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The self-consistent field (SCF) generation of the three-dimensional (3D) electron density distribution ($\rho$) represents a fundamental aspect of density functional theory (DFT) and related first-principles calculations, and how one can shorten or bypass the SCF loop represents a critical question from both practical and fundamental standpoints. Herein, a machine learning strategy DeepSCF is presented in which the map between the SCF $\rho$ and the initial guess density ($\rho_0$) constructed by the summation of neutral atomic densities is learned using 3D convolutional neural networks (CNNs). High accuracy and transferability of DeepSCF are achieved by expanding the input features to include atomic fingerprints beyond $\rho_0$ and encoding them on a 3D grid. The prediction of the residual density ($\delta\rho$) rather than $\rho$ itself is targeted, and, since $\delta\rho$ corresponds to chemical bonding information, a dataset of small-sized organic molecules featuring diverse bonding characters is adopted. After enhancing the fidelity of the method by subjecting the atomic geometries in the dataset to random strains and rotations, the effectiveness of DeepSCF is finally demonstrated using a complex large carbon nanotube-based DNA sequencer model. This work evidences that the nearsightedness in electronic structures can be optimally represented via the local connectivity in CNNs., Comment: 9 pages, 6 figures

While first-principles calculations of electrode-molecule binding play an indispensable role in obtaining atomic-level understanding in surface science and electrochemistry, a significant challenge remains because the adsorption energy is well-defined only in equilibrium. Herein, a theory to calculate the electric enthalpy for electrochemical interfaces is formulated within the multi-space constrained-search density functional theory (MS-DFT), which provides the nonequilibrium total energy of a nanoscale electrode-channel-electrode junction. An additional MS-DFT calculation for the electrode-only counterpart that maintains the same bias voltage allows one to identify the internal energy of the channel as well as the electric field and the channel polarization, which together determine the electric enthalpy and the nonequilibrium adsorption energy. Application of the developed scheme to the water-Au and water-graphene interface models shows that the Au and graphene electrodes induce very different behaviors in terms of the electrode potential-dependent stabilization of water configurations. The theory developed here will be a valuable tool in the ongoing effort to obtain an atomic-scale understanding of bias-dependent molecular reorganizations in electrified interfaces., Comment: 8 pages, 4 figures

While low-dimensional organometal halide perovskites are expected to open up new opportunities for a diverse range of device applications, like in their bulk counterparts, the toxicity of Pb-based halide perovskite materials is a significant concern that hinders their practical use. We recently predicted that lead triiodide (PbI$_3$) columns de-rived from trimethylsulfonium (TMS) lead triiodide (CH$_3$)$_3$SPbI$_3$ (TMSPbI$_3$) by stripping off TMS ligands should be semimetallic, and additionally ultrahigh negative differential resistance (NDR) can arise from the heterojunction composed of a TMSPbI$_3$ channel sandwiched by PbI$_3$ electrodes. Herein, we computationally explore whether similar material and device characteristics can be obtained from other one-dimensional halide perovskites based on non-Pb metal elements, and in doing so deepen the understanding of their mechanistic origins. First, scanning through several candidate metal halide inorganic frameworks as well as their parental form halide perovskites, we find that the germanium triiodide (GeI$_3$) column also assumes a semimetallic character by avoiding the Peierls distortion. Next, adopting the bundled nanowire GeI$_3$-TMSGeI$_3$-GeI$_3$ junction configuration, we obtain a drastically high peak current density and ultrahigh NDR at room temperature. Furthermore, the robustness and controllability of NDR signals under strain are revealed, establishing its potential for flexible electronics applications. It will be emphasized that, despite the performance metrics notably enhanced over those from the PbI$_3$-TMSPbI$_3$-PbI$_3$ case, these device characteristics still arise from the identical quantum hybridization NDR mechanism., Comment: 11 pages, 5 figures

