Zoran Djurić

Adsorption-desorption noise in nanowire FET biosensors

The adsorption-desorption (AD) noise in nanowire FET biosensors is analyzed, which manifests itself in the form of gate voltage fluctuations resulting from the fluctuations of the number of adsorbed particles. The derived theoretical model of the fluctuations takes into account the mass transfer effects. The numerical calculations show that when the mass transfer is slow, the low-frequency amplitude and the corner frequency of the Lorentzian AD noise spectrum obtained by derived expressions are significantly different compared to the same parameters predicted by the model that neglects mass transport effects. The presented analysis enables good estimation of these parameters from the measured noise spectrum, which is important because they are used as the source of information about the analyte concentration and the kinetics of biomolecular interactions.

RF MEMS and NEMS components and adsorption-desorption induced phase noise

Radio frequency micro- and nanoelectro-mechanical systems (RF MEMS and RF NEMS) and technologies have a great potential to overcome the constraints of conventional IC technologies in realization of fully integrated transceivers of next generation wireless communications systems. During the last two decades a considerable effort has been made to develop RF MEMS/NEMS resonators so that they could replace conventional bulky off-chip resonators in wireless transceivers. In MEMS, and especially in NEMS resonators, additional noise generating mechanisms exist that are characteristic for structures of small dimensions and mass, and high surface to volume ratio. One such mechanism is the adsorption-desorption (AD) process that generates the resonator frequency (phase) noise. In the first part of this paper a short overview of RF MEMS resonators is given, including comments on the necessary improvements and the direction of future research in this field (especially having in mind the need for NEMS resonators), with the intention to optimize RF MEMS and NEMS components according to requirements of both current and future systems. The main part of the paper presents a comprehensive theory of AD noise in MEMS/NEMS resonators. Apart from having a theoretical significance, the derived models of AD noise in multiple different cases of adsorption are also a useful tool for the design of optimal performance RF MEMS and NEMS resonators. The model of the MEMS/NEMS oscillator phase noise that takes into account the influence of AD noise is presented for the first time.

Highly sensitive graphene-based chemical and biological sensors with selectivity achievable through low-frequency noise measurement — Theoretical considerations

We have developed a theory of the low-frequency noise caused by interaction of the analyte with the active area of chemical and biological sensors. The main result is an analytical expression for the spectral density of the fluctuations of the number of particles adsorbed onto the sensing surface, taking into account the processes of mass transfer through the sensor reaction chamber, adsorption and desorption, and surface diffusion of adsorbed particles. The performed numerical calculations show good agreement with the experimental data from the literature, obtained for a graphene-based gas sensor. The derived theory contributes to the theoretical basis necessary for the development of a new method for the recognition and quantification of analytes, based on the measured noise spectrum.

Numerical simulation of transient response of chemical and biological micro/nanofabricated sensors operating in multianalyte environments

In different kinds of surface-based chemical and biological micro/nanosensors, interpretation of the experimentally obtained data is performed based on approximate solutions for the sensor response, valid for the case of single-analyte samples. A question arises whether or not the approximate models used for experimental data fitting in the case of a single analyte are also applicable for multianalyte environments. We test the validity of the two-compartment model for approximation of sensor transient response in the case of concurrent binding of multiple analytes on the sensing surface, by comparing the approximate response with the numerical simulation of the dynamics of a real adsorption experiment.

Characterization of adsorption-desorption processes on semiconductor surfaces using nanocantilever mass sensors

Characterization of adsorption-desorption (AD) processes of gas particles on semiconductors is necessary in order to investigate the influence of these processes on the micro/nano-devices performance. By applying a numerical computational method we determined the pressure dependence of the equilibrium coverage of the semiconductor surface by chemisorbed gas particles. We concluded that the pressure dependence of the total coverage of the surface by adparticles, and also of coverages by both the neutral and ionized adparticles, can be obtained by measuring the adsorbed mass. We propose the use of nanocantilever sensors for such extremely sensitive mass measurements. The obtained adsorption induced cantilevers resonant frequency shifts are higher than the detection threshold set by the termomechanical noise. This confirms the applicability of the used nanocantilever sensor for experimental characterization of AD processes at semiconductor surfaces.

Optimization of micro/nanooscillator parameters for analysis of transient adsorption processes

We consider optimization of parameters of micro/nanooscillators intended for investigation of adsorption-desorption (AD) processes, which are fast in comparison with the response rate of the oscillator itself. We consider the choice of the values of the Q-factor, the resonant frequency of the micro/nano-cantilever and the excitation frequency, in order to ensure that experimentally obtained micro/nanooscillator time response can be used for extraction of information about the AD process parameters. It is shown that a very small cantilever, which has a very high resonant frequency, can be used for investigation of AD processes whose time constant is as short as 10-5 s.

Adsorbed Mass Fluctuations of a Micro/Nanoresonator Surrounded by an Arbitrary Gas Mixture

Micro/nanoresonator mass and frequency fluctuations caused by sorption processes on a resonator surface are investigated. Arbitrary gas mixture is considered, assuming that particle arrivals at the surface are all poissonian in nature, independent from each other, and that sorption dynamics follows the Langmuir isotherm. Power spectral density for mass fluctuations are derived using the analytical Langevin approach. The results of simulations are given for a silicon micro/nanocantilever in the atmosphere of four gases. The presented analysis is useful for calculation of the ultimate performance of MEMS/NEMS sensors and oscillators, as well as for investigation of the possibilities of their parameters optimization by choosing the gas mixture in the cantilevers atmosphere. Another objective is to consider the possibility of identification of gases in the mixture, based on the power spectral density of the adsorbed mass fluctuation