arXiv subjects

Alexander A. Balandin

Publications and source records attributed to Alexander A. Balandin.

At least 73 records · Page 4Linked to original sources

arXiv2019

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductor Materials

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS3 and MnPS3 semiconductors. The Raman spectroscopy was conducted using three different excitation lasers with the wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). The resonant UV-Raman spectroscopy reveals new spectral features, which are not detectable via visible Raman light scattering. The thermal conductivity of FePS3 and MnPS3 thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS3 to be 1.35 W/mK at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS3 is 0.85 W/mK and 2.7 W/mK, respectively. The films of MnPS3 have higher thermal conductivity of 1.1 W/mK through-plane and 6.3 W/mK in-plane. The data obtained by both techniques reveal strong thermal anisotropy of the films and the dominant contribution of phonons to heat conduction. Our results are important for the proposed applications of the antiferromagnetic semiconductor thin films in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Low-Frequency Electronic Noise in Superlattice and Random-Packed Thin Films of Colloidal Quantum Dots

We report measurements of low-frequency electronic noise in ordered superlattice, weakly-ordered and random-packed thin films of 6.5 nm PbSe quantum dots prepared using several different ligand chemistries. For all samples, the normalized noise spectral density of the dark current revealed a Lorentzian component, reminiscent of the generation-recombination noise, superimposed on the 1/f background (f is the frequency). An activation energy of 0.3 eV was extracted from the temperature dependence of the noise spectra. The noise level in the ordered films was lower than that in the weakly-ordered and random-packed films. A large variation in the magnitude of the noise spectral density was also observed in samples with different ligand treatments. The obtained results are important for application of colloidal quantum dot films in photodetectors.

cond-mat.mes-hall↗

arXiv2019

Phononics of Graphene and Graphene Composites

I present a concise account concerning the emergence of a research field, which deals with the thermal properties of graphene, covering the refinement of understanding of phonon transport in two-dimensional material systems. The practical application of graphene and few-layer graphene in thermal interface materials are also discussed.

cond-mat.mes-hall↗

arXiv2019

Thermal Properties of the Binary-Filler Composites with Few-Layer Graphene and Copper Nanoparticles

The thermal properties of an epoxy-based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the "synergistic" filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size-dissimilar fillers, has a well-defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of ~15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches ~40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, ~40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. The electrical percolation is observed at low graphene loading, less than 7 wt.%, owing to the large aspect ratio of graphene. At all concentrations of the fillers, below and above the electrical percolation threshold, the thermal transport is dominated by phonons. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.

physics.app-ph↗

arXiv2019

Brillouin-Mandelstam Spectroscopy of Stress-Modulated Spatially Confined Spin Waves in Ni Thin Films on Piezoelectric Heterostructures

We report results of micro-Brillouin-Mandelstam light scattering spectroscopy of thermal magnons in the two-phase synthetic multiferroic structure consisting of a piezoelectric (PMN-PT) substrate and a Ni thin film with the thickness of 64 nm. The experimental data reveal the first two modes of the perpendicular standing spin waves (PSSW) spatially confined across the Ni thin film. A theoretical analysis of the frequency dependence of the PSSW peaks on the external magnetic field reveals the asymmetric boundary condition, i.e. pinning, for variable magnetization at different surfaces of the Ni thin film. The strain field induced by applying DC voltage to PMN-PT substrate leads to a down shift of PSSW mode frequency owing to the magneto-elastic effect in Ni, and corresponding changes in the spin wave resonance conditions. The observed non-monotonic dependence of the PSSW frequency on DC voltage is related to an abrupt change of the pinning parameter at certain values of the voltage. The obtained results are important for understanding the thermal magnon spectrum in ferromagnetic films and development of the low-power spin-wave devices.

cond-mat.mtrl-sci↗

arXiv2019

Low Resistivity and High Breakdown Current Density of 10-nm Diameter van der Waals TaSe3 Nanowires by Chemical Vapor Deposition

