Mimea

We have developed a low noise all-fiber interferometer for use as the deflection sensor in liquid environment frequency modulated atomic force microscopy (FM-AFM). A detailed description and rationale for the choice of the critical components are provided along with the design of a simple alignment assembly. The optimization of the deflection sensor toward achieving the highest possible sensitivity and lowest deflection noise density is discussed in the context of an ideal interference cavity. Based on the provided analysis we have achieved deflection noise densities of 2 fm/√ Hz on commercially available cantilevers in both ambient and liquid environments. The low noise interferometer works without the need for differential detection, special focusing lenses, or polarization sensitive optics, dramatically simplifying measurements. True atomic resolution imaging of muscovite mica by FM-AFM in water is demonstrated using the developed deflection sensor.

The formation of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) nanostrutures on Au(1 1 1)- (22 x √ 3) covered with NaCl islands has been studied using scanning tunneling microscopy (STM). Atomically resolved STM images show that NaCl grows as (1 0 0)-terminated layers on Au(1 1 1)- (22 x √ 3) . Local atomic hexagonal packing has also been observed in the NaCl(1 0 0) layer. At submonolayer NaCl coverage, PTCDA forms two-dimensional islands on the Au(1 1 1) surface and nucleate preferentially at the NaCl island step edges. When the Au surface is fully covered with NaCl layers, PTCDA molecules form three-dimensional molecular clusters decorating the step edges of NaCl layers.

Using a systematic method based on considering all possible hydrogen bond connections between molecules and subsequent density-functional theory (DFT) calculations, we investigated planar superstructures that the perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) molecules can form in one and two dimensions. Structures studied are mostly based on two molecule unit cells and all assemble in flat periodic arrays. We show that 42 different monolayer structures are possible, which can be split into eight families of distinct structures. A single representative of every family was selected and relaxed using DFT. We find square, herringbone and brick wall phases (among others) which were already observed on various substrates. Using scanning tunneling microscopy in ultrahigh vacuum, we also observed herringbone and square phases after sublimation of PTCDA molecules on the Au(111) surface at room temperature, the square phase being observed for the first time on this substrate. The square phase appears as a thin stripe separating two herringbone domains and provides a perfect structural matching for them. A similar structural formation serving as a domain wall between two other phases has been recently reported on the same surface formed by melamine molecules [F. Silly et al., J. Phys. Chem. C 112, 11476 (2008)]. Our theoretical analysis helps to account for these and other observed complex structures.

Engineering and tuning multi-component supramolecular self-assemblies on surfaces is one of the challenges of nanotechnology. We use scanning tunneling microscopy to investigate the influence of molecular ratio on the self-assembly of PTCDA–melamine structures on Au(111)-(22 x √ 3). Our observations reveal that three different chiral supramolecular networks having a PTCDA:melamine ratio of 3:2, 1:2, 1:4 can be selectively created by tuning the ratio of molecules deposited on the surface. The 1:2 ratio network having melamine in excess has been observed previously but the 1:4 network has not yet been reported. In comparison, the multi-component 3:2 network having PTCDA in excess is a completely new structure.

We describe in detail how atomic force microscopy (AFM) images can be routinely achieved with macroscopic silicon-based chips integrating mesoscopic tips, paving the way for the development of new near field devices combining AFM imaging with any kind of functionality integrated on a chip. The chips have been glued at the end of the free prong of 100 kHz quartz tuning forks mounted in Qplus configuration. Numerical simulations by modal analysis have been carried out to clarify the nature of the vibration modes observed in the experimental spectra. It is shown that two low frequency modes can be used to drive the system and scan the surface with a great stability in amplitude modulation as well as in frequency modulation AFM under ultrahigh vacuum. The AFM capabilities are demonstrated through a series of examples including phase and dissipation contrast imaging, force spectroscopy measurements, and investigations of soft samples in weak interaction with the substrate. The lateral resolution with the tips grown by focused ion beam deposition already matches the one achieved in standard amplitude modulation mode AFM experiments.

The acquisition rate of all scanning probe imaging techniques with feedback control is limited by the dynamic response of the control loops. Performance criteria are the control loop bandwidth and the output signal noise power spectral density. Depending on the acceptable noise level, it may be necessary to reduce the sampling frequency below the bandwidth of the control loop. In this work, the frequency response of a vacuum Kelvin force microscope with amplitude detection (AM-KFM) using a digital signal processing (DSP) controller is characterized and optimized. Then, the main noise source and its impact on the output signal is identified. A discussion follows on how the system design can be optimized with respect to output noise. Furthermore, the interaction between Kelvin and distance control loop is studied, confirming the beneficial effect of KFM on topography artefact reduction in the frequency domain. The experimental procedure described here can be generalized to other systems and allows to locate the performance limitations.

