Pradeep R. Nair

Performance limits of nanobiosensors

A simple analytical model, based on reaction-diffusion theory, is developed to predict the trade-off between average response (settling) time (ts) and minimum detectable concentration (ρ0) for nanobiosensors and nanochemical sensors. The model predicts a scaling relationship ρ0tsMD∼kD, where MD and kD are dimensionality dependent constants for one, two, and three dimensional nanosensors. We explore the performance limits of nanosensors using this analytical model and support its conclusions using detailed numerical simulation. Our results have obvious and significant implications for analyte density and response time reported in the literature and for design consideration of nanobiosensors and nanochemical sensors.

Design Considerations of Silicon Nanowire Biosensors

Biosensors based on silicon nanowires (Si-NWs) promise highly sensitive dynamic label-free electrical detection of biomolecules. Despite the tremendous potential and promising experimental results, the fundamental mechanism of electrical sensing of biomolecules and the design considerations of NW sensors remain poorly understood. In this paper, we discuss the prospects and challenges of biomolecule detection using Si-NW biosensors as a function of device parameters, fluidic environment, charge polarity of biomolecules, etc., and refer to experimental results in literature to support the nonintuitive predictions wherever possible. Our results indicate that the design of Si nanobiosensor is nontrivial and as such, only careful optimization supported by numerical simulation would ensure optimal sensor performance.

Screening-Limited Response of NanoBiosensors

Despite tremendous potential of highly sensitive electronic detection of biomolecules by nanoscale biosensors for genomics and proteomic applications, many aspects of experimentally observed sensor response (S) are difficult to understand within isolated theoretical frameworks of kinetic response or electrolyte screening. In this paper, we combine analytic solutions of Poisson-Boltzmann and diffusion-capture equations to show that the electrostatic screening within an ionic environment limits the response of nanobiosensor such that S(t) approximately c1(ln(rho0) - ln(I0)/2 + ln(t)/ D F + c2[pH]) + c3 where c i are geometry-dependent constants, rho0 is the concentration of target molecules, I0 the salt concentration, and D F the fractal dimension of sensor surface. Our analysis provides a coherent theoretical interpretation of a wide variety of puzzling experimental data that have so far defied intuitive explanation.

Anomalous resonance in a nanomechanical biosensor

The decrease in resonant frequency (−Δωr) of a classical cantilever provides a sensitive measure of the mass of entities attached on its surface. This elementary phenomenon has been the basis of a new class of bio-nanomechanical devices as sensing components of integrated microsystems that can perform rapid, sensitive, and selective detection of biological and biochemical entities. Based on classical analysis, there is a widespread perception that smaller sensors are more sensitive (sensitivity ≈ −0.5ωr/mC, where mC is the mass of the cantilever), and this notion has motivated scaling of biosensors to nanoscale dimensions. In this work, we show that the response of a nanomechanical biosensor is far more complex than previously anticipated. Indeed, in contrast to classical microscale sensors, the resonant frequencies of the nanosensor may actually decrease or increase after attachment of protein molecules. We demonstrate theoretically and experimentally that the direction of the frequency change arises from a size-specific modification of diffusion and attachment kinetics of biomolecules on the cantilevers. This work may have broad impact on microscale and nanoscale biosensor design, especially when predicting the characteristics of bio-nanoelectromechanical sensors functionalized with biological capture molecules.

