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An unresolved challenge facing electronics at a few-nm scale is that resistive channels start leaking due to quantum tunneling. This affects the performance of nanoscale transistors, with single-molecule devices displaying particularly low switching ratios and operating frequencies, combined with large subthreshold swings.1 The usual strategy to mitigate quantum effects has been to increase device complexity, but theory shows that if quantum effects are exploited correctly, they can simultaneously lower energy consumption and boost device performance.2-6 Here, we demonstrate experimentally how the performance of molecular transistors can be improved when the resistive channel contains two destructively-interfering waves. We use a zinc-porphyrin coupled to graphene electrodes in a three-terminal transistor device to demonstrate a >104 conductance-switching ratio, a subthreshold swing at the thermionic limit, a > 7 kHz operating frequency, and stability over >105 cycles. This performance is competitive with the best nanoelectronic transistors. We fully map the antiresonance interference features in conductance, reproduce the behaviour by density functional theory calculations, and trace back this high performance to the coupling between molecular orbitals and graphene edge states. These results demonstrate how the quantum nature of electron transmission at the nanoscale can enhance, rather than degrade, device performance, and highlight directions for future development of miniaturised electronics., 11 pages, 4 figures

Quantum interference in nano-electronic devices could lead to reduced-energy computing and efficient thermoelectric energy harvesting. When devices are shrunk down to the molecular level it is still unclear to what extent electron transmission is phase coherent, as molecules usually act as scattering centres, without the possibility of showing particle-wave duality. Here we show electron transmission remains phase coherent in molecular porphyrin nanoribbons, synthesized with perfectly defined geometry, connected to graphene electrodes. The device acts as a graphene Fabry-P\'erot interferometer, allowing direct probing of the transport mechanisms throughout several regimes, including the Kondo one. Electrostatic gating allows measurement of the molecular conductance in multiple molecular oxidation states, demonstrating a thousand-fold increase of the current by interference, and unravelling molecular and graphene transport pathways. These results demonstrate a platform for the use of interferometric effects in single-molecule junctions, opening up new avenues for studying quantum coherence in molecular electronic and spintronic devices., Comment: 14 pages, 3 figures

It is known that the electrical conductance of single molecules can be controlled in a deterministic manner by chemically varying their anchor groups to external electrodes., It is known that the electrical conductance of single molecules can be controlled in a deterministic manner by chemically varying their anchor groups to external electrodes. Here, by employing synthetic methodologies to vary the terminal anchor groups around aromatic anthracene cores, and by forming self-assembled monolayers (SAMs) of the resulting molecules, we demonstrate that this method of control can be translated into cross-plane SAM-on-gold molecular films. The cross-plane conductance of SAMs formed from anthracene-based molecules with four different combinations of anchors are measured to differ by a factor of approximately 3 in agreement with theoretical predictions. We also demonstrate that the Seebeck coefficient of such films can be boosted by more than an order of magnitude by an appropriate choice of anchor groups and that both positive and negative Seebeck coefficients can be realised. This demonstration that the thermoelectric properties of SAMs are controlled by their anchor groups represents a critical step towards functional ultra-thin-film devices for future molecular-scale electronics.

We report on the single-molecule electronic and thermoelectric properties of strategically chosen anthracene-based molecules with anchor groups capable of binding to noble metal substrates, such as gold and platinum. Specifically, we study the effect of different anchor groups, as well as quantum interference, on the electric conductance and the thermopower of gold/single-molecule/gold junctions and generally find good agreement between theory and experiment. All molecular junctions display transport characteristics consistent with coherent transport and a Fermi alignment approximately in the middle of the HOMO/LUMO gap. Single-molecule results are in agreement with previously reported thin-film data, further supporting the notion that molecular design considerations may be translated from the single- to the many-molecule devices. For combinations of anchor groups where one binds significantly more strongly to the electrodes than the other, the stronger anchor group appears to dominate the thermoelectric behaviour of the molecular junction. For other combinations, the choice of electrode material can determine the sign and magnitude of the thermopower. This finding has important implications for the design of thermoelectric generator devices, where both n- and p-type conductor are required for thermoelectric current generation.

