A 2D ferroelectric vortex pattern in twisted BaTiO3 freestanding layers.

The wealth of complex polar topologies1-10 recently found in nanoscale ferroelectrics results from a delicate balance between the intrinsic tendency of the materials to develop a homogeneous polarization and the electric and mechanical boundary conditions imposed on them. Ferroelectric-dielectric interfaces are model systems in which polarization curling originates from open circuit-like electric boundary conditions, to avoid the build-up of polarization charges through the formation of flux-closure11-14 domains that evolve into vortex-like structures at the nanoscale15-17 level. Although ferroelectricity is known to couple strongly with strain (both homogeneous18 and inhomogeneous19,20), the effect of mechanical constraints21 on thin-film nanoscale ferroelectrics has been comparatively less explored because of the relative paucity of strain patterns that can be implemented experimentally. Here we show that the stacking of freestanding ferroelectric perovskite layers with controlled twist angles provides an opportunity to tailor these topological nanostructures in a way determined by the lateral strain modulation associated with the twisting. Furthermore, we find that a peculiar pattern of polarization vortices and antivortices emerges from the flexoelectric coupling of polarization to strain gradients. This finding provides opportunities to create two-dimensional high-density vortex crystals that would enable us to explore previously unknown physical effects and functionalities.

Dielectrophoretic assembly of liquid-phase-exfoliated TiS3 nanoribbons for photodetecting applications.

Liquid-phase exfoliation is a technique capable of producing large quantities of two-dimensional materials in suspension. Despite many efforts in the optimization of the exfoliation process itself, not much has been done towards the integration of liquid-phase-exfoliated materials in working solid-state devices. In this article, we use dielectrophoresis to direct the assembly of liquid-phase-exfoliated TiS3 nanoribbons between two gold electrodes to produce photodetectors working in the visible region. Through electrical and optical measurements we characterize the responsivity of the device and we find values as large as 3.8 mA W-1, which are more than one order of magnitude higher compared to state-of-the-art devices based on liquid-phase-exfoliated two-dimensional materials assembled by drop-casting or ink-jet methods.

Gate Controlled Photocurrent Generation Mechanisms in High-Gain In2Se3 Phototransistors.

Photocurrent in photodetectors incorporating van der Waals materials is typically produced by a combination of photocurrent generation mechanisms that occur simultaneously during operation. Because of this, response times in these devices often yield to slower, high gain processes, which cannot be turned off. Here we report on photodetectors incorporating the layered material In2Se3, which allow complete modulation of a high gain, photogating mechanism in the ON state in favor of fast photoconduction in the OFF state. While photoconduction is largely gate independent, photocurrent from the photogating effect is strongly modulated through application of a back gate voltage. By varying the back gate, we demonstrate control over the dominant mechanism responsible for photocurrent generation. Furthermore, because of the strong photogating effect, these direct-band gap, multilayer phototransistors produce ultrahigh gains of (9.8 ± 2.5) × 10(4) A/W and inferred detectivities of (3.3 ± 0.8) × 10(13) Jones, putting In2Se3 among the most sensitive 2D materials for photodetection studied to date.

Pick-up and drop transfer of diamond nanosheets.

Nanocrystalline diamond (NCD) is a promising material for electronic and mechanical micro- and nanodevices. Here we introduce a versatile pick-up and drop technique that makes it possible to investigate the electrical, optical and mechanical properties of as-grown NCD films. Using this technique, NCD nanosheets, as thin as 55 nm, can be picked-up from a growth substrate and positioned on another substrate. As a proof of concept, electronic devices and mechanical resonators are fabricated and their properties are characterized. In addition, the versatility of the method is further explored by transferring NCD nanosheets onto an optical fiber, which allows measuring its optical absorption. Finally, we show that NCD nanosheets can also be transferred onto two-dimensional crystals, such as MoS2, to fabricate heterostructures. Pick-up and drop transfer enables the fabrication of a variety of NCD-based devices without requiring lithography or wet processing.

Optomechanical coupling between a multilayer graphene mechanical resonator and a superconducting microwave cavity.