Micron-scale single-crystal nanowires of metallic TaSe3, a material that forms -Ta-Se3-Ta-Se3- stacks separated from one another by a tubular van der Waals (vdW) gap, have been synthesized using chemical vapor deposition (CVD) on a SiO2/Si substrate, in a process compatible with semiconductor industry requirements. Their electrical resistivity was found unaffected by downscaling from the bulk to as little as 7 nm in width and height, in striking contrast to the resistivity of copper for the same dimensions. While the bulk resistivity of TaSe3 is substantially higher than that of bulk copper, at the nanometer scale the TaSe3 wires become competitive to similar-sized copper ones. Moreover, we find that the vdW TaSe3 nanowires sustain current densities in excess of 108 A/cm2 and feature an electromigration energy barrier twice that of copper. The results highlight the promise of quasi-one-dimensional transition metal trichalcogenides for electronic interconnect applications and the potential of van der Waals materials for downscaled electronics.

cond-mat.mtrl-sci↗

arXiv2019

Thermal and Electrical Properties of Hybrid Composites with Graphene and Boron Nitride Fillers

We report on the thermal and electrical properties of hybrid epoxy composites with graphene and boron nitride fillers. The thicknesses, lateral dimensions, and aspect ratios of each filler material were intentionally selected for geometric similarity to one another, in contrast to prior studies that utilized dissimilar filler geometries to achieve a "synergistic" effect. We demonstrate that the electrically-conductive graphene and electrically-insulating boron nitride fillers allow one to effectively engineer the thermal and electrical conductivities of their resulting composites. By varying the constituent fraction of boron nitride to graphene in a composite with ~44% total filler loading, one can tune the thermal conductivity enhancement from a factor of x15 to x35 and increase the electrical conductivity by many orders of magnitude. The obtained results are important for the development of next-generation thermal interface materials with controllable electrical properties necessary for applications requiring either electrical grounding or insulation.

physics.app-ph↗

arXiv2019

Low-Frequency Noise Spectroscopy of Charge-Density-Wave Phase Transitions in Vertical Quasi-2D Devices

We report results regarding the electron transport in vertical quasi-2D layered 1T-TaS2 charge-density-wave devices. The low-frequency noise spectroscopy was used as a tool to study changes in the cross-plane electrical characteristics of the quasi-2D material below room temperature. The noise spectral density revealed strong peaks - changing by more than an order-of-magnitude - at the temperatures closely matching the electrical resistance steps. Some of the noise peaks appeared below the temperature of the commensurate to nearly-commensurate charge-density-wave transition, possibly indicating the presence of the debated "hidden" phase transitions. These results confirm the potential of the noise spectroscopy for investigations of electron transport and phase transitions in novel materials.

cond-mat.mes-hall↗

arXiv2018

Anomalous Characteristics of the Generation - Recombination Noise in Quasi-One-Dimensional Van der Waals Nanoribbons

We describe the low-frequency current fluctuations, i.e. electronic noise, in quasi-one-dimensional ZrTe3 van der Waals nanoribbons, which have recently attracted attention owing to their extraordinary high current carrying capacity. Whereas the low-frequency noise spectral density reveals 1/f behavior near room temperature, it is dominated by the Lorentzian bulges of the generation - recombination noise at low temperatures (f is the frequency). Unexpectedly, the corner frequency of the observed Lorentzian peaks shows strong sensitivity to the applied source - drain bias. This dependence on electric field can be explained by the Frenkel-Poole effect in the scenario where the voltage drop happens predominantly on the defects, which block the quasi-1D conduction channels. We also have found that the activation energy of the characteristic frequencies of the G-R noise in quasi-1D ZrTe3 is defined primarily by the temperature dependence of the capture cross-section of the defects rather than by their energy position. These results are important for the application of quasi-1D van der Waals materials in ultimately downscaled electronics.

cond-mat.mes-hall↗

arXiv2018

Graphene Composites with Dual Functionality: Electromagnetic Shielding and Thermal Management

We report on the synthesis and characterization of the epoxy-based composites with the few-layer graphene fillers, which are capable of the duel functional applications. It was found that composites with the certain types of few-layer graphene fillers reveal an efficient total electromagnetic interference shielding, SE~45 dB, in the important X-band frequency range, f=8.2 GHz - 12.4 GHz, while simultaneously providing the high thermal conductivity, K = 8 W/mK, which is a factor of x35 larger than that of the base matrix material. The efficiency of the dual functional application depends on the filler characteristics: thickness, lateral dimensions, aspect ratio and concentration. Graphene loading fractions above the percolation threshold allow for strong enhancement of both the electromagnetic interference shielding and heat conduction properties. Interestingly, graphene composites can block the electromagnetic energy even below the percolation threshold, remaining electrically insulating, which is an important feature for some types of thermal interface materials. The dual functionality of the graphene composites can substantially improve the electromagnetic shielding and thermal management of the airborne systems while simultaneously reducing their weight and cost.