Aiming at boosting the photocatalytic activities of rutile and anatase TiO2 surfaces, an in-depth investigation of stoichiometric and reduced rutile TiO2 (1 1 0) and anatase TiO2 (1 0 1) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. In this study, density functional theory (DFT) plus a Hubbard correction (U) is implemented to explore the geometric and electronic properties of such systems. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a fivefold coordinated (Ti5c) atom or a sixfold coordinated (Ti6c) atom, indicating improved surface activity. Depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further by introducing a single oxygen vacancy into the studied surfaces, which means that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficient photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (1 0 1) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO2 (1 1 0). Moreover, the adsorption of both bipyramidal and trapezoidal Ag5 AQCs on both TiO2 surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy as compared to the pristine TiO2. This detailed investigation provides insight into new mechanisms for enhancing photocatalytic efficiency of both rutile TiO2 (1 1 0) and anatase TiO2 (1 0 1) surfaces.

Sorting out organic molecules with high thermopower is essential for understanding molecular thermoelectrics. The intermolecular coupling offers a unique chance to enhance the thermopower by tuning the bandgap structure of molecular devices, but the investigation of intermolecular coupling in bulk materials remains challenging. Herein, we investigated the thermopower of diketopyrrolopyrrole (DPP) cored single-molecule junctions with different coupling strengths by varying the packing density of the self-assembled monolayers (SAM) using a customized scanning tunneling microscope break junction (STM-BJ) technique. We found that the thermopower of DPP molecules could be enhanced up to one order of magnitude with increasing packing density, suggesting that the thermopower increases with larger neighboring intermolecular interactions. The combined density functional theory (DFT) calculations revealed that the closely-packed configuration brings stronger intermolecular coupling and then reduces the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, leading to an enhanced thermopower. Our findings offer a new strategy for developing organic thermoelectric devices with high thermopower.

Ordinary Portland cement (OPC) is the binding element in concrete materials and, CO2 emissions associated with its manufacturing and use is about 8% of the world's CO2 emissions. The engineering properties of hardened concrete depend on the amount of the hydrate phases in OPC. If the growth of the hydrate phases could be increased, the performance of concrete would be significantly improved, and the consumption of OPC will be decreased, and its environmental footprint will be reduced. In this paper, we present a new green approach for controlling the growth of the hydrate phases in OPC using bio flakes composed of staked carrot-based two-dimensional (2D) nanosheets (CNSs) synthesized from carrot waste. Density-functional theory and reactive molecular dynamics (DFT-MD) simulations were carried out in conjunction with analytical characterization to examine the interfacial interaction between CNS with tricalcium silicate Ca3SiO5 (C3S), the main constituent of OPC and understand how they influence the growth of the hydrate phases in OPC. The DFT-MD simulations results show the 2D CNS dissolves due to its interfacial interaction with the highly reactive C3S, leading to a series of fast proton exchange in C3S. This in return accelerates the dissolution rate of C3S thereby amplifying the growth of the hydrate phases. The DFT-MD simulations also show that the dissolution of the 2D CNS creates new several organic compounds that enhance the mobility and dynamics of protons that further amplify the dissolution rate of C3S. The analytical results from scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and thermography analysis (TGA) and differential scanning calorimetry (DSC) show a significant growth of the hydrate products in OPC due to interfacial dissolution of C3S and some CNS thus, confirming the DFT-MD results. This work demonstrates that the growth of the hydrate products in OPC can be amplified by the addition of green and renewable 2D bio-based nanomaterials. This green approach provides a base for the design and development of low-carbon cementitious materials.

Ordinary Portland cement (OPC) is the binding element in concrete materials and, CO2 emissions associated with its manufacturing and use is about 8% of the world's CO2 emissions. When OPC is mixed with water, a large quantity of OPC particles remain unhydrated or partially hydrated. If the dissolution and setting of calcium silicate phases of OPC could be increased, the performance of concrete would be improved, and the consumption of OPC will be decreased and its environmental footprint will be reduced. In this paper, we examine the origin of increased dissolution of tricalcium silicate (C3S) and dicalcium silicate (C2S) phases as a result of their interfacial chemical interactions with newly developed 2D carrot-based bio nanosheets (CNSs). Density-functional theory calculations and reactive molecular dynamics (DFT-MD) simulations were combined with experimental characterization to uncover the molecular chemical phenomena at the CNS/cement interface and understand how they influence the hydration of cement. The DFT-MD simulations results reveal that the observed increase in the degree of hydration of cement is due to the chemical interaction at the CNS/C3S interface. The dissolution of CNS by the highly reactive C3S leads to a series of proton exchange with higher diffusion rate in C3S. This increases the thickness of the hydration shell by as much as 3 nm. The DFT-MD simulations also show that the dissolution of CNS creates new organic compounds that enhance the mobility and dynamics of protons to further promote the dissolution of C3S. The experimental results support the DFT-MD simulation results. They confirm the dissolution of some CNS layers and the subsequent increased dissolution of C3S. The experimental results also show that CNS increases the degree of hydration of C3S and C2S by 6.3% at a maximum CNS concentration of 0.4-wt%. This work demonstrates that the kinetics of reaction of CNS in the cement environment amplify the hydration of OPC for enhanced performance and provides a base for the development of cementitious materials with low carbon footprint using green and renewable 2D nanomaterials.