Vertically stacked two-dimensional (2D) van der Waals (vdW) heterostructures based on graphene electrodes represent a promising architecture for next-generation electronic devices. However, their first-principles characterizations have been so far mostly limited to the equilibrium state due to the limitation of the standard non-equilibrium Green's function approach. To overcome these challenges, we introduce a non-equilibrium first-principles calculation method based on the recently developed multi-space constrained-search density functional formalism and apply it to graphene/few-layer hexagonal boron nitride (hBN)/graphene field-effect transistors. Our explicit finite-voltage first-principles calculations show that the previously reported negative differential resistance (NDR) current-bias voltage characteristics can be produced not only from the gating-induced mismatch between two graphene Dirac cones but from the bias-dependent energetic shift of defect levels. Specifically, for a carbon atom substituted for a nitrogen atom (C$_N$) within inner hBN layers, the increase of bias voltage is found to induce a self-consistent electron filling of in-gap C$_N$ states, which leads to changes in voltage drop profiles and symmetric NDR characteristics. On the other hand, with a C$_N$ placed on outer interfacial hBN layers, we find that due to the pinning of C$_N$ levels to nearby graphene states voltage drop profiles become bias-independent and NDR peaks disappear. Revealing hitherto undiscussed non-equilibrium behaviors of atomic defect states and their critical impact on device characteristics, our work points towards future directions for the computational design of 2D vdW devices, Comment: 11 pages, 5 figures

Halide perovskite light absorbers have great advantages for photovoltaics such as efficient solar energy absorption, but charge accumulation and recombination at the interface with an electron transport layer (ETL) remains a major challenge in realizing their full potential. Here we report the experimental realization of a zipper-like interdigitated interface between a Pb-based halide perovskite light absorber and an oxide ETL by the PbO capping of the ETL surface, which produces an atomically thin two-dimensional metallic layer that can significantly enhance the perovskite/ETL charge extraction process. As the atomistic origin of the emergent two-dimensional interfacial metallicity, first-principles calculations performed on the representative MAPbI$_3$/TiO$_2$ interface identify the interfacial strain induced by the simultaneous formation of stretched I-substitutional Pb bonds (and thus Pb-I-Pb bonds bridging MAPbI$_3$ and TiO$_2$) and contracted substitutional Pb-O bonds. Direct and indirect experimental evidences for the presence of interfacial metallic states are provided, and a non-conventional defect-passivating nature of the strained interdigitated perovskite/ETL interface is emphasized. It is experimentally demonstrated that the PbO capping method is generally applicable to other ETL materials including ZnO and SrTiO$_3$, and that the zipper-like interdigitated metallic interface leads to about two-fold increase in charge extraction rate. Finally, in terms of the photovoltaic efficiency, we observe a volcano-type behavior with the highest performance achieved at the monolayer-level PbO capping. The method established here might prove to be a general interface engineering approach to realize high-performance perovskite solar cells., Comment: 16 pages, 6 figures

Although single-layer transition-metal dichalcogenides with novel valley functionalities are promising candidate to realize valleytronic devices, the essential understanding of valley depolarization mechanisms is still incomplete. Based on pump-probe experiments performed for MoSe2 and WSe2 monolayers and corroborating analysis from density functional calculations, we demonstrate that coherent phonons at the K-point of the Brillouin zone can effectively mediate the valley transfer of electron carriers. In the MoSe2 monolayer case, we identify this mode as the flexural acoustic ZA(K) mode, which has broken inversion symmetry and thus can enable electron spin-flip during valley transfer. On the other hand, in the monolayer WSe2 case where spin-preserving inter-valley relaxations are preferred coherent LA(K) phonons with even inversion symmetry are efficiently generated. These findings establish that, while the specifics of inter-valley relaxations depend on the spin alignments of energy bands, the K-point phonons should be taken into account as an effective valley depolarization pathway in transition metal dichalcogenide monolayers., Comment: 8 pages, 5 figures

While the Landauer viewpoint constitutes a modern basis to understand nanoscale electronic transport and to realize first-principles implementations of the non-equilibrium Green's function (NEGF) formalism, seeking an alternative picture could be beneficial for the fundamental understanding and practical calculations of quantum transport processes. Herein, introducing a micro-canonical picture that maps the finite-bias quantum transport process to a drain-to-source or multi-electrode optical excitation, the multi-space constrained-search density functional theory (MS-DFT) formalism for first-principles electronic structure and quantum transport calculations is developed. Performing MS-DFT calculations for the benzenedithiolate single-molecule junction, it is shown that MS-DFT and standard DFT-NEGF calculations produce practically equivalent electronic and transmission data. Importantly, the variational convergence of "non-equilibrium total energy" within MS-DFT is demonstrated, which should have significant implications for in operando studies of nanoscale devices. Establishing a viable alternative to the Landauer viewpoint, the developed formalism should provide valuable atomistic information in the development of next-generation nanodevices., Comment: 9 pages, 3 figures