physics.app-ph↗

arXiv2018

Thermal Percolation Threshold and Thermal Properties of Composites with Graphene and Boron Nitride Fillers

We investigated thermal properties of the epoxy-based composites with a high loading fraction - up to f=45 vol.% - of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction f>20 vol.%. The graphene loading required for achieving the thermal percolation was substantially higher than the loading for the electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with the high filler loading, above the thermal percolation threshold, is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.

cond-mat.mtrl-sci↗

arXiv2018

Acoustic Phonon Dispersion Engineering in Bulk Crystals via Incorporation of Dopant Atoms

We report results of Brillouin - Mandelstam spectroscopy of transparent alumina crystals with Nd dopants. The ionic radius and atomic mass of Nd atoms are distinctively different from those of the host Al atoms. Our results show that even a small concentration of Nd atoms incorporated into the alumina samples produces a profound change in the acoustic phonon spectrum. The velocity of the transverse acoustic phonons decreases by ~600 m/s at the Nd density of only ~0.1 %. Interestingly, the decrease in the phonon frequency and velocity with the doping concentration is non-monotonic. The obtained results, demonstrating that modification of the acoustic phonon spectrum can be achieved not only by nanostructuring but also by doping have important implications for thermal management as well as thermoelectric and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Low-Frequency Noise and Sliding of the Charge Density Waves in Two-Dimensional Materials

There has been a recent renewal of interest in charge-density-wave (CDW) phenomena, primarily driven by the emergence of two-dimensional (2D) layered CDW materials, such as 1T-TaS2, characterized by very high transition temperatures to CDW phases. In the extensively studied classical bulk CDW materials with quasi-1D crystal structure, the charge carrier transport exhibits intriguing sliding behavior, which reveals itself in the frequency domain as "narrowband" and "broadband" noise. Despite the increasing attention on physics of 2D CDWs, there have been few reports of CDW sliding, specifically in quasi-2D rare-earth tritellurides and none on the noise in any of 2D CDW systems. Here we report the results of low-frequency noise (LFN) measurements on 1T-TaS2 thin films - archetypal 2D CDW systems, as they are driven from the nearly commensurate (NC) to incommensurate (IC) CDW phases by voltage and temperature stimuli. We have found that noise in 1T-TaS2 devices has two pronounced maxima at the bias voltages, which correspond to the onset of CDW sliding and the NC-to-IC phase transition. We observed unusual Lorentzian noise features and exceptionally strong noise dependence on electric bias and temperature. We argue that LFN in 2D CDW systems has unique physical origin, different from known fundamental noise types. The specifics of LFN in 2D CDW materials can be explained by invoking the concept of interacting discrete fluctuators in the NC-CDW phase. Noise spectroscopy can serve as a useful tool for understanding electronic transport phenomena in 2D CDW materials characterized by coexistence of different phases and strong CDW pinning.

cond-mat.mes-hall↗

arXiv2017

Raman Optothermal Technique for Measuring Thermal Conductivity of Graphene and Related Materials

We describe Raman spectroscopy based method of measuring thermal conductivity of thin films, and review significant results achieved with this technique pertinent to graphene and other two-dimensional materials. The optothermal Raman method was instrumental for the discovery of unique heat conduction properties of graphene. In this method, Raman spectroscopy is used to determine the local temperature of the sample while the excitation laser is utilized as a heat source. The success of Raman spectroscopy in investigating thermal conductivity of suspended graphene and graphene-based thin films motivated extension of this technique to other materials systems and films.

cond-mat.mes-hall↗

arXiv2016

Two-Dimensional Oscillatory Neural Network Based on Charge-Density-Wave Devices Operating at Room Temperature