We study the quantum interference (QI) effects in three-terminal Andreev interferometers based on polyaromatic hydrocarbons (PAH's) under non-equilibrium conditions. The Andreev interferometer consists of a PAH coupled to two superconducting and one normal conducting terminals. We calculate the current measured in the normal lead as well as the current between the superconducting terminals under non-equilibrium conditions. We show that both the QI arising in the PAH cores and the bias voltage applied to a normal contact have a fundamental effect on the charge distribution associated with the Andreev Bound States (ABS's). QI can lead to a peculiar dependence of the normal current on the superconducting phase difference that was not observed in earlier studies of mesoscopic Andreev interferometers. We explain our results by an induced asymmetry in the spatial distribution of the electron- and hole-like quasiparticles. The non-equilibrium charge occupation induced in the central PAH core can result in a $\pi$ transition in the current-phase relation of the supercurrent for large enough applied bias voltage on the normal lead. The asymmetry in the spatial distribution of the electron- and hole-like quasiparticles might be used to split Cooper pairs and hence to produce entangled electrons in four terminal setups., Comment: 16 pages

The single molecule conductance of a conjugated molecular wire is electrochemically switched upon oxidising or reducing a central bispyrrolotetrathiafulvalene unit., As the field of unimolecular electronics develops, there is growing interest in the development of functionalised molecular wires, such as switches, which will allow for more complex molecular-scale circuits. To this end, a three redox state single molecule switch, 1, based on bis(pyrrolo)tetrathiafulvalene (BPTTF) has been designed, synthesised and investigated using scanning tunnelling microscopy break junction (STM-BJ) studies and quantum transport calculations. Oxidising the BPTTF unit increases its conjugation, which was anticipated to increase the molecular conductance of 1. By changing the redox state of 1 electrochemically it was possible to vary the single molecule conductance by more than an order of magnitude (from 10–5.2 G 0 to 10–3.8 G 0). Simulations afforded a qualitatively similar trend. An additional, higher conductance feature is present in most traces at junction sizes of around 2.0 nm – further extension affords the switchable lower conductance feature at junction sizes closer to the molecular length (ca. 3.0 nm). Analysis of the conductance traces shows that these two conductance features occur sequentially in nearly all junctions. This behaviour is attributed to an alternative initial junction conformation in which one or more of the BPTTF sulfur atoms acts as an anchoring group. This hypothesis is supported by a computational study of binding conformations and STM-BJ studies on a model compound, 2, with only one thiol anchor. Our results indicate that the redox properties of BPTTF make it an excellent candidate for use in single molecule switches.

The compounds and complexes 1,4‐C6H4(C≡C‐cyclo‐3‐C4H3S)2 (2), trans‐[Pt(C≡C‐cyclo‐3‐C4H3S)2(PEt3)2] (3), trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2(dppe)2] (4; dppe=1,2‐bis(diphenylphosphino)ethane) and trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2{P(OEt)3}4] (5) featuring the 3‐thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3–5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break‐junction (STM‐BJ) and STM‐I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM‐I(s) methods, and a higher feature by the STM‐BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi‐parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non‐resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2–5.

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on constructive QI effects within the core. We also demonstrate that for molecules with thiol anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ~50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step towards functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.

Controlling the orientation of molecular conductors on the electrode surfaces is a critical factor in the development of single-molecule conductors. In the current study, we used the scanning tunnelling microscopy-based break junction (STM-BJ) technique to explore ‘bare-bones’ tripodal molecular wires, employing different anchor groups (AGs) at the ‘top’ and ‘bottom’ of the tripod. The triarylphosphine tris(4-(methylthio)phenyl)phosphine and its corresponding phosphine sulfide showed only a single high conductance feature in the resulting 1- and 2-dimensional conductance histograms, whereas analogous molecules with fewer than three thiomethyl AGs did not show clear conductance features. Thus, by systematic molecular modifications and with the aid of supporting DFT calculations, the binding geometry, with respect to the surface, was elucidated.

