Field emission in vacuum resonant tunneling heterostructures with high current densities

We analyse the steady-state thermal regime of a one-dimensional triode resonant tunnelling structure. The high currents generated by resonant tunnelling produce a large amount of heat that could damage the structure. Establishing the conditions under which it can operate at optimum efficiency is therefore a problem of great relevance for applications. The tunnel current is found via eigenvalues of the Schrödinger equation in quantum wells. By calculating the current generated in the device and using the energy conservation law in the electrodes, the temperature reached is obtained for different types of electrodes and the importance of heat conduction and thermal radiation is analysed. In the cases discussed, conduction is dominant. When the electrode material is copper, the temperature reached is similar to that of the thermostat for a wide range of electrode lengths, whereas when the cathode material is diamond-graphite and the anode material is copper, the temperature increases significantly as a function of length. The results obtained allow the temperature to be controlled for optimum performance of the field-emitting triode structures.

Island-Type Graphene-Nanotube Hybrid Structures for Flexible and Stretchable Electronics: In Silico Study

Using the self-consistent charge density functional tight-binding (SCC-DFTB) method, we study the behavior of graphene-carbon nanotube hybrid films with island topology under axial deformation. Hybrid films are formed by AB-stacked bilayer graphene and horizontally aligned chiral single-walled carbon nanotubes (SWCNTs) with chirality indices (12,6) and 1.2 nm in diameter. In hybrid films, bilayer graphene is located above the nanotube, forming the so-called "islands" of increased carbon density, which correspond to known experimental data on the synthesis of graphene-nanotube composites. Two types of axial deformation are considered: stretching and compression. It has been established that bilayer graphene-SWCNT (12,6) hybrid films are characterized by elastic deformation both in the case of axial stretching and axial compression. At the same time, the resistance of the atomic network of bilayer graphene-SWCNT (12,6) hybrid films to failure is higher in the case of axial compression. Within the framework of the Landauer-Buttiker formalism, the current-voltage characteristics of bilayer graphene-SWCNT (12,6) hybrid films are calculated. It is shown that the slope of the current-voltage characteristic and the maximum values of the current are sensitive to the topological features of the bilayer graphene in the composition of graphene-SWCNT (12,6) hybrid film. Based on the obtained results, the prospects for the use of island-type graphene-nanotube films in flexible and stretchable electronic devices are predicted.

Band Gap Opening in Borophene/GaN and Borophene/ZnO Van der Waals Heterostructures Using Axial Deformation: First-Principles Study

One of the topical problems of materials science is the production of van der Waals heterostructures with the desired properties. Borophene is considered to be among the promising 2D materials for the design of van der Waals heterostructures and their application in electronic nanodevices. In this paper, we considered new atomic configurations of van der Waals heterostructures for a potential application in nano- and optoelectronics: (1) a configuration based on buckled triangular borophene and gallium nitride (GaN) 2D monolayers; and (2) a configuration based on buckled triangular borophene and zinc oxide (ZnO) 2D monolayers. The influence of mechanical deformations on the electronic structure of borophene/GaN and borophene/ZnO van der Waals heterostructures are studied using the first-principles calculations based on density functional theory (DFT) within a double zeta plus polarization (DZP) basis set. Four types of deformation are considered: uniaxial (along the Y axis)/biaxial (along the X and Y axes) stretching and uniaxial (along the Y axis)/biaxial (along the X and Y axes) compression. The main objective of this study is to identify the most effective types of deformation from the standpoint of tuning the electronic properties of the material, namely the possibility of opening the energy gap in the band structure. For each case of deformation, the band structure and density of the electronic states (DOS) are calculated. It is found that the borophene/GaN heterostructure is more sensitive to axial compression while the borophene/ZnO heterostructure is more sensitive to axial stretching. The energy gap appears in the band structure of borophene/GaN heterostructure at uniaxial compression by 14% (gap size of 0.028 eV) and at biaxial compression by 4% (gap size of 0.018 eV). The energy gap appears in the band structure of a borophene/ZnO heterostructure at uniaxial stretching by 10% (gap size 0.063 eV) and at biaxial compression by 6% (0.012 eV). It is predicted that similar heterostructures with an emerging energy gap can be used for various nano- and optoelectronic applications, including Schottky barrier photodetectors.