Cu-phthalocyanine (CuPc) serves as a very efficient absorber molecule in organic solar cell devices. The authors have applied spatially resolved surface photovoltage spectroscopy in a Kelvin probe force microscope to investigate the absorption and exciton separation of CuPc molecules on Si(100) and ITO substrates. A positive surface photovoltage was observed for illumination energies above ~ 1.5 eV, indicating absorption of the light in the CuPc molecules and subsequent separation of holes into the substrate.

We demonstrate the possibility to map nanoscale charge transfers between quantum electronic levels at room temperature, using noncontact atomic force microscopy and Kelvin force microscopy in a regime of weak electromechanical coupling. A two-level system is studied, consisting of degenerately doped silicon nanocrystals on silicon substrates, with size in the 2–50 nm range, in which the energy spacing is tuned by the nanocrystal quantum confinement over a ≈1 eV range. The nanocrystal ionization is found to follow an energy compensation mechanism driven by quantum confinement, in quantitative agreement with parametrized tight-binding calculations of its band structure.

Porphyrin derivative molecules (MP-tBPP) on a Au(111) reconstructed surface have been investigated using scanning tunneling microscopy. The molecules were found to adsorb with two kinds of orientation at the elbow positions of the reconstructed surface. One orientation had the two di-tert-butylphenyl groups of a molecule aligning along a row of elbow positions, and the other was the same alignment rotated approximately 60°. Flipping behaviors from one orientation to the other were observed in successively obtained scanning tunneling microscopy images. The activation energy for the flipping behaviors was estimated to be approximately 60 meV from the flipping rates. The difference in the molecular populations with each of the orientations represents the potential energy of the adsorbed molecules having asymmetric double minima corresponding to the two orientations. The difference between the potential energies of the molecules in these orientations was approximately 12 meV, which was calculated from the ratio between the populations in the orientations and from the difference in the flipping rates.

We demonstrate frequency modulation Kelvin probe force microscopy operated in lift-mode under ambient conditions. Frequency modulation detection is sensitive to force gradients rather than forces as in the commonly used amplitude modulation technique. As a result there is less influence from electric fields originating from the tip's cone and cantilever, and the recorded surface potential does not suffer from the large lateral averaging observed in amplitude modulated Kelvin probe force microscopy. The frequency modulation technique further shows a reduced dependence on the lift-height and the frequency shift can be used to map the second order derivative of the tip–sample capacitance which gives high resolution material contrast of dielectric sample properties. The sequential nature of the lift-mode technique overcomes various problems of single-scan techniques, where crosstalk between the Kelvin probe and topography feedbacks often impair the correct interpretation of the recorded data in terms of quantitative electric surface potentials.

The three-dimensional force field above an NaCl(001) surface was measured on a fine grid by small amplitude dynamic force spectroscopy at room temperature. After careful drift corrections, maps in nominally equivalent symmetry planes as well as in sections parallel to the surface reveal distance-dependent shifts of characteristic atomic-sized features. These shifts reflect asymmetries of the probing tip apex but are mainly due to atomic displacements induced by short-range forces. In addition, weak decaying force oscillations with a period close to the interlayer spacing are detected at distances where no atomic-scale variations are expected. Stronger site-dependent changes appear in the interaction-induced energy dissipation.

Magnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the alignment of the individual atomic magnetic moments. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart, ranging from up to 100 Nm⁻¹ for atomic interactions down to 0.0001 Nm⁻¹ for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800 Nm⁻¹ (optimized for atomic interaction), which is sensitive enough to measure millihertz frequency contrast caused by magnetic dipole–dipole interactions. Thus we have succeeded in establishing a sensing technique that performs scanning tunneling microscopy, atomic force microscopy and MFM with a single probe.

Polythiophene molecules adsorbed on a highly oriented pyrolytic graphite surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) with a quartz tuning fork sensor operating in Qplus mode and equipped with a Pt/Ir tip. Upon completing a careful sub-angström oscillation amplitude calibration of the probe, experiments were conducted in an ultra high vacuum at room temperature. By selecting the tip/surface distance regulation parameter, one can select the type of simultaneous information obtained in an area. For distance regulation based on the mean tunneling current, dynamic STM images together with maps of tip/surface force gradient were obtained. FM-AFM images with maps of the tunnelingcurrent were also acquired when the distance regulation was based on the frequency shift. Comparison between these images reveals interesting features. For example the tip which operates in STM mode with ultra low current (<10 pA) generates different interaction forces above molecules or graphite. Changes in energy dissipation processes as small as tens of millielectronvolts per cycle were recorded when the tip oscillates above the polymer or on the graphite surface. Hence data demonstrates that a stiff piezoelectric tuning fork of several kilonewtons/meters working as an AFM/STM probe with sub-angström amplitude can characterize weakly adsorbed molecules.

