

STM study on the self-assembly of oligothiophene-based organic semiconductors

The self-assembly properties of a series of functionalized regioregular oligo(3-alkylthiophenes) were investigated by using scanning tunneling microscopy (STM) at the liquid–solid interface under ambient conditions. The characteristics of the 2-D crystals formed on the (0001) plane of highly ordered pyrolitic graphite (HOPG) strongly depend on the length of the π-conjugated oligomer backbone, on the functional groups attached to it, and on the alkyl substitution pattern on the individual thiophene units. Theoretical calculations were performed to analyze the geometry and electronic density of the molecular orbitals as well as to analyze the intermolecular interactions, in order to obtain models of the 2-D molecular ordering on the substrate

Plasmon-induced tunneling currents : the influence of tip modes

The interaction of propagating surface plasmons (PSP) excited in a silver film with the tunneling junction of a scanning tunneling microscope has been investigated. Under particular conditions we find strong plasmon-induced current (PIC) superimposed on the tunneling current. The generation of the PIC is in our view due to the rectification of the optical electric field by the nonlinearity of the tunneling junction. To obtain a PIC with detectable magnitude an enhancement of the optical field between tip and sample by several orders of magnitude is required. The observed dependence of the field enhancement (FE) on the tip material and the geometry of the tunneling junction will be discussed in terms of localized surface plasmons