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UPCommons - E-prints UPC Universitat Politècnica de Catalunya (67,473 recursos)
E-prints UPC cobreix dues finalitats: per una banda, és el dipòsit institucional de la UPC que recull els articles de revista, les comunicacions de congrés i els reports de recerca generats en les activitats de recerca del personal docent i investigador de la universitat; per l'altra, és una eina que permet accelerar la producció científica, allotjant versions de documents prèvies a la publicació en una revista o a les actes d’un congrés.

Articles de revista

Mostrando recursos 1 - 14 de 14

1. Phase contrast and operation regimes in multifrequency atomic force microscopy - Santos Hernández, Sergio
In amplitude modulation atomic force microscopy the attractive and the repulsive force regimes induce phase shifts above and below 90°, respectively. In the more recent multifrequency approach, however, multiple operation regimes have been reported and the theory should be revisited. Here, a theory of phase contrast in multifrequency atomic force microscopy is developed and discussed in terms of energy transfer between modes, energy dissipation and the kinetic energy and energy transfer associated with externally driven harmonics. The single frequency virial that controls the phase shift might undergo transitions in sign while the average force (modal virial) remains positive (negative). ©...

2. Unlocking higher harmonics in atomic force microscopy with gentle interactions - Santos Hernández, Sergio; Barcons Xixons, Víctor; Font Teixidó, Josep; Verdaguer Prats, Albert
In dynamic atomic force microscopy, nanoscale properties are encoded in the higher harmonics. Nevertheless, when gentle interactions and minimal invasiveness are required, these harmonics are typically undetectable. Here, we propose to externally drive an arbitrary number of exact higher harmonics above the noise level. In this way, multiple contrast channels that are sensitive to compositional variations are made accessible. Numerical integration of the equation of motion shows that the external introduction of exact harmonic frequencies does not compromise the fundamental frequency. Thermal fluctuations are also considered within the detection bandwidth of interest and discussed in terms of higher-harmonic phase contrast...

3. A superregenerative QPSK receiver - Palà Schönwälder, Pere; Bonet Dalmau, Jordi; Moncunill Geniz, Francesc Xavier; Águila López, Francisco del; Giralt Mas, Ma. Rosa
In this paper we present a description and experimental verification of a superregenerative receiver (SR) for QPSK signals. Exploiting the fact that a conventional SR generates pulses which preserve the input phase information, we take $N$ 1-bit samples of each generated pulse. A suitable choice of the sampling frequency gives as a result a bit vector containing a sub-sampled version of each PSK pulse. Extremely simple digital processing of the vectors from two consecutive pulses allows symbol decision, together with information on signal quality and frequency displacements. Although presented for the QPSK case, the principle may be applied to the...

4. Enhanced sensitivity and contrast with bimodal atomic force microscopy with small and ultra-small amplitudes in ambient conditions - Santos Hernández, Sergio
Here, we introduce bimodal atomic force microscopy operated with sub-nm and ultra-small, i.e., sub-angstrom, first and second mode amplitudes in ambient conditions. We show how the tip can be made to oscillate in the proximity of the surface and in perpetual contact with the adsorbed water layers while the second mode amplitude and phase provide enhanced contrast and sensitivity. Nonlinear and nonmonotonic behavior of the experimental observables is discussed theoretically with a view to high resolution, enhanced contrast, and minimally invasive mapping. Fractions of meV of energy dissipation are shown to provide contrast above the noise level.

5. Improved switching strategy for selective harmonic elimination in DC-AC signal generation via pulse-width modulation - Palacios Quiñonero, Francisco; Vicente Rodrigo, Jesús; Molina Hernández, M. Antonia; Karimi, Hamid Reza
We present an advanced design methodology for pulse-width-modulated (PWM) DC-AC signal generation. Using design methods based on the Walsh transform, AC sinusoidal signals can be approximated by suitable PWM signals. For different AC amplitudes, the switching instants of the PWM signals can be efficiently computed by using appropriate systems of explicit linear equations. However, the equation systems provided by conventional implementations of this approach are typically only valid for a restricted interval of AC amplitudes and, in general, a supervised implementation of several equation systems is necessary to cover the full AC amplitude range. Additionally, obtaining suitable equation systems for designs with a large number...

6. Stability, resolution, and ultra-low wear amplitude modulation atomic force microscopy of DNA: small amplitude small set-point imaging - Santos Hernández, Sergio; Barcons Xixons, Víctor; Christenson, Hugo K.; Billingsley, Daniel J.; Bonass, William A.; Font Teixidó, Josep; Thomson, Neil H.
A way to operate fundamental mode amplitude modulation atomic force microscopy is introduced which optimizes stability and resolution for a given tip size and shows negligible tip wear over extended time periods ( 24 h). In small amplitude smal l set-point (SASS) imaging, the cantilever oscillates with sub-nanometer am plitudes in the proximity of the sample, without the requirement of using large drive forces, a s the dynamics smoothly lead the tip to the surface through the water layer. SASS is demonstrat ed on single molecules of double-stranded DNA in ambient conditions where sharp silicon tips (R 2–5 nm) can resolve the right-handed double helix

