Non-Symmetrical Low-Voltage Negative Barrier Height Breast Cancer-Associated VOCs Sensing Different-Sized Gold Nanoflower-Functionalized ZnO Thin Films Prepared by a Low Cost Chemical Method

This work reports a high response (∼90,000) from a sol–gel synthesized ZnO thin film achieved through surface functionalization with gold nanoflowers (AuNFs). The device showed an asymmetric voltage response and exhibited higher sensitivity at very low voltages than at higher voltages. Previous studies used ZnO functionalized with symmetrical (spherical) gold nanoparticles, but their sensor response was nearly 1000 times lower than what we achieved. While standard gold nanoparticles are symmetrical, AuNFs grown with AgNO 3 are asymmetrical. This structural asymmetry creates an asymmetrical response to applied voltage, significantly boosting the sensor’s overall performance. For the first time, detached AuNF petals functionalized on ZnO showed a very high response to volatile organic compounds (VOCs). Thiol-free AuNFs were functionalized to the ZnO surface using a low-cost sol–gel drop coating method instead of expensive physical techniques. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and X-ray diffraction measurements were used to analyze AuNFs on the ZnO surface. These studies revealed that nanoflower size governed the loading, which increased up to a threshold value. Beyond this size, the nanoflowers were sparsely distributed, leaving mainly detached nanopetals on the ZnO surface. These sparse nanopetals were responsible for the exceptionally high response (∼90,000) compared to previously reported values. The nanoflower shape and size were controlled by the silver ion concentration in the gold colloidal solution. We fabricated low power gas sensors using AuNF-functionalized ZnO thin films with patterned aluminum electrodes. These sensors showed exceptionally high response and selectivity toward 2-ethyl-1-hexanol, a VOC associated with breast cancer. The sensor demonstrated high sensitivity at a low applied voltage of 0.05 V. Additionally, a negative contact barrier influenced by the size of the AuNFs and the VOCs was observed, which reflects the inter particle barrier height and conductivity.