In this colloquia review we discuss methods for thermal transport calculations for nanojunctions connected to two semi-infinite leads served as heat-baths. Our emphases are on fundamental quantum theory and atomistic models. We begin with an introduction of the Landauer formula for ballistic thermal transport and give its derivation from scattering wave point of view. Several methods (scattering boundary condition, mode-matching, Piccard and Caroli formulas) of calculating the phonon transmission coefficients are given. The nonequilibrium Green's function (NEGF) method is reviewed and the Caroli formula is derived. We also give iterative methods and an algorithm based on a generalized eigenvalue problem for the calculation of surface Green's functions, which are starting point for an NEGF calculation. A systematic exposition for the NEGF method is presented, starting with the fundamental definitions of the Green's functions, and ending with equations of motion for the contour ordered Green's functions and Feynman diagrammatic expansion. In the later part, we discuss the treatments of nonlinear effects in heat conduction, including a phenomenological expression for the transmission, NEGF for phonon-phonon interactions, molecular dynamics (generalized Langevin) with quantum heat-baths, and electron-phonon interactions. Some new results are also shown. We also briefly review the experimental status of the thermal transport measurements in nanostructures.PACS. 05.60.Gg quantum transport -44.10.+i heat conduction -65.80.+n thermal properties of small particles, nanocrystals, and nanotubes
van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next-generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe /SnS photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe /SnS vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe /SnS vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10 Jones (I /I ratio of ≈10 ) and photoresponsivity of 244 A W at a reverse bias under the illumination of 550 nm light (3.77 mW cm ).
We examine a molecular bridge connecting two metallic electrodes. We find that an electronic current passing across the bridge can cause a vibrational instability of the molecule, which ultimately can lead to a breakdown of the bridge. This instability is generated by a hitherto never considered mechanism, which surprisingly involves the quantum mechanical phase of the electronic waves, the "Berry phase". This mechanism works for highly conducting bridges, and contrary to breakdown by traditional Joule heating, this instability is deterministic and occurs at certain critical voltages. We demonstrate the new mechanism using state-of-the-art ab initio calculations on realistic molecular bridges.
Infrared light detection is generally limited by the intrinsic bandgap of semiconductors, which suppresses the freedom in infrared light photodetector design and hinders the development of high‐performance infrared light photodetector. In this work, for the first time infrared light (1030 nm) photodetectors are fabricated based on WS2/MoS2 heterostructures. Individual WS2 and MoS2 have no response to infrared light. The origin of infrared light response for WS2/MoS2 comes from the strong interlayer coupling which shrinks the energy interval in the heterojunction area thus rendering heterostructures longer wavelength detection ability compared to individual components. Considering the low light absorption due to indirect bandgap essence of few layers WS2/MoS2 heterostructures, its infrared responsivity is further enhanced with at most ≈25 times but the fast response rate is maintained via surface plasmon resonance (SPR). Such an interlayer coupling induced infrared light response and surface plasmon resonance enhancement strategy paves the way for high‐performance infrared light photodetection of infinite freedom in design.
Employing the nonequilibrium Green's function method, we develop a fully quantum mechanical model to study the coupled electron-phonon transport in one-dimensional atomic junctions in the presence of a weak electron-phonon interaction. This model enables us to study the electronic and phononic transport on an equal footing. We derive the electrical and energy currents of the coupled electron-phonon system and the energy exchange between them. As an application, we study the heat dissipation in current carrying atomic junctions within the self-consistent Born approximation, which guarantees energy current conservation. We find that the inclusion of phonon transport is important in determining the heat dissipation and temperature change of the atomic junctions.
We report the quantitative assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy based on a scanning tunneling microscope. The inelastic scattering cross section was resonantly enhanced by "gating" the frontier orbitals of water via a chlorine-terminated tip, so the hydrogen-bonding strength can be determined with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when a hydrogen bond is strongly coupled to the polar atomic sites of the surface.
Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS 2 and SnSe 2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe 2 with MoS 2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe 2 /MoS 2 based van der Waals heterostructures using MoS 2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallographic directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calculations, low-temperature photoluminescence (PL) spectra, and electrical transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 × 10 3 A W −1 which is higher by two orders of magnitude than those MoS 2 -only devices. The improved performance can be attributed to the efficient charge transfer from MoS 2 to SnSe 2 at the interface.
Van der Waals (vdW) heterostructures, stacking different two-dimensional materials, have opened up unprecedented opportunities to explore new physics and device concepts. Especially interesting are recently discovered two-dimensional magnetic vdW materials, providing new paradigms for spintronic applications. Here, using density functional theory (DFT) calculations, we investigate the spin-dependent electronic transport across vdW magnetic tunnel junctions (MTJs) composed of Fe3GeTe2 ferromagnetic electrodes and a graphene or hexagonal boron nitride (h-BN) spacer layer. For both types of junctions, we find that the junction resistance changes by thousands of percent when the magnetization of the electrodes is switched from parallel to antiparallel. Such a giant tunneling magnetoresistance (TMR) effect is driven by dissimilar electronic structure of the two spin-conducting channels in Fe3GeTe2, resulting in a mismatch between the incoming and outgoing Bloch states in the electrodes and thus suppressed transmission for an
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.