We propose an oscillatory neural network implemented with two-dimensional tantalum disulfide devices operating in the change density wave regime at room temperature. An elementary cell of the network consists of two 1T-TaS2 devices connected in series. Such a cell has constant output and oscillatory states. All cells have the same bias voltage. There is constant current flowing through the cell in the constant output mode. The oscillations occur at a certain bias voltage due to the electrical-field driven metal-to-insulator transition owing to the changes in the charge density wave phase in the 1T-TaS2 channel. Two 1T-TaS2 devices oscillate out-of-phase where one of the devices is in the insulator phase while the other one is in the metallic state. The nearest-neighbor cells are coupled via graphene transistors. The cells are resistively coupled if the graphene transistor is in the On state while they are capacitively coupled if the transistor is in the Off state. The operation of the oscillatory neural network is simulated numerically for the 30x30 node network. The results of our numerical modeling show the formation of artificial vortexes and cellular-automata type data processing. The two-dimensional 1T-TaS2 devices, utilized in the network, offer a unique combination of properties such as scalability, high operational frequency, fast synchronization speed, and radiation hardness, which makes them promising for both consumer electronic and defense applications.

cs.ET↗

arXiv2016

Low-Frequency Electronic Noise in Exfoliated Quasi-1D TaSe3 van Der Waals Nanowires

We report results of investigation of the low-frequency electronic excess noise in quasi-1D nanowires of TaSe3 capped with quasi-2D h-BN layers. Semi-metallic TaSe3 is a quasi-1D van der Waals material with exceptionally high breakdown current density. It was found that TaSe3 nanowires have lower levels of the normalized noise spectral density, compared to carbon nanotubes and graphene. The temperature-dependent measurements revealed that the low-frequency electronic 1/f noise becomes the 1/f^2-type as temperature increases to about 400 K, suggesting the onset of electromigration (f is the frequency). Using the Dutta- Horn random fluctuation model of the electronic noise in metals we determined that the noise activation energy for quasi-1D TaSe3 nanowires is approximately E_P=1.0 eV. In the framework of the empirical noise model for metallic interconnects, the extracted activation energy, related to electromigration, is E_A=0.88 eV, consistent with that for Cu and Al interconnects. Our results shed light on the physical mechanism of low-frequency 1/f noise in quasi-1D van der Waals semi-metals and suggest that such material systems have potential for ultimately downscaled local interconnect applications.

cond-mat.mes-hall↗

arXiv2016

Direct Observation of Acoustic Phonon Confinement in Free-Standing Semiconductor Nanowires

Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. Modification of acoustic phonon spectrum in structures with periodically modulated elastic constant or mass density - referred to as phononic crystals - has been proven experimentally and utilized in practical applications. A possibility of modifying acoustic phonon spectrum in individual nanostructures via spatial confinement would bring tremendous benefits for controlling phonon-electron interaction and thermal conduction at nanoscale. However, despite strong scientific and practical importance, conclusive experimental evidence of acoustic phonon confinement in individual free-standing nanostructures, e.g. nanowires, is still missing. The length scale, at which phonon dispersion undergoes changes and a possibility of the phonon group velocity reduction, are debated. Here, we utilize specially designed high-quality GaAs nanowires (NWs) with different diameters, D, and large inter-nanowire distances to directly demonstrate acoustic phonon confinement. The measurements conducted with Brillouin - Mandelstam spectroscopy reveal confined phonon polarization branches with frequencies from 4 GHz to 40 GHz in NWs with D as large as ~128 nm, i.e. at length scale, which exceeds the "grey" phonon mean-free path in GaAs by an almost an order of magnitude. The phonon dispersion modification and phonon energy scaling with D in individual nanowires are in excellent agreement with theory. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronics, thermoelectric energy conversion, thermal management, and novel spintronic technologies.

cond-mat.mes-hall↗

arXiv2016

Phonons and Thermal Transport in Graphene and Graphene-Based Materials

A discovery of the unusual thermal properties of graphene stimulated experimental, theoretical and computational research directed at understanding phonon transport and thermal conduction in two-dimensional material systems. We provide a critical review of recent results in the graphene thermal field focusing on phonon dispersion, specific heat, thermal conductivity, and comparison of different models and computational approaches. The correlation between the phonon spectrum in graphene-based materials and the heat conduction properties is analyzed in details. The effects of the atomic plane rotations in bilayer graphene, isotope engineering, and relative contributions of different phonon dispersion branches are discussed. For readers' convenience, the summaries of main experimental and theoretical results on thermal conductivity as well as phonon mode contributions to thermal transport are provided in the form of comprehensive annotated tables.

cond-mat.mtrl-sci↗