Radiative heat transfer between two carbon nanotubes

We analyze the radiative heat transfer between two parallel and infinitely long carbon nanotubes (CNTs). The radiative heat exchange is due to the difference between the Poynting vectors generated by the fluctuating currents when the CNTs are at different temperatures. The radiated and absorbed Poynting vectors are expressed in terms of the correlations of the electromagnetic fields obtained from the Green’s function and the fluctuation-dissipation theorem for the current density. The analysis takes into account the scattering of the fields by the nanotubes. We show that the radiative heat transfer depends not only on the distance between nanotubes, but also on their chiralities and thus on their semiconducting or metallic nature, which would allow the design of nanostructures for optimal radiative heat exchange.

Electronic properties and behavior of carbon network based on graphene and single-walled carbon nanotubes in strong electrical fields: quantum molecular dynamics study

Using the self-consistent-charge density-functional tight-binding method (SCC-DFTB) and extended lagrangian DFTB-based molecular dynamics, we performedin silicostudies of the behavior of graphene-nanotube hybrid structures that are part of a branched 3D carbon network in strong electrical fields. It has been established that strong fields with strength ranging from 5 to 10 V nm^-1cause oscillating deformations of the atomic framework with a frequency in the range from 1.22 to 1.38 THz. It has been revealed that the oscillation frequency is determined primarily by the topology of the atomic framework of graphene-nanotube hybrid, while the electric field strength has an effect within 1%-2%. A further increase in electric field strength reduces the oscillation frequency to 0.7 THz, which accompanies the partial destruction of the atomic framework. The critical value of the electric field strength when the graphene is detached from the nanotube is ∼20 V nm^-1.

In Silico Study of the Electrically Conductive and Electrochemical Properties of Hybrid Films Formed by Bilayer Graphene and Single-Wall Nanotubes under Axial Stretching

Using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method, we studied the effect of axial stretching on the electrical conductivity and quantum capacitance of hybrid films formed by AB-stacked bilayer graphene and horizontally oriented single-walled carbon nanotubes (SWCNTs) with indices chirality (12, 6). The paper discusses several topological models of hybrid graphene/SWCNT (12, 6) films, which differ in the width of the graphene layer in the supercell and in the value of the shift between the graphene layers. It is shown that axial stretching has a different effect on the electrical conductivity and quantum capacity of the hybrid graphene/SWCNT (12, 6) film depending on the width of the graphene layer. For a topological model with a minimum width of the graphene layer (2 hexagons) under a 10% stretching strain, the transformation of bilayer graphene from planar to wave-like structures is characteristic. This transformation is accompanied by the appearance of the effect of anisotropy of electrical conductivity and a sharp decrease in the maximum of quantum capacitance. For a topological model with a graphene layer width of 4 hexagons, axial stretching, on the contrary, leads to a decrease in the effect of anisotropy of electrical conductivity and insignificant changes in the quantum capacitance. Based on the obtained results, the prospects for using hybrid graphene/SWCNT (12, 6) films as a material for creating flexible electrodes of supercapacitors are predicted.

Thin Graphene-Nanotube Films for Electronic and Photovoltaic Devices: DFTB Modeling

Supercell atomic models of composite films on the basis of graphene and single-wall carbon nanotubes (SWCNTs) with an irregular arrangement of SWCNTs were built. It is revealed that composite films of this type have a semiconducting type of conductivity and are characterized by the presence of an energy gap of 0.43-0.73 eV. It was found that the absorption spectrum of composite films contained specific peaks in a wide range of visible and infrared (IR) wavelengths. On the basis of calculated composite films volt-ampere characteristics (VAC), the dependence of the current flowing through the films on the distance between the nanotubes was identified. For the investigated composites, spectral dependences of the photocurrent were calculated. It was shown that depending on the distance between nanotubes, the maximum photocurrent might shift from the IR to the optical range.