The effective mass approximation (EMA) could be an efficient method for the computational study of semiconductor nanostructures with sizes too large to be handled by first-principles calculations, but the scheme to accurately and reliably introduce EMA parameters for given nanostructures remains to be devised. Herein, we report on an EMA approach based on first-principles-derived data, which enables accurate predictions of the optoelectronic properties of quantum nanostructures. For the CdS/ZnS core/shell quantum rods, for which we recently reported its experimental synthesis, we first carry out density functional theory (DFT) calculations for an infinite nanowire to obtain the nanoscopic dielectric constant, effective mass, and Kohn-Sham potential. The DFT-derived data are then transferred to the finite nanorod cases to set up the EMA equations, from which we estimate the photoluminescence (PL) characteristics. Compared with the corresponding method based on bulk EMA parameters and abrupt potential, we confirm that our EMA approach more accurately describes the PL properties of nanorods. We find that, in agreement with the experimentally observed trends, the optical gap of nanorods is roughly determined by the nanorod diameter and the PL intensity is reduced with increasing the nanorod length. The developed methodology is additionally applied to CdSe nanoplatelets, where reliable experimental data became recently available. Here, we again obtain excellent agreements between calculated and measured optical gap values, confirming the generality of our approach. It is finally shown that the abrupt confinement potential approximation most adversely affects the accuracy of EMA simulations., Comment: 14 pages, 6 figures, 3 tables

The splitting of quasi-Fermi levels (QFLs) represents a key concept utilized to describe finite-bias operations of semiconductor devices, but its atomic-scale characterization remains a significant challenge. Herein, the non-equilibrium QFL or electrochemical potential profiles within single-molecule junctions obtained from the newly developed first-principles multi-space constrained-search density functional formalism are presented. Benchmarking the standard non-equilibrium Green's function calculation results, it is first established that algorithmically the notion of separate electrode-originated nonlocal QFLs should be maintained within the channel region during self-consistent finite-bias electronic structure calculations. For the insulating hexandithiolate junction, the QFL profiles exhibit discontinuities at the left and right electrode interfaces and across the molecule the accompanying electrostatic potential drops linearly and Landauer residual-resistivity dipoles are uniformly distributed. For the conducting hexatrienedithiolate junction, on the other hand, the electrode QFLs penetrate into the channel region and produce split QFLs. With the highest occupied molecular orbital entering the bias window and becoming a good transport channel, the split QFLs are also accompanied by the nonlinear electrostatic potential drop and asymmetric Landauer residua-resistivity dipole formation. Our findings underscore the importance of the first-principles extraction of QFLs in nanoscale junctions and point to a new direction for the computational design of next-generation electronic, optoelectronic, and electrochemical devices., Comment: 11 pages, 4 figures

The formation of an ideal van der Waals (vdW) contacts at metal/transition-metal dichalcogenide (TMDC) interfaces is a critical step for the development of high-performance and energy-efficient electronic and optoelectronic applications based on the two-dimensional (2D) semiconductors. In overcoming the key chal-lenges of the conventional metal deposition process that leads to an uncontrol-lable Schottky barrier height and high contact resistance, notable advances were recently made by transferring atomically flat metal thin films or thermally evapo-rating indium/gold alloy. However, the realization of an ideal vdW contact be-tween an elemental metal and TMDC through the evaporation process is yet to be demonstrated, and particularly the evidence of an Ohmic contact between three-dimensional metallic electrodes and TMDCs is still unavailable. Herein, we report the fabrication of atomically clean metal/TMDC contacts by evaporating metals at a relatively low thermal energy and subsequently cooling the substrate holder down to 100 K by liquid nitrogen, achieving for the indium (In)/molybdenum disulfide (MoS2) case an accumulation-type Ohmic contact with a metal-induced electron doping density of 10$^{12}$/cm$^2$. We find that the transport at the In/MoS2 contact is dominated by the field-emission mechanism over a wide temperature range from 2.4 to 300 K, and the contact resistance reaches 600 Ohm um and 1,000 Ohm um at cryogenic temperatures for the few-layer and monolayer MoS2 cases, respectively. Based on first-principles calculations, we find that the na-ture of the ideal In/MoS2 vdW contact is characterized by the formation of in-gap states within TMDC together with the abrupt and rigid shift of the TMDC band., Comment: 38 pages, 15 figures

