Mario S. Rodrigues

Probing the elastic properties of individual nanostructures by combining in situ atomic force microscopy and micro-x-ray diffraction

Atomic force microscopy (AFM) and micro-x-ray diffraction are combined to investigate nanostructures during in situ indentation. This technique allows the determination of elastic properties of individual nanoscale objects, particularly here SiGe∕Si(001) self-assembled islands. Using this novel technique it was possible to select a specific island, align it in the microfocused beam, and apply a pressure onto it, using the AFM tip. Simultaneously, the x-ray diffuse scattering map from the island and the surrounding substrate was recorded in order to probe the lattice parameter change during indentation. An elastic reduction of the island lattice parameter of up to 0.6% was achieved.

In situ observation of the elastic deformation of a single epitaxial SiGe crystal by combining atomic force microscopy and micro x-ray diffraction

An in situ combination of atomic force microscopy and micro x-ray diffraction was developed to study the elastic behavior of nanosized objects. This technique offers the means to locally access the Young elastic moduli and Poisson ratios of individual nanostructures. Here, we investigated the elastic behavior of a single self-assembled 450 nm high SiGe island. As pressure was applied on the island, the resonance frequency of the atomic force microscope tuning fork was tracked together with the x-ray diffraction stemming from this individual crystal. The change in the tip-island contact stiffness could be derived from the variation in the resonance frequency of the tuning fork, whereas the island mean lattice parameter was inferred from the center of mass of the island’s Bragg scattering. From this information, the reduced elastic modulus of the tip-island system could be directly determined, which is in very good agreement with literature values. The pressure needed to compress the island lattice to the S...

Local detection of X-ray spectroscopies with an in-situ Atomic Force Microscope

The in situ combination of Scanning Probe Microscopies (SPM) with X-ray microbeams adds a variety of new possibilities to the panoply of synchrotron radiation techniques. In this paper we describe an optics-free AFM/STM that can be directly installed on synchrotron radiation end stations for such combined experiments. The instrument can be used just for AFM imaging of the investigated sample or can be used for detection of photoemitted electrons with a sharp STM-like tip, thus leading to the local measure of the X-ray absorption signal. Alternatively one can can measure the flux of photon impinging on the sharpest part of the tip to locally map the pattern of beams diffracted from the sample. In this paper we eventually provide some examples of local detection of XAS and diffraction.

Why do atomic force microscopy force curves still exhibit jump to contact

The force between two particles as a function of distance is one of the most fundamental curves in physics. Here, we describe how the force feedback microscope can routinely measure the tip-surface interaction in the entire range of distances with a sensitivity of 1 pN and in different media. The method allows to measure simultaneously the force, force gradient, and damping from solely the knowledge of the lever spring constant. The jump to contact is avoided and thus it is possible to follow the brutal nucleation of a water bridge between the tip and the surface.The force between two interacting particles as a function of distance is one of the most fundamental curves in science. In this regard, Atomic Force Microscopy (AFM) represents the most powerful tool in nanoscience but with severe limits when it is to probe attractive interactions with high sensitivity. The Force Feedback Microscope (FFM) described here, removes from AFM the well known jump to contact problem that precludes the complete exploration of the interaction curve and the study of associated energy exchanges. The FFM makes it possible to explore tip-surface interactions in the entire range of distances with a sensitivity better than 1 pN. FFM stands out as a radical change in AFM control paradigms. With a surprisingly simple arrangement it is possible to provide the AFM tip with the right counterforce to keep it fixed at any time. The counterforce is consequently equal to the tip-sample force. The force, force gradient and damping are simultaneously measured independently of the tip position. This permits the measurement of energy transfer in thermodynamic transformations. Here we show some FFM measurement examples of the complete interaction force curve and in particular that the FFM can follow the nucleation of a water bridge by measuring the capillary attractive force at all distances, without jump to contact despite the large attractive capillary force. Real time combination of the measured parameters will lead to new imaging modalities with chemical contrast in different environments.