The combination of low mass density, high frequency and high quality factor, Q, of mechanical resonators made of two-dimensional crystals such as graphene make them attractive for applications in force/mass sensing and exploring the quantum regime of mechanical motion. Microwave optomechanics with superconducting cavities offers exquisite position sensitivity and enables the preparation and detection of mechanical systems in the quantum ground state. Here, we demonstrate coupling between a multilayer graphene resonator with quality factors up to 220,000 and a high-Q superconducting cavity. Using thermomechanical noise as calibration, we achieve a displacement sensitivity of 17 fm Hz(-1/2). Optomechanical coupling is demonstrated by optomechanically induced reflection and absorption of microwave photons. We observe 17 dB of mechanical microwave amplification and signatures of strong optomechanical backaction. We quantitatively extract the cooperativity C, a characterization of coupling strength, from the measurement with no free parameters and find C = 8, which is promising for the quantum regime of graphene motion.

Hydrogen termination of CVD diamond films by high-temperature annealing at atmospheric pressure.

A high-temperature procedure to hydrogenate diamond films using molecular hydrogen at atmospheric pressure was explored. Undoped and doped chemical vapour deposited (CVD) polycrystalline diamond films were treated according to our annealing method using a H2 gas flow down to ~50 ml∕min (STP) at ~850 °C. The films were extensively evaluated by surface wettability, electron affinity, elemental composition, photoconductivity, and redox studies. In addition, electrografting experiments were performed. The surface characteristics as well as the optoelectronic and redox properties of the annealed films were found to be very similar to hydrogen plasma-treated films. Moreover, the presented method is compatible with atmospheric pressure and provides a low-cost solution to hydrogenate CVD diamond, which makes it interesting for industrial applications. The plausible mechanism for the hydrogen termination of CVD diamond films is based on the formation of surface carbon dangling bonds and carbon-carbon unsaturated bonds at the applied tempera-ture, which react with molecular hydrogen to produce a hydrogen-terminated surface.

Laser-thinning of MoS2: on demand generation of a single-layer semiconductor.

Single-layer MoS(2) is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS(2) is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS(2) layers is a crucial step toward exploiting the large direct bandgap of monolayer MoS(2) in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS(2) single layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS(2) down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS(2) single layers.

Force-gradient-induced mechanical dissipation of quartz tuning fork force sensors used in atomic force microscopy.

We have studied the dynamics of quartz tuning fork resonators used in atomic force microscopy taking into account the mechanical energy dissipation through the attachment of the tuning fork base. We find that the tuning fork resonator quality factor changes even in the case of a purely elastic sensor-sample interaction. This is due to the effective mechanical imbalance of the tuning fork prongs induced by the sensor-sample force gradient, which in turn has an impact on dissipation through the attachment of the resonator base. This effect may yield a measured dissipation signal that can be different from the one exclusively related to the dissipation between the sensor and the sample. We also find that there is a second-order term in addition to the linear relationship between the sensor-sample force gradient and the resonance frequency shift of the tuning fork that is significant even for force gradients usually present in atomic force microscopy, which are in the range of tens of N/m.

Carbon fibre tips for scanning probe microscopy based on quartz tuning fork force sensors.

We report the fabrication and the characterization of carbon fibre tips for use in combined scanning tunnelling and force microscopy based on piezoelectric quartz tuning fork force sensors. We find that the use of carbon fibre tips results in a minimum impact on the dynamics of quartz tuning fork force sensors, yielding a high quality factor and, consequently, a high force gradient sensitivity. This high force sensitivity, in combination with high electrical conductivity and oxidation resistance of carbon fibre tips, make them very convenient for combined and simultaneous scanning tunnelling microscopy and atomic force microscopy measurements. Interestingly, these tips are quite robust against occasionally occurring tip crashes. An electrochemical fabrication procedure to etch the tips is presented that produces a sub-100-nm apex radius in a reproducible way which can yield high resolution images.

Dynamics of quartz tuning fork force sensors used in scanning probe microscopy.

We have performed an experimental characterization of the dynamics of oscillating quartz tuning forks which are being increasingly used in scanning probe microscopy as force sensors. We show that tuning forks can be described as a system of coupled oscillators. Nevertheless, this description requires knowledge of the elastic coupling constant between the prongs of the tuning fork, which has not yet been measured. Therefore, tuning forks have usually been described within the single oscillator or the weakly coupled oscillators approximation that neglects the coupling between the prongs. We propose three different procedures to measure the elastic coupling constant: an opto-mechanical method, a variation of the Cleveland method and a thermal noise based method. We find that the coupling between the quartz tuning fork prongs has a strong influence on the dynamics and the measured motion is in remarkable agreement with a simple model of coupled harmonic oscillators. The precise determination of the elastic coupling between the prongs of a tuning fork allows us to obtain a quantitative relation between the resonance frequency shift and the force gradient acting at the free end of a tuning fork prong.