Graphene provides a two-dimensional platform for contacting individual molecules, which enables transport spectroscopy of molecular orbital, spin, and vibrational states. Here we report single-electron tunneling through a molecule that has been anchored to two graphene leads. Quantum interference within the graphene leads gives rise to an energy-dependent transmission and fluctuations in the sequential tunnel-rates. The lead states are electrostatically tuned by a global back-gate, resulting in a distinct pattern of varying intensity in the measured conductance maps. This pattern could potentially obscure transport features that are intrinsic to the molecule under investigation. Using ensemble averaged magneto-conductance measurements, lead and molecule states are disentangled, enabling spectroscopic investigation of the single molecule.

Graphene and related two-dimensional (2D) materials possess outstanding electronic and mechanical properties, chemical stability and high surface area. However, to realize graphene’s potential for a range of applications in materials science and nanotechnology there is a need to understand and control the interaction of graphene with tailored high-performance surfactants designed to facilitate the preparation, manipulation and functionalization of new graphene systems. Here we report a combined experimental and theoretical study of the surface structure and dynamics on graphene of pyrene-oligoethylene glycol (OEG) -based surfactants, which have previously been shown to disperse carbon nanotubes in water. Molecular self-assembly of the surfactants on graphitic surfaces is experimentally monitored and optimized using a graphene coated quartz crystal microbalance in ambient and vacuum environments. Real-space nanoscale resolution nanomechanical and topographical mapping of sub-monolayer surfactant coverage, using ultrasonic and atomic force microscopies both in ambient and ultra-high vacuum, reveals complex, multi-length-scale self-assembled structures. Molecular dynamics simulations show that at the nanoscale these structures, on atomically-flat graphitic surfaces, are dependent upon the surfactant OEG chain length and are predicted to display a previously unseen class of 2D self-arranged ‘starfish’ micelles (2DSMs). Whilst three-dimensional micelles are well known for their widespread uses ranging from microreactors to drug-delivery vehicles, these 2DSMs possess the highly desirable and tunable characteristics of high surface affinity coupled with unimpeded mobility, opening up strategies for processing and functionalizing 2D materials.

Molecules due to their very small sizes, possess discrete energy levels and electrons can transmit from one side of the molecule to the other with high probability if their energy coincides with molecular energy levels. In the weak coupling limit such on-resonance electron transport is described by the simple Lorentzian-shaped Breit-Wigner formula. On the other hand, electrons with energy different than the molecular energy levels have to tunnel through the energy gap between two molecular energy levels (off resonance transmission). Consequently the electron transmission probability is much smaller than on resonance regime. Interesting phenomena including quantum interference could be observed in this regime at room temperature. In this thesis, I discuss both regimes though, my main aim is to introduce a new theory called ”mid- gap theory” to predict the conductance ratio between different connectivities driven by quantum interference (QI) in the tunneling regime. Both theory and experiment have focused primarily on elucidating the conditions for the appearance of constructive or destructive interference. In the simplest case, where electrons are injected at the Fermi energy EF of the electrodes, constructive QI arises when EF coincides with a delocalized energy level En of the molecule. Similarly a simple form of destructive QI occurs when EF coincides with the energy Eb of a bound state located on a pendant moiety. Unless energy levels are tuned by electrostatic, electrochemical or mechanical gating, molecules located within a junction rarely exhibit these types of QI, because EF is usually located in the HOMO-LUMO (H-L) gap. Furthermore few analytic formulas are available, which means that pre-screening of molecules often requires expensive numerical simulations. For this reason, discussions have often focused on conditions for destructive or constructive QI when EF is located at the centre of the H-L gap. In this thesis, based on a simple description of connectivity, I demonstrate that the conductance ratio between two different connectivities of a core molecule could be predicted simply by using the ratio between two magic numbers of the core molecule. This will be discussed in the chapters 4-6. This simple theory not only predicts conductance ratios, but it could be used also to propose new strategies for molecular electronic design and applications such as single molecule switches and thermoelectricity. In this thesis after an introduction to nano and molecular electronics, I discuss general ideas about nanoscale transport and the methods which could be applied to model nano and molecular scale devices. In chapter 3, on resonance transport is discussed. For a wide variety of molecules, the conductance G decays with length L as Aexp(−βL) and it is widely accepted that the attenuation coefficient β is determined by position of the Fermi energy of the electrodes relative to the energy gap of the molecular bridge, whereas the terminal anchor groups which bind the molecule to the electrodes contribute to A. In contrast with this expectation, in chapter 3, I demonstrate that gateway orbitals located on the anchor groups can significantly decrease the value of β, thereby creating a new design strategy for realizing low-conductance molecular wires. In chapters 4-6, I introduce mid-gap theory and drive a mid-gap ratio rule (MRR) which is an exact formula for conductance ratios of tight-binding representations of molecules in the weak coupling limit, when the Fermi energy is located at the centre of the HOMO-LUMO (H-L) gap. It does not depend on the size of the H-L gap and is independent of asymmetries in the contacts. I also show how conductance ratios change, when one of the carbon atoms within the parent polycyclic aromatic hydrocarbons (PAH) core is replaced by a heteroatom to yield a daughter molecule. I show that this heteroatom substitution could be used to enhance the conductance in a PAH molecule by several orders of magnitude. A good agreement between this new simple theory and experiment shows that, the MRR provides a useful tool to predict the conductances of PAH molecules prior to synthesis. Therefore it could be used to design molecules with desirable properties or to propose new molecular devices.

