Defects-assisted piezoelectric response in liquid exfoliated MoS2 nanosheets

MoS2 is an intrinsic piezoelectric material which offers applications such as energy harvesting, sensors, actuators, flexible electronics, energy storage and more. Surprisingly, there are not any suitable, yet economical methods that can produce quality nanosheets of MoS2 in large quantities, hence limiting the possibility of commercialisation of its applications. Here, we demonstrate controlled synthesis of highly crystalline MoS2 nanosheets via liquid phase exfoliation of bulk MoS2, following which we report piezoelectric response from the exfoliated nanosheets. The method of piezo force microscopy (PFM) was employed to explore the piezo response in mono, bi, tri and multilayers of MoS2 nanosheets. The effective piezoelectric coefficient of MoS2 varies from 9.6 pm/V to 25.14 pm/V. We attribute piezoelectric response in MoS2 nanosheets to the defects formed in it during the synthesis procedure. The presence of defects is confirmed by X-ray photoelectron spectroscopy (XPS).

Voltage controlled Néel vector rotation in zero magnetic field at CMOS-compatible temperatures

Multi-functional thin films of boron (B) doped Cr2O3 exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H. Toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. Boundary magnetization associated with the Néel vector orientation serves as state variable which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B: Cr2O3 film. Switching of the Hall voltage between zero and negative values implies Néel vector rotation by 90-degree. Magnetometry, spin resolved inverse photoemission, electric transport measurements and piezo force microscopy reveal B-dependent TN and resistivity enhancement, spin-canting, anisotropy reduction and dynamic polarization hysteresis. Their combined effect enables H=0, voltage controlled, nonvolatile Néel vector rotation at high-temperature. Theoretical modeling estimates switching speeds of about 100ps making B: Cr2O3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications.

Accurate Electromechanical Characterization of Soft Molecular Monolayers using Piezo Force Microscopy

We report a new methodology for the electromechanical characterization of organic monolayers based on the implementation of dual AC resonance tracking piezo force microscopy (DART-PFM) combined with a sweep of an applied DC field under a fixed AC field. This experimental design allows calibration of the electrostatic component of the tip response and enables the use of low spring constant levers in the measurement. Moreover, the technique is shown to determine both positive and negative piezo response. The successful decoupling of the electrostatic component from the mechanical response will enable more quantitative electromechanical characterization of molecular and biomaterials and should generate new design principles for soft bio-compatible piezoactive materials. To highlight the applicability, our new methodology was used to successfully characterize the piezoelectric coefficient (d₃₃) of a variety of piezoactive materials, including self-assembled monolayers made of small molecules (dodecane thiol, mercaptoundecanoic acid) or macromolecules (peptides, peptoids), as well as a variety of inorganic materials, including lead zirconate titanate [PZT], quartz, and periodically poled lithium niobate [PPLN]. Due to high differential capacitance, the soft organic monolayers demonstrated exceedingly large electromechanical response (as high as 250 pm/V) but smaller d₃₃piezocoefficients. Finally, we find that the capacitive electrostatic response of the organic monolayers studied are significantly larger than conventional inorganic piezoelectric materials (e.g., PZT, PPLN, quartz), suggesting organic electromechanical materials applications can successfully draw from both piezo and electrostatic responses.

Accurate Electromechanical Characterization of Soft Molecular Monolayers using Piezo Force Microscopy

We report a new methodology for the electromechanical characterization of organic monolayers based on the implementation of dual AC resonance tracking piezo force microscopy (DART-PFM) combined with a sweep of an applied DC field under a fixed AC field. This experimental design allows calibration of the electrostatic component of the tip response and enables the use of low spring constant levers in the measurement. Moreover, the technique is shown to determine both positive and negative piezo response. The successful decoupling of the electrostatic component from the mechanical response will enable more quantitative electromechanical characterization of molecular and biomaterials and should generate new design principles for soft bio-compatible piezoactive materials. To highlight the applicability, our new methodology was used to successfully characterize the piezoelectric coefficient (d₃₃) of a variety of piezoactive materials, including self-assembled monolayers made of small molecules (dodecane thiol, mercaptoundecanoic acid) or macromolecules (peptides, peptoids), as well as a variety of inorganic materials, including lead zirconate titanate [PZT], quartz, and periodically poled lithium niobate [PPLN]. Due to high differential capacitance, the soft organic monolayers demonstrated exceedingly large electromechanical response (as high as 250 pm/V) but smaller d₃₃piezocoefficients. Finally, we find that the capacitive electrostatic response of the organic monolayers studied are significantly larger than conventional inorganic piezoelectric materials (e.g., PZT, PPLN, quartz), suggesting organic electromechanical materials applications can successfully draw from both piezo and electrostatic responses.

