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 toward combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nanospin-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.

Switching modes in easy and hard axis magnetic reversal in a self-assembled antidot array.

We study the reversal mechanisms in a self-assembled, hexagonally ordered Fe antidot array with a period of 200 nm and an antidot diameter of 100 nm which was prepared by polystyrene nanosphere lithography. Direction-dependent information in such a self-assembled sample is obtained by measuring the anisotropic magnetoresistance (AMR) through constrictions processed by focused ion beam milling in nearest neighbor and next nearest neighbor directions. We show that such an originally integral method can be used to investigate the strong in-plane anisotropy introduced by the antidot lattice. The easy and hard axis reversal mechanisms and corresponding AMR signals are modeled by micromagnetic simulations. Additional in-field magnetic force microscopy studies allow the correlation of microscopic switching to features in the integral AMR. We find that the easy axis of magnetization is connected to a distinct periodic magnetic domain pattern, which can be observed during the whole magnetization reversal. While this process is driven by nucleation and propagation of reversed domains, the hard axis reversal is characterized by a (stepwise) rotation of the magnetization via the antidot lattice' easy axes.

Geometry-induced spin-ice structures prepared by self-organization on the nanoscale.

We use non-close packed colloidal lithography to prepare hexagonal magnetic antidot arrays with varying diameters at a period of 205 nm. Smaller antidots are attractive for applications as spin waveguides in magnonics. Larger antidots form a magnetically frustrated system, i.e. Kagome spin-ice, as we prove by magnetic force microscopy. The simple but effective approach successfully extends the limits of top-down lithography previously used to study spin-ice configurations and emergent magnetic monopoles towards smaller structures.

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.

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.

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.