Semiconductor-based gas sensors are of great interest in both industrial and research settings, but poor selectivity has hindered their further development. Current efforts including doping, surface modifications and facet controlling have been proved effective. However, the “methods-selectivity” correlation is ambiguous because of uncontrollable defects and surface states during the experiments. Here, as a case study, using a DFT method, we studied the adsorption features of commonly tested gases—CH₂O, H₂, C₂H₅OH, CH₃COCH₃, and NH_3—on facets of $\mathrm{ZnO}\left(000\overline{1}\right)$, $\mathrm{ZnO}\left(10\overline{1}0\right)$ and $\mathrm{ZnO}\left(10\overline{1}1\right)$. The adsorption energies and charge transfers were calculated, and adsorption selectivity was analyzed. The results show $\mathrm{ZnO}\left(000\overline{1}\right)$ has obvious CH₂O adsorption selectivity; $\mathrm{ZnO}\left(10\overline{1}0\right)$ has a slight selectivity to C₂H₅OH and NH₃; and $\mathrm{ZnO}\left(10\overline{1}1\right)$ has a slight selectivity to H₂, which agrees with the experimental results. The mechanism of the selective adsorption features was studied in terms of polarity, geometric matching and electronic structure matching. The results show the adsorption selectivity is attributed to a joint effort of electronic structure matching and geometric matching: the former allows for specific gas/slab interactions, the latter decides the strength of the interactions. As the sensing mechanism is probably dominated by gas–lattice interactions, this work is envisioned to be helpful in designing new sensing material with high selectivity.

It has been demonstrated that piezoresistive beams in a purely axial deformation state significantly enhance the performance of piezoresistive accelerometer. Current solution to realize purely axial deformation of piezoresistive beams for high performance micro-accelerometer relies heavily on beam positions, which limits its design flexibility and the error tolerance in the fabrication process. In this paper, a novel structure with position independent pure axially deformed piezoresistive beams is proposed. By controlling synchronous displacements at both ends of piezoresistive beams, the pure axially stress states of the piezoresistive beams can be easily realized at any beam position without tedious theoretical calculations. The theoretical model was developed to understand the relationship between the displacement and the axial stress of the piezoresistive beams, as well as the natural frequency of the whole structure. Then, the correctness of the theoretical model was verified by finite element simulation and experiments. The results demonstrated that the accelerometer had an extremely high sensitivity of 2.44 mV/g/5 V (without circuit amplification), and a high natural frequency of 11.4 kHz.

A novel structure was presented for a microelectromechanical system (MEMS) piezoresistive accelerometer. Taking advantage of the effects of tiny piezoresistive beam (TPB) to generate stress concentration region and microleverage mechanism to amplify the force, a piezoresistive accelerometer was proposed with improved sensitivity. The measured sensitivity was determined by the amplified inertia force applied to the TPBs. Based on the model analyses, the force amplification factor was influenced by the beams’ spring constants and the distance between support beam and TPB. The simulations were further carried out to verify the effects of amplifying force and find out the effects of dimensional parameters on amplification factor. The results suggested that the amplification factor was influenced by support and hinge beam widths dramatically, and also affected by the distance between support beam and TPB. Based on the simulation findings, three prototypes (denoted as XJTU_11, XJTU_12 and XJTU_32) were proposed with different support and hinge beam widths along with the distance between support beam and TPB to verify the amplification effect. Furthermore, the prototypes were successfully fabricated by MEMS technology with a size of 4.4 mm × 4.4 mm and packaged for experimental tests. The sensitivities were determined to be 1.03 mV/g/5V, 0.871 mV/g/5V and 0.736 mV/g/5V for XJTU_11, XJTU_12 and XJTU_32, respectively. The sensitivity of XJTU_11 increased by 18.4% and 40.1% compared with XJTU_12 and XJTU_32 respectively. The results indicated that the higher amplification factor was beneficial to the device to obtain the higher sensitivity. The figure of merit showed that proposed accelerometer obtained a more favorable comprehensive performance. This new design provided a good strategy and revealed the promising potential in improving the accelerometer’s sensitivity.