Alexander Tselev

Selective Growth of Well-Aligned Semiconducting Single-Walled Carbon Nanotubes

High-density arrays of perfectly aligned single-walled carbon nanotubes (SWNTs) consisting almost exclusively of semiconducting nanotubes were grown on ST-cut single crystal quartz substrates. Raman spectroscopy together with electrical measurements of field effect transistors (FETs) fabricated from the as-grown samples showed that over 95% of the nanotubes in the arrays are semiconducting. The mechanism of selective growth was explored. It is proposed that introducing methanol in the growth process, combined with the interaction between the SWNTs and the quartz lattice, leads to the selective growth of aligned semiconducting nanotubes. Such a high density of horizontally aligned semiconducting SWNTs can be readily used in high current nanoFETs and sensors. This method demonstrates great promise to solve one of the most difficult problems which limits application of carbon nanotubes in nanoelectronicsthe coexistence of metallic and semiconducting nanotubes in samples produced by most, if not all, growth methods.

Real Space Mapping of Li-Ion Transport in Amorphous Si Anodes with Nanometer Resolution

The electrical bias driven Li-ion motion in silicon anode materials in thin film battery heterostructures is investigated using electrochemical strain microscopy (ESM), which is a newly developed scanning probe microscopy based characterization method. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for studies on the sub-20 nm scale, and allows the relationship between Li-ion flow and microstructure to be established. The evolution of Li-ion transport during the battery charging is directly observed.

Mechanism of NO2 detection in carbon nanotube field effect transistor chemical sensors

We report an experimental method that clearly determines the sensing mechanism of carbon-nanotube field effect transistors. The nanotube/electrode contacts are covered with a thick and long passivation layer that hinders their exposure to chemicals in a controlled fashion, leaving only the midsection of the nanotube exposed. In the case of nitrogen dioxide, a considerably delayed response is fully consistent with the diffusion of the gas through the passivation layer. The results clearly indicate that nitrogen dioxide detection is due to changes at the interfaces between the nanotube and the electrodes and not to molecules adsorbed on the nanotube surface.

Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy

Ferroelectricity in functional materials remains one of the most fascinating areas of modern science in the past several decades. In the last several years, the rapid development of piezoresponse force microscopy (PFM) and spectroscopy revealed the presence of electromechanical hysteresis loops and bias-induced remnant polar states in a broad variety of materials including many inorganic oxides, polymers, and biosystems. In many cases, this behavior was interpreted as the ample evidence for ferroelectric nature of the system. Here, we systematically analyze PFM responses on ferroelectric and nonferroelectric materials and demonstrate that mechanisms unrelated to ferroelectricity can induce ferroelectric-like characteristics through charge injection and electrostatic forces on the tip. We will focus on similarities and differences in various PFM measurement characteristics to provide an experimental guideline to differentiate between ferroelectric material properties and charge injection. In the end, we apply the developed measurement protocols to an unknown ferroelectric material.

Symmetry relationship and strain-induced transitions between insulating M1 and M2 and metallic R phases of vanadium dioxide.

The ability to synthesize VO2 in the form of single-crystalline nanobeams and nano- and microcrystals uncovered a number of previously unknown aspects of the metal-insulator transition (MIT) in this oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg-Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO2 in the vicinity of the R-M2-M1 triple point using the Ginzburg-Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO2 nanostructures in desired phase states and to use of external fields in the control of the VO2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO2 nanoplatelets, and theoretically predict phases that have not been previously observed.

The role of electrochemical phenomena in Scanning Probe Microscopy of ferroelectric thin films

Applications of piezoresponse force microscopy and conductive atomic force microscopy to ferroelectric thin films necessitate understanding of the possible bias-induced electrochemical reactivity of oxide surfaces. These range from reversible ionic surface charging (possibly coupled to polarization) and vacancy and proton injection to partially reversible vacancy ordering, to irreversible electrochemical degradation of the film and bottom electrode. Here, the electrochemical phenomena induced by a biased tip are analyzed and both theoretical and experimental criteria for their identification are summarized.

Interplay between Ferroelastic and Metal−Insulator Phase Transitions in Strained Quasi-Two-Dimensional VO2 Nanoplatelets

Formation of ferroelastic twin domains in vanadium dioxide (VO(2)) nanosystems can strongly affect local strain distributions, and hence couple to the strain-controlled metal-insulator transition. Here we report polarized-light optical and scanning microwave microscopy studies of interrelated ferroelastic and metal-insulator transitions in single-crystalline VO(2) quasi-two-dimensional (quasi-2D) nanoplatelets (NPls). In contrast to quasi-1D single-crystalline nanobeams, the 2D geometric frustration results in emergence of several possible families of ferroelastic domains in NPls, thus allowing systematic studies of strain-controlled transitions in the presence of geometrical frustration. We demonstrate the possibility of controlling the ferroelastic domain population by the strength of the NPl-substrate interaction, mechanical stress, and by the NPl lateral size. Ferroelastic domain species and domain walls are identified based on standard group-theoretical considerations. Using variable temperature microscopy, we imaged the development of domains of metallic and semiconducting phases during the metal-insulator phase transition and nontrivial strain-driven reentrant domain formation. A long-range reconstruction of ferroelastic structures accommodating metal-insulator domain formation has been observed. These studies illustrate that a complete picture of the phase transitions in single-crystalline and disordered VO(2) structures can be drawn only if both ferroelastic and metal-insulator strain effects are taken into consideration and understood.

Interplay of Octahedral Tilts and Polar Order in BiFeO3 Films

Heterointerface stabilization of a distinct nonpolar BiFeO3 phase occurs simultaneously with changes in octahedral tilts. The resulting phase arises via suppression of polarization by a structural order parameter and can thus be identified as anti-ferroelectric (Fe displacements - bottom panel). The phase is metastable and can be switched into a polar ferroelectric state (top panel) under an applied electric bias.

Doping-Based Stabilization of the M2 Phase in Free-Standing VO2 Nanostructures at Room Temperature

A new high-yield method of doping VO(2) nanostructures with aluminum is proposed, which renders possible stabilization of the monoclinic M2 phase in free-standing nanoplatelets in ambient conditions and opens an opportunity for realization of a purely electronic Mott transition field-effect transistor without an accompanying structural transition. The synthesized free-standing M2-phase nanostructures are shown to have very high crystallinity and an extremely sharp temperature-driven metal-insulator transition. A combination of X-ray microdiffraction, micro-Raman spectroscopy, energy-dispersive X-ray spectroscopy, and four-probe electrical measurements allowed thorough characterization of the doped nanostructures. Light is shed onto some aspects of the nanostructure growth, and the temperature-doping level phase diagram is established.

Electromechanical actuation and current-induced metastable states in suspended single-crystalline VO2 nanoplatelets

Current-induced electromechanical actuation enabled by the metal-insulator transition in VO(2) nanoplatelets is demonstrated. The Joule heating by a sufficient current flowing through suspended nanoplatelets results in formation of heterophase domain patterns and is accompanied by nanoplatelet deformation. The actuation action can be achieved in a wide temperature range below the bulk phase transition temperature (68 °C). The observed current-sustained heterophase domain structures should be interpreted as distinct metastable states in free-standing and end-clamped VO(2) samples. We analyze the main prerequisites for the realization of a current-controlled actuator based on the proposed concept.