http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

/pdf/physics-of-thin-film-ferroelectric-oxides-3zmqphtpv1.pdf

Physics of thin-film ferroelectric oxides

Dynamic atomic force microscopy methods

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

/pdf/janus-monolayers-of-transition-metal-dichalcogenides-rrgl6npbig.pdf

Janus monolayers of transition metal dichalcogenides

One of the biggest challenges in estimating the elastic, transport and storage properties of shales has been a lack of understanding of their complete pore structure. The shale matrix is predominantly composed of micropores (pores less than 2 nm diameter) and mesopores (pores with 2–50 nm diameter). These small pores in the shale matrix are mainly associated with clay minerals and organic matter and comprehending the controls of these clays and organic matter on the pore-size distribution is critical to understand the shale pore network. Historically, mercury intrusion techniques are used for pore-size analysis of conventional reservoirs. However, for unconventional shale reservoirs, very high pressures (> 414 MPa (60 000 psi)) would be required for mercury to access the full pore structure, which has potential pitfalls. Current instrumental limitations do not allow reliable measurement of significant portions of the total pore volume in shales. Nitrogen gas-adsorption techniques can be used to
characterize materials dominated by micro- and mesopores (2–50 nm). A limitation
of this technique is that it fails to measure large pores (diameter >200 nm). We use a nitrogen gas-adsorption technique to study the micro- and mesopores in shales and clays and compare the results from conventional mercury porosimetry techniques. Our results on pure clay minerals and natural shales show that (i) they have a multiscale pore structure at different dimensions (ii) fine mesopores, with a characteristic 3 nm pore size obtained with N2 gas-adsorption are associated with an illite-smectite group of clays but not with kaolinite; (iii) compaction results in a decrease of pore volume and a reduction of pore size in the ‘inter-aggregate’ macropores of the illitesmectite clays while the fine ‘intra-tachoid’ mesopores are shielded from compaction; (iv) for natural shales, mineralogy controls the pore-size distributions for shales and the presence of micropores and fine mesopores in natural shales can be correlated
with the dominance of the illite-smectite type of clays in the rock. Our assessment of incompressible 3 nm sized pores associated with illite-smectite clays provides an important building block for their mineral modulus.

Specific surface area and pore‐size distribution in clays and shales

With an optical interferometer, the free vibration spectra and the local vibration amplitude of four rectangular atomic force microscope cantilevers made of silicon have been examined experimentally in a spectral range of 100 kHz to 10 MHz. A good agreement with the flexural wave theory of elastic beams was found. Coupling to torsional vibrations was also observed. When the sensor tip of the cantilever is in contact with a sample surface the resonances are shifted in frequency and the vibration amplitudes along the cantilever change. A method is presented to calculate this frequency shift using a linear approximation for the tip–sample interaction forces, and the results are compared with the frequency shift calculated from the point‐mass model. The measured resonance frequencies of a surface‐coupled cantilever do not correspond as well to the theoretical ones as in the free case even if the elastic‐beam model is used. The reason for the disagreement is found to be the geometry of the commercial cantileve...

Vibrations of free and surface‐coupled atomic force microscope cantilevers: Theory and experiment

We have constructed an atomic force microscope enabling one to image the topography of a sample, and to monitor simultaneously ultrasonic surface vibrations in the MHz range. For detection of the distribution of the ultrasonic vibration amplitude, a part of the position‐sensing light beam reflected from the cantilever is directed to an external knife‐edge detector. Acoustic images taken on the surface of a wafer show a lateral resolution of about 100 nm at an ultrasonic frequency of 20 MHz.

Acoustic microscopy by atomic force microscopy

Quantitative determination of contact stiffness using atomic force acoustic microscopy

In atomic force acoustic microscopy (AFAM) the cantilever of an atomic force microscope is vibrated at ultrasonic frequencies while a sample surface is scanned with the sensor tip contacting the sample. As a consequence, the amplitude and phase of the cantilever vibration as well as the shift of the cantilever resonance frequencies contain information about local tip-sample contact stiffness and can be used as imaging quantities. An appropriate theoretical description of the transfer of ultrasound in an atomic force microscope enables the measurement of the local mechanical material parameters of the sample surface by evaluating experimental cantilever vibration spectra. In the experiments presented here, we examine the sensitivity of the technique using silicon single crystals. Furthermore, we show that the ferroelectric domains of lead zirconate-titanate ceramics can be imaged by AFAM and that local elastic constants of the sample surface can be determined quantitatively. The lateral resolution of the technique is given by the contact area formed by the sensor tip and the sample surface, which can have a diameter of <10 nm.

Imaging and measurement of local mechanical material properties by atomic force acoustic microscopy

Recent advances in atomic-force microscopy have moved beyond the original quasistatic implementation into a fully dynamic regime in which the atomic-force microscope cantilever is in contact with an insonified sample. The resulting dynamical system is complex and highly nonlinear. Simplification of this problem is often realized by modeling the cantilever as a one degree of freedom system. This type of first-mode approximation (FMA), or point-mass model, has been successful in advancing material property measurement techniques. The limits and validity of such an approximation have not, however, been fully addressed. In this article, the complete flexural beam equation is examined and compared directly with the FMA using both linear and nonlinear examples. These comparisons are made using analytical and finite difference numerical techniques. The two systems are shown to have differences in drive-point impedance and are influenced differently by the interaction damping. It is shown that the higher modes must be included for excitations above the first resonance if both the low and high frequency dynamics are to be modeled accurately.

/pdf/high-frequency-response-of-atomic-force-microscope-3ybmquwdra.pdf

Ute Rabe

Papers

Vibrations of free and surface‐coupled atomic force microscope cantilevers: Theory and experiment

Acoustic microscopy by atomic force microscopy

Quantitative determination of contact stiffness using atomic force acoustic microscopy

Imaging and measurement of local mechanical material properties by atomic force acoustic microscopy

High-frequency response of atomic-force microscope cantilevers