Session V24: Electron Transport at Nanoscale Interfaces

Ligand engineering of nanoparticle solar cells

Semiconductor nanoparticles (NP) are promising materials to build cheap and efficient solar cells. One of the key challenges in their utilization for solar energy conversion is the control of NP surfaces and ligand-NP interfaces. Recent experiments have shown that by carefully choosing the ligands terminating the NPs, one can tailor electronic and optical absorption properties of NP assemblies, along with their transport properties.[1] By using density functional theory based methods, we investigated how the opto-electronic properties of lead chalcogenide NPs may be tuned by using diverse organic and inorganic ligands. We interpreted experiments, and we showed that an essential prerequisite to avoid detrimental trap states is to ensure charge balance at the ligand-NP interface, possibly with the help of hydrogen treatment. [1] R. Crisp et al., Scientific Reports 5, 9945 (2015); C. Giansante et al., J. Am. Chem. Soc. 137, 1875 (2015).   

The effect of Ta ''oxygen scavenger layer'' on HfO$_{2}$-based resistive switching behavior: termodynamic stability, electronic structure, and low-bias transport

Metal-oxide-metal heterostructures are promising candidates for next-generation random access memories, which exhibit reversible resistive switching between high- and low-conductance states. Recent experimental work showed that inserting a metallic `oxygen scavenger layer' between TiN electrode and HfO2 significantly improves device switching performance. We show, using atomistic modeling within the GGA$+$U scheme of Density Functional Theory, that a Ta oxygen scavenger layer significantly enhances the thermodynamic stability of depleting oxygen from the oxide. Furthermore, the presence of a Ta layer reduces the dependence of the Schottky barrier heights on the location of the oxygen removed from the oxide matrix. Finally, the Schottky barrier height has a very small effect on the on-state low-bias conductance; this is more sensitive to the location of the depleted oxygen. We gratefully acknowledge the computing resources provided on Blues, a high-performance computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. Work at Argonne was supported by U. S. DOE, Office of Science under Contract No. DE-AC02-06CH11357.   

Band alignment study on Al/SiO$_{\mathrm{2~}}$and Cu/SiO$_{\mathrm{2\thinspace }}$metal-oxide interface with the presence of point defect

Metal-Oxide interface has a wide use in electronic devices. Currently, technological development is aiming on the shrinkage of electronic devices' size. Based on the knowledge of electron tunneling effect, the reduction of dielectric thickness would cause an exponential increase on electron tunneling probability which contributes to current leakage. It might cause dielectric breakdown, which could make a severe and irreversible damage to the devices. Therefore, the main purpose of this study is to explore the possible factors that can lead to dielectric breakdown at metal-oxide interface. Density functional theory \textit{ab initio} calculation has been applied to study the Al/SiO$_{\mathrm{2~}}$and Cu/SiO$_{\mathrm{2\thinspace }}$metal-oxide interface. Results on oxygen (di)vacancies at the interface will be presented and compared with the defect free model. The band alignment has been constructed to describe the variation of potential barrier height due to defect at interface. Results show the oxygen (di)vacancies at interface might trap electron and reduce potential barrier height. Moreover, the potential barrier height has a significant dependence on defects charge states.   

Voltage mediated metal to insulator transition in VO$_{2}$ and V$_{2}$O$_{3}$ nanodevices

We investigate the mechanism of the voltage mediated MIT in a series of vanadium oxides (VO$_{2}$ and V$_{2}$O$_{3})$ nano-scopic devices. All films presented \textasciitilde 4 orders of magnitude resistance change at the MIT. The devices consist of in-plane gold electrodes with 200 nm wide tip on top of lithographically defined vanadium oxide films. The gap size between electrodes was fixed at 140 nm. Unlike micron-scale devices, the current-voltage characteristics in nano-scale V$_{2}$O$_{3}$ cannot be accounted solely by an inhomogeneous joule-heating model, suggesting additional mechanisms may be playing a role in the switching behavior. However, in the case of nano-scopic VO$_{2}$ devices, it may be possible to explain the results with only inhomogeneous heating. We perform detailed electrical and thermal Finite Element Method (FEM) calculations on both the VO$_{2}$ and V$_{2}$O$_{3}$ devices. We couple the FEM analysis with a variety of theoretical models, which can shed light on the nanoscopic nature of the MIT in VO$_{2}$ and V$_{2}$O$_{3}$.   

Ferroelectric Modulation of Two-dimensional Electron Gas Conductivity at Oxide Interfaces

In this report, by inserting a ferroelectric Ba$_{0.2}$Sr$_{0.8}$TiO$_{3}$ layer between LaAlO$_{3}$/SrTiO$_{3}$ heterostructure, a two-dimensional electron gas (2DEG) was found at LaAlO$_{3}$/ Ba$_{0.2}$Sr$_{0.8}$TiO$_{3\, }$interface. With electrical, optical, piezoresponse force microscopic measurements and first-principle calculations, we studied the impact of this ferroelectric Ba$_{0.2}$Sr$_{0.8}$TiO$_{3\, }$layer on the 2DEG. Both carrier density and mobility of the 2DEG can be modulated by changing the thickness of the ferroelectric layer. We also observed that Ba$_{0.2}$Sr$_{0.8}$TiO$_{3}$ layer can suppress oxygen vacancy formation, leading to observation of temperature-independent polarization-induced carrier density. These results indicate that the 2DEG at oxide interfaces can be ferroelectrically modulated.   

