Hao Jia

Various structural phases in two-dimensional (2D) crystal polytypes provide distinct material properties including electronic band structure, conductivity, optical properties, ferromagnetic and ferroelectric properties. However, the impact of phase on mechanical properties remains largely unexplored. Here, we quantitatively resolve the Young’s modulus of MoSe2 polytypes with different structural phases by leveraging the dynamic response of MoSe2 nanomechanical resonators. Through experimentally elucidating resonant behavior of MoSe2 drumheads with varying diameters and thicknesses, and developing theoretical models linking resonance behavior to phase-dependent mechanical properties, we determine the Young’s modulus for 2H (131 ± 10 GPa), 3R (77 ± 8 GPa), and 2H-3R mixed-phase (95 ± 5 GPa) MoSe2. We further show that structural phase can modulate the Young’s modulus by up to ~70%, which is effective for tuning device frequency and nonlinearity. Our findings establish phase as a powerful degree of freedom for tailoring mechanical properties of 2D polytypes and for exploiting polymorphism in designing strain-engineered multifunctional nanoelectromechanical devices.

Organic semiconductors are widely regarded as a key material platform for next generation flexible and wearable electronics. Realizing high-resolution, well-defined patterns is essential to scale organic field effect transistors for higher device density and greater electrical performance. Conventional photolithographic patterning technologies typically face the challenges of chemical contamination induced by photoresist, developers and chemical etchant. Here, we report a resist-free, maskless patterning approach for pentacene based on thermal scanning probe lithography (t-SPL), exploiting the localized thermochemical conversion of the soluble precursor 13,6-N-sulfinylacetamidopentacene into pentacene. By tuning key t-SPL parameters, including probe loading conditions and scanning velocity, well-defined pentacene nanostructures with a minimum full width at half maximum of 200 nm were achieved. Directly patterned pentacene channels were further integrated into field-effect transistors, demonstrating electrical functionality. This work establishes t-SPL as a viable route for direct, high-resolution patterning of organic semiconductors and provides a promising pathway toward further miniaturization and integration of organic electronic devices.

Highly sensitive and rapid detection of trace acetylene (C₂H₂) gas is crucial for industrial safety but challenging for existing sensors. Herein, we present a catalytic thermopile-based C₂H₂ sensor, combining the lowest detection limit and fastest response time/recovery time among reported C₂H₂ sensors. Facilitated by Pd NPs@Al₂O₃ (∼3.5 nm in diameter) decorated on the specially engineered differential thermopiles, the sensors can detect the C₂H₂ concentration down to 5 ppb. The sensors also exhibit the fastest response/recovery time of 0.34/0.40s. The structural design allows a power consumption of only 27.2 mW at 340 °C and a temperature variation of just ∼1.2%, surpassing reported thermopile sensors. Combining the neuroevolution potential (NEP) and multiphysics modeling, we reveal the mechanism of Pd NP-catalyzed cascade bond-breaking and exothermic reaction of C₂H₂ molecules in oxygen from atomic-scale and energy perspective. This study highlights the potential of noble nanoparticle-based catalytic thermopiles for high-performance C₂H₂ detection in various industrial applications.

Probing and understanding ultrahigh-frequency/gigahertz (UHF/GHz) vibrations in atomic layer nanomechanical resonators holds strong promise for fundamental studies and technological applications, such as sensing, signal processing, and quantum engineering. However, accessing GHz flexural-mode resonances in such devices at room temperature has been challenging. Here, we demonstrate the first measurement of GHz flexural vibrations in graphene and molybdenum disulfide (MoS ₂ ) resonators at room temperature, achieving multimode resonances ( f _{m , n} ) up to ~1.09 GHz and quality factors ( Q _{m , n} ) up to ~5400 in multilayer MoS ₂ resonators, and f _{m , n} up to ~1.03 GHz with high Q _{m , n} ~4500 in few-layer graphene resonators. Both f s and Q s are the highest among reported atomic layer nanomechanical resonators at room temperature, yielding f _{m , n} × Q _{m , n} ~ 5 × 10 ¹² Hz. We also reveal a Q scaling law with higher modes, which favors detecting GHz resonances. This study will enable exploiting multiple modes in atomic layer resonators toward resonant sensing and transduction functions at UHF/GHz.