In the rapidly progressing field of organometal halide perovskites, the dimensional reduction could open up new opportunities for device applications. Herein, taking the recently synthesized trimethylsulfonium lead triiodide (CH$_3$)$_3$SPbI$_3$ perovskite as a representative example, we carry out first-principles calculations and study the nanostructuring and device application of halide perovskite nanowires. We find that the one-dimensional (1D) (CH$_3$)$_3$SPbI$_3$ structure is structurally stable, and the electronic structures of higher-dimensional forms are robustly determined at the 1D level. Remarkably, due to the face-sharing [PbI$_6$] octahedral atomic structure, the organic ligand-removed 1D PbI$_3$ frameworks are also found to be stable. Moreover, the PbI$_3$ columns avoid the Peierls distortion and assume a semimetallic character, contradicting the conventional assumption of semiconducting metal-halogen inorganic frameworks. Adopting the bundled nanowire junctions consisting of (CH$_3$)$_3$SPbI$_3$ channels with sub-5 nm dimensions sandwiched between PbI$_3$ electrodes, we finally obtain high current densities and large room-temperature negative differential resistance (NDR). It will be emphasized that the NDR originates from the combination of the near-Ohmic character of (CH$_3$)$_3$SPbI$_3$-PbI$_3$ contacts and a novel NDR mechanism that involves the quantum-mechanical hybridization between channel and electrode states. Our work demonstrates the great potential of low-dimensional hybrid perovskites toward advanced electronic devices beyond actively-pursued photonic applications., Comment: 9 pages, 5 figures, & 1 table

Based on first-principles approaches, we study the ballistic phonon transport properties of finite monatomic carbon chains stretched between graphene nanoribbons, an $sp$-$sp^2$ hybrid carbon nanostructure that has recently seen significant experimental advances in its synthesis. We find that the lattice thermal conductance anomalously increases with tensile strain for the even-numbered carbon chains that adopt the alternating bond-length polyyne configuration. On the other hand, in the odd-numbered carbon chain cases, which assume the equal bond-length cumulene configuration, phonon conductance decreases with increasing strain. We show that the strong odd-even phonon transport effects originate from the characteristic longitudinal acoustic phonon modes of carbon wires and their unique strain-induced redshifts with respect to graphene nanoribbon phonon modes. The novel phonon transport properties and their atomistic mechanisms revealed in this work will provide valuable guidelines in de-signing hybrid carbon nanostructures for next-generation electronic, bio, and energy device applications., Comment: 12 pages, 5 figures

Based on a microcanonical picture that maps the steady-state quantum transport process to a drain-to-source excitation, we develop a constrained-search density functional formalism for finite-bias quantum transport calculations. By variationally minimizing the total energy of an electrode-channel-electrode system without introducing separate bulk electrode information, ambiguities in identifying its nonequilibrium electronic structure under a bias is reduced and finite electrode cases can be naturally treated. We apply the approach to vertically stacked van der Waals heterostructures made of a hexagonal boron nitride (hBN) channel sandwiched by single-layer graphene electrodes, which so far could not be treated within first-principles calculations. We find that the experimentally observed negative differential resistance originates from the hBN defect-mediated hybridizations between two graphene states, and concurrently obtain a high-bias linear current increase that was not captured in previous semiclassical treatments. Going beyond the capability of existing $ab\ initio$ nonequilibrium quantum transport simulation methods, the developed formalism will provide valuable atomistic information in the development of next-generation nanodevices., Comment: 7 pages, 4 figures

While one of the most promising applications of carbon nanotubes (CNTs) is to enhance polymer orientation and crystallization to achieve advanced carbon fibers, the successful realization of this goal has been hindered by the insufficient atomistic understanding of polymer-CNT interfaces. We herein theoretically study polyacrylonitrile (PAN)-CNT hybrid structures as a representative example of polymer-CNT composites. Based on density-functional theory calculations, we first find that the relative orientation of polar PAN nitrile groups with respect to the CNT surface is the key factor that determines the PAN-CNT interface energetics and the lying-down PAN configurations are much more preferable than their standing-up counterparts. The CNT curvature is identified as another important factor, giving the largest binding energy in the zero-curvature graphene limit. Charge transfer analysis explains the unique tendency of linear PAN alignments on the CNT surface and the possibility of ordered PAN-PAN assembly. Next, performing large-scale molecular dynamics simulations, we show that the desirable linear PAN-CNT alignment can be achieved even for relatively large initial misorientations and further demonstrate that graphene nanoribbons are a promising carbon nano-reinforcement candidate. The microscopic understanding accumulated in this study will provide design guidelines for the development of next-generation carbon nanofibers., Comment: 11 pages, 4 figures