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

Lateral profiling of trapped charge in SONOS flash EEPROMs programmed using CHE injection

The lateral profile of trapped charge in a silicon-oxide-nitride-oxide-silicon (SONOS) electrically erasable programmable read-only memory programmed using channel-hot-electron injection is determined using current-voltage (I/sub D/-V/sub G/) measurements along with two-dimensional device simulations and is verified using gate-induced-drain-leakage measurements, charge-pumping (CP) measurements, and Monte Carlo simulations. An iterative procedure is used to match simulated I/sub D/-V/sub G/ characteristics with experimental I/sub D/-V/sub G/ characteristics at different stages of programming, by sequentially increasing the trapped electron charge in simulations. Fresh cells are found to contain a high laterally nonuniform trapped charge, which (along with large electron injection during the program) make the conventional CP techniques inadequate for extracting the charge profile. This charge results in a nonmonotonous variation of threshold and flat-band voltages along the channel and makes it impossible to simultaneously determine interface and trapped charge profiles using CP alone. The CP technique is modified for application to SONOS cells and is used to verify the charge profile obtained using I/sub D/-V/sub G/ and to estimate the interface degradation. This paper enhances the study presented in our earlier work.

Device considerations for development of conductance-based biosensors

Design and fabrication of electronic biosensors based on field-effect-transistor (FET) devices require understanding of interactions between semiconductor surfaces and organic biomolecules. From this perspective, we review practical considerations for electronic biosensors with emphasis on molecular passivation effects on FET device characteristics upon immobilization of organic molecules and an electrostatic model for FET-based biosensors.

Flexure-FET biosensor to break the fundamental sensitivity limits of nanobiosensors using nonlinear electromechanical coupling

In this article, we propose a Flexure-FET (flexure sensitive field effect transistor) ultrasensitive biosensor that utilizes the nonlinear electromechanical coupling to overcome the fundamental sensitivity limits of classical electrical or mechanical nanoscale biosensors. The stiffness of the suspended gate of Flexure-FET changes with the capture of the target biomolecules, and the corresponding change in the gate shape or deflection is reflected in the drain current of FET. The Flexure-FET is configured to operate such that the gate is biased near pull-in instability, and the FET-channel is biased in the subthreshold regime. In this coupled nonlinear operating mode, the sensitivity (S) of Flexure-FET with respect to the captured molecule density (Ns) is shown to be exponentially higher than that of any other electrical or mechanical biosensor. In other words, while , classical electrical or mechanical biosensors are limited to Sclassical ∼ γ3NS or γ4 ln(NS), where γi are sensor-specific constants. In addition, the proposed sensor can detect both charged and charge-neutral biomolecules, without requiring a reference electrode or any sophisticated instrumentation, making it a potential candidate for various low-cost, point-of-care applications.

Device engineering of perovskite solar cells to achieve near ideal efficiency

Despite the exciting recent research on perovskite based solar cells, the design space for further optimization and the practical limits of efficiency are not well known in the community. In this letter, we address these aspects through theoretical calculations and detailed numerical simulations. Here, we first provide the detailed balance limit efficiency in the presence of radiative and Auger recombination. Then, using coupled optical and carrier transport simulations, we identify the physical mechanisms that contribute towards bias dependent carrier collection, and hence low fill factors of current perovskite based solar cells. Our detailed simulations indicate that it is indeed possible to achieve efficiencies and fill factors greater than 25% and 85%, respectively, with near ideal super-position characteristics even in the presence of Auger recombination.

Coupled heterogeneous nanowire-nanoplate planar transistor sensors for giant (>10 V/pH) nernst response

We offer a comprehensive theory of pH response of a coupled ISFET sensor to show that the maximum achievable response is given by ΔV/ΔpH = 59 mV/pH × α, where 59 mV/pH is the intrinsic Nernst response and α an amplification factor that depends on the geometrical and electrical properties of the sensor and transducer nodes. While the intrinsic Nernst response of an electrolyte/site-binding interface is fundamental and immutable, we show that by using channels of different materials, areas, and bias conditions, the extrinsic sensor response can be increased dramatically beyond the Nernst limit. We validate the theory by measuring the pH response of a Si nanowire-nanoplate transistor pair that achieves >10 V/pH response and show the potential of the scheme to achieve (asymptotically) the theoretical lower limit of signal-to-noise ratio for a given configuration. We suggest the possibility of an even larger pH response based on recent trends in heterogeneous integration on the Si platform.