Energy Level Alignment at the Interface between Linear-Structured Benzenediamine Molecules and Au(111) Surface

Using density functional theory with model self-energy corrections, we calculate the adsorption energetics and geometry, and the energy level alignment of benzenediamine (BDA) molecules adsorbed on Au(111) surfaces. Our calculations show that linear structures of BDA, stabilized via hydrogen bonds between amine groups, are energetically more favorable than monomeric phases. Moreover, our self-energy-corrected calculations of energy level alignment show that the highest occupied molecular orbital energy of the BDA linear structure is deeper relative to the Fermi level relative to the isolated monomer and agrees well with the values measured with photoemission spectroscopy. This work supported by DOE.   

Theory of work function tuning via mixed-monolayers on functional surfaces

Self-assembled monolayers (SAMs) provide both stability and functionality of surfaces useful in optoelectronic nanoscale devices. The work function, level alignment and other electronic properties of functionalized surfaces can be tuned with the choice of molecule and an even finer control of the properties can be obtained with a SAM comprised of multiple types of molecules [1]. Modeling the effect on electronic properties of mixed-monolayers via ab initio calculations poses a challenge due to the large supercell required to capture a range of relative concentrations between the two types of molecules. Here, we present an implicit model - fit from density functional theory calculations - capturing local electrostatic interactions within the SAM primarily due to depolarization of the induced dipoles formed upon binding [2]. This quantitative model allows us to explore supercells with a large number of molecules and, thus, surface concentrations that are inhomogeneous in nature. We compare to experimental results of thiol terminated carboranes on gold [1]. [1] Kim et al., Nano Lett. 14, 2946 (2014) [2] Kotiuga et al., Nano Lett. 15, 4498 (2015)   

Giant spatially-resolved self-assembled donor-acceptor molecular heterojunctions

Despite theoretical models predicting that rectification ratios (RR) $>$1000 should be achievable in molecular rectifiers, demonstrations of this have been rare. It has also been extremely challenging to unravel the structure-function relationships on the nanometer length scales that determine their behavior. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we show that RRs $>$1000 at biases $<$500~mV are realized in the two-molecule limit for self-assembled donor-acceptor bilayers of pentacene on C$_{60}$ on Cu. We show that the system behaves as a molecular analog to a Schottky diode due to strong electronic coupling of C$_{60}$ to the metallic substrate, and electronic transport is dominated by sequential tunneling from semiconducting pentacene to metallic C$_{60}$. Furthermore, we demonstrate the extreme sensitivity of the low-bias $I(V)$ characteristics to the molecularly-resolved structure of the heterojunction (HJ), which leads to negative differential resistance and $\sim 100\times$ variation in the rectification ratio within 2~nm of the edge of the molecular HJ.   

Transient magnetization of core excited organic molecules adsorbed on graphene

This work presents a density functional theory based computational investigation of electronic and magnetic properties of physisorbed and chemisorbed organic molecules on graphene in the ground state and core excited one at low molecular coverage. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, it is found that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the contrary, when graphene is covalently functionalized, the system is magnetic in the ground state presenting two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.   

\textbf{Probing the molecular structure of doped sites within crystals by Pyroelectricity and Dispersion Corrected DFT modeling}

We describe the analysis of polar architectures at the nanoscale by pyroelectric measurements and DFT. Controlled doping of crystals is a primary tool for modification of the properties of materials. Doping of non-polar molecular crystals with ``tailor-made'' auxiliaries often reduces their symmetry, and converts them into polar mixed crystals. Such crystals are pyroelectric, $i.e.$ they display temporary surface charge if subjected to a temperature change. When the non-polar crystals of the amino-acids are doped with different amino acids of concentrations as low as \textless 0.2{\%}, they display measurable pyroelectric effect. Since such minute amount of guest creates measurable macroscopic polarization, implies a local polar dislocation of the neighboring host molecules. We demonstrate here, that the value and the temperature dependence of the pyroelectric coefficient, provides intimate information on the molecular packing arrangement of the doped sites. Different amino acids as dopants induce different pyroelectric effects: The pyroelectric coefficient is a) not temperature dependent; b) changes its sign upon heating. These differences were explained by the determination of the polar domains at the molecular level by the DFT.   

Theory of Giant Rectification in Molecular Schottky Diodes

Following early theoretical models [1], efforts towards the synthesis and characterization of more efficient molecular diodes have consisted into attempts to increase the electron rich/poor characters of the donor/acceptor moieties, decrease their conjugation, and imbalance their coupling to the electrodes. The experimental poor performance of single-molecule diodes -- with the notable exception of environment-induced diodes [2] -- suggests that these physical parameters tend to be mutually exclusive in most molecular systems [3]. In this talk, inspired by recent observations of large rectification ratios at organic bilayers [4], we will show how molecules with a moiety strongly coupled to a metal electrode can, in principle, be used to optimize these different aspects simultaneously. Using first-principles calculations, we will show that this class of molecular systems --analog to macroscopic Schottky diodes, can display large rectification ratios at low operating voltages. [1] Taylor et al. Phys. Rev. Lett. 89, 138301 (2002); Andrews et al. JACS 130, 17309 (2008); [2] Capozzi et al. Nat. Nano 10, 522 (2015); [3] Mujica et al. Chem. Phys. 281, 147 (2002); Stokbro et al. JACS 125, 3674 (2003) ; [4] Smerdon et al., submitted.