Molecular bridge-mediated ultralow-power gas sensing

Abstract

We report the electrical detection of captured gases through measurement of the quantum tunneling characteristics of gas-mediated molecular junctions formed across nanogaps. The gas-sensing nanogap device consists of a pair of vertically stacked gold electrodes separated by an insulating 6 nm spacer (~1.5 nm of sputtered α-Si and ~4.5 nm ALD SiO₂), which is notched ~10 nm into the stack between the gold electrodes. The exposed gold surface is functionalized with a self-assembled monolayer (SAM) of conjugated thiol linker molecules. When the device is exposed to a target gas (1,5-diaminopentane), the SAM layer electrostatically captures the target gas molecules, forming a molecular bridge across the nanogap. The gas capture lowers the barrier potential for electron tunneling across the notched edge region, from ~5 eV to ~0.9 eV and establishes additional conducting paths for charge transport between the gold electrodes, leading to a substantial decrease in junction resistance. We demonstrated an output resistance change of >10⁸ times upon exposure to 80 ppm diamine target gas as well as ultralow standby power consumption of <15 pW, confirming electron tunneling through molecular bridges for ultralow-power gas sensing.

Keywords: Nanosensors; Sensors.

Fig. 1. Fabrication flow and high resolution SEM images of nanogap devices.

a Simplified fabrication flow of nanogap electrodes. The fabrication process began by (1) growing 500 nm of thermal oxide on a Si wafer. (2) Then, 200 nm of Au (with a Cr adhesive layer) was deposited and patterned to define the lower electrodes. (3) Next, a SiO₂ layer (~4.5 nm) was deposited using plasma-enhanced ALD at a 200 °C chuck temperature, and an additional α-Si layer (~1.5 nm) was deposited, which together determined the thickness of the spacer layer. The thickness of atomic layer deposition (ALD) SiO₂ and DC sputtered α-Si was verified using ellipsometry (Woollam Variable Angle Spectroscopic Ellipsometer (VASE)), and the thickness of the entire spacer layer was verified using SEM imaging. (4) On top of the spacer layer, an upper Au electrode layer (200 nm) was sputtered and then (5) subsequently patterned using standard lithographic techniques. Finally, (6) the spacer stack layers were etched away through SF6 plasma dry etching, thereby forming an air gap along the edges of the top electrode. b SEM images of the fabricated device, where the overlap area was reduced to suppress parasitic current flow. The device footprint was 0.36 mm², and the overlap area was ~16 μm² (middle). The nanometer-scale dimension of the air gap formed between the upper and lower electrodes was confirmed by high-resolution SEM imaging

Fig. 2. Chemical structures and molecule lengths of the target molecule (on the left) and linker molecule (on the right).

The IUPAC name of the linker molecule is (4-((4-((4-mercaptophenyl)ethynyl)phenyl)ethynyl)benzoic acid)

Fig. 3. SAM coating procedure, visual confirmation of optimum immersion parameters, and qualitative comparison of the conductivity of the synthesized linker molecule and commercially available alkane-thiol linker molecule.

a–c Cr/Au-coated samples functionalized for 12, 24, and 48 h and exposed to similar concentrations of fluorescent cadaverine for 2 min. The lighter spots in the image are representative of successful capture of fluorescent cadaverine molecules by the SAM coating. As shown in the figure, the sample immersed for 48 h displayed the highest density of cadaverine-to-linker capture, resulting in a 48-h immersion period as our standard functionalization protocol. d, e To compare the conductivity of our synthesized linker molecules (with a thiol end group) with that of commercially available alkane thiols (which are inherently nonconductive), we fabricated separate Cr/Au-coated glass samples functionalized with (1) our synthesized linker molecule and (2) 16-mercaptohexadecanoic acid, a commercially available alkanethiol purchased from Millipore Sigma. As shown in the figure, samples functionalized with our synthesized linkers qualitatively exhibited augmented conductivity compared to the samples functionalized with the commercially available alkane-thiol linker molecule. The conductivity was measured using Peak force tunneling atomic force microscopy (PF – TUNA, Bruker). The lighter color spots are indicative of a higher current reading by PF-TUNA and are hence an indication of higher conductivity

Fig. 4. Experimental setup for sensor measurements.

The nanogap sensor is placed within a steel gas testing chamber that has a septum seal and purging outlets. Cadaverine is injected into the chamber through the septum seal, using a syringe. The chamber has an electrical feedthrough, connected to the sensor, to measure the dynamic resistance of the sensor after exposure to analyte, using a Keithly 4200A-SCS parameter analyzer.

Fig. 5. Schematic representation of the working principle of the nanogap sensor, equivalent electrical model, and representative band diagram.

a Schematic of the nanogap device after fabrication and analyte capture. Successful target capture turns the switch ‘ON’. b Schematic representation of current conduction before and after analyte gas capture, and equivalent electrical model depicting the two current conducting paths. c Average edge energy barrier in the absence of a target analyte gas, and d average energy barrier at a molecular junction established by the capture of a target gas molecule. The average energy barrier is lowered, leading to a larger tunneling current. I_S is the electric current through the dielectric spacer stack, and I_E (C_g) is the net current through the molecular bridges formed due to target gas capture and is a function of the target gas concentration C_g. Φ_E(0) and Φ_E(C_g) are the average potentials barrier before and after the capture of the target gas, respectively. χ_target is the electron affinity of the captured target. ψ_SAM and ψ_Au are the work functions of the SAM layer and the metal layer, respectively

Fig. 6. Electrical characterization of the fabricated device and response of sensor vs. control device.

aI–V curve of the fabricated device without gas exposure showing complete electrical isolation between upper and lower gold electrodes. b Sensor analysis over one complete cycle of exposure to 40 ppm cadaverine gas and its removal. The sensor signal is compared to the response of a ‘control’ chip, which is our nanogap device without proper functionalization exposed to 40 ppm cadaverine for the entirety of the experiment. G/G₀ is the conductance of the sensor normalized to its lowest value, G₀ (prior to gas exposure)

Fig. 7. Curve fitting of sensor response to the modified Zimbovskaya model and Polanyi–Wigner equation.

aI–V measurements of the nanogap sensor device after successful capture of cadaverine molecules at various concentrations of the analyte curve-fitted to the established tunneling current models. The maximum root-mean-square error of the curve-fitting plots shown in the figure was found to be ~45%, 1%, and 0.6% of the average experimental data for the I–V characteristics of the nanogap junction after exposure to 0 ppm, 60 ppm, and 80 ppm cadaverine, respectively. b Normalized conductance response of the sensor to one period of exposure of the nanogap device to cadaverine and its subsequent removal, curve-fitted to adsorption-desorption models

Fig. 8. Selectivity studies of the nanogap sensor.

The sensor was exposed to multiple gases and analytes with different molecular end groups to determine its cross-selectivity. a A comparitive bar-graph showing the response of sensor to different analytes. b Device response to commonly found gases and analytes,their concentration and molecular molecular end groups.