Accurate Electromechanical Characterization of Soft Molecular Monolayers using Piezo Force Microscopy

We report a new methodology for the electromechanical characterization of organic monolayers based on the implementation of dual AC resonance tracking piezo force microscopy (DART-PFM) combined with a sweep of an applied DC field under a fixed AC field. This experimental design allows calibration of the electrostatic component of the tip response and enables the use of low spring constant levers in the measurement. Moreover, the technique is shown to determine both positive and negative piezo response. The successful decoupling of the electrostatic component from the mechanical response will enable more quantitative electromechanical characterization of molecular and biomaterials and should generate new design principles for soft bio-compatible piezoactive materials. To highlight the applicability, our new methodology was used to successfully characterize the piezoelectric coefficient (d₃₃) of a variety of piezoactive materials, including self-assembled monolayers made of small molecules (dodecane thiol, mercaptoundecanoic acid) or macromolecules (peptides, peptoids), as well as a variety of inorganic materials, including lead zirconate titanate [PZT], quartz, and periodically poled lithium niobate [PPLN]. Due to high differential capacitance, the soft organic monolayers demonstrated exceedingly large electromechanical response (as high as 250 pm/V) but smaller d₃₃piezocoefficients. Finally, we find that the capacitive electrostatic response of the organic monolayers studied are significantly larger than conventional inorganic piezoelectric materials (e.g., PZT, PPLN, quartz), suggesting organic electromechanical materials applications can successfully draw from both piezo and electrostatic responses.

Accurate electromechanical characterization of soft molecular monolayers using piezo force microscopy

We report a new methodology for the electromechanical characterization of organic monolayers based on the implementation of dual AC resonance tracking piezo force microscopy (DART-PFM) combined with a sweep of an applied DC field under a fixed AC field.

Melt Electrowriting of Electroactive Poly(vinylidene difluoride) Fibers

<p>Poly(vinylidene difluoride) (PVDF) has piezoelectric properties suitable for numerous applications such as flexible electronics, sensing and biomedical materials. In this study, individual fibers with diameters ranging from 17-55 μm were processed using melt electrowriting (MEW). Electroactive PVDF fibers can be fabricated via MEW, while the polymer could remain molten for up to ten hours without noticeable changes in the resulting fiber diameter. MEW processing parameters for PVDF were investigated, including applied voltage, pressure and temperature. A rapid fiber characterization methodology for MEW that automatically determines the fiber diameters from camera images taken of microscope slides was developed and validated. The outputs from this approach followed previous MEW processing trends already identified with different polymers, although overestimation of the < 25 micron fiber diameters was observed. The transformation of the PVDF crystalline phase to the electroactive beta-phase – without poling – was confirmed using piezo-force microscopy and revealed that the PVDF fibers possess piezoelectric responses showing d33~19 pm/V.</p>

Melt Electrowriting of Electroactive Poly(vinylidene difluoride) Fibers

<p>Poly(vinylidene difluoride) (PVDF) has piezoelectric properties suitable for numerous applications such as flexible electronics, sensing and biomedical materials. In this study, individual fibers with diameters ranging from 17-55 μm were processed using melt electrowriting (MEW). Electroactive PVDF fibers can be fabricated via MEW, while the polymer could remain molten for up to ten hours without noticeable changes in the resulting fiber diameter. MEW processing parameters for PVDF were investigated, including applied voltage, pressure and temperature. A rapid fiber characterization methodology for MEW that automatically determines the fiber diameters from camera images taken of microscope slides was developed and validated. The outputs from this approach followed previous MEW processing trends already identified with different polymers, although overestimation of the < 25 micron fiber diameters was observed. The transformation of the PVDF crystalline phase to the electroactive beta-phase – without poling – was confirmed using piezo-force microscopy and revealed that the PVDF fibers possess piezoelectric responses showing d33~19 pm/V.</p>