Using a first principles approach combined with analysis of heuristic tight-binding models, we examine the connectivity dependence of two forms of quantum interference in single molecules. Based on general arguments, Fano resonances are shown to be insensitive to connectivity, while Mach-Zehnder-type interference features are shown to be connectivity dependent. This behaviour is found to occur in molecular wires containing anthraquinone units, in which the pendant carbonyl groups create Fano resonances, which coexist with multiple-path quantum interference features.

The compounds and complexes 1,4-CH(C≡C-cyclo-3-CHS) (2), trans-[Pt(C≡C-cyclo-3-CHS)(PEt)] (3), trans-[Ru(C≡C-cyclo-3-CHS)(dppe)] (4; dppe=1,2-bis(diphenylphosphino)ethane) and trans-[Ru(C≡C-cyclo-3-CHS){P(OEt)}] (5) featuring the 3-thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3–5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break-junction (STM-BJ) and STM-I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM-I(s) methods, and a higher feature by the STM-BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi-parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non-resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2–5., C.J.L. and O.A.A. acknowledge financial support from the Ministry of Higher Education and Scientific Research of Iraq. C.J.L. acknowledges funding from the EU through the FP7 ITN MOLESCO (project number 212942) and EPSRC (EP/N017188/1, EP/M014452/1 and EP/N03337X/1). P. J.L. holds an ARC Future Fellowship (FT120100073) and gratefully acknowledge sfunding for this work from the ARC (DP140100855,LE15100148). S.B.holds an International Postgraduate Research Scholarship and gratefully acknowledges support from the University of Western Australia. S.G.E. held a Durham Doctoral Scholarship, further supported by the University of Western Australia. T.A.(EP/N032977/1) and R.J.N. and S.J.H. (EP/H035184/1 and EP/K007785/1) thank the EPSRC for funding. M.L.,N.J.L. and T.A. acknowledge the Leverhulme Trust (RPG 2012-754and RPG2014-225) for funding. H.M.O. is grateful for financial assistance from the Secretaría Nacional de Educación Superior, Ciencia,Tecnología e Innovación from Ministerio de Educación (Ecuador). P. C. and S.M. are grateful for financial assistance from Ministerio de Economia y Competitividad from Spain and fondos FEDER in the framework of projects CTQ2012-33198 and CTQ2013-50187-EXP. S.M.and P. C. also acknowledge DGA and fondos FEDER for funding the research group Platón(E-54).

Provided the electrical properties of electro-burnt graphene junctions can be understood and controlled, they have the potential to underpin the development of a wide range of future sub-10nm electrical devices. We examine both theoretically and experimentally the electrical conductance of electro-burnt graphene junctions at the last stages of nanogap formation. We account for the appearance of a counterintuitive increase in electrical conductance just before the gap forms. This is a manifestation of room-temperature quantum interference and arises from a combination of the semi-metallic band structure of graphene and a crossover from electrodes with multiple-path connectivity to single-path connectivity just prior to breaking. Therefore our results suggest that conductance enlargement prior to junction rupture is a signal of the formation of electro-burnt junctions, with a pico-scale current path formed from a single sp2-bond.

