arXiv subjects

Alexander A. Balandin

Publications and source records attributed to Alexander A. Balandin.

At least 91 records · Page 5Linked to original sources

arXiv2016

Breakdown Current Density in BN-Capped Quasi-1D TaSe3 Metallic Nanowires: Prospects of Interconnect Applications

We report results of investigation of the current-carrying capacity of nanowires made from the quasi-1D van der Waals metal tantalum triselenide capped with quasi-2D boron nitride. The chemical vapor transport method followed by chemical and mechanical exfoliation were used to fabricate mm-long TaSe3 wires with lateral dimensions in the 20 to 70 nm range. Electrical measurements establish that TaSe3/h-BN nanowire heterostructures have a breakdown current density exceeding 10 MA/cm2 - an order-of-magnitude higher than that in copper. Some devices exhibited an intriguing step-like breakdown, which can be explained by the atomic thread bundle structure of the nanowires. The quasi-1D single crystal nature of TaSe3 results in low surface roughness and the absence of grain boundaries; these features potentially can enable the downscaling of these wires to lateral dimensions in the few-nm range. These results suggest that quasi-1D van der Waals metals have potential for applications in the ultimately downscaled local interconnects.

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arXiv2016

Thermal Conductivity of Suspended Graphene with Defects

We investigate the thermal conductivity of suspended graphene as a function of the density of defects, ND, introduced in a controllable way. Graphene layers are synthesized using chemical vapor deposition, transferred onto a transmission electron microscopy grid, and suspended over ~7.5-micrometer size square holes. Defects are induced by irradiation of graphene with the low-energy electron beam (20 keV) and quantified by the Raman D-to-G peak intensity ratio. As the defect density changes from 2.0x10^10 cm-2 to 1.8x10^11 cm-2 the thermal conductivity decreases from ~(1.8+/-0.2)x10^3 W/mK to ~(4.0+/-0.2)x10^2 W/mK near room temperature. At higher defect densities, the thermal conductivity reveals an intriguing saturation behavior at a relatively high value of ~400 W/mK. The thermal conductivity dependence on defect density is analyzed using the Boltzmann transport equation and molecular dynamics simulations. The results are important for understanding phonon - point defect scattering in two-dimensional systems and for practical applications of graphene in thermal management.

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arXiv2016

Thermal Conductivity of Segmented Nanowires

We present a review of the phonon thermal conductivity of segmented nanowires focusing on theoretical results for Si and Si/Ge structures with the constant and periodically modulated cross-sections. We describe the use of the face-centered cubic cell and Born-von Karman models of the lattice vibrations for calculating the phonon energy spectra in the segmented nanowires. Modification of the phonon spectrum in such nanostructures results in strong reduction of the phonon thermal conductivity and suppression of heat transfer due to a trapping of phonon modes in nanowire segments. Possible practical applications of segmented nanowires in thermoelectric energy generation are also discussed.

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arXiv2016

An Integrated Tantalum Sulfide - Boron Nitride - Graphene Oscillator: A Charge-Density-Wave Device Operating at Room Temperature

The charge-density-wave (CDW) phase is a macroscopic quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or layered 2D metallic crystals. Several layered transition metal dichalcogenides, such as 1T-TaSe2, 1T-TaS2 and 1T-TiSe2, exhibit unusually high transition temperatures to different CDW symmetry-reducing phases. These transitions can be affected by environmental conditions, film thickness and applied electric bias. However, device applications of these intriguing systems at room temperature or their integration with other 2D materials have not been explored. Here we show that in 2D CDW 1T-TaS2, the abrupt change in the electrical conductivity and hysteresis at the transition point between nearly-commensurate and incommensurate charge-density-wave phases can be used for constructing an oscillator that operates at room temperature. The hexagonal boron nitride was capped on 1T-TaS2 thin film to provide protection from oxidation, and an integrated graphene transistor provides a voltage tunable, matched, low-resistance load enabling precise voltage control of the oscillator frequency. The integration of these three disparate two-dimensional materials, in a way that exploits the unique properties of each, yields a simple, miniaturized, voltage-controlled oscillator device. Theoretical considerations suggest that the upper limit of oscillation frequency to be in the THz regime.

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arXiv2016

Thermal Transport in Graphene, Few-Layer Graphene and Graphene Nanoribbons

The discovery of unusual heat conduction properties of graphene has led to a surge of theoretical and experimental studies of phonon transport in two-dimensional material systems. The rapidly developing graphene thermal field spans from theoretical physics to practical engineering applications. In this invited review we outline different theoretical approaches developed for describing phonon transport in graphene and provide comparison with available experimental thermal conductivity data. A special attention is given to analysis of the recent theoretical results for the phonon thermal conductivity of graphene and few-layer graphene, the effects of the strain, defects, isotopes and edge scattering on the acoustic phonon transport in these material systems.

cond-mat.mtrl-sci↗

arXiv2015

Acoustic Phonon Spectrum and Thermal Transport in Nanoporous Alumina Arrays

We report results of a combined investigation of thermal conductivity and acoustic phonon spectra in nanoporous alumina membranes with the pore diameter decreasing from D=180 nm to 25 nm. The samples with the hexagonally arranged pores were selected to have the same porosity of ~13%. The Brillouin-Mandelstam spectroscopy measurements revealed bulk-like phonon spectrum in the samples with D=180-nm pores and spectral features, which were attributed to spatial confinement, in the samples with 25-nm and 40-nm pores. The velocity of the longitudinal acoustic phonons was reduced in the samples with smaller pores. Analysis of the experimental data and calculated phonon dispersion suggests that both phonon-boundary scattering and phonon spatial confinement affect heat conduction in membranes with the feature sizes D<40 nm.