Custom AFM for X-ray beamlines: in situ biological investigations under physiological conditions

The performance of a custom atomic force microscope for grazing-incidence X-ray experiments on hydrated soft and biological samples is presented.

Giant resonance tuning of micro and nanomechanical oscillators

We present a method to tune the resonance frequency and the Q-factor of micro and nano-metric mechanical oscillators. A counteracting loop drives a capacitive force applied to the oscillator. The proportional and differential gains are used to shift the resonance frequency up to 75% and to tune the Q-factor of the oscillator, by changing its effective stiffness and damping ratio. The oscillator position is monitored in a large bandwidth with a fiber-optic based interferometer. We applied this simple operational scheme with different oscillators for modifying easily their dynamical properties. Compared to alternative methods requiring external fields, our method can either increase or decrease the resonance frequency in a frequency range much more extended. This opens up a wide range of applications, from force sensors with extremely low elastic constants but high quality factor to tunable energy harvesters or to high-frequency tuning of radio frequency filters. The control scheme can work in different media, and is then suitable to be applied to biological sensors and actuators.

Imaging material properties of biological samples with a force feedback microscope

Mechanical properties of biological samples have been imaged with a force feedback microscope. Force, force gradient, and dissipation are measured simultaneously and quantitatively, merely knowing the atomic force microscopy cantilever spring constant. Our first results demonstrate that this robust method provides quantitative high resolution force measurements of the interaction. The small oscillation imposed on the cantilever and the small value of its stiffness result in vibrational energies much smaller than the thermal energy, reducing interaction with the sample to a minimum. We show that the observed mechanical properties of the sample depend on the force applied by the tip and consequently on the sample indentation. Copyright

System analysis of force feedback microscopy

It was shown recently that the Force Feedback Microscope (FFM) can avoid the jump-to-contact in Atomic force Microscopy even when the cantilevers used are very soft, thus increasing force resolution. In this letter, we explore theoretical aspects of the associated real time control of the tip position. We take into account lever parameters such as the lever characteristics in its environment, spring constant, mass, dissipation coefficient, and the operating conditions such as controller gains and interaction force. We show how the controller parameters are determined so that the FFM functions at its best and estimate the bandwidth of the system under these conditions.

Spectroscopic investigation of local mechanical impedance of living cells.

We studied nanoscale mechanical properties of PC12 living cells with a Force Feedback Microscope using two experimental approaches. The first one consists in measuring the local mechanical impedance of the cell membrane while simultaneously mapping the cell morphology at constant force. As the interaction force is increased, we observe the appearance of the sub-membrane cytoskeleton. We compare our findings with the outcome of other techniques. The second experimental approach consists in a spectroscopic investigation of the cell while varying the tip indentation into the membrane and consequently the applied force. At variance with conventional dynamic Atomic Force Microscopy techniques, here it is not mandatory to work at the first oscillation eigenmode of the cantilever: the excitation frequency of the tip can be chosen arbitrary leading then to new spectroscopic AFM techniques. We found in this way that the mechanical response of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10 kHz range. In particular, we observe that the damping coefficient consistently decreases when the excitation frequency is increased.

Out of equilibrium anomalous elastic response of a water nano-meniscus

We report the observation of a transition in the dynamical properties of water nano-meniscus which dramatically changes when probed at different time scales. Using an AFM mode that we name Force Feedback Microscopy, we observe this change in the simultaneous measurements, at different frequencies, of the stiffness G′ (N/m), the dissipative coefficient G″ (kg/s) together with the static force. At low frequency we observe a negative stiffness as expected for capillary forces. As the measuring time approaches the microsecond, the dynamic response exhibits a transition toward a very large positive stiffness. When evaporation and condensation gradually lose efficiency, the contact line progressively becomes immobile. This transition is essentially controlled by variations of Laplace pressure.