Experimental correlation analysis and first-principles theory are used to probe the structure and evolution of Ag-CO-Ag single-molecule junctions, both before the formation, and after the rupture of the junctions. Two dimensional correlation histograms and conditional histograms demonstrate that prior to the single-molecule bridge configuration the CO molecule is already bound parallel to the Ag single-atom contact. This molecular precursor configuration is accompanied by the opening of additional conductance channels compared to the single-channel transport in pure Ag monoatomic junctions. To investigate the post-rupture evolution of the junction we introduce a cross-correlation analysis between the opening and the subsequent closing conductance traces. This analysis implies that the molecule is bound rigidly to the apex of one electrode, and so the same single-molecule configuration is re-established as the junction is closed. The experimental results are confirmed by ab initio simulations of the evolution of contact geometries, transmission eigenvalues and scattering wavefunctions.

We investigate the molecular sensing properties of carbon nanotube-boron nitride-carbon nanotube (CNT-BN-CNT) junctions. We demonstrate that the electrical conductance of such a junction changes in response to the binding of an analyte molecule to the region of BN. The change in conductance depends on the length of the BN spacer and the position of the analyte and therefore we propose a method of statistically analysing conductance data. We demonstrate the ability to discriminate between analytes, by computing the conductance changes due to three analytes (benzene, thiol-capped oligoyne and a pyridyl-capped oligoyne) binding to junctions with five different lengths of BN spacer.

We demonstrate how redox control of intra-molecular quantum interference in phase-coherent molecular wires can be used to enhance the thermopower (Seebeck coefficient) S and thermoelectric figure of merit ZT of single molecules attached to nanogap electrodes. Using first principles theory, we study the thermoelectric properties of a family of nine molecules, which consist of dithiol-terminated oligo(phenylene-ethynylenes) (OPEs) containing various central units. Uniquely, one molecule of this family possesses a conjugated acene-based central backbone attached via triple bonds to terminal sulfur atoms bound to gold electrodes and incorporates a fully conjugated hydroquinone central unit. We demonstrate that both S and the electronic contribution ZelT to the figure of merit ZT can be dramatically enhanced by oxidizing the hydroquinone to yield a second molecule, which possesses a cross-conjugated anthraquinone central unit. This enhancement originates from the conversion of the pi-conjugation in the former to cross-conjugation in the latter, which promotes the appearance of a sharp anti-resonance at the Fermi energy. Comparison with thermoelectric properties of the remaining seven conjugated molecules demonstrates that such large values of S and ZelT are unprecedented. We also evaluate the phonon contribution to the thermal conductance, which allows us to compute the full figure of merit ZT = ZelT/(1 + \k{appa}p/\k{appa}el), where \k{appa}p is the phonon contribution to the thermal conductance and \k{appa}el is the electronic contribution. For unstructured gold electrodes, \k{appa}p/\k{appa}el >> 1 and therefore strategies to reduce \k{appa}p are needed to realise the highest possible figure of merit.

We demonstrate the potential of perylene bisimide (PBI) for label-free sensing of organic molecules by investigating the change in electronic properties of five symmetric and asymmetric PBI derivatives, which share a common backbone, but are functionalised with various bay-area substituents. Density functional theory was combined with a Greens function scattering approach to compute the electrical conductance of each molecule attached to two gold electrodes by pyridyl anchor groups. We studied the change in their conductance in response to the binding of three analytes, namely TNT, BEDT-TTF and TCNE, and found that the five different responses provided a unique fingerprint for the discriminating sensing of each analyte. This ability to sense and discriminate was a direct consequence of the extended �� system of the PBI backbone, which strongly binds the analytes, combined with the different charge distribution of the five PBI derivatives, which leads to a unique electrical response to analyte binding., J. Mater. Chem. C, 2015

Congreso celebrado en Hungria del 13 al 17 de junio de 2011.-- et al.

Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH2, and CN) at a solid/liquid interface. The combination of current–distance and current–voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH2 > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively.

In this paper we use ab initio density functional theory (DFT) to calculate the electronic properties of C_50Cl_10. In comparison with the unstable C_50 which has a small t_lu(LUMO) - h_u(HOMO) energy gap and a high total free energy compared with C_60, the belt of chlorines atoms stabilize the C_50Cl_10 fullerene by increasing the energy gap to approximately that of C_60 and lowering the total free energy. We also examine the effects of inter-cage separation on the band structure for infinite periodic C_50Cl_10 chains where a high degree of dispersion is found to persist for separations beyond the predicted C_60 - C_60 distance of closest approach., 2pages, 2 figures, 1 table

If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons (PAHs) could be isolated and manipulated, then a significant step towards realizing the potential of single-molecule electronics would be achieved. Here we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO-LUMO gap (referred to as M-functions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers.