Improving the Sensory Properties of Layered Phospholipid-Graphene Films Due to the Curvature of Graphene Layers

This article is devoted to the in silico study of the sensory properties of mono- and bilayer phospholipid-graphene films with planar and curved graphene sheets. The DPPC (dipalmitoylphosphatidylcholine) molecules are considered as phospholipid structures. These molecules are part of lipid bilayers, liposomes and cell membranes. To find a way to improve the sensory properties of phospholipid-graphene films, we studied the effect of the curvature of the graphene sheet on the charge transfer and electrical conductivity of the films. The distribution of the electron charge density over the film atoms was calculated using the self-consistent-charge density-functional tight-binding method (SCC-DFTB). The calculation of the current through phospholipid-graphene films was carried out within the framework of the Landauer-Buttiker formalism using the Keldysh nonequilibrium Green function technique. As a result of the calculations, the optimal configuration of the arrangement of DPPC molecules between two graphene layers was established. This configuration provides the maximum possible increase in current to 1 μA at low voltages of ~0.2 V and is achieved for curved graphene with a radius of curvature of ~2.7 nm at individual points of graphene atomic network.

Controlling the electronic properties of 2D/3D pillared graphene and glass-like carbon via metal atom doping

We present the results of investigation of the nanopore filling of planar layered and bulk pillared graphene (PGR) as well as films and 3D samples of glass-like porous carbon (GLC) with potassium atoms. The patterns of charge transfer, electronic structure, and shift of the Fermi level during the filling of nanopores with potassium atoms are established. It is found that the greatest charge transfer from potassium atoms to the carbon framework is observed in PGR with a density of 1.1-1.4 g cm^-3 (that is, with a nanopore volume of 1300-1800 nm³) regardless of the framework topology. The maximum charge transfer occurs already when the mass fraction of potassium is 12 wt%. At the same potassium concentration, a maximum shift of the Fermi level to zero by ∼3 eV occurs in a bilayer PGR film with a density of 1.4 g cm^-3. Thus, our work shows for the first time that the electronic properties of nanoporous materials doped with alkaline earth metals (in particular, potassium) can be controlled by varying the volume of doped nanopores, i.e. by controlling the density of the nanoporous material. We first demonstrated that the potassium doping of PGR would be more effective than potassium doping of GLC. It is established that 2D samples of PGR and GLC completely reproduce the electronic properties of the bulk samples and even surpass them in some parameters. To carry out research, we developed a new method for nanopore filling with dopant atoms based on both the randomness of the nanopore filling and the energy advantage of this process. This method allows us to reliably determine the maximum possible mass fraction (wt%) of dopant atoms of any porous material.

Protein-Polymer Matrices with Embedded Carbon Nanotubes for Tissue Engineering: Regularities of Formation and Features of Interaction with Cell Membranes

This paper reveals the mechanism of nanowelding a branched network of single-walled carbon nanotubes (SWCNTs) used as a framework for the formation of protein-polymer matrices with albumin, collagen, and chitosan. It is shown that the introduction of certain point defects into the structure of SWCNTs (single vacancy, double vacancy, Stone-Wales defect, and a mixed defect) allows us to obtain strong heating in defective regions as compared to ideal SWCNTs. The wavelengths at which absorption reaches 50% are determined. Non-uniform absorption of laser radiation along with inefficient heat removal in defective regions determines the formation of hot spots, in which nanowelding of SWCNTs is observed even at 0.36 nm between contacting surfaces. The regularities of formation of layered protein-polymer matrices and the features of their interaction with cell membrane are revealed. All studies are carried out in silico using high-precision quantum approaches.

Stability of the thin partitioned carbon nanotubes

We report on the research of the stability of partitioned (bamboo-like) carbon nanotubes with different diameters. The stability of the partitioned carbon nanotubes of the smallest diameter were determined by the tight-binding method. For the prediction of the destruction regions of the bamboo-like nanotubes atomic framework subjected to strain the new original method of the calculation of the local stress of atomic network was developed. Using this method it was shown that partitioned carbon nanotubes with a diameter of 2.02 nm are stable. These partitioned carbon nanotubes with chirality (15,15) are the most stable partitioned carbon nanotubes with the smallest diameter.