Low-Power Negative-Differential-Resistance Device for Sensing the Selective Protein via Supporter Molecule Engineering

Abstract

Van der Waals (vdW) heterostructures composed of atomically thin two-dimensional (2D) materials have more potential than conventional metal-oxide semiconductors because of their tunable bandgaps, and sensitivities. The remarkable features of these amazing vdW heterostructures are leading to multi-functional logic devices, atomically thin photodetectors, and negative differential resistance (NDR) Esaki diodes. Here, an atomically thin vdW stacking composed of p-type black arsenic (b-As) and n-type tin disulfide (n-SnS₂ ) to build a type-III (broken gap) heterojunction is introduced, leading to a negative differential resistance device. Charge transport through the NDR device is investigated under electrostatic gating to achieve a high peak-to-valley current ratio (PVCR), which improved from 2.8 to 4.6 when the temperature is lowered from 300 to 100 K. At various applied-biasing voltages, all conceivable tunneling mechanisms that regulate charge transport are elucidated. Furthermore, the real-time response of the NDR device is investigated at various streptavidin concentrations down to 1 pm, operating at a low biasing voltage. Such applications of NDR devices may lead to the development of cutting-edge electrical devices operating at low power that may be employed as biosensors to detect a variety of target DNA (e.g., ct-DNA) and protein (e.g., the spike protein associated with COVID-19).

Keywords: broken bandgap; negative differential resistance; selective protein detection; van der Waals heterostructure.

Figure 1

Schematics and basic characterization of the NDR device. a) A schematic illustration of the NDR device composed of p‐type b‐As (dark grey color) and n‐type SnS₂ (sky blue color) sheets to form a vdW heterostructure. b) An optical image with a scale bar of 10 µm and c) atomic force microscope (AFM) image of the black marked region (5 × 5 µm²) show the smooth and sharp interface. d) The thickness of the bottom b‐As sheet is ≈17 nm and e) the thickness of the SnS₂ sheet is ≈14 nm, which was confirmed by the height profile graph extracted from the AFM image. f) Raman spectrum obtained for the multi‐layer b‐As and g) Raman spectrum obtained for the multi‐layer SnS₂. The calculated full width at half maximum (FWHM) of the main resonance peak (A _2g) for b‐As and (A _1g) for SnS₂ appears to be 17 and 9.85 cm⁻¹, respectively. The peaks were fitted by Gaussian functions.

Figure 2

Electrical characterization of the NDR device as a function of gate voltages. a) Output curves (I _ds −V _ds) obtained at various gate voltages varying from V _g = –10 to +40 V, illustrate an excellent NDR trend at room temperature. b) The I _ds−V _ds curves in the small range of applied bias voltage show a maximum value of the peak current at V _g = 40 V. c) The peak current (in cyan color) and valley current (in blue color) are plotted as a function of gate voltage. d) The peak‐to‐valley current ratio (PVCR) plotted at various gate voltages. The maximum PVCR value was obtained at V _g = 40 V. The data were taken from 3 similar devices.

Figure 3

Electrical transport across the NDR device as a function of temperature. a) At a fixed gate voltage of V _g = 40 V, the I _ds−V _ds curves for a b‐As/SnS₂ NDR device were plotted at various temperatures ranging from 100 to 300 K. When the temperature decreased from 300 to 100 K, the peak current increased. b) The peak current (in orange color) and valley current (in red color) plotted as a function of temperature, at V _g = 40 V. c) The ratio of peak current over valley current (PVCR) plotted at 100, 150, 200, 250, and 300 K. At 100 K, PVCR = 4.6 reaches its highest value. The PVCR value varies as a function of temperature, as seen by the red dotted line. d) The PVCR value of three devices plotted in a bar graph at 100 K, showing consistent characteristics of the NDR devices composed of b‐As/SnS₂ vdW heterostructures. The data were triplicated using three different devices (n = 3) and it was estimated that p ≤ 0.0005 and MSSD ≤ 0.00006 for all sets of measurements.

Figure 4

Band alignment of p‐type b‐As/n‐type SnS₂ vdW heterostructure. a) The electron affinity and band gap values for b‐As (SnS₂), obtained from the vacuum level before constructing a vdW heterostructure were 4.4 eV (5.06 eV) and 0.3 eV (2.24 eV), respectively. The Fermi‐level (black dotted line) of the p‐type b‐As exists near its valence band (E _v) while the Fermi‐level (blue dotted line) of n‐type SnS₂ lies near its conduction band (E _c). The difference between the energy bands indicates a broken band gap (type III) vdW heterostructure. b) The band alignment at V _ds < 0 V shows the band tunneling of carriers from the conduction band of SnS₂ to the valence band of b‐As, c) while at 0 V < V _ds < 0.85 V the carriers directly tunnel from SnS₂ to b‐As because of overlapping. d) Under a 0.85 V < V _ds condition, TE (orange arrow line) and FN (blue arrow line) tunneling (through the triangular region) are the major mechanisms for charge transport through the NDR device.

Figure 5

Configuration and detection mechanism of protein via NDR device. a) Schematic depiction of the supporter molecule (pyrene ring loaded with lysine and biotin) and target protein (streptavidin). The combined structure illustrates the coupling between the supporter molecule and streptavidin. Streptavidin is captured by biotin due to its inherent high‐affinity value (Ka = 2.5 × 10¹³ m ⁻¹). b) NDR device schematic showing the measurement configuration of the NDR device as a biosensor. After device functionalization, a small drop of the solution containing streptavidin is drop‐casted over the NDR device, which is captured by the supporter construct. c) The real‐time measurements at V _ds = 0.5 V (in blue color) and V _ds = 1 V (in cyan color) are presented with a normalized current to check the NDR device's response against streptavidin. d) Real‐time response of the NDR device at a fixed V _ds = 0.5 V. The concentration of the streptavidin was varied down to 1 pm. The NDR device response after functionalization with supporter molecules (PLB) is shown in green, while the real‐time response to detect streptavidin concentrations ranging from 1 to 100 pm is shown in cyan and navy‐blue colors, respectively. e) The ultimate real‐time response of the NDR device for 1 pm streptavidin recorded for up to 12.5 s. f) The percentage response of the NDR device recorded for various concentrations of the target protein (streptavidin) and non‐targeted biomolecule (BSA). The percentage response is equilibrated as a decrease in response current of the NDR device. For all measurements, the variance was significant (p ≤ 0.05) with σ ≤ 0.2, IQR ≤ 0.4, and MSSD ≤ 0.02.