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arXiv2015

Phonon Spectrum Engineering in Rolled-up Nano- and Micro-Architectures

We report on a possibility of efficient engineering of the acoustic phonon energy spectrum in multishell tubular structures produced by a novel high-tech method of self-organization of nano- and micro-architectures. The strain-driven roll-up procedure paved the way for novel classes of metamaterials such as single semiconductor radial micro- and nano-crystals and multi-layer spiral micro- and nano-superlattices. The acoustic phonon dispersion is determined by solving the equations of elastodynamics for InAs and GaAs material systems. It is shown that the number of shells is an important control parameter of the phonon dispersion together with the structure dimensions and acoustic impedance mismatch between the superlattice layers. The obtained results suggest that rolled up nano-architectures have potential for thermoelectric applications owing to a possibility of significant reduction of the thermal conductivity without degradation of the electronic transport.

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arXiv2015

Graphene Heat Spreaders and Interconnects for Advanced Electronic Applications

Graphene revealed a number of unique properties beneficial for electronics, including exceptionally high electron mobility and widely tunable Fermi level. However, graphene does not have an electron energy band gap, which presents a serious hurdle for its applications in digital electronics. A possible route for practical use of graphene in electronics is utilization of its exceptionally high thermal conductivity and electron current conducting properties. This invited review outlines the thermal properties of graphene and describes prospective graphene technologies that are not affected by the absence of the energy band gap. Specific examples include heat spreaders, thermal coatings, high-current density electrodes and interconnects. Our results suggest that thermal management of advanced electronic devices can become the first industry-scale application of graphene.

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arXiv2015

Suppression of 1/f Noise in Near-Ballistic h-BN-Graphene-h-BN Heterostructure Field-Effect Transistors

We have investigated low-frequency 1/f noise in the boron nitride - grapheme - boron nitride heterostructure field - effect transistors on Si/SiO2 substrates (f is a frequency). The device channel was implemented with a single layer graphene encased between two layers of hexagonal boron nitride. The transistors had the charge carrier mobility in the range from 30000 to 36000 cm2/Vs at room temperature. It was established that the noise spectral density normalized to the channel area in such devices can be suppressed to 5 x 10^-9 μm2 Hz^-1, which is a factor of x5 - x10 lower than that in non-encapsulated graphene devices on Si/SiO2. The physical mechanism of noise suppression was attributed to screening of the charge carriers in the channel from traps in SiO2 gate dielectric and surface defects. The obtained results are important for the electronic and optoelectronic applications of graphene.

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arXiv2015

Phonon Engineering of the Specific Heat of Twisted Bilayer Graphene: The Role of the Out-of-Plane Phonon Modes

We investigated theoretically the specific heat of graphene, bilayer graphene and twisted bilayer graphene taking into account the exact phonon dispersion and density of states for each polarization branch. It is shown that contrary to a conventional believe the dispersion of the out-of-plane acoustic phonons - referred to as ZA phonons - deviates strongly from a parabolic law starting from the frequencies as low as ~100 1/cm. This leads to the frequency-dependent ZA phonon density of states and the breakdown of the linear dependence of the specific heat on temperature T. We established that ZA phonons determine the specific heat for T<200 K while contributions from both in-plane and out-of-plane acoustic phonons are dominant for 200 K < T < 500 K. In the high-temperature limit, T>1000 K, the optical and acoustic phonons contribute approximately equally to the specific heat. The Debye temperature for graphene and twisted bilayer graphene was calculated to be around ~1861 - 1864 K. Our results suggest that the thermodynamic properties of materials such as bilayer graphene can be controlled at the atomic scale by rotation of the sp2-carbon planes.

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arXiv2015

Thermal Transport in Graphene and Graphene Multilayers

In this paper we review thermal properties of graphene and multilayer graphene and discuss the optothermal technique developed for the thermal conductivity measurements. We also outline different theoretical approaches used for the description of phonon transport in graphene and provide comparison with available experimental thermal conductivity data.

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arXiv2015

Review of Thermal Properties of Graphene and Few-Layer Graphene: Applications in Electronics

We review thermal properties of graphene and few-layer graphene, and discuss applications of these materials in thermal management of advanced electronics. The intrinsic thermal conductivity of graphene - among the highest of known materials - is dominated by phonons near the room temperature. The examples of thermal management applications include the few-layer graphene heat spreaders integrated near the heat generating areas of the high-power density transistors. It has been demonstrated that few-layer graphene heat spreaders can lower the hot-spot temperature during device operation resulting in improved performance and reliability of the devices.