7. Optimization of oxygen transfer through venturi-based systems applied to the biological sweetening of biogas - Rodriguez, Ginesta; Dorado Castaño, Antonio David; Bonsfills Pedrós, Anna; Sanahuja Moliner, Ricard; Gabriel, David; Gamisans Noguera, Javier

8. Investigation of nanoscale interactions by means of subharmonic excitation - Santos Hernández, Sergio; Phillips, M.A.; Verdaguer, Albert; Font Teixidó, Josep; Chiesa, Matteo; Gadelrab,, K.; Stefancich, M.; Armstrong, P.; Li, G.; Souier, T.; Thomson, Neil H.; Barcons Xixons, Víctor
Multifrequency atomic force microscopy holds promise as a method to provide qualitative and quantitative information about samples with high spatial resolution. Here, we provide experimental evidence of the excitation of subharmonics in ambient conditions in the regions where capillary interactions are predicted to be the mechanism of excitation. We also experimentally decouple a second mechanism for subharmonic excitation that is highly independent of environmental conditions such as relative humidity. This implies that material properties could be mapped. Subharmonic excitation could lead to experimental determination of surface water affinity in the nanoscale whenever water interactions are the mechanism of excitation.

9. Quantification of dissipation and deformation in ambient atomic force microscopy - Santos Hernández, Sergio; Gadelrab,, K.; Barcons Xixons, Víctor; Stefancich, M.; Chiesa, Matteo
A formalism to extract and quantify unknown quantities such as sample deformation, the viscosity of the sample and surface energy hysteresis in amplitude modulation atomic force microscopy is presented. Recovering the unknowns only requires the cantilever to be accurately calibrated and the dissipative processes occurring during sample deformation to be well modeled. The theory is validated by comparison with numerical simulations and shown to be able to provide, in principle, values of sample deformation with picometer resolution.

10. Spatial horizons in amplitude and frequency modulation atomic force microscopy - Font Teixidó, Josep; Santos Hernández, Sergio; Barcons Xixons, Víctor; Thomson, Neil H.; Verdaguer, Albert; Chiesa, Matteo
In dynamic atomic force microscopy (AFM) the cantilever is vibrated and its dynamics are monitored to probe the sample with nanoscale and atomic resolution. Amplitude and frequency modulation (AM and FM) atomic force microscopy have established themselves as the most powerful, robust and reliable techniques in the field. Nevertheless, it is still debatable whether one or the other technique is preferred in a given medium or experiment. Here, we quantitatively establish the limitations in resolution of one and the other technique by introducing the concept of space horizon SH and quantifying it. The SH is the limiting space boundary beyond which collective atomic interactions do not affect the...

11. A high-resolution UWB IR superregenerative receiver front end with an SRD quench shaper - Moncunill Geniz, Francesc Xavier; Bonet Dalmau, Jordi; Palà Schönwälder, Pere; Águila López, Francisco del; Giralt Mas, Ma. Rosa
We present a simple receiver front end that makes use of the baseband superregeneration principle to detect ultrawideband (UWB) impulse radio signals. The UWB antenna is directly connected to the core circuit consisting of a resistor-capacitor (RC) network coupled to a negative resistance that varies under the control of an external quench generator. Due to a step-recovery-diode quench shaper, 50-ps time-domain sensitivity windows are generated that filter the received pulses and reject noise and interference. The circuit achieves high gain, exhibits automatic gain control, and directly demodulates binary phase modulations.

12. The intrinsic resolution limit in the atomic force microscope: implications for heights of nano-scale features - Santos Hernández, Sergio; Barcons Xixons, Víctor; Christenson, Hugo K.; Font Teixidó, Josep; Thomson, Neil H.
Peer Reviewed

13. How localized are energy dissipation processes in nanoscale interactions? - Santos Hernández, Sergio; Barcons Xixons, Víctor; Verdaguer, Albert; Font Teixidó, Josep; Thomson, Neil H.; Chiesa, Mateo
We describe fundamental energy dissipation in dynamic nanoscale processes in terms of the localization of the interactions. In this respect, the areal density of the energy dissipated and the effective area of interaction in which each process occurs are calculated for four elementary dissipative processes. It is the ratio between these two, which we term M that provides information about how localized the interactions are. We show that neither the phase lag, nor the magnitude of the energy dissipated alone provide information about energy localization but M has to be considered instead.

14. Localization and Electrical Characterization of Interconnect Open Defects - Rodríguez Montañés, Rosa; Arumi Delgado, Daniel; Figueras Pàmies, Joan; Beverloo, Willem; Vries, Dirk K. de; Eichenberger, Stefan; Volf, Paul A. J.
A technique for extracting the electrical and topological parameters of open defects in process monitor lines is presented. The procedure is based on frequency-domain measurements performed at both end points of the line. The location as well as the resistive value of the open defect are derived from attenuation and phase shift measurements. The characteristic defect-free impedance of the line and its propagation constant are considered to be unknowns, and their values are also derived from the above measurements. In this way, the impact of process parameter variations on the proposed model is diminished. The experimental setup required to perform the characterization measurements and a simple graphical procedure to determine the defect...

 

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