We demonstrate a novel electronic readout for quadrant photodiode based optical beam deflection setups. In our readout, the signals used to calculate the deflections remain as currents, instead of undergoing an immediate conversion to voltages. Bipolar current mirrors are used to perform all mathematical operations at the transistor level, including the signal normalizing division. This method has numerous advantages, leading to significantly simpler designs that avoid large voltage swings and parasitic capacitances. The bandwidth of our readout is solely limited by the capacitance of the quadrant photodiode junctions, making the effective bandwidth a function of the intensity of photocurrents and thus the applied power of the beam deflection laser. Using commercially available components and laser intensities of 1–4 mW we achieved a 3 dB bandwidth of 20 MHz with deflection sensitivities of up to 0.5–1 V/nm and deflection noise levels below 4.5 fm/√Hz. Atomic resolution imaging of muscovite mica using FM–AFM in water demonstrates the sensitivity of this novel readout.

It has previously been shown that multimolecular organic nanostructures form on H-Si(100)-2×1 via a radical mediated growth process. In this mechanism, growth begins through the addition of a molecule to a silicon surface dangling bond, followed by the abstraction of a neighboring H atom and generation of a new dangling bond on the neighboring site. Nanostructures formed by this mechanism grow along one edge of a dimer row. Here, we explored the possibility of using lithographically prepared, biased metal contacts on the silicon surface to generate an electric field that orients molecules during the growth process to achieve growth in the perpendicular-to-row direction. The formation of some nanostructures in a direction that was nearly perpendicular to the dimer rows was achieved, whereas such features were not formed in the absence of the field. Analysis of the scanning tunneling microscopy images suggests that the formation of these nanostructures may involve self-templating effects in addition to dangling bond diffusion rather than a straightforward addition/abstraction mechanism. These initial results offer some indication that a molecular pattern writer can be achieved.

We report on the imaging of biological cells including living neurons by a dedicated fibered interferometric scanning optical microscope. The topography and surface roughness of mouse fibroblasts and hippocampal neurons are clearly revealed. This straightforward far-field technique allows fast, high resolution observation of samples in liquids without lengthy alignment procedures or costly components.

The atomic structure of graphene on polycrystalline copper substrates has been studied using scanning tunneling microscopy. The graphene overlayer maintains a continuous pristine atomic structure over atomically flat planes, monatomic steps, edges, and vertices of the copper surface. We find that facets of different identities are overgrown with graphene’s perfect carbon honeycomb lattice. Our observations suggest that growth models including a stagnant catalytic surface do not apply to graphene growth on copper. Contrary to current expectations, these results reveal that the growth of macroscopic pristine graphene is not limited by the underlying copper structure.

The atomic structures that develop as a function of coverage during deposition of Bi on Ag(111) have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, and ab initio calculations. The growth process involves two sequential stages. At low coverage, Bi atoms are incorporated into the topmost layer of Ag(111), resulting in the formation of an Ag2Bi alloy confined to the surface and ordered (√3×√3)R30° Ag2Bi islands supported on Ag(111). This mode of accommodation of Bi was found to be energetically favorable based on ab initio total-energy calculations. At coverage above a critical value of 0.55 monolayers, the Ag2Bi alloy phase gradually converts into an ordered Bi (p×√3) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface.

Scanning tunneling microscopy and dynamic force microscopy in the noncontact mode are used in combination to investigate the reversible switching between two stable states of a copper complex adsorbed on a NaCl bilayer grown on Cu(111). The molecular conformation in these two states is deduced from scanning tunneling microscopy imaging, while their charge is characterized by the direct measurement of the tip-molecule electrostatic force. These measurements demonstrate that the molecular bistability is achieved through a charge-induced rearrangement of the coordination sphere of the metal complex, qualifying this system as a new electromechanical single-molecular switch.

We describe the development of a novel setup, in which large stencils with suspended silicon nitride membranes are combined with atomic force microscopy (AFM) regulation by using tuning forks. This system offers the possibility to perform separate AFM and nanostencil operations, as well as combined modes when using stencil chips with integrated tips. The flexibility and performances are demonstrated through a series of examples, including wide AFM scans in closed loop mode, probe positioning repeatability of a few tens of nanometer, simultaneous evaporation of large (several hundred of micron square) and nanoscopic metals and fullerene patterns in static, multistep, and dynamic modes. This approach paves the way for further developments, as it fully combines the advantages of conventional stenciling with the ones of an AFM driven shadow mask.