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arXiv2014

Specific Heat of Twisted Bilayer Graphene

We have studied the phonon specific heat in single-layer, bilayer and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T<15 K, specific heat varies with temperature as T^n, where n = 1 for graphene, n = 1.6 for bilayer graphene and n = 1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pronounced at low temperature. The results suggest a possibility of phonon engineering of thermal properties of layered materials by twisting the atomic planes.

cond-mat.mtrl-sci↗

arXiv2013

Strong Enhancement of Thermal Properties of Copper Films after Chemical Vapor Deposition of Graphene

We demonstrated that chemical vapor deposition of graphene on Cu films strongly enhances their thermal diffusivity and thermal conductivity. Deposition of graphene increases the thermal conductivity of 9 micrometer (25 micrometer) thick Cu films by up to 24% (16%) near room temperature. Interestingly, the observed improvement of thermal properties of graphene coated Cu films is primarily due to changes in Cu morphology during graphene deposition and associated with it temperature treatment rather than graphene's action as an additional heat conducting channel. Enhancement of thermal properties of metal films via graphene coating may lead to applications in electronic circuits and metallurgy.

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arXiv2013

Graphene-Based Non-Boolean Logic Circuits

Graphene revealed a number of unique properties beneficial for electronics. However, graphene does not have an energy band-gap, which presents a serious hurdle for its applications in digital logic gates. The efforts to induce a band-gap in graphene via quantum confinement or surface functionalization have not resulted in a breakthrough. Here we show that the negative differential resistance experimentally observed in graphene field-effect transistors of "conventional" design allows for construction of viable non-Boolean computational architectures with the gap-less graphene. The negative differential resistance - observed under certain biasing schemes - is an intrinsic property of graphene resulting from its symmetric band structure. Our atomistic modeling shows that the negative differential resistance appears not only in the drift-diffusion regime but also in the ballistic regime at the nanometer-scale - although the physics changes. The obtained results present a conceptual change in graphene research and indicate an alternative route for graphene's applications in information processing.

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arXiv2013

Review of the Low-Frequency 1/f Noise in Graphene Devices

Low-frequency noise with a spectral density that depends inversely on frequency (f) has been observed in a wide variety of systems including current fluctuations in resistors, intensity fluctuations in music and signals in human cognition. In electronics, the phenomenon, which is known as 1/f noise, flicker noise or excess noise, hampers the operation of numerous devices and circuits, and can be a significant impediment to development of practical applications from new materials. Graphene offers unique opportunities for studying 1/f noise because of its 2D structure and carrier concentration tuneable over a wide range. The creation of practical graphene-based devices will also depend on our ability to understand and control the low-frequency 1/f noise in this material system. Here, I review the characteristic features of 1/f noise in graphene and few-layer graphene, and examine the implications of such noise for the development of graphene-based electronics including high-frequency devices and sensors.

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arXiv2013

Graphene-Enhanced Hybrid Phase Change Materials for Thermal Management of Li-Ion Batteries

Li-ion batteries are crucial components for progress in mobile communications and transport technologies. However, Li-ion batteries suffer from strong self-heating, which limits their life-time and creates reliability and environmental problems. Here we show that thermal management and the reliability of Li-ion batteries can be drastically improved using hybrid phase change material with graphene fillers. Conventional thermal management of batteries relies on the latent heat stored in the phase change material as its phase changes over a small temperature range, thereby reducing the temperature rise inside the battery. Incorporation of graphene to the hydrocarbon-based phase change material allows one to increase its thermal conductivity by more than two orders of magnitude while preserving its latent heat storage ability. A combination of the sensible and latent heat storage together with the improved heat conduction outside of the battery pack leads to a significant decrease in the temperature rise inside a typical Li-ion battery pack. The described combined heat storage - heat conduction approach can lead to a transformative change in thermal management of Li-ion and other types of batteries.

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arXiv2013

Effects of Functionalization on Thermal Properties of Single-Wall and Multi-Wall Carbon Nanotube - Polymer Nanocomposites

Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to improve their dispersion properties and increase the electrical conductivity of carbon-nanotube - polymer nanocomposites. We have studied experimentally the effects of this type of functionalization on the thermal conductivity of the nanocomposites. It was found that while even small quantities of carbon nanotubes (~1 wt%) can increase the electrical conductivity, a larger loading fraction (~3 wt%) is required to enhance the thermal conductivity of nanocomposites. Functionalized multi-wall carbon nanotubes performed the best as filler material leading to a simultaneous improvement of the electrical and thermal properties of the composites. Functionalization of the single-wall carbon nanotubes reduced the thermal conductivity enhancement. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix it also leads to formation of defects, which impede the acoustic phonon transport in the single wall carbon nanotubes. The obtained results are important for applications of carbon nanotubes and graphene flakes as fillers for improving thermal, electrical and mechanical properties of composites.

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