This work presents an integrated microsensor that combines the dual characterization capabilities of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). We integrated two pairs of thermocouples, heating resistors, and resonant drive/detection resistors into a single microcantilever, where the cantilever resonant frequency shifts respond to the mass change and the output voltage of the integrated thermocouples respond to the sample temperature. This integration enables programmable temperature control, temperature variation, and mass detection on a single chip. Our chip can achieve heating and cooling rates above 600 °C/min, which is significantly faster than commercial instruments with satisfactory measurement accuracy. The integrated polysilicon thermocouples bring high power responsivity of 6 V/W, making them suitable for highly sensitive DTA measurements on a chip. Moreover, the cantilever offers picogram (10^–12g) level mass resolution, reducing sample consumption from milligrams to nanogram levels. Additionally, the on-chip sample heating allows for easy observation of sample morphological evolution during heating under an optical microscope. We validated the dual functionality by conducting TGA measurements on a standard sample of calcium oxalate monohydrate (CaC₂O₄ ∙ H₂O) and DTA measurements on high-purity indium (In) and tin (Sn). The results indicate consistent measurements with the true values of the standard sample and high measurement efficiency. Our integrated cantilever chip is anticipated to have broad applications in high-performance and efficient TGA and DTA characterization.

A high quality (Q) factor is essential for enhancing the performance of resonant nanoelectromechanical systems (NEMS). NEMS resonators based on two-dimensional (2D) materials such as molybdenum disulfide (MoS₂) have high frequency tunability, large dynamic range, and high sensitivity, yet room-temperature Q factors are typically less than 1000. Here, we systematically investigate the effects of device size and surface nonidealities on Q factor by measuring 52 dry-transferred fully clamped circular MoS₂ NEMS resonators with diameters ranging from 1 μm to 8 μm, and optimize the Q factor by combining these effects with the strain-modulated dissipation model. We find that Q factor first increases and then decreases with diameter, with an optimized room-temperature Q factor up to 3315 ± 115 for a 2-μm-diameter device. Through extensive characterization and analysis using Raman spectroscopy, atomic force microscopy, and scanning electron microscopy, we demonstrate that surface nonidealities such as wrinkles, residues, and bubbles are especially significant for decreasing Q factor, especially for larger suspended membranes, while resonators with flat and smooth surfaces typically have larger Q factors. To further optimize Q factors, we measure and model Q factor dependence on the gate voltage, showing that smaller DC and radio-frequency (RF) driving voltages always lead to a higher Q factor, consistent with the strain-modulated dissipation model. This optimization of the Q factor delineates a straightforward and promising pathway for designing high-Q 2D NEMS resonators for ultrasensitive transducers, efficient RF communications, and low-power memory and computing.

This work presents an integrated microsensor that combines the dual characterization capabilities of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). We integrated two pairs of thermocouples, heating resistors, and resonant drive/detection resistors into one microcantilever to achieve programmable temperature control, temperature change, and mass detection in a single chip. Our chip can achieve heating and cooling rates above 600°C/min, which is significantly faster than commercial instruments with satisfactory measurement accuracy. The integrated polysilicon thermocouples bring high power responsivity of 6V/W, making them suitable for highly sensitive DTA measurements on a chip. Moreover, the cantilever offers picogram (10^-12g) level mass resolution, reducing sample consumption from milligrams to nanogram levels. Additionally, the on-chip sample heating allows for easy observation of sample morphological evolution during heating under an optical microscope. We validated the dual functionality by conducting TGA measurements on a standard sample of calcium oxalate monohydrate (CaC₂O₄∙H₂O) and DTA measurements on high-purity indium (In) and tin (Sn). The results indicate consistent measurements with the true values of the standard sample and high measurement efficiency. Our integrated cantilever chip is anticipated to have broad applications in high-performance and efficient TGA and DTA characterization.

Micromechanical resonators have aroused growing interest as biological and chemical sensors, and microcantilever beams are the main research focus. Recently, a resonant microcantilever with an integrated heater has been applied in on-chip thermogravimetric analysis (TGA). However, there is a strong relationship between the mass sensitivity of a resonant microcantilever and the location of adsorbed masses. Different sampling positions will cause sensitivity differences, which will result in an inaccurate calculation of mass change. Herein, an integrated H-shaped resonant beam with uniform mass sensitivity and temperature distribution is proposed and developed to improve the accuracy of bio/chemical sensing and TGA applications. Experiments verified that the presented resonant beam possesses much better uniformity of sensitivity and temperature distribution compared with resonant microcantilevers. Gas-sensing and TGA experiments utilizing the integrated resonant beam were also carried out and exhibited good measurement accuracy.