Ordinary Portland cement (OPC) is the binding element in concrete materials and, CO2 emissions associated with its manufacturing and use is about 8% of the world's CO2 emissions. When OPC is mixed with water, a large quantity of OPC particles remain unhydrated or partially hydrated. If the dissolution and setting of calcium silicate phases of OPC could be increased, the performance of concrete would be improved, and the consumption of OPC will be decreased and its environmental footprint will be reduced. In this paper, we examine the origin of increased dissolution of tricalcium silicate (C3S) and dicalcium silicate (C2S) phases as a result of their interfacial chemical interactions with newly developed 2D carrot-based bio nanosheets (CNSs). Density-functional theory calculations and reactive molecular dynamics (DFT-MD) simulations were combined with experimental characterization to uncover the molecular chemical phenomena at the CNS/cement interface and understand how they influence the hydration of cement. The DFT-MD simulations results reveal that the observed increase in the degree of hydration of cement is due to the chemical interaction at the CNS/C3S interface. The dissolution of CNS by the highly reactive C3S leads to a series of proton exchange with higher diffusion rate in C3S. This increases the thickness of the hydration shell by as much as 3 nm. The DFT-MD simulations also show that the dissolution of CNS creates new organic compounds that enhance the mobility and dynamics of protons to further promote the dissolution of C3S. The experimental results support the DFT-MD simulation results. They confirm the dissolution of some CNS layers and the subsequent increased dissolution of C3S. The experimental results also show that CNS increases the degree of hydration of C3S and C2S by 6.3% at a maximum CNS concentration of 0.4-wt%. This work demonstrates that the kinetics of reaction of CNS in the cement environment amplify the hydration of OPC for enhanced performance and provides a base for the development of cementitious materials with low carbon footprint using green and renewable 2D nanomaterials.

When a molecule is bound to external electrodes by terminal anchor groups, the latter are of paramount importance in determining the electrical conductance of the resulting molecular junction. Here we explore the electrical properties of a molecule with bidentate anchor groups, namely 4,4′-(1,4-phenylenebis(ethyne-2,1-diyl))bis(pyridin-2-amine), in both large area devices and at the single molecule level. We find an electrical conductance of 0.6 × 10−4G0 and 1.2 × 10−4G0 for the monolayer and for the single molecule, respectively. These values are approximately one order of magnitude higher than those reported for monodentate materials having the same molecular skeleton. A combination of theory and experiments is employed to understand the conductance of monolayer and single molecule electrical junctions featuring this new multidentate anchor group. Our results demonstrate that the molecule has a tilt angle of 30° with respect to the normal to the surface in the monolayer, while the break-off length in the single molecule junction occurs for molecules having a tilt angle estimated as 40°, which would account for the difference in their conductance values per molecule. The bidentate 2-aminepyridine anchor is of general interest as a contact group, since this terminal functionalized aromatic ring favours binding of the adsorbate to the metal contact resulting in enhanced conductance values., P. C. and J. L. S. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain and fondos FEDER in the framework of projects MAT2016-78257-R, CTQ2015-70174-P and PGC2018-097583-B-I00. J. L. S. also acknowledges the funded project Hierarchical Self Assembly of Polymeric Soft Systems, “SASSYPOL”, from the 7th Framework Programme (CEE, Ref-607602). L. H., S. M., J. L. S, and P. C. acknowledge support from DGA/Fondos FEDER (construyendo Europa desde Aragón) for funding PLATON (E31_17R) and CLIP (E47_17R) research groups. R. J. N. thanks EPSRC for funding (EP/M029522/1, EP/M005046/1 and EP/K007785/1). A. K. I. and C. J. L. acknowledge financial support from the UK EPSRC, through grant no. EP/M014452/1, EP/P027156/1 and EP/N03337X/1. This work was additionally supported by the European Commission is provided by the FET Open project 767187 – QuIET and the EU project Bac-to-Fuel. A. K. I. is grateful for financial assistance from Tikrit University (Iraq), and the Iraqi Ministry of Higher Education (SL-20).

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.

Η παρούσα διδακτορική διατριβή ασχολείται κυρίως με την επίδραση της αταξίας σε συγκεκριμένα συστήματα της Φυσικής Συμπυκνωμένης ‘Υλης. Η ισχυρή αταξία οδηγεί στο φαινόμενο του εντοπισμού Anderson όπου η κυματοσυνάρτηση περιορίζεται σε μία πεπερασμένη περιοχή του χώρου και χαρακτηρίζεται από εκθετική μείωση του πλάτους της. Πρώτον, μελετήθηκε η κατάσταση με ενέργεια Ε=0 σε ένα πλέγμα παρουσία αταξίας στους δεσμούς μεταξύ των ατόμων στη μία και στις δύο διαστάσεις. Με χρήση καθιερωμένων μεθόδων για τη στατιστική των ενεργειών και των καταστάσεων δείχνουμε μία άρτια-περιττή ασυμμετρία για το μέγεθος του συστήματος Ν και μία πολυμορφοκλασματική (multifractal) συμπεριφορά της ειδικής κατάστασης για ενέργεια Ε=0. Δεύτερον, εξετάζουμε το διπλό εκφυλισμό της ενέργειας Ε=0 και τις δύο καταστάσεις Majorana στην περίπτωση ενός τοπολογικού υπεραγωγού στη μία διάσταση. Απουσία αταξίας οι δύο καταστάσεις Majorana είναι εντοπισμένες στα δύο άκρα του μονοδιάστατου συστήματος. Στη συνέχεια μελετήθηκε η επίδραση της αταξίας στο συγκεκριμένο σύστημα που έχει προταθεί για χρήση σε κβαντικούς υπολογιστές. Παρουσία αταξίας εμφανίστηκε μία επέκταση των καταστάσεων στο εσωτερικό του συστήματος. Τρίτον, δείχνουμε το συνδυασμό αταξίας και αλληλεπιδράσεων σε ένα σύστημα πολλών σωματίων και συγκεκριμένα στον κβαντικό ΧΧΖ αντι-σιδηρομαγνήτη Heisenberg. Το βασικό κίνητρο στο πρόβλημα αυτό είναι να μελετηθεί με ποιο τρόπο η παρουσία αταξίας οδηγεί στο φαινόμενο του Εντοπισμού Πολλών Σωματίων (ή Many-Body Localization). Η στατιστική των ενεργειών έδειξε έναν διαχωρισμό μεταξύ μίας εργοδικής φάσης και μίας many-body εντοπισμένης φάσης. Η στατιστική των καταστάσεων φανέρωσε μια πολυμορφοκλασματική συμπεριφορά κοντά στην κρίσιμη περιοχή. Για ισχυρή αταξία, οι many-body εντοπισμένες καταστάσεις μπορούν δυνητικά να χρησιμοποιηθούν για κβαντικές μνήμες. Η σημασία αυτής της εργασίας είναι πως η παρουσία αταξίας στα κβαντικά συστήματα δεν έχει πάντα μία αρνητική επίδραση και μπορεί ενδεχομένως να οδηγήσει σε καλύτερη διαχείριση της κβαντικής πληροφορίας. The work in this thesis primarily adresses the effect of disorder in certain condensed matter systems. Disorder itself leads to the phenomenon of Anderson localization where the wavefunction due to disorder is confined in a finite region of space characterized by an exponential decay of its amplitude. First, we study the midgap state at energy E=0 in a lattice with hopping disorder in one and two dimensions. Using the well established methods of energy and wavefunction statistics, we show an even-odd system size asymmetry and the multifractal behavior of the special E=0 state. Second, we examine the doubly degenerate Majorana states at E=0 for the case of a topological superconductor in one dimension. In the absence of disorder, the two states are localized at the two ends of the chain. We ask how the disorder affects their behavior for this kind of system proposed for quantum computation. In the presence of disorder, a spreading of the Majorana states in the lattice occurs. Third, we show the interplay of disorder and interactions in a many-body system, namely the quantum XXZ Heisenberg anti-ferromagnet. The motivation is to study how disorder leads to the phenomenon of Many-Body Localization. The energy level statistics shows a distinction between an ergodic and a many-body localized phase and the eigenstate statistics reveals a multifractal behavior near the critical regime. For strong disorder the many-body localized states can be used as potential quantum memories. The significance of this work is that the ubiquitous presence of disorder in quantum systems has not always a negative impact and can lead to a better manipulation of quantum information. 3, xii, 90 σ.

This dissertation describes our recent efforts on the electrical characterization of graphene based molecular junctions. The properties of the junctions depend on both the graphene nanogap characteristics and the molecular structure. In this thesis, we study the different steps of making graphene-based molecular junctions.